Toward Real‐Life Applications of Fiber Lithium‐Ion Batteries

ABSTRACT

Fiber batteries have stimulated significant research interests in recent years, among the communities of energy storage, smart textiles, wearable electronics, biomedical sensing, and even soft robotics, primarily due to their high pliability to curved or irregular surfaces in various use scenarios. However, despite the numerous reports emerging, their widespread adoption in industry or in military applications are still far‐fetched. This review focuses on two key obstacles that hinder their fruition in the real world, namely poor encapsulation and questionable modeling. We also call for input from the packaging industry and electrochemical modeling community to further this nascent yet exciting technology.

Fiber‐shaped lithium‐ion batteries require effective encapsulation to achieve long‐term stability in wearable applications. This work discusses different encapsulation strategies and technologies, and summarizes how encapsulation affects capacity retention, efficiency, and internal resistance. Electrochemical modeling is further discussed to address current limitations in performance interpretation and to provide insights for the rational design of durable fiber‐based energy‐storage systems.

1. Introduction

The first report of a cable‐shaped lithium‐ion battery (LIB) came out in 2012 by Kwon et al. from LG Chem, Ltd [1]. Since then, significant efforts have been directed toward cable, thin ribbon, and fiber‐shaped batteries [2, 3, 4, 5, 6, 7]. The past decade has witnessed a surge in research and development of fiber batteries [8, 9, 10, 11, 12, 13], or in a more textile‐oriented expression, “yarn‐shaped batteries”. Although various phrases, such as fiber batteries, cable batteries, wire batteries, and yarn batteries, have been used in the literature to describe their linear geometry, this review adopts the term “fiber batteries” for simplicity and consistency. It is worthwhile to note that, in textile terminology, the word “yarn” denotes an assembly of fibers, which is a more accurate reflection of multiple components existing in these fiber batteries—fiber electrodes, separator wraps/strips, electrolytes, and encapsulation tubing and surface coatings—all within a single linear device.

In comparison to their thin‐film and ribbon‐shaped counterparts [14, 15], fiber batteries carry the potential to be seamlessly integrated into real garments with minimum compromises in fabric comfort or battery performances [16, 17, 18]. In certain scenarios, they may even become intangible as desired by some users. Driven by advancements in wearable devices, smart textiles, soft robotics, and biomedical sensors [19, 20, 21], numerous “fiber battery” chemistries, configurations, and applications have been explored, and these have been comprehensively summarized and carefully discussed in a few review articles recently [8, 9, 10, 11]. In geometry, many reported fiber‐battery prototypes remain only a few centimeters long with diameters ranging from a few millimeters to centimeters [2, 6, 7, 22]. These dimensions remain far from those of conventional fibers and yarns used in textiles. Among the most notable advancements, Peng's research group at the Fudan University, China has explored various chemistries and structural configurations for fiber‐shaped batteries [3, 22, 23, 24, 25, 26, 27]. In 2021, they demonstrated scalable manufacturing of continuous, meters‐long fiber LIBs [27], representing a significant step toward their textile compatibility and industrial adaptation.

In terms of battery chemistries, conventional high‐energy systems, such as Li‐ion, Li‐air, and silicon anode‐based batteries, have been extensively explored [9, 13, 28]. For wearable applications, safety is of paramount importance, which leads to significant interest in aqueous systems, including zinc‐ and sodium‐ion chemistries [29, 30, 31, 32, 33]. Despite these advancements, substantial technical challenges and knowledge gaps persist, particularly when compared to the well‐established chemistry of commercial LIBs. Therefore, this review will primarily focus on fiber LIBs, while borrowing some insights from other battery chemistries only when necessary.

In terms of manufacturing, critical parameters influencing battery performances have been thoroughly studied throughout the fabrication process of conventional LIB coin cells and pouch cells [34]. In contrast, investigation into fiber‐battery manufacturing remains in its early stage, with limited reports on systematic cell‐level assembly and evaluation. In addition, fiber batteries will have to overcome several technological obstacles, including reliability (calendar life), biosafety (toxicity and flammability), flexibility (comparable to textile yarns), washfastness (in smart garments), and scalable manufacturing, each of which encompasses specific structural and chemical challenges. Among these issues, some are more imperative since they hinder fiber batteries’ looming adoption by more advanced/trained users, such as those in military and intelligence communities. Herein, we will prioritize the list, and only focus on two major barriers that are blocking their initial acceptance by these imminent users [35, 36]. First of all, inferior encapsulation strategies in most reports have led to fiber batteries’ poor calendar life, questionable biosafety, and wearable discomfort, as well as unwanted increases in fiber diameters. Second, length‐dependent energy/power output has been observed and reported only by a few researchers in this field, and minimum modeling has been performed to unravel the underlying principles or to provide design guidance on fiber geometries (length, thickness, cross‐section, etc.) [27, 37, 38, 39, 40, 41, 42, 43, 44]. Therefore, we will dive deeper into these two subtopics, analyzing all the details reported in current literature, meanwhile calling for valuable input from relevant communities to further the technology of fiber LIBs.

2. Encapsulation Strategies

The practical use of fiber LIBs is fundamentally constrained by their environmental stability. In particular, exposure to oxygen and moisture accelerates electrolyte degradation, parasitic side reactions, and electrode decay [45]. Therefore, encapsulation is directly connected to their stability, performance, and safety. Unlike traditional batteries, fiber‐shaped devices require encapsulation that combines moisture and oxygen impermeability and mechanical flexibility (Figure 1a) [45]. In evaluating encapsulation performance, four major metrics are typically considered: water vapor transmission rate (WVTR), cyclic capacity retention, internal resistance, and calendar life. Over the past decade, several encapsulation approaches have been explored, including extrusion of polyolefin and fluoropolymer tubes, polymer/metal composites, thermally drawn structures, and liquid metal systems (Table 1). Each method provides unique trade‐offs between barrier performance and flexibility, and encapsulation has become a decisive factor for the long‐term viability of fiber LIBs.

FIGURE 1.

Schematics of soft encapsulation of fiber LIBs. (a) Soft‐tubing barrier is needed to block moisture and O₂ for key battery components to remain functioning. (b) WVTR ranking of various polymers adopted in literature for this purpose.

TABLE 1.

Fiber LIBs encapsulation materials and performances.

Materials adopted	Encapsulation protocol	WVTR	Cyclic capacity retention	Internal resistance	Calendar life (Capacity)
PP/Al‐tape [27]	Wrapped with aluminum–plastic tape and heat sealed at 300°C	< 0.005 g·m− 2·day− 1	90.5% (500 charge–discharge cycles)	N.A.	N.A.
FEP610 [46]	280°C to 330°C melt extrusion	0.5 mg·day− 1·pkg− 1 70% RH, 25°C	75.28% (150 cycles), 70.67% (200 cycles)	increased by 12.25% after 5 days	declined by 4.91% after 5 days
FEP810 [46]	280°C to 330°C melt extrusion	0.3 mg·day− 1·pkg− 1 70% RH, 25°C	77.77% (200 cycles) 80.05% (180 cycles)	N.A.	N.A.
PA12 [46]	190°C to 250°C melt extrusion	4.6 mg·day− 1·pkg− 170% RH, 25°C	1.04% (150 cycles)	increased by 218.65% after 5 days	dropped by 51.15% after 5 days
PE [46]	190°C to 250°C melt extrusion	1.1 mg·day− 1·pkg− 1 40–70% RH, 25°C	43.40% (150 cycles)	increased by 69.41% after 5 days	N.A.
PP [46]	190°C to 250°C melt extrusion	1.3 mg·day− 1·pkg− 140–70% RH, 25°C	33.11% (150 cycles)	increased by 126.82% after 5 days	N.A.
FEP [47, 48]	300°C to 370°C melt extrusion	0.382 mg·day− 1·pkg− 1 70% RH, 25°C	54.13% (350 cycles);	Increased by 135.13% after 50 days	Retain 80.05% after 27 days
PFA [47, 48]	300°C to 370°C melt extrusion	0.458 mg·day− 1·pkg− 1 70% RH, 25°C	37.65% (350 cycles)	Increased by 254.58% after 50 days	declined by 40.44% after 50 days
PCTFE [47, 48]	250°C to 280°C melt extrusion	0.123 mg·day− 1·pkg− 1 70% RH, 25°C	80.53% (350 cycles);	Increased by 36.92% after 200 days	Retain 72.73% after 100 days; 69.99% after 200 days
1%OMMT‐PCTFE [47]	250°C to 280°C melt extrusion	0.040 mg·day− 1·pkg− 1 70% RH, 25°C	80.03% (540 cycles)	N.A.	N.A.
2%OMMT‐PCTFE [47]	250°C to 280°C melt extrusion	0.006 mg·day− 1·pkg− 1 70% RH, 25°C	80.03% (870 cycles);	N.A.	over 80% retention after 200 days at 25°C, 50% SOC
COC/CPE (Multilayer Cladding) [49]	Preform‐to‐fiber, thermally drawn and hot‐pressed at125°C, laminated	N.A.	90% after 100 cycles at 5C	N.A.	N.A.
Polydimethylsiloxane (PDMS) [50]	Electrodes mounted on PDMS and sealed with another PDMS sheet to form inner cell	over 104 times higher than EGaIn; O2 permeability over 108 times higher	Fail after 160 cycles	Impedance increases as electrolyte dries, and electrode deteriorates	N.A.
EGaIn between PDMS shells and glass bead spacers [50]	Vacuum‐filling EGaIn into the chamber between inner cell and outer PDMS shells	H2O flux: 4.0×10− 7 cc·day− 1; O2 flux: 2.5×10− 6 cc·day− 1,	90% after 140 cycles; 72.5% after 500 cycles; Coulombic Efficiency ca. 98%.	Much lower impedance than the control at 100th cycle	stable over 227 cycles in 28 days.

WVTR is a key metric for evaluating moisture resistance of encapsulation systems. It is defined as the amount of water vapor permeating through the encapsulation structure under specific conditions within certain time. Two main descriptions of WVTR are reported in the literature: on the material level, WVTR is typically expressed in g·m^{− 2}·day^{− 1}, which means the mass of water vapor transmitted per square meter of material per day. This unit is adopted to compare the barrier properties of films or encapsulation tubes made of different materials. At the device level, WVTR is often expressed in the unit of mg·day^{− 1}·pkg^{− 1}, referring to the total amount of water vapor diffused into a single encapsulated package per day (Figure 1b). This provides a more practical evaluation of moisture resistance in real devices, and is thus used more often in the following discussions.

2.1. Polyolefins

The earliest encapsulation methods for fiber LIBs relied on polymeric tubes, including polypropylene (PP), polyethylene (PE), and polyamide (PA12), due to their low cost and excellent compatibility with industrialized melt‐extrusion processes [46]. In this approach, the selected polymer is melted at moderate temperatures (190°C–250°C) and extruded into long tubes, which later can be inserted with fiber electrodes and injected with liquid electrolytes. The extrusion process is attractive because of its simplicity, scalability, and compatibility with existing fiber manufacturing infrastructure. However, the intrinsic free volume in the amorphous regions of polyolefins, such as PP and PE, limits their barrier performances, making them less effective in blocking moisture and oxygen. Meanwhile, PA12 contains abundant amide groups along its backbone, which strongly interact with water molecules through hydrogen bonding. This intrinsic hydrophilicity induces severe moisture uptake, resulting in the poorest WVTR among the tested polymers [46]. Thus, the electrochemical performance of fiber LIBs encapsulated with those tubes are poor. The WVTR of PP is 1.3 mg·day^{− 1}·pkg^{− 1} in 40–70% RH/25°C, PE is 1.1 in 40–70% RH/25°C, and PA12 is 4.6 at 70% RH/25°C, all remain insufficient as long‐term moisture barrier (Table 1) [46]. For instance, at the device level, PA12‐based encapsulation retained only 1.04% of its capacity after 150 cycles, while PP‐based encapsulation retained 33.1% under similar conditions [27]. Also, after 150 cycles the capacity of PE‐based tubes remains 43.4% and after 5 days of storage the internal resistances increased by 69.41% [27]. These results demonstrate that polyolefins and polyamides are not suitable for achieving long‐calendar‐life in fiber LIBs, despite their low cost and ease of processing.

2.2. Fluoropolymers

To overcome the limitations of polyolefins, some researchers have turned to fluoropolymers, such as polychlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), and perfluoroalkoxy alkane (PFA), as well as OMMT (Organically Modified Montmorillonite)‐PCTFE composites [47, 48] (Figure 1).

Similar to polyolefins, melt extrusion of these fluoropolymers produces thin tubes that can be subsequently loaded with electrodes and electrolyte, but higher processing temperatures demand tighter thermal control within 280°C–300°C. Compared with polyolefins, they offer superior barrier performances while remaining compatible with scalable extrusion methods [45]. The advantage of fluoropolymers lie in their high crystallinity, strong C‐F bonds, and low free volume, which together prevent water and oxygen diffusion [46]. For instance, FEP tubes carry WVTR between 0.3 and 0.5 mg·day^{− 1}·pkg^{− 1} at 70% RH/25°C, enabling 75% to 78% capacity retention after 150 to 200 cycles. PFA shows WVTR of 0.458 mg·day^{− 1}·pkg^{− 1}, but retention is only 37.7% after 350 cycles, with a calendar life of 22 days. PCTFE performs substantially better, lowering the WVTR to 0.123 mg·day^{− 1}·pkg^{− 1} and supporting 88% retention after 100 days and 70% after 200 days, achieving a 130‐day calendar life [47]. Further enhancement has been achieved by incorporating 2% OMMT nanosheets, which reduces WVTR down to 0.006 mg·day^{− 1}·pkg^{− 1} and enables 80.03% retention after 870 cycles and a calendar life of 200 days. These findings establish fluoropolymers, especially PCTFE, as the better encapsulation material for long‐calendar‐life fiber LIBs.

2.3. Composite Structure Encapsulation

To prevent oxygen and moisture permeation, researchers also employed composite encapsulation strategies that combine polymers with metallic barrier layers. A representative approach is the use of polypropylene (PP) tubes laminated with external aluminum‐plastic films [27]. The heat‐sealing process integrates a metallic aluminum layer with the polymer tube, creating a double layer barrier structure. The inner PP layer provides structural support and electrical insulation, while the aluminum laminate specially reduces water vapor and oxygen permeability. This architecture achieves exceptional barrier performance. The WVTR drops below 0.005 g·m^{− 2}·day^{− 1}, nearly two orders of magnitude lower than that of conventional polyolefin tubes. Unfortunately, the authors did not provide its WVTR data in the unit of mg·day^{− 1}·pkg^{− 1}, so it is hard to directly compare this composite with all the polymeric systems discussed above. Its electrochemical stability is further confirmed in cyclic charge‐discharge tests, with 90.5% capacity retention after 500 cycles. Thus, this composite encapsulation offers outstanding impermeability at the cost of reduced textile compatibility [27].

2.4. Thermally Drawn Encapsulation

A fundamentally different approach is in situ thermally‐drawn encapsulation, which integrates encapsulation‐tube extrusion directly into the device fabrication process. In this method, macroscopic preforms are constructed by stacking multilayer polymers of cyclic olefin copolymer (COC), carbon‐loaded polyethylene (CPE), and/or poly caprolactone (PCL) [49]. The preforms are hot‐pressed to eliminate interfacial voids and then drawn under controlled heating into kilometers of continuous tubing with coaxial structures [49]. Electrodes are embedded inside the stack prior to thermal drawing, while their surrounding polymer claddings act as both mechanical support and encapsulation. The ends of the drawn fibers are sealed thermally to ensure hermeticity. This integrated approach yields seamless encapsulation without the need for post‐processing; it is thus inherently scalable. These thermally drawn fiber LIBs, encapsulated with COC/CPE claddings, are reported to retained over 90% of their initial capacity after 100 cycles at high charge–discharge rates (5C), with negligible impedance growth [49]. Moreover, the fiber LIBs exhibited remarkable mechanical durability, surviving repeated bending and even machine washing without compromising performance, according to the authors. However, unlike fluoropolymer systems, the WVTR values or calendar‐life data have not been reported for these thermally‐drawn encapsulations. Some potential users also noticed the reproducibility issue of this approach.

2.5. Liquid‐Metal Encapsulation

An unconventional but highly promising encapsulation strategy is the use of liquid metals, such as eutectic gallium–indium (EGaIn), integrated with elastomeric matrices such as polydimethylsiloxane (PDMS) [50]. Unlike conventional polymer barriers that suffer from intrinsic free volume and moderate H₂O/O₂ permeability, EGaIn provides metallic‐level hermeticity while preserving softness and stretchability. For example, Shen et al. first patterned PDMS into micro‐structured chambers supported by glass beads to prevent collapse under pressure. These chambers were then filled with EGaIn using vacuum‐assisted infiltration, followed by sealing of the inlets with additional PDMS layers. The resulting encapsulation exhibited orders of magnitude lower permeability compared with polymeric tubes, with the WVTR reduced to as low as 4.0 × 10^{− 7} cc/day. With this encapsulation, the batteries retained 90% of their initial capacity after 140 cycles and still maintained 72.5% after 500 cycles, with minimal impedance growth [50]. In addition, the elastomeric PDMS matrix allowed the encapsulated batteries to undergo repeated bending and stretching without failure, demonstrating unique adaptability to wearable conditions. However, this method is limited by the relatively high cost of gallium‐based alloys and the complexity of the vacuum‐assisted filling and sealing processes, which restricts its large‐scale manufacturing [50]. Nevertheless, liquid‐metal systems represent a decent solution to hermetic sealing of flexible LIBs, with unprecedented mechanical stretchiness in the final devices. Please note, we are yet to see this strategy to be applied to fiber LIBs.

2.6. Summary and Outlook

Encapsulation is one of the key issues blocking the practical viability of fiber LIBs. Extrusion‐based polyolefins are cost‐effective but fail to provide long‐term stability. Fluoropolymers, particularly PCTFE with 2% OMMT reinforcement, show better performance with WVTR as low as 0.006 mg·day^{− 1}·pkg^{− 1} and stable operation over 200 days. Composite systems achieve ultralow permeability but compromise device flexibility to some extent. Thermally drawn encapsulation offers scalable, textile‐compatible solutions but lacks in quantitative barrier data and reproducibility. Liquid metals provide unmatched combo of stretchiness and hermeticity but face challenges of complexity and cost.

Future research in this area can focus on balancing barrier properties and mechanical flexibility. Inspiration can be drawn from other industries facing similar challenges, such as organic light‐emitting diodes (OLEDs) [51] and medical implants [52]. OLED devices are highly sensitive to moisture and oxygen, requiring barrier materials with water vapor permeability as low as 10⁻⁶ g m⁻² day⁻¹ and oxygen permeability of 10⁻⁵ mL m⁻² day⁻¹ [53]. To address the tradeoffs between flexibility and barrier performance, the flexible OLED industry developed multilayer encapsulation strategies, alternating thin layers of impermeable materials such as Al₂O₃ with flexible polymers like PET. This approach has proven so effective that it is now widely commercialized [54]. Similarly, in biomedical applications, multilayer coatings combining atomic‐layer deposited (ALD) Al₂O₃ and biocompatible Parylene‐C have been successfully applied to protect neural implants [55]. Multilayer hybrid barriers, combing the excellent moisture‐blocking properties of inorganic Al₂O₃ with the mechanically flexibility of polymers, significantly outperform single‐material encapsulants. Adapting these multilayer strategies to the unique tubular geometry of fiber LIBs offers a highly promising route to achieving the necessary durability and barrier performance for their practical deployment.

3. Length Dependence and Electrochemical Modeling

3.1. Length‐Dependent Performances

One of the most peculiar features of fiber batteries is their length‐dependent performances, encompassing both capacity and internal resistance. Unfortunately, only a few groups reported the length‐dependent data in their reports [27, 37, 38, 39, 40, 41, 42, 43, 44], and many of them were on fiber supercapacitors instead. Peng's group discussed the length‐dependent performance of their fiber LIBs: as the length increased from 10 cm to 2 m, the capacity increased linearly, but no data beyond 2m is presented [27]. Yoel Fink's group presented linear energy and power output as a function of fiber length up to 25 meters (Figure 2g in ref. [37]), while stating that “the discrepancy of normalized capacities (i.e. length‐normalized capacity) between short fibers and long fibers generally becomes larger as fiber length increases” [37]. Interestingly, in the supporting document of ref. [37], the authors did present a detailed model for their fiber battery (Figure S11 in ref. [37]), with a conclusion that “the length of fiber battery which can be operated without ruining the battery performance is determined by the size and conductivity of the constituent materials” [37]. In addition, their model (Figure S11 in ref. [37]) suggests the voltage drop along fiber length becomes too large when less conductive materials (e.g., CNTs or 50µm‐diameter tungsten filament) are used as current collectors [37]. In the case of fiber supercapacitors, we have reported a capacitance plateau at ca. 3 meters of fiber length, with the extra length contributes little, if not zero, to the overall capacitance of the fiber devices [38, 40]. Similar trend has been reported in ref. [42], where the plateau is reached around 50 cm (Figure 5d in ref. [42]). Most of the remaining reports cover limited length range (≤ 1m) with only one exception [44]. In summary, the general conclusion here is somewhat perceived but rarely discussed by the researchers in the field: neither capacity (of fiber batteries) nor capacitance (of fiber supercapacitors) increases linearly as a function of fiber length in these fiber‐shaped energy‐storage devices; instead, a plateau typically emerges at certain length, depending on the device chemistry and configurations. A quite recent paper from Wang et al. [56], reported an interesting “dual‐end electron collection (DECT)” method, where both ends of the cathode current collector are shorted, and so as the anode one. This strategy can double the effective fiber length by homogenizing the electrical field distribution along the axial direction, offering a mitigation to the length‐dependent issue. In their case, a 10‐m‐long fiber LIB is reported with a 1 Ah capacity.

FIGURE 2.

Modeling of Fiber LIBs. (a) Schematic depicting the transmission‐line model Peng et al. used to simulate their fiber batteries; (b) voltage evolution of fiber LIBs along the fiber length direction, with a total fiber length of 2 meters, and a full‐charge voltage of 4.5 V at one end. The red curve is simulated according to Yoel Fink's model in ref. [37] and the dark curve according to Peng's model in ref. [27].

In summary, from a practical point of view, the length‐dependent performance limitation is not good news for us, since the original motive of making these devices is to exploit their linear, flexible, and pliable features. Now that their performance is not always proportional to their length, the desired power/energy output could be unachievable with only one individual fiber device, which makes interconnects among different devices necessary. Please note, interconnects among wearable electronics have been ranked as the top issue for smart e‐textiles recently, which requires another review article to elaborate on the conundrum there.

In terms of internal resistance of these devices, the behavior is also quite interesting. Peng's group reported a fast drop of internal resistance as the fiber length increases (Figure 1d, Extended Data Figures 1e and 2 in ref. [27]), where the length range reported expended up to 10 meters and the measured values matched pretty well with their model‐predicted values. Yoel Fink's report on thermally‐drawn LIB fibers only presented two Nyquist plots, at 10 meters and 140 meters, respectively, with the two presenting almost identical charge‐transfer resistance and the later only slightly higher in internal resistance [37]. In the case of our fiber‐supercapacitor system, we have observed a quite fast drop in the beginning, whereas a plateau was observed after passing the minimum point, which occurred around 50 cm of device length in our fiber‐supercapacitor system [40]. In the remaining literature, minimum discussion of internal resistance upon fiber length has been presented.

To sum up, fiber batteries (and supercapacitors) possess unique dependence of their performance on fiber length, which has been observed by researchers in this field but not thoroughly studied or modeled. This dependence is instrumental practically for their real‐life applications, since their positioning and incorporation in a real use scenario, whether in a fabric or not, relies highly on this dependence. Without clarifications on this aspect, fiber batteries can suffer from redundant length or poor interconnects among themselves.

3.2. Electrochemical Equivalent‐Circuit Modeling

To date, an accurate electrochemical model of fiber batteries (or supercapacitors) is still missing. In literature, few publications have included a modeling section, and problems exist in all these published models. As summarized in Table 2, two major correlations have been modeled in literature, with one focusing on the voltage evolution as a function of fiber length (Type I) [27, 37, 44, 57], and the other focusing on fitting the impedance spectra of their fiber batteries or supercapacitors (Type II) [40, 58], as a function of all structural and material parameters (fiber length, electrode configuration, electrode/electrolyte/current collector resistivities, etc.).

TABLE 2.

Summary of existing models of fiber batteries and supercapacitors in literature.

Device chemistry and configuration	Key correlation modelled	Correlation modelled	Refs.
Twisted LiCoO2/Graphite Lithium‐ion fiber battery	$V(x)=V^0\frac{\cosh [k(L-x)]}{\cosh (kL)};$ $R=\frac{{[({\rho }^{+}+{\rho }^{-})\times Z]}^{\frac{1}{2}}}{tanh(kL)}$	Type I	[27]
Thermally drawn fiber battery, lithium‐iron‐phosphate gel cathode/lithium titanate gel anode/electrolyte gel/cyclic olefin copolymer cladding, metal wires embedded in carbon‐loaded polyethylene	$V(z)=\frac{V_0\cosh (\frac{L-z}{\delta })}{\cosh (\frac{L}{\delta })};$ $\delta (z)=\sqrt{\frac{{\rho }_B(z)}{{\rho }_{Cu}}\frac{\pi }{2}{\mathrm{S}}_{Cu}}$	Type I	[37]
Parallel activated‐carbon based fiber supercapacitors	$V(z)=\frac{V_0\cosh (\frac{L-z}{\delta })}{\cosh (\frac{L}{\delta })};$ $\delta (z)=\sqrt{\frac{{\rho }_{Electrolyte}}{{\rho }_{Electrde}}\frac{\pi }{2}{S}_{Electrode}}$	Type I	[44]
Coaxial Carbon‐MnO2 based fiber supercapacitors	$\frac{\partial (V_i-V_o)}{\partial t}=A_3·\frac{{\partial }^2{V}_i}{\partial x^2}+A_4·\frac{\partial }{\partial t}(\frac{{\partial }^2{V}_i}{\partial x^2})$	Type I	[57]
Twisted rGO&MXene based fiber supercapacitors	ZTLM (ω)   = $\sqrt{\frac{R_i\times (R_{ct}+Z_W(\omega ))}{1+\frac{R_{ct}+Z_W(\omega )}{Z_{CP{E}_{ct}}(\omega )}}}coth(\sqrt{\frac{R_i}{R_{ct}+Z_W(\omega )}}(\frac{R_{ct}+Z_W(\omega )}{Z_{CP{E}_{ct}}(\omega )}+1))$	Type II	[58]
Twisted activated‐carbon based fiber supercapacitors	${\mathrm{Z}}_{\mathrm{p}}=\sqrt{{\mathrm{Z}}_{\mathrm{a}}{(\mathrm{R}}_{\mathrm{CC}}+{\mathrm{Z}}_{\mathrm{L}})}\coth \frac{{\mathrm{L}}_{\mathrm{sc}}{(\mathrm{R}}_{\mathrm{CC}}+{\mathrm{Z}}_{\mathrm{L}})}{\sqrt{{\mathrm{Z}}_{\mathrm{a}}{(\mathrm{R}}_{\mathrm{CC}}+{\mathrm{Z}}_{\mathrm{L}})}}$	Type II	[40]

Interestingly, most of the Type I models end up with the same kind of “cosh” expression for the voltage distribution along the fiber length. The key difference here lies in the definition of a geometric factor k in ref. [27] versus δ in ref. [37]. Factor ‘k’ takes into account of current‐collector resistance together with the measured polarization resistance from impedance spectrum, while ‘δ’ considers electrolyte resistance and copper wire resistance and geometry. We have adopted both models to fit the voltage drop of our fiber supercapacitors, and Peng's model (Figure 2a) [27] fits better with our length‐dependence data, when all the parameters are adjusted according to our fiber‐supercapacitor settings. Yoel Fink's model is mathematically quite close to Peng's model, but points to a much slower voltage decay upon length that mismatches our supercapacitor results. When it comes to fiber LIBs, neither model provides a close fit to the reality. We will dive into the details here and identify some remaining concerns in both models:

• 1) During typical LIB charge‐discharge cycles, the electrical current is controlled in magnitude according to the C‐rate defined, as opposed to the set voltage (V₀) in both models. In other words, Equation (1) below adopted in both models are describing a simple ohmic relationship, where current = voltage/resistance, but it is inaccurate for a typical charge/discharge cycle of LIB.

  $$-dI=\frac{V\left(x\right)}{\frac{Z}{dx}}$$ (1)
• For instance, in the charging step, LIB‐cell voltage (V_terminal ) starts at the open‐circuit voltage (OCV) and slowly rises to the targeted voltage by the charging current I ₀. OCV is primarily defined by the electrochemical potential difference between the cathode and anode; therefore, the simple ohmic correlation described in Equation (1) does not reflect electrochemical reality of batteries. A better model should start with a fixed charging current I(0) at x  =  0, and calculate the voltage change (dV) upon length as a function of x and other device parameters along the fiber length. If Equation (1) has to be updated to take that into consideration, and an OCV should be subtracted from the measured voltage V(x) at position x, which becomes:

  $$-dI=\frac{V\left(x\right)-OCV}{\frac{Z}{dx}}$$ (2)

• In other words, when fiber battery is being charged, the voltage V(x) must overcome the OCV to achieve the desired current. Therefore, the real voltage‐drop along the fiber length is much smaller than what's being calculated in Equation (1).

• Meanwhile, in the constant‐current discharging step, LIB‐cell voltage starts at “$OCV-I(0)\times R(internal)$” and slowly decreases to the targeted voltage by the discharging current. Therefore, the “$-dI$” expression can be written as:

  $$-dI=\frac{OCV-V\left(x\right)}{\frac{Z}{dx}}$$ (3)

• Again, the resulted “$-dI$” is much smaller than what is calculated according to Equation (1).

• 2) The defined shunt resistivity Z in Peng's model, in the unit of Ω · m, is the product of fiber‐battery length and the polarization resistance (the sum of R(SEI) and R(Charge‐Transfer) obtained from fitting of the Nyquist plot). According to their extended data Figure 3b, Z remains a constant with the fiber length up to 10 meters, while in reality Z, as an impedance, encompasses constant phase element (CPE), Warburg, resistance, and inductance terms. The accurate expression of Z, and the resulting voltage/current expressions should be more complicated.

• 3) On the same aspect, the “δ(Z)” parameter in Yoel Fink's model have taken more into consideration (Cu‐wire resistivity and geometry, CPE, electrode resistivity, and electrolyte resistivity), but surprisingly, they simplify the total battery‐component resistivity by adopting only the electrolyte term, since it is considered the largest among the later three. The resulted higher numerical value of “δ(z)”, versus that of its counterpart (1/k) in Peng's model (both “$\delta (Z)$” and (1/k) adopt a unit of meter), explains the much slower voltage decay in Yoel Fink's model versus that in Peng's model. To be more specific, it is the extremely high ρ _B(z)  ≈  ρ_Elect(z)  =  9 × 10⁴ Ω · m estimated in Yoel Fink's model (Figure S11 in ref. [37]), versus the much lower Z  =  7.61 Ω · m adopted in Peng's model (Table S2 in ref. [27]), which led to this large discrepancy between the two models in terms of voltage‐decay rate upon length. Please note, the electrolyte resistivity ρ_Elect(z)  =  9 × 10⁴ Ω · m adopted in Yoel Fink's model is extremely high for commercial LIB electrolytes, which typically lies around 1.0 Ω · m (converted from the reported value of 10.00 ± 0.5 mS/cm by a common supplier [59]). In our opinion, what really needs to be modified here, is the definition of V(x), instead of a super high ρ_Elect(z), to account for the fiber‐battery reality in practice.

• 4) According to Peng's model (Figure 2b dark curve), the total fiber length (L) is controlling the voltage‐drop rate along the fiber length; when L = 2 m and the charging‐end voltage of 4.5 V, the other end retains a measurable voltage of 3.2 V, close to the fully discharged state of LIBs (3.0 V). Therefore, “2 meters” is predicted to be close to the upper limit without redundant lengths that do not contribute to the battery capacity in Peng's model. The opposite extreme is expected according to Yoel Fink's model (Figure 2b red curve), where only 0.03 mV of voltage drop is supposed to be observed between the two ends of a fiber LIB (Figure 2b red curve), which is 2 m in length and 9 × 10⁴ Ω · m in electrolyte resistivity. However, among our lab‐made fiber LIBs (50–90 cm in lengths), the terminal‐end voltage is measured to be a few millivolts (∼5 mV) below that of the charging end, providing an experimental counter‐evidence to both models. Therefore, we believe a more accurate model is yet to be developed to describe the voltage evolution of fiber LIBs upon fiber length.

• 5) Due to the twisted configuration of their fiber LIBs, the contour length (L) in the model should be longer than the nominal fiber length measured, which has been ignored in all the models.

In summary, both models deviate from reality, and the true decay rate of voltage upon fiber length should be a function of charge/discharge current. In Figure3a of ref. [27], the capacity values have only been reported up to 2 meters of fiber length, and no data is mentioned beyond that point. On top of that, if this voltage distribution is correct, the stored‐energy dependence on fiber length should not be linear, but rather a ‘cosh’ style decay, just like what the voltage does here. Therefore, a more accurate model is needed to further our understanding of the fiber battery/supercapacitor systems.

In terms of Type II models, the focus gears toward Nyquist‐plot fitting of impedance spectra of fiber supercapacitors only. Instead of simulating voltage evolution upon fiber length, these models focus on developing the mathematical expression of total impedance Z as a function of device parameters, including fiber length, current‐collector specifications, electrode‐material factors, and electrolyte conductivity (Table 2). Once the full expression of Z is set, the real part and imaginary part of Z are calculated and used to fit the Nyquist plots experimentally obtained at different fiber lengths.

Due to the large number of device parameters taken into consideration in Type II models, e‐chem modelers typically consider it overfitting in these cases. On top of that, the transmission‐line model (TLM) adopted here follows the total impedance expression from Lasia's porous electrode models (Equation 4) [60], which are quite different from the fiber battery/supercapacitor cases on the following two aspects:

• 1) Lasia's TLM sets the current to flow to the pore where the pore walls are highly conductive, and the counter electrode is located in the solution outside the pore;

  $$\widehat{Z}=\frac{R_{\mathrm{\Omega },p}}{\sqrt{\mathrm{\Lambda }}}coth\sqrt{\mathrm{\Lambda }}$$ (4)

• 2) In Lasia's TLM, a 3‐electrode system is used, where a reference electrode is controlling the applied voltage accurately, while our fiber LIBs are mostly 2‐electrode systems.

Therefore, direct adoption of Lasia's TLM expression for total impedance Z is questionable, and a more reasonable impedance expression is needed to better describe the 2‐electrode fiber battery/supercapacitor geometry.

4. Other Remaining Challenges and Potentials

4.1. Energy Density

Energy density is a key parameter that governs the effectiveness and broad applicability of energy storage devices. For LIBs used in consumer electronics, typical gravimetric energy densities range from 170 to 250 Wh kg⁻¹, with advanced model reaching 250–300 Wh kg⁻¹ and next‐generation systems targeting values above 400 Wh kg⁻¹ [61]. In contrast, the best‐performing fiber LIBs reported to date carry energy densities around 128 Wh kg⁻¹ [62], approximately half that of state‐of‐the‐art film‐based counterparts. Although the primary advantage of fiber batteries lies in their unique one‐dimensional geometry, achieving competitive energy density is desirable for their wide adoption across diverse application scenarios.

Improving energy density while retaining a reasonable fiber thickness remains a significant challenge for these devices. Addressing this requires a combination of materials engineering and structural engineering. First, increasing the loading of active electrode materials within the fiber electrodes is essential for boosting capacity, although this must be carefully balanced to preserve mechanical robustness under deformation. Second, structural innovations beyond conventional yarn geometries can enhance active material utilization and shorten the ionic and electronic transport pathways, thereby improving overall electrochemical performance. Third, minimizing the proportion of non‐functional, or inactive components, such as excess binders, separators, or encapsulation materials, can further lead to higher gravimetric and volumetric energy densities without compromising mechanical durability or safety. Together, these strategies aim to push the performance limits of fiber batteries while retaining their unique form factor and compatibility with textile integration.

4.2. Mechanical Robustness and Integration

Fiber batteries are intended to be integrated into textiles via weaving, knitting, braiding, or embroidery. Thus, they must endure repeated bending, compressing, and stretching without significant degradation in performance. Existing studies often lack rigorous, standardized testing protocols for mechanical durability under realistic conditions, making it difficult to assess their long‐term reliability in textile applications. Another challenge lies in the size of the encapsulated devices. Currently, most encapsulated fiber batteries remain 1–2 mm in diameter, much larger than standard textile yarns. This “size mismatch” affects their seamless integration into fabrics, particularly when flexibility and wearability are key requirements. Efforts are also needed on the development of mechanically compliant electrical interconnects that can absorb or adapt to strain without compromising function. Advancing these aspects is critical to enabling durable, fabric‐integrated energy systems suitable for real‐world wearable and structural applications.

4.3. Broader Applications

While most reported applications of fiber batteries have focused on wearable electronics, their potential extends far beyond this domain. Exploring alternative applications can unlock new opportunities across various disciplines. In healthcare, fiber batteries can enable skin‐conformal vital‐sign shirts, wound‐monitoring dressings, and even biodegradable thread‐like implants for temporary cardiac pacing or neural stimulation, offering seamless power delivery inside and outside the body. However, for these bio‐oriented applications, material safety becomes of primary concern. The toxic electrolytes used in LIB fibers can potentially generate detrimental effects on the users’ health, which necessitates effective and robust encapsulation across entire devices. Military systems stand to gain from load‐sharing power fabrics that lighten the battery burden soldiers currently carry, while also feeding adaptive camouflage, heated uniforms and distributed radios. For aerospace and space exploration, fiber batteries imbedded in structural composites can both bear mechanical loads and energize drones, eVTOL cabins, or spacesuit assemblies, maximizing mass efficiency. In built environments, energy‐storing architectural textiles promise curtain walls, shade sails, and smart‐concrete elements that buffer renewable generation locally. Emerging niches such as soft robotics, drones, underwater‐communication cables, and haptic automotive interiors further illustrate how fiber‐shaped batteries can liberate designers from planar cell geometries, placing power precisely where flexibility, miniaturization, and mechanical integrity are simultaneously required. When incorporating LIB fibers into a variety of application systems mentioned above, the length setting and how they can seamlessly fit into a specific use case becomes an interesting challenge. For example, LIB fibers need be shorter but more flexible in soldier rucksacks as compared to those used in military tents or uniforms.

Funding

This research is based upon work supported by the officie of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), via N66001‐25‐C‐4501.

Conflicts of Interest

The authors declare no conflicts of interest.