A recent article published in Batteries proposed depositing a thin LixSny alloy layer over a Li foil as an anode in all-solid-state Li metal batteries. This would enable fast charging and improve overall electrochemical performance.
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Background
With a growing need for greater power and energy storage, Li metal is a promising anode material due to its high specific power and energy. However, safety concerns arise due to dendritic and mossy Li deposition, which can lead to short circuits and battery malfunctions.
Numerous methods, such as passivation and lithium anode modification, have been explored to address these issues and develop high-performance all-solid-state batteries (ASSBs). Passivating the Li surface with a polymer or polymer/ceramic mix layer can reduce dendritic formation.
Physical vapor deposition methods such as sputtering on lithium foil have been employed using various inorganic compounds and metals. Sputtering is an efficient method to form thin and uniform intermetallic coatings by controlling the deposition parameters. The coatings are almost devoid of contamination as the process is performed under a high vacuum.
This study employed sputtering to deposit a thin LixSny alloy layer on the Li metal anode. The researchers' choice of alloy was based on a previous investigation involving various metals, including Ag, Al, Sn, In, Bi, and W.
Methods
Sn was sputter-deposited on a 50 µm-thick Li metal foil using commercially procured Sn metal targets (>99.999 % purity). Subsequently, the Sn deposit on the Li surface was cleaned with plasma in situ without exposure to dry air.
A 3-4 µm-thick polymer/ceramic (polyethylene oxide (PEO) and Al2O3) layer was deposited on the Sn-coated Li metal anode using a doctor blade. LiFePO4 (LPF) electrodes were prepared on a carbon-coated aluminum foil, while the solid polymer electrolyte (SPE) was prepared on a polypropylene film.
LFP/SPE/Li batteries were assembled in pouch cells, obtaining an active surface area of approximately 7.48 cm2 for the anode and cathode. A reference cell was prepared using an unmodified Li metal anode. All the batteries were cycled at 80 °C using a potentiostat. A potential window of 2.0-3.8 V vs. Li/Li+ was selected for electrochemical investigations.
Grazing incident angle X-ray diffraction (GIXRD) was used to characterize Sn coatings on lithium and silicon substrates, while their optical images were captured using a laser microscope. Atomic force microscopy (AFM) was performed to examine the surface of Sn-deposited Li. Cross-sections of LFP/SPE/Li stackings were observed before and after cycling using a scanning electron microscope (SEM). Finally, nanoindentation curves of various coatings on Li were recorded.
Results and Discussion
SEM images of the Li foil revealed an average thickness of 1.41 μm, representing an expansion of about 380 % compared to the theoretical value. This expansion was attributed to the Li-rich LixSny alloy depicted in XRD data. However, the exact alloy composition could not be determined.
The AFM images of the lithium surface with the 10 nm-thick Sn deposits exhibited small spheres of about 50 nm. With the formation of the LixSny alloy, these spheres grew into a cauliflower shape, enhancing surface roughness. This high surface roughness, despite thin metal deposits, was explained using the island growth mechanism (Volmer-Weber model).
Nanoindentation measurements revealed a much higher hardness of the alloy layer than bare Li anode, which could impede dendrite progression. The hardness of pristine Li metal (~8.6 MPa) gradually increased with the thickness of Sn. The 600-nm thick Sn deposits increased the anode hardness by 16300 %, promising to counteract the progression of Li dendrites.
The SEM images of used LFP/SPE/Li from pouch cells revealed that the alloy layer remained intact after hundreds of fast cycles. Additionally, SnAl nanoparticles were observed at the Li/Sn interface, but no traces of Sn were identified in the SPE or the Li bulk anode.
Double modification by a ceramic/polymer layer over the LixSny layer enhanced the battery’s stability for 500 cycles at C/3. Moreover, the plasma treatment of the LixSny alloy layer after Sn deposition improved the cycling performance at 1C (1000 % compared to the Li metal anode for 300 cycles).
Conclusion
Overall, the proposed LixSny alloy coating over the Li metal anode helped realize an effective and inexpensive protection of Li metal while improving the battery performance. The new designs and combinations of interlayers exhibited good electrochemical results in the ASSB configuration at high cycling rates.
The in-situ generated three-dimensional Li-rich alloy favored fast Li+ ion transfer at the SPE/Sn interface. In addition, the alloy layer's higher electrochemical potential than the Li metal reduced its reactivity with the electrolyte. The chemically and electrochemically stable polymer/ceramic layer also allowed the battery to cycle at high C-rates for several hundred cycles.
The researchers claim the proposed process to be easily transposable at the industrial level to fabricate fast, cost-effective, and reliable ASSBs. Moreover, the alloy layer can be modified post-deposition to enhance applicability.
Journal Reference
Delaporte, N., et al. (2024). Designing a Stable Alloy Interlayer on Li Metal Anodes for Fast Charging of All-Solid-State Li Metal Batteries. Batteries. DOI: 10.3390/batteries1007025
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