Study On Chemical Modification And Interface Construction Of Lithium Manganese Cathode Conductive Agent

With the increasing exhaustion of fossil energy and environmental pollution,people have begun to vigorously develop new energy sources.Lithium-ion batteries（LIBs）have attracted much attention as a new type of clean energy;Lithium-ion batteries have the advantages of low self-discharge,large specific capacity,high energy density,no memory effect,and long cycle life;it is used in all aspects of life,such as new energy vehicles,mobile phones,computers and so on.As one of the key materials of lithium-ion batteries,lithium manganate（LMO）cathode materials are used as power batteries for automobiles due to their high safety performance,low price,and high voltage platform.However,the poor high and low temperature performance of LMO and the shortcomings of Mn dissolution and Jahn-Teller effect limit its large-scale application.Many experts and scholars use doping,coating and other methods to modify the lithium manganate cathode material;such as coating a passive film on the surface of the LMO to protect the active material from the electrolyte;this greatly reduces the conductivity of the active material and increases the interface impedance,which is not conducive to large-rate charge and discharge;Or use double cladding,but this will make the process cumbersome and costly.In addition to modifying the active material,we can also add high-conductivity conductive agents to improve the electrochemical performance of the battery,such as carbon nanotubes（CNTs）and graphene.However,carbon nanotubes（CNTs）and graphene are not only expensive but also difficult to disperse.Many companies add dispersants in the electrode manufacturing process,which not only increases the production cost but also does not conduct electricity.For the above problems,we can coat the active material to prevent it from being attacked by the electrolyte,and then modify the conductive additive to make it have good dispersibility and high conductivity,and can form with the active material.The assembly results in an excellent conductive network on the pole pieces.The research work of this paper is to investigate the influence of coating amount and sintering temperature by coating a cheap Al₂O₃ coating on the surface of LMO by sol-gel method,and then acidify the conductive agent Ketjen Black（KB）to improve its dispersion.Then without damaging the internal structure of Ketjen Black,the surface is oxidized to increase the oxygen-containing functional groups on the surface and self-assemble with the active material to form an excellent conductive network.Complementary,in addition to introducing oxygen-containing groups,we also introduced nitrogen-containing groups on the surface of KB and explored their effects on electrochemical performance.The main work is as follows:（1）Effect of Acidification Modification of Ketjen Black and its effect on the electrochemical performance of LiMn₂O₄/Al₂O₃ compositesThe sol-gel method was used to coat Al2O3 on the surface of LMO and the coating amount and sintering temperature were investigated.The composite material still has a spinel structure;it has the best electrochemical performance at room temperature when the coating amount is 1%and the sintering temperature is 600℃for 3 hours:at 1 C,300 cycles and a discharge of 85 mAh/g.Then acidifying Ketjen Black can increase the oxygen-containing functional groups on its surface to improve the dispersibility of Ketjen Black and explore the type and solubility of the acid.The results of the electrochemical test show that Ketjen Black acidified with 5M HCl is conductive.The agent has better cycle performance:the specific discharge capacity after cycling 300cycles at 1 C charge-discharge rate is 92 mAh/g.（2）Effect of Oxidative Modification of Ketjen Black on Chemical Properties of LiMn₂O₄/Al₂O₃ CompositeBecause the use of acid will generate waste liquid to pollute the environment;we use oxidation to add some oxygen-containing functional groups on the surface of Ketjen Black to improve the dispersibility of Ketjen Black.Due to the opposite charge between the active material and KBO,it self-assembles with the active substance.There is thus a good conductive network.The specific discharge capacity of KBO as a conductive agent at 0.1 C at room temperature can reach 126 mAh/g;after 400 cycles at 1 C,there are still 114 mAh/g,and the capacity retention rate is 90%;at 5 C charge-discharge rate,there is still a specific discharge capacity of 105.7 mAh/g;it shows good electrochemical performance.（3）Ketjen black nitrogen doping modification and its effect on the electrochemical performance of LiMn₂O₄/Al₂O₃ compositesIn addition to introducing oxygen-containing functional groups on the Ketjen black surface,other groups can also be introduced,such as nitrogen-containing groups,fluorine-containing groups,sulfur-containing groups,and the like.The high-temperature pyrolysis method is used to decompose the nitrogen-containing compound and combine with Ketjen Black,introduce nitrogen-containing groups on the surface of Ketjen Black,and explore the amount of addition.At room temperature,the charge-discharge rate of 1 C of LMO battery with 10%urea KB composite as the conductive additive still has a discharge specific capacity retention rate of 108.79 mAh/g after 500cycles of 91.2%;at a large rate of 5 C,the composite has a discharge specific capacity of 99.4 mAh/g.