LLNL postdoc Jianchao Ye (left) works on a lithium ion battery, while Morris Wang (right) looks on. The two are part of a team studying the use of hydrogen for longer-lasting batteries. Photo: Julie Russell.Scientists at the Lawrence Livermore National Laboratory (LLNL) have found that lithium-ion batteries operate longer and faster when their electrodes are treated with hydrogen.
Lithium-ion batteries are a class of rechargeable battery in which lithium ions move from a negative electrode to a positive electrode during discharge and then back again 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 bind to it.
Through experiments and calculations, the LLNL team have now discovered that hydrogen-treated graphene nanofoam electrodes show higher capacity and faster transport in lithium-ion batteries.
"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 that appears in Nature Scientific Reports.
Lithium-ion batteries are growing in popularity for electric vehicle and aerospace applications. For example, lithium-ion batteries are becoming a common replacement for the lead-acid batteries that have been used historically in golf carts and utility vehicles. Instead of heavy lead plates and acid electrolytes, the trend is to use lightweight lithium-ion battery packs that can provide the same voltage as lead-acid batteries without requiring modification of the vehicle's drive system.
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 that are frequently used leave behind significant amounts of atomic hydrogen, whose effect on the electrochemical performance of graphene derivatives is difficult to determine.
Yet Livermore scientists did just that. Their experiments and multiscale calculations reveal that deliberate low-temperature treatment of defect-rich graphene with hydrogen can actually improve rate capacity. Hydrogen interacts with defects in the graphene, opening 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, a postdoc staff scientist at LLNL'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. In this way, they were able to probe the electrochemical performance of three-dimensional (3D) graphene nanofoam (GNF) electrodes, which are comprised chiefly of defective graphene.
The team used 3D graphene nanofoams due to their numerous potential applications, including hydrogen storage, catalysis, filtration, insulation, energy sorbents, capacitive desalination, supercapacitors and lithium-ion batteries. The binder-free nature of graphene 3D foam also 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, " said LLNL scientist Brandon Wood, who directed the theory effort on the paper. "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."
The research suggests that controlled hydrogen treatment could be used as a strategy for optimizing lithium transport and reversible storage in other graphene-based anode materials.
This story is adapted from material from Lawrence Livermore National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.