Research progress on biomass-based hard-carbon anode materials for sodium storage

Sodium-ion batteries are expected to gradually replace lithium-ion batteries in large-scale energy storage and two-wheeled electric vehicles because of their excellent low-temperature performance, cost advantages, and high safety. The development of sodium-ion battery anode materials with low cost, high reversible capacity, and excellent cycling stability is challenging for the industry. Because sodium ions are larger than lithium ions, graphite materials with long-range ordered structures suitable for lithium-ion battery anodes cannot be applied to sodium-ion batteries. Instead, the graphite domains of hard carbon materials are short and chaotically aligned, exhibiting a short-range ordered structure with local graphite zones inside the carbon layer. Moreover, the layer spacing of hard carbon is larger than that of graphite, which is conducive to the storage of sodium ions. Hard carbon is easily accessible and environmentally friendly, with a high carbon yield. Biomass-based hard carbon has attracted considerable attention because of its abundant raw material sources, low cost, easy accessibility, high carbon yield, environmental friendliness, and the presence of various elements. Its unique microstructure exhibits notable advantages and great commercial potential among several anode materials for sodium-ion batteries. The storage mechanism of sodium ions in hard carbon is controversial. Herein, we first analyze the adsorption behavior of sodium ions at the active sites on the hard carbon surface and the sequential process of their entry into the graphite scale layer. Moreover, we review four controversial sodium-ion storage mechanisms. Furthermore, we explore the storage mechanism of sodium ions in hard carbon and the differences between various biomass-based precursors. The content of each component and microstructure of different precursors vary, and several differences exist between nutshells, woody plants, and herbs. Their internal structural characteristics and different component contents play a key role in the performance of hard carbon. We enumerate the structural and component differences among various biomass-based precursors and summarize the distinctions in sodium-ion storage properties among different precursor hard carbons. To enhance the sodium storage performance of biomass-based hard carbon, an optimization strategy is proposed for sodium-ion battery anodes. This strategy involves manipulating the microstructure of the hard carbon anode, including adjusting the carbon layer spacing, pore structure, and specific surface area. In addition, the doping of elements, introduction of functional groups, and optimization of the electrolyte can improve the sodium storage performance of hard carbon. These insights can provide significant guidance for developing sodium-ion batteries.