Synthesis of Porous Sn4P3-C Nanospheres As an Ultra- Stable and High Capacity Anode Material for Sodium-Ion Battery
Authors: Choi Jonghyun , Kim Won-Sik , Hong Seong-Hyeon 
Published: February 2020

Owing to the fast development of portable electronics and electric vehicles, the need for electrochemical energy storage has increased. Among various electrochemical energy storage devices, lithium ion batteries (LIBs) have been demonstrated as one of the most successful examples in many electric applications. Because of their distinguished features, LIBs have high energy density and good power performance. However, due to its limited resource, its price will grow high if LIBs are progressively used. In this regard, sodium ion batteries (SIBs) have been considered as a promising alternative due to its abundance source in earth crust. Due to its similar insertion chemistry, sodium can directly transform well-established electrode materials in LIBs to SIBs. Unlimited efforts have been devoted to find new anode materials for high performance SIBs, including carbonaceous materials, metal oxides and sulfides, and alloy-based materials. Recently, elemental phosphorus P has been found to be electrochemically active for sodium ion battery anode, forming sodium phosphide (Na3P). It gives a high theoretical specific capacity of 2,597 mAh/g. But, its electrochemical performance is unsatisfactory because phosphorus has a very low electrical conductivity (~10-14 S/cm) and suffer from structural pulverization problem caused by its large volume change. To overcome this problem, Sn4P3 material has been reported as a promising anode for SIBs. It has been recently reported that Sn4P3 showed a synergistic Na-storage reaction where Sn and P atoms can react with Na to form Na15Sn4 and Na3P, and that the resulting Na15Sn4 alloy acts as a conducting pathway to activate the reversible Na-storage reaction of low electrical conductive Na3P particles. Also, Sn4P3 anode material exhibits a high reversible capacity of 1,230 mAh/g. The architecture of a stable and robust protective layer on the metallic Sn anode is critically important to ensure the good electrochemical performance of SIBs. Up to now, most of SIBs anodes consisting of elemental P and Sn alloys are generally produced by ball milling, thus the morphology has not been controlled. Ball milling method have been widely used as SnxPy electrodes because their phases can be easily controlled in laboratory scale synthesis. However, ball milling method for SnxPy is usually hard to control the morphology and produce nano scale, which leads to the deterioration of an active material resulting in capacity decay by several cycles. Therefore, hydrothermal method has received significant attention because it can control the nanoscale morphology for improved electrochemical performance. Herein, we report the porous Sn4P3-C nano spheres synthesized by hydrothermal process as an anode for SIB with an ultra-long term cycle stability and a high capacity. First, SnO2 nano spheres was synthesized by hydrothermal method with glucose precursor. Glucose not only prevents the rapid precipitation of colloidal SnO2 nano spheres but also serves as a carbon precursor for SnO2-C nano spheres. Consecutively, reduction process from SnO2 to Sn was conducted. The SnO2 redox process under hydrogen atmosphere has a significant problem. The reduction of SnO2 at 600 °C generates a liquid metallic tin, which aggregates into Sn particles. To prevent this problem, we employed a thick carbon layer coating to prevent the aggregation. Finally, phosphorization process was carried out using solvothermal method. Compared with other methods, the solvothermal route is convenient, simple, mild, and the morphology can also be controlled. With suitable temperature and time, Sn nano spheres were chemically transformed into Sn4P3 nano spheres, resulting in porous Sn4P3-C nano spheres. The synthesized porous Sn4P3-C nano sphere electrode exhibited an ultra-long cycle stability (about 2,000 cycles at 2,000 mAh/g) and high reversible capacity (700 mAh/g after 100 cycles at 200 mA/g) (Fig. 1). And its original morphology is apparently retained and the characteristic sphere geometry was unchanged after cycling test. We suggest the design strategy for the next-generation SIBs, and it can be applied to other phosphorus-metal alloying composites.

Figure 1