Advantageous Al-ion intercalation in Mo₁₅S₁₉ nanosheets for aqueous symmetric supercapacitors

This work reports the anchoring of Mo₁₅S₁₉ nanosheets on conductive carbon cloth (CC) using a simple hydrothermal reaction. Based on the specific capacitance data for Mo₁₅S₁₉/CC electrode in 1 M AlCl₃, 1 M MgCl₂, 3 M LiCl, 1 M KCl, 1 M LiCl and 1 M NaCl electrolytes, the highest value (400 F g⁻¹) obtained for aluminum chloride is attributed to the fast redox reactions with short diffusion path. This is supported by the decrease in the acidity of the electrolyte and reduction in the relative intensity of the Raman peaks between A_1g and E_1/2g for the Al³⁺ intercalated Mo₁₅S₁₉ nanosheets. Moreover, ex-situ X-ray photoelectron spectroscopy displays the reduction of Mo⁴⁺ to Mo³⁺ for Mo₁₅S₁₉ nanosheets, confirming a charge compensation mechanism via Al³⁺ intercalation. Furthermore, an aqueous Al-ion capacitor is fabricated based on Mo₁₅S₁₉/CC positive/negative electrodes and 1 M AlCl₃ electrolyte for efficient energy storage. The device delivers a long cycling lifespan with over 90.1% capacitance retention over 10,500 cycles at 10 A g⁻¹. This work provides the possibility of aqueous capacitors based on trivalent Al-ion as a guest and offers a new direction for the future development of metal-ion capacitors.