A review of hard carbon anodes for rechargeable sodium-ion batteries

Abstract

Hard carbons (HCs) are recognized as potential anode materials for sodium-ion batteries (SIBs) because of their low cost, environmental friendliness, and the abundance of their precursors. The presence of graphitic domains, numerous pores, and disordered carbon layers in HCs plays a significant role in determining their sodium storage ability, but these structural features depend on the precursor used. The influence of functional groups, including heteroatoms and oxygen-containing groups, and the microstructure of the precursor on the physical and electrochemical properties of the HC produced are evaluated, and the effects of carbonization conditions (carbonization temperature, heating rate and atmosphere) are also discussed.

1. Introduction

Because of the rapid consumption of fossil fuels, severe global climate issues, and the growing need for electric or hybrid electric vehicles (EVs or HEVs), the demand for grid-scale energy storage (EES) systems with higher power/energy density and operability under extreme conditions is increasing[1,2]. Lithium-ion batteries (LIBs) have been widely applied in the fields of electronic devices and new energy vehicles due to their high energy density and cycling stability[3,4]. However, the widespread application of LIBs in large-scale EES systems faces significant obstacles due to the low abundance (0.0017%, mass fraction) and uneven distribution of Li resources[5,6]. Therefore, the development of a new type of secondary battery that is cost-effective, high-performing, and resource-abundant is an inevitable trend[7–9]. In this regard, sodium-ion batteries (SIBs) are ideal alternatives due to the high abundance of sodium resources (2.3%), which effectively reduces the cost of the batteries[10]. Additionally, sodium shares comparable physical and chemical characteristics with lithium, and the working mechanism of SIBs and LIBs are highly similar, enabling compatibility of SIB production facilities with those used for LIBs[11]. The comprehensive comparison between lithium and sodium is demonstrated in Fig. 1a[7].

The properties of anode materials greatly influence the energy density, stability and cost of SIBs. Because of the diversity, structural stability, and good electrochemical reversibility, carbon anodes have been widely studied in SIBs. Graphite, the most classic anode material, has been commercially used in LIBs for decades. Unfortunately, the interlayer distance of graphite (0.335 nm) is smaller than the recommended interlayer spacing (0.370 nm) required for Na+ storage[12]. In addition, density functional theory (DFT) calculations indicate that graphite is energetically unstable in the formation of sodium-graphite intercalation compounds (Na-GICs)[13]. As a result, graphite is commonly considered unsuitable for use as an anode material in SIBs[14]. Soft carbons (SCs) are a type of disordered carbon material that can be graphitized above 2500 °C[15]. SCs exhibit short-period structures in the planar or stacking direction, with a lateral size of 15 nm, 50 to 100 stacking layers per stack, and an interlayer spacing between 0.34 and 0.37 nm (Fig. 1b)[16,17]. However, SC anodes exhibit inferior available capacity due to the absence of effective storage sites[15]. Compared with SCs, hard carbons (HCs) have highly distorted structures, turbulent carbon layers, and numerous nanoscale pores and cannot be graphitized above 2500 °C[15,18–20] (Fig. 1c). The twisted carbon configuration enhances repulsive interactions among carbon nanosheets, leading to a greater interlayer spacing (0.3–0.42 nm) than SCs, which facilitates Na+ transport[21,22]. Additionally, the amorphous structures in HCs provide more storage space for Na+, leading to a higher Na+ storage capacity than SCs (Fig. 1c)[16,23]. Due to their highly disordered structure, they usually exhibit poor electronic conductivity, which reduces the rate performance. Moreover, the large specific surface areas (SSAs), plentiful defects, and micropores of HCs induce electrolyte decomposition and irreversible adsorption of Na+, resulting in lower initial Coulombic efficiency (ICE) and energy density[24,25]. Since 2000, the number of publications on HCs has grown exponentially (Fig. 1d), which indicates the great interest of the global research community in HCs[26].