Na3V2(PO4)2F3 is a positive electrode material for Na-ion batteries which is attracting strong interest due to its high capacity, rate capability, and long-term cycling stability. The sodium ...extraction mechanism from this material has been always described in the literature as a straightforward solid solution, but several hints point toward a more complicated phase diagram. In this work we performed high angular resolution synchrotron radiation diffraction measurements, realized operando on sodium batteries upon charge. We reveal an extremely interesting phase diagram, created by the successive crystallization of four intermediate phases before the end composition NaV2(PO4)2F3 is reached. Only one of these phases undergoes a solid solution reaction, in the interval between 1.8 and 1.3 Na per formula unit. The ability to resolve weak Bragg reflections allowed us to reveal differences in terms of symmetry among the phases, to determine their previously unknown space groups, and to correlate them with sodium (dis)ordering in the structure. Rietveld refinements enabled us to follow fine structural modifications in great detail. Intermediate identified phases are not simply described by their unit cell parameters, but bond-length variations can be tracked, as well as polyhedral distortions and site occupancy factors for mobile sodium ions. For NaV2(PO4)2F3 a full crystal structure determination was also carried out for the first time directly from operando measurements, assigning it to the Cmc21 space group and revealing two vanadium environments: V3+ and V5+. Our study demonstrates that improved angular resolution and high intensity diffraction data are key parameters for direct observation of fine reaction pathways in electrode materials and that the obtained insight is crucial for the understanding of (de)intercalation mechanisms in Na-ion batteries.
Lithium iron phosphate is one of the most promising positive-electrode materials for the next generation of lithium-ion batteries that will be used in electric and plug-in hybrid vehicles. Lithium ...deintercalation (intercalation) proceeds through a two-phase reaction between compositions very close to LiFePO(4) and FePO(4). As both endmember phases are very poor ionic and electronic conductors, it is difficult to understand the intercalation mechanism at the microscopic scale. Here, we report a characterization of electrochemically deintercalated nanomaterials by X-ray diffraction and electron microscopy that shows the coexistence of fully intercalated and fully deintercalated individual particles. This result indicates that the growth reaction is considerably faster than its nucleation. The reaction mechanism is described by a 'domino-cascade model' and is explained by the existence of structural constraints occurring just at the reaction interface: the minimization of the elastic energy enhances the deintercalation (intercalation) process that occurs as a wave moving through the entire crystal. This model opens new perspectives in the search for new electrode materials even with poor ionic and electronic conductivities.
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.
The structure of Li2MnO3 was investigated by the means of X-ray and electron diffraction as well as high resolution transmission electron microscopy experiments. Extra spots are present in the ...Li2MnO3 electron diffraction patterns, and their origin is fully understood and explained here. They result from the existence of diffuse scattering lines observed along the c* monoclinic axis, intercepted by the Ewald’s sphere, and not from double diffraction phenomenon nor from superstructure. Furthermore, the origin of these scattering lines is due to stacking faults of the ordered lithium/manganese layers along the c-axis that were observed in images obtained using high resolution transmission electron microscopy.
Na3V2(PO4)2F3 is a material that has been attracting great interest as a potential positive electrode for Na-ion batteries. Its crystal structure was determined from single-crystal X-ray diffraction ...in 1999 by Le Meins et al. in the tetragonal space group P42/mnm at 298 K. In this work, we show how the use of very high angular resolution synchrotron radiation diffraction reveals a subtle orthorhombic distortion with unit-cell parameters of a = 9.02847(3) Å, b = 9.04444(3) Å, c = 10.74666(6) Å in the Amam space group. Although this only slightly impacts the structural framework of the material, it reveals a significantly modified distribution of Na ions. Furthermore, the crystal structure of the high-temperature form of Na3V2(PO4)2F3 (at 400 K) was determined for the first time. This allowed comparing the totally disordered distribution of Na ions in this case with the partially ordered one of the room-temperature phase. We report here on an original structure and on an original electrochemical signature for stoichiometric Na3V2(PO4)2F3, and we propose that fluctuations in the O/F ratio are at the origin of discrepancies found in the literature.
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.
he thermal degradation mechanism of LixNi0.70Co0.15Al0.15O2 and LixNi0.90Mn0.10O2 (x = 0.50 and 0.30) was studied by thermal gravimetric analysis coupled with mass spectrometry. Correlation with in ...situ X-ray diffraction experiments was then achieved to determine the degradation mechanism and to explain the differences in thermal stability observed depending on the material composition. The degradation occurs in two steps: ...
Thermal evolution of the layered oxide Li2/3Co2/3Mn1/3O2, showing a T#2 stacking and prepared by a Na/Li ion exchange in P2-Na2/3Co2/3Mn1/3O2, was investigated by thermal analyses and X-ray ...diffraction. A thermal expansion of the T#2 orthorhombic unit cell is observed from 25 to 350 °C; from 350 °C the T#2 stacking is destabilized to the benefit of an O6-type stacking obtained from the former through slab gliding. The T#2 to O6 phase transformation is allowed to occur from a stacking with larger interlayer distances and the lithium ions in tetrahedral sites to a stacking with smaller interlayer distances and the lithium ions in octahedral sites. This phase transition from T#2 to O6 is reversible, even though its kinetic can be very slow: the thermal treatment of the T#2-type Li2/3Co2/3Mn1/3O2 phase at 450 °C with a quenching in air has shown to stabilize the O6HT-Li2/3Co2/3Mn1/3O2 phase. At temperatures higher than 450 °C, the layered oxide Li2/3Co2/3Mn1/3O2 is gradually decomposed into Li2MnO3 and Co3O4. First electrochemical tests performed in lithium batteries have revealed that O6HT-Li2/3Co2/3Mn1/3O2 delivers as positive electrode material a high reversible capacity of ∼230 mAh·g–1 over two voltage domains around 3 and 4 V vs Li+/Li.
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