Porous Crystalline Frameworks as Ion-Conducting Solid-State Electrolytes

Room-temperature Li⁺ conductors have been intensively revisited to develop high-safety solid-state batteries. While promising inorganic Li⁺ solid-state electrolytes (SSEs) with competitive ionic conductivity have been demonstrated, their practical applications are still hindered by manufacturing technology, cost constraints, and internal battery interfaces. Advances in the design and synthesis of periodic frameworks over the past decade have created a new platform for designing SSEs. These porous crystalline frameworks feature open channels that can be tailored into ion-hopping sites and guest-accessible voids, both essential for SSE construction. Framework-based SSEs uniquely merge the advantages of inorganic crystal-like ordered structures and the design flexibility of organic molecules, distinguishing them significantly from traditional inorganic or organic SSEs. Enhancing ionic conduction and exploring potential applications are two critical factors driving the rapid advancement of framework-based SSEs.

In this account, we summarize recent progress in framework-based SSEs and discuss factors influencing ion pair dissociation and free ion diffusion within these frameworks. An appropriately charged framework and guest assistance are key factors in enhancing ionic conductivity. We also highlight the importance of maximizing void utilization in porous frameworks to optimize framework ion conductivity. In the latter part of this account, we delve into the practical potential of framework-based SSEs, considering that practicality is crucial for ensuring rapid development of such SSEs. We start by discussing the processability of these framework materials, including their fabrication into SSE membranes or integration into battery configurations for practical application. Enhancing interfacial contact of framework-based SSEs is crucial for eliminating interfacial impedance and improving mechanical properties. In the subsequent discussion, we propose frameworks not as replacements for current SSEs but as novel SSEs with unique functionalities that complement traditional SSEs for various applications. These functionalities include enhancing interface contact, suppressing side reactions, and promoting uniform lithium deposition, among others. Understanding their conductive mechanisms, developing practical fabrication methods, and exploring new functionalities are key to advancing framework-based SSEs.

We propose the following: (1) Enhancing the conductivity of future framework-based SSEs should focus on synergistic “ionic framework + guest assistance” conduction, aiming for optimized porous frameworks that provide adequate free ions, excellent ion mobility, and high void utilization. (2) One potential application of framework-based SSEs is utilization as functional additives, offering specific functionalities that traditional SSEs lack or are less proficient at. (3) Developing orderly assembled frameworks compatible with large-scale manufacturing is technically valuable for developing practical applications. Interfacial engineering of these frameworks within the battery is essential to activate their functionality.