Ruqin Ma, Siyuan Pan, Hanyan Wu, Yuxi Deng, Yuqi Wu, Yu Luo, Ying Lin, Qin Wang, Pengzhan Chen, Zhengliang Gong, Yong Yang
High-energy-density all-solid-state lithium batteries (ASSLBs) require cathodes with exceptional mechanical integrity, interfacial compatibility, and long-term electrochemical stability. Single-crystal (SC) layered oxides, distinguished from polycrystalline (PC) counterparts by their grain-boundary-free architecture and crystallographic uniformity, exhibit enhanced structural and interfacial stability while providing an ideal model system for decoupling electro-chemo-mechanical interactions. These characteristics enable precise investigation of facet-dependent transport, reaction kinetics, and degradation pathways─insights that can inform the design of both SC and advanced PC cathodes. In this review, we examine the anisotropic lithium transport, mechanical responses, and interfacial behaviors of SC cathodes, and compare them systematically with PCs to clarify how microstructural differences influence performance in ASSLBs. We further summarize advances in intrinsic material optimization, interfacial engineering, and composite electrode architectures, alongside state-of-the-art characterization and modeling tools for probing degradation mechanisms and coupling effects. Finally, we outline key challenges and research directions to accelerate the practical deployment of SC cathodes in next-generation high-energy-density ASSLBs.