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 NaC 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)(2)C-20. 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 mAhg(-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.