Non-stoichiometric design for engineering oxygen vacancies and enhanced ionic transport in Cu–Mg-doped bismuth vanadate electrolytes

Abstract

BIVMEOX (Bi-bismuth, Me-dopant metal, V-vanadium, and O-oxide) solid electrolytes are promising candidates for low-temperature ionic conduction. However, the interplay between non-stoichiometric (ns) Bi : (V + Me₁ + Me₂) ratios under dual-element doping and the underlying conduction mechanisms remains largely unexplored. In this study, we report a range of non-stoichiometric Bi₄(V_0.9Cu_0.05Mg_0.05)_xO_6+2.45x (1.75 ≤ x ≤ 2.05) solid solutions synthesized through a solid-state reaction. Combined X-ray, neutron and synchrotron X-ray diffraction analyses reveal the precise crystal structure of these ns-BIVCUMGOX materials, which adopt the tetragonal I4/mmm space group (No.139) and consist of alternating Bi–O (Bi and O1) layers and partially occupied V/Cu/Mg–O (V/Cu/Mg, O2, O3 and O4) layers. Bond valence pathway analyzer screening reveals that O²⁻ migration predominantly occurs within the V/Cu/Mg–O layers and is primarily mediated by oxygen ions at the O2 site. The non-stoichiometric design can effectively modulate the occupancies of the equatorial (O2) and apical (O3 and O4) oxygen sites. Notably, the reduced unit cell volumes shorten the oxygen-ion hopping distance, while the increased vacancies at the O2 Wyckoff positions enhance the ion mobility, collectively yielding improved ionic conductivity. The conductivity of Bi₄(V_0.9Cu_0.05Mg_0.05)_2.05O_10.8175 reaches 7.8 × 10⁻³ S cm⁻¹ at 350 °C, representing a 23% increase relative to the x = 2 composition. This study presents a lattice-engineering approach to enhance high-performance solid electrolytes in BIVMEOX systems.