Financial support: DFG (Deutsche Forschungsgemeinschaft), SFB 595

Begin of the project: 2004

Rechargeable lithium batteries are used as power sources for nearly every mobile application. The main aspect which has to be taken into account for this field of application, is a high energy density. Typical values are 110-140 Wh/kg for commercial lithium batteries in comparison to 40-60 Wh/kg for NiCd- and 60-80 for NiMH-batteries.

In commercial batteries the most common cathode material is LiCoO₂, while different types of carbon are used as anode. During charging and discharging an intercalation process occurs. This is schematically shown in the following picture.

Operating lithium battery

While charging the battery, lithium ions are deintercalated from the cathode, move through the electrolyte (LiPF₆ in Ethylencarbonate/Dimethylcarbonate) and are intercalated in the anode. This process is reversed when the battery is discharged.

During intercalation of Li⁺ into the host material different phase transitions occur and the volume of the active materials change. Due to these - and some other - effects, lithium batteries show an irreversible capacity loss when they are cycled. Typically this leads to a remaining capacity of 80% after 500 cycles.

To study the processes leading to this degradation an in situ-cell is constructed (picture on the right side), which enables synchrotron HRXRD and EXAFS experiments in transmission geometry. It is essential to perform these experiments in situ, because if the cell is opened, even under inert atmosphere, the involved materials will change. The cell design must provide a high cycling stability to monitor the changes in the cathode material over a large number of cycles. To obtain good diffraction patterns a pellet (m=20-30 mg) is used as cathode. The collected diffraction patterns are analyzed by full-pattern Rietveld refinement. Mainly, materials like LiNi_1-xCo_xO₂ (x=0.33, 0.25, 0.2) will be used as cathodes. These materials are interesting for commercial application, because a partial substitution of Co by Ni reduces the material-costs significantly.

in situ-cell