In Situ Formed Metal-p-n Heterojunction for Ultrahigh Energy Density Supercapattery: Decelerates Ion Diffusion to Sustain Slow Discharge

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Supercapatteries combine both diffusion-controlled and capacitive charge storage mechanisms to simultaneously deliver exceptional power density and energy density. Though developing materials that combine both charge storage mechanisms for use as supercapattery electrodes is difficult, one solution has been to modify a material that inherently has one of these mechanisms so that it also exhibits the other form of charge storage. In the present study, we modulate the electrochemical reconstruction of a W5N4 electrode to modify its charge storage from pure battery-type to pseudocapacitive. This modulation was carried out by embedding Co in W5N4 (Co-W5N4), leading to the formation of W-doped CoOOH as an active phase. The incorporation of a thin layer of TiO2 (Co-W5N4/TiO2) through atomic layer deposition minimized tungsten dissolution during reconstruction, thus generating in-situ a dynamic metal–p-n junction heterostructure (Co-W5N4||CoOOH||TiO2) that modulated the charge flow, achieving high-rate capability (retaining 53.1% at 50 A g−1 compared to 2 A g−1)and cyclic stability (82.8% after 100,000 cycles at a high current density of 40 A g−1). As illustrated by operando electrochemical impedance spectroscopy and physicochemical analysis, the dynamic response of metal–p-n junction sustains the pseudocapacitive charge storage mechanism at high current rate. This p-n junction modulated the diffusion speed of OH− ions due to a decrease and an increase in the width of the depletion layer under charging and discharging, respectively. The solid-state supercapattery device assembled with a positive CoWN/TiO2 electrode and a negative NiMo2S4/Fe2O3 electrode attained an impressive energy density of 259.5 Wh kg− 1 at a power density of 2550 W kg− 1 and operated in a stable manner over 25,000 cycles. Our study thus highlights the importance of the band structure and its dynamic response to the operating conditions, and our approach can be extended to the design of a variety of energy-storage electrodes and energy-conversion catalysts.