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 composites from lithium dihydrogen phosphate and iron nitrate used as a precursor by a combination of spray pyrolysis and wet ball milling followed by heat treatment, and then investigated their electrochemical properties.

Experimental

Li₂FeP₂O₇/C composites were prepared by a combination of spray pyrolysis and wet ball milling followed by heat treatment. Stoichiometric amount of lithium dihydrogen phosphate and iron nitrate was dissolved into distilled water. Ascorbic acid was added to the precursor solution as a reduction agent. The precursor solution was introduced to an ultrasonic nebulizer that works at a frequency of 1.7MHz. The sprayed droplets were transported to a laminar flow aerosol reactor by a carrier gas and converted into solid particles through the process of evaporation of a solvent, precipitation of solute drying and thermal decomposition within the reactor. The obtained powders were collected by an electrostatic precipitator. The as-prepared powders were mixed with  acetylene black (AB) in ethanol by high-energy planetary ball milling and then annealed at 650°C  for 2 h in a 3%H₂+ Ar atmosphere. The obtained samples were identified as pure-crystallinity with monoclinic structure by the XRD analysis using Cu-Kα radiation. The electrochemical performance of the samples as cathode materials for lithium battery was investigated by galvanostatic measurements with a coin-type cell (CR2032). The cells were cycled between 2.0 and 4.2 V at different current densities from 0.05 C to 2 C (1C=110 mA g^-1) on multichannel battery testers (Hokuto Denko, HJ1010mSM8A). The current density and specific capacity were calculated from the mass of Li₂FeP₂O₇ in the electrode.

Results and discussion

Fig. 1 shows the XRD patterns of the samples which are prepared by spray pyrolysis with various temperatures and then annealed at 650°C for 2 h. Regardless of spray temperature, all of the XRD patterns are indexed to a monoclinic structure with pure phase of Li₂FeP₂O₇.

An electrochemical performance of the Li₂FeP₂O₇, which was prepared at 600°C by spray pyrolysis and then annealed at 650°C for 2 h, was tested at various charge-discharge rates ranging from 0.05 to 2 C, in which five cycles were carried out at each charge-discharge rate. The results were shown in Fig. 2.  The cell exhibited a first discharge capacity of 78.6 mAh g^-1 at 0.05C and dropped to 23.0 mAh g^-1 at 1C rate.

 The Li₂FeP₂O₇/C composite sample was also tested at the same condition and showed enhanced performance than the bare Li₂FeP₂O₇ sample. The detail results will be reported in the conference.

References

1. L. Adam, A. Guesdon, and B. Raveau, J. Solid State Chem., 181, 3110( 2008).

2. S. I. Nishimura, M. Nakamura, R. Natsui, and A. Yamada, J. Am. Chem. Soc., 132, 13596(2010).

3. J. Du, L. Jiao, Q. Wu, Y. Liu, Y. Zhao, L. Guo, Y. Wang, and H. Yuan, Electrochim. Acta, 103, 219(2013).

4. L. Tan, S. Zhang, and C. Deng, J. Power Sources, 275, 6(2015).

5. P. Barpanda, T. Ye, S.-C. Chung, Y. Yamada, S. Nishimura, and A. Yamada, J. Mater. Chem., 22, 13455(2012).