Revisiting Lithium‐ and Sodium‐Ion Storage in Hard Carbon Anodes

Abstract

The galvanostatic lithiation/sodiation voltage profiles of hard carbon anodes are simple, with a sloping drop followed by a plateau. However, a precise understanding of the corresponding redox sites and storage mechanisms is still elusive, which hinders further development in commercial applications. Here, a comprehensive comparison of the lithium‐ and sodium‐ion storage behaviors of hard carbon is conducted, yielding the following key findings: 1) the sloping voltage section is presented by the lithium‐ion intercalation in the graphitic lattices of hard carbons, whereas it mainly arises from the chemisorption of sodium ions on their inner surfaces constituting closed pores, even if the graphitic lattices are unoccupied; 2) the redox sites for the plateau capacities are the same as those for the closed pores regardless of the alkali ions; 3) the sodiation plateau capacities are mostly determined by the volume of the available closed pore, whereas the lithiation plateau capacities are primarily affected by the intercalation propensity; and 4) the intercalation preference and the plateau capacity have an inverse correlation. These findings from extensive characterizations and theoretical investigations provide a relatively clear elucidation of the electrochemical footprint of hard carbon anodes in relation to the redox mechanisms and storage sites for lithium and sodium ions, thereby providing a more rational design strategy for constructing better hard carbon anodes.

Hard Carbon Anode Synthesized by an In Situ Porous Strategy for Advanced Potassium-Ion Batteries

“…The I D / I G (intensity ratio of D and G-bands) value increases from 0.94 in B-HC to 1.03 in P-HC, indicating that the molecular-scale in situ porous strategy can cause more structural disturbances and defects in P-HC, which is also reflected in the more curved and disordered carbon layer structure of P-HC in HRTEM. As a result, Figure e summarizes the P-HC schematic diagram; the extensive pore structure and defective carbon in this design will give extra K-storage sites during later electrochemical procedures …”

“…As a result, Figure 2e summarizes the P-HC schematic diagram; the extensive pore structure and defective carbon in this design will give extra Kstorage sites during later electrochemical procedures. 34 XPS is used to analyze the surface properties and chemical content of B-HC and P-HC. The survey spectra of XPS demonstrate only the characteristic peaks of C 1s and O 1s, indicating that B-HC and P-HC are composed of C and O elements (Figure 2f).…”

The Progress of Hard Carbon as an Anode Material in Sodium-Ion Batteries

“…The difficulty in understanding the mechanism of sodium-ion storage in hard carbon has hindered the design of anode materials with excellent performance for sodium-ion batteries. Scientists have made many efforts to solve the controversial problems in developing sodium-ion batteries, and enormous achievements have been made during this period [ 74 , 75 , 76 ]. This review summarizes four different sodium-storage models for hard carbon anodes [ 77 , 78 ] ( Figure 1 ), including the “embedding-adsorption” model [ 79 , 80 ], the “adsorption-embedding” model [ 3 , 81 , 82 ], the “three-stage model” [ 83 ], and the “adsorption-filling” model [ 84 ].…”

Reappraisal of hard carbon anodes for practical lithium/sodium-ion batteries from the perspective of full-cell matters

“…For instance, an appropriate number of defects is necessary to facilitate intercalation behavior, which is more likely to occur near rich defects. 89 Furthermore, to extend the plateau-capacity, it is crucial to ensure that Na-ions can easily enter the internal structure after passing through defects. We recommend a more comprehensive review of the relationship between structure and electrochemical behaviors, focusing on the specific influence of functional group type, pore size, pore type, and pore-entrance diameter, and the synergistic effects of all the factors.…”