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Identification of Degradation Mechanisms for Lithium Titanate Oxide based Cells

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The cyclic and calendar aging of in total 43 same-typed prismatic LTO cells are investigated. Therefore, measurements over a period of more than 600 days were carried out. Regarding the calendar aging, 19 cells were stored at 8 different operating conditions, under variation of state of charge and temperature. Considering the cyclic aging, 24 cells were operated at 8 different operating conditions under variation of cycle range, depth of discharge and current rate. The results of these tests will be presented and examined with regards to the dependancy of capacity fade on the investigated parameters and operating ranges. Afterwards, the relative shift in the incremental capacity (IC) curve over the course of the measurements is compared and analysed in order to detect aging mechanisms and attribute them to their origin source. Our results show, that the cells exhibit a two stage aging mechanism with stagewise increasing degradation gradient. In the first ageing stage the LTO anode is limiting the amount of extractable capacity while the capacity fade mainly results from cathode degradation. After a certain level of degradation is reached the cathode starts limiting the amount of extractable capacity, initiating the second ageing stage with stronger occurring capacity fading gradient. However, loss of lithium inventory occurs only to an insignificant extend, which can be attributed to the high anode potential preventing the formation of a lithium consuming passivation layer.

Additionally, a capacity gain during the calendar aging measurements of up to 2.42 % compared to the initial capacity becomes visible for cells operated and stored in a range below 50 % SOC. For these cells, an extended three stage aging mechanism is more applicable. Both, the three stage aging mechanism and the capacity gain, as well as their correlation can be associated to characteristic shifts within the incremental capacity curve, which will be identified and characterized.

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