Phase change electrolytes for combined electrochemical and thermal energy storage

Inorganic salt-based phase change materials (PCMs) form the basis of next-generation thermal energy storage technologies that store and release energy at temperatures relevant for regulating energy usage in residential environments. Here, we detail a rational approach for designing a multifunctional electrolyte that stores latent heat using polymer-stabilized sodium sulfate and sodium thiosulfate mixture. This formulated PCM also allows rapid sodium ion transport in both the solid and liquid states. This PCM composite electrolyte was prepared by blending the components at elevated temperatures, forming a viscous liquid that can serve as a gel-electrolyte for Na-ion batteries. The addition of sodium borate as a nucleating agent resulted in a PCM composite electrolyte with fusion enthalpy (120 J/g) and a phase transition centered around 25.0 °C with 11 °C supercooling. This PCM electrolyte showed excellent thermal cycling stability (>10 cycles) that also maintains high ionic conductivity (>10 mS cm⁻¹). We show that the electrolyte enables Na ion cycling of the dual anode/cathode material, Na₂VTi(PO₄)₃. These properties make this composite electrolyte a promising material for multifunctional energy storage devices.