Synthesis And Modification Of Li_1.2[M_0.4Mn_0.4]O₂(M=Ni,Co,Ni_1/2Mn_1/2,Fe)as Li-rich Layered Cathode Materials For Li-ion Batteries

The cathode material of Li-ion battery is one of the key factors that affect the performance of the batteries.To explore cathode materals with higher energe density,higher working voltage and lower cost,a number of efforts have been made in the past decades.Among the cathode materials,Li-rich materials,which have lower cost and higher specific capacity have attracted much more attentions.However,due to the complexity of components and structures,the electrochemical reactions and the crystal structures of the the Li-rich materials have not been understood completely.The effects of temperature on the performance of Li-rich materials have been extensively studied.However,the influence of the components and temperature on the valence states of transition-ions is rarely reported.Therefore,a series of Li-rich materials Li_1.2[M_0.4Mn_0.4]O₂(M=Ni,Co,Ni_1/2Mn_1/2,Fe)were desined,synthesized and studyed.Besides,based on the above reasrch,a novel core-shelled structure Li-rich material was designed and prepared via co-precipation method.Firstly,to fully understand the effect of sintering temperature on the valence states of transition-ions and the different of redox couples among different Li-rich materials,a series of Li-rich materials Li_1.2[M_0.4Mn_0.4]O₂(M=Ni,Co,Ni_1/2Mn_1/2,Fe)were synthesized via sol-gel method and the effect of sintering temperature on the crystal structure were studied in detial.The results show that when M=Ni,higher sintering temperature may cause the formation of Li[Li_x(Ni_0.5Mn_0.5)_1-x]O₂+（0.1-0.5x）Li₂O（0￡x<0.2,Ni=+2 and+3）;For M=Co and Ni_1/2Mn_1/2 samples,single phase with layered structure can be obtained in the investigated temperature range and higher sintering temperature contributes to improving their crystallinity.Besides,when M=Fe,the mixture of cubica-LiFeO₂ and layered Li₂MnO₃ were always observed simultaneously.Electrochemical tests indicate that the reactivity of the redox couples in the M=Co sample are higher than that of the other two samples.By contrast,the redox couples in the M=Ni sample shows the lowest reactivity among the three samples.Besides,local phase transition from layered structure to spinel-like phase during cycling can be detected from the charge/discharge curves and XRD patterns for M=Co sample,while the layered structure can be maintained very well during cycling in the M=Ni and Ni_1/2Mn_1/2samples.Secondly,besed on the above study,which show that the Li_1.2(Co_0.5Mn_0.5)_0.8O₂ material has the higher initial specific capacity and worse cycling performance while the Li_1.2(Ni_0.5Mn_0.5)_0.8O₂ material has the better cycling performance,a novle core-shelled structure material Li1.2[（Co0.5Mn0.5）1/2（Ni0.5Mn0.5）1/2]0.8O2 was synthesized via co-precipation method.The novle structure was designed with Co-rich materials act as core and Ni-rich materials act as shell,and keep the Mn elements unchanged.It is expected that the synergetic effect of core Li1.2（Co0.5Mn0.5）0.8O2 and shell Li1.2（Ni0.5Mn0.5）0.8O2 can prevent the dissolution of Co element,and alleviate the capacity fading as well as enhance the rate capability of Li_1.2(Ni_0.25Co_0.25Mn_0.5)_0.8O₂.Electrochemicaltestsshowthatcomparedwith Li_1.2(Ni_0.25Co_0.25Mn_0.5)_0.8O₂,improved cyclability,rate capability and thermal stability are obtained in Li_1.2[(Co_0.5Mn_0.5)_1/2(Ni_0.5Mn_0.5)_1/2]_0.8O₂.