Is surface modification effective to stabilize high-voltage cycling for layered P2-Na_2/3Ni_1/3Mn_2/3O₂ cathodes?

Layered transition metal oxides (TMOs), like the P2-type Na_2/3Ni_1/3Mn_2/3O₂, are promising cathodes for sodium-ion batteries but suffer rapid capacity degradation at high voltages. Surface engineering is a popular strategy to modify the high-voltage stability of cathode materials, yet its efficacy for sodium layered TMOs remains elusive, especially given the deleterious layer-gliding phase transition during high-voltage operation. Here, we examined the effect of surface coatings on the high-voltage cycling stability of Na_2/3Ni_1/3Mn_2/3O₂, finding that they suppress high-voltage polarization but do not significantly affect capacity retention, which is mainly impacted by bulk structure degradation. Hence, surface engineering must be complemented with bulk structure modification to stabilize high-voltage cycling.