CA2130807A1

CA2130807A1 - Electrode and secondary battery using the same

Info

Publication number: CA2130807A1
Application number: CA002130807A
Authority: CA
Inventors: Jun Tsukamoto; Takeji Nakae; Tatsuhiko Suzuki; Mikio Nii; Masayuki Kidai
Original assignee: Individual
Current assignee: Toray Industries Inc
Priority date: 1992-12-25
Filing date: 1993-12-24
Publication date: 1994-06-26
Also published as: KR950700612A; WO1994015373A1

Abstract

ABSTRACT

The invention relates to an electrode suitable for a chargeable/dischargeable secondary battery in which a carbonaceous material capable of doping and dedoping of lithium ions is used as a negative electrode active material wherein carbon fibers are used as the carbonaceous material in a form of an uni-directionally arranged body or in combination of electrically conductive foil or fibers, and further relates to a secondary battery using the electrode. The invention enables to provide a secondary battery having high capacitance and high outputtng property.

Description

-- ~ILE P~N 1 ~ TR A N~ LAT I O N f~ r~

DESCRIPTION
ELECTRODE AND SECONDARY BATTERY USING THE SAME

Technlcal Fleld The present lnventlon relates to an electrode uslng carbon f-bers and a chargeable/dlschargeable secondary battery uslng the same.

Background Art In recent years, small secondary batteries havlng high capacltance have been remarkably demanded wlth the spread of portable devlces such as vldeo cameras and notebook-type personal computers. Most of the secondary batterles curren~iy used are nlckeL-cadmlum batterles whlch use alkallne electrolytic solutions. Such secondary batterles, however, show low ~attery voltages of about 1.2 V, and therefore are difflcult to be lmproved ln energy denslty. Under these clrcumstances, it has been :Lnvestlgated the hlgh energy-type secondary batterles uslng llthlum metal, whlch ls the basest metal, for negatlve electrode.
However, the secondary batterles ln whlch llthlum metal ~ -~
ls used for negatlve electrode have disadvantages such that the llthlum develops to dendrltes by the (re)charglng/discharglng cycle~ whlch may cause a short clrcult and further cause the danger of lgnltlon of the batterles. In addltlon, as llthlum me~al used ln a secondary battery ls very actlve, such a battery ltself lnvolves hlghly dangerous factors. Therefore, they are quest:Lonable ln domestlc appllcablllty. In order to solve the problems relatlng to safety descrlbed above, llthlum lon secondary batterles uslng varlous carbonaceous materials have been proposed recently, by whlch hlgh energy lnherent to the - 2 - ~ 8~7 llthlum electrode can be ylven. The secondary batterles of thls type are devlsed by utlllzlng the phenomenon that, slnce the carbonaceous materlal doped wlth llthlum lons at charglng comes to have the same electrlc potentlal as metal llthlum, the carbonaceous materlal doped wlth llthlum lons can be used for negatlve electrode in place of metal lithlum. In thls type of secondary battery, when dlscharged, the llthlum lons whlch have been doped to the carbonaceous material are dedoped from the negative electrode and go back to the carbonaceous materlal to whlch the llthlum lons have been doped orlginally. Therefore, the use of carbonaceous materlal doped wlth llthlum lons for negatlve electrode never causes the problem of dendrlte productlon, and furthermore glves excellent safety slnce metal llthlum ls not present; therefore has now been investlgated extenslvely.
As the secondary batterles utlllzlng the doplng of llthium lons to carbonaceous materlal, those have been known, for example, dlsclosed in Japanese Patent Appllcatlor. ~ald-open Nos.
90861/1987 and 122066/1987. The carbonaceous materlals used ln the references above are generally ln a form of powder, and therefore ls requlred to be lncorporated wlth a polymer as a blnder such as Teflon and poly(vlnylldene fluorlde) for moldlng lnto an electrode. That ls, an electrode can be prepared ln the manne!r that a powdery carbonaceous material ls mlxsd wlth a blnder and then adhered to a metal mesh, or applled on a metal foll as a slurry. On the contrary, as for carbon flbers, there has been no precedent ln whlch carbon flbers are practlcally used for electrodes of secondary batterles lndustrlally. Therefore, the form or structure of electrode to be preferably employed or the preparatlon technlque of such electrode has been quite - 3 _ ~13 08 Q7 unkn~wn. In particular, the most serious technical problems are how to shape carbon fibers into an electrode, how to take the electrica:L contact of the carbon fiber with a current collector, how to so.Lve a problem of electrlcal short circuit between a posltlve electrode and a negatlve electrode caused by the penetration of fluffs of the carbon fibers through a separator, and so on However, when carbon fibers are used in a form of non-woven fabric or woven fabric, the electrode can be prepared wlthout or, lf any, a trace amount of a binder. In addition, it is recognlzed that the use of carbon fibers for electrode is excellent with respect to chemical stability against electrolytes, structural stability against volume expansion caused by doping, cyclicity of (re)charging and discharging, and so on. As the secondary batteries using such electrodes, those have been known, for example, disclosed in Japanese Patent Application Lald-open Nos. 54181/1985 and 103991/1987. The electrode~; using carbon flbers as descrlbed above, however, have a defect that the electrlcal connectlon with a metal, l.e. a taklng-out. electrode~ becomes difficult. In case of a carbon powder electrode, since the carbon powder is mlxed with a binder and then adhered to a metal mesh or applied on a metal foil as a slurry as descrlbed above, the metal mesh or metal foll can be used as a collectlng electrode for the connection wlth a termlnal. On the contrary, in case of a carbon flber electrode, lt has been trled to lnsert the ends of the carbon flbers lnto a mesh-~haped or foll-shaped current collectlng metal electrode to be fixed. However, the carbon flber tend to become lnto pleces and to be broken easily, and consequently the use of the carbon flbers ls stlll dlsadvantageous ln workablllty in preparatlon of ~ 4 ~ 21~8~7 electrode, as well as mechanical strength and durablllty of the resul~ing electrode. Further, such problem also occurs that the fluffs, i.e. broken flbers, penetrate through a separator to cause the electrlcal contact between a positlve electrode and a negatlve electrode, whlch resultlng ln an lnternal short clrcult.
Furthermc,re, such problem occurs that, slnce the carbon flbers are merely~ lnserted lnto the current collecting metal electrode, the voltaqe to be applled to the carbon flbers dlffers from that applled to the carrent collecting metal electrode due ~o the contact reslstance of the carbon flbers. Thls can be detected by the phenomenon that the voltage returns lmmedlately to the lnltlal state when the appllcatlon of voltage ls stepped, ln other worcls, the lncrease in so-called overvoltage, and so on.
Stlll furt.her, such problem occurs that, when the surface area of the electrode ls lncreased, the dlfference ln potentlal at the polnts far from the current collectlng metal electrode becomes large due to the reslstance of the carbon flbers and, as the result, unlform doping and dedoping hardly occurs.

Brlef Descrlp_ on of Drawings FiLg. l ls a schematlc lllustratlon of an embodlment of the electrode according to the present lnvention, ln which an electroconductlng foll is applled to a carbon flber sheet.
FiLg. 2 ls a schematlc lllustratlon of another embodlment of the electrode accordlng to the present lnventlon, ln which an electroconductlng foil ls applled to a carbon flber sheet.
F:Lg. 3 ls a schematlc lllustratlon of another embodlment of the electrode accordlng to the present lnventlon, ln which electroconductlng wlres are arxanged ln carbon flbers ln the parallel dlrectlon ~o the flbers.

. .

- 5 - ~13~8~7 Flg. 4 is a schematlc illustration of another embodimen~
of the el,ectrode according to the present invention, in which the carbon fllbers are woven in networks of electroconductlng wlres.
In Flgs. 1 to 4, 1 stands for a carbon flber, 2 for a electroconductlng wlre, 3 for the dlrection of the taklng-out elect.rode, 4 for a carbon flber sheet, and 5 for an electroconductlng wlre, respectlvely.

Best Mode for Carrylng Out of the Invention In order to solve the problems descrlbed above, the present inventlon ls constructed as the followlngs.
That ls, the first lnventlon of the present appllcatlon ls to provlde an electrode comprlslng an unl-dlrectionally arranged body of carbon flbers and a secondary battery uslng the same. In the present inventlon, the form ln which the carbon flbers are arranged ln an unl-axlal dlrectlon can glve excellent packed density and handllng property of the carbon flbers. In thls case, lt ls preferable that the carbon fibers are placed ~
unlformly. If there exlst some uneven areas in the carbon flber arrangement, unlform doplng can sometlmes not be glven.
The electrode accordlng to the second lnventlon of the present appllcatlon ls characterlzed by comprlsing a carbon flber sheet and a foll or wlres havlng electrlcal conductlvlty.
The practlcal modes prefera~ly employed in the present lnventlon are lllustrated concretely ln the followings wlth reference to the drawlngs attached.
Flg. 1 lllustrates an embodlment of the electrode of the present lnventlon, ln whlch carbon flbers are arranged ln an unl-axlal dlrectlon and an electroconductlng foll ls applled thereto.
Flg 3 lllustrates another embodlment of the elec.rode of the - 6 - ~13 ~8a7 present invention, in whlch carbon flbers are arranged ln an uni-axlal dlrectlon and electroconducting wlres are also arranged ln the same dlrectlon as the carbon fibers. In Flg. 3, 1 stands for carbon flbers, 2 for electroconductlng wlres represented by metal flbers, and 3 for the dlrection connectlng wlth the taklng-out electrode, respectlvely. In the case where the carbon flbers are arranged iLn an unl-axlal direction as shown ln these drawlngs, by arranglng the electroconducting wlres ln the dlrec~lon vertlcal to the dlrectlon of the carbon flbers so that the carbon flbers are bound wlth the electroconductlng wlres, the carbon flbers come to be flxed to some extent; whlch ls the more preferable mode for practlce. In additlon, as shown ln Fig. ~, by weavlng the carbon flbers arranged ln an unl-axlal dlrectlon lnto the networks of electroconductlng wlre, not only the carbon flbers can be prevented from becomlng lnto pleces, but also the e~ectrlcal collectlon efflclency becomes well.
In order to lmprove the electrlcal conductlvlty wlth the carbon flbers, a method ln whlch a sheet-shaped carbon fibers arranged in an unl-axlal dlrectlon are closely adhered on an electroconductlng foll represented by a metal foll ls preferably employed. Thls method can be carrled out, for example, by adherlng a part or all of the carbon flbers on a metal foll under compression by means of roll press and the llke, or adherlng the carbon flbers on a metal foll uslng a small amount of a resln such as Teflon and poly(vlnylldene fluorlde) as a blnder.
In case where the electrode ls rolled up, The directlon of the carbon flbers to be arranged ls preferably approxlmately vertlcal agalnst the rolled dlrectlon. Thls ls because that, this arrangement can prevent the loosenlng of the carbon flbers placed lnslde of the metal foll and can make the carbon flbers to - 7 - ~ 8 07 be hardly broken when the electrode ls rolled up. Furthermore, by such arrangement, there can also be prevented the penetration of the broken carbon fiber edges through a separator or the sticking of the broken carbon fiber edges out of the boLh ends of the electrode by moving ln zlgzag directlons. The penetratlon through a separator and stlcklng out of the both ends of the electrode of the broken carbon flber edges are undesirable slnce they may cause the electrlcal short clrcuit with the positive electrode.
As described above, the electrode in which the carbon fibers are integrated with the metal foil enables to lower the contact resistance of the carbon fibers and to make the distance between the metal collector and the carbon fibers 4horter and, as the result, more uniform potential in the carbon flbers can be giverl. Therefore, the decrease in capacitance caused by overvoltage resulting from the contact reslstance and the non-unifc)rm doping caused by the non-uniform potential in the carbon fibers ca:n also be prevented.
T:he weight of the carbon fibers to be arranged ln an uni-axia]. direction is.preferably not smaller than 30 g/m2 and not larger than 200 g/m2, and more preferably not smaller than 50 g/m2 and not more larger than 150 g/m2. When the weight is too large, thle carbon fiber sheet itself becomes thick and the reslEItanc,e of the thickness dlrection becomes hlgk whlch results ln oc:caslonal non-unlform doping and dlfficulty in use of the resu].ting electrode at hlgh output current. On the other hand, when the weight is too small, the amount ratio of the carbon fibers, l.e. actlve material, based on the whole amount of the resul.ting negative electrode becomes small, which results in decre!ase .in the amount of the carbon fibers to be packed ln a ~:

battery and reductlon in energy density of the battery.
The carbon fiber ~o be used in the present lnventlon is not partlcularly llmited, but a fllament prepared by flrlng an organlc substance is generally used. Speclflc examples of such carbon fiber lnclude a PAN-based carbon flber prepared from polyacrylonltrlle (PAN), a pltch-based carbon flber prepared from pltch of coal, petroleum or ~he like, a cellulose-based carbon flber preparea from cellulose and a vapor phafie grown carbon flber prepared from gas of a low molecular organlc material. In addltlon, other carbon flbers prepared by flring poly(vinyl alcohol), lignin, poly(vinyl chloride), a polyamlde, a polyimide, a phenol resin, furfuryl alcohol and so on can also be employed.
The carbon flber to be used ls suitably selected f-om those llsted above dependlng on the lntended propertles vf the resultlng electrode or battery.
Among the carbon flbers llsted above, when used for a negatlve electrode of a secondary battery ln whlch a nonaqueous electrolyt.lc solutlon contalnlng an alkall metal salt ls used, preferably employed are a PAN-based carbon flber, a pltch-based carbon flber and a vapor phase grown carbon fiber. In partlcular, from the vlewpolnt of a good doplng property wlth llthium lons, a PAN-based carbon flber ls most preferable.
In the present lnventlon, the carbon flbers obtained by firlng may~ be sub~ected to any subsequent treatment and any type of carbon flber may be employed, so long as lt retalns a form of carbon flber. In partlcular, the carbon flber which ls sub~ected to the chalrglng/dlscharglng treatment ln an electrolytlc solutlon prlor to incorporatlng lnto a battery are effectlvely used slnce lt can recluce the lnltlal capaclty loss (l.e. retentlon) lnherent to a carbonaceous materlal. The lnltlal capaclty loss results ~13080'7 from the phenomenon that a part of dopants (e.g. llthlum ion) whlch are doped durlng the lnitial charging step remalns in the carbonaceous material and the residue i5 not dedoped in the subsequent. discharging step. In order to lmprove the capacity of a secondary battery, it is effective to reduce the initlal capaclty ].oss. The carbon fiber ltself has an electrlcal conductivity and is a continuous material, and therefore suitable for the prevlous charging/discharging treatment. Speclflc example of such prevlous treatment method ls that ln whlch the carbon flber ls doped or dedoped in an electrolytlc solu~lon contalning lithlum ions.
In the second invention of the present application, the form or shape of the carbon flber sheet ls not partlcularly llmlted, but is preferably a sheet-shaped structural in which the carbon fibers are arranged in an uni-axial direction. In the cloth-type or felt-type carbon fiber sheet, any form may be employed such as woven fabric, knit fabric, plalted fabric, lace, mesh, felt., paper, non-woven fabrlc and mat.
The dlameter of the carbon flber to be used ln the present invention should be determined so that the carbon flbers can be prepared ln the form as descrlbed above, and preferably l to 500 am. more preferably 3 to lO am. It is also preferable to use several klnds of carbon flbers havlng different diameters indlvldua]Lly.
A~; the metal to be used as an electrically conductive foil and wire, there can be employed gold, silver, copper, platinum, rhodium, aluminum, iron, nickel, chromium, manganese, lead, zinc, tungsten, titanium, and so on. In addition, alloys of the me1als listed above can also be employed, such as stainless steel. These metals may be coated of their surfaces 21308Q~I
wlth varlous substances so long as they are impalred of thelr electrica:L conductlvlty. The metal or coated one thereof ls made lnto a fo:Ll or a wlre, and then arranged wlth the carbon flbers ln the fo:rms shown ln the drawlngs. In case of metal foll, a thin foll ls preferably used, slnce the thlck metal foll causes to decrease ln the amount of the actlve material to be stored ln a battery. The thlckness of the metal foll ls preferably about 5 to 100 ~m. In partlcular, from the viewpolnts of electrlc reslstanc,s and thlckness and cost of the metal foll to be used, copper foil ls preferably employed. On the other hand, ln case of metal wlre, the dlameter should be determlned depending on propertles, dlameter and shape of the carbon flber used, so that the c:urrent collectlng effect ls enhanced or the carbon flbers are bundlled easlly, but ls preferably about 1 to 200 ~m, more preferably 5 to 100 ~m. In order to lncrease the lntenslty of bundllng of carbon flbers, lt ls preferable to bundle several flne metal wires ln a twlsted form.
The ratlo between the carbon flbers and the electroconductlng wlres ln an electrode of the present lnvention should be determlned sultably taklng into conslderatlon of the propertles and current collecting efficlency of the resulting electrode and so on. However, the ratlo of the electroconducting wlre~3 to the carbon fibers of the resulting electrode is preferably 1 to 10~ by weight and 0.2 to 2~ by volume, and more preferably 2 to 8~ by weight and 0.4 to 2~ by volume.
In the flbrous and cloth-shaped carbon flbers, the part:Lal breaking of the single flber ln the carbon flber bundle, l.e. fuzzlng, tends to occur. The fluffs sometlmes penetrate through a separator to contact wlth a posltlve electrode, whlch causes the internal short clrcult. In order to prevent thls ~130g~
defect, 11 is effectively carried out to paste and coat a part or all of carbon fibers with a resin. The resin to be used is not particularly limited, and a conventlonal thermoplastlc or thermoset1ing resin can be employed. In particular, preferably used are il fluororesln, an olefln resin, an epoxy resin, an urethane :resln, an acryl resln and a polyester resln slngly or in comblnatlon thereof, and a modified one thereof.
The method for pasting and coating carbon *ibers with a resin is ~lOt particularly limited. However, it is preferable to paste and coat carbon fibers by passing the carbon fibers through a polymer solution or emulsion vessel, or by spraying the solution or emulsion thereon. When the amount of the polymer to be cc,ated on the carbon fibers is too small, the fuzzing of the carbon flbers can not be depressed sufficiently. On the other hand, when the amount of the polymer is too large, the function of the carbon fibers themselves as active material tends to be reduced.
From these reasons, the amount of the polymer used for coating oE carbon fibers is preferably not less than 0.1 part by welght and not more than 15 parts by weight based on 100 parts by welght of the carbon flbers. When the amount is less than O.l part by w,elght, the fuzzing can not be prevented sufficiently.
On the ot~her hand, when the amount is over 15 parts by welght, the electrlcal propertles of the carbon flbers as negatlve electrode actlve materlal ls affected. In partlcuiar, when the dlscharglmg current becomes over 500 mA per l g of the negative elect.rode actlve materlal, the lnitial discharge capacity tends to be resuced to 70~ of that given when the carbon fibers are uncoclted.
From these reasons, the coating amount of a polymer is 213080 ~
most preferably 0.5 to 10 parts by welght, and partlcularly 0.S
to 8 parts by welght. In the method for coatlng a polymer on carbon flbers, ln the case where the polymer ls solved ln an water soluble organlc solvent such as N-methylpyrrolldone, lt ls more effectlve to preclpltate the polymer by wet solidlflcatlon ln wa~er cr a mlxed solutlon of an organlc solvent and water.
The separator to be used ln the present lnvention ls not partlcularly limlted, and a commerclally avallable product can also be emlployed, so long as lt ls a porous fllm, a woven fabrlc, a non-woven fabrlc and so on havlng lnsulatlng property, such as that made of polyolefln, polypropylene, polytetrafluoroethylene, polyethylene and polyacetal. The fllm thlckness o~ the separator ls prefera.bly not larger than 200 ~m, and more pre~erably not larger tha.n 50 ~m, for the purpose of reduclng the lnternal reslstance of the resultlng battery. More speclflcally, "
Cellguard" (a trade name produced by Dalcel kabushlkl Kaisha) and "Hlghpore" (a trade name produced by Asahl Kàsel Kogyo Kabushikl Kalsha) are preferably used.
As. the materlal used for a posltlve electrode as a const.ltuen.t of a secondary battery, a carbon flber can be used.
In ad~ltlon, there can also be used artlflcial or natural graph.lte, carbon fluorlde, an lnorganic compound such as a metal and a meta.l oxlde, and an organic hlgh molecular compound. When an lnorgan,lc compound such as a metal and a metal oxlde is used for a posltlve electrode, the charglng/dlscharglng reaction occuris utl.lizing the phenomenon of doping and dedoplng of catloms. On the other hand, when an organlc hlgh molecular compound is used for a posltlve electrode, the charg.lng/d!lscharglng reactlon occurs utlllzlng the phenomenon of doplng and~ dedoplng of anlons. Thus, the charglng~dlschargi;ng - 13 - 2130~ 0 ~

react.lon ~akes various manners according to the kinds of the subst.ances employed, and is suitable selected accordlng to the lntended propertles of the posltlve electrode of the resultlng battery.
Speclflc examples of the positive electrode material include lnorganlc compounds such as oxides and chalcogenldes of transltlon metals lnvolving alkall metals; con~ug~ted polymers .
such as polyacetylene, poly(para-phenylene), poly(phenylene :
vinyl.ene), polyaniline, polypyrrole and polythiophene; bridged polymers havlng dlsulfide bond(s); thionyl chlorlde; and so on;
whlch are compounds used in conventional secondary batterles.
Among thel3e, in case of a secondary battery using a nonaqueous elect.rolyLic solution containing lithlum ions, an ~xides or chalc:ogen:lde of a transltion metal such as cobalt, manganese, molybdenum, vanadlum, chromlum, lron, copper or tltanlu~ is prefe!rabl~y used. In partlcular, compounds LlCoO2 and LlN102 are most preferable slnce they exhlblt hlgh voltage and large energy densi.ty.
The electrolytlc solutlon to be used for the secondary battery ln whlch the electrode of the present invention is used 18 not pa:rtlcularly limlted, and a conventional one ls employed such as an acldlc or alkaline aqueous solutlon or non-aqueous solvont. In partlcular, as an electrolytlc solution for a secondary battery uslng a non-aqueous electrolytlc solutlon contalnlng an alkall metal salt llsted above, there are preferably used propylene carbonate, ethylene carbonate, 1-buty~olactone, N-methylpyrrolldone, acetonltrlle, N,N-dlmet.hylformamlde, dlmethylsulfoxlde, tetrahydrofuran, 1,3-dloxolane, methyl formate, sulfolane, oxazollne, thinyl chlorlde, 1,2-climetlhoxyethane, diethylene carbonate, derlvatlves thereof 2130813'`~
and mixtures of two or more of them. As the electrolyte to be contained in the electrolytlc solutlon, are preferably employed halldes of alkall metals, especlally of llthlum, perchlorates, thlcyanates, borofluorides, phosphofluorldes, arsenofluorldes, alumlnofluorldes, trlfluoromethylsulfates, and so on.
For the appllcatlon to a secondary battery whlch uses a non-aqueous electrolytic solutlon containing an alkall metal salt, the electrode comprlslng carbon fibers of thç present lnventlon utlllzes a phenomenon of doping of catiors or anlons to carbon fibers. Therefore, this electrode can be used for both of negatlve and posltlve electrodes, and preferably for a negatlve electrode of a secondary battery. In partlcular, when catlons represented by lithlum lons are doped, the carbon ,ibers show excellent propertles as a negatlve electrode material for high energy-type battery whlch exhlblts high capaclty and base potentlal. In addltlon, slnce the carbon flbers are used in a flbrous form, they can make thelr contact resistance lower compared with carbon powder, and as the result hlgh current dlscharge becomes possible.
The secondary battery using the electrode of the present lnventlon can be applled to various portable small electronlc devlces such as video cameras, personal computers, word processors, radlos wlth cassette, portable telephones and so on, due to lt~l characterlstlcs of llghtwelght, hlgh capacltance and hlgh enerqy denslty.

Examples The present lnventlon wlll be lllustrated ln more detail wlth reference to the followlng examples; however, these examples are lntencled to be understood to llmit the scope of the present invention.

Example 1 12000 pieces of carbon fibers "TORAYCA T300" (a trade name produced by Toray Industries, Inc.) were bundled into a tow-shaped structural body, fixed its ends with an electroconductive copper paste. Five of the resulting tow-shaped structural bodles are arranged ln an unl-dlrectlon, and then adhered of their ends with copper foils, to glve a sheet of 20 mm ln long, 50 mm ln wide and about 0.3 mm in thlck. The welght of the resultlng carbon flber sheet was 230 mg.
For the determlnation of the capacitance of the unl-dlrectlonally arranged body of the carbon flbers for a secondary battery, a trlode-type cell was prepared uslng llthlum folls as a counter electrode and a reference electrode and uslng a solutlon ln whlch 1 M of llthlum perchlorate had been dlssolved ln propylene carbonate as an electrolytlc solutlon. The resultlng cell was charged untll the voltage reached to 0 V at a constant current of 20 mA, and after restlng for 20 minutes, was dlschargeld untll the voltage reached to 1.5 V at a constant current of 20 mA to determlne the discharge capaclty. As the resu].t, the capacltance of the cell per welght was 304 mAh/g, and lt was proved that thLs method could glve a hlgh dlscharge capac:lty.

le 2 (1) Preparatlon of electrode On a copper foll of 15 ~m in thick, commercially available PAN-based carbon fibers "TORAYCA T-300" (a trade name produced by Toray Industrles, Inc.) were placed ln an unl-axlal 21308~7 directlon unlformly, to glve an electrode comprlslng a copper foll and carbon fibers, ln whlch the welght of the carbon flbers was 100 g,/m2.
(2) Prepara~lon of posltive electrode Commercially available lithium carbonate (Li2C03) and basic cobi~lt carbonate (2CoC03~Co(OH)2) were weighed so that the molar ratlo of these components became Li/Co = 1/1, and then mixedL wlth each other using a ball mill. The resulting mixture was treated by hea~ing at 900~ for 20 hours, to give LiCoO2. The resulting LiCoO2 was crushed using a ball mill. A slurry for a positive electrode was prepared by mixing the LiCoO2, artificial graph,lte as an electroconducting material, poly(vinylidene fluoride) (PVDF) as a binder and N-methylpyrrolidone as a solvent in a miximg ratlo of LlCoO2/ artiflcial graphite/ PVDF - 80/lS/5 by welght. The resultlng slurry was applled on an aluminum foil, drled and pressed; whereby a positive electrode was given.
(3) Preparation of battery Two kinds of electrodes prepared in steps (1) and (2) above!, respectively, were superposed upon each other wlth lnterposing a separator of a porous polypropylene fllm ("Cellguard #2500"; a trade name by Daicel Kagaku Kabushiki Xalsha) therebetween, and then rolled up, to give a cylindrical electrode body. The resul~ing electrode body was immersed ln~o a beake~r cell ln which an electrolytlc solution of propylene carbonate containing 1 M oE lithium perchlorate was put.
Termlnals were taken out from the copper foil and the aluminum foll, respectlvely; thus a secondary battery was prepared.
~4) Evaluation of battery The secondary battery thus prepared was evaluated for its charging property. That is, the secondary battery was charged - 17 - ~13~8Q7 un~ he voltage reached to 4.3 V at a constant current of 40 mA/g as the current density per weight of the carbon fibers, and was discharged. The dlscharge capacity of the secondary battery, whlch was determlned from the charge amount given by the dlschargln,g, was 320 mAh/g per weight of the carbon flbers used ln thls battery.

Example 3 As the carbon flbers, commerclally avallable PAN-based carbon flbers ("TORAYCA T-300"; a trade name produced by Toray Industrles, Inc.; 3K; 3000 flbers) were used. As the polymer for pastlng and coatlng of the carbon fibers, a commerclally avallable poly(vlnylidene fluoride) resln ("Neoflon VP-850"; a trade name produced by Dalkln Kagaku Kabushlki Kaisha) was used by dlssolvlng ln N-methyl-2-pyrrolldone.
The carbon flbers were lmmersed in the PVDF solu~ion, and then further lmmersed ln a solutlon havlng a compositlon of water: N-rnethyl-2-pyrrolldone = 1:1 (by welght) to solldlfy the polym,er. The resultant was dried at 150~ for 1 hour, to give carbon flbers pasted with the polymer. The PVDF polymer-adhered carbon flbers thus prepared had an adhesion amount of the polymer of 5~, by welght based on the welght of the carbon flbers and an avera,ge pore dlameter measured uslng a SEM photograph of about 15 ~m.
In order to examine the lnfluence of fluffs of the carbon flbers pasted and coated with the polymer agalnst a separator, the c:arbon flbers ls lnserted lnto polypropylene porous fllm ("Cellguard"; a trade name produced by Dalcel Kayaku Kabushlkl Kalsha) to be flxed and then wound up around a sta~nless steel - 18 ~ '~i30~'7 rod. The resultant was applied with a llne pressure of 2 kg/cm for 10 minutes to observe whether the carbon flbers penetrated through a separator or not. As the result, no penetration of carbon flbers was not observed.
Accordlng to the same method as Example 2, a trlode-type beaker cell was prepared uslng these carbon fibers woven into networks of nlckel fine wires as a working electrode, metal lithi.um as a counter electrode and a reference elec~rode, and 1 M-LlClC)4/propylene carbonate as an electrolytic solution. The resultlng cell was charged (i.e. doped with lithium ions) until the voltage reached to 0 V (vs Li+/Li) for the reference electrode at a constant current of 100 mM/g based on the weight of the carbon fibers, and after resting for 20 minutes, dlscharged (l.e. dedoped) under the same condltion until the voltage reached to 1.5 V (vs Lil/Li); thus the cell was charged and discharged to determine the discharge capacity.
As the result, the discharge capacity was 350 mAh/g, whlch was the same vale as that given when the carbon flbers were not pasted or coated with PVDF polymer. Accordingly, the reduction in discharge capacity caused by the pasting and coating wlth a polymer was not recognized.
On the other hand, when the carbon fibers were used as they were wlthout pasting or coating wlth a polymer, the dlscharge capacity given was 351 mAh/g. However, when the penetration of the carbon fibers through a separator was exam:Lned, 10 to 15 of penetrating ponts by the carbon flbers were obse:rved on the separator.

Example 4 (1) Preparation of electrode .

2() mg of commerclally avallable PAN-based carbon flbers "
TGRAYCA T--300" ~a trade name produced by Toray Industries, Inc.) were arranged ln an unl-axlal directlon, and woven thelr ends wlth nlckel flne wires ~diameter: l00 am) in the direction vertlcal 1:o the arranged direction of the carbon flbers, and bundled as shown in Fig. 3; thus an electrode was prepared. In the resul1:ing electrode, the welght ratio between ~he carbon fibers and the metal fine wires was l00:l.
(2) Evaluatlon of charging property The evaluation of charging property was carried out using the elect:rode prepared above. In this evaluation, a trlode-type liquid cell was used in which propylene carbonate containing l M
llthlum perchlorate was used as an electrolytic sclution and metal llthium foils were used as a counter electrode and a reference electrode, respectlvely, and the liquld cell was charged untll the voltage reached to 0 V (vs. Li /Li) at a constant current of 40 mA/g-as the current density per welght of the carbon flbers. As the result, the return of the voltage, l.e. overvoltage, glven after restlng for 20 mlnutes was l0 mV.

Example 5 (l) Preparatlon of electrode 20 mg of commercially avallable PAN-based carbon flbers "
TORAS'CA T-300" (a trade name produced by Toray Ind-1strles, Inc.) were arranged ln an unl-axlal dlrectlon, and woven thelr ends with nlckel fine wlres (dlameter: l00 ~m) ln a network form as shown ln Flg. 4; thus an electrode was prepared. In the resultlng electrode, the welght ratio between the carbon fibers and the metal flne wlres was l00:l.
(2) Evaluation of charglng property 2~30807 The evaluatlon of charglng property was carrled out uslng the electrode prepared above ln the same manner as Example 1. As the result., the overvoltage given after charglng was 0.5 mV.

Example 6 A coin-shaped secondary battery was prepared uslng the carbon flbers of Example 3, whlch had been pasted and coated with a PVDF polymer, as a negatlve electrode, the LiCoO / artificlal graphlte/ PVDF of Example 2 as a posltlve electrode; ln a manner that the posltlve electrode and the negative electrode were superposecl upon each other with lnterposlng a separator. In thls secondary battery, 1 M-LlClO4/propylene carbonate ~as used as an electrolyt.lc solutlon.
The charglng/dlscharglng test was carrled out using 100 pleces of the coln-shaped secondary batteries prepared above. As the result., no defectlve such as short clrcult was not observed, and all of the secondary batteries tested were operated normally.

Example 7 (1) Preparatlon of carbon fiber electrode and charglng/cllscharging thereof A beaker cell was prepared uslng commerclally avallable PAN-based carbon flbers "TORAYCA M40" (a trade name produced by Toray Industrles, Inc.) bundled wlth ~1 wlre of a current collector as a worklng electrode, and uslng metal llthlum as a count.er electrode and a reference electrode, and uslng 1 M-LlClO4/propylene carbonate as an electrolytlc solutlon.
In the resultlng cell, llthlum lons were doped untll the voltage re!ached to 0 V (vs. Ll /Ll) to the reference electrode and then cledoped under the same condition until the voltage reached to 1.5 V (vs. Ll+/Ll); thus the charging and discharglng of the ce:Ll was completed.
(:2) Preparatlon of secondary battery and evaluatlon thereof The carbon fibers which had been charged and discharged prevlously ln the step (1) above were arranged on a Ni mesh. The resultant was superposed upon the positlve electrode prepared in the same manner as Example 4 with interposing a separator; thus a coln shaped cell was prepa:red. In thls cell propylene carbonate contalnlng 1 M llthium perchlorate was used as an electrolytic solution. When this cell was charged and discharged the Coulomb s efficiency given was 96~. In this cell by the previous charging and discharging the inltlal volume loss was reduced from the value of 30 mAh/g glven when no treatment was carrled out to the value 5 mAh/g.

Industrlal Applicabllity As descrlbed above the electrode of the present lnven,tlon comprlses an unl~directlonally arranged body of carbon flbers or one comprlslng the carbon fibers and electrlcally conductlve foll or wlres. By using the electrode for a chargeable/dlschargeable secondary battery ln which a carbonaceous materlal capable of doplng and dedoplng of llthlum ions is u~3ed as a negatlve electrode actlve materlal there can be provlded a secondary battery havlng hlgh capacltance and hlgh outputtlng property.

Claims

1. An electrode which comprises an uni-directionally arranged structure of carbon fibers.

2. The electrode according to Claim 1, wherein the uni-directionally arranged structure has a sheet form.

3. The electrode according to Claim 1, wherein the uni-directionally arranged structure is placed on a metal foil.

4. The electrode according to Claim 3, which is rolled up and the arranged direction of the carbon fibers is approximately vertical against the rolled direction of the electrode.

5. The electrode according to Claim 3, which is rolled up and the arranged direction of the carbon fibers is approximately parallel to the rolled direction of the electrode.

6. The electrode according to Claim 3, wherein the metal foil is a copper foil.

7. The electrode according Claim 1 or 2, wherein the carbon fibers are arranged in an uni-axial direction and electroconducting wires are further arranged in the carbon fibers in a parallel or vertical direction against the axial direction of the fibers.

8. The electrode according to any of Claims 1 to 7, wherein the carbon fibers are pasted and coated with a resin.

9. The electrode according to Claim 8, wherein the resin is a thermoplastic resin.

10. The electrode according to Claim 8, wherein the resin is a thermosetting resin.

11. The electrode according to any of Claims 8 to 10, wherein the amount of the resin used is not smaller than 3% by weight and not larger than 17% by weight based on the amount of the carbon fibers.

12. The electrode according to any of Claims 8 to 11, wherein the amount of the resin used is not smaller than 5% by weight and not larger than 10% by weight based on the amount of the carbon fibers.

13. The electrode according to any of Claims 8 to 12, wherein the resin is poly(vinylidene fluoride).

14. The electrode according to any of Claims 1 to 13, wherein the carbon fibers which have been previously charged and discharged are used as active material.

15. The electrode according to any of Claims 1 to 14, which is used as a negative electrode.

16. A secondary battery which uses the electrode according to any of Claims 1 to 15.

17. The secondary battery according to Claim 16, which uses a non-aqueous electrolytic solution containing lithium salt and a positive electrode capable of taking in and out of lithium.

18. The secondary battery according to Claim 16 or 17, wherein a transition metal oxide is used for a positive electrode.

19. The secondary battery according to any of Claims 16 to 18, wherein the transition metal oxide is LiCoO2 or LiNiO2.

20. An electrode which comprises a carbon fiber sheet and an electrically conductive foil.

21. The electrode according to Claim 20, wherein the carbon fiber sheet is a woven fabric.

22. The electrode according to Claim 20 or 21, wherein the electrically conductive foil is a metal foil.

23. The electrode according to Claim 22, wherein the metal foil is a copper foil.

24. The electrode according to any of Claims 20 to 23, wherein the carbon fibers have the weight of not less than 30 g/m2 and not more than 200 g/m2.

25. The electrode according to any of Claims 20 to 24, wherein the carbon fibers have the weight of not smaller than 50 g/m2 and not larger than 150 g/m2.

26. The electrode according to any of Claims 20 to 25, wherein the carbon fibers are pasted and coated with a resin.

27. The electrode according to Claim 26, wherein the resin used for pasting and coating of the carbon fibers is in an amount of not smaller than 3% by weight and not larger than 17%
by weight based on the amount of the carbon fibers.

28. The electrode according to Claim 26 or 27, wherein the resin is poly(vinylidene fluoride).

29. A secondary battery which uses the electrode according to any of Claims 20 to 28.

30. The secondary battery according to Claim 29, which uses a non-aqueous electrolytic solution containing lithium salt and a positive electrode capable of taking in and out of lithium.

31. The secondary battery according to Claim 29 or 30, wherein a transition metal oxide is used for a positive electrode.

32. The secondary battery according to any of Claims 29 to 31, wherein the transition metal oxide is LiCoO2 or LiNiO2.