It is a pretty amazing time to be alive. We can video chat with friends halfway around the world while we wait in a queue for coffee. We can drive cars that don’t need fossil fuels. Instead of paper tickets, we use our phones to gain access to everything from airport departure terminals to concert halls.
However, many of us can’t get through a day without charging our mobile phones and we desperately want electric vehicles to have more range and require shorter charge times.
For all the technological advances of the 21st century, the thing that constantly trips us up is batteries. While you can video conference from Costa coffee, you might not want to because of the battery draw. Electric cars are still limited in range – few have more range than an equivalent petrol car. And many people have to monitor their phone’s battery usage over the course of an average day.
Batteries seemed always to have been the afterthought of the tech world, but that’s starting to change. Governments and corporations are putting resources into building bigger batteries that can store enough energy to power homes or even be an integral part of the grid. And there is an entire industry dedicated to ensuring that mobile devices last longer. The biggest advance in that field to date is a material that’s long been a critical player in semiconductors and solar power: Silicon.
Silicon, an abundant element found in the earth’s crust, is a semiconductor and a key component in the brains of basically every mobile electronic device. It has driven huge leaps in processing speed and today we now hold yesterday’s supercomputers in the palm of our hands. This rapid progress even has some experts believing that artificial intelligence will surpass human intelligence within the century. While this prediction, referred to as the technological singularity, is somewhat controversial, most experts agree that in the near future computers will be able to drive automobiles better than humans do. This will fundamentally change the way people are transported and how we look at cars. One additional thing that experts agree on is that devices and electric vehicles will continue to require lots of battery power. And on that front we haven’t made nearly as much progress, possibly because we haven’t really turned to silicon for help. Yet.
Scientists have known for a while that silicon could be a big factor in producing batteries that last longer. Lithium-ion batteries work in a “rocking chair” fashion, where energy is stored transferring lithium ions from one electrode to the other (typically from the positive to the negative). The negative is the electrode with a lower voltage and the difference in voltage between the two electrodes is the cell voltage. It’s at the negative electrode, or anode, where silicon can make a huge impact. Silicon can store more than 10 times as much energy as graphite, which is the material most commonly used today to store energy on the anode in lithium-ion batteries. Using silicon could increase the energy density of batteries by 25%.
So why hasn’t it been used sooner? It turns out that silicon’s greatest asset – its capacity to store energy – is also its greatest weakness.
Upon lithiation (the incorporation of lithium into an electrode in a lithium-ion battery), silicon suffers from severe volume expansion, which can approach 400%. In other words, the more energy a silicon particle stores, the bigger it gets and when that energy is used up, the particle shrinks again. The constant swelling and shrinking causes the particle to crack or pulverise. Over time, the cracks cause new surfaces to be exposed, which react and trigger battery performance degradation. Within a few hundred cycles of recharging and discharging, the battery is finished.
“With silicon anodes, a nice passivation layer is formed on the particles,” Dee Strand, chief scientific officer at Wildcat Discovery Technologies, explains. “But as the silicon expands and contracts, it essentially cracks apart that layer and then makes more. Over time it ends up with a very thick resistive film on the anode, which causes it to lose both capacity and power.”
Typical battery developers have used only very small amounts of silicon to avoid this issue. “It’s a race among battery makers to get more and more silicon in,” Jeff Dahn, lithium-ion battery researcher at Dalhousie University in Nova Scotia, says. ‘The number of researchers around the world working on silicon for lithium-ion cells is mind-boggling.”
We’re starting to see some solutions for silicon anodes. This is an industry where the active material (the material that stores lithium) has traditionally been in the form of a powder. Some companies are therefore working on making powders out of proprietary mixtures of active and inactive materials. Others are trying to shape silicon in different ways to enable the silicon to survive the volumetric changes that decrease battery lifespan. The active material is then typically mixed with an inactive polymer and sometimes other inactive additives to be coated on a metal foil current collector. These steps are an evolution, not a revolution.
Unlike these approaches, companies like Enevate, has developed a silicon technology for lithium-ion batteries that allows us to form an active material film without needing a polymer material to hold the film together. Instead of trying to glue the silicon together, they have made a harder film where silicon is stored within a porous conductive matrix. In a way, “balloons” of silicon have been put into a “concrete” matrix, so that the silicon can expand and contract repeatedly. And, more importantly, not crumble.
Unlike regular batteries that might contain a little silicon, cells like Enevate’s that are silicon-dominant have already begun testing by third parties and have shown improved features, such as high energy density, ultrafast charge, wide temperature tolerance and improved safety. Furthermore, the use of silicon in batteries is thought to be able to reduce the cost of batteries.
Inevitably, if the industry adopts silicon lithium-ion batteries, we’re going to see a revolution. Electric cars will become a road-tripper’s first choice. We won’t need to track mobile phone battery usage all the time. Solar power will expand as long-lasting batteries store and provide energy even after the sun stops shining.
The global silicon-anode battery market has been forecast to grow 43.4% over the next six years, which would boost its value to more than £1 billion, by 2022. These next-generation batteries will literally have the power to untether people from plug sockets, just as WiFi and smartphones untethered us from computers.