Tunable grain boundary conductivity in sodium doped high entropy oxides

Concerns with the safety and sourcing of lithium-ion batteries have prompted significant research into sodium-based systems. High entropy oxides (HEO), which contain five or more oxide components in equimolar amounts, are well suited for battery applications due to their ability to accommodate a substantial quantity of mobile charge carriers (such as sodium), while also demonstrating promising cycling stability, electrical conductivity, and battery capacity retention. Here we investigate the underexplored influence of sodium doping, processing, and microstructure on charge transport in bulk sintered (Co,Cu,Mg,Ni,Zn)_1-xNa_xO. We find that the conductivity increases with increasing dopant amount, up to 1.4 × 10⁻⁵ S∙cm⁻¹ at x = 0.33. Much of this increase is attributed to the high grain boundary conductivity, which originates from a Na_xCoO₂ layered structure that forms in the grain boundaries during processing. X-ray diffraction and transmission electron microscopy confirm the presence of the layered structure, while electrochemical impedance spectroscopy highlights the distinct contribution to the impedance response. The relative contributions of the grain boundaries and the bulk to the charge transport are discussed, along with how processing conditions and composition can be used to effectively engineer grain boundaries in doped HEO materials.