Will High-Speed Capacitors Replace Batteries?

The deliciously sci-fi sounding “ultracapacitor” has been getting buzz lately as a possible replacement for the batteries we use in everything from smartphones to city buses. The benefits of ultracapacitors seem like a dream: they can provide a charge much, much faster than lithium-ion batteries, and Tesla CEO Elon Musk has even said that he thinks ultracapacitors will be the future of his electric car business. But let’s start at the beginning: what even is an ultracapacitor?

Ultracapacitors, like batteries, are devices for storing and outputting energy. But that’s pretty much where the similarities end. “Unlike batteries, which produce and store energy by means of a chemical reaction, ultracapacitors store energy in an electric field,” says Dr. Kimberly McGrath, director of business development for Maxwell Technologies, a company that has lately positioned itself at the forefront of capacitor technology. A typical lithium-ion battery, like the one in your smartphone, has a layer of lithium metal oxide and a layer of (usually) graphite, separated by a layer called an electrolyte. The lithium layer and graphite layer pass ions back and forth between each other, through the electrolyte layer. When you charge a battery, the lithium layer lets loose with positively charged ions, which move through the electrolyte layer and are stored in the graphite layer. When you discharge (meaning, when you use the battery as an energy source, like when you use your smartphone), the process reverses, and the now-negatively charged ions move backwards from the graphite layer to the lithium layer. The movement of these ions provides the energy for your device.

But this isn’t a perfect system. The movement of all those ions isn’t totally efficient — you lose ions while doing it, which reduces the strength of your battery. That’s why, after a few hundred or thousand recharges, the battery in your phone or laptop doesn’t seem to last quite as long as it did at first.

A capacitor (we’ll get to the “ultra” part in a second) doesn’t have a chemical reaction at all. Instead, it has two metal plates, usually made of aluminum, separated, again, by a layer, this one called an insulator. One of the plates is positively charged, and the other is negatively charged. When you apply voltage to the capacitor, by, say, plugging it into an outlet, positive and negative ions stick, like gum on a wall, to their respective plates. These ions want to cross over the insulator layer; they want to balance each other out, to form an equal number of positive and negative ions on both sides. But the insulator layer prevents them from doing that, which creates an electrical field — think of static electricity, such as what you get when you rub a balloon on your head. That electrical field is stored in the capacitor until you want to use it for energy, in which case you simply crack open the insulator and all the ions fly together, like they’ve been dying to do since you charged them up.

Ultracapacitors are far smaller than regular capacitors; it turns out when you decrease the size of a magnetic field, you also increase its potency. And there are some other tricks involved, like a spongy layer on the capacitor’s plates that give the ions some more nooks and crannies to hide out on.

The benefit seems pretty clear: instead of slowly drip-drip-dripping ions through an electrolyte layer, as in batteries, a capacitor allows you to let out all the ions in a sudden gush. That magnetic field “enables ultracapacitors to charge and discharge in as little as fractions of a second,” says McGrath, as well as operating in a huge range of temperatures (unlike batteries, which have trouble in extreme cold or heat), and can handle many more charge/recharge cycles. Seems perfect, right?

But the big problem with ultracapacitors is the amount of storage they offer. “While ultracapacitors excel at power delivery, they store a relatively small amount of energy,” says McGrath. GigaOm, in an excellent explainer, compares an ultracapacitor to a small bucket with a large spout and a battery to a large bucket with a small spout. Ultracapacitors may be fast, but they don’t have much in the way of endurance.

The goal, for electronics companies tasked with creating our tech future, is not really to replace batteries with ultracapacitors. Instead, it’s to use them together. The ideal use case, one already in use, is in electric buses. Buses, like some electric cars, get energy back when they brake. Because buses brake so often, this can be a major source of energy for them. But batteries are slow to charge. “Ultracapacitors' rapid charge/discharge characteristics uniquely enable them to capture and store more energy during each braking event than battery-based systems, which have limited ability to absorb energy in the few seconds required to stop a vehicle,” says McGrath. So when a bus brakes, it can absorb a huge amount of energy via an ultracapacitor, which can then pass the energy to a battery, which can dispense it in the slower, longer way the bus needs to drive.

Ultracapacitors have the potential to radically change how we use electrical gadgets, but it’s not as simple as replacement, at least not yet. For some devices, it might mean having an ultracapacitor to handle peak output, like, say, a speaker system. When you need that huge punch of bass, an ultracapacitor would be better suited to the task than a slow-moving battery, but it couldn’t handle the day-to-day output of that system.

For smartphones, the ultracapacitor could be a huge benefit in terms of charging faster. Current charging systems are hamstrung by the slow charging capabilities of lithium-ion batteries: try to charge them too quickly, and they’ll fry. But what if you had an ultracapacitor in your phone to charge up quickly and feed the power to a battery at a speed the battery can handle? It could be the best of both worlds.

As for when that'll happen, well, nobody knows for sure. "Ultracapacitors are already in use in several applications," says McGrath, listing a few big-tech examples: green energy, transportation, and within the electrical grid. Given the constant demand for more power in small devices, it's an easy bet that we'll need energy solutions like ultracapacitors sooner rather than later. In only the past few years, we've seen devices require more intense battery power. For example, Apple increased the size of its iPhone considerably when it adopted the faster 4G LTE antenna. Why? Because the total device needed to be bigger, in order to house a larger battery to power that antenna.

More sensors and more power, whether it’s in faster processors, increased memory or new features like near field communication or heart rate monitors, will require more energy, and users won't want that to come at the expense of charging speed. Ultracapacitors could be coming soon to save the day.

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