[0001] The present invention relates to battery shells, i.e. the metallic outer sleeves of a battery, and to a method for the manufacture thereof .

BACKGROUND OF THE INVENTION

[0002] Conventional methods for the manufacture of battery shells of cylindrical and coin-shaped batteries , i.e. the me ^¬ tallic outer sleeves of the battery, apply a series of repeti ^¬ tive deep drawing steps using a press , wherein a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch.

[0003] Lubricants are used to reduce friction between the working material and the punch and die, cool the parts during deep drawing and cleaning the deep drawing tools. They also aid in removing the part from the punch after drawing. Lubrication plays an important role in the deep drawing process as it reduces friction at the tool-workpiece interface, thus en ^¬ hancing the ability to produce a good quality part. Since the quality of drawn parts significantly changes with lubrication, good lubrication condition lead to lower scrap rate and better quality of parts. Lubrication condition greatly affects the material flow during the drawing process. Examples of lubri ^¬ cants used in drawing operations are heavy-duty emulsions, phosphates, white lead, and wax films. After the deep drawing operation the lubricants must be removed.

[0004] Removing lubricant from a formed part after the deep drawing operation is important because any lubricant left be ^¬ hind can interfere with subsequent steps in the manufacturing of the part , especially in subsequent steps of battery form- ing . Mineral oils , animal fat , and vegetable oils can be re ^¬ moved with an organic solvent by emulsification or saponifica ^¬ tion, or with an aqueous alkaline cleaner . Greases can also be removed from sheet metal with an organic solvent or an alka ^¬ line cleaner . Solids are more difficult to remove because they are not readily soluble . The presence of solids often requires that additional cleaning methods be used . Petroleum oils can raise special issues from removal through disposal . These oils require the use of alkaline cleaners for removal , which can then contaminate cleaner tanks with oil , leading to potential disposal challenges . Vegetable oils can be removed with hot water if the parts are cleaned immediately and with a mildly to moderately alkaline cleaner if the parts are cleaned after they have been left standing for a few days .

[0005] In order to manufacture a battery from the battery shell obtained by said sequence of drawing steps, a conductive material is sprayed into the inside of the battery shell after removal of the lubricants to coat the battery shell with the conductive material. Thereafter cathode material, e.g. a mix ^¬ ture of manganese dioxide, graphite and others, is inserted into the battery shell coated with the conductive material. In alkaline Zn-Mn0 ₄ batteries, the cathode consists of manganese dioxide as active material and graphite as conducting addi ^¬ tive, both in powder form, which are pressed into rings and inserted into the coated battery shell. Then a separator is inserted to prevent short-circuit of a positive and a negative electrode, followed by injecting an electrolyte into the sepa ^¬ rator to impregnate the separator with the electrolyte, and injecting an anode gel into the impregnated separator. Finally a current collector is inserted into the anode gel and the battery is sealed to prevent leakage and drying. The current collector functions as an anode terminal, and the battery shell can function as cathode. [0006] As stated, the lubricant used during the drawing steps must be carefully removed in order to avoid any interference with subsequent process steps, for example coating the battery shell with a conductive material as lubricant remainders may interfere with subsequent coating steps. As removing the lub ^¬ ricant is not always easy, there is a need for improved pro ^¬ cesses, where lubricant removal may be avoided. Such processes would be of great commercial interest.

SUMMARY OF THE INVENTION

[0007] The object is achieved with a method for producing a battery shell which is coated with a conductive lacquer where ^¬ in the conductive lacquer comprises a resin component having graphite dispersed therein. The method comprises the steps of: a. coating a cleaned sheet metal blank with a conductive lac ^¬ quer which comprises a resin selected from a polyacrylate resin, a polymethacrylate resin or combinations thereof having graphite particles or graphite containing carbon black particles or both dispersed therein to produce a coated metal blank; b. thereafter radially drawing the coated metal blank into a forming die by the mechanical action of a punch to form a coated cup; c. further drawing the coated cup by a series of repetitive deep drawing steps using a press, wherein the cup is radi ^¬ ally drawn into a forming die by the mechanical action of a punch to form a battery shell which is coated with a conductive lacquer comprising a resin component having graphite particles or graphite containing carbon black particles or both dispersed therein, and d. wherein no lubricant comprising oil or grease is added be ^¬ fore or during the drawing processes.

[0008] According to an embodiment of the present invention, the content of graphite particles or graphite containing car ^¬ bon black particles, or, if applicable, both may be between 20 wt . % and 60 wt.%, preferably 30 wt . % to 50 wt . % of the dry weight of the resin component. The metal of the sheet metal blank is preferably chosen from steel, preferably a cold rolled steel. According to an embodiment the metal of the met ^¬ al sheet is chosen from ASTM A1008 CS Type A, ASTM A1008 CS Type B, ASTM A1008 CS Type C, ASTM A1008 DS Type A, ASTM A1008 DS Type B, ASTM A1008 DDS, ASTM A1008 EDDS steel. Most prefer ^¬ ably, however, the metal is of the metal sheet is a cobalt and/or nickel containing steel, most preferably a cobalt con ^¬ taining steel.

[0009] According to another embodiment of the present inven ^¬ tion, the conductive lacquer may further comprise a corrosion inhibitor. The corrosion inhibitor may comprise a compound selected from the group of 2-butin-l, 4-diol, polyether phos ^¬ phate, alkanolamine salts of nitrogen-containing organic ac ^¬ ids, propargyl alcohol alkoxylate, alkylphosphate, polyether phosphate or oleoyl sarcosine acid or mixtures thereof, pref ^¬ erably selected from polyether phosphate and alkylphosphate.

[0010] According to the present invention, the battery shell is produced by a method, wherein a sheet metal blank is first coated with a conductive lacquer which comprises a resin com ^¬ ponent having graphite particles or graphite containing carbon black particles dispersed therein, and which is thereafter radially drawn by a series of repetitive deep drawing steps first into a cup and then into a battery shell which is coated with a conductive lacquer comprising a resin component having graphite particles or graphite containing carbon black parti- cles or both dispersed therein. Said coating can be applied to the sheet metal blank by using a doctor blade, roll applica ^¬ tion or by a coil coating method, wherein a coating material is applied onto a metal strip in a continuous process which includes cleaning, if necessary, and chemical pre-treatment of the metal surface and either one-side or two-side, single or multiple application of (liquid) paints or coating powders by rollers, where the paints or coating powders are subsequently cured or/and laminating with permanent plastic films (EN 10169:2010) . Preferably, the coating is applied to the sheet metal blank by using a doctor blade or by roll application. More preferably, the sheet metal is cleaned before applying the coating.

[0011] When said coating is applied onto the sheet metal blank using a doctor blade, the coating is preferably cured at tem ^¬ peratures ranging from room temperature to 150°C, more prefer ^¬ ably from 100°C to 140°C. The coating thickness is preferably chosen between 10 pm and 30 pm, more preferably between 15 pm and 20 pm. Coating the sheet metal blank using a doctor blade and curing at temperatures ranging from room temperature to 150 °C has the advantage that no expensive additional equipment is necessary.

[0012] When said coating is applied using a roll application method, the coating is preferably cured at temperatures rang ^¬ ing from 150 to 300°C, more preferably from 200 to 250°C. The coating thickness is preferably chosen between 5 pm and 20 pm, more preferably between 9 pm and 15 pm.

[0013] Surprisingly, the conductive lacquer which comprises a resin component and a filler component which is formed at least in part by graphite particles or graphite containing carbon black particles act as a lubricant and at the same time as a conductive inner coating for the battery shell. Accord- ingly, one can dispense with the steps of adding a conventional lubricant and removing the lubricant after deep drawing by carefully removing oil or grease in the deep drawn battery shell and spraying a conductive material into the inside of the battery shell in order to form a battery from the battery shell .

DETAILED DESCRIPTION OF THE INVENTION

[0014] The battery shell of the present invention is coated with a conductive lacquer comprising a resin component having graphite particles or graphite containing carbon black parti ^¬ cles dispersed therein. The resin component of the conductive lacquer may be selected from the group consisting of polymers or copolymers of acrylate, polymers or copolymers of methacry ^¬ late having graphite particles or graphite containing carbon black particles dispersed therein. According to the present invention copolymers of acrylate are those that contain great ^¬ er or equal 50 wt . % acrylate, preferably greater or equal 80 wt . % acrylate. Polymers of acrylate are those composed of acrylate. According to the present invention copolymers of methacrylate are those that contain greater or equal 50 wt . % methacrylate, preferably greater or equal 80 wt . % methacry ^¬ late. Polymers of methacrylate are those composed of methacry ^¬ late. Examples of suitable resins having graphite dispersed therein are those available under the trade name Grapharol ^® , such as Grapharol ^® 3 acrylic resin, Grapharol ^® 3/II acrylic resin, Grapharol ^® 3K3 acrylic polymer or Grapharol ^® 114 acrylic poly,er available from Doduco GmbH. The graphite or graphite containing carbon black, or both, if applicable, content of the resin component may be between 20% and 60%, preferably 30% to 50%, such as about 35%, about 40% or about 42.5% of the dry weight of the resin component having graphite particles or graphite containing carbon black particles or both dispersed therein. Preferably, the resin component has a viscosity of from 1000 to 2500 mPas, more preferably from 1500 to 200 mPas .

[0015] In another embodiment of the present invention, the conductive lacquer comprising a resin component having graphite particles or graphite containing carbon black particles dispersed therein is applied as an aqueous dispersion thereof onto the metal sheet.

[0016] In still another embodiment of the present invention, the resin component or the aqueous dispersion thereof is ad ^¬ mixed with a corrosion inhibitor prior to coating. The corrosion inhibitor may comprise a compound selected from the group of 2-butin-1 , 4-diol , polyether phosphate, alkanolamine salts of nitrogen-containing organic acids , propargyl alcohol alkoxylate, alkylphosphate, polyether phosphate or oleoyl sarcosine acid or mixtures thereof, and is preferably selected from polyether phosphate and alkylphosphate . Suitable corro ^¬ sion inhibitors are those available from BASF SE under the trade name Korantin ^® , such as Korantin ^® LUB ( a corrosion inhibitor which contains polyether phosphate ) , Korantin ^® S K ( a corrosion inhibitor which contains alkyl phosphate ) , Korantin ^® TC- SH ( a corrosion inhibitor which contains oleoyl sacosine ac ^¬ id) , Korantin ^* MAT, and Korantin ^® PAT ( corrosion inhibitors which contain an alkanolamine salt of a nitrogen-containing organic acid) , or mixtures thereof . The most preferred are corrosion inhibitors which contain polyether phosphate and/or alkyl phosphate .

[0017] The resin component or the aqueous dispersion thereof may further be admixed with organofunctional or aminofunctional silanes , such as those available under the trade name Dynasylan ^® SIVO 110, 111, 112, 113, 121, 140, 160, 202, 203, or Dynasylan ^® HYDROSIL 1151, 1153, 2627, 2775, 2776, 2909, 2926, preferably Dynasylan ^® SIVO 160 or Dynasylan ^® HYDROSIL 2627 from Evonik Industries AG, and a cross-linker prior to coating such as melamine. The organofunctional or aminofunctional silanes are preferably admixed in amounts of up to 5% by weight of the resin component having graphite par ^¬ ticles or graphite containing carbon black particles or both dispersed therein, preferably in amounts ranging from 1 to 5 wt% .

[0018] The resin component or the aqueous dispersion thereof is coated onto a sheet metal blank, preferably by using a doc ^¬ tor blade. The coating thickness is preferably chosen between 5 pm and 20 pm, more preferably between 9 pm and 15 pm, such as about 12 pm. After coating, the resin component or the aqueous dispersion thereof is advantageously dried, preferably at a temperature between room temperature and 150 °C. Thereaf ^¬ ter, the coated metal blank is radially drawn into a forming die by the mechanical action of a punch to form a coated cup.

[0019] The coated cup is thereafter further drawn by a series of repetitive deep drawing steps using a press , wherein a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch to form a battery shell which is coated with a conductive lacquer which comprises a resin com ^¬ ponent having graphite dispersed therein. Surprisingly, the conductive lacquer which comprises a resin component having graphite dispersed therein is sufficiently flexible to with ^¬ stand the drawing process and adhere to the surface of the battery shell and acts as a lubricant due to the presence of graphite, even at the drawing conditions specified below. Fur ^¬ thermore, the coating is sufficiently conductive to act as an inner coating for a battery shell. Accordingly, one can dispense with the steps of adding a conventional lubricant before or during deep drawing and removing the lubricant after deep drawing by carefully removing oil or grease in the deep drawn battery shell and spraying a conductive material into the in ^¬ side of the battery shell.

[0020] Preferably, during deep-drawing the maximum temperature should not exceed 120°C, more preferably it should not exceed 100°C. Furthermore, the drawing velocity is preferentially chosen between 1 and 4 m/s, more preferentially between 2 and 3 m/s . It has been found that the conductive layer is suffi ^¬ ciently durable for up to 12 drawing steps and can sustain a total degree of deformation of up to 3.5. Additionally, the process prevents the creation of abrasion, which would ob ^¬ struct the forming process. The workpiece (battery can) can be carried to a battery production without supplementary production steps.

[0021] The battery can be formed in the usually manner by inserting cathode material, e.g. a mixture of manganese dioxide, graphite and into the battery shell coated with the conductive material. In alkaline Zn-Mn0 ₄ batteries, the cathode consists of manganese dioxide as active material and graphite as con ^¬ ducting additive, both in powder form, which are usually pressed into rings and inserted into the coated battery shell. Alternatively the manganese dioxide as active material and graphite as conducting additive may be inserted as a powder into the coated battery shell before the powder is radially and axially pressed into a ring. Then a separator is inserted to prevent short-circuit of a positive and a negative elec ^¬ trode, followed by injecting an electrolyte into the separator to impregnate the separator with the electrolyte, and inject ^¬ ing an anode gel into the impregnated separator. Finally, a current collector is inserted into the anode gel and the bat ^¬ tery is sealed to prevent leakage and drying. The current col ^¬ lector functions as an anode terminal, and the battery shell can function as cathode. [0022] The appended figures show photographs of coated deep drawn cups .

FIG. 1 is illustrative of the invention and shows coated cups after a first, a second and a third drawing step, respective ^¬ ly. A coil coating primer not comprising graphite was applied onto both sides of a steel metal blank with a coating thickness of 12 pm using a coil coating method, whereafter the coating was stoved at a temperature of about 240°C.

Fig. 2 is according to the invention and shows coated cups af ^¬ ter a first, a second and a third drawing step, respectively. The coating was applied on both sides a steel metal blank of "Type B" by applying 20 pm of a composition comprising 100 ppw of Grapharol ^® 3K3 and 10 ppw of Korantin ^® LUB onto the metal blank using a doctor blade, where the coating was cured at a room temperature.

Fig. 3 is according to the invention and shows coated cups af ^¬ ter a first, a second and a third drawing step, respectively. The coating was applied on both sides a steel metal blank of "Type C" by applying 20 pm of a composition comprising 100 ppw of Grapharol ^® 3K3 and 10 ppw of Korantin ^® LUB onto the metal blank using a doctor blade, where the coating was cured at a room temperature.

Fig. 4 is according to the invention and shows coated cups af ^¬ ter a first, a second and a third drawing step, respectively. The coating was applied on both sides a steel metal blank of "Type B" by applying 20 pm of a composition comprising 100 ppw of Grapharol ^® 3K3 and 10 ppw of Korantin ^® LUB onto the metal blank using a doctor blade, where the coating was cured 5 minutes at 140°C. Fig. 5 is according to the invention and shows coated cups af ^¬ ter a first, a second and a third drawing step, respectively. The coating was applied on both sides a steel metal blank of "Type C" by applying 20 pm of a composition comprising 100 ppw of Grapharol ^® 3K3 and 10 ppw of Korantin ^® LUB onto the metal blank using a doctor blade, where the coating was cured 5 minutes at 140°C.

[0023] Fig. 1 shows good adhesion of the coating after the entire drawing process. However, the coating according to Figure 1 is not conductive. Figures 2 to 5 show that also graphite coatings can withstand the drawing process and adhere to the metal thereafter. While the coating according to Fig. 2 still shows poor adhesion already after the first drawing, the adhe ^¬ sion of the coating according to Fig. 3 was already slightly improved. Further improvements were achieved when the coating was cured at 140 °C according to Figs. 4 and 5. It can also be derived that the quality of the coating after the drawing pro ^¬ cess is dependent on the metal. Metal sheets of "Type C" have improved coating properties over metal sheets of "Type B". Both metal sheets contain cobalt and were produced according the same specifications, however with different coarseness.

[0024] It has shown that the use of a resin component having graphite dispersed therein according to the present invention also possesses heat-removing properties which makes it possi ^¬ ble to perform the deep-drawing process without any additional lubricants. Accordingly one can dispense with a separate cleaning step after drawing and before coating, which will lead to savings in overall energy of at least 30% for the drawing process.