Why tantalum supercharges (oxy)halide electrolytes?

The incorporation of tantalum (Ta) into (oxy)halide solid electrolytes unlocks exceptional ionic conductivities, outperforming physicochemically similar Nb systems and other adjacent transition metal cations. However, the physical essence of this transformative “Ta-advantage” has remained elusive. Herein, we propose a synergistic classical-quantum explanation for this phenomenon. The +5 charge of Ta 5+ not only classically optimizes the lithium transport landscape via electrostatic frustration, but its intense polarization further acts as a driving force that enhances the orbital hybridization and electron delocalization between Ta and Cl, setting the stage for quantum effects, which are proposed as the key origin of Ta’s superior performance. Specifically, the 5d-destabilization, pronounced spin-orbit coupling, and spatial extension inherent to the significant relativistic effects of Ta’s 5d orbitals diminish bond rigidity, yielding a highly anharmonic and flexible potential energy surface of Ta-polyhedra. This triggers large-amplitude polyhedral vibrations and dynamic disorder, smoothing migration barriers and enabling liquid-like ion transport. By clarifying the critical role of relativistic effects in 5d orbitals, this explanation shifts the paradigm of solid electrolyte design from classical geometry to quantum chemical bonding, providing a crucial blueprint for next-generation superionic conductors.