In-Situ Cross-Sectional SEM Observations of Li Plating and Stripping on Oxide-Based-Solid-State Electrolytes

Abstract

Li-air, Li-S, and all-solid-state-lithium batteries presume the use of Li metal anodes thereby theroretically achieving much higher energy densities (> 2000 Wh kg−1) than that of Li-ion battery with graphite anode (< 300 Wh kg−1). However, Li dendrites easily grow toward the cathode to cause the short-circuiting of a battery when Li is plated on the anode surface. Thus, protective layers in Li-air and Li-S batteries and solid-state electrolyte in all-solid-state-lithium battery play important roles in preventing the growth of Li dendrites. However, it has been found that void formations and accumulations at Li/solid-electrolyte interfaces during thousands of Li plating-stripping cycles are critical problems in the solid-state-battery system. Hence, we in-situ observe Li plating and stripping processes on solid-state electrolytes using a scanning electron microscope (SEM) to understand how Li plating and stripping proceed at Li/solid-electrolyte interfaces. Oxide-based-Li+conductors such as lithium phosphorus oxynitride glass electrolyte (LiPON) were used as the electrolytes in this work. We fabricated LiPON/LATP/LiPON composite electrolyte. LATP denotes a Li1+x+y Al x Ti2−x Si y P3−y O12-based-glass-ceramic sheet (Ohara Inc.) with a thickness of 150 μm. Each of sputter-deposited LiPON layers on both sides of a LATP sheet has the thickness of approximately 2.5 μm. The detail of this cell was reported previously1. Li electrodes with thicknesses of 20 μm were deposited on both sides of a solid-electrolyte sheet by vacuum evaporation. Vacuum evaporation deposition of Li electrodes was carried out in an Ar-filled glove box with a dew point lower than 193 K. A field-emission-type SEM (SU8030, Hitachi High-Technologies Co.) was used for in-situ observations of Li plating and stripping on solid-state electrolytes. After fabricating a thin-film battery, we broke the cell to make cross-sections for SEM observations. A piece of fractured samples was installed in the customized SEM holder to align the cross-sectional plane in a direction normal to the electron beam direction of the SEM. The SEM holder was transferred into the SEM chamber without exposure to air. Figure 1 shows cross-sectional SEM images during in-situ observation for Li plating on a LiPON/LATP/LiPON composite electrolyte at 1.0 μA. In this cell, the thickness of the observed Li electrode was exceptionally thin for a test. The electrode area was unfortunately unknown for this observation. It is found that Li rods gradually grew with time without extending their diameters. This technique has been applied to observations of much thicker Li metal electrodes. We will further show how Li is electrodeposited and dissolved at solid/solid interfaces by in-situ movies and discuss the mechanism of Li plating/stripping processes in the solid-state-battery system. Reference M. Motoyama, M. Ejiri, and Y. Iriyama, J. Electrochem. Soc., 162, A7067 (2015). Fig. 1. In-situ SEM images during Li plating on a LiPON/LATP/LiPON composite electrolyte at 1.0 μA. The time intervals of the images were five minutes (time passes from the left to the right). The dotted line indicates the tip position of a Li rod in the leftmost image. Figure 1