The possibility of replacing lithium with sodium in lithium-ion batteries (LIBs) is extremely attractive as sodium is cheap and abundant. However, graphite anodes, which are widely used in LIBs, perform very poorly in the sodium-ion battery (NIB). Birte Jache and Philipp Adelhelm of Justus-Liebig-University Giessen address this problem in their article in the September 15, 2014, issue of Angewandte Chemie International Edition (DOI: 10.1002/ anie.201403734; p. 10169) and propose co-intercalation as a solution.
Sodium cannot be simply substituted for lithium in these batteries due to its larger ionic radius. A new approach is therefore required. In LIBs, lithium ions are reversibly intercalated (inserted) within layers of cathode or graphite anode. During discharge, the ions move from the anode to the cathode through an electrolyte, typically lithium hexafluorophosphate (LiPF6) in an organic solvent. They move back to the anode while charging. Graphite is successful as an anode in LIB because of its ability to reversibly intercalate lithium by forming a series of binary graphite intercalation compounds (b-GICs) Li Cx with the final stoichiometry LiC6. Unlike other alkali metals, sodium does not form b-GICs under room temperature and pressure probably because of the mismatch between the graphite structure and the size of the Na ion. Hence, a different chemistry for a NIB is needed if graphite is to be retained as the anode.
Jache and Adelhelm make use of the fact that sodium can form ternary graphite intercalation compounds (t-GICs) to get around this problem. They use a single-solvent electrolyte based on diglyme and sodium triflate (NaOTf) as the conductive salt. Sodium and solvent molecule co-intercalate graphite and a ternary compound, Na (diglyme)2 C20 is formed, which exhibits favorable electrode properties. The size mismatch between the graphite lattice and the intercalant is small. The electrode reaction is characterized by high-energy efficiency, small irreversible loss during the first cycle, capacities close to 100 mAh/g for 1000 cycles, and coulomb efficiencies greater than 99.87%. These properties make NIB attractive for stationary applications. Importantly, this work can also inspire a search for better electrode reactions for NIBs since t-GICs are also formed with other solvents.
According to Takeshi Abe, a professor at Kyoto University, Japan, who did not participate in the study, the reversibility of the system is good and the electrode properties are very interesting. The volume expansion of the anode, however, is large and requires further study.
Many questions have to be answered scientifically before widespread applications can be developed for NIBs, and the present work points a way forward.