Mitigating Anisotropic Changes in Classical Layered Oxide Materials by Controlled Twin Boundary Defects for Long Cycle Life Li-Ion Batteries

The classical layered NMC oxides LiNi_xMn_yCo_1-x-yO₂(0 < (x,y) < 1) are promising high energy density cathodes for Li-ion batteries. However, their inherent structure instability at the highly delithiated state causes capacity degradation as cycling proceeds. Here, we report a mitigating strategy for addressing the capacity decay problem in multiple classical NMC materials through the design of controlled twin boundary defects. The radially aligned twin boundary defects are engineered in nanosized NMC cathodes through polyol synthesis. The crystallographic orientation of each subgrain rotates across the twin boundaries, and the particles have maximum exposure to the electrolyte with the (003) planes (which are more stable than other planes). Increased cation disorder and the formation of rocksalt-like phase are consistently observed along the twin boundaries through scanning transmission electron microscopy (STEM), acting as a rigid framework that mitigates anisotropic changes in NMC during cycling. Operando X-ray diffraction confirms this hypothesis as the degree of anisotropic changes is minimized in NMC with twin boundaries. The synthesized NMC materials with twin boundary defects exhibits enhanced electrochemical performance compared to the corresponding microsized materials with identical composition. The twin boundary defects engineering in NMC structure can effectively suppress the phase transformation and material degradation, serving as a novel and universal approach in designing stable intercalation compounds for high voltage long-cycle life Li-ion batteries.