Manipulating Adsorption–Insertion Mechanisms in Nanostructured Carbon Materials for High‐Efficiency Sodium Ion Storage

Abstract

Hard carbon is one of the most promising anode materials for sodium‐ion batteries, but the low Coulombic efficiency is still a key barrier. In this paper, a series of nanostructured hard carbon materials with controlled architectures is synthesized. Using a combination of in situ X‐ray diffraction mapping, ex situ nuclear magnetic resonance (NMR), electron paramagnetic resonance, electrochemical techniques, and simulations, an “adsorption–intercalation” mechanism is established for Na ion storage. During the initial stages of Na insertion, Na ions adsorb on the defect sites of hard carbon with a wide adsorption energy distribution, producing a sloping voltage profile. In the second stage, Na ions intercalate into graphitic layers with suitable spacing to form NaC x compounds similar to the Li ion intercalation process in graphite, producing a flat low voltage plateau. The cation intercalation with a flat voltage plateau should be enhanced and the sloping region should be avoided. Guided by this knowledge, nonporous hard carbon material has been developed which has achieved high reversible capacity and Coulombic efficiency to fulfill practical application.

Electrode Materials for Sodium-Ion Batteries: Considerations on Crystal Structures and Sodium Storage Mechanisms

“…For battery applications, the cation intercalation corresponding to a flat voltage plateau should be enhanced, while the sloping region should be minimized. Qiu et al reported a new way to manipulate adsorption-insertion mechanisms in nanostructured carbon materials for Na-ion batteries [127]. They chose cellulose as a carbon precursor to synthesize high-performance Na storage anode material because it consists of d-glucose with carbon atoms in a hexa-atomic ring structure without N or S heteroatoms, which can easily form a layered graphene structure at high temperature.…”

“…Brunauer-Emmett-Teller (BET) surface area results for micropore volume and the average pore size are shown in Table 3. In terms of capacity, sodium storage capacity in the sloping region is 122, 133, 103 and 90 mAh g −1 for HC-900, HC-1100, HC-1300 and HC-1500, respectively, reflecting a decrease trend as temperature increases [127].…”

“…Li ions can be inserted into graphitized carbon, whereas Na ions cannot be inserted into graphitized carbon due to its large size (but it can be inserted in soft carbons). There are three factors that can affect the performance of hard carbon: pyrolysis temperature, carbon source and heteroatom doping modification Table 3 Physical parameters of hard carbon materials [127] d ) and a non-ideal Coulombic efficiency (14%) [129]. This problem had troubled researchers for many years.…”

Recent Advances in Sodium-Ion Battery Materials

“…Cao and co-workers proposed the "adsorption-insertion mechanism" that high potential slope region is associated with the adsorption/desorption of Na + ions, and low-potential plateau corresponds to the insertion/extraction of Na + between the carbon layers [155]. And very recently, they have used various techniques (such as, in situ XRD, NMR, electron paramagnetic resonance) to sufficiently evidence the sodium storage mechanism [181]. During discharge process, Na ions first adsorb on the surface active sites of hard carbon, which leads to a sloping voltage profile due to a wide distribution of adsorption energies; then, Na ions intercalate into graphene layers with suitable spacing to form NaC x compounds, which exhibits a flat voltage plateau similar to the Li-graphite counterpart [181].…”

“…And very recently, they have used various techniques (such as, in situ XRD, NMR, electron paramagnetic resonance) to sufficiently evidence the sodium storage mechanism [181]. During discharge process, Na ions first adsorb on the surface active sites of hard carbon, which leads to a sloping voltage profile due to a wide distribution of adsorption energies; then, Na ions intercalate into graphene layers with suitable spacing to form NaC x compounds, which exhibits a flat voltage plateau similar to the Li-graphite counterpart [181]. This mechanism suggests that the sodium storage capacity depends on appropriate carbon layer spacing rather than microporous structure.…”