固态电解质锂离子输运机制研究进展

全球环境问题推动了可充电锂电池技术的飞速发展.与液态电解液相比,固态电解质不易燃,构筑所得固态电池的安全性能得以提升.如果能够理解固态电解质中的离子输运行为,就能精准调控固态电池锂的动力学稳定性和倍率性能.随着计算机技术的快速发展,原子尺度模拟技术成为理解材料离子输运的重要手段.针对以上问题,本综合评述首先汇总了固体材料中的常见扩散机制;然后介绍了固态电解质中的锂离子输运机制,着重讨论了影响固态电解质锂离子输运的重要因素(晶体结构、电子结构、外部因素及晶界);最后对固态电解质锂离子输运机制研究进行了总结与展望.     

面向储能应用的多尺度电极材料结晶设计

电化学储能过程包含固/气/液界面反应、分层次的传质和电荷传递过程,并涉及微观-介观-宏观的跨尺度问题等.电极材料的宏观电化学性能是不同尺度影响因素的综合表现.结晶是电极材料合成的重要过程,与电极材料的功能特性紧密相关,涉及原子分子成核和晶体生长过程,具有多尺度、多因素、多层次特征.本文从结晶角度,在微观、介观、宏观尺度上分别介绍了构筑有利于离子、电子传输的晶体结构、界面结构、三维结构等,评述了热力学稳定、动力学增强的新型胶体电极材料,通过跨尺度设计方式强化电极材料的电化学性能.在原子、分子、材料、器件水平设计多尺度电极材料是解决电化学储能器件面临问题的重要策略.     

定制电解液或隔膜实现锂离子各向异性输运从而抑制枝晶生长：相场模拟研究

电池内部不可控的枝晶生长问题严重地影响着电池的循环性能和安全性能,这对于锂金属电池的实际应用是一个严峻的挑战。尽管已有较多的实验和理论研究工作聚焦于电极间锂离子各向异性输运特性对枝晶形貌的影响,但仍有一些开放性的问题有待进一步研究,例如,如何将枝晶生长的动态演变与电解液性质、电势分布或隔膜多孔结构诱导的锂离子各向异性输运关联起来。我们通过将锂离子在电解液中的扩散系数(DL)表示为二阶张量的形式并进行相场模拟,发现D_yy:D_xx比值的增加,以及电势诱导的电极/电解液界面锂离子快速扩散层均可以降低界面处锂离子浓度梯度和电势梯度,从而减弱枝晶生长的驱动力。我们还发现隔膜基体与y方向之间夹角的增大也会显著促进电解质中的锂离子各向异性输运特性,以利于抑制枝晶生长。籍此本文提出设计D_yy:D_xx=10:1的电解液和基体倾斜角为arctan (0.5)的隔膜用于锂金属电池。该相场研究有望为设计具有抑制枝晶能力的电解质或隔膜提供指导。     

分子电子学的新进展

分子尺度电子学通过构筑基于微尺度电极和单个分子或者少量分子聚集体的"电极-分子-电极"结，研究跨越分子的电荷输运性质.它将分子本征化学特性与器件构筑相结合，考察分子的理化特性与电荷输运的构效关系，揭示微尺度的量子输运动力学原理，并探索基于分子的功能电子器件.是一个集化学、物理学与微电子学为一体的交叉学科.总结整理了分子电子学近些年在器件制备、输运机理及应用方面部分有代表性的进展.     

影响电池性能的因素：金属离子溶剂化结构衍生的界面行为还是固体电解质界面膜?

通过电解液分解在电极上形成的固体电解质界面（SEI）层被认为是影响电池性能的最重要因素。 然而，我们发现金属离子溶剂化结构也会影响其电极性能，尤其可以阐明许多SEI无法解释的实验现象。基于该综述，本文总结了金属离子溶剂化结构和衍生的金属离子去溶剂化行为的重要性，并建立了相应的界面模型以展示界面行为和电极性能之间的关系，并将其应用于不同的电极和电池体系。我们强调了电极界面离子/分子相互作用对电极性能的影响，该解释与以往基于SEI的解释不同。该综述为理解电池性能和指导电解液设计提供了一个新的视角。     

润湿特性对超级电容器储能动力学的影响机理

润湿特性对超级电容器储能性能有着至关重要的影响。借助分子动力学模拟,本文研究了润湿特性对超级电容器储能动力学行为的影响。以石墨烯和晶体铜作为疏电解液和亲电解液电极材料。结果表明,在充电过程中,亲电解液铜电极呈现出非对称的U型微分电容曲线,负极电容是正极的～5.77倍,不同于经典双电层理论Gouy-Chapman-Stern(对称U型)和疏电解液型。该现象与离子自由能阻力分布密切相关,负极自由能阻力远小于正极(～2倍)和疏电解液电极,进而有利于强化双电层结构对电极电压的响应能力,导致更高微分电容。通过微分离子电荷密度,本文再现了微分电容演变规律,并发现改善润湿性会显著降低双电层厚度。最后,我们指出润湿性直接影响储能微观机理,将电荷储存机制从离子吸附和交换共同主导(疏电解液)转变到离子吸附主导(亲电解液)。本文所得结论揭示了润湿特性对储能动力学行为影响的原子层级机理,对超级电容器材料设计、构筑与润湿特性调控具有重要指导意义。     

拉曼光谱测试技术在可充电铝离子电池储能机理的研究进展

拉曼光谱是一种无损的分析技术,可以提供样品化学结构和分子相互作用的详细信息。由光谱学方法与常规电化学方法相结合产生的电化学原位光谱是一种动态探测电极材料结构和相组成的强大技术,能够方便地提供电极界面分子的微观结构信息,这使得其在储能领域中有广阔的应用前景。拉曼光谱能够有效地原位表征可充电铝离子电池氯化铝基电解液中络合离子、不同正极材料在充放电过程中的变化规律。结合X射线衍射技术（XRD）或X射线光电子能谱技术（XPS）等表征技术,拉曼光谱能够有效地揭示可充电铝离子电池的储能机理,包括对电池电解液和电极材料的研究以及电极表面反应的原位监测,对电池材料和界面结构性质的研究可以为电池材料和微观结构的优化设计提供指导,对电极表面反应的原位监测,有助于对电极界面反应的机理进行深入的研究,从而指导正极材料结构改进,促进可充电铝离子电池的发展。     

氟代溶剂在锂金属电池中的应用

近年来，锂金属电池由于具有较高的能量密度而成为储能领域的研究热点。电解液作为锂金属电池的“血液”发挥着至关重要的作用。在传统锂离子电池电解液中，锂金属负极与电解液之间的界面副反应严重并伴随着锂枝晶生长，从而导致安全隐患以及循环寿命缩短等问题。在解决锂金属负极问题上，电解液调控策略具有易操作性和有效性，因而在推动锂金属电池发展方面具有举足轻重的地位。氟代电解液是目前重要的研究方向，氟代电解液在循环过程中能够在电极表面形成富含LiF的固体电解质界面膜(SEI)；该界面膜不仅可以有效抑制负极锂枝晶的形成，并且在正极方面能够大幅提高电解液的氧化稳定性，从而提升高电压正极的适配性和锂金属电池的循环稳定性。氟代电解液中氟代溶剂/氟代锂盐的分子结构对电解液的溶剂化结构有重要影响。当氟代溶剂分子中氟原子的位置与数量不同时，氟代溶剂的物理化学性质也会随之发生变化，进而改变了电解液与电极的界面反应性。因此，氟代溶剂能够起到调制SEI膜成分和结构的作用，是决定电池性能的关键因素。本文总结了应用于锂金属电池的主要氟代溶剂，尤其是近几年来发展的新型氟代溶剂；着重介绍了高度氟代的溶剂分子作为局域超浓电解液的稀释剂，以及对溶剂进行精准分子设计得到的部分氟代溶剂等。此外，本文还分析探讨了氟代溶剂分子与电池性能之间的构效关系，展望了构建新型氟代溶剂分子的策略，希望能对电解液溶剂分子的结构设计以及构效关系的评估有一定的启发意义。     

钠离子电池电极材料的设计策略——固态离子学视角

钠离子电池是目前最有前景及可行性的新兴储能候选体系。对于钠离子电池而言，如何实现其电极材料的理性设计及构筑，是重要的科学问题。本文立足于钠离子/电子输运这一核心问题，从固态离子学视角探讨钠离子电池电极材料的设计策略。首先，对于体相电极材料，输运特性的明晰、调控以及缺陷化学模型的建立，是传统电极材料开发的关键。其次，对于纳米电极材料，随着尺寸的减小，电极材料的热力学性质、动力学特性以及钠离子微观储输机制都会发生相应变化，因此从纳米离子学视角，以尺寸效应调控电极材料具有重要的科学价值及现实意义。最后，无论对于体相材料还是纳米材料，从材料的本征输运特性出发，通过电化学电路的设计和构筑来优化电极动力学，可以为钠电电极材料的理性设计及可控制备提供理论指导。我们相信，通过本文系统地对钠离子电池电极材料设计策略的梳理，必将对钠离子电池的开发，提供有意义的指导，并为最终的产业化打下良好的基础。     

固态锂电池中正极/电解质界面的密度泛函计算研究

锂离子电池的广泛应用对储能器件的能量密度、安全性和充放电速度提出了新的要求. 全固态锂电池与传统锂离子电池相比具有更少的副反应和更高的安全性,已成为下一代储能器件的首选. 构建匹配的电极/电解质界面是在全固态锂电池中获得优异综合性能的关键. 本文采用第一性原理计算研究了固态电池中电解质表面及正极/电解质界面的局域结构和锂离子输运性质. 选取β-Li₃PS₄ (010)/LiCoO₂ (104)和 Li₄GeS₄ (010)/LiCoO₂ (104)体系计算了界面处的成键情况及锂离子的迁移势垒. 部分脱锂态的正极/电解质界面上由于Co-S成键的加强削弱了P/Ge-S键的强度,降低了对Li⁺的束缚,从而导致了更低的锂离子迁移势垒. 理解界面局域结构及其对Li+输运性质的影响将有助于我们在固态电池中构建性能优异的电极/电解质界面.     

Progress on predicting the electrochemical stability window of electrolytes

The advancement of novel electrical energy storage systems with high energy density encourages the development of electrolytes with wide electrochemical stability windows (ESWs). For the design of electrolytes, atomistic simulations have been used to investigate their electrochemical stability, providing a fast and economical approach for electrolytes screening, in which the simulation models are the key to predicting the electrolyte ESWs. Herein, the completing progress of the simulation models on predicting electrolyte ESWs is overviewed, which ranges in complexity from an isolated molecule/ion model to a solvation model and finally to a complex model of the electrode–electrolyte interface. We highlight the limitation and applicability of these models in detail and advocate a perspective of possible future research on the prediction of the electrolyte ESWs.     

The solvation structure,transport properties and reduction behavior of carbonate-based electrolytes of lithium-ion batteries

Despite the extensive employment of binary/ternary mixed-carbonate electrolytes (MCEs) for Li-ion batteries, the role of each ingredient with regards to the solvation structure, transport properties, and reduction behavior is not fully understood. Herein, we report the atomistic modeling and transport property measurements of the Gen2 (1.2 M LiPF₆ in ethylene carbonate (EC) and ethyl methyl carbonate (EMC)) and EC-base (1.2 M LiPF₆ in EC) electrolytes, as well as their mixtures with 10 mol% fluoroethylene carbonate (FEC). Due to the mixing of cyclic and linear carbonates, the Gen2 electrolyte is found to have a 60% lower ion dissociation rate and a 44% faster Li⁺ self-diffusion rate than the EC-base electrolyte, while the total ionic conductivities are similar. Moreover, we propose for the first time the anion–solvent exchange mechanism in MCEs with identified energetic and electrostatic origins. For electrolytes with additive, up to 25% FEC coordinates with Li⁺, which exhibits a preferential reduction that helps passivate the anode and facilitates an improved solid electrolyte interphase. The work provides a coherent computational framework for evaluating mixed electrolyte systems.

The different roles of the anion, cyclic and linear carbonates, and additive in mixed-carbonate electrolytes are revealed. The anion–solvent exchange mechanism and factors influencing the solid electrolyte interphase (SEI) formation are deciphered.     

Developing better ester- and ether-based electrolytes for potassium-ion batteries

Potassium-ion batteries (PIBs) have attracted extensive attention for next-generation energy storage systems because of the high abundance of potassium resources and low cost. However, the electrochemical performance of PIBs still cannot satisfy the requirements of practical application. One of the most effective strategies to improve the electrochemical performance of PIBs is electrolyte optimization. In this review, we focus on recent advances in ester- and ether-based electrolytes for high-performance PIBs. First, we discuss the requirements and components of organic electrolytes (potassium salts and solvents) for PIBs. Then, the strategies toward optimizing the electrolytes have been summarized, including potassium salt optimization, solvent optimization, electrolyte concentration optimization, and introducing electrolyte additives. In general, the electrolyte optimization methods can adjust the solvation energy, the lowest unoccupied molecular orbital energy level, and the highest occupied molecular orbital energy level, which are beneficial for achieving fast kinetics, stable and highly K⁺-conductive solid-electrolyte interphase layer, and superior oxidation resistance, respectively. Future studies should focus on exploring the effects of composition on electrolyte characteristics and the corresponding laws. This review provides some significant guidance to develop better electrolytes for high-performance PIBs.

A comprehensive summary on how to optimize ester- and ether-based electrolytes for high-performance potassium-ion batteries.     

Halide Layer Cathodes for Compatible and Fast-Charged Halides-Based All-Solid-State Li Metal Batteries

Insertion-type compounds based on oxides and sulfides have been widely identified and well-studied as cathode materials in lithium-ion batteries. However, halides have rarely been used due to their high solubility in organic liquid electrolytes. Here, we reveal the insertion electrochemistry of VX₃ (X=Cl, Br, I) by introducing a compatible halide solid-state electrolyte with a wide electrochemical stability window. X-ray absorption near-edge structure analyses reveal a two-step lithiation process and the structural transition of typical VCl₃. Fast Li⁺ insertion/extraction in the layered VX₃ active materials and favorable interface guaranteed by the compatible electrode-electrolyte design enables high rate capability and stable operation of all-solid-state Li-VX₃ batteries. The findings from this study will contribute to developing intercalation insertion electrochemistry of halide materials and exploring novel electrode materials in viable energy storage systems.     

Fluorinated Aromatic Diluent for High‐Performance Lithium Metal Batteries

In lithium metal batteries, electrolytes containing a high concentration of salts have demonstrated promising cyclability, but their practicality with respect to the cost of materials is yet to be proved. Here we report a fluorinated aromatic compound, namely 1,2‐difluorobenzene, for use as a diluent solvent in the electrolyte to realize the “high‐concentration effect”. The low energy level of the lowest unoccupied molecular orbital (LUMO), weak binding affinity for lithium ions, and high fluorine‐donating power of 1,2‐difluorobenzene jointly give rise to the high‐concentration effect at a bulk salt concentration near 2 m , while modifying the composition of the solid‐electrolyte‐interphase (SEI) layer to be rich in lithium fluoride (LiF). The employment of triple salts to prevent corrosion of the aluminum current collector further improves cycling performance. This study offers a design principle for achieving a local high‐concentration effect with reasonably low bulk concentrations of salts.     

Electrolyte Regulation towards Stable Lithium‐Metal Anodes in Lithium–Sulfur Batteries with Sulfurized Polyacrylonitrile Cathodes

Lithium–sulfur (Li–S) batteries are highly regarded as the next‐generation energy‐storage devices because of their ultrahigh theoretical energy density of 2600 Wh kg^?1. Sulfurized polyacrylonitrile (SPAN) is considered a promising sulfur cathode to substitute carbon/sulfur (C/S) composites to afford higher Coulombic efficiency, improved cycling stability, and potential high‐energy‐density Li–SPAN batteries. However, the instability of the Li‐metal anode threatens the performances of Li–SPAN batteries bringing limited lifespan and safety hazards. Li‐metal can react with most kinds of electrolyte to generate a protective solid electrolyte interphase (SEI), electrolyte regulation is a widely accepted strategy to protect Li‐metal anodes in rechargeable batteries. Herein, the basic principles and current challenges of Li–SPAN batteries are addressed. Recent advances on electrolyte regulation towards stable Li‐metal anodes in Li–SPAN batteries are summarized to suggest design strategies of solvents, lithium salts, additives, and gel electrolyte. Finally, prospects for future electrolyte design and Li anode protection in Li–SPAN batteries are discussed.     

Ab initio simulations of liquid electrolytes for energy conversion and storage

Understanding physicochemical properties of liquid electrolytes is essential for predicting and optimizing device performance for a wide variety of emerging energy technologies, including photoelectrochemical water splitting, supercapacitors, and batteries. In this work, we review recent progress and open challenges in predicting structural, dynamical, and electronic properties of the liquids using first-principles approaches. We briefly summarize the basic concepts of first-principles molecular dynamics (FPMD), and we discuss how FPMD methods have enriched our understanding of a number of liquids, including aqueous solutions, organic electrolytes and ionic liquids. We also discuss technical challenges in extending FPMD simulations to the study of liquid electrolytes in more complex environments, including the interface between electrolytes and electrodes, which is a key component in many energy storage and conversion systems.     

氟代溶剂在锂金属电池中的应用

Lithium metal batteries, which use lithium metal as the anode, have attracted tremendous research interest in recent years, owing to their high energy density and potential for future energy storage applications. Despite their advantages such as high energy density, the safety concerns and short lifespan significantly impede their practical applications in transportation and electronic devices. Tremendous efforts have been devoted to overcoming these problems, including materials design, interface modification, and electrolyte engineering. Among these strategies, electrolyte regulation plays a key role in improving the efficiency, stability, and safety of lithium metal anodes. As an important class of electrolyte components, fluorinated solvents, which can decompose to form LiF-rich interphase layers on both anode and cathode, have been proven to enhance the stability of lithium metal anodes and improve the oxidative stability of the electrolytes. Meanwhile, the spatial structure of fluorinated solvents, such as the number and sites of fluorine atoms, can influence the physicochemical properties of the electrolytes and the compositions/structure of the solid-electrolyte interphase, which eventually dictates the cycling performance of Li metal batteries. Recently, many fluorinated solvents with different molecular structures have been designed to regulate the solvation structure of electrolytes, and these solvents exhibit novel electrochemical properties in lithium metal batteries. However, there are few comprehensive reviews that summarize the fluorinated solvents used in Li metal batteries and discuss their functions in electrolytes and their physicochemical properties. This review summarizes the novel fluorinated solvents used in lithium metal batteries in recent years, which have been classified into three parts: diluents, traditional solvents, and novel molecules, based on their functions in the electrolytes. In every part, the understanding of the interactions between fluorinated solvents and Li ions, the decomposition mechanism of fluorinated solvents at the interface of the electrode, the functions of fluorinated solvents in the electrolytes, and the structure-activity relationship between the fluorinated solvents and battery performance have been comprehensively summarized and discussed. Moreover, the advantages and disadvantages of fluorinated solvents have been discussed, and the importance of precisely controlling the number of fluorine atoms and the structure of fluorinated solvents has been emphasized. At the end of this review, a perspective for designing new fluorinated solvents has been proposed. We believe that this review can provide insights on designing novel fluorinated solvents for high-performance Li metal batteries.      

Water-in-salt electrolytes: An interfacial perspective

Liquid electrolytes with high ionic conductivity, high transference number for the target ions, and excellent electrochemical, chemical, and thermal stability are essential for electrochemical energy storage devices. Water-in-salt (WIS) electrolytes, in which the salt–water ratio is larger than one, are gaining intensive attention in the electrochemical community. Here, we review the recent work on WIS electrolytes and the closely related water-in-ionic liquid electrolytes. We highlight the fact that many properties of these electrolytes, in bulk and at electrolyte–electrode interfaces, are underpinned by the physics and chemistry of the interfaces formed between water and ions (or aggregated water/ion clusters). Manipulating these interfaces by tailoring the selection of ions and water–ion ratio opens up new dimensions in the optimization of liquid electrolytes but also poses new challenges. We conclude the review by highlighting several directions for research on WIS electrolytes, in particular, the study of WIS electrolyte–electrode interfaces using surface force measurements.