Unlocking the multi-electron transfer reaction in NASICON-type cathode materials

The growing concern about scarcity and large-scale applications of lithium resources has attracted efforts to realize cost-effective phosphate-based cathode materials for next-generation Na-ion batteries (NIBs). In previous work, a series of materials (such as Na₄Fe₃(PO₄)₂(P₂O₇), Na₃VCr(PO₄)₃, Na₄VMn(PO₄)₃, Na₃MnTi(PO₄)₃, Na₃MnZr(PO₄)₃, etc) with 120 mAh g^-1 specific capacity and high operating potential has been proposed. However, the mass ratio of the total transition metal in the above compounds is only 22 wt%, which means that one-electron transfer for each transition metal shows a limited capacity (the mass ratio of Fe is 35.4 wt% in LiFePO₄). Therefore, a multi-electron transfer reaction is necessary to catch up to or go beyond the electrochemical performance of LiFePO₄. This review summarizes the reported NASICON-type and other phosphate-based cathode materials. On the basis of the aforementioned experimental results, we pinpoint the multi-electron behavior of transition metals and shed light on designing rules for developing high-capacity cathodes in NIBs.