The use of cathode material blending to exert the synergistic effect between different materials is an effective method for the design of lithium ion battery electrode materials. Material blending can affect both the battery open circuit voltage and lithium ion diffusion. The most direct manifestation is the influence on the crystal structure of the material, which causes the change of the material open circuit voltage. The thermodynamic and kinetic properties of blended materials mainly depend on the proportion of each component material and its own characteristics. Electrochemical reactions corresponding to each component material occur in different voltage ranges, which play a corresponding leading role in battery voltage, capacity, rate and cycle performance. . However, because different cathode materials can produce certain synergistic effects after blending, the thermodynamic and kinetic properties of the battery such as the voltage, capacity, and rate performance of the blended materials are not simply superimposed in strict accordance with the material mixing ratio, and often have spillover effects. An important reason why high-performance electrode designs can be achieved by blending. At the same time, the performance of the blended material is not only determined by the electrochemical properties of the cathode material used, but also related to the particle morphology and microstructure. Different blending of materials will increase the tap density, reduce the porosity of the mixed electrode material, and also act as a coating to achieve relative isolation from the electrolyte, thereby improving cycle performance, rate performance and safety performance.
1. LiCoO2 blend material
LiCoO2 is the earliest cathode material for commercial application of lithium-ion batteries. Due to its excellent electrochemical properties and simple production process, it has been widely used in electronic digital and drone products. LiCoO2 has the characteristics of layered rock salt structure of α-NaFeO2, and has poor overcharge resistance. With the gradual increase of charging voltage, the structural stability of the material decreases rapidly, and the life and safety deteriorate rapidly. In actual production, the structural integrity and electrochemical performance are improved by doping various metals or coating metal oxides, but the doping and coating methods have disadvantages such as high cost, complex process, and unstable product performance. The blending method can give full play to the characteristics of different materials, improve the overcharge resistance of LiCoO2, and reduce the manufacturing cost on the premise of improving the electrochemical performance. Using LiCoO2 and LiMn2O4 to reverse the crystal structure changes during lithium insertion/delithiation, the two materials can play a synergistic effect to improve structural defects and improve electrochemical performance after blending. The study found that the LiCoO2-LiMn2O4 blend material can inhibit the surface film resistance of the LiCoO2 electrode and the generation of Co3O4 under the condition of overcharge, slow down the Jahn-Teller phase transition of LiMn2O4 in the lithium-rich state and the dissolution of Mn at high potential, so that the material structure is stable. , resistant to overcharge and improve safety performance. The practical application of LiCoO2-LiMn2O4 mixed material was studied, and it was found that the battery capacity after 1:1 mixing was equivalent to that of the same type of liquid lithium-ion battery with pure LiCoO2 as the positive electrode material, and the overcharge resistance was better than that of the pure LiCoO2 positive electrode material. The electrochemical performance of LiCoO2 can also be well improved by blending with ternary cathode materials. LiCoO2 and LiNi1/3Co1/3Mn1/3O2 were coated with AlF3- and mixed respectively. The discharge capacity of the material was 180-188 mA∙h/g between 3.0 and 4.5 V at room temperature, and the capacity retention rate was as high as 95% after 50 cycles of cycling. Stability has also been greatly improved. LiCoO2-LiNi1/3Co1/3Mn1/3O2 was blended with graphite to form a 18650 battery for electrochemical performance testing. It was found that the electrochemical properties were similar to those of pure LiCoO2 under low rate conditions, and its rate performance was better than that of pure LiNi1 at 15 C. /3Co1/3Mn1/3O2 material, the safety performance is far better than pure LiCoO2 material. The electrochemical performance of LiCoO2 can be improved by exploiting the synergistic effect of cathode materials with different particle sizes. The research of 1:1 LiCoO2-LiFePO4 mixed material found that due to the small particle size of LiFePO4 distributed in the gaps of LiCoO2 particles, it plays a role of surface coating, which can prevent LiCoO2 from contacting the electrolyte during the charging process and cause the dissolution of Co4+ or the generation of inactive substances , showing good cycle performance, rate performance and safety performance.
2. LiMn2O4 blend material
LiMn2O4 has the advantages of abundant Mn resources in nature, low cost, simple synthesis process, high thermal stability, good overcharge resistance, high discharge voltage platform, and environmental friendliness. , electrolyte decomposition, Jahn-Teller effect, etc. lead to poor cycle stability, and it is difficult to obtain a good application in the manufacture of power batteries. The study found that by blending with Ni and Co cathode materials, the energy density of LiMn2O4 batteries can be increased and the cycle performance can be improved. Some people have studied and prepared LiMn2O4 coated with LiCoO2, which reduces the dissolution of Mn, and its high temperature stability is greatly improved, and the cycle stability and specific capacity are higher than those of LiMn2O4 before coating. The study found that adding a part of Li-Ni-Co-Mn oxide cathode material to the spinel cathode material can complex the hydrogen fluoride in the battery system, reduce the dissolution of divalent manganese, and improve its electrochemical cycle. The electrochemical and thermal stability of LiMn2O4-LiNi0.8Co0.15Al0.05O2 blend cathode materials were studied, and it was found that the addition of layered LiNi0.8Co0.15Al0.05O2 was beneficial to increase the discharge capacity, inhibit the dissolution of manganese and improve the material heat. stability. Adding LiNi0.8Co0.2O2 to Li1+xMn2-xO4 can prevent the generation of HF- and the loss of Li in the electrolyte. Adding 15% (mass fraction) LiNi0.8Co0.2O2 can almost completely inhibit the dissolution of manganese and improve the high temperature performance of the material. LiMn2O4-LiNi0.6Co0.2Mn0.2O2 blended cathode materials were studied, and it was found that its electrochemical performance was better than that of pure LiMn2O4. The energy density of LiMn2O4 played a leading role at high power, and the opposite was true at low power. The study found that adding an appropriate amount of LiNi1/3Mn1/3Co1/3O2 to LiMn2O4 can inhibit the dissolution of manganese and improve the capacity retention rate.
3. LiFePO4 blend material
The theoretical capacitance of LiFePO4 is 170 mA∙h/g, and the charge-discharge platform relative to lithium metal is about 3.45 V. It has good reversibility and is widely used in the manufacture of power batteries. However, LiFePO4 has low electrical conductivity and lithium ion diffusivity, resulting in poor high-current discharge capability and low-temperature electrochemical performance of the battery. At present, it is mainly solved by carbon coating, doping and particle size reduction. However, these modification methods have high cost, poor product stability, and difficult to control product consistency. By blending with a cathode material with higher conductivity, its electrochemical performance can be improved, and it has a good application prospect in power batteries. When adding 5% to 10% LiCoO2 to LiFePO4, the material has good thermal stability, high rate performance and cycle performance. The effect of Li3V2(PO4)3 as an additive on LiFePO4. After adding Li3V2(PO4)3 material, the electronic conductivity and electrochemical performance are improved to varying degrees. The discharge specific capacity of the blended material is close to 150 mA∙h/g, and the cycle No significant decay after 40 times. The spinel structure of LiMn2O4 has a high voltage plateau and relatively high tap density. Mixing the two can improve the tap density of LiFePO4 and the average voltage of the battery. The study found that adding LiMn2O4 to LiFePO4 can improve the working voltage of the battery, increase the tap density of the material, and improve the electrochemical performance. When the mixing mass ratio is 5:5, the average voltage can reach about 3.64 V. The electrochemical performance of LiFePO4-LiMn2O4 mixed cathode material, when the mixing ratio is 1:1, nano-LiFePO4 particles can well coat the surface of micron LiMn2O4 and fill the cavity, thus improving the electrochemical performance of the material. LiFePO4-xLi2MnO3∙(1-x) LiMO2 blending can greatly improve the electrochemical performance at low temperature.
4. Ternary blend materials
The ternary material LiNixCoyM1-x-yO2 (M is Mn, Al) has the advantages of high reversible capacity, good cycle performance, structural stability, etc. It is often used as a cathode material for high energy density power batteries, but its cycle stability, rate performance and other aspects There are still deficiencies, which limit its further development and application. In the ternary material, due to the disproportionation reaction of Ni3+, Ni2+ easily enters the lithium layer and mixes up, preventing the entry of lithium ions, resulting in irreversible capacity loss. At the same time, Mn ions are distorted under the John-Teller effect and enter the electrolyte, causing the structure of the material to be destroyed, resulting in the capacity attenuation of the battery. The study found that mixing spinel lithium manganate into the ternary material can not only reduce the material cost, but also improve the material rate performance and safety performance, and has developed into a relatively mature solution for foreign power battery companies. The blending of ternary materials with LiCoO2 can also improve the cycle stability and rate performance. The electrochemical properties of LiCoO2-LiNi1/3Mn1/3Co1/3O2 blends were studied, and it was found that the introduction of LiCoO2 can improve the rate performance of the material, and the effect is more obvious as the content increases, but the introduction of LiCoO2 will also cause The reversible capacity of the material decreases and the cycle performance deteriorates. After adding 3% LiCoO2 to LiNi0.8Co0.15Al0.05O2 to form a coating, the discharge capacity can reach 196.2 mA∙h/g, and the capacity retention rate after 50 cycles is as high as 98.7%. The NiO surface coating layer is the electrochemical performance. The main reason for the increase. The electrochemical performance of ternary materials can also be improved by utilizing the better cycle performance and safety performance of LiFePO4. LiNi0.7Co0.15Mn0.15O2-LiFePO4 was blended to make an all-solid-state battery. The addition of LiFePO4 reduced the polarization and showed better electrochemical cycle performance. The capacity retention rate was as high as 89% after 50 cycles of cycling. The electrochemical thermal stability of LiFePO4/LiNi0.8Co0.15Al0.05O2 mixed cathode material was studied, and it was found that nano-LiFePO4 particles coated on the surface of LiNi0.8Co0.15Al0.05O2 particles can reduce the charging voltage platform and improve the thermal decomposition of the material. temperature, reduce the heat during the charging and discharging process, and improve the stability and structural stability of the material during the charging and discharging process.
Summarize
Using the blending modification between multiple cathode materials, compared with a single cathode material, it can play a synergistic effect such as reducing capacity loss, improving battery life, and improving safety performance, and can provide a simple and controllable process for actual production and reduce costs. important method. The blending modification of cathode materials can be based on the following principles: First, the electrochemical thermodynamics and physical properties of the material are optimized. Through reasonable matching of material types and mass ratios, the working voltage and lithium ion diffusion rate can be effectively adjusted, thereby improving the thermodynamic properties of the material; the second is the safety performance of lithium ion batteries. By mixing in materials with better thermal stability, safety problems such as heat generation and overcharge during battery operation can be effectively improved; the third is to reduce manufacturing costs. The selection of power battery material system is a comprehensive consideration of energy density, safety, cyclability and manufacturing cost. Blending modification can be used as an important technical means to reduce manufacturing cost on the basis of meeting certain performance requirements of power battery.