Scalable synthesis of one-dimensional Na₂Li₂Ti₆O₁₄ nanofibers as ultrahigh rate capability anodes for lithium-ion batteries

Abstract

Carbon anode materials for Li-ion batteries have been operated close to their theoretical rate and cycle limits. Therefore, titanium-based materials have attracted great attention due to their high stability. Here, Na₂Li₂Ti₆O₁₄ nanofibers as anode materials were prepared through a controlled electrospinning method. The Na₂Li₂Ti₆O₁₄ nanofibers presented superior electrochemical performance with high rate capability and long cycle life and can be regarded as a competitive anode candidate for advanced Li-ion batteries. One-dimensional (1D) Na₂Li₂Ti₆O₁₄ nanofibers are able to deliver a capacity of 128.5 mA h g⁻¹ at 0.5C, and demonstrate superior high-rate charge–discharge capability and cycling stability (the reversible charge capacity is 77.8 mA h g⁻¹ with a capacity retention of 99.45% at the rate of 10C after 800 cycles). The 1D structure is considered to contribute remarkably to increased rate capability and stability. This simple and scalable method indicates that the Na₂Li₂Ti₆O₁₄ nanofibers have a practical application potential for high performance lithium-ion batteries.

---

1. Introduction

With rising interest in new energy vehicles, increasing attention has been paid to lithium-ion batteries (LIBs) due to their high energy density.¹ However, after being successfully commercialized for nearly 30 years, LIBs still need to be further improved to meet demands like higher power capability and longer cycle life for electric vehicles (EVs) or plug-in hybrid electric vehicles (PHEVs).² From this perspective, much effort has been paid to search for new electrode materials due to the widely used carbon anodes and oxide cathodes having been operated close to their theoretical limits.

The main drawbacks of carbon anode materials are low rate capability and the formation of unstable solid-electrolyte interphases (SEIs), which will cause the dendritic deposition of Li and hence, lead to short circuits or safety concerns for the whole batteries.^3,4 Some other potential anode materials, such as Sn-based and Si-based materials, have attracted much attention due to their very high theoretical specific capacity. Nevertheless, the practical applications of these materials have been hampered by drastic specific volume variation during Li insertion/extraction processes, which leads to poor cycle stability.^5,6 As a potential alternative, titanium-based materials such as TiO₂ ^7–9 and Li₄Ti₅O₁₂ ^10–17 have attracted much attention due to their structural stability. Moreover, the formation of SEI layers can be efficiently avoided because of the relatively high lithium insertion/extraction voltage of these materials (usually higher than 1.5 V vs. Li⁺/Li).^18–20 Therefore, compared with their carbon-based counterparts, the safety of the batteries can be greatly improved by adopting titanium-based materials as anodes. In order to further improve the energy density, M_xLi₂Ti₆O₁₄, (M = Sr,^21–25 Ba,^24–29 Pb,^25,30,31 and Na₂^32–39) with lowered lithium insertion/extraction potential have been proposed as anodes, which will enlarge the open cell voltage of the full LIBs. Among them, the introduction of Na (Na₂Li₂Ti₆O₁₄) is attractive due to its lower potential plateau (approximately 1.25 V vs. Li⁺/Li V),^32,39 low cost^40,41 and good recycling performance. However, the practical application of the Na₂Li₂Ti₆O₁₄ anode is still a challenge due to its low intrinsic electrical conductivity and limited diffusion dynamics which lead to poor rate capability.

In the past few years, efforts have been made to synthesize Na₂Li₂Ti₆O₁₄ composites. Li et al. synthesized Na₂Li₂Ti₆O₁₄ particles using a solid state method. The composite showed limited rate performance with a reversible capacity of 74 mA h g⁻¹ at a current density of 100 mA g⁻¹.³⁸ Yin et al. reported a melting salt method for the synthesis of pure and well-crystallized Na₂Li₂Ti₆O₁₄ particles. The composite could reach a capacity of 62 mA h g⁻¹ after 500 cycles at a current density of 100 mA g⁻¹.³⁹ However, the rate performance of these Na₂Li₂Ti₆O₁₄ particles was limited by the large particle size or particles’ self-aggregation. Consequently, modification of Na₂Li₂Ti₆O₁₄ anodes by morphology or structure design has attracted broad interest to obtain a suitable structure for fast electron and Li ion diffusion.^28,42–44 Among the various nanostructures, 1D nanofibers as active materials have been observed to effectively shorten the Li-ion and electron diffusion paths and enlarge the electrode/electrolyte interfacial area, which can effectively prevent the self-aggregation of the particles. There are a great number of methods to synthesize 1D nanofibers. However, their relatively complicated experimental procedures mean most of them are not easy to scale up. Therefore, electrospinning technology has received steadily increasing attention because it is easy to operate to synthesize 1D nanofibers. Utilizing electrospinning technology to synthesize anode materials with 1D nanostructures is urgently desirable.

In this paper, a simple method to synthesize a high-performance 1D Na₂Li₂Ti₆O₁₄ anode material via electrospinning is presented. Excitingly, our nanofibers exhibited wonderful cycling stability at large scale current densities when evaluated as anode materials for LIBs. Half Li cells with 1D Na₂Li₂Ti₆O₁₄ nanofibers as the cathode can be cycled 800 times at a current density as high as 10C. More importantly, this technique can also be used as a general method for preparing other kinds of anode material with 1D nanostructures.

2. Experimental

2.1 Sample preparation

A precursor sol of Na₂Li₂Ti₆O₁₄ nanofibers was obtained via the following steps: first, 0.0887 g anhydrous sodium acetate (CH₃COONa (NaAc), Macklin) was dissolved into 5 mL N,N-dimethylformamide (DMF) under vigorous stirring at room temperature (solution A). 0.0859 g lithium acetate (CH₃COOLi (LiAc), Macklin) and 1.0089 g tetrabutyl titanate (C₁₆H₃₆O₄Ti, (TBOT), Macklin) were dissolved in a mixed solvent of 5 mL anhydrous ethanol and 2 mL acetic acid (solution B). Solution A was added into solution B under vigorous stirring. After that, 1.2012 g of polyvinyl pyrrolidone (PVP K88-92, M_w: 1 300 000, Aladdin) was added and the resulting solution was further stirred for 1 h and additionally aged for 18 h. In a typical electrospinning experiment, the prepared spinnable sol was loaded into a plastic syringe equipped with a 22# needle made of stainless steel, and the needle was connected to a 17 kV applied voltage. The distance between the needle tip and the collector was about 17 cm. A 0.225 mL h⁻¹ flow rate was employed. The as-spun nanofibers were collected and dried overnight in a vacuum at 80 °C. The obtained nanofibers were further sintered at 750 °C for 6 h with a ramping rate of 2.5 °C min⁻¹ in air, resulting in the final products. In order to compare, Na₂Li₂Ti₆O₁₄ nanoparticles were prepared using a simple solid-state method.³⁴

2.2 Sample characterization

The crystal structures of the obtained samples were characterized using Bruker D8 Focus Advance X-ray diffraction (XRD, diffractometer with Cu-Kα radiation, λ = 0.15406 nm, receiving slit, 0.2 mm, scintillation counter, 40 mA, 40 kV) with scattering angles of 10°–80° in steps of 0.02°. In situ XRD tests were carried out using a special sample holder. The X-ray transmission window and current collector of the in situ XRD cell was a Be disc. Micro-structural properties were determined using HITACHI SU-70 field-emission scanning electron microscopy (FESEM) and JEOL JEM-2010 high-resolution transmission electron microscopy (HRTEM) at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a PerkinElmer PHI 550 spectrometer with Al Kα (1486.6 eV) as the X-ray source. The spinning process was accomplished using a FM1206 electrostatic spinning machine (Beijing Fuyouma Company, China).

2.3 Electrochemical tests

Swagelok-type cells were used to evaluate the electrochemical performance of Na₂Li₂Ti₆O₁₄ nanofibers and nanoparticles as anodes. For the Swagelok-type cells, the working electrode contained 70 wt% of active material, 20 wt% of carbon black and 10 wt% of polyvinylidene difluoride. The electrode slurry was evenly coated on a clean copper foil and dried at 100 °C for 12 hours. The loading of the active material on copper foil was 1.98 mg ± 0.2 mg cm⁻². The Li metal foil served as the counter electrode (16 mm in diameter). 1.0 M LiPF₆ solution in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1 : 1 by volume) was used as an electrolyte. The cells were assembled in an Ar-filled glove box. The Swagelok-type cells were tested at different current densities between 0.5C and 8C (1 C = 100 mA g⁻¹) within the voltage range of 1.0–3.0 V using a LAND-CT2001A battery test system (Jinnuo Wuhan Corp, China). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out on a CHI660D electrochemical working station.

3. Results and discussion

The synthesis strategy of Na₂Li₂Ti₆O₁₄ nanofibers is schematically depicted in Fig. 1. Tetrabutyl titanate, sodium acetate and lithium acetate were used as the titanium source, the sodium source and the lithium source, respectively. Sodium acetate was dissolved in DMF. Tetrabutyl titanate and lithium acetate were dissolved in a mixed solution of ethanol and acetic acid. After that, the two solutions were mixed and a certain amount of PVP was added to prepare the yellow precursor solution. After electrospinning and calcination, the 1D Na₂Li₂Ti₆O₁₄ nanofibers with an average diameter of 100 nm were obtained. This route is not only cost-effective, but it is also easy to control the 1D structure and morphology of the materials.

Fig. 1 Illustration of the preparation processes of Na₂Li₂Ti₆O₁₄ nanofibers.

A SEM image of the precursor fibers is shown in Fig. S1.† Without sintering, the precursor fibers with smooth surfaces align in random orientations. Most precursor fibers have diameters ranging from 100 nm–150 nm, although a few of them present a larger diameter of ∼300 nm, which is mainly affected by the electrospinning instrument. In order to ensure the formation of pure Na₂Li₂Ti₆O₁₄, the calcination process in our experiments was carried out at 750 °C, with the XRD results providing confirmation (Fig. 2a). The X-ray diffraction (XRD) patterns of Na₂Li₂Ti₆O₁₄ nanofibers and Na₂Li₂Ti₆O₁₄ nanoparticles are shown in Fig. 2a and 3a, respectively. All the characteristic peaks of the XRD patterns of both nanofibers and particles correspond to the feature planes of Na₂Li₂Ti₆O₁₄ (JCPDS Card No. 52-0690) which is in good agreement with the previous report.²⁰ No other peaks can be observed, suggesting that pure Na₂Li₂Ti₆O₁₄ nanofibers or particles were obtained after high temperature calcination. The final nanofibers retain an almost identical morphology to the precursor, with a homogeneous 1D structure as shown in Fig. 2b. The detailed structural characterization using TEM (Fig. 2c and d) reveals that the Na₂Li₂Ti₆O₁₄ nanofiber is composed of small nanoparticles with diameters ranging from 80–120 nm, which are connected to each other to form a 1D structure. The lattice fringes of 0.255 nm as shown in the high-resolution TEM image (HRTEM) in Fig. 2e correspond to the (022) plane of Na₂Li₂Ti₆O₁₄, which is in agreement with the XRD results. These Na₂Li₂Ti₆O₁₄ nanofibers are well crystallized, as indicated by the clear lattice fringes in the HRTEM image (Fig. 2e) and the sharp spots in the selected-area electron diffraction (SAED) patterns (Fig. 2f). The distribution of Na, Ti and O is homogeneous across the entire Na₂Li₂Ti₆O₁₄ nanofiber as shown in Fig. 2g. The high-resolution XPS spectra of the Na₂Li₂Ti₆O₁₄ nanofibers is shown in Fig. S2.† The binding energies of Ti 2p_3/2 and Ti 2p_1/2, which are at 458.06 eV and 463.86 eV, respectively, prove the presence of Ti(IV). The XPS spectra of Na, Li, and O further demonstrate the Na₂Li₂Ti₆O₁₄ nanofibers with pure crystallization.

Fig. 2 (a) XRD pattern of Na₂Li₂Ti₆O₁₄ nanofibers. (b) SEM image of Na₂Li₂Ti₆O₁₄ nanofibers. (c, d) TEM images of Na₂Li₂Ti₆O₁₄ nanofibers. (e) HRTEM image of Na₂Li₂Ti₆O₁₄ nanofibers. (f) SAED image of Na₂Li₂Ti₆O₁₄ nanofibers. (g) Dark-field TEM image of the single Na₂Li₂Ti₆O₁₄ nanofiber and corresponding Na, Ti, and O EDX mappings.

Fig. 3 (a) XRD pattern of Na₂Li₂Ti₆O₁₄ nanoparticles. (b) SEM image of Na₂Li₂Ti₆O₁₄ nanoparticles. (c, d) TEM images of Na₂Li₂Ti₆O₁₄ nanoparticles. (e) HRTEM image of Na₂Li₂Ti₆O₁₄ nanoparticles. (f) SAED image of Na₂Li₂Ti₆O₁₄ nanoparticles. (g) Dark-field TEM image of the single Na₂Li₂Ti₆O₁₄ nanoparticle and corresponding Na, Ti, and O EDX mappings.

As a comparison, Na₂Li₂Ti₆O₁₄ nanoparticles were synthesized using the traditional solid state method.³⁴ The detailed structure of the Na₂Li₂Ti₆O₁₄ nanoparticles is shown in Fig. 3. Fig. 3b shows that the Na₂Li₂Ti₆O₁₄ nanoparticles possess an irregular particle-like morphology with sizes ranging from 300–400 nm. Fig. 3c and d show the detailed structural characterization obtained from TEM. The lattice fringes of 0.463 nm as shown in the HRTEM image in Fig. 3e correspond to the (202) plane of Na₂Li₂Ti₆O₁₄, which is in agreement with the XRD results. These Na₂Li₂Ti₆O₁₄ nanoparticles crystallize well, as indicated by the sharp spots in the selected-area electron diffraction (SAED) pattern (Fig. 3f). The distribution of Na, Ti and O is homogeneous across the whole single Na₂Li₂Ti₆O₁₄ nanoparticle as shown in Fig. 3g. Although the Na₂Li₂Ti₆O₁₄ nanoparticles have good crystallinity and high purity, the same as the nanofibers, the particle-like structure aggregates severely more easily, which will influence the ionic and electronic conductivity. Therefore, 1D Na₂Li₂Ti₆O₁₄ nanofibers are expected to be excellent electrode materials for enlarging the electrode/electrolyte interfacial area as well as shortening the Li ion and electron diffusion paths. Experimental results showed that the surface areas exert weak influence on the electrochemical performance of the materials (BET data of the nanofibers and nanoparticles are shown in Table S1 and Fig. S3†).

The electrochemical performance of Na₂Li₂Ti₆O₁₄ nanofibers as an anode material for lithium ion batteries was evaluated as shown in Fig. 4. The cyclic voltammetry test was adopted to investigate the electrochemical redox behavior of Na₂Li₂Ti₆O₁₄ during lithium ion insertion and extraction. As shown in Fig. 4a, during the first three cycles, one pair of redox peaks located at 1.21 and 1.33 V can be found which indicates only one single-phase transition process in the lithiation–delithiation cycles. It also demonstrates that the Na₂Li₂Ti₆O₁₄ nanofibers possess high structural stability during the charge–discharge cycles. Fig. 4b shows the charge–discharge profiles of the Na₂Li₂Ti₆O₁₄ nanofibers at different current densities after initial activation. When the cell was tested at a current density of 0.5C, an obvious plateau is observed at 1.21 V in the discharge curve, and the corresponding delithiation plateau is perceived at 1.33 V in the charge curve, coincident with the CV results.

Fig. 4 (a) Cyclic voltammetry plots of the Na₂Li₂Ti₆O₁₄ nanofibers recorded using Swagelok-type cells at a scan rate of 0.1 mV s⁻¹ from 1.0 V to 3.0 V. (b) Charge–discharge curves at various current densities. (c) Rate performance. (d) Charge–discharge curves at current densities of 1C. (e) Cycling performances at current densities of 1C. (f) Cycling performances at current densities of 10C.

This phenomenon is similar to that reported for Na₂Li₂Ti₆O₁₄ and Li₄Ti₅O₁₂.^15–17 With the increase of current density to as high as 8C, the discharge plateaus shift to lower potentials while the charge plateaus move to higher potentials. This change in the potential curves can be attributed to the increase of the electrode polarization rate at high rates. Fig. 4c shows the Li⁺ extraction capability (charge) of Na₂Li₂Ti₆O₁₄ nanofibers at different rates of 0.5C–8C. At a current density of 0.5C, the initial reversible capacity of the Na₂Li₂Ti₆O₁₄ nanofibers is 128.5 mA h g⁻¹. Subsequently, the charge capacities decreased to 114.7, 105.5, 98.9, 92.8, 89.1, 85.8, 83.3 and 80.9 mA h g⁻¹, respectively, when the current densities gradually increase to 1, 2, 3, 4, 5, 6, 7 and 8C. Upon the current density returning back to 0.5C, the charge capacity of Na₂Li₂Ti₆O₁₄ can still recover to ∼128.3 mA h g⁻¹ after 90 cycles. As a comparison, Fig. S4a† shows the rate capability of Na₂Li₂Ti₆O₁₄ nanoparticles tested from 0.5C–8C with identical discharge/charge current density. When the current density gradually increased, the capacity of Na₂Li₂Ti₆O₁₄ nanoparticles faded quickly from 102.4 to only 49.2 mA h g⁻¹. At the high rate of 8C, the Na₂Li₂Ti₆O₁₄ nanofibers reached a capacity of 80.9 mA h g⁻¹ which was 64.4% higher than that of the Na₂Li₂Ti₆O₁₄ nanoparticles. Consequently, Na₂Li₂Ti₆O₁₄ nanofibers possess superior rate capability than their particle-like counterparts.

Fig. 4d displays representative charge and discharge-voltage profiles of Na₂Li₂Ti₆O₁₄ nanofibers at a current density of 1C with a potential range of 1.0–3.0 V. It can be seen that the Na₂Li₂Ti₆O₁₄ nanofibers delivered an initial discharge capacity of 222.2 mA h g⁻¹ and charge capacity of 93.9 mA h g⁻¹, with a first coulombic efficiency of about 42.3%. This phenomenon may be attributed to the activation process. With increasing the number of cycles to 100, the discharge/charge voltage profile exhibits good stability. Fig. 4e clearly displays the cycle performance and coulombic efficiency of one-dimensional Na₂Li₂Ti₆O₁₄ nanofibers at the current density of 1C. After 100 cycles, the Na₂Li₂Ti₆O₁₄ electrodes still have a reversible charge capacity of 116.49 mA h g⁻¹ with a capacity retention of 97.38%. The average coulombic efficiency of the electrode reaches 99.97% during the 100 cycles, showing excellent reversibility of the capacity. In contrast, Fig. S4b† shows the cycle performance and coulombic efficiency of Na₂Li₂Ti₆O₁₄ nanoparticles at a current density of 1C. After 100 cycles, the reversible charge capacity of the Na₂Li₂Ti₆O₁₄ nanoparticle electrode was only 92.43 mA h g⁻¹.

The Na₂Li₂Ti₆O₁₄ nanofibers also demonstrate highly stable cycling performance after being aged at 0.1C for 3 cycles, as shown in Fig. 4f. The half-cell using Na₂Li₂Ti₆O₁₄ nanofibers as a cathode was run at a high current density of 10C. The capacity retention was 99.45% after 800 cycles which means that the average capacity fading per cycle was as small as 0.0007%, hence the coulombic efficiency is nearly 100% every cycle. To further confirm the stability of Na₂Li₂Ti₆O₁₄ nanofibers under repeated cycling at high currents, TEM images of the pristine Na₂Li₂Ti₆O₁₄ nanofibers and Na₂Li₂Ti₆O₁₄ nanofibers after 800 cycles at 10C are presented in Fig. 5. As shown in Fig. 5b, the edge of the particle after ultra-long cycles is not as distinct as that of the pristine one. However, the Na₂Li₂Ti₆O₁₄ nanofibers still maintain a 1D structure with the same diameter. As a comparison, the Na₂Li₂Ti₆O₁₄ nanoparticles show obvious aggregation after cycling as shown in Fig. S5.† All the above results confirm that the Na₂Li₂TiO₁₄ nanofibers have good structural stability and excellent electrochemical performance. Compared with the reported results as shown in Table S2,† to the best of our knowledge, our Na₂Li₂Ti₆O₁₄ nanofibers demonstrate most excellent rate performance and outstanding cycle stability. The outstanding electrochemical performance can be attributed to the unique 1D nanostructures, which shorten the length of ion transport in Na₂Li₂Ti₆O₁₄, thereby improving the kinetics properties of the electrochemical reactions.

Fig. 5 TEM images of (a) the pristine Na₂Li₂Ti₆O₁₄ nanofibers, and (b) the Na₂Li₂Ti₆O₁₄ nanofibers after 800 cycles.

Electrochemical impedance spectroscopy (EIS) investigations were carried out to evaluate the electrochemical properties of as-prepared Na₂Li₂Ti₆O₁₄ nanofibers. The Nyquist plots of the Na₂Li₂Ti₆O₁₄ nanofibers and Na₂Li₂Ti₆O₁₄ nanoparticles before cycling are presented in Fig. 6. All the EIS curves are composed of a depressed semicircle in the high-frequency region and an inclined line in the low-frequency region. The plots can be satisfactorily simulated by an equivalent circuit shown in the inset of Fig. 6. In the equivalent circuit, R_e was denoted the electrolyte and contact resistance, and R_ct was denoted the charge transfer resistance. CPE and W represent the constant phase element and Warburg diffusion impedance, respectively. The simulated values of Na₂Li₂Ti₆O₁₄ nanofibers and Na₂Li₂Ti₆O₁₄ nanoparticles before cycling calculated from EIS spectra using an equivalent circuit are listed in Table 1. In Table 1, we can find that the value of the lithium ion diffusion coefficient (D_Li) of Na₂Li₂Ti₆O₁₄ nanofibers before cycling is calculated as approximately 6.48 × 10⁻¹⁶ cm² s⁻¹, while the D_Li is 8.56 × 10⁻¹⁷ cm² s⁻¹ for Na₂Li₂Ti₆O₁₄ nanoparticles composed using a solid-state method. This reveals that Na₂Li₂Ti₆O₁₄ nanofibers have higher ionic conductivity than Na₂Li₂Ti₆O₁₄ nanoparticles, which is beneficial for Li⁺ transfer. The significantly higher D_Li has validated the “high conductivity” of our materials. In addition, the lower charge-transfer resistance (R_ct) of Na₂Li₂Ti₆O₁₄ nanofibers (73.01 Ω) than that of Na₂Li₂Ti₆O₁₄ nanoparticles (175.86 Ω) also indicates that the nanofibers can significantly improve the conductivity.

Fig. 6 EIS spectra of Na₂Li₂Ti₆O₁₄ nanofibers and Na₂Li₂Ti₆O₁₄ nanoparticles.

Table 1 The simulated values of Na₂Li₂Ti₆O₁₄ nanofibers and Na₂Li₂Ti₆O₁₄ nanoparticles before cycling calculated from EIS spectra using an equivalent circuit

Sample	R e (Ω)	R ct (Ω)	D Li (×10^−17 cm^2 s^−1)
Na2Li2Ti6O14 nanofibers	6.07	73.01	64.8
Na2Li2Ti6O14 nanoparticles	6.16	175.86	8.56

---

To further trace the detailed information of the structural evolution of Na₂Li₂Ti₆O₁₄ upon the discharging and charging process, an in situ XRD experiment was carried out at a low current rate of 0.2C and in the potential range of 1.0–3.0 V. As shown in Fig. 7a, the main characteristic peaks of the Na₂Li₂Ti₆O₁₄ nanofibers located at 23.8°, 26.9°, 29.1°, 32.4°, 43.9° and 45.1° shift to lower angles gradually during the discharge process, and then reversibly return to their original positions during charging. Two diffraction peaks at 33.8° and 39.9° disappear gradually along with the lithiation process, and then reappear during delithiation. Additionally, the characteristic peak at 32.9° shifts towards to a higher angle firstly and then shifts towards to a lower angle gradually during charging and eventually returns to its initial position. The selected in situ XRD patterns are shown in Fig. 7b and the intensity and degree of change can be more intuitively observed from Fig. 7b. Upon the recharging process, all the characteristic peaks can reappear and revert back to their initial positions during delithiation, which shows a good reversibility of the Na₂Li₂Ti₆O₁₄ nanofibers.

Fig. 7 (a) Selected in situ XRD patterns of Na₂Li₂Ti₆O₁₄ nanofibers during the initial charge–discharge process. (b) Overall in situ XRD patterns.

4. Conclusions

In summary, a scalable and simple electrospinning method was first reported to synthesize pure Na₂Li₂Ti₆O₁₄ nanofibers which can be cycled at a high rate with a long service life. The 1D structure provided high conductivity, high structural stability, and improved kinetics properties and shortened the distance of ionic transport, resulting in excellent rate capability and cycling stability. Furthermore, compared to the other complex methods, our method only requires ambient reaction conditions and a mild reaction process, and can be expected to be a scalable method for exploring the high performance of not only Na₂Li₂Ti₆O₁₄, but also other potential anode materials for next-generation Li-ion batteries.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge financial support from the National Natural Science Foundation of China (21571110), Natural Science Foundation of Zhejiang Province (LY18B010003), and the Ningbo Key Innovation Team (2014B81005), and sponsorship by the K. C. Wong Magna Fund in Ningbo University.

Notes and references

1. J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587–603.
2. R. Sun, G. Liu, S. Cao, B. Dong, X. Liu, M. Hu, M. Liu and X. Duan, Dalton Trans., 2017, 46, 17061–17066.
3. T. Yi, Y. Xie, Y. Zhu, R. Zhu and H. Shen, J. Power Sources, 2013, 222, 448–454.
4. S. S. Zhang, J. Power Sources, 2006, 161, 1385–1391.
5. N.-S. Choi, Y. Yao, Y. Cui and J. Cho, J. Mater. Chem., 2011, 21, 9825–9840.
6. Z. P. Guo, J. Z. Wang, H. K. Liu and S. X. Dou, J. Power Sources, 2005, 146, 448–451.
7. H. Wang, H. Yang, L. Lu, Y. Zhou and Y. Wang, Dalton Trans., 2013, 42, 8781–8787.
8. S. Liu, H. Jia, L. Han, J. Wang, P. Gao, D. Xu, J. Yang and S. Che, Adv. Mater., 2012, 24, 3201–3204.
9. L. Sun, F. Wang, T. Su and H. Du, Dalton Trans., 2017, 46, 11542–11546.
10. Y. Shi, L. Wen, F. Li and H. Cheng, J. Power Sources, 2011, 196, 8610–8617.
11. Y. Wang, L. Gu, Y. Guo, H. Li, X. He, S. Tsukimoto, Y. Ikuhara and L. Wan, J. Am. Chem. Soc., 2012, 134, 7874–7879.
12. L. Shen, X. Zhang, E. Uchaker, C. Yuan and G. Cao, Adv. Energy Mater., 2012, 2, 691–698.
13. N. Zhu, W. Liu, M. Xue, Z. Xie, D. Zhao, M. Zhang, J. Chen and T. Cao, Electrochim. Acta, 2010, 55, 5813–5818.
14. Y. Sun, L. Zhao, H. Pan, X. Lu, L. Gu, Y. Hu, H. Li, M. Armand, Y. Ikuhara, L. Chen and X. Huang, Nat. Commun., 2013, 4, 1870.
15. M. Hirayama, K. Kim, T. Toujigamori, W. Cho and P. Kanno, Dalton Trans., 2011, 40, 2882–2887.
16. Y. He, M. Liu, Z. Huang, B. Zhang, Y. Yu, B. Li, F. Kang and J. Kim, J. Power Sources, 2013, 239, 269–276.
17. X. Jia, Y. Kan, X. Zhu, G. Ning, Y. Lu and F. Wei, Nano Energy, 2014, 10, 344–352.
18. H. Ge, L. Chen, W. Yuan, Y. Zhang, Q. Fan, H. Osgood, D. Matera, X. Song and G. Wu, J. Power Sources, 2015, 297, 436–441.
19. M. Z. Kong, W. L. Wang, J. Park and H. Gu, Mater. Lett., 2015, 146, 12–15.
20. H. Zhang, Q. Deng, C. Mou, Z. Huang, Y. Wang, A. Zhou and J. Li, J. Power Sources, 2013, 239, 538–545.
21. J. Liu, Y. Li, X. Wang, Y. Gao, N. Wu and B. Wu, J. Alloys Compd., 2013, 581, 236–240.
22. D. Dambournet, I. Belharouak, J. Ma and K. Amine, J. Power Sources, 2011, 196, 2871–2874.
23. J. Liu, X. Sun, Y. Li, X. Wang, Y. Gao, K. Wu, N. Wu and B. Wu, J. Power Sources, 2014, 245, 371–376.
24. I. Belharouak and K. Amine, Electrochem. Commun., 2003, 5, 435–438.
25. I. Koseva, J. P. Chaminade, P. Gravereau, S. Pechev, P. Peshev and J. Etourneau, J. Alloys Compd., 2005, 389, 47–54.
26. L. L. Garza-Tovar and L. M. Torres-Martínez, Mater. Sci. Forum, 2006, 510–511, 102–105.
27. X. Lin, P. Li, L. Shao, M. Shui, D. Wang, N. Long, Y. Ren and J. Shu, J. Power Sources, 2015, 278, 546–554.
28. X. Lin, P. Wang, P. Li, H. Yu, S. Qian, M. Shui, D. Wang, N. Long and J. Shu, Electrochim. Acta, 2015, 186, 24–33.
29. X. Wu, X. Li, C. Zhu, P. Li, H. Yu, Z. Guo and J. Shu, Mater. Today Energy, 2016, 1–2, 17–23.
30. P. Li, S. Qian, H. Yu, L. Yan, X. Lin, K. Yang, N. Long, M. Shui and J. Shu, J. Power Sources, 2016, 330, 45–54.
31. X. Sun, S. Yin and C. Feng, J. Solid State Electrochem., 2017, 21, 1625–1630.
32. S. Y. Yin, L. Song, X. Y. Wang, Y. H. Huang, K. L. Zhang and Y. X. Zhang, Electrochem. Commun., 2009, 11, 1251–1254.
33. K. Wu, J. Shu, X. Lin, L. Shao, P. Li, M. Shui, M. Lao, N. Long and D. Wang, J. Power Sources, 2015, 275, 419–428.
34. J. Shu, K. Wu, P. Wang, P. Li, X. Lin, L. Shao, M. Shui, N. Long and D. Wang, Electrochim. Acta, 2015, 173, 595–606.
35. K. Wu, D. Wang, X. Lin, L. Shao, M. Shui, X. Jiang, N. Long, Y. Ren and J. Shu, J. Electroanal. Chem., 2014, 717–718, 10–16.
36. S. Fan, H. Zhong, H. Yu, M. Lou, Y. Xie and Y. Zhu, Sci. China Mater., 2017, 60, 427–437.
37. K. Wu, J. Shu, X. Lin, L. Shao, M. Lao, M. Shui, P. Li, N. Long and D. Wang, J. Power Sources, 2014, 272, 283–290.
38. P. Li, K. Wu, P. Wang, X. Lin, H. Yu, M. Shui, X. Zheng, N. Long and J. Shu, Ceram. Int., 2015, 41, 14508–14516.
39. S. Y. Yin, C. Q. Feng, S. J. Wu, H. L. Liu, B. Q. Ke, K. L. Zhang and D. H. Chen, J. Alloys Compd., 2015, 642, 1–6.
40. L. M. Torres-Martínez, J. Ibarra, J. R. Loredo, L. L. Garza-Tovar and O. Martínez-Bruno, Solid State Sci., 2006, 8, 1281–1289.
41. V. B. Nalbandyan, Solid State Sci., 2007, 9, 329–330.
42. C. Zhu, J. Shu, X. Wu, P. Li and X. Li, J. Electroanal. Chem., 2015, 759, 184–189.
43. L. Mai, L. Xu, C. Han, X. Xu, Y. Luo, S. Zhao and Y. Zhao, Nano Lett., 2010, 10, 4750–4755.
44. L. Qie, W. Chen, Z. Wang, Q. Shao, X. Li, L. Yuan, X. Hu, W. Zhang and Y. Huang, Adv. Mater., 2012, 24, 2047–2050.

---

Footnote

† Electronic supplementary information (ESI) available: SEM of the precursors, XPS of Na₂Li₂Ti₆O₁₄ nanofibers and the performance of rechargeable Na₂Li₂Ti₆O₁₄ reported in recent literature. See DOI: 10.1039/c8qi00973b

---