Abstract:
The invention relates to a method and apparatus for coating both sides of a thin metal strip with electrochemically active material by the use of coating rod mechanism acting on a single side of the metal strip. The method is preferably carried out by applying the coating to one face of the strip drawn from the supply roll, changing the orientation of the strip, coating the opposite side of the strip and drying the coated sides. The dried and coated strip may be wound up on a take up reel. In the preferred embodiment, the strip from the feed roll first passes a web guide which reverses the direction of travel of the strip before the first side is coated. The strip then passes around a second web guide which again reverses the strip direction of travel before the second side of the strip is coated and the coated strip is dried.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/081,400 filed on Apr. 10, 1998. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The field of invention is electrochemical batteries, more particularly, a method and apparatus for applying an electrochemically active material onto an ultra-thin metal film for use in electrochemical batteries. 
     Developments in the field of electrochemical batteries have produced batteries having electrochemical cells that have both excellent charge and discharge characteristics. These cells require ultra-thin films of a metal such as lead, nickel or metal alloys having a thickness of 2-5 mils coated with extremely thin layers of an electrochemically active material, having a thickness of approximately 1-5 mils. 
     A typical electrochemical cell, as shown in FIGS. 2 and 3, includes an ultra-thin strip  2  of lead alloy of approximately 2 inches in width and 2-5 mils thick. The electrochemically active material  3  is a compound that includes lead monoxide, sodium sulfate and other inactive materials. The compound  3  is applied to the top and bottom surfaces of the metal strip  2 , coating the entire width of the strip  2  except for an uncoated strip edge  4  approximately 0.2 inches wide on both surfaces of the strip to provide an uncoated electrical connection surface. 
     Methods have been developed to produce lead and other suitable metals and alloys in the form of an ultra-thin film that is suitable for use in these electrochemical cells. The use of these ultra-thin strips is disclosed in U.S. Pat. No. 5,677,078. 
     The electrochemically active material can be applied to the metal strip in a fluid form and then dried to produce a solid coating. Various means and methods to coat the ultra-thin metal films have proved less than satisfactory. One particular method sprays the aqueous solution onto the sides of the metal films. Due to environmental hazards that exist with spraying a solution containing metals such as lead, this method is unacceptable for use. 
     Other methods coat a side of the thin film using a roller apparatus that applies the fluid solution to a film that is supported by a backing roller to prevent the ultra-thin film from breaking. The use of a backing roller only allows coating a single side of the film at a time with an intermediate drying step, thus lengthening the time necessary to manufacture the electrochemical cell. The use of a coating roller also does not typically provide a clean edge that is suitable for an electrical connection without masking the edge at the point of application or a subsequent step of wiping. 
     A method used to coat both sides of a thin film simultaneously is to dip the entire film in the aqueous solution. This particular method does not provide an uncoated edge without a subsequent step of wiping or other method to remove some of the coated material. Additionally, this particular method requires that the same coating material is applied to both sides of the film, which is unacceptable for certain applications, such as in use with bi-polar batteries. 
     Providing a clean edge for an electrical connection is important to ensure the performance of the assembled battery. The steps described above such as wiping and masking have proven unacceptable for various reasons. Wiping does not always completely remove the coated material leaving a residue that affects the battery performance. Additionally, the thin film has a low tensile strength that is subject to tearing during the wiping process. 
     Masking is a process of covering the area that is not to be coated during the coating process. Typically, a roller covers the edge as the remaining portion of the strip is coated. This roller tends to become fouled, that is coating material can be splattered on the masking roller which is then transferred to the edge that should be left clean, thus reducing the battery performance. 
     BRIEF SUMMARY OF THE INVENTION 
     In one of its aspects, the invention relates to a method and apparatus for coating both sides of a thin metal strip with electrochemically active material by the use of coating rod mechanism acting on a single side of the metal strip. The method is preferably carried out by applying the coating to one face of the strip drawn from the supply roll, reversing the direction of travel of the strip coating the opposite side of the strip and drying the coated sides. The dried and coated strip may be wound up on a take up reel. 
     In the preferred embodiment, the strip from the feed roll first passes a web guide which reverses the direction of travel of the strip before the first side is coated. The strip then passes around a second web guide which again reverses the strip direction of travel before the second side of the strip is coated and the coated strip is dried. The web guides, supply reel, and take-up reel controls the speed and tension in the strip so that back up rolls are not needed at the coating mechanisms. 
     The invention also resides in a coating mechanism that includes a reservoir, a mixing rod relating through fluid coating material in the reservoir, and a coating rod receiving coating material from the mixing rod and applying the coating material to the strip. The mixing rod is longitudinally grooved and the coating rod is circumferentially grooved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a ultra-thin strip coating apparatus incorporating the present invention; 
     FIG. 2 is a plan view of a portion of a coated ultra-thin strip that can be produced by the apparatus of FIG. 1; 
     FIG. 3 is a cross sectional view of the strip in FIG. 2; 
     FIG. 4 is a plan view of a coating mechanism of the apparatus of FIG. 1; 
     FIG. 5 is a cross sectional view of the coating mechanism of FIG. 4; 
     FIG. 6 is a front sectional view of the coating mechanism of FIG. 4; 
     FIG. 7 is an elevational view of a mixing rod of FIG. 4; 
     FIG. 8 is an end view of the mixing rod of FIG. 7; 
     FIG. 9 is an elevational view of a coating rod of FIG. 4; 
     FIG. 10 is an end view of the coating rod of FIG. 9; 
     FIG. 11 is an exploded view of the coating rod drum of FIG. 9; and 
     FIG. 12 is an alternative coating rod and doctor blade configuration for the apparatus of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an embodiment of the present invention is a coating method and apparatus  10  that coats both sides of a thin strip  12  of lead foil with a thin layer of electrochemically active paste having a density of approximately 3.5-4 grams/cm 3  for use in batteries, such as described in U.S. Pat. Nos. 5,047,300 and 5,677,078 which are incorporated herein by reference. The coating apparatus  10  includes a foil supply reel  14 , a first web guide assembly  16 , a first coating mechanism  18 , a second web guide assembly  20 , a second coating mechanism  22 , a drying assembly  24 , and a take up reel  26 . The entire apparatus is mounted on a side of a vertical base (not shown). 
     As shown in FIG. 1, the supply reel  14  has a thin lead foil strip  12  wound thereon and is rotably mounted to the vertical base. The thin lead foil  12  unwinds from the supply reel  14  and travels underneath an idler roller  28  that helps maintain a constant tension in the foil strip  12  in cooperation with the supply reel  14  and the first web guide assembly  16 . 
     The first web guide assembly  16  is disposed along a foil strip path  30  and changes the foil strip  12  orientation by reversing the foil strip direction of travel, exposing a downwardly facing first side  32  of the strip  12  to the first coating mechanism  18 . The web guide assembly  16 , such as an AccuGlide II Model 060802 narrow web guide assembly available from AccuWeb, Inc., Madison, Wis., has a pair of cork-wrapped rollers  32  and  33  ratably mounted between a pair of opposing sidewalls  34 , one roller  33  being disposed above the other  32 . The sidewalls  34  are rigidly mounted to a bracket (not shown) which is mounted to the apparatus base (not shown) by bolts or other methods known in the art. 
     The foil strip  12  having an upwardly facing first side engages the web guide lower roller  32  and changes direction upwardly 90 degrees to engage the upper roller  33  which changes the foil direction rearwardly 90 degrees, thus reversing the original travel direction of the foil  12  and facing the foil first side  32  downwardly. The web guide dual roller assembly  16  prevents over bending of the foil  12  causing breakage. 
     The first web guide assembly  16  also accurately guides the foil strip  12  over the first coating assembly  18  using a microprocessor controlled web guide control system (not shown), such as an AccuGuide III Micro 1000 microprocessor-pulsed compensated ultrasonic web guide control system available from AccuWeb, Inc. of Madison, Wis. The control system receives inputs from an ultrasonic edge detector (not shown) that is part of the web guide control system and assembly as provided by AccuWeb, Inc., to determine the foil edge and shifts the rollers  32  and  33  using actuators in the desired direction to properly align the foil  12  above the coating mechanism  18 , with respect to the detected edge. Other methods known in the art may be used to guide the foil without departing from the present invention. 
     The first coating mechanism  18  is disposed along the foil path  30  following the first web guide assembly  16  and coats the downwardly facing side  32  of the foil  12  with an electrochemically active solution. The coating mechanism  18  coats the foil strip  12  using a method and apparatus not previously known in the art to provide a consistent coating thickness having well defined coating edges that terminate inward from the foil edges. Although the coating mechanism  18  is novel as described herein, any coating apparatus or method that can coat a foil strip  12 , such as used to coat a single side of a strip, may be used without deviating from the intent of the present invention. 
     As shown in FIGS.  4 - 6 , the coating mechanism  18  has a base assembly  40 , mixing rod  42 , coating rod  44 , and a doctor blade  46 . The base assembly  40  has a base  48  that is a nonreactive material block with a front  50 , a rear  52 , a top  54 , a bottom  56  and two sides  58 . A cavity  60  formed in the block top  54  defines a bottom wall  63  of a reservoir. Opposing nonreactive material sidewalls  62  mounted to each side  58  of the base  48  by bolting or other means known in the art, define substantially straight reservoir sidewalls  64 . The reservoir holds the electrochemically active coating material prior to coating. 
     The bottom wall  63  of the reservoir has a feed portion  66  and a coating portion  68 . The feed portion  66  slopes inwardly and downwardly having a rounded bottom  70  to collect the coating for feeding into the rotating mixing rod  42 . The curved coating portion  68  is disposed above and rearward of the feed portion  66  and accommodates the rotably mounted coating rod  44  that is fed coating material by the mixing rod  42 . 
     A countersunk bore  72  in the base front  50  communicates with the feed portion  66  of the reservoir. The countersunk portion of the bore  72  is threaded to receive a fitting (not shown) on a coating material supply line. The supply line supplies the reservoir with coating material for use during the coating process. 
     Clean out holes,  74  and  75 , in the base bottom  56  communicate with feed portion  66  and coating portion  68  of the reservoir, respectively, to facilitate cleaning the reservoir. The holes,  74  and  75 , each have a threaded countersunk portion for receiving threaded plugs (not shown) to prevent the coating material from leaking out during the coating process. Four threaded mounting holes  76  in the base bottom  56  receive bolts to mount the coating mechanism  18  to a bracket (not shown) rigidly mounted to the apparatus base. 
     The mixing rod  42  is rotably mounted to the coating mechanism base sidewalls  62  and has a cylindrical drum  78  that feeds coating material to the coating rod  44  and is supported by an axially extending shaft  80 . The drum  78  extends substantially the entire width of the reservoir in close proximity to the rounded bottom  70  of the reservoir feed portion  66 . 
     As shown in FIGS.  7 - 8 , the drum  78  outer surface has twelve axial grooves  82  forming scallops in the drum surface that agitate the coating material in the reservoir and scoops up the coating material, feeding it to the coating rod  44 . 
     The mixing rod shaft  80  extends axially along a central axis of the mixing rod  42 . Each end of the shaft  80  extends into one of the base assembly sidewalls  66  and is rotably mounted therein. One end  86  of the shaft  80  extends through the sidewall  66  and has a gear  88  mounted thereon for engaging an adjacent gear  90  mounted on a coating rod shaft  92 . 
     The coating rod  44 , shown in FIGS.  9 - 11 , is disposed above and rearward of the mixing rod  42  and receives coating material fed by the mixing rod  42 . The coating material is upwardly applied by the rotating coating rod  44  to the downwardly facing side  32  of the lead foil  12  traveling overhead. Preferably, the foil direction of travel is opposing to the coating rod  44  rotation providing a smooth consistent coating thickness on the foil  12 . 
     The coating rod  44  has a drum  94  that extends substantially the entire width of the reservoir and is supported by an axially extending shaft  92 . The coating rod drum  94  applies the coating layer to the foil  12  substantially equal in width to the drum  94 . The foil  12  is aligned by the web guide  16  with the respect to the coating rod drum  44  in order to leave at least one edge and a portion inward from the edge of the foil uncoated for attachment to electrical connectors. If the foil  12  is wider than the drum  94  both edges and portions inward from the edge of the foil can remain uncoated. 
     Referring to FIG. 11, the coating rod drum  94  has a plurality of circumferential grooves  96  on its outer surface. The circumferential grooves  96  carry the coating, fed by the mixing rod  42 , to the downwardly facing foil surface providing a substantially consistent coating thickness. The grooves  96  are circumferential, as opposed to helical as found in a wire wound coating apparatus, to provide a sharp clean coating edge on the foil  12  that is inward of the foil edge. 
     Preferably, the grooves  96  are cut into the drum surface and substantially equally spaced along the drum length. The groove width, depth, and shape have an effect on the coating thickness applied to the foil. For example, equally spaced circumferential V-shaped grooves  96  cut into the drum surface approximately 0.040 inches apart having a depth of approximately 0.040 inches provide a coating thickness of approximately 0.005 for the coating material having a viscosity of approximately 2000-12000 cps. The V-shape of the grooves forms an angle A of 60 degrees. 
     The coating rod shaft  92  extends axially along a central axis  98  of the coating rod  44 . Each end,  100  and  102 , of the shaft  92  extends into one of the base assembly sidewalls  62  and is rotably mounted therein. One shaft end  100  extends through the sidewall  62  has a gear  90  mounted thereon for engaging the adjacent gear  88  mounted on the mixing rod shaft  80 . The opposite coating rod shaft end  102  extends through the opposite coating mechanism base sidewall  62  and is adapted to mate with a drive motor (not shown) for rotably driving the coating rod shaft  92  and counter rotating feed rod shaft  80 . 
     As shown in FIGS.  4 - 5 , a doctor blade  46  adjustably mounted to the coating mechanism base top  54  with an edge  108  adjacent to the coating rod  44  prevents splattering and helps provide a consistent coating thickness. The blade  46  is nonreactive material having a pair of parallel slots  104  formed therein for slidable adjustable movement. The blade  46  is mounted by bolts (not shown) inserted through the slots  104 . The bolts are received by threaded holes  106  in the base top  54 . 
     The doctor blade edge  108  should be adjusted as close as possible toward the coating rod  44  without causing the material to build up on the blade  46 . Preferably, the blade  46  is adjusted to define a gap of 0.002-0.004 inches between the blade edge  108  and the coating rod drum  44 . 
     Referring back to FIG. 1, once the downwardly facing first side  32  of the foil  12  has been coated, the foil  12  travels along its path  30  toward the second web guide assembly  20 . The second web guide assembly  20  is essentially identical to the first web guide assembly  16 . The second web guide assembly  16  changes the foil strip  12  orientation by reversing the strip  12  direction of travel, thus the downwardly facing first foil side  32  having a freshly coated surface faces upwardly and the second foil side  36  faces downwardly, exposed to the second coating mechanism  22 . 
     The second web guide  20  guides the foil  12  over the second coating mechanism  22  as described for the first web guide  16 . The second coating mechanism  18  is substantially identical to the first coating mechanism  18 . Tension in the foil  12  maintained by the web guides  16  and  20 , idler rollers  28  and  38 , and reels  14  and  26 , eliminates the need for a backing roller at the first and second coating mechanisms  18  and  22  as the coating is applied. 
     The use of two independent coating mechanisms  18  and  22  advantageously allows application of the coating on the second side  36  of the foil  12  while the coating on the first side  32  is still wet, thus eliminating an intermediate drying step. Additionally, using two independent coating mechanisms  18  and  22 , allows the user to apply a coating on the first side  32  of the foil  12  that is different from the coating applied to the second side  36  of the foil  12  for use in bi-polar batteries and the like. 
     The foil  12  having a moist coating on both sides is then passed through a drying apparatus  24  to reduce the moisture content in the coatings to approximately 7-8%. Preferably, the drying apparatus  24  consists of infra red heating elements (not shown) disposed above and below the foil path  30  that are directed at the coated foil  12  as it travels along the foil path  30 . The infra red heating elements provide a consistent dry heat. 
     The dried coated foil  12  then engages an idler roller  38  prior to being wound up on the take up reel  26 . The idler roller  38  in cooperation with the second web guide assembly  20  and take up reel  26  helps maintain the tension in the coated foil. 
     The coating apparatus components described above are controlled by a microprocessor control system (not shown) that coordinates the operation of each component during the coating process. Sensors (not shown) such as laser micrometers and moisture sensing equipment provide inputs to the microprocessor for processing to adjust foil travel speed, coating rod rotational speed, heating levels and the like, in order to meet set criteria. 
     Furthermore, stopping rotation of the coating rod, effectively stops the application of the coating material to the foil. Therefore, the control system allows the user to stop rotation of a coating mechanism coating rod in order to provide the entire width of a portion of the traveling foil without coating material. 
     An alternative feed coating rod  44  and doctor blade  46  configuration shown in FIG. 12, is a coating rod  44  with a drum  110  having circumferential grooves  112  formed on a central portion  114  of the drum surface and two nongrooved portions  116  axially extending from the central portion  114  to the drum ends  118 . The grooved central portion  114  has an outside drum diameter larger than the nongrooved portions  116  creating a diameter differential between the grooved and nongrooved portions,  114  and  116 . 
     A doctor blade  46  has an edge  108  with a notched central portion  120  to receive the grooved central portion  114  of the coating rod drum  110 . The notched portion  120  length is slightly larger than the length of the grooved portion  114  on the drum  110  and the depth is slightly larger than the diameter differential of the two drum portions  114  and  116 , allowing the nonnotched portion  122  of the doctor blade  46  to be set closer to the nongrooved portion  116  of the coating drum  110  than the grooved portion  116 . 
     The notched doctor blade in combination with a partially grooved coating rod drum allows precise doctoring of the coating edge on the foil strip. The portion of the doctor blade adjacent to the grooved portion of the drum provides a smooth consistent thickness coating as described in the first embodiment. The nonnotched portion of the doctor blade cleanly defines the coating edge at the edge of the grooved section of the coating rod and prevents the deposition of excess material on the foil edge that is to remain uncoated. 
     While there has been shown and described what are at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention.