Impact of Mg and Al Substitutions on the Structural and Electrochemical Properties of NASICON-Na VMn M (PO ) (M = Mg and Al) Cathodes for Sodium-Ion Batteries.

Sodium superionic conductor (NASICON)-Na VMn(PO ) (NVMP) cathode is attractive for sodium-ion battery application due to its reduced cost and toxicity, and high energy density (≈425 Wh kg ). However, it exhibits significant polarization, limited rate and cycling performances due to its lower electronic conductivity and formation of Jahn-Teller active Mn during cycling. In this report, a chemical approach is presented to partially replace Mn of the NVMP framework by Mg and Al substitutions. The Mg- and Al-substituted NVMP cathodes present smoother voltage profiles, facile sodium (de)intercalation, enhanced rate performances (80 mA h g at 5C rate) and capacity retention (≈96% after 100 cycles) in comparison with the unsubstituted sample. Their enhanced performances are attributed to suppressed Jahn-Teller effect, increased covalent character and sodium ion vacancies of the NASICON framework. These results highlight the significance of fine tuning the chemical compositions to attain high performance NASICON cathodes.