Clarification of Electron Orbitals Contributing to Electrode Reaction of Lithium Ion Batteries -New design direction for lithium manganese oxide-based cathode materials- (Press Release)
- Release Date
- 04 Feb, 2015
- BL08W (High Energy Inelastic Scattering)
Gunma University
Kyoto University
Japan Synchrotron Radiation Research Institute (JASRI)
Japan Science and Technology Agency
Key points
• Specific electrons in oxygen were experimentally found to govern the cathode reaction of lithium manganese oxide.
• The valence of manganese atoms changed negligibly, unlike the conventionally accepted idea.
• The achievements of this research are expected to lead to the clarification of the mechanism of the electrode reaction and provide a new design direction for the electrode materials of lithium ion batteries.
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The research group precisely measured the change in the electron momentum distribution*1 (Compton profile) of lithium manganese oxide upon lithium ion insertion by Compton scattering*2 measurement using the highly brilliant and high-energy synchrotron radiation X-rays (BL08W beamline) of SPring-8*3 and compared the results with those obtained by the first-principles calculation*4. The group found that the change in the valence of manganese atoms is negligible even when lithium ions enter the base manganese oxide, although the number of oxygen 2p electrons increases. This finding indicates that the change of the manganese atom from tetravalent to trivalent, which conventionally has been accepted as the cathode reaction of the lithium manganese oxide, does not occur. The achievements of this research are expected not only to clarify the mechanism of the electrode reaction in detail, but also to provide a new design direction for the electrode materials of lithium ion batteries. Publication: |
<<Figures>>
Charging and discharging of lithium ion batteries occur through the migration of lithium ions between the cathode and anode. In the base manganese oxide (Mn2O4), a manganese atom exists at the center of an octahedron consisting of oxygen atoms. Thus, the manganese 3d orbital or oxygen 2p orbital is expected to be the orbital that electrons enter upon lithium insertion (the orbital contributing to the electrode reaction of batteries).
The left figure shows the difference of Compton profiles obtained by experiment, by Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA). The experimental result agrees fairly well with the calculation result. The right figure shows the change in the calculated number of electrons at the manganese site (red), the oxygen site (blue), and between lattices (green) with respect to the amount of lithium. With increasing amount of lithium, the number of electrons increases between lattices (oxygen 2p orbital) and slightly increases at oxygen sites; however, it remains unchanged at the manganese site.
The left figure shows the difference of Compton profiles obtained by experiment. The number of electrons for the electron momentum of 1.5 atomic units or lower increases, which indicates that the number of electrons between lattices (oxygen 2p orbital) increases upon lithium insertion. The right figure shows the Compton profiles of manganese 3d and oxygen 2p electrons on the basis of the atomic model. The shape of the difference of Compton profiles obtained by experiment is close to that of the Compton profile of the oxygen 2p electrons.
<<Glossary>>
※1 Electron momentum distribution
Electrons in crystals are classified by momentum (i.e., velocity) in accordance with quantum mechanics. The electron momentum distribution is a physical quantity describing how many electrons exist for a certain momentum. The electron momentum density is the electron density in a momentum space and the electron momentum distribution (Compton profile) is obtained by projecting the electron momentum density in a three-dimensional distribution to that in a one-dimensional distribution.
※2 Compton scattering
Light (X-ray) has particles called photons. When X-ray photons and electrons collide similarly to billiard balls, the photons are scattered by the electrons, and the directions of the electrons are also changed. It has been observed that the energy of photons is lower after a collision than before the collision. This phenomenon is called Compton scattering. In many textbooks, Compton scattering is often explained to be the elastic collision between stationary electrons and X-ray photons. However, the electrons in actual materials are constantly moving. Therefore, the energy distribution of Compton-scattered X-ray photons reflects the electron momentum (Doppler effect). The measured scattering intensities of X-rays with respect to the energy give the Compton profile, which reflects the momentum of the electrons in materials. The electronic state of materials can be examined using the Compton profile.
※3 SPring-8
SPring-8 (Harima Science Park, Hyogo, Japan) is a synchrotron radiation facility that provides the world's highest brilliance radiation. It’s owned by Riken and run by JASRI. The name "SPring-8" is from "Super Photon ring-8 GeV". Synchrotron radiation is a electromagnetic wave radiated when charged particles are forced to bend in magnetic fields. Synchrotron radiation from SPring-8 is widely used for the studies of nano-technology, bio-technology and industrial purposes.
※4 First-principles calculation
First-principles calculation is a method of calculating physical quantities using only theories based on the fundamental laws of quantum mechanics without existing experimental data.
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