Revealing the Effects of Polymer Additives on Zn Dendrite Suppression in Aqueous Zn Batteries via In-Situ EC-AFM

Polyethylene glycol (PEO) is a commonly used polymer for achieving flat and uniform electrodes to enhance the performance of batteries, such as those based on lithium or zinc (Zn). However, the impact of PEO on the electrochemical deposition of Zn metal on electrodes and the mechanism by which it maintains electrode flatness remains uncertain. In this study, we addressed these knowledge gaps by using <i>in situ</i> electrochemical atomic force microscopy (EC-AFM) to observe the nucleation and growth of Zn metal plates on copper (Cu) substrates in the presence of different concentrations of ZnSO₄ and PEO. Here the ZnSO₄ solution provided the electrolyte and the Cu substrates served as the electrodes, both of which are widely utilized in Zn batteries.<br/>Our results indicate that PEO biases the crystallographic orientation of the initially deposited Zn metal nuclei, but does not have an obvious influence on subsequent growth of the resulting Zn platelets. The consistent aspect ratio of the Zn plates combined with the lack of an effect on growth rates suggests that PEO does not interact significantly with the surface of the newly formed Zn plates. High-speed and high-resolution <i>in-situ</i> AFM, along with <i>in-situ</i> Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) reveal that Zn metal undergoes rapid surface reorganization in a mildly acidic aqueous solution due to oxidation, which is not affected by PEO adsorption. Adhesion force maps, obtained through contact fast force mapping under flowing AFM, demonstrate real-time PEO adsorption and distribution on Cu and Zn surfaces, confirming a strong Cu-PEO interaction and a weak PEO-Zn interaction after oxidation. Based on these findings, we hypothesize that PEO primarily interacts with the Cu substrate to adjust the interfacial structure and energy of the Cu-electrolyte interfaces. To test this hypothesis, we conducted molecular dynamics (MD) simulations to simulate the electric double-layer structure in the presence and absence of PEO. We also calculated the Cu-Zn and Zn-solvent interfacial energies under both conditions.<br/>Our findings provide a clear picture of how PEO flattens the electrode: PEO first adsorbs onto the Cu substrate, then Zn metal nucleates as Zn (002) beneath the PEO, followed by PEO detachment as the Zn nuclei grow. Subsequently, the Zn platelets grow layer-by-layer using the first layer of Zn (002) as a template. The results suggest key design and engineering principles for flat electrode synthesis in energy-related applications, emphasizing the use of polymer additives that exhibit appropriately strong binding to the substrate metal and weak binding to the deposited metal.<br/>Work by C.O. was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.