In recent years, with the rapid development of wearable devices, the need for flexible flexible battery also increase quickly, all kinds of flexible battery design is also emerge in endlessly, but most of the design of flexible battery will result in significantly lower energy density of lithium ion battery, wearable device's battery life. In order to solve this problem, Columbia University, Guoyu Qian et al., inspired by biological backbone structure, developed a flexible lithium-ion batteries, while ensuring good flexible at the same time, also keep the advantages of high volumetric energy density.
Flexible battery structure as shown in the above, the basic structure of flexible battery can be divided into a backbone, are connected to the trunk and branch, the branch will be winding around the trunk and eventually form a similar animal vertebrae structure, by the main and external circuit connection. This structure not only guarantees the battery has good flexibility, also greatly improve the energy density of flexible batteries, when using LiCoO2 / graphite as active material, the structure of the cell volume energy can reach 242 wh/L.
In electrochemical performance tests, both bending (20 mm bending radius, 10000) or reverse turn round 90 degrees (1000) under the condition of the structure of the cell cycle performance are not affected, in 0.2 C ratio in loop 100 times, capacity retention rate of 94.3%, the coulomb efficiency reached 99.9%. Can also see from the below battery voltage of the c curve, both bending and torsion are all not significantly affect the performance of the battery.
Flexible battery in the whole life cycle, may experience thousands of bending and twisting, so Guoyu Qian adopts the structure of the mechanical structure of the flexible battery under the mechanical load stability is assessed. C you can see from the image below, even in the case of the battery up to 20 mm bending radius, the battery can still stable charge and discharge, only slightly lower capacity, suggesting that most of the active substances in the process of bending and are able to maintain contact, no obvious active material loss.
In order to study in the bending process, the strain distribution inside the battery, Guoyu Qian chose one of the most common situations - as a wrist strap on your wrist, using finite element analysis tool for its internal strain distribution were analyzed, the results as shown in the figure below. Guoyu Qian found that USES the bionic structure of flexible battery in bending is only 0.08%, the maximum strain square cell strain has reached 1.8%, and Al foil and Cu foil the yield strain of 0.47% and 0.73%, respectively, indicating that the bionic structure of flexible battery strain in Al foil and Cu foil can be affordable, and square battery is beyond the yield limit of Al foil and Cu foil. Suggesting that Guoyu Qian bionic structure design can greatly reduce the electrode in the battery bending strain, reduce deformation for collection of fluid and the destruction of the active material, improve circulation performance of the battery.
Guoyu Qian this design fully absorb the animal the characteristics of the spine, with the structure characteristics of a lithium ion battery, bionic structure greatly reduces the bending and torsional deformation for the damage caused by fluid and active substances, in a battery of flexible at the same time, also greatly improved the flexible battery energy density, improves the practicability, the battery in the future of wearable devices, especially smart watches, etc have broad application prospects.