Improved Cycling Performance of P2-Na0.67Ni0.33Mn0.67O2 Based on Sn Substitution Combined with Polypyrrole Coating

P2-Na_0.67Ni_0.33Mn_0.67O₂ presents high working voltage with a theoretical capacity of 173 mAh g^-1. However, the lattice oxygen on the particle surface participates in the redox reactions when the material is charged over 4.22 V. The resulting oxidized oxygen aggravates the electrolyte decomposition and transition metal dissolution, which cause severe capacity decay. The commonly reported cation substitution methods enhance the cycle stability by suppressing the high voltage plateau but lead to lower average working voltage and reduced capacity. Herein, we stabilized the lattice oxygen by a small amount of Sn substitution based on the strong Sn-O bond without sacrificing the high voltage performance and further protected the particle surface by polypyrrole (PPy) coating. The obtained Na_0.67Ni_0.33Mn_0.63Sn_0.04O₂@PPy (3.3 wt %) composite showed excellent cycling stability with a reversible capacity of 137.6 (10) and 120.0 mAh g^-1 (100 mA g^-1) with a capacity retention of 95% (10 mA g^-1, 50 cycles) and 82.5% (100 mA g^-1, 100 cycles), respectively. The present work indicates that slight Sn substitution combined with PPy coating could be an effective approach to achieving superior cycling stability for high-voltage layered transition metal oxides.