Cracking vs. surface reactivity in high-nickel cathodes for lithium-ion batteries

Summary

High-nickel layered oxide cathodes LiNi_xMn_yCo_zO₂ (NMC) experience microcracks during cycling. This can expose fresh cathode surfaces for parasitic reactions and isolate active cathode material from the conductive electrode matrix, resulting in impedance increase and capacity fade. The commonly held belief attributes microcracks to anisotropic lattice volume changes of primary particles during cycling. Nevertheless, recent reports suggest that certain electrolytes might reduce microcracks in NMC cathodes during deep cycling. This raises a crucial question on the origin of microcracks: do microcracks exacerbate surface stability, or does poor surface stability contribute to microcrack formation? This perspective aims to provide context and expound on this "chicken or egg" dilemma. We contend that the consequence of surface reactivity on the cycle life of high-Ni cathodes is more pronounced than that of particle cracking. We hypothesize that particle cracking is more of a symptom of severe surface reactivity rather than a cause of capacity fade.