The influence of hafnium impurities on the electrochemical performance of tantalum substituted Li7La3Zr2O12 solid electrolytes

Abstract

Garnet-based Li7La3Zr2O12 (LLZO) is considered one of the most promising oxide-ceramic solid electrolyte materials for inorganic all-solid-state batteries. Dopants and substituents like Al, Ta, Nb, Ga, and W were shown to have a high impact on the total ionic conductivity, increasing it from 10−6 S/cm up to 10−3 S/cm. However, natural zirconium sources always contain a small amount of hafnium which could also act as dopant, but the separation of these two elements is complicated and expensive. In this work, we investigate the influence of various Hf-impurity concentrations on the performance of tantalum-doped LLZO. We synthesised Li6.45Al0.05La3Zr1.6−xHfxTa0.4O12 (LLZHO with x = 0 – 1.6) via conventional solid-state synthesis and have demonstrated that up to x = 0.1, hafnium impurities do not have a significant impact on the performance of the material. Above this concentration, the Li-ion conductivity is steadily reduced to around 70% when zirconium is fully substituted by hafnium resulting in Li6.45Al0.05La3Hf1.6Ta0.4O12. As the purity of Zr precursors has a great impact on their price, these findings can help to reduce the price of LLZO in general, as lower grade zirconium can be used in industrial scale applications.

All-Solid-State Li Batteries with NCM–Garnet-Based Composite Cathodes: The Impact of NCM Composition on Material Compatibility

“…45 Al 0.05 La 3 Zr 1.6 Ta 0.4 O 12 ), hereafter abbreviated as LLZO:Ta, was synthesized via a modified four-step solid-state reaction (SSR) as described before. 32,33 ) and H 3 BO 3 (Alfa Aesar, 99+%) were mixed in a molar ratio. The raw material mixture was homogenized and thoroughly ground in an agate mortar and calcined in an Al 2 O 3 crucible at 500 °C for 1 h and at 600 °C for 2 h in air.…”

“…Al- and Ta-substituted LLZO (nominal composition: Li 6.45 Al 0.05 La 3 Zr 1.6 Ta 0.4 O 12 ), hereafter abbreviated as LLZO:Ta, was synthesized via a modified four-step solid-state reaction (SSR) as described before. , The starting materials LiOH·H 2 O (Applichem, 99.00%), La 2 O 3 (Merck, 99.90%, predried for 10 h at 900 °C), ZrO 2 (Treibacher, 99.50%), Ta 2 O 5 (Treibacher, 99.99%), and Al 2 O 3 (Inframat, 99.90%) were mixed in a stoichiometric ratio with an excess of 20% LiOH·H 2 O. The homogenized powder mixture was pressed into pellets and calcined at 850 °C and two times at 1000 °C for 20 h in air.…”

Towards economic processing of high performance garnets – case study on zero Li excess Ga-substituted LLZO

“…The results were calculated based on a fixed stoichiometry of three La per formula unit. A typical Hf-impurity of 1% in Zr is assumed, 4 and Al impurities were found in all samples at long dwell times caused by alumina crucibles during sintering (see extended Table SI1 in ESI†). Unfortunately, the Zr content in all samples is systematically lower than expected and cannot be readily explained.…”

“…The low-frequency tail is a clear sign of ion-blocking electrodes, typical for gold electrodes, and can be described, for example, by a CPE-element, as previously reported. 4,9 …”

“…To achieve these conductivities, dopants and substituents are required to stabilize the cubic LLZO structure and increase the number of lithium vacancies. [1][2][3][4][5] Al, Ga, Ta and Nb are typical substituents. Trivalent substituents, such as Al and Ga ions, occupy the lithium position ðAl Li ; Ga Li Þ, while higher-valent Ta and Nb ions are incorporated into the Zr sub-lattice ðTa Zr ; Nb Zr Þ.…”