Porous Red Phosphorus and Reduced Graphene Oxide Composite Anode with High Capacity and Stability in a Sodium-Ion Battery
Authors: Lee Hwon-gi , Chang UiJin , Song Hayong , Eom KwangSup 
Published: February 2020

Rechargeable lithium-ion batteries (LIBs) with high energy density have been widely applied in portable electronic devices and electric vehicles. However, considering the increasing cost of lithium for LIBs due to geologically uneven distribution and limited supply of lithium resources, sodium-ion batteries (SIBs) have recently received attention as a candidate to replace LIBs because the sodium is the abundant element and low cost. Although the sodium and sodium ion have similar chemical/electrochemical properties to lithium/lithium ion, the major challenges still exist in developing anode materials because ionic radius of sodium ion (1.02 °A) is 34% larger than that of lithium ion (0.76 °A), which leads to difficulty in the intercalation of sodium ion into conventional anode materials for LIBs, such as graphite. Therefore, it is urgent and challenging to explore a suitable material with enough capacity and stable cycling performance for SIB anode.
Recently, red phosphorous (Red P) has been considered as a new anode material for the SIBs due to its high theoretical specific capacity (2596 mAh g-1), low cost, and environmental friendliness. Nevertheless, it is difficulty to use the red P in practical applications, because the red P undergoes tremendous volume expansion (~300%) during sodiation/desodiation, resulting in the pulverization of the particles and formation of unstable solid electrolyte interphase (SEI).
In this work, to solve the problematic behaviors of red P, we prepared the porous red P@RGO (Reduced Graphene Oxides) composite anodes to increase electrical/ionic conductivities and buffer the volume expansion. Figure 1 shows the TEM images of porous red P and porous red P@RGO composite. The red P exhibits porous surface morphology (Figure 1a), and the porous characteristics is expected to alleviate the volume expansion and offer fast sodium-ion transport. Additionally, to improve electrical conductivity, porous red P was loaded on the RGO in the form of composite. It was observed that the porous red P particles were successfully loaded and well distributed on RGO sheets (Figure 1b). The X-ray diffraction (XRD) pattern of commercial red P, porous red P, RGO, and porous red P@RGO composites are shown in Figure 2. The porous red P shows three peaks at 12.1-18°, 27-39°, and 45-67°, which is consistent with those of commercial red P.[2] Notably, the porous red P@RGO exhibits four broadened peaks at 12.5−17°, 19−27°, 28−41°, and 46−68°, corresponding to both of porous red P and RGO.[
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Figure 3 represents the cycling performance of porous red P@RGO and commercial red P at 0.1 A g-1 within the voltage range of 0.005−2.5 V. It is notable that the porous red P@RGO anodes shows much higher (more than five times) initial cycling performance (1214.6 mAh g-1) compared to the commercial red P anodes (185.4 mAh g-1) during 6 cycles. To elucidate the enhanced performance, the electrochemical, surface-chemical, and morphological analyses were performed using EIS, CV, XPS, XRD, HR-TEM, SEM/EDS and STEM/mapping.

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Figure 1