Optimized storage principle and new material increase lithium storage density in cathode material
16 March 2015
|
|
| A new intercalation compound Li2VO2F with disordered rock-salt structure enables up to ≈1.8 Li+ storage (420 mAh g−1) at ≈2.5 V with a lattice volume change of only ≈3%. A high capacity of 300 mAh g−1 at 1C rate is observed. Chen et al. Click to enlarge. |
An interdisciplinary team of researchers of Karlsruhe Institute of Technology (KIT) and KIT-founded Helmholtz Institute Ulm (HIU) has developed a new Li-ion cathode material based on a new storage principle, resulting in increased energy storage density.
The new material, presented in a paper in the journal Advanced Energy Materials, allows for the reversible storage of 1.8 Li+ per formula unit. With a material of the composition Li2VO2F, storage capacities of up to 420 mAh/g were measured at a mean voltage of 2.5 V. As a result of the comparably high density of the material, a storage capacity of up to 4600 Wh/L relative to the active material was obtained.
Mainstream electrode materials used so far in Li-ion batteries are based on intercalation storage of lithium in small cavities (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 problematic, as the structure then is no longer stable and collapses. It is therefore be desirable to increase the packing density of lithium in the stable structure and to exceed the upper limits reached so far.
A team led by Professor Maximilian Fichtner and Dr. Ruiyong Chen of KIT has now presented a new storage principle and a material on this basis. Contrary to the materials used so far, the new 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 significantly.
|
|
| New storage material with (left) and without lithium (right). (Photo: HIU) Click to enlarge. |
Surprisingly, 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 3 % only—this is what makes the new 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.
—Professor Fichtner
Resources
R. Chen, S. Ren, M. Knapp, D. Wang, R. Witter, M. Fichtner, and H. Hahn (2015) “Disordered Lithium-Rich Oxyfluoride as a Stable Host for Enhanced Li+ Intercalation Storage” Advanced Energy Materials doi: 10.1002/aenm.201401814
What would be the end results, at the cell level?
Posted by: HarveyD | 19 March 2015 at 08:48 AM
Buena noticia pero a día de hoy ¿Donde están las celdas con una energía especifica de 300-400wh/kg?.
Posted by: Centurion | 19 March 2015 at 11:27 PM