Dokko-2005-Preparation of micro

Electrochimica Acta51(2005)

966–971

Preparation of micro-dot electrodes of LiCoO2and

Li4Ti5O12for lithium micro-batteries

Kaoru Dokko a,b,Jun-ichi Sugaya a,b,Hirokazu Munakata a,b,Kiyoshi Kanamura a,b,?

a Department of Applied Chemistry,Graduate School of Engineering,Tokyo Metropolitan University,Minami-Ohsawa,

Hachioji,Tokyo192-0397,Japan

b CREST,Japan Science and Technology Agency,4-1-8,Honcho,Kawaguchi,

Saitama332-0012,Japan

Received17October2004;received in revised form18February2005;accepted15April2005

Available online2August2005

Abstract

Fabrications of micro-dot electrodes of LiCoO2and Li4Ti5O12on Au substrates were demonstrated using a sol–gel process combined with a micro-injection technology.A typical size of prepared dots was about100?m in diameter,and the dot population on the substrate was2400dots cm?2.The prepared LiCoO2and Li4Ti5O12micro-dot electrodes were characterized with scanning electron microscopy,X-ray diffraction,micro-Raman spectroscopy,and cyclic voltammetry.The prepared LiCoO2and Li4Ti5O12micro-dot electrodes were evaluated in an organic electrolyte as cathode and anode for lithium micro-battery,respectively.The LiCoO2micro-dot electrode exhibited reversible electrochemical behavior in a potential range from3.8to4.2V versus Li/Li+,and the Li4Ti5O12micro-dot electrode showed sharp redox peaks at1.5V.

?2005Elsevier Ltd.All rights reserved.

Keywords:Rechargeable lithium batteries;Micro-dot electrode;Sol–gel;LiCoO2;Li4Ti5O12

1.Introduction

Rechargeable lithium batteries have been widely utilized as power sources for electronic devices such as mobile phones and laptop computers.In commercialized lithium batteries, cathode materials are transition metal oxides such as LiCoO2 and LiMn2O4,and an anode is graphite[1,2].These cath-odes and anodes operate in non-aqueous liquid electrolytes. The replacement of liquid electrolyte with solid electrolyte improves safety of lithium batteries.This is the so-called “all solid state rechargeable lithium battery”.The all solid-state battery does not suffer from?ammability or leak of electrolyte.Recently,many researchers have studied micro lithium batteries[3–8].If all solid-state lithium batteries are realized in micron size,it will be utilized in various application?elds related to micro-systems,such as micro-?Corresponding author.Tel.:+81426772828;fax:+81426772828.

E-mail address:kanamura-kiyoshi@c.metro-u.ac.jp(K.Kanamura).sensors,micromechanics,and microelectronics.The tech-nologies for fabrication of all solid-state lithium batteries have been explored intensively in our group.By using sol-gel process,we have demonstrated the preparation of thin-?lm cathode and anode for all solid-state lithium batteries[9,10]. In sol–gel processes,ceramics can be prepared from precur-sor solutions.The precursor solution may be applicable as an ink for printers.A combination of the sol–gel process and printing technologies will make it possible that micro-pattern of ceramics is prepared on appropriate substrates.

In this work,micro-dot electrodes for all solid-state rechargeable lithium batteries were prepared by sol–gel pro-cess combined with a micro-injection https://www.360docs.net/doc/7214508641.html,ing this technique,the micro-dot pattern can be prepared on conductive substrates.We selected LiCoO2and Li4Ti5O12 as cathode and anode for the micro-battery,respectively, because these materials can be prepared via sol–gel pro-cesses[9,10].The LiCoO2has a layered structure,and Li4Ti5O12has a spinel related structure.The Li+ion can

0013-4686/$–see front matter?2005Elsevier Ltd.All rights reserved. doi:10.1016/j.electacta.2005.04.061

K.Dokko et al./Electrochimica Acta 51(2005)966–971967

be extracted/inserted reversibly from/into crystallographic structures of these materials.The micro-dot electrodes of LiCoO 2and Li 4Ti 5O 12were fabricated on Au substrates,and their electrochemical properties were characterized.

2.Experimental

LiCoO 2and Li 4Ti 5O 12were prepared via a PVP sol–gel process [9–11].A precursor sol for LiCoO 2was prepared from CH 3COOLi,Co(CH 3COO)2,CH 3COOH,H 2O,and poly(vinylpyrrolidone)(PVP,Mw:55,000).These starting materials were mixed with molar ratio of 1.1:1:1:50:0.5(VP monomer unit).The excess amount CH 3COOLi of 10at%was added to the precursor in order to avoid the lack of Li due to evaporation of lithium during heat treatments at high temperatures [12].A precursor sol for Li 4Ti 5O 12was similar to that of our previous report [10].The sol was prepared from CH 3COOLi,[(CH 3)2CHO]4Ti,PVP,CH 3COOH,i-C 3H 7OH,and HOCH 2CH(OH)CH 2OH,and the molar ratio of these starting materials was 4.5:5:2:60:100:12.5.The glyc-erin (HOCH 2CH(OH)CH 2OH)was added to the Li–Ti–O sol in order to increase the viscosity of sol.

Schematic illustration of a micro-injection system is shown in Fig.1.By using the micro-injection system (Inject-Man NI2,Eppendorf),micro-dots were drawn on Au sub-strates with a glass capillary (inner diameter:0.5?m)under an optical microscope observation (BX51W1,Olympus).The precursor sol was used as an ink for the drawing.The injection pressure of the sol was set to be 1000hPa.The dot popula-tion on the substrate was 2400dots cm ?2.The sol converted into gel on the Au substrate in air at room temperature.Then,it was calcinated at high temperatures in air.The calcina-tion of LiCoO 2was carried out at 800?C for 10–60min,and Li 4Ti 5O 12was heat-treated at temperatures between 700and 900?C for 20min.Their surface morphologies were observed with scanning electron microscope (SEM,JSM-5310,JEOL),and crystallographic structures were characterized by X-ray diffraction (XRD,RINT-UltimaII,Rigaku)with Cu K ?radia-tion and micro-Raman spectroscopy (NRS-1000,Jasco)with 532nm laser radiation (spot size:ca.5?

m).

Fig.1.Schematic illustration of micro-injection system for fabrication of micro-dot electrode.

A three-electrode cell with lithium metal foil and wire as the counter and reference electrodes was used for cyclic voltammetry.The electrolyte was a mixed solvent of ethy-lene carbonate (EC)and diethyl carbonate (DEC)(1:1in volume)containing 1.0mol dm ?3LiClO 4.The electrochemi-cal cell was fabricated in an argon-?lled glove box.The

cyclic

Fig.2.Optical microscope images for micro-dots of Li-Co-PVP gel (a),annealed LiCoO 2on Au substrates (b),and SEM image for surface of LiCoO 2dot annealed at 800?C (c).

968K.Dokko et al./Electrochimica Acta 51(2005)966–971

voltammetry measurements were carried out with a potentio-stat (Model 660B,ALS)at room temperature.

3.Results and discussion 3.1.LiCoO 2micro-dot electrodes

Fig.2a and b display the optical microscope images of micro-dots of Li-Co-PVP gel and LiCoO 2prepared on a gold substrate.The size of a dot was about 100?m in diameter.Fig.2c shows the SEM image of a LiCoO 2dot heat-treated at 800?C for 20min.The surface of the dot was rough,and the grain growth was observed.Fig.3shows the XRD patterns of LiCoO 2micro-dots prepared by heat treatment at 800?C for various durations.The peaks due to Au substrate can be seen at 38.3?and 44.5?.The peaks of LiCoO 2corresponding to the (003),(101),and (104)planes were observed for all samples,and this pattern agrees well to that of previous report [9].The additional peaks were observed at 31.4?and 36.9?,and these were attributed to Co 3O 4generated during heat treatments at high temperatures [13].The Raman spectra of LiCoO 2dots are shown in Fig.4.Raman

measurements

Fig.3.XRD patterns of LiCoO 2micro-dot electrodes fabricated on Au sub-strates annealed at 800?C for 10min (a),20min (b),40min (c),and 60min

(d).

Fig.4.Micro-Raman spectra for LiCoO 2micro-dots annealed at 800?C for 20min.Raman spectra were measured for 25LiCoO 2dots,and 15dots exhibited spectra similar to (a),8dots similar to (b),and 2dots similar to (c).

were carried out for 25LiCoO 2dots.The Raman spectrum of the hexagonal phase (space group:R 3m)with two Raman active modes at 592cm ?1and 482cm ?1arising from E g and A 1g ,respectively [14,15].However,additional Raman peaks at 686and 518cm ?1of an impurity phase were observed for some dots.These peaks originated from Co 3O 4[16].These Raman results are consistent with the results of XRD.The three spectra in Fig.4suggested that the amounts of Co 3O 4in the dots were different from each other,despite the dots were fabricated on the same substrate.In fact,the amount of impurity depended on the heat treatment condition,and the optimized condition to minimize the impurity was annealing at 800?C for 20min.

Fig.5shows the cyclic voltammogram of the LiCoO 2micro-dot electrodes.The cyclic voltammogram was mea-sured for 400dots of LiCoO 2,and the current was divided by the number of dots.The prepared micro-dot electrodes exhibited intrinsic electrochemical properties of the LiCoO 2.The main redox peaks were observed at 3.92and 3.88V versus Li/Li +during anodic and cathodic scan,respectively.Two small redox peaks were also clearly observed at 4.06and 4.16V .These redox peaks are in good agreement with reported ones [9].The Li +ion was deintercalated/intercalated

K.Dokko et al./Electrochimica Acta 51(2005)966–971

969

Fig. 5.Cyclic voltammogram of LiCoO 2micro-dot electrode.Cyclic voltammogram was measured for 400dots of LiCoO 2.Current was nor-malized by both the number of dots and the area of Au substrate.Scan rate was 0.167mV s ?1

.

Fig.6.Optical microscope image for micro-dots of Li 4Ti 5O 12(a),and SEM image for surface of Li 4Ti 5O 12dot on Au substrate annealed at 800?C.from/into the layered structure of Li x CoO 2in the course of anodic scan and cathodic scan,respectively.These redox peaks are correlated with structural changes of Li x CoO 2during intercalation and deintercalation of Li +ions [17].The Co 3O 4is electrochemically inactive in this potential range.The average current for a dot was on the nA level,and the discharge capacity of a LiCoO 2dot calculated from the integration of cathodic current was 14?C.The x in Li x CoO 2was 0.45at 4.2V ,and 14?C corresponded to 2.8×10?11mol of LiCoO 2.The size of a dot is important for designing a micro-battery.In order to control the dot size,viscosity of the precursor sol should be further opti-mized,and modi?cation of the substrate surface may also be effective.

3.2.Li 4Ti 5O 12micro-dot electrodes

Li 4Ti 5O 12micro-dot electrodes were heat-treated at vari-ous temperatures.Fig.6a and b display the optical microscope image and the SEM image of micro-dots of Li 4Ti 5O 12heat-treated at 800?C for 20min,respectively.The typical size of a Li 4Ti 5O 12micro-dot was 100?m in diameter.The Li 4Ti 5O 12dots were very thin and transparent.The surface of Li 4Ti 5O

12

Fig.7.XRD patterns of Li 4Ti 5O 12micro-dot electrodes fabricated on Au substrates annealed at 700?C (a),800?C (b),and 900?C (c)for 20min.

970K.Dokko et al./Electrochimica Acta 51(2005)966–971

was smooth,although there were several micro-cracks in a dot as shown in Fig.6b.XRD patterns of Li 4Ti 5O 12samples are shown in Fig.7.A peak corresponding to (111)plane was observed at 18.5?for all samples.However,the peaks due to impurity phases were detected for the samples prepared at 700and 900?C,and these peaks were attributed to anatase and rutile TiO 2.The Raman spectra of Li 4Ti 5O 12micro-dots prepared at various temperatures are shown in Fig.8.The Raman spectra of the Li 4Ti 5O 12changed depending on the heat treatment temperature.The sample prepared at 700and 900?C showed characteristic peaks of anatase and rutile TiO 2,respectively [18],which agreed well with the results of XRD.The Raman spectrum of the sample annealed at 800?C showed good agreement to that reported by Liu et al.[19],however,the peak due to rutile-TiO 2,which was generated as an impurity,was observed at 614cm ?1.

Fig.9shows the cyclic voltammogram of the Li 4Ti 5O 12micro-dot electrodes prepared at 800?C.The cyclic voltam-mogram was measured for 1000dots of Li 4Ti 5O 12,and the current was divided by the number of dots.The Li 4Ti 5O 12sample showed reversible redox peaks at 1.55V .However,the background currents were observed in both

cathodic

Fig.8.Micro-Raman spectra of Li 4Ti 5O 12micro-dots fabricated on Au substrates annealed at 700?C (a),800?C (b),and 900?C (c)for 20

min.

Fig.9.Cyclic voltammogram of Li 4Ti 5O 12micro-dot electrode.Cyclic voltammogram was measured for 1000dots of Li 4Ti 5O 12.Current was nor-malized by both the number of dots and the area of Au substrate.Scan rate was 0.167mV s ?1.

and anodic scans in the potential range from 2.0to 1.2V .This background was considered due to irreversible reduc-tion of electrolyte components on Au substrate.In the course of the cathodic peak,Li +ion was inserted into Li 4T 5O 12which converted into Li 7Ti 5O 12[20–22].The reversible CV peaks suggest that the Li +ion insertion proceeds reversibly.The discharge capacity of a Li 4Ti 5O 12dot calculated from the integration of anodic current was 1.4?C,which corre-sponded to 4.8×10?12mol of Li 4Ti 5O 12.In spite that the sizes of LiCoO 2dot and Li 4Ti 5O 12dot were similar,the dis-charge capacity of a Li 4Ti 5O 12dot was smaller than that of a LiCoO 2dot.It was considered that the thickness of a Li 4Ti 5O 12dot was thinner than that of LiCoO 2.The opti-mization of the Li–Ti–O sol composition is now underway in order to control the thickness of the dot.

4.Conclusions

Fabrications of micro-dot electrodes of LiCoO 2and Li 4Ti 5O 12on Au substrates were demonstrated using a sol–gel process combined with a micro-injection technol-ogy.The typical size of a prepared dot was about 100?m in diameter,and the dot population on the substrate was 2400dots cm ?2(i.e.20%of the gold was covered by micro-dots).The LiCoO 2micro-dot electrode exhibited reversible electrochemical behavior in a potential range from 3.8to 4.2V versus Li/Li +,and the Li 4Ti 5O 12micro-dot electrode showed sharp redox peaks at 1.55V .

As preliminary experiment,a micro-injection device was utilized in this work to draw the micro-dot patterns of LiCoO 2and Li 4Ti 5O 12.Developments of fabrication tech-niques of micro-battery using ink-jet printing technology are in progress in our group.

K.Dokko et al./Electrochimica Acta51(2005)966–971971

Acknowledgment

This work was partially supported by a Grant-in-Aid for Scienti?c Research(A)(Grant no.14205097)from the Japan Society for the Promotion of Science.

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