Unveiling the impact of residual Li conversion and cation ordering on electrochemical performance of Co-free Ni-rich cathodes

The residual Li and Li⁺/Ni²⁺ cation mixing play essential roles in the electrochemical properties of Ni-rich cathodes. However, a general relationship between the residual Li conversion, cation mixing, and their effects on the Li⁺ kinetics and structural stability has yet to be established, due to the presence of cobalt in the cathode. Here, we explore the synergistic impact of the residual Li conversion and cation ordering on a Co-free Ni-rich cathode (i.e., LiNi_0.95Mn_0.05O₂). It discloses that the rate capability is mainly affected by residual Li contents and operating voltage. Specifically, residual Li can be electrochemically converted to cathode electrolyte interphase (CEI) below 4.3 V, thus inducing high interphase resistance, and decomposes to produce CO₂-dominated gas at 4.5 V, causing temporary enhancement of Li⁺ diffusivity but severe surface degradation during cycling. Moreover, the cycling performance of Co-free Ni-rich cathode is not only determined by Li⁺/Ni²⁺ cation-ordered superlattice, which enhances the structural stability as it functions as the pillar to impede lattice collapse at a highly charged state, but also by the robust CEI layers which protect the bulk from electrolyte attack under 4.3 V. These findings promote an in-depth understanding of residual Li conversion and Li⁺/Ni²⁺ cation ordering on Co-free Ni-rich cathode.