Sputtered Thin Films for Lithium Ion Microbatteries: Recent Results on TiN Lithium Barrier Diffusion Layer, Au Negative and C-LiFePO4 Positive Electrodes

With the development of energy autonomous systems (sensors, connected objects, active RFID…), the interest for energy storage devices as microbatteries is growing. Such batteries are based on lithium technologies. Classically, a thin film lithium microbattery consists of the deposition on a substrate of several functional layers such as 2 current collectors, a positive and a negative (lithium metal) electrodes separated by a solid electrolyte (fig. 1). Our study focuses on sputtered thin films deposited on a silicon substrate (figure 1).

To prevent the lithium diffusion from the active layers to the silicon substrate, a diffusion barrier layer should be integrated in the structure. Titanium Nitride (TiN) is really developed in microelectronics and TiN could be used both as a current collector and as lithium diffusion barrier (1) for the negative electrode (2, 3). The purpose of this study is to understand the influence of deposition parameters on the titanium nitride characteristics in order to get low resistivity and low electrochemically active thin film against lithium ion insertion. To do so, experiments have been led to get resistivity and Li-capacity as function of the deposition parameters. Structural properties such as crystalline texture, roughness and microstructure have been also studied (figure 2).

Prior to the thin film deposition of gold materials and their electrochemical characterization, in operando X ray diffraction measurement has been performed on a gold foil in order to clearly understand the structural evolution of the lithium-gold alloys (4). As in the lithium-silicon alloys, the understanding is really complicated and the number of publications is limited (5-7): ours conclusion and results will be presented. Then, a negative gold electrode has been deposited on the TiN current collector by sputtering means. It has been shown that the electrochemical study is quite difficult if the gold thin film is deposited directly on the silicon wafer owing to the lithium-silicon alloys. It has been demonstrated the benefit from the TiN layer by TOF-SIMS measurement where the lithium ion bas been blocked in the gold electrode due to the TiN barrier layer. Galvanostatic cycling tests has been carried out on the gold electrode and two plateaus have been highlighted corresponding to the lithium-poor and lithium-rich gold phases. The reversibility of these two plateaus have been studied in liquid (1M LITFSI/EC/DEC) as well as in solid (sputtered LIPON) electrolytes and will be presented.

Finally, the C-LiFePO₄ positive electrode has been studied by RF and pulsed DC magnetron sputtering deposition means. The electrochemical (cyclic voltammetry, galvanostatic cycling) experiments on the thin films as a function of the deposition parameters (figure 3) will also be reported. Micro-patterning of the C-LiFePO₄ layer has been realized for the first time by deep reactive ion etching. All the building blocks mixing material deposition/characterization and microelectronic fabrication have been developed in this study and paves the ways to the technological fabrication of thin film lithium-ion microbattery based on this technology.

Acknowledgments: The authors want to thank the French network of the electrochemical energy storage (RS2E) for this support. This research is financially supported by the ANR and the DGA within the MECANANO project (ANR-12-ASTR-0032-01). The French RENATECH network and the CPER CIA are greatly acknowledged.

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