Elsevier

Solid State Ionics

Volume 294, 15 October 2016, Pages 15-20
Solid State Ionics

Improved electrochemical properties of solvothermally synthesized Li2FeSiO4/C nanocomposites: A comparison between solvothermal and sol-gel methods

https://doi.org/10.1016/j.ssi.2016.06.014Get rights and content

Highlights

  • Porous Li2FeSiO4/C nanocomposites have been synthesized by sol-gel and solvothermal methods using polymer P123.

  • Solvothermal synthesis of Li2FeSiO4/C results in a smaller average particle size, large surface area and large porosity.

  • Solvothermally prepared Li2FeSiO4/C exhibits excellent stability at high rates.

Abstract

We have synthesized porous Li2FeSiO4/C nanocomposites by two different routes such as sol-gel (Li2FeSiO4/C-SG) and solvothermal (Li2FeSiO4/C-ST) using block copolymer pluronic (P123) as both in-situ carbon source and structure directing agent. Various techniques, like, powder x-ray diffraction, BET nitrogen adsorption–desorption measurement, scanning electron microscopy, transmission electron microscopy, galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the nanocomposites. A comparative study of their structural, electronic and electrochemical properties show that the solvothermally synthesized nanocomposite sample Li2FeSiO4/C-ST-600 (annealed at 600 °C) shows better electrochemical performance compared to the corresponding sol-gel synthesized (Li2FeSiO4/C-SG-600) sample. At a rate of C/30, Li2FeSiO4/C-ST-600 nanocomposite delivered a discharge capacity of ~ 276 mA h g 1 (84% of theoretical capacity) and also exhibited excellent stability at high rates. It retained 95% of its initial discharge capacity (140 mA h g 1) after 100 cycles at 1C, comparable to the recently published data on Mg-doped Li2FeSiO4/C composites. We attribute this enhanced electrochemical behavior of Li2FeSiO4/C-ST-600 due to the formation of porous nanocrystalline (~ 15 nm) composite material with a large BET surface area (~ 100 m2 g 1) resulting in a lower charge transfer resistance (~ 30 Ω) and a higher Li-ion diffusion coefficient (~ 5 × 10 14 cm2 s 1). The present study demonstrates that solvothermal synthesis of Li2FeSiO4/C nanocomposites using P123 as a carbon source is an effective method for improving its electrochemical properties.

Introduction

Rechargeable lithium ion batteries (LIBs) are a key technology in the present energy scenario addressing global energy requirements. Apart from the small scale applications in portable electronics, LIBs have the potential of being used in hybrid electric vehicles and renewable power stations as intermediate energy storage devices [1]. For applications in LIBs, cathode materials with large energy density, high safety and low cost are highly desired. Even though Li-based layered oxide materials such as LiCoO2, Li[Ni0.85Co0.15Al0.05]O2, Li[Ni1/3Co1/3Mn1/3]O2 and the 3-D spinel LiMn2O4 are currently used as cathode materials, there is a great demand for safe and low-cost alternatives to these conventional materials as they pose safety risks due to the release of activated oxygen when they are heavily charged [2]. New compounds containing (XO4)n  polyanions like LiMPO4 and Li2MSiO4 (M = Mn, Fe) are being studied and proposed as cathode materials due to their high thermal stability due to strong Xsingle bondO covalent bond and also due to less release of activated oxygen. Though Li2FeSiO4 has been considered as a cathode material with great potential for use in the next generation LIBs by virtue of its high specific theoretical capacity (330 mA h g 1), low cost, and eco-friendliness [3], [4], [5], [6], [7], its application is still plagued due to its poor electronic and ionic conductivities and low lithium ion diffusion coefficient [8], [9]. Some of the strategies proposed to overcome these limitations are: coating the material with carbon [10], [11], doping with hetero-atoms [12], [13], [14], [15] and scaling of Li2FeSiO4 particle size to nanoregime [10], [16].

Up to now, various synthesis methods have been tried to synthesize Li2FeSiO4 based materials [5], [10], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Recently, Qu et al. [30] synthesized Mg-doped Li2FeSiO4/C using Fe2O3 nanoparticles as Fe source. They have shown that Mg-doped Li2FeSiO4/C can deliver discharge capacity of 190 mA h g 1 at 0.1C and it retains 96% of its capacity after 100 charge-discharge cycles. They claimed that Mg-doping could help decrease the charge-transfer resistance and increase the Li+ diffusion. The enhanced electrochemical performance observed by Qiu et al. [31], in Y doped Li2FeSiO4/C composite system was attributed to its enhanced electronic conductivity, higher lithium ion diffusion coefficient and better structural stability due to proper amount of Y doping in Fe sites and carbon coating.

The carbon coating using surfactants or polymers is one of the most simple and common methods to enhance electronic conductivity and hence the electrochemical performance of Li2FeSiO4 [6], [32], [33], [34], [35], [36]. Nanostructured Li2FeSiO4/C cathode material is successfully synthesized by Du et al. [37] by co-precipitation method using Fe3 + salt as iron source and polyethylene glycol as surfactant. The improved electrochemical performance has been attributed to fast transport of electron and lithium ion due to the formation of nanocrystals of Li2FeSiO4 with in-situ formed carbon network. Reducing the cathode material to nanoscale with large surface area is known to decrease the Li ion diffusion path length [38]. Preparation of hierarchically structured morphologies, such as mesoporous structure, is another effective approach for improving the electrochemical properties [38], [39]. It is known that solvothermal treatment plays a key role in controlling the crystallite size of the Li2FeSiO4 particles and to form the porous structure [40].

The Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) tri-block copolymer P123 (EO20PO70EO20) is used to provide the carbon coating and also to make meso-porous structures of Li2FeSiO4 [6], [32], [33], [34], [35], [36]. In the present work, we have synthesized the porous Li2FeSiO4/C nanocomposites by sol-gel and solvothermal methods with tri-block copolymer P123 as an in-situ carbon source and a structure directing agent, and compare their structural and electrochemical properties. The composites synthesized by solvothermal method (Li2FeSiO4/C-ST) show better electrochemical performance compared to the sol-gel prepared composites (Li2FeSiO4/C-SG). We believe that uniform carbon coating of particles, formed by the in-situ combustion of the surfactant during the heating process, increases the electronic conductivity and also limits the particle growth leading to production of porous Li2FeSiO4/C nanoparticles. Further, the reduction of particle size to nanometers (~ 15 nm) shortens the lithium ion diffusion length and thereby improves the electrochemical performance of the material. Cyclic voltammetry and electrochemical impedance spectroscopy are used to understand the lithium diffusion process.

Section snippets

Materials and methods

Li2FeSiO4/C was synthesized by two different methods: sol-gel and solvothermal. All chemicals used in the synthesis were procured from Sigma-Aldrich and were used without further purification. In a typical synthesis of Li2FeSiO4/C by the sol-gel method, lithium acetate (1.0202 g), ferric nitrate (2.02 g), silicon acetate (1.32 g), and P123 (1 g) were dissolved separately in ~ 20 ml of absolute ethanol and then transferred to a three neck flask to form precursor solution. The whole precursor solution

Characterization

The phase purity and the structure of the samples were confirmed using x-ray diffraction (XRD). A Rikagu miniflex table top x-ray diffractometer was used to record XRD patterns using Cu-Kα radiation (λ = 1.54 Å). The morphology was determined using a scanning electron microscope (JSM - 6510LV, SEM) and transmission electron microscope (JEOL 2010 TEM) operated at 200 kV. The surface area of the composites was determined by Brunauer–Emmett–Teller (BET) N2 adsorption–desorption measurement

Results and discussion

The XRD patterns of Li2FeSiO4/C nanocomposites studied in this work are shown in Fig. 1. No evidence of any secondary phase is observed in samples heated at 600 and 650 °C for 9 h. However, a weak diffraction peak at ~ 44.8° due to the presence of small amount of Fe metal is observed in the XRD pattern of 700 °C (9 h) annealed samples, synthesized by both sol-gel and solvothermal methods. This may be due to reducing nature of polymer P123 at high temperature. XRD profiles of all heated samples are

Conclusions

In summary, we have synthesized nanoparticles of porous Li2FeSiO4/C composites by sol-gel and solvothermal methods using tri-block copolymer (P123) as both carbon source and surfactant and compared their structural, morphological and electrochemical properties. The heating of the Li2FeSiO4-polymer (P123) composite at high temperature creates porous Li2FeSiO4/C cathode materials with uniform carbon coating which improves the electron mobility leading to improved electrochemical properties. At

Acknowledgments

This work was supported by Richard Barber Foundation.

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