Optimization Of The Synthesis Technology And Investigation On The Performance Of Layered LiNi_1/3Co_1/3Mn_1/3O₂ Used For Lithium Ion Battery

Recently, Layered Li[Ni_(1-x)/2Co_xMn_(1-x)/2]O₂ is considered to be one of the best candidates to replace LiCoO₂. In this paper, we optimized the composition of Li[Ni_(1-x)/2Co_xMn_(1-x)/2]O₂ system firstly, and layered LiNi_1/3Co_1/3Mn_1/3O₂ was selected to be the object of our research. Layered LiNi_1/3Co_1/3Mn_1/3O₂ was synthesized by mechanical activation and co-precipitation route. The investigations of XRD, SEM, TG-DTA, laser particle size distribution, galvanostatic charge-discharge and cyclic voltammetry were introduced to characterize the structure, micro-morphology and electrochemical properties of the layered LiNi_1/3Co_1/3Mn_1/3O₂. The interfacial process and diffusion capability of Li-ion in layered LiNi_1/3Co_1/3Mn_1/3O₂ were investigated by ac impedance and potential step methods.With ball-milling activation, uniform precursor with high reacting activity was obtained. The layered LiNi_1/3Co_1/3Mn_1/3O₂ was then synthesized by one-step high temperature heating in air. And the synthesizing conditions such as activation time, synthesizing temperature and period were optimized. Ball-milling activation decreases the particle size of the active material. And the active material is able to participate in the intercalation and de-intercalation reactions completely. So the charge and discharge capacities of the materials increase with the extension of activation time. However, the specific area of the avtive material increases with the decrease of particle size after long time ball milling. Thus the irreversible capacity loss increases and the charge-discharge efficiency decreses. Raising synthesizing temperature and prolonging heating time appropriately can improve the crystallinity and the layered structure of the active materials. The optimal technological conditions for preparing layered LiNi_1/3Co_1/3Mn_1/3O₂ are: the atomic ratio of Li to M(Ni+Co+Mn)=1.02, the synthesizing temperature=980℃, the heating time=14 hours, and the time duration of ballmilling=4 hours. The initial charge and discharge capacity are 189.5 and 153.2mA.h·g^-1, respectively, with charge/discharge efficiency of 80.8% in the potential range of 2.75-4.3V (vs.Li/Li⁺). The discharge capacity remains 98.3% in the 30^th cycle. The tap density of LiNi_1/3Co_1/3Mn_1/3O₂ was improved remarkably by compress the precursors together, and the optimized pressure was 30MPa.The relationship of concentration and pH value in M²⁺(Ni²⁺+Co²⁺+Mn²⁺)-NH₃-CO₃^2--H₂O system was calculated. Co-precipitation pH value in the system was optimized according to the precipitation efficiencies. The optimal pH value is 8.0, and the precipitation efficiencies of Ni, Co and Mn are 99.72%, 99.85%, 99.48% respectively. The particle size of precipitate as-prepared is in the range of 15-20μm. The pre-decomposition process of the precipitate was investigated, and the optimal pretreatment temperature is 600℃. The oxide preserves the spherical morphology of carbonate precursor. Using the mixture of oxide and Li₂CO₃, layered LiNi_1/3Co_1/3Mn_1/3O₂ was synthesized by one-step high temperature heating in air. The optimized technological conditions for preparing layered LiNi_1/3Co_1/3Mn_1/3O₂ are: synthesizing temperature=980℃, and the time duration=12 hours. The initial charge/discharge capacity are 185.7 and 156.3mAh·g^-1, respectively, with the charge/discharge efficiency of 84.1% in the potential range of 2.75-4.3V (vs.Li/Li⁺). The discharge capacity remains 98.8% in the 40^th cycle. The above results show excellent electrochemical performances. Comparing these synthesizing methods mentioned above, the mechanical activation route is more promising for industrialization.The Li⁺ ion transmission mechanism in the interfacial region of LiNi_1/3Co_1/3Mn_1/3O₂ electrode was investigated by electrochemical impedance spectroscopy (EIS) technique. Equivalent circuit of layered LiNi_1/3Co_1/3Mn_1/3O₂ electrode during intercalation and de-intercalation process was suggested. The smaller particle size and larger specific area, the larger surface film impedance. Near to the intercalation/de-intercalation potential platform, the electrochemical impedance is the smallest. The electrochemical impedance of layered LiNi_1/3Co_1/3Mn_1/3O₂ material synthesized by co-precipitation route is smaller than that by mechanical activation route, and it increases more slowly during cycling process, which indicates better cycling stability. The diffusion coefficients (D_Li+), at different potentials during the intercalation/de-intercalation process were determined by potential stepmethod. D_Li+ reaches the maximum value of 1.54×10^-12 cm²Â·s^-1 near 3.8Vduring de-intercalation, and 6.08×10^-12 cm²Â·s^-1 near 3.6V during intercalation. Larger diffusion coefficient indicates that, the layered LiNi_1/3Co_1/3Mn_1/3O₂ synthesized by co-precipitation route has better reversibility during intercalation/de-intercalation process than that synthesized by mechanical activation method.