BATTERY ANODES Silicon-based vs. carbon-based battery anodes
While lithium-ion batteries have long since used graphite as an anode material, its lack of density is a problem for next-gen high energy applications like electric vehicles. One potential replacement material is silicon, and significant research efforts are underway to commercialize so-called lithium-silicon batteries.
Powerful and portable lithium-ion (Li-ion) batteries will be a crucial building block of modern technology, especially in next-gen applications like electric vehicles.
While Li-ion batteries have been around for years and can be found in everything from your bedside clock to your smartphone and beyond, we have only in recent years truly began to realize their potential to revolutionize how we might consume power in the future.
This is perhaps best illustrated by the fact that one of the original scientists central to the development of the Li-ion battery, Dr Akira Yoshino, was co-awarded a Nobel Prize just two years ago.
Graphite in Li-ion batteries
Traditionally, graphite (a crystalline form of carbon) is used to build the anode of a Li-ion battery. This is because graphite can reversibly place lithium ions between its many layers. This reversible electrochemical capability can be maintained by the material over several thousands of cycles in batteries with optimized electrodes.
But there are major limitations, however. When a battery is being charged, lithium ions 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 and discharged, the lithium ions move back into the cathode from the anode and an electrical current is released by the battery.
In graphite anodes, however, six atoms of carbon are required to store a single lithium ion. The resulting energy density of these batteries is therefore inherently low. But with science and industry currently exploring the use of Li-ion batteries to power high-energy applications such as renewable energy storage, electric vehicles, and electric aircraft, improving energy density of Li-ion batteries is of high importance.
Silicon to the rescue?
As a result, researchers are looking for new materials that could replace graphite and increase the nimbler of lithium ions stored in the anode. One of the most promising materials is silicon.
While the first laboratory experiments involving lithium-silicon materials took place in the 1970s, there has been much research progress in this field of battery research in recent years, with the term “lithium-silicon battery” being coined and subsequently by many to identify lithium-ion batteries with a silicon anode as a subclass of Li-ion battery technology.
Silicon can bind four lithium ions per silicon atom. This means that silicon-based Li-ion battery anodes could store ten times as much charge in each volume than graphite anodes.
Silicon-based materials also generally have a much larger specific capacity. For instance, pristine silicon has a capacity of 3600 mAh/g while graphite is limited to a maximum theoretical capacity of 372 mAh/g.
While there are some challenges standing in the way of the commercialization of these anodes, namely the fact that silicon’s large volume change and high reactivity can cause the battery to break, significant research efforts are underway to overcome these. There are also research teams investigating the possibility of inserting small amounts of silicon into graphite anodes to provide a slight performance boost, but information surrounding these is scarce.
Given the huge amount of research interest surrounding silicon as a potential replacement for graphite in Li-ion battery anodes, it’s likely that we will see further promising developments in lithium-silicon batteries very soon.