In-Situ Scanning Electron Microscope Observations of Lithium Nucleation and Growth at Solid/Solid Interfaces for All-Solid-State-Lithium Battery

The all-solid-state-lithium battery (SSLB) is an attractive candidate for the next generation rechargeable batteries.  Solid-state electrolytes are non-flammable and offer excellent calendar life performance.  The non-flammability is a great advantage over lithium ion batteries with organic liquid electrolytes.  Additionally, the container and separator volumes can be reduced or totally removed with solid electrolytes, leading to a dramatic increase of overall energy density.  Moreover, solid-state electrolytes block Li dendrites in contrast to liquid electrolytes.  Li metal anodes have attracted a great deal of interest due to the ten times larger theoretical capacity compared to graphite anodes.  Although a great deal of effort has been devoted, Li dendrites have remained critical problems for Li metal anodes.  Hence, SSLB provides the innovative changes for not only increasing battery energy density but also developing “rechargeable” Li metal anodes.

The Oak Ridge National Laboratory pioneered thin-film-solid-state batteries with Li metal anodes using lithium phosphorus oxynitride (LiPON) glass electrolyte.  They demonstrated that the solid-state electrolyte blocked Li growth toward the cathode, and Li grew in the opposite direction to penetrate thick Cu current collectors (CCs).  Still, the control factors of Li electrodeposition on a solid electrolyte are obscure.  Hence, it is important to study the mechanisms of Li nucleation and dissolution on solid electrolytes to improve the stability of Li plating/stripping reactions for a SSLB.

Nucleation sites are located at solid/solid interfaces for metal nucleation in a solid electrolyte system.  Hence, the nuclei must push either electrode or electrolyte to create their own spaces.  This process is associated with generation of strain energies around metal nuclei.  This study focuses on the modeling of Li nucleation on solid electrolyte under the influence of strains at solid/solid interfaces through the in-situscanning electron microscope (SEM) observations.

The top and bottom surfaces of a Li_1.3Al_0.3Ti_1.7(PO₄)₃(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) 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.

Figure 1A shows the all-solid CC/LiPON/LATP/LiPON/Li cell for in-situ SEM observations.  An all-solid-state cell was sandwiched with a Cu plate and a brass plate.  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.

Figure 1B shows in-situ surface SEM images during Li electrodeposition at 50 μA cm^-2 with Cu CCs of 30 nm−1.0 μm in thickness and Ni, W CCs of 90 nm in thickness.  The number of seconds in each SEM image expresses the duration after the Li nucleation.  Numerous small bumps with diameters (d_c) of 1−2 μm soon appear on the Cu CC surface after starting the electrodeposition.  Since LiPON is an amorphous material, there are no grain boundaries, whereby the nucleation probability is basically equal for any locations on the LiPON surface.  With a 30-nm-thick Cu CC, Li nuclei create bumps of ~1 μm in d_c.  They eventually break the Cu layer forming Li rods penetrating through the cracks as shown in Fig. 1B.  The d_c no longer increases once the Li cracks the CC.  No bumps grow with a 1.0-μm-thick Cu CC, but significantly large cracks eventually appear on the surface indicating that Li grow underneath.  Li particles grow to be larger under Ni and W CCs than under Cu CCs in Fig. 1B.  The d_c with Ni and W CCs more rapidly increases than with Cu CCs.  The number density of bumps naturally decreases with increasing the mean d_cwith the same charge.  The relationship between mechanical properties of CC and overpotential for expansion work of Li nuclei will be discussed.

Acknowledgements

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

Figure 1.  (A) All-solid electrochemical cell for in-situ SEM observation.  (B) In-situ SEM images during Li electrodeposition at 50 μA cm^-2 with Cu CCs of 30 nm−1.0 μm in thickness and Ni, W CCs of 90 nm in thickness.  (t: CC thickness)