Tuesday, 27 September 2016

Rechargeable batteries half the size, or double the charge

There's an interesting story here in all about circuits about new battery technology that could double the density. In a lot of applications, battery weight is critical. Remote control helicopters is one example, since the power inside the battery must be used to lift the battery itself. These first appeared in the mid 1990s with nickel-cadmium batteries, but later became popular as toys when lithium-ion polymer batteries provided sufficient current for the weight. The development in battery technology marches on, and now a solar-powered plane has flown around the world. It takes a while to find information about the batteries  on the Solar Impulse website, but without the four lithium polymer batteries, which make up a quarter of the plane's weight, it would have struggled to stay up through the nights, or get through any clouds on the way to or from the stratosphere. 

Although solar power is the biggest part of the solar impulse story in the media, the batteries may be more significant, making possible hydro-electric or nuclear-fusion powered flight. Where toys and explorers play, the industrial economy often follows, and I will not be surprised to see some commercial battery-powered flight in the next ten years. It will certainly start off expensive and short-haul, perhaps flying from airports in built-up areas where noise and pollution are bigger issues. Flexible solar panels may be used cosmetically, but the batteries will be the power source.

Anyway, one of the reasons why lithium is a more popular battery choice than nickel-cadmium is that it's lighter. Anyone who has seen the periodic table knows that lithium comes in at number three, right after hydrogen and helium. Electricity is basically stored in the outer electron of each atom, so the light weight of the lithium atom means more free electrons per weight. The first lithium batteries were made with mixtures of other metals, like iron or manganese. In lithium-ion batteries, lithium atoms freed from their out electrons float through the electrolyte from one side of the battery to the other. Once these were put in plastic cases, they started to be called lithium polymer batteries. In its more technical meaning, lithium-ion polymer batteries have a polymer electrolyte.

Image from of Business Wire.
As far as the travelling ions are concerned, batteries have an anode on one side and a cathode on the other. The first lithium batteries used the metal case as the cathode, and a large chunk of lithium as the anode. Lithium-ion batteries use graphite for the anodes. This new technology uses a thin film of lithium as the anode, which means it can hold extra charge with less weight.

These batteries are still using the ions to carry the charge rather than the electrons. Looking at the relative size of the electrons and ions, this is a bit like playing tennis where the players have to go back and forth over the net, rather than the ball. Battery technology is progressing, but still has a long way to go!

You can see an infographic of how other kinds of batteries work here.