Dyer, R. Edayathumangalam, C. White, Y. Bao, S.
CSIR – National Institute for Interdisciplinary Science and Technology (NIIST)
Chakravarthy, U. Muthurajan, K. Luger, Reconstitution of nucleosome core particles from recombinant histones and DNA, in: Methods in enzymology, Elsevier, , pp. Gasgnier, A. Petit, Effects of microwave and heating treatments on the crystallographic properties of a potassium acetate powder, Journal of materials science, 29 Ferloni, M.
Sanesi, P. Franzosini, Phase Transitions in the Alkali C1—n. Jenkins, P.
O'Brien, A thermal and Raman investigation of the phase transitions above room temperature in anhydrous potassium acetate, Journal of Physics and Chemistry of Solids, 44 Frisch, G. Trucks, H. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, G. Scalmani, V.
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Barone, B. Electronic structural evolution of antiphase boundaries and transition-metal ion migration front after delithiation. Light blue dashed line rectangle indicates the position of two antiphase boundaries in a. We can see the Mn edge shifts to left lower energy and Ni edge shifts to right higher energy , which means the valence state of Mn decreased and Ni increased. The results show the O and Mn content decreased, Ni content increased at antiphase domain boundaries.
Light blue dash line rectangle indicates the position of the migration front in g. Due to the edge shift of Ni and Mn, the valence state of Mn decreased and Ni increased. In area A2, we see antiphase boundaries and a bright area perpendicular to the electric field, which connects every two antiphase boundaries. Here, the boundary is formed by losing one transition-metal-rich layer along the crystal plane; there is a half unit-cell length mismatch along the crystal plane between the two parts divided by the boundary.
The structure model of this type of antiphase boundary is demonstrated in Fig. The distribution of the antiphase boundaries becomes clearly visible in GPA scale image see inset of Fig. The linking area with the bright contrast is similar to the A1 region where transition-metal ions occupying 4a sites. The A3 region is the forefront part of all these three regions.
We can see bright single lines Fig. In the magnified image, this line is bright because the 4a sites are occupied by the transition-metal ions. The presence of both the dislocation and the linking areas between two antiphase boundaries explained the formation of both the antiphase boundaries and the migration front. The electronic structure of the antiphase boundary and migration front are studied. We found that the L-edge of Mn shift to the left, and the L-edge of Ni shift to the right. This indicates a decrease of the valence state of Mn and an increase of the valence state of Ni.
We also find evidence for a fluctuation in the concentration of each transition metal via EELS spectrum. The content distribution of O, Mn, and Ni along the arrow in Fig. We obtained almost the same results with respect to antiphase boundaries. A loss of Mn and enrichment of Ni are evident at the migration-front area as shown in Fig. However there is no obvious change in oxygen content. EELS spectra in Fig.
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Overall, Ni is more likely to occupy the 4a site than Mn during delithiation along with the electronic structural evolution at the same time. In Fig. To help understanding the experimental results, density functional theory DFT -based first-principles calculations was further applied to study the behavior of the transition-metal ions and the formation of antiphase boundaries during delithiation.
Both primary and reconstruction supercells with five different Li concentrations x x in Li x Ni 0. It suggests that extraction of Li tends to diminish the energy discrepancy between the reconstructed and primary phases, making the reconstructed phase energetically competitive.
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Theoretical calculation result of the interphase energy and the extraction of lithium ions. The structure models for the calculations are shown at each point. The red dashed line indicates the result without considering oxygen vacancies. The green line shows the result considering oxygen vacancies at a high delithiation state.
The pink line shows the result of the bare structure. However, the energy gap increased to 1. Moreover, the concentration of transition-metal ion is low in the dislocation layers in the reconstructed supercell. Consequently, oxygen escapes from the unstable frame.
Since stoichiometric oxygen calculation models are not suitable for delithiated conditions, we fabricated the oxygen deficient delithiated supercells by removing two oxygen atoms from both the reconstructed and the primary phases. Due to oxygen vacancies, the defect formation energy was reduced to 1. By combining in situ STEM results and theoretical calculations, the dynamic picture of the atomic and electronic structural evolution of LNMO cathode during delithiation is constructed. In liquid electrolyte lithium-ion batteries, transition-metal ions dissolve into electrolyte.
However, in solid electrolyte lithium-ion batteries, transition-metal ions are more confined within the solid electrolyte. At the beginning, lithium ions were extracted due to the electrochemical force. However, the contact condition between electrode and solid electrolyte are not constant. Because of the compatibility and complexity between and within electrode and solid electrolyte, lithium ions leave the LNMO lattice with different speeds 33 , 34 , resulting in the formation of different zones with different delithiation levels along the electric field.
It has been shown that transition-metal ions would migrate into the 4a 16c in Fd-3m space group sites of spinel LNMO during the battery cycle. As a result, an area with high-level delithiation will have more transition-metal ions migration than areas with lower-level delithiation. Here, the high-level delithiation zone represents the bright line area of the A1 region in our results.
In addition, practical defects exist in LNMO cathode too as shown in the feature at the top of migration front region in Fig. Together with the extraction of lithium ions, lattice parameters change, which introduces strain into the LNMO crystal. The energy hidden in the dislocation tends to be released to reduce strain and then antiphase boundaries are formed. Furthermore, dislocations can absorb transition-metal ions to dislocation core.
However, the formation sequence of antiphase boundary and the migration front is hard to determine. Presumably with the help of dislocations and uneven level of delithiation, transition-metal ions can migrate along a particular path. As also suggested by previous studies, the migration of transition-metal, oxygen ions and the existence of antiphase boundaries in the cathode severely affect the electrochemical properties such as cycle ability, rate ability, and battery capacity 8 , 9.
In this study, by applying theoretical calculation, it is found that doping low valence-state cations would hinder the formation of antiphase boundaries and enhance the structural stability. Different valence-state cations were tested. M-doped supercells, i. For the doped primary supercells, M ions were used to obtain the minimum energies. For the doped reconstructed supercells, M ions were placed in the interchanged layers. Thus, they do not reject the reconstruction process.
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In conclusion, we used state-of-the-art atomic scale in situ STEM methodology and theoretical calculations to visualize the delithiation of a single-crystal LNMO cathode in three dimensions.