Poor intrinsic conductivity and huge volume expansion during charge/discharge process greatly limit the development of Ge-based ternary oxide as anode material for both lithium-ion batteries and sodium-ion batteries. To alleviate these issues, an ideal strategy is developed to achieve active particle nanocrystallization and composite with conductive carbon materials, simultaneously. Therefore, ultrasmall Fe2GeO4 nanodots (similar to 4.6 nm) uniformly and tightly anchored on 3D interconnected N-doped ultrathin carbon nanosheets (3D Fe2GeO4/N-CNSs) were constructed via one-step high temperature calcination process. This unique hybrid nanostructure can not only effectively enhance electron conductivity but also restrict the aggregation and volume fluctuation of Fe2GeO4 during the charge/discharge process. As a result, the 3D Fe2GeO4/N-CNSs electrode exhibited excellent electrochemical performances for both lithium-ion and sodium-ion battery anodes. When utilized for lithium-ion battery anode, the electrode delivered a highly reversible specific capacity (1280 mA h g(-1) at 0.4 A g(-1) after 180 cycles). It is the first time that Fe2GeO4 was applied for sodium-ion battery anode, which showed a remarkable rate capability (350 mA h g(-1) at 0.1 A g(-1) and 180 mA h g(-1) at 22.8 A g(-1)), and ultralong cycling stability (similar to 86% reversible capacity retention after 6000 cycles). (C) 2017 Elsevier B.V. All rights reserved.