First principles investigation of cation substituted Li4P2O7 frameworks for enhanced ionic conductivity in all solid-state batteries

Oxide-based solid electrolytes offer a leak-free, non-toxic alternative for next-generation lithium-ion batteries. However, their wide-spread use is hampered by the poor ambient ionic conductivity which has stimulated intensive research efforts. This study uses first-principles density functional theory (DFT) to systematically explore the structural evolution, interfacial electro and transport-kinetics of the pyrophosphate Li 4 P 2 O 7 framework via targeted aliovalent substitution of Mg 2 + , Al 3 + at the Li-side and Si 4 + at the P--side; respectively. All doped configurations demonstrate the wide-bandgap insulating character of the host matrix 4.67 ≤ E g ≤ 5.08 eV while the pristine unitcell meet macroscale Born stability conditions with the introduction of desirable ductility to cope with volume changes. To gain insight into the physics of these underlying processes, we develop a localized charge density difference (CDD) mapping approach to visualize the atomic-scale electronic rearrangements and interfacial distortions that lead to these changes in transport phenomena. The climbing-image nudged elastic band (CI-NEB) simulations show that the migration energy barrier is 0.93 eV in the pristine material while it is reduced to an extraordinarily low value of 0.27 eV in the Si 4+ -doped configuration. This reduction brings better ion transport properties, resulting in room-temperature ionic conductivity of 3.62 × 10 −4 S/cm when doped with Al 3+ and 10 −3 S/cm at 400 K when doped with Si 4+ . The results provide a macroscopic-to-microscopic ion-transport design roadmap for developing high-performance polyanionic solid electrolytes via a direct quantitative correlation between the local electronic interface distortions and ion transport.