First author: Jinkwang Hwang
Corresponding author: Jinkwang Hwang, Kazuhiko Matsumoto
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18 span_strong _strong_span_strong. Solid electrolyte has excellent electrochemical performance and safety performance, and is one of the important components of next-generation battery devices. Among them, some solid electrolytes with excellent ionic conductivity, electrochemical stability and high safety performance, such as lithium-filled garnet type oxides, sulfides, etc., have been widely reported. In addition, suitable for metal anodes. Further maximize the battery energy density. However, the formation of metal dendrites severely limits the actual performance of the solid electrolyte, which is usually manifested as high resistance between the metal electrode/solid electrolyte interface or the grain boundary, thus limiting their practical application. The latest research points out that the use of ionic liquids to modify solid electrolytes to form "quasi-solid electrolytes" with unique properties is expected to make great progress in improving battery performance and other uses, and is regaining people's interest.
[Introduction]
In view of this, Jinkwang Hwang, Kazuhiko Matsumoto , Kyoto University, Japan, and others. Explained the corresponding design strategy. Based on realistic prospects and future challenges,The author believes that the development of quasi-solid electrolyte materials requires an in-depth understanding of their properties, so as to stimulate far-reaching exploration of the properties and functions of their materials. The review was published in the top international journal " Pseudo-solid-state Electrolytes Utilizing the Ionic Liquid Family for Rechargeable Batteries " on the topic of " Pseudo-solid-state Electrolytes Utilizing the Ionic Liquid Family for Rechargeable Batteries ".
[Article Guide]
1. Ionic liquid electrolyte
1.1 Basic properties
Ionic liquids (ILs) are composed of cations and anions. They can form a variety of compounds through different combinations. It has many characteristics, such as low flammability, low vapor pressure, excellent thermal and chemical stability, wide electrochemical window, high intrinsic ionic conductivity, etc., which help to realize a safe, wide operating temperature battery. Figure 1 shows the ionic species of common ionic liquids or ionic plastic crystals. The choice of anion is relatively limited, usually including fluorine coordination anions (PF6− and BF4−) and sulfonamide anions (TFSA− and FSA) −) Compared with traditional organic solvent electrolytes, they have a lower melting point (wide liquid range) and higher conductivity.
The melting point of ionic liquids is related to the volume of cations,The introduction of larger or longer alkyl chains on the cations can lower the melting point, even if it will increase the viscosity and sacrifice a certain ionic conductivity. Therefore, cations with asymmetric structures with long alkyl chains are generally used to achieve a wide range of liquids. Procedure. In addition, ionic liquids based on inorganic cations (such as K+, Li+, etc.) have higher melting points and are suitable for high-temperature applications.
Figure 1 Ionic species of representative ionic liquids or ionic plastic crystals
1.2 The effect of ILS on the behavior of lithium metal interface
Interface instability is the most serious problem faced by lithium metal electrodes One of the problems is that compared with traditional organic solvent electrolytes, the use of ILs electrolytes helps to achieve lithium anodes with excellent performance, such as good cycle stability, high Coulomb efficiency, small overpotentials and fewer dendrites. This is largely due to the good thermal and chemical stability of ILs, as well as their remarkable dendrite inhibition ability, which enables them to form a strong solid electrolyte interface layer (SEI) on the negative electrode. As shown in Figure 2, the use of [C3C1pyrr][FSA] ionic liquid or Li[FSA]-[C2C1im][FSA] ionic liquid helps to form a dense and strong SEI film and improves the cycle stability of the lithium anode.
Figure 2 Lithium metal SEM and mechanism diagram after ILs surface treatment.
2. Ionic liquid composite solid electrolyte
2.1 Interface properties
The preparation method of the ionic liquid composite solid electrolyte is to encapsulate the solid electrolyte in ILs to form a solid electrolyte containing two types of ionic conductors The composite, Figure 3 is a schematic diagram,In this system, the solid electrolyte is the main ionic conductor, and the ionic liquid acts as an auxiliary ionic conductor, and plays an important interface wetting effect by modifying the electrode/solid electrolyte or solid electrolyte/solid electrolyte (grain boundary) interface. The limit potential of the composite solid electrolyte is usually determined by the ionic liquid, and the SEI of the solid electrolyte/electrode interface layer is mainly the decomposition product of ILs.
composite solid electrolyte preparation methods mainly include direct dropping method, ball milling method, ultraviolet polymerization method, etc., aiming to improve the interface wettability. Traditional organic electrolytes also have a similar effect, but their highly flammable properties cannot guarantee the safety performance of electrolytes, and ILs have a huge advantage in this regard due to their non-flammability.
Figure 3 Interfacial wetting characteristics of composite solid electrolyte prepared by ionic liquid.
2.2 Application in lithium/sodium secondary batteries
Sulfur-based solid electrolyte has excellent room temperature conductivity and is one of the solid materials with very promising applications, but it is very easy to interact with lithium metal It reacts and induces degradation, increases interface impedance, and deteriorates battery performance . After the ionic liquid is combined with this type of solid electrolyte, the wettability and interface stability of the lithium metal and the electrolyte can be improved. The specific example is shown in Figure 4. Whether it is in the lithium metal or sodium metal system, the sulfur group after the composite iLs Electrolytes have lower interface impedance and better cycle stability.
Figure 4 Application of ionic liquid composite sulfur-based solid electrolyte in lithium/sodium secondary batteries
ion gel is a kind of stable solid-liquid mixed system,It has a continuous solid network spanning the entire liquid phase. Compared with ionic liquids, ionic gels have many prospects and advantages: 1) Enhanced transport properties, due to stronger ability to dissociate charged ions or anchor anions; 2) Wider usable temperature The range is attributed to the low glass transition temperature or thermal decomposition temperature; 3) solid-like properties, such as leakproofness and mechanical robustness; 4) other complementary functions, such as adjusting the deposition behavior of the metal anode.
ion gel has high process compatibility and portability, and can reduce the battery's demand for separators, thus simplifying the battery assembly and being very cost-effective. In addition, some ionic gels can be easily recovered by dissolving the solid phase in a solvent, which has good sustainability.
3.2 Application in lithium metal battery
Physics by combining ionic liquid with solid host materials (porous or two-dimensional layered materials such as MOF or hexagonal boron nitride material) After mixing and aging at room temperature or appropriate temperature to make it gel, ionic gel electrolyte can be obtained. This kind of material exhibits high shear modulus, sufficient ion conductivity, high ion mobility and other characteristics. , And can infiltrate the lithium metal/electrolyte interface, solve the problem of lithium dendrites, and realize a lithium metal battery with wide operating temperature and high performance. An example of ion gel is shown in Figure 5.
Figure 5 Schematic diagram of the structure and performance of ionic gel solid-state battery and the mechanism of wetting interface
4. Ion plastic crystal
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Plastic crystals (IPCs) are usually composed of long-range ordered crystals and partially oriented disordered liquid phases.The highly disordered components of will cause ions to rotate, thereby increasing the room temperature ionic conductivity of the material . In addition to the ionic species listed in Figure 1, the ionic species that make up plastic crystals also include spherical aliphatic alkyl ammonium and phosphorus with a ring structure. These ring structures can perform specific movements with temperature changes, making the plastic crystals have A variety of temperature change properties, so as to be able to adapt to different working temperatures.
Similarly, similar to ILs, plastic crystals also have low volatility and non-flammability, which is conducive to battery safety. At the same time, the material also has a certain degree of plasticity, which is beneficial to improve the contact problem between the electrolyte/electrode.
4.2 Ionic conductivity
Generally speaking, the ionic conductivity of plastic crystals is related to temperature, defects, interactions between ions, radius of curvature, chirality and other properties. Doping with metal ions (such as Li+) can increase the ionic conductivity of plastic crystals. This is due to the formation of a solid solution phase between the plastic crystal and the lithium salt, thereby forming defects.
4.3 Application in lithium metal secondary batteries
The application of plastic crystals in lithium secondary batteries has been extensively studied. Early research mainly focused on the ion conduction behavior of Li-doped plastic crystals . As shown in Figure 6, the appropriate combination of cations and anions can achieve highly reversible lithium metal deposition/dissolution, while obtaining high ion conductivity and ion mobility, indicating that this material has a wide range of applications in lithium metal batteries prospect.
Figure 6 Application of plastic crystals in lithium/sodium secondary batteries
4.4 Application in sodium metal secondary batteries
Excellent heat observed in IPCs The conductivity and ion transport characteristics have prompted people to continue to explore its application in sodium metal secondary batteries.Although the literature on these electrolytes is still limited, it can be seen from the current reports that the thermal, electrical conductivity and ion transport properties of the sodium-based IPCs system have similar trends to those of the lithium-based system. The article lists some prominent examples of these systems. It has good compatibility with sodium metal and high ion migration number.
4.5 polymer composites
Although ionic plastic crystals have good properties, their low mechanical strength is still one of the key factors that limit its application in batteries. In order to solve this problem, some studies have combined polymers and plastic crystals to form a composite electrolyte to form an independent film. Among them, the most widely used polymer material is PVDF, which gives the composite material high ionic conductivity and good mechanical properties.
[Conclusion Outlook]
solid electrolyte is considered to be the material of choice for the design of next-generation rechargeable batteries. However, their current applications are severely hindered by severe dendrite formation and high interfacial resistance. In contrast, due to the presence of liquid or flexible solid phases, quasi-solid electrolytes can provide better performance than solid electrolytes. The review concludes The characteristics and development direction of three types of ionic liquid type quasi-solid electrolytes are discussed.
1) Ionic liquid: Compared with organic solvents, it has many advantages, such as low volatility and flammability, so it has better compatibility with solid electrolytes. Composite solid electrolyte is its most promising application direction. In this system, ionic liquid can reduce interfacial resistance and inhibit dendrite growth.
2) Ion gel: It has an ionic conductivity equivalent to that of ionic liquids, and at the same time has higher mechanical properties, and has a good effect in inhibiting dendrite growth. Its future research direction is mainly to develop ionic gel formulations. In order to further enhance the interface wettability, and improve its interface stability, reduce side reactions. At the same time, its host materials are currently relatively expensive, and it is necessary to develop host materials that are more conducive to commercialization.
3) Plastic crystal: It has good ion transport performance and high toughness and plasticity. It can be used as a solid electrolyte alone or combined with a polymer as a composite solid electrolyte, helping to realize high-performance secondary batteries.At present, there are few studies on the mechanical properties of this material, and the corresponding viewpoints are very important for its practicality.
J. Hwang, K. Matsumoto, C. Chen and R. Hagiwara, Pseudo-solid-state Electrolytes Utilizing the Ionic Liquid Family for Rechargeable Batteries. Energy Environ. Sci., 2021, DOI: 10.1039/D1EE02567H
https: //pubs.rsc.org/en/Content/ArticleLanding/2021/EE/D1EE02567H
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