From Ternary to Quatenary Graphite Intercalation Compound in Na-Ion Batteries
Authors: Son Youhyun , Åvall Gustav , Alvarez Ferrero Guillermo , Freytag Annica , Janßen Knut , Adelhelm Philipp 
Published: December 2024

Sodium-ion batteries (SIBs) have emerged alongside the prominence of lithium-ion batteries (LIBs), capturing significant attention. By sharing chemical similarities with LIBs, SIBs have experienced notable growth, not only in research but also in industrial applications throughout 2023, particularly in the development of large-scale battery systems.[1] Graphite, a common anode material in LIBs, has unfavorable thermodynamic properties that limit the formation of binary graphite intercalation compounds (b-GICs). Consequently, this results in a poor specific capacity of approximately 20 mA h g-1 when conventional carbonate-based electrolytes are used in SIBs.[2] However, in 2014, it was confirmed that employing ether-based electrolytes, such as diglyme (2G), could significantly enhance the specific capacity to 110 mA h g-1 through a solvent co-intercalation mechanism. This mechanism involves the intercalation of solvated sodium ions into graphite layers, leading to the formation of ternary graphite intercalation compounds (t-GICs).[3] Despite causing a large expansion of graphite layers to accommodate the solvated ions, co-intercalation shows high reversible cycling behavior and rapid kinetic properties due to the absence of desolvation steps.[4]
Previous studies have predominantly focused on the electrochemical solvent co-intercalation in graphite using linear ethers such as mono-, di-, tri-, tetra-, and pentaglyme.[5] Herein, we investigate the potential for reversible electrochemical co-intercalation with alternative solvents, broadening the range of usable electrolytes. Especially, we examine cyclic ethers like tetrahydrofuran (THF) and 1,3-dioxolane (DOL), which show no indication of co-intercalation in graphite.[6] Surprisingly, incorporating diglyme at an additive level facilitates reversible co-intercalate THF and DOL. However, explicit acknowledgment of this phenomenon has been infrequent, and the effect of THF and DOL mixed electrolytes has never been fully explored.
When employing mixed electrolytes, i.e., THF/2G and DOL/2G, they show different voltage profiles compared to the pure glyme-based electrolytes, while having equivalent specific capacities of ~90 mA h g-1 and good long-term durability. Operando X-ray diffraction and ex-situ solid-state NMR techniques are extensively utilized to gain comprehensive insights into the co-intercalation reaction of these cyclic ethers, along with Na+-glymes in graphite during the cycling process.
The present study demonstrates for the first time the concept of using diglyme as a "gate opener" for the electrochemical synthesis of GICs. Since diglyme serves as a gate opener within the graphite layers, other solvent molecules are capable of co-intercalating, a task they were unable to achieve independently before. According to the changes in voltage profiles and the results of structural characterization, it is indicated that the quaternary graphite intercalation compounds (q-GICs) have formed. The finding of the new role of diglyme as a gate opener will enable the development of various compounds even higher-order GICs. This approach could be applied to design new intercalation electrodes for rechargeable batteries.

References:

[1] P. K. Nayak, L. Yang, W. Brehm, P. Adelhelm, Angew. Chem. Int. Ed.57 (2018) 102.
[2] O. Lenchuk, P. Adelhelm, D. Mollenhaur, Phys. Chem. Chem. Phys., 21 (2019) 19378-19390.
[3] B. Jache, P. Adelhelm, Angewandte Chemie., 126 (2014) 10333-10337.
[4] K. Xu, A. von Cresce, U. Lee, Langmuir 26 (2010), 11538-11543.
[5] M. Goktas, B. Akduman, P. Huang, A. Balducci, P.Adelhelm, J. Phys. Chem. C, 122 (2018) 26816-26824.
[6] Y. Son, G. Åvall, G. A. Ferrero, A. I. Freytag, I. Escher, K. A. Janßen, M. Jauregui, D. Saurel, M. Galceran, P. Adelhelm, Battereis & Supercaps 7 (2024) e202300506.

Figure 1