Computational Design of Inorganic Solid-State Electrolyte Materials for Lithium-Ion Batteries

Solid-state electrolytes hold great promise for advancing electrochemical energy storage devices. Advanced batteries based on solid electrolytes, particularly all-solid-state lithium-metal batteries, hold the potential to simultaneously address both high energy density and safety concerns associated with traditional lithium-ion batteries. Ideally, solid electrolytes should exhibit a high ionic conductivity at room temperature. In practical applications, other properties, such as electrochemical stability and compatibility with electrodes, are equally important. However, the pursuit of a single solid electrolyte possessing all of these properties remains challenging. Simulation techniques play an important role in the design of solid electrolyte materials, bypassing the difficulty of chemical synthesis and structural characterization. In these simulations, ionic conduction within bulk electrolytes and the ion deposition and stripping processes at the charged electrode-electrolyte interface can be investigated. By providing the flexibility to construct electrolyte models and explore structural evolution at multiple scales, simulation techniques facilitate the rational design of advanced solid electrolytes that maximizes their advantages and mitigates limitations. This account is initiated by introducing fundamental theories and simulation techniques to investigate the ionic conductivity of an inorganic solid electrolyte. Subsequently, we present our recent progress in designing high ionic conductivity electrolytes by increasing the concentration of Li vacancies, by tuning the type of defects, by constructing diffusion pathways, and by avoiding ion crowding. At last, the electrochemical stability of inorganic solid electrolytes and their compatibility with lithium-metal electrodes are addressed. In bulk electrolytes, increasing the defect concentration can often enhance the ionic conductivity. For instance, to surpass the upper limit of Li vacancy concentration without compromising structural stability, we adopt an antispinel crystal structure, enhancing Li mobility within the Li3OBr electrolyte. The type of defects also matters. Instead of O doping, we propose to introduce Li interstitials effectively through S doping, significantly reducing lattice distortion and eliminating the anchoring effect of Li around the O dopant. Compared to the electrolyte with vacancy defects, introducing Li interstitials boosts the ionic conductivity of Li3OCl by 3 orders of magnitude. In addition to defect engineering, designing a three-dimensional diffusion pathway for Li ions enhances bulk ionic conductivity. While LaCl3-based electrolytes exhibit good compatibility with Li-metal electrodes, their intrinsic low ionic conductivity poses limitations. We propose constructing a three-dimensional diffusion pathway by connecting neighboring one-dimensional channels through the introduction of La vacancies, significantly enhancing the ionic conductivity of LaCl3-based electrolytes at room temperature. Furthermore, we investigate ion diffusion in the space charge layer (SCL) near charged solid interfaces. We observe that the mobility of Li interstitials in the SCL is close to that of Li vacancies in bulk electrolytes. However, a defect-deficient region within the SCL may induce high ionic resistance. These studies demonstrate that material design based on simulation techniques offers promise for the development of solid electrolytes and the advancement of electrochemical energy storage devices.