Effects of non-stoichiometry on the ground state of the frustrated system Li0.8Ni0.6Sb0.4O2

The non-stoichiometric system Li0.8Ni0.6Sb0.4O2 is a Li-deficient derivative of the zigzag honeycomb antiferromagnet Li3Ni2SbO6. Structural and magnetic properties of Li0.8Ni0.6Sb0.4O2 were studied by means of X-ray diffraction, magnetic susceptibility, specific heat, and nuclear magnetic resonance measurements. Powder X-ray diffraction data shows the formation of a new phase, which is Sb-enriched and Li-deficient with respect to the structurally honeycomb-ordered Li3Ni2SbO6. This structural modification manifests in a drastic change of the magnetic properties in comparison to the stoichiometric partner. Bulk static (dc) magnetic susceptibility measurements show an overall antiferromagnetic interaction (Θ = -4 K) between Ni2+ spins (S = 1), while dynamic (ac) susceptibility reveals a transition into a spin glass state at a freezing temperature TSG ~ 8 K. These results were supported by the absence of the λ-anomaly in the specific heat Cp(T) down to 2 K. Moreover, combination of the bulk static susceptibility, heat capacity and 7Li NMR studies indicates a complicated temperature transformation of the magnetic system. We observe a development of a cluster spin glass, where the Ising-like Ni2+ magnetic moments demonstrate a 2D correlated slow short-range dynamics already at 12 K, whereas the formation of 3D short range static ordered clusters occurs far below the spin-glass freezing temperature at T ~ 4 K as it can be seen from the 7Li NMR spectrum

Laboratory Operando XAS Study of Sodium Iron Titanite Cathode in the Li-Ion Half-Cell

Electrochemical characterization of the novel sodium iron titanate Na0.9Fe0.45Ti1.55O4 was performed upon cycling in the Li-ion half-cell. The material exhibited stable cycling in the voltage range 2–4.5 V, and the number of alkali ions extracted per formula unit was approximately half of the Na stoichiometry value. Using laboratory X-ray absorption spectrometry, we measured operando Fe K-edge X-ray absorption spectra in the first 10 charge–discharge cycles and quantified the portion of charge associated with the transition metal redox reaction. Although 3d metals are commonly accepted redox-active centers in the intercalation process, we found that in all cycles the amount of oxidized and reduced Fe ions was almost 20% less than the total number of transferred electrons. Using density functional theory (DFT) simulations, we show that part of the reversible capacity is related to the redox reaction on oxygen ions

Laboratory Operando XAS Study of Sodium Iron Titanite Cathode in the Li-Ion Half-Cell

Electrochemical characterization of the novel sodium iron titanate Na0.9Fe0.45Ti1.55O4 was performed upon cycling in the Li-ion half-cell. The material exhibited stable cycling in the voltage range 2–4.5 V, and the number of alkali ions extracted per formula unit was approximately half of the Na stoichiometry value. Using laboratory X-ray absorption spectrometry, we measured operando Fe K-edge X-ray absorption spectra in the first 10 charge–discharge cycles and quantified the portion of charge associated with the transition metal redox reaction. Although 3d metals are commonly accepted redox-active centers in the intercalation process, we found that in all cycles the amount of oxidized and reduced Fe ions was almost 20% less than the total number of transferred electrons. Using density functional theory (DFT) simulations, we show that part of the reversible capacity is related to the redox reaction on oxygen ions

Laboratory X-ray Microscopy Study of Microcrack Evolution in a Novel Sodium Iron Titanate-Based Cathode Material for Li-Ion Batteries

The long-term performance of batteries depends strongly on the 3D morphology of electrode materials. Morphological changes, i.e., particle fracture and surface deterioration, are among the most prominent sources of electrode degradation. A profound understanding of the fracture mechanics of electrode materials in micro- and nanoscale dimensions requires the use of advanced in situ and operando techniques. In this paper, we demonstrate the capabilities of laboratory X-ray microscopy and nano X-ray computed tomography (nano-XCT) for the non-destructive study of the electrode material’s 3D morphology and defects, such as microcracks, at sub-micron resolution. We investigate the morphology of Na0.9Fe0.45Ti1.55O4 sodium iron titanate (NFTO) cathode material in Li-ion batteries using laboratory-based in situ and operando X-ray microscopy. The impact of the morphology on the degradation of battery materials, particularly the size- and density-dependence of the fracture behavior of the particles, is revealed based on a semi-quantitative analysis of the formation and propagation of microcracks in particles. Finally, we discuss design concepts of the operando cells for the study of electrochemical processes

Laboratory X-ray Microscopy Study of Microcrack Evolution in a Novel Sodium Iron Titanate-Based Cathode Material for Li-Ion Batteries

The long-term performance of batteries depends strongly on the 3D morphology of electrode materials. Morphological changes, i.e., particle fracture and surface deterioration, are among the most prominent sources of electrode degradation. A profound understanding of the fracture mechanics of electrode materials in micro- and nanoscale dimensions requires the use of advanced in situ and operando techniques. In this paper, we demonstrate the capabilities of laboratory X-ray microscopy and nano X-ray computed tomography (nano-XCT) for the non-destructive study of the electrode material’s 3D morphology and defects, such as microcracks, at sub-micron resolution. We investigate the morphology of Na0.9Fe0.45Ti1.55O4 sodium iron titanate (NFTO) cathode material in Li-ion batteries using laboratory-based in situ and operando X-ray microscopy. The impact of the morphology on the degradation of battery materials, particularly the size- and density-dependence of the fracture behavior of the particles, is revealed based on a semi-quantitative analysis of the formation and propagation of microcracks in particles. Finally, we discuss design concepts of the operando cells for the study of electrochemical processes

Effects of Non-Stoichiometry on the Ground State of the Frustrated System Li0.8Ni0.6Sb0.4O2

The non-stoichiometric system Li0.8Ni0.6Sb0.4O2 is a Li-deficient derivative of the zigzag honeycomb antiferromagnet Li3Ni2SbO6. Structural and magnetic properties of Li0.8Ni0.6Sb0.4O2 were studied by means of X-ray diffraction, magnetic susceptibility, specific heat, and nuclear magnetic resonance measurements. Powder X-ray diffraction data shows the formation of a new phase, which is Sb-enriched and Li-deficient with respect to the structurally honeycomb-ordered Li3Ni2SbO6. This structural modification manifests in a drastic change of the magnetic properties in comparison to the stoichiometric partner. Bulk static (dc) magnetic susceptibility measurements show an overall antiferromagnetic interaction (Θ = −4 K) between Ni2+ spins (S = 1), while dynamic (ac) susceptibility reveals a transition into a spin glass state at a freezing temperature TSG ~ 8 K. These results were supported by the absence of the λ-anomaly in the specific heat Cp(T) down to 2 K. Moreover, combination of the bulk static susceptibility, heat capacity and 7Li NMR studies indicates a complicated temperature transformation of the magnetic system. We observe a development of a cluster spin glass, where the Ising-like Ni2+ magnetic moments demonstrate a 2D correlated slow short-range dynamics already at 12 K, whereas the formation of 3D short range static ordered clusters occurs far below the spin-glass freezing temperature at T ~ 4 K as it can be seen from the 7Li NMR spectrum

Synthesis and Characterization of MnCrO₄, a New Mixed-Valence Antiferromagnet

A new orthorhombic phase, MnCrO₄, isostructural with MCrO₄ (M = Mg, Co, Ni, Cu, Cd) was prepared by evaporation of an aqueous solution, (NH₄)₂Cr₂O₇ + 2 Mn­(NO₃)₂, followed by calcination at 400 °C. It is characterized by redox titration, Rietveld analysis of the X-ray diffraction pattern, Cr K edge and Mn K edge XANES, ESR, magnetic susceptibility, specific heat and resistivity measurements. In contrast to the high-pressure MnCrO₄ phase where both cations are octahedral, the new phase contains Cr in a tetrahedral environment suggesting the charge balance Mn²⁺Cr⁶⁺O₄. However, the positions of both X-ray absorption K edges, the bond lengths and the ESR data suggest the occurrence of some mixed-valence character in which the mean oxidation state of Mn is higher than 2 and that of Cr is lower than 6. Both the magnetic susceptibility and the specific heat data indicate an onset of a three-dimensional antiferromagnetic order at <i>T</i>_N ≈ 42 K, which was confirmed also by calculating the spin exchange interactions on the basis of first principles density functional calculations. Dynamic magnetic studies (ESR) corroborate this scenario and indicate appreciable short-range correlations at temperatures far above <i>T</i>_N. MnCrO₄ is a semiconductor with activation energy of 0.27 eV; it loses oxygen on heating above 400 °C to form first Cr₂O₃ plus Mn₃O₄ and then Mn_1.5Cr_1.5O₄ spinel

Synthesis and Characterization of MnCrO₄, a New Mixed-Valence Antiferromagnet

A new orthorhombic phase, MnCrO₄, isostructural with MCrO₄ (M = Mg, Co, Ni, Cu, Cd) was prepared by evaporation of an aqueous solution, (NH₄)₂Cr₂O₇ + 2 Mn­(NO₃)₂, followed by calcination at 400 °C. It is characterized by redox titration, Rietveld analysis of the X-ray diffraction pattern, Cr K edge and Mn K edge XANES, ESR, magnetic susceptibility, specific heat and resistivity measurements. In contrast to the high-pressure MnCrO₄ phase where both cations are octahedral, the new phase contains Cr in a tetrahedral environment suggesting the charge balance Mn²⁺Cr⁶⁺O₄. However, the positions of both X-ray absorption K edges, the bond lengths and the ESR data suggest the occurrence of some mixed-valence character in which the mean oxidation state of Mn is higher than 2 and that of Cr is lower than 6. Both the magnetic susceptibility and the specific heat data indicate an onset of a three-dimensional antiferromagnetic order at <i>T</i>_N ≈ 42 K, which was confirmed also by calculating the spin exchange interactions on the basis of first principles density functional calculations. Dynamic magnetic studies (ESR) corroborate this scenario and indicate appreciable short-range correlations at temperatures far above <i>T</i>_N. MnCrO₄ is a semiconductor with activation energy of 0.27 eV; it loses oxygen on heating above 400 °C to form first Cr₂O₃ plus Mn₃O₄ and then Mn_1.5Cr_1.5O₄ spinel

New Phase of MnSb₂O₆ Prepared by Ion Exchange: Structural, Magnetic, and Thermodynamic Properties

A new layered trigonal (<i>P</i>3̅1<i>m</i>) form of MnSb₂O₆, isostructural with MSb₂O₆ (M = Cd, Ca, Sr, Pb, and Ba) and MAs₂O₆ (M = Mn, Co, Ni, and Pd), was prepared by ion-exchange reaction between ilmenite-type NaSbO₃ and MnSO₄–KCl–KBr melt at 470 °C. It is characterized by Rietveld analysis of the X-ray diffraction pattern, electron microprobe analysis, magnetic susceptibility, specific heat, and ESR measurements as well as by density functional theory calculations. MnSb₂O₆ is very similar to MnAs₂O₆ in the temperature dependence of their magnetic susceptibility and spin exchange interactions. The magnetic susceptibility and specific heat data show that MnSb₂O₆ undergoes a long-range antiferromagnetic order with Néel temperature <i>T</i>_N = 8.5(5) K. In addition, a weak ferromagnetic component appears below <i>T</i>₁ = 41.5(5) K. DFT+U implies that the main spin exchange interactions are antiferromagnetic, thereby forming spin-frustrated triangles. The long-range ordered magnetic structure of MnSb₂O₆ is predicted to be incommensurate as found for MnAs₂O₆. On heating, the new phase transforms to the stable <i>P</i>321 form via its intermediate disordered variant