Publications of Frédéric Boschini
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See detailSpray drying synthesis of Na3V2(PO4)2F3/C cathode material for Na-ion batteries: study of the effect of the carbon source on electrochemical performance
Eshraghi, Nicolas ULiege; Bodart, Jérôme ULiege; Mahmoud, Abdelfattah ULiege et al

Poster (2018, November 25)

Lithium-ion batteries have enjoyed great success and have outperformed other rechargeable battery system since 1980. However, Li-ion batteries face many challenges and limitations: safety, the low ... [more ▼]

Lithium-ion batteries have enjoyed great success and have outperformed other rechargeable battery system since 1980. However, Li-ion batteries face many challenges and limitations: safety, the low abundance of lithium in the Earth’s crust. Recently, Sodium-ion batteries attracted a lot of interest as a potential alternative to lithium-ion batteries for large-scale energy storage applications, due to the large natural abundance and lower cost of sodium. In recent years, fluorophosphates with the NASICON (Na Super-Ionic Conductor) type structure are considered among the most interesting series of cathode materials for Li/Na-ion batteries, because they exhibit rich chemistry, attractive lithium/sodium insertion properties and thus offer promising electrochemical properties [1]. Na3V2(PO4)2F3 (NVPF) attracted high attention thanks to its promising electrochemical properties. The inductive effects of both PO43- and F- allow for a high working potential combined with a high theoretical specific capacity due to the multiple oxidation states of vanadium[1-2]. One of the key drawbacks of Na3V2(PO4)2F3 electrodes is their low intrinsic electronic conductivity. NVPF and NVPF/carbon composite materials were prepared by spray-drying method using the same conditions used in our previous work [2]. Spray drying is a cost-effective and easily up-scalable route to prepare homogeneous multi-component powders, thus making it a suitable method to incorporate carbon in the composite powder. We used different carbon sources like conductive carbons (MWCNTs, Carbon Black, etc) and organic sources (PVA, Citric Acid, Ascorbic acid, etc) to prepare NVPF/carbon composite powders. the structural, electrochemical, and morphological properties of the synthesized Na3V2(PO4)2F3/C samples were systematically investigated in order to understand the influence of carbon source on structural and morphological properties and most importantly electrochemical performance of NVPF and NVPF/carbon composite cathode materials for Na-ion batteries. The chemical diffusion of Na ions was studied using results obtained by varying scan rates in cyclic voltammetry measurements. Raman spectroscopy is used to evaluate the quality in disordered carbon materials and its electronic conductivity [3] and compared the results with the results from EIS and cycling performance of different samples. Acknowledgments The authors are grateful to University of Liege and FRS-FNRS for grants. Part of this work was supported by the Walloon Region under the “PE PlanMarshall2.vert” program (BATWAL – 1318146). A. Mahmoud thanks to the Walloon region for a Beware Fellowship Academia 2015-1, RESIBAT n° 1510399. References: [1] R. A. Shakoor, D-H. Seo, H. Kim, Y-U. Park, J. Kim, S-W. Kim, H. Gwon, S. Leec and K. Kang, Mater. Chem. 22 (2012) 20535–20541. [2] N. Eshraghi, S. Caes, A. Mahmoud, R. Cloots, B. Vertruyen, F. Boschin, Electrochimica Acta, 228, (2017), 319-324. [3] A. Mahmoud, S. Caes, M. Brisbois, R. P. Hermann, L. Berardo, A. Schrijnemakers, C. Malherbe, G. Eppe, R. Cloots, B. Vertruyen, F. Boschin J Solid State Electrochem. 22 (2018) 103-112. [less ▲]

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See detailElectrochemical mechanism and effect of carbon addition during hydrothermal synthesis to improve the electrochemical performance of Fe1.19(PO4)(OH)0.57(H2O)0.43 cathode material for Li-ion batteries
Mahmoud, Abdelfattah ULiege; karegeya, Claude; Sougrati, Moulay Tahar et al

Poster (2018, November 25)

Since the introduction of lithium-ion batteries (LIBs) to market in 1991, their performance has improved significantly, which has been achievable through development in materials technologies. However ... [more ▼]

Since the introduction of lithium-ion batteries (LIBs) to market in 1991, their performance has improved significantly, which has been achievable through development in materials technologies. However, further breakthroughs are still needed to ameliorate cycle-life, safety and energy density of LIBs. This requires new electrode materials and a detailed understanding of the electrochemical mechanisms during cycling. Transition metal phosphates are interesting candidates as cathode materials for LIBs [1]. In this work, we report the electrochemical performance of FPHH/C and FPHH/CNT composites where FPHH represents Fe1.19(PO4)(OH)0.57(H2O)0.43 while carbon black and carbon nanotubes (CNT) were used as precursors in the one-pot hydrothermal synthesis, respectively. We show that the addition of conducting carbon black into the solution has a strong influence on reducing the particle size and tailoring their morphology, but does not interfere with the formation of the FPHH phase. Thanks to its favorable microstructural characteristics, the FPHH-10 wt% C and FPHH-20 wt% C materials exhibited good performance [2]. The CNT also improve the performance of FPHH such as capacity retention (500 cycles at 1 C). The mechanisms of lithiation-delithiation were investigated by combining operando X-ray diffraction and 57Fe Mössbauer spectroscopy. FPHH undergoes a monophasic reaction based on Fe3+/Fe2+ redox process. However, the variations of the lattice parameters and 57Fe quadrupole splitting indicate a more complex mechanism than a random occupation of the vacant sites within FPHH. This can be related to the peculiar structure of FPHH formed by chains of face sharing (Fe0.6•0.4)O6 octahedra connected by PO4 tetrahedra and by channels for Li diffusion along [100] and [010] directions. The existence of Fe vacancies provide interconnections between the one-dimensional channels, improving lithium diffusion within FPHH. This mechanism, combined with the addition carbon black or nanotubes in the solution prior to hydrothermal treatment as a simple and effective way to reduce particle size and improve electronic conductivity, provides good cycle life and rate capability for FPHH. Acknowledgements A. Mahmoud is grateful to University of Liege and FRS-FNRS for the grants and thanks to the Walloon region for a Beware Fellowship Academia 2015-1, RESIBAT n° 1510399. Part of this work was supported by the Walloon Region under the “PE PlanMarshall2.vert” program (BATWAL – 1318146). References 1. C. Karegeya, A. Mahmoud, F. Hatert, B. Vertruyen, R. Cloots, P.E. Lippens, F. Boschini, Journal of Power Sources 388 (2018) 57-64. 2. C. Karegeya, A. Mahmoud, R. Cloots, B. Vertruyen, F. Boschini, Electrochim. Acta 250 (2017) 49-58. [less ▲]

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See detailCrystal chemistry and polymorphism of an iron-tungsten mixed oxide
Caubergh, Stéphane ULiege; Jungers, Thomas ULiege; Mahmoud, Abdelfattah ULiege et al

Conference (2018, October 17)

Among the complex metal oxides, the crystal chemistry of the iron-tungsten oxide Fe2WO6 is of interest because of the existence of several crystal structures with different organizations of the FeO6 and ... [more ▼]

Among the complex metal oxides, the crystal chemistry of the iron-tungsten oxide Fe2WO6 is of interest because of the existence of several crystal structures with different organizations of the FeO6 and WO6 octahedra. The conditions of polymorphism are still unclear: structures were reported in the literature for only two of the three polymorphs (α and γ) and very little is known about the third polymorph (β). For the first time, we could synthesize a complete set of the three polymorphs as single-phase products by spray-drying. We proposed a new structural model for α-polymorph. We confirmed the existence of β-polymorph and solved its structure. With this procedure of synthesis, we were also able to control and tune the Fe/W ratio and investigate the possibility for a solid solution domain. The final products were characterized by means of X-Ray Diffraction (synchrotron data), Rietveld refinement, Scanning Electron Microscopy and 57Fe Mossbauer spectroscopy. [less ▲]

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See detailCrystal chemistry and polymorphism of an iron-tungsten mixed oxide
Caubergh, Stéphane ULiege; Jungers, Thomas ULiege; Mahmoud, Abdelfattah ULiege et al

Poster (2018, October 17)

Among the complex metal oxides, the crystal chemistry of the iron-tungsten oxide Fe2WO6 is of interest because of the existence of several crystal structures with different organizations of the FeO6 and ... [more ▼]

Among the complex metal oxides, the crystal chemistry of the iron-tungsten oxide Fe2WO6 is of interest because of the existence of several crystal structures with different organizations of the FeO6 and WO6 octahedra. The conditions of polymorphism are still unclear: structures were reported in the literature for only two of the three polymorphs (α and γ) (image A) and very little is known about the third polymorph (β). In this work, our aim is to synthesize the different polymorphs of Fe2WO6 as pure products in order to be able to investigate the crystal structures and properties in more detail. We report about the synthesis procedures that we have developed starting from aqueous solutions to minimize the effects of diffusion length and maintain a homogeneous distribution of the cations during the phase formation. The stable solutions were spray-dried in order to obtain spherical particles of dried powder as small and homogeneous as possible (around 1 µm). With this technique, we were also able to control and tune the Fe/W ratio and investigate the possibility for a solid solution domain. The dried powders underwent heat treatments at various temperatures and durations before characterization by means of X-Ray Diffraction, Rietveld refinement, Scanning Electron Microscopy and 57Fe Mossbauer spectroscopy. [less ▲]

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See detailFlexible thin-films for battery electrodes
Piffet, Caroline ULiege; Boschini, Frédéric ULiege; Cloots, Rudi ULiege

Patent (2018)

A composition comprising: a. An at least partially hydrolysed polyvinyl acetate component having an hydrolysation degree of at least 5%, b. A polyalkylene glycol component having a number average ... [more ▼]

A composition comprising: a. An at least partially hydrolysed polyvinyl acetate component having an hydrolysation degree of at least 5%, b. A polyalkylene glycol component having a number average molecular mass Mn lower than 9000 g/mol and consisting of one or more substances selected from the group consisting of polyethylene glycol, polypropylene glycol, copolymers of ethylene glycol and propylene glycol, and their derivatives, c. a positive or negative electrode active component, and d. a conductive component, wherein the mass ratio between the at least partially hydrolysed polyvinyl acetate component and the positive or negative electrode active component equals at least 0.12 and at most 0.30, and wherein the mass ratio between the polyalkylene glycol component and the positive or negative electrode active component equals at least 0.012 and at most 0.10 [less ▲]

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See detailCrystal chemistry and polymorphism of an iron-tungsten mixed oxide
Caubergh, Stéphane ULiege; Jungers, Thomas ULiege; Mahmoud, Abdelfattah ULiege et al

Poster (2018, July 09)

Among the complex metal oxides, the crystal chemistry of the iron-tungsten oxide Fe2WO6 is of interest because of the existence of several crystal structures with different organizations of the FeO6 and ... [more ▼]

Among the complex metal oxides, the crystal chemistry of the iron-tungsten oxide Fe2WO6 is of interest because of the existence of several crystal structures with different organizations of the FeO6 and WO6 octahedra. The conditions of polymorphism are still unclear: structures were reported in the literature for only two of the three polymorphs (α and γ) (image A) and very little is known about the third polymorph (β). In this work, our aim is to synthesize the different polymorphs of Fe2WO6 as pure products in order to be able to investigate the crystal structures and properties in more detail. We report about the synthesis procedures that we have developed starting from aqueous solutions to minimize the effects of diffusion length and maintain a homogeneous distribution of the cations during the phase formation. The stable solutions were spray-dried in order to obtain spherical particles of dried powder as small and homogeneous as possible (around 1 µm). With this technique, we were also able to control and tune the Fe/W ratio and investigate the possibility for a solid solution domain. The dried powders underwent heat treatments at various temperatures and durations before characterization by means of X-Ray Diffraction, Rietveld refinement, Scanning Electron Microscopy and 57Fe Mossbauer spectroscopy. [less ▲]

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See detailSpray-Drying Synthesis of Na2FePO4F as Positive Electrode for Na-ion Batteries
Bodart, Jérôme ULiege; Mahmoud, Abdelfattah ULiege; Vertruyen, Bénédicte ULiege et al

Conference (2018, July 05)

The goal of this works is minimized post-treatment of batteries material synthesis, as grinding or carbon addition. We have add during synthesis carbon nanotubes to enhance the conductivity of the ... [more ▼]

The goal of this works is minimized post-treatment of batteries material synthesis, as grinding or carbon addition. We have add during synthesis carbon nanotubes to enhance the conductivity of the phosphate based material Na2FePO4F. This addition of carbon induce a modifications of the cristallisation and lead to impureties. By modifying the attack of iron(0) and adapting the heat treatment we have enhance the purity of the material. We have also compare the influence of the injection mode on the size which is drastically reduce (8micron to 1.5micron); on the electrochemical performance ( 110mAh/g for rotative nozzle, 140mAh/g for the bifluid) and on the oxidation state of the iron in the product analysed by mossbauer spectroscopy (16% of Fe(III) with rotative nozzle; 12% of Fe(III) with the bifluid nozzle). [less ▲]

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See detailNa1.25Ni1.25Fe1.75(PO4)3 nanoparticles as a janus electrode material for Li-ion batteries
Karegeya, Claude; Mahmoud, Abdelfattah ULiege; Hatert, Frédéric ULiege et al

in Journal of Power Sources (2018), 388

Abstract A solvothermal method was used to prepare Na1.25Ni1.25Fe1.75(PO4)3 nanoparticles, a new promising electrode material for lithium-ion batteries. The composition and the crystal structure were ... [more ▼]

Abstract A solvothermal method was used to prepare Na1.25Ni1.25Fe1.75(PO4)3 nanoparticles, a new promising electrode material for lithium-ion batteries. The composition and the crystal structure were determined by 57Fe Mössbauer spectroscopy and powder X-ray diffraction Rietveld refinements and confirmed by magnetic measurements. The structural formula □0.75Na1.25Ni1.25Fe1.75(PO4)3 was obtained showing a significant amount of Na vacancies, which enhances Li diffusion. Na1.25Ni1.25Fe1.75(PO4)3 was used as negative and positive electrode material and shows excellent electrochemical performances. As negative electrode in the voltage range 0.03-3.5 V vs. Li+/Li, the first discharge at current density of 40 mA g−1 delivers a specific capacity of 1186 mAh g−1, which is almost three times its theoretical capacity (428 mAh g−1). Then, reversible capacity of 550 mAh g−1 was obtained at 50 mA g−1 with high rate capability (150 mAh g−1 at 500 mA g−1) and capacity retention of 350 cycles. As positive electrode material, specific capacities of about 145 and 99 mAh g−1 were delivered at current densities of 5 and 50 mA g−1, respectively, in the voltage range of 1.5–4.5 V vs. Li+/Li. In addition, we show that the use of solvothermal synthesis contributes to the synthesis of small sized particles leading to good electrochemical performances. [less ▲]

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See detailComposites Fe1.19(PO4)(OH)0.57(H2O)0.43/C comme matériaux d'électrode positive pour batteries Li-ion
Mahmoud, Abdelfattah ULiege; karegeya, Claude; Sougrati, Moulay Tahar et al

Conference (2018, May 17)

Les matériaux composites Fe1.19(PO4)(OH)0.57(H2O)0.43-C (FPHH-C) ont été obtenus par synthèse hydrothermale en une seule étape. L'addition du noir de carbone dans la solution contenant les précurseurs ... [more ▼]

Les matériaux composites Fe1.19(PO4)(OH)0.57(H2O)0.43-C (FPHH-C) ont été obtenus par synthèse hydrothermale en une seule étape. L'addition du noir de carbone dans la solution contenant les précurseurs avant la réaction hydrothermale (2, 10 et 20 %m) a conduit à une réduction de la taille des particules de FPHH avec une bonne dispersion du carbone conducteur électronique autour et entre les particules. La caractérisation des composites par diffraction des rayons X et spectroscopie Mössbauer du 57Fe montre que l'ajout de carbone ne modifie pas la pureté des matériaux mais améliore significativement les propriétés électrochimiques pour FPHH - 10 %m et FPHH - 20 %m par rapport à FPHH et FPHH - 2 %m [1]. Une étude du mécanisme d'intercalation et de désintercalation du lithium a été effectuée en associant diffraction des rayons X et spectroscopie Mössbauer operando [2]. L’analyse de l’ensemble des résultats montre que ce mécanisme est une réaction d'intercalation monophasique réversible associée au couple redox Fe3+/Fe2+. L’excellente réversibilité observée sur plus de 500 cycles avec des variations volumiques de 10% confirme la bonne stabilité de ce matériau. Remerciements Ab. Mahmoud remercie la région wallonne pour le projet RESIBAT n°1510399. Références 1. C. Karegeya, A. Mahmoud, R. Cloots, B. Vertruyen, F. Boschini, Electrochim. Acta 250 (2017) 49-58. 2. K. Lasri, A. Mahmoud, I. Saadoune, M-T. Sougrati, L. Stievan, P-E. Lippens, R. P. Hermann, H. Ehrenberg, Sol. Energ. Mater. & Sol. Cells148 (2016) 11–19. [less ▲]

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See detailStudy of the Properties of Co-Substituted Ba2Mg2Fe12O22 Hexaferrites
Koutzarova, Tatyana; Georgieva, Borislava; Kolev, Svetoslav et al

in Proceedings of The 3rd International Electronic Conference on Materials Sciences (2018, May 17)

Multiferroic materials, in which long-range magnetic and ferroelectric orders coexist, have recently been of great interest in the fields of both basic and applied sciences. The Y-type hexagonal ferrite ... [more ▼]

Multiferroic materials, in which long-range magnetic and ferroelectric orders coexist, have recently been of great interest in the fields of both basic and applied sciences. The Y-type hexagonal ferrite Ba2Mg2Fe12O22 is an example of a multiferroic material. Its single crystals have a relatively high spiral-magnetic transition temperature (~200 K), show multiferroic properties at zero magnetic field, and the direction of the ferroelectric polarization can be controlled by a weak magnetic field (< 0.02 T) [1]. We present a study of the influence of substituting the Mg2+ cations in the Y-type Ba2Mg2Fe12O22 hexaferrites with a magnetic cation, such as Co2+, on the structural and magnetic properties. The Ba2Mg0.4Co1.6Fe12O22 powder was synthesized by sonochemical co-precipitation. High-power ultrasound was applied to assist the co-precipitation process. The precursors produced were synthesized at 1170°С. The XRD spectra of the powders showed the characteristic peaks corresponding to the Y-type hexaferrite structure as a main phase and some CoFe2O4 impurity (< 2%) as second phase. This was also confirmed by Mössbauer spectroscopy measurements. The magnetization values at 50 kOe were 30 emu/g and 26.6 emu/g at 4.2 and 300 K, respectively. The ZFC and FC magnetization curves were obtained at a magnetic field of 500 Oe. The magnetic measurements revealed a magnetic phase transition at 200 K from ferrimagnetic-to-helical spin order. Such a transition is considered as a precondition for the material to exhibit multiferroic properties. [less ▲]

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See detailStructural, magnetic and magnetocaloric properties in Rare earth orthochromite (Sm, Ndand La)CrO3 for cooling product
Fkhar, Lahcen; Mounkachi, Omar; Hamedoun, Mohammed et al

Poster (2018, April 25)

Polycrystalline phase of SmCrO3, NdCrO3 and LaCrO3 orthochromites were synthetized with solid state reaction. The pure orthorhombic phases with space group Pnma (62) were been confirmed by X-Ray ... [more ▼]

Polycrystalline phase of SmCrO3, NdCrO3 and LaCrO3 orthochromites were synthetized with solid state reaction. The pure orthorhombic phases with space group Pnma (62) were been confirmed by X-Ray diffraction. In addition, the FTIR spectroscopy proved the formation Sm-O, Nd-O and La-O liaisons in SmCrO3, NdCrO3 and LaCrO3 respectively, and Cr-O and Cr-O-Cr liaisons in all our samples. The particles morphology and composition were seeing using the Scanning Electron Microscopy (SEM). The magnetic characterization was been performed by the Quantum Design XL-SQUID magnetometer in order to determine the magnetic transition type and transition temperature of each sample. We found that our samples exhibit a transition at 195 K, 65 K and 35 K for SmCrO3, NdCrO3 and LaCrO3 respectively. The magnetic entropy change (∆Sm) as the key parameter to evaluate the magnetocaloric effect is calculated. Our samples present a ∆Sm in the range of 0.11 and 0.25 J.kg-1.K-1 for an external magnetic field of 5 Tesla. [less ▲]

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See detailSpray-drying of electrode materials for lithium- and sodium-ion batteries
Vertruyen, Bénédicte ULiege; Eshraghi, Nicolas ULiege; Piffet, Caroline ULiege et al

in Materials (2018), 11(7), 1076

The performance of electrode materials in lithium-ion (Li-ion), sodium-ion (Na-ion) and related batteries depends not only on their chemical composition but also on their microstructure. The choice of a ... [more ▼]

The performance of electrode materials in lithium-ion (Li-ion), sodium-ion (Na-ion) and related batteries depends not only on their chemical composition but also on their microstructure. The choice of a synthesis method is therefore of paramount importance. Amongst the wide variety of synthesis or shaping routes reported for an ever-increasing panel of compositions, spray-drying stands out as a versatile tool offering demonstrated potential for up-scaling to industrial quantities. In this review, we provide an overview of the rapidly increasing literature including both spray-drying of solutions and spray-drying of suspensions. We focus, in particular, on the chemical aspects of the formulation of the solution/suspension to be spray-dried. We also consider the post-processing of the spray-dried precursors and the resulting morphologies of granules. The review references more than 300 publications in tables where entries are listed based on final compound composition, starting materials, sources of carbon etc. © 2018 by the authors. [less ▲]

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See detailLi4Ti5O12 powders by spray-drying: influence of the solution concentration and particle size on the electrochemical properties
Jamin, Claire; Jungers, Thomas ULiege; Piffet, Caroline ULiege et al

in Journal of Physics. Conference Series (2018), 1081

The lithium battery electrode compound Li4Ti5O12 was synthesized by calcination of precursor powders obtained through spray-drying of solutions prepared with titanium isopropoxide and lithium nitrate. X ... [more ▼]

The lithium battery electrode compound Li4Ti5O12 was synthesized by calcination of precursor powders obtained through spray-drying of solutions prepared with titanium isopropoxide and lithium nitrate. X-ray diffraction and thermal analysis coupled to mass spectrometry show that single phase crystalline Li4Ti5O12 particles can be obtained after calcination at 800 °C for 2 hours. Decreasing the solution concentration leads to smaller particle sizes but also to an unexpected decrease of the electrochemical capacity, probably related to the presence of residual Li2TiO3. On the contrary, the capacity of the Li4Ti5O12 powder prepared with the high concentration solution can be increased from 150 mAh/g to 165 mAh/g (C/4 rate) by grinding. These results highlight the fact that smaller particles do not systematically display better performances for Li+ intercalation/desintercalation and confirm the need for a comprehensive approach including parameters such as crystallinity, phase purity or agglomeration. [less ▲]

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See detailFacile solvothermal synthesis of Na1.5□0.5Mn1.5Fe1.5(PO4)3: Electrochemical study as a dual electrode material for lithium-ion batteries
Karegeya, C.; Mahmoud, Abdelfattah ULiege; Hatert, Frédéric ULiege et al

in Solid State Ionics (2018), 326

Na1.5□0.5Mn1.5Fe1.5(PO4)3 (where □ denotes vacancies) microrods were obtained through a solvothermal synthesis in ethylene glycol. The combination of the XRD, Mössbauer and magnetic analyses confirm that ... [more ▼]

Na1.5□0.5Mn1.5Fe1.5(PO4)3 (where □ denotes vacancies) microrods were obtained through a solvothermal synthesis in ethylene glycol. The combination of the XRD, Mössbauer and magnetic analyses confirm that the sodium vacancies in the Na1.5□0.5Mn1.5Fe1.5(PO4)3 structure are linked to the oxidation of Mn and Fe transition metals. The electrochemical tests have shown that Na1.5□0.5Mn1.5Fe1.5(PO4)3 is a dual electrode material for Li-ion batteries. The electrochemical study in the potential range of 1.5–4.5 V indicate that such material can be used as 3 V cathode with specific capacities of 109, 97, and 80 mAh·g−1 at current densities of 5, 10, and 20 mA·g−1, respectively. When it is tested in the potential range of 0.03–3.0 V as negative electrode material, it delivers a reversible capacity of about 170 mAh·g−1 at 200 mA·g−1 current density during >100 cycles. © 2018 Elsevier B.V. [less ▲]

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See detailPreparation of p-type Bi2Sr2Co2Ox and n-type doped CaMnO3 polycrystalline thermoelectric bulk materials by Pilot Spray Drying processing way
Combe, Emmanuel ULiege; Bastin, Frédérique; Caes, Sébastin et al

Conference (2017, December 13)

Since decades, thermoelectricity is deeply studied as no-waste energy source. Thermoelectric (TE) materials allow the direct transformation of a heat source into an electrical current (Seebeck effect). TE ... [more ▼]

Since decades, thermoelectricity is deeply studied as no-waste energy source. Thermoelectric (TE) materials allow the direct transformation of a heat source into an electrical current (Seebeck effect). TE conversion is very attractive as it is the strongest candidate for producing electricity from waste heat energy sources (automobiles, incinerators, boilers, …). For TE applications at high temperatures, oxide compounds have attracted attention, in particular due to their natural oxidation resistance. However, for large scale utilization of TE conversion systems, performances of actual TE oxides compounds must be improved. Among the different studied oxide compositions, misfit cobaltite Bi2Sr2Co2Ox and doped calcium manganate CaMnO3- have attracted much attention, as p and n-type oxide TE elements, respectively. Bi2Sr2Co2Ox (abbreviated BSC-222) material exhibits a layered structure, composed of CoO2 layers which alternate with Bi2Sr2O4 block layers along the c-axis. Such crystallographic structure allows interesting electrical and thermal properties, suitable for reaching high TE performances. On the other hand, CaMnO3- compound (abbreviated as CMO) has an orthorhombic perovskite-type structure with interesting TE properties. Reaching high TE performances requires using new processing way. Indeed, conventional solid-state reaction does not allow controlling the microstructure of materials, i.e. the shape, size, or alignment of grains, size and distribution of porosity of samples. Those parameters play a major role on the electrical and thermal transport properties. With the aim of developing a processing method which can be easily transposable to an industrial level in terms of simplicity, speed, large-scale production and cost, a pilot spray-drying processing method has been developed for the production of BSC-222 and CMO thermoelectric powders. Spray-drying is a processing way in which preparing powders is achieved by passing a solution, or a suspension, in a stream of hot air. By using solutions, or suspensions, of precursor compounds, a more homogeneous dispersion of the cations into the material is reaching. Spray-dried powders are so more reactive, which can help reducing the sintering temperature or duration. This study deals with the preparation of BSC-222 and doped CMO thermoelectric powders by the development of spray-drying method. Contrary to CMO powders, prepared from solution of cations, BSC-222 powders have been prepared from Bi2O3-Co3O4-SrCO3 suspensions. For that purpose, a preliminary study of the stability of the precursors in the suspension’s solvent has been conducted in order to favor a homogeneous distribution of precursors in prepared spray-dried powders. After calcination thermal treatment, spray dried powders with BSC-222 or CMO compositions have been successfully obtained. Polycrystalline BSC-222 and CMO bulk materials have been then prepared by conventional sintering. In this study, we report also on the thermoelectric properties, carried out up to 1000 K, of such prepared polycrystalline bulk materials and compare these with those reported in literature. [less ▲]

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