Sizing up red phosphorus for use in future battery technologies

A step forward in the search for better anodes for sodium-ion batteries

In 2015, the world used around 16 TW of energy, and this is predicted to rise to about 24 TW by 2035. The need for high-performing energy storage is growing, with the increased use of both intermittent, renewable power sources and electric vehicles. The current technology of choice is lithium-ion batteries (LIBs), which have high specific energies, rate capabilities, and cycle lives. However, LIBs rely on lithium and cobalt, two elements with an uneven geographical distribution. Disruptions to supply can cause price spikes, and there are concerns that the world's total cobalt reserves may not meet future demand. Scientists are therefore investigating the potential of other battery technologies, which use cheap and widely available materials, such as sodium-ion batteries (SIBs). Although operation and manufacturing processes for SIBs are similar to those for LIBs, they cannot use the graphite anodes that are common in LIS. In research recently published in Energy Fuels, a team of researchers from the University of Oxford investigated how the particle-size distribution of red phosphorus affects the performance of composite anodes for SIBs.

Alloying materials

: Scheme 1. Shown Here Is a Schematic of the Division of the Milling Procedure for Preparing the RP–Carbon Composite.

In the first step (gold arrows), commercial RP is wet-ball-milled in EG for time t1 to reduce the particle size, which mitigates pulverization on sodiation. Milling the RP alone allows for the accurate measurement of its particle-size distribution before formation of the composite. In the second step (silver arrows), the RP from step one is combined with graphite in a 7:3 ratio and dry-ball-milled for time t2 to form an electronically conductive RP–carbon composite suitable for use as an anode.

Although hard, carbon - which has a disordered structure - has a limited charge/discharge rate, and it suffers from a high level of irreversible capacity loss. Previous research has investigated composite anodes made of carbon and materials such as tin, antimony, and bismuth, but models suggest that their capacity is too low to compete with LIBs.

In the continuing search for an inexpensive, scalable, and high-performing anode material for SIBs, researchers from the University of Oxford have turned their attention to phosphorus. Phosphorus occurs in several forms, but most are too expensive or dangerous to use as battery materials. (For example, white phosphorus reacts violently with air). Red phosphorus (RP) is a more promising candidate, but it has two disadvantages. Firstly, its volume expands greatly during sodiation, as sodium ions insert in the structure to form the alloy Na₃P, and this causes cracks and reductions in particle size, and substantially reduces the battery lifespan. Secondly, red phosphorus has very low electrical conductivity. Both of these disadvantages can be overcome through ball milling.

Ball mining

In a ball mill, balls in a rotating cylinder grind up the target material. For battery anodes, red phosphorus can be milled with a carbon matrix, which forms a conducting composite material with smaller RP particles that suffer less damage during expansion. However, the presence of carbon particles makes it difficult to determine the resulting particle-size distribution of red phosphorus.

According to lead author Isaac Capone,

"Reducing the particle size leads to lower forces on the particles and fewer cracks. How much do we need to reduce the particle size? That is the fundamental question."

: (a) TEM image of the composite material made by mixing phosphorus (Dv90 = 0.79 μm) with graphite for 48 h in which graphene planes can be seen on the surface of the phosphorus particle. (b) Plotting the ratio between the integrated areas of the peaks fitted on the photoelectron spectra collected from the composite versus the probing depth shows that surficial P–C chemical bonds gradually decrease and P–P bonds increase as we move deeper toward the particle bulk. The areas are calculated from the fit shown in panels c–e, with the photoelectron spectra of the P 2p region acquired using increasing incident radiation energy.

To investigate the relationship between RP particle size and the life cycle of batteries, the research team separated the ball milling process into two steps. After wet-milling the RP reduces the particle size, the researchers were able to determine the particle-size distribution using dynamic light scattering (DLS). They then used a second, dry-milling, step to produce the RP-graphite composite material. As the milling time gives different properties, they analysed samples that had been milled for one, 24 and 48 hours.

The team used a variety of techniques to analyse the samples: DLS, X-ray diffraction (XRD), Raman spectroscopy, both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and galvanostatic cycling. They used energy-tuned photoelectron spectroscopy (PES) measurements on the Surface and Interface Structural Analysis beamline (I09) to investigate the carbon coating on the particles. By using both hard X-ray photoelectron spectroscopy (HAXPES) and soft X-ray photoelectron spectroscopy (SOXPES), they were able to probe the material at different depths. Isaac Capone said;

"The results from Diamond showed what we had expected to see - more phosphorus-carbon bonds on the surface, and an increase of phosphorus-phosphorus bonds as we sampled the bulk, confirming the presence of a carbon coating."

Size matters

The results of this research showed that an RP particle size of below two microns offers better capacity and a significantly longer life cycle for sodium-ion batteries. The team were able to confirm that their process led to the successful formation of a carbon coating and that longer milling times lead to a more uniform layer. These are important steps towards the development of high-capacity anodes for SIBs, but also apply to other battery chemistries.

The next step in this research is a more fundamental study to investigate why this is the critical particle size, why it prevents cracks, and whether cracking can be prevented in another way.

To find out more about the I09 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Tien-Lin Lee tien-lin.lee@diamond.ac.uk.

Related publication:

Capone, I et al. Effect of the Particle-Size Distribution on the Electrochemical Performance of a Red Phosphorus−Carbon Composite Anode for Sodium-Ion Batteries. Energy Fuels 33, 5, 4651-4658 (2019). DOI:10.1021/acs.energyfuels.9b00385.

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