If there is one thing stalling electric car development, preventing renewable energy sources from plugging into electricity grids and causing grief to smartphone users, it is poorly performing batteries. The first high-resolution video to show exactly how battery electrodes twist and distort during charging could be an important step towards designing the batteries that today's technologists are waiting for.
A US-Chinese team of nanotechnologists used a specially outfitted transmission electron microscope to capture the footage, allowing the effect of electrical charging on nanostructures to be seen in action for the first time.
For the experiment, the team built a tiny battery with a lithium-cobalt anode and a cathode made from tin oxide nanowires just 200 nanometres wide. Tin oxide is not much use in commercial batteries because it can only take a single charging, but in that charging it experiences dramatic effects. This makes it an excellent material for studying the subtle battery fatigue experienced by more commercially useful materials.
In the time-lapse video above, taken as the battery is charged by electrons pumping into the tin oxide, lithium ions can be seen streaming though the hollow nanowire to pair with the electrons. As they do, the nanowire contorts, lengthening by 90 per cent and swelling to two-and-a-half times its former volume. Further microscopy of the nanowire showed that the structure of the tin oxide atoms had changed from an ordered crystalline arrangement to an amorphous glass-like form. However, nanowires were able to withstand the effects of charging better than bulk tin oxide, a finding that may influence the design of future batteries.
Electronics manufacturers have long known that repeated charging cycles damage a battery's electrodes, but the findings shed light on how this wear and tear accumulates at the nano scale. "Cyclic charge and discharge of a battery induces structural evolution of the active materials, and such a structural evolution is attributed to the failure of the battery," says team member Chongmin Wang at the Pacific Northwest National Laboratory in Richland, Washington. "What we have done is to pioneer a way to observe how this structural evolution may occur."
Wang told New Scientist that the same method could be used to study commercial batteries. The team plans to create fully functioning nano-sized rechargeable batteries to observe the effect of repeated charging cycles.