Highly efficient, fast and reversible multi-electron reaction of Na₃MnTi(PO₄)₃ cathode for sodium-ion batteries

The sodium super ionic conductor (NASICON) structured Na₃MnTi(PO₄)₃ is attractive due to the eco-friendly and low-cost Na–Mn–Ti–P–O system. However, Na₃MnTi(PO₄)₃ suffers from low Coulombic efficiency, inferior electronic conductivity and limited specific capacity basing on two-sodium-exchange. To address these multifaceted issues, herein, an interpenetrating graphene encapsulated Na₃MnTi(PO₄)₃ particles with carbon-shell covering material (rGO@NMTP-C) was synthesized. By regulating cut-off voltages, a three-electron reaction was realized and Coulombic efficiency of the rGO@NMTP-C was found to increase from <96% to 100% due to the fast kinetics that enhanced by Ti³⁺/Ti⁴⁺ redox, which was verified by EIS and GITT. Sodium storage mechanism of the multielectron reaction was deeply studied by ex-situ XRD and DFT calculations. It shows that the enhanced reversibility synergizing with multifunctional 3D conductive network and the contribution of pseudocapacitance, makes rGO@NMTP-C a promising cathode candidate, which can exhibit a remarkable discharge capacity of 173 mA h g⁻¹. Additionally, for the first time, the rGO@NMTP-C achieves outstanding high rate capability of 92.4 mA h g⁻¹ at 50C, and cycling stability of 74.5% of capacity retention after 3500 cycles at 20C. High performance full cells were also realized for practical utilization of SIBs.