Electrochemical Preparation Of Graphene And Its Application As A Conductive Agent For Lithium Ion Batteries

Graphene is a two-dimensional sheet carbon material with good electrical conductivity,high thermal conductivity,large specific surface area and high strength.It has good potential application value in electrochemical fields such as lithium-ion batteries,super capacitors,fuel cells,and electrolyzed water catalysts.the premise of realizing the industrialization and commercialization of graphene is to obtain a graphene material with excellent structure.Therefore,the large-scale preparation of graphene has very important significance for the practical application of graphene.In terms of energy storage,graphene as a lithium ion battery conductive agent is the most industrially applicable application.In this dissertation,graphene was synthesized by electrochemical intercalation and ultrasonic shearing methods.Meanwhile,the effect of graphene conductive agent on lithium ion battery cathode material was also investigated.The main contents are as follows:First,electrochemical stripping of graphite rods in organic electrolytes?NMP+TBA·HSO₄?was studied.The amount of peeling increases with the augments of voltage and electrolyte concentration at the different conditions.The maximum amount of graphene dispersion is 0.65 mg mL^-1 after shear and ultrasonication at 0.2 mol L^-1,40 V.The resulting graphene sheet is thicker and within 20 layers.the interlayer spacing of graphite is increased from 0.335 nm to 0.825 nm by electrochemical intercalation.Then the electrochemical stripping of graphite paper in organic electrolyte?NMP+TBA·HSO₄?was studied to avoid premature flaking of the graphite particles and a few layers of graphene products were obtained.Finally,the electrochemical stripping of graphite paper at a concentration of 0.2 mol L^-1 and the 10 V DC voltage in an aqueous electrolyte?H₂O+TBA·HSO₄?was studied.A large number of bubbles generated are beneficial to separate the graphite sheets.The graphene size is mainly distributed in 2-5?m,accounting for about 69%of the total number of graphenes,and the thickness is less than 10 layers accounting for about 80%of the total.Although there are partial defects,the defect level is significantly lower than graphene oxide.The prepared graphene was dispersed in NMP through a shear-ultrasonic process to obtain a conductive paste having a certain fluidity and moderate viscosity.The graphene paste was mixed with a cathode material of commercial lithium ion battery and coated on an aluminum foil.The cathode material particles were evenly distributed on the graphene sheet by SEM characterization,and some particles were in the middle of two sheets of graphene,avoiding interparticles.It can form the conductive network,which is used to improve the conductivity of electrode material so that lithium ions are more effortless to insert and deinsert into/from the host materials.It is apparent from the electrochemical tests that graphene conductive paste is superior to Super P in its conductive effect.Through the comparison of the peak-to-peak and peak currents in the cyclic voltammetry?CV?diagrams,the reversibility of graphene as the conductive agent in battery material was found to be better;the graphene conductive paste decreased the polarization of cathode electrode material and increased the rate performance.The specific capacity of lithium iron phosphate in graphene as conductive agent is 44.6 mAh g^-1 at 5 C,which is higher than that of Super P.For NCM,the specific capacitance of graphene slurry as the conducting agent at 2 C is 100.6 mAh.g^-1,which is 22.1 mAh g^-1 higher than that of Super P as a conductive agent.Meanwhile,the potentio electrochemical impedance spectroscope results show that the semicircle diameter?Rs?and the charge-transfer resistance?Rct?of graphene conductive paste are smaller,which is more beneficial to the transfer of lithium ions between the electrode materials.