Zinc Oxide Nanoplatelet-Coated Polypropylene Separators as a Bifunctional Tool for Enabling Dendrite-Free Lithium Metal Batteries: A Binder-Free Approach

The commercialization of lithium metal as an anode is challenged by its poor electrochemical reversibility and short cycle life. Both issues stem from the fragile solid electrolyte interphase (SEI) and dendritic growth during continuous stripping-plating cycles. Here, we report ZnO-decorated polypropylene separators as a bifunctional tool to suppress or mitigate dendritic proliferation. The ZnO-coated separators, prepared via a binder-free strategy, delivers two benefits. First, the formation of a Zn-rich in situ artificial SEI is expected due to the spontaneous reduction of ZnO on the lithium surface. Second, the lithiophilic ZnO will provide a uniform ion flux to counter Li+ depletion/nonuniform Li+ distribution near the anode surface. The modified separators were explicitly characterized to support the role of these benefits. The ZnO-coated separators exhibited improved wettability, higher electrolyte uptake percentage, improved ionic conductivity, larger transference number, and higher exchange current density. During electrochemical characterization, a substantially lower nucleation overpotential of 70 mV, superior cycle life of more than 500 cycles at a current density of 1 mA/cm2 in aggressive carbonate solvents, and better reversibility at elevated current densities up to 5 mA/cm2 were demonstrated by ZnO modified separators. With comprehensive electrochemical and postcycling characterization, it is shown that ZnO-coated separators are potentially promising for mitigating dendritic growth and improving interfacial instability, as observed from the HR-SEM and EDS analysis. The in situ formation of Zn and Li x Zn y SEI was confirmed from the postcycling XRD and XPS analysis of the metallic anode. Ascribed to the synergistic effect of lithiophilicity and in situ formation of anion-dominant, inorganic-rich SEI, the proposed strategy enabled stable lithium metal deposition-stripping behavior and can further be applied for anode-free batteries.