(418d) Interfacial Stability of Al and Ga Dopants at the Lithium Garnet | Lithium Metal Interface

All-solid-state lithium-ion batteries (ASSB) are a critical technology for the future development and expansion of lightweight, energy dense, and safer battery technologies. The move to an ASSB would allow for the integration of higher energy density electrode components in particular Li metal as an anode. Of the potential solid electrolyte materials, the lithium garnet, Li₇La₃Zr₂O₁₂ (LLZO) has been touted as a leading candidate due to its high ionic conductivity (10^-3 S/cm), toughness (shear moduli ~60GPa) and purportedly strong electrical and chemical stability (0.02-4.0 V vs Li/Li⁺). However, progress in advancing the deployment of ASSB has been slow due to the difficulty of engineering a stable solid-solid interface between the solid electrolyte and lithium metal. This is further complicated in the garnet system where a wide range of dopants are commonly used further expanding the possibility of impurity phase formation at these interfaces.

Previous works looking at the stability of dopants on the Zr site (Zr, Ta, Nb), observed that reduction of these transition metals is possible the presence of Li metal. This can result in the formation of impurity phases as the reduction of dopant species is propagated through the microstructure. Luckily, methods to control lithium stoichiometry, and stabilize cubic LLZO, are not limited to Zr site doping. Strong electrochemical performance has been obtained by doping the Li_24d site via Al and Ga substitution. These alternatives to Ta or Nb allow for the removal of three Li ions per Al/Ga ion insertion and are commonly believed to be chemically equivalent to each other. Interestingly, the ionic conductivity for gallium doped samples is reported to be 2x-5x greater than aluminum doped samples. One possible explanation for this is the formation of Li-Ga eutectics that enhance the interfacial contact between the Li metal and solid electrolyte. In this study we look to investigate the stability of aluminum and gallium dopants at the LLZO|Li^o interface through a combined effort of theoretical modeling and experimental techniques including, XPS, impedance spectroscopy, neutron diffraction.

During the structural optimization of Al/Ga-LLZO|Li interface simulation cells, regardless of the distance from the interface, Al doped structures optimized to relatively the same energy. However, using the same initial atomic distributions and replacing Al with Ga, results in two regions of energy convergence. The Ga that are located at the Li metal interface or on the free surface have converged to energies lower than configurations where Ga is in the “bulk” of LLZO. Additional simulations where Ga is imbedded in the Li metal layer, result in even lower energies, suggesting a stable Li-Ga alloy formation at the interface. The difference surface/interface stability between the two dopants appears to be related to their relative bonding affinity in oxides verses native metal or alloy compositions.

When in contact with E-beam deposited Li metal, XPS measurements on Ga and Al doped LLZO samples show that there are additional peaks characteristic to Ga reduction, while no additional peaks are evident in Al doped samples. Unlike the observed reduction in Zr site species, the reduction of Ga occurs under low energy E-beam deposition, indicating that the Ga is easily reduced in this system. The reactivity of Ga dopants in LLZO is further supplemented by our impedance spectroscopy work looking at the aging of Ga doped LLZO samples in contact with Li metal. During the aging process we can divide the observed degradation of the cell into three regimes. Region one where the initial Li-Ga allow is formed resulting in lowered interfacial resistance. Region two, when the reduction of Ga is slowed, and there is an increase in the charge transfer resistance. Finally, once all easily accessible Ga is depleted, region three is entered where further degradation of the cell has halted with the formation of a equilibrium interface.

The observed instability of the Ga-LLZO compared to Al-LLZO is critical to progressing our understanding of engineered interfaces in solid-state batteries. Despite the improved ionic conductivity of Ga doped samples, the chemical instability of Ga makes it a poor choice compared to Al, which forms a more stable interfacial structure. Further, techniques that look to form interfacial Li-Ga alloys through liquid metal processing at the Li|LLZO interface may be in vain, where the interphase compositions are similar to the naturally formed interface structures observed in this work. The formation of Li dendrites in the microstructure during the cycling of LLZO batteries may be directly related to the preference of Ga at particle surfaces. Because Ga is more stable at the surface of a particle, it is more likely to be in contact with lithium metal, increasing the probability that the dopant to be reduced. As Ga is reduced and incorporated in a Li-Ga alloy, it is uncertain whether Li is reintercalated into LLZO or precipitated out to the interface.