Binder Chemistry of Na ₃ V ₂ (PO ₄ ) ₂ F ₃ Cathodes in Aqueous Sodium-Ion Batteries: From “Salt-in-Water” to “Water-in-Salt”

Aqueous sodium-ion batteries are emerging as a promising and sustainable alternative to lithium-ion batteries due to the abundance and low cost of sodium. However, optimizing electrode formulation, particularly the choice of binder and electrolyte, remains crucial for maximizing electrochemical performance. In this work, Na3V2(PO4)2F3 (a so-called “NVPF” nanoflowers) were employed as a model cathode to systematically investigate the influence of different binders. Polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), and polytetrafluoroethylene (PTFE) were evaluated in NaClO4 electrolytes across a wide concentration range, from conventional “salt-in-water” to highly concentrated “water-in-salt” regimes. The influence of operating temperature was explored from 10 to 60 °C. This study introduces the concept of binder selection tailored for “water-in-salt” electrolytes across a wide range of temperature range in aqueous batteries. Remarkably, tuning the binder led to more than a 5-fold improvement in capacity at a low scan rate (1 mV s–1). Among the tested binders, PAA delivered the highest specific capacity of 91.65 mAh g–1, attributed to its strong hydrophilicity and abundant carboxyl functional groups, which enhanced electrode wettability and facilitated rapid Na+ transport at the electrode–electrolyte interface. Meanwhile, PVDF, PTFE, and CMC exhibited capacities of 44.09, 25.03, and 15.83 mAh g–1, respectively. These findings highlight the critical role of binder chemistry in enabling the effective use of highly concentrated electrolytes (e.g., 17 m NaClO4) coupled with the PAA binder for next-generation sodium-ion batteries.