From Atomistic to Continuum Models of Interfaces in Lithium-Ion Batteries

模拟锂离子固体电解质中锂的传输对于预测它们的性能是很重要的。这些材料通常是多晶的，并且具有显著影响锂在材料中的传输的界面。因此，为了充分发掘锂离子电池中潜在的新材料，能够模拟这些颗粒电池的效应是至关重要的。传统的模拟跨晶界离子输运的方法是求解泊松-玻尔兹曼方程来寻找一维连续介质模型中点缺陷(如Li间隙和空位)的平衡分布。这种方法假定缺陷只通过平均场静电场相互作用，并对应于对缺陷浓度的稀释极限进行建模。这里，结构缺陷包括离子空位和掺杂离子。在缺陷浓度可能很高的电池材料中，这些缺陷之间的相互作用比稀薄极限平均场描述要复杂得多。为了建立晶界对锂离子在这些材料中传输的影响的准确模型，因此有必要超越稀释极限的近似。这个项目将涉及到研究这个问题的热力学，以便尝试在化学势中加入一个额外的浓度相关项。已经有人尝试建立一个包含固体电解质中结构缺陷之间相互作用的模型，但由于缺乏计算这些模型的关键参数的方法，这些方法目前还没有被广泛接受[1]。在确定这个附加项的形式之后，将需要计算单个晶界和材料的这种项的系数的方法。这将需要进一步研究问题的基本热力学，并确定需要哪些计算才能先验地获得这些参数的值。通过这个项目，现有的代码(由以前的博士生Georgina Wellock编写和开发)将被改进和修改，以满足问题的新热力学的要求或提高效率。参考文献[1]Mebane D.S.和de Souza R.A.；能源环境。SCI。2015年，8,2935-2940。

Modelling Li transport through Li-ion solid-state electrolytes is important for predicting their performances. These materials are often polycrystalline and have interfaces which significantly affect Li transport through the material. Therefore, it is vital to be able to model the effect of these grain batteries in order to fully explore potential new materials to be used in Li-ion batteries.The conventional method of modelling ionic transport across grain boundaries involves solving the Poisson-Boltzmann equation to find the equilibrium distri- bution of point defects (e.g. Li interstitials and vacancies) in a 1D continuum model. This approach assumes defects interact only through mean-field electro- statics, and corresponds to modelling the dilute limit of defect concentrations. Where here, structural defects include ion vacancies and dopant ions. In bat- tery materials, where defect concentrations can be high, interactions between these defects are more complex than the dilute limit mean-field description. To develop accurate models of the effects of grain boundaries on lithium-ion trans- port in these materials, it is therefore necessary to go beyond the dilute limit approximation.The project will involve investigating the thermodynamics of the problem in order to try to incorporate an additional concentration dependent term to the chemical potential. Attempts of producing a model that incorporates the inter- actions between structural defects in solid state electrolytes have been made, however these methods are not widely accepted at the present time, due to the lack of a method for calculating the key parameters for these models [1]. After the form of this additional term is determined, methods of calculating coefficients for such a term for individual grain boundaries and materials will be required. This will require investigating the fundamental thermodynamics of the problem further and determining which calculations are required in order to obtain the values of these parameters a priori. An alternative approach whereby chemical activities are used in order to minimise the free energy of the entire system will also be explored.Throughout the project, the existing code (written and developed by the pre- vious PhD candidate, Georgina Wellock) will be improved and altered to meet the requirements of the new thermodynamics of the problem or to improve efficiency.References[1] Mebane D. S. and De Souza R. A.; Energy Environ. Sci. 2015, 8, 2935-2940.