Pristine Li-rich layered cathodes, such as Li1.2Ni0.2Mn0.6O2 and Li1.2Ni0.1Mn0.525Co0.175O2, were identified to exist in two different structures: LiMO2R3m and Li2MO3C2/m phases. Upon 300 cycles of charge/discharge, both phases gradually transform to the spinel structure. The transition from LiMO2R3m to spinel is accomplished through the migration of transition metal ions to the Li site without breaking down the lattice, leading to the formation of mosaic structured spinel grains within the parent particle. In contrast, transition from Li2MO3C2/m to spinel involves removal of Li+ and O2-, which produces large lattice strain and leads to the breakdown of the parent lattice. The newly formed spinel grains show random orientation within the same particle. Cracks and pores were also noticed within some layered nanoparticles after cycling, which is believed to be the consequence of the lattice breakdown and vacancy condensation upon removal of lithium ions. The AlF3-coating can partially relieve the spinel f...

Formation Of The Spinel Phase In The Layered Composite Cathode Used In Li-Ion Batteries

High voltage cathode materials Li-excess layered oxide compounds Li[NixLi1/3−2x/3Mn2/3−x/3]O2 (0 < x < 1/2) are investigated in a joint study combining both computational and experimental methods. The bulk and surface structures of pristine and cycled samples of Li[Ni1/5Li1/5Mn3/5]O2 are characterized by synchrotron X-Ray diffraction together with aberration corrected Scanning Transmission Electron Microscopy (a-S/TEM). Electron Energy Loss Spectroscopy (EELS) is carried out to investigate the surface changes of the samples before/after electrochemical cycling. Combining first principles computational investigation with our experimental observations, a detailed lithium de-intercalation mechanism is proposed for this family of Li-excess layered oxides. The most striking characteristics in these high voltage high energy density cathode materials are 1) formation of tetrahedral lithium ions at voltage less than 4.45 V and 2) the transition metal (TM) ions migration leading to phase transformation on the surface of the materials. We show clear evidence of a new spinel-like solid phase formed on the surface of the electrode materials after high-voltage cycling. It is proposed that such surface phase transformation is one of the factors contributing to the first cycle irreversible capacity and the main reason for the intrinsic poor rate capability of these materials.

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Identifying surface structural changes in layered Li-excess nickel manganese oxides in high voltage lithium ion batteries: A joint experimental and theoretical study

New and improved materials for energy storage are urgently required to make more efficient use of our finite supply of fossil fuels, and to enable the effective use of renewable energy sources. Lithium ion batteries (LIB) are a key resource for mobile energy, and one of the most promising solutions for environment-friendly transportation such as plug-in hybrid electric vehicles (PHEVs). Among the three key components (cathode, anode and electrolyte) of LIB, cathode material is usually the most expensive one with highest weight in the battery, which justifies the intense research focus on this electrode. In this review, we present an overview of the breakthroughs in the past decade in developing high energy high power cathode materials for lithium ion batteries. Materials from six structural groups (layered oxides, spinel oxides, olivine compounds, silicate compounds, tavorite compounds, and borate compounds) are covered. We focus on their electrochemical performances and the related fundamental crystal structures, solid-state physics and chemistry are covered. The effect of modifications on both chemistry and morphology are discussed as well.

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Recent progress in cathode materials research for advanced lithium ion batteries

Lithium- and manganese-rich layered electrode materials, represented by the general formula xLi2MnO3·(1 − x)LiMO2 in which M is Mn, Ni, and Co, are of interest for both high-power and high-capacity lithium ion cells. In this paper, the synthesis, structural and electrochemical characterization of xLi2MnO3·(1 − x)LiMn0.333Ni0.333Co0.333O2 electrodes over a wide compositional range (0 ≤ x ≤ 0.7) is explored. Changes that occur to the compositional, structural, and electrochemical properties of the electrodes as a function of x and the importance of using a relatively high manganese content and a high charging potential (>4.4 V) to generate high capacity (>200 mAh/g) electrodes are highlighted. Particular attention is given to the electrode composition 0.3Li2MnO3·0.7LiMn0.333Ni0.333Co0.333O2 (x = 0.3) which, if completely delithiated during charge, yields Mn0.533Ni0.233Co0.233O2, in which the manganese ions are tetravalent and, when fully discharged, LiMn0.533Ni0.233Co0.233O2, in which the average manganese ...

Synthesis, Characterization and Electrochemistry of Lithium Battery Electrodes: xLi2MnO3·(1 − x)LiMn0.333Ni0.333Co0.333O2 (0 ≤ x ≤ 0.7)

Lithium-rich layered oxide materials xLi2MnO3·(1-x)LiMO2 (M = Mn, Ni, Co, Fe, Cr, etc.) have attracted much attention for the use of cathode materials in lithium-ion batteries in recent years. However, there are many issues still unclear (the structure and reaction mechanism are ambiguous until now), and numerous scientific challenges (low initial Coulombic efficiency, poor rate capability, and voltage degradation during cycling) of these materials that must be overcome to realize their utilization in commercial lithium-ion batteries. This Perspective focuses on the challenges and prospects associated with the current researching results of these lithium-rich layered cathode materials. Specifically, their average/local structures, reaction mechanisms, and electrochemical properties are discussed.

High-Energy Cathode Materials (Li2MnO3-LiMO2) for Lithium-Ion Batteries.

Local structure and composition studies of Li1.2Ni0.2Mn0.6O2 by analytical electron microscopy

Detailed crystallographic data on high-quality Li2MnO3 material has been obtained using a combination of X-ray diffraction (XRD), selected-area electron diffraction (SAED), high-resolution electron microscopy (HREM), and 0.1 nm probe high-angle annular dark-field imaging (HAADF) in a scanning transmission electron microscope. A high-purity Li2MnO3 powder was annealed at 950 °C for 3 days to obtain predominantly defect-free grains which average size was 3.0 ± 1.5 μm. Rietveld refinement indicated that the C2/m spacegroup provided the best fit for the XRD data. Electron diffraction patterns obtained along various zone axes, on defect-free oxide particles, could be uniquely indexed to the monoclinic structure. HREM and HAADF images of defect-free grains were consistent with a Li–Mn–Mn– arrangement, i.e., lithium ordering in the transition metal planes. Low-magnification TEM images occasionally revealed stacking defects within oxide particles. HREM images of sample areas containing defects revealed a low density of stacking faults within the monoclinic sequence, resulting in a trigonal P3
 1
 12 local arrangement.

Structural study of Li2MnO3 by electron microscopy

A microchemical analysis study is reported comparing the grain and grain boundary chemistries of Cu(In,Ga)Se2 (CIGS) films deposited by three different laboratories by different processes. An analysis of a GaAs wafer is described to provide a calibration of the accuracy and precision of the energy dispersive spectroscopy nanoprobe analysis as applied in the current instruments. When averaged over many measurements the precision of the instrument is ±0.1at.% and the accuracy is ±1.0at.% for individual points. The analysis of the CIGS shows less than 0.5at.% composition difference for each constituent element between grain and grain boundary compositions when averaged over hundreds of data points for most samples. One sample deposited at 400°C as a bilayer shows different grain and grain boundary compositions with grain boundaries being In deficient and Se rich. Both grain and grain boundary data in this sample scatter along a line between the Cu(In,Ga)Se2 and CuSe2 phases, which is a nonequilibrium behavio...

Grain boundary compositions in Cu(InGa)Se2

The spontaneous formation of voids has been observed in Cu(In,Ga)Se2 films deposited by multistage evaporation processes but was not found commonly in single-stage evaporated material. These voids may be isolated in the grains or may occur at the grain boundaries. The voids exhibit strongly faceted geometries with interior surfaces being close-packed {112} planes of the chalcopyrite structure. A model based on the Kirkendal effect [Trans. AIME 171, 130 (1947)] and the interdiffusion and phase reaction in the bilayer process is proposed to explain the observation. The void shape is analyzed and an approximate Wulff construction [Z. Kristallogr. Mineral. 34, 449 (1901)] is used to estimate the relative surface energies of the predominant facets. Certain dislocation cores are also found to be unstable and convert to nanovoids in the presence of sufficient atomic vacancies moving in the lattice.

Void formation and surface energies in Cu(InGa)Se2

Cu(In1−xGax)Se2 (CIGS) films were grown on (001) GaAs at 570 or 500°C by means of metal organic vapor-phase epitaxy. All films were Cu-rich [Cu∕(In+Ga)>1] with pseudomorphic Cu2Se second phase particles found only on the growth surface. During growth, diffusion of Ga from the substrate and vacancies generated by the formation of CIGS from Cu2Se at the surface occurred. The diffusion processes lead to the formation of Kirkendall voids at the GaAs/CIGS interface. Transmission electron microscopy and nanoprobe energy dispersive spectroscopy were used to analyze the diffusion and void formation processes. The diffusivity of Ga in CIGS was found to be relatively low. This is postulated to be due to a comparatively low concentration of point defects in the epitaxial films. A reaction model explaining the observed profiles and voids is proposed.

C. Lei

Papers

Local structure and composition studies of Li1.2Ni0.2Mn0.6O2 by analytical electron microscopy

Structural study of Li2MnO3 by electron microscopy

Grain boundary compositions in Cu(InGa)Se2

Void formation and surface energies in Cu(InGa)Se2

Interface reactions and Kirkendall voids in metal organic vapor-phase epitaxy grown Cu(In,Ga)Se2 thin films on GaAs