Germanium oxide (GeO2) films with two different grain sizes of 10 nm (GeO2 (10 nm)) and 100 nm (Ge02 (100 nm)) were grown via reactive radio frequency sputtering at different temperatures. Electrochemical measurements of the GeO2 (10 nm) thin film used as an anode in Li ion rechargeable batteries show superior capacity retention and rate capability compared to those of GeO2 (100 nm). The GeO2 (10 nm) thin film possesses an initial capacity of 930 mAh g(-1) with 89% capacity retention after 100 cycles, compared with 455 mAh g(-1) with 53% of GeO2 (100 nm) and 225 mAh g(-1), 30% (10 cycles) of common macro-size GeO2. Microstructural studies reveal that the Ge02 (10 nm) thin film can better accommodate volume changes during Li-Ge alloying and de-alloying processes. (C) 2011 Elsevier Ltd. All rights reserved.