Analysis and prediction of the state of health (SOH) is essential for the improvement of lithium-ion batteries. For this purpose, non-invasive techniques are an interesting tool. Electrochemical impedance spectroscopy (EIS) and differential voltage analysis (DVA) provide detailed information about ageing mechanism occurring in lithium-ion batteries [1, 2]. DVA indicates how lithium-ion batteries lose capacity. It is possible to differentiate between loss of active lithium and loss of active material for the cathode and the anode material . EIS is used to determine specific physical parameters, which are needed to model the battery behaviour and ageing [4, 5, 6].
In this work, we focus on the ageing mechanism of two different high power lithium-ion batteries. The compared batteries are a 22 Ah lithium iron phosphate battery (LFP/graphite) and a 27 Ah lithium nickel cobalt aluminium oxide battery (NCA/graphite) which are provided by the same cell manufactor. Both battery types are cycled with 1C at 25 °C and 45 °C. The test protocol includes a check up at 25 °C to record the DVA curves.
The results of the DVA show that the ageing phenomena of the investigated batteries differ significantly from each other. The main ageing mechanism of the LFP battery is loss of active lithium. In contrast to the LFP battery, the ageing behaviour of the NCA battery is complex because the DVA shows that the battery loses both active lithium and active material of anode and cathode. These results show that the degradation of the cathode plays a major role in the ageing behaviour of the investigated batteries and it influences the ageing of the anode material. Understanding these cross talk phenomena between the electrodes can help to improve the performance and the prediction of failure mechanism of lithium ion batteries.
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