publications

Submitted

1. 75

   Design principle for anode stable solid-state electrolytes Tan-Lien Pham, Lin Wang, Bin Ouyang* , Submitted
2. 74

   Charting the Electronic Structure for Discovering Low-cost Intermetallic Catalysts Zhengda He, Bin Ouyang* , Submitted

2024

1. 73

   Phase selection rules of multi-principal element alloys Lin Wang, Bin Ouyang* Advanced Materials, 2024 [Abs] [HTML] [PDF]

   Computational prediction of phase stability of multi-principal element alloys (MPEAs) holds a lot of promise for rapid exploration of the enormous design space and autonomous discovery of superior structural and functional properties. Regardless of many plausible works that rely on phenomenological theory and machine learning, precise prediction is still limited by insufficient data and the lack of interpretability of some machine learning algorithms, e.g., convolutional neural network. In this work, we have presented a comprehensive approach, encompassing the development of a complete dataset that contains 72,387 density functional theory calculations, as well as a predictive global phenomenological descriptor. The phase selection descriptor, based on atomic electronegativity and valence electron concentration, significantly outperforms the widely used valence electron concentration, excelling in both accuracy (with an F1 score of 63% compared to 47%) and its ability to predict the HCP phase (0.48 recall compared to 0). The comprehensive data mining on the global design space of 61,425 quaternary MPEAs made from 28 possible metals, together with the phenomenological theory and physical interpretation, will set up a solid computational science foundation for data-driven exploration of MPEAs.

2. 72

   Design Principle of Carbon-Supported Single-Atom Catalysts – Interplay between d-Orbital Periodicity and Local Hybridization Zhengda He, Jingyang Wang*, Bin Ouyang* Chemistry of Materials, 2024 [Abs] [HTML] [PDF]

   Carbon-based single-atom catalysts (SACs) have been widely investigated as a potential alternative for noble-metal-based catalysts for the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR). The rational design of such catalysts requires not only physical intuitions but also practical descriptors that can be directly applied in experiments. In this work, we establish a practical theoretical framework based on a comprehensive data set of SACs compromising 28 metals, 5 types of local environments, and adsorption calculations for 4 adsorbates (e.g., H/O/OH/OOH). We disentangle the complex trend of H/OH adsorption as an interplay between d-orbital periodicity and local hybridization, allowing for the estimation of the catalytic performance solely on the basis of the number of valence electrons. By utilizing this data set and theoretical framework, we have also identified several promising catalyst candidates and overlooked design strategies.

3. 71

   Selective formation of metastable polymorphs in solid-state synthesis Yan Zeng, Nathan Szymanski, Tanjin He, KyuJung Jun, Leighanne Gallington, Haoyan Huo, Christopher Bartel, Bin Ouyang, Gerbrand Ceder* Science Advances, 2024 [Abs] [HTML] [PDF]

   Metastable polymorphs often result from the interplay between thermodynamics and kinetics. Despite advances in predictive synthesis for solution-based techniques, there remains a lack of methods to design solid-state reactions targeting metastable materials. Here, we introduce a theoretical framework to predict and control polymorph selectivity in solid-state reactions. This framework presents reaction energy as a rarely used handle for polymorph selection, which influences the role of surface energy in promoting the nucleation of metastable phases. Through in situ characterization and density functional theory calculations on two distinct synthesis pathways targeting LiTiOPO4, we demonstrate how precursor selection and its effect on reaction energy can effectively be used to control which polymorph is obtained from solid-state synthesis. A general approach is outlined to quantify the conditions under which metastable polymorphs are experimentally accessible. With comparison to historical data, this approach suggests that using appropriate precursors could enable the synthesis of many novel materials through selective polymorph nucleation.

4. 70

   Promoting Fast Ion Conduction in Li-Argyrodite through Lithium Sublattice Engineering Po-Hsiu Chien, Bin Ouyang*, Xuyong Feng*, Lei Dong, David Mitlin, Jagjit Nanda, Jue Liu* Chemistry of Materials, 2024 [Abs] [HTML] [PDF]

   Fundamental understanding of ionic transport plays a pivotal role in designing and optimizing fast ionic conductors. Here, through a systematic neutron scattering and theoretical investigation, we discovered new insights about how anion sublattice affects Li+ distribution and transport in Li-argyrodite. We found that the promotion of Li+ conductivities is strongly correlated with a previously overlooked Li+ interstitial site (16e), which is critical for realizing intercage Li+ migration. More isotropic Li+ migration pathways with higher Li+ occupancies on the interstitial 16e site are found to be the underlying reason for the much higher Li+ conductivity in Li6PS5Cl relative to the Br- and I-based analogues. We further confirm that they are also the universal driving force for the ultrahigh Li+ conductivities in both anion-substituted Li-poor (Li6–aPS5–aXa, X = Cl and Br) and cation-substituted Li-rich argyrodite (e.g., Li6+aGeaP1–aS5I and Li6+aSiaSb1–aS5I). It is expected this strategy can be generally adopted to improve the ionic conductivity of the broad family of Li-rich argyrodite and beyond.

5. 69

   The rise of high-entropy battery materials Bin Ouyang*, Yan Zeng* Nature Communications, 2024 [Abs] [HTML] [PDF]

   The emergence of high-entropy materials has inspired the exploration of novel materials in diverse technologies. In electrochemical energy storage, high-entropy design has shown advantageous impacts on battery materials such as suppressing undesired short-range order, frustrating energy landscape, decreasing volumetric change and reducing the reliance on critical metals. This comment addresses the definition and potential improper use of the term “high entropy” in the context of battery materials design, highlights the unique properties of high-entropy materials in battery applications, and outlines the remaining challenges in the synthesis, characterization, and computational modeling of high-entropy battery materials.

6. 68

   Unlocking Li superionic conductivity in face-centred cubic oxides via face-sharing configurations Yu Chen, Zhengyan Lun, Xinye Zhao, Krishna Koirala, Linze Li, Yingzhi Sun, Christopher O’Keefe, Xiaochen Yang, Zijian Cai, Chongmin Wang, Huiwen Ji*, Clare Grey*, Bin Ouyang*, Gerbrand Ceder* Nature Materials, 2024 [Abs] [HTML] [PDF]

   The emergence of high-entropy materials has inspired the exploration of novel materials in diverse technologies. In electrochemical energy storage, high-entropy design has shown advantageous impacts on battery materials such as suppressing undesired short-range order, frustrating energy landscape, decreasing volumetric change and reducing the reliance on critical metals. This comment addresses the definition and potential improper use of the term “high entropy” in the context of battery materials design, highlights the unique properties of high-entropy materials in battery applications, and outlines the remaining challenges in the synthesis, characterization, and computational modeling of high-entropy battery materials.

2023

1. 67

   Atomic-scale probing of short-range order and its impact on electrochemical properties in cation-disordered oxide cathodes Linze Li, Bin Ouyang*, Zhengyan Lun, Haoyan Huo, Dongchang Chen, Yuan Yue, Colin Ophus, Wei Tong, Guoyin Chen, Gerbrand Ceder*, Chongmin Wang* Nature Communications, 2023 [Abs] [HTML] [PDF]

   Earth-abundant cathode materials are urgently needed to enable scaling of the Li-ion industry to multiply terawatt hours of annual production, necessitating reconsideration of how good cathode materials can be obtained. Irreversible transition metal migration and phase transformations in Li-ion cathodes are typically believed to be detrimental because they may trigger voltage hysteresis, poor kinetics and capacity degradation. Here we challenge this conventional consensus by reporting an unusual phase transformation from disordered Li- and Mn-rich rock salts to a new phase (named δ), which displays partial spinel-like ordering with short coherence length and exhibits high energy density and rate capability. Unlike other Mn-based cathodes, the δ phase exhibits almost no voltage fade upon cycling. We identify the driving force and kinetics of this in situ cathode formation and establish design guidelines for Li- and Mn-rich compositions that combine high energy density, high rate capability and good cyclability, thereby enabling Mn-based energy storage.

2. 66

   Weak Correlation between the Polyanion Environment and Ionic Conductivity in Amorphous Li–P–S Superionic Conductors Byungju Lee, KyuJung Jun, Bin Ouyang, Gerbrand Ceder* Chemistry of Materials, 2023 [Abs] [HTML] [PDF]

   Amorphous Li–P–S materials have been widely used as solid-state electrolytes for all-solid-state batteries because of their high ionic conductivity (10–4 to 10–3 S cm–1) as well as good synthetic accessibility and processability. Despite the potential of these materials, their amorphous structures have made it challenging to quantify the relation between the structure and conductivity. In this paper, we use ab initio molecular dynamics simulations to investigate the role of the local structure and density in determining the conductivity of amorphous Li–P–S structures with different polyanion units. We observe similar rates for Li-ion hopping regardless of the local P–S polyanion environment in these amorphous materials, indicating that the path connectivity at a larger length scale may be controlling the overall Li conductivity. This finding will serve as an important guideline in the continued development of amorphous solid electrolytes for advanced all-solid-state batteries.

3. 65

   Semigrand-canonical Monte-Carlo simulation methods for charge-decorated cluster expansions Fengyu Xie, Peichen Zhong, Luis Barroso-Luque, Bin Ouyang, Gerbrand Ceder* Computational Materials Science, 2023 [Abs] [HTML] [PDF]

   Monte-Carlo sampling of lattice model Hamiltonians is a well-established technique in statistical mechanics for studying the configurational entropy of crystalline materials. When the species to be distributed on the lattice carry charge, the charge balance constraint on the overall system prohibits single-site Metropolis exchanges in MC. In this article, we propose two methods to perform MC sampling in the semigrand-canonical ensemble in the presence of a charge-balance constraint. The table-exchange method (TE) constructs small charge-conserving excitations, and the square-charge bias method (SCB) allows the system to temporarily drift away from charge neutrality. We illustrate the effect of internal hyper-parameters on the efficiency of these algorithms and suggest practical strategies on how to employ these algorithms in real applications.

4. 64

   Mitigating the High-Charge Detrimental Phase Transformation in LiNiO2 Using Doping Engineering Jianlin Chen, Bin Ouyang, Kristin Persson* ACS Energy Letters, 2023 [Abs] [HTML] [PDF]

   Cobalt-free layered LiNiO2 has gained increased interest due to the scarcity and high cost of cobalt. However, LiNiO2 suffers from poor cycling stability, which is mainly due to oxygen loss and structural instability, especially when operating at high voltages. Herein, we present a doping strategy to mitigate the detrimental O3-to-O1 phase transformation in LiNiO2 from first-principles calculations. Temperature–composition phase diagrams of pristine and doped Li1–xNiO2 are obtained using a cluster-expansion and Monte Carlo simulation approach. We investigate the effects of dopant oxidation states, sizes, and concentrations on the dopant distribution in LiNi1–yMyO2 (M = Sb, Ti, Si, Al, and Mg) as well as the phase transitions during delithiation. We find that introducing high-valence dopants with ionic radii similar to that of Ni3+ into LiNiO2 stabilizes the O3-phase cathode bulk structure at high charge. Our results provide a general guidance on using doping engineering to realize Ni-rich, Co-free cathodes for lithium-ion batteries.

5. 63

   Modeling Short-Range Order in Disordered Rocksalt Cathodes by Pair Distribution Function Analysis Nathan Szymanski, Zhengyan Lun, Jue Liu, Ethan Self, Christopher Bartel, Jagjit Nanda, Bin Ouyang*, Gerbrand Ceder* Chemistry of Materials, 2023 [Abs] [HTML] [PDF]

   Cobalt-free layered LiNiO2 has gained increased interest due to the scarcity and high cost of cobalt. However, LiNiO2 suffers from poor cycling stability, which is mainly due to oxygen loss and structural instability, especially when operating at high voltages. Herein, we present a doping strategy to mitigate the detrimental O3-to-O1 phase transformation in LiNiO2 from first-principles calculations. Temperature–composition phase diagrams of pristine and doped Li1–xNiO2 are obtained using a cluster-expansion and Monte Carlo simulation approach. We investigate the effects of dopant oxidation states, sizes, and concentrations on the dopant distribution in LiNi1–yMyO2 (M = Sb, Ti, Si, Al, and Mg) as well as the phase transitions during delithiation. We find that introducing high-valence dopants with ionic radii similar to that of Ni3+ into LiNiO2 stabilizes the O3-phase cathode bulk structure at high charge. Our results provide a general guidance on using doping engineering to realize Ni-rich, Co-free cathodes for lithium-ion batteries.

6. 62

   Data driven design of compositionally complex energy materials (special issue for rising star in computational materials science) Lin Wang, Zhengda He, Bin Ouyang* Computational Materials Science, 2023 [Abs] [HTML] [PDF]

   Compositionally complex materials have emerged as new frontier for sustainable energy storage and conversion. There are many unique features of compositional complex energy materials (CCEMs), include but not limited to less dependency of critical elements, the potential for enhanced ionic conduction, and the capability to prevent chemo-mechanical degradation. However, the design space of CCEMs is intricate due to higher dimensionality of compositional space, as well as convoluted interplay between long range order and short-range structures. This review aims at providing a concise summation of research frontier in CCEMs, covering aspects from synthesis, manipulation of local structures and property control. Given the rapid advancement of the battery field in recent years, this review will particularly emphasize on battery related CCEMs as a key area of representation. Furthermore, the common challenges and benefits of CCEMs will also be extrapolated to other fields of energy storage and conversion. Among the various prospects for utilizing data science in studying CCEMs, this work will primarily concentrate on the physical interpretation based on massive data generated by high-throughput computation. However, it will also encompass the cutting-edge progress of machine learning algorithms and their potential applications in the study of CCEMs.

7. 61

   Computational investigation of MAX as intercalation host for rechargeable aluminum-ion battery Lin Wang, Jingyang Wang*, Bin Ouyang* Advanced Energy Materials, 2023 [Abs] [HTML] [PDF]

   Layered carbides and their analogs with MAX phase (general formula AMn+1Xn) have emerged as promising candidates for energy storage and conversion applications. One frontier for energy storage is using MAX as an Al-ion intercalation electrode. Given that many MAXs have Al as the A sites, the structure can potentially serve as a stable host for Al intercalation. Here in this work, 425 ternary MAX Al-ion battery electrodes are computationally enumerated. Specifically, first principal phase diagram calculations are performed on the combinatorial space of 17 types of typical transition metals, five types of anions (C, N, B, Si, and P), three types of stoichiometries (n = 1, 2, and 3) and two types of layered stackings (α and β). Among all the ternary MAX materials, 44 candidates show reasonable synthetic accessibility, and six with extraordinary performance are predicted to be promising Al-ion battery electrodes. With the phase stability, and electrochemical performance (average voltage, theoretical capacity, energy density, and Al diffusion barrier), the work provides a comprehensive computational assessment of the great opportunities behind MAX-based Al-ion batteries.

8. 60

   BaTa0.5Li0.5O2.5: A New Highly Oxygen Deficient Perovskite Oxide-Ion Conductor with Disordered Oxygen Vacancies Jungu Xu*, Qin Cao, Lin Wang, Bin Ouyang*, Tianjie Wei, Jiazheng Hao, Jie Chen, Lunhua He*, Laijun Liu, Kevin Huang* Energy Advances, 2023 [Abs] [HTML] [PDF]

   High oxide-ion or proton conductivity in perovskite oxides has been well documented in the literature. Herein, we report on a highly oxygen deficient perovskite oxide BaTa0.5Li0.5O2.5 (BTLO) as a new oxide-ion conductor. The material exhibits a simple primitive cubic perovskite structure (S.G.: Pm m) with lattice parameter a = 4.1024(1) Å and completely disordered Ta and Li cations 3 on the B-site. The ionic conduction is predominated by oxide-ion over a wide range of oxygen partial pressure and temperature, with an ionic conductivity comparable to the state-of-the-art yttria-stabilized zirconia. Ab initio molecular dynamics (AIMD) simulations reveal that oxide-ion diffusion follows three-dimensional pathways, involving oxygen vacancy hopping along the edges of (Ta/Li)O4 tetrahedra and (Ta/Li)O6 octahedra. This work may inspire future work to discover new oxide-ion conductors in similar systems.

9. 59

   Design principles for NASICON super-ionic conductors Jingyang Wang, Tanjin He, Xiaochen Yang, Zijian Cai, Yan Wang, Valentina Lacivita, Haegyeom Kim, Bin Ouyang*, Gerbrand Ceder* Nature Communications, 2023 [Abs] [HTML] [PDF]

   Na Super Ionic Conductor (NASICON) materials are an important class of solid-state electrolytes owing to their high ionic conductivity and superior chemical and electrochemical stability. In this paper, we combine first-principles calculations, experimental synthesis and testing, and natural language-driven text-mined historical data on NASICON ionic conductivity to achieve clear insights into how chemical composition influences the Na-ion conductivity. These insights, together with a high-throughput first-principles analysis of the compositional space over which NASICONs are expected to be stable, lead to the successful synthesis and electrochemical investigation of several new NASICONs solid-state conductors. Among these, a high ionic conductivity of 1.2 mS cm−1 could be achieved at 25 °C. We find that the ionic conductivity increases with average metal size up to a certain value and that the substitution of PO4 polyanions by SiO4 also enhances the ionic conductivity. While optimal ionic conductivity is found near a Na content of 3 per formula unit, the exact optimum depends on other compositional variables. Surprisingly, the Na content enhances the ionic conductivity mostly through its effect on the activation barrier, rather than through the carrier concentration. These deconvoluted design criteria may provide guidelines for the design of optimized NASICON conductors.

10. 58

    Impact of the energy landscape on the ionic transport of disordered rocksalt cathodes Shashwat Anand, Bin Ouyang, Tina Chen, Gerbrand Ceder Physical Review Materials, 2023 [Abs] [HTML] [PDF]

    Traditional approaches to identify ion-transport pathways often presume an equal probability of occupying all hopping sites and focus entirely on finding the lowest migration barrier channels between them. Although this strategy has been applied successfully to solid-state Li battery materials, which historically have mostly been ordered frameworks, in the emerging class of disordered electrode materials some Li sites can be significantly more stable than others due to a varied distribution of transition metal (TM) environments. Using kinetic Monte Carlo simulations, we show that in such cation-disordered compounds only a fraction of the Li sites connected by the so-called low-barrier “0-TM” channels actually participate in Li diffusion. The Li-diffusion behavior through these sites, which are determined primarily by the voltage applied during Li extraction, can be captured using an effective migration barrier larger than that of the 0-TM barrier itself. The suppressed percolation due to cation disorder can decrease the ionic diffusion coefficient at room temperature by over two orders of magnitude.

11. 57

    Patterns Lead the Way to Far-from-equilibrium Materials Pamela Knoll*, Bin Ouyang*, Oliver Steinbock* Acs Physical Chemistry Au, 2023 [Abs] [HTML] [PDF]

    The universe is a complex fabric of repeating patterns that unfold their beauty in system-specific diversity. The periodic table, crystallography, and the genetic code are classic examples that illustrate how even a small number of rules generate a vast range of shapes and structures. Today, we are on the brink of an AI-driven revolution that will reveal an unprecedented number of novel patterns, many of which will escape human intuition and expertise. We suggest that in the second half of the 21st century, the challenge for Physical Chemistry will be to guide and interpret these advances in the broader context of physical sciences and materials-related engineering. If we succeed in this role, Physical Chemistry will be able to extend to new horizons. In this article, we will discuss examples that strike us as particularly promising, specifically the discovery of high-entropy and far-from-equilibrium materials as well as applications to origins-of-life research and the search for life on other planets.

12. 56

    In situ formed partially disordered phases as earth-abundant Mn-rich cathode materials Zijian Cai+, Bin Ouyang+, Han-Ming Hau, Tina Chen, Raynald Giovine, Krishna Koirala, Linze Li, Huiwen Ji, Yang Ha, Yingzhi Sun, Jianping Huang, Yu Chen, Vincent Wu, Wanli Yang, Chongmin Wang, Raphaële Clément, Zhengyan Lun*, Gerbrand Ceder Nature Energy, 2023 [Abs] [HTML] [PDF]

    Earth-abundant cathode materials are urgently needed to enable scaling of the Li-ion industry to multiply terawatt hours of annual production, necessitating reconsideration of how good cathode materials can be obtained. Irreversible transition metal migration and phase transformations in Li-ion cathodes are typically believed to be detrimental because they may trigger voltage hysteresis, poor kinetics and capacity degradation. Here we challenge this conventional consensus by reporting an unusual phase transformation from disordered Li- and Mn-rich rock salts to a new phase (named δ), which displays partial spinel-like ordering with short coherence length and exhibits high energy density and rate capability. Unlike other Mn-based cathodes, the δ phase exhibits almost no voltage fade upon cycling. We identify the driving force and kinetics of this in situ cathode formation and establish design guidelines for Li- and Mn-rich compositions that combine high energy density, high rate capability and good cyclability, thereby enabling Mn-based energy storage.

13. 55

    Inhibiting collective cation migration in Li-rich cathode materials as a strategy to mitigate voltage hysteresis Jianping Huang+, Bin Ouyang+, Yaqian Zhang, Liang Yin, Deok-Hwang Kwon, Zijian Cai, Zhengyan Lun, Guobo Zeng, Mahalingam Balasubramanian, Gerbrand Ceder* Nature Materials, 2023 [Abs] [HTML] [PDF]

    Lithium-rich cathodes are promising energy storage materials due to their high energy densities. However, voltage hysteresis, which is generally associated with transition metal migration, limits their energy efficiency and implementation in practical devices. Here we reveal that voltage hysteresis is related to the collective migration of metal ions, and that isolating the migration events from each other by creating partial disorder can create high-capacity reversible cathode materials, even when migrating transition metal ions are present. We demonstrate this on a layered Li-rich chromium manganese oxide that in its fully ordered state displays a substantial voltage hysteresis (>2.5 V) associated with collective transition metal migration into Li layers, but can be made to achieve high capacity (>360 mAh g−1) and energy density (>1,100 Wh kg−1) when the collective migration is perturbed by partial disorder. This study demonstrates that partially cation-disordered cathode materials can accommodate a high level of transition metal migration, which broadens our options for redox couples to those of mobile cations.

2022

1. 54

   Alternate Synthesis Method for High-Performance Manganese Rich Cation Disordered Rocksalt Cathodes Shripad Patil, Devendrasinh Darbar, Ethan Self, Thomas Malkowski, Vincent Wu, Raynald Giovine, Nathan Szymanski, Rebecca McAuliffe, Bo Jiang, Jong Keum, Krishna Koirala, Bin Ouyang, Katharine Page, Chongmin Wang, Gerbrand Ceder, Raphaële Clément, Jagjit Nanda Advanced Energy Materials, 2022 [Abs] [HTML] [PDF]

   Advances in solid-state batteries have primarily been driven by the discovery of superionic conducting structural frameworks that function as solid electrolytes. We demonstrate the ability of high-entropy metal cation mixes to improve ionic conductivity in a compound, which leads to less reliance on specific chemistries and enhanced synthesizability. The local distortions introduced into high-entropy materials give rise to an overlapping distribution of site energies for the alkali ions so that they can percolate with low activation energy. Experiments verify that high entropy leads to orders-of-magnitude higher ionic conductivities in lithium (Li)–sodium (Na) superionic conductor (Li-NASICON), sodium NASICON (Na-NASICON), and Li-garnet structures, even at fixed alkali content. We provide insight into selecting the optimal distortion and designing high-entropy superionic conductors across the vast compositional space.

2. 53

   Machine-learning rationalization and prediction of solid-state synthesis conditions Haoyan Huo, Christopher Bartel, Tanjin He, Amalie Trewartha, Alexander Dunn, Bin Ouyang, Anubhav Jain, Gerbrand Ceder* Chemistry of Materials, 2022 [Abs] [HTML] [PDF]

   There currently exist no quantitative methods to determine the appropriate conditions for solid-state synthesis. This not only hinders the experimental realization of novel materials but also complicates the interpretation and understanding of solid-state reaction mechanisms. Here, we demonstrate a machine-learning approach that predicts synthesis conditions using large solid-state synthesis datasets text-mined from scientific journal articles. Using feature importance ranking analysis, we discovered that optimal heating temperatures have strong correlations with the stability of precursor materials quantified using melting points and formation energies (, ). In contrast, features derived from the thermodynamics of synthesis-related reactions did not directly correlate to the chosen heating temperatures. This correlation between optimal solid-state heating temperature and precursor stability extends Tamman’s rule from intermetallics to oxide systems, suggesting the importance of reaction kinetics in determining synthesis conditions. Heating times are shown to be strongly correlated with the chosen experimental procedures and instrument setups, which may be indicative of human bias in the dataset. Using these predictive features, we constructed machine-learning models with good performance and general applicability to predict the conditions required to synthesize diverse chemical systems. Codes and data used in this work can be found at: https://github.com/CederGroupHub/s4.

3. 52

   Design principles for zero-strain Li-ion cathodes Xinye Zhao+, Yaosen Tian+, Zhengyan Lun, Zijian Cai, Tina Chen, Bin Ouyang*, Gerbrand Ceder* Joule, 2022 [Abs] [HTML] [PDF]

   Cycling of cathode materials for Li-ion batteries is often accompanied by a severe volume change, posing a challenge to the integrity of cathode particles and to the electrolyte/cathode interface in solid-state batteries. To enhance capacity retention, it is thus crucial to design materials that undergo small volume changes or even remain structurally invariant during electrochemical cycling. Here, we use well-calibrated firstprinciples calculations with controlled variables to systematically investigate the effect of transition metal chemistry, cation ordering, Li site occupancy, redox-inactive species, anion substitution, and cation migration on the volume change upon delithiation in cathodes with an FCC anion framework. Our findings point to the critical role of transition metal selection and crystal structure isotropy and show multiple other factors that can be used to lower the volume change upon Li removal. Combining first-principles calculations with cluster expansion Monte Carlo simulation in the Li+-V3+-Nb5+-O2–F- system, we identify and confirm experimentally Li1.3V0.4Nb0.3O2 and Li1.25V0.55Nb0.2O1.9F0.1 as nearly zero-strain cathodes. Our study establishes a fundamental understanding of the important physical descriptors that determine the dimensional change of materials during cycling and provides general guidelines for designing zero-strain cathodes.

4. 51

   Understanding the Fluorination of Disordered Rocksalt Cathodes through Rational Exploration of Synthesis Pathways Nathan Szymanski+, Yan Zeng+, Tyler Bennett, Shripad Patil, Jong Keum, Ethan Self, Jianming Bai, Zijian Cai, Raynald Giovine, Bin Ouyang, Feng Wang, Christopher Bartel, Raphaële Clément, Wei Tong, Jagjit Nanda, Gerbrand Ceder* Chemistry of Materials, 2022 [Abs] [HTML] [PDF]

   We have designed and tested several synthesis routes targeting a highly fluorinated disordered rocksalt (DRX) cathode, Li1.2Mn0.4Ti0.4O1.6F0.4, with each route rationalized by thermochemical analysis. Precursor combinations were screened to raise the F chemical potential and avoid the formation of LiF, which inhibits fluorination of the targeted DRX phase. MnF2 was used as a reactive source of F, and Li6MnO4, LiMnO2, and Li2Mn0.33Ti0.66O3 were tested as alternative Li sources. Each synthesis procedure was monitored using a multi-modal suite of characterization techniques including X-ray diffraction, nuclear magnetic resonance, thermogravimetric analysis, and differential scanning calorimetry. From the resulting data, we advance the understanding of oxyfluoride synthesis by outlining the key factors limiting F solubility. At low temperatures, MnF2 consistently reacts with the Li source to form LiF as an intermediate phase, thereby trapping F in strong Li−F bonds. LiF can react with Li2TiO3 to form a highly lithiated and fluorinated DRX (Li3TiO3F); however, MnO is not easily incorporated into this DRX phase. Although higher temperatures typically increase solubility, the volatility of LiF above its melting point (848 °C) inhibits fluorination of the DRX phase. Based on these findings, metastable synthesis techniques are suggested for future work on DRX fluorination.

5. 50

   Equilibrium Particle Shape and Surface Chemistry of Disordered Li-excess, Mn-rich Li-ion Cathodes through First-principles Modeling Jordan Burns*, Bin Ouyang, Jianli Cheng, Matthew Horton, Martin Siron, Oxana Andriuc, Ruoxi Yang, Gerbrand Ceder, Kristin Persson* Chemistry of Materials, 2022 [Abs] [HTML] [PDF]

   novel methodology for calculating the surface energy of a disordered material was developed and is described here. The method was used to calculate the range of surface energies for 100, 110, 111, and 112 type facets of the disordered rock salt (DRX) cathode material Li2MnO2F, as a function of surface cation and anion decoration. Boltzmann averaging was used to determine average surface energies for each facet which were then used to calculate the equilibrium particle shape. It was found that Li2MnO2F displays predominantly 100 type lithium/fluorine-rich facets favoring a cubic particle shape. The density of states along with electronic structure-based bonding analyses are calculated to rationalize differences observed in surface energy. Importantly, it is found that surface lithium and fluorine lower the surface energy of the majority facets, suggesting that surfaces of Li2MnO2F are likely enriched in lithium and fluorine and display less oxygen and manganese, which has implications for capacity and rate retention.

6. 49

   Enhanced ionic conductivity and lack of paddle-wheel effect in pseudohalogen-substituted Li argyrodites Yingzhi Sun+, Bin Ouyang+, Yan Wang, Zhang Yaqian, Shuo Sun, Zijian Cai, Valentina Lacivita, Yinsheng Guo, Gerbrand Ceder* Matter, 2022 [Abs] [HTML] [PDF]

   Superionic conductors are key to the development of safe and high-energy-density all-solidstate batteries. Using a combined theoretical and experimental approach, we explore the feasibility of increasing the ionic conductivity through pseudohalogen substitution in the Li argyrodite structure. Under the guidance of calculated thermodynamic stability, BH4- substituted Li argyrodite, Li5.91PS4.91(BH4)1.09, was successfully synthesized via a mechanochemical method. As-synthesized BH4-substituted Li argyrodite displays an ionic conductivity of 4.8 mS/cm at 25 oC. Ab initio molecular dynamics simulation trajectory analysis was used to investigate how BH4 facilitates Li-ion diffusion and indicates only a weak correlation with the B-H bond motion. We find that the enhanced conductivity mainly originates from the weak interaction between Li and BH4 and find no evidence of a paddle-wheel effect from the polyanion. This work provides insight on how cluster ions enhance Li diffusion and systematically describes how to explore superionic conductors with pseudohalogen substitution.

7. 48

   Cluster Expansions of Multicomponent Ionic Materials: Formalism & Methodology Luis Barroso-Luque*, Peichen Zhong, Yang Julia, Fengyu Xie, Tina Chen, Bin Ouyang, Gerbrand Ceder* Physical Review B, 2022 [Abs] [HTML] [PDF]

   The cluster expansion (CE) method has seen continuous and increasing use in the study of configuration-dependent properties of crystalline materials. The original development of the CE method along with the underlying mathematical formalism and assumptions was focused on the study of metallic alloys. The methodology has been actively and successfully used in the study of ionic materials as well. In this work, we present the mathematical formalism underlying the CE based on a synthesis of its original formulation and several additions and extensions that have been proposed since. We then proceed to describe some of the formal implications of using the methodology for charge neutral configurations in ionic systems. In particular, we discuss the reduction of the size of configuration spaces and the resulting linear dependencies that arise among correlation functions that span the larger unconstrained configuration space. Additionally, we explore the effects of long-range electrostatic interactions. We demonstrate how the previously proposed use of a point electrostatic term successfully accounts for the majority of the longer range electrostatic interactions, and leaves the cluster expansion terms to capture mostly short-range interactions. Finally, we present and discuss a variety of recently developed and novel methods, including training structure selection, oxidation state assignment, structure mapping, and regression algorithms, that are necessary to address these formal mathematical notions for a practical implementation of the CE method in the study of multi-component ionic materials.

8. 47

   High-entropy mechanism to boost ionic conductivity Yan Zeng+, Bin Ouyang+*, Jue Liu, Young-Woon Byeon, Zijian Cai, Lincoln Miara, Yan Wang, Gerbrand* Ceder Science, 2022 [Abs] [HTML] [PDF]

   Advances in solid-state batteries have primarily been driven by the discovery of superionic conducting structural frameworks that function as solid electrolytes. We demonstrate the ability of high-entropy metal cation mixes to improve ionic conductivity in a compound, which leads to less reliance on specific chemistries and enhanced synthesizability. The local distortions introduced into high-entropy materials give rise to an overlapping distribution of site energies for the alkali ions so that they can percolate with low activation energy. Experiments verify that high entropy leads to orders-of-magnitude higher ionic conductivities in lithium (Li)–sodium (Na) superionic conductor (Li-NASICON), sodium NASICON (Na-NASICON), and Li-garnet structures, even at fixed alkali content. We provide insight into selecting the optimal distortion and designing high-entropy superionic conductors across the vast compositional space.

9. 46

   Thermodynamically Driven Synthetic Optimization for Cation-Disordered Rock Salt Cathodes Zijian Cai+, Ya-Qian Zhang+, Zhengyan Lun+, Bin Ouyang, Leighanne Gallington, Yingzhi Sun, Han-Ming Hau, Yu Chen, Mary Scott*, Gerbrand Ceder* Advanced Energy Materials, 2022 [Abs] [HTML] [PDF]

   Relating the synthesis conditions of materials to their functional performance has long been an experience-based trial-and-error process. However, this methodology is not always efficient in identifying an appropriate protocol and can lead to overlooked opportunities for the performance optimization of materials through simple modifications of the synthesis process. In this work, the authors systematically track the structural evolution in the synthesis of a representative disordered rock salt (a promising next-generation Li-ion cathode material) at the scale of both the long-range crystal structure and the short-range atomic structure using various in situ and ex situ techniques, including transmission electron microscopy, X-ray diffraction, and pair distribution function analysis. An optimization strategy is proposed for the synthesis protocol, leading to a remarkably enhanced capacity (specific energy) of 313 mAh g−1 (987 Wh kg−1) at a low rate (20 mA g−1), with a capacity of more than 140 mAh g−1 retained even at a very high cycling rate of 2000 mA g−1. This strategy is further rationalized using ab initio calculations, and important opportunities for synthetic optimization demonstrated in this study are highlighted.

2021

1. 45

   Cation-disordered rocksalt-type high-entropy cathodes for Li-ion batteries Zhengyan Lun+, Bin Ouyang+, Deok-Hwang Kwon, Yang Ha, Emily E. Foley, Tzu-Yang Huang, Zijian Cai, Hyunchul Kim, Mahalingam Balasubramanian, Yingzhi Sun, Jianping Huang, Yaosen Tian, Haegyeom Kim, Bryan D. McCloskey, Wanli Yang, Raphaële J Clément, Huiwen Ji*, Gerbrand Ceder* Nature Materials, 2021 [Abs] [HTML] [PDF]

   High-entropy (HE) ceramics, by analogy with HE metallic alloys, are an emerging class of solid solutions composed of a large number of species. These materials offer the benefit of large compositional flexibility and can be used in a wide variety of applications, including thermoelectrics, catalysts, superionic conductors and battery electrodes. We show here that the HE concept can lead to very substantial improvements in performance in battery cathodes. Among lithium-ion cathodes, cation-disordered rocksalt (DRX)-type materials are an ideal platform within which to design HE materials because of their demonstrated chemical flexibility. By comparing a group of DRX cathodes containing two, four or six transition metal (TM) species, we show that short-range order systematically decreases, whereas energy density and rate capability systematically increase, as more TM cation species are mixed together, despite the total metal content remaining fixed. A DRX cathode with six TM species achieves 307 mAh g−1 (955 Wh kg−1) at a low rate (20 mA g−1), and retains more than 170 mAh g−1 when cycling at a high rate of 2,000 mA g−1. To facilitate further design in this HE DRX space, we also present a compatibility analysis of 23 different TM ions, and successfully synthesize a phase-pure HE DRX compound containing 12 TM species as a proof of concept.

2. 44

   Synthetic accessibility and stability rules of NASICONs type oxides Bin Ouyang+, Jingyang Wang+, Tanjin He, Christopher Bartel, Haoyan Huo, Yan Wang, Valentina Lacivita, Haegyeom Kim, Gerbrand Ceder* Nature Communications, 2021 [Abs] [HTML] [PDF]

   In this paper we develop the stability rules for NASICON-structured materials, as an example of compounds with complex bond topology and composition. By first-principles high-throughput computation of 3881 potential NASICON phases, we have developed guiding stability rules of NASICON and validated the ab initio predictive capability through the synthesis of six attempted materials, five of which were successful. A simple two-dimensional descriptor for predicting NASICON stability was extracted with sure independence screening and machine learned ranking, which classifies NASICON phases in terms of their synthetic accessibility. This machine-learned tolerance factor is based on the Na content, elemental radii and electronegativities, and the Madelung energy and can offer reasonable accuracy for separating stable and unstable NASICONs. This work will not only provide tools to understand the synthetic accessibility of NASICON-type materials, but also demonstrates an efficient paradigm for discovering new materials with complicated composition and atomic structure.

3. 43

   Computational and experimental search for potential polyanionic K-ion cathode materials Jingyang Wang+, Bin Ouyang+, Hyunchul Kim Kim, Yaosen Tian, Gerbrand Ceder*, Haegyeom Kim* Journal of Materials Chemistry A, 2021 [Abs] [HTML] [PDF]

   Discovering high-energy cathode materials is critical to constructing K-ion batteries for practical applications. Owing to the great success of layered oxides in Li- and Na-ion systems, K layered cathodes have also been investigated in recent years. However, the much larger size of K+ compared to Li or Na introduces strong K+–K+ interaction within the layer, which results in a sloped voltage profile thereby limiting the specific capacity and operating voltage. In contrast, polyanionic materials with a three-dimensional K+ arrangement can effectively mitigate K+–K+ interaction. In this work, ten K polyanionic compounds with theoretical capacity >100 mAh g−1 are screened from the Inorganic Crystal Structure Database as potential cathode materials for K-ion batteries. Among the ten proposed compounds, K2MnP2O7, K2Mn2P2O7F2, K2Fe2P2O7F2, and K6V2(PO4)4 with average voltage <4.5 V are synthesized and evaluated electrochemically. While the re-insertion of K into these compounds is not fully reversible, it may be related to the very high migration barrier that we compute for K ions. In addition, we show the successful synthesis of a series of K3V3−xCrx(PO4)4 (x = 0, 1, 2, 3) compounds. Among these, K3V2Cr(PO4)4 exhibits the largest reversible capacity, as revealed by the in situ investigation. Finally, we find that the redox couples in many of these compounds sit at remarkably high potential, even higher than in equivalent Li compounds, which brings both opportunities and challenges in the future research of K polyanion cathodes.

2020

1. 42

   Effect of Fluorination on Lithium Transport and Short-Range Order in Disordered-Rocksalt-Type Lithium-Ion Battery Cathodes Bin Ouyang+, Nongnuch Artrith+, Zhengyan Lun+, Zinab Jadidi, Daniil A Kitchaev, Huiwen Ji, Alexander Urban, Gerbrand Ceder* Advanced Energy Materials, 2020 [Abs] [HTML] [PDF]

   Fluorine substitution is a critical enabler for improving the cycle life and energy density of disordered rocksalt (DRX) Li‐ion battery cathode materials which offer prospects for high energy density cathodes, without the reliance on limited mineral resources. Due to the strong Li–F interaction, fluorine also is expected to modify the short‐range cation order in these materials which is critical for Li‐ion transport. In this work, density functional theory and Monte Carlo simulations are combined to investigate the impact of Li–F short‐range ordering on the formation of Li percolation and diffusion in DRX materials. The modeling reveals that F substitution is always beneficial at sufficiently high concentrations and can, surprisingly, even facilitate percolation in compounds without Li excess, giving them the ability to incorporate more transition metal redox capacity and thereby higher energy density. It is found that for F levels below 15%, its effect can be beneficial or disadvantageous depending on the intrinsic short‐range order in the unfluorinated oxide, while for high fluorination levels the effects are always beneficial. Using extensive simulations, a map is also presented showing the trade‐off between transition‐metal capacity, Li‐transport, and synthetic accessibility, and two of the more extreme predictions are experimentally confirmed.

2. 41

   Computational investigation of halogen-substituted Na argyrodites as solid-state superionic conductors Bin Ouyang, Yan Wang, Yingzhi Sun, Gerbrand Ceder* Chemistry of Materials, 2020 [Abs] [HTML] [PDF]

   The discovery of promising inorganic superionic conductors for use as solid-state electrolytes can enable the design of safe and high-energy-density solid-state batteries. Li argyrodites with the composition Li–P–S–X (X = Cl, Br, or I) have been found to have an ionic conductivity of up to 14.8 mS/cm, indicating the ability of the argyrodite framework to conduct alkali-metal ions. However, the sodium version of argyrodites remains unexplored. In this work, using first-principles density functional theory calculations, we examine the phase stability, electrochemical stability, and ionic conductivity of 48 potential Na argyrodites with different pnictogen, chalcogen, and halogen chemistries and site occupancies. We find that for sulfide-based Na argyrodites, the compounds with halogens occupying both the 4a and 4c sites typically display better ionic conductivity. However, the larger size of the Se2– anion makes selenide-based Na argyrodites less-dependent on the exact halogen-site occupancy. Most of the Na argyrodites have positive zero-K formation energy but are within the typical energy where compounds, including the Li argyrodite, can be synthesized. We find that compounds within the Na–P–Se–X (X = Cl, Br, or I) chemical space may form a good compromise between ionic conductivity, phase stability, and synthesizability.

3. 40

   Na+ redistribution by electrochemical Na+/K+ exchange in layered NaxNi2SbO6 Haegyeom Kim, Deok-Hwang Kwon, Jae Chul Kim, Bin Ouyang, Hyunchul Kim, Julia Yang, Gerbrand Ceder* Chemistry of Materials, 2020 [Abs] [HTML] [PDF]

   This work investigates the electrochemical Na+/K+ ion exchange mechanism occurring in layered Na3Ni2SbO6. Structural characterizations using X-ray diffraction and transmission electron microscopy uncover a remarkable and rich phase evolution as K is inserted in partially desodiated NaxNi2SbO6. Rather than simple addition of K to the structure, we see significant Na rearrangement, with discrete Na phases traversing the full range of Na compositions, even though the overall Na content in the sample is not changed. At any given time as much as three distinct phases can be present in the sample, consistent with thermodynamic equilibrium rules. Using DFT computations we demonstrate that this remarkable phase behavior during ion exchange is due to the repulsion between Na+ and K+ which creates distinct phases of them. Our analysis should be applicable to other ion-exchange systems where the exchanged ions are not well miscible in the host structure.

4. 39

   The interplay between thermodynamics and kinetics in the solid-state synthesis of layered oxides Matteo Bianchini, Jingyang Wang, Raphaële J Clément, Bin Ouyang, Penghao Xiao, Daniil Kitchaev, Tan Shi, Yaqian Zhang, Yan Wang, Haegyeom Kim, Gerbrand Ceder* Nature Materials, 2020 [Abs] [HTML] [PDF]

   In the synthesis of inorganic materials, reactions often yield non-equilibrium kinetic byproducts instead of the thermodynamic equilibrium phase. Understanding the competition between thermodynamics and kinetics is a fundamental step towards the rational synthesis of target materials. Here, we use in situ synchrotron X-ray diffraction to investigate the multistage crystallization pathways of the important two-layer (P2) sodium oxides Na0.67MO2 (M = Co, Mn). We observe a series of fast non-equilibrium phase transformations through metastable three-layer O3, O3′ and P3 phases before formation of the equilibrium two-layer P2 polymorph. We present a theoretical framework to rationalize the observed phase progression, demonstrating that even though P2 is the equilibrium phase, compositionally unconstrained reactions between powder precursors favour the formation of non-equilibrium three-layered intermediates. These insights can guide the choice of precursors and parameters employed in the solid-state synthesis of ceramic materials, and constitutes a step forward in unravelling the complex interplay between thermodynamics and kinetics during materials synthesis.

5. 38

   Promises and Challenges of Next-Generation “Beyond Li-ion” Batteries for Electric Vehicles and Grid Decarbonization Yaosen Tian+, Guobo Zeng+, Ann Rutt+, Tan Shi+, Haegyeom Kim+, Jingyang Wang, Julius Koettgen, Yingzhi Sun, Bin Ouyang, Tina Chen, Zhengyan Lun, Ziqin Rong, Kristin Persson*, Gerbrand Ceder* Chemical Reviews, 2020 [Abs] [HTML] [PDF]

   The tremendous improvement in performance and cost of lithium-ion batteries (LIBs) have made them the technology of choice for electrical energy storage. While established battery chemistries and cell architectures for Li-ion batteries achieve good power and energy density, LIBs are unlikely to meet all the performance, cost, and scaling targets required for energy storage, in particular, in large-scale applications such as electrified transportation and grids. The demand to further reduce cost and/or increase energy density, as well as the growing concern related to natural resource needs for Li-ion have accelerated the investigation of so-called “beyond Li-ion” technologies. In this review, we will discuss the recent achievements, challenges, and opportunities of four important “beyond Li-ion” technologies: Na-ion batteries, K-ion batteries, all-solid-state batteries, and multivalent batteries. The fundamental science behind the challenges, and potential solutions toward the goals of a low-cost and/or high-energy-density future, are discussed in detail for each technology. While it is unlikely that any given new technology will fully replace Li-ion in the near future, “beyond Li-ion” technologies should be thought of as opportunities for energy storage to grow into mid/large-scale applications.

6. 37

   Effect of interstitial oxygen and nitrogen on incipient plasticity of NbTiZrHf high-entropy alloys Youxiong Ye*, Bin Ouyang, Chenze Liu, Gerd Duscher, Tai-Gang Nieh* Acta Materialia, 2020 [Abs] [HTML] [PDF]

   In this work, instrumented nanoindentation was employed to investigate the effect of interstitial oxygen or nitrogen addition on the incipient plasticity and dislocation nucleation in a body-centered cubic NbTiZrHf high-entropy alloy (HEA) at loading rates of 10–1000 µN/s. We conducted quantitative statistical analysis and density functional theory (DFT) calculations to identify the role of interstitial oxygen/nitrogen during the onset of plasticity. Synchrotron X-ray diffraction and transmission electron microscopy were also performed to confirm that the oxygen/nitrogen atoms were indeed present as interstitial solutes. These interstitial solutes could increase the critical shear stress required to initiate plasticity, and nitrogen yielded a larger hardening effect than oxygen. The activation volumes were evaluated to be about 2–3 atomic volumes, indicating cooperative migration of multiple atoms during the dislocation nucleation, and neither oxygen nor nitrogen appeared to significantly affect this activation process. Hardness tests were also carried out and the result demonstrated that the enhancement of the critical shear stress for incipient plasticity was not caused by the traditional solid-solution strengthening mechanism. DFT calculations revealed that oxygen/nitrogen interstitials induced local charge transfer and improved the lattice cohesion, which was probably responsible for the enhanced pop-in load/stress in the current interstitially alloyed HEAs.

7. 36

   Increasing Capacity in Disordered Rocksalt Cathodes by Mg Doping Peichen Zhong+, Zijian Cai+, Yaqian Zhang, Raynald Giovine, Bin Ouyang, Guobo Zeng, Yu Chen, Raphaële Clément, Zhengyan Lun+, Gerbrand Ceder+ Chemistry of Materials, 2020 [Abs] [HTML] [PDF]

   Using both computations and experiments, we demonstrate that the performance of Li-excess cation-disordered rocksalt cathodes can be improved by Mg substitution. Mg reduces the amount of Li in the compound that is strongly bound to F and thereby increases the capacity. This enables the use of fluorination as a tool to improve stability of the compounds without significant loss of capacity. Mg emerged as the most optimal substitution element from a systematic computational study aimed at identifying inactive doping elements with a strong enough bonding strength to fluorine to displace Li from the F environments. Our results also show that capacity can be traded for cycle life depending on whether Mg is substituted for Li or for the redox metal. This design strategy should be considered in fluorinated cathodes, which will facilitate the design of optimized disordered rocksalt oxyfluoride cathodes.

2019

1. 35

   Improved Cycling Performance of Li-Excess Cation-Disordered Cathode Materials upon Fluorine Substitution Zhengyan Lun+, Bin Ouyang+, Daniil A Kitchaev, Raphaële J Clément, Joseph K Papp, Mahalingam Balasubramanian, Yaosen Tian, Teng Lei, Tan Shi, Bryan D McCloskey, Gerbrand Ceder* Advanced Energy Materials, 2019 [Abs] [HTML] [PDF]

   The recent discovery of Li‐excess cation‐disordered rock salt cathodes has greatly enlarged the design space of Li‐ion cathode materials. Evidence of facile lattice fluorine substitution for oxygen has further provided an important strategy to enhance the cycling performance of this class of materials. Here, a group of Mn3+–Nb5+‐based cation‐disordered oxyfluorides, Li1.2Mn3+0.6+0.5x Nb5+0.2−0.5x O2−x Fx (x = 0, 0.05, 0.1, 0.15, 0.2) is investigated and it is found that fluorination improves capacity retention in a very significant way. Combining spectroscopic methods and ab initio calculations, it is demonstrated that the increased transition‐metal redox (Mn3+/Mn4+) capacity that can be accommodated upon fluorination reduces reliance on oxygen redox and leads to less oxygen loss, as evidenced by differential electrochemical mass spectroscopy measurements. Furthermore, it is found that fluorine substitution also decreases the Mn3+‐induced Jahn–Teller distortion, leading to an orbital rearrangement that further increases the contribution of Mn‐redox capacity to the overall capacity.

2. 34

   Thermal Transport Engineering in Graphdiyne and Graphdiyne Nanoribbons Yingchun Wan, Shiyun Xiong, Bin Ouyang, Zhihui Niu, Yuxiang Ni, Yu Zhao, Xiaohong Zhang* ACS Omega, 2019 [Abs] [HTML] [PDF]

   Understanding the details of thermal transport in graphdiyne and its nanostructures would help to broaden their applications. On the basis of the molecular dynamics simulations and spectrally decomposed heat current analysis, we show that the high-frequency phonons in graphdiyne can be strongly hindered in nanoribbons because of the boundary scattering. The isotropic transport in graphdiyne can be switched to anisotropic along the armchair and zigzag directions. Adding side chains onto the nanoribbon edges further reduces the thermal conductivity (TC) along both armchair and zigzag directions thanks to the reduction of heat current carried by low-frequency modes, a mechanism that arises from the phonon resonances. The uniaxial tensile strain plays a different role in the TC of graphdiyne, armchair nanoribbons, and zigzag nanoribbons. Tensile strain causes the thermal conductivities of graphdiyne, and armchair nanoribbons increase first and then get reduced, whereas for zigzag nanoribbons, the TC decreases with strain first and reaches to a plateau. The different low-frequency phonon response on strain is the main reason for the different TC behavior. For graphdiyne and armchair nanoribbons, the low-frequency heat current is enhanced gradually first and then get reduced with the increase of strain, while that of zigzag nanoribbons decreases with strain and then increases slightly. The current studies could help us understand the phonon transport in graphdiyne and its nanoribbons, which is useful for their TC engineering.

3. 33

   Cluster Expansion Framework for the Sr (Ti1–x Fe x) O3–x/2 (0< x< 1) Mixed Ionic Electronic Conductor: Properties Based on Realistic Configurations Bin Ouyang, Tanmoy Chakraborty, Namhoon Kim, Nicola H Perry, Tim Mueller, Narayana R Aluru, Elif Ertekin* Chemistry of Materials, 2019 [Abs] [HTML] [PDF]

   Several mixed ionic/electronic conductors (MIECs) used as fuel or electrolysis cell electrodes may be thought of as solid solutions of perovskite oxides and ordered oxygen vacancy compounds. For example, the model MIEC SrTi1–xFexO3–x/2+δ (STF) can be described as a mixture of the perovskite SrTiO3 and the brownmillerite Sr2Fe2O5 that can accommodate some degree of oxygen off-stoichiometry δ. The large configurational space for these nondilute, disordered mixtures has hindered atomic scale modeling, limiting in-depth understanding and predictive analysis. We present a cluster expansion framework to describe the energetics of the disordered STF system within the full solid solution composition space Sr(Ti1–xFex)O3–x/2, 0 < x < 1, δ = 0. Cluster expansion Monte Carlo (CEMC) simulations are performed to identify low-energy configurations and to investigate the origin and degree of lattice disorder. Using realistic configurations obtained from CEMC, the electronic structure, band gap, and optical properties of STF and their sensitivity to the stoichiometry are examined and compared to those of hypothetical ordered structures and special quasirandom structures. The predicted evolution of the band gap and optical absorption with composition is consistent with the experiment. Analysis of the electronic structure elucidates that electronic transport within the mixture benefits from the simultaneous presence of Fe/Ti disorder on the B cation sublattice and a tendency for oxygen vacancies to cluster around Fe atoms. The modeling framework adopted here for the SrTiO3/Sr2Fe2O5 mixture can be extended to other MIEC materials.

4. 32

   Design Principles for High-Capacity Mn-Based Cation-Disordered Rocksalt Cathodes Zhengyan Lun+, Bin Ouyang+, Zijian Cai, Raphaële J Clément, Deok-Hwang Kwon, Jianping Huang, Joseph K Papp, Mahalingam Balasubramanian, Yaosen Tian, Bryan D McCloskey, Gerbrand Ceder* Chem, 2019 [Abs] [HTML] [PDF]

   Mn-based Li-excess cation-disordered rocksalt (DRX) oxyfluorides are promising candidates for next-generation rechargeable battery cathodes owing to their large energy densities, the earth abundance, and low cost of Mn. In this work, we synthesized and electrochemically tested four representative compositions in the Li-Mn-O-F DRX chemical space with various Li and F content. While all compositions achieve higher than 200 mAh g−1 initial capacity and good cyclability, we show that the Li-site distribution plays a more important role than the metal-redox capacity in determining the initial capacity, whereas the metal-redox capacity is more closely related to the cyclability of the materials. We apply these insights and generate a capacity map of the Li-Mn-O-F chemical space, LixMn2-xO2-yFy (1.167 ≤ x ≤ 1.333, 0 ≤ y ≤ 0.667), which predicts both accessible Li capacity and Mn-redox capacity. This map allows the design of compounds that balance high capacity with good cyclability.

5. 31

   Unveiling the mechanism of improved capacity retention in Pmn 2 1 Li 2 FeSiO 4 cathode by cobalt substitution Yan Zeng, Hsien-Chieh Chiu, Bin Ouyang, Jun Song, Karim Zaghib, George P Demopoulos* Journal of Materials Chemistry A, 2019 [Abs] [HTML] [PDF]

   Li2FeSiO4 (LFS) is a sustainable Li-ion cathode material composed of earth-abundant elements with potentially high energy density but suffers from limited reversible storage capacity (less than one Li), low intrinsic conductivities, and cycling instability. In search of deeper understanding of the structural chemistry of LFS towards overcoming some of these challenges, the viability of Fe-site doping by Co and other dopants in Pmn21 LFS is investigated using both first-principles calculations and experimental testing. Computational results suggest that the formation of Li2Fe1−xCoxSiO4 is energetically favorable, predictions confirmed by successful hydrothermal synthesis. Substitution of Co in LFS is revealed to have a dual effect; firstly catalyzing faster electrochemically induced phase transformation to the stable inverse-Pmn21 phase; and secondly enabling the formation of a low-resistant, uniform and stable fluorine-rich cathode-electrolyte interphase (CEI) layer that inhibits detrimental reactions between the cathode and electrolyte. As a result, Co-substituted LFS displays enhanced reversibility with 95% capacity retention after 50 cycles as compared to 80% of the undoped LFS.

6. 30

   Defect Engineering of Iron-Rich Orthosilicate Cathode Materials with Enhanced Lithium-Ion Intercalation Capacity and Kinetics Yan Zeng, Hsien-Chieh Chiu, Bin Ouyang, Karim Zaghib, George P Demopoulos* ACS Applied Energy Materials, 2019 [Abs] [HTML] [PDF]

   Defect engineering via nonstoichiometric composition control can serve as an effective strategy to tune the electronic and crystal structures of intercalation compounds, as has been recently evidenced in Li-rich cathode materials. To extend this strategy in another direction, Fe-richness as opposed to Li-richness is investigated in improving the electrochemical performance of a promising cathode material, Li2FeSiO4 (LFS). Nonstoichiometric LFS compounds in orthorhombic Pmn21 phase with up to 8% Fe-excess are successfully synthesized via controlled hydrothermal synthesis. It is demonstrated that, in addition to the higher electron capacity from the accessible Fe2+/Fe3+ redox couple, the presence of excess Fe enhances the intercalation kinetics vis-à-vis the stoichiometric composition. From combined electrochemical evaluation and first-principles DFT calculations, the enhanced kinetics are rationalized by the introduction of the FeLi• + VLi′ defect pair and newly generated electron conducting states from the creation of local Fe–O–Fe configurations. Moreover, the Fe-rich structure facilitates Fe migration to the Li-site due to a lower energy barrier than that of stoichiometric LFS, hence apparently leading to faster phase transformation from Pmn21 toward the cycled inverse Pmn21 phase. More generally, this study opens alternative defect and compositional engineering approaches in designing next generation intercalation materials with improved electrochemical performance.

7. 29

   Control of the metal-to-insulator transition by substrate orientation in nickelates J. J. Peng, B. Ouyang, H. Y. Liu, C. S. Hao, S. S. Tang, Y. D. Gu, Y. Yan AIP Advances, 2019 [Abs] [HTML] [PDF]

   We proved that the critical thickness for metal-to-insulator transition (MIT) of LaNiO3 could be controlled by substrate orientation. By means of density functional theory calculations, films grown on SrTiO3 substrates with (001), (110) and (111) orientations have different amount of charge transfer across the interface. Different charge transfer induces different interfacial conductivity behavior and at the same time modifies the carrier density of adjacent LaNiO3 films. The manipulation of MIT by substrate orientation can be achieved through interfacial charge transfer induced interfacial conductive layer with the modified conductivity of LNO layer.

2018

1. 28

   Phonon Transport of Zigzag/Armchair Graphene Superlattice Nanoribbons Jianjun Liu, Yang Liu, Yuhang Jing*, Yufei Gao, Junqing Zhao, Bin Ouyang International Journal of Thermophysics, 2018 [HTML] [PDF]
2. 27

   Conjugated π electron engineering of generalized stacking fault in graphene and h-BN Bin Ouyang*, Cheng Chen, J Song* Nanotechnology, 2018 [Abs] [HTML] [PDF]

   Generalized-stacking-fault energy (GSFE) serves as an important metric that prescribes dislocation behaviors in materials. In this paper, utilizing first-principle calculations and chemical bonding analysis, we studied the behaviors of generalized stacking fault in graphene and h-BN. It has been shown that the π bond formation plays a critical role in the existence of metastable stacking fault (MSF) in graphene and h-BN lattice along certain slip directions. Chemical functionalization was then proposed as an effective means to engineer the π bond, and subsequently MSF along dislocation slips within graphene and h-BN. Taking hydrogenation as a representative functionalization method, we demonstrated that, with the preferential adsorption of hydrogen along the slip line, π electrons along the slip would be saturated by adsorbed hydrogen atoms, leading to the moderation or elimination of MSF. Our study elucidates the atomic mechanism of MSF formation in graphene-like materials, and more generally, provides important insights towards predictive tuning of mechanic properties in two-dimensional nanomaterials.

3. 26

   Controllable Phase Stabilities in Transition Metal Dichalcogenides through Curvature Engineering: First-Principles Calculations and Continuum Prediction Bin Ouyang*, Pengfei Ou, Jun Song* Advanced Theory and Simulations, 2018 [Abs] [HTML] [PDF]

   A controllable phase transition between the semiconducting 2H phase and metallic/semimetallic T (1T, 1T′, and 1T″) isomorphs provides an effective route to tune and even switch physical and electronic properties within 2D transition metal dichalcogenides (TMDs). In this study, the feasibility of curvature engineering in manipulating the structural phase is investigated employing first‐principles density functional theory (DFT) calculations and continuum mechanics analysis. With WSe2 and MoTe2 as TMD representatives, it is found that bending deformation can not only energetically induce 2H → T (1T, 1T′, or 1T″) phase transformation, but also kinetically facilitate the phase transition by lowering transition activation barriers. Moreover, a phase stability diagram is constructed which suggests ways of achieving both uniformly 2H → T phase transition and programming printing of T phases in 2H‐TMD membranes. The theoretical results not only suggest a new feasible experimental design strategy of phase engineering, but also sheds light on novel device design with the patterning of T phases on the 2H‐TMD membrane.

4. 25

   Tunable phase stability and contact resistance of monolayer transition metal dichalcogenides contacts with metal Bin Ouyang*, Shiyun Xiong, Yuhang Jing* npj 2D Materials and Applications, 2018 [Abs] [HTML] [PDF]

   Monolayer transition metal dichalcogenides/metal (MX2/metal) based transistors have been widely studied. However, further development is hindered by the large contact resistance between MX2 and metal contact. In this paper, we demonstrated that interfacial charge transfer between MX2 and metal is the key for tuning contact resistance. With the lattice misfit criterion applied to screen combination of MX2s and metals, it has been found out that both phase stability of MX2 and contact nature between MX2 and metal will be sensitively affected by interfacial charge transfer. Additionally, we have identified seven MX2/metal systems that can potentially form zero Schottky barrier contacts utilizing phase engineering. On base of interfacial charge calculations and contact resistance analysis, we have presented three types of MX2/metal contacts that can be formed with distinguished contact resistance. Our theoretical results not only demonstrate various choice of MX2/metal designs in order to achieve different amounts of interfacial charge transfer as well as manipulate contact resistance, but also shed light on designing ohmic contacts in MX2/metal systems.

5. 24

   Wafer-scale synthesis of monolayer WSe2: A multi-functional photocatalyst for efficient overall pure water splitting Yongjie Wang, Songrui Zhao, Yichen Wang, David Arto Laleyan, Yuanpeng Wu, Bin Ouyang, Pengfei Ou, Jun Song, Zetian Mi* Nano Energy, 2018 [Abs] [HTML] [PDF]

   A multi-functional photocatalyst, that can combine the catalytic functions of water oxidation and proton reduction together with light harvesting capacity, is highly desired for low cost, high efficiency, and highly stable solar fuel production. Monolayer WSe2, with a direct energy gap of   1.65 eV is a nearly ideal light absorber to convert sunlight to hydrogen fuels through solar water splitting. To date, however, the controlled synthesis of monolayer WSe2 on a wafer scale and the realization of overall water splitting on WSe2 have remained elusive. Here, we report the van de Waals epitaxy of crystalline monolayer WSe2 on large area amorphous SiOx substrates. We have demonstrated, for the first time, the multi-functionality of monolayer WSe2 in solar water splitting, including extraordinary capacities for efficient light harvesting, water oxidation, and proton reduction. The absorbed photon conversion efficiency exceeds 12% for a single monolayer WSe2. This work provides a viable strategy for wafer-scale synthesis of multi-functional photocatalysts for the development of efficient, low cost, and scalable solar fuel devices and systems.

6. 23

   Enhanced thermoelectric performance of two dimensional MS2 (M= Mo, W) through phase engineering Bin Ouyang*, Shunda Chen, Yuhang Jing, Tianran Wei, Shiyun Xiong*, Davide Donadio Journal of Materiomics, 2018 [Abs] [HTML] [PDF]

   The potential application of monolayer MS2 (M = Mo, W) as thermoelectric material has been widely studied since the first report of successful fabrication. However, their performances are hindered by the considerable band gap and the large lattice thermal conductivity in the pristine 2H phase. Recent discoveries of polymorphism in MS2s provide new opportunities for materials engineering. In this work, phonon and electron transport properties of both 2H and 1T′ phases were investigated by first-principle calculations. It is found that upon the phase transition from 2H to 1T′ in MS2, the electron transport is greatly enhanced, while the lattice thermal conductivity is reduced by several times. These features lead to a significant enhancement of power factor by one order of magnitude in MoS2 and by three times in WS2. Meanwhile, the figure of merit can reach up to 0.33 for 1T′MoS2 and 0.68 for 1T′WS2 at low temperature. These findings indicate that monolayer MS2 in the 1T′ phase can be promising materials for thermoelectric devices application. Meanwhile, this work demonstrates that phase engineering techniques can bring in one important control parameter in materials design.

7. 22

   Atomistic Simulations on the Tensile Deformation Behaviors of Three-Dimensional Graphene (Phys. Status Solidi B 7/2018) Yang Liu, Jianjun Liu, Shaofeng Yue, Junqing Zhao, Bin Ouyang, Yuhang Jing* physica status solidi (b), 2018
8. 21

   Thermodynamic assessment of the Mo-S system and its application in thermal decomposition of MoS2 Senlin Cui*, Biao Hu, Bin Ouyang, Dongdong Zhao Thermochimica Acta, 2018 [Abs] [HTML] [PDF]

   The Mo-S system is crucial to extractive metallurgy, tribology, and various possible applications in design and fabrication of novel two-dimensional (2D) materials. First-principles calculations were utilized to compute the enthalpies of formation of molybdenum sulfides at 0 K and the heat capacity of Mo2S3 up to 960 K. A critical evaluation of the thermodynamic properties, phase equilibria, and phase diagram information in the literature was carried out to facilitate the thermodynamic optimization of the Mo-S system. The obtained thermodynamic description of the Mo-S system can satisfactorily represent most of the reliable thermodynamic and phase diagram information. Pressure-temperature (P-T), pressure-composition (P-x), and temperature-composition (T-x) diagrams of the Mo-S system were utilized to study the thermal decomposition of MoS2 for raw Mo production. Reduced pressure and high temperature are thermodynamically favorable conditions for the MoS2 reduction process.

2017

1. 20

   Microstructure evolution of Inconel 625 with 0.4 wt% boron modification during gas tungsten arc deposition Y Tian, B Ouyang, A Gontcharov, R Gauvin, P Lowden, M Brochu* Journal of Alloys and Compounds, 2017 [Abs] [HTML] [PDF]

   Gas tungsten arc deposits were made on substrate of stainless steel 304 using IN625 wires modified with 0.4 wt% B in shielding argon. The temperature profiles were simulated by the modified Rosenthal 3D equation. The re-melting boundary correlated well with the hardness profile and corresponding microstructure evolution along the as-manufactured sample. The upper section had higher hardness than the lower section. This high hardness was attributed to the existence of continuous eutectics (Laves phase and NbC) in the inter-dendritic regions. These eutectics in the lower section partially re-melted in this multi-pass process to form non-continuous features contributing to a lower hardness. Moreover, fine isolated borides precipitated out from the γ matrix in lower section as a result of the multiple thermal cycles. The eutectic phases were identified to be Laves phase with few NbC. The isolated borides were confirmed as M5B3 type of boride by TEM. Modeling of the segregation behavior of the major alloying elements according to Clyne-Kurz equations was conducted and the results were in good agreement with chemical concentration profiles.

2. 19

   MoS2 heterostructure with tunable phase stability: strain induced interlayer covalent bond formation Bin Ouyang*, Shiyun Xiong*, Zhi Yang, Yuhang Jing, Yongjie Wang Nanoscale, 2017 [Abs] [HTML] [PDF]

   The structural phase transition in MoS2 promises applications in novel nanoelectronic devices. Elastic strain engineering can not only serve as a potential route for phase transition engineering, but also reveal potential ferroelastic behavior of MoS2 nanostructures. However, the elastic strain required for phase transition in monolayer MoS2 is far beyond its elastic limit, thus inhibiting the experimental realization. In this study, employing density functional theory calculations, we uncover that by forming heterostructure with buckled 2D materials, such as silicene, germanene and stanene, the critical phase transition strain required in monolayer MoS2 can be drastically reduced. Particularly when MoS2 forms sandwiched structures with silicene or stanene, the uniaxial and biaxial critical strain can be reduced to ∼0.06 and ∼0.03, respectively, which is well below the experimental elastic limit. This theoretical study not only proposes an experimental achievable strategy for flexible phase transition design in MoS2 nanostructure, but also identifies those MoS2 heterostructures as 2D candidates for potential shape memory devices and pseudoelasticity applications.

2016

1. 18

   The tunneling magnetoresistance and spin-polarized optoelectronic properties of graphyne-based molecular magnetic tunnel junction Zhi Yang*, Bin Ouyang, Guoqiang Lan, Li-Chun Xu, Ruiping Liu, Xuguang Liu Journal of Physics D: Applied Physics, 2016 [Abs] [HTML] [PDF]

   Using density functional theory and the non-equilibrium Green’s function method, we investigate the spin-dependent transport and optoelectronic properties of the graphyne-based molecular magnetic tunnel junctions (MMTJs). We find that these MMTJs exhibit an outstanding tunneling magnetoresistance (TMR) effect. The TMR value is as high as 106%. When the magnetization directions of two electrodes are antiparallel under positive or negative bias voltages, two kinds of pure spin currents can be obtained in the systems. Furthermore, under the irradiation of infrared, visible or ultraviolet light, spin-polarized photocurrents can be generated in the MMTJs, but the corresponding microscopic mechanisms are different. More importantly, if the magnetization directions of two electrodes are antiparallel, the photocurrents with different spins are spatially separated, appearing at different electrodes. This phenomenon provides a new way to simultaneously generate two spin currents.

2. 17

   Bandgap Transition of 2H Transition Metal Dichalcogenides: Predictive Tuning via Inherent Interface Coupling and Strain Bin Ouyang, Zetian Mi, Jun Song* The Journal of Physical Chemistry C, 2016 [Abs] [HTML] [PDF]

   Phase transitions within two-dimensional transition metal dichalcogenides (TMD) promise new possibilities for engineering their properties. Using first-principles density functional theory (DFT) calculations, we systematically examined the interfacial electronic coupling between the 2H phase monolayer with its polymorphic phases in several group IV TMD, i.e., MoS2 (MoSe2) and WS2 (WSe2), inherent bilayer heterostructures. It is found that the interface coupling, augmented by in-plane strain, can greatly modify the band structure of the 2H phase to induce bandgap transition (either indirect-to-direct or direct-to-indirect). Moreover, the effects of strain on the band structure can be well understood and predicted within the framework of deformation potential theory. The present study provides important insights toward engineering optoelectronic properties of TMD-based devices.

3. 16

   Predictions of thermal expansion coefficients of rare-earth zirconate pyrochlores: A quasi-harmonic approximation based on stable phonon modes Guoqiang Lan, Bin Ouyang, Yushuai Xu, Jun Song*, Yong Jiang Journal of Applied Physics, 2016 [Abs] [HTML] [PDF]

   Rare-earth (RE) pyrochlores are considered as promising candidate materials for the thermal barrier coating. In this study, we performed first-principles calculations, augmented by quasi-harmonic phonon calculations, to investigate the thermal expansion behaviors of several RE2Zr2O7 (RE = La, Nd, Sm, Gd) pyrochlores. Our findings show that RE2Zr2O7 pyrochlores exhibit low-lying optical phonon frequencies that correspond to RE-cation rattling vibrational modes. These frequencies become imaginary upon volume expansion, preventing correct determination of the free energy versus volume relation and thereby quantification of thermal expansion using QH phonon calculations. To address this challenge, we proposed a QH approximation approach based on stable phonon modes where the RE-cation rattling modes were systematically eliminated. This approach is shown to provide accurate predictions of the coefficients of thermal expansion (CTEs) of RE2Zr2O7 pyrochlores, in good agreement with experimental measurements and data from first-principles molecular dynamics simulations. In addition, we showed that the QH Debye model considerably overestimates the magnitudes and wrongly predicts the trend for the CTEs of RE2Zr2O7 pyrochlores

4. 15

   Atomistic investigation of the influence of hydrogen on dislocation nucleation during nanoindentation in Ni and Pd Xiao Zhou, Bin Ouyang, WA Curtin, Jun Song* Acta Materialia, 2016 [Abs] [HTML] [PDF]

   The effects of hydrogen charging on the mechanical response of FCC Ni and Pd under nanoindentation are systematically investigated by molecular dynamics simulations. Simulations consider a random H distribution and time scales prevent any diffusion of H during the simulations. Hydrogen charging is then found to reduce the threshold load for dislocation nucleation, i.e., the pop-in load, but only to a limited extent. After pop-in, the indentation response is largely independent of the presence of H. Furthermore, the influence of hydrogen charging on the pop-in load originates only from the hydrogen-induced swelling of the lattice. That is, H does not directly influence dislocation nucleation, either in terms of facilitating initial slip or interacting with the nascent dislocation(s). These findings suggest that rate-dependent processes, either associated with fluctuating nucleation or H transport, are necessary to interpret experimental observations of hydrogen-influenced

5. 14

   The thermoelectric performance of bulk three-dimensional graphene Zhi Yang*, Guoqiang Lan, Bin Ouyang, Li-Chun Xu, Ruiping Liu, Xuguang Liu, Jun Song Materials Chemistry and Physics, 2016 [Abs] [HTML] [PDF]

   The electronic and thermoelectric properties of a new carbon bulk material, three-dimensional (3D) graphene, are investigated in this study. Our results show that 3D graphene has unique electronic structure, i.e., near the Fermi level there exist Dirac cones. More importantly, the thermoelectric performance of 3D graphene is excellent, at room temperature the thermoelectric figure of merit (ZT) is 0.21, an order of magnitude higher than that of graphene. By introducing line defects, the ZT of 3D graphene could be enhanced to 1.52, indicating 3D graphene is a powerful candidate for constructing novel thermoelectric materials.

6. 13

   Tuning Magnetic States of Planar Graphene/h-BN Monolayer Heterostructures via Interface Transition Metal-Vacancy Complexes Bin Ouyang, Jun Song* The Journal of Physical Chemistry C, 2016 [Abs] [HTML] [PDF]

   Planar graphene/h-BN (GPBN) heterostructures promise low-dimensional magnetic semiconductor materials of tunable bandgap. In the present study, interplay between 3d transition metal (TM) atoms and single vacancies (SVs) at the armchair interface in a planar GPBN monolayer was investigated through first principle density functional theory calculations. The TM-SV complexes were found to give rise to a rich set of magnetic states, originated from the interactions between valence electrons of the TM atom with dangling orbitals at the SV. The magnetic state at a TM-SV complex was further shown to be tunable upon the application of strain and electric field. The present study suggests a route to enrich and engineer the magnetic states of planar GPBN heterostructures, providing new insights for the design of tunable low-dimensional spintronic devices.

7. 12

   Phase engineering of MoS 2 through GaN/AlN substrate coupling and electron doping Bin Ouyang, Pengfei Ou, Yongjie Wang, Zetian Mi, Jun Song* Physical Chemistry Chemical Physics, 2016 [Abs] [HTML] [PDF]

   The polymorphism of two dimensional MoS2 promises new possibilities for nanoelectronics. The realization of those possibilities necessitates techniques to enable flexible and controllable phase engineering of MoS2. In the present study, based on first-principles calculations, a new and flexible route to engineer the phase stability of MoS2 by interfacing it with a GaN or AlN substrate is reported. Depending on the surface termination of the underlying substrate, MoS2 may exhibit either the 2H or 1T′ (1T′′) phase. The interface coupling between MoS2 and the substrate also affects the phase transition kinetics. In addition, electron doping can act as another means to influence MoS2–substrate interactions and enable further phase engineering of MoS2. The present findings contribute to new knowledge towards phase engineering of MoS2 and the design of hybrid nanodevices comprising both 2D and 3D optoelectronic materials.

8. 11

   Covalent pathways in engineering h-BN supported graphene Bin Ouyang, Jun Song* Carbon, 2016 [Abs]

   Cross-planar di-vacancies (CPDVs) within stacked graphene hexagonal boron nitride (h-BN) heterostructures provide stabilized covalent links to bridge adjacent graphene and h-BN sheets. Through a first principle theoretical study based on density functional theory (DFT), it was shown that the CPDVs serve as focal points for cross-planar atom diffusion between graphene and h-BN, and the chemical nature of interlayer links along with associated cross-planar migration pathways at these defects can be predictively manipulated through modulation of the chemical environment and charge engineering, to achieve consistent B or N doping and simultaneous healing of graphene. The present study proposed a viable approach integrating irradiation, chemical and charge engineering, to produce high-quality graphene with tunable electronic and electrochemical properties, using the h-BN substrate.

2015

1. 10

   First-Principles Study of Dislocation Slips in Impurity-Doped Graphene Fanchao Meng, Bin Ouyang, Jun Song* The Journal of Physical Chemistry C, 2015 [Abs] [HTML] [PDF]

   Employing density-functional theory (DFT) calculations, the generalized-stacking-fault energy (GSFE) curves along two crystallographic slips, glide and shuffle, for both pristine graphene and impurity of boron (B) or nitrogen (N) doped graphene were examined. The effects of B and N doping on the GSFE were clarified and correlated with local electron interactions and bonding configurations. The GSFE data were then used to analyze dislocation dipole and core structure and subsequently combined with the Peierls–Nabarro (P–N) model to examine the role of doping on several key characteristics of dislocations in graphene. We showed that the GSFE curve may be significantly altered by the presence of dopants, which subsequently leads to profound modulations of dislocation properties, such as increasing spontaneous pair-annihilation distance and reducing resistance to dislocation slip. Our results indicate that doping can play an important role in controlling dislocation density and microscopic plasticity in graphene, thereby providing critical insights for dopant-mediated defect engineering in graphene.

2. 9

   The role of low-lying optical phonons in lattice thermal conductance of rare-earth pyrochlores: A first-principle study Guoqiang Lan, Bin Ouyang, Jun Song* Acta Materialia, 2015 [Abs] [HTML] [PDF]

   Rare-earth pyrochlores, commonly exhibiting remarkably low lattice thermal conductivities, are considered as promising topcoat materials for thermal barrier coatings. However the structural origin underlying their low thermal conductivities remains unclear. In the present study, we investigated the phonon properties of two groups of RE pyrochlores, Ln2Zr2O7 (Ln = La, Nd, Sm, Gd) and Gd2T2O7 (T = Zr, Hf, Sn, Pb) employing density functional theory and quasi harmonic approximation. Through the relaxation time approximation (RTA) with Debye model, the thermal conductivities of those RE pyrochlores were predicted, showing good agreement with experimental measurements. The low thermal conductivities of RE pyrochlores were shown to largely come from the interference between the low-lying optical branches and acoustic branches. The structural origin underlying the low-lying optical branches was then clarified and the competition between scattering processes in transverse and longitude acoustic branches was discussed.

3. 8

   Synthesis of MoO2 hierarchical peony-like microspheres without a template and their application in lithium ion batteries Shasha Tang, Bin Ouyang, Linyu Yang, Wenhai Ji* RSC Advances, 2015 [Abs] [HTML] [PDF]

   Three dimensional hierarchical structures constructed with low dimensional nanoscale building blocks have become a new research focus for lithium storage. Some hierarchical structured compounds, such as V2O5, Co3O4 and MoS2 micro/nanoflowers, have been synthesized for energy storage applications. However, it still remains a challenge to fabricate well-designed hierarchical MoO2 microflowers. This paper reports on a morphology-controlled synthesis of hierarchical peony-like MoO2 microspheres as an anode material for lithium-ion batteries. This hierarchical structure is fabricated via a simple template-free solvothermal method. The as-derived hierarchical MoO2 microspheres exhibit high specific capacity, high capacity retention, and remarkable cycling stability, when evaluated as an anode material for LIBs.

4. 7

   Probing the dynamics of the metallic-to-semiconducting structural phase transformation in MoS2 crystals Yinsheng Guo, Dezheng Sun+, Bin Ouyang+, Archana Raja, Jun Song, Tony F Heinz, Louis E Brus* Nano Lett, 2015 [Abs] [HTML] [PDF]

   We have investigated the phase transformation of bulk MoS2 crystals from the metastable metallic 1T/1T′ phase to the thermodynamically stable semiconducting 2H phase. The metastable 1T/1T′ material was prepared by Li intercalation and deintercalation. The thermally driven kinetics of the phase transformation were studied with in situ Raman and optical reflection spectroscopies and yield an activation energy of 400 ± 60 meV (38 ± 6 kJ/mol). We calculate the expected minimum energy pathways for these transformations using DFT methods. The experimental activation energy corresponds approximately to the theoretical barrier for a single formula unit, suggesting that nucleation of the phase transformation is quite local. We also report that femtosecond laser writing converts 1T/1T′ to 2H in a single laser pass. The mechanisms for the phase transformation are discussed.

5. 6

   Phase engineering of monolayer transition-metal dichalcogenide through coupled electron doping and lattice deformation Bin Ouyang, Guoqiang Lan, Yinsheng Guo, Zetian Mi, Jun Song* Applied Physics Letters, 2015 [Abs] [HTML] [PDF]

   First-principles calculations were performed to investigate the phase stability and transition within four monolayer transition-metal dichalcogenide (TMD) systems, i.e., MX2 (M ¼ Mo or W and X ¼ S or Se) under coupled electron doping and lattice deformation. With the lattice distortion and electron doping density treated as state variables, the energy surfaces of different phases were computed, and the diagrams of energetically preferred phases were constructed. These diagrams assess the competition between different phases and predict conditions of phase transitions for the TMDs considered. The interplay between lattice deformation and electron doping was identified as originating from the deformation induced band shifting and band bending. Based on our findings, a potential design strategy combining an efficient electrolytic gating and a lattice straining to achieve controllable phase engineering in TMD monolayers was demonstrated.

2014

1. 5

   Energetics and kinetics of vacancies in monolayer graphene boron nitride heterostructures Bin Ouyang, Fanchao Meng, Jun Song* 2D Materials, 2014 [Abs] [HTML] [PDF]

   Graphene and boron nitride (GPBN) heterostructures provide a viable way to realize tunable bandgap, promising new opportunities in graphene-based nanoelectronic and optoelectronic devices. In the present study, we investigated the interplay between vacancies and graphene/h-BN interfaces in monolayer GPBN heterostructures. The energetics and kinetics of monovacancies and divacancies in monolayer GPBN heterostructures were examined using first-principle calculations. The interfaces were shown to be preferential locations for vacancy segregation. Meanwhile the kinetics of vacancies was found to be noticeably modified at interfaces, evidenced by the minimum energy paths and associated migration barriers calculations. The role of interfacial bonding configurations, energy states and polarization on the formation and diffusion of vacancies were discussed. Additionally we demonstrated that it is important to recognize the dissimilarities in the diffusion prefactor for different vacancies for accurate determination of the vacancy diffusion coefficient. Our results provide essential data for the modeling of vacancies in GPBN heterostructures, and important insights towards the precise engineering of defects, interfaces and quantum domains in the design of GPBN-based devices.

2013

1. 4

   Strain engineering of magnetic states of vacancy-decorated hexagonal boron nitride Bin Ouyang, Jun Song* Applied Physics Letters, 2013 [Abs] [HTML] [PDF]

   Nanomaterials with tunable magnetic states play a significant role in the development of next-generation spintronic devices. In this paper, we examine the role of biaxial strain on the electronic properties of vacancy-decorated hexagonal boron nitride (h-BN) monolayers using density functional theory calculations. We found that the strain can lead to switching of the magnetic state for h-BN monolayers with boron vacancy or divacancy. Our findings promise an effective route for the operation of low-dimensional spintronic devices.

2. 3

   Energetics and kinetics of Li intercalation in irradiated graphene scaffolds Jun Song*, Bin Ouyang, Nikhil V Medhekar* ACS applied materials & interfaces, 2013 [Abs] [HTML] [PDF]

   In the present study, we investigate the irradiation-defects hybridized graphene scaffold as one potential building material for the anode of Li-ion batteries. Designating the Wigner V22 defect as a representative, we illustrate the interplay of Li atoms with the irradiation defects in graphene scaffolds. We examine the adsorption energetics and diffusion kinetics of Li in the vicinity of a Wigner V22 defect using density functional theory calculations. The equilibrium Li adsorption sites at the defect are identified and shown to be energetically preferable to the adsorption sites on pristine (bilayer) graphene. Meanwhile, the minimum energy paths and corresponding energy barriers for Li migration at the defect are determined and computed. We find that, while the defect is shown to exhibit certain trapping effects on Li motions on the graphene surface, it appears to facilitate the interlayer Li diffusion and enhance the charge capacity within its vicinity, because of the reduced interlayer spacing and characteristic symmetry associated with the defect. Our results provide critical assessment for the application of irradiated graphene scaffolds in Li-ion batteries.

3. 2

   Predicting the Size-and Shape-Dependent Cohesive Energy and Order-Disorder Transition Temperature of Co-Pt Nanoparticles by Embedded-Atom-Method Potential Chenze Liu, Weihong Qi*, Bin Ouyang, Xing Wang, Baiyun Huang Journal of nanoscience and nanotechnology, 2013 [Abs] [HTML]

   The cohesive energy (CE) of CoPt nanoparticles (NPs) with different sizes and shapes have been calculated by embedded-atom-method (EAM) potential. It is shown that CE of NPs with order or disorder structures decreases with the decrease of particle size, while the shape effects become obvious only at small size. The CE difference per atom between order and disorder structures decreases with the decrease of particle size, indicating that the possibility of order-disorder transition in small size becomes larger compared with these in large size. Significantly, the CE difference varies in proportion to order-disorder transition temperature (Tc ), which suggests that one can predict order-disorder transition of NPs by calculation the cohesive energy. The present calculated Tc of CoPt NPs is consistent with recent experiments, simulation and theoretical predictions, and the method can also be applied to study the order-disorder transition of FePt, FePd, and so on.

2012

1. 1

   Size and shape dependent order–disorder phase transition of Co–Pt nanowires Bin Ouyang, Weihong Qi*, Chenze Liu, Xing Wang, Lanying Wei, Chang Q Sun Computational Materials Science, 2012 [Abs] [HTML] [PDF]

   Monte Carlo simulation of the order–disorder transition revealed that the transition temperature of Co–Pt nanowires increases with wire diameter, approaching the bulk value if the size is large enough. The transition temperature is affected by the shape of cross-section, though the shape effect is less significant than the size effect. It is showed that the rise of transition temperature in nanowires is largely due to the decrease of surface area compared with nanoparticles. The phase separation and tetragonalization are discussed by introducing mixing parameter and asphericity parameter. It is also found that the order–disorder transition starts from the surface and then to the core, indicating that the order–disorder transition of nanowires is a surface-dominant phenomenon, governed by the atomic under coordination.