Symmetric Sodium-Ion Battery Based on Dual-Electron Reactions of NASICON-Structured Na₃MnTi(PO₄)₃ Material

Symmetric sodium-ion batteries possess promising features such as low cost, easy manufacturing process, and facile recycling post-process, which are suitable for the application of large-scale stationary energy storage. Herein, we proposed a symmetric sodium-ion battery based on dual-electron reactions of a NASICON-structured Na₃MnTi(PO₄)₃ material. The Na₃MnTi(PO₄)₃ electrode can deliver a stable capacity of up to 160 mAh g^-1 with a Coulombic efficiency of 97% at 0.1 C by utilizing the redox reactions of Ti^3+/4+, Mn^2+/3+, and Mn^3+/4+. This is the first time to investigate the symmetric sodium-ion full cell using Na₃MnTi(PO₄)₃ as both cathode and anode in the organic electrolyte, demonstrating excellent reversibility and cycling performance with voltage plateaus of about 1.4 and 1.9 V. The full cell exhibits a reversible capacity of 75 mAh g^-1 at 0.1 C and an energy density of 52 Wh kg^-1. In addition, both ex situ X-ray diffraction (XRD) analysis and first-principles calculations are employed to investigate the sodiation mechanism and structural evolution. The current research provides a feasible strategy for the symmetric sodium-ion batteries to achieve high energy density.