Patent Application
20150372338
The present invention relates to a non-aqueous electrolyte secondary battery including an electrode wound body and a method for manufacturing the non-aqueous electrolyte secondary battery, and particularly to a method for manufacturing a positive electrode sheet of the battery. More particularly, the present invention relates to a non-aqueous electrolyte secondary battery configured to prevent elution of metal component from a width-direction end portion of a mixture layer in an outermost circumferential portion of an electrode wound body, a method for manufacturing a positive electrode sheet of the non-aqueous electrolyte secondary battery, and a method for manufacturing the non-aqueous electrolyte secondary battery.
Heretofore, such a non-aqueous electrolyte secondary battery disclosed in Patent Document 1 for example generally employs an electrode wound body composed of a positive electrode sheet and a negative electrode sheet wound in overlapping relation with separators interposed therebetween. An electrode sheet of this type of non-aqueous electrolyte secondary battery is formed of a current collector sheet (a metal foil) formed with a mixture layer made of electrode active material. For forming the mixture layer on the current collector sheet, commonly, mixture paste made by kneading powder of the electrode active material and other components for the mixture layer with solvent is used. In other words, the mixture paste which is fluid material is applied or coated onto the current collector sheet and dried to form the mixture layer.
Patent Document 1: JP-A-2009-283270
However, the foregoing conventional art would cause the following problems. Specifically, in an end portion of the formed mixture layer, a thin layer region having a slant surface, resulting in a thin layer thickness, is inevitably generated due to fluidity and surface tension of the mixture paste. This thin layer region leads to a disadvantage that a battery capacity of the non-aqueous electrolyte secondary battery could not be sufficiently obtained. In this thin layer region, furthermore, metal elements are eluted due to a local increase in potential during charging. Elution of the metal elements would be especially problematic in the end portion of the positive electrode mixture layer for the following reason. In the electrode wound body of the non-aqueous electrolyte secondary battery of the above type, as shown in a cross-sectional schematic diagram of
The above problem is especially serious on an outer surface side of an outermost circumferential portion of a positive electrode sheet in the electrode wound body. This is because the electrode wound body of this type of non-aqueous electrolyte secondary battery is configured such that a negative electrode sheet is located as an outermost circumferential electrode sheet. Thus, a mixture layer 21E on the outer surface side of the outermost circumferential portion of the negative electrode sheet does not face the mixture layer 31 of the positive electrode sheet. Even in this portion 21E, lithium ions escaped from the thin layer region 31R in the end portion of the positive electrode mixture layer 31 on the outer surface side of the outermost circumferential portion of the positive electrode sheet will diffuse by detouring around the negative electrode sheet (arrow B). This also additionally influences the local increase in potential in the thin layer region 31R, leading to elution of metal elements therefrom.
As above, the mixture layer end portion of the positive electrode sheet is apt to release a large amount of lithium ions during charging. However, the above-described mixture-layer end portion is formed as the thin layer region, which originally has a smaller amount of active material than other portions of the mixture layer. This leads to the foregoing problems. The technique of Patent Document 1 is also configured to address a negative effect resulting from the thin layer region. However, this technique of Patent Document 1 could still generate a thin layer region with a width on the order of millimeter. Thus, it would be insufficient to prevent the negative effect by the thin layer region.
The present invention has been made to solve the above problems and has a purpose to provide a non-aqueous electrolyte secondary battery configured to enable effectively preventing a problem due to a thin layer region in an end portion of a mixture layer, a method for manufacturing a positive electrode sheet of the non-aqueous electrolyte secondary battery, and a method for manufacturing the non-aqueous electrolyte secondary battery.
To achieve the above purpose, a first aspect of the invention provides a non-aqueous electrolyte secondary battery having an electrode wound body including a positive electrode sheet and a negative electrode sheet, which are wound in overlapping relation with separators interposed therebetween, wherein an outermost circumferential electrode sheet of the electrode wound body is the negative electrode sheet, and the positive electrode sheet is configured such that a mixture layer formed on an outer surface of an outermost circumferential portion has an end portion in a width direction having a steep cross sectional shape in which a portion as thin as or thinner than 50% of a thickness of a flat portion of the mixture layer at a center in the width direction has a width of 100 μm or less. Since the width-direction end portion of the mixture layer has such a steep cross sectional shape, it is possible to effectively prevent any problem due to a thin layer region.
Further, a second aspect of the invention provides a method for manufacturing a positive electrode sheet of a non-aqueous electrolyte secondary battery having an electrode wound body including a positive electrode sheet and a negative electrode sheet, which are wound in overlapping relation with separators interposed therebetween, wherein the method comprises a coating step of coating positive electrode mixture paste onto a current collector sheet to form a mixture layer, and the mixture layer to be formed in the coating step has an end portion in a width direction having a steep cross sectional shape in which a portion as thin as or thinner than 50% of a thickness of a flat portion of the mixture layer at a center in the width direction has a width of 100 μm or less in at least the outermost circumferential region on a surface which will be an outer surface of the electrode wound body.
Herein, according to the first method for manufacturing a positive electrode sheet of a non-aqueous electrolyte secondary battery in the second aspect, prior to the coating step, wettability adjusting treatment is performed on at least an outer surface of an outermost circumferential region of the current collector sheet in a longitudinal direction, the outermost circumferential region corresponding to a range which will be placed on an outermost circumference of the electrode wound body, to adjust a wettability value NA of a width-direction end portion to be formed as an uncoated portion and a wettability value NB of a width-direction central portion to be formed as a coated portion at a ratio NA/NB expressed by
0.5<NA/NB<1.
By this wettability adjusting treatment, a steep cross sectional shape of the end portion of the foregoing mixture layer in the width direction is achieved. This is because the positive electrode mixture paste is uniformly applied onto the coated portion having high wettability, while the positive electrode mixture paste is repelled in the uncoated portion having low wettability.
The wettability adjusting treatment may include at least one of a treatment of decreasing wettability of the width-direction end portion of the current collector sheet and a treatment of increasing wettability of the width-direction central portion of the current collector sheet. The treatment of decreasing the wettability may include an oil coating process or a water-repellent material coating process in the case where the decreasing treatment is performed. The treatment of increasing the wettability may include a corona discharge treatment, a roughening treatment, and a cleaning treatment using solvent in the case where the increasing treatment is performed. The wettability adjusting treatment may be performed over an entire region of the current collector sheet in the longitudinal direction or may performed on only a range of an entire region of the current collector sheet in the longitudinal direction, the range being to be disposed on the outermost circumference of the electrode wound body.
In a second method for manufacturing a positive electrode sheet of a non-aqueous electrolyte secondary battery according to the second aspect, the coating step uses positive electrode mixture paste having a TI value falling within a range of 1.7 to 4.6, the TI value being a ratio between viscosity at a shear rate of 2 s−1 and viscosity at a shear rate of 100 s−1 at 20° C. Accordingly, the steep cross sectional shape of the end portion of the mixture layer in the width direction is achieved. This is because the viscosity of the positive electrode mixture paste is low during coating in which the shear rate or velocity is fast, while the viscosity is high after coating in which the shear rate is slow.
According to the method for manufacturing a positive electrode sheet of a non-aqueous electrolyte secondary battery in the second aspect of the invention, preferably, a drying step of drying the mixture layer formed in the coating step is performed. On an entrance side in the drying step, it is preferable to make the end portion of the mixture layer in the width direction lower in temperature than the central portion in the width direction. This is because drying can be promoted while suppressing viscosity reduction due to a temperature rise in the width-direction end portion of the mixture layer. For this purpose, a back surface of the current collector sheet after the coating step is supported by a supporting roller, and the supporting roller is an end-portion cooling roller having cooling zones in end portions in a width direction and a non-cooling zone between the cooling zones. Alternatively, the supporting roller may be a central-portion heating roller having a heating zone in a central portion in a width direction and non-heating zones at both ends.
A method for manufacturing the non-aqueous electrolyte secondary battery of the invention uses a positive electrode sheet manufactured by any one of the foregoing manufacturing methods, together with a negative electrode sheet and separators, and comprises a winding step of winding the positive electrode sheet and the negative electrode sheet in overlapping relation with the separators interposed therebetween to form an electrode wound body. In this winding step, the negative electrode sheet is disposed as an outermost circumferential electrode sheet of the electrode wound body, and the portion having the steep cross sectional shape, of the end portion of the mixture layer in the width direction, is placed on at least an outer surface of an outermost circumferential portion of the positive electrode sheet.
According to the foregoing structure, there are provided a non-aqueous electrolyte secondary battery capable of effectively preventing a problem due to a thin layer region in an end portion of a mixture layer, a method for manufacturing a positive electrode sheet of the non-aqueous electrolyte secondary battery, and a method for manufacturing the non-aqueous electrolyte secondary battery.
A detailed description of a preferred embodiment of the present invention will now be given referring to the accompanying drawings. In the present embodiment, the invention is applied onto a positive electrode sheet of a battery 1 as shown in
The electrode wound body 3 is an assembly formed by overlapping and winding a positive electrode sheet and a negative electrode sheet with separators interposed therebetween. Furthermore, the electrode wound body 3 is impregnated with electrolyte. This electrode wound body 3 is a power generating element of the battery 1. At both ends of the electrode wound body 3 in a direction parallel to a winding axis direction, there are provided a region 20 in which only a negative electrode sheet is present and a region 30 in which only a positive electrode sheet is present. The region 20 and the external terminal plate 6 are connected through a current collecting member 13. Further, the region 30 and the external terminal plate 7 are connected through a current collecting member 12.
The electrode wound body 3 will be further explained. The electrode wound body 3 is constituted of a negative electrode sheet 22 and a positive electrode sheet 32 which are wound in overlapping relation as shown in a cross-sectional schematic view of
As seen in
The cross-sectional schematic diagram shown in
A cross sectional view of the positive electrode sheet 32 of the present embodiment is shown in
The positive electrode mixture layer 31 is not formed over the entire region of the current collector sheet 33. Near a right end in
As is clear from
The negative electrode sheet 22 of the present embodiment is also configured in a similar manner to the positive electrode sheet 32 shown in
Next, a method for manufacturing the positive electrode sheet 32 including the end edge region 31S with a small width L as described above will be explained. In the present embodiment, similarly, the positive electrode sheet 32 is manufactured in such a way that mixture paste of positive active material is applied onto an aluminum foil which is the current collector sheet 33 to form the positive electrode mixture layer 31. Herein, two ways are available for forming the end edge region 31S with a small width L; one is to subject the current collector sheet 33 to surface treatment in advance and the other is to use special mixture paste.
Of the foregoing ways, the way of subjecting the current collector sheet 33 to surface treatment in advance will be explained below as the first embodiment. In this first embodiment, prior to a coating treatment, the current collector sheet 33 made of an aluminum foil is subjected to a treatment for producing a wettability difference between a portion which is to be formed thereon with the positive electrode mixture layer 31 and a portion which is to be formed as the uncoated portion 34. Naturally, the wettability of the portion to be formed with the positive electrode mixture layer 31 is made high and the wettability of the portion to be formed as the uncoated portion 34 is made low. This prevents the mixture paste applied on the portion to be formed with the positive electrode mixture layer 31 from flowing onto the portion to be formed as the uncoated portion 34.
If the wettability of the portion to be formed as the uncoated portion 34 is high, the width L of the end edge region 31S of the positive electrode mixture layer 31 to be formed is apt to be large. Even if the mixture paste is applied onto only on the portion to be formed with the positive electrode mixture layer 31, the coated mixture paste could flow and move to the portion to be formed as the uncoated portion 34. Accordingly, the amount of mixture per area is decreased at and around an edge of the positive electrode mixture layer 31. On the other hand, if the wettability of the portion to be formed with the positive electrode mixture layer 31 is low, it conversely causes a negative effect that is likely to generate pinhole(s) in the positive electrode mixture layer 31. The surface of the current collector sheet 33 is given a wettability difference in advance, so that the positive electrode mixture layer 31 can be formed with the end edge portion having a good cross sectional shape and having no pinhole.
To be concrete, the wettability difference is produced as shown in
The foregoing differentiating into the regions 134 and the region 135 may be performed on both, front and back, surfaces of the aluminum foil 133 for forming the current collector sheet 33 or may be performed on only one surface. In the case of performing the differentiating on only one surface, the surface subjected to the differentiating will be an outward facing surface of the electrode wound body 3. The differentiating on only one surface is more advantageous in terms of cost. However, this needs to manage the orientation of the front and back surfaces of the positive electrode sheet 32 in a winding step. The differentiating on both the surfaces may cost more, but does not need to manage the orientation of the front and back surfaces of the positive electrode sheet 32 in a winding step.
A concrete method for making a wettability difference between the regions 134 and the region 135 is classified into two ways; decreasing the wettability of the regions 134 and increasing the wettability of the region 135. Only either one of the two ways may be performed or both the ways may be carried out. The way of decreasing the wettability of the regions 134 may include coating the relevant portion with a low wettability component. This low wettability component may include various kinds of grease and water repellent material (fluorine contained resin, silicone, etc.). The way of increasing the wettability of the region 135 may include corona discharge treatment, roughening treatment, cleaning treatment using solvent, and others.
Herein, an explanation is given to the extent of a wettability difference between the regions 134 and the region 135. The wettability is represented by a lower value for lower wettability and a higher value for higher wettability. Thus, the magnitude relation between the wettability NA of the regions 134 and the wettability NB of the region 135 is NA<NB.
In the present embodiment, furthermore, the ratio of the wettability NA to the wettability NB (NA/NB) is set in a range of
0.5<NA/NB<1.
Measuring the wettability can be performed by any known method. In examples mentioned later, the wettability is evaluated in terms of a repellent degree of wettability evaluation reagent. The evaluation reagent used was a wetting tension test liquid mixture made by Wako Pure Chemical Industries, Ltd. As another way, there is also an evaluation method by measurement of a contact angle.
In
The positive electrode sheet 32 will be further cut along lines 137 extending in a lateral direction in
Layout of the regions 134 and the region 135 in the aluminum foil 133 is not limited to one shown in
In
Layout of the regions 134 and the region 135 in the aluminum foil 133 may be performed as shown in
In
The treatment of intermittently differentiating into the end portions and the central portion as shown in
Next, an apparatus structure to achieve the differentiating into the region(s) 134 and the region(s) 135 as shown in
In the corona discharge treatment section 203, the aluminum foil is partially subjected to the corona discharge treatment. Of the surface of the aluminum foil, a portion subjected to the corona discharge treatment is increased in wettability. Thus, the portion subjected to the corona discharge treatment forms the region 135 and the remaining portions not subjected thereto form the regions 134. The corona discharge treatment section 203 may employ for example a “Corona Master” by Shinko Electrical Instrumentation Co., Ltd. and any device having the equivalent function thereto. In the examples mentioned later, a “Corona Mater PS-1” was used.
The corona discharge treatment section 203 has a mask 213 shown in
The surface of the aluminum foil 133 having passed through the corona discharge treatment section 203 as above is thus differentiated into the regions 134 and the region 135 as shown in
The aluminum foil 133 with the surface differentiated into the regions 134 and the region 135 in the corona discharge treatment section 203 is then subjected to coating process of active material mixture paste in the die-coating section 205. The mixture paste coating process is performed on the surface having been subjected to the corona discharge treatment in the corona discharge treatment section 203. In the drying furnace 206, the mixture paste is dried. Consequently, the positive electrode mixture layer 31 is formed. The aluminum foil 133 formed with the positive electrode mixture layer 31 is wound up once in the wind-up roll 207. The aluminum foil 133 is then passed again through the apparatus of
The above apparatus of
The apparatus in
Subsequently, Examples according to the first embodiment will be explained. Firstly, common subject matters between Examples according to the first embodiment and Comparative Examples are listed below.
An aluminum foil for forming a current collector sheet for positive electrode was subjected to the corona discharge treatment (the treatment apparatus in
Specifically, in Example 1, the wettability ratio, NA/NB, is about 0.59.
An aluminum foil for forming a current collector sheet for positive electrode was subjected to the roughening, instead of the corona discharge treatment, to execute the continuous differentiating treatment shown in
Without roughening: 32 dyne/cm (corresponding to the wettability NA of the region 134)
With roughening: 36 dyne/cm (corresponding to the wettability NB of the region 135)
Specifically, in Example 2, the wettability ratio, NA/NB, is about 0.89.
An aluminum foil for forming a current collector sheet for positive electrode was subjected to oil coating, instead of the corona discharge treatment, to execute the continuous differentiating treatment shown in
Specifically, in Example 3, the wettability ratio, NA/NB, is about 0.88.
An aluminum foil for forming a current collector sheet for positive electrode was subjected to coating of water repellent material, instead of the corona discharge treatment, to execute the continuous differentiating treatment shown in
With coating: 22.6 dyne/cm (corresponding to the wettability NA of the region 134)
Without coating: 32 dyne/cm (corresponding to the wettability NB of the region 135)
Specifically, in Example 4, the wettability ratio, NA/NB, is about 0.71.
An aluminum foil for forming a current collector sheet for positive electrode was subjected to the corona discharge treatment and the oil coating in combination to execute the continuous differentiating treatment shown in
Specifically, in Example 5, the wettability ratio, NA/NB, is about 0.52.
An aluminum foil for forming a current collector sheet for positive electrode was subjected to the corona discharge treatment and the coating with water repellent material in combination to execute the continuous differentiating treatment shown in
Specifically, in Example 6, the wettability ratio, NA/NB, is about 0.42.
An aluminum foil for forming a current collector sheet for positive electrode was subjected to intermittent corona discharge treatment to execute the differentiating shown in
An aluminum foil for forming a current collector sheet for positive electrode was not subjected to any wettability adjusting treatment such as the corona discharge treatment and was directly subjected to coating of a positive electrode mixture layer. The wettability of the aluminum foil of the current collector sheet was equal to a value measured before the corona discharge treatment in Example 1. Specifically, in Comparative example 1, the differentiating for producing a wettability difference was not performed, and the wettability ratio, NA/NB, is 1.0.
An aluminum foil for forming a current collector sheet for positive electrode was subjected to the corona discharge treatment on an entire surface. The wettability of the aluminum foil of the current collector sheet after treatment was equal to a value measured after the corona discharge treatment in Example 1. Specifically, in Comparative example 2, similarly, the differentiating for producing a wettability difference was not performed, and the wettability ratio, NA/NB, is 1.0.
Each of the foregoing Examples and Comparative examples was subjected to the following three evaluation tests.
The measurement of voltage failure generation rate was conducted according to the following method. Specifically, two hundred batteries were produced in each of Examples and Comparative examples, and tested by the following sequence.
An average value Vave of voltage and a standard deviation σ of two hundred batteries in Comparative example 1 were calculated. Accordingly, a voltage value given by the following expression was set as criteria voltage:
Criteria voltage=Vave−3σ
The batteries lower in voltage than the criteria voltage were evaluated as failure, and the failure rate in each of Examples and Comparative examples was calculated.
The stability test of coating width was conducted as follows. Specifically, a width-direction end portion of the mixture layer of a produced electrode sheet was observed through a microscope over the length of 1 m in the longitudinal direction. It was then checked whether or not the portion of the mixture layer greatly indented more than 0.6 mm in the width direction. That is, it was checked whether the linearity of the end portion was good or not.
The evaluation of the cross sectional shape of the width-direction end portion of the mixture layer was executed as below. Specifically, a produced electrode sheet was embedded in resin and the cross section was observed through a microscope. And, the dimension L explained in
The measurement results are shown along with the wettability values in Table 1. As seen in the column of L-dimension in Table 1, the L-dimension in each of Comparative examples 1 and 2 exceeds 100 μm and thus is too large. This is conceived because differentiating of wettability was not conducted in Comparative examples 1 and 2, that is, the ratio of NA/NB is 1. Thus, the comprehensive evaluation of Comparative examples 1 and 2 is rated as “x (no-good)” in Table 1. In each of Examples 1 to 7 (NA/NB is 0.42 to 0.89) other than Comparative examples 1 and 2, the L-dimension is below 100 μm. Comparing Examples 1 to 7 in detail, it is found that the smaller the ratio of NA/NB, the smaller the L-dimension.
As seen in the column entitled “Voltage failure rate” in Table 1, all the values are 0% except for the values, 1.5 to 2%, in Comparative examples 1 and 2. The reason why the voltage failure occurred in Comparative examples 1 and 2 is conceived that the width of each portion defined by L is too large as described above. The reason why no voltage failure occurred in Examples 1 to 7 is conceived that the width of each portion defined by L is 100 μm or less, which is a good result.
As seen in the column entitled “Stability of coating width” in Table 1, only Example 6 is rated as “x (no-good)” and others are rated as “◯(good)”. This is because in Example 6 there was found one portion indented by a distance slightly exceeding 0.6 mm in the end portion of the mixture layer. In other Examples than Example 6, there was not found any portion indented by a distance exceeding 0.6 mm. Thus, Example 6 is understood that the stability of coating width is poorer than those in other Examples. This is conceivably because the wettability ratio of NA/NB between the region 134 and the region 135 is 0.42, which is extremely lower than those in other Examples. In other words, it is understood that a wettability difference between the region 134 and the region 135 is slightly excessive.
However, such an extent of the generation condition of the indented portion could not be always assessed as a defective product depending on use of a battery. Accordingly, Example 6 is rated as “◯”, not “x”, in the comprehensive evaluation in Table 1. On the other hand, Examples 1 to 5 and 7 exhibiting nonproblematic results in both of the L-dimension and the stability of coating width are rated as “⊚(very good)” in the comprehensive evaluation. From the foregoing results, the ratio of NA/NB has to be a value less than 1. In addition, also considering the coating width stability, the ratio of NA/NB is preferably a value larger than 0.5.
Table 1 also reveals that Example 7 in which the corona discharge treatment was intermittently performed could obtain the equivalent result to Example 1. Example 1 is identical to Example 7 except for continuous execution of the corona discharge treatment. Accordingly, it is confirmed that differentiating into the region 134 and the region 135 is required only to be performed to only a portion which will be disposed on an outermost circumference of an electrode wound body as explained in
Next, a second embodiment, that is, a method using specially prepared mixture paste will be explained. The reason why a thin layer region is generated in an end portion of a mixture layer is in short that the mixture paste is fluid material. Naturally, as the viscosity of the mixture paste is lower, the thin layer region is more likely to be remarkably generated. This is because the mixture paste low in viscosity is likely to flow. In this sense, it is preferable that the viscosity of mixture paste is higher in order to prevent the thin layer region from being generated widely in the end portion of the mixture layer.
However, the high viscosity of the mixture paste will conversely cause difficulty in coating the mixture paste onto the aluminum foil (the process in the foregoing die-coating section 205). To flatly apply the mixture paste onto the aluminum foil, the fluidity of the mixture paste has to be high to some extent.
Meanwhile, it is known that the viscosity of fluid material such as the mixture paste depends on a shear rate or velocity in a stirring operation. The mixture paste of positive active material to be used for forming the positive electrode mixture layer 31 in the positive electrode sheet 32 has generally a viscosity characteristic as shown in a graph of
In the case of the mixture paste used to form the positive electrode mixture layer 31 as in the present embodiment, a state thereof during coating is equivalent to a state under being stirred at a shear rate of the order of approximately 100 sec−1 and a state after coating is equivalent to a state under being stirred at a shear rate of the order of approximately 2 sec−1. To perform the coating process without difficulty, therefore, the viscosity at the shear rate of 100 sec−1 (a star mark P in
Specifically, the mixture paste used to form the positive electrode mixture layer 31 is desired to make a somewhat remarkable difference between the star marks P and Q in
In the second embodiment, the positive electrode mixture paste prepared to have a high TI value to some extent is used. Accordingly, the viscosity of the mixture paste is low, thus easy to coat, during coating, and, is high to some extent on the aluminum foil, thus difficult to flow, after coating. Thus, the thin layer region will not be formed largely in the end portion of the mixture layer. However, if the TI value is too high, the flatness of the flat region 31F of the positive electrode mixture layer 31 to be finished will be poor. This is because the fluidity of the coated mixture paste is low on the current collector sheet. For this reason, the TI value of the mixture paste has a preferable range; that is, a range of 1.7 to 4.6 as mentioned later.
In the second embodiment, furthermore, also in the drying process following the coating process, special heating process is performed to avoid the generation of a thin layer region in the end portion of the mixture layer. This is because, in the drying process, when the temperature of the mixture paste just coated rises, this rise in temperature causes the viscosity of the mixture paste to go down. A graph of
In the drying process of the second embodiment, therefore, a temperature difference is produced between the end portion and the central portion of the mixture layer in a coating width before entering in the drying furnace or at an initial stage of drying. This prevents the temperature of the end portion from greatly rising, but causing mainly the temperature of the central portion in the coating width to rise. The mixture paste is dried in this manner. Accordingly, the drying treatment is promoted while keeping the viscosity of the mixture paste in the end portion in the coating width from greatly declining. Even in the course of the drying treatment, the thin layer region in the end portion of the mixture layer will not become so wide.
In the second embodiment, in order to achieve a temperature difference between the end portion and the central portion, a special designed supporting roller can be used to support an aluminum foil after coating. The special supporting roller is as shown in
A supporting roller 140 in
When the aluminum foil after coating is supported and delivered by the supporting roller 140 of
A supporting roller 150 in
The second embodiment employs an apparatus configured by removing the corona discharge treatment section 203 or the roughening treatment section 223 from the apparatus shown in
The positive electrode sheet 32 given the foregoing temperature difference in the width direction thus enters the drying furnace 206. In the course of the drying process, particularly at an initial stage in the drying furnace 206, the thin layer region in the end portion of the mixture layer is prevented from widening as described in [0083]. At a middle stage and subsequent in the drying course, the temperature difference in the width direction gradually diminishes. At that time, the solvent amount in the mixture paste has already decreased to some extent. Therefore, even when the temperature of the mixture paste rises in the end portion in the width direction, it does not lead to widening of the thin layer region, because the mixture paste has already started to harden.
In a lower half of
In a pre-heating period 216 in which the positive electrode sheet 32 having just entered the drying furnace 206, the temperature of the mixture layer 31 sharply rises, but the solvent amount does not so decrease. This is because the temperature itself in this period is not so high. At that time, the mixture layer 31 has been given the temperature difference by the supporting roller 140, so that the thin layer region does not widen. In a constant-rate drying period 226 following the pre-heating period 216, the temperature gets nearly saturated to a constant value. Thus, the solvent amount nearly linearly decreases. For this period, even though the temperature difference by the supporting roller 140 has significantly weakened, the decrease in solvent amount makes the mixture paste less likely to flow. Accordingly, the thin layer region also does not expand.
In a decreasing drying period 236 corresponding to an end stage of the drying furnace 206, the speed of decreasing the solvent amount slows down, because the remaining solvent amount comes to zero. Simultaneously, the temperature of the mixture layer 31 slightly rises again. This is because vaporization heat caused by the evaporating solvent decreases. At this time, the temperature difference by the supporting roller 140 has already nearly disappeared, but the mixture layer 31 itself is no longer hardly fluid. Accordingly, the thin layer region also does not widen. In the second embodiment, the generation of a large thin layer region in the end portion of the mixture layer 31 can be thus prevented.
Instead of providing the supporting roller 140 or 150 in front of an entrance of the drying furnace 206, it may be provided within the region for the pre-heating period 206 (first one-sixth to first one-quarter of the total length of the drying furnace 206) in the drying furnace 206. However, it is meaningless to provide the supporting roller 140 or 150 in a position corresponding to the constant-rate drying period 226 or the decreasing-rate drying period 236. Further, instead of providing the supporting roller 140 or 150, a heater 217 (or a hot-air blowing port) may be provided within the region for the pre-heat period 216 in the drying furnace 206 so that the heater 217 faces to only the central portion of the mixture layer 31 as shown in
Next, examples of the second embodiment will be explained. The common subject matters of the examples of the first embodiment (see [0052]-[0055]) are also basically common for each of the examples and comparative examples in the second embodiment. In the second embodiment, similarly, two hundred batteries were produced in each of Examples and Comparative examples, and subjected to tests.
In the second embodiment, additionally, the mixture paste for the positive electrode was adjusted in TI value as described in [0080] according to kneading time. Specifically, a longer kneading time produces a mixture paste having a lower TI value, while a shorter kneading time produces a mixture paste having a higher TI value. For kneading, a planetary mixer was used. The positive electrode mixture paste after kneading was subjected to measurement of the viscosity at two-level shear rates, of the order of 100 sec−1 and 2 sec−1, and the TI values were calculated from those measurement results. For this viscosity measurement, a “Physica MCR301” by Anton Paar was used.
The kneading time of the positive electrode mixture paste was set to 90 min. to prepare a mixture paste having a TI value of 1.8. As the supporting roller located before the drying furnace 206, a normal roller, not a special one shown in
The kneading time of the positive electrode mixture paste was set to 60 min. to prepare a mixture paste having a TI value of 2.7. Other conditions were the same as those in Example 8.
The kneading time of the positive electrode mixture paste was set to 40 min. to prepare a mixture paste having a TI value of 3.6. Other conditions were the same as those in Example 8.
The kneading time of the positive electrode mixture paste was set to 30 min. to prepare a mixture paste having a TI value of 4.5. Other conditions were the same as those in Example 8.
As the supporting roller placed before the drying furnace 206, the end-portion cooling supporting roller 140 shown in
As the supporting roller placed before the drying furnace 206, the central-portion heating supporting roller 150 shown in
The kneading time of the positive electrode mixture paste was set to 120 min. to prepare a mixture paste having a TI value of 1.3. Other conditions were the same as those in Example 8.
The kneading time of the positive electrode mixture paste was set to 20 min. to prepare a mixture paste having a TI value of 5.5. Other conditions were the same as those in Example 8.
Each of the foregoing Examples and Comparative examples was subjected to the following three evaluation tests.
The cycle characteristic test was conducted according to the following method. The batteries were firstly charged and discharged at 25° C. as below, and respective initial battery capacities were calculated.
Subsequently, the following steps was defined as one cycle, and charge and discharge were repeated by 1000 cycles at 60° C.
The batteries having undergone 1000 cycles were subjected again to charge and discharge as described in [0105], and then the battery capacity after cycles were calculated. Capacity maintenance ratios were calculated by the following expression and an average of them was determined as the capacity maintenance ratio in each of Examples and Comparative examples.
Capacity maintenance ratio=Battery capacity after cycles/Initial battery capacity
Measurement results are shown together with TI values of mixture paste and others in Table 2.
In every example (Examples 8-13, Comparative example 4, TI value: 1.8 to 5.5) other than Comparative example 3, the L-dimension was below 100 μm. Of them, Example 8 exhibited a lowest TI value and a large L-dimension (TI value: 1.8, L-dimension: 73 μm). Since a large difference in L-dimension is found between Example 8 and Comparative example 3 by comparison, an allowable lower limit of the TI value is conceived to be about 1.7, slightly lower than the TI value in Example 8.
As seen in the column entitled “Voltage failure rate” in Table 2, all the values are 0% except for 2% in Comparative example 3. The reason why the voltage failure occurred in Comparative example 3 is conceived that the width of the L-portion is too large as described above. The reason why no voltage failure occurred in other examples than Comparative example 3 is conceived that the width of the L-portion is 100 μm or less and the relevant examples were good in this respect.
As seen in the column entitled “Capacity maintenance ratio” in Table 2, all results was as good as 88% or better except for Comparative example 4 exhibiting a value as low as 73%. The reason why the capacity maintenance rate was not good in Comparative example 4 is conceived that the TI value of the used positive electrode mixture paste is 5.5, which is too high. From this, it is presumed that the flatness of the flat region 31F of the finished positive electrode mixture layer 31 was poor, causing non-uniform reaction of a battery to charge and discharge. Thus, Comparative example 4 is rated as “x (no-good)” in the comprehensive evaluation in Table 2.
On the other hand, the capacity maintenance ratio in each of the examples (Example 8-13, Comparative example 3, TI value: 1.3 to 4.5) other than Comparative example 4 was good because the TI values of respective positive electrode mixture pastes were not excessively large. Accordingly, it is presumed that the flatness of the flat region 31F of the positive electrode mixture layer 31 was good, causing no non-uniform reaction of a battery to charge and discharge. Of them, Example 11 exhibits a highest TI value and a lowest capacity maintenance ratio (TI value: 4.5, Capacity maintenance ratio: 88%). Since a relatively large difference in capacity maintenance ratio is found between Example 11 and Comparative example 4 by comparison, an allowable upper limit of the TI value is conceived to be about 4.6, slightly higher than the TI value in Example 11.
From the above results, except for Comparative example 3 with a poor result in L-dimension of the end portion shape and Comparative example 4 with a poor result in capacity maintenance ratio, Examples 8-13 are rated as “◯ (good)” in the comprehensive evaluation in Table 2. Consequently, a preferable range of the TI value of the mixture paste is a range of 1.7 to 4.6.
Of Examples 8 to 13, Examples 12 and 13 each employing the special roller as the supporting roller disposed before the drying furnace 206 are further studied below. These Examples 12 and 13 are similar to Example 9 in terms of the TI value of the used mixture paste. However, in Examples 12 and 13, the obtained L-dimension was better than that in Example 9. In Examples 12 and 13, specifically, the L-dimension equivalent to those in Examples 10 and 11 employing the mixture paste having higher TI values could be obtained. In addition, in Examples 12 and 13, the capacity maintenance ratios are more excellent than those in Examples 10 and 11. Example 13 employing the supporting roller of the central-portion heating type, especially, exhibits the capacity maintenance ratio surpassing that in Example 8 employing the mixture paste having a lower TI value.
Such superior characteristics in Examples 12 and 13 are conceived to result from the use of the end-portion cooling type supporting roller shown in
According to the foregoing embodiments explained in detail above, prior to the coating treatment of the positive electrode mixture layer, the aluminum foil for forming the current collector sheet for positive electrode is given a wettability difference between a portion to be formed as a coated portion and a portion to be formed as an uncoated portion. Alternatively, a positive electrode mixture paste to be used for coating is one prepared so that the TI value is a value falling within a predetermined high range to some extent. These achieve a width of 100 μm or less in the end edge region of the thin layer region in the width-direction end portion of the positive electrode mixture layer 31. Consequently, there are realized a non-aqueous electrolyte secondary battery configured to prevent a problem due to current concentration in an outermost circumferential portion of a positive electrode, a method for manufacturing a positive electrode sheet of the non-aqueous electrolyte secondary battery, and a method for manufacturing the non-aqueous electrolyte secondary battery.
The above-described embodiments are mere examples and do not give any limitations to the present invention. Thus, the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. For instance, each part or component may be made of any material and a battery may have any outer shape as long as they can function as a non-aqueous electrolyte secondary battery. Further, producing a temperature difference on the positive electrode mixture paste layer after coating in the width direction, which is explained in the second embodiment, may also be applied to the first embodiment. Further, the first embodiment and the second embodiment may be combined.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-023345 | Feb 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/082924 | 12/9/2013 | WO | 00 |
