Nuclear Magnetic Resonance (NMR) Characterization of a Polymerized Ionic Liquid Electrolyte Material
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Nuclear Magnetic Resonance (NMR) Characterization of a Polymerized Ionic Liquid Electrolyte Material A.L. Michan1, G.T.M. Nguyen2, O. Fichet2, F. Vidal2, C. Vancaeyzeele2 C.A. Michal1 1 Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada. 2 Laboratoire de Physicochimie des Polymères et des Interfaces (LPPI), Institut des Matériaux, University of Cergy-Pontoise, France. ABSTRACT Solid electrolyte materials have the potential to improve performance and safety characteristics of lithium-ion batteries by replacing conventional solvent-based electrolytes. A candidate solid polymer electrolyte, AMLi/PEGDM, has been synthesized by crosslinking an anionic monomer AMLi, with poly(ethylene glycol) dimethacrylate. The main goal of the synthesis is to produce a single-ion conducting polymer network where lithium cations can move freely and fluorinated anions are immobilized as part of the polymer network. A comprehensive characterization of anion and cation mobility in the resulting material is therefore required. Using pulsed-field gradient nuclear magnetic resonance (PFG-NMR), we are able to measure and quantify the individual diffusion coefficients of mobile species in the material (19F and 7Li) and confirm the extent to which the fluorinated anionic component is immobilized. We have characterized dry (σ~3.0 x10-7 S/cm at 30°C) and propylene carbonate (PC) saturated gel (σ~1.0x10-4 S/cm at 30°C) samples. Experimental results include NMR spin-spin and spin-lattice relaxation times in addition to diffusion coefficient measurements over a temperature range up to 100°C. Practically, the diffusion measurements are extremely challenging, as the spin-spin (T2) relaxation times are very short, necessitating the development of specialized pulsed-field gradient apparatus. Diffusion coefficients for the most mobile components of the lithium cations and fluorinated anions at 100°C in dry membranes have been found to be 3.4 x10-8 cm2/s and 2.1 x10-8 cm2/s respectively. These results provide valuable insight into the conduction mechanisms in these materials, and will drive further optimization of solid polymer electrolytes. INTRODUCTION Solid polymer membranes have the potential to improve lithium-ion battery safety by replacing conventional liquid electrolytes with a relatively non-volatile alternative. During battery cell operation, concentration gradients develop as lithium cations shuttle between the electrodes and across the electrode/electrolyte interfaces. Mobile anions can accumulate and take part in unwanted side reactions, impeding current flow [1]. Single-ion conductive polymers are therefore expected to deliver more current than bi-ionic conductors. A candidate solid polymer electrolyte, AMLi/PEGDM, has been synthesized by crosslinking an anionic monomer AMLi, Fig 1, with poly(ethylene glycol) dimethacrylate (PEGDM, M=750 g/mol) by free-radical polymerization initiated with AIBN in ethanol. The AMLi lithium pair of the anionic monomer (AMLi) was synthesized using the precursor AMH, described previously
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