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Nanomaterials 2022, 12, 3529 5 of 29 enhances ion conductivity; thus, NFP@C obtained ideal electrochemical performance below ambient temperatures. Combined with the hard carbon anode, the full cell delivered a specific capacity of 115 mAh/g at −10 ◦C and 100 mAh/g at −20 ◦C. Even after 200 cycles at 0.5 C, 85.5% and 75.8% capacity retentions were obtained with a coulombic efficiency around 100%. Furthermore, when the rates increased to 2 C, the full cell still exhibited a favorable performance at −10 ◦C and −20 ◦C. In such low temperatures, the capacity retentions could still reach 87.0% and 75.8%, respectively, when compared with specific retentions at ambient temperatures, as shown in Figure 3b. Na3VCr (PO4)3 (NVCP) was proven to have better electrochemical performance at low temperatures of −15 ◦C than that at ambient temperatures of 30 ◦C. Due to V3+/V4+ and V4+/V5+ redox couples in the structure, 1.5-electron cell can be transported during the redox reaction. In 2016, Yong Yang’s group [29] reported that the NVCP three-electrode cell can obtain a high voltage of 3.4 V and an outstanding specific capacity of 93 mAh/g at 0.1 C under −15 ◦C, as shown in Figure 3c. Later in 2020, the same group [30] further investigated the migration of V in the polyanionic cathode during the Na+ storage process by using various in situ/ex situ characterization tools. After a comprehensive analysis, they challenged the traditional common view of the stable framework in this system and proposed that the V migration was associated with the irreversible long-range structural trans- formation and capacity decay of polyanion-based cathode materials, as shown in Figure 3d. They further demonstrated that V migration could be effectively inhibited at low temperatures (−15 ◦C) and restored at room temperature via a low-voltage discharge (<1.7 V). Shanqing Zhang et al. [31] proposed nanocomposite NASICON-structured Na3V2(PO4)3 with low-cost organic carbon derived from sucrose which was abbreviated as NVP@C. They introduced a 3D Na+ transportation system in NASICON which maintained a slight voltage fluctuation of 170 mV. When the temperature changed from 23 ◦C to −10 ◦C, a specific retention of NVP@C could reach 108 mAh/g, as shown in Figure 3e. Simultaneously, after 500 cycles at a high rate of 10 C at an extremely low temperature of −20 ◦C, a high capacity retention of 75.8% could be obtained, which was almost the same with that at room temperature. Yunhai Wang et al. [32] reported the Na3V2(PO4)3/CNT composite as a cathode for SIBs. In their work, NVP was cross-linked by CNTs, and a full SIB was fabricated with the Na3V2(PO4)3/CNT cathode, the Bi anode, and the NaPF6-diglyme electrolyte. Such fabrication enabled the full cell to obtain satisfactory cycling stability in the temperature from −15 ◦C to 45 ◦C. In addition, Kazuhiko Matsumoto et al. [33] prepared a carbon-coated NASICON-type Na3V2(PO4)3 through a sol–gel method and investigated it as a cathode material in ionic liquids. Because the cathode has merits of traditional NASICON-type material, which corner-share the arrangement of the polyhedral units, promoted by carbon coating, it is manifested as an ideal performance at low temperatures. In another paper, Yan Wang et al. [34] synthesized the NASICON-structured Na4MnCr(PO4)3 cathode through sol–gel-assisted solid-based way. Then, the system was tested with the Na counter electrode. This half-cell delivered a favorable specific capacity of 100 mAh/g and a high charge–discharge stage around 4.0 V at a rate of 0.1 C under −10 ◦C. Although these above cathode materials showed relatively high operating voltage under low temperatures, the use of toxic and expensive V and Cr elements remains a critical issue in real applications. The Fe-based NASICON-type cathode is more environmentally friendly. Yongyao Xia et al. [35] prepared a flaky porous Na3Fe2(PO4)3 cathode via a spraying and drying method. In the microscopic characterization stage, they found that formed [Fe2(PO4)3] was shaped as cylindrical lanterns. The cathode manifested as a NASICON type underwent two main processes when charge–discharge begun. In the first Na+ transfer reaction, there is a phase changing process which is a transportation from original Na3Fe2(PO4)3 to Na4 Fe2 (PO4 )3 . Then, as Na+ keeps transferring, the Na4 Fe2 (PO4 )3 in cathode turned into Na5Fe2(PO4)3. Assembled with a hard carbon anode, the full cell delivered an ideal specific capacity of 74.6 mAh/g in 0 ◦C and 40 mAh/g at −20 ◦C with rate of 1 C, as shown in Figure 3f. Meanwhile, MNVP@C nano tubes were designed and synthesized by Changzhou Yuan et al. [36]. They first processed the raw material using a large-scalePDF Image | Na Ion Batteries Used at Low Temperatures
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