Electron Microscopy Characterization of the Aluminum Anode of a Novel Rechargeable Aluminum-Sulfur Battery David Moser1, Sandra Steiner3, Bernhard Gollas3, Gerald Kothleitner1,2 1. Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria 2. Graz Centre for Electron Microscopy (ZFE), Steyrergasse 17, 8010 Graz, Austria 3. Institute for Chemistry and Technology of Materials, Graz University of Technology, Stremayergasse 9, 8010 Graz, Austria
Introduction Current energy storage devices have significant issues in capacity, safety and availability of raw materials [1]. In this context, new technologies are intensively researched. As a part of the SALBAGE (Sulfur-Aluminum Battery with Advanced Polymeric Gel Electrolyte) consortium (EU FETOPEN project), our working group at Graz University of Technology focuses on the electrochemical behavior of metallic aluminum and aluminum alloy electrodes in combination with a deep eutectic solvent consisting of AlCl3 and urea as electrolyte. Electrochemical methods cannot provide information on the morphological changes at electrode surfaces. Electrodeposition and electrodissolution caused by repetitive charging and discharging is therefore necessary to be studied by electron microscopy.
Issue: Surface Passivation • The native oxide layer present at aluminum surfaces has a passivating effect during electrochemical experiments. • It is formed not even at ambient conditions, but also at low oxygen pressures, and is therefore expected to be present at all times [2]. • Literature reports imply that the layer is compact, amorphous and reaches a limiting thickness during its growth [3,4]. • Thickness at the surface of high purity Al measured by Transmission Electron Microscopy (TEM): 3 to 5 nm.
Oxide Layer: 3-5 nm
• When depositing Al on a passivated surface, dendritic growth is observed at a limited number of sites. The native oxide layer electrically shields the metallic aluminum underneath. The corrosive electrolyte locally disrupts the passivation. At these sites, the local current density is very high, which leads to different growth morphologies (flakes or dendrites).
Surface Activation
•
The electrochemical response of cells and the morphology present at electrodes are dependent on the time between cell assembly and start of measurement (stand-by time). To gain a better understanding of this, stand-by times of 2 h, 6 h, 18 h, 54 h and 168 h in combination with low current galvanostatic cycling were tested.
•
Two morphological features, which are predominantly present after short stand-by, could be identified: Flake like deposits and large passivated areas. 54 hours was the shortest stand-by time without flakes or large passivated areas. It is suspected that after a long stand-by time, enough active sites were available to avoid the dendritic growth regime during cycling.
•
2h
• Stand-by causes local disruption of the native oxide layer. At these sites metallic aluminum is in direct contact with the electrolyte. • Short stand-by: few active sites, cycling causes excessive pitting or formation of flakes; high local current density. • Long stand-by: more active sites, improved dissolution/deposition morphology; lower local current density. Stand-by
Short stand-by time
Cycling
Stand-by
Long stand-by time
Cycling
54h
• First investigations revealed that impurities from the electrolyte are deposited at these active sites (cementation).
References [1] SALBAGE proposal, No. 766581, FETOPEN-01-2016-2017, (17.01.2017). [2] Cai, N.; Zhou, G.; Müller, K.; Starr, D. E., Phys. Rev. Lett., 2011, 107 (3), 035502. [3] Baran, J. D.; Grönbeck, H.; Hellman, A., Phys. Rev. Lett., 2014, 112 (14), 146103. [4] Yang, Y.; Kushima, A.; Han, W.; Xin, H.; Li, J., Nano Lett., 2018, 18 (4), 2492–2497.
Contact World Wide Web: E-Mail:
www.felmi-zfe.at david.moser@felmi-zfe.at
EDS: 79% Al 18% O 2% Cl 1% N
EDS: 51% Al 32% O 1% Cl 3% N 4% Ga 6% Cu 3% Ni
• An in-cell activation method to disrupt the native oxide layer present at aluminum surfaces is proposed. • A two-step approach: • At least 54h stand-by time to create a sufficient number of active sites by corrosion • Low current galvanostatic cycling to enlarge the active sites. • Further investigations to fully understand the processes taking place during stand-by: • Time resolved surface characterization to investigate formation of local disruptions (exsitu and in-situ). • TEM-EELS and EFTEM to achieve locally resolved, compositional information of the native oxide layer after stand-by • XPS: information on chemical species present at the surface after stand-by
Acknowledgements The Institute for Electron Microscopy and Nanoanalysis (FELMI) and the Center for Electron Microscopy Graz (ZFE) as well as the Institute for Chemistry and Technology of Materials (ICTM) are thanked for provision of the infrastructure. This project has received funding from the European Union’s Horizon 2020 research and innovation program FET-OPEN-1-20162017 under grant agreement Nº 766581.