Study On Preparation And Properties Of Polyvinylidene Fluoride(PVDF)/Polymethyl Methacrylate(PMMA) Separators For Lithium Ion Batteries(LIBS)

The increasing popularity of the concept of energy conservation and environmental protection,as well as the rapid progress of new energy vehicles and other emerging fields,lithium-ion batteries(LIBs)have attracted more and more attention in recent years,because of the competitive advantage of renewable,green energy and cost effective.Polyvinylidene fluoride(PVDF)and polymethyl methacrylate(PMMA)are ideal materials for the production of separators for LIBs due to their perfect performance.For the reason of getting LIB separators with outstanding properties and high safety,it is really matter to investigate the preparation process of PVDF/PMMA films and their performance.In this study,a series of PVDF/PMMA blend films were prepared by thermally induced phase separation(TIPS),and nano-silica particles(Si O₂)were also used to modify the above-mentioned films,and the influence of Si O₂on microstructure and performance of the synthetic films were explored.The main research contents of this paper are described briefly as below:With PVDF and PMMA as film materials,dioctyl phthalate(DOP)and dimethyl phthalate carbonate(DMP)as mixed diluents,growth mechanism,film microstructure and performance were intensively explored by varying the polymer concentration,coagulation bath temperature and PMMA content.The temperature distributions inside the films,which were built by altering quenching temperature,were analysed.The results showed that the cooling rate in the films displayed position dependence and affected the phase separation process during film forming process,resulting in the hierarchical film microstructure.Lower cooling rate tended to induce the formation of larger pore size,while higher cooling rate produced smaller pore size.Polymer concentration on PVDF/PMMA films heavily influenced the pore microstructure through the change of the viscosity of the film-forming system.With increasing polymer concentration,the fluidity of the system decreased,and the crystallization of polymer and the aggregation of diluent were hindered during phase separation,narrowing the liquid-liquid(L-L)phase separation region and unconducive to the formation of microporous structure.The existence of PMMA significantly inhibited the crystallization process of PVDF,improved the competitiveness of L-L phase separation and promoted the transition from solid-liquid(S-L)to L-L phase separations,leading to the variations in film structure from spherulite to double continuous structure as well as improvement on film porosity,water vapor transmittance and tensile properties.In order to further improve the comprehensive performance of PVDF/PMMA LIBs separator,nano-Si O₂particles were also added to modify the as-prepared films.Various proportions of Si O₂particles were incorporated to the PVDF/PMMA/Si O₂composite films.The results of DSC,XRD and FITR characterizations clearly showed that the Si O₂nanoparticles significantly reduced the crystallinity of PVDF films without changing the crystal forms of PVDF.With the increase of Si O₂content,the porosity and electrolyte uptake rate of the composite film increased,but excessive loading of Si O₂particles tended to agglomerate in the films.As a result,porosity and electrolyte absorption rate of the film were both reduced.The porosity and electrolyte uptake rate of the composite films containing 3%Si O₂were found to be the highest.The PVDF/PMMA/Si O₂composite films showed good thermal stability primarily owing to the relatively good thermal stability of Si O₂.With the increase of Si O₂loading,the first charge-discharge capacity and efficiency of the composite film were both enhanced.And the discharge cycle stability was also improved.When 3%Si O₂was incorporated,the PVDF/PMMA/Si O₂composite film displayed the best comprehensive performance of the battery among all film samples examined.