Technical status and development prospects of batteries for electric vehicles

　　The rapid development of the automobile industry has promoted the progress of global machinery, energy and other industries as well as the development of economy, transportation and other aspects. It has also greatly facilitated people's lives. However, the inherent shortcomings of traditional internal combustion engine vehicles that consume energy and pollute the environment have always affected and troubled people's lives and the development of society. With the progress of society and the development of science and technology, with the call to protect the environment and save resources As a new generation of electric vehicles with no pollution and diversified energy configuration, it has attracted widespread attention and achieved great development in recent years. Beijing wants to make the 2008 Olympic Games a green Olympics, and one of its tasks is to replace the current internal combustion engine vehicles with environmentally friendly electric vehicles.

　　Electric vehicles are driven by electricity, have no emissions (or low emissions), low noise, and much higher energy conversion efficiency than internal combustion engine vehicles. At the same time, electric vehicles also have the advantages of simple structure, low operating costs, and are safer than internal combustion engine vehicles. However, electric vehicles still have problems such as higher prices, shorter driving range, and poor power performance. These problems are closely related to power supply technology. The difficulty in making electric vehicles practical still lies in power supply technology, especially batteries ( chemical power) technology. At present, the key factor restricting the development of electric vehicles is the unsatisfactory power battery. The most important thing in the competition to develop electric vehicles is the competition to develop on-board power batteries.

　　The power battery for electric vehicles is different from the general starting battery. It mainly discharges continuously with a medium current for a long time, occasionally discharges with a large current (when starting and accelerating), and is mainly used in deep cycles. The basic requirements for batteries of electric vehicles can be summarized as follows: 1. High energy density; 2. High power density; 3. Long cycle life; 4. Good charge and discharge performance; 5. Good battery consistency; 6. Lower price; 7. Easy to use and maintain.

　　Current research and development of electric vehicle power batteries mainly include lead-acid batteries, nickel metal batteries, lithium-ion batteries, high-temperature sodium batteries, metal-air batteries, supercapacitors, flywheel batteries, as well as fuel cells and solar cells with better development prospects.

　　1. Lead-acid battery

　　Lead-acid batteries have a history of more than 100 years and are widely used as starting power sources for internal combustion engine vehicles. They are also mature batteries for electric vehicles. The positive and negative electrodes of lead-acid batteries are lead dioxide and lead respectively, and the electrolyte is sulfuric acid. Lead-acid batteries can be divided into two categories, namely water-filled lead-acid batteries and valve-regulated lead-acid batteries. The former is cheap, but requires regular maintenance and electrolyte replenishment; the latter uses a safety control valve to automatically adjust the excess gas generated in the sealed battery body when charging or working abnormally, is maintenance-free, and is more in line with the requirements of electric vehicles. Generally speaking, lead-acid batteries have the advantages of good reliability, easy availability of raw materials, and low price, and their specific power can basically meet the power requirements of electric vehicles. But it has two major shortcomings; one is that the specific energy is low, the mass and volume are too large, and the mileage on a single charge is short; the other is that the service life is short and the cost of use is too high. As the technology of lead-acid batteries is relatively mature, the further improved lead-acid batteries will still be the main power source for electric vehicles in the near future. The advanced lead-acid batteries being developed for electric vehicles mainly include the following types: horizontal lead-acid batteries, bipolar batteries Sealed lead-acid batteries, rolled electrode lead-acid batteries, etc.

　　2. Nickel metal battery

　　The nickel metal batteries currently used in electric vehicles mainly include cadmium-nickel batteries and nickel-hydrogen batteries. Compared with lead-acid batteries, nickel-cadmium batteries can achieve a specific energy of 55Wh/kg, a specific power of 200W/kg, a cycle life of 2,000 times, and can be charged quickly. Although its price is 4 to 5 times that of lead-acid batteries, due to its It has advantages in specific energy and service life, so its long-term actual use cost is not high. However, because it contains the heavy metal cadmium, if you do not pay attention to recycling during use, it will cause environmental pollution. Currently, many developed countries have restricted the development and use of cadmium-nickel batteries. The nickel-hydrogen battery is a green nickel metal battery. Its positive and negative electrodes are nickel hydroxide and hydrogen storage alloy materials respectively. There is no problem of heavy metal pollution, and there will be no increase or decrease in the electrolyte during the working process. , the battery can achieve a sealed design. Nickel-metal hydride batteries are better than cadmium-nickel batteries in terms of specific energy, specific power and cycle life. The driving range of electric vehicles using nickel-hydrogen batteries once reached 600 kilometers after a single charge. Currently, they have been implemented in batches in Europe and the United States. production and use. Nickel-hydrogen batteries are suitable for electric vehicles due to their working principles and characteristics. They have been listed as the preferred power batteries for electric vehicles in the near and mid-term. However, they are still too expensive and have poor uniformity (especially at high speeds, The capacity and voltage difference between batteries under deep discharge is large), the self-discharge rate is high, and there is still a gap between performance levels and practical requirements. These problems all affect the widespread use of nickel-hydrogen batteries in electric vehicles.

　　3. Lithium-ion battery

　　Lithium-ion batteries are high-capacity rechargeable batteries developed in the 1990s. They can store more energy than nickel-hydrogen batteries, have larger specific energy, longer cycle life, small self-discharge rate, no memory effect and environmental pollution. They are the leading energy storage battery in various countries today. The hot spots in storage technology research mainly focus on the three aspects of large capacity, long life and security. In a lithium-ion battery, lithium ions can diffuse freely in the crystal lattice of the positive and negative electrode materials. When the battery is charged, the lithium ions are released from the positive electrode and embedded in the negative electrode. On the contrary, the lithium ions are in the discharge state, that is, during the battery charge and discharge cycle, With the help of electrolyte, lithium ions move back and forth between the two poles of the battery to transfer electrical energy. The electrodes of lithium-ion batteries are lithium metal oxide and lithium-storage carbon materials. According to different electrolytes, lithium-ion batteries can generally be divided into two types: lithium-ion batteries and lithium polymer batteries.

　　4. High temperature sodium battery

　　High-temperature sodium batteries mainly include sodium nickel chloride batteries (NaNiCl2) and sodium-sulfur batteries. The sodium nickel chloride battery was invented in 1978. Its positive electrode is solid NiCl2, the negative electrode is liquid Na, and the electrolyte is solid β-Al2O2 ceramic. During charging and discharging, sodium ions drift between the positive and negative electrodes through the ceramic electrolyte. Sodium nickel chloride battery is a new type of high-energy battery. It has the advantages of high specific energy (more than 100Wh/kg), no self-discharge effect, resistance to overcharge and over-discharge, fast charging, safety and reliability, etc. However, its operating temperature is high (250-350℃), and the internal resistance is related to the operating temperature, current and charging status, so a heating and cooling management system is required. Sodium-sulfur batteries are also generally optimistic about electric vehicle batteries in the near future. They have been listed as mid-term development electric vehicle batteries by the United States Advanced Battery Consortium (USABC). Sodium-sulfur batteries have high specific energy, but their peak power is low. Moreover, the operating temperature of this battery is approximately 300°C, the molten sodium and sulfur are potentially toxic, and corrosion also limits the reliability and life of the battery.

　　5. Zinc-air battery (Zinc-air)

　　Zinc-air battery is a high-energy battery that mechanically replaces the off-car charging method. The positive electrode is Zinc, the negative electrode is Carbon (absorbing oxygen in the air), and the electrolyte is KOH. Zinc-air batteries have the advantages of high specific energy (200Wh/kg), maintenance-free, resistant to harsh working environments, clean, safe and reliable, but they have small specific power (90W/kg), cannot store regenerative braking energy, and have a long lifespan. It is short, cannot output large current and is difficult to charge. Generally, in order to make up for its shortcomings, electric vehicles using zinc-air batteries will also be equipped with other batteries (such as nickel-cadmium batteries) to help start and accelerate.

　　6. Supercapacitor

　　Supercapacitor is an energy storage device proposed to meet the real-time changes in energy and power requirements of hybrid electric vehicles. It is an electrochemical capacitor that combines the advantages of batteries and traditional physical capacitors. Supercapacitors are often used in conjunction with other batteries as the power source of electric vehicles, which can meet the power requirements of electric vehicles without reducing the performance of the battery. The use of supercapacitors will reduce the vehicle's requirements for high-current discharge of the battery and reduce the size of the battery. and extend battery life. Developing supercapacitors with high specific energy, high specific power, long life, high efficiency and low cost can improve the power (especially acceleration), economy and driving range of commercial electric vehicles. According to different electrode materials, supercapacitors can be divided into two types: carbon supercapacitors (electric double layer electrochemical capacitors) and metal oxide supercapacitors.

　　7. Flywheel battery

　　Flywheel battery is a new concept battery proposed in the 1990s. It breaks through the limitations of chemical batteries and uses physical methods to achieve energy storage. Flywheel battery is a mechanical battery that stores energy in the form of kinetic energy. It is composed of electric motor/generator, power conversion, electronic control, flywheel, magnetic bearing and vacuum shell. It has high power ratio, high energy ratio and high efficiency. , long life and good environmental adaptability. The motor in the flywheel battery operates as an electric motor when charging. Driven by an external power source, the motor drives the flywheel to rotate at high speed (up to 200,000 rpm). That is, charging the flywheel battery with electricity increases the speed of the flywheel and thereby increases its kinetic energy. ; When discharging, the motor operates as a generator, outputting electrical energy driven by the flywheel to complete the conversion of mechanical energy (kinetic energy) into electrical energy. To develop a practical flywheel battery suitable for electric vehicles, it is necessary to further improve its safety and reduce costs.

　　8. Fuel cell

　　A fuel cell is a power generation device that directly converts chemical energy stored in fuel and oxidant into electrical energy through electrode reactions. Its basic chemical principle is the reverse process of water electrolysis reaction, that is, the reaction of hydrogen and oxygen to produce electricity, water and heat. It does not require combustion, has no rotating parts, has no noise, has a long operating life, high reliability, and good maintenance performance. Its actual efficiency can reach 2 to 3 times that of ordinary internal combustion engines. In addition, its final product is water, making it truly clean and renewable. , no emission requirements, and is the preferred energy source in the 21st century. Moreover, fuel cells do not need to be charged for a long time like other batteries. They only need to refill fuel like refueling a car. According to the forecast of the American ABI research company, the global production of fuel cell vehicles will reach 2.4 million units in 2011, accounting for 4.3% of the world's total automobile production. The Japanese government also plans to popularize fuel cells within ten years. In December 2002, Japan's Toyota Corporation delivered the first batch of commercial fuel cell electric vehicles to the Japanese government. Fuel cells are composed of positive and negative electrodes, catalytic layers and electrolytes. According to different electrolytes, fuel cells can be divided into phosphoric acid type, proton exchange membrane type, alkaline type, molten carbonate type and solid oxide type. At present, Only proton exchange membrane fuel cells are most suitable for electric vehicles. China's first hydrogen-powered car successfully developed in our country uses proton exchange membrane fuel cells. A relatively complete fuel cell system consists of the following parts: fuel processing part, fuel cell, DC-AC converter and thermal energy management part.

　　9. Solar cells

　　Solar cell is a device that converts light energy into electrical energy. Solar energy has been widely used in lighting, household appliances, power generation, traffic signals, geology, aerospace and other fields. At present, some institutions have also developed electric vehicle prototypes using solar cells. However, due to problems such as low photoelectric conversion efficiency, high price, and complicated battery system configuration, solar cells can only be used as the basis for electric vehicles in the near future. Supplementary power supply is not yet available for large-scale production and application, but as the cleanest and inexhaustible energy source, solar energy will surely make great progress in its research and application.

　　Currently, electric vehicles are at another development climax. The comprehensive development of electric vehicle technology focuses on two aspects: energy storage technology and power drive system technology. Electric vehicle power drive system technology develops relatively quickly. Therefore, with the development and breakthrough of energy storage technology, low-cost, high-energy-density, high-power-density power batteries and low-cost, light-weight, small-volume fuel cell products With the realization of globalization, electric vehicles will surely become the mainstream means of transportation in the 21st century.

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