Influences Of Carbon-based Conductive Additives On Cathode Performance Of Lithium Iron Phosphate

With the large-scale popularization of battery equipment such as new energy vehicles,the requirements for lithium-ion batteries are getting higher and higher,and traditional lithium-ion batteries cannot meet the requirements of high-power output.The development of lithium-ion batteries is constrained by cathode materials.At present,the most widely used cathode materials are lithium iron phosphate（Li Fe PO₄,LFP）.In this paper,carbon black（SP）and Li Fe PO₄were used as conductive agent and active material,respectively,and carbon nanotubes（CNTs）and graphene（G）with excellent propertieswere used as conductiveadditives.The electrodeslurry with unitary,binary,and ternary conductive compositeswere preparedby mechanical stirring methods,respectively,then coated on aluminumfoilas lithium-ion batterieselectrodes.By adjusting the proportion of the conductive agent component in the electrode slurryand the multiple compounding,observing and comparing the morphology of the electrode,the influence of the conductive agent on the electrochemical performance of the battery was discussed.This work will provide theory and practice basis for the application of high-performance conductive agents in lithium-ion batteries.The specific conclusions of this paper are as follows:（1）Unitary conductive agent composite electrodes with Li Fe PO₄as active material,carbonblack,carbon nanotubesand graphene as conductive agentswere prepared by mechanical stirring methods.The dimension and additive amount were discussed.The effect of the dimension and additive amount of unitary conductive agent on the electrochemical performance were discussed.The results indicate that LFP-G electrode shows a relatively higher specific capacity and better cycling performance than LFP-CNT and LFP-SP.After 500cycles,the capacity of LFP-G electrode still keeps 97.4 m Ah g^-1 with a capacity retention rate of 74.5%.The CNTs were not easily dispersed in the electrode but it can increase the lithium ion diffusion rate.When 3%G or CNTswere added,both electrodesshowed excellent electrochemical performance.At 1 Cafter 140 cycles,LFP-0.3CNT delivered a capacity of163.3m Ah g^-1 with a capacity retention rate of 93.2%.But withthe increasing of G,surface defects of the electrode also increase.For LFP-SP electrode,too much SP cannot improve the cycle performance of the battery,but it can reduce the ion diffusion resistance.（2）Combination of the advantages of three conductive agents,binary conductive agent composite electrodes with Li Fe PO₄as active material,SP,CNTsand Gascompositeconductive agentswere prepared by mechanical stirring methods.The effects of the binary conductive network on the electrochemical performance of the battery were discussed.Results indicate that SP is beneficial to the dispersion of CNTs in the electrodes,but if the proportion of CNTs continuesto increase,severe entanglement of CNTs in the electrodes was observed.When the current density was changed from 0.2C to 5C,LFP-0.3SP-0.7CNT delivered a capacity of53.2%,which was lower than that of LFP-0.3SP-0.7G.However,the cycling performance of the battery was improved,which can be attributed to the superior adhesion of CNTs.Furthermore,the addition of graphene was beneficial to improve the rate performance of the battery,which can beascribed to the excellent conductivity of G.（3）Based on the binary conductive network,the SP-CNT-G ternary conductive electrodes with Li Fe PO₄as active material,SP,CNTsand Gascompositeconductive agentswere prepared by mechanical stirring methods.And the effects of the ternary conductive network on the electrochemical performance of the battery were discussed.The results show that the SP-CNT-G conductive network constructs a good conductive path.In 3S-CGxy electrodes,CNTs and G have a high proportion in the electrode,which causes serious entanglement of CNTs,while graphene stacking also increases the defects of the electrode.But in 5S-CGxy electrodes,the entanglement of CNTs becomes slight due to the increasing of SP content.Electrochemical tests indicate that when the current density increases from 0.2 C to 5 C,the5S-CG73 shows the highest capacity retention rate（73.5%）.And 5S-CG73 also shows the best cycle performance at different current densities.This is probably because increasing the content of SP makes CNTs easily dispersed in the electrode.And because the adhesion of CNTs,5S-CG73 is not easy to fall off during cycling,hence shows the best cycling performance.