Group IV element-based nanostructures have been actively investigated as high-capacity anodes for Li-ion batteries. Among them, germanium (Ge)-based materials become newly promising because it shows a high theoretical capacity (ca. 1600 mAh·g^-1) and faster Li transport properties compared to the other materials in the same group. However, the long-term cycling stability is still a challenging issue, which is resulted from volume expansion during cycling. To overcome the limitation, the way to maintain stable structures against the volume changes of Ge should be considered.

This study proposes a new Ge-based structure by incorporating Nb₂O₅, which is stable during Li insertion and extraction processes. An Nb₂O₅ nanoparticle core-Ge/GeO₂ shell structure material was prepared by a simple hydrothermal method and the following heat-treatment. First, a microscale NbO_x-GeO_x precursor was synthesized by the hydrothermal process with copolymer producing core-shell structure. Subsequently, it was heat-treated to obtain pyrolysis of copolymer and to form a reduced Ge/GeO₂ thin shell on the nanoscale Nb₂O₅ core structure without additional carbon sources. Electrochemical test results exhibited that the synthesized electrode materials had a high reversible capacity of 700 mAh·g^-1 after 100 cycles with an initial coulombic efficiency of 74%. In the long-term cycling test with a high rate, it exhibited an excellent cycling stability with a capacity retention ratio of 97% even after 1000 cycles.