The chemical formula of the product prepared on the basis of LiNiO2 type material can be basically described as Li1-xNi1+xO2 (x>0). Because part of the nickel ions will occupy the lithium ion layer, the control of material preparation conditions is very strict. The synthetic methods that can achieve the mixing of raw materials at the molecular level are as follows (solid-phase methods are basically not suitable for the preparation of pure substances):
①Spray drying synthesis method;
③ Mixed metal carbonate method.
The above methods are all co-precipitation methods. By using this co-precipitation technique, it is possible to mix several species at the atomic level. Compared with solid-phase methods, these methods have an advantage in controlling the particle size of primary and secondary particles, that is, the particle size of fine particles becomes controllable; another advantage is the control of crystallinity and surface morphology. However, in some cases, these methods are too elaborate for practical mass production.
The sol-gel method is often reported as a laboratory synthesis method, but it is often neglected due to its expensive manufacturing cost and its impracticality for industrial mass production and limited to experimental use.
1. Spray drying method
The spray drying method invented by Toda Kogyo Co., Ltd. (formerly Fuji Chemical Co., Ltd.) is well known in the industry, and their samples are used by companies all over the world, and their excellent properties have been proven. In 1999, Saft in France and Samsung in South Korea completed safety tests, and the results were presented to the public through international conferences. As far as I know, the French company Saft uses nickel-based materials as cathode materials for EV batteries because they think the high-temperature volume decay problem of manganese-based materials cannot be overcome. So far, this material has been used as a battery material. Some patents describe a method in which metal salts and lithium salts are reacted in aqueous solvents to form suspensions, which are spray-dried and then calcined at suitable temperatures.
Aluminum doping is a feature of Toda Kogyo Co., Ltd. products. The disadvantage of the product is the poor thermal stability of the nickel-based material itself, but it has been overcome by the doping of cobalt and aluminum. Through doping, the composition structure of the compound is converted to Li0.8Co0.15Al0.05O2, and the thermal stability of the charged state can reach the level of LiCoO2. The thermal stability data for this compound were provided by Binsan et al., Soft Company, France, and are shown in Table 1.
Due to the reduction of nickel content and the doping of aluminum, the temperature of the exothermic peak accompanying the oxidation process ranges from 200 °C for LiCoO2 to 310 °C for Li0.8Co0.15Al0.05O2, and the thermal stability of Li0.8Co0.15Al0.05O2 is better. Great improvement, no oxidation reaction occurs below 300 °C, and its thermal stability is better than LiCoO2 and spinel LiMn2O4. The SEM image of this material is shown in Figure 1. The properties of the powder can also be controlled using the coprecipitation method. It can be seen from Table 1 that Li0.8Co0.15Al0.05O2 has higher capacity than the traditional cobalt-based cathode material. As shown in Figure 2, the discharge specific capacity of this material at 4.3V can reach more than 180mA · h/g.
Recently, Toshiba Corporation reported a laminated battery fabricated with this material and a high-capacity graphite material. The prismatic cell (thickness X width X length = 3.8mmX35mmX62mm) has a capacity of 920mA·h and an energy density of 200W·h/g, which is 16% higher than the cobalt-based cathode material, which should be attributed to the use of nickel-based materials. The rate performance of this battery is also very excellent. As shown in Figure 3, the residual capacity after 500 charge-discharge cycles at 1C is 70% of the initial capacity, which is comparable to that of cobalt-based batteries.
2. Hydroxide coprecipitation method
The co-precipitation method developed by Tanaka Chemical Co., Ltd. is the most advanced method of this kind, and the company has a monopoly as a supplier of cathode materials for nickel-cadmium batteries and nickel-metal hydride batteries. Metal hydroxides are prepared by amine complexation, and spherical materials can be prepared by adjusting the pH value of the aqueous solution, the standing temperature, and the ratio of reactants. This technology has been applied to the manufacture of cathode materials for lithium-ion batteries, and a patent has been applied for. Precipitates with uniform distribution of various elements can be obtained by co-precipitation, which is considered a good choice. Aluminum hydroxide can be obtained through the particle shape control technology developed by Tanaka Company. Various particles of different shapes, such as agglomerated fine particles, agglomerated flake crystals, agglomerated large particles, independent large particles, etc., can be obtained by Controlled co-precipitation conditions. The cathode material of lithium ion battery is synthesized by the mixed calcination of lithium salt and hydroxide. In this case, it is very important to control the heating temperature and maintain the particle shape of the raw material, and Tanaka Corporation provides these hydroxides for synthesizing cathode materials. In Canada, Ohzuku and Dahn synthesized LiNi0.5Mn0.5O2 and LiNi1/3Mn1/3Co1/3O2 using nickel ingot co-precipitation and nickel-cobalt-manganese co-precipitation technology pioneered by the company. This material has attracted more attention due to the higher capacity of batteries using this material. The data provided by Ohzuku is shown in Figure 4. Both materials can reach 200mA • h/g, and they will be promising materials in the future. However, its performance in high-rate discharge is not satisfactory, so improving the rate performance is a top priority. In addition, nickel-cobalt-manganese coprecipitates with higher tap densities of 2.0 to 2.3 g/cm3 have been used.
3. Mixed metal carbonate method
As mentioned above, the hydroxide coprecipitation method is an excellent synthetic method. However, the manufacture of hydroxide coprecipitates containing manganese ions can be difficult in terms of processing technology. Since the hydroxide co-precipitate is formed under alkaline conditions, manganese ions may be +2, +3, +4 valence ions, and oxygen in the air will aggravate the oxidation of manganese ions, so the repeated synthesis of the same coprecipitate material is quite difficult. According to Tanaka’s patent, in order to overcome this problem, manganese ions should be co-precipitated from manganese ammine complexes such as Mn(OH)2 under the action of a reducing agent.
On the other hand, the mixed metal carbonate method is a mixed coprecipitation method developed by Chuo Denki Kogyo Company. These metal salts are co-precipitated into carbonate or bicarbonate salts in an alkaline solution of bicarbonate. This method does not have the disadvantages of the hydroxide co-precipitation method mentioned earlier and may be a very good method. Since this method is also a co-precipitation method, the shape of the particles can be controlled to obtain spherical particles with high density. The initial capacity of LiNi0.56MnxCo0.44-xO2 synthesized by this method and measured by DSC under electrolyte coexistence conditions The peak temperature is shown in Table 2. Obviously, with the increase of manganese doping amount and the sharp decrease of heat production, the peak temperature related to the degree of oxidation shifts towards higher temperature. That is to say, through the doping of manganese, the safety of the battery can be improved, and its initial capacity is higher than that of lithium cobalt oxide, and its energy density (determined by voltage and capacity) is also higher than that of lithium cobalt oxide due to the higher discharge voltage.
Battery manufacturers use this material as an active material to produce large batteries of the 100W·h and 400W·h classes. Its cycle life, thermal stability, high rate performance, and storage performance under charge have all been evaluated, and the battery capacity retention rate was 83% after 1,000 cycles. Studies have confirmed that with the increase of manganese content, its thermal stability is enhanced, but due to the extreme expansion of the c-axis, the manganese content cannot exceed 0.35, and the content of manganese and cobalt has no effect on the charge-discharge curve. The ternary phase diagram drawn by Terasaki et al. in Figure 5 shows the relationship between the battery capacity and the composition of the compound LiNi1-x-yCoxMnyOz. The samples they reported were prepared from mixed-metal carbonate precursors that enabled the preparation of a broad composition of LiNi1-x-yCoxMnyOz.