Effect of organic electrolyte and voltage on electrochemical performances and capacity degradation for Li-ion hybrid capacitors based on Nb2O5 anode

Lithium-ion capacitor (LIC), as a hybrid energy storage device, integrates a capacitive electrode with a battery electrode, enabling concurrent high energy and power density performance. In addition to electrode materials, the optimization of electrolyte composition and operating voltage parameters are also crucial for practical LIC implementation. Here, we fabricated pouch-type LICs utilizing commercial Nb2O5 and activated carbon (AC) as anodic and cathodic materials, respectively. The correlation between electrolyte, operating voltage, and capacity degradation behavior was comprehensively investigated. The acetonitrile-based (AN-based) electrolyte demonstrated superior performance compared to ester-based alternatives, achieving a remarkable capacitance of 160.7 F and a maximum power density of 15.74 kW kg(-1) in the Nb2O5//AC LICs. This system exhibited exceptional rate performance, maintaining 78.3% capacity retention from 1 to 100 C. Expanding the operating voltage to 2.85 V from 2.7 V yielded an enhanced energy density of 51.5 Wh kg(-1), albeit at the expense of cyclic stability. Cycling testing revealed 89.7% capacity retention after 10,000 cycles at 1.0 similar to 2.7 V, while operation at 2.85 V resulted in severe capacity deterioration to 28.6%. Through incremental capacity (dQ/dV) and electrochemical impedance spectroscopy (EIS) analysis, it is concluded that conductivity loss primarily accounts for capacity loss within 1.0 similar to 2.7 V operation for the AN-based LIC, while elevated voltages trigger more severe active material degradation and gas generation.