High‐Coulombic‐Efficiency Hard Carbon Anode Material for Practical Potassium‐Ion Batteries

Biomass‐derived hard carbon (HC) has emerged as a promising candidate for anode materials of potassium‐ion batteries because of low cost and abundant raw materials. Whereas, the large specific surface area and high porosity of this type of HC often lead to inferior initial Coulombic efficiency (ICE) and unsatisfactory cycling stability. Herein, we report a coconut shell‐derived HC (CS‐HC) featuring an expanded interlayer spacing and small specific surface area. The CS‐HC delivers a reversible specific capacity of 280 mAh g‐1 and an impressive ICE of 87.32% at 50 mA g−1. In addition, it exhibits stable cycling performance (92.8% capacity retention after 100 cycles at 50 mA g−1) and fast rate capability (∼280 mAh g‐1 at 300 mA g‐1). The ex situ Raman spectra characterization combined with cyclic voltammetry tests elucidate that the storage of potassium ions in the present HC is mainly achieved by (pseudo)capacitive behavior at the disordered defect sites along with minor contribution from the interlayer intercalation process. Finally, a full‐cell constructed with unprecycled CS‐HC anode and high‐voltage K2Mn[Fe(CN)]6 cathode demonstrates exceptional electrochemical stability and retains 90.6% capacity after 100 cycles. This work reports a high‐performance HC anode material derived from low‐cost and sustainable biomass for practical potassium‐ion batteries.