Sulfonyl chloride-intensified metal chloride intercalation of graphite for efficient sodium storage

Abstract

Metal chloride-intercalated graphite with excellent conductivity and a large interlayer spacing is highly desired for use in sodium ion batteries. However, halogen vapor is usually indispensable in initiating the intercalation process, which makes equipment design and experiments challenging. In this work, SO2Cl2 was used as a chlorine generator to intensify the intercalation of BiCl3 into graphite (BiCl3-GICs), which avoided the potential risks, such as Cl2 leakage, in traditional methods. The operational efficiency in the experiment was also improved. After the reaction of SO2Cl2, BiCl3, and graphite at 200 °C for 20 h, the synthesized BiCl3-GICs had a large interlayer spacing (1.26 nm) and a high amount of BiCl3 intercalation (42%), which gave SIBs a high specific capacity of 213 mAh g−1 at 1 A g−1 and an excellent rate performance (170 mAh g−1 at 5 A g−1). In-situ Raman spectra revealed that the electronic interaction between graphite and intercalated BiCl3 is weakened during the first discharge, which is favorable for sodium storage. This work broadly enables the increased intercalation of other metal chloride-intercalated graphites, offering possibilities for developing advanced energy storage devices.

1. Introduction

Sodium-ion batteries (SIBs) have garnered significant interest from researchers due to the practically abundant sodium supplies, good reversibility, and respectable energy density[1–4]. Therefore, significant research has been conducted to explore advanced anode materials for SIBs such as phosphorus[5], metal oxides[6], and carbon-based materials[7–12]. Among these materials, carbon materials have been proposed as prospective anode materials since carbon is abundantly available, and exhibits high specific capacity, low redox potential and excellent chemical stability[13–15]. Benefiting from the weak interlayer interaction and the distinctive layered structure, graphite with excellent conductivity and satisfactory stability, has been extensively employed as the common natural carbon material in energy storage devices[16]. However, due to the mismatch of graphite lattices and sodium ions, graphite can only deliver a low capacity of 35 mAh g−1[17]. As such, the development of high-performance carbon-based electrode materials require critical attention to boost the energy storage capacity of SIBs[18].

As a typical example of graphite-based materials, metal chloride-intercalated graphite possesses exceptional physical and chemical properties[19]. First, the conductivity of metal chloride-intercalated graphite is improved due to the hole-doping effect of metal chloride towards graphite layers. Furthermore, the energy barrier of sodium ions diffusion can be reduced due to the enlarged interlayer spacing and the adjusted ion migration path. In addition, the dense structure of original graphite is maintained, which can be of great benefit to improve the volumetric performance. Especially, some metal chlorides, such as those based on Mo, Sn, Bi and Ge metal chlorides, usually possess high theoretical capacities due to the multi-electron conversion or conversion-alloying reaction with the sodium ions. Intercalating graphite with these metal chlorides is an effective method to produce advanced carbon-based materials for SIBs[20,21]. Li et al. demonstrated that the molybdenum pentachloride-graphite intercalation compounds (MoCl5–GICs), used as the anode material for SIB, delivered a capacity retention of 27%. Specifically, they achieved a specific capacity of 271 at 0.2 A g−1, while at a higher current density of 5 A g−1, the specific capacity decreased to 72 mAh g−1[21]. Li et al. proposed that an AlCl3-GICs as the anode material of SIB exhibited a capacity retention of 32% upon increasing the current density; 259 mAh g−1 at 0.2 A g−1 quickly reduced to 83 mAh g−1 at 5 A g−1[22]. Therefore, the rate performance of these metal chloride-intercalated graphite need further improvement. Moreover, the pressure threshold of halogen vapor always is a crucial factor to determine the operational efficiency in experiment[23,24]. Consequently, the synthesis of metal chloride-intercalated graphite is restricted by th