Study Of Two Layers Coating,Co-doping,Coating And La-doping Combined Effect In Lithium-rich Layered Oxide Material

Lithium-rich layered oxide material is considered to be the most promising next-generation cathode material because of its high specific discharge capacity and low cost.However,the lithium-rich layered oxide cathode material is facing some problems.Such as the high irreversible capacity loss in the first cycle,the poor rate capability,as well as the serious capacity fading and voltage decay,which hinder its commercial application.In recent years,a lot of research has been done on lithium-rich layered oxide material,but it is mainly concentrated on single ion doping or single shell coating,and little research has been done on the joint mechanism between them.In this paper,we will take up the CaF2 and graphite two-layers coating,La-ion and Mg-ion co-doping,La-doping and CaF2-coating,and research the joint modification mechanism among coating and coating,doping and doping,doping and coating,which hope to offer a reference for further improving the lithium-rich layered oxidized material by using multiple mechanisms.CaF2 has a fluorite structure,which can provide a large number of channels for lithium ion migration,and fluoride ions can induce excellent SEI film on the surface of lithium-rich layered oxide;graphite has excellent electronic conductivity and it is beneficial for reducing interface impedancebetween between active particles.Therefore,in this paper,CaF2 and graphite are used as coating reagents to study the joint modification mechanism between coating and coating.It is found that although the first specific discharge capacity of CaF2 and graphite two-layers coating lithium-rich layered oxide material is only 215.2 mAh g-1 at 0.5 C rate,the specific discharge capacity increase to 195.4 mAh g-1 after 150 cycles.Besides,compared with CaF2-coating and graphite-coating,its rate performance and interface impedance are also significantly improved.The lithium-rich layered oxide material mainly has three ion positions:oxygen ion-site,transition metal ion-site,and lithium ion-site.They play a different role in the entire lithium-rich layered oxide material.When the La ion is doped into the lithium-rich layered oxide material,it generally replaces the Mn-ion position in the transition metal layer,which is favorable for inhibiting the migration of the transition metal ion toward the lithium layer;when the magnesium ion is doped into the lithium-rich layered oxide,it generally replaces the lithium ion position in the transition metal layer,which is advantage for suppressing excessive deintercalation of lithium ions in the transition metal layer.Therefore,the joint mechanism between doping and doping is studied by using La(NO3)3 and MgO as dopant.After La-ion and Mg-ion co-doping,the discharge specific capacity and capacity retention rates after the cycle of 150 weeks are 196.3 mAh g-1 and 86.16%,respectively.Moreover,we find that it also exhibits the highest lithium ion migration rate after 150 weeks of cycling,which are 1.11 × 10-11 cm2 s-1 and 2.94 × 10-12 cm2 s-1,respectively.In addition,compared with the La-doping and Mg-doping,other electrochemical performance have also been significantly improved.Ion doping can improve the stability of crystal structure and surface coating can protect the active material from electrolyte erosion.They can improve the electrochemical performance of lithium-rich layered oxide material.Therefore,at the end of this paper,CaF2 is used as the coating agent and La(NO3)3 is used as the dopant to study the joint mechanism between coating and doping.The study find that the first discharge specific capacity at a rate of 0.5 C is 227.1 mAh g-1,and the discharge specific capacity retention is still 93.9%after 100 cycles,Besides,compared with the simple ion-doping and surface coating,other electrochemical properties have also been significantly improved.