Sputtered LiFePO4 for Solid State Batteries

Abstract

The progress in microelectronics and micro/nanoelectromechanical technologies allows for miniaturization and optimization of the electronic devices used in our daily life. To assure the energy supply to such devices, this progress should be accompanied by the development of micro power generators able fulfil the energy demand when being integrated on the microchips.The working principle of that kind of micro storage systems, namely, micro batteries, would not be different from the existing ones. By means of an electrochemical reaction, the chemical energy stored in the electrodes of the battery is converted to electrical energy. Lithium-ion batteries (LIBs) are the state-of-the-art configuration that allow for the best compromise between the amount of energy stored, and the power at which this energy can be delivered. When referring to lithium-ion micro batteries, the working principle is not different from the traditional one. However, there are some main differences. The first is that organic liquid electrolytes could hardly be included, due to their leakage risk. Moreover, these electrolytes are flammable and could constitute a safety issue when included in microdevices. Additionally, the miniaturization of the battery device involves thin layer deposition for all the battery components [1]. In this regards, physical deposition techniques (sputtering, atomic layer deposition, etc.) have been demonstrated to be promising techniques for on-chip micro batteries fabrication [2,3].LiFePO4 thin film layers have been attracting some attention due to their compromise between cost, performance, and availability.In this study, LiFePO4 thin film cathodes have been obtained on a platinum coated silicon wafer by sputtering, which allows to deposit a well-controlled thickness and composition. Due to LiFePO4 ‘s acceptable electrical conductivity, it was possible to carry out the deposition both in direct current and radiofrequency mode. Both processes have been used with the aim of understanding if any difference arises in the deposited film because of the sputtering method. The LiFePO4 film was characterized by means of Atomic Force Microscope (AFM) to measure the thickness of the deposited layer, X-Ray Diffraction (XRD) to investigate the crystalline phases present, Scanning Electron Microscopy (SEM) to analyse the morphology and the homogeneity of the composition and Raman spectroscopy to assess the chemical and physical structure. Finally, the deposition condition as well as the post-treatments were correlated to the electrochemical behaviour of the LiFePO4-coated Pt-Si wafer by cyclic voltammetry, demonstrating the feasibility of the as-obtained LiFePO4 for the application as positive electrode in on-chip Li-ion micro batteries.