Understanding the Impact of Composition and Processing on Tortuosity and Electrochemical Performance for Li-Ion Battery Electrodes

Abstract

Li-ion batteries (LIBs) are considered as the suitable technology for electric vehicles, as they offer a combination of power and energy density. Research principally aims to improve the energy density, e.g. by increasing the specific energy density of the active material. Another approach to enhance the cells energy density is to increase the ratio of active material to inactive parts, for example by increasing the electrodes coating thickness. However, by enhancing the thickness the diffusion length of Li-ions extends which impairs the cells rate capability significantly. To tackle this problem the electrode design has to be optimized The diffusion path in electrodes is described by the tortuosity parameter, which is defined by the deviation from the shortest diffusion path. By knowing the electrodes tortuosity, the rate capability of thicker electrodes can be optimized within the production process. In the past the tortuosity was only accessible by tomographic measurements, which are time consuming and have a boundary in resolution or sample size. Newly developed electrochemical methods have been described in the literature, that allow a simple measurement of tortuosity, enabling comprehensive analysis of influences on tortuosity We present a systematic study of the influence of composition and processing on the tortuosity. For the composition different active materials and conductive additives are evaluated. Furthermore, the impact of different processing of materials is evaluated, for example a change from tape casting with low viscosity slurries to an extrusion based coating process with high viscosity pastes. Main aim is to improve the rate capability of thick electrodes. This is done by comparing the acquired tortuosity with the measured electrochemical test. Results show small changes in composition can lead to better rate capability with minimal sacrifice of capacity. Lastly, potential impact on the cycle life will be considered, to avoid a sacrifice in cycle life time.