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

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.

Automated summary

This study compared the Li-ion transport behavior in primitive NCA and ball-milled NCA to reveal the critical role of grain boundaries in facilitating efficient Li-ion transport. The grain boundaries in NCA can construct an interconnected Li-ion transport network, contributing to excellent high-rate performance.