For several decades, the miniaturization of nomadic electric systems has made the world of energy storage evolve. Indeed, these devices are getting progressively smaller and smaller, and more powerful. This makes more difficult the integration of conventional batteries into these modern devices including; incompatibilities due to their sizing, life-time and limited cyclability, risks of inflammation, risk of liquid electrolyte leakage. In response to these difficulties, the "lithium-free" all-solid-state thin film microbatteries provide answers to the manufacturers. 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. These micro-devices can be integrated into diverse applications as connected watches, SmartCards, RFID Tags, or even be integrated into contact lenses to power the autofocus system.

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 initially contained in the cathodic insertion material is electroplated on the copper current collector during the initial charge, to create a metallic lithium anode and make the battery operational. However, this technology, offering a lot of advantages, still remains not fully understood and consequently difficult to control.

The thin-film design makes individual characterization difficult after cycling. Analysis of the surface with X-ray Photoelectron Spectrometry (XPS) and Auger Electron Spectroscopy (AES) enable determining the chemical composition of the Li-Free microbatteries after cycling. The use of Electrochemical Impedance Spectroscopy (EIS), can also provide information about the operating behaviour and ageing of these technologies and finally help in better manufacturing. 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 necessary mechanisms during the first charges and discharges of the battery for ulterior good performances upon cycling. With this adapted protocol [2], the battery can now exhibit a lifetime and cyclability similar to conventional 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)