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In electric vehicles (EVs) and other applications, lithium-ion batteries experience variable load profiles with frequencies up to several kilohertz, as caused by power electronics. It is crucial to know if certain frequencies accelerate battery degradation and should be avoided. This applies in particular for EV batteries with an expected lifetime of more than ten years. This study investigates the influence of alternating current (ac) profiles on the lifetime of lithium-ion batteries. High-energy battery cells were tested for more than 1500 equivalent full cycles to practically check the influence of current ripples. The applied load profiles consisted of a constant current with superimposed ac frequencies. The frequencies were chosen with regards to the impedance spectra and ranged from 30 kHz to 1 Hz. The measurement results are compared to published results and to a second series of experiments conducted with high-power battery cells. The results of the experiments indicate that lithium-ion battery cells cycled with low frequencies experience a 1% to 2% higher impedance increase and capacity fade than battery cells cycled with high ac frequencies. These differences in aging characteristics established within the first 50 cycles of the brand-new battery cells and remained almost constant thereafter. For these experiments, the antithesis is disproved since no accelerated degradation occurred at higher frequencies. Therefore, with regards to battery lifetime, high frequencies can be tolerated as long as temperatures are considered as well. This new finding may help us to reduce the costs of products with complex battery systems, such as EVs. [ABSTRACT FROM AUTHOR]

Electric vehicles based on high-energy lithium-ion batteries often exhibit a substantial loss in performance at subzero temperatures: Due to slower electrochemical kinetics, the internal resistances of the batteries rise and diminish available power and capacity. Hybrid energy storage systems (HESSs) can be used to overcome these weaknesses. In this paper, the performance of two HESSs, combining a high-energy lithium-ion battery with either a high-power lithium-ion battery or a lithium-ion capacitor, has been investigated experimentally for a driving scenario at various temperatures. Both configurations enable driving at -20 °C, which was not possible without hybridization. The HESS using the high-power lithium-ion battery provides a substantially higher driving range due to its higher energy density. An analysis of different operating strategies has helped to maximize the driving range: Discharging the high-energy battery with a constant current and keeping the high-power cell at a higher state of charge (SoC) extend the driving duration, as the requested driving power can still be provided at a lower SoC of the high-energy battery. In addition to the HESSs, two energy storage systems without hybridization, consisting of different generations of high-energy lithium-ion cells, have been examined to disclose improvements in battery technology. These improvements narrow the benefits of HESSs, as the high-energy batteries have become less reliant on the support of an additional high-power device. Although HESSs lose importance for current lithium-ion battery systems, they can be a valuable option for next-generation lithium batteries, which are expected to provide higher energy densities but exhibit reduced rate capability. [ABSTRACT FROM AUTHOR]