The development of high-energy-density and safe lithium-metal batteries (LMBs) is crucial for various applications, but challenges remain. Traditional liquid electrolytes suffer from flammability, narrow electrochemical windows, and the formation of dendrites on the lithium anode, leading to safety hazards and poor cycle life. Solid-state electrolytes offer improved safety but often have low ionic conductivity and poor interfacial contact with electrodes. Quasi-solid electrolytes (QSEs), an intermediate state between liquid and solid electrolytes, aim to combine the advantages of both while mitigating their drawbacks. This research focuses on creating a safe and highly efficient QSE for LMBs that can operate reliably even in harsh environments. The key idea is to confine a small amount of liquid electrolyte within the sub-nanoscale channels of a porous, flexible metal-organic framework (MOF), thereby altering the properties of the confined electrolyte and enhancing battery performance and safety. The stability and performance of high-voltage lithium-ion batteries are significantly influenced by the properties of the electrolyte, making the development of advanced electrolytes a critical area of research in energy storage. High-energy-density batteries are essential for the widespread adoption of electric vehicles and portable electronic devices, driving the search for solutions that improve energy density and safety. The ability of the proposed QSE to operate under extreme conditions – such as high temperatures and physical damage – significantly broadens the potential application of LMBs. The work investigates if sub-nanoscale confinement can modify liquid electrolyte behavior to create a QSE with enhanced properties, thus addressing the critical challenges associated with current battery technology. This would enable the creation of safer and more efficient LMBs suitable for a range of demanding applications. The use of MOFs, with their tunable pore sizes and functionalities, presents an exciting avenue for electrolyte design and modification, offering a potential pathway towards next-generation energy storage solutions. The study examines how confining the liquid electrolyte within the MOF structure impacts its properties such as boiling point, ionic conductivity, electrochemical window, and flammability. Furthermore, the researchers explore the performance of LMBs utilizing this innovative QSE under both standard conditions and under harsh conditions to fully evaluate its potential.

Existing literature highlights the limitations of both liquid and solid-state electrolytes for LMBs. Liquid electrolytes, while possessing high ionic conductivity, suffer from flammability and safety concerns, particularly at elevated temperatures. The formation of dendrites on the lithium anode during cycling is another significant challenge, leading to short circuits and battery failure. Solid-state electrolytes, on the other hand, offer enhanced safety but typically exhibit lower ionic conductivity and poor interfacial contact with the electrodes, limiting their performance. Several strategies have been explored to overcome these limitations, including the use of electrolyte additives, novel solvent systems, and the development of solid-state electrolytes with improved ionic conductivity. However, these approaches often involve trade-offs between safety, performance, and cost. The concept of quasi-solid electrolytes has emerged as a potential solution, aiming to combine the advantages of both liquid and solid electrolytes. Previous studies have investigated different approaches to create QSEs, including the use of polymer gels and nanoconfined electrolytes. However, the challenges of achieving both high ionic conductivity and good mechanical stability remain. This study builds upon previous work in nanoconfined electrolytes and MOF-based materials, aiming to create a novel QSE with enhanced properties to overcome the limitations of existing electrolyte technologies.

The researchers synthesized a quasi-solid electrolyte by confining a small amount of liquid electrolyte (1 M LITFSI in propylene carbonate) within the sub-nanoscale channels of a modified metal-organic framework (MOF). The MOF used was CuBTC (copper benzene-1,3,5-tricarboxylate) functionalized with poly(sodium 4-styrene-sulfonate) (PSS), providing a flexible and porous structure with 6.5 Å channels. The MOF was prepared by a solution-based method, involving the synthesis of copper hydroxide nanorods, followed by the addition of PSS and subsequent reaction with BTC. The resulting MOF was then processed to form a film. The amount of liquid electrolyte incorporated into the MOF was carefully controlled to minimize the total electrolyte weight and achieve a quasi-solid state. The physical and electrochemical properties of the QSE were then characterized using a variety of techniques. Powder X-ray diffraction (XRD) was employed to confirm the presence of the liquid electrolyte within the MOF channels and to assess changes in pore size. Thermogravimetric analysis (TGA) was used to determine the thermal stability of the QSE, comparing its decomposition temperature to that of a typical liquid electrolyte. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) and Raman spectroscopy were utilized to investigate the configuration and interactions of the electrolyte molecules within the MOF channels. Linear sweep voltammetry (LSV) was used to measure the electrochemical stability window of the QSE. The lithium-ion conductivity was measured using electrochemical impedance spectroscopy (EIS) at both room temperature and 90 °C. The mechanical and flammability properties of the QSE were also assessed. The compatibility of the QSE with both the cathode (NCM-811) and anode (lithium metal) was evaluated using half-cells and Li//Li symmetric cells. Scanning electron microscopy (SEM) was used to analyze the morphology of the cycled electrodes, while depth-profiling techniques such as etching FTIR and X-ray photoelectron spectroscopy (XPS) were used to characterize the solid electrolyte interphase (SEI) layer on the cathode surface. Finally, the electrochemical performance of high-voltage NCM-811//Li pouch cells assembled with the QSE was tested under various conditions, including room temperature, 90 °C, and after intentional damage (bending and cutting) to simulate real-world scenarios. The pouch cells had a high NCM-811 mass loading of 20 mg cm-2. The comparison between the performance of the pouch cells using this QSE and those using a traditional liquid electrolyte highlights the advantages of the proposed QSE. The detailed characterization of the QSE and its performance in pouch cells provide a comprehensive understanding of its properties and potential for practical applications.

The sub-nanoscale confinement of the liquid electrolyte within the MOF channels resulted in several key improvements. The boiling point of the electrolyte was significantly increased (nearly 100°C higher than the bulk liquid electrolyte), indicating enhanced thermal stability. Spectroscopic analysis (ATR-FTIR and Raman) revealed a more aggregated electrolyte configuration compared to both dilute and concentrated liquid electrolytes, suggesting stronger Li-PC solvent and Li-TFSI interactions. This aggregation contributed to a wider electrochemical stability window of approximately 5.4 V vs. Li/Li+, significantly exceeding that of conventional liquid electrolytes. The QSE exhibited good ionic conductivity, slightly lower than the liquid electrolyte but considerably higher than a commercial LAGP solid-state electrolyte at both room temperature and 90 °C. Importantly, the QSE was found to be nonflammable, offering a significant safety advantage over traditional liquid electrolytes. When used in LMBs, the QSE led to a remarkably improved cathode/electrolyte interface, with nearly no SEI layer formation on the NCM-811 cathode after 300 cycles, in contrast to the significant SEI layer observed in cells using liquid electrolytes. This was attributed to the reduced contact between the electrolyte solvent and the cathode due to the aggregated electrolyte configuration. Similarly, the QSE effectively suppressed lithium dendrite formation on the lithium anode, resulting in smoother morphology and improved cycling stability in Li//Li symmetric cells. The high-voltage NCM-811//Li pouch cells assembled with the QSE demonstrated exceptional stability and performance at both room temperature and 90 °C. At 90 °C, the pouch cell exhibited a high initial capacity of 191.5 mAh g-1 and maintained 171.2 mAh g-1 (89% retention) after 300 cycles. Even after intentional damage (bending and cutting), the pouch cell continued to operate effectively at 90 °C, achieving a capacity of 164 mAh g-1 after 100 cycles. These results represent a substantial improvement over pouch cells with conventional liquid electrolytes, which showed significantly faster capacity decay and failure under similar conditions. The capacity retention and stability of the cells with the QSE highlights the advantages of this new electrolyte in creating highly durable and robust high-energy batteries.

The findings demonstrate the effectiveness of sub-nanoscale confinement within a flexible MOF as a strategy for enhancing the properties of liquid electrolytes and creating a safe, high-performance QSE. The significant improvement in the thermal stability, electrochemical stability window, and flammability of the QSE directly addresses the major limitations of conventional liquid electrolytes. The improved interfacial compatibility with both the cathode and anode materials, resulting in a CEI-free cathode and suppressed dendrite formation, is a critical factor contributing to the enhanced cycle life and performance of the LMBs. The exceptional performance of the high-voltage pouch cells at 90°C and even after physical damage strongly suggests the suitability of this QSE for a wide range of applications, including high-temperature environments and situations where mechanical robustness is paramount. The superior performance of the QSE-based pouch cells compared to those using liquid electrolytes is striking and underscores the potential impact of this technology on the next generation of energy storage solutions. The ability to maintain high capacity and stability even under extreme conditions represents a significant advancement in battery technology, opening doors to the development of more reliable, safe, and efficient batteries. The aggregated electrolyte configuration, caused by the sub-nanoscale confinement and interaction with the MOF channels, is a key aspect of this enhancement, significantly impacting the electrolyte's behavior and interaction with the electrode materials. The work provides clear evidence that controlling the molecular-level interactions within the electrolyte can significantly improve battery performance and safety.

This study successfully demonstrates a novel quasi-solid electrolyte based on a sub-nanoscale confined liquid electrolyte within a flexible metal-organic framework. The resulting electrolyte exhibits superior thermal stability, a wider electrochemical window, nonflammability, and excellent compatibility with both high-voltage cathodes and lithium-metal anodes. High-voltage pouch cells utilizing this electrolyte demonstrate exceptional stability and performance, even under harsh conditions, including high temperatures and physical damage. This approach represents a significant advance in battery technology, offering a promising pathway for developing safe and high-energy-density lithium-metal batteries for various applications. Future research could explore different MOF structures and liquid electrolyte compositions to further optimize the electrolyte properties and expand its applicability to other battery chemistries.

While this study demonstrates significant advancements in QSE technology, some limitations exist. The current synthesis method for the MOF-based QSE may require further optimization to improve scalability and reduce production costs. The long-term stability of the QSE under extreme conditions needs to be further investigated through extended cycling tests. A thorough investigation into the cost-effectiveness of this method compared to traditional electrolyte production is also necessary before widespread adoption. Furthermore, while the study focuses on NCM-811 cathodes, evaluating the performance of this QSE with other cathode and anode materials would enhance its general applicability.