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Photothermally reduced graphene papers as high-rate capable Li-ion anode material
23 August 2012
Researchers at Rensselaer Polytechnic Institute led by Nikhil Koratkar reported the use of photoflash- and laser-reduced free-standing graphene papers as high-rate capable anodes for lithium-ion batteries in a paper published in the journal ACS Nano.
Photothermal reduction of graphene oxide—by photoflash or laser—results in an expanded structure with micrometer-scale pores, cracks, and intersheet voids. This open-pore structure enables access to the underlying sheets of graphene for lithium ions and facilitates efficient intercalation kinetics even at ultrafast charge/discharge rates of >100 C, they reported. The photothermally reduced graphene anodes are also structurally robust and display outstanding stability and cycling ability. At charge/discharge rates of 40 C, the anodes delivered a steady capacity of 156 mAh/ganode continuously over 1,000 charge/discharge cycles, providing a stable power density of 10 kW/kganode.
In addition, the team suggests that such electrodes could be mass scalable with relatively simple and low-cost fabrication procedures, thereby providing a clear pathway toward commercialization.
Although standard Li-ion batteries can provide very high energy densities, they are unable to provide high power densities, the authors note. A lithium-ion battery provides capacities through lithium intercalation with an active electrode material; its high-rate performance is thus largely governed by Li+ diffusivity and electron conductivity.
It should be noted that achieving high capacities at elevated C rates is particularly challenging since the time available for lithium ions to diffuse through the anode and intercalate is now significantly shorter. As a result, only a partial lithiation is achieved, if at all, and the capacities are often very low. Another limitation with ultrahigh charge/discharge rates is the electron transfer mechanism. The electron conductivity of the anode material directly influences the charge transfer mechanism and thus largely governs the achievable C rates. Finally, a high surface area is desirable for operating at high rates since it is of prime importance that the lithium ions have sufficient active sites for intercalation to make up for the diffusivity constraints.
...Here we describe the photothermal reduction of free-standing graphene oxide paper to obtain graphene anodes with a unique “open-pore” structure. The energy from a camera flash or laser causes instantaneous and extensive heating of graphene oxide and induces a deoxygenation reaction. We show that this rapid outgassing creates microscale pores, cracks, and voids in graphene paper, which enhances lithium intercalation kinetics at ultrafast charge/discharge rates. We attribute this to better ion diffusivity, greater access to the underlying graphene sheets through the micropores, and improved electrolyte wetting of the electrode.—Mukherjee et al.
They found a stable capacity at 5 C of of ∼370 mAh/g, which is the highest capacity reported at 5 C for a pure carbon anode (without any additives) in a Li-ion cell. The capacity drops with increasing C rate, but the electrode is still capable of delivering ∼156 mAh/g at 40 C and ∼100 mAh/g at 100 C.
...these are by far the highest capacities reported so far for pure carbon-based anodes (without additives) at 40 and 100 C and are an order of magnitude higher than conventional graphitic anodes. Most importantly, these capacities were highly stable and could be maintained for over a thousand cycles of continuous high-rate charge/discharge. While laser-scribed graphene has recently been demonstrated in an electrochemical capacitor, to our knowledge, this is the first demonstration of photoreduced graphene electrodes in lithium-ion batteries.—Mukherjee et al.
Along with Koratkar, co-authors of the paper are Rensselaer graduate students Rahul Mukherjee, Abhay Varghese Thomas, and Ajay Krishnamurthy, all of the Department of Mechanical, Aerospace, and Nuclear Engineering (MANE).
The study was funded by the National Science Foundation, and supported by Koratkar’s John A. Clark and Edward T.Crossan Endowed Chair Professorship at Rensselaer.
Koratkar is a professor in MANE and the Department of Materials Science and Engineering at Rensselaer. He is also a faculty member of the university’s Center for Future Energy Systems and the Rensselaer Nanotechnology Center.
Rahul Mukherjee, Abhay Varghese Thomas, Ajay Krishnamurthy, and Nikhil Koratkar (2012) Photothermally Reduced Graphene as High-Power Anodes for Lithium-Ion Batteries. ACS Nano doi: 10.1021/nn303145j
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