A universal electrochemical lithiation-delithiation method to prepare low-crystalline metal oxides for high-performance hybrid supercapacitors

The electrochemical performance of transition metal oxides (TMOs) for hybrid supercapacitors has been optimized through various methods in previous reports. However, most previous research was mainly focused on well-crystalline TMOs. Herein, the electrochemical lithiation-delithiation method was performed to synthesise low-crystallinity TMOs for hybrid supercapacitors. It was found that the lithiation-delithiation process can significantly improve the electrochemical performance of "conversion-type" TMOs, such as CoO, NiO, etc. The as-prepared low-crystallinity CoO exhibits high specific capacitance of 2154.1 F g(-1) (299.2 mA h g(-1)) at 0.8 A g(-1), outstanding rate capacitance retention of 63.9% even at 22.4 A g(-1) and excellent cycling stability with 90.5% retention even after 10 000 cycles. When assembled as hybrid supercapacitors using active carbon (AC) as the active material of the negative electrode, the devices show a high energy density of 50.9 W h kg(-1) at 0.73 kW kg(-1). Another low-crystallinity NiO prepared by the same method also possesses a much higher specific capacitance of 2317.6 F g(-1) (302.6 mA h g(-1)) compared to that for pristine commercial NiO of 497.2 F g(-1) at 1 A g(-1). The improved energy storage performance of the low-crystallinity metal oxides can be ascribed to the disorder of as-prepared low-crystallinity metal oxides and interior 3D-connected channels originating from the lithiation-delithiation process. This method may open new opportunities for scalable and facile synthesis of low-crystallinity metal oxides for high-performance hybrid supercapacitors.