Unraveling the Grain Boundary Effects in Lithium Lanthanum Titanate (LLTO) Solid Electrolyte

To meet the high energy density and safety requirements of next-generation batteries, all-solid-state batteries (ASSBs) are promising candidates. A lithium metal anode with a high theoretical specific capacity (3860 mAh g^-1) and the lowest electrochemical potential (-3.04 V vs. standard hydrogen electrode) is proving to be an essential component in ASSBs. However, solid electrolytes (SEs) in ASSBs still have hurdles to overcome such as lithium metallic phase accumulation, interfacial instability, and unsatisfactory ionic conductivity. Furthermore, solid electrolytes are generally powder-sintered into the polycrystalline phase, and the nature of grain boundaries (GBs) affects the overall performance of solid electrolytes. The polycrystalline solid electrolytes and the effects of GBs are not fully understood yet. In this work, we present a first-principles calculation-based theoretical study that discovers the effect of grain boundaries on ionic and electronic properties in perovskite Li_3xLa_(2/3)-xTiO₃ (0 < x < 0.167) (LLTO). Since the grain boundaries of SEs have various microstructure configurations, we consider the experimentally observed grain boundary configurations: i) grain boundaries without pore and ii) grain boundaries with pore space. The density functional theory (DFT) studies have uncovered the following properties of LLTO GBs. In pore-free grain boundaries, lithium-ion can enter the grain boundary region without an energy barrier in the aspect of thermodynamics. Moreover, the grain boundary region has an electron insulating feature comparable to bulk LLTO, even differences in A-site composition (0 < x < 0.167) have a negligible effect on electronic conductivity for stoichiometric grain boundaries. Besides, when the A-site distortion occurs at grain boundaries, the grain boundary region exhibits a p-type conductive property but has a high energy barrier for lithium-ion to cross into the grain boundary region. These findings show that the internal lithium metallic phase nucleation does not occur at the interface of firmly connected crystalline grains. On the contrary, in the grain boundaries with pore structure, the pore space has a p-type conductive feature and lithium-ion tends to reside in the pore space in a neutral charge state. This indicates that the nucleation of the lithium metallic phase begins at the intergranular pore space of solid electrolytes. Based on the thorough investigation of the ionic and electronic properties of various LLTO grain boundaries, consequently, we confirm that the lithium metallic phase nucleates in the solid electrolytes crystalline grains interface rather than at the densely connected solid electrolyte grain boundaries. Our understanding provides an essential insight into the effect of grain boundary on the electrochemical performance of solid electrolytes as well as the crucial properties to be considered in the manufacturing of high electrochemical performance and safety improved solid electrolytes for advanced secondary batteries.