Is surface modification effective to stabilize high-voltage cycling for layered P2-Na2/3Ni1/3Mn2/3O2 cathodes?

Layered transition metal oxides (TMOs), like the P2-type Na2/3Ni1/3Mn2/3O2, 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 Na2/3Ni1/3Mn2/3O2, 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.