Alkali metal ion-doped GaN for ultrafast electrochemical capacitor: Doping mechanism, structural adjustment, and structure–performance relationship

Abstract

Microstructure adjustment plays a crucial role in improving the surface reactivity of materials. However, surface reactivity improved by alkali metal doping for electrode materials applied in energy storage is usually overlooked by researchers. Herein, we report on facile synthesis of Li⁺/Na⁺-doped GaN microcrystals and their structure–electrochemical performance relationship. The dosage concentration is optimized, and the doping mechanism is proposed in terms of the metathesis reaction of Li₂O/Na₂O with GaN under carbothermal reduction conditions using LiNO₃/NaNO₃ as the dopants. Both macro- and microstructures of GaN can be tuned through Li⁺/Na⁺ doping. In addition to the increased specific surface area and total pore volume, the reduced d band center and band gap caused by Li⁺/Na⁺ doping present an anti-d band center phenomenon and contribute to enhanced ion storage and transport, leading to excellent electrochemical performance of the target materials. In addition, symmetric electrochemical capacitors with the target materials can deliver output voltage of 2.4, 1.5, and 1.0 V at –60, 25, and 120 °C by using phosphoric acid–water–dimethyl sulfoxide mixed solution as the electrolyte. The symmetric electrochemical capacitors with Na⁺-doped GaN can deliver specific energy of 45.9 mW h cm⁻³ at 344 mW cm⁻³ and –60 °C, 36.1 mW h cm⁻³ at 215 mW cm⁻³ and 25 °C, and 16.7 mW h cm⁻³ at 287 mW cm⁻³ and 120 °C, respectively, holding considerable promise for practice in all-temperature environment.