Do you know what a lithium-ion battery is?
First of all, to correct a statement, many people think that lithium battery is the abbreviation of lithium ion battery, which is actually a misunderstanding. The true identity of lithium battery is the primary battery with lithium metal as the negative electrode. Its origin can be traced back to Edison. However, lithium metal is no longer widely used because of its active chemical characteristics and easy combustion and explosion.
Modern Blue used in electric vehicles has applied high-energy polymer lithium-ion battery with the capacity of 16.4kwh. Polymer lithium-ion battery is a new generation of high specific energy battery developed from liquid lithium-ion battery. At the same capacity as liquid lithium ion battery, polymer lithium ion battery has the advantages of smaller volume, lighter weight, wider working range and longer service life. More importantly, its electrolyte is solid or gel, and there is no danger caused by electrolyte leakage of liquid battery.
◆ Error correction:
Myth: Lithium-ion batteries also have a "memory effect" and need to be fully charged and discharged to activate the battery capacity.
Correction: "Memory effect" is the "patent" of Ni-Cd and Ni-MH batteries. Its principle is that incomplete charging and discharging leads to crystallization, and lithium-ion batteries hardly produce this effect. However, although there is no "memory effect" in lithium-ion batteries, with the increase of service time, the cavity structure of lithium-ion on the anode and cathode will gradually collapse and block. Chemically, the active passivation of anode and cathode materials leads to the formation of other stable compounds through side reactions, which leads to the reduction of movable lithium ions, which are reflected in the reduction of capacitance.
Full charge and discharge will reduce the life of lithium-ion batteries! Intuitively, it can be understood that overdischarge will lead to the excessive release of lithium ions in the negative carbon, leading to the collapse of its lamellar structure, and overcharge will force too many lithium ions into the negative carbon structure, so that some lithium ions cannot be released.
The "charge and discharge once a month" mentioned in the battery manual is not to eliminate the "memory effect". Lithium-ion batteries are usually equipped with management chips, which contain a series of registers, such as storage capacity, temperature, charging state and discharging times. These values will change gradually during use. The main function of the instruction "You should charge and discharge all data once a month or so" should be to correct the inappropriate values in these registers, so that the charging control and nominal capacity of the battery can match the actual situation of the battery.
Wide application of lithium ion battery
● Wide application of lithium-ion batteries
◆ Nissan Feng Ling
"Nissan Leaf Pure Electric Vehicle"
Feng Ling is a pure electric zero-emission vehicle launched by Nissan. It is driven by stacked compact lithium-ion batteries with a battery capacity of 24 kWh and a maximum output power of 90 kW. After fully charged, it can reach the cruising range of 160 km. In addition, Ling Feng offers a variety of charging methods. If you charge Feng Ling at home, it will take 8 hours to fully charge the car. If you charge at the fast charging station, it only takes 30 minutes to charge 80% of the electricity.
◆ Mercedes S400 Hybrid Power
"Mercedes-Benz S400 Hybrid Vehicle"
Mercedes-Benz made it clear that S400 is the first large-scale production model using lithium batteries. It consists of 35 batteries, which can provide 19 kW output power and 6.5 ampere-hours capacity. Small size, can be placed directly in the engine compartment.
◆ Chevrolet Volt
"Chevrolet Volanda Extended Range Electric Vehicle"
"Volanda's" T "lithium-ion battery pack"
Different from many electric vehicles and hybrid vehicles, Volanda adopts extended program design, which can drive the vehicle reliably by generating electricity from gasoline engine and charging the battery, or it can run only by battery. Volda's battery pack is a lithium-ion battery jointly developed by General Motors and American LG Chemical Company, with a capacity of 16 kWh. However, due to spontaneous combustion three weeks after the crash test, it is reported that GM will switch to lithium iron phosphate batteries.
◆ BYD E6
"BYD E6 Pure Electric Vehicle"
BYD E6 electric vehicle adopts lithium iron phosphate battery pack, and its cruising range can reach 300 kilometers. The battery pack of E6 is very interesting. As long as there is enough wire load at home, it can be charged by directly plugging in 220v household electricity, and the full charging time is about 7-8 hours. Using special charging equipment, 15 minutes can fill 80%.
Keywords lithium iron phosphate battery, accident mechanism and standard
● Lithium iron phosphate battery is a lithium-ion battery named after cathode material.
There are many cathode materials for lithium ion batteries, such as lithium cobaltate and lithium manganate. However, due to various shortcomings, it was gradually replaced by lithium ferrous phosphate. Lithium ferrous phosphate is the most popular cathode material at present.
Lithium iron phosphate battery is a lithium ion battery named after cathode material. The structure and working principle of all lithium batteries are roughly the same. The lithium iron phosphate battery takes Ferrous lithium phosphate as the positive electrode, which is connected with the positive electrode through aluminum foil, and a polymer diaphragm separates the positive electrode from the negative electrode in the middle, but the lithium ion Li+ can pass through, but the electron e- cannot. The battery cathode made of carbon is connected with the battery cathode through copper foil. Between the upper and lower ends of the battery is the electrolyte of the battery, and the battery is sealed by a metal shell. When lithium iron phosphate battery is charged, lithium ion Li+ in the positive electrode migrates to the negative electrode through the polymer diaphragm; During the discharge process, Li+ ions in the negative electrode migrate to the positive electrode through the separator.
BYD calls its products iron batteries for two reasons. On the one hand, there is a saying that iron batteries can be divided into high-speed iron batteries and lithium iron batteries, so in theory, this name cannot be overemphasized. On the other hand, at present, only A 123, Phostech and Aleees hold the patents of Ferrous lithium phosphate compounds. Not all the batteries produced by people can be called lithium iron phosphate batteries, so they are called iron batteries. But BYD's battery is essentially a lithium-ion battery.
◆ Real iron battery
Real iron battery is a battery with stable ferrate as the positive electrode and zinc, aluminum, iron, cadmium and magnesium as the negative electrodes, which are charged and discharged through the movement of electrons between them. At present, no manufacturer claims to be able to put iron batteries into practical use on a large scale.
● Combustion and explosion: all caused by short circuit.
Combustion conditions are both combustible and igniter, which can be intuitively understood as dry wood and fire. The safety accidents of lithium-ion batteries are mainly caused by the reaction between electrodes and electrolyte. Electrolyte contains organic solvent, which is a highly flammable substance. This is "dry wood". In order to prevent abuse, the cathode material has poor stability and is easy to release oxygen. Just because organic solvents easily react with oxygen to release a lot of heat and gas, the generated heat will further aggravate the decomposition of the cathode, resulting in a vicious circle; When the temperature reaches the ignition point of the electrolyte, the electrolyte will burn, and this exothermic reaction is "fire". At this time, if the battery is in a closed state, a large amount of gas will be generated, and the internal pressure of the battery will rise rapidly, leading to an explosion. The following are some common causes of accidents.
◆ Overcharging leads to combustion and explosion.
More exciting videos are on car home Video Channel.
The first 25 seconds of this video are security warnings. At 26 seconds, the lithium-ion battery was overcharged by high voltage, and at 40 seconds, the battery exploded.
Charging with a damaged or mismatched charger may lead to overcharging. When the voltage is too high, a large number of lithium ions will overflow from the positive electrode, and the lithium ions that cannot be absorbed by the negative electrode will form dendrites on its surface, making the battery short-circuited. Short-circuit current will generate a lot of heat, and the rapid increase of temperature will lead to the combustion of electrolyte as an organic solvent, and in serious cases, it will lead to the decomposition reaction of anode or the reaction between cathode and electrolyte, resulting in a lot of gas. In such a closed loop, the only result is explosion. Even if the battery safety valve works normally, it may spray electrolyte and cause a wider fire.
◆ Burning and explosion caused by puncture and impact.
More exciting videos are on car home Video Channel.
From the third second of this video, the test needle is inserted into the lithium-ion battery, and then a lot of heat is generated inside the battery, and a lot of gas is generated by the internal reaction.
In the process of driving, you will inevitably encounter collisions and dirty roads. Rear-end collision, bottom dragging and slag splashing will all pose a great threat to the battery. Puncture will short-circuit the battery, generate a lot of heat, and cause the electrolyte to burn. The increase of temperature will lead to the decomposition of cathode material or the reaction with electrolyte to produce a large amount of gas, which will lead to explosion. When the lithium ion battery is impacted, the overvoltage loss on the electrode will generate heat, which will promote the reaction between the solvent and the negative electrode. The released heat will further heat the battery, and the positive electrode will undergo thermal decomposition reaction, resulting in the explosion of the battery.
More exciting videos are on car home Video Channel.
In this video, the positive and negative electrodes of the battery are short-circuited within 4 seconds, gas explosion occurs instantly, and the electrolyte burns 17 seconds.
It can be seen that the essence of these hazards is the internal reaction caused by short circuit, which reflects the battery combustion and explosion caused by high temperature and high pressure. Therefore, the heat resistance of the internal materials of the battery should be strictly controlled.
● How to certify the safety of lithium-ion batteries?
At present, UL is the most authoritative third-party certification body in the world. In other words, if a series of UL experiments of lithium-ion batteries pass, it will be recognized by global electric vehicle manufacturers.
High temperature, impact and puncture will all cause changes in the internal environment of lithium-ion batteries, leading to various harmful reactions inside the batteries. Therefore, ul has designed a series of cruel tests to control the safety performance of lithium-ion batteries. Among lithium cobaltate, lithium manganate and lithium iron phosphate batteries, Ferrous lithium phosphate has the best overcharge and overheating resistance and is not easy to burn and explode.
Even if the battery itself is safe, it is not necessarily safe to install it in the car. It is difficult to simulate the harsh environment of the battery in a driving accident by battery testing. At present, neither China nor the United States has a set of independent and comprehensive crash standards for electric vehicles. From the American FMVSS305 and GB18384.1-20065438+0, they are only limited to the regulations of battery electrolyte leakage and battery position after collision, but they have not seen the regulations of combustion and explosion that the public cares about. Although we are not backward in this respect, I believe that everyone, like me, wants to be standardized like developed countries, and not all countries are not standardized.
The National Traffic Safety Administration conducted a crash test on the Chevrolet Volanda. After the test, the vehicle was placed in the parking lot and spontaneously ignited three weeks later. The official explanation is that the battery is short-circuited, and the electrolyte leaks due to collision and spontaneously ignites. Although the battery burned out eventually, it can be seen that the battery did not pose a threat to the safety of the vehicle during the collision.
As for the fire of BYD E6 after the impact, we won't discuss the cause of the accident, but everyone who knows it knows: First of all, the power battery of an electric vehicle is a battery pack, that is, dozens or hundreds of single batteries are connected in series and parallel. Assuming that a single battery is safe, the connection circuit and control circuit between them are also to blame. What's more, the unity of each battery is not easy to judge, just like two identical memories have no quality problems, and there will be incompatibility. Secondly, high-speed impact may not escape the bad luck of gasoline vehicles. Thirdly, collision safety is determined by many factors such as body structure and material strength. So don't blame the battery.
Full text summary:
Battery safety has naturally become the focus of electric vehicle safety, but the reliability of the connection circuit and control circuit of each battery in the battery pack can not be ignored, and the importance of body structure and material strength can not be ignored because of overemphasis on battery safety. In all fairness, the safety of diesel locomotives is less than 100%, so we should learn to look at a comprehensive problem with bold assumptions and careful arguments.
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