Continuous Hydrothermal Synthesis of Metal Germanates (M₂GeO₄; M = Co, Mn, Zn) for High-Capacity Negative Electrodes in Li-Ion Batteries

Nanosized metal germanates (M₂GeO₄; M = Co, Mn, Zn) are synthesized using a continuous hydrothermal flow synthesis process for the first time. The electrochemical properties of all samples as active materials for negative electrodes in Li-ion half cells are explored. The galvanostatic and potentiodynamic testing is conducted in the potential range of 3.00–0.05 V versus Li/Li⁺. The results suggest that both alloying and conversion reactions associated with Ge contribute to the stored charge capacity; Zn₂GeO₄ shows a high specific capacity of 600 mAh g⁻¹ (ten cycles at 0.1 A g⁻¹) due to alloying and conversion reactions for both Ge and Zn. Mn₂GeO₄ is studied for the first time as a potential negative electrode material in a Li-ion half cell; an excellent specific charge capacity of 510 mAh g⁻¹ (10 cycles per 0.1 A g⁻¹) is obtained with a significant contribution to charge arising from the conversion reaction of Mn to MnO upon delithiation. In contrast, Co₂GeO₄ only shows a specific capacity of 240 mAh g⁻¹, after ten cycles at the same current rate, which suggests that cobalt has little or no benefit for enhancing stored charge in germanate.