Highly Conducting 3D-Hybrid Polymer Electrolytes for Lithium Batteries Based on Siloxane Networks and Cross-Linked Organic Polar Interphases

Abstract

The development of polymer electrolytes with high ionic conductivity, high lithium transference number, and high electrochemical stability is one of the main aims in the field of lithium battery research. In this work, we describe the synthesis and the characterization of new electrolyte systems, composed of three-dimensional hybrid inorganic−organic networks doped with LiClO 4 . The preparation route comprises only three steps, namely a sol−gel reaction, salt dissolution, and an epoxide polymerization reaction. The lithium concentration, and thus the lithium transference number, was modulated by adding lithium hydroxide in the sol−gel step. In this way, seven electrolytes with varying salt concentrations were prepared. The hybrid electrolytes are characterized by good ionic conductivities (up to 8·10 −5 S/cm at room temperature) and high thermo-mechanical and electrochemical stabilities. Stability tests versus lithium metal via galvanostatic polarization showed that this material is superior with respect to reference poly(ethylene oxide) based electrolytes. W ith the demand for higher energy densities, coupled with the need of increased safety, the electrolyte is considered to be the key component for the development of improved lithium batteries. 1 In particular, much effort is currently spent on the development of solid polymer electrolytes (SPEs) which provide higher thermal stability with respect to standard liquid electrolytes. They also offer better resistance to dendrite formation, thus paving the way for the use of lithium metal anodes and to high energy density batteries such as Li/air and Li/S batteries. 2 The most studied class of polymer electrolytes consists of complexes of poly(ethylene oxide) (PEO) with various lithium salts, as described by Wright and Armand already in the 1970s. 3 These materials are of great interest because of their low cost and toxicity, but the conductivity at room temperature is restricted to ca. 10 −6 S/cm only, which is too low for practical purposes. 4 One of the causes of the low conductivity is the semicrystalline morphology of PEO: ionic conduction occurs predominantly in the amorphous domains, with the crystalline domains playing an impeding role by increasing the tortuosity of the conduction pathways. 5,6 Other drawbacks are the decrease of the mechanical and electrochemical stability at high temperatures 2 and the low Li + transport number. 7 This point is particularly critical for electrolyte application, and it has been variously addressed, either by varying the lithium salt 8 or by developing single-ion conduction electrolytes. 2,9−11 In the field of the binary polyether based electrolytes, one of the easiest and most rewarding research strategies consists of the preparation of composites, in which inorganic particles are dispersed in the PEO matrix. These systems usually benefit from improved mechanical properties and from an increased ionic conductivity. 12,13 The latter effect is attributed to the hindering of the crystallization process or to Lewis acid−ba...

Conductivity and properties of polysiloxane-polyether cluster-LiTFSI networks as hybrid polymer electrolytes

“…For y ¼ 0.1 and T ¼ 30 C, s DC ¼ 3.8$10 À5 S cm À1 . This decrease is observed also in other similar systems [41] and is attributed to a progressive hindering of the polymer dynamics, related to the increase of the matrix's viscosity. As the salt concentration increases, the conductivity profiles become also steeper, thus suggesting an increase of the conductivity activation energy.…”

“…concentrations (y < 0.02) a mechanical transition at about 0 C is observed. This transition, which is also observed in other homologue hybrid polymer electrolytes [41], is coupled with the first order transition in the DSC profiles, and is attributed to the melting of micro-crystalline polyether domains. This results in a drop of the storage modulus, which is particularly drastic for the least concentrated sample.…”

“…With respect to previously reported materials [41], the synthetic procedure is simpler and more controllable. In addition, thanks to the substitution of the cross-linkable epoxy groups with vinyl moieties, the curing process occurs at a milder temperature (70 C, respect to 90 C) and in a considerably reduced time (less than 15 h), thus allowing the coating of electrodes under milder conditions.…”

“…In a previous report [41], we discussed the synthesis and the properties of ORMOCER ® -like hybrid polymer electrolytes based on partially cross-linked polyether chains covalently bound to tridimensional polysiloxane clusters. The materials showed high conductivity, and good thermo-mechanical and electrochemical stabilities.…”

“…With respect to the previously reported ORMOCER ® -based electrolytes [28,29,41], the newly synthesized materials are obtained by sol-gel reaction of dialkoxysilanes, rather than of trialkoxysilanes. Dialkoxysilanes are preferred since their condensation products are viscous liquids, even without adding condensations limiters such as LiOH.…”