Impedance Characterization of the Film Formation Process at the Graphite Anodes

Chinese Chemical Letters Vol. 11, No.10, pp. 915 -918, 2000915Impedance Characterization of the Film Formation Process at theG raphite AnodesBo Hua DENG, Yong Fang LI*Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences,Beijing100080Hua Quan YANGDepartment of Chemistry, Peking University, Bejing 100875Abstract In this paper, the formation mechanism of the passive SEI film at the natural graphiteanodes was investigated with the electrochemical impedance spectroscopy (EIS). A characteristicsemicircle was observed in the lower frequency range of the EIS spectrum for the irreversiblecharge process (lithium intercalation) at ca.0.75V, 0.40V and 0.20V.Keywords: Electrochemical impedance spectroscopy, surface film, graphite anodes, Li-ion battery.A passive SEI (Solid electrolyte interface) film had been suggested to be formed on thegraphite anodes with the irreversible capacity loss at ca. 1.20V in the first charge processby Fong et al. And the capacity loss at ca. 0.75V was originated from the electrolytedecomposition on the new surface created by the exfoliation of the graphite structure forsolvent co- intercalation'. Aurbach et al studied the surface chemistry of the insulatingfilm on the graphite electrodes in different electrolyte solutions?. But the formationprocess of the passive film on the graphite surface is still not very clear, especially for thevariation of the electrode/electrolyte interface in the first cycle. For example, if a stablepassive film (SEI) had been formed on the graphite surface, no exfoliation should occur.The film formation is a temporary process and in-situ study is not easy. Theelectrochemical impedance technique is a very useful tool for indirect study and analysisof the mechanism of this process3-7The natural graphite investigated in this paper was obtained from Fujian, China(average size of the graphite powder is 15um). The graphite electrodes were prepared bywet mixing the graphite powders with wt.5% PTFE (binder) and pressing the mixtureunder ca. 5 MP pressure into a thin tablet (ca. 0.30 cm2 in geometry area). Half cellswere constructed in a parallel plate configuration using the carbon tablets as the workingelectrodes, lithium foil as counter electrodes miernngrnne. nnlunrnylene as separators中国煤化工and 1 mol/L LiClO4 EC/DEC solutionhe electrolyte. Toavoid the effect of the overvoltage at theTYHC NM H G lithium foil wasplaced in the cell and used as the reference electrode. All the potentials were referred tothe Li electrode. And the electrochemical cells were constructed in a VAC glove box(filled with dry argon gas) at room temperature. Electrochemical impedance measure-916Bo Hua DENG et al.ments were performed with Zahner Elektrik IM6e (Electrochemical workstation,Germany). After the working electrode was charged or discharged galvanostaticly to thepotential desired (in the rate of 20 hrs for△x=1 in Li C), the impedance was measuredunder potentiostatic condition with 10 mV ac voltage (signal) in the frequency rangefrom 1 MHz to 5 mHz. (8 points per decade above 66 Hz and 4 points per decade below66 Hz).The EIS of the natural graphite electrode in the first cycle are shown in Figure 1 (aand b). Three types of the EIS spectra can be distinguished in the first charge process(see Figure 1 a), the EIS at ca. 3.05V(type D, the EIS at 0.75V, 0.40V and 0.20V(type II),and the EIS at 0.01V(type I). The EIS spectra in the first discharge process (Figure 1 b)all belong to type II.Figure 1 The EIS spectra of the natural graphite electrodes at different potentials in the firstcycle (in a, the value of -Im in the EIS for 0.20V was offset by +4002, 0.40V by + 10002,0.75V by +180092, 3.05V by +260092).14001st chargeaI1st dischargeb14000，364Hz12003.05V10003000483Hz800 t-0.75V点r 483Hz600E 20000.60Hz0.40V00 t156Hz207Hz1000 ._0.37Hz364Hz00上.75V+ 0.20po 0.01V200 400 600 800 1000 1200 1400300Re(ohm)In the EIS spectrum of type I (at ca. 3.05V for the freshly prepared graphite anode),there are a depressed semicircle in the high and middle frequency range. No lithiumintercalation should be expected at 3.05V for the freshly prepared electrodes. Thesemicircle should be corresponding to the electrode/electrolyte interface with a doublelayer capacitance in parallel to a charge transfer resistance, which may result from somesurface reaction. And the depressed semicircle may be caused by the rough surface of thecarbon electrodes.Figure 2 The equivalent circuit used for the中国煤化工xcra in Figure1.CrTHCNMHGFC]Re口-0RRaImpedance Characterization of the Film Formation Process917at the Graphite Anodesunsymmetric flattened semicircle in the impedance plots. It is well known that apassive film should have been formed on the carbon electrode surface after theirreversible charge process is finishedl,8. The equivalent circuit for the electrode systemwith a passive film is shown in Figure 2. The equivalent circuit consists of two parallelcircuits in series. R. is the ohmic resistance of the electrolyte, Rc and R; are the chargetransfer resistance and the resistance associated with the film respectively. W is theWarburg impedance, and C。and C are the double layer capacitance and the filmcapacitance corresponding to Rct and Rp respectively. The unsymmetric semicircle in theEIS of type III indicates that the time constants of the two parallel circuit of the electrodestudied here are not well separated and the two semicircles emerged together. The highfrequency part should mainly be contributed from R and C, because Cp should be muchsmaller than Cg.The EIS of type II for the electrode at 0.75V, 0.40V and 0.20V in the first chargeprocess is special and interesting. Different from the spectrum of type I and type II,there are two separated semicircles appeared in the higher and lower frequency rangerespectively. Compared with the EIS spectra of type II, the semicircle in the higherfrequency range should result from CJ and Rc and no passive film had been formed atthis potential. Galvanostatic charge/discharge results show that there is still someirreversible capacity loss at ca. 0.40V and 0.20V, except at 0.75V for the graphiteelectrodes in the first charge process. So the second semicircle in the lower frequencyrange could be related to the irreversible reaction associated with the capacity loss at thegraphite anodes. The irreversible reaction is the electrolyte decomposition on thegraphite electrode surface and some gas (eg. ethane) is evoluted with the solventdecomposition9-10. Probably, a new interface between the carbon particles in theelectrodes appears by the perturbation of the gas product, which results in the lowerfrequency semicircle. This is confirmed by the large capacitance value for the secondsemicircle estimated from the spectra. The approximate capacitance value was about1mF, which is much larger than the value of Cg mentioned before (in the magnitude of20uF)". .If the film was not very stable, or the electrolyte decomposition product on thegraphite electrode surface can be easily dissolved by the electrolyte, the solventmolecules could be co-intercalated into the graphite with the Li+ ions, and exfoliationoccurred. So the irreversible capacity loss at ca. 0.75V in the first charge was resultedfrom the electrolyte decomposition both on the electrode surface before exfoliation andon the new surface exfoliated from solvent co- intercalation. As seen from Figure 1(a),the film formation process may begin at above 0.75V, and the SEI film could be formedwith the irreversible capacity loss at ca. 0.75V, 0.40v and 0.20V until charging to 0.01V.In conclusion, a passive SEI film may be formed gradually with the decomposition of theelectrolyte solvent on the graphite surface.中国煤化工MHCNMHGAcknowledgmentThis work was financially supported by the National 863 project (KJ951-A1-501). The authorsthank Dr. Yuping Wu and Wenfeng Huang of Tsinghua University for their useful discussions.