LI-ION BATTERIES Researchers discover nanostructure for improving Li-ion
Lithium-ion batteries are used globally in everything from basic consumer electronics to autonomous vehicles and aerospace and military applications. Now, researchers discovered a nanostructure, which enhances battery performance.
In most Lithium-ion (Li-ion) batteries, the positive electrode is made from an intercalated lithium compound and the negative electrode is made from graphite. During charging cycles, lithium ions move from the positive electrode (the cathode) to the negative electrode (the anode) through an electrolyte solution which acts as a conductor.
While Li-ion batteries have enabled a great deal of technological progress, there’s substantial room for improvement. Scientists have been trying to improve Li-ion batteries and exploring potential replacements to them for decades, so as to ensure that future applications won’t be hindered by their drawbacks and limitations.
In a study published in the journal Communication Materials on February 5th, researchers from the Okinawa Institute of Science and Technology (OIST) say that they have discovered a new, unique nanostructure that delivers dramatic improvements in anode performance in Li-ion batteries.
A numbers game
In a press release, former OIST researcher and first author of the study Dr Marta Haro 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.” He added that in graphite anodes, however, six atoms are needed to store one lithium ion, meaning that the energy densities of these batteries are low. When it comes to increasing energy density, it’s very much a numbers game. Silicon atoms can hold more charge than carbon atoms. For every single silicon atom, four lithium ions can bind with them. This is why the OIST researchers chose to work with this material in their research.
There’s a catch, though. The stability of silicon-based anodes is much lower than carbon-based graphite anodes, and the four-fold increase in volume causes the electrode to fracture and break. This volume increase can also prevent the formation of a protective layer between the anode and the electrolyte, reducing the battery’s lifespan.
The unique nanostructure
In this study, the OIST researchers built upon previous work from 2017. In this earlier work, OIST researchers built a multi-layer structure where silicon was layered in a way where it was placed between tantalum metal nanoparticles. This, they found, improved the silicon anode’s integrity and stopped it from swelling up.
In the new study, the OIST team experimented with the silicon layer’s thickness to see if any change in the material’s elastic properties took place. However, they observed something else: One of the PhD students working on the study noticed that there “was a point at a specific thickness of the silicon layer where the elastic properties of the structure completely changed.” According to the researchers, the material became gradually stiffer but lost its stiffness when the thickness of the silicon layer increased. “We had some ideas, but at the time, we didn’t know the fundamental reason behind why this change occurred.”
This video shows that as silicon atoms are deposited in the presence of nanoparticles, columns grow in the shape of an inverted cone:
The researchers carried out microscopy and computer simulations to reveal the formation of a vaulted structure as silicon atoms deposited onto a nanoparticle layer. Column-like structures were formed as inverted cones, and strong vaulted structures formed when these cones touched. The strength of this formation led to enhanced battery performance. Before the cones touched, the anode structure was much less stable.
Following electrochemical tests, increased charge capacity of the Li-ion battery was observed in addition to greater stability of the protective layer between the electrolyte and anode. According to the researchers, these properties will enable the commercialization of silicon anodes and bring us one step closer to more stable Li-ion batteries that are vital for meeting the demands of emerging applications.