NMR-Detected Dynamics of Sodium Co-Intercalation with Diglyme Solvent Molecules in Graphite Anodes Linked to Prolonged Cycling

Following the success of Li-ion batteries, Na-ion batteries are becoming an important economical alternative, particularly where weight and density considerations are not of primary importance. Graphite, the anode of choice for nearly all commercial Li-ion battery applications, has only recently been successfully used as such in Na systems. This unprecedented success was due to the proper choice of solvent, e.g., diglyme. Interestingly, lithium performs poorly under such conditions, which is the converse of their respective behavior in standard carbonate solvents. These phenomena have been attributed to co-intercalation of the alkali ions upon their complexation with the smaller solvent molecules. In the case of Li, the use of such solvents leads to deterioration, while in the case of Na, it improves its electrochemical performance so substantially as to make the previously irrelevant Na-graphite system viable. Several studies have since followed, mainly focusing on the Na-diglyme intercalation; however, a thorough understanding of the mechanisms of ternary intercalation into graphite for both Na and Li is still lacking. In particular, the characteristic differences in location and dynamics of the guest complexes in the host material upon electrochemical cycling are not yet fully understood. In this study, the co-intercalation mechanisms of Na and Li in diglyme into graphite were explored via solid-state NMR spectroscopy. Direct evidence for the atomic proximity of both Na-(diglyme)(2) and Li-(diglyme)(2) complexes to the graphite planes in discharged electrodes was observed. Reduced mobility and stronger coupling of the Li-(diglyme)(2) complex to the graphene electrons are seen, whereas higher mobility and weaker coupling to the host are detected for the Na-(diglyme)(2) complexes in the galleries, providing molecular cues for the difference in cycling performance of the two systems.