The design of flexible, solvent-free polymer electrolytes for solid-state batteries requires an understanding of the fundamentals that control ion transport. Researchers at Oak Ridge National Laboratory established a correlation between composite structure, polymer segmental dynamics, and lithium-ion transport in ceramic-polymer composites, elucidating that this structure-property relationship will adjust the Li+ conductivity by optimizing the macroscopic electrochemical stability of the electrolyte. By controlling the morphology and function of the polymer/ceramic interface, it was found that the dissociation of ions from the slow polymer segment dynamics was enhanced, and the chemical structure of lithium salts in the composite electrolyte was related to the size of the ion cluster domain, the conduction mechanism, and the electrochemical stability of the electrolyte. Polyethylene oxide (PEO) filled with bis(trifluoromethanesulfonyl)imide (LiTFSI) or bis(fluorosulfonyl)imide (LiFSI) salts was used as a matrix, garnet electrolyte, aluminum-substituted lithium lanthanum zirconium oxide (Al-LLZO) with planar geometry for ceramic nanoparticle fractions. Dielectric relaxation spectroscopy was used to study the dynamics of strongly bound and highly fluidized Li+, and the incorporation of Al-LLZO increased the quantitative density of more mobile Li+. The structure of nanoscale ion agglomeration was investigated by small-angle X-ray scattering, and molecular dynamics (MD) simulations were performed to obtain the basic mechanism of desorption of Li+ from long PEO chains in LiTFSI and LiFSI salts. The research results were published in ACS Nano under the title "Nanoscale Ion Transport Enhances Conductivity in Solid Polymer-Ceramic Lithium Electrolytes".
Original link:
https://doi.org/10.1021/acsnano.3c03901
The use of ceramic electrolytes in solid-state batteries (SSBs) is still challenging, and from a material processing perspective, polymer electrolytes can be a solution for manufacturing solid-state batteries due to their flexibility, roll-to-roll processing, and excellent interfacial properties. To achieve this, the next generation of lightweight, flexible, solvent-free, and electrochemically stable polymer electrolyte materials with ultra-fast ion transport properties is required. Ion transport can be quantitatively expressed by three basic parameters: ion mobility, free ion concentration, and number of mobility. In order to adjust these parameters, several electrolyte structures have been proposed and studied, and among the great structural variability of the electrolytes studied, the representative electrolyte types are related to single ion conductors, cross-linking, gels, plasticizers, nanostructured block copolymers, and composite electrolytes. Among the different electrolyte types, the polymer composite electrolyte has performance advantages, the ceramic oxide phase has high conductivity and dendrite resistance, while the polymer phase, although less conductive, provides a flexible and easy-to-process matrix for dispersing the ceramic phase and synthesizing a separate thin film electrolyte with excellent interface properties with the positive and negative electrodes.
The focus of this work is to elucidate the rationale for nanoscale Li+ movement to control mesoscale Li+ ion transport and conductance in polyethylene oxide (PEO) electrolytes filled with bis(trifluoromethanesulfonyl)imide (LiTFSI) or bis(fluorosulfonyl)imide (LiFSI) salts. To further enhance Li+ transport, a garnet-type electrolyte, aluminum-substituted lithium lanthanum zirconium oxide (Al-LLZO), is used in the ceramic phase. According to the interfacial properties of PEO/Al-LLZO, Al-LLZO fillers with planar geometry were synthesized. This configuration results in better adsorption of PEO segments on the Al-LLZO surface, where Li+ movement is highly correlated at the nanoscale, which affects the mesoscopic transport mechanism due to the strong coupling between Li+ and PEO, the formation of cationic and anionic ion cluster domains, and the presence of ionic conductive ceramic interfaces. Nanoscale diffusion of Li+ can occur through PEO segmental movement or through interfacial diffusion at the Al LLZO/PEO boundary, or through diffusion processes within salt clusters (intra-cluster diffusion) or between adjacent clusters (inter-cluster diffusion).
In this work, the authors investigated the molecular mechanisms responsible for long-range conductivity at the nanoscale. Li+ in solid electrolytes may exist in different phases, and the number density of different states depends on the electrostatic interaction between Li+ and the coordination site. Studies have shown that by adjusting the morphology and function of the polymer/ceramic interface, Li+ ions can be dissociated (decoupled) from the local polymer environment and diffuse over the long-range macrodomain. The kinetics (synergistic kinetics) of the rearrangement of polymer segments, which are responsible for facilitating the conductive mechanism, are very slow and prevent the rapid transport of ions through the polymer phase. LiTFSI and LiFSI are two of the most typical salts used in solid-state electrolytes, and the relationship between their chemical structure and conduction mechanism has been extensively studied. Li+ will exhibit higher diffusivity and conductivity in the LiFSI system compared to the LiTFSI system, and although the LiFSI electrolytes are more conductive, they are found to be more prone to cluster formation, while LiTFSI shows a better permeable structure than the PEO matrix. The formation of ionic cluster domains in the electrolyte bulk structure correlates with the chemical structure of Li+ salts, and the size of these clusters is estimated by small angle X-ray scattering (SAXS) measurements. Molecular dynamics (MD) simulation studies are used to model the Li+ ion transport mechanisms involved in PEO segmental motion and ion cluster domain formation. The focus of this work is to fundamentally understand the correlation between motion processes at different time and length scales that superimpose and determine the conductive mechanisms of Li+ ions. Better Li+ ionic solvation in the nanoscale domain and the incorporation of Al-LLZO lamellae lead to improvements in the critical current density and long-term cycling of the LiTFSI electrolyte. (Text: Li Shu)
Figure 1: SEM image of electrospun Al-LLZO lamellae
图2填充Al LLZO的复合LiTFSI和LiFSI-PEO电解质的Arrhenius图
Figure 3: Dielectric loss spectra and fitting analysis of the original PEO/LiTFSI electrolyte and the 7wt%, 15wt% Al-LLZO composite electrolyte at 60°C
Figure 4 (a, b) SAXS model and model fit for PEO/LiFSI electrolytes at 25 °C and 60 °C, (c, d) PEO/LiTFSI electrolytes at 25 °C and 60 °C
Fig.5 Structural behavior of Li+ ions
Fig.6 Mean square shift (MSD) and diffusivity of Li+, FSI/TFSI anions, and PEO chains
图7(a,b)LiFSI和LiTFSI电解质,(c,d)填充7 wt% Al-LLZO的LiTFSI和LiTFI复合材料的恒电流循环对比
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