Multi-electron redox reactions with iron and vanadium ions at a mixed phosphate–sulfate electrode during sodium intercalation†

In order to improve the specific capacity of intercalation electrodes for sodium-ion batteries, it is necessary to identify materials capable of storing Na⁺ ions by activating multi-electron redox reactions. Herein, we report a NaFeVPO₄(SO₄)₂ compound as a multi-electron electrode that combines the most abundant Fe and V ions, having multiple oxidation states, with a stable mixed phosphate–sulfate matrix. Na_xFeVPO₄(SO₄)₂ reversibly intercalates a total of 3 moles of Na⁺ ions (corresponding to a specific capacity of 175 mA h g⁻¹) within a potential range of 1.5–4.2 V, which is concomitant with a limited variation in the lattice volume (up to 5.2%). NaFeVPO₄(SO₄)₂ interacts with rGO, resulting in rGO covering the phosphate–sulphate particles, and the thickness of the covering varies between 5 and 10 nm. The NaFeVPO₄(SO₄)₂/rGO composite stores Na⁺ ions via a hybrid mechanism involving faradaic and capacitive reactions. In sodium half-cells, the NaFeVPO₄(SO₄)₂/rGO composite displays high capacity (about 90 mA h g⁻¹), and it exhibits an excellent long-term cycling stability at elevated temperatures (about 96–97% after 100 cycles at 20 °C, followed by the next 100 cycles at 40 °C). The improved electrochemical performance is discussed based on the structural robustness of NaFeVPO₄(SO₄)₂ and the surface interaction of NaFeVPO₄(SO₄)₂/rGO with an electrolyte salt and electrolyte solvent. The information from this study will be relevant to the design of high energy polyanionic electrodes for practical application in sodium-ion batteries.