Materials Science & Engineering

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Author: search.filters.author.Zhu, YizhouEnd date: 2019

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    Atomistic Modeling of Solid Interfaces in All-solid-state Li-ion Batteries
    (2018) Zhu, Yizhou; Mo, Yifei; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    All-solid-state Li-ion battery based on solid electrolyte is a promising next-generation battery technology, providing intrinsic safety and higher energy density. Despite the development of solid electrolyte materials with high ionic conductivity, the high interfacial resistance and interfacial degradation at the solid electrolyte–electrode interfaces limit the electrochemical performance of the all-solid-state batteries. Fundamental understanding about the solid-solid interfaces is essential to improve the performance of all-solid-state batteries. In this dissertation, I perform first principles computation to bring new understanding about these solid interfaces. Using our developed computation approach based on large materials database, I calculated the intrinsic electrochemical stability window of solid electrolytes and predicted interphase decomposition products. I revealed the effects of different types of interphase layers on the interface stability and battery performance, and also provided interfacial engineering strategies to improve interface compatibility. Lithium metal anode can provide significantly higher energy density of Li-ion batteries. However, only a limited number of materials are known to be stable against lithium metal due to its strong reducing nature. Using first-principles calculations and large materials database, I revealed the general trend of lithium reduction behavior in different material chemistry. Different from oxides, sulfides, and halides, nitride anion chemistry exhibits unique stability against lithium metal, which is either thermodynamically intrinsic or a result of stable passivation. Therefore, many nitrides materials are promising candidate materials for lithium metal anode protection. Since solid electrolytes in all-solid-state batteries are often polycrystalline, the grain boundaries can have an important impact on the ion diffusion in solid electrolytes. I performed molecular dynamics simulations to study the ion diffusion at grain boundaries in solid electrolyte materials, and showed the distinct diffusion behavior at grain boundaries different from the facile ion transport in the bulk. In addition, I studied the order-disorder transition induced by mechanical strain in lithium garnet. Such transition can lead to orders of magnitude change in ionic diffusivity. This series of work demonstrated that computational modeling techniques can help to gain critical fundamental understandings of the solid interfaces in all-solid-state Li-ion battery, and to provide practical engineering strategies to improve the battery performance.
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    Statistical variances of diffusional properties from ab initio molecular dynamics simulations
    (Nature Publishing Group, 2018-04-03) He, Xingfeng; Zhu, Yizhou; Epstein, Alexander; Mo, Yifei
    Ab initio molecular dynamics (AIMD) simulation is widely employed in studying diffusion mechanisms and in quantifying diffusional properties of materials. However, AIMD simulations are often limited to a few hundred atoms and a short, sub-nanosecond physical timescale, which leads to models that include only a limited number of diffusion events. As a result, the diffusional properties obtained from AIMD simulations are often plagued by poor statistics. In this paper, we re-examine the process to estimate diffusivity and ionic conductivity from the AIMD simulations and establish the procedure to minimize the fitting errors. In addition, we propose methods for quantifying the statistical variance of the diffusivity and ionic conductivity from the number of diffusion events observed during the AIMD simulation. Since an adequate number of diffusion events must be sampled, AIMD simulations should be sufficiently long and can only be performed on materials with reasonably fast diffusion. We chart the ranges of materials and physical conditions that can be accessible by AIMD simulations in studying diffusional properties. Our work provides the foundation for quantifying the statistical confidence levels of diffusion results from AIMD simulations and for correctly employing this powerful technique.
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    Strategies Based on Nitride Materials Chemistry to Stabilize Li Metal Anode
    (John Wiley & Sons Ltd., 2017-03-03) Zhu, Yizhou; He, Xingfeng; Mo, Yifei
    Lithium metal battery is a promising candidate for high-energy-density energy storage. Unfortunately, the strongly reducing nature of lithium metal has been an outstanding challenge causing poor stability and low coulombic efficiency in lithium batteries. For decades, there are significant research efforts to stabilize lithium metal anode. However, such efforts are greatly impeded by the lack of knowledge about lithium-stable materials chemistry. So far, only a few materials are known to be stable against Li metal. To resolve this outstanding challenge, lithium-stable materials have been uncovered out of chemistry across the periodic table using first-principles calculations based on large materials database. It is found that most oxides, sulfides, and halides, commonly studied as protection materials, are reduced by lithium metal due to the reduction of metal cations. It is discovered that nitride anion chemistry exhibits unique stability against Li metal, which is either thermodynamically intrinsic or a result of stable passivation. The results here establish essential guidelines for selecting, designing, and discovering materials for lithium metal protection, and propose multiple novel strategies of using nitride materials and high nitrogen doping to form stable solid-electrolyte-interphase for lithium metal anode, paving the way for high-energy rechargeable lithium batteries.