How much energy can a battery hold? If your smartphone struggles to make it through the day, you may have wondered why we can’t pack even more juice into a palm-sized device. Our expert M. Rosa Palacín looks at the limits of power. Energy © Przemek Klos, Shutterstock “There is no absolute answer to this, it depends on the materials you use,” says Palacín. “But the energy in a battery cannot increase to infinity – there are physical limitations.” Palacín says there are no established standards over how to estimate the theoretical maximum energy content of a battery, which depends on a host of competing factors. The most obvious constraint is set by the chemistry involved. Batteries are typically made of two dissimilar materials connected by an electrolyte medium. As electrons flow from one end of the circuit to the other, ions travel through the electrolyte in the opposite direction, allowing the process to continue. In the 1990s, many consumer electronic devices relied on bulky nickel-cadmium batteries. The toxic cadmium was then replaced with alloys to create nickel-metal hydride batteries, which offered better energy density. Nowadays, smartphones and other portable devices typically use lithium in their batteries, which can hold even more energy for the same given volume. Just how much power you can pack into a battery also depends on the physical design of the battery itself, which is shaped by the desired application. Speaking of desired applications, what’s stopping us making a phone battery that lasts a week? “We could with the first generation of mobile phones, which had very small black and white screens,” Palacín recalls. But modern smartphones face a slew of challenges, including large, energy-hungry colour screens, slim battery compartments, and the need to be connected to the internet at all times. Thus despite being bigger and more evolved technology, modern phone batteries last a fraction of the time. “There is always a trade-off, the performance of lithium-ion (Li-ion) batteries has improved, but devices are always requesting more energy,” explains Palacín. Future power A research professor at the Institute of Materials Science of Barcelona, Spain, Palacín is working on a new generation of batteries based on calcium. “One of the issues with lithium metal is that repeated charging cycles lead to the growth of dendrites, small protrusions, on the negative electrode. This eventually may create a short circuit and can result in the battery exploding.” For this reason, the most common lithium batteries use a graphite electrode, reducing their energy density. Replacing the lithium with calcium could solve this problem, as well as offering higher energy densities, as metal electrodes could be used. Every ion of calcium transported across the battery also moves two electrons, compared to a Li-ion’s one. Finally, calcium batteries make use of a metal that is cheaper and far more abundant than lithium. Is there anything that can dethrone lithium when it comes to portable power? Sodium has a similar chemistry to lithium, so that the know-how learnt from Li-ion technology can be imported and is also very abundant. “But there is no metal as good as lithium,” adds Palacín. “One could imagine a fluorine-lithium battery, as fluorine is very reactive. We can speculate about the energy capacity of that, but fluorine is gas, and is corrosive… so how would it work, exactly?” Packing even more joules into each kilogram of battery will likely mean a revolution in technology, namely a move to ‘solid-state’ batteries which don’t have a liquid electrolyte and where lithium metal could possibly be used as an electrode. That is easier said than done, and currently they only exist as prototypes. “As you can imagine, moving ions through a solid is not as easy as a liquid,” says Palacín. Nonetheless, solid-state batteries will offer a range of benefits, such as much higher energy densities, and much less chance of bursting into flames. They might even keep your smartphone running until the end of the day. Click here to find out more about M. Rosa Palacín’s research: Building a better battery Keywords Ask, expert, EU, research, battery, calcium, lithium, ion, smartphone, energy
