ZnO quantum dots anchored in multilayered and flexible amorphous carbon sheets for high performance and stable lithium ion batteries

Owing to their high reversible capacity, cycling stability, low cost and safety, ultrafine ZnO particles embedded in carbonaceous materials are promising as electrode materials for lithium ion batteries (LIBs). Although there are a number of recent studies on ZnO/C based materials, direct evidence on the role played by the carbonaceous component, which is essential for the rational design of these nanomaterials with optimal electrochemical performance, is still missing. Herein, we systematically investigate the LIB performance of a novel ZnO quantum dot (ZnO-QD)/carbon composite using conventional electrochemical methods as well as the in situ transmission electron microscopy (TEM) technique. A composite of ZnO-QDs anchored in amorphous carbon multilayered sheets (ZnO-QDs@CMS) was synthesized using a pre-prepared zinc glycolate complex as the precursor. After low temperature annealing in Ar, the glycolate moieties carbonized and directly formed homogeneously distributed ZnO QDs embedded in ultrathin amorphous carbon layers. The ZnO-QDs@CMS delivered a high reversible capacity of 1015 mA h g(-1) after 80 cycles at a specific current of 50 mA g(-1). Even at 1000 mA g(-1), a reversible capacity of 565 mA h g(-1) was maintained after 350 cycles, with a capacity fading of only 5.7% (with respect to the second cycle). Comparative in situ TEM (de)lithiation studies of ZnO-QDs@CMS and pristine ZnO uncovered the role played by the amorphous carbon network in LIB performance. In addition to mitigating the volume expansion, during lithiation the flexible and conductive amorphous carbon network of the ZnO-QDs@CMS electrode suppressed the formation of adversely large Zn crystals and favoured the formation of a LiZn alloy, which contributed to the high reversible capacity and long-term stability.