|The Ragone plots of graphene surface-enabled Li ion-exchanging cells with different electrode thicknesses. Credit: ACS, Jang et al. Click to enlarge.
A team from Nanotek Instruments and Angstrom Materials reports on a new strategy for the design of high-power and high energy-density devices based on the massive exchange of lithium ions between surfaces (not the bulk) of two nanostructured electrodes. This approach obviates the need for lithium intercalation or deintercalation—the basic process used in Li-ion batteries.
In a paper published in the ACS journal Nano Letters, the team reports that such surface-enabled, lithium ion-exchanging cells—based on unoptimized materials and configuration—are already capable of storing an energy density of 160 Wh/kgcell, which is some 30 times higher than that (5 Wh/kgcell) of conventional symmetric supercapacitors and comparable to that of Li-ion batteries. They are also capable of delivering a power density of 100 kW/kgcell, which is 10 times higher than that (10 kW/kgcell) of supercapacitors and 100 times higher than that (1 kW/kgcell) of Li-ion batteries.
In both electrodes, massive graphene surfaces in direct contact with liquid electrolyte are capable of rapidly and reversibly capturing lithium ions through surface adsorption and/or surface redox reaction.
Both the cathode and the anode are porous, having large amounts of graphene surfaces in direct contact with liquid electrolyte, thereby enabling fast and direct surface adsorption of lithium ions and/or surface functional group-lithium interaction, and obviating the need for intercalation. When the cell is made, particles or foil of lithium metal are implemented at the anode [upper portion of figure below] which are ionized during the first discharge cycle, supplying a large amount of lithium ions.
These ions migrate to the nanostructured cathode through liquid electrolyte, entering the pores and reaching the surfaces in the interior of the cathode without having to undergo solid-state intercalation. [lower left in diagram below] When the cell is recharged, a massive flux of lithium ions are quickly released from the large amount of cathode surface, migrating into the anode zone. The large surface area of the nanostructured anode enable concurrent and high-rate deposition of lithium ions [lower right in diagram below] re-establishing an electrochemical potential difference between the lithium-decorated anode and the cathode.—Jang et al.
The team prepared both oxidized and reduced single-layer and multilayer graphene from natural graphite (N), petroleum pitch-derived artificial graphite (M), micrometer-scaled graphite fibers (C), exfoliated graphite (G or EG), AC, carbon black (CB), and chemically treated carbon black (t-CB). They then constructed coin-size cells to test these nanostructured carbon materials. Electrodes were prepared with 85% active material, 5% conductive additive, and 10% binder.
Among their observations were:
For the fully surface-mediated cells, the electrode thickness is a dominating factor. In the case of using functionalized NGP as the electrodes, the total migration time of Li ions in liquid electrolyte is 1.27 s if the cathode and anode are each 200 μm thick and separator is 100 μm thick. The migration time is reduced to 0.318 s if the anode = cathode thickness = 100 μm and separator thickness = 50 μm.
The surface-enabled cells should have an extraordinary power density, particularly when the electrodes are ultrathin. The power densities observed with graphene-enabled, fully surface-mediated cells are comparable or slightly superior to those of LBL f-CNT-based batteries (thickness of 3 μm) at comparable current densities.
...the surface-enabled cells are a class of energy storage cells by itself, distinct from both supercapacitors and lithium-ion batteries. More work is needed to more clearly differentiate the dominant lithium-storage mechanism(s) between surface redox, surface adsorption, and surface defect trapping.—Jang et al.
Bor Z. Jang, Chenguang Liu, David Neff, Zhenning Yu, Ming C. Wang, Wei Xiong, Aruna Zhamu (2011) Graphene Surface-Enabled Lithium Ion-Exchanging Cells: Next-Generation High-Power Energy Storage Devices. Nano Letters Article ASAP doi: 10.1021/nl201849