Synthesis of Hollow Geo2 nanostructures, Transformation into Ge@C, and Lithium Storage Properties

In terms of capacity, group IVA elements are the most promising as anode material for LIBs, especially silicon and germanium, which have theoretical capacities of 4200 and 1600 mAh/g, respectively. However, compared to silicon, germanium has higher lithium diffusivity and electrical conductivity, but lower volume change. In this work, we have designed mesoporous germanium@carbon hollow ellipsoids (Ge@C) through the formation of a hollow GeO₂ ellipsoid precursor and subsequent simultaneous carbon coating and reduction of the GeO₂ precursor. The GeO₂ hollow ellipsoid precursor was firstly prepared through a facile one-pot ultrasonication method, and then after carbon coating and reduction by decomposition of acetylene and H₂/Ar, the obtained mesoporous Ge@carbon exhibited excellent cycling stability at the 0.2 C and 1 C rates (1 C = 1600 mAh/g), with no significant capacity fading upon 200 cycles, indicating high reversible cycling stability. The Ge@carbon electrodes also presented a high couloumbic efficiency of 80% for the initial cycle, as well as excellent rate capability that the capacity retention value at the rate of 20 C is 66% (805 mAh/g) of the capacity at 1 C, and when the rate was returned to the 1 C rate, the capacity recorded is the same as that in the initial cycle at the 1 C rate. The excellent electrochemical performance can be ascribed to the unique hollow structure, which provides more voids and pores for accommodation of volume expansion, as well as the uniform carbon film on the surfaces of the Ge nanoparticles, which maintains their structural integrity and enhances the electronic conductivity of the whole electrode.