BACKGROUND

FIELD OF THE INVENTION

The present invention relates to current collector tabs and a method of connecting the current collector tabs to electrode substrates most particularly to porous foam based electrode substrates. The porous electrode substrate can either be metallic or polymeric foam serving as positive or negative electrode for both primary as well as secondary electrochemical cells.

DISCUSSION OF PRIOR ART

Depending on the battery type, different batteries have different electrode substrate or support. For example, in the case of lead-acid batteries, lead grid plates provide mechanical support, hold the active materials and act as current collectors. The grid plates contain tabs which are welded to a common bus-bar to constitute the main positive or negative terminal. In most of the commercial lithium-ion batteries, thin metal foils are used as electrode substrates which also act as the current collectors. Current collector tabs in the form of small rectangular metal strips are welded at regular intervals on the top portion of the current collector foil, which are in turn welded together to constitute the main positive or negative terminal. Instead of metal foils, porous foams can also be used as electrode substrates and provide numerous advantages over metal foil substrates. However, joining current collector tabs to foam based substrates is not straight forward and often requires additional operations to prepare the surface for welding as conventional welding is not suitable for porous substrates.

A number of prior arts are available demonstrating the use of porous current collectors and establishing their superior performance as compared to metallic foil current collectors. US 2012/0288757 Al titled "Three-dimensional network aluminum porous body for current collector, electrode using the aluminum porous body, non-aqueous electrolyte battery, capacitor and lithium-ion capacitor" by Hosoe et.al, demonstrates the use of a 3D porous aluminium current collector in the case of a battery, capacitor and lithium-ion capacitor containing non-aqueous electrolytes. US 2013/0065122 titled "Semi-solid electrode cell having a porous current collector and methods of manufacture" by Chiang et.al, demonstrates an electrochemical cell that uses a porous current collector both for cathode and anode which are disposed into the respective semi solid suspension of active material. Here a portion of the porous substrate coming out of their respective electrode cavities is folded and compressed to constitute the electrode lead.

A paper in the Journal of Power Sources titled "LiFeP04 based electrode using micro-porous current collector for high power lithium-ion battery" by Yoa et
al., demonstrates the use of a 3D micro-porous current collector made of foamed polyurethane and nickel-chromium alloy as the positive electrode substrate for a LiFeP04 battery cell. The resulting cell demonstrated a significant reduction in the charge transfer resistance as well as superior high-rate discharge capabilities as compared to cells containing Al-foil as current collector. A number of ways including soldering, welding, compression joining, use of conductive adhesives, etc. are suggested in the literature for joining the current collection tabs to the porous substrate. Some have demonstrated the use of an extension of the foam-based substrate itself as the current collector tab or used an additional piece of metallic foam as current collection tab that is connected to the main body by compression. US 5667915 titled "Electrode including a metal connection and a core having a fibre or foam-type structure for an electrochemical cell" by Loustau et.al discloses methods of connecting a metal conductor in the form of metal strip to an electrode substrate having foam or fibre kind of structure. One way where a single metal strip is used, the said metal strip is placed between a retaining core and a main core and the entire assembly is compressed to witness reduced thickness.

Later, the metal strip is secured at its place by performing electric seam welding. In another embodiment where two metal strips are used both the strips are placed on opposite edges across the entire width of the main core and the retaining core. The spot welding is performed to fix the conductors onto the porous substrate. The main and the retaining core may or may not have similar composition and/or physical characteristics. The core material mostly consists of either nickel foam or polyurethane felt. US 2004/0002006 Al titled "Battery including carbon-foam current collector" by Kelley et.al, demonstrates the use of porous carbon foam with a total porosity of 60% as a current collector for both positive as well as negative electrode in a lead-acid battery that has demonstrated significantly lower electrical resistivity. The invention also discloses a way of making electrical connections to the graphite based porous carbon current collector with tabs also made of carbon foam and are actually extensions of the original porous current-collector. In order to make the tabs conductive to high currents, a carbon-metal interface is established by thermally spraying a conductive metal (e.g. Ag) onto the tab such that the conductive metal penetrates through the porous carbon. Thereafter, a second conductive material (e.g. Pb) is coated onto the carbon-metal surface to complete the electrical connection.

US 5456813 titled "Method of joining a metal connection tab to an electrochemical cell electrode having a foam-type support and an electrode obtained by the method" by Grange-Cossou et.al, discloses various types of connection tabs to be connected to a foam type of electrode support. The connection tabs consists of a single or multiple branches to be attached on opposite sides of the foamed support and is either disposed on a portion of the edge or along the entire edge of the support. The foam-based electrode support is basically made of nickel foam with > 90% porosity prior to compression whereas the connection tabs are made of either expanded metal or perforated metal foil with perforations occupying > 40% of the surface area and are made of nickel, nickel plated steel or stainless steel. The foam support and the connection tab are joined by compression leading the three-dimensional porous structure of both the support and the tab to interpenetrate and bind together. This method has many advantages including simple tab designs, joining connection tabs to the electrode support without any extra operational stage and minimal reduction of the active surface area.

US 5518840 titled "Electrode plate for an electrochemical cell and having a metal foam type support, and a method of obtaining such an electrode" by Verhoog et.al, discloses an electrode plate with porous foam type support particularly for electrochemical cells with alkaline electrolyte. The electrode plate consists of two main portions, namely the active portion where the active material is pasted onto the porous substrate and the head portion which serves as the electrical connection or the tab of the electrode plate and is devoid of active material. Upon the application of active material to the active portion of the plate, entire plate is compressed to obtain a reduced thickness. Later, two porous metal strips of similar or different composition as that of the electrode substrate is placed on its either sides of the head portion and the entire head portion assembly consisting of three layers is then compressed to the thickness of the rest of the electrode plate. The assembly is further maximally compressed to obtain close to 0% porosity in order to provide maximum mechanical strength and good electrical connection to the plate.

Later, some part of the head portion is cut-off such that only a small part of it remains to serve as the connection tab. US 6238819 Bl titled "Metalfoam support, electrode and method of making the same" by Cahill et.al, discloses the use of a metal foam support either as positive or negative electrode made of metal-hydride, nickel-cadmium or lithium-ion secondary batteries. The three-dimensional support foam made of porous metals, alloys or intermetallic mixtures is rolled at the edges into a coiled or S-shaped edge or at the centre as pleats or grooves that can hold connection tabs between its folds. The connection tab mainly in the form of thin metal foils of nickel, copper or alloys of nickel and copper are secured in position by compression in combination with other operations such as soldering, brazing, welding or using conductive adhesives.

SUMMARY OF THE INVENTION
The present invention discloses a way of joining current collector tabs in the form of thin metal strips by stapling them to the porous foam based electrode substrate. One of the important aspects of the present invention is the use of a connection pin with a flat rectangular face and four or more legs to make the connection. Another important aspect of the present invention is a two-way current collector tab which can attach to both the top and bottom faces of the foam based substrate at the same time and is secured at its position by using a connection pin. A conductive adhesive is used to join the current collector tab to the foam based substrate in addition to performing the stapling operation.
The stapling operation for joining the current collector tabs to the foam-based substrate is easy to implement in the present invention wherein the electrode surface does not require additional preparation prior to tab connection. The connection pin used for joining the tabs by stapling has a flat rectangular surface and four or more legs that firmly holds the tab in position and provides reliable electrical connection. The two way current collector tabs connects both the top and bottom faces of the foam-based substrate and provides better electrical connection. The invention is applicable to both metallic as well as polymeric foam-based electrode substrates and the resultant porous electrode can serve as either positive or negative electrode for primary or secondary batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1A shows the schematic representation of a 4-leg connection pin. Figure IB shows the schematic representation of a 6-leg connection pin.

Figure 2A shows the top face of a foam-based electrode to which a thin metal strip current collector tab is attached.

Figure 2B shows the bottom face of a foam-based electrode to which a thin metal strip current collector tab is attached.

Figure 3A shows the schematic representation of a two-way metal strip current collector tab.

Figure 3B shows the side-view of a two-way metal strip current collector tab.

Figure 4A shows the schematic representation of the top face of a foam-based electrode to which a two-way metal strip current collector tab is attached.

Figure 4B shows the bottom face of a foam-based electrode to which a two-way metal strip current collector tab is attached.

Figure 5 shows a flowchart illustrating the method of preparation of a pouch cell with porous electrode substrate and connection pin to join the current collector tabs to the porous substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Descriptions of various embodiments of the present invention are made with reference to the accompanying drawings and are shown by way of illustration in which the invention may be practiced. The following description is only for the sake of understanding and is not to limit the present invention. By adding or utilizing other embodiments, structural or dimensional changes are possible without eliminating the scope of present invention.

Figure 1A shows the schematic representation of a 4-leg connection pin with a flat rectangular face 2 and 4-legs 1.

Figure IB shows the schematic representation of a 6-leg connection pin with a flat rectangular face 2 and 6-legs 3. The connection pins are made of metallic sheets having a thickness of about 0.1-0.5 mm depending on the specific application.

The flat surfaces 2 of the connection pins provide sufficiently large contact area with the current collector tab and can withstand high currents. The legs 1 and 3 of the 4- leg and the 6-leg connection pins respectively secure the current collector tab firmly in its position. The material used for the current collector tab and the connection pin may or may not be same. Aluminium and nickel sheets are generally used for cathodic connection pin whereas copper sheet is used for anodic connection pin.

Figure 2A shows the top face of a foam-based electrode to which a thin metal strip current collector tab 13 is attached using the connection pin 14. The top face shows the flat rectangular face 15 of the connection pin 14.

Figure 2B shows the bottom face of a foam-based electrode to which a thin metal strip current collector tab 13 is attached using the connection pin 14.

The bottom face shows the legs 16 of the connection pin 14. In this case, a 6-leg connection pin has been used. In Figure 2A and Figure 2B the region 11 corresponds to the portion of the foam-based electrode substrate that is coated with active material whereas the portion of the foam-based electrode substrate devoid of active material is denoted as 12. The foam-based electrode substrate has either metallic or polymeric three-dimensional porous structure. Metallic substrates may include nickel foam, nickel-plated steel foam, copper foam or the like whereas polymeric substrates may constitute polyurethane foam, polyester foam, etc. Various foam-based electrode substrates are commercially sold as rolls. In the case of porous electrode substrate, the entire depth of the active material is in contact with the current collector which significantly reduces the travelling distance of the electrons from the current collector to the active material particles. This results in much reduced charge transfer resistance thereby aiding in faster charging, higher power output and improved charge-discharge capacity of the cell. Porous electrode substrate also helps in effective utilization of the active materials and more uniform current distribution across the entire depth of the electrode active material. The porous substrate is electronically conductive and hence eliminates the need to use any extra current collector. It also provides mechanical stability to the active material against structural deformations.

In the case of most conventional batteries using metallic foil type current collectors, the tabs are connected to the current collector foils by spot welding. However, in the case of porous foam-based electrode substrates; performing any kind of welding is not a very effective option and the surface of the porous substrate needs additional preparation prior to welding. In the present invention, the current collection tab 13 in the form of a thin metal strip is attached to the foam based substrate and is secured in its position by stapling a connection pin, 14. The connection pin used may have four or more legs depending upon face width of the connection pin which in turn depends on the application and the current collector tab width. This method of connecting the current collector tab to the foam-based substrate is practical and easy to implement both at lab scale as well as in a continuous production line.

Figure 3A is the schematic representation of the two-way metal strip current collector tab used in the invention. The metallic foil type current collectors used in conventional batteries have thickness in the range of 10-30um whereas the foam-based electrode substrates have an initial thickness of 1.6-2mm which upon compression reduces to 0.08-1 mm. As the foam-based substrates are much thicker than metallic foil substrates, the use of thin strip metallic current collector tabs may not be very efficient. A two-way current collector tab as shown in Figure 3A has two flat faces 22 and 23 connected to a joint face 21. The flat faces 22 and 23 can simultaneously attach to both the top and the bottom faces of the foam-based substrate ensuring better electrical and mechanical support to the foam-based substrate. Further, a two-way current collector tab would result in more efficient current distribution, load sharing as well as better utilization of active materials. Figure 3B shows the side-view of a two-way metal strip current collector tab with two flat faces 22 and 23 connected to a joint face 21. The two-way current collector tab can be made either by bending a long metal strip or joining two separate metal strips at the one end either by using some conductive adhesive or welding to obtain a joint face, 21 and leaving the other ends free to obtain two free flat faces 22 and 23.

The material of construction of the two-way current collector tab is the same as the thin strip metallic current collector tab. Figure 4A shows the top face of a foam type electrode to which a two-way current collector tab 27 is attached using the connection pin 14. The top face shows the flat rectangular face 15 of the connection pin 14. Figure 4B shows the bottom face of a foam-based electrode to which a two-way current collector tab 27 is attached using the connection pin 14. The bottom face shows the legs 30 of the connection pin 14. In this case, a 6-leg connection pin has been used. In both the figures, i.e. Figure 4A and Figure 4B; region 11 corresponds to the portion of the foam-based electrode substrate that is coated with active material whereas the portion of the foam-based electrode substrate devoid of active material is denoted as 12. The active material paste coated onto the foam-based substrate depends on the battery type; for example, the cathode slurry of lithium-ion batteries would consist of LiFeP04, LiMn204, Li(NiCoMn)i/302, LiCo02, Li(Nio.5Coo.2Mno.3)02 etc., for lead-acid batteries the active material mainly contains PbO paste, the positive electrode paste of nickel based batteries contains nickel(III) oxide-hydroxide or nickel oxy-hydroxide depending on whether it is a nickel-cadmium or nickel-metal hydride battery. Apart from the main active ingredient, the active material slurry would also contain electrolyte, conductive additives, binders, extenders etc. depending on the battery type and the electrode (positive or negative).

Figure 5 illustrates a method of preparation of a pouch cell considering both the electrodes consist of porous substrates of dissimilar materials. A desired length of the anodic and cathodic foam-based electrode substrate is coated with the respective active material slurry 41. Slurry coating can be performed either manually, by using a film coating machine or by spray application. The active material is coated only upto a certain height of the foam based substrate to leave some place at the top to attach the current collector tabs. This results in two distinct portions of the electrode substrate, one that is filled with active material and another portion at the top which is devoid of active material. Then the electrode substrate is hot pressed so as to compress the assembly 42 and to ensure uniform distribution of the active material slurry throughout the three-dimensional porous matrix of the electrode substrate. The current collector tabs are stapled at regular intervals to the foam-based substrate on its top portion that is devoid of active material 43. A 4- or higher leg connection pin can be used to make the connection using a small press which may either be manually, pneumatically, hydraulically or electrically operated.

In addition to stapling operation, some amount of conductive adhesive can also be applied to the foam-based electrode substrate prior to placing the current collector tab. A number of conductive adhesives filled with conductive particles like silver, copper or graphite are available commercially. At block 44, it is checked whether a jelly roll assembly is desired. If YES; i.e. in the case of jelly-roll assembly, alternate layers of cathode and anode separated by separator films are stacked over one another at 45 and are wounded over a prismatic or cylindrical blade with the help of a winding machine at 46. If NO; i.e. in the case if a stacked assembly, the coated electrode substrates after compression and current collector tab attachment are cut into desired sizes using a pouch cell die cutter or manually using scissors 47. The cut-out coated electrodes are alternately stacked over one another separated by separator films 48. The entire cell assembly is then placed between two formed aluminium laminated films 49 and the side and bottom edges are heat sealed 50. The electrolyte is added into the electrode assembly from the unsealed top edge using a syringe or electrolyte dispenser 51 and finally the top edge is vacuum sealed to make the final cell 52.

REFERENCES

1. US2012/0288757A1 11/2012 Hosoeet.al.

2. US2004/0002006 Al 01/2004 Kelleyet.al.

3. US2013/0065122 03/2013 Chiang et.al.

4. US 5667915 09/1997 Loustau et.al.

5. US 5456813 10/1995 Grange-Cossou et.al.

6. US 5518840 05/1996 Verhoogetal.

7. US 6238819 Bl 05/2001 Cahill et.al.

8. Masaru Yao et.al, "LiFePC»4 based electrode using micro-porous current collector for high power lithium-ion battery", Journal of Power Sources, 173 (2007) 545-549

WE CLAIM

1. A current collector tab connecting to a porous foam-based electrode substrate, further used as a two-way tab which simultaneously attaches both the top and the bottom faces of the substrate to ensure better electrical and mechanical support comprising (a) four or more legs 1, 3,16, 30, (b) one or more flat rectangular faces 2,15, 22, 23, (c) a portion of the foam- based electrode substrate coated with active material 11, (d) a portion of the foam-based electrode substrate devoid of active material 12, (e) one or more current collector tabs 13,27, and (f) a connection pin 14 wherein:

a) The current collection tabs 13, 27 are in the form of thin metal strips attached to the foam-based substrate and secured in its position by stapling a connection pin 14;

b) The connection pin 14 comprises of four or more legs 1, 3, 16, 30 depending on face width of the connection pin 14;

c) the legs 1, 3, 16, 30 of the connection pins 14 secure the current collector tab firmly in its position

d) The flat rectangular faces 2, 15, 22, 23 of the connection pin 14 provide sufficiently large contact area with the current collector tab and can withstand high currents;

e) The foam-based electrode substrate has the region coated with active material 11 which is in contact with the current collector for more uniform current distribution across the entire depth of the electrode active material; and

f) The foam-based electrode substrate has a region devoid of the active material 12.

2. The current collector tab as claimed in Claim 1 wherein the two-way tab comprises of two flat faces 22, 23 connected to a joint face 21 which simultaneously attaches to both the top and the bottom faces of the substrate for more efficient current distribution, load sharing and better utilization of the active materials.

3. The current collector tab as claimed in Claim 1 wherein the joint face 21 and two free flat faces 22 and 23 are obtained by one or more ways of:

a) Bending a long metal strip;

b) Joining two separate metal strips at one end by using a conductive adhesive; and

c) Joining two separate metal strips at one end by welding.

4. The current collector tab as claimed in Claim 1 wherein the foam-based electrode substrate is a metallic structure which includes nickel foam, nickel-plated steel foam and copper foam.

5. The current collector tab as claimed in Claim 1 wherein the foam-based electrode substrate is a polymeric three-dimensional porous structure made of polyurethane foam and polyester foam.

6. The current collector tab as claimed in Claim 1 wherein the connection pins 14 are made of metallic sheets having a thickness ranging from 0.1-0.5 mm depending on the specific application.

7. The current collector tab as claimed in Claim 1 wherein aluminium and nickel sheets are used for cathodic connection pin 14 and copper sheet is used for anodic connection pin 14.

8. The current collector tab as claimed in Claim 1 wherein (a) the flat rectangular face 15 of the connection pin 14 is attached to the top face of the foam-based electrode and (b) the legs 16, 30 of the connection pin 14 hold to the bottom face of a foam-based electrode.

9. The current collector tab as claimed in Claim 1 wherein the entire depth of the active material of the porous electrode substrate remains in contact with the current collector whereby:

a) Reducing the travelling distance of the electrons from the current collector to the active material particles;

b) Reducing charge transfer resistance to aid in faster charging, higher power output and improved charge-discharge capacity of the cell;

c) Effectively utilizing the active materials along with more uniform current distribution across the entire depth of the electrode active material; and

d) Eliminating the need for an extra current collector by providing mechanical stability to the active material against structural deformations.

10. The current collector tab as claimed in Claim 1 wherein the active material slurry contains electrolyte, conductive additives, binders and extenders in addition to the electrode active material based on the battery type and the electrode.

11. The current collector tab as claimed in Claim 1 wherein the resultant electrode containing the porous electrode substrate serves as positive electrode for primary and secondary batteries.

12. The current collector tab as claimed in Claim 1 wherein the resultant electrode containing the porous electrode substrate serves as negative electrode for primary and secondary batteries.

13. A method of preparation of a pouch cell with the electrodes made of porous substrates of dissimilar materials to ensure better electrical and mechanical support comprising (a) one or more legs 1,3,16,30, (b) one or more flat rectangular face 2, 15, 22, 23, (c) a portion of the foam-based electrode substrate coated with active material 11, (d) a portion of the foam-based electrode substrate devoid of active material 12, (e) a thin metal strip current collector tab 13, 27, and (f) a connection pin 14 having steps of:

a) Coating a desired length of the anodic and cathodic foam-based electrode substrate with the respective active material slurry 41;

b) Pressing the electrode substrate to compress the assembly and ensure uniform distribution of the active material slurry throughout the three-dimensional porous matrix of the electrode substrate 42;

c) Stapling the current collector tabs at regular intervals to the foam-based substrate on the portion that is devoid of active material 43;

d) Checking whether a jelly roll assembly is desired 44:

i. If a jelly roll assembly is desired: I. Stacking alternate layers of cathode and anode separated by separator films over one another 45, and II. Wounding alternate layers of cathode and anode over a prismatic and cylindrical blade with the help of a winding machine 46, ii. If jelly roll assembly is not-desired and is a stacked assembly: I. Cutting the coated electrode substrates after compression and current collector tab attachment into desired sizes using a pouch cell die cutter or manually using scissors 47, and II. Stacking the cut-out coated electrodes alternately over one another separated by separator films 48;

e) Placing the entire cell assembly between two formed aluminium laminated films 49;

f) Heat sealing the side and bottom edges 50;

g) Adding the electrolyte into the electrode assembly from the unsealed top edge using a syringe or electrolyte dispenser 51; and h) Vacuum sealing the top edge to make the final cell 52.

14. The method of preparation of a pouch cell as claimed in Claim 13 wherein the slurry coating is performed manually, by using a film coating machine.

15. The method of preparation of a pouch cell as claimed in Claim 13 wherein the slurry coating is performed by spray application.

16. The method of preparation of a pouch cell as claimed in Claim 13 wherein the active material is coated only upto a certain height of the substrate to leave some place at the top to attach the current collector tabs resulting in two distinct portions of the electrode substrate such that one that is filled with active material 11 and another portion at the top which is devoid of active material 12.

17. The method of preparation of a pouch cell as claimed in Claim 13 wherein four or more legs connection pin 14 is used to make the connection using a small press which is operated in one or more ways.