With the rapid development of new energy power vehicles and energy storage technology industry, higher requirements are put forward for lithium-ion batteries in terms of energy density, cycle stability and price. As the most critical material of lithium-ion batteries, the cathode material determines the battery performance and cost. High nickel ternary cathode materials is one of the most commercially valuable cathode materials at present.
1. Problems with using high nickel ternary cathode materials
Although the high-nickel ternary cathode material is considered by most scientific researchers and the electric vehicle industry as the most promising cathode material for lithium-ion batteries, there are still many deficiencies in the actual production application and scientific research. place. In order to pursue higher performance, the Ni content of high-nickel ternary cathode materials needs to be continuously increased, and the following four problems arise;
The mixed arrangement of cations between Li and transition metals, mainly Li+/Ni2+, will seriously interfere with the migration of Li ions;
The formation of micro-cracks during long-term cycling;
The problem of surface side reactions, resulting in a variety of side reaction products;
The thermal stability is poor, and the electrochemical performance declines significantly under high temperature environmental conditions.
These problems seriously restrict the commercialization process and large-scale application of high-nickel ternary cathode materials. In order to improve their performance, they are mainly modified by bulk doping, surface coating, single crystallization and gradient structure at this stage. Research.
2. What is physique?
Bulk doping stabilizes the layered structure of the material by incorporating other elements, and enhances the thermal stability of the H3 phase from the microstructure, thereby improving the long-term cycling and electrochemical performance of the material under high current density. The choice of different doping elements can play different roles in the material, so the doping methods can be divided into three types: cation, anion and anion-cation synergistic doping.
Cationic doping is to preferentially occupy Li or Ni sites by doping elements to stabilize the layered structure of the material and improve the cycle stability of the material. Low-valence cations (such as K+, Mg2+) are preferentially doped into Li sites to stabilize the lattice structure by preventing Li+/Ni2+ intermixing, but adversely affect the high nickel ternary cathode materials discharge capacity.
Anion doping is to increase the bonding strength between transition metal elements and anions by replacing oxygen elements with more electronegative elements. Commonly used doping anions are F-, Br-, BO33-polyanions, etc. Anion-cation co-doping is to select one or more anions and cations at the same time, such as Na-F, Mg-F, P-F, etc. The method combines the advantages of doping anions and cations, and can simultaneously improve the high temperature, cycle and rate performance of the material.
Bulk doping can stabilize the material structure from the microscopic level, improve the electrochemical performance, and has less difficulty in operation and obvious modification effect.
3. What is surface coating?
Surface coating is to physically or chemically treat the surface of the positive electrode material to isolate anode electrode material from the electrolyte and prevent positive electrode material from directly contacting the electrolyte and cause side reactions, thereby improving the dispersibility, thermal stability and stability of anode electrode material. Electrochemical properties such as discharge rate.
If you want to improve the electrochemical performance of anode electrode material by surface coating, it is necessary to select appropriate materials and coating methods. At the same time, it is necessary to protect the structure of the positive electrode material to prevent the lattice of the active material from being damaged during the charging and discharging process. The most commonly used surface coating materials are: carbon materials, phosphates, fluorides, metal oxides, and the like. In this way, the quality of high nickel ternary cathode can be improved.
4. What is single crystallization?
Single crystallization is a modification strategy proposed for the secondary particles of high nickel ternary cathode materials that are easily broken, pulverized, and easily reacted with the electrolyte. At present, high nickel ternary cathode products are secondary particles formed by agglomeration of primary particles, which have low compaction density and are easily broken under excessive pressure or high-pressure working conditions in the production of battery pole pieces, thereby increasing the contact and reaction between the internal primary particles and the electrolyte. , accelerating capacity decay and causing gas production problems.
If single crystals can be grown purposefully by controlling the reaction parameters, it can not only improve the compaction density, but also ensure that the material achieves better safety and cycle performance. Commonly used single crystal preparation methods are co-precipitation-high temperature solid phase method and co-solvent method.
Although single-crystal materials show excellent characteristics in terms of cycle stability and capacity retention, the experimental parameters are difficult to control, and it is difficult to prepare single-crystal pure phases. It is difficult to meet the actual requirements, so it is still necessary to explore breakthroughs in the preparation of single crystal cathode materials.
5. What is a gradient structure?
The gradient structure is derived from the core-shell structure. In traditional core-shell structural materials, the core (rich Ni) and shell (rich Mn) play the roles of providing specific capacity and stabilizing the material structure, respectively, but the content of the internal and external components is large, and faults will appear between the core and shell after long-term cycling, resulting in destroy the Li+ transport channel and cause severe capacity fading.
On the other hand, the concentration gradient structure makes the core-shell composition change gradient from the inside to the outside on this basis, which improves the structural fracture caused by the large difference between the inner core and the outer shell. Among many modification methods, bulk doping and surface coating strategies are simple and easy to control, and are mostly used in industry. However, the effect of single modification is still not ideal.
At present, two methods are used simultaneously to achieve co-modification. The single-crystal material exhibits significantly improved structural stability and electrochemical performance, but is still in the research stage due to factors such as low specific capacity and difficult control of process parameters. In short, there is still a long path to high nickel ternary cathode materials.