Fabricating stable protective layer on orthorhombic tungsten oxide anode for long-lifespan pseudocapacitors by trace of aluminum ion electrolyte additives.

Orthorhombic tungsten oxide (WO·HO) has been considered as a promising anode material due to its layered crystal structure and high capacity. However, the instability of its crystal structure usually results in poor cyclic stability of the battery/capacitor. Herein, a novel strategy of introducing aluminum ion (Al) additives for designing hybrid electrolyte is demonstrated to enhance the cycling stability of WO·HO. Aluminum ions may participate in the redox reaction and contribute extra capacitance. More importantly, the Al ions can facilitate fast phase transformation of metastable orthorhombic tungsten oxides to stable monoclinic phase, but also participate in the construction of stable protective layer during the long-term cycling, forming a protective layer at the surface of tungsten oxide for alleviating the dissolution and structural damage. The WO·HO is electrodeposited on the exfoliated graphite foil (Ex-GF) and shows a specific capacitance of 395 mF cm at 2 mA cm in the LiCl + AlCl hybrid electrolyte (pH = 2.93), and the capacitance remains 91.8 % after 10,000 charge/discharge cycles, indicating that the WO·HO/Ex-GF electrode material exhibits excellent stability in supercapacitors. The density functional theory (DFT) calculations further demonstrate whether the adsorption energy or intercalation energy of Li at the monoclinic WO is lower than the orthorhombic WO·HO. This result suggests that the electrochemical performance of WO·HO, which operates on a pseudocapacitive reaction mechanism, can be enhanced through the phase transformation from the orthorhombic phase to the monoclinic phase during the cycling. Hence, this ion additive approach can adjust the interface composition and protect internal active material, and can be extended to the stability improvement of other metal oxide electrodes.