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Nanomaterials 2022, 12, 3529 4 of 29 2.2. Layered Oxides In latest research, layered oxides, including NaxTMO2 (x ranges from 0 to 1; TM is a transition metal), are being increasingly noticed by researchers because they have superior electrochemical performance. These layered oxides also are environmentally friendly and easy to prepare. In NaxTMO2, there are different Na+ conditions and oxide layering ways, and scientists summarized them and divided them into two categories (P2 and O3 types). For now, the main obstacle of layered oxide application in SIB cathodes is that a phase transition is required when charge and discharge take place. Some of them have unfavorable electrochemical behaviors when the temperature goes down, leading to potential safety hazards. Therefore, the use of transition metal oxide cathode materials at low temperatures is more challenging. In the study of Yufeng Zhao et al. [22], researchers coated a NaTi2(PO4)3 nano-shell on the surface of P2-type manganese-based layered oxide Na0.67Co0.2Mn0.8O2 (NCM) to improve its electrochemical performance. Compared with uncoated NCM, the NCM@NTP (7 wt%) sample significantly improved the kinetics of Na+ migration and the structural stability of NCM. Therefore, when combined with the sodium metal anode, at −20 ◦C, the NCM@NTP7 sample exhibited discharge capacities of 122.7 mAh/g at 0.2 C, 110.8 mAh/g at 0.5 C, 104.8 mAh/g at 1 C, 72.7 mAh/g at 5 Cm and 56.1 mAh/g at 10 C. It obtained a capacity retention of 92.3% after 100 cycles at 0.2 C. O3-type oxide is another layered structure cathode for SIBs. Changzhou Yuan et al. [23,24] fabricated 1D ultralong NaCrO2 nanowires with a continuous and interconnected framework, which guaranteed large sur-/interfaces for the contacting of electrolyte and active material to provide convenient electronic/ionic migration pathways for rapid charge transfer. Moreover, 1D NaCrO2 shows remarkable performance compared with traditional NaCrO2 bulk cathodes at low temperatures. When coupled with a hard carbon anode, it exhibited a specific capacity of 108 mAh/g and a stable voltage stage of 3.1 V at a current density of 0.2 C under −15 ◦C. Even at super high rate of 15 C, its voltage and capacity stayed almost unchanged. Furthermore, 1D NaCrO2 obtained a 78.3% specific capacity retention over long charge–discharge life of 100 cycles. It exhibited superior stability than that of bulk NaCrO2 cathodes (only a 16.7% specific capacity retention at −15 ◦C) [25]. Yang-Kook Sun et al. [26] designed a hierarchical columnar structure by assembling Na(Ni0.75Co0.02Mn0.23)O2 with another proportion Na(Ni0.58Co0.06Mn0.36)O2. This layered structure effectively avoids unnecessary side reactions of the materials with the electrolyte because it expands surface contact proportion by adding more pores. Therefore, this system combined with a hard carbon anode enclosed outstanding electrochemical performance at low temperatures with the rate of 0.5 C through 100 cycles, which delivered specific capacities of 128.8 and 114 mAh/g at 0 ◦C and capacity retentions of 89% and 92% and −20 ◦C, respectively. Hou, Yanglong et al. [27] proposed P2-type Na0.67Ni0.1Co0.1Mn0.8O2 material prepared through reasonable structure modulation. The material offers an excellent Na+ conduction rate at −20 ◦C. At this extremely low temperature, combined with the Na counter electrode, it still delivered a specific capacity of 148.1 mAh/g at the 0.2 C rate. Moreover, when assembled as a full cell, it also had an extraordinary energy density at −20 ◦C. 2.3. Polyanionic-Type Cathodes Polyanionic-type material is another kind of attractive cathode material for SIBs due to its high operating potential, stable structural framework, and superior safety. According to the type of polyanions, polyanionic-type cathodes can be categorized into six types: phosphates, fluorophosphates, pyrophosphates, mixed phosphates, sulfates, and silicates. So far, mainly phosphates and fluorophosphates are reported for low-temperature SIBs. 2.3.1. Phosphates In a recent study, Chungang Wang et al. [28] coated a nanoparticle NaFePO4 (NFP) with a carbon shell and produced a NFP@C composite, as shown in Figure 3a. The NFP in NFP@C effectively helps to establish ion–electron transfer pathway. Carbon coatingPDF Image | Na Ion Batteries Used at Low Temperatures
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