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Plug-in hybrid bus with LpTO battery pack and fast charging with pantograph
In the development of hybrid technology, with the continuous improvement of the system requirements for energy storage systems, the energy storage unit has also experienced the process from lead-acid batteries, nickel-hydrogen 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 supercapacitors are also widely used in the field of hybrid buses.
1. Requirements for energy storage systems for hybrid and plug-in hybrid vehicles Power batteries are the basic energy storage unit of hybrid vehicles, and their performance directly affects the performance of the drive motor, thus affecting the performance of the vehicle. The use of power batteries in hybrid vehicles is different from that of pure electric vehicles. In operation, the power battery is often in a non-periodic charge and discharge cycle, and the capacity of the battery pack is usually small, requiring the charge and discharge rate of the battery and High efficiency requires high specific power density (W/Kg) of the battery. At the same time, the hybrid vehicle needs to charge and discharge the battery frequently, which means that the battery is charged and discharged more frequently, and the battery is charged and discharged at a large rate. The cycle life underneath also has higher requirements. For plug-in hybrid vehicles, the specific energy density (Wh/Kg) and specific power density of the battery are required due to both pure electric driving conditions and hybrid driving conditions.
2. Common energy storage units a. Nickel-metal hydride batteries Before the large-scale application of lithium-ion batteries, most of the battery systems on hybrid vehicles were nickel-metal hydride batteries. Domestic hybrid buses also used nickel-hydrogen batteries as energy storage in the initial stage. unit. Compared with lead-acid batteries, nickel-metal hydride batteries have the characteristics of large-magnification charge and discharge, better cycle life, and higher specific power density. However, under the mixed power condition, the charge and discharge rate of the battery is high, and the temperature of the battery is extremely easy to rise. When the ambient temperature is higher than 35 degrees Celsius, the overall efficiency is greatly reduced, and the more serious problem is the oxygen at high temperature. The precipitation is easier, and the Coulomb efficiency of the battery charging is greatly reduced. At 45 degrees Celsius, the charging efficiency of the high-power nickel-hydrogen battery is only about 30%, and in this state, the self-discharge will also be intensified, which is not easy at present. Solve the problem. In addition, the "memory effect", cycle life, thermal management, and battery management of Ni-MH battery packs for hybrid vehicles are also difficult to solve. Therefore, with the technical update of lithium-ion batteries, Nickel-metal hydride batteries are gradually fading out of the hybrid vehicle market.
. b super capacitor is a supercapacitor capacity up to thousands of farads, the theoretical charging and discharging cycle life is very long, generally can reach 50 million times or 90,000 hours; super capacitor can be in the tens of seconds to several It can be quickly charged in minutes, and it can also provide a high discharge current. It is difficult to achieve such a requirement in a general battery . A big disadvantage of supercapacitors is that their energy density is very low, generally 5% or lower of batteries. Hybrid buses with supercapacitors are often used in mountainous cities or frequently accelerated. The exhaustion of power has affected the vehicle's power and fuel economy. At the same time, its current price is also a bottleneck.
Due to the low volumetric energy density, most of the supercapacitors are currently used in hybrid buses, but there are no practical applications for plug-in hybrid buses and hybrid passenger cars with high energy density requirements.
c. Lithium-ion battery Lithium-ion battery has higher voltage platform, energy density, power density and longer cycle life than nickel-hydrogen battery; higher energy density and cost performance than super-capacitor, in hybrid car and plug-in type Hybrid vehicles have a good application prospect. In China, the current use of lithium-ion batteries is more than a hybrid city bus, although the performance has been greatly improved relative to nickel-metal hydride batteries, but due to the harsh conditions of hybrid power conditions, in the high charge Under the test of discharge rate and high temperature, the life of lithium-ion battery system and the safety problem in extreme cases are all facing challenges. Battery manufacturers are also actively looking for more suitable lithium-ion batteries.
3. LpTO lithium-ion battery and its application prospects in hybrid vehicles and plug-in hybrid vehicles. Lithium titanate (LTO) materials used in battery development began in the 1990s, and they are used as anode materials, relative to carbon anodes. The big advantage has received a lot of attention in recent years. LTO is a zero-strain material. Lithium ions have small changes in lattice constant and volume during embedding and deintercalation, making them excellent cycle performance. The LTO material does not react with the electrolyte, and no SEI film is formed; more importantly, the potential of the LTO is higher than that of the lithium metal, and it is difficult to precipitate lithium dendrites, so that it has higher safety performance than the lithium ion battery of the graphite negative electrode. Micro-macro modified the existing LTO materials, and simultaneously modified the positive electrode materials, separators and electrolytes. After system integration, LpTO batteries with safer and longer cycle life were developed. 
1. Long cycle life under high rate charge and discharge conditions, can be used with the vehicle;
2. It has high power density and meets the energy density requirements of hybrid and plug-in hybrids for battery packs;
3. A safer negative electrode material makes the battery pack more secure overall.
4. Various complicated working conditions that can economically meet the power demand and energy recovery of the vehicle;
5. Lower system cost.
Compared with lithium iron phosphate batteries, LpTO batteries have ten times cycle life under high rate charge and discharge conditions, higher low temperature charge and discharge efficiency, and higher safety performance. The long cycle life allows the battery pack to last the same life as the vehicle, reducing the cost of late maintenance of the vehicle. Compared with supercapacitors, LpTO batteries have energy densities of more than ten times, which can avoid the situation that electric energy is easily exhausted in mountainous areas and frequent start acceleration, and at the same time, the competitiveness is increased at a relatively lower cost.
At present, LpTO batteries have been applied in batches on plug-in hybrid buses. Table 2 lists the basic parameters of a 12-meter plug-in hybrid bus using LpTO batteries. This model can be charged from the roof with a pantograph, and the battery can be quickly charged. Charging, fully utilizing the pure electric mode to achieve the goal of high fuel economy. Since April 2012, the car has been in operation in Chongqing with more than 300 vehicles, and by the end of 2012, the total will reach 500. 
Table 2. Basic parameters of a plug-in hybrid bus using an LpTO battery pack
The LpTO battery pack has been rigorously tested in Chongqing for a long period of severe winter and heat. On a fast-charging pure electric bus, the accumulated mileage of bicycles exceeds 60,000 Km and the number of cycles exceeds 2,500. It is also beginning to be scaled up on plug-in hybrid buses.
Application Prospect of LpTO Battery in Hybrid and Plug-in Hybrid Electric Vehicles
In recent years, the number of hybrid electric vehicles (HEVs) for the purpose of saving fuel consumption and reducing carbon dioxide emissions has been increasing, and from mass transit vehicles to public transportation, bulk applications have been realized. The Toyota Pres Hybrid sedan, which was officially launched in 1997, has achieved a global sales of 2.87 million units. In Europe, hybrid buses have been operated in cities such as London and Barcelona. In the domestic public transportation sector, hybrid buses are used. The number of demonstration cities in the demonstration cities has also accounted for more than 85% of the total number of “Ten Cities and Thousand Vehicles†demonstration vehicles. Hybrid vehicles can use smaller displacement engines, shut down the engine when the car is idling, use batteries or capacitors as energy storage units to recover braking energy, and improve fuel economy more effectively; compared with pure electric vehicles , motors The power is small, the capacity of the vehicle power battery pack can be reduced, and the driving range is long. The plug-in hybrid electric vehicle (PHEV) adopts a larger capacity battery pack, and the vehicle's pure electric driving mileage is longer and more energy-saving. It is also mentioned in the National Energy Conservation and New Energy Industry Plan (2012-2020). A development object that is juxtaposed with pure electric vehicles.
a. Refer to the well-known domestic manufacturers of hybrid nickel-metal hydride battery data b. Refer to the well-known manufacturers of hybrid power supercapacitor data c. Reference domestic famous manufacturers hybrid lithium iron phosphate battery data d.6C charge / 6C release, 100% DOD, Cycle to 80% at room temperature
Table 1. Comparison of main performance parameters of LpTO battery and other energy storage units for hybrid vehicles
Table 1 lists the main performance parameters of several commonly used energy storage units for hybrid vehicles. In general, LpTO battery technology has the following advantages in the field of hybrid and plug-in hybrid vehicles:
     4. Summary Hybrid vehicles and plug-in hybrid vehicles are favored by the market for their lower initial investment and lower fuel consumption than pure electric vehicles. With the continuous advancement of battery technology, the new LpTO battery can be quickly charged and discharged, and has an extremely long service life, which can achieve the same life as the vehicle, taking into account the advantages of lithium-ion batteries and super capacitors, which can make up for the existing lithium ion. The battery system has shortcomings in terms of service life and safety, and has higher energy density and cost performance than supercapacitors. It is a better choice for hybrid vehicles and plug-in hybrid vehicle energy storage systems.