The development of all solid state lithium microbatteries has been widely investigated over the last decade. Indeed, they can fit with the requests of portable devices and microelectronic components and especially with the ability of the power source to be integrated directly on/in the component. These microbatteries consist in stacks of thin film materials elaborated by physical vapour deposition techniques.
Lithium cobalt oxide, synthesized at temperature above 700°C, is used as a positive electrode in bulk commercial secondary batteries. It has been chosen due to its high electrochemical voltage (E~4V vs. Li+/Li), good cycling performance and high capacity. This work deals with the elaboration of LiCoO2 thin films by radio frequency magnetron sputtering for use in microbatteries, with the objective of the lowest thermal budget (one of the integration requirements). The deposition parameters such as working pressure, Ar/O2 ratio, power and substrate bias have been studied to obtain thin films with optimal electrochemical properties. The films deposited on aluminium foils have been annealed at 500°C and cycled in liquid electrolyte. These results are correlated with data obtained by Raman Spectroscopy, Scanning Electron Microscopy and elementary analysis. A LiCoO2/LiPON/Li microbattery is then fabricated in order to evaluate electrochemical behaviour of these thin films with a solid electrolyte.
Optimal parameters have been determined to get high capacity values for the LiCoO2 films (around 80% of the theoretical capacity with liquid electrolyte and 55% with solid electrolyte at i=10µA/cm2 between 3 and 4.2V) even for films crystallized at 500°C. Excellent cycling performance has been observed for all solid state microbatteries. Future work will be focused on the improvement of interfaces (LiCoO2/LiPON and LiCoO2/Pt) before and after annealing treatment, with the aim of obtaining maximal capacity values.
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