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See detailSpray-drying as a versatile synthesis method for fluorophosphate-based compounds as cathodes material for Na-ion batteries
Bodart, Jérôme ULiege; Eshraghi, Nicolas ULiege; Mahmoud, Abdelfattah ULiege et al

Conference (2019, July 22)

Fluorophosphate compounds attract much attention as cathode materials for Na-ion batteries. Indeed, thanks to the inductive effect of the fluoride and phosphate groups, such compounds exhibit a relatively ... [more ▼]

Fluorophosphate compounds attract much attention as cathode materials for Na-ion batteries. Indeed, thanks to the inductive effect of the fluoride and phosphate groups, such compounds exhibit a relatively high working potential and good theoretical capacity [1]. In this work, we focus on the iron (Na2FePO4F (NFPF)) and vanadium (Na3V2(PO4)3F3 (NVPF)) based fluorophosphate materials which are characterized by a high theoretical capacity of 124 mAh/g and 128 mAh/g, respectively. In addition, the iron-based phase is also more environmental friendly due to the low toxicity of the iron. The preparation of such complex phases using conventional solid-state route requires at least two steps with long heat treatment durations. Here, we report a facile aqueous synthesis using the spray-drying method that can be used for both NFPF and NVPF materials and that can be tuned in order to optimize the composition and the morphology of the desired materials. The major limitation of phosphate-based electrodes is their low intrinsic electronic conductivity. The synthesis of composites with carbon is an effective strategy for the enhancement of the electronic conductivity of these compounds. Spray-drying is a suitable method to incorporate carbon in composite compounds due to the high homogeneity of the obtained particle [2]. It is also a cost effective and up-scalable technique which can produce large quantities of the desired material. Here, we report the influence of the carbon addition on the structural, morphological and electrochemical properties of NFPF and NVPF compounds. The prepared composite materials were investigated by combining several characterisation techniques such as XRD, SEM and TEM. The electrochemical properties were evaluated by galvanostatic cycling and electrochemical impedance spectroscopy in Na-ion batteries. [less ▲]

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See detailXPS and Raman study of spray-dried Na3V2(PO4)2F3/C as cathode material for sodium ion batteries
Eshraghi, Nicolas ULiege; Bodart, Jérôme ULiege; Mahmoud, Abdelfattah ULiege et al

Poster (2019, May 28)

Na3V2(PO4)2F3 (NVPF) has attracted much attention as cathode material for Na-ion batteries thanks to the inductive effects allowing for a high working potential combined with a high theoretical specific ... [more ▼]

Na3V2(PO4)2F3 (NVPF) has attracted much attention as cathode material for Na-ion batteries thanks to the inductive effects allowing 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 limitations of NVPF electrodes is their low intrinsic electronic conductivity; therefore, we have prepared composites of NVPF with conductive carbon sources to ensure higher electronic conductivity of the electrodes. NVPF and NVPF/carbon composite materials were prepared by spray-drying as in our previous work [2]. Spray drying is a cost-effective and up-scalable route to prepare homogeneous multi-component powders, thus making it a suitable method to incorporate carbon in the composite powder [4]. The influence of different carbon sources on structural and morphological properties of NVPF and NVPF/carbon composite was investigated by combining several characterization techniques (XRD, SEM and TEM). The chemical composition at the surface of the powders were studied using XPS. Raman spectroscopy was used to evaluate the quality in disordered carbon materials and its electronic conductivity [3] and compare the results with the results from EIS and cycling performance of different samples. References: [1] Shakoor et al., Chem. 22,2012, 20535. [2] Eshraghi et al., Electrochimica Acta, 228, 2017, 319. [3] Mahmoud et al., J Solid State Electrochem. 22, 2018,103. [4] Vertruyen et al., Materials 2018, 11, 1076. [less ▲]

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See detailElectrode materials for Li-ion and Na-ion batteries prepared by hydrothermal and spray drying methods
Mahmoud, Abdelfattah ULiege; Bodart, Jérôme ULiege; karegeya, claude et al

Conference (2019, May 21)

Li-ion batteries are widely used energy storage; however they are limited by availability of materials, price and safety. Regarding economic reasons and preservation of Earth resources, it is necessary to ... [more ▼]

Li-ion batteries are widely used energy storage; however they are limited by availability of materials, price and safety. Regarding economic reasons and preservation of Earth resources, it is necessary to anticipate the lithium substitution by more abundant elements. Sodium-ion batteries are potential candidates to replace Li-ion batteries because the sodium sources are “unlimited”, attainable at low-cost, and geographically distributed. The electrode materials optimization plays a key role in the development of advanced rechargeable batteries. Indeed, the electrode properties have a great influence on the safety, performance and durability of alkali-ion batteries [1]. Spray-drying and hydrothermal (solvothermal) methods are very appropriate to obtain electrode materials for alkali-ion batteries [2-4]. Both techniques are easily adaptable to new materials and can be applied to a wide range of potential electrode materials. These two synthesis methods, suitable for a large-scale industrial production, allow forming spherical aggregates at low temperatures. A variety of synthetic conditions can be tested including different calcination temperatures, dilution of the starting slurry, milling the material in the dry state and in various nonaqueous solvents. In addition, it is possible to mix the precursors with conductive additives during the material synthesis, leading to carbon/active material composites blends at the nanometric level [3]. In this presentation, we report the synthesis and the study of the iron-based phosphate materials (Na1.25Ni1.25Fe1.75(PO4)3, Na2Mn1.5Fe1.5(PO4)3 [4], Na2FePO4F...) for Li/Na-ion batteries. Mössbauer spectroscopy analyses was combined with other characterization techniques (X-ray diffraction, SEM, TEM, impedance spectroscopy, etc.) to study and investigate the structural, magnetic and electrochemical properties of electrode materials prepared by spray-drying and hydrothermal synthesis methods. References [1]. N. Yabuuchi, K.Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (23) (2014) 11636-11682. [2] B. Vertruyen, N. Eshraghi, C. Piffet, J. Bodart, A. Mahmoud, F. Boschini; Materials 11 (7) (2018) 1076. [3] C. Karegeya, A. Mahmoud, R. Cloots, B. Vertruyen, F. Boschini, Electrochimica Acta 250 (2017) 49-58. [4] C. Karegeya, A. Mahmoud, B. Vertruyen, F. Hatert, R. Cloots, F. Boschini. Solid State Ionics, 312 (2017) 88-96 ACKNOWLEDGMENTS The authors are grateful to the University of Liège and FRS-FNRS for the financial support. Part of this work was supported by the Walloon Region under the “PE PlanMarshall2. vert”program (BATWAL 1318146). A.M. is grateful to the Walloon region for a Beware Fellowship Academia 2015-1,RESIBAT no. 1510399 and University of Liége. [less ▲]

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See detailLead-free double perovskite materials for photovoltaic application
Daem, Nathan ULiege; Dewalque, Jennifer ULiege; Spronck, Gilles ULiege et al

Poster (2019, May 13)

This work consists in studying the implementation of double perovskite materials in the form of thin film and showing the effect of the deposition conditions on the structural properties (uniformity ... [more ▼]

This work consists in studying the implementation of double perovskite materials in the form of thin film and showing the effect of the deposition conditions on the structural properties (uniformity, coverage rate, roughness, thickness, crystallinity, crystallite size) and on the optoelectronic properties (light absorption, electron-hole pair generation efficiency, charge diffusion length, recombination...). Before considering ultrasonic spray pyrolysis deposition, preliminary spin-coating tests are ongoing in order to verify the formation of Cs2AgBiBr6 phase according to the protocol reported by Greul et al. (J. Mater Chem A (2017), 19972-19981). The effect of precursors concentration, thermal post-treatment (hot plate/oven, temperature/duration), antisolvent dripping, relative humidity - which are reported as critical parameters for the preparation of high efficiency lead halides perovskite solar cells - on the layer morphology have been studied. The (micro)structural properties of the layers (uniformity, coverage rate, crystallite size, crystalline phase) have been characterized by scanning electron microscopy and X-ray diffraction. To improve charge collection within the lead-free double perovskite photoactive material, we have also investigated a mesoporous TiO2 network as an electron collecting material filled by Cs2AgBiBr6. Efficiencies of 1.7 % have been reached for our best PV cells which are very promising results. [less ▲]

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See detailOpal-like CH3NH3PbI3 perovskite solar cells : effect of the 3D structuration on the conversion efficiency
Dewalque, Jennifer ULiege; Daem, Nathan ULiege; Spronck, Gilles ULiege et al

Poster (2019, May 13)

In this work, the 3D structuration of perovskite films is studied in order to highlight the effect of a periodic porous structure on the optical properties of the films (light harvesting, optical ... [more ▼]

In this work, the 3D structuration of perovskite films is studied in order to highlight the effect of a periodic porous structure on the optical properties of the films (light harvesting, optical coloration, semi-transparency…) and on the PV efficiency, in comparison with dense perovskite films usually used in planar solar cells configuration. The opal-like perovskite scaffold is obtained from templating fabrication method, with polystyrene beads as structuring agent. Five PS bead diameters are studied: 300 nm, 540 nm, 810 nm, 1.0 µm and 2.1 µm, to highlight the effect of the PS bead diameter on the optical properties of the films and on the PV efficiency. PbI2/CH3NH3I 0.7M in DMSO leads to the most covering, homogeneous and overlayer-free porous films. The PV efficiency of the corresponding cells increases with the bead diameter. A significant improvement in the PV conversion efficiency is observed thanks to the 3D structuration compared to a dense reference, due to the improvement of charge separation at the Spiro-OMeTAD/perovskite interface and thus to the reduction of charge recombination. In addition, CH3NH3PbI3 porous films prepared with 810 nm, 1000 nm and 2100 nm PS bead diameter respectively, are coloured, which is very interesting for building-integrated applications (BIPV). [less ▲]

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See detailInfluence of ZnO Surface Modification on the Photocatalytic Performance of ZnO/NiO Thin Films
Periyannan, Shanmugapriya ULiege; Manceriu, Laura ULiege; Nguyen, Ngoc Duy ULiege et al

in Catalysis Letters (2019)

Charge carrier separation is considered as a key factor in enhancing the photocatalytic process and can be maximized by mitigating surface recombination. Following this idea, the surface of zinc oxide ... [more ▼]

Charge carrier separation is considered as a key factor in enhancing the photocatalytic process and can be maximized by mitigating surface recombination. Following this idea, the surface of zinc oxide (ZnO) was modified by thermal treatment and nickel oxide (NiO) deposition. The influence of the ZnO thermal treatment and NiO deposition conditions on the ZnO surface chemistry and heterostructure interface properties were investigated by in situ X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) and correlated to the dye photodegradation efficiency. The XPS analysis confirmed a change of doping of ZnO after thermal treatment, which mainly influenced the developed band bending, and has led to an improved photocatalytic activity. For the same reason, the heterostructures based on the surface cleaned ZnO surface had higher photocatalytic efficiency than the ones based on non-cleaned ZnO. The temperature input during NiO deposition had negligible effect on the heterostructure interface properties. The photocatalytic efficiency did not follow the band bending evolution because of a dominant contribution of charge recombination across the NiO layer as indicated by PL analysis. [less ▲]

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See detailHydrothermal synthesis and electrochemical properties of Fe1.19(PO4)(OH)0.57(H2O)0.43/C cathode material for Li-ion batteries
Mahmoud, Abdelfattah ULiege; Karegeya, Claude; Sougrati, Moulay Tahar et al

Conference (2019, January 29)

Recent rapid development of the portable electronic devices, growing interest in the electric vehicles and increasing integration the renewable energies required the development of cost-effective and high ... [more ▼]

Recent rapid development of the portable electronic devices, growing interest in the electric vehicles and increasing integration the renewable energies required the development of cost-effective and high energy storage systems. Lithium-ion batteries are considered as system of the choice for variety of mobile and stationary applications. However, new electrode materials are demanded to increase the energy density of Li-ion batteries. This presentation will report on the preparation and study of Fe1.19(PO4)(OH)0.57(H2O)0.43/C ((FPHH/C) composite as positive electrode material with high capacity and long cycle-life [1, 2]. FPHH/C (C= carbon black (CB) and carbon nanotubes (CNT)) composites were obtained by one-step the hydrothermal synthesis route. These cathode materials showed an excellent reversible capacity corresponding to 1.19 Li reaction. This is attributed to the stable and open structure of FPHH and also to the effect of carbon addition (CB and CNT) that improves the electronic percolation of the composite. The study of the reaction mechanism of FPHH/CNT during cycling by combining operando XRD and 57Fe Mossbauer spectroscopy (Figure 1) shows that the insertion mechanism is a monophasic reaction with 10% volume variations associated to the Fe3+/Fe2+ redox reaction [2]. References 1. C. Karegeya, A. Mahmoud, R. Cloots, B. Vertruyen, F. Boschini. Electrochimica Acta 250 (2017) 49-58. 2. A. Mahmoud, C. Karegeya, M. T. Sougrati, J. Bodart, B. Vertruyen, R. Cloots, P-E. Lippens, F. Boschini. ACS Applied Materials and interfaces 10 (2018) 34202-34211. [less ▲]

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See detailOne Step Hydrothermal Synthesis of Fe1.19(PO4)(OH)0.57(H2O)0.43 composite cathode material and Its Electrochemical Properties in Li-Ion Batteries
Mahmoud, Abdelfattah ULiege; Karegeya, Claude; Sougrati, Moulay Tahar et al

Conference (2018, December 13)

Lithium-ion batteries (LIBs) have been widely applied as a power source for portable and stationary energy storage systems. In this work, we report the electrochemical performance of FPHH/CB and FPHH/CNT ... [more ▼]

Lithium-ion batteries (LIBs) have been widely applied as a power source for portable and stationary energy storage systems. In this work, we report the electrochemical performance of FPHH/CB and FPHH/CNT cathode materials for Li-ion batteries where FPHH represents Fe1.19(PO4)(OH)0.57(H2O)0.43 while carbon black (CB) and carbon nanotubes (CNT) were used as precursors in the one-pot hydrothermal synthesis. 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. Thanks to its favorable microstructural characteristics, the FPHH-10 wt% C and FPHH-20 wt% C materials exhibited good performance [1]. The CNT also improve the performance of FPHH such as capacity retention. The study of the reaction mechanism of FPHH/CNT during cycling by combining operando XRD and 57Fe Mössbauer spectroscopy (Figure 1) shows that the insertion mechanism is a monophasic reaction with 10% volume variations associated to the Fe3+/Fe2+ redox reaction [2]. 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, R. Cloots, B. Vertruyen, F. Boschini, Electrochimica Acta 250 (2017) 49-58. 2. A. Mahmoud, C. Karegeya, M. T. Sougrati, J. Bodart, B. Vertruyen, R. Cloots, P-E. Lippens, F. Boschini. ACS Applied Materials and interfaces 10 (2018) 34202-34211. [less ▲]

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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 detailDispersion of conductive carbon allotropes inside Na2FePO4F particles by spray-drying
Berardo, Loris ULiege; Mahmoud, Abdelfattah ULiege; Caes, Sebastien et al

Poster (2018, October 02)

Among all phosphate based compounds, Na2FePO4F (NFPF) is identified as a promising low-cost material for Li and Na-ion batteries [2]. Beside a relatively high voltage (3.6 V vs. Li+/Li0), Na2FePO4F ... [more ▼]

Among all phosphate based compounds, Na2FePO4F (NFPF) is identified as a promising low-cost material for Li and Na-ion batteries [2]. Beside a relatively high voltage (3.6 V vs. Li+/Li0), Na2FePO4F exhibits a good theoretical capacity (~135 mAh/g) and an excellent stability of the structure during the intercalation process. However, despite these powerful advantages, the main drawback of Na2FePO4F cathode material is its low conductivity leading to mediocre cycling performance. The aim of this work is to enhance the electronic conductivity of Na2FePO4F particles by addition of carbon nanotubes (CNT) or carbon black (CB) during the spray-drying synthesis [3]. The structural, electrochemical and morphological properties of the synthesized Na2FePO4F-xCB and Na2FePO4F-xCNT (x = wt% of carbon) were investigated by various techniques in order to compare the effect of CB and CNT. [less ▲]

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See detailElectrochemical Mechanism and Effect of Carbon Nanotubes on the Electrochemical Performance of Fe1.19(PO4)(OH)0.57(H2O)0.43 Cathode Material for Li-Ion Batteries
Mahmoud, Abdelfattah ULiege; Karegeya, Claude; Moulay Tahar, Sougrati et al

in ACS Applied Materials and Interfaces (2018), 10(40), 3420234211

A hydrothermal synthesis route was used to synthesize iron(III) phosphate hydroxide hydrate–carbon nanotube composites. Carbon nanotubes (CNT) were mixed in solution with Fe1.19(PO4)(OH)0.57(H2O)0.43 ... [more ▼]

A hydrothermal synthesis route was used to synthesize iron(III) phosphate hydroxide hydrate–carbon nanotube composites. Carbon nanotubes (CNT) were mixed in solution with Fe1.19(PO4)(OH)0.57(H2O)0.43 (FPHH) precursors for one-pot hydrothermal reaction leading to the FPHH/CNT composite. This produces a highly electronic conductive material to be used as a cathode material for Li-ion battery. The galvanostatic cycling analysis shows that the material delivers a specific capacity of 160 mAh g–1 at 0.2 C (0.2 Li per fu in 1 h), slightly decreasing with increasing current density. A high charge–discharge cyclability is observed, showing that a capacity of 120 mAh g–1 at 1 C is maintained after 500 cycles. This may be attributed to the microspherical morphology of the particles and electronic percolation due to CNT but also to the unusual insertion mechanism resulting from the peculiar structure of FPHH formed by chains of partially occupied FeO6 octahedra connected by PO4 tetrahedra. The mechanism of the first discharge–charge cycle was investigated by combining operando X-ray diffraction and 57Fe Mössbauer spectroscopy. FPHH undergoes a monophasic reaction with up to 10% volume changes based on the Fe3+/Fe2+ redox process. However, the variations of the FPHH lattice parameters and the 57Fe quadrupole splitting distributions during the Li insertion–deinsertion process show a two-step behavior. We propose that such mechanism could be due to the existence of different types of vacant sites in FPHH, including vacant “octahedral” sites (Fe vacancies) that improve diffusion of Li by connecting the one-dimensional channels. [less ▲]

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