Solid-state lithium (Li) batteries are theoretically superior to liquid-based Li-ion batteries due to their higher energy density and improved safety. Research has primarily focused on developing solid-state electrolytes (SSEs) and enhancing their interfaces with the cathode and anode. However, efficient Li⁺ ion transport within solid-state cathodes, particularly in thick cathodes with high areal loadings, remains a significant challenge. Current strategies for constructing Li⁺ transport networks in solid-state cathodes include adding liquid electrolytes, ionic liquids, or plastic crystals to porous structures. However, these often compromise long-term stability and electrochemical window. The use of inorganic SSEs as catholytes, such as LLZO and LGPS, suffers from aggregation issues and requires high weight percentages, reducing energy density. Polymer electrolytes offer an alternative but frequently lack sufficient ionic conductivity and wide electrochemical window at room temperature. These limitations result in few reports demonstrating satisfactory room temperature SSLB performance suitable for practical applications. This study aims to address these limitations by introducing a novel soluble organic cage-based Li⁺ conductor as a catholyte for room temperature SSLBs. Porous organic cages (POCs) are promising because their discrete molecular covalent structures are solution-processable, enabling facile incorporation into solid-state cathodes using conventional slurry coating methods.

Extensive research has explored porous solids like metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) for ion conduction. Unlike extended, insoluble MOFs and COFs, organic cages offer the advantage of solution processability due to their discrete molecular structures. This solubility enables the use of conventional slurry coating techniques for cathode fabrication, a significant advantage for scalability and industrial relevance. While crystalline porous amine cages have demonstrated proton conductivity comparable to MOFs, Li⁺ ion conductors based on porous organic cages have not been reported prior to this work.

The study begins with the synthesis of the solid-state electrolyte (SSE), Li-RCC1-ClO₄. This SSE is derived from a three-dimensional porous organic cage, RCC1-Cl, where chloride counterions are exchanged with perchlorate ions. The resulting RCC1-ClO₄ is then blended with LiClO₄ salt. The researchers characterized the material using various techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy to confirm the successful ion exchange and the structural properties of the material. The electrochemical performance of Li-RCC1-ClO₄ was evaluated by electrochemical impedance spectroscopy (EIS) to determine ionic conductivity as a function of LiClO₄ content and temperature. Linear sweep voltammetry (LSV) was used to assess the electrochemical window, while DC polarization was employed to determine the Li⁺ ion transference number. The Li-RCC1-ClO₄ was incorporated into solid-state cathodes using the conventional slurry coating method with LiFePO₄ as the active material, alongside acetylene black (AB) and carbon nanotubes (CNTs) as electronic conductors, polyvinylidene fluoride (PVDF) as a binder. Methanol/N-methylpyrrolidone (NMP) solvent was used. The resulting cathodes were analyzed using SEM, EDX, and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to visualize the distribution of Li-RCC1-ClO₄ within the cathode structure. FTIR and XRD were also used to study the stability and structural changes of the cage. Finally, all-solid-state batteries (SSLBs) were assembled using the modified LiFePO₄ cathodes, a lithium foil anode, and a P(IL-PEGDA) solid polymer electrolyte. Electrochemical performance, including cycling performance and rate capability, was evaluated using a battery tester. Similar procedures were followed for SSLBs with LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) and LiCoO₂ (LCO) cathodes and with LLZO solid electrolytes. Detailed synthesis procedures for RCC1, RCC1-Cl, RCC1-ClO₄ and Li-RCC1-ClO₄ are also provided in the methods section.

The synthesized Li-RCC1-ClO₄ SSE exhibits a room temperature ionic conductivity of 5.13 × 10⁻⁵ S cm⁻¹, a wide electrochemical window up to 5.0 V, and a high Li⁺ transference number of ~0.7. Its solubility in polar solvents allows for easy integration into solid-state cathodes via slurry coating. SEM and EDX mapping confirmed the uniform distribution of Li-RCC1-ClO₄ throughout the LiFePO₄ cathode, forming a continuous ion-conducting network. TOF-SIMS further confirmed the complementary distribution of LiFePO₄ and Li-RCC1-ClO₄ components. The optimized Li-RCC1-ClO₄ content in the LiFePO₄ cathode is 20 wt%, significantly lower than the amounts typically required for inorganic SSEs. SSLBs using this optimized cathode exhibited a high initial discharge capacity (~147 mAh g⁻¹ at 0.1 C) and excellent long-term cycling stability at room temperature. Even at a higher discharge rate of 1.0 C, the SSLBs displayed excellent cycle life with 88.2% capacity retention after 750 cycles. The study also demonstrated the effectiveness of Li-RCC1-ClO₄ in SSLBs with NCM523 and LCO cathodes, showcasing a stable performance. The researchers also demonstrated the efficacy of the Li-RCC1-ClO₄ catholyte with a Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte. The researchers note that no electrolyte diffused into the cathode during cycling.

The results demonstrate that the soluble organic cage-based ionic conductor, Li-RCC1-ClO₄, successfully addresses the key challenge of efficient Li⁺ ion transport in solid-state cathodes. The solution-processibility of the organic cage allows for compatibility with current industrial cathode manufacturing methods, making this approach highly practical for scaling up. The significantly reduced amount of SSE needed (20 wt% vs. 30-60 wt% for inorganic SSEs) leads to improved energy density compared to previous approaches. The excellent room-temperature performance and long-term cycling stability observed in SSLBs with LiFePO₄, NCM523, and LCO cathodes highlight the versatility and potential of this new catholyte. The comparable performance with LLZO further supports the broader applicability of this method.

This work introduces a novel soluble organic cage-based ionic conductor, Li-RCC1-ClO₄, as a highly effective catholyte for room-temperature all-solid-state lithium batteries. The solution-processibility of Li-RCC1-ClO₄ enables the use of conventional slurry coating methods for cathode fabrication, resulting in superior electrochemical performance, especially in terms of cycle life and room-temperature operation. Future research will focus on enhancing the air/moisture stability, mechanical properties, and ionic conductivity of the cage-based SSEs.

While the study demonstrates excellent performance with LiFePO₄, NCM523, and LCO cathodes, further investigation is needed to assess the performance with other cathode materials. The long-term stability at higher temperatures and under various operating conditions requires further examination. The current study focuses on coin cell configurations; scaling up to larger formats and exploring the impact on cell performance needs further exploration.