Environmentally responsible synthesis of high-performance P2-Na2/3[Ni1/3Mn2/3]O2 sodium-ion battery cathodes

Abstract

P2-type Na_2/3[Ni_1/3Mn_2/3]O₂ (NNMO) is a promising Na-ion battery cathode for its high operation voltage, high gravimetric capacity, and low critical element content. Current methods to make NNMO, such as sol–gel, solid-state reaction, and spray pyrolysis, are not environmentally friendly, as they often use acetate and nitrate metal salt precursors, which release undesirable gaseous byproducts such as CO₂ and HNO₃ when these metal salts are heat-treated at high temperatures. Decarbonizing our industries and the transportation sector requires not only using batteries to store intermittent renewable energy, but also decarbonizing the synthesis methods to make key components of these batteries, i.e., the electrode materials. Here, we demonstrate an environmentally responsible synthesis of NNMO by direct conversion of metallic Ni powder with various particle sizes (10 nm, 100 nm, 1 μm, 5 μm), a process that only releases O₂ and H₂O as byproducts, as opposed to the harmful gases released when nickel salts are used. The resulting NNMO made from ≈1 μm commercial Ni powder exhibits an excellent cycling performance as a Na-ion battery cathode. When cycled between 2-4 V vs. Na/Na⁺, the material exhibits a rate-capability of ≈83 mA h g⁻¹ at 10C (i.e., > 96% theoretical capacity) and ≈ 78 mA h g⁻¹ at 20C (i.e., >90% theoretical capacity), together with a remarkable stability of 93% capacity retention after 500 cycles at 10C. When cycled between 1.3–4 V vs. Na/Na⁺, the material shows a high initial capacity of ≈134 mA h g⁻¹ at 2C, with a capacity retention of 57% after 500 cycles.