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Power lithium battery principle and material selection

With the hot sales of lithium-ion electric vehicles in large and medium-sized domestic cities such as Beijing, Shanghai, Suzhou and Hangzhou, more and more electric vehicle manufacturers have begun to launch lithium-ion battery projects. However, what kind of lithium-ion battery to choose becomes their face. Right first question. Although the protection circuit of lithium-ion batteries is relatively mature, for power lithium batteries, to truly ensure safety, the choice of cathode material is very important. Currently, the most widely used cathode materials in lithium-ion batteries are as follows: lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel cobalt manganese oxide (LiCoxNiyMnzO2) and lithium iron phosphate (LiFepO4). Which cathode material lithium-ion battery to choose? A detailed analysis will be done below.



To test the safety of lithium-ion batteries, overcharging (referring to the charging voltage exceeding its charge cut-off voltage, for lithium-ion batteries, generally 10V/cell can be set as the overcharging voltage) is a good method. Speaking of 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 escapes 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 discharge process is the opposite. An important test to measure the safety of cathode materials:



A: It is not easy to form dendrites during charging.



The charging process of lithium-ion batteries is that Li escapes from the positive electrode, swims to the negative electrode through the electrolyte, and 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 inserted into the positive electrode material through the electrolyte.



Based on cyclical considerations, the actual use capacity of lithium cobalt oxide (LiCoO2) material is only one-half of its theoretical capacity, that is, lithium-ion batteries using lithium cobalt oxide as the positive electrode material will be charged to the cut-off voltage after the end of normal charging. 4.2V or so), the Li in the LiCoO2 cathode material will still have surplus. It can be expressed in the following short form: LiCoO2→0.5LiLi0.5CoO2 (end of normal charging). At this time, if the charging voltage continues to rise, the remaining Li in the LiCoO2 cathode material will continue to deintercalate and swim 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 metal The form precipitates on its surface. On the one hand, the surface deposition of metallic lithium is very easy to coalesce into branched lithium dendrites, which pierce the diaphragm and cause a direct short circuit between the positive and negative electrodes; on the other hand, metallic lithium is very active and will directly react with the electrolyte to generate heat; at the same time, the metal The melting of lithium is quite low. Even if the surface metal lithium dendrites do not pierce the diaphragm, as long as the temperature is slightly higher, such as the battery heating caused by discharge, the metal lithium will melt, which will short-circuit the positive and negative electrodes, causing safety accidents. In short, when the charging voltage of lithium cobalt oxide is too high, for example, when the protective plate fails, there are great safety hazards, and the high capacity of power lithium-ion batteries will cause great destructiveness.



Lithium nickel cobalt manganese oxide (LiCoxNiyMnzO2) is the same as lithium cobalt oxide. In order to ensure its cycleability, the actual use capacity is much lower than its theoretical capacity. If the charging voltage is too high, there is a safety risk of internal short circuit.



The difference is that after the normal charging of the lithium manganese oxide (LiMn2O4) battery, all Li has been inserted into the negative electrode from the positive electrode. The reaction formula can be written as: LiMn2O4→Li2MnO2. At this time, even if the battery enters an overcharged state, the positive electrode material has no Li to deintercalate, so the precipitation of metallic lithium is completely prevented, thereby reducing the hidden danger of internal short circuit of the battery and enhancing safety.



B: oxidation-reduction temperature.



Oxidation temperature refers to the temperature at which the material undergoes redox exothermic reaction, and is an important indicator to measure the oxidizing ability of the material. The higher the temperature, the weaker the oxidizing ability. The following table lists the oxidation exothermic temperature of four important cathode materials:



As can be seen from the table, lithium cobalt oxide (including lithium 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 reducing properties and will undergo redox reactions with the positive electrode material and release heat. The stronger the oxidation ability of the positive electrode material, the more violent the reaction will be. , The easier it is to cause safety accidents. Lithium manganese oxide and lithium iron phosphate have high redox exothermic stability, and their oxidizing properties are weak. In other words, their thermal stability is much better than lithium cobalt oxide and lithium nickel cobalt oxide, and they 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-type lithium-ion batteries; the safety of lithium-ion batteries with lithium manganate (LiMn2O4) and lithium iron phosphate (LiFepO4) as cathode materials is Recognized at home and abroad.



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



Lithium iron phosphate is not the mainstream cathode material. Power lithium-ion batteries require high-rate charging and discharging, that is, large currents and short-time discharge of electrical energy; another requirement of power lithium-ion batteries is low-temperature performance. From the perspective of the material itself, lithium iron phosphate is currently unable to balance the requirements of high current discharge, low temperature performance, and lightness and compactness.



1. From the point of view of material characteristics: 1) The energy density of lithium iron phosphate is relatively low, resulting in larger batteries and heavier weight; 2) The electronic conductivity of lithium iron phosphate materials is low, and carbon black must be added or modified It can improve the conductivity, but this will cause the volume to increase and add electrolyte; 3) The electronic conductivity of lithium iron phosphate materials is lower at low temperatures, and its low temperature performance is another obstacle to its application in power lithium batteries.



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



2. From the perspective of technological maturity, phosphate is the development trend of lithium-ion battery cathode materials due to safety clearance. However, because the application time of lithium iron phosphate and lithium ion batteries is much shorter than that of lithium cobalt oxide and lithium manganese oxide, they are still in the initial stage of product application, and they have to go through a small to large development process, so it is impossible to become a driving force at present The mainstream cathode material for lithium-ion batteries.



3. From the perspective of battery cost, the production of lithium iron phosphate requires lithium carbonate as an important material, as well as protective gases such as argon and nitrogen. The manufacturing cost is very high. At present, the best price of lithium iron phosphate in the international market is more than 300,000 yuan/ton, but the output is 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. Currently, the price of lithium manganate is 80,000-100,000 yuan/ton.

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