A gradient heat management structure integrated manufacturing method of a square metal shell lithium ion battery

Abstract

A kind of square metal shell lithium ion battery's gradient heat management structure integrated manufacturing method, method includes: obtaining cell temperature gradient distribution data;Establish thermal conductivity gradient distribution model;Prepare gradient heat conduction composite powder;Establish the corresponding relationship between cold spraying process parameters and thermal conductivity;Provide core mold and end baffle;Layered gradient deposition forming shell;Demoulding and machining obtain shell;Assemble battery.The present application integrates gradient heat management structure in the shell, the thermal conductivity is continuously gradient change along the thickness direction, matches with the temperature gradient distribution in the cell, eliminates the interface thermal resistance of split assembly, realizes the synchronous manufacturing of shell and heat management structure.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery thermal management technology, and in particular to a method for integrating and manufacturing a gradient thermal management structure for a square metal-cased lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and low self-discharge rate, have become the core power source for new energy vehicles, energy storage systems, and portable electronic devices. Among the various battery packaging forms, prismatic metal-cased lithium-ion batteries have been widely used in the fields of power batteries and energy storage batteries due to their high space utilization, good structural strength, and high packing efficiency.

[0003] During the charging and discharging process, prismatic metal-cased lithium-ion batteries generate a significant amount of heat through electrochemical reactions within the cell. Studies have shown that the temperature distribution within a prismatic cell exhibits a significant non-uniformity: the temperature is highest in the central region and gradually decreases radially outwards, forming a steep temperature gradient along the thickness direction. This temperature gradient leads to the following problems: overheating due to heat accumulation in the central region accelerates the degradation of the cathode material structure, electrolyte decomposition, and thickening of the solid electrolyte interface film, significantly shortening the battery cycle life; non-uniform temperature also causes uneven current density distribution within the cell, increasing local overpotential, accelerating lithium dendrite growth, and increasing the risk of internal short circuits and thermal runaway. Furthermore, the traditional prismatic metal casing only serves as a mechanical protective container and sealing barrier for the cell, with limited and uniform thermal conductivity. It cannot actively regulate the heat flow distribution along different directions within the cell, resulting in uniform heat conduction along the casing and an inability to preferentially remove heat from the central region along the shortest path.

[0004] Existing thermal management solutions for square lithium-ion batteries mainly fall into two categories. The first category involves adding cooling devices to the outside of the battery casing, such as liquid cooling plates, air-cooled channels, or heat dissipation fins. Patent CN222233714U from Liyang Xingbo Light Materials Technology Co., Ltd. discloses a square battery casing including a first cell housing cavity and at least two heat exchange cavities, achieving multi-sided cooling through these cavities. Patent CN223181234U from Xiaogan Chuneng New Energy Innovation Technology Co., Ltd. discloses a battery casing with a hollow channel penetrating the casing in the middle of the outer casing's accommodating space, serving as a cooling channel for the heat exchange medium to pass through. While these solutions can remove heat generated by the battery to some extent, there is interfacial thermal resistance between the cooling device and the battery casing. Heat transfer from the cell to the cooling medium requires passing through multiple heat transfer stages, including the inside of the cell, the gap between the cell and the casing, the casing wall thickness, and the contact surface between the casing and the cooling device. This results in a long heat transfer path, high thermal resistance, and low heat dissipation efficiency. The second type of solution involves filling the battery casing with phase change material (PCM) or attaching it to the outside of the casing, utilizing the latent heat of the PCM to absorb the heat generated by the battery cell. Patent CN210403966U from Nanchang University discloses a battery pack thermal management system based on prismatic batteries, employing alternating bonding of prismatic batteries with composite PCM. The composite PCM is formed by mixing paraffin wax and expanded graphite in a molten state and then hot-pressing. Patent CN119275412A from Jiangxi Ganfeng Lithium Battery Technology Co., Ltd. discloses a prismatic aluminum-cased battery sandwich insulation structure, using heat insulation plates and heat-conducting plates to separate multiple prismatic aluminum-cased batteries for heat conduction and insulation. In this type of solution, the PCM exists as an independent component, and there is still interfacial thermal resistance between it and the battery casing. Furthermore, the PCM itself has a low thermal conductivity, limiting the rate at which heat is transferred from the battery cell to the interior of the PCM, making it difficult to meet the rapid heat dissipation requirements under high-rate charge and discharge conditions.

[0005] The two existing technical solutions mentioned above share the following drawbacks: the thermal management structure and the battery casing are manufactured separately and then assembled, and the thermal management function is achieved through "external superposition," making it impossible to organically integrate the thermal management function with the casing's load-bearing function; the thermal properties of the thermal management structure are spatially uniformly distributed, making it impossible to differentiate the design based on the actual temperature gradient distribution within the cell, resulting in uniform heat flow distribution along the casing and the inability to form a directional heat conduction channel from the central area of ​​the cell to the surface of the casing; the manufacturing process of the thermal management structure is independent of the battery casing manufacturing process, resulting in numerous manufacturing steps, long cycles, and high costs.

[0006] Therefore, there is an urgent need for a method that can integrate thermal management functions into the square metal casing itself, design the spatial distribution of thermal conductivity according to the temperature gradient distribution law inside the battery cell, and realize the synchronous integrated manufacturing of thermal management structure and casing. Summary of the Invention

[0007] To achieve the above objectives, this invention provides an integrated manufacturing method for a gradient thermal management structure of a square metal-cased lithium-ion battery, comprising the following steps: Step 1: Conduct thermal characteristic tests on the square metal-cased lithium-ion battery cell. Arrange at least 5 temperature measurement points along the thickness direction on the cell surface. Apply a 1C constant current discharge load to the cell and record the temperature change data of each temperature measurement point over time. Calculate the temperature gradient distribution function T(x) and temperature gradient value ▽T(x) in the thickness direction of the cell based on the collected steady-state temperature values. Step 2: Based on the temperature gradient distribution function T(x) and temperature gradient value ▽T(x) obtained in Step 1, establish a thermal conductivity gradient distribution model for the square metal shell. The thermal conductivity of each sidewall of the shell changes continuously along the thickness direction from the inner sidewall to the outer sidewall. The direction of the thermal conductivity gradient change is consistent with the direction of the internal temperature gradient of the battery cell measured in Step 1. The gradient change rate of the thermal conductivity of each sidewall of the shell along the thickness direction is positively correlated with the internal temperature gradient value ▽T(x) of the battery cell measured in Step 1. Step 3: Based on the thermal conductivity gradient distribution model established in Step 2, prepare a gradient thermally conductive composite powder material for cold spraying. The gradient thermally conductive composite powder material is made by mixing metal matrix powder and thermally conductive reinforcing phase powder in a gradient ratio with at least 5 gradient levels. Step 4: Establish the correspondence between cold spraying process parameters and the thermal conductivity of the deposited layer through process calibration experiments. Select the first cold spraying process parameters and the second cold spraying process parameters that satisfy the target thermal conductivity value of the inner wall and the target thermal conductivity value of the outer wall in the thermal conductivity gradient distribution model in Step 2 from the process parameter-thermal conductivity mapping database. Step 5: Provide a square metal shell core mold. All four sidewall surfaces of the core mold are coated with a release agent coating. Fix four end baffles to the edge of the open end face of the core mold. Step 6: Install the core mold and end baffle assembled in Step 5 onto the workpiece rotating table of the cold spraying equipment. Spray the gradient thermally conductive composite powder material prepared in Step 3 using a layered gradient deposition method. Divide the thickness of the shell sidewall into at least 5 deposition layers, depositing them sequentially from the 1st layer to the Nth layer. The 1st layer corresponds to the inner sidewall, and the Nth layer corresponds to the outer sidewall. The gradient level of the gradient thermally conductive composite powder material in the i-th deposition layer is determined according to the position of the layer in the thickness direction. The cold spraying gas pressure and cold spraying gas temperature of the i-th deposition layer linearly transition from the first cold spraying process parameter to the second cold spraying process parameter. After all N layers are deposited, a gradient thermally conductive composite deposition layer with a total thickness of t is formed on the surface of the core mold. The gradient thermally conductive composite deposition layer is the square metal shell. Step 7: Cool the core mold assembly after deposition in Step 6, remove the end baffle, demold using the difference in thermal expansion coefficient between the core mold and the gradient thermally conductive composite deposition layer, remove the gradient thermally conductive composite deposition layer, and machine the open end face of the removed gradient thermally conductive composite deposition layer to obtain a square metal shell with a gradient thermal management structure. Step 8: Place the square battery cell into the square metal casing with gradient thermal management structure obtained in Step 7. Coat the outer surface of the square battery cell with thermal interface material between the outer surface of the square battery cell and the inner wall of the square metal casing with gradient thermal management structure. Weld the cover plate assembly to the open end face of the square metal casing with gradient thermal management structure. After welding, perform an airtightness test. Inject electrolyte through the injection hole on the cover plate and seal the injection hole. Perform aging, formation and capacity testing on the assembled battery to obtain the finished lithium-ion battery with square metal casing and gradient thermal management structure.

[0008] Preferably, the process of testing the thermal characteristics of the square metal-cased lithium-ion battery cell in step 1 is as follows: The square metal-cased lithium-ion battery cell to be tested is placed in a constant temperature environment set at 25℃±2℃. At least five temperature measurement points are arranged along the thickness direction on the cell surface. The locations of these temperature measurement points are: the center point of the cell thickness, 1 / 4 of the cell thickness, 1 / 8 of the cell thickness, the inner surface of the cell near the casing, and the outer surface of the cell. K-type thermocouples are used for temperature measurement, with a measurement accuracy of ±0.5℃ and a sampling frequency of 1 Hz. A 1C constant current discharge load is applied to the cell, and the temperature readings are continuously recorded. The temperature change data of each measurement point over time is collected until the cell voltage drops to the discharge cutoff voltage. The steady-state temperature values ​​of each measurement point at the end of discharge are extracted from the recorded temperature data. The process of calculating the temperature gradient distribution function along the cell thickness direction based on the collected steady-state temperature values ​​is as follows: Let the cell thickness direction be the x-direction, the thickness center point be the origin x=0, and the cell surface position be x=±L / 2, where L is the cell thickness. Substitute the position coordinates xi of each temperature measurement point and the corresponding steady-state temperature value Ti into the one-dimensional steady-state heat conduction equation, and use the least squares method to fit the temperature gradient distribution function T(x). The calculation formula for the least squares fitting is:

[0009] Where n is the number of temperature measurement points and n≥5, Ti is the measured steady-state temperature value at the i-th temperature measurement point, and T(xi) is the fitted value of the temperature gradient distribution function at the i-th temperature measurement point; the temperature gradient value ▽T(x) is calculated based on the fitted temperature gradient distribution function T(x):

[0010] Preferably, the mathematical expression for the thermal conductivity gradient distribution model of the square metal shell established in step 2 is:

[0011] Where k(y) is the thermal conductivity at position y in the thickness direction of the shell sidewall, kin is the thermal conductivity at the inner sidewall of the shell sidewall, kout is the thermal conductivity at the outer sidewall of the shell sidewall, y is the thickness distance measured from the inner sidewall and 0≤y≤t, t is the thickness of the shell sidewall, α is the gradient exponent and its value ranges from 0.5≤α≤3.0; the value of the gradient exponent α is determined based on the maximum value of the internal temperature gradient value ▽T(x) of the cell measured in step 1.

[0012] Where |▽T|max is the maximum absolute value of the temperature gradient measured in step 1, |▽T|min is the minimum absolute value of the temperature gradient measured in step 1, and |▽T|ref is the reference temperature gradient value with a value of 5℃ / mm.

[0013] Preferably, the process of preparing gradient thermally conductive composite powder material in step 3 is as follows: the metal matrix powder is pure aluminum powder, the particle size of the pure aluminum powder is 15 micrometers to 45 micrometers, and the purity of the pure aluminum powder is not less than 99.5%; the thermally conductive reinforcing phase powder is high thermal conductivity graphite powder, the particle size of the high thermal conductivity graphite powder is 5 micrometers to 20 micrometers, and the thermal conductivity of the high thermal conductivity graphite powder is not less than 800 W / m Kelvin; the method for determining the mass fraction of thermally conductive reinforcing phase powder corresponding to each gradient level is as follows: based on the target thermal conductivity value of the thermal conductivity gradient distribution model k(y) in step 2, the corresponding mass fraction of thermally conductive reinforcing phase powder is obtained by reverse lookup through the pre-established thermal conductivity-mass fraction calibration curve; the thermal conductivity-mass fraction calibration curve is obtained through the following calibration experiment: the prepared thermally conductive reinforcing phase powder mass fractions are 0 Seven sets of standard samples with concentrations of 5%, 10%, 15%, 20%, 25%, and 30% were formed into standard test blocks using a cold spray coating process under the same process parameters. The thermal conductivity of each standard test block was measured using the laser flash method. A calibration curve was plotted with the mass fraction of the thermally conductive reinforcing phase powder as the abscissa and the thermal conductivity as the ordinate. A continuous thermal conductivity-mass fraction calibration curve was obtained using cubic spline interpolation. The metal matrix powder and the thermally conductive reinforcing phase powder were respectively put into a ball milling mixing device according to the mass fraction corresponding to each gradient level for mixing. The ball milling mixing process parameters were: ball-to-material ratio 5:1, rotation speed 200 rpm, mixing time 30 minutes, and argon protective atmosphere. After mixing, the gradient thermally conductive composite powder materials of each gradient level were sealed and packaged and labeled with the corresponding gradient level.

[0014] Preferably, the process of establishing the correspondence between the cold spraying process parameters and the thermal conductivity of the deposited layer in step 4 is as follows: Select the grade with the middle mass fraction of the thermally conductive reinforcing phase powder in the gradient thermally conductive composite powder material prepared in step 3 as the calibration powder material. Set the range of cold spraying process parameters as follows: gas pressure 1.5 MPa to 4.5 MPa, gas temperature 250°C to 550°C, spraying distance 15 mm to 35 mm, powder feeding rate 15 g / min to 45 g / min, and spray gun moving speed 200 mm / s to 600 mm / s. Within the above parameter range, design at least 16 different combinations of process parameters using orthogonal experimental design. Use each combination of process parameters to perform cold spraying deposition experiments on an aluminum substrate, with a deposition thickness of 2 mm. After deposition, use wire cutting to remove the deposited layer from the aluminum substrate. Standard test blocks were obtained by separation; the thermal conductivity of each set of standard test blocks was measured using the laser flash method, and the microscopic cross-sectional morphology of the deposited layer was observed and the porosity of the deposited layer was measured using a scanning electron microscope; a process parameter-thermal conductivity mapping database was established with cold spraying process parameters as independent variables and the thermal conductivity of the deposited layer as dependent variables; process parameter combinations that can meet the requirements of the inner wall target thermal conductivity value and the outer wall target thermal conductivity value in the thermal conductivity gradient distribution model in step 2 were selected from the process parameter-thermal conductivity mapping database. During the selection, parameter combinations with higher gas pressure and higher gas temperature were given priority. The parameters that meet the inner wall target thermal conductivity value among the selected process parameter combinations were recorded as the first cold spraying process parameters, and the parameters that meet the outer wall target thermal conductivity value were recorded as the second cold spraying process parameters.

[0015] Preferably, the external dimensions of the square metal shell core mold provided in step 5 are consistent with the internal cavity dimensions of the target square metal shell. The length of the square metal shell core mold is 0.05 mm to 0.10 mm smaller than the internal cavity length of the target square metal shell, the width of the square metal shell core mold is 0.05 mm to 0.10 mm smaller than the internal cavity width of the target square metal shell, and the thickness of the square metal shell core mold is 0.05 mm to 0.10 mm smaller than the internal cavity thickness of the target square metal shell. The square metal shell core mold is made of mold steel, and its surface roughness Ra is not higher than 0.8 micrometers. All four sidewall surfaces are coated with a release agent coating, which is a boron nitride spray coating with a thickness of 5 to 10 micrometers. The dimensions of the four end baffles correspond to the four edges of the opening end of the square metal shell. The material of the four end baffles is the same as that of the square metal shell core mold. The surfaces of the four end baffles are coated with a boron nitride release agent coating. The four end baffles are fixed to the edge of the opening end face of the square metal shell core mold with fasteners. The inner surface of the four end baffles is flush with the corresponding sidewall surface of the square metal shell core mold, and the outer surface of the four end baffles extends 1 to 2 millimeters beyond the sidewall surface of the square metal shell core mold.

[0016] Preferably, in step 6, the cold spraying equipment adopts a high-pressure cold spraying system, the working gas is nitrogen, and the rotation speed of the workpiece rotary table is set to 30 to 60 revolutions per minute; the specific method for dividing the shell sidewall thickness t into at least 5 deposition layers is as follows: assuming the total number of deposition layers is N and N≥5, the target thickness of the i-th deposition layer is t / N, where i=1,2,...,N; the method for determining the gradient level corresponding to the i-th deposition layer is as follows: calculate the normalized thickness position yi / t corresponding to the center position of the i-th deposition layer, substitute the normalized thickness position yi / t into the thermal conductivity gradient distribution model in step 2 to calculate the target thermal conductivity value of the i-th deposition layer, and then determine the gradient level of the gradient thermal conductivity composite powder material to be used in the i-th deposition layer according to the thermal conductivity-mass fraction calibration curve in step 3; the cold spraying gas pressure of the i-th deposition layer is determined according to the following formula:

[0017] Where Pi is the cold spray gas pressure of the i-th deposition layer, P1 is the gas pressure of the first cold spray process parameter, and P2 is the gas pressure of the second cold spray process parameter; the cold spray gas temperature of the i-th deposition layer is determined according to the following formula:

[0018] Where Tgas,i is the cold spray gas temperature of the i-th deposition layer, Tgas,1 is the gas temperature of the first cold spray process parameter, and Tgas,2 is the gas temperature of the second cold spray process parameter; the spraying distance and powder feeding rate of the i-th deposition layer remain constant throughout all deposition layers. The spraying distance and powder feeding rate are respectively taken from the spraying distance value with the lowest sensitivity of thermal conductivity to spraying distance changes and the powder feeding rate value with the lowest sensitivity of thermal conductivity to powder feeding rate changes in the process parameter-thermal conductivity mapping database of step 4; after each deposition layer is completed, the actual deposition thickness of the deposition layer is measured using a laser thickness gauge. When the deviation between the actual deposition thickness and the target thickness exceeds ±5%, the powder feeding rate of the next deposition layer is adjusted for thickness compensation. The adjustment rule for thickness compensation is: if the actual deposition thickness is less than the target thickness, the powder feeding rate of the next deposition layer is increased by 10%; if the actual deposition thickness is greater than the target thickness, the powder feeding rate of the next deposition layer is decreased by 10%.

[0019] Preferably, the demolding process in step 7 is as follows: After the cold spray deposition is completed, the square metal shell core mold assembly with the gradient thermally conductive composite deposition layer is placed in a room temperature environment and naturally cooled to 25℃±5℃. After cooling, the fasteners of the four end baffles are removed and the four end baffles are removed. The square metal shell core mold assembly with the gradient thermally conductive composite deposition layer is heated to 120℃ to 150℃ and kept at that temperature for 15 minutes to 20 minutes. The difference in thermal expansion coefficient between the aluminum gradient thermally conductive composite deposition layer and the steel square metal shell core mold creates a gap between the gradient thermally conductive composite deposition layer and the surface of the square metal shell core mold. The square metal shell core mold is then removed from the gradient thermally conductive composite deposition layer. The machining method for the open end face of the removed gradient thermally conductive composite deposition layer is CNC milling. The machining allowance is 0.2 mm to 0.5 mm. The machining removes the irregular edge parts formed during the cold spray deposition process to make the open end face flat.

[0020] Preferably, in step 8, before placing the square battery cell into the square metal casing with the gradient thermal management structure, a thermally conductive interface material is coated between the outer surface of the square battery cell and the inner wall of the square metal casing with the gradient thermal management structure. The thermally conductive interface material is thermally conductive silicone grease, with a thermal conductivity of not less than 3 watts per meter Kelvin, and a coating thickness of 0.1 mm to 0.2 mm. The welding process used to weld the cover plate assembly to the open end face of the square metal casing with the gradient thermal management structure is laser welding. The laser power of the laser welding is 1.5 kW to 2.5 kW, the laser welding speed is 30 mm / s to 50 mm / s, and the shielding gas for laser welding is argon. The helium leakage rate standard for airtightness testing is not higher than 1 × 10⁻⁶. -7Pa·m / s; the amount of electrolyte injected through the injection hole on the cover plate is 105% to 110% of the theoretical electrolyte absorption capacity of the cell; after sealing and welding the injection hole, the assembled battery is left to stand and age for 24 to 48 hours at 45℃±2℃.

[0021] Preferably, in step 2, the square metal shell includes four sidewalls, consisting of two large-area sidewalls and two small-area sidewalls. The thermal conductivity gradient distribution model is applied to each of the four sidewalls simultaneously. In step 6, when spraying the gradient thermal conductivity composite powder material prepared in step 3 using a layered gradient deposition method, the spraying path of the spray gun is executed in the following order: first, the first to Nth layers are deposited on one large-area sidewall of the square metal shell core mold; then, the first to Nth layers are deposited on the other large-area sidewall of the square metal shell core mold; then, the first to Nth layers are deposited alternately on the two small-area sidewalls of the square metal shell core mold. When spraying any sidewall of the square metal shell core mold, the travel distance of the spray gun in the length direction of the sidewall exceeds the edges of both ends of the sidewall by 5 mm to 10 mm, and the travel step of the spray gun in the width direction of the sidewall is 1 / 2 to 2 / 3 of the spray gun outlet diameter, so as to ensure the uniformity of the thickness of the deposited layer in the edge region.

[0022] The beneficial effects of this invention are: 1. This invention integrates thermal management functions into the square metal shell itself. The shell simultaneously has three functions: mechanical load-bearing, sealing protection, and gradient heat conduction. The thermal management structure and the shell are manufactured synchronously in the same cold spray deposition process, eliminating the interfacial thermal resistance caused by the separate manufacturing and assembly of the thermal management structure and the shell in the traditional solution. The heat transfer path is greatly shortened and the number of manufacturing steps is significantly reduced.

[0023] 2. This invention establishes a thermal conductivity gradient distribution model based on measured temperature gradient distribution data inside the battery cell, so that the thermal conductivity of each sidewall of the casing changes continuously along the thickness direction, and the gradient direction is consistent with the temperature gradient direction inside the battery cell, forming a directional heat conduction channel from the central region of the battery cell to the surface of the casing, effectively alleviating the problem of uneven temperature inside the battery cell.

[0024] 3. This invention utilizes a layered gradient deposition method to synergistically apply the gradient variation of powder material ratio with the gradient control of cold spraying process parameters, establishes a thermal conductivity-mass fraction calibration curve and a process parameter-thermal conductivity mapping database, precisely controls the thermal conductivity of each deposition layer, and achieves a continuous and adjustable gradient distribution of the shell's thermal conductivity along the thickness direction. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in this invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without creative effort.

[0026] Figure 1 This is a flowchart of the steps of the method of the present invention. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. It should also be noted that, to make the embodiments more comprehensive, the following embodiments are the best and preferred embodiments, and those skilled in the art can use other alternative methods to implement some well-known technologies; moreover, the accompanying drawings are only for more specific description of the embodiments and are not intended to specifically limit the present invention.

[0028] Please see Figure 1 This invention provides a method for integrating a gradient thermal management structure into a square metal-cased lithium-ion battery. This method integrates thermal management functionality into the square metal casing itself. Based on the temperature gradient distribution within the battery cell, the thermal conductivity is designed to spatially distribute along the casing thickness. A square metal casing with gradient thermal conductivity is then formed in a single step on a core mold using a cold-spray additive manufacturing process. This achieves the organic integration and simultaneous manufacturing of the thermal management structure and the casing's load-bearing function. The following detailed description of the method follows.

[0029] Step 1: Obtain the internal temperature gradient distribution data of the square metal-cased lithium-ion battery cell.

[0030] First, thermal characteristic tests were conducted on the square metal-cased lithium-ion battery cell to obtain internal temperature gradient distribution data under actual operating conditions. This data provides input for subsequent design of the casing's thermal conductivity gradient. The specific procedure was as follows: The square metal-cased lithium-ion battery cell to be tested was placed in a constant temperature environment, set at 25℃±2℃. Five temperature measurement points were arranged along the thickness direction on the cell surface: the center point of the cell thickness, one-quarter of the cell thickness, one-eighth of the cell thickness, the inner surface of the cell near the casing, and the outer surface of the cell. K-type thermocouples were used for temperature measurement, with a measurement accuracy of ±0.5℃, and a sampling frequency of 1 Hz. A 1C constant current discharge load was applied to the cell, and the temperature change data of each measurement point over time was continuously recorded until the cell voltage dropped to the discharge cutoff voltage. The steady-state temperature values ​​corresponding to each measurement point at the end of the discharge were extracted from the recorded temperature data.

[0031] The temperature gradient distribution function along the cell thickness is calculated based on the collected steady-state temperature values. Let the cell thickness direction be the x-direction, the thickness center point be the origin x=0, and the cell surface position be x=±L / 2, where L is the cell thickness. Substitute the position coordinates xi of each temperature measurement point and the corresponding steady-state temperature value Ti into the one-dimensional steady-state heat conduction equation, and use the least squares method to fit the temperature gradient distribution function T(x). The formula for the least squares fitting is:

[0032] Where n is the number of temperature measurement points and n≥5, Ti is the measured steady-state temperature value at the i-th temperature measurement point in degrees Celsius, and T(xi) is the fitted value of the temperature gradient distribution function at the i-th temperature measurement point in degrees Celsius.

[0033] Calculate the temperature gradient value ▽T(x) based on the fitted temperature gradient distribution function T(x):

[0034] Step 2: Establish a gradient distribution model for thermal conductivity.

[0035] Based on the temperature gradient distribution function T(x) and temperature gradient value ▽T(x) obtained in step 1, a thermal conductivity gradient distribution model for a square metal shell is established. The square metal shell contains four sidewalls, namely two large-area sidewalls and two small-area sidewalls. The thermal conductivity gradient distribution model is applied to each of the four sidewalls simultaneously. The core design principle of the thermal conductivity gradient distribution model is as follows: the thermal conductivity of each sidewall of the shell changes continuously along the thickness direction from the inner sidewall to the outer sidewall, and the direction of the thermal conductivity gradient change is consistent with the direction of the internal temperature gradient of the battery cell measured in step 1; the thermal conductivity value at the inner sidewall of each sidewall of the shell is determined according to the heat flux density requirement of the inner surface of the battery cell near the shell, and the thermal conductivity value at the outer sidewall is determined according to the heat exchange requirement between the shell and the environment; the gradient change rate of the thermal conductivity of each sidewall of the shell along the thickness direction is positively correlated with the internal temperature gradient value ▽T(x) of the battery cell measured in step 1.

[0036] The mathematical expression for the thermal conductivity gradient distribution model is:

[0037] Where k(y) is the thermal conductivity at position y in the thickness direction of the shell sidewall, in watts per meter Kelvin; kin is the thermal conductivity at the inner sidewall of the shell sidewall, in watts per meter Kelvin; kout is the thermal conductivity at the outer sidewall of the shell sidewall, in watts per meter Kelvin; y is the thickness direction distance measured from the inner sidewall, 0≤y≤t, in millimeters; t is the thickness of the shell sidewall, in millimeters; α is the gradient exponent, with a value range of 0.5≤α≤3.0.

[0038] The value of the gradient exponent α is determined based on the maximum value of the internal temperature gradient ▽T(x) of the battery cell measured in step 1, and the calculation formula is as follows:

[0039] Wherein, |▽T|max is the maximum absolute value of the temperature gradient measured in step 1, in degrees Celsius per millimeter; |▽T|min is the minimum absolute value of the temperature gradient measured in step 1, in degrees Celsius per millimeter; and |▽T|ref is the reference temperature gradient value, which is 5℃ / mm.

[0040] This model establishes a quantitative mapping relationship between the actual temperature gradient distribution inside the battery cell and the spatial distribution of the thermal conductivity of the casing, enabling the design of the casing's thermal conductivity to accurately match the heat source distribution characteristics inside the battery cell, thereby forming a directional heat conduction channel from the central region of the battery cell to the surface of the casing.

[0041] Step 3: Prepare gradient thermally conductive composite powder material.

[0042] Based on the thermal conductivity gradient distribution model established in step 2, a gradient thermal conductivity composite powder material for cold spraying is prepared. The gradient thermal conductivity composite powder material is composed of a metal matrix powder and a thermally conductive reinforcing phase powder mixed in a gradient ratio, serving as the raw material basis for achieving a gradient distribution of thermal conductivity.

[0043] The metal matrix powder is made of pure aluminum powder with a particle size of 15 to 45 micrometers and a purity of not less than 99.5%. Pure aluminum powder has good plastic deformation ability, which facilitates the formation of a dense deposition layer during cold spraying, while providing the basic mechanical properties required for the shell. The thermally conductive reinforcing phase powder is made of high thermal conductivity graphite powder with a particle size of 5 to 20 micrometers and a thermal conductivity of not less than 800 W / m Kelvin. The introduction of high thermal conductivity graphite powder can effectively control the thermal conductivity of the deposition layer. Graphite particles form a thermally conductive network in the deposition layer, and the thermal conductivity of the deposition layer shows a regular change with the increase of graphite content.

[0044] A series of gradient thermally conductive composite powder materials with different proportions were prepared, with at least five gradient levels. The method for determining the mass fraction of the thermally conductive reinforcing phase powder corresponding to each gradient level was as follows: based on the target thermal conductivity value of the thermal conductivity gradient distribution model k(y) in step 2, the corresponding mass fraction of the thermally conductive reinforcing phase powder was obtained by reverse lookup from a pre-established thermal conductivity-mass fraction calibration curve. The thermal conductivity-mass fraction calibration curve was obtained through the following calibration experiment: seven groups of standard samples with thermally conductive reinforcing phase powder mass fractions of 0%, 5%, 10%, 15%, 20%, 25%, and 30% were prepared. They were formed into standard test blocks using a cold spraying process under the same process parameters. The thermal conductivity of each standard test block was measured using the laser flash method. A calibration curve was plotted with the mass fraction of the thermally conductive reinforcing phase powder as the abscissa and the thermal conductivity as the ordinate. A continuous thermal conductivity-mass fraction calibration curve was obtained using cubic spline interpolation.

[0045] Pure aluminum powder and high thermal conductivity graphite powder were separately added to a ball mill mixing device according to the mass fraction corresponding to each gradient grade. The ball milling process parameters were: ball-to-powder ratio 5:1, rotation speed 200 rpm, mixing time 30 minutes, and argon protective atmosphere. After mixing, the gradient thermal conductivity composite powder materials of each grade were sealed and labeled with the corresponding gradient grade for subsequent cold spray deposition.

[0046] Step 4: Establish the correspondence between cold spraying process parameters and the thermal conductivity of the deposited layer.

[0047] Before formally performing cold spraying molding of the square metal shell, a process calibration experiment was conducted to establish the correspondence between the cold spraying process parameters and the thermal conductivity of the deposited layer, providing data support for the subsequent control of process parameters in the layered gradient deposition process.

[0048] The gradient grade with the middle mass fraction of thermally conductive reinforcing phase powder in the gradient thermally conductive composite powder material prepared in step 3 was selected as the calibration powder material. The range of cold spraying process parameters was set as follows: gas pressure 1.5 MPa to 4.5 MPa, gas temperature 250℃ to 550℃, spraying distance 15 mm to 35 mm, powder feed rate 15 g / min to 45 g / min, and spray gun movement speed 200 mm / s to 600 mm / s. At least 16 different combinations of process parameters were designed using orthogonal experimental design within the above parameter range. Cold spraying deposition experiments were performed on an aluminum substrate using each combination of process parameters, with a deposition thickness of 2 mm. After deposition, the deposited layer was separated from the aluminum substrate using wire cutting to obtain standard test blocks. The thermal conductivity of each standard test block was measured using laser scintillation, and the microscopic cross-sectional morphology of the deposited layer was observed and the porosity of the deposited layer was measured using scanning electron microscopy. A process parameter-thermal conductivity mapping database was established with the cold spraying process parameters as independent variables and the thermal conductivity of the deposited layer as the dependent variable.

[0049] The process parameter combinations that meet the target thermal conductivity values ​​for both the inner and outer walls in the thermal conductivity gradient distribution model of step 2 are selected from the process parameter-thermal conductivity mapping database. Priority is given to parameter combinations with higher gas pressure and temperature to ensure the bonding strength between the deposited layer and the substrate. The parameters that meet the target thermal conductivity value for the inner wall are designated as the first cold spray process parameter, and the parameters that meet the target thermal conductivity value for the outer wall are designated as the second cold spray process parameter. The first cold spray process parameter corresponds to the higher thermal conductivity required for the inner wall of the shell, and the second cold spray parameter corresponds to the lower thermal conductivity required for the outer wall of the shell.

[0050] Step 5: Provide a square metal housing core mold and end baffles.

[0051] A set of square metal shell core molds is provided, the external dimensions of which are consistent with the internal cavity dimensions of the target square metal shell. The length of the core mold is 0.05 mm to 0.10 mm smaller than the internal cavity length of the target square metal shell, the width of the core mold is 0.05 mm to 0.10 mm smaller than the internal cavity width of the target square metal shell, and the thickness of the core mold is 0.05 mm to 0.10 mm smaller than the internal cavity thickness of the target square metal shell. This dimensional difference is used to allow for machining allowance for the cold spray deposition layer, ensuring that a shell with accurate internal cavity dimensions can be obtained after simple end face machining after deposition. The core mold is made of mold steel, and the surface roughness Ra of the core mold is not higher than 0.8 micrometers. All four sidewall surfaces of the core mold are coated with a release agent coating, which is a boron nitride spray coating with a coating thickness of 5 micrometers to 10 micrometers. The boron nitride coating has good high-temperature lubricity and release properties, ensuring smooth separation of the cold spray deposition layer from the core mold during subsequent demolding.

[0052] Four end baffles are provided, each corresponding to one of the four edges of the opening of the square metal shell. The material of the four end baffles is the same as that of the square metal shell core mold, and their surfaces are coated with a boron nitride release agent. The four end baffles are fixed to the edge of the opening face of the square metal shell core mold using fasteners. The inner surfaces of the four end baffles are flush with the corresponding sidewall surfaces of the square metal shell core mold, while the outer surfaces extend 1 mm to 2 mm beyond the sidewall surfaces of the square metal shell core mold. The function of the end baffles is to constrain the end boundaries of the deposited layer during cold spray deposition, preventing powder material from overflowing at the edge of the opening face and ensuring a neat opening face of the shell.

[0053] Step 6: Perform cold spray gradient deposition to form a square metal casing.

[0054] The assembled square metal shell core mold and end baffle from step 5 are installed on the workpiece rotary table of the cold spraying equipment. The cold spraying equipment uses a high-pressure cold spraying system, and the working gas is nitrogen. The rotation speed of the workpiece rotary table is set to 30 to 60 revolutions per minute to ensure the uniformity of the deposition layer thickness on all circumferential sidewalls.

[0055] The spraying strategy employs a layered gradient deposition method. The thickness t of the shell sidewall is divided into at least 5 deposition layers, with a total number of layers N ≥ 5. Deposition occurs sequentially from the innermost layer (y = 0) to the outermost layer (y = t). Layered deposition allows each layer to use gradient thermally conductive composite powder materials with different proportions and different cold spraying process parameters, thereby achieving a continuous gradient change in thermal conductivity along the thickness direction.

[0056] The method for determining the gradient level of the i-th deposition layer is as follows: Calculate the normalized thickness position yi / t corresponding to the center position of the i-th deposition layer. Substitute the normalized thickness position yi / t into the thermal conductivity gradient distribution model in step 2 to calculate the target thermal conductivity value of the i-th deposition layer. Then, determine the gradient level of the gradient thermal conductivity composite powder material to be used in the i-th deposition layer according to the thermal conductivity-mass fraction calibration curve in step 3. The first layer corresponds to the inner wall surface, using the powder material with the lowest mass fraction of the thermally conductive reinforcing phase powder to obtain the highest thermal conductivity; the N-th layer corresponds to the outer wall surface, using the powder material with the highest mass fraction of the thermally conductive reinforcing phase powder to obtain a lower thermal conductivity.

[0057] The cold spray gas pressure of the i-th deposition layer is determined according to the following formula:

[0058] Where Pi is the cold spray gas pressure of the i-th deposition layer, in megapascals; P1 is the gas pressure of the first cold spray process parameter, in megapascals; P2 is the gas pressure of the second cold spray process parameter, in megapascals; and N is the total number of deposition layers.

[0059] The temperature of the cold spray gas for the i-th deposition layer is determined according to the following formula:

[0060] Wherein, Tgas,i is the cold spray gas temperature of the i-th deposition layer, in degrees Celsius; Tgas,1 is the gas temperature of the first cold spray process parameter, in degrees Celsius; and Tgas,2 is the gas temperature of the second cold spray process parameter, in degrees Celsius.

[0061] The gas pressure and gas temperature transition linearly from the first cold spraying process parameters corresponding to the inner wall to the second cold spraying process parameters corresponding to the outer wall. This parameter gradient control, in conjunction with the gradient ratio of powder materials, can precisely control the thermal conductivity of each deposition layer and ensure good interlayer bonding.

[0062] The spraying distance and powder feeding rate of the i-th deposition layer remain constant throughout all deposition layers. The spraying distance is taken from the process parameter-thermal conductivity mapping database in step 4, which has the lowest sensitivity of thermal conductivity to changes in spraying distance. The powder feeding rate is also taken from the powder feeding rate, which has the lowest sensitivity of thermal conductivity to changes in powder feeding rate. This ensures that during changes in gas pressure and gas temperature gradients, the thermal conductivity is mainly controlled by the preset gradient control parameters, avoiding interference from fluctuations in other parameters on the thermal conductivity.

[0063] After each layer is deposited, a laser thickness gauge is used to measure the actual thickness of that layer. When the deviation between the actual thickness and the target thickness exceeds ±5%, the powder feed rate of the next layer is adjusted for thickness compensation. The adjustment rule for thickness compensation is as follows: if the actual thickness is less than the target thickness, the powder feed rate of the next layer is increased by 10%; if the actual thickness is greater than the target thickness, the powder feed rate of the next layer is decreased by 10%. Through layer-by-layer thickness monitoring and compensation, the final total thickness of the shell is ensured to meet the design tolerance requirements.

[0064] When spraying any sidewall of a square metal shell core mold, the spray gun's travel distance along the length of the sidewall extends 5 to 10 millimeters beyond each end edge. The spray gun's travel distance along the width of the sidewall is 1 / 2 to 2 / 3 of the spray gun's outlet diameter to ensure uniform thickness of the deposited layer in the edge region. The spraying path is executed in the following order: first, complete the deposition of layers 1 to N on one large-area sidewall of the square metal shell core mold; then, complete the deposition of layers 1 to N on another large-area sidewall of the square metal shell core mold; finally, alternately complete the deposition of layers 1 to N on the two small-area sidewalls of the square metal shell core mold.

[0065] After all N layers are deposited, a gradient thermally conductive composite deposition layer with a total thickness of t is formed on the surface of the square metal shell core mold. This deposition layer is the square metal shell.

[0066] Step 7: Remove the core mold and end baffles to obtain a square metal shell with a gradient thermal management structure.

[0067] After the cold spray coating is completed, the square metal shell core mold assembly with the gradient thermally conductive composite deposition layer is placed in a room temperature environment to cool naturally to 25℃±5℃. After cooling, the fasteners of the four end baffles are removed and the four end baffles are removed.

[0068] Demolding is achieved by utilizing the difference in thermal expansion coefficients between the aluminum gradient thermally conductive composite deposition layer and the steel square metal shell core mold. The thermal expansion coefficient of the aluminum gradient thermally conductive composite deposition layer is approximately 23 × 10⁻⁶. -6 The coefficient of thermal expansion of a steel square metal shell core mold is approximately 12 × 10⁻⁶ degrees Celsius. -6 At a temperature of 120°C to 150°C, the square metal shell core mold assembly with the gradient thermally conductive composite deposition layer is heated and held for 15 to 20 minutes. During heating, the thermal expansion of the aluminum gradient thermally conductive composite deposition layer is significantly greater than that of the steel square metal shell core mold, creating a gap between them. This allows the gradient thermally conductive composite deposition layer to be easily separated from the surface of the square metal shell core mold. After heating and holding, the square metal shell core mold is removed from the gradient thermally conductive composite deposition layer.

[0069] The open end face of the extracted gradient thermally conductive composite deposition layer is machined. CNC milling is used for machining, with a machining allowance of 0.2 mm to 0.5 mm. The purpose of machining is to remove irregular edges formed during the cold spray deposition process, making the open end face flat to meet the flatness requirements of the subsequent cover plate welding. After machining, a square metal shell with a gradient thermal management structure is obtained.

[0070] Step 8: Assemble the square metal-cased lithium-ion battery.

[0071] The square battery cell is placed into the square metal casing with a gradient thermal management structure obtained in step 7. Before placing the square battery cell, a thermally conductive interface material is applied between the outer surface of the square battery cell and the inner wall of the square metal casing with the gradient thermal management structure. The thermally conductive interface material is thermal grease with a thermal conductivity of not less than 3 W / m Kelvin, and the coating thickness is 0.1 mm to 0.2 mm. The function of the thermally conductive interface material is to fill the microscopic gaps between the square battery cell and the inner wall of the casing, reduce the interfacial contact thermal resistance, and enable the heat generated by the battery cell to be efficiently transferred to the inner wall of the casing.

[0072] The cover plate assembly was welded to the open end face of a square metal shell with a gradient thermal management structure. Laser welding was used, with a laser power of 1.5 kW to 2.5 kW and a welding speed of 30 mm / s to 50 mm / s. Argon was used as the shielding gas. After welding, an airtightness test was performed, with a helium leakage rate not exceeding 1 × 10⁻⁶. -7 Pa·m / s, ensuring the casing seal meets battery usage requirements.

[0073] Electrolyte is injected through the injection hole on the cover plate, with the injection volume being 105% to 110% of the theoretical electrolyte absorption capacity of the cell. After injection, the injection hole is sealed by welding. The assembled battery is then aged at 45℃±2℃ for 24 to 48 hours to ensure the electrolyte fully wets the electrode plates. After aging, formation and capacity testing are performed to finally obtain a square metal-cased lithium-ion battery product with a gradient thermal management structure.

[0074] Using the methods described in steps 1 to 8 above, this invention integrates a gradient thermal management structure into the interior of a square metal casing. The casing itself serves three functions: mechanical load-bearing, sealing protection, and thermal management. The thermal conductivity of the casing exhibits a continuous gradient along its thickness, and this gradient distribution matches the actual temperature gradient distribution within the battery cell. This forms a directional, efficient heat conduction channel from the center of the battery cell to the surface of the casing, effectively reducing the temperature gradient within the battery cell and improving its temperature uniformity. The cold-spray additive manufacturing process enables the simultaneous, integrated molding of the casing and the thermal management structure, eliminating the interfacial thermal resistance and complex assembly processes present in traditional separate manufacturing methods, significantly simplifying the manufacturing process.

[0075] Example 1; This embodiment provides a method for integrating and manufacturing a gradient thermal management structure for a square metal-cased lithium-ion battery. The specific process is as follows.

[0076] Step 1: Obtain the internal temperature gradient distribution data of the square metal-cased lithium-ion battery cell.

[0077] In this embodiment, the target cell for the square metal-cased lithium-ion battery to be manufactured is a square cell based on lithium iron phosphate, with a nominal capacity of 100 Ah, a cell length of 148 mm, a cell width of 52 mm, and a cell thickness of 27 mm.

[0078] First, thermal characteristics of the square metal-cased lithium-ion battery cell were tested. The cell was placed in a constant-temperature chamber at 25°C, with temperature fluctuations controlled within ±2°C. Five temperature measurement points were arranged along the thickness direction on the cell surface: at the center of the cell thickness, at a distance of 3.375 mm from the center, at a distance of 5.063 mm from the center, near the casing on the inner surface of the cell at a distance of 13.5 mm from the center, and on the outer surface of the cell. K-type thermocouples were used for temperature measurement, with the thermocouple tips fixed to the surface of each measurement point using high-temperature tape. The K-type thermocouples had a temperature measurement accuracy of ±0.5°C, and the data acquisition instrument sampling frequency was set to 1 Hz. A 1C constant-current discharge load was applied to the cell, with a discharge current of 100 amps. The temperature change data at each measurement point over time was continuously recorded until the cell voltage dropped to the discharge cutoff voltage of 2.5 volts. The steady-state temperature values ​​corresponding to each temperature measurement point at the end of the discharge were extracted from the recorded temperature data. The measured data are shown in Table 1.

[0079] Table 1. Measurement locations and steady-state temperatures along the cell thickness direction.

[0080] The temperature gradient distribution function along the cell thickness direction is calculated based on the collected steady-state temperature values. Let the cell thickness direction be the x-direction, the thickness center point be the origin x=0, and the cell surface position be x=±L / 2, where L is the cell thickness of 27 mm. Substitute the position coordinates xi and the corresponding steady-state temperature value Ti of each temperature measurement point in Table 1 into the one-dimensional steady-state heat conduction equation, and use the least squares method to fit the temperature gradient distribution function T(x). The formula for the least squares fitting is:

[0081] Where n is the number of temperature measurement points, and in this embodiment n=5; Ti is the measured steady-state temperature value at the i-th temperature measurement point, in °C; T(xi) is the fitted value of the temperature gradient distribution function at the i-th temperature measurement point, in °C.

[0082] The fitting uses a quadratic polynomial form T(x) = a0 + a1x + a2x 2 The coefficients obtained by least squares calculation are a0 = 58.2, a1 = -0.687, and a2 = -0.0318. The fitted temperature gradient distribution function is T(x) = 58.2 - 0.687x - 0.0318x. 2 Goodness of fit R 2 =0.993.

[0083] Calculate the temperature gradient value ▽T(x) based on the fitted temperature gradient distribution function T(x): Calculate the absolute value of the temperature gradient at each measurement point: at x=0, |▽T|=0.687℃ / mm; at x=13.5, |▽T|=1.546℃ / mm. The maximum absolute value of the temperature gradient is |▽T|max=1.546℃ / mm, and the minimum absolute value is |▽T|min=0.687℃ / mm.

[0084] Step 2: Establish a gradient distribution model for thermal conductivity.

[0085] Based on the temperature gradient distribution function T(x) and temperature gradient value ▽T(x) obtained in step 1, a thermal conductivity gradient distribution model for the square metal shell is established. The square metal shell contains four sidewalls: two large-area sidewalls and two small-area sidewalls. The thermal conductivity gradient distribution model is applied to each of the four sidewalls. The thermal conductivity of each sidewall of the shell exhibits a continuous gradient change along the thickness direction from the inner sidewall to the outer sidewall. The direction of the thermal conductivity gradient change is consistent with the direction of the internal temperature gradient of the battery cell measured in step 1, i.e., the thermal conductivity changes layer by layer from the center of the battery cell towards the surface of the shell. The thermal conductivity value at the inner sidewall of each sidewall is determined based on the heat flux density requirement near the shell on the inner surface of the battery cell, while the thermal conductivity value at the outer sidewall is determined based on the heat exchange requirements between the shell and the environment. The rate of change of the thermal conductivity gradient along the thickness direction of each sidewall of the shell is positively correlated with the internal temperature gradient value ▽T(x) of the battery cell measured in step 1.

[0086] The mathematical expression for the thermal conductivity gradient distribution model is:

[0087] Wherein, k(y) is the thermal conductivity at position y in the thickness direction of the shell sidewall, in watts per meter Kelvin; kin is the thermal conductivity at the inner sidewall of the shell sidewall, in watts per meter Kelvin; kout is the thermal conductivity at the outer sidewall of the shell sidewall, in watts per meter Kelvin; y is the thickness direction distance measured from the inner sidewall, 0≤y≤t, in millimeters; t is the thickness of the shell sidewall, in this embodiment t is taken as 1.2 millimeters; α is the gradient exponent, with a value range of 0.5≤α≤3.0.

[0088] The value of the gradient exponent α is determined based on the maximum value of the internal temperature gradient ▽T(x) of the battery cell measured in step 1:

[0089] Where |▽T|max is the maximum absolute value of the temperature gradient measured in step 1, which is 1.546℃ / mm in this embodiment; |▽T|min is the minimum absolute value of the temperature gradient measured in step 1, which is 0.687℃ / mm in this embodiment; and |▽T|ref is the reference temperature gradient value, which is 5℃ / mm. Substituting these values ​​into the calculation, we get α = 0.5 + 2.5 × 0.1718 = 0.9295, and we take α = 0.93.

[0090] In this embodiment, based on thermal management design requirements, the target thermal conductivity of the inner wall surface is determined to be kin = 180 W / m Kelvin, and the target thermal conductivity of the outer wall surface is determined to be kout = 120 W / m Kelvin. Substituting α = 0.93, kin = 180, kout = 120, and t = 1.2 mm into the thermal conductivity gradient distribution model expression, the distribution function of the thermal conductivity of the shell sidewall along the thickness direction in this embodiment is obtained as follows:

[0091] Step 3: Prepare gradient thermally conductive composite powder material.

[0092] Based on the thermal conductivity gradient distribution model established in step 2, a gradient thermally conductive composite powder material for cold spraying is prepared. The gradient thermally conductive composite powder material is composed of a metal matrix powder and a thermally conductive reinforcing phase powder mixed in a gradient ratio.

[0093] The metal matrix powder is pure aluminum powder with a particle size of 15 to 45 micrometers, a median particle size D50 of 28 micrometers, and a purity of not less than 99.5%. The thermally conductive reinforcing phase powder is high thermal conductivity graphite powder with a particle size of 5 to 20 micrometers, a median particle size D50 of 12 micrometers, and a thermal conductivity of not less than 800 W / m Kelvin. In this embodiment, natural flake graphite powder with a thermal conductivity of 1200 W / m Kelvin is selected.

[0094] In this embodiment, a series of gradient thermally conductive composite powder materials with different ratios were prepared, with five gradient levels. The method for determining the mass fraction of the thermally conductive reinforcing phase powder corresponding to each gradient level is as follows: based on the target thermal conductivity value of the thermal conductivity gradient distribution model k(y) in step 2, the corresponding mass fraction of the thermally conductive reinforcing phase powder is obtained by reverse lookup using a pre-established thermal conductivity-mass fraction calibration curve.

[0095] The thermal conductivity-mass fraction calibration curve was obtained through the following calibration experiments: Seven standard samples with thermally reinforcing phase powder mass fractions of 0%, 5%, 10%, 15%, 20%, 25%, and 30% were prepared. These samples were formed into standard test blocks using a cold spraying process under the same process parameters: gas pressure 3.0 MPa, gas temperature 400℃, spraying distance 25 mm, and powder feed rate 30 g / min. The thermal conductivity of each standard test block was measured using a Netzsch LFA467 laser thermal conductivity meter at a measurement temperature of 25℃. The calibration curve was plotted with the thermally reinforcing phase powder mass fraction as the abscissa and thermal conductivity as the ordinate. The measured data points are shown in Table 2. A continuous thermal conductivity-mass fraction calibration curve was obtained using cubic spline interpolation.

[0096] Table 2. Relationship between mass fraction of thermally conductive reinforcing phase powder and thermal conductivity

[0097] In this embodiment, the shell sidewall thickness t=1.2 mm is divided into 5 deposition layers, each with a thickness of 0.24 mm. The center positions y of the 5 deposition layers are 0.12 mm, 0.36 mm, 0.60 mm, 0.84 mm, and 1.08 mm, respectively. The y values ​​of each center position are substituted into the thermal conductivity distribution function in step 2 to calculate the target thermal conductivity of each layer. Then, the mass fraction of thermally conductive reinforcing phase powder corresponding to each layer is obtained by interpolation of the calibration curves in Table 2. The results are shown in Table 3.

[0098] Table 3 Thermal conductivity and powder ratio of each deposition layer

[0099] Pure aluminum powder and high thermal conductivity graphite powder were added to a ball mill mixing device according to the mass fractions corresponding to each gradient grade in Table 3. The ball mill mixing device used was a planetary ball mill with a grinding jar volume of 500 ml and zirconium oxide grinding balls. The ball milling process parameters were: ball-to-powder ratio 5:1, rotation speed 200 rpm, mixing time 30 minutes, and a protective argon atmosphere with argon purity of 99.99%. After mixing, the gradient thermal conductivity composite powder materials of each grade were separately placed into sealed bags, vacuum-sealed, and labeled with the corresponding gradient grade numbers 1 to 5.

[0100] Step 4: Establish the correspondence between cold spraying process parameters and the thermal conductivity of the deposited layer.

[0101] The gradient grade with the middle mass fraction of thermally conductive reinforcing phase powder in the gradient thermally conductive composite powder material prepared in step 3 is selected as the calibration powder material. In this embodiment, gradient grade 3 is selected, and the mass fraction of thermally conductive reinforcing phase powder is 18.1%.

[0102] The cold spray process parameters were set within the following ranges: gas pressure 1.5 MPa to 4.5 MPa, gas temperature 250°C to 550°C, spraying distance 15 mm to 35 mm, powder feed rate 15 g / min to 45 g / min, and spray gun movement speed 200 mm / s to 600 mm / s. Within these parameter ranges, 16 different combinations of process parameters were designed using orthogonal experimental design. Specific parameter combinations are shown in Table 4.

[0103] Table 4. Combination of process parameters for orthogonal experiments

[0104] Cold spray deposition experiments were conducted on aluminum substrates with process parameter combinations of 100 mm × 100 mm × 3 mm. The deposition thickness was 2 mm. After deposition, the deposited layer was separated from the aluminum substrate using wire cutting to obtain standard test blocks with dimensions of 10 mm × 10 mm × 2 mm. The thermal conductivity of each standard test block was measured using laser scintillation, and the microscopic cross-sectional morphology and porosity of the deposited layer were observed using scanning electron microscopy. A process parameter-thermal conductivity mapping database was established with cold spray process parameters as independent variables and the thermal conductivity of the deposited layer as the dependent variable. The measured data show that gas pressure and gas temperature have the most significant impact on the thermal conductivity of the deposited layer. Under the conditions of gas pressure of 3.5 MPa, gas temperature of 450℃, spraying distance of 25 mm, powder feeding rate of 25 g / min, and spray gun moving speed of 400 mm / s, the thermal conductivity of the deposited layer is 162.5 W / m Kelvin, and the porosity is 1.8%, which is closest to the target value of 161.3 W / m Kelvin.

[0105] The process parameter-thermal conductivity mapping database was used to select process parameter combinations that met the target values ​​of kin = 180 W / m Kelvin and kout = 120 W / m Kelvin in the thermal conductivity gradient distribution model of step 2. During the selection process, parameter combinations with higher gas pressure and higher gas temperature were preferred to ensure the bonding strength between the deposited layer and the substrate. The selection results are as follows: the first cold spraying process parameters that meet the target thermal conductivity value for the inner wall are a gas pressure of 4.5 MPa and a gas temperature of 550°C; the second cold spraying process parameters that meet the target thermal conductivity value for the outer wall are a gas pressure of 2.5 MPa and a gas temperature of 350°C.

[0106] Step 5: Provide a square metal housing core mold and end baffles.

[0107] A set of core molds for a square metal shell is provided. The external dimensions of the core molds are consistent with the internal cavity dimensions of the target square metal shell. The internal cavity of the target square metal shell has a length of 148 mm, a width of 52 mm, and a thickness of 27 mm. The length of the core mold is 0.08 mm smaller than the internal cavity length, so it is taken as 147.92 mm; the width of the core mold is 0.08 mm smaller than the internal cavity width, so it is taken as 51.92 mm; the thickness of the core mold is 0.08 mm smaller than the internal cavity thickness, so it is taken as 26.92 mm. The 0.08 mm dimensional difference is reserved to compensate for the subsequent processing allowance of the cold spray deposition layer.

[0108] The core mold is made of Cr12MoV mold steel. All four sidewalls of the core mold are precision ground to a surface roughness Ra of 0.4 micrometers. All four sidewalls are coated with a release agent coating, which is a boron nitride spray coating applied using plasma spraying. The boron nitride spray coating has a thickness of 8 micrometers.

[0109] Four end baffles are provided, each corresponding to one of the four edges of the opening of the square metal shell. Two end baffles corresponding to the length edge are 148 mm × 5 mm × 2 mm, and two end baffles corresponding to the width edge are 52 mm × 5 mm × 2 mm. The end baffles are made of Cr12MoV mold steel and coated with a boron nitride release agent coating with a thickness of 8 micrometers. The four end baffles are fixed to the edge of the opening face of the core mold using M3 hexagonal socket fasteners. The inner surface of the end baffles is flush with the corresponding sidewall surface of the core mold, while the outer surface of the end baffles extends 1.5 mm beyond the sidewall surface of the core mold to constrain the end boundary of the cold-sprayed deposition layer and prevent the deposited material from overflowing at the edge of the opening face.

[0110] Step 6: Perform cold spray gradient deposition to form a square metal casing.

[0111] The assembled core mold and end baffle from step 5 are installed on the workpiece rotary table of the cold spraying equipment. The cold spraying equipment uses a high-pressure cold spraying system, with nitrogen as the working gas and a purity of 99.9%. The rotation speed of the workpiece rotary table is set to 45 revolutions per minute to ensure the circumferential uniformity of the deposited layer.

[0112] The spraying strategy employs a layered gradient deposition method. The shell sidewall thickness of t=1.2 mm is divided into 5 deposition layers, with a total of N=5 deposition layers. Deposition occurs sequentially from the innermost layer (1st layer, corresponding to y=0) to the outermost layer (5th layer, corresponding to y=t).

[0113] The target thickness of the i-th deposition layer is t / N = 1.2 mm / 5 = 0.24 mm. The method for determining the gradient level of the i-th deposition layer is as follows: calculate the normalized thickness position yi / t corresponding to the center position of the i-th deposition layer, substitute the normalized thickness position yi / t into the thermal conductivity gradient distribution model in step 2 to calculate the target thermal conductivity value of the i-th deposition layer, and then determine the gradient level of the gradient thermal conductivity composite powder material to be used in the i-th deposition layer according to the thermal conductivity-mass fraction calibration curve in step 3. The specific correspondence is given in Table 3.

[0114] The cold spray gas pressure of the i-th deposition layer is determined according to the following formula:

[0115] Where Pi is the cold spray gas pressure of the i-th deposition layer, in megapascals; P1 is the gas pressure of the first cold spray process parameter, in this embodiment P1=4.5 megapascals; P2 is the gas pressure of the second cold spray process parameter, in this embodiment P2=2.5 megapascals; N is the total number of deposition layers, N=5.

[0116] The calculated gas pressures for each layer are: Layer 1 P1 = 4.5 MPa, Layer 2 P2 = 4.0 MPa, Layer 3 P3 = 3.5 MPa, Layer 4 P4 = 3.0 MPa, and Layer 5 P5 = 2.5 MPa.

[0117] The temperature of the cold spray gas for the i-th deposition layer is determined according to the following formula:

[0118] Wherein, Tgas,i is the cold spray gas temperature of the i-th deposition layer, in °C; Tgas,1 is the gas temperature of the first cold spray process parameter, in this embodiment Tgas,1=550 °C; Tgas,2 is the gas temperature of the second cold spray process parameter, in this embodiment Tgas,2=350 °C.

[0119] The calculated gas temperatures for each layer are as follows: Layer 1 Tgas,1 = 550℃, Layer 2 Tgas,2 = 500℃, Layer 3 Tgas,3 = 450℃, Layer 4 Tgas,4 = 400℃, and Layer 5 Tgas,5 = 350℃.

[0120] The spraying distance and powder feed rate of the i-th deposition layer remain constant throughout all deposition layers. Based on the analysis results from the process parameter-thermal conductivity mapping database in step 4, the sensitivity of thermal conductivity to changes in spraying distance is lowest in the range of 20 mm to 25 mm, and the sensitivity of thermal conductivity to changes in powder feed rate is lowest in the range of 25 g / min to 30 g / min. Therefore, in this embodiment, the spraying distance is set to 25 mm, and the powder feed rate is set to 25 g / min. The spray gun movement speed is set to 400 mm / s.

[0121] When spraying any sidewall of a square metal shell core mold, the spray gun travels 8 mm beyond each of the two edges of the sidewall along its length. The spray gun moves 1 / 2 of the nozzle diameter along its width. The nozzle diameter is 6 mm, and the moving step is 3 mm, to ensure the uniformity of the deposited layer thickness in the edge area.

[0122] The spray gun operates in the following sequence: first, layers 1 through 5 are deposited on one large sidewall of the square metal housing core mold; then, layers 1 through 5 are deposited on the other large sidewall of the square metal housing core mold; finally, layers 1 through 5 are deposited alternately on the two small sidewalls of the square metal housing core mold. After each sidewall is deposited, the workpiece rotary table rotates 90 degrees to begin depositing on the next sidewall.

[0123] After each layer is deposited, the actual deposition thickness is measured using a laser thickness gauge. The laser thickness gauge is a Keyence LK-G5000 series, with a measurement accuracy of ±1 micrometer. Five measurement locations are taken evenly along the sidewall length, and the average thickness is recorded as the actual deposition thickness of that layer. When the deviation between the actual deposition thickness and the target thickness of 0.24 mm exceeds ±5% (0.012 mm), the powder feed rate for the next deposition layer is adjusted for thickness compensation. The adjustment rule for thickness compensation is: if the actual deposition thickness is less than the target thickness, the powder feed rate for the next deposition layer is increased by 10%; if the actual deposition thickness is greater than the target thickness, the powder feed rate for the next deposition layer is decreased by 10%.

[0124] The actual deposition thickness and powder delivery rate adjustments for each layer in this embodiment are shown in Table 5.

[0125] Table 5 Adjustment of Thickness and Powder Feeding Rate of Each Deposition Layer

[0126] After all five layers of deposition are completed, a gradient thermally conductive composite deposition layer with a total thickness of 1.2 mm is formed on the surface of the core mold, which is the square metal shell.

[0127] Step 7: Remove the core mold and end baffles to obtain a square metal shell with a gradient thermal management structure.

[0128] After the cold spray coating is completed, the core mold assembly with the gradient thermally conductive composite deposition layer is placed in a room temperature environment to cool naturally to 25℃±3℃. After cooling, the M3 hexagonal socket fasteners of the four end plates are removed, and the four end plates are removed.

[0129] Demolding is achieved by utilizing the difference in thermal expansion coefficients between the aluminum gradient thermally conductive composite deposition layer and the steel core mold. The thermal expansion coefficient of the aluminum gradient thermally conductive composite deposition layer is approximately 23 × 10⁻⁶. -6 The coefficient of thermal expansion of steel core molds is approximately 12 × 10⁻⁶ per degree Celsius. -6 At a temperature of 135°C, the core mold assembly with the gradient thermal conductivity composite deposition layer is placed in a heating furnace and heated to 135°C, then held for 18 minutes. During heating, the expansion of the aluminum gradient thermal conductivity composite deposition layer is greater than that of the steel core mold, creating a gap between them. After heating and holding, a special demolding fixture is used to smoothly remove the core mold from the gradient thermal conductivity composite deposition layer, obtaining a square metal shell blank with a gradient thermal management structure.

[0130] The open end face of the extracted gradient thermally conductive composite deposition layer was machined. CNC milling was used for machining, with a machining allowance of 0.3 mm, to remove irregular edges formed during the cold spray deposition process, ensuring a smooth open end face. After machining, the inner and outer surfaces of the square metal casing were cleaned with anhydrous ethanol using ultrasonic cleaning for 5 minutes, and then dried for later use.

[0131] Step 8: Assemble the square metal-cased lithium-ion battery.

[0132] The square battery cell is placed into the square metal casing with the gradient thermal management structure obtained in step 7. Before placing the square battery cell, a thermal interface material is applied between the outer surface of the square battery cell and the inner wall of the square metal casing with the gradient thermal management structure. The thermal interface material is thermal grease with a thermal conductivity of 3.5 W / m Kelvin, and the coating thickness is controlled at 0.15 mm. The thermal grease is uniformly applied to the inner wall of the square metal casing with the gradient thermal management structure using a screen printing method.

[0133] The cover plate assembly was welded to the open end face of a square metal shell with a gradient thermal management structure. Laser welding was used at a power of 2.0 kW and a welding speed of 40 mm / s. Argon was used as the shielding gas at a flow rate of 15 L / min. After welding, an airtightness test was performed using a helium mass spectrometer leak detector, and the helium leakage rate was found to be 5 × 10⁻⁶. -8 Pa·m / s, satisfying a value not exceeding 1×10 -7 The requirement is Pa·m³ / s.

[0134] Electrolyte is injected through the injection hole on the cover plate, with the injection volume being 108% of the theoretical electrolyte absorption capacity of the cell. After injection, the injection hole is sealed by welding. The assembled battery is then aged at 45℃±2℃ for 36 hours, followed by formation and capacity testing processes to finally obtain a square metal-cased lithium-ion battery product with a gradient thermal management structure.

[0135] Comparative Example 1 Comparative Example 1 employs a traditional square metal-cased lithium-ion battery manufacturing method. The specific process is as follows: a square metal casing is formed from aluminum alloy sheet through a multi-pass stamping and stretching process. The casing material is 3003 aluminum alloy, and the casing thickness is 1.2 mm. After the battery cell is placed inside the casing, the same thermally conductive silicone grease as in Example 1 is applied between the battery cell and the inner wall of the casing, with a coating thickness of 0.15 mm. An aluminum alloy liquid cooling plate is installed on the outside of the casing. The liquid cooling plate is bonded to the large surface of the casing using thermally conductive adhesive with a thermal conductivity of 2.0 W / m Kelvin and a bonding thickness of 0.5 mm. After battery assembly, the same formation and capacity testing processes as in Example 1 are performed to obtain a conventionally structured square metal-cased lithium-ion battery.

[0136] Comparative Example 2 Comparative Example 2 employs a uniformly thermally conductive shell design. The shell material is pure aluminum, and it is formed in a single deposition process on a core mold using cold spraying. The shell thickness is 1.2 mm. The cold spraying powder material is a single-component powder with a thermally conductive reinforcing phase powder mass fraction of 18.1%. The cold spraying process parameters are fixed as follows: gas pressure 3.5 MPa, gas temperature 450°C, spraying distance 25 mm, and powder feed rate 25 g / min. The thermal conductivity of the shell is uniformly distributed along the thickness direction, with a measured value of 162 W / m Kelvin. Other assembly steps are the same as in Example 1.

[0137] Performance comparison test The performance of the square metal-cased lithium-ion battery with a gradient thermal management structure prepared in Example 1 was compared with that of the conventional structure battery in Comparative Example 1 and the battery with a uniform thermally conductive casing in Comparative Example 2. The test conditions were: ambient temperature 25°C, constant current discharge at 1C rate to a cutoff voltage of 2.5 volts, and the temperature at the center and corners of the cell surface during discharge was recorded to calculate the maximum temperature difference. The capacity retention rate of the battery after 500 1C charge-discharge cycles was also tested. The test results are shown in Table 6.

[0138] Table 6 Performance comparison of Example 1 with Comparative Examples 1 and 2

[0139] As shown in Table 6, the battery prepared in Example 1 had a maximum temperature difference of only 4.2℃ under 1C discharge conditions, which is 57.1% lower than that of Comparative Example 1 and 40.8% lower than that of Comparative Example 2. Regarding capacity retention after 500 cycles, Example 1 achieved 94.3%, an increase of 6.8 percentage points compared to Comparative Example 1 and 4.1 percentage points compared to Comparative Example 2. In terms of the number of manufacturing steps, Example 1 requires only 6 steps, a reduction of 8 steps compared to Comparative Example 1. These comparative results fully demonstrate the technical advantages of the present invention in integrating a gradient thermal management structure into a square metal casing and manufacturing it simultaneously. By matching the thermal conductivity gradient distribution with the internal temperature gradient distribution of the battery cell, the temperature uniformity and cycle life of the battery cell are effectively improved, while significantly simplifying the manufacturing process.

[0140] This invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of this invention. To provide the public with a thorough understanding of this invention, specific details are described in detail in the following preferred embodiments; however, those skilled in the art will fully understand the invention even without these details. Furthermore, to avoid unnecessary misunderstanding of the essence of this invention, well-known methods, processes, procedures, components, and circuits are not described in detail.

[0141] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for integrating and manufacturing a gradient thermal management structure for a square metal-cased lithium-ion battery, characterized in that, Includes the following steps: Step 1: Conduct thermal characteristic tests on the square metal-cased lithium-ion battery cell. Arrange at least 5 temperature measurement points along the thickness direction on the cell surface. Apply a 1C constant current discharge load to the cell and record the temperature change data of each temperature measurement point over time. Calculate the temperature gradient distribution function T(x) and temperature gradient value ▽T(x) in the thickness direction of the cell based on the collected steady-state temperature values. Step 2: Based on the temperature gradient distribution function T(x) and temperature gradient value ▽T(x) obtained in Step 1, establish a thermal conductivity gradient distribution model for the square metal shell. The thermal conductivity of each sidewall of the shell changes continuously along the thickness direction from the inner sidewall to the outer sidewall. The direction of the thermal conductivity gradient change is consistent with the direction of the internal temperature gradient of the battery cell measured in Step 1. The gradient change rate of the thermal conductivity of each sidewall of the shell along the thickness direction is positively correlated with the internal temperature gradient value ▽T(x) of the battery cell measured in Step 1. Step 3: Based on the thermal conductivity gradient distribution model established in Step 2, prepare a gradient thermally conductive composite powder material for cold spraying. The gradient thermally conductive composite powder material is made by mixing metal matrix powder and thermally conductive reinforcing phase powder in a gradient ratio with at least 5 gradient levels. Step 4: Establish the correspondence between cold spraying process parameters and the thermal conductivity of the deposited layer through process calibration experiments. Select the first cold spraying process parameters and the second cold spraying process parameters that satisfy the target thermal conductivity value of the inner wall and the target thermal conductivity value of the outer wall in the thermal conductivity gradient distribution model in Step 2 from the process parameter-thermal conductivity mapping database. Step 5: Provide a square metal shell core mold. All four sidewall surfaces of the core mold are coated with a release agent coating. Fix four end baffles to the edge of the open end face of the core mold. Step 6: Install the core mold and end baffle assembled in Step 5 onto the workpiece rotating table of the cold spraying equipment. Spray the gradient thermally conductive composite powder material prepared in Step 3 using a layered gradient deposition method. Divide the thickness of the shell sidewall into at least 5 deposition layers, depositing them sequentially from the 1st layer to the Nth layer. The 1st layer corresponds to the inner sidewall, and the Nth layer corresponds to the outer sidewall. The gradient level of the gradient thermally conductive composite powder material in the i-th deposition layer is determined according to the position of the layer in the thickness direction. The cold spraying gas pressure and cold spraying gas temperature of the i-th deposition layer linearly transition from the first cold spraying process parameter to the second cold spraying process parameter. After all N layers are deposited, a gradient thermally conductive composite deposition layer with a total thickness of t is formed on the surface of the core mold. The gradient thermally conductive composite deposition layer is the square metal shell. Step 7: Cool the core mold assembly after deposition in Step 6, remove the end baffle, demold using the difference in thermal expansion coefficient between the core mold and the gradient thermally conductive composite deposition layer, remove the gradient thermally conductive composite deposition layer, and machine the open end face of the removed gradient thermally conductive composite deposition layer to obtain a square metal shell with a gradient thermal management structure. Step 8: Place the square battery cell into the square metal casing with gradient thermal management structure obtained in Step 7. Coat the outer surface of the square battery cell with thermal interface material between the outer surface of the square battery cell and the inner wall of the square metal casing with gradient thermal management structure. Weld the cover plate assembly to the open end face of the square metal casing with gradient thermal management structure. After welding, perform an airtightness test. Inject electrolyte through the injection hole on the cover plate and seal the injection hole. Perform aging, formation and capacity testing on the assembled battery to obtain the finished lithium-ion battery with square metal casing and gradient thermal management structure.

2. The integrated manufacturing method for a gradient thermal management structure of a square metal-cased lithium-ion battery according to claim 1, characterized in that, The specific process of conducting thermal characteristic testing on the square metal-cased lithium-ion battery cell in step 1 is as follows: The square metal-cased lithium-ion battery cell to be tested is placed in a constant temperature environment set at 25℃±2℃. At least five temperature measurement points are arranged along the thickness direction on the cell surface. The locations of these temperature measurement points are: the center point of the cell thickness, 1 / 4 of the cell thickness, 1 / 8 of the cell thickness, the inner surface of the cell near the casing, and the outer surface of the cell. K-type thermocouples are used for temperature measurement, with a measurement accuracy of ±0.5℃ and a sampling frequency of 1 Hz. A 1C constant current discharge load is applied to the cell, and the temperature readings are continuously recorded. The temperature change data of each measurement point over time is collected until the cell voltage drops to the discharge cutoff voltage. The steady-state temperature values ​​of each measurement point at the end of discharge are extracted from the recorded temperature data. The process of calculating the temperature gradient distribution function along the cell thickness direction based on the collected steady-state temperature values ​​is as follows: Let the cell thickness direction be the x-direction, the thickness center point be the origin x=0, and the cell surface position be x=±L / 2, where L is the cell thickness. Substitute the position coordinates xi of each temperature measurement point and the corresponding steady-state temperature value Ti into the one-dimensional steady-state heat conduction equation, and use the least squares method to fit the temperature gradient distribution function T(x). The calculation formula for the least squares fitting is: ； Where n is the number of temperature measurement points and n≥5, Ti is the measured steady-state temperature value at the i-th temperature measurement point, and T(xi) is the fitted value of the temperature gradient distribution function at the i-th temperature measurement point; the temperature gradient value ▽T(x) is calculated based on the fitted temperature gradient distribution function T(x): 。 3. The integrated manufacturing method for a gradient thermal management structure of a square metal-cased lithium-ion battery according to claim 1, characterized in that, The mathematical expression for the thermal conductivity gradient distribution model of the square metal shell established in step 2 is as follows: ； Where k(y) is the thermal conductivity at position y in the thickness direction of the shell sidewall, kin is the thermal conductivity at the inner sidewall of the shell sidewall, kout is the thermal conductivity at the outer sidewall of the shell sidewall, y is the thickness distance measured from the inner sidewall and 0≤y≤t, t is the thickness of the shell sidewall, α is the gradient exponent and its value ranges from 0.5≤α≤3.0; the value of the gradient exponent α is determined based on the maximum value of the internal temperature gradient value ▽T(x) of the cell measured in step 1. Where |▽T|max is the maximum absolute value of the temperature gradient measured in step 1, |▽T|min is the minimum absolute value of the temperature gradient measured in step 1, and |▽T|ref is the reference temperature gradient value with a value of 5℃ / mm.

4. The integrated manufacturing method for a gradient thermal management structure of a square metal-cased lithium-ion battery according to claim 1, characterized in that, The specific process for preparing gradient thermally conductive composite powder materials in step 3 is as follows: the metal matrix powder is selected from pure aluminum powder, with a particle size of 15 micrometers to 45 micrometers and a purity of not less than 99.5%; the thermally conductive reinforcing phase powder is selected from high thermally conductive graphite powder, with a particle size of 5 micrometers to 20 micrometers and a thermal conductivity of not less than 800 W / m Kelvin; the method for determining the mass fraction of thermally conductive reinforcing phase powder corresponding to each gradient level is as follows: based on the target thermal conductivity value of the thermal conductivity gradient distribution model k(y) in step 2, the corresponding mass fraction of thermally conductive reinforcing phase powder is obtained by reverse lookup through the pre-established thermal conductivity-mass fraction calibration curve. The thermal conductivity-mass fraction calibration curve was obtained through the following calibration experiments: Seven standard samples with thermally conductive reinforcing phase powder mass fractions of 0%, 5%, 10%, 15%, 20%, 25%, and 30% were prepared. These samples were formed into standard test blocks using a cold spraying process under the same process parameters. The thermal conductivity of each standard test block was measured using the laser flash method. A calibration curve was plotted with the thermally conductive reinforcing phase powder mass fraction as the abscissa and the thermal conductivity as the ordinate. A continuous thermal conductivity-mass fraction calibration curve was obtained using cubic spline interpolation. The metal matrix powder and the thermally conductive reinforcing phase powder were respectively added to a ball mill mixing device according to the mass fractions corresponding to each gradient level. The ball milling process parameters were: ball-to-material ratio 5:1, rotation speed 200 rpm, mixing time 30 minutes, and argon protective atmosphere. After mixing, the gradient thermally conductive composite powder materials of each gradient level were sealed and packaged and labeled with the corresponding gradient level.

5. The integrated manufacturing method for a gradient thermal management structure of a square metal-cased lithium-ion battery according to claim 1, characterized in that, Step 4, establishing the correspondence between cold spraying process parameters and the thermal conductivity of the deposited layer, specifically involves selecting the gradient grade with a moderate mass fraction of thermally conductive reinforcing phase powder in the gradient thermally conductive composite powder material prepared in Step 3 as the calibration powder material. The range of cold spraying process parameters is set as follows: gas pressure 1.5 MPa to 4.5 MPa, gas temperature 250℃ to 550℃, spraying distance 15 mm to 35 mm, powder feed rate 15 g / min to 45 g / min, and spray gun moving speed 200 mm / s to 600 mm / s. Within the above parameter range... At least 16 different combinations of process parameters were designed using orthogonal experimental design. Cold spray deposition experiments were performed on aluminum substrates using each combination of process parameters, with a deposition thickness of 2 mm. After deposition, the deposited layer was separated from the aluminum substrate by wire cutting to obtain standard test blocks. The thermal conductivity of each standard test block was measured using laser scintillation. The microscopic cross-sectional morphology of the deposited layer was observed and the porosity of the deposited layer was measured using scanning electron microscopy. A process parameter-thermal conductivity mapping database was established with the cold spray process parameters as independent variables and the thermal conductivity of the deposited layer as dependent variable. The process parameter-thermal conductivity mapping database is used to select process parameter combinations that meet the requirements of the target thermal conductivity values ​​of the inner wall and the outer wall in the thermal conductivity gradient distribution model in step 2. During the selection, parameter combinations with higher gas pressure and higher gas temperature are given priority. The parameters that meet the target thermal conductivity value of the inner wall are recorded as the first cold spraying process parameters, and the parameters that meet the target thermal conductivity value of the outer wall are recorded as the second cold spraying process parameters.

6. The integrated manufacturing method for a gradient thermal management structure of a square metal-cased lithium-ion battery according to claim 1, characterized in that, The external dimensions of the square metal shell core mold provided in step 5 are consistent with the internal cavity dimensions of the target square metal shell. The length of the square metal shell core mold is 0.05 mm to 0.10 mm smaller than the internal cavity length of the target square metal shell, the width of the square metal shell core mold is 0.05 mm to 0.10 mm smaller than the internal cavity width of the target square metal shell, and the thickness of the square metal shell core mold is 0.05 mm to 0.10 mm smaller than the internal cavity thickness of the target square metal shell. The square metal shell core mold is made of mold steel, and the surface roughness Ra of the square metal shell core mold is not higher than 0.8 micrometers. The four... The sidewall surfaces are all coated with a release agent coating, which is a boron nitride spray coating with a thickness of 5 to 10 micrometers. The dimensions of the four end baffles correspond to the four edges of the opening end of the square metal shell. The material of the four end baffles is the same as that of the square metal shell core mold. The surfaces of the four end baffles are coated with a boron nitride release agent coating. The four end baffles are fixed to the edge of the opening end face of the square metal shell core mold with fasteners. The inner surface of the four end baffles is flush with the corresponding sidewall surface of the square metal shell core mold, and the outer surface of the four end baffles extends 1 to 2 millimeters beyond the sidewall surface of the square metal shell core mold.

7. The integrated manufacturing method for a gradient thermal management structure of a square metal-cased lithium-ion battery according to claim 1, characterized in that, In step 6, the cold spraying equipment uses a high-pressure cold spraying system, with nitrogen as the working gas. The rotation speed of the workpiece rotary table is set to 30 to 60 revolutions per minute. The specific method for dividing the shell sidewall thickness t into at least 5 deposition layers is as follows: Assume the total number of deposition layers is N and N≥5, and the target thickness of the i-th deposition layer is t / N, where i=1,2,...,N. The method for determining the gradient level corresponding to the i-th deposition layer is as follows: calculate the normalized thickness position yi / t corresponding to the center position of the i-th deposition layer, substitute the normalized thickness position yi / t into the thermal conductivity gradient distribution model in step 2 to calculate the target thermal conductivity value of the i-th deposition layer, and then determine the gradient level of the gradient thermal conductivity composite powder material to be used in the i-th deposition layer according to the thermal conductivity-mass fraction calibration curve in step 3. The cold spraying gas pressure of the i-th deposition layer is determined according to the following formula: ； Where Pi is the cold spray gas pressure of the i-th deposition layer, P1 is the gas pressure of the first cold spray process parameter, and P2 is the gas pressure of the second cold spray process parameter; the cold spray gas temperature of the i-th deposition layer is determined according to the following formula: ； Where Tgas,i is the cold spray gas temperature of the i-th deposition layer, Tgas,1 is the gas temperature of the first cold spray process parameter, and Tgas,2 is the gas temperature of the second cold spray process parameter; the spraying distance and powder feeding rate of the i-th deposition layer remain constant throughout all deposition layers. The spraying distance and powder feeding rate are respectively taken from the spraying distance value with the lowest sensitivity of thermal conductivity to spraying distance changes and the powder feeding rate value with the lowest sensitivity of thermal conductivity to powder feeding rate changes in the process parameter-thermal conductivity mapping database of step 4; after each deposition layer is completed, the actual deposition thickness of the deposition layer is measured using a laser thickness gauge. When the deviation between the actual deposition thickness and the target thickness exceeds ±5%, the powder feeding rate of the next deposition layer is adjusted for thickness compensation. The adjustment rule for thickness compensation is: if the actual deposition thickness is less than the target thickness, the powder feeding rate of the next deposition layer is increased by 10%; if the actual deposition thickness is greater than the target thickness, the powder feeding rate of the next deposition layer is decreased by 10%.

8. The integrated manufacturing method for a gradient thermal management structure of a square metal-cased lithium-ion battery according to claim 1, characterized in that, The specific demolding process in step 7 is as follows: After the cold spray deposition is completed, the square metal shell core mold assembly with the gradient thermally conductive composite deposition layer is placed in a room temperature environment to cool naturally to 25℃±5℃. After cooling, the fasteners of the four end baffles are removed and the four end baffles are removed. The square metal shell core mold assembly with the gradient thermally conductive composite deposition layer is heated to 120℃ to 150℃ and held for 15 minutes to 20 minutes. The difference in thermal expansion coefficient between the aluminum gradient thermally conductive composite deposition layer and the steel square metal shell core mold creates a gap between the gradient thermally conductive composite deposition layer and the surface of the square metal shell core mold. The square metal shell core mold is then removed from the gradient thermally conductive composite deposition layer. The machining method for the open end face of the removed gradient thermally conductive composite deposition layer is CNC milling. The machining allowance is 0.2 mm to 0.5 mm. The machining removes the irregular edge parts formed during the cold spray deposition process to make the open end face flat.

9. The integrated manufacturing method for a gradient thermal management structure of a square metal-cased lithium-ion battery according to claim 1, characterized in that, Before placing the square battery cell into the square metal casing with a gradient thermal management structure in step 8, a thermal interface material is applied between the outer surface of the square battery cell and the inner wall of the square metal casing with the gradient thermal management structure. The thermal interface material is thermally conductive silicone grease with a thermal conductivity of not less than 3 W / m Kelvin, and the coating thickness is 0.1 mm to 0.2 mm. Laser welding is used to weld the cover plate assembly to the open end face of the square metal casing with the gradient thermal management structure. The laser power is 1.5 kW to 2.5 kW, the welding speed is 30 mm / s to 50 mm / s, and the shielding gas is argon. The helium leakage rate standard for airtightness testing is not higher than 1 × 10⁻⁶. -7 Pa·m / s; the amount of electrolyte injected through the injection hole on the cover plate is 105% to 110% of the theoretical electrolyte absorption capacity of the cell; after sealing and welding the injection hole, the assembled battery is left to stand and age for 24 to 48 hours at 45℃±2℃.

10. The integrated manufacturing method for a gradient thermal management structure of a square metal-cased lithium-ion battery according to claim 1, characterized in that, In step 2, the square metal shell contains four sidewalls: two large-area sidewalls and two small-area sidewalls. The thermal conductivity gradient distribution model is applied to each of the four sidewalls simultaneously. In step 6, when spraying the gradient thermally conductive composite powder material prepared in step 3 using a layered gradient deposition method, the spraying path of the spray gun is executed in the following order: first, complete the deposition of layers 1 to N on one large-area sidewall of the square metal shell core mold; then, complete the deposition of layers 1 to N on the other large-area sidewall of the square metal shell core mold; then, alternately complete the deposition of layers 1 to N on the two small-area sidewalls of the square metal shell core mold. When spraying any sidewall of the square metal shell core mold, the travel distance of the spray gun in the length direction of the sidewall exceeds the edges of both ends of the sidewall by 5 mm to 10 mm, and the travel step of the spray gun in the width direction of the sidewall is 1 / 2 to 2 / 3 of the spray gun outlet diameter to ensure the uniformity of the thickness of the deposited layer in the edge region.

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