Realizing the Storage Mechanisms Potentials Involving Multi-Ion Reactions for NASICON Cathodes in Rechargeable Hybrid...

NASICON-based sodium superionic conductors are promising cathode materials for sodium-ion batteries and hybrid aqueous batteries. However, a common NASICON cathode, Na₃V₂(PO₄)₃ (NVP), uses vanadium, which is expensive and toxic, so more sustainable alternatives are needed. Despite efforts to explore alternative NASICON-type materials, few viable cathodes can offer a comparable electrochemical performance while reducing or eliminating V. Dual-substituent Na₃Ti_0.5VMn_0.5(PO₄)₃ (NTVMP) and Na₃Ti_0.5VFe_0.5(PO₄)₃ (NTVFP) have been suggested as a means to reduce the use of V in NASICON-based materials. One major challenge in battery design is understanding how the cathode materials store energy. In this study, we examined how NTVMP and NTVFP store energy to help develop future rechargeable hybrid aqueous batteries. In the galvanostatic charge-discharge curves of NTVMP and NTVFP, two voltage plateaus appeared in a hybrid electrolyte (2 M CH₃COONa + 1 M (CH₃COO)₂ Zn). The analysis showed that insertion/extraction at a high voltage of ∼1.50 V is dominated by Zn^{2 +} ions, and Na⁺ ion insertion/extraction is dominant at a lower voltage of ∼0.30 V, indicating the presence of multi-electron and multi-ion reactions. In situ X-ray diffraction and ex situ X-ray showed a reversible lattice parameter and transition metal transformation during cycling, indicating a reversible insertion/extraction ion process.