A Concept for Direct Deposition of Thin Film Batteries on Flexible Polymer Substrates
Authors: Glenneberg Jens , Andre Felix , Bardenhagen Ingo , Langer Frederieke , Schwenzel Julian , Kun Robert 
Published: February 2020

Regarding the increased number of small electronic appliances, miniaturized energy storage devices currently receive much attention in different markets. One of the most important and still growing fields of application is the Internet-of-Thing (IoT), where small, reliable, self-powered systems enable new functions for private and professional users. Another exciting issue is addressed to consumer electronics. Currently, this scope of application is strongly influenced by the persisting trend for wearable electronics. Since these devices became part of the everyday life, safety and security issues are paramount. Moreover, based on the various forms electronic devices can have, flexible designs are of huge importance and interest. One critical component in the development of fully flexible devices is the battery, which currently typically needs a rigid, stiff housing to hold the functional battery layers in place and also to prevent leakage of the liquid electrolyte. A promising approach to overcome these limitations is the deployment of solid state electrolytes.
As the ionic conductivity of typical solid electrolyte materials is significantly lower than in their liquid counterparts, thin electrolyte layers (<5 µm) are needed to achieve a sufficiently low resistance. One possibility is the use of sputtered electrolyte layers in combination with PVD deposited electrodes in a thin film battery layout (TFB). A significant number of reports on TFBs have been presented in the literature and TFBs were also commercially introduced.
Most TFBs use a lithium cobalt oxide (LCO) thin film as cathode active material due to its attractively high voltage versus lithium and excellent stability. However, as-deposited layers of LCO are amorphous and suffer from a low diffusion rate. To improve the diffusion of Li+ ions through the LCO layer, the electrodes are typically annealed at 650 °C after deposition to increase the crystallinity. However, the used temperatures make the utilization of thermally stable substrates, but rigid such as silicon wafers, aluminate, or metal foils inevitable. Deposition on flexible polymer substrates is not possible with this approach. One method to enable the use of a polymer substrate is to first deposit the cathode layer on a thermally stable substrate. Afterwards the tempered and crystalline layer is transferred onto a polymer film. Nevertheless, this procedure still requires high temperatures and subsequently to the high energies heavy expenses.
We present a divergent process, where the complete battery cell is deposited directly on a flexible polymer substrate. Therefore, we exclusively use room temperature processes with no further heat treatment and consequently amorphous materials. Following, the electrochemical parameters under relaxed and mechanically stressed conditions will be discussed. In order to acquire these information concerning the electrochemical performance of the respective battery cells galvanostatic cycling and electrochemical impedance spectroscopy (EIS) is carried out. Furthermore, changes in the microstructure during cycling are visualized using scanning electron microscopy (SEM). Based on our investigations assessments to the mechanical properties and microstructure in context to the electrochemical performance are allowed.