Structure formation and surface chemistry of ionic liquids on model electrode surfaces-Model studies for the electrode | electrolyte interface in Li-ion batteries

Highly Conductive Imidazolate Covalent Organic Frameworks with Ether Chains as Solid Electrolytes for Lithium Metal Batteries

Poly(ethylene oxide) (PEO)-based electrolytes are often used for Li+ conduction as they can dissociate the Li salts efficiently. However, high entanglement of the chains and lack of pathways for rapid ion diffusion limit their applications in advanced batteries. Recent developments in ionic covalent organic frameworks (iCOFs) showed that their highly ordered structures provide efficient pathways for Li+ transport, solving the limitations of traditional PEO-based electrolytes. Here, we present imidazolate COFs, PI-TMEFB-COFs, having methoxyethoxy chains, synthesized by Debus-Radziszewski multicomponent reactions and their ionized form, Li+@PI-TMEFB-COFs, showing a high Li+ conductivity of 8.81 mS cm-1 and a transference number of 0.974. The mechanism for such excellent electrochemical properties is that methoxyethoxy chains dissociate LiClO4, making free Li+, then those Li+ are transported through the imidazolate COFs' pores. The synthesized Li+@PI-TMEFB-COFs formed a stable interface with Li metal. Thus, employing Li+@PI-TMEFB-COFs as the solid electrolyte to assemble LiFePO4 batteries showed an initial discharge capacity of 119.2 mAh g-1 at 0.5 C, and 82.0 % capacity and 99.9 % Coulombic efficiency were maintained after 400 cycles. These results show that iCOFs with ether chains synthesized via multicomponent reactions can create a new chapter for making solid electrolytes for advanced rechargeable batteries.

Model Studies on the Formation of the Solid Electrolyte Interphase: Reaction of Li with Ultrathin Adsorbed Ionic-Liquid Films and Co3 O4 (111) Thin Films

In this work we aim towards the molecular understanding of the solid electrolyte interphase (SEI) formation at the electrode electrolyte interface (EEI). Herein, we investigated the interaction between the battery‐relevant ionic liquid (IL) 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP‐TFSI), Li and a Co₃O₄(111) thin film model anode grown on Ir(100) as a model study of the SEI formation in Li‐ion batteries (LIBs). We employed mostly X‐ray photoelectron spectroscopy (XPS) in combination with dispersion‐corrected density functional theory calculations (DFT‐D3). If the surface is pre‐covered by BMP‐TFSI species (model electrolyte), post‐deposition of Li (Li⁺ ion shuttle) reveals thermodynamically favorable TFSI decomposition products such as LiCN, Li₂NSO₂CF₃, LiF, Li₂S, Li₂O₂, Li₂O, but also kinetic products like Li₂NCH₃C₄H₉ or LiNCH₃C₄H₉ of BMP. Simultaneously, Li adsorption and/or lithiation of Co₃O₄(111) to Li_nCo₃O₄ takes place due to insertion via step edges or defects; a partial transformation to CoO cannot be excluded. Formation of Co⁰ could not be observed in the experiment indicating that surface reaction products and inserted/adsorbed Li at the step edges may inhibit or slow down further Li diffusion into the bulk. This study provides detailed insights of the SEI formation at the EEI, which might be crucial for the improvement of future batteries.

Density Functional Theory and Thermodynamics Modeling of Inner-Sphere Oxyanion Adsorption on the Hydroxylated α-Al2O3(001) Surface

The inner-sphere adsorption of AsO₄^3-, PO₄^3-, and SO₄^2- on the hydroxylated α-Al₂O₃(001) surface was modeled with the goal of adapting a density functional theory (DFT) and thermodynamics framework for calculating the adsorption energetics. While DFT is a reliable method for predicting various properties of solids, including crystalline materials comprised of hundreds (or even thousands) of atoms, adding aqueous energetics in heterogeneous systems poses steep challenges for modeling. This is in part due to the fact that environmentally relevant variations in the chemical surroundings cannot be captured atomistically without increasing the system size beyond tractable limits. The DFT + thermodynamics approach to this conundrum is to combine the DFT total energies with tabulated solution-phase data and Nernst-based corrective terms to incorporate experimentally tunable parameters such as concentration. Central to this approach is the design of thermodynamic cycles that partition the overall reaction (here, inner-sphere adsorption proceeding via ligand exchange) into elementary steps that can either be fully calculated or for which tabulated data are available. The ultimate goal is to develop a modeling framework that takes into account subtleties of the substrate (such as adsorption-induced surface relaxation) and energies associated with the aqueous environment such that adsorption at mineral-water interfaces can be reliably predicted, allowing for comparisons in the denticity and protonation state of the adsorbing species. Based on the relative amount of experimental information available for AsO₄^3-, PO₄^3-, and SO₄^2- adsorbates and the well-characterized hydroxylated α-Al₂O₃(001) surface, these systems are chosen to form a basis for assessing the model predictions. We discuss how the DFT + thermodynamics results are in line with the experimental information about the oxyanion sorption behavior. Additionally, a vibrational analysis was conducted for the charge-neutral oxyanion complexes and is compared to the available experimental findings to discern the inner-sphere adsorption phonon modes. The DFT + thermodynamics framework used here is readily extendable to other chemical processes at solid-liquid interfaces, and we discuss future directions for modeling surface processes at mineral-water and environmental interfaces.

Surface Science and Electrochemical Model Studies on the Interaction of Graphite and Li-Containing Ionic Liquids

The process of solid-electrolyte interphase (SEI) formation is systematically investigated along with its chemical composition on carbon electrodes in an ionic liquid-based, Li-containing electrolyte in a combined surface science and electrochemical model study using highly oriented pyrolytic graphite (HOPG) and binder-free graphite powder electrodes (Mage) as model systems. The chemical decomposition process is explored by deposition of Li on a pre-deposited multilayer film of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) under ultrahigh vacuum conditions. Electrochemical SEI formation is induced by and monitored during potential cycling in [BMP][TFSI]+0.1 m LiTFSI. The chemical composition of the resulting layers is characterized by X-ray photoelectron spectroscopy (XPS), both at the surface and in deeper layers, closer to the electrode|SEI interface, after partial removal of the film by Ar⁺ ion sputtering. Clear differences between chemical and electrochemical SEI formation, and also between SEI formation on HOPG and Mage electrodes, are observed and discussed.

Adsorption of Ultrathin Ethylene Carbonate Films on Pristine and Lithiated Graphite and Their Interaction with Li

Aiming at a better understanding of the solid-electrolyte interphase formation in Li-ion batteries, we have investigated the interaction of ultrathin films of ethylene carbonate (EC), which is a key solvent of battery electrolytes, with pristine and lithiated highly oriented pyrolytic graphite (HOPG) and with postdeposited Li. Employing X-ray and ultraviolet photoelectron spectroscopy as well as Fourier transform infrared spectroscopy under ultrahigh-vacuum conditions, in combination with density functional theory (DFT)-based calculations, we find that EC adsorbs molecularly intact on pristine HOPG in the entire temperature range between 80 K and desorption at 200 K. Features in the ultraviolet photoelectron spectra could be related to the molecular orbitals of EC obtained from DFT calculations, and a similar adsorption/desorption behavior is obtained also on lithiated HOPG. In contrast, stepwise postdeposition of ∼0.5 and one monolayer of Li⁰ on a preadsorbed EC adlayer leads not only to stabilization of Li⁺/Li^δ+ at the surface, possibly as adsorbed Li⁺(EC) _n species, but also to EC decomposition, forming products such as Li₂CO₃, ROCO₂Li (CH₂OCO₂Li)₂, and Li₂O. Consequences on the electronic surface properties and on the stabilization of the resulting adlayer are discussed. Upon annealing up to room temperature, some residual Li-containing decomposition products remain on the surface, which is considered as the initial stage of the solid|electrolyte interphase formation at the electrode|electrolyte interface.