Grain boundaries contribute to highly efficient lithium‐ion transport in advanced LiNi0.8Co0.15Al0.05O2 secondary sphere with compact structure

Abstract LiNi0.8Co0.15Al0.05O2 (NCA) secondary particles with high tap density have a great potential for high volumetric energy density lithium (Li)‐ion power battery. However, the ionic conductivity mechanism of NCA with compact structure is still a suspense, especially the function of grain boundaries. Herein, we systematically investigate the Li‐ion transport behavior in both the primitive NCA (PNCA) secondary sphere densely grown by single‐crystal primary grains and ball‐milled NCA (MNCA) nanosized particle to reveal the role of grain boundaries for Li‐ion transport. The PNCA and MNCA have comparable Li‐ion diffusion coefficients and rate performance. Moreover, the graphene nanosheet conductive additive only mildly affects the Li‐ion diffusion in PNCA cathode, while which severely blocks the Li‐ion transport in MNCA cathode. Through high‐resolution transmission electron microscopy and electron energy loss spectroscopy, we clearly observe Li‐ion depletion at lower state of charge (SOC) and Li‐ion aggregation at high SOC along the grain boundaries of PNCA secondary particles during high‐rate lithiation process. The grain boundaries can construct an interconnected Li‐ion transport network for highly efficient Li‐ion transport, which contributes to excellent high‐rate performance of compact PNCA secondary particles. These findings present new strategy and deep insight in designing compact materials with excellent high‐rate performance.