Electrochimica Acta 52 (2006) 675–680 An in situ Raman study of the intercalation of supercapacitor-type electrolyte into microcrystalline graphite Laurence J. Hardwick a,∗ , Matthias Hahn a , Patrick Ruch a , Michael Holzapfel a , Werner Scheifele a , Hilmi Buqa a , Frank Krumeich b , Petr Nov´ ak a ,R¨ udiger K ¨ otz a a Paul Scherrer Institut, Electrochemistry Laboratory, 5232 Villigen PSI, Switzerland b ETH Zurich, Laboratory of Inorganic Chemistry, 8093 Z¨ urich, Switzerland Received 7 April 2006; received in revised form 26 May 2006; accepted 26 May 2006 Available online 11 July 2006 Abstract An initial Raman study on the effects of intercalation for aprotic electrolyte-based electrochemical double-layer capacitors (EDLCs) is reported. In situ Raman microscopy is employed in the study of the electrochemical intercalation of tetraethylammonium (Et 4 N + ) and tetrafluoroborate (BF 4 - ) into and out of microcrystalline graphite. During cyclic voltammetry experiments, the insertion of Et 4 N + into graphite for the negative electrode occurs at an onset potential of +1.0 V versus Li/Li + . For the positive electrode, BF 4 - was shown to intercalate above +4.3 V versus Li/Li + . The characteristic G-band doublet peak (E 2g2 (i) (1578 cm -1 ) and E 2g2 (b) (1600 cm -1 )) showed that various staged compounds were formed in both cases and the return of the single G-band (1578 cm -1 ) demonstrates that intercalation was fully reversible. The disappearance of the D-band (1329 cm -1 ) in intercalated graphite is also noted and when the intercalant is removed a more intense D-band reappears, indicating possible lattice damage. For cation intercalation, such irreversible changes of the graphite structure are confirmed by scanning electron microscopy (SEM). © 2006 Elsevier Ltd. All rights reserved. Keywords: In situ Raman microscopy; Graphite; Electrochemical intercalation; Et 4 N + ; BF 4 - ; Supercapacitors; SEM 1. Introduction Energy storage in electrochemical double-layer capacitors (EDLCs), also referred to as supercapacitors, is based on charge separation at the interface between an electronic conductor and an electrolyte solution with ionic conductivity. Energy is stored in the electric field established in the double-layer consist- ing of electronic charges in the electrode and the according counter-ions in the electrolyte. The use of high surface area carbon electrodes results in a considerable charge storage capa- bility and therefore can provide high specific capacitances and power densities [1]. When the energy storage occurs solely via double-layer charging and discharging, the lifetime of an EDLC device is expected to be considerable. However, it can be shown that processes other than those related to double-layer effects may readily occur under working conditions. In particular, ion intercalation may lead to degradation of the carbon electrodes, ∗ Corresponding author. Tel.: +41 56 310 2174; fax: +41 56 310 4415. E-mail address: laurence.hardwick@psi.ch (L.J. Hardwick). which can be anticipated to reduce the lifetime of EDLCs [2–4]. Raman microscopy is pertinent in the examination of car- bonaceous materials [5] and is surface sensitive to an order of 100 nm and for this reason was selected to follow changes in the exterior of the electrode during charge/discharge. In this first study, microcrystalline graphite is used as a model sys- tem, since the intercalation of a wide variety of anions and cations into graphite is well studied [6]. A splitting of the G- band (1578 cm -1 ) into a doublet peak (E 2g2 (i) (1578 cm -1 ) and E 2g2 (b) (1600 cm -1 )), observed in the Raman spectrum, is an unmistakable indication of intercalation [7–9]. A quantitative measure of the intercalation stage index, n can be derived from the relative intensities of the Raman doublet, R by the following equation [10]: R = I i I b = σ i σ b n - 2 2 (n> 2) where I i and I b represent the intensities of the interior E 2g2 (i) and bounding E 2g2 (b) layer modes, respectively and σ i /σ b is the ratio of the cross section for Raman scattering 0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.05.053