Are Electrospun Fibrous Membranes Relevant Electrode Materials for Li-Ion Batteries? The Case of the C/Ge/GeO2 Composite Fibers

Self-supporting paper-like membranes consisting of carbon/germanium dioxide (C/GeO2) fibers are prepared via electrospinning of solutions with different germanium load (2.50−4.25 wt%), followed by carbonization at 550−700 °C, and are evaluated as anode materials in lithium ion batteries. The investigation of the physicochemical properties of the membranes by several characterization techniques shows that, as expected, with increasing carbonization temperature better graphitized and less nitrogen-rich C fibers are obtained, containing Ge0 and/or reduced oxide phases along with GeO2 nanoparticles. These characteristics, combined with the cold pressing of the as-spun membrane that noticeably reduces the hollow space within the fibres giving rise to a more compact and tight structure, lead to initial discharge volumetric capacities (≈1390–3580 mAh cm−3) much higher than commercial graphite anodes. In particular, the membrane prepared from solution with 4.25 wt% Ge-load by cold-pressing and carbonization at 700 °C, is able to deliver ≈1500 mAh cm−3 after 50 cycles at 50 mA g−1 with a Coulombic efficiency close to 100%. Nevertheless, the anodes exhibit poor rate capability. This is because the carbonization at high temperature promotes outward diffusion and subsequent coalescence of Ge-clusters in larger particles, with the structure of the fibers made fragile by the formation of voids within them.