Improved pouch cell and method for producing improved pouch cells

Abstract

A pouch cell formed from wall layers, each wall layer having a metal film outer layer and a polymeric inner layer. An outer cathode conductor tab having a polymeric sealing film containing an electrically conductive filler on at least one side of the outer conductor tab is heat-sealed between the edges of two wall layers. A method of making a pouch cell comprises disposing a metallic outer cathode conductor tab having a polymeric sealing film strip containing an electrically conductive filler between the edges of pouch wall layers, each pouch wall layer having a metal film outer layer and a polymeric inner layer, and applying heat and pressure to the wall layers to heat-seal the outer cathode conductor tab and provide an electrical connection between the metal film layer of the wall layer and the outer cathode conductor tab.

Description

INITIATIONThis invention relates to a pouch cell having an improved connection between outer tabs and the wall material and to a method of making such a pouch cell.For example, DE 10 2021 214 965 A1 discloses a pouch cell having an outer metal foil which is coated with an electrically conductive polymer layer for protection against corrosion.Pouch cells generally comprise an anode, a cathode and an electrolyte enclosed in a pouch formed from portions of a multi-layer wall material whose edge edges are heat sealed together. These wall sheets often comprise at least one metal film outer layer containing the cell contents and an inner layer of a heat sealable and electrolyte resistant polymer such as modified polypropylene which protects the outer layer from the electrolyte and serves as a thermal adhesive layer for bonding the wall sheets to form the sealed pouch. Outer tabs electrically connected to anode and cathode extend between opposing portions of the wall sheet and are sealingly secured to the edge edges of the portions of the wall sheets.Minor defects in the wall sheets, particularly in the inner layer of the wall sheets, may occasionally cause leaks. Due to the nature of the pouch forming process, the general manufacturing process tolerances and control, and the repeated handling and use of the cell, defects may form in the inner layer, exposing the outer metal film layer to the electrolyte in the pouch, and forming a circuit between the anode of the cell and the intermediate metal film layer of multi-layered wall material that erodes the metal film. In particular, aluminum, which frequently constitutes the multi-layer wall material, forms a lithium-containing alloy in the region exposed to the electrolyte, since the naturally formed Al 2 O 3 is easily attacked by the acid in the electrolyte. Over time, these undesirable side reactions can damage the metal film interlayer and ultimately lead to leakage of electrolyte from the pouch cell.SUMMARYEmbodiments of this invention provide an improved pouch cell for a battery, particularly a pouch cell having an improved connection to outer tabs. Generally, one embodiment of the pouch cell according to this invention comprises a pouch made from a multi-layered wall material. The multilayer wall material comprises an intermediate metal film layer and an inner polymeric layer that protects the intermediate metal film layer and heat seals portions of the wall material together into a pouch containing the contents of the cell. The metal film interlayer may optionally include an outer protective insulating layer.The pouch contains an anode, a cathode, and an electrolyte. An outer anode lead tab and an outer cathode lead tab are connected to the anode and cathode, respectively, and extend from the pouch between opposing portions of the wall material and sealingly engage therebetween. The outer cathode tab has a strip of polymeric sealing film on at least one side containing an electrically conductive filler. The outer cathode tab is heat sealed between the edges of the opposing inner layers of two wall portions. The electrically conductive filler in the strip of the sealing film forms part of the heat seal and provides electrical conductivity between the metal film interlayer of the wall material and the cathode which is connected to the outer cathode lead tab.The electrical connection between the cathode and the metal film interlayer of the wall material enables a protective layer to be formed on the metal film interlayer of AlF 3, when the metal film is exposed to the electrolyte in the cell and the cell operates at over 3.4V. The AlF 3 provides protection from the lithium alloying reaction that would otherwise occur if the electrolyte were to contact the aluminum interlayer in the event of failure of the inner layer.Further, the formation of a circuit between the anode electrode and the aluminum interlayer of the pouch wall material means that the cell parasitically discharges the energy. The resistance in the circuit between the cathode, the aluminum interlayer, and the cathode is so high that a battery management system (BMS) in the battery pack can identify the defective pouch by comparing the voltages of the cells and issue a warning before the electrolyte leaks from the cell in the battery pack.The polymeric sealing films on the outer lead tab may optionally comprise at least one of polypropylene and polyphthalamide that can be sealed to the inner layer of the wall plies, and may optionally be made of the same material as the inner layer of the wall plies. The metal film of the pouch layers and the metal of the outer lead tab may be made of aluminum or an aluminum alloy or another suitable electrically conductive and optionally relatively light metal.The electrically conductive filler in the polymeric sealing film may be metallic, for example, aluminum, nickel, stainless steel or aluminated steel. The electrically conductive filler can be powder, granulate, needle-shaped particles or spike-shaped particles. Alternatively, the conductive filler in the polymeric sealing film may also be non-metallic, for example diamond, fullerenes, graphite, carbon black, carbon fibers and nanofibers, carbon nanotubes and graphene. Due to the electrically conductive filler, the multimeter electrical resistance between the exposed aluminum layer of the pouch film and the center of the outer cathode tab is between about 1 kOhm and about 20 kOhm.In some embodiments, conductive fill material may also be present in the inner layer of the wall material. The conductive filler in the inner layer of the wall material may be concentrated in the vicinity of the intermediate layers of the wall layers. This location may reduce the electrical interaction between the electrolyte in the cell and the interlayer.According to a second embodiment of this invention, embodiments of methods of making a pouch cell are provided. According to such an embodiment, a metallic outer cathode tab having a polymeric sealing film containing an electrically conductive filler is disposed on at least one side of the outer cathode tab between the edge edges of two portions of the wall material, each portion of the wall material having a metal film outer layer and a polymeric inner layer; heat and pressure are applied to the wall portions to heat seal the outer cathode tab between the wall portion, the sealing film is compressed, and thereby the concentration of the electrically conductive filler between the metal film outer layer in the wall material and the outer cathode tab is increased.The sealing film can be compressed between about 30% and about 40% of its original thickness, thereby achieving sufficient concentration to provide a conductive path between the cathode connected to the outer cathode tab and the metal film outer layers of the wall material.Alternatively to the first embodiment, the filler material may be placed in relatively higher melting polymer bodies which do not substantially soften at the temperature and pressure used to heat seal the wall material around the outer tabs. Upon heating and pressure during heat sealing, the film softens and flows onto the outer cathode tab and the inner layer of the wall material, while the relatively higher melting polymer bodies remain substantially intact and increase the concentration of the conductive material between the outer cathode tab and the outer metal layer of the wall material.In both alternatives to the first embodiment, the polymeric inner layer of the wall material may optionally also comprise an electrically conductive filler, optionally concentrated adjacent to the metallic outer layer, to reduce the electrical interaction between the electrolyte in the cell and the metallic intermediate layer.Further areas of applicability of the present invention will become apparent from the detailed description, claims and drawings. The detailed description and specific examples are provided for illustrative purposes only.BRIEF DESCRIPTION OF THE DRAWINGSThe present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: FIG. 1 is a plan view of a pouch cell constructed in accordance with the principles of this invention; FIG. 2 is an enlarged cross-sectional view of the bag cell; FIG. 3 is an enlarged plan view of an outer lead tab; FIG. 4 is a plan view of a schematic diagram illustrating the metallic outer lead tab positioned between the edge edges of two wall plies prior to heat sealing; FIG. 5 is a schematic diagram illustrating the metallic outer lead tab disposed between the edge edges of two wall plies after heat sealing; FIG. 6A is a schematic diagram illustrating the location at which the electrically conductive filler has been introduced into the sealer strip; and FIG. 6B is a schematic diagram illustrating the location of the electrically conductive filler introduced into the sealer strip and liner of the pouch material after heat sealing and mixing of the materials; FIG. 7A is a schematic diagram illustrating the location of the electrically conductive filler disposed in the higher melting point support bodies; FIG. 7B is a schematic diagram illustrating the location of the conductive filler in the higher melting point support bodies after heat sealing and mixing the lower melting point materials.In the drawings, reference numerals may be reused to identify similar and / or identical elements.DETAILED DESCRIPTIONEmbodiments of this invention provide an improved pouch cell 20 for a battery, and more particularly for a pouch cell having an improved connection to an outer cathode tab 22, which in some cases may reduce the likelihood of electrolyte leakage and / or make the potential for such leakage detectable before it actually occurs. Generally, one embodiment of pouch cell 20 according to this invention comprises a pouch 24 made from one or more sections of a multi-layered wall material. The multi-layer wall material includes a metal film outer layer 26 and a polymeric inner layer 28.The metal film outer layer 26 of the wall material may be made of aluminum or an aluminum alloy or another metal film that is a relatively good conductor, and optionally is relatively lightweight. The inner layer 28 may be made of polypropylene and / or polyphenylene sulfide or other suitable thermoplastic material that can be heat sealed and resistant to the components of battery chemistry and, in particular, the electrolytes, typically a lithium salt dissolved in a solvent. The insulative cover 30 of the outer layer 26 may be polyethylene terephthalate or other suitable flexible insulative polymeric material.The cell includes an anode, a cathode, and an electrolyte to facilitate ion exchange. There is an outer anode lead tab and an outer cathode lead tab for making an outer electrical connection to the anode and cathode of the cell. The outer anode and cathode tabs extend between opposite portions of the wall material and are sealingly secured between the edge edges of the portions of the wall material. In accordance with the principles of this invention, the outer cathode lead tab 22 comprises a strip 34 of polymeric sealing film containing an electrically conductive filler (indicated in Figures 6A, 6B and 7A, 7B).If the pouch is made from one piece of wall material, it may be sufficient to have only one strip 34 on one side of the outer cathode lead tab 22, but if the pouch is made from two opposing pieces of wall material, there are desirably two strips 34, one on each side of the outer cathode lead tab, to contact each of the pieces of wall material.The outer cathode lead tab 22 is configured to be heat sealed between the edge edges of opposing inner layers 28 of two portions of the wall material, with an inner tab portion 36 located within the pouch cell 20 and an outer tab portion 38 located outside the pouch cell. The electrically conductive filler in the strips 34 of the sealing film provides electrical conductivity between the film outer layers 26 of the wall material and the outer cathode lead tab 22, and thus the cathode connected thereto.The outer tabs, particularly the outer cathode tab 22, may be made of aluminum or an aluminum alloy or other metal film that is a relatively good conductor, and optionally relatively lightweight. The polymeric seal film strips 34 may be comprised of polypropylene and / or polyphenylene sulfide or other suitable thermoplastic that can be heat sealed to the inner layers 28 of the portions of the wall material and is resistant to the components of battery chemistry, particularly the electrolyte.The electrically conductive filler in the polymeric sealing film 34 may be metallic, for example, aluminum, nickel, stainless steel, or aluminated steel, in the form of powder, granules, acicular particles, and / or pointed particles. Alternatively, the conductive filler in the polymeric sealing film 34 may also be non-metallic, for example diamond, fullerenes, graphite, carbon black, carbon fibers and nanofibers, carbon nanotubes, and graphene. The electrically conductive filler is preferably at a higher concentration near the outer surface of the polymer sealing films (i.e., the exposed surface opposite the tabs 22) prior to heat sealing. The electrically conductive filler may be dispersed in the sealing film 34 such that when the wall material and the outer lead tab are compressed and heat sealed, at least a portion of the electrically conductive filler is dispersed into the polymeric inner layers of the wall material, thereby increasing the concentration of the electrically conductive filler between the tab 22 and the outer layer 26 of the wall material.Due to the electrically conductive filler in the strips 34 of sealing film, the electrical resistance between the outer cathode lead tab 22 and the outer layer 26 of the wall layer is between about 1 kOhm and about 20 kOhm. This creates an electrical connection or circuit between the cathode of the cell, which is connected to the outer cathode tab 22, and the outer film layer 26 of the wall material. This connection or circuit may help form a protective layer of AlF 3 on the outer film layer 26 if a defect or crack in the inner layer 28 exposes the outer film layer to the electrolyte of the cell. The protective layer protects the naturally formed Al 2 O 3 on the intermediate layer 28 from attack by hydrofluoric acid in the electrolyte and at least prevents or slows the erosion of the intermediate layer and the resulting leakage of electrolyte from the pouch.Instead or additionally, the generation of a circuit between the anode and outer film layer 26 of the pouch wall material (as is the case with a defect or crack in the inner layer 28) means that the cell has a separate circuit that discharges the energy from the battery cell in a parasitic manner that a battery management system (BMS) in the battery pack can detect and use to identify that defective housing cell by comparing the voltages of the cells in the battery and generate a warning before electrolyte leaks from the cell into the battery pack.The electrical connection of the outer aluminum layers of the wall material to the cathode may prevent or slow the erosion of the outer metal film layer when the inner layer of the wall material fails, and may facilitate detecting the failure of the inner layer. This can be achieved with minimal change in the overall bag cell design or manufacturing process by enclosing a strip 34 of sealing film with electrically conductive filler.In some alternative constructions of the first embodiment of this invention, the conductive filler may also be present in the inner layers 28 of the wall layers 24. This requires some change in the conventional construction and manufacturing method, but may facilitate the electrical connection between the metal film outer wall and the cathode. The conductive filler in the inner layers of the wall sheets 24 may be concentrated near the intermediate layers 26 of the wall sheets 24 to reduce the electrical interaction between the electrolyte and the intermediate layers.In another alternative construction of the first embodiment of this invention, the electrically conductive filler material may be disposed in relatively higher melting polymer bodies 36 (Figs. 7A, 7B) which do not substantially soften at the temperature and pressure used to heat seal the wall material about the outer cathode lead tab. Upon heating and compression pressure during heat sealing, the film 34 on the outer cathode tab 22 and the inner layer 28 of the wall material soften and flow while the relatively higher melting polymer bodies 36 remain substantially intact and increase the conductive material concentration between the outer cathode tab 22 and the outer metal layer 26 of the wall material.According to a second embodiment of this invention, embodiments of methods of making a pouch cell are provided. According to such an embodiment, a metallic outer cathode lead tab 22 having a strip 34 of polymer sealing film containing electrically conductive filler is disposed on each side of the outer lead tab between the edge edges of two portions of the wall material, each portion having an intermediate metal film layer 26 and an inner polymeric layer 28. Heat and pressure are applied to the wall plies 24 to heat seal the outer lead tab 22 to the wall plies, compressing the strips 34 of sealing film, thereby increasing the concentration (volume fraction) of the electrically conductive filler between each wall ply 24 and the outer lead tab 22.The strips 34 of the sealing film can be compressed between about 30% and about 40% of their original thickness, thereby achieving sufficient concentration of the electrically conductive filler to provide a conductive path between the metallic outer cathode lead tab 22 and the metallic outer film layers 26 of the wall material. The electrically conductive filler can be placed in higher melting point polymer bodies 36 so that by heating and compressing the sealing film, a portion of the electrically conductive filler is driven into the polymeric inner layers 28 of the wall material.The polymeric inner layer 28 of each wall layer 24 may optionally comprise an electrically conductive filler, which may be disposed adjacent the intermediate film layer 26. Thus, when the wall material and metallic outer lead tab 22 are heat sealed together, the electrically conductive filler concentrates in the polymer layer 28 and in the strips 34 so that a conductive path is formed between the outer lead tab and the outer metal film layer 26 by the seal formed between the sheet walls and metallic outer lead tab.In an exemplary embodiment, the outer layer 26 is aluminum of 50 μm thick, the inner layer 28 is at least one of polypropylene, polyphthalamide, and polyphenylene sulfide is 80 μm thick, and the insulating cover layer 30 of polyethylene terephthalate 24 is 80 μm thick. The outer tabs including the outer cathode tab 22 are made of aluminum or an aluminum alloy that is 400 μm thick, and the polymeric sealing film strips 34 are each at least one of polypropylene, polyphthalamide, and polyphenylene sulfide, 150 μm thick. After compression and heat sealing, the inner layers 28 and the strips 34 of polymer sealing film soften and flow, thereby increasing the density or concentration of the conductive filler between the outer layer 26 and the outer lead tab 22.Due to the electrically conductive filler in the sealing film 34, the electrical resistance between the outer cathode tab 22 and the outer layers 26 of the wall material is between about 1 kOhm and about 20 kOhm. This creates an electrical connection or circuit between the cathode of the cell, which is connected to the outer lead tab 22, and the intermediate film layer 26 of the wall material. This connection or circuit may help form a protective layer ALF 3 on the outer film layer 26 if a defect or crack in the inner layer 28 exposes the outer film layer to the electrolyte of the cell. The protective layer protects the naturally formed Al 2 O 3 on the outer layer 28 from attack by hydrofluoric acid in the electrolyte and at least prevents or slows the erosion of the intermediate layer and the resulting leakage of electrolyte from the pouch.Instead or additionally, the generation of a circuit between the anode and the film interlayer 26 of the pouch film means that the cell has a separate circuit that parasitically discharges the energy from the battery cell, which can detect and utilize the battery management system (BMS) in the battery pack to identify this defective housing cell by comparing the voltages of the cells in the battery and generate a warning before electrolyte leaks from the cell into the battery pack.

Claims

A pouch cell (20) comprising an anode, a cathode, and an electrolyte disposed in a pouch (24) formed from at least one wall sheet (24) having a metal film outer layer (26) and a polymeric inner layer (28); and metallic outer anode and cathode tabs (22) connected to the anode and cathode, respectively, each outer tab (22) having a portion extending out of the pouch (24) between opposing portions of the at least one wall layer (24) and being sealingly secured to a heat seal between the edge edges of the opposing portions of the at least one wall layer (24), the heat seal securing the outer tab (22) comprising a polymer sealing film (34) on the outer tab (22) containing an electrically conductive filler sufficient to establish an electrical connection between the outer metal film layer (26) of the at least one wall layer (24) and the cathode connected to the outer tab (22).The pouch cell (20) of claim 1, wherein the electrically conductive filler in the sealing film (34) on the outer cathode tab (22) is metallic.The pouch cell (20) of claim 2, wherein the electrically conductive filler in the sealing film (34) on the outer cathode tab (22) comprises at least one of aluminum, nickel, stainless steel, or aluminated steel.The pouch cell (20) of claim 2, wherein the electrically conductive filler in the sealing film (34) is disposed on the outer cathode lead tab (22) in polymer bodies (36) having a higher melting point than the sealing film (34).The pouch cell (20) of claim 4, wherein the polymeric bodies (36) are sufficiently sized to provide electrical conductivity between the outer cathode lead tab (22) and the outer layer of the wall sheet (24) upon heat sealing.The pouch cell (20) of claim 1, wherein the electrically conductive filler in the sealing film (34) on the outer cathode tab (22) is metallic.The pouch cell (20) of claim 6, wherein the electrically conductive filler comprises at least one of diamond, fullerenes, graphite, carbon black, carbon fibers, carbon nanofibers, carbon nanotubes, and graphene.The pouch cell (20) of claim 6, wherein the electrically conductive filler in the sealing film (34) is disposed on the outer cathode lead tab (22) in polymer bodies (36) having a higher melting point than the sealing film (34).The pouch cell (20) of claim 8, wherein the polymeric bodies (36) are sufficiently sized to provide electrical conductivity between the outer cathode lead tab (22) and the outer layer (26) of the wall sheet (24) upon heat sealing.A method of making a pouch cell (20) comprising: disposing a metallic outer cathode lead tab (22) having a polymeric sealing film strip (34) containing an electrically conductive filler on at least one side of the outer cathode lead tab (22) between the edge edges of two portions of pouch wall plies (24), each portion of the pouch wall ply (24) having a metal film outer layer (26) and a polymeric inner layer (28); applying heat and pressure to the portions of the wall plies (24) to heat seal the outer cathode lead tab (22) to the portions of the wall ply (24); compressing the sealing film (34); and thereby providing an electrical connection between the metal film outer layer (26) of the wall ply (24) and the cathode connected to the outer cathode lead tab (22).

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