In recent years, the number of hybrid electric vehicles (HEVs) that have the advantages of saving fuel consumption and reducing carbon dioxide emissions has been increasing. From passenger cars to public transportation, mass applications have been realized.
Toyota Prius hybrid cars, which came out in 1997, currently have a cumulative global sales volume of 2.87 million units; hybrid buses have also been operated in European cities such as London and Barcelona; in the domestic public transport field, hybrid buses have been demonstrated in various demonstration cities The number of vehicles has already accounted for more than 85% of the total of 1,000 engineering demonstration vehicles in ten cities.
Hybrid vehicles can use smaller displacement engines, turn off the engine when the vehicle is idling, etc., use batteries or capacitors as energy storage units to recover braking energy, and improve fuel economy more effectively; compared to pure electric vehicles, motors The smaller power can reduce the capacity of the vehicle's power battery pack and the driving range is longer. The plug-in hybrid electric vehicle (PHEV) uses a larger-capacity battery pack, and the pure electric mileage is longer and more energy-saving. It is also mentioned in the national "Energy Saving and New Energy Industry Planning (2012-2020)". Parallel development objects.
In the development of hybrid technology, as the system's requirements for energy storage systems continue to increase, the energy storage unit has also experienced a leap from lead-acid batteries, nickel-metal hydride batteries, super capacitors to lithium-ion batteries. At present, hybrid vehicles, especially plug-in hybrid vehicles, mostly use lithium ion batteries as energy storage systems, and super capacitors are also widely used in the field of hybrid buses.
1. Requirements for energy storage systems of hybrid vehicles and plug-in hybrid vehicles
The power battery is the basic energy storage unit of a hybrid vehicle, and its performance directly affects the performance of the drive motor, thereby affecting the performance of the entire vehicle. The use of power batteries in hybrid electric vehicles is different from that of pure electric vehicles. They are often in non-periodic charge and discharge cycles during operation, and usually have a small battery pack capacity. The battery needs to have a high charge and discharge rate and efficiency, and a high ratio Power density (W / Kg); In addition, hybrid vehicles need to charge and discharge batteries frequently, so there are more charge and discharge times, and higher requirements are placed on the cycle life under large charge and discharge ratios. For plug-in hybrid vehicles, due to both pure electric driving conditions and hybrid driving conditions, the specific energy density (Wh / Kg) and specific power density of the battery are required.
2. Comparison of common energy storage units
Domestic hybrid buses used nickel-metal hydride batteries as energy storage units in the early days, but they have more difficult problems to solve in terms of high-temperature discharge efficiency, memory effect, cycle life, thermal management and battery pack management; super capacitors theoretically charge The discharge cycle life is long, it can be charged quickly, and it can also provide a high discharge current, but its shortcoming is that the energy density is low, and hybrid buses and buses often run out of power in mountainous cities or under frequent acceleration conditions. Vehicle power and fuel efficiency; lithium-ion batteries have a higher voltage platform, energy density, power density and longer cycle life than nickel-metal hydride batteries. Compared with supercapacitors, they have higher energy density and cost-effectiveness. However, due to hybrid power The harsh conditions, the life of the lithium-ion battery system, and the safety issues in extreme cases are all facing challenges.
3. LpTO lithium-ion battery characteristics and application prospects
The use of lithium titanate (LTO) materials for battery development began in the 1990s. As a negative electrode material, it has great advantages over carbon negative electrodes and has received much attention in recent years. LTO is a zero-strain material. When lithium ions are inserted and extracted, the lattice constant and volume change are very small, and the cycle performance is excellent. The LTO material does not react with the electrolyte and no SEI film is formed. The potential of LTO is also higher than that of lithium metal, and it is not easy to precipitate lithium dendrites. It has higher safety performance than graphite anode lithium ion batteries.
Comparison of main performance parameters of LpTO battery and other energy storage units for hybrid vehicles (Table 1):
Ni-MH battery supercapacitor lithium iron phosphate battery LpTO battery voltage platform (V) 1.2 2.5 3.2 2.3 Cell weight energy density (Wh / Kg) 45 6 90 75 Cell volume energy density (Wh / L) 140 10 200 140 Peak ratio Power density (W / Kg) 800 20,000 1,800 2,300 Monomer cycle life 600 500,000 2,000 25,000 d High and low temperature performance is poor, good, safe performance is generally good, good
Source: Data from well-known domestic manufacturers of hybrid nickel-metal hydride batteries, data from well-known Korean manufacturers of hybrid supercapacitors, data of domestic well-known manufacturers of lithium iron phosphate batteries for hybrid power, 6C charge / 6C discharge, 100% DOD, room temperature cycle to 80% .
The modified macro-power will be used to modify the existing LTO material, and at the same time, the cathode material, separator and electrolyte will be modified accordingly. After system integration, a safer and ultra-long cycle life LpTO battery has been developed. The application of LpTO battery technology in the field of hybrid and plug-in hybrid vehicles has the following advantages: long cycle life under high-rate charge and discharge conditions, which can have the same service life as the vehicle; high power density, while meeting hybrid and plug-in The requirements of battery-type hybrid power on the energy density of the battery pack; safer negative electrode materials make the overall safety of the battery pack higher; it can economically meet the complex dynamic conditions of vehicle power requirements and energy recovery; and the system cost is also lower.
Compared with lithium iron phosphate batteries, LpTO batteries have a ten-fold cycle life under high-rate charge-discharge conditions, higher low-temperature charge-discharge efficiency, and higher safety performance. The long cycle life allows the battery pack to have the same life as the vehicle, reducing the later maintenance costs of the vehicle. Compared with supercapacitors, LpTO batteries have an energy density of more than ten times, which can avoid the situation that the electric power is easily depleted in mountains and frequent starting acceleration, and the relatively lower cost also increases its competitiveness.
At present, LpTO batteries have been used in batches on plug-in hybrid buses. Table 2 lists the basic parameters of a 12-meter plug-in hybrid bus that uses LpTO batteries. This model can use a pantograph to charge from the roof, and the battery can be quickly charged. The gap between vehicle stops Charging, make full use of pure electric mode to achieve the goal of high fuel efficiency.
Basic parameters of plug-in hybrid bus with LpTO battery pack (Table 2):
Vehicle length 12 meters hybrid form parallel plug-in engine CNG engine, 170 kW motor 94 kW permanent magnet synchronous motor battery pack 20 kWh LpTO battery pack, 334V 60Ah
The plug-in hybrid buses and plug-in hybrid buses and plug-in hybrid buses equipped with LpTO battery packs and using pantographs for fast charging have low initial investment and low fuel consumption, and are increasingly favored by the market. With the continuous advancement of battery technology, the new LpTO battery is expected to have the same life as the vehicle, taking into account the advantages of lithium-ion batteries and supercapacitors, which can make up for the shortcomings of the existing lithium-ion battery system in terms of service life and safety. The higher energy density and cost performance of supercapacitors are a better choice for energy storage systems of hybrid vehicles and plug-in hybrid vehicles.
The LpTO battery pack has undergone a rigorous test of severe winter and scorching heat in Chongqing for a long time. The cumulative mileage of bicycles on fast-charging pure electric buses exceeds 60,000Km and the number of cycles exceeds 2,500. It is also beginning to be applied on a large scale in plug-in hybrid buses.
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