Synthesis of Li2FeSiO4/C nanocomposite via a hydrothermal-assisted sol–gel process
Introduction
Since the pioneering work by Nyten and co-workers [1], [2], lithium iron orthosilicate (Li2FeSiO4) has been regarded as a promising candidate of cathode materials for lithium-ion batteries due to its high theoretical capacity (166 mAh·g− 1 for one Li+ exchange and 332 mAh·g− 1 for two Li+ exchange per formula unit), high safety and environmental benignity [3], [4], [5]. Moreover, silicon and iron are among the most abundant elements on earth [6]. However, their poor electronic conductivity and low ionic diffusivity have hampered their extensive applications [7], [8]. To tackle these issues, many strategies have been taken including coating conductive materials (carbon or conducting polymer) [9], [10], doping with transition metal ions [11] or reducing the particle size [12]. Apparently, a combination of an interconnected carbon framework (electronic conductor) and porous structure (high surface area contact with electrolyte) could significantly enhance the electron and Li+ ion transport in the electrode material and improve its electrochemical performance. However, with the addition of a carbon source it is difficult to achieve a uniform carbon dispersion on the surface of nanoparticles [13]. Furthermore, heating treatment usually leads to undesirable particle growth and agglomeration [14], leading to a low reversible capacity especially at high current rates. Thus, it is necessary to explore a facilitated and effective route for the synthesis of Li2FeSiO4 materials with carbon coating to improve electronic and ionic conductivity of the composite.
Among various carbon coating techniques on Li2FeSiO4/C, chemical vapor deposition (CVD) is considered as a good choice to deposit carbon onto the surface of cathode materials which can form a homogeneous carbon layer [13], [15]. Xia et al. [16] reported a solvothermal method in combination with CVD for a Li2FeSiO4/C composite using toluene vapor as the carbon source. At the current rate of 166 mA·g− 1, the Li2FeSiO4/C composite delivers a stable discharge capacity of 158 mAh·g− 1 up to 100 cycles. Recently, our group [17] has successfully synthesized a Li2FeSiO4/C/graphene composite by a novel CVD assisted solid-state route using graphene and ethanol vapor as the conductive network and carbon source, which delivers a discharge capacity of 153.0 mAh·g− 1 at 0.2 C and remains stable. Apparently, the properties of cathode materials are strongly affected by the synthetic method. Compared with the solid-state based methods, the solution-based methods allow the precursors to be homogeneously mixed at the molecular level in the solution, which facilitate the morphology and particle size control of the cathode materials [18], [19], [20]. For instance, Yang et al. [19] synthesized a Li2FeSiO4/C composite via a solution-polymerization approach by using ethylene glycol as a template and cross-linking agent, which shows micron sized particles that consist of nanosized spherical Li2FeSiO4 with uniform size distribution. Mu et al. [20] synthesized hierarchical shuttle-like Li2FeSiO4 using ethylene glycol assisted hydrothermal method and the shuttle particles show an aggregate structure consisting of a large amount of primary nanocrystallites. During their study, the organic additive is the key factor to control the phase and morphology of Li2FeSiO4/C nanocomposite.
In this study, porous Li2FeSiO4/C composite was synthesized via a hydrothermal-assisted sol–gel process. The mechanism of formation of the porous Li2FeSiO4/C was proposed based on the results of Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). Meanwhile, the effects of the hydrothermal process on the structure, morphology and electrochemical performance of the composite were also investigated.
Section snippets
Experimental
The Li2FeSiO4/C was synthesized through a hydrothermal-assisted sol–gel method. First, 0.02 mol lithium hydroxide was dissolved in 20 mL distilled water and the solution was neutralized with acetic acid to a pH value of 7. Afterwards, stoichiometric amounts of tetraethyl orthosilicate (TEOS) (0.01 mol) and iron(III) nitrate hydrate (0.01 mol) were dissolved in 20 mL ethanol and added to the above solution with continuous stirring for 2 h to form a brown, homogeneous sol. During this process the TEOS
Results and discussion
X-ray diffraction (XRD) was used to determine the crystallinity and phase structure of the synthesized materials. Fig. 1 presents the XRD patterns of the Li2FeSiO4/C-HT and Li2FeSiO4/C-NHT. All the diffractions of the Li2FeSiO4/C-HT and Li2FeSiO4/C-NHT can be indexed on the basis of a monoclinic structure with P21 space group without any impurity phase, which is in good agreement with those literatures reported previously [21], [22]. The lattice parameters of Li2FeSiO4/C-HT and Li2FeSiO4/C-NHT
Conclusions
In summary, we have synthesized a nanostructured Li2FeSiO4/C composite via a hydrothermal-assisted sol–gel method. The effects of hydrothermal treatment on the structure, morphology and electrochemical characteristics of Li2FeSiO4/C were investigated comparatively. The composite synthesized by this method exhibits a large surface area and porous nanostructure. Furthermore, a homogeneous nitrogen-doped carbon layer with a thickness of 2–3 nm is coated on the surface of the composite. It is
Acknowledgments
This work was mainly supported by the National Nature Science Foundation of China (Grant No. 21071026).
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