(166ag) Investigating the Effects of Lithium Phosphorous Oxynitride Coating on Blended Solid Polymer Electrolytes

Solid state electrolytes are very promising to enhance the safety of lithium ion batteries. Two classes of solid electrolytes, polymer and ceramic, can be combined to yield a hybrid electrolyte that can synergistically combine the properties of both materials. For instance, the chemical and thermal stability and high mechanical modulus of ceramic electrolytes against dendrite penetration can be combined with the flexibility and ease of processing of polymer electrolytes. By laminating a polymer electrolyte with a ceramic electrolyte, the stability of the solid electrolyte is expected to improve against lithium metal, and the ionic conductivity could remain close to the value of the original polymer electrolyte as long as an appropriate thickness of the ceramic electrolyte is applied. Here we report a bilayered lithium-ion conducting hybrid solid electrolyte consisting of a blended polymer electrolyte (BPE) laminated with a thin layer of the inorganic solid electrolyte lithium phosphorous oxynitride (LiPON). The hybrid system is thoroughly studied. First, we investigated the influence of polymer chain length and lithium salt ratio on the ionic conductivity of the BPE based on poly(ethylene oxide) (PEO) and poly(propylene carbonate) (PPC) with the salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The optimized BPE consists of 100k molecular weight PEO, 50k molecular weight PPC, and 25(w/w) % LiTFSI, (denoted as PEO100PPC50LiTFSI25) which exhibits an ionic conductivity of 2.11x10^-5S/cm, and the ionic conductivity shows no thermal memory effects as the PEO crystallites are well disrupted by PPC. Secondly, the effects of LiPON coating on the BPE were evaluated as a function of thickness down to 20 nm. The resulting bilayer structure shows an increase in the voltage window from 5.2 to 5.5V (vs Li/Li⁺) and thermal activation energies that approach the activation energy of the BPE when thinner LiPON layers are used, resulting in similar ionic conductivities for 30nm LiPON coatings on PEO100PPC50LiTFSI25. Coating BPEs with a thin layer of LiPON is shown to be an effective strategy to improve the long-term stability against lithium of the developed solid-state lithium ion conductors.