(596d) Lithium Batteries Enabled By Robust Thin Film Composite Solid Electrolyte Separators

The meteoric growth in electrochemical energy storage research over the past decade has led to the discovery of solid-state lithium ion conductors with conductivities on par and even exceeding the conductivities of conventional liquid electrolytes. However, practical solid state lithium metal batteries have not yet been realized due to several challenges associated with fabricating and integrating solid electrolyte separators into electrochemical cells. Solid electrolyte layers with low specific surface areas and thicknesses on the order of 20 μm or less are required. One potential solution is the development of thin film composite separators where a dense solid electrolyte film is coated onto a porous scaffold. The function of the thin film electrolyte is to enable reversible cycling of Li metal by isolating it from a catholyte and inhibiting the growth of dendritic structures. The porous scaffold provides mechanical support for the thin film and hosts the catholyte within its porosity for efficient ion transport.

In this work, lithium phosphate oxynitride (Lipon) thin films on the order of 50 - 100 nm thick were deposited onto microporous polypropylene separators. The composite separator was robust and flexible – the Lipon film didn’t fracture as the separator was handled – which enabled 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. A low interfacial resistance between the Lipon and liquid electrolyte was observed. The total ASR of the composite separator was 40 Ω-cm² in alkyl carbonate electrolytes, which is lower than typical separators consisting of ceramic electrolytes. The Lipon layer was shown to be chemically and physically stable in several common liquid electrolyte formulations. The development of interfacial resistance and possible reaction products at the Lipon/liquid electrolyte was studied over extended storage durations in 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. The solid electrolyte membrane also was also shown to inhibit the polysulfide shuttle between anode and cathode in Li-S cells. The Li-S cell enabled by the new solid electrolyte membranes showed high coulombic efficiency and stable cycling performance. 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.