The joint intercalation of ions and solvent molecules into a layered host structure ("co-intercalation") has gained increasing attention in the last years with the reversible formation of ternary graphite intercalation compounds, especially in graphite electrodes for the sodium-ion batteries (SIBs).[1] Co-intercalation allows graphite as an anode material for this next generation battery technology. This process overcomes the poor specific capacity caused by unfavorable thermodynamic properties that limit the formation of binary graphite intercalation compounds (b-GICs) through the formation of ternary graphite intercalation compounds (t-GICs) by using ether-based electrolytes. In other words, while conventional carbonate-based electrolytes with graphite show low  specific capacity of 20 mA h g^-1, the use of ether-based electrolytes with graphite exhibits a reversible and stable specific capacity of 110 mA h g^-1.[2,3]  Although co-intercalation drives the large volume expansion of graphite layers to accommodate the solvated ions, interestingly, the reaction manifests rapid charge transfer kinetics as a result of the absence of desolvation steps which could enhance performance in low temperature environment.[4] Based on this, it becomes clear that employing different types of electrolyte solvents can improve the electrochemical properties.

Most of research on co-intercalation reaction in graphite electrode has focused on linear ethers such as mono-, di-, tri-, tetra- and penta-glyme.[5] Herein, we investigate the potential candidates of electrolyte solvents for the reversible electrochemical co-intercalation with enhancing these advantages. Cyclic ethers such as tetrahydrofuran (THF) and 1,3-dioxolane (DOL) exhibit no indication of co-intercalation in graphite on their own. However, this reaction is feasible by incorporating diglyme (2G) as an additive. When using THF/2G and DOL2G electrolytes, they display the change of voltage profiles compared to conventional 2G electrolyte while having equivalent specific capacity of 90 mA h g^-1 and good long-term cyclability.[6] According to the results of structural characterization, it is indicated that two different types of solvent intercalants can be contained together in graphite layers (quaternary graphite intercalation compounds (q-GICs)). Furthermore, it is confirmed that the concept of the use of mixed electrolytes can reduce significantly the undesired volume expansion of graphite lattice compared to using diglyme alone.

Apart from ether solvents, amine solvents have recently considered as candidates of electrolyte solvents.[7] Unlike the cyclic ethers are not able to co-intercalate themselves, 1-propylamine (PN)  electrolytes show the adequate specific capacity of 80 mA h g^-1 without requiring any other additives. Especially, when 2G mixed with PN,  PN/2G electrolytes display enhanced rate capability and cycle life. Taking into account the lower freezing temperature of PN compared to 2G, along with its co-intercalation behavior, the PN/2G electrolytes demonstrate the improvement of electrochemical properties at low temperature.

Last but the least, This present study provides the comprehensive overview of the advantage of using mixed solvents for the electrolytes based on co-intercalation. 2G serves as a key for activating the inert solvents of co-intercalation with suppression of volume expansion of graphite electrode. In addition, we demonstrate for the first time the concept of using 2G in combination with PN for the low temperature application.