Investigations of Ionic Transport Mechanism of Polyether-Based Polymer Electrolytes for All-Solid-State Batteries

Abstract

Introduction Recently, renewable energy is expected as new power resource, development of secondary batteries such as high safety and high energy density is desired. Lithium-ion secondary batteries containing the organic electrolyte solution are also desired to improve safety owing to their ignition and liquid properties. Therefore, from the viewpoint of safety, all-solid-state battery have attracted attention. Solid electrolytes are mainly classified into polymer electrolyte (SPE) and inorganic electrolyte. The inorganic electrolytes exhibit relatively high ionic conductivity at room temperature, even though their low mechanical properties. In contrast, ionic conduction of SPE is considered as coupling model from segment motion of ether chain. Furthermore, it has high flexibility and adhesiveness, and achieves suitable interface formation at electrolyte the electrode. However, SPE exhibits relatively low ionic conductivity at room temperature as compared to electrolyte solution. In general, in the case of below glass transition temperature (T g), segmental motion of polymer chain is freezed and also should be stopped ionic motion. Introducing of side chain has reported as method for improving ionic conductivity. The mobility of the side chain is faster and has smaller temperature dependence than segmental motion of main chain. The motion of main chain and side chain are decoupled, carrier ion coordinating to side chain may show the ionic conduct ion below T g. In this study, to investigate ionic conduction behavior below T g, SPE was prepared having side chain with difference chain length. Prepared SPEs were evaluated by physicochemical and electrochemical measurements, respectively. Exerimental SPEs were prepared with LiTFSA as an alkaline salt, polyether-based tri-acrylate macromonomer (P(EO/PO), TA), mono-acrylate monomer (MA, M w=350) in Ar-filled globe box. The mixed solution of TA and MA and LiTFSA ([Li]/[O]=0.1 per molar of oxygen units of P(EO/PO) and MA) were mixed and the solution changed weight ratio to xTA+(10-x)MA (x=10, 8, 6, 4, 2 and 1, respectively). SPE was synthesized by UV irradiation for 5 minutes after addition of DMPA as a photo initiator. The prepared SPEs are expressed as xTA:(10-x)MA (x=10, 8, 6, 4, 2, 1). The prepared SPEs were evaluated for thermal properties by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC), and ionic conductivity by the AC impedance method. Result and Discussion Fig.1 shows DSC thermogram of prepared SPE having various segmental compositions. It was confirmed decreasing trend of T g with amount of introduce side chain. Increase of free volume into SPE might be affected for T g changes with introducing side chain. Fig.2 shows the temperature dependence of ionic conductivity of SPE as the Arrhenius-type plots. Ionic conductivity improved by introduction the side chain, and absolutely exsisted ionic conductivity below T g. Temperature dependence of ionic conductivity below T g is extremely smaller than their ionic conductivity over T g. These results suggest that ionic conduction by the motion of the side-chain (β-relaxation) owing to their quite small temperature dependence of mobility below the T g. Figure 1