Insights Into the Kinetics and Storage Mechanism of Quasi-Metallic Lithium Clusters in the Closed-Pore Structure of Microcrystalline Cellulose-Derived Hard Carbon.

Hard carbon (HC) is a promising negative electrode material whose multiscale microstructure enables high theoretical capacity. However, its Li-storage mechanism remains insufficiently understood, particularly the origin of the low-potential plateau. Here, microcrystalline cellulose-derived HCs exhibiting pronounced low-potential plateau are systematically investigated. Multi-gas adsorption isotherms reveal that the transition in galvanostatic profiles correlates with the porosity. The sloping capacity arises from Li-ion adsorption at defects and open pores, whereas the plateau capacity originates from Li ions filling into closed pores. Notably, cyclic voltammetry and ex situ EIS show that although higher calcination temperatures will promote closed-pore formation, but will also increase kinetic barriers and hinder access to closed pores. Adoption of a constant current-constant voltage (CC-CV) protocol provides extra time for Li-ion to reach an equilibrium state and unlocks additional capacity. Consequently, the optimized HC delivers 434 mAh g-1 and 580 mAh g-1 under CC and CC-CV modes, respectively. Moreover, ex situ EPR, ssNMR, and SAXS provided mechanical evidence for quasi-metallic Li cluster formation within closed pores, and in situ XRD and Raman analyses further supported the proposed adsorption-filling mechanism. This work elucidates the relationship between microstructure, electrochemical kinetics, and Li-storage, highlighting that maximizing closed-pore capacity requires balancing the closed-pore volume against excessive graphitic ordering.