Abstract:Structural changes in a 0.5Li 2 MnO 3 $0.5LiCoO 2 cathode material were investigated by X-ray absorption spectroscopy. It is observed that both Li 2 MnO 3 and LiCoO 2 components of the material exist as separate domains, however, with some exchange of transition metal (TM) ions in their slab layers. A large irreversible capacity observed during activation of the material in the 1 st cycle can be attributed to an irreversible oxygen release from Li 2 MnO 3 domains during lithium extraction. The average valence … Show more
“…36,37 The inclusion of this latter dataset is motivated by our desire to improve the diversity of the test data, especially with regards to transition metal species.…”
X-ray absorption spectroscopy (XAS) is a widely used materials characterization technique to determine oxidation states, coordination environment, and other local atomic structure information. Analysis of XAS relies on comparison of measured spectra to reliable reference spectra. However, existing databases of XAS spectra are highly limited both in terms of the number of reference spectra available as well as the breadth of chemistry coverage. In this work, we report the development of XASdb, a large database of computed reference XAS, and an Ensemble-Learned Spectra IdEntification (ELSIE) algorithm for the matching of spectra. XASdb currently hosts more than 800,000 K-edge X-ray absorption near-edge spectra (XANES) for over 40,000 materials from the open-science Materials Project database. We discuss a high-throughput automation framework for FEFF calculations, built on robust, rigorously benchmarked parameters. FEFF is a computer program uses a real-space Green's function approach to calculate X-ray absorption spectra. We will demonstrate that the ELSIE algorithm, which combines 33 weak "learners" comprising a set of preprocessing steps and a similarity metric, can achieve up to 84.2% accuracy in identifying the correct oxidation state and coordination environment of a test set of 19 K-edge XANES spectra encompassing a diverse range of chemistries and crystal structures. The XASdb with the ELSIE algorithm has been integrated into a web application in the Materials Project, providing an important new public resource for the analysis of XAS to all materials researchers. Finally, the ELSIE algorithm itself has been made available as part of veidt, an open source machine-learning library for materials science.npj Computational Materials (2018) 4:12 ; doi:10.1038/s41524-018-0067-x INTRODUCTION X-ray absorption spectroscopy (XAS) is a widely used technique in the study of the properties, physical states, and local environments of materials.1-3 When incident X-ray photons with energy greater than the binding energy are absorbed by an atom, a corelevel electron is removed from its quantum level. In XAS, the absorption coefficient, μ(E) is measured as a function of X-ray energy E. Detailed descriptions of X-ray absorption theory and equation have been included in many excellent books and review papers.
“…36,37 The inclusion of this latter dataset is motivated by our desire to improve the diversity of the test data, especially with regards to transition metal species.…”
X-ray absorption spectroscopy (XAS) is a widely used materials characterization technique to determine oxidation states, coordination environment, and other local atomic structure information. Analysis of XAS relies on comparison of measured spectra to reliable reference spectra. However, existing databases of XAS spectra are highly limited both in terms of the number of reference spectra available as well as the breadth of chemistry coverage. In this work, we report the development of XASdb, a large database of computed reference XAS, and an Ensemble-Learned Spectra IdEntification (ELSIE) algorithm for the matching of spectra. XASdb currently hosts more than 800,000 K-edge X-ray absorption near-edge spectra (XANES) for over 40,000 materials from the open-science Materials Project database. We discuss a high-throughput automation framework for FEFF calculations, built on robust, rigorously benchmarked parameters. FEFF is a computer program uses a real-space Green's function approach to calculate X-ray absorption spectra. We will demonstrate that the ELSIE algorithm, which combines 33 weak "learners" comprising a set of preprocessing steps and a similarity metric, can achieve up to 84.2% accuracy in identifying the correct oxidation state and coordination environment of a test set of 19 K-edge XANES spectra encompassing a diverse range of chemistries and crystal structures. The XASdb with the ELSIE algorithm has been integrated into a web application in the Materials Project, providing an important new public resource for the analysis of XAS to all materials researchers. Finally, the ELSIE algorithm itself has been made available as part of veidt, an open source machine-learning library for materials science.npj Computational Materials (2018) 4:12 ; doi:10.1038/s41524-018-0067-x INTRODUCTION X-ray absorption spectroscopy (XAS) is a widely used technique in the study of the properties, physical states, and local environments of materials.1-3 When incident X-ray photons with energy greater than the binding energy are absorbed by an atom, a corelevel electron is removed from its quantum level. In XAS, the absorption coefficient, μ(E) is measured as a function of X-ray energy E. Detailed descriptions of X-ray absorption theory and equation have been included in many excellent books and review papers.
“…5a and Supplementary Fig. 22), which originated from irreversible Ni/Co ion migration (intra-layer to O Li in the inter-layer) and evolution of layered MnO 2 -type structure caused by lithium extraction and oxygen loss from Li 2 MnO 3 and LiTMO 2 phases 11,42–45 . In contrast, overall peak features in all RDFs for D-MNC (discharged state) were relatively constant, compared with OH-MNC, indicating the relative preservation of TM local structure surrounded by oxygen and TM even during 100th cycle.Fig.…”
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.
“…[ 18,21 ] This MnO 2 component can intercalate Li + in the following discharge. According to Rana et al, [ 19,20 ] this MnO 2 -type structure changes to an Li 2 MnO 3 -type structure when Li + are inserted into it during discharge. Moreover, it is reported that the Mn ions remain in its tetravalent oxidation state throughout the whole cycle.…”
Section: Introductionmentioning
confidence: 96%
“…Moreover, it is reported that the Mn ions remain in its tetravalent oxidation state throughout the whole cycle. [ 20 ] Even though, these materials deliver high capacity, the fi rst cycle Coulombic effi ciency is lower than that of the conventional layered or spinel cathode materials due to the activation process. [ 21 ] The material exhibits a poor rate capability in most cases due to surface damage caused by the activation process.…”
Section: Introductionmentioning
confidence: 99%
“…[ 1,14,15 ] The layered lithium-rich (Li-rich) materials with a general composition x Li 2 MnO 3 · (1 -x ) LiMO 2 (M = Mn, Co, Ni) are known to deliver capacities >250 mAh g −1 when cycled within the voltage range 2.0-4.8 V. [ 16 ] This material has a complex structure which is reported either as composites with nanodomains of Li 2 MnO 3 -and LiMO 2 -like features or as their solid solutions. [17][18][19][20] The powder diffraction patterns of this material
…”
the LiNi 0.5 Mn 1.5 O 4 material is intensively investigated due to its high-voltage electrochemical activity, excellent rate capability as well as cycling stability. [2][3][4][5][6][7] LiNi 0.5 Mn 1.5 O 4 exists mainly in two crystallographic structures according to the oxygen stoichiometry in the material. [ 2,[8][9][10] The cation-ordered spinel (space group P 4 3 32) which is oxygen-stoichiometric, contains all the Mn ions in their tetravalent form. [ 11 ] At the same time in the cation-disordered structure (space group Fd 3 m) , in addition to the tetravalent Mn species, some of the Mn ions exist in the trivalent form as a result of oxygen deficiency from the crystal lattice. [ 12 ] This is mainly associated with the synthesis temperature. According to Pasero et al. [ 10 ] when the synthesis temperature exceeds ≈650 °C, the structure of LiNi 0.5 Mn 1.5 O 4 transforms gradually from the cationordered to cation-disordered. In the cation-ordered structure, the only electrochemically active species is Ni 2+ . The electrochemical reaction takes place at ≈4.7 V with two plateaus corresponding to Ni 2+ / Ni 3+ and Ni 3+ /Ni 4+ reactions, respectively. [ 2,12 ] Meanwhile, in the cation-disordered structure, a slight electrochemical activity is observed around 4.0 V versus Li/Li + as a result of Mn 3+ /Mn 4+ electrochemical reaction. [ 12 ] However, this material offers only a theoretical capacity of ≈148 mAh g −1 in the usual cycling voltage range 3.5-5.1 V. [ 13 ] It is possible to intercalate a second Li + into the material at voltage <3.0 V which in turn increases the capacity delivered. [ 1,14 ] For this purpose a Li excess electrode must be used as the counter electrode during cycling. Moreover, this process is believed to induce Jahn-Teller distortion of the structure due to the existence of excessive amount of Mn 3+ which in turn results in an average oxidation state of Mn less than +3.5. [ 1,14,15 ] The layered lithium-rich (Li-rich) materials with a general composition x Li 2 MnO 3 · (1 -x ) LiMO 2 (M = Mn, Co, Ni) are known to deliver capacities >250 mAh g −1 when cycled within the voltage range 2.0-4.8 V. [ 16 ] This material has a complex structure which is reported either as composites with nanodomains of Li 2 MnO 3 -and LiMO 2 -like features or as their solid solutions. [17][18][19][20] The powder diffraction patterns of this material
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