New energy car power batteries can be divided into two categories: storage batteries and fuel cells. Storage batteries are used in pure electric cars (EV), hybrid electric cars (HEV) and plug-in hybrid electric cars ( PHEV); fuel cells are dedicated to fuel cell cars (FaV).
Storage batteries suitable for pure electric cars can be classified as lead-acid batteries, nickel-based batteries (nickel-hydrogen and nickel-metal hydride batteries, nickel-cadmium and nickel-zinc batteries), sodium-based batteries (sodium- Sulfur batteries and sodium-nickel chloride batteries), secondary lithium-ion batteries, air batteries and other types.
In a pure electric car equipped only with a storage battery, the role of the storage battery is the only power source for the car’s drive system. In a hybrid car equipped with a traditional engine (or fuel cell) and storage battery, the storage battery can not only play the role of the main power source of the car drive system, but also act as an auxiliary power source. At low speeds and starting of electric cars, the storage battery plays the role of the main power source of the car drive system; when accelerating at full load, it acts as an auxiliary power source; during normal driving or deceleration or braking, It plays the role of storing energy.
The comparison of specific energy, specific power, safety and other basic performance of lead-acid batteries, nickel-hydrogen batteries and lithium-ion batteries (including lithium polymer batteries) is shown in Figure 2. Through comparison, it can be found that none of the current storage battery technologies can occupy an advantageous position in every aspect of performance. This is the reason for the coexistence of these different types of storage batteries in the field of electric car applications, and the defects of various storage battery technologies to varying degrees have caused the development of electric cars to be restricted and not large-scale industries. Reasons for change.
As can be seen from Figure 2 , among the storage batteries currently on the market, lithium ion batteries (including lithium polymer batteries) are in an absolute leading position in addition to price and safety. Power lithium-ion batteries are in a stage of rapid development. Power lithium-ion batteries are used in new energy cars such as Nissan Leaf, Toyota Prius plug-in, Tesla Models, General Volt, Ford Focus EV and BMW i3. In addition, lithium resources are relatively abundant and the price is not expensive. It can be said that among pure electric car storage batteries, power lithium-ion batteries are currently the most promising storage batteries in the market.
Fuel cell is also called “continuous battery”, which is a type of battery that can discharge continuously for a long time as long as the active material is continuously injected into the battery. A fuel cell can be regarded as a battery that sends reactants into the battery from the outside when electricity is needed. Fuel cells dedicated to electric cars are divided into alkaline fuel cells (AFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), proton exchange membrane fuel cells (PEMFC) ), direct methanol fuel cell (DMFC) and other types.
The fuel cell consists of an anode, a cathode, an electrolyte and a diaphragm. The fuel is oxidized at the anode and the oxidant is reduced at the cathode. If gaseous fuel (hydrogen) is continuously supplied on the anode (ie, the negative electrode of the external circuit, also called the fuel electrode), and oxygen (or air) is continuously supplied on the cathode (ie, the positive electrode of the external circuit, also called the air electrode), Then the electrochemical reaction can take place continuously on the electrode and generate electric current. It can be seen that the fuel cell is different from the conventional battery. Its fuel and oxidant are not stored in the battery, but stored in a storage tank outside the battery. When it is working (outputting current and doing work), it needs to continuously input fuel and oxidant into the battery and discharge the reaction products at the same time. Therefore, in terms of working mode, it is similar to a conventional gasoline or diesel generator. Because the fuel cell needs to continuously feed fuel and oxidant into the battery when it is working, the fuel and oxidant used by the fuel cell are both fluid (gas or liquid). The most commonly used fuels are pure hydrogen, various hydrogen-rich gases (such as reformed gas) and certain liquids (such as methanol aqueous solution). The commonly used oxidants are pure oxygen, purified air and other gases and certain liquids (such as peroxide Aqueous solutions of hydrogen and nitric acid, etc.).
The fuel cell does not need to be charged, and has the advantages of high specific energy, long service life, low maintenance workload, and continuous high-power power supply. In addition, fuel cell cars can reach the same driving range as fuel cars.
According to different electrolytes, fuel cells can be divided into five categories: alkaline fuel cells, phosphoric acid fuel cells, proton exchange membrane fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. At present, proton exchange membrane fuel cells are widely used in fuel cell cars, and they are extremely competitive batteries in the field of storage batteries for new energy cars in the future.
The function of the fuel cell anode is to provide a common interface between the fuel and the electrolyte, and to catalyze the oxidation of the fuel. At the same time, the electrons generated in the reaction are transmitted to the external circuit or first to the collector plate and then to the external circuit. The role of the cathode (oxygen electrode) is to provide a common interface for oxygen and electrolyte, catalyze the reduction of oxygen, and transport electrons from the external circuit to the reaction site of the oxygen electrode. Since most of the reactions that occur on the electrodes are multiphase interface reactions, in order to increase the reaction rate, the electrodes generally use porous materials and are coated with electrocatalysts.
The role of the electrolyte is to transport the ions produced in the electrode reaction between the fuel electrode and the oxygen electrode, and to prevent the direct transfer of electrons between the electrodes. The function of the diaphragm is to conduct ions, prevent the direct transfer of electrons between the electrodes, and separate the oxidant and the reducing agent.
Therefore, the diaphragm must be resistant to electrolyte corrosion and insulation, and have good wetting resistance.