Nano-graphitic crystallites effect on the improved K-ion intercalation properties of Kigelia Africana fruit-derived hard carbon for potassium-ion batteries

The development of intermittent renewable energy storage technology is essential for building a sustainable, low-carbon society. Potassium-ion (K-ion) batteries are being extensively explored as alternate energy storage devices due to their closest redox potential and similarities to Lithium (Li) and Sodium (Na) batteries. Identifying alternate carbonaceous materials is, therefore, crucial to overcome the shortcomings of graphite. In this context, hard carbon derived from cellulose-rich Kigelia Africana fruit (KAP) was pyrolyzed at 1000 °C and 1100 °C and characterized using various techniques. The employed facile direct carbonization retains the inherent oxygen atoms (6.6 %). It restricts the growth of graphitic crystallites in the ab plane, thereby reducing the d-spacing revealed by X-ray Diffraction (XRD) and Raman Spectral analysis. The enhanced K-ion storage through insertion, as corroborated by Cyclic Voltammetry (CV) through the increased kinetics at the low potential < 0.5 V vs K/K⁺. The Galvanostatic Charge-Discharge (GCD) analysis further substantiates the role of graphitic nanocrystallites combined with the adsorption of O-atoms through the improved rate capability of KAP-1100 (110 mAh/g at 200 mA/g). On the other hand, KAP-1000 provides 10 mAh/g at 200 mA/g, which is almost ten times lower. Galvanostatic Intermittent Titration Technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) analysis further prove the efficient diffusion coefficient (1.5 × 10^-8 cm²/s) of K-ions in the hard carbon and low charge transfer resistance (R_ct). This study reveals the effect of local structure and the inherently available heteroatom for developing better anodes for K-ion batteries.