Charge Transport Regulating Films on Mg Anodes and Implications for Mg-Battery Electrolyte Evaluation
Authors: Hahn Nathan T , Zavadil Kevin R 
Published: February 2020

The development of rechargeable Mg batteries, driven by the desire to surpass the limiting specific energy density of Li ion batteries, is currently limited by the lack of stable, functional electrolytes compatible with high voltage cathodes. A functional electrolyte must deliver the Mg cation to the anode surface at nearly 100% Coulombic efficiency, requiring cation desolvation and accommodation without formation of a cation blocking film. Successful electrolytes rely on Lewis acid – base reactions to form Mg cation-solvent complexes that include the traditional Grignard based and more recently reported inorganic Mg salt based electrolytes. As the search for functional magnesium battery electrolytes with wider electrochemical windows and better compatibility with high voltage oxide cathodes proceeds, improved understanding of the anode interfacial phenomena at work in these systems must be attained and applied. We seek to provide a better understanding of anode/electrolyte interfacial processes and their impact on the electrodeposition and dissolution of Mg in both traditional Grignard and novel, inorganic salt electrolyte systems.
We demonstrate that films form and evolve on the Mg surface in these electrolytes. These films regulate the deposition and dissolution kinetics by restricting cation transport, contrary to the commonly held view that Mg surfaces are film-free during deposition in traditional electrolytes (D. Aurbach et al., Electrochem Solid St, 2000). We demonstrate that a stable film forms in an All Phenyl Complex (APC) Grignard electrolyte as evidenced by a transient polarization observed in the chronopotentiometric stripping response of a Mg adlayer deposited in the presence of such a film (Figure 1a). TOF-SIMS depth profiling (Figure 1b) confirms that a film comprised of electrolyte constituents is present at this interface and is responsible for the restricted cation transport observed in figure 1a. We find this behavior also occurs in non-traditional, newly developed electrolytes and correlate cation transport with the structure and composition of these films.
Mg nucleation and growth can also be impacted by film formation on alternate deposition substrates. Electrolyte activity is often evaluated by depositing Mg onto inert substrates such as platinum, glassy carbon, and copper. However, it has been demonstrated for the case of traditional magnesium electrolytes, that the substrate identity can affect the observed electrodeposition kinetics (F. Zhang et al., Energy & Environ Sci,2012). Figure 1c demonstrates that for certain nontraditional electrolytes this effect is magnified such that Mg deposition can be drastically impeded on a non-magnesium substrate. Such observations can be ascribed to the transport properties of surface films formed by adsorbed electrolyte species and cathodic decomposition products. We demonstrate that use of a non-magnesium substrate can prevent accurate assessment of the viability of electrolytes for Mg-battery applications, since such a battery would undoubtedly cycle Mg onto and off of itself rather than a bare current collector. We will demonstrate that utilization of active, uniform magnesium substrates produced by electrodeposition ensures the relevant evaluation of the magnesium electrodeposition kinetics in several novel electrolytes based on simple Mg-salt complexes and will relate said kinetics to the nature of the magnesium-complex formed in solution.
This work is supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science. Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. DOE’s NNSA under contract DE-AC04-94AL85000.