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 LiNi_0.6Co_0.2Mn_0.2O₂ (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.