Investigation of Li-Ion Diffusion Rates Using Gitt in Cells with 3D Structured Intercalation Cathode Materials

The public demand for high power energy storage systems is on a continuous ascending path. Efforts are directed towards the development of advanced lithium-ion electrodes, e.g. modified surface architectures. Direct laser patterning of thin and thick film electrode materials is a fairly new technical approach which enables an increase of capacity retention especially for high charging and discharging rates (Figure 1). It is assumed that an appropriate three dimensional surface topography influences the diffusion kinetics of Li-ions in the electrode materials through the electrolyte, whereas critical mechanical tensions during charging/discharging can be avoided and ohmic losses are significantly reduced inside the electrochemical cell.

The goal of this work was to enable quantitative studies of lithiation/delithiation rates which contribute to a better understanding of electrochemical intercalation/deintercalation processes in laser modified electrodes. The simplest quantitative approach to determine the rate of Li-ion insertion in the active material and the rate of Li-ion transport in the electrolyte is expressed by diffusion coefficient values. For this purpose, one of the most common coulometric titration technique, the galvanostatic intermittent titration technique (GITT) has been involved.

Electrochemical measurements were performed using the Swagelok® cell design for both, full and half-cell types, using graphite and metallic lithium as counter electrodes, respectively. The results of Li-ions diffusion rates are presented for laser structured and unstructured lithium -metal oxide cathode materials. The focus is set on composite thick film electrode materials containing binder, conductive carbon and layered intercalation lithium cobalt oxide (LiCoO₂), which is the main cathode material used nowadays in many commercial lithium-ion cells. A main challenge for GITT was to evaluate suitable measurement parameters such as current pulse length, charge/discharge rate and relaxation time for each specific cell system. The results obtained were evaluated and compared to literature data.