Nickel Hydride Batteries

　　Wanxiang made a breakthrough in the synthesis of ternary high-nickel single crystal materials

　　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%. While the nickel content in the ternary material is increased, the gram capacity of the positive electrode material is significantly improved; however, the structural stability, thermal stability, safety, and processing performance of the positive electrode material will deteriorate, making its application severe. 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%. While the nickel content in the ternary material is increased, the gram capacity of the positive electrode material is significantly improved; however, the structural stability, thermal stability, safety, and processing performance of the positive electrode material will deteriorate, making its application severe. challenge.

　　Another effective way to increase the energy density of batteries is to increase the charging voltage of batteries/materials. However, the increase of the charging voltage will make the structural stability of the material itself worse; in addition, the oxidative decomposition of the electrolyte will intensify with the increase of the charging voltage. Accompanied by the dissolution of transition metals and the increase of side reactions at the electrode/electrolyte interface.

　　In view of the above problems such as poor thermal stability, poor cycle life and structural instability of high-nickel ternary materials, the lithium-ion battery industry generally adopts the technical route of coating or doping.

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charge, -40℃ 3C discharge capacity ≥70%

　　Charging temperature: -20~45℃ -Discharging temperature: -40~+55℃ -40℃ supports maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate≥70%

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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 some extent can inhibit the oxidative decomposition of the electrolyte and the transition metal. Dissolution problem. The cladding layer is generally made of inorganic oxides, which will affect the migration/diffusion kinetics 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 lattice of high-nickel ternary materials, stabilize the crystal structure of the material, and can significantly inhibit the crystal structure of high-nickel ternary materials due to stress during cycling. rupture or phase transition. Although element doping has the effect of stabilizing the lattice structure, it often affects the discharge capacity of the material and increases the production cost.

　　Recently, some foreign R&D teams have reported that medium-nickel ternary materials, such as NCM523/622 single crystals, have certain advantages in terms of heat production and high-temperature cycles compared to aggregates/polycrystalline materials. 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. The number of grain boundaries is small, the stress concentration point is small, and the interface is stable, which can reduce the generation and spread of microcracks caused by the phase change of the positive electrode material during the charging and discharging process of the battery, reduce the contact interface between the active material and the electrolyte, and thus reduce the cycle time. Gas production. The development of single-crystal high-nickel materials is considered to be an effective approach to improve battery safety and cycle life.

　　Due to the lower sintering temperature of the traditional secondary pellet aggregate high nickel ternary, the surface residual alkali content is higher. Therefore, washing process must be adopted in industry to reduce residual alkali, and the cost is higher. Compared with the secondary pellet aggregate, due to the difference in sintering temperature, the surface residual alkali of the single crystal high-nickel material is lower, which is expected to avoid the consistency and cost problems caused by the water washing process and reduce the material cost. Due to the low surface residual alkali and good crystallinity, single crystal materials have great advantages in high temperature storage and gas production, and are suitable for application in pouch battery systems.

　　For the synthesis of high-nickel ternary materials, the selection of suitable precursors is crucial. The study found that the physical and chemical properties of the precursor, such as crystal structure, microscopic morphology and particle size distribution, determine the comprehensive performance of the cathode material in the later stage. For the synthesis of single-crystal high-nickel materials, the team's researchers screened precursors with a specific particle size range. It has been verified that the precursor with uniform, coarse and ordered primary grain morphology is more conducive to the formation of single crystals during sintering.

　　Low temperature high energy density 18650 3350mAh-40℃ 0.5C discharge capacity ≥60%

　　Charging temperature: 0~45℃ Discharging temperature: -40~+55℃ Specific energy: 240Wh/kg -40℃ discharge capacity retention rate: 0.5C discharge capacity≥60%

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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 acts as the driving force and determines the growth of the primary grains of the single crystal. Lower sintering temperature cannot sufficiently drive the development and growth of grains. Excessively high sintering temperature will lead to over-burning problems, resulting in increased mixing of lithium and nickel (see Figure 1).

　　In addition, lithium salt, as a fluxing agent, can promote grain growth and achieve better single crystal morphology. However, an excessively high amount of lithium compounding will also result in higher residual lithium on the surface and increase production costs.

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

　　For single crystal materials, it is necessary to coat the surface to further improve its cycle life. Wanxiang 123 has selected several elements to coat the surface of the single crystal material. After heat treatment, the surface of the single crystal material can form a fast ion conductor coating layer. On the one hand, this technology reduces the residual alkali by reacting the residual lithium with the coating material, and on the other hand improves the cycle life through the formed coating layer.