Elucidating and controlling phase integration factors in Co-free Li-rich layered cathodes for lithium-ion batteries

Li- and Mn-rich layered oxides (LLOs) with a Co-free composition are promising candidates for next-generation cathodes in low-cost and high-energy-density lithium-ion batteries. Despite their potential, the commercialization of Co-free LLOs encounters several electrochemical challenges, such as low activity and initial coulombic efficiency of the first activation cycle and compromised cycle retention, which are primarily attributed to the poor phase integrity between LiTMO₂ and Li₂MnO₃ domains. In this study, we identified that the compromised phase integrity in Co-free LLOs can be driven by the sticking Ni²⁺ compositional design, which induces Li-Ni site-exchange defects in the LiTMO₂ domain, leading to severe TMO₂ slab mismatches between phases and resulting in a penalty in enthalpy mixing energy. To address this, we proposed a rational off-stoichiometric compositional design. By introducing a slight excess of Li, the Ni valence state shifts slightly from +2 to +3, reducing the superexchange interaction and significantly suppressing site exchange formation. The off-stoichiometric Co-free LLO shows highly integrated domains, markedly improving all electrochemical parameters, including coulombic efficiency, cycle stability, and voltage decay. These findings deepen the understanding of designing domain structures to enhance the redox chemistry of the LLO cathode class.