Electrochemical Properties of Li2FeP2O7/C Nanocomposites Prepared from LiH2PO4 and Fe(NO3)3·9 H2o used As a Precursor

Introduction

Application of lithium-ion battery has been extended from mobile phones, laptop, and portable electronic devices to large scale electric devices with its improvement of capacity and cycle life. Adapting to the electric vehicle (EV) or energy storage system (ESS) as an electrical power source, there are required cathode materials which are safety, low cost and environmentally benign. Thus far, several studies on crystal structure of poly-anion compounds with Fe (II) as cathode materials for lithium-ion battery have been reported [1,2]. Furthermore, synthesis of Li₂FeP₂O₇/C composites has been introduced with various synthesis methods, such as solid state reaction [3], sol-gel [4] and splash combustion method [5]. In this study, we have prepared Li₂FeP₂O₇/C nanocomposites  from LiH₂PO₄ and Fe(NO₃)₃¥9H₂O used as a precursor by a novel preparation method, and then investigated their electrochemical properties.

 

Experimental

The precursor solution was prepared by dissolving stoichiometric amounts of LiH₂PO₄ and Fe(NO₃)₃¥9H₂O in distilled water and then atomized at a frequency of 1.7MHz using an ultrasonic nebulizer. The sprayed droplet were transported to a reactor using a 3% H₂+N₂ gas with a gas flow rate of 1 L min^-1 , heated in the range of 600 to 800 ^oC and converted into solid particles through the evaporation of the solvent, the precipitation of the solute,  drying and  thermal decomposition within a laminar flow aerosol reactor. The resulting powder was collected at the reactor exit using an electrostatic precipitator operated at 150 ^oC. The powder was then milled with acetylene black (AB) in ethanol by high-energy planetary ball milling, and then annealed to obtain the crystalized Li₂FeP₂O₇.

The crystalline phase of the samples was studied by X-ray diffractionanalysis using Cu-Kα radiation. The surface morphology of the samples was examined by scanning electron microscopy (SEM, Keyence YE-8800) at 8 kV.

The electrochemical performance of Li₂FeP₂O₇/C nanocomposite was investigated using coin-type cells (CR2032). A 1 mol dm^-3 LiPF₆ solution in a solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in 1:1 volume ratio was used as the electrolyte. The cathode consisted of 70 wt. % Li₂FeP₂O₇/C, 10 wt. % polyvinylidene fluoride (PVdF) as a binder and 20 wt. % acetylene black. The cells were cycled in a constant current-constant voltage mode at a 0.05 C rate (where 1 C = 110 mA g⁻¹) to 4.3 V, held at 4.3 V until C/100, and then discharged to 2.0 V at a 0.05C rate.

Results and discussion

Fig. 1 shows the XRD patterns of the samples prepared by spray pyrolysis (SP) at different synthesis temperatures and then annealed at 600 ^oC. The XRD patterns of the samples prepared by SP above 700 ^oC were indexed to the pure-phase of Li₂FeP₂O₇.

SEM images in Fig. 2 shows the morphology of the Li₂FeP₂O₇/C composite sample prepared at 800 ^oC by SP with wet ball milling (WBM) and then annealed at 600 ^oC for 2 h. While the spray pyrolysis sample consisted of spherical particles with approximately 1 μm in size, primary particles with several hundred nm could be observed, as shown in Fig. 2, which may indicate the reducing particle size of Li₂FeP₂O₇ in the WBM process. As a result, we could conclude from these results that Li₂FeP₂O₇/C nanocomposite was synthesized by a novel preparation method, i.e., a combination of SP and WBM with heat treatment.

The Li₂FeP₂O₇/C nanocomposite prepared at 800 ^oC by SP with WBM and then annealed at 600 ^oC for 2 h was used as a cathode active material of lithium battery and then the cell performance test was carried out in a room temperature. Fig. 3 shows the charge-discharge curves of the cell. The cell exhibited an initial discharge capacity of 108 mAh g^-1 at the discharge rate of 0.05C, which corresponds to 98% of its theoretical capacity (110 mAh g^-1). Also, it shows a discharge capacity of 101 mAh g^-1 at 5th cycle. The Li₂FeP₂O₇/C composite cathode exhibited a wide potential plateau at approximately 3.4 V vs Li.

References

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