Poly(ethylene oxide) (PEO) is the most widely used compound as a solid-state (solvent-free) polymer electrolyte for Li batteries, mainly due to its low glass transition temperature (T g ) and ability to dissolve Li salts. It is also frequently suggested that its cathodic stability renders it possible to operate with Li metal anodes in the design of high energy density storage devices. However, little is still known about the true interfacial chemistry between Li metal and PEO and how these two materials interact with each other. We are here exploring this relationship by the means of density functional theory (DFT)-based modeling. Using bulk structures and isolated PEO chains, we have found that there is a strong thermodynamic driving force to oxidize Li metal into lithium oxide (Li 2 O) when PEO is decomposed into C 2 H 4 and H 2 , irrespectively of the PEO oligomer length. Explicit modeling of PEO on a Li(100) surface reveals that all steps in the decomposition are exothermic and that the PEO/Li metal system should have a layer of Li 2 O between the polymer electrolyte and the metal surface. These insights and the computational strategy adopted here could be highly useful to better tailor polymer electrolytes with favorable interfacial properties.

Conducting polymers have become standard engineering materials, used in many electronic devices. Despite this, there is a lack of understanding of the microscopic origin of the conducting properties, especially at realistic device field strengths. We present simulations of doped poly(p-phenylene) (PPP) using a Su-Schrieffer-Heeger (SSH) tight-binding model, with the electric field included in the Hamiltonian through a time-dependent vector potential via Peierls substitution of the phase factor. We find that polarons typically break down within less than a picosecond after the field has been switched on, already for electric fields as low as around 1.6 mV/Å. This is a field strength common in many flexible organic electronic devices. Our results challenge the relevance of the polaron as charge carrier in conducting polymers for a wide range of applications.

We present simulation results, computed with the Car-Parrinello molecular dynamics method, at zero and ambient temperature (300 K) for poly (3,4-ethylenedioxythiophene) [PEDOT] and its selenium and tellurium derivatives PEDOS and PEDOTe, represented as 12-oligomer chains. In particular, we focus on structural parameters such as the dihedral rotation angle distribution, as well as how the charge distribution is affected by temperature. We find that for PEDOT, the dihedral angle distribution shows two distinct local maxima whereas for PEDOS and PEDOTe, the distributions only have one clear maximum. The twisting stiffness at ambient temperature appears to be larger the lighter the heteroatom (S, Se, Te) is, in contrast to the case at 0 K. As regards point charge distributions, they suggest that aromaticity increases with temperature, and also that aromaticity becomes more pronounced the lighter the heteroatom is, both at 0 K and ambient temperature. Our results agree well with previous results, where available. The bond lengths are consistent with substantial aromatic character both at 0 K and at ambient temperature. Our calculations also reproduce the expected trend of diminishing gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital with increasing atomic number of the heteroatom.

PostprintThis is the accepted version of a paper published in Journal of Chemical Physics. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the original published paper (version of record):Mirsakiyeva, A., Hugosson, H W., Linares, M., Delin, A. (2017) Temperature dependence of band gaps and conformational disorder in PEDOT and its selenium and tellurium derivatives: density functional calculations. Journal of Chemical PhysicsAccess to the published version may require subscription. N.B. When citing this work, cite the original published paper. Permanent link to this version:http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-214934Temperature dependence of band gaps and conformational disorder in PEDOT and its selenium and tellurium derivatives: density functional calculations The conducting polymer poly(3,4-ethylenedioxythiophene), or PEDOT, is an attractive material for flexible electronics. We present combined molecular dynamics and quantum chemical calculations, based on density functional theory, of EDOT oligomers and isoelectronic selenium and tellurium derivatives (EDOS and EDOTe) to address the effect of temperature on the geometrical and electronic properties of these systems.With finite size scaling, we also extrapolate our results to the infinite polymers, i.e. PEDOT, PEDOS and PEDOTe. Our computations indicate that the most favourable oligomer conformations at finite temperature are conformations around the flat trans-conformation and a non-flat conformation around 45• from the cis-conformation. Also, the dihedral stiffness increases with the atomic number of the heteroatom. We find excellent agreement with experimentally measured gaps for PEDOT and PEDOS. For PEDOT, the gap does not increase with temperature whereas this is the case for its derivatives. The conformational disorder as well as the choice of basis set both significantly affect the calculated gaps.

Synthesis routes of novel piperidine-containing acetylenes are presented. The new molecules are expected to exhibit plant growth stimulation properties. In particular, the yield in a situation of drought is expected to increase. Our synthesis makes use of the Favorskii reaction between cyclohexanone/piperidone and triple-bond containing alcohols. The structures of the obtained molecules were determined using nuclear magnetic resonance (NMR). The electronic structure and geometries of the molecules were studied theoretically using first-principles calculations based on density functional theory. The calculated geometries agree very well with the experimentally determined ones, and also allow us to determine bond lengths, angles and charge distributions inside the molecules.