Mechanical characterization of lithium-ion batteries with different chemistries and formats

Lithium-ion batteries undergo structural deformation during operation because of the electrochemical-induced strain caused by the insertion of lithium ions inside the active material of the electrodes. In this work, the mechanical characteristic, i.e. thickness change, of batteries with different chemistries (lithium iron phosphate and lithium cobalt oxide) and formats (prismatic and pouch) is measured in-operando and with different current rates. A dedicated test bench is built to carry out the measurements with contactless triangulation optical sensors, sensing the displacement of the battery surfaces. The results are critically discussed, justifying the trajectory of the thickness change observed experimentally with the mechanical properties of the electrode’s active materials. Furthermore, the results obtained with the batteries of different chemistries are compared, discussing the reasons underlying these differences. Statistical considerations on the results, such as repeatability errors and cell-to-cell differences, are provided as well. The mechanical characteristic appears as a three-stage linear curve, with two deflection points between the linear sections, and proportional to the state of charge of the battery. In fact, the mechanical characteristic tells the amount of lithium ions stored in each electrode, which together with the stoichiometric range defining the electrodes balancing, is closely related to the state of charge. Then, these measurements reveal an interesting tool for evaluating the state of charge. The two deflection points of the mechanical characteristic are linked to graphite stage changes. Then, these measurements carry the same information about battery degradation as the peaks in differential voltage analysis. The advantage is that the deflection points remain visible at any current rate, but the peaks in the differential voltage analysis vanish at high current rates. Then, these preliminary studies of the mechanical characteristic evidence it can become a promising alternative tool for evaluating the state of health as well.