Revisit Electrolyte Chemistry of Hard Carbon in Ether for Na Storage

Abstract

Hard carbons (HCs) as an anode material in sodium ion batteries present enhanced electrochemical performances in ether-based electrolytes, giving them potential for use in practical applications. However, the underlying mechanism behind the excellent performances is still in question. Here, ex situ nuclear magnetic resonance, gas chromatography–mass spectrometry, and high-resolution transmission electron microscopy were used to clarify the insightful chemistry of ether- and ester-based electrolytes in terms of the solid–electrolyte interphase (SEI) on hard carbons. The results confirm the marked electrolyte decomposition and the formation of a SEI film in EC/DEC but no SEI film in the case of diglyme. In situ electrochemical quartz crystal microbalance and molecular dynamics support that ether molecules have likely been co-intercalated into hard carbons. To our knowledge, these results are reported for the first time. It might be very useful for the rational design of advanced electrode materials based on HCs in the future.

Eco‐Friendly Synthesis of 3D Disordered Carbon Materials for High‐Performance Dual Carbon Na‐Ion Capacitors

“…From a practical viewpoint, it is worth mentioning that the ICE of AGNa‐600‐5 can be substantially improved from 49 % up to 85 % (value typical of low surface area hard carbons) by using an ether‐based electrolyte (i.e., 1 m NaPF 6 in diglyme). Thus, as can be seen in Figure S11a, electrolyte decomposition during the first sodiation process is suppressed to a large extent, in agreement with previous studies [33,34] . Furthermore, the coulombic efficiency already reaches a value of 97 % in the second cycle.…”

“…thinner and more compact SEI layer, as shown in previous studies. [33][34][35] Finally, the cycling stability is also considerably improved in the ether electrolyte (Figure S11c), the capacity loss per cycle decreasing from 0.070 mAh g À 1 in the carbonate electrolyte to 0.015 mAh g À 1 in the ether electrolyte after 1400 cycles at 1 A g À 1 . These results show that both rate capability and long-term stability could be enhanced by optimizing the electrolyte composition.…”

Improved Low‐Temperature Performance of Rocking‐Chair Sodium‐Ion Hybrid Capacitor by Mitigating the De‐Solvation Energy and Interphase Resistance

“…The higher capacity of DEGDME could be contributed by the co‐intercalation of Na + –G2 into the graphitic domain of the hard carbon, in accordance with previous reports. [ 35–38 ] Both coulombic efficiency and capacity retention of hard carbon in the EC/DMC electrolyte are lower than DEGDME electrolyte, indicating that more irreversible electrolyte decomposition was happening in the EC/DMC electrolyte. As for the activated carbon cathode, similar capacities and voltage profiles were delivered in both electrolytes at the low current density of 0.0625 A g –1 , indicating a similar reaction process.…”

“…These structures prevent the exfoliation of graphene layers and, more importantly, provide the host for co‐intercalation of Na + –solvent complex. [ 35 ] At the voltage below 0.2 V versus Na/Na + , the Na + –G2 cluster was proved to be able to reversibly co‐intercalate into graphitic structures in the hard carbon anode by ex situ FTIR, high‐resolution transmission electron microscope, (HRTEM) and 1 H nuclear magnetic resonance. [ 22,35–38 ] The co‐intercalation phenomenon not only increased the utilization of the hard carbon materials with a high capacity, more importantly, it improved the rate performance by promoting the kinetics of the de‐solvation process.…”

Recent advances for SEI of hard carbon anode in sodium-ion batteries: A mini review

“…Pan et al used ex-situ nuclear magnetic resonance (NMR), gas chromatography-mass spectrometry (GC-MS) and HR-TEM to elucidate the structure and evolution process of SEI on HC in ether-based and ester-based electrolytes, and obtained the structure of SEI in ester-based electrolytes consistent with the mosaic model. More advanced cryo-TEM has also been used to observe the SEI ( Pan et al, 2021 ), by freezing the sample, the damage and the deformation to the sample by the electron beam can be reduced, thereby a more realistic morphology could been obtained. Using ex-situ cryo-TEM, Cui Yi et al discovered that inorganic materials like Li 2 O formed dense SEI, and carbonates on the dense layer formed inhomogeneous extended SEI in cycling ( Huang et al, 2019 ).…”