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Polymers 2018, 10(12), 1350; https://doi.org/10.3390/polym10121350

Boron Trifluoride Anionic Side Groups in Polyphosphazene Based Polymer Electrolyte with Enhanced Interfacial Stability in Lithium Batteries

Institute of Inorganic and Analytical Chemistry, University of Münster, Corrensstraße 28/30, 48149 Münster, Germany
These authors contributed equally to this work
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Received: 10 November 2018 / Revised: 26 November 2018 / Accepted: 3 December 2018 / Published: 5 December 2018
(This article belongs to the Special Issue Polymeric Materials for Electrochemical Energy Conversion and Storage)
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Abstract

A modified polyphosphazene was synthesized using a mixed substitution at phosphorus consisting of 2-(2-methoxyethoxy)ethoxy side groups and anionic trifluoroborate groups. The primary goal was to increase the low lithium ion conductivities of the conventional lithium salt containing poly[2-(2-methoxyethoxy)ethoxy-phosphazene] (MEEP) by the immobilized anionic groups. As in previous studies, the mechanical stability was stabilized by UV induced radiation cross linking. By variation of the molar ratio between different side groups, mechanical and electrochemical properties are controllable. The polymer demonstrated large electrochemical stability windows ranging between 0 and 4.5 V versus the Li/Li+ reference. Total and lithium conductivities of 3.6 × 10−4 S·cm−1 and 1.8 × 10−5 S·cm−1 at 60 °C were revealed for the modified MEEP. When observed in special visualization cells, dendrite formation onset time and short-circuit time were determined as 21 h and 90 h, respectively, under constant current polarization (16 h and 65 h for MEEP, both with 15 wt % LiBOB), which hints to a more stable Li/polymer interface compared to normal MEEP. The enhanced dendrite suppression ability can be explained by the formation of a more conductive solid electrolyte interphase (SEI) and the existence of F-contained SEI components (such as LiF). With the addition of ethylene carbonate–dimethyl carbonate (EC/DMC) to form MEE-co-OBF3P gel polymer, both total and lithium conductivity were enhanced remarkably, and the lithium transference numbers reached reasonable values (σtotal = 1.05 mS·cm−1, σLi+ = 0.22 mS·cm−1, t Li + = 0.18 at 60 °C). View Full-Text
Keywords: Polyphosphazene; polymer electrolyte; electrochemical stability; lithium metal battery Polyphosphazene; polymer electrolyte; electrochemical stability; lithium metal battery
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Schmohl, S.; He, X.; Wiemhöfer, H.-D. Boron Trifluoride Anionic Side Groups in Polyphosphazene Based Polymer Electrolyte with Enhanced Interfacial Stability in Lithium Batteries. Polymers 2018, 10, 1350.

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