Storage Mechanism of Alkali Metal Ions in the Hard Carbon Anode: an Electrochemical Viewpoint

The storage mechanisms of Li, Na, and K in hard carbon anodes are investigated through systematically exploring their electrochemical behaviors. Two charge/discharge voltage regions are observed for all the Li, Na, and K storage, a slope at a high voltage, and a plateau in a low-voltage range. Considerably different behaviors are revealed by the galvanostatic intermittent titration technique and electrochemical impedance spectroscopy measurements, and accordingly different storage mechanisms are proposed. The sloping region is mainly attributed to the adsorption at defects/heteroatoms for all the Li, Na, and K storage. In the plateau region, pore filling contributes very little to Li storage but much to Na and K storage. Furthermore, significant effects of ionic sizes on the storage behavior in hard carbons are revealed by the electrochemical performance from Li to Na to K. These findings not only offer a fundamental understanding of storage mechanisms of alkali metal ions in hard carbons but also help develop and design innovative electrode materials for low-cost and large-scale energy storage systems.