Effect of zwitterion on the lithium solid electrolyte interphase in ionic liquid electrolytes

An understanding of the solid electrolyte interphase (SEI) that forms on the lithium-metal surface is essential to the further development of rechargeable lithium-metal batteries. Currently, the formation of dendrites during cycling, which can lead to catastrophic failure of the cell, has mostly halted research on these power sources. The discovery of ionic liquids as electrolytes has rekindled the possibility of safe, rechargeable, lithium-metal batteries. The current limitation of ionic liquid electrolytes, however, is that when compared with conventional non-aqueous electrolytes the device rate capability is limited. Recently, we have shown that the addition of a zwitterion such as N-methyl-N-(butyl sulfonate) pyrrolidinium resulted in enhancement of the achievable current densities by 100%. It was also found that the resistance of the SEI layer in the presence of a zwitterion is 50% lower. In this study, a detailed chemical and electrochemical analysis of the SEI that forms in both the presence and absence of a zwitterion has been conducted. Clear differences in the chemical nature and also the thickness of the SEI are observed and these may account for the enhancement of operating current densities.     

A search for a single-ion-conducting polymer electrolyte: Combined effect of anion trap and inorganic filler

Lithium trifluoromethanesulfonate:polyethylene oxide (PEO) polymer electrolytes modified by 1,1,3,3,5,5-meso-hexaphenyl-2,2,4,4,6,6-meso-hexamethyl-6-pyrrole (C6P) and nanosize silica filler were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and electrochemical means. An increase in the lithium transference number is observed upon incorporation of even a small amount of C6P. Silica facilitates interchain ion hopping in polymer electrolytes and possibly introduces an additional interfacial ion-conduction path without decreasing t₊. Stable solid electrolyte interphase resistance (SEI) was achieved in the polymer electrolytes containing calix[6]pyrrole and silica. It was found that lithium single-ion-conductive polymers with good electrochemical stability and ion transport properties have the potential for considerably boosting the performance of lithium/molybdenum oxysulfide all-solid-state thin film batteries.     

Studies on the enhancement of solid electrolyte interphase formation on graphitized anodes in LiX-carbonate based electrolytes using Lewis acid additives for lithium-ion batteries

The new electrolyte systems utilizing one type of Lewis acids, the boron based anion receptors (BBARs) with LiF, Li₂O, or Li₂O₂ in carbonate solutions have been developed and reported by us. These systems open up a new approach in developing non-aqueous electrolytes with higher operating voltage and less moisture sensitivity for lithium-ion batteries. However, the formation of a stable solid electrolyte interphase (SEI) layer on the graphitized anodes is a serious problem needs to be solved for these new electrolyte systems, especially when propylene carbonate (PC) is used as a co-solvent. Using lithium bis(oxalato)borate (LiBOB) as an additives, the SEI layer formation on mesophase carbon microbeads (MCMB) anode is significantly enhanced in these new electrolytes containing boron-based anion receptors, such as tris(pentafluorophenyl) borane, and lithium salt such as LiF, or lithium oxides such as Li₂O or Li₂O₂ in PC and dimethyl carbonate (DMC) solvents. The cells using these electrolytes and MCMB anodes cycled very well and the PC co-intercalation was suppressed. Fourier transform infrared spectroscopy (FTIR) studies show that one of the electrochemical decomposition products of LiBOB, lithium carbonate (Li₂CO₃), plays a quite important role in the stablizing SEI layer formation.     

Effect of vinylene carbonate (VC) as electrolyte additive on electrochemical performance of Si film anode for lithium ion batteries

The effect of VC as electrolyte additive on the electrochemical performance of Si film anode was studied in this paper. The charge/discharge test, scanning electron microscopy (SEM), electrochemical impedance spectrum (EIS), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to investigate the cycle performance and SEI layer of Si film anode. It was found that the SEI layer formed in VC-containing electrolyte possessed better properties. It was impermeable to electrolyte and its impedance kept almost invariant upon cycling. The presence of VC in electrolyte brought out the VC-reduced products and decreased the LiF content in SEI layer. The major components of SEI layer were similar in VC-free and VC-containing electrolytes, which contained lithium salt (e.g. ROCO₂Li, Li₂CO₃, LiF), polycarbonate and silicon oxide. It was newly found that silicon oxide could be formed in SEI layer of Si film anode due to the reaction of lithiated silicon with permeated electrolyte in both VC-free and VC-containing electrolytes.     

Research progress on lithium based Water-in-salt electrolytes

与传统的商用有机锂离子电池相比,水系锂离子电池具有高安全性、成本低、环境友好等优点,但由于水的热力学窗口较窄（1.23 V）,从而大大限制了其输出电压和能量密度。Water-in-salt电解液的提出将水溶液的电化学窗口拓宽到3.0 V以上,为实现新型高电压水系锂离子电池提供了有利前提保证。本综述意在介绍Water-in-salt电解液及其相关衍生体系以及其在锂离子电池、锂硫电池以及混合离子电池中的相关应用拓展。与此同时,对该新体系中所引出的新的基础科学问题,包括水系固态电解质界面（SEI）膜的生长机理及锂离子的传输机制做了简单归纳和总结。     

Performance improvement of lithium ion battery using PC as a solvent component and BS as an SEI forming additive

The electrochemical behavior of propylene carbonate (PC)-based electrolytes with and without butyl sultone (BS) on graphite electrode and the performance of lithium ion batteries with these electrolytes were studied with cyclic voltammetry (CV), energy dispersive spectroscopy (EDS), as well as density functional theory (DFT) calculation. It is found that the co-insertion of PC with lithium ions into graphite electrode can be inhibited to a great extent by adjusting the composition of solvent in electrolytes. With the application of PC in the electrolyte without any additive, the discharge capacity of lithium ion battery is improved under high temperature or low temperature, however it decays under room temperature compared with the battery without PC. This drawback can be overcome by using BS as a solid electrolyte interphase (SEI) forming additive. BS has a lower LUMO energy and can be more easily electro-reduced than other components of solvent in electrolyte on a graphite electrode, forming a stable SEI film. With the application of BS in the electrolyte, the discharge capacity and cyclic stability of lithium ion battery is improved significantly under room temperature.     

Differential pulse effects of solid electrolyte interface formation for improving performance on high-power lithium ion battery

Solid electrolyte interface (SEI) formation is a key that utilizes to protect the structure of graphite anode and enhances the redox stability of lithium-ion batteries before entering the market. The effect of SEI formation applies a differential pulse (DP) and constant current (CC) charging on charge-discharge performance and cycling behavior into brand new commercial lithium ion batteries is investigated. The morphologies and electrochemical properties on the anode surface are also inspected by employing SEM and EDS. The electrochemical impedance spectra of the anode electrode in both charging protocols shows that the interfacial resistance on graphite anodes whose SEI layer formed by DP charging is smaller than that of CC charging. Moreover, the cycle life result shows that the DP charging SEI formation is more helpful in increasing the long-term stability and maintaining the capacity of batteries even under high power rate charge-discharge cycling. The DP charging method can provide a SEI layer with ameliorated properties to improve the performance of lithium ion batteries.     

Tri(ethylene glycol)-substituted trimethylsilane/lithium bis(oxalate)borate electrolyte for LiMn2O4/graphite system

Silane-based electrolyte is a promising candidate for safer electrochemical energy storage devices because it is thermally and electrochemical stable, less flammable and environmental benign. In this paper, electrochemical properties of one of the silane-based electrolytes, tri(ethylene glycol)-substituted trimethylsilane (1NM3)-lithium bis(oxalate)borate (LiBOB) was studied using LiMn₂O₄ as cathode and MAG graphite as anode. When combined with LiBOB as lithium salt, the 1NM3-LiBOB electrolyte can provide solid electrolyte interface (SEI) formation due to the reductive decomposition of LiBOB at first charging cycle. Compared to the electrolyte used in the conventional lithium-ion batteries, 1NM3-LiBOB electrolyte showed compatible battery performance in LiMn₂O₄/MAG chemistry. The AC impedance measurement indicates that the activation energy (E_a) obtained from the charge transfer impedance for 1NM3-LiBOB was higher than that of the state-of-the-art electrolyte. Due to its low conductivity, the rate capability of 1NM3-LiBOB electrolyte needs to be improved.     

Ionic liquids as electrolytes for Li-ion batteries—An overview of electrochemical studies

The paper reviews properties of room temperature ionic liquids (RTILs) as electrolytes for lithium and lithium-ion batteries. It has been shown that the formation of the solid electrolyte interface (SEI) on the anode surface is critical to the correct operation of secondary lithium-ion batteries, including those working with ionic liquids as electrolytes. The SEI layer may be formed by electrochemical transformation of (i) a molecular additive, (ii) RTIL cations or (iii) RTIL anions. Such properties of RTIL electrolytes as viscosity, conductivity, vapour pressure and lithium-ion transport numbers are also discussed from the point of view of their influence on battery performance.     

Analysis of SEI formed with cyano-containing imidazolium-based ionic liquid electrolyte in lithium secondary batteries

Two kinds of cyano-containing imidazolium-based ionic liquid, 1-cyanopropyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)imide (CpMI-TFSI) and 1-cyanomethyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)imide (CmMI-TFSI), each of which contained 20 wt% dissolved LiTFSI, were used as electrolytes for lithium secondary batteries. Compared with 1-ethyl-3-methylimidazolium-bis(trifluoromethane-sulfonyl)imide (EMI-TFSI) electrolyte, a reversible lithium deposition/dissolution on a stainless-steel working electrode was observed during CV measurements in these cyano-containing electrolytes, which indicated that a passivation layer (solid electrolyte interphase, SEI) was formed during potential scanning. The morphology of the working electrode with each electrolyte system was studied by SEM. Different dentrite forms were found on the electrodes with each electrolyte. The SEI that formed in CpMI-TFSI electrolyte showed the best passivating effect, while the deposited film formed in EMI-TFSI electrolyte showed no passivating effect. The chemical characteristics of the deposited films on the working electrodes were compared by XPS measurements. A component with a cyano group was found in SEIs in CpMI-TFSI and CmMI-TFSI electrolytes. The introduction of a cyano functional group suppressed the decomposition of electrolyte and improved the cathodic stability of the imidazolium-based ionic liquid. The reduction reaction route of imidazolium-based ionic liquid was considered to be different depending on whether or not the molecular structure contained a cyano functional group.     

Novel ionic plastic crystal-polymeric ionic liquid all-solid-state electrolytes for lithium ion batteries

离子塑性晶体作为一类新型的固态电解质材料,近年来受到研究人员的极大关注。本文合成了一种新型离子塑性晶体：N,N-二甲基吡咯双氟磺酰亚胺（P₁₁FSI）,并将其与吡咯阳离子离子液体聚合物-聚二甲基二烯丙基铵双氟磺酰亚胺（PILFSI）和锂盐（LiFSI）复合制备了P₁₁FSI-PILFSI-LiFSI全固态电解质。采用差示扫描量热法、热重分析、阻抗测试、线性扫描伏安法及对称锂电池测试等一系列表征技术对全固态电解质的热性能和电化学性能进行了系统研究。所制备的电解质膜具有好的柔韧性和热稳定性,高的离子电导率和电化学稳定性,以及与金属锂良好的界面相容性。将全固态电解质应用于Li/LiFePO₄电池中,在50℃、0.2 C充放电倍率时,电池放电比容量在60次循环后仍可达151.1 mA·h/g,容量保持率为96.8%;且在0.5 C、1.0 C倍率下放电比容量仍然高达138.1 mA·h/g和128.1 mA·h/g,展现出高的放电比容量,好的循环性能和倍率性能,有望应用于全固态锂离子电池中。     

Electrolyte infiltration in phosphazene-based dye-sensitized solar cells

We report here a study of phosphazene polymer and oligomer electrolyte infiltration into high surface area titanium dioxide electrodes and its effect on the performance of dye-sensitized solar cells. The effects of different cell assembly procedures on the electrochemical properties are examined, as well as the infiltration of electrolytes based on poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (MEEP), hexakis(2-(2-methoxyethoxy)ethoxy)cyclotriphosphazene (MEE trimer), and a linear short chain analogue into conventional titanium dioxide electrode mesoporous (nanosphere) films, microcolumns and nanowires. The effects of temperature, co-solvents, and the order of addition of the electroactive components are found to affect both the conductivity of the electrolytes and the electrochemical performance of the cells. Cross-sectional scanning electron microscopy (SEM) imaging is employed to examine the degree of electrolyte infiltration into the nanostructured electrodes as a function of filling conditions. Using these techniques, conditions are identified for achieving a high degree of pore filling by the three electrolyte systems. Increased power conversion efficiency is obtained when iodine is introduced after the heating and evacuation procedures required for maximum infiltration.     

Reviews of selected 100 recent papers for lithium batteries (Dec.1,2018 to Jan.31,2019)

该文是一篇近两个月的锂电池文献评述,以“lithium”和“batter^*”为关键词检索了Web of Science从2018年12月1日至2019年1月31日上线的锂电池研究论文,共有2472篇,选择其中100篇加以评论。正极材料主要研究了三元材料、富锂相材料和尖晶石材料的结构和表面结构随电化学脱嵌锂变化以及掺杂和表面包覆及界面层改进对其循环寿命的影响。硅基和锡基复合负极材料研究侧重于嵌脱锂机理以及SEI界面层,金属锂负极的研究侧重于通过集流体和表面覆盖层的设计以及电解液添加剂来提高其循环性能。固态电解质、电解液添加剂、固态电池、锂硫电池的论文也有多篇。原位分析偏重于界面SEI和电极反应机理,理论模拟工作涵盖储锂机理、动力学、界面SEI形成机理分析和固体电解质等。除了以材料为主的研究之外,还有多篇针对电池分析、电池管理系统技术的研究论文。     

Reviews of selected 100 recent papers for lithium batteries (Feb. 1, 2018 to Mar. 31, 2018)

该文是一篇近两个月的锂电池文献评述,以“lithium”和“batter^*”为关键词检索了Web of Science从2018年2月1日至2018年3月31日上线的锂电池研究论文,共有2731篇,选择其中100篇加以评论。正极材料主要研究了三元材料、富锂相材料和尖晶石材料和有机物正极材料,材料结构和表面结构随电化学脱嵌锂变化以及掺杂和表面包覆及界面层改进对其循环寿命的影响以及富锂材料的氧参与氧化还原反应的机制受到重视。硅基复合负极材料研究侧重于嵌脱锂机理以及SEI界面层,金属锂负极的研究侧重于通过表面覆盖层的设计来提高其循环性能。电解液添加剂、新型固态电解质、固态电池、锂硫电池的论文也有多篇。原位分析偏重于界面SEI和电极反应机理,理论模拟工作涵盖储锂机理、动力学、界面SEI形成机理分析和固体电解质等。除了以材料为主的研究之外,还有多篇关于电池界面及材料分析方法的研究论文。     

Progress of electrolyte additives for high-capacity power lithium ion batteries

动力电池是新能源汽车的核心部件,而电解液是制约动力电池发展的关键。电解液一般由碳酸酯类溶剂、锂盐和添加剂组成,其性质对电池的高低温、倍率、寿命等性能有显著影响。高比能动力电池所需电解液的主要开发策略是利用功能添加剂在电池正、负极同时形成稳定的保护膜,同时稳定界面。文章回顾了近年来匹配高压正极材料和高容量硅碳负极材料所需添加剂的组成和基本功能,论述了添加剂作用机理和发展趋势,认为300 W·h/Kg高能量密度电池电解液的关键在于开发新型多功能添加剂。     

Surface chemistry and morphology of the solid electrolyte interphase on silicon nanowire lithium-ion battery anodes

Silicon nanowires (SiNWs) have the potential to perform as anodes for lithium-ion batteries with a much higher energy density than graphite. However, there has been little work in understanding the surface chemistry of the solid electrolyte interphase (SEI) formed on silicon due to the reduction of the electrolyte. Given that a good, passivating SEI layer plays such a crucial role in graphite anodes, we have characterized the surface composition and morphology of the SEI formed on the SiNWs using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). We have found that the SEI is composed of reduction products similar to that found on graphite electrodes, with Li₂CO₃ as an important component. Combined with electrochemical impedance spectroscopy, the results were used to determine the optimal cycling parameters for good cycling. The role of the native SiO₂ as well as the effect of the surface area of the SiNWs on reactivity with the electrolyte were also addressed.     

Potential low-temperature application and hybrid-ionic conducting property of ceria-carbonate composite electrolytes for solid oxide fuel cells

Ceria-carbonate composite materials have been widely investigated as candidate electrolytes for solid oxide fuel cells operated at 300-600 °C. However, fundamental studies on the composite electrolytes are still in the early stages and intensive research is demanded to advance their applications. In this study, the crystallite structure, microstructure, chemical activity, thermal expansion behavior and electrochemical properties of the samaria doped ceria-carbonate (SCC) composite have been investigated. Single cells using the SCC composite electrolyte and Ni-based electrodes were assembled and their electrochemical performances were studied. The SCC composite electrolyte exhibits good chemical compatibility and thermal-matching with Ni-based electrodes. Peak power density up to 916 mW cm⁻² was achieved at 550 °C, which was attributed to high electrochemical activity of both electrolyte and electrode materials. A stable discharge plateau was obtained under a current density of 1.5 A cm⁻² at 550 °C for 120 min. In addition, the ionic conducting property of the SCC composite electrolyte was investigated using electrochemical impedance spectroscopy technique. It was found that the hybrid-ionic conduction improves the total ionic conductivity and fuel cell performance. These results highlight potential low-temperature application of ceria-carbonate composite electrolytes for solid oxide fuel cells.     

Effect of succinic anhydride as an electrolyte additive on electrochemical characteristics of silicon thin-film electrode

The effect of an electrolyte additive, succinic anhydride (SA), on the electrochemical performances of a silicon thin-film electrode, which is prepared by radio-frequency magnetron sputtering, is investigated. The introduction of SA into a liquid electrolyte consisting of ethylene carbonate/diethyl carbonate/1 M LiPF₆ significantly enhances the capacity retention and coulombic efficiency of the electrode. This improvement in the electrochemical performance of the electrode is attributed to modification of the solid/electrolyte interphase (SEI) layer by the introduction of SA. The differences in the characteristic properties of SEI layers, with or without SA, are explained by analysis with scanning electron microscopy, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy.     

Pr-doped ceria nanoparticles as intermediate temperature ionic conductors

One of the most important challenges for further development of solid oxide fuel cells is to develop new electrolytes which operate at intermediate temperatures. In this sense, the Ce_0.8Pr_0.2O₂ material has been synthesized for the first time using polyol-mediated synthesis route, obtaining well crystallized nanoparticles with a particle size of 2-3 nm. In order to study the thermomechanical properties of the sample, dilatometric measurements have been carried out. The Ce_0.8Pr_0.2O₂ nanostructured sample exhibits a thermal expansion coefficient of 22.1 × 10⁻⁶ K⁻¹, making this material one of the first electrolytes compatible with cobaltites and double perovskites, the most promising cathodes for these devices. The electrochemical behavior of these nanoparticles has been studied and the influence of the particle size on the material properties has been also analyzed. The nanostructured electrolyte presents bulk conductivity much higher than microstructured samples, giving rise to an improvement of the electrolyte performance. Experimental results indicate that Ce_0.8Pr_0.2O₂ nanoparticles exhibit a lot of advantages for their employment as electrolyte material in terms of smaller particle size and better electrochemical performance.     

Possible use of non-flammable phosphonate ethers as pure electrolyte solvent for lithium batteries

Dimethyl methyl phosphonate (DMMP) was selected and tested as a non-flammable solvent for primary and secondary lithium batteries, because of its non-flammability, good solvency of lithium salts and appropriate liquidus properties. Experimental results demonstrated that DMMP can solvate considerable amount of commonly used lithium salts to form non-flammable and Li⁺-conducting electrolyte, which has very wide electrochemical window (>5 V vs. Li) and excellent electrochemical compatibility with metallic lithium anode and oxide cathodes. Primary Li–MnO₂ cells using DMMP-based electrolyte showed almost the same discharge performances as those using organic carbonate electrolytes, and also, Li–LiMn₂O₄ cells using DMMP electrolyte exhibited greatly improved cycleability and dischargeability, suggesting a feasible application of this new electrolyte for constructing high performance and non-flammable lithium batteries.