Heterogeneous Photocatalysis and Sensitized Photolysis for Enhanced Degradation of Bisphenol A and its Analogues

This work deals with elucidation of bisphenols transformations occurring in the processes of TiO2 photocatalysis and sensitized photolysis. A special attention is paid to the clarification of the mechanisms of the reactive oxygen species generation. In particular, the work investigates the sources and mobility of photocatalytically generated OH radicals by isotopic labeling experiments and explores the role of natural organic matter as a sensitizer in the sunlight photolysis of bisphenols

Charge Transfer-Induced Lattice Collapse in Ni-Rich NCM Cathode Materials during Delithiation

Ni-rich LiNixCoyMnzO2 (NCM) cathode materials have great potential for application in next-generation lithium-ion batteries owing to their high specific capacity. However, they are subjected to severe structural changes upon (de)lithiation, which adversely affects the cycling stability. Herein, we investigate changes in crystal and electronic structure of NCM811 (80% Ni) at high states of charge by a combination of operando X-ray diffraction (XRD), operando hard X-ray absorption spectroscopy (hXAS), ex situ soft X-ray absorption spectroscopy (sXAS), and density functional theory (DFT) calculations, and correlate the results with data from galvanostatic cycling in coin cells. XRD reveals a large decrease in unit cell volume from 101.38(1) Å3 to 94.26(2) Å3 due to collapse of the interlayer spacing when x(Li) < 0.5 (decrease in c-axis from 14.469(1) Å at x(Li) = 0.6 to 13.732(2) Å at x(Li) = 0.25). hXAS shows that the shrinkage of the transition metal-oxygen layer mainly originates from nickel oxidation. sXAS, together with DFT-based Bader charge analysis, indicates that the shrinkage of the interlayer, which is occupied by lithium, is induced by charge transfer between O 2p and partially filled Ni eg orbitals (resulting in decrease of oxygen-oxygen repulsion). Overall, the results demonstrate that high-voltage operation of NCM811 cathodes is inevitably accompanied by charge transfer-induced lattice collapse

Between Scylla and Charybdis: Balancing Among Structural Stability and Energy Density of Layered NCM Cathode Materials for Advanced Lithium-Ion Batteries

Two major strategies are currently pursued to improve the energy density of lithium-ion batteries using LiNi_<i>x</i>Co_<i>y</i>Mn_<i>z</i>O₂ (NCM) cathode materials. One is to increase the fraction of redox active Ni (≥80%), which allows larger amounts of Li to be extracted at a given cutoff voltage (<i>U</i>_max). The other is to increase <i>U</i>_max, in particular for medium-Ni content NCM materials. However, the accompanying lattice changes ultimately lead to capacity fading in both cases. Here the structural changes occurring in Li_1.02Ni_<i>x</i>Co_<i>y</i>Mn_<i>z</i>O₂ (with <i>x</i> = ¹/₃, 0.5, 0.6, 0.7, 0.8 and 0.85) during cycling operation in the voltage range between 3.0 and 4.6 V vs Li are quantified by means of <i>operando</i> X-ray diffraction combined with detailed Rietveld analysis. All samples show a large decrease in unit cell volume upon charging, ranging from 2.4% for NCM111 (33% Ni) to 8.0% for NCM851005 (85% Ni). To make a fair comparison of the structural stability of the different NCM materials, energy densities as a function of <i>U</i>_max are estimated and correlated with X-ray diffraction results. It is shown that NCMs with a lower Ni content allow for specific energies similar to that of, e.g., Ni-rich NCM811 (80% Ni) when operated at sufficiently high <i>U</i>_max, but still undergo less pronounced changes in structure. Nevertheless, as indicated by charge/discharge tests, the capacity retention of low- and medium-Ni content NCMs cycled to high <i>U</i>_max is also strongly affected by factors other than stability of the layered crystal lattice (electrolyte decomposition etc.). Overall, it is demonstrated that the complexity of the degradation processes needs to be better understood to identify optimal cycling conditions for specific cathode compositions

Anisotropic Lattice Strain and Mechanical Degradation of High- and Low-Nickel NCM Cathode Materials for Li-Ion Batteries

In the near future, the targets for lithium-ion batteries concerning specific energy and cost can advantageously be met by introducing layered LiNi_<i>x</i>Co_<i>y</i>Mn_<i>z</i>O₂ (NCM) cathode materials with a high Ni content (<i>x</i> ≥ 0.6). Increasing the Ni content allows for the utilization of more lithium at a given cell voltage, thereby improving the specific capacity but at the expense of cycle life. Here, the capacity-fading mechanisms of both typical low-Ni NCM (<i>x</i> = 0.33, NCM111) and high-Ni NCM (<i>x</i> = 0.8, NCM811) cathodes are investigated and compared from crystallographic and microstructural viewpoints. In situ X-ray diffraction reveals that the unit cells undergo different volumetric changes of around 1.2 and 5.1% for NCM111 and NCM811, respectively, when cycled between 3.0 and 4.3 V vs Li/Li⁺. Volume changes for NCM811 are largest for <i>x</i>(Li) < 0.5 because of the severe decrease in interlayer lattice parameter <i>c</i> from 14.467(1) to 14.030(1) Å. In agreement, in situ light microscopy reveals that delithiation leads to different volume contractions of the secondary particles of (3.3 ± 2.4) and (7.8 ± 1.5)% for NCM111 and NCM811, respectively. And postmortem cross-sectional scanning electron microscopy analysis indicates more significant microcracking in the case of NCM811. Overall, the results establish that the accelerated aging of NCM811 is related to the disintegration of secondary particles caused by intergranular fracture, which is driven by mechanical stress at the interfaces between the primary crystallites