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.     

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.     

Structure Sensitivity in the Decomposition of Ethylene Carbonate on Si Anodes

The interaction of ethylene carbonate (EC) with Si surfaces is studied by density functional theory. The results show a strong structure sensitivity in the adsorption of EC on Si surfaces. While the adsorbed EC molecule readily decomposes on the Li/Si(111) surface, it does not dissociate on the Li/Si(100) and Li/Si(110) surfaces. On Si(111), the O atom at the top of EC is detached from the EC molecule and binds to the Li adatom, forming Li?O molecules. The mechanism of EC decomposition is the transfer of 2.4 electrons from the surface to the EC molecule, as well as the formation of a covalent bond between the Li adatom and the EC molecule. This result shows that in lithium‐ion batteries with Si anodes, dissociation of the solvent and formation of a solid electrolyte interphase layer start as soon as the Li atoms cover the anode surface.     

Studies of the interfacial properties of an electroplated Sn thin film electrode/electrolyte using in situ MFTIRS and EQCM

Sn thin film electrodes were prepared by electroplating in an acidic sulfate bath containing SnSO4. During charge/discharge processes, the interfacial properties between a Sn thin film electrode and an electrolyte of 1 mol.L(-1) LiPF6 in a mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 vol %) were investigated by using cyclic voltammetry (CV), electrochemical quartz crystal microbalance (EQCM), and in situ microscope Fourier transform infrared reflection spectroscopy (in situ MFTIRS). The processes of alloying/dealloying of lithium with Sn and the decomposition of the electrolyte on the Sn electrode were characterized quantitatively by surface mass change and at the molecule level. EQCM studies demonstrated that the mass accumulated per mole of electrons (mpe) was varied in different electrochemical processes. In the process of electrolyte decomposition, the measured mpe is smaller than the theoretical value, whereas it is higher than the theoretical value in the process of alloying/dealloying. The reduction products, ROCO2Li, of the electrolyte involved in charge/discharge processes were determined by in situ MFTIRS. The solvation/desolvation of lithium ion with solvent molecules, which is induced by the alloying/dealloying of lithium with Sn, was evidenced by shifts of relevant IR bands of C=O, C-O, and C-H. The current studies clearly revealed the details of interfacial reactions involved in lithium ion batteries employing a Sn thin film as the anode.     

A new piece in the puzzle of lithium/air batteries: computational study on the chemical stability of propylene carbonate in the presence of lithium peroxide

The electrolyte role in non-aqueous lithium/air batteries is attracting a lot of attention in several research groups, because of its fundamental importance in producing the appropriate reversible electrochemical reduction. While recent published works identify the lithium superoxide as the main degrading agent for propylene carbonate (PC), there is no clear experimental evidence that the oxygen at the cathode interface layer does not reduce further to peroxide before reacting with PC. Here, we investigate the reactivity of lithium peroxide versus propylene carbonate and find that Li(2)O(2) irreversibly decomposes the carbonate solvent, leading to alkyl carbonates. We also show that, compared with a single Li(2)O(2) unit in PC, a crystalline surface of Li(2)O(2) exhibits an enhanced reactivity. Our findings support the possibility that in lithium/air cells, oxygen may still be reduced to peroxide, with the formation of solid Li(2)O(2), which degrades by decomposing PC.     

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.     

Influence of adsorbed polar molecules on the electronic transport in a composite material Li(1.1)V3O8-PMMA for lithium batteries

The broadband dielectric spectroscopy (BDS) technique (40 to 10(10) Hz) is used here to measure the electronic transport across all observed size scales of a Li(1.1)V(3)O(8)-polymer-gel composite material for lithium batteries. Different electrical relaxations are evidenced, resulting from the polarizations at the different scales of the architecture: (i) atomic lattice (small-polaron hopping), (ii) particles, (iii) clusters of particles, and finally (iv) sample-current collector interface. A very good agreement with dc-conductivity measurements on a single macro-crystal [M. Onoda and I. Amemiya, J. Phys.: Condens. Matter, 2003, 15, 3079.] shows that the BDS technique does allow probing the bulk (intrinsic) electrical properties of a material in the form of a network of particles separated by boundaries in a composite. Moreover, this study highlights a lowering of the surface electronic conductivity of Li(1.1)V(3)O(8) particles upon adsorption of polar ethylene carbonate (EC) and propylene carbonate (PC) that trap surface polarons. This result is meaningful as EC and PC are typical constituents of a liquid electrolyte of lithium batteries. It is thus suggested that interactions between active material particles and the liquid electrolyte play a role in the electronic transport within composite electrodes used in a lithium battery.     

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。     

全氟烷基膦酸锂电解液的制备与性能研究

选取氯代二异丙基膦(C6H14PCl)为原料, 利用电化学氟化方法, 得到全氟烷基膦酸[(C3F7)2PF3], (C3F7)2PF3与氟化锂(LiF)反应得到全氟烷基膦酸锂(Li[(C3F7)2PF4]), 将其溶于碳酸乙烯酯(EC)和碳酸二甲酯(DMC)质量比为1∶1的混合溶剂中得到电解液, 考察电解液的电导率、抗水性及氧化分解电位. 以LiCoO2为正极, 锂片为负极组装两电极模拟电池体系, 测试得到电池的放电平台为3.7 V; 电池的首次放电比容量为107 mA•h•g－1; 当循环放电40次后, 容量衰减较快, 电池循环50周后, 效率仍保持102%. 交流阻抗图谱表明电解液放电时的阻抗约为140 Ω. 研究结果表明, 全氟烷基膦酸锂有望成为新型锂离子二次电池的电解质盐.     

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.     

新型锂盐Li[CF3BF3]电解液的制备与性能

制备了1种高纯度的新型锂盐三氟甲基三氟硼酸锂(Li[CF3BF3]),通过核磁共振(NMR)、元素分析(EA)及离子色谱(IC)对其结构进行表征和杂质分析.采取示差扫描量热(DSC)、交流阻抗(EIS)、循环伏安(CV)和扫描电镜(SEM)等方法研究了1 mol/L Li[CF3BF3]-EC/EMC/DMC(体积比5∶3∶2)电解液的物化和电化学性质.结果表明,Li[CF3BF3]基电解液的电导率和Li+迁移数远高于LiBF4,氧化电位高达5.91 V(vs.Li+/Li),在镍电极表面能观察到可逆的锂沉积-溶出过程,并对Al箔表现出优良的钝化性能.研究了Li[CF3BF3]基电解液的电导率与温度和浓度、黏度与浓度的变化规律,以及一系列浓度电解液的相变规律.Li/C半电池测试结果表明,—CF3取代LiBF4的1个F原子后,其衍生产物Li[CF3BF3]明显改善了电解液与人造石墨的相容性.     

新型导电盐(三氟甲基磺酰)(三氟乙氧基磺酰)亚胺锂及其非水电解液的制备与性能

制备了一种新型含氟磺酰亚胺锂盐(三氟甲基磺酰)(三氟乙氧基磺酰)亚胺锂{Li[(CF₃SO₂)·(CF₃CH₂OSO₂)N], Li[TFO-TFSI]}及其与碳酸乙烯酯(EC)/碳酸甲乙酯(EMC)混合溶剂(3∶7, 体积比)组成的非水电解液. 采用核磁共振波谱(NMR)、 红外光谱(IR)、 质谱(MS)、 元素分析(EA)和离子色谱(IC)等手段对合成锂盐Li[TFO-TFSI]进行了结构表征及纯度分析. 通过差示量热扫描(DSC)和热重分析(TG)对Li[TFO-TFSI]及其电解液1.0 mol/L Li[TFO-TFSI]-EC/EMC(3∶ 7)的热学性质进行了表征. 采用交流阻抗(EIS)、 循环伏安(CV)、 计时安培法及扫描电子显微镜(SEM)等对Li[TFO-TFSI]/碳酸酯电解液的基础物化和电化学性质进行了表征. 结果表明, Li[TFO-TFSI]/碳酸酯电解液具有较好的电化学稳定性; 在4.2 V(vs. Li/Li⁺)以下Al箔不发生腐蚀; 室温下基于Li[TFO-TFSI]/碳酸酯电解液的Li/人造石墨和人造石墨/LiCoO₂电池均保持较好的循环性能, 特别是人造石墨/LiCoO₂锂离子电池循环100周后, 其比容量保持率明显高于相应的基于LiPF₆/碳酸酯电解液体系的电池.     

Li_2CO_3添加剂对石墨电极性能的影响

运用电化学阻抗谱(EIS)和循环伏安法(CV)研究了在1mol/LLiPF6-EC(碳酸乙烯酯):DMC(碳酸二甲酯)电解液中添加Li2CO3对石墨电极性能的影响及机制.CV研究结果表明,在1mol/LLiPF6-EC:DMC电解液中添加Li2CO3能够有效抑制石墨电极首次充放电过程中碳酸乙烯酯(EC)的单电子还原过程,即还原分解产生乙烯和碳酸锂的过程,进而改善石墨电极的电化学循环性能.EIS研究结果表明,在添加Li2CO3的1mol/LLiPF6-EC:DMC电解液中,石墨电极表面的固体电解质相界面膜(SEI膜)具有较强的黏弹性,可以更好地适应锂离子嵌入过程中石墨颗粒体积的微小变化,从而使锂离子的嵌入过程更容易进行.     

Protective layer with oligo(ethylene glycol) borate anion receptor for lithium metal electrode stabilization

The gel polymer electrolyte based on semi-IPN (interpenetrating polymer network) structure for the protection of lithium metal electrode was successfully developed by ultraviolet (UV) radiation-curing method. A curable mixed solution consists of linear polymer (Kynar 2801), crosslinking agent (1,6-hexanediol diacrylate), liquid electrolyte (ethylene carbonate (EC)/propylene carbonate (PC)/1 M LiClO₄), oligo(ethylene glycol) borate (OEGB) anion receptor, and photoinitiator (methyl benzoylformate). The OEGB was synthesized by the dehydrocoupling reaction of hydroxyl group in di(ethylene glycol) methyl ether with hydrogen in BH₃ and characterized by ¹H NMR. The presence of OEGB anion receptor in the protection layer could lead to an enhancement in the ionic conductivity, electrochemical stability, and the interfacial properties. The deposited lithium exhibited particle-like shape resulting from the introduction of the protection layer onto the lithium electrode surface. The unit cell based on the lithium anode protected with gel polymer electrolyte containing OEGB showed higher discharge capacity than that of the unit cell without OEGB after 100 cycles at C/2 rate (1.25 mA cm⁻²).     

P(VdF-HFP)-基离子液体复合聚合物电解质的阴极稳定性及热稳定性

以聚偏氟乙烯-六氟丙烯P(VdF-HFP)聚合物为基体, 制备了含离子液体1-甲基-3-乙基咪唑六氟磷酸盐(EMIPF6)、用于锂离子电池的离子液体复合聚合物电解质[P(VdF-HFP)/LiPF₆/EMIPF₆/EC(碳酸乙烯酯)-PC(碳酸丙烯酯)]. 采用热重分析法以及燃烧实验测试了复合聚合物电解质的热稳定性. 离子电导率测试表明, 离子液体的存在显著改善了复合聚合物电解质的离子传输; 循环伏安测试表明, 添加剂EC和PC的加入提高了复合电解质的阴极稳定性, 制得的离子液体复合聚合物电解质在0.3-4.3 V 电压范围内稳定存在. Li₄Ti₅O₁₂ 和LiCoO₂为电极材料、P(VdF-HFP)/LiPF₆/EMIPF₆/EC-PC 为电解质的半电池表现出优良的循环性能, 0.1C充放电倍率下, Li/LiCoO₂和Li/Li₄Ti₅O₁₂半电池的可逆容量分别为130和144 mAh·g^-1. 但EC、PC在一定程度上降低了离子液体复合聚合物电解质的热稳定性.     

Stabilizing Lithium Plating by a Biphasic Surface Layer Formed In Situ

The dendritic growth of Li metal leads to electrode degradation and safety concerns, impeding its application in building high energy density batteries. Forming a protective layer on the Li surface that is electron‐insulating, ion‐conducting, and maintains an intimate interface is critical. We herein demonstrate that Li plating is stabilized by a biphasic surface layer composed of a lithium‐indium alloy and a lithium halide, formed in situ by the reaction of an electrolyte additive with Li metal. This stabilization is attributed to the fast lithium migration though the alloy bulk and lithium halide surface, which is enabled by the electric field across the layer that is established owing to the electron‐insulating halide phase. A greatly stabilized Li‐electrolyte interface and dendrite‐free plating over 400 hours in Li|Li symmetric cells using an alkyl carbonate electrolyte is demonstrated. High energy efficiency operation of the Li₄Ti₅O₁₂ (LTO)|Li cell over 1000 cycles is achieved.     

Theoretical studies to understand surface chemistry on carbon anodes for lithium-ion batteries: reduction mechanisms of ethylene carbonate.

Reductive decomposition mechanisms for ethylene carbonate (EC) molecule in electrolyte solutions for lithium-ion batteries are comprehensively investigated using density functional theory. In gas phase the reduction of EC is thermodynamically forbidden, whereas in bulk solvent it is likely to undergo one- as well as two-electron reduction processes. The presence of Li cation considerably stabilizes the EC reduction intermediates. The adiabatic electron affinities of the supermolecule Li(+)(EC)n (n = 1-4) successively decrease with the number of EC molecules, independently of EC or Li(+) being reduced. Regarding the reductive decomposition mechanism, Li(+)(EC)n is initially reduced to an ion-pair intermediate that will undergo homolytic C-O bond cleavage via an approximately 11.0 kcal/mol barrier, bringing up a radical anion coordinated with Li(+). Among the possible termination pathways of the radical anion, thermodynamically the most favorable is the formation of lithium butylene dicarbonate, (CH2CH2OCO2Li)2, followed by the formation of one O-Li bond compound containing an ester group, LiO(CH2)2CO2(CH2)2OCO2Li, then two very competitive reactions of the further reduction of the radical anion and the formation of lithium ethylene dicarbonate, (CH2OCO2Li)2, and the least favorable is the formation of a C-Li bond compound (Li carbides), Li(CH2)2OCO2Li. The products show a weak EC concentration dependence as has also been revealed for the reactions of LiCO3(-) with Li(+)(EC)n; that is, the formation of Li2CO3 is slightly more favorable at low EC concentrations, whereas (CH2OCO2Li)2 is favored at high EC concentrations. On the basis of the results presented here, in line with some experimental findings, we find that a two-electron reduction process indeed takes place by a stepwise path. Regarding the composition of the surface films resulting from solvent reduction, for which experiments usually indicate that (CH2OCO2Li)2 is a dominant component, we conclude that they comprise two leading lithium alkyl bicarbonates, (CH2CH2OCO2Li)2 and (CH2OCO2Li)2, together with LiO(CH2)2CO2(CH2)2OCO2Li, Li(CH2)2OCO2Li and Li2CO3.     

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.     

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.     

锂金属负极的可逆性与沉积形貌的关联

高能量密度二次电池的商业化将会推动便携式电子设备和电动车的飞速发展。锂金属电池因具有较高的理论能量密度而受到研究者的广泛关注。然而,锂金属负极较低的库仑效率(CE)和枝晶生长等问题,严重制约了锂金属电池的发展。库仑效率是衡量电池体系可逆性的关键参数之一,锂金属负极的库仑效率在不同电解液中存在较大的差异,本文以四种常见的电解液为例,包括1 mol·L^-1六氟磷酸锂-碳酸乙烯酯/碳酸二甲酯电解液,1 mol·L^-1六氟磷酸锂-碳酸乙烯酯/碳酸二甲酯+5%(w)氟代碳酸乙烯酯电解液,1 mol·L^-1双(三氟甲烷磺酰)亚胺锂-乙二醇二甲醚/1,3二氧戊环+2%(w)硝酸锂电解液,以及4 mol·L^-1双氟磺酰亚胺锂-乙二醇二甲醚电解液,利用原子力显微镜研究了不同电解液体系中锂金属的生长行为,探讨了锂金属沉积形貌与其库仑效率之间的联系,为发展高效的锂金属负极提供了参考依据。