Study On Preparation Of Mesoporous Carbon By Hard Template Method And Its Electrochemical Performance

Lithium-ion batteries have the advantages of high charge-discharge specific capacity,low self-discharge rate,wide operating temperature window,high energy density,and good environmental protection effect.They have been widely used in smart grids,electric vehicles,and portable smart devices.At present,commercial lithium-ion batteries generally use graphite as the negative electrode material.Graphite stores lithium ions through lithium insertion reaction.Only one lithium can be combined with every six carbons.Therefore,the theoretical capacity of graphite negative electrode is only 372m Ah g^-1,which does not meet the current requirements for high energy density and rapid charge and discharge capabilities.The introduction of pores,defect sites and functional groups into the carbon material can effectively improve the electrochemical performance.This is because the pores can store lithium by adsorption,and the functional groups and defect sites can store lithium by inducing capacitive behavior.Pores can be generated by template or gas volatilization during carbonization,and functional groups and defects can be introduced by heteroatom doping.In this thesis,a hard template synthesis method is used to effectively increase the specific surface area and porosity of the material.Amorphous porous carbon materials are prepared using glucose and ionic liquid as carbon sources,and other elements of the raw materials are used for in-situ heteroatom doping.The specific research content and conclusions are as follows:（1）Using glucose as the carbon source and zinc chloride as the template,an amorphous mesoporous carbon material with ultra-high specific surface area was prepared at low temperature through a simple mixing-preforming-carbonization-washing process.The specific surface area of mesoporous carbon prepared with zinc chloride as a template can reach 1710.6m² g^-1,and the average pore diameter is 4.2nm.After the ICP-MS test,the zinc content was only 0.249wt%after the first wash,indicating that one wash with hydrochloric acid basically removed all the template.The mesoporous carbon exhibits excellent electrochemical performance.After 200charge-discharge cycles at a current density of 0.1A g^-1,it can still provide a discharge specific capacity of 536.1m Ah g^-1 with a coulombic efficiency of 97%,which is far better than commercial graphite.In addition,the mesoporous carbon exhibits excellent rate performance.It still maintains a discharge specific capacity of 129.4m Ah g^-1at a high current density of 2A g^-1,and when the current density returns to 0.1A g^-1,the discharge specific capacity rises to 544.4m Ah g^-1.（2）1-ethyl-3-methylimidazole dicyandiamide salt was used as the carbon source,and MCM-41 molecular sieve was used as the template.After a simple mixing-carbonation-etching process,in-situ nitrogen-doped mesoporous carbon was synthesized.Furthermore,cobalt nitrate was added as a template and catalyst to the precursor,the effect of cobalt nitrate additives on the electrochemical performance of the synthetic material was investigated,and the influence of the amount of cobalt nitrate added was discussed.The specific surface area of D-M-700 can reach 259.61m²g^-1,the average pore diameter is 4.16nm and the nitrogen content is 5.31%.The addition of cobalt nitrate further increases the specific surface area of the carbon material and widens the pore size(D-M-0.5-700:476.46m² g^-1,5.07nm).D-M-700（prepared at 700°C）has a specific capacity of 475.52m Ah g^-1 after 100 cycles at a current density of 0.1A g^-1,which is significantly better than D-700 without template added(the discharge specific capacity of the 100th cycle is 214.99m Ah g^-1)and commercial graphite(theoretical specific capacity 372m Ah g^-1).After adding cobalt nitrate,the nitrogen doping content of D-M-0.5-700 is 6.35%,and the discharge specific capacity of the 100th cycle can reach 627.86m Ah g^-1 at a current density of0.1A g^-1,and it can still provide a discharge specific capacity of 190.95 m Ah g^-1 at a high current density of 2A g^-1.When the current density returns to 0.1A g^-1,the specific capacity also returns to 614.86m Ah g^-1,showing good rate performance.（3）Ionic liquid transition metal salts have the characteristics of uniform element distribution,and it is hopeful that they can be used to synthesize mesoporous carbon with uniform pores and in-situ nitrogen doping.This chapter uses 1-butyl-3-methylimidazole cobalt tetrachloride salt and 1-butyl-3-methylimidazole nickel tetrachloride salt as the carbon source,and silica sol（model:LUDOX HS-40）as the template agent.After a simple mixing-carbonization-etching process,in-situ nitrogen-doped porous carbon was prepared at a series of temperatures.The results showed that the porous carbon material prepared by the ionic liquid transition metal chloride salt as a carbon source is a typical amorphous carbon.Among mesoporous carbons prepared with temperature gradients of 700,800,and 900°C,the electrochemical performance of mesoporous carbon at 800°C is the best.The nitrogen doping amounts of samples Co-H-800 and Ni-H-800 are 4.66%and 5.54%,respectively.The discharge specific capacity of Co-H-800 at the 50th cycle under the current density of 0.1A g^-1 can reach767.6m Ah g^-1,the coulombic efficiency is 101.83%,and the electrochemical performance is better than Ni-H-800(381.6m Ah g^-1,98.98%)and Co-800 without template agent(367.73m Ah g^-1,99.59%)under the same current density.This is largely because Co-H-800(1104.9m² g^-1)has a specific surface area far exceeding Co-800(263.9m² g^-1)and Ni-H-800(574.3m² g^-1).The uniform and abundant pore structure provides abundant lithium storage sites.At a current density of 2A g^-1,the discharge specific capacity of Co-H-800 still maintains 194.60m Ah g^-1.When the current density returns to 0.1A g^-1,the discharge specific capacity rises to 737.25m Ah g^-1,showing excellent rate performance.