Origin of Vanadium Site Sequential Oxidation in K_<i>x</i>VPO₄F_1–<i>y</i>O_<i>y</i>

M-ion batteries (M = Li, Na, K ...) positive electrode materials most often operate through the reversible oxidation of transition-metal ions. In complex materials involving many transition metals or many redox centers, understanding the sequence in which they participate to the reaction is not trivial but is often necessary to explain the electrochemical properties. Mixed anion vanadium phosphates, such as KVPO4F0.5O0.5, are known to contain two different redox entities that are V3+O4F2 and V3+O5F “ionic” entities on the one hand and {V4+O}O5 and {V4+O}O4F “covalent” vanadyl-type units on the other hand. However, their participation to the redox mechanism occurring during the charge of this material has never been studied. Here, we use V K-edge X-ray absorption spectroscopy to unveil the redox mechanism of KVPO4F1–yOy (y = 0, 0.5, 1), performing data analysis via a chemometric approach. With XAS being very sensitive to the oxidation state and bond length, it was found that the ionic V3+–F units oxidize at a lower potential than the covalent {V4+O} ones, which is surprising considering the high electronegativity of fluoride anions but is consistent with the redox potential observed for KVPO4F and KVOPO4. Further, ab initio calculations and ex situ X-ray diffraction analyses allowed an atomistic description of the redox mechanism with the sequential oxidation of the cis V site before the trans V site in KVPO4F upon charge. Finally, the complete atomically resolved redox mechanism of KVPO4F0.5O0.5 is proposed

Simulation of NMR Fermi Contact Shifts for Lithium Battery Materials: The Need for an Efficient Hybrid Functional Approach

In the context of the development of NMR Fermi contact shift calculations for assisting structural characterization of battery materials, we propose an accurate, efficient, and robust approach based on the use of an all electron method. The full-potential linearized augmented plane wave method, as implemented in the WIEN2k code, is coupled with the use of hybrid functionals for the evaluation of hyperfine field quantities. The WIEN2k code uses an autoadaptive basis set that is highly accurate for the determination of the hyperfine field. Furthermore the implementation of an onsite version for the Hartree–Fock exchange offers the possibility to use hybrid functional schemes at no additional computational cost. In this paper, NMR Fermi contact shifts for lithium are studied in different classes of paramagnetic materials that present an interest in the field of Li-ion batteries: olivine LiMPO₄ (M = Mn, Fe, Co, and Ni), anti-NASICON type Li₃M₂(PO₄)₃ (M = Fe and V), and antifluorite-type Li₆CoO₄. Making use of the possibility to apply partial hybrid functionals either only on the magnetic atom or also on the anionic species, we evidence the role played by oxygen atoms on polarization mechanisms. Our method is quite general for an application on various types of materials. Furthermore, it is very competitive compared to the other methods recently proposed that are based either on a plane wave basis set with a PAW implementation or on an LCAO one with a full potential description

P2-Na_<i>x</i>Mn_1/2Fe_1/2O₂ Phase Used as Positive Electrode in Na Batteries: Structural Changes Induced by the Electrochemical (De)intercalation Process

The electrochemical properties of the P2-type Na_<i>x</i>Mn_1/2Fe_1/2O₂ (<i>x</i> = 0.62) phase used as a positive electrode in Na batteries were tested in various voltage ranges at <i>C</i>/20. We show that, even if the highest capacity is obtained for the first cycles between 1.5 and 4.3 V, the best capacity after 50 cycles is obtained while cycling between 1.5 and 4.0 V (120 mAh g^–1). The structural changes occurring in the material during the (de)­intercalation were studied by operando in situ X-ray powder diffraction (XRPD) and ex situ synchrotron XRPD. We show that a phase with an orthorhombic P′2-type structure is formed for <i>x</i> ≈ 1, due to the cooperative Jahn–Teller effect of the Mn³⁺ ions. P2 structure type stacking is observed for 0.35 < <i>x</i> < 0.82, while above 4.0 V, a new phase appears. A full indexation of the XRPD pattern of this latter phase was not possible because of the broadening of the diffraction peaks. However, a much shorter interslab distance was found that may imply a gliding of the MO₂ slab occurring at high voltage. Raman spectroscopy was used as a local probe and showed that in this new phase the MO₂ layers are maintained, but the phase exhibits a strong degree of disorder

DFT+U Calculations and XAS Study: Further Confirmation of the Presence of CoO₅ Square-Based Pyramids with IS-Co³⁺ in Li-Overstoichiometric LiCoO₂

LiCoO₂, one of the major positive electrode materials for Li-ion batteries, can be synthesized with excess Li. Previous experimental work suggested the existence of intermediate spin (IS) Co³⁺ ions in square-based pyramids to account for the defect in this material. We present here a theoretical study based on density functional theory (DFT) calculations together with an X-ray absorption spectroscopy (XAS) experimental study. In the theoretical study, a hypothetical Li₄Co₂O₅ material, where all the Co ions are in pyramids, was initially considered as a model material. Using DFT+U, the intermediate spin state of the Co³⁺ ions is found stable for U values around 1.5 eV. The crystal and electronic structures are studied in detail, showing that the defect must actually be considered as a pair of such square-based pyramids, and that Co–Co bonding can explain the position of Co in the basal plane. Using a supercell corresponding to more diluted defects (as in the actual material), the calculations show that the IS state is also stabilized. In order to investigate experimentally the change in the electronic structure in the Li-overstoichiometric LiCoO₂, we used X-ray absorption near edge structure (XANES) spectroscopy and propose an interpretation of the O Kedge spectra based on the DFT+U calculations, that fully supports the presence of pairs of intermediate spin state Co³⁺ defects in Li-overstoichiometric LiCoO₂

Iron(III) Phosphates Obtained by Thermal Treatment of the Tavorite-Type FePO₄·H₂O Material: Structures and Electrochemical Properties in Lithium Batteries

Thermal treatment of the Tavorite-type material FePO₄·H₂O leads to the formation of two crystallized iron phosphates, very similar in structure. Their structural description is proposed taking into account results obtained from complementary characterization tools (thermal analyses, diffraction, and spectroscopy). These structures are similar to that of the pristine material FePO₄·H₂O: iron atoms are distributed between the chains of corner-sharing FeO₆ octahedra observed in FePO₄·H₂O and the octahedra from the tunnels previously empty, in good agreement with the formation of a Fe_4/3PO₄(OH)-type phase. The formation of an extra disordered phase was also proposed. These samples obtained by thermal-treatment of FePO₄·H₂O also intercalate lithium ions through the reduction of Fe³⁺ to Fe²⁺ at an average voltage of ∼2.6 V (vs Li⁺/Li), with a good cyclability and a reversible capacity around 120 mA h g^–1 (>160 mA h g^–1 during the first discharge)

Self-Discharge Mechanism of High-Voltage KVPO₄F for K‑Ion Batteries

Current performances of Li-, Na-, or K-ion batteries are mainly limited by the specific capacity of the positive electrode. Therefore, it is important to reach the highest capacity possible for a given electrode material. Here, we investigate the performance limitation of KVPO4F, a prospective material for K-ion batteries, which can deliver only 80% of its theoretical capacity. We discover that the capacity limitation of KVPO4F is related to a kinetic competition between K+ deinsertion and side reactions ascribed to the electrolyte degradation at high potentials. Homeotypic VPO4F can be obtained from KVPO4F through a chemical deintercalation process, which disproves a possible structural limitation or instability. The deintercalated compound was characterized by electron and X-ray diffraction, X-ray absorption spectroscopy, and nuclear magnetic resonance spectroscopy. Despite the structural stability, a spontaneous reaction occurs between the deintercalated KxVPO4F (x 6 in ethylene carbonate/diethylene carbonate), with an electron transfer to vanadium compensated by K+ intercalation. This reaction leads to self-discharge until the open circuit potential is lower than 4.7 V versus K+/K, corresponding to the K0.5VPO4F composition

V^IV Disproportionation Upon Sodium Extraction From Na₃V₂(PO₄)₂F₃ Observed by Operando X‑ray Absorption Spectroscopy and Solid-State NMR

Among the series of polyanionic positive electrodes for sodium-ion batteries having the general formula Na₃V₂(PO₄)₂F_3–<i>y</i>O_<i>y</i> (0 ≤ <i>y</i> ≤ 2), the composition Na₃V₂(PO₄)₂F₃ (<i>y</i> = 0) has the highest theoretical energy that offers competitive electrochemical performances compared to sodium transition metal oxides. Recently, the structural phase diagram from Na₃V₂(PO₄)₂F₃ to Na₁V₂(PO₄)₂F₃ has been thoroughly investigated by operando synchrotron X-ray diffraction revealing an unexpected structural feature for the end member composition. In fact, the crystal structure of Na₁V₂(PO₄)₂F₃ has two very different vanadium environments within each bioctahedron that suggests a charge disproportionation of two V^IV into V^III and V^V. This work shows an operando X-ray absorption spectroscopy at vanadium K edge during the electrochemical extraction of Na⁺ in order to monitor the redox processes involved in this compound. The large data set provided by this experiment has been processed by the principal component analysis combined with multivariate curve resolution. The results suggest that the bioctahedra have to be considered as the basic structural unit. The peculiar geometry of this material combined with the mixed vanadium valence, directly investigated here along the reaction, seems to allow original electronic configurations. In particular, the two vanadium sites into the basic bioctahedra unit evolve from V^III–V^III to V^III–V^IV and to a final V^III–V^IV configuration. These observations are completed with ⁵¹V NMR sensitive to diamagnetic V^V

Simultaneous Reduction of Co³⁺ and Mn⁴⁺ in P2-Na_2/3Co_2/3Mn_1/3O₂ As Evidenced by X‑ray Absorption Spectroscopy during Electrochemical Sodium Intercalation

Sodium intercalation in P2-Na_2/3Co_2/3Mn_1/3O₂ (obtained by a coprecipitation method) was investigated by ex situ and in situ X-ray absorption spectroscopy. The electronic transitions at the O K-edge and the charge compensation mechanism, during the sodium intercalation process, were elucidated by combining Density Function Theory (DFT) calculations and X-ray absorption spectroscopy (XAS) data. The pre-edge of the oxygen K-edge moves to higher energy while the integrated intensity dramatically decreases, indicating that the population of holes in O 2p states is reduced with increasing numbers of sodium ions. From the K-edge and L-edge observations, the oxidation states of pristine Co and Mn were determined to be +III and +IV, respectively. The absorption energy shifts to lower positions during the discharging process for both the Co and the Mn edges, suggesting that the redox pairs, that is, Co³⁺/Co²⁺ and Mn⁴⁺/Mn³⁺, are both involved in the reaction

Oxidation under Air of Tavorite LiVPO₄F: Influence of Vanadyl-Type Defects on Its Electrochemical Properties

Tavorite-type compositions offer a very rich crystal chemistry, among which LiV^IIIPO₄F has the highest theoretical energy density (i.e., 655 Wh/kg). In this article, an in-depth study of vanadyl-type defects generated by temperature-controlled oxidation of LiV^IIIPO₄F under air is proposed, and the influence of the defects on the electrochemical properties is demonstrated. A combination of high resolution synchrotron diffraction, infrared spectroscopy, and magic angle spinning nuclear magnetic resonance was used to fully characterize the materials thus generated, from their average long-range structure to their local structure with the presence of defects. The increase of the annealing temperature tends to substitute oxygen for fluorine with the formation of a series of LiVPO₄F_1–<i>x</i>O_<i>x</i> compositions. The miscibility domains appear to be narrow at the two ends of the solid solution (i.e., in the composition ranges LiVPO₄F_[1‑0.9]O_[0‑0.1] and LiVPO₄F_[0‑0.1]O_[1‑0.9]). The presence of vanadyl-type defects obtained as localized or more extended ones, depending on the annealing conditions, affects drastically the electrochemical properties of these Tavorite LiVPO₄F-type materials

Vanadium Clustering/Declustering in P2–Na_1/2VO₂ Layered Oxide

The new layered phase P2–Na_1/2VO₂ has been synthesized by sodium electrochemical deintercalation. Its structure has been studied by high resolution powder diffraction, pair distribution function analysis, and nuclear magnetic resonance spectroscopy between 300 and 350 K. An increase of 2 orders of magnitude in its electronic conductivity has been observed at approximately 322 K, and a structural transition has been found to occur simultaneously. The arrangement of sodium ordering in P2–Na_1/2VO₂, which maximizes sodium–sodium distances to lower electrostatic repulsions between alkali ions, is found to be unchanged across this transition. At room temperature, high resolution powder diffraction and pair distribution function analysis reveal the triangular lattice formed by vanadium ions to be distorted by the formation of pseudotrimers clusters with vanadium–vanadium distances as short as 2.581 Å. Above the transition, the pseudotrimers disappear and the triangular vanadium lattice becomes more regular with a mean vanadium–vanadium distance of ∼2.88 Å. At 350 K, the increase in P2–Na_1/2VO₂ electronic conductivity is due to enhanced charge transport resulting from the declustering of vanadium ions. These results highlight how sodium ordering between the MO₂ layers and the electronic transport within the MO₂ layers are intimately correlated in Na_<i>x</i>MO₂-type sodium-layered oxides