Abstract

Polyanion-based electrode materials are important for rechargeable sodium (Na)-ion batteries owing to their structural stability and their high redox voltage. The redox voltages and gravimetric capacities of polyanion electrodes can be tuned by the choice of transition metal and/or polyanion groups. In this work we explore the effect of changing polyanion groups on the redox voltages and the gravimetric capacities of polyanion electrodes. Using first-principles calculations, we examine the influence of polyanionic substitutions on the stability and electrochemical behavior of the Na₃V₂(PO₄)₃ electrode material, which adopts the prototype structure of the Natrium Super Ionic Conductor (NaSICON). Starting from the Na₃V₂(PO₄)₃ structure, we explore the partial or total substitution of PO₄^3- groups by SiO₄^4- or SO₄^2- moieties, unveiling the uncharted multicomponent Na-V-P-(Si/S)-O phase diagram. We show that small amounts of SiO₄^4- can activate the V^V/IV redox couple, which is not experimentally accessible in the PO₄^3- NaSICON analogue, thereby increasing the average Na intercalation voltage. In the case of SO₄^2- substitution, we observe an increase in voltage of each V redox couple (i.e., V^III/II, V^IV/III and V^III/II) but at the expense of the maximum amount of Na⁺ intercalated. We show that exploiting the varying inductive effects of different polyanion groups can be effective for tuning the energy density of NaSICON electrode materials.