Introduction to batteries used in 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, and has also greatly facilitated people's lives. However, the inherent defects of traditional internal combustion engine vehicles, such as energy consumption and environmental pollution, have always affected and troubled people's lives and social development. With the progress of society and the development of science and technology, and with the growing calls for protecting the environment and saving resources, the new generation of electric vehicles, as a new type of transportation with no pollution and diversified energy configuration, has attracted widespread attention and has been greatly developed in recent years. Beijing wants to make the 2008 Olympic Games a green Olympic Games, and one of the tasks is to replace the current internal combustion engine vehicles with environmentally friendly electric vehicles.

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

The power battery for electric vehicles is different from the general starting battery. It is mainly used for long-term medium current continuous discharge, occasionally large current discharge (when starting and accelerating), and deep cycle use. The basic requirements of electric vehicles for batteries 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. Low price; 7. Easy to use and maintain, etc.

The electric vehicle power batteries currently under research and development 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 electric vehicle batteries. 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 frequent maintenance and replenishment of electrolyte; the latter automatically adjusts the excess gas generated in the sealed battery body during charging or abnormal operation through a safety control valve, 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 prices, and their specific power can basically meet the power requirements of electric vehicles. However, it has two major disadvantages; one is that the specific energy is low, the mass and volume occupied are too large, and the driving mileage on a single charge is short; the other is that the service life is short and the cost of use is too high. Since the technology of lead-acid batteries is relatively mature, the lead-acid batteries after further improvement will still be the main power source for electric vehicles in the near future. The advanced lead-acid batteries for electric vehicles under development mainly include the following: horizontal lead-acid batteries, bipolar sealed lead-acid batteries, and rolled electrode lead-acid batteries.

2. Nickel metal battery

At present, the nickel metal batteries used in electric vehicles are mainly nickel-cadmium 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 quickly charged. Although its price is 4 to 5 times that of lead-acid batteries, due to its advantages in specific energy and service life, its long-term actual use cost is not high. However, since it contains heavy metal cadmium, if it is not recycled during use, it will cause environmental pollution. At present, many developed countries have restricted the development and use of nickel-cadmium batteries. Nickel-hydrogen batteries are a green nickel metal battery. Its positive and negative electrodes are nickel hydroxide and hydrogen storage alloy materials respectively. There is no heavy metal pollution problem, and there will be no increase or decrease in electrolyte during its operation. The battery can be sealed. Nickel-hydrogen batteries are better than nickel-cadmium batteries in terms of specific energy, specific power and cycle life. The driving range of electric vehicles using nickel-hydrogen batteries has reached 600 kilometers after a single charge. At present, they have been mass-produced and used in Europe and the United States. Nickel-hydrogen batteries are suitable for use in 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 medium term. However, they still have problems such as high prices, poor uniformity (especially under high-rate and deep discharge, the capacity and voltage differences between batteries are large), high self-discharge rates, and a gap between performance levels and actual requirements. These problems 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 a large specific energy, a long cycle life, a low self-discharge rate, no memory effect and no environmental pollution. They are a hot topic in energy storage technology research in various countries today, and the research mainly focuses on three aspects: large capacity, long life and safety. In lithium-ion batteries, lithium ions can diffuse freely in the lattices of positive and negative electrode materials. When the battery is charged, lithium ions are removed from the positive electrode and embedded in the negative electrode. On the contrary, it is a 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 oxides and lithium storage carbon materials. According to the different electrolytes, lithium-ion batteries can generally be divided into 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. The 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 overdischarge, 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 state, so a heating and cooling management system is required. The sodium sulfur battery is also a popular electric vehicle battery in the near future. It has been listed as a medium-term electric vehicle battery by the United States Advanced Battery Consortium (USABC). The sodium sulfur battery has a high specific energy, but its peak power is low, and the operating temperature of this battery is approximately 300℃. The molten sodium and sulfur are potentially toxic, and corrosion also limits the reliability and life of the battery.

[page]5. Zinc-air battery

Zinc-air battery is a high-energy battery that is mechanically replaced and charged off-vehicle. The positive electrode is Zinc, the negative electrode is Carbon (absorbs oxygen in the air), and the electrolyte is KOH. Zinc-air battery has the advantages of high specific energy (200Wh/kg), maintenance-free, resistant to harsh working environment, clean, safe and reliable, but it has the disadvantages of low specific power (90W/kg), inability to store energy for regenerative braking, short life, inability to output high current and difficulty in charging. In general, in order to make up for its shortcomings, electric vehicles using zinc-air batteries are also equipped with other batteries (such as nickel-cadmium batteries) to help start and accelerate.

6. Supercapacitor

Supercapacitors are energy storage devices proposed to meet the real-time energy and power requirements of hybrid electric vehicles. They are electrochemical capacitors that have the advantages of both batteries and traditional physical capacitors. Supercapacitors are often used in conjunction with other batteries as power sources for electric vehicles. They can meet the power requirements of electric vehicles without reducing the performance of batteries. The use of supercapacitors will reduce the requirements of large current discharge of batteries by cars, thereby reducing the size of batteries and extending the life of batteries. The development of supercapacitors with high specific energy, high specific power, long life, high efficiency and low cost can improve the power (especially acceleration capability), economy and driving range of commercial electric vehicles. Depending on the electrode material, supercapacitors can be divided into two categories: carbon supercapacitors (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 consists of electric/generator, power conversion, electronic control, flywheel, magnetic bearing and vacuum shell. It has the advantages of high power ratio, high energy ratio, high efficiency, long life and good environmental adaptability. The motor in the flywheel battery operates in the form of an electric motor when charging. Driven by an external power source, the motor drives the flywheel to rotate at a high speed (up to 200,000 rpm), that is, "charging" the flywheel battery with electricity increases the speed of the flywheel and thus increases its kinetic energy; when discharging, the motor operates in the state of a generator, outputs electrical energy to the outside under the drive of the flywheel, and completes the conversion of mechanical energy (kinetic energy) to electrical energy. To develop a practical flywheel battery suitable for electric vehicles, it is necessary to further improve its safety and reduce its cost.

8. Fuel Cells

A fuel cell is a power generation device that converts chemical energy stored in fuel and oxidant into electrical energy directly through electrode reaction. Its basic chemical principle is the reverse process of water electrolysis, that is, hydrogen and oxygen react to produce electricity, water and heat. It does not require combustion, has no rotating parts, is noiseless, has a long service 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, which truly meets the requirements of clean, renewable and emission-free. It 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. It only needs to refuel like refueling a car. According to the forecast of the US ABI Research Company, the global production of fuel cell vehicles will reach 2.4 million 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, Toyota Motors of Japan delivered the first batch of commercial fuel cell electric vehicles to the Japanese government. Fuel cells are composed of positive and negative electrodes, a catalyst layer and an electrolyte. Depending on the electrolyte, fuel cells can be divided into phosphoric acid type, proton exchange membrane type, alkaline type, molten carbonate type and solid oxide type. Currently, only proton exchange membrane fuel cells are most suitable for electric vehicles. The "China's No. 1 Hydrogen Powered Vehicle" successfully developed in my 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 cells are devices that convert 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 car prototypes using solar cells. However, due to the low photoelectric conversion efficiency, high price and complex battery system configuration of solar cells, they can only be used as supplementary power sources for electric vehicles in the near future and cannot be mass-produced and applied. However, as the cleanest and inexhaustible energy source, the research and application of solar energy will surely make great progress.

At present, electric vehicles are in another development climax. The comprehensive development of electric vehicle technology focuses on two aspects: energy storage technology and power drive system technology. The technology of electric vehicle power drive system is developing relatively fast. Therefore, with the development and breakthrough of energy storage technology, and with the commercialization of low-cost, high energy density, high power density power batteries and low-cost, light weight, small size fuel cells, electric vehicles will surely become the mainstream means of transportation in the 21st century.

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