Abstract
Garnet Li7La3Zr2O12 is one of the most promising solid electrolytes used for solid-state lithium batteries. However, low ionic conductivity impedes its application. Herein, we report Ta-doping garnets with compositions of Li7-xLa3Zr2-xTaxO12 (0.1 ≤ x ≤ 0.75) obtained by solid-state reaction and free sintering, which was facilitated by graphene oxide (GO). Furthermore, to optimize Li6.6La3Zr1.6Ta0.4O12, Mg2+ was select as a second dopant. The dual substitution of Ta5+ for Zr4+ and Mg2+ for Li+ with a composition of Li6.5Mg0.05La3Zr1.6Ta0.4O12 showed an enhanced total ionic conductivity of 6.1 × 10−4 S cm−1 at room temperature. Additionally, spark plasma sintering (SPS) was applied to further densify the garnets and enhance their ionic conductivities. Both SPS specimens present higher conductivities than those produced by the conventional free sintering. At room temperature, the highest ionic conductivity of Li6.5Mg0.05La3Zr1.6Ta0.4O12 sintered at 1000 °C is 8.8 × 10−4 S cm−1, and that of Li6.6La3Zr1.6Ta0.4O12 sintered at 1050 °C is 1.18 × 10−3 S cm−1.
Highlights
Nowadays, commercial lithium ion batteries generally employ organic liquid electrolytes, which have caused a series of problems and hindered development of the batteries
It has been shown that LLZO exists in two phases, namely a tetragonal phase with an inferior Li-ion conductivity of about 10−6 S cm−1 at room temperature, and a cubic phase with a superior Li-ion conductivity of about 10−4 S cm−1 at room temperature
The most common method for preparing garnet-type solid electrolytes is through the solid state method (SSR) and free sintering, which is widely applied in industry due to its low cost
Summary
Commercial lithium ion batteries generally employ organic liquid electrolytes, which have caused a series of problems and hindered development of the batteries. As one of the most promising solid electrolytes, the lithium-rich garnet-type compound with a nominal composition of Li7La3Zr2O12 (LLZO) has many advantages, especially in Li-ion conductivity [3,4]. Mg2+ has been reported as a sintering aid which can lower the sintering temperature and improve the relative density. Combining with the uniaxial high pressure, the contact area between particles starts to fuse, resulting in rapid densification [9,10] This principle can effectively improve the relative density and lower the sintering temperature. It greatly shortens the sintering time and simplifies the process because the sinter does not need the mother powders to compensate the loss of lithium in the vacuum environment [11]. In order to compare with the conventional free sintering method and to further improve the ionic conductivity, the optimized Ta-doping and Ta/Mg dual substituted samples were sintered via spark plasma sintering
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