Stress-driven Li dendrite growth on Li metal anodes. Li dendrite has two growth stages, i.e., the root-growing Li filaments at the early-growth-stage and the tip-growing ramified fractal structure growth stage. The tip-growing dendrite follows the mechanism of diffusion-induced interface instability, where ion depletes in the electrolyte at growth front. The growth of root-growing Li filaments is, however, due to stress relief, which is same to whisker growth in tin thin films. A mesoscopic phase-field model is developed to study the stress-driven Li dendrite growth. This multi-grain model includes grain growth, rigid-body motion, long-range diffusion, elastic energy and stress relaxation at grain boundaries. Our collaborator conducted novel experiments and confirmed that dendrite growth is mitigated on soft substrates through surface-wrinkling-induced stress relaxation in deposited Li film. Our calculated dendrite growth rate agrees with experimental results.
Li intercalation kinetics of multi-particle electrode. Coherent strain energy inside the phase-changing electrodes promotes particle-by-particle intercalation (right figure), while almost concurrent intercalation process (left figure) occurs when strain energy is totally relaxed. The multi-particle model is executed by implementing smoothed boundary approach within a highly scalable code for solving Cahn-Hilliard equation.
2D Li diffusion behavior and hybrid phase transformation kinetics in LiFePO4 micro-rod is reported in a collaborated paper with researchers from Wisconsin Univ., Brookhaven National Lab and MIT. We combine operando hard X-ray spectroscopic imaging and phase-field modeling to understand the delithiation dynamics of single-crystalline LiFePO4 microrods with long-axis along the [010] direction. The results not only provide direct evidence of the previously predicted surface-reaction-limited phase boundary migration mechanism for the first time, but reveal a new hybrid mode of phase growth in energy storage compounds.
Three-dimensional phase-field simulations have been carried out to examine the thermodynamic stability and formation kinetics of domain structure in free standing, partially (de)lithiated LiFePO4 particles.The results show that a novel checkboard domain structure is stable with the strongly anisotropic Li diffusivity along [010]. However, when Li intra- or inter-channel transport is permitted, the checkboard pattern will destabilize and transform to stripe domain structure that has lower elastic energy and is resistant to crack propagation. This study shows the important role of kinetic constraints in domain structure evolution in LiFePO4, which provides new insights on potential avenues to improve control of domain morphology via engineering the kinetic properties of olivine materials.
A spectral smoothed boundary phase-field model (SSBPF) is developed to study anisotropci Li intercalation in a LiFePO4 nanoparticle immersed in a Li+ rich electrolyte. Contrary to the traditional shrinking core-shell mechanism without a preferred phase boundary orientation, the coherent phase boundary drives anisotropic Li intercalation at low to moderate charging/discharging rates. SSBPF is powerful in examining the compositionally, elastically and structurally heterogeneous 3D system. The detailed implementation of SSBPF and derivation of the heterogeneous nucleation behaviors and mechanical equilibrium conditions have been re-visited in the Appendix of the paper published in PCCP.
Electronic and ionic transport across the cathode/electrolyte interface in solid oxide fuel cells (SOFC) is studied by an electrochemical phase-field model. The local inhomogeneity in concentration and electrostatic potential around the interface is suspected to influence cation inter-diffusion and coarsening in electrodes. This model presents an efficient approach to couple the electrochemical effects with ionic/electronic transportation through the diffuse interface. The phase-field approach is also capable of examining the heterogeneity of the cell structure and the phase transitions in electrodes. Therefore, this electrochemical phase-field model presents a promising strategy to establish a full cell model, including cathode, electrolyte, anode and all associated interfaces, which is advantageous to exploring some essential problems that impede the development of SOFC. Details about this model can be found in the paper published in JPS.
More works on energy materials:
- Phase-field model coupled with large elasto-plastic deformation in silicon anode.
- Nanovoid formation and annihilation in gallium nanodroplets under lithiation–delithiation cycling.
Upcoming works:
- Li dendrite formation kinetics.
- Coupled lithium-electron diffusion in olivine LiFePO4 cathode.
- Thermodynamic and kinetics study on intermediate solid solution phase in LiMnyFe1-yPO4 for ultrahigh rate battery cathodes.
- Defects are good thing for high-rate LiFePO4 cathode?
Liang Hong 