Research On Preparation And Doping Modification Of Spinel LiMn₂O₄ Cathode Materials With High Power

The lithium-ion battery, as a new type of green secondary power supply with advantages such as high energy density, high power density and high operating voltage, has been developing rapidly. Nowadays, it has already become the main power of the portable electronic devices such as mobile phones, digital cameras and notebook computers. However, the fast expending of its applications, especially in 3G communication technologies, electric vehicles and mobile energy storage devices, requires that the lithium-ion battery could have better performances, especially higher power density and operating voltage.Currently, the performances such as power density and operating voltage of lithium-ion battery are mainly constrained by the properties of cathode materials. Therefore, the spinel LiMn₂O₄ system with three-dimensional lithium-ion diffusion channels was chosen as the research object in this work. The nano-sized spinel LiMn₂O₄, doped LiNi_xMn_2-xO₄ (0.4≤x≤0.55) and LiCr_0.2Ni_0.4Mn_1.4O₄ materials were successfully prepared by a resorcinol-formaldehyde sol-gel method, which has effectively increased the power density and operating voltage of cathode materials and laid a good foundation for further improving the performances of lithium-ion batteries.The impacts of the preparation conditions of the resorcinol-formaldehyde sol-gel method on the structure, composition and electrochemical properties of spinel LiMn₂O₄ cathode materials were systematically investigated. By the gel pre-heating and resorcinol-formaldehyde ratio optimization, the impurities and reunion of the materials were resolved and the nano-sized spinel LiMn₂O₄ cathode material with high phase purity, uniform particle size and excellent electrochemical properties was obtained.To overcome the phase purity issues of the nano-sized spinel LiMn₂O₄ material during the preparation process, we have systematically studied the heat treatment process of gel precursor and found that pre-heating of gel precursor could effectively improve the phase purity and avoid impurity phase. In comparison to the samples prepared by the one-step method, the LiMn₂O₄ materials prepared by the two-step method have better crystallinity, higher phase purity, better dispersion, uniformity and electrochemical performances.To resolve the dispersion and uniformity issues of the nano-sized spinel LiMn₂O₄ material during the preparation process, we have systematically studied the impacts of resorcinol-formaldehyde ratios on the morphology and particle size of the materials. The study shows that as-prepared spinel LiMn₂O₄ with R/L ratio of 5:1 has good crystallinity, high phase purity, loose porous structure, good dispersion and uniformity.To improve the crystallization and control particle size of the nano-sized spinel LiMn₂O₄, we have systematically investigated the influences of the reheating temperature and time on the properties of the materials. With the increase of the reheating temperature and time, the crystallinity of the materials was improved, the grain size was increased, the proportion of lithium and oxygen was decreased and the average valence of manganese was decreased. The sample reheated at 650℃for 10 h has good crystallinity, small particle size, uniform distribution and the best electrochemical performances.The optimized preparation conditions are R/L ratio of 5:1, two-step heating method, preheated the gel precursor at 360℃and then reheated at 650℃for 10 h. The nano-sized spinel LiMn₂O₄ under above conditions has the lattice constant of 8.2386 (A|。), the average grain size of 47 nm, the chemical composition of Li_0.993Mn_1.997O₄, the initial discharge capacity of 137 mAh/g and coulombic efficiency of 96.3% at 0.2 C, the capacity retention of 85% after 100 cycles. Its initial discharge capacity is higher than the reported values.The relationship between the high rate performances (above 20 C) of the spinel LiMn₂O₄ cathode materials and the preparation conditions was systematically investigated in this work and the optimized conditions were obtained. We have studied the relationship between the lattice constants and grain sizes of the materials and its capacity retention, voltage retention and the energy density at high rate, so as to provide a good guide for developing the cathode materials with high power.By systematically investigating the impacts of the resorcinol-formaldehyde sol-gel process on the high-rate performances of the nano-sized spinel LiMn₂O₄ cathode materials, we have found that the effects of the heating temperature on the high-rate performances is greater in comparison with the heating time and ratio of reactants. With the increase of the heating temperature, the capacity retention, voltage retention and the energy density of the spinel LiMn₂O₄ materials first increased and then decreased. The sample prepared by the optimized conditions (R/L ratio 5:1, reheated at 650℃for 10 h) has the best high-rate performances. The capacity retention and the voltage retention rate is 87% and 94% at 40 C repectively, the energy density at 30 kW/kg is up to 400 Wh/kg.In order to explore the links between the structures and the high-rate performances of the spinel LiMn₂O₄ material, we have firstly systematically studied the effects of the lattice constants on the lithium-ion diffusion coefficients and high-rate performances of the spinel LiMn₂O₄ material. The study shows that the lithium-ion diffusion coefficients first increase and then decease with the raise of the lattice constants. The samples with the lattice constants (8.235⁸.245 (A|。)) have high lithium-ion diffusion coefficients. The study also shows that the impacts of the lattice constants on the capacity retentions and voltage retentions increase as the discharge rate raises. The spinel LiMn₂O₄ materials with the lattice constants (8.236⁸.240 (A|。)) have high capacity retentions and voltage retentions at high rate.In order to explore the links between the grain size and the high-rate performances of spinel LiMn₂O₄ materials, we have systematically studied the effects of the grain sizes on the high-rate performances of spinel LiMn₂O₄. The study shows that the effects of the grain sizes on the electrochemical performances of the materials, which increase with the raise of the discharge rate, are lesser below 20 C than those over 20 C. The spinel LiMn₂O₄ materials with grain size (35⁵0 nm) have high capacity and voltage retentions.The impacts of the doping process with the resorcinol-formaldehyde sol-gel method on the structure, composition and electrochemical properties of spinel LiNi_xMn_2-xO₄ cathode materials were systematically investigated, and the effects of the doping process on the voltage characteristics were revealed. The spinel LiNi_0.5Mn_1.5O₄ materials, which own high voltage platform, good high-rate performance and cycle ability, were prepared. Because the oxidation potential of Cr³⁺/Cr⁴⁺ is high and the structural stability of the Cr-O bond is strong, we have studied the effects of binary nickel-chromium doping on the performances of spinel LiMn₂O₄ materials and prepared the spinel LiCr_0.2Ni_0.4Mn_1.4O₄ with higher operating voltage and better cycle ability.In order to investigate the influence of the resorcinol-formaldehyde sol-gel process on the structure, composition and electrochemical properties, especially the voltage characteristics of the spinel LiNi_0.5Mn_1.5O₄ cathode materials, we have studied the impacts of the reheating temperature on the properties of the materials. The study shows that the spinel LiNi_0.5Mn_1.5O₄ materials prepared at various temperatures have two different structures. The sample prepared at 650℃, which is ordered LiNi_0.5Mn_1.5O₄, has poor electrochemical performance, especially the high-rate performances. With the increase of the heating temperature, the materials change from the ordered structure to disordered structure, its lattice constants increase, particle size increases, 4 V voltage plateau area increases, the initial discharge capacities and high-rate performances first increase and then decrease. By the systemic optimization and analysis, we have found that the doping didn't affect the capacity of the material obviously. The sample prepared at 750℃has the best electrochemical performances: its initial discharge capacity (134 mAh/g) is close to the spinel LiMn₂O₄ materials (137 mAh/g), its average discharge voltage (4.60 V) is significantly higher than the latter (4.02 V) at 0.2 C, its capacity retention (93%) after 100 cycles is obviously higher the latter (84%), its average discharge voltage (4.27 V) is significantly higher than the latter (3.70 V) at 40 C, its energy density (380 Wh/kg) at the power density of 40 kW/kg is obviously higher than the latter (320 Wh/kg).By further studying the effects of Ni-doping ratio on the performances of the spinel LiNi_xMn_2-xO₄ (0.4≤x≤0.55) material, we have found that the Ni-doping ratio had significant impacts on the structure, composition and electrochemical properties, especially the voltage characteristics, of the materials. With the raise of Ni-doping ratios, the lattice constants of the LiNi_xMn_2-xO₄ decrease, the average valence of Mn increases, the platform area around 4 V decreases, the electrochemical performances first increase and then decrease. The sample with the ratio of LiNi_0.5Mn_1.5O₄ has the best electrochemical properties.In order to further enhance the operating voltage and cycle abilities of cathode materials, we have doped the spinel LiMn₂O₄ with nickel and chromium by resorcinol-formaldehyde sol-gel method and successfully prepared the spinel LiCr_0.2Ni_0.4Mn_1.4O₄ cathode material, which has good crystallinity, high phase purity, uniform distribution and excellent performances. Because of the high oxidation potential of Cr³⁺/Cr⁴⁺ and the strong structural stability of the Cr-O bond, the spinel LiCr_0.2Ni_0.4Mn_1.4O₄ material has higher voltage platform around 4.8 V, its average discharge voltage (4.65 V) at 0.2 C is higher than the spinel LiNi_0.5Mn_1.5O₄ (4.60 V), its capacity retention (95%) after100 cycles is higher the latter (93%). So the spinel LiCr_0.2Ni_0.4Mn_1.4O₄ material has good research and application prospects.