Investigating the Effects of Lithium Phosphorus Oxynitride Coating on Blended Solid Polymer Electrolytes

Abstract

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 yeild a hybrid electrolyte that can syngergistically combine the properties of both materials. Chemical stability, thermal stability, and high mechanical moduli of ceramaic 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 eletrolyte 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 was 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 consisted of 100k molecular weight PEO, 50k molecular weight PPC, and 25(w/w) % LiTFSI, (denoted as PEO100PPC50LiTFSI25) which exhibited an ionic conductivity of 2.11x10-5 S/cm, and the ionic conductivity showed no thermal memory effects as the PEO crystallites were well distrupted by PPC and LiTFSI. Secondly, the effects of LiPON coating on the BPE were evaluated as a function of thickness down to 20 nm. The resulting bilayer structure showed an increase in the voltage window from 5.2 to 5.5V (vs Li/Li+) and thermal activation energies that approached the activation energy of the BPE when thinner LiPON layers were 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.