Analysis of Vinylene Carbonate Derived SEI Layers on Graphite Anode

142

179

The gas evolution during the formation of graphite electrodes is quantified by On-line Electrochemical Mass Spectrometry (OEMS) for dry electrolyte (< 20 ppm H 2 O) and 4000 ppm H 2 O containing electrolyte to mimic the effect of trace water during the formation process. While the formation in dry electrolyte mainly shows ethylene (C 2 H 4 ) from the reduction of ethylene carbonate (EC) and small amounts of hydrogen (H 2 ), the formation in water-containing electrolyte yields large amounts of H 2 and considerable amounts of CO 2 in addition to the expected C 2 H 4 evolution. We could show that a protective solid-electrolyte interphase (SEI) layer formed by pre-cycling the graphite electrode in 2% vinylene carbonate (VC) containing electrolyte can reduce the H 2 evolution in watercontaining electrolyte by a factor of 7.5 compared to a pristine graphite electrode. Consequently, the ability of graphite electrodes to form an SEI prevents excessive gassing from trace water, which, e.g., is observed for non-SEI forming lithium titanate ( Today's Li-ion batteries usually employ graphite or lithium titanate Li 4 Ti 5 O 12 (LTO) as anode material. Graphite electrodes are known to have an irreversible capacity loss upon the first charge, i.e., during the first Li + -ion intercalation into the layered graphite structure. 1 This effect is related to the formation of the so called solid-electrolyte interphase (SEI) and depends strongly on the electrolyte composition and the electrode potential.2 The capacity loss is explained by the reduction of electrolyte components on the graphite surface, which irreversibly consumes Li + -ions and forms the protective and passivating SEI layer on the negative electrode. On the one hand, the SEI inhibits further reduction reactions on the anode surface due to its electronically insulating nature, on the other hand it is still permeable for Li + -ions, allowing for Li + intercalation. [3][4][5][6][7][8][9][10][11] Recent work by the group of Brett Lucht showed that lithium ethylene dicarbonate (LEDC) and LiF are the main constituents of the SEI layer on graphite, when ethylene carbonate-based electrolytes are used, independent of the type of lithium salt. 12 The main gaseous components formed during electrolyte reduction on graphite anodes are ethylene (C 2 H 4 ), hydrogen (H 2 ) and other hydrocarbons like propylene (depending on the cyclic carbonate, which is used in the electrolyte mixture).13-15 When employing the common alkyl carbonate electrolytes containing ethylene carbonate (EC), dimethylcarbonate (DMC), diethylcarbonate (DEC) and/or ethyl methyl carbonate (EMC) with LiPF 6 , no carbon dioxide (CO 2 ) formation is observed during the SEI formation. 10In contrast to graphite, LTO is generally believed to possess no SEI protection, since its lithiation potential (1.55 V Li ) is too high to allow reduction of electrolyte components. Thus, LTO is more prone to gassing from trace water and other impurities in the alkyl carbonate electrolyte.16 Gassing studies with LTO as anode material...

Section: Gas Evolution At An Sei-protected Electrode In Electrolyte Cmentioning

confidence: 99%

Gas Evolution at Graphite Anodes Depending on Electrolyte Water Content and SEI Quality Studied by On-Line Electrochemical Mass Spectrometry

Bernhard

Metzger

Gasteiger

2015

142

179

“…8, the coulombic efficiencies of these electrolyte solutions do not reach that of EC+EMC. However, this study will be a valuable source for developing electrolytes of lithium-ion batteries because the additives used in this study can suppress co-intercalation of Li + with a high-donor solvent like TMP without conventional additives such as vinylene carbonate 19,29,30 and vinyl ethylene carbonate, 19,30,31 which can form a protective film on the surface of a graphite negative electrode. In the near future, we will find new factors that enable the coulombic efficiency of electrolyte solution including TMP to reach that of EC+EMC.…”

Section: Chemical Shift / Ppmmentioning

confidence: 99%

Effect of Lewis Acids on Graphite-Electrode Properties in EC-Based Electrolyte Solutions with Organophosphorus Compounds

Tsubouchi

Suzuki

Nishimura

et al. 2018

Compatibility of self-extinguishing property and electrochemical stability of electrolyte solutions containing organophosphorus compounds as flame retardants has been required for practical application of electrolyte solutions to lithium-ion batteries. By adding a Lewis acid, Ca 2+ , Mg 2+ , or Na + , to ethylene carbonate (EC)-based electrolyte solutions containing 50 vol.% trimethylphosphate or dimethyl methylphosphonate, the electrolyte solutions are able to suppress the co-intercalation of the organophosphorus compounds with Li + into graphite. However, the coulombic efficiency in the first charge-discharge in a graphite/Li metal half-cell with an electrolyte solution depends on the species of the added Lewis acid and organophosphorus compound. The difference between the chemical-shift changes of the oxygen of the C=O bond in EC and the oxygen of the P=O bond in organophosphorus compounds in the oxygen-17 nuclear magnetic resonance ( 17 O NMR) spectra of the electrolyte solutions was investigated. The investigation revealed that the difference in the acidities of the Lewis acids, electron-donating abilities of the organophosphorus compounds, and solvated structures with a Lewis acid affect the electrochemical stability of self-extinguishing electrolyte solutions. Safety, specifically concerning flammability, is still an important concern with lithium-ion batteries, especially as they are now widely used in high-power and -energy applications such as electric-load leveling and electric-vehicle systems. Organophosphorus compounds, such as phosphate, 1-5 phosphonate, 6,7 and phosphazene, 8,9 are widely known as excellent flame retardants. These organophosphorus compounds are used either as co-solvents or additives in the conventional alkyl-carbonate-based electrolyte solution used in lithium-ion batteries. Trimethylphosphate (TMP) suppresses the flammability of electrolyte solutions because of its self-extinguishing property; that is, it can selectively scavenge the active hydrogen and oxygen radicals that contribute to the combustible chain reaction.1,10,11 However, TMP, which has a higher electron-donating ability than ethylene carbonate (EC), 12 tends to co-intercalate with Li + into the graphite negative electrode, resulting in continuous decomposition of the electrolyte solution and large, irreversible capacity loss during the initial chargedischarge of a lithium-ion battery. 1,[13][14][15][16]20 A solution to this co-intercalation problem was devised by Takeuchi et al. That is, the compatibility of TMP with graphite was improved by introducing a salt composed of stronger Lewis acids than Li + , such as Ca 2+ . 16 Moreover, the charge-discharge efficiency in the case of TMP in graphite was improved by adding calcium bis(trifluoromethanesulfonyl) amide (Ca(TFSA) 2 ) to an electrolyte containing TMP. This suppression of co-intercalation is derived from the solvation structure of Li + being altered by the stronger Lewis acid, namely, Ca 2+ . Fluorinated alkyl phosphates, such as tris(2,2,2-trifluoroethyl) pho...

“…2,3 Vinylene carbonate (VC) is one of the most effective additives to modify the SEI on graphite anodes, as it is reduced at potentials more positive than 1.0 V vs. Li/Li + and hence suppresses the reduction of EC. 4,5 Aurbach et al have used VC as electrolyte additive in an EC/DMC (dimethyl carbonate) based electrolyte and that time reported a reduction of the irreversible capacity in the first cycles and an improved cycling stability at elevated temperatures for graphite anodes. The SEI resulting from the reduction of VC consists mainly of poly (vinylene carbonate) (poly(VC)).…”

mentioning

confidence: 99%

Analysis of Vinylene Carbonate (VC) as Additive in Graphite/LiNi_0.5Mn_1.5O₄Cells

Pritzl

Solchenbach

Wetjen

et al. 2017

124

Vinylene Carbonate (VC) is an effective electrolyte additive to produce a stable solid electrolyte interphase (SEI) on graphite anodes, increasing the capacity retention of lithium-ion cells. However, in combination with LiNi 0.5 Mn 1.5 O 4 (LNMO) cathodes, VC drastically decreases cell performance. In this study we use on-line electrochemical mass spectrometry (OEMS) and electrochemical impedance spectroscopy (EIS) with a micro-reference electrode to understand the oxidative (in-)stability of VC and its effect on the interfacial resistances of both anode and cathode. We herein compare different VC concentrations corresponding to VC to graphite surface area ratios typically used in commercial-scale cells. At low VC concentrations (0.09 wt%, corresponding to 1 wt% in commercial-scale cells), an impedance increase exclusively on the anode and an improved capacity retention is observed, whereas higher VC concentrations (0.17 wt -2 wt%, corresponding to 2 wt -23 wt% in commercial-scale cells) show an increase in both cathode and anode impedance as well as worse cycling performance and overcharge capacity during the first cycle. By considering the onset potentials for VC reduction and oxidation in graphite/LNMO cells, we demonstrate that low amounts of VC can be reduced before VC oxidation occurs, which is sufficient to effectively passivate the graphite anode. During the first charge of a lithium ion battery (LiB), the so called solid electrolyte interphase (SEI) 1 is formed on the surface of the negative electrode. The standard electrolyte for LiBs consists of a mixture of cyclic and linear carbonates, e.g., ethylene carbonate (EC) and ethyl methyl carbonate (EMC), typically with lithium hexafluorophosphate (LiPF 6 ) as salt. Starting from a potential of ∼0.8 V vs. Li/Li + , EC is reduced electrochemically into ethylene gas and lithium ethylene dicarbonate (LEDC), which is a key component of the SEI.2,3 Vinylene carbonate (VC) is one of the most effective additives to modify the SEI on graphite anodes, as it is reduced at potentials more positive than 1.0 V vs. Li/Li + and hence suppresses the reduction of EC. 4,5Aurbach et al. have used VC as electrolyte additive in an EC/DMC (dimethyl carbonate) based electrolyte and that time reported a reduction of the irreversible capacity in the first cycles and an improved cycling stability at elevated temperatures for graphite anodes. The SEI resulting from the reduction of VC consists mainly of poly (vinylene carbonate) (poly(VC)). 4,6Important studies on the impact of different VC concentrations in graphite/NMC pouch cells have been carried out by the Dahn group. For example, Burns et al. 7 investigated the effect of different concentrations of VC (0, 1 and 2 wt%) on cycle life and impedance growth of full-cells with graphite anodes and either LCO (LiCoO 2 ) or NMC (Li(Ni 0.42 Mn 0.42 Co 0.16 )O 2 ) cathodes, employing galvanostatic cycling experiments coupled with high precision coulombic efficiency and electrochemical impedance spectroscopy (EIS) measurements. For cel...