Electrochemical behaviors of solid LiFePO4 and Li0.99Nb0.01FePO4 in Li2SO4 aqueous electrolyte

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Abstract

LiFePO4 and Li0.99Nb0.01FePO4 with olivine structure were synthesized by an in situ synthesis technique. Their electrochemical behaviors in Li2SO4 aqueous electrolytes were investigated by cyclic voltammetry (CV). It shows that both compounds undergo lithium ion extraction and intercalation upon oxidation and reverse reduction at the safe potential window without causing the kinetic electrolysis of water. For LiFePO4, only one pair of symmetrical redox peaks, which are associated with the Li+ ion extraction/insertion upon the oxidation/reduction of Fe2+/Fe3+ redox couple, appears on its CV curves at both low (0.10–1.0 mV s−1) and medium (5.0–50 mV s−1) scan rate ranges. For Li0.99Nb0.01FePO4, various electrochemical behaviors were observed at different scan rate ranges. Two pairs of redox peaks, which are broad and sharp, respectively, appeared at the low scan rate range, but the broad peak pair disappeared at the medium scan rate range. Further study found that the reactions happening at the sharp peak pair are independent of the reactions occurring at the broad one. Various scan rate experiments revealed a linear relationship between the peak current and the square root of scan rate for all peak pairs, indicating that the Li+ deintercalation/intercalation processes occurred in both compounds are diffusion-controlled. The corresponding diffusion coefficients were calculated in the range of 10−11–10−12 cm2 s−1.

Introduction

Rechargeable lithium ion batteries have been considered as an attractive power source for a wide variety of applications, such as cellular phones, notebook computers, camcorders and hybrid electric vehicles (HEV) due to the high energy density, high working potential and long life [1]. On the other hand, the lithium ion batteries are limited by several drawbacks such as the severe safety problems, the economic and environmental problems [2], [3], [4]. The flammable organic electrolytes used in lithium ion batteries may cause smoke or fire in the case of improper use such as overcharge or short-circuit. Moreover, lithium ion batteries are relatively expensive, because of the special cell manufacturing and the costly nonaqueous electrolytes. These disadvantages as mentioned above urge the development of less expensive and environmentally benign energy-storage materials and devices, such as rechargeable lithium ion batteries with an aqueous electrolyte [5], [6] and the new concept hybrid aqueous supercapacitors [7], i.e., the carbon/LiMn2O4 aqueous system [8].

With the development of electrochemical energy-storage devices using aqueous electrolytes, research on the aqueous electrochemistry of different intercalation compounds, such as LiV3O8 [9], LiMn2O4 [10], [11], LiCoO2 [12], LiNiO2 [13], and MnO2 [14], [15], [16], has attract attention recently to evaluate their fundamental performances as electrode materials in aqueous devices. These intercalation compounds behave various electrochemical properties in different aqueous electrolytes. Kohler et al. [9] reported the reversible Li+ deintercalation/intercalation from/into LiV3O8 in 1 M Li2SO4 aqueous electrolyte. Scholz et al. [10], [11], [12], [13], [16] found the proton insertion behavior of LiNiO2, LiCoO2, and LiMn2O4 in alkaline solutions and the reversible insertion of K+, Li+, and NH4+ ions for LiMn2O4 in neutral aqueous electrolytes. More recently, Minakshi et al. [17] investigated the redox behavior of LiFePO4 in LiOH aqueous electrolyte and found that LiFePO4 undergoes partially reversible oxidation/reduction with lithium ion extraction from LiFePO4 upon oxidation.

Olivine-structured lithium iron phosphate (LiFePO4), an intercalation compound proposed recently by Goodenough and co-workers [18], [19], has been gaining particular interest as a promising cathode material for rechargeable lithium-ion batteries from both economic and environmental points of view [20], [21], [22]. Recent results suggest that, Nb-doped LiFePO4 electrode has good electrochemical performance, due to the 8-fold order of magnitude increase in conductivity [30]. Moreover, the striking structure similarity and smaller volume change of 6.81% between the LiFePO4 and FePO4 bring excellent reversibility of cells with LiFePO4 cathode [18]. By far, the structure and electrochemistry of lithium iron phosphate with nonaqueous electrolytes has been extensively investigated [23], [24], [25]. Lithium can be extracted from or inserted into olivines by electrochemical or chemical redox reactions [26], [27]. Reversible Li-ion deintercalation/intercalation occurs upon oxidation/reduction of Fe2+/Fe3+ redox couple in LiFePO4/FePO4 along with a flat potential plateau at 3.5 V vs. Li/Li+ in nonaqueous lithium-ion battery electrolytes.

More recently, the electrochemistry of LiFePO4 in aqueous electrolytes has attracted the attention of only a few authors [17]. To our knowledge, there is no report on the electrochemistry of solid LiFePO4 and Li0.99Nb0.01FePO4 in Li2SO4 aqueous electrolyte by cyclic voltammetry in detail. In this paper, LiFePO4 and Li0.99Nb0.01FePO4 were synthesized by an in situ synthesis technique [28] and their electrochemical behaviors in Li2SO4 aqueous electrolytes were investigated by cyclic voltammetry. Study on the cyclic voltammogram behavior of lithium iron phosphate is essential, because it is a preliminary step to understand the extent of electrochemical reversibility of a redox reaction and to evaluate the feasibility of the material as an active battery electrode.

Section snippets

Experimental

LiFePO4 and Li0.99Nb0.01FePO4 were prepared by an in situ synthesis technique described in [28]. The synthesis procedures are described simply as follows: stoichiometric amounts of FePO4 · 4H2O (11.14 g) and LiOH · H2O (2.098 g) were ballmilled for 2 h in a planetary miller with nylon vessels and nylon balls, using a ball-to-powder weight ratio of 20:1 and a rotation speed of 200 rpm. The milled powders mixed with polypropylene (1.840 g) were fired in a tube furnace at 700 °C for 10 h and LiFePO4 was

Results and discussion

The typical XRD patterns of LiFePO4 and Li0.99Nb0.01FePO4 samples synthesized at 700 °C are shown in Fig. 1. Comparison with the diffractograms shows that there is almost no difference between doped and undoped samples. Main peaks for these prepared samples are labeled with h k l indexes. Two diffraction profiles are identified to the ordered olivine structure and indexed by the space group of orthorhombic Pnma, in which the Li ions occupy the octahedral sites (4a); Fe atoms occupy the

Conclusion

The detailed cyclic voltammetry investigation revealed that solid LiFePO4 and Li0.99Nb0.01FePO4 undergo lithium ion deintercalation/intercalation in Li2SO4 aqueous electrolytes at the safe potential window without H2O decomposition. For LiFePO4, only one pair of redox peaks, which are associated with the Li+ ion extraction/insertion upon the oxidation/reduction of Fe2+/Fe3+ redox couple, appears on its CV curve at both low and medium scan rate range. The magnitude of scan rate has an important

Acknowledgement

This work was supported by the National Natural Science Foundation of China (No. 60471014, No. 20403014, No. 20633040), Natural Science Foundation of Jiangsu Province (BK2006196), and Jiangsu Planned Projects for Postdoctoral Research Funds (No. 0601008A).

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