Multiple Functions of Carbon Additives in NASICON-Type Electrodes for Stabilizing the Sodium Storage Performance

Recently, there has been increased interest in NASICON-type electrodes for sodium-ion batteries due to their unique combination of intercalation properties, low cost, and safety. However, their commercialization is hindered by the low electrical conductivity. One strategy to overcome this issue is to integrate NASICON materials with carbon additives. This study shows that carbon additives improve the sodium storage performance of a NASICON-type electrode in various ways, depending on the additives' functional groups, texture, and conductivity properties. The proof-of-concept is based on a multi-electron phospho-sulphate electrode, NaFeVPO₄(SO₄)₂ (NFVPS) mixed with carbon black (C) and reduced graphene oxide (rGO). Carbon-coated samples are obtained via a simple ball milling procedure followed by thermal treatment in an argon flow. Sodium storage in the composites occurs through capacitive and Faradaic reactions. The Faradaic reaction is facilitated at the carbon black composite, while the capacitive reaction dominates for the rGO composite. NFVPS operates through two-electron reactions at 20 °C, while the increased temperatures favor the three-electron reaction. The rGO composite outperforms the carbon black composite in terms of cycling stability and rate capability at 20 and 40 °C. The role of the rGO and carbon black in electrochemical performance is discussed based on the different reactivity of hydroxyl/epoxide and carbonyl functional groups with the electrolyte salt, NaPF₆, and the solvent, polypropylene carbonate.