Abstract
We report on the synthesis of cubic-phase garnet-type solid-state electrolytes based on Bi-doped Li7La3Zr2O12 (LLZO). Bi aliovalent substitution of Zr in LLZO utilizing the Pechini processing method is employed to synthesize Li7−xLa3Zr2−xBixO12 compounds. A strong dependence of the ionic conductivity on Bi content is observed, and under our synthesis and sintering conditions, a >100-fold increase over the un-doped sample is observed for x=0.75. Cubic-phase Li6La3Zr1BiO12 compounds are generated upon annealing in air in the temperature range 650 °C–900 °C. In contrast, in the absence of Bi, the cubic garnet phase of Li7La3Zr2O12 is not formed below 700 °C and a transformation to the tetragonal phase is observed at ∼900 °C for this un-doped compound. The role of Bi in lowering the formation temperature of the garnet cubic phase and in the ionic conductivity improvements is investigated in this work. We ascribe the effect of Bi-doping on ionic conductivity increments to changes in Li+-site occupancy and lattice parameters and the reduction in the formation temperature for the cubic-phase formation to rate enhancements of the solid-state reaction. To identify the site occupancy of Bi in the garnet structure, we employ synchrotron extended x-ray absorption fine structure spectroscopy. Our results indicate that Bi additions occupy the Zr-type sites exclusively, to within the accuracy of the measurements.
Highlights
Conventional liquid electrolyte–salt combinations in lithium batteries present inherent safety concerns due to dendritic growth and thermal runaway issues.1,2 Solid-state electrolytes provide increased functionality to the cell in terms of enhanced stability, cyclability, and safety.3–5 ionic conductivity through solid electrolyte materials is in general orders of magnitude lower than that in liquid electrolytes.6 If the ionic conductivity of lithium-ion conducting solid materials can be significantly increased, battery safety and performance can be improved significantly
We report on the synthesis of cubic-phase garnet-type solid-state electrolytes based on Bi-doped Li7La3Zr2O12 (LLZO)
Li6La3Zr1BiO12 [Fig. 1(b)] transforms almost 100% into the garnet cubic phase at around 650 ○C, with only trace amounts of BiLa2O4.5 present. This doped sample forms a mixture of La2Zr2O7 along with R-3mH BiLa2O4.5 when heat-treated at 600 ○C and readily starts to convert to the cubic garnet phase at 650 ○C, a lower temperature than for L7L3Z2O12
Summary
Conventional liquid electrolyte–salt combinations in lithium batteries present inherent safety concerns due to dendritic growth and thermal runaway issues. Solid-state electrolytes provide increased functionality to the cell in terms of enhanced stability, cyclability, and safety. ionic conductivity through solid electrolyte materials is in general orders of magnitude lower than that in liquid electrolytes. If the ionic conductivity of lithium-ion conducting solid materials can be significantly increased, battery safety and performance can be improved significantly. If the ionic conductivity of lithium-ion conducting solid materials can be significantly increased, battery safety and performance can be improved significantly. Li7La3Zr2O12 (LLZO) garnet-type oxides have been shown to be promising materials for electrolyte applications on account of their relatively high ionic conductivity and good chemical stability.. Dopants have been used to modify the Li+ content in LLZO through atomic substitution onto the 24c and 16a sites of the La3+ and Zr4+ ions, respectively.. Dopants have been used to modify the Li+ content in LLZO through atomic substitution onto the 24c and 16a sites of the La3+ and Zr4+ ions, respectively.9,20–26 These aliovalent dopants modify the geometry of the Li+ conduction channels.. These aliovalent dopants modify the geometry of the Li+ conduction channels.16,25,26 Such studies confirm that there is an optimized Li+ occupancy ratio providing the highest ionic conductivity.. Optimization of the Li+-site occupancy and lattice parameter modifications of LLZO has been attributed to reported improvements of ionic conductivity in this garnet structure. high-temperature heat treatments were originally employed to achieve the cubic-phase stabilization and densification required for electrolyte applications. Subsequently, site-specific aliovalent doping has proven to be an effective approach for reducing the temperature stabilization of the garnet cubic phase. Dopants have been used to modify the Li+ content in LLZO through atomic substitution onto the 24c and 16a sites of the La3+ and Zr4+ ions, respectively. These aliovalent dopants modify the geometry of the Li+ conduction channels. Such studies confirm that there is an optimized Li+ occupancy ratio providing the highest ionic conductivity. The cubic garnet lattice has a maximum of 7.5 Li+
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