DRYING SYSTEM, CONTINUOUS STRIP, BATTERY, AND BATTERY

MANUFACTURING SYSTEM

FIELD OF THE INVENTION

[OOOl] The invention relates to a drying system, continuous strip, battery, and a battery manufacturing system. More specifically, the invention is concerned with a drying system for drying a solution containing an active material, which is applied by coating onto an electrode base material, in the process of manufacture of electrodes carrying the active material.

BACKGROUND OF THE INVENTION

[0002] In the manufacture of lithium ion batteries, a solution containing an active material is applied by coating onto an electrode base material in the form of a metal sheet, and the resulting coating is dried in a drying step. In the drying step, the coating is dried so that the final moisture content of the coating becomes equal to a specified value. If the moisture content of the coating deviates from the specified value, the electrode active material may chemically react with an electrolyte to generate gas, and the resulting batteries may suffer from slow reactions and insufficient power that is less than a specified level, due to the generation of the gas. If a command for generating power is given to the batteries incapable of providing sufficient power, heat may be grown higher than usual.

[0003] Japanese Patent Application Publication No. 5-50023 (JP-A-5-50023) describes method and system for drying a solution containing an active material, which is applied by coating to a continuous strip of metal sheet, so that the resulting coating has a specified moisture content. In the system as described in JP-A-5-50023, hygrometers are provided in an air feeding duct and an exhaust duct, and the amount of water vapors from the coating is measured by calculating a difference between the amount of water vapors in air fed into the system and the amount of water vapors in air discharged from the system. While

monitoring the measurement value, the system controls the amount of water vapors from the coating in a feedback manner, so as to control the final moisture content to the specified value.

[0004] In the system as described in JPα-5-50023, the hygrometer for measuring, water vapors from the coating is installed in the exhaust duct. Since the amount of water vapors needs to be accurately measured in order to control the final moisture content to the specified value, there may be great advantages in the installation of the hygrometer in the exhaust duct and the measurement of the amount of vapors in the exhaust air. However, where the hygrometer is installed in the exhaust duct, the distance from the coating to the hygrometer is long, and it takes time for water vapors to reach the hygrometer. As a result, feedback control based on the measurement values may be too late to provide the final moisture content equal to the specified value. Also, a region of the electrode base material in which the moisture content of the coating deviates from the specified value is not suitable for use as electrodes. When the amount of water in exhaust air is measured in the exhaust duct at successive points in time, the amount of water evaporating from a large area or range of the coating is ultimately measured. Thus, it is difficult to specify a region of the electrode base material which is out of specifications, thus making it difficult to improve yields of batteries.

SUMMARY OF THE INVENTION

[0005] It is an object of the invention to provide a drying system that adequately and accurately measures a dried condition of a coating, to provide a dried continuous strip that is within specifications.

[0006] A first aspect of the invention is concerned with a drying system. A continuous strip having at least one line of a coating applied by coating to a continuous strip of a base material is continuously fed into the drying system, and the- drying system is adapted to dry the coating to achieve a predetermined moisture content while the continuous strip is conveyed along a predetermined

travelling path. The drying system includes a plurality of drying furnaces disposed along the travelling path of the continuous strip, and a water vapor sensor provided in each of at least two of the drying furnaces for detecting water v , - vapors from the coating. The water vapor sensor includes an air intake having an air intake opening through which air is taken in, a duct through which the air from the air intake is passed, and a water detector provided in the duct for detecting the amount of water contained in the air. The air intake is mounted at a position adjacent to the coating for each line of the coating formed on the continuous strip that is being conveyed through the drying system.

[0007] With the arrangement as described above, the continuous strip to which the coating is applied is conveyed successively through the drying furnaces of the drying system. During the conveyance, evaporation of water from the coating is promoted, and the coating is dried so that its moisture content becomes equal to the specified value. The water vapor sensors are provided in some of the drying furnaces through which the continuous strip passes while being conveyed in the drying system, and water vapors from the coating are detected by the water vapor sensors. While most of the water vapors from the coating diffuses in the drying furnace, a part of the vapors flows along with the airflow formed on a surface of the continuous strip, and reaches the air intake of the water vapor sensor located adjacent to the coating. Then, that part of the vapors is taken in through the air intake opening of the air intake, along with the air, and is detected by the water detector while passing through the duct.

[0008] In the drying system constructed as described above, the air intake of the vapor sensor is located at a position adjacent to the corresponding line of the coating, and therefore, the water vapors can be immediately taken in and detected with virtually no time lag. Since the water evaporating from the coating is immediately taken in and detected at a location right above the coating, the amount of water is detected with high accuracy and stability, and the moisture content is detected based on the detected amount of water with high accuracy and

high resolution. Then, drying of the continuous strip is controlled with high accuracy, based on detection data of the moisture content obtained with high accuracy and high resolution, so as to provide a dried continuous strip that is precisely within specifications.

[0009] Also, since the water vapors are immediately taken into the water vapor sensor and detected with virtually no time lag, the drying system can be feedback-controlled without delay, and the moisture content of the coating can be controlled, with higher reliability, to its specified value. Furthermore, since a path of air flow from the location where water evaporates from the coating to the location where the water vapors are detected by the water vapor sensor is clear or obvious, and there is virtually no time lag between evaporation and detection, it is possible to accurately specify a portion of the coating of which the moisture content is reflected by the detection value of the sensor. Accordingly, even if the coating has a portion where the moisture content deviates from the specified value, this portion can be specified in detail.

[OOIO] In the case where a hygrometer is mounted in an exhaust duct, as in the system of the related art as described above, water vapors are detected with a considerable time lag after diffusing along with complex airflow in the drying furnace. Therefore, even if it is found from the detection value that the moisture content deviates from the specified value, it is impossible or difficult to precisely specify a region where the deviation occurs, and a large portion of the product may have to be discarded so as to make sure that the moisture content is within a specified range. In this respect, it is possible, according to the invention, to precisely specify a portion of the coating of which the moisture content is reflected by the detection value of the sensor, which leads to an improvement in the manufacturing efficiency of the products. Also, since the water vapor sensor of this invention is provided for each line of the coating, detection of the moisture content can be performed with respect to each line of the coating.' Thus, the entire area or all regions of the continuous strip need not be discarded, but disposition

may be determined with, respect to each line of the coating, thus assuring further improved manufacturing efficiency (yields).

[OOll] While it is difficult to achieve sufficiently high levels of both of the quality and the manufacturing efficiency in the drying system of the related art, the drying system according to the first aspect of the invention is able to provide a continuous strip that is dried to achieve a specified moisture content with high accuracy and high efficiency.

[0012] In the drying system according to the first aspect of the invention, the distance between the air intake and the coating may be equal to or smaller than a thickness of a boundary layer.

[0013] With the above arrangement, the water vapor sensor takes in water evaporating from the coating via the air intake while the water vapors are flowing along with the airflow of the boundary layer as the continuous strip is conveyed in the system, and detects the water vapors. If a sensor is installed in the exhaust duct or at some location in the drying furnace, as in the drying system of the related art, water vapors reach the sensor after diffusing in complex airflow, thus causing a time lag in sensing the vapors. Further, the water vapors may collide with the coating again while diffusing in complex airflow, which makes it impossible or difficult for the sensor to provide a detection value that accurately reflects the moisture content of the coating. In this respect, according to the above arrangement, the amount of water contained in the airflow in the boundary layer reflects the moisture content of the coating quite precisely, and thus the moisture content of the coating can be accurately determined by taking in the air of the boundary layer and detecting water contained in the air.

[0014] The distance between the air intake and the coating may be within the thickness of the boundary layer. However, if it is difficult to control the distance in this manner, as a matter of design, the distance between the air intake and the coating may be within twice the thickness of the boundary layer. In this case, the detection accuracy may be somewhat reduced, but the drying system

yields satisfactory effects, as compared with that of the related art.

[0015] In the drying system according to the first aspect of the invention, the air intake may be located on a downstream side of the travelling path of the continuous strip in a corresponding one of the drying furnaces.

[0016] With the above arrangement, the moisture content of the coating after it undergoes drying in the drying furnace can be detected. Also, since a large portion of water evaporating from the coating flows in the travelling direction of the continuous strip, along with the airflow of the boundary layer caused by the travelling continuous strip, it is possible to detect water vapors with high accuracy and high resolution, by taking in air and vapors at the downstream side of the travelling path.

[0017] In the drying system according to the first aspect of the invention, the water detector is placed, along with the air intake, in a corresponding one of the drying furnaces.

[0018] With the above arrangement, the distance between the air intake and the water detector is extremely shortened, and therefore, the time lag from evaporation of water to detection of vapors is extremely reduced.

[0019] In the drying system according to the first aspect of the invention, the air intake opening of the air intake may have a flat, rectangular shape, and the length of the air intake opening may be larger than the width of the corresponding line of the coating. Further, the air intake may be mounted at a position adjacent to the coating such that a longitudinal direction of the air intake opening of the air intake is parallel to a direction of the width of the coating.

[0020] With the above arrangement, the water vapor sensor is able to surely take in air containing water evaporating from a corresponding line of the coating, so as to provide a detection value that precisely reflects the moisture content of the coating.

[0821] In the drying system according to the first aspect of the invention, the air intake may be positioned such that the air intake opening faces in a

direction opposite to the travelling direction of the continuous strip, and such that a center axis of the air intake opening is inclined from a direction parallel to the travelling direction of the continuous strip.

[0022] With the above arrangement, the water vapor sensor is able to smoothly take, in air. lowing along with the travelling continuous strip without suffering from resistance, so as to provide more accurate detection values.

[0023] In the drying system according to the first aspect of the invention, the duct may be formed of a magnetic material. Also, the duct may be formed of a material that is unlikely to react with the continuous strip. For example, the duct may be formed of stainless steel as the material that is unlikely to react with the continuous strip.

[0024] With the above arrangement, even if particles of the material of the duct are deposited on the continuous strip, they have almost no influence on the product quality of the continuous strip. In the system of the invention in which the water vapor sensor is installed at a position adjacent to the coating, there is a possibility that particles produced from the water vapor sensor are deposited on the coating. In particular, flow of air through the duct of the water vapor sensor is likely to cause particles to be produced at a joint between the duct and the air intake when vibrations occur. In this case, the drying itself may be adequately accomplished, but the quality of the product may deteriorate. In this respect, where the duct is formed of stainless steel, it has a significantly reduced influence on the quality since the material is ^* clearly identified. Also, where the duct is formed of a magnetic material, particles of this material can be removed by a magnetic remover (magnet) after the drying step.

[0025] A second aspect of the invention is concerned with a continuous strip. The continuous strip is subjected to drying in the above-described drying system. In the continuous strip subjected to drying in the drying system, the coating may contain an aclive material that provides electrodes of batteries, and the continuous strip may provide the electrodes of the batteries. A third aspect of

the invention is concerned with a battery. The battery has an electrode formed from the above-described continuous strip. A fourth aspect of the invention is concerned with a battery manufacturing apparatus. The battery manufacturing apparatus includes the above-described drying system.

[0026] Accordk' g to the present invention, the contini ous strip is dried so that its moisture content, is controlled to a specified value with high accuracy and high efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a view schematically showing the construction of a drying system!

FIG. 2 is a perspective view of an electrode base material that is dried in the drying system,"

FIG. 3 is a view showing the interior arrangement of one drying furnace,"

FIG. 4 is a perspective view of an air intake of a water vapor sensor;

FIG. 5 is a schematic view showing the water vapor sensor installed in the drying furnace >"

FIG. 6 is a side view showing a condition in which the water vapor sensor is installed in the drying furnace; and

FIG. 7 is a graph indicating the relationship between the moisture content of a coating (vertical axis) and a detection value of a water detector (horizontal axis).

DETAILED DESCRIPTION OF THE EMBODIMENTS [G028] A drying system according to a Sx st embodiment of the invention will be described. FIG. 1 schematically shows the construction of the drying

system 100. In FIG. 1, a continuous strip of an electrode base material (continuous strip) 800 is introduced or fed into the drying system by means of a feeding device (not shown), and is delivered out of the drying system after being conveyed in the drying system. The electrode base material 800, when delivered from the drying system, 100, is taken up into the form of a roll by a take-up device (not shown). The feeding device and the take-up device are equipped with rotary encoders, which make it possible to trace the electrode base material 800 to find which portion of the electrode base material 800 was located at a certain point in the drying system 100 at a certain point in time.

[0029] As shown in FIG. 1, the drying system 100 consists of a plurality of furnaces 110. Some of the furnaces 110 are electric heating furnaces 111 for heating the electrode base material 800, and other furnaces 110 are drying furnaces 112 for promoting drying of the electrode base material 800 with the aid of air fed into and exhausted from the furnaces. The electric heating furnaces 111 and the drying furnaces 112 may be selected from the furnaces 110 as desired. For example, the first furnace or the first and second furnaces into which the electrode base material 800 is introduced may be electric heating surfaces 111, and the other furnaces may be drying furnaces 112. If the electrode base material 800 is heated at first and then passed through the drying furnaces 112, drying is promoted with high efficiency. (In FIG. 1, the reference numerals (ill, 112) are assigned to the furnaces at the bottom of the drying system 100, as an example of arrangement of the electric heating furnaces and drying furnaces.)

[0030] FIG. 2 is a perspective view of the electrode base material 800 as a dried object to be dried by the drying system 100. The electrode base material 800 has a metal sheet 810 in the form of a continuous strip, and two stripes or lines of coatings 820 that are applied by coating onto the metal sheet 810. The metal sheet 810 is formed of Al, Cu, or an alloy of Al and Cu. The coating 820 is formed by applying a -solution containing an active material to the metal sheet 810 by a coater (coating device) that is not illustrated. The active material contains a

metallic material including Li, carbon as an electrically conductive material, and a binder for binding these materials and the metal sheet 810 together, which are dispersed in water to provide a coating solution. The drying system 100 dries the electrode base material 800 so as to reduce its moisture content to a specified value, and the electrode base material 800 thus dried is then cut irλtό a predetermined size to provide electrodes of secondary batteries. In the case where two or more lines of coatings 820 containing the active material are formed on the metal sheet 810, the electrode base material 800 may be cut along the line(s) of the coating(s) 820, so that two or more electrodes can be obtained from a single region. Namely, if two or more (for example, two) coatings 820 are formed as shown in FIG. 2, electrodes are manufactured with significantly improved efficiency, as compared with the case where a single line of coating is formed on the metal sheet 810.

[0031] Next, the drying furnaces 120 of the drying system 100 will be described. FIG. 3 shows the interior arrangement of one of the drying furnaces 120. The drying furnace 112 includes partition walls 113 that define each furnace, air feeders 114 for feeding air into the furnace, and air exhausters 115 for exhausting the air in the furnace. In addition, some of the drying furnaces 112 are each provided with a water vapor sensor 200 for detecting water evaporating from the coating 820, i.e., vapors from the coating 820.

[0032] Two or more air feeders 114 are provided on the upstream side of a travelling path of the electrode base material 800, for feeding air into the drying furnace. Two or more air exhausters 115 are provided on the downstream side of the travelling path of the electrode base material 800, for exhausting the air in the drying furnace. Thus, the air feeders 114 are located on the upstream side while the air exhausters 115 are located on the downstream side, and the electrode base material 800 is conveyed from the upstream end to the downstream end in the drying furnace. With this arrangement, air flows in substantially the same direction _' as the electrode base material' 800 from the upstream end to the downstream end in the drying furnace.

[0033] While the water vapor sensors 200 need not be provided in all of the drying furnaces 112, the number of the water vapor sensors 200 installed in the drying system 100 should be large enough to enable a host computer (which will be described) to keep track of changes in the moisture content of each coating 820 during a period from the. time when the electrode base material 800 enters the drying system 100 to the time when the electrode base material 800 leaves the drying system 100. While the final moisture content of the coating 820 of the electrode base material 800 is to be controlled to the specified value, as a matter of course, it is also necessary to control the drying speed to a desired speed in the course of drying of the coating 820. For example, if water is rapidly vaporized from the coating 820, the active material of the coating 820 floats along with the water, and the active material and the metal sheet 810 are not sufficiently bound together, but are likely to peel off or separate from each other. It is thus necessary to achieve the final moisture content while adequately controlling the drying speed, in the drying step for vaporizing water from the coating 820. To this end, the water vapor sensors 200 may be installed in alternate ones of the drying furnaces 112, or in every two drying furnaces 112, for example, so as to keep track of the drying speed.

[0034] As shown in FIG. 3, the water vapor sensor 200 is disposed on the downstream side of the travelling path of the electrode base material 800 in the drying furnace. The water vapor sensor 200 includes an air intake 210 for taking in air, a duct 220 through which the air from the air intake 210 passes, and a water detector 230 provided in the duct 200 for detecting the amount of water (vapors) in the air.

[0035] FIG. 4 is a perspective view of the air intake 210. The air intake 210 has a through-hole 212 that extends from an opening (air intake opening) 211 formed at one end for taking in air, to an opening (not shown) formed at the other end connected to the duel 220. The one -end opening 211' has a generally flat, rectangular shape, and the length (i.e., horizontal dimension) of the opening 211 is

somewhat larger than the width of one line of coating 820. For example, the length of the one-end opening 211 is larger by about 10% than the width of the coating 820. The through-hole 212 is tapered from the one-end opening 211 to the other-end opening (not shown), and air taken into the relatively wide one-end opening 211 is naturally collected toward the duct 220 connecteά,to the other-end opening, and is introduced into the duct 220.

[0036] FIG. 5 is a perspective view of a condition in which the water vapor sensors 200 are installed in the drying furnace, and FIG. 6 is a side view of the condition in which the water vapor sensors 200 are installed in the drying furnace. The air intake 210 of each of the water vapor sensors 200 is located right above the corresponding coating 820 such that the longitudinal direction of the one-end opening 211 of the air intake 210 is parallel to the width direction of the coating 820. In this condition, the one-end opening 211 of the air intake 210 is located close to the corresponding coating 820 of the electrode base material 800, and the distance between the one-end opening 211 and the electrode base' material 800 is equal to or smaller than the thickness of a boundary layer. Assuming that airflow in the boundary layer is laminar flow, the thickness 5L of the boundary layer is defined by the following equation (l), where L is the width of the electrode base material, and Re is the Reynolds number.

[0037]

[0038] In the above equation (l), the Reynolds number Re is defined as follows, where U is the travelling speed of the electrode base material 800, L is the width of the electrode base material, p is the density as a physical property of an atmosphere or gas in the drying furnace, and μ is the viscosity as another physical property of the atmosphere in the drying furnace.

[CC33]

Re = -- μ/p

[0040] Here, since the Reynolds number is normally equal to or smaller than 5 x 10 ⁵ (Re ≤ 5 x 10 ⁵ ), and the airflow of the boundary layer is assumed to be laminar flow, the above -indicated equation (l) may be applied; • However, in exceptional cases where the electrode base material 800 is designed such that Re > 5 x 10 ⁵ , the airflow in the boundary layer becomes turbulent, and therefore, the thickness 5L of the boundary layer is defined by the following equation, instead of the above-indicated equation (l).

[0041]

[0042] If, on the other hand, the air intake 210 is located too close to the electrode base material 800, the air intake 210 may collide with the electrode base material 800. Thus, the air intake 210 is located at a position that is spaced apart from the electrode base material 800 by a distance larger than the amplitude of vertical oscillation of the electrode base material 800 during travelling through the drying furnace. Although the distance between the air intake 210 and the electrode base material 800 is not specified by a specific numerical value, it may be controlled to, for example, within the range of 5 mm to 10 mm. It is also preferable that the entire area of the one-end opening 211 of the air intake 210 is contained in the boundary layer.

[0043] Also, the air intake 210 is positioned such that the one-end opening 211 faces in a direction opposite to the travelling direction of the electrode base material 800, and such that the axis of the through-hole 212 is inclined by a certain angle θ from a direction parallel to the travelling direction of the electrode base material 800. The angle θ of the inclination may be within the range of, for example, 10° to 50°, and is desirably around 30°.

[0044] The duct 220, which is connected or joined to the other-end opening

of the air intake 210, is extended or led to the outside of the drying furnace 112, and is connected to an exhaust pump (not shown). The duct 220 is preferably formed of a magnetic material. It is also preferable that the duct 220 is formed of a material, such as stainless steel, which is unlikely to react with the electrode base material 800.

[0045] The water detector 230 is provided at some midpoint in the duct 220. It is preferable that the length of a portion of the duct 230 between the water detector 230 and the air intake 210 is as short as possible. Also, the water detector 230 is located within the drying furnace. Thus, the water detector 230 preferably has sufficiently high heat resistance (up to, for example, about 200 ⁰ C) so that it can endure the heat of the atmosphere in the drying furnace even when the temperature in the drying surface rises to a high level. A detection value that represents the amount of water (vapors) detected by the water detector 230 is transmitted from the water detector 230 to the outside and processed.

[0046] The processing of detection values obtained by the water detector 230 will be explained. A detection value representing the amount of water detected by the water detector 230 is transmitted to an exterior host computer (not shown), for example, and is used for calculation of the moisture content of the coating 820, feedback control of the drying system 100 and the quality control of products. In the following, a moisture content calculator 910 and a central control unit 920, which are provided in the host computer, will be described.

[0047] Initially, a detection value obtained by the water detector 230 is transmitted to the moisture content calculator 910. The moisture content calculator 910 calculates the moisture content of the coating 820 at the time of detection, using the detection value received from the water detector 230. In this connection, a data table indicating the relationship between the moisture content of the coating 820 and the detection value of the water detector 230 is set and stored in the moisture content calculator 910, and the calculator 910 calculates the moisture content of the coating 820 from the detection value, with reference to the

data table. FIG. 7 is an example of graph indicating the relationship between the moisture content of the coating 820 (the vertical axis) and the detection value of the water detector 230 (the horizontal axis). As a matter of course, the detection value of the water detector 230 increases as the moisture content of the coating 820 increases, and the detection value of the water detector 230 decreases as the moisture content of the coating 820 decreases. If the water vapor sensor 200 has a high resolution, the detection value (the horizontal axis) changes by a large degree in response to a small change in the moisture content (the vertical axis). The graph as described above is obtained in advance through preliminary experiments, and set in the moisture content calculator 910. The moisture content calculated by the moisture content calculator 910 is further transmitted to the central control unit 920.

[0048] The central control unit 920 stores therein a desirable drying speed of the coatings 820 of the electrode base material 800, a specified value of the final moisture content, and so forth, and calculates the drying speed from values of the moisture content sequentially transmitted from the moisture content calculator 910, for comparison with the desired drying speed. Then, the central control unit 920 controls the temperature of the electrical heating furnaces and controls the travelling speed of the electrode base material 800, depending on whether the drying speed is higher or lower than the desired speed. If the drying speed or the final moisture content measured with respect to a certain region deviates from its permissible value or is outside a permissible range thereof, the region is specified from values of the rotary encoders, and a flag is set which indicates that the line of the coating 820 located in this region is out of specifications. Where the electrode base material 800 is provided with two or more lines (e.g., two lines) of coatings 820, the above-mentioned flag set with respect to a particular region of the electrode base material 800 does not mean that all of the lines of the coatings 820 located in this region- are deemed as being out of specifications, but the moisture content is determined with respect to each line of the coating 820, and a line (or lines) of the

coating 820 that is out of specifications is specified based on the result of the determination. The region that is finally determined as being out of specifications is not used as products, but will be discarded.

[0049] In the drying system 100 constructed as described above, the process from the feeding of the electrode base material 800 to take-up of the material 800 will be generally explained. Initially, the metal sheet 810 wound in the form of a roll is delivered, and two lines of coatings 820 are applied to the metal sheet 810 with a coater (not shown) (see FIG. 2). The electrode base material 800 formed with the coatings 820 is continuously carried or fed into the drying system 100. More specifically, the coatings 820 are applied by coating to one surface of the metal sheet 810, and the surface is dried in an upstream stage (a lower array of furnaces in FIG. l) of the drying system 100. Subsequently, the coatings 820 are also applied to the other surface (rear surface) of the metal sheet 810, and the rear surface is dried in a middle stage (a middle array of furnaces in FIG. l) of the drying system 100. Then, final drying of the front and rear surfaces of the metal sheet 810 is performed in a downstream stage (an upper array of furnaces in FIG. l) of the drying system 100, and the electrode base material 800 that has been finally dried is taken up by the take-up device.

[0050] The electrode base material 800 fed into the drying system 100 is first heated in the electric heating furnaces. Then, the heated electrode base material 800 is successively conveyed through the two or more drying furnaces 112 where the material 800 is dried. In each of the drying furnaces, air is fed from the air feeders 114 located on the upstream side, and is discharged from the air exhausters 115 located on the downstream side. Most of water evaporating from the coating 820 is discharged, along with air, from the air exhausters 115. Apart of water evaporating from the coating 820 flows along with the boundary layer that flows along the surface of the electrode base material 800, and reaches the air intake 210 of the water vapor sensor 280 where that part of water is taken into the water vapor sensor 200 via the air intake 210, together with the air of the

boundary layer. The water that enters the water vapor sensor 200 via the air intake 210 passes through the duct 220, and is detected by the water detector 230. The detection value obtained by the water detector 230 is transmitted to the moisture content calculator 910. Then, the moisture content of the coating 820 at the time of detection is calculated from the detection value, and the result of the calculation is generated to the central control unit 920. In the central control unit 920, data of the moisture content thus transmitted from the moisture content calculator 910 is recorded, together with the corresponding region of the electrode base material 800 and the corresponding line of the coating 820, and the history of the drying speed and the final moisture content are calculated with respect to each point in the electrode base material 800. On the basis of the drying speed thus obtained, the temperature of the electric heating furnaces and the travelling speed of the electrode base material 800 are controlled in a feedback manner. Also, the central control unit 920 flags the region(s) and line(s) of the coating 820 that fail to meet with the desired drying-speed history or the specified final moisture content as those that are out of specifications.

[0051] Of the electrode base material 800 that has gone through the drying process, the line or lines of the coating in the region(s) flagged as being out of specifications is/are discarded, and the other portions that are within specs are processed into electrodes of batteries, and ultimately shipped in the form of batteries.

[0052] The embodiment as described above yields the following effects. Since the air intake 210 of the water vapor sensor 200 is located right above the coating 820 for each line of the coating 820 of the electrode base material 800 that is conveyed through the drying system 100, water vapors from the coating 820 are immediately taken into the air intake 210 and detected without delay. With this arrangement in which water vapors from the coating 820 are immediately taken ixiix) the air intake 210 located right above the coating 820 and detected, the amount of water is detected with high accuracy and stability, and the moisture

content in the coating 820 is detected with high accuracy and high resolution, based on the detection value of the water vapor sensor 200.

[0053] With the above arrangement in which the distance between the air intake 210 and the coating 820 is equal to or smaller than the thickness of the boundary layer, water that evaporates from the coating 820 is taken into the air intake 210 when it is carried with the airflow of the boundary layer while the electrode base material 800 is being conveyed through the system. The amount of water contained in the airflow in the boundary layer quite correctly reflects the moisture content of the coating 820. Thus, the moisture content of the coating 820 can be correctly determined by detecting water contained in the air of the boundary layer which is taken into the air intake 210.

[0054] The water vapor sensor 200, which is installed on the downstream side of the travelling path in the drying furnace, is able to detect the moisture content of the coating 820 after it undergoes drying in the drying furnace. Also, since a large proportion of water evaporating from the coating 820 flows along with the airflow of the boundary layer caused by the travelling electrode base material 800, in the travelling direction of the electrode base material 800, it is possible to detect water vapors with high accuracy and high resolution, by taking in air and water vapors at the downstream side of the travelling path.

[0055] With the above arrangement in which the water detector 230 as well as the air intake 210 is mounted in the drying furnace, the distance from the air intake 210 to the water detector 230 is considerably shortened, so that a time lag from evaporation of water to detection of vapors can be considerably reduced.

[0056] The one-end opening 211 of the air intake 210 is formed in a flat rectangular shape, with its length (horizontal dimension) being larger than the width of the corresponding line of the coating 820, and the air intake 210 is located right above the coating 820 such that the longitudinal direction of the one-end opening 211 is parallel to the width direction of the coating 820. ^" With this arrangement, the air intake 210 is able to surely take in air containing water

evaporating from the line of the coating 820, and the water detector 230 provides a detection value that accurately reflects the moisture content of the coating 820.

[0057] The air intake 210 is positioned such that the one-end opening 211 faces in the direction opposite to the direction of the travelling direction of the electrode base material 800, and such that the axis of the through-hole 212 is inclined from the direction parallel to the travelling direction of the electrode base material 800. Thus, the airflow of the boundary layer can be smoothly taken into the air intake 210 with virtually no resistance. Consequently, the water detector 230 provides more accurate detection values.

[0058] In the illustrated embodiment, the duct 220 is formed of stainless steel or a magnetic material. Therefore, even if particles of the material of the duct 200 are deposited on the electrode base material 800, they have virtually no influence on the product quality. Namely, where the dust 220 is formed of stainless steel, for example, the influence on the quality can be significantly reduced since the material is clearly identified. Where the duct 220 is formed of a magnetic material, the particles of this material can be removed by a magnetic remover (magnet) after the drying step.

[0059] While the electrode base material is subjected to drying in the illustrated embodiment, an object to be dried in the drying system of the invention is not limited to the electrode base material. While two lines of coatings are formed on the electrode base material in the illustrated embodiment, the number of lines of coatings is not limited to two, but three or four lines of coatings may be formed on the electrode base material. In this case, the invention may be applied such that a water vapor sensor is provided for each line of the coatings. The water vapor sensors may be installed in all of the drying furnaces. However, the required minimum number of water vapor sensors may be provided from the viewpoint of reduction of the manufacturing cost. The shape of the one-end opening of the air intake is not limited to the shape of a rectangle, but may be selected from other shapes provided that the length of the opening is larger than

the width of the corresponding line of the coating.

[0060] While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. • In addition, while the various elements of the disclosed invention are shown in various example combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the appended claims.