First principles study on Li metallic phase nucleation at grain boundaries in a lithium lanthanum titanium oxide (LLTO) solid electrolyte

Abstract

Solid electrolytes (SEs) are critical for next-generation all solid-state batteries with high energy density and fire safety. However, recent studies observed that the Li metallic phase nucleates at the electrode interfaces as well as the interfaces between crystalline grains of SEs. Many studies have revealed the origins and control methods for Li metallic phase formation at the anode interface, but a thorough understanding of metallic Li formation at intergranular regions in SEs has not been developed yet. Through systematic DFT simulations, we present a thorough atomistic study that reveals the impact of intergranular regions on Li-metallic phase formation in SEs using the perovskite Li_3xLa_(2/3)−x□_(1/3)−2xTiO₃ (0 < x < 0.167) (LLTO) as a model SE. We investigated the three representative model structures for intergranular regions, which are experimentally observed with various microstructure configurations: (i) stoichiometric grain boundary (GB), (ii) A-site deficient GB, and (iii) intergranular pore space. In the stoichiometric GB, the GB region has an electron insulating feature regardless of A-site compositions (0 < x < 0.167). In the A-site deficient GB, however, the GB region has electronic conductivity, but it has a high repulsive force against Li-ions moving into the GB region. However, in the intergranular pore structure, Li-ions prefer to move with a neutral charge state into the pore space which shows a p-type conductive property. Accordingly, Li metallic phase nucleation starts in the intergranular pore space of the SE. These results elucidate the critical role of pore space in SEs for Li metallic phase nucleation and provide an insight into the design of Li metallic phase-free SEs and further studies on SE materials.