Marco Scheuermann

Electrochemical insertion of lithium in mechanochemically synthesized Zn2SnO4

We studied the electrochemical insertion of Li in mechanochemically prepared Zn(2)SnO(4). The mechanism of the electrochemical reaction was investigated by using X-ray diffraction, nuclear magnetic resonance spectroscopy, and Mössbauer spectroscopy. Changes in the morphology of the Zn(2)SnO(4) particles were studied by in situ scanning electron microscopy. The results were compared with mixtures of SnO(2) + ZnO and with Zn(2)SnO(4) prepared by conventional solid-state synthesis and showed that the mechanochemically prepared Zn(2)SnO(4) exhibits the best cyclic stability of these samples.

Electrochemical insertion of Li into nanocrystalline MnFe2O4: a study of the reaction mechanism

The study of the mechanism of Li insertion into nanosized partially inverse spinel MnFe2O4 applying X-ray diffraction, in situ quick X-ray absorption spectroscopy, Mossbauer spectroscopy, high resolution transmission electron microscopy, 7Li MAS NMR, and electrochemical measurements yields a comprehensive picture of the individual steps occurring during Li uptake. At the very early beginning of the reaction Fe3+ on the tetrahedral site is reduced and moves to empty octahedral sites. Increasing the amount of Li to 0.7 per MnFe2O4, further Fe3+ is reduced and Mn2+ residing on the tetrahedral site moves to empty octahedral sites thus forming a defect NaCl-type structure. At least for 2 Li per MnFe2O4 reflections of the spinel disappeared in the X-ray powder pattern and only those of a monoxide are observed. No indications were found for a phase separation and Fe and Mn are homogeneously distributed over the sample. Further Li uptake leads to a stepwise conversion of the material and after insertion of 8 Li/MnFe2O4 only nanosized Mn, Fe, and Li2O are detected. After a capacity loss at the beginning of Li insertion, a constant capacity of about 266 mA h g−1 is reached after 100 cycles discharging–charging the material.

Structural and morphological study of mechanochemically synthesized tin diselenide

Mechanochemical synthesis of tin diselenide, SnSe2, was performed by high-energy milling of tin and selenium powder in a planetary ball mill. The mechanosynthesized product was characterized by X-ray diffraction, 119Sn MAS NMR and 119Sn Mossbauer spectroscopy, which confirmed the presence of the hexagonal SnSe2 phase after 100 min of milling. The size and morphology of tin diselenide particles were studied by specific surface area measurements, and transmission electron microscopy. The specific surface area of powders was found to increase with increasing time of mechanochemical synthesis. Electron diffraction revealed reflections that correspond to hexagonal SnSe2 modification. TEM observations show that the mechanochemical preparation route results in the formation of nanosized thick barrel-shaped SnSe2 platelets. SnSe2 nanoparticles show good absorption in the visible region of the UV-Vis optical spectrum and they evidence direct and indirect types of transitions in the lattice.

Study of local structure and Li dynamics in Li4+xTi5O12 (0≤x≤5) using 6Li and 7Li NMR spectroscopy

We studied the local structure and the Li ion dynamics in electrochemically and chemically prepared Li(4+x)Ti(5)O(12) with x = 0…5. We used magic-angle spinning (7)Li NMR on samples with different Li contents to investigate the sites that are occupied/emptied during Li insertion/removal. While the electrochemical measurements show a lithium insertion in two steps, 1D MAS NMR as a function of the lithium content shows that the overall spectral evolution observed during lithium insertion is inverted during lithium removal. Thereby the second insertion step is associated with an increased structural disorder. For samples with x = 0, 2, 3, and about 5, we performed temperature-dependent measurements of the (7)Li NMR relaxation rates T(1)(-1), T(2)(-1), and T(1ρ)(-1) to study the dynamics of the Li ions. For the samples with x = 0, 2, and 3, activation energies of (0.45 ± 0.1)eV were obtained. The highest mobility of the Li ions is observed for the samples with x = 2 and 3. Results from (6)Li and (7)Li 2D exchange MAS NMR spectroscopy on samples with x = 2 and 4 show that magnetization transfer for (7)Li below 323K is dominated by spin diffusion.

Is there a universal reaction mechanism of Li insertion into oxidic spinels: a case study using MgFe2O4

Structural and electronic changes during Li insertion into spinel-type MgFe2O4 particles with different sizes were studied applying a multi-method approach yielding a detailed picture about distinct reaction steps during Li uptake. A small amount of Li is intercalated into the smaller particles (8 nm) at the beginning of the reaction while no such reaction step occurs for the large crystallites (ca. 100 nm). Li uptake is accompanied by a reduction of Fe3+ ions and simultaneous movement from the tetrahedral to an empty octahedral site. After uptake of 2 Li per formula unit all ions have moved from tetrahedral to free octahedral voids resulting in the formation of a disordered NaCl-type material. Insertion of 4 further Li atoms transforms the crystalline material to an amorphous and inhomogeneous product consisting of a Li2O matrix with embedded nanosized metallic Fe and MgO particles. During the charge process Fe is oxidized to FeO and Li2O is converted to Li.

Elucidation of the Conversion Reaction of CoMnFeO4 Nanoparticles in Lithium Ion Battery Anode via Operando Studies

Conversion reactions deliver much higher capacities than intercalation/deintercalation reactions of commercial Li ion batteries. However, the complex reaction pathways of conversion reactions occurring during Li uptake and release are not entirely understood, especially the irreversible capacity loss of Mn(III)-containing oxidic spinels. Here, we report for the first time on the electrochemical Li uptake and release of Co(II)Mn(III)Fe(III)O4 spinel nanoparticles and the conversion reaction mechanisms elucidated by combined operando X-ray diffraction, operando and ex-situ X-ray absorption spectroscopy, transmission electron microscopy, (7)Li NMR, and molecular dynamics simulation. The combination of these techniques enabled uncovering the pronounced electronic changes and structural alterations on different length scales in a unique way. The spinel nanoparticles undergo a successive phase transition into a mixed monoxide caused by a movement of the reduced cations from tetrahedral to octahedral positions. While the redox reactions Fe(3+) ↔ Fe(0) and Co(2+) ↔ Co(0) occur for many charge/discharge cycles, metallic Mn nanoparticles formed during the first discharge can only be oxidized to Mn(2+) during charge. This finding explains the partial capacity loss reported for Mn(III)-based spinels. Furthermore, the results of the investigations evidence that the reaction mechanisms on the nanoscale are very different from pathways of microcrystalline materials.

Study of local structure and Li dynamics in () using 6Li and 7Li NMR spectroscopy

We studied the local structure and the Li ion dynamics in electrochemically and chemically prepared Li(4+x)Ti(5)O(12) with x = 0…5. We used magic-angle spinning (7)Li NMR on samples with different Li contents to investigate the sites that are occupied/emptied during Li insertion/removal. While the electrochemical measurements show a lithium insertion in two steps, 1D MAS NMR as a function of the lithium content shows that the overall spectral evolution observed during lithium insertion is inverted during lithium removal. Thereby the second insertion step is associated with an increased structural disorder. For samples with x = 0, 2, 3, and about 5, we performed temperature-dependent measurements of the (7)Li NMR relaxation rates T(1)(-1), T(2)(-1), and T(1ρ)(-1) to study the dynamics of the Li ions. For the samples with x = 0, 2, and 3, activation energies of (0.45 ± 0.1)eV were obtained. The highest mobility of the Li ions is observed for the samples with x = 2 and 3. Results from (6)Li and (7)Li 2D exchange MAS NMR spectroscopy on samples with x = 2 and 4 show that magnetization transfer for (7)Li below 323K is dominated by spin diffusion.

Study of local structure and Li dynamics in Li4+xTi5O12Li4+xTi5O12 (0≤x≤50≤x≤5) using 6Li and 7Li NMR spectroscopy

We studied the local structure and the Li ion dynamics in electrochemically and chemically prepared Li(4+x)Ti(5)O(12) with x = 0…5. We used magic-angle spinning (7)Li NMR on samples with different Li contents to investigate the sites that are occupied/emptied during Li insertion/removal. While the electrochemical measurements show a lithium insertion in two steps, 1D MAS NMR as a function of the lithium content shows that the overall spectral evolution observed during lithium insertion is inverted during lithium removal. Thereby the second insertion step is associated with an increased structural disorder. For samples with x = 0, 2, 3, and about 5, we performed temperature-dependent measurements of the (7)Li NMR relaxation rates T(1)(-1), T(2)(-1), and T(1ρ)(-1) to study the dynamics of the Li ions. For the samples with x = 0, 2, and 3, activation energies of (0.45 ± 0.1)eV were obtained. The highest mobility of the Li ions is observed for the samples with x = 2 and 3. Results from (6)Li and (7)Li 2D exchange MAS NMR spectroscopy on samples with x = 2 and 4 show that magnetization transfer for (7)Li below 323K is dominated by spin diffusion.