Mitigating Sodium Ordering for Enhanced Solid Solution Behavior in Layered NaNiO 2 Cathodes.

The O-type layered nickel oxides suffer from undesired cooperative Jahn-Teller distortion stemming from Ni ³⁺ 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 (Mg ²⁺ ) or trivalent (Al ³⁺ or Co ³⁺ ) ions for Ni ³⁺ 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 NiO ₆ octahedra from Jahn-Teller distortion and a reduced electron density at the interlayer with Mg ²⁺ 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.
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