A PEG assisted sol–gel synthesis of LiFePO4 as cathodic material for lithium ion cells
Introduction
The olivine LiFePO4 with a large theoretical capacity of 170 mAh g−1, represents a prospective lithium-intercalating cathode material for lithium ion batteries. LiFePO4 offers benefits such as inexpensive, natural abundance and environmentally benign, thermal stability in the fully charged state and good cycle stability. Reversible electrochemical extraction of lithium ions from LiFePO4 proceeds at about 3.5 V versus Li. The voltage maintains the integrity of electrolyte as well as the appropriate energy density. However, it was reported that this cathode has very low electronic conductivity and diffusion-controlled kinetics associated with the two-phase character of the insertion/extraction process [1], which decreases utilization in the charge–discharge capacity of active material. Recently, the synthesis of a LiFePO4/electronic conductor composite compound [2], [3], [4], [5], [6], [7], [8], [9], [10] and doping [11], [12], [13] have been used to increase the electronic conductivity. As pointed out by Huang et al. [14], poor Li+ ions diffusion can be overcome by decreasing the particle size of the active material.
The electrochemical behavior of LiFePO4 is strongly influenced by preparation method, which includes different precursors and heat treatment protocols. LiFePO4 has conventionally been synthesized by diffusion-limited solid-state reactions with their associated repeated grinding and a longer period of high temperature operations. Because of several disadvantages of this method such as larger particle size, broader particle size distribution, and the inhomogeneous and random distribution of carbon surrounding the active particles, numerous synthetic approaches were developed to obtain the title compound in its pure and conductive form [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32].
In this sol–gel synthesis, citric acid was used as a chelate agent to promote metal ions dispersion on atomic or molecular level. In addition, citric acid also acted as a reduce agent, which could convert iron(III) salts to iron(II) salts, in situ. The simplified process avoid pre-sintering as well as oxidation of Fe2+. Due to outstanding physio-chemical and biological properties such as hydrophilicity, solubility in water and in organic solvents, lack of toxicity, and absence of antigenicity and immunogenicity, PEG has been used for many biomedical and biotechnological applications [33]. In this paper, PEG was firstly used to prepare fine-particle LiFePO4 material and PEG played key influence on the morphology of LiFePO4 products, brought the superior electrochemical performance of LiFePO4.
Section snippets
Experimental
The stoichiometric amounts of H3PO4, Fe(NO3)3, CH3COOLi and citric acid (mole ratio of citric acid and total cations is 1:2) were dissolved in distilled water. After addition of required amount of PEG (mean molecular weight of PEG is about 400, mole ratio of PEG and product LiFePO4 described as nPEG/nLFP varying from 0.5:1 to 2:1), the pH of the mixture was adjusted to 8.5–9.5 by adding ammonia. Then this mixture was stirred slowly until a clear solution formed and the solution was heated under
Influence of synthetic conditions on LiFePO4 capacities
Fig. 1 shows the first charge–discharge curves of samples synthesized under 600 °C with and without PEG (nPEG/nLFP = 1:1) at the current of 15 mA g−1. All profiles exhibited extremely flat operating voltage around 3.45 V (versus Li/Li+). A flat charge–discharge profile over a large range indicates that a two-phase Fe3+/Fe2+ redox reaction proceeds via a first-order transition between FePO4 and LiFePO4 [1]:As shown in Fig. 1, the PEG synthesis sample delivers an initial
Conclusion
A PEG assisted sol–gel method is used to manufacture LiFePO4/C composite with uniform and fine particles. Owing to the addition of PEG, colloid grains are inhibited from coalescence during the formation of the gel. The LiFePO4/C composite particles possess nanometric dimension size, approximately regular global structure as well as uniform carbon coating, which brings the good capability and the high capacity retention upon cycling. The benign electrochemical performance indicates that the
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