Cation-Substituted Na3SbS4 Electrolyte with High Sodium-Ion Conductivity
Authors: Hayashi Akitoshi , Tsuji Fumika , Masuzawa Naoki , Sakuda Atsushi , Tatsumisago Masahiro 
Published: December 2020

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-Na3PS4 metastable phase having the conductivity of 10-4 S cm-1 [2]. Studies on increasing Na+ conductivity in sulfides have been widely done and Na3SbS4 [3] with higher Na+ conductivities of 10-3 S cm-1 have been developed. In particular, the Na3SbS4 electrolyte has an advantage of high safety in air atmosphere because of the formation of a hydrate with suppressing the generation of harmful H2S gas.
In order to increase conductivity, cation-substituted Na3SbS4 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 Na3-xSb1-xWxS4 in which a part of Sb was replaced by W (Na vacancy doping) and the other is Na3+ySb1-ySiyS4 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 Na3SbS4, 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 Na2.88Sb0.88W0.12S4 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 Li9.54Si1.74P1.44S11.7Cl0.3 [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 Na3SbS4, and thus Na vacancy doping is effective for increasing the conductivity of Na3SbS4.
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 Na3SbS4 electrolyte was mixed to form sulfur composite positive electrodes. Na3Sb 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 25oC [6].

Acknowledgements: This work was supported by Element Strategy Initiative of MEXT, Grant Number JPMXP0112101003 and JSPS KAKENHI Grant Number 18H01713 and 19H05816.

References

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[6] T. Ando, A. Sakuda, M. Tatsumisago, A. Hayashi, Electrochem. Commun., 116 (2020) 106741.