advances in wearable fiber‐shaped lithium‐ion batteries

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim4524 wileyonlinelibrary.com

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Ye Zhang , Yang Zhao , Jing Ren , Wei Weng , * and Huisheng Peng *

Y. Zhang, Y. Zhao, J. Ren, Dr. W. Weng, Prof. H. Peng State Key Laboratory of Molecular Engineering of Polymers Department of Macromolecular Science and Laboratory of Advanced Materials Fudan University Shanghai 200438 , China E-mail: [email protected] ; [email protected]; [email protected]

DOI: 10.1002/adma.201503891

made smaller and be woven into textiles to satisfy soft substrates such as our bodies.

Here, recent advances in these novel fi ber-shaped LIBs are summarized. The working mechanism, material and struc-ture of the LIB are fi rst introduced, and the realization of fl exibility is further emphasized to highlight the importance of fi ber-shaped LIBs. Then, a description is given of the fabrication, structure, mecha-nism, and properties of wearable fi ber-shaped LIBs with a focus on the electrode material, refl ecting the key achievements at each step. The remaining challenges

and future directions are then discussed to provide some useful insights from the viewpoint of practical applications.

2. Lithium-Ion Batteries for Wearable Applications

An LIB is typically composed of a cathode, an anode, and an electrolyte. While they are discharging, Li + ions are removed from the anode, transferred into the electrolyte and inserted into the cathode, along with the electrons fl owing from the anode to the cathode at the external circuit. The above process is reversed during charging. The LIB realizes energy delivery and storage through the reversible movement of Li + ions between the cathode and anode. [ 12,13 ]

Conventionally, lithium metal oxides such as LiCoO 2 , LiFePO 4 and LiMn 2 O 4 and carbonaceous materials such as graphite are used as the cathode and anode materials, respec-tively. [ 14,15 ] These active materials are made into a slurry and coated on the surfaces of copper or aluminum foils to work as electrodes that are not fl exible. An organic solution dissolved with lithium salt is typically used as an electrolyte. When the liquid electrolyte is assembled with the rigid electrode, it may give rise to leakage or produce a short circuit in the resulting LIB under deformation. In other words, the LIBs are not fl ex-ible and should be carefully protected during application. [ 16 ]

Wearable electronics require the use of fl exible, miniatur-ized, and lightweight LIBs with high energy densities. Tremen-dous effort has been made toward developing fl exible planar LIBs, [ 17–24 ] and fl exibility has been found to be mainly deter-mined by the electrode. [ 25–28 ] Currently, fl exible electrodes are typically prepared by incorporating electrochemically active materials onto fl exible and conductive substrates. [ 29,30 ] A strong interaction between the active material and the substrate is gen-erally required to maintain the structural integrity during defor-mation. To this end, a variety of nanomaterials are primarily used as the components in composite electrodes because the large specifi c surface areas from the nanostructured interfaces are favorable for high stability. A variety of methods, such as

It is highly desirable to develop fl exible and effi cient energy-storage sys-tems for widely used wearable electronic products. To this end, fi ber-shaped lithium-ion batteries (LIBs) attract increasing interest due to their combined superiorities of miniaturization, adaptability, and weavability, compared with conventional bulky and planar structures. Recent advances in the fabrication, structure, mechanism, and properties of fi ber-shaped LIBs are summarized here, with a focus on the electrode material. Remaining challenges and future directions are also highlighted to provide some useful insights from the view-point of practical applications.

1. Introduction

Wearable electronic products such as smart clothes, the Apple Watch, and the Samsung bracelet are emerging in the main-stream and represent promising directions for future life-styles. [ 1–3 ] Conceivably, these products would be directly worn on the human body and work stably under complex deforma-tion in use, such as bending, folding and even stretching. The rapid development of fl exible and wearable electronic prod-ucts strongly demands indispensable power systems that can be miniaturized, fl exible, and adaptable. Lithium-ion batteries (LIBs) have been used as one of the most ubiquitous types of power supplies due to their superior features such as high energy density, no memory effect, long life cycle, high working voltage, and environmental benignity. [ 4–9 ] However, commer-cially available LIBs are typically rigid and cannot effectively sat-isfy the above requirements, which has become a bottleneck for the further development of wearable electronics.

Advances in textile technology offer general and useful routes to solve the problems described above. It is well recog-nized that our clothes can bear various severe deformations, including twisting during use, and building chemical fi bers is key to their high performance. Inspired by this simple yet effi -cient strategy, a new family of fi ber-shaped LIBs has recently been investigated. [ 10,11 ] Compared with conventional 3D bulky or 2D planar structures, these unique 1D fi bers allow LIBs to be deformable in all dimensions. Fiber-shaped LIBs can also be

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coating [ 31 ] and hydrothermal reactions, [ 32 ] have been used to introduce active materials onto supporting substrates. Carbon nanomaterials such as carbon cloth, [ 32 ] carbon nanotubes (CNTs), [ 33 ] and graphene [ 34 ] have primarily been explored and will be discussed below.

LiCoO 2 and Li 4 Ti 5 O 12 were separately incorporated into CNT networks to form hybrid electrodes by coating with solutions containing their respective mixtures. The two hybrid electrodes were laminated with a paper between them to act as a separator to make an LIB that was thin, lightweight, and fl exible. [ 31 ] Based on a similar fabrication process, the mechanical properties of the LIB were further enhanced by fi rst embedding active mate-rials into a porous membrane and then coating a CNT layer to prepare the hybrid electrode ( Figure 1 a). [ 35 ] The resulting elec-trode showed increasing tensile strengths of ca. 5.5–7.0 MPa and maintained a high storage capability after repeated bending at a radius of 4 mm (Figure 1 b).

These active metal-oxide materials can also be anchored to carbonaceous materials via a hydrothermal reaction. Graphene foams consisting of 3D interconnected networks have been widely studied as promising substrates and current collectors due to their high fl exibility and electrical conductivity. Li 4 Ti 5 O 12 and LiFePO 4 were incorporated to act as an anode and cathode, respectively, through an in situ hydrothermal deposition (Figure 1 c). [ 34 ] Because of the high contact area between the active material and the graphene foam, these hybrid electrodes demonstrated remarkable electrical and mechanical properties. The resulting LIB had an energy density of 110 W h kg −1 and

a specifi c capacity of 117 mA h g −1 at a rate of 10 C. In addi-tion, the specifi c capacity of the LIB varied by less than 1% after bending to a radius of 5 mm for many cycles (Figure 1 d).

Electrolytes, which are also a key component of fl exible LIBs, can be divided into liquid and solid electrolytes. For a liquid electrolyte, good contact between the electrode and electrolyte with high ionic conductivity offers a fl exible LIB with a rela-tively high storage capability. However, the liquid nature may give rise to leakage or short-circuit under deformation. In con-trast, a solid electrolyte can effectively prevent both leakage and short-circuiting, but the poor contact between the electrode and electrolyte and the low ionic conductivity lowers the storage capability. [ 36,37 ] The selection of liquid or solid electrolytes relies on the application requirements and balancing the above parameters.

Although a variety of fl exible LIBs have been obtained, typical bulky or planar structures demonstrate limited fl ex-ibility in nature, and electrochemical performance is largely decreased under severe deformation. To realize good fl exibility, planar LIBs are made to be as thin as possible. Punctures and cracks often occur in such a thin-fi lm structure, which leads to a decrease in electrochemical performance or even the destruc-tion of the LIB, with severe safety issues. Many efforts have been made to enhance the fl exibility and stability of thin-fi lm LIBs by improving the sealing process. However, they tend to break and fail to work in many cases under 3D twisting; thin-fi lm LIBs cannot effectively adhere to soft substrates such as our bodies under deformation; and they are not breathable,

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Figure 1. a) Schematic of a fl exible battery laminated within an aluminum-lined pouch. b) The battery is able to power a green LED after fl exing 50 times (with a bending radius of 4 mm). a,b) Reproduced with permission. [ 35 ] Copyright 2013, Wiley. c) Scanning electron microscopy (SEM) image of a Li 4 Ti 5 O 12 /graphene foam. d) Galvanostatic charging/discharging curves of the battery. The red and blue lines represent the fabricated fl at battery and the bent battery after bending (with a radius of 5 mm) 20 times. c,d) Reproduced with permission. [ 34 ] Copyright 2012, National Academy of Sciences.

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A revolution in LIB structure is necessary to ultimately solve these problems.

3. Lithium-Ion Batteries with a Fiber Shape

Fiber-based clothes have been used by human beings for many years because they are soft, fl exible, breathable, deformable, washable, and durable, which also represents an ideal para-digm for the development of wearable electronic products. [ 38 ] A variety of fi ber-shaped devices such as sensors, [ 39 ] solar cells, [ 40 ] and supercapacitors [ 41 ] have been widely explored. Recently, several attempts have been made to produce fl exible LIBs with fi ber shapes, highlighting a new direction in wearable electronics. [ 42–44 ]

Compared with the conventional planar structure, the fi ber shape offers many unique and promising advantages. i) In addition to the typical bending deformation, a fi ber-shaped LIB can effectively work under twisting and even stretching, which are critically important for fl exible devices. ii) The LIB fi ber can be woven into textiles with porous structures to allow water vapor and air to transport through them, which is highly desirable for wearable applications. iii) Both LIB fi bers and textiles can effectively adapt to a variety of soft or curved substrates.

Currently, a fi ber-shaped LIB is generally constructed by twisting two fi ber electrodes. In other words, the realization of a fi ber-shaped LIB with high performance requires the use of effective fi ber electrodes. It is well recognized that an elec-trode in a planar LIB typically includes four components: an electrochemically active material, a current collector, a binder, and a conductive additive. Therefore, several key parameters should be carefully considered to prepare effective fi ber elec-trodes. i) They are expected to exhibit high electrical conductivi-ties to rapidly transfer electrons. ii) The electrochemically active materials can be stably anchored on the supporting substrate. iii) They should be highly fl exible and stable to satisfy both fab-rication and application.

3.1. Fiber Electrodes

Common metal wires can provide excellent electronic proper-ties, but they are relatively heavy and rigid, which has limited their applications in fi ber-shaped devices. In contrast, com-mercial polymer fi bers are lightweight and fl exible but they are less conductive or even insulating. Although some conducting layers have been coated on the surface of a polymer fi ber to offer high electrical conductivity through physical vapor deposi-tion, it remains challenging to continuously prepare such com-posite fi ber electrodes on a large scale. Recently, there has been increasing interest in developing a new family of nanostruc-tured fi ber electrodes based on the well-explored CNTs. [ 45–48 ] These CNT fi bers have typically been spun from CNT arrays, and the aligned structure for the CNT provides remarkable chemical and physical properties. i) CNT fi bers show high electrical conductivities on the level of 10 2 –10 4 S cm −1 and tensile strengths on the order of 10 2 –10 3 MPa. ii) CNT fi bers

are highly fl exible and lightweight. iii) Active materials can be stably anchored on the surfaces of aligned CNTs to favor the reversible intercalation and deintercalation of Li + ions. iv) CNTs can act as a skeleton to support active materials and a current collector for charge transport. As a result, no binder or metal current collector is required, so specifi c capacities are largely enhanced.

The fi rst CNT fi ber-based electrode appeared just a few years ago, and it was synthesized by depositing MnO 2 nanoparticles on the surface of CNT fi bers through electrochemical deposi-tion in an aqueous solution. [ 49 ] The weight percentages of the MnO 2 nanoparticles ranged from 0.5% to 8.6% and were accu-rately controlled by the cycle number of the electrochemical deposition. Figure 2 a,b show typical scanning electron micro-scopy (SEM) images of an aligned CNT/MnO 2 hybrid fi ber. The fl ower-shaped MnO 2 nanoparticles are uniformly strewn on the surface of the CNT fi ber. The hybrid fi ber electrode was fl ex-ible, and no distinct damage in terms of structural integrity or decay of electrical conductivity was observed during deforma-tion. The aligned CNT/MnO 2 hybrid fi ber was further investi-gated with a lithium wire as the negative electrode in 1 M LiPF 6 electrolyte solution. A specifi c capacity of 109.62 mA h cm −3 or 218.32 mA h g −1 could be achieved based on the whole elec-trode (Figure 2 c).

To further improve the electrochemical performance, sil-icon, which is widely studied because of its high theoretical capacity of 4200 mA h g −1 , has also been incorporated to pre-pare hybrid fi ber electrodes. [ 44 ] The silicon was fi rst deposited onto the aligned CNT sheet through an electron-beam evapora-tion method, and the weight percentage of the silicon was con-trolled by varying the deposition time. The amorphous silicon was coated on the outer surfaces of aligned CNTs with a core–sheath structure (Figure 2 d,e). The resulting hybrid sheet was then scrolled into a hybrid fi ber electrode. For the hybrid fi ber, the unique core–sheath structure combined the high electrical conductivity of CNTs in the core and the high specifi c capacity of silicon in the sheath. The aligned CNT/Si hybrid fi bers were also fl exible, and no obvious damage was observed under SEM after bending, twisting, or other deformations. The aligned CNT/Si electrode was also paired with a lithium wire to test the electrochemical performance. For a silicon weight percentage of 38.1%, a voltage plateau of approximately 0.4 V and a specifi c capacity of 1670 mA h g −1 were obtained at a current density of 1.0 A g −1 (Figure 2 f). However, the cyclic performance still needed to be enhanced, as the capacity decayed rapidly, possibly due to a limited counterbalance of the Si volume change. It was diffi cult to uniformly coat silicon on the surfaces of CNTs via electron-beam evaporation.

Active materials have also been incorporated into an aligned CNT fi ber via a co-spinning process. [ 11,42,51 ] In a typical prepara-tion, suspensions of active nanoparticles such as LiMn 2 O 4 are dropped onto the aligned CNT sheets, followed by a scrolling process to transform 2D sheets to 1D fi bers. This method can enable a high concentration of electrochemically active nano-particles (e.g., 90% for LiMn 2 O 4 in weight), and the active nanoparticles are also uniformly dispersed in the aligned CNTs (Figure 2 g,h). As expected, the specifi c capacity of the CNT/LiMn 2 O 4 hybrid fi bers can be easily maintained at over 80% after 200 cycles. The hybrid fi ber also showed a high discharge

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plateau of ca. 4.0 V (Figure 2 i), which was a promising candi-date as the cathode in fi ber-shaped LIBs.

3.2. Assembly of Fiber-Shaped Lithium-Ion Batteries

Current fi ber-shaped LIBs include two typical structures: heli-cally coaxial and parallel structures ( Figure 3 ). A cable-type LIB was made into a helically coaxial structure with fi ve main parts: a Ni–Sn hollow-spiral anode, a separator, a LiCoO 2 cathode, liquid electrolytes, and a packaging insulator (Figure 3 a). [ 50 ] For a typical fabrication, the Ni–Sn active materials are fi rst depos-ited on a hollow-spiral Cu wire to act as the anode; a modifi ed poly(ethylene terephthalate) separator and an aluminum wire are then wound on the anode, followed by coating with LiCoO 2

slurry to form the cathode. The cable-type LIB had to be sealed in a heat-shrinkable tube and fi nally injected with a liquid electrolyte. It was relatively too large, with a diameter in the millimeter range, and cannot effectively meet future require-ments regarding weavability. It also had a low specifi c capacity of 1 mA h cm −1 (Figure 3 b). Fortunately, this cable-type LIB was fl exible and exhibited stable discharge characteristics with increasing bending angle (Figure 3 c).

To further meet the application requirements for wearable electronics, the metal current collector was removed to reduce the weight and volume of the LIB. As a result, a helically coaxial fi ber-shaped battery was developed with an aligned CNT/Si hybrid fi ber as an anode and an aligned CNT/LiMn 2 O 4 hybrid fi ber as the cathode. [ 10 ] The fi ber electrodes were sequentially wound on a cotton fi ber while separated by a layer of electrolyte

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Figure 2. SEM images and electrochemical properties of fi ber electrodes. a,b) SEM images of a CNT/MnO 2 fi ber electrode at low and high magni-fi cation, respectively. c) Voltage profi les of the CNT/MnO 2 fi ber electrode versus Li/Li + . a–c) Reproduced with permission. [ 49 ] Copyright 2013, Wiley. d,e) SEM images of a CNT/Si fi ber electrode at low and high magnifi cation, respectively. f) Voltage profi les of the CNT/Si fi ber electrode versus Li/Li + . d–f) Reproduced with permission. [ 44 ] Copyright 2014, Wiley. g,h) SEM images of a CNT/LiMn 2 O 4 fi ber electrode at low and high magnifi cation, respectively. i) Voltage profi les of the CNT/LiMn 2 O 4 fi ber electrode versus Li/Li + . g–h) Reproduced with permission. [ 51 ] Copyright 2014, Royal Society of Chemistry. i) Reproduced with permission. [ 11 ] Copyright 2014, Wiley.


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gel, followed by covering with a shrinkable tube as a protec-tive layer (Figure 3 d). The initial specifi c capacity of the fi ber-shaped LIB reached 106.5 mA h g −1 with an output voltage of 3.4 V, which was consistent with the voltage difference between LiMn 2 O 4 and Si (Figure 3 e,f). Although this modifi ed fi ber-shaped LIB showed much improved electrochemical perfor-mance, the high specifi c capacity of Si was not fully used. In addition, the tendency of the Si layer to be fragile weakened the stability of the LIB. Furthermore, the fabrication process was complex because the operation had to be carried out in a glove box.

A fi ber-shaped LIB with better performance was developed from a CNT/LiMn 2 O 4 hybrid fi ber cathode and a CNT/Li 4 Ti 5 O 12 hybrid fi ber anode in a parallel structure (Figure 3 g). [ 42 ] Spinel

Li 4 Ti 5 O 12 showed a high cyclic stability due to the zero-strain structure for lithium intercalation, i.e., no volume change occurred during the lithiation and delithiation processes. In addition, Li 4 Ti 5 O 12 exhibited a higher lithiation potential of ca. 1.5 V (vs Li/Li + ) to effectively avoid the growth of dendritic lithium on the anode surface. The two fi ber electrodes were separated by a poly(vinylidene fl uoride) membrane and then assembled into a heat-shrinkable tube. The resulting fi ber-shaped LIB exhibited a discharge plateau voltage of 2.5 V at 0.1 mA, and the specifi c capacity reached 138 mA h g −1 , which could be retained at 85% after 100 cycles. As expected, the fi ber-shaped LIB delivered a high energy density of 17.7 W h L −1 and a power density of 560 W L −1 based on the entire volume of both the anode and cathode. This fi ber-shaped LIB was highly

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Figure 3. Flexible LIBs in a one-dimensional shape. a) Schematic illustration of the structure of a helically coaxial cable-shaped LIB. b) Voltage profi les of the cable-shaped LIB. c) Discharge characteristics of the cable-shaped LIB under bending. a–c) Reproduced with permission. [ 50 ] Copyright 2012, Wiley. d) Schematic illustration of the fabrication of a helically coaxial fi ber-shaped LIB from the CNT/Si and CNT/LiMn 2 O 4 electrode. e) Voltage profi les of the coaxial fi ber-shaped LIB. f) Photographs of the coaxial fi ber-shaped LIB lit up a light-emission dioxide. d–f) Reproduced with permission. [ 10 ] Copyright 2014, American Chemical Society. g) Schematic illustration of the parallel structure of a fi ber-shaped LIB from the CNT/Li 4 Ti 5 O 12 and CNT/LiMn 2 O 4 electrode. h) Voltage profi les of the parallel fi ber-shaped LIB before and after bending. i) Photographs of the parallel fi ber-shaped LIB being deformed into different shapes. g–i) Reproduced with permission. [ 42 ] Copyright 2014, Wiley.


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fl exible and could be bent into various formats. The galvano-static charge and discharge curves remained almost unchanged after repeated bending for 1000 cycles (Figure 3 h,i).

3.3. Stretchable Fiber-Shaped Lithium-Ion Batteries

The above fl exible fi ber-shaped LIBs could bear various defor-mations such as bending, tying, and twisting, but their struc-tures could be damaged under stretching, which is unfavorable for practical application in a variety of wearable electronic devices. To this end, an elastic fi ber-shaped LIB was created by winding an aligned CNT/LiMn 2 O 4 fi ber cathode and CNT/Li 4 Ti 5 O 12 fi ber anode around a poly(dimethylsiloxane) (PDMS) fi ber, which served as the elastic substrate, followed by a coating of a thin layer of lithium bis(trifl uoromethane)sulfona-mide (LiTFSI)/succinonitrile (SCN)/poly(ethyleneoxide) (PEO) gel electrolyte ( Figure 4 a). [ 51 ] The elastic substrate, stretchable gel electrolyte, and winding structure made the fi ber-shaped LIB stretchable. During the stretching process, the pitch dis-tance between the two fi ber electrodes was increased while the winding structure of the fi ber electrode remained unchanged. The elastic gel electrolyte played an important role in anchoring the winding structure during stretching. A red light-emitting diode was powered by the elastic fi ber-shaped LIB, and no obvious change in brightness was observed under stretching (Figure 4 b). This stretchable fi ber-shaped LIB showed a specifi c capacity of 91.3 mA h g −1 and could be maintained up to 88% after stretching by 600% (Figure 4 c).

Achieving stretchability is marked as a great advance, as it makes fi ber-shaped LIBs more adaptable to the concept of wearable and stretchable electronics. However, the introduc-tion of elastomeric polymer substrates increases the volume and weight of the LIB, and the poor mechanical properties and

low operating temperature of polymer substrates have also lim-ited their practical applications. To this end, a new and effective strategy has been discovered to achieve stretchable fi ber-shaped LIBs using hybrid fi ber springs as electrodes (Figure 4 d). [ 11 ] A spring-like fi ber was prepared by over-twisting several aligned CNT fi bers together. The spring-like fi ber exhibited uniform coiled loops that enabled high stretchability with elongation of over 300% (Figure 4 e). An elastic fi ber-shaped LIB was then produced from two spring-like fi bers bearing LiMn 2 O 4 and Li 4 Ti 5 O 12 nanoparticles as the cathode and anode, respec-tively. This design avoided the use of non-capacitive elastomeric polymer. Compared with previous stretchable fi ber-shaped LIBs based on a polymer substrate, [ 51 ] the volume and weight of the LIB were decreased by appropriately 400% and 300%, respec-tively. The elastic fi ber-shaped LIBs with the spring structure were highly stretchable and stable. The specifi c capacities were well maintained at 85% after a large strain of 100%, and they varied by less than 1% after stretching for 300 cycles (Figure 4 f).

4. Lithium-Ion-Battery Textiles

Fiber-shaped LIBs can be further woven into various fl ex-ible textiles ( Figure 5 ). [ 10,42,51 ] An areal energy density of 4.5 mW h cm −2 can be achieved from these LIB textiles. [ 10 ] Moreover, stretchable fi ber-shaped LIBs can be easily woven into a knitted sweater to achieve better adaptability to the move-ment of the human body (Figure 5 d–f). [ 51 ] For instance, the resulting LIB in the sweater can be well accommodated with severe deformations such as folding and stretching.

Note that research on LIB textiles just began last year, and much effort should be exerted to develop general and effi cient weaving methods that are aimed at continuous and large-scale fabrication. For instance, it is highly desired, although

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Figure 4. a–f) Stretchable fi ber-shaped LIBs with (a–c) and without (d–f) an elastomeric polymer substrate. a) Schematic illustration of the structure of a stretchable fi ber-shaped LIB on an elastomeric polymer. b) Photograph of the stretchable fi ber-shaped LIB being used to power a light-emission dioxide before and after stretching. c) Voltage profi les of the stretchable fi ber-shaped LIB before and after stretching. a–c) Reproduced with permis-sion. [ 51 ] Copyright 2014, Royal Society of Chemistry. d) Schematic illustration of the structure of a stretchable fi ber-shaped LIB based on CNT fi ber springs. e) SEM images of the CNT fi ber springs before and after stretching for 100%. f) Dependence of specifi c capacitance on stretching number. d,e) Reproduced with permission. [ 11 ] Copyright 2014, Wiley.


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it remains challenging, to effectively connect numerous fi ber electrodes by building fi ber-shaped LIBs. It is also necessary to carefully consider many factors such as washability, comfort, and safety, which are very important for practical applications. In addition, such LIB textiles will become more useful if they can be integrated with an energy-harvesting functionality to achieve self-powering systems.

5. Conclusion and Outlook

State-of-the-art fi ber-shaped LIBs are described from the view-point of the fi ber electrode, with their electrochemical perfor-mances being summarized in Table 1 . Although research in this direction began just a few years ago, obvious advances have been made by introducing new materials and optimizing structures for high electrochemical performance. In particular, these fi ber-shaped LIBs exhibit many unique advantages such as fl exibility, weavability, and wearability. Despite these great achievements, current fi ber-shaped LIBs are still far away from satisfying the requirements for wearable electronics, and much effort should be made to further improve them for practical applications.

Aligned CNT fi bers exhibit both remarkable electrical and mechanical properties for fl exible and stretchable electrodes.

However, some challenges remain for future development. It is expensive to continuously prepare aligned CNT fi bers based on current technology, and the electrical conductivity must be further enhanced as it is relatively low for use in LIBs. For instance, fi brous LIBs cannot be made with an applicable length because the electrical resistance is too high and the electrical performance drops even at a length comprising tens of centim-eters. Therefore, a number of new fi ber electrodes should be investigated and compared for large-scale applications.

The power and energy densities of fi ber-shaped LIBs must be increased by a large amount as well. Silicon has been intro-duced to improve the fi ber anode, but the current cathode cannot effectively match it in capacity. Sulfur may be proposed as a promising cathode candidate because of the superiority its high theoretical capacity and working voltage, but this has not been attempted yet. Other materials may also be incorporated into fi ber electrodes and systematically compared for electro-chemical performance by designing different structures, which offer an effective route to optimize LIBs.

Safety is also a critical issue for practical applications, par-ticularly in wearable electronic devices that are in direct con-tact with the human body. The materials used should be non-toxic, biocompatible, and safe, and the fi ber-shaped LIB should be able to stably work even under severe deformations and strict weather conditions. A major concern comes from the

Figure 5. LIB textiles. a-c) Fiber-shaped LIBs woven into fl exible textiles. a) Reproduced with permission. [ 42 ] Copyright 2014, Wiley. b,c) Reproduced with permission. [ 10 ] Copyright 2014, American Chemical Society. d–f) Stretchable fi ber-shaped LIBs woven into a knitted sweater (d) under deformation by folding (e) and stretching (f). Reproduced with permission. [ 51 ] Copyright 2014, Royal Society of Chemistry.

Table 1. Fiber-shaped LIBs.

Cathode Anode Electrolyte Voltage [V] Capacity [mA h g −1 ] Ref.

CNT/MnO 2 Li liquid 1.5 218.32 [49]

CNT/Si Li liquid 0.4 1,670 [44]

CNT/LiMn 2 O 4 Li liquid 4.0 60 [42]

CNT/Li4Ti 5 O 12 Li liquid 1.5 150 [42]

CNT/LiMn 2 O 4 CNT/Si gel 3.4 106.5 [10]

CNT/LiMn 2 O 4 CNT/Li4Ti 5 O 12 liquid 2.5 138 [42]

CNT/LiMn 2 O 4 CNT/Li4Ti 5 O 12 gel 2.3 91.3 [51]


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fl ammability and toxicity of the organic electrolyte. In addition, the use of liquid electrolytes may cause leakage and short-cir-cuit during use, and appropriate gel or solid electrolytes with high performance are urgently needed to solve these problems.

To summarize, although much effort is still required to opti-mize fi ber-shaped LIBs in terms of material, structure, and property, they represent a new family of energy-storage systems based on unique fi ber confi gurations that have many attractive properties, and which may change our life in the near future.

Acknowledgements This work was supported by NSFC (21225417, 51403038), MOST (2011CB932503), STCSM (15XD1500400), the Changjiang Chair Professor Program and the Program for Outstanding Young Scholars from the Organization Department of the CPC Central Committee.

Received: August 10, 2015 Revised: September 15, 2015

Published online: December 8, 2015

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advances in wearable fiber‐shaped lithium‐ion batteries

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