Impact of Mg2+ and Al3+ Substitutions on the Structural and Electrochemical Properties of NASICON-Nax VMn0.75 M0.25 (PO4 )3 (M = Mg and Al) Cathodes for Sodium-Ion Batteries.

Sodium superionic conductor (NASICON)-Na4 VMn(PO4 )3 (NVMP) cathode is attractive for sodium-ion battery application due to its reduced cost and toxicity, and high energy density (≈425 Wh kg-1 ). However, it exhibits significant polarization, limited rate and cycling performances due to its lower electronic conductivity and formation of Jahn-Teller active Mn3+ during cycling. In this report, a chemical approach is presented to partially replace Mn2+ of the NVMP framework by Mg2+ and Al3+ substitutions. The Mg- and Al-substituted NVMP cathodes present smoother voltage profiles, facile sodium (de)intercalation, enhanced rate performances (80 mA h g-1 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.