Synthesis of TiN@C Nanocomposites for Enhanced Electrochemical Properties

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2. Materials and Methods

2.1. Preparation of TiN@C Nanocomposites

All the chemicals were analytical grade and used without further purification. In a typical procedure, 0.5 g of commercial TiN was dispersed in 100 mL of oleic acid solution at room temperature. After being stirred for 8 hours, the precipitate was collected by centrifugation, and then washed with ethanol for two times. Finally, the product was dried completely in vacuum at 80°C, then annealed in a muffle furnace at 500°C for 2 h in Ar atmosphere with heating rate of 2°C min^-1. A brief summary of the synthesis procedure is shown in Scheme 1.

2.2. Characterization

The crystal structure of the products were determined by X-ray powder diffraction (XRD, Cu Kα radiation; λ = 1.5408 Å) with a SIEMENS D5000 X-ray diffractometer. The morphology and microstructure were characterized by transmission electron microscopy (TEM; JEOL, 2010) and scanning electron microscopy (SEM; Hitachi S4800) equipped with an energy-dispersive X-ray spectrometer (EDX).

2.3. Electrochemical Measurement

The assembly of the test cells (CR 2025-type) was performed in an argon-ﬁlled glove box with water and oxygen contents less than 1ppm before measurement. The anode electrode consisted of 80 wt% active material, 10 wt% conductivity agents (super carbon black), and 10 wt% binder (carboxyl methyl cellulose). One molar LiPF₆ in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (EC : DMC : EMC = 1 : 1 : 1) was used as the electrolyte. The electrochemical measurements were carried out with a multichannel current static system (Arbin Instruments BT 2000, ) after the cells were aged for 8 h. The electrochemical performances of the cells were evaluated within the potential range of 1.0 – 3.0 V versus Li/Li⁺.

3. Results and Discussion

3.1. Structural and Morphological Characterization

Figure 1a and b display low- and high-magnification TEM images of the commercial TiN nanoparticles. In Figure 1a and b, it can be found that the particles have a average size of about 20 nm. Figure 1c shows the low-magnification TEM image of TiN@C nanocomposites, which displays particle-like morphology with coating shells as indicated with black arrows. The high-magnification TEM image of TiN@C nanoparticle Figure 1d clearly exhibits that TiN nanoparticles are coated, and the coating carbon shell has a thickness of about 3 nm, as shown in the inset of Figure 1d. In Figure 1d, the intimate contact between the amorphous carbon layer and TiN nanocrystals could be observed, ruling out the possible increasing of contact resistance.

To confirm the crystal structure and phase of our TiN samples, the XRD characterization was performed. The XRD patterns of TiN and TiN@C samples are shown in Figure 2, respectively. In Figure 2a, the XRD pattern of commercial TiN nanoparticles can be indexed to the cubic crystal structure, which is well accorded with the standard card (JCPDS card No: 38-1420) without any obvious impurities. The XRD pattern of TiN@C nanocomposites in Figure 2b is similar with that of the commercial TiN sample. The carbon diffraction peaks could not be found in the pattern of TiN@C, which might be of the low content of C in TiN@C and its inferior crystallinity. EDX characterization of TiN@C Figure S1 shows the peaks of C, Ti, N, O and Si (arising from the silicon substrate), which further confirm the existence of carbon element. The content of carbon element is about 20% in TiN@C nanocomposites.

Figure 1. TEM images of the samples: (a and b) commercial TiN nanoparticles; (c and d) TiN@C nanoparticles

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Figure 2. XRD patterns of the samples: (a) commercial TiN nanoparticles; (b) TiN@C nanoparticles

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3.2. Electrochemical Properties

Inspired by the excellent characteristics of both titanium-based materials and carbon coating structures, we tested the electrochemical properties of TiN@C nanocomposites as anodes for LIBs. In this study, the specific capacity and cycling stability of electrodes were measured by constant current charge/discharge testing. The cycling stability for TiN and TiN@C electrodes are shown in Figure 3c. Compared with TiN electrodes, it is clear that the TiN@C electrodes deliver better cycling performance. The capacity of TiN@C has nearly no lost after 20 cycles while the capacity of TiN decrease gradually in all 200 cycles. The corresponding voltage profiles are plotted in Figure 3a and b. The initial capacity of TiN@C is 157 mA h g^-1, and decreases gradually to 76 mA h g^-1 after 200 cycles, while the capacity of TiN decreases from 127 to 54 mA h g^-1. The above results demonstrate that the cycling performance of TiN@C nanocomposite is much better than TiN.

Figure 3d shows rate performances of TiN and TiN@C electrodes at different current densities between 1.0 V and 3.0 V. It can be seen that TiN@C electrodes exhibit a stable capacity and good rate behaviors, while the capacity of TiN is lower than TiN@C electrodes. For the TiN@C electrodes, a specific discharge capacity of 87 mA h g^-1 is obtained at a rate of 50 mA g^-1 after 6 cycles; the discharge capacities of 80, 75, 70, 64, 56 mA h g^-1 is observed at 100 mA g^-1, 200 mA g^-1, 500 mA g^-1, 1000 mA g^-1, 2500 mA g^-1, respectively. For comparison, the discharge capacity of TiN electrodes at the rate of 100 mA g^-1, 200 mA g^-1, 500 mA g^-1, 1000 mA g^-1, 2500 mA g^-1 are 85, 71, 60, 53, 45, 34 mA h g^-1, separately. When the rate was brought back to 50 mA g^-1 after 6 cycles at 2500 mA g^-1, the discharge capacity of TiN@C electrode could recover to 80 mA h g^-1 and remain. On the contrary, the commercial TiN electrodes show lower capacity (69 mA h g^-1), and their capacity displays the trend of decay. The above results demonstrate that TiN@C composites are excellent anode materials for LIBs.

Figure 3. Electrochemical performance of TiN and TiN@C materials: (a) voltage profiles of TiN and TiN@C; (c and d) the corresponding cycling performance test at the 50 mA g^-1 rate and rate capability

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In order to understand the effects of carbon shells on the electrochemical properties of TiN nanoparticles, we examined their morphology change after electrochemical cycles. They were investigated on the copper foils by SEM. Figure 4a and 4c show the SEM photos of TiN before and after 200 cycles testing, respectively. The SEM images of TiN@C before and after 200 cycles are shown in Figure 4b and Figure 4d, respectively. Nearly all the TiN@C nanoparticles are unchanged on the battery electrodes and their SEI films are dense and slick, indicating that the composites are stable and strong enough to withstand the casting. On the other hand, an obvious change in morphology is deserved for TiN nanoparticles, which are coated by rough films Figure 4c. Hence, the SEI films on TiN@C are more stable than those on TiN nanoparticles. These results indicate that the carbon shells of TiN@C nanocomposites could effectively protect the active materials.

Here, the excellent lithium storage properties of TiN@C could be explained as follows: i) carbon shells enable the composite electrode to show a long cycle life by preventing the TiN from volume change and pulverization during the charge-discharge process. ii) the carbon coating layer can effectively enhance the electronic conductivity of electrode material, which results in improved rate performance. The enhanced cycling and rate performance implies that this type of electrode can be a promising candidate for LIBs.

4. Conclusion

In summary, TiN@C nanocomposites have been synthesized by an annealing approach using the oleic acid as carbon resource. When used as the anode materials of rechargeable LIBs, TiN@C nanocomposites could exhibit a high specific capacity, the improved cycling performance and enhanced rate capabilities. The excellent properties could be due to the effective carbon coating. Our results suggest coating TiN nanoparticles with carbon is a reliable strategy to improve the electrochemical properties of TiN anode material.

Figure 4. (a and b) SEM images of the commercial TiN and TiN@C materials as anode materials on copper foils; (c and d) SEM images of the commercial TiN and TiN@C in a fully lithiated state after 200 cycles

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