Towards rechargeable zinc–air batteries with aqueous chloride electrolytes

Abstract

This paper presents a combined theoretical and experimental investigation of aqueous nearneutral electrolytes based on chloride salts for rechargeable zinc-air batteries (ZABs). The resilience of near-neutral chloride electrolytes in air could extend ZAB lifetime, but theory-based simulations predict that such electrolytes are vulnerable to other challenges including pH instability and the unwanted precipitation of mixed zinc hydroxide chloride products. In this work, we combine theory-based simulations with experimental methods such as full cell cycling, operando pH measurements, ex-situ XRD, SEM, and EDS characterization to investigate the performance of ZABs with aqueous chloride electrolytes. The experimental characterization of near-neutral ZAB cells observes the predicted pH instability and confirms the composition of the final discharge products. Steps to promote greater pH stability and control the precipitation of discharge products are proposed. IntroductionRechargeable zinc-air batteries (ZABs) are a promising post-Lithium-Ion battery technology 1-3 for applications ranging from renewable energy storage, to electric vehicles 4 and flexible electronics 5 . Current state-of-the-art ZABs feature an alkaline electrolyte like KOH for its high conductivity, good electrochemical reaction kinetics, and moderate Zn solubility 6,7 . Unfortunately, the absorption of CO 2 from air into the electrolyte leads to the parasitic formation of carbonates (CO 2 -3 ), which slowly poisons the electrolyte 8,9 . For this reason, the lifetime of alkaline ZABs is cut short by a few weeks of continuous exposure to air.Engineering solutions to the carbonation challenge have been proposed 10 . The use of CO 2 filters to scrub the feed-gas could delay the onset of carbonation 11 , but to reach competitive lifetimes, the CO 2 concentration would need to be reduced by two ordersof-magnitude 8 . Mechanically rechargeable Zn-air fuel cells and Zn-air flow batteries, in which the electrolyte is routinely replaced, have also been demonstrated 12-17 . These solutions are effective for some applications, but they add cost and complex-Aqueous electrolytes with near-neutral pH values are resilient towards carbonation and could improve ZAB lifetime 18 . The most common near-neutral electrolyte (NNE) is ZnCl 2 −NH 4 Cl, which has been used in zinc-based LeClanché batteries for over 100 years 19,20 . In this tradition, we refer to zinc-air batteries with aqueous ZnCl 2 −NH 4 Cl as LeClanché zinc-air batteries (L-ZABs).The L-ZAB concept was first proposed in the 1970s 21 , but it has only recently become a broadly pursued topic in industry and research. Start-up companies are beginning to commercialize L-ZAB technology for grid-scale stationary applications 22 , and recent experimental research 23,24 has verified the favorable cycling stability and lifetime of these systems. Although the future of L-ZABs is hopeful, there are some factors limiting their further development.We recently performed a theoretical investigation of L-ZAB cell operatio...

Theory of Impedance Spectroscopy for Lithium Batteries

“…We describe the interface reaction rate j I with the linearized Butler-Volmer expression given by eq. (11). In the electroneutral model, we evaluate the electrochemical potential at the interface δϕ bulk = δϕ(L).…”

Designing Aqueous Organic Electrolytes for Zinc–Air Batteries: Method, Simulation, and Validation

“…Table 2 lists the properties of the as‐prepared electrolyte compared with previously reported values for aqueous ZAB electrolytes KOH and NH 4 Cl–ZnCl 2 . The properties of the proposed electrolyte are measured at three pH values: 8, 9, and 12.…”

“…Ammonium, NH 4 + , is one of the most common pH buffers and requires a negatively charged counter‐ion, often a halide like Cl − , Br − , F − , or I − . The solubility of zinc‐halide‐hydroxide solids in the near‐neutral pH regime is low, which suppresses ZnO as the dominant discharge product . Furthermore, halide anions in electrolyte solutions are known to corrode non‐noble metals and poison Pt/C oxygen reduction reaction (ORR) catalyst materials, and elemental halogens, especially Cl 2 and F 2 , are toxic.…”

“…The first is a thermodynamic model of ion speciation and solubility, based on the law of mass action. The modeling method is derived in existing works . The second is a cell‐level continuum model based on the quasi‐particle method derived in our previous works .…”

“…The modeling method is derived in existing works . The second is a cell‐level continuum model based on the quasi‐particle method derived in our previous works . Assuming that the homogeneous acid–base and Zn‐complexing reactions in the electrolyte occur instantly, the quasi‐particle continuum model supports the efficient calculation of the dynamic concentration profiles in the electrolyte and gives insight into the pH stability in the cell during operation.…”