Lithium Conducting, Polar-Polar Polymer Blend Electrolytes with Systematically Varied Mechanical Properties and Ion Conductivity
Authors: Sun Che-Nan , Zawodzinski Thomas A. , Ren Fei , Keum Jong Kahk , Chen Jihua 
Published: February 2020

Highly polar polymers such as PEO (polyethylene oxide)1-5 and PVDF (polyvinylidene difluoride)6-10 are well known as potential candidates for solid -state and gel-type electrolytes in rechargeable lithium-ion batteries, although the high crystallinity of these semi-crystalline, polar polymers leads to large polymer crystallites or crystalline domains which significantly lower their room-temperature ion conductivities.1-10 Hybrid polar-polar polymer blends, on the other hand, were previously reported to reduce the crystallinity of both polymer component, if a proper window of blend ratio is chosen. 11-13 This approach is more favored than an inert additive approach, in which inactive components that are not beneficial to ion transport are introduced to the electrolyte systems. Abraham, et al., reported the high ion conductivity of a polymer blend electrolyte made up with poly(vinylidene fluoride-co-hexafluoropropylene) or PVDF-HFP, oligomeric poly(ethylene glycol) dimethyl ether and lithium salts. In addition, Han, et al, studied ion transport of nanoparticle modified PEO/PVDF blends for dye-sensitized solar cell13. Several other reports examined ion transport behaviors in PEO/P(VDF-HFP)/lithium perchlorate (LiClO4) blend electrolytes.12
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14
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15 However, none of these reports provided a systematic and detailed nanostructure-ion transport correlation of polar-polar polymer hybrid electrolytes.

Herein we studied PEO-PVDF- LiTFSI (lithium bis(trifluoromethanesulfonyl) imide) based hybrid electrolytes with systematically tunable mechanical properties and room temperature ion conductivities. We examined the nano- morphology dependent ion transport of these important type of composite electrolytes and their behaviors in solid polymer electrolyte systems. Based on results from a variety of experiments including nanoidentation, wide-angle X-ray diffraction, energy filtered TEM, and temperature-dependent impedance spectroscopy, we demonstrate that the low temperature ion conductivity of PEO/PVDF/LiTFSI blend can be 3 times higher than their parent electrolytes because of the largely optimized nanomorphology and mitigated polymer crystallinity. At the same time, we also show that, the modulus and hardenss of these blend electrolytes can be systematically tuned with the polymer blend ratio.

(The attached energy filtered composite TEM images show oxygen, fluorine, and nitrogen distribution as a function of PVDFwt% in PEO/LiTFSi electrolytes.)

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Acknowledgements

A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy.