To effectively harvest the energy from renewable sources such as solar and wind, cheap and long-lasting electric energy storage (EES) devices are essential.¹ Among various EES technologies, lithium ion batteries (LIBs) are the most mature technologies with high energy density, high coulombic efficiency and long cycle life. Unfortunately, their high cost and the unevenly distributed global lithium source are major concerns. ² Thus, beyond lithium technologies based on cheap and naturally abundant elements such as sodium (Na), magnesium (Mg), and aluminum (Al) ion batteries ^{3, 4}  have been intensively studied during the last few years. Among different metals, Al is one of the most earth abundant elements, only second to silicon, resulting in its low price of $0.52/kg, 100 times lower than Li. In addition, Al with three electrons redox couple has distinct advantage over one electron redox couple such as Li and Na and two electrons redox couple such as Mg and Ca, providing a high theoretical specific capacity of 2980 mAh g^-1 and a high volumetric capacity of 8040 mAh cm^-3.

The development of rechargeable Al ion batteries has been hampered by the electrolyte and cathode materials. Due to the low reduction potential of Al (-1.68 V vs. SHE), aqueous electrolytes cannot be used when Al is being used as the anode, since hydrogen will be released before Al can be plated during the reduction process. As an alternative, ambient room temperature ionic liquids with high ionic conductivity and wide electrochemical windows are good candidates for rechargeable Al batteries. Currently, the ionic liquids used for Al deposition are still based on mixtures of anhydrous AlCl₃ and organic halide salts.⁵  It is well-known that only the acidic mixtures show reversible Al deposition/stripping, which poses stringent requirement for the hardwares of the Al batteries.⁶ To avoid the corrosion issue, pouch cell and special cell configuration have been used.⁷ Besides electrolytes, Al batteries also face challenges from the cathodes due to the high charge density of the Al³⁺ ion, making its insertion/extraction into/from the cathode host very difficult.

An alternative way to take advantage of metallic Al anode but bypasses the need for efficient Al insertion cathodes is hybrid Al battery.⁷ It was demonstrated that high voltage and high capacity were possible in a hybrid Al battery with LiFePO₄ as the cathode in an acidic ionic liquid electrolyte.⁷ However, a switch to the more earth-abundant sodium chemistry is more desirable due to the limited Li resources. In this presentation, we report a new hybrid battery using earth-abundant Na and Al chemistry, that is, Al as the anode, Na₃V₂(PO₄)₃ (NVP) as the cathode and NaAlCl₄ dissolved in acidic ionic liquid as the electrolyte.⁸ The hybrid battery exhibits a discharge voltage of 1.25 V and a cathodic capacity of 99 mAh g^-1 under a current rate of C/10. In addition, the hybrid battery exhibits good rate performance and long 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 the Na extraction/insertion.

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