Nanographite And Its Fluorinated Derivatives:Preparation And Application In Electrochemical Energy Storage

Carbon based materials are widely used in electrochemical energy storage devices.Graphite is the most common negative electrode material for commercial lithium-ion batteries.However,the theoretical specific capacity of graphite is low and cannot meet the requirements of power-type energy storage devices.As a method to effectively adjust the electronic structure and surface topography of materials,defect engineering has attracted much attention.The novel physicochemical properties and strong synergistic effects can effectively improve the material performance.Herein,we use a magnetic grinding method for the preparation of nanographite materials,and study the effect of structure defect on the lithiuim storage performance of the nanographite materials.Fluorinated graphite is a kind of positive electrode material with high energy density,but it has poor conductivity and voltage hysteresis.We further prepare new type of fluorinated nanographite by using the nanographite as precursor to relieve voltage lag.The main contents and conclusions are detailed as follows:1.Preparation of nanographite by magnetic grinding method and its lithium storage performance:Using natural flake graphite?LC?as raw material,nanographite?NC?materials with particle size of about 20 nm are prepared by a new magnetic grinding method.Nanographite anode materials with different defect degrees are prepared by heat treatment in different temperatures,and their specific surface area,pore structure,surface chemical properties and crystal structure are systematically characterized and their lithium storage properties are also studied.The magnetic grinding method uses a high-speed transformed magnetic field to drive the permanent magnet steel needle to generate high-speed shearing force,thereby grinding the graphite into nano-sized particles.The method overcomes the disadvantage that the graphite self-lubricating property is difficult to be nano-sized in the conventional ball milling method,and the prepared nanographite material is a hard carbon/graphite composite material.When used as a negative electrode of lithium ion batteries,the reversible discharge specific capacity of the NC800 could be reached to 525.5 mAh g^-1 after 100 charge/discharge cycles at 0.5C.Under the same test conditions,the commercial anode material BTR and the original flake graphite material only have reversible discharge specific capacities of 295.5 mAh g^-1 and 181.4 mAh g^-1.After500 cycles at 5C,it shows a reversible specific capacity of 225.2 mAh g^-1,and the cycle retention rate is as high as 85%.The performance improvement of nanographite materials depends on edge defects to provide more lithium-rich active sites and nano particle size to increase the diffusion rate of lithium ions,so it exhibits higher lithium storage capacity and better rate performance as a negative electrode material for lithium ion batteries.2.Controllable preparation of fluorinated nanographite and its application in lithium fluorocarbon batteries:Fluorinated nanographite material is prepared by direct fluorination method and using nanographite as precursor.Their fluorine content C-F bonding properties,surface area and pore structure,and crystal stucture are systematically studied.The unique nanoscale structure can effectively improve the contact area between the electrode material and the electrolyte,shorten the lithium ion diffusion path,thus effectively improve the rate performance of the battery system and alleviate the voltage lag problem.The highest specific discharge capacity is 837.4 mAh g^-1 for the FG-450 sample at 10 mA g^-1,and still maintains a high value of 589.3 mAh g^-1 at 3000 mA g^-1,and the corresponding energy density was 2004.5.Wh kg^-1 and 1030.5 Wh kg^-1.The FG-450 sample also has good power density and can reach5460 W kg^-1 at 3000 mA g^-1.EIS and GITT methods are used to analyze the causes of the difference in electrode material properties and the diffusion coefficient of lithium ions.The diffusion coefficient of lithium ion of fluorinated nanographite material is 5 orders of magnitude higher than that of the commercial fluorinated graphite material,which proves the faster reaction kinetics of fluorinated nanographite material.The main reason for the excellent electrochemical performance of the FG-450 sample is its synergistic effect on the nanometer particle size,large specific surface area and continuous mesopores.