Characterizing Zn Corrosion in Printed Zn-Air Batteries Via Operando Dems, XRD, and XAS
Authors: Kumar Rajan , Kedzie Elyse , McCloskey Bryan D. , Subramanian Vivek 
Published: February 2020

Developing low cost energy storage for integrated electronics depends on the ability to increase energy densities while reducing materials and manufacturing costs. Among the several next generation battery chemistries currently being explored, Zn-air batteries are well suited to tackle these challenges and power a wide variety of electronics applications. Zn-air batteries provide a high theoretical energy density, exhibit high performance at high discharge rates, and use low cost, earth abundant materials. Additionally, Zn-air batteries utilize aqueous, non-flammable electrolytes and can be processed entirely in air, making them an emerging candidate to replace Li-ion batteries.

Despite their significant promise, side reactions at the Zn anode limit Zn-air battery performance and reduce practical energy densities. Zn corrosion follows a complex series of intermediate reactions in an alkaline environment and is highly dependent on properties of the electrolyte (OH- concentration and use of additives), properties of the electrode (film porosity and active particle size), and cell operating conditions (temperature, current density, depth of discharge, and cell geometry). Thus, each of these factors can alter the fundamental Zn corrosion mechanism and need to be studied systematically to understand their influence on Zn-air battery stability and performance. Previous studies on Zn corrosion have identified single factors that influence reactions at the anode, but typically rely on ex-situ characterization methods that are not representative of the transient and non-equilibrium nature of electrochemical interfaces.

In this work, we investigate corrosion in printed Zn-air batteries through the use of operando characterization techniques including differential electrochemical mass spectroscopy (DEMS), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). This approach couples electrochemical measurements with chemical and structural information to directly probe the reaction products at an electrochemical interface and observe non-equilibrium reactions in real time. Moreover, Zn-air batteries are rapidly processed using additive manufacturing in order to investigate several experimental factors in parallel and determine their impact on Zn corrosion. Using this combined approach, we aim to understand which experimental factors significantly influence Zn corrosion in order to identify methods for mitigating self-discharge and improving Zn utilization in Zn-air batteries.

DEMS experiments, coupled with operando pressure decay analysis, are used to quantify hydrogen evolution rates at the Zn anode and are used to compare corrosion rates across various cell designs. Through this technique, we examine the effects of current collector type, electrode mass loading, electrode size, and electrolyte concentration on Zn corrosion rates. Based on our results, electrolyte concentration and current collector material have the greatest influence on corrosion rate among the factors studied, while electrode size and electrode mass loading have little effect on corrosion rates. Through operando XRD, we confirm the formation of a passivating oxide layer at the Zn anode and observe that higher electrolyte concentrations lead to faster passivation of the anode under both discharge and open-circuit conditions. Cell potential is also measured during these experiments and is correlated to the fractional amounts of Zn and oxide species in the anode, suggesting that cell potential can be used as a marker for battery lifetime. In addition, operando XAS experiments show characteristic shifts in the Zn K-edge during the passivation process, corresponding to a change in the Zn valence state during oxidation. These shifts are highly dependent on electrolyte concentration, but show little dependence on electrode geometry or mass loading.