In situ fabrication of Li₄Ti₅O₁₂@CNT composites and their superior lithium storage properties

Abstract

In this paper, Li₄Ti₅O₁₂@CNT composites are fabricated by a controlled in situ growth of CNTs on the surface of Li₄Ti₅O₁₂. The formation of coiled CNTs occurs by the electroless loading of Ni–P catalysts on the surface of Li₄Ti₅O₁₂. Coiled CNTs provide electron bridges interconnecting the Li₄Ti₅O₁₂ particles to form a nano/micro-structured conductive hyper-network. The electronic conductivity of Li₄Ti₅O₁₂@CNT composites is better than that of the sample before coating. As a result, Li₄Ti₅O₁₂@CNT composites show superior lithium storage properties comparable to the pristine Li₄Ti₅O₁₂. Based on electrochemical analysis, it is obvious that Li₄Ti₅O₁₂@CNT composites can deliver reversible charge capacities of 149.2, 102.6, 73.3 and 47.5 mA h g⁻¹ at 0.2 C, 10 C, 20 C and 50 C, respectively.

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1 Introduction

Lithium titanate oxide (Li₄Ti₅O₁₂) is a high performance anode material for high power lithium-ion batteries. Based on a reversible conversion reaction between Li₄Ti₅O₁₂ and Li₇Ti₅O₁₂, a theoretical capacity of 175 mA h g⁻¹ can be delivered by the pristine sample, which corresponds to the redox platforms at around 1.5 V in the charge–discharge curves. Attributed to the low electronic conductivity (10⁻⁹ S cm⁻¹) of Li₄Ti₅O₁₂, the pristine sample shows poor lithium storage properties. Therefore, various doping, mixing and coating techniques are used to improve the electronic conductivity and electrochemical properties of Li₄Ti₅O₁₂.^1–5

For fabricating an effective conductive network, carbon nanotubes (CNTs) or nanofibers (CNFs) have been introduced as electron bridges interconnecting the Li₄Ti₅O₁₂ particles in recent years. X. Li⁶ described the preparation of Li₄Ti₅O₁₂/CNT composites by a simple solid state technique. It is found that the presence of CNTs has great effect on the particle size, surface morphology and electrochemical properties of Li₄Ti₅O₁₂. To fabricate similar nano/micro-structure, commercial CNTs can be dispersed into the Li₄Ti₅O₁₂ particles or precursors by a simple sol–gel method, as reported by J. J. Huang⁷ and H. I. Kim.⁸ Cycled at 5 C, the as-prepared Li₄Ti₅O₁₂/CNT composites show a reversible lithium storage capacity of 142 mA h g⁻¹ after 500 cycle. Besides, K. Naoi⁹ and H. S. Choi¹⁰ also reported the preparation of CNFs@Li₄Ti₅O₁₂ composites with nano-Li₄Ti₅O₁₂ loaded onto commercial CNFs by a sol–gel method and electrospinning technique, respectively. Used as a negative electrode in hybrid supercapacitor, they show higher energy density and power densities comparable to conventional electric double-layer capacitors. In these previous reports, the CNTs are purchased and added into the pristine Li₄Ti₅O₁₂ samples to form nano/micro-structure composites. However, it is difficult to form a well-dispersed conductive hyper-network in the Li₄Ti₅O₁₂/CNT composites with the ex situ introduction of CNTs.

In this paper, we report the in situ fabrication of Li₄Ti₅O₁₂@CNT composites and research on their lithium storage properties. Good conductive pathways interconnecting Li₄Ti₅O₁₂ particles are constructed by in situ formed CNTs with high electronic conductivity. These novel nano/micro-structured composites show lower impedance, higher reversible capacity and better rate performance than those of the pristine Li₄Ti₅O₁₂.

2 Experimental section

Pristine Li₄Ti₅O₁₂ samples are prepared by a citric acid assisted sol–gel method. Tetrabutyl titanate and lithium oxalate are used as the precursors for Li₄Ti₅O₁₂ preparation in the experiment. Li₄Ti₅O₁₂ is formed after a calcination process at 850 °C for 10 h. Li₄Ti₅O₁₂@CNT composites are prepared by a pre-deposition Ni–P alloys on Li₄Ti₅O₁₂ by electroless plating as catalysts and then a thermal hydrolysis process of acetylene at 850 °C for 1 h for CNTs growth. It is similar to the preparation technique to fabricate Si/CNFs composites described in a previous report.¹¹

The phase characteristics of as-prepared samples are identified by a Riguku X-ray diffraction (XRD) instrument. The surface morphologies and structures of samples are observed by a Hitachi S-3400 scanning electron microscopy (SEM) and a JEOL JEM-2100 transmission electron microscopy (TEM). The electronic conductivities of the pristine Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂@CNT composites are measured by a HP4329A high frequency resistance meter at room temperature. Pellets with symmetric silver film coated on both sides as sandwich blocking electrodes are served for electronic conductivity measurements.

The working electrodes for lithium storage analysis are prepared by mixing and casting the as-prepared materials, acetylene black and polytetrafluoroethylene, onto the copper mesh. The working electrode is sealed in a simulated battery and cycled in a LiPF₆-based electrolyte with metal lithium as the counter electrode at room temperature. Galvanostatic charge–discharge cycles are recorded on a multichannel Land battery test system in a potential range between 0.0 and 3.0 V. Electrochemical impedance spectroscopy (EIS) patterns are recorded by a CHI660D electrochemical working station in the frequency range between 100 kHZ and 0.005 HZ with an amplitude of 5 mV.

3 Results and discussion

Fig. 1 shows the XRD patterns of pristine Li₄Ti₅O₁₂, Li₄Ti₅O₁₂@catalysts and Li₄Ti₅O₁₂@CNTs. Based on the JCPDS card No. 26-1198, it is known that the primary Li₄Ti₅O₁₂ sample is a pure spinel-structured compound. After catalyst loading, Ni₈P₃ can be detected on the surface of Li₄Ti₅O₁₂ by a weak diffraction peak appearing at 44.7° in the XRD pattern of the Li₄Ti₅O₁₂@catalysts, which is similar to a previous study.¹² By using Ni₈P₃ as a catalyst, graphitized carbon materials can be formed on the surface of Li₄Ti₅O₁₂. It is confirmed by the new diffraction peak at 26.2° in the XRD pattern of Li₄Ti₅O₁₂@CNTs. This Bragg position contributes to the (111) plane of graphite. In addition, some anatase TiO₂ can be observed after a thermal hydrolysis process of acetylene at 850 °C, which is attributed to the extra diffraction peaks at 25.4°, 38.0°, 48.1°, 54.0° and 55.1°. It indicates that partial Li₄Ti₅O₁₂ can decompose into anatase TiO₂ and Li₂O according to the following equation:

Li₄Ti₅O₁₂ → 5TiO₂ + 2Li₂O (1)

Fig. 1 XRD patterns of pristine Li₄Ti₅O₁₂, Li₄Ti₅O₁₂@catalysts and Li₄Ti₅O₁₂@CNTs.

As reported, carbon coated Li₄Ti₅O₁₂ with high purity can prepared by a high temperature solid state reaction.^13–16 It is found that the decomposition of Li₄Ti₅O₁₂ will not take place using carbon nanotubes as a carbon coating layer.^7,8,16–18 Therefore, the appearance of anatase TiO₂ is probably associated with the existence of catalysts during the thermal hydrolysis process. It induces and increases the evaporation of Li₂O during the high temperature chemical vapor deposition (CVD) reaction.

Fig. 2 shows the surface morphologies of pristine Li₄Ti₅O₁₂, Li₄Ti₅O₁₂@catalysts and Li₄Ti₅O₁₂@CNTs. It is found that pristine Li₄Ti₅O₁₂ sample displays irregular polyhedral particles, as shown in Fig. 2a with an average particle size of 500 nm. With electroless plating of catalysts, a large number of Ni₈P₃ particles with an average particle size of 20–50 nm are attached to the surface of Li₄Ti₅O₁₂, as shown in Fig. 2b. Attributed to the nano-catalysts, coiled CNTs are grown on the surface of Li₄Ti₅O₁₂ with the Ni–P alloys on the top, as shown in Fig. 2c–2f. These coiled CNTs are multi-walled hollow tubes with different diameters. As a result, Li₄Ti₅O₁₂ particles are tightly wrapped by crosslinked CNTs to form Li₄Ti₅O₁₂@CNT composites, which provide a conductive hyper-network for electron transportation. There are lots of macro-sized channels built by crosslinked CNTs for lithium ion transportation in Li₄Ti₅O₁₂@CNTs. Therefore, CNTs coated Li₄Ti₅O₁₂ may show better kinetic properties than Li₄Ti₅O₁₂@C composites with a continuous carbon coating layer. Based on the XRD and SEM results, the formation process of Li₄Ti₅O₁₂@CNTs can be described by the schematic view in Fig. 3. It is expected that in situ fabricated CNTs can provide electron bridges interconnecting the Li₄Ti₅O₁₂ particles to improve the lithium storage properties. As shown in Fig. 3, rapid lithium ion transportation between Li₄Ti₅O₁₂ and Li₇Ti₅O₁₂ can be achieved by crosslinked electron pathways and porous nano/micro-structure.

Fig. 2 SEM images of pristine Li₄Ti₅O₁₂ (a), Li₄Ti₅O₁₂@catalysts (b) and Li₄Ti₅O₁₂@CNTs (c–f).

Fig. 3 Schematic view of in situ formation process of Li₄Ti₅O₁₂@CNTs and their lithium storage mechanisms.

EIS results in Fig. 4a show that the introduction of CNTs on the surface of Li₄Ti₅O₁₂ decreases the direct current resistance and charge transfer impedance for active materials. Furthermore, based on the formula σ = d/AR (where σ is the conductivity, R is the resistance, d is the diameter and A is the specific surface area), the electronic conductivities of Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂@CNT composites are 1.6 × 10⁻⁸ and 3.4 × 10⁻⁵ S cm⁻¹, respectively. As a result, the redox polarization decreases from 90 mV to 30 mV based on the charge–discharge curves at 0.2 C. Furthermore, the kinetic properties of Li₄Ti₅O₁₂@CNTs are also greatly improved by the CNT coating. Cycled at 10 C, the insertion/extraction polarization reduces from 284 mV to 86 mV as shown in Fig. 4b. Usually, the pristine Li₄Ti₅O₁₂ sample always shows poor lithium storage properties at different charge–discharge rates due to the low electronic conductivity. As shown in Fig. 4c, the reversible charge capacities of pristine Li₄Ti₅O₁₂ are 86.2, 34.6, 25.7 and 6.5 mA h g⁻¹ at 0.2 C, 10 C, 20 C and 50 C, respectively. After coated with CNTs, Li₄Ti₅O₁₂@CNT composites show superior electrochemical performance as shown in Fig. 4b and 4c. Cycled at 0.2 C, Li₄Ti₅O₁₂@CNT composites deliver a reversible charge capacity of 149.2 mA h g⁻¹ after 10 cycles. Considering the 15.2 weight percent for electrochemically inactive CNTs in Li₄Ti₅O₁₂@CNT composites, it is obvious that Li₄Ti₅O₁₂ particles deliver an actual capacity same as the theoretical one. Cycled at high rates, Li₄Ti₅O₁₂@CNT composites can show excellent high power behaviors, as shown in Fig. 4b. The reversible charge capacities for Li₄Ti₅O₁₂@CNT composites are 102.6, 73.3 and 47.5 mA h g⁻¹ at 10 C, 20 C and 50 C, respectively. These values are higher than those obtained at the same rates (10 C and 20 C) by H. I. Kim.⁸ This indicates that the existence of CNTs significantly improves the electrochemical properties of Li₄Ti₅O₁₂. Therefore, nano/micro-structured Li₄Ti₅O₁₂@CNT composites fabricated by in situ CNT growth are promising anode materials for high power lithium-ion batteries. It is a general nano/micro-structured M@CNTs system (M = oxide, metal, alloy, etc.) for lithium storage, which is suitable to fabricate other composites as anode or cathode materials for high power lithium-ion batteries. Therefore, this nano/micro-structured system and its preparation technique may have great effect on the rapid development of novel active materials as anode or cathode for high power lithium-ion batteries.

Fig. 4 EIS patterns (a), charge–discharge curves (b) and cycling properties (c) of pristine Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂@CNTs.

4 Conclusions

In the Li₄Ti₅O₁₂@CNT composites, pristine Li₄Ti₅O₁₂ particles are prepared by a citric acid assisted sol–gel method. Based on the loading of nano-catalysts, the coiled CNTs are grown in situ on the surface of Li₄Ti₅O₁₂ by a thermal hydrolysis of acetylene. The crosslinked CNTs build a hyper-networked conductive pathway in the Li₄Ti₅O₁₂@CNT composites. As a result, Li₄Ti₅O₁₂@CNT composites show a lower redox polarization than pristine Li₄Ti₅O₁₂. Attributed to its low electronic conductivity, the pristine Li₄Ti₅O₁₂ sample shows poor lithium storage properties. The cycled charge capacities are merely 86.2, 34.6, 25.7 and 6.5 mA h g⁻¹ at 0.2 C, 10 C, 20 C and 50 C, respectively. After coating with electronically conductive CNTs, Li₄Ti₅O₁₂@CNT composites show superior electrochemical properties. They can deliver reversible lithium storage capacities of 149.2, 102.6, 73.3 and 47.5 mA h g⁻¹ at 0.2 C, 10 C, 20 C and 50 C, respectively.

Acknowledgements

This project is sponsored by National Science Foundation of China (No. 51104092), Key Project of Chinese Ministry of Education (No. 210083) and Qianjiang Talent Project of Zhejiang Province (2011R10089). The work is also supported by K. C. Wong Magna Fund in Ningbo University.

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