(Invited) Lithium Batteries Enabled By Thin Film Composite Solid Electrolyte Separators
Authors: Hu Fei , Ioanniti Marina , Wang Shaofei , Tenhaeff Wyatt E. 
Published: May 2020

Over the past decade, solid-state lithium ion conductors with conductivities on par and even exceeding the conductivities of conventional liquid electrolytes have been developed. However, practical solid-state lithium metal batteries have not yet been realized due to challenges associated with fabricating and integrating solid electrolyte separators into electrochemical cells. To achieve the power metrics and effective energy densities of existing lithium ion cells, solid electrolytes must be fabricated with comparable area specific resistances and thicknesses on the order of 10 μm, without a significant increase in cost. Thin film composite separators, where a dense solid electrolyte film is coated onto microporous membranes (separators), are one possible solution. The solid thin film electrolyte enables reversible cycling of Li metal by isolating it from the catholyte and inhibiting growth of dendritic structures, while the porous scaffold mechanically supports the thin film and hosts the catholyte within its porosity for efficient ion transport.
In this work, lithium phosphate oxynitride (Lipon) thin films with thicknesses on the order of 50 - 100 nm were deposited onto microporous polypropylene separators. These composite separators were robust and flexible; the Lipon film didn’t fracture as the separator was handled, enabling the integration into standard lithium cells, such as coin cells. Due to its thinness, the area specific resistance (ASR) of the solid electrolyte layer was 5 - 10 Ω-cm2, and a relatively low interfacial resistance of 2.5 Ω-cm2 between the Lipon and liquid electrolyte was observed. The Lipon layer was shown to be chemically and physically stable in several common liquid electrolyte formulations. Negligible evolution of interfacial resistance and reaction products at the Lipon/liquid electrolyte were observed after extended exposure to these electrolytes. In Li cycling tests, the Lipon layer promoted smooth, dense Li morphologies in electrodeposits with areal capacities exceeding 15 mAh cm-2. Without the dense Lipon layer, the morphology of the Li electrodeposits was rough – consistent with “mossy” lithium morphology typically observed in liquid electrolytes. In operando impedance spectroscopy showed that a stable resistance of the Lipon bulk was maintained during Li plating, suggesting that the Lipon layer remains intact. The solid electrolyte membrane was also shown to inhibit the polysulfide shuttle in Li-S cells, preventing the reaction of the polysulfide with Li metal. The Li-S cell using the composite separators showed high coulombic efficiency and stable cycling performance. In LiMn2O4-Li cells, the thin film composite separators protected the Li metal from reactions with Mn ions dissolved in liquid electrolyte. The development of these composite solid electrolyte separator offers a promising path to energy dense lithium metal batteries. This work was supported by the ARPA-E IONICS program, U.S. Department of Energy, award DE-AR0000775.