Investigation of Mg^2 +, Sc^3 + and Zn^2 + doping effects on densification and ionic conductivity of low-temperature sintered Li₇La₃Zr₂O₁₂ garnets

Highlights

• LLZ’s densification is greatly enhanced by doping with Mg²⁺ and Sc³⁺.

• A high relative density (96.6%) is obtained in the low-temperature sintered LLZ-Mg_0.1.

• A high total conductivity (0.291 mS/cm at 293 K) is obtained in the LLZ-Mg_0.1.

Abstract

Garnet-type solid electrolytes Li₇La₃Zr_2 − xMg_2xO₁₂, Li₇La₃Zr_{2 − 0.75x}Sc_xO₁₂ and Li_7 − 2xZn_xLa₃Zr₂O₁₂ with x = 0, 0.025, 0.05 and 0.1 were synthesized at 1075 °C by using the traditional solid-state reaction method. It is found that substitution of Mg^2 + and Sc^3 + for Zr^4 + is more effective in improving the Li₇La₃Zr₂O₁₂ (LLZ)'s densification and ionic conductivity than the substitution of Zn^2 + for Li⁺. As a result, the highest room-temperature total conductivity and the lowest activation energy are obtained in LLZ-Mg-2x with x = 0.05, being 0.291 mS/cm at 293 K and 0.254 eV, respectively.

Introduction

The liquid organic electrolytes are widely used in lithium-ion batteries at present, but their flammability may cause serious safety issues; hence, inorganic solid-state electrolytes are now attracting much attention [1]. In addition, solid-state electrolytes show excellent storage stability and very long cycle life, revealing that they are highly reliable [2].

Since the first preparation of Li₇La₃Zr₂O₁₂ (abbreviated as LLZ) in 2007 [3], its high total conductivity, 0.24 mS/cm at 298 K, has implied that it is a promising solid electrolyte candidate to replace organic electrolytes, which is also ascribed to its high chemical stability and wide potential window. However, two phases were found to exist in LLZ, in which the conductivity of tetragonal phase is approximately two or three orders of magnitude lower than that of the cubic phase [4]. Therefore, formation and stabilization of cubic phase is an effective approach for increasing the total conductivity of LLZ [1]. Furthermore, the total conductivity of cubic phase LLZ at 298 K could be reduced to half that of the bulk conductivity because of grain boundary resistance; hence, minimizing the grain boundary resistance is also an effective approach for enhancing the total conductivity in this solid electrolyte [5].

More importantly, a dense microstructure related to the grains and grain boundaries is crucial for achieving a high total conductivity in LLZ [6]. On one hand, several sintering methods were used to get a dense microstructure, such as flowing oxygen sintering [7] and field-assisted sintering technology [8]. On the other hand, ionic doping was adopted to obtain a higher density or relative density in LLZ, such as a sole ionic doping like Al^3 + [9], Sr^2 + [10], Ga^3 + [11], Ce^4 + [12], Ta^5 + [13], Sb^3 + [14], Te^4 + [15] and co-doping like Y^3 + and Ta^5 + [6], Al^3 + and Te^4 + [15], Al^3 + and Ta^5 + [16], Ga^3 + and Ta^5 + [16], Sb^3 + and Ba^2 + [17].

Very recently, first-principles studies by Lincoln J. Miara et al. [18] indicate that several novel dopants emerge in the LLZ material system, such as subvalent Sc^3 + and Mg^2 + on the Zr site and Zn^2 + on the Li site. Aroused by this intriguing theoretical finding, we report here our experimental improvement of lithium-ion conductivity in Li₇La₃Zr₂O₁₂ by Mg^2 +, Sc^3 + and Zn^2 + dopings. Unlike Yuki Kihira et al. [19] co-doped unsuccessfully Mg^2 + on the La site, Mg^2 + is substituted on the Zr site in this work based on the aforementioned theoretical work [18]. Also different from the high content of Zn^2 + doping effect in LLZ by Yan Chen et al. [20], we demonstrate that a small amount of Zn^2 + doping on Li site can improve the LLZ's total conductivity slightly. Furthermore, through comparison among these three dopants Mg^2 +, Sc^3 + and Zn^2 +, it is concluded that the maximum room-temperature total conductivity and relative density as well as the minimum activation energy occur in LLZ-Mg-2x at x = 0.05.