The nanostructure of the silicon anode creates new potential for lithium-ion batteries

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In chamber 1, metal nanoparticles made of tantalum grow. Within this chamber, individual tantalum atoms come together, similar to the formation of raindrops. In chamber 2, the nanoparticles are filtered by mass, removing those that are too large or too small. In chamber 3, a layer of nanoparticles accumulates. This layer is “sprayed” with isolated silicon atoms, forming a layer of silicon. This process can then be repeated to create a multi-layered structure. Credit: Scheme created by Pavel Puchenkov, OIST Scientific Informatics and Data Analysis Section

Scientists are revealing a new nanostructure that can overturn technology in batteries and beyond.

  • A new study has identified a nanostructure that improves the anode in lithium-ion batteries
  • Instead of using graphite for the anode, the researchers turned to silicon, a material that stores more load but can withstand fracture.
  • The group has made a silicon anode by placing silicon atoms on top of metal nanoparticles
  • The resulting nanostructure created arches, increasing the strength and structural integrity of the anode
  • According to electrochemical tests, lithium-ion batteries with better silicon anodes have a higher charge capacity and a longer life.

New research from the Okinawa Institute of Science and Technology Graduate University (OIST) has identified a specific building block that enhances the anode in lithium-ion batteries. The special properties of the structure constructed using nanoparticle technology are revealed and explained today (February 5, 2021) here: Communication materials.

Powerful, portable and rechargeable lithium-ion batteries are crucial components of modern technology found in smartphones, laptops and electric vehicles. In 2019, their potential to revolutionize the way energy is stored and consumed in the future, as we move away from fossil fuels, was recognized when Akira Yoshino, a new member of the OIST Governing Board, was awarded the Nobel Prize. his work is developing a lithium-ion battery.

Traditionally, graphite is used in the anode of a lithium-ion battery, but this carbon material has major limitations.

“When the battery is charging, lithium ions are forced to move from one side of the battery – the cathode – through an electrolyte solution to the other side of the battery – the anode. Then, when a battery is used, the lithium ions return to the cathode and electric current is released from the battery.” Dr. Haro, former OIST researcher and first author of the research. “But at graphite anodes, six carbon atoms are needed to store a lithium ion, so the energy density of these batteries is low.”

Since science and industry are currently studying the use of lithium-ion batteries to power electric vehicles and aerospace vessels, improving energy density is key. Researchers are now looking for new materials that can increase the number of lithium ions stored in the anode.

One of the most promising candidates is silicon, which can bind lithium ions to each silicon. atom.

Phases of silicon film growth and mechanical strength

In the first phase, the silicon film structure is found as a rigid but turbulent column. In the second stage, the columns touch the top, forming a vaulted structure that is strong as a result of arch action. In the third stage, further deposition of silicon atoms results in a sponge-like structure. The red lines show how silicon is deformed as a force is applied. Credit: Scheme created by Dr. Panagiotis Grammatikopoulos, OIST Nanoparticles Design Unit and Particle Technology Laboratory, ETH Zürich

“Silicon anodes can store ten times the charge at a certain volume than graphite anodes – the order of magnitude is much higher in terms of energy density,” Dr. Haro said. “The problem is that as lithium ions enter the anode, there is a tremendous change in volume, about 400%, which causes the electrodes to break and break.”

The high-volume change has stabilized the formation of a protective layer between the electrolyte and the anode. Each time the battery is charged, this layer must be constantly reformed, using a limited supply of lithium ions and reducing battery life and rechargeability.

“Our goal was to try to create a stronger anode that is able to withstand these stresses, to ensure as much lithium as possible and to ensure as many charge cycles as possible before it deteriorates,” said Dr. Grammatikopoulos. “And the approach we took was to build a structure using nanoparticles.”

In a previous article, published in 2017 Advanced Science, the now dismantled OIST Nanoparticles Design Unit developed a cake-like layer structure where each silicon layer was positioned between tantalum metal nanoparticles. This improved the structural integrity of the silicon anode, preventing excessive swelling.

To see how the elastic properties of the material were being experimented with with different thicknesses of the silicon layer, the researchers discovered something strange.

“There was a point at a specific thickness of the silicon layer, the elastic properties of the structure were completely changed,” said Theo Bouloumis, a current PhD student at OIST who was conducting this experiment. “The material gradually stiffened, but then the stiffness quickly decreased as the thickness of the silicon layer increased further. We had some ideas, but at the time, we didn’t know why that change was the main reason. “

Now, this new article finally sheds light on the sudden rise in stiffness to a critical thickness.

Through microscopy techniques and computer simulations at the atomic level, the researchers showed that as silicon atoms accumulate in the nanoparticle layer, they do not form a uniform, uniform film. Instead, they form columns in the shape of inverted cones as more and more expanding silicon atoms accumulate. Eventually, the individual silicon columns touch each other, forming a vaulted structure.

“The vault structure is strong, as is a strong arch in civil engineering,” Dr. Grammatikopoulos said. “The same concept applies, only at the nanoscale.”

It is important to note that the greater strength of the structure also coincided with the better performance of the battery. When scientists performed electrochemical tests, they found that the lithium-ion battery had a higher charge capacity. The protective layer was also more stable, as the battery could withstand more charge cycles.

These improvements are only visible at the exact moment the columns are touched. Prior to this moment, the individual columns are curved and therefore cannot give the anode structural integrity. And if the silicon deposition continues after touching the column, it creates a porous film with many gaps, resulting in a weak sponge-like behavior.

This shows how it achieves its vaulted structure and its unique properties as an important step forward in the commercialization of silicon anodes in lithium-ion batteries, as well as many other potential applications within the material sciences.

Dr. Grammatikopoulos said that “strong vault structures and materials capable of withstanding various stresses can be used when needed, such as for bioimplants or to store hydrogen.” “The exact type of material you need – stronger or softer, more flexible or less flexible – can be made exactly by just changing the thickness of the layer. That’s the beauty of nanostructures.”

Reference: February 5, 2021, Communication materials.
DOI: 10.1038 / s43246-021-00119-0

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