An Adsorption-Insertion Mechanism of Potassium in Soft Carbon

Soft carbon (SC) has become a promising anode for potassium ion batteries (PIBs) benefiting from its structural flexibility. However, the evolution of potassium storage behavior with the microstructure (the average size of the crystallites L_a and the average interlayer spacing a₃ ) is still unclear, which hinders the understanding of the potassium storage mechanism. Herein, a series of soft carbon with different microstructures is prepared through pyrolysis of petroleum pitch. Based on the analysis of the relationship between electrochemical behavior and microstructure, an adsorption-insertion mechanism is proposed: the capacity in the voltage range of 0.45-1.1 V is originated from the adsorption of potassium ions on edge-defect sites whereas the capacity below 0.45 V is attributed to the insertion of potassium ions into interlayers. When L_a equals to 10.56 Å, SCs exhibit an adsorption-controlled mechanism. However, as L_a increases to 120.98 Å, the insertion process is dominant. With L_a increasing from 21.9 to 93.02 Å, SCs have two mixed behaviors. The initial insertion coefficients do not change until a₃ decreases to 3.46 Å. These findings highlight the relation of potassium storage behavior with different microstructures and the adsorption-insertion mechanism can provide insights into the design of SC anodes for PIBs.