Project
Understanding the Detrimental Nature of Dislocations in Layered Cathode Active Materials
Figure 1: Gliding barriers for LiCoO₂ from a pure O3 stacking to a structure containing a O1 stacking fault for different lithium contents. The barriers have been obtained by performing solid-state nudged elastic band calculations within density function theory. The barriers and stacking fault energies are insightful to understand stacking sequence changes (i.e., phase transitions) as a function of the lithium content as well as the mechanical behaviour of the material. Layered transition metal oxides are employed as cathode materials in high-voltage, rechargeable lithium ion batteries. The continuous delithiation and lithiation during charge and discharge, however, puts the material literally under stress: By changing the lithium content, phase transitions (i.e., certain lithium orderings or stacking sequence changes) as well as contraction and elongation of the lattice constants are induced. This couples to already present defects (e.g., Ni ions occupying the Li layer) and influences the generation of new defects such as oxygen vacancies or Ni migration, ultimately resulting in phases that resemble spinel or rock salt structures. These phases heavily deteriorate the properties of the material and are not well understood, yet. Therefore, we apply atomistic simulations to improve the understanding of the chemomechanical processes to derive optimization schemes for layered transition metal oxides.
Project Details
Project term
February 1, 2022–November 30, 2022
Affiliations
TU Darmstadt
Institute
Materials Modeling Division
Project Manager
Principal Investigator
Methods
We have mainly applied density functional theory (DFT) calculations using the Vienna ab initio simulation package (VASP) to optimize atomistic models and to calculate their electronic structure, total energies, forces and stresses. Initial models containing dislocations have been generated using the Babel package.
Results
Different models containing a dislocation dipole of screw dislocations for the two layered transition metal oxides LiCoO₂ and LiNiO₂ were generated and treated within density functional theory. Additionally, also the fully delithiated materials were taken into account. Based on the mechanical properties and elasticity theory, the excess energies of the dislocations could be explained. Interestingly, the Jahn-Teller active LiNiO₂ was found to couple with the high strain around the dislocation core: A reorientation of the Jahn-Teller distortions is capable to take a portion of the strain, which lowers the excess energies considerably. A full dislocation is able to split into two partial dislocations which reduces the energy of the system. The two partial dislocations then span a stacking fault, typically having a positive formation energy. We approached this topic by performing solid-state nudged elastic band simulations to study the gliding barriers of a rigid shift of the material. We find that both the gliding barrier (similar to the unstable stacking fault energy) as well as the stacking fault energy decreases with decreasing lithium content and becomes even negative towards 0% lithium content. A splitting of the dislocations was then also observed after an advanced structural identification tool had been developed. In the case of LiNiO₂, we also investigated how point defects influence the stacking fault energy by introducing an extra nickel in the lithium layer. At 0% Li content, 2% extra Ni has a similar effect on the barriers and stacking fault energies as a Li content of 11% to 15%. Furthermore, we calculated defect binding energies between the dislocation and lithium and oxygen vacancies. For LiCoO₂, both defects show overall negative binding energies. This indicates that an agglomeration of defects in the vicinity of the dislocation could occur, potentially accelerating unwanted phase transitions. A comparison to elasticity theory (for which binding energies are obtained from strain and calculated defect dipole tensors) shows that inelastic contributions (only properly represented in density functional theory calculations) are responsible for the non-zero binding energies. For LiNiO₂, the Jahn-Teller distortions complicate the situation considerably and mostly alternating patterns between positive and negative formation energies are found, again with the highest values close to the cores.
Finally, as a first step towards understanding the generation of dislocations calculated
surface energies of differently oriented surfaces. Based on these, a surface phase diagram
was constructed and the particle morphology is predicted as a function of the chemical
potentials for the atomic species. The different chemical potentials are motivated by
different synthesis conditions (mainly temperature and oxygen partial pressure).
Discussion
Our efforts on understanding the chemomechanical degradation of layered transition metal oxides will be continued in a future project and we want to include edge dislocation in our analysis. However, it is most likely that the system sizes are too large to be handled with DFT. Therefore, we will work on generating a machine learning potential to describe the interatomic interactions.
Additional Project Information
DFG classification: 406
Software: VASP, GULP
Cluster: Lichtenberg 2
Publications
M. Sadowski, L. Koch, K. Albe and S. Sicolo, “Planar gliding and vacancy condensation:
The role of dislocations in the chemomechanical degradation of layered transition metal
oxides”, Chemistry of Materials 2023 35 (2), 584-594 https://doi.org/10.1021/acs.chemmater.2c03069