The diffusion coefficient and conductivity of the chemically cross-linked polymer gel electrolyte composed of poly(ethylene glycol) dimethacrylate and LiBF 4 -EC/EMC were measured in order to investigate the effect of the polymer on the nature of carrier migration in the gel. With an increase of the polymer fraction in the gel, the dissociation degree of the salt in the gel increased. This indicates that the polymer accelerated the dissociation of the salt in the progress of gelation. The dissociated cation and anion showed a different manner of change in the activation energy of diffusion with gelation. The cation, through Coulombic interaction, showed a decrease in activation energy, revealing a change in the migration mechanism to hopping on the oxygen sites linked with the segmental motion of the chains. The anion, on the other hand, showed an increase in the activation energy of diffusion. This means that the conduction mechanism essentially follows, as in solution, the mobility of the solvent correlated with enhanced viscosity due to gelation.
The development of new materials for energy conversion systems is a pressing need for dealing with the energy problem and environmental preservation of the earth. On the basis of these demands, a new type of polymer gel electrolyte for lithium secondary batteries was prepared in this research using the concept of restricting the anion mobility with the chemical interactive effect of a specific site of the polymer in the gel. The polymer having the urea group CBMEU was designed to use the electron donating and withdrawing effect of the urea group for attracting the cation and anion, respectively, for the demands of promoting the dissociation of the salt and reducing the anion mobility. To quantitatively confirm the interactive effect of the polymer site, a theoretical model was set up based on the observed dynamic values to estimate the dissociation degree of the salt and interactive force. Application of the model to the new polymer gel electrolyte (CBMEU gel) and the PEO-type gel for comparison showed that, during the progress of gelation, the interactive effect of the polymer on the ionic species promoted the dissociation of the salt and reduced the ionic mobility. The absolute value of the interactive force of the cation, γ cation, was greater than that of the anion, γ anion, for both gels. The ratio, γ cation/γ anion, of the PEO-type gel was three times larger than that of the new polymer gel electrolyte. This is attributed to the anion-attracting effect of the urea group of the CBMEU gel in contrast to only the cation-attracting behavior of the ether oxygen of the PEO-type gel. From this investigation, we proposed an idea to design the polymer gel electrolyte which provides a high dissociation degree and cation transport number based on the investigation of the dynamic properties.
Carrier diffusivity of a lithium electrolyte solution in porous membranes was evaluated by observation of diffusion behavior of ionic species using NMR spectroscopy. We performed analysis on the basis of the restricted diffusion model and a new idea of diffusion distribution. That new concept is engendered in the assumption that observed diffusion values are distributed according to pore size distribution and nonuniformity of pore and polymer density in a membrane. Carrier diffusivity in polyethylene (PE) porous membranes showed characteristic echo attenuation, which differs from echo behavior as a result of random walk migration. We first tried to simulate the echo change on the basis of the restricted diffusion model in porous structure, resulting in disagreement with the observed echo change. As a result, the idea that diffusion values are distributed concurrent with porous geometry was applied to interpret the situation. Introduction of a Gaussian function to represent the distributed condition yielded highly reproducible results for the membranes. Carrier diffusivity in porous PVDF membranes showed a narrow distribution compared with that of PE membrane. The swelling feature of the PVDF polymer would contribute to averaging the pore size and preparing the uniform network for carrier transport pathway.
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