Advantages, Disadvantages and Safety of Ternary Materials

Habitually, we say that ternary materials generally refer to lithium nickel cobalt manganese oxide NCM cathode materials (in fact, there are also negative ternary materials), Ni, Co, Mn, three metal elements can be obtained according to different ratios of different types of three Meta material.
The general formula is LiNi1-x-yCoxMnyO2. The common ratios are 111,424,523,622,811. Please note that the order of the above ratios is N:C:M, which is different from Chinese and foreign countries.
In addition, one point to say is that although NCA materials are often mentioned together with NCM, they are accurately binary high Ni materials and cannot be classified as ternary materials.
Comparison of synthetic methods of ternary materials
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 the solution is first precipitated in the solution. An intermediate product is obtained, which is then calcined and decomposed to prepare a fine powder.
The chemical coprecipitation method is divided into a direct chemical coprecipitation method and an indirect chemical coprecipitation method.
In the direct chemical co-precipitation method, salts of Li, Ni, Co, and Mn are simultaneously co-precipitated, filtered, washed, and then subjected to high-temperature baking. Indirect chemical co-precipitation is to synthesize Ni, Co, Mn ternary mixed coprecipitation, then filter and dry, and then mix and sinter with lithium salt; or after Ni, Co, Mn ternary mixed coprecipitation is not filtered The solution containing the lithium salt and the mixed coprecipitate is 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 achieve 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 synthesis product component is obtained. Uniform, reproducible, easy to control conditions, simple to operate, commercial production using this method.
Solid phase synthesis method: generally, nickel hydroxide manganese and lithium hydroxide or carbonate or oxide are used as raw materials, mixed according to the amount of the corresponding substance, and calcined at 700 to 1000 ° C to obtain a product. The method mainly uses mechanical means to mix and refine the raw materials, which tends to cause uneven microscopic distribution of raw materials, which makes the diffusion process difficult to carry out smoothly. At the same time, impurities are easily introduced in the mechanical refining process, and the calcination temperature is high and the calcination time is long. The reaction steps are many, the energy consumption is large, the lithium loss is serious, the stoichiometric ratio is difficult to control, the heterophase is easily formed, and the product has large differences in composition, structure, particle size distribution, etc., and thus the electrochemical performance is unstable.
Sol-gel method: Firstly, the raw material solution is uniformly mixed to form a uniform sol, which is gelled, molded or dried in the gel process or after gelation, and then calcined or sintered to obtain a desired powder material. The sol-gel technology requires simple equipment and easy control of the process. Compared with the conventional solid-phase reaction method, it has a lower synthesis and sintering temperature, and can produce materials with high chemical uniformity and high chemical purity, but the synthesis cycle is relatively long. The synthesis process is relatively complicated, high in cost, and difficult to industrialize.
The role and advantages and disadvantages of the three elements
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.
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.
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?
From the perspective of energy density, ternary materials have advantages over LFP and LMO, but safety performance has always been a difficult problem for large-scale applications.
A pure ternary battery with a large capacity is difficult to pass safety tests such as needle punching and overcharging, which is why a large capacity battery is generally used in combination with lithium manganate. From the situation I have learned, there are mainly the following solutions to solve the ternary security problem:
1. Select the ternary material with the best ratio of safety performance
It is well known that the higher the nickel content in the ternary material, the worse the stability of the material and the worse the safety. The current safety ** mainstream ternary nickel-cobalt-manganese ratio is 1:1:1, which is usually Said 111 ternary, 111 ternary stability **, mainly because:
1) The proportion of nickel is relatively low (relative to 422/523, etc.), and it is easier to form a complete layered structure during material preparation, taking into account the energy density.
2) The proportion of manganese is relatively high (relative to 422/523, etc.), and manganese is an important element of structural stability in ternary materials.
3) The ratio of nickel to manganese is 1:1, and both nickel and manganese are both positive and positive tetravalent of stability**. (To say more here, 111 ternary is the most suitable ternary material for high voltage. If the high voltage electrolyte bottleneck breaks through, its energy density will not be inferior to any high nickel ternary, cycle and electrode processing performance will be A few high grades.)
In summary, in the large-capacity pure ternary battery, 111 ternary has the security of **.
2. Improve from the ternary material itself
The ternary material itself is a new type of material developed from doping. We believe that if other elements are doped in the ternary, it will not only affect its electrochemical performance, but also impose more requirements on the preparation process. The improvement will also limit the application of ternary power in the power, and the coating process will have an impact on the consistency of the product. Therefore, we believe that the improvement of the safety performance of the material can ensure that the product is suitable for industrialization. Yuan is really applied to the ** method in the power battery.
So here we only talk about our improvement program. We have said it many times before. Our ternary material is a primary particle similar to lithium cobaltate. In addition to the great advantages in compaction density and electrode processing performance, There are also improvements to security for the following reasons:
1) Micron-sized primary particles have a more complete layered structure. The more complete the layered structure, the better the stability of the material, which is reflected in the improvement of cycle performance and safety performance.
2) The primary particles with larger particle size have better kinetic stability. I heard that a joint venture company in China claimed that the safety performance of the power battery made by Japanese nano-scale ternary materials is the same, at least in my opinion. Come, the effect of such propaganda is negative. Since the promotion of nanomaterials should focus on the performance of the rate and avoid safety, because the nano-scale materials themselves have high activity, and the nano-materials make the stability and safety of the materials different. The reduction, the reason I mentioned the micron level is to distinguish it from the nano level.
3) Another advantage of making the primary particle size larger is to reduce the specific surface area and reduce the damage of the material due to the side reaction caused by the contact with the electrolyte, which is helpful for circulation and material stability.
Nevertheless, we believe that the safety of ternary materials in batteries is its own nature, just like the high temperature of lithium manganate. Even after thorough modification, the 3V platform of lithium manganate is completely eliminated, and the morphology control is also done. Many optimizations still require matching of the electrolyte and the negative electrode to fully meet the high temperature performance requirements.
Reducing the upper limit voltage of the battery At present, a domestic enterprise has solved the safety problem of the 35Ah pure ternary battery well, and its charging upper limit voltage is 4.1V, which is a good improvement for the stability of the entire battery system.
Improve battery safety by making polymer pure ternary batteries. Here is a lithium-ion battery that is really a solid polymer electrolyte, not a soft-pack battery in the usual sense.
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 believe that 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.
Every material of the lithium battery and the lithium battery itself are complex, so there is no perfect material, no perfect process, only continuous optimization and continuous communication progress.

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