Determination of Space-Charge-Layer Width in Li-Ion Solid Electrolyte Under the Application of Voltages

Abstract

Distribution of conducting ions in solid electrolytes is key information for revealing operation mechanisms of ionics-based devices, such as Li-ion batteries and fuel cells. The distributions of conducting ions in liquid electrolytes are well understood, especially near liquid-electrolyte/electrode interface; an electric double layer is formed in nano-meter scale. In contrast, as for solid electrolytes, although the quantitative study of physics in space charge layer is of crucial importance, few researches have addressed the depth profile of the density of conducting ions. Here we report applied-voltage-dependent Li-ion distributions in a Li-ion solid electrolyte, measured by using Rutherford backward scattering (RBS) and nuclear reaction analysis (NRA). Combining RBS and NRA, we were able to obtain the depth profile of chemical compositions for both light and heavy elements, in a sample under the application of bias voltage. Thus, quantitative study of Li-density distribution is possible. For the RBS and NRA measurements, 200 nm-thick Ni thin films were deposited as blocking electrodes using DC magnetron sputtering method on both sides of a Li1.5Al0.5Ge1.5P3O12 (LAGP, Toshima Co. Ltd.) solid electrolyte substrate. The sputtering conditions of Ar pressure, substrate temperature and DC output power were 1 Pa, room temperature and 30 W, respectively. H+ beam accelerated at 1 MeV was used for both the RBS and NRA measurements. We developed an original sample holder that would enable us to apply voltages (V= 0 V, +5 V and -5V) during the measurements. AC impedance spectroscopy was also measured before and after the RBS and NRA measurements. Surprisingly, the NRA measurements revealed that the space charge layer in Li-ion solid electrolyte is in the order of micro-meters. Depth profiles of Li composition obtained from the RBS and NRA measurements are summarized in the Figure. It should be stressed here that the Li composition is depressed more than 40% at V = +5 V compared to that at V= 0 V. The width of Li-depressed region exceeds 8 μm. This result is in striking contrast to the liquid electrolyte case; the electric double layer is in the order of nano-meters. This study was supported by JST-CREST, JST-ALCA and Kakenhi. Figure: Depth profiles of the Li composition at 0 V (blue line) and at +5 V (red line). Li composition is normalized by the bulk Li composition at 0 V. The inset shows an experimental layout of RBS, NRA and electrochemical (Ec) measurements. Figure 1