Nb5+-doped P2-type Mn-based layered oxide cathode with an excellent high-rate cycling stability for sodium-ion batteries

Abstract

P2-Na0.67Co0.25Mn0.75-xNbxO2 (x = 0, 0.015, 0.030, 0.045) cathode materials were obtained by a simple solid-state method. The Nb5+-doped effects on ameliorating the structural and electrochemical properties of the Na0.67Co0.25Mn0.75O2 parent material for sodium-ion batteries have been investigated. XRD measurements confirm a hexagonal main phase (P63/mmc) and some trace of orthorhombic NaNbO3 (P21ma). Rietveld refinements exhibit the higher lattice parameters and the bigger c/a values due to Nb5+ doping into the parent Na0.67Co0.25Mn0.75O2. The optimized 3.0 at.% Nb5+-doped P2-Na0.67Co0.25Mn0.75O2 sample delivers the initial discharge capacity of 126.7 mAh g−1 at 0.1 C in the voltage range of 1.8–4.0 V, and its capacity retention is 75.3% after 100 cycles, which is 63.7% higher than that of the pristine. Whereas at 10 C rate, the optimized sample presents the maximal discharge capacity of 72.8 mAh g−1 with an excellent high-rate cycling stability of 91.6% retention after 100 cycles, correspondingly the pristine shows 61.4 mAh g−1 and 73.1% retention. These excellent electrochemical performances are ascribed to better structural stability, and lower charge transfer resistance and higher Na+ diffusion coefficient after Nb5+ doping. Therefore, the Nb5+-doped P2-type cathode materials can be deemed as a beneficial improvement for researches on sodium-ion batteries.