Correlating the effect of dopant type (Al, Ga, Ta) on the mechanical and electrical properties of hot-pressed Li-garnet electrolyte

Abstract

The effect of cubic-phase stabilizing dopant (Al, Ga, Ta) on the mechanical and electrochemical properties of Li garnet solid electrolyte was studied. Dense Li_6.25La₃Al_0.25Zr₂O₁₂, Li_6.50La₃Ta_0.50Zr_1.5O₁₂, Li_6.25La₃Ga_0.25Zr₂O₁₂ were prepared by conventional solid-state synthesis of powder and densified using hot pressing. Ga-LLZO exhibited the highest fracture stress (∼143 MPa), fracture toughness (∼1.22 MPa m^1/2) and total conductivity (∼1 mS/cm) of the three materials; however, the bulk conductivity was about 1.5 mS/cm. We believe that the weak grain-boundaries, as evidenced by a predominately intergranular fracture, correlates with a relatively high grain-boundary impedance, thus reducing the value of the total conductivity by about 30 % lower than that for the bulk. Based on the combined mechanical and electrical properties, overall, Li_6.25La₃Ga_0.25Zr₂O₁₂ exhibits the most favorable combination of some of the most salient properties of the three dopants. We believe the results of this study will facilitate the commercialization of Li metal batteries using Li-garnet ceramic electrolyte.

Introduction

Owing to its potential to improve battery performance compared to state-of-the-art Li-ion, there has recently been a resurgence in the development of Li metal anode technology [[1], [2], [3]]. However, to date, efforts to cycle Li metal using liquid electrolytes with sufficient coulombic efficiency and safety have been largely insufficient to demonstrate commercial viability for electric vehicles [[4], [5], [6]]. However, Li-ion conducting solid electrolytes are attracting considerable attention to overcome the issues associated with liquid electrolytes [1,7]. The major requirements for the solid Li-ion electrolyte are: high Li-ion conductivity, low electronic conductivity, high relative density, and chemical stability with the electrodes, in particular for metallic Li, sufficient mechanical integrity and adequate rate capability [7,8]. One such solid electrolyte that meets many of these requirements is cubic Li-garnet of the nominal composition Li₇La₃Zr₂O₁₂ (LLZO) [9,10], in which the room temperature phase of Li₇La₃Zr₂O₁₂ is tetragonal [10]. It must be doped with aliovalent cations to form the high conductivity cubic phase at room temperature [11]. A large variety of dopants have been considered, with Al and Ta receiving the most attention [12]. Recently, another dopant Ga has been investigated [13]. In contrast, there is limited information on the mechanical properties of doped LLZO. Most of the mechanical property studies have focused on the elastic properties of Al or Ta doped LLZO. However, to achieve widespread adoption, LLZO will likely be in the form of dense thin (< 30 μm) membranes in which case the more important mechanical properties will be fracture stress and fracture toughness. There are very limited studies on the fracture stress and fracture toughness of Al, Ta, Ga-doped LLZO and how these properties depend on microstructure [14]. This information is required if LLZO is to be used as an electrolyte in solid-state batteries. No data is available for the case where Li garnet consisting of Al, Ta or Ga dopants have been prepared using the same densification method.

It is the purpose of this study to form dense (> 97 % relative density) single-phase cubic Al, Ta, Ga-doped LLZO by uniaxial hot-pressing and compare both their mechanical (fracture stress, characteristic strength, hardness and fracture toughness) and the electrical (bulk conductivity, grain boundary and total conductivity) properties. It was observed that each dopant had a unique effect on both the mechanical and electrochemical properties. Overall, Ga-doping exhibits the most favorable combination of some of the most salient properties of the three dopants in this study.