Understanding voltage decay in lithium-excess layered cathode materials through oxygen-centred structural arrangement

Lithium-excess 3d-transition-metal layered oxides (Li1+xNiyCozMn1-x-y-zO2, > 250 mAh g(-1)) suffer from severe voltage decay upon cycling, which decreases energy density and hinders further research and development. Nevertheless, the lack of understanding on chemical and structural uniqueness of the material prevents the interpretation of internal degradation chemistry. Here, we discover a fundamental reason of the voltage decay phenomenon by comparing ordered and cation-disordered materials with a combination of X-ray absorption spectroscopy and transmission electron microscopy studies. The cation arrangement determines the transition metal-oxygen covalency and structural reversibility related to voltage decay. The identification of structural arrangement with de-lithiated oxygen-centred octahedron and interactions between octahedrons affecting the oxygen stability and transition metal mobility of layered oxide provides the insight into the degradation chemistry of cathode materials and a way to develop high-energy density electrodes

Unravelling structural ambiguities in lithium- and manganese-rich transition metal oxides

Although Li- and Mn-rich transition metal oxides have been extensively studied as high-capacity cathode materials for Li-ion batteries, the crystal structure of these materials in their pristine state is not yet fully understood. Here we apply complementary electron microscopy and spectroscopy techniques at multi-length scale on well-formed Li1.2(Ni0.13Mn0.54Co0.13)O2 crystals with two different morphologies as well as two commercially available materials with similar compositions, and unambiguously describe the structural make-up of these samples. Systematically observing the entire primary particles along multiple zone axes reveals that they are consistently made up of a single phase, save for rare localized defects and a thin surface layer on certain crystallographic facets. More specifically, we show the bulk of the oxides can be described as an aperiodic crystal consisting of randomly stacked domains that correspond to three variants of monoclinic structure, while the surface is composed of a Co- and/or Ni-rich spinel with antisite defects

Dynamics of Ti, N, and TiNx (x=1-3) admolecule transport on TiN(001) surfaces

We use classical molecular dynamics and the modified embedded atom method formalism to investigate the dynamics of atomic-scale transport on a low-index model compound surface, TiN(001). Our simulations, totaling 0.25 mu s for each case study, follow the pathways and migration kinetics of Ti and N adatoms, as well as TiNx complexes with x = 1-3, which are known to contribute to the growth of TiN thin films by reactive deposition from Ti, N-2, and N precursors. The simulations are carried out at 1000 K, within the optimal range for TiN(001) epitaxial growth. We find Ti adatoms to be the highest-mobility species on TiN(001), with the primary migration path involving jumps of one nearest-neighbor distance d(NN) between adjacent fourfold hollow sites along in-plane andlt; 100 andgt; channels. Long jumps, 2d(NN), are also observed, but at much lower frequency. N adatoms, which exhibit significantly lower migration rates than Ti, diffuse along in-plane andlt; 110 andgt; directions and, when they intersect other N atoms, associatively form N-2 molecules, which desorb at kinetic rates. As expected, TiN and TiN3 complexes migrate at even lower rates with complex diffusion pathways involving rotations, translations, and rototranslations. TiN2 trimers, however, are shown to have surprisingly high diffusion rates, above that of N adatoms and almost half that of Ti adatoms. TiN3 motion is dominated by in-place rotation with negligible diffusion.Funding Agencies|Swedish Research Council (VR)|2008-6572|Swedish Government Strategic Research Area Grant in Materials Science|Mat-LiU 2009-00971|</p

Structural Evolution of Reversible Mg Insertion into a Bilayer Structure of V₂O₅·<i>n</i>H₂O Xerogel Material

Functional multivalent intercalation cathodes represent one of the largest hurdles in the development of Mg batteries. While there are many reports of Mg cathodes, many times the evidence of intercalation chemistry is only circumstantial. In this work, direct evidence of Mg intercalation into a bilayer structure of V₂O₅·<i>n</i>H₂O xerogel is confirmed, and the nature of the Mg intercalated species is reported. The interlayer spacing of V₂O₅·<i>n</i>H₂O contracts upon Mg intercalation and expands for Mg deintercalation due to the strong electrostatic interaction between the divalent cation and the cathode. A combination of NMR, pair distribution function (PDF) analysis, and X-ray absorption near edge spectroscopy (XANES) confirmed reversible Mg insertion into the V₂O₅·<i>n</i>H₂O material, and structural evolution of Mg intercalation leads to the formation of multiple new phases. Structures of V₂O₅·<i>n</i>H₂O with Mg intercalation were further supported by the first principle simulations. A solvent cointercalated Mg in V₂O₅·<i>n</i>H₂O is observed for the first time, and the ²⁵Mg magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to elucidate the structure obtained upon electrochemical cycling. Specifically, existence of a well-defined Mg–O environment is revealed for the Mg intercalated structures. Information reported here reveals the fundamental Mg ion intercalation mechanism in a bilayer structure of V₂O₅·<i>n</i>H₂O material and provides insightful design metrics for future Mg cathodes

Large scale bioinformatics data mining with parallel genetic programming on graphics processing units

Abstract A suitable single instruction multiple data GP interpreter can achieve high (Giga GPop/second) performance on a SIMD GPU graphics card by simultaneously running multiple diverse members of the genetic programming population. SPMD dataflow parallelisation is achieved because the single interpreter treats the different GP programs as data. On a single 128 node parallel nVidia GeForce 8800 GTX GPU, the interpreter can out run a compiled approach, where data parallelisation comes only by running a single program at a time across multiple inputs. The RapidMind GPGPU Linux C++ system has been demonstrated by predicting ten year+ outcome of breast cancer from a dataset containing a million inputs. NCBI GEO GSE3494 contains hundreds of Affymetrix HG-U133A and HG-U133B GeneChip biopsies. Multiple GP runs each with a population of five million programs winnow useful variables from the chaff at more than 500 million GPops per second. Sources available via FTP.