Solid state battery electrolyte technology route-Lithium - Ion Battery Equipment

Solid state battery electrolyte technology route -Lithium - Ion Battery Equipment

1、 Why study solid state battery?

At present, lithium ion batteries based on liquid electrolyte are increasingly difficult to meet the long-term requirements of consumers in some application scenarios, such as electric vehicles and intelligent electronic products. For this reason, researchers combine Li metal as negative electrode with sulfur, air (oxygen) or positive electrode with high content of layered nickel oxide to prepare batteries with higher energy density. At the same time, due to the potential safety hazards of organic solvent electrolytes, people have accelerated the research on solid electrolytes, ionic liquids, polymers and their combinations. In addition, the current lithium ion technology requires complex cooling and control modules to ensure that the operating temperature is kept below 60 ℃. If the temperature is always high, the battery performance and life will be seriously damaged. Therefore, the core of solid electrolyte research is still for safety, whether for existing lithium ion batteries or future lithium metal batteries.(Lithium - Ion Battery Equipment)

2、 Type of solid electrolyte

In addition to inorganic ceramic or glass electrolytes, there are four kinds of electrolyte materials that have recently shown their superiority over Li ion energy storage technologies: solid polymer electrolytes (SPE), ionic liquids, gel and nanocomposites, and organic ionic plastic crystals. The batteries based on SPE and Li metal still have good performance at high temperature (50-100 ℃). Among them, ionic liquids as electrolytes in high energy density batteries can provide many ideal and customizable features. For example, they can achieve large electrochemical performance of metal negative electrode and high-voltage positive electrode, minimum volatility and zero flammability, and higher temperature stability. In this paper, the progress and prospect of the next generation solid electrolytes based on polymers and ionic liquids will be discussed.

3、 Development Status and Trend of Solid Electrolyte Technology

Recently, Professor Maria Forsyth (corresponding author) of Deakin University in Australia and others published a review article entitled "Innovative Electronics Based on Ionic Liquids and Polymers for Next Generation Solid State Batteries" on Account of Chemical Research. In this article, the author discusses some of his team's work in these areas. Metals such as sodium, magnesium, zinc and aluminum are also considered as substitutes for lithium metal energy storage technology. However, the research based on these materials for energy storage applications is still in a relatively preliminary stage. Secondly, electrolytes play an important role in realizing these devices, which is similar to Li technology to a large extent. The authors also discussed their recent progress in these fields and their views on the future development direction of this field. This paper also demonstrated that the block copolymer can provide both mechanical properties and high ionic conductivity when used in combination with ionic liquid electrolyte. The final electrolyte material will enable all high-performance solid-state batteries to have the mechanical properties of ion transport decoupling.

3.1. Anionic single ion conductor

Single ion conductor: refers to the polymer electrolyte where anions attach to the polymer skeleton and restrict its free movement, resulting in cations becoming the only ions that can move freely, and the cation transport number is almost equal to 1. Recently, research by Armand team shows that the ionic conductivity of single ion conductor is generally lower than that of double ion conductor. Then, based on their research on the single ion conductor system, the author proposes two main strategies to improve the ionic conductivity of the single ion conductor: I. Design the system with lower glass transition temperature and higher polymer segment fluidity; 2. The ionic conductivity is decoupled from the dynamics of the polymer skeleton.

1) Copolymerization method increases the kinetics of segmental migration

At present, a large number of anionic polymers can be used as single ion conductors of lithium and sodium. In the author's research, he mainly focused on the single ion conductors of vinyl, acrylamide and methacrylate groups. As free radical polymerization is not only simple and economical, but also highly resistant to ionic functional groups, free radical polymerization technology was selected. The general chemical structure of these polymers is shown in Figure 1a. For example, the single ion conductor of poly (lithium 1 - [3 - (methacryloyloxy) - propylsulfonyl] - 1 - (trifluoromethylsulfonyl) imide) (PMTFSI) developed by the author shows a high glass transition temperature (Tg>90 ℃), so it has a low room temperature conductivity. At the same time, the addition of epoxyethane increases the flexibility of the polymer skeleton.

In order to improve the mechanical properties of the polymer, a single ion triblock copolymer electrolyte was prepared by using linear PEO and branched poly (methyl methacrylate) (PMTFSI) blocks and PMTFSI-b-PEO-b-PMTFSI. The crystallinity of PEO was controlled by changing the composition of the block copolymer. Recently, in the presence of propylene carbonate, the author's team prepared cross-linked electrolytes through copolymerization of polyethylene glycol dimethacrylate (PEGDM), PEGM and LiMTFSI. In addition to plasticizing the polymer, the high dielectric constant of propylene carbonate is conducive to the dissociation of Li ions and covalent bond anions to increase the proportion of mobile ions. The material has high lithium ion migration number and high conductivity (about 10-4S/cm) at ambient temperature.

2) Decoupling Li/Na Ion Migration from Segmental Migration by Mixed Co cation Method

By designing a polymer electrolyte, the ions can also be moved below the glass transition temperature, so that the segmental motion of the polymer can be decoupled from the ionic conductivity. Firstly, the author selected poly (2-acrylamide-2-methyl-1-propanesulfonic acid) homopolymer (PAMPS) as the polymer skeleton of Na single ion conductor. Then, large volume quaternary ammonium salt cation is used to replace the sodium ion in the polymer skeleton, and its chemical structure is shown in Figure 2. The ionic conductivity was measured below Tg of the polymer: when the molar ratio of Na+/triethylmethylammonium (N1222) cation in the system was 10:90%, the conductivity was the best. Subsequently, the author tested different polymer skeletons, such as the copolymer of PAMPS and polyvinyl alcohol sulfonate (PVS), and the copolymer of polystyrene sulfonimide and ethyl acrylate. Similar decoupling phenomena were observed in these polymers. However, the ionic conductivity measured in these systems is still too low to be applied to practical equipment.

The author also used the above method to prepare PAMPS polymer containing Li ion, and the polymer was mixed with N1222 or dimethylbutylmethoxyethylamine (N114 (2O1)) cation. The temperature dependence of the conductivity of the two samples is independent of Tg. In addition, the linewidth analysis of LiNMR shows that the main mechanism of lithium diffusion in the polymer system is ion hopping. Compared with Na based ionomers, the addition of a small amount of tetracyandiamide does not increase the ionic conductivity. The MD simulation of lithium ion and sodium ion polymers with different co cation compositions illustrates the possible decoupling mechanism of organic cations and the optimal composition and size. When the ratio of co cation of lithium ion and ammonia ion increases to 1:1, the formation of a hopping mechanism containing lithium coordination environment rearrangement in an interconnected cluster is considered to be the origin of the decoupling of alkali metal cations and polymer bulk dynamics. The comparison of mixed cationic polymer systems based on Li and Na shows that different coordination environments are complex and important for promoting alkali metal cation transport, which provides an opportunity to design materials to optimize the decoupling between ion dynamics and polymer dynamics.

Based on organic and IL (ionic liquids) based systems, another strategy to decouple lithium or sodium ion migration from volumetric dynamics has recently been reported. At a concentration close to the saturation limit of salt in the solvent, the ionic form can permeate the complexes and aggregates of electrolyte. Then ion diffusion is supported not only by carrier movement but also by a structural diffusion mechanism similar to proton transport through the Grotthus mechanism in acids. Among them, MD simulation has clarified the obvious difference between the coordinated environment and lithium/sodium transport under the more traditional low salt concentration compared with the ultra concentrated concentration. These super concentrated electrolytes can be used for dendrite free lithium and sodium metal electroplating at very high current density. In addition, these systems have been successfully used for high energy density positive electrodes. Recently, it is also used for sodium metal device batteries operating at 50 ℃. They have excellent stability, so they can be safely used at moderate temperatures, avoiding flammability problems and electrode degradation.

3.2. Ionic gel and composite electrolyte

Although the super concentrated ionic liquid electrolytes obviously support the stability of the alkali metal cycle, they still have some challenges in the full battery, because they need the compatible membrane materials as described above. The current diaphragm technology design is a carbonate based system, which is not necessarily suitable for these new electrolytes, nor for high-temperature operation. However, ionic gel polymers formed by combining ionic liquids with polymer matrix to solidify ionic liquids can not only provide mechanical integrity, but also participate in the conduction process. The ionic conductive polymer materials of polyDADMA have different anti anions and are combined with IL based electrolytes. Because of its commercial availability, low cost, high dielectric constant, high thermal stability and electrochemical stability. The chemical properties of these materials for Li based electrolytes and some recent examples of alternative chemicals such as Zn and Na are summarized in Figure 5.

As mentioned above, the high concentration IL system shows good performance, such as higher lithium ion migration number and the ability to improve battery charging and discharging performance. Recently, the author's team proposed a new type of composite ion gel electrolyte, which combines high molecular weight PIL and polyDADMATFSI as host polymers with super concentrated IL electrolyte composed of LiFSI. The most important is the measurement of diffusion coefficient, which shows that compared with lithium ion, the addition of PIL in ionic liquid can significantly reduce the diffusion of anions and effectively increase the transport times of lithium ion. The results highlight the advantages of PILs as a potential host or solvent of salts in polymer electrolyte materials. In PILs materials, PILs can dissociate lithium salts, and their coordination ability with lithium ions is weak, which is conducive to the transportation of lithium ions. Although the ionic conductivity of electrolyte was improved by adding ionic drag reducer, the mechanical stability of salt plasticized system was reduced.

1) A New Method for Achieving Mechanical Strength and High Ion Migration in the Same Polymer Electrolyte Block Copolymer

The stability of PIL materials is improved while maintaining high conductivity by preparing block copolymers, wherein polystyrene blocks provide mechanical strength while PIL allows ion transport. Inspired by this, the author developed a new phase separated ionic gel electrolyte with high Li transfer number by combining PIL block copolymer with high LiFSI salt concentration and low ionic liquid content. Enhanced lithium transport performance is achieved by using a method similar to the ultra concentrated ionic liquid electrolyte, in which high LiFSI salt content is used to keep the molar ratio of total anions to Li below 1.5. The ionic gel electrolyte works well in LiFePO4/lithium metal battery at 50 ℃, and the positive load is close to the actual level. Recently, the nanostructure block copolymer filled with ethyl carbonate composed of polystyrene block and perfluorosulfonamide anion block has been proved to be a safer electrolyte, which can perform well in complete lithium batteries even when using NCM positive electrode with high energy density. However, although the method of using PIL as the main body of ionic liquids and inorganic salts for various battery chemistry is very promising from the perspective of application and new scientific understanding, the polymer skeleton with conductive process and electrification is still unclear.

4、 Summary and outlook

To sum up, it is obvious that there are several methods suitable for developing ionic polymer conductors for advanced energy storage technology, including Li metal and alternative chemicals such as Zn or Na batteries. The polymerizable IL has replaceable anions (or cations) to improve solubility or compatibility with salts and IL. In view of the fact that binding more delocalized anions can promote the cation dissociation of alkali metal ions, we can design anion PIL that can promote the single ion conduction. If combined with the co cation method of the author's team, the larger organic cations can be combined into the main chain. Among them, the latter method has well proved the decoupling of alkali metal cation diffusion and ionic conductivity from polymer dynamics. Higher conductivity is achieved through the chemical design of new polymer frameworks and anions. Although many different methods have been studied to realize the single ion conductive polymer for lithium, this field is still in the early stage of Na+(or other metal cations). Considering the difference of the size and coordination of the latter ions, there are many places worth exploring. Preliminary studies on mixed anions in ILs and early studies on zwitterionic additives showed that by using the mixed coordination environment around metal ions, a conduction mechanism through structural rearrangement can be designed to promote ion jump transfer.



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