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Molecules 2022, 27, 6516 23 of 32 area, and could change the formation dynamics of SEI films. Herein, rGO with a high specific surface area was selected as the carrier for the performance comparison between ester and ether solvents, and the coulomb efficiency difference in the first cycle could be nearly doubled. Although HC materials were not chosen for testing, the results of this comparative test have significant implications for the optimization strategy of electrolytes with an HC anode and open up a general SEI-modifying strategy that is not limited to carbon microstructure. 4.4.2. Anode Pre-Treatment Another important means to optimize battery conditions is pre-treatment. Pre-oxidation can improve the electrochemical properties of HC materials from the perspective of elec- trode materials. Hu pioneered a simple pre-treatment method that could be extended to other precursors [26] through pre-oxidation. Crosslinking in a low temperature oxidation process was ensured, while rearrangement in a high temperature carbonization process was inhibited, maintaining a high degree of disorder in HC (shown in Figure 17). The reversible capacity increased from 94.0 mAh g−1 to 300.6 mAh g−1, and the ICE rose up to 88.6% from 64.2%. Recently, it has been proven that HC materials with expanded carbon interlayers could be made by the controlled introduction of oxygen-containing functional groups [131]. This pre-treatment greatly improved the capacity and performance of the raw commercial HC from 270 mAh g−1 to 341 mAh g−1 in 20 mA g−1. Kinetic measure- ment and theoretical calculation results show that the introduction of oxygen-containing functional groups expands the distance between the carbon layers, promotes the diffusion of sodium-ions, and enhances the adsorption capacity of sodium-ions. On the other hand, Wang [132] considered that the influence of unremoved heteroatoms and defects on HC lacks an experimental confirmation. Therefore, they used a pre-treatment method in which a mixture of organic vapor and argon is introduced during the annealing process of the precursor, to precisely regulate the heteroatoms and defects in the HC layer by controlling the atmosphere. If the vapor of cyclohexane is introduced, for example, the decomposition products react with the oxygen-containing functional groups on the surface and fill the defects of the carbon layer. Following this engineering treatment, this kind of HC anode exhibited a high ICE up to 85%, and the full-cell consists of HC//NVPF with an energy density of 239 Wh kg−1. Pre-lithium is a pre-treatment method that significantly improves lithium capacity in LIBs. Balbuena and Li [133] applied pre-lithium to SIBs by ball milling Si with Li powder and contacting them with a tetraglyme-based electrolyte. It formed a SEI film via a reaction with the active substances, hindering the transfer of Sodium-ions and reducing the open-circuit voltage. This operation avoids the danger and instability of Na powder. Meanwhile, the lithium replenishment strategy in LIBs is adopted and the ICE in the HC anode of SIB is improved. The pre-treated HC anode has a specific capacity of about 150 mAh g−1 in 1 A g−1 and an ICE greater than 92%. This method provides practical significance by combining theory with experiment. However, the process is complex and resource problems still exist, so an independent “pre-sodiation” strategy belonging to SIBs still needs to be explored. In 2012, Ai and Lai jointly constructed a new pre-sodiation method [134] using a three-electrode method. A sodium foil was introduced as a reference electrode to complete the sodiation of the HC anodes (as shown in Figure 18a), and the ICE was increased to 73%. It provides the feasibility to achieve the “pre-sodiation” of SIBs in spite of the difficulty of controlling the unstable sodium metal electrode, the complexity of the method and the difficulty in achieving industrialization. Qian’s Group [135] has proposed a method of chemical pre-sodiation using a sodium diphenyl reagent to achieve a mild and efficient sodium supplement, which makes up for the problem of irreversible capacity loss. Com- bined with an Na3V2(PO4)3 cathode to construct a full battery, it shows a high ICE of 95.0% and an energy density up to 218 Wh kg−1. As shown in Figure 18b, the pre-sodiation depth can be precisely controlled by adjusting the pre-treatment time by immerging thePDF Image | Hard Carbons as Anodes in Sodium-Ion Batteries
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