Franck Ferreira Gomes1 2 Sylvain Franger2 Delphine Guy-Bouyssou1

1, STMicroelectronics, Orsay, , France
2, Institut de Chimie Moléculaire et des Matériaux d’Orsay, Orsay, , France

For several decades, the miniaturization of nomadic electric systems has made the world of energy storage evolve. Indeed, the designs of these devices are getting progressively smaller and smaller, yet more powerful. This of course, makes it more difficult integrating these conventional batteries into these modern devices including; integration incompatibilities due to their sizing, life-time and limited cyclability, risks of inflammation due to metallic lithium, risk of solvents leaking into liquid electrolytes. All of the above factors limit the development of these new technologies. In response to these difficulties, the all-solid-state thin film microbatteries "lithium-free" have provided answers to the needs of the industrialists. They can reach a potential of 0V [1], far below the capability of lithium and lithium-ion batteries. Furthermore, these micro batteries are thinner, more flexible, safer due to the absence of liquid electrolyte and metallic lithium electrode. These micro-devices can be integrated into diverse applications as connected watches, SmartCards, RFID Tags, or even be integrated into state of the art contact lenses to order to power the autofocus system for medical applications.

Li-Free microbatteries are composed of a platinum current collector, LiCoO2 as positive electrode, LiPON glass as solid electrolyte and a current collector of copper. The lithium contained in insertion material of cathode is electroplated on the current collector during the initial charge, to create an anode of metallic lithium and make the battery operational. However, this technology offering a lot of advantages still remains misunderstood and consequently difficult to control.

The thin-film design makes individual separation of elements difficult after cycling. Therefore, it is complicated to characterize every layer individually. Analysis of the surface with X-ray Photoelectron Spectrometry (XPS) and Auger Electron Spectroscopy (AES) determines the chemical composition of the surface of Li-Free microbatteries after cycling. Characterizing techniques by the use of Electrochemical Impedance Spectroscopy (EIS), can provide answers to the questions on Li-Free, and finally, can control the manufacturing process and understand the functioning and ageing of these technologies. This study is focused on the non-destructive characterization of lithium-free microbatteries by means of the Electrochemical Impedance Spectroscopy coupled with X-ray Photoelectron Spectrometry and Auger Electron Spectroscopy, and shows mechanisms necessary during the first charges and discharges of the battery for good performances in cycling. With this adapted protocol to start the Li-Free [2], this battery can now exhibit a life duration and cyclability similar to lithium metallic and lithium-ion microbatteries.

References :
[1] Neudecker and al., J. Electrochem. Soc., 147 (2) 517-523 (2000)
[2] Larfaillou and Guy-Bouyssou. U.S. Patent 2015325878 (2015)