Silica depleted rice hull ash (SDRHA), an agricultural waste, as a high-performance hybrid lithium-ion capacitor†
Abstract
Rice hull ash (RHA, an agricultural waste) produced during combustion of rice hulls to generate electricity consists (following dilute acid leaching) of high surface area SiO2 (80–90 wt%) and 10–20 wt% carbon (80 m2 g−1 total). RHA SiO2 is easily extracted by distillation (spirosiloxane) producing SDRHA, which offers an opportunity to develop “green” hybrid lithium-ion capacitors (LICs) electrodes. SDRHA consists of 50–65 wt% SiO2 with the remainder carbon with a specific surface area of ≈220 m2 g−1. SDRHA microstructure presents a highly irregular and disordered nanocomposite composed of nanosilica closely connected via graphene layers enhancing Li-ion mobility during charge/discharge process. SDRHA electrochemical properties were assessed by assembling Li/SDRHA half-cells and LiNi0.6Co0.2Mn0.2O2 (NMC622)-SDRHA full-cells. The half-cell delivered a high specific capacity of 250 mA h g−1 at 0.5C and retained a capacity of 200 mA h g−1 at 2C for 400 h. In contrast to the poor cycle performance of NMC based batteries at high C-rates, the hybrid full-cell demonstrated a high specific capacitance of 200 F g−1 at 4C. In addition, both the half and full hybrid cells demonstrate excellent coulombic efficiencies (∼100%). These results suggest that low cost and environmentally friendly SDRHA, may serve as a potential alternative electrode material for LICs.