Lithium-Ion Conductivity of Mixed-Anion Chloride Solid Electrolyte
Authors: Yamagishi Mariya , Tachibana Shintaro , Shimizu Takeshi , Orikasa Yuki 
Published: July 2021

Lithium-ion batteries are required to have high energy density, high cycle characteristics, and safety to be used for electric vehicles. Current lithium-ion batteries use a flammable organic electrolyte as electrolyte suffers from safety issues. All-solid-state batteries using non-flammable solid electrolytes are attracting attention because they can improve safety and energy density. The practical use of all-solid-state batteries requires solid electrolytes having higher conductivity, atmospheric stability, electrochemical stability, and moldability than conventional liquid electrolytes. However, no material that combines everything has been found at present.

Solid electrolytes are divided into two main types: oxide-based and sulfide-based materials. As oxide-based solid electrolytes, garnet-type Li7La3Zr2O12, which shows the room-temperature ionic conductivity is 5.1 × 10-4 S / cm, has been developed [1]. Generally, oxide-based materials have high atmospheric stability and are stable to oxide-based cathode materials, but the ionic conductivity is lower than sulfide-based materials, and a high-temperature process is required for synthesis. As sulfide-based solid electrolytes, crystalline Li10GeP2S12, which shows the room-temperature ionic conductivity is 1.2 × 10-2 S / cm, has been developed [2]. Sulfide-based materials have a relatively higher ionic conductivity than oxide-based materials. Moreover, sulfide-based materials have high moldability and can be synthesized at a lower temperature than oxide-based materials. However, sulfide-based materials have the disadvantages of low atmospheric stability, are at risk of reacting with water to generate H2S, and have low stability to oxide-based cathode materials.

High ionic conductivity solid electrolytes have been discovered mainly in oxide-based and sulfide-based materials. In addition, recently, it has been reported that lithium conductive solid electrolytes of halide-based materials have high ionic conductivity, and the material-searching region is expanding. Halide-based materials are considered to have high ionic conductivity due to the properties of halogen anions. Since the monovalent halogen anion has a weaker interaction with lithium ions than the divalent sulfur and oxygen anions, high-speed transport of lithium ions can be expected. In addition, the ionic radius of the halogen anion is relatively large, and the halogen anion has high polarizability. Li3YCl6, Li3YBr6 and Li3InCl6, which show the room-temperature ionic conductivity is 10-3 S / cm have been reported as lithium halide solid electrolyte materials [3], [4].

Most of these lithium halide solid electrolyte materials are single-anion compounds, and little research has been conducted on mixed-anion compounds as lithium-ion conductors. In this study, we focused on the chloride-based solid electrolyte Li3InCl6, which is reported to have high lithium-ion conductivity comparable to oxide and sulfide-based materials and is expected to have higher chemical stability than sulfide-based materials. The characteristics of the solid electrolyte when Li3InCl6 was doped with a group 16 element were investigated. A composite anion compound in which oxygen, sulfur, and selenium, which have higher polarizabilities than chlorine, were dissolved was synthesized, and the conductivity characteristics were analyzed by electrochemical measurement. Li3InCl6, Li3InCl5.4O0.3, Li3In0.9Cl5.4O0.15, Li3In0.82Cl5.4O0.03, Li3InCl5.4S0.3, Li3InCl5.4S0.6 and Li3InCl5.4Se0.6 samples were synthesized by mechanochemical method and solid-phase reaction in an argon atmosphere. The crystal structure of the synthesized samples was characterized by X-ray diffraction (XRD), and the ionic conductivity was measured by AC impedance measurements. Comparing the XRD patterns of the Li3InCl6 sample and each sample, a peak of impurities was confirmed for the oxygen solid solution sample, and a decrease in crystallinity was confirmed for the sulfur and selenium solid solution samples. The conductivity of each sample decreased in the order of Li3InCl6> Li3In0.82Cl5.4O0.03 > Li3In0.9Cl5.4O0.15 > Li3InCl5.4O0.3 > Li3InCl5.4S0.3 > Li3InCl5.4Se0.6 > Li3InCl5.4S0.6. In conclusion, addition of mixed anion to the chloride-based solid electrolytes causes negative effects for the lithium-ion conduction. From these results, it is considered that if a divalent anion is dissolved in a crystal constituting a sublattice by the monovalent anion, an increase in Coulomb interaction has a greater effect on lithium-ion conductivity than an action due to polarizability, decreasing conductivity.

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