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Chem Soc Rev Review Article especially improve the reversible capacity and achieve a longer cycle life for black P electrodes with NaPF6 in EC/DEC. The outermost surface layer of the SEI in the VC additive is composed of both inorganic and organic compounds. According to their report, the organic compounds that contain fragments (m/z = 55 (C5H7+), 57 (C4H9+), 67 (C5H7+), 69 (C5H9+), 71 (C5H11+), and 83 (C6H11+) in TOF-SIMS result) play an important role as components of SEI layers in the electrode, which results in the improvement of electrochemical performance and suppression of SEI dissolution. On the other hand, VC did not form efficient surface layer films on hard-carbon electrodes with NaClO4 in PC.577 As mentioned above, a solid electrolyte interphase (SEI) is one of the crucial determinants of battery life. The final SEI composition can be affected by a multitude of different factors, such as the order of the bond breaking events, the relative stability of resulting radical ions, and the thermodynamic feasibility of different decomposition reactions.582 Previous studies suggested that prolonged decomposition of electrolytes forming a thick and unstable solid–electrolyte interphase continues to be a major drawback in designing sodium- ion batteries. Recently, Kumar et al. investigated the funda- mental mechanisms of solvent decomposition involved in SEI formation in SIBs using the density functional theory (DFT) method.583 According to their calculations, in EC based elec- trolytes, the high reduction potential and a low barrier for the ring opening of EC is the main cause for the continuous growth of a SEI in SIBs. They also studied the effect of additive molecules on the decomposition pathways of the EC. On the other hand, Dahbi et al. reported the formation mecha- nism and the main component of surface films on the phos- phorus and hard carbon electrode with different electrolyte solutions.522,588 In the NaPF6 based electrolyte, they proposed the decomposition mechanism of NaPF6 in the electrolyte in the following four step. (1) NaPF6 2 NaF + PF5, (2) PF5 + nNa+ + ne -NaxPFy +NaF,(3)NaPF6 +H2O-NaF+POF3 +2HFand (4) POF3 + nNa+ + ne - NaxPOFy + NaF. In other words, NaPF6 is reduced to a gaseous product, phosphorus pentafluoride (PF5), according to step 1. F ions are also released and react with Na+ ions in the electrolyte to form NaF on the electrode surface. They also suggested the possible mechanism for additive (FEC and VC) reduction reactions in Na cells. When FEC is added in a Na cell, it is believed that FEC reduces first, then polymerizes through the opening of a five-membered ring, and forms very thin and stable protective films which may contain NaF, Na polycarbonate and polyenes (oxygen-free poly- mers). In contrast, VC-based electrolytes are also reduced, resulting in the formation of oxygen-containing species. The electrochemical reduction of VC forms CHCHOCO2 radical anions, which could be stabilized by Na+ cations in Na cells. They speculate that such organic compounds that contain fragments play an important role as components of the SEI in the electrode. Such results clearly show that the additive molecules protect solvent molecules from reductive decompo- sition but also they can promote alternate pathways for decom- position, leading to qualitatively different and potentially stable SEI products.584–587 formation on the hard carbon electrode surface without the need of FEC additives. Recently, Dahbi et al. reported the electrochemical sodium insertion for hard carbon in a cyclic alkylene carbonate based (PC, PC/EC) solution containing a NaClO4 or NaPF6 salt with or without a fluoroethylene carbo- nate (FEC) additive to study electrolyte dependency for sodium- ion batteries.588 According to their report, by adding a FEC additive in both NaPF6 and NaClO4 based electrolytes, the hard carbon electrode exhibited an additional voltage plateau at approximately 0.7 V vs. Na/Na+ during the first reduction due to decomposition of FEC and delivered a higher reversible capacity than without a FEC additive electrolyte. Namely, the irreversible reaction during the initial cycle in both NaPF6 and NaClO4 based electrolytes was effectively suppressed by the FEC additive, which enhanced the reversibility of hard carbon anodes. The passivation layer would be further stabilized and become a thinner surface layer in the presence of FEC because the fluorine compounds dissolved in the electrolyte would efficiently induce the formation of an electronically insulating, ionically conducting, and insoluble surface layer to effectively passivate the electrode surface. On the other hand, the significant influence of the electro- lyte additive is clearly observed in alloying and/or conversion reaction based anodes.384,460,539,561,564 Such electrode materials accompany dramatic volume changes, which has led to severe electrode pulverization upon the sodiation–desodiation process. Therefore, a stable passivation film on an electrode surface is required for effectively preventing such drawback. The formation of stable SEI films can stabilize the electrode and improve the migration kinetics of Na by using FEC addi- tives. The main components of FEC-containing electrolytes were proposed as important compounds in effectively protect- ing surface films. The final SEI composition can be affected by a multitude of different factors, such as the order of the bond breaking events, the relative stability of resulting radical ions, and the thermodynamic feasibility of different decomposition reactions.582 When FEC free electrolytes were used in Na cells, the XPS results suggest that the SEI in SIBs has a higher percentage of inorganic salts, alkyl carbonates, and CQO rich compounds, which are unstable upon electrochemical cycling.352,486 Recently, Dahbi et al. reported the possible mechanism for FEC reduction reactions in Na cells.522 In addition, Kumar et al. proposed the reaction pathways for the reduction induced decomposition of FEC–Na+ in one electron reduction states.583 When FEC is added in a Na cell, it is believed that FEC reduces first, then polymerizes through the opening of a five-membered ring, and forms very thin and stable protective films which may contain NaF, Na polycarbonate and polyenes (oxygen-free polymer).584–587 As a result, previous reports have indicated that FEC addition in organic electrolyte solutions leads to a significant improvement of the cyclability. Kim et al. and Li et al. examined and compared electrochemical performances of tin–phosphide electrodes with/without FEC additives460–462 (Fig. 42f). More recently, Dahbi et al. investigated the electrode/ electrolyte interface and reported the positive effect of VC additives in black phosphorous electrodes.522 VC added electrolytes View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3593 Open Access Article. Published on 28 March 2017. Downloaded on 7/1/2019 3:41:21 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.PDF Image | Sodium-ion batteries present and future
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