Synthesis Of LiNi_xCo_yMn_1-x-yO₂ Cathode Material By Carbonate Co-precipitation Method And Its Characterization

Cobalt in LiCoO₂ has economic and environmental problems that leave thedoor open to exploit alternative cathode materials. Among several alternatives,layered Li-Ni-Co-Mn-O composites have been regarded as one of the mostprospective lithium intercalation materials. In this paper, layered LiNi_xCo_yMn_1-x-yO₂is synthesized by carbonate co-precipitation method. The relationship amongtransition metal ratio, lattice parameters, electrochemical capacity, cycling stabilityand improvement of physical and chemical performance were investigatedsystematically.In our previous studies, LiNi_0.6Co_0.2Mn_0.2O₂ was synthesized by the mixedhydroxide method. However, Mn(OH)₂ is not stable in the solution with high pHvalues which brings about secondary phase of MnO_x to make negative influence onthe electrochemical characters of the final product and limit the commercializationof this new type of cathode material. In order to avoid the appearance of the secondphase, LiNi_0.6Co_0.2Mn_0.2O₂ was synthesized by carbonate co-precipitation method.The results showed that the precursor had a smaller particle size and the finalproduct had a higher discharge capacity compared with the products synthesized byhydroxide co-precipitation method when cycled in the same voltage window.As pH value up to 9, the phase of Ni_0.6Co_0.2Mn_0.2(OH)₂ appeared, while thecontent of Mn decreased sharply when pH fell to 7. Therefore, the optimal pH valuefor the basic carbonate was between 8 and 9.Synthesis mechanism of LiNi_0.6Co_0.2Mn_0.2O₂ cathode material starting fromprecursor and Li₂CO₃ was investigated by Differential thermal analysis (DTA), SEMand Ex-situ X-Ray diffraction (XRD) analysis at high temperature. The results showthat the synthesis of LiNi_0.6Co_0.2Mn_0.2O₂ undergoes a solid-solid process, and theoptimum reaction temperature for hexagonal layered structure is 900℃. The CVcurves of LiNi_0.6Co_0.2Mn_0.2O₂/Li illustrate that only one couple of peaks appears between 2.8-4.4V, implying that there is no structural transition from hexagonal tomonoclinic in this voltage range.In the optimized condition, between the voltage field of 2.8～4.3 V, at 0.2 C rate,the initial charge and discharge capacity reached to 210.3mAh/g and 179.8mAh/grespectively, meanwhile, the initial charge-discharge efficiency is 83%.In order to decrease the cost of LiNi_xCo_yMn_1-x-yO₂, LiNi_0.4Co_0.2Mn_0.4O₂ isregarded as the object by carbonate co-precipitation. In the operated condition of2.8～4.3 V, at 0.2C rate, the initial charge and discharge capacity reached to172.7mAh/g and 152.1mAh/g respectively, meanwhile, the initial charge-dischargeefficiency is 88.1%, and the discharge plateau is 3.75V. After 50 cycles, thedischarge capacity remains 136mAh/g. At 0.2C rate, between 2.8～4.3 V and 2.8～4.5V, the initial discharge capacity is from 150 mAh/g to 169 mAh/g. After 50 cycles,the charge-discharge efficiency is from 91.3% to 88.2%.In order to improve the physical and chemical properties ofLiNi_0.4Co_0.2Mn_0.4O₂, additives of A and B were used during the reaction. After usingadditive A, the tap density increases to 2.05g/cm³.The results show that both LiNi_0.6Co_0.2Mn_0.2O₂ and LiNi_0.4Co_0.2Mn_0.4O₂ arepromising alternatives instead of LiCoO₂ for their high capacity, good cyclingstability and excellent safety performance.