Hydrothermal Synthesis And Modification Study Of Spherical Micro-nano Structure Lithium-rich Cathode Materials

Compared with other cathode materials,layered structure lithium-rich(LLR)cathodes exhibit extremely high specific energies.So far,these materials have attracted much attention since it was reported firstly.However,the practical applications of LLR cathodes are still hindered by several significant challenges,including voltage decay,large initial irreversible discharge capacity,unsatisfactory rate performance and limited cycle life.In order to speed up LLR material,s procession of commercialization,researchers have put into many efforts to overcome above technical obstacles.In this paper,solvothermal method was used to fabricate LLR material 0.4Li2MnO3·0.6LiMn1/3Ni1/3Co1/3O2(Li1.4Mn0.6Ni0.2Co0.2O2.4)with optimized synthetic processes.In order to further improve the combination properties of material,some strategies such as structural design and surface modification were adopted in this dissertation.And the major research contents are as follow:1.The spherical carbonated precursor Mn0.6Ni0.2CO0.2CO3 was firstly prepared via solvothermal method,where its properties were detected by various analysis instruments.Next,how the total Li content and lithiation temperature affect the structure and electrochemical performance of material were studied deeply in this chapter.The research results demonstrate that the proper ratio of Li/TM(TM=transition metal Mn,Co,Ni)is 1.4 and the desired lithiation temperature is between 750-850 ?,where the samples with well crystallization and excellent performance can be obtained:1)when the lithiation temperature stays at 750 ?,the material exhibits highest initial discharge capacity as high as 298.5 mAh g-1 at 0.1 C(1 C=200 mA g-1)during 2.0-4.6V versus Li/Li+;2)when the lithiation temperature stays at 800 ?,the material exhibits optimum long term cycle performance,which can obtain a capacity retention rate of 85%at 0.5 C for 200th;3)when the lithiation temperature stays at 850 ?,the material exhibit excellent rate performance with the retention rate of 58%at 10 C,but the capacity declines with the increase of lithiation.And the crystallinity and electrochemical performance would be reduced at higher or lower calcined temperature.2.Based on the fundamental research of fabricating LLR material above,ternary self-template method was adopted to develop hollow Multi-component LLR cathode material.By adjusting the weight of NH4HCO3 precipitant during preparing spherical carbonate precursor,metal oxides with more porous can be obtained after the heat treatment.The further lithiation results demonstrate that the metal oxide with more porosities is beneficial for the formation of hollow structure,which extend the approach for designing hollow structure material.When the molar ratio of participant and total transition elements is 5:1,the hollow structure material and excellent electrochemical performance can be obtained.The target product displays a capacity of 151.3 mAh g-1 at 10 C.The capacity retention is 89%at 0.5 C for 200th,and exhibits a capacity of 86%even at high rate of 2 C cycling for 200th.Besidies,the result of galvanostatic intermittent titration technique(GITT)indicate a faster diffusion rate of Li+.3.Because of the intrinsic instability of the layered structure of LLR materials and its 2D Li+ transport channel with relatively lower diffusion rate,the method of surface modification was applied to change its surface structure.Ammonia solution(3 mol/L)was used to modify the surface of carbonate precursor and a small part of Ni element on the subsurface was removed,which process resulted in the formation of surface spinel structure with 3D Li+ channel.This special surface structure not only stabilize material's structure,but increase the transportation ability of Li+ between the substrate material and electrolyte and improve the cycle and rate performance.The study results demonstrate that the LLR material modified with ammonia solution for 10 min exhibit optimum electrochemical performance and faster Li+ conductivity.Moreover,the thin hetero surface can not sacrifice the high capacity of LLR materials,which optimize the combination performance in the maximum.