All-solid-state lithium secondary batteries using nonflammable inorganic solid electrolytes have been expected as next-generation batteries with greater safety and reliability. In order to improve the electrochemical properties of the all-solid-state batteries, the studies on sulfide-based solid electrolytes with high ionic conductivities and favorable mechanical properties have been actively carried out. Especially, argyrodite-type Li₆PS₅X (X=Cl, Br) crystals are attracting much attention as sulfide-based solid electrolytes with a high lithium-ion conductivities of more than 10^-3 S cm^-1 at room temperature [1]. In general, it is difficult to prepare solid-solid interfaces between electrodes and solid electrolytes with large contact areas in bulk-type all-solid-state batteries. In order to form favorable interfaces, a coating of active material particles with solid electrolytes is an effective process. We have reported that LiCoO₂ (LCO) particles were coated with Li₂S-P₂S₅ solid electrolytes by the pulsed laser deposition (PLD) method and the all-solid-state batteries using electrolyte-coated LCO showed the higher capacities than those using non-coated LCO [2]. Liquid-phase methods are simpler and more cost-effective than the gas-phase methods such as PLD. We have reported that Li₂S-P₂S₅ solid electrolytes were synthesized using N-methylformamide (NMF) via homogeneous solution from starting materials [3]. Moreover, the sulfide solution was used for the formation of interfaces between LCO and Li₂S-P₂S₅ solid electrolytes with the large contact areas in all-solid-state batteries. However, the prepared electrolyte showed a low ionic conductivity of 2.3×10^-6 S cm^-1 at room temperature. Recently, we have reported that argyrodite-type Li₆PS₅X (X=Cl, Br) solid electrolyte prepared by mechanical milling was dissolved in ethanol solution, and the ionic conductivity of the reprecipitated electrolyte was 10^-5-10^-4 S cm^-1 at room temperature [4]. Moreover, all-solid-state batteries using Li₆PS₅Br-coated LCO prepared using this process showed the same capacities as the batteries using LCO coated with Li₂S-P₂S₅ solid electrolytes by PLD. However, this approach needs the multi-step processes of mechanical milling and dissolution-reprecipitation. In this study, the Li₆PS₅Br solid electrolyte was synthesized from Li₂S, LiBr and Li₃PS₄ by liquid-phase reaction using ethanol. In addition, Li₆PS₅Br solid electrolyte was coated on LiNi_1/3Mn_1/3Co_1/3O₂ particles (Li₆PS₅Br-coated NMC) using ethanol solution and then the all-solid-state batteries using Li₆PS₅Br-coated NMC were fabricated and characterized.

   Li₂S and LiBr as the starting materials were completely dissolved into dehydrated ethanol and then Li₃PS₄ glass were added into the solution. The solution was dried at 150^oC under vacuum to obtain solid powders. On the other hand, NMC particles were coated with Li₆PS₅Br solid electrolyte using the obtained ethanol solution. The all-solid-state batteries using the mixture of NMC and Li₆PS₅Br powders or only Li₆PS₅Br-coated NMC were fabricated and characterized. The weight ratios of NMC to Li₆PS₅Br solid electrolyte were 90/10.

   The homogeneous solution was prepared by dissolution of Li₂S, LiBr and Li₃PS₄ glass into ethanol. The white powder was obtained after removing ethanol by drying at 150^oC under vacuum. X-ray diffraction (XRD) measurements and Raman spectroscopy suggested that the obtained powder was mainly Li₆PS₅Br crystal. Ionic conductivity of the Li₆PS₅Br solid electrolyte at room temperature was 1.9×10^-4 S cm^-1 and the activation energy for conduction was 34 kJ mol^-1. Li₆PS₅Br-coated NMC was prepared by mixing the ethanol solution and NMC particles and subsequent removing the solvent. Scanning electron microscopy (SEM) image and energy dispersive X-ray analysis (EDX) mapping of Li₆PS₅Br-coated NMC indicated that Li₆PS₅Br coating layer on NMC was formed with the large contact areas. The all-solid-state batteries using only Li₆PS₅Br-coated NMC showed the reversible capacity of about 120 mAh g^-1, although the batteries using the mixture of NMC and Li₆PS₅Br powders showed the reversible capacity of about 75 mAh g^-1.

References

[1] R.P. Rao et al., Phys. Status Solidi A, 8 (2011) 1804.

[2] A. Sakuda et al., J. Am. Ceram. Soc., 93 (2010) 765.

[3] S. Teragawa et al., J. Mater. Chem. A, 2 (2014) 5095.

[4] S. Yubuchi et al., J. Powder Sources, 293 (2015) 941.

Acknowledgement

   This research was financially supported by the Japan Science and Technology Agency (JST), “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries program (ALCA-SPRING)”.