Ionic Conduction and Dielectric Relaxation in Poly(ethylene carbonate)-Li Salt Electrolytes
In this study, we report on the use of poly(ethylene carbonate) (PEC) as a matrix for novel ion-conductive polymer electrolytes . We found that the addition of small amount of TiO2 nano-particulates to the PEC-based electrolytes can enhance lithium transference numbers, t+, which are measured using an electrochemical combination method of DC polarization and AC impedance measurements for Li│electrolyte│Li cells. Moreover, we undertook lithium-7 and fluoride-19 NMR spectroscopic and pulsed field gradient (pfg) diffusion measurements on these electrolytes. The values of self-diffusion coefficients of the lithium cation (DLi) and the estimated t+ values for PEC-LiFSI-TiO2 were more than 10-7 cm2/s and 0.8 at 60 oC. The Li-ion conductivities (σ at 60 oC × t+ estimated from the electrochemical method) of samples PEO20LiFSI, PEC0.53LiFSI and PEC0.53LiFSI-TiO2 (1 wt%) were calculated to be 5.6×10-5, 2.2×10-4 and 4.3×10-4S/cm, which are superior as a structure to polyether as an electrolyte for flexible batteries.
On the other hand, there are no detailed information on the ion-conductive behavior of PEC-based electrolytes. So, we used broadband dielectric spectroscopy (BDS) measurement for the investigation of correlation between ionic conduction and dielectric relaxation behavior over the broad range of salt concentration. We measured BDS data and analyzed dielectric spectra using the Havriliak-Negami functions. Fig. 1 shows salt concentration dependence of relaxation peak for PEC-LiTFSI electrolytes at 40 oC. A strong peak indicates α relaxation derived from segmental motion of PEC main chains. As the salt concentration increased, the α relaxation peak shifted to the high frequency region with increasing the peak area, which is opposite to the behavior of typical PEO-based electrolytes (the peak shifts to the lower frequencies) . This means that the PEC-LiTFSI electrolytes show good conductivities at high salt concentrations because of the fast segmental motion with local ion-rich structures.
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Acknowledgment: This work was financially supported by a Grant-in-Aid for Scientific Research (B) of JSPS KAKENHI (25288095), Japan.