Solvent-directed sol-gel assembly of 3-dimensional graphene-tented metal oxides and strong synergistic disparities in lithium storage

Abstract

Graphene/metal oxide (GMO) nanocomposites promise a broad range of utilities for lithium ion batteries (LIBs), pseudocapacitors, catalysts, and sensors. When applied as anodes for LIBs, GMOs often exhibit high capacity, improved rate capability and cycling performance. Numerous studies have attributed these favorable properties to a passive role played by the exceptional electronic and mechanical properties of graphene in enabling metal oxides (MOs) to achieve near-theoretical capacities. In contrast, the effects of MOs on the active lithium storage mechanisms of graphene remain enigmatic. Via a unique two-step solvent-directed sol-gel process, we have synthesized and directly compared the electrochemical performance of several representative GMOs, namely Fe₂O₃/graphene, SnO₂/graphene, and TiO₂/graphene. We observe that MOs can play an equally important role in empowering graphene to achieve large reversible lithium storage capacity. The magnitude of capacity improvement is found to scale roughly with the surface coverage of MOs, and depend sensitively on the type of MOs. We define a synergistic factor based on the capacity contributions. Our quantitative assessments indicate that the synergistic effect is most achievable in conversion-reaction GMOs (Fe₂O₃/graphene and SnO₂/graphene) but not in intercalation-based TiO₂/graphene. However, a long cycle stability up to 2000 cycles was observed in TiO₂/graphene nanocomposites. We propose a surface coverage model to qualitatively rationalize the beneficial roles of MOs to graphene. Our first-principles calculations further suggest that the extra lithium storage sites could result from the formation of Li₂O at the interface with graphene during the conversion-reaction. These results suggest an effective pathway for reversible lithium storage in graphene and shift design paradigms for graphene-based electrodes.