Use of graphite as a highly reversible electrode with superior cycle life for sodium-ion batteries by making use of co-intercalation phenomena.

Although being the standard anode material in lithium-ion batteries (LIBs), graphite so far is considered to fail application in sodium-ion batteries (NIBs) because the Na-C system lacks suitable binary intercalation compounds. Here we show that this limitation can be circumvented by using co-intercalation phenomena in a diglyme-based electrolyte. The resulting compound is a stage-I ternary intercalation compound with an estimated stoichiometry of Na(diglyme)2C20. Highlights of the electrode reaction are its high energy efficiency, the small irreversible loss during the first cycle, and a superior cycle life with capacities close to 100 mAh g(-1) for 1000 cycles and coulomb efficiencies >99.87%. A one-to-one comparison with the analogue lithium-based cell shows that the sodium-based system performs better and also withstands higher currents.