Organocatalytic Synthesis of Vinylene Carbonates Using Dimethyl Carbonate as a Carbonyl Source

Capacity loss and solid electrolyte interphase (SEI) buildup are accompanied by continuous electrolyte consumption due to electrochemical decomposition reactions[1],[2],[3]. The quantitative analysis of electrolytes in battery cells usually consists in the determination of concentrations or quantity ratios. As the total electrolyte amount in the cell decreases during cell aging due to electrochemical decompositions, concentrations do not reveal reliable information about the dominant decomposition reactions. Hence, the total electrolyte mass and the absolute consumption of the electrolyte components need to be determined to get reliable insights into cell chemistry. For this purpose, a method for electrolyte extraction from pouch cells and subsequent analysis by high-performance liquid chromatography (HPLC) coupled to an electrospray ionization mass spectrometer (ESI/MS) and an ultraviolet/visible light (UV/Vis) detector is presented that enable a quantification of the absolute amounts of the electrolyte components in battery cells[4].The investigated pouch cells are initially filled with 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) 1:1 by weight with 3 wt% vinylene carbonate (VC). For the electrolyte extraction, diethyl carbonate (DEC) is injected into the cell, followed by a sealing of the puncture and a storage of the cell for 8 days to enable an intermixing between DEC and the original electrolyte. Afterwards, the electrolyte is extracted through a puncture in the cell. The UV/Vis detector is used for the determination of the concentrations of EC, DMC, VC, and DEC in the extracts, while the quantification of Li+ and PF6 − is performed by the ESI/MS. Based on these concentrations and the weighed mass of the injected DEC, the total mass of the original electrolyte and the absolute amounts of EC, DMC, VC, Li+, and PF6 − in the cell is determined. The investigated electrolytes are extracted from fresh cells, cells after formation, 25 cycles, and about 2000 cycles (with a remaining capacity of 80%). Based on the absolute amounts of EC, DMC, VC, Li+, and PF6 − in the cells at different aging stages, the absolute consumption of the electrolyte components by electrochemical decomposition is eventually calculated.From the concentrations shown in Figure (a), only a VC decomposition during formation and cycling can be concluded. The consideration of the absolute, consumed amounts of substance during formation and long-term cycling in Figure (b), however, suggests that the EC decomposition is dominant and thus much more pronounced than the VC degradation. Besides, a slight salt and DMC decomposition during long-term cycling is observed. Simultaneously, the quantified electrolyte mass is diminished during formation and cycling, as can be seen in Figure (c).The pronounced VC consumption during formation and cycling is expected, as VC is an additive for the SEI buildup. As a high fraction of the initially present VC is decomposed, its decomposition can be identified by the concentrations. The decomposition of the other components cannot be detected based on the concentrations, as lower fractions of the initially present amounts are consumed and the total electrolyte amount in the cell decreases during formation and cycling. The strong EC degradation might be related to the tendency of EC to contribute to the Li+ solvation shell, which is associated with a preferential reduction[5]. These results demonstrate that only the determination of the absolute consumed amounts of substance allows for identification of the decompositions in battery electrolytes. Figure: (a) Overview of the average concentrations of EC, DMC, and VC (quantified by HPLC-UV/Vis), as well as the average concentrations of Li+ and PF6 − (quantified by HPLC-ESI/MS) in cells at different stages of electrochemical aging[4]. (b) The amounts of substance consumed during formation and prolonged cycling altogether determined by HPLC analyses[4]. (c) The electrolyte masses in the fresh cells, the cells after formation, and the cells after prolonged cycling determined by HPLC analyses related to the corresponding initial electrolyte masses[4].

The electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) improve the lifetime of lithium‐ion batteries with silicon‐containing anodes by their reduction yielding a more stable solid electrolyte interphase (SEI). However, the reductive decomposition mechanism of FEC and VC has not yet been fully clarified. For this purpose, we investigate the electrolyte decomposition in LiNi0.6Co0.2Mn0.2O2 (NCM622)/silicon‐graphite pouch cells containing either 1 M LiPF6 in FEC:dimethyl carbonate (DMC) or 1 M LiPF6 in VC:DMC using high‐performance liquid chromatography, gas chromatography, X‐ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectrometry. Based on the molar consumptions of FEC and VC, and the cumulative irreversible capacities, we show that three electrons are consumed for every reduced FEC molecule, and that one electron is consumed for every reduced VC molecule. Based on the results, reactions of the FEC reduction are proposed yielding LiF, Li2CO3, Li2C2O4, HCO2Li, and a PEO‐type polymer. Furthermore, the reaction of the VC reduction is proposed yielding lithium‐containing, polymerized VC. During formation, the capacity loss of the cells is induced by lithium trapping in LixSiy/LixSiOy under the SEI and by lithium trapping in the SEI. During subsequent cycling, only lithium trapping in the SEI triggers the capacity loss.

The growing demand for advanced portable electronics and electric vehicles calls for the development of Li-ion batteries with enhanced performance and safety. Among the major goals still to achieve is the improvement of cycling stability and safety, where electrolyte and electrode interfacial properties play a central role. It is generally known that during the first battery charge, a thin film called solid electrolyte interphase (SEI) is formed on the negative electrode due to the decomposition of the electrolyte components. The chemical nature and the morphology of the SEI are important factors for the battery performance. Ideally, the SEI layer is stable and prevents further electrolyte decomposition by blocking the electron transfer through the interface, while concomitantly preserving Li+transport. The most reliable way to control the SEI formation is via electrolyte additives, which have a positive impact on the interface properties without affecting the main electrolyte functions. There is extensive research available on polymerizable additives, where vinylene carbonate (VC) is most widely researched and commercialized. In the last years, however, battery safety is of increasing concern, still limiting the implementation of Li-ion batteries in some industrial fields. In relation to the safety issues one appropriate solution is the design of electrolytes with low flammability. The application of diphenyloctyl phosphate (DPOF) as an additive with a twofold input, acting as a SEI improving and additionally as flame-retarding component was recently reported [1]. However, the structural aspects of the functional improvement of electrode interfacial properties are not fully understood and require further analysis. The central aim of this paper is to correlate the electrical and structural properties of the SEI layer built on the graphite anode under the influence of DPOF and comparison with its commercial analogue - VC. Galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy (EIS) of graphite anodes were performed in 1M LiPF6in ethylene carbonate (EC) / dimethyl carbonate (DMC) / diethyl carbonate (DEC) (vol. 1:1:1), containing VC or DPOF as additives. The cells with DPOF additive showed the best performance in terms of capacity and rate capability. EIS analysis was performed in symmetric cell configuration, allowing individual interpretation of the impedance parameters for both electrodes [2,3]. After five initial cycles at C/20 used for the formation of the SEI the cells were stopped at 50% SoC and disassembled. The graphite electrodes were re-assembled in symmetric cells, using the same electrolyte type. The EIS spectra of the graphite symmetric cells consist of at least two overlapping semicircles for higher and a Warburg line at low frequencies. They can be fitted by the equivalent circuit proposed in the literature [3], (Fig.1A). In general, the electrical parameters extracted in the presence of the VC closely resemble these of the control cell (without electrolyte additives). After addition of 2% VC EIS showed a slight decrease of SEI resistance R1 and at the same time a minimal increase of SEI capacitance C1. In contrast, the addition of DPOF to the electrolyte resulted in a substantial decrease in R1 and C1. The structural reason for the lower resistance and capacitance of DPOF formed SEI was analysed by means of X-Ray Photoelectron Spectroscopy (XPS). The analysis showed the presence of typically visible SEI features for all samples (Fig. 1B). C1s peaks at around 285eV and 287eV are attributed to a lithium alkyl carbonates. The O1s peaks at 533eV, 532.5eV and 534eV are assigned to σC-O bond in carbonates (Li2CO3 and non–lithiated alkyl carbonates) and O2C=O groups. Beside the discussed C1s components, a low energy peak (dominant for DPOF and less pronounced for VC and control samples) related to the σC-C bonds from the graphite substrate, suggests a formation of much thinner SEI. The F1s core peaks of all samples consist of two main components at 687.0eV and 685.0eV, related to LiPF6 and LiF, respectively. The P2p spectra are composed of one unresolved doublet (2p3/2 and 2p1/2), common for all three samples and attributed to LiPF6. Additionally, the DPOF samples have a component at 136.8eV, originating from decomposed oxidized phosphorous compounds [1]. The correlation of EIS and XPS analysis indicates that the formation of low-resistive and stable SEI by the assistance of DPOF is related to the growth of much thinner and compact structure, containing oxidized phosphorous compounds.

Introduction Li-ion batteries are used in a wide range of applications, ranging from consumer electronics to electric vehicles. However these batteries suffer from limited lifetime and high cost. The development of long-lived batteries often necessitates the use of elaborate additives that can further increase their manufacturing cost. In this report, we demonstrate that phenyl carbonates can be very competitive additives and can even perform as well as vinylene carbonate (VC). This class of additives can be very inexpensive and can bring many advantages. Experimental The effect of phenyl carbonates including methyl carbonate (MPC), ethyl carbonate (EPC), and diphenyl carbonate (DPC) as additives was studied in machine-made 220 mAh graphite/Li[Ni1/3Mn1/3Co1/3]O2 pouch cells using a wide range of techniques. These techniques included ultra-high precision coulometry,1,2 open circuit voltage storage,3 electrochemical impedance spectroscopy (EIS), EIS on symmetric cells,4 gas chromatography coupled with mass spectrometry for the measurement of additive consumption5,6 as well as gas composition and liquid reaction by-products.7,8 Results and discussion Figure 1 shows the compounds detected in the gas formed after the first charge to 3.5 V of NMC(111)/graphite pouch cells filled with 1M LiPF6 EC:EMC (3:7) base electrolyte containing no additive, containing 1% MPC or 1% DPC. The compounds detected for cells containing no additive can be rationalized with the multiple reduction pathways EC and EMC undertake.7 Figure 1 shows that small loadings of phenyl carbonates yield very similar gas composition as cells without additives. Figure 1 also shows that cells containing MPC and DPC produced a small quantity of benzene. At the same time, cells containing MPC seemed to produce more CH4 than cells filled with control electrolyte, while cells containing DPC produced less CH4 than cells filled with control electrolyte. The presence of benzene, the variation of CH4 and CO2 indicate that the phenyl carbonates get reduced at the graphite surface to some extent. In addition, the transesterification of EMC, to DMC and DEC, that occurs during formation in cells with control electrolyte is completely eliminated when 1% MPC or 1%DPC is added to the electrolyte just as it is when 1% VC is added. Figure 2 shows the results of 40°C, 4.2 V open circuit voltage storage experiments of NMC(111)/graphite pouch cells containing no additive (control), 2% VC, different loadings of MPC or 2% VC + 2% MPC. Figure 2a shows that all cells containing either VC or MPC had a much lower voltage drop during storage than cells without any additive. This is strong evidence that both MPC and VC slow the parasitic reactions at the positive electrode.3 The similarity in voltage drop between cells with VC or phenyl carbonates also indicates that phenyl carbonates are competitive with VC in terms of parasitic reaction reduction at the positive electrode. Figure 2b shows the impedance spectra, measured at 10°C and 3.8 V, of the same cells after 1000 h of storage at 4.2 V and 40°C. Figure 2c shows that small loadings of phenyl carbonates give rise to cells with very small impedance compared to VC. Conclusion Phenyl carbonates are very promising additives. While they slow down parasitic reactions at the negative electrode7 and at the positive electrode (Figure 2a and 2b) as much as VC, they give rise to cells with very small impedance. Diphenyl carbonate is a very inexpensive chemical whose bulk price is less than half of that of VC. The use of phenyl carbonates as additives can then help yield Li-ion cells with long lifetime, good power performance and reduced manufacturing cost. The detailed reduction mechanism of this class of additive will be discussed as well as their effect on the impedance of the positive electrode and negative electrode. Reference 1. A. J. Smith, J. C. Burns, D. Xiong, and J. R. Dahn, J. Electrochem. Soc., 158, A1136–A1142 (2011). 2. A. J. Smith, J. C. Burns, S. Trussler, and J. R. Dahn, J. Electrochem. Soc., 157, A196–A202 (2010). 3. N. N. Sinha et al., J. Electrochem. Soc., 158, A1194–A1201 (2011). 4. R. Petibon et al., J. Electrochem. Soc., 160, A117–A124 (2013). 5. R. Petibon, J. Xia, J. C. Burns, and J. R. Dahn, J. Electrochem. Soc., 161, A1618–A1624 (2014). 6. R. Petibon et al., J. Electrochem. Soc., 161, A1167–A1172 (2014). 7. R. Petibon, L. M. Rotermund, and J. R. Dahn, J. Power Sources, 287, 184-195 (2015) 8. J. Self, C. P. Aiken, R. Petibon, and J. R. Dahn, J. Electrochem. Soc., 162, A796–A802 (2015). Figure 1

On the use of vinylene carbonate (VC) as an additive to electrolyte solutions for Li-ion batteries

Alkyl dicarbonates are known electrolyte degradation products produced in Li-ion cells that use ethylene carbonate (EC) and dimethyl carbonate (DMC) as electrolyte components. Also referred to as dimerization compounds, dimethyl 2,5-dioxahexanedioate (DMOHC) and diethyl 2,5-dioxahexanedioate (DEOHC), were investigated as a possible sole electrolyte solvent or and one component of a blended solvent mixture, when mixed with linear carbonates. The viscosities of DMOHC and DEOHC were measured in this report and compared to the predictions of the Advanced Electrolyte Model1, along with the two alkyl dicarbonates mixed with lithium salts and other common electrolyte solvents. Electrolytes based on DMOHC or DEOHC alone exhibit much higher viscosity than conventional EC-based electrolytes at room temperature. Thus, for testing, LiNi0.5Mn0.3Co0.2O2/graphite (NMC532), LiNi0. 83Mn0. 6Co0. 11O2/graphite (Ni83) and LiFePO4(LFP)/graphite cells with DMOHC were tested at 70°C and 85°C using C/20 charge and discharge rates.DMOHC and DEOHC were mixed with two different electrolyte salts. First, the common LiPF6 salt and the second, lithium bis(fluorosulfonyl)imide (LiFSI), both with 2% vinylene carbonate (VC). These two electrolytes were tested in NMC532/graphite cells cycled to 4.3V at a C/20 charge/discharge rate and 70°C. Cells with DMOHC and LiFSI showed considerable improvements in capacity retention compared to those filled with an EC-based electrolyte with LiPF6 salt, also cycled at 70°C. The same conclusion was found with NMC532 cells filled with DEOHC with LiFSI salt or LiPF6. Figure 1a shows the fractional capacity versus time for NMC532/graphite, Ni83/graphite and LFP/graphite pouch cells with 1.0 M LiFSI in DMOHC with 2% VC (vinylene carbonate) and 1% DTD (ethylene sulfate) additives tested at C/20 and 85°C. These cells were tested to upper cut-off potentials of 3.8, 3.9 and 3.65 V respectively. In addition, Figure 1b shows the corresponding voltage polarization results. Results show that Ni-containing cells experience exceptional lifetimes and low impedance growth despite the high cycling temperature. DMOHC-containing cells have since been tested up to 100°C. DMOHC was most advantageous for mitigating severe gassing at high temperatures. Ex-situ gas experiments show DMOHC-containing Ni83 cells produce the least amount of gas even at high voltage (4.0 V) compared to cells using EC-based electrolytes. Ni-containing cells operating to low voltage limits show unprecedented cycling lifetimes when using DMOHC electrolytes containing LIFSI while also exhibiting limited gassing. To mitigate the high viscosity of DMOHC, various cells were filled with electrolytes that used DMOHC and various percentages of diethyl carbonate (DEC) and dimethyl carbonate (DMC) with LiFSI as the salt. DMC/DEC was chosen for its low viscosity and its ionic conductivity. Even with DMC, the cycling performance showed improvements over traditional EC-based electrolytes. There were slight decreases in performance compared to a cell with a pure DMOHC electrolyte. In addition, various additives were tested, including prop-1-ene-1,3-sultone (PES), to see if gassing could be further limited. We propose DMOHC and DEOHC as new solvents for Li-ion cell electrolytes, possibly replacing EC, to enable long-lasting, high-temperature tolerant Li-ion cells. REFERENCES E. R. Logan, E. M. Tonita, K. L. Gering, and J. R. Dahn, J Electrochem Soc, 165, A3350–A3359 (2018). Figure 1. (a) Normalized capacity versus time for LFP (blue), NMC532 (red) and Ni83 (black) artificial graphite cells, using 1M LIFSI DMOHC 2%VC 1%DTD electrolyte cycling at C/20 and at 85°C. (b) corresponding voltage polarization data versus time. Figure 1

Effects of vinylene carbonate on high temperature storage of high voltage Li-ion batteries

Introduction We recently proposed a novel lithium-ion battery system based on a redox reaction between oxide ions and peroxide ions at a cathode, and demonstrated the system with a Co-doped Li2O cathode, in which Co ions were located at the same sites as Li+ ions.1 In these earlier studies, the Co-doped Li2O exhibited a specific capacity of 270 mAh g−1 with 50-cycle durability.2 However, the specific capacity of 270 mAh g−1 was much smaller than the theoretical specific capacity of 897 mAh g–1 on the basis of Li2O2 + 2Li = 2Li2O. In the present study, we aimed at improving the cell performance with the Co-doped Li2O cathode and clarifying the mechanism of reactions during charging/discharging. Experimental The sample was prepared by pulverization of mixture of Li2O and LiCoO2 with a planetary ball mill at 600 rpm for 10 h. The starting materials were set so that the atomic ratio (Co/(Co+Li)) in the mixture was 0.09, which had been optimized in our previous study.2 XRD analysis revealed that the prepared sample contained (Li0.82Co0.058□0.12)2O (Co-doped Li2O) and LiCoO2 at the molar ratio of 95:5, where □ represents a Li+-site vacancy. The sample was mixed with ketjen black and polytetrafluoroethylene at the weight ratio of 75:20:5 and typically 10 mg of the mixtures were pressed onto an Al mesh current collector. Thus fabricated electrode was immersed in vinylene carbonate (VC) for 1 h and dried in vacuum. The VC-treated cathode, Li metal as an anode, an electrolyte, and a glass-fiber filter as a separator were assembled in a 2032 coin-type cell. The electrolyte solution was 1 M LiPF6 dissolved in a mixture (1:1 by volume) of ethylene carbonate (EC) and dimethyl carbonate (DMC), with a 5vol% VC additive. The aforementioned operations were conducted in an Ar-filled glove box. Charge/discharge tests between 1.7 and 3.5 V were carried out at 25 °C in a constant-current mode at the current density of 50 mA g–1. Pressure change in a cell was monitored using a pressure gauge connected to the cell. The lower detection limit of the pressure was 5×102 Pa, which corresponded to 6 mmol per 1 mol of the empirical formula of Co-doped Li2O. Results and discussion Figure 1(a) shows voltage curves of the cathode during charge/discharge and pressure in the cell during charge. The charging voltage increased to 3.2 V at 0–150 mAh g–1, and reached a 3.2 V plateau region at 150–400 mAh g–1. Pressure started increasing at around 320 mAh g–1, indicating that an irreversible O2 evolution reaction occurred. During the following discharge, the voltage curve contained a plateau region at 2.7–3.1 V and a slope region at 1.7–2.7 V, and showed capacity of 410 mAh g–1. This capacity was higher than that of the preceding first charge, during which the irreversible O2 evolution reaction occurred. This is because the capacity by the reduction from Co3+ to Co2+ was greater than the irreversible capacity due to the O2 evolution. The capacity 410 mAh g–1 was much higher than our previous reported value 270 mAh g–1, which is due to the pretreatment with VC. Although the mechanism enhancing the specific capacity by the VC treatment has not been clarified, we confirmed from separately conducted experiments that VC was more effective as a pretreatment agent than an additive into an electrolyte. After the first cycle, the charge/discharge was carried out with the charge depth reset to 410 mAh g–1, which was the capacity of the first discharge. Fig. 1(b) exhibits second charge/discharge profiles and pressure in the cell during second charge. The second charge curve differed from the first one; a slope region at around 2.3 V due to oxidation of Co2+ appeared. No change in pressure was observed in the whole region, indicating that the evolved O2 gas during the second charge was below the detection limit (5×102 Pa). The following discharge curve was similar to that of the first discharge. The coulombic efficiency (= discharge capacity/charge capacity) was 98.8%. The voltage curves for a multi-cycle test are shown in Fig. 1(c). While the charge/discharge curves present little changes in shape, the discharge capacity gradually decreased with cycles to be 400 mAh g–1at 10th cycle. Acknowledgments A part of this work was conducted with the support of JSPS Grant-in-Aid for Scientific Researches (B) Grant Number 26289371 and the support of JSPS Grant-in-Aid for Young Scientists (B) Grant Number 15K18326.

Self- and cross-associations of cyclic as well as linear carbonates such as ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), and dimethyl carbonate (DMC) are investigated with ab initio (MP2) and density functional theory (DFT) methods. The results show that cyclic and linear carbonates associate mainly through the intermolecular interactions of C-H‚‚‚O. The basis set superposition error and zero-point energy-corrected binding energy, D0(BSSE), for the global minimum of the linear carbonate DMC dimer (1.7 kcal/mol at B3LYP/6-311++G**) is much lower than those of the cyclic carbonates, and for the involved cyclic carbonates, it decreases in the following order: 5.1 (EC) 5.1 (EC/PC) > 4.7 (PC) > 3.9 (PC/VC) 3.8 (EC/VC) > 3.0 (VC). The dimer of DMC with EC (D0(BSSE) ) 2.8 kcal/mol) is also much less stable than the EC and PC dimers. Consistent with experimental findings, the results indicate that PC may also associate with EC molecules in their mixture; therefore, the cyclic carbonate molecules apparently still behave like associates instead of free molecules. However, EC molecules would be more free in a DMC/EC mixture because of weak intermolecular interactions. On the basis of the atoms-in-molecules (AIM) calculations, the C-H‚‚‚O interactions may be classified as hydrogen bonds, although the C-H‚‚‚O interaction exhibits a bond contraction and a blue vibrational frequency shift as compared with the monomer when the proton donor is linked with saturated molecules such as EC/PC/DMC, whereas it shows a C-H stretch and a red shift for the case of a proton linking with an unsaturated carbon such as VC. Such a difference perhaps arises from the opposite effect of a C-H bond stretch upon the monomer dipole moment. In line with the AIM analysis, the electron localization function technique (ELF) nicely demonstrates the existence of clear C -H‚‚‚ O interactions for VC dimers and trimers, which demonstrates the ability of the ELF procedure to characterize C-H‚‚‚O systems.

The reduction potentials of five organic carbonates commonly employed in lithium battery electrolytes, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and vinylene carbonate (VC) were determined by cyclic voltammetry using inert (Au or glassy carbon) electrodes in tetrahydrofuran/ supporting electrolyte. The reduction potentials for all five organic carbonates were above 1 V (vs. PC reduction was observed to have a significant kinetic hindrance. The measured reduction potentials for EC, DEC, and PC were consistent with thermodynamic values calculated using density functional theory (DFT) assuming one-electron reduction to the radical anion. The experimental values for VC and DMC were, however, much more positive than the calculated values, which we attribute to different reaction pathways. The role of VC as an additive in a PC-based electrolyte was investigated using conventional constant-current cycling combined with ex situ infrared spectroscopy and in situ atomic force microscopy (AFM). We confirmed stable cycling of a commercial li-ion battery carbon anode in a PC-based electrolyte with 5 mol % VC added. The preferential reduction of VC and the solid electrolyte interphase layer formation therefrom appears to inhibit PC cointercalation and subsequent graphite exfoliation. © 2001 The Electrochemical Society. All rights reserved.

The density functional theory (DFT) calculations have been performed for the reduction decompositions of solvents widely used in Li-ion secondary battery electrolytes, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonates (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), including a typical electrolyte additive, vinylene carbonate (VC), at the level of B3LYP/6-311+G(2d,p), both in the gas phase and solution using the polarizable conductor calculation model. In the gas phase, the first electron reduction for the cyclic carbonates and for the linear carbonates is found to be exothermic and endothermic, respectively, while the second electron reduction is endothermic for all the compounds examined. On the contrary, in solution both first and second electron reductions are exothermic for all the compounds. Among the solvents and the additive examined, the likelihood of undergoing the first electron reduction in solution was found in the order of EC > PC > VC > DMC > EMC > DEC with EC being the most likely reduced. VC, on the other hand, is most likely to undergo the second electron reduction among the compounds, in the order of VC > EC > PC. Based on the results, the experimentally demonstrated effectiveness of VC as an excellent electrolyte additive was discussed. The bulk thermodynamic properties of two dilithium alkylene glycol dicarbonates, dilithium ethylene glycol dicarbonate (Li-EDC) and dilithium 1,2-propylene glycol dicarbonate (Li-PDC), as the major component of solid-electrolyte interface (SEI) films were also examined through molecular dynamics (MD) simulations in order to understand the stability of the SEI film. It was found that film produced from a decomposition of EC, modeled by Li-EDC, has a higher density, more cohesive energy, and less solubility to the solvent than the film produced from decomposition of PC, Li-PDC. Further, MD simulations of the interface between the decomposition compound and graphite suggested that Li-EDC has more favorable interactions with the graphite surface than Li-PDC. The difference in the SEI film stability and the behavior of Li-ion battery cycling among the solvents were discussed in terms of the molecular structures.

The use of ethyl acetate and methyl propanoate in combination with vinylene carbonate as ethylene carbonate-free solvent blends for electrolytes in Li-ion batteries

A matrix of LiNi0.5Mn0.3Co0.2O2/graphite cells filled with 1.33 molal LiPF6 in EC:EMC:DMC (ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate) (25:5:70 by volume) electrolyte and different weight percentages of vinylene carbonate (VC) and ethylene sulfate (DTD) electrolyte additives underwent prolonged charge-discharge cycling at 20 °C and 40 °C. The volume of gas produced during formation and cycle testing was measured. The impedance spectra of the cells before and after cycling was measured. After testing, the electrolyte was extracted for study by nuclear magnetic resonance spectroscopy (NMR) and gas chromatography/mass spectroscopy (GC-MS) to determine what changes in electrolyte composition had occurred. Some cells had their negative electrodes studied by scanning micro-X-ray fluorescence to quantify the amount of transition metals that transferred from the positive electrode to the negative electrode during the testing. Cells containing 1% VC or 2% VC with an additional 1% DTD by weight had the best capacity retention and lowest impedance growth. NMR and GC-MS suggest that these additive combinations promote increased electrolyte salt consumption which may represent a source of lithium to replenish the lithium inventory. Only a small amount of transition metals (0.03% or less) originating from the positive electrode active material was found on the negative electrode after testing. Most cells had over 1500 cycles at both 20 °C and 40 °C conditions.

Li-ion batteries intended to operate over extremes in temperature or at cell voltages approaching 5 V exceed the fundamental capabilities of the electrolytes presently available. The most promising solvents that do meet the fundamental requirements exhibit exceptional instability at the low potentials found with negative electrodes of Li-ion batteries today. Herein, nitrile and linear carbonate electrolytes were stabilized with only the use of a small percentage of additives to enable formulations that may be of use for low temperature and high voltage operating conditions, respectively. In this work, the electrochemical characteristics of Li-ion cells were explored for a variety of electrolytes, with promising performance identified in systems composed predominantly of ethyl methyl carbonate (EMC), 3-methoxypropionitrile (3MPN), or adiponitrile (ADN). Vinylene carbonate (VC) and monofluoroethylene carbonate (FEC) were added in low concentrations (≤5 vol%) to stabilize the interface of the carbon negative electrode and the electrolyte, with FEC proving to be effective across all electrolytes examined herein. The fluoride decomposition products of FEC contributing to the SEI have been identified for the first time without the presence of lithium hexafluorophosphate (LiPF6) in the electrolyte, thereby leading to a clearer explanation of its exceptional protective effect within the SEI.

Measurement, correlation, and analysis of the solubility of triethylamine hydrochloride in ten pure solvents