New Barium Vanadate Ba_xV₂O₅ (x ≈ 0.16) for Fast Lithium Intercalation: Lower Symmetry for Higher Flexibility and Electrochemical Durability

Na_0.33V₂O₅-type metal vanadates generally show better cycling stability than α-V₂O₅ as a cathode in Li-ion batteries, because they contain enough crystallographic voids that allow for lithium intercalation without significant structural deformation, but metal leaching upon cycling deteriorates their capacity retention. Here a new barium vanadate Ba_0.16(1)V₂O₅ is reported that does not undergo Ba leaching during cycling and shows a great promise for fast Li intercalation. Sr_0.15(1)V₂O₅ is isostructural to Na_0.33V₂O₅ (space group C2/m), while Ba_0.16(1)V₂O₅ adopts a new structure (space group P2₁/c), a derivative of Na_0.33V₂O₅. The framework of Ba_0.16(1)V₂O₅ consists of edge-shared VO₆ octahedral layers and the VO5 pyramidal bridge, generating unidirectional tunnels. Unlike the in-plane arrangement of atoms in Sr_0.15(1)V₂O₅, the nonplanar arrangement in Ba_0.16(1)V₂O₅ makes the host framework more flexible and creates larger voids for Ba. Compared with the combination of displacement and intercalation mechanisms in the Sr_0.15(1)V₂O₅ cathode, Li intercalation looks dominant in the Ba_0.16(1)V₂O₅ cathode due to its increased flexibility. Hence, Ba_0.16(1)V₂O₅ exhibits improved cycling stability compared to Sr_0.15(1)V₂O₅, suggesting that lower symmetry leads to higher cyclic stability in the V₂O₅-related compounds. This study illustrates how guest cations can tune the structural symmetry to modulate the reaction mechanism of electrode materials toward superior performances.