|Structure of the flexible wire-shaped lithium-ion battery. The aligned MWCNT/LTO and MWCNT/LMO composite yarns are paired as the anode and cathode, respectively. Ren et al. Click to enlarge.|
A team led by Huisheng Peng from Fudan University in Shanghai has developed a stretchable wire-shaped lithium-ion battery produced from two aligned multi-walled carbon nanotube/lithium oxide composite yarns as the anode and cathode without extra current collectors and binders. As the researchers report in the journal Angewandte Chemie, they were able to weave their batteries into light, flexible, elastic, and safe textile batteries with a high energy density.
The two composite yarns can be well paired to obtain a safe battery with energy densities of 27 Wh kg−1 or 17.7 mWh cm−3 and power densities of 880 W kg−1 or 0.56 W cm−3, which are an order of magnitude higher than the densities reported for lithium thin-film batteries. These wire-shaped batteries are flexible and light, and 97% of their capacity was maintained after 1,000 bending cycles.
|Capacity retention and coulombic efficiency of the wire-shaped full cell with a length of 1 cm after 100 charge–discharge cycles at 0.05 mA. Ren et al. Click to enlarge.|
They are also very elastic as they are based on a modified spring structure; 84% of the capacity was maintained after stretching for 200 cycles at a strain of 100%. These novel wire-shaped batteries have been woven into lightweight, flexible, and stretchable battery textiles, which reveals possible large-scale applications.
Previous methods for producing wire-shaped electrochemical supercapacitors by twisting two fiber electrodes together resulted in systems with inferior performance that prevented them from being brought to the market.
Lithium-ion batteries can attain significantly higher energy density, but have not previously been produced in wire form. In addition to other barriers, the safety problems associated with lithium-ion batteries are a factor. The source of the safety problem is dendritic lithium, which can form during over-charging, “growing” out of the anode and causing a short circuit. This can cause the battery to ignite. This seems especially critical for wire-shaped batteries that can be stretched, twisted, and bent during use.
The Fudan team succeeded in producing wire-shaped lithium ion batteries that have a high energy density and are also safe. Their success results from the special structure as well as the materials used.
The anode and cathode are two fibers made of parallel multi-walled carbon nanotubes that contain either lithium titanium oxide (LTO) or lithium manganese oxide (LMO) particles, respectively. Because of the aligned nanostructure and high electrical conductivity, no current collector and binders are required during the fabrication.
When the battery is charging, lithium ions are transferred from the LMO lattice to the electrolyte and then into the LTO lattice of the anode. The reverse process occurs as the battery is being discharged. Because the Li insertion takes place at ~1.5 V (vs. Li/Li+) for the applied LTO composite electrode, the chance of short circuit caused by dendritic lithium would be small and therefore the batteries are safe.
The parallel arrangements of continuous carbon nanotubes hold the nanoparticles; they are also efficient pathways for charge transport and serve as current collectors. The two electrode yarns are arranged in parallel, separated by a layer of insulator, and enclosed in a heat-shrinkable tube. To make the wires elastic, they can be wrapped around an elastic fiber such as polydimethylsiloxane and coated with a thin-layer gel electrolyte. Neither repeated stretching to twice its original length nor thousands of deformation cycles reduces the battery capacity.
The wire-shaped batteries can be spun into long fibers and woven into a fabric that can be incorporated into textiles.
Jing Ren, Ye Zhang, Wenyu Bai, Xuli Chen, Zhitao Zhang, Xin Fang, Wei Weng, Dr. Yonggang Wang, and Prof. Huisheng Peng (2014) “Elastic and Wearable Wire-Shaped Lithium-Ion Battery with High Electrochemical Performance,” Angew. Chem. Int. Ed.. doi: 10.1002/anie.201402388