Structure, Lithium-Ion Conductivity Coupled with Second-Order Jahn–Teller Effect, and Electrochemical Stability of Sr-Based Perovskite-Type Solid Electrolytes

Abstract

We synthesized polycrystalline perovskite-type Lii o n -c o n d u c t i n g o x i d e s ( g e n e r a l f o r m u l a : A B O 3 ) , Sr 0.5−x Li 0.3+2x Ti 0.3 Ta 0.7 O 3 (x = 0.030−0.100), and assessed their crystal structure, microstructure, ionic conductivity, and electrochemical stability. Based on first-principles calculations, local structure changes accompanied by Li-ion diffusion were discussed. It was found that the average structure of Sr 0.5−x Li 0.3+2x Ti 0.3 Ta 0.7 O 3 (x = 0.030−0.100) is a cubic perovskite-type one, and at x = 0.042, i.e., Sr 0.458 Li 0.384 Ti 0.3 Ta 0.7 O 3 , the highest bulk ionic conductivity and the total ionic conductivity at 300 K were observed to be 1.87 × 10 −3 and 1.05 × 10 −3 S cm −1 , respectively, which are greater than those of La 2/3−x Li 3x TiO 3 (LLTO). The first-principles calculations suggested that BO 6 octahedra are distorted, and the Li-ion diffusion is assisted by the dynamic distortion of BO 6 octahedra coupled with the second-order Jahn−Teller effect. The reduction potential of Sr 0.458 Li 0.384 Ti 0.3 Ta 0.7 O 3 was 1.6−1.7 V vs Li/Li + , which is comparable to that of LLTO. A cell using a Sr 0.458 Li 0.384 Ti 0.3 Ta 0.7 O 3 pellet with a deposited thin film LiCoO 2 cathode on one side was successfully operated as a secondary battery at room temperature, indicating that the compound can be applied as a solid electrolyte for Li-ion batteries.

High Li-Ion Conductivity in Pyrochlore-Type Solid Electrolyte Li2–xLa(1+x)/3M2O6F (M = Nb, Ta)

“…Figures and S5 show the temperature dependence of the σ all and Nyquist plots, respectively, of Li 1.25 La 0.58 Nb 2 O 6 F synthesized with 91% excess LiF and Li 1.25 La 0.58 Ta 2 O 6 F synthesized with 100% excess LiF. The activation energies for Li 1.25 La 0.58 Nb 2 O 6 F and Li 1.25 La 0.58 Ta 2 O 6 F are 0.22 and 0.26 eV, respectively, which are comparable with that of LGPS (0.25 eV) and superior to those of oxide-based superionic conductors [e.g., perovskite-type Sr 0.458 Li 0.384 Ti 0.3 Ta 0.7 O 3 (0.31 eV) and garnet-type Li 7 La 3 Zr 2 O 12 (0.31 eV)] …”

“…The Δ E calculated by BVEL ignores dynamic structural changes . Perovskite-type Sr 0.458 Li 0.384 Ti 0.3 Ta 0.7 O 3 reportedly contains Ti 4+ and Ta 5+ with a d 0 electron coordination, and the dynamic distortion of the (Ti, Ta)­O 6 octahedra originating from the second-order Jahn–Teller effect assists Li + diffusion . Pyrochlore-type Li 1.25 La 0.58 M 2 O 6 F (M = Nb, Ta) also contains second-order Jahn–Teller active Nb 5+ and Ta 5+ .…”

“…However, sulfide-type electrolytes easily react with moisture in air to generate toxic H 2 S . Although their conductivity is inferior to that of sulfide-based electrolytes, oxide-based solid electrolytes have attracted attention because they are highly stable in air and do not generate H 2 S. Oxide-based solid electrolytes with high ionic conductivity such as perovskite-type Li 1–3 x La x TiO 3 (1.0 mS cm –1 ), Sr 0.458 Li 0.384 Ti 0.3 Ta 0.7 O 3 (1.9 mS cm –1 ), , and garnet-type Li 7 La 3 Zr 2 O 12 (0.5 mS cm –1 ) , have been developed. However, an even higher conductivity is needed.…”

Research and Development of Novel Secondary Batteries in Japan

“…First, all-solid-state batteries with inorganic solid electrolytes do not use liquid electrolytes, resulting in no risk of solution leakage, and they are considered highly safe. The VSI discusses elemental techniques such as interface construction, basic technology for materials processing, and advanced analyses for all-solid-state batteries using sulfide-based − and oxide-based solid electrolytes. − …”