On enhancing the Li-ion conductivity of quasi-solid-state electrolytes by suppressing the flexibility of zeolitic imidazolate framework-8 via a mixed ligand strategy.

Zeolitic imidazole frameworks (ZIFs) have emerged as potential conductive materials for Li ion-transport in polymer solid state electrolytes. However, developing ZIFs with high Li ionic conductivity is rather limited due to their flexible frameworks allowing dual ion conduction. Herein, we have used a mixed ligand strategy for fine-tuning the aperture and enhancing the rigidity of ZIF-8, which restricts the passage of large size anions. Poly(ethylene oxide)-based quasi-solid state electrolytes utilizing mixed ligand ZIF-7-8 frameworks as passive fillers show a continuous enhancement in Li ion-conductivity exclusively attributed to modifications in the flexibility and pore architecture of ZIF-8 as confirmed through broadband dielectric spectroscopy and positron annihilation spectroscopy. This study shows that polymer segmental relaxation and conductivity relaxation processes are decoupled in these electrolytes. Consequently, our proposed approach provides a new strategy for manufacturing a polymer-based electrolyte with enhanced ionic conductivity.

High ionic conductivity and ion conduction mechanism in ZIF-8 based quasi-solid-state electrolytes: a positron annihilation and broadband dielectric spectroscopy study.

The low ionic conductivity and electrode-electrolyte interface instability issues with solid polymer electrolytes jeopardize their electrochemical performances in lithium-ion batteries (LIBs). The use of quasi-solid-state electrolytes (QSSEs) with concentrated Li salt embedded inside the pore networks of metal organic frameworks (MOFs) can successfully address the aforementioned issues. Owing to the sieve effect of zeolitic imidazolate framework-8 (ZIF-8) towards selective cation permeability over anions through the interconnected pore network, a unique QSSE with LiTFSI salt concentrated in the ZIF-8 skeleton used as a filler in poly(ethylene oxide) has been synthesized. LiTFSI gets embedded inside the interconnected pore network of ZIF-8 that furnishes unhindered pathways for Li+ ion migration leading to a very high ionic conductivity of ∼6 × 10-4 S cm-1. The higher ionic conductivity is directly related to the Li+ ion conduction through the pore network of ZIF-8 which has been experimentally evident from complementary methods viz. Positron annihilation and broadband dielectric spectroscopy. The design route towards these types of QSSEs encompassing porous MOFs paves the way for realizing Li superionic conductors suitable for practical application in commercial LIBs with high safety and stability.

Role of free volumes and segmental dynamics on ion conductivity of PEO/LiTFSI solid polymer electrolytes filled with SiO2 nanoparticles: a positron annihilation and broadband dielectric spectroscopy study.

The limited ionic conductivity of polymer electrolytes is a major issue for their industrial application. Enhancement of ionic conductivity in the poly(ethylene oxide), PEO, based electrolyte has been achieved by loading passive nanofillers such as SiO2 nanoparticles (NPs). To investigate the role of modifications in free volume characteristics and the polymer chain dynamics induced by the loading of passive fillers on the ionic conductivity of the PEO based ternary electrolyte, a systematic investigation has been carried out using positron annihilation and broadband dielectric spectroscopy. As a result of interfacial interactions, the loading of SiO2 NPs alters the semi-crystalline morphology of PEO resulting in a higher crystallinity at lower loadings due to the surface confinement of PEO chains, and the formation of smaller PEO crystallites at higher loadings due to interparticle nanoconfinement. These modifications are accompanied by a decrease in free volume fraction at the lowest loading (0.5 wt%) followed by an increase at higher loadings (≥2.0 wt%). The Almond-West formalism considering two different universalities in different temperature and frequency ranges has been used to explain the ion-conduction process at different NP loadings. The Li ion conductivity is observed to be maximum for a 5.0 wt% loading of SiO2 NPs. The enhancement in ionic conductivity is observed to be directly correlated with the free volume characteristics and segmental dynamics of the PEO matrix, confirming their role in ion transport in polymer electrolytes.