Abstract
Red phosphorus (RP) is a promising candidate as an anode for sodium-ion batteries because of its low potential and high specific capacity. It has two main disadvantages. First, it experiences 490% volumetric expansion during sodiation, which leads to particle pulverization and substantial reduction of the cycle life. Second, it has an extremely low electronic conductivity of 10–14 S cm–1. Both issues can be addressed by ball milling RP with a carbon matrix to form a composite of electronically conductive carbon and small RP particles, less susceptible to pulverization. Through this procedure, however, the resulting particle-size distribution of the RP particles is difficult to determine because of the presence of the carbon particles. Here, we quantify the relationship between the RP particle-size distribution and its cycle life for the first time by separating the ball-milling process into two steps. The RP is first wet-milled to reduce the particle size, and then the particle-size distribution is measured via dynamic light scattering. This is followed by a dry-milling step to produce RP–graphite composites. We found that wet milling breaks apart the largest RP particles in the range of 2–10 μm, decreases the Dv90 from 1.85 to 1.26 μm, and significantly increases the cycle life of the RP. Photoelectron spectroscopy and transmission electron microscopy confirm the successful formation of a carbon coating, with longer milling times leading to more uniform carbon coatings. The RP with a Dv90 of 0.79 μm mixed with graphite for 48 h delivered 1354 mA h g–1 with high coulombic efficiency (>99%) and cyclability (88% capacity retention after 100 cycles). These results are an important step in the development of cyclable, high-capacity anodes for sodium-ion batteries.
Highlights
The rate of global energy use in 2015 was about 16 TW and has been estimated to grow to about 24 TW by 2035.1 Increasingly, this energy is stored electrochemically in batteries for transportation or later use
Sodium-ion batteries (SIBs) are one such technology.[10,11]. Their operation and manufacturing process are largely similar to Lithium-ion batteries (LIBs), but the charge-carrying lithium is substituted with sodium, a ubiquitous element
In the first step, commercial Red phosphorus (RP) is ball-milled in ethylene glycol (EG) under argon in a zirconium oxide jar to decrease the particle size
Summary
The rate of global energy use in 2015 was about 16 TW and has been estimated to grow to about 24 TW by 2035.1 Increasingly, this energy is stored electrochemically in batteries for transportation or later use. To minimize the further decrease in the RP particle size, we used a smaller number of larger balls.[36] To ensure that all of the capacity measures in later electrochemical tests were from the RP (and not the carbon additive), graphite was chosen because it does not intercalate sodium It does have high electrical conductivity, lubricating properties (which mitigate further pulverization of the phosphorus particles), and the ability to exfoliate to form graphene layers. The composite ball milled for 48 h, shows a charge capacity on the first cycle of 1354 mA h g−1 and a high capacity retention after 100 cycles, delivering 88% of the initial capacity with a coulombic efficiency above 99%, as shown in panels d and e of Figure 3 These samples deliver most of the initial capacity at C/5, but a slow capacity decay is observed when higher rates are used (Figure 3f). This further confirms the presence of a carbon coating on the surface of the phosphorus particles
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