CR2032 battery

Nanotechnology may solve the bottleneck of electric vehicle energy storage

Professor Cui Yi of Stanford University uses nanotechnology to control the chemical reactions inside the battery. He said, I want to change the world, and I want to be rich, but mainly to change the world.

"In April, on a drizzling morning, Cui Yi drove his red Tesla through the busy traffic to Silicon Valley. Cui Yi is a materials scientist at Stanford University. His destination is Amprius, a battery company he founded six years ago. Coincidentally, he was driving a battery-powered car, but it was rented, not bought, and he hoped that in the next few years "our batteries can be used in this car." ”

Cui Yi and his battery company

Cui Yi and his company are trying to take lithium-ion batteries, the best commercial technology today, to a new level. Currently, many companies, such as Panasonic, Samsung, LG Chem, Apple and Tesla, are competing to make batteries smaller and lighter while having greater power storage capacity. But among these strong competitors, Cui Yi's company is still the most groundbreaking.

Unlike his peers, who mainly focus on adjusting the chemical composition of battery electrodes or conductive electrolytes, Cui Yi is combining battery chemistry with nanotechnology. He is currently creating complex battery electrodes that can absorb and release a large number of charged ions more and faster than standard electrodes without producing harmful chemical side reactions.

He is using innovative techniques from nanotechnology to manipulate chemistry." said Wei Luo, a materials scientist and battery expert at the University of Maryland.

In a series of experimental demonstrations, Cui Yi showed how he solved a series of battery chemical reaction problems that have long plagued researchers through uniquely structured electrodes. These include:

? Lithium-ion batteries with silicon replacing standard graphite;

? Batteries using bare lithium metal as an electrode material;

? Batteries that offer a lithium-sulfur chemistry that is more powerful than lithium-ion batteries.

Nanostructured batteries currently being explored include:

? Silicon nanowires that expand and contract as they absorb and release lithium ions, and tiny egg-shaped structures with a "carbon shell" that protects the "yolk" of silicon particles containing lithium ions.

Amprius has already started supplying silicon-electrode cellphone batteries that store 10% more energy than the best conventional lithium-ion batteries on the market, and another prototype in development could store 40% more. So far, Cui's company has not yet produced batteries for electric vehicles (EVs), but if the technology it is developing one day works out, such car batteries could store 10 times more energy than today's best products. Affordable electric cars could travel as far as conventional gasoline cars, and large numbers of vehicles could be powered by solar and wind power, revolutionizing the auto industry and significantly reducing global carbon emissions.

"I want to change the world and get rich, but mainly I want to change the world," Cui said when he began his research.

His pursuit has gone beyond the battery industry, and the new nanotechnology his lab is exploring will give rise to a group of startups that provide cheaper and more efficient air and water purification systems. But so far, Cui's most impressive achievements are in the battery industry. Luo Wei described Cui's success as "subversive and shocking." Jun Liu, a materials scientist at the Pacific Northwest National Laboratory, was more straightforward: Cui's nanotechnology has made a "huge" contribution to the battery industry.

Current status of the battery industry

Achieving a leap in battery technology is difficult. Compared with the exponential growth of computer chip performance in Silicon Valley over the past few decades, battery development has lagged behind. The current best lithium-ion battery has an energy density of about 700Wh/L, five times that of nickel-cadmium batteries in the 1980s. Although this result is good, it is not a real breakthrough. In the past decade, the energy density of the best commercial batteries has doubled, but it still cannot meet the growing needs of users. According to some market research reports, the market share of lithium-ion batteries is expected to reach $30 billion by 2020. With the increase in electric vehicle production by Tesla, General Motors, Nissan and other auto companies, the battery market has expanded dramatically.

Today's electric vehicles also have a lot of room for development. Take Tesla's S-type electric vehicle as an example. Its 70-90 kWh battery weighs 600 kilograms. For a $100,000 car, the battery price accounts for $30,000. And the range on a single charge is only 400 kilometers, far inferior to traditional cars. Nissan Leaf is much cheaper, with a price tag of about $29,000, but its battery pack is smaller and its maximum range is only 1/3 of Tesla's.

Battery technology innovation will have a significant impact on electric vehicles. Every time the battery energy density doubles, automakers can reduce the battery size and cost by half while keeping the range unchanged, or choose to keep the battery size and cost unchanged and double the range. "The era of electric vehicles is coming," but in order for electric vehicles to replace traditional vehicles, "we have to do better!" Cui Yi said.

Nanobattery Startup

In his early research career, Cui Yi realized this need. After graduating from the University of Science and Technology of China in 1998, he first came to Harvard University in the United States, and then completed his doctorate at the University of California, Berkeley, and conducted postdoctoral research in the then most cutting-edge nanomaterial synthesis laboratory. At that time, nanotechnology was in its early stages of development, and researchers were still trying to find reliable ways to make the materials they wanted, and the application of nanotechnology had just taken shape.

At the University of California, Berkeley, Cui Yi worked with colleagues at the Lawrence Berkeley National Laboratory (LBNL). At that time, LBNL's director Steven Chu was promoting the development of renewable energy technologies in the laboratory to combat climate change, including the development of better batteries to store clean energy (Chu served as Secretary of Energy in the US President Barack Obama's administration from 2009 to 2013.)

"At the beginning, I didn't pay much attention to energy issues, and I had never done research on batteries before," Cui Yi said.

But the efforts of Chu and other laboratory colleagues gave him a great shock, and he realized that nanotechnology could bring new ways out for batteries. As Chu said, nanotechnology has brought a "new starting point" to the field of batteries. Researchers can not only control the chemical composition of battery materials at the smallest scale, but also control the chemical reactions of batteries by rearranging the atoms in the materials.

After arriving at Stanford University, Cui Yi quickly began to study the combination of nanotechnology and battery electrochemistry.

Take lithium-ion rechargeable batteries as an example. In principle, the structural principle of these batteries is simple: two electrodes are separated by a thin film as a "separator", and the liquid electrolyte allows ions to slide back and forth between the electrodes.

When the battery is charged, lithium ions are released from the positive electrode or cathode. The cathode material is a lithium alloy, usually lithium cobalt oxide or lithium iron phosphate. The released lithium ions are attracted to the negatively charged electrode (also called the anode, which is usually made of graphite) and tightly gathered between the carbon atoms of the graphite. The power supply voltage from the outside drives the entire ion group to move on a large scale, thereby achieving the purpose of storing electrical energy.

When a device, such as a power tool or car, needs energy to start, the battery discharges, and the lithium atoms gathered between the carbon atoms of graphite release electrons through the external circuit to the cathode. At the same time, the lithium ions released from the graphite pass through the electrolyte and the "separation membrane" to the cathode, where they meet the electrons and complete the battery circuit cycle.

Graphite is the most ideal negative electrode material today. Its high conductivity can easily transfer electrons to the metal wires in the circuit. But graphite's ability to collect lithium ions during discharge is very average. It takes six carbon atoms to "grab" one lithium ion. The weak absorption ability limits the amount of lithium that can be accommodated in the electrode, that is, the battery's ability to store energy.

Silicon is better in this regard. Each silicon atom can "bind" four lithium ions, which means that the energy storage capacity of silicon-based negative electrodes can reach 10 times that of graphite negative electrodes. For decades, electrochemists have been working tirelessly to develop this potential of silicon-based negative electrodes, but have been unsuccessful.

It is simple to make negative electrodes using silicon materials, but the problem is that such negative electrodes cannot exist continuously and stably. When the battery is charged, lithium ions flow in and combine with silicon atoms, and the negative electrode material can expand by 300%. Then, during the discharge process, as the lithium ions flow out, the negative electrode material shrinks rapidly. Silicon electrodes can't withstand a few twists and turns before they break and split into tiny particles. The negative electrode of the battery, or the entire battery, is scrapped.

Cui Yi felt that he could solve this problem. His experience at Harvard University and Berkeley, California, told him that nanomaterials behave differently from ordinary materials. First, the proportion of atoms contained on the surface of nanomaterials is higher than that in their interior. At the same time, the atoms on their surface are rarely bound by neighboring atoms, and they can move more freely when subjected to pressure and stress.

Nanobattery innovation

In 2008, Cui Yi proposed using nanosilicon to make silicon negative electrodes, which can reduce the pressure and stress that cause the block silicon negative electrode to disintegrate. His idea was indeed feasible. In a paper published in Nature Nanotechnology, Cui Yi and his colleagues showed their research results. After experiencing multiple lithium ion inflows and outflows in silicon nanowires, the nanowires were almost undamaged. Even after 10 cycles of charge and discharge, the negative electrode still had 75% of the theoretical power storage capacity.

Unfortunately, silicon nanowires are more difficult to prepare and more expensive than bulk silicon. So Cui Yi and his colleagues began to study ways to reduce the cost of silicon negative electrode materials. First, they found a way to use spherical silicon nanoparticles to prepare lithium-ion battery negative electrodes. Although this solved the cost problem, they had to face another problem. As lithium atoms flow in and out, the nanoparticles also shrink and expand, causing the glue that binds the nanoparticles to crack. The liquid electrolyte penetrates between the nanoparticles through these cracks, producing a chemical reaction and forming a non-conductive layer on the surface of the silicon nanoparticles, called a solid electrolyte membrane (solid-electrolyte interphase, SEI). As this layer of membrane accumulates, the charge collection ability of the negative electrode is gradually destroyed. "It's like scar tissue," said a graduate student in Cui's lab.

A few years later, Cui's team tried another nanotech solution. They made egg-shaped nanoparticles, wrapping these tiny silicon nanoparticles (the "yolk") with a highly conductive carbon shell, through which lithium ions can pass freely, and the carbon shell gives the silicon atoms in the "yolk" enough space to expand and contract, while protecting them from the SEI formed by electrolyte chemical reactions. In a paper published in NanoLetters in 2012, Cui's research team reported that after 1,000 charge and discharge cycles, the yolk-shell electrode still retained 74% of its storage capacity.

Two years later, the "yolk-shell" nanoparticles were further improved, and they were assembled into a micron-sized composite structure, like a miniature pomegranate. This new silicon nanosphere structure can increase the lithium storage capacity of the negative electrode and reduce harmful side reactions in the electrolyte. In February 2014, Cui Yi published new progress in nanobatteries in Nature Nanotechnology. The battery based on the new material maintained up to 97% of its battery capacity after 1,000 charge and discharge cycles.

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With the battery company up and running, Cui Yi also plans to apply nanotechnology to air purification and water purification projects

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Earlier this year, Cui Yi's team reported that they had a better solution than the "micro-pomegranate" combined nanostructure. They hammered larger silicon nanoparticles into micron-sized particles and then wrapped them in an extremely thin graphene carbon layer. The silicon nanoparticles made in this way are larger than the previous "micro-pomegranates". Such a large volume usually breaks after a few charges and discharges, but the graphene wrapping layer prevents the electrolyte from contacting the silicon nanomaterial, while easily transferring the charge to the metal wire. The relevant results have been published in the journal Nature Energy. The larger the silicon nanoparticles, the more electrical energy they can contain in a certain volume. Compared with the "micro-pomegranate" nanostructure, it is cheaper and simpler to make.

He really found the right direction this time. "Liu Jun said.

Inspired by these innovative ideas, Amprius raised $100 million for the commercial development of silicon negative electrode lithium-ion batteries. Amprius has sold 1 million mobile phone batteries produced in China. The company's chief technology officer, Song Han, said that this battery based on simple silicon nano-nano has a low production cost, but the capacity is only 10% higher than today's lithium-ion batteries. But at the headquarters of Amprius, Han Song demonstrated the technical prototype of the nanowire-silicon battery, which can increase the storage capacity by 40%. He said that this is just the beginning, and the future prospects of silicon negative electrode batteries are unlimited.

Now, Cui Yi's vision has gone beyond silicon materials. One research focus is to use pure metallic lithium Pure metallic lithium has always been regarded as the ultimate negative electrode material because it has better potential for greater energy storage and lighter weight than silicon materials.

But there are still some major technical problems to be solved. First, a SEI layer that lithium ions can pass through is usually formed around the lithium metal electrode, so the SEI layer can act as a protective layer for the lithium negative electrode, which is a good thing. But with the repeated cycles of battery charging and discharging, metallic lithium also expands and contracts like silicon nanoparticles, eventually destroying the SEI protective layer, and lithium ions accumulate at the fractures, forming many metal spikes called "dendrites" in the electrode.

Such dendrites can pierce the battery separator, causing the battery to short-circuit and catch fire. "Said another graduate student in Cui Yi's research team.

Traditional processes have not been able to solve this problem, but nanotechnology may be able to. To prevent the formation of metal dendrites, one method adopted by Cui Yi's team is to stabilize the SEI layer by adding a layer of interconnected nano-carbon spheres to the negative electrode; another method is to add a new "yolk shell" particle composed of gold nanoparticles in a larger carbon shell. The gold nanoparticles absorb lithium ions, and the shell provides space for the expansion and contraction of lithium, thereby solving the problem of cracks in the SEI layer and the formation of metal dendrites.

Improving the negative electrode of the battery is only half of the success. Cui Yi's team also used similar nanotechnology to improve the positive electrode materials, especially sulfur. Just as silicon is regarded as the best negative electrode material, sulfur has long been regarded as the best choice for positive electrode materials. Each sulfur atom can bind two lithium ions, which can theoretically increase the storage capacity of the positive electrode several times. Equally important, sulfur materials are quite cheap. But there are also some problems in specific practice. Sulfur has a very general conductivity and will react with ordinary electrolytes to produce harmful chemicals that harm the battery, resulting in several chargingAfter discharge, the battery is scrapped. In addition, during discharge, the sulfur cathode tends to accumulate charge rather than release it.

In the process of seeking nano solutions, Cui Yi's team wrapped sulfur particles in a shell of highly conductive titanium dioxide, which increased the battery capacity by 5 times compared with traditional batteries, while also preventing the formation of harmful chemicals that damage the battery. The researchers also made a sulfur-based version of "micro pomegranates" that fixed sulfur in long and thin nanofibers. These innovative measures not only increased battery capacity, but also increased the Coulomb efficiency (referring to the battery discharge efficiency) from 86% to 99%.

Today, both the positive and negative electrodes of nano batteries have reached the high capacity requirements. "Cui Yi said.

Cui Yi's next goal is to merge these two innovations into one. He hopes to successfully combine the silicon anode with the sulfur cathode to create a high-capacity, low-cost battery, which will fundamentally change the world's energy landscape.

I believe that if our nano batteries can achieve the final success, it will have a great impact on the world." Cui Yi said.

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