How Lithium Batteries Challenge Large Capacity

We are looking forward to a new era of widespread use of clean energy. As one of the hallmarks of this era, people want to see new cars like the Tesla electric car running all over the street. Instead of needing gasoline, it is powered by lithium batteries that are full of electricity. By that time, filling stations on both sides of the road will be replaced by charging stations. The latest news is that Shanghai has announced that it will grant Tesla electric vehicles a licence free of charge and support them in building supercharging stations in China faster.

However, when you realize that the batteries used in electric vehicles are not fundamentally different from the batteries in your phone, the elation of the bright future may be covered with a cloud. What the cell phone party often grieves about is battery life. Many people's mobile phones are full in the morning and full in the afternoon. They must be filled once a day. Laptops also have this problem, and they may run out of power in a few hours. The utility of electric vehicles is questioned by the fact that they need to be recharged frequently when they are not far away. Tesla's only Model S electric car on the market, the highest-equipped model that claims to be 480 kilometers after a full charge, is already a remarkable achievement.

Why are batteries "not durable"? In a certain space, the size of the energy that the material can store is called "energy density." Judging from the ranking of energy density, the battery is almost at the bottom. Using the energy generated per kilogram as an indicator, our daily gasoline use can reach 50 MJ, while the average level of lithium batteries is less than 1 MJ. Other types of batteries also hover at very low levels. Obviously, we can not make the battery's volume infinitely large; In order to increase the capacity of the battery, we can only focus on increasing the energy density of the battery, but it is difficult. What is so hard about this technology? The reporter interviewed Liurun, an associate professor of chemistry at Zhejiang University, and took the lithium ion battery(lithium battery) as an example to dissect the secret of the internal structure of the battery.

Electrolytes are important.

The battery can provide energy because of the transfer of electrons. The battery is connected to a circuit, the switch is switched on, and the current is turned on. At this point, the electrons will run out of the negative pole and go through the circuit to the positive pole. In the process, electronics will do the work to support your mobile phone, or drive a Tesla electric car.

The electrons of a lithium battery are supplied by lithium. So, with lithium in the battery, wouldn't the energy density go up? Unfortunately, in order for a lithium-ion battery to be rechargeable, its internal structure must be at the expense of energy density. Liurun pointed out that the internal structure of lithium batteries includes electrolytes, negative electrode materials, positive electrode materials, and diaphragm. Each has its own special process requirements and plays a unique function and is indispensable. This structure limits the increase in energy density of lithium batteries.

First, the electrolyte, which is indispensable in the battery transport pipeline. "When the battery discharges, the lithium atom loses electrons and becomes a lithium ion. At this time, it runs from one pole of the battery to the other and runs back when the battery is charged. " said Liurun. The movement of lithium ions at the two poles of the battery constitutes the key to the recycling of lithium batteries, and the electrolyte guarantees its free travel. Electrolytic fluid is like river water, lithium ion is like fish. If the riverbed is dry and the fish can not reach the other side, the lithium battery will not work properly.

The electrolyte is also wonderful because it only carries lithium ions, not electrons, which ensures that the battery will only discharge when the circuit is connected. At the same time, depending on the electrolyte, the movement of lithium ions is orderly and clear, so that electrons always move in one direction, so that electric currents are formed.

Positive and negative poles for stability

The electrolyte does not provide any energy, and it is not small enough, but it is absolutely indispensable in lithium batteries. So why can't there be less negative materials based on graphite? Graphite is the material that makes pencil cores, and it is not responsible for providing electrons. ` It's to make sure the charge doesn't go wrong, 'Mr. Liurun said.

When charging, lithium ions jump from the positive pole into the electrolyte and swim back to the negative, where the electrons become lithium atoms and gather to prepare for the next discharge. However, lithium itself is a good conductor of electrons. Later lithium ions may not have reached the negative pole, and they have obtained electrons from their predecessors who have turned into lithium atoms. In this way, lithium atoms may grow like weeds in the battery and appear crystals. This process is called "crystallization." The brutal growth of lithium crystals will eventually pierce the diaphragm and short-circuit the battery.

In order to solve this trouble, scientists made negative materials using graphite, using its surface voids, like a small room, so that lithium atoms stay safely inside and do not interact with each other. As a result, lithium atoms settle down, but the battery's energy density drops again.

Similar reasons apply to the positive electrode of the battery. In order to maintain the stability and order of the battery, the positive electrode material has also been specially designed. Not all lithium ions will swim back to the negative pole when charging. About half of the lithium ions will stay. This also weakens the energy density of the battery.

Large capacity challenge

Objectively, lithium batteries are small in size, light in weight, long in life, low in cost, safe and environmentally friendly, and can be used for a longer period of time in a shorter charging time ... It is already an extremely high quality battery that humans have found at present.. However, scientists always strive for excellence. Liurun said the key to improvement is to find better positive and negative materials to increase the energy density of the battery.

Possible alternatives to negative materials are lithium Silicon alloys, which have energy density nearly ten times higher than graphite. Finding more stable positive materials is even more important. Lithium cobalt is now used mainly. This material is very expensive, but stability and energy density are not satisfactory. Scientists are studying the possibility of replacing it with an oxide of iron or manganese.

Is it possible to get a larger battery capacity in the foreseeable future? Scientists are still working around lithium. Because lithium participates in the reaction with a high proportion of electrons, metallic lithium has a large energy density. Both lithium-sulfur batteries and lithium-air batteries have theoretical possibilities. They are batteries, but they can provide as much energy as gasoline fuel. However, practical development and promotion still face many challenges.

It seems that you are interested in electric cars, still need to temporarily accept the fact that running for a while will be charged. Scientists in New York recently developed a new invention that uses solar panels to create a manhole cover that allows electric cars to park on the road at any time to charge. That's good news for Tesla, isn't it?

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