Revealing the Intercalation Mechanisms of Lithium, Sodium, and Potassium in Hard Carbon

Abstract

Hard carbon is the most promising anode material for sodium-ion batteries and potassium-ion batteries owing to its high stability, widespread availability, low-cost, and excellent performance. Understanding the carrier-ion storage mechanism is a prerequisite for developing high-performance electrode materials; however, the underlying ion storage mechanism in hard carbon has been a topic of debate because of its complex structure. Herein, it is demonstrated that the Li+-, Na+-, and K+-ion storage mechanisms in hard carbon are based on the adsorption of ions on the surface of active sites (e.g., defects, edges, and residual heteroatoms) in the sloping voltage region, followed by intercalation into the graphitic layers in the low-voltage plateau region. At a low current density of 3 mA g(-1), the graphitic layers of hard carbon are unlocked to permit Li+-ion intercalation, resulting in a plateau region in the lithium-ion batteries. To gain insights into the ion storage mechanism, experimental observations including various ex situ techniques, a constant-current constant-voltage method, and diffusivity measurements are correlated with the theoretical estimation of changes in carbon structures and insertion voltages during ion insertion obtained using the density functional theory.