Analysis of the advantages and disadvantages of CR927 battery technology routes

Currently, there are four main technology routes for power sources for transportation: lithium-ion batteries, hydrogen fuel cells, supercapacitors and aluminum-air batteries. Among them, lithium-ion batteries, supercapacitors and hydrogen fuel cells are widely used, while aluminum-air batteries are still in the laboratory research stage. In terms of energy supply, lithium-ion batteries and supercapacitors are suitable for pure electric vehicles, but they require external charging, while hydrogen fuel cell vehicles require external hydrogen refueling, and aluminum-air batteries require replenishment of aluminum plates and electrolytes. 1. Characteristics of hydrogen fuel cells (1) Good environmental compatibility Hydrogen fuel cells provide efficient and clean energy. The water they emit is not only small in amount, but also very clean, so there is no water pollution problem. At the same time, because fuel cells do not need to convert heat energy into mechanical energy like engines, but directly convert chemical energy into electrical energy and heat energy, the energy conversion efficiency is high and the noise is low. (2) Good operating performance Hydrogen fuel cell power generation does not require complex and large configuration equipment, and the battery stack can be assembled modularly. For example, a 4.5MW power generation device can be composed of 460 battery components, and its power plant occupies a much smaller area than a thermal power plant. Hydrogen fuel cells are suitable as distributed power generation devices. In addition, compared with thermal, hydropower and nuclear power generation, hydrogen fuel cell power plants have a short construction period and are easy to expand. They can be built in phases according to actual needs. At the same time, hydrogen fuel cells have high operating quality and excellent characteristics in dealing with rapid changes in load (such as peak load). They can be converted from low power to rated power within seconds. (3) Efficient output performance When working, hydrogen fuel cells convert the energy stored in the fuel into electricity and heat. The efficiency of converting electrical energy is over 40%, while only 1/3 of the steam turbine can be converted into electricity. (4) Flexible structural characteristics Hydrogen fuel cells are very flexible to assemble and the power size is easy to adjust. Compared with traditional engines, hydrogen fuel cells have good modularity. On the basis of not increasing infrastructure investment, the output power and voltage can be easily adjusted by increasing or decreasing the number of single cells. Therefore, it is also easy to build and it is relatively easy to control the power grid. This feature of fuel cells improves system stability. (5) Wide sources of hydrogen Hydrogen, as a secondary energy source, can be obtained in many ways, such as coal-to-hydrogen, natural gas reforming hydrogen, water electrolysis hydrogen, etc. When fossil energy is exhausted, hydrogen will become the world's main fuel and energy. Hydrogen is produced by electrolyzing water with solar energy, and there is no carbon emission in the process. Therefore, hydrogen can be considered as the ultimate energy source. (6) Existing bottlenecks From the current development perspective, the popularization of hydrogen fuel cells has encountered certain bottlenecks, such as the high cost of the battery itself and the lack of popularization of infrastructure. 2. Characteristics of lithium-ion batteries (1) Voltage platform Due to the different positive and negative electrode materials used in lithium-ion batteries, the operating voltage range of its single cells is 3.7~4V. Among them, the operating voltage of lithium iron phosphate single cells with a larger application scale is 3.2V, which is 3 times that of nickel-hydrogen batteries and 2 times that of lead-acid batteries. (2) Large specific energy The energy density of lithium-ion power batteries for passenger cars is currently close to 200Wh/kg, and is expected to reach 300Wh/kg in 2020. (3) Short battery life Due to the constraints of electrochemical material characteristics, the number of cycles of lithium-ion batteries has not made a breakthrough. Taking lithium iron phosphate as an example, the number of cycles of a single cell can reach more than 2,000 times, but only more than 1,000 times after grouping. It cannot meet the requirements of the 8-year operation period of public transportation. (4) A greater impact on the environment Lithium-ion batteries use light metal lithium. Although they do not contain harmful heavy metals such as mercury and lead, they are considered green batteries with less environmental pollution. However, in fact, because their positive and negative electrode materials and electrolytes contain metals such as nickel and manganese, the United States has classified lithium-ion batteries as batteries that are toxic and harmful, including flammable, leaching toxicity, corrosiveness, and reactivity. They are the batteries that contain the most toxic substances among all types of batteries. In addition, because their recycling and reuse processes are relatively complex, the cost is relatively high. Therefore, the current recycling and reuse rate is not high, and discarded batteries have a greater impact on the environment.

(5) The cost is still high The initial purchase cost of lithium-ion batteries is high. Taking lithium iron phosphate batteries, the mainstream CR927 battery product for buses, as an example, the price is about 2,500 yuan/kWh. With the popularization of electric vehicles, it is expected to drop to less than 1,000 yuan/kWh in 2020. Due to the limitation of the number of cycles after the single battery is assembled, buses usually need to replace batteries in about 3 years, which puts a great pressure on the cost of the operating unit. (6) A large impact on the power grid First, the large-scale application of pure electric vehicles will cause a large demand for charging, and the harmonic interference of charging equipment on the power grid will be prominent, affecting the power supply quality of the power grid; secondly, during fast charging, due to the high rate of charging, the charging power is high (passenger cars are 50kW, buses are about 150~250kW), which has a large impact on the load of the power grid. Therefore, based on the current technical level of lithium-ion batteries, its application in electric vehicles is mainly in short-distance pure electric vehicles with a mileage of less than 200km. 3. Characteristics of supercapacitors (1) Extremely high charge and discharge rates Supercapacitors have a high power density and can discharge hundreds to thousands of amperes of current in a short time. They charge quickly and can complete the charging process within tens of seconds to minutes. Supercapacitor buses and trams use this feature to complete charging in a short time and drive the vehicle forward. (2) Long cycle life The loss of supercapacitors in the charging and discharging process is extremely small, so in theory its cycle life is infinite, and in reality it can reach more than 100,000 times, which is 10~100 times higher than that of batteries. (3) Good low-temperature performance Most of the charge transfer that occurs during the charging and discharging process of supercapacitors takes place on the surface of the electrode active material, so the capacity decay with temperature is very small, while the capacity decay of lithium-ion batteries at low temperatures is as high as 70%. (4) Energy density is too low One of the bottlenecks in the application of supercapacitors is that the energy density is too low, which is only about 1/20 of that of lithium-ion batteries, about 10Wh/kg. Therefore, it cannot be used as the main power source for electric vehicles. It is mostly used as an auxiliary power source, mainly for quick start devices and brake energy recovery devices. 4. Characteristics of aluminum-air batteries (1) Low material cost and high energy density The negative electrode active material of aluminum-air batteries is abundant metal aluminum, which is cheap and environmentally friendly. The positive electrode active material is oxygen in the air, and the positive electrode capacity is virtually infinite. Therefore, aluminum-air batteries have the advantages of light weight, small size and long service life. (2) Key technologies have not made breakthroughs and have not yet left the laboratory. Problems such as air electrode polarization and aluminum hydroxide precipitation are important obstacles to the marketization of metal-air batteries. The improvement of aluminum-air battery performance has encountered a great bottleneck. It is still in the laboratory stage and there is still a long way to go before commercial promotion.

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