Lithium ion batteries are a class of rechargeable battery types in which lithium ions move from the negative electrode to the positive electrode during discharge and back when charging.
Several key characteristics of lithium ion battery performance - capacity, voltage and energy density - are ultimately determined by the binding between lithium ions and the electrode material. Subtle changes in the structure, chemistry and shape of an electrode can significantly affect how strongly lithium ions bond to it.
Through experiments and calculations, the Livermore team discovered that hydrogen-treated graphene nanofoam electrodes in the lithium ion batteries showed higher capacity and faster transport capabilities.
“These findings provide qualitative insights in helping the design of graphene-based materials for high-power electrodes,” said Morris Wang, an LLNL materials scientist and co-author of a paper appearing in November 5 edition of 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 the 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.
The experiments and multiscale calculations made by Livermore scientists reveal 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.
“The performance improvement we’ve seen in the electrodes is a breakthrough that has real world applications,” said Jianchao Ye, who is a postdoc staff scientist at the Lab’s Materials Science Division, and the leading author of the paper.
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 lithium ion batteries.