Riding the Waves: The Effect of Current Ripple on Lithium-Ion Cells Characterised via Operando Synchrotron Techniques

Abstract

This thesis covers the work undertaken to investigate the effect that current ripple has on lithiumion cells, specifically with regards to automotive applications. Testing of lithium-ion cells is typically performed using highly filtered signals that are not representative of the noisy signals that exist in
real world situations due to the high frequency switching of power electronics.

Attempts to modify a standard 2032 coin cell for use in operando conditions are detailed, and the lessons learnt in the process illuminate the criticality of stack pressure. A nine-point scan was taken for three separate coin cells while cycling, showing the variation in the movement of the lithiumions to be extremely significant. A lateral move of just 0.75 mm significantly changed the ability of the graphite electrode to successfully pass through the relevant graphite stages. The only cause for this difference was the additional deflection of a 0.5 mm thick polypropylene window over this 0.75 mm distance. This finding has large implications for the manufacture of lithium-ion cells, which are currently plagued by manufacturing reliability during the scale-up of giga factories.

A novel method for identifying major and minor problems through the incremental capacity analysis of cells during formation was shown. This will aid in speeding up the feedback loop when manufacturing cells by exposing the failing cells sooner, helping reduce the time to profitability for the afore mentioned giga factory scale up procedures.

Furthermore, the effect of current ripple on cells during normal operation was shown to be independent of frequency when the magnitude is 25 % of the DC value. The results also show that when that magnitude is increased to 100 % at the tested frequency of 10 kHz, the variation in the state of lithiation across the electrode was reduced, but the variation through the thickness of the electrode was increased. This is proposed as a likely reason for the current disagreement in literature on the effect of current ripple, as it has both positive and negative effects that have a finely balanced relationship.

Finally, the benefits of deliberately applying a current ripple during formation were shown, where success rates and cycle life were increased significantly. Specifically for the 10 kHz cases, the success rate went up from 50 % for DC to 67 % and 84 % for waveforms with 50 % and 100 % magnitude respectively. Concurrently cycle life was shown to increase from circa 110 cycles for the best performing DC formed cells to almost 300 for the best cells formed under a 10 kHz ripple.

Date of Award	25 Jun 2025
Original language	English
Awarding Institution	University of Bath
Sponsors	AVL List GmbH & Engineering and Physical Sciences Research Council
Supervisor	Christopher Vagg (Supervisor), Alexander Lunt (Supervisor) & Frank Marken (Supervisor)