Breakthroughs in battery technology are relatively rare. The last one came in the 1990s, with the advent of rechargeable lithium-ion batteries. “Batteries are very complex objects,” said Michael Toney, a materials scientist at the SLAC National Accelerator Laboratory. “The complexity makes it very hard to get any sort of sustained improvement.”

The prospect of doubling an iPhone’s battery lifetime within the next five years is not unreasonable, Toney says, but anything more than that would require a dramatic development. He’s part of a team that observes batteries as they charge and discharge as a way to better predict how long they’ll last—and how battery technology might be improved.

In addition to looking for ways to increase the amount of time between charges, there’s also cycle life to consider—that is, how many times a battery can be charged and discharged completely until a battery starts to diminish, typically to 80 percent of its original capability. According to Apple, an iPhone can handle 500 charge cycles before this point, while MacBooks and iPads can complete about 1,000 cycles apiece.

Improving the batteries of the future will require optimizing the amount of energy stored in the battery, while also accounting for portability, weight, and volume. One way to make a more powerful battery that’s still relatively small would be to find new materials that can hold more ions by volume. (The conventional battery in a cell phone or laptop has a liquid electrolyte that shuttles lithium ions and electrons back and forth between the two materials as the battery is charged and discharged.) Silicon is one candidate to replace anodes that are today composed of graphite. “But we don’t just dump silicon into the battery and hope it works because it doesn’t work,” said Srinivasan. “Instead we introduce silicon bit by bit into existing anode material.” The thinking goes: Additions will slowly improve the battery’s energy density until researchers get to the point where there is significantly more silicon in the battery and less graphite in it.

On the cathode end, sulfur will most likely replace currently used metal oxides, such as cobalt oxide. Another area for improvement, according to Srinivasan, would involve using magnesium instead of lithium in a battery. Recent research shows magnesium can have double the charge-speed and can hold twice the amount of charge as lithium. (Aluminum is another likely contender as well because it can hold three times the amount of electrons as lithium.) But the use of these materials is still being explored in lab settings, and it will take a while before they are introduced into the market and then widely used.

Researchers are also exploring how they might add more power to batteries by squeezing out the components that do not store energy so that the active parts can be made bigger, increasing energy density and giving consumers a longer-lasting charge.