Facile Process and Electrochemical Characterizations of Carbon Nanotube-LiFePO4 Composite As a Cathode for High Rate Lithium Ion Batteries

Abstract:

LiFePO₄ nanoparticles were incorporated with multi-walled carbon nanotubes (CNTs) via a facile one-step at low temperature polyol process. The CNTs were fund to embed into the LiFePO₄ particles that form a network to enhance the electrochemical performance of LiFePO₄ electrode. Structural morphologies of these CNT-LiFePO₄ composites were investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The electrochemical properties were analyzed by charge/discharge testing and cyclic voltammetry. Primary results showed that the CNT-LiFePO₄ composites exhibited enhanced electrochemical performance with a good reversibility and the electronic conductivity.

Introduction:

Lithium-ion batteries have been considered to play an important role in the electric vehicles (HEV) and plug-in HEV (PHEV) in the near future, which require high power density and high energy density. Thus, cathode materials with high rate capability are in demand [1, 2]. LiFePO₄ shows promise as a cathode material for HEVs owing to its high specific capacity and excellent structural stability [3, 4]. However, its poor electronic conductivity and lithium diffusion significantly limit its performance at high rates [5-9]. Therefore, studies on LiFePO₄ have focused on improving its rate capability. Coatings of conductive materials [5-10] has been reported to improve the electronic conductivity. Recent efforts have been made to improve rate performance of the LiFePO₄ by the synthesized nanocomposites using carbon nanotubes (CNTs) [11]. These nano-carbons improve the high rate capability of LiFePO₄, when they are uniformly dispersed in the composite. Herein we report the facile synthesis of CNT-LiFePO₄composites with the CNT network providing a conduction path.

Experimental:

Multi-walled carbon nanotubes (5 wt.% ) were functionalized via the mixed acid method in order to remove the impurities and acquire excellent dispersion in the polyol medium. CNT-LiFePO₄ nanocomposites were obtained by using a polyol process as follows: first, the CNTs were added into 100 ml of ethylene glycol and a homogeneous dispersion was achieved by an ultrasonic process. Then, pristine LiFePO₄ particles (~50 nm) were directly dissolved into the dispersion by mechanical agitation in a three-neck round flask. Lastly, the mixed dispersion was heated to above its boiling point and maintained for 10 h under stirring and refluxing. After cooling down, the products were separated from the solvent via centrifugation. In order to remove the organic residue impurities, the products were washed with deionized water several times. Finally, the CNT-LiFePO₄composite powders were dried at 100 °C for 24 h.  Structural morphology were characterized by X-ray diffraction, scanning and transmission electron microscopy (SEM, TEM). Electrochemical performances were investigated by galvanostatic charge-discharge and cyclic voltammetry (CV) over the voltage between 2.0 and 4.0 V using coin cells (CR2032) at a LAND-CT2001A battery-testing system.

Results:

The morphology of the composites were investigated by scanning electron microscopy (SEM), as showed in Fig.1 that the LiFePO₄ nanoparticles are dispersed uniformly with the CNT network. The electrochemical CVs tests were performed in the voltage of 2.0 and 4.0 V at the scanning rate of 0.1 mV s⁻¹.  A pair of redox peaks is clearly exhibited in the CNT-LiFePO₄ composite cathode, which correspond to the two-phase charge-discharge reaction of the Fe²⁺/Fe³⁺ redox couple. The peak curves are very symmetric. The oxidation and reduction peaks appear at around 3.5 and 3.3 V with the potential gap of 0.2 V, which is less than the value of pristine LiFePO₄ cathode as reported, and suggested that electrode reaction reversibility and the electronic conductivity of these composites are greatly enhanced owing to the incorporation of CNTs.

Acknowledgements:

Financial supports from University of Waterloo President’s Award, Natural Sciences and Engineering Research Council of Canada (NSERC) and Waterloo Institute for Nanotechnology (WIN) are greatly appreciated.

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