Effect of Mo6+ substitution on microstructure and lithium ionic conductivity of garnet-type Li7La3Zr2O12 solid electrolytes by field assisted sintering technology

Abstract

Advanced rechargeable lithium batteries with high energy density are required as power sources for electric or modern storage systems. Solid-state batteries with non-flammable inorganic solid electrolytes are one candidate to replace the currently-used liquid electrolyte. In recent years, a novel class of inorganic ceramic solid electrolyte with garnet structure Li7La3Zr2O12, known as LLZO, has been developed, which has several superior advantages, such as high Li-ion conductivity, high chemical and electrochemical stability in air. However, Li-ion conductivity for LLZO solid electrolyte is still too low to be applied in the industry. In order to obtain high Li-ion conductivity of LLZO, the priority is to stabilize its cubic phase, because the conductivity of tetragonal phase is two orders of magnitude lower than that of the cubic phase. Previous studies indicated that elemental doping was an effective means to stabilize the cubic phase as well as increase the density of ceramic samples, especially with Al3+. It can not only substitutes the corresponding element in the lattice but also helps expel pores through low melting-point phase formed at grain boundaries, which leads to a good connection between the cubic grains. In addition, Mo (normally Mo6+) substitution for Zr4+ may lead to more Li vacancies in LLZO, which is beneficial to the enhancement of ionic conductivity. In this study, Mo6+ doped Li6.5La3Zr1.75Mo0.25O12 (LLZM) solid electrolytes are successfully prepared via field assisted sintering technology (FAST). The effect of sintering temperature on the microstructure and lithium ionic conductivity is mainly investigated. The results show that pure cubic phase LLZM can be obtained at the range of temperatures from 1050 to 1150 °C for no more than 10 min. For the sample sintered at 1150 °C, a maximum relative density of >95% with a total ionic conductivity as high as 1.3 × 10−4 S cm−1 are obtained at room temperature. The change of ionic conductivity is ascribed to the smaller ionic size of Mo6+ (0.62 Å) to Zr4+ (0.72 Å). The higher valence of Mo6+ to Zr4+ can reduce the Li+ concentration and stabilize the cubic phase.