In-situ synthesis of Ge/reduced graphene oxide composites as ultrahigh rate anode for lithium-ion battery

Abstract

Germanium (Ge), as an anode material for lithium-ion batteries (LIBs), has received considerable attention because of its high theoretical capacity and excellent electrochemical performance. However, Ge anode usually undergoes large volume expansion during lithium insertion/extraction, resulting in severe structural pulverization and fast capacity fading. Here, we report an efficient cationic polymer-assisted in-situ reduction method for the preparation of highly uniform Ge/RGO composites (RGO = reduced graphene oxide) using GeO2, instead of expensive and toxic GeXn (X = Cl, Br and I), as Ge source. In this protocol, GeO32− anions are well adsorbed on cationic polymer-modified graphene oxide sheets by electrostatic interaction, followed by in-situ reduction, which results in the growth of ultrafine Ge nanoparticles (ca. 5 nm) on RGO sheets to form highly uniform sandwich-like structure. Benefiting from the tiny size of Ge nanoparticles and the well-conductive RGO sheets, the Ge/RGO anodes exhibit a high reversible capacity of 960 mAh g−1 after 100 cycles at 0.2 A g−1, an excellent rate capability of 631 mAh g−1 at 5 A g−1, and 705 mAh g−1 after 350 cycles at 1 A g−1 (with the capacity fading of only 0.07% per cycle based on the initial discharge capacity of 940 mAh g−1). More impressively, by coupling with LiFePO4 cathode, the full battery also shows a good electrochemical performance in terms of specific capacity and cycling stability. The simple, low-cost and environmental friendly synthesis strategy together with the outstanding lithium-storage performance of Ge/RGO composites affords a promising anode material for next-generation LIBs.