| Rechargeable lithium ion batteries are widely used in portable electronic devices andhybrid electric vehicles because of high energy density, long cycle life and environmentalbenign. Nowadays the development for LIBs has encountered the bottleneck, which demandedpeople to design and synthesize new materials for solving the problems, especially cathodematerials. In cathode materials, the layered Rich-Li Li1+zM1-zO2(M is one or more than onetype of transition metal elements, z≥0) has been the most potential materials due to its highcapacity (>250mAh/g). In our precious work, we have found that the change of lithum contenthas effect on the structure, morphology and electrochemical properties of materials, at thesame time, when a little Co element was doped, the structure, morphology andelectrochemical properties of materials have also changed. This series materials could benamed as Liδ[Ni0.25-zMn0.75-zCo2z]Oy（0≤δ≤2.0,0≤z≤0.25）Li+–insertion materials, which thestructure, morphology and electrochemical properties of materials have changed as the δ and zchanged, and the composite of this series materials (0.5<δ<1.5) has exhibited more excellentcombination property, so Liδ[Ni0.25-zMn0.75-zCo2z]Oy（0≤δ≤2.0,0≤z≤0.25）Li+–insertionmaterials provided more assistance and possibility for exploring new materials for LIBs. Butthe previous research was primary understand for LIBs, and this thesis studied the followingthree contents centered on the tetrahedral phase diagram of Li-Ni-Mn-Co in order tounderstand the series materials deeply:Firstly, the precursors were synthesized by carbonate coprecipitation methode. BecauseLiδ[Ni0.25-zMn0.75-zCo2z]Oy(0≤δ≤2.0,0≤z≤0.25)series materials are Mn-based materials.Mn(OH)2was easy to oxidated and Mn2+could be combined stably with CO32+. The ICP andXRD results suggested that the prepared precursors were consistent with designed precursorscomponent. In order to determine the synthesis route of Liδ[Ni0.25-zMn0.75-zCo2z]Oy, theprecursors were calcined to oxide firstly. The XRD results showed that it was not pure phaseM3O4with the calcined temperature of500-600oC, so the Liδ[Ni0.25-zMn0.75-zCo2z]Oy(0≤δ≤2.0,0≤z≤0.25) materials were synthesized by precursors and Li2CO3.Secondly, the effect of Li and Co contents on structure, morphology andelectrochemical properties of Liδ[Ni0.25-zMn0.75-zCo2z]Oy(0≤δ≤2.0,0≤z≤0.25) series materialswas studied. The results revealed that, for each certain z, as the lithium contents increased, thestructure of materials evolved from spinel (Fd-3m)(0≤δ≤0.5) to a mixed spinel and layeredstructure (Fd-3m+R-3c)(1.0≤δ≤1.2), and then to a pure layered Rich-Li (R-3c)(1.5≤δ≤2.0).The size of primary particles and roughness of second particles changed, and theelectrochemical properties of materials also exhibited remarkable changes. For each certain δ,Co has not caused more impacts on structure and morphology of materials, except for threecompositions (z=0.15, δ=0.25),(z=0.25, δ=0.25/0.5), which the structure of materials was not pure spinel, but the space group was Fd-3m and I41/amd. At the same time, theelectrochemical properties of materials were also influenced by Co contents.Finally, based on the results above, we designed and synthesized composite materialswith core-shell structure {[Li1.0Ni0.5Mn1.5O4]1-x}core·{[Li1.2Ni0.2Mn0.6O2]x}shell. First, wepreliminary studied core-shell structure materials with high proportion (x=10/14, δ=1.0), andcompared with easy composite materials {[Li1.0Ni0.5Mn1.5O4]1-x}·{[Li1.2Ni0.2Mn0.6O2]x}, thenstudied the effect of temperature(800oC,850oC,900oC) on these two structure materials.XRD showed that temperature had seldom effects on these two composite materials; TEMdemonstrated that the core-shell materials had already formed; SEM showed that thecrystallinity increased on the surface of the materials with the temperature increasing. Theelectrochemical properties of materials exhibited remarkable changes due to the temperature.The core-shell materials synthesized at800oC displayed the best electrochemical properties.Second, when the x value was10/209,10/106,10/56(δ=0.52，0.54，0.58), it could be regaredas spinel materials coated with Rich-Li due to x was very small. XRD results revealed that thestructure of coated and uncoated materials was similar, and some Li2MnO3phase was formedwith the increasing of coating materials; SEM results displayed that the surface of particlesfor noncoated materials was more smooth than coated materials; The results fromelectrochemical properties of materials showed that, the first discharge capacity was higherthan noncoated material, and the first discharge capacity was gradually decreasing, and thecycling life of coated materials was seldom better than noncoated material. |
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