Defect Enabled Room Temperature Superionicity in 3D Covalent Mixed Ionic Electronic Conductor LiB₃

Mixed ionic-electronic conductors (MIECs) are essential materials for solid-state energy storage and conversion systems, yet intrinsic MIECs remain limited by low ionic conductivity and narrow operating temperature windows. Here, combining first-principles calculations with machine learning molecular dynamics (MLMD), we systematically explore the ionic diffusion behavior and coupled ionic-electronic transport properties of LiB₃. LiB₃ is found to undergo a superionic state at 800 K, delivering a high ionic conductivity of 0.254 S/cm while retaining an exceptional electronic conductivity on the order of 10³-10⁴ S/cm. Notably, LiB₃ demonstrates robust structural stability, exhibiting a volume expansion of only 2.93% over the temperature range of 0-800 K. Furthermore, defect engineering enables substantial modulation of the transport properties. By introducing a 6.25% vacancy concentration, the superionic transition temperature of LiB₃ is reduced to 300 K while maintaining an ionic conductivity of 0.249 S/cm. Therefore, LiB₃ is an intrinsic MIEC that simultaneously achieves fast ionic transport and high electronic conductivity, highlighting its strong potential for solid-state energy storage applications.