Study shows paper-folding concepts can compact a Li-ion battery and increase its areal energy density
|Areal discharge capacities for Miura-folded versus unfolded cells. Credit: ACS, Cheng et al.Click to enlarge.|
Researchers at Arizona State University have shown that paper-folding concepts can be applied to Li-ion batteries in order to realize a device with higher areal energy densities. In a paper published in the ACS journal Nano Letters, the team reported that folded cells showed higher areal capacities compared to the planar versions with a 5 × 5 cell folded using the Miura-ori pattern displaying a 14× increase in areal energy density.
In their paper, they suggested that advances in geometric folding algorithms and computational tools to determine folding patterns for making complex 3D structures from planar 2D sheets may lead to numerous other configurations possible for 3D batteries. Furthermore, with advances in robot manipulation including paper folding by robots, the manufacturability of folded batteries at scale may be possible in the near future, they suggested.
Recently, there has been much interest in the development of electronic and energy storage devices using paper and textile components. The low cost, roll-to-roll fabrication methods, flexibility, and bendability of these substrates are attractive for high-performance devices....Specifically for energy storage and conversion applications, the ability for the power source to be intimately integrated to unconventional substrates has motivated research in paper-based flexible devices such as batteries, supercapacitors, nanogenerators, solar cells, and fuel cells.
...The use of paper as substrates for Li-ion battery electrodes creates a natural opportunity to exploit paper folding to achieve energy storage devices with higher areal energy density using conventional active materials. As a proof-of-concept, we have chosen to apply simple paper folding as well as the more complicated Miura-ori pattern to paper-based Li-ion battery electrodes. Miura folding consists of dividing a sheet into parallelograms with interdependent folds and has been used to fold maps, solar panels, and recently, metamaterials.—Cheng et al.
To prepare their batteries, the researchers used carbon nanotube (CNT) coated papers as the current collectors and deposited conventional active material layers (Li4Ti5O12 and LiCoO2) on top of them. They used Laboratory Kimwipes as substrates because the thin and porous nature of the paper allowed the CNT ink to diffuse easily both inside and outside of the paper. This resulted in CNT-coated papers that were conductive on either side.
Polyvinylidene difluoride (PVDF) was used as a binder to improve the CNT adhesion by coating an additional CNT/PVDF layer onto the CNT-coated papers prior to depositing the active materials.
|(A) Schematic of folding procedures for batteries with one fold, two folds, and three folds; (B) Schematic showing planar, unfolded full cell; and (C) full cell with one fold. Credit: ACS, Cheng et al. Click to enlarge.|
First using a simple folding procedure (shown in the schematic above), they found that the batteries with one fold, two folds, and three folds had approximately 1.9×, 4.7×, and 10.6× the areal capacity compared to a planar battery.
|Comparison of areal discharge capacities for planar, 1-fold, 2-fold, and 3-fold batteries. Credit: ACS, Cheng et al. Click to enlarge.|
The Coulombic efficiencies (CE) for the folded cells also were higher than for the unfolded cell. Noting that the reason for the higher CE in the folded cells is not understood, the authors suggested that it may be due to improved contact between the active materials layers and the CNTs after folding. These initial results showed that the Li-ion batteries can still exhibit good electrochemical performance even after multiple folds, they said.
To increase the areal capacities further, they used Miura folding to compact and to fold the paper more efficiently. In these Miura batteries, additional Cu or Al current collectors were not used because they could not be folded that many times; the CNT/PVDF-coated papers served as the sole current collector.
Although the areal density increased with the Miura folding, the battery in the folded state showed a slightly lower discharge capacity of 103 mAh/g compared to 113 mAh/g when unfolded. Compared to the results for the cell with three folds, the gravimetric capacity was lower in the Miura folded cell. The lower specific capacity in the folded cell may indicate that some of the active material was inaccessible to the electrolyte after folding, the team suggested. It could also be due to delamination at the intersections of perpendicular folds.
...internal resistance losses can also be responsible for why the gravimetric capacities are lower in the Miura folded batteries compared to the simple folded cells, which had Cu and Al foil backing layers underneath the CNT/PVDF-coated papers. However, the Miura folding still resulted in a significant increase in areal capacity compared to the planar cells...the areal capacity was ∼14× higher for the folded Miura cell at the 20th cycle, indicating that the Miura folding could be used to increase the energy density of the Li-ion battery.—Cheng et al.
Qian Cheng, Zeming Song, Teng Ma, Bethany B. Smith, Rui Tang, Hongyu Yu, Hanqing Jiang, and Candace K. Chan (2013) Folding Paper-Based Lithium-Ion Batteries for Higher Areal Energy Densities. Nano Letters doi: 10.1021/nl4030374