Coupling Particle Size and Composition to Enable Synergistic Enhancement in Ni‐Rich LiNi <i> _x </i> Co <i> _y </i> Mn _{(1‐ <i>x</i> ‐ <i>y</i> )} O ₂ Cathodes

Ni‐rich layered Li(Ni, Co, Mn)O 2 (NCM) oxides are important cathode materials for high‐energy large‐format Li‐ion batteries (LIBs). The urge for higher energy densities at a lower cost has continuously driven the use of NCM with higher Ni contents exceeding 80%. This work presents a coupling strategy of Ni‐rich layered oxide cathodes, consisting of large NCM (LNCM; LiNi 0.83 Co 0.12 Mn 0.05 O 2 , d 50 = 9.4 µm) particles having a relatively lower Ni but a higher Co content than the small NCM (SNCM, LiNi 0.88 Co 0.06 Mn 0.05 Al 0.01 O 2 , d 50 = 3.5 µm), to balance the intrinsic trade‐offs in lithium‐ion battery performance. The LNCM particles enhance structural stability and lithium‐ion diffusion, while SNCM particles provide high specific capacity and mitigate cracking issues associated with large particles. By strategically coupling particle size with transition‐metal composition, this approach achieves more than conventional packing‐density optimization. Optimized large‐to‐small ratios (70/30 and 50/50) deliver up to 38% higher volumetric capacity, &gt;20% enhancement in capacity retention after 200 cycles, and a 32.7% improvement in fast‐charging capability. Importantly, using the coupling strategies can significantly suppress both surface side reactions and mechanical degradation, offering a mechanistic pathway to simultaneously improve energy density, rate performance, and cycling stability.