Improved Electrochemical Properties of MoP2 Anode By Substitution of Si for Lithium Ion Batteries

Introduction

  Recently, metal phosphides have received much attention as anode material for lithium ion batteries due to high reversible capacity at relatively low potential.[1] However, most MP_x suffers from a relatively large irreversible capacity due to the decomposition of Li_xM, Li_xP, and metal during cycling. In order to improve the electrochemical properties, ternary M-M’-P system have been suggested and some of them prove to be effective in improving the cycle performance. [2] Cho et. al. reported the electrochemical properties and reaction mechanism of nanoparticle cluster MoP₂ anode. [3] It showed high reversible capacity and good cycle performance at limited voltage range. However, upon increasing voltage, the capacity decreases rapidly, suggesting that MoP₂ decomposed to Mo and LiP_nphase.  Here, we developed a novel Si-substituted MoP₂ (Mo_0.8Si_0.2P₂) alloy to increase the reversible capacity and enhance the cycle performance. We investigated the differences in physical and electrochemical properties between MoP₂ and Mo_0.8Si_0.2P₂ anode, examined using powder X-ray diffraction, SEM, and TEM, and galvanostatic charge-discharge test. We also investigated the volume expansion of MoP₂ and Mo_0.8Si_0.2P₂ alloys during cycling, considering the influence of substitution of Si into MoP layers in MoP₂alloy.

Experimental 

  Orthorhombic Molybdenum Phosphide (MoP₂) and Si-substituted Molybdenum Phosphide (Mo_0.8Si_0.2P₂) were synthesized by mechanical-ball milling method. The physical properties were examined by powder XRD, SEM, HR-TEM and EDX. The electrochemical characterizations were performed using a CR2032 coin-type cell. The electrodes were prepared by mixing 75 wt% active material, 10 wt% Super P conducting agent, and 15 wt% PAA binder dissolved in NMP. The cells were made of a lithium metal anode separated by polypropylene film. The electrolyte was a mixture of 1M LiPF6-ethylene carbonate (EC)/diethylene carbonate (DEC)/dimethyl carbonate (DMC) (3:5:2 by vol) with 10 wt% fluroethylene carbonate (FEC). The charge and discharge rate was both 0.1C (0.3 mA/cm²) with a cut-off voltage of 5 ~ 1200 mV.

  Results and Discussion

  Fig. 1 shows the typical XRD results of MoP₂ and Mo_0.8Si_0.2P₂ alloy powders prepared by mechanical milling method. All peaks of MoP₂ and Mo_0.8Si_0.2P₂ could be indexed as the Orthorhombic MoP2 phase of Cmc21 (JCPDS#00-016-0499). The lattice constants of Mo_0.8Si_0.2P₂ show smaller than those of pristine MoP₂ because of substitution of smaller Si ion. The broad peak at 2θ = 22^oindicated the residue amorphous red P.  Fig. 2 shows the cycle performance of the Li/MoP₂ and Li/Mo_0.8Si_0.2P₂ half cells at 0.1C rate with voltage region from 1200 to 5 mV. The Mo₀P₂ anode showed a specific capacity of 655 mAhg^-1 in the first charge process and showed a capacity retential rate of 88 % after 100 cycles. Conversely, the Mo_0.8Si_0.2P₂exhibited much higher initial capacity of 783 mAhg^-1better cyclic retention rate of 93.2 % after 100 cycles.

  Acknowledgement

This study was supported by a National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2011-C1AAA001-0030538).

 References

1. D. C. S. Souza, V. Pralong, A. J. Jacobson, L. F. Nazar, Science 296, 2012 (2002).

2. G. Park, C. Lee, J. Lee, J. Choi, Y. Lee, S. Lee, J. Alloys compd. 585, 534 (2014).

3. M. Kim, S. Lee, C. Cho, J. Electrochem. Soc. 156, A89 (2009).