A high energy-density P2-Na2/3[Ni0.3Co0.1Mn0.6]O2 cathode with mitigated P2–O2 transition for sodium-ion batteries

Abstract

High voltage P2-Na_2/3[Ni_1/3Mn_2/3]O₂ is regarded as a promising cathode for high-energy sodium-ion batteries (SIBs). However, the undesired P2–O2 phase transition at high voltages above 4.0 V leads to a large volume change and further causes the rapid decay of capacity. Herein, an electrochemical-active Co³⁺ substitution is introduced to suppress the P2–O2 phase transformation but not at the cost of capacity. The spherical, Co³⁺ substituted P2-Na_2/3[Ni_0.3Co_0.1Mn_0.6]O₂ with a high tap-density of 1.86 g cm⁻³ is successfully synthesized by co-precipitation and solid-state reactions. As anticipated, it delivers a large specific capacity of 161.6 mA h g⁻¹ with a high median-voltage of 3.64 V (vs. Na/Na⁺), translated into a high energy-density of ∼590 W h kg⁻¹, which is comparable to that of the commercialized LiCoO₂ cathode in lithium-ion batteries. Apart from improved cycling stability ascribed to the mitigated P2–O2 transition, this cathode also shows a better rate property compared with those modified by Li⁺, Mg²⁺, Zn²⁺, Cu²⁺, and Ti⁴⁺ doping and Al₂O₃ coating. Besides, the P2-Na_2/3[Ni_0.3Co_0.1Mn_0.6]O₂|hard carbon full-cells deliver a reversible capacity of 150.6 mA h g⁻¹ and have enhanced cycle-life and high-rate capability. These gratifying achievements indicate that this P2-Na_2/3[Ni_0.3Co_0.1Mn_0.6]O₂ is a very promising candidate as a high energy-density cathode for SIBs.