BATTERY CELL HOUSING INCLUDING QUENCH-HARDENED STEEL

Abstract

A method for producing a tubular housing for a battery cell comprises roll forming a steel sheet into a tubular body. The steel contains carbon in a range of 0.02 to 0.3 wt.%, manganese in a range of 0.2 to 2.0 wt.%, chromium and / or molybdenum in a range of 0.5 wt.% to 3.0 wt.%, silicon in a range of 0.2 wt.% to 2.0 wt.%, niobium, titanium and / or vanadium in a range of 0.01 wt.% to 0.2 wt.%, and iron. The method comprises welding the sides of the tubular body to form a weld, heating the tubular body to a temperature in a range of 900°C to 950°C, and quenching the tubular body.

Description

INTRODUCTION

[0001] The information contained in this section is intended to provide a general context for the disclosure. Work by the presently named inventors, to the extent described in this section, as well as aspects of the description that might not otherwise be considered prior art at the time of filing, are not expressly or impliedly admitted as prior art against this disclosure.

[0002] The present disclosure relates to battery cells and, more particularly, to quench-hardened steel battery cell casings.

[0003] Electric vehicles (EVs), such as battery electric vehicles (BEVs), hybrid vehicles, and / or fuel cell vehicles, comprise one or more electric motors and a battery system with one or more battery cells, modules, and / or packs. A power control system is used to control the charging and / or discharging of the battery system during charging and / or driving.

[0004] Battery cells comprise one or more cathode electrodes, anode electrodes, and separators arranged in a battery cell housing. The cathode electrodes comprise a cathode active material layer arranged on a cathode current collector. The anode electrodes comprise an anode active material layer arranged on an anode current collector. SUMMARY

[0005] A method for producing a tubular housing for a battery cell comprises roll forming a steel sheet into a tubular body. The steel contains carbon in a range of 0.02 to 0.3 wt.%, manganese in a range of 0.2 to 2.0 wt.%, chromium and / or molybdenum in a range of 0.5 wt.% to 3.0 wt.%, silicon in a range of 0.2 wt.% to 2.0 wt.%, niobium, titanium and / or vanadium in a range of 0.01 wt.% to 0.2 wt.%, and iron. The method comprises welding the sides of the tubular body to form a weld, heating the tubular body to a temperature in a range of 900°C to 950°C, and quenching the tubular body.

[0006] In other features, the method includes mechanically crimping or welding a base portion to one end of the tubular body. The tubular body has a martensite microstructure after quenching. The tubular body comprises one or more chromium carbides with a size in a range of 50 to 500 nm.

[0007] For other features, the proportion of the one or more chromium carbides in the tubular body after quenching is in a range from 1.0 vol.% to 20 vol.%. The weight of the one or more chromium carbides in the tubular body after quenching is in a range from 5 wt.% to 52 wt.%. The tubular body has a minimum tensile strength of 800 MPa at room temperature and a minimum tensile strength of 300 MPa at 600 °C. The tubular housing has a cylindrical or prismatic cross-section.

[0008] For other characteristics, the maximum hardness difference between a weld seam and the steel of the tubular body, which was not heat-affected during welding, is less than 50 HV. The steel comprises a nickel coating and an iron-nickel alloy layer that, after quenching, is sandwiched between the nickel coating and the steel.

[0009] A tubular housing for a battery cell comprises a tubular steel body with a weld seam. The steel contains carbon in a range of 0.02 to 0.3 wt.%, manganese in a range of 0.2 to 2.0 wt.%, chromium and molybdenum in a range of 0.5 to 3.0 wt.%, silicon in a range of 0.2 to 2.0 wt.%, niobium, titanium, and / or vanadium in a range of 0.01 to 0.2 wt.%, and iron. A base portion is attached to one end of the tubular body.

[0010] Other features include a martensite microstructure after austenitizing and quenching. After austenitizing and quenching, the tubular body comprises one or more chromium carbides with a size range of 50 to 500 nm.

[0011] For other features, the proportion of the one or more chromium carbides in the tubular body after austenitizing and quenching is in a range of 1.0 vol% to 20 vol%. The weight of the one or more chromium carbides in the tubular body after austenitizing and quenching is in a range of 5 wt% to 52 wt%. The tubular body has a minimum tensile strength of 800 MPa at room temperature and a minimum tensile strength of 300 MPa at 600 °C. The tubular housing has a prismatic cross-section.

[0012] Other features include a cylindrical cross-section of the tubular housing. The maximum hardness difference between the weld seam and the steel of the tubular body, which was not heat-affected during welding, is less than 50 HV. The steel comprises a nickel coating and an iron-nickel diffusion layer located between the nickel coating and the steel.

[0013] Further areas of applicability of the present disclosure will become apparent from the detailed description, claims, and drawings. The detailed description and specific examples are for illustrative purposes only and are not intended to limit the scope of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present disclosure will be better understood from the detailed description and the accompanying drawings, in which: Fig. 1 is a functional block diagram of an exemplary battery cell having a battery cell stack including anode electrodes, cathode electrodes, and separators arranged in a housing according to the present disclosure; Fig. 2A and Fig. 2B are perspective views of exemplary prismatic battery cell housings according to the present disclosure; and Fig. 3 is a side cross-sectional view of an exemplary cylindrical battery cell according to the present disclosure; Fig. 4A is a side view of an exemplary tube being induction welded in accordance with the present disclosure; Fig. 4B is a side view of an exemplary welded pipe undergoing induction heating in accordance with the present disclosure; Fig. 5 is a flowchart of an exemplary method for manufacturing a tubular housing according to the present disclosure; Fig. 6A is a scanning electron microscope image of the exemplary tubular housing prior to curing; Fig. 6B and Fig. 6C Scanning electron microscope images of the example tubular housing after curing; Fig. 7A and Fig. 7B Simulations indicating the estimated volume fractions of the phases as a function of temperature for the exemplary package according to the present disclosure; Fig. 8A and Fig. 8B are side cross-sectional views showing nickel-plated steel before and after age hardening in accordance with the present disclosure; and Fig. 9A, Fig. 9B and Fig. 9C Improvements in weld quality after welding, after heating to austenitization, and after quench hardening, respectively, according to the present disclosure.

[0015] Reference symbols may be reused in the drawings to identify similar and / or identical elements. DETAILED DESCRIPTION

[0016] Although battery enclosures according to the present disclosure are shown in connection with electric vehicles, the battery enclosures may also be used in stationary applications and / or other applications.

[0017] Battery cells consist of a stack of anode electrodes, cathode electrodes, and separators arranged in a battery cell stack. The battery cell stack is arranged in a housing, which may be made of metal. For cylindrical and prismatic battery cells, the housings are typically made of metal such as aluminum or steel.

[0018] The melting point of steel is about 2.5 times higher than that of aluminum. Using steel to manufacture the enclosures helps maintain the integrity of the enclosure during thermal runaway, when the enclosure temperature exceeds the melting point of aluminum.

[0019] During thermal runaway, the sidewalls of steel casings can still fail. For example, in battery cells with thin steel walls (e.g., with a thickness in the range of 0.2 mm to 0.3 mm), the sidewalls can fail. Although thermal runaway temperatures are typically lower than the melting temperature of steel, if the temperature inside the battery cell rises above 800°C, the casing can fracture due to softening of the steel during thermal runaway. The battery casing is exposed to temperatures in the range of 500°C to 800°C during thermal runaway. At these high temperatures, mild steel softens, which can lead to sidewall fracture (e.g., due to high gas pressure). Sidewall failure can be mitigated by increasing the casing wall thickness. However, as the casing thickness increases, the gravimetric energy density (Wh / kg) of the battery cells decreases.

[0020] The present disclosure relates to a housing for cylindrical and prismatic battery cells made of a steel alloy that gradually softens with increasing temperature than mild steel. Lower softening (e.g., allowing for greater strength at higher temperatures) helps prevent sidewall failure.

[0021] In some examples, a cold-rolled and annealed steel sheet with a nickel coating is used. In some examples, the steel sheet contains carbon in a range of 0.02 to 0.3 wt.%, manganese in a range of 0.2 to 2.0 wt.%, chromium and molybdenum in a range of 0.5 wt.% to 3 wt.%, silicon in a range of 0.2 wt.% to 2.0 wt.%, niobium, titanium, and / or vanadium in a range of 0.01 wt.% to 0.2 wt.%, and iron and other materials making up the remainder.

[0022] In some examples, the housing has a martensitic microstructure for strength at room temperature and one or more chromium carbides (e.g., fine and / or undissolved) for properties at elevated temperatures. In some examples, a carbide content ranges from 1.0 vol% to 20 vol%. In some examples, the one or more chromium carbides have a size range from 50 nm to 500 nm. In some examples, the chromium content of the one or more chromium carbides ranges from 5 wt% to 52 wt%. In some examples, a minimum tensile strength of the housing is 800 MPa at room temperature and 300 MPa at 600°C.

[0023] In some examples, a method for manufacturing a housing comprises roll forming cold-rolled and annealed steel sheet (having the composition described herein) into a tube having a rectangular or cylindrical tubular shape. The opposing sides of the tubular shape are welded to form an open, tubular housing (e.g., by high-frequency welding or laser welding).

[0024] The tubular casing is rapidly austenitized by induction heating in a temperature range of 900 °C to 950 °C. After heating, the tubular casing is quenched (with air, water, or a cooled nozzle) to room temperature to form a martensite microstructure with a fine dispersion of one or more chromium carbides for strength. After induction heating and quenching, the bottom section of the casing is welded to one end of the tubular casing.

[0025] The battery casings described here are less expensive than the aluminum casings currently used in prismatic cells. The steel casings are lighter due to their relatively thinner steel walls. The casings also contribute to greater safety due to their increased temperature resistance. The increased strength suppresses sidewall fractures, allowing more time for pressure to dissipate during a thermal event.

[0026] With reference now to Fig. 1, a battery cell 10 comprises C cathode electrodes 20, A anode electrodes 40, and S separators 32 arranged in a predetermined order in a battery cell stack 12, where C, A, and S are integers greater than zero. The C cathode electrodes 20-1, 20-2, ..., and 20-C comprise cathode active material layers 24 arranged on one or both sides of a cathode current collector 26. The A anode electrodes 40-1, 40-2, ..., and 40-A comprise anode active material layers 42 arranged on one or both sides of the anode current collectors 46.

[0027] In some examples, the A anode electrodes 40 and the C cathode electrodes 20 exchange lithium ions during charging / discharging.

[0028] In some examples, the cathode active material layers 24 and / or the anode active material layers 42 comprise coatings comprising one or more active materials, one or more conductive additives, and / or one or more binders applied to the current collectors (e.g., via a roll-to-roll wet or dry process).

[0029] In some examples, the cathode current collector 26 and / or the anode current collector 46 comprises a metal foil, a metal mesh, perforated metal, three-dimensional (3D) metal foam, and / or expanded metal. In some examples, the current collectors are made from one or more materials selected from the group consisting of copper, stainless steel, brass, bronze, zinc, aluminum, and / or their alloys. The outer tabs 28 and 48 are connected to the current collectors of the cathode and anode electrodes, respectively, and can be arranged on the same or different sides of the battery cell stack 12. The outer tabs 28 and 48 are connected to terminals of the battery cells.

[0030] With reference now to Fig. 2A and Fig. 2B, a battery cell 58 includes a housing 60. In some examples, the housing 60 has a prismatic shape with rectangular cross-sections in the x-, y-, and z-axis planes. In some examples, the housing 60 includes a housing body 61 having sides 80 corresponding to narrow faces and sides 82 corresponding to wide faces. The housing body 61 forms an open, rectangular prism. In some examples, the housing 60 includes a lid portion 84 and a bottom portion 86. In other examples, the bottom portion 86 is attached after the housing 60 is formed. Edges 83 are disposed between the sides 80 and 82, the sides 80 and 82 and a lid portion 84, the sides 80 and 82, and the bottom portion 86.

[0031] The lid portion 84 and optionally the bottom portion 86 are attached to the housing body 61 to close the upper and lower openings of the housing body 61, respectively. The battery cell 58 includes outer terminals 62 and 64 that extend through the lid portion 84. The battery cell stack 12, comprising the C cathode electrodes 20, the A anode electrodes 40, and the S separators 32, is arranged in the housing 60.

[0032] The outer poles 62 and 64 are connected to the outer tabs 28 and 48 of the C cathode electrodes 20 and A anode electrodes 40, respectively. Fig. 2A, the cover portion 84 does not include a pressure-dependent vent cap. In Fig. 2B, the lid portion 84 (and / or the bottom portion 86) includes a pressure-dependent vent cap 66. The pressure-dependent vent cap 66 is configured to release vent gases when the pressure in the inner housing is greater than a predetermined pressure.

[0033] With reference now to Fig. 3, a cylindrical battery cell 110 comprises a tubular housing 114, a lid portion 118 with a positive terminal 120, and a bottom portion 122. A battery cell stack 126 (e.g., a jellyroll) is arranged in the tubular housing 114. Outer tabs 128 and 132 connect the cathode and anode electrodes to the positive terminal 120 and the negative terminal (e.g., at the bottom portion 122).

[0034] With reference now to Fig. 4A and Fig. 4B, the steel sheet is rolled or formed into a tube 210 (e.g., with an open cylindrical or prismatic shape), a seam is welded, and the tube 210 is hardened. In Fig. 4A, during induction welding of pipe 210, current flows through an induction coil 224 wound around pipe 210. The current creates a time-varying magnetic field that heats pipe 210. Welding rollers 226 press opposing sides 212 of pipe 210 together to form a weld 230 enclosing opposite sides of pipe 210.

[0035] In Fig. 4B, the welded tube 260 is hardened after seam welding. For example, the welded tube 260 is inductively heated by an induction coil 270 wound around the welded tube 260. The welded tube 260 is heated to a predetermined temperature in a range of 900°C to 950°C. After heating, the welded tube 260 is quenched (e.g., with air, water, or a cooled nozzle) to room temperature.

[0036] In some examples, roll forming is performed using cold-rolled and annealed steel with high formability. In some examples, the cold-rolled and annealed steel sheet includes an outer coating, e.g., pure nickel.

[0037] In some examples, the cold rolled and annealed steel sheet has a lean alloy composition including carbon in a range of 0.02 wt% to 0.3 wt%, manganese in a range of 0.2 wt% to 2.0 wt%, chromium and / or molybdenum in a range of 0.5 wt% to 3.0 wt%, silicon in a range of 0.2 wt% to 2.0 wt%, niobium, titanium and / or vanadium in a range of 0.01 wt% to 0.2 wt%, and iron (and optionally other materials making up the remainder).

[0038] In some examples, the housing has a martensite structure for strength at room temperature and one or more chromium carbides for properties at elevated temperatures. In some examples, the carbide content is between 1.0 vol% and 20 vol%. In some examples, particles of the chromium carbides have a size range of 50 nm to 500 nm. In some examples, the chromium content of the carbides is between 5 wt% and 52 wt%.

[0039] In some examples, a weld of the welded pipe has a hardness difference of less than 50 HV (Vickers Pyramid Number) between the weld and the bulk steel. In some examples, the strength of the weld is as high as the strength of the bulk steel. In some examples, the casing has high strength at room temperature. In some examples, a minimum tensile strength of the casing is 800 MPa at room temperature and 300 MPa at 600°C. In some examples, the casing has a strength in a range of 800 MPa to 2 GPa at room temperature. In some examples, the casing has a strength in a range of 1200 MPa to 2 GPa at room temperature. In some examples, the casing has a strength in a range of 1500 MPa to 2 GPa at room temperature.

[0040] In some examples, the housing can function at elevated temperatures without significant loss of strength. In some examples, the steel contains Cr / Mo-rich alloy carbides (e.g., ranging in size from 50 nm to 500 nm) for high-temperature reinforcement. Cr and / or Mo are added for hardenability, allowing the use of air (with a slower cooling rate) as the quench medium instead of water to minimize distortion. In some examples, the lean alloy composition maintains a total thermal conductivity of > 30 W / mK.

[0041] With reference now to Fig. Figure 5 illustrates a method for manufacturing a battery housing. At 310, cold-rolled and annealed steel sheet is rolled and / or formed into a cylindrical or prismatic tube. At 314, edges of the tube are welded to form a weld seam of a tubular housing.

[0042] At 318, the tubular housing is hardened. In some examples, the tubular housing is austenitized by heating the tubular housing using inductive heating and holding for a predetermined holding time. In some examples, the predetermined holding time is in a range of 1 s to 60 s. In some examples, the predetermined holding time is in a range of 4 s to 20 s. At 320, after rapid heating, the tubular housing is quenched to form a martensite microstructure with finely dispersed chromium carbides. At 322, a bottom portion of the housing is mechanically crimped, brazed, or welded (e.g., by radio frequency or laser welding) to one end of the tubular housing. The battery cell stack is then arranged within the housing, the terminals are connected, and the lid portion is attached.

[0043] With reference now to Fig. 6A to 6C show scanning electron microscope images of the tubular housing during production. Fig. 6A shows the tubular casing before curing (~ 600 MPa final strength). Fig. 6B shows the tubular casing after curing (~ 1700 MPa final strength). Fig. 6C, the steel contains Cr-rich M7C3 carbides to improve high-temperature strength.

[0044] With reference now to Fig. 7A and Fig. 7B show simulations (e.g. using Thermo-Calc ® ) the estimated volume fractions of the phases as a function of temperature for the casing. The simulations show that alloy carbides No. 1 and No. 2 remain in the matrix up to elevated temperatures of 800 °C.

[0045] With reference now to Fig. 8A and Fig. 8B, the steel 410 may have a coating 414 (e.g., nickel) on its outer surface to improve corrosion resistance to the electrolyte during use. Fig. 8A, pinholes may occur in the coating 414. Without the inductive heating and hardening steps described herein, the electrolyte comes into contact with the steel 410 and causes corrosion. After the inductive heating and hardening steps, the coating 414 diffuses into the steel 410 (see Fig. 8B) to form an alloy diffusion layer 416 (e.g., an iron-nickel alloy) near an interface therebetween. The iron-nickel diffusion layer provides increased corrosion resistance at the coating pinholes 420. Therefore, hardening the steel 410 improves the coating coherence and corrosion resistance (e.g., particularly at the coating pinholes 420).

[0046] With reference now to Fig. Figures 9A to 9C show the improvements in weld quality after welding the tubular casing and heating to austenitization or after quench hardening. After welding (see Fig. 9A), the material in the center of the weld is fusion hardened. The areas adjacent to the weld are heat-affected (e.g., softened by heat). Outer areas, including the bulk steel, show little or no change in hardness. In Fig. 9B, the tubular housing is heated to austenitization. After quench hardening (see Fig. 9C) is the hardness of the tubular housing compared to Fig. 9A more uniform. Austenitizing before quench hardening improves the microstructure of the weld, including the weld zone and the heat-affected zone, resulting in greater toughness and hardness.

[0047] The foregoing description is merely illustrative and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure may be embodied in a variety of forms. While this disclosure includes specific examples, its true scope should not be limited thereto, since other modifications will become apparent upon review of the drawings, the specification, and the following claims. It is understood that one or more steps within a method may be performed in different orders (or simultaneously) without altering the principles of the present disclosure.Furthermore, although the embodiments are each described above as having specific features, any one or more of these features described with respect to one embodiment of the disclosure may be implemented and / or combined with features of any of the other embodiments, even if such combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and interchanging one or more embodiments for one another remains within the scope of this disclosure.

[0048] Spatial and functional relationships between elements (e.g., between modules, circuit elements, semiconductor layers, etc.) are described using various terms, including "connected," "engaging," "coupled," "adjacent," "beside," "on top of," "over," "below," and "disposed." If a relationship between first and second elements is not explicitly described as "direct" in the above disclosure, that relationship may be a direct relationship, with no other intervening elements between the first and second elements, or an indirect relationship, with one or more intervening elements (either spatial or functional) between the first and second elements.As used herein, the phrase “A, B, and / or C” should be construed as logical (A ORed with B ORed with C) using a non-exclusive logical OR, rather than as “at least one of A, at least one of B, and at least one of C.”

Claims

[1] A method for producing a tubular housing for a battery cell, comprising: Roll forming a steel sheet into a tubular body, the steel comprising: Carbon in a range of 0.02 to 0.3 wt.%, Manganese in a range of 0.2 to 2.0 wt.%, Chromium and / or molybdenum in a range of 0.5 wt% to 3.0 wt%, Silicon in a range of 0.2 wt% to 2.0 wt%, niobium, titanium and / or vanadium in a range of 0.01 wt% to 0.2 wt% and Iron; Welding the sides of the tubular body to form a weld seam; Heating the tubular body to a temperature in a range of 900 °C to 950 °C; and Quenching the tubular body. [2] The method of claim 1, further comprising attaching a bottom portion to one end of the tubular body. [3] The method of claim 1, wherein the tubular body has a martensite structure after quenching. [4] The method of claim 1, wherein the tubular body comprises one or more chromium carbides having a size in a range of 50 nm to 500 nm. [5] The method of claim 4, wherein the proportion of the one or more chromium carbides in the tubular body after quenching is in a range of 1.0 vol% to 20 vol%. [6] The method of claim 4, wherein a weight of the one or more chromium carbides in the tubular body after quenching is in a range of 5 wt% to 52 wt%. [7] The method of claim 1, wherein the tubular body has a minimum tensile strength of 800 MPa at room temperature and a minimum tensile strength of 300 MPa at 600 °C. [8] The method of claim 1, wherein the tubular housing has a cylindrical or prismatic cross-section. [9] A method according to claim 1, wherein a maximum hardness difference between a weld seam and the steel of the tubular body which was not heat affected during welding is less than 50 HV. [10] The method of claim 1, wherein the steel comprises a nickel coating and an iron-nickel alloy layer disposed between the nickel coating and the steel after quenching.

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