Journal of Electroanalytical Chemistry 687 (2012) 30–34

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Journal of Electroanalytical Chemistry

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Rapid SECM probing of dissolution of LiCoO2 battery materials in an ionic liquid

Graeme A. Snook a,⇑, Thuy D. Huynh b, Anthony F. Hollenkamp b, Adam S. Best b

a CSIRO Process Science and Engineering, Box 312, Clayton South, Vic 3169, Australiab CSIRO Energy Technology, Box 312, Clayton South, Vic 3169, Australia

a r t i c l e i n f o

Article history:Received 6 August 2012Received in revised form 16 August 2012Accepted 21 August 2012Available online 6 October 2012

Keywords:Scanning Electrochemical Microscope(SECM)Lithium cobalt oxide (LiCoO2)Battery cathodeCycle-lifeDissolutionRoom temperature ionic liquid

1572-6657/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jelechem.2012.08.021

⇑ Corresponding author. Tel.: +61 395458863; fax:E-mail address: graeme.snook@csiro.au (G.A. Snoo

a b s t r a c t

Attempts to utilise LiCoO2 as the cathode when the electrolyte is based on an ionic liquid, e.g. 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, have been severely limited by significantcapacity fade. Given that some form of instability of this material in ionic liquids is widely suspectedas the principal cause, the aim of the current work is to determine the efficacy of using the scanning elec-trochemical microscope (SECM), to detect solubilisation of species leaving the LiCoO2 electrode duringcharging and/or discharging. The SECM microelectrode probe tip was lowered to within 10–20 lm ofthe platinum substrate electrode, onto which solid LiCoO2 had been coated. This substrate materialwas charged and discharged to different voltages, whilst probing any solubilised species at the microelec-trode tip. The detection of solubilised Co2+ and oxygen during over-charge and more so under deep dis-charge was observed. Possible mechanisms for degradation of the electrode are proposed. From thesepreliminary results, strategies are suggested for mitigating the degradation of LiCoO2 when used in aroom temperature ionic liquid electrolyte.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

The mechanism of failure of LiCoO2, a common Li-ion batterycathode material, is hotly contested. Difficulty in monitoring thecondition of cathode materials in situ has helped to continue de-bate over the possible failure modes. The likely causes are: (i) dis-solution of cobalt from the LiCoO2 [1,2]; (ii) damage from straininduced by cation disorder [3–5]; (iii) structural instability dueto a hexagonal-to-monoclinic phase change at higher voltages[6]; (iv) side reactions involving oxygen loss from the LiCoO2 [7].Some of these papers [1,2,6,7] utilise a metal-oxide coating suchas Al2O3 or ZrO2 to reduce the degradation of the electrodes (pre-sumably by limiting contact with the underlying cobalt oxide)and all of them relate the degradation to a high voltage process(i.e. overcharging effect). The ability to measure the degradationof this material, during charge and discharge, is essential in ascer-taining the real mechanism of failure and for designing new mate-rials with improved performance.

The particular focus of this paper was to investigate the behav-iour of LiCoO2 in the presence of an emerging family of batteryelectrolyte materials that are based on room temperature ionic liq-uids (RTILs). These electrolytes, generally, have low flammabilityand wide electrochemical windows, which gives them a distinctadvantage over traditional battery electrolytes. The characteristicsof RTILs are often complicated, leading to unexpected behaviours.

ll rights reserved.

+61 395628919.k).

This is certainly the case when utilised as a battery electrolyte,such that the behaviour of LiCoO2, and other electrode materials,in RTILs is even less understood. LiCoO2 has been cycled in RTILsin the literature, for example in imidazolium based ILs [8]. In thispaper, a pyrrolidinium based IL is investigated; namely, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide(C4mpyrTFSI).

Most techniques used for studying the degradation of batterymaterials involve post-mortem or ex situ analysis. Such techniquesinclude, Infra-red Spectroscopy (IR), Nuclear Magnetic ResonanceSpectroscopy (NMR), X-ray diffraction (XRD) and Scanning Elec-tron Microscopy (SEM). An alternative to these techniques is de-scribed in this paper; namely Scanning ElectrochemicalMicroscopy (SECM). This technique is an in situ electrochemicalanalysis technique capable of measuring solubilised species re-leased from the cathode electrode and can be used to distinguishbetween structural failure mechanisms and dissolution mecha-nisms. It has been used before [9] to examine LiCoO2 behaviourbut only focussing on the Li+ ion transport.

The SECM technique involves positioning a microelectrodeprobe within 20–30 lm of the substrate electrode (containingthe battery material). The positioning of the microelectrode ismade complicated in RTILs due to their viscosity [10–13]. This vis-cosity (85 mPas in this particular IL [15]) means that in order toreach a steady-state (a requirement of the technique), either verysmall electrodes or very long approach times are required. Lower-ing of the electrode could, nonetheless, be done at slow approachspeeds (�0.25 lm/s) to give a rough separation of electrodes. Qual-

G.A. Snook et al. / Journal of Electroanalytical Chemistry 687 (2012) 30–34 31

itative identification of soluble species released from the cathodeduring charge or discharge to various voltages was performed.

As many proposed mechanisms for the failure or degradation ofLiCoO2 cathodes involve the solubilisation of the constituents (suchas Co2+) we have developed a method based on SECM for monitor-ing the stability of battery materials. In our research, we investi-gate the degradation of LiCoO2 in the RTIL (C4mpyrTFSI) andshow that solubilisation is a major contributor to the loss of dis-charge capacity.

Fig. 1. Cyclic voltammogram of 5 mM Ferrocene in C4mpyrTFSI at 5 mV s�1 at thePt (10 lm diameter) microelectrode probe tip (in the bulk solution), measuredusing a Ag|Ag+ (10 mM AgTriflate) reference electrode.

2. Experimental

2.1. Materials

The room temperature ionic liquid (RTIL) used for the measure-ments was 1-butyl-1-methylpyrrolidinium bis(trifluoromethane-sulfonyl)imide (C4mpyrTFSI); was purchased from Merck andcontained <100 ppm water and <100 ppm chloride. This was driedat 40 �C under vacuum for 7 days prior to use and stored in the ar-gon atmosphere glove box (�60 ppm remaining water). LiTFSI waspurchased from 3M.

Cobalt deposition onto platinum was achieved utilising 0.1 MCoSO4 (anhydrous, BDH Chemicals) and 1.5 M Na2SO4 (BDH Chem-icals) in a distilled water solution.

The LiCoO2 coated platinum electrodes were prepared, outsidethe glove box, for the Scanning Electrochemical Microscope (SECM)measurements, utilising an ethanol slurry. The LiCoO2 was pur-chased from Hohsen, Japan. The 1.6 mm diameter platinum work-ing electrode (BASi electrode) was immersed in this dilute slurry,removed and allowed to dry. The slurry was made reasonably di-lute to limit the amount of resistive material coated on theelectrode.

2.2. Equipment

All SECM and electrochemical measurements were carried outwith a CH Instruments model CHI 900 SECM. The SECM setupwas located within the argon-atmosphere glove box (less than1 ppm O2 and 5 ppm water) with the potentiostat located outsidethe box connected via leads.

The substrate electrode was a 1.6 mm diameter platinum elec-trode (purchased from BASi) and the SECM probe tip was a 10 lmdiameter platinum microelectrode (purchased from CHI Instru-ments). According to the supplier, the probe tip had an Rg valueof 6–7 (ratio of glass diameter to electrode diameter). The SECMmeasurements were run in substrate generation–tip collection(SG–TC) mode [14], such that the substrate electrode (containingLiCoO2) was charged and discharged as the tip detected species sol-ubilised from this working electrode. An oversized platinumwound auxiliary electrode and a Ag|Ag+ (10 mM Agtriflate in C4-

mpyrTFSI) reference electrode were used in these measurements[15]. The potentials quoted in this paper were converted to Li|Li+

by measuring the potential of the ferrocene|ferrocenium couple(Fc|Fc+) vs Ag|Ag+. The Fc|Fc+ couple according to the literature[16] is 3.44 V vs Li|Li+.

3. Results and discussion

3.1. SECM approach curves

A 5 mM solution of ferrocene in the RTIL, C4mpyrTFSI, was usedto position the microelectrode (10 lm diameter) probe tip to with-in 10–20 lm of the substrate electrode. The downward facingprobe electrode was initially positioned at around 1–2 mm abovethe upward facing substrate electrode. The electrode was lowered

using positive feedback mode with a microelectrode probe tip volt-age of �0.10 V vs Ag|Ag+ (oxidising the ferrocene to ferrocenium)and a substrate potential of �0.65 V vs Ag|Ag+ (reducing the ferro-cenium produced at the tip). The cyclic voltammetry (5 mV s�1) ofthe probe was run prior to lowering the electrode (Fig. 1) and re-run (with a bias of �0.65 V on the substrate) once the electrodewas positioned close to the substrate to ensure that there was aninteraction between the two electrodes.

Examples of slow approach curves recorded for these measure-ments (lowering at 0.05 lm per 0.2 s) show that, in initial stages ofthe approach, the current was still reaching a steady-state (Fig. 2a–c). It was near steady state once it had been moved down 100 lm.When the electrodes started to approach to a close proximity(within around 30 lm), the current began to rise. The probe wasstopped, typically, at the position shown in Fig. 1a. In some in-stances, due to the viscous nature of the IL and a true steady-statenot being reached, the current began to dip down (see Fig. 2b).Using Eq. (1) below, the rough distance between the probe andthe substrate could still be calculated for positive feedback mech-anism and incorporating the effect of diffusion around the insulat-ing glass on the microelectrode (Rg � 5) [17]:

itip

itip;1¼ k1 þ

k2

L

� �þ k3 exp

k4

L

� �ð1Þ

where L = d/a or distance from substrate/radius of electrode,k1 = 0.72035, k2 = 0.75128, k3 = 0.26651, k4 = �1.62091, for Rg = 5.1(valid in the range of L = 0.2–20).

Even though, the distance was calculated between the two elec-trodes to a reasonable confidence, only a qualitative identificationof solubilised species was attempted. For the particular results dis-cussed later (in Fig. 6) the separation of the two electrodes wasroughly 35 lm (approach curve in Fig. 2c). For the other measure-ments, carried out in this work, typically the two electrodes wereestimated to be 20–30 lm apart.

The ferrocene/RTIL solution was rinsed (3 times) out of the cellusing the test electrolyte, C4mpyrTFSI containing 0.5 mol/kg LiTFSI.A series of experiments were carried out to determine the identityof the species expelled from the LiCoO2 under different chargingand discharging conditions in this electrolyte. The substrate(LiCoO2) was held at different potentials while sweeping the probetip potential; as well as sweeping the substrate potential whileholding the probe tip at a constant potential.

(a)

(b)

(c)

Fig. 2. Slow approach curves and fittings via Eq. (1) for the SECM measurementwith an increment distance of 0.05 lm and an increment time of 0.2 s, a biaspotential on the probe tip of �0.1 V vs Ag|Ag+ and a bias potential on the substrateelectrode of �0.65 V vs Ag|Ag+. (a) Typical approach curve, (b) unusual approachcurve with current decrease at close distance, and (c) approach curve used forresults in Fig 6.

Fig. 3. Cyclic voltammogram of 0.1 M CoSO4 and 1.5 M Na2SO4 at a Pt (1.6 mmdiameter) electrode in a distilled water solution. Scan rate of 20 mV s�1.

32 G.A. Snook et al. / Journal of Electroanalytical Chemistry 687 (2012) 30–34

3.2. Co2+ model experiments

A series of model experiments were carried out to determinethe electrochemical behaviour of Co2+ as probed by the microelec-trode tip. Once the behaviour of Co2+ in the RTIL was known, thecharge–discharge of the LiCoO2 was monitored using the SECMtechnique.

Previous work on cobalt in RTILs [18–22] has shown that thebehaviour can be complicated due to the speciation of the ion.The reduction potential can vary significantly with the choice of io-nic liquid, much like the voltammetry of Ag|Ag+ in ionic liquids[23,24]. Consequently, in order to confidently match the responsesobserved with the electrochemical characteristics of solubilisedCo2+, a series of preliminary experiments were needed.

These Co2+ model experiments utilised Pt substrate electrodescoated with cobalt. This coating was done in a controlled fashion,outside the glove box, utilising a sulphate based aqueous Co2+ solu-tion [25]. This deposition was found to be easiest utilising this sul-phate solution rather than a chloride medium [26–28]. The cyclicvoltammogram (CV) of this solution (Fig. 3) was run at 20 mV s�1

prior to running the deposition at a constant voltage of �0.8 V vsAg|AgCl (3 M KCl[aq]). This voltage was selected well beyond thereversible potential or crossover point (�0.630 V) and near thereduction peak position of the CV. A total charge of close to 0.5Cwas passed in 1 h to produce a thick layer of cobalt metal (dull-grey in appearance) on the platinum electrode.

Two experiments were carried out to determine the behaviourof Co2+ in the RTIL. Firstly, an SECM experiment, utilised the coatedelectrode (0.515C coating) as a substrate. In substrate generation–tip collection (SG–TC) mode, the Co2+ was generated from the sub-strate at +5.1 V vs Li|Li+, while the probe electrode collected thisproduct by scanning at 20 mV s�1 between 5.3 V and 2.6 V vs Li|Li+

(see Fig. 4a). Here, the cobalt was stripped from the substrate elec-trode (at the positive voltage as a cobalt ion) and reduced at theprobe tip (Fig. 4a) at a potential between 3.5 and 3.0 V vs Li|Li+.This reduction product was then oxidatively stripped at a potentialbetween 3.5 and 4.0 V vs Li|Li+.

The second experiment involved stripping large amounts ofCo2+ from the cobalt coated Pt electrode (0.525C) into 2 ml of theionic liquid. This gave a final concentration in the RTIL of close to1 mM according to Faraday’s Law and the charge passed. The CVwas run using the Pt microelectrode and at two different scanrates, 50 mV s�1 (Fig. 4b) and 500 mV s�1 (Fig. 4c). This showedthe complex behaviour of the Co2+ in this RTIL. There was no re-verse peak when ran at 50 mV s�1, whereas at 500 mV s�1 the re-verse peak was enhanced. This was partly due to the micro-sizeof the electrode (fast radial diffusion to and from the electrode giv-ing a quasi steady-state behaviour) and partly due to the complexbehaviour of cobalt in the IL. The behaviour of the detected Co2+ inthese two experiments was very different to the classical behav-iour observed in aqueous solution [25]. The difference between

(a)

(b)

(c)

Fig. 4. (a) Cyclic voltammogram of Co2+ in C4mpyrTFSI at 20 mV s�1 at the Pt(10 lm diameter) microelectrode probe tip (positioned within 10–20 lm of thesubstrate electrode), produced from cobalt metal coated Pt substrate electrode(utilising a bias of 5.1 V vs Li|Li+). (b) Separate experiment with the probe tip in thebulk solution (�1 mM Co2+ dissolved into RTIL solution by biasing the macro Ptworking electrode coated with 0.5C of cobalt metal at 5.1 V vs Li|Li+) and the cyclicvoltammogram ran at 50 mV s�1. (c) Cyclic voltammogram of the same solution at500 mV s�1.

Fig. 5. Cyclic voltammogram of LiCoO2 coated Pt substrate electrode in C4mpyrTFSIat 20 mV s�1.

G.A. Snook et al. / Journal of Electroanalytical Chemistry 687 (2012) 30–34 33

Fig. 4a when compared to Fig. 4b is partly due to the different scanrate but mostly due to different concentrations involved, changingfluxes of species during the sweep and the distinct possibility ofregistering responses for any breakdown products of the IL whichwere liberated from the substrate at the potential held (i.e., 5.1 V vsLi|Li+). The CV at 50 mV s�1 (Fig. 4b) was much better resolved andnot as broad as the CV run at 500 mV s�1 (Fig. 4c). Consequently,this scan rate was chosen as the optimum value in terms of speedof measurement and resolution of peaks and was used as the probetip scan rate in subsequent measurements. The CV run at50 mV s�1 was used for comparison in later figures to match thisscan rate used for the SECM measurements.

In this series of experiments, the oxidative electrochemical win-dow (>6 V vs Li|Li+) is reached prior to the oxidation of Co2+ to Co3+;therefore not allowing this particular process to be observed. Thisfact becomes important in later discussion.

3.3. Behaviour of the LiCoO2 powder on cycling

LiCoO2/ethanol pastes were utilised rather than LiCoO2/binder/carbon mixes so as to examine the behaviour of the LiCoO2 by itself

without interferences from the other materials. Preliminary testsprogressively lowered the amount of active material adhered tothe electrode so as to minimise the ohmic drop, caused by thelow electronic conductivity of the LiCoO2 that adhered to the elec-trode. An example of scans 4–6 of LiCoO2 powder on the platinumelectrode (Fig. 5) showed the E1/2 potential of the LiCoO2 charge/discharge process was �4.2 V vs Li|Li+. Even using this low cover-age, there was still a large ohmic distortion. Consequently, the ac-tual potentials experienced by the positive material weresignificantly lower than that applied.

The blank probe tip cyclic voltammogram (black dashed line)sometimes showed a small amount of residual ferrocene response(e.g. Fig. 6a). There was also a significant response in the blank scanat �2.2 V vs Li|Li+, which was likely to be residual oxygen which isshown in the literature [29,30] to reduce reversibly (under theright conditions) at �2.6–2.7 V vs Li|Li+ and will possibly shift inthe presence of lithium ions.

These preliminary results showed, during both overchargingand deep discharging conditions, that there were significant re-dox-active species evolved from the LiCoO2 electrode. Firstly, dur-ing overcharging conditions, a small amount of what appeared tobe Co2+ was released into the ionic liquid solution from the LiCoO2

electrode (Fig. 6a). Overcharging also appeared to shift the sup-posed oxygen process to more positive potentials (perhaps dueto changing lithium ion concentration near the electrode). How-ever, in this deep charging regime, only relatively low concentra-tions of (redox-active) solubilised species were detected.

It was a different story on the deep discharge. As shown inFig. 6b, deep discharge, a large concentration of solubilised cobaltis detected (as well as a post-peak at 3.05 V vs Li|Li+). Also, therewas an enhanced peak at �2.3 V vs Li|Li+ (assumed to be additionaloxygen solubilised into the solution near the probe tip). Finally,there was also a large oxidative process occurring at >4.6 V vsLi|Li+. As discussed previously, the oxidation of Co2+ to Co3+ didnot occur in this region during the Co2+ model experiments. Theprocess may be a cobalt containing species, monovalent cobalt,or the electrochemistry of the breakdown products of the ionic li-quid itself.

Little cobalt was detected in the other experiments (i.e. lowerover potentials or shallow discharge as indicated in Fig. 6b). How-ever, the oxidative process at >5.0 V vs Li|Li+ was observed whencharging above 4.5 V vs Li|Li+ then shallow discharging to 3.7 Vvs Li|Li+.

(a)

(b)

Fig. 6. (a) Cyclic voltammogram of the Pt (10 lm) microelectrode probe tip inC4mpyrTFSI at 50 mV s�1, showing the effect of overcharging by utilising a biaspotential of 5.2 V vs Li|Li+ and (b) showing the effect of shallow discharge and deepdischarge by utilising a bias potential of 3.3 V and 2.3 V vs Li|Li+ respectively.Superimposed on both plots is the cyclic voltammogram (50 mV s�1) of Co2+

solubilised in the experiments shown in Fig. 4b.

34 G.A. Snook et al. / Journal of Electroanalytical Chemistry 687 (2012) 30–34

4. Conclusions

Shown in this paper, was the successful utilisation of SECM torapidly monitor dissolution from the battery material, LiCoO2, inan RTIL. Interestingly, in this work, the majority of the dissolu-tion of the battery material (LiCoO2) occurred during deep dis-charge conditions. This is contrary to popular belief of thefailure mechanisms of this cathode material in traditional bat-tery electrolytes [1–7]. The redox state of cobalt should switchbetween Co3+ and Co2+ during discharge. The further reductionof Co2+ to Co+, however, could be the reason for the degradation.This Co+ is likely to be unstable in the structure and easily re-moved into the surrounding solution, taking with it some ofthe charge balancing O2. The other possibility is that the excessnegative charge in the structure could be balanced by insertingexcess Li+, which in turn could mechanically destabilise thestructure. The former explanation is more likely, due to thedetection of appreciable concentrations of O2 at the tip electrodeon deep discharge.

The advantage of using the SECM technique, is that the detec-tion limit is low. In these measurements, the signal due to the sol-ubilised species was found to be very small. This suggests that onlythe outer layers of the battery material (LiCoO2) were being ac-cessed in the dissolution process. This is, however, enough to de-

grade the material in the long term and seriously reduce batterycycle-life.

In future work, our aim is to develop the quantitative aspects ofthis methodology. This can be achieved by utilising pasted elec-trodes of LiCoO2/conducting filler/binder mixture on aluminiumfoil as the substrate. This would better simulate the actual batteryelectrode configuration. Of particular interest would be the ten-dency for corrosion of the aluminium current collector, which isthe subject of much interest to the battery community. Finally,coin cells made from these pasted aluminium foils utilising a lith-ium metal anode and the ionic liquid electrolyte will be cycled inthe future to ascertain whether limiting the discharge voltagehas an effect on the cycle life of the active material.

Acknowledgements

The authors would like to thank Dr. Noel Duffy for his assistancewith providing the SECM setup. Dr. Anand Bhatt for assistance withSECM setup and Prof. Clare Grey for helpful discussions. Theauthors would like to thank the US ARL via International Technol-ogy Centre Pacific (ITC-PAC) under contract FA5209-10-P-0259 forfunding. We also acknowledge CSIRO’s National Flagship EnergyTransformed.

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