Manipulation of Transition Metal Migration via Cr-Doping for Better-Performance Li-Rich, Co-Free Cathodes

The irreversible migration of transition metals is a primary issue, resulting in detrimental structural changes and poor battery performance in Li-rich layered oxide (LLO) cathodes. Herein, we propose that manipulating the migration of transition metals between octahedral and tetrahedral sites effectively inhibits undesirable phase transitions by stabilizing the delithiated structure of LLOs at high potential. This is demonstrated by introducing Cr into the Co-free LLO, Li_1.2Ni_0.2Mn_0.6O₂. A new spinel-like phase, accompanied by significant lattice variation, was observed in the heavily cycled Co-free LLO at high potential by using operando synchrotron characterizations. Benefiting from a well-maintained solid-solution reaction after long-term cycling, Cr-doped Li_1.2Ni_0.2Mn_0.6O₂ delivers up to 99% of its initial discharge capacity after 200 cycles at 1C (∼200 mAh g^-1), far surpassing the pristine material (∼74%). The work provides valuable insights into the structural degradation mechanisms of LLOs and underscores the importance of stabilizing the delithiated structure at high potential.