High-performance fiber-shaped lithium-ion batteries

Introduction

With the boost in interdisciplinary integration and technological convergence, there have appeared a lot of new application fields such as flexible medical devices, wearable devices and electronic fabrics that may revolutionize the current society and shape the future life [1], [2]. Conceivably, these products would be directly worn on the human body and work stably under complex deformations, such as bending, folding and even stretching in use [3]. As a result, there is an urgent need to develop corresponding power systems that should be miniaturized, flexible, and adaptable. However, conventional lithium-ion batteries, including both rigid bulk and flexible film architectures, cannot meet the above requirements [4]. Here, a new family of fiber-shaped batteries have been developed which exhibit excellent performances and can be further woven into breathable, flexible, stretchable and shape-memory textiles to effectively meet these requirements.

Fiber electrodes

The realization of fiber-shaped batteries requires the use of effective fiber electrodes. Aligned carbon nanotube (CNT) fibers spun from CNT arrays can effectively act as a skeleton to support active materials and a current collector for charge transport [5]. Spinnable CNT arrays were first synthesized by chemical vapor deposition, and aligned CNT fibers had been then dry-spun from the array [6]. As the CNTs were highly aligned along the axis direction, the resulting fibers well maintained the remarkable properties of individual CNTs. The aligned CNT fibers were lightweight, flexible, electrically conductive and mechanically strong. We had then developed two effective methods to introduce active materials into the CNT fiber. The first method was to co-spin them by a physical process [7]. Active materials such as LiMn₂O₄ and Li₄Ti₅O₁₂ suspensions were first deposited onto aligned CNT sheets, followed by twisting into hybrid fibers [8] (Fig. 1a). The nanoparticles were uniformly dispersed in the CNT fiber with high electrochemical properties. To better optimize the structure and morphology of active materials in the hybrid fibers, we had also developed in situ synthesis. A variety of active materials such as polymers [9], metallic compounds [10], [11], and inorganic components [12] can be uniformly deposited on the surface of CNT after chemical reaction (Fig. 1b and c). No binder or metal current collector was required, so specific capacities are largely enhanced. These hybrid fiber electrodes were very stable. CNT worked as skeletons to support active materials for both cathode and anode. They showed high loading capability up to 90% by weight. Besides, the designed aligned structure favored rapid transport of electrons based on a unique three-dimensional hopping conduction mechanism, and the nanoscale and micrometer-scale gaps from the hierarchically aligned helical fiber structure greatly enhanced the infiltration of electrolyte.

Fig. 1:

The fabrication, morphology and property of fiber electrode. (a) Schematic illustration of the fabrication of CNT hybrid fiber electrode by co-spinning method. Active materials suspensions are first deposited onto aligned CNT sheets, followed by twisting into hybrid fibers. (b) Schematic illustration of the fabrication of CNT hybrid fiber electrode by in situ synthesis. A variety of active materials can be uniformly deposited on the surface of CNT sheets after chemical reaction and then the hybrid sheets are twisted into hybrid fibers. (c) Scanning electron microscopy (SEM) images of CNTs before and after in site polymerization. Left: The CNT sheets are highly aligned. Middle: After in situ polymerization, polyimide nanosheets with widths of ~300 nm and thicknesses of ~25 nm are uniformly coated on the CNT sheets. Right: The aligned structure is well maintained in the CNT/polyimide hybrid fiber. (c) Adapted from ref. [9]. Copyright Royal Society of Chemistry (2016).

Fiber-shaped batteries

A flexible fiber-shaped lithium-ion battery was developed from a CNT/LiMn₂O₄ hybrid fiber cathode and a CNT/Li₄Ti₅O₁₂ hybrid fiber anode in a parallel arrangement [13]. However, owing to the low theoretical energy density, this fiber-shaped lithium-ion battery showed an energy density of 27 Wh/kg, which was much lower than that of commercial lithium-ion batteries. Lithium air batteries exhibit a high theoretical energy density of 3500 Wh/kg, 5–10 times higher than that of commercial lithium-ion batteries. To further improve the energy density, we discovered a lithium-ion air battery fiber in a solid-state coaxial architecture with the CNT/lithiated silicon hybrid fiber as inner anode, a polymer gel as middle electrolyte and a bare CNT sheet as outer cathode [14] (Fig. 2). The fiber-shaped battery exhibited an energy density of 512 Wh/kg based on the total weight of the two electrodes. The as-fabricated lithiated silicon/CNT hybrid fiber not only avoided dendrite formation that occurred to lithium metal with a safety problem but also showed ultra-high flexibility. The fiber battery could effectively work after 20 000 bending cycles.

Fig. 2:

Flexible fiber-shaped batteries with high energy density. (a) and (b) Schematic illustration to the structure and working mechanism of the lithium-ion air battery, respectively. During the discharge process, lithium ions dealloy from the inner lithiated silicon/CNT fiber, and then transfer through the gel electrolyte. Oxygen diffuses into the voids of aligned CNT sheets from all directions and reacts with lithium ions to form lithium peroxide (Li₂O₂) at the CNT cathode, while the electrons flow from the anode to the cathode through the external circuit. The process is reverse during the charge process. (c) The lithium-ion air battery under various deformations. The battery can bear multiple deformations including bending, tying, twisting and looping, which shows its ultra-high flexibility. (d) Charge and discharge curves of the lithium-ion air battery before and after different bending cycles. The voltage profiles are perfectly maintained even after repeating the bending tests for 20 000 cycles. For a typical bending test, the lithium-ion air battery with length of 8 cm was bent into circle with the distance between two ends as 1 cm. (e) Energy density and bending cyclic stability of the ultra-flexible lithium-ion air battery compared with the previous energy storage systems including LIB fiber [13], LIB film [24], sodium-ion battery tube [25], aqueous LIB film [26], Al-ion battery [27], Sony Li-ion polymer battery, aqueous LIB fiber [9], hybrid battery fiber [18], Li-O₂ film battery [28], and Li-O₂ cable battery [29]. LIB means lithium-ion battery. Adapted from ref. [14]. Copyright John Wiley and Sons (2017).

Stretchable batteries

Besides flexibility, for many applications such as wearable facilities, the batteries are also required to work under stretching. Traditionally, stretchable devices are based on elastic polymer substrates [15], [16]. However, these polymer substrates make the battery heavy and contribute nothing to the energy storage. Based on the unique fiber shape, we developed a new and general strategy by designing a spring fiber structure (Fig. 3). The spring-like CNT fiber was prepared by over-twisting several aligned CNT fibers together [17]. The formed coiled loops made them highly stretchable. The resulted fiber battery can be stretched up to 100%. Compared with the previous studies [8], as no substrates were needed, the volume and weight were decreased by 400% and 300%, respectively. The capacity had been thus enhanced by 600%.

Fig. 3:

Stretchable fiber-shaped batterries. (a) Schematic illustration of the fabrication of stretchable spring-like CNT fiber electrode. The spring-like CNT fiber was prepared by over-twisting several aligned CNT fibers together. (b–d) SEM images of a spring-like CNT fiber at different magnifications. The spring-like CNT fiber has uniform coiled loops aligning along the fiber axis and CNTs are highly aligned along the helical direction in the coiled loops. (e) SEM images of a fiber at different strains (0%, 50%, and 100%). The fiber can be easily stretched and released up to 100%, and the coiled loops were gradually elongated with the CNTs remaining highly aligned during stretching, and the spring-like fiber returned to the original coiled structure after release. (f) Structure of the stretchable fiber-shaped battery. Spring-like CNT/LiMn₂O₄ hybrid fiber cathode and CNT/Li₄Ti₅O₁₂ hybrid fiber anode were first coated with elastic gel electrolyte and then assembled in a parallel arrangement to produce stretchable fiber-shaped battery. (g) and (h) Evolution of specific capacitance with strain and stretch cycles, respectively. The battery was highly stretchable, and the capacity remained at 85% at 100% strain. In addition, the capacity varied within less than 1% after repeated stretching for 300 cycles at the strain of 50%. Adapted from ref. [17]. Copyright John Wiley and Sons (2014).

Integration into power textiles

The fiber shape makes it easy for integration. We had designed a three-electrode-twisted structure to integrate the properties of the lithium-ion battery and the supercapacitor to give both high energy and power densities [18] (Fig. 4a and b). We can also integrate fiber-shaped solar cells with the fiber battery to harvest and store energy at the same time [19], [20] (Fig. 4c). A variety of sensors needs power from the battery and they can also be integrated with the battery [21]. These fiber batteries can be further woven into flexible, stretchable and breathable clothes for large-scale applications [14], [22], [23] (Fig. 5). The energy density was well maintained after washing.

Fig. 4:

Integration of fiber-shaped batteries. (a) A hybrid energy storage device formed by twisting three fibre electrodes together. The multi-walled CNT/Li₄Ti₅O₁₂ (MWCNT/LTO) electrode serves as a common electrode for a lithium-ion battery and supercapacitor. (b) During the discharge process, the MWCNT/LiMn₂O₄ (MWCNT/LMO) and MWCNT/ordered mesoporous carbon (MWCNT/OMC) electrodes are connected to function as positive electrodes, and the MWCNT/LTO electrode serves as the common negative electrode, by which the hybrid device can discharge at high current densities. (a) and (b) Adapted from ref. [18]. Copyright John Wiley and Sons (2015). (c) Integrated energy harvesting and storage devices with an in-series architecture. Photoelectric conversion and energy storage components are produced on the left and right of a fiber, respectively. (c) Adapted from ref. [19]. Copyright John Wiley and Sons (2012).

Fig. 5:

Weavability of fiber-shaped batteries. (a) Schematic diagram of energy storage fabric. The method can accurately control the number of fiber-shaped batteries and their serial or parallel connection modes, thereby effectively regulating the output voltage, current and storage capacity of the energy fabric. (b) A Photograph showing the progress of fiber-shaped batteries continuously being woven into a textile. (c) Photographs of the textile under twisting. The resulting textile is breathable and soft, which can bear various deformations such as bending, folding, stretching and twisting. (b) and (c), Adapted from ref. [14]. Copyright John Wiley and Sons (2017).

Summary

In summary, we have developed aligned CNT composite fibers as effective electrodes for fabricating high-performance fiber-shaped batteries that were highly flexible and stretchable with high energy densities. The main efforts were made to understand the charge separation and transport along the aligned CNT fiber electrode and develop continuous fabrication methods for large-scale production, which paved the way for the future application of fiber-shaped batteries and open up a new direction in the advance of the next-generation electronics.

Fiber-shaped batteries have made a series of important advances in recent years. However, to truly realize the practical application, further research is needed in the following aspects. First, electrochemical performance needs to be further improved. Although fiber-shaped batteries have achieved high energy and power density, the actual energy and power are low for these micro-batteries. Increasing the length of the battery is an effective way to improve energy and power. Second, currently reported fibrous lithium-ion batteries are limited to centimeter length. The next research should focus on improving the electrical conductivity of the fiber electrode and optimizing its electrochemical performance to achieve a fiber-shaped lithium-ion battery with a length of up to meters. Besides, the development of continuous preparation method is essential to realize large-scale production of fibrous lithium-ion batteries.