The enlarged interlayer spacing in NaNi_0.5Mn_0.5O by doping with Sn⁴⁺ prevents TMO₂ slips and eliminates irreversible multiphase transitions during cycling, achieving a high capacity of 191 mA h g⁻¹ at 0.1C for half cells, as well as 1000 long cycles at 1C and high power ability at 50C for the full cell.