Ion Transport Numbers of Gel Polymer Electrolytes Based on Block Copolymers Containing Ethylene Oxide Sidechains

Abstract

Nowadays, lithium ion batteries (LIBs) play a key role in the field of portable electronic devices. In the last decades, LIBs have also been considered for large scale applications like in electric vehicles (EVs) or hybrid electric vehicles (HEVs). In state of the art LIBs, mainly liquid electrolytes, consisting of two or more carbonate solvents (e. g. ethylene carbonate and dimethyl carbonate) and a lithium salt, are used. But despite their advantage of displaying a high conductivity at room temperature, the disadvantages like a low flash and boiling point and the danger of leakage in case of cell damage, show that an improvement in safety is necessary.As alternative solid polymer electrolytes were developed, but their low room temperature conductivity is still an obstacle to their use in commercial LIBs. Another approach is the use of gel polymer electrolytes (GPEs), which combine the advantages of liquid and solid polymer electrolytes. They display a high conductivity at room temperature while having a sufficient mechanical stability to be used as separator. Another advantage is that GPEs are able to prevent leakage, as the liquid electrolyte is immobilized in the polymer matrix.1 In this work we report on the development of novel gel polymer electrolyte systems based on non-commercial available amphiphilic block copolymers matrices comprising a norbonene backbone. The synthesis of monomers with different side chains, and the use of ring opening metathesis polymerization2, a living polymerization method allow the tailoring of the properties of the polymer. The polymer matrix, comprising ethylene oxide side chains with different lengths, is thermally stable up to 180 °C and shows microphase separation, proven by DSC measurements. The resulting GPEs were investigated regarding their conductivity and electrochemical stability. Particular attention was put on the investigation of the transport properties of the GPEs, using PFG-NMR and Raman-spectroscopy. The influence of the length of the ethylene oxide side chains on the diffusion coefficients of the ions and the Li+ complexation was studied. [1] P. Isken, M. Winter, S. Passerini, A. Lex-Balducci J. Power Sources 2013, 225, 157-162.² C. W. Bielawski, R.H. Grubbs, Prog. Polym. Sci. 2007, 32, 1 - 29.