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Increasing energy storage capacity of Li-ion batteries

Posted: 19 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Karlsruhe Institute of Technology? lithium ion battery? energy storage? interstitial?

Karlsruhe Institute of Technology (KIT) and KIT-founded Helmholtz Institute Ulm (HIU) researchers have created a cathode material based on an innovative storage principle that boosts energy storage densities. The team has reported details of the material in the journal Advanced Energy Materials.

The lithium ion battery as of the present is the most widespread battery technology. Existing research activities are aimed at reaching higher lithium storage densities in order to increase the amount of energy stored in a battery. Lithium storage should be quick for energy supply of devices with high power requirements. This requires the detailed understanding of the electrochemical processes and new development of battery components.

The materials used so far are based on intercalation storage of lithium in small cavities (so-called interstitials), in a host structure that usually consists of metal oxides. This method works well, but the storage densities reached 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, as the structure then is no longer stable and collapses.

A team led by Maximilian Fichtner and Ruiyong Chen of KIT has presented a storage principle and a material that allows for the reversible storage of 1.8L per formula unit. With a material of the composition Li2VO2F, storage capacities of up to 420mAh/g were measured at a mean voltage of 2.5V. As a result of the comparably high density of the material, a storage capacity of up to 4600Wh/L relative to the active material is obtained.

Storage material for Li ion batteries

Novel storage material with (left) and without lithium (right). (Photo: HIU)

Contrary to the materials used so far, the system no longer stores lithium at the interstitials, but directly at the lattice sites of a cubic close packed structure. As a result, packing densities are increased.

The lithium ions are highly mobile in this structure 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, a novelty in such storage materials. The structure has a high defect mobility, such that the lattice can stabilize itself.

"The high stability of the structure at a high defect mobility, associated with a very small volume change of three per cent only, this is what makes the novel system unusual. The storage principle appears to be transferable to other compositions. Using other compounds of similar structure, we presently measure even higher energy densities than for the vanadium-based system," explained Fichtner.

- Paul Buckley
??EE Times Europe

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