The Science

Polymer additives, such as polyethylene oxide (PEO), are widely used to enable smooth electrode deposition for electrochemical systems, including zinc-based batteries. However, exactly how these additives regulate morphology is unclear. Researchers used a combination of in situ electrochemical atomic force microscopy and simulations to study the 2D electrodeposition of zinc (Zn) onto copper (Cu). They found that the PEO affects the system by tuning the energy of the Zn/Cu/electrolyte interface and slows the diffusion of Zn ions to the interface while maintaining minimal interaction with growing Zn crystals. Combined, these effects direct smooth layer-by-layer deposition.

The Impact

Electrodeposition is an approach to fabricating films of functional materials with tightly controlled atomic structures and orientations on a substrate. Additionally, controlled electrodeposition is central to directing cyclic deposition/stripping processes in memristors and energy storage devices. This work provides interlayer design principles, based on molecular-level mechanistic insights, for the dynamic control of interface topography during electrodeposition processes and electrodeposition-based synthesis of hierarchical structures across multiple types of materials.

Summary

Controlling the layer-by-layer electrodeposition is critical for the reliable and safe operation of energy storage devices and the synthesis of functional materials. However, the precise mechanisms of how films electrodeposit remain unclear. This study focused on determining the mechanism of 2D electrodeposition at metal surfaces modified with PEO. The team used in situ electrochemical atomic force microscopy to directly observe interfacial evolution during Zn electrodeposition on a Cu substrate in the presence of varying concentrations of PEO. Contrary to previous assumptions, which have focused on binding to Zn, these results demonstrate that the polymer smooths Zn films by promoting nucleation of (002)-oriented Zn platelets through interactions with the Cu substrate. Simulations based on quantum and classical density functional theory revealed that PEO tunes the interfacial energy and the kinetics of Zn²⁺ ion diffusion to the interface. Through a thermodynamic effect, polymer adsorption on Cu modifies the interfacial energy of Zn/Cu/electrolyte interfaces and favors the stabilization of Zn (002) on the Cu substrate. From a kinetic standpoint, the organic adsorbate creates a dielectric barrier that confines Zn electrodeposition to a narrow near-surface region and promotes the layer-by-layer growth of flat-lying plates. These findings establish a novel design principle for metal/additive interfaces for electrode smoothing, emphasizing the importance of substrate selection paired with additives that exhibit an attractive interaction with the substrate but minimal interaction with growing crystals and offer a mechanistic perspective for improved battery performance.

Contact

Maria Sushko, Pacific Northwest National Laboratory, maria.sushko@pnnl.gov

Funding

This work was supported by the Department of Energy (DOE), Office of Science, Basic Energy Sciences program, Materials Sciences and Engineering Division, Synthesis and Processing Science Program (FWP 12152). The AFM and SEM were conducted at Pacific Northwest National Laboratory and the University of Washington’s Molecular Analysis Facility (MAF). MAF is a National Nanotechnology Coordinated Infrastructure site at the University of Washington, supported in part by the National Science Foundation (grant NNCI-1542101), the University of Washington, the Molecular Engineering & Sciences Institute, and the Clean Energy Institute. The PiFM was conducted at the University of Washington MAF. TEM imaging, FTIR, and XPS were performed in the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility supported by the Biological and Environmental Research program. Simulations were performed using resources of the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. Work by CO was performed under the auspices of the DOE by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. J. S. D. acknowledges support from a Washington Research Foundation Postdoctoral Fellowship.