In-Situ Scanning Electron Microscope Observations of Electrochemical Li Deposition and Dissolution at Lipon/Pt, Au Interfaces

Abstract

The theoretical capacity of Li metal (3860 Ah kg–1) is much greater than those of rechargeable anodes in the present lithium-ion batteries (e.g. graphite, 372 Ah kg–1). Controlling Li plating/stripping reactions is thus important for next-generation batteries using Li metal anodes. However, the growth of Li dendrites is always a critical problem because it leads to the short-circuiting of a battery. It is expected that the use of inorganic-solid-state electrolytes for Li-S, Li-air, and all-solid-state-lithium batteries (SSLB) mechanically blocks the growth of Li dendrites. Amorphous electrolyte especially plays a key role in suppressing the Li growth from the anode surface toward the cathode because there are no grain boundaries. The nucleation sites for Li metal are supposed to be located at solid/solid interfaces in SSLB. Hence, the nuclei must push either electrode or electrolyte to create their own spaces. This process is associated with generation of significant strains [1]. Previous work studied the Li plating/stripping reactions with lithium phosphorous oxynitride (LiPON) electrolytes coated with current collector (CC) films of Cu, Ni, and W [1,2]. These metals are unable to form any alloy phases with Li. As a next step, this study investigates how the Li nucleation/growth and dissolution occur with CCs of Pt and Au that form several alloy phases with Li in the phase diagrams, using an in-situ scanning-electron microscope (SEM) observation technique. The top and bottom surfaces of a Li1+x+y Al x (Ti,Ge) 2-x Si y P 3-y O12(LATP) sheet (1.25 cm × 1.25 cm, Ohara Co.) were coated with 2.5-μm-thick LiPON layers by radio frequency magnetron sputtering. A current collector film (Pt, Au, Cu) was deposited on the top LiPON surface by pulsed laser deposition (PLD). The CC area was controlled to be 5.0 mm in diameter. A several-μm-thick Li film with a diameter of 9.0 mm was deposited on the LiPON surface on the bottom by vacuum evaporation deposition. A fabricated all-solid CC/LiPON/LATP/LiPON/Li cell was sandwiched with Cu and brass plates. The Cu plate has a viewport with a diameter of 3.0 mm in the center. Electrochemical impedance measurements were performed with amplitude of 20 mV in the frequency range from 3×106 Hz to 1 Hz. Li electrodeposition was performed under galvanostatic conditions. Applied current densities were estimated for the whole area of a CC film with a diameter of 5.0 mm. Figure 1A shows potential transients during Li electrodeposition at 100 μA cm−2 with Pt, Au, and Cu CCs with thicknesses of 30 nm. The potentials were positive for the initial 510-610 seconds (shaded region) when Pt and Au CCs were used. Subsequently, the potentials decreased to be negative and then achieved steady states at −0.6 V to −0.8 V. On the other hand, the potential quickly decreased from the open circuit potential (OCP > 2 V) to negative values after starting the electrodeposition with a Cu CC. Based on in-situ SEM results with Pt and Au CCs, Li began to grow only after the potentials became negative. It is thus considered that Pt and Au form alloys with Li when the potentials showed positive values. The Li nucleation subsequently occurred after the Li concentrations in Pt and Au exceeded the critical supersaturation. Figure 1B shows SEM images of Li particles electrodeposited at 100 μA cm−2for 3600 s with Pt, au, and Cu CCs. It is found that Li particles distribute with more uniform interdistances with Pt and Au CCs. We will further discuss the observed phenomena with the analyzed results. Acknowledgements The authors gratefully acknowledge JST-ALCA and JSPS, 26870272 for the financial support.