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Advantages and disadvantages of several cathode materials for lithium batteries
The performance of a lithium-ion battery depends primarily on the structure and properties of the materials used in the battery used. These battery internal materials include a negative electrode material, an electrolyte, a separator, a positive electrode material, and the like. The choice and quality of the positive and negative materials directly determine the performance and price of the lithium ion battery. Therefore, the research of low-cost, high-performance positive and negative materials has always been the focus of the development of the lithium-ion battery industry. The anode material is generally made of carbon material, and the current development is relatively mature. The development of cathode materials has become an important factor restricting the further improvement of lithium ion battery performance and further price reduction. In the current commercial production of lithium-ion batteries, the cost of the cathode material accounts for about 40% of the total battery cost, and the reduction in the price of the cathode material directly determines the price of the lithium-ion battery. This is especially true for lithium-ion power batteries. For example, a small lithium-ion battery for a mobile phone requires only about 5 grams of positive electrode material, and a lithium-ion battery that drives a bus may require up to 500 kilograms of positive electrode material.
To measure the quality of the positive electrode material of lithium ion battery, it can be evaluated from the following aspects: (1) The positive electrode material should have a high oxidation-reduction potential, so that the battery has a higher output voltage; (2) lithium ion can A large amount of reversible embedding and deintercalation in the positive electrode material to make the battery have a high capacity; (3) in the process of lithium ion intercalation/deintercalation, the structure of the positive electrode material should be changed as little as possible or small, Ensure good cycle performance of the battery; (4) The oxidation-reduction potential of the positive electrode should be changed as little as possible during the insertion/deintercalation process of lithium ions, so that the voltage of the battery does not change significantly to ensure the battery is charged and discharged smoothly; (5) The positive electrode material should have a high electrical conductivity, which can make the battery charge and discharge at a large current; (6) the positive electrode does not chemically react with the electrolyte; (7) the lithium ion should have a large diffusion in the electrode material. The coefficient is convenient for the battery to be quickly charged and discharged; (8) the price is cheap and the environment is not polluted.
Lithium ion battery cathode materials are generally lithium oxides. Among the more studied ones are LiCoO2, LiNiO2, LiMn2O4, LiFePO4 and vanadium oxides. Conductive polymer cathode materials have also attracted great interest.
1, LiCoO2
In the current commercial lithium ion battery, a layered structure of LiCoO 2 is basically selected as a positive electrode material. Its theoretical capacity is 274mAh/g, and the actual capacity is about 140mAh/g. It has also been reported that the actual capacity has reached 155mAh/g. The main advantages of the positive electrode material are: high working voltage (average working voltage is 3.7V), stable charge and discharge voltage, suitable for large current charge and discharge, high specific energy, good cycle performance, high electrical conductivity, simple production process and easy preparation. Wait. The main disadvantages are: high price, poor overcharge resistance, and further improvement in cycle performance.
2, LiNiO2
LiNiO2 used for a positive electrode material of a lithium ion battery has a layered structure similar to that of LiCoO2. Its theoretical capacity is 274mAh / g, the actual capacity has reached 190mAh / g ~ 210mAh / g. The operating voltage range is 2.5 to 4.2V. The main advantages of the positive electrode material are: low self-discharge rate, no pollution, good compatibility with various electrolytes, and low price compared with LiCoO2. However, LiNiO2 has a fatal disadvantage: the preparation conditions of LiNiO2 are very demanding, which brings considerable difficulty to the commercial production of LiNiO2; the thermal stability of LiNiO2 is poor, and the heat of LiNiO2 is compared with LiCoO2 and LiMn2O4 cathode materials under the same conditions. Decomposition temperature ** (about 200 ° C), and the most heat release, which brings great safety hazards to the battery; LiNiO2 is prone to structural changes during charging and discharging, making the cycle performance of the battery worse. These shortcomings make LiNiO2 a considerable step as a positive electrode material for lithium-ion batteries.
3, LiMn2O4
LiMn2O4 used for a positive electrode material of a lithium ion battery has a spinel structure. The theoretical capacity is 148 mAh/g and the actual capacity is 90-120 mAh/g. The operating voltage range is 3 to 4V. The main advantages of the positive electrode material are: abundant manganese resources, low price, high safety, and relatively easy to prepare. The disadvantage is that the theoretical capacity is not high; the material will dissolve slowly in the electrolyte, that is, the compatibility with the electrolyte is not so good; in the process of deep charge and discharge, the material is prone to lattice stagnation, resulting in rapid decay of battery capacity, especially This is especially true when used at higher temperatures. In order to overcome the above shortcomings, a layered structure of trivalent manganese oxide LiMnO2 has been newly developed in recent years. The theoretical capacity of the positive electrode material is 286 mAh/g, and the actual capacity is about 200 mAh/g. The operating voltage range is from 3 to 4.5V. Although LiMnO2 has a significant increase in both theoretical capacity and actual capacity compared with LiMn2O4 with spinel structure, there are still structural instability problems during charge and discharge. During the charging and discharging process, the crystal structure is repeatedly changed between the layered structure and the spinel structure, thereby causing repeated expansion and contraction of the electrode volume, resulting in deterioration of the cycle performance of the battery. Moreover, LiMnO2 also has a dissolution problem at a higher working temperature. The solution to these problems is doping and surface modification of LiMnO2. Gratifying progress has been made so far.
4, LiFePO4
The material has an olivine crystal structure and is one of the hot cathode materials for lithium ion batteries studied in recent years. Its theoretical capacity is 170 mAh/g, and its actual capacity has reached 110 mAh/g without doping modification. By surface modification of LiFePO4, its actual capacity can be as high as 165 mAh/g, which is very close to the theoretical capacity. The operating voltage range is around 3.4V. Compared with the positive electrode materials described above, LiFePO4 has high stability, is safer, more environmentally friendly and less expensive. The main disadvantage of LiFePO4 is that the theoretical capacity is not high and the room temperature conductivity is low. Based on the above reasons, LiFePO4 has a very good application prospect in large lithium ion batteries. However, in order to show strong market competitiveness in the field of lithium-ion batteries, LiFePO4 faces the following disadvantages: (1) low-cost competition from LiMn2O4, LiMnO2, and LiNiMO2 cathode materials; (2) people may use it in different application fields. Priority is given to the specific battery material that is more suitable; (3) LiFePO4's battery capacity is not high; (4) People in high-tech fields may pay more attention not to cost but to performance, such as to mobile phones and notebook computers; (5) LiFePO4 urgently needed Improve the conductivity of the deep discharge at 1C speed to increase its specific capacity. (6) In terms of safety, LiCoO2 represents the current safety standards of the industry, and the safety of LiNiO2 has also been greatly improved. Only LiFePO4 shows higher safety performance, especially in electric vehicles. Application can guarantee its full competitive advantage in terms of security. The table below compares the performance of different lithium ion battery cathode materials.
Although theoretically it can be used as a cathode material for lithium ion batteries, the most widely used cathode material in commercial lithium ion batteries is still LiCoO2. Although the layered structure of LiNiO2 has a higher specific capacity than LiCoO2, due to structural changes and safety problems caused by its thermal decomposition reaction, there is still a considerable distance for directly applying LiNiO2 as a positive electrode material. However, the replacement of Ni by Co to obtain LiNi1-xCoxO2 with higher safety as a positive electrode material may be an important development direction in the future. The spinel-structured LiMn2O4 and the layered LiMnO2 are considered to be one of the most competitive candidates for the market due to their abundant raw material resources, obvious price advantages and high safety performance. However, the problem of structural instability during the charging and discharging process will be an important research topic in the future. The current actual discharge capacity of LiFePO4 with olivine structure has reached about 95% of the theoretical capacity, and it has the advantages of low price, high safety, stable structure and no environmental pollution. It is considered to be ideal for large-scale lithium-ion batteries. Cathode material.