In-Situ Observations of Electrochemical Li Growth and Dissolution on a Lipon Electrolyte By High Resolution Scanning Electron Microscopy

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 energy storage technologies using Li metal anodes such as all-solid-state-lithium batteries (SSLB) and Li-air batteries.  A great advantage of using an inorganic solid electrolyte is that the Li dendrite growth can be blocked without any separators, which is always a critical problem in charging/discharging Li metal anodes in organic liquid electrolytes.  Amorphous electrolyte especially plays an important role in suppressing the Li growth from the anode surface toward the cathode because there are no grain boundaries.  Previous work studied the Li plating/stripping reactions with lithium phosphorous oxynitride (LiPON) electrolyte coated with Cu current collector (CC) films [1].

   Fundamental studies on metal electrodeposition in solid electrolyte systems have hardly been reported in literature compared to liquid electrolyte systems.  Nucleation sites exist at solid/solid interfaces for metal nucleation with a solid electrolyte.  Hence, the nuclei must push either electrode or electrolyte to create their own spaces.  This process is associated with generation of strain energies and the unique aspect of solid electrolyte systems.  This study investigates how Li nucleates, grows, and dissolves across solid-electrolyte/metal-CC interfaces.  We also in-situ observe Li electrodeposition and dissolution using a field-emission-scanning-electron microscope (FE-SEM).

   The top and bottom surfaces of a Li_1+x+yAl_x(Ti,Ge)_2-xSi_yP_3-yO₁₂(LATP) sheet (1.25 cm × 1.25 cm, Ohara Co.) were coated with 2.5-μm-thick LiPON layers by magnetron sputtering.  A current collector film (Cu, Ni, W, Pt) 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 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 1 shows a FE-SEM (SU8030, Hitachi) image of a single Li island electrodeposited under a 90-nm-thick Cu CC and a voltage transient during Li dissolution fot this Li island at 50 μA cm⁻².  The excellent spatial resolution of a FE-SEM allowed us to observe transient behaviors of the single Li island growth and dissolution.  We will present in-situ movies showing how Li islands gradually dissolve back to a solid electrolyte under a metal CC at high magnifications by the help of a FE-SEM.  We will also discuss the  mechanisms of Li deposition and dissolution on a LiPON electrolyte based on the observed results.

Acknowledgements

The authors gratefully acknowledge JST-ALCA and JSPS, 26870272 for the financial support.

Reference

[1] M. Motoyama, M. Ejiri, and Y. Iriyama, Electrochemistry,82, 364 (2014).

Figure 1.  (Front)  Potential transient during Li dissolution at 50 μA cm⁻² under a 90-nm-thick Cu CC.  (Back) FE-SEM image of a single Li island with a broken Cu lid due to the Li growth.