Investigating the Effects of Lithium Phosphorous Oxynitride Lamination on PEO/PPC Blended Hybrid Electrolytes

Abstract

Solid-state electrolytes play an important role in the development of new generations of technology including lithium metal batteries. Lithium conducting solid materials offer a solution to the safety issues that are present in lithium-ion batteries (LIBs). Conventional LIBs typically use highly flammable organic solvents as a transport medium for lithium ions, potential risks that arise due to these solvents include fire, volatilization, and explosions. Solid polymer electrolytes (SPEs) are great as electrolyte material because fabrication of the electrolyte films is cheap, and processing is simple. However, SPEs can have stability issues when in contact with lithium metal for extended periods which is a problem for cells required to offer long lifetimes. Poly (ethylene oxide) (PEO) has been extensively studied since the conception of SPEs, due to its ability to solvate ions due to its polar ether groups. However, PEO, being a semi-crystalline polymer, exhibits thermal history effects due to recrystallization behavior. Amorphous SPEs are highly desirable because there are no recrystallization kinetics at play, and the properties are consistent regardless of the thermal history. Poly(propylene carbonate) (PPC), an amorphous, biodegradable polymer, can offer high ionic conductivities. Blending PEO and PPC offers a solution to resolve the thermal history effects of PEO by interfering with the crystalline structures in blended electrolyte systems with the salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Inorganic solid electrolytes (ISEs) can offer better chemical stability and excellent mechanical properties compared to SPEs; however, brittleness may decrease the structural stability over time. Thin films ISE’s also tend to cost much more than SPEs due to the fabrication methods required such as electron beam evaporation, and chemical vapor deposition. Lithium phosphorous oxynitride (LiPON), produced through RF magnetron sputtering, was developed in the 1990s at Oakridge National Laboratories and offers great electrochemical stability against lithium and relatively high ionic conductivities on the order of 10-6 S/cm at room temperature. A new bilayer lithium-ion conducting hybrid solid electrolyte (HSE) consisting of a blended polymer electrolyte (BPE) coated with a thin layer of the ISE LiPON, has been prepared. PEO and PPC are blended in a 1:1 mass ratio, then the salt LiTFSI is incorporated. BPE films are then produced through a facile solution casting method using acetonitrile as the solvent. Structure-property relationships are then systematically evaluated to understand the effects of molecular weight and mass loading of LiTFSI on the BPE systems. Influences of molecular weight and salt loading on crystallinity is observed though X-ray diffraction and optical microscopy of the BPE films. Ionic conductivity is evaluated through impedance spectroscopy between two stainless steel blocking electrodes, thermal history measurements were taken by heating and cooling the SPE while recording EIS at specified temperatures. The best BPE based on ionic conductivity, consists of 100k molecular weight PEO, 50k molecular weight PPC, and 25(w/w)% LiTFSI, (denoted as PEO100PPC50LiTFSI25) and exhibits an ionic conductivity of ~2x10-5S/cm. Temperature dependent EIS shows that the lowest molecular weight polymers offer lower activation energies as a result of the higher degree of free volume and molecular motion of the polymer chain. The optimized BPE system is then coated with various thicknesses of LiPON. The effects of LiPON coating are evaluated and show promise for the development of lithium stable solid electrolytes. Linear sweep voltammetry (LSV) is used to assess the anodic voltage stability against lithium metal. Incorporating very thin layers of LiPON down to 20nm on the BPE shows an increase in the voltage window of the electrolyte system from 5.2 to 5.5V (vs Li/Li+). However, the incorporation of LiPON decreases the ionic conductivity of the BPE. Thinner layers of LiPON show the ionic conductivity approaches that of the BPE, with thicker LiPON layers offering lower ionic conductivies. To assess the stability against lithium metal over time, impedance spectroscopy is used. Cells consisting of the solid electrolyte against lithium metal are prepared, and impedance measurements are taken over a 20-day period to assess the changes in charge transfer resistance. From these measurements, it has been shown that only a 30nm thin layer of LiPON is required to improve the stability of the resulting HSE. Therefore, coating BPEs with a thin layer of LiPON is shown to be an effective strategy to improve the long-term stability of solid-state lithium ion conductors.