Thermal Transients to Accelerate Cyclic Aging of Lithium‐Ion Batteries

Cyclic aging tests of lithium-ion batteries are very time-consuming. Therefore, it is necessary to reduce the testing time by tightening the testing conditions. However, the acceleration with this approach is limited without altering the aging mechanisms. In this paper, we investigate whether and how thermal transients accelerate the aging. The tests are performed on NMC/graphite pouch cells by applying temperatures in a range of 5 °C to 45 °C to the cell surface. The results show, that an accelerated capacity loss can be achieved in comparison to the reference cell at a steady-state temperature of 25 °C. However, capacity difference analysis (CDA) prognoses a covering layer for the transient cells, which is confirmed upon post-mortem analysis. We suspect the origin to lie in the dynamics of temperature fields and current distribution during temperature changes when charging. More specifically, areas of higher temperature in the cell lead to high local current densities and plating. Subsequently, high temperatures promote the reaction of the plated lithium with electrolyte. The results show that thermal transients are a critical condition for lifetime and safety and should be treated with caution as they can occur during real life operation

Non-invasive identification of calendar and cyclic ageing mechanisms for lithium-titanate-oxide batteries

Lithium-titanate-oxide (LTO) batteries are one of the most promising technologies for various types of future applications in electric mobility, stationary storage systems and hybrid applications with high-power demands due to their long cyclic stability and superior safety. This paper investigates the cyclic and calendar ageing of 43 same-typed LTO cells considering 16 different operation conditions under variation of state of charge (SOC), temperature, depth of discharge, cycle SOC range and current rate. The ageing results are presented and the relative shift in incremental capacity is analysed in order to detect degradation mechanisms, separate the influence of degradation enhancing parameters and attribute them to their origin source. Our results show that the cells exhibit a two-stage ageing mechanism with stagewise increasing degradation gradient. In the first ageing stage the 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. A capacity gain of up to 2.42% becomes visible for cells operated and stored in a range below 50% SOC. For these cells an extended three-stage ageing mechanism is shown to be more applicable. The degradation behaviour is then estimated using a machine learning approach based on a recurrent neural network with long short-term memory, for which the presented incremental capacity data is used as training input

A Comprehensive Electric Vehicle Model for Vehicle-to-Grid Strategy Development

A comprehensive electric vehicle model is developed to characterize the behavior of the Smart e.d. (2013) while driving, charging and providing vehicle-to-grid services. To facilitate vehicle-to-grid strategy development, the EV model is completed with the measurement of the on-board charger efficiency and the charging control behavior upon external set-point request via IEC 61851-1. The battery model is an electro-thermal model with a dual polarization equivalent circuit electrical model coupled with a lumped thermal model with active liquid cooling. The aging trend of the EV’s 50 Ah large format pouch cell with NMC chemistry is evaluated via accelerated aging tests in the laboratory. Performance of the model is validated using laboratory pack tests, charging and driving field data. The RMSE of the cell voltage was between 18.49 mV and 67.17 mV per cell for the validation profiles. Cells stored at 100% SOC and 40 °C reached end-of-life (80% of initial capacity) after 431–589 days. The end-of-life for a cell cycled with 80% DOD around an SOC of 50% is reached after 3634 equivalent full cycles which equates to a driving distance of over 420,000 km. The full parameter set of the model is provided to serve as a resource for vehicle-to-grid strategy development