Ilie Hanzu

Nanostructured negative electrodes based on titania for Li-ion microbatteries

This work reviews recent developments on Li-ion microbatteries. After a short literature overview, use of TiO2 as an alternative anode for Li-ion batteries and enhanced electrochemical performances of nanostructured titania electrodes is introduced. Principle and formation mechanism of self-organized TiO2 nanotubes by electrochemical anodization and electrochemical fabrication of metallic nanowires are discussed in detail. Electrochemical performance of negative electrodes for Li-ion microbatteries composed of self-organized TiO2 nanotubes and composite TiO2 nanotubes–oxide nanowires is presented.

A novel architectured negative electrode based on titania nanotube and iron oxide nanowire composites for Li-ion microbatteries

We report a novel procedure for the fabrication of vertical iron oxide nanowires with quite regular form and diameters ranging between 20 and 150 nm grown from a matrix of self-organized TiO2 nanotubes. The 3 μm thick nanocomposite electrode presented here shows relatively high areal capacities of 468 μAh cm−2 (1st reversible discharge) and 200 μA h cm−2 over 45 cycles at a rate of 25 μA cm−2 (specific capacities of 1190 and 510 mAh g−1, respectively). Additionally, studies performed at kinetics of 6, 12.5, and 50 μA cm−2 suggest that this architectured nanocomposite material reveals excellent electrochemical performance with promising potential applications as nano-architectured negative electrodes for Li-ion microbatteries.

Short-range Li diffusion vs. long-range ionic conduction in nanocrystalline lithium peroxide Li2O2—the discharge product in lithium-air batteries

Understanding charge carrier transport in Li2O2, the storage material in the non-aqueous Li-O2 battery, is key to the development of this high-energy battery. Here, we studied ionic transport properties and Li self-diffusion in nanocrystalline Li2O2 by conductivity and temperature variable 7Li NMR spectroscopy. Nanostructured Li2O2, characterized by a mean crystallite size of less than 50 nm as estimated from X-ray diffraction peak broadening, was prepared by high-energy ball milling of microcrystalline lithium peroxide with μm sized crystallites. At room temperature the overall conductivity σ of the microcrystalline reference sample turned out to be very low (3.4 × 10−13 S cm−1) which is in agreement with results from temperature-variable 7Li NMR line shape measurements. Ball-milling, however, leads to an increase of σ by approximately two orders of magnitude (1.1 × 10−10 S cm−1); correspondingly, the activation energy decreases from 0.89 eV to 0.82 eV. The electronic contribution σeon, however, is in the order of 9 × 10−12 S cm−1 which makes less than 10% of the total value. Interestingly, 7Li NMR lines of nano-Li2O2 undergo pronounced heterogeneous motional narrowing which manifests in a two-component line shape emerging with increasing temperatures. Most likely, the enhancement in σ can be traced back to the generation of a spin reservoir with highly mobile Li ions; these are expected to reside in the nearest neighbourhood of defects generated or near the structurally disordered and defect-rich interfacial regions formed during mechanical treatment.

Nanocomposite Electrode for Li-Ion Microbatteries Based on SnO on Nanotubular Titania Matrix

A nanocomposite electrode made by electrochemical deposition of Sn on TiO 2 nanotube (ntTiO 2 ) layers and subsequent thermal oxidization to SnO is proposed. X-ray diffraction patterns confirmed the presence of SnO. The spongelike structure of SnO combined with the presence of titania nanotubes is beneficial to buffer large volume changes during reaction with lithium. Galvanostatic discharge/charge tests have been carried out to characterize the electrochemical properties. The electrochemical performance shows that nanocomposite SnO-ntTiO 2 is a promising alternative negative electrode for Li-ion microbatteries.

Order vs. disorder—a huge increase in ionic conductivity of nanocrystalline LiAlO2 embedded in an amorphous-like matrix of lithium aluminate

Coarse grained, well crystalline γ-LiAlO2 (P43212) is known as an electronic insulator and a very poor ion conductor with the lithium ions occupying tetrahedral voids in the oxide structure. The introduction of structural disorder such as point defects or higher-dimensional defects, however, may greatly affect ionic conduction on both short-range as well as long-range length scales. In the present study, we used high-energy ball milling to prepare defect-rich, nanocrystalline LiAlO2 that was characterized from a structural point of view by powder X-ray diffraction, TEM as well as small angle X-ray scattering (SAXS). Temperature-dependent conductivity spectroscopy revealed an increase of the room-temperature ionic conduction by several orders of magnitude when going from microcrystalline γ-LiAlO2 to its nanocrystalline form. The enhanced ion transport found is ascribed to the increase of Li ions near defective sites both in the bulk as well as in the large volume fraction of interfacial regions in nano-LiAlO2. The nanocrystalline ceramic prepared at long milling times is a mixture of γ-LiAlO2 and the high-pressure phase δ-LiAlO2; it adapts an amorphous like structure after it has been treated in a planetary mill under extremely harsh conditions.

Electrochemical fabrication of Sn nanowires on titania nanotube guide layers

We describe a novel approach for the fabrication of tailored nanowires using a two-step electrochemical process. It is demonstrated that self-organized TiO(2) nanotubes can be used to activate and guide the electrochemical growth of Sn crystallites, leading to the formation of vertical features with a high aspect ratio. We show that the dimensions and the density of Sn crystallites depend on the electrodeposition parameters.

An Electrolyte for Reversible Cycling of Na Metal and Na Intercalation Compounds

Na battery chemistries show poor passivation behavior of low voltage Na storage compounds and Na metal with organic carbonate-based electrolytes adopted from Li-ion batteries. Therefore, a suitable electrolyte remains a major challenge for establishing Na batteries. Here we report highly concentrated sodium bis(fluorosulfonyl)imide (NaFSI) in dimethoxyethane (DME) electrolytes and investigate them for Na metal and hard carbon anodes and intercalation cathodes. For a DME/NaFSI ratio of 2, a stable passivation of anode materials was found owing to the formation of a stable solid electrolyte interface, which was characterized spectroscopically. This permitted non-dentritic Na metal cycling with approximately 98 % coulombic efficiency as shown for up to 300 cycles. The NaFSI/DME electrolyte may enable Na-metal anodes and allows for more reliable assessment of electrode materials in Na-ion half-cells, as is demonstrated by comparing half-cell cycling of hard carbon anodes and Na3 V2 (PO4 )3 cathodes with a widely used carbonate and the NaFSI/DME electrolyte.

Long-Cycle-Life Na-Ion Anodes Based on Amorphous Titania Nanotubes--Interfaces and Diffusion.

Amorphous self-assembled titania nanotube layers are fabricated by anodization in ethylene glycol based baths. The nanotubes having diameters between 70-130 nm and lengths between 4.5-17 μm are assembled in Na-ion test cells. Their sodium insertion properties and electrochemical behavior with respect to sodium insertion is studied by galvanostatic cycling with potential limitation and cyclic voltammetry. It is found that these materials are very resilient to cycling, some being able to withstand more than 300 cycles without significant loss of capacity. The mechanism of electrochemical storage of Na(+) in the investigated titania nanotubes is found to present significant particularities and differences from a classical insertion reaction. It appears that the interfacial region between titania and the liquid electrolyte is hosting the majority of Na(+) ions and that this interfacial layer has a pseudocapacitive behavior. Also, for the first time, the chemical diffusion coefficients of Na(+) into the amorphous titania nanotubes is determined at various electrode potentials. The low values of diffusion coefficients, ranging between 4 × 10(-20) to 1 × 10(-21) cm(2)/s, support the interfacial Na(+) storage mechanism.

Novel fabrication technologies of 1D TiO2 nanotubes, vertical tin and iron–based nanowires for Li–ion microbatteries

We present the combination of anodisation, sputtering and electrodeposition processes as a novel technology to fabricate nanoarchitectured materials. Titania nanotubes are successfully fabricated using Ti foils and Ti film on Si wafers; by simply varying the anodisation parameters a 600–900 nm range of tube length and a 50–150 nm range of tube diameter can be obtained. Iron and tin oxides nanowires, microballs, microcubes or with sponge–like morphology are obtained showing that the crystallinity can be tuned by optional heat treatment, but the initial morphology is preserved. We investigate all these materials as alternative electrodes for lithium–ion batteries and microbatteries. It should be highlighted the fabrication of vertical nanowires using a template–free approach exhibits some advantages because the electroactive species are fabricated directly onto the current collector, ensuring good electrical contact between titania nanotube layers and the current collector, and tackle the use of additives such as binder and conductive agents. Thus, Sn on an amorphous titania matrix and SnO nanowires on a crystalline titania matrix with a particular geometry (2 µm of tin/tin oxide length) have a remarkable reversible capacity of about 140 µA h cm−2 (675 mA h g−1 and 70 µA h cm−2 µm−1) which is kept about 85% over 50 cycles. The matrix presented here can allow the volume expansion of lithium–tin alloys and thus enhances the electrochemical performances as compared with usual tin–based electrodes. In the text is also described the electrochemistry of a series of samples such as a 3 µm thick nanocomposite made of vertical iron oxide nanowires with quite regular form and diameters ranging between 20 nm and 150 nm grown on a matrix of self–organised TiO2 nanotubes. The obtained capacities compare very favourably with the best literature data for Li–ion microbatteries.

Nanostructured Ceramics: Ionic Transport and Electrochemical Activity

Abstract Ceramics with nm-sized dimensions are widely used in various applications such as batteries, fuel cells or sensors. Their oftentimes superior electrochemical properties as well as their capabilities to easily conduct ions are, however, not completely understood. Depending on the method chosen to prepare the materials, nanostructured ceramics may be equipped with a large area fraction of interfacial regions that exhibit structural disorder. Elucidating the relationship between microscopic disorder and ion dynamics as well as electrochemical performance is necessary to develop new functionalized materials. Here, we highlight some of the very recent studies on ion transport and electrochemical properties of nanostructured ceramics. Emphasis is put on TiO2 in the form of nanorods, nanotubes or being present as mesoporous material. Further examples deal with nanocrystalline peroxides such as Li2O2 or nanostructured oxides (Li2TiO3, LiAlO2, LiTaO3, Li2CO3 and Li2B4O7). These materials served as model systems to explore the influence of ball-milling on overall ionic transport.