Electrolytes vinylene carbonate

Numerous research activities have focused on the improvement of the protective films and the suppression of solvent cointercalation. Beside ethylene carbonate, significant improvements have been achieved with other film-forming electrolyte components such as C02 [156, 169-177], N20 [170, 177], S02 [155, 169, 177-179], S/ [170, 177, 180, 181], ethyl propyl carbonate [182], ethyl methyl carbonate [183, 184], and other asymmetric alkyl methyl carbonates [185], vinylpropylene carbonate [186], ethylene sulfite [187], S,S-dialkyl dithiocarbonates [188], vinylene carbonate [189], and chloroethylene carbonate [190-194] (which evolves C02 during reduction [195]). In many cases the suppression of solvent co-intercalation is due to the fact that the electrolyte components form effective SEI films already at potential which are positive relative to the potentials of solvent co-intercalation. An excess of DMC or DEC in the electrolyte inhibits PC co-intercalation into graphite, too [183]. 

The lithium-storage properties of these Si SiOx/C nanocomposite electrodes were investigated in different electrolyte systems and compared to pure Si nanoparticles. From all the analyzed systems, the Si SiOx-C nanocomposite in conjunction with the solvent vinylene carbonate (VC) to form the solid-electrolyte interface showed the best lithium storage performance in terms of a highly reversible lithium-storage capacity (1100 mAh g-1), excellent cycling performance, and high rate capability (Fig. 7.9). 

Chen L, Wang KE, Xie X, Xie J. Enhancing electrochemical performance of silicon film anode by vinylene carbonate electrolyte additive. Electrochem Solid-State Lett 2006 9 A512-A515. 

Fig. 2.10 Cycling performance of lithium-ion battery composed of oxygen stoichiometric Lij q AIq j Mn and graphite (MCMB6-28) at RT(open circle) and 60°C (filled circle). Electrolyte contains vinylene carbonate. EC MEC(3 7), 1-M LiPF6, 4.2-S.3 V...

When PC is used as a solvent, the electrolyte continues to decompose at about 0.9 V Vi. LP/Li, as shown in Fig. 4.4, and graphitic (crystalline) carbons cannot be charged due to their exfohation caused by the solvent co-intercalation. This is the reason why EC is dominantly used instead of PC, which was adopted for the first commercial lithium-ion cells with anongraphitic (amorphous) carbon anode. However, the addition of some compounds such as vinylene carbonate (VC) prevents the graphite exfoliation and enables the charge of graphitic carbons, as shown in Fig. 4.4. 

Because these sulfone derivatives have no ability to form an effective SEI, 2 wt% of vinylene carbonate (VC) was added in 1 mol dm LiPFe in EMES or MEMS for graphite/LiCoOa full cells [174]. Imol dm LiPFg in EMES+2 wt% VC showed a capacity drop, when the rate was raised to 0.50 mA cm" (C/4.6), whereas 1 mol dm LiPFg in MEMS + 2 wt% VC exhibited almost the same performance as the reference electrolyte, 1 mol dm LiPFg in EC-DMC (50-50 vol.%), at 0.46 mA cm (C/5). These results indicated that more flnid versions of the sulfone-based electrolytes are necessary. 

Ota, H. Sakata, Y. Otake, Y. Shima, K. Ue, M. Yamaki, J., Structural and functional analysis of surface film on Li anode in vinylene carbonate-containing electrolyte, J. Electrochem. Soc., 2004,151 (11), A1778-A1788. 

Wang, Y Nakamura, S. Tasaki, K. Balbuena, P.B. Theoretical studies to understand surface chemistry on carbon anodes for lithium-ion batteries how does vinylene carbonate play its role as an electrolyte additive J. Am. Chem. Soc., 2002,124,4408-4421. 

Comparison of the common electrolyte solvents (EC, propylene carbcaiate [PC], dimethyl carbonate [DMC], EMC, vinylene carbonate [VC], dimethoxyethane [DME]) oxidative stability with experiments was reported by Zhang et al. [3]. While trends of the oxidative stability were reasonably captured in this study, typical deviations between experiments and simulations were reported to be around 0.5-1.0 V. Note that Zhang et al. [3] did not use the value of 1.4 V to convert from the absolute to Li /Li potential scale, instead they used the Li/LF and M/M" cycles with a number of calculated/estimated quantities, resulting in the absolute potential versus LF/Li being around 2.2 V. Application of the value of 1.4 or 1.54 V derived from SHE potential in water and acetonitrile and using the standard LF/Li vs. SHE potential will result in an improved agreement between QC-based values reported by Zhang et al. [3] and experiments. 

Chen LB, Wang K, Xie XH, Xie JY (2007) Effect of vinylene carbonate (VC) as electrolyte additive on electrochemical performance of Si film anode for lithium ion batteries. J Power Sources 174 538-543... 

Functional electrolyte additives are included in the electrolyte solution to improve battery performance. This concept has been around for some time, tind the basic technology is well established. One early example is the addition of propane sultone to the nonaqueous electrolyte solution of a rechargeable battery using a metallic lithium anode. Although this technology was initially developed for metallic lithium batteries, the use of such additives for LIBs began around 1994. Since then a wide range of additives have been developed. So many different compounds have been used as additives that they are too numerous to mention, but notable examples include vinylene carbonate, propane sultone, phenylcyclohexane, and fluoroethylene carbonate. The selection of additives and determination of their appropriate formulations have become a key aspect of the proprietary know-how of each battery manufacturer, and the search for new additives continues apace. 

Figure 1.5. Vinylene carbonate (VC) andfluoroethylene carbonate (FEC) two electrolyte additives for SEIformation currently used in Li-ion batteries...

Lee HH et al (2005) The function of vinylene carbonate as a thermal additive to electrolyte in lithium batteries. J Appl Electrochem 35 615-623. doi 10.1007/sl0800-005-2700-x... 

SEI formation-boosting additives. There has been a significant interest in the development of electrolyte additives to enhance the stabihty of LiPFe-based electrolytes for lithium-ion batteries [28]. One class of additives, which includes vinylene carbonate (VC) [29] and LiBOB [30, 31], react on the surface of the anode to generate a more stable SEI. Another class of additives includes N,N-dimethylacetamide (DMAc). DMAc reduces the reactivity of LiPFe and inhibits the reactions between the electrolyte and the electrode materials [32-34], Stabilizing the anode SEI and inhibiting the detrimental reactions of the electrolyte with the surface of the electrode materials will extend calendar life and enhance the thermal stability of lithium-ion batteries. 

This irreversible consumption of lithium can be optimized by adding adjuvants into the electrolyte. The use of an adjuvant such as VC Vinylene Carbonate) is often cited in the composition of the electrolyte. It is, like other molecules, considered to be a precursor of the SEI, because it decomposes early on (at high potential), before the other solvents in the electrolyte, forming a fine, homogeneous film on the surface of the graphite particles. The number of lithium ions involved in its decomposition is therefore low, and the irreversible capacity reduced. 

PC is also a very useful solvent of LIBs because of its superior ionic conductivity over a wide temperature range. However, despite the close structural similarity between EC and PC, PC cannot form as effective SEI films as EC does, for LIBs that employ graphite as negative electrodes. " To enable to use PC in these batteries, there have been a lot of efforts focusing on the identification of proper additives and/or co-solvents for PC-based electrolytes, which would help to generate an efficient SEI layer. The typical liquid additives include chloroethylene carbonate (CEC), other halogen-substituted carbonates, a variety of unsaturated carbonates such as vinylpropylene carbonate and vinylene carbonate, and ethylene/propylene sulfite (ES/PS). The most common co-solvents are DMC, DEC, EMC, y-butyrolactone (y-BL), dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), dimethyl amide (DMA), 1,2-dimethoxy-ethane (DME) and 1,2-dimethoxy-methane (DMM). To explore the role of these additives and co-solvents, it is necessary to understand their structures and some properties that may affect the SEI formation on graphite anodes. 

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