Potassium-ion batteries are considered a promising option for large-scale energy storage because potassium is abundant and low-cost. However, their development is limited by the large size of potassium ions, which makes it difficult for anode materials to store them quickly and stably. Biomass-derived carbon is an attractive alternative, but in most cases its structure is tuned largely by trial and error.

In a study published in Small Methods, researchers from the Xinjiang Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences used cotton stalk as a biomass precursor and deep eutectic solvents (DESs) as a controllable reaction medium to construct carbon materials with enlarged interlayer spacing, hierarchical porosity, and enhanced nitrogen doping, thereby accelerating K⁺ diffusion kinetics and improving rate capability and long-term cycling stability.

Researchers introduced different metal ions into the solvent system and compared the effects of calcium, magnesium, and zinc. They found that calcium played a unique role during carbon formation. It helped guide the decomposition of biomass and promoted the formation of a carbon structure with more suitable pores, richer nitrogen active sites, and wider layer spacing, which made potassium ions easier to move through the material and to be stored more efficiently.

The calcium-regulated carbon anode, denoted as cotton stalk derived carbon (CCS)-DES_Ca, exhibited the best electrochemical performance among all studied samples under optimized conditions. It delivered a reversible capacity of 320 mAh g^-1 at 0.05 A g^-1 and maintained 212 mAh g^-1 at 1 A g^-1. It also showed excellent long-term stability, retaining 89% of its capacity after 2000 cycles.

Further kinetic analysis demonstrated a higher pseudocapacitive contribution and faster potassium ions diffusion, suggesting that the calcium ions-induced multiscale carbon structure is beneficial for achieving both high capacity and rapid potassium storage.

In addition, researchers revealed how different metal ions in DES affect biomass carbonization and the formation of the final carbon structure. They established a clear relationship among metal-ion identity, carbonization behavior, structural characteristics, and potassium-storage performance, providing a basis for the rational design of biomass-derived carbon anodes through DES chemistry. The precursor treatment strategy was also found to be applicable to other lignocellulosic precursors, such as pear wood and bamboo powder, demonstrating its versatility and scalability.