Key technologies of high temperature NiMH batteries-EEWORLD

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1. Overview

Nickel-cadmium (Ni-Cd) batteries have always been one of the batteries that environmental protection workers have criticized because they contain the highly toxic element cadmium. The EU and other organizations have continuously issued policies and directives (the Waste Electrical and Electronic Equipment Directive WEEE and the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment Directive RoHS), which has accelerated the process of nickel-cadmium batteries being replaced by other batteries. Nickel-metal hydride (Ni-HM) batteries are the most promising replacement. How to solve the performance problems of nickel-metal hydride batteries in high temperature environments is the key to whether they can be applied in a wider range of fields. In the charging and discharging process and the use environment of nickel-metal hydride batteries, temperature must be involved in the problem of battery performance and service life. The high-capacity mobile power supplies urgently needed by the military, aerospace, navigation, petroleum, coal, geological exploration and operations, ice and mountaineering sports, and secondary mobile power supplies have strong strategic significance, scientific value and economic value. In addition, nickel-metal hydride power batteries also have important application value prospects in hybrid vehicles such as fuel cells + nickel-metal hydride batteries (electric-electric hybrid) and gasoline + nickel-metal hydride batteries (oil-electric hybrid).

During the charging and discharging process of rechargeable batteries, changes in ambient temperature, etc., affect the battery performance. Although all battery materials have a certain impact on battery performance, for high-temperature batteries, improving and optimizing positive and negative electrode materials is a better method. In addition to a small number of patents that disclose improvements to hydrogen storage alloys, the main technology is still in the positive electrode material, including the use of mechanical mixing to add rare earths, rare metals, alkaline earth elements, etc., such as Mg, Ca, Sr, Sc, Y, La, lanthanide elements, Ti, Zr, Cr, Mo, W, Mn, Fe, Co, Cu, Zn, Cd, B, Al, Ga, In, Si, P, As, Sb, Bi, one or more oxides and hydroxides. Since it is difficult to achieve complete uniformity of several materials of different properties when preparing the positive electrode, it is considered to use co-precipitation to dope the above elements when manufacturing spherical nickel hydroxide, and it is also considered to coat the spherical nickel with a layer of hydroxide of the above elements.

Although the above methods play a certain role in improving the performance of high-temperature batteries, there are still many deficiencies and shortcomings. The main method to solve the problem of reduced battery performance is to improve the internal structure of spherical nickel to prevent the generation of γ-NiOOH, hoping that β-NiOOH can be easily converted with β-Ni(OH)2 during charging and discharging (the interlayer spacing of γ-NiOOH is 0.69nm, the interlayer spacing of β-Ni(OH)2 is about 0.46nm, and the interlayer spacing of β-NiOOH is about 0.48nm. The presence of γ-NiOOH causes electrode expansion, loss of active materials, reduced conductivity, and seriously reduces the cycle life and efficiency of the electrode); another method is to add conductive materials to improve the conductivity, such as adding CoO or Co(OH)2. However, during the charging and discharging process, cobalt hydroxide, as a raw material powder, dissolves in an alkaline aqueous solution while precipitating again, and undergoes a sharp structural change, with some cobalt compounds being free, resulting in changes in the amount of cobalt and reducing battery performance. Although the coated spherical nickel has improved the above phenomenon to a certain extent, there is still a phenomenon that the coating is not strong enough and the surface layer dissolves and falls off after charging and discharging.

Functional gradient materials (FGM) are high-performance materials with step-wise changes in microscopic components, structures, and performance. They have the characteristics of high mechanical strength, thermal shock resistance, and high temperature resistance. They are widely used in electronic components, artificial teeth, automobile engines, brakes, chemical components, etc. The author believes that combining the principles of gradient materials with spherical nickel manufacturing will become the development trend of high-temperature battery positive electrode materials.

2. Key technologies for high-temperature nickel-hydrogen batteries

2.1. Improvement of positive electrode materials

2.1.1. Mechanical mixing method of positive electrode materials

When mixing batteries, adding the main group elements of ⅡA, ⅢB, ⅣB, ⅦB, ⅧB, ⅡB in the periodic table and elements, oxides or hydroxides of periods 3, 4, and 5 through mechanical mixing can better improve or improve the high-temperature performance of nickel-hydrogen. Many of them are introduced in the patents applied for authorization in China by world-renowned battery manufacturers, such as Japan's Panasonic and Sanyo; China's BYD; Germany's HC Stark and other companies. See the table below for details:

2.1.2. Chemical co-precipitation method for positive electrode materials

The above elements are doped into the layered structure nickel hydroxide during the production process of the positive electrode material spherical nickel hydroxide to replace some nickel ions and form a solid solution, so that the uniformity between the elements is better; a layer of cobalt hydroxide is coated on the outside of the spherical nickel hydroxide, etc., which can improve the overall performance of the battery. Representative patents are shown in the table below:

2.2. Improvement of negative electrode materials

The negative electrode material of nickel-hydrogen batteries uses hydrogen storage alloys, the main components of which are M(NiCoMnAl)5, i.e. AB5. M is rare earth La, Ce, Pr, Nd.
Liu Huafu used a chemical formula of Mm0.95~1.05Ni4.08~4.40Co0.38~0.95Mn0.25~0.399Al0.32~0.49M0.04~0.999, where Mm is a rare earth alloy of lanthanum, cerium, praseodymium, and neodymium, and M is two or three or four elements of vanadium, bismuth, iron, gallium, zinc, silicon, boron, tungsten, molybdenum, chromium, titanium, lithium, tin, and copper. It is used in MH-Ni secondary batteries. It can be quickly charged under high temperature conditions and has a hydrogen storage alloy material with high electrochemical capacity.

The composition (atomic %) of the negative electrode material of Li Rong et al. is: AB5, which is composed of the negative electrode material for high-temperature nickel-hydrogen batteries, A is La, Ce, Pr, Nd, Y elements; B is Ni, Co, Mn, Al elements;

Sichuan University has developed a low-temperature hydrogen storage alloy with excellent performance, and combined it with unique nickel-hydrogen battery manufacturing technology to produce a D-type nickel-hydrogen battery with a rated capacity of 8Ah. The battery was tested by the Ministry of Information Industry's Tianjin 18th Institute, Changhong Power Supply Company, Chengdu Jianzhong Lithium Battery Factory and Sichuan University itself and found that the room temperature performance is 9.2Ah at 0.2C and 9.0Ah at 1C, its high rate performance is about 98%, the small current (0.2C~0.4C) charge and discharge cycle life is more than 500 times, the 1C large current charge and discharge cycle life is more than 300 times, and the self-discharge after 28 days at room temperature is less than 10%; the low temperature performance is that the discharge capacity reaches 80% of the rated capacity under -40℃, 0.2C and -40℃, 0.4C conditions, and the discharge capacity reaches 70% of the rated capacity under -45℃, 0.2C conditions; the high temperature performance is that after charging at 55℃/0.2C for 6.5 hours, the 0.2C discharge capacity is greater than 90% of the rated capacity, and the 0.2C discharge at 55℃ for 8 hours has a discharge capacity greater than 90% of the actual capacity, and there is no capacity loss after storage at 50℃ for 30 days. Under the leadership of Academician Tu Mingjing, Professor Chen Yungui, a former doctoral supervisor at the School of Materials Science and Engineering of Sichuan University, completed the development of a neodymium-free nickel-metal hydride power battery. Its comprehensive performance was at the forefront of competitive tests with major domestic and foreign brands of batteries, and it was awarded four national invention patents and one of the top ten rare earth science and technology news in China in 2003. Academician Tu Mingjing and Professor Chen Yungui are actively promoting this high-performance wide-temperature range nickel-metal hydride battery, and developing a nickel-metal hydride starting power source with excellent low-temperature and high-current discharge performance for aircraft and a wide-temperature range, long-life, and low-cost nickel-metal hydride battery for electric vehicles [1].

3. Doped and plated gradient composite spherical nickel hydroxide

Spherical nickel hydroxide has been industrialized for more than ten years. The commercialization of Cd+Co doped and Zn+Co doped spherical nickel is relatively mature, and cobalt-coated (or cobalt-coated) is gradually moving towards commercialization. So much so that some people say [2] "The current development of β-Ni(OH)2 has reached its limit; the research and development prospects of nano-Ni(OH)2 and α-Ni(OH)2 materials will be very broad."

Functionally Gradient Materials (FGM) are a new type of functional material developed to meet the needs of high-tech fields such as modern aerospace industry and to meet the needs of repeated normal operation in extreme environments (ultra-high temperature, large temperature drop) [3]. It is currently the main frontier technology field for the development of international composite functional materials.

Doped and plated gradient composite spherical nickel hydroxide should be divided into two concepts:
1. Doped spherical nickel hydroxide, which is based on the traditional doped Cd+Co and doped Zn+Co spherical nickel, optimizes the selection of II group elements, rare earth elements, etc., to prepare spherical nickel hydroxide with uniform composition, small microstructure grain size, large interlayer spacing, large half-height width, specific surface area and particle size distribution that meet the requirements, and stable quality. In this regard, the author believes that the "system microcrystal online ternary control method" developed by the author is in the leading position in China in terms of product stability and uniformity; simplicity of process re-line control, parameter precision and reliability; low equipment investment and overall product cost. In the batch supply of nearly 1,000 tons of products to Panasonic Battery Company for one year, there has been no quality complaint, which has set a precedent for similar products in China [4][5].

2. Gradient composite spherical nickel hydroxide, which is similar to the current cobalt-coated spherical nickel, but also has great differences. Cobalt-coated spherical nickel is simply depositing a single layer of cobalt hydroxide in spherical nickel hydroxide; gradient composite spherical nickel hydroxide is to place the material to be infiltrated (cobalt, yttrium, titanium, calcium, magnesium or other rare earth elements) and the material to be repaired (doped spherical nickel) under strictly controlled conditions. The ions of the infiltrated ions and hydroxides gathered on the surface of the substrate (doped spherical nickel) under the action of additives continue to diffuse rapidly into the substrate along the crystal defects of the substrate. Finally, the metal element to be infiltrated is enriched and crystallized on the surface of the substrate and penetrates into the substrate to a certain depth. From the surface to the inside, the concentration of the element to be infiltrated decreases gradually, and its organizational structure also changes gradually, forming a gradient functional material in which the outer surface of the substrate has the properties of the metal to be infiltrated, the core of the substrate still maintains the original properties, and the performance of the middle layer gradually changes. The densification of the gradient material with continuously changing components makes the infiltration material and the matrix firmly bonded, and the infiltration material and the matrix are not easy to fall off during the reaction process of the battery material, which ensures the consistency of the battery cycle performance life. By adding selected group II elements, rare earth elements, etc., doped infiltration gradient composite spherical nickel hydroxide is prepared to obtain the effect of high-temperature nickel-hydrogen battery.

4. Conclusion

Adding rare earth, rare, alkaline earth elements or oxides to the positive electrode ingredients of nickel-hydrogen batteries can improve the performance of nickel-hydrogen batteries in high temperature states. Representative elements include: such as Mg, Ca, Sr, Sc, Y, La, lanthanide elements, Ti, Zr, Cr, Mo, W, Mn, Fe, Co, Cu, Zn, Cd, B, Al, Ga, In, Si, P, As, Sb, Bi, one or more oxides and hydroxides. Among them, there are many introductions to the research and application of zirconium in new energy materials [6][7][8]. Except for the industrialized practical application of zirconium in nuclear power plants (nuclear energy, zirconium plates and pipes), other industrialized practical applications in nickel-hydrogen positive and negative electrode materials and lithium battery positive electrode material additives are not yet available. The surface mixing of mechanical mixing method has the problem of uniformity, which affects the performance; chemical precipitation doping and coating have certain advantages over mechanical mixing method, but there are still technologies for production process control; doping and infiltration plating gradient composite spherical nickel hydroxide may be an effective way to solve the above defects.

References
[1] New wide temperature range nickel-hydrogen battery has excellent performance China Science and Technology Information Network
[2] Su Linghao Fan Shaohua The current status and prospects of research on improving the electrochemical performance of Ni(OH)2 Journal of Henan University of Science and Technology (Natural Science Edition) 2005.26(1).-100-104
[3] Gong Changsheng Zhang Keli ed. New functional materials Beijing: Chemical Industry Press 2001;
[4] Building a nest to attract phoenixes, spherical nickel leads the country's first power technology 2005 (2)
[5] Spherical nickel's stable quality is praised by Panasonic and domestic orders increase greatly Power Technology 2005 (3)
[6] Lv Wenguang Zheng Jingyi The production of zirconium oxychloride (zirconium dioxide) and its application prospects in batteries Mineral Resources and Geology, 2001 15 (supplement)
[7] Lv Wenguang Zheng Jingyi Modern high-tech batteries and zirconium powder materials Proceedings of the 25th China Chemical and Physical Power Academic Conference, 2002, Huizhou, Guangdong;
[8] Lv Wenguang Application status and trend of zirconium in secondary battery materials International Battery 2004 (11); 28-31

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