Al and cation vacancies promote reversibility of Cobalt-free layered cathodes

This work emphasizes that structural irreversibility and phase transitions should be as important as Li/Ni intermixing in the rational design of Co-free cathodes, which, however, has been neglected in previous considerations. Al is able to suppress Li/Ni intermixing in Co-free layered cathodes. However, we find that Al exacerbates the structural irreversibility and significantly expands the two-phase region, resulting in a large initial irreversible capacity and poor Li conductivity. Remarkably, cation vacancy-assisted Al substitution exhibits a remarkable effect in enhancing the structural reversibility and suppressing the two-phase region in Co-free layered cathodes. Moreover, cation vacancies greatly reduce the energy barrier for Li migration by an order of magnitude, and the detrimental strain between H2 and H3 phases is significantly reduced by 78%. The newly designed Co-free layered cathode achieves the advantages of small irreversible capacity, high Coulombic efficiency, and enhanced cycling stability through the synergistic effect of Al and cation vacancies. These insights reveal the rational design principles for Co-free cathodes, and we believe that cation vacancy-assisted metal substitution will be an effective design strategy to enrich the chemistry of Co-free cathodes to meet the expectations of next-generation Li-ion batteries.