(597h) Structure and Properties of Nanocomposite Gel Polymethyl Methacrylate Electrolytes

Gel polymer electrolyte (GPE) has been attractive for developing plastic Li-ion batteries due to its combined advantages of liquid electrolyte (high ionic conductivity) and solid electrolyte (free of leaking).With the existence of liquid electrolyte as the plasticizer, the GPE still has some disadvantages such as thermal stability and mechanical properties. Recently, researchers found that it could enhance the ion-conductivity and the thermal stability by adding some ceramic powders into the GPE to form the nanocomposite gel polymer electrolyte (NGPE).

Rectorite is a kind of mineral which has exchangeable cation between its layers like the Montmorillonite. The distance between layers (d-spacings) enlarged after organic modification. It will enhance the stability of the GPE by adding the organic rectorite, which brings the exfoliated layer structure in the gel system. SiO₂ could absorb amount of organic plasticizer for its specific open cell structure, at the meantime hydroxide radicals with different bonding status on the surface of SiO₂ could exist strong interactions with the other components such as polymer matrix or organic plasticizer, which is favorable for forming the stable GPE was made.

In this study, Organic-modified rectorite/poly(methyl methacrylate) OREC/PMMA and SiO₂/PMMA gel polymer electrolyte were synthesized through in situ polymerization with free radical initiator(AIBN) in propylene carbonate (PC) which is simultaneously act as an organic plasticizer in GPE. The micro-structure, interaction between different components, electrochemical properties, gel thermal-stability were all investigated. The influence of structure and properties of NGPE with different nano-partials was studied. It is also explained the mechanism of the conductivity of NGPE.

The results show that Organic-modified rectorite/poly(methyl methacrylate) OREC/PMMA and SiO₂/PMMA gel polymer electrolyte can be successfully synthesized through in situ polymerization with one step. The microstructure of NGPE was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that the fine OREC layers are exfoliated and dispersed well in the gel system. The cell structure on the sphericity surface of SiO₂ could enhance its absorption ability of organic plasticizer.

It is found that both OREC and SiO₂ have interactions between the PC and LiClO Li₄respectively by Fourier transform infrares(FTIR). For the sake of the existence of that kind of interaction, the different properties of NGPE have been improved. Especially the hydroxide radicals on the surface of nano-particles are both favorable to exfoliation of the LiClO₄ and transferring of Li⁺, which may be the main reason to increase ionic conductivity of GPE.

The electrical properties of the NGPE were analyzed by the AC impedance. It shows the same trend that ionic conductivity (&alpha) is firstly enhanced and then decreased by the increasing of addition of both OREC and SiO₂. When the content of nano-particles is under 5wt%, ionic inductivity of the NGPE increased with the increasing of addition of nano-particles. As well as he proportion of nano-particles over a certain extent, there will present the decreasing of conductivity for obstructing Li⁺ transferring by the cluster of gathered particles. At the same extent of nano- particles, &alpha of NGPE with OREC is lower than that of SiO₂, which might due to the specific layer structure which could obstruct Li⁺ transferring.

The thermal stability of the gel system was studied by the differential scanning calorimetry (DSC) and thermogravimetric analysis(TGA). When the ratio of nano-particles is below 5wt%, thermogravimetric temperature of 5% weight loss of the NGPE increased with the increasing of addition of nano-particles. As well as he proportion of nano-particles over a certain extent, there will presents the decreasing of thermogravimetric temperature of 5% weight loss. It can found that NGPE with OREC presents well gel thermal stability compare with that with SiO₂. In the gel system we studied, the silicate layers rearranged and dispersed in the gel system with an extent of exfoliated layers which improving the thermo stability a lot and assuring the stable, homogeneous gel state during a wide temperature range.

The different nano-particles also lead different influence on ionic conductivity. The open cell structure on the surface of SiO₂ can absorb more PC to provide channel for Li⁺ transferring, while the layer structure of OREC could enlarging the path some extent which make the Li⁺ to go through longer distance, conducting poor conductivity. Therefore ionic conductivity of NGPE with SiO₂ is better than that with OREC.