Structure-Electrochemical Property Relationships of Lipons Thin-Films Electrolytes for All Solid-State Microbatteries

Abstract

All-solid-state microbatteries have interested many researchers and companies for the last decade, in order to supply in electrical energy small sensors and connected object. In these systems, Lithium Phosphorus Oxy-Nitride (LiPON) is generally used as solid-state electrolyte since its discovery in the early 1990s by Bates et al. [1]. However, LiPON presents a quite low ionic conductivity around 10-6 S/cm at room temperature. To increase this value, Sulfur can be added to the LiPON structure [2,3]. The present work focuses on the deposition of LiPONS thin-film electrolytes, and their chemical, structural, and electrochemical properties. LiPONS glassy films were deposited by reactive radio-frequency magnetron sputtering from a pure Li3PO4 target at room temperature in a mixed N2:H2S plasma, with H2S gas flow varying from 0 to 5% to fine tune the S concentration in the LiPONS thin films. LiPOS materials were also obtained by sputtering the same target under a Ar:H2S plasma. Constant power, pressure and deposition time were used. Potentiostatic Electrochemical Impedance Spectroscopy (PEIS), Scanning Electron Microscopy (SEM) and X-Ray Photoelectron Spectroscopy (XPS) measurements were performed to probe the changes in morphology and composition, and understand the structure-electrochemical property relationships. Whereas an improvement of the electrochemical properties of LiPON with the addition of Sulfur was expected, the opposite occurred (Fig. 1). In order to correlate the chemical structure to the ionic conductivity and identify the causes for the performance degradation, the electronic structure of LiPONS and LiPOS materials will be discussed based on XPS core level and valence band structure analyses, and compared with the classical LiPON electronic structure. The observed decrease in ionic conductivity when the S content in the LiPONS thin-films increases from 0 to 15 at% (Fig. 1) can be related with the appearance of two groups of species in the High-resolution S2p XPS spectra (Fig 2). For low S concentrations, doublets from 160 to 164 eV assigned to S-P structures are dominant whereas signals from 165 to 168 eV, corresponding to more oxidized S species that can originate from reactions between Sulfur and Nitrogen, become non-negligible at higher S-contents and may be the cause of the electrochemical properties degradation. [1] J. B. Bates, N. J. Dudney, G. R. Gruzalski et al, Solid State Ionics, vol. 53–56, pp. 647–654, 1992. [2] J. B. Bates, “Thin film battery and electrolyte therefor,” US 6,818,356 B1, 2004. [3] N. Mascaraque, H. Takebe, G. Tricot et al, J. Non. Cryst. Solids, vol. 405, pp. 159–162, Dec. 2014. Figure 1