Abstract
Lithium-ion battery systems have dynamic diffusivity properties that continuously change as a function of state of charge due to lithium concentration gradients and material phase changes in both electrodes. Additionally, external factors such as pressure and temperature can further influence the ion-diffusion-rate profile with respect to the open-circuit potential resulting in different performance characteristics in different environments. Therefore it is critical to understand the impact of the external enviornment on diffusivity in order to optimize battery performance during usage in extreme conditions, particularly in the case of thin film batteries which are especially susceptible to changes in external pressure. This work investigates the combined effect of low to ambient external pressures and open-circuit potential on electrode diffusion-rate properties. Commercial thin film batteries were subjected to tests in a low-pressure chamber and in a dynamic materials analyzer simulating hydrostatic pressures between 0 and 115 KPa. Under each hydrostatic pressure condition, galvanostatic intermittent titration technique (GITT) was performed to measure the lithium diffusion rate at various states of charge and a method to extract the pressure-dependent component of the diffusivity measurements was developed and evaluated for statistical significance. The results show that the relationship between pressure and the aggregate cell diffusivity does not remain constant throughout the battery’s operating voltage limits. Three regions were found in the open ciricut voltage range in which diffusion displays a positive relation to pressure, no relation to pressure, and an inverse relation to pressure. It’s theorized that the pressure/diffusivity relationship is governed by individual electrode diffusivity and that depending on the state of charge, the effects of either the anode or cathode become more pronouced in the over-all aggregate diffusion rate measurement.
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