Stable Li-ion storage in Ge/N-doped carbon microsphere anodes

The development of environmentally benign, low-cost and high-performance Ge-based materials for lithium-ion batteries (LIBs) has remained a great challenge. Herein, the synthesis of Ge/N-doped carbon microspheres (Ge/NC) is firstly performed using N-(2hydroxyethyl)ethylenediamine (AEEA) and ethanediamine (EDA) as solvents, ligands and carbon sources. The three-dimensional Ge/NC microspheres prepared with AEEA (Ge/NC-A) are constructed from nanosheets with a thickness of about 20 nm. Such a hierarchically structured material not only allowed sufficient contact between the nanosheets and electrolyte, but also provided sufficient void space and uniform conductive sites. At the same time, N-doped carbon in the Ge/NC-A microspheres can greatly improve the electrical conductivity and the structural stability. This material exhibited a superior rate performance (633.1 mA h g-1 at 20 A g-1), favorable reversible capacity (1113.2 mA h g-1 at 0.2 A g-1) and good cycling stability (a high reversible capacity of 965.0 mA h g-1 after 1000 cycles) when examined as an anode for LIBs. A full cell was fabricated using Ge/NC-A as an anode and LiFePO4 as a cathode and delivered a capacity of 100.7 mA h g-1 after 100 cycles. Furthermore, the lithiation/delithiation mechanisms in the Ge/NC-A microspheres were revealed by in situ Raman and in situ XRD measurements, indicating that the crystalline Ge was firstly converted into amorphous Li-Ge phases and transformed into amorphous Ge during the discharge/charge process. Therefore, the repeated transition between the amorphous and crystalline phases can be avoided, thus improving the cycling stability.