A sodium–aluminum hybrid battery
Novel hybrid batteries are fabricated using an aluminum anode, a sodium intercalation cathode Na3V2(PO4)3 (NVP), and a sodium/aluminum dual salt electrolyte based on NaAlCl4 and an eutectic mixture of 1-ethyl-3-methylimidazolium chloride (EMImC) and aluminum chloride. Cyclic voltammograms indicate that increasing the molar concentration of AlCl3 in the electrolyte is beneficial to high coulombic efficiency of aluminum deposition/stripping, which, unfortunately, results in lower coulombic efficiency of sodium extraction/insertion in the cathode. Therefore, EMImC–AlCl3 with a molar ratio of 1–1.1 is used for battery evaluation. The hybrid battery with 1.0 M NaAlCl4 exhibits a discharge voltage of 1.25 V and a cathodic capacity of 99 mA h g−1 at a current rate of C/10. In addition, the hybrid battery exhibits good rate performance and long-term cycling stability while maintaining a high coulombic efficiency of 98%. It is also demonstrated that increasing salt concentration can further enhance the cycling performance of the hybrid battery. X-ray diffraction analysis of the NVP electrodes under different conditions confirms that the main cathode reaction is indeed Na extraction/insertion. Based on all earth-abundant elements, the new Na–Al hybrid battery is very attractive for stationary and grid energy storage applications.