Ternary graphite intercalation compounds for the reversible storage of alkali ions in electrochemical cells

Abstract

This study is focused on graphite as anode material for sodium-ion batteries (NIBs). Graphite electrodes, prepared by lithium-ion technology standards, are implemented into sodium-ion cells without further structural modification, using different kind of electrode reaction (co-intercalation) to store ions inside the graphite structure. These co-intercalation electrodes might be implemented into NIBs and allow the simple construction of reliable, low-cost NIBs that may be suitable for stationary applications, where the price is more relevant than energy density. These NIBs may thereby lower the demand and price of lithium compounds for lithium-ion batteries for mobile applications.At first, the inability to intercalate "bare" sodium ions into a graphite electrode from a carbonate based electrolyte (no co-intercalation) is reproduced and compared to analogously constructed lithium cells. Afterwards, measurements in a diglyme based electrolyte (co-intercalation occurs) are conducted and characterized by electrochemical measurements and X-ray diffraction, proving that the solvent molecules participate in the electrode reaction. The effect of different chain lengths of the linear ethers (glymes) is tested to make a statement about coordination effects that may affect the co-intercalation process. Furthermore, the steric hindrance for the co-intercalation electrode reaction is increased by using linear ethers with side groups. Concluding this study, first electrochemical measurements on sodium-ion full cells based on graphite/diglyme/P2-NaNiMnO2 are shown.