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The Limits of Your Battery Explained

It was the great terror of hanging on to my cellphone well past its expired warranty. With no warning at all, the phone would inexplicably flash that it had one percent battery left, leading to a mad dash to my destination or to make a call before the phone inevitably died.

But for all our griping, there’s a reason the lithium-ion batteries in our phones are also found in just about every laptop, electric vehicle, and most anything else portable. (For more on how batteries work, check out the first piece in this series, “Why Batteries Die.”)

“You can get very good energy density from a lithium-ion battery, more so than pretty much anything else that’s on the market,” says Greg Less, manager of the University of Michigan’s Battery Lab. High energy density means the battery can store a lot of energy in a relatively small space. That’s useful for enabling our gadgets to be slim and compact and our electric cars to be as light as possible.

But phone and laptop design doesn’t sit still, and these rechargeable batteries must keep pace with constantly upgraded devices that have ever-increasing power requirements. And that’s to say nothing of users who want to be able to drive farther or talk longer on a charge.

“How much energy a battery holds—its energy density—depends upon the material choice,” explains Venkat Srinivasan, a researcher at Argonne National Laboratory. “There are two materials, the cathode and anode, with an electrolyte between them. These dictate how much energy it stores.”

The cathode and anode are the two electrodes inside the battery that the lithium ions flow between, charging or discharging depending on their direction. The cathode, or positive electrode, is made from a compound of lithium, various metals, and oxygen. For about a decade, the preferred cathode design has been lithium nickel manganese cobalt oxide, though other, somewhat simpler metal oxide compounds are also used.

The best material for the anode, or negative electrode, has been graphite — the same carbon mineral found in pencils — though in the past decade researchers have also experimented with an alloy of tin and cobalt and, more recently, a mix of carbon and silicon. The latter is a good example of the challenges involved in trying to increase energy density. Silicon can theoretically hold far more lithium ions and, by extension, hold more electrical charge, but such anodes have had trouble taking in lithium without their volumes expanding past the battery’s breaking point.

When looking at the present state of battery research, it’s tempting to say that lithium-ion technology is already close to optimized and that any real improvements to batteries will come with figuring out a whole new chemistry that would make today’s batteries obsolete. But that’s only half true, according to Less. “Right now, we’re probably at or approaching a fundamental science limit, but there’s a lot of engineering space left.”

A simple example of that is the one part of the battery’s design we haven’t mentioned yet: The anode and cathode can never touch, because bringing two electrodes together can cause an explosive reaction. To prevent this, batteries include a separator through which only the lithium ions can pass. Making the separator thinner can free up more space for energy storage, but the trade-off is an increased risk of the battery exploding.

Because lithium-ion batteries are generally more volatile than other battery types, safety is something designers have to take seriously. But improved understanding of the chemical environment within the battery can allow engineers to scale back redundant safety features.

Less points to the example of the Chevy Volt, General Motors’ big entrance into the electric vehicle market. “GM massively overengineered the cooling for the battery to make sure it met its warranty specifications,” he says. “If you don’t have too much cooling, not only would it be less expensive, but you’d also have less parasitic mass, so you can put in more material, better performance.”

That term “parasitic mass” refers to any part of the battery not specifically geared toward energy storage. In theory, the ideal battery would have no such parasitic mass, but things like internal sensors play vital roles in assuring the battery’s safety and measuring its performance.

Our ability to measure how batteries perform remains far more basic than you might think, given their ubiquity. Maybe my phone’s sudden drop to the brink of death wasn’t so unusual after all.

“How you predict the lifetime or how much charge is in the battery is pretty crude now, because there are only a few things you can measure,” says Michael Toney, a battery researcher at the SLAC National Laboratory in California. “You can measure voltage, the temperature, and that’s kind of about it. You can measure how much current is flowing instantaneously. Those predictors are not that good for figuring out how much charge is left.”

Without that ability to accurately measure the remaining charge, designers often can only guess at how to get peak performance from a battery. Developing better hardware that can be put inside the battery is one possible solution, though that too has its problems.

“From a hardware standpoint, it’s very hard to put small probes in a battery and have them last a long time, because the environment is very chemically aggressive,” says Toney. Another problem, he says, is that you’re taking up space that could be used to derive energy for the powered device.

The final possibility to make batteries work better and longer lies in designing software to better manage the battery, conserving each charge cycle and extending its overall lifespan.

“Companies have gotten really good at turning off processors they don’t need, turning on the ones they want, and going into sleep mode,” says Srinivasan. Making more energy-efficient chips inside our phones is part of the puzzle. “How could I optimize my chipsets to take the battery energy only when I need it? That’s happening for sure.”

But the basic challenge of battery design always goes back to the chemistry. It’s not just a question of picking the right materials, but also of grappling with the fact that no matter how you design a lithium-ion battery, it means placing a volatile, uncertain chemical universe inside your phone, laptop, or car.

That uncertainty requires the careful design of safety features inside the battery, but it also means lithium-ion batteries still guard some of their secrets. Researchers know enough to feel fairly confident that there’s not a ton that can be done to improve the overall structure of lithium-ion batteries, but there could still be plenty left to do on the margins. It’s still an open question of how clearly we can see what those margins are.

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