Synthesis and Characterization of Network Single Ion Conductors Based on Comb-Branched Polyepoxide Ethers and Lithium Bis(allylmalonato)borate

Abstract

Network single ion conductors (NSICs) based on comb-branch polyepoxide ethers and lithium bis(allylmalonato) borate have been synthesized and thoroughly characterized by means of ionic conductivity measurements, electrochemical impedance, and cycling in symmetrical Li/Li half cells, Li/V6O13 full cells in which a NSIC was used as both binder and electrolyte in the cathode electrode and as the electrolyte separator membrane, and by dynamic mechanical analysis (DMA). The substitution of the trimethylene oxide (TMO) unit into the side chains in place of ethylene oxide (EO) units increased the polymer−ion mobility (lower glass transition temperature). However, the ionic conductivity was nearly one and half orders of magnitude lower than the corresponding pure EO-based single ion conductor at the same salt concentration, which may be ascribed to the lower dielectric constant of the TMO side chains that result in a lower concentration of free conducting lithium cations. For a highly cross-linked system (EO/Li = 20), only 47 wt % plasticizing solvent (ethylene carbonate (EC)/ethyl methyl carbonate (EMC), 1/1 by wt) could be taken up, and the ionic conductivity was only increased by 1 order of magnitude over the dry polyelectrolyte, while for a less densely cross-linked system (EO/Li = 80), up to 75 wt % plasticizer could be taken up and the ionic conductivity was increased by nearly 2 orders of magnitude. A Li/Li symmetric cell that was cycled at 85 °C at a current density of 25μA cm-2 showed no concentration polarization or diffusional relaxation, which was consistent with a lithium ion transference number of 1. However, both the bulk and interfacial impedance increased after 20 cycles, which was apparently due to continued cross-linking reactions within the membrane and on the surface of the lithium electrodes. A Li/V6O13 full cell constructed using a single ion conductor gel (propylene carbonate (PC)/EMC, 1/1 in wt) was cycled at 25 °C at a current density of 25μA cm-2 and showed an initial capacity of 268 mAh g-1 of V6O13, which stabilized at around 200 mAh g-1 after the first 20 cycles. During the DMA measurements on the NSICs, it was found that besides the main glass transition (α transition) there was a distinct secondary glass transition (β transition) for NSICs having five EO units in the side chains, while this (the secondary transition) was not clearly visible in the network single ion conductors (NSICs) with shorter side chains (two, three, and four EO units). The main glass transition (α transition) was attributed to the whole network structure of the single ion conductors and secondary glass transition (β transition) appeared to be due to the complexation of lithium by the side-chain chains. Both the main glass transition and the secondary transition were found to shift to higher temperature with increasing salt concentration.