In situ Raman study of nickel bicarbonate for high-performance energy storage device

Highlights: • The Ni(HCO₃)₂/rGO nanocomposite was successfully prepared, demonstrating high capacity and excellent rate performance. • The energy storage mechanism of the Ni(HCO₃)₂ is investigated by using in situ Raman spectroscopy. • The hybrid supercapacitor demonstrates ultrafast energy storage capability. -- Abstract: In situ Raman spectroscopy is a powerful technique for probing the structure and phase composition of the electrode materials that are undergoing charge-discharge process. Herein, the charge storage mechanism of as-prepared Ni(HCO₃)₂ nanomaterial is successfully studied by using the in situ Raman spectroscopy. The charge storage can be attributed to the deep oxidation of Ni²⁺ into Ni³⁺, and the irreversible phase transformation of γ-NiOOH into disordered β-Ni(OH)₂ damages the crystal structure of Ni(HCO₃)₂, arousing the capacity loss of the electrode during the long-term cycling process. Under the guidance of the experimental investigations, a porous Ni(HCO₃)₂/reduced graphene oxide (rGO) nanocomposite is designed and synthesized, exhibiting ultrahigh specific capacity (846 C g⁻¹) and excellent rate capability (618 C g⁻¹ at 20 A g⁻¹). When coupled with an negative electrode based on rGO, the resulting hybrid supercapacitor shows an ultrahigh energy density of 66 Wh kg⁻¹ at power density of 1.9 kW kg⁻¹ and good cycling stability. These findings provide important insight into the mechanism of charge storage, and scientific basis for design of high-performance energy storage materials.