BESSY II: How pulsed charging enhances the service time of batteries

An improved charging protocol might help lithium-ion batteries to last much longer. Charging with a high-frequency pulsed current reduces ageing effects, an international team demonstrated. The study was led by Philipp Adelhelm (HZB and Humboldt University) in collaboration with teams from the Technical University of Berlin and Aalborg University in Denmark. Experiments at the X-ray source BESSY II were particularly revealing.

Lithium-ion batteries are powerful, and they are used everywhere, from electric vehicles to electronic devices. However, their capacity gradually decreases over the course of hundreds of charging cycles. The best commercial lithium-ion batteries with electrodes made of so-called NMC532 (molecular formula: LiNi_0.5Mn_0.3Co_0.2O₂) and graphite have a service life of up to eight years. Batteries are usually charged with a constant current flow. But is this really the most favourable method? A new study by Prof Philipp Adelhelm's group at HZB and Humboldt-University Berlin answers this question clearly with no. The study in the journal Advanced Energy Materials analyses the effect of the charging protocol on the service time of the battery.

Ageing effects analysed

Part of the battery tests were carried out at Aalborg University. The batteries were either charged conventionally with constant current (CC) or with a new charging protocol with pulsed current (PC). Post-mortem analyses revealed clear differences after several charging cycles: In the CC samples, the solid electrolyte interface (SEI) at the anode was significantly thicker, which impaired the capacity. The team also found more cracks in the structure of the NMC532 and graphite electrodes, which also contributed to the loss of capacity. In contrast, PC-charging led to a thinner SEI interface and fewer structural changes in the electrode materials.

Synchrotron experiments at BESSY II and PETRA III

HZB researcher Dr Yaolin Xu then led the investigation into the lithium-ion cells at Humboldt University and BESSY II with operando Raman spectroscopy and dilatometry as well as X-ray absorption spectroscopy to analyse what happens during charging with different protocols. Supplementary experiments were carried out at the PETRA III synchrotron. "The pulsed current charging promotes the homogeneous distribution of the lithium ions in the graphite and thus reduces the mechanical stress and cracking of the graphite particles. This improves the structural stability of the graphite anode," he concludes. The pulsed charging also suppresses the structural changes of NMC532 cathode materials with less Ni-O bond length variation.

The pulse current frequency is crucial

However, it depends on the frequency of the pulsed current: the series of measurements with a high-frequency pulsed current extended the service life of the commercial lithium-ion battery analysed the most, up to doubling the cycle life (with 80% capacity retention). Co-author Prof. Dr Julia Kowal, an expert in electrical energy storage technology at TU Berlin, emphasises: "A good understanding of the influence of pulsed charging at different frequencies on the SEI layer will be very helpful for the development of more gentle charging processes."