Abstract
Correlations among the structure, thermal properties, and ionic conductivity of solid polymer electrolytes (SPEs) were studied using a ureasil–polyethylene oxide (U-PEO) organic–inorganic hybrid prepared according to a simple sol–gel route, employing a low molecular weight PEO macromer (Mw = 1900 g mol−1). The behavior of an amorphous sample loaded with lithium triflate (LiTFSI) at an optimum ratio between ether oxygen and lithium (EO/Li+ = 15) was compared with that of a semicrystalline sample prepared without salt loading. The temperature range investigated by differential scanning calorimetry (DSC), Raman spectroscopy, small angle X-ray scattering (SAXS), and complex impedance spectroscopy covered both the glass transition and the melting temperature of the U-PEO. The gauche to trans conformational transformation of the (O–C–C–O)Li+ sequence showed similarity between the temperature evolution of the semi-crystalline U-PEO and amorphous U-PEO:Li+ samples, providing an indication of the local structural memory of crystalline state in the amorphous SPE. The linear thermal expansion of the average correlation distance between the siloxane crosslink nodes and the long-distance period of the lamellar semi-crystalline edifice were determined by SAXS. Comparison of the expansion curves suggested that although the siloxane nodes were excluded from the PEO crystalline edifice, the sharp expansion of the amorphous region between the lamellae during melting permitted modulation of the free volume of the hybrid network. In addition, the temperature-induced Li+-EO decomplexation observed by Raman spectroscopy explained the change of the average activation energy of the conduction process revealed by the different Arrhenius regimes. These results evidence the key role of the ionic conductivity decoupling from the segmental motion of chain pair channels on the improvement of ion mobility through the free volume between chains. This concept may inspire materials chemistry researchers to design optimized structures of polymer electrolytes with minimized structural memory of crystaline building blocks and improved ionic conductivity.
Highlights
The research and development concerning solid-state electrolytes (SEs) for electrochemical devices such as lithium-ion batteries (LIBs) has remained popular, due to their better performance in terms of safety and fabrication, compared to liquid electrolytes that can have associated safety concerns due to possible leakage and the explosive nature of volatile organic electrolytes.[1,2] Most liquid electrolytes used in commercial LIBs are non-aqueous solutions, typically containing a lithium salt such as lithium tri ate (LiTFSI), due to the “so ” characteristic of the CF3SO3À anion, which has low ion-dipole stabilization (PEO)
We investigated the correlation among the structural, thermal, and ion conduction properties, by means of a comparative study of the temperature evolution of semi-crystalline ureasil–polyethylene oxide (U-PEO) (PEO molecular weight (Mw) 1⁄4 1900 g molÀ1) and an solid polymer electrolytes (SPEs) based on U-PEO:LiTFSI with [EO]/[Li+] 1⁄4 15
An in-depth thermal, conformational, and nanostructural analysis was made of organic–inorganic hybrid monoliths based on ureasil–poly(ethylene oxide) (U-PEO, Mw 1⁄4 1900 g molÀ1), unloaded and loaded with Li+ (U-PEO:Li+, [EO]/[Li+] 1⁄4 15)
Summary
The research and development concerning solid-state electrolytes (SEs) for electrochemical devices such as lithium-ion batteries (LIBs) has remained popular, due to their better performance in terms of safety and fabrication, compared to liquid electrolytes that can have associated safety concerns due to possible leakage and the explosive nature of volatile organic electrolytes.[1,2] Most liquid electrolytes used in commercial LIBs are non-aqueous solutions, typically containing a lithium salt such as lithium tri ate (LiTFSI), due to the “so ” characteristic of the CF3SO3À anion, which has low ion-dipole stabilization (PEO). Renewed interest in novel SPEs has been stimulated by the development of lithium metal batteries (LMBs) of high speci c capacity, due to the increasing demand for electric vehicles, portable electronic equipment, and implantable medical devices.[8,9] In PEO, the high segmental motion of the chains at temperatures above the glass transition results in high ionic mobility, but harms the mechanical properties of SPEs.[8] In addition to the concurrence between these properties, PEO is semi-crystalline and the Li+ transport is limited to the mobility of its amorphous phase. Unmodi ed PEO must be heated to around 80 C to achieve conductivity higher than 10À4 S cmÀ1, which is a limitation for many applications.[10]
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