Development of new test protocols for testing and evaluating of process parameters effects on performance of lithium ion cell and batteries

Test protocols include cycling and self discharge at room temperature and at 55 °C, power rate measurement using signature curves, internal resistance and self discharge at 55 °C. Power rate plots show the capacity change of a plastic cell as a function of various discharge rates. Determination of the power rate through measuring a signature curve is faster than performing a complete cell discharge at various currents, while the outcome is very similar. The internal cell resistance is comprised of a rate-dependent ionic portion and a rate independent electronic portion. At low power rates, the electronic portion dominates, while at higher rates the ionic pan overwhelms the electronic resistance (e.g. becomes the rate limiting process) [1]. Here we concentrate solely on electronic resistance and review ways to minimize contributions from electrode bulk, electrode surface and tab connection. The main impetus for the present work was provided by the poor reproducibility of self discharge tests at elevated temperatures, followed by the observation of the current collector grid separation in some plastic Li-ion batteries tested for extended periods of time at elevated temperatures (55 C). We believe this adhesive failure is caused by an excessive softening of the plasticized polymer matrix in the DMC-rich liquid electrolyte component, and that it results in irreproducible self-discharge and other battery performance characteristics at elevated temperature. Used two different approaches to overcome this problem:1) We embedded (buried) the current collector grids between an additional, external separator film and an electrode film, or between two thinner electrode films. 2) We identified copolymer compositions with improved thermomechanical properties in the presence of the electrolyte solutions and reduced extraetability in DMC ]2[. The use of the embedded grid design has led not only to a dramatic improvement in the reproducibility of the self-discharge results at elevated temperatures, but also to a very significant decrease in capacity loss. Cell performance measured with cycle life, power rate, pulse performance, temperature effect on cycling and rate, self-discharge at 25°C and at 55°C, gas evolution, cell performance using other active materials and optimization of cell capacity, electrode thickness and composition, grid treatment, activation time, liquid electrolyte composition and choice of electrode material effect. The power rate of the plastic Li-ion cells can be controlled by adjusting the electrode composition and the electrode thickness.