Introduction
Since the commercialization of lithium-ion batteries in the 1990s, they have been widely utilized for energy storage in mobile electric devices owing to their large gravimetric and volumetric energy densities. Recently, attention has turned to the use of them as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs). Large-scale batteries require safety, low-cost, long cycle-life, and high energy and power densities. Furthermore, it is well known that the cathode materials have a significant impact on battery capacity, cycle life, safety and cost. Recently, pyrophosphates (Li2MP2O7, M=transition metal), the polyanion compounds with 3D crystal structure, have come into sight as a new candidate of cathode materials [1, 2].
Thus far, we have investigated the synthesis of LiMPO4/C (M=Fe, Mn and Co) nanocomposites by a combination of aerosol and powder technologies followed by heat treatment, and then reported that the composite electrodes showed a good electrochemical performance [3-5]. In this work, the synthesis of Li2FeP2O7/C nanocomposites has been prepared by a combination of spray pyrolysis (SP) and wet ball milling (WBM) followed by heat treatment and their physical and electrochemical properties have been also studied.
Experimental
The precursor solution used in this study was prepared by dissolving stoichiometric amounts of LiNO3,H3PO4 and Fe(NO3)3¥9H2O in distilled water and then atomized at a frequency of 1.7MHz using an ultrasonic nebulizer. Ascoribic acid was added into the precursor solution as a reducing agent. The sprayed droplet were transported to a reactor using a 3% H2+N2 gas with a gas flow rate of 4 L min-1 , heated at 800 oC and converted into solid particles. The resulting particles were then milled with acetylene black (AB) in ethanol by high-energy planetary ball milling, and then annealed at 600 oC for 2 h in a 3% H2+N2 atmosphere to obtain the desired material.
The crystalline phase of the samples was studied by X-ray diffraction (XRD) analysis using Cu-Kα radiation. The surface morphology of the samples was examined by scanning electron microscopy (SEM) 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 (Tomiyama Pure Chemical Co., Ltd.) 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 (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 800 oC (a) and then annealed at 600 oC for 2 h in a 3%N2+H2 gas atmosphere (b). The XRD pattern of the sample prepared by SP with annealing was indexed to the pure-phase of Li2FeP2O7.
Fig. 2a shows the SEM image of Li2FeP2O7 prepared by SP and then annealed at 600 oC. The final sample consists of spherical particles with approximately several micrometers in size. The charge-discharge curves of the Li2FeP2O7/Li cell were shown in Fig. 2b. The discharge capacity at 1st cycle was 64 mAh g-1, which corresponds to 58% of its theoretical capacity (110 mAh g-1). This fact may be due to its low electronic and iron conductivities.
The synthesis of Li2FeP2O7/C nanocomposite was carried out using a combination of SP at 800 oC and WBM with heat treatment annealed at 600 oC. Fig. 3 shows the charge-discharge curves of the Li2FeP2O7/C nanocomposite /Li cell. The cell exhibited an initial discharge capacity of 100 mAh g-1, which corresponds to 91% of its theoretical capacity. Also, it shows a discharge capacity of 96 mAh g-1 at 5th cycle. The Li2FeP2O7/C composite cathode exhibited a wide and flat potential plateau at approximately 3.5 V vs Li. These facts may indicate that the WBM process is an effective way to improve the electrochemical properties of Li2FeP2O7 electrode.
References
L. Adam, A. Guesdon, and B. Raveau, J. Solid State Chem., 181, 3110( 2008).
P. Barpanda, T. Ye, S.-C. Chung, Y. Yamada, S. Nishimura, and A. Yamada, J. Mater. Chem., 22, 13455(2012).
M. Konarova and I. Taniguchi, J. Power Sources, 195, 3661-3667(2010).
Z.Bakenov and I. Taniguchi, Electrochem. Commun., 12, 75-78(2010)
T. N. L. Doan and I. Taniguchi, J. Power Sources, 195,5679-5684(2011).
Figure 1
