New research conducted by OIST, Japan has identified a specific building block that
improves the anode in lithium-ion batteries.
The unique properties of the structure, which was built using nanoparticle technology, are
revealed and explained in the journal Communications 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 how we store and consume power in the future,
as we move away from fossil fuels, was notably recognized, with the Nobel Prize awarded for
the development of the lithium-ion battery.
Traditionally, graphite is used for the anode of a lithium-ion battery, but this carbon
material has major limitations.
When a battery is being charged, 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 being used, the lithium ions move back into the cathode and an electric
current is released from the battery.
But in graphite anodes, six atoms of carbon are needed to store one lithium ion, so the
energy density of these batteries is low.
With science and industry currently exploring the use of lithium-ion batteries to power
electric vehicles and aerospace craft, improving energy density is critical.
Researchers are now searching 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 four lithium ions for every
one silicon atom.
Silicon anodes can store ten times as much charge in a given volume than graphite anodes
– a whole order of magnitude higher in terms of energy density.
The problem is, as the lithium ions move into the anode, the volume change is huge, up to
around 400%, which causes the electrode to fracture and break.
The large volume change also prevents stable formation of a protective layer that lies
between the electrolyte and the anode.
Every time the battery is charged, this layer therefore must continually reform, using up
the limited supply of lithium ions and reducing the lifespan and rechargeability of the battery.
In a previous paper, published in 2017 in Advanced Science, a research team developed
a cake-like layered structure, where each layer of silicon was sandwiched between tantalum
metal nanoparticles.
This improved the structural integrity of the silicon anode, preventing over-swelling.
While experimenting with different thicknesses of the silicon layer to see how it affected
the material’s elastic properties, the researchers noticed something strange.
There was a point at a specific thickness of the silicon layer where the elastic properties
of the structure completely changed.
The material became gradually stiffer, but then quickly decreased in stiffness when the
thickness of the silicon layer was further increased.
The researchers had some ideas, but at the time, they didn’t know the fundamental reason
behind why this change occurred.
Now, this new paper finally provides an explanation for the sudden spike in stiffness at one critical
thickness.
Through microscopy techniques and computer simulations at the atomic level, the researchers
showed that as the silicon atoms are deposited onto the layer of nanoparticles, they don’t
form an even and uniform film.
Instead, they form columns in the shape of inverted cones, growing wider and wider as
more silicon atoms are deposited.
Eventually, the individual silicon columns touch each other, forming a vaulted structure.
The vaulted structure is strong, just like an arch is strong in civil engineering.
The same concept applies, just on a nanoscale.
Importantly, the increased strength of the structure also coincided with enhanced battery
performance.
When the scientists carried out electrochemical tests, they found that the lithium-ion battery
had an increased charge capacity.
The protective layer was also more stable, meaning the battery could withstand more charge
cycles.
These improvements are only seen at the precise moment that the columns touch.
Before this moment occurs, the individual pillars are wobbly and so cannot provide structural
integrity to the anode.
And if silicon deposition continues after the columns touch, it creates a porous film
with many voids, resulting in a weak, sponge-like behavior.
This reveal of the vaulted structure and how it gains its unique properties not only acts
as an important step forward towards the commercialization of silicon anodes in lithium-ion batteries,
but also has many other potential applications within material sciences.
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.