On the possibility of polystyrene-derived carbon coating for NASICON-type Na₃V₂(PO₄)₃ composites as cathode materials for sodium-ion batteries

Abstract

We here prepared polystyrene-derived carbon-coated Na 3 V 2 (PO 4 ) 3 composite (NVP/C) cathode materials via a solid-phase reaction using a mixture of NVP precursor and polystyrene (as the carbon source), followed by calcination at high temperatures. Using this method, we were able to obtain active NVP materials with relatively high purity while allowing a uniform surface coating with carbon layers about 34 nm thick. This structure significantly prevented NVP particle growth even after calcination at high temperatures. The NVP/C material calcined at 700°C exhibited the best chargedischarge performance among the samples studied.©2017 The Ceramic Society of Japan. All rights reserved.Key-words : NASICON compounds, Solid state reaction, Polystyrene, Carbon-coating method, Sodium-ion secondary battery [Received November 12, 2016; Accepted December 22, 2016] Currently, development of valuable materials for energy storage and efficient conversion is urgently required from the viewpoint of both the effective use of limited resources and prevention of global warming. Regarding energy storage systems, lithiumor sodium-ion rechargeable batteries, which mostly comprise transition-metal oxides and oxoacids as well as lithium or sodium ions as cathode materials, have gained significant attention in the field of chemistry and in industrial applications. 1)3) Among the lithium-or sodium-ion battery materials, phosphates of vanadium(III) such as NASICON-type Na 3 V 2 (PO 4 ) 3 (NVP) and NASICON-related compounds such as Li 3 V 2 (PO 4 ) 3 have been well-known since the pioneering work of Goodenough et al. 4) and have been extensively investigated. 5)7) However, as in the case of LiFePO 4 , NVP has low electronic conductivity, which limits its electrochemical performance. With the aim of overcoming this limitation, approaches based on surface carboncoated active materials [e.g., NVP, Li 3 V 2 (PO 4 ) 3 and LiFePO 4 ] have been frequently used by many researchers. 8)15) In this study, we synthesized carbon-coated NVP (NVP/C) materials via a solid-phase reaction using polystyrene as a carbon source (a detailed procedure is described in supplementary file), and their crystallinity, structural morphologies and electrochemical properties were investigated. Although various carbon sources such as sugars, 9),13) polyols 10),12) and citric acid 7),11),14),15) have been previously used, this work includes, to the best of our knowledge, the "first attempt at using polystyrene" as a carbon precursor for the synthesis of NVP/C composites. Figure 1 shows the powder XRD patterns of T-NVP and T-NVP/C (T: final calcination temperature in°C) calcined at various temperatures. These samples were first calcined at 300°C before being subjected to calcination at the final temperature (Fig. S1). According to the ICSD, NVP with an ordered rhombohedral NASICON structure (ICSD #248140) 16) was identified as the main phase present in all samples. In the case of uncoated carbon-free NVP samples, diffraction peaks responsible for an impurity phase are c...