In-situ construction of a crystalline LiAlO₂ ion-conducting layer via residual lithium conversion for ultra-stable Ni-Rich NCM811 cathodes

The commercialization of Ni-rich layered oxide cathodes is critically impeded by interfacial instability, primarily caused by spontaneous reactions with ambient moisture/CO₂ that generate electrochemically inert residual lithium species (Li₂CO₃/LiOH). These residues not only impede Li⁺ transport, but also trigger electrolyte decomposition, accelerating capacity fade. While converting these residues into protective coatings is an attractive mitigation strategy, the resulting layers are often amorphous, lacking the structural integrity and ionic conductivity required for long-term cycling stability. Herein, we transform this inherent drawback into an advantage by in situ synthesizing a crystalline LiAlO₂ coating (5–10 nm) from the residual lithium. Advanced electron microscopy confirms its near-epitaxial growth on NCM811 primary particles, which is facilitated by a minimal lattice mismatch. This coherent interface consumes electrochemically harmful residual lithium and reduces interface resistance. It can also act as a physical barrier to ion conduction, inhibit parasitic reactions, and mitigate the structural failure during cycle process. Crystal LiAlO₂ is a rapid Li⁺ conductor with a 3D diffusion path, thereby enhancing reaction kinetics. The modified cathode delivers ultra-stable cycling (95.9% capacity retention after 100 cycles at 1 C) and superior rate performance (124.0 mAhg^− 1 at 10 C). This study provides a modified strategy for constructing crystalline LiAlO₂ coating on the surface of LiNi_0.8Co_0.1Mn_0.1O₂ cathode, fundamentally highlighting the critical role of coating crystallinity in governing interfacial ion transport and structural integrity.