Structure–Interface Coupling in Li-Rich Mn-Based Cathodes for All-Solid-State Batteries: Single-Crystal Versus Polycrystalline Architectures

Abstract

Li-rich Mn-based layered oxides (LRM) are attractive high-energy cathodes for all-solid-state batteries (ASSBs) because of their high theoretical capacity, low cost, and Mn-rich composition. However, their practical implementation is constrained by oxygen-redox instability, voltage decay, chemo-mechanical degradation, and parasitic interfacial reactions with solid-state electrolytes (SSEs). These issues become more pronounced in ASSBs, where rigid solid–solid contact, limited interfacial wetting, and mechanically induced contact loss strongly affect cathode utilization and cycling stability. This review critically compares polycrystalline LRM (PC-LRM) and single-crystal LRM (SC-LRM) cathodes in ASSBs from the perspective of structure–interface coupling. PC-LRM provides high surface area and favorable electrolyte access in liquid cells, but its grain-boundary network and internal porosity accelerate lattice-oxygen release, electrolyte penetration, intergranular cracking, and electrochemical isolation in solid-state systems. In contrast, SC-LRM suppresses grain-boundary-induced cracking and reduces reactive interfacial area, thereby improving mechanical integrity and interfacial stability, although particle-size control, anisotropic Li-ion transport, exposed facets, and limited solid–solid contact remain important challenges. We further summarize recent progress in matching LRM cathodes with sulfide, halide, and oxide SSEs, focusing on surface coatings, elemental doping, oxygen-vacancy regulation, particle-size engineering, and interlayer design. Finally, we discuss electrolyte-specific design principles and future opportunities based on operando characterization, multiscale simulation, and machine-learning-guided materials optimization for high-energy ASSBs.