Interplay of local and long-range structure underpins electrochemical behaviour in disordered rock salt LiNiO2

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Abstract
Lithium nickel oxide (LiNiO2, LNO) is a promising Co-free cathode but suffers structural degradation at high and low voltages. Here we report the synthesis of a cation-disordered rock-salt LiNiO2 (DRS-LNO; Li:Ni ≈ 1:1) from mechanochemical treatment of layered LNO. Its electrochemical behaviour arises from the interplay between local and average structure: reverse Monte Carlo modelling of total scattering data reveals pronounced Li–Li and Ni–Ni clustering (Warren–Cowley α (NiLi) = 0.054(4)) and expanded Li–O distances, enabling Li diffusion pathways that allow ~70% of Li to participate in long-range transport. This accounts for the unexpectedly high first-cycle charge capacity of 185(9) mAh g -1 at 10 mA g -1 despite the absence of Li excess. The disordered average structure leads to small isotropic volume changes (~2.1%) over large electrochemical windows, also accessing reversible capacity from low-voltage Ni 3+ /Ni 2+ redox at low voltages. Operando total scattering suggests that reversible Ni migration takes place at high delithiation, forming regions of α-NaFeO2-like local order. On discharge, the reduction from Ni 3+ to Ni 2+ is associated with the reversible formation of a secondary phase whose formation can be partially suppressed at higher current densities, with implications in the cycling stability of DRS-LNO. These results show that mechanochemistry can access unique local structural motifs that tune electrochemical behaviour, expanding the design space for next-generation cathodes.