Self-Limiting Phase Transition Enabling Reversible Overstoichiometric Li Storage in Ni-Rich Cathodes

Ni-rich cathodes are some of the most promising candidates for advanced lithium-ion batteries, but their available capacities have been stagnant due to the intrinsic Li⁺ storage sites. Extending the voltage window down can induce the phase transition from O3 to 1T of LiNiO₂-derived cathodes to accommodate excess Li⁺ and dramatically increase the capacity. By setting the discharge cutoff voltage of LiNi_0.6Co_0.2Mn_0.2O₂ to 1.4 V, we can reach an extremely high capacity of 393 mAh g^-1 and an energy density of 1070 Wh kg^-1 here. However, the phase transition causes fast capacity decay and related structural evolution is rarely understood, hindering the utilization of this feature. We find that the overlithiated phase transition is self-limiting, which will transform into solid-solution reaction with cycling and make the cathode degradation slow down. This is attributed to the migration of abundant transition metal ions into lithium layers induced by the overlithiation, allowing the intercalation of overstoichiometric Li⁺ into the crystal without the O3 framework change. Based on this, the wide-potential cycling stability is further improved via a facile charge-discharge protocol. This work provides deep insight into the overstoichiometric Li⁺ storage behaviors in conventional layered cathodes and opens a new avenue toward high-energy batteries.