Sn/Ti Co-Substitution Boosting NASICON-Type Symmetric Cell With Enhanced Electrochemical Performance

To address the growing industrial demand for sodium-ion batteries (SIBs) with high energy density, this work designed and synthesized a novel manganese-based NASICON-structure Na₃MnTi_0.5Sn_0.5(PO₄)₃ (NMTSP). Leveraging the multi-step reversible redox reactions of Mn²⁺/Mn³⁺/Mn⁴⁺ and Ti³⁺/Ti⁴⁺, NMTSP exhibits favorable cathodic performance, delivering a reversible specific capacity of 137.4 mAh g^-1 along with favorable cycling stability. When evaluated as an anode, the Sn-rich NMTSP also shows considerable sodium-storage capability, with a reversible capacity of 219.0 mAh g^-1 and a retained capacity of 99.4 mAh g^-1 at 571 mA g^-1. On this basis, an NMTSP||NMTSP symmetric full-cell was further constructed by taking advantage of the complementary redox characteristics of Mn, Ti, and Sn. The assembled cell delivers an operating voltage of 1.48 V and an energy density of 134.5 Wh kg^-1, while retaining 76.0% of its capacity after 100 cycles at 315 mA g^-1. These results demonstrate that Ti/Sn co-substitution is an effective strategy for tuning the electrochemical behavior of Mn-based NASICON electrodes and highlight the potential of NMTSP for application in high-performance symmetric SIBs.