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.

硬碳阳极具有吸引人的性能，因为它有丰富的活性位点来承载电荷。在多种电荷存储机制中，由于在低电压下感应出高容量，孔隙填充在新兴的 Na/K 离子电池中特别受关注。尽管 Na 离子孔隙填充被广泛接受，但 K 离子是否可以填充纳米孔隙仍不清楚。我们以开心果壳衍生的硬碳为模型，探讨了与不同电压区域相关的钾离子存储行为。除了已报道的相对高电位的吸附和嵌入机制外，低温透射电子显微镜和电子顺磁共振表明，一旦以 5 mV相对于K ⁺连续放电，就会存在准金属钾纳米团簇/K，明确地证明了硬碳阳极中的 K 离子孔隙填充。我们还讨论了促进这种行为的策略，并表明化学蚀刻诱导的开孔可以提高动力学但不利于容量。开发高容量硬碳负极依赖于闭孔的合理设计。