WEB Atomistic simulations of pore formation & evolution at Li metal | solid electrolyte interfacesWednesday (01.01.2020) 00:30 - 00:45 Part of:
It has been experimentally observed that pores form at Li metal | solid electrolyte interfaces during electrochemical cycling. Hence, it is necessary to apply mechanical pressure to the cell in order to close these pores through plastic deformation of the lithium metal .
This behaviour is not obvious, since the activation barrier for lithium vacancy diffusion is only 50 meV  which means that any pores should readily close at room temperature allowing the system to reach the thermodynamically beneficial state without any inner surfaces.
To elucidate why this annihilation is not observed experimentally, we conducted Kinetic Monte Carlo (KMC) simulations supported by Density Functional Theory (DFT) calculations.
These revealed that, while the activation energy for vacancy diffusion is indeed very low, the barrier to move the vacancy away from a pore surface is above 700 meV. Au contraire, the activation energy for vacancy diffusion at the pore surface is only 25 meV.
Hence, the vacancy keeps jumping on the inner surface and before the rare event of an escape from the pore can occur, a new vacancy arrives and expands the pore even more.
This means, that any pores or interface artefacts act as sinks for vacancies, trapping them and hindering their transport into the metal bulk. Hence, no pore formation is observed for an initially ideally flat interface, because each vacancy produced by a charge transfer process is transported into the bulk and to the outer surface before a new vacancy is generated. Considering the experimentally relevant case of a non-ideal interface with residual pores due to the synthesis process, however, these initial pores will act as sinks for vacancies and will expand during battery operation.
If, however, open circuit conditions (OCV) are applied and no new vacancies are formed, existing pores annihilate over time. Now, the time scale is sufficient to allow the rare event of a vacancy escaping from the pore and eventually the thermodynamically favourable state with no inner surfaces is obtained. This observation is in line with the experimental finding that an increased interface resistance upon cycling is negated after storing the battery at OCV conditions.
 Krauskopf, T. et al., ACS Appl. Mater. Interfaces 11, 2019, doi.org/10.1021/acsami.9b02537
 Frank W., et al., Phys. Rev. Lett. 77, 1996, doi.org/10.1103/PhysRevLett.77.518