(565c) Solid State Electrolytes with Low-Temperature Plasticizer Optimization

Solid State Batteries (SSBs) are the next generation of commercial energy storage devices, as needs for battery innovation grow, owing to their safety, high energy density, wider potential stability, and high mechanical strength. However, SSBs have had historically poor performance at subzero temperatures, due to poor interfacial contact, charge transfer kinetics, and freezing of electrolyte. A solid-state electrolyte with good performance at low temperatures is of great interest to space and defense applications. PVDF-based polymer electrolytes have been recently explored as polymer matrixes for electrolytes due to their excellent stability, yet have been less explored compared to PEO^1,2,3. Furthermore, it has been shown that a garnet-type ceramic, LLZTO, optimized at 5% wt. in a composite polymer electrolyte (CSPE), improves the ionic conductivity and thermal stability⁴. Here, we present a systematic mechanistic elucidation of a wide-range Quasi Solid-State Electrolyte (QSSE) optimized for low-temperature operation (-25 C to 25 C) with a binary solvent system via modification of a polar aprotic low-melting plasticizer as a wetting agent. Additionally, we provide insight into the transport phenomenon regulating the low temperature performance at varying concentrations of LiTFSI salt in the wetting agent and composite polymer film.

We show that a 2M LiTFSI/THF wetting agent introduced into the DMF/PVDF/LiTFSI/LLZTO composite electrolyte displays 146 mAh/g capacity in a Li|CSPE|LFP battery with 99% CE at room temperature. Low temperature studies show 95 mAh/g capacity at -20 C with 99.9% CE, and 85 mAh/g at -25 C with 96% CE. The ionic conductivity is reported at 1.2 x 10^-3 s/cm at 25 C without ambient heating. We also report a transference number of 0.42. An in situ-safety study also demonstrated improved thermal safety with lower exothermic heat flux and more stable thermal degradation relative to traditional commercial liquid electrolytes. Additionally, we show the ionic conductivity profiles and performances at varying temperatures from 25 C to -30 C, providing activation energies and Arrhenius behavior. Finally, density functional theory (DFT) calculations and molecular dynamics (MD) simulations provide potential insight into stable configurations and mechanisms governing the complex system.

This work precedes further exploration into a low temperature QSSE, with exploration into alternative methods to polymer wetting such as in-situ polymerization and solution casting with higher boiling organic co-solvents, such as CPME. Ionic liquids are also a potential direction for nonvolatile solvents towards low temperature function. Further DFT and MD work and other transport characterization in these directions can magnify the research community’s understanding of low temperature challenges for solid-state electrolytes.

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