Habitually, the ternary material we refer to generally refers to lithium nickel cobalt manganese oxide NCM cathode material (in fact, there is also a negative ternary material), Ni, Co, Mn three metal elements can be different according to different ratios The ternary material.
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The general formula is LiNi1-x-yCoxMnyO2, the common ratio is 111,424,523,622,811. Please note that the order of the above ratio is N:C:M, which is different from Chinese and foreign. In addition, it should be noted that although NCA materials are often mentioned together with NCM, it is accurate to say that NCA materials are binary high Ni materials and cannot be listed in ternary materials.
Comparison of synthetic methods of ternary materials
The chemical coprecipitation method is further divided into a direct chemical coprecipitation method and an indirect chemical coprecipitation method. Generally, the chemical raw materials are mixed in a solution state, and a suitable precipitating agent is added to the solution, so that the components which have been uniformly mixed in the solution are coprecipitated in a stoichiometric ratio, or an intermediate product is precipitated in the solution first. Then, it is calcined and decomposed to prepare a fine powder.
In the direct chemical co-precipitation method, Li, Ni, Co, and Mn salts are simultaneously coprecipitated, filtered, washed, and then subjected to high-temperature baking. The indirect chemical co-precipitation method is to synthesize Ni, Co and Mn ternary mixed coprecipitation, filter and dry, and then mix and sinter with lithium salt; or after the Ni, Co, Mn ternary mixed coprecipitation is formed, it will be included without filtering. The lithium salt and the mixed coprecipitated solution are evaporated or freeze-dried, and then the dried product is subjected to high-temperature baking.
Compared with the traditional solid phase synthesis technology, the coprecipitation method can make the material reach the stoichiometric ratio of molecular or atomic linearity, and it is easy to obtain a precursor with small particle size and uniform mixing, and the calcination temperature is low, and the synthetic product group The unit is uniform, the reproducibility is good, the conditions are easy to control, the operation is simple, and the commercial production adopts this method. In addition, there are other methods such as solid phase synthesis, sol-gel method and the like.
The role and advantages and disadvantages of the three elements
NCM622 material structure diagram
Introducing 3+Co: reducing the cation mixing occupancy, stabilizing the layered structure of the material, reducing the impedance value, improving the conductivity, and improving the cycle and rate performance.
The introduction of 2+Ni: can increase the capacity of the material (increasing the volumetric energy density of the material), and due to the similar radius of Li and Ni, excessive Ni will also cause lithium nickel to be mixed due to dislocations with Li, in the lithium layer. The greater the concentration of nickel ions, the more difficult it is to deintercalate lithium in the layered structure, resulting in poor electrochemical performance.
Figure (b) shows the schematic diagram of the mixing of Ni and Li.
The introduction of 4+Mn: not only can reduce the cost of materials, but also improve the safety and stability of materials. However, an excessively high Mn content tends to occur in the spinel phase and destroy the layered structure, resulting in a decrease in capacity and cycle decay.
What is the high PH effect of ternary materials?
We all know that the high Ni ternary material is the future application direction of high energy density power battery, but why has it been used badly? One of the most important reasons is that the material is alkaline, and the slurry is easy to cause jelly after water absorption. Our requirements for the production environment and process control capabilities are not good at all. Reducing the surface residual alkali content is very important for the application of ternary materials in batteries.
Where is the high Ph? This is because the lithium salt is excessive in the synthesis of the ternary material. The product of the excess lithium salt after calcination at high temperature is mainly the oxide of Li, which reacts with H2O and CO2 in the air to regenerate LiOH and Li2CO3. On the surface of the material, the pH of the material is higher.
In addition, in the high Ni system, due to the limitation of valence balance, a part of Ni in the material exists in the form of 3+, and excess Li easily forms LiOH and Li2CO3 on the surface of the material. The higher the Ni content, the larger the alkali content on the surface. The more easily the water is absorbed during the homogenization and coating process, the jelly is formed.
At the same time, it should be noted that these residual lithium salts are not only electrochemically active, but also cause gas production of the battery during charge and discharge of the battery due to decomposition of lithium carbonate or the like under high pressure.
How to reduce the PH of ternary materials?
Generally, the pH and production environment of the precursor are controlled from the source, the proportion of lithium salt is lowered, and the sintering system is adjusted so that lithium can rapidly diffuse into the interior of the crystal. The material is washed with water and then sintered twice to reduce the surface residual alkali content, but a corresponding loss of electrical properties is lost. Surface coating is also an effective method to reduce the residual alkali content on the surface of ternary materials.
Ternary material modification method?
The surface of the ternary material is modified with metal oxides (Al2O3, TiO2, ZnO, ZrO2, etc.) to mechanically separate the material from the electrolyte, reduce the side reaction between the material and the electrolyte, inhibit the dissolution of metal ions, and the ZrO2, TiO2 and Al2O3 oxides. The coating can prevent the impedance from increasing during the charging and discharging process and improve the cycle performance of the material. The surface resistance of the coating of ZrO2 is the smallest, and the coating of Al2O3 does not reduce the initial discharge capacity.
How to improve the safety of ternary materials?
The safety of the ternary battery, especially the ternary battery above the 111 system, has been plaguing the industry. From the beginning of last year, the power battery route was chosen to suppress the ternary battery, and the ban on the ternary battery at the end of the year. These are all related to the safety of the material system used in the future.
And as the NCM energy density continues to increase, the thermal stability of the material will become worse and worse. The graph below shows that the decomposition temperature of the material gradually decreases as the Ni content increases.
How to improve the safety of ternary materials? Simply put, the points are more important. First of all, from the ternary material itself:
1. Coating of ceramic alumina, Al2O3 can consume HF in the battery system by forming Al-OF and Al-F layers, and the charging voltage can be increased to 4.5V;
2, control the content of Ni in a reasonable range (811 is of course more unstable than 622);
3. Performing appropriate doping coatings on other metal elements (Al, Mg, Ti, Zr) can improve the structural stability, thermal stability and cycle stability of the material.
Secondly, we must work hard on the cooperation with other materials in the battery system:
1. A high-boiling point and flash point flame retardant additive is added to the electrolyte, and a common organic phosphorus or fluorophosphate series is used;
2. Selection of ceramic separator, increase the thickness of the separator substrate and coating, and use a new type of non-woven fabric with low temperature shrinkage.
In addition, it is common to use a mixture of different positive electrode materials to achieve complementary advantages, such as ternary mixed lithium manganate to improve the safety of the battery. Personally, the ternary materials that can be applied in large scale in the short term in China are 622 systems. Higher systems and even NCA use power battery systems are difficult to control with the existing domestic technical level.
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