High Throughput Studies of Li-La-Zr-O Garnet Solid Electrolytes

Development of next-generation Li-ion batteries has increasingly focused on all-solid batteries employing either ceramic, polymer, or glass electrolytes in order to address shortcomings in currently commercialized liquid electrolyte Li-ion batteries including safety, limited lifetime, and lower energy densities resulting from instability with respect to Li metal anodes. [1] Lithium lanthanum zirconium oxide (LLZO) is a leading candidate for solid Li-batteries due to its high Li-ion conductivity, stability in air and against Li metal, and compatibility with high-voltage cathodes. [2] , [3] Despite significant progress being made, our understanding of LLZO is limited by the relatively small number of compositions which have been studied; especially considering the leading contender is a pseudo-quaternary oxide (Ga-doped LLZO) and many studies are now utilizing multiple dopants. [4] Herein, we have applied a high-throughput methodology for synthesizing, characterizing, and testing sets of 64 LLZO electrolytes at the mg-scale. We employ a citrate sol-gel synthesis method whereby reagent solutions are dispensed across a well-plate to give a composition gradient. After drying the samples and burning off the citrates, the resulting powders are pressed into pellets using a custom-made 64-pellet die and the pellets are sintered at the desired temperature. The high-throughput characterization techniques utilized include powder X-ray diffraction, electrochemical impedance spectrometry and electrochemical cycling in order to test electrolyte stability; with each method performed on up to 64 samples simultaneously. Using our methodology, we have studied over 700 samples to produce a full phase stability diagram for the Li-La-Zr-O pseudoternary system. We find that within the Li-La-Zr-O system, there is significant solubility of Li into the La2Zr2O7 pyrochlore structure commonly found as an impurity in LLZO synthesis due to lithium loss. We also find that LLZO appears as both tetragonal and cubic forms throughout the system, with the cubic LLZO appearing in an extremely restricted region that is difficult to access as pure phase due to lithium loss, while excess lithium leads to the tetragonal LLZO. Li conductivity measurements show that both cubic and tetragonal undoped LLZO have similar bulk conductivities, but there is only a limited region near the formal Li7La3Zr2O12 composition where grain boundary conductivity is high. Our methodology is also applied to a comprehensive doping study where over 40 different dopants are evaluated and promising dopants are tested in co-doped compositions. [1] D. Aurbach et al., Electrochimica Acta 2004, 50, 247-254. [2] Q. Liu, et al., J. Power Sources 2018, 389, 120-134. [3] T. Thompson, et al., ACS Energy Letters 2017, 2, 462-468. [4] F. Zheng, et al., J. Power Sources 2018, 389, 198-213.

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