Nitrogen-doped CNT on CNT hybrid fiber as a current collector for high-performance Li-ion capacitors

Carbon 149 (2019) 407e418

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Nitrogen-doped CNT on CNT hybrid fiber as a current collector for high-performance Li-ion capacitors Sathya Narayan Kanakaraj a, Yu-Yun Hsieh a, Paa Kwasi Adusei a, Bradley Homan b, Yanbo Fang a, Guangqi Zhang a, Siddharth Mishra a, Seyram Gbordzoe a, Vesselin Shanov a, b, * a b

Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, 45221, USA Department of Biomedical, Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH, 45221, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 December 2018 Received in revised form 17 March 2019 Accepted 8 April 2019 Available online 9 April 2019

In this work, we describe a scalable synthesis process of binder-free, nitrogen-doped carbon nanotubes (CNTs) on CNT fibers combining a solvothermal process with chemical vapor deposition (CVD). Li4Ti5O12 was selected as an example active material to evaluate the performance of the obtained current collector electrode, which achieved 100% capacity retention after 1000 cycles at a 15C rate and a stable specific capacity of 144 mAhg
1 at 5C. We also report here the fabrication of an asymmetrical hybrid capacitor that exhibited a maximum specific energy of 0.296 mWhcm
2/0.019 Whcm
3/68 Whkg
1 at a specific power of 0.172 mWcm
2/0.011 Wcm
3/126 Wkg-1. It maintained specific capacitance of 0.0779 mWhcm
2/0.005 Whcm
3/17 Whkg
1 at a high specific power of 57.05 mWcm
2/3.719 Wcm
3/12,500 Wkg-1. The device exhibited a very stable cycling performance, retaining 100% of its specific energy after 2000 cycles at 4 Ag-1 current density. The increase in specific power, energy and cycling performance was attributed to the porous network afforded by the nitrogen-doped CNTs and their strong binding with the active material Li4Ti5O12. The porous network enabled fast Li-ion diffusion paths while the pristine CNT allowed for fast electron transfer all in a fiber format, making it attractive as an electrode for wearable energy storage devices. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction Currently, there has been an increased interest in research towards more efficient energy storage devices due to the rising fuel prices and the shift towards renewable clean energy production [1e4]. Nowadays, these devices have begun to be used in many practical applications including portable and wearable electronics, hybrid electric vehicles and other conventional appliances [5,6]. Another technological trend in energy storage devices is the increased emphasis on fast charging devices. Conventionally, batteries are known for their high specific energy and electrochemical capacitors for their high specific power [7e11]. However, with the development of new intercalation based active materials, batteries have begun to shift towards high specific power [12e15]. However, there is a wide disparity in reported performance. This is in part due to the performance of the active material itself [16e19], but even

* Corresponding author. Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, 45221, USA. E-mail address: [email protected] (V. Shanov). https://doi.org/10.1016/j.carbon.2019.04.032 0008-6223/© 2019 Elsevier Ltd. All rights reserved.

more so due to the limitations of the current collector employed. There is still a heavy reliance on metal-based current collectors that inhibit the flexibility, rate capability, and the active material loading. This is caused in part to their stiffness, inclination to corrode and tendency to dissolve at high cut off voltages that can potentially cause short circuiting [20e24]. Metal current collectors also increase the total mass of the electrodes which needs to be accounted for in a full device thus considerably lowering its gravimetric performance metrics. Efforts to solve these issues have led to the fabrication of electrodes that utilize a combination of high ratio of binders to active materials [25e27]. This, however, negatively affects the conductance homogeneity of the material and in some cases even lead to delamination, while adding mass to the electrode [26,27]. As a result, 100% carbon-based current collectors started gaining interest recently. The current trend in energy storage devices can be broadly divided into two categories based on the form factor of the electrodes: films/sheets and fibers. With wearable electronic technology becoming more prevalent, research on fiber/yarn devices is becoming more common. The use of binders or additives in this

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form factor is much more accepted than in the case of sheets so long the binders preserve the flexibility of the fibers. However, due to substantial limitations of material conductivity and structural strength, the electrochemical properties of these devices fall far shorter than those based on sheets/films, which is the reason that more work has been done on sheets than fibers [28,29]. Nevertheless, fibrous devices are still attractive due to their small dimension, low weight, flexibility and ease of integration into textiles or structural composite panels. A common approach to improve the electrochemical device performance is to introduce porous structures which can both load active material and provide fast ion diffusion channels, thereby increasing the ion transfer efficiency [30e33]. Similarly, in the case of a macro-fiber electrode (diameter dimension in the micron range), in order to enable high mass loadings of active material it would have to be introduced into the bulk of the housing material (fiber). This would require the electrode to be porous to facilitate a fast Lie ion diffusion. However, the latter results in a decrease of the strength of the material due to the excess porosity. Another method to achieve fast Li-ion diffusion channels is the use of surface nanoparticle decoration, thus reducing the diffusion path for the Li þ ions [34e37]. This, in the case of macro fibers, limits the nanoparticle deposition to the outer curved surface, which lowers the areal loading of active material, as over-deposition would result in increased resistance to ionic diffusion. In this study, we report an easy to implement and scalable process of synthesizing a novel “hairy” Carbon Nanotube (CNT) structure on a CNT core fiber. This approach helped to overcome the limitation on the quantity of the active material loading without introducing porosity into the bulk electrode material, and thus maintaining its strength (comparable to that of the pristine fiber). We believe that the main achievement here is the proposed versatile, scalable process that is capable of fabricating a new design and surface morphology of the fiber electrode. Such an approach enables the base CNT fiber to be potentially replaced by other more conventional macro-fiber electrodes for similar applications. We report the use of the hybrid fiber as an efficient current collector and scaffolding template for active materials, primarily Li4Ti5O2 (LTO). For the electrode fabrication, we designed a unique combination of three synthesis routes such as hydrothermal, solvothermal and CVD process. The resulting fiber was an assemblage of pristine CNT (PC) decorated with nitrogen-doped CNTs (N-CNT), thus yielding a hybrid N-CNT on CNT fiber (NCH). The resulting composite LTO electrode fiber exhibited excellent cyclic stability and rate performance. We addressed the issue of low active material loading, with our devices housing 2.1 mgcm
2 of LTO, which was comparable to some 3D format devices from the literature [37,38]. This data was much higher than that reported for fiberbased devices [40]. Finally, this work presents results on a full hybrid device based on LTO and nitrogen CNT electrodes. To the best of our knowledge, there is a scarcity of publications on cylindrical CNT thread-based LTO capacitors. This is the reason for comparing our fiber electrochemical data with those reported in the literature for devices using sheet form electrodes, which are more available for accessing. The achieved supercapacitor performance is comparable and, in some instances, exceeding that of present literature data. It is worth noting that the length of the hybrid fiber reported here is limited only by the size of the used autoclave container, making the device fabrication truly scalable.

reactor ET3000 from CVD Equipment Corporation. Oxidized single crystal silicon with aluminum oxide as a buffer layer and FeeCo catalyst film on top was used as a substrate in the CVD process. The growth process parameters and properties of the resulting array have already been published in our previous work [41]. A CNT ribbon was drawn from the end of the array and a continuous fiber was spun using a home-made spinning apparatus [42]. The “asobtained” pristine CNT fiber is designated here as PC fiber. The latter was then fed through a tubular plasma source (SurFx AtomFlo model 400-V2.0HE) operating at atmospheric pressure with parameters: 100 W power, 0.3 L/min flow of oxygen and 15 L/min flow of helium. The functionalized fiber is designated as oxygen functionalized CNT (OC). 2.2. Nickelecobalt catalyst deposition CNT fibers were decorated with NiCoOH via a modified solvothermal reaction. A solution containing 0.04 M cobalt (II) nitrate, 0.02 M nickel (II) nitrate, and 0.2 M urea, dissolved in a 40 mL mixture of ethanol and de-ionized water at a 4:1 ratio, was prepared, following a similar recipe reported by Wang et al. [43]. Once all the ingredients were completely dissolved, it was transferred to a Teflon lined autoclave which contained the CNT fibers. To increase the area of fiber exposure to the solution, the samples were coiled around two quartz tubes roughly half an inch apart from one another. The autoclave was then heated in a furnace at 126  C for 3.5 h and allowed to cool down to room temperature. The treated fibers were then transferred into a water bath followed by an ethanol bath and then dried at room temperature. Annealing of the dried fibers in air yielded the NiCo2O4 decorated fibers. 2.3. N-CNT growth N-CNTs were grown via CVD on CNT fibers decorated with NiCo2O4 catalyst to form the hybrid NCH fiber. The carbon source (acetonitrile) was initially introduced at 800  C. After a 30-min soak, the temperature was ramped up to 1000  C where it was exposed again to acetonitrile. Once removed from the reactor, the fiber was placed into a 10 M HCl bath for 12 h to remove any remaining catalyst. The post acid treated fiber was transferred to a water bath for 6 h followed by an ethanol bath and finally dried at room temperature. The reaction temperature profiles are detailed in Fig. S1. 2.4. LTO deposition

2. Experimental

The LTO fiber composites were synthesized via a modified hydrothermal route [44]. A 0.7 M LiOH solution was prepared in 40 mL of distilled water. After mixing the solution with a stir bar for 30 min, 1.02 mL of hydrogen peroxide was added and mixed for 5 min. Finally, 0.88 mL of titanium isopropoxide was introduced to create a yellow translucent solution. It was transferred into a Teflon lined autoclave containing the NCH fibers. The autoclave was then heated at 137  C for 12 h and allowed to cool to room temperature. The fibers were then washed in a water bath followed by an ethanol bath and dried at room temperature. The final NCH LTO fiber was synthesized through a 6-h argon annealing in a furnace to increase the crystallinity [45,46] and ensure complete transformation of the LTO to its spinel phase. The XRD pattern of LTO prior to annealing is shown in Fig. S2 [47,48]. A schematic representing the entire synthesis process is depicted in Fig. 1.

2.1. CNT fiber spinning and oxygen functionalization

2.5. Materials characterization

VACNT arrays were synthesized in a modified commercial CVD

The surface morphology of the CNT fibers was observed by

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Cg ¼

It*1000 m

409

(1)

where I is the constant current (A), t is the time (sec) and m is mass of half-cell (g). The specific energy Ex (Whcm
2/Whcm-3/Whkg
1) of NCH LTO/ NCH hybrid full cell was calculated using the equation:

ð t2 I

VðtÞdt t1

Ex ¼

3600*x

(2)

where x represents m’ (mass of both electrodes combined), A (curved surface of both electrodes combined) or V (cylindrical volume of both electrodes combined) depending on the parameters used for normalizing the specific power and specific energy. The specific power (Wcm
2/Wcm
3/Wkg
1) of the hybrid full cell was obtained from the equation:

Px ¼

Fig. 1. Schematic illustrating the steps in the fabrication of the hybrid fiber electrode from PC fiber to NCH fiber and finally to NCH LTO fiber. (A colour version of this figure can be viewed online.)

Ex t

(3)

3. Results and discussion 3.1. Catalyst deposition

scanning electron microscopy (FEI XL30). Raman spectroscopy (Renishaw inVia e 514 nm Ar ion laser) was employed to evaluate the nanotube quality and other bonds that were present. X-ray photoelectron spectroscopy (XPS) (VG Thermo-Scientific MultiLab 3000 ultra-high vacuum surface analysis system) was conducted using an Al Ka source of 1486.6 eV excitation energy. A Sartorius micro-analytical balance model ME5 with microgram resolution was utilized to measure precisely the mass of the CNT fibers and related composites. Strength characterizations were done via an Instron tensile testing instrument Model 5948. Specific surface area and pore size distribution analysis was carried through nitrogen adsorption at
196  C using a Micromeritics ASAP 2060 accelerated surface area and porosity measurement instrument. 2.6. Electrochemical characterization The half-cell electrochemical performances of NCH and PC fibers were evaluated with a Potentiostat (Gamry E1000) using a 3electrode setup with platinum as the counter electrode and Ag/ AgCl as the reference electrode. Aqueous electrolyte (1 M Na2SO4) was employed providing an operating voltage window range of 0e1 V. The “as prepared” NCH LTO fiber was evaluated in a coin cell with lithium foil as the counter and reference electrodes. 20 cm of the NCH LTO fiber was coiled into a ball and placed on a stainlesssteel spacer for use as the working electrode. 40 ml of 1 M LiPF6 in 1:1, (v:v, volume ratio) mixture of ethylene carbonate and dimethyl carbonate was employed as the electrolyte between the fiber and lithium electrode. A propylene film (Celgard 2400) was used as a device separator. The hybrid full cell was made in a similar manner with NCH LTO as the anode and NCH as the cathode. All cells were assembled in an argon-filled glove box. The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry of the cells were conducted using a Potentiostat (Gamry Interface 1000E) and the galvanostatic charge-discharge was done in LAND Battery testing system CT2001A. The specific capacity Cg (mAhg
1) of NCH LTO fibers for the halfcell testing was calculated according to the equation:

The SEM images provided in-depth information on the deposition uniformity and size distribution of the catalyst. As is seen in Fig. 2A, the deposition of NiCo2O4 catalyst on the PC fiber is sparse and non-uniform. This is attributed to the poor wettability of the sp2 bonded carbon surface that the PC is composed of. The latter issue was overcome through oxygen plasma functionalization, as is seen in Fig. 2B. The OC fiber showed very uniform deposition with nanosized catalysts completely coating the surface. A significant increase in mass loading of NiCo2O4catalyst from 0.01 mg/cm to 0.4 mg/cm was also observed. Due to the strict correlation between the size of the catalyst particles and the properties the CNTs grown, the parameters of the deposition process were significantly optimized. The SEM images showed a clear change in the deposition morphology as the procedure was moved from a strictly hydrothermal process 1:0 of water:ethanol (w:e) to a 1:4 (w:e) modified solvothermal process and finally to a strictly solvothermal process 0:1 (w:e). As seen in Fig. 2C(i-iii), there was a severe inhomogeneity in coating produced by the strictly hydrothermal and solvothermal processes. The 1:4 ratio w:e hydrothermal process showed a highly uniform catalyst coating. This corresponds with results reported by Wang et al. [43]. Very fast and very slow hydrolysis led to bulky and non-uniform deposition. The optimal deposition was achieved with a slow hydrolysis leading to a uniform coating of the tubes without allowing for agglomeration. We further optimized the catalyst size by controlling the molar ratio of the precipitating agent Urea to the nitrate salts based on the work by Ameri et al. [49]. As shown in Fig. 2D(iiii), the smallest and most uniform nano-sized catalysts were grown with a 5:1 M ratio Urea: Nitrate (u:n). Both the 10:1 ratio and 2.5:1 ratio revealed larger sizes and agglomeration. XRD analysis of the post-annealed fiber showed distinctive peaks of NiCo2O4 indexed based on ICDD PDF NO. 02e1074 (Fig. 2E). 3.2. Nitrogen-doped CNT hybrid (NCH) fiber It has been reported that growth of well graphitized CNT growth is more pronounced at higher temperatures with acetonitrile as a

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Fig. 2. SEM images of A) NiCo2O4 deposition on PC fiber and B) on OC fiber. C) NiCo2O4 deposition on OC fibers with varying water: ethanol (w:e) volume ratios: Ci) 1:0; Cii) 1:4; Ciii) 0:1. D) NiCo2O4 deposition on OC fibers with varying u:n molarity ratios: Di) 10; Dii) 5:1; Diii) 2.5:1. E) XRD pattern of NiCo2O4 on OC fiber indexed based on ICDD PDF NO. 02e1074.

carbon source [50e52]. Therefore, as a starting point, we set the initial growth temperature at 1000  C with acetonitrile used as both carbon and nitrogen precursor. As evidenced by the SEM image in Fig. 3A, the growth of large diameter graphitic carbon bulbs occurred when synthesis of NCNT was initiated at 1000  C (Fig. S1). Lowering the temperature (800  C) resulted in a fine and short CNT growth (Fig. 3B). The graphitic clump growth can be attributed to Ostwald ripening of the catalyst at higher temperatures (1000  C). Since the growth mechanism was tip based (as observed in backscattering SEM image in Fig. S3), we devised a two-step growth process - Initiating the CNT growth at 800  C, where no clumps

were formed, followed by growth at an elevated temperature of 1000  C for faster reaction kinetics and graphitic growth of long CNTs. Thus, uniform and long N-CNTs (average diameter of 60 nm) coated CNT fiber was synthesized. The NCH fiber was heavily coated with N-CNTs to the point that the core fiber was no longer visible (Fig. 3C and inset of Fig. 3Di). The high temperature grown tubes were significantly longer than the low temperature grown CNTs. From the catalyst deposition to the CVD growth, the obtained NCH fiber followed a very scalable process pathway. Using the proposed approach, one can produce meters of flexible and strong & conductive fibers in a single run (Fig. 3Di and 3Dii), as evidenced by

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411

Fig. 3. SEM images illustrating NCT growth at: A) 1000  C, B) 800  C and C) 800  C þ 1000  C. Di) Picture of “as prepared” NCH fiber wound in the bobbin and pulled out of it (scale measures at 3.5 feet), with low magnification SEM image of NCH fiber as an inset. Dii) Bent NCH fiber. E) XPS analysis of NCH fiber: i) survey spectra; ii) C1s spectra and N1s spectra. F) Raman spectra comparing PC, NCH grown with H2 and without H2. (A colour version of this figure can be viewed online.)

the tensile strength and conductivity data in Fig. S4. After multiple exposure times at higher temperatures, the optimal recipe was reached e 30 min at 800  C and 30 min at 1000  C (Fig. S5.) XPS analysis of the NCH fiber revealed the following elemental composition through the survey scan (Fig. 3Ei): N e 1.51 atm%, O e 4.99 atm% and C e 93.49 atm%. About 50 cm of pre-weighed pristine fiber was used for NCNT growth and weighed again after the nanotube deposition. The obtained difference provided an approximate of the mass of the grown NCNT. Having in mind that the N-CNT accounts for less than 8% of the total mass of the NCH fiber, the nitrogen content was found to be quite high when solely considering the carbon content of the N-CNT. The C1s (Fig. 3Eii) showed a major peak at 284.8 eV belonging to sp2 carbon [53], which accounts for the majority of the NCH fibers. A small peak belonging to sp3 type carbon, which includes CeN and CeOH type species, is seen at 286.5 eV [53]. High-resolution N1s (Fig. 3Eii) indicated presence of two nitrogen functional groups - pyridinic (398 eV) and graphitic nitrogen (401 eV) [54e56]. These nitrogen functional groups have been shown to significantly increase electrochemical performance owing to their higher electronegativity [57]. In addition, Raman analysis (Fig. 3F) of the NCH samples showed

a narrow and high intensity D peak corresponding to the presence of defects, which can be attributed to the introduced nitrogen groups. An increase in the ratio of the D to G peak intensities was observed as well. This is in line with the study done by Ito et al. [58], which found a higher percentage of nitrogen group incorporation into the carbon material when using hydrogen during the CVD process and lesser amorphous defects, due to the etching behavior of hydrogen on both carbon and nitrogen species. The small intensity of the D peak observed for the NCH fiber grown without H2 could be interpreted as a low deposition of defective (nitrogen doped) CNTs. Whereas, the high D peak of the hybrid fiber grown in the presence of H2 gas could be indicative of a substantial growth of defective nitrogen doped CNTs. We expect that H2 plays a key role in increasing the catalytic activity for growing CNTs by restricting the oxidation of the catalyst or by directly reducing the oxidized catalyst [59e61]. This requires further investigation that will be done in the future. The electrochemical results of the NCH fiber with and without hydrogen are illustrated in Fig. S6. The incorporation of the nitrogen groups was also evidenced by the improved electrochemical half-cell performance of the NCH fiber over the PC fiber. The cyclic voltammetry (CV) curves shown in Fig. 4A revealed a 10 time increase in gravimetric capacity of NCH

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Fig. 4. A) CV curve of PC vs. NCH fibers in a half-cell measurement arrangement at 200 mV/s scan rate. B) EIS of PC and NCH fibers showing the Rct and Rs values. (A colour version of this figure can be viewed online.)

over the PC fiber. This can be explained by the electrochemical impedance spectroscopy (EIS) data that showed a significant drop in the ionic charge transfer resistance (Rct) values, from 620.3U for

PC to 83.5U for NCH, as is seen in Fig. 4B. The “hairy”-type CNTs provided easy access for the ions and the presence of nitrogen groups increased the specific capacity of the material [62,63].

Fig. 5. A) SEM image of LTO deposition on NCH fiber. B) XRD pattern of the NCH LTO fiber indexed according to ICDD PDF NO. 00-049-0207. C) XPS analysis of NCH LTO fiber survey spectra. High-resolution Di) O1s spectra and Dii) Ti2p spectra. (A colour version of this figure can be viewed online.)

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3.3. NCH LTO composite fiber The SEM image in Fig. 5A shows the LTO coating very finely decorated around each individual N-CNT, thus creating a stable porous network. The XRD patterns (Fig. 5B) of the annealed NCH

413

LTO fiber revealed a close match to the peaks indexed according to ICDD PDF NO. 00-049-0207. The latter confirmed the synthesis of pure spinel LTO with no impurities. This was substantiated by the XPS analysis, a survey scan of which (Fig. 5C) showed Li1s, Ti3p and Ti2p peaks that were indicative of LTO. A high-resolution scan for Ti

Fig. 6. A) Voltage profile of PC LTO, CCH LTO and NCH LTO fibers at 5C rate. B) Schematic representation of the faster Li þ ion and electron e
transfer pathways and mechanisms in the NCH LTO fiber. C) EIS plots of PC LTO and NCH LTO fibers. D) Rate performance (specific capacity vs. cycles and efficiency percentage vs. cycles) at different C rates for PC LTO, CCH LTO and NCH LTO fibers. E) Voltage profiles of NCH LTO fiber from 0.5C to 10C rates. F) Cyclic performance of PC LTO, CCH LTO and NCH LTO fibers at 15C (capacity retention percentage vs. cycles and efficiency percentage vs. cycles). (A colour version of this figure can be viewed online.)

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displayed mainly two peaks corresponding to Ti4þ species around 464 eV (Ti 2p1/2) and 458eV (Ti 2p3/2) [46] (Fig. 5Dii). The O1s (Fig. 5Di) showed TieO at 530.1 eV [46], and CeOH at 532.5 eV [53] species, which represented the oxidized state of Ti and carbon in the LTO-NCNT material, respectively. The CV curves of the fiber showed a characteristic cathodic peak at about 1.5 V and an anodic peak at 1.6 V (Fig. S7), which was mirrored by the plateaus in the voltage profiles (Fig. 6A). There were no redox peaks for nickel or cobalt indicating that catalyst has been completely removed. A comparison of the voltage profiles of PC and NCH LTO fibers at a 5C rate (Fig. 6A) showed a great performance difference, with the NCH LTO at 144 mAhg
1 and PC LTO at 42 mAhg
1 supplemented by the cyclic voltammetry curve in Fig. S7. The PC LTO fiber exhibited plateau potentials at about 1.45 V and 1.72 V, while the NCH LTO fiber still showed very stable potentials of 1.5 V and 1.6 V, which is indicative of low polarization effects. The N-CNTs on the fiber surface created a porous network provides easy access and short transport paths for Li þ ions and electrons e
(Fig. 6B), thus increasing the specific power [30e33,64e67]. To validate this claim, we have conducted specific surface area and pore volume analysis of both the PC LTO and NCH LTO fibers using the BET method (Fig. S8). Based on this study, a very appreciable specific surface area 2.5 times in magnitude to that of PC LTO fiber was observed. The NCH fiber also showed a high porosity in the mesoporous range, which was much less pronounced and some pores even absent for the PC LTO fiber. Apart from the network, the N-CNTs also provided a strong nitrogen doped (hydrophilic) substrate for easy material deposition. The faster lithium ion diffusion can be electrochemically quantified as a lower Rct, dropping from 95U to 10U with the introduction of NCNT (Fig. 6C). The two-fold advantage manifested itself in the rate performance. The NCH LTO at 20C rate showed higher specific capacity than the PC LTO at 0.5C rate. This can be seen in the rate performance where the samples were cycled from 0.5Ce30C e 5C rate between 1 Ve2.5 V (Fig. 6D). The NCH LTO fiber maintained low polarization even at a 10C rate, as observed by the voltage profile measurements (Fig. 6E), maintaining 123 mAhg
1 of specific capacity. These results are higher compared to the reported fiber based LTO electrodes [40,68] and comparable to some 2D/3D electrodes [32,33,67]. The lower capacitance with higher C rates relative to some references can be attributed to the 1D form factor our electrode and the limited conductivity of the material, since the longer the fiber the higher the resistance of the electrode. Further work is being done to improve the conductivity of the pristine CNT fiber core to overcome these issues. We also report here a very high areal loading of LTO (2.1 mgcm
2) which is comparable with current published work on many 2D-3D electrodes [39,46], making the NCH fiber ideal substrate for micro and wearable energy storage applications. The electrode reported in this work also exhibited excellent cyclic performance, retaining 100% of its initial discharge capacity after being cycled at 15C rate for 1000 cycles, whereas the

PC LTO dropped to 79.6% (Fig. 6F). We attribute this high cyclic stability to the presence of the nitrogen groups, which anchor and bind the LTO firmly. Further, undoped CNT on PC fiber using ethylene as a precursor has been used to synthesize undoped CNT on CNT hybrid fiber (CCH) (discussed in Fig. S9). This experiment was performed to study whether the “hairy” undoped CNTs on the fiber surface had a stabilizing effect on LTO. From Fig. 6A, D and F it can be concluded that the CCH LTO fiber measured in between the NCH LTO and the PC LTO fiber in terms of overpotential and rate performance. Regarding its cyclic capacity retention, it fell short of even the PC LTO fiber, dropping to 75% after 1000 cycles. Thus, it is fair to attribute the observed stabilizing effect to the nitrogen functional groups. We also found reduced areal loading of LTO due to the hydrophobic nature of CCH (Fig. S9), which further proved the advantage of the nitrogen groups presence in the NCH fiber. As can be seen in Table 1, our electrode compares well with many of the current work on LTO electrodes, most of which are in a 2D/3D format. Although some of the current literature data exceed our results in rate performance (which can be attributed to the 1D nature of our electrode), the NCH LTO fiber outperforms other works in its excellent cyclic behavior at high C rates. The key differences here are in the current collector. Many groups in literature choose to adopt the practice of coating binder slurry mixtures onto metal foils [25,69]. The electrode for these materials must be considered as a composite with the said metal current collector as an integrated component. This makes them prone to fatigue, oxidation and increases the overall weight. The latter is not usually accounted for, especially when calculating gravimetric energy storage parameters. Another point of significance is that the latter is also not taken into consideration when calculating the active material mass loading of the electrode e a parameter of great importance when considering actual full device performances. In our case, the NCH LTO fiber acts as a current collector, thus dramatically reducing the weight of the material in a full device. In this way, the actual loading of active material becomes far higher for our electrode setup as compared to that of slurry-metal current collector composite electrodes. There has been other work done on nitrogen templates or coatings on top of active materials helping to facilitate volume expansion and preventing solid electrolyte interphases (SEI) layer degradation [70e73]. However, when considering intercalation materials, it is important to have a fast channel/ion pathway which is provided by the hairy N-CNT network. In our electrode, the entirety of the active material rests anchored to the N-CNT surface. This enables the bulk fiber core to remain strong and highly flexible. Such a quality allows for further modification of the core via ion doping for higher conductivity, while the strength of the overall electrode is not affected. We also further anticipate that the N-CNT scaffolding template synthesized in this work can house a variety of high volume expansion materials, which would only require a further layer of carbon coating to prevent SEI layer degradation

Table 1 Comparison with our work of specific capacity, current rate and cyclic stability of different types, recently published LTO electrodes. Type of LTO Electrode/Current Collector

Current rate (C)

Capacity (mAhg
1) @ mass loading w/o metal mass

Cyclic Stability

Ref.

LTO powder/Copper foil LTO in C Matrix/Al foil CNT- LTO/Cu foil Graphene LTO e CNT/Carbon cloth LTO Fiber/Pt wire LTO-C/Cu foil LTO-C/Cu foil Hollow LTO/CNF-CNT NCH LTO Fiber/CNT

1 5 20 20 2 20 20 10 5/10

113 @80 wt% 130 @76 wt% 112 @85 wt% 150 @62.5 wt% 20 @— 147@80 wt% 106@85 wt% 142 @70 wt% 144/123@78 wt%

93% @50 cycles @1C 94.4% @1000 cycles @20C 98% @100 cycles @5C 89.5% @10000 cycles @20C 85% @20 cycles 74.2%@3000 cycles @5C e 96.4%@ 500 cycles @10C 100.1% @1000 cycles @15C

[74] [33] [75] [76] [68] [45] [77] [78] This work

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[71,73]. This makes our current collector highly tunable for various applications. 3.4. NCH LTO e NCH hybrid capacitor To evaluate the practical application of the created current collector we fabricated an asymmetrical hybrid supercapacitor. The NCH LTO fiber was used as anode and NCH fiber was employed as

415

cathode with 1 M LiPF6 in 1:1, (v:v, volume ratio) mixture of ethylene carbonate and dimethyl carbonate employed as the electrolyte. Unlike symmetrical conventional capacitors that show rectangular CV curves, the hybrid capacitor demonstrated a distinct non-rectangular peak (Fig. 7A). This is reflective of the mechanisms in work here. The cathode stores ions via the classical Electrochemical Double Layer Capacitance (EDLC) whereas the anode stores the Liþ ions via insertion [79e85]. Such a mechanism is

Fig. 7. A) CV curves of NCH LTO e NCH hybrid device at scan rates from (5 mVe100 mV). B) Galvanostatic charge-discharge curves of the hybrid device at current densities ranging from 0.25 A/g to 8 A/g). C) Cyclic performance of the hybrid device at 4 Ag-1. D) Ragone plot of current work in comparison with conventional energy storage devices (from Ref. [86]). E) Picture of green LED being powered by the hybrid device created in this work. (A colour version of this figure can be viewed online.)

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Table 2 Comparison of specific energy, specific power and cyclic stability of recently reported different types of Li-ion capacitors with our device. Type of Li-ion Capacitor

Specific Energy (Whkg
1)

Specific Power (Wkg
1)

Cyclic Stability

Ref.

Graphene-wrapped LTO LTO Spheres LTO-AC LTO/C microspheres LTO-Graphene TPO-AC TiO2-CNT LCTO-AC rGO LTO - Graphene NCH LTO e NCH fiber

15 41.7 38 20 14 13 12.5 23 95 38/27/22/17

2500 469 7964 800 2700 371 1300 4000 3000 312/555/3385/12500

e 93% @500 cycles 90.8% @2000 cycles 96% @1000 cycles 97% @3000 cycles 100% @500 cycles e 84% @1000 cycles 87% @500 cycles 100% @2000 cycles

[87] [83] [82] [84] [81] [88] [89] [90] [91] This work

responsible for the device's high specific energy and specific power leading to a hybrid behavior between that of supercapacitors and batteries. The galvanostatic charge-discharge (Fig. 7B) was obtained to calculate the specific energy at various specific powers which also showed a non-triangular curve, indicating the presence of Liþ ion insertion mechanics. It exhibited a 100% capacity retention after 2000 Cycles (Fig. 7C). The performance of the full device as seen in the Ragone plot (Fig. 7D) places our device higher than that of conventional supercapacitors, exceeding battery performance in terms of specific power, while remaining comparable to its high specific energy at lower specific power. To portray the device's practical application, an LED was connected to the fully charged device Fig. 7E. After charging to 2.5 V it was capable of powering and lighting a green LED for 40 min. The device showed a maximum specific energy of 0.296 mWhcm
2 corresponding to 0.019 Whcm
3 or 68 Whkg
1 at 0.172 mWcm
2 specific power equivalent to 0.011 Wcm
3 or 126 Wkg-1. This capacitor maintained specific capacitance of 0.0779 mWhcm
2 corresponding to 0.005 Whcm
3 or 17 Whkg
1 at a high specific power of 57.05 mWcm
2 equivalent 3.719 Wcm
3 or 12,500 Wkg-1. Since a potential application for the device presented here, is powering micro-sized and wearable electronics, we have included Ragone plots using areal and volumetric normalization in the Supplementary InformationFig. S10. As mentioned previously, the fabricated electrodes have been tested in a coiled state, which resulted in a low effective density due to inclusion of air gaps within the coil. Thus volumetric data reported here does not fare well when compared with some high energy density devices [81]. However, since the primary application of the reported capacitor is a micro-sized and wearable device, which fully utilizes the flexible and fibrous nature of the electrodes, presenting areal normalized data is more appropriate. Despite the large curved surface area of the fiber electrodes, it has been demonstrated that a significant loading of active material is achieved. The obtained areal energy densities vs. power densities are highly competitive with other fiber devices as shown in Fig. S10ii. As can been seen in Table 2 our device performs better in capacity retention than most 2D electrodes. The specific power and corresponding specific energy far exceeded that of other 1D electrode devices and compared very well to recently reported 2D/3D electrode-based devices. Further studies are ongoing to create a higher specific area material that can act as the cathode to further increase the performance of the device and alleviate the initial drop in specific energy with an increase in specific power. 4. Conclusions CNT hybrid fiber has been designed and fabricated as a current collector for high-performance Li-ion capacitor. The synthesis approach involved catalyst deposition on dry spun CNT fibers through a modified hydrothermal process followed by CVD grown nitrogen-doped CNTs on their surface. The “as prepared” NCH fiber

remained as flexible and strong as the pristine one, allowing it to be potentially used for micro-sized and wearable energy storage applications. The created hybrid fiber electrode surpassed 10 times the specific capacity of pristine (PC) fiber owing to its highly accessible porous network and presence of nitrogen-containing groups. In addition, a significant drop in Rct from 620U to 83U was observed. Further, LTO was successfully deposited on the NCH thus fabricating a fast charging, intercalation-type battery electrode. The hybrid composite fiber electrode exhibited an excellent rate performance compared to other 1D or in some cases 2D electrodes and showed an energy density of 144 mAhg
1 at 5C rate. This high rate performance can be explained by the faster ion transfer and shorter pathway facilitated by the N-CNTs substantiated by porosity measurements. The cyclic stability of the NCH LTO fiber significantly exceeded that reported in the literature due to the N-CNTs ability to bind firmly with the LTO, thus retaining 100.9% capacitance after 1000 cycles at 15C rate. The N-CNT template accommodating the active material also enabled high areal loading of LTO reaching 2.1 mgcm
2. Finally, a lithium-ion hybrid capacitor was fabricated, with NCH LTO as anode and NCH as a cathode, to facilitate a fully functional device. The hybrid capacitor revealed a high specific energy of 0.296 mWhcm
2 corresponding to 0.019 Whcm
3 or 68 Whkg
1 at 0.172 mWcm
2 specific power equivalent to 0.011 Wcm
3 or 126 Wkg-1. This device maintained specific capacitance of 0.0779 mWhcm
2 corresponding to 0.005 Whcm
3 or 17 Whkg
1 at a high specific power of 57.05 mWcm
2 equivalent 3.719 Wcm
3 or 12,500 Wkg-1. These values are much higher than those reported for conventional 1D pseudocapacitors and can be compared directly with 2D devices. The fully assembled supercapacitor showed excellent cyclic stability, retaining 100% of its initial capacity after cycling at 4 Ag-1 for 2000 runs. This work demonstrated that the synthesized NCH fiber behaved as a strong, versatile and flexible current collector that could be considered as an outstanding template for easy accommodation of many active battery materials thus enhancing their performance. In addition, a scalable synthesis route has been proposed and successfully employed through which meters long fiber electrodes can be easily fabricated. Acknowledgment This work was funded by NASA through grant # NNC16CA17C and by the National Institute for Occupational Safety and Health through the Pilot Research Project Training Program at the University of Cincinnati (ERC grant # 1013735). Appendix A. Supplementary data Supplementary data to this article can be found online at

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