Fabrication and characterization of SiO2/PVDF composite nanofiber‐coated PP nonwoven separators for lithium‐ion batteries

SiO₂/polyvinylidene fluoride (PVDF) composite nanofiber‐coated polypropylene (PP) nonwoven membranes were prepared by electrospinning of SiO₂/PVDF dispersions onto both sides of PP nonwovens. The goal of this study was to combine the good mechanical strength of PP nonwoven with the excellent electrochemical properties of SiO₂/PVDF composite nanofibers to obtain a new high‐performance separator. It was found that the addition of SiO₂ nanoparticles played an important role in improving the overall performance of these nanofiber‐coated nonwoven membranes. Among the membranes with various SiO₂ contents, 15% SiO₂/PVDF composite nanofiber‐coated PP nonwoven membranes provided the highest ionic conductivity of 2.6 × 10^?3 S cm^?1 after being immersed in a liquid electrolyte, 1 mol L^?1 lithium hexafluorophosphate in ethylene carbonate, dimethyl carbonate and diethyl carbonate. Compared with pure PVDF nanofiber‐coated PP nonwoven membranes, SiO₂/PVDF composite fiber‐coated PP nonwoven membranes had greater liquid electrolyte uptake, higher electrochemical oxidation limit, and lower interfacial resistance with lithium. SiO₂/PVDF composite fiber‐coated PP nonwoven membrane separators were assembled into lithium/lithium iron phosphate cells and demonstrated high cell capacities and good cycling performance at room temperature. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1719–1726     

An enhanced poly(vinylidene fluoride) matrix separator with high density polyethylene for good performance lithium ion batteries

A sponge-like poly(vinylidene fluoride)/high density polyethylene (PVDF/HDPE) separator exhibiting high ionic conductivity and transference number of Li⁺ ion for lithium ion battery has been prepared by non-solvent induced phase separation (NIPS) method. HDPE fillers with size smaller than 250 nm are prepared with moderated reverse phase emulsion. The ion conductivity of PVDF/HDPE separator saturated with 1.0 M LiPF₆–ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (1:1:1, v/v/v) can be up to 2.54 mS cm^?1 at 25 °C, which is higher than that of pristine PVDF separator (1.85 mS cm^?1). The transference number of lithium ion with PVDF/HDPE separator is 0.495, better than that with commercial PP separator (0.33) and pristine PVDF separator (0.27). What is more, LiCoO₂/Li cells assembled with PVDF/HDPE separator show good C-rate and cycling performance which indicates great potential in serving as a good candidate of polymer separator for lithium ion batteries application.     

Electrochemical performances of two kinds of ternary electrolyte mixtures with lithium bis(oxalate)borate

Conductivities (??) of PC (propylene carbonate)/EMC (ethyl methyl carbon ate)/DMC (dimethyl carbonate) and EC (ethylene carbonate)/EMC/DMC solutions of lithium bis(oxalate)borate (LiBOB) were experimentally determined at a temperature (??) range from ?40.0 to 60.0°C. Under such experimental conditions, the effect factors on the ??, such as the salt molar concentrations (m), and the volume ratio of solvent compositions, were also investigated. The results showed that, in wide ?? range, the higher ?? were obtained with 0.7 mol L^?1 LiBOB in PC/EMC/DMC and 0.6 mol L^?1 LiBOB in EC/EMC/DMC and with a volume ratio of 1: 1: 1 and 1: 1: 2, respectively. When used in LiFePO₄/Li cells, compared to the cell with the electrolyte system of 1.0 mol L^?1 LiPF₆-EC/EMC/DMC (1: 1: 1), LiBOB cells with PC/EMC/DMC and EC/EMC/DMC electrolyte systems with the same volume mixture solvent compositions exhibit several advantages, such as more stable cycle performance, higher mean voltage, excellent large current discharge capability, more capacity retention at high temperature, and more stable storage performance, etc. This study not only shows that LiBOB is a very promising alternative salt for lithium ion chemistry, but also provides appropriate solvent to improve LiBOB??s electrochemical performance.     

Study of Carbamate‐Modified Disiloxane in Porous PVDF‐HFP Membranes: New Electrolytes/Separators for Lithium‐Ion Batteries

A gel electrolyte membrane is obtained through the absorption of a carbamate‐modified liquid disiloxane‐containing lithium bis(trifluoromethane)sulfonimide (LiTFSI) by using macroporous poly(vinylidene fluoride–hexafluoropropylene) (PVDF‐HFP) membranes. The porous membranes are prepared by means of a phase inversion technique. The resulting gel electrolyte membrane is studied by using differential scanning calorimetry, Fourier‐transform infrared (FTIR) spectroscopy, and microscope mapping through coherent anti‐Stokes Raman scattering (CARS) confocal microscopy and impedance spectroscopy. The ionic conductivity of the gel electrolyte is 10^?4 S cm^?1 at 20 °C. FTIR spectroscopy reveals interactions between LiTFSI and the carbonyl moiety of the disiloxane. No interactions between LiTFSI and PVDF‐HFP or between disiloxane and PVDF‐HFP are detected by FTIR spectroscopy. Furthermore, the distribution of the α and β/γ phases of PVDF‐HFP and the homogeneous distribution of disiloxane/LiTFSI in the gel electrolyte membranes are examined by FTIR mapping. CARS confocal microscopy is used to image the three‐dimensional interconnectivity, which reveals a reticulated structure of macrovoids in the porous PVDF‐HFP framework. Owing to properties such as electrochemical and thermal stability of the disiloxane‐based liquid electrolyte and the mechanical stability of the porous PVDF‐HFP membrane, the gel electrolyte membranes presented herein are promising candidates for applications as electrolytes/separators in lithium‐ion batteries.     

Li[B(OCH2CF3)4]: Synthesis,Characterization and Electrochemical Application as a Conducting Salt for LiSB Batteries

A new Li salt with views to success in electrolytes is synthesized in excellent yields from lithium borohydride with excess 2,2,2‐trifluorethanol (HOTfe) in toluene and at least two equivalents of 1,2‐dimethoxyethane (DME). The salt Li[B(OTfe)₄] is obtained in multigram scale without impurities, as long as DME is present during the reaction. It is characterized by heteronuclear magnetic resonance and vibrational spectroscopy (IR and Raman), has high thermal stability (T_{decomposition}>271 °C, DSC) and shows long‐term stability in water. The concentration‐dependent electrical conductivity of Li[B(OTfe)₄] is measured in water, acetone, EC/DMC, EC/DMC/DME, ethyl acetate and THF at RT In DME (0.8 mol L ^?1) it is 3.9 mS cm^?1, which is satisfactory for the use in lithium‐sulfur batteries (LiSB). Cyclic voltammetry confirms the electrochemical stability of Li[B(OTfe)₄] in a potential range of 0 to 4.8 V vs. Li/Li⁺. The performance of Li[B(OTfe)₄] as conducting salt in a 0.2 mol L ^?1 solution in 1:1 wt % DME/DOL is investigated in LiSB test cells. After the 40th cycle, 86 % of the capacity remains, with a coulombic efficiency of around 97 % for each cycle. This indicates a considerable performance improvement for LiSB, if compared to the standard Li[NTf₂]/DOL/DME electrolyte system.     

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。     

Polymer electrolyte membranes by in situ polymerization of poly(ethylene carbonate‐co‐ethylene oxide) macromonomers in blends with poly(vinylidene fluoride‐co‐hexafluoropropylene)

Salt‐containing membranes based on polymethacrylates having poly(ethylene carbonate‐co‐ethylene oxide) side chains, as well as their blends with poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP), have been studied. Self‐supportive ion conductive membranes were prepared by casting films of methacrylate functional poly(ethylene carbonate‐co‐ethylene oxide) macromonomers containing lithium bis(trifluorosulfonyl)imide (LiTFSI) salt, followed by irradiation with UV‐light to polymerize the methacrylate units in situ. Homogenous electrolyte membranes based on the polymerized macromonomers showed a conductivity of 6.3 × 10^?6 S cm^?1 at 20 °C. The preparation of polymer blends, by the addition of PVDF‐HFP to the electrolytes, was found to greatly improve the mechanical properties. However, the addition led to an increase of the glass transition temperature (T_g) of the ion conductive phase by ～5 °C. The conductivity of the blend membranes was thus lower in relation to the corresponding homogeneous polymer electrolytes, and 2.5 × 10^?6 S cm^?1 was recorded for a membrane containing 10 wt % PVDF‐HFP at 20 °C. Increasing the salt concentration in the blend membranes was found to increase the T_g of the ion conductive component and decrease the propensity for the crystallization of the PVDF‐HFP component. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 79–90, 2007     

新型锂盐LiBC2O4F2在EC＋DMC溶剂中的电化学行为

采用差热-热重(TG-DTA)、恒电流充放电和交流阻抗(EIS)分析了二氟草酸硼酸锂(LiODFB)的热稳定性,研究了LiODFB/碳酸乙烯酯(EC)+碳酸二甲酯(DMC)电解液的电化学性能及界而特征.实验结果表明,LiODFB不仅具有更高的热稳定性,而且在EC+DMC溶剂中具有较好的电化学性能.与使用LiPF6/EC+DMC的电解液相比,锂离子电池应用LiODFB基电解液在55℃的高温具有更好的容量保持能力;以0.5C、1C(1C=250 mA·g-1)倍率循环放电,两种电池间的倍率性能差别较小;LiODFB能够在1.5 V(vs Li/Li+)左右在石墨电极表面还原形成一个优异稳定的保护性固体电解质相界面膜(SEI膜);交流阻抗表明,使用LiODFB基电解液的锂离子电池仅具有稍微增加的界面阻抗.因此LiODFB是一种非常有希望替代LiPF6用作锂离子电池的新盐.     

Rational Design of an Ionic Liquid‐Based Electrolyte with High Ionic Conductivity Towards Safe Lithium/Lithium‐Ion Batteries

It is a very urgent and important task to improve the safety and high‐temperature performance of lithium/lithium‐ion batteries (LIBs). Here, a novel ionic liquid, 1‐(2‐ethoxyethyl)‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR_1(2o2)TFSI), was designed and synthesized, and then mixed with dimethyl carbonate (DMC) as appropriate solvent and LiTFSI lithium salt to produce an electrolyte with high ionic conductivity for safe LIBs. Various characterizations and tests show that the highly flexible ether group could markedly reduce the viscosity and provide coordination sites for Li‐ion, and the DMC could reduce the viscosity and effectively enhance the Li‐ion transport rate and transference number. The electrolyte exhibits excellent electrochemical performance in Li/LiFeO₄ cells at room temperature as well as at a high temperature of 60 °C. More importantly, with the addition of DMC, the IL‐based electrolyte remains nonflammable and appropriate DMC can effectively inhibit the growth of lithium dendrites. Our present work may provide an attractive and promising strategy for high performance and safety of both lithium and lithium‐ion batteries.     

新型锂盐LiBC2O4F2在EC+DMC溶剂中的电化学行为

采用差热-热重(TG-DTA)、恒电流充放电和交流阻抗(EIS)分析了二氟草酸硼酸锂(LiODFB)的热稳定性, 研究了LiODFB/碳酸乙烯酯(EC)+碳酸二甲酯(DMC)电解液的电化学性能及界面特征. 实验结果表明, LiODFB不仅具有更高的热稳定性, 而且在EC+DMC溶剂中具有较好的电化学性能. 与使用LiPF6/EC+DMC的电解液相比, 锂离子电池应用LiODFB基电解液在55 ℃的高温具有更好的容量保持能力; 以0.5C、1C(1C=250 mA·g-1)倍率循环放电, 两种电池间的倍率性能差别较小; LiODFB能够在1.5 V(vs Li/Li+)左右在石墨电极表面还原形成一个优异稳定的保护性固体电解质相界面膜(SEI膜); 交流阻抗表明, 使用LiODFB基电解液的锂离子电池仅具有稍微增加的界面阻抗. 因此LiODFB是一种非常有希望替代LiPF6用作锂离子电池的新盐.     

Evaluating the electrochemical properties of PEO‐based nanofibrous electrolytes incorporated with TiO2 nanofiller applicable in lithium‐ion batteries

In the present work, nanofibrous composite polymer electrolytes consist of polyethylene oxide (PEO), ethylene carbonate (EC), propylene carbonate (PC), lithium perchlorate (LiClO₄), and titanium dioxide (TiO₂) were designed using response surface method (RSM) and synthesized via an electrospinning process. Morphological properties of the as‐prepared electrolytes were studied using SEM. FTIR spectroscopy was conducted to investigate the interaction between the components of the composites. The highest room temperature ionic conductivity of 0.085 mS.cm^?1 was obtained with incorporation of 0.175 wt. % TiO₂ filler into the plasticized nanofibrous electrolyte by EC. Moreover, the optimum structure was compared with a film polymeric electrolyte prepared using a film casting method. Despite more amorphous structure of the film electrolyte, the nanofibrous electrolyte showed superior ion conductivity possibly due to the highly porous structure of the nanofibrous membranes. Furthermore, the mechanical properties illustrated slight deterioration with incorporation of the TiO₂ nanoparticles into the electrospun electrolytes. This investigation indicated the great potential of the electrospun structures as all‐solid‐state polymeric electrolytes applicable in lithium ion batteries.     

Physical and electrolytic properties of difluorinated dimethyl carbonate

The physical and electrolytic properties of difluorinated dimethyl carbonate (DFDMC) synthesized using F₂ gas (direct fluorination) were examined. The dielectric constant and viscosity of DFDMC are higher than those of monofluorinated dimethyl carbonate (MFDMC) and dimethyl carbonate (DMC). The oxidative decomposition voltage of DFDMC is higher than those of DMC and MFDMC. The specific conductivity in DFDMC solution is considerably lower than those in MFDMC and DMC solutions. The ethylene carbonate (EC)-DFDMC equimolar binary solution containing 1 mol dm⁻³ LiPF₆ shows a moderate conductivity of 6.91 mS cm⁻¹ at 25 °C. The lithium electrode cycling efficiency (charge-discharge coulombic cycling efficiency of lithium electrode) in EC-DFDMC equimolar binary solution containing 1 mol dm⁻³ LiPF₆ is higher than 80%. The EC-DFDMC solution is a good electrolyte for rechargeable lithium batteries.     

Enhanced Performance of a Lithium–Sulfur Battery Using a Carbonate‐Based Electrolyte

The lithium–sulfur battery is regarded as one of the most promising candidates for lithium–metal batteries with high energy density. However, dendrite Li formation and low cycle efficiency of the Li anode as well as unstable sulfur based cathode still hinder its practical application. Herein a novel electrolyte (1 m LiODFB/EC‐DMC‐FEC) is designed not only to address the above problems of Li anode but also to match sulfur cathode perfectly, leading to extraordinary electrochemical performances. Using this electrolyte, lithium|lithium cells can cycle stably for above 2000 hours and the average Coulumbic efficiency reaches 98.8 %. Moreover, the Li–S battery delivers a reversible capacity of about 1400 mAh g^?1_sulfur with retention of 89 % for 1100 cycles at 1 C, and a capacity above 1100 mAh g^?1_sulfur at 10 C. The more advantages of this cell system are its outstanding cycle stability at 60 °C and no self‐discharge phenomena.     

Synthesis and characterization of poly(ethylene oxide‐co‐ethylene carbonate) macromonomers and their use in the preparation of crosslinked polymer electrolytes

Methacrylate‐functionalized poly(ethylene oxide‐co‐ethylene carbonate) macromonomers were prepared in two steps by the anionic ring‐opening polymerization of ethylene carbonate at 180 °C, with potassium methoxide as the initiator, followed by the reaction of the terminal hydroxyl groups of the polymers with methacryloyl chloride. The molecular weight of the polymer went through a maximum after approximately 45 min of polymerization, and the content of ethylene carbonate units in the polymer decreased with the reaction time. A polymer having a number‐average molecular weight of 2650 g mol^?1 and an ethylene carbonate content of 28 mol % was selected and used to prepare a macromonomer, which was subsequently polymerized by UV irradiation in the presence of different concentrations of lithium bis(trifluoromethanesulfonyl)imide salt. The resulting self‐supportive crosslinked polymer electrolyte membranes reached ionic conductivities of 6.3 × 10^?6 S cm^?1 at 20 °C. The coordination of the lithium ions by both the ether and carbonate oxygens in the polymer structure was indicated by Fourier transform infrared spectroscopy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2195–2205, 2006     

Fluoroethylene carbonate as an important component in organic carbonate electrolyte solutions for lithium sulfur batteries

The benefits of fluoroethylene carbonate (FEC)-based electrolyte solution (1 M LiPF₆ in FEC/dimethyl carbonate (DMC)) over ethylene carbonate (EC)-based electrolyte solution (1 M LiPF₆ in EC/DMC) for the cycling of sulfur/carbon (S/C) composite cathodes were demonstrated for S/C composites prepared with two drastically different types of carbon hosts, micrometer-sized activated carbon powder (AC1) and carbonized polyacrylonitrile (PAN) cloth. The formation of solid electrolyte interphase (SEI) on the surface of the cycled S/C electrodes was demonstrated using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS).     

Controlled grafting from poly(vinylidene fluoride) films by surface‐initiated reversible addition–fragmentation chain transfer polymerization

A reversible addition–fragmentation chain transfer (RAFT) polymerization technique was applied to graft polymerize brushes of poly(methyl methacrylate) (PMMA) and poly(poly(ethylene glycol) monomethacrylate) (PPEGMA) from poly(vinylidene fluoride) (PVDF) surfaces. PVDF surfaces were exposed to aqueous LiOH, followed by successive reductions with NaBH₄ and DIBAL‐H to obtain hydroxyl functionality. Azo‐functionalities, as surface initiators for grafting, were immobilized on the PVDF surfaces by esterification of 4,4′‐azobis(4‐cyanopentanoic acid) and the surface hydroxyl groups. The chemical composition and surface topography of the graft‐functionalized PVDF surfaces were characterized by X‐ray photoelectron spectroscopy, attenuated total reflectance‐FTIR spectroscopy, and atomic force microscopy. Kinetics studies revealed a linear increase in the graft concentration of PMMA and PPEGMA with the reaction time, indicating that the chain growth from the surface was consistent with a “controlled” or “living” process. The living chain ends were used as the macroinitiator for the synthesis of diblock copolymer brushes. Water contact angles on PVDF films were reduced by surface grafting of PEGMA and MMA. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3071–3082, 2006     

新型PMMA基聚合物电解质的研制

制备了聚甲基丙烯酸甲酯（PMMA）基聚合物电解质，通过加入交联剂使其形成网状结构，提高了聚合物电解质的机械性能.对MMA以及交联剂的含量作了优化，并测试了聚合物电解质的温度特性.测试结果表明,MMA、EGD(二甲基丙烯酸乙二醇酯)和电解液(LiBF4/EC DMC)含量分别为25％、2％、73％(质量分数）时，所制备的聚合物电解质具有较高的电导率，室温条件下可以达到2×10－3 S•cm－1，电化学窗口为4.8 V.用其作为电解质组装的聚合物锂离子电池具有较好的充放电性能.     

Effects of organophilic clay on the solvent‐maintaining capability,dimensional stability,and electrochemical properties of gel poly(vinylidene fluoride) nanocomposite electrolytes

Four quaternary alkyl ammonium salts were used in an organophilic procedure, performed on montmorillonite clay, and resulted in intercalation in dimethylformamide (DMF) or ethylene carbonate (EC)/propylene carbonate (PC) as a cosolvent between poly(vinylidene fluoride) (PVdF) and the organophilic clay. An examination using X‐ray diffraction revealed that PVdF entered galleries of montmorillonite clay, and it exhibited exfoliation and intercalation phenomena when it was analyzed with transmission electron microscopy. Gel PVdF nanocomposite electrolyte materials were successfully prepared by the addition of the appropriate percentages of DMF or PC/EC as a cosolvent, organophilic clay, and lithium perchlorate to PVdF. The maximum ionic conductivity was 1.03 × 10^?2 S/cm, and the materials exhibited better film formation, solvent‐maintaining capability, and dimensional stability than electrolyte films without added organophilic clays. The results of cyclic voltammetry testing showed that the addition of the organophilic clays significantly enhanced the electrochemical stability of the polymer electrolyte system. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3873–3882, 2002     

Enhanced electrochemical properties of a novel polyvinyl formal membrane supporting gel polymer electrolyte by Al2O3 modification

Polyvinyl formal (PVFM)‐based dense polymer membranes with nano‐Al₂O₃ doping are prepared via phase inversion method. The membranes and also their performances as gel polymer electrolytes (GPEs) for lithium ion battery are studied by field emission scanning electron microscope, X‐ray diffraction, differential scanning calorimetry, mechanical strength test, electrolyte uptake test, electrochemical impedance spectroscopy, cyclic voltammetry, and charge–discharge test. The polymer membrane with 3 wt % nano‐Al₂O₃ doping shows the improved mechanical strength of 12.16 MPa and electrolyte uptake of 431.25% compared with 10.47 MPa and 310.59% of the undoped sample, respectively. The membrane absorbs and swells liquid electrolyte to form stable GPE with ionic conductivity of 4.92 × 10^?4 S cm^?1 at room temperature, which is higher than 1.77 × 10^?4 S cm^?1 of GPE from the undoped membrane. Moreover, the Al₂O₃‐modified membrane supporting GPE exhibits wide electrochemical stability window of 1.2–4.8 V (vs. Li/Li⁺) and good compatibility with LiFePO₄ electrode, which implies Al₂O₃‐modified PVFM‐based GPE to be a promising candidate for lithium ion batteries. © 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014 , 52, 572–577     

High ionic conductive PVDF-based fibrous electrolytes

The high ionic conductive polymer electrolytes were prepared based on poly(vinylidenefluoride) (PVDF) fibers modified via preirradiation grafting poly(methyl methacrylate) (PMMA). In these polymer electrolytes, the PVDF fibers served as the supporting phase providing dimensional stability, and PMMA acted as the gel phase helping for the trapping liquid electrolyte and substituting the nonconductive PVDF phase to provide contact with electrodes well thus increasing conductive area. The modified PVDF fibrous membranes were used as a polymer electrolyte in lithium ion battery after they were activated by uptaking 1 M LiPF₆/ethylene carbonate–dimethyl carbonate (1:1 vol) liquid electrolyte, which showed a much higher room-temperature ionic conductivity than the pristine PVDF fibrous membrane. The LiCoO₂-mesocarbon microbead coin cells containing the dual-phase fibrous membrane (degree of graft, 111.8%) demonstrated excellent rate performance, and the cell still retained about 86% of discharge capacity at 4C rate, as compared to that at 0.1C rate. The prototype cell showed good cycle performance.