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.
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.
The Li reactivity of NiP2 is investigated by means of electrochemical tests, in situ XRD, and 31P NMR characterizations as well as first principles DFT calculations. A two-step insertion/conversion ...reaction is shown to transform the NiP2 starting electrode into an intermediate Li2NiP2 single phase and then to convert into the Li3P/Ni° nanocomposite. The ternary phase is fully characterized and is shown to be structurally very close to the starting NiP2 regarding the Ni ions environment. This demonstrates that its formation results from a P-redox insertion mechanism associated with a very good reversibility. However, its nucleation upon delithiation from the fully converted Li3P/Ni composite is shown to be kinetically limited (poor structural relationship) which strongly suggests that restricted lithiation is required for best cycleability of the NiP2/Li cell.
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.
Aluminum-based coatings are commonly used in lithium-ion batteries to modify the surface of LiCoO2 particles, to limit cobalt dissolution in the electrolyte at high voltage. It was shown that the ...formation of a LiCo1−xAlxO2 solid solution occurs at the interface between the coating and the core material. In this paper, we investigated the surface properties of LiCo1−xAlxO2 materials by X-ray photoelectron spectroscopy. We explored the surface acid−base properties of these materials by adsorption of gaseous probe molecules (NH3 and SO2) followed by XPS analyses. We showed that the basic character of the LiCo1−xAlxO2 surface strongly decreases when x increases, which makes these materials less reactive than LiCoO2 toward acidic species (such as HF) that are present in LiPF6-based electrolytes. This is a possible explanation for the efficiency of Al-based coatings to protect LiCoO2 against cobalt dissolution in the electrolyte.
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.
Lithium nickel oxide derivatives are promising positive electrode materials for the next generation of lithium-ion batteries. Partial substitution of certain cations for nickel in this family of ...oxides significantly modifies their properties and is therefore an attractive route to develop an optimised oxide electrode which satisfies the demanding requirements for rechargeable battery applications. In this paper the interest is focused on the effect of cobalt, iron, aluminium and magnesium for a general discussion of the effect of cationic substitution on the properties based on a review of results mostly obtained in our laboratories. Although iron substitution does not seem interesting for the practical aspect, iron Mössbauer spectroscopy allows very precise characterisations, interesting to understand the general behaviour of this family of materials. We deal with the optimisation of the synthesis conditions in order to obtain the most electrochemically active materials. The relations between the nature of the substituting cation, the presence of foreign cations in the lithium site, the electrochemical behaviour and the redox processes upon electrochemical cycling are discussed in detail. A new view of the relation between this latter point and the cationic distribution formed during the material synthesis is proposed.
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.
Li(Ni,Mn,Co)O2/carbon lithium-ion batteries designed to work at high temperature exhibit good performances for cycling at 85 °C but a strong impedance increase for cycling or storage at 120 °C. The ...effects of high temperature on the aging process of positive electrode's binder, electrodes/electrolyte interfaces and positive active material were investigated by bulk and surface analysis techniques: X-ray diffraction, 7Li and 19F Nuclear Magnetic Resonance, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy. The main phenomenon observed at 120 °C is a migration of PVdF binder from the outer part of the positive electrode which thus shows a bad Li reintercalation. Binder migration results in the formation of a PVdF layer at the extreme surface of the positive electrode, and in its diffusion and deposition on the surface of the negative electrode. Several changes in the composition of the Solid Electrolyte Interphase (SEI) were also evidenced between a cycle at 60 °C and cycling at 85 °C and 120 °C. Disappearance of carbonate species disappearance and increase of inorganic species, which both together may also affect battery's performance. Though, since the battery can undergo 25 cycles at 120 °C, Li(Ni,Mn,Co)O2 seems to be an appropriate material for high temperature cycling.
► Li(Ni,Mn,Co)O2/carbon lithium-ion batteries were cycled at +85 °C and +120 °C for 15 days. ► Good stability of chosen electrochemistry at +85 °C. ► XPS, XRD, SEM and NMR were used to identify main aging mechanisms. ► Positive electrode binder PVDF was identified as the main aging root cause. ► Evolution of the SEI composition was quantified between +85 °C and +120 °C.
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.
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