3D‐Printed All‐Fiber Li‐Ion Battery toward Wearable Energy ...

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3D-Printed All-Fiber Li-Ion Battery toward Wearable Energy Storage

Yibo Wang, Chaoji Chen, Hua Xie, Tingting Gao, Yonggang Yao, Glenn Pastel, Xiaogang Han, Yiju Li, Jiupeng Zhao,* Kun (Kelvin) Fu,* and Liangbing Hu*

Conventional bulky and rigid power systems are incapable of meeting flex-ibility and breathability requirements for wearable applications. Despite the tremendous efforts dedicated to developing various 1D energy storage devices with sufficient flexibility, challenges remain pertaining to fabrication scalability, cost, and efficiency. Here, a scalable, low-cost, and high-efficiency 3D printing technology is applied to fabricate a flexible all-fiber lithium-ion battery (LIB). Highly viscous polymer inks containing carbon nanotubes and either lithium iron phosphate (LFP) or lithium titanium oxide (LTO) are used to print LFP fiber cathodes and LTO fiber anodes, respectively. Both fiber electrodes demonstrate good flexibility and high electrochemical performance in half-cell configurations. All-fiber LIB can be successfully assembled by twisting the as-printed LFP and LTO fibers together with gel polymer as the quasi-solid electrolyte. The all-fiber device exhibits a high specific capacity of ≈110 mAh g−1 at a current density of 50 mA g−1 and maintains a good flex-ibility of the fiber electrodes, which can be potentially integrated into textile fabrics for future wearable electronic applications.

DOI: 10.1002/adfm.201703140

Y. Wang, Prof. J. ZhaoSchool of Chemistry and Chemical EngineeringHarbin Institute of TechnologyHarbin 150001, P. R. ChinaE-mail: [email protected]. Wang, Dr. C. Chen, H. Xie, T. Gao, Y. Yao, G. Pastel, Dr. X. Han, Y. Li, Dr. K. (Kelvin) Fu, Prof. L. HuDepartment of Materials Science and EngineeringUniversity of Maryland College ParkCollege Park, MD 20742, USAE-mail: [email protected]; [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201703140.

and smart designs including 1D fibers, 2D papers, and 3D frameworks to enable sufficient flexibility. For instance, Peng and co-workers have successfully devel-oped various flexible 1D fiber-shaped LIBs by combining aligned multiwall carbon nanotube (MWCNT) fibers with electrode materials including MnO2,[13] Silicon,[14] and LiMn2O4.[15] Yu and co-workers[16] constructed a 2D flexible planar LIBs by directly growing Li4Ti5O12 (LTO) nanotube arrays on stainless steel foil by a template-based solution route integrated with carbon coating. Cui and co-workers[17] demonstrated a smart 3D sponge-inspired structure design to con-struct highly stretchable LIBs by using a sugar template. Lee and co-workers[18] developed a hetero-nanonet rechargeable paper batteries with feasible flexibility even with high mass loading. Among these strategies, most of the 2D and 3D

flexible batteries designs are less able to be woven into flexible textile for wearable applications.[12,19,20] In this context, devel-oping 1D fiber-shaped energy storage devices with the ability to be twisted and woven into flexible, wearable, and breathable textiles is highly desirable, yet remains challenging.

Recently, tremendous efforts have been dedicated to devel-oping 1D energy storage devices with coaxial or twisted struc-tures.[12,21–27] The former structure is capable of coating various active materials into the core–shell architecture. However, it remains challenging to achieve a thin yet uniform coating layer on a curved fiber. The latter structure involves the twisting of two fiber electrodes (anode and cathode) at a certain angle into an integrated fiber-shaped device. Due to the similarity between commercial textile and the twisted fiber structures, it is much easier to incorporate twisted fiber energy storage devices into a commercial textile, thereby making it more attractive for wear-able electronic textile applications. However, major limitations persist in the fabrication cost, efficiency, and scalability, which has hindered their practical application.

3D printing, as one of the most advanced additive manu-facturing technologies, has recently attracted interest due to its high manufacturing efficiency, scalability, low cost, and complexity capability.[28–33] Specifically, 3D extrusion-based printing methods are the easiest and most efficient among 3D fabrication technologies.[29,34–40] They have been widely used in conventional energy storage devices,[34,37] 3D bioscaffolds,[38] 3D evaporators,[29] and functional hydrogels.[39,40] With suitable ink

Batteries

1. Introduction

The growing demand for smart garments,[1] biosignal moni-tors,[2] electronic textiles,[3] and epidermal electronics[4] has fueled a search for powerful, flexible, and wearable energy storage devices.[5–8] As one of the most promising electrochem-ical systems, lithium-ion batteries (LIBs) have been widely used in portable electronics due to their high energy density, high safety, and long cycling life.[9–11] However, conventional LIBs cannot meet the needs of high flexibility and breathability for wearable electronics.[12] Over the past decades, many efforts have been dedicated to developing various advanced materials

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materials, diverse sophisticated patterns can be printed in a fast and scalable way.

Here, for the first time, we demonstrate the design and fabrication of all-fiber quasi-solid-state LIBs using the extrusion-based direct-write 3D printing method. The uniqueness or nov-elty of our work lies in the following aspects: (1) This is the first work that 3D extrusion-based printing technology was used as a fast, scalable, and universal method to prepare fiber-shape cathode and anode electrodes. Although several prior works[34,37] have successfully demonstrated that 3D microbatteries can be fabricated by 3D printing, challenge remains in achieving fiber-shape electrode with good mechanical strength and flexibility, as well as excellent electrochemical performance. (2) The composi-tion of the prepared inks for 3D printing in this work is sim-ilar to the generally used electrode slurry both in the scientific research and industry of lithium-ion batteries, with lithium iron phosphate (LFP) or LTO particles (electrode active materials), poly(vinylidene fluoride) (PVDF) dissolved in n-methyl-2-pyrro-lidone (NMP) , and CNTs (conductive additive) mixing together. In this context, the mature techniques in the lithium-ion bat-teries industry can be directly applied to the 3D printing fabrica-tion of fiber electrode, positioning the developed approach as a highly promising method for efficient, scalable, and cost-effec-tive electrode fabrication. The preparation of inks in the prior works either involves uncommon materials or complicated process, less competitive to our developed one-step approach. (3) The flexible and strong all-fiber quasi-solid-state LIBs design is unique and interesting. For the first time, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) soaked with LiPF6 in EC/DEC as gel electrolyte was successfully integrated (coated) onto the surface of the fiber electrode, contributing to a highly integrated all-fiber LIB structure. Meanwhile, due to the abundant pore structure of the PVDF-co-HFP coated layer, sufficient liquid electrolyte can be absorbed, leading to fast ion transport. On the other hand, the interconnected CNTs facilitate the transport of electrons. As a consequence, excellent electro-chemical performance can be achieved.

Electrode materials of LFP and LTO, were incorporated into the PVDF solution with CNT conductive additive to make the cathode and anode inks, respectively, for printing LFP (cathode) and LTO (anode) fibers (Figure 1a). PVDF-co-HFP was coated

both on the surfaces of LFP and LTO fibers and soaked with limited LiPF6 in EC/DEC solution to serve as quasi-solid-state electrolyte and electronic insulating separator layer. The coated LFP and LTO fibers were then twisted together into one inte-grated fiber with the protection of an outer heat shrink tube. Subsequently, this type of all-fiber LIBs can be integrated into current textiles or woven to fabric directly due to their advan-tageous mechanical flexibility (Figure 1b). The combination of all-fiber flexible 1D device design and the fast, low-cost, and scalable advanced 3D printing fabrication can offer a promising solution for wearable electronics.

2. Results and Discussion

To prepare ideal inks for extrusion 3D printing, the rheological properties should be taken into account. The apparent viscosity as a function of shear stress for cathode and anode inks is given in Figure 2a. Each ink constitutes of PVDF as a binder, CNT as a conductive additive and LFP or LTO nanoparticles as active materials with a mass ratio of 10:3:6 and a total solid materials concentration of 200 mg mL−1. The apparent viscosities of the two inks decrease as shear rate decreases, which characterizes typical shear-thinning or non-Newtonian behavior and is well suited for extrusion 3D printing.[29,41] The high concentration of solid materials results in high apparent viscosity ranging from 104 to 105 Pa s at a shear rate of 10−2 s−1 for both the LFP and LTO inks. A higher apparent viscosity value contributes to a better printability for forming a fiber shape.[42] Figure 2b shows the storage modulus (G′) and loss modulus (G″) of the LFP and LTO inks as a function of shear stress. Storage mod-ulus describes the solidification behavior of the ink in low shear stress conditions, while the loss modulus reflects the liquid-like response. For both inks, the storage modulus plateaus above 103 Pa under low shear stress which is higher than the relevant loss moduli ranging from 102 to 103 Pa. In this region, inks reflect a solid-like response owing to elastic modulus domi-nation, which is crucial for solidification. Note that both inks have a relatively high yield stress of around 1300 Pa, which corresponds to the intersection of the elastic modulus and vis-cous modulus. After reaching the yield point, the relationship

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Figure 1. Schematic of the design concept and fabrication process of 3D printed all-fiber flexible LIBs. a) 3D printing fabrication process. b) The potential application of fiber-shaped batteries for wearable applications.



between G′ and G″ reverses at higher shear stresses. In this region, inks exhibit viscous deformation and a good fluidity, which can ensure reliable flow through fine deposition nozzles. The high elastic moduli and high yield stress are essential for favorable printing and solidification of the fiber electrodes.

For comparison, we also measured the apparent viscosity as a function of shear stress of a conventional LFP electrode slurry. The apparent viscosity value of the conventional slurry is less than 103 Pa s, which is one order of magnitude lower than the printable ink we used, indicating inferior printability (Figure 2c). This can be further confirmed by the flow behavior of the conventional slurry and the printable ink. As shown in Figure S1 (Supporting Information), we put similar quantities of the printable LFP ink and conventional LFP slurry on one upright plate. Slurry can easily flow down under the influence of gravity while our ink remains attached due to its distinct vis-cosity and properties. The storage modulus (G′) and loss mod-ulus (G″) as a function of shear for the conventional LFP slurry are further evaluated and plotted in Figure 2d. Compared with the printable inks, the conventional slurry exhibits a lower pla-teau value of less than 100 Pa for its storage modulus (G′) and a smaller yield stress of ≈40 Pa, respectively.

The above results emphasize that our optimized printing inks are more suitable for 3D printing than the conventional slurry. Figure 2e,f presents the plots of G′ and G″ as a function of time. It shows that G′ and G″ of the LFP and LTO inks are almost unchanged and relatively independent with time change, indicating their stable rheological properties under shear.

The appearance and morphology of the printed fibers are shown in Figure 3. Figure 3a shows the extrusion of the fiber

into ethanol solution from the 3D printing syringe. Typically, the as-prepared ink was ejected into a coagulation bath under 80 psi by an air-powered fluid dispenser. When the ink is extruded into ethanol, it is wrapped by the PVDF fibrous gel with nanoparticles. This facile process produces smooth, con-tinuous and long fibers within a short period of time. After soaking in ethanol for 1 min the fiber is completely formed and exhibits sufficient mechanical strength to be removed from solution and handled with tweezers, as indicated in Figure 3b. Figure 3c shows photographs of the dried LFP fibers with a length of almost 23 cm, which indicates the scalability of the 3D printing process. Figure 3d–g shows the ability of the pris-tine, stretched, spring-shape fibers to lift a heavy ring, thereby demonstrating the excellent flexibility and mechanical strength of the printed fibers, which can be further confirmed by the mechanical tensile test results (Figure S2, Supporting Informa-tion). The morphology of the LFP fibers was characterized by scanning electron microscopy (SEM) as shown in Figure 3g–m. Figure 3g shows the SEM image of a single LFP fiber with a diameter of 200 µm. The cross-sectional SEM images in Figure 3h show that the fiber has a relatively dense structure without big pores. Figure S3 (Supporting Information) gives the elemental mapping data, revealing the uniform distribu-tion of LFP nanoparticles and CNTs in the PVDF scaffold pro-viding mechanical intensity. Magnified SEM images from the outer surface of the LFP fiber in Figure S4 in the Supporting Information show the rough and porous surface of the fiber. More magnified SEM images show that the LFP nanoparticles are uniformly and tightly wrapped by the PVDF binder and CNT conductor, forming interconnected porous networks

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Figure 2. Rheological properties of LFP/CNT/PVDF, LTO/CNT/PVDF inks and traditional LFP slurry ink. a) Apparent viscosity as a function of shear rate for LFP/CNT/PVDF and LTO/CNT/PVDF inks. b) Storage modulus, G′, and loss modulus, G″, as a function of shear stress for LFP/CNT/PVDF inks and LTO/CNT/PVDF inks (upper data sets), respectively. c) Apparent viscosity as a function of shear rate for conventional LFP slurry. d) Storage modulus, G′, and loss modulus, G″, as a function of shear stress for conventional LFP slurry. e) Storage and loss modulus aging as a function of time for LFP/CNT/PVDF ink. f) Storage and loss modulus aging as a function of time for LTO/CNT/PVDF ink.



(Figure S4c, Supporting Information). This effect is attributed to the surface tension caused by the concentration gradient between the solvent and coagulation bath. When the gel fluid flows into the coagulation bath, the concentration of NMP sol-vent is higher than the coagulation bath, so a large concentra-tion gradient occurs at the interface between the fibrous gel and liquid. As a result, NMP flows away from regions with a higher concentration, which is the driving force to enable the solidi-fication process. Accordingly, during fiber formation, many fluidic cross-channels are formed through the fiber structures. These micro channels can facilitate the flow of liquid electrolyte into the inner areas of fibers and enable good ion transfer. A twisted yarn structure is shown in Figure 3i with a tight con-nection. Figure 3j shows the PVDF-co-HFP coated yarn struc-ture. This porous PVDF-co-HFP (Figure 3k) layer with a thick-ness of 8–16 µm can absorb and hold liquid electrolyte locally. Meanwhile, the electrode fibers can contact together intimately (Figure 3l), enabling good electrolyte penetration. Similar morphology for the LTO composite fiber can also be observed (Figure S5, Supporting Information). To determine the porous structure of the printed fiber electrodes, we carried out nitrogen

adsorption/desorption measurements. As shown in Figure S6 in the Supporting Information, both fiber electrodes demon-strate a Type-IV behavior, suggesting the mesoporous nature of the fiber electrodes. The Brunauer-Emmett-Teller (BET) surface areas of the LFP and LTO fiber electrodes are 6.30 and 6.26 m2 g−1, respectively. Additionally, the phase structures of the LFP and LTO composite fibers can be confirmed by Raman spectra and X-ray diffraction (XRD) results (Figures S7–S10, Supporting Information).

Electrochemical performance of the printed LFP fiber and LTO fiber half-cells and the LFP fiber/LTO fiber full cell is shown in Figure 4. Coin cell configurations were used to dem-onstrate and evaluate the electrochemical properties of these cells. The charge and discharge curves for the LFP electrode at a current density of 50 mA g−1 (Figure 4a) give initial charge and discharge capacities of 160.4 and 164.8 mAh g−1, respec-tively, close to the theoretical capacity of LFP (170 mAh g−1) with an initial Coulombic efficiency of ≈97%. The overpotential is small with an average discharge plateau of around 3.37 V. In the following 10th, 20th, and 30th cycles, charge and discharge capacities remain 161 and 160 mAh g−1 (68.8 mAh cm−3 based

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Figure 3. Morphological characterization of the fiber-shaped LFP electrode. a) The optical image of a wet fiber during the printing process. b) The optical image of a wet fiber being removed from the coagulation bath. c) The optical image of a dry fiber. d,e) The optical image of the original and stretched fiber, respectively. f) The optical image of a fiber bearing a burden. g) SEM image of the LFP fiber. h) Cross-sectional view of the LFP fiber. i) SEM image of yarn composed of three LFP fibers. j) SEM image of polymer layer coated on the yarn fibers. k) Magnified SEM image of the gel elec-trolyte coating layer. l) Magnified cross-sectional SEM image of polymer coating on the fibers.



on volume), respectively, suggesting a tiny capacity drop of less than 3% over 30 cycles. Meanwhile, the charge and discharge plateaus from the selected cycles almost overlap with a constant overpotential of ≈0.18 V, indicating the excellent stability of the LFP fiber electrode (Figure 5b).

The LTO fiber half-cell delivers an initial discharge capacity of 189.3 mAh g−1 and charge capacity of 148.1 mAh g−1 (65.5 mAh cm−3 based on volume), suggesting an initial Coulombic efficiency of ≈78%. The initial irreversible capacity loss should be attributed to side reactions between the LTO nanoparticles and electrolyte, as well as the possible forma-tion of a solid electrolyte interphase on the surface of the electrode.[43–45] We should also note that the obtained initial dis-charge capacity is a little higher than the theoretical capacity of LTO (175 mAh g−1), which should be due to the additional inter-facial lithium storage of the abundant solid–solid and solid–liquid interfaces.[46] The charge and discharge curves show flat plateaus at the potential of 1.59 and 1.54 V versus Li/Li+, ascrib-able to the redox reaction between the Ti4+/Ti3+ couple.[47–49] The capacity of the LTO fiber electrode shows significant decay in the initial few cycles (decreases to 120.1 and 125.2 mAh g−1 after 10 cycles) and remains relatively stable in the subsequent cycles (116.7 and 120.4 mAh g−1 after 30 cycles, Figure 4d), sug-gesting an acceptable cyclability from the LTO fiber electrode.

Based on the evaluation of electrochemical performance of the LFP and LTO fiber electrodes, we assembled coin-type full cell and evaluated their performance. Figure 4e,f shows the charge/discharge curves from selected cycles and the cycling stability of the LTO/LFP full cell over 30 cycles at a current density of 50 mA g−1. The combination of LFP and LTO gives an average discharge potential plateau of 1.9 V, consistent with the potential difference between the LFP and LTO fibers. The full cell exhibits reasonable cyclability over 30 cycles without

serious fading. Meanwhile, the charge/discharge plateaus show negligible variation, indicating the stable lithium insertion/extraction process occurring in the full cell.

To further expand the applications of the printed fiber elec-trodes for wearable devices, we constructed an all-fiber LIB device by applying the printed LFP fiber as the positive elec-trode, LTO fiber as the negative electrode, and PVDF-co-HFP gel soaked with LiPF6-EC/DEC as gel electrolyte (Figure 5a). Typically, the all-fiber device was made by twisting the LFP and LTO fibers with gel electrolyte into a single yarn, which was then sealed in a heat-shrinkable tube in an Ar-filled glove box, followed by injecting trace amounts of LiPF6-EC/DEC elec-trolyte. The fiber cell with 9 cathode and 9 anode fibers has a mass loading of 7.6 and 8.2 mg cm−1, respectively. The assem-bled fiber battery was taken outside of the glove box for battery testing. Figure 5b shows the selective charge–discharge curves of the all-fiber device at a current density of 50 mA g−1. The ini-tial charge and discharge capacities are 141.3 and ≈110 mAh g−1, respectively, with an initial Coulombic efficiency of 77.1%. In the 10th cycle, the charge and discharge capacities slightly decrease to 107.4 and ≈100 mAh g−1, respectively, suggesting a significantly improved Coulombic efficiency of 93%. In the fol-lowing cycles, the charge and discharge capacities slowly stabi-lize at 91.7 and 89 mAh g−1, respectively, demonstrating a high capacity retention of 81% over 30 cycles (Figure 5c). Due to the highly integrated twisted fiber configuration, good flexibility and stability are expected. As shown in Figure 5d, the charged all-fiber device can light up a light emitting diode (LED) con-tinuously for several minutes without fading. Moreover, no failure in the LED brightness occurs while manually bending the device for several minutes (Figure 5e). Since the printed fibers have good flexibility and proper mechanical strength, they can be woven into fabric, as demonstrated in Figure 5f,g.

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Figure 4. Performance of half-cell and full cell based on coin cell with liquid electrolyte (1 m LiPF6 in EC/DEC). a) Charge and discharge profiles of the LFP fiber half-cell. b) Cycling stability of the LFP fiber half-cell. c) Charge and discharge profiles of the LTO fiber half-cell. d) Cycling stability of the LTO fiber half-cell. e) Charge and discharge profiles of the full cell. f) Cycling stability profiles of the full cell.



This indicates the feasibility of incorporating the 3D printed all-fiber LIB into wearable or portable devices for a wide range of applications.

3. Conclusion

In conclusion, the smart design and fast fabrication of an all-fiber flexible lithium-ion battery are confirmed using advanced 3D printing technology. Polymer-based inks with electroactive materials exhibit shear-thinning behavior and high apparent viscosity. The inks are also tailored to have a high storage mod-ulus for extended durations and maintain a stable loss modulus. These excellent rheological properties are realized to ensure smooth printing and good solidification. The LFP and LTO fiber electrodes demonstrate high capacity and good cyclability, which in turn enables excellent electrochemical performance of the LFP fiber/LTO fiber full cell. The all-fiber quasi-solid-state LIB device assembled by twisting the as-printed LFP and LTO fibers together with gel polymer electrolyte, exhibits a high discharge specific capacity of ≈110 mAh g−1 at 50 mA g−1, and good flexibility for wearable applications. The fast, low-cost, and scalable 3D printing method developed in this work will open

up new opportunities for smart, flexible, and wearable all-fiber energy storage devices.

4. Experimental SectionPreparation of Inks: LFP nanoparticles (50–100 nm, density =

2.9 g cm−3, Pulead technology Industry Co., Ltd) were used as cathode active material and LTO nanoparticles (<200 nm, density = 3.5 g cm−3, Sigma-Aldrich) were used as anode active material. P3-single-wall carbon nanotube powder (Carbon Solution, Inc.) was used as electric conductive additive. PVDF powder (Mw = 600 000, MTI Corporation) is served as binder and NMP (99.5%, Sigma-Aldrich) acts as solvent.

Highly concentrated gel was prepared by dispersing PVDF powder (1000 mg) in NMP (9.5 mL) and grinding for half an hour under a fume hood. Then, active material (LFP or LTO powder) (600 mg) and CNT powder (300 mg) were added to the resultant PVDF gel to form a homogenized paste by grinding for 30 min. Finally, the inks had a solid mass ratio of 200 mg mL−1.

3D Printing: The 3D printing was conducted by a desktop robot (Fisnar F4200n). The LFP and LTO inks were housed in separate syringe barrels (30 mL, Fisnar Inc.), which were attached by a stainless steel nozzle (500 µm in diameter, Mcmaster-carr). DSP501N air-powered fluid dispenser (Fisnar Inc.) was used to pressurize the barrel and control the ink flow rate. The typical printing pressure for both LFP and LTO inks were 80 psi. The nozzle was immersed in a coagulation bath (ethanol)

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Figure 5. Electrochemical performance of the all-fiber LIB device with gel electrolyte. a) Schematic of the all-fiber LIB device. b) Charge and discharge profiles of the all-fiber LIB device. c) Cycling stability profiles. d,e) Demonstration of the all-fiber device lighting up a LED in an unstrained and bent state. f) Graphical illustration of fiber batteries weaved into textiles. g) A photo image to demonstrate the integration of fiber electrodes into textile fabrics.


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with a move speed of 15 mm s−1. After printing, the fiber was immersed in coagulation bath for 1 min to remove solvent followed by drying at room temperature.

Characterization: The morphology of the samples was characterized by a field emission scanning electron microscopy (JEOL 2100F) operated at 1 kV. Phase structure of the samples was determined by XRD (Rigaku Ultima III using Cu Kα radiation (λ = 1.5406 Å)) and Raman spectrum (Labram Aramis model manfuctured by Horiba Jobin Yvon). The rheological study of inks was conducted on a stress controlled AR 2000 rheometer (TA instruments). All experiments were conducted using 20 mm steel flat plate geometry with solvent trap to prevent evaporation. The temperature was maintained at 25 °C throughout the whole experiment. Two tests were run on each sample: shear rate sweep to get viscosity as a function of shear rate (0.01–100 s−1) and oscillatory stress sweep at a frequency of 1 Hz to get storage (G′) and loss modulus (G″) as a function of shear stress (0.1 to 1000 Pa). Nitrogen adsorption/desorption isotherms were recorded on Micromeritics ASAP 2020 adsorption instrument. The mechanical tensile tests were carried out on an electromechanical universal testing machine.

Assembly of All-Fiber Battery: The gel electrolyte was prepared by dissolving PVDF-co-HFP (Mw = 400 000, Sigma-Aldrich) (0.25 g) in acetone (4.75 g) at room temperature and stirring for 2 h. Fiber-shaped electrodes were prepared by twisting three fibers with a length of 5 cm into a yarn, followed by twisting multiple yarns together. To assemble a fiber-shaped battery, the fiber-shaped electrodes were first coated with the gel polymer, and then twisted together before encasing in a heat-shrinkable tube. At last, 1 m LiPF6 in EC/DEC electrolyte was injected in an Ar-filled glove box. Once the cell was well packaged, it was taken out of the glove box for battery tests under ambient lab conditions.

Battery Performance Evaluation: All the cells were assembled in a high-purity argon-filled glove box. The liquid electrolyte was 1 m LiPF6 in EC/DEC (1:1 by volume). Two types of cells, coin cells and fiber-shape cells were assembled. For coin cell assembling, LFP or LTO composite fiber was directly used as current-collector-free electrode with liquid electrolyte and Cegard 2300 separator. Cells were tested by using a LAND-CT 2001A battery test system. The galvanostatic charge–discharge tests were conducted in a voltage range of 4.0–2 V versus Li/Li+ for the LFP fiber half-cells testing, and a range of 2.5–1 V versus Li/Li+ for the LTO fiber half-cells and full cells testing.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsY.W. and C.C. contributed equally to this work. The authors acknowledge the support of the Maryland NanoCenter and its FabLab and AIMLab. Y.W. would like to acknowledge the financial support from China Scholarship Council (CSC).

Conflict of InterestThe authors declare no conflict of interest.

Keywords3D printing, all-fiber batteries, flexible batteries, lithium-ion batteries, wearable energy storage devices

Received: June 10, 2017Revised: August 1, 2017

Published online: September 14, 2017

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