Composite electrode and secondary battery therefrom

The present invention relates to an improved secondary battery having high weight energy density and good reversibility, more specifically, a battery containing i) a positive electrode comprising a reversible positive electrode material containing an organosulfur compound wherein sulfur--sulfur bond is formed upon oxidation and sulfur--sulfur bond is cleaved upon electrolytic reduction, a metallic compound, and a current collector containing copper metal; ii) a polymer electrolyte having lithium salt; and iii) a negative electrode made of lithium metal, lithium alloy or lithium intercalating carbon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electrode having high capacity and good 
reversibility, and a secondary battery comprising i) a positive electrode 
comprising an organosulfur compound which is capable of reversible 
formation of S--S bond upon oxidation and a metallic compound selected 
from transition metals, and a current collector containing copper metal; 
ii) a polymer electrolyte having lithium salt; and iii) a negative 
electrode made of lithium metal, lithium alloy or lithium intercalation 
compounds. 
2. Description of Prior Art 
Batteries have a wide spectrum of applications as key component, of modern 
portable electronic devices. Especially, secondary batteries are essential 
to the development of hand-carrying devices such as cellular 
telecommunication tools and notebook computers. A series of development of 
nickel-cadmium, nickel-metal hydride, and lithium ion types have provided 
advantages in reducing the size and weight of secondary batteries. 
However, the rapid advancement of electronic technology and the widespread 
use of mobile devices has been continuously demanding a next generation 
battery which has higher capacity than existing systems. 
The secondary battery using organosulfur compound as a positive electrode 
material has been disclosed in U.S. Pat. No. 4,833,048. In this patent, an 
S--S bond of organic disulfide compound consisting of positive electrode 
is cleaved by electrolytic reduction to form organic thiolate and organic 
disulfide is reformed by electrolytic oxidation of organic thiolate. 
Especially, in case of two or more thiolate groups present in a molecule, 
polymeric form of organic disulfide is formed. The redox couple of organic 
disulfide and organic thiolate accounts for theoretical energy density of 
350 to 800 Wh/kg in combination with metal negative electrode. A 
rechargeable metal-sulfur battery described in the invention provides 
practically higher energy density of 150 Wh/kg than conventional secondary 
battery. 
To increase the practical capacity of organic disulfide electrode, U.S. 
Pat. No. 5,324,599 suggested the addition of .pi. electron conjugated 
conductive polymer like polyaniline to cathodic composition containing 
organic disulfide. According to the report of same inventors disclosed in 
Nature, 373, 598(1995), the electron transfer of organic disulfide was 
catalytically accelerated in the presence of polyaniline. Accordingly, the 
composite electrode from organic disulfide and polyaniline mixed together 
in molecular level shows the enhanced energy density in excess of 600 
Wh/kg when coupled with lithium metal as negative electrode. However, in 
order to maintain the high energy density, the cell required high charging 
voltage up to 4.75 V, which is too high to ensure the electrochemical 
stability of cell components such as polymeric electrolyte and other 
organic parts. Lower charging potential which is practically required 
results in the decline of energy density of the cell. 
In order to increase the cycle life of organic disulfide electrode, 
approaches to immobilize organic disulfide have been made since diffusive 
loss of soluble form of organic disulfide, such as mercaptan or thiolate, 
eventually results in the decrease of capacity over the repeated cycle of 
charge and discharge. Addition of metal such as copper, or silver to bind 
organic disulfide species was diclosed in U.S. Pat. 5,665,492. Addition of 
copper ion to organic disulfide and use of the resulted complex was 
described in Eur. Pat. No. 799,264, A2. Improvement of cycle life was also 
suggested in U.S. Pat. No. 5,516,598, when metal salt of broad range of 
multivalent metallic complex of organic disulfide was used. In these 
disclosures, the role of metal as a coordinating center of sulfur 
containing ligand were suggested to improve the cycle life of organic 
disulfide electrode, but functions of metal such as redox reaction and 
activation of sulfur containing compound were not described. Consequently, 
above mentioned approaches provide only limited level of energy densities 
which is at maximum the sole capability of organic disulfide. 
Accordingly, the improvement for enhancing capacity and extended cycle life 
has yet to be realized to the secondary battery employing organosulfur 
compound as positive electrode material. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a positive electrode with 
high capacity and good reversibility comprising 
1) a positive electrode material comprising; 
i) an organosulfur compound which is capable of forming sulfur--sulfur bond 
by electrolytic oxidation and reversibly regenerated by reductive cleavage 
of sulfur--sulfur bond; 
ii) one or more metallic components selected from the group consisting of 
transition metals, their alloys, their ionic salts, and combinations 
thereof; and 
iii) one or more electrically conductive ingredients selected from the 
group consisting of conductive carbon and electrically conductive polymer 
and 
2) an electrically conductive current collector made of copper or copper 
alloy on which said positive electrode material is placed. 
Preferred organosulfur compound has active functional group containing 
sulfur atom in which electrochemical activity is accompanied with the 
formation of sulfur--sulfur bond upon oxidation and the cleavage of 
sulfur--sulfur bond and generation of S--M bond (M is alkali metal, 
alkaline earth metal or transition metal and includes proton) to form 
mercaptan or thiolate group upon electrolytic reduction. 
Preferred metallic compounds of the composite electrode of present 
invention are selected from the group of transition metals and their 
alloys, their ionic salts, and their combinations. Of the transition 
metals, preferred are metals having multiple oxidation states, which 
include scandium(Sc), titanium(Ti), vanadium(V), chromium(Cr), 
manganese(Mn), iron(Fe), cobalt(Co), nickel(Ni), Zinc(Zn) in the first 
row, molybdenum(Mo), ruthenium(Ru), rhodium(Rh) in the second row, and 
tungsten(W) in the third row. More preferred are chromium(Cr), 
manganese(Mn), iron(Fe), molybdenum(Mo), tungsten(W) and cobalt(Co) in 
terms of high equivalent capacity. 
Another object of the present invention is to provide a positive electrode 
in which said positive electrode material of the present invention is 
coupled with conductive current collector. Preferred conductive material 
for current collector is made of metal or metal alloy. More preferred 
conductive material for current collector is copper or copper alloy. 
It is still another object of the present invention to provide an improved 
secondary battery having high energy density and good cycle life 
comprising; 
i) a positive electrode selected from one of the positive electrodes 
described above 
ii) a polymer electrolyte having lithium salt; and 
iii) a negative electrode made of lithium metal, lithium alloy, or lithium 
intercalation materials selected from the group consisting of graphite, 
hard carbon, carbon fiber and polyacene.

DETAILED DESCRIPTION OF THE INVENTION 
A new type of positive electrode material containing organosulfur compound 
and transition metal has been developed. The positive electrode of the 
invention is combined with a suitable negative electrode such as lithium 
metal, lithium alloy, or lithium intercalating compounds like carbon and a 
suitable polymer electrolyte to provide a secondary battery with high 
capacity and extended cycle life. 
The composite positive electrode material of the present invention contains 
metallic component as an active material participating in electrode 
reaction in combination with organosulfur compound. When battery is 
charged, metal becomes metal ion by oxidation, and when battery is 
discharged, metal or metal ion with lower oxidation state is regenerated 
by reduction. Transition metals usually have multiple oxidation states and 
undergo more than one redox reactions. Selected metals of the invention 
have high equivalent capacity as enhanced by the number of electrons 
involved in redox chemistry. If the redox couples of metal and metal ion 
are utilized for electrode reaction, electrochemical equivalent capacity 
is as high as 2630 mAh/g for vanadium, 1550 mAh/g for chromium, 976 mAh/g 
for manganese, 1440 mAh/g for iron, 910 mAh/g for cobalt, and 1670 mAh/g 
for molybdenum (D. Linden Ed., Handbook of Batteries and Fuel Cells, 
McGraw-Hill, pp. C-3, 1984). Theoretical capacity is far above that of 
metal oxide electrodes used for conventional secondary lithium ion 
battery. On the other hand, the metallic component activates the electron 
transfer reaction of organosulfur compound coexisting in electrode so that 
the high capacity of organosulfur electrode can be fully utilized. 
Furthermore, the interaction between sulfur atom contained in organosulfur 
compound and metal species holds each other and prevents the loss of 
capacity caused by the diffusion of soluble forms of active material such 
as organic thiolate and metal ion into electrolyte. 
Positive electrode material of the invention contains organosulfur compound 
which is capable of forming sulfur--sulfur bond by electrolytic oxidation 
and reversibly regenerated by reductive cleavage of sulfur--sulfur bond. 
Examples of functional groups which are capable of forming sulfur--sulfur 
bond includes mercaptan, thiolate, thioacid, thioester and thioketone. 
Formation of sulfur--sulfur bond takes place in intermolecular or 
intramolecular mode. Oxidation of an organosulfur compound which has a 
single functional group capable of forming sulfur--sulfur bond leads to 
formation of dimeric compound. If an organosulfur compound has two or more 
capable functional groups, a polymeric compound which has sulfur--sulfur 
linkage is generated. Regarding the organosulfur compound used for 
electrode material, an example include disulfide compound represented by 
the formula of (R(S).sub.y).sub.n which was disclosed in U.S. Pat. No. 
4,833,048. This material can be represented by R(SM).sub.y when reduced. 
In these formulas, R represents aromatic or aliphatic hydrocarbons;y is an 
integer from 1 to 6; and n is an integer of 2 or more. Examples of organic 
disulfide include 2,5-dimercapto-1,3,4-thiadiazole and trithiocyanuric 
acid. As another example of organic disulfide, the compound having two or 
more organic thiolale groups in a molecule and having capability of 
intramolecular formation of disulfide bond is included. An example of such 
compound is represented by 1,8-disulfide naphthalene as described in U.S. 
Pat. No. 5,324,599. 
Positive electrode material of the invention contains electrically 
conductive ingredients such as carbon, or electrically conductive polymer. 
Electrically conductive carbon includes graphite and acetylene black. 
Among electrically conductive polymers used as conductive ingredients for 
electrode material, the compound having nitrogen or sulfur atom in the 
polymer skeleton or side chain is preferred. Interaction of nitrogen or 
sulfur atom in conducting polymer with other active components, 
organosulfur compound and metal species, helps all the active components 
combine each other and minimize the loss of active contents from the 
electrode. Further, accelerated electron transfer of organic disulfide in 
the presence of conducting polymer is described in Nature, 373, 598(1995). 
Examples of such conductive polymer include polyaniline, polypyrrole and 
polythiophene and their derivatives. 
To prepare the electrode, binder material can be added. The polymer used as 
binder is preferably the same kind of polymer used for polymer 
electrolyte. However, a suitable polymer which is not ionically conductive 
can be also selected. In order to dissolve binder material, organic 
solvent, generally, aprotic solvent can be used. The metallic component is 
added in powder form and dispersed in the mixture of electrode material. 
The particle size of powder is preferably smaller than 10 .mu.m. and the 
surface of metal can be activated by treating with diluted weak acid such 
as acetic acid. The preferred content of metallic part is 5.about.95 wt. % 
of total active components in electrode. The slurry of composite mixture 
is homogenized using suitable means such as magnetic stirring, mechanical 
stirring, sonication, or ball milling. 
The well dispersed composite electrode material is pasted on the conductive 
current collector. Preferred condutive material consisting current 
collector is copper metal. According to the present invention, the 
capacity of positive electrode is fully utilized by using current 
collector containing copper metal. The current collector made of copper 
metal maintains the stability of each components of the battery by 
preventing the excess elevation of charging potential by fixing the upper 
limit of voltage as the oxidation potential of copper metal. 
According to the invention, a secondary battery with a said positive 
electrode is provided. Typical structure of the cell includes a said 
positive electrode of the invention, polymer electrolyte, and a negative 
electrode from lithium metal, lithium alloy or materials capable of 
lithium intercalations. 
Applicable polymer electrolyte used for lithium secondary battery of the 
present invention is ion conductive polymer electrolyte capable of 
solvating lithium ion and can be prepared either in a solid type or in a 
gel type. Polymer electrolyte essentially consists of base polymer and 
lithium salt. Lithium salt can be selected from the group of LiClO.sub.4, 
LiBF.sub.4, LiPF.sub.6, LiAsF.sub.6, LiAsCl.sub.6, LiCF.sub.3 SO.sub.3, 
LiN(SO.sub.2 CH.sub.3).sub.2 and their combinations. Base polymer suitable 
for application has a functional group containing hetero atom in repeating 
unit and has a certain degree of chemical affinity with lithium salt. 
Examples include poly(ethylene oxide), poly(propylene oxide), 
polyacrylonitrile, poly(acrylonitrile-co-methyl acrylate), poly(vinylidene 
fluroride), and poly(vinylidene fluoride-co-hexafluoropropylene). The 
content of lithium salt in polymer electrolyte is in the range of 
5.about.50 mol % relative to monomer unit of base polymer. In order to 
plasticize polymer electrolyte, organic solvent can be added. The polar 
organic solvent having carbonate group, for example, one or combination of 
more from propylene carbonate, ethylene carbonate, dimethyl carbonate, 
diethyl carbonate and methyl ethyl carbonate can be used. The content of 
plasticizer is 10.about.90 wt. % of polymer electrolyte. 
Negative electrode, lithium metal or lithium alloy which is capable of 
lithium striping and plating upon charge and discharge. Another example is 
a group of materials which are capable of intercalations. Examples include 
carbon materials such as graphite, amorphous carbon, coke pitch, 
polyacene, etc. Upon charging the battery, lithium is intercalated into 
the carbon structure and lithium ion is, in turn, ejected from negative 
electrode when the battery is discharged. 
Positive electrode of the invention provides high capacity and good 
reversibility, since the redox system of metal in combination with 
organosulfur compound is effectively utilized. Furthermore, the invention 
provides a rechargeable battery with light weight and high capacity, which 
is most advantageous in the application of portable electronic devices 
such as cellular phone or notebook computer. Further, since the lithium 
secondary battery of the present invention consists of all solid 
components, it dose not raise any problems relating to the use of liquid 
such as leakage or pressure development and can be easily fabricated in a 
suitable shape according to the various application purpose of battery. 
The present invention can be explained more concretely by following 
examples. However, the scope of the present invention shall not be limited 
to these examples. 
EXAMPLE I 
Preparation of Polymer Electrolyte 
A mixture of 3.0 g of poly(acrylonitrile-co-methyl acrylcite)(94:6) and 2.3 
g of lithium tetrafluoroborate was added to a mixed solvent of propylene 
carbonate and ethylene carbonate(10.5:7.9 wt./wt.). The mixture was heated 
at 120.about.140.degree. C. under a nitrogen atmosphere. Then, the polymer 
was cast on glass plate, and dried at 60.about.80.degree. C. under vacuum. 
The ionic conductivity of resulted electrolyte film was 10.sup.-3 
.about.10.sup.-4 S/cm by impedance measurement. 
EXAMPLE II 
A powdery mixture of 1.0 g of trithiocyanuric acid(TTCA), 0.6 g of iron 
metal powder (particle size, 1.about.10 .mu.m), 0.7 g of acetylene black, 
and 1.6 g of polyaniline (Versicon, Allied Signal Inc.) was prepared and 
mixed for 1 day using ball-mill. The mixture was added to a solution of 
0.4 g of poly(vinylidene fluoride) and 0.1 g of Brij 35(Aldrich) in 20 mL 
of N-methyl-2-pyrrolidone and again homogenized using ball-mill for three 
days. The resulted slurry was pasted on copper metal sheet under an argon 
atmosphere. The electrode was dried at 60.about.80.degree. C. under vacuum 
and pressed under pressure of 0.1.about.3 ton/cm.sup.2. The test cell A 
was prepared by combining the electrode and a negative electrode from 
lithium metal foil with nickel mesh current collector and gel polymer 
electrolyte layer. 
COMATIVE EXAMPLE I 
Test cell A' was prepared in the same manner as example II except that SUS 
316 foil was used instead of copper metal sheet as a current collector of 
positive electrode. 
EXAMPLE III 
Positive electrode was prepared in the same manner as example II except 
that polyaniline additive was not used. Thus a powdery mixture of 1.0 g of 
TTCA, 0.6 g of iron metal powder (particle size, 1.about.10 .mu.m) and 0.7 
g of acetylene black was mixed for 1 day using ball-mill and then added to 
the solution of 0.4 g of poly(vinylidene fluoride) and 0.1 g of Brij 35 
(Aldrich) in 20 mL of N-methyl-2-pyrrolidone. The mixture was treated to 
prepare the test cell B in the same manner as described in example II. 
COMATIVE EXAMPLE II 
Test cell B' was prepared according to the procedure in example III without 
iron metal in the positive electrode. Thus, 1.0 g of TTCA, 0.4 g of 
poly(vinylidene fluoride), 0.1 g of Brij35 were dissolved in 20 mL of 
N-methyl-2-pyrrolidone. And then, 0.7 g of acetylene black was added. The 
mixture was used to prepare the test cell B' in the same manner as example 
II. 
EXAMPLE IV 
To a solution of 1.0 g of TTCA, 0.4 g of poly(vinylidene fluoride), 1.6 g 
of polyaniline(Versicon, Allied Signal Inc.) in 20 mL of 
N-methyl-2-pyrrolidone were added 0.5 g of tungsten metal powder (particle 
size, 1.about.10 .mu.m) and 0.7 g of acetylene black. The mixture was used 
to prepare the test cell C in the same manner as example II. 
COMATIVE EXAMPLE III 
Test cell C' was prepared in the same manner as example IV except that 
graphite sheet was used instead of copper metal sheet as a current 
collector of positive electrode. 
EXAMPLE V 
To a solution of 1.5 g of TTCA, 0.4 g of poly(vinylidene fluoride), 1.2 g 
of polyaniline(Versicon, Allied Signal Inc.) in 20 mL of 
N-methyl-2-pyrrolidone were added 0.5 g of molybdenum metal powder 
(particle size, 1.about.10 .mu.m), 0.3 g of acetylene black and 0.5 g of 
graphite. The mixture was used to prepare the test cell D in the same 
manner as example II. 
COMATIVE EXAMPLE IV 
Test cell D' was prepared in the same manner as example V except that 
titanium metal foil was used instead of copper metal sheet as a current 
collector of positive electrode. 
EXAMPLE VI 
To a solution of 1.5 g of TTCA, 0.4 g of poly(vinylidene fluoride), 1.6 g 
of polyaniline(Versicon, Allied Signal Inc.) in 20 mL of 
N-methyl-2-pyrrolidone were added 0.6 g of chromium metal powder (particle 
size, 1.about.10 .mu.m), 0.7 g of acetylene black and 0.1 g of Brij 
35(Aldrich). The resulted mixture was used to prepare the test cell E in 
the same manner as example II. 
COMATIVE EXAMPLE V 
Test cell E' was prepared in the same manner as example VI except that 
graphite sheet was used instead of copper metal sheet as a current 
collector of positive electrode. 
EXAMPLE VII 
To a solution of 1.5 g of TTCA, 0.4 g of poly(vinylidene fluoride), 1.6 g 
of polyaniline(Versicon, Allied Signal Inc.) in 20 mL of 
N-methyl-2-pyrrolidone were added 0.6 g of cobalt metal powder (particle 
size, 1.about.10 .mu.m), 0.7 g of acetylene black and 0.1 g of Brij 
35(Aldrich). The resulted mixture was used to prepare the test cell F in 
the same manner as example II. 
COMATIVE EXAMPLE VI 
Test cell F' was prepared in the same manner as example VII except that 
graphite sheet was used instead of copper metal sheet as a current 
collector of positive electrode. 
COMATIVE EXAMPLE VII 
Test cell G was prepared without metallic additive in the positive 
electrode. Thus, 1.0 g of TTCA, 0.4 g of poly(vinylidene fluoride), 1.6 g 
of polyaniline(Versicon, Allied Signal Inc.), 0.1 g of Brij35 were 
dissolved in 20 mL of N-methyl-2-pyrrolidone. And then, 0.7 g of acetylene 
black was added. The mixture was used to prepare the test cell G in the 
same manner as example II. 
Evaluation of the Test Cells 
Charging and discharging test was galvanostatically carried out using test 
cells A.about.G and A'.about.F'. FIG. 1 demonstrates the discharge 
profiles of test cell A and C which contains iron and tungsten, 
respectively, in addition to TTCA. Capacity of test cells A and B is well 
beyond the capacity of test cell G which contains only TTCA. It indicates 
that metallic components contained in positive electrode actively 
participates in electrode reaction and contributes to the capacity of the 
electrode. FIG. 2 shows the discharge profile of test cell B which does 
not contain polyaniline in positive electrode and yet exceeds the capacity 
of organosulfur electrode. FIG. 3 and 4 illustrates the stable cycling 
behavior of composite electrode containing organosulfur and metal as 
active components in positive electrode. Charging and discharging profiles 
are more or less reproducible during 25 cycles. FIG. 5 and 6 shows the 
charge and discharge profiles of test cell A' and C' which has the same 
composition of positive electrode material as test cell A and C, but with 
current collector made of SUS and graphite, respectively, instead of 
copper metal. Charging potential in both cells rapidly increases and 
discharging capacity accordingly decreases in the initial cycles. The 
comparison of FIG. 3 and 4 with FIG. 5 and 6, indicates that the composite 
positive electrode having current collector made of copper metal provides 
better stability and consequently ensures higher capacity. Enhanced 
capacity of the composite positive electrode is further demonstrated in 
FIG. 7 which shows the discharging profiles of the test cells D, E, and F. 
Capacities of test cells with the composite electrodes having metallic 
components such as molybdenum, chromium, and cobalt in addition to TTCA 
are much higher than what is accounted for by TTCA. Also the enhanced 
capacity is supported by employing current collector consisted of copper 
metal when compared with the discharging profiles of test cell D', E' and 
F' which have other conductive current collectors. FIG. 8 shows the 
variations of Coulombic efficiencies of test cells E and F. Coulombic 
efficiencies higher than 80% are maintained until 30 cycles. From the 
results demonstrated by the above examples, it is clearly shown that the 
positive electrode prepared following the present invention provides high 
energy density and good reversibility for secondary battery.