Solid-State NMR Study on the Structure and Dynamics of Graphite Electrodes in Sodium-Ion Batteries with Solvent Co-Intercalation

The possibility to co-intercalate sodium ions together with various glymes in graphite enables its use as a negative electrode material in sodium-ion batteries (SIBs). However, the storage mechanism and local interactions appearing during this reaction still needs further clarification. H-1, C-13 and Na-23 ex situ solid-state NMR (ss-NMR) experiments are performed to obtain insights into the storage mechanism depending on the state of charge (SOC) and the electrolyte solvent used. Distinct differences could be seen depending on the SOC, indicating a possible change of the solvation shell, differences in the mobility as well as a phase transition at the voltage plateau. Furthermore, exchange experiments reveal information on the sodium ion transport process in the graphitic lattice. The inferior cycling performance of triglyme (3G) (compared to diglyme (2G) and pentaglyme (5G)) is also reflected in the ss-NMR spectra, showing a reduced mobility and stronger interactions between sodium ions, 3G and graphite already at room temperature (RT).