Graphene – metal oxide (GMO) nanocomposites have attracted enormous attention for their potentials in energy storage and conversion, including capacitors, lithium-ion batteries (LIBs), catalysis (for fuel cells, water splitting, and air cleaning), and sensors. For the majority of these applications (e.g., catalysis, sensing, capacitors), high surface-to-volume ratio nanoporous architectures such as those built from interconnected 3-dimensional (3D) graphene networks are preferable in order to maximize the potentials of both metal oxides and graphene. For applications in LIBs, nanosized metal oxide (MO) particles and highly conductive graphene are considered beneficial for shortening Li⁺-diffusion pathways and reducing ohmic polarization in the electrode, leading to enhanced performance. In addition, porous space is critical in order to accommodate the relatively large volume expansion of many MOs upon lithiation. A vast number of studies have indeed demonstrated the effectiveness of these design principles. From the viewpoint of composite-electrodes, however, there have been few attempts to optimize the pore space and maximize the exposure of MO active materials to electrolyte.

… Most importantly, the total capacity achieved in some GMO nanocomposites is observed to be larger than the sum of each constitutive component—a synergistic effect that has not been fully understood in the literature. One chief aim of our work is to investigate this interesting capacity synergistic effect. From the literature, the quantitative contribution of graphene to the overall capacities of GMOs and the related storage mechanisms remain highly controversial. For instance, it is unclear whether this synergistic effect exists ubiquitously in all GMOs, and whether MOs affect graphene storage capacity. The broad range of reported synthetic methods and wide variations in graphene quality (different defects and impurities) 2make it nearly impossible to compare directly the performance of various GMOs. It remains challenging to elucidate the true origins of the synergistic mechanisms observed in some GMO systems.

—Ye et al.

For the study, the team used a two-step solvent-directed solgel method to synthesize the GMOs. They said that this approach has two unique advantages:

• Because the solgel method dips prefabricated 3D porous graphene in metal ion solutions, all metal oxide nanoparticles appear to be anchored on the surface of graphene that is fully accessible to electrolyte (i.e., open pore space). This result helps to optimize the system-level performance by ensuring that most metal oxides are active.

• The approach can deposit most types of MOs onto the same prefabricated 3D graphene structure, allowing for direct comparison of electrochemical performance of a wide range of GMOs.

We found that the experiments showed large reversible lithium storage capacities of graphene sheets, enabled by the unheralded roles of metal oxides. Surprisingly, we saw the magnitude of capacity contributions from graphene is mainly determined by active materials and the type of MO bound onto the graphene surface.

—Morris Wang, corresponding author

We observed that MOs can play an 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.

Based on the capacity contributions, they defined a synergistic factor. Their quantitative assessments indicated 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.

They proposed a surface coverage model to rationalize qualitatively the beneficial roles of MOs to graphene. Their first-principles calculations also suggested 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.

—Ye et al.

Other Livermore researchers include Jianchao Ye, Yonghao An, Elizabeth Montalvo, Patrick Campbell, Marcus Worsley, Ich Tran, Yuanyue Liu, Brandon Wood and Juergen Biener. The Laboratory’s Research and Development funded the work.

Resources

• Jianchao Ye, Yonghao An, Elizabeth Montalvo, Patrick G. Campbell, Marcus A. Worsley, Ich C. Tran, Yuanyue Liu, Brandon C. Wood, Juergen Biener, Hanqing Jiang, Ming Tang and Y. Morris Wang (2016) “Solvent-directed sol-gel assembly of 3-dimensional graphene-tented metal oxides and strong synergistic disparities in lithium storage” J. Mater. Chem. A doi: 10.1039/C5TA10730J