Preparation And Properties Of Epoxy/COFs Composite Electrolytes

Lithium metal with high theoretical capacity（3860 m A h/g）,low redox potential（-3.04 V）,and low density（0.53 g/cm³）has been widely used as anode materials for secondary batteries.However,lithium metal batteries are faced with safety challenges such as inextricable dendrite growth,leakage of liquid electrolytes and inflammability.Gel polymer electrolyte can solve the problems of liquid electrolyte volatilization and leakage,and has the advantages of wide operating voltage range,high ionic conductivity,good interface compatibility and flexible design,so it is considered as a promising lithium metal battery electrolyte.However,gel polymer electrolytes can’t completely inhibit the growth of lithium dendrites due to their low mechanical strength.Therefore,it is necessary to select a polymer matrix with good mechanical properties and further strengthen it by adding fillers to prepare composite gel polymer electrolytes,so as to effectively inhibit the growth of lithium dendrites.In this work,bisphenol A epoxy resin（DGEBA）with excellent mechanical properties was selected as polymer matrix with cross-linked network structure,and covalent organic frameworks（COFs）with high chemical and thermal stability was used as the filler to carry out the preparation and property study of epoxy resin/COFs composite electrolyte.Firstly,in order to save energy and improve reaction efficiency,TpPa-1and TpPa-2 were prepared by different methods（mechanochemical synthesis,mechanical grinding synthesis and sonochemical synthesis）,respectively.The TpPa-1 prepared by mechanochemical synthesis shows stacked lamellar microstructure,with the pore size of 2.25 nm and the specific surface area of765.73 m2/g.The average particle size is small（1120.28 nm）,and the average particle size is only 231.60 nm after treatment.The agglomerated structure of TpPa-1 was obtained by mechanical grinding synthesis,with a small specific surface area of 158.84 m²/g,a pore size of 3.40 nm,and a large particle size（1277.21nm）.The TpPa-2 prepared by sonochemical synthesis shows a fragrant snowball-shape morphology,with a large specific surface area（100.90 m²/g）and a pore size of 6.28 nm;TpPa-2 obtained by mechanical grinding synthesis method has a lower specific surface area（34.83 m²/g）,the pore size is 8.52 nm,and the particle size of TpPa-2 obtained by both methods are larger（1528.19 nm and 1578.39 nm）.In summary,the TpPa-1 prepared by mechanochemical synthesis has the smallest particle size and can be used as a electrolyte filler.Moreover,the ultra-high specific surface area and good pore structure provide the possibility for lithium-ion transport channel.Secondly,the TpPa-1 material with the smallest particle size and the largest specific surface area was selected as the filler to prepare the Epoxy@x TpPa-1 composite polymer electrolyte,and then its structure and performance were analyzed.The results show that with the increase of TpPa-1,the surface morphology of the Epoxy@x TpPa-1 composite electrolyte changes from the initial pore structure to the spherical structure plus the covered pore structure（the pore structure is embedded with small spheres）.Both structures exhibit excellent mechanical properties.The prepared composite electrolytes have excellent mechanical properties and heat stability,and begin to lose weight at about 200℃Finally,composite electrolytes with different contents of TpPa-1 were fully activated in liquid electrolyte to form gel electrolytes.When the content is 11%,the comprehensive performance of the composite electrolyte is the best.The Epoxy@11%TpPa-1 composite electrolyte has a liquid absorption rate of 357%,an ionic conductivity of 2.55×10^-3S cm^-1,the lithium-ion migration number of 0.87,and a first-cycle specific capacity of 135.5 m A h/g at a rate of 0.5 C.Even at a current density of 2 m A/cm²,there is still a good inhibition of lithium dendrites.