Multi-layer type nonaqueous electrolyte secondary cell

Internal shorting of large capacity multi-layer type nonaqueous electrolyte secondary cells is minimized by inclusion of heat resistant porous film layers adjacent thermally fusible resin microporous films disposed between negative electrodes and positive electrodes in an electrode assembly and between adjacent electrode assemblies stacked together to provide an electrode stack. The heat resistant porous film layers may be organic or inorganic temperature resistant sheet materials exhibiting heat resistance to temperatures of at least about 600.degree. C.

BACKGROUND OF THE INVENTION 
The present invention relates to a multi-layer type nonaqueous electrolyte 
secondary cell such as a large-capacity lithium ion secondary cell 
suitably used for electric vehicles, uninterruptible power supplies (UPS), 
load leveling machines, and the like. 
Lithium ion secondary cells including multi-layer type nonaqueous 
electrolyte secondary cells have been under research and development in a 
number of fields as a possible solution to environmental problems such as 
for example as a power source for electric vehicles, UPS and road leveling 
machines. There is a great demand for a lithium ion secondary cell having 
a large capacity, high output, high voltage and long shelf life. 
In a charged lithium ion secondary cell, lithium ions within an active 
material of the positive electrode dissolve in the electrolyte, pass 
through a separator, and penetrate into the active material of the 
negative electrode. During discharging, the lithium ions penetrated into 
the active material of the negative electrode dissolve to the electrolyte 
and return to the active material of the positive electrode. In this way, 
the charging-discharging operation is performed. 
In most of the conventional small-sized lithium ion secondary cells, in 
order to improve the energy density, an active material is coated on the 
two sides of a metal foil collector to provide a sheet-like positive 
electrode and a sheet-like negative electrode. A plurality of separators 
of thermally-fusible polyethylene or polypropylene resin are disposed 
between a cathode and an anode to provide an electrode pair and a 
multiplicity of electrode pairs of predetermined size are stacked to form 
a rectangular cell. Alternatively, long positive and negative electrodes 
may be wound together with a plurality of polyethylene or polypropylene 
separators to form a cylindrical cell structure. 
Generally, the above-mentioned small-sized lithium ion cells have a 
capacity of not more than several Ah. If a shorting occurs in or outside 
of the cell, the internal temperature of the cell increases, and the 
separators constituting a micro-porous film of polyethylene or 
polypropylene are fused by heat. As a result, the pores are closed and 
shut off the flow of ions between the electrodes. After a time, the 
shorting current is reduced and heat generation is suppressed. 
In the case of large capacity lithium-ion secondary cells constructed of a 
plurality of layers of positive and negative electrodes each including a 
collector with active material coated on the sides thereof, similar to the 
above-mentioned small-sized lithium ion secondary cell, heat may be 
generated by an internal shorting. The separator of thermally fusible 
resin between adjacent positive and negative electrodes is thermally 
fused, thereby enlarging the internal shorting. As a result, a great 
amount of heat is released to the environment, often blowing out a great 
amount of gas. 
Generally, a test simulating an internal shorting is conducted in which a 
nail is pierced from outside of the cell to artificially short the 
positive and negative electrodes. The present inventor has found that when 
a large-capacity lithium ion secondary cell is pierced by nail, a great 
amount of gas is blown out and the heat due to the resistance of the 
pierced portion constitutes a fire source. Consequently, the separator 
between adjacent positive and negative electrodes is thermally fused. The 
direct reaction between the positive and negative electrodes generates 
heat, followed by thermal fusion of the separator between the adjacent 
electrodes. In this way, heat is generated sequentially, finally leading 
to a great amount of heat being generated by the reaction among all the 
electrodes. 
SUMMARY OF THE INVENTION 
In view of these problems, an object of the present invention is to provide 
a multi-layer type nonaqueous electrolyte secondary cell, in which an 
internal shorting of a cell, such as the large-capacity lithium ion 
secondary cell, is prevented from affecting adjacent positive and negative 
electrodes thereby minimizing the damage to the cell and the environment. 
In accordance with its first embodiment, the present invention provides a 
multi-layer type nonaqueous electrolyte secondary cell comprising a 
negative electrode, a thermally fusible resin micro-porous film, and a 
positive electrode, in stacked relationship to form an electrode stack. A 
heat resistant porous film comprising at least one layer of an organic or 
an inorganic material is disposed adjacent to said thermally fusible resin 
micro-porous film. 
According to this invention, there is provided a multi-layer type 
nonaqueous electrolyte secondary cell, in which a heat-resistant porous 
film of, for example, an organic or an inorganic material is disposed 
adjacent to the whole or part of a thermally fusible resin micro-porous 
film. The heat resistant porous layer has a heat-resistance temperature of 
600.degree. C. or higher. The heat resistant porous layer is not thermally 
fused or decomposed when an internal shorting occurs in a large-capacity 
multi-layer type nonaqueous electrolyte secondary cell. As a result, the 
enlargement of shorting is prevented, thereby minimizing the damage to the 
cell and the environment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, explanation will be made about a multi-layer type nonaqueous 
electrolyte secondary cell according to an embodiment of the invention as 
applied to a lithium ion secondary cell with reference to FIGS. 1, 2 and 
3. 
In FIGS. 2 and 3, numeral 10 designates a flat-type rectangular cell case, 
which is made of a stainless steel plate having a thickness of, for 
example, 300 .mu.m. The flat rectangular cell case 10 includes a cell case 
body 10a having length dimension, l, of about 300 mm, a height dimension, 
h, of about 115 mm and a width dimension, w, of about 22 mm, and an upper 
cover 10b made of a stainless steel plate having a thickness of 1.5 mm. 
In the flat-type rectangular cell case 10, as shown in FIG. 1, a plurality 
of positive electrode units each including a sheet of positive electrode 2 
encased in a bag-shaped separator 8 of a micro-porous film of a thermally 
fusible resin, and a plurality of negative electrode units each including 
a sheet of negative electrode 3 encased in the bag-shaped separator 8 of a 
micro-porous film of a thermally fusible resin. The positive electrode 
units and the negative electrode units are alternately stacked with a 
plurality of intermediate heat-resistant porous films 20 placed 
therebetween. The resulting electrode stack 14 (refer to FIG. 3) is 
encased in the flat-type rectangular cell case 10. 
The positive electrode 2 is fabricated in the manner described below. 
Lithium carbonate and cobalt carbonate are mixed together to provide a mol 
ratio Li/Co=1, and baked in air for 5 hours at 900.degree. C. thereby to 
synthesize a positive active material (LiCoO.sub.2). This positive active 
material is crushed in an automatic crushing bowl thereby to produce 
LiCoO.sub.2 powder. 95 weight % of the prepared LiCoO.sub.2 powder and 5 
weight % of lithium carbonate are mixed. 91 weight % of this mixture is 
further mixed with 6 weight % of graphite as a conductive material and 3 
weight % of polyvinylidene fluoride as a binder thereby to produce an 
active cathode material. The active cathode material is dispersed in 
N-methyl-2-pyrrolidone to form a slurry. The slurry is coated on the both 
sides of a band-shaped aluminum foil constituting a positive collector 5 
(refer to FIG. 1) except for the leads thereof. This positive electrode 
assembly, after being dried, is formed by compression in a roller press 
thereby to prepare a sheet of positive electrode 2 coated with the active 
cathode material 4 on both sides of a positive collector 5. 
The negative electrode 3 is prepared as described below. A petroleum pitch, 
is used as a starting material, then oxygen is introduced into the 
petroleum to provide about 10 to 20% of functional groups (in what is 
called the oxygen cross-linking). After that, the material is baked at 
1000.degree. C. in an inactive gas, thus producing a non-graphitizable 
carbon material similar in property to glass carbon. 
90 weight % of this carbon material constituting a negative active material 
is mixed with 10 weight % of polyvinylidene fluoride as a binder to 
prepare an active anode material for the negative electrode. The active 
anode material is dispersed into N-methyl-2-pyrrolidone to form a slurry. 
This slurry is coated on the both sides of a band-shaped copper foil 
constituting a negative collector 7 (refer to FIG. 1) except for the lead 
sections. This anode assembly, after being dried, is formed by compression 
in a roller press, thereby producing a sheet of negative electrode 3 with 
the active anode material 6 coated on the two sides of the negative 
collector 7. 
This sheet of positive electrode 2 is cut or punched in such a manner that 
the size of the portion coated with the positive active cathode material 4 
is, for example, 107 mm.times.265 mm. The portion of the punched positive 
electrode 2 coated with the active cathode material 4 is encased in a 
bag-shaped separator 8 (refer to FIG. 1) made from two mutually-attached 
sheets of polyolefin micro-porous films such as polypropylene constituting 
a thermally fusible resin micro-porous film having a thickness of 25 .mu.m 
and a size of 112 mm by 273 mm, thus completing a positive electrode unit. 
In this case, the lead section 5a (refer to FIG. 3) of the positive 
electrode 2 is exposed from the separator 8. 
The negative electrode 3 in sheet form is punched in such a manner that the 
portion thereof coated with the active anode material 6 sections is, for 
example, 109 mm.times.270 mm in size. The portion of the negative 
electrode 3 coated with the active anode material 6 is encased in a 
bag-shaped separator 8 having two mutually-attached thermally fusible 
resin micro-porous films of polyolefin group such as polypropylene 25 
.mu.m thick and 112 mm by 273 mm in size, thereby constituting a negative 
electrode unit. In this case, the lead section 7a of the negative 
electrode 3 is exposed from the separator 8. 
The heat-resistant porous film 20, shown in FIG. 1, is prepared as 
described below. In this example, a polyimide film (Capton.RTM. brand, for 
example) about 25 .mu.m thick having a heat-resistance temperature of 
about 800.degree. C. is punched in a press to provide an apertured 
polyimide punched film having a plurality of holes of about 0.8 mm in size 
arranged at 1.27 mm pitch or spacing over the entire surface. The 
apertured film in turn is punched into the size of 109 mm by 270 mm, 
thereby producing a heat-resistant porous film 20. 
47 negative electrode units, 46 positive electrode units and 92 
heat-resistant porous films 20 are sequentially stacked in the order of a 
negative electrode unit, a heat-resistant porous film 20, a positive 
electrode unit, a heat-resistant porous film 20, a negative unit, and so 
on. In this way, a rectangular parallelopipedal electrode stack 14 is 
formed as shown in FIG. 3. In this case, the lead sections 5a of the 
positive electrodes 2 extend from one side of the electrode stack and the 
lead sections 7a of the negative electrodes 3 extend from the other side 
of the electrode stack. 
Also, as shown in FIG. 3, one side of the electrode stack 14, i.e., the 
lead sections 5a exposed from the separator 8 of the positive electrodes 2 
are welded by ultrasonic methods to the positive terminal 11 of aluminum 
in parallelopipedal form. Further, the other side of the electrode stack 
14, i.e., the lead sections 7a of the negative electrodes 3 exposed from 
the separator 8 are welded by ultrasonic methods to the negative terminal 
12 of copper in parallelopipedal form. 
The outer periphery of the electrode stack 14 with the positive terminal 11 
and the negative terminal 12 welded thereto as shown in FIG. 3 is covered 
with an insulating sheet of a 125 .mu.m thick polyimide film. The 
resulting assembly is bolted to the upper cover 10b by means of the 
positive terminal 11 and the negative terminal 12 through an O-ring and an 
insulating ring (not shown in FIG. 3). After that, the assembly is 
inserted in the cell case body 10a, and the upper cover 10b is fixedly 
laser-welded to the cell case body 10a. 
After assembly, a non-aqueous organic electrolyte including a solvent 
mixture of propylene carbonate and diethyl carbonate in which LiPF6 is 
dissolved in the ratio of 1 mol/l is injected into the flat rectangular 
cell case 10. 
Also, a safety valve 13 is arranged on the upper cover 10b to drain off 
internal air in case the internal pressure of the enclosed flat 
rectangular cell case 10 increases beyond a predetermined level. 
In the lithium ion secondary cell in accordance with this embodiment, the 
heat-resistant porous film 20 with a plurality of holes 0.8 mm in size 
regularly spaced at 1.27 mm pitch has an ion transmissibility, and thus it 
is possible to produce a lithium ion secondary cell of a capacity as large 
as 53 Ah. 
In accordance with this embodiment, a heat-resistant porous film 20 
comprising polyimide having a heat-resistance temperature of 800.degree. 
C. is arranged between each pair of the negative electrode units and the 
positive electrode units. Even when an internal shorting occurs, 
therefore, the polyimide heat-resistant porous film 20 is not thermally 
fused or decomposed, so that an enlargement of the internal shorting is 
prevented, thereby minimizing the damage to the cell and the environment. 
Research performed by the inventors on the heat-resistance temperature of 
the heat-resistant porous film 20 has shown that if the internal shorting 
is to be prevented from spreading, the large-capacity lithium ion 
secondary cell as described above desirably requires a heat resistance 
temperature of at least 600.degree. C. for the practical type having a 
thickness of 200 .mu.m or less, or preferably 50 .mu.m or less, depending 
on the thickness of the heat-resistant porous film 20. The heat-resistance 
temperature of 800.degree. C. or higher is preferred. 
Further, in the above-mentioned example, the heat-resistant porous film 20 
is disposed between separators 8 composed of a micro-porous film of 
thermally-fusible polyolefin resin. When an external shorting occurs, the 
minute pores of the separator 8 of polyolefin resin are closed (shut down) 
with the increase of the battery temperature to prevent an enlargement of 
the internal shorting. As a result, the charge transfer of lithium ions is 
prevented between the electrodes for a reduced discharge current, thereby 
leading to the advantage that an excess energy is not released. 
A nailing test was conducted on the above-mentioned lithium ion secondary 
cell. The result of this nailing test is shown in Table 1, as follows: 
TABLE 1 
______________________________________ 
Comparative Weight Reductions for 
Secondary Cells After External Shorting 
Heat Resistant 
EXAMPLE Porous Film Weight Reduction, % 
______________________________________ 
Embodiment 1 
Polyimide punching 
18 
film 
Embodiment 2 
Polyamide fibrous 
24 
paper 
Embodiment 3 
Fluoric resin 23 
film + almina particles 
Embodiment 4 
Alumina fiber cloth 
17 
Embodiment 5 
Mica porous film 
20 
Embodiment 6 
Glass fiber cloth 
18 
Reference 128 
______________________________________ 
In Table 1, the weight reduction represents the percentage of the weight 
reduction of the cell after nailing against the weight of cell before 
nailing. The smaller the weight reduction, the smaller amount of gas that 
blows out. According to the first embodiment, the gas blows out in a 
comparatively small amount of 18%. As a comparative embodiment, 46 
positive electrode units and 47 negative electrode units are stacked 
sequentially as in the first embodiment, without the heat-resistant porous 
film, as shown in FIG. 7 to form an electrode stack 14. The other parts of 
the configuration are the same as the corresponding parts of the first 
embodiment. In this way, a lithium ion secondary cell with a capacity of 
53 Ah was obtained and subjected to a nailing test. 
The result of the nailing test conducted on the comparative embodiment is 
shown in Table 1. As shown, the weight reduction is large at 128%. An 
internal shorting occurs and the heat generated reaches the adjacent 
electrodes. Not only the electrolyte but also a part of the cell materials 
including the separator are blown out. 
The second embodiment shown in Table 1 is a lithium ion secondary cell, in 
which each of a plurality of heat-resistant porous films 20 according to 
the first embodiment is made of aromatic polyamide fiber such as 
polyaramid fiber (Technora) with a heat-resistance temperature of 
600.degree. C. or higher and heat-treated to produce a wet unwoven fiber 
web or paper 50 .mu.m thick. The structure of the other parts are the same 
as those of the lithium ion secondary cell of the first embodiment. 
The polyaramid fibrous paper constituting the heat-resistant porous film 20 
has a heat-resistance temperature of 600.degree. C. or higher. The nailing 
test conducted on the second embodiment, as seen from Table 1, shows that 
the weight reduction is a comparatively small 24% with substantially the 
same effect as in the first embodiment. 
In the third embodiment shown in Table 1, the heat-resistant porous film 20 
is a porous film containing powder with a 30% porosity and 50 .mu.m thick, 
obtained by heating, hardening and rapidly extending the suspension of the 
polytetrafluoroethytene powder constituting a thermoplastic fluoropolymer 
and alumina powder 10 .mu.m in average particle size. The heat-resistance 
temperature of the fluoropolymer porous film containing the alumina powder 
is 600.degree. C. or higher. The other parts of the configuration are 
identical to those for the lithium ion secondary cell in the first 
embodiment. 
The fluoropolymer porous film containing alumina powder constituting the 
heat-resistant porous film 20 mentioned above has a heat-resistance 
temperature of 600.degree. C. or higher. A nailing test conducted on the 
third embodiment shows as in Table 1 that the weight reduction is a 
comparatively small 23%. The effect similar to the first embodiment thus 
is obtained for the third embodiment. 
The fourth embodiment shown in Table 1, on the other hand, represents the 
heat-resistant porous film 20 of the first embodiment made up of an 
alumina fiber cloth providing an inorganic fiber with a heat-resistance 
temperature of at least 800.degree. C. This alumina fiber cloth is 
obtained in such a manner that powder mixture in the ratio of Al.sub.2 
O.sub.3 :SiO.sub.2 :B.sub.2 O.sub.3 =68:27:5 is dispersed in water, and 
the resulting suspension is dropped through a nozzle, dried and baked. In 
this way, a bundle of 80 filaments 11 .mu.m in diameter is obtained. The 
filament bundle thus obtained is woven into an alumina fiber cloth 80 
.mu.m in thickness. The other component parts of the lithium ion secondary 
cell are identical to the corresponding parts in the first embodiment. 
The alumina fiber cloth constituting this heat-resistant porous film 20 has 
a heat-resistance temperature of at least 800.degree. C., A nailing test 
conducted on the fourth embodiment shows, as seen from Table 1, that the 
weight reduction is a comparatively small 17%, indicating that 
substantially the same effect and advantage are obtained as in the first 
embodiment. 
In the fifth embodiment shown in Table 1, an electrode stack 14 inserted 
into the flat rectangular cell case 10 is configured, as shown in FIG. 4, 
in such a manner that the positive electrode 2 and the negative electrode 
3 in sheet form are laid one on another through a heat-resistant porous 
film unit including heat-resistant porous films 20 encased in the 
bag-shaped separator 8 made up of a thermally fusible resin micro-porous 
film. 
According to this fifth embodiment, the positive electrode 2 and the 
negative electrode 3 are formed in a manner similar to the first 
embodiment. Also, the heat-resistant porous film 20 is fabricated in the 
manner mentioned below. A hard mica block is crushed in a wet environment 
to a grain thickness of 0.49 .mu.m. The resulting powder is classified 
using a strainer. The powder of grain size of 60M to 80M (M: mesh) in 
4.5%, the grain size of 80M to 140M in 40.8%, the grain size of 140M to 
200M in 7.2% and the grain size of 200M or more in 47% are used to make a 
composite mica sheet 34 .mu.m thick. 
The mica sheet thus obtained is soaked with the N-methyl-2-pyrrolidone 
solution of polyvinylidene fluoride (PVDF), so that the 
N-methyl-2-pyrrolidone is rapidly vaporized as a foaming process, thereby 
producing a mica micro-porous film 20 having a thickness size of 50 .mu.m. 
This mica micro-porous film 20 is punched to the size of 109 mm.times.270 
mm. The resulting assembly is encased in a bag-shaped separator 8 with two 
mutually-attached polyolefin micro-porous films such as polypropylene 
constituting a thermally fusible resin micro-porous film having a 
thickness of 25 .mu.m and a size of 112 mm by 273 mm, thus completing a 
heat-resistant porous film unit. 
In this fifth embodiment, the negative electrode 3, the heat-resistant 
porous film unit, the positive electrode 2, the heat-resistant porous 
unit, the negative electrode 3 and so on, are sequentially stacked for a 
total of 41 negative electrodes, 40 positive electrodes 2 and 80 
heat-resistant porous film units thereby to form a parallelopipedal 
electrode stack 14 as shown in FIG. 3. In the process, the assembly is 
formed in such a manner that the lead section 5a of the positive electrode 
2 is situated on one side, and the lead section 7a of the negative 
electrode 3 on the other side. 
The configuration of the other component parts is similar to the 
corresponding one of the first embodiment. In this way, a lithium ion 
secondary cell having a capacity of 46 Ah is obtained. 
The mica micro-porous film constituting this heat-resistant porous film 20 
according to the fifth embodiment has a heat-resistance temperature of at 
least 800.degree. C. A nailing test conducted on the fifth embodiment, as 
seen from Table 1, shows that the weight reduction is comparatively small 
at 20%. It will be easily understood that a similar effect is obtained in 
this fifth embodiment as in the first embodiment described above. 
The sixth embodiment, as seen from Table 1, shows an example of a 
cylindrical lithium ion secondary cell. For fabricating the lithium ion 
secondary cell according to the sixth embodiment, an active cathode 
material 4 is coated on the both sides of a positive collector 5, 280 mm 
by 1745 mm in size, to make a band-shaped positive electrode 40 as in the 
first embodiment. Then, a band-shaped negative electrode 41 is fabricated 
in a similar manner as the first embodiment by coating an active anode 
material 6 on the sides of a negative collector 7, 283 mm by 1750 mm in 
size. 
Also, in this example, a heat-resistant porous film unit 20a having a size 
of 287 mm by 1755 mm is prepared in a form disposed between two sheets of 
polyolefin resin micro-porous film such as separators 8 comprising 
polypropylene constituting a thermally fusible resin micro-porous film 25 
.mu.m thick. 
According to the sixth embodiment, the heat-resistant porous film 20 is 
made of glass fiber woven with the density of 78 strands per inch (warp) 
and 73 strands per inch (weft). In this way, a glass fiber cloth having a 
thickness of 51 .mu.m and a porosity of 11% is obtained. 
In the sixth embodiment, as shown in FIG. 5, the negative electrode 41, the 
heat-resistant porous unit 20a including a separator layer 8, heat 
resistant porous layer 20 and another separator layer 8, the positive 
electrode 40, a heat-resistant porous unit 20a and so on, are laid on one 
another in this order. The resulting assembly is wound by a predetermined 
number of turns in spiral form along the longitudinal direction thereby to 
form a spiral stack 44. 
Also, according to this example, as shown in FIG. 6, an end of the negative 
lead 45 made of nickel is fused by resistance welding to the lead section 
on one side of the negative electrode 41. At the same time, the lead 
section on one side of the positive electrode 40 is fused by resistance 
welding to an end of the positive lead 46 made of aluminum. 
A nickel-plated cylindrical cell can 47a of iron having a diameter of 50 mm 
and a height of 300.5 mm is prepared. After an insulating plate is 
inserted into the bottom of the cell can 47a, as shown in FIG. 6, a spiral 
stack 44 is inserted into the cell can 47a. In the process, the negative 
terminal 49 and the positive terminal 50 arranged on the cell cover 47b 
are welded to the other end each of the negative lead 45 and the positive 
lead 46, respectively. 
An electrolyte dissolved with LiPF6 at the rate of one mol/1 in a solvent 
mixture of 50 vol % of polypropylene carbonate and 50 vol % of diethyl 
carbonate is injected into the cell can 47a. After that, the cell cover 
47b is caulked to the cell can 47a through an insulating seal gasket 
coated with asphalt. With the cell cover 47b fixed this way, a cylindrical 
large-capacity lithium ion secondary cell having a capacity of 20 Ah is 
completed. 
Also, the cell cover 47b may have a safety valve 48 for draining off the 
internal air when the internal pressure of the hermetic cell case 47 rises 
beyond a predetermined level. 
As will be readily understood, according to the sixth embodiment, the 
heat-resistant porous film 20 is interposed between the positive electrode 
40 and the negative electrode 41 along the diameter of the spiral stack 
44. The heat-resistant porous film 20 is made of glass fiber cloth having 
a heat-resistance temperature of at least 800.degree. C. A nailing test 
conducted on this sixth embodiment shows, as seen from Table 1, that the 
weight reduction is a comparatively small 18%. It is thus seen that 
substantially the same effect is obtained in this embodiment as in the 
first embodiment. 
The heat-resistant porous film according to the invention is not limited to 
the one used in the above-mentioned embodiments, but any type of 
heat-resistant porous film may be used so long as it has a heat-resistance 
temperature of 600.degree. C. or higher and can be formed comparatively 
thin to form multi-layer type nonaqueous cell. 
Also, although the heat-resistant porous film is inserted between every 
pair of the positive electrodes and the negative electrodes according to 
this embodiment, each heat-resistant porous film may be interposed at 
intervals of every other pair or several pairs of the positive and 
negative electrodes. In such a case, too, substantially the same effect 
can be obtained as in the aforementioned embodiments, as will be easily 
understood. 
Further, instead of applying the invention to a lithium ion secondary cell 
as described in the foregoing embodiments, the invention is of course 
applicable to alternative multi-layer type nonaqueous electrolyte 
secondary cells. 
Furthermore, the present invention may assume various modifications without 
departing from the spirit and scope thereof. 
It will thus be understood that according to the present invention, a 
heat-resistant porous film of polyimide, polyamide, inorganics or organics 
can be arranged adjacently to the whole or part of a thermally fusible 
resin micro-porous film. The heat-resistance temperature of these 
components is at least 600.degree. C., so that an internal shorting, which 
may occur in a large-capacity multi-layer type nonaqueous electrolyte 
secondary cell, is prevented from spreading for lack of thermal fusion and 
decomposition. Furthermore, in the process, the thermally fusible resin 
micro-porous film is fused to enclose (shut down) the micro-probes. 
Therefore, the charge transfer of lithium ions between the electrodes is 
prevented for a reduced discharge current with a smaller release of excess 
energy, thereby minimizing the damage to the cell and the effect on the 
environment.