Ball-Milling Synthesis of Fep: A New, Cheap, Productive Anode Material for Sodium-Ion Batteries

Nowadays, how to effectively utilize the renewable energy is a great challenge for researchers. An energy system for renewable energy requires low cost, high power capability, and safety. In terms of low-cost, lithium ion batteries (LIBs) are not suitable for practical application, because lithium source in earth is low abundance causing growing price. Recently, the Na-ion batteries (SIBs) have been attracting more attention due to their low cost, and the abundant supply and widespread reserves of the Na mineral salts. They could potentially be the most promising candidate for stationary batteries.

In the last ten years, a large variety of  host materials have been demonstrated as sodium ion storage cathodes, in comparison, there are few investigations about anode materials for SIBs.^[1-5] Since the Na ion (radius 0.95 Å) is about 55% larger than the Li ion (0.6 Å), this seriously limits the range of potential candidate materials. Many good anodic host materials for Li-ion batteries, such as graphite and silicon, are not suitable for Na-ion storage. It has been well established that only a small amount of sodium ions can intercalate into graphite, and no Na ions can insert themselves into Si.^[6-7] More recently, the metals which can alloy with sodium have been reported, because they can deliver higher theoretical capacity as the anode in SIBs than non-graphitic carbons with reversible capacities of less than 300 mAh g^-1, such as, Sn (847 mAh g^-1),^[8] Sb (664 mAh g^-1),^[9] and P (2596 mAh g^-1).^[10-11]However, the fatal defect for these anode candidates is their huge volume change during the sodiation/desodiation, resulting in a poor cycling performance.

It is reported that alloy compounds could reduce the volume change with better cycling stability than alloy in Li-ion batteries.^[12-15] There have been very little research studies on alloy compounds for reversible negative electrodes for Na-storage up to now. Manthiram A.’s group reported M_xSb (M=Cu,Fe,Ni) in a matrix of Al₂O₃-C had a better cycling performance than Sb in the same matrix of Al₂O₃-C for sodium-ion batteries.^[16] Lin Y. suggested that Sn_0.9Cu_0.1 as anode for SIBs can stably cycle for 100 cycles with capacity of 420 mAh g^-1.^[17] SnSb/C alloy anode for Na-ion batteries has been reported with a capacity retention of 80% after 50 cycles.^[18] As for the element, P has a higher theoretical capacity than Sn and Sb, however, it is not clear that whether the phosphide (M_xP) will have a higher capacity than that of M_xSn and M_xSb because no papers have been reported about M_xP up to now. Fe has a very abundant reserve in earth, and is none-toxic, cheap.  Thus, in the present work, we prepared iron phosphide (FeP) in large amounts by a simple ball milling method, which can meet the demand for large-scale applications. The as-obtained FeP compound was investigated as the anode in SIBs in half cells and showed high capacity of 780 mAh g^-1, which offers a new alternative to carbonaceous anode materials in Na cells. In addition, the improvement of its cycling stability was also achieved by combining it with an appropriate binder and additive to the electrolyte, with 68.8% retention of 321 mAh g^-1 capacity in the second cycle after 60 cycles.

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