Thin film all-solid-state secondary microbatteries are extensively studied since the 1990’s [1-3]. These systems present specific features such as reduced surface, high thermal resistance, excellent cycle life, absence of liquid electrolyte, etc. that make them the most suitable energy sources for some emerging applications (real time clock, medical implants, secured smart cards, etc.). All-solid-state microbatteries are realized by successive deposition of thin films with a total thickness of less than 20 µm. The corresponding manufacturing steps are mainly carried out using physical vapor deposition processes such as thermal evaporation or sputtering, which are commonly used in the microelectronics industry. Each deposited layer is patterned either with shadow mask or photo-lithography step. Shadow masking process involves restrictive steps such as mask alignment and cleaning. Moreover, standard photolithography processes, commonly used in the microelectronics industry, are actually difficult to adapt to microbattery systems considering the high reactivity of active materials, mainly lithium metal and lithiated compounds like LiPON. The use of such techniques, for deposition as well as patterning steps, involves sophisticated equipment leading to high production and maintenance costs. Furthermore, deposition rates of each processing step remains rather low (~ 10 nm mn^-1 for sputtering techniques).

In this study, to increase throughput and limit production costs, Lithium anode thin films are deposited via an easy electrochemical route [4-5] using standard carbonate based electrolyte with LiPF₆ salt. Deposited films exhibit self-aligned nanorods structure without any dendritic growth. The electrochemical performance of such lithium electrodes is evaluated in coin cell configuration versus silicon thin films and no obvious difference in behavior is observed between electrodeposited lithium thin films and bulk lithium metal foils.

The process is then used to manufacture all-solid-state thin film battery stacks. The lithium anode electrodeposition on non-conductive LiPON layer is possible through the use of very thin Cu layer (Fig1).  The all solid state micro-battery stack is then composed of a silicon cathode, LiPON as solid electrolyte, copper as conductive layer for the electrodeposition process and electrodeposited lithium as anode material (Fig2). Finally, the electrochemical performance (cycle life, specific capacity and curve shape (Fig3)) is close to those obtained with standard evaporated lithium (without Cu layer). This new lithium film deposition technique is suitable for 3D textured or high roughness substrates and limited to electronic conductive surfaces. In the end, this enables a direct patterning of the lithium anode layer. Therefore, considering that all other microbattery materials can be structured by photolithography, the Li electrodeposition could avoid using shadow masking technique during the all-solid state microbatteries manufacturing process.

The authors thank ST-Microelectronics and the French Directorate General for Enterprise (DGE) for financial support through the Tours2015 project.

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[2] J.B. Bates, N.J. Dudney, B. Neudecker, A. Ueda, C.D. Evans, Solid State Ionics 2000, 135, 33-45.

[3] B. Fleutot, B. Pecquenard, F. Le Cras, B. Delis, H. Martinez, L. Dupont, D. Guy-Bouyssou, J. Power Sources 2011, 196, 10289-10296.

[4] X. Yang, Z. Wen, X. Zhu, S. Huang, Solid State Ionics 2005, 176, 1051-1055.

[5] J. Qian, W. Xu, P. Bhattacharya, M. Engelhard, W.A. Henderson, Y. Zhang, J.-G. Zhang, Nano Energy 2015, 15, 135-144.