What is the stumbling block in the development of pure electric vehicles? There is more than one answer to this question, but among the many answers, one of the most important is the safety of electric vehicles, because it is closely related to the safety of our lives. Among the many problems that affect the safety of electric vehicles, battery safety is the most difficult and the most influential. So this time, we plan to use four topics to trace back to the source, and analyze the safety problems of pure electric vehicles from the development, current situation and future of power batteries.
This is the first issue of the series. We will take you back to the Middle Ages to see how battery was born, what it experienced as the energy source of driving vehicles, and when it became a “dangerous person”?
If there is a time machine in the world, let’s go back to the day in 1786 when Luigi Galvani, an Italian anatomist, and his assistant were preparing to dissect a dead frog in a damp and fishy dissecting room in Italy. When he touched the frog’s leg with his scalpel in his hands, the frog’s leg muscles twitched.
This young man named volt is not only an aristocrat, but also a physicist. Inspired by Benjamin Franklin, he has repeatedly demonstrated through experiments that it is not the frog’s muscles that generate electric current, but some liquid and two heterogeneous metals. Although Galvani’s conclusion is not accurate, people have started a series of researches on storable electric energy from him.
Maybe volt was too addicted to applause. He didn’t deeply study these electrochemical reactions. He didn’t know the basic principle of current generation. Just as the bottle opener was invented many years after the invention of the can, for a long time, people did not know the principle of electricity generated by the stack.
In fact, the silver and zinc of the “volt stack” are the positive and negative electrodes of the battery, and the paper soaked in salt water is the electrolyte. At that time, scientists tried chemical batteries in many ways. People found that when the battery generated electricity, there would be many bubbles (hydrogen). If the bubbles were not well controlled, the battery would easily expand and explode.
At this time, the battery was low voltage and easy to explode. Because the electrolyte was sulfuric acid, it was inconvenient to carry. So in 1888, chemist casnier added starch to the electrolyte to make paste. Since then, this kind of battery has been called “dry cell”. It should be noted here that the electrolyte in power batteries is also paste like, but in order to distinguish it from the solid electrolyte in solid-state batteries, we usually call the former electrolyte.
Later, nickel cadmium and nickel iron batteries were invented. However, because these materials were much more expensive than other batteries at that time, their practical application was hindered. However, the batteries with nickel as cathode material system began to enter people’s field of vision. Since then, scientists have been working hard on how to make battery power more durable, safer and cheaper.
If you can recite the periodic table of elements, you will know that lithium is a relatively active metal element. Scientists have found that the battery made of composite materials mixed with lithium ion can emit the most electricity, and the weight of lithium is very light. The volume of the battery made of it is very small, which lays the foundation for the wide promotion and application of batteries. However, things always have two sides, and lithium is too small In the active, prone to short circuit, there are more security risks.
Because of its high activity, lithium will react with water or air, leading to combustion and explosion. How can it be stabilized? Scientists found that lithium ion has the property of embedding graphite, so they tried to use this property to make rechargeable batteries. The first available lithium ion graphite electrode was successfully developed by Bell laboratory.
Because this idea was not consistent with the direction of lithium-ion battery research at that time, no enterprise dared to accept goodinaf’s invention at that time, and even his alma mater Oxford University was not willing to apply for a patent for it. But at this time, a company from Japan extended an olive branch, it is Sony. So far, the “Scepter” of lithium-ion battery development has shifted from Europe to Asia.
In fact, since Sony commercialized lithium-ion batteries in 1992, lithium-ion batteries began to appear in a large number of consumer electronics products. The wide application not only reduced its cost, but also promoted the development of lithium-ion batteries. Lithium ion batteries with higher energy density, longer driving range, higher charging rate and various material systems appeared: lithium cobalt oxide, lithium iron phosphate, lithium iron phosphate, lithium iron oxide Lithium ion batteries emerge one after another.
LiFePO4 has a stronger crystal structure than licoo4, which means it is more durable and safe, and its main constituent elements are iron and phosphorus, so the production cost is much lower than licoo4. Later, people were not satisfied with the energy density of the battery, so the ternary lithium battery with higher voltage and higher energy density appeared in people’s field of vision.
From the development process of electrochemical battery, it originated in Europe and was carried forward in Asia. Through years of development, lead-acid battery, nickel chromium battery, nickel hydrogen battery and lithium-ion battery are applied in all aspects of life because of their “personality” characteristics. So, among so many battery types, why does lithium battery stand out as the preferred vehicle power battery? With its own instability, why can it make car companies rush to it?
The early electric vehicles were very popular for a short time. Because the lead-acid battery used for power was large in volume, large in mass and small in energy density, and the battery weighed 1200 pounds, it was necessary to remove the whole battery during maintenance, and sediment would form at the bottom of the battery, which could easily cause short circuit when cleaning from time to time, and electrolyte such as sulfuric acid would flow out, causing poisoning or explosion. Charles duriea, the pioneer of gasoline vehicles, once joked that “a set of batteries is more difficult to serve than a hospital full of patients”. At that time, there was another important reason why the electric car didn’t develop. At that time, the electric car was still a tool of the nobility, because in many parts of Europe, there was no electricity at home.
EV1 uses a lead-acid battery pack with a endurance of only 96 km. The poor endurance of the lead-acid battery has become a barrier that electric vehicles cannot overcome. Although GM later used a nickel alloy battery pack to replace the lead-acid battery, the endurance mileage is only 260 km, which is not comparable to that of gasoline vehicles. Moreover, the materials in the battery are expensive and seriously polluted.
Ford also tried to use sodium sulfur battery as power battery, which has three times the endurance of lead-acid battery, but this kind of battery caught fire frequently in the laboratory, and finally failed to be applied to mass production vehicles. Although the development of pure electric vehicles has been hindered, hybrid electric vehicles have made some progress at this stage. Nickel hydrogen battery is also widely used at this time. It has high stability and resistance to overcharge and overdischarge. Even if the energy density is low and the range is small, its disadvantages can be accepted by people because the vehicle is not only driven by electricity.
Since its birth, automotive power battery has been looking for a balance in cycle performance, cost, safety, energy density and capacity. In fact, it’s not only automotive power battery, these aspects even run through the development of the whole battery for more than 200 years. People are pushing things forward between mutual pulling and choice. The foreseeable future is that we will continue to explore and advance in these aspects. As for automotive power battery, how to achieve the most perfect balance may require the attention of all mankind strive.
Editor in charge: Tzh