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Nanotube-Constructed Li4Ti5O12-TiO2 Hybrid Spheres for Lithium Ion Batteries
Spinel Li4Ti5O12 and anatase TiO2 have been regarded as two typical alternative anode materials for rechargeable lithium-ion batteries (LIBs) to the commercial graphite because of their better safety and low volume expansion.1-3 The flat lithium insertion potential plateau of spinel Li4Ti5O12 locates at 1.55 V, which is above the solid electrolyte interface (SEI) and the lithium dendrite formation potentials, thus making it much safer. Also, the spinel Li4Ti5O12 is “zero strain” insertion, which endows the material a high potential of long cycle life. Similarly, anatase TiO2 features a potential plateau of ~1.7 V along with a low volume expansion of 3-4%. More importantly, the reduction of the particle size to nanoscale allows nearly 1 Li+ insertion into the anatase host structure, resulting in a high capacity of more than 300 mA h g-1 theoretically. By combining the two materials, high safety and long cycle life as well as high capacity are expected. However, the instinct electronic conductivity for both materials is very low.4-7
Here, we report on the large-scale fabrication of nanotube-constructed Li4Ti5O12-TiO2 hybrid spheres (L-T-SPs), and the evaluation of their lithium storage properties. The hybrid exhibits a high capacity and long cycle life, benefitting from the tiny tubular morphology, defects in the grain boundary of the Li4Ti5O12-TiO2 hybrid and the synergistic effects of the two phases.
Experimental
Typically, the L-T-SPs product was prepared by a modified hydrothermal route, followed by an ion-exchange and post annealing processes. The microstructure of the L-T-SPs product were determined by scanning electron microscopy (SEM). The working electrodes were fabricated by mixing the L-T-SPs material, super P and polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone (8:1:1), and then coated onto a Cu foil. A lithium foil was used as the counter and reference electrode, Celgard 2300 as the separator, and 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1) as the electrolyte. Galvanostatic charge-discharge measurements were performed on a Land Battery Measurement System (Land, China) at various current densities.
Results and Discussion
The synthesis process is schematically illustrated in Fig. 1a. The titanic acid (HTO) precursor was firstly treated by LiOH to exchange the H+ ions with Li+ ions, forming an intermediate containing Li, Ti and O elements (Li-Ti-O spheres). The Li4Ti5O12-TiO2 product was obtained by post annealing the intermediate. The SEM images in Fig. 1b and 1c shows the spherical morphology of the product. Each individual urchin-like microsphere consists of numerous tiny nanotubes.
The lithium storage performance of the as-prepared L-T-SPs product was evaluated using lithium half-cells. Fig. 1d demonstrates the initial two cycles of the galvanostatic charge-discharge profiles. There are two distinct voltage plateaus located at ~1.7 and 1.55 V in the discharge process, assigned to the insertion of lithium into the anatase and the Li4Ti5O12 host structures, respectively. The cycle performances of the L-T-SPs electrode are presented in Fig. 1e. Capacities of ~140 mA h g-1 at 5 C and ~110 mA h g-1 at 20 C can be obtained. Moreover, the capacity at 20 C retained for 100 cycles without obvious decay, indicating excellent cyclability of the L-T-SPs electrode. The tiny tubular microstructure integrated with the defects in the grain boundary and the synergy between the spinel Li4Ti5O12 and anatase TiO2 contribute to the enhanced lithium storage performance of the L-T-SPs electrode.
Figure captions: Figure 1. (a) Schematic illustration of the formation process for the L-T-SPs product, (b,c) SEM images of the L-T-SPs product, (d,e) The charge-discharge profiles of the L-T-SPs electrodes in the initial two cycles and their cycle performances at 5 and 20 C.
References
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