Impact of Mg2+and Al3+Substitutions on the Structural and Electrochemical Properties of NASICON-NaxVMn0.75M0.25(PO4)3(M = Mg and Al) Cathodes for Sodium-Ion Batteries

Sodium superionic conductor (NASICON)-Na4VMn(PO4)(3)(NVMP) cathode is attractive for sodium-ion battery application due to its reduced cost and toxicity, and high energy density (approximate to 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 Mn(3+)during cycling. In this report, a chemical approach is presented to partially replace Mn(2+)of the NVMP framework by Mg(2+)and Al(3+)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 (approximate to 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.