Synthesis, Structrure And Electrochemical Performance Of Doped Spinel LiMn₂O₄ And Layered LiNiO₂ As Cathodic Materials For Lithium-ion Batteries

Lithium ion batteries have become the primary choice of the green batteries in the 21th century due to its high voltage, high energy density, no memory and non-pollution. At present, the lithium cobaltate, LiCoO₂, is the major cathode active material of commercial lithium ion rechargeable batteries. However, because of its expensive costs and toxicity, it couldn't meet lithium ion batteries development. Many attempts have been made to develop new cathode materials with lower cost, safety and excellent electrochemical performance. This work aims at developing spinel LiMn₂O₄ and layered LiNiO₂. New synthesized methods, and doping or substituting by metal element are employed to enhance their electrochemical performance. The structure and morphology of these products were investigated by the means of TG/DTA, XRD, SEM and TEM. The electrochemical performances of the compounds have been evaluated by the means of galvanostatic charge-discharge cycling, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS).Spinel LiMn₂O₄ powder was prepared by a spray-drying method, in which the corresponding acetate salts as the reagents. The product showed a high purity, good crystallinity, small particle size and homogenous size distribution. The effects of synthesis temperature and time on the morphology, structure and electrochemical performances of LiMn₂O₄ had been discussed. It was found that well-crystallized LiMn₂O₄ prepared at 750°C for 24 h in air exhibited the best electrochemical performance. In contrast with LiMn₂O₄ prepared by solid-state reaction with the same sintered process, the LiMn₂O₄ prepared by spray-drying method showed better particle characteristics and better electrochemical performances. Its initial charge capacity and rate capability were improved. The initial capacity was 123 mAh g^-1 at 0.1 C, and at 2 C it also delivered 107 mAh g^-1. Spinel LiMn₂O₄ powder had been successfully synthesized by a hydrothermal method directly, which was no any pretreatment and following treatment in the process. The LiMn₂O₄ delivered reversible capacity of about 121 mAh g^-1 at a current density of 1/10 C. Cycled the cell to 40 cycles, the capacity remained at about 111 mAh g^-1 at 1/2 C.Ni-doped spinel LiMn_2-xNi_xO₄ (0.01≤x≤0.06) was prepared and the dopant effects on structures and electrochemical performances were investigated. Compared with the pure LiMn₂O₄, with increasing the value x of Ni-doped, the Mn average valence in LiMn_2-xNi_xO₄ increased, and the amount of Mn³⁺ decreased, which could restrain the Jahn-Teller effect and reduce the Mn³⁺ dissolution. Through some electrochemical tests, it showed that the capacity of spinel LiMn_2-xNi_xO₄ (0.01≤ x ≤0.06) decreased with the increase of the amounts ofNi-doped, but the cycle capability improved.Abundant Ni-substitated LiMn_2-xNi_xO₄ powders were prepared by spray-drying method, and the x of Ni content was controlled from 0.1 to 0.5. The Ni-substitated LiMn_2-xNi_xO₄ still showed single spinel phase (Fd-3m) without any impurity. The change of lattice parameter was accord to a linearity relation with the Ni-substitated amount. The cell shrunk with the decrease in the lattice parameter. The electrochemical performance would change with the variation of Ni content. When the value x of Ni increased, the amount of Mn³⁺ decreased, which determined the theoretical capacity of 4 V range, and also the amount of the redox pair of Ni²⁺/Ni⁴⁺ increased, which was relevant to the theoretical capacity of 4.6 V plateau. The results showed the factual capacity of LiMn_2-xNi_xO₄ (0.1≤x ≤0.5) had a linearity relation with the Ni-substitated amount. When x was 0.5, the product LiMn_1.5Ni_0.5O₄ still had little Mn³⁺ due to the oxygen absence, and the main capacity happened in 4.6 V plateau. At room temperature, the LiMn_1.5Ni_0.5O₄ exhibited good cycle performance. The initial capacity of LiMn_1.5Ni_0.5O₄ was 124 mAh g^-1, after 50 cycles, it still retained over 110 mAh g^-1 Through ex situ XRD measurement, there were only the shrinkage and enlargement of the lattice parameter, and the structure of LiMn_1.5Ni_0.5O₄ did not change during the charge and discharge process between 3.2-4.95 V.Co³⁺ was doped to stabilize the structure of LiMn_1.5Ni_0.5O₄ due to stronger Co-O bond being than Mn-O and Ni-O. Spinel LiMn_1.5Ni_0.5-xCo_xO₄ (0.1≤x ≤0.5), LiMn_1.5-xNi_0.5Co_xO₄ (0.1≤ x ≤0.5) and LiMn_1.5-xCo_2xNi_0.5-xO₄ (0.05≤ x ≤0.25) compounds were prepared by spray-drying method. The XRD results showed that lattice parameter of three series compounds were accord to a linearity relation with the Co-doped amount respectively. With increasing the amount of Co, the lattice parameter diminished. After electrochemical tests, it is found that the redox pair of Mn³⁺/Mn⁴⁺ corresponds to the 4 V plateau, Ni²⁺/Ni⁴⁺ corresponds to the 4.6 V plateau and Co³⁺/Co⁴⁺ corresponds to the plateau above 5 V during the charge/discharge process. With increasing the Co³⁺ amount, the valances of the transition metals (Mn, Ni, Co) in the compounds would bring variety, which arose the change of the charge/dischage plateaus. The upper voltage was limited to 4.95 V for fear of possible electrolyte decomposition, especially upon extended cycling, which led to the capacity related to Co³⁺/Co⁴⁺ (which is above 5 V from the CV results) abandon. In the range of 3.2-4.95 V, the discharge capacity would decrease, while all of three series compounds showed good cycle stability at high temperature (55℃). For the spinel LiMn_1.5Ni_0.5-xCo_xO₄ (0.1≤x ≤0.5), when the amount x of Co³⁺ increased, the amount of Ni²⁺ decreased, and the amount of Mn³⁺ increased, which led to the theoretical capacity of 4.6 V diminished and the theoretical capacity of 4 V increased. When x =0.5, the capacity of LiMn_1.5Co_0.5O₄ only happened in 4 V range between 3.2-4.95 V. The initial capacity was 66 mAh g^-1, and after 20 cycles thecapacity didn't any obviously fade. About spinel LiMn_1.5-xNi_0.5Co_xO₄ (0.1≤ x ≤0.5), when Co³⁺ doped Mn⁴⁺, the valence of Mn didnot change, and partial Ni²⁺ changed to Ni³⁺, which induced the theoretical capacity of 4.6 V decreased. The spinel LiMn_1.5-xNi_0.5Co_xO₄(0.1≤ x ≤0.5) delivered the capacity of 119, 111, 95, 80 and 63 mAh g^-1 (x=0.1, 0.2, 0.3, 0.4 and 0.5). For the spinel LiMn_1.5-xCo_2xNi_0.5-xO₄ (0.05≤ x ≤0.25), Co substituted the Mn and Ni site at same time. When the amount of Co³⁺ increased, the valence of both of Mn and Ni retained stability, the amount of Mn⁴⁺ and Ni²⁺ decreased, and the theoretical capacity of 4.6 V decreased. In the series of LiMn_1.5-xCo_2xNi_0.5-xO₄ compounds, LiMn_1.35Co_0.3Ni_0.35O₄ showed the initial capacity value of 99 mAh g^-1 , and after 20 cycles, the capacity obtained over 90 mAh g^-1 at high temperature (55℃).The layered structure LiNi_0.8Co_0.2O₂ cathode material for lithium-ion batteries was synthesized by sintering the precursor at 750℃ for 24 h in oxygen, which was obtained from the corresponding metal acetate solution via a spray-drying method. Compared with the product prepared in air, the LiNi_0.8Co_0.2O₂ particles obtained in oxygen were fine, narrowly distributed and well crystallized. The product had better layered structure with obvious (108)/ (110) and (006)/ (102) peak splitting and higher ratio of I(003)/I(104) than that prepared in air. As a result, the LiNi_0.8Co_0.2O₂ prepared in oxygen had excellent electrochemical properties. The initial discharge capacity reached 176 mAh g^-1, and its 20th cycle capacity kept to be 172 mAh g^-1 at a current density of 0.1 C.Some Mn⁴⁺ were introduced to layered LiNi_1-xCo_xO₂, the valence of Ni would decreased. The layered LiNi_1/3Co_1/3Mn_1/3O₂ cathode was prepared by spray-drying method. It was found that at at 850℃ for 24 h in air, the obtained layered LiNi_1/3Co_1/3Mn_1/3O₂ showed better crystal structure and better particle characteristics. When the cut-off voltage was controlled at 4.8 V, the initial discharge capacity was 183 mAh g^-1, when the cut-off voltage decreased to 4.3 V, the initial discharge capacity was 151 mAh g^-1, and it showed excellent cycle performance, after 10 cycles, the capacity was over 97% of initial capacity.