Abstract
Lithium cation solvation structures [Li(S)(n=1-4)](+) with ligands of cyclic or noncyclic carbamate-modified disiloxanes are optimized at B3LYP level of theory and compared to their corresponding simplified carbamates and to the organic carbonates ethylene carbonate (EC) and dimethyl carbonate (DMC). The electrostatic potentials (ESP) of these investigated carbonyl-containing solvents are mapped on the electron density surface. The maximum ESP is located at the C=O-oxygen, whereas the disiloxane functionality represents an unpolar residue. Natural Bond Orbitals (NBO) analysis reveals strong n(N) →π(C[double bond, length as m-dash]O) donor-acceptor interactions in carbamates which outrun dipolar properties. As a result, higher total binding energies (ΔE(B)) for solvation of Li(+) in carbamates (-148 kcal mol(-1)) are found than for carbonates (-137 kcal mol(-1)). Furthermore, the disiloxane moiety with its Si-O bond is stabilized by n(O) →σ*(Si-C) hyperconjugation that provides additional electron density to a nearby SiCH3 methyl group thus supporting an additional SiCH2-H...Li(+) coordination. The formation of all investigated solvation structures is exothermic. Owing to steric hindrance of noncyclic carbonyl-containing ligands and the bulky disiloxane functionality, the solvation structure [Li(S)3](+) is the preferred structure according to Gibbs free energy ΔG(B) results.
Highlights
Further improvement of safety issues may be achieved by application of solid polymer electrolytes (SPE) since they replace flammable liquids completely.[20,21] But so far, they suffer from poor ionic conductivity in a magnitude of 10À5 S cmÀ1 at ambient temperature
Beside their application as alternative liquid electrolytes, we considered carbamate-modified disiloxanes as model compounds for the development of future SPEs, due to the decreased dielectric constant e of their lead structure N-methyloxazolidin-2-one (NMO, 3; e = 78).[1]
As visualized by electrostatic potentials (ESP) mapping, for all investigated solvents 1–6 the highest electron density is located at the CQO-oxygen, where the coordination to the lithium cation occurs
Summary
Further improvement of safety issues may be achieved by application of SPEs since they replace flammable liquids completely.[20,21] But so far, they suffer from poor ionic conductivity in a magnitude of 10À5 S cmÀ1 at ambient temperature. In order to increase the ionic conductivity of polysiloxane-based SPE systems, mixed ethylene glycol- and carbonate-modified polysiloxanes were prepared to introduce a high ability to dissociate lithium salts and increase the concentration of free ions in a polymer matrix.[22,23]. We have reported the synthesis of novel liquid carbamate-modified disiloxanes,[24] as well as their application in porous PVDF-HFP membranes.[25] These liquid materials reach an ionic conductivity in the magnitude of 10À4 S cmÀ1 at ambient temperature and are electrochemically stable up to 4.5 V vs Li/Li+ These liquid materials reach an ionic conductivity in the magnitude of 10À4 S cmÀ1 at ambient temperature and are electrochemically stable up to 4.5 V vs. Li/Li+
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