Researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) uncovered a unique nanostructure that enhances anode performance in lithium-ion batteries (LIBs). The research study was published in the journal Communications Materials

 

A strong vaulted silicon nanostructure results at the precise moment that inverted comes touch. Image used courtesy of OIST 
A strong vaulted silicon nanostructure results at the precise moment that inverted comes touch. Image used courtesy of OIST 

 

LIBs are used globally in electric consumer wearables, electric vehicles (EVs), and are growing in use within aerospace and military sectors. Typically, the positive electrode of a LIB is made up of an intercalated lithium compound and the negative electrode or anode is made from graphite. When a battery is being charged, lithium ions move from the cathode (positive electrode) to the anode (negative electrode) through an electrolyte solution. This solution acts as a conductor. 

In a news release from earlier this month Dr. Marta Haro, a former researcher at OIST and first author of the study, explained that “when a battery is being used, the lithium ions move back into the cathode and an electric current is released from the battery.” Dr. Haro added: “But in graphite anodes, six atoms of carbon are needed to store one lithium-ion, so the energy density of these batteries is low.”

When it comes to energy density and improving it, it is a numbers game. Unlike carbon atoms, silicon atoms can hold more charge. For every one silicon atom, four lithium ions can be bound to them. This is one of the reasons why the OIST team chose to work with this material in their research.

Although silicon anodes can store much more charge than graphite anodes, the stability of these anodes is low. A large volume change of 400% results as lithium ions move into the anode. This causes the electrode to fracture and break. The volume change can also prevent the stable formation of a protective layer between the anode and the electrolyte. This means that the layer must continually reform itself every time the battery is charged. This exhausts the limited supply of lithium ions and consequently reduces the battery’s lifespan and rechargeability. 

In the same news release, senior author of the paper, Dr. Grammatikopoulos, said: “Our goal was to try and create a more robust anode capable of resisting these stresses, that can absorb as much lithium as possible and ensure as many charge cycles as possible before deteriorating.” Dr. Grammatikopoulos added: “And the approach we took was to build a structure using nanoparticles.”

 

A Vaulted Nanostructure

The OIST research team expanded upon experimentation conducted in an earlier study, published in the journal Advanced Science in 2017. In this earlier study, a multi-layered cake-like structure was developed at the now-disbanded OIST Nanoparticles by Design Unit. Dr. Haro and colleagues layered silicon in a way where it was placed between tantalum metal nanoparticles. This improved the silicon anode’s integrity and prevented over-swelling.

The team experimented with the silicon layer’s thickness to investigate any change in the material’s elastic properties and observed something they could not quite understand.

OIST Ph.D. student, Theo Bouloumis, was conducting the experiment and observed that there “ was a point at a specific thickness of the silicon layer where the elastic properties of the structure completely changed.” Bouloumis added: “The material became gradually stiffer, but then quickly decreased in stiffness when the thickness of the silicon layer was further increased.  We had some ideas, but at the time, we didn’t know the fundamental reason behind why this change occurred.”

 

Image captured from a video taken by the OIST researchers shows the growth of the silicon structure (inverted cone) as silicon atoms are deposited in the presence of nanoparticles. Image used courtesy of OIST 

 

In the recent study by the OIST team highlighted in this article, microscopy and computer simulations were run to reveal the formation of a vaulted structure as silicon atoms deposited onto a layer of nanoparticles. Column-like structures were formed in the shape of inverted cones. Strong vaulted structures resulted when the cones touched. The structural strength of this formation resulted in enhanced battery performance. Before the cones touch, the anode structure is wobbly and less stable. 

Electrochemical tests were run on the anode (with the vaulted structure), and this revealed increased charge capacity of the LIB and greater stability of the protective layer between the electrolyte and anode. The research team believes that these properties will bring the commercialization of silicon anodes one step closer to realization. 

“The vaulted structure could be used when materials are needed that are strong and able to withstand various stresses, such as for bio-implants or for storing hydrogen,” said Dr. Grammatikopoulos. “The exact type of material you need – stronger or softer, more flexible or less flexible – can be precisely made, simply by changing the thickness of the layer. That’s the beauty of nanostructures.”


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