Synthesis And Modification Of LiNi_1/3Co_1/3Mn_1/3O₂ Cathode Material And Its Electrochemical Performance

Layer-structured LiNi_1/3Co_1/3Mn_1/3O₂ compound has been considered as a promising candidate of next-generation cathode materials to replace LiCoO₂ for rechargeable lithium ion batteries due to its large capacity and stable structure. In this dissertation, the recent research progresses in various synthesizing methods and modification in Li[Ni,Co,Mn]O₂ are generalized and analyzed in detail. The current main problems and further R&D trend of the material are also discussed. Li[Ni,Co,Mn]O₂ cathode material has been traditionally synthesized by hydroxide co-precipitation. The co-precipitation method could give phase-pure oxide products and spherical powders with high tap-density and uniform distribution. But this synthesis method must control the pH value of the solution at certain a range and precipitated Mn(OH)2 is easily oxidized to MnOOH or MnO₂ without inert gas protection in preparing precursor. Furthermore, filtering, washing and precalcination procedures are also required. Thus, it is very difficult to reproduce the material with little difference. So, it is necessary to develop a new simple synthesis route for LiNi_1/3Co_1/3Mn_1/3O₂ compound.To solve above various problems, a simple but novel route was firstly proposed for preparing LiNi_1/3Co_1/3Mn_1/3O₂ cathode material. Technological conditions of this synthesis method were investigated systematically and optimized. The electrochemical characteristics of the prepared LiNi_1/3Co_1/3Mn_1/3O₂ samples were investigated. The effects of fluorine substitution and sucrose on performance of LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials were also studied.Submicron-sized LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials were synthesized using a simple self-propagating solid-state metathesis method with the help of ball milling and the following calcination. A mixture of Li(Ac)·2H2O, Ni(Ac)2·4H2O, Co(Ac)2·4H2O, Mn(Ac)2·4H₂O(ac=acetate) and excess H2C2O4·2H2O was used as starting material without any solvent. The mixtures were ballmilled in a planetary mill. The ball milling would cause a metathesis reaction between the acetate salts and H2C2O4·2H2O. It is apparent that the crystalline (or coordinated) water in hydrated metal salts plays an important role in the solvent-free solid-state reactions. The obtained ballmilled product was the mixture of transition metal oxalates and Li2C2O4. The product was calcined at 900°C in air atmosphere for 20h to obtain final LiNi_1/3Co_1/3Mn_1/3O₂ material without any precalcination procedure. XRD analyses indicate that the LiNi_1/3Co_1/3Mn_1/3O₂ was formed with hexagonalα–NaFeO₂ structure (space group: 166, R 3? m). XPS studies show that the predominant oxidation states of Ni, Co and Mn in the LiNi_1/3Co_1/3Mn_1/3O₂ compound are 2+, 3+ and 4+, respectively with small content of Ni3+ and Mn3+ ions. The steady discharge capacities of the material cycled at 1C (160mA/g) rate are at about 140mAh/g after 100 cycles in the voltage range 3-4.5V (vs. Li+/Li) and the capacity retention is about 87% at the 350th cycle. The excellent rapid charge-discharge cyclic performance of LiNiCo1/3Mn1/3O₂ cathode could be attributed to its special microstructure developed in the self-propagating solid-state metathesis method using the ballmilling procedure.The influence of fluorine substitution on the microstructure and electrochemical properties of LiNi_1/3Co_1/3Mn_1/3O₂ was also studied. LiNi_1/3Co_1/3Mn_1/3O₂-zFz (z=0, 0.04, 0.08 and 0.12) samples were fabricated by adding LiF in the starting mixture. XRD analyses indicate that LiF was completely introduced into LiNi_1/3Co_1/3Mn_1/3O₂ matrix in the samples with small amount of LiF additive. All samples have a phase-pure hexagonalα–NaFeO₂ structure with a space group of R 3? m and no obvious impurity phase peaks. As the fluorine content increases, the primary particle size of LiNi_1/3Co_1/3Mn_1/3O₂-zFz gradually increases as confirmed by FESEM which in turn results in high tap density。Although the fluorine-substituted materials have lower initial discharge capacities, a small amount of fluorine-substituted LiNi_1/3Co_1/3Mn_1/3O₂-zFz (z=0.04 and 0.08) exhibit excellent cycling stability and rate capability compared to fluorine-free LiNi_1/3Co_1/3Mn_1/3O₂.The effects of sucrose on performance of LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials.have been studied. It is found that adding a small amount of sucrose in precursor materials could improve the microstructure and electrochemical characteristics of LiNi_1/3Co_1/3Mn_1/3O₂ products in the following ways: 1) Sucrose acts as a blending agent to keep homogeneous mixing and tight contact of starting materials. 2) Sucrose functions as the source of a mesoporous carbon during its decomposition, which could prevent the agglomeration of the particles. 3) The cation mixing in the layer structure was prevented and the specific surface area of LiNi_1/3Co_1/3Mn_1/3O₂ was increased. As a result, sucrose-added samples exhibit much better electrochemical performance than the sucrose-free sample.