A Novel Anode Material Li₂ FeGeO₄ with Lithium Storage Mechanism Controlled by Both Iron and Germanium Elements

To obtain anode materials with high capacity/energy density for lithium-ion batteries, a polyanionic compound Li₂ FeGeO₄ is prepared, which combines the conversion-type Fe-based oxide and the alloy-type Ge-based oxide at the atomic scale. The influence of citric acid in the sol-gel process on the structure and performance of the calcined products (LFG0, LFG1, and LFG2) is investigated. The results demonstrate that citric acid does not affect the phase of Li₂ FeGeO₄ . However, with the increase of citric acid, the crystallinity and grain size of the final product are reduced and its dispersion becomes better. Among the as-prepared samples, LFG1 exhibits moderate particle size and more uniform dispersion, providing a high discharge capacity of 669.7 mAh g^-1 at 0.5 A g^-1 after 200 cycles. Based on the ex situ XPS and operando XRD tests, it is found that the electrochemical reaction process of LFG1 is controlled by both the conversion of iron/germanium and the alloying of germanium. In addition, it is verified that the reaction mechanism of LFG1 for aqueous lithium-ion capacitors is also controlled by iron and germanium elements. Importantly, Li₂ FeGeO₄ is first proved to be a novel anode for lithium-ion batteries and lithium-ion capacitors.