The effects of high-rate cycling on battery cells
Researchers from WMG, University of Warwick have investigated the impacts on battery cell ageing from high current operation using commercial cells, in an effort to find the most suitable batteries for use in electric vehicles.
The researchers used two tests to establish the maximum current limits before cell failure, with the results published in the journal Batteries. Testing was performed to determine how far cycling parameters could progress beyond the manufacturer’s recommendations.
During testing, current fluxes were increased up to 100 C cycling conditions. Charge and discharge current capabilities were possible at magnitudes of, respectively, 1.38 and 4.4 times more than that specified by the manufacturers’ claims. This increased current was applied for 500 charge/discharge cycles.
The application of these currents did, however, result in a rapid decrease in capacity in the first 60 cycles as well as an increase in resistance. Furthermore, the application of such currents resulted in an increase of cell temperature during both charge and discharge, with natural convection during the rest step cooling the cell. Batteries operate in an optimum temperature range, and any deviations outside this can cause components and chemicals to start decomposing inside them.
The researchers also identified deformation of the ‘jelly roll’ (coiled electrodes and separator) with formation of lithium plating from testing and ageing. These deformations emanate from the centre of the cell in an axial direction towards the outside of the cell, suggesting the core of the cell was the hottest.
“The testing showed there is a window for operating batteries above manufacturer-stated current limits … whilst maintaining manufacturer-stated voltage limits,” said lead engineer Justin Holloway. “We need to ensure that batteries operate in the safest manner possible, and for an appropriate practical lifetime, which is why the manufacturers have these limits.
“We also identified thermal fatigue as the driving mechanism for jelly roll deformation. With each cycle of charge and discharge, the cell experienced thermal stresses causing deformation of its components. These deformations grew progressively with cycle number, while the jelly roll was constrained mechanically by the rigid outer can and centre pin.
“If convection cooling could be applied to the centre of the cell where the cell was the hottest, these deformations could be mitigated and controlled, allowing the cell to maintain capacity and resistance criteria for longer.”
WMG’s Battery Forensic Group is keen to engage with industry and academia alike to grow advances in understanding new materials, battery performance and degradation modes.
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