Combination of solid state NMR and DFT calculation to elucidate the state of sodium in hard carbon electrodes

Abstract

We examined the state of sodium electrochemically inserted in HC prepared at 700-2000 C using solid state Na magic angle spinning (MAS) NMR and multiple quantum (MQ) MAS NMR. The 23 Na MAS NMR spectra of Na-inserted HC samples showed signals only in the range between +30 and À60 ppm. Each observed spectrum was ascribed to combinations of Na + ions from the electrolyte, reversible ionic Na components, irreversible Na components assigned to solid electrolyte interphase (SEI) or nonextractable sodium ions in HC, and decomposed Na compounds such as Na 2 CO 3 . No quasi-metallic sodium component was observed to be dissimilar to the case of Li inserted in HC. MQMAS NMR implies that heat treatment of HC higher than 1600 C decreases defect sites in the carbon structure. To elucidate the difference in cluster formation between Na and Li in HC, the condensation mechanism and stability of Na and Li atoms on a carbon layer were also studied using DFT calculation. Na 3 triangle clusters standing perpendicular to the carbon surface were obtained as a stable structure of Na, whereas Li 2 linear and Li 4 square clusters, all with Li atoms being attached directly to the surface, were estimated by optimization. Models of Na and Li storage in HC, based on the calculated cluster structures were proposed, which elucidate why the adequate heat treatment temperature of HC for high-capacity sodium storage is higher than the temperature for lithium storage. † Electronic supplementary information (ESI) available: XRD patterns of HC samples, wide range 23 Na NMR spectra, Na NMR spectra of some inorganic sodium compounds and NaPF 6 /PC solutions, charge/discharge curves of reassembled cells, and DFT optimizations of an alkali atom (Li or Na) set at the center of C 150 H 30 . See

From Charge Storage Mechanism to Performance: A Roadmap toward High Specific Energy Sodium‐Ion Batteries through Carbon Anode Optimization

“…This mechanism has been further confirmed for lithium by 7 Li solid state NMR, both in situ (operando) and ex situ, by Gotoh et al in 2006 and Chevallier et al in 2003, respectively. The clustering mechanism was also demonstrated for sodium using operando and ex situ 23 Na solid state NMR by Stratford et al and Morita et al in 2016 . In these later cases, however, a notable discrepancy between the results can be observed.…”

Operando visualisation of battery chemistry in a sodium-ion battery by 23Na magnetic resonance imaging

“…This suggests these sodium species are less mobile, but in a comparable chemical environment to the electrolyte within the separators, indicating the formation of additional diamagnetic species, through degradation of the electrolyte, possibly on the surface of the electrode and forming part of the solid-electrolyte interphase (SEI) 13 . The presence of similar additional peaks was not observed previously during galvanostatic cycling in the in situ 23 Na NMR study of a Na symmetrical cell with Na bis(trifluoromethylsulfonyl)imide (NaTFSI) in propylene carbonate (PC) electrolyte 10 , but has been observed in ex situ 23 Na NMR measurements of NIB hard carbon electrodes 28 with 1 M NaPF 6 / PC and fluoroethylene carbonate (FEC, 2 wt%) as an additive. Moreover, the appearance of a similar peak, believed to arise from species within the glass fibre separator, has been observed for solvated lithium species in an EC/DMC electrolyte of a lithiumion battery following charge cycling, which was assigned to the formation of lithiated degradation products in the electrolyte 29 .…”

Revealing the Intercalation Mechanisms of Lithium, Sodium, and Potassium in Hard Carbon

“…This indicates that the additional capacity achieved in the CV mode at near 0 V is associated with the Li + ion storage in an unexplored site. The ionic interaction between the Li + ions and misaligned graphene sheets was weaker than those of the Li + ions in the adsorption sites; [ 30 ] this caused the peak to shift toward the high‐frequency region when Li + ions intercalated into the graphitic layer. In addition, the peak at 9 ppm was much sharper than that at 4 ppm, indicating that the Li + ion insertion sites in the CV mode are more homogeneously distributed chemical environments as compared to the adsorption sites.…”

“…[ 15t ] This is because the interaction between the Na + ions and graphene layers is weaker than that between the Na + ions and micropore surfaces; thus, the Na + ′ ions in the graphitic region have higher ionic characteristics than those in the micropores, which results in a high frequency in the NMR spectra. [ 30b ] The peak at –14 ppm is not indicative of the Na metal plating because the characteristic peak of metallic Na appeared at 1132 ppm (Figure S11b, Supporting Information). When the discharge voltage dropped from 0.05 V to the cutoff voltage, the peak at –4 ppm shifted to –7 ppm and peak broadening occurred, which could be attributed to the additional electron shielding originating from the further adsorption of Na + ions in the micropores.…”