Burst noise in rechargeable LiFePO4 batteries and its changes after long-term calendar aging

Noise analysis can provide insight into both internal electrochemical processes and the health of batteries. Here we show noise measurements taken in 2017, during discharging, both in the frequency and in the time domains for lithium iron phosphate (LiFePO4) cells manufactured by Hailei. The low freuency noise was substantially higher when the cell voltage and the discharge current exceeded certain thresholds. Moreover, the mean square voltage spectral density was remarkably constant until the cell voltage decreased below the noise threshold voltage and these results were almost unaffected by up to 500 charge-discharge cycles. The same cells, characterized again in 2023, exhibited increased noise, lower noise threshold voltages and much more variable mean square voltage spectral densities. For comparison, measurements on different LiFePO4 cells (Enix) and lithiumions batteries (RS PRO, Samsung and Ansmann) confirmed that, unlike for other lithium-ion batteries, for new LiFePO4 cells the mean square voltage spectral density was almost constant until the cell voltage decreased below the noise threshold voltage. This is the first report on the peculiar characteristics of the noise of LiFePO4 cells and its dramatic changes after long-term calendar aging, thus paving the way to the age estimation of LiFePO4 cells by noise analysis.