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Molecules 2022, 27, 6516 14 of 32 4. Modification and Optimization Strategies of HC The practical application of HC is limited by its low-rate capability, cycle stability and the low initial coulombic efficiency, especially. Due to the high specific surface area of HC, the contact between the electrolyte and electrode material surface will be strengthened, forming an SEI film, which will block the path of Sodium-ion transportation and lead to an irreversible capacity after the first cycle of charge and discharge. The effect of SEI films on the first cycle electrochemical properties of materials has been studied and reported as early as the 1990s [35,75]; the same problem exists in LIBs, where the decomposition and coating of the electrolyte on the surface of the material impedes the movement of electrons and ions, which is also directly related to the specific surface area of the material. Subsequently, Ji’s group [76] characterized the surface area and structure of the material by means of XRD and BET, proving that the increase of measurable porosity was closely related to the decrease of reversible capacity, and proposed that the carbon atoms contributed to the surface area and porosity and were “exposed carbon atoms”. The contribution of these atoms to Sodium-ion storage is very low, probably because the exposed carbon atoms become nucleation sites for SEI phase formation, forming a passivation layer on its surface and leading to an irreversible storage in the first cycle. Cao [42] predicted through theoretical simulation of the experimental results that defects in the carbon layer capture Sodium-ions and generate a repulsion electric field. High defect sites and sodium binding energy will not only hinder the further entry of Sodium-ions, but also lead to the increase of the irreversible capacity in the first cycle. Therefore, in order to synthesize high-performance HC anode materials that can be put into practical applications, the modification and optimization strategies of HC materials are particularly important. Various optimization strategies reported in recent years are summarized as follows. 4.1. Heteroatom Doping Heteroatom doping can change the electron/ion state of the active material and favor Sodium-ion storage. For carbon-based anode materials, the main dopants are non-metallic atoms, such as B [65], N [77], F [78], P [79], S [80], and O [81]. The heteroatom doping can not only adjust the intrinsic structure of carbon materials, but also introduces a variety of different functions due to its own properties. For example, fluorine, because of its strong electronegativity, can weaken the repulsive force of Sodium-ion insertion, and thus lower the energy barrier of Sodium-ion insertion. Wang [78] prepared an F-doped HC derived from lotus petioles, which had a reversible capacity of 228 mAh g−1 and remained at 99.1% capacity retention for up to 200 cycles. Early in 2015, Yu [82] prepared N-doped porous carbon fibers using polypyrrole as a carbon source, focusing on the effect of nitrogen function on the electrochemical performance of the carbon anode. In the porous structure of fibrosis, N atoms provided enough active sites and showed a high capacity of 296 mAh g−1 at 0.05 A g−1, however, its ICE reached only 46% because of the irreversible adsorption of defects and heteroatom sites along with the formation of SEI films. Recently, N-doped HC anode materials with low ICE and a high capacity were also reported. Ma [77] prepared a porous carbon material rich in N using bamboo leaves as a precursor. Yang [83] synthesized a structurally stable high nitrogen-doped carbon material by combining metal-organic framework (MOF) materials. The anode made from this nitrogen-doped carbon material showed a high capacity of 198.2 mAh g−1 at 5 A g−1, but its coulomb efficiency was still less than 60% in the first cycle. Carbon sheets with a high N-content were prepared with okara [39], exhibiting an ultrahigh rate capability after carbonization and subsequent exfoliation. It showed a capacity of 258.9 mAh g−1 in the first cycle and remained at 247.5 mAh g−1 after 50 cycles. Its ICE is much higher than that of other reported heteroatom doped materials [40,84,85]. Its excellent performance was attributed to its special structures. However, the underlying causes need to be studied more deeply. In short, the synergistic effect of N-doping and defects in the structure [86] can increase the sodium storage performance of HC materials,PDF Image | Hard Carbons as Anodes in Sodium-Ion Batteries
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