Interfacial Properties of Alloying Interlayers in Anode-Free Sodium Metal Batteries

Due to the low price and abundance of sodium, researchers have gained interest in developing sodium-ion batteries as a more sustainable energy storage option to lithium-ion batteries. Although the energy density of sodium-ion batteries is relatively lower than lithium, the “anode-free” battery (AFB) has the potential to be competitive with lithium-ion energy densities. In the anode-free configuration, sodium is entirely sourced from the cathode and is plated/stripped directly on the aluminum current collector (CC). However, plating of sodium on the current collector can be inhomogeneous in morphology, lowering the coulombic efficiency and decreasing cycle life. The incorporation of a metaling alloying interlayer on the CC has been previously shown to promote homogeneous deposition due to the enhanced sodium wetting on the substrate, resulting in reduced plating overpotentials and improved cycle life.[1,2] However, there has been a general lack of in situ/operando observations of the morphological evolution of sodium in the presence of an alloying interlayer, or quantitative analysis of sodium contact angles on these interlayers. Additionally, these interlayers are reported to delaminate from the CC throughout cycling and expose the underlying CC surface, which is detrimental to battery performance over time. In this work, we explore the sodium morphological evolution and adhesion on nanoscale gold interlayers that were deposited on an aluminum CC, to better understand how the interlayer improves sodium AFB performance. We further introduce an intermediate adhesion bi-layer structure to mitigate delamination and promote improved stability. To observe the morphological evolution of sodium on the interlayers compared to an uncoated CC, we utilized operando video microscopy,[3] together with complimentary post-mortem optical imaging and scanning electron microscopy (SEM). Sodium exhibited increased adhesion and uniformity on the gold interlayer compared to the uncoated CC; however, sodium preferentially plates onto microstructural features on the CC surface, suggesting there is current focusing in these regions. The alloying interlayer elements were also observed to migrate along the surface during plating, exposing the underlying aluminum foil. To further analyze sodium adhesion, quantitative contact angle measurements between molten sodium and these substrates were performed,[4] which revealed increased sodiophilicity and a higher work of adhesion on the interlayer. We further performed quantitative peel tests, which suggests that the interfacial toughness of gold on the CC substrate is poor. Therefore, to increase the adhesion of the gold to the CC to prevent interlayer delamination during subsequent cycling we introduce a bi-layer structure. Preliminary results suggest that the bilayer provides stronger interfacial toughness of the interlayer on the CC, which can circumvent the limitations of a pure gold layer. These fundamental studies will inform design requirements for metallic interlayers for sodium AFBs, and highlight the coupled importance of wetting, adhesion, and nucleation. [1] Tang, S.; Qiu, Z.; Wang, X.-Y.; Gu, Y.; Zhang, X.-G.; Wang, W.-W.; Yan, J.-W.; Zheng, M.-S.; Dong, Q.-F.; Mao, B.-W. A Room-Temperature Sodium Metal Anode Enabled by a Sodiophilic Layer. Nano Energy 2018 , 48 , 101–106. https://doi.org/10.1016/j.nanoen.2018.03.039. [2] Tang, S.; Zhang, Y.; Zhang, X.; Li, J.; Wang, X.; Yan, J.; Wu, D.; Zheng, M.; Dong, Q.; Mao, B. Stable Na Plating and Stripping Electrochemistry Promoted by In Situ Construction of an Alloy‐Based Sodiophilic Interphase. Adv. Mater. 2019 , 31 (16), 1807495. https://doi.org/10.1002/adma.201807495. [3] Sanchez, A. J.; Kazyak, E.; Chen, Y.; Chen, K.-H.; Pattison, E. R.; Dasgupta, N. P. Plan-View Operando Video Microscopy of Li Metal Anodes: Identifying the Coupled Relationships among Nucleation, Morphology, and Reversibility. ACS Energy Lett. 2020 , 5 (3), 994–1004. https://doi.org/10.1021/acsenergylett.0c00215. [4] Yoon, J. S.; Liao, D. W.; Greene, S. M.; Cho, T. H.; Dasgupta, N. P.; Siegel, D. J. Thermodynamics, Adhesion, and Wetting at Li/Cu(-Oxide) Interfaces: Relevance for Anode-Free Lithium–Metal Batteries. ACS Appl. Mater. Interfaces 2024 , 16 (15), 18790–18799. https://doi.org/10.1021/acsami.3c19034.