Synthesis And Characterization Of Lithium-Nickel Based Oxides

Presently, the lithium-ion battery, fuel cell technology and the nickel metal hydride battery have been acclaimed as advanced power sources, slowly replacing several versions of conventional systems such as the lead acid and nickel cadmium batteries. Of all the possible systems, the lithium-ion battery stands a forerunner and market leader with its high energy density in miniature batteries. It is a compact, rechargeable power source stable to over 500 cycles. It can be fabricated in size ranging from a few microns to a large-scale battery capable of providing power for computer memory chips, communication equipment and electrical vehicles.The theoretical capacity of LiNiCh giving as high as 274 mAh/g is a hot issue in research. Mg-doped lithium nickelate can effectively enhance cyclability, and Mg²⁺ stabilize the layered structure by pillaring effect as Li⁺ deintercalation. Therefore, doping small amount of Mg is a good way to improve the electrochemical performance of LiNiO₂. The x-ray diffraction pattern of synthesized Mg-doped LiNiO₂ was used to investigate the function of parameters, including calcination temperature, length of time of calcination, heating rate, pelletizing pressure and atmosphere. LiNio.95Mgo.05O2 could be formed with I₀₀₃/I₁₀₄ ratio 1.03-1.09, R-factor (I₀₀₆+I(102))/I(101) ratio 0.68-0.94, and (108)/(110) splitted peak, da ratio 4.915-4.926, in R(3|-)m space group under the conditions of larger than 210 MPa pelletized powder, heating rate 2℃/min to precalcined temperature 600℃ for 9-15 hours, crushed and repelletized, heated till 600℃ in 2-5℃/min, then switched to 1 ℃/min till 750℃ isothermally for 20 hours all in oxygen flow. Nevertheless, the capacity is unsatisfactory.The capacity of synthesized LiNio.95Mgo.05O2 is really uncomfortable even though it is difficult to prepare. An increased ordering have been proved by Delmas group in lithium nickel cobalt oxide (LiNi_1-yCo_yO₂) system as the cobalt content increases. It is necessary to synthesize lithium nickel cobalt oxides in order to improve capacity and cyclability. Pure, cation-doped and anion-doped LiNi_1-yCo_yO₂. which are Lini_0.8Co_0.2O₂, Lini_0.8Co_0.2O_1.95F_0.05 and LiNio.75Co_0.2Mg_0.05O₂, respectively, were selected to perform tests. Each sample was prepared by precalcined at 600℃ for 10 hours, then calcined at 750℃ for 15 hours after reground and repelletized. It is found that dopants F, Mg can not affect the Li-ion diffusion, characterized by their chemical diffusion coefficients being in the same order as that of LiNi_0.8Co_0.2O₂. Through electrochemical impedance test, charge transfer resistance is increased as a result of the lattice distortion, volume change and microcracking in cathodematerial upon cycling while the random variation of SEI resistance relates the deteriorationand reformation process to the SEI layer. Upon cycling, the cathode materials reacted with the electrolyte and formed different functional groups, such as C=O, CH2,CH3, C-0 andOCO^,etc. The difference between the Fourier transform infrared spectroscopic (FTIR) patterns for pure and doped lithium nickel cobalt oxides is little. The cyclability of well-crystallized cathode material is somewhat improved by the dopants, but its capacity was still unsatisfactory, below 90 mAh/g initial capacity, even though the capacity is enhanced with doping Co.The use of solution chemistry based techniques, such as sol-gel, for synthesizing materials can yield molecularly homogeneous intermediate precursors to further improve capacity and cyclability. Xerogels were prepared utilizing maleic acid as the chelating agent, de-ionized (DI) water or ethanol as the solvent together with metal nitrates. After calcined at 750 °C for 12 hours, the Dl-water-prepared LiNio.8Coo.2O2 offers better capacity 135 mAh/g and cyclability and its activation energy is about one-half that of ethanol. The amount of Ni-ion occupancy on Li-ion site is smaller upon the investigation of Rietveld refinement. The reason of higher activation energy for ethanol-prepared xerogel is probably because it has been through the process of reduction that occurred at sol-gel, in which metals nickel and cobalt were found, and reoxidation which happened at calcine. The more stages the higher the activation energy. The coincidence is that nitrate ion itself plays the role for catalyzing the esterification of maleic acid accompanied by the reduction of metal salts.Citric acid has three carboxylic and one hydroxyl group for coordinating metal ions and supplies intimate blending among the ions. Use citric acid as the chelating agent and DI water as the solvent along with the same synthesis condition as that of maleic acid to prepare LiNio.8Coo.2O2. Its functional groups are completely same as those of using maleic acid upon studied by FTIR. The cyclability is good and the maximum capacity is 122 mAh/g lower than that of maleic acid.Coprecipitation like sol-gel is also a wet chemistry method. Sol-gel has been verified to improve both capacity and cyclability of the material. It is worth to conduct coprecipitation in conjunction with two-step calcining same as the previous solid-state reaction for synthesizing and testing LiNio.sCoo.2O2. P-(Ni,Co)(OH)2 was coprecipitated and calcined with LiOH. There is oxygen deficiency observed and the occupancy of Ni ions on Li-ion site is almost the same as that of citric acid through Rietveld refinement. Its Li-ion diffusion coefficients at roomtemperature and 55 °C are in the same order ¹0¹0 cm2/sec as those of LiNio.sCoo.2O2 prepared by maleic or citric acid.The capacity and cyclability of samples LiNio.8Coo.2O2 synthesized by sol-gel combined with calcining, coprecipitation combined with calcining and pure solid-state calcining were correlated to their Rietveld refinement. It is inferred that the smaller amount of Ni-ion on Li-ion site the better the capacity and cyclability. The reason is the longer bond length of Li-0 could be obtained with the less Ni occupancy, and this can facilitate Li-ion migration during intercalation and deintercalation. In addition, Ni3+/4+ ions tend to migrate from the octahedral sites (3a sites) of the nickel planes to the octahedral sites (3b sites) of the lithium planes via the neighboring empty tetrahedral sites (6c sites) at high temperature. Hence, the degradation of capacity fading is more serious for each sample at 55 °C.Hydrothermal route combined with calcining failed to synthesize pure and doped lithium nickel cobalt oxides. However, it is confirmed again that a well-mixed Ni with Co is indispensable to synthesizing lithium nickel cobalt oxides.