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LLNL team finds hydrogen treatment improves performance of graphene nanofoam anodes in Li-ion batteries

Lawrence Livermore National Laboratory researchers have found, through experiments and calculations, that hydrogen-treated graphene nanofoam (GNF) anodes in lithium-ion batteries (LIBs) show higher capacity and faster transport. The research suggests that controlled hydrogen treatment may be used as a strategy for optimizing lithium transport and reversible storage in other graphene-based anode materials. An open-access paper on their work is published in Nature Scientific Reports.

Commercial applications of graphene materials for energy storage devices, including lithium ion batteries and supercapacitors, hinge critically on the ability to produce these materials in large quantities and at low cost. However, the chemical synthesis methods frequently used leave behind significant amounts of atomic hydrogen, whose effect on the electrochemical performance of graphene derivatives is difficult to determine.

… impurities—especially atomic hydrogen—exist ubiquitously in graphene materials made by chemical methods, and their effect on lithium storage capability remains little understood despite some rather important roles that these cross contaminants could play. Theory and experiments have suggested that hydrogen adsorbents impact the electronic structures of graphene and are expected to play a significant role in influencing the lithium storage capacity when applied as anodes in LIBs. Nonetheless, the conclusive evidence for this behaviour has not been forthcoming.

Earlier studies on various carbon materials (e.g., most are soft carbon from pyrolysis of organic precursors) generally pointed at an increased lithium storage capacity with increasing hydrogen content. The exact mechanisms underpinning this empirically observed behaviour remain a subject of ongoing debate and it is unclear whether this phenomenon also occurs for graphene. A main challenge has been to control the hydrogen content and location in graphene materials, a subject that is also of great interest to hydrogen storage applications. Control experiments on graphene single sheets, on the other hand, indicate that the hydrogenation of graphene could convert highly conductive zero-overlap semimetal graphene into an insulator (graphane)11. If this occurs, hydrogen may adversely impact lithium storage due to the loss of electrical conductivity

… Computer simulations in fact showed that perfect graphene lacks lithium storage mechanisms, and that defect structures are prerequisite for lithium storage. In practice, however, defect sites of graphene tend to bind functional groups that often contain hydrogen (e.g., hydroxyl, carboxyl, amine, hydrogen). This underscores the universal significance of understanding the roles of hydrogen in influencing the electrochemical behaviour of graphene.

—Ye et al.

The Livermore experiments and multiscale calculations revealed that deliberate low-temperature treatment of defect-rich graphene with hydrogen can actually improve rate capacity. Hydrogen interacts with the defects in the graphene and opens small gaps to facilitate easier lithium penetration, which improves the transport. Additional reversible capacity is provided by enhanced lithium binding near edges, where hydrogen is most likely to bind.

Srep16190-f2
Electrochemical characteristics of 3D GNFs. (a) Charge/discharge rate jump experiments show the improved rate performance after H2 treatment. (b) The percentage capacity enhancement at different charge/discharge rates before and after H2 treatment. The inset is the anodic differential capacity curves at various current densities at fifth cycle. (c) Coulombic efficiency of three representative GNF samples. Note that enhancement of Coulombic efficiency after H2 treatment. (d) Nyquist plots in impedance measurement imply easier charge transfer after H2 treatment. Ye et al. Click to enlarge.

The performance improvement we’ve seen in the electrodes is a breakthrough that has real world applications.

—Jiancho Ye, lead author

To study the involvement of hydrogen and hydrogenated defects in the lithium storage ability of graphene, the team applied various heat treatment conditions combined with hydrogen exposure and looked into the electrochemical performance of 3-D graphene nanofoam (GNF) electrodes, which are comprised chiefly of defective graphene. The team used 3-D graphene nanofoams due to their numerous potential applications, including hydrogen storage, catalysis, filtration, insulation, energy sorbents, capacitive desalination, supercapacitors and LIBs.

The binder-free nature of graphene 3-D foam makes them ideal for mechanistic studies without the complications caused by additives.

We found a drastically improved rate capacity in graphene nanofoam electrodes after hydrogen treatment. By combining the experimental results with detailed simulations, we were able to trace the improvements to subtle interactions between defects and dissociated hydrogen. This results in some small changes to the graphene chemistry and morphology that turn out to have a surprisingly huge effect on performance.

—Brandon Wood

Still to be answered are questions as to how to optimize defect density and hydrogen incorporation in graphene materials in order to achieve high energy density and high power density for LIB applications, the team noted in the paper.

Other Livermore researchers include co-lead author Mitchell Ong, Tae Wook Heo, Patrick Campbell, Marcus Worsley, Yuanyue Liu, Swanee Shin, Supakit Charnvanichborikarn, Manyalibo Matthews, Michael Bagge-Hansen and Jonathan Lee.

The work was funded by LLNL’s Laboratory Directed Research and Development program.

Resources

  • Jianchao Ye, Mitchell T. Ong, Tae Wook Heo, Patrick G. Campbell, Marcus A. Worsley, Yuanyue Liu, Swanee J. Shin, Supakit Charnvanichborikarn, Manyalibo J. Matthews, Michael Bagge-Hansen, Jonathan R.I. Lee, Brandon C. Wood & Y. Morris Wang (2015) “Universal roles of hydrogen in electrochemical performance of graphene: high rate capacity and atomistic origins” Scientific Reports 5, Article number: 16190 doi: 10.1038/srep16190

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