Lithium-ion batteries have been put on the market since 1990. The negative electrode material has been developed from the initial hard carbon to graphite, while the positive electrode material has always been lithium cobalt oxide. The manufacturing process of the battery has also been greatly improved, and the battery capacity has been increased by more than three times. However, in order to meet the high-current pulse discharge requirements of modern mobile phones, the performance of lithium cobalt oxide has also been greatly improved in recent years, and its development cannot be ignored.
First, let’s take a look at the main indicators of LiCoO2. Electrode density is its most important property, which is related to packing density as well as pole piece density. These data are very important for manufacturers to manufacture batteries with the highest possible specific capacity by volume. At present, LiCoO2 accounts for 96% (mass fraction) in the positive electrode sheet, and the remaining 4% (mass fraction) is glue and conductive agent (such as carbon). Therefore, even if the filling amount of LiCoO2 is increased by 1% (mass fraction), it is extremely meaningful, and by increasing the electrode density, the battery capacity can be improved.
In the process of preparing LiCoO2, it is important to control the Li/Co ratio, which is usually less than 1. The positive electrode slurry is usually formed by dispersing LiCoO2 and conductive agent in N-methylpyrrolidone solution containing polyvinylidene fluoride (PVDF). It is inevitable that the slurry contains some moisture. When some unreacted lithium ions in LiCoO2 exist in the form of Li2O, they react with the water in the slurry, making the cathode slurry alkaline. Under such alkaline conditions, the positive electrode slurry becomes a gel state and cannot be coated on the aluminum current collector. Therefore, in order to prevent the positive electrode slurry from forming a gel state, traditional LiCoO2 is washed with warm water. Taking the product provided by a company as an example, the pH value of 10% (mass fraction) LiCoO2 aqueous solution is required to be adjusted to 9.5~11.0, and the recommended pH value is less than 10.5. The Li/Co ratio of LiCoO2 provided by the company is less than 1, which exists in the product. The probability of unreacted lithium salts is close to zero. However, in order to reduce costs, manufacturers generally cancel the washing process of LiCoO2.
Specific surface area (SSA) is also one of the important properties. Because the specific surface area is large, the reaction area is large. Obviously, the use of a cathode material with a higher specific surface area can improve the rate performance and improve the high-current discharge capacity of the battery. When the specific surface area increases, the decrease of material density is inevitable, and then the electrode density decreases accordingly, so the increase of the specific surface area of the material is not unlimited. Of course, if nano-scale electrode active materials are used, the electrode density will be significantly reduced. Although the rate performance is improved, the safety performance of the battery will be affected, so the use of nano-scale cathode materials is not recommended. Nanomaterials are more likely to catch fire in acupuncture experiments (a short-circuit test) and hot box experiments (battery heated to 150°C), so such materials are rarely used in practice.
Different battery manufacturers choose materials with different average particle sizes and particle size distributions according to different coating processes. Even from the same battery manufacturer, the average particle size and particle size distribution of the positive electrode materials used in different types of batteries such as square, cylindrical, and polymer are different. It is unrealistic to apply the same material to all batteries. These factors are closely related to battery performance. Battery manufacturers should choose suitable materials according to their uses and equipment. This is why raw material manufacturers mass-produce LiCoO2 with different characteristics.
Impurities that cause redox side reactions in the battery should be removed. In recent years, people have been skeptical about the residual CO3O4 in the positive electrode material. During the repeated discharge process, CO3O4 is dissolved in the electrolyte in an ionic state, reduced to metallic Co at the negative electrode, and precipitated on the surface of the electrode, and the impedance of the negative electrode increases. The intercalation of Li is hindered. The above-mentioned specifications of cathode materials provide a great deal of raw information.
In the following, the synthesis conditions of LiCoO2 and its physical and electrochemical properties will be described from the perspective of industrial products. Generally speaking, the easiest way to control the physical properties of LiCoO2 is to change the Li/Co ratio. It has been reported that when the Li/Co ratio is greater than 1, the structure of LiCoO2 near 4.1V from rhombohedral to monoclinic disappears. Here, the characteristics of LiCoO2 with different Li/Co ratios will be elaborated.
As shown in Figure 1, as the Li/Co ratio increases, the specific surface area (SSA) decreases. That is, at a high Li/Co ratio, LiCoO2 crystals are easier to form and have larger grains. Figure 2 shows Li/Co. Electron micrographs of LiCoO2 with ratios of 1 (a) and 1.05 (b). The particle size of samples with Li/Co ratio of 1 is 2 to 3 μ/m, and the particle size of samples with Li/Co ratio of 1.05 is about 10 μm. The study confirmed that the SSA decreased at high Li/Co ratios. Obviously, higher Li/Co. It is more suitable for the preparation of dense materials with higher bulk and tap densities (see Figure 3). It should be pointed out that in some cases the particle size distribution of the wet method is different from that shown in the SEM image, and the Li/Co ratio is different from the particle size (D10, D50, D90) measured by the Microtrack method (wet method). The relationship is shown in Figure 4. It can be seen from the SEM image that when the Li/Co ratio is 1.0, the crystal particle size is 2 to 3 μm, but the average particle size D50 measured by the particle size distribution method is 10 μm. Agglomeration of fine particles. In other words, in the wet test, the fine particles agglomerate into large particles in the solution, so the wet test does not reflect the size of the primary particles, but the size of the agglomerated secondary particles.
On the other hand, when the particles grow continuously and reach 10 μm under the SEM image, the average particle size D50 measured by the wet method is 10 μm, as shown in Figure 4. Both methods can provide the same particle size. As the particle size increases, particle agglomeration is suppressed, and in this case, the particle size distribution reflects the size of the primary particles. For the synthesis of LiCoO2 at higher Li/Co ratios, once the particle size decreases, excess lithium exists in the form of Li2O. The amount of remaining Li2O can be determined by dissolving LiCoO2 in an aqueous solution and measuring the pH.
The relationship between 10% (mass fraction) LiCoO2 suspension and Li/Co ratio is shown in Figure 5. When the pH value is greater than 10.5~10.6, the positive electrode slurry becomes a gel state, and the pH value is adjusted by washing the product with warm water. value is required.
In addition, the effect of Li/Co ratio on the properties of LiCoO2 such as particle size and pH of suspension was first published by Nippon Chemical Corporation. LiCoO2 with high Li/Co ratio has advantages and disadvantages for battery performance, and the product synthesis method needs to be optimized.
Now, review the history of LiCoO2 synthesis. According to Nishi (Sony Corporation), early synthesis methods are as follows. The raw materials CO3O4 and Li2CO3 are mixed in an aqueous solution containing polyvinyl alcohol to form a slurry, and then calcined. In this case, the Li/Co ratio of the mixture is greater than 1. In order to improve the safety of the battery, the calcination temperature is set to above 900 °C. Due to the high Li/Co ratio, excess Li2CO3 will remain in the product. When it is used as a positive electrode material for lithium-ion batteries, once the battery is overcharged, the residual Li2CO3 CO2 will be generated, and the pressure generated will destroy the explosion-proof valve, thereby improving the safety performance of the battery. Studies have confirmed that at higher Li/Co ratios, the particle size becomes larger and easier to sinter. The results of acupuncture experiments show that the use of large particle size cobalt compounds can effectively improve the safety of batteries, and large particle LiCoO2 has been used in the early stage of battery development. With the advancement of battery manufacturing technology, the necessity of using large particle size cathode materials has decreased. In order to improve the performance of the battery, especially the high rate performance, people have recently used the small particle size cathode materials shown in Table 2.4. If the maximum calcination temperature is set below 900°C, the calcination will be insufficient, and part of the excess Li2CO3 will exist in the form of Li2O after calcination. When the Li/Co ratio is close to or slightly lower than 1, the residual amount of Li2O after calcination decreases. In other words, a low-cost process without a washing process can be used. These technical information can be known from the LiCoO2 specification provided by a company. When the Li/Co ratio decreases very close to 1, the sintering temperature also seems to decrease accordingly. Co3O4 and Li2CO3 are still used as raw materials until now.
However, in the past two or three years, due to the development of mobile phones, the preparation method of LiCoO2 has changed. It was originally expected that this new type of LiCoO2 would be used after 2003, but it has not been used by battery suppliers in Korea and China. use.
The pulse discharge mode of the third-generation mobile phone is shown in Figure 6. In this system, when the mobile phone is talking, in order to capture the communication signal, a 0.6ms current pulse with a maximum discharge current of 2A will be generated. This large current is quite excessive for the battery, equivalent to a 500mA h battery with 4C The high current is discharged for a short time. The demand for high-current usage of mobile phones continues to increase, such as the use of color screens, digital cameras, and transoceanic telephones using satellite transmission. In order for the battery to discharge at a high current of 3~4C, the preparation method of LiCoO2 must be improved. In order to obtain a large discharge current, the particle size of LiCoO2 The secondary particle should be controlled at 1~2 μm, and in order to maintain the high density of LiCoO2 and its electrode density, the secondary particle size should be controlled at 5~10 μm. This increase in particle size effectively avoids the increase in SSA due to the decrease in primary particle size. To synthesize this high-rate discharge LiCoO2, a hydroxide co-precipitation method was developed based on the traditional solid-phase synthesis method. At present, this new type of LiCoO2 is mainly synthesized by co-precipitation method. The raw material is changed from traditional cobalt oxide to cobalt hydroxide. Using this method, the precipitate of hydroxide is synthesized in cobalt solution with pH value greater than 7. In this way, controlling the precipitation conditions, pH, standing temperature, etc., can obtain particles of the desired size and shape. Subsequently, the precipitate and Li2CO3 were mixed and calcined to synthesize LiCoO2. A spherical product is specially introduced below, and its SEM image is shown in Figure 7(b). of secondary particles. This uniform spherical LiCoO2 cannot be used as a cathode material for batteries due to insufficient packing density. Lithium onium oxide formed from loose secondary particles as described above is shown in Figure 7a).
Active materials whose physical and chemical properties meet battery standards, electrode specifications, and match other excipients have been adopted, and the accumulation of these data is very important.