A series of “Li
1+
z
/2(Ni
0.425Mn
0.425Co
0.15)
1−
z
/2O
2−
z
F
z
” materials was prepared by a coprecipitation route and their structure was characterized using X-ray diffraction (XRD), as well as
...7Li and
19F Magic Angle Spinning (MAS) NMR spectroscopy. Two hypotheses were considered: (i) formation of layered oxyfluoride materials and (ii) formation of a mixture between the layered material and LiF. Structural parameters were refined by the Rietveld method, using XRD diffraction data. The refinement results did not allow us to choose between these two hypotheses: no significant change in crystallinity and structural parameters was observed irrespective of the fluorine ratio.
7Li and
19F MAS NMR analyses showed signals with isotropic positions characteristic of LiF, but envelopes characteristic of very strong dipolar interactions with the electron spins of the material, demonstrating that LiF was not incorporated into the layered oxide structure but was instead present as a coating.
7Li and
19F MAS NMR show signals with isotropic positions characteristic of LiF, but with spinning sidebands envelopes characteristic of very strong dipolar interactions with the electron spins of the layered oxide, demonstrating that F is not a part of the material but is present as an LiF coating.
A series of “Li1+z/2(Ni0.425Mn0.425Co0.15)1−z/2O2−zFz” materials was prepared by a coprecipitation route and their structure was characterized using X-ray diffraction (XRD), as well as 7Li and 19F ...Magic Angle Spinning (MAS) NMR spectroscopy. Two hypotheses were considered: (i) formation of layered oxyfluoride materials and (ii) formation of a mixture between the layered material and LiF. Structural parameters were refined by the Rietveld method, using XRD diffraction data. The refinement results did not allow us to choose between these two hypotheses: no significant change in crystallinity and structural parameters was observed irrespective of the fluorine ratio. 7Li and 19F MAS NMR analyses showed signals with isotropic positions characteristic of LiF, but envelopes characteristic of very strong dipolar interactions with the electron spins of the material, demonstrating that LiF was not incorporated into the layered oxide structure but was instead present as a coating.
Complementing our work on the experimental measurements and DFT calculations of the NMR Fermi contact shifts for 7Li, 31P, and 1H in the tavorite LiFePO4·OH and homeotypic FePO4·H2O, LiMnPO4·OH, ...MnPO4·H2O, and VPO4·H2O phases, we aim to understand the origin of those shifts. Using FP-LAPW calculations that were validated by reproducing correctly the experimental shifts, we analyze the electronic spin-transfer mechanisms from the transition metal M to the probed nucleus in relation with the electronic configuration of the M3+ ions and the nature of the chemical bonds in the compounds, by plotting the spin DOS and 2D or 3D spin density maps in selected energy domains. These mechanisms are analyzed in detail for the two Fe3+ phases, and the differences occurring for the Mn and V phases are discussed further.
The fluorination of La2CuO4 was achieved for the first time under normal conditions of pressure and temperature (1 MPa and 298 K) via electrochemical insertion in organic fluorinated electrolytes and ...led to lanthanum oxyfluorides of general formula La2CuO4F x . Analyses showed that, underneath a very thin layer of LaF3 (a few atomic layers), fluorine is effectively inserted in the material’s structure. The fluorination strongly modifies the lanthanum environment, whereas very little modification is observed on copper, suggesting an insertion in the La2O2 blocks of the structure. In all cases, fluorine insertion breaks the translation symmetry and introduces a long-distance disorder, as shown by electron spin resonance. These results highlight the efficiency of electrochemistry as a new “chimie douce” type fluorination technique for solid-state materials. Performed at room temperature, it additionally does not require any specific experimental care. The choice of the electrolytic medium is crucial with regard to the fluorine insertion rate as well as the material deterioration. Successful application of this technique to the well-known La2CuO4 material provides a basis for further syntheses from other oxides.
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.
A series of binary transition metal phosphides (Ni3P, Ni12P5, Ni2P, Ni5P4, NiP, NiP2, FeP, FeP2, FeP4, VP2, CoP) were investigated by solid state 31P MAS NMR, leading to rather different lineshapes, ...shifts, relaxation times, and temperature dependences. The electronic structures of these compounds were computed using various DFT codes, based either on plane wave PAW potentials (VASP) or on all-electron basis sets in the FPLAPW formalism (Wien2K). Depending on the electronic features of the phosphide, self-interaction corrected formalisms (DFT+U or PBE0 hybrid functional) were also used to reach a better description of the electronic ground state and to establish a correlation with the shape and the nature of the NMR signals. As a result of the analysis, the main categories are diamagnetic compounds (FeP4, NiP2) and metallic ones, either real (VP2) or with some electronic localization in band tails (Ni12P5, Ni2P, Ni5P4, NiP) or with spin-polarized conduction bands (CoP, FeP). FeP2 appears somewhat ambiguous, both based on the various computational results and on the NMR characteristics. Besides, FeP4 is the only compound for which very clear J couplings resulting from P−P bonds were observed.
The electrochemical reaction mechanisms of FeP x (x = 1, 2, 4) as electrode materials in Li batteries were analyzed by 57Fe Mössbauer spectroscopy, magnetic and NMR measurements. Depending on their ...stoichiometry, iron phosphides react with lithium along different pathways. FeP and FeP2 are fully or partially converted to a composite electrode of Li3P and Fe. For both, the same intermediate phases, namely FeP and Li x FeP, were identified in cycling and FeP is partially regenerated on charge. Surprisingly, FeP4 reacts with Li through an insertion type process previously identified for earlier transition metal phosphides (Ti, V, and Mn), and no conversion reaction to Li3P/Fe is observed. The long-term stability of the composite electrode Li3P/Fe0 and of the intermediate phase Li x FeP is analyzed.
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.
