Electrode deterioration processes in lithium ion capacitors monitored by in situ X-ray radiography on micrometre scale

Gas bubble formation and the related mechanical and chemical deterioration of electrodes are processes limiting performance and lifetime of lithium ion capacitors and batteries. Moreover, the increase of internal pressure caused by gas formation constitutes a severe safety problem. The authors were able to monitor the degradation process of capacitor cells for the first time by in situ synchrotron and laboratory X-ray radiography in through-plane and in-plane geometry with high optical resolution of 1-5 mu m. The decomposition is found to be dependent only on the electrolyte system but not on the applied voltage, current or the state of charge. The standard electrolyte system for lithium ion batteries is a solution of LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate (EC/DMC) and leads to bubble formation because of decarboxylation, increase of internal pressure, material ablation and particle migration on the cathode layer, as could be shown by radiography studies on working capacitor cells. As an alternative, the scarcely studied electrolyte system LiBF4 in N, N-dimethylformamide (DMF) shows a good performance and high chemical stability even at high charge voltages.