Authors: Harish Gudla, Yunqi Shao, Supho Phunnarungsi, Daniel Brandell, and Chao Zhang
Ion pairing is commonly considered as a culprit for the reduced ionic conductivity in polymer electrolyte systems. However, this simple thermodynamic picture should not be taken literally, as ion pairing is a dynamical phenomenon. Here we construct model poly(ethylene oxide)–bis(trifluoromethane)sulfonimide lithium salt systems with different degrees of ion pairing by tuning the solvent polarity and examine the relation between the cation–anion distinct conductivity σ+–d and the lifetime of ion pairs τ+– using molecular dynamics simulations. It is found that there exist two distinct regimes where σ+–d scales with 1/τ+– and τ+–, respectively, and the latter is a signature of longer-lived ion pairs that contribute negatively to the total ionic conductivity. This suggests that ion pairs are kinetically different depending on the solvent polarity, which renders the ion-pair lifetime highly important when discussing its effect on ion transport in polymer electrolyte systems.
J. Phys. Chem. Lett. 2021, 12, 35, 8460–8464
Authors: Cleber F. N. Marchiori, Daniel Brandell, and C. Moyses Araujo
Organic electroactive materials are promising candidates to be used as lithium insertion electrodes in the next generation of environmentally friendly battery technologies. In this work, evolutionary algorithms at interplay with density functional theory calculations have been employed to predict the crystal structure for both delithiated and lithiated phases of dilithium thiophene dicarboxylate (Li2TDC). On the basis of the resulting crystals, electronic structure modifications and voltage profiles for the lithiation process have been calculated. The obtained structure for the delithiated phase showed a well-defined salt layer intercalating the organic components, forming a so-called lithium organic framework (LOF). Upon lithiation, new structures appear which deviate from the LOF as a consequence of the reduction of the S atoms, which coordinate with the additional Li ions. The calculated average potential of ∼1.00 V vs Li/Li+ is found to be in good agreement with experimental findings. An additional study at the molecular level has also been conducted aiming at gaining insight into the importance of the crystallographic environment on the structural and thermodynamics properties. This strategy is suitable for an initial assessment of the electrochemical process that underlies the lithiation mechanism of electrode materials. Moreover, the employed evolutionary algorithm emerges as a promising tool to predict crystal structures during lithiation, which are otherwise difficult to resolve experimentally.
J. Phys. Chem. C, 2019 123, 8, p. 4691-4700
Authors: Amina Mirsakiyeva, Mahsa Ebadi, C. Moyses Araujo, Daniel Brandell, Peter Broqvist, Jolla Kullgren
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 (Tg) 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 (Li2O) when PEO is decomposed into C2H4 and H2, 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 Li2O 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.
J. Phys. Chem. C, 2019, 123, 37, p. 22851-22857