Driven by the rapidly-growing of hybrid and electric vehicles, there is an increasing demand on lithium ion batteries (LIBs) with high energy density, long cycle life, and low cost. One of the most widely used anode materials in LIBs, graphite, only possesses a theoretical specific capacity of about 370 mAh/g, greatly limiting the specific energy of LIBs.¹ Recently, conversion-type of electrode materials, i.e. metal oxide and metal sulfide, has been regarded as one of the most promising anode materials for high performance LIBs, due to the low cost and exceptionally high specific capacity (>1000 mAh/g).² However, conversion-type anode materials suffer from several issues, i.e. relatively low conductivity and rapid capacity fade due to irreversible phase change and particle pulverization during cycling. For example, the poor performance of tin oxide is mainly caused by its low intrinsic conductivity, formation of irreversible insulating Li₂O phase during discharging, and large volume change.³ Tin phosphide (Sn₄P₃) has a layered structure and can be prepared by a mechanochemical or hydrothermal method. It has shown promising electrochemical performance as the anode in Li-ion batteries with high specific capacity of 1255 mAh/g.^4,5 As the lithium was inserted, Sn₄P₃ was first converted into lithium phosphides and Sn, followed by lithium-tin alloy formation. The advantage of Sn₄P₃ is its high capacity and Li⁺ conductivity of the Li₃P. However, it suffers from similar drawbacks as tin oxide, i.e. large volume change and irreversible Li₃P phase formation.

In this work, we prepared Sn₄P₃ by either hydrothermal reaction or high-energy ball milling of red phosphorus and tin. Then, graphite/Sn₄P₃ composites were prepared by high-energy ball milling under Ar atmosphere. Graphite and Sn₄P₃ were uniformly distributed in the composites. When tested as the anode materials, the composites showed superior performance compared to Sn₄P₃ alone. The capacity of Sn₄P₃ rapidly decreased to around 200 mAh/g after 15 cycles. In comparison, graphite/Sn₄P₃ composites exhibited excellent electrochemical performance and durability. The electrode can maintain a specific capacity more than 610 mAh/g at 500 mA/g (~0.4 C) after 100 cycles, which is close to 905 of initial capacity. In addition, the coulombic efficiency was also much higher in the graphite/Sn₄P₃ composites, indicating a reversible Li insertion/deinsertion behavior and more stable SEI layer.  The rate performance of graphite/Sn₄P₃ composites was also much better than pure Sn₄P_3. It is hypothesized that Sn₄P₃ was uniformed distributed within graphite matrix. During the discharge process, the formed Li₃P and Li_xSn phases were tightly confined in the conductive graphite matrix. Thus, these formed Li₃P and Li_xSn phases were able to recover to Sn₄P₃ phase during charge process.

To understand the origin of significantly improved performance, EXAFS and high-resolution TEM were carried out. From the EXAFS, it is evident that the local Sn environment in the graphite/Sn₄P₃ composite is fully reversible on lithiation/ delithiation while the local Sn environment of pure Sn₄P₃ changes irreversibly after several cycles. Our hypothesis was further supported by TEM images which show that during the high-energy ball milling process, Sn₄P₃ was uniformly embedded in graphite and was wrapped by few-layer graphene sheets. These highly conductive and mechanically strong graphene sheets could be one of the major causes of enhanced performance in graphite/Sn₄P₃ composites. In conclusion, we report a facile preparation of graphite/Sn₄P₃ composites with excellent electrochemical performance as anode material in Li-ion batteries.

Figure 1. Electrochemical results, TEM image, and EXAFS of graphite/Sn₄P₃ composites.

Reference

1. Tarascon, J. M.; Armand, M. Nature 2001, 414, 359.

2. Monconduit, L. The Journal of Physical Chemistry C 2014, 118, 10531.

3.  Oro, S.; Urita, K.; Moriguchi, I. Chem. Commun. 2014, 50, 7143.

4. Kim, Y.-U.; Lee, C. K.; Sohn, H.-J.; Kang, T. J. Electrochem. Soc. 2004, 151, A933.

5. León, B.; Corredor, J. I.; Tirado, J. L.; Pérez-Vicente, C. J. Electrochem. Soc. 2006, 153, A1829.