Abstract：In recent years, the huge demand for energy has stimulated the development of high-energy-density lithium-ion battery materials, especially in the field of elec
In recent years, the huge demand for energy has stimulated the development of high-energy-density lithium-ion battery materials, especially in the field of electric vehicles. The current commercial lithium-ion batteries still cannot meet the low-cost and high-energy density requirements of electric vehicles.
Lithium nickel manganese oxide (LiNi0.5Mn1.5O4, LNMO) is developed based on spinel lithium manganate (LiMn2O4, LMO). High voltage spinel can be obtained by replacing part of Mn in LMO material with 0.5 ratio of Ni. Type LNMO. The addition of Ni makes the average valence of Mn become +4, and there is only a voltage plateau near 4.7V, which basically eliminates the 4V plateau corresponding to Mn3+/Mn4+, that is, reduces the Mn3+ ion content, avoids the Jahn-Teller effect of Mn3+ and The dissolution of Mn2+ enhanced the cycling stability of the material.
The theoretical discharge specific capacity of LNMO material can reach 146.7mA h/g, the upper limit of lithium voltage is as high as 5V, and the voltage platform is as high as 4.7V. It has ultra-high energy density (650W h/kg) and power density. , No pollution to the environment, good safety performance, good cycle stability and other advantages have become one of the hot spots in the research of cathode materials for lithium-ion batteries.
High Temperature Solid Phase Method
High temperature solid-phase method is widely used because of its simple operation process, low cost and easy industrialization.
In the method, different kinds of lithium sources, manganese sources and nickel sources are mechanically mixed uniformly according to the stoichiometric ratio, and then calcined at high temperature to obtain LNMO positive electrode materials. High temperature calcination is usually carried out between 600-1000°C. The research shows that by controlling the calcination temperature and calcination time, LNMO cathode materials with different types of crystal structures (ordered/disordered) can be obtained. Although the high-temperature solid-phase method is relatively mature and easy to achieve industrialization, it has disadvantages such as uneven mixing of raw materials, many impure synthetic materials (producing LixNi1-xO impurities), and irregular morphology. Materials synthesized by this method tend to have poor electrochemical performance.
Hydrothermal/solvothermal
The hydrothermal method has been widely used due to its advantages of simple operation, easy manipulation of material morphology, and atomic-level mixing. In the method, the metal salt solution is mixed with the precipitant and then transferred to a high-temperature reaction kettle, and the product is obtained after reacting at a certain temperature for a period of time. The method can realize chemical reactions that cannot be reacted or are difficult to react under general conditions, the reaction conditions are easy to control, and materials with high crystallinity and good uniformity can be obtained. However, this method has poor safety performance, high equipment requirements, and cannot be used to prepare water-sensitive compounds, which has serious limitations.
The solvothermal method is a preparation method in which the water in the hydrothermal method is replaced with an organic solvent. On the one hand, organic solvents can reduce the reaction temperature, accelerate the dissolution and dispersion of solutes, and improve the reactivity. On the other hand, the high viscosity, low dielectric constant and low boiling point of organic solvents can be used to increase the reaction pressure, which is beneficial to the crystallization of the product. The solvothermal preparation process is simple, the system closure is easy to control, the crystal orientation adjustment and particle size control can be realized, and the final product has good dispersibility.
Total Precipitation
The co-precipitation method is to drop the precipitant solution into the solution in which the transition metal salt is dissolved to obtain the precursor precipitate (mostly hydroxide or carbonate) with the target morphology. The proportion of lithium salt is mixed uniformly, and then calcined at high temperature to obtain LNMO material. The advantage is that the operation is simple, and the particle size and microscopic morphology of the material can be controlled by controlling the reaction conditions; however, in the actual co-precipitation process, it is difficult to achieve stoichiometric precipitation of nickel and manganese elements.
Sol-gel Method
The basic process of the sol-gel method is to prepare a sol containing lithium, nickel and manganese, dry and calcine to form a lithium nickel manganate cathode. The advantages are high crystallinity of particles and good dispersibility; the disadvantages are high cost and slow reaction speed.
Problems With Lithium Nickel Manganate Cathode Materials
Lack Of Oxygen
In order to ensure the good crystallinity of the lithium nickel manganate material, the calcination temperature is generally greater than 700℃, and the higher sintering temperature is conducive to the rapid diffusion of lithium element, which also leads to the inevitable lack of oxygen. Appropriate loss can make it a disordered Fd-3m structure, but at the same time, it also leads to the material being easily mixed with impurity phases (LixNi1-xO and manganese-based oxides, etc.), resulting in a decrease in the discharge specific capacity of LNMO materials.
Electrolyte Decomposition
The reaction between the electrolyte and the LNMO material seriously hinders the migration of Li+ and reduces the Coulombic efficiency and energy utilization of the material during cycling. In addition, when the operating voltage is higher than 4.5V, the lithium hexafluorophosphate salt in the electrolyte will decompose to generate PF5 and Li
Manganese Dissolution
The LiF produced by the decomposition of the electrolyte and the HF produced by the reaction of trace water will dissolve the manganese ions in the material, destroy the material structure, cause the battery capacity to decay, and reduce the service life.
Study on Modification of Lithium Nickel Manganate Cathode Material
Surface Coating
The cathode material can be protected from HF corrosion by cladding. The method of surface coating is generally to first synthesize pure phase LNMO, then use the sol method to uniformly mix with the coating material, and obtain the coated LNMO after subsequent heat treatment. The materials currently used for coating are mostly oxides, such as ZnO, ZrO2, SiO2, etc.
FE-SEM images of pristine LNMO (a1, a2), FE-SEM images of SiO2-coated LNMO (b1) and TEM images (b2), FE-SEM images of PI-coated LNMO (c1, c2)
Ion Doping
Doping elements mainly use active metal ions such as Cu, Fe, Cr, Co or inactive metal ions such as Mg, Al, Ti to replace part of Ni and Mn, which can improve the capacity and cycle performance while improving the single charge and discharge platform. In particular, Cr2+ has the most significant improvement on the electrical properties of materials than other elements.
Topography Design
By preparing core-shell, nanoscale and porous cathode materials, the electrical properties of the materials can be improved, especially nanomaterials can greatly improve the electrical properties of the materials under high-rate charge/discharge. Reducing the particle size is mainly because it is beneficial to reduce the diffusion path of Li+ in the material, improve the diffusion ability of Li+, and finally achieve the purpose of improving the electrical properties of the cathode material.
It is generally believed that the ionic and electronic conductivity of LNMO cathode materials with disordered Fd-3m structure is higher than that of ordered P4332 structure. However, the excess Mn3+ will not only cause the Jahn-Teller distortion to cause lattice expansion and structural collapse, but also cause a disproportionation reaction (2Mn3+→Mn4++Mn2+), the Mn2+ produced by the reaction will dissolve in the electrolyte, and the Mn4+ will remain in the solid phase of the cathode material. When Mn2+ continues to dissolve and deposit on the negative electrode in the form of metallic Mn, it will cause the SEI of the negative electrode to destabilize, increase the thickness of the SEI, and cause irreversible loss of Li+ and battery capacity decay. Therefore, it is particularly important to control the content of Mn3+ in the disordered Fd-3m phase.
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