How to choose cathode material and safety analysis of power lithium battery

With the sales of lithium-ion electric vehicles in Beijing, Shanghai, Suzhou, Hangzhou and other large and medium-sized cities in China, more and more electric vehicle manufacturers have begun to launch lithium battery projects. However, what kind of lithium battery to choose has become their primary priority problem. Although the protection circuit of the lithium battery is relatively mature, for the power battery, to truly ensure safety, the choice of the cathode material is very critical.

At present, the most commonly used cathode materials in lithium-ion batteries are the following: lithium cobaltate (LiCoO2), lithium manganate (LiMn2O4), lithium nickel cobalt manganate (LiCoxNiyMnzO2) and lithium iron phosphate (LiFePO4). Which kind of positive electrode material is the lithium battery? The following will do a detailed analysis.

To test the safety of lithium-ion batteries, overcharging (meaning that the charging voltage exceeds its charging cut-off voltage, for lithium-ion batteries, generally 10V / cell can be set as an overcharging voltage) is a good method. 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 a lithium-ion battery is that Li runs out from the positive electrode, swims through the electrolyte to the negative electrode and gets electrons, is embedded in the negative electrode material, and the process of discharging is the opposite.

The main test for measuring the safety of cathode materials:

A: It is easy to form dendrites during charging.

The charging process of a lithium ion battery is that Li runs out from the positive electrode, travels to the negative electrode through the electrolyte to be reduced and intercalated into the negative electrode material; the process of discharge is the opposite, the lithium in the negative electrode material is oxidized, and the positive electrode material is intercalated through the electrolyte.

Based on cyclical considerations, the actual use capacity of lithium cobaltate (LiCoO2) material is only one-half of its theoretical capacity, that is, the lithium-ion battery using lithium cobaltate as the positive electrode material after the normal charge is completed (ie, charged to the cut-off voltage 4.2 V or so), Li in LiCoO2 cathode material will remain. It can be expressed by the following abbreviation: LiCoO2 → 0.5Li Li0.5CoO2 (the end of normal charging). At this time, if the charging voltage continues to increase, the remaining Li in the LiCoO2 cathode material will continue to deintercalate and swim toward the anode, and the position of the anode material that can accommodate Li has been filled at this time, and Li can only be The form precipitates on its surface. On the one hand, the deposition of lithium metal on the surface is very easy to coalesce into bifurcated lithium dendrites, which punctures the separator and causes a direct short circuit between the positive and negative electrodes; in addition, the lithium metal is very active and will directly react with the electrolyte to emit heat; The fuse of lithium is quite low. Even if the surface metal lithium dendrites do not pierce the separator, as long as the temperature is slightly higher, such as the battery temperature rise due to discharge, the metal lithium will melt, thereby shorting the positive and negative electrodes, causing a safety accident. In short, when the charging voltage is too high, for example, when the protection plate fails, there is a great potential safety hazard, and the high capacity of the power lithium-ion battery will cause great damage.

Lithium cobalt manganese oxide (LiCoxNiyMnzO2) is the same as lithium cobalt oxide. In order to ensure its cyclicity, the actual use capacity is also far lower than its theoretical capacity. In the case of excessive charging voltage, there is a potential safety hazard of internal short circuit.

The difference is that after the lithium manganate (LiMn2O4) battery is charged normally, all Li has been inserted from the positive electrode to the negative electrode. The reaction formula can be written as: LiMn2O4 → Li 2MnO2. At this time, even if the battery enters an overcharged state, the positive electrode material has no Li to be intercalated. Therefore, the precipitation of metallic lithium is completely avoided, thereby reducing the hidden danger of short circuit inside the battery and enhancing safety.

B: Oxidation-reduction temperature.

The oxidation temperature refers to the temperature at which the redox exothermic reaction of the material occurs, and is an important indicator to measure the oxidation ability of the material. The higher the temperature, the weaker the oxidation ability. The following table lists the oxidation heat release temperatures of the four main cathode 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 have reducibility and will undergo a redox reaction with the cathode material and release heat. The stronger the oxidation ability of the cathode material, the more violent the reaction will be , The easier it is to cause a safety accident. Lithium manganate and lithium iron phosphate have higher redox exothermic stability and weak oxidizability, or thermal stability is much better than lithium cobalt oxide and lithium nickel cobalt oxide, and has better safety.

From the above comprehensive performance, it can be seen that lithium cobaltate (LiCoO2) is extremely unsuitable for use in the field of power lithium-ion batteries; the safety of lithium batteries with lithium manganate (LiMn2O4) and lithium iron phosphate (LiFePO4) as the cathode material is domestic Outside recognized.

Suzhou Xingheng Power Co., Ltd. uses lithium manganate treated with nano-coating on the surface as the positive electrode material. The oxidizability of the surface-modified lithium manganate is reduced, which can further improve safety.

Lithium iron phosphate is not the mainstream cathode material power lithium-ion battery requires high rate charge and discharge, that is, large current, short-term discharge of electrical energy; another requirement of power lithium-ion battery is low temperature performance. From the point of view of the material itself, lithium iron phosphate currently cannot meet the requirements of large current discharge, low temperature performance and lightness and compactness.

1. From the perspective of material characteristics 1) The energy density of lithium iron phosphate is relatively low, resulting in a battery with a large volume and heavy weight; 2) The electronic conductivity of lithium iron phosphate material is low, and carbon black must be added or modified It can improve the conductivity, but this will lead to a larger volume and increase the electrolyte; 3) The lithium iron phosphate material has a lower electronic conductivity at low temperatures, and its low temperature performance is another obstacle to its application to power batteries.

At present, international-level companies such as Valence Technology, A123, and Phostech in Canada can provide samples and batteries of lithium iron phosphate. However, these samples have voltage, density, high current, and low temperature performance compared with the current mature lithium manganate. The difference is more. There is a data to show that the capacity of 18650 battery with lithium iron phosphate as the positive electrode can only reach 1300mAh / g;

2. From the technical maturity point of view, due to safety clearance, phosphate is the development trend of lithium battery cathode materials. However, because the application time of lithium iron phosphate and lithium ion batteries is much shorter than that of lithium cobaltate and lithium manganate, and it is still in the initial stage of product application, it needs to go through a development process from small to large, so it is currently impossible to become a driving force. Mainstream cathode material for lithium-ion batteries.

3. From the perspective of battery cost, the manufacture of lithium iron phosphate requires lithium carbonate as the main material, as well as protective gases such as argon and nitrogen, which is very expensive to manufacture. At present, the best price of lithium iron phosphate in the international market is more than 300,000 yuan / ton, but the output is very small and the volume is unstable; the domestic price is 15-16 million yuan / ton, within 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 feasibility of mass production, the cost of cathode materials is only a part of the cost of batteries. The decline in the price of cathode materials will not have an essential impact on the overall cost of batteries. In the battery manufacturing, the cathode material only accounts for 15% -20% of the raw materials. The electrolyte, manufacturing process, and low yield rate also need to be considered. Among them, the manufacturing process of lithium iron phosphate batteries has yet to be resolved. At present, it is possible to make a power lithium iron phosphate battery from the laboratory, but the material stability of lithium iron phosphate is poor, the material process is more complicated, the coating film is difficult, the preparation process is difficult, and it will take time to enter mass production.

In summary, lithium iron phosphate has defects in terms of technical maturity, performance, cost, and manufacturing process. Although it is a choice for future research and development, it is not suitable for current market applications.

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 no doubt. The modified lithium manganate developed by Suzhou Xingheng Power Co., Ltd. has better performance in terms of capacity and cycle performance. At the same time, Xingheng products that use lithium manganese oxide as the cathode material are 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 special group, Xingheng's safety, cycle, high and low temperature performance and other tests have all passed the test and become the only selected unit.

The figure below is the capacity cycle attenuation diagram of Xingheng modified lithium manganate battery at 55 ℃. This graph shows that Xingheng's modified lithium manganate still has good cycle performance at high temperature of 55 ℃. After 200 charge-discharge cycles, the capacity retention rate is still above 90%, showing excellent high-temperature cycle stability and structural stability, which can meet the use requirements of high-temperature environments for electric bicycle power lithium-ion batteries

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

The experiment also shows that Xingheng modified lithium manganate reduces the oxidation reaction of the material with the electrolyte caused by the temperature increase, and has better thermal stability.

It can be seen that Xingheng's modified lithium manganate has better overcharge resistance, stronger high-rate discharge endurance, and better safety performance. It also overcomes many shortcomings of general lithium manganate and is very suitable for Power type large-capacity lithium-ion battery.

2. No. 1 in sales, market inspection application.

In the domestic market, Suzhou Xingheng's lithium manganate batteries have been mass-produced and have been used in the field of electric bicycles for more than 40,000 sets. Overseas sales exceeded 10,000 sets, accounting for more than 80% of the domestic electric bicycle lithium battery market share. After more than one year of market inspection, the comprehensive customer complaint rate of Xingheng Lithium Manganese Oxide battery is no more than 3%, and there is no case of safety problem, which shows the stable performance and excellent quality of Xingheng Lithium Manganese Oxide battery.

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

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

In summary, although lithium iron phosphate has its unique advantages, as far as the current technical level is concerned, it is not the first choice for the positive electrode material of power lithium-ion batteries, and its maturation requires a longer time research investment, so manganese Lithium acid is also currently the first choice for cathode materials for power lithium-ion batteries.

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