Indian Journal of Engineering & Materials SciencesVol. 5, February 1998, pp.33-37

Impedance and corrosion behaviour of the carbon fibre electrode inaqueous battery electrolytes-A preliminary study

D P Bhatt & 0 P Bahl

Carbon Technology Unit.National Physical Laboratory, Dr. K S Krishnan Marg, New Delhi 110 012,. India

Received 26 February 1997; accepted 23 May 1997

Electrochemical aspects of polyacrylonitrile ( PAN ) based carbon fibre electrodes in differentconcentrations of aqueous lithium salts, viz., lithium perchlorate and lithium chloride have beenexamined using the impedance technique and potential versus time profile. The corrosion rates of theelectrode are determined for the constant time ( 600 h ) by traditional weight loss method. Scanningelectron microscopy (SEM) was performed on post-immersed electrode samples to follow themorphological changes in the anodic material of the rocking chair rechargeable lithium battery.

Even though carbon has been employed as anadditive or a conductive material in batterycathodes for about two centuries!", its' role as anexclusive special anode material as a replacementof lithium metal has been emerging in the globalscenario only recently'". Reactivity of the alkalimetals and their related passivation phenomena atthe interface account for the reduction in the cyclelife and the commercial limitation of suchrechargeable batteries. The risk of dendritepenetration in causing internal short circuits andthe safety of the batteries with metallic lithiumanode have been presently less than satisfactory. Inthe course of the cycling operations, the area of thelithium electrode surface increases and themetal/electrolyte interface becomes thermo-dynamically less stable. Cells, in turn, becomesusceptible to shock followed by mechanical andelectrical abuse. Even in the first 1-2 cycles of theoperation of these batteries using non-aqueoussolvents, the passivation continued to persistleading to the reduction in the coulombicefficiency. Import of the sophisticated glove-boxesto handle the organic solvents has been all alongpracticed in the country which in a way makes thedevelopmental prospects as uneconomical andnon-indigenous. Indigenisation of this sophis-ticated facility has thus been one of the concern ofthe development of rechargeable lithium batterywithin the country.

In order to circumvent these problems it has

been, therefore, thought worthwhile to employcarbon as the prospective anode material in theconditions of aqueous lithium salts. Theperformance of the half cell has been evaluatedelectrochemically in terms of open circuitpotential, weight loss and impedancemeasurements. Scanning electron microscopy(SEM) has been employed to characterize thepost-immersed carbon electrodes.

Experimental ProcedureKnown quantity of the PAN based carbon fibres

(NPL's grade) is used after reduction to powder inpresence of acetone. A binder of phenolic resin ismixed with the carbon fibre, stirred well andheated upto 40°C for a period of 2-3 h. This fol-lowed the curing step at 150°C for about 4 h.Pressure ranging from 25-50 kg/ern" was employedto mould into the 1 crrr' carbon electrodes of 2.15mm thickness. The electrode prepared in this waywas then subjected to the electrical resistivitymeasurements using the standard four-probetechnique. The electrical conductivity of theelectrode has been found to be 95.22 S em". Oneside of these electrodes was masked by lacquor toenable the 1 em' area alone to be exposed for theexperiment. The test electrodes were polished withemery papers and degreased with acetone. Metallicwire leads were provided to the top ends of thecarbon electrodes, for electrical connection.

34 INDIAN 1. ENG. MATER. set, FEBRUARY 1998

Larger area platium foil was used as the counterelectrode and the saturated calomel electrode(SCE), fabricated in-house, was employed as thereference electrode. Aqueous solutions of lithiumchloride (AR grade, Loba Chemie) and lithiumperchlorate (AR grade, Loba Chemie) rangingfrom 0.5 to 2.0 M were prepared in double distilledwater. Impedance measurements were obtainedwith Solartron model 1174 frequency responseanalyser and Solartron model 1186 electro-chemistry interface equipment. The real andimaginary parts of the cell impedance at differentfrequencies (10 mHz-I kHz) of AC wave formwere obtained for different concentrations oflithium chloride and perchlorate at differentfrequencies. -Weight loss method was adopted forthe study of corrosion of the electrode. All theexperiments were conducted at 30°C.

Potential/Time studies-The open circuitimmersion potential of the chosen carbon fibreelectrodes in different concentrations (0.5-2.0 M)of LiCI04 electrolytes are noted to be 0.15-0.16 V(vs. SCE). The electrodes are then subjected to themeasurement of potential with respect to time.Data have been monitored continuously for 600 h.It is interesting to note that the PAN based carbonfibre electrodes give rise to the constant potentialof 0.15 V (vs SCE) which was found more or lessunaffected even after 600 h. The weight losses aredetermined for the time duration of 600 h afterremoving the corrosion products from the carbonelectrodes by dipping in 10% NH40H for 5-10 min. The corrosion rate has been determined inmpy (mils penetration per year units) using theformula: mpy=534 WIDAT where W, D, A and Thave their usual meanings. The values obtained forthe exposure time of 600 h are found to be 0.0042,

Results and Discussion 0.0065, 0.0106 and 0.0114 g for 0.5, 1.0 , 1.5 andThe current and voltage developed in the 2.0 M LiCI04 electrolyte. The corresponding

rocking chair energy system is governed by the corrosion rate values thus calculated for 0.5, 1.0,uptake of lithium ions during charge and their 1.5 and 2.0 M concentrations of LiCI04 are 14.19,release during discharge at the negative electrode 21.95, 35.80 and 38.50 mpy, respectively. It isin presence of lithium salts as the electrolytes. An noticed that the lower concentrations do give riseinnovation of the introduction of carbon as a novel to the marginal loss of the electrode and increasingnegative electrode material in place of the lithium the concentrations has led to the gradual increasemetal gives expectations of improvements in of the corrosion rate till 1.5 M. Beyond 1.5 M, thecyclability (1200 deep cycles), energy density corrosion rate is not substantially increased which(over three times to that of nickel-cadmium battery is suggestive of the fact that 1.5 M could be thedue to the low equivalent weight of carbon ) and optimum concentration of LiCIO .•. Similar trend incost". Employment of the lithium-insertion the corrosion rate has been observed with respectcathodic compounds capable of maintaining the to LiCl. However, the half-cell potential readingsLi' activity as close to 1 as possible could exhibit a of the PAN based carbon fibres in LiCI have beenpotential approaching that of the metal. Petroleum observed to be erratic for the first 100 h ofcoke has been one of the earliest materials in the immersion. Thereafter a fairly constant valuefamily of such carbon anodes 10. Charging process ranging between 0.16 and 0.17 V (vs. SCE) hasin the cell thus involves the initial transfer of Li" been recorded. However, in comparison to theions to the anode forming the intercalation in the metal chloride and/or perchlorate electrolytecarbon lattices through van der Waals' forces. systems':", the weight loss has been significantlyThese lithium ions leave the lattice during the low in the present case. This indicates that thedischarge operations and join their parent carbon fibre electrode could be an ideal anode forcompound. The electrochemical process, thus, long term use in aqueous battery environments.involves the transfer or the rocking of the lithium ' Impedance measurements-Since a simpleions across the cell as

electrochemical system consists of double layer+ charge . luti . d'6C+ xLi + x e rt.: .•. Li

XC

6capacitance, so ution resistance, an activation

Discharge polarisation resistance, the employment of an a.c.where the cyclable charge x is around 0.5 Faraday signal can provide supplementary information toper mole. that obtained from a d.c. polarisation technique.

BHA IT & BAHL: CARBON FIBRE ELECfRODE IN AQUEOUS BA ITERY ELECfROL YTES 35

The impedance diagram is obtained by applying a20 mV rms sinusoidal potential between theworking and reference electrodes through apotentiostatic circuit, the potential/current functiongives the impedance value. The magnitude(Z)/phase {O) relation of the impedance is given by: Z'=Z cosO for the real part and 2"=2 sinOfor theimaginary part. The diameter of the circle ( 2" 'vs2') gives the 'polarisation resistance, Rp and theintercept on the x-axis at the higher frequenciesgives the solution resistance, R •. The nature of thecurves indicate whether the system is activationcontrolled, diffusion controlled, or a combinationof both. Typical impedance diagrams of PANbased carbon fibre in 1.5 M LiCI and 1.5 M LiCI04

have been represented in Figs 1 and 2,respectively. Using these figures, and extrapolatingthe curves to cut the x-axis, the polarisationresistance of the electrode-electrolyte interface isdetermined approximately from the chord of thearc thus obtained. The exchange current density, iohas been calculated from polarisation resistancedata using the following expression 12:

io = (RTlnF).lIRp.

2 .000E4 ....--------::----------,

Es:o

4.000 E4

0-0 0.5M- 1.0M0-0 1 5Mb-O 2.0M

z', ohmFig. I-Impedance plot of PAN based carbon fibre electrodein lithium chloride solution of different concentrations

These values are given in Table 1. It can be seenthat increase of the electrolyte concentration bringsabout the gradual decrease in the polarisationresistance till 1.5 M beyond which the trend isreversed. This suggests that 1.5 M could be theoptimum concentration of either of the two lithiumsalts. The low Rp values observed in the case ofLiCI system when compared with the LiCI04

system could be envisaged based on the concept ofspecific anion (Cl") adsorption in the surface layerof the carbon electrode wherein a field assistedtransport of ions through the surface deposit isintroduced to account for the ion current densityincrease. Ion current density increase wouldobviously correspond to the enhancement of thecharge. These chloride ions could have beenincorporated into the outermost layer of say thesemi-passive electrode film by occupying anionvacancies in the film and thereby lead to the filmbreakdown process, responsible for the decrease ofthe polarisation resistance. Slightly lower value ofthe polarisationresistance for LiCI system overLiCI04 system could still give an edge to the latterone for it's being preferred as the compromise

4.000E4 r---------,.-----------,6

E.c:o

o-c 0.5 M- 1.0M0-01.5M6-d 2.0M

NI

z ', ohm 8000E4

Fig. 2-Impedance plot of PAN based carbon fibre electrodein lithium perchlorate solution of different concentrations

Table I-Polarisation resistance, Ry, (Ohm), exchange current density, io (A.cm·2) of PAN based carbon fibre electrodellithiumsalts systems

LiClO.Concentration ofthe electrolyte

M

0.5

LiCIRy, io Rp io

12.25xW 2.13xlO·7 8.1 Ix 10· 3.22xlO·7

10.OOxlQ4 2.6IxlO·7 5.85xI0· 4.46xlO·7

8.16xW 3.20x 10.7 4.30xlO· 6.07x 10.7

8.41 X 104 3.lOxlO·7 4.50xl04 5.80x 10.7

1.0

1.5

2.0

36 INDIAN J. ENG. MATER. sci, FEBRUARY 1998

electrolyte, no matter the latter electrolyte has tosacrifice a little in terms of exchange currentdensity. Expectancy of the longevity of the cellscould be perhaps another reason of the preferenceof LiCI04 over LiCI. It may also be evident fromthe Figs 1 and 2 that the plots using theconcentrations down to 1.0 M approximate tostraight lines indicating the largely diffusion-controlled mechanism of the interfacial reaction,whereas those in 1.5 M LiCI04 and even beyondare predominantly activation controlled. This factcan be readily understood as lower concentrationsare bound to involve diffusion-controlledprocesses. It may be pertinent to mention that theabsolute values of R, could not be determinedusing the lowest concentration say 0.5 M.However, the maximum Rp as obtained at 0.5 M

Fig. 3-SEM micrograph of PAN based carbon fibres

Fig. 4-SEM micrograph of post-immersed PAN basedcarbon fibres in lithium perchlorate (before treatment)

concentration justifies the trend of the reportedresults. It may also be noted that the solutionresistance, R, is apparently visible in the lowerconcentrations thereby giving a clue not to employlow concentrations of electrolyte for the carbonanode.

SEM analysis-Fig. 3 is the SEM micrograph ofthe as received PAN based carbon fibres, showingonly the striations on the surface as observed byother authors as well. Figs 4 and 5 represent thetypical micrographs of the post-immersed carbonfibre samples [in LiCI04] prior to the treatmentand after the treatment, respectively. Emergence ofthe Non-uniform surface deposition in the formercase is indicative of the fact that the interaction isgoverned predominantly through the more defect

Fig. 5-SEM micrograph of post-immersed PAN basedcarbon fibres in lithium perchlorate (after treatment)

Fig. 6--SEM micrograph of post-immersed PAN basedcarbon fibres in lithium chloride

BHA IT & BAHL: CARBON FIBRE ELECTRODE IN AQUEOUS BA ITERY ELECTROLYTES 37

zones rather than through the basal planes and/orthrough the availability of more free carbon atomsplus small percentage of 0, H, or N groups in thechosen carbon fibres. In the latter case after thetreatment (Fig. 5), only the adhered depositpersists in a way qualitatively suggesting themorphological changes of the carbon surface. Itmay, however, be noted that the post-immersedcarbon fibres in LiCI (Fig. 6) do not showappreciable deposits presumably due to the break-down of the film. These results also substantiatethe impedance measurements of this study.

ConclusionsElectrolyte concentrations <1.5M yield a high

polarisation, resulting in apparently low exchangecurrent density. Also the solution resistance hasbeen found to be distinctly visible in 0.5 and 1.0 Mconcentrations of the electrolytes. This has beenexplained predominantly due to the influence ofdiffusion controlled mechanism in the interfaceusing low electrolyte concentrations. In view ofthese facts, it may thus be concluded that theconcentration of the electrolyte has been optimisedas 1.5 M with respect to say lithium perchlorate.Lithium perchlorate has been found to be thepreferred compromise electrolyte over lithiumchloride. PAN based carbon fibres could ,therefore , be considered as a prospective anodematerials in the aqueous medium of lithiumperchlorate. The use of the aqueous .electrolyteover' non-aqueous electrolytes is encouraging interms of ruling out the import of expensive glovebox facility for an indigenous R & D programmefor the development of suitable lithium batteries ..

The electrochemical intercalation studies of thepartially graphitised PAN and other carbon fibresin the aqueous electrolyes are being pursued in thelaboratory primarily with an objective ofimprovising the exchange current density/reversibility of the system which will be publishednext.

AcknowledgementsGrateful thanks are due to the Director, NPL,

New Delhi for his constant encouragement. One ofthe authors (DPB) also acknowledges thecooperation and the help rendered by Dr R BMathur, NPL in carrying out the electricalresistivity measurements.

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