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

模拟锂离子在锂离子固态电解质中的输运对预测其性能具有重要意义。这些材料通常是多晶的，并且具有显著影响锂通过材料输运的界面。因此，为了充分探索用于锂离子电池的潜在新材料，能够模拟这些颗粒电池的效果是至关重要的。模拟离子跨晶界输运的传统方法包括求解泊松-玻尔兹曼方程，以找到一维连续体模型中点缺陷（如Li间隙和空位）的平衡分布。这种方法假设缺陷仅通过平均场静电相互作用，并对应于缺陷浓度的稀释极限的建模。在这里，结构缺陷包括离子空位和掺杂离子。在电池材料中，缺陷浓度可能很高，这些缺陷之间的相互作用比稀释极限平均场描述更复杂。因此，为了建立晶界对这些材料中锂离子输运影响的精确模型，有必要超越稀释极限近似。这个项目将包括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态电解质结构缺陷之间相互作用的模型，但是由于缺乏计算这些模型关键参数的方法，这些方法目前还没有被广泛接受。在确定了这一附加项的形式之后，将需要为单个晶界和材料计算这一附加项的系数的方法。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。还将探讨另一种方法，即利用化学活动来使整个系统的自由能最小化。在整个项目中，现有的代码（由之前的博士候选人Georgina Wellock编写和开发）将被改进和修改，以满足问题的新热力学要求或提高效率。参考文献[1]Mebane D. S.和De Souza R. A.；能源环境。科学通报，2015,35(4):635 - 639。

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.