Cation-Substituted Na3SbS4 Electrolyte with High Sodium-Ion Conductivity

Solid electrolytes are key materials for developing all-solid-state rechargeable batteries. In particular, developing all-solid-state Na/S batteries is desired because of using abundant Na and S resources and their high energy density. To realize all-solid-state Na batteries, superior solid electrolytes with both high conductivity and good ductility are needed. Sulfide glass electrolytes are useful as a precursor for precipitating metastable crystalline phases which tend to have considerably high ionic conductivity [1]. We reported glass-ceramic electrolytes with cubic-Na 3 PS 4 metastable phase having the conductivity of 10 -4 S cm -1 [2]. Studies on increasing Na + conductivity in sulfides have been widely done and Na 3 SbS 4 [3] with higher Na + conductivities of 10 -3 S cm -1 have been developed. In particular, the Na 3 SbS 4 electrolyte has an advantage of high safety in air atmosphere because of the formation of a hydrate with suppressing the generation of harmful H 2 S gas. In order to increase conductivity, cation-substituted Na 3 SbS 4 electrolytes were focused on. The electrolytes were prepared via mechanochemistry, followed by heat-treatment to enhance their crystallinity. Two types of cation substitution were done; one is Na 3-x Sb 1-x W x S 4 in which a part of Sb was replaced by W (Na vacancy doping) and the other is Na 3+y Sb 1-y Si y S 4 in which a part of Sb was replaced by Si (additional Na doping). The structure and conductivity of the prepared electrolytes were evaluated. The prepared electrolytes were solid-solution based on Na 3 SbS 4 , and their conductivities were affected by the substituted elements and their contents. A partial substitution of Si for Sb decreased the conductivity, while W substitution increased the conductivity. The sulfide superionic conductor with the composition of Na 2.88 Sb 0.88 W 0.12 S 4 exhibits a room temperature conductivity of 3.2 × 10 −2 S cm −1 in a sintered body [4], which is higher than the best Li + conductivity of 2.5 × 10 −2 S cm −1 in LGPS-type Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 [5]. Partial substitution of Sb with W induced the generation of Na vacancies and tetragonal to cubic phase transition. The substitution of Mo instead of W also increased the conductivity of Na 3 SbS 4 , and thus Na vacancy doping is effective for increasing the conductivity of Na 3 SbS 4 . We applied the prepared sulfide electrolytes to all-solid-state Na/S batteries. For improving battery performance, the sulfur-carbon composites were prepared by melt-diffusion process of sulfur, and then the Na 3 SbS 4 electrolyte was mixed to form sulfur composite positive electrodes. Na 3 Sb alloy was used as a negative electrode. Developed all-solid-state Na/S cells showed a full reversible capacity of 1560 mAh per gram of S and good cyclability at 25 o C [6]. Acknowledgements: This work was supported by Element Strategy Initiative of MEXT, Grant Number JPMXP0112101003 and JSPS KAKENHI Grant Number 18H01713 and 19H05816. References [1] A. Hayashi, A. Sakuda and M. Tatsumisago, Front. Energy Res., 4 (2016) 25. [2] A. Hayashi, K. Noi, A. Sakuda and M. Tatsumisago, Nat.Commun ., 3 (2012) 856. [3] A. Banerjee et al., Angew. Chem. Int. Ed ., 55 (2016) 9634. [4] A. Hayashi et al, Nat.Commun ., 10 (2019) 5266. [5] Y. Kato et al ., Nat. Energy , 1 (2016) 16030. [6] T. Ando, A. Sakuda, M. Tatsumisago, A. Hayashi, Electrochem. Commun ., 116 (2020) 106741.