Graphite As Anode Material for Sodium-Ion Batteries: Making an Inactive Electrode Active to Compete with Li-Ion Batteries

Being the standard anode material in lithium-ion batteries (LIBs), graphite is considered to fail application in sodium-ion batteries (NIBs) because no suitable binary intercalation compounds (b-GICs) exist. In this study, we demonstrate that this limitation can be successfully circumvented by using co-intercalation phenomena in a diglyme-based electrolyte. Results have been published in Angewandte Chemie International Edition: B. Jache, P. Adelhelm, Angew. Chemie 2014, 53, 10169–10173. The resulting compound is a stage-I ternary intercalation compound (t-GICs) with an estimated stoichiometry of Na(diglyme)₂C₂₀.

Beneficial properties of the electrode reaction are its low overpotentials and stable capacities of around 100 mAh/g for more than 1000 cycles. The initial irreversible capacity of around 10 % is, to the best of our knowledge, the smallest one reported so far for NIB negative electrodes. The overall coulomb efficiency exceeds > 99.87 %. The intercalation process proceeds over several intermediated stages and occurs in narrow potential window below 1 V vs. Na/Na⁺. A one-to-one comparison with the analogue lithium system shows the more suitable performance of the sodium based system. X-ray diffraction was used to study the electrode reaction during different stages of sodiation/lithiation. The reversible formation of ternary GICs is unequivocally proven by the shift in the (002) diffraction line and the appearance of additional reflexes.

The peculiar properties of ternary GICs combined with the advantage of using relatively low cost substances might make this electrode reaction attractive for stationary applications. As t-GICs are also formed with other solvents, our approach provides an alternative route to search for new electrode reactions for NIBs.