Synthesis Of Boron Nitride Fiber/MgO Composite Separator For Lithium Thermal Battery

In this work,a membrane was composited by boron nitride fiber cloth that was as a carrier material and nano MgO particles which were inserted inside the BN cloth.By the invasion process,MgO was synthesized by precursor conversion method.The membrane was aiming to apply inside the long-life lithium battery,in order to improve the safety of it.Firstly,the preparation methods and technological parameters of boron nitride fiber cloths were studied.By comparing the paper making,knitting and plain weave knitting,plain weave technology was selected and optimized for the preparation of the boron nitride fiber cloths.The synthesized BN fiber cloths were characterized by the analysis of SEM,XRD,FT-IR etc..Influence of the diameter of precursor fiber and number of boron nitride fiber on the thickness of cloth was studied in details,and the influence of heat treatment process on the nitride content and of boron nitride fiber cloth studied as well.Heat resistance properties of the fiber cloth below650? were also studied.Secondly,the process of improving boron nitride fiber fabric wettability with magnesia powder was studied.It was shown that the wetting ability of the fiber cloth in precursor solution could be significantly enhanced by addition of isopropyl alcohol in the precursor solution.the optimized concentration of the precursor solution was determined as 25%~30% after a serious examinations.The precursor solution absorbed into the internal diaphragm of the BN fiber cloth will be transformed into MgO nanoparticles after heat treatment.The electrolyte adsorption capacity of the boron nitride fiber membrane composed by the MgO nanoparticles was greatly improved reaching upto 8 times of that of the pure boron nitride fiber cloth without MgO nanoparticles.Finally,the measurements of physical parameters of the synthesized BN fiber/MgO composite membrane,such as thickness,temperature resistance,electrolyte retention capacity and discharge behavior of a single cell,were carried out.SEM and electronic spectrum analysis were also performed,The results showed that the average thickness of the synthesized membranes was about 350?m,50?m decrease than that of the magnesium oxide separator bringing benefits in thereduction of the internal resistance of the battery and the improvement of the battery energy densities in both weight and volume.After 2h heat treatment at 700?,the weight loss of boron fiber composite membrane was less than 1% indicating a high stability in diaphragm temperature resistant.This is because that the adsorption of MgO nanoparticles acting as a protector could,in some degree,postpone the attenuation performance of fiber cloth in oxidizing atmosphere.The SEM images and energy spectrum analysis indicate that the main components of the synthesized membrane were BN and MgO.The electrolyte retention ability of the diaphragm was tested at 500? and 2kg weight loading.No detectable changes could be found in the SEM images of the diaphragm before and after discharge indicating that the electrolyte maintaining ability of composite membrane was good.Using boron nitride fiber composite diaphragm to assemble the monomer battery,after the pulse discharged 600 s,the open-circuit voltage reaches to 1.7V,its discharge curve declined steadily.Under no-load condition,after testing for 5000 s,the voltage has been maintained 1.8V,no jitter and micro short circuit phenomenon were occurred in the battery curves.The above results indicate that the present boron nitride fiber composite membrane meets the requirement for applying in the lithium thermal battery.