How to choose the positive electrode material for power lithium-ion batteries

With the hot sales of lithium-ion electric vehicles in large and medium-sized cities in China such as Beijing, Shanghai, Suzhou, and Hangzhou, more and more electric vehicle manufacturers have begun to launch lithium battery projects. However, the choice of lithium battery has become the primary problem they face. Although the protection circuit of lithium batteries is relatively mature, for power batteries, the selection of positive electrode materials is very critical to truly ensure safety. At present, the most commonly used positive electrode materials in lithium-ion batteries are the following: lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel cobalt manganese oxide (LiCoxNiyMnzO2) and lithium iron phosphate (LiFePO4). Which lithium battery with positive electrode material should be selected? The following will be analyzed in detail.

Overcharging (referring to charging voltage exceeding the charging cut-off voltage, for lithium-ion batteries, 10V/cell can generally be defined as overcharging voltage) is a good method to test the safety of lithium-ion batteries. When it comes to overcharging, we should first understand the charging principle of lithium-ion batteries (as shown in Figure 1). The charging process of lithium-ion batteries is that Li runs out from the positive electrode, swims to the negative electrode through the electrolyte and obtains electrons , which are embedded in the negative electrode material, while the discharging process is the opposite.

The main tests for measuring the safety of positive electrode materials are:

A: Is it easy to form dendrites during charging?

The charging process of lithium-ion batteries is that Li runs out from the positive electrode, travels to the negative electrode through the electrolyte, is reduced and embedded in the negative electrode material; the discharge process is the opposite, the lithium in the negative electrode material is oxidized, and is embedded in the positive electrode material through the electrolyte.

Based on cyclic considerations, the actual use capacity of lithium cobalt oxide (LiCoO2) material is only half of its theoretical capacity, that is, after the normal charging of lithium-ion batteries using lithium cobalt oxide as the positive electrode material (i.e. charging to a cut-off voltage of about 4.2 V), there will be some Li remaining in the LiCoO2 positive electrode material. This can be expressed by the following simplified formula: LiCoO2→0.5Li Li0.5CoO2 (normal charging end). At this time, if the charging voltage continues to increase, the remaining Li in the LiCoO2 positive electrode material will continue to be deintercalated and move to the negative electrode. At this time, the position in the negative electrode material that can accommodate Li has been filled, and Li can only be precipitated on its surface in the form of metal. On the one hand, the surface deposits of metallic lithium are very easy to aggregate into branch-like lithium dendrites, which can pierce the diaphragm and cause a direct short circuit between the positive and negative electrodes. In addition, metallic lithium is very active and will directly react with the electrolyte to release heat. At the same time, the melting point of metallic lithium is quite low. Even if the surface metallic lithium dendrites do not pierce the diaphragm, as long as the temperature is slightly higher, such as the battery heating caused by discharge, the metallic lithium will melt, thus short-circuiting the positive and negative electrodes and causing safety accidents. In short, when the charging voltage of lithium cobalt oxide materials is too high, such as when the protection board fails, there are great safety hazards, and the high capacity of power lithium-ion batteries will cause great damage.

Like lithium cobalt oxide, lithium nickel cobalt manganese oxide (LiCoxNiyMnzO2) has an actual usage capacity far lower than its theoretical capacity to ensure its cyclability. When the charging voltage is too high, there is a safety hazard of internal short circuit.

In contrast, after normal charging of lithium manganese oxide (LiMn2O4) batteries, all the Li has been embedded from the positive electrode to the negative electrode. The reaction equation can be written as: LiMn2O4→Li 2MnO2. At this point, even if the battery enters an overcharged state, there is no Li to be deintercalated from the positive electrode material, so the precipitation of metallic lithium is completely avoided, thereby reducing the hidden danger of internal short circuits in the battery and enhancing safety.

B: Oxidation-reduction temperature.

Oxidation temperature refers to the temperature at which the material undergoes an exothermic redox reaction. It is an important indicator for measuring the oxidation ability of a material. The higher the temperature, the weaker its oxidation ability. The following table lists the oxidation exothermic temperatures of the four main positive electrode materials:

As can be seen from the table, lithium cobalt oxide (including nickel cobalt manganese oxide) is very active and has strong oxidizing properties. Due to the high voltage of lithium-ion batteries, non-aqueous organic electrolytes are used. These organic electrolytes are reducing and will undergo redox reactions with the positive electrode materials and release heat. The stronger the oxidation ability of the positive electrode material, the more violent the reaction, and the more likely it is to cause safety accidents. Lithium manganese oxide and lithium iron phosphate have higher redox exothermic stability, weak oxidizing properties, or thermal stability is far better than lithium cobalt oxide and nickel cobalt oxide, and have better safety.

From the above comprehensive performance, it can be seen that lithium cobalt oxide (LiCoO2) is extremely unsuitable for use in the field of power lithium-ion batteries; the safety of lithium batteries with lithium manganese oxide (LiMn2O4) and lithium iron phosphate (LiFePO4) as positive electrode materials is recognized both at home and abroad.

Suzhou Xingheng Power Co., Ltd. uses lithium manganese oxide with surface nano-coating treatment as the positive electrode material. The surface-modified lithium manganese oxide has reduced oxidation, which can further improve safety.

Lithium iron phosphate is not a mainstream positive electrode material. Power lithium-ion batteries require high-rate charge and discharge, that is, large current and short-term release of electrical energy; another requirement for power lithium-ion batteries is low-temperature performance. From the perspective of the material itself, lithium iron phosphate currently cannot meet the requirements of high-current discharge, low-temperature performance and light weight.

1. From the perspective of material properties: 1) The energy density of lithium iron phosphate is relatively low, resulting in a larger and heavier battery. 2) The electronic conductivity of lithium iron phosphate materials is low, and carbon black must be added or modified to increase the conductivity, but this will increase the volume and increase the electrolyte. 3) The electronic conductivity of lithium iron phosphate materials is even lower at low temperatures, and its low temperature performance is another obstacle to its application in power batteries.

At present, international companies such as Valence Technology, A123 and Phostech in Canada can provide samples and batteries of lithium iron phosphate, but these samples are much different from the mature lithium manganese oxide in voltage, density, high current and low temperature performance. There is a data showing that the capacity of 18650 battery with lithium iron phosphate as positive electrode can only reach 1300mAh/g; 2. From the perspective of technical maturity, phosphate is the development trend of positive electrode materials for lithium batteries due to safety. However, since the application time of lithium iron phosphate and lithium-ion batteries is much shorter than that of lithium cobalt oxide and lithium manganese oxide, it is still in the primary stage of product application and needs to go through a development process from small to large, so it is impossible to become the mainstream positive electrode material for power lithium-ion batteries at present.

3. From the perspective of battery cost, the manufacturing of lithium iron phosphate requires lithium carbonate as the main material, and also requires protective gases such as argon and nitrogen, so the manufacturing cost is very high. At present, the price of the best lithium iron phosphate in the international market is more than 300,000 yuan/ton, but the output is very small and the batch is unstable; the domestic price is 150,000-160,000 yuan/ton. In the next 3-5 years, the price of lithium iron phosphate will remain high. At present, the price of lithium manganese oxide is 80,000-100,000 yuan/ton.

4. From the perspective of the feasibility of mass production, the cost of positive electrode materials is only a part of the battery cost, and the price drop of positive electrode materials will not have a substantial impact on the overall cost of the battery. In the production and manufacturing of batteries, positive electrode materials only account for 15%-20% of the raw materials, and other issues such as electrolyte, manufacturing process, and low yield rate need to be considered. Among them, the problem of lithium iron phosphate battery manufacturing process has yet to be solved. At present, power lithium iron phosphate batteries can be made in the laboratory, but the material stability of lithium iron phosphate is poor, the material process is relatively complex, the coating is difficult, and the preparation process is difficult. It will take some time to enter mass production.

In summary, lithium iron phosphate has defects in terms of technology maturity, performance, cost, and manufacturing process. Although it is an option for future research and development, it is not suitable for market applications at this stage.

Lithium manganate is unanimously recognized by leading manufacturers at home and abroad. 1. The technology is mature and the safety is guaranteed.

The safety of lithium manganese oxide is beyond doubt. The modified lithium manganese oxide developed by Suzhou Xingheng Power Co., Ltd. performs better in capacity and cycle performance. At the same time, Xingheng's products using lithium manganese oxide as positive electrode materials are also the first high-power lithium-ion batteries used in electric vehicles in China. In the unified test of the National "863" Plan Electric Vehicle Major Project Group, Xingheng passed all the tests on safety, cycle, high and low temperature performance, and became the only selected unit.

The figure below is the capacity cycle attenuation diagram of Xingheng's modified lithium manganese oxide battery at 55°C. This figure shows that Xingheng's modified lithium manganese oxide still has good cycle performance at a high temperature of 55°C. After 200 charge and discharge cycles, the capacity retention rate still reaches more than 90%, showing excellent high-temperature cycle stability and structural stability, which can meet the use requirements of power lithium-ion batteries for electric bicycles in high temperature environments.

The above figure is a comparison of the rate characteristics of two lithium manganese oxide lithium-ion batteries. This figure shows that Xingheng modified lithium manganese oxide significantly improves the charge and discharge rate of the material, almost close to 100%.

The experiment also showed that Xingheng modified lithium manganese oxide reduced the oxidation reaction of the material with the electrolyte caused by increased temperature and had better thermal stability.

It can be seen that Xingheng's modified lithium manganese oxide has better overcharge resistance, stronger high-rate discharge tolerance, better safety performance, and has overcome many shortcomings of general lithium manganese oxide. It is very suitable for use in power-type large-capacity lithium-ion batteries.

2. Sales volume first, market tested application.

In the domestic market, Suzhou Xingheng's lithium manganese oxide batteries have been mass-produced, with more than 40,000 sets used in the field of electric bicycles, and overseas sales have exceeded 10,000 sets, accounting for more than 80% of the domestic electric bicycle lithium battery market share. After more than a year of market testing, the comprehensive customer complaint rate of Xingheng's lithium manganese oxide batteries does not exceed 3%, and there is no safety problem, which shows the stable performance and excellent quality of Xingheng's lithium manganese oxide batteries.

3. Lithium manganese oxide is the common choice of international high-level manufacturers.

Internationally, Japan's power lithium battery technology was developed the earliest and has the highest technical level. Lithium battery manufacturers represented by Sanyo and Hitachi all choose lithium manganese oxide as the positive electrode material for power lithium-ion batteries, and it is widely used in electric bicycles and electric vehicles. This shows that only lithium manganese oxide is the current mainstream positive electrode material.

In summary, although lithium iron phosphate has its unique advantages, it is not the first choice for positive electrode materials of power lithium-ion batteries at the current level of technology. Its maturity still requires a longer period of research and investment, so lithium manganese oxide is still the first choice for positive electrode materials of power lithium-ion batteries.