Lithium deintercalation of Li x CoO2 from x = 1 to x ≈ 0 has been carried out electrochemically. The changes in the electronic structure from LiCoO2 to CoO2 have been investigated by X-ray ...photoelectron spectroscopy (XPS) to bring some new developments about the electron transfer mechanisms upon lithium deintercalation. All available XPS core peaks (Co 2p, Co 3p, Co 3s, O 1s, F 1s, P 2p, C 1s) and valence spectra have been analyzed. The contributions of the electrode material and of the electrode/electrolyte interface have been clearly distinguished. We show that cobalt and oxygen simultaneously undergo a partial oxidation process and that the sole participation of oxygen atoms to the charge transfer process, as it is sometimes assumed, can be excluded. The surface film consists of organic and inorganic species resulting from degradation of the electrolyte.
Liy(Ni0.425Mn0.425Co0.15)0.88O2 materials were synthesized by a slow rate electrochemical deintercalation from Li1.12(Ni0.425Mn0.425Co0.15)0.88O2 during the first charge and the first discharge in ...order to study the structural modifications occurring during the first cycle and especially during the irreversible “plateau” observed in charge at 4.5 V vs Li+/Li. Chemical Li titrations showed that the lithium ions are actually deintercalated from the material during the entire first charge process, excluding the possibility that electrolyte decomposition causes the “plateau”. Redox titrations revealed that the average transition metal oxidation state is almost constant during the “plateau”, despite further lithium ion deintercalation. 1H MAS NMR data showed that no Li+/H+ exchange was associated to the “plateau” itself. Rietveld refinement of the XRD pattern for a material reintercalated after being deintercalated at the end of the “plateau”, as well as redox titrations, revealed an M/O ratio larger than that of the pristine material, which is consistent with the oxygen loss proposed by Dahn and coauthors for the LiNi x Li(1/3−2x/3)Mn(2/3−x/3)O2 materials to explain the irreversible overcapacity phenomenon observed upon overcharge. X-ray and electron diffraction showed that the transition metal ordering initially present within the slabs is lost during the “plateau” due to a cation redistribution. To explain this behavior a cation migration to the vacancies formed by the lithium deintercalation from the transition metal sites (3a) is assumed, leading to a material densification.
In a recent study, we showed by solid-state NMR that LiVPO4F, which is a promising material as positive electrode for Li-ion batteries, often exhibits some defects that may affect its electrochemical ...behavior. In this paper, we use DFT calculations based on the projector augmented-wave (PAW) method in order to model possible defects in this (paramagnetic) material and to compute the Fermi contact shifts expected for Li nuclei located in their proximity. The advantage of the PAW approach versus FP-LAPW we have been previously using is that it allows considering large supercells suitable to model a diluted defect. In the first part of this paper, we aim to validate the Fermi contact shifts calculation using the PAW approach within the VASP code. Then we apply this strategy for modeling possible defects in LiVPO4F. By analogy with the already existing homeotypic LiVOPO4 phase, we first replace one fluoride ion, along the VO2F4 chains, by an oxygen one and consider, in a second step, an association with a lithium vacancy. As a result, the agreement between the calculated NMR spectra and the experimental one is satisfying. In both cases, the local electronic structure and the spin transfer mechanisms from V3+ or V4+ ions to the Li nuclei are analyzed.
XPS analyses (core peaks and valence spectra), under highly controlled conditions, have been carried out on stoichiometric LiCoO2 and lithium-overstoichiometric Li1+y Co1−y O2−y (y ∼ 0.05) materials, ...with significant changes observed in the oxygen peaks. Indeed, beside the component attributed to the O2− anions of the crystalline network, a second one with variable intensity has been observed on the high binding energy side. With the support of ab initio biperiodical calculations on LiCoO2, we propose that this peculiar oxygen signature is partially associated, for LiCoO2, to undercoordinated oxygen atoms coming from (0 0 1) oriented surfaces. These surface oxygen anions are significantly less negative than the ones of the lattice. These results, in conjunction with SEM analyses for the lithium overstoichiometric material (as prepared and thermally treated), show that the presence of defects (oxygen vacancies) has to also be considered in the overstoichiometric case. As in battery material, all reactions (the intercalation but also the parasitic ones) occur through the surface; characterization of its crystallographic nature (as well as its electronic properties) is a key point to a better understanding and optimization of Li ion batteries.
Operando X-ray absorption spectroscopy investigations have been carried out to follow changes in the atomic and electronic local structures of all three transition metals for the ...Li1.20Mn0.54Co0.13Ni0.13O2 layered oxide during the first and second charges and discharges of lithium batteries. The experiments were performed using a Quick-XAS monochromator on the SAMBA beamline at Synchrotron SOLEIL to record the three K-edges by edge-jumping between two energy ranges (Mn, Co and Co, Ni) every 3 min during the cycling of the battery. The results obtained especially at the Mn K-edge fully support the participation of oxygen in the reversible charge–discharge reaction of this Li- and Mn-rich layered material as a redox center and not only with oxygen loss, as was proposed previously.
Phase Change Materials as those of the Ge-Sb-Te ternary system are of great interest for technological applications. Properties of these compounds are strongly related to presence of vacancies and ...structural investigations remain challenging. In this paper we evidence that 125Te NMR in natural abundance and using commercial systems at intermediate field (14.1 T) together with NMR parameters prediction can contribute to improve understanding of electronic structure of such systems. GeTe is a typical phase change material, whose structure contains germanium vacancies, even in its stoichiometric form, giving it metallic properties. Here, we use nominal Ge50Te50 and Ge48Te52 crystalline samples as an example to optimize the WURST-CPMG technique, a powerful technique to record wide NMR spectra which has not yet been used on 125Te. The goal was to minimize the time devoted to experiments as well as maximize the signal-to-noise ratio in order to detect small intensity signals directly linked to vacancies. Virtual Crystal Approximation (VCA) calculations performed with WIEN2K helped to interpret the NMR spectra. For Te-based crystalline conducting samples the best experimental results were obtained using 3.2 mm thin wall rotors with diluted samples 40 vol% GeTe-60 vol% SiO2. In addition to the WURST-CPMG technique, high resolution spectra using MAS as implemented in the pj-MAT technique allowed us to identify the distributions of chemical shift parameters in the high intensity contribution of the 1D spectra. The NMR spectra recorded on the samples showed that an addition of Tellurium in the stoichiometric Ge50Te50 sample leads to an important broadening of the spectrum together with a shift of the lines. According to VCA calculations it could be attributed to a distribution of concentrations of germanium vacancies in the sample and it would appear that Knight Shift but also Chemical Shift could contribute in similar proportion to the NMR line position when metavalent bonding is invoked.
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•125Te NMR Investigation of Ge–Te crystalline materials, a model for phase-change materials.•125Te WURST-CPMG sequence optimized for conducting Te-containing materials, using commercial systems at intermediate fields.•VCA calculations in WIEN2K linking lineshape of the spectra to NMR parameters and charge carriers concentration distribution.•Use of the NMR pj-MAT sequence, relating the decrease of the anisotropy of 125Te environment with the amount of vacancies.
The lithium-ion battery electrode material TiSnSb shows excellent electrochemical performance related to its high capacity (550 mA h g–1) and rate capability over 210 cycles. To discriminate between ...the role of active material and the role of the electrode formulation in the good electrochemical features of the TiSnSb electrodes, a full study comparing the electrochemical mechanisms of TiSnSb and a Ti/Sn/Sb composite vs Li is undertaken by combining X-ray diffraction (XRD), 121Sb, 119Sn Mössbauer and 7Li NMR spectroscopic in situ measurements. During the first discharge, TiSnSb undergoes a direct conversion reaction while Ti/Sn/Sb composites proceed by a stepwise alloying process, both leading to a mixture of lithium antimonide, lithium stannides, and titanium. More surprisingly the charge occurs differently with a reformation of the “TiSnSb” phase in the first case and the formation of Sn and Sb in the second case. The key role of the interfaces in conversion type reactions is discussed. The nature of the interfaces is linked to the long-range order of elements in the starting material. Furthermore, the length scales of the interfaces between Li3Sb, Li x Sn and Ti appear to control the reactions that occur on charge.
7Li, 31P, and 1H MAS NMR spectra and magnetic properties are reported for LiFePO4·OH and FePO4·H2O. The former shows no Curie–Weiss-type behavior up to room temperature, while the latter tends to ...such a behavior in a restricted temperature range. Calculation strategies are discussed for the NMR shifts that result from Fermi contact interaction with the high spin Fe3+ ions. Zero Kelvin electron spin densities obtained by averaging over the ion size using VASP (with PAW potentials) range with those obtained at the nucleus from WIEN2k, with the GGA and GGA+U methods. The latter values have been scaled with the temperature of the NMR measurement by using the experimental magnetic susceptibility, yielding calculated NMR shifts. The agreement is quite satisfactory, but very much dependent on the exchange correlation potential used for the calculation. Possible reasons for this are discussed, also considering the difference in magnetic behaviors.
M2+-doped aluminate spinels (M=Co or Ni) were prepared by a polymeric route leading to pure phases for synthesis temperatures equal to 800 or 1200°C and characterized by UV–vis–NIR spectroscopy, 27Al ...NMR and XRD refinements. Coloration of the synthesized pigments is clearly sensitive to the distribution of doping ions in the aluminate spinel lattice. As the synthesis temperature increased, a color shift from green to blue has been observed for Zn1−xCoxAl2O4 compound while coloration of Zn1−xNixAl2O4 compound remains greenish-gray. Hence, to improve pigment coloration and/or synthesis cost, two different strategies have been proposed: (i) the synthesis of aluminum over-stoichiometric spinel with Zn0.9Co0.1Al2.2O4+δ formal composition in order to force Co2+ to be located in tetrahedral sites and (ii) changing from ZnAl2O4 to MgAl2O4 as host lattices for Ni2+ doping ions in order to force Ni2+ to be located in octahedral sites.
We report on the Li electrochemical reactivity of amorphous and crystalline VP
2, synthesized by ball-milling and by 600
°C heat treatment of a ball-milled sample, respectively. The amorphous sample ...can reversibly react with 3.5 Li per formula unit as compared to solely 2.5 for the crystalline one. However in both cases there is a rapid capacity decay upon cycling that is more pronounced in the case of the crystalline sample. Complementary X-rays, HTREM and NMR tend to show that the Li reactivity mechanism differs from the classical conversion reactions since neither V nanoparticles nor the formation of Li
3P were detected, as opposed to some of the other MP
2 compounds (M
=
Ni or Cu). Besides structural phase variations within the 3d metal-based binary phosphide series, the possibility of a change in the nature of the redox centre upon lithiation from cation (M) to anion (P) is evoked.
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