High-capacity K-storage operational to −40 °C by using RGO as a model anode material

Low operating temperatures (< −30 °C) result in slow ion transport (e. g. Li⁺, Na⁺, K⁺) in electrolytes and electrodes and severe decay in capacity of batteries. Here, we demonstrate a capacity of 171 mAh g⁻¹ at −40 °C with a high capacity retention rate of 58% compared to room temperature by using the reduced graphene oxide (RGO) as the anode for potassium ion batteries (PIB). This high retention at low operational temperatures is one of the record high low-temperature capacities among all the carbon anode materials. RGO was prepared and optimized as a representative defective nanocarbon material with a moderate amount of surface defects, large interlayer spacing, and relatively high electrical conductivity. These factors are known to shorten the ions’ transport distance for storage for the ions and reduce the difficulty of ion intercalation, especially at low temperatures. For these reasons, the capacity of RGO at −40 °C is ten times that of graphite, hard carbon and GO. In addition, optimized RGO shows much higher K-storage capacity (171 mAh g⁻¹) than Li and Na-storage capacities (68 and 56 mAh g⁻¹, respectively), which reveals the importance of the coordination between the high mobility of K⁺ ion in the electrolyte and the surface-dominated ion storage mechanism on the energy storage performance at low temperatures.