As one of the green high-energy secondary batteries, nickel / metal hydride secondary battery, generally referred to as nickel hydrogen (MH / Ni) battery, is a kind of high-energy green environmental protection battery. The battery uses hydrogen storage alloy material to replace metal cadmium, eliminates environmental pollution, and has the characteristics of high energy density, high power, high rate discharge, fast charging ability and no obvious memory effect, It is one of the key development directions of secondary battery in recent 20 years.
- Working principle and characteristics of MH / Ni battery
Ni MH battery uses Ni oxide as positive electrode and hydrogen storage metal as negative electrode. KOH solution is generally used as electrolyte. The main difference between the electrochemical principle of MH / Ni battery and the traditional cadmium nickel battery is that the hydrogen storage alloy replaces the cadmium negative electrode. The reaction formula of the battery composed of Ni (OH), positive material and negative material of hydrogen storage alloy (represented by M) can be expressed as follows:
It can be seen from the equation that during charging, hydrogen is precipitated from the negative electrode and stored in the container, and the positive electrode changes from nickel hydroxide to hydroxyl nickel oxide (NiOOH) and H2O; During discharge, hydrogen is consumed on the negative electrode, and the positive electrode changes from hydroxyl nickel oxide to nickel hydroxide.
The normal charge discharge reaction of MH / Ni battery shows that the hydrogen atom in the negative electrode is transferred to the positive electrode to become proton during discharge, and the proton in the positive electrode is transferred to the negative electrode to become hydrogen atom during charging, without producing hydrogen.
There are two kinds of abnormal use of MH / Ni battery: overcharge and overdischarge. When charging the battery, overcharge may occur because there is no appropriate charging control method to strictly control the charging, or the control fails, and the charging is not stopped in time after the battery is fully charged; When the battery is used in deep discharge, the battery with small capacity in the series battery pack may have reverse over discharge driven by other batteries. During overcharge, all Ni (OH) on the positive electrode is transformed into nio0h, and the charging reaction is transformed into the oxygen evolution reaction of electrolytic water on the positive electrode, and the hydrogen evolution reaction of electrolytic water is generated on the negative electrode. Due to the large area of hydrogen electrode with catalyst and the hydrogen can diffuse to the surface of hydrogen electrode at any time, hydrogen and oxygen can easily recombine inside the battery to generate water, so as to keep the gas pressure in the container unchanged, This recombination rate is very fast, which can make the oxygen concentration in the battery not more than a few thousandths, and basically keep the KOH concentration and the total amount of water unchanged. This is the overcharge protection mechanism of Ni MH battery.
During over discharge, all NiOOH with electrochemical activity on the positive electrode is converted into Ni (OH) 2, and the electrode reaction becomes the electrolytic water reaction to generate H2. At this time, hydrogen in the battery is generated from the positive electrode and recombined on the negative electrode. The voltage between the positive and negative electrodes is about -0.2v. This phenomenon is called battery reverse pole, and – 0.2V is reverse pole voltage. During over discharge, the battery will automatically reach the equilibrium state, and the battery temperature is much lower than that during over charging with the same current, which is the over discharge protection mechanism of Ni MH battery.
It can be seen from the battery reaction that MH / Ni battery has the ability of long-term over discharge and over charging protection, and Ni MH battery can be made into a sealed structure. The electrolyte of Ni MH battery mostly adopts KOH aqueous solution and adds a small amount of LiOH. The diaphragm adopts porous vinylon non-woven fabric or nylon non-woven fabric. In order to prevent the internal pressure of the battery from being too high in the later stage of the charging process, an explosion-proof device is installed in the battery.
MH / Ni battery takes nickel hydroxide as the positive electrode and high-energy hydrogen storage alloy material as the negative electrode, which makes MH / Ni battery have more energy. Compared with CD / Ni battery, MH / Ni battery has the following significant advantages.
(1) High energy density, same size battery, capacity is 1.52 times that of CD / Ni battery.
(2) With good environmental compatibility and no cadmium pollution, MH / Ni battery is also known as green battery.
(3) It can charge and discharge quickly with high current and high charge and discharge efficiency. When the discharge rate is 0.2C, the discharge capacity of Ni MH battery can reach 90% of the nominal capacity. When the discharge rate is 1C, the discharge capacity of Ni MH battery can also reach more than 85% of the nominal capacity. The self discharge rate of Ni MH battery is very small. At room temperature, after the Ni MH battery is fully charged and placed for 28 days, the battery capacity can still be maintained between 75% ~ 85% of the nominal capacity.
(4) The working voltage of the battery is 1.2V. Ni MH battery is a replacement product of Ni Cd battery. The physical parameters of the battery, such as size, quality and appearance, can be interchanged with Ni Cd battery, and the electrical performance is basically the same. The charge discharge curve is similar, and the discharge curve is very smooth. The voltage will drop suddenly when the electricity is about to be consumed. Therefore, it can completely replace Ni Cd battery without any modification to the equipment.
(5) There was no significant memory effect.
(6) Good low temperature performance and strong resistance to overcharge and discharge.
The disadvantage of MH / Ni battery is that its service life is not as long as that of cadmium nickel battery, but it can also reach the service life of 500 cycles and the recommended standard of the International Electrotechnical Commission. The use of metal hydride (such as rare earth alloy or TiNi alloy) hydrogen storage material as negative active material replaces the carcinogen cadmium, which not only makes this new battery a green battery integrating energy, material, chemistry and environmental protection, but also increases the specific energy of the battery by nearly 40%, reaching 6080w · H / kg and 210240w · H / L. MH / Ni battery gradually realized industrialization in the early 1990s.
- Naming and classification of MH / Ni Batteries
According to IEC standard (iec614361998.1) and national standards (GB / t15100-1994 and GB / t18288-2000), the identification of MH / Ni battery consists of five parts.
(1) Type of battery: HR refers to cylindrical MH / Ni battery and HF refers to square MH / Ni battery.
(2) Battery size data, including diameter and height of circular battery; The width, thickness and height of square battery are separated by slash, and the unit is mm. In cylindrical batteries, the two numbers on the left of the slash indicate that it is equal to or slightly greater than the maximum diameter specified by the battery, and the number on the right indicates that it is equal to or slightly greater than the maximum height specified by the battery. In a square battery, the number on the left of the first slash represents the maximum width specified by the battery, the number in the middle represents the maximum thickness specified by the battery, and the number on the right of the second slash represents the maximum height specified by the battery. These three numbers are represented by the next integer.
(3) Discharge characteristic symbol, l indicates that the appropriate discharge current ratio is within 0.5c.
(4) The symbol of high temperature battery is represented by T.
(5) The battery connecting piece indicates that CF represents no connecting piece, HH represents the connecting piece for battery series connection, and Hb represents the connecting piece for battery belt side by side series connection.
For example, hr15 / 51 indicates that the diameter of cylindrical MH / Ni battery is 15mm and the height is 51mm; The width of the battery is 49mm / mm, and the thickness of the battery is 18mm / mm. At present, the shape of commercial MH / Ni batteries can be cylindrical, square, button and other types.
- Composition of MH / Ni battery
The design of MH / Ni battery comes from cadmium nickel battery. The main change is that the negative electrode replaces the original cadmium with hydrogen storage alloy. Various types of MH / Ni batteries are composed of nickel hydroxide positive electrode, hydrogen storage alloy negative electrode, diaphragm paper, electrolyte, positive and negative collector, safety valve, sealing ring, top cover, shell, etc. The small MH / Ni battery is mainly composed of positive electrode, negative electrode and diaphragm, and a certain residual space is reserved at the same time.
1) Positive pole
The positive electrode of MFH/Ni battery uses high porosity nickel foam or nickel fiber as conducting skeleton, and high density nickel hydroxide powder is coated. According to the positive electrode manufacturing technology of the battery, it can be divided into two types: sintered nickel foam and nickel foam type. At present, the thickness of the foam nickel electrode suitable for MH/Ni battery cathode has three specifications: 1.7mm, 2.0mm and 2.4mm. 2.0mm is generally used. The manufacturing of fiber nickel electrode is made of active substances, conductive agents and additives, and then treated by electrochemical impregnation or paste coating. There are many manufacturing methods of active material nickel hydroxide, among which the high-capacity nickel hydroxide with spherical structure has the best quality. The nickel electrode has been transformed from a sintered electrode of nickel cadmium battery to a nickel foam electrode.
2) Negative electrode
The negative electrode of MH/Ni battery is made up of two parts: skeleton and hydrogen storage alloy. It is made up of paste paste material mixed with hydrogen storage alloy powder and adhesive, and then coated with foam nickel matrix and skeleton to form an integrated body, which is made by drying and rolling.
3) Diaphragm and electrolyte
The membrane of MH / Ni battery and nickel cadmium battery also use nylon non-woven fabric or polypropylene (PP) non-woven fabric. Due to the dissociation of nylon non-woven fabric in alkaline electrolyte, it is less used in the production of MH / Ni battery. At present, more than 90% of PP non-woven fabrics are used. Due to the problem of mechanical strength, the thickness of PP non-woven fabrics used in MH / Ni batteries must be more than 0.1mm, generally 0.12mm ~ 0.13MM, which is much thicker than the film used in lithium-ion secondary batteries. In order to improve the capacitance of MH / Ni batteries, the thinning of diaphragm has become the most important topic at present.
The electrolyte is adsorbed in the middle of each pole piece and diaphragm. MH / Ni batteries generally use KOH aqueous solution as the electrolyte, and some add a small amount of LiOH or NaOH.
4) Safety valve and housing
At the top of MH / Ni battery, there is a reusable safety exhaust device, namely safety valve, which is an important part of MH / Ni battery. Under normal circumstances, the gas generated by overcharging can be recombined to maintain the internal pressure balance of the battery. However, under incorrect charging or incorrect operating conditions, the generation rate of oxygen and even hydrogen is greater than the rate of recombination. At this time, the exhaust hole of the battery will be opened to reduce the pressure and prevent the battery from bursting. When the pressure decreases, the vent will return to its original state for reuse.
The shell is mostly made of nickel plated steel sheet, i.e. high-quality low-carbon steel. The outer layer is plated with 35mm thick nickel, and the inner layer is greater than or equal to 0.2mm. The coating is required to be uniform and dense, free of rust spots, scratches, scratches and other mechanical defects. Plastic shells are mostly used in square batteries used in electric vehicles. Sometimes, the negative electrode lug is omitted. The negative electrode conducts electricity only by contacting with the inside and bottom of the shell, and its conductivity is worse than that of the negative electrode with the lug. When the battery is required to discharge with high current, this kind of electrode ear is indispensable, otherwise it will seriously affect the high-power discharge performance. In addition, the bleeder valve in the positive cap now adopts rubber balls, which can also achieve the same effect as the spring.
- Development direction of MH / Ni battery
MH / Ni battery is a new development direction of CD / Ni battery. It has high volume energy density, no pollution to the environment and small memory effect, which is welcomed by the majority of users. It has high capacity, can discharge with high current, allows recharging times up to 5001000 times, the price is becoming more and more reasonable, and can use the current nickel cadmium battery facilities, so it is widely used.
The main technical development trends in the field of MH / Ni batteries are as follows:
(1) Research on hydrogen storage alloy materials with low cost, high performance and high stability, so that they can significantly improve the specific energy and specific power, such as the development of hydrogen storage alloy for low-cost power battery and the research on the new production process of quasi rapid condensation hydrogen storage alloy.
(2) Study the high surface conductivity Ni (OH) 2 material and current collector material to further improve the utilization rate and high rate discharge performance of the positive electrode.
(3) Study the manufacturing technology of new positive and negative electrodes.
(4) The technical ways of thin battery shell and thin diaphragm.
(5) Battery structure and internal optimization design technology of battery pack.
(6) Various technical ways to reduce the internal resistance of battery.
(7) Research on key preparation technology of battery.
(8) Battery management system research, battery pack comprehensive performance test, battery condition simulation system research.
(9) Comprehensive battery recovery technology.
MH / Ni battery integrates materials, energy and information technology, and is listed as a key research and development field by developed countries such as Japan and the United States. The development of MH / Ni battery industry can improve the urban environment, make the national economy sustainable, contribute to the development of high-tech industries such as portable electronic products and pollution-free electric vehicles, and drive the development of upstream raw material industry. With the demand of the market, the new green and environment-friendly MH / Ni battery is developing in the direction of low cost, high capacity, lightweight, miniaturization, new variety, long life and safer. MH / Ni battery industry will become one of the major industries in the energy field in the 21st century.