Li metal has a much greater theoretical capacity (3860 Ah kg^–1) than rechargeable anodes in the present lithium-ion batteries (graphite, 372 Ah kg^–1).  Controlling Li plating/stripping reactions is thus important for next-generation-battery technologies with Li metal anodes such as Li-air battery, Li-S battery, and all-solid-state-lithium battery (SSLB).  A great advantage of using an inorganic-solid-state electrolyte is that the Li dendrite growth can be blocked without any specific separators, which is always a critical problem in charging/discharging Li metal anodes in organic liquid electrolytes.  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.  Previous work studied the Li plating/stripping reactions with lithium phosphorous oxynitride (LiPON) electrolyte layers 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 are located 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, Au) was deposited on the top LiPON surface by pulsed laser deposition (PLD).  Only a 1.0-μm-thick Cu CC film was deposited by magnetron sputtering.  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 (50 μA cm⁻²).  Applied current densities were estimated for the whole area of a CC film with a diameter of 5.0 mm.

   The contribution of the CC deformation to the overpotential for the Li nucleation and growth at CC/LiPON interfaces is expressed by the following equation in previous work [2].

η_s = ε_θEM{ρF(1–ν}/{3(r_i+t)³/[2{(r_i+t)³–r_i³]–ν/(1–ν)}

This model is based on an elastic process that a dome-shaped Li nucleus initially pushes and deforms a CC layer.  The calculated voltage profiles consistently explain the initial stages of the experimental voltage profiles.

  Figure 1 shows a high-resolution SEM (SU8030, Hitachi) image of a single Li island electrodeposited under a 90-nm-thick Cu CC and a voltage transient during Li dissolution of the Li island at 50 μA cm⁻².  Our observed results support the above model that deformations of a CC are associated with the Li nucleation and growth on LiPON.  The excellent spatial resolution of a FE-SEM allowed us to observe transient behaviors of single Li island growth and dissolution.  We will present in-situFE-SEM movies showing how Li islands gradually dissolve back to a solid electrolyte under a metal CC at high magnifications by the help of a SEM.  We will further 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).

[2] M. Motoyama, M. Ejiri, and Y. Iriyama, J. Electrochem. Soc., 162, A7067 (2015).