First-principles view of the interaction between Li and Bi4Ge3O12 anodes

Abstract

As a novel anodic electrode for Li-ion storage, the cubic Bi₄Ge₃O₁₂ phase can experimentally deliver a remarkably high reversible specific capacity of 586 mA h g⁻¹ at 200 mA h g⁻¹ with a coulombic efficiency of 99.8% after 500 cycles, and has recently attracted attention for its stable electrochemical performance. Here we calculated its lithiation/delithiation reactions by using density functional theory studies, through the structural changes as the conversion and alloying reaction takes place during the Li-ion insertion and extraction process. The obtained theoretical capacity of Li is 48.75 mol (∼1043 mA h g⁻¹) for 1 mol Bi₄Ge₃O₁₂. The decomposed Bi₂O₃ (Pm1) and GeO₂ (P3₁21) in the lithiation process of Bi₄Ge₃O₁₂ are the active materials to react with the Li atoms via a conversion reaction. Besides Li₂O with both Fmm and Pnma phases, the final lithiation products of Bi₄Ge₃O₁₂ should include Li₃Bi (Fmm) and Li_4.25Ge (F3m), through the alloying reactions of multi-valence elements of Bi and Ge with Li. Bi and Ge metals are also helpful in the decomposition of Li₂O into Li during the delithiation process, increasing the reversibility of the conversion reactions. Our research provides theoretical support to understand the working mechanism of Bi₄Ge₃O₁₂ and related mixed-metal anode materials.