Cu-catalyzed Si-NWS grown on “carbon paper” as anodes for Li-ion cells

The very high theoretical capacity of the silicon (4200mAh/g more than 10 times larger than graphite), environmental-friendly, abundant and low-cost, makes it a potential candidate to replace graphite in high energy density Li-ion batteries. As a drawback, silicon suffers from huge volume changes (300%) on alloying and dealloying with Li, leading a structural deformation that induces disruption. The use of nanostructured silicon materials has been shown to be an effective way to avoid this mechanical degradation of the active material. In this paper the synthesis of silicon nanowires, grown on a highly porous 3D-like carbon paper substrate by CVD using Cu as the catalyst, is presented. The use of carbon paper allows to achieve remarkable loadings of active material (2-5 mg/cm2) and, consequently, high capacity densities. The silicon electrode was investigated both morphologically and electrochemically. To improve the electrochemical performance various strategies have been carried out. It was observed that a very slow first cycle (C/40), which helps the formation of a stable solid electrolyte interphase on the silicon surface, improves the performance of the cells; nevertheless, their cycle life has been found not fully satisfactory. Morphological analysis of the Si-NWs electrodes before and after cycling showed the presence of a dense silicon layer below the nanowires which could reduce the electrical contact between the active material and the substrate

A lithium-ion battery based on LiFePO4 and silicon/reduced graphene oxide nanocomposite

In this paper, the preparation and chemical–physical characterization of a composite material made of silicon nanoparticles (nSi) and reduced graphene oxide (RGO) for using as an anode for lithium-ion batteries are report- ed. The nSi/RGO composite was synthesized by microwave irradiation followed by a thermal treatment under reducing atmosphere of a mixture of nSi and graphene oxide, and characterized by XRD, SEM, and TGA. The nano- structured material was used to prepare an electrode, and its electrochemical performance was evaluated in a lithium cell by galvanostatic cycles at various charge rates. The electrode was then coupled with a LiFePO4 cathode to fabricate a full lithium-ion battery cell and the cell performance evaluated as a function of the discharge rate and cycle number

Phase Separation in Lix_xFePO4_4 Induced by Correlation Effects

We report on a significant failure of LDA and GGA to reproduce the phase stability and thermodynamics of mixed-valence Li$_x$FePO$_4$ compounds. Experimentally, Li$_x$FePO$_4$ compositions ($0 \leq x \leq 1$) are known to be unstable and phase separate into Li FePO$_4$ and FePO$_4$. However, first-principles calculations with LDA/GGA yield energetically favorable intermediate compounds an d hence no phase separation. This qualitative failure of LDA/GGA seems to have its origin in the LDA/GGA self-interaction which de localizes charge over the mixed-valence Fe ions, and is corrected by explicitly considering correlation effects in this material. This is demonstrated with LDA+U calculations which correctly predict phase separation in Li$_x$FePO$_4$ for $U-J \gtrsim 3.5$eV. T he origin of the destabilization of intermediate compounds is identified as electron localization and charge ordering at different iron sites. Introduction of correlation also yields more accurate electrochemical reaction energies between FePO$_4$/Li$_x$FePO$_ 4$ and Li/Li$^+$ electrodes.Comment: 12 pages, 5 figures, Phys. Rev. B 201101R, 200

The Li intercalation potential of LiMPO4 and LiMSiO4 olivines with M = Fe, Mn, Co, Ni

The Li intercalation potential of LiMPO4 and LiMSiO4 compounds with M = Fe, Mn, Co, and Ni is computed with the GGA+U method. It is found that this approach is considerably more accurate than standard LDA or GGA methods. The calculated potentials for LiFePO4, LiMnPO4 and LiCoOPO4 agree to within 0.1 V with experimental results. The LiNiPO4 potential is predicted to be above 5 V. The potentials of the silicate materials are all found to be rather high, but LiFeSiO4 and LiCoSiO4 have negligible volume change upon Li extraction.Comment: 10 pages, 2 figure

Easy and Scalable Syntheses of Li1.2Ni0.2Mn0.6O2

Solid-state and sol-gel syntheses were selected as easy and scalable methods to prepare a lithium-rich cathode material for lithium-ion batteries. Among the extended family of layered oxides, Li1.2Ni0.2Mn0.6O2 was chosen for its low nickel content and the absence of cobalt. Both synthesis methods involved two heating steps at different temperatures, 600 and 900 °C. The first step is needed to decompose the metal acetates, which were selected as precursors, and the second step is needed to crystallise the material. To obtain a material with well-defined defects, the rate of heating and cooling was carefully controlled. The materials were characterised by X-ray diffraction, SEM coupled with EDS analysis, and thermal analysis and were finally tested as cathodes in a lithium semi cell. The solid-state synthesis allowed us to obtain better structural characteristics with respect to the sol-gel one in terms of a well-formed hexagonal layer structure and a reduced Li+/Ni2+ disorder. On the other hand, the sol-gel method produced a material with a higher specific capacity. The performance of this latter material was then evaluated as a function of the discharge current, highlighting its good rate capabilities

Impact of air oxygen presence on the stability of the V3+ ions in the vanadium redox flow batteries

Questo lavoro intende definire il ruolo dell'ossigeno sulla stabilità dello ione vanadio V(III) nelle batterie di flusso redox vanadio. La spettrofotometria UV-visibile ha evidenziato che, non solo la presenza di ossigeno nella soluzione anodica limita fortemente la riduzione da V(IV) a V(II) a causa della rapida ossidazione del V(II) da parte dell’ossigeno disciolto. Più importante, la presenza di ossigeno nella stessa soluzione rende anche gli ioni V(III) molto instabili e soggetti ad ossidazione in tempi brevi. Questo effetto, mai precedentemente ben quantificato, mostra che la presenza di O2 deve essere assolutamente esclusa nel serbatoio di accumulo per mantenere stabile la concentrazione di V(III).This work is intended to define the role of oxygen on the stability of vanadium ions V(III) in the vanadium redox flow batteries. The UV-visible spectrophotometry revealed that, not only the oxygen presence in the anolyte solution strongly limits the V(IV) to V(II) reduction in the negative half-cell electrolyte, due to the rapid oxidation of the V(II) by dissolved oxygen. Most important, the oxygen presence in the same solution also makes very unstable the V(III) ions that tend to be oxidized in short time. This effect, never previously well quantified, shows that the presence of O2 must be absolutely excluded in the storage reservoir in order to keep stable the concentration of V(III)

Unveiling Oxygen Redox Activity in P2-Type NaxNi0.25Mn0.68O2 High-Energy Cathode for Na-Ion Batteries

Na-ion batteries are emerging as convenient energy-storage devices for large-scale applications. Enhanced energy density and cycling stability are key in the optimization of functional cathode materials such as P2-type layered transition metal oxides. High operating voltage can be achieved by enabling anionic reactions, but irreversibility of O2–/O2n–/O2 evolution still limits this chance, leading to extra capacity at first cycle that is not fully recovered. Here, we dissect this intriguing oxygen redox activity in Mn-deficient NaxNi0.25Mn0.68O2 from first-principles, by analyzing the formation of oxygen vacancies and dioxygen complexes at different stages of sodiation. We identify low-energy intermediates that release molecular O2 at high voltage, and we show how to improve the overall cathode stability by partial substitution of Ni with Fe. These new atomistic insights on O2 formation mechanism set solid scientific foundations for inhibition and control of this process toward the rational design of new anionic redox-active cathode materials

Realizzazione e prova di celle complete da laboratorio. Test elettrochimici di miscele elettrolitiche miste in semicelle e celle complete

Il presente rapporto descrive la caratterizzazione di miscele elettrolitiche miste, sviluppate nell’ambito dell’Accordo di Programma ENEA-MSE per celle litio-ione destinate ad applicazioni alle reti elettriche, in semicelle catodiche e anodiche. Queste ultime sono state realizzate utilizzando gli elettrodi costituiti dai materiali attivi LiFePO4 e TiO2, selezionati nel corso delle attività svolte nell’ambito degli Obiettivi A e B, congiuntamente ad elettroliti misti (sviluppati nell’ambito dell’Obiettivo C) non volatili ed ininfiammabili contenenti liquidi ionici. I test elettrochimici, eseguiti a differenti regimi di corrente, hanno mostrato prestazioni, in termini di capacità e cicli di vita, prossime rispetto a quelle esibite in elettroliti organici convenzionali sino a regimi di corrente pari a C/10. Ad elevati regimi di corrente si osserva un decremento in capacità erogata dovuto alla maggiore viscosità degli elettroliti a base di liquidi ionici. È necessario tenere presente che questi risultati si riferiscono a prototipi realizzati in scala di laboratorio (suscettibili di notevole ottimizzazione) che, tuttavia, dimostrano la possibilità di realizzare celle litio-ione ad alta energia ma dotate, al contempo di elevata sicurezza

Unveiling Oxygen Redox Activity in P2-Type NaxNi0.25Mn0.68O2 High-Energy Cathode for Na-Ion Batteries

Na-ion batteries are emerging as convenient energy-storage devices for large-scale applications. Enhanced energy density and cycling stability are key in the optimization of functional cathode materials such as P2-type layered transition metal oxides. High operating voltage can be achieved by enabling anionic reactions, but irreversibility of O2–/O2n–/O2 evolution still limits this chance, leading to extra capacity at first cycle that is not fully recovered. Here, we dissect this intriguing oxygen redox activity in Mn-deficient NaxNi0.25Mn0.68O2 from first-principles, by analyzing the formation of oxygen vacancies and dioxygen complexes at different stages of sodiation. We identify low-energy intermediates that release molecular O2 at high voltage, and we show how to improve the overall cathode stability by partial substitution of Ni with Fe. These new atomistic insights on O2 formation mechanism set solid scientific foundations for inhibition and control of this process toward the rational design of new anionic redox-active cathode materials

Ricerca su materiali e processi per la realizzazione di materiali catodici con prestazioni migliorate. Analisi morfologica dei prodotti intermedi

Lo scopo di questo lavoro è stato quello di trovare le migliori condizioni di preparazione del LiFePO4 a partire dal (NH4)FePO4 (FAP). Come fonte di ferro è stato utilizzato il solfato ferroso eptaidrato. Il solfato è stato trasformato in fosfato e precipitato come sale di ammonio. In particolare sono state effettuate quattro sintesi del sale ammonico distinte in precipitazione in fase omogenea, precipitazione da sali sodici e precipitazione stechiometrica e sopra stechiometrica da sali ammonici. I materiali così preparati sono stati caratterizzati da un punto di vista morfologico tramite microscopia elettronica a scansione. L’analisi composizionale dei materiali è stata effettuata mediante tecniche di spettroscopia di diffrazione elettronica mentre le caratteristiche termiche sono state valutate tramite analisi termo gravimetrica ed analisi termica differenziale