High-energy density graphite/AC capacitor in organic electrolyte

A high-energy density hybrid capacitor has been designed in organic electrolyte (1 mol L⁻¹ LiPF₆ in 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC)) using commercial grades of graphite and activated carbon for negative and positive electrodes, respectively. Different approaches have been explored for assembling the hybrid capacitor in order to achieve an optimum ratio between the energy and power density, while keeping a long cycle-life capability. In the optimized hybrid capacitor, the potential of the positive electrode ranges from 1.5 up to 5 V vs. Li/Li⁺, being extended to the whole stability window of the activated carbon in the organic electrolyte, whereas the potential of the negative electrode remains almost constant at around 0.1 V vs. Li/Li⁺. After balancing carefully the respective masses of the electrodes and appropriately formatting the system, it was found that a voltage of 4.5 V is the optimal value for avoiding a capacitance fading of the hybrid capacitor during cycling. Gravimetric and volumetric energy densities as high as 103.8 Wh kg⁻¹ and 111.8 Wh L⁻¹, respectively, were obtained. The noticeable value of volumetric energy density is 10 times higher than for symmetric or asymmetric capacitors built with the same activated carbon.