Mechanochemically Desodiated Na₄Fe₃(PO₄)₂P₂O₇ as a Lithium and Sodium Storage Material

Searching for an affordable class of intercalation electrode materials for rechargeable ion batteries, Na4Fe3(PO4)2P2O7, has recently been identified. Herein, we demonstrate first results on sodium deintercalation from Na4Fe3(PO4)2P2O7 during ball milling with carbon additives at room temperature. The desodiated Na4–xFe3(PO4)2P2O7 displays good Na+ and Li+ storage performance when it is used as an electrode in both Li- and Na-ion cells. The storage performance of Na4Fe3(PO4)2P2O7 electrodes is monitored and discussed on the basis of ex situ X-ray diffraction. To achieve stable alkali-ion intercalation in Na4Fe3(PO4)2P2O7, the advantages of using lithium and sodium imide salts (LiTFSI and NaTFSI) in organic electrolytes over the conventional LiPF6 and NaPF6 ones are first reported. It has been found that the imide-based electrolyte salts favor the consecutive intercalation of the alkali ions into the different crystallographic sites of the Na4Fe3(PO4)2P2O7 structure.