In-situ spectroelectrochemical analysis of the passivating surface film formed on a carbon film electrode as a function of the water content in 1 M LiPF6-EC/DEC solution

The in-situ spectroelectrochemical technique has been applied to investigate the role of water in the formation of a passivating surface film on a plasma enhanced chemical vapor deposited (PECVD) carbon film electrode in 1 M LiPF₆-ethylene carbonate (Li-EC) and diethyl carbonate (DEC) solution, combined with cyclic voltammetry. In-situ Fourier transform infra-red (FTIR) spectra of the surface film showed that all the peak intensities of the Li₂CO₃, ROCO₂Li, and LiPF₆ constituents significantly increase with increasing water content under application of the negative potentials with respect to open circuit potential (OCP). It is suggested that the reduction of Li-EC to ROCO₂Li runs via a one-electron transfer pathway with the help of the unrestricted supply of the electron transfer path as a result of diffusion of water through the surface film; then Li₂CO₃ formation proceeds concurrently by the chemical reaction of ROCO₂Li with water. Moreover, the compact sedimentation of ROCO₂Li in the presence of water in the electrolyte is subjected to severe interference of the salt reduction product, Li_xPF_y, than in the absence of water in the electrolyte. These FTIR results coincide well with those of cyclic voltammetry. From the combined results of in-situ FTIR spectroscopy and cyclic voltammetry, it is indicated that, unlike other salt and solvent reaction products, ROCO₂Li, Li₂CO₃ and Li_xPF_y simultaneously increase to constitute the outer layer of the surface film with equal amounts.     

In situ analysis of interfacial reactions between negative MCMB,lithium electrodes,and gel polymer electrolyte

The interfacial properties of mesocarbon-microbeads (MCMB) and lithium electrodes during charge process in poly (vinylidenefluoride-co-hexafluoropropylene)-based gel electrolyte were investigated by in situ Raman microscopy, in situ Fourier transform-infrared (FTIR) spectroscopic methods, and charge–discharge, electrochemical impedance spectroscopy techniques. For MCMB electrode, the series phase transitions from initial formation of the dilute stage 1 graphite intercalation compound (GIC) to a stage 4 GIC, then through a stage 3 to stage 2, and finally to stage 1 GIC was proved by in situ Raman spectroscopic measurement. The formation of solid electrolyte interface (SEI) films formed on MCMB and metal lithium electrode was studied by in situ reflectance FTIR spectroscopic method. At MCMB electrode surface, the solvent (mostly ethylene carbonate) decomposed during charging process and ROCO₂Li may be the product. ROCO₂Li, ROLi, and Li₂CO₃ were the main composites of SEI film formed on lithium electrode, not on electrodeposited lithium electrode or lithium foil electrode.     

Investigation of interfacial processes in graphite thin film anodes of lithium-ion batteries by both in situ and ex situ infrared spectroscopy

Graphite thin film anodes with a high IR reflectivity have been prepared by a spin coating method. Both ex situ and in situ microscope FTIR spectroscopy (MFTIRS) in a reflection configuration were employed to investigate interfacial processes of the graphite thin film anodes in lithium-ion batteries. A solid electrolyte interphase layer (SEI layer) was formed on the cycled graphite thin film anode. Ex situ MFTIRS revealed that the main components of the SEI layer on cycled graphite film anodes in 1 mol L -1 LiPF6 /ethylene carbonate + dimethyl carbonate (1:1) are alkyl lithium carbonates (ROCO2 Li). The desolvation process on graphite anodes during the initial intercalation of lithium ion with graphite was also observed and analyzed by in situ MFTIRS.     

Surface film formation on a graphite electrode in Li‐ion batteries: AFM and XPS study

The formation of a passivation film (solid electrolyte interphase, SEI) at the surface of the negative electrode of full LiCoO₂/graphite lithium‐ion cells using LiPF₆ (1M ) in carbonate solvents as electrolyte was investigated by means of x‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The analyses were carried out at different potentials of the first and the fifth cycles, showing the potential‐dependent character of the surface‐film species formation. These species were mainly identified as Li₂CO₃ up to 3.8 V and LiF up to 4.2 V. This study shows the formation of the SEI during charging and its partial dissolution during discharge. Copyright © 2005 John Wiley & Sons, Ltd.     

电解液添加剂硫酸亚乙酯对锂离子电池性能的影响

在1 mol/L LiPF₆/碳酸乙烯酯+碳酸二甲酯+碳酸甲乙酯（体积比1∶1∶1）电解液中,采用恒流充放电测试、循环伏安法（CV）、扫描电子显微镜（SEM）、能量散射光谱（EDS）、电化学阻抗谱（EIS）等测试技术,研究了添加剂硫酸亚乙酯（DTD）对锂离子电池性能及石墨化中间相碳微球（MCMB）电极/电解液界面性质的影响。 结果表明,在电解液中引入体积分数0.01%DTD后,MCMB/Li电池可逆放电容量从300 mA·h/g提高至350 mA·h/g,电池总阻抗降低,循环稳定性提高。CV测试发现,在首次还原过程中,DTD在电极电位1.4 V左右（vs Li/Li⁺）发生电化学还原,参与了MCMB电极表面固体电解质相界面膜（SEI膜）的形成过程。 同时,DTD对LiMn₂O₄电极性能无不良影响。     

Properties of passivation film on carbon electrode in LiPF6/EC+DEC electrolytes

The co-solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) was used to investigate the decomposition of electrolyte in Li-ion batteries. The electrolyte solutions were prepared by mixing in various volume ratios from pure DEC to 7:3 (EC:DEC). The potentials at which they are decomposed on the anodic electrode were examined using cyclic voltammetry. It was found that some kinds of reduction reactions proceeded and a film on the surface of the anode was formed. The film showed different properties, which were dependent on the mixing ratio of the solvents. From our results, we concluded that the best composition ratio of EC:DEC in 1 M LiPF₆/(EC+DEC) system was approximately 4:6 (EC:DEC, volume ratio).     

Hybrid Polymer/Garnet Electrolyte with a Small Interfacial Resistance for Lithium‐Ion Batteries

Li₇La₃Zr₂O₁₂‐based Li‐rich garnets react with water and carbon dioxide in air to form a Li‐ion insulating Li₂CO₃ layer on the surface of the garnet particles, which results in a large interfacial resistance for Li‐ion transfer. Here, we introduce LiF to garnet Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZT) to increase the stability of the garnet electrolyte against moist air; the garnet LLZT‐2 wt % LiF (LLZT‐2LiF) has less Li₂CO₃ on the surface and shows a small interfacial resistance with Li metal, a solid polymer electrolyte, and organic‐liquid electrolytes. An all‐solid‐state Li/polymer/LLZT‐2LiF/LiFePO₄ battery has a high Coulombic efficiency and long cycle life; a Li‐S cell with the LLZT‐2LiF electrolyte as a separator, which blocks the polysulfide transport towards the Li‐metal, also has high Coulombic efficiency and kept 93 % of its capacity after 100 cycles.     

On the use of LiPF3(CF2CF3)3 (LiFAP) solutions for Li-ion batteries. Electrochemical and thermal studies

Electrolyte solutions comprising a mixture of LiPF₆ and LiPF₃(CF₂CF₃)₃ (LiFAP) in alkyl carbonates (ethylene, dimethyl and diethyl carbonate) were found to be superior to single salt LiFAP or LiPF₆ solutions for lithium–graphite anodes at elevated temperatures. Graphite electrodes could be cycled (Li insertion–deinsertion) more than hundred times at 80 °C with high and stable capacity in the two-salt solutions, while in the single-salt solutions this was impossible. Preliminary studies by voltammetry and impedance spectroscopy indicate that the combination of the two salts in solution has a unique influence on the electrodes surface (not yet defined). Thermal studies by accelerating rate and differential scanning calorimetry show that thermal decomposition of LiFAP solutions has a higher onset, but very high heat and pressure developing rates, compared to LiPF₆ solutions. The presence of LiPF₆ in LiFAP solutions decreased their self-heating and pressure-developing rates pronouncedly. From product analysis of the thermal reactions by NMR, FTIR and MS, we can suggest possible unique bulk reactions that occur in LiPF₆–LiFAP solutions. One of these is a nucleophilic reaction between F⁻ and PF₃(CF₂CF₃)₃⁻, which may neutralize the effect of trace HF in solutions (thus forming new P–F bonds and HCF₂CF₃). Such a reaction should have a positive effect on both the performance of the Li–graphite electrodes and the thermal behavior of the solutions.     

Polythiophenes via thiophene, bithiophene and terthiophene in propylene carbonate: an electrochemical and in-situ FTIR study

Polythiophene films obtained by the galvanostatic polymerization of thiophene (Th), bithiophene (BTh) and terthiophene (TTh) in 0.5 M LiClO₄ + propylene carbonate have been investigated by cyclic voltammetry and in-situ Fourier transform IR (FTIR) spectroscopy. The polymerization potential in the propylene carbonate electrolyte decreases in the order Th BTh TTh. Both the charge capacity and the doping level of the resulting polymers increase in the order polythiophene (PTh) < polybithiophene (PBTH) polyterthiophene (PTTh). For PTTh, a doping level of 37% is obtained during cycle 10 and 31% during cycle 1000.In-situ FTIR spectra of the neutral PBTh and PTTh films show a single band at 3063 cm⁻¹ (corresponding to aromatic β-(C---H)-groups) which suggests a regular α,α′-linking of the monomeric units. For thin PTh films the intensity of this band is very weak, indicating a disordered cross-linked polymer structure. During the electrochemical oxidation of the three polythiophenes investigated from 3.3 V up to 4.4 V vs. Li/Li⁺ the intensity of the electronic absorption band (above 2000 cm⁻¹) increases with increasing potential, but it strongly decreases at potentials above about 4.4 V, indicating a strong irreversible oxidation of the polymer films. Furthermore, strong CO₂ evolution is observed at potentials above 4.2 V.     

Ionic processes in solid-electrolyte passivating films on lithium

The electrochemical behaviour of a Li electrode in solutions of LiAlCl₄ in thionyl chloride, LiBF₄ in γ-butyrolactone and LiClO₄ in the mixed solvent propylene carbonate (PC) + dimethoxyethane (DME) in the process of cell storage has been investigated by the methods of electrode impedance spectroscopy and pulse voltammetry. Analogous studies have been carried out in PC + DME solution with the Li electrode coated with a specially formed protecting film of Li₂CO₃. The results have been compared with those obtained earlier for other lithium electrochemical systems. The general regularities of the Li electrode electrochemical kinetics attributed to the process of Li⁺ ion transport through a passivating film coating a lithium surface have been discussed. Received: 22 February 1999 / Accepted: 20 June 1999     

Fabrication of a novel Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MoO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> film modified electrode and its application as an electrochemical sensor of iodate

A novel organic gel film modified electrode was simply and conveniently fabricated by casting Li_xMoO_y and polypropylene carbonate (PPC) onto the surface of a gold electrode. The cyclic voltammetry and amperometry studies demonstrated that the Li_xMoO_y film modified electrode has a high stability and a good electrocatalytic activity for the reduction of iodate. In amperometry, a good linear relationship between the steady current and the concentration of iodate was obtained in the range from 3×10^–7 to 1×10^–4 mol L^–1 with a correlation coefficient of 0.9997 and a detection limit of 1×10^–7 mol L^–1.     

FTIR spectroscopy study of the electrochemical reduction of CO2 on various metal electrodes in methanol

The electrochemical reduction of CO₂ on Sn, Cu, Au, In, Ni, Ru and Pt electrodes in methanol containing 0.1 M sodium perchlorate was studied by cyclic voltammetry and in-situ FTIR spectroscopy. Dissolved CO₂ increases the cathodic current at potentials below −1.3 V vs. Ag|0.01 M Ag⁺ with Sn, Au, Cu, In and Ni electrodes. It is concluded from the FTIR spectra obtained that there is no reduction of CO₂ on any of the metals studied, and that the only reaction product detected by Fourier transform (FT) IR spectroscopy, i.e. CO²⁻₃, is formed by reaction of CO₂ with hydroxyl anions produced in the electroreduction of residual water.In order to identify the electroreduction products of CO₂ it was necessary to obtain the FTIR spectra of sodium oxalate and sodium carbonate in methanol. They were obtained by the electroreduction of oxalic acid and the alkalinization of CO₂-saturated methanol respectively. It could be proved that the electroreduction of carboxylic acids to carboxylate anions in organic solvents does not require either a H-chemisorbing metal electrode, or the presence of water in the solvent.     

Investigation of the gas evolution in lithium ion batteries: effect of free lithium compounds in cathode materials

The evolution of gas in lithium ion batteries (LIBs) was investigated. The large amount of gas emission related to a charged cathode has been a critical issue because it causes deformation and performance degradation of LIBs. This study examined the effect of free lithium compounds such as Li₂CO₃ or LiOH on gas generation, which revealed several different features comparing with gas generation related to the cathode active materials themselves: CO₂ was the main gas generated, chain-structured carbonate solvents such as dimethyl carbonate or ethyl methyl carbonate generated more gas than cyclic-structured ethylene carbonate, and the gas generation did not occur without LiPF₆ in the electrolyte solution. These were found to be the main reason for the different gas-generating behaviors between LiCoO₂ (LCO) and LiNi_0.85Co_0.12Al_0.03O₂ (NCA) cathodes. For LCO, which has a very small amount of free lithium compounds on the surface, the gas was generated mainly by a reaction between delithiated LCO itself and the electrolyte solution, whereas a considerable amount of gas was generated by surface free lithium for NCA. Therefore, the removal of free lithium compounds is essential, particularly for NCA, to prevent the swelling of LIBs.     

Lithium ethylene dicarbonate identified as the primary product of chemical and electrochemical reduction of EC in 1.2 M LiPF6/EC:EMC electrolyte

Lithium ethylene dicarbonate ((CH2OCO2Li)2) was chemically synthesized and its Fourier transform infrared (FTIR) spectrum was obtained and compared with that of surface films formed on Ni after cyclic voltammetry (CV) in 1.2 M lithium hexafluorophosphate (LiPF6)/ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7, w/w) electrolyte and on metallic lithium cleaved in-situ in the same electrolyte. By comparison of IR experimental spectra with that of the synthesized compound, we established that the title compound is the predominant surface species in both instances. Detailed analysis of the IR spectrum utilizing quantum chemical (Hartree-Fock) calculations indicates that intermolecular association through O...Li...O interactions is very important in this compound. It is likely that the title compound in the passivation layer has a highly associated structure, but the exact intermolecular conformation could not be established on the basis of analysis of the IR spectrum.     

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.     

Enhanced electrochemical performance of LiFePO4 cathode with the addition of fluoroethylene carbonate in electrolyte

The effect of the fluoroethylene carbonate (FEC) addition in electrolyte on LiFePO₄ cathode performance was investigated in low-temperature electrolyte LiPF₆/EC/PC/EMC (0.14/0.18/0.68). Cyclic voltammetry, electrochemical impedance spectroscopy, and charge/discharge tests were conducted in this work. In the presence of FEC, the polarization of LiFePO₄ electrode decreased both at room and low temperatures. Meanwhile, the exchange current density increased. The rate capability of LiFePO₄ electrode was greatly enhanced as well. The morphology of the solid electrolyte interphase (SEI) on LiFePO₄ surface was modified with the addition of FEC as confirmed by scanning electron microscopy measurement. A compact film with small impedance was formed on LiFePO₄ surface compared to the case of FEC-free. The compositions of the film were analyzed by X-ray photoelectron spectroscopic measurement. The contents of Li _x PO _y F _z , LiF, and the carbonate species generated from solvents decomposition were reduced. The modified SEI promoted the migration of lithium ion through the electrode/electrolyte interphase and enhanced the electrochemical performance of the cathode.     

Realizing Stable Carbonate Electrolytes in Li–O2/CO2 Batteries†

The increasing demand for high-energy storage systems has propelled the development of Li-air batteries and Li-O₂/CO₂ batteries to elucidate the mechanism and extend battery life. However, the high charge voltage of Li₂CO₃ accelerates the decomposition of traditional sulfone and ether electrolytes, thus adopting high-voltage electrolytes in Li-O₂/CO₂ batteries is vital to achieve a stable battery system. Herein, we adopt a commercial carbonate electrolyte to prove its excellent suitability in Li-O₂/CO₂ batteries. The generated superoxide can be captured by CO₂ to form less aggressive intermediates, stabilizing the carbonate electrolyte without reactive oxygen species induced decomposition. In addition, this electrolyte permits the Li metal plating/stripping with a significantly improved reversibility, enabling the possibility of using ultra-thin Li anode. Benefiting from the good rechargeability of Li₂CO₃, less cathode passivation, and stabilized Li anode in carbonate electrolyte, the Li-O₂/CO₂ battery demonstrates a long cycling lifetime of 167 cycles at 0.1 mA·cm^–2 and 0.25 mAh·cm^–2. This work paves a new avenue for optimizing carbonate-based electrolytes for Li-O₂ and Li-O₂/CO₂ batteries.      

In situ AFM studies of SEI formation at a Sn electrode

Early stages of the solid electrolyte interphase (SEI) formation at a tin foil electrode in an ethylene carbonate (EC) based electrolyte were investigated by in situ AFM and cyclic voltammetry (CV) at potentials >0.7 V, i.e., above the potential of Sn–Li alloying. We detected and observed initial steps of the surface film formation at ～2.8 V vs. Li/Li⁺ followed by gradual film morphology changes at potentials 0.7 < U < 2.5 V. The SEI layer undergoes continuous reformation during the following CV cycles between 0.7 and 2.5 V. The surface film on Sn does not effectively prevent the electrolyte reduction and a large fraction of the reaction products dissolve in the electrolyte. The unstable SEI layer on Sn in EC-based electrolytes may compromise the use of tin-based anodes in Li-ion battery systems unless the interfacial chemistry of the electrode and/or electrolyte is modified.     

Effect of sulfolane on the morphology and chemical composition of the solid electrolyte interphase layer in lithium bis(oxalato)borate‐based electrolyte

To discuss the source of sulfolane (SL) in decreasing the interface resistance of Li/mesophase carbon microbeads cell with lithium bis(oxalate)borate (LiBOB)‐based electrolyte, the morphology and the composition of the solid electrolyte interphase (SEI) layer on the surface of carbonaceous anode material have been investigated. Compared with the cell with 0.7 mol l^?1 LiBOB‐ethylene carbonate/ethyl methyl carbonate (EMC) (1 : 1, v/v) electrolyte, the cell with 0.7 mol l^?1 LiBOB‐SL/EMC (1 : 1, v/v) electrolyte shows better film‐forming characteristics in SEM (SEI) spectra. According to the results obtained from Fourier transform infrared spectroscopy, XPS, and density functional theory calculations, SL is reduced to Li₂SO₃ and LiO₂S(CH₂)₈SO₂Li through electrochemical processes, which happens prior to the reduction of either ethylene carbonate or EMC. It is believed that the root of impedance reduction benefits from the rich existence of sulfurous compounds in SEI layer, which are better conductors of Li⁺ ions than analogical carbonates. Copyright © 2013 John Wiley & Sons, Ltd.     

Elucidating the Beneficial Effect of Vinylene Carbonate on the Electrochemistry of Antimony Electrodes in Lithium Batteries

Vinylene carbonate (VC) is an effective electrolyte additive for improving electrode performance in lithium‐ion batteries. We confirm its beneficial effects in a nanosized Sb–cellulose fiber composite electrode and examine its mechanism of action by X‐ray photoelectron spectroscopy, the most suitable technique for characterizing active particle surfaces (especially electrode/electrolyte interfaces). Vinylene carbonate clearly increases electrolyte stability (particularly that of LiPF₆) by avoiding the formation of a thick layer of LiF that conceals Sb and prevents its reaction with Li. Decreasing the layer thickness therefore increases the reversibility of the electrochemical reaction and improves capacity retention on cycling. The presence of VC afforded high‐quality Sb 4d and Li 1s photoemissions, fitting of which clearly exposed a previously unreported low‐energy component. We believe that this component is an Li_xSb alloy, which for the first time is identified spectroscopically.