Mitigating the P2–O2 phase transition of high-voltage P2-Na2/3[Ni1/3Mn2/3]O2 cathodes by cobalt gradient substitution for high-rate sodium-ion batteries

Abstract

High-voltage P2-Na_2/3[Ni_1/3Mn_2/3]O₂ as a high energy density cathode for sodium-ion batteries (SIBs) has attracted considerable attention. But the unfavorable P2–O2 phase transition and electrode/electrolyte side reactions easily occur when charged above 4.2 V (vs. Na/Na⁺), resulting in the rapid decay of capacity. Here, for the first time, a nanoscale cobalt gradient substitution is introduced to build a Co-enriched surface and cobalt-substituted interior, in which the cobalt-enriched surface is expected to reduce the side reactions and improve the Na⁺ kinetics while the cobalt substitution is supposed to mitigate the P2–O2 transition. Correspondingly, this gradient cobalt substituted P2-Na_2/3[Ni_1/3Mn_2/3]O₂ delivers a large reversible capacity of 164.6 mA h g⁻¹ with a high median potential of 3.55 V, achieving a high energy density of ∼585 W h kg⁻¹ and is comparable to the LiCoO₂ cathode in lithium-ion batteries. As anticipated, it shows a mitigated P2–O2 transition and thus exhibits an improved cycling stability. Besides, it delivers a much higher capacity of 110 mA h g⁻¹ at a high rate of 10C than reported pristine and modified electrodes by bulk doping and surface coating, indicating better high-rate properties. These gratifying achievements make this nanoscale gradient substitution an effective approach for suppressing the P2–O2 transition and improving the Na⁺ kinetics of P2-Na_2/3[Ni_1/3Mn_2/3]O₂ cathodes.