Search

Physical review / B 104(18), 184113 (2021). doi:10.1103/PhysRevB.104.184113, The century-old Vegard���s law has been remarkably accurate in describing the evolution of the lattice parameters of almost all solid solutions. Contractions or expansions of lattice parameters of such systems depend on the size of the guest atom being smaller or larger than the host atom it replaces to form the solid solution. This has given rise to the concept of ���chemical pressure��� in analogy to the physical pressure. We have investigated using EXAFS the evolution of the local structure in terms of atom-pair distances extending up to the third-nearest neighbors in the family of compounds, ZnSe$_x$S$_{1���x}$ as an example of an anionic solid solution, in contrast to all previous studies focusing on cationic solid solutions. Our results establish several common features between these two types of solid solutions, while strongly suggesting that the concept of a chemical pressure is inaccurate and misleading. Most interestingly, we also find a qualitative difference between the cationic solid solutions, reported earlier, and the anionic solid solution., Published by Inst., Woodbury, NY

Physical review / B 102(9), 094428 (1-6) (2020). doi:10.1103/PhysRevB.102.094428, [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], a two dimensional misfit metallic compound, is famous for its rich phases accessed by temperature, i.e., high temperature spin-state transition, metal-insulator transition (MIT) at intermediate temperature (∼100K), and low temperature spin density wave (SDW). It enters into a SDW phase below $T_{MIT}$ which becomes long range at 27 K. Information on the independent role of misfit layers (rocksalt/Ca$_2$CoO$_3$ and triangular/CoO$_2$) in these phases is scarce. By combining a set of complementary macroscopic (DC magnetization and resistivity) and microscopic (neutron diffraction and x-ray absorption fine structure spectroscopy) measurements on pure (CCO) and Tb substituted in the rocksalt layer of CCO (CCO1), magnetic correlations in both subsystems of this misfit compound are unraveled. CCO is found to exhibit glassiness, as well as exchange bias (EB) effects, while CCO1 does not exhibit glassiness, albeit it shows weaker EB effect. By combining local structure investigations from extended x-ray absorption fine structure (EXAFS) spectroscopy and neutron diffraction results on CCO, we confirm that the SDW arises in the CoO$_2$ layer. Our results show that the magnetocrystalline anisotropy associated with the rocksalt layer acts as a source of pinning, which is responsible for EB effect. Ferromagnetic clusters in the Ca$_2$CoO$_3$ layer affects the SDW in CoO$_2$ and ultimately glassiness arises., Published by Inst., Woodbury, NY

In this work, the effect of Li+ substitution in Li3V2(PO4)3 with a large divalent ion (Ca2+) toward lithium insertion is studied. A series of materials, with formula Li3−2xCaxV2(PO4)3/C (x = 0, 0.5, 1, and 1.5) is synthesized and studied in the potential region 3–0.01 V versus Li+/Li. Synchrotron diffraction demonstrates that Li3V2(PO4)3/C has a monoclinic structure (space group P21/n), while Ca1.5V2(PO4)3/C possesses a rhombohedral structure (space group R-3c). The intermediate compounds, Li2Ca0.5V2(PO4)3/C and LiCaV2(PO4)3/C, are composed of two main phases, including monoclinic Li3V2(PO4)3/C and rhombohedral Ca1.5V2(PO4)3/C. Cyclic voltammetry reveals five reduction and oxidation peaks on Li3V2(PO4)3/C and Li2Ca0.5V2(PO4)3/C electrodes. In contrast, LiCaV2(PO4)3/C and Ca1.5V2(PO4)3/C have no obvious oxidation and reduction peaks but a box-type voltammogram. This feature is the signature for capacitive-like mechanism, which involves fast electron transfer on the surface of the electrode. Li3V2(PO4)3/C undergoes two solid-solution and a short two-phase reaction during lithiation and delithiation processes, whereas Ca1.5V2(PO4)3/C only goes through capacitive-like mechanism. In operando X-ray absorption spectroscopy confirms that, in both Li3V2(PO4)3/C and Ca1.5V2(PO4)3/C, V ions are reduced during the insertion of the first three Li ions. This study demonstrates that the electrochemical characteristic of polyanionic phosphates can be easily tuned by replacing Li+ with larger divalent cations., publishedVersion

Advanced energy materials 9(33), 1901864 (2019). doi:10.1002/aenm.201901864, In this work, the effect of Li$^+$ substitution in Li$_3$V$_2$(PO$_4$)$_3$ with a large divalent ion (Ca$^{2+}$) toward lithium insertion is studied. A series of materials, with formula Li$_{3−2x}$Ca$_x$V$_2$(PO$_4$)$_3$/C (x = 0, 0.5, 1, and 1.5) is synthesized and studied in the potential region 3–0.01 V versus Li$^+$/Li. Synchrotron diffraction demonstrates that Li$_3$V$_2$(PO$_4$)$_3$/C has a monoclinic structure (space group P2$_1$/n), while Ca$_{1.5}$V$_2$(PO$_4$)$_3$/C possesses a rhombohedral structure (space group R‐3c). The intermediate compounds, Li$_2$Ca$_{0.5}$V$_2$(PO$_4$)$_3$/C and LiCaV$_2$(PO$_4$)$_3$/C, are composed of two main phases, including monoclinic Li$_3$V$_2$(PO$_4$)$_3$/C and rhombohedral Ca$_{1.5}$V$_2$(PO$_4$)$_3$/C. Cyclic voltammetry reveals five reduction and oxidation peaks on Li$_3$V$_2$(PO$_4$)$_3$/C and Li$_2$Ca$_{0.5}$V$_2$(PO$_4$)$_3$/C electrodes. In contrast, LiCaV$_2$(PO$_4$)$_3$/C and Ca$_{1.5}$V$_2$(PO$_4$)$_3$/C have no obvious oxidation and reduction peaks but a box‐type voltammogram. This feature is the signature for capacitive‐like mechanism, which involves fast electron transfer on the surface of the electrode. Li$_3$V$_2$(PO$_4$)$_3$/C undergoes two solid‐solution and a short two‐phase reaction during lithiation and delithiation processes, whereas Ca$_{1.5}$V$_2$(PO$_4$)$_3$/C only goes through capacitive‐like mechanism. In operando X‐ray absorption spectroscopy confirms that, in both Li$_3$V$_2$(PO$_4$)$_3$/C and Ca$_{1.5}$V$_2$(PO$_4$)$_3$/C, V ions are reduced during the insertion of the first three Li ions. This study demonstrates that the electrochemical characteristic of polyanionic phosphates can be easily tuned by replacing Li$^+$ with larger divalent cations., Published by Wiley-VCH, Weinheim

Inorganic chemistry frontiers 6(7), 1861 - 1872 (2019). doi:10.1039/C9QI00356H, As a conversion-type electrode material, ZnCo2O4 (ZCO) is intensively researched due to its attractive high specific capacity. Much effort to study ZCO supported on a conductive matrix has been successful to overcome the inherent drawbacks of low conductivity and dramatic volume variation during the (de)lithiation process. Despite many reported studies, the lithiation storage mechanism in the ZCO electrode is not yet clearly elucidated. In this work, in operando synchrotron radiation diffraction and in operando X-ray absorption spectroscopy are used to study the lithium storage mechanism in the ZCO material. The initial conversion process of ZnCo2O4, involving multiple reactions based on intercalation, conversion and alloying is deeply elucidated. During the 1st lithiation intermediate phases such as LiCo2O3, CoO and ZnO are formed. On the other hand, upon delithiation, the conversion to ZnO and CoO (and not to the pristine ZnCo2O4) occurs. This is different from the previous conclusion, which claims that Co3O4 forms after the initial delithiation. Furthermore, a binder-free ZnCo2O4/carbon cloth composite electrode is also prepared, which exhibits higher rate performance and capacity retention, compared to the bare ZCO electrode, Published by RSC, Cambridge

Journal of the American Chemical Society 141(6), 2305 - 2315 (2019). doi:10.1021/jacs.8b08998, Orthorhombic $\mathrm{V_{2}O_{5}}$ nanowires were successfully synthesized via a hydrothermal method. A cell-configuration system was built utilizing $\mathrm{V_{2}O_{5}}$ as the cathode and 1 M Mg(ClO$_4$)$_2$ electrolyte within acetonitrile, together with Mg$_x$Mo$_6$S$_8$ (x ≈ 2) as the anode to investigate the structural evolution and oxidation state and local structural changes of $\mathrm{V_{2}O_{5}}$. The $\mathrm{V_{2}O_{5}}$ nanowires deliver an initial discharge/charge capacity of 103 mAh g$^{–1}$/110 mAh g$^{–1}$ and the highest discharge capacity of 130 mAh g$^{–1}$ in the sixth cycle at C/20 rate in the cell-configuration system. In operando synchrotron diffraction and in operando X-ray absorption spectroscopy together with ex situ Raman and X-ray photoelectron spectroscopy reveal the reversibility of magnesium insertion/extraction and provide information on the crystal structure evolution and changes of the oxidation states during cycling., Published by American Chemical Society, Washington, DC

ACS applied materials & interfaces 11(33), 29888 - 29900 (2019). doi:10.1021/acsami.9b08539, The carbon coating strategy is intensively used in the modification of conversion-type anode materials to improve their cycling stability and rate capability. Thus, it is necessary to elucidate the modification mechanism induced by carbon coating. For this purpose, bare ZnMn2O4, carbon-derivative-coated ZnMn2O4, and carbon-coated ZnO–MnO composite materials have been synthesized and investigated in-depth. Herein, high-temperature synchrotron radiation diffraction is used to monitor the phase transition from ZnMn2O4 to ZnO–MnO composite during the carbonization process. The electrochemical performance has been evaluated by cyclic voltammetry, galvanostatic cycling, and electrochemical impedance spectroscopy. The carbon- and carbon-derivative-coated samples display well-improved cycling stability in terms of suppressed electrode polarization, a moderate increase in resistance, and slight capacity variation. The influence of carbon coating on the intrinsic conversion process is investigated by ex situ X-ray absorption spectroscopy, which reveals the evolution of Zn and Mn oxidation states. This result confirms that the strong capacity variation of the bare ZnMn2O4 is induced not only by the reversible charge storage in the solid electrolyte interphase but also by the phase evolution of active materials. Carbon coating is an effective method to prevent the additional oxidation of MnO to Mn3O4, which leads to a stabilization of the main conversion reaction., Published by Soc., Washington, DC

Physical chemistry, chemical physics 19(47), 32105 - 32115 (2017). doi:10.1039/C7CP06826C, The 6Mg(NH$_2$)$_2$–9LiH–LiBH$_4$ composite system has a maximum reversible hydrogen content of 4.2 wt% and a predicted dehydrogenation temperature of about 64 °C at 1 bar of H$_2$. However, the existence of severe kinetic barriers precludes the occurrence of de/re-hydrogenation processes at such a low temperature (H. Cao, G. Wu, Y. Zhang, Z. Xiong, J. Qiu and P. Chen, J. Mater. Chem. A, 2014, 2, 15816–15822). In this work, Li$_3$N and YCl$_3$ have been chosen as co-additives for this system. These additives increase the hydrogen storage capacity and hasten the de/re-hydrogenation kinetics: a hydrogen uptake of 4.2 wt% of H$_2$ was achieved in only 8 min under isothermal conditions at 180 °C and 85 bar of H$_2$ pressure. The re-hydrogenation temperature, necessary for a complete absorption process, can be lowered below 90 °C by increasing the H$_2$ pressure above 185 bar. Moreover, the results indicate that the hydrogenation capacity and absorption kinetics can be maintained roughly constant over several cycles. Low operating temperatures, together with fast absorption kinetics and good reversibility, make this system a promising on-board hydrogen storage material. The reasons for the improved de/re-hydrogenation properties are thoroughly investigated and discussed., Published by RSC Publ., Cambridge

Q2XAFS 2017, Harwell Science and Innovation Campus, Deutsches Elektronen Synchrotron, United Kingdom, 14 Aug 2018 - 15 Aug 2018, A number of more or less strict protocolls were introduced in analytical chemistry to ensure that results from analytical procedures reflect the true value and maybe even more important to make them comparable between different laboratories in different places. An integral part of the protocolls are so called standard operation procedures (SOP) which contain detailed descriptions how to perform an analytical task. The same arguments are valid for XAFS spectroscopy which is often used as an as an analytical method in catalyis research and other fields of science which are also of enormous commercial interest . Without some level of standardisation the results of XAFS spectroscopy would not be comparable between different beamlines and even beamtimes. Today most XAFS useres are already following certain protocols for measurment and evaluation of spectra. Examples are the use of edge energies from the “orange book” (1) for the calibration of monochromators or the widly followed practice of using the first maximum of the first derivative of the measured spectrum as origin of the k-scale. The quality of XAFS results is mainly determined by three factors, the quality of the sample, the evaluation procedure, and the performance of the beamline. The latter is relativly easy to check using for instance standardised test samples. An obvious example is the XANES spectrum of a Cu-foil which can be used to check the energy resolution of the monochromator. The talk will discuss protocolls (SOPs) that should be applicable at the beamline and enable beamline users and scientists to confirm that an experiment is set-up properly. References1. Thompson A. C. editor, X-RAY DATA BOOKLET, LBN Lab., Berkeley, USA

GeCats Infoday 2017 - Synchrotron Radiation and Neutrons for Catalysis and Materials Science, GeCats 2017, Franfurt a.M., Deutsches Elektronen Synchrotron, Germany, 23 Oct 2017 - 23 Oct 2017, EXAFS spectroscopy is one of the most often used analytical methods in chemistryrelated research at synchrotron radiation sources. After a gap of 3 years DESY isagain operating 2 EXAFS beamlines in regular user operation. Both are equippedwith infrastructure for in-situ experiments with gases etc. A sample preparation lab,with a fume hood and an Ar filled glove box is available directly adjacent to thebeamlines. One beamline, is designed for classical EXAFS experiments on mm sizedsamples, the second offers a higher brilliance and an optional smaller focus foradvanced methods.