CA2265075A1

CA2265075A1 - Lithium secondary battery

Info

Publication number: CA2265075A1
Application number: CA002265075A
Authority: CA
Inventors: Hiroshi Nemoto
Original assignee: NGK Insulators Ltd
Current assignee: NGK Insulators Ltd
Priority date: 1998-03-11
Filing date: 1999-03-08
Publication date: 1999-09-11
Also published as: US6632565B2; US20020061439A1; EP0942485A1; JPH11260416A

Abstract

A lithium secondary battery uses an organic electrolyte solution and includes a battery case, an internal electrode body contained in a battery case and including a positive electrode, a negative electrode and a separator made of porous polymer. The positive electrode and the negative electrode are wound or laminated so that the positive electrode and negative electrode are not brought into direct contact with each other via the separator. A zeolite having a moisture absorption characteristic, has been incorporated in the battery case so that the zeolite is brought into contact with the organic electrolyte solution within the battery case. The lithium secondary battery achieves suppression of deterioration of a charge-discharge cycle characteristic of a battery caused by decomposition of an electrolyte by limiting moisture mixed into an organic electrolyte solution to a considerably lower level as well as improvement of its self-discharge characteristic.

Description

ï»¿101520CA 02265075 1999-03-09Title of the InventionLI THIUM SECONDARY BATTERYBackground of the Invention and Related Art StatementThe present invention relates to a lithium secondarybattery which achieves suppression of deterioration in thecharge-discharge cycle characteristic of a battery caused.bydecompositMn1ofanelectrolytebyljndtingthemoisturendxedin an organic electrolyte solution to a considerably lowerlevel as well as to improvement of its selfâdischargecharacteristic.In recent years, realization of practical use of lithiumsecondary batteries is being planned, as secondary batterieswith a large energy density, and which can be used as a powersource for electric equipment that is small, such as portablecommunication devices and.notebook-sizeclpersonal computers.Moreover, concerns about resource saving and energy savingare raised.in the background to international protection ofthe earth's environment, which is one of the reasons why thelithium secondary battery is expected to serve as a motor drivebattery for electric Vehicles and hybrid electric vehicles,which are under consideration for active introduction on themarket in the automobile industry, etc. Thus, it is eagerlydesired to put large capacity lithium secondary batteries,suitable for these uses into early practical use.ï»¿lO152025CA 02265075 1999-03-09In a lithium secondary battery, a lithium transitionmetal compound oxide or the like is used.as a positive activematerial, while a carbon material such as hard carbon orgraphiteisusedaseanegativeactivematerial. Uponcharging,lithium ions in the positive active material are transferredto and captured by the negative active material through anelectrolyte solution obtained by dissolving a lithiumelectrolyte ineanonaqueous organic solvent. In discharging,the reverse battery reaction occurs.Here, as an organic electrolyte solution, the carbonicacid ester family such as ethylene carbonate (EC), diethylecarbonate(DEC),ordimethylecarbonate(DMC),isnminlyused,while as an electrolyte, lithium fluoride complex compounds,particularly LiBFâ LiPFw LiAsFw LiSbFw etc. are used. Itis known that these electrolytes dissolve well into theaforementionedorganicsolvent,andshowrelativelyhighionicconductivity.However, the above-mentioned electrolytes are highlyhygroscopic, and there are those, like LiPFH which decomposedue to moisture absorption. In addition, these electrolytesarehandledcarefullyineadrynitrogenatmosphere,etc.sincemany of them do not dehydrate easily once they have beenmoisturized, even if it does not result in decomposition.Eveniftheelectrolytesarestrictlycontrolled,however,when. moisture exists in the electrolyte solution, thismoisture causes decomposition of electrolytes.For example,in the case where LiPF6 is adopted as an electrolyte, itsï»¿10152025CA 02265075 1999-03-09decomposition separates out HF (hydrogen fluoride) so thatHF affects the positive active material to elute a transitionmetalixxthepositiveactivenaterial. Thus,batterycapacitydecreases due to a chemical change in the positive activematerial, causing problems such as Â«deterioration (of thecharge-discharge cycle characteristic.The control of moisture contained in such.an electrolytesolutionrequiresnotonlyqualitycontrolbythenmnufacturer,etc., producing the electrolyte solutions but also strictcontrol at the site where batteries are assembled. Sinceother battery parts, e.g. the battery case, electrodes,electrode active material, etc., are usually handled underan air atmosphere prior to assembly, the moisture absorbedinto these parts will come out and mix with the electrolytesolution.Furthermore, the present inventors have obtained anexperimentalresultinwhichndxingofnwistureintotheinsideof aloatteryraffects the selfâdischarge characteristic:badly.Fig. 4, shows the selfâdischarge characteristic capturedaccording to changes in open circuit voltage in case wherethe experimental coin cells were formed under variousconditions using an electrolyte solution in which LiPE5wasdissolved in.a mixed solvent of EC and DEC. Having been leftalone after full charging, a battery D, which was formed andcharged inside a globe box replaced with and filled with drynitrogen, shows the least selfâdischarge, while aIbatteryZLwhidhwasformedinsideaaï¬mularglobebox,andwasthereafterï»¿1O152025CA 02265075 1999-03-09charged.inside a tight box containing a silica gel, proceedswith selfâdischarge a little bit faster than the battery D.In comparison with the above, a battery B, which wascharged in a tight box which was assembled in an air atmosphereandinumichsilicagelwasput,showajasteepvoltagedecreaseinabouthalfthetimeofthekmtteryI)orthebatteryA,spends,and in addition, a battery C, which was formed inside saidglobe box using an electrolyte solution where water drops wereintentionallyadded,andchargedwithinsaidtightbox,showeda steep voltage drop immediately after finishing the charge.It may thus be considered that the moisture within a batterygreatly affects the self-discharge characteristic.Therefore, there is a possibility that the admixture ofmoisture takes place not only from the materials with whichthe above-mentioned battery and each member are made, but alsofrom the ndxture of moisture inside a battery under theenvironment where a battery is being produced. Usually, toavoid such an event, the assembly of a battery is performedunder a dryznitrogen atmosphere, etc., resulting in, however,considerable cost for production facilities to producelarge-capacity large-sized lithiunlsecondaryrbatteries undersuch an atmosphere.Summary of the InventionThe present invention was achieved, considering theproblemsofthepriorartnentionedabove,thepurposeofmmichï»¿10152O25CA 02265075 1999-03-09istopmovidealithiumsecondarybatterythatremovesmoisturethat mixes easily within the battery and that has a goodcharge-discharge cycle characteristic and a self-dischargecharacteristic without requiring large-scale productionfacilities.That is, according to the present invention, a lithiumsecondary battery comprising a battery case, an internalelectrode body contained in the battery case and includinga positive electrode, a negative electrode and a separatorfilm made of porous polymer, the positive electrode and thenegative electrode being wound or laminated so that thepositiveelectrodeandnegativeelectrodeenxanotbroughtintodirect Contact with each other via the separator film, anorganic electrolyte solution contained in the battery case,and a zeolite having a moisture absorption characteristic,having been incorporated in the battery case so that thezeolite is brought into contact with the organic electrolytesolution within the battery case.In.a lithium.secondaryloattery of the present invention,itisrueferredthatthezeolitebeincorporatadinthebatterycase, using at least one of the following means, namely: (1)a means to dispose the zeolite to be contained in a bagpermeable to electrolyte solution inside the battery case,(2) a means to mix the zeolite with an electrode active materialstructuring the positive electrode and/or the negativeelectrode, (3) a.means to disperse the zeolite on the surfaceof the separator film, and (4) a means to make the zeoliteï»¿101520CA 02265075 1999-03-09into a fine powder and to disperse it by suspension in theelectrolyte solution. Thus, it is also preferred to use thesemeans together in plurality.Here, as the zeolite, the zeolite of an aluminosilicatefamily having at least a structure of the LTA type, FAU type,CHA.type, or MOR type, and.having anZÂ¥l/Si ratio in the zeoliteframe equal to or less than 10, is preferably used. Suchzeolite does not contribute to battery reaction and exhibitsa good moisture absorption under low moisture pressure.Brief Description of the DrawingsFig. 1 is a perspective View showing the structure of awound-type internal electrode body.Fig. 2 is a perspective View showing one embodiment ofthe structure of a laminationâtype internal electrode body.Fig. 3 is a (cross-)section view showing anotherembodiment of the structure of a laminationâtype internalelectrode body.Fig. 4 is a graph showing self-discharge characteristicofalithiumsecondarybatterywithvariousamountsofnmisturemixed therein.Detailed Description of Preferred EmbodimentAccordingixnthelithiumsecondarybattery(MEthepresentinvention, deterioration of the charge-discharge cycleï»¿10152025CA 02265075 1999-03-09characteristic of a battery due to decomposition ofelectrolyte is suppressed and the selfâdischargecharacteristic is improved since the moisture in the organicelectrolyte solution is limited.to a considerably low level.While the embodiments of the present invention aredescribed below, it goes without saying that the presentinvention is not limited to the following embodiments.The internal electrode body of the lithium secondarybattery of the present invention comprises a positiveelectrode, a negative electrode and a separator made of porouspolymer film, the positive electrode and the negativeelectrode being wound or laminated so that the positiveelectrode and negative electrode are not brought into directcontact with each other via the separator. In.particular, asshown in Fig. 1, an internal electrode body 1 of a windingtype is produced by winding a positive electrode 2 and anegative electrode 3 having two sheets of separator 4 inbetween, with lead lines 5 provided for electrode 2 and 3respectively.Ontheotherhand,thelaminationâtypeinternalelectrodebody 7 laminates the positive electrode 8 and the negativeelectrode 9 alternately via the separator 10 with lead lines6 being connected to each of electrodes 8 and 9 respectivelyas shown in Fig. 2. Such internal electrode bodies 1, 7 arebasically configured to have a plurality of element batteriesbeing connected in parallel, an element battery consistingof positive electrodes and negative electrodes facing eachï»¿10152O25CA 02265075 1999-03-09other. Incidentally the positive electrodes 2, 8 and thenegative electrodes 3, 9 are formed in the shape of a thinplate with an electrode active material being coatedrespectively onto aluminum foil and copper foil as substratematerials.In contrast, the internal electrode body 19 with alaminate configuration shown in Fig. 3 is structured so thata positive active material layer 14 is formed on one surfaceof a positive substrate material 11 in the shape of a plateor a foil, while a negative active material layer 15 is formedon one surface of a negative substrate material 12, connectingelectrically respective surfaces without electrode activematerial layers being formed thereon. The surface of apositive active material layer 14 and the surface of a negativeactive material layer 15 are laminated so as to oppose eachother via a separator 17 or a solid electrolyte 18 to configurea plurality of steps. In this case, unlike the above-described internal electrode body 1, 7, the configuration ofthe internal electrode body 19 has element batteries connectedin series.For a battery with any of the aboveâdescribed structures,lithium transition metal compound oxides such as lithiumcobalt oxide (LiCoO2) , lithium nickel oxide (LiNiO2) , orlithium manganese oxide (LiMn2O4) , etc. are generally used aspositive active materials. In addition, in order to improvethe conductivity of these positive active materials, it ispreferable to mix with the electrode active material a carbonï»¿10152025CA 02265075 1999-03-09powder such as acetylene black, graphite powder, etc. On theother hand, for the negative active electrode, an amorphouscarbon.material such as soft carbon or hard carbon, or carbonpowder such as natural graphite, etc. is used.In addition, as a separator, it is preferable to use athreeâlayer structural separator in which.a polyethylen filmwith lithium ion permeability and including micropores issandwiched.between.porous polypropylene films having lithiumion permeability. This serves also as a safety mechanism inwhich when the temperature of the internal electrode body israised, the polyethylene film is softened at about 130Â°C sothat the micropores collapse to suppress the movement oflithium ions, that is, battery reaction. When thispolyethylene film is sandwiched between polypropylene filmshaving a softening temperature higher than said polyethylenefilm, it is possible to prevent contact/welding between thepositive and negative electrodes.Such various internal electrode bodies 1, 7, and 19 aremounted within battery cases in accordance with theirrespective shapes. Here, as the electrolyte solution withwhich the internal electrode bodies 1, 7, and 19 areimpregnated.and which is filled in the battery cases, otherthan the above-mentioned EC, DEC, DMC, propylene carbonate(PC),Y-butyrolactone,tetrahydrofuran,andacetonitrile,etc.can be nominated. A.nonaqueous organic electrolyte solutionis preferably used, including a single solvent or a mixturesolvent of these organic solvents, and one or more of theï»¿10152025CA 02265075 1999-03-0910aboveâmentioned LiPFm etc., and lithium halide, etc., suchasLiClO4asenielectrolytedissolvediJ1thesolvent. Further,it is also possible to use alnacromolecular solid.electrolyteor the like formed by gelating the thus formed electrolytesolution.Next, in the present invention, decomposition ofelectrolyte is suppressed by making the moisture in theelectrolyte solution be absorbed by zeolite, which featuresexcellent absorption of moisture without being reactive withmembers or materials configuring a battery. The zeolite isincorporated. within the battery case, within. the thus-structured lithium secondary battery, so as to contact theorganic electrolyte solution. Here, in many cases, zeoliteis preferably used in a powder state in order to make theContact area with the electrolyte solution larger.As one means to incorporate this zeolite into a batterythere is, a means where the zeolite is contained in a bag havingelectrolyte solution.as well as moisture permeability and thezeolite is never allowed to spill out, and said bag is disposedwithin the battery case. Here, if the bag where the zeoliteis contained has the shape of a band, it is possible to fitit in a part of the internal electrode body. In that case,however,thedistancebetweenpositiveandnegativeelectrodesis made longer, and thus there is a disadvantageous aspectsuch that the internal resistance of the battery increases.On the other hand, in the case of not fitting it into theinternal electrode body, there is a disadvantage in that itï»¿10152025CA 02265075 1999-03-0911is inferior to the means described below in terms of efficiencyinabsorbingnmisturesincetheplacewherethebagischsposedis set at a local position such as the circumference of theinternal electrode body. Thus, the present means arepreferablyusednotcnitheirownbuttogetherwithotherneans.As another means to incorporate zeolite into a battery,it is possible to mention a means whereby it is mixed intomnelectrodeactivenaterialstructuringeapositiveelectrodeand/or a negative electrode. In the case where the method ofcoating an electrode active material in a paste state froman original powder state onto a substrate plate of metal foilis adopted as a way of forming a positive electrode or anegative electrode, it is possible to easily incorporatezeolite uniformly within the internal electrode body that isconsidered to requirelnoisture absorption in the battery casemerely by adding a necessary amount of zeolite powder to theelectrode active material when this paste is being formed.Moreover,ii:ispossibleixbincorporatezeoliteuniformlywithin the internal electrode body, which is considered torequire moisture absorption, by a means whereby zeolite isdispersed/stuckontothesurfaceofeaseparatorfilm,andalsoby'a means whereby zeolite is made into a fine powder to sucha degree that it does not settle due to gravity and is insteaddispersed in the electrolyte solution. Incidentally, themeans to incorporate the above-described zeolite into theinside of a battery case can be adopted on their own or incombination.ï»¿10152025CA 02205075 1999-03-0912Now, absorption of moisture in an electrolyte solutionusing the above-described zeolite is to be absorptionfundamentally at a very low level of around less than severalten.ppH1of.moisture density in the electrolyte solution, thatis, under a low moisture pressure. Thus, it is necessary toselect a zeolite which exhibits good absorption of moistureunder such conditions.Therefore, as a zeolite to be used in the presentinvention a zeolite of an aluminosilicate family having atleast any structure of LTA type, FAU type, CHA type, or MORtype, and having an.Al/Si ratio in the zeolite frame of equalto or less than 10, is preferably used.Among these zeolites, for LTA.types, the 3A(K~A) type and4A(NaâA) type, or 5(CaâA) type are preferably used, and forFAUtypes,theX(Na-X)typeorYWNaâY)typeispmeferablyused.In addition, the zeolite frame having an Al/Si ratio equalto<n:under]I)ispreferable,sinceii:isgenerallyhydrophilicand has a superior absorption characteristic under a lowmoisture pressure.Although the present invention is described in furtherdetail by way of examples as follows, needless to say thepresent invention is not to be limited to the followingexamples.ExamplesA.paste has been formed.with a LiMngL powder body havingï»¿10152025CA 02265075 1999-03-0913surface area of l.4m2/g based on the BET method as a positiveactive material, to which 4wt% of acetylene black has beenadded to provide conductivity to it . Further, a polyvinylidenfluoride (PVDF) as a binder and a normal methylpyrrolidone(NMP) as a solvent are mixed therein. With this paste coatedon both sides of 25âE|m aluminum foil, a positive electrodeis formed having an electrode planar shape with a lengthtowards the winding direction of 3400mm x a width of 200mm.On the other hand, a paste has been formed with a highlygraphitized carbon fiber in the shape of a fiber with a diameterof approximately 5pm, and a length of approximately 10um asa negative active material, to which 2wt% of artificialgraphite has been added to improve conductivity. Further aPVDF as a binder and an NMP as a solvent are mixed therein,and it is coated on both sides of 20âum copper foil, and therebya negative electrode is formed having an electrode planar shapewith a length towards the winding direction of 3400mm x a widthof 200mm.The thusâformed positive electrode and negativeelectrode are wound with insulation provided by employing2lOmmâwide separators made of polyprophylene to from aninternal electrode body. During formation of this internalelectrode body, as a lead tab for electricity collection, analuminum foil lead tub is connected to the positive electrodeand a copper foil lead tab is connected to the negativeelectrode by ultrasonicâwelding with an appropriate distancein between and on the respective side surfaces of the internalï»¿10152025CA 02265075 1999-03-0914electrode bodies so that one of the electrodes is formed atone end. of the internal electrode body. Incidentally,production of the foregoing positive and negative electrodeand production of an internal electrode body proceed undera normal air atmosphere without taking any measures againstlow moisture.Subsequently the above-described wound.body is insertedinto an aluminun1pipe (a.battery case) with an inside diameterof 48mm.and an outside diameter of 50mm and a length of 260mmwithin a globe box having a dew point of -80Â°C and a bag madeof the same substance as that forming the separator where inLTAâtype zeolite powder (zeolunt Ar3 bulb-shape productproduced by Tosoh Corp.) of 5g at the lead tub was disposedat both ends of the internal electrode bodies. The negativelead tab is fitted into a battery terminal, the cap of whichis further attached to the battery case, and the side of thenegative terminal of the battery case is sealed.Next, from the open side of positive terminal of thebattery case, the electrolyte solution, a mixed solvent ofEC and DEC where in electrolyte LiPEg is dissolved to yield1 mol% density, is injected and the internal volume of theglobe box is kept at a vacuous for two hours, allowingpermeation of electrolyte into the battery. Thereafter thepositive lead.tub is fitted into the positive terminal of thebattery, to which the cap of positive terminal is attachedand sealed.Ten batteries according to this example were formed.ï»¿1015CA 02265075 1999-03-0915Discharge capacity, internal resistance, and selfâdischargeamount were measured for each battery. Here, charge anddischarge were measured, employing a constant-current powersource as the power source with a current intensity ofapproximately O.2C covering a range of 2.5V to 4.2V. Theinternal resistance was calculated.based on the voltage dropat the terminals at the time of alternating tordischarge afterthe initial charge. The selfâdischarge amount is calculatedafter measuring the initial discharge capacity at dischargeimmediately after the initial charge, and rechargingimmediately and leaving it stand at room temperature for 28to 30 days. Thereafter, the discharge capacity'wasrneasured,with the difference between the discharge amounts before andafter being let stand at room temperature being divided bythe number of days of being left to stand. The test resultsare shown in Table 1.[Table .1]Characteristic of Battery Related to ExampleBattery Discharge Internal Self-discharge Battery Quantity ofNo. Capacity Resistance Amount Weight Electrolyte(Ah) (mil) (%/ day) (g) (g)1 218 475 042 813 1782 224 525 058 805 1653 223 55 052 808 1674 22.6 5.75 0.48 815 1715 219 5 052 825 1786 213 525 031 852 2097 223 65 068 860 2158 21.7 6.75 0.69 872 2309 225 625 034 878 23410 22.2 6.75 0.36 868 226Average 22.1 5.78 0.49 840 19710 04 074 013 29 7 0'28ï»¿CA 02265075 1999-03-0916In comparison with the above-described example, a testsimilar to the example was conducted as an example forcomparison adopting the same method as in the example ofproducing ten batteries with the exception that the zeolitepowder was incorporated inside the battery case.results are shown in Table 2.[Table 2]Characteristic of Battery Related to Example for ComparisonThe testBattery Discharge Internal Self-discharge Battery Quantity ofNo. Capacity Resistance Amount Weight Electrolyte(Ah) (H113) (%/ day) (g) (g)11 22.5 5.5 1.01 822 17712 22.5 5.75 1.01 858 21313 22.3 6.25 1.1 823 18214 22.5 6.75 0.78 847 20215 22.5 8.25 1.01 851 20816 22.4 8 0.93 864 22017 22.4 6.5 0.93 844 19918 22.3 6.5 0.93 879 23319 22.5 7.5 0.92 838 19420 22.6 6.25 0.84 853 208Average 22.5 6.73 0.95 848 20410 0.1 0.92 0.09 18 17ï»¿10152025CA 02265075 1999-03-0917As shown in Table 1 and Table 2, concerning the self-discharge amount, a result such that the amount for the batteryfor the comparative example reached approximately twice thelevel as that for the battery for the example in spite of thefact that the discharge capacity is almost similar in thebatteries in the example and in the comparative example . Sucha difference in selfâdischarge amount can be considered tohave taken place as a result in which is the battery for theexample, moisture mixed in various materials used to form thebattery as well as moisture inside the battery case duringthe forming process of the battery was absorbed by zeolite,resulting in preventing the decomposition of the electrolyte,that is, maintaining the charge/discharge characteristicwithout degrading ionic conductivity.As described above, according to the lithium secondarybattery of this invention, zeolite performing well in termsof its moisture absorption characteristic even under a lowermoisture pressure is incorporated inside the battery so asto be in contact with the organic electrolyte solution by asimple means, so that any moisture mixed into the electrolytesolution is limited to an extremely lower level. Thusdeterioration of the chargeâdischarge cycle characteristicof the battery due to decomposition of electrolyte issuppressed, and the selfâdischarge characteristic is improved.Moreover, concerning the formation of a large-sized lithiumsecondary battery, there is an advantage in that a productionfacility, etc. having a large-scale dry nitrogen atmosphereï»¿CA 02265075 1999-03-0918becomes unnecessary and it becomes possible to suppress theproduction cost.

Claims

What is claimed is:

1. A lithium secondary battery comprising:
a battery case;
an internal electrode body contained in the battery case and including a positive electrode, a negative electrode and a separator film made of porous polymer, the positive electrode and the negative electrode being wound or laminated so that the positive electrode and negative electrode are not brought into direct contact with each other via the separator film, an organic electrolyte solution contained in the battery case, and a zeolite having a moisture absorption characteristic, which is incorporated in the battery case so that the zeolite is brought into contact with the organic electrolyte solution within the battery case.

2. A lithium secondary battery according to claim 1, wherein the zeolite is incorporated in the battery case, using at least one of the following means:
(1) a means that disposes the zeolite to be contained in a bag permeable to electrolyte solution inside the battery case, (2) a means that mixes the zeolite with the electrode active material structuring the positive electrode and/or the negative electrode, (3) a means that disperses the zeolite on the surface of the separator film, and (4) a means that makes the zeolite into a fine powder and disperses it by suspension in the electrolyte solution.

3. A lithium secondary battery according to claim 1, wherein;
the zeolite of an aluminosilicate family having at least one structure of the LTA type, FAU type, CHA type, or MOR type, and having an Al/Si ratio in the zeolite frame equal to or less than 10 is used.

4. A lithium secondary battery according to claim 2, wherein;
the zeolite of an aluminosilicate family having at least one structure of the LTA type, FAU type, CHA type, or MOR type, and having an Al/Si ratio in the zeolite frame equal to or less than 10 is used.