
==== Front
Natl Sci Rev
Natl Sci Rev
nsr
National Science Review
2095-5138
2053-714X
Oxford University Press

10.1093/nsr/nwae262
nwae262
RESEARCH ARTICLE
MATERIALS SCIENCE
Special Topic: Functional and Smart Fibers
Nsr/4
AcademicSubjects/MED00010
AcademicSubjects/SCI00010
Interface-reinforced high-capacity fiber cathode for wearable Li–S batteries
Huang Lei Data curation Resources Writing - original draft School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore

Zhou Tianzhu Investigation Writing - review & editing School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore

Zhu Siyu Formal analysis School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore

Yang Tianqi Formal analysis Department of Physics, City University of Hong Kong, Hong Kong 999077, China

Zhou Xuhui Writing - review & editing School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore

He Bing Investigation School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore

Wang Shuai Investigation School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore

Yan Wei Funding acquisition State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China

https://orcid.org/0000-0003-0819-8325
Wei Lei Funding acquisition Project administration Writing - review & editing School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore

Corresponding authors. E-mails: weiyan@dhu.edu.cn
Corresponding authors. E-mails: wei.lei@ntu.edu.sg
10 2024
30 7 2024
30 7 2024
11 10 nwae26227 6 2024
14 7 2024
25 7 2024
09 9 2024
© The Author(s) 2024. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.
2024
https://creativecommons.org/licenses/by/4.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Fiber-shaped Li–S batteries are attractive for constructing smart textiles as flexible power solutions due to their high theoretical specific capacity, flexibility and wearability. However, severe interfacial issues, such as the shuttle effect of polysulfides on the cathode side, lead to capacity decay and poor lifespan of the batteries. Herein, we report a fiber-shaped composite cathode with collaborative interface interactions to maintain electrode integrity and boost electrochemical performance. In this architecture, nanosulfur-polyvinylpyrrolidone (nanoS-PVP) particles are uniformly implanted into the few-layer Ti3C2Tx with outstanding electrical conductivity and then coated on aluminum (Al) fiber current collectors. Impressively, nanoS and soluble polysulfides are restricted to the cathode side via synergy physical confinement and chemical adsorption of Ti3C2Tx. The PVP chains on the surface of the nanoS prevent the sulfur from agglomeration and bridge the Ti3C2Tx by abundant hydrogen bonds. The enhanced interface endows the cathode with excellent mechanical flexibility, good adsorption of polysulfides and fast reaction kinetics. Consequently, the prepared Ti3C2Tx/nanoS-PVP@Al cathode exhibits excellent cycling performance (capacity retention of 92.8% after 1000 cycles at 1 C), high-rate capacity (556.2 mAh g−1 at 2.0 C) and high linear capacity (22.9 mAh m−1). Additionally, the fiber-shaped Li–S battery works effectively under deformation and high/low-temperature conditions. It can be integrated into the fabric to power light emitting diodes or charge a smartphone wirelessly.

This work reports a fiber-shaped composite cathode with collaborative interface interactions to maintain electrode integrity and boost electrochemical performance for wearable Li–S batteries.

Li–S batteries
flexible electrodes
interface interactions
fiber-shaped devices
energy storage
Singapore Ministry of Education Academic Research Fund Tier 2 MOE2019-T2-2-127 MOE-T2EP50120-0002 MOE-T2EP50123-0014 Singapore Ministry of Education Academic Research Fund Tier 1 RG62/22 National Natural Science Foundation of China 10.13039/501100001809 52450017 52202167 A*STAR 10.13039/501100001348 A2083c0062 I2001E0067 Institute for Digital Molecular Analytics and Science NTU-PSL Joint Lab collaboration
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pmcINTRODUCTION

Future smart electronic textiles, capable of communicating, sensing, displaying and supplying electricity, have driven research and development in recent years [1,2]. Integrating energy-storage devices into textiles offers exciting opportunities for smart electronic textiles, poised to be self-powered without external power-supply equipment [3]. Conventional planar energy devices exhibit limited flexibility due to their bulky and rigid structures [4]. Consequently, fiber-shaped energy-storage devices are emerging to meet an ideal paradigm for energy-storage textiles that are soft, flexible, deformable and durable [5]. Compared with planar structures, the fiber shape endows them with promising advantages such as flexibility, light weight and feasibility for integration into large-scale textile systems [6,7]. However, these fiber-shaped devices face challenges primarily due to their limited energy-storage capacities [8]. Energy storage of conventional cathode materials such as lithium iron phosphate (LFP), lithium cobalt oxide and lithium nickel cobalt manganese oxide is intrinsically limited by the theoretical ceilings, implying the difficulty in meeting the energy supply needs of power-consuming wearable devices [9,10]. To seek capacity breakthroughs, researchers are exploring next-generation flexible batteries with high theoretical specific capacity and low cost, contributing to the fast growth of the wearable electronics market [11]. For instance, lithium–sulfur (Li–S) batteries, known for their exceptional theoretical specific capacity (1675 mAh g−1) and energy density (2600 Wh kg−1), represent a promising alternative [12–15]. Nevertheless, attempts to develop flexible sulfur cathodes face severe interfacial issues, such as the polysulfide shuttle effect and the formation of lithium dendrites, which result in capacity decay and poor lifespan of the batteries [16,17].

Several improvements have been made to develop eligible flexible sulfur cathodes to improve the geometric restrictions of planar devices. Fibrous materials are typically considered to be ideal hosts for sulfur due to their large surface area, excellent flexibility, good mechanical properties and lightweight characteristics. In particular, designing 1D Li–S fiber batteries can achieve maximum flexibility and wearability, which is beneficial for their successful application as power accessories in wearable devices. Previously, Peng et al. reported a 1D cable-shaped Li–S battery assembled with a sulfur-containing carbon nanostructured hybrid fiber cathode and Li wire [8]. The cable-shaped battery delivered an initial discharge capacity of 1051 mAh g−1 at 0.1 C and the capacity remained at ∼600 mAh g−1 after 100 cycles. Subsequently, two more fibrous Li–S fiber batteries with carbon-based hosts were developed, improving the electrochemical performance by constructing a composite carbon/sulfur cathode using wet spinning technology [18,19]. Except for carbon-based hosts, Liu et al. produced fibrous sulfur composite cathodes using stainless steel fibers as supports and current collectors, while the fiber battery exhibited a fast capacity decay compared with coin cells [20]. Despite several explorations, interface issues of fiber Li–S batteries have rarely been effectively addressed, especially in inhibiting the shuttle of polysulfides, which is crucial for improving the performance of Li–S batteries.

In this work, we present the construction of composite fiber sulfur cathodes with significant interface interactions, in which nanosized sulfur-polyvinylpyrrolidone (nanoS-PVP, ∼500 nm) are uniformly implanted into the few-layer Ti3C2Tx and then uniformly coated on Al fiber current collectors. Note that nanoS and the intermediate soluble polysulfides can synergistically be hosted by Ti3C2Tx, manifested as a synergistic effect of physical confinement and chemical adsorption. Meanwhile, the PVP chains on the surface of nanoS not only prevent the sulfur from agglomeration, but also form hydrogen bonding (H-bond) with surface groups (e.g. –OH) of Ti3C2Tx, effectively suppressing sulfur detachment and maintaining cathode integrity. Density functional theory (DFT) calculations further demonstrate the strong interfacial interaction of lithium polysulfides with Ti3C2Tx, as well as enhanced reaction kinetics. As a result, our prepared Ti3C2Tx/nanoS-PVP@Al cathodes exhibit noteworthy flexibility, competitive electrochemical cycling performance, good high-rate capacity and high linear capacity. Moreover, fiber Li–S batteries work efficiently under deformation and high/low-temperature conditions, and can be integrated into commercial textiles to power light emitting diodes (LEDs) or woven into a phone bag for wirelessly charging a smartphone. Our work provides a general strategy to design advanced flexible cathodes for energy storage and conversion.

RESULTS AND DISCUSSION

The design strategy of composite fiber cathode

The two-step preparation of the Ti3C2Tx/nanoS-PVP material is shown in Fig. 1a. First, the synthesis of nanoS-PVP was achieved through chemical reactions between Na2S2O3 and HCl, with PVP acting as nucleation sites and dispersants, effectively controlling the average size of the sulfur at the nanoscale. Due to the poor conductivity of sulfur, reducing its size was considered an effective way to improve its utilization [21]. Next, few-layer Ti3C2Tx solutions (5 g L−1) were mixed with the collected nanoS-PVP and stirred, resulting in the formation of Ti3C2Tx/nanoS-PVP composites due to group interactions on the surfaces of the nanoS-PVP and Ti3C2Tx. Insight into the composite interfaces between nanoS-PVP and Ti3C2Tx is proposed in Fig. 1b, highlighting its potential as an ideal cathode material for Li–S batteries: (i) nanoS-PVP particles are encapsulated by few-layer Ti3C2Tx materials, improving the conductivity of the cathode and the utilization of active materials. (ii) Interface interaction ensures cathode integrity. The H-bond interaction between PVP chains wrapped in nanoS and Ti3C2Tx prevents the active sulfur from detaching during cathode deformation, reducing the production of ‘dead S’. (iii) Shuttle effect inhibition—the synergistic effect of physical confinement and chemical adsorption (Ti3C2Tx-polysulfides) suppresses the diffusion of polysulfides and reduces side reactions. (iv) The significant catalytic conversion ability of Ti3C2Tx, reported in several battery systems, accelerates the catalytic conversion kinetics of polysulfides [22,23]. As a demonstration, fibrous Ti3C2Tx/nanoS-PVP-based composite S cathodes were prepared for fiber-shaped Li–S batteries. Figure 1c illustrates the construction of the composite S cathode. In this architecture, Al fiber is applied as the current collector and Ti3C2Tx/nanoS-PVP slurries are uniformly coated via the dip-coating method. A Ti3C2Tx/nanoS-PVP@Al cathode with a length of 5 m can be easily fabricated (Fig. 1d), revealing the potential of scalable production. Furthermore, the abundant interface interactions endow fibers with a robust flexible structure that withstands rigorous deformations (e.g. bent, knotted, enwound and bundled, Fig. 1e–h) and an excellent tensile strength of ∼66.7 MPa (Fig. 1i and Fig. S1).

Figure 1. (a) Schematic fabrication process of Ti3C2Tx/nanoS-PVP. (b) Schematic illustration of composite interfaces between nanoS-PVP and Ti3C2Tx. (c) Structural diagram of fiber Ti3C2Tx/nanoS-PVP@Al cathode. Photos of Ti3C2Tx/nanoS-PVP@Al fibers: (d) collected on a spindle, (e) bent, (f) knotted, (g) enwound, (h) bundled and (i) loaded.

Characterizations of fiber Li–S batteries

The morphology and microstructure of the prepared Ti3C2Tx/nanoS-PVP@Al fiber cathode were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-solution transmission electron microscopy (HRTEM). The average diameter of the Ti3C2Tx/nanoS-PVP@Al fibers is ∼360 μm, featuring a slightly wrinkled and crack-free surface (Fig. 2a), which is superior to the carbon/nanoS-PVP@Al cathode that shows obvious cracks (Fig. S2). The enlarged view of Ti3C2Tx/nanoS-PVP reveals the accurate distribution of nanoS-PVP and Ti3C2Tx (Fig. 2b). The nanoS-PVP particles (∼500 nm) are well encapsulated by the few-layer Ti3C2Tx (Fig. S3a), providing more effective dispersion compared with pure nanoS-PVP (Fig. S3b). Moreover, a continuous electron-transfer network is formed, improving the overall conductivity of the cathode and enhancing sulfur utilization efficiency. The constructed Ti3C2Tx/nanoS-PVP@Al fibers show a high conductivity of ∼8.2 S cm−1, verifying good electron transfer of the Ti3C2Tx/nanoS-PVP@Al electrode (Fig. S4). TEM characterization further confirms the microstructure of Ti3C2Tx/nanoS-PVP (Fig. 2c). HRTEM images of Ti3C2Tx (Fig. 2d) show a few-layer structure with a monolayer thickness (without residues) of ∼0.98 nm, consistently with previous reports [24]. As shown in Fig. 2e, nanoS-PVP displays a typical amorphous configuration. PVP chains are captured on the surface of nanoS (Fig. 2f) to anchor and stabilize nanoS on Ti3C2Tx. A Cu/Li anode and gel electrolyte were also prepared to assemble full Li–S fiber batteries. As shown in Fig. S5, the Cu/Li fiber anode was fabricated by coating molten Li on the surface of Cu fiber (∼100 μm). After loading Li, the diameter of the composite Cu/Li fiber increases to 350 μm. The gel electrolyte was prepared by dissolving polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) into the commercial Li–S liquid electrolyte at 80°C and then cooling it down (Fig. S6). The cross-sectional SEM image (Fig. 2g) and the corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping (Fig. 2h–k) demonstrate the integrity of the architecture and uniform element dispersion of electrode fibers.

Figure 2. (a and b) Scanning electron microscopy (SEM) and (c) transmission electron microscopy (TEM) images of Ti3C2Tx/nanoS-PVP-based fiber cathode. High-resolution TEM (HRTEM) images of (d) few-layer Ti3C2Tx, (e and f) nanoS-PVP. (g) Cross-sectional SEM image and (h–k) corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mappings of a Li–S fiber battery.

Fourier transform infrared spectrometer (FTIR) measurement further demonstrates the interfacial interaction between nanoS-PVP and Ti3C2Tx (Fig. 3a). Ti3C2Tx/nanoS-PVP composites exhibit characteristic peaks of nanoS-PVP and Ti3C2Tx without new chemical bond formation. The C–N stretching vibration (∼1281.8 cm−1) and C=O vibration (∼1662.0 cm−1) of nanoS-PVP are weakened and have a certain shift (∼1278.2 cm−1 of C–N and ∼1619.4 cm−1 of C=O) after being combined with Ti3C2Tx, indicating the H-bond interaction of C=O, C–N with Ti3C2Tx residues (e.g. –OH) [25]. Thermogravimetric (TG) analysis (Fig. 3b) of Ti3C2Tx/nanoS-PVP reveals the content of nanoS-PVP to be as high as 90.9 wt%. Considering the mass ratio of PVP to nanoS, it can be inferred that the sulfur loading of the composite is ∼81.8 wt%. Additionally, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements were conducted to investigate the chemical compositions of the samples. As shown in Fig. 3c, the XRD pattern of Ti3C2Tx/nanoS-PVP exhibits characteristic sulfur peaks (JCPDS 53–1109) and a (002) peak of few-layer Ti3C2Tx (characterizations of multi-layer Ti3C2Tx are supported in Fig. S7) [26], verifying the successful incorporation of sulfur on few-layer Ti3C2Tx. As depicted in high-resolution Ti 2p spectra (Fig. 3d), the binding energies at 454.5/460.5, 455.6/462.1 and 458.4/464.2 eV belong to Ti–C, Ti–O–C and Ti–O, respectively [27]. The peaks of O 1s at 529.3, 530.0 and 531.2 eV correspond to Ti–O, C–Ti–O and C–Ti–OH, respectively (Fig. 3e) [28]. In the S 2p spectrum, two peaks at 163.2 and 168.3 eV are consistent with S0 and a small amount of sulfate (Fig. 3f) [29,30]. The above results indicate that the nanoS-PVP is bridged with few-layer Ti3C2Tx to fabricate Ti3C2Tx/nanoS-PVP through the H-bond.

Figure 3. (a) Fourier transform infrared spectrometer (FTIR) spectra and (b) thermogravimetric (TG) analysis of Ti3C2Tx/nanoS-PVP, nanoS-PVP and Ti3C2Tx. (c) X-ray diffraction (XRD) patterns of Ti3C2Tx/nanoS-PVP and few-layer Ti3C2Tx. High-resolution X-ray photoelectron spectroscopy (XPS) spectra of (d) Ti 2p, (e) O 1s and (f) S 2p of Ti3C2Tx/nanoS-PVP. (g) Top and front view of Li2S6 adsorbed on Ti3C2Tx–OH. (h) Adsorption energies for sulfur species on Ti3C2Tx–OH and pure carbon. (i) Gibbs free energy plots of S8-to-Li2S conversion on Ti3C2Tx–OH and pure carbon. (j) Electronic density of states (DOS) analysis before and after Ti3C2Tx–OH and Li2S6 interaction.

Determination of adsorption and catalytic conversion of polysulfides

To illustrate the strong interfacial interactions between Ti3C2Tx and various sulfur species (Li2S8, Li2S6, Li2S4, Li2S2, Li2S and S8), and to simulate the immobilization and catalytic conversion kinetics, DFT calculations were conducted. In our case, Ti3C2Tx–OH, of which the content is abundant through FTIR and XPS analysis, is employed as the modeling molecule to calculate the binding energies (ΔE) with different states of sulfur species. Figure 3g displays the top and front views of Li2S6 adsorbed on the Ti3C2Tx–OH. Other adsorption structures of Ti3C2Tx–OH and carbon absorbed with lithium polysulfides are exhibited in Figs S8 and S9. The summary bar chart of adsorption energies (Fig. 3h) shows that the ΔE of Li2S, Li2S2, Li2S4, Li2S6, Li2S8 and S8 with Ti3C2Tx–OH is −3.26, −2.73, −3.80, −3.67, −3.69 and −2.37 eV, respectively, which is significantly higher than the ΔE absorbed with pure carbon surfaces (−0.82, −0.80, −0.53, −0.76, −0.86 and −0.68 eV). This demonstrates that the Ti3C2Tx–OH possesses a much stronger affinity to the sulfur species, thereby enhancing its adsorption capacity for soluble lithium polysulfides and improving electrochemical performance. The enhanced adsorption capabilities of Ti3C2Tx are further verified by ex situ visible adsorption measurements and ultravioletvisible (UV–vis) spectroscopy (Fig. S10). The pure Li2S6 solution was applied as the representative for soluble long-chain polysulfide species [31]. Equal amounts of Ti3C2Tx and carbon samples were immersed into the Li2S6 solution for 24 h and the corresponding optical images were recorded (Fig. S10a). After 24 h, the solution with the Ti3C2Tx became clear and transparent, while the solution with carbon was still yellowish, indicating that Ti3C2Tx has stronger polysulfide adsorption ability than carbon. UV–vis spectroscopy after adsorption further confirmed these results (Fig. S10b). The densities of characteristic Li2S6 adsorption peaks of Ti3C2Tx and carbon samples both decrease. Especially for the Ti3C2Tx sample, the characteristic peak has almost disappeared, suggesting that soluble polysulfides are adsorbed by Ti3C2Tx.

Figure 3i illustrates that Ti3C2Tx–OH has a lower Gibbs free energy than carbon during the reaction from S8 to Li2S, demonstrating easier reduction processes of polysulfides on the surface of Ti3C2Tx–OH. Notably, the overall free energy on the surface of Ti3C2Tx–OH and carbon is −6.16 and −0.72 eV, respectively, suggesting that the whole reaction process on the surface of Ti3C2Tx–OH is more inclined towards being thermodynamically spontaneous [32]. In the rate-limiting steps of the reaction path with positive free energy, the Ti3C2Tx–OH reduces the overall positive energy from 4.17 to 2.40 eV, indicating improved reaction kinetics for lower reaction barriers. Moreover, electronic density of state (DOS) analysis of the Ti3C2Tx–OH on the redox and transformation of polysulfides was performed to reveal interfacial charge regulation mechanisms. The total DOS and the Ti 3d orbital projected DOS of Ti3C2Tx–OH before and after Li2S6 probe molecule adsorption were investigated. Due to the chemical interaction between Ti3C2Tx–OH and Li2S6, the DOS curves show a certain deviation and significant electron concentration enhancement peaks are observed near Ef. More charge compensation from the d-band of the Ti atom enters the adsorbed Li2S6, resulting in more d–p orbital hybridization and enhancing the charge compensation effect, thus promoting interfacial redox and conversion kinetics [33]. Furthermore, the accelerated polysulfide catalytic conversion ability of the Ti3C2Tx host is validated via cyclic voltammogram (CV) curves of Li2S6 symmetric batteries (Fig. S11). Compared with the pure carbon sample, the Ti3C2Tx assembled battery shows higher current density and sharper redox peak, verifying that the Ti3C2Tx has better polysulfide catalytic conversion efficiency and faster reaction kinetics.

Electrochemical performance of fiber Li–S batteries

The composite interface interaction and remarkable sulfur species encapsulation and adsorption ability make the Ti3C2Tx/nanoS-PVP@Al an ideal cathode for forming flexible Li–S batteries. The electrochemical performance of Ti3C2Tx/nanoS-PVP@Al, Ti3C2Tx/S@Al and C/nanoS-PVP@Al as cathodes for fiber Li–S batteries was evaluated. CV curves of different cathodes with a scan rate of 0.1 mV s−1 are displayed in Fig. 4a. The Ti3C2Tx/nanoS-PVP@Al cathode exhibits two pairs of redox peaks, corresponding to the serial oxidation/reduction of solid S8 to soluble long-chain Li2Sx (4 ≤ x ≤ 8) and solid Li2S2/Li2S [34]. Compared with Ti3C2Tx/S@Al and C/nanoS-PVP@Al, the Ti3C2Tx/nanoS-PVP@Al cathode delivers the lowest voltage polarization and highest peak current density, revealing the increased capacity and enhanced reaction kinetics that are further confirmed by using galvanostatic charge/discharge plots (Fig. 4b) and electrochemical impedance spectroscopy (EIS) analysis (Fig. S12). As shown in Fig. 4b, the Ti3C2Tx/nanoS-PVP cathode presents a polarization of ∼0.41 V at a constant current density of 0.1 C, which is much lower than those of the Ti3C2Tx/S@Al and C/nanoS-PVP@Al counterparts, which are >0.45 V. EIS analysis of assembled cells shows a semicircle in the high-frequency range, which contains charge-transfer resistance (Rct) and overlapped solid–electrolyte interphase resistance (Rs) [35]. The Rct values of Ti3C2Tx/nanoS-PVP@Al, Ti3C2Tx/S@Al and C/nanoS-PVP@Al are approximately 64.2, 72.6 and 107.1 Ω, respectively, demonstrating the fastest electrochemical reaction kinetics of the Ti3C2Tx/nanoS-PVP@Al cathode. The above results indicate that the Ti3C2Tx/nanoS-PVP@Al cathode has enhanced electrochemical redox reaction kinetics, largely attributed to the hierarchical electrode structure and excellent catalytic conversion performance. First, few-layer Ti3C2Tx links charge transport paths and provide space for the loading of nanoS, while composite PVP chains on nanoS particles prevent the agglomeration and detachment of nanoS via interface interactions such as H-bond forces. Second, soluble lithium polysulfides can be effectively limited to the cathode side due to multiscale physical barriers and plentiful chemisorption sites for polysulfides, thus decelerating the ‘shuttle effect’ during the reaction process. Accordingly, the Ti3C2Tx/nanoS-PVP@Al cathode exhibits higher rate performance (Fig. 4c) and more stable cycling performance (Fig. 4d). The rate superiority is revealed with higher discharge capacities of 1210.9 (0.1 C), 990.2 (0.2 C), 812.8 (0.5 C), 716.8 (1.0 C), 556.2 (2.0 C) and 1054.2 (0.1 C) mAh g−1 being achieved, which are much better than those of the Ti3C2Tx/S@Al and C/nanoS-PVP@Al cathodes. For long-term cycling performance, the Ti3C2Tx/nanoS-PVP@Al cathode maintains a capacity of 912.6 mAh g−1 after 100 cycles at 0.1 C, with a capacity retention of 74.8%, which is superior to those of the Ti3C2Tx/S@Al (549.2 mAh g−1, 45.0%) and C/nanoS-PVP@Al (434.5 mAh g−1, 36.8%, 50 cycles) cathodes. Furthermore, 99.1% of the overall coulombic efficiency of the Ti3C2Tx/nanoS-PVP@Al cathode can be achieved during the cycling, which is higher than those of the Ti3C2Tx/S@Al (98.7%) and C/nanoS-PVP@Al (96.6%) counterparts. Impressively, even at a high current rate of 1.0 C, the Ti3C2Tx/nanoS-PVP@Al cathode preserves an exceptional cycling performance with an initial specific discharge capacity of 674.9 mAh g−1 and capacity retention of 92.8% after 1000 cycles (Fig. S13).

Figure 4. Electrochemical performances of Ti3C2Tx/nanoS-PVP@Al, Ti3C2Tx/S@Al and C/nanoS-PVP@Al fiber cathodes. (a) Cyclic voltammogram (CV) curves at a scan rate of 0.1 mV s−1, (b) galvanostatic charge/discharge plots, (c) rate performance, (d) cycling performance at 0.1 C. (e) The specific capacities of the fiber Li–S battery with different bending angles. (f) Rate performances of Ti3C2Tx/nanoS-PVP@Al cathode at 0°C, 30°C and 60°C. (g) The capacity and specific capacity of fiber Li–S battery with different lengths. (h) Comparisons of the linear and specific capacity of this fiber Li–S battery with previously reported flexible fiber batteries.

Fiber-shaped batteries may work under deformation and high/low-temperature adversity. The electrochemical performances of the fiber Li–S battery assembled with a Ti3C2Tx/nanoS-PVP@Al cathode under different bending states and temperatures were investigated (Fig. 4e and f, and Fig. S14). The specific capacity of the fiber Li–S battery showcases almost negligible capacity degradation after bending from 0° to 60°, 90°, 120° and back to 0° (Fig. 4e), indicating excellent flexible energy-storage capability. Meanwhile, even after repetitive bending for 1000 cycles, the fiber battery retains ∼98.8% capacity (Fig. S14), demonstrating superior flexibility and durability, which are highly desired for wearable electronic devices. Furthermore, the high/low-temperature tolerance of the fiber Li–S battery is shown in Fig. 4f. Under conditions of 0°C and 60°C, the fiber battery operates normally at different current densities (0.1–2.0 C) and displays initial specific capacities of 1116.4 and 1024.7 mAh g−1 at 0.1 C, respectively, reaching 93.0% and 85.4% of the battery performance at 30°C (1200.4 mAh g−1). In addition to excellent gravimetric capacity (attributed to the cathode), the fiber battery achieved a high linear capacity, as shown in Fig. 4g. Within a certain range (1–10 cm), a linear relationship between the capacity and fiber length is proved, with a slope of the fitted line indicating a linear capacity of ∼22.9 mAh m−1. Compared with representative fiber batteries measured in specific and linear capacity, our prepared fiber Li–S battery exhibits great advantages (Fig. 4h), paving a promising path for next-generation wearable electronics [3,36–42].

Applications of fiber Li–S batteries

The open-circuit voltage of a single Li–S fiber battery is ∼2.94 V (Fig. 5a). When two fibers are assembled in series, the obtained output voltage doubles, as shown in the CV curve (Fig. 5b) and galvanostatic charge/discharge plots (Fig. 5c). Fiber Li–S batteries can be integrated into commercial textiles to power wearable devices. In this case, the fiber Li–S batteries were woven into a soft cloth to light up an LED. As shown in Fig. 5d and e, two fiber batteries in series easily light up a yellow LED and function normally even when bent, crumpled or submerged in water. Moreover, the fiber batteries were woven into a phone bag for wireless charging. As demonstrated in Fig. 5f, fiber Li–S batteries and a wireless transmitting coil were connected and embedded in a commercial phone bag. The charging process can be triggered when the smartphone is stuffed into the bag and reaches the wireless coil sensing area (Fig. 5g and Video S1). These results highlight the promising potential of fiber Li–S batteries for application in next-generation wearable electronic devices and smart fabrics.

Figure 5. (a) Photograph of the output voltage of the fiber Li–S battery. (b) CV curve and (c) charge/discharge plots of two fiber batteries in series connection. Two fiber Li–S batteries were woven into a soft cloth (d) to light a yellow LED and (e) light an LED while bent, crumpled or submerged in water. (f) Schematic of the design strategy for wireless charging phone bag. (g) Photographs of a smartphone before and after wireless charging by the phone bag.

CONCLUSION

In summary, a highly flexible and mechanically robust composite fiber sulfur cathode with significant interface interactions was prepared by cascading assemblies of nanoS-PVP and few-layer Ti3C2Tx. Notably, nanoS and polysulfides were synergistically hosted by Ti3C2Tx with a synergistic effect of physical confinement and chemical adsorption. Meanwhile, the H-bond interaction between PVP chains wrapped in nanoS and Ti3C2Tx prevented active sulfur from falling off during cathode deformation. The fabricated Ti3C2Tx/nanoS-PVP@Al fiber cathode exhibited excellent cycling performance (capacity retention of 92.8% after 1000 cycles at 1 C), high-rate capacity (556.2 mAh g−1 at 2.0 C) and high linear capacity of ∼22.9 mAh m−1. Moreover, fiber Li–S batteries based on a Ti3C2Tx/nanoS-PVP@Al fiber cathode showed sufficient flexibility and stability to endure 1000 deformation cycles and work effectively under high and low temperatures. The fiber batteries were integrated into commercial textiles to power LEDs or woven into a phone bag for wirelessly charging a smartphone. This work provides new insights into the preparation of high-performance wearable energy-storage devices.

Supplementary Material

nwae262_Supplemental_Files

FUNDING

This work was supported by the Singapore Ministry of Education Academic Research Fund Tier 2 (MOE2019-T2-2-127, MOE-T2EP50120-0002 and MOE-T2EP50123-0014), the Singapore Ministry of Education Academic Research Fund Tier 1 (RG62/22), the National Natural Science Foundation of China (52450017 and 52202167), A*STAR under AME IRG (A2083c0062), A*STAR under IAF-ICP Programme I2001E0067 and the Schaeffler Hub for Advanced Research at NTU, the IDMxS (Institute for Digital Molecular Analytics and Science) by the Singapore Ministry of Education under the Research Centres of Excellence scheme, and the NTU-PSL Joint Lab collaboration.

AUTHOR CONTRIBUTIONS

L.H. and L.W. proposed the original idea. L.W. and W.Y. supervised the project. L.H. conducted the fabrication and carried out battery tests. S.Z. assisted in the HR-TEM test. L.H. and T.Y. performed the mechanism analysis. T.Z., S.W., B.H. and X.Z. assisted in the demonstration. L.H. and L.W. co-wrote the manuscript. All the authors discussed the results and commented on the manuscript.

Conflict of interest statement. None declared.
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REFERENCES

1. Zeng  K, Shi  X, Tang  C  et al.  Design, fabrication and assembly considerations for electronic systems made of fibre devices. Nat Rev Mater  2023; 8 : 552–61.10.1038/s41578-023-00573-x
2. Qian  S, Liu  M, Dou  Y  et al.  A ‘Moore's law’ for fibers enables intelligent fabrics. Natl Sci Rev  2022; 10 : nwac202.10.1093/nsr/nwac202 36684517
3. Liao  M, Wang  C, Hong  Y  et al.  Industrial scale production of fibre batteries by a solution-extrusion method. Nat Nanotechnol  2022; 17 : 372–7.10.1038/s41565-021-01062-4 35058651
4. Khudiyev  T, Grena  B, Loke  G  et al.  Thermally drawn rechargeable battery fiber enables pervasive power. Mater Today  2022; 52 : 80–9.10.1016/j.mattod.2021.11.020
5. Chen  C, Feng  J, Li  J  et al.  Functional fiber materials to smart fiber devices. Chem Rev  2023; 123 : 613–62.10.1021/acs.chemrev.2c00192 35977344
6. He  J, Lu  C, Jiang  H  et al.  Scalable production of high-performing woven lithium-ion fibre batteries. Nature  2021; 597 : 57–63.10.1038/s41586-021-03772-0 34471277
7. Chen  L, Li  R, Yuan  S  et al.  Fiber-shaped artificial optoelectronic synapses for wearable visual-memory systems. Matter  2023; 6 : 925–39.10.1016/j.matt.2022.12.001
8. Fang  X, Weng  W, Ren  J  et al.  A cable-shaped lithium sulfur battery. Adv Mater  2016; 28 : 491–6.10.1002/adma.201504241 26585740
9. Huang  X, Wang  C, Li  C  et al.  Braided fiber current collectors for high-energy-density fiber lithium-ion batteries. Angew Chem Int Ed  2023; 62 : e202303616.10.1002/anie.202303616
10. Wang  L, Zhang  Y, Bruce  PG. Batteries for wearables. Natl Sci Rev  2022; 10 : nwac062.10.1093/nsr/nwac062 36684516
11. Huang  L, Guan  T, Su  H  et al.  Synergistic interfacial bonding in reduced graphene oxide fiber cathodes containing polypyrrole@sulfur nanospheres for flexible energy storage. Angew Chem Int Ed  2022; 61 : e202212151.10.1002/anie.202212151
12. Kim  J-H, Lee  Y-H, Cho  S-J  et al.  Nanomat Li-S batteries based on all-fibrous cathode/separator assemblies and reinforced Li metal anodes: towards ultrahigh energy density and flexibility. Energy Environ Sci  2019; 12 : 177–86.10.1039/C8EE01879K
13. Gao  Y, Guo  Q, Zhang  Q  et al.  Fibrous materials for flexible Li-S battery. Adv Energy Mater  2021; 11 : 2002580.10.1002/aenm.202002580
14. Zhou  S, Shi  J, Liu  S  et al.  Visualizing interfacial collective reaction behaviour of Li–S batteries. Nature  2023; 621 : 75–81.10.1038/s41586-023-06326-8 37673990
15. Zhou  C, Li  Z, Xu  X  et al.  Metal-organic frameworks enable broad strategies for lithium-sulfur batteries. Natl Sci Rev  2021; 8 : nwab055.10.1093/nsr/nwab055 34987837
16. Huang  L, Li  J, Liu  B  et al.  Electrode design for lithium-sulfur batteries: problems and solutions. Adv Funct Mater  2020; 30 : 1910375.10.1002/adfm.201910375
17. Fang  M, Han  J, He  S  et al.  Effective screening descriptor for MXenes to enhance sulfur reduction in lithium-sulfur batteries. J Am Chem Soc  2023; 145 : 12601–08.10.1021/jacs.3c01834 37276342
18. Li  L, Hou  L, Cheng  J  et al.  A flexible carbon/sulfur-cellulose core-shell structure for advanced lithium-sulfur batteries. Energy Storage Mater  2018; 15 : 388–95.10.1016/j.ensm.2018.08.019
19. Chong  WG, Huang  J-Q, Xu  Z-L  et al.  Lithium-sulfur battery cable made from ultralight, flexible graphene/carbon nanotube/sulfur composite fibers. Adv Funct Mater  2017; 27 : 1604815.10.1002/adfm.201604815
20. Liu  R, Liu  Y, Chen  J  et al.  Flexible wire-shaped lithium-sulfur batteries with fibrous cathodes assembled via capillary action. Nano Energy  2017; 33 : 325–33.10.1016/j.nanoen.2016.12.049
21. Meng  T, Gao  J, Zhu  J  et al.  Unearth the understanding of interfacial engineering techniques on nano sulfur cathodes for steady Li-S cell systems. J Mater Chem A  2020; 8 : 11976–85.10.1039/D0TA04592F
22. Zhou  Y, Yin  L, Xiang  S  et al.  Unleashing the potential of MXene-based flexible materials for high-performance energy storage devices. Adv Sci  2024; 11 : 2304874.10.1002/advs.202304874
23. Huang  Z, Zhu  Y, Kong  Y  et al.  Efficient synergism of chemisorption and wackenroder reaction via heterostructured La2O3-Ti3C2Tx-embedded carbon nanofiber for high-energy lithium-sulfur pouch cells. Adv Funct Mater  2023; 33 : 2303422.10.1002/adfm.202303422
24. Rong  C, Su  T, Li  Z  et al.  Elastic properties and tensile strength of 2D Ti3C2Tx MXene monolayers. Nat Commun  2024; 15 : 1566.10.1038/s41467-024-45657-6 38378699
25. Jiang  M, Li  M, Cui  C  et al.  Molecular-level interfacial chemistry regulation of MXene enables energy storage beyond theoretical limit. ACS Nano  2024; 18 : 7532–45.10.1021/acsnano.3c12329 38412072
26. Wang  M, Li  D, Xu  H  et al.  Flexible Ti3C2Tx MXene bonded bio-derived carbon fibers support tin disulfide for fast and stable sodium storage. Small  2023; 20 : 2305530.10.1002/smll.202305530
27. Wang  X, Mathis  TS, Li  K  et al.  Influences from solvents on charge storage in titanium carbide MXenes. Nat Energy  2019; 4 : 241–8.10.1038/s41560-019-0339-9
28. Tang  H, Li  W, Pan  L  et al.  A robust, freestanding MXene-sulfur conductive paper for long-lifetime Li-S batteries. Adv Funct Mater  2019; 29 : 1901907.10.1002/adfm.201901907
29. Cantrell  KJ, Yabusaki  SB, Engelhard  MH  et al.  Oxidation of H2S by iron oxides in unsaturated conditions. Environ Sci Technol  2003; 37 : 2192–9.10.1021/es020994o 12785525
30. Cha  BJ, Choi  JY, Ji  Y  et al.  Fe-oxide/Al2O3 for the enhanced activity of H2S decomposition under realistic conditions: mechanistic studies by in-situ DRIFTS and XPS. Chem Eng J  2022; 443 : 136459.10.1016/j.cej.2022.136459
31. Huang  L, Shen  S, Zhong  Y  et al.  Multifunctional hyphae carbon powering lithium-sulfur batteries. Adv Mater  2022; 34 : 2107415.10.1002/adma.202107415
32. Gu  H, Yue  W, Hu  J  et al.  Asymmetrically coordinated Cu-N1C2 single-atom catalyst immobilized on Ti3C2Tx MXene as separator coating for lithium-sulfur batteries. Adv Energy Mater  2023; 13 : 2204014.10.1002/aenm.202204014
33. Han  Z, Gao  R, Wang  T  et al.  Machine-learning-assisted design of a binary descriptor to decipher electronic and structural effects on sulfur reduction kinetics. Nat Catal  2023; 6 : 1073–86.10.1038/s41929-023-01041-z
34. Zhang  X, Ni  Z, Bai  X  et al.  Hierarchical porous N-doped carbon encapsulated fluorine-free MXene with tunable coordination chemistry by one-pot etching strategy for lithium-sulfur batteries. Adv Energy Mater  2023; 13 : 2301349.10.1002/aenm.202301349
35. Shen  S, Huang  L, Tong  X  et al.  A powerful one-step puffing carbonization method for construction of versatile carbon composites with high-efficiency energy storage. Adv Mater  2021; 33 : 2102796.10.1002/adma.202102796
36. Ling  S, Li  X, Zhou  T  et al.  Densifiable ink extrusion for roll-to-roll fiber lithium-ion batteries with ultra-high linear and volumetric energy densities. Adv Mater  2023; 35 : 2211201.10.1002/adma.202211201
37. Hoshide  T, Zheng  Y, Hou  J  et al.  Flexible lithium-ion fiber battery by the regular stacking of two-dimensional titanium oxide nanosheets hybridized with reduced graphene oxide. Nano Lett  2017; 17 : 3543–9.10.1021/acs.nanolett.7b00623 28535338
38. Cheng  X, Gao  H, Tian  X  et al.  Tunable fabric zinc-based batteries utilizing core-shell like fiber electrodes with enhanced deformation durability. Nano Energy  2024; 125 : 109501.10.1016/j.nanoen.2024.109501
39. Li  Y, Wang  Y, Liu  Y  et al.  Polymer engineering enables high linear capacity fiber electrodes by microenvironment regulation. Adv Sci  2024; 11 : 2309461.10.1002/advs.202309461
40. Zhang  Y, Bai  W, Ren  J  et al.  Super-stretchy lithium-ion battery based on carbon nanotube fiber. J Mater Chem A  2014; 2 : 11054–9.10.1039/c4ta01878h
41. Zhang  Y, Bai  W, Cheng  X  et al.  Flexible and stretchable lithium-ion batteries and supercapacitors based on electrically conducting carbon nanotube fiber springs. Angew Chem Int Ed  2014; 53 : 14564–8.10.1002/anie.201409366
42. Zhang  Y, Bi  Z, Liang  Y  et al.  Ultrahigh line-capacity and flexible graphene/carbon nanotube/tin oxide fibers as sodium ion battery anodes. Energy Storage Mater  2022; 48 : 35–43.10.1016/j.ensm.2022.03.002
