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 Na 3 MnTi 0.5 Sn 0.5 (PO 4 ) 3 (NMTSP). Leveraging the multi‐step reversible redox reactions of Mn 2+ /Mn 3+ /Mn 4+ and Ti 3+ /Ti 4+ , 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.