Electron holography (EH) is one of the phase contrast imaging techniques by transmission electron microscopy (TEM). We have so far used EH to observe not only magnetic flux in magnetic materials [ 1] but also electric potential in semiconductor and battery materials [ 2, 3]. Here, we report recent results of EH applied to an all-solid-state Li-ion battery (ASS-LIB). ASS-LIBs consist of cathode and anode electrodes and nonflammable solid electrolyte instead of liquid electrolyte. Thus, they have a high possibility to replace the conventional LIBs because of safety, high energy density, low cost and so on. To develop high performance ASS-LIBs, it is important to understand how and where Li-ions move and electric potentials are formed in the batteries. We have already succeeded in visualizing Li-ion movement by operando electron energy-loss spectroscopy (EELS) [ 4]. In this presentation, we focus on the observation of electric potential changes during charge and discharge cycle.
Figure 1(a) shows a schematic configuration of thin-film-type ASS-LIB for our EH observation. A commercially available Ge-doped-LATSPO sheet, (Li 1+x+yAl y(Ge,Ti) 2-ySi xP 3-xO 12) was used as the solid electrolyte. The LiCoO 2 (LCO) cathode and Fe 2(MoO 4) 3 (FMO) anode were deposited on the sheet via pulsed laser deposition. The Pt and Pt/Au current collectors were also deposited by using a common sputtering coater. In this study, we observed the potential distribution around the cathode/solid-electrolyte interface indicated by dotted lines in Fig. 1(a). For reliable operando observation of electric potentials in ASS-LIBs, sample preparation is so important. To suppress additional potential fields generated by the applied voltage and the electron charging in the solid electrolyte, the sample was coated by amorphous AlO x insulator and amorphous C conductive films termed “nano-shield” [ 5].
Fig. 1.
Electric potential changes in a thin-film-type all-solid-state Li-ion battery during charge and discharge cycle. (a) Schematic configuration of the battery sample. (b) TEM image around the cathode/solid-electrolyte interface. (c) Electric potential maps during the cycle and (d) corresponding potential profiles in the horizontal direction of (c).
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