Secondary cell

Abstract

A cylindrical secondary cell (100) is provided comprising a housing (102) with an end surface (103). The cell further comprises an electrode assembly (204) arranged in the housing (102), and a current collector plate (106) connected to an electrode (105) of the electrode assembly (204). The end surface (103) is provided with at least one fracturable portion (124), which is configured to break when the pressure in the housing (102) reaches a threshold value. The current collector plate (106) is provided with at least one flap (202), which is configured to bend toward the end surface when the fracturable portion (124) breaks, such that one or more openings are formed in the current collector plate (106).

Description

[0001] Secondary cell

[0002] Technical field

[0003] The present disclosure relates to a secondary cell, and to an energy storage system and a vehicle comprising a secondary cell.

[0004] Background

[0005] The transition from fossil fuels towards renewable energy has gained considerable momentum. One of the most important contributing factors is the development of better and cheaper rechargeable battery cells, also referred to as secondary cells. Currently, lithium-ion battery cells are becoming increasingly popular. They represent a type of cell in which lithium ions move from the negative electrode to the positive electrode during discharge and back when charging.

[0006] An electric vehicle typically comprises a large number of secondary cells (often hundreds or even thousands) arranged in a module and / or pack. In case of a short circuit or other malfunction, e.g. due to external crushing due to a crash, the temperature may increase inside the cell causing development of gases and an increase of pressure. If this reaction is not controlled, it may lead to thermal runaway which could also propagate to neighbouring cells, in the worst case causing a fire and / or explosion.

[0007] To prevent such events from occurring, secondary cells are generally provided with safety features. One commonly applied safety feature is to provide the cell with a vent for releasing gas and / or other ejecta when the pressure inside the cell rises above a threshold value. Venting contributes to slowing down or stopping a thermal runaway event as well as directing the ejected material away from neighbouring cells.

[0008] Such a vent may be provided as a weakened portion, for example a notch, designed to fracture at a defined threshold pressure and provide an opening for gases and material. The weakened portion may be stamped into the housing or lid of the cell, or the vent may be provided as a separate component inserted into an opening of the cell. This may however require additional solutions, such as additional cell components, to ensure efficient venting. There is a general need in the art for secondary cells that provide adequate safety without sacrificing cell performance.

[0009] Summary

[0010] According to some examples, a cylindrical secondary cell 100 is provided comprising a housing with an end surface. The cell further comprises an electrode assembly arranged in the housing, and a current collector plate connected to an electrode of the electrode assembly. The end surface is provided with at least one fracturable portion which is configured to break when the pressure in the housing reaches a threshold value. The current collector plate is further provided with at least one flap configured to bend toward the end surface when the fracturable portion breaks, such that one or more openings are formed in the current collector plate. The opening in the current collector plate may optionally be formed at a location corresponding to the fracturable portion.

[0011] Since the flap forms part of the current collector plate, it will contribute to the electrical connection with the electrode assembly during normal operation of the cell. The flap doesn't open until the fracturable portion breaks, and therefore the flap opening can be made larger without affecting the cell performance in normal operation. This enables more efficient venting and thereby improved safety. The flap opening can also be shaped to direct ejecta in a desired direction.

[0012] The end surface may comprise a lid plate. A portion of the lid plate may be configured for connecting an external load.

[0013] In some variants, a plurality of flaps are provided on the current collector plate. The flaps may be arranged symmetrically around the center of the current collector plate.

[0014] In some variants, the at least one flap may be formed by one or more weakened portions on the current collector plate, such as notches. The depth of the notch, or other properties thereof, may be designed so as to control a pressure at which the flap will open. In other variants, at least one flap is formed by one or more cut-out portions on the current collector plate. The fracturable portion may comprise one or more weakened portions forming an annular shape around the center of the end surface.

[0015] Brief summary of the drawings

[0016] Different aspects are now described, by way of example, with reference to the accompanying drawings, in which:

[0017] Fig. la shows a secondary cell according to some examples.

[0018] Fig. lb shows a secondary cell according to some examples.

[0019] Figures 2a is a cross section view of the secondary cell shown in Fig. la.

[0020] Figures 2b is a cross section view of the secondary cell shown in Fig. la.

[0021] Fig. 3a - 3e shows top views of different arrangements of a current collector plate according to some examples.

[0022] Fig. 4 shows an energy storage system according to some examples.

[0023] Fig. 5 shows a vehicle according to some examples.

[0024] Detailed description

[0025] The disclosure is not limited to the embodiments disclosed but may be varied and modified within the scope of the claims.

[0026] In the context of this disclosure, "terminal" refers to a portion of the cell which is configured to be connected to an external load.

[0027] "Electrode" refers to a foil or other substrate coated with active material. However, when this disclosure refers to connecting to an electrode, this means making an electrical connection to an electrode tab, which may either be an uncoated portion of the foil, or a separate tab attached to the foil.

[0028] Figure 1 shows a cylindrical secondary cell 100 wherein some embodiments may be implemented. The cell 100 comprises a housing 102 - in this example, a cylindrical can - with an end surface. The housing may be made of steel, nickel plated steel, aluminium, or any other suitable material. The end surface comprises a lid, which comprises a lid plate 210 and a terminal portion 212. The terminal portion 212 functions as the positive terminal of the cell 100 and can be attached to an external load by connecting a bus bar or other connector to the outer surface of the terminal portion 212, for example by laser welding.

[0029] The negative terminal may be provided by an additional terminal structure on the other end of the can, which is electrically connected to the negative electrode. Alternatively, the negative electrode may be directly connected to the can, such that the entire surface of the can 102 functions as the negative terminal. In the latter case, the positive terminal structure or the lid may be insulated against the negative can e.g. by applying a gasket 222 or other insulating material.

[0030] As mentioned above, secondary cells are typically provided with a safety vent to allow gases to escape the cell in a controlled manner in case of a thermal event. In the example of Figure 1, the end surface is provided with at least one fracturable portion 124 configured to break when the internal pressure in the cell reaches a predetermined threshold level. The fracturable portion 124 may for example be a weakened portion, specifically a notch, arranged in an annular shape on the lid plate 210. Preferably, the fracturable portion surrounds the terminal portion 212.

[0031] Figure lb illustrates the same cell as in Figure la after fracturing of the vent. As shown, a part 104 of the fracturable portion 124 has opened, providing an opening for gases and other ejecta to vent outside the cell 100. It should be appreciated that depending on the cell pressure and other factures, the fracturable portion 124 could break either partially or completely, potentially causing a portion of the lid plate 210 and / or the terminal portion 212 to deform and / or bend in the process.

[0032] Figure 2a is a cross section view of the secondary cell illustrated in Figure la and lb. Further details of the internal components of this cell will now be described.

[0033] As shown in Figure 2a, an electrode assembly 204 is arranged in the cell housing. The electrode assembly 204, which can alternatively be referred to as a jelly roll or electrode roll, comprises a negative electrode and a positive electrode. The negative electrode will be interchangeably referred to as the "anode electrode" in this disclosure, and the positive electrode will be interchangeably referred to as the "cathode electrode". The anode and cathode electrode sheets are rolled together with a separator sheet (not shown) in between. The separator is a thin sheet of insulating material, typically a polyolefin, which prevents physical contact between anode and cathode which would lead to a short circuit. Each electrode comprises a foil substrate (typically copper foil on the anode side, and aluminium foil on the cathode side) which is coated with active material. The type of active material can vary as different cell chemistries may be applied depending on the desired cell characteristics. One commonly used anode active material is graphite, which may be mixed with a silicon-based material. Commonly used cathode active materials are NMC (Lithium nickel manganese cobalt oxides) and LFP (Lithium iron phosphates).

[0034] The edge of the foil is uncoated and functions as a tab 105 for electrical connection to an external terminal of the cell, which will be further described below. It is however also possible to attach a separate tab to the foil, e.g. by welding, in which case the separate tab will provide the electrical connection.

[0035] The positive electrode (sometimes referred to as the cathode side) is connected to a current collector plate 106, which in this example is disc shaped. However, it should be understood that the current collector plate 106 could have any other physical shape, for example an oval or polygonal disc, or a structure with one or more arms extending from a central portion. Advantageously, the current collector plate 106 is large enough to cover the electrode assembly 204, in order to maximize electrical contact. When the current collector plate 106 is a circular disc, the diameter of the disc may be at least the same size as the diameter of the electrode assembly 204.

[0036] Physical contact between the electrode 105 and the current collector plate 106 can be achieved by pressing the plate 106 against the electrode 105 and then welding in one more positions from the top of the current collector plate 106, for example by laser welding. Optionally, the electrode 105 may be slit in one or more positions before applying the current collector plate 106, to improve physical contact and reduce resistance. To prevent the electrode assembly 204 from moving inside the cylindrical can 102, a beading groove 218 may optionally be provided around the circumference of the can 102. The beading groove 218 also provides a shelf for positioning the lid plate 210. Hence, the beading groove 218 is advantageously provided near the can opening, adjacent to or just below the lid plate 210.

[0037] The can 102 is closed by crimping the can edge over the lid plate 210 and gasket 222. It should be noted that if the negative electrode is connected to the can 102, the crimped edge will also form part of the negative terminal. This provides the possibility to connect the external load to both terminals on the same side of the cell 100, which may facilitate the assembly of large numbers of cells into e.g. modules, packs or vehicles.

[0038] Figure 2a schematically illustrates the notch or weakened portion 124 as provided on the upper side of the terminal part 212. It should however be understood that it is equally possible to provide the notch on the underside, or on both sides, as long as the properties of the notch (e.g. depth and width) are adapted to provide the desired opening behaviour.

[0039] In case of a thermal event inside the cell, gases and heat will develop inside the electrode assembly 204. If the current collector plate 106 would be a solid metal plate, there may not be sufficient room for gas to escape in a controlled manner through the fractured vent. One possible solution is to provide one or more openings in the current collector plate. These openings need to be made large enough to allow gas or debris to pass through. However, at the same time, large openings reduce the available area for electrical connection toward the electrode assembly and increase resistance, which will negatively impact cell performance.

[0040] According to some examples, a cylindrical secondary cell 100 is therefore provided according to Figure 2b. This cell comprises a housing 102 with an end surface 103, an electrode assembly 204 arranged in the housing 102, and a current collector plate 106 connected to an electrode 105 of the electrode assembly 204. The end surface 103 is provided with at least one fracturable portion 124 configured to break at a threshold pressure value. These components are analogous to what was described above in connection with Figures la, lb and 2a. However, in this example the current collector plate 106 is provided with at least one flap 202 which is configured to bend toward the end surface 103 when the fracturable portion 124 breaks, such that an opening is formed in the current collector plate 106. More specifically, the bending of the flap 202 is triggered by the flow of gases and / or material being ejected from the cell 100 as the vent opens.

[0041] Before the fracturable portion 124 breaks, the flap 202 forms part of the current collector plate 106, or stated differently, the flap 202 is arranged in the same plane as the current collector plate 106 and hence contributes to the electrical connection with the electrode assembly 204. Because the flap 202 does not open until the fracturable portion 124 breaks, the flap opening can be made larger to enable more efficient venting without affecting the cell performance in normal operation.

[0042] The at least one flap 202 may be formed by one or more weakened portions on the current collector plate 106. This is illustrated in Figure 3a, where each flap 202a is provided by two notches which meet at an angle, forming a triangular shape. The pointed end of the flap will bend toward the lid. Another variant is shown in Figure 3c, where the flaps 202c are instead formed as rounded tongues. Yet further variants are illustrated in Figures 3d and 3e. Figure 3d shows triangular flaps 202d similar to Figure 3a, but with the pointed end facing toward the can wall. In Figure 3e, triangular flaps 202e are formed by several notched lines which intersect in the center of the current collector plate 106. In this variant, all the flaps 202e are adjacent to each other thereby providing an even larger opening in the current collector plate 106.

[0043] The at least one flap 202 may alternatively be formed by one or more cut-out portions on the current collector plate 106. An example of this flap configuration is illustrated in Figure 3b. In this variant, some of the current collector plate area is sacrificed to provide the cut-outs. However an advantage may be that the flaps 202b bend more easily and that venting can start earlier, since some gas will be able to pass through the cut-outs even before the flap 202b start to bend.

[0044] It should further be appreciated that it's possible to combine these variants such that some flaps

[0045] 202 are formed by weakened areas (such as notches) while other flaps 202 are formed by cutouts. As is clear from Figures 3a-3e, different numbers, orientations, and shapes of the flaps are possible, for example but not limited to 4, 6, or 8 flaps. It may however be advantageous to provide a plurality of flaps as this ensures a larger total venting area as well as reduces the impact of one flap failing to open.

[0046] In the variants shown in Figures 3a-3e the flaps are arranged symmetrically around the center of the current collector plate. However, it should be appreciated that this is not necessary. The flaps could be arranged at any location on the current collector plate, and it's also possible to provide a single flap as long as its placement and size is dimensioned to provide sufficient venting area.

[0047] It should further be noted that, as shown in Figures 3a-3d, the flaps may be arranged such that the openings in the current collector plate will be formed at a location corresponding to the fracturable portion 124. This arrangement may contribute to more efficient venting. However, it's also possible to provide the flaps in a different arrangement, for example as illustrated in Figure 3e where the flaps are arranged inside the vent notch.

[0048] Figure 4 illustrates an energy storage system comprising a plurality of secondary cells as described above in connection with Figures 1-3. The cells may be arranged in modules and / or packs within the energy storage system (not shown) and connected to a battery management system (not shown) which controls the operation of the cells.

[0049] Figure 5 illustrates an electric vehicle comprising a plurality of secondary cells as described above in connection with Figures 1-3. The cells may be arranged in modules and / or packs (not shown) within the vehicle and connected to a battery management system (not shown) which controls the operation of the cells.

[0050] While the above examples have described the solution in the context of a positive (cathode side) terminal structure and current collector plate, it should be appreciated that the polarity of the cell terminals could also be reversed. Hence, the examples apply equally well to an end side with a negative terminal part, and a corresponding anode current collector plate. The terminology used herein is for the purpose of describing particular aspects of the disclosure only and is not intended to limit the invention. The singular forms "a", "an" and "the" are intended to include plural forms as well, unless the context clearly indicates otherwise. Furthermore, the word "comprising" does not exclude the presence of other elements or steps than those listed.

Claims

Claims1. A cylindrical secondary cell (100) comprising: a housing (102) with an end surface (103); an electrode assembly (204) arranged in the housing (102); a current collector plate (106) connected to an electrode (105) of the electrode assembly (204); wherein the end surface (103) is provided with at least one fracturable portion (124) configured to break when the pressure in the housing (102) reaches a threshold value; and wherein the current collector plate (106) is provided with at least one flap (202; 202a-d) configured to bend toward the end surface when the fracturable portion (124) breaks, such that one or more openings are formed in the current collector plate (106).

2. The cylindrical secondary cell (100) of claim 1, wherein the opening in the current collector plate (106) is formed at a location corresponding to the fracturable portion (124)3. The cylindrical secondary cell (100) of any one of claims 1-2, wherein at least one flap (202; 202a-d) is formed by one or more weakened portions on the current collector plate (106).

4. The cylindrical secondary cell (100) of any one of claims 1-2, wherein at least one flap (202; 202a-d) is formed by one or more cut-out portions on the current collector plate (106).

5. The cylindrical secondary cell (100) of any one of claims 1-4, wherein the current collector plate (106) is provided with a plurality of flaps (202; 202a-d).

6. The cylindrical secondary cell (100) of claim 5, wherein the flaps (202; 202a-d) are arranged symmetrically around the center of the current collector plate (106).

7. The cylindrical secondary cell (100) of any one of claims 1-6, wherein the fracturable portion (124) comprises one or more weakened portions forming an annular shape around the center of the end surface (103).

8. The cylindrical secondary cell (100) of any one of claims 1-7, wherein the end surface (103) comprises a lid plate (210).

9. The cylindrical secondary cell (100) of claim 8, wherein a portion of the lid plate (210) is configured for connecting an external load.

10. An energy storage system comprising the secondary cell (100) of any one of claims 1-9.

11. A vehicle comprising the secondary cell (100) of any one of claims 1-9.

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