Lithium Alkyl Carbonates Research Articles

Molecular Dynamics Simulations and Experimental Study of Lithium Ion Transport in Dilithium Ethylene Dicarbonate

Understanding the properties of the solid electrolyte interphase (SEI) of lithium batteries is important for minimizing interfacial resistance and improving battery safety and cycling. Ion transport has been investigated in the dilithium ethylene dicarbonate (Li2EDC) component of the SEI by impedance spectroscopy and molecular dynamics (MD) simulations employing a revised many-body polarizable APPLE&P force field. The developed force field accurately described the binding energies in LiCH3CO3, its dimer, and Li2EDC calculated at the G4MP2 and MP2 levels. M05-2X and LC-ωPBE functionals predicted too high binding energy in lithium alkyl carbonates compared to the G4MP2 results, while the MP2 and M06-L predictions agreed well with the G4MP2 data. The conductivity of Li2EDC at room temperature was found to be 10–9 S/cm from impedance measurements and extrapolation of MD simulation results. A near Arrhenius temperature dependence of Li2EDC’s conductivity was found in the MD simulations with an activation energy ranging from 64 to 84 kJ/mol. At room temperature, the lithium transport was subdiffusive on time scales shorter than ∼10–2 s in MD simulations corresponding to the onset of the plateau of resistivity vs frequency occurring at frequencies lower than 102 Hz. The influence of Li2EDC ordering on the ion transport was investigated by contrasting supercooled amorphous melts and ordered material. At 393 K Li+ transport was heterogeneous, showing chainlike and looplike Li+ correlated displacements. The non-Gaussianity of Li+ transport was examined. The influence of polarization on the structure of the lithium coordination shell and ion transport has been investigated in the molten phase of Li2EDC and contrasted with the previous results obtained for room-temperature ionic liquids (RTILs). Nonpolarizable Li2EDC exhibited orders of magnitude slower dynamics below 600 K and a higher activation energy for the Li+ diffusion coefficient. Initial simulations of Li2EDC dissolved in an EC:DMC(3:7)/LiPF6 liquid electrolyte were performed at 450 K and showed a strong aggregation of Li2EDC consistent with its phase separation from the electrolyte. The plasticizing effects of carbonate electrolyte on Li2EDC dynamics were examined.

In-situ Fourier transform infrared spectroscopic analysis on dynamic behavior of electrolyte solution on LiFePO4 cathode

The dynamic behavior of 1.0 mol dm−3 LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) mixed solvent (1:1 in volume) on lithium iron phosphate (LiFePO4) thin film electrode fabricated by RF-sputtering was investigated by in-situ Fourier transform infrared (FT-IR) spectroscopy. The solvation and desolvation reactions of EC and DEC with Li+ ion were observed in the charge and discharge processes of thin film electrode, respectively. In addition, the adsorption of solvent molecules on the electrode surface was suggested by polarized IR analyses. Lithium alkylcarbonate, lithium carboxylate and lithium carbonate were formed as decomposition products of the electrolyte solution at the anodic potential more than 4.5 V vs. Li/Li+.

Understanding of Electrolyte Stability and Its Impact to Lifespan of Li-O2 Battery

The impact to the Li-O2 battery performance from the insoluble lithium salts formed from the electrolyte decomposition during discharge-charge cycle was investigated by a microfocused synchrotron X-ray diffraction (m-XRD) technique, together with the conventional imaging and spectroscopic methods. Lithium alkyl carbonate deposit was found throughout the battery. Surprisingly, the concentration of Li2CO3 in the separator is significantly higher than that in both electrodes. Imaging method such as Scanning electron microscopy revealed that the precipitates grew on the separator fiber surface, ultimately obstructing the pores serving as the ion-transport channel. A model based on finite-element analysis was developed to qualitatively illustrate the possible chemical/physical processes leading to high accumulation of insoluble precipitates in the separator region.

Relationships between the crystal/interfacial properties and electrochemical performance of LiNi0.33Co0.33Mn0.33O2 in the voltage window of 2.5–4.6 V

LiNi0.33Co0.33Mn0.33O2 which was synthesized by co-precipitation method has been used to investigate the relationships between crystal/interfacial structures and electrochemical performance of the material in the voltage range of 2.5–4.6V. X-ray diffraction showed that the material maintained high structural stability with charge–discharge cycling. Formation of SEI film was observed by transmission electron microscopy. The chemical compositions of the SEI film were analyzed by X-ray photoelectron spectroscopy, which were mainly composed of some organic compounds such as lithium alkyl carbonate RCH2OCO2Li, as well as some inorganic compounds such as LixPFy and LixPOyFz. Transmission electron microscopy showed that the SEI film gradually grew to a thickness of about 40nm after 30 cycles. This caused larger and larger interfacial resistance with charge–discharge cycling, which has been attributed to the continuous capacity fading of LiNi0.33Co0.33Mn0.33O2 in the voltage window of 2.5–4.6V.

The Cathode Surface Composition of a Cycled Li–O2 Battery: A Photoelectron Spectroscopy Study

A layer of reaction products, dominantly built up of C and O in the form of ethers and lithium alkyl carbonates, is formed on the surface of the carbon cathode during discharge of a Li–O2 battery in an electrolyte of 1 M LiPF6 in PC. The results are based on a detailed surface analysis combining the use of in house X-ray photoelectron spectroscopy (XPS) and synchrotron based hard X-ray photoelectron spectroscopy (HAXPES). The Li–O2 batteries were investigated at uncycled state (stored), after the first discharge, after the first charge, and at the end of life (discharge state). The results showed little to no Li2O2 and/or Li2O among the discharge products. The surface layers on the cathode were dominantly removed during charging of the battery. At the end of battery life, no complete discharge product layer is formed. The cathodes showed a strong indication of binder decomposition during cycling of the Li–O2 cell. Overall, the results obtained in this investigation show that the whole cathode formulation as well as the electrolyte composition need a completely new approach for the realization of a recyclable Li–O2 battery.

Effects of lithium salts on thermal stabilities of lithium alkyl carbonates in SEI layer

With the aim of attaining a better understanding of the thermal stabilities of solid electrolyte interphase (SEI) layers with lithium salts, a series of lithium alkyl carbonates, including lithium methyl carbonate (LMC) and lithium ethyl carbonate (LEC), were synthesized as model reference SEI components. The effects of the lithium salts, lithium hexafluorophosphate (LiPF6) and lithium bis(oxalate) borate (LiBOB), on the thermal stabilities of the lithium alkyl carbonates were then investigated via the differential scanning calorimetry (DSC) method. It was demonstrated that small and broad exothermic peaks near 100°C for pure lithium alkyl carbonates were drastically increased, likely due to vigorous synergic exothermal reactions between the lithium alkyl carbonates and LiPF6. In contrast, LiBOB did not show increased peaks.

Spectroscopic Characterization of Surface Films Formed on Edge Plane Graphite in Ethylene Carbonate-Based Electrolytes Containing Film-Forming Additives

Surface films formed on edge plane pyrolytic graphite electrodes were characterized by attenuated total reflection (ATR)-Fourier transform infrared spectroscopy (FTIR). FTIR-ATR spectra revealed that the surface films consisted mainly of LiOCO2R (R = alkyl or lithium alkyl carbonate), Li2CO3 and (CH2CH2O)n on both the edge and basal planes, while the formation reactions at the edge plane proceeded more rapidly than those at the basal plane. The major constituents of the surface films formed at 0.1 and 0.8 V were LiOCO2R and Li2CO3, respectively. We discuss this difference based on the results in the literature. In addition, surface films derived from film-forming additives, such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC), were investigated by ATR-FTIR and X-ray photoelectron spectroscopy (XPS), and the formation mechanisms are discussed by considering the reactivity of intermediate radical compounds.

Reaction mechanisms for the limited reversibility of Li–O 2 chemistry in organic carbonate electrolytes

The Li–O 2 chemistry in nonaqueous liquid carbonate electrolytes and the underlying reason for its limited reversibility was systematically investigated. X-ray diffraction data showed that regardless of discharge depth lithium alkylcarbonates (lithium propylenedicarbonate (LPDC), or lithium ethylenedicarbonate (LEDC), with other related derivatives) and lithium carbonate (Li 2CO 3) are constantly the main discharge products, while lithium peroxide (Li 2O 2) or lithium oxide (Li 2O) is hardly detected. These lithium alkylcarbonates are generated from the reductive decomposition of the corresponding carbonate solvents initiated by the attack of superoxide radical anions. More significantly, in situ gas chromatography/mass spectroscopy analysis revealed that Li 2CO 3 and Li 2O cannot be oxidized even when charged to 4.6 V vs. Li/Li +, while LPDC, LEDC and Li 2O 2 are readily oxidized, with CO 2 and CO released from LPDC and LEDC and O 2 evolved from Li 2O 2. Therefore, the apparent reversibility of Li–O 2 chemistry in an organic carbonate-based electrolyte is actually an unsustainable process that consists of (1) the formation of lithium alkylcarbonates through the reductive decomposition of carbonate solvents during discharging and (2) the subsequent oxidation of these same alkylcarbonates during charging. Therefore, a stable electrolyte that does not lead to an irreversible by-product formation during discharging and charging is necessary for truly rechargeable Li–O 2 batteries.

Investigation on the charging process of Li 2O 2-based air electrodes in Li–O 2 batteries with organic carbonate electrolytes

The charging process of Li 2O 2-based air electrodes in Li–O 2 batteries with organic carbonate electrolytes was investigated using in situ gas chromatography/mass spectroscopy (GC/MS) to analyze gas evolution. A mixture of Li 2O 2/Fe 3O 4/Super P carbon/polyvinylidene fluoride (PVDF) was used as the starting air electrode material, and 1-M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in carbonate-based solvents was used as the electrolyte. We found that Li 2O 2 was actively reactive to 1-methyl-2-pyrrolidinone and PVDF that were used to prepare the electrode. During the first charging (up to 4.6 V), O 2 was the main component in the gases released. The amount of O 2 measured by GC/MS was consistent with the amount of Li 2O 2 that decomposed during the electrochemical process as measured by the charge capacity, which is indicative of the good chargeability of Li 2O 2. However, after the cell was discharged to 2.0 V in an O 2 atmosphere and then recharged to ∼4.6 V, CO 2 was dominant in the released gases. Further analysis of the discharged air electrodes by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy indicated that lithium-containing carbonate species (lithium alkyl carbonates and/or Li 2CO 3) were the main discharge products. Therefore, compatible electrolytes and electrodes, as well as the electrode-preparation procedures, need to be developed for rechargeable Li-air batteries for long term operation.

Concatenation of electrochemical grafting with chemical or electrochemical modification for preparing electrodes with specific surface functionality

Surface modified electrodes are used in electro-analysis, electro-catalysis, sensors, biomedical applications, etc. and could also be used in batteries. The properties of modified electrodes are determined by the surface functionality. Therefore, the steps involved in the surface modification of the electrodes to obtain specific functionality are of prime importance. We illustrate here bridging of two routes of surface modifications namely electrochemical grafting, and chemical or electrochemical reduction. First, by electrochemical grafting an organic moiety is covalently immobilized on the surface. Then, either by chemical or by electrochemical route the terminal functional group of the grafted moiety is transformed. Using the former route we prepared lithium alkyl carbonate (–O(CH 2) 3OCO 2Li) modified carbon with potential applications in batteries, and employing the latter we prepared phenyl hydroxyl amine (–C 6H 4NHOH) modified carbon which may find application in biosensors. Benzyl alcohol (–C 6H 4CH 2OH) modified carbon was prepared by both chemical as well as electrochemical route. We report combinations of conjugating the two steps of surface modifications and show how the optimal route of terminal functional group modification depends on the chemical nature of the moiety attached to the surface in the electrochemical grafting step.

Investigating the solid electrolyte interphase using binder-free graphite electrodes

Binder-free (BF) electrodes simplify interpretation of solid electrolyte interphase (SEI) data obtained from studies of graphite surfaces. In this work, we prepared BF-graphite electrodes by electrophoretic deposition (EPD), and the SEI layers formed on the electrode in lithium cells containing LiPF 6- and LiF 2BC 2O 4-bearing electrolytes were examined by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results showed that the dominant SEI species were lithium alkyl carbonates (ROCO 2Li) and lithium alkoxides (ROLi); Li 2CO 3 was conspicuously absent. Trigonal borate oligomers are most likely present in the SEI of graphite samples cycled in LiF 2BC 2O 4 electrolyte, while lithium fluorophosphates are present on graphite samples cycled in LiPF 6 electrolyte. The SEI layer coverage was greater on graphite samples cycled in LiF 2BC 2O 4 electrolyte than in the LiPF 6 electrolyte. Our results demonstrate that BF-graphite electrodes prepared by EPD are suitable for the study of SEI layer formed in various electrolyte systems.

Investigation of positive electrodes after cycle testing of high-power Li-ion battery cells: III: An approach to the power fade mechanism using FT-IR-ATR

Surface materials on positive electrodes of lithium-ion batteries which have been degraded are characterized by ATR technique of FT-IR. Lithium carbonate, P O bond based material, lithium alkyl carbonate (ROCO 2Li), and polycarbonate-type compound are detected which have been formed by decomposition reactions of electrolyte solvent and LiPF 6 as lithium salt. Stability of lithium carbonate for charging to SOC = 100% is depended on the condition during degradation. Especially, it changes to unstable by long time cycling at low temperatures. Formation of polycarbonate-type compound during cycling is remarkable at elevated temperatures. Degradation of batteries is larger in higher temperatures, and it is suggested that the variation of surface structure has relation to degradation of batteries.

Effects of anode active materials to the storage-capacity fading on commercial lithium-ion batteries

Thermal storage of prismatic Li-ion cell with different types of anodes has been performed at 60 °C for 15 days to 30 days. The results were compared for two anodes: natural-like graphite (NLG) with styrene–butadiene rubber (SBR, 2.5 wt.%) binder and artificial graphite (AG) with polyvinylidene fluoride (PVdF, 6 wt.%) as binder. The storage-capacity fading behavior of the commercial Li-ion cell was studied by dissection the storage cells and analyzing their electrodes and solid electrolyte interphase (SEI), allows lithium-ion transfer but prevents electron migration using SEM, DSC, FT-IR, XRD and impedance analysis. Side-reaction and transformation of the passivation film on NLG anode contributed the capacity loss. Self-discharge of NLG cell due to high specific surface area was one of the main factors for capacity fading. Impedance analysis revealed that the interfacial resistance at NLG anode was larger than that of the AG anode. The increase of lithium alkylcarbonate and lithium carbonate due to reductive decomposition of electrolyte with storage time decreased the charge and increased the interfacial resistance.

Analyses of Capacity Loss and Improvement of Cycle Performance for a High-Voltage Hybrid Electrochemical Capacitor

A high-voltage hybrid electrochemical capacitor (HEC) which uses nongraphitizable carbon for a negative electrode and activated carbon for a positive electrode has been constructed with lithium salt for the electrolyte. Under high-voltage conditions, large capacity loss during charge-discharge cycling was observed when the positive electrode was highly polarized at a potential over vs . This capacity loss was induced by the increase of the interfacial resistance at the negative electrode. A large amount of lithium fluoride (LiF) was observed on the surface of the negative electrode after charge-discharge cycling. It is considered that the LiF was generated by a reaction of hydrofluoric acid (HF), generated by the decomposition of hexafluorophosphate anion , with lithium alkyl carbonate (semicarbonate, ROCOOLi) or lithium carbonate on the surface of the negative electrode. The generation of LiF and the capacity loss were suppressed by providing lithium metal between the positive electrode and the negative electrode. It was thus confirmed that the suppression of LiF generation, caused by high polarization of activated carbon positive electrode, can realize a high-voltage HEC with good cycleability.

Ageing of V 2O 5 thin films induced by Li intercalation multi-cycling

Cyclic voltammetry, XPS, RBS and AFM have been combined to study the ageing mechanism of Li intercalation in V 2O 5 thin films prepared by thermal oxidation of vanadium metal. Multi-cycling tests were performed in 1 M LiClO 4-PC in the potential range E ∈ [3.8, 2.8 V] versus Li/Li +, corresponding to the α-to-δ phase transition. XPS and AFM were performed using direct anaerobic and anhydrous transfer. Capacity fading remains inferior to 20% during ∼2500 cycles. XPS shows slight modifications of the oxide composition with a V 4+ concentration increasing from ∼5% prior to cycling to ∼16–27% after cycling, due to Li trapped in the oxide film and to the loss of V 2O 5 active material. The presence of lithium carbonate and lithium-alkyl carbonate species evidences the formation of the so-called SEI layer. AFM evidences the loss of crystalline material by grain exfoliation from the outer V 2O 5 layer of the oxide film. By further exfoliation, the inner VO 2 layer of the oxide film is reached and pits are formed, occupying ∼9–13% of the surface. This de-cohesion at grain boundaries is attributed to the strain generated by repeated lattice distortions. After 3300 cycles, the disappearance of lithium carbonates, whereas Li-alkyl carbonates and/or Li-alkoxides remain on the surface, indicates the dissolution and/or conversion of the SEI layer. After 4500 cycles, the oxide film became very labile and could be stripped away by rinsing to reveal the vanadium metal substrate.

TG-MS analysis of solid electrolyte interphase (SEI) on graphite negative-electrode in lithium-ion batteries

The thermal stability and chemical structure of solid electrolyte interphase (SEI) formed on a natural-graphite negative-electrode in ethylene carbonate (EC) and dimethyl carbonate (DMC)-based electrolyte was investigated by thermogravimetry-differential thermal analysis combined with mass spectrometry (TG-DTA/MS) and X-ray photoemission spectroscopy (XPS). Due to the decomposition of SEI, two CO 2 evolution peaks at around 330 and 430 °C were detected in TG-MS studies with continuous CO 2 background. The continuous CO 2 background was attributed to the gradual decomposition of oxygen-containing polymeric species of SEI. Another two dominant components of SEI, lithium alkyl carbonate and lithium oxalate, were found to contribute to the CO 2 peaks at 330 and 430 °C separately. The effects of charging-depth, current density and cycle number on the CO 2 distribution and XPS spectra were studied. It was found that lithium oxalate was reduction product of lithium alkyl carbonate during the intercalation of lithium ions. The reduction reaction could be accelerated by elevated temperature. The transformation of SEI chemical structure showed direct effect on the thermal stability of SEI. At the same time, lithium carbonate was also found in SEI on the graphite electrode after long cycles, while it was negligible in the electrode subjected to short cycles.

Electrochemical quartz crystal microbalance analysis of the cathodic process in a lithium-ion battery

The surface processes at carbon and platinum electrodes have been studied using the electrochemical quartz crystal microbalance technique in organic electrolyte solutions for lithium ion batteries. The changes in resonance frequency were analyzed as a function of the electrode potential, indicating that the process depended not only on the electrode material but also on the cathode potential. In the solution containing LiBF4 as the electrolyte, the main product at the platinum surface was Li2CO3 and LiF, whereas formation of lithium alkylcarbonates was the primary process at the platinum and carbon electrodes in LiPF6 solution.

Characterization of Lithium Alkyl Carbonates by X-ray Photoelectron Spectroscopy: Experimental and Theoretical Study

Lithium alkyl carbonates ROCO2Li result from the reductive decomposition of dialkyl carbonates, which are the organic solvents used in the electrolytes of common lithium-ion batteries. They play a crucial role in the formation of surface layers at the electrode/electrolyte interfaces. In this work, we report on the X-ray photoelectron spectroscopy (XPS) characterization of synthesized lithium methyl and ethyl carbonates. Using Hartree-Fock ab initio calculations, we interpret and simulate the valence spectra of both samples, as well as several other Li alkyl carbonates involved in Li-ion batteries. We show that Li alkyl carbonates can be identified at the electrode's surface by a combined analysis of XPS core peaks and valence spectra.

Effects of solvents and salts on the thermal stability of LiC 6

Accelerating rate calorimetry (ARC) was used to study the thermal stability of Li 0.81C 6 in dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), and an EC/DEC mixture as well as in LiPF 6- and LiBOB-based electrolytes. ARC results show that linear carbonates like DMC or DEC react strongly with Li 0.81C 6 and that robust passivating layers do not form. By contrast, the cyclic carbonate, EC, creates a robust passivating film that limits the rate of reaction between Li 0.81C 6 and EC as the temperature increases. X-ray diffraction shows that the addition of LiPF 6 to EC/DEC changes the surface film that forms on Li 0.81C 6 at elevated temperature to one dominated by LiF instead of lithium-alkyl carbonate or lithium carbonate. This increases the thermal stability of Li 0.81C 6 in LiPF 6 electrolyte compared to pure EC/DEC solvent. By an apparently similar mechanism, the addition of only 0.2 M LiBOB to EC/DEC greatly improves the thermal stability of Li 0.81C 6. ARC results for Li 0.81C 6 in pure and mixed salt LiPF 6 and LiBOB EC/DEC electrolytes of various molarities shed light on the reasons for the beneficial effect of the salts.

Study on the decomposition mechanism of alkyl carbonate on lithium metal by pyrolysis-gas chromatography-mass spectroscopy

The surface films formed on deposited lithium in electrolyte solutions based on ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were analyzed by pyrolysis-gas chromatography-mass spectroscopy (Py-GC-MS). In 1 M LiClO 4/EC, the main component of the surface film was easily hydrolyzed to give ethylene glycol after exposure to air, and hence was considered to have a chemical structure of ROCH 2CH 2OR′, of which OR and OR′ are OLi or OCO 2Li. Ethylene oxide, acetaldehyde, and 1,4-dioxane were detected in decomposition products, and they were considered to have been formed by pyrolysis of ROCH 2CH 2OR′ in the pyrolyzer. The presence of ethanol in decomposition products confirmed that ring cleavage at the CH 2O bonds of EC occurs by one electron reduction. In addition, the presence of methanol implied the cleavage of the CC bond of EC upon reduction. From the surface films formed in 1 M LiClO 4/DEC and /DMC, ethanol and methanol, respectively, were detected, which suggested that corresponding lithium alkoxides and/or lithium alkyl carbonates were the main components. In 1 M LiClO 4/EC+DEC (1:1), EC dominantly decomposed to form the surface film. The surface film formed in 1 M LiPF 6/EC+DEC (1:1) contained a much smaller amount of organic compounds.