Nickel Hydride No. 5 battery.Breakthrough in the synthesis of Wanxiang ternary high-nickel single crystal material

With the continuous development of power lithium-ion batteries, consumers have put forward higher requirements for the energy density and safety of power batteries. In order to reduce costs and increase energy density, power battery companies currently tend to use high-nickel and low-cobalt ternary materials, such as Ni≥80%. As the nickel content in the ternary material increases, the gram capacity of the cathode material is significantly increased; however, the structural stability, thermal stability, safety, processing performance and other aspects of the cathode material will become worse, making its application more serious. challenge.

With the continuous development of power lithium-ion batteries, consumers have put forward higher requirements for the energy density and safety of power batteries.


In order to reduce costs and increase energy density, power battery companies currently tend to use high-nickel and low-cobalt ternary materials, such as Ni≥80%. As the nickel content in the ternary material increases, the gram capacity of the cathode material is significantly increased; however, the structural stability, thermal stability, safety, processing performance and other aspects of the cathode material will become worse, making its application more serious. challenge.


Another effective way to increase battery energy density is to increase the charging voltage of batteries/materials. However, increasing the charging voltage will worsen the structural stability of the material itself; in addition, the oxidative decomposition of the electrolyte will intensify as the charging voltage increases. It is accompanied by problems such as the dissolution of transition metals and the increase in side reactions at the electrode/electrolyte interface.


In response to the above-mentioned problems such as poor thermal stability, poor cycle life and structural instability of high-nickel ternary materials, the lithium-ion battery industry generally adopts coating or doping technical routes.
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Coating materials are generally metal oxides, phosphides, etc. The existence of the coating layer can block the direct contact between the electrolyte and the high-nickel ternary material, and to a certain extent can inhibit the oxidation and decomposition side reactions of the electrolyte and the transition metal Dissolution problem. The coating layer generally uses inorganic oxides, which will affect the migration/diffusion dynamics of lithium ions;


In addition, because it is an inorganic oxide layer with poor conductivity, it will increase the DCR of the battery. Element doping technology can introduce a small amount of metal atoms into the crystal lattice of high-nickel ternary materials to stabilize the crystal structure of the material, and can significantly suppress the crystal structure of high-nickel ternary materials caused by stress during cycling. rupture or phase change. Although element doping has the effect of stabilizing the lattice structure, it often affects the discharge capacity of the material and increases production costs.


Recently, some foreign R&D teams have reported that medium-nickel ternary materials, such as NCM523/622 single crystal, have certain advantages over agglomerate/polycrystalline materials in terms of heat generation and high-temperature cycling. Due to its unique microscopic morphology, single crystal materials have good mechanical strength and pressure resistance, so they are not easy to break during electrode rolling and charging and discharging processes. There are few grain boundaries, few stress concentration points, and stable interfaces, which can reduce the generation and spread of microcracks in the positive electrode material due to phase changes during the battery charge and discharge process to a certain extent, reduce the contact interface between the active material and the electrolyte, thereby reducing cycles Produces gas. The development of single-crystal high-nickel materials is considered an effective method to improve battery safety and cycle life.


Due to the lower sintering temperature of traditional secondary pellet agglomerates with high nickel content, the residual alkali content on the surface is high. Therefore, water washing process must be used in industry to reduce residual alkali, which is costly. Compared with secondary pellet agglomerates, due to the difference in sintering temperature, the surface residual alkali of single crystal high-nickel materials is lower, which is expected to avoid the consistency and cost problems caused by the water washing process and reduce material costs. Due to the lower surface residual alkali content and better crystallinity, single crystal materials have great advantages in high-temperature storage and gas production, and are suitable for application in soft-pack battery systems.


For the synthesis of high-nickel ternary materials, the selection of appropriate precursors is crucial. Research has found that the physical and chemical properties such as the crystal structure, micromorphology and particle size distribution of the precursor determine the comprehensive performance of the later cathode material. For the synthesis of single crystal high-nickel materials, team researchers screened precursors in a specific particle size range. It has been verified that precursors with uniform, coarse and orderly grain morphology are more conducive to the formation of single crystals during sintering.
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In the sintering of single crystal materials, the core technology is the sintering system and the amount of lithium. The sintering temperature serves as the driving force and determines the primary grain growth of single crystals. Lower sintering temperatures cannot fully drive grain development and growth. Excessively high sintering temperatures will lead to over-burning problems and intensify the mixing of lithium and nickel (see Figure 1).


In addition, lithium salt, as a flux aid, can promote grain growth and achieve better single crystal morphology. However, too high a lithium content will also result in high residual lithium on the surface and increase production costs.


After optimizing the sintering process, scientists from Wanxiang 123 successfully prepared a single crystal high-nickel ternary material with a 0.1C (2.75-4.3V half cell) gram capacity of greater than 210mAh/g, which is in line with the domestic technology-leading mainstream materials. Supplier products are at the same level.


For single crystal materials, it is necessary to coat their surfaces to further improve their cycle life. Wanxiang 123 selects several elements to coat the surface of single crystal materials. After heat treatment, a fast ion conductor coating layer can be formed on the surface of the single crystal material. On the one hand, this technology reduces residual alkali through the reaction between residual lithium and coating materials, and on the other hand, it improves cycle life through the formed coating layer.


In addition, in order to reduce costs, Wanxiang 123 independently developed a special treatment process that can effectively reduce surface residual alkali and improve cycle life through one-time sintering. As can be seen from Figure 2, the cycle performance of the battery can be improved to varying degrees through surface coating of single crystal materials or special heat treatment.


Compared with single crystal materials from a certain domestic supplier, specially treated single crystals and coated single crystals show relatively large advantages in cycle performance. After analysis, scientists from Wanxiang 123 found that the cyclicity of single crystal materials is related to the residual lithium on the surface and the crystal structure to a certain extent. These R&D accumulations have pointed the way for the company to develop cathode materials in the future.


Cyclicity after special heat treatment or surface coating technology


Can surpass ordinary 811 materials


Compared with low-nickel materials, high-nickel materials have certain advantages in raw material costs, but their complex preparation processes, oxygen-rich sintering conditions, and strict temperature and humidity control increase raw material costs.


Future research and development will focus mainly on achieving hundred-kilogram scale scale-up experimental exploration and further developing low-cost material manufacturing processes and equipment.


In the future, Wanxiang 123 plans to mass-produce high-nickel ternary materials with its own IP through cooperation with suppliers, which is expected to further improve the cell cycle and safety performance and reduce the cost of cell products. The company's 811 high-energy product will be launched in 2020 .