Elsevier

Journal of Power Sources

Volume 194, Issue 2, 1 December 2009, Pages 1029-1035
Journal of Power Sources

Physical and electrochemical properties of LiFePO4 nanoparticles synthesized by a combination of spray pyrolysis with wet ball-milling

https://doi.org/10.1016/j.jpowsour.2009.06.046Get rights and content

Abstract

A novel preparation technique was developed to synthesize LiFePO4 nanoparticles through a combination of spray pyrolysis (SP) with wet ball-milling (WBM). Using this technique, the preparation of LiFePO4 nanoparticles was investigated for a wide range of process parameters such as ball-milling time and sintering temperature. The effect of process parameters on the physical and electrochemical properties of LiFePO4 was then discussed through analysis using by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), the Brunauer–Emmet–Teller (BET) method, Raman spectroscopy and using an electrochemical cell of Li|1 M LiClO4 in EC:DEC = 1:1|LiFePO4. LiFePO4 nanoparticles with a geometric mean diameter of 58 nm were prepared at a rotating speed of 800 rpm and a ball-milling time of 12 h in an Ar atmosphere followed by heat treatment at 500 °C for 4 h in a N2 + 3% H2 atmosphere. The sample delivered first discharge capacities of 164 and 100 mAh g−1 at charge–discharge rates of 0.1 and 10 C in the test cells, respectively. The electrochemical properties of LiFePO4 nanoparticles were strongly affected by the formation of Fe2P, Fe3P and α-Fe2O3 at higher charge–discharge rates.

Introduction

In recent years, the demand for the development of new cathode materials for high-performance lithium-ion batteries has increased continuously. LiFePO4 is one of the most promising cathode materials for lithium-ion batteries owing to its relative lack of toxicity and the low cost and abundance of the raw materials. It also has a high lithium intercalation voltage of 3.4 V versus lithium metal and a high theoretical capacity of 170 mAh g−1 [1], [2]. However, the performance of this material is limited by its poor electronic conductivity, which is a barrier preventing its large-scale application such as in hybrid electric vehicles (HEVs). Much effort has been developed to improving the electrochemical property of LiFePO4 by reducing the particle size [3], [4] or by coating LiFePO4 particles with carbon [5], [6], [7] or a LiFePO4/C composite [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. As a result, the procedure of synthesis is becoming increasingly important, particularly when a reduced LiFePO4 particle size and a carbon coating are required. Also, the formation of impurity phases such as α-Fe2O3 must be avoided to achieve full theoretical capacity [20]. To overcome the poor electronic conductivity, many synthesis routes have been developed, involving high-temperature solid-state reactions [1], [2], [9], [10], [11], [12], [21], [22], [23], [24], [25], the polyol process [3], the sol–gel process [8], [13], [21], [26], hydrothermal synthesis [20], [21], [27], [28], [29], mechanical activation [6], [18], [30], [31], [32], [33], [34], [35], [36] and co-precipitation [37]. The majority of these synthesis routes require high sintering temperatures, long sintering times and several grinding steps. It is also difficult to precisely control the chemical composition of the as-prepared materials for the hydrothermal synthesis.

SP is a well-known continuous and single-step method for the preparation of fine homogeneous and multicomponent powders. Compared with particles obtained by conventional ceramic preparation methods, the particle size distribution is narrow and controllable from micrometer to submicrometer order, the purity of the products is high and the composition of the powders is easy to control. Even if the post-annealing of as-prepared powders by SP is required to obtain the desired materials, a shorter annealing time of the as-prepared powders can be expected in comparison with conventional ceramic preparation methods. In the contrary, several long sintering and regrinding procedures are needed to obtain the final product in the solid-state reaction method [25].

In our previous studies [38], [39], a LiFePO4/C composite and carbon-coated LiFePO4 were prepared via SP and a combination of SP with dry ball-milling (DBM), respectively. However, the electrochemical performance of the prepared materials was not sufficient for large-scale application because the size of the LiFePO4 particles was approximately 300 nm [39]. The reduction of particle size is a key factor for obtaining LiFePO4 with a high rate capability. Thus, we present a novel preparation technique for LiFePO4 nanoparticles in this work.

Section snippets

Preparation of LiFePO4 nanoparticles

The precursor solution was prepared by dissolving the correct amount of Li(HCOO)·H2O, FeCl2·4H2O and H3PO4 in distilled water in a stoichiometric ratio. The concentrations of Li+, Fe2+and PO43− were all 0.2 mol dm−3. The pH of the precursor solution was adjusted to 1.9 by adding HCl.

A schematic diagram of the SP setup was provided in our previous paper [40]. The precursor solution was atomized at a frequency of 1.7 MHz using an ultrasonic nebulizer. The sprayed droplets were transported to the

Preparation and characterization of LiFePO4 nanoparticles

Fig. 2 shows the XRD patterns of samples synthesized by SP at 500 °C and then ball-milled at 800 rpm for various times from 6 to 24 h followed by heat treatment at 500 °C for 4 h in a N2 + 3% H2 atmosphere. The JCPDS standard LiFePO4 patterns are also shown in the figure. The diffraction peaks of all samples are identified as those of the orthorhombic structure with space group Pnma without any secondary phases such as Fe2P and Fe3P.

Fig. 3 shows the variation of the specific surface area of the LiFePO4

Conclusions

LiFePO4 nanoparticles were successfully prepared by the combination of SP with WBM at 800 rpm for 12 h in Ar gas followed by heat treatment at 500 °C for 4 h in a N2 + 3% H2 atmosphere. XRD patterns of LiFePO4 nanoparticles were assigned to an ordered olivine structure indexed by orthorhombic Pnma. TEM observation demonstrated that LiFePO4 particles with a geometric mean diameter of 58 nm were obtained by the present method. The LiFePO4 nanoparticles exhibited first-discharge capacities of 164 mAh g−1

Acknowledgments

This research was partially supported by Development of an Electric Energy Storage System for Grid-connection with New Energy Resources in New Energy and Industrial Technology Development Organization. The authors are grateful to the staff members (Mr. A. Genseki and Mr. J. Koki) of the Center for Advanced Materials Analysis (Tokyo Institute of Technology, Japan) for help in the TEM observation of as-prepared powders. Moreover, the authors gratefully acknowledge the assistance of Ms. M. Saigou

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