Approche théorique et expérimentale combinée dans l’exploration de LiFeV2O7 et son application comme matériau d’électrode positive pour batterie aux ions lithium

Cotutelle en électrochimie du solide et chimie computationnelleLes batteries aux ions lithium sont développées pour alimenter un nombre grandissant d’applications portables et subvenir aux besoins d’une société de plus en plus mobile. Le succès de ces batteries repose sur les performances des matériaux d’électrodes et d’électrolyte qui n’ont cessé d’être optimisées ces dernières décennies. Le processus qui a mené à leur industrialisation et commercialisation a toujours été initié par la découverte de nouveaux matériaux. La recherche de matériaux est une approche qui peut paraître incertaine, lorsque comparée à l’ingénierie et l’optimisation de matériaux déjà existants, mais représente le moteur de l’innovation. Dans cette optique, nous avons revisité le diagramme de phases ternaire Li2O-V2O5-Fe2O3 à la recherche de nouveaux composés pour électrode positive. L’étude systématique du diagramme a mené à la synthèse par voie solide d’un nouveau matériau de composition LiFeV2O7. Le présent manuscrit porte sur l’étude de ce composé. Après une caractérisation détaillée de LiFeV2O7, une étude électrochimique a été menée afin d’évaluer son potentiel en tant que matériau d’électrode positive. Les résultats ont mis en avant un comportement électrochimique complexe que nous avons tenté de comprendre en menant des analyses DRX et Mössbauer ex situ sur le composé LiFeV2O7 pendant le cyclage. Cette partie expérimentale est complétée par une étude théorique sur le comportement de LiFeV2O7 en électrochimie notamment la modélisation de la courbe électrochimique et l’évolution des magnétisations du composé.Lithium ion batteries are developed to power a growing amount of portable applications and meet the needs of an increasingly mobile society. These batteries owe their success to the performances of electrode materials and electrolytes which were continuously optimized these last decades. The process leading to their industrial and commercial application was always initiated by the discovery of new materials. The research of new materials can be seen as an uncertain approach, when compared to the engineering and optimization of existing ones, but remains the driving force behind innovation. In this context, we revisited the Li2O-V2O5-Fe2O3 ternary phase diagram on the lookout for new positive electrode materials. The systematic study of the diagram led to the synthesis by solid state reaction of a new material with the composition LiFeV2O7. The following manuscript will be covering the study of this material. After a detailed characterization of LiFeV2O7, an electrochemical study was carried out to evaluate the material’s potential as a positive electrode material. The results displayed a rather complex electrochemical behavior. ex situ XRD and Mössbauer analyses were carried out on LiFeV2O7 upon cycling to try to comprehend this behavior. The experimental section was complemented with a theoretical study on LiFeV2O7 behavior in electrochemistry. It includes the modelling of LiFeV2O7 electrochemical curve and the evolution of its magnetizations upon discharge

Using experiment and first-principles to explore the stability of solid electrolytes for all-solid-state lithium batteries

Lithium-ion batteries (LIBs) are considered the most promising energy storage technology. LIBs electrode materials have the highest known energy densities, allowing the constant miniaturization of commercial electronic devices. Research in the field of LIBs has more recently turned to their implementation in electric vehicles, which will require higher energy and power densities4. A concrete way to increase the energy density of LIBs is to increase the cell voltage. To do so, the new generation of batteries will be composed of high potential positive electrode materials (such as LiMn1.5Ni0.5O4 with a potential of 4.7 V vs. Li+/Li) and metallic lithium in the negative electrode. Nevertheless, the introduction of these high potential positive electrode materials is limited by the electrochemical stability of conventional liquid electrolytes, composed of a lithium salt and organic solvents (LiPF6 + EC/DEC), which gets oxidized around 4.2 V vs. Li+/Li5,6. The use of metallic lithium as the negative electrode is also hindered by the liquid nature of the conventional electrolyte, which does not offer enough mechanical resistance to prevent the formation of lithium dendrites, ultimately causing a short-circuit of the battery. Such short-circuits are likely to lead to thermal runaway because liquid electrolytes are composed of organic solvents that are flammable at low temperature, posing a serious safety issue. Solid electrolytes, based on ceramics or polymers, are developed as an alternative to liquid electrolytes. They contain no flammable solvents and are stable at high temperatures. They are the key element of a new generation of lithium batteries called all-solid-state lithium batteries. These are developed to meet high expectations in terms of safety, stability and high energy density. Solid electrolytes must satisfy a number of requirements before they can be commercialized, including possessing a high ionic conductivity, a wide electrochemical stability window and negligible electronic conductivity. These properties are the most important criteria to consider when selecting solid electrolyte materials. However, the majority of studies found in the literature focuses on the ionic conductivity of solid electrolytes, overshadowing the exploration of their electrochemical stability and electronic conductivity. The electrochemical stability window has long been reported to be very wide in ceramic solid electrolytes (at least from 0 to 5 V vs. Li+/Li). Nevertheless, more recent studies tend to show that the value of this window depends greatly on the electrochemical method used to measure it, and that it is often overestimated. In this context, the first objective of this thesis was to develop a relevant method to determine the stability window of solid electrolytes with precision. This method was optimized and validated on flagship ceramic solid electrolytes such as Li1.5Al0.5Ge1.5(PO4)3, Li1.3Al0.3Ti1.7(PO4)3 and Li7La3Zr2O12. As for the electronic conductivity, it is scarcely studied in solid electrolytes, which are considered as electronic insulators given their wide band gaps. That being said, more recent studies on this subject proved that despite their band gap, solid electrolytes can generate electronic conductivity through defects, and that electronic conductivity, even if it is weak, can eventually cause the failure of the electrolyte. For this reason, the second objective of this thesis project was to explore the formation of defects in solid electrolytes in order to determine their effect on the generation of electronic conductivity. To get a better overview, first-principles were used to investigate six widely used ceramic solid electrolytes, including LiGe2(PO4)3, LiTi2(PO4)3, Li7La3Zr2O12, and Li3PS4.(SC - Sciences) -- UCL, 202

Assessing the Electrochemical Stability Window of NASICON-Type Solid Electrolytes

All-Solid-State Lithium Batteries (ASSLBs) are promising since they may enable the use of high potential materials as positive electrode and lithium metal as negative electrode. This is only possible through solid electrolytes (SEs) stated large electrochemical stability window (ESW). Nevertheless, reported values for these ESWs are very divergent in the literature. Establishing a robust procedure to accurately determine SEs’ ESWs has therefore become crucial. Our work focuses on bringing together theoretical results and an original experimental set up to assess the electrochemical stability window of the two NASICON-type SEs Li1.3Al0.3Ti1.7(PO4)3 (LATP) and Li1.5Al0.5Ge1.5(PO4)3 (LAGP). Using first principles, we computed thermodynamic ESWs for LATP and LAGP and their decomposition products upon redox potentials. The experimental set-up consists of a sintered stack of a thin SE layer and a SE-Au composite electrode to allow a large contact surface between SE and conductive gold particles, which maximizes the redox currents. Using Potentiostatic Intermittent Titration Technique (PITT) measurements, we were able to accurately determine the ESW of LATP and LAGP solid electrolytes. They are found to be [2.65–4.6 V] and [1.85–4.9 V] for LATP and LAGP respectively. Finally, we attempted to characterize the decomposition products of both materials upon oxidation. The use of an O2 sensor coupled to the electrochemical setup enabled us to observe operando the production of O2 upon LAGP and LATP oxidations, in agreement with first-principles calculations. Transmission Electron Microscopy (TEM) allowed to observe the presence of an amorphous phase at the interface between the gold particles and LAGP after oxidation. Electrochemical Impedance Spectroscopy (EIS) measurements confirmed that the resulting phase increased the total resistance of LAGP. This work aims at providing a method for an accurate determination of ESWs, considered a key parameter to a successful material selection for ASSLBs. © Copyrigh

Synthesis, structure, and electrochemical properties of LiFeV2O7

The structure of a novel compound, LiFeV2O7, has been determined from single-crystal X-ray diffraction data. The phase crystallizes in the non-centrosymmetric monoclinic Cc space group. The structure can be described as a layered type compound alternating (Li,Fe)–O sheets and V–O chains that are perpendicular to the [101] direction. Within the (Li,Fe)–O sheets, “hexagonal” holes are formed and assembled into tunnels running parallel to the [201] direction and hosting the vanadium atoms. Original (V4O14)8– strings are observed within the structure in association with well-known (V2O7)4– pyrovanadate units. Both units alternate parallel to the [−101] direction. LiFeV2O7 displays a reversible insertion–deinsertion mechanism for Li+ ions. The theoretical capacity for the insertion of one Li+ into LiFeV2O7 reaches 97 mAh/g. When the compound is cycled between 3.50 and 2.35 V versus Li+/Li, the electrochemical curve displays an initial capacity of 100 mAh/g, with 85% of this capacity retained after 60 cycles. No evidence of the formation of Fe4+ upon oxidation to a high voltage was observed. LiFeV2O7 represents the first reported phase in the Li2O–Fe2O3–V2O5 ternary diagram with electrochemical activities

Mg3(BH4)4(NH2)2 as Inorganic Solid Electrolyte with High Mg2+ Ionic Conductivity

Mg3(BH4)4(NH2)2 compound was synthesized through the investigation of the Mg(BH4)2-Mg(NH2)2 phase diagram; its crystal structure was solved in a tetragonal unit cell with the space group I4̅. Interestingly, Mg3(BH4)4(NH2)2 has a high thermal stability with a decomposition temperature above 190 °C and exhibits a high Mg2+ ionic conductivity of 4.1 × 10−5 S·cm−1 at 100 °C with a low activation energy (0.84 eV). The reversible Mg deposition/stripping was demonstrated at 100 °C when using Mg3(BH4)4(NH2)2 as solid electrolyte. Thus, Mg3(BH4)4(NH2)2 is a compound that could help to develop rechargeable Mg-ion solid-state batteries

Synthesis, Structure, and Electrochemical Properties of LiFeV₂O₇

The structure of a novel compound, LiFeV₂O₇, has been determined from single-crystal X-ray diffraction data. The phase crystallizes in the non-centrosymmetric monoclinic <i>Cc</i> space group. The structure can be described as a layered type compound alternating (Li,Fe)–O sheets and V–O chains that are perpendicular to the [101] direction. Within the (Li,Fe)–O sheets, “hexagonal” holes are formed and assembled into tunnels running parallel to the [201] direction and hosting the vanadium atoms. Original (V₄O₁₄)^8– strings are observed within the structure in association with well-known (V₂O₇)^4– pyrovanadate units. Both units alternate parallel to the [−101] direction. LiFeV₂O₇ displays a reversible insertion–deinsertion mechanism for Li⁺ ions. The theoretical capacity for the insertion of one Li⁺ into LiFeV₂O₇ reaches 97 mAh/g. When the compound is cycled between 3.50 and 2.35 V versus Li⁺/Li, the electrochemical curve displays an initial capacity of 100 mAh/g, with 85% of this capacity retained after 60 cycles. No evidence of the formation of Fe⁴⁺ upon oxidation to a high voltage was observed. LiFeV₂O₇ represents the first reported phase in the Li₂O–Fe₂O₃–V₂O₅ ternary diagram with electrochemical activities

Synthesis, Structure, and Electrochemical Properties of LiFeV₂O₇

The structure of a novel compound, LiFeV₂O₇, has been determined from single-crystal X-ray diffraction data. The phase crystallizes in the non-centrosymmetric monoclinic <i>Cc</i> space group. The structure can be described as a layered type compound alternating (Li,Fe)–O sheets and V–O chains that are perpendicular to the [101] direction. Within the (Li,Fe)–O sheets, “hexagonal” holes are formed and assembled into tunnels running parallel to the [201] direction and hosting the vanadium atoms. Original (V₄O₁₄)^8– strings are observed within the structure in association with well-known (V₂O₇)^4– pyrovanadate units. Both units alternate parallel to the [−101] direction. LiFeV₂O₇ displays a reversible insertion–deinsertion mechanism for Li⁺ ions. The theoretical capacity for the insertion of one Li⁺ into LiFeV₂O₇ reaches 97 mAh/g. When the compound is cycled between 3.50 and 2.35 V versus Li⁺/Li, the electrochemical curve displays an initial capacity of 100 mAh/g, with 85% of this capacity retained after 60 cycles. No evidence of the formation of Fe⁴⁺ upon oxidation to a high voltage was observed. LiFeV₂O₇ represents the first reported phase in the Li₂O–Fe₂O₃–V₂O₅ ternary diagram with electrochemical activities