US boffins are overturning the understanding of how Li-ion batteries charge, by watching the behaviour of individual molecules as they absorb charges.
It's long been assumed that a battery charges fairly uniformly – in other words, that electrons are distributed evenly across the charge-carrying material as the battery charges. However, the work led by Stanford's Institute for Materials and Energy Sciences at the US Department of Energy's (DOE's) SLAC Accelerator Laboratory says this isn't the case.
As the SLAC explains , charge and discharge behaviours are vastly different at the nano-scale.
When charging, regardless of the charging rate, only a small percentage of the nanoparticles in the batteries tested actually absorbed ions. Those ions would then pass their charge onto others, and absorb new ions.
Under discharge, how the particles behave depends on whether the battery is being discharged fast or slowly. The researchers find that when a battery is being discharged quickly, the process is relatively uniform, but under a slow discharge, only a few nanoparticles are discharging at any given time, as shown in the video below.
To get their understanding of the inside of a battery during charge and discharge, the researchers had to pull apart individual coin cell batteries at different stages of the cycle, slice up the bits, and examine them under the Berkeley Lab / DoE Advanced Light Source Synchrotron.
The hope is that by getting a better understanding of what's going on at the nano-scale, the SLAC research can help design better batteries.
In particular, if electrodes can be designed so that charge is always evenly distributed, it might be possible to create batteries that can always be fast-charged without exhibiting the heat-stress damage that's such a problem in the world of Li-ion batteries.
The next round of research will be to try and examine batteries intact while they charge and discharge.
The work has been published in Nature Materials (abstract here). ®