AVS 72 Session BT+AS+CA+TF-FrM: Solid-State Batteries

Lithium metal is a critical anode material for next-generation, high-energy batteries. To realize these batteries, the lithium metal must be between thick, depending on the cathode chemistry. Furthermore, lithium metal must have controlled surface chemistry and high purity for optimal performance. Thermal evaporation is one of the most effective methods for producing thin lithium metal films that meet these criteria. In this work, we present our efforts to understand how to produce lithium metal films as thin as , and we explore the ability to fabricate lithium metal thin films with controlled microstructures. We have developed a process to modify the as-deposited lithium metal grain size in evaporated films from . Using these thin films with standard liquid electrolytes, localized high-concentration electrolytes, and solid electrolytes lead to significant differences in the cycling performance of lithium metal films. This work demonstrates the critical importance of microstructure control for realizing next-generation, high-energy lithium metal batteries.

This work was funded by the United States Department of Energy Transportation Technology Office Battery Materials Research Program and German-US Collaboration on Energy Storage under Simon Thompson and Tien Duong.

Sulfide superionic conductors with the argyrodite structure (e.g., Li₆PS₅Cl, LPSCl) are extremely promising for all solid-state batteries, but poor atmospheric stability and high interfacial reactivity limit their widespread adoption. Coating LPSCl powders with ultrathin, metal oxide coatings using atomic layer deposition (ALD) mitigates these problems, protecting LPSCl against atmospheric degradation¹ and reducing reactivity with Li metal, yielding more stable cycling². Despite significant promise, the ALD mechanism is unknown, hampering the development of new coating chemistries.

In this study, we elucidate the mechanism for Al₂O₃ ALD on LPSCl using trimethyl aluminum (TMA) and H₂O by combining in situ Fourier transform infrared spectroscopy measurements and ex situ solid-state magic angle spinning nuclear magnetic resonance, UV Raman spectroscopy, and X-ray photoelectron spectroscopy measurements with density functional theory calculations. We determine that ALD Al₂O₃ nucleates promptly via TMA reaction with native -OH, -SH, and PS₃-OH groups to form transient C-Al-O(S) species that are rapidly hydrolyzed during the subsequent H₂O exposure. This reversible transformation maintains surface nucleophilicity and prevents sulfide decomposition. The resulting layer-by-layer growth leads to highly conformal Al₂O₃ coatings on LPSCl.

This detailed understanding of ALD surface reactions provides critical insights guiding the selection of future ALD chemistries with greater enhancement in cycling performance.

1. Acs Materials Letters 2024, 6, (12), 5409-5417.

1. Advanced Materials 2023, 35, (21), 13.

In solid-state batteries, stresses resulting from electrode material chemomechanics are strongly coupled to solid electrolyte-electrode interface failures. Such failures are significant barriers to realization of practical Li metal solid-state batteries. While high external pressures can reduce these failures, the required pressures are impractical, and thus a different approach to reducing interface failures is required. We show that control of cathode chemomechanical stress provides an important path for realization of solid-state batteries which operate at commercially relevant low (e.g., <1 MPa) stack pressures. Using a series of cathodes with different chemomechanics we provide experimental evidence of the role of electrode chemomechanics in solid-state battery failure. Our model systems reveal that electrode chemomechanics significantly alter Li metal plating and stripping behavior at low stack pressure. We utilize these learnings to build long cycle-life solid-state batteries with practical areal capacity (5 mAh/cm²) operating under a 1 MPa stack pressure and at room temperature. Our findings highlight the importance of controlling positive electrode chemomechanics to realize low stack pressure solid-state batteries.

Vanadium (V) Oxide (V₂O₅) is a well-researched layered oxide with high capacity (375 mAh/g), often cited for its applications in energy storage, electrochromic, and neuromorphic applications. In the field of thin-film ionic devices, we can sputter Lithium Phosphorus Oxynitride (LiPON) on top of crystalline V₂O₅, creating our electrolyte/electrode stack. This sequence simplifies the fabrication of thin-film ionic batteries as sputtering LiPON can insert Lithium up to 2 Li⁺ per V₂O₅. We wanted to address the roughness of the LiPON/V₂O₅ interface by altering the fabrication steps to form a smooth interface, but unintentionally reduced the V₂O₅ to a VO₂ phase.

This smooth interface was achieved by sputtering amorphous V₂O₅ thin films in Ar/O₂ environment, then coated with LiPON, and subjected to post-deposition annealing in N₂ gas environment at 300 ◦C. The N₂ gas environment for the post-deposition annealing would crystallize the V₂O₅ and preserve the LiPON composition. We tracked the formation of a VO₂ phase, which was not observed for N₂ annealed V₂O₅ without LiPON, which remains VO₂ free. We used electrochemical cycling to probe the presence and evolution of the VO₂ phase, illustrating VO₂ reaction peaks instead of characteristic crystalline V₂O₅ peaks in the 2.2 V – 4.0 V vs Li+/Li window. (Seen in Figure 1). We will discuss how this behavior may be caused by LiPON-enabled reduction pathways at the buried interface. We confirm this reduction is limited to the interface as analysis via Raman identifies lithiated V₂O₅ phases. Systematic variation of LiPON sputtering parameters and annealing conditions, while holding LiPON thickness constant, reveal processing regimes that drive VO₂ formation versus preserving V₂O_­5. Ultimately, preserving the morphological benefits of a smooth LiPON/ V₂O₅ interface while constraining VO₂ formation, providing guidance for designing desired application-specific vanadium-oxide phases.