Discovering the pore-filling of potassium ions in hard carbon anodes: Revisit the low-voltage region

Hard carbon anodes deliver attractive performance because of abundant active sites for hosting the charge. Among diverse charge storage mechanisms, pore-filling is of particular interest in emerging Na/K ion batteries owing to the induced high capacity at a low voltage. Despite the widely accepted Na ion pore-filling, whether K ion could fill in the nanopores remains vague. We explore the K ion storage behavior associated with different voltage regions taking pistachio shell-derived hard carbon as a model. Besides reported adsorption and intercalation mechanisms at relatively high potentials, cryo-transmission electron microscopy and electron paramagnetic resonance indicate the presence of quasi-metallic potassium nanoclusters once discharged continuously at 5 mV vs. K⁺/K, unambiguously demonstrating the K ion pore-filling in hard carbon anodes. We also discuss the strategies to promote such behavior, and show that chemical etching-induced open pores could boost the kinetics but not benefit the capacity. Developing high-capacity hard carbon anodes relies on the rational design of closed pores.