Linking structure to performance of Li1.2Mn0.54Ni0.13Co0.13O2 (Li and Mn rich NMC) cathode materials synthesized by different methods

Li and Mn-rich Li1+xNiyCozMnwO2 (LMR-NMC, 0 < x < 0.2; w > 0.5) materials remain commercially relevant owing to their high specific capacity. Due to this stoichiometry, their synthesis forms always at least two phases: monoclinic Li2MnO3 and rhombohedral LiNiaCobMncO2 (a = b = c = 1) layered moieties. However, a complete understanding of their complex crystal structure has not yet been fully realized. The monoclinic phase may become electrochemically active only at high potentials (>4.6 V vs. Li). To complicate matters even more, it has been shown that the electrochemical performance of these materials, having formally the same stoichiometry, can vary with the chosen method of material synthesis. Identification of the chemical and/or structural reasons for these variations in performance is crucial to ensure the promotion of these important cathode materials towards a practical use. Yet most methods of analysis cannot distinguish the subtle, localized variations that account for such differences. Here, solid state 6,7Li NMR was found to be successful in identifying several distinctions between compounds with identical chemical formulae. Many distinctions can be made, and even suggested to account for some of the differences in the electrochemical behaviors noted for the differently prepared materials.

High performance of thick amorphous columnar monolithic film silicon anodes in ionic liquid electrolytes at elevated temperature

The cycling performance of thick (about 7 μm) amorphous columnar monolithic film silicon anodes was studied in ionic liquid based electrolyte solutions. More than 1000 cycles at 60 °C were achieved.

Recent Studies of Interfacial Phenomena which Determine the Electrochemical Behavior of Lithium and Lithiated Carbon Anodes with the Emphasis on In Situ Techniques

This paper reports on some new results on the application of surface sensitive techniques for the study of the correlation of surface chemistry, morphology and electrochemical behavior of lithium and lithiated graphite as anodes for rechargeable batteries. Surface sensitive FTIR spectroscopy, atomic force microscopy (AFM), electrochemical quartz crystal microbalance (EQCM) were applied to Li and Li-graphite electrodes in a variety of electrolyte solutions of interest, in conjunction with standard electrochemical techniques. The similarity in the surface chemistry developed on Li and lithiated graphite in solutions is demonstrated and discussed. We demonstrate the strong impact of the surface chemistry on the morphology of Li deposition-dissolution processes, and the use of in situ EQCM measurements for the choice of optimal electrolyte solutions for rechargeable batteries with Li metal anodes.

On the Electroanalytical Characterization of LixCoO2, LixNiO2 and LiMn2O4 (Spinel) Electrodes in Repeated Lithium Intercalation-Deintercalation Processes

This paper reports on electroanalytical studies of the intercalation-deintercalation of lithium into lithiated transition metal oxides which are used as cathodes for Li ion batteries. These include LixCoO2 LixNiO2 and LixMn2O4 spinel. The basic electroanalytical response of these systems in LiAsF6 1M/EC-DMC solutions was obtained from the simultaneous use of slow and fast scan cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT) (from which D vs. E was calculated), and impedance spectroscopy (EIS). Surface sensitive FTIR spectroscopy and XRD were also used for surface and 3D characterization, respectively. A large and important denominator was found in the electrochemical behavior of lithium intercalation-deintercalation into these transition metal oxides and graphite. The use of the electroanalytical response of these systems as a tool for the study of stabilization and failure mechanisms of these materials as cathodes in rechargeable Li batteries is demonstrated and discussed.

Phase diagram of Mg insertion into Chevrel phases, MgxMo6T8 (T = S, Se). 3. The crystal structure of triclinic Mg2Mo6Se8

This series of papers is devoted to unique cathode materials for Mg batteries, MgxMo6T8 (T = S, Se, x = 1 and 2) Chevrel phases (CPs). In this part, a combination of neutron and high-resolution synchrotron X-ray diffractions was used to study the crystal structure of Mg2Mo6Se8, which is triclinic at room temperature (space group P1, a = 6.868 A, b = 6.921 A, c = 6.880 A, alpha = 93.00 degrees , beta = 94.40 degrees , gamma = 96.22 degrees ). In contrast to other members of the MgxMo6T8 family, this compound does not follow the classic scheme of successive cation insertion into so-called inner and outer sites: Both the Mg(2+) ions per formula are located in the tetrahedral sites of the outer ring. This surprising cation location, predicted previously for Mg-containing CPs by ab initio calculations, provides the uniform distribution of the cation charge in the triclinic structure, which is similar to that of rhombohedral CPs. A mapping of the cation sites was widely used to demonstrate the variety of cation arrangement in CPs and the factors affecting this arrangement, as well as to clarify the origin of the exceptionally high mobility of the Mg(2+) ions in Mg2Mo6Se8.

On the Mechanism of Triclinic Distortion in Chevrel Phase as Probed by In-Situ Neutron Diffraction

This work presents, for the first time, a general mechanism of a rhombohedral (R)-triclinic (T) phase transition in Chevrel Phases (CPs) with small cations (radius<1 A), which was unclear in spite of intensive studies of these important materials in the past. In contrast to previous interpretation of the R<-->T transition in some CPs as cation ordering, T-distortion is regarded here as a particular case of general adaptation of the framework to cation insertion, which includes the deformations of the coordination polyhedra and their tilting. The research is based on a combination of experimental studies (in-situ neutron diffraction at different temperatures) for one model compound, MgMo6Se8, and structural analysis for a variety of known CPs. This analysis shows that the structure flexibility is fundamentally different for the R and T forms. As a result of the lower flexibility, in the R form, a strict correlation exists between the compression of the framework along the -3 symmetry axis and the cation position in the structure (the so-called 'delocalization'). The decreasing delocalization in the R-CPs, which occurs on cooling, leads to excessive repulsion within the cations pairs (R-Cu1.8Mo6S8 case) or undesirable asymmetry in the cation polyhedra (R-MgMo6Se8 case). The higher flexibility of the T framework allows for relaxation of these structural strains by increasing the cation-cation distances and forming a more symmetric cation environment, sometimes with higher coordination number (CN), like CN=5 in the T-Fe2Mo6S8 type. Thus, this work also proposes possible driving forces for T-distortion in CPs.

The Effect of Slow Interfacial Kinetics on the Chronoamperometric Response of Composite Lithiated Graphite Electrodes and on the Calculation of the Chemical Diffusion Coefficient of Li Ions in Graphite

This paper deals with a study of the shape of the chronoamperometric response (current, I, vs time, t) and, eventually, the mechanism of Li-ions insertion and deinsertion to/from composite graphite electrodes obtained by a small-amplitude (incremental) technique, such as potentiostatic intermittent titration (PITT). The dependences of log I, the Cottrell parameter It(1/2), and the differential parameter d log I/d log t on the process duration (vs log t) were carefully examined both for single- and two-phase coexistence domains. log I vs log t curves for single-phase domains were characterized by a single monotonic curve with a gradually increasing slope. In contrast, the same curves for two-phase domains consist of two sequential downward concave lines. Both types of response were explained by using the cell-impedance-control model. To separate the contributions of solid-state diffusion, Ohmic drops, and slow interfacial charge-transfer kinetics to the chronoamperometric response, the data were presented in the form of the inverse Cottrell parameter, (It(1/2))(-1) vs t(-1/2), from which the chemical diffusion coefficient (D) could be obtained. Refined values of D for Li insertion into graphite obtained herein agree very well with values of the component diffusion coefficient obtained from quasielastic neutron scattering for Li insertion into HOPG, reported in the literature.

Distinction between Energetic Inhomogeneity and Geometric Non-Uniformity of Ion Insertion Electrodes Based on Complex Impedance and Complex Capacitance Analysis

Electrochemical insertion of Mg ions into Mo6S8 Chevrel phase is a unique, model system for studying the nature of the energetic inhomogeneity of the host sites suitable for ions accommodation. We show that the two energy state model can be successfully used for describing the very specific Mg ions insertion kinetics into the host, in particular, as relates to a drastic increase of Mg ions mobility in the vicinity of the critical potential of 1.25 V (vs Mg). This is accompanied by very pronounced changes of the impedance spectra. On the other hand, similar behavior of impedance spectra could be obtained for geometrically nonhomogeneous intercalation electrodes, comprising a distribution of thicknesses. One can frequently meet both these cases in practice for a vast variety of intercalation electrodes (e.g., for lithiated graphite, composite Li(x)MO2, M = Mn, Ni, Co, etc.). In this paper, we developed a methodology aimed at a reliable distinction between the two alternatives, based on complex impedance and complex capacitance analysis.

The use of tin-decorated mesoporous carbon as an anode material for rechargeable lithium batteries

A sonochemical process has been developed for the insertion of tin nanoparticles into mesoporous carbon, giving a product which was used as a building block for the anode of a rechargeable Li battery; the electrode could deliver a reversible capacity of 400 mAh gr(-1)(C + Sn) at 100% cycling efficiency, which is higher than that of graphite electrodes.