Current lithium ion battery technology is based on intercalation – the storage of lithium in small cavities in a host structure that usually consists of metal oxides. This method is said to work well, but storage densities are limited as lithium cannot be packed very densely in the structure. In addition, intercalation storage of more than one lithium ion per formula unit is generally not possible.
Researchers from Karlsruhe Institute of Technology have developed a new approach in which it is said to be possible to store 1.8 Li ions per formula unit. Using Li2VO2F, the team says it has stored 420mAhr/g, with a mean voltage of 2.5V. This is said to translate to a storage capacity of 4600Whr/litre.
Instead of intercalated storage of lithium ions, the new system stores the ions at the lattice sites of a cubic close packed structure. As a result, says the team, packing densities are increased significantly.
The lithium ions are said to be highly mobile and can be incorporated into the lattice and removed again easily. Vanadium takes up two charges or releases them again, while the lattice as a whole remains stable. The structure has a high defect mobility, such that the lattice can stabilise itself.
"The high stability of the structure at a high defect mobility, associated with a volume change of only 3%, is what makes the new system unusual," said team leader Professor Maximilian Fichtner. "The storage principle appears to be transferable to other compositions. Using compounds of a similar structure, we have measured even higher energy densities than for the vanadium based system."
Author
Graham Pitcher
Source: www.newelectronics.co.uk