Electrochemical Performance and Failure Mechanism of Phosphorus/Graphite Composite as an Anode for Na-ion Batteries

Abstract

These days, there are large applications of lithium ion batteries (LIBs) as energy conversion and storage devices, from portable devices (e.g. mobile devices) to large scale energy storage devices (e.g. power devices – electrical vehicles (EVs), military parts, and stationary electrical energy storage systems – energy storage system (ESS)). As expanding applications of the LIBs, the amount of demanding the LIBs is growing fast. But lithium reserves are not so much in Earth’s crust and distributed in limited locations. Therefore, in the not too distant future, lithium prices will be raised inevitably. At this point in time, Na-ion batteries (NIBs) are re-spotlighted and nominated for one of the post LIBs. The NIBs have typical two advantages of cost reduction. One is that sodium resources are abundant materials in Earth’s crust. The other is that using Aluminum current collector is available on the anode for NIBs. However, at the present time, there are limitations for practical using NIBs. Many materials for NIBs have not been developed up to now. On the cathode materials, comparing with cathode materials for LIB, the cathode materials for NIBs have lower operating voltage and lower specific capacity. And on the cathode side, it has limitations for the intercalation chemistry. Hence, the development of the anode materials for NIBs should be conducted. Among them, the anode materials for NIBs should have lower operating voltage and higher specific capacity to increase energy density for NIBs. In this situation, an amorphous red phosphorus/carbon composite was introduced as a promising anode material for NIBs. Because the red phosphorus has poor electronic conductivity, the carbon acts as a conductive material. It showed the highest reversible capacity and suitable operating voltage for NIBs. But the composite has poor cycle-ability in terms of long cycling. Because alloying compounds (in this case, phosphorus) have severe volume expansion and shrinkage that cause contact loss, and then capacity fading is happened. The carbon couldn’t sufficiently act as a buffer matrix in this case. Therefore, to improve cycle-ability for phosphorus composite materials, a new composite structure is adopted and an electrolyte additive is used in this research. Firstly, a phosphorus/graphite composite is used as an anode material for NIBs. The composite is obtained via a facile and simple mechanochemical ball milling process. The composites are tested by two kinds of different weight ratio. The two composites show that high reversible capacities and good cycle-ability comparing with previously reported the amorphous red phosphorus/carbon composite. And by having different graphite contents, the two composites have different structures which affect the reversible capacity, the cycle-ability, low changing ratio of electrode thickness, surface area, and pore volume. Hence, optimizing weight ratio of the phosphorus/graphite composite, higher reversible capacity and good cycle-ability can be achieved simultaneously. Secondly, fluoroehtylene carbonate (FEC) is used as an electrolyte additive, surfaces of electrodes and Na metals are analyzed. The FEC used case shows better electrochemical performances than the FEC unused case. To define different points between the two cases, EIS and XPS analysis are carried out on the electrodes and the Na metals. In EIS result, charge transfer resistances have different behaviors between the electrodes and the Na metals. To investigate these phenomena, XPS analysis is conducted, and the XPS result shows that compositions of solid electrolyte interphase (SEI) layers and surface are different between the cases. The FEC additive makes stable SEI layers and minimizes oxidation of phosphorus on the electrode. Through the results in this research, the phosphorus/graphite composite can fulfill increasing electronic conductivity and buffer matrix structure. Phosphorus is coated by graphite, exposure of phosphorus is minimized. And the composite can be one of the promising anode materials for NIBs by having high reversible capacity and low operating voltage. Additionally, using the FEC additive can further enhance the electrochemical performances by making stable surface for the composite.