Mechanical Processing Aggravates Cycling Degradations of LiCoO 2

Abstract Intrinsic and cycling‐induced defects in cathode materials of lithium‐ion battery are revealed to play conflicting roles in either boosting or deteriorating the electrochemical performance. However, the impact of mechanical defects introduced during the battery manufacturing process is not systematically investigated, nor is their evolution upon electrochemical cycling. Herein, taking high‐energy LiCoO 2 cathode material as an example, it is shown that mechanical defects can be easily generated during the industrial manufacturing process. Electron microscopy characterizations reveal five major mechanical defects: slip, dislocation, lattice distortion, kink boundary, and crack, which are proven detrimental to cycling stability and rate capability. Upon low‐voltage cycling, aggravated interfacial degradations are the major cause of the fast performance decay, which is primarily due to crack propagation and consequential cathode electrolyte interphase (CEI) growth, leading to high interfacial polarization and reduced interfacial reaction kinetics. Upon high voltage cycling, all kinds of mechanical defects evolve significantly, which together lead to severe structural degradations by forming a degraded spinel structure, voids, and microcracks. This work highlights that manufacturing process‐induced mechanical defects can significantly aggravate material degradation during subsequent electrochemical cycling, suggesting that minimizing pre‐cycling mechanical defects and enhancing mechanical strength can effectively improve cathode material performance.