Next-generation CR1625 battery electrolyte technology route based on ionic liquids and polymers

1. Why study solid-state batteries? At present, lithium-ion batteries based on liquid electrolytes are increasingly difficult to meet consumers' long-range requirements in some application scenarios, such as electric vehicles and smart electronic products. To this end, researchers use Li metal as a negative electrode combined with sulfur, air (oxygen) or a high-content layered nickel oxide positive electrode to prepare batteries with higher energy density. At the same time, due to the safety risks of organic solvent electrolytes themselves, people have accelerated the research on solid electrolytes, ionic liquids, polymers and their combinations. In addition, current lithium-ion technology requires complex cooling and control modules to ensure that the operating temperature remains below 60°C. If the temperature is often at high temperatures, the battery performance and life will be seriously damaged. Therefore, the core of studying solid electrolytes is still for safety, both for existing lithium-ion batteries and possible lithium metal batteries in the future. 2. Types of solid electrolytes In addition to inorganic ceramics or glass electrolytes, there are four electrolyte materials that have recently shown to surpass Li-ion energy storage technology: solid polymer electrolytes (SPE), ionic liquids, gels and nanocomposites, and organic ion plastic crystals. Batteries based on SPE and Li metal still have good performance under high temperature (50-100 ° C). Among them, ionic liquids can provide many ideal and customizable features as electrolytes in high energy density batteries. For example, they have large electrochemical performance, minimal volatility, zero flammability and higher temperature stability that can achieve metal anodes and high-voltage cathodes. In this article, the author will discuss the progress and prospects of the next generation of solid electrolytes based on polymers and ionic liquids. 3. Current status and trends in the development of solid electrolyte technology Recently, the team of Professor Maria Forsyth (corresponding author) of Deakin University, Australia, and others published a review article entitled "Innovative Electrolytes Based on Ionic Liquids and Polymers for Next-Generation Solid-State Batteries" in Account of Chemical Research. In this article, the author discusses some of the work of his team in these areas. Metals such as sodium, magnesium, zinc and aluminum are also considered to be alternatives to lithium metal energy storage technologies. However, research on materials required for these alkali metal-based energy storage applications is still in a relatively early stage. Secondly, electrolytes play an important role in enabling these devices, largely similar to Li technology. The authors also discuss some of their recent progress in these areas and their views on future development directions in this field. This paper also demonstrates that block copolymers can provide both mechanical properties and high ionic conductivity when used in combination with ionic liquid electrolytes. The ultimate electrolyte material that will enable all high-performance solid-state batteries will have mechanical properties with decoupled ion transport. 3.1. Anionic single-ion conductors Single-ion conductors: that is, polymer electrolytes in which anions are attached to the polymer backbone to restrict their free movement, resulting in cations becoming the only ions that can move freely, and the cation transport number is almost equal to 1. Recently, the research of Armand's team showed that the ionic conductivity of single-ion conductors is generally lower than that of dual-ion conductors. Next, based on their research on single-ion conductor systems, the authors proposed two main strategies to improve the ionic conductivity of single-ion conductors: I. Designing systems with lower glass transition temperatures and higher polymer segment mobility; II. Decoupling ionic conductivity from the kinetics of the polymer backbone. 1) Copolymerization method to increase the kinetics of segmental migration Currently, there are a large number of anionic polymers that can be used as single-ion conductors for lithium and sodium. In the authors' research, the focus is mainly on single-ion conductors based on vinyl, acrylamide and methacrylate. Free radical polymerization technology was chosen because it is not only simple and economical, but also has high tolerance to ionic functional groups. 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 authors exhibits a high glass transition temperature (Tg>90℃), so it has a low room temperature conductivity, and the addition of ethylene oxide increases the flexibility of the polymer backbone. In order to improve the mechanical properties of the polymer, single-ion triblock copolymer electrolytes were prepared using linear PEO and branched polymethyl methacrylate (PMTFSI) blocks, PMTFSI-b-PEO-b-PMTFSI, and the composition of the block copolymer was changed to control the crystallinity of PEO. Recently, in the presence of propylene carbonate, the author team prepared a cross-linked electrolyte by copolymerization of polyethylene glycol dimethacrylate (PEGDM), PEGM and LiMTFSI. Among them, the high dielectric constant of propylene carbonate, in addition to plasticizing the polymer, helps the dissociation of Li ions and covalent anions to increase the proportion of mobile ions. The material has a high lithium ion migration number and high conductivity (about 10-4S/cm) at ambient temperature. 2) Decoupling Li/Na ion migration from segmented migration using a mixed co-cation method By designing a polymer electrolyte so that ions can move below the glass transition temperature, the segmental motion of the polymer is decoupled from ionic conductivity. First, the authors selected poly (2-acrylamide-2-methyl-1-propanesulfonic acid) homopolymer (PAMPS) as the polymer backbone of the Na single ion conductor. Then, bulky quaternary ammonium cations were used to replace sodium ions in the polymer backbone. Its chemical structure is shown in Figure 2. The ionic conductivity was measured below the Tg of the polymer: the best conductivity was achieved when the molar ratio of Na+/triethylmethylammonium (N1222) cations in the system was 10:90%. Subsequently, the authors tested different polymer backbones, such as copolymers of PAMPS with polyvinyl alcohol sulfonate (PVS) and copolymers of polystyrene sulfonimide with ethyl acrylate, and similar decoupling phenomena were observed in all of these polymers. However, the ionic conductivity measured in these systems was still too low to be applied in practical devices. The authors also used the above method to prepare PAMPS polymers containing Li ions and mixed the polymers with N1222 or dimethylbutylmethoxyethylamine (N114(2O1)) cations. The temperature dependence of the conductivity of the two samples prepared was independent of Tg. In addition, LiNMR line width analysis showed that the main mechanism of lithium diffusion in this polymer system is ion hopping. Compared with Na-based ions, the addition of a small amount of melamine did not increase the ionic conductivity. Possible decoupling mechanisms and optimal compositions and sizes of organic cations have been elucidated by MD simulations of Li- and Na-based polymers with different cocation compositions. When the cocation ratio of Li- to ammonium-based ions increases to 1:1, the formation of a hopping mechanism involving rearrangement of the lithium coordination environment within an interconnected cluster is considered to be the origin of the decoupling of alkali metal cations from the polymer bulk dynamics. Comparison of Li- and Na-based mixed cation polymer systems shows the complexity and importance of different coordination environments in facilitating alkali metal cation transport, providing opportunities for designing materials to optimize the decoupling between ion dynamics and polymer dynamics. Another strategy to decouple Li- or Na-ion transport from bulk dynamics has been recently reported based on organic and IL (ionic liquid)-based systems. At concentrations close to the saturation limit of salt in the solvent, complexes and aggregates resulting in ionic species can permeate through the electrolyte. Ion diffusion is then supported not only by carrier motion but also by a structural diffusion mechanism similar to the transport of protons via the Grotthus mechanism in acids. Among them, MD simulations have elucidated the clear differences between the coordination environment and Li/Na transport at more traditional low salt concentrations compared to superconcentrated concentrations. These ultra-concentrated electrolytes allow for dendrite-free lithium and sodium metal plating at very high current densities. In addition, these systems have been successfully used for high energy density cathodes. Recently, they have also been used for sodium metal device batteries operating at 50 °C. They have excellent stability and can therefore be safely used at moderate temperatures, avoiding flammability issues and electrode degradation. 3.2. Ion gel and composite electrolytes Although ultra-concentrated ionic liquid electrolytes clearly support the stability of alkali metal cycling, they still present some challenges in full cells because they require compatible separator materials as mentioned above. Current separator technology designs are based on carbonate systems, which are not necessarily suitable for these new electrolytes or for high temperature operation. However, the ion gel polymer formed by curing the ionic liquid by combining the ionic liquid with a polymer matrix can not only provide mechanical integrity, but also may participate in the conduction process. The ion-conducting polymer material of polyDADMA has different counter anions and is combined with IL-based electrolytes. Because of its commercial availability, low cost, high dielectric constant and high thermal and electrochemical stability. As shown in Figure 5, the chemical properties of these materials for Li-based electrolytes and some recent examples of alternative chemistries such as Zn and Na are summarized. As mentioned above, high-concentration IL systems have shown promising properties, such as higher lithium ion transference numbers and the ability to improve battery charge and discharge performance. Recently, the author team proposed a new type of composite ion gel electrolyte, combining PIL with high molecular weight and polyDADMATFSI as host polymers with super-concentrated IL electrolyte composed of LiFSI. Most importantly, the diffusion coefficient measurement showed that the addition of PIL to the ionic liquid can more significantly reduce the diffusion of anions compared with lithium ions, effectively increasing the number of lithium ion transports. The results of this study highlight the advantages of PILs as potential hosts or solvents for salts in polymer electrolyte materials. In PILs materials, PILs can dissociate lithium salts and have weak coordination ability with lithium ions, which is conducive to the transport of lithium ions. Although the addition of ionic drag reducers improves the ionic conductivity of the electrolyte, the mechanical stability of the salt plasticized system is reduced. 1) A new method to achieve mechanical strength and high ion transport in the same polymer electrolyte-block copolymer The stability of PIL materials is improved while maintaining high conductivity by preparing block copolymers, in which the polystyrene block provides mechanical strength and the PIL allows ion transport. Inspired by this, we developed novel phase-separated ion gel electrolytes with high Li transfer numbers by combining PIL block copolymers with high LiFSI salt concentration and low ionic liquid content. Enhanced lithium transport performance was achieved by using a similar approach to superconcentrated ionic liquid electrolytes, where the total anion to Li molar ratio was kept below 1.5 using high LiFSI salt content. This ion gel electrolyte performed well in LiFePO4/lithium metal cells at 50 °C with cathode loading close to practical levels. Recently, ethylene carbonate-filled nanostructured block copolymers composed of polystyrene blocks and perfluorosulfonamide anion blocks were demonstrated to be a safer electrolyte that performs well in full lithium cells even when using high energy density NCM cathodes. However, the approach of using PILs as hosts for ionic liquids and inorganic salts for a variety of different battery chemistries, while very promising from the perspective of applications and new scientific understanding, the role of the polymer backbone in conducting processes and carrying charge remains unclear. IV. Summary and Outlook In summary, it is clear that there are several approaches suitable for the development of ionic polymer conductors for advanced energy storage technologies, including Li metal as well as alternative chemistries such as Zn or Na batteries. Polymerizable ILs have alternative anions (or cations) to improve solubility or compatibility with salts and ILs. Given that tethering more delocalized anions can promote cationic dissociation of alkali metal ions, anionic PILs that promote single-ion conduction can be designed. If combined with the co-cation approach of the author team, in which larger organic cations can be incorporated into the backbone. Among them, the latter approach has well demonstrated the decoupling of alkali metal cation diffusion and ionic conductivity from polymer dynamics. Higher conductivity is achieved through chemical design of novel polymer backbones and anions. Although many different approaches have been studied to achieve single-ion conducting polymers for lithium, the field is still in its infancy for Na+ (or other metal cations), and given the differences in the size and coordination of the latter ions, there is much to explore. Initial studies on mixed anions in ILs and early investigations on zwitterionic additives suggested that by invoking a mixed coordination environment around metal ions, conduction mechanisms via structural rearrangements could be designed to facilitate ion hopping transfer.

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