Electrochemical Stability Discovered in Halide Electrolytes

Beijing Institute of Technology Press Co., Ltd

They published their work on February in Energy Material Advances.

“Constructing an efficient ionic/electronic framework is crucial for the development of high-performance solid-state batteries,” said Dr. Chuang Yu, a professor at the State Key Laboratory of Advanced Electromagnetic Engineering and Technology at Huazhong University of Science and Technology. “Currently, the application of solid-state batteries with inorganic electrolytes is challenging because most solid electrolytes have an unsatisfactory low oxidation potential.”

According to Dr. Yu, halide electrolytes have been found to be cathode-stable materials with relatively wide electrochemical stability windows and good compatibility with cathodes. “Halide electrolytes are stable materials that have an upper oxidation potential higher than 4.0 V, with some even reaching 4.8 V,” he said. “Additionally, their ionic conductivities are generally in the range of 10-3 S cm-1, which is suitable for high voltage cathodes and produces fewer side reactions.”

A good solid electrolyte should have both high Li-ion conductivity and low electronic conductivity. The high electronic conductivity provided by carbon conductive additives in the cathode mixture can cause degradation of the solid electrolyte and damage its electrochemical stability.

However, there has been little research into the stable voltage window and the electrochemical redox reaction of halide electrolytes themselves. In addition, many factors can influence the redox potential of solid electrolytes in actual battery systems. Dr. Yu explained that while halide electrolytes have a high oxidation potential, this potential is close to the working voltage of traditional layer cathodes such as LiCoO2. Therefore, additional strategies are needed to reduce the reaction of the electrolytes.

“Electronic additives such as carbon have been shown to increase side reactions in sulfide and polymer systems,” said Yu. “We have found that halide electrolyte Li2.5Zr0.5Y0.5Cl6 also follows this rule, as the oxidation potential decreases with increasing carbon nanotube (CNT) content in a Li2.5Zr0.5Y0.5Cl6-CNT combined cathode. The electrolyte is even chemically nonreactive with a small amount of carbon additive.”

“This reversible redox process can provide stable charging/discharging capacity greater than 200 mAh g-1 for over 100 cycles,” Yu added. “We divide this process into two parts: the reaction of Cl/Clx at high potential and the redox of Zr4+/Zr0 and Y3+/Y0 at low potential. This reaction and its products have been confirmed by other data, such as X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD).”

This research provides guidelines for designing solid-state batteries using high-voltage cathode materials in combination with lithium halide solid electrolytes, resulting in high energy density and long lifespan.

“While this work presents one strategy for regulating the stable voltage window of halide Li2.5Zr0.5Y0.5Cl6, many factors can impact the dynamics and thermodynamics of electrolyte reactions,” said Dr. Yu. “Changes in these properties can result in different reactions for the same solid electrolytes. A deeper understanding of these mechanisms can provide new insights and strategies for assembling all-solid-state batteries and advancing their applications.”

Other contributors include Shuai Chen, Chaochao Wei1, Ziling Jiang, Ziqi Zhang, Linfeng Peng, Shijie Cheng, Jia Xie, State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan.

The following authors have additional affiliations: Shuai Chen, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan

The National Key Research and Development Program (2021YFB2500200, 2021YFB2400300) and the National Natural Science Foundation of China (Nos. 52177214, 51821005) supported this work. This work use resources of Analytical and Testing Center of Huazhong University of Science and Technology.

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