Polymer Electrolytes for Lithium-Metal Batteries: PFG Diffusion and Relaxation NMR Measurements to Study Ionic Transport Properties Tan-Vu Huynh,1 Robert Messinger,1 Vincent Sarou-Kanian,1 Renaud Bouchet,2,3 Michaël Deschamps1, Trang Phan4 CEMHTI, CNRS UPR 3079, Orléans University, Orléans, France LEPMI, CNRS UMR 5279, Grenoble University, Grenoble, France 3 MADIREL, CNRS UMR 7246, Aix Marseille University, Marseille, France 4 ICR, CNRS UMR 7273, Aix Marseille University, Marseille, France 1 2 Lithium polymer batteries have many advantages, as they withstand the contact of reductive lithium metal and benefit from the large capacity and higher operating voltage provided by the lithium metal. As liquid-state electrolytes can leak and catch fire or react easily with reactive electrode materials, electrolytes made with polyethylene oxide (PEO) with added lithium trifluorosulfonylimide salts (LiTFSI) are used as a substitute even if they intrinsically display lower lithium conductivities.1 However, lithium-polymer batteries are prone to short circuits and explosion: dendrites tend to appear on the lithium metal surface during charging, creating a short circuit through the polymer separator between the negative electrode (metallic lithium) and the positive electrode. To increase the mechanical strength and prevent dendrite formation, M. Singh et al.1 showed that block copolymers with PEO and polystyrene (PS) could provide the desired increase in mechanical strength and prevent dendrite formation. In our research, we focused on characterizing and understanding the differences in ionic dynamics and transport properties between the PEO(LiTFSI) and tri-block PS-b-PEO(LiTFSI)-b-PS polymer electrolytes. In particular, we conducted variable temperature 7Li and 19F Pulsed Field Gradient (PFG) and 7Li, 19F and 1H relaxation NMR measurements to characterize the mobilities of the Li + and TFSI- anions and of the polymer chains. PFG experiments were also combined with magic-angle spinning (MAS) to retrieve self-diffusion coefficients at lower temperatures, using the Bruker Micro-2.5 triple axis gradient system that generate 170 G/cm along the MAS axis of a 3.2 mm double resonance MAS probe in our 750 MHz WB Bruker spectrometer. In our samples, the PEO and PS domains are phase separated and the diffusion of lithium occurs through the ca. 5 nm wide PEO channels. Using PFG diffusion measurements, probing self-diffusion on a constant characteristic length scale of 2 m, we have been able to retrieve the activation energies of the Arrhenius-like diffusion behavior, the ionic transport numbers and the tortuosity of the PEO network in the tri-block copolymer and to compare them with data obtained from conductivity measurements. 7Li longitudinal relaxation (T1) can be interpreted with a simple model to retrieve the correlation times of the fluctuations of the lithium environment (Figure) and compare them with results obtained from molecular dynamics simulations2 or quasielastic neutron scattering.3 Figure: 7Li and 19F self-diffusion coefficients and longitudinal T1 relaxation times, obtained by PFG and inversion recovery experiments, respectively, of the PEO(LiTFSI) and the PS-b-PEO(LiTFSI)-b-PS polymer electrolytes. 1. M. Singh et al., Macromolecules 40, 4578-4585 (2007). 2. O. Borodin et al., Macromolecules 40, 1252-1258 (2007). 3. G. Mao et al., Nature 405, 163 (2000).