磷酸三甲酯和碳酸亚乙烯酯对锂离子电池的复合作用

应用循环伏安、交流阻抗、扫描电子显微镜和锂离子电池性能检测装置研究了阻燃添加剂磷酸三甲酯(TMP)和成膜添加剂碳酸亚乙烯酯(VC)对锂离子电池的复合作用.结果表明,复合使用TMP和VC不仅能提高电池的安全性而且能改善电池的循环性能,原因可能是在电池首次充放电过程中VC优先还原,还原产物在负极表面聚合形成良好的SEI膜,有效地制约了因TMP在石墨负极表面的分解而造成负极石墨的脱落,同时提高了SEI膜的稳定性.     

Comparative surface analysis study of the solid electrolyte interphase formation on graphite anodes in lithium‐ion batteries depending on the electrolyte composition

The composition of the solid electrolyte interphase (SEI) on graphite anodes is characterized within a comparative surface analytical study varying systematically the electrolyte composition and the cycling conditions. In particular, the conducting salts lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)imide as well as vinylene carbonate and 1‐fluoroethylene carbonate as different electrolyte additives are compared regarding the SEI formation under different cycling conditions. A comprehensive study using X‐ray photoelectron spectroscopy revealed pronounced differences of the SEI compositions at different aging stages. Both additives significantly influence the SEI composition and are able to prevent from parasitic side reactions as well as from decomposition of the conducting salt lithium hexafluorophosphate. This study suggests a promising approach to improve the SEI properties to enhance long‐term stability of lithium‐ion batteries by changing the electrolyte composition. Copyright © 2016 John Wiley & Sons, Ltd.     

Solvation Rule for Solid‐Electrolyte Interphase Enabler in Lithium‐Metal Batteries

Despite the exceptionally high energy density of lithium metal anodes, the practical application of lithium‐metal batteries (LMBs) is still impeded by the instability of the interphase between the lithium metal and the electrolyte. To formulate a functional electrolyte system that can stabilize the lithium‐metal anode, the solvation behavior of the solvent molecules must be understood because the electrochemical properties of a solvent can be heavily influenced by its solvation status. We unambiguously demonstrated the solvation rule for the solid‐electrolyte interphase (SEI) enabler in an electrolyte system. In this study, fluoroethylene carbonate was used as the SEI enabler due to its ability to form a robust SEI on the lithium metal surface, allowing relatively stable LMB cycling. The results revealed that the solvation number of fluoroethylene carbonate must be ≥1 to ensure the formation of a stable SEI in which the sacrificial reduction of the SEI enabler subsequently leads to the stable cycling of LMBs.     

Vinylene carbonate–LiNO3: A hybrid additive in carbonic ester electrolytes for SEI modification on Li metal anode

A hybrid solid electrolyte interphase (SEI) formation additive, vinylene carbonate (VC)–LiNO₃, was investigated in carbonic ester electrolytes. An efficiency of lithium plating/stripping as high as nearly 100% and spherical Li deposits were obtained. The electrochemical impedance spectroscopy (EIS) results demonstrate that the modified SEI is very stable and of good conductivity. X-ray photoelectron spectroscopy (XPS) results indicate that VC–LiNO₃ dominates the surface chemistry of the Li anode. The formation of Li₃N in the SEI contributes to the enhancement of the anode performance.     

Spatiotemporal Changes of the Solid Electrolyte Interphase in Lithium‐Ion Batteries Detected by Scanning Electrochemical Microscopy

The solid electrolyte interphase (SEI) in lithium‐ion batteries separates the highly reductive lithiated graphite from reducible electrolyte components. It is critical for the performance, durability, and safe operation of batteries. In situ imaging of the SEI is demonstrated using the feedback mode of scanning electrochemical microscopy (SECM) with 2,5‐di‐tert‐butyl‐1,4‐dimethoxy benzene as mediator. The formation of the SEI is indicated by a decrease of the mediator regeneration rate. Prolonged imaging of the same region revealed fluctuation of the passivating properties on time scales between 2 min and 20 h with an inhomogeneous distribution over the sample. The implications of the approach for in situ assessment of local SEI properties on graphite electrodes are discussed with respect to studying the influence of mechanical stress on SEI reliability and the mode of action of electrolyte additives aiming at improving SEI properties.     

二氟草酸硼酸钠作为电解液添加剂对石墨负极性能的影响

锂离子电池日益广泛的应用对其性能提出越来越高的要求,而在电解液中加入适当的添加剂能够显著提升电极材料的电化学性能. 本文首次在1 mol·L^-1 LiPF₆/EC + DMC + EMC（体积比1:1:1）的电解液中添加一定量的二氟草酸硼酸钠(NaDFOB),并通过循环伏安（CV）、电化学阻抗图谱（EIS）和扫描电子显微镜（SEM）等分析考察了其对石墨负极材料性能的具体影响. 结果显示,添加NaDFOB的电解液显著提高了石墨材料在常温下的可逆充放电容量和循环性能,同时明显改善了石墨材料的高温循环性能. 其机理在于NaDFOB的阴阳离子同时参与了石墨表面固体电解质界面膜（SEI）的形成,形成高稳定性的电解液/电极界面.     

Fluoroethylene Carbonate Enabling a Robust LiF‐rich Solid Electrolyte Interphase to Enhance the Stability of the MoS2 Anode for Lithium‐Ion Storage

As a high‐capacity anode for lithium‐ion batteries (LIBs), MoS₂ suffers from short lifespan that is due in part to its unstable solid electrolyte interphase (SEI). The cycle life of MoS₂ can be greatly extended by manipulating the SEI with a fluoroethylene carbonate (FEC) additive. The capacity of MoS₂ in the electrolyte with 10 wt % FEC stabilizes at about 770 mAh g^?1 for 200 cycles at 1 A g^?1, which far surpasses the FEC‐free counterpart (ca. 40 mAh g^?1 after 150 cycles). The presence of FEC enables a robust LiF‐rich SEI that can effectively inhibit the continual electrolyte decomposition. A full cell with a LiNi_0.5Co_0.3Mn_0.2O₂ cathode also gains improved performance in the FEC‐containing electrolyte. These findings reveal the importance of controlling SEI formation on MoS₂ toward promoted lithium storage, opening a new avenue for developing metal sulfides as high‐capacity electrodes for LIBs.     

Microgravimetric and Spectroscopic Analysis of Solid−Electrolyte Interphase Formation in Presence of Additives

Electrochemical quartz crystal microbalance (EQCM) with damping monitoring is applied for real-time analysis of solid−electrolyte interphase (SEI) formation in diphenyl octyl phosphate (DPOP) and vinylene carbonate (VC) modified electrolytes. Fast SEI formation is observed for the DPOP containing electrolyte, whereas slow growth is detected in VC-modified and reference electrolytes. QCM measurements in a dry state show considerable reduction of the mass quantity for DPOP and reference samples and minor mass decrease for the SEI layer formed in the presence of VC. The results indicate that VC enhances SEI stability, whereas the addition of DPOP or no additive results in incorporation of loosely attached species, leadubg to SEI instability. Resonance frequency damping, Δw, and dissipation factor, D, are used for analyzing mechanical properties of the SEI layers. The apparent increase of Δw and D during SEI formation in presence of DPOP suggests a pronounced viscoelasticity of the layer. QCM results are compared with surface morphology and chemical composition, revealing excellent agreement of the applied characterization approaches.     

温度对石墨电极性能的影响

运用电化学阻抗谱(EIS)并结合循环伏安法(CV)研究了石墨电极25和60 ℃时在1 mol·L-1 LiPF6-EC(碳酸乙烯酯):DEC(碳酸二乙酯):DMC(碳酸二甲酯)电解液中, 以及60 ℃时在1 mol·L-1 LiPF6-EC:DEC:DMC+5%VC(碳酸亚乙烯酯)电解液中的首次阴极极化过程. 发现高温下(60 ℃)石墨电极在1 mol·L-1 LiPF6-EC:DEC:DMC电解液中可逆循环容量衰减的主要原因在于其表面无法形成稳定的固体电解质相界面(SEI)膜. 实验结果显示, VC添加剂能够增进高温下石墨电极表面SEI膜的稳定性, 进而改进石墨电极的循环性能.     

锂离子电池电解液成膜添加剂乙烯基亚硫酸乙烯酯的电化学行为

研究了具有不饱和双键和亚硫酸酯双官能团的乙烯基亚硫酸乙烯酯(VES)作为锂离子电池电解液成膜添加剂对中间相碳微球(CMS)和LiFePO4电极电化学性能的影响. 结果表明: 在1 mol/L LiClO4/PC电解液体系中, 少量的VES (5%)能够在电化学过程中先于PC在CMS表面还原, 形成稳定的SEI膜, 明显抑制PC和溶剂化锂离子共嵌入石墨层间, 改善了电池的循环性能. 此外, 电解液1 mol/L LiClO4/PC＋5%VES (V∶V)在LiFePO4电极中展现出良好的电化学稳定性.     

四丁基六氟磷酸铵作为锂离子电池阻燃添加剂的研究

近年来关于锂离子电池造成的安全问题甚至事故的报道屡见不鲜,锂离子电池的安全问题已经成为人们关注的焦点. 我们用四丁基六氟磷酸铵（TBAPF₆）作为锂离子电池电解液阻燃添加剂,研究发现添加了TBAPF₆的电解液具有明显的阻燃效果,同时电解液电导率下降并不明显. LiCoO₂/Graphite全电池在添加了TBAPF₆的电解液中可逆容量会略有降低,但具有更优异的循环稳定性. 主要是由于TBAPF₆添加量的增加会影响石墨电极的库伦效率,延长活化时间. 通过对LiCoO₂/Graphite全电池绝热加速量热仪（ARC）测试,表明添加TBAPF₆对电池的燃烧有明显的抑制作用. 在TBAPF₆添加量至5%时,电池在300 ^oC内自放热速率不超过0.1^oC/min,电池的安全性显著提高.     

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.     

氟代碳酸乙烯酯添加剂对钠离子电池正极的影响

使用电解液成膜添加剂是一种简单高效的提高电池循环稳定性的方法。氟代碳酸乙烯酯(FEC)的最低未被占据分子轨道(LUMO)能量较低,易被还原,通常被认为是很好的负极成膜添加剂,但因其最高占据分子轨道(HOMO)能量也较低,抗氧化性较好,故其被认为不在正极上发生作用。本工作结合电化学,形貌分析,化学成分表征,原位结构分析等方法研究了FEC添加剂在钠离子电池中的作用。我们发现适量的FEC添加剂不仅可以显著抑制电解液溶剂碳酸丙烯酯(PC)的分解,而且会在正极上形成一层富NaF的保护层,提高循环过程中正极晶格结构稳定性,从而提高电池的循环稳定性。密度泛函理论(DFT)计算表明,FEC之所以能在正极上形成保护层,可能与其容易在正极界面与钠盐阴离子ClO_4~-结合反应有关。     

Self‐Stabilized and Strongly Adhesive Supramolecular Polymer Protective Layer Enables Ultrahigh‐Rate and Large‐Capacity Lithium‐Metal Anode

Constructing a solid electrolyte interface (SEI) is a highly effective approach to overcome the poor reversibility of lithium (Li) metal anodes. Herein, an adhesive and self‐healable supramolecular copolymer, comprising of pendant poly(ethylene oxide) (PEO) segments and ureido‐pyrimidinone (UPy) quadruple‐hydrogen‐bonding moieties, is developed as a protection layer of Li anode by a simple drop‐coating. The protection performance of in‐situ‐formed LiPEO–UPy SEI layer is significantly enhanced owing to the strong binding and improved stability arising from a spontaneous reaction between UPy groups and Li metal. An ultrathin (approximately 70 nm) LiPEO–UPy layer can contribute to stable and dendrite‐free cycling at a high areal capacity of 10 mAh cm^?2 at 5 mA cm^?2 for 1000 h. This coating together with the promising electrochemical performance offers a new strategy for the development of dendrite‐free metal anodes.     

Solvent decompositions and physical properties of decomposition compounds in Li-ion battery electrolytes studied by DFT calculations and molecular dynamics simulations

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.     

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.     

Thermal reactions of lithiated graphite anode in LiPF6-based electrolyte

The thermal reactions of a lithiated graphite anode with and without 1.3 M lithium hexafluorophosphate (LiPF₆) in a solvent mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) were investigated by means of differential scanning calorimetry (DSC). The products of the thermal decomposition occurring on the lithiated graphite anode were characterized by Fourier transform infrared (FT-IR) analysis. The lithiated graphite anode showed two broad exothermic peaks at 270 and 325 °C, respectively, in the absence of electrolyte. It was demonstrated that the first peak could be assigned to the thermal reactions of PF₅ with various linear alkyl carbonates in the solid electrolyte interphase (SEI) and that the second peak was closely related to the thermal decomposition of the polyvinylidene fluoride (PVdF) binder. In the presence of electrolyte, the lithiated graphite anode showed the onset of an additional exothermic peak at 90 °C associated with the thermal decomposition reactions of the SEI layer with the organic solvents.     

Initial solid electrolyte interphase formation process of graphite anode in LiPF6 electrolyte: an in situ ECSTM investigation

Understanding the structure and formation dynamics of the solid electrolyte interphase (SEI) on the electrode/electrolyte interface is of great importance for lithium ion batteries, as the properties of the SEI remarkably affect the performances of lithium ion batteries such as power capabilities, cycling life, and safety issues. Herein, we report an in situ electrochemical scanning tunnelling microscopy (ECSTM) study of the surface morphology changes of a highly oriented pyrolytic graphite (HOPG) anode during initial lithium uptake in 1 M LiPF(6) dissolved in the solvents of ethylene carbonate plus dimethyl carbonate. The exfoliation of the graphite originating from the step edge occurs when the potential is more negative than 1.5 V vs. Li(+)/Li. Within the range from 0.8 to 0.7 V vs. Li(+)/Li, the growth of clusters on the step edge, the decoration of the terrace with small island-like clusters, and the exfoliation of graphite layers take place on the surface simultaneously. The surface morphology change in the initial lithium uptake process can be recovered when the potential is switched back to 2.0 V. Control experiments indicate that the surface morphology change can be attributed to the electrochemical reduction of solvent molecules. The findings may lead to a better understanding of SEI formation on graphite anodes, optimized electrolyte systems for it, as well as the use of in situ ECSTM for interface studies in lithium ion batteries.     

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.     

Effects of current densities on the formation of LiCoO<Subscript>2</Subscript>/graphite lithium ion battery

The activation characteristics and the effects of current densities on the formation of a separate LiCoO₂ and graphite electrode were investigated and the behavior also was compared with that of the full LiCoO₂/graphite batteries using various electrochemical techniques. The results showed that the formation current densities obviously influenced the electrochemical impedance spectrum of Li/graphite, LiCoO₂/Li, and LiCoO₂/graphite cells. The electrolyte was reduced on the surface of graphite anode between 2.5 and 3.6 V to form a preliminary solid electrolyte interphase (SEI) film of anode during the formation of the LiCoO₂/graphite batteries. The electrolyte was oxidized from 3.95 V vs Li⁺/Li on the surface of LiCoO₂ to form a SEI film of cathode. A highly conducting SEI film could be formed gradually on the surface of graphite anode, whereas the SEI film of LiCoO₂ cathode had high resistance. The LiCoO₂ cathode could be activated completely at the first cycle, while the activation of the graphite anode needed several cycles. The columbic efficiency of the first cycle increased, but that of the second decreased with the increase in the formation current of LiCoO₂/graphite batteries. The formation current influenced the cycling performance of batteries, especially the high-temperature cycling performance. Therefore, the batteries should be activated with proper current densities to ensure an excellent formation of SEI film on the anode surface.