NaBH4-Poly(Ethylene Oxide) Composite Electrolyte for All-Solid-State Na-Ion Batteries

A disordered sodium borohydride (NaBH4) environment to facilitate Na+ mobility was achieved by partially hydrolyzing NaBH4 and this significantly improved Na+ ionic conductivity to 10−3 S cm−1 at 75 °C. The reaction rate of NaBH4 self-hydrolysis, however, is determined by several parameters, including the reaction temperature, the molar ratio between NaBH4 and H2O, and the pH value; but these factors are hard to control. In this paper, poly(ethylene oxide) (PEO), capable of retaining H2O through hydrogen bonding, was used in an attempt to better control the amount of H2O available for NaBH4 self-hydrolysis. This strategy led to the ionic conductivity of 1.6 × 10−3 S cm−1 at 45 °C with a Na+ transference number of 0.54. The amorphous nature of the PEO matrix in hydrolyzed NaBH4 is also believed to provide a conduction path for fast Na+ conduction.