In Situ study of Novel Sustainable Dual Carbon Sodium-Ion Battery

Abstract

The booming development of electric vehicles (EVs) raised concerns about sustainability of Li-ion batteries (LIBs). Some of their constituents, such as Li and Co, are of scarce and highly localised resources. Furthermore, LIB’s building materials are difficult to recycle raising questions on the sustainability of the technology. These issues inspired research on chemistries based on more abundant elements, i.e. Na, Mg or Ca. Among them the most promising are sodium-ion batteries (SIBs), which are currently heavily researched as a viable and sustainable alternative to LIBs for medium to large scale energy storage applications. The mostly researched electrode materials for SIBs are hard carbons for anode and intercalation compounds for cathodes. The latter contain transition metals (including cobalt) and share the same concerns as cathode materials for LIBs. Additionally, having in mind recyclability problems of LIBs, it is necessary to take the circular economy approach when designing new technologies such as Na-ion batteries. Therefore, more sustainable and greener materials are needed. A promising solution is the use of carbon as an active electrode material. Carbonaceous materials are inexpensive, environmentally friendly and highly conductive. So called ‘dual carbon’ batteries utilise carbon for both electrodes – in case of LIBs it’s usually graphite for cathode and anode.1 Literature on this field regarding SIBs is sparse, particularly when it comes to electrochemical mechanisms and full cells performance. For instance Fan et al. reported on soft carbon-graphite Na-ion full cell with remarkable long cycle life.2 However, graphite cathode works via anion intercalation mechanism, which delivered small capacity of around 40 mAh g-1 in a full cell. Recently Ali et al. reported on electrochemical performance of reduced graphene oxide (rGO) as a suitable cathode for SIBs3. The cathode showed high specific capacity of 235 mAh g-1 and stable behaviour over 1000 cycles. Nevertheless, the exact electrochemical energy storage mechanism in rGO is unclear and its performance in a full cell wasn’t yet verified. In this work we investigated electrochemical behaviour of reduced graphene oxide based cathode for SIBs. Techniques such as in situ Raman spectroscopy and X-ray Pair Distribution Function (PDF) provided insightful look into electrochemical reactions of rGO cathodes for SIBs. It has been concluded that processes responsible for high gravimetric capacity are surface Faradaic reactions between the sodium ions and the functional groups, as well as the bipolar behaviour of rGO resulting in capacitive storage of anions from electrolyte salt (Figure 1a). Furthermore, as a proof-of-concept a full cell with the rGO cathode and hard carbon (HC) as anode was tested. It showed high energy density of 80 Wh kg-1 and outstanding stability over 1000 cycles (Figure 1b), proving that rGO is a viable SIBs cathode material candidate. Replacing expensive transition metal based oxides with carbonaceous cathodes would solve sustainability and environmental impact concerns and lead to significant reduction of overall SIBs manufacturing costs. Placke, A. Heckmann, R. Schmuch, P. Meister, K. Beltrop and M. Winter, Joule, 2018, 1–23.Fan, Q. Liu, S. Chen, Z. Xu and B. Lu, Adv. Energy Mater., 2017, 7, 1602778.G. Ali, A. Mehmood, H. Y. Ha, J. Kim and K. Y. Chung, Sci. Rep., 2017, 7, 40910. Figure 1