Regulating the Interlayer Spacings of Hard Carbon Nanofibers Enables Enhanced Pore Filling Sodium Storage

Abstract

Hard carbon (HC) represents an attractive anode material for sodium‐ion batteries. However, most HC materials deliver limited capacity and the sodium storage mechanisms in the slope and plateau regions are controversial. Herein, a series of hard carbon nanofibers (HCNFs) with tunable interlayer spacings are designed to understand the sodium storage manners in HC. The optimized HCNFs featuring short‐range graphitic layers with sufficient interlayer spacings (0.37–0.40 nm) for Na+ intercalation deliver a high reversible capacity (388 mAh g–1 at 30 mA g–1) and good rate capability. In‐situ X‐ray diffraction and Raman characterizations reveal a revised adsorption/insertion–filling sodium storage mechanism. Combined with the density functional theory (DFT) calculation, the detailed relationship between pore‐filling plateau capacity and interlayer spacing is disclosed. It is found that sufficient interlayer spacings (>0.37 nm) provide diffusion channels for Na+ to reach the pores for further filling. Additionally, the reason for plateau‐region capacity degradation of the HCNFs is completely demonstrated. This contribution provides insights into the sodium storage mechanism and rational construction of high‐performance HC anode materials.

Non-negligible Influence of Oxygen in Hard Carbon as an Anode Material for Potassium-Ion Batteries

“…27−29 As the annealing temperature increases, the area ratio of the D 1 peak and G peak decreases gradually, indicating that the graphene carbon sheet structure becomes more orderly (the data in Table 2), which is consistent with the TEM results. 30 In addition, the ratio of I D3 / I G decreases from 1.90 to 0.29 with increasing annealing temperature, which means that the content of oxygen atoms and defects in HCs is reduced significantly, implying a more ordered carbon network. 31 N 2 adsorption/desorption measurements were performed to analyze the Brunauer−Emmett−Teller (BET) surface area and pore structure of HCs.…”

“…The Raman spectra of HCs can be divided into four peaks at 1220, 1343, 1500, and 1595 cm –1 , which correspond to carbon atoms bonded by sp 2 –sp 3 bonds (D 4 ), carbon atoms at edges and defects of graphite sheets (D 1 ), the integrated heteroatoms (oxygen atoms) or five-numbered rings in carbon layers (D 3 ), and stretching vibrations of carbon atoms in sp 2 -hybridized graphite (G), respectively (Figure b). − As the annealing temperature increases, the area ratio of the D 1 peak and G peak decreases gradually, indicating that the graphene carbon sheet structure becomes more orderly (the data in Table ), which is consistent with the TEM results . In addition, the ratio of I D3 / I G decreases from 1.90 to 0.29 with increasing annealing temperature, which means that the content of oxygen atoms and defects in HCs is reduced significantly, implying a more ordered carbon network …”

Pore Structure Modification of Pitch‐Derived Hard Carbon for Enhanced Pore Filling Sodium Storage

“…Since the sodium ions diffuse through the graphite layers to reach the closed pores, the structure order degree of the graphitic domains is decreased due to the interlayer expansion upon Na + intercalation. [ 31 ] Thus, it is concluded that the sodiation behavior of hard carbon below 0.07 V consists of Na + intercalation into the graphitic layers and Na deposition into the micropores.…”

Understanding of Sodium Storage Mechanism in Hard Carbons: Ongoing Development under Debate

“…An “ideal” carbon was thus supposed to combine large pores with an expanded interlayer distance of ≈3.65 Å, which could allow the diffusion of Na ions into the pores but not the electrolyte molecules. Similarly, Mai et al [ 134 ] investigated the Na‐storage property of hard carbon nanofibers (HCNFs) with tunable interlayer spacing, and reported that sufficient interlayer spacing (>0.37 nm) can provide diffusion channels for Na + to reach the enclosed pores for further filling.…”