Intercalation compounds form the basis of essentially all lithium rechargeable batteries. They exhibit a wide range of electronic and crystallographic structures. The former varies from metallic conductors to excellent insulators. Today’s lithium batteries are limited in capacity, because less than one lithium ion is reversibly intercalated per transition metal redox center. There may be an opportunity to increase the storage capacity by utilizing redox centers that can undergo multi-electron reactions. This might be accomplished by intercalating multiple monovalent cations or one multivalent cation. In this talk we will review the key theoretical and experimental results on lithium and magnesium reversible intercalation into two prototypical materials: titanium disulfide, TiS2, and vanadyl phosphate, VOPO4. Both of these materials exist in two or more phases, which have different molar volumes and/or dimensionalities and thus are expected to show a range of diffusion opportunities for battery active guest ions such as lithium, sodium, and magnesium. A major conclusion of this research is that reversibly intercalating two lithium or sodium ions into a host lattice whilst maintaining it’s crystal structure is possible. A second major conclusion is that theoretical studies are now sufficiently mature that they can be relied upon to predict the key free energy values of simple intercalation reactions, i.e. the energy that might be stored. This could help to focus future choices of battery couples.
This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012583.