Influence of the Formulation on the Microstructure and Thus Performance of Li-Ion Batteries

The research in the field of Li-ion batteries is stronger than ever, with a common aim of increasing the energy and power density. More and more studies pinpoint the limitation of power-related performance for Li-ion batteries to the microstructure of each electrode. This random arrangement of the active material and carbon filling particles bound by the binder is typically characterized through physical values such as tortuosity, porosity and MacMullin Number. To investigate the relationship between microstructure and performance in details, we used a design of experiment to evaluate close to seventy different formulations with LiFePO4 and Li4Ti5O12 as the active materials (AM), carbon black (CB) and carbon nanofibers (CNF) as the conductive fillers, and two different binders: polyvinylidene fluoride and a thermoplastic elastomer. By keeping the loading and porosity constant throughout the study, all formulations are homogenously dispersed in a regular tetrahedron of coordinates (AM, CB, CNF, Binder). First, the microstructure was characterized in order to extract data related to the tortuosity, porosity and electrical conductivity. Then, galvanostatic measurements, in charge and discharge, from C/25 to 30C allowed to gather information such as the low- and high-rate capacity, energy and power and the Peukert constant for intermediate rates. The statistical analysis of all these results showed some clear correlation between the formulation and the microstructure and performance of the cells. Thus, it was made possible to draw the optimal formulation to get the best compromise depending on the application requirements (energy, rate or power). Finally, trends between the physical properties and performance were deciphered. This could be used to fine-tune the best possible electrode. Figure 1