Mild-Expanded Graphitic Onion-like Carbon with Nonflammable Electrolytes for Safety and High-Performance Potassium Storage

Potassium-ion batteries (PIBs) are attractive energy storage devices due to the abundant low-cost potassium resources, and graphite is one of the most competing anodes of PIBs. Nevertheless, graphite often suffers from sluggish kinetics, large volume variations, and high activity of K+, leading to poor rate and low-temperature performance, insufficient cyclability, and safety issues. Herein, mild-expanded graphitic onion-like carbon (MEGOC) with an enlarged interlayer distance is prepared from confined graphitic onion-like carbon by liquid-phase oxidation and slow thermal reduction, which can accommodate volume variations and promote potassium diffusion. Meanwhile, using triethyl phosphate, a nonflammable and low-melting-point electrolyte ensures safety and low-temperature operation performance. In the nonflammable potassium-ion electrolyte, MEGOC exhibits long cycle life at a high rate (175 mA h g–1 at 2C after 1500 cycles) and superior low-temperature performance (165 mA h g–1 at −40 °C). Furthermore, an assembled nonflammable potassium-ion capacitor with MEGOC as the anode exhibits high reversibility and superior energy/power density at ambient and low temperatures. Accordingly, combining structural engineering and electrolyte engineering is an effective strategy for improving the safety and high performance of potassium-based electrochemical energy storage devices.