Mitigating Sodium Ordering for Enhanced Solid Solution Behavior in Layered NaNiO₂ Cathodes

The O-type layered nickel oxides suffer from undesired cooperative Jahn-Teller distortion stemming from Ni³⁺ ions and undergo multiple biphasic structural transformations during the insertion/extraction of large Na⁺ ions, posing a significant challenge to stabilize the structural integrity. We present here a systematic investigation of the impact of substituting 5 % divalent (Mg²⁺) or trivalent (Al³⁺ or Co³⁺) ions for Ni³⁺ to alleviate Na⁺ion ordering and perturb the Jahn-Teller effect to enhance structural stability. We gauge a fundamental understanding of the Mg-O and Na-O or Mg-O-Na bonding interactions, noting that the ionicity of the Mg-O bond deshields the electronic cloud of oxygen from Na⁺ ions. Furthermore, calculations of the Van Vleck distortion modes reveal a relaxation of NiO₆ octahedra from Jahn-Teller distortion and a reduced electron density at the interlayer with Mg²⁺ substitution. Long-range (operando X-ray diffraction) and short-range (magic angle spinning nuclear magnetic resonance) structural analyses provide insights into reduced ordering, allowing a stable continuous solid solution. Overall, Mg-substitution results in a high-capacity retention of ~96 % even after 100 cycles, showcasing the potential of this strategy for overcoming the structural instabilities and enhancing the performance of sodium-ion batteries.