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

Sodium-ion batteries are expected to gradually become a replacement for lithium-ion batteries due to their excellent low-temperature performance, cost advantages and high safety, while the main factor limiting the development of sodium-ion batteries is the anode material. Because the size of sodium ions is larger than lithium ions, the graphite material applicable to the anode of lithium-ion batteries with long-range ordered structure can not be applied to sodium-ion batteries, while the hard carbon material has short graphite domains and chaotic arrangement, the internal short-range ordered structure of the carbon layer with local graphite area, the layer spacing is larger compared to the graphite, which is conducive to the storage of sodium ions, and the hard carbon is easier to obtain, high carbon yield, and environmental friendly. Biomass-based hard carbon has attracted much attention because of its abundant raw material sources, low cost, easier accessibility, high carbon yield, environmental friendliness, and multi-element content, etc. Its unique microstructure shows obvious advantages and great commercial potential among many anode materials for sodium-ion batteries. The storage mechanism of sodium ions in hard carbon is still controversial. In this paper, we firstly analyse the adsorption behaviour of sodium ions at the active sites on the hard carbon surface, the process sequence of entering the graphite flake layer, and review the four controversial sodium ion storage mechanisms. In this paper, not only the storage mechanism of sodium ions in hard carbon is deeply analysed, but also the differences of hard carbon with different biomass-based precursors are further discussed. The content of each component and microstructure varies among different precursors, and there are many differences between nut shells, woody and herbaceous stems, whose internal structural features and different component contents may play a key role in the hard carbon performance.This paper enumerates the structural and component differences between different biomass-based precursors, and then summarises the differences in the sodium storage performance of the hard carbons from different precursors. In order to enhance the sodium storage performance of biomass-based hard carbon, the optimisation strategy of sodium ion battery anode is proposed through the microstructure of hard carbon anode, such as the adjustment of carbon layer spacing, the adjustment of pore structure and its specific surface area, in addition to the elemental doping and the introduction of functional groups, which can also enhance the sodium storage performance of hard carbon, which is of some significance in guiding the development of sodium ion batteries.