Effects of alternating current on Li-ion battery performance: Monitoring degradative processes with in-situ characterization techniques

With the rapidly growing markets for electric vehicles and renewable energy systems, the complex duty cycles imposed by electric machines and power electronics components are now a common feature of battery service. As a result, lithium-ion, increasingly the battery of choice, must cope with superimposed alternating current (AC) across a broad range of frequencies. To advance understanding of how AC may influence the ageing of Li-ion batteries (LIBs), this work uses electrochemical impedance spectroscopy to investigate the interaction of AC with key aspects of LIB operation (charge-transfer, properties of the solid electrolyte interphase, and lithium-ion diffusion), along with analysis of capacity performance to quantify the main modes of degradation. For a set of fresh LiFePO4 cells, subjected to nearly 200 days of service, comparison of DC profile with coupled DC + AC duty reveals that high-frequency superimposed AC does not contribute to ageing, with some evidence that it can actually enhance the battery life. In contrast, low-frequency superimposed AC accelerates both loss of lithium inventory and loss of active material (as the most pertinent degradation modes of LIBs) and consequently induces greater capacity loss. This important correlation between AC frequency and extent of degradation was confirmed in AC-only studies, which showed that the voltage polarization induced by AC perturbation is the key indicator of any likely effect. These findings can serve as a guide for deciding whether a given combination of frequencies and amplitudes is likely to have an adverse impact.