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Electrospinning of Porous Li4Ti5O12@C Nanofibers for High-Rate Lithium Ion Batteries
Lithium ion batteries (LIBs) have attracted continuous attention in both scientific and industrial fields because of their excellent properties like high energy density and long cycle life. Recently, Li4Ti5O12 (LTO) has been widely investigated due to its excellent cyclability and safety performance.1-3 However, the main problems preventing its scalable application lie in the intrinsically slow kinetics of Li+diffusion and low electronic conductivity. To address these issues, various strategies have been developed by facilitating the kinetics through decreasing the particle size and coating the particles with conductive agents, most commonly carbon. Herein, we demonstrate an efficient and scalable strategy for the synthesis of porous LTO@C nanofibers with ultra-high rate capability and excellent cycling performance.
Experimental
The porous LTO@C nanofibers were prepared by an electrospinning technique followed by subsequent heat treatment. Typically, stoichimetric amounts of lithium acetylacetonate, titanium isopropoxide and poly(vinyl pyrrolidone) were dissolved in the mixed ethanol/acetic acid solution to get a clear precursor solution for electrospining. At a high voltage of 15 kV, the precursor nanofibers were formed. Then, the as-spun nanofibers were subjected to a heat-treatment to obtain the final porous LTO@C nanofibers. Electrochemical measurements were carried out between 1.0 and 3.0 V vs. Li+/Li0 with CR2032 coin cells. The obtained materials were mixed with C-black and PTFE (70:20:10 wt %) to fabricate the electrodes. In the coin-cell tests, metallic lithium foil was used as the counter and reference electrodes; The electrolyte was 1 M LiPF6 solution in a mixture of EC and DMC (v/v=1:1).
Results and Discussion
Figure 1. (a) Representative XRD pattern, (b) Raman spectrum, (c) SEM image, (d) EDX spectrum and (e, f) TEM images of the as-prepared porous LTO@C nanofibers. (g) Cyclic voltammogram of the porous LTO@C electrode obtained at a scan rate of 0.1 mV s-1. (h) The discharge capacity as a function of C-rate for the porous LTO@C, nonporous LTO@C nanofibers prepared by calcinating the as-spun fibers in an inert atmosphere, and bulk LTO obtained by traditional solid-state method, respectively.
Figure 1a displays the typical XRD pattern for the product. All the diffraction peaks can be indexed to the cubic spinel structure of LTO. Raman spectrum of the as-formed porous LTO@C fibers are shown in Figure 1b. The G-band at 1590 cm-1 for sp2-hybridized carbon and the D-band at 1350 cm-1 for disordered carbon can be observed in the product spectrum, which confirms the existence of carbon after thermally treating. SEM image in Figure 1c reveals that the porous LTO@C nanofibers have a 1D geometry with rough surfaces and an average diameter of ~200 nm, indicating the pyrolysis of PVP and the crystallization of LTO precursor after annealing. EDX analysis (Figure 1d) further confirms the formation of carbon and LTO in the composite fibers. The TEM images depicted in Figure 1e and f shows that the porous LTO@C fibers are actually a hierarchically porous nanofiber constructed from well-interconnected LTO@C nanocrystals and wormhole-like pores sizing a few nanometers.
Figure 1g shows the CV curves of the porous LTO@C electrodes at a rate of 0.1 mV s-1. One set of well-defined sharp redox peaks at 1.63/1.50 V are observed, and should be ascribed to the Ti4+/Ti3+ redox couple reaction. Figure 1h shows the rate capability of the porous LTO@C sample in comparison to LTO@C and bulk LTO. Clearly, porous LTO@C exhibits a much higher storage capacity and better rate capability. The discharge capacities are 161, 157, 154, 151, 150, 144 mAh g-1 when cycled at the C-rates of 0.5, 1, 2, 5, 10 and 20 C. The high capacity and excellent rate capacity of the as-prepared porous LTO@C electrodes should be ascribed to the synergistic effects of the nanocomposites: Uniformly distributed carbon can not only serve as continuous channels for electron transfer but also restrict the growth of LTO nanoparticles during the long-time calcination process. Also, the obtained LTO nanoparticles can effectively boost Li+ and e-transfer in nanostructured electrodes. In addition, the porous structure provides a large surface area that would create more exposed active sites between electrode materials and electrolyte, and thus promote rapid charge-transfer reaction.
In conclusion, the electrospinning method combined with post annealing treatment was explored to prepare the porous LTO@C nanofibers with superb electrochemical performances. The present synthesis method can be extended to prepare other porous carbon-coated fibers for energy-storage applications. References
1. L. Yu, H. B. Wu and X. W. Lou, Adv. Mater., 2013, 25, 2296-2300.
2. L. Shen, X. Zhang, E. Uchaker, C. Yuan and G. Cao, Adv. Energy Mater., 2012, 2, 691-698.
3. Y. Sun, X. Hu, W. Luo and Y. Huang, J. Mater. Chem., 2012, 22, 425.