Co-pyrolysis synthesis of Fe₃BO₆ nanorods as high performance anodes for lithium-ion batteries

Abstract

The high capacity, negligible toxicity, environmentally benign nature and abundant reserves (low cost of elements contained) of the Fe₃BO₆ nanomaterial enable it to be a highly promising anode material for lithium-ion batteries. In this study, Fe₃BO₆ nanorods encapsulated in graphite (defined as “Fe₃BO₆@C”) core–shell like composites have been produced in situ firstly via a co-pyrolysis approach in a stainless-steel autoclave. After subsequent calcinations, Fe₃BO₆ nanorods with diameters in the range of 20–50 nm were obtained with high yield, which display a first discharge capacity of 1192 mA h g⁻¹ (with a coulombic efficiency of 70%). It is found that at the current density of 100 mA g⁻¹, the specific capacity of the Fe₃BO₆ nanorods can remain at 873.2 mA h g⁻¹ after 100 cycles; it is worth noting that their specific capacity can still remain at 710 mA h g⁻¹ even if the current density was set at 1000 mA g⁻¹, indicating the excellent cycle stability and promising applications of the as-obtained Fe₃BO₆ nanorods utilized as anode material at high power field.

---

Introduction

Fe₃BO₆ is a new kind of developing anode polyanion material for lithium-ion batteries, which has many merits such as higher capacity, negligible toxicity, environmentally benign nature and abundant reserves (inexpensive) of the iron and boron elements contained. The olivine-related Fe₃BO₆ is orthorhombic phase, with space group Pnma.^1,2 During the past few years, Fe₃BO₆ has been extensively investigated and widely applied as materials in color pigments, catalysts, barcodes, drugs, or biological tools owing to their special electronic, magnetic and biocompatible properties.^3–5

The electrochemical property of microscale Fe₃BO₆ potentially applied as anode material for lithium-ion battery was firstly studied by Nazar et al. in 2001, which had a specific capacity of 450 mA h g⁻¹ at an average potential of 1.6 V.¹ Recently, increasing attention has been paid on the controllable synthesis and property investigation of nanoscaled Fe₃BO₆ materials as anode materials for lithium-ion batteries. For instance, Fe₃BO₆ particles with discharge capacity of 250 mA h g⁻¹ at the 12 cycle were prepared via a solid state reaction route above 900 °C using Fe₂O₃ and H₃BO₃ as raw materials;⁶ Fe₃BO₆ nanospherical particles with diameters of about 40 nm (with discharge capacity of above 500 mA h g⁻¹ in the voltage range of 0.8–3.1 V at the current density of 100 mA g⁻¹) were fabricated via rheological phase reaction route at 800 °C.⁷ Though much progress has been made, further property improvement (such as the capacity and the current density of Fe₃BO₆ should be further improved or adjusted to fulfil its future practical application) are still the major challenge faced by Fe₃BO₆ nanomaterials.

It is generally accepted that the electrochemical properties (such as the specific capacitance and cycle stability performance) of the electrode materials are greatly influenced by their sizes and dimensions, for example, the smaller of the particles (∼5 μm versus 70 nm), the higher specific capacity (450 mA h g⁻¹ versus 700 mA h g⁻¹) of the Fe₃BO₆ could be obtained;¹ one-dimensional nanomaterials not only have a large surface-to-volume ratio but also provide efficient one-dimensional electron transport pathways and facile strain relaxation during battery charge and discharge process.^6–9 In order to achieve high capacity and excellent cycle stability of target nanoscaled products, convenient synthesis is one of the crucial and effective strategies for the further improvement of the electrochemical properties of Fe₃BO₆. To the best of knowledge, up to now, there are few reports about the convenient synthesis of Fe₃BO₆ nanorods of high yields, good cycle stability and rate performances.

In this study, Fe₃BO₆ nanorods were obtained via a co-pyrolysis method in autoclaves at 700 °C following a subsequent calcination treatment. In the absence of the subsequent calcination process, Fe₃BO₆ nanorods encapsulated in graphite core–shell like powders were obtained firstly. The rate performances at varied current densities (100, 200, 500, 1000 mA g⁻¹) and long cycle stability (at 100 mA g⁻¹ within 0.01–3.0 V) of the Fe₃BO₆ powders were systematically investigated. The as-obtained Fe₃BO₆ nanocrystals mainly composed of nano-scaled nanorods have enhanced cycling stability and improved specific capacity. The high electrochemistry performances (with enhanced cycling stability and high improved specific capacity) of the Fe₃BO₆ nanorods indicating their promising applications as an anode material for lithium-ion battery.

Experimental

Preparation of Fe₃BO₆

The Fe₃BO₆ nanocrystals were prepared via a co-pyrolysis method in stainless-steel autoclaves of 20 ml capacity. In a typical synthesis process, the mixtures of 1.86 g FeC₁₀H₁₀, 0.62 g H₃BO₃ and 5 ml ethanol were transferred into the autoclave after constantly stirring. The autoclave was then tightly sealed and put into an electronic furnace at room temperature. The temperature of the furnace was increased from room temperature to 700 °C in 100 minutes and maintained at 700 °C for 15 h, and then the autoclave was cooled down to room temperature naturally. The dark precipitates in the autoclave were collected and alternately washed with absolute ethanol and ultra-pure water for several times. The dried product was calcined in a muffle furnace at 700 °C for 5 h to remove carbon for further characterization.

Characterization

The X-ray diffraction (XRD) analysis was measured on a Bruker D8 advanced X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 10–80° at the scan rate of 0.08° per minute. The Raman spectrum was recorded at ambient temperature on a LABRAM-HR confocal laser MicroRaman spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. The transmission electron microscopy (TEM, JEM-2100) and field emission scanning electron microscope (FESEM; JEOL JSM-6700F) were used to observe the morphology and size of the samples. High-resolution transmission electron microscope (HRTEM) was carried out on a JEOL 2100 transmission electron microscope with an accelerating voltage of 200 kV to observe the morphology and inter structure of the samples. The BET surface area (SBET) and Barrett–Joyner–Halenda (BJH) pore size distribution (PSD) were measured by using a QuadraSorb SI surface area analyzer (version 5.06).

Electrochemical measurements

The charge–discharge performance of button batteries was tested on a Land battery test system (CT2001A, China) at room temperature (25 °C). The working electrodes were mixed slurry consisted of 60 wt% Fe₃BO₆, 30 wt% carbon black, 10 wt% poly(vinylidene fluoride) (PVDF). The mass of the active Fe₃BO₆ material was calculated based on 60% of the total mass of the electrode piece. And n-methylpyrrolidone (NMP) was used as the solvent. The mixed slurry was scribbled onto a copper foil of 12 mm diameter. The fabricated working electrodes were dried in vacuum oven at 80 °C for 12 h. Nickel foam was used as current collector and Celgard 2300 microporous polypropylene membrane was used as the separator. The electrolyte was the mixture of LiPF₆ (1 mol l⁻¹) and the solution of ethylene carbonate–dimethyl carbonate–diethyl carbonate (EC–DMC–DEC, 1 : 1 : 1 w/w). The assembly of button batteries were conducted in a dry argon-filled glove box, the pressures of water and oxygen were both lower than 1 ppm. Lithium foils were used as counter electrodes. The tested batteries were cycled at different current densities (100, 200, 500, 1000 mA g⁻¹) in the voltage range of 0.01–3.0 V. Cyclic voltammetry (CV) experiments were measured between 0.01 V and 3.0 V at a scan rate of 0.1 mV s⁻¹ by using a LK2005A Electrochemical Workstation.

Results and discussion

Fig. 1a shows a typical XRD pattern of the product after calcinations. All the diffraction peaks can be indexed to Fe₃BO₆ (JCPDS card no. 18-0636), and no obvious diffraction peaks associated with impurities were observed.¹⁰ Fig. 1b displays the XRD pattern of the product before calcinations. The broad diffraction peaks can be indexed to hexagonal graphite (JCPDS card no. 41-1487). And the peaks marked with asterisk correspond the (111), (121) and (311) planes of Fe₃BO₆, respectively, indicating the product is consisted of graphite and Fe₃BO₆. The Raman spectrum (Fig. 1c) indicates that there are two obvious peaks at ∼1320 (D-band) and ∼1590 cm⁻¹ (G-band), which corresponds to the vibrations of carbon atoms with dangling bonds in planar terminations of disordered graphite and the vibration in all sp²-band carbon atoms in a two-dimensional hexagonal lattice, respectively.¹¹ The I_D/I_G value of the precursor (0.92) reveals the relative low crystallinity of the outer graphite layers.^11,12

Fig. 1 Typical XRD patterns of the products with (a) and without (b) the calcinations. (c) Raman spectrum of the sample before calcinations.

The morphology and structure of the as-obtained Fe₃BO₆ product has been examined by FESEM (Fig. 2a and b), TEM (Fig. 2d) and HRTEM analyses. It is observed from Fig. 2a and b that high yield of nanorods with diameters in the range of 20–50 nm have been obtained. The corresponding HRTEM image of a Fe₃BO₆ nanorod has an average lattice spacing of 0.37 nm (Fig. 2c), which is consistent with the (111) plane of Fe₃BO₆. Besides the Fe₃BO₆ nanorods, small amount of Fe₃BO₆ nanoparticles were also observed co-existed with the Fe₃BO₆ nanorods (Fig. 2e). The average lattice distance spacing between the planes (0.224 nm, 0.214 nm, see Fig. 2f) were consistent with the values of the (002) and (231) planes of Fe₃BO₆. The clear resolved and regular arranged lattice fringes reveal the well crystalline of the Fe₃BO₆ product.

Fig. 2 Typical FESEM images (a and b) TEM (d and e) and HRTEM images (c and f) of the Fe₃BO₆ sample.

The morphology and structure of product without calcination process were also investigated. Fig. 3 show the typical TEM and HRTEM images of core–shell like uniform raw product obtained before calcination process. From the different contrasts of the images together with the analyses result of XRD pattern (Fig. 1), it is clear that the outer light grey layer is graphite (originates from the decomposition of ferrocene and ethanol; with thickness of 10–30 nm), and the inner part is Fe₃BO₆ nanorods (see Fig. 3a; with width of 20–100 nm and length of 100–200 nm). This core–shell product is defined as “Fe₃BO₆@C”. Fig. 3b and c depict HRTEM images of part area of a core–shell like nanorod, in which the clear resolved fringes were separated by 0.406 nm, which correspond to the (101) crystal spacings of a Fe₃BO₆ nanocrystal. Their formation process and thermal stability of the as-obtained products have also been investigated (see Fig. S1and S2, ESI†).

Fig. 3 (a) TEM image of the raw product (Fe₃BO₆@C) without calcinations; (b and c) HRTEM images of Fe₃BO₆@C.

The nitrogen cryo-adsorption desorption isotherm and the corresponding pore size distribution curve of the Fe₃BO₆ and Fe₃BO₆@C are shown in Fig. 4a. The curve displays an adsorption–desorption hysteresis at a relative pressure of 0–1.0, which can be attributed to the formed inhomogeneous mesopores. The nitrogen sorption isotherm of Fe₃BO₆ exhibited a type IV curve with a H₃ hysteresis loop.¹² The specific surface area value of the Fe₃BO₆ nanocrystals is 12.76 m² g⁻¹ and their average pore diameter is 3.68 nm with a narrow distribution of 3–5 nm (inset in Fig. 4a). As nanoscaled size and apposite surface area of the Fe₃BO₆ nanorods with certain open, porous structure can provide ideal lithium ion and electrolyte paths, shorten the required diffusion distances and enhance the interface contact,^12–15 therefore, it is likely that the high cycle performance of the present Fe₃BO₆ nanorods that superior to the previous reported ones are mainly determined by their nanostructures. Fig. 4b displays the nitrogen adsorption–desorption isotherm of Fe₃BO₆@C. The specific surface area value of the Fe₃BO₆@C nanocrystals is 20.97 m² g⁻¹ and the pore size distribution has a relative wide peak at 17 nm (inset in Fig. 4b).

Fig. 4 Nitrogen adsorption–desorption isotherms of the products: (a) Fe₃BO₆; (b) Fe₃BO₆@C.

Fig. 5 shows the cyclic voltammograms (CV) of the as-prepared Fe₃BO₆ sample versus Li⁺/Li in the voltage range of 0.01–3.0 V. Different behaviors have been found for the CV in the first, second and third cycle. In the oxidation process of the three cycles, it is obvious that two low current peaks locate at 1.5 V and 2.1 V; whereas in the reduction process, it is found that two main potential stages locate at 0.8 V and 1.3 V in the first cycle, respectively, while in the subsequent two cycles, the stage is conducted at a slight higher potential (∼1.5 V), indicating the discontinuity in the cyclability among the initial three cycles. The appearance of the first voltage stage different from the following ones phenomenon might be partly attributed to the progressively formed amorphous materials that cycled reversibly against lithium as the crystallized Fe₃BO₆ can not allow to topological intercalate lithium,⁵ but the exact reason still needs further research.

Fig. 5 Cyclic voltammograms of Fe₃BO₆ versus Li⁺/Li at a scanning rate of 0.1 mV s⁻¹ in the voltage of 0.01–3.0 V at room temperature.

Fig. 6a displays the charge–discharge capacities versus cycle number curves of Fe₃BO₆ electrode in the potential range of 0.01–3.0 V at the current density of 100 mA g⁻¹. The first discharge capacity of the product is up to 1192 mA h g⁻¹ (equivalent to ∼12.2 mol Li per mol Fe₃BO₆), corresponding to a coulombic efficiency of about 70%. The irreversible capacity may be attributed to the formation of a solid electrolyte interface (SEI) layer at the Fe₃BO₆/electrolyte interface or the decomposition of electrolyte.^9,10 There is an obvious capacity decreasing and then increasing process in Fig. 6a, which might be largely attributed to the structure evolution of Fe₃BO₆ from crystalline to amorphous phase that can be cycled reversibly.^1,6 It is impressive that the electrode exhibits good cycle stability and the discharge capacity can maintain at 873.2 mA h g⁻¹ after 100 cycles. The retention of discharge capacity is 95.5% from the second cycles. And the coulombic efficiency is near 100%. The above results confirm that the Fe₃BO₆ material is a promising anode material for lithium-ion rechargeable batteries. The charge–discharge curves of the Fe₃BO₆ material are presented in Fig. 6b, in which two different discharge and charge stages consistent with those of the CV results. These curves overlap very well except the initial few cycles, which also confirms that Fe₃BO₆ electrode can possess excellent cycle stability. Fig. 6c displays the corresponding cycling performance of the Fe₃BO₆/Li coin cell at larger density (500 mA g⁻¹). It can be observed that it had a high initial discharge capacity as 1170 mA h g⁻¹ (∼12 mol Li). After 110 cycles, the discharge capacity can stabilize at 489 mA h g⁻¹ (∼5 mol Li), which is still higher than that of commercial graphite. Fig. 6d exhibit the rate performances of Fe₃BO₆ and Fe₃BO₆@C electrodes, in which the discharge capacities of Fe₃BO₆ nanorods at 100, 200, 500, 1000 mA g⁻¹ are 1013, 850, 673 and 550 mA h g⁻¹, respectively; when the current density decreased to 100 mA g⁻¹ after 40 cycles, the discharge capacity can be restored to 940 mA h g⁻¹, indicating the superior advantage of the present material than Fe₃BO₆@C and those of previous reported Fe₃BO₆ nanoparticle and they might be potentially applied in rapid charge–discharge batteries. Compared to Fe₃BO₆ electrode, the capacity of Fe₃BO₆@C electrode obtained here is relatively low. It is tentatively considered that the high percentage of graphite content (almost 44.7% through the analyses of TGA). Fig. 6e displays the charge–discharge curves of Li/Fe₃BO₆ at different current densities (100, 200, 500, 1000 mA g⁻¹). It is clear that the larger of the charge–discharge current density, the less voltage stage of the product is performed. The above date reveal the improved rate capability and capacity retention ability of the as-obtained Fe₃BO₆ nanorods compared with those of the previously reported ones.¹⁶ The present synthesis strategy is believed can be applied to fabricate series of many other target one-dimensional boride nanomaterials with promissing properties.^16,17

Fig. 6 (a) The cycle performance of Fe₃BO₆ at current density of 100 mA g⁻¹; (b) charge–discharge curves of Fe₃BO₆; (c) cycling performance of the Fe₃BO₆/Li coin cell at 500 mA g⁻¹; (d) the rate performance of Fe₃BO₆ and Fe₃BO₆@C at room temperature; (e) the charge–discharge curves of Fe₃BO₆ at different current densities.

Conclusions

In this study, core–shell like Fe₃BO₆ nanorods encapsulated in graphite composites and Fe₃BO₆ nanorods were conveniently fabricated via a co-pyrolysis method in stainless-steel autoclaves at 700 °C with or without the subsequent calcinations. The discharge capacity of Li/Fe₃BO₆ battery can maintain at 873.2 mA h g⁻¹ after 100 cycles at the current density of 100 mA g⁻¹, and the discharge specific capacity is up to 710 mA h g⁻¹ at high current density of 1000 mA g⁻¹, indicating its excellent cycle stability and good rate performances as anode material at high power field.

Acknowledgements

This work was supported by the National Nature Science Found of China and Academy of Sciences large apparatus United Found (no.11179043, 20971079), and the 973 Project of China (no. 2011CB935901).

Notes and references

1. J. L. C. Rowsell, J. Gaubicher and L. F. Nazar, J. Power Sources, 2001, 97–98, 254–257.
2. A. I. Palos, M. Morcrette and P. J. Strobel, J. Solid State Electrochem., 2002, 6, 134–138.
3. L. A. Bauer, N. S. Birenbaum and G. J. Meyer, J. Mater. Chem., 2004, 14, 517–526.
4. J. Dobson, Nat. Nanotechnol., 2008, 3, 139.
5. Handbook on Borates: Chemistry, Production and Applications, ed. M. P. Chung, Nova Publishers, Huntington, New York, USA, 2009.
6. A. Ibarra-Palos, C. Darie, O. Proux, J. L. Hazemann, L. Aldon, J. C. Jumas, M. Morcrette and P. Strobel, Chem. Mater., 2002, 14, 1166–1173.
7. X. X. Shi, C. X. Chang, J. F. Xiang, Y. Xiao, L. J. Yuan and J. T. Sun, J. Solid State Chem., 2008, 181, 2231–2236.
8. F. Y. Cheng, Z. L. Tao, J. Liang and J. Chen, Chem. Mater., 2008, 20, 667–681.
9. F. M. Wang, H. Y. Wang, M. H. Yu, Y. J. Hsiao and Y. Tsai, J. Power Sources, 2011, 196, 10395–10400.
10. J. Cabana, L. Monconduit, D. Larcher and M. R. Palacín, Adv. Mater., 2010, 22, 170–192.
11. H. Wu, M. Xu, H. Y. Wu, J. J. Xu, Y. L. Wang, Z. Peng and G. F. Zheng, J. Mater. Chem., 2012, 22, 19821–19825.
12. Y. Liu, C. H. Mi, L. H. Su and X. G. Zhang, Electrochim. Acta, 2008, 53, 2507–2513.
13. N. Yan, L. Hu, Y. Li, Y. Wang, H. Zhong, X. Y. Hu, X. K. Kong and Q. W. Chen, J. Phys. Chem. C, 2012, 116, 7227–7235.
14. K. Zaghib, X. Song, A. Guerfi, R. Kostecki and K. Kinoshita, J. Power Sources, 2003, 124, 505–512.
15. Y. J. Mai, X. H. Xia, R. Chen, C. D. Gu, X. L. Wang and J. P. Tu, Electrochim. Acta, 2010, 67, 73–78.
16. M. V. Reddy, G. V. Subba Rao and B. V. R. Chowdari, Chem. Rev., 2013, 113, 5364–5457.
17. L. Q. Xu, S. L. Li, Y. X. Zhang and Y. J. Zhai, Nanoscale, 2012, 4, 4900–4915.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra46482b

---