Delineating the Impact of Ti/Mg Substitution in P2-type Na2/3Ni1/3Mn2/3O2 with Advanced Electrolyte for Sodium-ion Batteries

Abstract

Sodium layered oxide cathodes are drawing interest globally as a potential alternative to lithium layered oxides, but they suffer from egregious capacity fade and have intrinsically lower capacity. P2-type Na2/3Ni1/3Mn2/3O2 is a particularly relevant chemistry as it demonstrates up to 550 Wh kg-1 at high operating potentials, although this can only be maintained for a handful of cycles with industrial electrolytes. Here, a localized saturated electrolyte (LSE) is shown to significantly improve the cycle life of Na2/3Ni1/3Mn2/3O2 by suppressing the surface reactivity, despite large volume changes during cycling. The demonstrated influence of surface stability on cycle life in this work challenges the prevailing notion of a popular capacity stabilization strategy with titanium/magnesium co-doping, which is primarily thought to improve cycle life via improved structural stability. Single crystals of Na2/3Ni1/3-xMgxMn2/3-2xTi2xO2 (x = 0, 1/48, 1/24, 1/12) materials are cycled with a traditional electrolyte and the LSE to demonstrate that despite eliminating the phase transition with dopants in Na2/3Ni1/4Mg1/12Mn1/2Ti1/6O2, the predominant role of the dopants is in reducing the parasitic oxygen reactivity at the cathode surface. The different roles these dopants play is systematically disambiguated, and this work can guide future research to focus on reducing the parasitic cathode/electrolyte reactivity further.