Phosphorus-based High Capacity Negative Electrode Materials for Sodium-ion Batteries

Abstract

In recent years, the demand for large scale energy storage devices has increased with the development of electric vehicles (EVs) and energy storage system (ESS), and Li-ion batteries are considered promising candidates because of their high energy density. However, there is growing concern that lithium resources are insufficient to meet the demands of large scale applications due to the limited reserve and too geographically constrained. Actually, the recent price of lithium raw materials has shown sharp increases. Cosquently, Na-ion batteries are an alternative to Li-ion batteries because sodium resources are much more abundant and inexpensive than lithium. However, there is a critical obstacle to their development. The energy density of Na-ion batteries is slightly lower than that of Li-ion batteries because the reversible capacity and operating voltage of currently reported electrode materials in Na-ion batteries are lower in comparison. This implies that it is not easy to replace Li-ion batteries with Na-ion batteries because the cost per energy stored ($/Wh) of Na-ion batteries does not provide much advantage. Therefore, new negative electrode materials having higher reversible capacities are necessary if one is to increase the energy density of Na-ion batteries. In this study, firstly, exploring the possibility and investigating electrochemical reaction mechanism of red phosphorus as negative electrode materials for sodium-ion battery are performed based on thermodynamic information. In order to improve electrical conductivity, carbon coating via ball-milling is carried out. This carbon coating is enhanced electrochemical reactivity including the highest specific capacity among any reported negative electrode materials. The understanding is that during the sodiation process amorphous red phosphorus undergoes a phase change to crystalline sodium phosphide (Na3P) by electrochemical reaction and structure analysis. The corresponding to Na3P peaks in dQ/dV plots are shown at the ca. 0.2 and 0.55 V vs. Na/Na+ during sodiation and desodiation, respectively. However, poor cycle performance is observed due to volume change during charge/discharge. Secondly, the study on failure mechanism and enhanced cycle performance of phosphorus electrode materials are carried out. The failure mechanism of this material comes from volume change. The large volume change of electrode materials is attributed to be the main reason for their rapid capacity loss. It increases internal resistance by contact loss between phosphorus and conducting agent. The instability of solid electrolyte interphase (SEI) at the electrode surface via repetitive volume expansion is also considered to failure mechanism. In order to improve cell performance, tried to change the phosphorus to carbon ratio and add electrolyte additive such as fluoroethylene carbonate (FEC). In case of increasing active materials, it is shown alleviating volume expansion. In addition, excellent cycle performance is observed for 100 cycles with FEC additives in electrolytes. It is attributed that NaF-like SEI film is mechanically strong and EC decomposition is minimized. Finally, the volumetric capacity is improved by Sn4P3 electrode materials. Phosphorus has poor electrical conductivity. Accordingly, a large amount of carbon has to be used in order to enhance their electrical conductivity, leading to decreases their gravimetric and volumetric capacity. The volumetric capacity in a cell is one of the most key factors that determine its feasibility as a post battery. Therefore, it is important to develop new negative electrode materials delivering high volumetric capacity for sodium-ion batteries. Sn4P3 is one of the candidates for high volumetric capacity electrode materials. Sn4P3 showed excellent electrochemical performance for Na-ion batteries. Sn4P3 delivered a high volumetric capacity and exhibited very stable cycle performance with a negligible capacity fading over 100 cycles. The redox potential of Sn4P3 is lower than that of phosphorus, indicating that the energy density of full cells with Sn4P3 can be higher than those of cells with phosphorus when the same reversible capacity is utilized.