New Iron-Based Mixed-Polyanion Cathodes for Lithium and Sodium Rechargeable Batteries: Combined First Principles Calculations and Experimental Study

New iron-based mixed-polyanion compounds LixNa4-xFe3(PO4)(2)(P2O7) (x = 0-3) were synthesized, and their crystal structures were determined. The new compounds contained three-dimensional (3D)-sodium/lithium paths supported by P2O7 pillars in the crystal. First principles calculations identified the complex 3D paths with their activation barriers and revealed them as fast ionic conductors. The reversible electrode operation was found in both Li and Na cells with capacities of one-electron reaction per Fe atom, 140 and 129 mAh g(-1), respectively. The redox potential of each phase was similar to 3.4 V (vs Li) for the Li-ion cell and similar to 3.2 V (vs Na) for the Na-ion cell. The properties of high power, small volume change, and high thermal stability were also recognized, presenting this new compound as a potential competitor to other iron-based electrodes such as Li2FeP2O7, Li2FePO4F, and LiFePO4.