Structure and Chemical Composition of Soil C-Rich Al-Si-Fe Coprecipitates at Nanometer Scale.

Soil carbon stabilization is mainly driven by organo-mineral interactions. Coprecipitates, of organic matter with short-range order minerals, detected through indirect chemical extraction methods, are increasingly recognized as key carbon sequestration phases. Yet the atomic structure of these coprecipitates is still rather conceptual. We used transmission electron microscopy imaging combined with energy-dispersive X-ray and electron energy loss spectroscopy chemical mappings, which enabled direct nanoscale characterization of coprecipitates from Andosols. A comparison with reference synthetic coprecipitates showed that the natural coprecipitates were structured by an amorphous Al, Si, and Fe inorganic skeleton associated with C and were therefore even less organized than short-range order minerals usually described. These amorphous types of coprecipitates resembled previously conceptualized nanosized coprecipitates of inorganic oligomers with organics (nanoCLICs) with heterogeneous elemental proportions (of C, Al, Si, and Fe) at nanoscale. These results mark a new step in the high-resolution imaging of organo-mineral associations, while shedding further light on the mechanisms that control carbon stabilization in soil and more broadly in aquatic colloid, sediment, and extraterrestrial samples.

First evidence of manganese-nickel segregation and densification upon cycling in Li-rich layered oxides for lithium batteries.

Lithium-rich manganese-based layered oxides Li[Li(x)Mn(y)TM(1-x-y)]O2 with TM standing for Ni, Co, or Fe are of great interest as cathode materials for lithium ion batteries. Indeed, among all of the materials, they offer the highest rechargeable capacity and energy density. However, when used, they suffer from complex evolutions that need to be understood before their practical use. Here we report on such evolutions studied using advanced transmission electron microscopy. Structural modifications are directly observed at the atomic scale using Cs corrected STEM HAADF imaging technique, and the chemical modifications are probed by the means of STEM EELS experiments. For the first time, segregation between nickel and manganese close the particle surface is pointed out. Finally, observed evolutions are correlated within a proposed mechanism that leads to the densification of the material. Our results allow understanding the link between the decrease of electrochemical performance and these evolutions occurring into the material upon cycling.

Crystal structure of a new polymorph of Li2FeSiO4.

We report on the crystal structure of a new polymorph of Li(2)FeSiO(4) (prepared by annealing under argon at 900 degrees C and quenching to 25 degrees C) characterized by electron microscopy and powder X-ray and neutron diffraction. The crystal structure of Li(2)FeSiO(4) quenched from 900 degrees C is isostructural with Li(2)CdSiO(4), described in the space group Pmnb with lattice parameters a = 6.2836(1) A, b = 10.6572(1) A, and c = 5.0386(1) A. A close comparison is made with the structure of Li(2)FeSiO(4) quenched from 700 degrees C, published recently by Nishimura et al. (J. Am. Chem. Soc. 2008, 130, 13212). The two polymorphs differ mainly on the respective orientations and alternate sequences of corner-sharing FeO(4) and SiO(4) tetrahedra.

Stabilization of Li-Rich Disordered Rocksalt Oxyfluoride Cathodes by Particle Surface Modification.

Promising theoretical capacities and high voltages are offered by Li-rich disordered rocksalt oxyfluoride materials as cathodes in lithium-ion batteries. However, as has been discovered for many other Li-rich materials, the oxyfluorides suffer from extensive surface degradation, leading to severe capacity fading. In the case of Li2VO2F, we have previously determined this to be a result of detrimental reactions between an unstable surface layer and the organic electrolyte. Herein, we present the protection of Li2VO2F particles with AlF3 surface modification, resulting in a much-enhanced capacity retention over 50 cycles. While the specific capacity for the untreated material drops below 100 mA h g-1 after only 50 cycles, the treated materials retain almost 200 mA h g-1. Photoelectron spectroscopy depth profiling confirms the stabilization of the active material surface by the surface modification and reveals its suppression of electrolyte decomposition.

Study of Immersion of LiNi0.5Mn0.3Co0.2O2 Material in Water for Aqueous Processing of Positive Electrode for Li-Ion Batteries.

The understanding of the phenomena occurring during immersion of LiNi0.5Mn0.3Co0.2O2 (NMC) in water is helpful to devise new strategies toward the implementation of aqueous processing of this high-capacity cathode material. Immersion of NMC powder in water leads to both structural modification of the particles surface as observed by high-resolution scanning transmission electron microscopy and the formation of lithium-based compounds over the surface (LiOH, Li2CO3) in greater amount than after long-time exposure to ambient air, as confirmed by pH titration and 7Li MAS NMR analysis. The surface compounds adversely affect the electrochemical performance and are notably responsible for the alkaline pH of the aqueous slurry, which causes corrosion of the aluminum collector during coating of the electrode. The corrosion is avoided by adding phosphoric acid to the slurry as it lowers the pH, and it also enhances the cycling stability of the water-based electrodes due to the phosphate compounds formed at the particles surface, as evidenced by X-ray photoelectron spectroscopy analysis.

Dissolution Mechanisms of LiNi1/3Mn1/3Co1/3O2 Positive Electrode Material from Lithium-Ion Batteries in Acid Solution.

The sustainability through the energy and environmental costs involve the development of new cathode materials, considering the material abundance, the toxicity, and the end of life. Currently, some synthesis methods of new cathode materials and a large majority of recycling processes are based on the use of acidic solutions. This study addresses the mechanistic and limiting aspects on the dissolution of the layered LiNi1/3Mn1/3Co1/3O2 oxide in acidic solution. The results show a dissolution of the active cathode material in two steps, which leads to the formation of a well-defined core-shell structure inducing an enrichment in manganese on the particle surface. The crucial role of lithium extraction is discussed and considered as the source of a "self-regulating" dissolution process. The delithiation involves a cumulative charge compensation by the cationic and anionic redox reactions. The electrons generated from the compensation of charge conduct to the dissolution by the protons. The delithiation and its implications on the side reactions, by the modification of the potential, explain the structural and compositional evolutions observed toward a composite material MnO2·Li xMO2 (M = Ni, Mn, and Co). The study shows a clear way to produce new cathode materials and recover transition metals from Li-ion batteries by hydrometallurgical processes.

One-step synthesis of Si@C nanoparticles by laser pyrolysis: high-capacity anode material for lithium-ion batteries.

Carbon-covered silicon nanoparticles (Si@C) were synthesized for the first time by a one-step continuous process in a novel two stages laser pyrolysis reactor. Crystallized silicon cores formed in a first stage were covered in the second stage by a continuous shell mainly consisting in low organized sp(2) carbon. At the Si/C interface silicon carbide is absent. Moreover, the presence of silicon oxide is reduced compared to materials synthesized in several steps, allowing the use of such material as promising anode material in lithium-ion batteries (LIB). Auger Electron Spectroscopy (AES) analysis of the samples at both SiKLL and SiLVV edges proved the uniformity of the carbon coating. Cyclic voltammetry was used to compare the stability of Si and Si@C active materials. In half-cell configuration, Si@C exhibits a high and stable capacity of 2400 mAh g(-1) at C/10 and up to 500 mAh g(-1) over 500 cycles at 2C. The retention of the capacity is attributed to the protective effect of the carbon shell, which avoids direct contact between the silicon surface and the electrolyte.

Multiscale phase mapping of LiFePO4-based electrodes by transmission electron microscopy and electron forward scattering diffraction.

LiFePO4 and FePO4 phase distributions of entire cross-sectioned electrodes with various Li content are investigated from nanoscale to mesoscale, by transmission electron microscopy and by the new electron forward scattering diffraction technique. The distributions of the fully delithiated (FePO4) or lithiated particles (LiFePO4) are mapped on large fields of view (>100 × 100 µm(2)). Heterogeneities in thin and thick electrodes are highlighted at different scales. At the nanoscale, the statistical analysis of 64 000 particles unambiguously shows that the small particles delithiate first. At the mesoscale, the phase maps reveal a core-shell mechanism at the scale of the agglomerates with a preferential pathway along the electrode porosities. At larger scale, lithiation occurs in thick electrodes "stratum by stratum" from the surface in contact with electrolyte toward the current collector.

Thermal stability of Li2MnO3: from localized defects to the spinel phase.

The thermal stability of Li(2)MnO(3) has been investigated by the means of coupled differential thermal analysis and thermogravimetric analysis associated with powder X ray diffraction. Various experiments performed in air and in argon allowed us to propose a mechanism of spinel-type defects formation in intergrowth with Li(2)MnO(3) when treated in air above 900 °C. The fidelity of the DIFFaX simulations performed led to the understanding of the influence of the existence of spinel type defects intergrowth on X ray and electron diffraction patterns. The formation of these defects occurs during cooling and is preceded by the formation of LiMnO(2) defects in heating. With sufficiently long thermal treatments, defects expand such that a spinel type phase can be observed after cooling.

Polymorphism and structural defects in Li(2)FeSiO(4).

Li(2)FeSiO(4), an interesting material with potential applications as the positive electrode in lithium batteries, shows complex crystal chemistry due to the versatility of cation ordering (Li(+), Fe(2+), Si(4+)) within tetrahedral sites of buckled hexagonal close packed layers of oxygen atoms. This study, conducted through X-ray and electron diffraction experiments, focuses on three samples of Li(2)FeSiO(4) (obtained from ceramic synthesis at 700 degrees C, 800 degrees C and 900 degrees C) which may contain significant amounts of structural defects. Two polymorphs of Li(2)FeSiO(4) were isolated and investigated through X-ray diffraction and electron microscopy. A new form of Li(2)FeSiO(4) (space group Pmnb with a = 6.2853(5), b = 10.6592(8) A and c = 5.0367(4) A or alternatively P2(1)/n with a = 6.2819(1) A, b = 10.6575(2) A, c = 5.0371(1) A, beta = 90.032(7) degrees ) prepared at 900 degrees C, shows cooperative small displacements of lithium cations from one tetrahedral site (up) to another (down). Attempts to prepare the second, low-temperature, polymorph (space group P2(1)/n, a = 8.2253(5) A, b = 5.0220(1) A, c = 8.2381(4) A, beta = 99.230(2) degrees ) previously reported by Nishimura et al., lead to crystals exempt of structural defects (at 700 degrees C) or built up by an intergrowth between the low temperature polymorph and a residue of the high temperature one.