Corrosion and Passivation Mechanisms of Li-Ion Battery Cathodes from Ab Initio Interfacial Reaction Dynamics

氧化还原活性材料的腐蚀和表面钝化是电化学储能和催化中的一个交叉主题，但其过程及其基本的分子基础仍然知之甚少。活性金属的溶解是导致锂离子电池正极性能下降的主要原因之一，与锂离子电池的晶体结构和化学成分无关。这一研究项目的结果将为解释和预测降解行为所需的腐蚀和钝化机制的分子起源提供基本的见解。所获得的知识将使改进的方法学基础的发展能够攻击与电化学界面相关的其他缓慢和罕见的事件现象。为了捕捉LiB溶解过程的瞬时性质并动态地解释电子结构的变化，将使用原子从头计算分子动力学(AIMD)模拟。这样的计算与增强的自由能采样技术相结合，可以提供对单一事件水平上统计上罕见事件的动力学、机制和动力学的关键见解。所获得的结果可以用来解释通常难以解释的实验结果，并提出抑制表面降解和钝化的方法。这项研究的主要目标是建立一个详细的原子学理解在LiB阴极/电解液界面的反应动力学，重点是过渡金属的溶解和电解液的分解。研究计划集中在三类重要的氧化物基锂正极材料上，即尖晶石型LiM2O4、层状氧化物LiMo2和磷橄榄石LiMPO4(M=Mn，Co，Ni，Fe)化合物。重点将放在研究富化学环境中的界面反应，包括多相电解液、多组分阴极材料、结构不均一性和高温效应。研究将集中于了解以下因素的作用：1)阴极材料的表面化学，包括过渡金属歧化反应；2)在有水分的情况下涉及两个或两个以上组分的复杂混合电解液化学；以及3)局部结构不均匀，如氧空位的存在和电荷状态(Li含量)的变化。该项目的科学目标是通过对基本反应步骤的详细描述，理清阴极化学/结构和混合电解液化学之间的密切关系。将确定一组简单的描述符，这些描述符可用于预测阴极在复杂的LiB环境中的腐蚀倾向。该项目将为本科生和研究生，包括STEM中代表性不足的群体的成员，提供独特的机会，研究第一原理技术在界面反应动力学中的应用。该项目的主要成果将被纳入课程材料，并在纳米物理高中夏令营向高中生展示。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Corrosion and surface passivation of redox-active materials is a crosscutting theme in electrochemical energy storage and catalysis, but the processes and their fundamental molecular underpinnings remain poorly understood. Active metal dissolution is one of the main reasons for degradation of Li-ion battery (LIB) cathodes, regardless of their crystal structure and chemical composition. The results of this research project will provide fundamental insights into the molecular origins of corrosion and passivation mechanisms necessary to explain and predict degradation behavior. The knowledge obtained will enable the development of an improved methodological foundation to attack other slow and rare event phenomena related to electrochemical interfaces.To capture the transient nature of the LIB dissolution process and account for changes in the electronic structure dynamically, atomistic ab initio molecular dynamics (AIMD) simulations will be employed. Such calculations in combination with enhanced free-energy sampling techniques can provide key insights into the dynamics, mechanism and kinetics of statistically rare events at a single event level. The obtained results can be used to explain experimental findings that are often challenging to interpret and to propose ways to inhibit surface degradation and passivation. The primary objective of the proposed research is to build a detailed atomistic understanding of reaction dynamics at LIB cathode/electrolyte interfaces focusing on transition-metal dissolution and electrolyte decomposition. The research plan focuses on three important classes of oxide-based LIB cathode materials, namely, spinel-type LiM2O4, layered-oxide LiMO2 and phospho-olivine LiMPO4 (M = Mn, Co, Ni, Fe) compounds. The emphasis will be on studying interfacial reactions in the chemically rich environments considering multiphase electrolyte solutions, multi-component cathode materials, structural heterogeneities and elevated temperature effects. The research will focus on understanding the role of: 1) surface chemistry of cathode materials including transition-metal disproportionation reactions; 2) complex mixed electrolyte chemistry involving two or more components in the presence of moisture; and 3) local structural heterogeneities such as the presence of oxygen vacancies and varying state of charge (Li content). The scientific goal of the project is to untangle the intimate relationships between cathode chemistry/structure and mixed electrolyte chemistry through a detailed characterization of elementary reaction steps. A set of simple descriptors will be determined that can be used to predict the propensity of the cathodes to corrode in complex LIB environments. The project will provide unique opportunities for undergraduate and graduate students, including members of underrepresented groups in STEM, to work on the application of first-principles techniques in interfacial reaction dynamics. Key results from the project will be incorporated into course materials and presented to high school students at a Nanophysics High School Camp.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.