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Nanomaterials 2022, 12, 3529 15 of 29 Among all the above-mentioned anode materials, HC anode materials are most valued due to their low potential versus Na+/Na and high cycling stability during de/sodiumization. The smooth electrochemical plateau of HC is attributed to the intercalation and deinter- calation of Na+ into graphite interlayer gap which is larger than 0.37 nm. HC, which has been widely used as a node material, also has higher stability and better electrochemical performance than other anode materials at low temperatures. The most important thing is that the precursor of HC is cheap and environmentally friendly, which has incomparable practical advantages compared to other anode materials. General HC precursor materials are composed of various biopolymers, including hemicellulose, cellulose, lignin, pectin, protein, free sugars, etc. In our article, both longan peel and tissue paper used as precursor materials showed satisfactory low-temperature performance. The main progress now lies in annealing the precursor and further improving the porosity inside the HC to increase the Na+ diffusion rate and improve its electrochemical performance at low temperatures. 4. Low-Temperature SIB Electrolytes As an important component of SIBs, the electrolyte is a medium for ion conduction that provides pathway to support electrochemical reactions on electrodes [56]. The elec- trolyte not only affects the migration rate of Na+, but also participates in the formation of the SEI [57]. Therefore, the electrolyte is a key factor determining the electrochemical performance of SIBs. The unfavorable electrochemical character of electrolytes at low tem- peratures is also a considerable bottleneck to the application of SIBs [58]. Low-temperature conditions seriously affect the performance of the electrolyte. Firstly, the viscosity of the electrolyte increases, which not only leads to a decrease in ion migration rate and conduc- tivity [59], but also causes the poor wettability of the electrode and the membrane [60]. In addition, because the resistance of SEI between the electrode and the electrolyte increases, the electrolyte becomes less compatible with electrode materials at low temperatures [61]. All these unfavorable factors damage the energy density and cycle stability of the battery. Therefore, electrolytes that can be applied under low-temperature conditions should have the following characteristics: (a) favorable stability; (b) good ionic conductivity; (c) a wide electrochemical window; (d) no reaction with other battery components; (e) high safety and low toxicity, and (f) can meet the cost requirements for practical applications. The above characteristics of electrolytes essentially depend on the property and selection of the electrolyte salt [62]. While a limited number of efforts are being directed to the search for new electrode materials for SIBs at low temperatures, studies dealing with the electrolytes are much scarcer. Herein, we summarize recent research on SIBs at low temperatures and divide these electrolytes into four sections according to their composition: organic electrolytes, aqueous electrolytes, ionic liquid electrolytes, and solid/quasi-solid electrolytes. 4.1. Organic Electrolytes Organic electrolytes are usually a mixture of sodium salts (ca. NaClO4, NaPF6) with differ- ent carbonic acid derivative (ca. PC = polycarbonate, EC = ethylene carbonate, DMC = dimethyl carbonate, DEC = diethyl carbonate, EMC = ethyl methyl carbonate, etc.). Meanwhile, the performance of electrochemical SIBs can be significantly improved by changing the composition and ratio of carbonic acid derivatives or slightly adding some other additives. At the same time, matched with suitable anode and cathode materials, the SIB can be manifested given its favorable electrochemical capacity and stability. Among sodium salts, NaClO4 is more abundantly studied at low temperatures. In addition, the PC solvent has a freezing point of −48 ◦C and an outstanding dielectric constant which effectively avoids any solidification of the electrolyte system and improves the low-temperature performance of SIBs by increasing the ionic conductivity. However, the single-component PC is less compatible with the electrodes, so efforts are devoted to solving the problem with addi- tives [63]. It is proved by Haoshen Zhou et al. [64] that the addition of fluoroethylene carbonate (FEC) hugely improved the PC electrolyte low-temperature performance. The electrolyte of 1 M NaClO4 dissolving in PC with the addition of 2 vol% FEC exhibited aPDF Image | Na Ion Batteries Used at Low Temperatures
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