Studies on Full Na-Ion Batteries with a Hard Carbon Anode and Oxide Cathode Materials

Abstract

The limited abundance of lithium on the Earth forces the search of alternative battery systems for mobile and stationary applications. Sodium-ion batteries could be a reasonable replacement for lithium-ion ones if a compromise between electrochemical performance and cost can be found. Since the ionic radius of Na+ is larger than of Li+, the choice for electrode materials is more restricted compared to Li-ion batteries, despite the similar chemistry of Na- and Li-compounds in many cases. The reason for that is a lower stability of crystalline phases with Na in fully charged or discharged states because of a larger volume change upon sodiation/desodiation. Anode materials like metallic alloys on the basis of Sn, Sb or Ge represent the most crucial point in Na-ion batteries because of a huge volume difference between sodiated and desodiated states, resulting in fast capacity fading due to reducing the particle size and contact loss. Hence, nowadays solely hard carbons (HC) can exhibit electrochemical performance suitable for a potential application. The structure of hard carbon consists of amorphous areas (sp 3-hybridization of carbon) embedding and partially crosslinking layered graphitic segments of low dimensional graphene layers (sp 2-hybridization of carbon). Electrochemical insertion of sodium into hard carbon involves two steps: a slope potential region resulting from very stable intercalation of Na-ions between graphitic layers, and a plateau in potentials from adsorption of ions in micropores. Upon desodiation, a large irreversible capacity up to 70 % of the total first discharging capacity is usually observed, which includes the solid-electrolyte interface (SEI) formation, and Na-ions being stably trapped between graphitic layers [1]. In our present work, we studied electrochemical performance of full Na-ion batteries with olivine-like NaFePO4 or layered-type NaNi0.5Ti0.5O2 cathodes, and HC anode materials. In order to overcome the problem with irreversible capacity loss, hard carbon materials were partially or fully sodiated (discharged) prior to electrochemical tests. A NaPF6-containing electrolyte modified with 1-butyl-1-methylpyrrolidinium hexafluorophosphate was used. The observed specific capacities of about 150 mAh g-1 for NaFePO4/HC batteries after 20 cycles are comparable with half NaFePO4/Na cells [2], while NaNi0.5Ti0.5O2/HC batteries show about 75 mAh g-1 (Fig. 1). The reason for a lower capacity retention of NaNi0.5Ti0.5O2 upon cycling is discussed. Figure 1. Specific capacity of full Na-ion batteries with NaFePO4 (NFPO) and NaNi0.5Ti0.5O2 (NNTO) cathodes and hard carbon (HC) anodes, normalized on the cathode weight, at C/10 rate meaning 1Na intercalated during 10 h.