pubs.acs.org/NanoLett
The Interaction of Li+ with Single-Layer and Few-Layer Graphene Elad Pollak,† Baisong Geng,‡,§ Ki-Joon Jeon,† Ivan T. Lucas,† Thomas J. Richardson,† Feng Wang,‡,§ and Robert Kostecki*,† †
Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA, and § Department of Physics, University of California, Berkeley, Berkeley, California 94720, USA ‡
ABSTRACT The interaction of Li+ with single and few layer graphene is reported. In situ Raman spectra were collected during the electrochemical lithiation of the single- and few-layer graphene. While the interaction of lithium with few layer graphene seems to resemble that of graphite, single layer graphene behaves very differently. The amount of lithium absorbed on single layer graphene seems to be greatly reduced due to repulsion forces between Li+ at both sides of the graphene layer. KEYWORDS graphene, lithium intercalation, CVD, Raman spectroscopy.
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current diminishes significantly with cycling but still dominates the electrochemical response even after four cycles. The intercalation of lithium into graphite usually manifests itself by the appearance of well-defined cathodic peaks16 that correspond to the formation of different intercalation stages.17 Close examination of the fourth cycle reveals the
he realization of graphene has inspired a considerable research effort toward the better understanding of the physicochemical properties of this new material. Ballistic electronic transport,1 unusual magnetic properties2,3 and quasi-relativistic behavior2,4-6 are just a few of the intriguing properties of graphene. The potential of graphene with regard to possible applications in energy storage devices has been considered but not yet realized.7-9 The physicochemical properties of lithiated carbonaceous materials have been studied rigorously over the last three decades, yet, there is still an ongoing debate regarding the exact mechanism of intercalation/deintercalation processes and the nature of Li-C interactions.10,11 The mechanism of charge transfer between lithium and graphene has been the subject of a few theoretical10,12 and experimental studies.9 Recent work by Kaskhedikar and Maier suggests that singlelayer graphene (SLG) might adsorb lithium on both sides, doubling the sorption capacity toward lithium as compared to standard graphite13 and shortening the lithium diffusion distance.14 We employed standard electrochemical techniques and in situ Raman spectroscopy to investigate the mechanism of Li-C interactions upon electrochemical lithiation/delithiation of SLG and few-layer graphene (FLG) produced by chemical vapor deposition (CVD). The results and conclusions of this work provide a new insight into the mechanism of Li+ intercalation into graphitic materials. Figure 1A shows the first and fourth cyclic voltammograms (CV) of the FLG on a Ni substrate (Ni/FLG) in polymer electrolyte. The CVs display a significant cathodic current that corresponds to the irreversible reduction of the polymer electrolyte on the graphene and Ni substrate.15 The cathodic
FIGURE 1. (A) First and fourth cyclic voltammograms of the Ni/FLG electrode in polymer electrolyte. (B) In-situ Raman spectra of the Ni/FLG electrode at selected potentials during electrochemical lithiation (dash line represents Lorentzian fit).
* To whom correspondence should be addressed: E-mail.
[email protected]. Received for review: 4/7/2010 Published on Web: 08/02/2010 © 2010 American Chemical Society
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DOI: 10.1021/nl101223k | Nano Lett. 2010, 10, 3386–3388
presence of at least two cathodic peaks (0.05 and 0.025 V) and three anodic peaks (0.088, 0.12, and 0.21 V). The cathodic peaks most likely correspond to the formation of stages “2” and “1” (0.085 and 0.044 V, respectively, in the case of bulk graphite). The oxidation peaks are attributed to the decomposition of stages 1, 2, and 4. The formation of stage 4 is not seen during the cathodic scan due to the high background current from the Ni substrate. In situ Raman spectra of the carbon G-band recorded during potentiostatic polarization of the FLG electrode are shown in Figure 1B. The Raman spectra collected at 1.5 and 0.5 V show the typical graphite G-band at 1582 cm-1. However, a peak shoulder located at 1592 cm-1 begins to emerge at 0.4 V. This peak originates from graphene layers adjacent to intercalated lithium, so-called interior layers.16,18 Upon further polarization to lower potentials and higher Li+ concentration in FLG, the vibration energy of the interior layers shifts to a higher frequency (∼1600 cm-1), and the peak intensity increases, while the position of the exterior layers peak (not adjacent to intercalated lithium) is shifted to a lower frequency (1570 cm-1), and its intensity gradually decreases.18 This is a clear indication of stage formation with low indices (n > 2).19 During polarization at 10 mV, the graphite-related Raman bands disappear altogether due to the interference of the discrete E2g2 mode with a Raman active continuum, which is typical of stage 1 formation.20-22 A Breit-Wagner-Fano (BWF) line shape should have appeared at ∼1500 cm-1, however, it was not observed due to significant losses of the backscattered radiation through the polymer electrolyte. CVs of SLG on a Cu substrate (Cu/SLG) electrode in polymer electrolyte (Figure 2A) display irreversible behavior similar to that observed for the Ni/FLG electrode. Interestingly, the CVs do not exhibit any peaks that may be correlated with Li+ intercalation/deintercalation processes. The electrochemical response of the SLG particle on the SiO2 substrate (Figure 2B) reveals the true behavior of SLG unobstructed by the background current created by the Cu substrate. Similarly to the Cu/SLG electrode, the SiO2-supported SLG electrode does not exhibit any sharp anodic and cathodic peaks related to reversible redox reactions. Instead, the CV displays a quasi-capacitive23 behavior with a broad cathodic current minimum between 0.5 and 1.4 V that indicates a different mechanism for the lithiation of SLG compared to the lithiation of FLG and graphite. To investigate the nature of Li+-SLG interaction upon electrochemical cathodic polarization, in situ Raman spectra of the SLG were collected (Figure 2B). The G-band at 1586 cm-1 gradually shifts to 1602 cm-1 upon cathodic polarization that is attributed to stiffening of the G-band due to electron doping.24-26 Interestingly, the G-band shift correlates with the onset of cathodic current at 1.5 V in the cyclic voltammogram (Figure 2B), and it is observed at a much higher potential than the FLG electrode. The G-band shift to 1602 cm-1 is completed at 0.5 V, and its position © 2010 American Chemical Society
FIGURE 2. (A) First and fourth CVs of a Cu/SLG electrode in polymer electrolyte. (B) CV of SiO2/SLG. (C) In situ Raman spectra of SiO2/ SLG at selected potentials during lithiation (dash line represents Lorentzian fit).
remains virtually constant upon polarization at potentials
