Phosphorus and Carbon Nanotubes Composite As Anode for Sodium-Ion Batteries

Recently, the Na-ion batteries (SIBs) have attracted more attention due to their low cost and the abundant supply and widespread reverses of the Na mineral salts. It could potentially be the promising candidate for stationary batteries. In the last ten years, in the light of the successful experience with carbon anodes in lithium ion batteries (LIBs), carbonaceous materials, such as hard carbons,^[1-3] carbon nanospheres^[4] and nanowires,^[5] and graphene,^[6] have been introduced as anode materials for SIBs. Subsequently, Sn and Sb-based materials^[7-10] have been studied as anode for sodium ion batteries, as they have high reversible capacity, but their specific capacities are still relatively low (500 – 800 mAh g^-1).

Elemental phosphorus (P) is an attractive anode material, which can give a high theoretical specific capacity of 2596 mAh g^-1 to form Na₃P phase. Phosphorus has three allotropes, white, black, and red. Among these allotropes, white phosphorus is not chemically stable, and synthesis of black phosphorus is not facile, as it needs an inert atmosphere under high pressure. In comparison, red phosphorus is commercially available with ease. For sodium ion storage, recently, Qian et al.^[11] and Kim et al.^[12] reported that an amorphous phosphorus composite with carbon that was obtained by high-energy mechanical milling could deliver a high capacity of 1764 mAh g^-1 at the current density of 250 mA g^-1 and 1890 mAh g^-1 at current density of 143 mA g^-1, respectively. Our group find commercially bulk red phosphorus also can reversibly cycle in SIBs through improving the electronic conductivity by simply hand grinding with carbon nanotube (CNT).^[13] The factors restrict the electrochemical performance of P are the electronic conductivity and the huge volume change (391% for Na₃P formation) during the charging/discharging process. Up to now, how these factors affect its reversibility and cycling stablility has not been investigated. Thus, in this present work, the effect of particle size, electronic conductivity and the milling time on the electrochemical performance of P were studied. The results shown that the poor conductivity of phosphorus is the key factor on the irreversible cycle, and P can charge and discharge reversibly after improvement of conductivity by hand grinding with CNT. However, the cycling stability is not good resulting from huge volume change. When increase the milling time,  CNT covered the surface of P well , as a result, the P/CNT composite milled with CNT  for 20h delivered a capacity of 862 mAh g^-1, with a retention of 73.5% over 50 cycles.

Acknowledgments

The work is supported by the Australian Research Council through a Discover project (DP110103909) and a Linkage Project (LP120200432).

 References

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