One-pot sol-gel synthesis of Li4Ti5O12/C anode materials for high-performance Li-ion batteries
Introduction
In recent years, rechargeable Li-ion batteries (LIBs) have been considered as one of the most promising technologies for the next generation electric vehicles such as electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs). As a result, lots of critical issues including low cost, high energy/power density, durability, and safety for LIBs should be addressed [1], [2], [3], [4]. Li-intercalated graphite is the most commonly used as an anode material in commercial LIBs. However, one of the safety issues is the formation of lithium dendrites on the graphite anode surface, thus resulting in the short circuits after long-term charge/discharge processes, especially when operated below 0.2 V vs. Li+/Li [5], [6], [7].
Li4Ti5O12 (LTO) has attracted extensive attention as a promising anode material due to its zero-strain feature during the charge/discharge process, and high and flat operation potential plateau at 1.55 V vs. Li+/Li, at which can restrain the deposition of metallic lithium, therefore improving the safety of LIBs. Unfortunately, the development of high-rate performance of LTO suffers from its inherent poor electronic conductivity (ca. 10−13 S cm−1) and moderate Li+ ion diffusion coefficient (10−8 cm2 s−1), which could result in the serious polarization of the electrode when operated at high current densities [8]. Up to date, a great deal of strategies have been employed to overcome these issues, including synthesis of nano-sized LTO particles to reduce the Li+ ion diffusion path [9], [10], [11], doping with other metal ions [12], [13], [14], [15], [16], [17], surface modification with metals or metal oxides [18], [19], coating with conductive materials [20], [21].
Among them, carbon coating of LTO particles as LTO/C has been considered as the most cost-effective way to significantly enhance the electrochemical performance of LTO owing to the improvement of surface electronic conductivity and the electrical contact with electrolyte. According to the previous studies, most of LTO/C composites were synthesized via coating the as-prepared LTO particles with desired carbon sources followed by calcinations under inert atmosphere [22], [23], [24], [25]. Nevertheless, until now, the one-pot in-situ synthesis of carbon coated LTO as LTO/C composite is supposed to be the relatively facile and effective approach to improve the electrochemical properties of LTO [26], [27], [28]. Zheng et al. [26] prepared spinel LTO/C by one-step solid-state reaction via lithium citrate as carbon source and found that the in situ carbon coating can not only improve the surface electronic conductivity of LTO, but also restrict the particle-size growth during the following heat treatment. Therefore, the LTO/C with reduced particle size can demonstrate an impressive initial discharge capacity of up to 121.1 mAh g−1at a relatively high charge/discharge current density of 20 C [29]. Yang et al. [27] also reported that the LTO/C synthesized by a simple solid-state reaction, in which citric acid was employed as the carbon source, revealed an improved electrochemical performance due to the improved electronic conductivity via carbon coating. They also found that the improved electronic conductivity can be ascribed to the reduction of the partial Ti4+ in the LTO to Ti3+ during the carbonization of citric acid in the N2 atmosphere [30]. In these studies; however, LTO/C was synthesized via solid-state reaction. Although this way is simple, there are some shortcomings of the solid-state reaction such as larger particle size with inhomogeneous distribution, and higher temperature and longer duration of subsequent heat treatment. These issues can be addressed by sol-gel method [28], [31], [32]. For instance, Hao et al. [31] proposed a sol-gel method to synthesize LTO with a homogeneous distribution of sub-micro size particles. Wang et al. [32] further obtained LTO/C by means of a sol-gel synthesis, followed by sintering under Ar atmosphere. The resultant LTO/C with in situ carbon coating exhibited improved rate capability and cycling stability compared to the pristine LTO. The authors only explained it by the enhanced electronic conductivity of the LTO/C due to the coating of highly conductive carbon.
In this current work, we successfully reported a facile one-pot sol-gel method to synthesize LTO/C composite as an anode material for high-performance LIBs. LTO/C composite with merely ca.1.3 wt% carbon content indeed possessed improved electronic conductivity, and therefore exhibited the improved rate capability and cycling stability. For instance, the as-synthesized LTO/C composite anode revealed an impressive first discharge capacity of 147.9 mAh g−1 at a high charge/discharge current density of 20 C and the reversible discharge capacity still retained up to 98% of that at first discharge. Additionally, we found that the improved electronic conductivity of LTO/C can be ascribed to not only the coating of highly conductive carbon, but also the existence of mixed valence of Ti4+/Ti3+. This is the first time that the mixed valence of Ti4+/Ti3+ was found within the LTO/C synthesized by sol-gel method. In view of the synergistic effect of the conductive carbon coating and the mixed valence of Ti4+/Ti3+, the LTO/C prepared by the proposed one-pot sol-gel method can be considered as a promising anode material for high-performance LIBs.
Section snippets
Synthesis of LTO and LTO/C
To synthesize the spinel LTO/C composite, we basically used a modified sol-gel method according to our previous work [33]. Spinel LTO/C was synthesized using a sol-gel method with lithium acetate dehydrate (CH3COOLi·2H2O) (98%, Acros)、tetrabutyl titanate [Ti(OC4H9)4] (99%, Acros) and citric acid (99.5%, Acros) as chelating agent. First, two solutions, A and B, were prepared by dissolving CH3COOLi·H2O (98%, Acros) and Ti(OC4H9)4 (99%, Acros) in ethanol solution (99 wt%, Shimakyu) with Li: Ti
Material characterizations
The crystalline structure and morphology of the synthesized materials was analyzed by scanning electron microscope (SEM, JSM-7600, JEOL) and high-resolution transmission electron microscope (HR-TEM, Philips, Technai G2). The carbon content was obtained by an elemental analyzer (EA, Heraeus Vario EL III). Powder X-ray diffraction (XRD) was performed using a Lab XRD 6000 (Shimadzu Corporation) with Cu Kα radiation. The diffraction results were recorded at 2θ= 10 to 80o. X-ray photoelectron
Electrochemical tests
Electrochemical tests were performed using CR2032 coin-type cells, as described in our previous work [33]. In brief, the anodes were made of either the LTO or the LTO/C active material with acetylene black (99.99%, Strem Chemicals Inc.), and polyvinylidene fluoride (Kynar 7200, ELF) in a weight ratio of 83: 10: 7. After these materials were thoroughly dispersed in N-methyl-2-pyrrolidine solution (ultra, ISP Technologies Inc.), the as-prepared slurry was coated on a Cu foil to a thickness of ca.
Results and discussion
Fig. 1 shows the SEM micrographs of the LTO and LTO/C samples. As indicated in Fig. 1 a and c, the LTO/C sample displays not only more regular and uniform in particle shape and size distribution but also a slightly smaller particle size of ca. 100-300 nm than the pristine LTO sample. In the enlarged SEM images, in contrast to the pristine LTO (Fig. 1b) with well-crystallized spinel structure and smooth surface morphology, the LTO/C sample (Fig. 1d) with the slightly coarse and reunion surface
Conclusions
In this current work, spinel LTO/C composites were successfully synthesized by a facile one-pot sol-gel method using citric acid as carbon source. It was found that the uniformly coated highly conductive carbon layer on LTO particles not only improved the electrical contact between LTO particles but also resulted in the generation of mixed valence of Ti4+/Ti3+ after sintering in N2 atmosphere, thus significantly enhancing the electrochemical performance of the LTO/C electrode, especially on the
Acknowledgements
This work is financially supported by National Science Council Taiwan (99-2632-E-036-001-MY3 and 102-2632-E-036-001-MY3) and Tatung University (B101-C09-025). The authors are also grateful to Prof. She-huang Wu in Tatung University for his helpful discussion and partial supports in materials and instruments. The instrumental support by National Synchrotron Radiation Research Center (NSRRC) of Taiwan is also appreciated by the authors.
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