A battery pack and an electric device using the same

Abstract

The present application relates to a battery pack, which includes at least a first type of battery cell and a second type of battery cell, wherein the battery pack includes an A region and a B region, wherein the A region is an area with poor thermal insulation ability and is located around and / or at the bottom of the battery pack, and the remaining area is the B region, wherein the number of the first battery cells in the battery cells included in the A region accounts for 10% to 100%, and the number of the second battery cells in the battery cells included in the B region accounts for 5% to 100%, wherein the volume energy density D of the first type of battery cell and the second type of battery cell are D1 and D2 respectively, and the ionic conductivity σ of the electrolyte of each type at -10°C is σ1 and σ2 respectively, and K is defined as D 2 ×σ, then K1=D1 2 ×σ1 and K2=D2 2 ×σ2, wherein: when D1=D2, the K value satisfies 0.2<K2 / K1≤0.99; and when D1≠D2, the K value satisfies 1.01≤K2 / K1≤10. The present application also relates to an electrical device comprising the battery pack.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of secondary batteries, in particular to a battery pack and an electric device using the same. BACKGROUND

[0002] Secondary batteries have become the most popular energy storage system due to their low cost, long service life, and good safety, and have been widely used in pure electric vehicles, hybrid electric vehicles, and smart grids. A battery pack formed by a plurality of secondary batteries arranged in a certain space can directly serve as a power source for electric vehicles. Secondary batteries are increasingly widely used due to their cleanliness and renewable characteristics, and in order to meet the needs of different environments and application scenarios, the performance requirements of secondary batteries are continuously improved. For example, as a driving energy source for new energy vehicles, it needs to ensure safety performance in high-temperature environments and normal operation in low-temperature extremely cold regions. Lithium-ion batteries have a large polarization at low temperatures, which leads to early reaching of the cut-off voltage, reduction of discharge energy, and reduction of power. Therefore, as an inherent bottleneck of commonly used lithium-ion batteries, low-temperature performance has been difficult to break through for a long time.

[0003] In the prior art, this problem is mainly solved by including two types of battery monomers with different cold resistance capabilities in the battery pack. However, although these technical solutions in the prior art improve the low-temperature discharge capability of the entire pack, the safety performance of batteries of different types of chemical systems has a large difference. Therefore, there is still a need for a battery pack and an electric device that takes into account both low-temperature discharge capability and overall safety performance in the field of secondary batteries. SUMMARY

[0004] The present application is made in view of the above-mentioned problems, and aims to provide a battery pack to solve the technical problem that low-temperature discharge capability and overall safety performance cannot be taken into account.

[0005] In order to achieve the above-mentioned purpose, the first aspect of the present application provides a battery pack, which includes at least a first type of battery monomer and a second type of battery monomer, and the battery pack includes an A region and a B region, wherein the A region is a region with poor heat preservation capability, located at the periphery and / or the bottom of the battery pack, and the remaining region is the B region, wherein the number of the first battery monomers in the battery monomers contained in the A region accounts for 10% to 100%, and the number of the second battery monomers in the battery monomers contained in the B region accounts for 5% to 100%,

[0006] wherein the volume energy density D of the first type of battery monomer and the second type of battery monomer is D1 and D2 respectively, and the ionic conductivity σ of the electrolyte matched with each of them at -10℃ is σ1 and σ2 respectively, and K is defined as K=D 2 ×σ, then K1=D1 2Xσ1 and K2=D2 2 Xσ2, wherein:

[0007] When D1=D2, the K value satisfies 0.2

[0008] When D1≠D2, the K value satisfies 1.01≤K2 / K1≤10.

[0009] By matching and differentially arranging the batteries with the same or different volume energy densities D and electrolyte ion conductivities σ, the low-temperature weak area short board is supplemented, and the overall safety performance of the battery pack is considered.

[0010] In any embodiment, when D1=D2, the K value satisfies 0.25≤K2 / K1≤0.98; optionally, 0.3≤K2 / K1≤0.96. When D1≠D2, the K value satisfies 1.02≤K2 / K1≤5; optionally, 1.03≤K2 / K1≤3. By setting the volume energy densities D of the two types of battery monomers to be the same and different, and further selecting the preferred range of the K value ratio, the balance between the low-temperature performance and the safety performance of the battery pack can be continuously improved.

[0011] In any embodiment, when D1=D2, the ion conductivities σ1 and σ2 of the electrolytes of the first type of battery monomer and the second type of battery monomer at -10°C satisfy 0mS / cm

[0012] The second aspect of the present application provides another battery pack, which includes at least a first type of battery monomer and a second type of battery monomer, and the battery pack includes an A region and a B region, wherein the A region is a region with poor heat preservation ability, located at the periphery and / or bottom of the battery pack, and the remaining region is the B region, wherein the number of the first battery monomers in the battery monomers contained in the A region accounts for 10% to 100%, and the number of the second battery monomers in the battery monomers contained in the B region accounts for 5% to 100%,

[0013] wherein the volume energy density D of the first and second battery cells is D1 and D2 respectively, and the viscosity of the electrolyte of each battery cell at -10℃ is η1 and η2 respectively, and M=D / η, then M1=D1 / η1 and M2=D2 / η2, wherein:

[0014] When D1=D2, the M value satisfies 0.2

[0015] When D1≠D2, the M value satisfies 1.01≤M2 / M1≤10.

[0016] By matching and differentiating the arrangement of the batteries with the same or different volume energy density D and electrolyte viscosity η, the battery pack can have good low-temperature performance and excellent overall safety performance at the same time.

[0017] In any embodiment, when D1=D2, the M value satisfies 0.25≤M2 / M1≤0.98; optionally, 0.3≤M2 / M1≤0.96. When D1≠D2, the M value satisfies 1.02≤M2 / M1≤9; optionally, 1.03≤M2 / M1≤8. By assuming that the volume energy density D of the two battery cells is the same or different, and further optimizing the ratio of the M values of the two, the best balance of the low-temperature performance and overall safety performance of the battery pack can be achieved.

[0018] In any embodiment, when D1=D2, the viscosity η1 and η2 of the electrolyte of the first and second battery cells at -10℃ satisfies 0

[0019] In any embodiment, the number of the first battery cells in the battery cells contained in the A area accounts for 10% to 100%, optionally 20% to 90%, and further optionally 20% to 80%, and the number of the second battery cells in the battery cells contained in the B area accounts for 20% to 100%, optionally 40% to 100%, and further optionally 60% to 100%.

[0020] In any embodiment, the battery pack has a diagonal length Lc in the horizontal direction, and the area surrounded by connecting four points at 1 / 5Lc apart from each end of the diagonal is defined as the B area, and the remaining area is defined as the A area. In some embodiments, when the battery cells in the battery pack are placed in multiple layers, the vertical height of the battery pack is defined as Ld, and the area below 1 / 5Ld at the bottom is defined as the A area, and the area above 4 / 5Ld is defined as the B area. By dividing the space of the battery pack in the horizontal direction and the vertical direction, the area with poor heat preservation capacity and the area with relatively good heat preservation capacity can be reasonably defined, and battery cells with different performances can be correspondingly arranged, so as to balance the low-temperature performance and the overall safety performance of the battery pack.

[0021] In any embodiment, the battery pack has different heat preservation capacities in different areas, and at least two types of areas satisfy the temperature difference ΔT in the range of 0<ΔT<10℃; optionally, 0.1<ΔT<9℃; further optionally, 0.2<ΔT<8℃. By setting the temperature difference of at least two areas in the battery pack in the range, the effect of differential arrangement is embodied, and the uniformity of the temperature of the battery pack is improved.

[0022] In any embodiment, the film formation resistance of the positive electrode tab in the first type of battery cell and the second type of battery cell is Rct1 and Rct2 respectively, and they satisfy the relationship: 1≤Rct1 / Rct2≤4; optionally, 1.01≤Rct1 / Rct2≤3.5; further optionally, 1.02≤Rct1 / Rct2≤3. Rct is an important parameter in the use scenario of low-temperature discharge, and the smaller the value, the better the low-temperature performance. Controlling the Rct ratio within a reasonable range ensures the low-temperature improvement effect while avoiding overdesign.

[0023] In any embodiment, the freezing point of the electrolyte of all battery cells in the battery pack is ≤-20℃, optionally ≤-30℃. By setting the upper limit of the freezing point of the electrolyte of the battery cell, it can be ensured that the battery pack has good discharge capacity at low temperature, and the lower the freezing point, the wider the window of low-temperature application.

[0024] In any embodiment, the volume energy density D of the first type of battery cell and the second type of battery cell satisfies 100Wh / L<D<1000Wh / L; optionally, 200Wh / L<D<900Wh / L; further optionally, 300Wh / L<D<800Wh / L. The numerical range of the volume energy density D of the two types of battery cells can further adjust the low-temperature performance of the battery pack, and better match the endurance and power demand of various application scenarios.

[0025] In any embodiment, when D1≠D2, a third type of battery cell is also included, which is formed by mixing the primary material system of the first type of battery cell and the second type of battery cell, and has a volumetric energy density D3, wherein D3 is between D1 and D2. In some embodiments, the ionic conductivity of the electrolyte of the third type of battery cell at -10°C is σ3, and K3=D3 / σ3 is defined, wherein K3 is between K1 and K2; or the viscosity of the electrolyte of the third type of battery cell at -10°C is η3, and M3=D3 / η3 is defined, wherein M3 is between M1 and M2. By setting a third type or other type of battery cell in addition to the above two types of battery cells, the low-temperature performance and overall safety performance of the battery pack can be more accurately controlled and adjusted, making it easier to achieve a balance. 2 2 2

[0026] In any embodiment, the battery cells are independently selected from lithium ion batteries, lithium metal batteries, and sodium ion batteries. In some embodiments, the chemical system of the positive active material of the battery cells is independently selected from lithium nickel cobalt manganese, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium iron phosphate, lithium vanadium phosphate, lithium cobaltate, lithium nickelate, lithium-rich manganese, lithium nickel cobalt aluminum, and lithium manganate systems, as well as Prussian blue, polyanion, oxide, and dual-ion battery systems. The selection of the system of the battery cells can make the performance of the battery pack more abundant and adjustable.

[0027] In any embodiment, the battery pack comprises a first type of battery cell and a second type of battery cell, wherein a and b are both natural numbers ≥1, and 0.01

[0028] The third aspect of the present application provides a power device comprising a battery pack selected from the first aspect and / or the second aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings needed to be used in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can also be obtained according to the drawings without creative labor for those skilled in the art.

[0030] Figure 1 is a schematic diagram of a lithium ion secondary battery in an embodiment of the present application.

[0031] Figure 2 is Figure 1An exploded view of a lithium-ion secondary battery in one embodiment of the present application.

[0032] Figure 3 A schematic view of a battery pack in one embodiment of the present application.

[0033] Figure 4 A schematic view of a battery pack in one embodiment of the present application. Figure 3 An exploded view of a battery pack in one embodiment of the present application.

[0034] Figure 5 A schematic view of a battery pack in one embodiment of the present application. Figure 3 A schematic view of a battery pack in one embodiment of the present application.

[0035] Figure 6 A schematic view of a device using a battery pack as a power source in one embodiment of the present application.

[0036] Reference sign list

[0037] 1 Battery pack

[0038] 2 Upper case

[0039] 3 Lower case

[0040] 4 Battery module

[0041] 5 Lithium-ion secondary battery

[0042] 51 Case

[0043] 52 Electrode assembly

[0044] 53 Cover plate DETAILED DESCRIPTION

[0045] For the sake of brevity, the present application often discloses one or more specific embodiments of the application, however, it is understood that individual characteristics of one embodiment can be combined with individual characteristics of another embodiment unless explicitly stated otherwise. Also, it is understood that features of the disclosed embodiments can be interchanged even though not explicitly stated. For example, the use of a specific compound or material is not limited to a particular embodiment, but can be used in any embodiment unless explicitly stated otherwise. Furthermore, the use of a specific compound or material in one embodiment does not exclude the use of the same compound or material in another embodiment. For the sake of brevity, the present application specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly recited; and any lower limit can be combined with other lower limits to form a range not explicitly recited, likewise any upper limit can be combined with any other upper limit to form a range not explicitly recited. Furthermore, each individual disclosed point or single numerical value can itself be combined as a lower limit or upper limit with any other point or single numerical value or with other lower limits or upper limits to form a range not explicitly recited.

[0046] The prior art improves the low-temperature performance of the battery pack by placing battery cells with different low-temperature discharge capabilities in different areas of the battery pack, but this solution has substantial risk problems in the overall safety performance of the battery pack. Differentiating different types of battery cells by discharge capability alone can result in battery cells with high discharge performance but poor safety performance being placed at the periphery of the battery pack, and the peripheral area of the battery pack is more prone to risk of thermal runaway in the event of extrusion collision, high-temperature heating, falling, etc. Thus, the problem of not being able to balance the low-temperature discharge performance and overall safety performance of the battery pack constructed therefrom arises because the battery cells with poor safety performance are placed in the high-risk area.

[0047] The present inventors have found that by selecting and setting specific parameters of different types of battery cells in the battery pack to be within a specific range, the low-temperature discharge performance and overall safety performance of the battery pack can be effectively balanced.

[0048] Specifically, the first aspect of the present application provides a battery pack comprising at least a first type of battery cell and a second type of battery cell, the battery pack comprising an A region and a B region, wherein the A region is a poor heat preservation capability region, located at the periphery and / or bottom of the battery pack, and the remaining region is the B region, wherein the number of the first battery cells in the battery cells contained in the A region accounts for 10% to 100%, and the number of the second battery cells in the battery cells contained in the B region accounts for 5% to 100%,

[0049] wherein the volume energy density D of the first type of battery cell and the second type of battery cell is D1 and D2 respectively, and the ion conductivity σ of the electrolyte matched with each of them at -10℃ is σ1 and σ2 respectively, and K=D 2 ×σ, then K1=D1 2 ×σ1 and K2=D2 2 ×σ2, wherein:

[0050] When D1=D2, the value of K satisfies 0.2

[0051] When D1≠D2, the value of K satisfies 1.01≤K2 / K1≤10.

[0052] For the battery pack defined in the first aspect of the application, when the volume energy density D of the two types of battery cells contained therein is the same, it means that the main materials such as cathode, anode, separator and their proportions used by the two types of battery cells are the same. At this time, only the low-temperature conductivity σ of the electrolyte of the battery is changed, so that the battery cells placed in the weak heat preservation area of the battery pack have higher low-temperature discharge capacity, while the safety performance of the battery pack is not deteriorated. In addition, the difference in low-temperature conductivity is controlled within a reasonable range to avoid large waste in capacity performance, cost, etc., so as to achieve the technical effect of supplementing the endurance and power short board of the weak low-temperature area while taking into account the overall safety performance of the battery pack.

[0053] When the volume energy density D of the two types of battery cells contained therein is different, it means that the main material system or the proportion of main material components used by the two types of battery cells is different. According to experience in the lithium ion battery system, the battery with higher volume energy density D usually has relatively poor safety performance. Because the volume energy density D affects the severity of the battery thermal runaway and the speed of heat radiation to the surrounding, it is usually inversely proportional to the safety performance, that is, the larger the 1 / D, the safer it is. At the same time, the low-temperature discharge capacity of the high-energy-density system is often more advantageous, so the low-temperature conductivity requirement of the electrolyte system used is relatively low. Based on the above rules, the use of different system batteries in the battery pack, the smaller the K value of the system, the more suitable it is to be placed in the weak heat preservation area. Through the range of K value ratio, the short board of the weak low-temperature area is supplemented while ensuring that the overall battery pack has high safety performance.

[0054] In summary, by selecting the volume energy density D of the two types of battery cells and the ionic conductivity σ of the electrolyte thereof at low temperature, and setting the product K=D 2 ×σ within a suitable range, the best balance between the low-temperature discharge capacity and the overall safety performance of the battery pack can be achieved.

[0055] In some embodiments, when D1=D2, the K value satisfies 0.25≤K2 / K1≤0.98; optionally, 0.3≤K2 / K1≤0.96. When D1≠D2, the K value satisfies 1.02≤K2 / K1≤5; optionally, 1.03≤K2 / K1≤3. By setting the volume energy density D of the two types of battery cells to be the same and different, and further selecting the preferred range of the ratio of K value, the balance of the low temperature performance and safety performance of the battery pack can be continuously improved. The volume energy density D of the battery cell can be obtained by measuring the discharge capacity of the battery cell and dividing the volume of the battery cell, i.e., D=discharge capacity of the battery cell / volume of the battery cell, the basic unit is Wh / L (watt-hour / L). The volume energy density D of the battery cell is mainly related to the main materials such as cathode, anode, separator and their proportions, so the size of the volume energy density D can be adjusted by selecting specific main materials and their proportions. The ionic conductivity σ of the electrolyte of the battery cell at -10°C can be directly measured using a conductivity meter at the corresponding temperature. For a given system of battery cells, the ionic conductivity σ of the electrolyte can be adjusted by selecting the type of ionic species and the type and proportion of solvents, cosolvents and additives of the specific electrolyte.

[0056] In some embodiments, when D1=D2, the ionic conductivities σ1 and σ2 of the electrolytes of the first and second types of battery cells at -10°C satisfy 0<|σ1-σ2|<8 mS / cm; optionally, 0.05 mS / cm<|σ1-σ2|<7 mS / cm; further optionally, 0.1 mS / cm<|σ1-σ2|<6 mS / cm. When the volume energy densities D of the two types of battery cells are the same, by limiting the absolute value range of the difference of the ionic conductivities σ of the two types of battery cells, the balance of the low temperature performance and safety performance of the battery pack can be advantageously adjusted, and the waste in capacity, performance and cost caused by overdesign can also be avoided. In some embodiments, the total range of the ionic conductivity σ of the electrolyte at -10°C is limited to 0.5-15 mS / cm, optionally 1-10 mS / cm.

[0057] The second aspect of the present application provides another battery pack, which comprises at least a first type of battery cell and a second type of battery cell, and the battery pack comprises an A region and a B region, wherein the A region is a region with poor heat preservation ability, located at the periphery and / or bottom of the battery pack, and the remaining region is the B region, wherein the proportion of the number of the first battery cells in the battery cells contained in the A region is 10% to 100%, and the proportion of the number of the second battery cells in the battery cells contained in the B region is 5% to 100%,

[0058] Wherein the volume energy density D of the first and second battery cells is D1 and D2 respectively, and the viscosity η of the electrolyte matched with each battery cell at -10℃ is η1 and η2 respectively, define M = D / η, then M1 = D1 / η1 and M2 = D2 / η2, wherein:

[0059] When D1 = D2, the M value satisfies 0.2 < M2 / M1 ≤ 0.99; and

[0060] When D1 ≠ D2, the M value satisfies 1.01 ≤ M2 / M1 ≤ 10.

[0061] For the battery pack defined in the second aspect of the present application, when the volume energy densities of the two types of battery cells contained therein are the same, i.e., the main materials such as cathode, anode and separator used by the two types of batteries and their proportions are the same. At this time, only the low-temperature viscosity of the electrolyte of the two types of battery cells is changed, so that the battery cells placed in the weak heat preservation area of the battery pack have higher low-temperature discharge capacity, while the safety performance of the battery pack is not deteriorated. In addition, by controlling the low-temperature viscosity ratio within a reasonable range, the waste in capacity performance, cost, etc. is avoided, so as to achieve the technical effect of supplementing the endurance and power short board of the low-temperature weak area while taking into account the overall safety performance of the battery pack.

[0062] When the volume energy densities of the two types of battery cells contained therein are different, i.e., the main material system or the proportion of main material components used by the two types of batteries is different. According to the experience in the lithium ion battery system, the battery with higher volume energy density D usually has relatively poor safety performance. Because the volume energy density D affects the severity of the battery thermal runaway and the speed of heat radiation to the surrounding, it is usually inversely proportional to the safety performance, i.e., the larger the 1 / D, the safer it is. At the same time, the low-temperature discharge capacity of the high-energy-density system is usually more advantageous, so the electrolyte system usually used has relatively low demand for reducing the viscosity at low temperature. Based on the above rules, the use of different system battery cells in the battery pack is set as M = D / η, the smaller the M value of the system, the more suitable it is for placement in the weak heat preservation area. By limiting the range of M value ratio, the low-temperature weak area short board is supplemented while ensuring that the overall battery pack has high safety performance.

[0063] Therefore, by selecting the volume energy density D of the two types of battery cells and the viscosity η of the electrolyte at low temperature, and setting the ratio M = D / η within a suitable range, the two types of battery cells can be matched and arranged differently, so that the battery pack can have good low-temperature performance and excellent overall safety performance at the same time.

[0064] In some embodiments, when D1=D2, the M value satisfies 0.25≤M2 / M1≤0.98; alternatively, 0.3≤M2 / M1≤0.96. When D1≠D2, the M value satisfies 1.02≤M2 / M1≤9; alternatively, 1.03≤M2 / M1≤8. By assuming the volume energy density D of the two battery monomers is the same or different, and further optimizing the ratio of the two M values, an excellent balance of low-temperature performance and overall safety performance of the battery pack can be achieved.

[0065] In some embodiments, when D1=D2, the viscosity η1 and η2 of the electrolyte of the first and second battery monomers at -10°C satisfies 0<|η2-η1|<5mPa·s; alternatively, 0.05mPa·s<|η2-η1|<4mPa·s; further alternatively, 0.1mPa·s<|η2-η1|<3mPa·s. By setting the absolute value of the difference of the viscosity η of the two battery monomers, the balance of the low-temperature performance and overall safety performance of the battery pack can be conveniently adjusted, while avoiding the waste of capacity performance / cost, etc. caused by over-design. In some embodiments, the total range of the viscosity η of the electrolyte at -10°C is limited to 3-20mPa s, alternatively 5-15mPa s.

[0066] In some embodiments, the number of the first battery monomers in the battery monomers contained in the A region accounts for 10% to 90%, alternatively 20% to 90%, further alternatively 20% to 80%, and the number of the second battery monomers in the battery monomers contained in the B region accounts for 20% to 100%, alternatively 40% to 100%, further alternatively 60% to 100%. The number of the first and second battery monomers in the A and B regions can be adjusted according to the K and M values of the battery monomers.

[0067] In some embodiments, the battery pack has a length-width diagonal Lc in the horizontal direction, and the region surrounded by connecting the four points at the distance of 1 / 5Lc from the end points of the two diagonals is defined as the B region, and the remaining region is defined as the A region. The A region formed in this way is in the relative periphery of the battery pack, and its heat preservation capacity is relatively poor compared to the internal region. When the battery pack works at low temperature, the battery monomers in the A region need to have relatively stronger low-temperature discharge capacity. However, the A region is in the periphery, and its safety is also poor, so the low-temperature discharge capacity and safety performance of the battery monomers in the A region need to be overall planned to achieve a balance. Correspondingly, the B region is in the relative interior, and its heat preservation capacity is good, and its safety is also good. The division of the A and B regions in the horizontal direction reasonably defines the spatial distribution of the two battery monomers with different performances.

[0068] In some embodiments, when the battery cells in the battery pack are placed in multiple layers in an overlapping manner, the vertical height of the battery pack is defined as Ld, the area below 1 / 5 Ld at the bottom is defined as the A area, and the area above the remaining 4 / 5 Ld is defined as the B area. By also dividing the space in the vertical direction of the battery pack, a three-dimensional battery pack performance distribution can be constructed, which can be combined with the A area and B area division method in the horizontal direction described above, so as to achieve the best balance of the low-temperature performance and overall safety performance of the battery pack. The numerical range of Lc and Ld is not the focus, and both can be selected by the skilled person according to the needs and requirements of reasonable configuration.

[0069] In some embodiments, the battery pack has a cuboid structure, which is a rectangle in the horizontal plane, with a length of La, a width of Lb, a diagonal of Lc, and a vertical height of Ld.

[0070] In some embodiments, different regions in the battery pack have different heat preservation capabilities, and at least two types of regions satisfy the temperature difference AT in the range of 0 < AT < 10°C; optionally, 0.1 < AT < 9°C; further optionally, 0.2 < AT < 8°C. By setting the temperature difference range of at least two regions in the battery pack, the effect of differential arrangement is embodied, and the uniformity of the battery pack temperature is improved. There is a temperature difference AT greater than 0°C between at least two types of regions in the battery pack, so as to form a temperature gradient to balance the different performance of the battery cells; when the temperature difference AT is too large, it will make it difficult to balance the battery pack.

[0071] In some embodiments, the film formation resistance of the positive electrode tab in the first type of battery cell and the second type of battery cell is Rct1 and Rct2, respectively, and they satisfy the relationship: 1 < Rct1 / Rct2 < 4; optionally, 1.01 < Rct1 / Rct2 < 3.5; further optionally, 1.02 < Rct1 / Rct2 < 3. The ratio range of the film formation resistance of the two types of battery cells can further distinguish their discharge capacity, which helps to improve the performance balance of the battery pack. The film formation resistance reflects the discharge capacity of the battery cell, and the ratio of the two must be within a reasonable range in order to adjust the spatial arrangement of different battery cells to achieve precise balance. Rct is an important parameter in the use scenario of low-temperature discharge, and the smaller the value, the better the low-temperature performance. Controlling the Rct ratio within a reasonable range ensures the low-temperature improvement effect while avoiding overdesign.

[0072] In some embodiments, the freezing point of the electrolyte of all the battery cells in the battery pack is ≤ -20°C, optionally ≤ -30°C. Setting the freezing point of the electrolyte of the battery cells in this range can ensure that the battery pack has good discharge capacity at low temperatures. When the freezing point is too high, the ion migration ability in the electrolyte will be severely reduced after the battery pack works for a long time at low temperatures, which is not conducive to the performance at low temperatures. By setting the upper limit of the freezing point of the electrolyte of the battery cells, the battery pack can have good discharge capacity at low temperatures, and the lower the freezing point, the wider the window for low-temperature applications.

[0073] In some embodiments, the volume energy density D of the first type of battery cell and the second type of battery cell both satisfy 100 Wh / L < D < 1000 Wh / L; optionally, 200 Wh / L < D < 900 Wh / L; further optionally, 300 Wh / L < D < 800 Wh / L. The numerical range of the volume energy density D of the two types of battery cells can further adjust the low-temperature performance of the battery pack. At the same time, selecting a material system with a suitable energy density range can make it have good floor application value and product competitiveness, so as to better match the endurance and power requirements of various application scenarios.

[0074] In some embodiments, when D1 ≠ D2, a third type of battery cell is further included, which is formed by mixing the main material system of the first type of battery cell and the second type of battery cell, and has a volume energy density D3, wherein D3 is between D1 and D2. In some embodiments, the ionic conductivity of the electrolyte of the third type of battery cell at -10°C is σ3, and K3 = D3 2 × σ3, K3 is between K1 and K2; or the viscosity of the electrolyte of the third type of battery cell at -10°C is η3, and M3 = D3 / η3, M3 is between M1 and M2. By setting a third type or other battery cell in addition to the above two types of battery cells, the low-temperature performance and overall safety performance of the battery pack can be more accurately controlled and adjusted, making it easier to achieve a balance. The setting of the third type of battery cell can expand the performance adjustment range of the battery pack, so that the selection of battery cells is not limited to the pairing of different pure systems, but can also cover the pairing between various mixed systems and pure systems.

[0075] In some embodiments, the battery cells are independently selected from lithium ion batteries, lithium metal batteries, and sodium ion batteries, respectively. In some embodiments, the chemical system of the positive active material of the battery cells is independently selected from lithium nickel cobalt manganese, lithium iron phosphate, lithium manganese iron phosphate, lithium iron vanadium phosphate, lithium vanadium phosphate, lithium cobaltate, lithium nickelate, lithium-rich manganese, lithium nickel cobalt aluminum, and lithium manganate systems, as well as Prussian blue type, polyanion type, oxide type, and dual-ion battery systems. The selection of the system of the battery cells can make the performance of the battery pack more rich and adjustable.

[0076] In some embodiments, the battery pack comprises a first type of battery cell and a second type of battery cell, wherein a and b are both natural numbers ≥1, and 0.01 < a / b ≤ 200; optionally, 0.05 ≤ a / b ≤ 180; further optionally, 0.1 ≤ a / b ≤ 150. The ratio of the number of the two types of battery cells is controlled within a reasonable range, which can effectively adjust the low-temperature performance and safety performance of the battery pack.

[0077] The battery cells contained in the battery pack are described in detail below. In some embodiments, the first type of battery cell and the second type of battery cell are both lithium ion secondary batteries. As described above, the battery cells can also select other system battery cells, such as sodium ion batteries.

[0078] Generally, a lithium ion secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. During the charging and discharging process of the battery, active ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet and serves as a separator. The electrolyte serves as an ion conductor between the positive electrode sheet and the negative electrode sheet.

[0079] [Electrolyte]

[0080] The electrolyte serves as an ion conductor between the positive electrode sheet and the negative electrode sheet. The electrolyte includes an electrolyte salt and a solvent.

[0081] In the present application, the electrolyte salt can be a commonly used electrolyte salt in lithium ion secondary batteries, such as a lithium salt, which can be a lithium salt as described above as a high-thermal-stability salt, a lithium salt as a low-impedance additive, or a lithium salt that suppresses corrosion of an aluminum foil. As an example, the electrolyte salt can be selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorobis(oxalato)borate (LiDFOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalato)phosphate (LiDFOP), lithium fluorosulfonate (LiSO3F), bis(difluoro)oxalato phosphate (NDFOP), Li2F(SO2N)2SO2F, KFSI, CsFSI, Ba(FSI)2, and LiFSO2NSO2CH2CH2CF3.

[0082] The type of the solvent is not particularly limited and can be selected according to the actual needs. In some embodiments, the solvent is a non-aqueous solvent. Alternatively, the solvent can include one or more of a chain carbonate, a cyclic carbonate, and a carboxylic acid ester. In some embodiments, the solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), tetrahydrofuran, sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0083] In some embodiments, the electrolyte solution can further optionally include other additives. For example, the additives can include a negative electrode film-forming additive, a positive electrode film-forming additive, and an additive capable of improving certain properties of the battery, such as an additive capable of improving overcharge performance of the battery, an additive capable of improving high-temperature performance of the battery, and an additive capable of improving low-temperature performance of the battery, etc. As an example, the additive is selected from at least one of a cyclic carbonate compound containing an unsaturated bond, a halogen-substituted cyclic carbonate compound, a sulfate compound, a sulfite compound, a sulfonolactone compound, a disulfonic acid compound, a nitrile compound, an aromatic compound, an isocyanate compound, a phosphazene compound, a cyclic anhydride compound, a phosphite compound, a phosphate compound, a borate compound, and a carboxylic acid ester compound.

[0084] [Positive electrode sheet]

[0085] The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode material layer includes a positive electrode active material and carbon.

[0086] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode material layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0087] In the lithium-ion secondary battery of the present application, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (e.g., aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (e.g., a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0088] The positive electrode material layer provided on the surface of the positive electrode current collector includes a positive electrode active material. The positive electrode active material used in this application may have a structure of formula (I), formula (II), formula (III) or formula (IV) as described above and the various numerical definitions defined therein. Optionally, the positive electrode active material of formula (I), formula (II), formula (III) or formula (IV) in each case accounts for 60-100% by weight of the total weight of the positive electrode active material of the battery cell, optionally 80-100% by weight. In some embodiments, in addition to the above substances, the positive electrode active material may also include one or more other selected from lithium transition metal oxides, olivine-structured lithium-containing phosphates and their respective modified compounds. Examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and their modified compounds. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, a composite material of lithium iron manganese phosphate and carbon, and modified compounds thereof. These materials are all commercially available. The surface of the positive electrode active material may be coated with carbon.

[0089] The positive electrode material layer can optionally include a conductive agent. However, the type of the conductive agent is not particularly limited, and a person skilled in the art can select it according to actual needs. As an example, the conductive agent for the positive electrode material can be selected from one or more of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0090] The positive electrode material layer can also optionally include a binder. As an example, the binder can be one or more of styrene butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0091] The positive electrode tab can be prepared according to a method known in the art in the present application. As an example, the carbon-coated positive active material, the conductive agent, and the binder can be dispersed in a solvent (for example, N-methyl pyrrolidone (NMP)) to form a uniform positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after processes such as drying, cold pressing, etc., the positive electrode tab is obtained.

[0092] [Negative electrode tab]

[0093] The negative electrode tab includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, the negative electrode material layer including a negative active material.

[0094] As an example, the negative electrode current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode material layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector.

[0095] In the lithium ion secondary battery of the present application, the negative electrode current collector can be a metal foil or a composite current collector. For example, as a metal foil, a copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base. The composite current collector can be formed by forming a metal material (for example, copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base (such as a base of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0096] The negative electrode material layer in the lithium ion secondary battery of the present application generally comprises a negative electrode active material and optionally a binder, an optional conductive agent and other optional additives, and is generally formed by coating and drying a negative electrode slurry. The negative electrode slurry is generally formed by dispersing the negative electrode active material and optionally the conductive agent and the binder in a solvent and stirring until uniform. The solvent can be N-methyl pyrrolidone (NMP) or deionized water.

[0097] The specific type of the negative electrode active material is not limited, and any active material known in the art that can be used in the negative electrode of a lithium ion secondary battery can be used, and the person skilled in the art can select according to the actual needs. As an example, the negative electrode active material can be selected from one or more of graphite, soft carbon, hard carbon, mesocarbon microbeads, carbon fiber, carbon nanotube, elemental silicon, silicon oxide compound, silicon-carbon composite, and lithium titanate.

[0098] As an example, the conductive agent can be selected from one or more of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0099] As an example, the binder can be selected from one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0100] Other optional additives are, for example, thickening agents (such as sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0101] [Separator]

[0102] The lithium ion secondary battery using an electrolyte further comprises a separator. The separator is arranged between the positive electrode sheet and the negative electrode sheet and serves as a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used. In some embodiments, the material of the separator can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0103] In some embodiments, the positive electrode sheet, the negative electrode sheet and the separator can be used to make an electrode assembly by a winding process or a stacking process.

[0104] In some embodiments, the lithium ion secondary battery can comprise an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0105] In some embodiments, the outer package of the lithium ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the lithium ion secondary battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic, and as plastic, polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), and the like can be listed.

[0106] The present application does not have a particular limitation on the shape of the lithium ion secondary battery, which can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 is a square structure of the lithium ion secondary battery 5 as an example.

[0107] In some embodiments, referring to Figure 2 , the outer package can include a shell 51 and a cover plate 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the lithium ion secondary battery 5 can be one or more, which can be selected by those skilled in the art according to the specific actual needs.

[0108] In some embodiments, the lithium ion secondary battery can be assembled into a battery module 4, and the number of lithium ion secondary batteries contained in the battery module 4 can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module 4. In the battery module 4, a plurality of lithium ion secondary batteries 5 can be arranged in sequence along the length direction of the battery module. Of course, it can also be arranged in any other arbitrary manner. Further, the plurality of lithium ion secondary batteries 5 can be fixed by fasteners. Alternatively, the battery module 4 can also include a housing having a receiving space, and the plurality of lithium ion secondary batteries 5 are received in the receiving space.

[0109] In some embodiments, the above-mentioned lithium ion secondary battery 5 or battery module 4 can be assembled into a battery pack 1, and the number of lithium ion secondary batteries 5 or battery modules 4 contained in the battery pack 1 can be selected by those skilled in the art according to the application and capacity of the battery pack 1.

[0110] Figure 3 and Figure 4 is a battery pack 1 as an example. Referring to Figure 3 and Figure 4In the battery pack 1, a battery case and a plurality of battery cells disposed in the battery case can be included. The battery case includes an upper case 2 and a lower case 3, and the upper case 2 can be disposed on the lower case 3 to form an enclosed space for accommodating the battery cells.

[0111] Figure 5 is a schematic diagram of the area division and the arrangement of the battery cells in the battery pack in one embodiment of the present application. In Figure 5 In (a), the length La in the horizontal direction of the battery pack, the width Lb, and the diagonal Lc of the length and the width are defined. The area surrounded by connecting the four points each 1 / 5Lc apart from the end points of the two diagonals Lc is defined as the B area, and the remaining area is defined as the A area. In Figure 5 In (b), the vertical height of the battery pack is defined as Ld, and the area below 1 / 5Ld at the bottom is defined as the A area, and the remaining area above 4 / 5Ld is defined as the B area. The battery cells are placed in the corresponding areas according to the types and proportions described in the present application, so as to adjust the discharge performance of different areas.

[0112] In addition, the present application also provides a device, which includes the battery pack provided by the present application. The battery pack can be used as a power supply of the device, or can be used as an energy storage unit of the device. The device can be, but is not limited to, a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc.

[0113] As the device, the battery pack can be selected according to the use requirements thereof.

[0114] Figure 6 is a device as an example. The device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the lithium ion secondary battery for the device, a battery pack or a battery module can be used.

[0115] Embodiments

[0116] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and cannot be understood as a limitation of the present application. If the specific technology or condition is not specified in the embodiments, it is performed according to the technology or condition described in the literature in the art or according to the product instruction. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained by purchase.

[0117] 1. The parameters of the battery cell and the performance test method of the battery pack are as follows:

[0118] 1) Volume energy density determination:

[0119] D = initial discharge capacity of battery cell x discharge voltage plateau / volume of battery cell

[0120] The initial discharge capacity of battery cell is defined as the capacity at 25°C discharged from the upper cut-off voltage of each battery cell to the lower cut-off voltage of each battery cell at a rate of 0.33C;

[0121] The discharge voltage plateau is defined as the average discharge voltage at 25°C discharged from the upper cut-off voltage of each battery cell to the lower cut-off voltage of each battery cell at a rate of 0.33C.

[0122] 2) Electrolyte conductivity determination:

[0123] The electrolyte conductivity is tested by using the equipment model Leici DDSJ-318 conductivity tester. First, the electrode probe is placed in the standard electrolyte for calibration, then placed in the electrolyte to be tested, and measured after the ambient temperature is stable. Repeat the measurement three times, take the average value, and keep two decimal places.

[0124] 3) Electrolyte viscosity determination:

[0125] According to GB / T 10247-2008, the temperature is set to -10°C.

[0126] 4) Battery pack capacity retention rate test:

[0127] 25°C discharge capacity test: Place the battery pack in a 25°C environment, and charge and discharge the lithium ion battery at a fixed rate of 1 / 3C (1C = rated capacity of battery cell). Repeat this step three times, and take the third discharge capacity as the nominal capacity of the battery pack C1;

[0128] -20°C discharge capacity test: Place the battery pack in a -20°C environment, and discharge the lithium ion battery at a fixed rate of 1 / 3C1 to the lower cut-off voltage. Record the discharge capacity at this time as C2;

[0129] -20°C battery pack discharge capacity retention rate: The ratio of C2 / C1 is the discharge capacity retention rate of the battery pack at -20°C.

[0130] 5) Thermal propagation test: whether a certain battery cell in the test battery will spread to the adjacent battery cell after thermal runaway due to needle puncture. Two or more battery cells to be tested are assembled into a test battery module. The test battery module is fully charged, and a two-plate steel clamp with holes is used to fix the test battery module. A high-temperature-resistant stainless steel needle with a diameter of 8 mm (the angle of the needle angle cone is 45°, the surface of the needle is smooth, and there is no rust, oxidation layer and oil stain) is used to penetrate the first battery cell at a speed of 25 mm / s from the direction perpendicular to the pole plate of the battery cell to trigger thermal runaway. The time of thermal runaway of the adjacent second battery cell is observed and recorded; if the battery cell triggering thermal runaway does not cause fire or explosion of the adjacent battery cell, it is determined that thermal propagation is blocked, otherwise it is determined that thermal propagation occurs.

[0131] 2. Preparation of battery cell

[0132] 1) NCM:

[0133] Positive electrode sheet: the positive active material LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are dissolved in solvent N-methyl pyrrolidone (NMP) at a weight ratio of 90:5:5, and after being fully stirred and mixed uniformly, a positive electrode slurry is obtained; then the positive electrode slurry is uniformly coated on the positive electrode current collector, and after drying, cold pressing, and slitting, a positive electrode sheet is obtained.

[0134] Negative electrode sheet: the active material artificial graphite, conductive agent acetylene black, binder styrene-butadiene rubber (SBR), and thickening agent sodium carboxymethyl cellulose (CMC) are dissolved in deionized water at a weight ratio of 90:4:4:2, and after being uniformly mixed with the solvent deionized water, a negative electrode slurry is prepared; then the negative electrode slurry is uniformly coated on the negative electrode current collector copper foil one or more times, and after drying, a negative electrode film is obtained, which is then cold-pressed and slitted to obtain a negative electrode sheet.

[0135] Electrolyte: in an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), organic solvents EC / EMC are mixed uniformly at a volume ratio of 3 / 7, 12.5% LiPF6 lithium salt is dissolved in the organic solvent, and after being stirred uniformly, the corresponding electrolyte is obtained.

[0136] Battery preparation: using the positive electrode sheet and negative electrode sheet prepared as described above, a polypropylene film is used as a separator, and the positive electrode sheet, separator, and negative electrode sheet are stacked in order with the separator between the positive and negative electrode sheets to play a separating role, and then wound to obtain an electrode assembly. The electrode assembly is placed in a battery case, dried, and then injected with electrolyte, and after formation and standing, a lithium ion secondary battery is prepared as a battery cell.

[0137] Mixed NCM: (for example only, the mixing ratio can be adjusted as needed) positive electrode sheet: the positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811) and carbon-coated lithium iron phosphate (LFP), conductive agent acetylene black, binder polyvinylidene fluoride (PVDF) are dissolved in solvent N-methyl pyrrolidone (NMP) at a weight ratio of 60:30:5:5, after fully stirring and mixing uniformly, the positive electrode slurry is obtained; then the positive electrode slurry is uniformly coated on the positive electrode current collector, and then dried, cold-pressed, and cut to obtain the positive electrode sheet.

[0138] Negative electrode sheet: the active material artificial graphite, conductive agent acetylene black, binder styrene-butadiene rubber (SBR), thickening agent sodium carboxymethyl cellulose (CMC) are dissolved in deionized water at a weight ratio of 90:4:4:2, and then uniformly mixed with the solvent deionized water to prepare the negative electrode slurry; then the negative electrode slurry is uniformly coated on the negative electrode current collector copper foil one or more times, and then dried to obtain the negative electrode film, and then cold-pressed and cut to obtain the negative electrode sheet.

[0139] Electrolyte: in an argon atmosphere glove box (H2O <0.1 ppm, O2 <0.1 ppm), organic solvents EC / EMC are mixed uniformly at a volume ratio of 3 / 7, 12.5% LiPF6 lithium salt is dissolved in the organic solvent, and then stirred uniformly to obtain the corresponding electrolyte.

[0140] Battery preparation: using the positive electrode sheet and the negative electrode sheet prepared as described above, polypropylene film is used as the separator film, the positive electrode sheet, the separator film, and the negative electrode sheet are stacked in order, the separator film is between the positive electrode sheet and the negative electrode sheet to play a separating role, and then the electrode assembly is obtained by winding. The electrode assembly is placed in the battery shell, dried, and then injected with the electrolyte, and then formed, and then placed to obtain the lithium ion secondary battery as the battery monomer.

[0141] 2) LFP:

[0142] Positive electrode sheet: carbon-coated lithium iron phosphate (LFP) as the positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are dissolved in solvent N-methyl pyrrolidone (NMP) at a weight ratio of 96:2:2, and then fully stirred and mixed uniformly to obtain the positive electrode slurry. In addition, a particle sizer is used to sieve the selected lithium iron phosphate particle material and the lithium manganese iron phosphate particle material, and the various materials are further sieved into fractions with different volume average particle sizes D50 values, and are respectively applied in the preparation of the battery monomer. The positive electrode slurry is uniformly coated on the aluminum positive electrode current collector, and then dried, cold-pressed, and cut to obtain the positive electrode sheet.

[0143] Negative electrode sheet: Dissolve the negative electrode active material (artificial graphite), the conductive agent (acetylene black), the binder (styrene-butadiene rubber (SBR), and the thickener (sodium carboxymethyl cellulose (CMC-Na)) in deionized water at a weight ratio of 95:2:2:1. Mix thoroughly to prepare a negative electrode slurry. Apply the slurry evenly to the negative electrode current collector copper foil, dry it, and then cold press and slit it to produce the negative electrode sheet.

[0144] Electrolyte: In an argon atmosphere glove box (H2O <0.1ppm, O2 <0.1ppm), the organic solvents ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) were uniformly mixed according to the weight ratios listed in Tables 1 and 2, respectively, lithium hexafluorophosphate (LiPF6) was added and adjusted to a concentration of 1M / L, and stirred evenly to obtain the corresponding electrolyte.

[0145] Battery Preparation: Using the positive and negative electrode sheets prepared as described above, and using a polypropylene film as a separator, stack the positive, separator, and negative electrode sheets in order, placing the separator between the positive and negative electrode sheets to provide insulation. The assembly is then wound to form an electrode assembly. The electrode assembly is placed in a battery casing, dried, and then injected with electrolyte. The battery is then formed and allowed to stand to form a lithium-ion secondary battery cell.

[0146] 3) Lithium manganese iron phosphate (LMFP):

[0147] Positive electrode sheet: Lithium manganese iron phosphate LiMn will be used as the positive electrode active material 0.6 Fe 0.4 PO4 (LMFP), conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are dissolved in N-methylpyrrolidone (NMP) at a weight ratio of 96:2:2 and thoroughly stirred to form a positive electrode slurry. Separately, a particle sieving machine is used to sieve the selected lithium iron phosphate and lithium manganese iron phosphate granules. Each material is further sieved into fractions with different volume average particle sizes (D50) and used in the preparation of different battery cells. The positive electrode slurry is evenly coated on an aluminum positive electrode current collector, followed by drying, cold pressing, and slitting to produce positive electrode sheets.

[0148] Negative electrode sheet: Dissolve the negative electrode active material (artificial graphite), the conductive agent (acetylene black), the binder (styrene-butadiene rubber (SBR), and the thickener (sodium carboxymethyl cellulose (CMC-Na)) in deionized water at a weight ratio of 95:2:2:1. Mix thoroughly to prepare a negative electrode slurry. Apply the slurry evenly to the negative electrode current collector copper foil, dry it, and then cold press and slit it to produce the negative electrode sheet.

[0149] Electrolyte: In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), organic solvents ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl ethyl carbonate (EMC) were mixed according to the weight ratio listed in Table 1 and Table 2, lithium hexafluorophosphate (LiPF6) was added and adjusted to a concentration of 1 M / L, stirred uniformly to obtain the corresponding electrolyte.

[0150] Battery preparation: using the positive electrode sheet and negative electrode sheet prepared as described above, polypropylene film as a separator film, the positive electrode sheet, the separator film, the negative electrode sheet were stacked in order, the separator film was between the positive and negative electrode sheets to play a role of isolation, then wound to obtain an electrode assembly. The electrode assembly was placed in a battery shell, after drying, the electrolyte was injected, then formed, and stood to prepare a lithium ion secondary battery as a battery monomer.

[0151] 4) SIB:

[0152] Positive electrode sheet: the positive electrode active material sodium oxide Na2FeO2, conductive agent acetylene black, binder polyvinylidene fluoride (PVDF) were dissolved in solvent N-methyl pyrrolidone (NMP) according to the weight ratio of 90:5:5, after fully stirring and mixing uniformly, the positive electrode slurry was obtained; then the positive electrode slurry was uniformly coated on the positive electrode current collector, and then dried, cold pressed and cut to obtain the positive electrode sheet.

[0153] Negative electrode sheet: the active material artificial graphite, conductive agent acetylene black, binder styrene butadiene rubber (SBR), thickening agent sodium carboxymethyl cellulose (CMC) were dissolved in deionized water according to the weight ratio of 90:4:4:2, and then uniformly mixed with deionized water to prepare a negative electrode slurry; then the negative electrode slurry was uniformly coated on the negative electrode current collector copper foil one or more times, and the negative electrode film was obtained after drying, and then cold pressed and cut to obtain the negative electrode sheet.

[0154] Electrolyte: in an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), organic solvents EC / EMC were mixed according to the volume ratio of 3 / 7, 12.5% LiPF6 lithium salt was dissolved in organic solvents, stirred uniformly to obtain the corresponding electrolyte.

[0155] Battery preparation: using the positive electrode sheet and negative electrode sheet prepared as described above, polypropylene film as a separator film, the positive electrode sheet, the separator film, the negative electrode sheet were stacked in order, the separator film was between the positive and negative electrode sheets to play a role of isolation, then wound to obtain an electrode assembly. The electrode assembly was placed in a battery shell, after drying, the electrolyte was injected, then formed, and stood to prepare a lithium ion secondary battery as a battery monomer.

[0156] 3. Assembly of the battery pack

[0157] The main materials of each battery cell, the types, compositions, amounts of solvents and additives in the electrolyte, and the types and concentrations of electrolyte salts were adjusted to obtain two types of battery cells with the conductivity and viscosity ratios as described in Table 1 and the volumetric energy density. The battery cells with different main material systems prepared as described above were placed in the amounts and areas shown in Table 1 to obtain the battery packs with different battery cell arrangements in Examples 1-6 and Comparative Examples 1-3. In Examples 1-6 and Comparative Examples 1-3, each battery pack contains only two types of battery cells selected from the battery cells prepared as described above, which are labeled as the first type of battery cell and the second type of battery cell, respectively, as shown in the following table; wherein the A area is the area surrounded by connecting the four points at the respective 1 / 5 Lc points of the horizontal long width diagonal Lc of the battery pack and the area below the 1 / 5 vertical height Ld from the bottom, and the remaining area of the battery pack is the B area. The number of all battery cells contained in the A area is 48, of which the number of the first type of battery cell is 40 and the number of the second type of battery cell is 8; the number of all battery cells contained in the B area is 72, of which the number of the first type of battery cell is 27 and the number of the second type of battery cell is 45.

[0158] The battery packs prepared in each example were tested, and the test results are shown in Table 1.

[0159] Table 1: Test results of the corresponding parameters of the battery packs of Examples 1-6 and Comparative Examples 1-3

[0160]

[0161] As can be seen from Table 1, when the volumetric energy densities D of the two types of battery cells in the battery pack are equal, the value of K2 / K1 or M2 / M1 less than 1 can make the battery pack have a higher capacity retention rate, while meeting the requirement of thermal propagation barrier, as shown in Example 1. In contrast, as shown in Comparative Example 1, for the same NCM battery cells, when the value of K2 / K1 or M2 / M1 is 1, the capacity retention rate of the battery pack decreases, and the battery pack cannot pass the thermal propagation barrier test. Similarly, for the LFP system battery cells, when the volumetric energy densities D of the two types of battery cells in the battery pack are equal, the value of K2 / K1 or M2 / M1 being 1 makes the battery pack have a severely reduced capacity retention rate (Comparative Example 2).

[0162] In addition, when using different battery monomer compositions to form a hybrid battery pack, the volume energy densities of the two are different, and the value of K2 / K1 or M2 / M1 greater than 1 brings a relatively high capacity retention rate, while both meet the requirement of thermal propagation barrier, as shown in Examples 2-6. For different battery systems, the capacity retention rate is related to the specific battery system used, and does not change in proportion to the value of K2 / K1 or M2 / M1. However, when the value of K2 / K1 or M2 / M1 in the hybrid system is less than 1, the capacity retention rate of the battery pack decreases, and it cannot pass the thermal propagation barrier test (Comparative Example 3).

[0163] Although the present application has been described with reference to the embodiments, various modifications can be made to it without departing from the scope of the application, and equivalent components therein can be replaced. In particular, the technical features mentioned in each embodiment can be combined in any manner as long as there is no structural conflict. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A battery pack, characterized in that: It includes at least a first type of battery cell and a second type of battery cell. The battery pack includes an A area and a B area. The length, width, and diagonal of the battery pack in the horizontal direction is Lc. The area enclosed by connecting four points at the 1 / 5Lc equal division points from the endpoints of the two diagonals is defined as the B area, and the remaining area is defined as the A area; when the battery cells in the battery pack are stacked in multiple layers, the vertical height of the battery pack is defined as Ld, and the area below 1 / 5Ld at the bottom is defined as the A area, and the remaining upper 4 / 5Ld area is defined as the B area; the A area is an area with poor heat preservation ability, located around and / or at the bottom of the battery pack, and the remaining area is the B area, where the proportion of the number of the first type of battery cells in the battery cells included in the A area is 10% to 100%, and the proportion of the number of the second type of battery cells in the battery cells included in the B area is 5% to 100%; The volume energy density D of the first type of battery cell and the second type of battery cell are D1 and D2 respectively, and the ionic conductivity σ of their respective electrolytes at -10°C are σ1 and σ2 respectively, and K is defined as D 2 ×σ, then K1=D1 2 ×σ1 and K2=D2 2 ×σ2, where: When D1 = D2, the K value satisfies 0.2 < K2 / K1 ≤ 0.99; and When D1 ≠ D2, the K value satisfies 1.01 ≤ K2 / K1 ≤ 10.

2. The battery pack according to claim 1, wherein: When D1 = D2, the K value satisfies 0.25 ≤ K2 / K1 ≤ 0.

98.

3. The battery pack according to claim 1, wherein: When D1 = D2, the K value satisfies 0.3 ≤ K2 / K1 ≤ 0.

96.

4. The battery pack according to claim 1, wherein: When D1 ≠ D2, the K value satisfies 1.02 ≤ K2 / K1 ≤ 5.

5. The battery pack according to claim 1, wherein: When D1 ≠ D2, the K value satisfies 1.03 ≤ K2 / K1 ≤ 3.

6. The battery pack according to claim 1, characterized in that: When D1 = D2, the ionic conductivities σ1 and σ2 of the electrolytes of the first type of battery cell and the second type of battery cell at -10 °C satisfy 0 < |σ1 - σ2| < 8 mS / cm.

7. The battery pack according to claim 1, characterized in that: 0.05 mS / cm < |σ1 - σ2| < 7 mS / cm.

8. The battery pack according to claim 1, wherein: 0.1 mS / cm < |σ1 - σ2| < 6 mS / cm.

9. The battery pack according to claim 1, wherein: The proportion of the number of the first type of battery cells in the battery cells included in the A area is 10% to 90%.

10. The battery pack according to claim 1, wherein: The proportion of the number of the first type of battery cells in the battery cells included in the A area is 20% to 80%.

11. The battery pack according to claim 1, wherein: The proportion of the number of the second type of battery cells in the battery cells included in the B area is 20% to 100%. According to the battery pack according to claim 1, wherein The proportion of the number of the second type of battery cells in the battery cells included in the B area is 40% to 100%.

13. The battery pack according to claim 1, wherein: The proportion of the number of the second type of battery cells in the battery cells included in the B area is 60% to 100%.

14. The battery pack according to claim 1, wherein: Different areas in the battery pack have different heat preservation abilities, and at least two types of areas satisfy the temperature difference ΔT within the range of 0 < ΔT < 10 °C.

15. The battery pack according to claim 1, wherein: Different areas in the battery pack have different heat preservation abilities, and at least two types of areas satisfy the temperature difference ΔT within the range of 0.1 < ΔT < 9 °C.

16. The battery pack according to claim 1, wherein: Different areas in the battery pack have different heat preservation abilities, and at least two types of areas satisfy the temperature difference ΔT within the range of 0.2 < ΔT < 8 °C.

17. The battery pack according to claim 1, wherein: The film formation resistances of the positive electrode plates in the first type of battery cell and the second type of battery cell are Rct1 and Rct2 respectively, and the two satisfy the relationship: 1 ≤ Rct1 / Rct2 ≤ 4.

18. The battery pack according to claim 1, wherein: The film formation resistances of the positive electrode plates in the first type of battery cell and the second type of battery cell are Rct1 and Rct2 respectively, and the two satisfy the relationship: 1.01 ≤ Rct1 / Rct2 ≤ 3.

5.

19. The battery pack according to claim 1, wherein: The film formation resistances of the positive electrode plates in the first type of battery cell and the second type of battery cell are Rct1 and Rct2 respectively, and the two satisfy the relationship: 1.02 ≤ Rct1 / Rct2 ≤ 3.

20. The battery pack according to claim 1, wherein: The freezing point of the electrolyte of all battery cells in the battery pack ≤ -20 °C.

21. The battery pack according to claim 1, wherein: The freezing point of the electrolyte of all battery cells in the battery pack ≤ -30 °C.

22. The battery pack according to claim 1, wherein: When D1 ≠ D2, it further includes a third type of battery cell, which is formed by physically mixing the main material systems of the first type of battery cell and the second type of battery cell. The third type of battery cell has a volume energy density D3, where D3 is between D1 and D2.

23. The battery pack according to claim 22, characterized in that: The first type of battery cell, the second type of battery cell, and the third type of battery cell are each independently selected from lithium-ion batteries and sodium-ion batteries.

24. The battery pack according to any one of claims 1 to 23, characterized in that: It includes a first type of battery cells and b second type of battery cells, where both a and b are natural numbers ≥ 1, and 0.01 < a / b ≤ 200.

25. The battery pack according to any one of claims 1 to 23, characterized in that: It includes a first type of battery cells and b second type of battery cells, 0.05 ≤ a / b ≤ 180.

26. The battery pack according to any one of claims 1 to 23, characterized in that: It includes a first type of battery cells and b second type of battery cells, 0.1 ≤ a / b ≤ 150.

27. A battery pack, characterized in that: It includes at least the first type of battery cell and the second type of battery cell. The battery pack includes area A and area B. The length, width, and diagonal of the battery pack in the horizontal direction is Lc. The area enclosed by connecting four points at the 1 / 5Lc equal division points of each diagonal distance from the endpoints is defined as area B, and the remaining area is defined as area A; when the battery cells in the battery pack are stacked in multiple layers, the vertical height of the battery pack is defined as Ld, and the area below 1 / 5Ld at the bottom is defined as area A, and the remaining upper 4 / 5Ld area is defined as area B; area A is the area with poor heat preservation ability, located around and / or at the bottom of the battery pack, and the remaining area is area B. The proportion of the number of the first type of battery cells in the battery cells included in area A is 10% to 100%, and the proportion of the number of the second type of battery cells in the battery cells included in area B is 5% to 100%; Where the volume energy densities D of the first type of battery cell and the second type of battery cell are D1 and D2 respectively, and the viscosities η of the respective paired electrolytes at -10 °C are η1 and η2 respectively. Define M = D / η, then M1 = D1 / η1 and M2 = D2 / η2, where: When D1 = D2, the M value satisfies 0.2 < M2 / M1 ≤ 0.99; and When D1 ≠ D2, the M value satisfies 1.01 ≤ M2 / M1 ≤ 10.

28. The battery pack according to claim 27, wherein: When D1 = D2, the M value satisfies 0.25 ≤ M2 / M1 ≤ 0.

98.

29. The battery pack according to claim 27, wherein: 0.3 ≤ M2 / M1 ≤ 0.

96.

30. The battery pack according to claim 27, wherein: When D1≠D2, the M value satisfies 1.02≤M2 / M1≤9.

31. The battery pack according to claim 27, wherein: 1.03≤M2 / M1≤8.

32. The battery pack according to claim 27, wherein: When D1=D2, the viscosities η1 and η2 of the electrolytes of the first and second types of battery cells at -10°C satisfy 0<|η2-η1|<5 mPa·s.

33. The battery pack according to claim 27, wherein: 0.05mPa·s<|η2-η1|<4mPa·s.

34. The battery pack according to claim 27, wherein: 0.1mPa·s<|η2-η1|<3mPa·s.

35. The battery pack according to claim 27, wherein: The first type of battery cells accounts for 10% to 90% of the battery cells included in the area A.

36. The battery pack according to claim 27, characterized in that The first type of battery cells accounts for 20% to 80% of the battery cells included in the area A.

37. The battery pack according to claim 27, characterized in that: The second type of battery cells account for 20% to 100% of the battery cells included in the B area.

38. The battery pack according to claim 27, wherein: The second type of battery cells account for 40% to 100% of the battery cells included in the B area.

39. The battery pack according to claim 27, wherein: The second type of battery cells account for 60% to 100% of the battery cells included in the B area.

40. The battery pack according to claim 27, wherein: Different areas in the battery pack have different heat preservation capabilities, and at least two types of areas satisfy the temperature difference ΔT range of 0<ΔT<10°C.

41. The battery pack according to claim 27, wherein: Different areas in the battery pack have different heat preservation capabilities, and at least two types of areas satisfy the temperature difference ΔT range of 0.1<ΔT<9°C.

42. The battery pack according to claim 27, wherein: Different areas in the battery pack have different heat preservation capabilities, and at least two types of areas satisfy the temperature difference ΔT range of 0.2<ΔT<8°C.

43. The battery pack according to claim 27, wherein: The film forming resistances of the positive electrode sheets in the first type of battery cells and the second type of battery cells are Rct1 and Rct2 respectively, and the two satisfy the relationship: 1≤Rct1 / Rct2≤4.

44. The battery pack according to claim 27, wherein: The film forming resistances of the positive electrode sheets in the first type of battery cells and the second type of battery cells are Rct1 and Rct2 respectively, and the two satisfy the relationship: 1.01≤Rct1 / Rct2≤3.

5.

45. The battery pack according to claim 27, wherein: The film forming resistances of the positive electrode sheets in the first type of battery cells and the second type of battery cells are Rct1 and Rct2 respectively, and the two satisfy the relationship: 1.02≤Rct1 / Rct2≤3.

46. ​​The battery pack according to claim 27, characterized in that The freezing point of the electrolyte of all battery cells in the battery pack is ≤-20°C.

47. The battery pack according to claim 27, characterized in that The freezing point of the electrolyte of all battery cells in the battery pack is ≤-30°C.

48. The battery pack according to claim 27, wherein: The volume energy density D of the first type battery cell and the second type battery cell both meet 100Wh / L <D<1000Wh / L。 49. The battery pack according to claim 27, wherein: The volume energy density D of the first type battery cell and the second type battery cell both meet 200Wh / L <D<900Wh / L。 50. The battery pack according to claim 27, wherein: The volume energy density D of the first type battery cell and the second type battery cell both meet 300Wh / L <D<800Wh / L。 51. The battery pack according to claim 27, wherein: When D1≠D2, a third type of battery cell is also included, which is formed by physically mixing the main material systems of the first type of battery cell and the second type of battery cell. The third type of battery cell has a volume energy density D3, wherein D3 is between D1 and D2.

52. The battery pack according to claim 51, characterized in that The ionic conductivity σ of the electrolyte of the third type of battery cell at -10°C is σ3, and K3=D3 is defined 2 ×σ3, K3 is between K1 and K2; or The viscosity η of the electrolyte of the third type of battery cell at -10°C is η3, and M3 is defined as D3 / η3, where M3 is between M1 and M2.

53. The battery pack according to any one of claims 51-52, characterized in that: The first type of battery cells, the second type of battery cells, and the third type of battery cells are independently selected from lithium-ion batteries and sodium-ion batteries.

54. The battery pack according to any one of claims 51-52, characterized in that: The chemical systems of the positive electrode active materials of the first type of battery cells, the second type of battery cells, and the third type of battery cells are independently selected from lithium nickel cobalt manganese, lithium iron phosphate, lithium iron manganese phosphate, lithium iron vanadium phosphate, lithium vanadium phosphate, lithium cobalt oxide, lithium nickel oxide, lithium-rich manganese, lithium nickel cobalt aluminum, and lithium manganese oxide systems.

55. The battery pack according to any one of claims 51-52, characterized in that: The battery cell comprises a first type battery cells and b second type battery cells, wherein a and b are both natural numbers ≥1, and 0.01<a / b≤200.

56. The battery pack according to any one of claims 27 to 52, characterized in that: It comprises a first-category battery cells and b second-category battery cells, wherein a and b are both natural numbers ≥1, and 0.05≤a / b≤180.

57. The battery pack according to any one of claims 27 to 52, characterized in that: It comprises a first-category battery cells and b second-category battery cells, wherein a and b are both natural numbers ≥1, and 0.1≤a / b≤150.

58. An electrical device, characterized in that: A battery pack comprising any one of claims 1-26; or a battery pack comprising any one of claims 27-57.

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