Synthesis-Dependent Grain Boundary Density Governs Low-Temperature Lithium Diffusion in Single-Crystal NCM Cathodes

Mid-nickel LiNi_0.6Co_0.1Mn_0.3O₂ (NCM613) single-crystal cathodes were synthesized via co-precipitation and spray pyrolysis routes and systematically compared to elucidate the relationship between synthesis-driven single-crystallization efficiency, internal grain boundary characteristics, and electrochemical performance. Although both materials exhibit comparable particle sizes and chemical compositions, pronounced differences in internal grain boundary density were observed depending on the synthesis route. The spray pyrolysis-derived NCM613 forms a highly integrated single-crystal-like structure with a substantially reduced density of internal grain boundaries compared to its co-precipitation-derived counterpart. Electrochemical and kinetic analyses reveal that residual internal grain boundaries disrupt continuous Li-ion diffusion, resulting in pronounced performance deterioration that becomes particularly severe under low-temperature operation. At -10°C, increased grain boundary density induces significant transport heterogeneity and persistent surface microcracking, ultimately accelerating electrochemical degradation.