Mixed ionic–electronic conductor based on B16 framework

Abstract

Mixed ionic–electronic conductors (MIECs) are essential materials for next-generation energy storage and conversion systems. However, several limitations—including the scarcity of intrinsic MIEC materials, low ionic conductivity, and a narrow operational temperature range—restrict their practical use. Based on the experimentally synthesized MgB4 crystal, this study designs a B16 covalent framework with three-dimensionally connected channels by theoretically removing Mg atoms through computational methods. Using first-principles calculations and machine learning molecular dynamics simulations, we systematically explore its compounds' potential as intrinsic MIEC materials. The results indicate that the B16 framework can stably host and transport Li+, Be2+, and Al3+ ions with different valences. The formations of LiB2, BeB4, and AlB8 enter superionic states at 1200, 1700, and 1400 K, respectively, with ionic conductivities reaching 10−2 S/cm, and the optimal electronic conductivities approaching 104 S/cm. The framework shows good structural stability: the volume change after ion insertion is less than 12.4%, and its mechanical properties (for example, a shear modulus of 47.8 GPa) are comparable to those of traditional electrode materials like LiFePO4 and NCM. Through defect engineering, LiB2 with 12.5% crystal defects can lower the superionic transition temperature to 400 K while maintaining a high ionic conductivity of 1.65 × 10−1 S/cm. This study offers a viable approach to designing MIEC materials and demonstrates significant potential for applications such as high-temperature solid-state batteries and electrochemical sensors.