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Nanomaterials 2022, 12, 3529 11 of 29 to extremely low cycle stability. Researchers improved their low-temperature performance by designing the structure of the unique Se anode material or modifying it with carbon material to accelerate the transfer of electrons and ions during charge and discharge, while increasing the active site. Xing-Long Wu et al. [46] coated a-Se on a three-dimension network structure with reduced graphene oxide nanolayers to produce the Se/graphene (3DSG) anode. The three- dimension network based on graphene oxide nanolayers in 3DSG, as shown in Figure 7a, provided ideal transport paths for Na+. When cycled in half-cells with the Na counter electrode, it exhibited favorable electrochemical performance when the temperature went down. For example, it showed a reversable specific capacity of 250 mAh/g at −5 ◦C and 180 mAh/g at −15 ◦C at a rate of 2 C. Moreover, it still had satisfactory capacity retentions of 96.2% and 98% after 1000 cycles, as shown in Figure 7b. Luo Wen-Bin et al. [47] prepared NbSSe nanosheets through calcination as the SIB anode material. The two-dimensional nanosheets stabilizes interlayer band gap and improves electronic transformation. One prominent advantage of this system is the combination of two different kinds of anionic ligand characteristics which improved electrochemical performance and stability at low temperatures. At a low temperature of 0 ◦C, it showed a specific capacity of 136 mAh/g. Furthermore, after a long life span of 500 cycles at 0 ◦C, it exhibited a satisfactory specific retention of 92.67% at a rate of 0.2 C. Compared with amorphous selenium, metal selenides own higher electrical conduc- tivity; thus, they are more favorable in low-temperature SIBs. From the SEM image in Figure 7c,d, it can be easily concluded that selenide carbon composite has a one-dimensional structure which represents considerable superficial area, resulting in an ideal Na+ diffu- sion efficient. Moreover, as a widely acknowledged semiconductor, SnSe is abundant and environmentally friendly. In addition, it has a high theoretical capacity of 780 mAh/g for sodium-ion storage. In the research of Ming Zhang et al. [48], a SnSe@carbon nanofiber (SnSe@CNF) was designed by adding selenium powder into a tin solution. After annealing and covering with a carbon nanofiber, SnSe@CNF was formed which improved the tradi- tional method using a thermal method and its nanofiber structure, as shown in Figure 7e. The half-cell showed a stable capacity of 267 mAh/g at 0 ◦C after 100 cycles at a 1 C rate. In another report, Chunming Zheng et al. [49] also chose carbon nanotubes to integrate ZnSe. Through a unique hydrothermal method, ZnSe was perfectly combined with carbon nanofibers to form the anode material and provide a network for Na+ diffusion. In addition to the merits of carbon nanotubes mentioned above, the nanofiber ZnSe structure can also play a supporting role because of its superior bearing ability. It can stretch when the pressure is reduced and prevent the structure from collapsing when the pressure increases, resulting in improved electrochemical performance at low temperatures. For instance, the half-cell delivered a discharge capacity of 267.0 mAh/g at a rate of 1 C. Moreover, when it came to 600 cycles, an 83.3% capacity was still retained, demonstrating better cycling performance than pure ZnSe at −10 ◦C, as shown in Figure 7f. In another work, Jing-Ping Zhang et al. [50] constructed a coral-like cl-Fe7Se8@C structure with Fe and Se material. One of the advantages of this system is that one- dimensional carbon nanotubes in the three-dimensional structure facilitates the transfer of zero-dimensional Fe7Se8 nanospheres, as shown in Figure 7g, resulting in favorable performance. To former test its low-temperature stability, the cl-Fe7Se8@C anode was assembled with a Na3V2(PO4)2O2F cathode which already showed an ideal capacity when the temperature decreased. The full cell exhibited a specific capacity of 166 mAh/g after 440 long-term cycles at 0.5 C. This means that its capacity fading per cycle was reduced to about 0.15% at an extremely low temperature of −25 ◦C, as shown in Figure 7h. In another study, FeSe2/rGO hybrids with a 3D hierarchical structure, prepared in a traditional hydrothermal way, were developed by Ye, Zhizhen et al. [51]. Not only did the unique anode show impressive performance at ambient temperatures, but it also exhibited ideal electrochemical performance and stability at low temperatures. At low temperatures of 0 ◦C, −10 ◦C, and −20 ◦C when they were compared with room temperature, itsPDF Image | Na Ion Batteries Used at Low Temperatures
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