Development of Novel High Li-Ion Conductivity Hybrid Electrolytes of Li10GeP2S12 (LGPS) and Li6.6La3Zr1.6Sb0.4O12 (LLZSO) for Advanced All-Solid-State Batteries

A lithium superionic conductor of Li10GeP2S12 that exhibits the highest lithium ionic conductivity among the sulfide electrolytes and the most promising oxide electrolytes, namely, Li6.6La3Sr0.06Zr1.6Sb0.4O12 and Li6.6La3Zr1.6Sb0.4O12, are successfully synthesized. Novel hybrid electrolytes with a weight ratio of Li6.6La3Zr1.6Sb0.4O12 to Li10GeP2S12 from 1/1 to 1/3 with the higher Li-ion conductivity than that of the pure Li10GeP2S12 electrolyte are developed for the fabrication of the advanced all-solid-state Li batteries.


Introduction
All-solid-state battery electrolyte has received increasing attention because of its advantages such as safety (nonexplosive) and excellent electrochemical properties (high conductivity and wide potential window). A lithium superionic conductor of Li 10 GeP 2 S 12 that exhibits an extremely high lithium ionic conductivity of 12 mS cm −1 at room temperature was first found by Canno et al. [1], which represents the highest conductivity achieved in the sulfide solid electrolyte, exceeding even those of liquid organic electrolytes. On the other hand, Murugan [2] has reported that Li 6.6 La 3 Zr 1.6 Sb 0.4 O 12 exhibits the maximum total (bulk + grain boundary) ionic conductivity of 7.7 × 10 −4 S·cm −1 at 30 • C, which represents the highest conductivity achieved in the solid oxide electrolyte. All-solid-state batteries include a metal-anode and solid-state battery with considerable potential improvements in safety and lifetime, as well as higher energy and power densities [3]. Solid-state Li-ion electrolytes (SSEs) are the key materials for the fabrication of next-generation all-solid-state batteries. The lower reactivity of solids compared with liquids leads to expectations of longer lifetimes for solid-state batteries. Inorganic solid electrolytes could support battery operation at low and high temperatures in which conventional liquid electrolytes would freeze, boil, or decompose. A prominent disadvantage of solid-state systems is the reliance of ionic diffusion on the contact of solid particles. Garnet-type Li 7 La 3 Zr 2 O 12 (LLZO) has been considered a promising candidate because of its superior chemical and electrochemical stability with air and metallic Li anode. LLZO with the cubic phase exhibits a Li-ion conductivity of two orders of magnitude higher than that of tetragonal LLZO [4,5]. Many metal elements have been employed to stabilize the cubic phase, among which Ga has been found to be effective in enhancing the lithium-ion conductivity [6]. In the present study, A lithium superionic conductor of Li 10 GeP 2 S 12 that exhibits the highest lithium ionic conductivity among the sulfide electrolytes and the most promising oxide electrolytes, namely, Li 6.6 La 3 Sr 0.06 Zr 1.6 Sb 0.4 O 12 (LLZSSO) and Li 6.6 La 3 Zr 1.6 Sb 0.4 O 12 (LLZSO) are successfully synthesized. Novel hybrid electrolytes with a weight ratio of Li 6.6 La 3 Zr 1.6 Sb 0.4 O 12 (LLZSO) to Li 10 GeP 2 S 12 from 1/1 to 1/3 with a higher Li-ion conductivity than that of the pure Li 10 GeP 2 S 12 electrolyte are developed for the fabrication of the advanced all-solid-state Li batteries.

Experimental Procedure
LGPS (Li 10 GeP 2 S 12 ) was synthesized with the starting materials of Li 2 S, P 2 S 5, and GeS 2 , which were weighed, mixed in the Li 2 S/P 2 S 5 /GeS 2 molar ratio of 5/1/1 in an Arfilled glove box, placed into a stainless-steel pot, and mixed for 30 min using a vibrating mill. The specimens were then pressed into pellets, placed in a quartz tube, and heated under flowing N 2 at a reaction temperature of 550 C for 8 h in a furnace. After reacting, the tube was slowly cooled to room temperature under the stream of flowing N 2 . The high ionic conductivity and stability were quantified by positive formation energies and challenging synthesis. In the synthesis of Li 10 GeP 2 S 12 , high Li mobility often seems to occur at the expense of stability.
The ionic conductivity measurements of all the solid electrolyte samples were performed by AC electrochemical impedance spectroscopy using a frequency response analyzer (Solartron 1260, AMETEK Scientific Instruments, Oak Ridge, TN, USA) with a frequency range of 0.1 Hz-1 MHz with an applied voltage of 20-100 mV at 295 K. All electrolyte pellets were polished, and Au was applied by coating at both sides of the pellets, or a gold paste was painted onto each side of the sample as a blocking electrode. The pellets (5 mm diameter and about 1 mm thickness) were heated at 583 K for 5 min under an argon atmosphere to obtain dry samples for carrying out the measurements. All the full batteries were evaluated on the LAND CT2001A battery test system. Charge and discharge tests of the all-solid-state batteries were performed with the figuration of Li-In//solid electrolyte//[LiNbO 3 -coated LiCoO 2 +solid electrolyte] and at 295 K.

Results and Discussion
A highly pure crystal of Li 10 GeP 2 S 12 that exhibits an extremely high lithium ionic conductivity is successfully synthesized and identified by XRD analysis, as shown in Figure 1. Li-ion conductivity of synthesized Li 10 GeP 2 S 12 solid electrolyte was measured; it is about 1.8 × 10 −3 S/cm, as shown in Table 1. The ion conductivity calculation formula is as follows: where σ: ion conductivity; d: sample sickness; R: resistance; A: sample area. The high Li-ion conductivity of the Li 10 GeP 2 S 12 seems to occur at the expense of stability. The more stable oxide-type electrolytes such as LLZSSO (Li 6.6 La 2.94 Zr 1.6 Sr 0.06 Sb 0.4 O 12 ) and LLZSO (Li 6.6 La 3 Zr 1.6 Sb 0.4 O 12 ) are also successfully synthesized and identified by their XRD patterns, as shown in Figures 2 and 3.  Li-ion conductivity of synthesized Li 6 Figure 4. The Liion conductivity of synthesized Li 6.6 La 3 Zr 1.6 Bi 0.4 O 12 and Li 6.6 La 3 Zr 1.6 Ga 0.4 O 12 are about 1.3 × 10 −4 S/cm and 1.4 × 10 −4 S/cm, respectively, at room temperature, as shown in Table 1. Other oxide electrolytes such as Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 , LiGe 2 (PO 4 ) 3 , LiTa 2 PO 8 Li 5 La 3 Nb 2 O 12 , and Li 5 La 3 Ta 2 O 12 are also successfully synthesized. The Li-ion conductivity of synthesized Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 , Li 5 La 3 Nb 2 O 12, and Li 5 La 3 Ta 2 O 12 are 1.3 × 10 −5 S/cm, 6.5 × 10 −5 S/cm, and 1.0 × 10 −5 S/cm, respectively, at room temperature, as shown in Table 1. The Li-ion conductivity of synthesized Li 6.6 La 3 Zr 1.8 Sb 0.2 O 12 , Li 6.6 La 3 Zr 1.6 Sb 0.4 O 12, and Li 6.6 La 3 Zr 1.4 Sb 0.6 O 12 are compared in Table 2. The Li-ion conductivity of synthesized Li 6.6 La 3 Zr 1.8 Sb 0.2 O 12 , Li 6.6 La 3 Zr 1.6 Sb 0.4 O 12, and Li 6.6 La 3 Zr 1.4 Sb 0.6 O 12 are 2.5 × 10 −4 S/cm, 4.7 × 10 −4 S/cm, and 3.7 × 10 −4 S/cm, respectively, at room temperature, as shown in Table 2. To develop the novel solid electrolyte with high Li-ion conductivity and the higher stability, the hybrid electrolytes of LGPS and LLZSO were prepared by mechanically mixing LGPS with LLZSO at the different weight ratio of Li 6.6 La 3 Zr 1.6 Sb 0.4 O 12 to Li 10 GeP 2 S. The Li-ion conductivity of the prepared hybrid electrolytes of LGPS and LLZSO with different compositions at room temperature (295 K) are listed in Table 3. The Li-ion conductivity of hybrid solid electrolytes of sulfide (LGPS) and oxide (LLZSO) as a function of LGPS/(LGPS + LLZSO) ratio at room temperature (295 K) is shown in Figure 5. It has been accepted that the Li 10 GeP 2 S 12 is the highest Li-ion conductivity so far. We can infer from Table 3 and Figure 5 that the Li-ion conductivity of hybrid electrolytes with a weight ratio of Li 6.6 La 3 Zr 1.6 Sb 0.4 O 12 to Li 10 GeP 2 S 12 from 1/1 to 1/3 is higher than the pure LGPS (Li 10 GeP 2 S 12 ) electrolyte. It is of significance that the novel hybrid electrolytes with a weight ratio of Li 6.6 La 3 Zr 1.6 Sb 0.4 O 12 to Li 10 GeP 2 S 12 from 1/1 to 1/3 exhibit a higher Li-ion conductivity than the pure LGPS (Li 10 GeP 2 S 12 ) electrolyte. The pure solid oxide electrolytes and pure solid sulfide electrolytes have been extensively studied. However, the novel solid hybrid electrolytes of oxide (LLZSO) and sulfide (LGPS) are first reported in the present paper, which opens a door for developing the more advanced hybrid solid electrolytes different from pure oxides and sulfides. Further studies on the mechanism of the high-ion conductivity of the novel hybrid electrolytes and the characterization of the hybrid electrolytes will be conducted in our future research.

Conclusions
Novel hybrid electrolytes with a weight ratio of Li 6.6 La 3 Zr 1.6 Sb 0.4 O 12 to Li 10 GeP 2 S 12 from 1/1 to 1/3 with the higher Li-ion conductivity than that of the pure Li 10 GeP 2 S 12 electrolyte are found for the fabrication of advanced all-solid-state Li batteries.
Funding: This research received no external funding.