Finding ways to increase battery capacity has been a prior task of many research teams around the world. This is especially the case with paper based Li-ion batteries, due to their flexibility and relatively low cost. Now a team from Arizona State University have discovered that by folding these batteries, the energy density and capacity is increased 14 times.
The study published in the latest issue of Nano Letters with a lead author Qian Cheng, reveals how folding batteries can be used to power future folding electronic devices, or provide energy supply in limited spaces.
The batteries that the team tested consist of current collectors made of carbon nanotube ink. As paper substrates, the scientists used porous KimwipesTM, while the electrodes were made of conventional lithium-based powders. The new addition was a coating made of polyvinylidene difluoride (PVDF), which was applied in order to straighten the attraction between the ink and the paper substrate. The team measured the battery conductivity and stable capacity, before experimenting with different folding.
To establish the performance characteristics after folding, the team used areal density, which is essentially the energy density per foot print area. They found that the more complicated the folding, and the higher the number of folds, the greater the areal energy density capacity. In this sense, the highest energy density and capacity for the smallest total area, were measured when the battery is folded in a Miura-ori pattern into a stack of 25 layers.
Folding the batteries by using simple patterns did not improve the performance greatly in comparison with planar batteries. The team noted that the Coulombic efficiency of the folded batteries was higher, which could be due to the better contact between the electrodes and the ink. Folding the batteries in the Miura-ori pattern, resulted in a lower discharge capacity, which the researchers explain with the delamination at the intersections of perpendicular folds.
The team is very proud of their findings, as these reveal great potential for future mass production of high areal energy density and capacity Li-ion batteries, which can power small or folding electronic devices. The researchers are now testing different and more complex folding patterns, which could open up the scope of using paper-based energy storage for new and exciting applications.