Electrochimica Acta 50 (2004) 535–539

Surface characterization of emulsified lithium powder electrode

Seung-Taek Hong, Jin-Suk Kim, Suk-Jun Lim, Woo Young Yoon∗

Division of Material Science and Engineering, Korea University, 1, 5-Ka, Anam-dong, Sungbuk-gu, Seoul 136-701,Republic of Korea

Received 12 June 2003; received in revised form 11 March 2004; accepted 11 March 2004Available online 20 August 2004

Abstract

The surface modification of Li powder was attempted in order to enhance the electrochemical properties of the electrodes used in rechargeablebatteries. To create a “native” LiF film on Li powder, Li powder was manufactured using a droplet emulsion technique (DET) process byintroducing fluoride as a surfactant. The surface modification of the Li powder was confirmed by means of X-ray photoelectron spectroscopy(XPS) and energy dispersive X-ray analysis (EDX). Two-electrode cells (Li symmetric cells) were prepared for the purpose of impedancea und to havea ssed whent reover, thes©

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nalysis and scanning electron microscope (SEM) observation. From the impedance analysis, the modified surface film was fosmaller resistance than the bare Li powder surface. SEM images showed that dendritic formations were more highly suppre

he compacted Li powder and surface-modified Li powder electrodes were used than when the Li foil electrode was used. Mourface-modified Li powder electrodes suppressed dendritic formations more effectively than regular Li powder electrodes.2004 Published by Elsevier Ltd.

eywords:Lithium metal battery; Lithium powder; DET; Surface modification; XPS

. Introduction

Lithium metal is an attractive material for use as a neg-tive electrode in rechargeable batteries because of its high

heoretical specific energy density (3860 mAh/g). However,he poor cycle efficiency and safety problems caused by theendritic deposition of lithium metal have prevented lithiumetal from being used in rechargeable batteries. It is well

nown that the characteristics of the Li metal/electrolyte in-erface greatly affect the charge–discharge performance ofithium metal anodes[1,2]. Previous studies have suggestedhat the uneven current distribution caused by the nonunifor-ity of the surface film on Li is the most probable reason

or dendritic formations during charging[3,4]. In order touppress dendritic deposition, many researchers have tried toodify the morphology and properties of the surface film on

ithium. For example, they investigated the effects of lithium

∗ Corresponding author. Tel.: +82 2 3290 3274; fax: +82 2 928 3584.E-mail addresses:wyyoon@korea.ac.kr, hsteak@korea.ac.kr

W.Y. Yoon).

salts[5], solvent systems[6], and additives such as CO2 [7],HF[8], iodide[9], benzene[10] and organic compounds[11].

In previous papers, the compacted Li powder electwas proposed as a new anode material[12]. The lithium pow-der was manufactured using the droplet emulsion techn(DET). Compared with Li foil electrodes, the Li powder eltrode has a higher surface area, which is about 4.5 times lIn our previous work, the BET (Brunauer–Emmett–Telmeasurement results revealed that surface area of apacted lithium powder (size:∼20�m) electrode was lagthan that of a lithium foil electrode by about 4.5 times[13].This larger surface area probably lowers the effectiverent density in the electrode surface, which may supprespotential for the dendritic formation of lithium.

Since liquid lithium is emulsified during the DET procethe bare surface of the lithium is exposed to the siliconThrough the addition of a surfactant to the silicon oil, theface film on the lithium powder can be modified throughinteraction of the surfactant with the bare surface of lithiIt is widely known that the Li electrode with the LiF film oits surface has enhanced electrochemical properties[8].

013-4686/$ – see front matter © 2004 Published by Elsevier Ltd.oi:10.1016/j.electacta.2004.03.065

536 S.-T. Hong et al. / Electrochimica Acta 50 (2004) 535–539

In this study, surface modification was attempted in orderto obtain a suitable LiF film on the Li powder by introducingfluoride during the DET process. The surface properties ofthe Li electrode were investigated in order to confirm that thesurface of the Li powder was well modified.

2. Experimental

Three kinds of Li electrodes were used as the workingelectrodes in this study. One was a commercially availableLi foil which was extruded from an ingot (Cyprus Co. USA,99.9%). The other two electrodes were made of Li powderand surface-modified Li powder, respectively. The Li pow-ders were produced using the DET process.Fig. 1shows theschematic diagram of the DET apparatus used. Details onthe DET process have been presented elsewhere[12,13]. Amixture of molten Li and silicon oil (Shin-Etsu, Japan) wassheared at about 25,000 rpm to produce an emulsion. As theemulsion cooled down to room temperature, the liquid Liwas solidified to form powders. These Li powders were thenseparated from the silicon oil and cleaned with hexane. Toobtain the surface-modified Li powder, surfactant materialswere dissolved in the silicon oil until full saturation occurred.In this work, LiPF6 was used as a surfactant to form the LiFfi ce-m tely1 is al-m eval-u werec re ofa ionso

t us-i derh ret ed tot seda th aird C 1sh used

as an internal standard to calibrate the binding energy scale.The binding energies for all compounds are provided in pre-vious papers[1,14]. The Mg K� line was used as an X-raysource. An XPS depth analysis was also performed with anAr ion beam (accelerating voltage, 2 keV; ion beam current,7–8�A). The etching rate was estimated to be approximately5A/min based on a theoretical calculation.

To investigate the electrochemical properties of the Lielectrode, two-electrode cells (Li symmetric cells) were pre-pared. The electrolyte used was a 1:1 mixture of ethylenecarbonate (EC) and dimethyl carbonate (DMC) containing1 M LiPF6 (LP30 Selectipur, Merck). The water content ofthe electrolytes was <30 ppm and the HF content was a max-imum of 100 ppm.

The electrochemical behaviors of the Li electrodes werealso analyzed through impedance measurements (Solatron1255), which were performed at open-circuit potential. Theamplitude of the applied alternating potential was±5 mV,and its frequency was varied from 100 kHz to 0.01 Hz at roomtemperature.

To observe the morphology of Li deposition in a 1 MLiPF6, EC:DMC (1:1) solution on different Li electrodes,lithium was electrodeposited (1 C/cm2) at a uniform currentdensity (0.5 mA/cm2). The morphology of Li deposition wasinvestigated using SEM (Horiba 7200-H).

3

ETp wass w-d olor,w lor.O thatt sti-g DXa c-t ivedL ione theLoa thp ofL vely.T wdere eL foileC r ione rre-s afterA howt ckert rface

lm on the Li powder surface. The Li powders and surfaodified Li powders both have a diameter of approxima0–20�m. The surface area of both powder electrodesost the same because of their similar powder size. Toate their electrochemical performance, the Li powdersompacted into the form of a coin by applying a pressuround 15 MPa. All of the Li electrodes had equal dimensf 15 mm in diameter and 1.6 mm in thickness.

The surface analysis of the Li electrode was carried oung XPS (PHI 5700 ESCA) and EDX (Horiba 7200-H) unigh vacuum conditions (<5× 10−7 Pa). The samples we

ransferred from a glove box to a subchamber connecthe XPS equipment filled with Ar. A glass bottle was us a transfer vessel to avoid undesirable reactions wiuring the transfer process. The binding energy of theydrocarbon peak in the XPS chamber (285.0 eV) was

Fig. 1. Schematic diagram of the DET apparatus.

. Results and discussion

The surface film of the Li powder was modified in the Drocess, and the color of the surface-modified Li powderhown to be outwardly different from that of the Li poer. The surface-modified Li powder had a dark beige chereas the unmodified Li powder is bright beige in cone possible explanation for this variation in color was

he surface-modified Li powder might be different. To inveate the surface modification on the Li powder, XPS and Enalyses were performed.Fig. 2(a–c) shows the XPS spe

ra of Li 1s, C 1s, and O 1s, respectively, for the as-recei foil electrode. These spectra were obtained after Artching for various durations. The surface film formed oni foil electrode is illustrated schematically inFig. 3(a). Theuter part of the surface film consists of Li2CO3 and LiOHnd the inner part consists of Li2O. This result agrees wirevious reports[3,14]. Fig. 2(d–f) shows the XPS spectrai 1s, C 1s, and O 1s for the Li powder electrode, respectihese spectra indicate that the surface film on the Li polectrode consists of the Li2CO3/LiOH outer layer and thi2O inner layer, which is similar to the as-received Lilectrode. The peaks corresponding to Li2CO3 in the Li 1s,1s, and O 1s spectra, however, diminished during A

tching performed for a duration of 41 min. The peak coponding to Li metal in the Li 1s spectra was observedr ion etching performed for 121 min. These results s

hat the surface films on the Li powder electrode are thihan those on the as-received Li foil electrode. The su

S.-T. Hong et al. / Electrochimica Acta 50 (2004) 535–539 537

Fig. 2. XPS spectra of Li 1s, C 1s, O 1s and F 1s for Li foil (a–c), Li powder (d–f), modified Li powder (g–j) after Ar sputtering.

conditions of the Li powder electrode are schematically il-lustrated inFig. 3(b).

Fig. 2(g–j) shows the Li 1s, C 1s, O 1s, and F 1s XPS spec-tra, respectively, for the Li powder electrode modified usingLiPF6. From these spectra, four different Li compounds –Li2CO3, LiOH, Li2O and LiF – were found to exist on themodified Li powder electrode, and these compounds made upthree quarters of the surface film. In the Li 1s XPS spectra, apeak attributed to Li2CO3/LiOH appeared at 55.5 eV. Afteretching, a peak was observed at 56 eV, which corresponded

Fig. 3. Schematic illustration of the surface films on the as-received foil (a),as-compacted Li powder (b), and modified Li powder (c).

to F in LiF. The formation of LiF may be explained by thereaction of molten Li with LiPF6 during the DET process.The intensity of this peak decreased as the duration of the Arion etching increased. After 51 min of etching, a peak corre-sponding to Li2O appeared at 53.7 eV in the Li 1s spectra.In the C 1s spectra, a peak was observed at 285.0 eV, whosebinding energy corresponded to the C atom of a hydrocarbon.A weak peak corresponding to Li2CO3 at 290.2 eV was alsoobserved. After 11 min of etching, the peak assigned to thecarbon atom of a hydrocarbon disappeared. The O 1s XPSspectra at various Ar ion etching durations showed that LiOHor Li2CO3 was present in the outer part of the surface filmand that Li2O existed in the inner part. However, in the Li 1Sspectra, during the period of Ar ion etching from 11 to 51 min,the peak corresponding to LiF at 56.0 eV was a major peak.Also, in the F 1s spectra, a strong peak corresponding to LiFwas observed at 685.9 eV during the period of etching from11 to 51 min. Therefore, the outer part of the surface film con-sists mainly of Li2CO3 and LiOH. The middle part consistsmostly of LiF, but with a small amount of Li2CO3. Finally,

538 S.-T. Hong et al. / Electrochimica Acta 50 (2004) 535–539

Fig. 4. EDX analysis for a single modified-powder in a compacted electrode.

the inner part of the surface film is composed of Li2O. Thesurface state of the Li powder electrode modified with LiPF6is schematically illustrated inFig. 3(c).

Because the spatial resolution of X-rays is larger than themodified Li powder size, it was difficult to confirm that theentire surface of the electrode was covered by the LiF filmusing this technique. Therefore, EDX analysis was performedto prove that all of the modified Li powder, as well as the entiresurface of the electrode, contained the LiF film. Accordingto the F 1s XPS spectra, there was a small amount of fluorideon the outer film of the modified powder.Fig. 4 shows thatfluoride exists on all of the modified Li powder. This resultshows that all of the powders were modified to LiF and sothe entire surface of the electrode was covered with the LiFfilm.

Fig. 5 shows the resistance of the surface film on the Lipowder and modified Li powder electrodes. The resistancesof the surface film on the Li electrode were determined fromthe cole–cole plot of the impedance analysis in accordancewith a previous report[15]. The resistance of the surface-modified Li powder electrode was lower than that of the Lipowder electrode. This result can be explained by the fact thatthe surface film of the modified powder electrode is thinnerthan that of the Li powder electrodes. The thickness of the

ode.

surface film was roughly proportional to the duration of Arion etching in XPS.

Fig. 6 shows SEM images of the Li foil, Li powder andsurface-modified Li powder electrodes, on which lithiumwas electrochemically deposited at 1 C/cm2 in EC:DMC(1:1), 1 M LiPF6. Lithium was deposited galvanostatically at0.5 mA/cm2. The white, irregular phase shown inFig. 6(a–c)consists of clusters of Li dendrites, which were electrode-posited on the electrode. From the results shown inFig. 6(aand b) it was found that the use of Li powder electrodes de-creases the dendritic formation of lithium. Compared withthe Li foil electrode, the Li powder electrode has a surfacearea, which is about 4.5 times larger[13]. A modified powderelectrode has almost the same surface area as the Li powderelectrode, because both powders have the same dimensions.This larger surface area probably lowers the effective currentdensity of the electrode surface, and thus suppresses the den-dritic formation of lithium.Fig. 6(c) shows that the films ofthe surface-modified Li powder electrodes suppress the for-mation of dendrite of lithium more effectively than those ofthe unmodified Li powder electrode. This result can be ex-plained by the fact that the surface film of a modified powderelectrode has the LiF film. It is well known that the Li elec-trode with the LiF film on its surface has enhanced electro-chemical properties[8]. Fig. 6(d–f) shows the magnificationo g-u thes dere theL Lip thant sultsi s ag s Lit dere

allers ow-d eforem nse-q int s the

Fig. 5. The resistance of the Li powder and modified Li powder electr

f the black regions ofFig. 6(a–c), respectively. These fires show the distribution of the Li deposit formed onurface of the Li electrodes. The Li deposit on the Li powlectrodes has a more uniform distribution than that oni foil electrodes. In fact, the Li deposit on the modifiedowder electrodes has a more homogeneous distribution

hat on any of the other electrodes in this study. These rendicate that the LiF film on the Li powder electrode hareater ability to suppress dendritic formation and allow

o be electrodeposited more uniformly than the Li powlectrode.

From these results, it can be concluded that a smized powder (as in the case of the compacted Li per used in this study) has a larger area and is therore effective on suppressing dendritic deposition. Couently, through surface film modification of Li powder

he DET process, the use of a smaller sized powder allow

S.-T. Hong et al. / Electrochimica Acta 50 (2004) 535–539 539

Fig. 6. SEM images for the surface of (a and d) lithium foil electrode (300× and 1500×); (b and e) lithium powder electrode (300× and 1500×); (c and f)modified lithium powder electrode (300× and 1500×) after electrodeposit until 1 C/cm2 at uniform current density (0.5 mA/cm2).

production of the Li electrode with enhanced electrochemicalproperties.

4. Conclusion

Using the DET process 10–20�m diameter Li powderscan easily be synthesized. Also, a surface-modified powdercan be obtained by simply introducing surfactant agents dur-ing the DET process. In this study, the presence of the LIFfilm on modified Li powder was identified using XPS andEDX analysis. From impedance analysis and SEM observa-tion, the surface film of the modified powder electrodes wasfound to exhibit a lower resistance and a greater ability tosuppress dendritic formation than Li powder electrodes.

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