Li-ion thin film solid state batteries are capable of battery life of more than 10000 cycles¹. “Li-free” anodes in solid-state batteries eliminate lithium metal in the manufacturing stage, which could improve energy density, simplify manufacturing process, and lower cost. Attempts to fabricate “Li-free” cells have been challenging because of the concomitant reduction in cycle stability^{2, 3}. Replacing the Cu current collector with metals like Au and Pt, which alloy with Li, shows improvement in cycle stability, probably because it creates a more stable interface^4-6. The microstructural changes associated with such improvement, especially at the interfaces, have not been fully understood, hampering future development of all-solid-state “lithium-free” batteries.

We fabricate LiCoO₂ (35 µm)/LiPON (4 µm)/M (~15 nm) thin film solid state cells, where M is Cu, Au, or Pt, and study the electrochemical properties and microstructure changes within each thin film during cycling using an in-situ SEM. Figure 1 shows the nucleation of lithium through Cu, Au and Pt current collectors upon charging the cell for 1.5 µAh. In the case of Cu current collector, lithium penetrates the thin Cu layer and preferentially nucleates along the grain boundaries, which is a result of the LiPON solid electrolyte following the contour of the underneath LiCoO₂ layer. Assisted by fast lithium diffusion in the alloy, Li-Au and Li-Pt alloy layers probably homogenize the chemical potential across the surface, leading to well-spaced nucleation sites. We further discuss the growth and stripping of lithium at the solid electrolyte/current collector interface and pay close attention to the interactions of the metals with lithium. Finally, we correlate these microstructural details with coulombic efficiency and cycle stability of the solid state cells. These results provide insights into the fundamental electrochemistry and microstructure evolution that occurs in thin film solid-state Li-ion batteries during cycling and may have significant impact on developing “Li-free” solid-state batteries.

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