Preparation And Performance Study Of Lithium-ion Battery Separators By Modified Polyethylene Membrance

As an important electric energy storage device,batteries are widely used in various fields of production and life.Among them,lithium batteries have higher energy storage density and longer cycle life,and stand out among all secondary batteries which are widely studied and applied.Lithium batteries are mainly composed of a positive electrode,a negative electrode,a separator,and an electrolyte.The separator does not participate in any reaction in the battery,but will provide a channel for the transmission of Li+.The performance of the diaphragm directly determines the interface structure and resistance of the battery,and affects the capacity,cycle and safety performance of the battery.At present,most lithium batteries use polyolefin separators that are chemically stable.However,polyolefin separators have disadvantages such as low melting point,large polarity difference with electrolyte and uneven pore size,which makes polyolefin separators have poor thermal stability and low electrolyte wettability,and the uneven distribution of Li+ flow.The existence of the above problems restricts the application of polyolefin separators in batteries with high energy density and high safety.In this work,in order to improve the heat resistance of the battery separator,the electrolyte wettability and the ability to filter Li+ flow,we prepared a series of composite separators for Lithium-ion batteries with high thermal stability and high ion conductivity.These composite separators are based on polyethylene(PE)separators which were coated with phenolic resin(AF),AF/silica(SiO2)composite particles or molybdenum trioxide modified carbon materials.The main conclusions are as follows:1.In order to improve the thermal stability and electrolyte compatibility of commercial polyolefin separators,we used 3-aminophenol containing amino groups to prepare AF-modified PE composite separator through a simple immersion in-situ reaction process.The PE@AF separators have lower heat shrinkage and higher electrolyte wettability than PE membrane.The thickness of the modified layer can be effectively adjusted by simply changing the concentration of the reaction solution.The shrinkage rate of the prepared PE@AF separator is only 6%after heat-treated at 145? for 30 min,which is much smaller than that of the PE separator(77%).Moreover,the ion conductivity of PE@AF composite separator increases from 0.206 mS cm-1 for the pure one to 0.604 mS cm-1.After 450 cycles,the batteries assembled with PE@AF separators showed higher coulombic efficiency and capacity retention rate(96.7%and 86.0%),which were much higher than that of the PE base separators(67.4%and 72.7%).2.The coating layer on the surface of membrane often leads to a decrease in the porosity of the PE base separator.In order to reduce the influence of the modification layer on the porosity of the separator,we adopt a simple "one-pot" in-situ reaction method to graft the AF/SiO2 composite nanoparticles(NPs)onto the surface of the PE separator.By simply adjusting the concentration of the reaction solution,composite separators with different particle sizes and grafting densities of NPs can be obtained.NPs can form a modified layer with self-supporting connected pores on the surface of the base separator,and therefore the porosity of the separator will not be decreased.In addition,the modified layer contains a large number of polar functional groups,and the PE@AF/SiO2 separator has better electrolyte wettability and compatibility.The lithium ion transfer number and ion conductivity of PE@AF/SiO2 separator were increased from 0.42 and 0.212 mS cm-1 for the pure one to 0.62 and 0.722 mS cm-1 for the composite separator.After heat treatment at 140? for 30 min,the thermal shrinkage rate of PE@AF/SiO2 separator is 12.43%,lower than pure PE separator(20.13%).Although the pores of both pure PE and PE@AF/SiO2 separators are completely closed at 150?,the AF/SiO2 layer has better heat resistance,which can reduce the thermal shrinkage caused by the thermally closed pores of the base PE separator.The batteries assembled with PE@AF/SiO2 separators have higher cycle rate,discharge capacity,capacity retention rate and coulombic efficiency compared with PE separator.3.Commercial polyolefin membranes are prepared by unidirectional or biaxial stretching processes,and their pore distribution is uneven,resulting in uneven distribution of Li+ shuttles.In order to improve the pore distribution of separators,we prepared composite PE separators modified with molybdenum trioxide(MoO3)/carbon materials.The stack of carbon nanoparticles can form ordered pore structures and narrow pore distribution,and the randomly arranged MoO3 nanobelt layer has sufficient porous network,which makes the composite membrane an effective ion filter,and stabilizes the ion current.The ion conductivity of composite separator increases from 0.167 mS cm-1 for the pure one to 1.013 mS cm-1.Batteries assembled with composite separators showed higher initial discharge capacity,discharge capacity after cycling,and coulombic efficiency.