An international team led by scientists from the Institute for Superconducting and Electronic Materials at the University of Wollongong in Australia has demonstrated that introducing novel molecular orbital interactions can improve the structural stability of cathode materials for lithium-ion batteries.
In open access research published in applied ChemistryFirst author Dr. Gemeng Liang, Prof. Zaiping Guo, A/Prof. Wei Kong Pang and collaborators, leveraged multiple capabilities from the Australian Organization for Nuclear Science and Technology (ANSTO) and other techniques to provide evidence that doping a promising cathode material – spinel LiNi – is possible0.5Mn1.5O4 (LNMO) – strengthened with germanium the 4ths-2p orbital interaction between oxygen and metal cations.
The 4s-2p orbital is relatively uncommon, but we have found a compound in the literature where germanium has a valence state of +3, allowing for an electron configuration ([Ar] 3d104s1), in which 4s transition-metal orbital electrons are available to interact with unpaired electrons in the oxygen 2p orbital, creating the hybrid 4s-2p orbital.
– dr Gemeng Liang
The 4s-2p Orbital creates structural stability in the LNMO material, as determined by synchrotron and neutron experiments at ANSTO’s Australian Synchrotron and Australian Center for Neutron Scattering, as well as other methods.
The team used neutron and (laboratory-based) X-ray powder diffraction and microscopy to pinpoint the position of the doped germanium on June 16c and 16i.e crystallographic sites of the LNMO structure with Fd3m space group symmetry.
Since it was important to study the valence state of the germanium dopants, laboratory measurements using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) were carried out at the Australian synchrotron. They confirmed that germanium dopants have an average valence state of +3.56, with germanium at 16c and 16i.e Sites are +3 and +4, respectively. The results of density functional theory (DFT) calculations supported this observation.
Researchers evaluated the electrochemical performance of batteries containing LNMO and compared them to those containing LNMO at 4s-2p orbital hybridization (known as 4s-LNMO). These evaluations found that the 2% germanium doping contributed to superior structural stability as well as reduced battery voltage polarization, improved energy density, and high output voltage.
We aimed to understand the lithium diffusion kinetics in the two materials and found that the diffusion of lithium into the material is faster after the introduction of the germanium into the system, allowing for a faster charge capability.
– dr long
OK edge NEXAFS spectra of a) LNMO and b) 4s LNMO at OCV, fully charged and fully discharged states; Mn L-edge NEXAFS spectra of c) LNMO and d) 4s LNMO before, and after 500 and 1000 cycles.
After the performance tests, Dr. Liang synchrotron-based near-edge X-ray absorption spectroscopy (NEXAFS) at the soft X-ray beamline to get more detailed information about the electronic structures of active materials during the cycle. Spectroscopic data at the battery open circuit voltage revealed a significant increase in the intensity of the peaks of the 4s-LNMO material at the position corresponding to 4s-2p orbital hybridization – a further validation of the successful introduction of the novel 4s-2p orbital interaction.
Because we can see the unfilled orbitals, these are related to the filled orbitals in a unique but intricate way, and we can use these to better characterize the chemistry of the system, either through quantum mechanical calculations or by comparison with similar materials.
– Co-author instrument scientist Dr. Bruce Cowie
The NEXAFS data was also useful to evaluate the behavior of manganese in the material.
We know that preventing manganese from dissolving in the electrolyte and inhibiting the formation of manganese +2 and +3 in the structure helps prevent structural degradation.
– dr long
The NEXAFS results indicated that only a small amount of Mn3 was present+ and no appreciable Mn2+ in the 4s LNMO, which further increases the structural stability of the material.
The structural behavior of the material within a battery during cycling was investigated in operando experiments at the powder diffraction beamline at the Australian Synchrotron. Using these data, the team confirmed the suppression of an unfavorable two-phase response at high operating voltage in the 4s LNMO.
Orbital hybridization is a fairly new concept in battery research, but holds great promise for solving battery performance problems. Even better – this approach is extendable to other battery materials.
– dr long
Other co-authors from ANSTO were Dr. Anita D’Angelo, Dr. Bernt Johannessen, Dr. Lars Thomsen and Prof Vanessa Peterson. The collaborating institutions included the University of Adelaide, the University of Surrey (UK) and the Industrial Technology Research Institute (Taiwan).
dr Liang, who is currently at the University of Adelaide, received a Post Graduate Research Award from the Australian Institute of Nuclear Science and Engineering (AINSE).
G. Liang, E. Olsson, J. Zou, Z. Wu, J. Li, C.-Z. Lu, AM D’Angelo, B Johannessen, L Thomsen, B Cowie, VK Peterson, Q Cai, WK Pang, Z Guo (2022) “Introduction of 4s–2p Orbital Hybridization to Stabilize Spinel Oxide Cathodes for Lithium-Ion Batteries” Angew. Chem. Int. Ed. doi: 10.1002/anie.202201969