Copper-substituted Na0.67Ni0.3−xCuxMn0.7O2 cathode materials for sodium-ion batteries with suppressed P2–O2 phase transition

Abstract

P2-type sodium layered oxides Na_xMO₂ (M = transition metal) are considered as one kind of promising cathode material for sodium-ion batteries because of their known structures, superior electrochemical properties, and their ease of synthesis. The Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ redox reactions endow the P2–Na_2/3Ni_1/3Mn_2/3O₂ electrode with a relatively high operating voltage and high specific capacity. However, the phase transition from P2 to O2 and Na⁺/vacancy ordering make P2–Na_2/3Ni_1/3Mn_2/3O₂ susceptible to severe voltage and capacity decay. Herein, we propose to employ the electrochemically active copper(II) as a unique substituent to stabilize the P2 phase, forming Na_0.67Ni_0.3−xCu_xMn_0.7O₂ (x = 0, 0.1, 0.2 and 0.3). Our work highlights the importance of Cu(II) in the structural engineering of high performance cathode materials, whose existence can not only stabilize the P2 phase against the notorious phase transition, but also contribute to the rechargeable capacity due to the high potential Cu²⁺/Cu³⁺ redox. We identified that the cathode formulated as P2-type Na_0.67Ni_0.1Cu_0.2Mn_0.7O₂ shows favorable battery performance with much-alleviated structural degradation.