Ti2Nb10O29 core-shell arrays

Journal Pre-proof Ultrafast and durable lithium ion storage enabled by intertwined carbon nanofiber/ Ti2Nb10O29 core-shell arrays Yulin Tang, Shengjue Deng, Shaohua Shi, Lihong Wu, Guizhen Wang, Guoxiang Pan, Shiwei Lin, Xinhui Xia PII:

S0013-4686(19)32305-9

DOI:

https://doi.org/10.1016/j.electacta.2019.135433

Reference:

EA 135433

To appear in:

Electrochimica Acta

Received Date: 30 October 2019 Revised Date:

30 November 2019

Accepted Date: 2 December 2019

Please cite this article as: Y. Tang, S. Deng, S. Shi, L. Wu, G. Wang, G. Pan, S. Lin, X. Xia, Ultrafast and durable lithium ion storage enabled by intertwined carbon nanofiber/Ti2Nb10O29 core-shell arrays, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2019.135433. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Ultrafast and Durable Lithium Ion Storage Enabled by Intertwined

Carbon

Nanofiber/Ti2Nb10O29

Core-Shell

Arrays Yulin Tanga, Shengjue Dengb, Shaohua Shia, Lihong Wua, Guizhen Wanga,*, Guoxiang Panc, Shiwei Lina, Xinhui Xiab,*

a

Key Laboratory of Advanced Materials of Tropical Island Resources (Hainan

University), Ministry of Education, Haikou 570228, P. R. China b

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials

and Applications for Batteries of Zhejiang Province, and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China. c

Department of Materials Chemistry, Huzhou University, Huzhou, 313000, P. R.

China

*Corresponding authors. Tel: +86-137 0040 8893. E-mail: [email protected] (Guizhen Wang). Tel: +86-139 5716 7165. E-mail: [email protected] (Xinhui Xia).

Abstract Ti2Nb10O29 (TNO) has been extensively regarded as a promising material for lithium-ion batteries (LIBs) in view of its large theoretical capacity and high structural stability. Nevertheless, accomplishing ultrafast and durable lithium storage meets a challenge of designing TNO-based nanostructure with improved electronic conductivity and favorable kinetics. Herein, the binder-free hierarchical carbon nanofibers@TNO arrays grown on carbon cloth (CNFs@TNO/CC) are prepared by a facile electrophoretic deposition & solvothermal method. The intertwined CNF arrays as both an excellent bridge and a strong support can endow TNO with high conductivity, good mechanical stability and large surface area. TNO particles attached to CNF surface are also limited at a narrow size of distribution (20–50 nm), which decreases the Li-ion transport pathways and improves the Li-ion diffusion coefficients. Thus, the remarkably high Li-ion diffusion coefficients are achieved for the present CNFs@TNO/CC. CNFs@TNO/CC arrays manifest an excellent capacity of 308 mA h g–1 at 1 C with a high initial Coulombic efficiency of 91%. A capacity retention of 192.6 mA h g–1 (91% initial capacity) is achieved after 1000 cycles at a rate of 10 C. Our work illustrates that the CNFs@TNO/CC as a promising anode material, exhibits great potential application for ultrafast LIBs.

Keywords: Ti2Nb10O29, core-shell arrays, Li-ion diffusion coefficient, high rate capability, long cycle life

Introduction Ti2Nb10O29 (TNO) has been appeared as a highly promising candidate for lithium-ion batteries (LIBs) owing to its high theoretical capacity, safe operating potential, superior cycling stability and impressive intercalation pseudocapacitive behavior.1–3 With the specific 22-electron transfer reactions of Nb3+/Nb4+, Nb4+/Nb5+ and Ti3+/Ti4+, TNO shows a relatively high theoretical capacity of 396 mA h g–1, which is higher than graphite and almost two times higher than Li4Ti5O12.4 Its high working potential (~1.6 V) can also restrain the electrolyte decomposition and prohibit the growth of lithium dendrites. Meanwhile, TNO shows a Wadsley-Roth shear structure constructed by sharing corners or edges of 3 × 4 × ∞ octahedron-blocks. The edge sharing polyhedrons significantly improve its structural stability, and thus guarantee its high cycling stability.5, 6 Moreover, TNO presents a pseudocapacitive Li-ion intercalation behavior without any crystallographic changes during Li-ion insertion/extraction.7 Such feature leads to that Li-ion transport in TNO material is unrestricted from solid-state diffusion. However, its intrinsic low electronic conductivity still prevents the sufficient diffusion of Li-ion in aggregated and bulk-size TNO, which limits its practical capability and rate performance. Numerous efforts have been devoted to increasing the charge transfer kinetics and electronic conductivity of TNO such as synthesizing TNO in nano scale or incorporating it with conductive carbon materials.8–19 Constructing hierarchical nanostructure has been proved to be an effective way to improve the rate capability of TNO materials for LIBs, because hierarchical nanostructure can not only shorten path lengths for electrons/ions transfer, but also provide enough void space to accommodate

volume expansions.20–22 In addition, the capability property can also be further improved by the significant pseudocapacitive effects generated from hierarchical nanostructure.9, 23, 24 Incorporating TNO with carbonaceous materials is also a powerful way to elevate the electronic conductivity of TNO materials in the course of charging and discharging process.25, 26 Recently, Lou et al. prepared a three-dimensional ordered macroporous structure of TiNb2O7 by employing polystyrene colloidal microspheres as a hard template.9 The as-prepared honeycomb-like construction showed a reversible capacity of 120 mA h g–1 at 50 C and capacity retention of 188 mA h g–1 after 1000 cycles at 10 C. Wang et al. synthesized an ultrathin N-doped carbon coated porous TNO microspheres through an atomic layer deposition assisted method.3 The ultrathin carbon layers could elevate electron/ion transport performance and limit its grain size. The N-doped carbon coated TNO microspheres displayed a lithium storage capacity of 199 mA h g–1 at 40 C and 241 mA h g–1 capacity retention over 500 cycles at 10 C. Apparently, synthesizing hierarchical TNO nanostructure and combining it with carbonaceous material are conducive to improving cycling stability and rate capability of TNO anodes. However, all the above methods are generally based on powder-form electrodes, and the introduction of insulating binders also hinders its electrochemical performance especially at high current rates.27 For these reasons, approaches to design an integrated TNO anode with both nanocrystalline and high-conductivity characteristics, and without a binder utilization are extremely important to maximize lithium ion storage performance. In this work, we report carbon nanofibers/TNO core-shell arrays grown on carbon cloth (CNFs@TNO/CC) as anode material to promote lithium-ion diffusion kinetics, enhance the capability property, and ameliorate the rate performance and cycling stability. Such architecture as LIB anode has several obvious advantages. First, the

unique core-shell structure of nanosized TNO particles uniformly anchored on CNF surface improves the lithium-ion transfer efficiency. Next, the CNF arrays with an optimized length served as a bridge increase the conductivity of TNO nanoparticles, and also as a strong support enhance the mechanical stability of composite arrays. Most important of all, compared to the conventional linear arrays, the intertwined and interconnected CNFs@TNO arrays can form a tridimensional conductive network, which is conductive to fully boosting their LIB performance. Consequently, CNFs@TNO/CC exhibits a high reversible capacity (308 mA h g–1 at 1 C), exceptionally fast Li-ion diffusion coefficients (2.27 × 10–11 cm2 s–1 for insertion and 2.59 × 10–11 cm2 s–1 for extraction), a high rate capacity (182 mA h g–1 at 80 C) and a superior cycling stability (high capacity retention of 78% over 1000 cycles at 10 C). These results show that the present CNFs@TNO/CC electrode has potential application in LIBs, and it may shed light on the development of other anode materials with a similar configuration.

Fig. 1 Schematic illustrations of the synthesis of CNFs@TNO/CC.

Experimental Section

Preparation of CNFs/CC The CNFs used in this work were synthesized according to our previous report with some minor modifications.28–31 Then, CNFs/CC array films were synthesized by a simple electrophoretic deposition process. The as-prepared CNFs were functionalized using a mixture of H2SO4 and HNO3 in a 3:1 volume ratio and then ultrasonicated at 60 ºC for 5 hours. After that, CNFs were washed and collected. 5 mg of functionalized CNFs were first dispersed in 50 mL of isopropanol in a beaker and then sonicated for 2 h. Pt plate and clean CC were served as the positive electrode and the negative electrode, respectively. The electrodes were vertically oriented and separated by 2.5 cm in the beaker containing the CNFs solution. Then a voltage of 60 V was applied for 6 min to obtain the CNFs/CC films. The mass loading for CNFs was calculated as approximately 0.7 mg cm–2. Preparation of CNFs@TNO/CC and CC@TNO electrodes The CNFs@TNO/CC electrodes were fabricated by a facile solvothermal method (Fig. 1). Typically, titanium isopropoxide (0.284 g) was dissolved in ethanol (60 mL) with an ultrasonic treatment for 5 min. Then, niobium chloride (1.35 g) was added into the above solution. After vigorous stirring for 30 min, the mixed precursor solution and CNF/CC films was transferred to a 100mL Teflon-linked steel autoclave and kept at 200 ºC for 6 h. The obtained sample was rinsed with deionized water and ethanol for several times, then dried in the oven at 60 ºC for 12 h. In order to obtain high-crystallinity TNO, the CNFs@TNO/CC composites were further annealed at 800 ºC for 2 h in an Ar atmosphere. The CC@TNO composites were synthesized by the same procedure for comparison. Materials characterizations The X-ray diffraction (XRD) patterns were collected on a RigakuD/Max–2550

X-ray diffractometer using Cu Kα radiation source. Chemical state measurements were performed by X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD), using a monochromic Al X-ray source. Scanning electron microscopy (SEM) images were obtained by using a field emission scanning electron microscope (FESEM, Hitachi SU8100 operating at 5 kV). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were captured by a JEOL JEM–2100F microscope instrument operating at 200 kV accelerating voltage. Raman spectra were obtained on a Renishaw inVia Raman Microscopy under 514 nm laser excitation. BET surface areas and pore size distributions of CC@TNO and CNFs@TNO/CC electrodes were investigated via JW–BK112 physical adsorption instrument, and N2 was used as the desorption/adsorption gas. Electrochemical characterization Electrochemical performance of the CC@TNO and CNFs@TNO/CC electrodes were tested by using assembled CR2025 coin cells operated in an Ar-filled glovebox (O2 < 0.1 ppm, H2O < 0.1 ppm). Generally, the dried CC@TNO and CNFs@TNO/CC films were used directly as the work electrode and the pure Li metal foil was used as the counter and reference electrode. The Celgard 2400 microporous polypropylene film was employed as separator. The electrolyte is LB–006 (Suzhou Qianmin Chemical Reagent Co.) which contains 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by weight). Cyclic voltammetry (CV) was performed on CHI660E electrochemistry workstation (Chenhua Instrument, Shanghai, China) at different scan rates from 1.0–2.5 V versus Li/Li+. Rate performance and cyclability were tested by using a multichannel battery testing system (Neware CT–3008, China) under the working voltage of 2.5–1.0 V. The 1 C current rate is equal to 396 mA h g–1, which corresponds to the theoretical capacity of Ti2Nb10O29. The galvanostatic

intermittent titration (GITT) curves were collected on the above battery testing system, by applying a constant small current flux of 0.1 C for 10 min with a rest time interval of 20 min in the voltage range of 1.0–2.5 V. The electrochemical impedance spectroscopy (EIS) tests were conducted on a Princeton Applied Research advanced electrochemical system over a frequency range of 100 kHz to 0.01 Hz with amplitude of the sine perturbation signal of 10 mV. Before EIS measurements, the cells were cycled for two cycles at 1 C, discharged to 50% state of charge, and then equilibrated at open circuit (~1.6 V) for 2 h to allow the potential to stabilize.

Results and discussion

Fig. 2 (a) XRD patterns of CNFs/CC, CC@TNO and CNFs@TNO/CC. (b) Raman spectra of the CNFs@TNO/CC and CC@TNO electrodes. (c, d) XPS spectra of Ti 2p and Nb 3d of the CNFs@TNO/CC electrode. Fig. 2a displays the XRD patterns of CNFs/CC, CC@TNO and CNFs@TNO/CC samples. The CNFs/CC shows two peaks at ~26° and 43° in the XRD pattern, related

to the (002) and (004) crystal planes of graphitic carbon (JCPDS No.75–1621), respectively. In the case of CC@TNO and CNFs@TNO/CC samples, all peaks show broad features, indicating the nanosized primary particles. It can also be observed from the SEM and TEM results. The XRD patterns can be indexed well with A2/m space group of monoclinic Wadsley-Roth shear structure (JCPDS No.72–0159), which indicates that the pure Ti2Nb10O29 is obtained.32 Besides, the diffraction peaks at ~26° and 43° were also observed, resulting from the presence of CC substrate. The crystal structures of the CC@TNO and CNFs@TNO/CC samples were also investigated by Raman spectra (Fig. 2b). For the CC@TNO composites, the two peaks at 1590 cm–1 and 1349 cm–1 are correspond to the G and D bands of CC.33 The ID/IG band intensity ratio of CC@TNO composites is 0.998, demonstrating the low crystallinity of CC. As for the CNFs@TNO/CC composites, the intensity ratio (ID/IG) is 1.002. The increased intensity ratio (ID/IG) value of CC versus CNFs/CC is due to the introduction of CNFs. These functionalized CNFs are expected to increase the conductivity of CNFs@TNO/CC composites. Apart from peaks induced by CC and CNFs, five new peaks at 1002, 893, 648, 543 and 210 cm–1 observed in both samples can be typically attributed to the edge-shared NbO6 octahedral and corner shared NbO6 octahedral, respectively.34 No impurity phases (Nb2O5 or TiO2) are detected in the range of 200–2000 cm−1, which is in line with our XRD results, indicating the acquisition of phase-pure TNO. The investigation on surface chemical composition of CC@TNO and CNFs@TNO/CC was performed by utilizing X-ray photoelectron spectroscopy (XPS). The photoelectron peaks confirm the abundance of Nb and O, as well as small amounts of Ti. The similar high resolution XPS spectra of Ti 2p and Nb 3d regions are detected in Fig. 2c and d. For the Ti 2p region, photoelectron peaks at ~459 eV and

~465 eV correspond to the binding energies of 2p3/2 and 2p1/2 of oxidized Titanium (Ti(IV)).35 While the photoelectron spectrum of Nb 3d region is decomposed into two peaks at ~207.5 eV and ~210.5 eV, which is well indexed with the binding energies of Nb 3d5/2 and Nb 3d3/2, revealing that the existence of Nb5+.36

Fig. 3 FESEM images of (a–c) CC@TNO and (d–f) CNFs@TNO/CC electrodes.

SEM and TEM images were recorded to investigate the microstructure and morphologies of the samples. The CC is uniformly coated by layers of radially dispersed, random oriented, cross-linked CNFs forming a highly conductive and porously network (Fig. S1). For CC, after the solvothermal reaction and calcination process, TNO nanoparticles with diameters of 30–100 nm are homogeneously adhered to the surface of carbon fibers (Fig. 3a–c). For CNFs@TNO/CC electrodes, the dense and uniform of TNO nanoparticles in size of 20–80 nm is homogeneously grown on the surface of CNFs. The legible CNFs@TNO core-shell structure can be easily recognized due to their different contrast (Fig. 3 d–f). Such structure is beneficial for the electron conduction among the TNO nanoparticles.

Fig. 4 (a–c) TEM and HRTEM images of CNFs@TNO/CC. (d–h) EDS elemental mapping images of C, Ti, Nb and O in the CNFs@TNO/CC.

Further characterization of the microstructures is conducted by TEM-HRTEM. For the CNFs/TNO core-shell nanofibers, it can be seen that TNO nanoparticles of 20–50 nm are uniformly anchored on the CNF surface with their average diameter increasing from ~100 nm to ~150 nm (Fig. 4a–b). The measured lattice space for Ti2Nb10O29 particle is about 0.37 nm (Fig. 4c), corresponding to the (011) crystal planes of the Ti2Nb10O29 phase (JCPDS 72–159). Energy-dispersive X-ray spectroscopy (EDX) measurements were performed to observe the contents and distributions of various elements. The obtained mapping images in Fig. 4d–h confirm that Ti, Nb, O and C elements are uniformly distributed in the final CNFs@TNO composite nanofibers, further verifying the successful synthesis of CNFs@TNO core-shell structure.

Fig. 5 CV profiles of CC@TNO|Li and CNFs@TNO/CC|Li cells at (a) 0.2 mV s–1 and (b, c) various sweep rates. (d) Relationship between peak currents of cathodic/anodic relationship and square roots of scan speed v0.5. (e) Separation of the total current (solid line) and capacitive currents (shaded regions) at 0.4 mV s–1. (f) Contribution ratio of the capacitive and diffusion-controlled charge at different scan rates. The Brunauer-Emmett-Teller (BET) analysis were conducted to investigate the specific surface area of CNFs@TNO/CC and CC@TNO hierarchical nanostructures. As shown in Fig. S2a, BET surface area of CNFs@TNO/CC and CC@TNO are

measured to be 8.98 m2 g–1 and 2.84 m2 g–1, respectively. Obviously, the addition of CNFs leads to a notably increased surface area for CNFs@TNO/CC composite structure. The increased specific surface area can improve cycling stability by increasing the total number of active sites for Li-intercalation. The pore size of CNFs@TNO/CC

and

CC@TNO

electrodes

were

investigated

by

Barrett-Joyner-Halenda (BJH) method. The BJH pore size distribution results (Fig. S2b) indicate that both CNFs@TNO/CC and CC@TNO show a typical mesoporous characteristic (2–50 nm). Cyclic voltammetry (CV) tests were performed on the CC@TNO|Li and CNFs@TNO/CC|Li half cells to clarify the redox kinetic properties of TNO composites. The CV curves of the CC, CNFs/CC, CC@TNO and CNFs@TNO/CC electrodes were obtained at sweep rate of 0.2 mV s–1 in the voltage window of 1–2.5 V (vs. Li/Li+). As shown in Fig. 5a, CC and CNFs/CC substrates have no obvious redox peaks and little current flow compared to CC@TNO and CNFs@TNO/CC, which suggests that they provide little capacity during the cyclic process. Thus, the main capacity contribution relies on the intercalation pseudocapacitve capacity from TNO nanoparticles. For the CC@TNO electrode, two intensive peaks centered at 1.61 V (cathodic) and 1.71 V (anodic) are related to the Nb4+/Nb5+ redox couple. A pair of weak peaks, located at ~1.9 V (cathodic) and 1.92 V (anodic) are attributed to the redox couple of Ti3+/Ti4+. A broad bump range from 1–1.4 V can be assigned to the redox reaction of Nb3+/Nb4+ redox couple. The average working potential for CC@TNO electrode was calculated to be ~1.66 V, which can be attributed to the couple of intermediate potential of cathodic/anodic peaks at 1.61/1.71 V. Similar to those TNO anode materials (~1.66 V for TiNb24O62,37 ~1.67 V for Ti2Nb14O39,38 ~1.7 V for TiNb6O17,39 ~1.65 V for TiNb2O740, 41), this rationally high operating potential of

TNO avoids the safety issues caused by lithium dendrite formation and the decomposition of electrolyte at high current densities. The CNFs@TNO/CC electrode shows similar CV peaks with CC@TNO, but exhibits a higher current flow mainly attributed to the large surface area and reasonably high porosity, which leads to a reduced Li-ion diffusion path into TNO and facilitates the Li-ion flux at the interface between electrolyte and electrode material. In addition, the decrease of polarization is also observed due to the enhancement of Li-ion diffusion kinetics and electronic conductivity. Table 1. DLi values of MNbO–based ReO3 shear structured electrodes calculated from CV curves obtained at different scan rates. Active Materials

Anodic DLi (cm2 s–1) Cathodic DLi (cm2 s–1)

Ti2Nb14O39 microspheres

9.07 × 10–14

5.52 × 10–14

TiNb6O17 microspheres

5.48 × 10–14

4.28 × 10–14

CrNb11O29 nanorods

3.57 × 10–13

1.51 × 10–13

Ti2Nb10O29 nanofibers

1.49 × 10–12

1.45 × 10–12

CC@TNO nanoarrays

1.84 × 10–11

1.67 × 10–11

CNFs@TNO/CC nanoarrays

2.59 × 10–11

2.27 × 10–11

To further characterize the Li-ion diffusion coefficient of CC@TNO and CNFs@TNO/CC electrodes, the CV curves of CC@TNO and CNFs@TNO/CC were collected at different sweep rates (0.2, 0.4, 0.7 and 1.1mV s–1), as shown in Fig. 5b–c. The cathodic/anodic peak current (Ip) exhibits a linear relationship with the square root of the sweep rate (v0.5) during the intensive Nb4+/Nb5+ redox peaks at ~1.61/1.71 V under the scan rates ranging from 0.2 to 1.1 mV s–1 (Fig. 5d). Therefore, the

apparent Li-ion diffusion coefficients D of CC@TNO and CNFs@TNO/CC can be calculated from the slopes in Fig. 5c through the Randles-Sevcik equation (Equation (1))42, 43: Ip = 2.69 × 105 × n1.5 × SCD0.5v0.5

(1)

where n, S and C represent to the number of electrons transfer involved in the redox reactions, the effective electrode area in cm2 and the molar concentration of Li-ions in the TNO crystals, respectively. The obtained Li-ion diffusion coefficients of TNO@CC are 1.67 × 10–11 cm2 s–1 for lithiation and 1.84 × 10–11 cm2 s–1 for delithiation respectively, indicating that lithiation is the rate-limiting step in the whole electrochemical reaction of TNO as reported by previous work.19,

38

As for

CNFs@TNO/CC, the larger insertion and extraction Li-ion diffusion coefficients are obtained. It can reach 2.27 × 10–11 cm2 s–1 (insertion) and 2.59 × 10–11 cm2 s–1 (extraction), respectively. The Li-ion diffusion coefficients of CC@TNO and CNFs@TNO/CC are 1–3 order of magnitude larger than that of TiNb6O17 (5.48 × 10−14 and 4.28 × 10–14 cm2 s–1),11 Ti2Nb10O29 (1.49 × 10−12 and 1.45 × 10 –12 cm2 s–1),19 Ti2Nb14O39 (9.07 × 10−14 and 5.52 × 10–14 cm2 s–1)38 and CrNb11O29 (3.57 × 10–13 cm2 s–1 and 1.51 × 10 –13 cm2 s–1) (Table 1).44 It is noteworthy that the Li-ion diffusion coefficients of nanoarrays (CC@TNO and CNFs@TNO/CC) are higher than TiNb6O17 microspheres11, Ti2Nb10O29 nanofibers19, CrNb11O29 nanorods44 and Ti2Nb14O39 microspheres38. Compared with CNFs@TNO nanoarrays, these as referred MNbO–based ReO3 structures have larger grain sizes and lower length/diameter ratios, resulting in longer Li-ion diffusion pathways. In addition, during Li-ion insertion/extraction, cracks and SEI layer may be induced due to the stress generated between the particles and at defective sites of MNbO surface. Meanwhile, the indirect current pathways between the particles may hinder electron

transport, and thus cause sluggish Li-ion diffusion kinetics. The excellent electrochemical characteristic of TNO composite anodes is mainly due to that the intertwined and interconnected CNFs@TNO arrays can form a tridimensional conductive network, comprehensively improving the electron transport from TNO to TNO particles, TNO to CNFs and CNFs to CC. Moreover, the TNO particles anchored on the CNFs have a narrow size distribution under nanometers, which leads to short Li-ion diffusion pathways and facilitates the Li-ion transport within them. The details on charge storage process of CC@TNO and CNFs@TNO/CC electrodes are investigated based on the CV results. According to Conway et al., the capacitive or diffusion-controlled process can be distinguished through estimating the peak current (ip) change with potential sweep rate (v), which can obey a power law relationship45, 46: ip = avb

(2)

where a and b are adjustable parameters relevant to sweep rates. The b value can be calculated from the slope of linear fitting of the log (ip) vs. log (v) plot. It indicates how capacitive the process is. b = 1 represents that the reaction process is completely controlled by the pseudocapacitive effect, while b = 0.5 symbolizes the dominant position of ion diffusion process. log (ip) versus log (v) was plotted and shown in Fig. S3a–b. It can be seen that the CC@TNO shows large b values of 0.76 and 0.74 for cathodic and anodic process, respectively. For CNFs@TNO/CC, b values are 0.79 and 0.74. The results suggest that the reaction process is determined by the Li-ion insertion/extraction diffusion and pseudocapacitive effect together. A quantitative analysis method proposed by Dunn et al. was applied to distinguish the pseudocapacitive contribution to the current response.47 The whole current response is composed of surface capacitive (k1v) and diffusion-controlled intercalation process (k2v1/2) at any given potential (V), leading to Equation (3)

i(V) = k1v + k2v1/2

(3)

i(V)/v1/2 = k1v1/2 + k2

(4)

where k1 and k2 are two constants. They can be calculated by using Equation (4) by plotting the fitting lines of i(V)/v1/2 vs v1/2. As shown in Fig. 5e, the proportion of capacitive contribution accounts for as high as 69.97% of the total observed current at the sweep rate of 0.4 mV s–1. Using the same methodology, the capacitive contributions at different sweep rates are calculated (Fig. 5f). With the increasing of scan rates (0.2– 1.1 mV s–1), the ratio of the capacitive contribution gradually increases (63.85–82.45%) and dominates. As a result of the significantly high capacitive contribution, the CNFs@TNO/CC electrode presents an excellent rate capability. It mainly originates from the TNO nanocrystals in the composites together with its porous structure which could afford a short-length mass diffusion process and abundant active sites.

Fig. 6 (a) Charge/discharge profiles of the CC@TNO|Li and CNFs@TNO|Li cells. (b) Rate capabilities of the CC@TNO|Li and CNFs@TNO/CC|Li cells. (c) Comparison of

the electrochemical performances of different TNO-based electrodes at the rate of 10 C. (d) Nyquist plots of the CC@TNO|Li and CNFs@TNO/CC|Li cells (inset: selected equivalent circuit). Electrochemical performance of the CC@TNO and CNFs@TNO/CC arrays was further conducted on galvanostatic conditions at lithium half cells. Fig. 6a shows their galvanostatic charge/discharge profiles at current density of 5 C and 10 C (1 C = 396 mA h g–1) for 1.0–2.5 V, respectively. The shape of galvanostatic charge-discharge curves of the two examples are similar, verifying that the presence of CNFs has no effects on the lithium storage but enhances the conductivity and reduces the particle size. It can be seen that both electrodes show similar sloping regions and plateau regions on their charge/discharge curves. The sloping line from 2.5 V to 1.7 V is a solid-solution process, and the plateau region at 1.6–1.7 V corresponds to a two-phase transformation process.6 The following sloping region in the range of 1.6–1.0 V can be rooted in another solid-solution process.6 In addition, the charge/discharge plateau voltage corresponds well to the intermediate potential of anodic/cathodic peaks of CV curves. Furthermore, compared with CC@TNO electrode, the CNFs@TNO/CC anode exhibits a smaller polarization, which signifies a better charge transfer kinetics. Both samples exhibit excellent rate capability, even at ultra-large current rates. When the current density increases, the capacities of the CC@TNO and CNFs@TNO/CC samples gradually decrease due to the increased electrode polarization (Fig. 6b). At 1, 2, 5, 10, 20, 40 and 60 C, the capacities of the CC@TNO sample remain at 289, 245, 210, 193, 178, 161, and 151 mA h g–1, respectively. After combined with CNFs/CC, the electrochemical performance of the CNFs@TNO/CC sample is further improved. Consistent with CV results, the CNFs@TNO/CC sample always shows lower polarizations, higher capacities and less capacity degradation than the CC@TNO

sample at all the current rates. This tendency becomes more significant with the increase of current rates. The CNFs@TNO/CC sample is capable of delivering high capacities of 308, 280, 261, 241, 227, 203 and 188 mA h g–1 at 1, 2, 5, 10, 20, 40 and 60 C, respectively. In addition, a high ICE of 91% are achieved at 1 C with an initial discharge capacity of 338 mA h g–1(Fig. S4). Even at an ultra-large current rate of 80 C (only 45 s to full discharge/charge), it can still offer 182 mA h g–1, which is up to 59.3% of its capacity at 1 C, and 119% higher than that of CC@TNO at 60 C. This superior rate capability surpasses most of the reported TiNbxO2+2.5x-based anode materials (including macroporous TiNb2O7,23 Ti2Nb10O29 microspheres,8 bulk Ti2Nb10O29 particles,34 nano-TiNb2O7/CNTs,48 Mo-doped TiNb2O7,49 hollow TiNb2O7@C composites50) (Fig. 6c). As mentioned above, the exceptionally excellent rate capability for CNFs@TNO/CC should originate from their outstanding three-dimensional core-shell hierarchical architecture. This architecture composed of largely exposed cross-linked CNFs@TNO nanonet enables electron/ion transport efficiently along the skeleton and provides numerous easily accessible active sites. EIS was further applied to characterize the enhanced reaction kinetics of the CC@TNO and CNFs@TNO/CC electrodes. Fig. 6d shows the Nyquist plots of CC@TNO and CNFs@TNO/CC. It can be seen that each plot shows a depressed semicircle and a straight line. The semicircle at the high frequencies can be ascribed to the charge transfer resistance (Rct), while the straight line at the low frequencies represents the Li-ion diffusion within active materials. A reduced Rct (95.67 Ω) of the CNFs@TNO/CC can be achieved compared to that (268.2 Ω) of the CC@TNO, indicating that the core-shell hierarchical architecture can effectively reduce the charge transfer impedance of TNO. The improved charge transfer of the CNFs@TNO/CC can be mainly ascribed to the high electric conductivity of CC

decorated with CNFs as well as the three-dimensional configuration serving as electron highways. According to the impedance data, we calculated the apparent Li-ion diffusion coefficients of CC@TNO and CNFs@TNO/CC electrodes by using the equation (5) and (6) as follows: Z' = RΩ + Rct + σω–1/2

(5)

D = R2T2/(2S2F4C2σ2)

(6)

where Z', ω, D, R, T, S, F and C represent the real part of the impedance, the angular frequency in the low frequency region (0.1–1 Hz), the Li-ion diffusion coefficient, the gas constant (8.314 mol–1 K–1), the absolute temperature (298 K), the area of the electrodes surface, the Faraday constant (96500 C mol–1), and the Li-ion molar concentration in the crystal, respectively. σ corresponds to the Warburg factor, which can be derived from the slop for the plot of Z'–ω–1/2 (eqn (6)) in the low frequency region (Fig. S5). Consistent with DLi values derived from CV results, the calculated Li-ion diffusion coefficients by EIS method of CNFs@TNO/CC (1.58 × 10–14 cm² s–1) are higher than CC@TNO (1.35 × 10–15 cm² s–1). DLi deviations between EIS and CV are mainly due to the limitations involved in each measurement.51, 52 In addition, the apparent DLi of insertion and extraction calculated from CV data are presumed as two constants in the potential window, while the DLi values calculated from EIS measurements are dependent on the potential and vary greatly during potential changes. In our work, EIS measurement were tested at a fixed potential. Thus, the derived DLi values have difference with apparent DLi measured from CV at various potentials. The GITT test was conducted to further investigate the Li-ion diffusion behaviour of TNO. Fig. 7a presents the GITT potential curves of the CNFs@TNO/CC electrode

during the first Li-ion charge/discharge process. A typical single step of GITT is clearly depicted in Fig. 7b. As is shown in Fig. S6, the average DLi of CNFs@TNO/CC shows a remarkably high value of 2.78 × 10–11 cm2 s–1 (extraction) and 2.12 × 10–11 cm2 s–1 (insertion). The average DLi derived from GITT method are very close to those obtained from CV data (2.27 × 10–11 cm2 s–1 for insertion and 2.59 × 10–11 cm2 s–1 for extraction). Meanwhile, consisted with DLi derived from CV data, CNFs@TNO/CC electrode still displays a higher DLi for extraction than insertion. Compared with EIS derived DLi, the average DLi calculated from GITT are much higher but similar with CV results. All of these excellent DLi calculated from different methods adequately demonstrate the fast Li-ion diffusion in CNFs@TNO/CC.

Fig. 7 (a) GITT charge/discharge curves for CNFs@TNO/CC. (b) A single step in the GITT test of CNFs@TNO/CC. (c) Cycling stability of CC@TNO|Li and CNFs@TNO/CC|Li cells at 10 C. Clearly, the outstanding rate capability of CNFs@TNO/CC can be ascribed to its

unique structure and intrinsic material characteristics. First, the tridimensional conductive network from well-design array structure and nanosized TNO improve the Li-ion diffusion of the electrode. Among all the investigated niobium-based anode materials, CNFs@TNO/CC manifests the largest Li-ion diffusion coefficient of 2.59 × 10–11 cm2 s–1. Second, the binder-free structure ensures the structural integrity of the electrode, facilitates electrolyte penetration, and enables efficient electron transport along the CNFs. Third, the hierarchical core-shell structure formed by numerous cross-linked nanosized TNO particles coating on the intertwined porous CNFs layers can shorten the mass transfer length and provide abundant active sites for the Li-ions intercalation in view of the increased interfacial area between electrode and electrolyte. Finally, a significant pseudocapacitive effect due to the improved electronic conductivity and favorable Li-ions diffusion kinetics also promote the electrochemical reaction and storage efficiency. Fig. 7c shows the cyclic performances of the CC@TNO|Li and CNFs@TNO/CC|Li cells at 10 C. The CNFs@TNO/CC sample possesses excellent cyclic stability with a largely retained capacity of 192 mA h g–1 after 1000 cycles, which corresponds to a capacity loss of 22%. Despite 22% of capacity loss after 1000 cycles at 10 C, the retained capacity of CNFs@TNO/CC electrode is still higher than LTO (theoretical capacity, 175 mA h g–1) and some TNO based materials8, 9, 12, 15. SEM image (Fig. S7) of CNFs@TNO/CC electrode after cycling stability test also demonstrates its good cyclic stability. The Coulombic efficiencies of the two samples quickly increase to near 100% within five cycles and remain stable after a long cycling test. For comparison, the CC@TNO presents a relatively low capacity retention of 160 mA h g–1 at the end of cycling test.

Conclusion

In summary, CNFs@TNO/CC with a large Li-ion diffusion coefficient has been successfully synthesized as a promising intercalating pseudocapacitive anode material for high-performance LIBs. Benefited from the well-design hierarchical architecture, Li-ion diffusion coefficients for present CNFs@TNO/CC arrays can reach 2.27 × 10– 11

cm2 s–1 for insertion and 2.59 × 10–11 cm2 s–1 for extraction, which is much higher

than the reported niobium-based anode materials. Such desirable architecture for CNFs@TNO/CC with the large Li-ion diffusion coefficients intrigues its significant pseudocapacitive effect, and thus results in excellent electrochemical performance. CNFs@TNO/CC exhibits a high reversible capacity of 308 mA h g–1 at 1 C and maintains a high capacity of 182 mA h g–1 even at an ultra-large rate (80 C). After 1000 cycles, it still achieves a significant capacity retention of 78% at 10 C, suggesting its promising application in high-power LIBs for electric vehicles.

Acknowledgements This work is supported by the National Natural Science Foundation of China (11564011, 21706046, 51875318, 51362010), the Graduate Student Innovation Research Project of Hainan Province (Hys2018–80, Hys2018–81, Hys2018–82, Hys2018–83, Hys2018–84), and the Natural Science Foundation of Hainan Province (514207, 514212).

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Figure Captions Fig. 1 Schematic illustrations of the synthesis of CNFs@TNO/CC Fig. 2 (a) XRD patterns of CNFs/CC, CC@TNO and CNFs@TNO/CC. (b) Raman spectra of the CNFs@TNO/CC and CC@TNO electrodes. (c, d) XPS spectra of Ti 2p and Nb 3d of the CNFs@TNO/CC electrode. Fig. 3 FESEM images of (a–c) CC@TNO and (d–f) CNFs@TNO/CC electrodes. Fig. 4 (a–c) TEM and HRTEM images of CNFs@TNO/CC. (d–h) EDS elemental mapping images of C, Ti, Nb and O in the CNFs@TNO/CC. Fig. 5 CV profiles of CC@TNO|Li and CNFs@TNO/CC|Li cells at (a) 0.2 mV s–1 and (b, c) various sweep rates. (d) Relationship between peak currents of cathodic/anodic relationship and square roots of scan speed v0.5. (e) Separation of the total current (solid line) and capacitive currents (shaded regions) at 0.4 mV s–1. (f) Contribution ratio of the capacitive and diffusion-controlled charge at different scan rates. Fig. 6 (a) Charge/discharge profiles of the CC@TNO|Li and CNFs@TNO|Li cells. (b) Rate capabilities of the CC@TNO|Li and CNFs@TNO/CC|Li cells. (c) Comparison of the electrochemical performances of different TNO-based electrodes at the rate of 10 C.

(d) Nyquist plots of the CC@TNO|Li and CNFs@TNO/CC|Li cells (inset: selected equivalent circuit). Fig. 7 (a) GITT charge/discharge curves for CNFs@TNO/CC. (b) A single step in the GITT test of CNFs@TNO/CC. (c) Cycling stability of CC@TNO|Li and CNFs@TNO/CC|Li cells at 10 C.

Highlights


hierarchical carbon nanofibers@Ti2Nb10O29 arrays grown on carbon cloth (CNFs@TNO/CC) are prepared.




The intertwined and interconnected carbon nanofiber arrays can endow Ti2Nb10O29 with tridimensional conductivity, fast Li-ion transfer efficiency and good mechanical stability.




CNFs@TNO/CC shows remarkably high Li-ion diffusion coefficients.




CNFs@TNO/CC exhibits a high reversible capacity of 308 mA h g–1 at 1 C and maintains a high capacity of 182 mA h g–1 at 80 C.




A capacity retention of 78% at 10 C are achieved after1000 cycles.