Synthesis of ZnO quantum dot/graphene nanocomposites by atomic layer deposition with high lithium storage capacity

Zinc oxide, as an inexpensive anode material, has attracted less attention than other metal oxides due to its poor cycling stability. A rational design of ZnO nanostructures with well-controlled particle sizes and microstructures is essential in order to improve their stability and performance as electrodes for lithium ion batteries (LIBs). Here, we demonstrate a simple approach via atomic layer deposition (ALD) to synthesize ZnO quantum dots (QDs) on graphene layers, in which the size of the ZnO QDs can be controlled from 2 to 7 nm by ALD cycles. A strong relationship between size and electrochemical performance is observed, in which smaller sized QDs on graphene display enhanced electrochemical performance. A high reversible specific capacity of 960 mA h g(-1) is achieved at a current density of 100 mA g(-1) for 2 nm ZnO QDs, approaching to the theoretical value of ZnO as the LIB anode. The greatly enhanced cycling stability and rate performance of the ALD ZnO QD/graphene composite electrode can be attributed to the well-maintained structural integrity without pulverization upon electrochemical charge/discharge for ZnO QDs with the grain size below a critical value.