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. In contrast to the general view, graphite can be used as an electrode material in sodium‐ion batteries by taking advantage of the formation of ternary graphite intercalation compounds. The important features of this electrode reaction are the small irreversible capacity, the low overpotentials, and the superior cycle life. With a capacity close to 100 mAh g−1, the electrode is attractive for stationary applications.