Thermally and Oxidatively Stable Polymer Electrolyte for Lithium Batteries Enabled By Phthalate Plasticization

Abstract

Solid-state lithium batteries are desirable alternatives to lithium ion batteries given their high theoretical energy densities associated with Li metal anodes and the potentially superior safety from the elimination of flammable liquid electrolytes. Compelling ceramic lithium conductors with ionic conductivities surpassing those of conventional liquid electrolytes have been developed, but they suffer from high fabrication and processing costs. Moreover, achieving intimate solid-solid contact at the electrolyte-electrode interface is challenging. Polymer electrolytes are alternative materials that offer processing ease and the ability to fully infiltrate and wet composite electrodes. Poly(ethylene oxide) (PEO) mixed with lithium salt is the archetypical polymer electrolyte and solid-state cells using LiFePO4 cathode and lithium metal anode have been successfully commercialized. However, a significant limitation of PEO is its low oxidative stability, which restricts its application to lower voltage cathode (e.g. LiFePO4). Thus, polymer electrolytes with higher oxidative stabilities are desperately needed. In previous work, our group investigated hydrogenated nitrile butadiene rubber (HNBR) as a candidate for high-voltage application because of its superior oxidative stability of 5.3 V vs. Li/Li+. This polymer readily dissolves lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt and has a high lithium transference number (0.59 compared to 0.37 for PEO). However, HNBR-LiTFSI suffers from low ionic conductivity (3.6×10-8 S·cm-1 at 20°C), which severely limits transport in full solid-state cells. In this work, phthalates were employed as the plasticizer to increase the conductivity of the HNBR-based electrolyte. FTIR shows that phthalates not only enhance the segmental mobility of the polymer chains but also coordinate with Li+ and participate in the solvation of lithium salt. Phthalates have a broad liquid-phase window preventing potential phase separation in the polymer electrolyte and evaporation at elevated temperatures. Their low HOMOs (e.g. -10.7 eV for di-ethyl-phthalate) should translate to higher oxidative resistance. A series of di-alkyl-phthalate homologues differing in the length of the alkyl branches were investigated in this report. Plasticized polymer electrolytes were prepared by blending HNBR-LiTFSI and a given amount of phthalate in acetone solution, and then casting into a mold and evaporating the volatile acetone. The plasticized electrolyte can withstand 159°C without significant evaporation; the decomposition of HNBR occurs at over 400°C even in a pure O2 environment. The conductivity of HNBR-LiTFSI with di-ethyl-phthalate (up to 20 wt.%) was 7.3×10-7 S·cm-1 at 20°C and 4.4×10-5 S·cm-1 at 70°C. The electrochemical oxidative stability of both HNBR-LiTFSI and plasticized-HNBR-LiTFSI was assessed by linear sweep voltammetry. The oxidation potential was reduced from 5.3 V to 5.2 V vs. Li/Li+ after introducing phthalates. Electrochemical impedance analysis showed that a resistive interface developed between HNBR-based electrolytes and lithium, indicating a chemical incompatibility between HNBR and lithium metal. However, this problem was readily addressed through a laminated electrolyte structure where a layer of PEO/LiTFSI physically separated plasticized HNBR/LiTFSI from direct contact with lithium metal. The effectiveness of this structure was confirmed by over 2000 hours of reversible galvanostatic cycling of symmetric lithium metal electrodes at 70°C.