Over the past three years, the idea of using silicon in lithium-ion batteries to greatly increase energy storage capacity has been an on-again, off-again proposition. But some new announcements by different groups of researchers working on silicon-based lithium-ion batteries indicate that silicon may well be a viable charge-holding battery material.
If so, electric cars with a thousand miles of range are in striking distance.
Today’s lithium ion batteries use graphite anodes (in general terms, the anode is the part of the battery that holds the charge). Graphite is cheap and durable, meaning that batteries can be expected to last many thousands of charge cycles before breaking. But graphite is also not the best material for holding a charge — its relatively large size and structure mean that it takes a long time to charge and it doesn’t have many spots for the charge to “grab onto.”
If that graphite could be replaced with silicon, your average lithium-ion battery could theoretically hold 10 times the charge of a graphite-based one and charge much faster. The problem: silicon is very brittle when compared to graphite and the constant charging and discharging inside a battery quickly cracks the silicon into a pulverized mess — ultimately making the battery useless. Also, unless a corresponding cathode material can be found that holds just as much of a charge as the anode, that extra capacity will go unused.
Back in 2007, a group of researchers at Stanford headed by professor Yi Cui, showed that by using “silicon nanowires” they could make the silicon anodes more durable; since that time we’ve heard little from them. But now, newly published research shows the group has been hard at work. They’re still using the silicon nanowires, and they’ve also found a high density cathode. Not only does their lithium sulfide cathode store a huge amount of discharged energy, it also greatly reduces the potential for the battery to explode — a problem that plagues today’s lithium-ion batteries.
Although Yi Cui’s group has now found a sufficient cathode, their battery still doesn’t last more than 40-50 charge/discharge cycles — clearly not enough to be used in anything but an experimental setting. Cui says his group thinks they have a solution to that problem that involves potentially putting additives in the battery that stop the formation of polysulfides, which can reduce the batteriy’s efficiency.
Meanwhile, another research group at the Georgia Institute of Technology has announced a low-cost method to make nanocomposite silicon anodes that self assemble and are extremely resistant to wear and tear. Gleb Yushin, an assistant professor in the School of Materials Science and Engineering at the Georgia Institute of Technology, said that because their silicon anode builds itself from the bottom up, it avoids many of the problems associated with previous silicon anodes by fine-tuning its own properties.
So far, the researchers have tested the material through more than a hundred charge/discharge cycles, but Yushin has not seen any degradation in the material as of yet and believes it is durable enough to last the thousands of cycles needed to be viable for vehicle use. One of the major benefits of their process is that it was designed to be easily scalable and can be implemented on an assembly line style operation so that it could potentially just be inserted in current battery manufacturing operations without much retooling. Currently they’re only able to make the self-assembled carbon-silicon nanocomposite perform 5X better than a graphite anode, but hey, that’s pretty damn good in and of itself.
To me, it sounds like these two groups of researchers need to get together and combine their two processes into an ultimate silicon nanocomposite lithium-sulfide battery. Let’s get on with it, no?
Sources: As listed in the text
Image Credits: Gleb Yushin