ToF-SIMS Li Depth Profiling of Pure and Methylated Amorphous Silicon Electrodes After Their Partial Lithiation.

Pure (a-Si:H) and methylated [a-Si0.95(CH3)0.05:H] amorphous silicon thin films were analyzed by time-of-flight secondary ion mass spectrometry after partial lithiation. Depth profiling gives insights into the lithiation mechanism of the material, enabling us to study the detailed biphasic process in the first lithiation process. Lithiation induces swelling and roughening of the active layer. In both a-Si:H and a-Si0.95(CH3)0.05:H, no measurable Li diffusion was observed after stopping current-induced lithiation. After applying the same lithiation charges, distinct Li profiles were observed for these two materials. Unlike a-Si:H, the Li concentration drops slowly from the heavily lithiated region to the non-lithiated region in a-Si0.95(CH3)0.05:H. This apparent progressive transition between the lithiated and lithium-free regions is attributed to the presence of nanovoids inside the material. When their concentration is high enough, these nanovoids constitute favorable quasi-percolating paths for lithium during the first lithiation. A specific model was developed to simulate the Li depth profiles, fully supporting this hypothesis.

Influence of Carbonate Solvents on Solid Electrolyte Interphase Composition over Si Electrodes Monitored by In Situ and Ex Situ Spectroscopies.

A solid electrolyte interphase (SEI) layer on Si-based anodes should have high mechanical properties to adapt the volume changes of Si with low thickness and good ionic conductivity. To better understand the influence of carbonate solvents on the SEI composition and mechanism of formation, systematic studies were performed using dimethyl carbonate (DMC) or propylene carbonate (PC) solvent and LiPF6 as a salt. A 1 M LiPF6/EC-DMC was used for comparison. The surface chemical composition of the Si electrode was analyzed at different potentials of lithiation/delithiation and after a few cycles. Ex situ X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry results demonstrate that a thinner and more stable SEI layer is formed in LiPF6/DMC. The in situ Fourier transform infrared spectroscopy proves that the coordination between Li+ and DMC is weaker, and fewer DMC molecules take part in the formation of the SEI layer. The higher capacity retention during 60 cycles and less significant morphological modifications of the Si electrode in 1 M LiPF6/DMC compared to other electrolytes were demonstrated, confirming a good and stable interfacial layer. The possible surface reactions are discussed, and the difference in the mechanisms of formation of SEI in these three various electrolytes is proposed.

Hydrothermal Synthesis of TiO2 Aggregates and Their Application as Negative Electrodes for Lithium-Ion Batteries: The Conflicting Effects of Specific Surface and Pore Size.

TiO2 aggregates of controlled size have been successfully prepared by hydrothermal synthesis using TiO2 nanoparticles of different sizes as a building unit. In this work, different techniques were used to characterize the as-prepared TiO2 aggregates, e.g., X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer, Emmett and Teller technique (BET), field emission gun scanning electron microscopy (FEGSEM), electrochemical measurements etc. The size of prepared TiO2 aggregates varied from 10-100 nm, and their pore size from around 5-12 nm; this size has been shown to depend on synthesis temperature. The mechanism of the aggregate formations was discussed in terms of efficiency of collision and coalescence processes. These newly synthetized TiO2 aggregates have been investigated as potential negative insertion electrode materials for lithium-ion batteries. The influence of specific surface areas and pore sizes on the improved capacity was discussed-and conflicting effects pointed out.

Aluminum-Based Metal-Organic Frameworks Derived Al2O3-Loading Mesoporous Carbon as a Host Matrix for Lithium-Metal Anodes.

Li-metal anode attracts great focus owing to its ultra-high specific capacity and the lowest redox potential. However, the uncontrolled growth of Li dendrite leads to severe security issues and limited cycle life. Herein, Al2O3 loading mesoporous carbon (Al2O3@MOF-C) derived from Al-based metal-organic frameworks (Al-MOFs) was investigated as the stable host matrix for Li metal, in which, Al2O3 was served as nano seeds for the Li deposition and decrease the Li nucleation overpotential. Except that, the high specific surface area and wide pore distribution can also buffer the volume changes of Li and fasten electron transfer, hence a dendrite-free morphology was observed even after 50 cycles at 2 mA cm-2. High Li coulombic efficiency of 97.9% after 100 cycles at 1 mA cm-2, 1 mAh cm-2, and 97.6% after 50 cycles at 1 mA cm-2 and 6 mAh cm-2 were performed by Al2O3@MOF-C electrodes. Good performances were also obtained for Li-sulfur and LiFePO4 batteries. The performances of Al2O3@MOF-C@Li were compared with Li foil and Cu@Li in full cell configurations. The electrochemical tests of full cells based on Al2O3@MOF-C@Li indicated that this Al-based functional host matrix can enhance the Li-utilization and lead to significant enhancement of the cycling performance of Li anodes.

Red-Shifted Absorptions of Cation-Defective and Surface-Functionalized Anatase with Enhanced Photoelectrochemical Properties.

Manipulating the atomic structure of semiconductors is a fine way to tune their properties. The rationalization of their modified properties is, however, particularly challenging as defects locally disrupt the long-range structural ordering, and a deeper effort is required to fully describe their structure. In this work, we investigated the photoelectrochemical properties of an anatase-type structure featuring a high content of titanium vacancies stabilized by dual-oxide substitution by fluoride and hydroxide anions. Such atomic modification induces a slight red-shift band gap energy of 0.08 eV as compared to pure TiO2, which was assigned to changes in titanium-anion ionocovalent bonding. Under illumination, electron paramagnetic resonance spectroscopy revealed the formation of TiIII and O2 - radicals which were not detected in defect-free TiO2. Consequently, the modified anatase shows higher ability to oxidize water with lower electron-hole recombination rate. To further increase the photoelectrochemical properties, we subsequently modified the compound by a surface functionalization with N-methyl-2-pyrrolidone (NMP). This treatment further modifies the chemical composition, which results in a red shift of the band gap energy to 3.03 eV. Moreover, the interaction of the NMP electron-donating molecules with the surface induces an absorption band in the visible region with an estimated band gap energy of 2.25-2.50 eV. Under illumination, the resulting core-shell structure produces a high concentration of reduced TiIII and O2 -, suggesting an effective charge carrier separation which is confirmed by high photoelectrochemical properties. This work provides new opportunities to better understand the structural features that affect the photogenerated charge carriers.

Novel Sulfur Host Composed of Cobalt and Porous Graphitic Carbon Derived from MOFs for the High-Performance Li-S Battery.

A composite consisting of cobalt and graphitic porous carbon (Co@GC-PC) is synthesized from bimetallic metal-organic frameworks and employed as the sulfur host for high-performance Li-S batteries. Because of the presence of a large surface area (724 m2 g-1) and an abundance of macro-/mesopores, the Co@GC-PC electrode is able to alleviate the debilitating effect originating from the volume expansion/contraction of sulfur species during the cycling process. Our in situ UV/vis analysis indicates that the existence of Co@GC-PC promotes the adsorption of polysulfides during the discharge process. Density functional theory calculations show a strong interaction between Co and Li2S and a low decomposition barrier of Li2S on Co(111), which is beneficial to the following Li2S oxidation in the charge process. As a result, at 0.2C, the discharge capacity of the S/Co@GC-PC cathode is stabilized at 790 mAh g-1 after 220 cycles, much higher than that of a carbon-based cathode, which delivers a discharge capacity of 188 mAh g-1.

Structural and Morphological Description of Sn/SnOx Core-Shell Nanoparticles Synthesized and Isolated from Ionic Liquid.

The potential application of high capacity Sn-based electrode materials for energy storage, particularly in rechargeable batteries, has led to extensive research activities. In this scope, the development of an innovative synthesis route allowing to downsize particles to the nanoscale is of particular interest owing to the ability of such nanomaterial to better accommodate volume changes upon electrochemical reactions. Here, we report on the use of room temperature ionic liquid (i.e., [EMIm+][TFSI-]) as solvent, template, and stabilizer for Sn-based nanoparticles. In such a media, we observed, using Cryo-TEM, that pure Sn nanoparticles can be stabilized. Further washing steps are, however, mandatory to remove residual ionic liquid. It is shown that the washing steps are accompanied by the partial oxidation of the surface, leading to a core-shell structured Sn/SnOx composite. To understand the structural features of such a complex architecture, HRTEM, Mössbauer spectroscopy, and the pair distribution function were employed to reveal a crystallized ß-Sn core and a SnO and SnO2 amorphous shell. The proportion of oxidized phases increases with the final washing step with water, which appeared necessary to remove not only salts but also the final surface impurities made of the cationic moieties of the ionic liquid. This work highlights the strong oxidation reactivity of Sn-based nanoparticles, which needs to be taken into account when evaluating their electrochemical properties.

The influence of the electrolyte on chemical and morphological modifications of an iron sulfide thin film negative electrode.

The chemical and morphological modifications of FeS thin film as anode material for LiBs have been studied in detail in two classical electrolytes usually used in Li-ion batteries: 1 M LiClO4-PC and 1 M LiPF6-EC/DMC. The X-ray photoelectron spectroscopic (XPS) analysis evidenced the formation of a solid electrolyte interphase (SEI) that contains a more significant amount of inorganic salt residues formed in LiPF6-EC/DMC than in LiClO4-PC, which is likely to increase the ionic resistivity of the SEI, thus impeding the lithiation-delithiation in the first cycles while improving its reversibility. Ion depth profiles performed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) show volume expansion-shrinkage of the thin film leading to cracking and pulverization of the electrode material, which is also confirmed by scanning electron microscopy (SEM) analysis. The prolonged cycling results in penetration and accumulation of the electrolyte in a bulk electrode with accumulation of the inorganic species in the inner part of the SEI enhanced in a fluoride-containing electrolyte. Cycling in these two different electrolytes leads also to formation of two different electrode morphologies: with a compact electrode structure formed in LiClO4-PC and a foam-like, porous structure in LiPF6-EC/DMC. A model of this conversion-type thin film electrode modification based on these thorough spectroscopic and microscopic analyses induced by cycling in two different electrolytes is proposed.

Effect of lithiation potential and cycling on chemical and morphological evolution of Si thin film electrode studied by ToF-SIMS.

Si thin films obtained by plasma enhanced chemical vapor deposition (PECVD) were used to investigate chemical and morphological modifications induced by lithiation potential and cycling. These modifications were thoughtfully analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling, which allows to distinguish the surface and bulk processes related to the formation of the solid electrolyte interphase (SEI) layer, and Li-Si alloying, respectively. The main results are a volume expansion/shrinkage and a dynamic behavior of the SEI layer during the single lithiation/delithiation process and multicycling. Trapping of lithium and other ions corresponding to products of electrolyte decomposition are the major reasons of electrode modifications. It is shown that the SEI layer contributes to 60% of the total volume variation of Si electrodes (100 nm). The apparent diffusion coefficient of lithium (DLi) calculated from the Fick's second law directly from Li-ion ToF-SIMS profiles is of the order of ∼5.9 × 10(-15) cm(2).s(-1). This quite low value can be explained by Li trapping in the bulk of electrode material, at the interfaces, continuous growth of the SEI layer and increase of SiO2 quantity. These modifications can result in limitation the ionic transport of Li.

Aging-induced chemical and morphological modifications of thin film iron oxide electrodes for lithium-ion batteries.

Spectroscopic (XPS, ToF-SIMS) and microscopic (SEM, AFM) analytical methods have been applied to iron oxide (∼Fe2O3) using a thin film approach to bring new insight into the aging mechanisms of conversion-type anode materials for lithium-ion batteries. The results show that repeated lithiation/delithiation causes both chemical and morphological modifications affecting the electrochemical performance. The SEI layer formed by reductive decomposition of the electrolyte remains stable in composition (mostly Li2CO3) but irreversibly thickens upon multicycling. Irreversible swelling of the material accompanied by penetration of the SEI layer and accumulation of non-deconverted material in the bulk of the oxide thin film occurs upon repeated conversion/deconversion. After initial pulverization of the thin film microstructure, grain growth and aggregation are promoted by multicycling. This leads to capacity increase in the first few cycles, but upon further cycling volume expansion and accumulation of non-deconverted material lead to deterioration of the electrode performances.

Sealing of hard CrN and DLC coatings with atomic layer deposition.

Atomic layer deposition (ALD) is a thin film deposition technique that is based on alternating and saturating surface reactions of two or more gaseous precursors. The excellent conformality of ALD thin films can be exploited for sealing defects in coatings made by other techniques. Here the corrosion protection properties of hard CrN and diamond-like carbon (DLC) coatings on low alloy steel were improved by ALD sealing with 50 nm thick layers consisting of Al2O3 and Ta2O5 nanolaminates or mixtures. In cross sectional images the ALD layers were found to follow the surface morphology of the CrN coatings uniformly. Furthermore, ALD growth into the pinholes of the CrN coating was verified. In electrochemical measurements the ALD sealing was found to decrease the current density of the CrN coated steel by over 2 orders of magnitude. The neutral salt spray (NSS) durability was also improved: on the best samples the appearance of corrosion spots was delayed from 2 to 168 h. On DLC coatings the adhesion of the ALD sealing layers was weaker, but still clear improvement in NSS durability was achieved indicating sealing of the pinholes.

Interface stability of a TiO2/3-methoxypropionitrile-based electrolyte: first evidence for solid electrolyte interphase formation and implications.

We report an in-depth study focusing on the stability of a benchmark electrolyte composition based on a low-volatile 3-methoxypropionitrile (MPN) solvent employed in dye-sensitized solar cells. In the presence of TiO2, the semi-conductor surface plays a catalytic role in the thermal degradation of the electrolyte, which induces, among other effects, the nucleation and growth of a uniform solid electrolyte interphase (SEI) layer that wraps TiO2. On the basis of our actual understanding, we argue that SEI formation is responsible for triiodide depletion in the electrolyte during ageing and also has a simultaneous impact on TiO2 optoelectronic properties through the onset of a visible-light absorption tail, energy modification of intraband trap states, and the induction of an increase in both electron lifetime and transport time in TiO2. In-depth characterization of this layer by using XPS and ToF-SIMS indicates that the chemical composition of this SEI results from solvent and additive degradation, that is, iodide, sulfur, cyano, nitrogen, carbon, and imidazolium rings. The SEI thickness, its content, and the concentration profile strongly vary depending on the ageing conditions. The outcome of this new finding is discussed in comparison with literature observations and stresses the difficulties in reaching long-term stability at 85 °C by using MPN-based electrolytes unless new interfacial engineering is accomplished to impede pinholes between dye molecules on TiO2.

Diaminoethane adsorption and water substitution on hydrated TiO2: a thermochemical study based on first-principles calculations.

Epoxy-amines are used as structural adhesives deposited on Ti. The amine adhesion to a Ti surface depends highly on the surface state (oxidation, hydroxylation). Amines may adsorb above preadsorbed water molecules or substitute them to bind directly to surface Ti(4+) Lewis acid sites. The adsorption of a model amine molecule, diaminoethane (DAE), on a model surface, hydrated TiO2-anatase (101) surface, is investigated using Density Functional Theory including Dispersive forces (DFT-D) calculations. DAE adsorption and water substitution by DAE are exothermic processes and turn nearly isoenergetic at high coverage with adsorption-substitution energies around -0.3 eV (including dispersion forces and ZPE). Complementary ab initio molecular dynamics studies also suggest that the formation of an amine-water interaction induces water desorption from the surface at room temperature, a preliminary step towards the amine-Ti bond formation. An atomistic thermodynamic approach is developed to evaluate the interfacial free energy balance of both processes (adsorption and substitution). The main contributions to the energetic balance are dispersive interactions between molecules and the surface on the exergonic side, translational and rotational entropic contributions on the endergonic one. The substitution process is stabilized by 0.55 eV versus the adsorption one when free solvation, rotational and vibrational energies are considered. The main contribution to this free energy gain is due to water solvation. The calculations suggest that in toluene solvent with a water concentration of 10(-4) M or less, a full DAE layer replaces a preadsorbed water layer for a threshold concentration of DAE ≥ 0.1 M.