Method to Synthesize Conformal Thin Films of Li7La3Zr2O12 on Surfaces and Three-Dimensional Substrates

3D battery electrodes with solid-state electrolytes (SSE) are an attractive architecture for next-generation Li-ion energy storage due to their superior safety characteristics and potential for high power/energy densities. These electrodes are structured as arrays of plates or pillars with high aspect ratios (~100) onto which SSE must be conformally deposited.

Among the most promising SSE types in terms of Li-ion conductivity and chemical stability is Li₇La₃Zr₂O₁₂, or LLZO. However, these properties are only exhibited in the crystalline form of LLZO, requiring high temperatures (>1000 degrees C) and long sintering times (24+ hours) to produce. Furthermore, the 3D electrode geometries present unique challenges for SSE deposition: Although atomic layer deposition (ALD) can achieve desirable conformality, the required thickness of the SSE deposition—between 1 and 10 microns—exceeds typical ALD limits by 1-2 orders of magnitude. Recent efforts to integrate the ALD process with crystalline LLZO required complex layering sub-cycles and were unable to produce quality films. Therefore, in order for LLZO to confer its advantages to 3D electrodes, a process must be able to effectively form and conformally deposit crystalline LLZO at thicknesses up to 10 microns.       

Researchers at Arizona State University have developed a novel process that combines the conformal nature of ALD but incorporates a synthetic method that enables the formation of high-quality, crystalline LLZO. First, ALD is used to deposit a coat of ZrO₂ onto the substrate electrode. This thin film of ZrO₂ serves as a seed layer for a topochemical, molten-salt-mediated conversion process which results in the formation of a thin film of crystalline LLZO. Further film growth in the salt melt can yield thicknesses on the order of several microns.

Performing ALD for ZrO₂ is well established and bypasses the need for multiple ALD sub-cycles. Since the ZrO₂ seed layer does not need to be crystalline, ALD can be performed at a lower temperature which can preserve the integrity of the interface between the ZrO₂ and the active electrode material. The crystalline LLZO that results from molten salt synthesis has ionic conductivity comparable to materials prepared by solid-state reaction.

Related publications:

Synthesis of Fine Cubic Li₇La₃Zr₂O₁₂ Powders in Molten LiCl–KCl Eutectic and Facile Densification by Reversal of Li⁺/H⁺ Exchange

Reduction in Formation Temperature of Ta-Doped Lithium Lanthanum Zirconate by Application of Lux–Flood Basic Molten Salt Synthesis

Potential Applications:

• Solid-state electrolytes
• 3D battery electrodes
• Microbatteries

Benefits and Advantages:

• Innovative – Greatly simplifies current LLZO formation processes by hybridizing ALD with molten salt synthesis.
• Practical – Formed LLZO is highly crystalline with ionic conductivity ~10-4 to 10-3 S/cm, and controlled thickness from ~0.1 to 10 microns. 
• Superior – Process requires less reaction time than ALD-based LLZO deposition, while delivering crystallinity and ionic conductivity that outperforms ALD, chemical vapor deposition (CVD), and sputtering methods.