Why some battery materials expand without cracking?
It is a rule of nature that metals, when heated, expand and when cooled, they contract. Frequent changes in temperature can cause this expansion and contraction, which can potentially lead to their cracking. But it is really surprising how electrode materials present in battery sustain these cycles of expansion and contraction without any signs of cracking.
Mechanism of charging of Battery
When a battery is charged, the ions carrying electrical charge travel from one electrode to the other. When the battery starts charging, ions or electrons move from the cathode to the anode. This process is responsible for the expansion and contraction of the electrodes.
Why batteries do not crack?
Lithium-ion batteries regularly expand and shrink when heated and cooled, respectively, during charging and discharging. If the material fails to accommodate this particular strain, the battery may fail to function. However, it is the molecular composition of some electrodes that primarily protect them from cracking in spite of expansion and contraction.
The electrodes are generally made of crystalline materials. The atoms present in these materials are always arranged very neatly in consecutive arrays. While being charged or discharged, the arrangement of the array is disturbed and the atoms take a disordered shape of a glass to withstand the changes occurred in dimension. This helps them to take the stress while expanding and contracting and they remain intact without cracking.
These research findings have been made possible by a team from Rice University, MIT, the University of Southern Denmark, and Argonne National Laboratory. They have found out this secret in the material of the electrodes and these new findings can open up new ways of creating high capacity batteries with longer lives.
The team dealt with sodium ion batteries for their research and used sodium-iron-phosphate (NaFePO4) and phospho-olivines as potential electrodes or battery cathodes. They found that the expansion can be gradual or rapid depending upon the composition of the electrodes. For instance, the change comes very rapidly for the sodium-iron-phosphate electrodes and it changes the volume drastically by 17%.
Usually, under normal theoretical conditions, soft materials would crack with even 1% change in volume. But in this case, the atoms change their neat order and assume the disorderly shape of a glass to adapt to the volume changes. This is precisely the reason why some battery materials do not crack in spite of regular cycles of heating and cooling.
These findings from the team have opened up new potentials for creating applications of electrode materials where the volume changing property can be put to use. The olivine compounds can also be used in various compositions to devise new ways of creating crystalline materials that have this wonderful property of adapting to volume change.
This may lead to creation of glassy battery materials in future. It may also lead to the use of such materials for batteries that were previously thought to be incapable of handling volume change during the process of charging. Therefore, new generation batteries may have greater capacities to function for long durations while perfectly withstanding the volume change brought about by expansion and contraction.
Ref: Battery Cracking