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Model studies for the electrode electrolyte interface

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Joachim Bansmann
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Employing density functional theory (DFT) calculations and x-ray photoelectron spectroscopy (XPS), we identify products of the reaction of the ionic liquid N,N - butylmethylpyrrolidinum bis(triuoromethylsulfonyl)imide (BMP-TFSI) with lithium in order to model the initial chemical processes contributing to the formation of the solid electrolyte interphase in batteries. Besides lithium oxide, sulfide, carbide and fluoride, we find lithium cyanide or cyanamide as possible, thermodynamically stable products in the Li-poor regime, whilst Li$_{\textrm{3}}$N is the stable product in the Li-rich regime. The thermodynamically controlled reaction products as well as larger fragments of TFSI persisting due to kinetic barriers could be identified by a comparison of experimentally and computationally determined core level binding energies.
Joachim Bansmann
added a research item
Employing density functional theory (DFT) calculations and x-ray photoelectron spectroscopy (XPS), we identify products of the reaction of the ionic liquid N,N - butylmethylpyrrolidinum bis(trifluoromethylsulfonyl)imide (BMP-TFSI) with lithium in order to model the initial chemical processes contributing to the formation of the solid electrolyte interphase in batteries. Besides lithium oxide, sulfide, carbide and fluoride, we find lithium cyanide or cyanamide as possible, thermodynamically stable products in the Li-poor regime, whilst Li3N is the stable product in the Li-rich regime. The thermodynamically controlled reaction products as well as larger fragments of TFSI persisting due to kinetic barriers could be identified by a comparison of experimentally and computationally determined core level binding energies.
Joachim Bansmann
added a research item
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(trifluoromethyl-sulfonyl)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 LinCo3O4 takes place due to insertion via step edges or defects; a partial transformation to CoO cannot be excluded. Formation of Co 0 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.
Joachim Bansmann
added a research item
Aiming at a molecular level understanding of the processes at the electrode|electrolyte interface (EEI), we investigated the interaction between the battery-relevant Ionic Liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]‒), Li and CoO(111) thin films on Ru(0001) as a model study of the solid|electrolyte interphase (SEI) in Li-ion batteries (LIBs). Employing mainly angle-dependent X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), in combination with dispersion-corrected density functional calculations (DFT-D) for characterization of the CoO(111) surface, we found that vapor deposition of metallic Li on CoO(111) at 300 K results in the conversion of Co2+ to Co0, together with the formation of Li2O and adsorbed surface Li2O2. The conversion starts in the near surface region (1-2 nm) and proceeds in the extended surface region (6-8 nm). If the surface is precovered by molecularly adsorbed anion-cation pairs of [BMP][TFSI] (solvent / electrolyte), stepwise postdeposition of small amounts of Li results in gradual decomposition of [TFSI] and [BMP] (= SEI formation ), forming products such as Li3N, Li2S, LiF, LiCyHyNz and other Li-bound fragments of the anion. For higher amounts of Li deposition, relative to the IL precoverage, IL decomposition is followed by conversion of CoO(111). Hence, the SEI resulting from IL decomposition is permeable for Li, which is essential for the storage of Li in the CoO(111) anode. This study demonstrates the potential of model studies for a molecular scale understanding of the initial stages of SEI formation at the EEI, and its role in Li storage in a CoO(111) model anode.
Joachim Bansmann
added a research item
We report results of a combined experimental and computational model study on the interaction of the battery-relevant Ionic Liquid (IL) 1-butyl-1-methylpyrrolidinium bis(tri-fluoromethylsulfonyl)imide ([BMP]+[TFSI]‒) with Li on pristine highly oriented pyrolytic graphite (HOPG), which aims at a molecular/atomic level understanding of the processes at the electrode|electrolyte interface of Li-ion batteries. Employing mainly X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) as well as dispersion-corrected density functional calculations (DFT-D), we find intact anion-cation pairs for adsorbed [BMP]+[TFSI]‒ (sub-)monolayers on HOPG at 300 K, and also on lithiated HOPG at 80 K, i.e., under conditions where the mobility of Li+ in the bulk is low. Vapor deposition of [BMP]+[TFSI]- on lithiated HOPG at 300 K results in rapid accumulation of Liδ+ at the surface or in the surface region, indicating that de-intercalation is activated under these conditions. This is explained by a dynamic equilibrium between bulk Li+ and surface Liδ+, which is established independent of whether Li is deposited as metallic Li0 from the vacuum side or segregates as Li+ from the bulk of lithiated HOPG to the surface, and which is shifted to the side of surface Liδ+ by stabilization of these species. Stabilization occurs either by formation of stable Li-containing surface compounds by reactive decomposition mainly of the [TFSI]‒ anions (Li3N, Li2S, LiF, etc.), or by interaction of partially charged Liδ+ species with [TFSI]‒ anions in the adlayer. DFT-D calculations reveal that a possible initial step in the reactive decomposition is the transfer of electrons from the HOPG surface covered with Liδ+ into the lowest unoccupied molecular orbital of [TFSI]‒, resulting in elongation and cleavage of the S‒N bond and finally insertion of Li into it. Alternatively, stabilization of Liδ+ is possible by formation of a polar bond with the oxygen atoms of [TFSI]‒ within the IL adlayer. The resulting calculated work function decrease ΔΦ with respect to that of the bare graphite(0001) surface is in excellent agreement with experimental observations. The interaction of [BMP]+[TFSI]‒ and Li at the HOPG interface is considered as the initial stage of the solid|electrolyte interphase formation at the electrode|electrolyte interface in Li-ion batteries.
Joachim Bansmann
added 9 research items
The ionic liquid (IL) 1‒butyl‒1‒methyl¬pyrrolidinium bis (tri‒fluoro‒methyl¬sulfonyl) imide [BMP][TFSA] is a promising candidate for improved next‒generation rechargeable lithium‒ion batteries. We here report results of a model study of the reactive interaction of (sub‒) monolayers and multilayers of [BMP][TFSA] with lithium (Li) on Cu(111), employing scanning tunnelling microscopy (STM), X‒ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIRS) under ultrahigh vacuum (UHV) conditions. Upon post‒deposition of Li on [BMP][TFSA] multilayers at 80 K we identified changes in the chemical state of the [TFSA] anion and the [BMP] cation, as well as in the IR absorption bands related to the anion. These changes are most likely due to the decomposition of the IL adlayer into a variety of products like LiF, Li2S, Li2O upon anion decomposition and LiN3, LiCxHyN and / or LixCHy upon cation decomposition, where the latter includes cracking of the pyrrolidinium ring. Deposition of Li on [BMP][TFSA] (sub‒) monolayer covered surfaces led to similar decomposition patterns, and the same was observed also for the reverse deposition order. Addition of the corresponding amounts of Li to a [BMP][TFSA] adlayer resulted in distinct changes in the STM images, which must be due to the surface reaction. After annealing to 300 K, the core level peaks of the cation lose most of their peak area. Upon further heating to 450 K, the anion is nearly completely decomposed, resulting in LiF and Li2S decomposition products which dominate the interface.
Large-scale STM images after vapor deposition of a [BMP]+[TFSI]− (sub-)monolayer on Ag(111) (a), Au(111) (b), HOPG(0001) (c), Cu(111) (d), and TiO2(110) (e) at RT and subsequent cooling to 100 K. On the more reactive Cu(111) surface, the IL was adsorbed at 200 K to avoid decomposition. STM imaging was also conducted at 100 K. A molecular representation of [BMP]+[TFSI]− is inserted in the figure.