Visualization of Electrochemical Reactions in All-Solid-State Li-Ion Batteries by Operando Transmission Electron Microscopy

All-solid-state Li-ion batteries (ASS-LIBs) having incombustible solid-electrolytes are expected to be promising energy-storage devices because they have advantages of safety, long life-time, low cost and high energy density, compared with common liquid-based LIBs. However, there is a serious issue that needs to be solved for practical use, namely, large resistance of Li-ion transfer at the electrode/solid-electrolyte interfaces and/or in the electrodes. The high resistance of Li-ions drastically reduces the power density of the batteries. To overcome this issue, it is necessary to understand how Li-ions are transferred across the solid-solid interfaces during battery operation. We have so far developed a technique to operate ASS-LIBs in a transmission electron microscope (TEM) and succeeded in visualizing the electric potential distribution using electron holography [1, 2]. Moreover, to directly visualize the Li distribution, we have used electron energy-loss spectroscopy (EELS) in a scanning/transmission electron microscope (S/TEM) [3, 4]. Here, we report recent results of <i>operando</i> observation using S/TEM techniques.<br/>We prepared a thin-film-type ASS-LIB having LiCoO₂-cathode/Al-, Si-, Ge-doped LiTi₂(PO₄)₃-solid-electrolyte (LATP)/<i>in-situ-</i>formed-anode structures. We used <i>operando</i> STEM-EELS to visualize the 2-dimensional Li maps in LiCoO₂ region [4, 5]. In a pristine state, the Li concentration was not uniform and the low concentration layers (20 nm) were observed near the interface. We found that mixture of Co₃O₄ and LiCoO₂ was formed, which is the origin of the large interfacial resistance of Li-ion transfer. In charge/discharge states, the Li concentration was changed in the LiCoO₂ as expected. To improve the time-resolution of <i>operando</i> STEM-EELS, we applied machine learning technique to obtain Li signals from EELS data set by reducing random noises. We succeeded in observing that Li-ions moved in not only perpendicular direction but also parallel direction to the interface [5]. If presentation time is permitted, we will show another result of Li observation in a bulk-type ASS-LIB having sulfide-based solid-electrolyte [6].<br/>We would like to thank Dr. M. Fujii, Dr. T. Hirayama, Ms. E. Igaki, Prof./Dr. K. Saitoh and Prof./Dr. Y. Iriyama for valuable discussion in this study. This work was partially supported by JSPS KAKENHI (JP17H02792, JP19H05814).<br/><br/>[1] K. Yamamoto et al., <i>Angew. </i><i>Chem. Int. Ed.</i> <b>49</b> (2010) 4414-4417.<br/>[2] K. Yamamoto et al., <i>Electrochem. </i><i>Commun.</i> <b>20</b> (2012) 113-116.<br/>[3] K. Yamamoto et al., <i>Microscopy</i> <b>66</b> (2017) 50-61.<br/>[4] Y. Nomura et al., <i>Nano Lett.</i> <b>18</b> (2018) 5892-5898.<br/>[5] Y. Nomura et al., <i>Nat. Commun.</i> <b>11</b> (2020) 2824.<br/>[6] Y. Nomura et al., <i>ACS Energy Lett.</i> <b>5</b> (2020) 2098-2105.