| Nowadays,the research of high energy cathode material has been the key to realize the next generation of lithium ion batteries.The manganese-based cathode materials with low cost,especially the layered phase Lit+zM1-zO2(M=Mn1-x-yNixCoy,z>0)and spinel phase LiMxMn2-xO4(x>0)with high voltage have been considered to be two of the most promising cathode materials for next-generation lithium-ion battery.Li-rich layered materials with high specific capacity and excellent stability have attracted much attention.However,they have large initial irreversible capacity loss in the initial charging of high potential.Spinel materials have drawn much attention because of its high operating voltage,low cost and satisfactory environmental friendliness.What's more,its three dimensional diffusion channels of lithium ions insure the excellent rate performance.However,the spinel material suffers from low specific capacity and poor cycle life.The composites of Li1.2Nio.2Mno.6O2 layered material-LiNio.5Mn1.5O4 spinel material have been extensively studied,for they can make best use of the advantages and bypass the disadvantages of each material,achieving the complementary performance or synergistic effect.In this paper,we consider that the valences of transition metal elements in Li1.2Nio.2Mno.6O2 and LiNi0.5Mn1.5O4 materials are +2(Ni)and +4(Mn),and the ratios of transition metal ions are same.The change of Li content only influences the ratio of layered and spinel phases.Because the valences of Co and Cr are +3,so there exists a substitution equation(2Co3+/Cr3+?Ni2++Mn4+)and the ratios of transition metal ions are same in Li1.2Mo.4Mno.4O2 and LiMMnO4(M=Co,Cr)materials.Therefore,we designed new composite systems of Li1.2Co0.4Mn0.402-LiCoMnO4,Li1.2Cr0.4Mn0.4O2-LiCrMnO4 and Li-Co-Cr-Mn-O,and studied their structures and electrochemical performances.Firstly,the composites of Li1.2CoO.4Mn0.4O2-LiCoMnO4 were studied systematically.It is known that Li-rich layered Li1.2Co0.4Mn0.4O2 can be regarded as the Li2MnO3·LiCoO2 solid solutions,so the composites can be rewritten as{x[0.5Li(CoMn)04]·(1-x)[0.5Li2MnO3·0.5LiCoO2](x=0,0.25,0.5,0.75 and 1)}.The effects of different spinel contents(x)to the structure and performance of composites were studied.The results showed that the structures of composites changed from a layered structure(x=0)to mixed layered-spinel structures(x=0.25,0.50 and 0.75),and then to a spinel structure(x=1)with the increase of spinel content.All samples have uniform particles with smooth surfaces,and primary particle sizes become larger with the increase of spinel content.Electrochemical tests show that the layered-spinel composites present improved cycling performances and rate capabilities compared with those of Li-rich materials.The composite of x=0.25 exhibits the best electrochemical performances.Secondly,the composites of Lii.2Cr0.4Mn0.4O2-LiCrMnO4,{x[0.5Li(CrMn)O4]·(1-x)[0.5Li2MnO3·0.5LiCrO2](x=0,0.25,0.5,0.75 and 1)} were studied further.The results reveal that the structures of composites vary from layered(x=0)to hybrid layered-spinel(x=0.25,0.50 and 0.75),then to spinel(x=1).Electrochemical tests show that Coulombic efficiencies and rate capacities are remarkably improved of hybrid layered-spinel composites compared with pristine Li-rich material,and the samples of x=0.25 and 0.5 display better electrochemical performances.Finally,based on the above study,Li1.25(Co0.5Mn0.5)O2.375 and Li1.25(Cr0.5Mn0.5)O2.375 were chosen to design the Li1.25CoyCr0.5-yMn0.5O2.375(y=0.1,0.2.0.3,0.4)composites.The best contents of Co and Cr in composites were researched,as well as the structure,morphology and electrochemical performance were studied.The results show that the formed Li-Co-Cr-Mn-O composites have improved the cycling stabilities and rate performances compared with those of single phase,and the electrochemical performance of Li1.25Co0.3Cro.2Mn0.5O2.375(y=0.3)is the best. |
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