Hydrochloric acidic surface treatment for multivalent battery metal electrode

Description (excerpt)

BACKGROUND Commercial applications of batteries with metal electrodes are ubiquitous. However, some metals, such as Zn, are currently generally limited to primary (non-rechargeable) systems. The majority of electrolytes for these metal electrode batteries are water-based, and suffer from reactivity between, e.g., Zn and water, leading to corrosion, self-discharge, hydrogen evolution, dendrite growth, and other parasitic reactions, which reduce the efficiency of the battery. Even state of the art optimized aqueous/non-aqueous electrolyte formations are not capable of cycling metal batteries at reasonable areal capacities (e.g., >1 mAh/cm 2 ) and current densities (e.g., >0.25 mA/cm 2 ) with high efficiency (e.g., >95%). In order for rechargeable batteries with metal electrodes to be commercially viable, electrolytes with significantly higher efficiencies (e.g., improvements of >10-15%) must be developed. The embodiments herein may be manufactured, used, and/or licensed by or for the United States Government without the payment of royalties thereon. BRIEF SUMMARY The present disclosure is drawn to providing such improved efficiencies through a novel acid-based surface treatment for metal anodes in multivalent (MV) battery metal anodes (e.g., Zn, Mg, Ca, Al, etc.). Through the use of this technique, the acid chemically etches metal oxides on the surface and forms, e.g., a chloride enriched interphase (when using HCl as the acid), which greatly enhances charge transfer kinetics and is favorable for plating/stripping of MV cations during cycling. A first aspect of the present disclosure is drawn to the disclosed method for preparing or treating metal electrodes (preferably, anodes) prior to use in an electrochemical cell (such as a battery). The method involves applying a solution comprising an acid to a surface of a metal electrode, then removing excess solution and drying the surface of the metal electrode. While any multivalent metal can be utilized, this method is advantageously directed towards metal electrodes comprising Mg, Al, Ca, Zn, or combinations thereof. The acid is advantageously a single organic or inorganic acid (that is, the solution comprises only a single acid), while in some embodiments, a mixture of organic and inorganic acids are used. Such acids may include hydrochloric acid, sulfonic acid, acetic acid, sulfuric acid, trifluoroacetic acid, bis(trifluoromethane)sulfonic acid, fluorosulfonic acid, or a combination thereof. In a preferred embodiment, the solution consists of hydrochloric acid and water. Advantageously, the concentration of the hydrochloric acid is at least 30% by weight of the solution. In preferred embodiments, the concentration is between 35% and 40% by weight of the solution. In preferred embodiments, the solution is applied via spin coating, spray coating, or dip coating. In preferred embodiments, the removal of excess solution comprising the acid begins very shortly after the solution is applied, such as within no more than 30 seconds, preferably no more than 5 seconds, and still more preferably no more than 1 second. Advantageously, heat may be applied to the multivalent metal electrode after applying the solution comprising an acid. Preferably, any excess solution is removed under vacuum conditions, preferably under a pressure of 100 millitorr or less, and/or at a temperature between 50° C. and 70° C. A second aspect of the present disclosure is drawn to an electrode having undergone this treatment. Specifically, the electrode comprises (i) at least one metal layer comprising a multivalent metal selected from Mg, Al, Ca, Zn, or a combination thereof; and (ii) an interphase layer external to the at least one metal layer, the interphase layer comprising a hydrated hydroxychloride, the hydrated hydroxychloride comprising the multivalent metal. The interphase layer will generally have a thickness less than 5 μm, preferably less than 2 μm, and more preferably between 0.1 μm and 1.5 μm. In some embodiments, the hydrated hydrochloride may have the formula ZnO—ZnCl 2 -2H 2 O or Zn(OH) 2 —ZnCl 2 —H 2 O or a related material assumed to have the same space group as ZnOHCl (Pbca), or otherwise have a crystal structure belonging to the orthorhombic crystal family. A third aspect of the present disclosure is drawn to an electrochemical cell for repeatedly plating and stripping an electrode, where the electrode underwent the disclosed treatment. For repeatedly plating and stripping at electrode. The electrochemical cell comprises an electrode as described above, at least one additional electrode, and an electrolyte. BRIEF DESCRIPTION OF DRAWINGS FIG.