In Situ Pore Formation in Graphite Through Solvent Co‐Intercalation: A New Model for The Formation of Ternary Graphite Intercalation Compounds Bridging Batteries and Supercapacitors

Abstract

For Li‐ion and Na‐ion batteries, the intercalation behavior of graphite anodes is quite different. While Li‐ions intercalate, Na‐ions only co‐intercalate with solvent molecules from the electrolyte solution leading to ternary graphite intercalation compound (t‐GIC) formation along with an expansion of the graphite interlayer spacing to 1.2 nm. This large interlayer spacing represents a micropore with parallel slit geometry. Little is known about t‐GIC formation, but it is commonly believed that throughout the reaction the ion is accompanied by either a full or partial solvation shell. Here, it is elucidated for the first time, using two independent methods – mass measurements and electrochemical impedance spectroscopy – supplemented by operando microscopy, entropymetry and simulations, that the storage mechanism is far more complex. A new model for the electrochemical solvent co‐intercalation process is proposed: As soon as solvated ions enter, the graphite structure is flooded with free solvents, which are subsequently replaced by solvated ions. Close to full sodiation, few free solvents remain and structural rearrangement take place to reach the full storage capacity. Thus, t‐GICs represent a unique case of switchable microporous systems and hence appear as a bridge between ion storage in the bulk phase and in micropores, i.e., between batteries and supercapacitors.

Diglyme as a Promoter for the Electrochemical Formation of Quaternary Graphite Intercalation Compounds Containing two Different Types of Solvents

“…This is in agreement with the pore formation model previously proposed by us, where a t-GIC formed using diglyme should act as a slit pore which free solvents can enter. [18] From these experiments it can be inferred that a minuscule amount of 2G solvated ions in the graphite layer is sufficient to enable further co-intercalation with THF molecules. Even the desodiation of 2G solvated ions seems to activate the graphite to allow THF solvated ions to co-intercalate.…”

“…Yet this is not observed, (109 mAh g À 1 for 10 vol%, 92 mAh g À 1 for 5 vol%) indicating that not only 2G is participating in the reaction. [18] This strongly indicates that not only 2G, but also THF, is taking part in the reaction to sodiate the graphite.…”

“…Several NMR studies have indicated that at full sodiation, there are both free and bound solvents inside the graphite galleries, and a recent paper by Escher et al indicate that there are 2-3 2G molecules per Na + in the graphite at full sodiation. [18,42] This would indicate that roughly 1.2-1.8 μl of 2G is needed per mg of graphite. Thus, an electrode with a radius of 12 mm and a loading of 9 mg/cm 2 requires in the range of 12-18 μl of 2G at full sodiation.…”

“…However, even more 2G is required to reach full sodiation as measurement indicate that the largest amount of 2G in the system occurs on the main voltage plateau, after which there is a decrease of solvent molecules in the graphite. [18] Thus, 100 μl of electrolyte with less than 10 vol% 2G should not be able to fully sodiate such an electrode.…”

“…Son, Dr. G. Åvall when diethylene glycol dimethyl ether (2G, diglyme) solvated ions intercalate into graphite the interlayer spacing is turned into a slit pore which fills up with the free diglyme solvents of the electrolyte, it appears reasonable that once the slit pores are formed other, previously thought inert, solvents could enter the graphite structure. [18] We therefore investigate whether small additions of diglyme to two different cyclic ether-based electrolytes, where previous attempts to electrochemically form a t-GIC has failed, can allow also the cyclic ethers to reversibly co-intercalate into graphite. Although several theoretical studies on the intercalation of Na + in graphite with cyclic ethers such as tetrahydrofuran (THF) and 1,3-dioxolane (DOL) have been conducted [17,19] , their experimental validation remains incomplete.…”