Why Charge Rate Capability Tests Fail to Characterize Structured Electrodes in Lithium-Ion Batteries and How to Modify Them
Authors: Keilhofer Josef , Oppermann Niklas , Hadawi Kiyan Vincent , Dorau Filip Adam , Daub Rüdiger 
Published: August 2024

Fast charging is one of the most essential criteria for the user acceptance of battery electric vehicles. Especially for high-loaded electrodes used in automotive applications, the rate capability is limited due to the transport limitations of lithium ions in the pores.1 Electrode structuring is a powerful tool to overcome those limitations by creating additional directed diffusion channels for lithium ions.2 Despite the frequently described reduction of the transport limitation of lithium-ion batteries,3 there are hardly any charge rate capability tests in the literature that demonstrate the improvement through structuring. Instead, most studies present discharge rate capability tests showing the desired performance increase due to structuring.4-8 This study will explain why charge rate capability tests fail to characterize electrodes in the transport-limiting C-rate range. For this purpose, graphite anodes underwent “structure calendering”, where electrodes are simultaneously mechanically structured and calendered using an embossing roller. Thus, identical electrode properties, except for the lower tortuosity, can be achieved compared to the non-structured graphite anodes that were only calendered. Despite their different tortuosities, we will show that structured and non-structured graphite anodes exhibit minimal capacity differences in the charge rate capability test. This is attributed to lithium plating, which shifts the anode potential to similar values, regardless of the transport properties of the investigated electrode. We will modify the test to characterize the charge rate capability of electrodes in the transport-limiting regime where lithium plating occurs. For this purpose, in the charge rate capability test, the current is superimposed with a sinusoidal signal and the voltage response is measured. This allows the continuous estimation of the cell impedance during charging. Previous studies have shown that this method allows to track the onset of the lithium plating reaction.9-10 Applying the modified charge rate capability test will demonstrate that this detrimental side reaction occurs significantly later in structured electrodes.

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