Continuous Lithium exchange causes battery fatigue

Scientists at the Research Neutron Source Heinz Maier-Leibnitz (FRM II) of Technische Universität München in collaboration with the Institute of Applied Materials – Energy Storage Systems (IAM-ESS) at Karlsruhe Institute of Technology (KIT) and the Materials Science Department at Darmstadt University of Technology used neutron diffraction to observe an important process that contributes to fatigue in current Li-ion batteries. By means of neutron scattering it was found that during a continuous use some lithium-ions are irreversibly intercalated into the graphite structure of the anode and are therefore lost for subsequent cycles within a constant voltage window. This causes a decrease of the cell capacity.

The researchers compared commercially available batteries in fresh state and after up to 1,000 cycles of charge and discharge. A typical lithium-ion battery has a carbon anode (negative electrode), whereas the cathode (positive electrode) usually consists of lithium cobalt oxide (LiCoO₂), lithium manganese oxide (LiMn₂O₄) or lithium iron phosphate (LiFePO₄). The cathode material is known to be of primary importance for the battery performance. “By using neutrons we now could prove, that there are processes at the anode, too, which affect the performance of the storage system”, said physicist Anatoliy Senyshyn, who conducted the experiments together with his colleagues at the instrument SPODI at the Research Neutron Source Heinz Maier-Leibnitz (FRM II).

As the researchers observed, this process is temperature-dependent: Electrochemical analysis showed that with an increasing number of charge/discharge cycles the battery capacity decreases so that after 1,000 cycles only about 80 percent of the original capacity is available. At 25 degrees Celsius, the effect was much stronger than at 50 degrees Celsius. Batteries, that had been operated at the higher temperature only showed about two third of the capacity loss observed at room temperature after 1,000 cycles. Apparently, the Li-ions show a higher mobility at elevated temperatures and can be released more easily from the anode material even if barriers for their transport have built up due to fatigue.

Today rechargeable batteries like the ones investigated by the scientists at the Research Neutron Source Heinz Maier-Leibnitz (FRM II) are used in multiple portable electronic devices like cell phones, digital cameras and notebooks, whenever limited weight and long operation time are demanded. Currently, there is no other modern battery system offering comparable energy and power density. The use of improved Li-ion batteries in electric and hybrid cars is also intended to make them more competitive. But in order to do so there has to be a further increase of their lifetime and energy density. Therefore science and industry are currently making great efforts to optimize Li-ion battery systems. A detailed knowledge of fatigue mechanisms is essential for this purpose.