3.0 V High Energy Density Symmetric Sodium-Ion Battery: Na₄V₂(PO₄)₃∥Na₃V₂(PO₄)₃

Symmetric sodium-ion batteries (SIBs) are considered as promising candidates for large-scale energy storage owing to the simplified manufacture and wide abundance of sodium resources. However, most symmetric SIBs suffer from suppressed energy density. Here, a superior congeneric Na₄V₂(PO₄)₃ anode is synthesized via electrochemical preintercalation, and a high energy density symmetric SIB (Na₃V₂(PO₄)₃ as a cathode and Na₄V₂(PO₄)₃ as an anode) based on the deepened redox couple of V⁴⁺/V²⁺ is built for the first time. When measured in half cell, both electrodes show stabilized electrochemical performance (over 3000 cycles). The symmetric SIBs exhibit an output voltage of 3.0 V and a cell-level energy density of 138 W h kg^-1. Furthermore, the sodium storage mechanism under the expanded measurement range of 0.01-3.9 V is disclosed through an in situ X-ray diffraction technique.