Johann Chable

Ce travail porte sur la synthèse, la mise en forme et la caractérisation desolutions solides de type tysonite RE1-xMxF3-x (RE = La, Sm, Ce et M = Ba, Ca, Sr). Dans unpremier temps, une démarche d’étude rigoureuse est mise en place pour la solution solide ditede référence, La1-xBaxF3-x. Les synthèses menées à l’état solide aboutissent à une maîtrise dela composition chimique et à l'établissement de lois de variations des paramètres structuraux.Une meilleure compréhension de l’influence de la structure sur la mobilité des ions F- estégalement acquise. L’influence du frittage dans l’obtention de bonnes valeurs de conductivitéionique est également à souligner. Dans un second temps, les effets de la nanostructurationpar mécanobroyage sur les propriétés de conductivité sont évalués. L’utilisation de laméthodologie des plans d’expériences mène à la mise au point des réglages optimums debroyage. Il apparaît alors que la synthèse des électrolytes peut être accélérée et mise àl’échelle tout en gardant des valeurs optimales de conductivité. Enfin, la démarche déterminéeest appliquée à d'autres solutions solides de type tysonite et à la recherche du conducteurionique le plus performant. Si les composés issus de la substitution Ce/Sr ou encore Sm/Casemblent les plus prometteurs, la plus grande stabilité chimique de la solution solide La1-xSrxF3-x est le compromis idéal pour l'utiliser comme électrolyte solide lors des mesuresélectrochimiques des batteries.

The recycling of LiNi_0.6Mn_0.2Co_0.2O₂ cathode active material (CAM) from Li-ion batteries cannot avoid the presence of impurities in the recycled Ni-, Co-, or Mn sulfates. The precise understanding of the impurities influence on the resynthesis is of uttermost importance. To tackle this goal, this study simulated the resynthesis process by adding on purpose and separately Al-, Fe-, and Li-sulfate in the regular CAM synthesis. Their effects on the morphology, elemental composition, crystal structure, and electrochemical performances of Ni_0.6Mn_0.2Co_0.2M_x(OH)₂ and LiNi_0.6Mn_0.2Co_0.2M_xO₂ were systematically investigated for x = 0.0005, 0.005, and 0.05. The in-house reference material synthesized via a coprecipitation reaction allowed to produce a well-characterized basis. Above x_Al = 0.0005, the hydroxide secondary particles lost their spherical form and above x_Fe = 0.005, the crystal structure was affected. Both effects led to cell capacity decay, contrary to the discrete influence of Li. The absence of any positive effects reported by similar studies, the complexity of the various influences of these foreign ions and the limitations of the coin cells tests were pointed out. These results are a solid groundwork for future academic and industrial studies in the field of circular economy, by determining a tolerance threshold for each impurity.

Among the different factors that influence lithium-ion batteries (LIB) performances, the properties of the cathode active material (CAM) play a crucial role. The composition, the crystallinity and also the particle size and morphology of these materials are part of the major parameters governing the CAM behaviour. These parameters are strongly dependent of the synthesis method, especially in the case of the hydroxide precursor of the layered LiNi_xMn_yCo_zO₂ ^[1]. One of the most widely spread synthesis method of such precursors is the synthesis by co-precipitation, where metal salts of Ni, Mn and Co are precipitated together in a continually-stirred tank reactor (CSTR) in carefully controlled pH- and temperature conditions^[2]. Any parameter of the co-precipitation must be precisely tuned to obtain the desired material, as highlighted in the figure here after. Considering the increasing presence of wanted (coating/doping)^[3] and unwanted (impurities after recycling)^[4] elements in the co-precipitation process, extensive knowledge of their impact on it is of great interest. Therefore, the synthesis of Ni_xMn_yCo_z(OH)₂ was conducted by co-precipitation with the controlled addition of increasing amounts of elements such as Cu and Al. The effects of these additions were investigated by thoroughly characterizing the morphology of the obtained hydroxides with the help of SEM, BET, particle size distribution and porosity measurements. The composition of the synthesized hydroxides was determined by the ICP-OES technique. The possible influence on the further step of CAM synthesis, i.e. calcination, was followed by in-situ X-Ray thermodiffraction and the electrochemical performances were assessed by galvanostatic cycling the obtained CAM in lithium half-cells. This helped to evidence the varying impact of the added elements, depending on their concentration in the synthesis process. This work is supported by the German Ministry for Economy and Climate Protection (BMWK). References [1] Malik, M., et al., Materials Today Energy, 2022, 28, 101066 [2] Van Bommel, A., Dahn, J.R., Chemistry of Materials, 2009, 21, 1500-1503 [3] Feng, Z. et al., ACS Applied Materials and Interfaces, 2021, 13, 43039-43050 [4] Kim, S. et al., Journal of Alloys and Compounds, 2021, 857, 157581 Figure 1

The stability of the cathode is of crucial importance for the service life of lithium-ion batteries (LIB). In the cathode, the lattice parameter usually changes during (de-)insertion of lithium, which causes various signs of aging that have a negative effect on the capacity of the cell. This problem can be tackled using a specific type of materials called zero-strain (ZS) materials. These materials do not show any (de-)intercalation-induced structural changes and stresses. Such a zero-strain effect was already demonstrated with fluorine-containing materials ^[1] and even for the similar material K_0.6FeF₃ ^[2] for sodium-ion batteries (SIB). In this case, potassium ions are removed in the first cycle and the vacant lattice sites can be used for the incorporation of sodium ions, while keeping the structural integrity of the host material. Complementary to experiments, theoretical atomistic simulations are powerful to develop design criteria for further ZS-materials. Combining experiments and simulations in this work, the structural behavior of the material K_0.5FeF₃ was investigated when used as a cathode material in a LIB. The synthesis of K_0.5FeF₃ was achieved by a Rapid Microwave-Enhanced Solvothermal Process, which allowed to obtain the material in a TTB-type (Tetragonal Tungsten Bronze) phase with a high degree of purity. The material was then processed into electrodes, which were galvanostatically cycled against lithium metal. It was shown that the material is suitable for LIB as a cathode material. X-ray diffraction experiments showed that the lattice parameter changed by less than 1% after the incorporation of Li-Ions, in good agreement with the theoretical calculations conducted in parallel. Lattice Parameter Charged Discharged a[Å] 12.74±0.024 12.73±0.006 c[Å] 3.99±0.008 4.00±0.002 Volume [ų] 647.57±2.67 647.75±0.76 This work was supported by the German Research Foundation (DFG) BI 1636/7-1 and EL 155/29-1. References [1] De Biasi, L., et al. Journal of Power Sources, 2017, 362. Jg., S. 192-201. [2] Han, Y., et al. Journal of Materials Chemistry A, 2016, 4. Jg., Nr. 19, S. 7382-7389. Figure 1: Diffractograms of an electrode with K_0.5FeF₃, which was measured before use and an electrode after the first successful discharge, down to a voltage of 1.8 V. Figure 1

Promising theoretical capacities and high voltages are offered by Li-rich disordered rocksalt oxyfluoride materials as cathodes in lithium ion batteries. However, as has been discovered for many other Li-rich materials, the oxyfluorides suffer from extensive surface degradation, leading to severe capacity fading. In the case of Li₂VO₂F, we have previously determined this to be a result of detrimental reactions between an unstable surface layer and the organic electrolyte. Herein, we present the protection of Li₂VO₂F particles with AlF₃ surface modification, resulting in a much enhanced capacity retention over 50 cycles. While the specific capacity for the untreated material drops below 100 mA h g^-1 after only 50 cycles, the treated materials retain almost 200 mA h g^-1. Photoelectron spectroscopy depth profiling confirms the stabilisation of the active material surface by the surface modification and reveals its suppression of electrolyte decomposition.

Facile and reversible insertion and extraction of Li+ was recently demonstrated to be possible with a new cation-disordered rock salt (DRS) LiVO₂ compound (Space Group (SG): Fm3 ̅m). Further structural analyses describing the links between the well-known layered trigonal structure of LiVO₂ (space group:R3 ̅m) and DRS LiVO₂ synthesized by mechanical ball milling are presented. Optimized Rietveld refinements of the X-ray diffraction patterns confirm the formation of LiVO₂ in disordered rock salt phase. We observed the phase transition from the trigonal to cubic structure and the evolution of cell parameters during the ball-milling process using ex situ X-ray diffraction. The reverse phase transition from cubic to trigonal structure is achieved via thermal treatment, which was confirmed via differential scanning calorimetry and in situ X-ray diffraction experiments. The phase transition to trigonal structure suggests the metastability of the cubic structure. An exhaustive computational analysis based on a cluster expansion model trained by density functional theory calculations is conducted to confirm the metastability. The presented methods and results show that different (meta-)stable phases can be achieved depending on the synthesis method, which is an important aspect to consider in investigating the properties of the new DRS cathode materials for Li-ion batteries.

Lithium-rich transition metal disordered rock salt (DRS) oxyfluorides have the potential to lessen one large bottleneck for lithium ion batteries by improving the cathode capacity. However, irreversible reactions at the electrode/electrolyte interface have so far led to fast capacity fading during electrochemical cycling. Here, we report the synthesis of two new Li-rich transition metal oxyfluorides Li₂V_0.5Ti_0.5O₂F and Li₂V_0.5Fe_0.5O₂F using the mechanochemical ball milling procedure. Both materials show substantially improved cycling stability compared to Li₂VO₂F. Rietveld refinements of synchrotron X-ray diffraction patterns reveal the DRS structure of the materials. Based on density functional theory (DFT) calculations, we demonstrate that substitution of V³⁺ with Ti³⁺ and Fe³⁺ favors disordering of the mixed metastable DRS oxyfluoride phase. Hard X-ray photoelectron spectroscopy shows that the substitution stabilizes the active material electrode particle surface and increases the reversibility of the V³⁺/V⁵⁺ redox couple. This work presents a strategy for stabilization of the DRS structure leading to improved electrochemical cyclability of the materials.

The strive for increased energy density in Li-ion batteries is particularly dependent on the cathode materials, that so far have been limiting for the overall battery performance. A new class of materials, Li-rich disordered rock-salts, has recently been brought forward as promising candidates for next-generation cathodes due to their ability to reversibly cycle more than one Li-ion per transition metal. Several variants of these Li-rich cathode materials have been developed recently and show promising initial capacities, but challenges concerning capacity fade and voltage decay during cycling need yet to be overcome. Mechanisms behind the significant capacity fade of some materials must be understood to allow for the design of new materials in which detrimental reactions can be mitigated. In this study the origin of the capacity fade in the Li-rich material Li₂VO₂F is investigated and it is shown to begin with a degradation of the particle surface that spreads inwards with continued cycling.

Recently, the ternary spinel selenide MgSc₂Se₄ was proposed to have high magnesium ion mobility and is therefore an interesting potential candidate as a solid electrolyte in magnesium secondary batteries. To test the properties of the material, a modified solid‐state reaction was used to synthesize pure MgSc₂Se₄. Electrochemical characterizations identified detrimental high electronic conductivities, which limit its application as a Mg‐conducting solid electrolyte. Two methods were attempted to lower electronic conductivities, including the implementation of Se‐rich phases and aliovalent doping, however, with no sufficient improvement. Based on the mixed conducting properties of MgSc₂Se₄, a reversible insertion/extraction of Mg²⁺ into the spinel structure could be demonstrated.

A thin layer of amorphous carbon on the surface of CaMoO₄synthesized at a lower temperature suitable for sodium devices storing energy.