Exploring Advanced Polymeric Binders and Solid Electrolytes for Energy Storage Devices

Abstract

Intermittent electricity generation from renewable energy sources, such as wind energy, ocean energy, and solar energy, has significantly intensified the demand for high-energy-density, high-power, and low-cost energy storage devices. In this regard, tremendous efforts have been devoted to the development of electrode materials, electrolytes, and separators of energy-storage devices to address the fundamental needs of emerging technologies such as electric vehicles, artificial intelligence, and virtual reality. Polymer materials are ubiquitous in fabricating these energy storage devices and are widely used as binders, electrolytes, separators, and other components. However, binders, as an important component in energy-storage devices, are yet to receive sufficient attention. Polyvinylidene fluoride (PVDF) has been the dominant binder in the battery industry for decades despite several well-recognized drawbacks, i.e., limited binding strength due to the lack of chemical bonds with electroactive materials, insufficient mechanical properties, and low electronic and lithium-ion conductivities. The limited binding function cannot meet the inherent demands of emerging electrode materials with high capacities such as silicon anodes and sulfur cathodes. Polymers are also used as electrolyte matrices because they offer the advantages of low cost, lightweight, easy processability, excellent mechanical deformation, and better interfacial contact and compatibility with electrodes. However, the practical implementation of solid polymer electrolytes has been hindered by several challenging issues including low ionic conductivity, low ion transfer number, high-voltage instability, and lithium dendrite growth. Because of the increasingly growing demand for higher performance of energy storage devices, it is necessary to develop novel polymeric binders and solid electrolytes with advanced functionalities to help improve the operation of the currently existing energy storage systems. In the first study, we synthesized a novel self-healing poly(ether-thioureas) (SHPET) polymer with balanced rigidity and softness for the silicon anode. The as-prepared silicon anode with the self-healing binder exhibits excellent structural stability and superior electrochemical performance, delivering a high discharge capacity of 3744 mAh g−1 at a current density of 420 mA g−1, and achieving a stable cycle life with a high capacity retention of 85.6% after 250 cycles at a high current rate of 4200 mA g−1. The success of this work suggests that the proposed SHPET binder facilitates fast self-healing, buffers the drastic volume changes and overcomes the mechanical strain in the course of the charge/discharge process, and could subsequently accelerate the commercialization of the silicon anode. Binders could play crucial or even decisive roles in the fabrication of low-cost, stable, and high-capacity electrodes. This is especially the case for the silicon (Si) anodes and sulfur (S) cathodes that undergo large volume change and active material loss in lithium-ion batteries during prolonged cycles. In the second study, a hydrophilic polymer poly(methyl vinyl ether-alt-maleic acid) (PMVEMA) was explored as a dual-functional aqueous binder for the preparation of high-performance silicon anodes and sulfur cathodes. Benefiting from the dual functions of PMVEMA, i.e., the excellent dispersion ability and strong binding forces, the as-prepared electrodes exhibit improved capacity, rate capability, and long-term cycling performance. In particular, the as-prepared Si electrode delivers a high initial discharge capacity of 1346.5 mAh g-1 at a high rate of 8.4 A g-1 and maintains 834.5 mAh g-1 after 300 cycles at 4.2 A g-1, while the as-prepared S cathode exhibits enhanced cycling performance with high remaining discharge capacities of 711.44 mAh g-1 after 60 cycles at 0.2 C and 487.07 mAh g-1 after 300 cycles at 1 C, respectively. These encouraging results suggest that PMVEMA could be a universal binder to facilitate the green manufacture of both anodes and cathodes for high-capacity energy storage systems. Stable and seamless interfaces among solid components in all‐solid‐state batteries (ASSBs) are crucial for high ionic conductivity and high rate performance. This can be achieved by the combination of functional inorganic material and flexible polymer solid electrolytes. In the third study, a flexible all‐solid‐state composite electrolyte is synthesized based on oxygen‐vacancy‐rich Ca‐doped CeO2 (Ca-CeO2) nanotube, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and poly(ethylene oxide) (PEO), namely Ca-CeO2/LiTFSI/PEO. Ca-CeO2 nanotubes play a key role in enhancing ionic conductivity and mechanical strength while the PEO offers flexibility and assures the stable seamless contact between the solid electrolyte and the electrodes in ASSBs. The as‐prepared electrolyte exhibits high ionic conductivity of 1.3 × 10−4 S cm−1 at 60 °C, a high lithium ion transference number of 0.453, and high‐voltage stability. More importantly, various electrochemical characterizations and density functional theory (DFT) calculations reveal that Ca-CeO2 helps dissociate LiTFSI, produces free Li-ions, and therefore enhances ionic conductivity. The ASSBs based on the as‐prepared Ca-CeO2/LiTFSI/PEO composite electrolyte deliver high‐rate capability and high‐voltage stability. Offering high energy density and high safety, all-solid-state lithium-sulfur batteries (ASSLSBs) have emerged as one of the most promising next-generation energy storage systems. However, there are a series of barriers to their practical applications, including insufficient sulfur utilization, low ionic conductivity and unstable interfaces. In the fourth study, we adopt acetamide to construct a deep eutectic system to suppress electrode passivation, and therefore address the issues of sulfur utilization, and improve the ionic conductivity of the solid polymer electrolytes. Furthermore, we establish a lithium bis(trifluoromethanesulfonyl)imide - lithium oxalyldifluoroborate (LiTFSI-LiDFOB) dual-salt system to facilitate the establishment of a stable and uniform passivation layer, a favorable interface on lithium anode, to prevent lithium dendrite formation and the polysulfide shuttling. Consequently, the as-prepared ASSLSBs deliver a high initial discharge specific capacity of 1012 mAh g-1 at 0.05 C and a stable capacity of 234.84 mAh g-1 after 1000 cycles at 0.1 C. This work suggests that the simultaneous adoption of the deep eutectic system and dual-salt electrolyte could accelerate the practical applications of ASSLSBs. In summary, the high performance of the as-prepared silicon anodes demonstrates potential for addressing the challenges for next-generation anodes by designing self-healing polymers and aqueous hydrophilic polymers. Moreover, the success of the aqueous hydrophilic polymer in lithium-sulfur batteries suggests that such a binder system can be extended to other high-capacity energy storage materials that suffer from severe volume changes. As for the polymer electrolytes, the design of functional inorganic/polymeric composite electrolyte presents a promising strategy to resolve the stubborn barriers (i.e., insufficient contact at the interfaces and ionic conductivity) of ASSBs. Additionally, combining the merits of the deep eutectic system and the dual-salt system, long-term cycling stability and high capacity retention of ASSLSBs can be achieved. These polymeric binders and electrolytes can be further optimized to realize high performance for various energy storage systems.