Effect of electrolyte transport properties and variations in the morphological parameters on the variation of side reaction rate across the anode electrode and the aging of lithium ion batteries

In this work, for the first time, we model the variation of solid electrolyte interface (SEI) across the depth of anode electrode of lithium ion battery. It is anticipated that due to higher thickness of SEI layer at the electrode side connected to the separator, a more critical condition prevails there. The present work also investigates the effects of variations in the morphological parameters including porosity, interfacial surface area and active particle radius across anode electrode on the uniformity of side reaction. Moreover, the sensitivity of the side reaction uniformity to electrolyte parameters, such as diffusion and ionic conductivity, is studied. Results show that the ionic conductivity has a major role on the uniformity, and could reduce critical conditions in the part of electrode next to the separator. Moreover, simulation results show that increasing ionic conductivity could significantly prolong the lifetime of the battery. An increase in electrolyte diffusion improves side reaction uniformity. Results also show that positive gradients of morphological parameters across anode electrode, when parameters are changed independently, have considerable effects on uniformity of side reaction. This could be a criterion in choosing new morphologies for the part of anode electrode connected to separator.     

锌镍单液流电池正极内部流动与传质过程的格子Boltzmann方法数值模拟

本文针对锌镍单液流电池多孔正极，基于格子玻尔兹曼方法从孔隙尺度对多孔正极内的流动传质及化学反应过程进行了模拟，获得了多孔电极孔隙内部电解液渗流速度场，浓度场和电流密度变化。从孔隙内渗流与传质的角度分析了不同充电电流、电解液流速和多孔电极孔隙率对电极电化学反应的影响。     

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.     

High safety electrolyte for lithium-ion battery

锂离子电池安全性问题的本质是电池内部发生了热失控,热量不断的累积,造成电池内部温度持续上升,其外在的表现是燃烧、爆炸等。因此,锂离子电池的安全性与比能量、使用温度和倍率性能等存在一定的矛盾。电池能量密度越高、倍率性能越快和使用环境越恶劣,其能量剧烈释放时对电池体系的影响就越大,安全问题也越突出。当前锂离子电池电解液一般由低闪点的碳酸酯、对痕量水和温度敏感的LiPF₆和其它添加剂组成,本身具有高度可燃性。同时,电解液与正负极材料之间形成界面膜被认为是电池热失控的起点。因此,电解液改性是提升电池安全性的重要措施。本文分析了离子液体和氟代溶剂等溶剂对电解液安全性的提升效果,对比了多种锂盐对电解液安全性的影响,介绍了阻燃剂、过充保护剂、锂枝晶抑制剂和成膜稳定剂等电解液添加剂对锂电池安全性的改善。最后,从电池整体应用性能的角度出发,讨论了今后高安全性锂离子电池电解液的研发方向。     

锂离子电池氧化物固态电解质研究进展

可充电锂离子电池（LIB）是移动和固定存储系统中最具潜力的电池体系。然而,传统锂离子电池中不稳定的电沉积和不可控的界面反应会在液体电解质中发生,导致电池存在安全隐患。采用固态电解质（SSE）的全固态锂离子电池因具有高安全性、高可靠性和高能量密度可满足许多方面对储能的要求。但要实现商业化,SSE依然面临诸多挑战,如室温离子电导率较低（1×10^-5 ~ 1×10^-3 S/cm）以及电极和电解质之间的界面稳定性差等。为加快SSE的研究与开发,分别对无机钙钛矿（LLTO）型、石榴石（LLZO）型和钠快离子导体（NASICON）型固态电解质的结构和电导率改性进行了综述,特别强调了电解质与电极界面的重要性及其对电池性能的影响。     

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.     

锂离子电池电解液添加剂的研究进展

综述了锂离子电池电解液添加剂的发展现状,根据作用功能,添加剂主要可以分为以下几类:改善SEI膜性能添加剂、过充电保护添加剂、提高电解液低温性能添加剂和改善电解液热稳定性添加剂等,分别从作用机理进行了探讨,展望了添加剂在锂离子电池未来发展中的前景。     

锂离子电池容量衰减机理研究进展

容量衰减与电池循环寿命直接相关。导致锂离子电池容量衰减的原因主要包括：固体电解质界面膜（SEI）的增长、电解液的分解、电极材料结构破坏、活性物质的溶解和相转变等。过充过放电、不良的储存或使用温度等外部因素也会导致电池容量衰减。本文综述了近年来锂离子电池容量衰减机理的研究进展。     

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.     

Investigation and application of lithium difluoro(oxalate)borate (LiDFOB) as additive to improve the thermal stability of electrolyte for lithium-ion batteries

Lithium difluoro (oxalate) borate (LiDFOB) is used as thermal stabilizing and solid electrolyte interface (SEI) formation additive for lithium-ion battery. The enhancements of electrolyte thermal stability and the SEIs on graphite anode and LiFePO₄ cathode with LiDFOB addition are investigated via a combination of electrochemical methods, nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared-attenuated total reflectance (FTIR-ATR), as well as density functional theory (DFT). It is found that cells with electrolyte containing 5% LiDFOB have better capacity retention than cells without LiDFOB. This improved performance is ascribed to the assistance of LiDFOB in forming better SEIs on anode and cathode and also the enhancement of the thermal stability of the electrolyte. LiDFOB-decomposition products are identified experimentally on the surface of the anode and cathode and supported by theoretical calculations.     

Gamma-crotonlatone as an electrolyte additive for improving the cyclability of MCMB electrode

In this paper, gamma-crotonlatone (GCL) was tested as an additive to electrolyte solutions of 1 M LiPF₆ EC:DMC (1:1, vol) for lithium-ion batteries. Smaller volume amount in the order of 10⁻⁶ GCL improved the cyclability of MCMB electrode, and decreased the impedance of MCMB/Li cell. The results of cyclic voltammetry show that the GCL has higher reduction decomposition potential at about 2.1 V. The surface morphologies and chemistry of the solid electrolyte interphase (SEI) film formed on MCMB electrodes cycled in 1 M LiPF₆ EC:DMC with and without GCL were studied by SEM, FTIR and XPS analyses, and the results show that a uniform, stable and low-resistive SEI film was formed on the surface of MCMB electrode, which cause the excellent cyclability of electrode.     

Recent progress in rechargeable nickel/metal hydride and lithium-ion miniature rechargeable batteries

VARTA is searching for alternative battery solutions for memory back-up and bridging applications, and for this, it is developing nickel/metal hydride and lithium-ion button cells. Presented are the results on different sizes and forms of lithium-ion cells (621, 1216 and 2025) containing different electrode materials and shapes. Presently, the most favoured cathode material is lithiated manganese dioxide. The electrodes are made from both solid and porous materials and, together with an organic electrolyte, result in a cell system with a voltage level of approximately three. Included are results, both from these lithium-ion cells, and also from ones using the nickel/metal hydride system.     

Analysis of Toyota's patents on lithium ion solid electrolyte

采用易燃有机溶剂的液体电解质的锂离子电池存在安全隐患,固体电解质在安全性、热稳定性等方面具有明显优势,因此发展固体电解质是提高动力电池安全性能的有效途径。使用锂离子动力电池的新能源汽车目前强势发展,丰田作为汽车领域的领军企业,其对锂离子固体电池的发展对汽车行业具有重要参考价值。本文主要以CNABS专利数据库以及DWPI专利数据库中的检索结果为分析样本,从专利文献的视角对丰田在锂离子固体电解质的专利进行了全面统计。结果表明,丰田关于锂离子固体电池的专利申请主要分布在日本、美国和中国,在固体电解质上的重点研究方向为硫化物固体电解质,并且重点围绕提高其锂离子传导率、减少硫化物气体的产生、降低界面电阻等方面。     

Fundamental scientific aspects of lithium batteries (V)----Interfaces

电池中固液界面的性质对锂离子电池充放电效率,能量效率,能量密度,功率密度,循环性,服役寿命,安全性,自放电等特性具有重要的影响.对界面问题的研究是锂离子电池基础研究的核心.本文小结了 锂离子电池电极表面固体电解质中间相(SEI)形成机理及对其组成结构的认识,介绍了近年来对锂离子输运机制,SEI膜改性研究以及透射电镜(TEM)及原子力显微镜(AFM)中力曲线等实验技术来分析SEI膜的形貌,厚度,覆盖度及力学性能等实验方法.     

Thermal safety study of Li-ion batteries under limited overcharge abuse based on coupled electrochemical-thermal model

Many fire accidents of electric vehicles were reported that happened during the charging process. In order to investigate the reasons that lead to this problem, this paper studies the thermal safety of Li-ion batteries under limited overcharge abuse. A 3D electrochemical-thermal coupled model is developed for modeling thermal and electrochemical characteristics from normal charge to early overcharge state. This model is validated by experiment at charge rates of 0.5C, 1C, and 2C. The simulation results indicate that irreversible heat contributes most to temperature rise during the normal charge process, but the heat induced by Mn dissolution and Li deposition gradually dominates heat generation in the early overcharge period. Based on this, a threshold selection method for multistage warning of batteries overcharge is proposed. Among them, level 1 should be considered as a critical stage during the early overcharge process due to the deposited lithium starts to react with electrolyte at the end of level 1, where temperature rate increases to 0.5°C min⁻¹ for 1C charge. While the thresholds of levels depend on charge rate and composition of battery. Furthermore, several critical parameters are analyzed to figure out their effects on thermal safety. It is found that the temperature at the end of overcharge is significantly influenced by the change of positive electrode thickness and solid electrolyte interface (SEI) film resistance. The final temperature increases by 17.5°C and 7.9°C, respectively, with positive electrode thickness ranging from 50 to 80 μm and SEI film resistance increasing from 0.002 to 0.03 Ω.     

Simulation of capacity loss in carbon electrode for lithium-ion cells during storage

A mathematical model was developed which simulates the self-discharge capacity losses in the carbon anode for a SONY 18650 lithium-ion battery. The model determines the capacity loss during storage on the basis of a continuous reduction of organic solvent and de-intercalation of lithium at the carbon/electrolyte interface. The state of charge, open circuit potential, capacity loss and film resistance on the carbon electrode were calculated as a function of storage time using different values of rate constant governing the solvent reduction reaction.     

An advanced lithium-ion battery based on a nanostructured Sn-C anode and an electrochemically stable LiTFSi-Py24TFSI ionic liquid electrolyte

We report here a detailed impedance analysis of the interface between a selected IL electrolyte, i.e. a solution of lithium N,N-bis(trifluoromethane sulfon) imide in N-n-butyl-N-ethyl pyrrolidinium and a N,N-bis(trifluoromethane sulfon) imide, LiTFSI-Py₂₄TFSI, and two electrodes, namely a conventional lithium metal and an advanced nanostructured Sn-C alloy, respectively. We show that the Sn-C alloy, in virtue of a specific formation of a surface protecting film, has an interface much more stable than that of the Li electrode. This favourable property is exploited for using Sn-C as a new anode for the development of an advanced lithium-ion battery based on LiTFSI-Py₂₄TFSI as the electrolyte and on olivine LiFePO₄ as the cathode. The results demonstrated that this battery has very promising performances in terms of cycle life and rate capability.     

A three-dimensional tin-coated nanoporous copper for lithium-ion battery anodes

Nanoporous copper (NPC), as a new kind of porous metal prepared by dealloying, is introduced into the lithium-ion battery as both the current collector and substrate of active material. The nanoporous copper has three-dimensional structure composed of large channels (hundreds of nanometers) and small pores (tens of nanometers) on the channel walls. Anodes were prepared by electroless depositing of a thin layer of tin on NPC and copper foil. By comparing the electrochemical performance of both electrodes, the nanostructured electrode exhibits much higher areal capacity and better Coulombic efficiency than planar electrode.     

Surface film formation on electrodes in a LiCoO2/graphite cell: A step by step XPS study

In this paper, we report on a study of the electrode/electrolyte interfaces of a LiCoO₂/C cell using XPS (X-ray photoelectron spectroscopy). The originality of our work lies in detailed investigations step by step during the first cycle. The results have shown that the formation process of the SEI takes place in different successive stages that are dependent on the potential of the cell. It clearly appears that SEI formation continues in the potential range where lithium ion intercalation proceeds in the carbon electrode. Salt degradation products are formed after solvent decomposition ones. Concerning the LiCoO₂/electrolyte interface, the results obtained have shown the formation of a very thin film on the active material but not on the binders. In all cases, the novel XPS approach applied in the lab, combining core peaks and valence band analyses, allows a precise characterization of the main chemical species of the interface layers. On the basis of these results and in conjunction with literature data, the degradation mechanisms have been discussed.     

Model-based distinction and quantification of capacity loss and rate capability fade in Li-ion batteries

This work is focused on an easy-to-handle approach for estimating the residue power and capacity of a lithium-ion cell during operation. For this purpose, an earlier presented lumped parameter electrochemical battery model is employed. By means of the parameters accounting for the cathode capacity and the electrolyte conductivity, the cell degradation is successfully reproduced. Moreover, the method enables the distinction of capacity fade due to impedance rise and due to active material loss. High discharge rates together with the correlated self-heating of the cell enable a model-based quantification of SEI and electrolyte contributions to the overpotential.