Using Nanoscale Metal Oxide Films to Investigate the Influence of Anode Interfaces on Lithium Battery Performance
Authors: Sanroman Gutierrez Kenzie Marie , Shuchi Sanzeeda Baig , Cui Yi , Bent Stacey F. 
Published: August 2024

As renewable energy and battery-powered technologies become more widespread, high energy-density and high-performing batteries are required to meet society’s needs. While lithium-ion batteries have been the staple, their energy density development has been slow, and at gravimetric energy densities of around 250 Wh/kg, they are approaching their theoretical limit.1 The lithium-metal battery (LMB) is considered a next-generation battery due to high theoretical energy densities (ex: Lithium-Sulfur, 2600 Wh/kg), and lithium metal’s high gravimetric capacity (3860 mAh/g) and low reduction potential (-3.04 V vs S.H.E.). However, commercialization of LMBs has been limited due to various issues.

Two properties that dictate the performance of the LMBs are: 1. The composition of the solid electrolyte interphase (SEI), an electronically conducting, yet passivating, layer at the anode-electrolyte interface, and 2. The morphology of the lithium deposited at the anode. Morphological studies focus on creating dense, low surface area, deposits that limit SEI fracturing and the formation of electronically disconnected lithium. SEI quality is improved by incorporating inorganic or anion-rich SEIs, formed from the electrolyte salt, to promote SEIs that are mechanically robust, ionically conducting, and homogeneous. Our group’s previous work aimed to improve LMB performance via both routes by growing resistive Al2O3 coatings on the copper (Cu) current collector using atomic layer deposition (ALD). Defects in the thin film create areas of low resistance that behave like ultramicroelectrodes which encourage radial diffusion of lithium ions to the nuclei promoting lateral growth and producing low surface area, dense, planar lithium deposits. Simultaneously, the Al2O3 coating’s Lewis acidic properties promote the decomposition and subsequent incorporation of inorganic electrolyte species into the SEI.

We present a study of how the different interfaces present at the anode in LMBs impact SEI composition and battery performance. To hold the Li nucleation morphology at the anode constant, we deposit metal oxide films of a fixed resistance onto the Cu current collector, generating planar, low surface area, lithium deposits. This experimental design allows us to decouple the effect of the metal oxide thin film-electrolyte interface on SEI quality and battery performance from that of the lithium morphology. First, as shown in Fig. 1a, different metal oxide films of equal resistance, but different thicknesses, are grown via ALD, confirmed by measuring the first-cycle nucleation overpotential. The resistive metal oxides investigated are Al2O3, HfO2, AlHfxOy, and ZrO2. Scanning electron microscopy results indicate that all cells modified with equal resistance metal oxide films exhibit the same low surface area, dense, planar Li nucleation morphology. However, long-term cycling tests show differences in cell performance and life cycle. Subsequently, the composition of the SEI is investigated using X-ray photoelectron spectroscopy (XPS). Through XPS, an elemental atomic ratio analysis is conducted to investigate the relative number of species derived from the anion of the electrolyte versus the solvent of the electrolyte to reflect the anion-derived nature of the SEI. Additionally, high-resolution peaks are used to determine the relative abundance of high-quality SEI species such as lithium fluoride. This analysis confirms that the metal oxide thin film tunes the SEI composition. Additionally, we identify trends present between the SEI quality of the ALD-modified cell and its performance.

To further investigate the thin film-electrolyte interface, a set of cells modified with binary stacked metal oxide thin films with two different stack orders were created, as shown in Fig. 1b. Investigating SEI composition and LMB performance, with the morphology fixed by the nucleation overpotential, allowed for deconvolution of the impact of the Cu-thin film interface and thin film-electrolyte interface. Both interfaces can in principle impact the battery’s function because 1. The Cu-thin film interface controls the electron transport to form Lio from Li+ ion during electrodeposition, and 2. The thin film-electrolyte interface can modulate the SEI. Our results indicated that the SEI and performance were both primarily regulated by the thin film-electrolyte interface. Based on our fixed-resistance and binary stack experimental design, we propose that modulation of the SEI composition by the metal oxide coating controls LMB performance when the Li nucleation morphology is fixed. This result suggests the potential for tuning LMB performance by careful selection of the metal oxide interfacial coating.

References

(1) Cheng, X.-B., et al. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chem. Rev.
2017, 117 (15), 10403–10473. https://doi.org/10.1021/acs.chemrev.7b00115.

(2) Oyakhire, S. T., et al. Electrical Resistance of the Current Collector Controls Lithium Morphology. Nature Communications
2022, 13 (1), 3986. https://doi.org/10.1038/s41467-022-31507-w.

Figure 1