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• Studying the effects of AC perturbation on degradation mechanisms of lithium-ion batteries. • High-frequency AC has negligible ageing effects; slightly improved cell lifetime. • Low-frequency AC of sufficient amplitude accelerates degradation rate. • Voltage polarization induced by AC is the key indicator of any likely ageing effect. 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 LiFePO 4 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. [ABSTRACT FROM AUTHOR]

• Studying the ageing effects of AC preheating methods on LiFePO 4 (LFP) cells. • The ageing impact of shallow charge-discharge cycles, microcycles, on LFP cells. • High-frequency microcycling has a minor degradation impact on LFP cells. • Low-frequency microcycling causes accelerated degradation. • Voltage polarization induced by AC is the key indicator of any likely ageing effect. Superimposed Alternating Current (AC) imposed by electric machines and power electronics components in renewable energy systems and electric vehicles (EVs) exposes Li-ion batteries (LIBs) to high-amplitude AC harmonics (i.e., ripple currents). These AC perturbations make LIBs experience shallow charge-discharge cycles or microcycles, resulting in significantly increased battery charge throughput. Recently high amplitude AC-only waveforms have also attracted significant attention as they can be used to warm up/preheat EV batteries in low-temperature environments. To understand what happens when the amplitude of AC-bearing duty cycles is pushed to higher levels, beyond the recommended constant current value set by the manufacturer, a set of fresh LiFePO 4 (LFP) cells were subjected to high-amplitude AC-only profiles at low and high frequencies for extended periods. Subsequently, LFP cells were cycled on dynamic current profiles (superimposed AC profiles) for ∼ 200 days, representing real situations encountered by LIBs in renewable energy and electromobility systems. Long-term aging results revealed that as long as the AC current induces a voltage oscillation that remains below the calculated threshold polarization overpotential, the cycle-life of LFP cells is not significantly affected for AC duties as high as the recommended constant current values. Furthermore, for high-frequency AC, a significant amount of microcycling is effectively filtered out by the battery's double-layer capacitance. However, for low-frequency superimposed AC, for which most of the AC current flows through the charge-transfer resistance, higher charge throughput induced by microcycles causes accelerated degradation. [ABSTRACT FROM AUTHOR]