Rational Development of Neutral Aqueous Electrolytes for Zinc–Air Batteries

Abstract

Neutral aqueous electrolytes have been shown to extend both the calendar life and cycling stability of secondary zinc–air batteries (ZABs). Despite this promise, there are currently no modeling studies investigating the performance of neutral ZABs. Traditional continuum models are numerically insufficient to simulate the dynamic behavior of these complex systems because of the rapid, orders‐of‐magnitude concentration shifts that occur. In this work, we present a novel framework for modeling the cell‐level performance of pH‐buffered aqueous electrolytes. We apply our model to conduct the first continuum‐scale simulation of secondary ZABs using aqueous ZnCl2–NH4Cl as electrolyte. We first use our model to interpret the results of two recent experimental studies of neutral ZABs, showing that the stability of the pH value is a significant factor in cell performance. We then optimize the composition of the electrolyte and the design of the cell considering factors including pH stability, final discharge product, and overall energy density. Our simulations predict that the effectiveness of the pH buffer is limited by slow mass transport and that chlorine‐containing solids may precipitate in addition to ZnO.

A Review of Model-Based Design Tools for Metal-Air Batteries

“…For some systems, such as KOH−ZnO, this behavior is rather straightforward and well-documented [80,106], while for others, such as ZnCl 2 −NH 4 Cl, it is very complex and sensitive [3,107]. Figure 3a shows the speciation of the Zn 2+ ion in ZnCl 2 −NH 4 Cl.…”

“…A framework for such a model [112,113] was presented by Limpo, et al, in the 1990s, and was expanded upon in more recent research [106,114]. Clark, et al, recently presented a framework for predicting the discharge characteristics of a Zn-air cell with an aqueous near-neutral electrolyte based on equilibrium thermodynamic considerations [3], and showed how they can be incorporated into a dynamic model. Models of equilibrium thermodynamics can also be used to generate potential-pH, or Pourbaix, diagrams [115].…”

“…A detailed derivation of these terms based on non-equilibrium thermodynamics and their applicability to metal-air systems can be found in existing works [3,30,55,134]. In their most general form, the continuity equations describe the local conservation of mass and charge due to transport across the boundaries of a control volume and the presence of a source/sink within the bulk of the control volume.…”

“…While the fundamental components of electrolyte transport models are universal, their exact form can vary based on the ionic strength [116] and pH [3] of the electrolyte. For low ionic strength electrolytes, a simplified dilute solution theory (DST) approach can be applied to model the diffusion and migration transport of solutes [116].…”

“…Figure 1 compares the theoretical energy density and specific energy of metal-air systems. In some non-ideal cases the precipitation of the solid discharge product can consume active electrolyte components, reducing the achievable energy density [3]. Research into a variety of metal-air chemistries is ongoing.…”