Tracking Sodium Cluster Dynamics in Hard Carbon with a Low Specific Surface Area for Sodium‐Ion Batteries

Abstract

Here, the sodium storage mechanism in commercial grade hard carbon with a low surface area is comprehensively investigated using electrochemical impedance spectroscopy (EIS), the galvanostatic intermittent titration technique, and in situ Raman spectroscopy for fresh and cycled electrodes. The reversible shift of the carbon G‐band peak on Raman spectra and substantial change of the charge‐transfer resistance in the sloping region of the voltage profile indicates the intercalation of sodium ions into hard carbon, whereas the low‐voltage plateau is associated with the pore‐filling process. In situ Raman analysis at low frequencies reveals that the pore filling is progressed via formation of small sodium clusters in closed pores. Prolonged cycling demonstrates that intercalation is stable and consistent throughout multiple charge–discharge cycles. The transition from intercalation to pore filling strongly affects the diffusion behavior, leading to considerably slower diffusivity at low voltage. The EIS effectively differentiates the contribution of adsorption to charge storage. The gradual growth of the solid‐electrolyte interphase layer affects the rise of the interfacial resistance as cycling progresses. In combination with the slower diffusivity, the low‐voltage plateau region strictly impedes fast de/sodiation and eventually causes capacity fade.

Design of the Particle Size and Morphology of Hard Carbon Anode Materials for Sodium-Ion Batteries Through Hydrothermal Carbonization

“…This decrease in capacity can be related with early cut-off of potential at the plato region due to the high overpotential, wich related with decrease of diffusivity, representing a transition from intercalation to inner pore filling. 49 Figure S5 shows data on cycling at a current density of 50 mA g −1 (0.2 C) for hard carbon obtained through hydrothermal carbonization with the following parameters: 6 h, 2.6 M, 200 °C (Fig. S8c) and 12 h, 2.6 M, 200 °C (Fig.…”

Internal-Parallel Hybrid Hard Carbon/TiO2 Electrode with a Boosted High-Rate Capability and Volumetric Capacity for Sodium-Ion Capacitors

“…Owing to the rich pores, HC has an inherently low compaction density of ∼1 mg cm –3 . The sodium-ion storage of HC is based on Na + adsorption with a slope region of 1.5–0.1 V vs Na + /Na and the intercalation and pore-filling process in a plateau region of 0.1–0.01 V vs Na + /Na. − The reaction in the plateau region is a typical diffusion-controlled process and is very close to the deposition potential of sodium metal. The potential shifts caused by fast sodiation would lead to large capacity loss or even risk of dendritic growth .…”

“…The sodium-ion storage of HC is based on Na + adsorption with a slope region of 1.5–0.1 V vs Na + /Na and the intercalation and pore-filling process in a plateau region of 0.1–0.01 V vs Na + /Na. − The reaction in the plateau region is a typical diffusion-controlled process and is very close to the deposition potential of sodium metal. The potential shifts caused by fast sodiation would lead to large capacity loss or even risk of dendritic growth . The Na + adsorption reaction of HC particles has capacitor-like behavior at the high potentials of 1.5–0.1 V vs Na + /Na, enabling fast (dis)charging. , Therefore, taking advantage of capacitive-controlled regions is beneficial for the assembly of high-power SICs.…”