Multilayer Separator for Lithium Batteries
Authors: de Vries Henrik , Joost Mario , Passerini Stefano 
Published: February 2020

Lithium metal provides the highest specific energy density and would be a perfect anode material for a lithium ion battery [1]. But so far only polymer batteries based on PEO show a stable interaction with lithium metal [2-3]. However, these electrolytes feature only a low ionic conductivity (up to 10-3 S cm-1) at room temperature. To use lithium metal in combination with liquid electrolytes, a thin layer (10 μm) of polyethylene oxide (PEO) was coated on conventional separators. A simple, solvent free technique was used to combine the polymer electrolyte to the separator (patent pending). To enhance the mechanical stability during the swelling with electrolyte, the polymer was cross-linked after the coating step [4].

An application of the polymer/separator system is possible with established positive electrode materials like lithium iron phosphate (LFP) or lithium nickel manganese cobalt oxide (NMC) to enhance the energy density of the system. Even more promising is the use in next generation battery systems like lithium air and lithium-sulfur. The thin layer of PEO optimizes the lithium-electrolyte interface and therefore acts as a protective layer against parasitic reactions like electrolyte decomposition [5]. In Li-O2and Li-S batteries, the PEO could additionally act as a barrier against, respectively, oxygen or polysulfide diffusion to the lithium anode.

The PEO layer was found to be mechanically stable during cell assembling and electrolyte application. Due to the cross-linking, even an excess of electrolyte does not dissolve the polymer. By chemical bonding between the PEO and polyethylene (PE) or polypropylene (PP) a loss of connection between the two separator layers is prevented. Partly impregnation of PEO into porous ceramic separators during the coating process shows the same effect after cross-linking. Differential scanning calorimetry (DSC) tests show that PEO and ionic liquids form an amorphous layer on the lithium surface, similar to ternary polymer electrolytes [6].

To verify the electrochemical stability of the PEO on the lithium metal surface, stripping-plating tests in symmetrical lithium cells were performed with a broad range of electrolytes. For established cathode materials, carbonate-based solvents like ethylene carbonate (EC) and dimethyl carbonate (DMC) with lithium hexafluorophosphate (LiPF6) were investigated. For next generation batteries or cycling at higher temperatures, tests with tetraethylene glycol dimethyl ether (TEGDME), 1,3-dioxolane and ionic liquids like 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were performed. Finally, cycling tests with NMC and sulfur cathodes were performed.

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