Mesoporous In₂O₃ nanofibers assembled by ultrafine nanoparticles as a high capacity anode for Li-ion batteries

Abstract

As a lithium storage material, In₂O₃ has been hindered by its rapid capacity degradation due to the very large volume change during the repeated lithiation and delithiation process, although an initial discharge capacity of more than 1600 mA h g⁻¹ has been reported. In this work, the mesoporous In₂O₃ nanofibers with ultrafine In₂O₃ nanoparticles were synthesized by a cost-effective method. The mesopores within the nanofiber matrix allow more Li⁺ to enter the interface of the material, achieving a high utilization of the In₂O₃ electrode. The volume change during alloying and de-alloying of lithium and indium has been greatly alleviated within this unique mesoporous nanofiber structure, which kept a better integrity of the electrode and a high cycle stability of the LIBs as well. The ultrafine In₂O₃ nanoparticles shorten the diffusion path of the electrons and lithium ions and exhibited excellent cycle stability and a very encouraging specific capacity (526.7 mA h g⁻¹ in the 200th cycle at a current density of 100 mA g⁻¹) as an anode for LIBs.

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1. Introduction

The ever-growing demands for higher energy and power densities of Li-ion batteries (LIBs) have stimulated rapid exploration of innovative electrode materials as well as electrode engineering technologies in the last few decades, since both the commonly used cathode and anode materials have reached their property limits.¹ According to the work of Poizot et al.,² metal oxides, especially some transition-metal oxides, can deliver extraordinarily high capacity for Li-ion storage via either insertion/extraction or conversion mechanisms. Therefore, various transition-metal oxides, such as TiO₂,^3,4 Fe₃O₄,^5,6 SnO₂,⁷ NiO,⁸ and MnO₂,⁹ etc., have been extensively investigated as anode of LIBs, because of their high theoretical capacities and natural abundance. However, concerning to In₂O₃, a well-known transition-metal oxide that has been extensively used in microelectronic industries like window heaters, solar cells, liquid crystal displays, and ultrasensitive gas detectors, its applications in LIBs have encountered difficulties due to its very poor cycle stability and low rate capability.^10,11 According to the few available reports, mechanical failure and loss of electrical contact caused by the large volume change in repetitive charging and discharging are responsible for the failure of In₂O₃ electrode, similar to the behaviours of other electrode materials like Si and SnO₂.¹⁰

To alleviate the negative influence of volume change of the electro-active materials on their cycle stability, one commonly adopted strategy is to use nano-sized electrode materials, which is expected to enhance both the ion and electron transport with the greatly shortened transport length as well as the increased contact area between the electrolyte and electrode.^12,13 Meanwhile, the loosely packed nanoparticles are helpful for accommodation of the volume change. However, the low thermodynamic stability, low packing density and unexpected side reactions between the electrode and electrolyte are the disadvantages of nano-sized electrode materials for practical applications.

Concerning to these problems, hierarchical micro/nanostructures which appear overall as micro-sized assemblies but composed of nano-sized building blocks have been recognized as the potential candidates to achieve both high energy densities and long cycle life, by decreasing the current density per unit surface area, lessening the compounded internal stresses, and preventing the nanoparticles from agglomerating.¹⁴ Various hierarchical structures, such as hollow spheres,¹⁵ nanoplates,^16,17 nanowires¹⁸ and nanotubes,¹⁹ have been synthesized and tested as electrodes of LIBs. Among them, one-dimensional (1D) nanostructures have drawn particular attention as they can take advantages of better electronic kinetics due to the higher probability of contact and higher integrity of the electrode by restricting the nanoparticles within a local space.²⁰

Electrospinning is so far the most popularly utilized technique for preparation of 1D nanostructure in the past two decades.^21–23 Regarding In₂O₃, nanofibers have also been synthesized by electrospinning process, but they were mostly used as gas sensors, not for LIBs.²⁴ To date, a work related to alleviation of the volume change induced capacity degradation of In₂O₃ anode for LIBs by using mesoporous nanofibers has not been reported.

In this paper, mesoporous In₂O₃ nanofibers composed of ultrafine nanoparticles were prepared by a modified electrospinning process followed by a post-synthesis calcination step. Benefited from the ultrafine In₂O₃ nanoparticles embedded in the mesoporous nanofibers, the anode exhibited a very stable cycle performance after the initial capacity drop, namely 526.7 mA h g⁻¹ in the 200th cycle at a current density of 100 mA g⁻¹, much higher than that of the commercial graphite anode that with a theoretical capacity of 372 mA h g⁻¹.

2. Experimental

2.1 Synthesis of mesoporous In₂O₃ nanofibers

All the chemicals were of analytical grade and used as received. In a typical process, 0.002 mol indium oxide (In(NO)₃·5H₂O, Tianjin Chemical Corp, China) and 0.8 g polyvinyl pyrrolidone (PVP, Sigma Aldrich, M_w ∼ 1 300 000) were dissolved in 4.4 g ethanol and 4.4 g N,N-dimethyl formamide under magnetic stirring for 3 h at room temperature. The prepared mixture was used as precursor solution for electrospinning. The tip needle was made of stainless steel and connected to a high-voltage supply. The flow rate and voltage were 0.5 ml h⁻¹ and 20 kV, respectively. The nanofibers were collected by a collector made of stainless steel with the distance between the needle and the collector of 20 cm. The as-spun fibers were annealed at 450 °C for 2 h with the heating rate of 4 °C min⁻¹ in air.

2.2 Material characterizations

Scanning electron microscope (SEM, Hitachi S-4800, 5 kV), transmission electron microscope (TEM, JEOL JEM-2100F) and high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F) were used to characterize the morphologies of the samples. X-ray diffraction (XRD, Philips, X'pert pro, Cu Kα, 1.54056 nm) was used to measure the crystal structure. Brunauer–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore size distributions were determined based on the nitrogen adsorption/desorption isotherms measured at 77 K by a surface area and porosity analyzer (NOVA 2200e, Quanthachrome, USA).

2.3 Electrochemical measurements

The working electrode was prepared by mixing the as-prepared In₂O₃ nanofibers, acetylene black and sodium carboxymethyl cellulose (CMC) binder in a weight ratio of 80 : 10 : 10 in a mixed solvent of deionized water and ethanol (volume ratio of water to ethanol is 1 : 1) with vigorous stirring for 8 h. The obtained slurry was then coated onto Cu foil and dried in vacuum at 80 °C overnight. CR2025 coin cells were assembled in an argon-filled glove box with Li metal as the counter electrode, 1 M LiPF₆ dissolved in ethylene carbonate and dimethyl carbonate (EC : DMC = 1 : 1 in volume) as the electrolyte and Celgard 2400 polypropylene as the separator, respectively. An electrochemical workstation (CHI 660E, Shanghai, China) was used to measure the cyclic voltammetry (CV) at a scanning rate of 0.5 mV s⁻¹ within the potential range of 0.01–3.0 V (vs. Li⁺/Li). Galvanostatic charge/discharge cycles were measured at room temperature using a battery test system (Land CT 2001A) at a current density of 100 mA g⁻¹ within the voltage window of 0.01–3.0 V (vs. Li⁺/Li).

3. Results and discussion

The morphologies of the as-spun In(NO)₃·5H₂O/PVP nanofibers, the annealed In₂O₃ nanofibers are shown in Fig. 1. As can be seen from Fig. 1a and b, the as-spun In(NO)₃·5H₂O/PVP nanofibers are continuous and smooth, which have an average diameter of about 190 nm (see Fig. S1a†). After being dried in vacuum for 24 h and calcined at 450 °C for 2 h in air, the diameter of the nanofibers decreases to about 165 nm and remained continuous structure, which might be caused by selective removal of PVP (see Fig. 1c and d and S1b†). Fig. 1e and f show TEM image of the In₂O₃ nanofibers at different magnifications. Obviously, the ultrafine In₂O₃ nanoparticles disperse uniformly within the nanofiber matrix.

Fig. 1 SEM images of the as-spun In(NO₃)₃/PVP nanofibers (a and b), SEM (c and d) and TEM (e and f) images of the nanofibers composed of In₂O₃ nanoparticles as obtained by annealing the In(NO₃)₃/PVP nanofibers at 450 °C for 2 h in air.

Fig. 2a and b show the high resolution transmission electron microscope (HRTEM) images. Apparently, the In₂O₃ nanoparticles in the range from 5 to 10 nm uniformly embed in the mesoporous nanofiber matrix. Fig. 2c and d display the XRD pattern and nitrogen adsorption and desorption isotherms of In₂O₃ nanofibers, respectively. As seen from Fig. 2c, all the diffraction peaks can be indexed to the rutile phase In₂O₃ (JCPDS no. 06-0416), indicating the high crystallinity of the calcined nanofibers. The d-spacing of individual In₂O₃ crystals as marked in Fig. 2a is about 0.292 nm, corresponding to the (2 2 2) planes of the cubic In₂O₃ phase, which is consistent with the XRD result. According to Fig. 2d, the average pore diameter of the sample as determined by the BJH method is about 4.64 nm, which is accordance to the bright regions marked in Fig. 2b which indicate the pores with sizes ranging from 2 to 5 nm. The specific surface area of the mesoporous In₂O₃ nanofibers is about 15.7 m² g⁻¹. Such a relatively high surface area may enable a better contact between the electrolyte and active material and more complete reaction during the repeated charge and discharge process.

Fig. 2 (a and b) HRTEM images, (c) XRD pattern and (d) nitrogen adsorption and desorption isotherms of the as-obtained In₂O₃ nanofibers. The inset in (d) is the pore diameter distribution of the In₂O₃ nanofibers.

The electrochemical performance of the mesoporous In₂O₃ nanofibers as anode of LIBs were studied by both cyclic voltammograms (CV) and galvanostatic charge–discharge measurements. The CV curves of the In₂O₃ electrode was obtained at a scan rate of 0.5 mV s⁻¹ over a voltage range of 0.01–3.00 V versus Li⁺/Li, as shown in Fig. 3. Four pairs of oxidation and reduction peaks were identified, indicating that there were multi-step electrochemical reactions associated with the lithiation and delithiation process. A strong cathodic peak at 0.49 V versus Li⁺/Li was observed in the first cycle, which was then transferred to two peaks (0.37, 0.83 V) in subsequent cycles. This strong cathodic peak in the first cycle can be attributed to the formation of Li_xIn alloy in a multi-step electrochemical reduction reactions of In₂O₃ with Li following the reactions (1) and (2).^9,10,15,25 After these irreversible reactions, the reaction of electrode with electrolyte is close to stable, therefore, both of the cathodic and anodic peaks were verge to duplication after 3 cycles.²⁶ The corresponding oxidation peaks at about 0.5 and 0.7 V could be ascribed to the de-alloying of Li/In which formed during the anodic scan. The broad oxidation hump at 1.86 V observed in the anodic scan might come from subsidiary de-alloying reaction of the In_xLi and the formation of In. According to these CV curves, it can be concluded that the alloying and de-alloying during charge–discharge should be the dominant contributions to the lithium storage capacity.

In₂O₃ + 6Li → 2In + 3Li₂O (1)

In + 3Li⁺ + 3e ↔ Li₃In (2)

Fig. 3 Cyclic voltammogram (CV) of the anode assembled by In₂O₃ nanofibers.

Fig. 4a shows the galvanostatic charge–discharge curves of In₂O₃ nanofibers at selected cycles. The In₂O₃ nanofibers presented an initial discharge capacity of 1323.8 mA h g⁻¹, which gradually degraded to 512.6 mA h g⁻¹ after 20 cycles. The large irreversible capacity can be ascribed to intense surface reactions with the Li-In compounds and the formation of amorphous Li₂O matrix.^27,28 The potential plateau at about 1.5 V (vs. Li/Li⁺) in the first charge curve is attributed to allowing of In with Li. Such alloying and de-alloying processes will proceed in the following charge and discharge cycles, producing the cycle ability. However, due to gradual loss of the reversibility of these alloying and de-alloying processes, the potential plateau at about 1.5 V in the charge curve becomes unclear after a certain cycles. After the initial rapid capacity degradation, the In₂O₃ nanofiber electrode exhibited stable cycle performance in the voltage range of 0.01–3.0 V at a current density of 100 mA g⁻¹ over 200 cycles.

Fig. 4 (a) Charge and discharge curves at the 1st, 5th, 10th, 30th and 50th cycles, (b) cycle performance of the In₂O₃ nanofibers anode at a current density of 100 mA g⁻¹ over 200 cycles, and (c) rate performance of the In₂O₃ nanofibers anode at different current densities.

The discharge capacities at the 20th, 40th, 60th, 80th, 100th 150th and 200th cycles are 512.6, 488.7, 530.4, 579.9, 567.1, 548.3 and 524.5 mA h g⁻¹, respectively. These values are obviously better than that of the hollow spherical and thin film In₂O₃ electrodes as were reported previously,^10,28 and comparable to the vertically aligned In₂O₃ nanoblades.²⁹ As an attractive battery feature, rate performance was investigated at different current densities. As shown in Fig. 4c, the mesoporous In₂O₃ electrode presents an excellent rate performance that delivers specific capacities of 605, 482, 392, 318, 155 mA h g⁻¹ at the current densities of 100, 200, 500, 1000, and 2000 mA g⁻¹, respectively. It is worth to note that the In₂O₃ electrode displays a high specific capacity of 589 mA h g⁻¹ when the current density returns to 100 mA g⁻¹. It is believed that by integrating the ultrafine In₂O₃ nanoparticles within a mesoporous nanofiber matrix, the aggregation of In₂O₃ nanoparticles is prohibited, which is very important for fully utilization of the active material for lithium storage. Meanwhile, the mesoporous structure provides sufficient void space derived from numerous mesoporous to buffer the volume change of In₂O₃ nanoparticles during the lithium insertion and extraction reactions, which results in an excellent cyclic stability.

4. Conclusions

Mesoporous In₂O₃ nanofibers assembled by ultrafine In₂O₃ particles were fabricated by electrospinning and post-synthesis calcination process. The unique 1D mesoporous structure not only prevents aggregation of In₂O₃ nanoparticles, so that enables fully use of them for lithium storage, but provide enough space to buffer the large volume change during the repeated alloying and de-alloying between Li and In in a multistep electrochemical reactions. As a consequence, stable cycle performance and quite encouraging capacity were obtained when the obtained material was tested as anode of LIBs. The results present in this work illustrate that by integrating In₂O₃ nanoparticles into mesoporous nanofibers with the well-established electrospinning technique, the In₂O₃ could be a promising high capacity anode material for electrochemical energy storage in LIBs.

Acknowledgements

The authors gratefully acknowledge the financial supports from the Natural Science Foundation Guidance Project of Fujian Province, China (Grant No. 2012D131) and the Industry Leading Key Projects of Fujian Province, China (Grant No. 2014H0046).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07804d

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