Calcium-Doping for Structure Stabilization of Sodium Transition Metal Oxide Cathodes in Sodium Ion Batteries

NaNi_0.5Mn_0.5O₂ (NNMO), which has a layered structure, is a promising cathode material for sodium ion batteries. The stack of Mn(Ni)O₂ slabs in a layered structure of NaiNi_0.5Mn_0.5O₂, are composed of edge-shared Mn(Ni)O₆ octahedra, forming metal-occupied octahedral sites and empty tetrahedral sites. The octahedral sites in Na slabs are also occupied by Na⁺ ions, which are sandwiched by edge-sharing with Mn(Ni)O₂ slabs. In this work, O3-Ca_xNa_1-2x[Ni_0.5Mn_0.5]O₂ (0 < x ≤ 0.1) was synthesized by incorporating Ca²⁺ into NaO₆ layer of Na[Ni_0.5Mn_0.5]O₂. Both the significant difference in ionic radii between Ca²⁺ (1.12Å) and Ni²⁺ (0.69Å) or Mn⁴⁺ (0.53 Å) and the reduction of Na⁺ content allowed the preparation of phase-pure and cation-mixing-free (i.e., absence of Ca²⁺ in transition metal oxide layers) Ca_xNa_1-2x[Ni_0.5Mn_0.5]O₂ up to x = 0.05. It was expected that, while no noticeable change was observed in a dimension of MnO₆ octahedron, NaO₆ octahedron continuously shrank as the increase of Ca²⁺ content. The latter effect was expected to prevent the structural degradation during repeated charge-discharge, eventually contributing to the improvement of capacity retention. Preliminary study indicated that, while pristine Na[Ni_0.5Mn_0.5]O₂ showed a continuous decrease in discharge capacity, Ca_0.05Na_0.90[Ni_0.5Mn_0.5]O₂ maintained a significant fraction of an initial capacity with a small capacity fading during 50 cycles. The excellent capacity retention of Ca²⁺-doped one is believed because the strong interaction of immobile Ca²⁺ with O^2- helps restore of the initial O3 structure, which leads to the maintenance of distinctive phase transition steps on discharge profiles after repeated cycles in contrast to pristine one.