A large international team led by scientists from the Institute for Superconducting and Electronic Materials at the University of Wollongong has verified that the introduction of novel molecular orbital interactions can improve the structural stability of cathode materials for lithium-ion batteries.
The production of better cathode materials for high-performance lithium-ion batteries is a major challenge for the electric car industry.
In research published in Angewandte Chemie, first author Dr. Gemeng Liang, Prof Zaiping Guo and associates, used multiple capabilities at ANSTO and other techniques to provide evidence that doping a promising cathode material, spinel LiNi0.5 Mn1.5 O4 (LNMO), with germanium significantly strengthens the 4s-2p orbital interaction between oxygen and metal cations.
"The 4s-2p orbital is relatively uncommon, but we found a compound in the literature in which germanium has a valence state of + 3, enabling an electron configuration ([Ar] 3d104s1) in which 4s transition metal orbital electrons are available to interact with unpaired electrons in the oxygen 2p orbital, producing the hybrid 4s-2p orbital."
The 4s-2p orbital creates structural stability in the LNMO material, as determined using synchrotron and neutron experiments at ANSTO's Australian Synchrotron and the Australian Center for Neutron Scattering, as well as other methods.
The team used neutron and (lab-based) X-ray powder diffraction, as well as microscopy, to confirm the location of the doped germanium at the 16c and 16d crystallographic sites of the LNMO structure with Fd3 ̅m space group symmetry.
As the valence state of the germanium dopants was important to investigate, laboratory X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements at the Australian Synchrotron were carried out. They confirmed that germanium dopants have an average valence state of +3.56, with germanium at the 16c and 16d sites being +3 and +4, respectively. The results of density functional theory (DFT) calculations supported this observation.
The researchers evaluated the electrochemical performance of batteries containing LNMO and compared that with those containing LNMO with 4s-2p orbital hybridization (known as 4s-LNMO). These assessments found that doping with 2% germanium contributed to superior structural stability, as well as reduced battery voltage polarization, improved energy density, and high voltage output.
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