How Bulk Sensitive is Hard X-ray Photoelectron Spectroscopy: Accounting for the Cathode-Electrolyte Interface when Addressing Oxygen Redox.

Sensitivity to the "bulk" oxygen core orbital makes hard X-ray photoelectron spectroscopy (HAXPES) an appealing technique for studying oxygen redox candidates. Various studies have reported an additional O 1s peak (530-531 eV) at high voltages, which has been considered a direct signature of the bulk oxygen redox process. Here, we find the emergence of a 530.4 eV O 1s HAXPES peak for three model cathodes-Li2MnO3, Li-rich NMC, and NMC 442-that shows no clear link to oxygen redox. Instead, the 530.4 eV peak for these three systems is attributed to transition metal reduction and electrolyte decomposition in the near-surface region. Claims of oxygen redox relying on photoelectron spectroscopy must explicitly account for the surface sensitivity of this technique and the extent of the cathode degradation layer

Evidence of a second-order Peierls-driven metal-insulator transition in crystalline NbO2

The metal-insulator transition of NbO2 is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed NbOx that may be inherently different to crystalline NbO2. We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial NbO2 thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at ~810⁰C. The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline NbO2 as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap

Untersuchung von strukturellen Veränderungen in Kathodenmaterialien für wiederaufladbare Lithium-Ionen-Batterien mittels X-ray Absorption Spektroskopie

Lokale strukturelle Änderungen in Li2MnO3, LiMn0.4Ni0.4Co0.2O2, 0.5Li2MnO3·0.5LiCoO2 und 0.5Li2MnO3·0.5LiMn0.4Ni0.4Co0.2O2 Kathodenmaterialien wurden mit Röntgenabsoptionsspektroskopie (XAS) untersucht. Qualitative Informationen über den mittleren Valenzzustand der absorbierenden Atome und Änderungen in der lokalen Symmetrie wurden aus den kantennahen Regionen (XANES) der Absorptionsspektren abgeschätzt, wohingegen geometrische Änderungen in der lokalen Struktur um die absorbierenden Atome durch Anpassung theoretischer Modelle an die kantenfernen Gebiete der Absorptionsspektren (EXAFS) quantifiziert wurden. Das Fitten der EXAFS-Daten liefert direkte Evidenz für die Ladungskompensation durch jedes der Übergangsmetall Ionen (TM) in dem Material, und den damit verbundenen strukturellen Änderungen. Die elektrochemischen Prozesse in Li2MnO3 sind komplex und ungewöhnlich. Sowohl der Sauerstoff-Verlust als auch der Ionen-Austausch-Mechanismus spielen eine wichtige Rolle beim Aktivieren der Kathode. Li-Extraktion geschieht gleichzeitig mit dem Entzug von Sauerstoff, was zur Bildung von Schichten vom Typ MnO2 führt, wohingegen die Anwesenheit von Protonen zwischen den Schichten als Ergebnis des Li+-H+ Austausches die Änderung der Stapelfolge der Sauerstoff-Schichten vom Typ O3 zum Typ P3 zur Folge hat. Der Wiedereinbau von Li in MnO2-Schichten kehrt die Stapelfolge von Sauerstoff um von P3-Typ zum O3-Typ, das die Bildung einer Struktur vom Li2MnO3-Typ mit Sauerstoff-Defizit zur Folge hat. Sauerstoff-Entzug geschieht nur während der Aktivierung des Materials und bei kleineren Raten. Beim nachfolgenden Zyklieren geschieht die Ladungskompensation nur durch Li+-H+ Austausch, mit dem strukturellenÜbergang zwischen P3-Typ und O3-Typ. Die wiederholten Änderungen in der Stapelfolge während jedes Zyklus sind verantwortlich für die strukturelle Degradierung und vermindern die elektrochemische Leistung beim Zyklieren. Die Li-Extraktion und der Wiedereinbau aus bzw. in LiMn0.4Ni0.4Co0.2O2 basiert auf konventionellen Redox-Reaktionen. Starkes Delithiieren führt jedoch zu einem irreversiblen Übergang von der O3-Struktur zur O1-Struktur. Es gibt keine Löslichkeit zwischen den beiden Komponenten der Komposit-Kathode 0.5Li2MnO3·0.5LiCoO2. Es gibt jedoch einige qualitative Hinweise die auf die Löslichkeit zwischen den beiden Komponenten der 0.5Li2MnO3·0.5LiMn0.4Ni0.4Co0.2O2 Kathode hinweisen. Jede der beiden Komponenten der Komposit-Kathode reagiert auf die elektrochemische Aktivierung auf ihre eigene Art und Weise. Die Li2MnO3 Komponente agiert als Quelle für zusätzliches Lithium oberhalb von 4.4 V und schützt die LiMO2 (M=Mn, Ni or Co) Komponente vor vollständigem Delithiieren und vor Verschlechterung der strukturellen Eigenschaften.Local structural changes in Li2MnO3, LiMn0.4Ni0.4Co0.2O2,0.5Li2MnO3·0.5LiCoO2 and 0.5Li2MnO3·0.5LiMn0.4Ni0.4Co0.2O2 cathode materials are investigated by X-ray absorption spectroscopy (XAS). Qualitative information about the average valence state of absorbing atoms and changes in their local coordination symmetry are estimated from the near-edge region (XANES) of the absorption spectra, while geometrical changes in the vicinity of absorbing atoms are quantified by fitting the extended region of the absorption spectra (EXAFS) with theoretical models. Fitting the EXAFS data provides a direct evidence for charge compensation by each of the transition metal (TM) ions present in the material and associated structural changes. The electrochemical processes in Li2MnO3 are complex and non-conventional. Both oxygen-loss and ion-exchange mechanisms play an important role during the activation of the cathode. Li extraction occurs with the concurrent removal of oxygen, giving rise to the formation of a layered MnO2-type structure, while the presence of protons in the interslab region, as a result of Li+-H+ exchange, alters the stacking sequence of oxygen layers from O3-type to P3-type. Li re-insertion into layered MnO2 reverts the oxygen stacking from P3-type back to O3-type, which gives rise to the formation of a Li2MnO3-type structure which is oxygen deficient. Oxygen removal occurs only during the activation of the material and at slower rates. Upon subsequent cycling, charge compensation occurs by the Li+-H+ exchange only, with the structural flip-over between P3-type and O3-type. The repetitive changes in the oxygen stacking sequence during each cycle are responsible for the structural degradation, and in turn fading electrochemical performance of Li2MnO3 upon cycling. Li extraction/re-insertion from/into LiMn0.4Ni0.4Co0.2O2 involves the conventional redox reactions. However, deep delithiation results in an irreversible transition of O3 structure to O1-type structure. There is no solubility between the two components of the composite 0.5Li2MnO3·0.5LiCoO2 cathode. However, there are qualitative indications suggesting some solubility between the two components of the 0.5Li2MnO3·0.5LiMn0.4Ni0.4Co0.2O2 cathode. Both components of the composite cathode respond to the electrochemical activation in their own unique ways. The Li2MnO3 component acts as a source of excess Li above 4.4 V and protects LiMO2 (M=Mn, Ni or Co) component from complete delithiation and deteriorating structural changes

Structural Changes in a Li-Rich 0.5Li 2 MnO 3 * 0.5LiMn 0.4 Ni 0.4 Co 0.2 O 2 Cathode Material for Li-Ion Batteries: A Local Perspective

Local structural changes in a Li-rich 0.5Li2MnO3*0.5LiMn0.4Ni0.4Co0.2O2 cathode material are investigated using X-ray absorption spectroscopy (XAS). The element-selective nature of XAS revealed the composite structure of the material, where both Li2MnO3 and LiMn0.4Ni0.4Co0.2O2 components exist as separate domains and also exhibit a distinct electrochemical response. An irreversible oxygen release from Li2MnO3 domains contributes to a large irreversible capacity delivered by the material during activation and gives rise to the formation of a layered MnO2-type structure. Lithium reinsertion into this layered MnO2-type structure during discharge reforms the original Li2MnO3-type structure, which is lithium and oxygen deficient. The average valence state of Mn in Li2MnO3 domains remains unchanged at 4+ during charge and discharge, suggesting an unusual participation of oxygen anions of Li2MnO3 domains in redox processes. On the contrary, electrochemical processes in LiMn0.4Ni0.4Co0.2O2 domains involve conventional redox processes of transition-metal (TM) ions. In addition to Ni2+/Ni4+ and Co3+/Co4+ redox reactions, a small amount of Mn3+ detected in LiMn0.4Ni0.4Co0.2O2 domains also participates in electrochemical processes via a Mn3+/Mn4+ redox reaction. All structural modifications introduced into the material during activation are recovered upon discharge to 2.5 V, except those caused by the permanent removal of oxygen from Li2MnO3 domains

An investigation of the electrochemical delithiation process of carbon coated α−Fe2O3α-Fe_{2}O_{3} nanoparticles

The electrochemical lithiation–delithiation of iron oxide is a rather complex process, which is still not fully understood. In this study we investigated the electrochemical lithiation–delithiation mechanism of hematite by means of X-ray diffraction (XRD), 57Fe Mössbauer spectroscopy, high-resolution transmission electron microscopy (HRTEM) and X-ray absorption spectroscopy (XAS). Since the delithiation process has been so far less investigated, particular attention was dedicated to the characterization of the chemical species that are formed during this process. The results of this investigation indicated that at the end of the delithiation process lithium iron oxide α-LiFeO2 is formed. The formation of this compound may be the explanation for the irreversible capacity loss in the first cycle, which is usually assigned to the formation of an organic gel-like layer. Based on these results a new charge–discharge mechanism of hematite in lithium-ion batteries (LIBs) is proposed and discussed

On the structural integrity and electrochemical activity of a 0.5Li(2)MnO(3)center dot 0.5LiCoO(2) cathode material for lithium-ion batteries

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Structural changes in a 0.5Li2MnO3·0.5LiCoO2cathode material were investigated by X-ray absorption spectroscopy. It is observed that both Li2MnO3and LiCoO2components of the material exist as separate domains, however, with some exchange of transition metal (TM) ions in their slab layers. A large irreversible capacity observed during activation of the material in the 1stcycle can be attributed to an irreversible oxygen release from Li2MnO3domains during lithium extraction. The average valence state of manganese ions remains unchanged at 4+ during charge and discharge. In the absence of conventional redox processes, lithium extraction/reinsertion from/into Li2MnO3domains occurs with the participation of oxygen anions in redox reactions and most likely involves the ion-exchange process. In contrast, lithium deintercalation/intercalation from/into LiCoO2domains occurs topotactically, involving a conventional Co3+/Co4+redox reaction. The presence of Li2MnO3domains and their unusual participation in electrochemical processes enable LiCoO2domains of the material to sustain a higher cut-off voltage without undergoing irreversible structural changes.EC/EFRE/200720132-35/EU//BATMA

Local structural changes in LiMn1.5Ni0.5O4LiMn_{1.5}Ni_{0.5}O_{4} spinel cathode material for lithium ion batteries

Local structural changes in LiMn1.5Ni0.5O4 cathode material were investigated by X-ray absorption spectroscopy in-operando using a specially designed electrochemical cell. The average structure of the starting material determined by neutron powder diffraction confirmed partial ordering of Mn and Ni cations on the octahedral sites in the spinel structure. It is observed that the electrochemical activity of the material between 3.5 V and 5.0 V is largely attributed to a two-step Ni2+/Ni4+ redox reaction. However, a small fraction of Mn3+ present in the pristine material also participates in electrochemical processes via a Mn3+/Mn4+ redox reaction. The excess lithium inserted into the material during deep discharge of the cell down to 2.0 V causes a further reduction of Mn4+ to Mn3+, while Ni remains electrochemically inactive. An increased proportion of Mn3+ in the material increases the distortion of MnO6 octahedra by the Jahn-Teller effect, which locally reduces the crystal symmetry from cubic to tetragonal, giving rise to the formation of domains of a Li2Mn2O4-type tetragonal phase. The fraction of this tetragonal phase was found to be directly related to the excess lithium inserted into the material. Upon subsequent charging to 2.9 V, the tetragonal phase tends to revert back to the original cubic spinel phase. The observed decline in the electrochemical performance of the material when cycled between 2.0 V and 5.0 V may be attributed to repetitive structural changes associated with the cubic ↔ tetragonal phase transition