Studies On The Synthesis And Electrochemical Performance Of Oxygen-deficient Li₂MnO_3-?

Lithium-rich layered oxides xLi₂MnO₃·?1-x?LiMO₂(M=Ni_1/3Co_1/3Mn_1/3)have been recognized as one of the ost promising cathode materials due to their high specific discharge capacities of 230-300 mAh·g^-1.However,these cathode materials have fatal drawbacks of discharge voltage decay and irreversible capacity loss occurring during galvanostatic charge-discharge cycles,and their high-capacity mechanism is still ambiguous up until now.As a generally recognized“inert”component of the lithium-rich layered oxides,Li₂MnO₃ has been treated as an ideal structural model?LiMO₂,M = Li1/3Mn2/3?and may possess a high theoretical specific capacity of 459 mAh·g^-1 if there exist four-electron conversion mechanism.How to prepare the crystallographically phase-pure sample of Li₂MnO₃ and then to investigate its possibly high-capacty feature are of great importance,which becomes one of hot research topics in reacent years.Herein,both the phase-pure sample of Li₂MnO₃ and its oxygen-deficient form of Li₂MnO_3-? have been prepared at first,then the much higher electrochemical activity of the oxygen-deficient form proves the anionic-cathodic redox mechanism of the pristine form,and then the electrochemical properties of Li₂MnO₃(or Li₂MnO_3-?)and LiNi_1/3Mn_1/3Co_1/3O₂ mixtures have been conducted.That is,this dissertation deals with two aspects,shown as below.Li₂MnO_3-? crystallites was prepared by the solid-state NaBH₄-assisted heat treatment of nanocrystalline Li₂MnO₃,the resulting material acquire distinguishable appearances in color and shape and slight differences in surface composition and lattice structure.As a LIB cathode within the potential range of 2.5-4.7 V,at 20 mA·g^-1,pristine Li₂MnO₃ gives the specific discharge capacities of 3.3,5.0 and 7.4 mAh·g^-1 in the 1st,10th and 100th cycles,while the derivative Li₂MnO_3-? delivers the relatively high values of 64.8,103.8 and 140.2 mAh·g^-1 in the 1st,10th and 120th cycles,respectively.Aside from the similar phenomenon of gradual electrochemical activation,substituting Li₂MnO_3-? for Li₂MnO₃ means the great enhancements of charge-transfer ability and electrochemical performances.Especially,the cationic-anionic redox mechanisms of Li₂MnO₃ and Li₂MnO_3-? are similar to each other,suggesting a possible solution to prepare high-performance xLi₂MnO₃-s·?1-x?LiMO₂ solid solutions for application purposes.LiNi_1/3Co_1/3Mn_1/3O₂ powder material was prepared by co-precipitation method.The material has good crystallization with particle size of 100-500 nm.A mixed electrode was prepared by physically mixing LiNi_1/3Co_1/3Mn_1/3O₂ and Li₂MnO_3-? or Li₂MnO₃ according to different mass ratios,and the effect of different mixing ratios on electrochemical performance of mixed electrodes were investigated.When LiNi_1/3Co_1/3Mn_1/3O₂ is mixed with Li₂MnO_3-? at a mass ratio of 5:5,the mixed electrode has a discharge capacity of 123.0 mAh·g^-1 during the 1st cycle,and capacity retention rate can reach 87.1%after 100 cycles,which is higher than that of pure LiNi1/3Co2/3Mn1/3O₂?61.7%?.After mixing LiNi_1/3Co_1/3Mn_1/3O₂ and Li₂MnO₃ at different mass ratios,the addition of Li₂MnO₃ can improve capacity retention rate of mixed electrodes.The effect of“inert component”Li₂MnO₃ and its content on electrochemical cycling stability of ternary material was studied.Besides,by substituting Li₂MnO_3-? for Li₂MnO₃ in Li-rich layered oxides,the mixed electrode realized an enhancement in both capacity and cycling stability.