A high energy-density P2-Na[NiCoMn]O cathode with mitigated P2-O2 transition for sodium-ion batteries.

High voltage P2-Na2/3[Ni1/3Mn2/3]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-Na2/3[Ni0.3Co0.1Mn0.6]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-Na2/3[Ni0.3Co0.1Mn0.6]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-Na2/3[Ni0.3Co0.1Mn0.6]O2 is a very promising candidate as a high energy-density cathode for SIBs.