Toyota Mirai fuel cell electric vehicle
Toyota recently launched the Mirai, a lithium-ion battery-powered fuel cell vehicle, directly competing with pure electric vehicles. The term "lithium-ion battery" is likely familiar to most, as it's a technology that scientists worldwide have researched for decades without significant breakthroughs, at least not with a mature technology cost-effective for the consumer market. Less than a year ago, Toyota boasted of a technological breakthrough in lithium-ion batteries, claiming it could reduce the cost of automotive lithium-ion batteries from $1 million to $50,000, a 95% reduction! In less than a year, Toyota delivered on its promise with the Mirai. While the Mirai is currently priced at approximately 380,000 RMB, it's important to understand that this is the first mass-produced lithium-ion battery-powered vehicle to hit the market.
Toyota Mirai Core Structure Notes
The origin of cosmic black technology
The Toyota Mirai's structure differs from traditional gasoline cars or pure electric vehicles. If one had to find a similar structure, perhaps the best-selling Toyota Prius would share some similarities. The Mirai's powertrain is called TFSC (Toyota FCStack), a hybrid system with a lithium-ion battery stack as its core component. TFSC lacks a traditional gasoline engine and transmission; the engine compartment houses the electric motor and its control unit.
Toyota Mirai structural diagram
The lithium-ion battery stack located at the bottom of the cockpit is the core of the entire system, and this article will focus on analyzing it. A nickel-metal hydride lithium-ion battery pack and two high-pressure hydrogen storage tanks are located at the rear axle. That's right, without a fuel tank or a large lithium-ion battery, the Mirai's only "fuel" is hydrogen. It doesn't need refueling or charging; a full 5 kg of hydrogen allows it to travel 640 km! Why is the title referring to lithium-ion fuel cells as "cosmic black technology"? Because hydrogen is the most abundant element in the universe and also the most abundant on Earth. Hydrogen combines with oxygen in the lithium-ion fuel cell, and the only "waste" produced is pure water! Therefore, hydrogen fuel cell batteries have long been considered "alien technology," one of the most suitable backup energy sources for space stations or space probes.
Toyota Mirai core structure layout diagram
Working principle of fuel cell lithium batteries
Although the name "fuel cell" includes the word "fuel," and hydrogen can indeed burn violently with oxygen, fuel cell batteries do not obtain energy through combustion. Instead, they generate an electric current through charge transfer during the chemical reaction between hydrogen and oxygen. The most crucial technology in this process is the use of a special "electrolyte film" to break down hydrogen. This process can be understood as a mosquito being unable to pass through a window screen, but a smaller dust particle can... The electrolyte film is also the most difficult technological barrier to overcome in the field of fuel cell batteries.
External details and key parameters of fuel cell lithium-ion battery stack
Because hydrogen molecules are small, they can travel through the tiny pores of the thin film to the other side. However, during this process, electrons are stripped from the molecule, leaving only positively charged hydrogen protons to pass through. These protons are attracted to the electrode on the other side of the film and combine with oxygen molecules. The electrode plates on both sides of the electrolyte film split hydrogen gas into hydrogen ions (positively charged) and electrons, and oxygen gas into oxygen ions (negatively charged) and electrons. Electrons form a current between the electrode plates, and two hydrogen ions and one oxygen ion combine to form pure water, which is the waste product of the reaction. So, essentially, the entire process is a power generation process. Therefore, the Mirai is a pure electric vehicle, using a fuel cell lithium battery stack instead of a bulky and inefficient lithium-ion battery pack.
Toyota's 2008 fuel cell lithium-ion battery
Toyota Mirai's innovative fuel cell lithium-ion battery
The Toyota Mirai's lithium-ion battery stack consists of 370 thin-film lithium-ion battery cells, hence the term "stack," and can output a total of 114 kilowatts of power. We previously analyzed the Volkswagen Group's lithium-ion battery technology, which has a basically similar structure. Toyota's lithium-ion battery stack has undergone more than a decade of technological optimization, resulting in its own unique structure, such as 3D microfluidic technology. This technology effectively improves power generation efficiency by better expelling byproduct water and allowing more air to flow in. Therefore, the entire stack's power generation efficiency has reached a world-class level of 3.1 kilowatts per liter, a 2.2-fold improvement over Toyota's technology in 2008.
Mirai fuel cell lithium-ion battery stack technology iteration
Since the voltage generated by each cell in the fuel cell lithium battery stack is approximately between 0.6V and 0.8V, and the total voltage will not exceed 300V, a boost converter is also needed to increase the voltage to 650V in order to better drive the electric motor.
Fuel cell lithium battery iteration
Storing hydrogen at 700 atmospheres
Anyone familiar with the physical properties of hydrogen knows that it differs from gasoline. At room temperature, hydrogen is a gas with a very low density and is extremely difficult to liquefy; in fact, it cannot be liquefied at room temperature. Therefore, the safe storage and transportation of hydrogen is not easy. Thus, hydrogen cannot be directly injected into ordinary fuel tanks like gasoline. Toyota designed two hydrogen storage tanks, one large and one small, to fill them with as much hydrogen as possible using high pressure. Using current mainstream storage technology, Toyota chose a 700 MPa (700 atmospheres) high-pressure storage tank, similar to a common gas cylinder, but much thicker and heavier. The two storage tanks have a combined capacity of 122.4 liters, and at 700 atmospheres, they can only hold about 5 kilograms of hydrogen. Therefore, the actual weight of the fuel is not significant; rather, the hydrogen storage tanks are extremely bulky.
Hydrogen storage performance
To maintain driving safety under 700 atmospheres of pressure, we certainly don't want to be driving on top of two bombs, right? Therefore, the hydrogen storage tank is designed with a four-layer structure. The aluminum alloy tank is lined with a plastic inner liner, wrapped with a protective layer of carbon fiber reinforced plastic, and then an additional shock-absorbing protective layer of glass fiber material is added outside the protective layer. In addition, the fiber texture of each layer has been further optimized according to its location on the tank body, so that the fibers are in the direction of pressure distribution to improve the effectiveness of the protective layer.
Fuel cell lithium-ion battery stack + nickel-metal hydride battery hybrid power
The electric motor directly driving the Mirai's wheels has a power output of 113 kW and a peak torque of 335 Nm, roughly equivalent to the power level of a 2.0-liter naturally aspirated family sedan. Besides generating electricity from the fuel cell stack, the 1.6 kWh nickel-metal hydride battery pack located above the Mirai's rear axle also serves a crucial purpose—a combined lithium-ion battery and energy storage battery. This battery pack is essentially identical to the one in the Camry Hybrid. When the vehicle load is low, it can power the vehicle independently, while simultaneously charging the fuel cell stack, with the nickel-metal hydride battery acting as a "buffer."
Working principle of fuel cell lithium battery vehicles
When the vehicle has a greater power demand, the nickel-metal hydride battery pack will be depleted quickly. At this time, the fuel cell lithium battery stack will directly supply power to the electric motor, achieving dual power supply with the nickel-metal hydride battery pack to meet the demand. When the vehicle decelerates, the electric motor will turn into a generator to recover kinetic energy, and the electricity will be directly transferred to the nickel-metal hydride battery pack for storage.
summary:
Throughout the entire operation of a hydrogen fuel cell lithium-ion battery, besides consuming hydrogen and air, no other energy is consumed; there is no need for refueling or charging. Compared to pure electric vehicles, even the fastest charging Tesla Model S Supercharger takes 1.25 hours to fully charge. Hydrogen refueling is much faster, requiring only 3 minutes to fill two hydrogen tanks, and its range of over 600 kilometers is even superior to that of ordinary gasoline cars. Although hydrogen refueling stations are still extremely rare, the cost of converting an ordinary gas station into a hydrogen refueling station is far lower than the cost of converting it into a fast-charging station. Therefore, we can predict that if breakthroughs can be achieved in cost control for fuel cell lithium-ion battery vehicles, the market potential will actually be larger than that of pure electric vehicles.
