LI-ION BATTERY New silicon anode could prolong Li-ion battery life

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The importance of lithium-ion batteries cannot be understated, and their growing role in the constantly evolving electronics market means that researchers are constantly looking for ways to make them perform better, last longer, and work more efficiently.

Now, a research team from the Okinawa Institute of Science and Technology (OIST) has reported findings relating to a new “building block” that could drastically improve the anode of your typical lithium-ion (Li-ion) battery.

A silicon-based anode

According to a press release published on the OIST website, the research team has used a silicon-based nanostructure as a replacement for the traditional graphite-based anode. To understand why this is such a potentially groundbreaking innovation, we must first understand how Li-ion batteries work.

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During the charging process in a Li-ion battery, lithium ions travel from the cathode of the battery to the anode through an electrolyte solution that separates the two sides. During normal operation when the battery is discharging, these ions move in the opposite direction from the anode to the cathode back through the electrolyte solution.

To make these processes more efficient, the anode needs to be able to store as many lithium ions as possible. The more ions that are stored, the higher a Li-ion battery’s density and the longer it lasts between charging cycles. However, graphite is not the ideal material for this because it requires six carbon atoms to store a single lithium ion. With silicon, a single atom can store four lithium ions, which is a huge increase in potential energy density.

Structural integrity problems

While this may make it sound as if the switch from graphite to silicon is an obvious one - and begs the question of why it hasn’t already been done - there are major structural integrity issues that have stopped researchers in their tracks.

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When silicon is used as an anode and lithium ions begin to move into it, a huge volume change takes place. According to Dr. Marta Haro, a former researcher of OIST, this change can be as high as 400 percent and can cause the silicon anode to fracture and fray, degrading battery performance repeatedly over charge cycles and hampering the battery’s lifespan.

To get around this problem, the researchers looked at a previous OIST research paper - which utilized a “cake-like” layered structure of silicon sandwiched between tantalum metal nanoparticles - with the goal of building upon it.

The researchers found that when they made the silicon layer thicker, the material would grow gradually stiffer. This was expected. What wasn’t expected, however, was that at a specific point, the silicon would begin to decrease in stiffness while it continued to grow thicker.

This, they discovered, is down to the formation of columns of “inverted cones” that grow as more and more silicon atoms are deposited onto the nanoparticle layer. These cones then grow to touch one another, forming a “vault” structure, much like an archway. If silicon atoms continue to be deposited once this archway is formed, however, pores will form within the structure and become very weak. Thus, there is a precise moment when the structure of the silicon anode is maximized in terms of stability.

Importantly, the thickness that leads to maximum stability also gives the battery the best electrochemical characteristics. This, the researchers note, means that their silicon nanostructure solves all the problems that were previously faced with using a silicon anode while simultaneously providing all of the benefits.

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