Inductive drying of lithium-ion graphite anodes

Summary:

Aiming an efficient lithium-ion battery production all process steps have to be enhanced in terms of quality and performance. In this regard, the drying process of electrodes holds great potential for improvement. The main disadvantage of state-of-the-art convective drying methods is their negative impact electrode properties on the cyclic stability due to binder and carbon black segregation when higher drying rates are applied. Therefore, conventional drying processes are limited to a certain drying rate, which limits the overall battery production speed.

This study focuses on the development and prototypical implementation of a new method for drying electrodes through inductive heating. By means of induction, the conductive material of the coated current collector can be heated specifically using due to resistance heating based on eddy currents that are induced by an alternating magnetic field. This way, the coating is dried from the bottom up. Furthermore, induction allows the generation of complex heating zones with individual heat distributions, for example to prevent the formation of tensions inside the active material. Therefore, this method enables the effective drying of the coating through direct and specific energy input into the metallic substrate. This in turn offers the opportunity to significantly reduce the heating time and accordingly allows a significant energy saving compared to conventional convective methods respectively.

Within this study, inductive drying experiments were carried out on coated SMG-A5 anodes. The recently coated anodes were placed above a pair of inductors with a fixed distance of 3 mm. Afterwards the anodes were dried at a constant frequency of 30 kHz as well as varied pulse width modulations (PWM). The temperature distribution of the generated heating zone was observed using an infrared imaging camera. Initially, the drying time was held at a constant 200 s and was adjusted accordingly based on the temperature measurements. To examine the influence of the inductive drying process on the electrical and mechanical properties of the electrodes, the electrical conductivity as well as the adhesion strength of the coatings were measured.

The temperature measurements show that the inductively generated heating zone leads to a homogeneous temperature distribution along the coating width during the drying process, although slight temperature maxima can be observed at higher PWM at the center of the inductors. Furthermore, higher steady-state temperatures can be observed compared to the convective drying process suggesting shorter drying times can be achieved. The adhesion strength of all inductively dried electrodes is within a satisfactory range. However, a decrease in adhesive strength was observed compared to conventionally dried electrodes. At a constant drying time of 200 s the adhesion strength slightly decreases with increasing drying intensity which is probably due to binder degradation as a result of temperatures above 130 °C being reached. This effect is not observed when the drying time is reduced indicating that binder degradation did not take place. The electrical conductivity of all inductively dried anodes is also acceptable showing no negative impact of the drying process on the electrical properties of the electrodes. Overall the results suggest that the drying process can be accelerated while mechanical and electrical electrode properties remain comparable to conventional dried electrodes. Further drying experiments on cathodes as well as investigations regarding the influence of the drying process on the electrode structure and component distribution inside the coating have to be conducted. Additionally, the influence of inductively dried electrodes on the performance of battery cells has to be investigated to exclude any negative impact regarding the cyclic stability or capacity.