Research On PVDF-HFP Based Nanocomposite Polymer Electrolytes For Li-ion Batteries

Abstract:Li-ion polymer batteries have attracted great attention as promising next generation batteries for their superior performance compared to the traditional liquid Li-ion batteries in terms of low safety failure probability, good flexibility in the cell geometry design and high mass specific energy. Developing high performance polymer electrolyte is an effectively approach to obtain superior performance polymer batteries. One of the most promising ways to obtain high performance polymer electrolyte is to prepare high properties of nano-sized inorganic particle packed composite polymer electrolyte. However, as results of high surface energy and low Zeta potential of the inorganic nanoparticles, it is very difficult to disperse nanoparticles into polymer matrix. The aggregated nano-particles will negate any benefits associated with the nano scale dimension. In this work, according to the general strategy for nanopartice dispersion, the nano-TiO2orginic-inorganic composites, which are used to enhance the performance of PVDF-HFP based polymer electrolyte membrane, have been designed to prevent the nano-TiO2from conglomerating. The nanocomposite preparation process, the structure and properties of composite polymer electrolyte membranes and the conductive mechanism for nanoparticle to enhance the performance of polymer electrolyte membranes are studied, and the obtained results are showed as following:1) A high dispersiblity nano-crystalline TiO2-PMMA hybrid is prepared successfully by in situ polymerization and crystallization. PMMA layer is grafted onto the surface of TiO2particles by in-situ polymerization with vinyl triethoxy silane as coupling agent to form TiO2-PMMA hybrid suppresses the conglomeration of nano-TiO2.2) The PE separator as supporting layer, PVDF-HFP and nano-crystalline TiO2-PMMA hybrid blends are fixed on both sides of PE separator to form sandwich-type composite polymer electrolyte membranes. The sturcture and properties of membranes are investigated by SEM, linear sweep voltammetry (LSV), charge-discharge cycle testing and AC impedance measurements and so on. The results show that, as the high dispersion nano-TiO2-PMMA addition in PVDF-HFP matrix, the cell performance of PVD-HFP based poymer electrolyte membrane are improved remarkably, especially the c-rate performance. Increasing the dispersity of nano-TiO2in PVDF-HFP composite polymer electrolyte membranes, the ionic conductivity and electrode compatibility of membranes are improved effectively, so that the cell performance are enhanced, and the improved electrode compatibility is the main reason for the cell performance.3) A monodispersed nano-crystalline TiO2@Li+single ionic conductor (TiO2-PAALi-PMMA) is prepared successfully by in situ polymerization and crystallization. Under the synergistic effect of acid base reaction and polar solvent DMF, the dispersion energy consumption of nano-TiO2particle is decreased, the dispersion efficiency is improved.4) Electrospun PVDF-HFP membrane as supporting laymer, PVDF-HFP and nano-crystalline TiO2@Li+single ionic conductor blends are fixed on one side of the electrospun membrane to form composite polymer electrolyte membranes, and the thick of composite membranes is controlled at about45um. The composite membranes exhibit good mechanical strength, excellent accithermal shrinkage resistance, superior electrochemical properties and cell performance. As the content of single ionic conductor TiO2@Li+in composite layer is up to50wt%, the breaking strength of the composite membrane is up to39MPa, the ionic conductivity and ionic transferance number of the activated membrane is up to3.63×10-3S cm-1and5.15at room temperature, respectively, and the cycle performance and c-rate performance are improved significantly compared to the cell assembled PE separator.5) Due to the fact that the monodispersed TiO2nanoparticle can keep uniformly distribution in membrane composite layer, it brings about a phenomenon of "self-assembly", in which the mono-dispersed TiO2@Li+single ionic conductor in membrane composite layer will segregate from the membrane body to the interface between the electrodes and polymer electrolyte membrane or the pore inner walls within the composite polymer electrolyte membrane. Enriched nano-TiO2on the interface between the electrodes and the polymer electrolyte membrane will enhance interfacial compatibility of electrodes and polymer electrolyte membrane, and reduces the value of interface resistance of cells. Due to the electronic double laymer effect of nano-TiO2particles, enriched nano-TiO2on the pore inner walls within the composite polymer electrolyte membrane will generate a fast ionic conduction pathway, in which the space charge layers of the individual particles overlap to form a continuous network of favorable conduction pathways. As a results, the ionic conductivity of composite polymer electrolyte membranes is improved significantly.