Acid- and Alkali-Resistant Microporous Na₄Cu₈Ge₃S₁₂·xH₂O Exhibiting Fast Mixed Sodium Ion and Electron Conduction

Mixed ion-electron conductors (MIECs) capable of simultaneously transporting ions and electrons possess critical applications in a large variety of electrochemical devices. However, the development of MIECs with desirable conduction and stability remains a great challenge. Herein, we present the first example of an open-framework metal chalcogenide for use as a MIEC with mixed sodium ion and electron conduction. The open-framework metal chalcogenide, Na₄Cu₈Ge₃S₁₂·xH₂O (NCGS), comprised of a {[Cu₈Ge₃S₁₂]^4-}_∞ anion framework, Na⁺ counter cations, and lattice water molecules, displays good thermal stability and remarkable resistance to acid and alkali. Importantly, NCGS exhibits fast mixed sodium ion and electron conduction under high humidity conditions, with a sodium ionic conductivity of 4.05 × 10^-2 S cm^-1 and electronic conductivity of 4.03 × 10^-3 S cm^-1 at room temperature and achieves the maximum value of 1.16 × 10^-1 S cm^-1 and 8.12 × 10^-2 S cm^-1 at 328 K, respectively. This study provides new opportunities for the development of high-performing MIECs toward practical application in electrochemical devices.