Lithium-ion era: current input and output quadrupled!

　　The input and output of current is 4 times

　　Similarly, the parallel hybrid system with a clutch between the engine and the motor is also found in the HEVs of Volkswagen's "Touareg" and Porsche's "Cayenne". In addition to the clutch, both cars also retain the torque converter originally equipped on the gearbox, which is what makes them different from the Fengya hybrid car.

　　The impact of depressing the clutch cannot be absorbed by the torque converter, so the Fuga Hybrid suppresses the drastic torque fluctuation by instantly increasing and decreasing the motor torque. Since the motor torque needs to be instantly increased, the battery also needs to have corresponding output characteristics.

　　Another reason for the high output characteristics is to balance fuel efficiency and output power . In order to achieve a strong acceleration feeling when stepping on the accelerator, Nissan uses a 3.5L V-type 6-cylinder engine in the hybrid system.

　　"In order to improve fuel efficiency with this system structure, the frequency of motor driving has been increased, so the current input and output of the battery is larger." According to Nissan's evaluation, the Fuga Hybrid requires about four times the current input and output compared to the "Altima Hybrid" equipped with Toyota's hybrid system (Figure 5).

　　Figure 5: Comparison of current input and output of Fuga Hybrid and Altima Hybrid

　　The cumulative input and output of the Fuga Hybrid (red line) is approximately four times that of the Altima Hybrid (blue line).

　　The battery capacity of the Fuga Hybrid is 1.4kWh, which is comparable to the Ni-MH rechargeable battery (1.31kWh) of the Prius. If a lithium battery with high output density is used, the output power can be equal or higher even if the capacity is reduced, so the capacity can be reduced. On the other hand, this shows how much importance the Fuga Hybrid places on output power, and why it is equipped with a large lithium battery.

　　The current input and output of this hybrid system is higher than that of the previous hybrid system, so there is a problem that the battery temperature is easy to rise. The battery pack cooling system takes in the air in the car from behind the rear seat and sends it to the battery pack. The cold air flows from top to bottom through the tiny gaps between the modules, ensuring cooling performance (Figure 6).

　　Figure 6: Cooling system for battery pack

　　Air drawn from above flows through the battery pack from top to bottom for cooling. A dedicated cooling path is provided for the DC-DC converter.

　　One of the characteristics of Nissan's laminated cell is that it uses LiMnO2 as the positive electrode material. Mn-based positive electrode materials have more resources than Co and Ni, and have cost advantages. In addition, LiMnO2 has high thermal stability and strong resistance to overcharge, so it also has the advantage of excellent safety. However, as mentioned above, it has shortcomings in terms of energy density. Therefore, Nissan also added Ni, but did not disclose the amount of addition.

　　Nissan also uses Mn-based positive electrode materials in batteries for electric vehicles (EVs) that emphasize capacity, which is different from batteries for hybrid vehicles that emphasize output power. Compared with batteries for hybrid vehicles, the batteries used in Nissan's EV "LEAF" (Chinese name: 葉风) have a higher energy density by thickly coating the positive and negative active materials on the plates, which is about twice the density of HEV batteries.

　　The battery structure looks like a simple laminated unit, but it actually took a lot of effort to reach mass production levels. Take the packaging technology as an example. When the terminal is exposed to the outside and clamped from both sides with a film, only a triangular gap is slightly opened next to the terminal. For this reason, a resin packaging technology was developed. In addition, in the structure of each unit in electrical series, two different metals, Al and Cu, need to be joined, and for this purpose, a technology using ultrasonic bonding has been developed.

　　Distinguish between Ni-MH and Li-ion batteries

　　Toyota's Prius α places particular emphasis on battery miniaturization, and therefore uses lithium batteries. The Prius α is available in 5-seater and 7-seater models. The 5-seater model, like the regular Prius, is equipped with a Ni-MH rechargeable battery under the trunk floor behind the rear seats. However, if the battery is installed in this position, the third row of seats in a 7-seater car cannot be installed. Therefore, the 7-seater model installs the battery pack in the center console between the driver's seat and the front passenger seat (Figure 7). Ni-MH rechargeable batteries are not feasible in this position to install a battery of the required capacity.

　　Figure 7: Battery placement in the 7-seater Prius α

　　Built into the center console between the driver and passenger seats.

　　"If we compare them simply on a unit basis, lithium-ion batteries are about half the weight and half the volume of Ni-MH rechargeable batteries. However, since lithium-ion batteries place an emphasis on safety, thicker materials are used for the outer casing, so if we compare the battery pack, the weight and volume are only reduced by about 20%" (Ryo Mano, New Battery Control Group 2, HV Battery Cell Development Department, Toyota 2nd Technical Development Headquarters).

　　The lithium battery pack has 56 square cells built into it (Figure 8). The 56 cells are configured to match the voltage of the Ni-MH rechargeable battery used in the 5-seat model. 3.6V×56＝201.6V, which is exactly the same as the battery pack voltage of the 5-seat Ni-MH rechargeable battery. However, the current capacity of the Ni-MH rechargeable battery is 6.5Ah, while the lithium battery is smaller, only 5Ah. The battery pack power capacity of the Ni-MH rechargeable battery is 1.31kWh, while the lithium battery is only 1.0kWh.

　　Figure 8: Lithium battery pack of the 7-seater Prius α

　　The battery pack is composed of 28 square cells stacked in two layers.

　　Unlike the Civic Hybrid and Fuga Hybrid, the Prius α focuses on compatibility with Ni-MH rechargeable batteries and does not use the output characteristics of lithium batteries to improve power performance. Acceleration performance and other aspects are also kept at the same level as Ni-MH rechargeable batteries.

　　Surprisingly, the peripheral circuit of the inverter is also common to Ni-MH rechargeable batteries, and is said to be common to Prius. Honda was forced to change the inverter, etc. in the new Civic hybrid. It is speculated that the peripheral circuit of the current Prius was designed with the possibility of being used in the future with lithium batteries in mind.

　　The lithium battery equipped in the Prius α uses NCA-type [Li (Ni-Co-Al) O2] materials with LiNiO2 as the main component for the positive electrode. This is because LiNiO2 has advantages in energy density, but has problems in thermal stability.

　　Toyota has made great efforts to ensure safety. Specifically, a conductive resin polymer PTC (Positive Temperature Coefficient ) layer is formed on the positive electrode, which increases resistance as the temperature rises, and a "HRL (Heat Resistance Layer)" ceramic layer with high heat resistance is formed on the negative electrode (Figure 9). In addition, multiple configurations are made for unit status monitoring based on sensors , "thoroughly ensuring safety."

　　Figure 9: Structure of the Prius α's lithium battery

　　To improve safety, a polymer PTC layer and a heat-resistant ceramic layer are provided.

The production of lithium batteries is handled by Primearth EV Energy (PEVE), 　　a joint venture between Toyota and Panasonic , on a production line at Toyota's Zhenbao plant. The production capacity of the production line can provide 1,000 vehicles per month, and it is estimated that it will be difficult to increase the production of the 7-seater Prius α.

　　On June 12, one month after the Prius α was launched, the order volume reached 52,000 units, of which 17,000 were three-row seat models, accounting for about one-third. If 1,000 units are produced per month, it will take nearly one and a half years to deliver the vehicles. However, this is the first time Toyota has mass-produced lithium batteries, so the production increase plan is cautious. Yu Chuanhong, the person in charge of the development of the Prius α and the current head of Toyota's product planning department, said, "We want to increase production slowly."