Non-aqueous liquid electrolyte secondary cell

A non-aqueous liquid electrolyte secondary cell capable of preventing bursting or explosion thereof even when a current breaking device or a relief valve for pressure release fails in operation thereof due to any trouble or failure thereof. The cell includes a wound-up body formed by laminatedly spirally winding up a positive electrode and a negative electrode together while interposing a separator therebetween. The wound-up body thus formed is received in a cell can. The negative electrode is constructed by forming a negative active material layer containing amorphous carbon on each of both surfaces of a negative collector. The positive electrode is constructed by forming a positive active material layer containing LixCoO.sub.2 on each of both surfaces of a positive collector. The negative electrode is electrically connected to the cell can through a negative electrode lead. The positive electrode is electrically connected to the cell lid through a positive electrode lead joined to a connection plate of the cell lid by welding.

BACKGROUND OF THE INVENTION 
This invention relates to a non-aqueous liquid electrolyte secondary cell, 
and more particularly to a non-aqueous liquid electrolyte secondary cell 
having a lithium double oxide and a material which is capable of occluding 
and releasing lithium ions or has lithium ion occluding and releasing 
properties incorporated therein so as to act as a positive active material 
and a negative active material, respectively. 
There has been conventionally known a non-aqueous liquid electrolyte 
secondary cell in the art which includes a positive electrode and a 
negative electrode wherein the positive electrode has a positive active 
material layer containing a positive active material consisting of a 
lithium double oxide formed on a positive collector and the negative 
electrode has a negative active material layer containing a negative 
active material having lithium ion occluding and releasing properties 
formed on a negative collector. Such a non-aqueous liquid electrolyte 
secondary cell has been extensively used for a potable electric/electronic 
equipment such as a VTR camera, a note-type personal computer, a portable 
telephone or the like, because it is increased in energy density. Active 
materials incorporated in the non-aqueous liquid electrolyte secondary 
cell are chemically active, so that a non-aqueous liquid electrode of the 
cell is deteriorated in performance when water intrudes thereinto. In 
order to eliminate the disadvantage, the non-aqueous liquid electrolyte 
secondary cell is constructed into a hermetically sealed structure. 
Unfortunately, such a hermetically sealed structure causes an internal 
pressure of the cell to be increased due to gas generated by decomposition 
of the electrolyte, leading to bursting of a cell case of the non-aqueous 
liquid electrolyte secondary cell, resulting in damage to a peripheral 
equipment, when it falls into a supercharged state due to any trouble or 
failure of a charging circuit. In order to solve such a problem, it was 
proposed that the cell case of the non-aqueous liquid electrolyte 
secondary cell is provided therein with a current breaking device such as 
a pressure switch for interrupting electrical connection between the 
electrodes and cell terminals when the internal pressure is increased, as 
detailedly discussed in U.S. Pat. No. 5,567,539, which corresponds to 
Japanese Patent Application Laid-Open Publication No. 102331/1996 
(8-102331), and the like. 
Nevertheless, arrangement of such a current breaking device often leads to 
generation of heat in the cell case which causes a rapid increase in 
temperature of the cell rather than generation of gas therein in the 
supercharged state, before an increase in internal pressure of the cell 
due to generation of gas by decomposition of the electrolyte permits 
operation of the current breaking device to be carried out. In the worst 
case, the heat generation often causes uselessness of the current breaking 
device, leading to breakage or explosion of the cell. In order to avoid 
such a situation, techniques of incorporating an additive such as lithium 
carbonate or lithium oxalate in the positive active material were proposed 
as disclosed in Japanese Patent Application Laid-Open Publication No. 
329269/1992 (4-329269) and U.S. Pat. No. 5,427,875 which corresponds to 
Japanese Patent Application Laid-Open Publication No. 328278/1992 
(4-328278). Incorporation of the additive in the positive active material 
permits carbon dioxide to be generated due to electrochemical 
decomposition of lithium carbonate or lithium oxalate contained in the 
positive active material, when the cell is subject to supercharging. It 
would be considered that carbon dioxide thus generated acts to not only 
restrain any abnormal reaction which causes heat generation leading to a 
rapid increase in temperature of the cell, but increase an internal 
pressure of the cell to ensure positive actuation of the current breaking 
device. However, actually, the additive thus incorporated in the positive 
active material fails to fully restrain such a temperature increase of the 
cell depending on supercharged conditions thereof. In particular, in 
techniques disclosed in Japanese Patent Application Laid-Open 329269/1992 
described above, in order to effectively decompose lithium oxalate, the 
positive active material is mixed with lithium carbonate to prepare a 
mixture, which is then subject to a heat treatment, to thereby permit 
lithium carbonate to be contained in the positive active material. 
Unfortunately, this causes particles of the positive active material to be 
increased in size, leading to a decrease in specific surface area of the 
active material, resulting in current density being increased, so that the 
non-aqueous liquid electrolyte secondary cell may be deteriorated in both 
high-rate discharge characteristics and low-temperature discharge 
characteristics. 
In order to solve the problem, it was proposed to incorporate an additive 
such as manganese carbonate, cobalt carbonate, nickel carbonate, sodium 
carbonate, potassium carbonate, rubidium carbonate, magnesium carbonate, 
calcium carbonate, barium carbonate or the like in the positive active 
material layer, as disclosed in Japanese Patent Application Laid-Open 
Publication No. 338323/1994 (6-338323) and U.S. Pat. No. 5,567,539, which 
corresponds to Japanese Patent Application Laid-Open Publication No. 
102331/1996 (8-102331). Use of such an additive solves the problem 
described above. 
Nevertheless, incorporation of the additive in the positive active material 
very rarely causes a failure in actuation of the current breaking device 
when an internal pressure of the cell is increased due to generation of 
gas by decomposition of the electrolyte owing to overcharging of the cell, 
even if it restrains a rapid increase in temperature of the cell. This 
would be due to materials for components for the current breaking device, 
assembling of the device and quality of welding for joining between the 
components. Such a failure in actuation of the current breaking device 
further promotes supercharging of the cell to increase a voltage across 
the cell, to thereby increase a charging current correspondingly, when the 
additive for restraining an increase in temperature of the cell as 
described above is incorporated in the positive active material. Such an 
increase in rate of increase in charging current causes abnormal or 
excessive heat generation, so that a rate at which the electrolyte is 
decomposed is increased correspondingly. This possibly causes early 
bursting or explosion of the cell when it is not provided with a relief 
valve for releasing an internal pressure of the cell. When the relief 
valve is arranged, operation of the valve reduces an internal pressure of 
the cell; however, it possibly causes an electrolyte to leak to an outside 
of the cell, leading to damage to a peripheral equipment. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the foregoing disadvantage 
of the prior art. 
Accordingly, it is an object of the present invention to provide a 
non-aqueous liquid electrolyte secondary cell which is capable of 
effectively preventing bursting or explosion of the cell even in the worst 
situation wherein a current breaking device or a relief valve is kept from 
operating during supercharging of the cell. 
In accordance with the present invention, a non-aqueous liquid electrolyte 
secondary cell is provided. The non-aqueous liquid electrolyte secondary 
cell includes a positive electrode including a positive collector on which 
a positive active material layer containing a positive active material 
consisting of a lithium double oxide and any one of a phosphate compound 
and strontium carbonate is formed, a negative electrode including a 
negative collector on which a negative active material layer containing a 
negative active material having lithium ion occluding and releasing 
properties is formed, and a current breaking device for interrupting 
electrical connection between the electrodes and cell terminals. 
The inventors made a study of an additive which restrains substantial 
decomposition of an electrolyte causing excessive heat generation and a 
substantial increase in charging current, even when a current breaking 
device fails in operation thereof due to any trouble or failure, to 
thereby cause supercharging of the cell to proceed, resulting in a cell 
voltage being continuously increased. As a result, the inventors found 
that a phosphate compound or strontium carbonate acts as the additive. 
Incorporation of such an additive in the positive active material layer 
effectively restrains an increase in charging current after an internal 
pressure of the cell exceeds a pressure at which a current breaking device 
operates, in the case that the device fails in operation due to any 
trouble or failure. Thus, the non-aqueous liquid electrolyte secondary 
cell of the present invention prevents a significant increase in charging 
current to keep excessive or abnormal heat generation from occurring, even 
when the current breaking device fails in operation due to any trouble or 
failure thereof to cause supercharging to further continue. Thus, it 
effectively prevents decomposition of the electrolyte. Also, the cell of 
the present invention effectively prevents bursting or explosion thereof 
even when the relief valve for pressure release is not arranged therein or 
fails in operation due to any failure or trouble. 
The phosphate compounds suitable for use in the present invention 
preferably include lithium phosphate (Li.sub.3 PO.sub.4) and cobalt (II) 
phosphate Co.sub.3 (PO.sub.4).sub.2 !. Also, any other suitable phosphate 
compounds such as K.sub.3 PO.sub.4, Ca PO.sub.4).sub.2, Na.sub.3 PO.sub.4, 
Na.sub.2 HPO.sub.4, NaH.sub.2 PO.sub.4, MgNH.sub.4 PO.sub.4, 
(NH.sub.4).sub.2 HPO.sub.4, (NH.sub.4)H.sub.2 PO.sub.4, Na.sub.2 
HPO.sub.4, NaH.sub.2 PO.sub.4, Fe.sub.3 (PO.sub.4).sub.2, (CH.sub.3 
C.sub.6 H.sub.4).sub.3 PO.sub.4, (C.sub.6 H.sub.5).sub.3 PO.sub.4 and the 
like may be likewise conveniently used for this purpose. Further, the 
phosphate compounds suitable for use in the present invention may include 
hydrated phosphate compounds. 
The phosphate compound may be contained in an amount of 0.2 to 15% by 
weight based on the positive active material. A decrease in content of the 
phosphate compound to a level below 0.2% by weight leads to an increase in 
occurrence of bursting or explosion of the cell. Whereas, an increase in 
the content to a level above 15% by weight causes a deterioration in 
discharge capacity of the cell. 
The phosphate compound preferably has an average particle diameter of 30 
.mu.m or less. An average particle size of the phosphor compound above 30 
.mu.m causes an increase in a ratio of specific surface area of the 
compound to a weight thereof, to thereby delay generation of gas with 
respect to an increase in cell voltage, leading to a failure in effective 
operation of the current breaking device. 
The lithium double oxides include LixMO.sub.2 such as LixCoO.sub.2, 
LixNiO.sub.2, LixMnO.sub.2 and the like, wherein M is at least one 
transition metal and x is between 0.05 and 1.10 
(0.05.ltoreq.x.ltoreq.1.10). 
The non-aqueous liquid electrolyte secondary cell of the present invention 
may further include a relief valve for releasing a pressure in the cell 
when an internal pressure of the cell reaches a predetermined level or 
above. The positive electrode and negative electrode may be spirally wound 
up while interposing an electrolyte layer containing the non-aqueous 
liquid electrolyte therebetween, to thereby form the cell into a 
cylindrical shape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now, a non-aqueous liquid electrolyte secondary cell according to the 
present invention will be described hereinafter with reference to the 
accompanying drawings. 
Referring first to FIG. 1, an embodiment of a non-aqueous liquid 
electrolyte secondary cell according to the present invention which is 
equipped with a current breaking device is generally illustrated. A 
non-aqueous liquid electrolyte secondary cell of the illustrated 
embodiment includes a wound-up body 4 formed by laminatedly spirally 
winding up a positive electrode 1 and a negative electrode 2 together 
while interposing a separator 3 therebetween. The wound-up body 4 thus 
formed is arranged or received in a cell can 5. The negative electrode 2 
is constructed by forming a negative active material layer 7 on each of 
both surfaces of a negative collector 6. The positive electrode 1 is 
constructed by forming a positive active material layer 9 on each of both 
surfaces of a positive collector 8. The cell can 5 is mounted thereon with 
a cell lid 10 through an insulating sealing gasket 14. The negative 
electrode 2 is electrically connected to the cell can 5 through a negative 
electrode lead 11. Also, the positive electrode 1 is electrically 
connected to the cell lid 10 through a positive electrode lead 12 joined 
to a recess 13a of a connection plate 13 of the cell lid 10 by welding. 
The connection plate 13 is constructed so as to be upwardly forced or 
pushed, to thereby be deformed as shown in FIG. 2, when an internal 
pressure of the cell is excessively or abnormally increased. This results 
in the positive electrode lead 12 being broken at a portion thereof joined 
to the connection plate 13 by welding, to thereby interrupt flowing of a 
current therethrough. Thus, in the cell shown in FIG. 1, the positive 
electrode lead 12 and connection plate 13 cooperate with each other to 
provide a current breaking device. The cell lid 10 is formed with a 
through-hole 15 for selectively releasing a pressure in the cell. The 
connection plate 13 is formed with a cross notch (not shown) of a depth 
reduced to a degree sufficient to permit the cutout to be broken when a 
pressure in the cell is increased to a predetermined level higher than an 
internal pressure of the cell which permits operation of the current 
breaking device, so that gas generated in the cell may be outwardly 
discharged from the cell via the through-hole 15 and the notch broken. 
Thus, in the illustrated embodiment, the notch of the connection plate 13 
and the through-hole 15 of the cell lid 10 cooperate with each other to 
provide a relief valve. 
The present invention will be understood more readily with reference to the 
following examples; however, these examples are intended to illustrate the 
invention and are not to be construed to limit the scope of the invention. 
EXAMPLE 1 
A non-aqueous liquid electrolytic secondary cell of the present invention 
which is constructed in such a manner as shown in FIG. 1 was prepared. 
First, the positive electrode 1 was formed. For this purpose, three 
gradients or a positive active material consisting of LixCoO.sub.2 (x=1.0) 
powder of 1 to 2 .mu.m in average particle diameter, graphite powder of 
0.5 .mu.m in average particle diameter and a binder consisting of 
polyvinylidene fluoride (PVDF) were fully kneaded together at a weight 
ratio of 80:10:10, to thereby prepare a kneaded mixture. Then, a suitable 
amount of dispersing solvent consisting of N-methyl-2-pyrrolidone was 
added to the kneaded mixture and then both were fully kneaded together to 
form an ink-like intimate mixture. Subsequently, to the ink-like mixture 
thus formed was added lithium phosphate (Li.sub.3 PO.sub.4) powder in an 
amount of 5% by weight based on the positive active material 
(LixCoO.sub.2) prepared by Mitsuwa Kagaku Yakuhin Kabushiki Kaisha (a 
Japanese corporation), to thereby prepare an ink-like kneaded material for 
a positive active material. Thereafter, the kneaded material was coated on 
each of both surfaces of the positive collector 8 made of an aluminum foil 
having a size of 20 .mu.m.times.50 mm.times.450 mm, followed by drying of 
the kneaded material, resulting in the positive active material layer 9 of 
100 .mu.m in thickness being formed on each of both surfaces of the 
positive collector 8, to thereby provide the positive electrode 1. 
Then, the negative electrode 2 was formed. First, two gradients or 
amorphous carbon having lithium ion occluding and releasing properties and 
a binder consisting of polyvinylidene fluoride (PVDF) were fully kneaded 
together at a weight ratio of 90:10, to thereby prepare a kneaded mixture. 
Then, a suitable amount of dispersing solvent consisting of 
N-methyl-2-pyrrolidone was added to the kneaded mixture and then both were 
fully kneaded together to form an ink-like kneaded mixture for the 
negative active material. Thereafter, the kneaded mixture was coated on 
each of both surfaces of the negative collector 6 made of a copper foil 
having a size of 10 .mu.m.times.50 mm.times.490 mm, followed by drying of 
the kneaded material, resulting in the negative active material layer 7 of 
100 .mu.m in thickness being formed on each of both surfaces of the 
negative collector 6, to thereby provide the negative electrode 2. 
Then, the positive electrode 1 and negative electrode 2 were wound up 
together while interposing therebetween the separator 3 made of a 
microporous polyethylene film, to thereby provide the wound-up body 4, 
which was then received in the cell can 5 of a cylindrical shape. The 
negative electrode lead 11 connected to the negative electrode 2 was 
electrically connected to the cell can 5 by welding and then the positive 
electrode lead 12 was connected to the recess 13a of the connection plate 
13 of the cell lid 10 by welding. Subsequently, a non-aqueous liquid 
electrolyte was prepared by dissolving 1 mol/l of LiPF.sub.6 in a solvent 
obtained by mixing propylene carbonate, dimethyl carbonate and diethyl 
carbonate with each other, followed by charging of the thus-prepared 
electrolyte in the cell can 5. Mixing of propylene carbonate, dimethyl 
carbonate and diethyl carbonate was carried out at a volume ratio of 
30:55:15. 
Thereafter, the cell lid 10 was placed on the cell can 5 through the 
insulating sealing gasket 14. Then, the cell can 5 was subject to caulking 
in a such a manner that an opening surrounds a peripheral edge of the cell 
lid 10, to thereby hermetically close the cell can 5, resulting in the 
non-aqueous liquid electrolyte secondary cell being assembled. The current 
breaking device constituted by the positive electrode lead 12 and 
connection plate 13 was constructed so as to operate when an internal 
pressure of the can 5 reaches a level of 6 to 8 kg/cm.sup.2. Also, the 
relief valve was constructed so as to openably operate at a pressure of 
from 10 kg/cm.sup.2 to 15 kg/cm.sup.2 higher than the inner pressure of 
the cell which permits operation of the current breaking device. 
EXAMPLE 2 
The procedure described in Example 1 was substantially repeated except that 
cobalt (II) phosphate Co.sub.3 (PO.sub.4)! powder of 5 .mu.m in average 
particle diameter was added in an amount of 5% by weight based on 
LixCoO.sub.2 to the positive active material in place of the lithium 
phosphate powder. 
EXAMPLE 3 
The procedure described in Example 1 was substantially repeated except that 
strontium carbonate (SrCO.sub.3) powder of 5 .mu.m in average particle 
diameter was added in an amount of 5% by weight based on LixCoO.sub.2 to 
the positive active material in place of the lithium phosphate powder. 
Comparative Example 1 
The procedure described in Example 1 was substantially repeated except that 
lithium phosphate powder was not added to the positive active material 
layer. 
Comparative Examples 2 to 12 
Comparative Example 2 was executed according to the procedure described in 
Example 1 except that lithium carbonate (Li.sub.2 CO.sub.3) powder was 
added to the positive active material layer in place of the lithium 
phosphate powder. Comparative Example 3 was carried out according to the 
procedure described in Example 1 except that lithium oxalate (LiC.sub.2 
O.sub.4) powder was added to the positive active material layer in place 
of the lithium phosphate powder. Comparative Example 4 was practiced 
according to the procedure described in Example 1 except that manganese 
carbonate (MnCO.sub.3) powder was added to the positive active material 
layer in place of the lithium phosphate powder. Comparative Example 5 took 
place according to the procedure described in Example 1 except that cobalt 
carbonate (CoCO.sub.3) powder was added to the positive active material 
layer in place of the lithium phosphate powder. Comparative Example 6 was 
executed according to the procedure described in Example 1 except that 
nickel carbonate (NiCO.sub.3) powder was added to the positive active 
material layer in place of the lithium phosphate powder. Comparative 
Example 7 was executed according to the procedure described in Example 1 
except that sodium carbonate (Na.sub.2 CO.sub.3) powder was added to the 
positive active material layer in place of the lithium phosphate powder. 
Comparative Example 8 was executed according to the procedure described in 
Example 1 except that potassium carbonate (K.sub.2 CO.sub.3) powder was 
added to the positive active material layer in place of the lithium 
phosphate powder. Comparative Example 9 was executed according to the 
procedure described in Example 1 except that rubidium carbonate (Rb.sub.2 
CO.sub.3) powder was added to the positive active material layer in place 
of the lithium phosphate powder. Comparative Example 10 was executed 
according to the procedure described in Example 1 except that calcium 
carbonate (CaCO.sub.3) powder was added to the positive active material 
layer in place of the lithium phosphate powder. Comparative Example 11 was 
executed according to the procedure described in Example 1 except that 
magnesium carbonate (MgCO.sub.3) powder was added to the positive active 
material layer in place of the lithium phosphate powder. Comparative 
Example 12 was executed according to the procedure described in Example 1 
except that barium carbonate (BaCO.sub.3) powder was added to the positive 
active material layer in place of the lithium phosphate powder. In each of 
the comparative examples, the additive incorporated in the positive active 
material layer in place of lithium phosphate had an average particle 
diameter of 5 .mu.m which is the same as that of the lithium phosphate 
powder in Example 1. Also, the amount in which each of the additives was 
incorporated in the positive active material layer was 5% by weight based 
on LixCoO.sub.2 as in Example 1. 
Then, the cell obtained in each of the comparative examples was subject to 
full charging to a voltage level of 4.2 V. Then, the positive electrode of 
each of the cells and the negative electrode thereof were cut into a size 
of 20 mm.times.20 mm and that of 21 mm.times.21 mm, respectively. 
Subsequently, the positive and negative electrodes thus cut were arranged 
opposite to each other with the non-aqueous liquid electrolyte being 
interposed therebetween, to thereby provide a test cell. Thereafter, each 
of the test cells was subject to potential sweep wherein a charging 
voltage was applied to the test cell at a rate of 0.1 mV/sec, to thereby 
obtain relationship between a current of the test cell and a voltage 
thereof. The results were as shown in FIGS. 3 to 17. 
In each of FIGS. 3 to 17, a peak appearing near a voltage of 4.6 V is an 
oxidation peak occurring due to separation of a large amount of Li from 
the positive active material (LixCoO.sub.2) and is not due to 
electrochemical decomposition of the electrolyte and/or additive. 
Occurrence of electrochemical decomposition of the electrolyte and/or 
additive is indicated by a small current peak appearing near a voltage of 
5 to 5.1 V. At the time of occurrence of the small current peak, the 
additive is electrochemically decomposed to promote decomposition of the 
electrolyte. Thus, the electrolyte is decomposed to generate gas when the 
small current peak occurs, so that the current breaking device is actuated 
for interruption of flowing of a charging current. A voltage which causes 
occurrence of the small current peak is called a decomposition voltage. 
When the current breaking device fails in operation thereof due to any 
trouble or failure, a charging current flowing after the small current 
peak occurs causes further decomposition of the electrolyte, leading to 
further generation of the gas. Therefore, a charging current flowing 
beyond the decomposition voltage promotes decomposition of the 
electrolyte. It will be noted that the cell of each of FIGS. 6 to 17 
causes a current to be substantially increased when a voltage thereof 
rises more than about 5 to 5.1 V, to thereby exceed the decomposition 
voltage. 
On the contrary, the cells obtained in Examples 1 to 3, as shown in FIGS. 3 
to 5, each prevent a substantial increase in current thereof even when the 
cell voltage exceeds a level of about 5 to 5.1 V. In particular, the cell 
of each of Examples 1 and 2 wherein lithium phosphate and cobalt (II) 
phosphate are respectively added as the phosphate compound to the positive 
active material substantially restrains an increase in current thereof 
when the voltage reaches a decomposition voltage of the electrolyte. Such 
restraint of an increase in current flowing after the voltage exceeds the 
decomposition voltage effectively prevents the electrolyte from being 
decomposed even when the current breaking device fails in operation 
thereof due to any trouble or failure. Thus, it will be noted that the 
cell of each of the examples prevents breakage and/or explosion thereof 
even when the relief valve for pressure releasing fails in operation. 
Such supercharging as described above occurs when any trouble or failure of 
a charging circuit causes constant-current charging to continue after full 
charging (4.2 V) as well, in the case that a cell is charged by means of a 
charging device which shifts charging of the cell from constant-current 
charging to constant-voltage charging when it exceeds full charging. 
Then, cells were prepared under the same conditions as described above 
while varying the amount of the additive between 0.05% by weight and 20% 
by weight based on the positive active material (LixCoO.sub.2). The cells 
each were subject to constant-voltage charging at a constant voltage of 
4.2 V (upper limit current: 100 mA) at an ambient temperature of 
25.degree. C. for 20 hours. Then, each of the cells was discharged to a 
final voltage of 2.8 V at a constant current of 100 mA, resulting in a 
discharge capacity (mAh) of the cell being measured. The results were as 
shown in Table 1. Then, the cells thus charged and discharged each were 
subject to continuous charging at a charging current of 2.8 A, to thereby 
be supercharged, so that a rate (%) of occurrence of bursting or explosion 
in each of the cells was measured. Measurement of the rate was carried out 
using fifty (50) samples. The results were likewise as shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Amount of Addition (% by weight) 
Cell 0.05 
0.1 
0.2 
1 5 10 15 20 
__________________________________________________________________________ 
Ex. 1 (mAh) 1225 
1224 
1222 
1220 
1195 
1175 
1145 
985 
Li.sub.3 PO.sub.4 (%) 
6 2 0 0 0 0 0 0 
Ex. 2 (mAh) 1220 
1218 
1222 
1219 
1198 
1164 
1140 
992 
Co.sub.3 (PO.sub.4).sub.2 (%) 
5 2 0 0 0 0 0 0 
Ex. 3 (mAh) 1224 
1223 
1221 
1220 
1194 
1173 
1144 
979 
SrCO.sub.3 (%) 
6 3 1 0 0 0 0 0 
Comp. Ex. 1 (mAh) 
1230 
1230 
1230 
1230 
1230 
1230 
1230 
1230 
No additive 22 22 22 22 22 22 22 22 
Comp. Ex. 2 (mAh) 
1213 
1211 
1211 
1207 
1188 
1166 
1133 
978 
Li.sub.2 CO.sub.3 (%) 
8 4 2 2 0 2 0 0 
Comp. Ex. 3 (mAh) 
1218 
1217 
1217 
1210 
1190 
1171 
1135 
975 
Li.sub.2 C.sub.2 O.sub.4 (%) 
16 12 18 12 14 16 16 16 
Comp. Ex. 4 (mAh) 
1216 
1215 
1215 
1208 
1188 
1169 
1132 
972 
MnCO.sub.3 (%) 
18 12 12 12 16 14 14 12 
Comp. Ex. 5 (mAh) 
1219 
1218 
1217 
1210 
1290 
1171 
1133 
972 
CoCO.sub.3 (%) 
10 6 6 4 4 2 2 2 
Comp. Ex. 6 (mAh) 
1220 
1220 
1218 
1211 
1191 
1172 
1134 
973 
NiCO.sub.3 (%) 
16 12 14 14 12 14 12 14 
Comp. Ex. 7 (mAh) 
1210 
1210 
1207 
1200 
1181 
1162 
1125 
966 
Na.sub.2 CO.sub.3 (%) 
18 18 16 12 14 12 18 16 
Comp. Ex. 8 (mAh) 
1215 
1215 
1213 
1205 
1186 
1168 
1129 
970 
K.sub.2 CO.sub.3 (%) 
18 16 16 16 16 16 14 12 
Comp. Ex. 9 (mAh) 
1220 
1219 
1218 
1211 
1192 
1173 
1135 
975 
Rb.sub.2 CO.sub.3 (%) 
14 12 16 16 14 16 16 12 
Comp. Ex. 10 (mAh) 
1210 
1209 
1208 
1201 
1182 
1163 
1125 
966 
CaCO.sub.3 (%) 
16 18 16 14 16 14 16 12 
Comp. Ex. 11 (mAh) 
1209 
1208 
1208 
1200 
1181 
1161 
1123 
961 
MgCO.sub.3 (%) 
12 12 10 6 8 4 4 4 
Comp. Ex. 12 (mAh) 
1213 
1212 
1212 
1203 
1185 
1166 
1128 
969 
BaCO.sub.3 (%) 
10 6 6 4 4 2 4 2 
__________________________________________________________________________ 
Table 1 clearly indicates that the cells of Examples 1 to 3 each 
significantly prevent bursting or explosion thereof due to supercharging 
as compared with those of Comparative Examples 1 to 12. This is due to the 
fact that the cells of Examples 1 to 3 restrain an increase in charging 
current to substantially prevent decomposition of the electrolyte, even 
when the cell voltage exceeds the decomposition voltage. This is also 
noted from FIGS. 3 to 5. 
Also, Table 1 indicates that a decrease in the amount of addition of 
lithium phosphate, cobalt (II) phosphate or strontium carbonate to a level 
below 0.2% by weight causes a rate of bursting or explosion of the cell to 
be increased. This is due to a reduction in generation of gas in the cell. 
Further, an increase in the amount of addition of the additive to a level 
above 15% by weight causes a reduction in discharge capacity of the cell. 
This is due to an increase in internal resistance of the cell due to low 
conductivity of lithium phosphate, cobalt (II) phosphate or strontium 
carbonate. 
Then, cells were prepared under the same conditions as those of Examples 1 
to 3 while varying an average particle diameter of lithium phosphate, 
cobalt (II) phosphate or strontium carbonate within a range of from 5 
.mu.m to 40 .mu.m. The cells were subject to measurement of a rate of 
occurrence of bursting or explosion thereof under the same conditions as 
described above. The measurement was carried out using fifty samples. The 
results were as shown in Table 2. 
TABLE 2 
______________________________________ 
Average Particle Diameter (.mu.m) 
Cell 5 10 15 20 25 30 35 40 
______________________________________ 
Ex. 1 0 0 0 0 0 0 1 2 
Li.sub.3 PO.sub.4 
Ex. 2 0 0 0 0 0 0 2 2 
CO.sub.3 (PO.sub.4).sub.2 
Ex. 3 0 0 0 0 0 1 2 2 
SrCO.sub.3 
______________________________________ 
Table 2 indicates that an increase in average particle diameter of the 
additive to a level above 30 .mu.m causes an increase in rate of the 
bursting or explosion of the cell. This is due to the fact that such an 
increase in average particle diameter causes a specific surface area of 
the additive based on a weight thereof to be increased, to thereby delay 
generation of gas with respect to an increase in cell voltage. 
In each of the examples, lithium phosphate or cobalt (II) phosphate was 
used as the phosphate compound to be added in the positive active material 
layer. It was found that any other phosphate compound likewise exhibits 
substantially the same advantage as the above. 
In addition, in each of the examples, LixCoO.sub.2 was used as the positive 
active material. It was found that any other lithium double oxide such as 
LixNiO.sub.2, LixMnO.sub.2 or the like likewise exhibits substantially the 
same advantage. 
As can be seen from the foregoing, the non-aqueous liquid electrolyte 
secondary cell of the present invention prevents a significant increase in 
charging current to keep the electrolyte from being decomposed, even when 
the current breaking device fails in operation due to any trouble or 
failure thereof to cause supercharging to further continue. Also, the cell 
of the present invention effectively prevents bursting or explosion 
thereof even when the relief valve for pressure release is not arranged 
therein or fails in operation thereof due to any failure. 
While a preferred embodiment of the invention has been described with a 
certain degree of particularity with reference to the drawings, obvious 
modifications and variations are possible in light of the above teachings. 
It is therefore to be understood that within the scope of the appended 
claims, the invention may be practiced otherwise than as specifically 
described.