Structural regulation of P2-typed cathode via multi-cation doping for high-rate and stable sodium-ion batteries

Abstract

Mn, Ni-based P2-type layered Na0.7Mn0.65Ni0.35O2 has been extensively studied because of its good air stability and electrochemical performance. However, it usually suffers from phase transitions at high voltage and different Na-vacancy ordering during sodium (de)intercalation, bringing about rapid capacity decline and poor rate capability. In this work, an entropy-modulation strategy based on multiple-cation doping is used to design a P2-type Na0.7Mn0.65Ni0.15Cu0.12Li0.03Fe0.05O2 (NMNCLFO) cathode. Fe3+, Cu2+ and Li+ are successfully doped into the transition metal layers, which can synergistically regulate the lattice parameters, structure entropy and anionic redox activity, leading to the improved structure stability and reaction kinetics. As a result, NMNCLFO has smoothed electrochemical curves, higher capacities (126.72 mAh g−1 at 50 mA g-1), superior cycling stability (capacity retention of 76.2% at 1.0 A g-1 within 1000 cycles) and rate capability (76.2 mAh g−1 at 2.0 A g-1) compared with the undoped cathode and mono-cation doped cathodes. Based on the ex-situ X-ray diffraction and in-situ electrochemical impedance spectroscopy measurements, it is found P2 NMNCLFO is stable during the (de)intercalation process, predicating the regulated lattice and lower Gibbs free energy are able to effectively inhibit the migration of transition metal (TM) ions and severe sliding of the TMO2 layers.