Multifunctional Zeolite Coated Separators for Improved Performance and Safety of Lithium Metal Batteries

Abstract

Lithium (Li) metal batteries are attractive due to their high gravimetric and volumetric energy densities. However, they can fail catastrophically due to dendritic nucleation, growth, and penetration through the polypropylene (PP) or polyethylene (PE) separators. Poor electrolyte wetting and non-uniform Li ion flux are known to affect Li dendrite formation, especially since the PP/PE separators have broad gaussian pore size distribution and typically organic electrolytes do not wet them well. In this work, we demonstrate that a multifunctional zeolite coating on a commercial PP separator (Z-PP) can improve electrolyte wettability and in-plane ionic conductivity, giving rise to more uniform Li flux. Consequently, Z-PP can effectively delay dendrite penetration and enhance cell performance and safety. Firstly, electrochemical impedance spectroscopy (EIS) was used to establish that the thin zeolite coating on PP separator had little effect on through-plane ionic conductivity. For electrochemical cycling behavior, PP and Z-PP separator based symmetric cells and half-cells were studied by monitoring the cell overpotential (symmetric cell) and discharge capacity (half-cell). For the Z-PP separator, lower overpotential and better discharge capacity retention were observed in the respective symmetric and half-cell studies, suggesting improved electrolyte wetting and relatively uniform Li flux. Scanning electron microscopy (SEM) equipped with energy dispersive x-ray spectroscopy (EDS) studies helped in understanding coating microstructure and cycled electrode morphologies. SEM images of cycled Li metal harvested from the Z-PP cells show much smoother Li surface for Z-PP cell as compared to PP cells. This work suggests that overall cell performance and safety can be improved by employing Z-PP separators.