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1.
Characterization of electrochemical processes in metal-organic batteries by X-ray Raman spectroscopy
Ava Rajh, Iztok Arčon, Klemen Bučar, Matjaž Žitnik, Marko Petric, Alen Vižintin, Jan Bitenc, Urban Košir, Robert Dominko, Hlynur Gretarsson, Martin Sundermann, Matjaž Kavčič, 2022, original scientific article

Abstract: X-ray Raman spectroscopy (XRS) is an emerging spectroscopic technique that utilizes inelastic scattering of hard Xrays to study X-ray absorption edges of low Z elements in bulk material. It was used to identify and quantify the amount of carbonyl bonds in a cathode sample, in order to track the redox reaction inside metal−organic batteries during the charge/ discharge cycle. XRS was used to record the oxygen K-edge absorption spectra of organic polymer cathodes from different multivalent metal−organic batteries. The amount of carbonyl bond in each sample was determined by modeling the oxygen K-edge XRS spectra with the linear combination of two reference compounds that mimicked the fully charged and the fully discharged phases of the battery. To interpret experimental XRS spectra, theoretical calculations of oxygen K-edge absorption spectra based on density functional theory were performed. Overall, a good agreement between the amount of carbonyl bond present during different stages of battery cycle, calculated from linear combination of standards, and the amount obtained from electrochemical characterization based on measured capacity was achieved. The electrochemical mechanism in all studied batteries was confirmed to be a reduction of double carbonyl bond and the intermediate anion was identified with the help of theoretical calculations. X-ray Raman spectroscopy of the oxygen K-edge was shown to be a viable characterization technique for accurate tracking of the redox reaction inside metal−organic batteries.
Keywords: X-ray Raman spectroscopy, meta-organic batteries, oxygen K-edge XANES, electrochemical processes
Published in RUNG: 24.03.2022; Views: 1807; Downloads: 20
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2.
Spectroscopic insights into the electrochemical mechanism of rechargeable calcium/sulfur batteries
Antonio Scafuri, Romain Berthelot, Klemen Pirnat, Alen Vižintin, Jan Bitenc, Giuliana Aquilanti, Dominique Foix, Rémi Dedryvère, Iztok Arčon, Robert Dominko, Lorenzo Stievano, 13, original scientific article

Abstract: Calcium batteries represent a promising alternative to lithium metal systems. The combination of the low redox potential and low cost and the energy-dense calcium anode (2073 mAh/cm3, similar to 2044 mAh/cm3 for Li) with appropriate low-cost cathode materials such as sulfur could produce a game-changing technology in several fields of applications. In this work, we present the reversible activity of a proof-of-concept Ca/S battery at room temperature, characterized by a surprising medium-term cycling stability with low polarization, promoted by the use of a simple positive electrode made of sulfur supported on an activated carbon cloth scaffold, and a state-of-the-art fluorinated alkoxyborate-based electrolyte. Insights into the electrochemical mechanism governing the chemistry of the Ca/S system were obtained for the first time by combining X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The mechanism implies the formation of different types of soluble polysulfide species during both charge and discharge at room temperature, and the formation of solid CaS at the end of discharge. The reversible electrochemical activity is proven by the reformation of elemental sulfur at the end of the following charge. These promising results open the way to the comprehension of emerging Ca/S systems, which may represent a valid alternative to Mg/S and Li/S batteries.
Keywords: Calcium/Sulfur Batteries EXAFS, XANES
Published in RUNG: 17.10.2020; Views: 3004; Downloads: 0
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3.
A mechanistic study of metal sulfur batteries
Robert Dominko, Alen Vižintin, Sara Drvarič Talian, Ana Robba, Iztok Arčon, 2018, published scientific conference contribution abstract (invited lecture)

Keywords: Li-sulphur batteries, Sulphur XANES, magnesium
Published in RUNG: 15.10.2020; Views: 2473; Downloads: 0

4.
Ceramic synthesis of disordered lithium rich oxyfluoride materials
Jean-Marcel Ateba Mba, Iztok Arčon, Gregor Mali, Elena Tchernychova, Ralf Witte, Robert Kruk, Miran Gaberšček, Robert Dominko, 2020, original scientific article

Abstract: Disordered lithium-rich transition metal oxyfluorides with a general formula Li1þxMO2Fx (M being a transition metal) are gaining more attention due to their high specific capacity which can be delivered from the facecentered cubic (fcc) structure. The most common synthesis procedure involves use of mechanosynthesis. In this work, ceramic synthesis of lithium rich iron oxyfluoride and lithium rich titanium oxyfluoride are reported. Two ceramic synthesis routes are developed each leading to the different level of doping with Li and F and different levels of cationic disorder in the structure. Three different Li1þxMO2Fx samples (x ¼ 0.25, 0.3 and 1) are compared with a sample prepared by mechanochemical synthesis and non-doped LiFeO2 with fcc structure. The obtained lithium rich iron oxyfluoride are characterized by use of M€ossbauer spectroscopy, X-ray absorption spectroscopy, NMR and TEM. Successful incorporation of Li and F have been confirmed and specific capacity that can be obtained from the samples is in the correlation with the level of disorder introduced with doping, nevertheless oxidation state of iron in all samples is very similar. Conclusions obtained from lithium rich iron oxyfluoride are validated by lithium rich titanium oxyfluoride.
Keywords: Lithium batteries Face centered-cubic Oxyfluoride Li-rich Disorder
Published in RUNG: 05.06.2020; Views: 4547; Downloads: 0
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5.
Effects of a Mixed O/F Ligand in the Tavorite-Type LiVPO4O Structure
Sorour Semsari Parapari, Jean-Marcel Ateba Mba, Elena Tchernychova, Gregor Mali, Iztok Arčon, Gregor Kapun, Mehmet Ali Gülgün, Robert Dominko, 2020, original scientific article

Abstract: We report the synthesis and detailed structural and chemical characterization including electrochemical properties of a lithium vanadium oxy/fluoro-phosphate material. To the best of our knowledge, we have for the first time synthesized a LiVPO4O-type phase with a mixed O/F ligand. In the synthesis procedure, the LiVPO4O precursor compound was fluorinated via LiF incorporation, with preservation of the LiVPO4O framework structure. The operating potential of the synthesized material is increased compared to that of the LiVPO4O precursor (4.12 V vs 3.95 V versus metallic lithium, respectively). The related increase in operating potential was assigned to the effect of the intermixing O/F ligand, which is attained via the successful fluorine incorporation into the LiVPO4O structure. A characterization of the investigated materials was performed using microscale-covering XRD, XANES, and NMR techniques as well as nanoscale spatially resolved imaging and analytical STEM techniques. The obtained oxy/fluoro-phosphate phase is isostructural to LiVPO4O; however, the presence of the mixed O/F ligand promoted a higher symmetry of vanadium octahedra. These variations of the vanadium local environment along with the observed inhomogeneous distribution of the incorporated fluorine gave rise to the minor local deviations in vanadium valence. Our results clearly emphasize the connection among the fluorine ligand incorporation, its local distribution, and the electrochemical properties of the material.
Keywords: LiVPO4O, XRD, SEM, V XANES, Tavorite-Type, electrochemical properties
Published in RUNG: 17.02.2020; Views: 3009; Downloads: 0
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6.
Role of Cu current collector on electrochemical mechanism of Mg–S battery
Ana Robba, Maja Mežnar, Alen Vižintin, Jan Bitenc, Jernej Bobnar, Iztok Arčon, Anna Randon-Vitanova, Robert Dominko, 2020, original scientific article

Abstract: Development of magnesium sulfur battery is accompanied with all known difficulties present in Li–S batteries, however with even more limited choice of electrolytes. In the present work, the influence of current collector on electrochemical mechanism was investigated in light of different reports where improved behavior was ascribed to electrolyte. Notable differences in cycling behavior are reported when Al current collector is replaced by Cu current collector independent of electrolyte. The initial reduction of sulfur follows similar reaction path no mater of current collector, but formation of MgS can be in competition with formation of CuS in the presence of Cu cations. With the subsequent cycling cells prepared from cathodes deposited on Cu current collector show decrease in the voltage and formation of single plateau during cycling. The change corresponds to the involvement of Cu into the reaction and formation of redox couple Mg/CuS as determined by Cu K-edge XANES measurements. Corrosion of Cu foil is identified by SEM and serves as a source of Cu cations for the chemical reaction between Cu and polysulfides. Mg/CuS redox couple shows improved cycling stability, but theoretical energy density is severely reduced due to substitution of S with CuS as cathode active material.
Keywords: Magnesium Sulfur Rechargeable batteries Current collector Copper Corrosion
Published in RUNG: 16.01.2020; Views: 3095; Downloads: 0
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7.
Sulfur based batteries studied by in-operando S K-edge RIXS and XAS spectroscopy
Matjaž Kavčič, Ana Robba, Janez Bitenc, Alen Vižintin, Iztok Arčon, Matjaž Žitnik, Klemen Bučar, Robert Dominko, 2018, published scientific conference contribution abstract

Abstract: Sulfur based batteries are considered as very attractive energy storage devices. Sulfur is one of the most abundant elements in the earth, it is electrochemically active material which can accept up to two electrons per atom. In combination with alkali metals, sulfur forms electrochemical couples with much higher theoretical energy density compared to Li-ion batteries commonly available today. At the moment, the electrochemical couple with Li is most extensively studied. While the main principle of operation is known the relevant operation mechanism(s) is not completely clear. Even more promising is the electrochemical couple with Mg providing almost twofold higher volumetric energy density due to its ability to provide two electrons during oxidation. However, Mg-S batteries are still in the very early stage of research and development and the complex mechanism of sulfur conversion has been less extensively studied. In order to improve the understanding of sulfur electrochemical conversion and its interactions within electrode, we need to apply new experimental approaches capable to provide precise information about local environment of S in the cathode during battery operation. In our work, resonant inelastic X-ray scattering (RIXS) and XAS measurements at the sulfur K-edge performed in operando mode were used to study the lithium-polysulfide formation during the discharge process. Measurements were performed at ID26 beamline of the ESRF synchrotron using tender X-ray emission spectrometer [1]. Resonant excitation condition enhanced the sensitivity for the lithium−polysulfide detection. On the other hand, the sulfate signal from the electrolyte was heavily suppressed and the self-absorption effects minimized due to fixed excitation energy. This experimental methodology was used to provide quantitative analysis of sulfur compounds in the cathode of a Li−S battery cell during the discharge process [2]. The high-voltage plateau in the discharge curve was characterized by a rapid conversion of solid sulfur into liquid phase Li polysulfides reaching its maximum at the end of this plateau. At this point the starting point for the precipitation of the Li2S from the liquid polysulfide phase was observed. The same approach has been used also for the Mg-S battery revealing similar mechanism as in case of Li-S battery [3]. The electrochemical conversion of sulfur with magnesium proceeds through two well-defined plateaus, which correspond to the equilibrium between sulfur and Mg polysulfides (high-voltage plateau) and polysulfides and MgS (low-voltage plateau).
Keywords: Mg-Sulphur batteries, XANES, RIXS
Published in RUNG: 13.09.2018; Views: 4578; Downloads: 0
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8.
9.
Polysulfides formation in different electrolytes from the perspective of X-ray absorption spectroscopy
Robert Dominko, Alen Vižintin, Giuliana Aquilanti, Lorenzo Stievano, Maria Joseph Helen, Anji Reddy Munnangi, Maximilian Fichtner, Iztok Arčon, 2018, original scientific article

Abstract: Li-S batteries are promising energy storage technology for the future, however there two major problems remained which need to be solved before successful commercialization. Capacity fading due to polysulfide shuttle and corrosion of lithium metal are directly connected with the type and quantity of electrolyte used in the cells. Several recent works show dependence of the electrochemical behavior of Li-S batteries on type of the electrolyte. In this work we compare and discuss a discharge mechanism of sulfur conversion in three different electrolytes based on measurements with sulfur K-edge XAS. The sulfur conversion mechanism in the ether based electrolytes, the most studied type of solvents in the Li-S batteries, which are enabling high solubility of polysulfides are compared with the fluorinated ether based electrolytes with a reduced polysulfide solubility and in carbonate based electrolytes with the sulfur confined into a ultramicroporous carbon. In all three cases the sulfur reduction proceeds through polysulfide intermediate phases with a difference on the type polysulfides detected at different steps of discharge.
Keywords: Li-S batteries, operando sulphur K-edge XANES, EXAFS, Li-polysulphides
Published in RUNG: 01.06.2018; Views: 3682; Downloads: 0
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10.
A Mechanistic Study of Magnesium Sulfur Batteries
Ana Robba, Alen Vižintin, Jan Bitenc, Gregor Mali, Iztok Arčon, Matjaž Kavčič, Matjaž Žitnik, Klemen Bučar, Giuliana Aquilanti, Charlotte Martineau-Corcos, Anna Randon-Vitanova, Robert Dominko, 2017, original scientific article

Abstract: Magnesium sulfur batteries are considered as attractive energy storage devices due to the abundance of electrochemically active materials and high theoretical energy density. Here we report the mechanism of a Mg-S battery operation, which was studied in the presence of simple and commercially available salts dissolved in a mixture of glymes. The electrolyte offers high sulfur conversion into MgS in the first discharge with low polarization. The electrochemical conversion of sulfur with magnesium proceeds through two well-defined plateaus, which correspond to the equilibrium between sulfur and polysulfides (high-voltage plateau) and polysulfides and MgS (low-voltage plateau). As shown by XANES, RIXS and NMR studies, the end discharge phase involves MgS with Mg atoms in a tetrahedral environment resembling the wurtzite structure, while chemically synthesized MgS crystalizes in the rock-salt structure with octahedral coordination of magnesium.
Keywords: magnesium, sulfur, rechargeable batteries, XAS, NMR
Published in RUNG: 19.10.2017; Views: 4503; Downloads: 0
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