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

Abstract Co‐intercalation using ether‐based electrolytes renders graphite as a promising anode material in sodium‐ion batteries (SIBs). While most research on electrochemical solvent co‐intercalation in graphite has focused on linear ethers such as mono‐, di‐, tri‐, tetra‐, and penta‐glyme, we herein investigate the possibility of reversible electrochemical co‐intercalation with alternative solvents, especially cyclic ethers tetrahydrofuran (THF) and 1,3‐dioxolane (DOL), which show no signs of co‐intercalation on their own. We demonstrate, however, that this reaction becomes feasible when incorporating diethylene glycol dimethyl ether (2G, diglyme) as an additive. Operando X‐ray diffraction and ex‐situ ss‐NMR techniques are employed comprehensively to understand the co‐intercalation reaction of these cyclic ethers, along with Na + ‐glymes, into graphite during cycling. When using these mixed electrolytes i. e., THF/2G and DOL/2G, the voltage profiles changes compared to the pure glyme‐based electrolyte, while showing comparable specific capacities and good long‐term durability. Overall, we propose that even trace amounts of diglyme prompt the co‐intercalation of THF and DOL into graphite layers. This leads to the formation of quaternary graphite intercalation compounds (q‐GICs), expanding beyond the realm of ternary graphite intercalation compounds (t‐GICs).