Preparation And Performance Study Of Polyvinylidene Fluoride Polymer Electrolytes

High specific energy and high safety energy storage technology is an important support to realize the lightweighting of equipment,electrification of transportation,and intelligence of power grid.At present,commercial lithium-ion batteries are approaching the limit of their theoretical energy density.To cope with the increasing requirements of energy storage technology for various applications,high specific energy lithium metal batteries（LMBs）have been given renewed attention.However,their lithium metal anodes is highly reactive and prone to electrolyte side reactions,leading to continuous electrolyte consumption.At the same time,uneven lithium deposition/exfoliation and easy to produce dendrites will not only form dead lithium and reduce the battery coulomb efficiency,but also short circuit inside the battery and cause thermal runaway or even explosion.Gel polymer electrolyte can reduce the risk of electrolyte leakage and thermal runaway,but it also has problems such as low ionic conductivity at room temperature,dendrite and dead lithium formation.In order to solve the above problems,this thesis uses high temperature resistant zirconia（ZrO₂）nanoceramics as filler to explore the effects of filler ratio and dispersion on key parameters such as transport characteristics and thermal stability of inorganic polymer composite gel electrolyte lithium ion,so as to improve the rate performance,safety and cycle lifespan of lithium metal batteries.The details are as follows:（1）The ZrO₂ nanofiber fillers were prepared by electrostatic spinning technique,and the electrolytes of polyvinylidene fluoride（PVDF）gel polymers（PVDF-XZrO₂）with different ZrO₂ doping ratios were prepared by a combination of mechanical blending and phase transfer method,and their electrochemical properties were tested.The results showed that the best performance of the electrolyte membrane was achieved at 5 wt%of ZrO₂ nanofiber filler addition,and the ionic conductivity of PVDF-5ZrO₂ could reach5.86×10^-3 S cm^-1 at room temperature.The full battery assembled with the electrolyte can maintain 98.2%of the initial specific capacity at a multiplicity of 1 C at room temperature for 100 cycles,and the charge/discharge plateau is stable.The doping of ZrO₂ reduces the crystallinity of PVDF,which in turn increases the ionic conductivity.Although this method enhances the ionic conductivity and thermal stability of the gel polymer electrolyte,the filler is prone to precipitation and agglomeration,which weakens the modifying effect of ZrO₂ continuously.To address the problem of easy agglomeration of nanofillers,PVDF-based polymer electrolytes（PIZX）doped with different percentages of ZrO₂ in situ were prepared by electrostatic spinning technique.The ionic conductivity of PIZ7 prepared by this method at room temperature(1.051×10^-2 S cm^-1)was much higher than that of the undoped polymer electrolyte(3.29×10^-3 S cm^-1)and the polymer electrolyte obtained by mechanical co-blending of ZrO₂ with PVDF substrate(5.86×10^-3 S cm^-1).The Li/PIZ7/Li symmetric battery was charged and discharged at a high current density of 0.7 m A cm^-2for 600 h without short circuit.The reason for its excellent performance may be firstly because the in situ generated ZrO₂ filler is uniformly distributed in the PVDF substrate,which maximizes the advantages such as excellent chemical resistance and thermal stability of the ZrO₂ filler,and the Lewis acid-base interaction between it and the polymer is enhanced,thus accelerating the dissociation of lithium salts and reducing the crystallinity of the polymer matrix to promote rapid lithium ion transfer.Secondly,because the electrostatic spinning membrane has a three-dimensional network structure,it can absorb a large amount of electrolyte and thus activate the gel polymer electrolytes（GPEs）,resulting in a significant increase in the ionic conductivity of GPEs.