90 research outputs found
Li-7 Nmr and Ionic-conductivity Studies of Gel Electrolytes Based On Poly(methylmethacrylate)
No full textGel electrolytes synthesized from poly(methylmethacrylate), ethylene carbonate, propylene carbonate and various lithium salts [LiClO4, LiAsF6, or LiN(CF3SO2)(2)] have been investigated by differential scanning calorimetry, electrical conductivity, and Li-7, F-19 and As-75 NMR spectroscopy. Although the ionic conductivities of the gels approach those of liquid electrolytes above room temperature, the NMR results indicate that the immediate environments of both the cations and anions differ significantly in the gel and in the liquid. Thus the presence of microscopic regions of pure liquid electrolyte in the gel can be ruled out
Tin-coated graphite electrodes as composite anodes for Li-ion batteries. Effects of tin coatings thickness toward intercalation behavior.
No full textTheelectrochemicalbehaviorofgraphiteanodes,coatedby50–500A ̊-thickSnlayers,isdiscussedin the present paper. Morphology and structure of the modified electrode surfaces are described, and the charge/discharge behavior is evaluated by galvanostatic cycles at temperatures down to −30◦C. The enhanced kinetics of the intercalation/deintercalation process is studied by cyclic voltammetry and electrochemical impedance spectroscopy, focusing on the role played by the Sn coatings in the inter- calation/deintercalation mechanism.
The results show that the metal layers modify and stabilize the electrode/electrolyte interphase and that the intercalation process is mediated by reversible Li–Sn alloys formation. Moreover, all the Sn coatings are effective in modifying the energy barriers related both to the Li+ desolvation step and to the migration of the desolvated Li+ ion through the modified surface layers. As a consequence, the overall polarization for the charge-transfer process is reduced, and enhanced low-temperature intercalation performances are obtained
Studio dell'effetto del ball milling sulla reattività electtrochimica del NaAlH4
No full textGli idruri metallici complessi sono composti ampiamente studiati come materiali per l'accumulo d'idrogeno. Tra questi, il sodio alluminio idruro (NaAlH4) è tra i più studiati [1] grazie alle favorevoli proprietà termodinamiche e all'elevata densità volumetrica di idrogeno (7.5 wt % H2 and 94 gH2/L). Recentemente, è stato dimostrato che il suo uso può essere esteso all'accumulo di energia elettrica. Nello specifico, il sodio alluminio idruro si è dimostrato un promettente candidato da utilizzare come materiale anodico in batterie litio-ione [2, 3].
Studi elettrochimici in celle al litio hanno messo in luce la sua elevata reattività. Il sodio alanato è in grado di scambiare più di 3.5 equivalenti di litio durante il primo ciclo di scarica, corrispondente a una capacità specifica quasi pari a quella teorica (1985 mAh/g). Il processo redox, che avviene attraverso reazione di conversione [4], consiste di step multipli e prevede l'iniziale formazione di LiNa2AlH6 e Na3AlH6 intermedi e, poi, la loro successiva decomposizione in Na e Al metallico e LiH [2].
Purtroppo, tale tipo di materiale è caratterizzato da un'elevata irreversibilità: alla fine della prima ricarica, viene scambiato solo 1 dei quasi 4 equivalenti di litio scambiati durante la scarica. E' noto che i materiali elettrodici che agiscono mediante un meccanismo a conversione siano caratterizzati da una grande isteresi di potenziale tra scarica e carica. La causa di tale fenomeno è l'elevata espansione volumetrica in seguito alla consistente riorganizzazione strutturale a cui il materiale è sottoposto durante il processo di conversione. In questo modo, il volume aumenta durante l'incorporazione del litio e diminuisce in seguito all'estrazione di esso. La conseguenza è la disgregazione delle particelle, il disfacimento dell'elettrodo e la conseguente perdita del contatto elettrico. Nel caso di NaAlH4, se si considera la completa riduzione secondo la reazione: NaAlH4 + 4Li+ + 4e- → 4LiH+ Al +Na, si può stimare una variazione del volume di circa il 72 %.
Importanti miglioramenti sulla reversibilità del processo elettrodico e, quindi, sull'efficienza di cella sono stati ottenuti sottoponendo il materiale di partenza a macinazione, mediante High Energy Ball Milling. Soprattutto, la macinazione in mulino del NaAlH4 con un carbone conduttivo, ha portato ad un significativo miglioramento dell'efficienza di cella, passando da un 30 % per il materiale tal quale al 70 % per il materiale sottoposto a trattamento meccanochimico [2].
Infatti, la riduzione delle dimensioni delle particelle di materiale attivo e, soprattutto, la macinazione con un opportuno carbone conduttivo (e.g., il SuperP) migliora l'efficienza del processo di conversione [2, 5], in quanto il carbone distribuendosi sulla superficie dell'idruro, porta alla formazione di un vero e proprio composito che, oltre a migliorarne la conducibilità, previene l'agglomerazione delle particelle di idruro durante i processi redox e attenua le variazioni volumetriche.
Considerati i notevoli miglioramenti ottenuti sulle performance del sodio alluminio idruro in celle al litio, sono state analizzate le principali differenze tra il campione di NaAlH4 sottoposto a macinazione in mulino e il materiale tal quale. Nello specifico, le caratteristiche strutturali e morfologiche sono state valutate mediante misure statiche di NMR allo stato solido e microscopia elettronica a trasmissione. Il contenuto d'idrogeno è stato calcolato mediante esperimenti di desorbimento termico in TPD. Infine, la reversibilità del processo elettrochimico è stata confermata mediante misure di MAS NMR allo stato solido.
[1] T. K. Nielsen, M. Polanski, D. Zasada, P. Javadian, F. Besenbacher, J. Bystrzycki, J. Skibsted, and T. R. Jensen, “Improved Hydrogen Storage Kinetics of Nanoconfined NaAlH4 Catalyzed with TiCl3 Nanoparticles,” ACS Nano, vol. 5, no. 5, pp. 4056–4064, Maggio 2011.
[2] L. Silvestri, L. Farina, D. Meggiolaro, S. Panero, F. Padella, S. Brutti, P. Reale. "The Reactivity of Sodium Alanates in Lithium Batteries". J. Phys. Chem. C, 119 (52), pp 28766–28775, Novembre 2015.
[3] J. A. Teprovich, J. Zhang, H. Colón-Mercado, F. Cuevas, B. Peters, S. Greenway, R. Zidan, and M. Latroche, “Li-Driven Electrochemical Conversion Reaction of AlH3, LiAlH4, and NaAlH4,” J. Phys. Chem. C, Febbraio 2015.
[4] Y. Oumellal, A. Rougier, G. A. Nazri, J.-M. Tarascon, and L. Aymard, “Metal hydrides for lithium-ion batteries,” Nat. Mater., vol. 7, no. 11, pp. 916–921, Nov. 2008.
[5] Brutti S., Mulas G., Piciollo E., Panero S., Reale P. "Magnesium hydride as a high capacity negative electrode for lithium ion batteries". Journal of Materials Chemistry, Vol. 22, p. 14531-14537, Maggio 2012
Inorganic-organic membranes based on Nafion, [(ZrO2)(HfO2)0.25] and [(SiO2)(HfO2)0.28] nanoparticles. Part II: Relaxations and conductivity mechanism
No full textTwo classes of hybrid inorganic–organic proton-conducting membranes consisting of Nafion and either [(ZrO2)·(HfO2)0.25] or [(SiO2)·(HfO2)0.28] nanofiller are investigated to elucidate their relaxations and conductivity mechanism and are labeled [Nafion/(ZrHf)x] and [Nafion/(SiHf)x], respectively. The membranes are studied by dynamic mechanic analysis (DMA) and broadband electric spectroscopy (BES). The latter technique allows a determination of the direct current ionic conductivity (σDC) and the proton diffusion coefficient (DH+). Pulse-field-gradient spin-echo nuclear magnetic resonance experiments (PFGSE-NMR) are carried out to determine the water self-diffusion coefficients (DH2O). DH+ and DH2O are correlated to obtain insight on the conductivity mechanism of the proposed materials. Results indicate that the nanofiller particles play a major role in the proton conduction mechanism of the proposed materials. It is demonstrated that the basic [(ZrO2)·(HfO2)0.25] nanoparticles form Nafion–nanofiller dynamic cross-links with high ionic character. These cross-links improve the mechanical properties and enhance the overall proton conductivity of the membranes at low humidification levels owing to an efficient delocalization of the protons. In [Nafion/(SiHf)x] membranes, the dynamic cross-links occur due to dipole–dipole interactions between the side groups of the Nafion host polymer and the quasi-neutral [(SiO2)·(HfO2)0.28] nanoparticles. These cross-links significantly reduce the delocalization of the protons, which decreases the overall conductivity of materials
Probing the potential of type V Deep eutectic solvents as sustainable electrolytes
No full textThe increasing interest within the scientific community in environmentally friendly solvents has led to a focus on Deep Eutectic Solvents (DES), which have natural components. DES are viewed as alternatives to traditional organic solvents and have the potential to be used as electrolytes. For the first time, transport properties of four Type V Deep Eutectic Salt Solutions (DESS) were accessed to investigate the potential of this technology, selecting precursors ranked as excellent in Eco-Scale metrics. The DESS were composed of terpene and trioctylphosphine oxide (TOPO), and varying concentrations of lithium bis(trifluoromethane)sulfonimide (LiTFSI),
and their properties were assessed through self-diffusion, viscosity, density, and conductivity measurements.
While Type V DESS are capable of dissolving significant amounts of LiTFSI (up to 30 % molar), their ionic
conductivity is low, with values ranging from 3.6⋅10 3 to 9.3⋅10 2 mS⋅cm 1 at 25 ◦C, thus limiting their suitability
as electrolytes, for instance, for lithium-ions batteries applications. Similar diffusion coefficients for Li+
and TFSI ions suggest the formation of long-lived ion pairs moving as a neutral species. Therefore, future
research aims to introduce additives to disrupt contact ion pairs and enhance transport properties, leveraging the
sustainable appeal of DES and their use in advanced energy storage technologies
Investigation of the Effects of Mechanochemical Treatment on NaAlH4 Based Anode Materials for Li-Ion Batteries
No full textSodium alanate has proven to be a feasible candidate for electrochemical applications. Within a lithium cell, NaAlH4 closely approaches its theoretical capacity of 1985 mAhg−1 upon the first discharge. Despite its high specific capacity, NaAlH4 suffers from poor cycle efficiency, mostly due to the severe volume expansion following the conversion reaction and resulting in damage to electrode mechanical integrity with loss of electrical contact. Synthesis of an appropriate composite alanate/carbon by high energy ball milling demonstrates an ability to mitigate these deleterious effects, whereby large improvements in terms of electrochemical reversibility can be achieved. In order to highlight the effects of mechanochemical treatment on the electrochemical properties of NaAlH4, new insights on such NaAlH4/C composites are reported. Solid state NMR has been used to study the impact of ball milling on the NaAlH4 crystal structure, while, the hydrogen content and associated desorption properties have been evaluated by thermal programmed desorption measurements. Also, electrochemical features have been analyzed via the combined application of potentiodynamic cycling with galvanostatic acceleration and electrochemical impedance spectroscopy measurements. Finally, new evidence concerning the reversibility of the conversion processes has been obtained by ex-situ NMR measurements on cycled electrodes
Polymeric delta-MgCl2 nanoribbons
No full textdelta-MgCl2 has relevant applications in the field of electrochemical energy storage and Ziegler–Natta catalysis. Here, we clarify the short-range structural peculiarities that make the disordered phase delta-MgCl2 extremely chemically active relative to the higher lattice energy phases, alpha-MgCl2 and beta-MgCl2. X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and nuclear magnetic resonance (NMR) results are included. These findings, demonstrate the existence of [MgCl2]n nanoribbons and active nanosurfaces in delta-MgCl2 and provide new insight about the nature of the bonding in the allotropic forms of MgCl2
Lithium-7 nuclear magnetic resonance and Ti K-edge X-ray absorption spectroscopic investigation of electrochemical lithium insertion in Li4/3+xTi5/3O4
No full textThe spinel compound Li 4/3+xTi 5/3O 4 is known to undergo reversible lithium intercalation up to x=1 with almost no change in lattice parameters, hence its designation as a "zero strain" intercalation compound. Structural changes that accompany electrochemical Li intercalation into this compound were studied by both 7Li nuclear magnetic resonance (NMR) and Ti K-edge X-ray absorption fine structure (XAFS). The NMR results demonstrate that Li occupancies do not follow a simple distribution between two possible sites, one tetrahedral and one octahedral. The presence of at least one additional octahedral site is suggested. Line width measurements show that the Li + ions do not return to their original distribution after cycling. XAFS results indicate the presence of modest static disorder in Ti-O and Ti-Ti distances above x = 0.5. Both methods thus reveal subtle structural details previously unobserved by X-ray diffraction (XRD)
Polymer electrolytes based on protic ionic liquids with perfluorinated anions for safe lithium-ion batteries
No full textThe quest for safe and high-performance polymer electrolytes in lithium-ion batteries (LIBs) has led researchers to explore protic ionic liquids (PILs) as potential candidates to be entrapped in polymer matrices. In this context, we present an investigation into solid polymeric systems based on poly(methyl methacrylate) (PMMA) as a host for PILs, featuring 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU) cation paired with three different anions: bis(trifluoromethanesulfonyl)imide (TFSI−), trifluoromethanesulfonate (TFO−), and (trifluoromethanesulfonyl-nonafluorobutylsulfonyl)imide (IM14−). Additionally, we explore the lithium-doped IM14-gel-like system to broaden our understanding of these intriguing materials. Through comprehensive thermal analysis, solid-state NMR, and diffusion NMR techniques, we delve into the interactions and structural features of these binary and ternary polymeric systems. Our investigation reveals unique dynamics and ion interactions within the PMMA matrix, shedding light on the potential of these materials for advanced energy storage technologies. Particularly, we highlight the distinctive features of DBUH-IM14 and its specific interaction with the polymeric matrix and the lithium ions, underscoring its significance in advancing safer and more efficient energy storage devices
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