Battery pack, preparation method and vehicle

Abstract

The invention provides a battery pack, a preparation method and a vehicle, and relates to the technical field of power batteries, the battery pack comprises a plurality of single cells connected in series, and the positive electrode total active material of each single cell comprises a lithium iron phosphate material and a nickel cobalt lithium manganate material. At least two single battery cells with different lithium iron phosphate material mass ratios exist in the plurality of single battery cells; the number of types of the single cells with different mass ratios of the lithium iron phosphate materials is determined based on the bearable single voltage abrupt change amplitude of the battery management system of the battery pack and influences the number of voltage steps formed at the voltage abrupt change position in the operation process of the battery pack; and the number of the types of the single cells is negatively correlated with the amplitude of single voltage jump bearable by the battery management system and is positively correlated with the number of voltage steps. According to the battery pack, the single voltage abrupt change amplitude caused by series connection of the single battery cells of different systems is reduced through structural improvement, and smooth processing of abrupt change voltage is realized, so that the requirement of the battery pack on a battery management system is reduced.

Description

Technical Field

[0001] This disclosure belongs to the field of power battery technology, specifically relating to a battery pack, a preparation method, and a vehicle. Background Technology

[0002] Battery packs based on a hybrid cathode of lithium nickel manganese cobalt oxide (NCM) and lithium iron phosphate (LFP) suffer from voltage spikes during discharge. While the voltage spike in a single cell is approximately 0.3V, for a battery pack composed of numerous cells connected in series, the voltage spike can reach tens to hundreds of volts. Taking a current mainstream 600V battery pack as an example, assuming a single cell operates at 3.6V, this pack would require approximately 167 cells connected in series, resulting in a voltage spike of around 50V. When such significant voltage fluctuations occur, the battery management system struggles to distinguish whether the spike is caused by a battery malfunction or the inherent electrochemical characteristics of the battery materials. Therefore, battery packs based on a hybrid cathode of NCM and LFP have not yet been commercially available.

[0003] One type of dual-system battery system in related technologies includes a battery module comprising multiple ternary lithium-ion cells, multiple lithium iron phosphate cells, and a busbar, with the ternary lithium-ion cells and lithium iron phosphate cells arranged alternately. This hybrid integration of ternary lithium-ion cells and lithium iron phosphate cells improves the battery system's low-temperature performance, rate capability, and safety. However, the battery pack requires two battery management systems, increasing the complexity of the battery management system and the manufacturing cost of the battery pack.

[0004] One hybrid cell solution in related technologies involves a single cell comprising both lithium iron phosphate (LFP) and ternary lithium (LiPF6) cells, separated by a separator, with the individual cells connected in series. This hybrid integration of LFP and LiPF6 improves the energy density and safety of the individual cells. However, even when using the same hybrid cell type in a battery pack, the cells still need to be connected in series, which can still cause significant voltage fluctuations.

[0005] In summary, the related technologies involve the mixed use of two different types of individual cells. However, whether it is a scheme that forms a hybrid cell by connecting individual cells in series and parallel and then connecting the same type of hybrid cells in series and parallel to form a battery pack, or a scheme that forms a battery pack by connecting individual cells of different systems in series and parallel and using cell layout to improve battery pack performance, none of them have solved the problem of excessive voltage fluctuation caused by connecting individual cells of different systems in series. Summary of the Invention

[0006] This disclosure provides a battery pack, a manufacturing method, and a vehicle, aiming to at least partially solve the technical problem of excessive voltage fluctuations in battery packs composed of individual cells from different systems connected in series in related technologies.

[0007] At least one embodiment of this disclosure provides a battery pack, including:

[0008] Multiple individual battery cells connected in series, wherein the total active material of the positive electrode of each of the multiple individual battery cells contains lithium iron phosphate (LFP) material and lithium nickel cobalt manganese oxide (NCM) material; Among the plurality of individual cells, there are at least two types of individual cells with different mass ratios of lithium iron phosphate material. Furthermore, the number of individual cell types with different mass ratios of lithium iron phosphate material is determined based on the single voltage change amplitude that the battery management system of the battery pack can withstand, and its influence on the number of voltage steps formed at the voltage change points during the operation of the battery pack. The number of individual cell types is negatively correlated with the single voltage change amplitude that the battery management system can withstand and positively correlated with the number of voltage steps.

[0009] The above solution offers the following technical advantages: It proposes a battery pack based on an NCM+LFP hybrid cathode, achieving smooth voltage fluctuations by combining individual cells with different proportions of NCM and LFP hybrid cathodes. This reduces the requirements for the battery management system and promotes the commercialization of battery packs based on NCM+LFP hybrid cathodes. Compared to battery packs composed of cells with a single LFP ratio, the above solution uses a series of cells with different LFP ratios to achieve smooth voltage fluctuations, thus reducing the technical requirements of the battery management system. Furthermore, the above solution does not require any upgrades to the battery management system and introduces new parameters to the battery pack, namely the number of voltage steps (also known as the number of battery steps), increasing system detection parameters and facilitating accurate identification of battery faults and inherent battery characteristics.

[0010] In the battery pack provided in at least one embodiment of this disclosure, the mass ratio of lithium iron phosphate material in the plurality of individual cells generally decreases along their series connection direction.

[0011] The above solution has the following technical effects: using individual cells with different LFP ratios, the reasonable layout between individual cells is conducive to improving the safety and durability of the battery pack.

[0012] In the battery pack provided in at least one embodiment of this disclosure, the mass ratio of lithium iron phosphate material in each of the plurality of individual cells is different.

[0013] The above scheme has the following technical effects: the number of different types of individual cells connected in series is large, the voltage fluctuation range of the battery pack is smaller, and the number of voltage steps is greater.

[0014] In a battery pack provided in at least one embodiment of this disclosure, each of the plurality of individual cells has the same individual cell capacity.

[0015] The above solution has the following technical effects: it ensures the performance of the battery pack and makes full use of the active materials of each individual cell during use.

[0016] In the battery pack provided in at least one embodiment of this disclosure, the molar ratio of nickel, cobalt and manganese in the lithium nickel cobalt manganese oxide material of each of the plurality of individual cells is the same and they are the same lithium nickel cobalt manganese oxide material.

[0017] The above solution has the following technical effects: different types of lithium nickel cobalt manganese oxide materials have significant performance differences, and using the same lithium nickel cobalt manganese oxide material can improve the battery pack life.

[0018] In at least one embodiment of the method provided in this disclosure, the number of different types of battery cells is m, where m is greater than 2, and the mass percentage of lithium iron phosphate material in the m types of battery cells is uniformly distributed within a set range.

[0019] The above solution has the following technical effects: it optimizes the overall temperature field distribution of the battery pack and reduces the risk of local overheating.

[0020] In at least one embodiment of the battery pack provided in this disclosure, each of the plurality of individual battery cells has the same internal resistance; and, The number of different types of individual cells is greater than or equal to the ratio of the total number of individual cells in the battery pack to the voltage step parameter, wherein the voltage step parameter is the ratio of the voltage fluctuation amplitude that the battery management system can withstand in a single voltage change to the voltage fluctuation amplitude of an individual cell.

[0021] The above solution has the following technical effects: ensuring that the actual voltage fluctuation of the battery pack does not exceed the single voltage fluctuation range that the battery management system can withstand.

[0022] At least one embodiment of this disclosure provides a method for preparing a battery pack, comprising: The number of different types of individual cells in the battery pack with different mass proportions of lithium iron phosphate materials is determined based on the single voltage surge amplitude that the battery management system can withstand. The number of different types of individual cells is negatively correlated with the single voltage surge amplitude that the battery management system can withstand, and the number of different types of individual cells is ≥2. Based on the aforementioned types and quantities of individual battery cells, battery cells with different mass ratios of lithium iron phosphate material are prepared, wherein the total active material of the positive electrode of each individual battery cell contains both lithium iron phosphate material and lithium nickel cobalt manganese oxide material; and, All the individual battery cells are connected in series according to a pre-set mass ratio trend of lithium iron phosphate material to assemble the required battery pack. The number of different types of individual battery cells affects the number of voltage steps formed at voltage abrupt changes during the operation of the battery pack, and the number of different types of individual battery cells is positively correlated with the number of voltage steps.

[0023] The above solution has the following technical effects: it makes the voltage change of each individual cell in the assembled battery pack more stable during the charging and discharging process, effectively reducing the probability that the battery management system will trigger the protection mechanism due to excessive voltage fluctuations, thereby improving the charging and discharging efficiency and cycle life of the battery pack.

[0024] The method for preparing a battery pack provided in at least one embodiment of this disclosure further includes: Obtain the actual discharge voltage curve of the battery pack; Extract the actual discharge characteristics contained in the actual discharge voltage curve, wherein the actual discharge characteristics include the number of voltage steps, the voltage step height, and the voltage step spacing; and, The battery pack is identified as qualified based on the comparison between the actual discharge characteristics and the baseline characteristics under the health state of the battery pack, and if the battery pack is unqualified, a corresponding process improvement plan is generated based on the current actual discharge characteristics.

[0025] The above solution has the following technical effects: it introduces a new parameter, namely the number of voltage steps, into the battery pack manufacturing process, thereby enriching the diagnostic parameters of the system and helping to improve the identification accuracy between battery faults and inherent battery characteristics.

[0026] At least one embodiment of this disclosure also provides a vehicle including a battery management system and a battery pack as provided in any embodiment of this disclosure.

[0027] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of this disclosure, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this disclosure. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the discharge curve of a battery pack composed of individual cells with the same LFP ratio connected in series. Figure 2 for Figure 1 A schematic diagram illustrating the voltage fluctuation amplitude of the battery pack. Figure 3 This is a schematic diagram of the structure of a battery pack provided in at least one embodiment of the present disclosure; Figure 4 A schematic diagram of a single cell with different LFP ratios in a battery pack provided in at least one embodiment of this disclosure; Figure 5 A schematic diagram of the discharge curve of a battery pack provided in at least one embodiment of this disclosure; Figure 6 A flowchart illustrating a method for preparing a battery pack according to at least one embodiment of this disclosure; Figure 7 A flowchart illustrating another method for preparing a battery pack according to at least one embodiment of this disclosure; Figure 8 A structural block diagram of a vehicle provided for at least one embodiment of this disclosure.

[0030] Figure label: 1- Individual battery cell; 10- Vehicle; 11- Battery pack; 12- Battery management system; NCM- Lithium nickel cobalt manganese oxide; LFP- Lithium iron phosphate; ΔU- Voltage drop amplitude of the battery pack; ΔU1- Voltage drop in the first stage; ΔU2- Voltage drop in the second stage; ΔU3- Voltage drop in the third stage; ΔU4- Voltage drop in the fourth stage; ΔU5- Voltage drop in the fifth stage; ΔU N - Voltage drop in stage N. Detailed Implementation

[0031] The present disclosure will now be described in further detail with reference to the accompanying drawings and embodiments. It should be particularly noted that the following embodiments are for illustrative purposes only and do not limit the scope of the disclosure. Similarly, the following embodiments are only some, not all, embodiments of the present disclosure, and all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure.

[0032] The terms "first," "second," and "third" used in the embodiments of this disclosure are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first," "second," and "third" may explicitly or implicitly include at least one of that feature.

[0033] In the description of this disclosure, "multiple" means at least two, such as two or three, unless otherwise expressly and specifically limited.

[0034] In this disclosure, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of this disclosure. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0035] The terms “comprising” and “having”, and any variations thereof, used in this disclosure are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or components inherent to such processes, methods, products, or devices.

[0036] The term "lithium nickel cobalt manganese oxide" in this disclosure, abbreviated as NCM, also known as ternary lithium, refers to a cathode material composed of three transition metal elements: nickel, cobalt, and manganese. Common examples include NCM111, NCM523, NCM622, NCM712, and NCM811.

[0037] The term "lithium iron phosphate" (LFP) used in this disclosure is a commonly used cathode material.

[0038] The term "lithium manganese iron phosphate" in this disclosure, abbreviated as LFMP, is also a commonly used cathode material.

[0039] In the embodiments of this disclosure, the term "lithium iron phosphate material mass percentage" (LFP percentage for short) refers to the proportion of lithium iron phosphate material in the total active material of the positive electrode of a single battery cell.

[0040] In the embodiments of this disclosure, the term "voltage plateau" refers to the voltage range corresponding to the stage during which the voltage of the battery pack remains relatively stable and changes little during the charging and discharging process.

[0041] The technical approach adopted in this disclosure will be explained below.

[0042] Figure 1 This is a schematic diagram of the discharge curve of a battery pack composed of individual cells with the same LFP ratio connected in series. Figure 2 for Figure 1A schematic diagram illustrating the voltage fluctuation range of the battery pack. The battery pack has N individual cells with a mixed NCM+LFP cathode connected in series, and each individual cell has the same mass percentage of lithium iron phosphate material. Assuming a mass ratio of LFP / (NCM+LFP) of 40%, a molar ratio of nickel, cobalt, and manganese of 5:2:3, an average voltage of 3.7V per cell, and a voltage fluctuation range of 0.3V per cell, as shown... Figure 2 As shown, the voltage fluctuation amplitude of the battery pack is ΔU = 0.3. N The unit of ΔU is V. N This indicates the number of individual battery cells contained in the battery pack.

[0043] The aforementioned battery pack exhibits a significant problem during discharge: voltage fluctuations in individual cells are linearized, resulting in drastic system-level voltage spikes. These drastic system-level voltage spikes can easily lead to false protection or failure of the battery management system.

[0044] Based on this, this disclosure proposes to connect individual cells (also known as single cells) of NCM+LFP mixed cathode with different LFP ratios in series to form a battery pack. This changes the battery pack composed of single cells with a single LFP ratio in the original related technology into a new battery pack composed of a series of single cells with mixed cathodes of different LFP ratios, thereby achieving smooth processing of sudden voltage changes and reducing the technical requirements of the battery pack on the battery management system.

[0045] This disclosure does not require any upgrades to the battery management system. By improving the structure, it gives the battery pack new parameters, namely several voltage steps, and increases the system detection parameters, which is beneficial for accurately distinguishing battery faults and inherent battery characteristics.

[0046] Since cells with a high LFP (Liquidity-to-Flattening) ratio have higher thermal stability than cells with a high NCM (Natural Mass-to-Metal) ratio, the arrangement of cells with different LFP ratios, while not affecting voltage smoothing, can impact the thermal safety of the battery pack. This disclosure further optimizes the layout between cells, for example, by arranging them sequentially from low to high (increasing trend) or from high to low (decreasing trend). Using cells with different LFP ratios and a reasonable layout between them helps improve the safety and durability of the battery pack.

[0047] This disclosure can be extended to applications within battery cells, namely, by manufacturing a series of individual battery cells (cell units) with different LFP ratios, and connecting each individual battery cell in series within the total battery cell, a hybrid battery cell is obtained, and thus a battery pack based on the hybrid battery cell is obtained.

[0048] In this disclosure, when multiple individual cells with different LFP ratios are connected in series, the capacity of the multiple individual cells connected in series should be consistent. The cycle stability of NCM material is not as good as that of LFP material. Therefore, the lifespan of individual cells with different LFP ratios is different. At present, it is not possible to technically compensate for the lifespan difference between LFP material and NCM material. In this disclosure, the lifespan of the battery pack is determined by the individual cell with the highest proportion of NCM material.

[0049] Based on this, this disclosure aims to ensure that the internal resistance of each individual cell is as uniform as possible. Internal resistance affects the heat generated during battery charging and discharging. The internal resistance of ternary lithium batteries is generally lower than that of lithium iron phosphate batteries. For the thermal management of the battery management system, it is important to ensure that the internal resistance is uniform as much as possible. Although there are no requirements for whether the battery pack uses a single NCM material, considering that the performance differences between different types of NCM may be significant, it is recommended to use a single NCM material within the same battery pack.

[0050] Figure 3 This is a schematic diagram of the structure of a battery pack provided in at least one embodiment of the present disclosure. Figure 3 As shown, the battery includes multiple individual cells 1 connected in series, and the total active material of the positive electrode of each of the multiple individual cells 1 contains lithium iron phosphate material and lithium nickel cobalt manganese oxide material.

[0051] Among them, there are at least two types of cells with different mass ratios of lithium iron phosphate material. The number of cell types with different mass ratios of lithium iron phosphate material is determined based on the single voltage change amplitude that the battery management system of the battery pack can withstand. The number of cell types affects the number of voltage steps formed at voltage change points during battery pack operation. The number of cell types is negatively correlated with the single voltage change amplitude that the battery management system can withstand, and positively correlated with the number of voltage steps.

[0052] In the market deployment plan, the greater the voltage fluctuation range that the battery management system can withstand in a single instance, the fewer types of individual battery cells are required, and the fewer voltage steps are formed at voltage fluctuation points during battery pack operation. Conversely, the smaller the voltage fluctuation range that the battery management system can withstand in a single instance, the more types of individual battery cells are required, and the more voltage steps are formed at voltage fluctuation points during battery pack operation.

[0053] The above-described scheme does not limit the arrangement of individual cells with different mass percentages of lithium iron phosphate material in the battery pack. In practical applications, a suitable arrangement can be selected based on the specific design requirements of the battery pack, such as space utilization, heat dissipation efficiency, and the difficulty of voltage equalization control. For example, individual cells with the same mass percentage of lithium iron phosphate material can be grouped together in the same area to facilitate later maintenance and voltage monitoring; alternatively, an alternating arrangement can be used to evenly distribute individual cells with different mass percentages, thereby optimizing the overall temperature field distribution of the battery pack and reducing the risk of local overheating.

[0054] In the above-mentioned scheme, this disclosure does not limit the composition of the total active material of the positive electrode, excluding lithium iron phosphate and lithium nickel cobalt manganese oxide materials. In practical applications, the supplementary components can be flexibly adjusted according to the specific performance goals of the battery pack, such as improving energy density, extending cycle life, and controlling manufacturing costs. For example, a small amount of conductive carbon black or carbon nanotubes can be added to enhance the electronic conductivity of the positive electrode and improve the rate charge and discharge performance; a very small amount of lithium aluminate can also be introduced as a structural stabilizer to suppress the volume expansion of the active material during cycling and improve the cycle stability of the battery; by increasing the proportion of lithium manganese iron phosphate, its higher voltage platform characteristics can be utilized to effectively improve the single-cell energy density of the battery without significantly sacrificing cycle stability, thereby helping the battery pack achieve a higher overall energy density index to meet the vehicle's demand for long driving range. In addition, lithium manganese iron phosphate can also be introduced in appropriate amounts according to the low-temperature performance optimization requirements in specific scenarios, taking advantage of its relatively better electrochemical activity in low-temperature environments to improve the battery's discharge capacity retention and rate performance in cold regions.

[0055] Some embodiments of this disclosure also provide a method for preparing the battery pack described above and a vehicle.

[0056] The method provided in at least one embodiment of this disclosure is applicable to any existing application scenario that requires preventing excessive voltage fluctuations in the battery pack at the end of discharge. For example, in the final stage of discharge of a new energy vehicle when the remaining charge is below 10%, excessive voltage fluctuations in the battery pack can easily lead to a sudden drop in vehicle power output or a jump in the range display on the dashboard, affecting driving continuity and safety. The method of this disclosure can effectively smooth the voltage curve at the end of discharge by dynamically adjusting the balancing strategy of individual cells in the battery pack, combined with the optimized electrochemical performance of the supplementary components in the cathode material, ensuring that the vehicle can still output power stably in a low-charge state until it safely reaches the charging station. Similarly, in the final stage of grid-connected discharge of an energy storage power station, excessive voltage fluctuations may interfere with the grid frequency stability. This method can monitor the voltage change trend of each individual cell in real time, trigger an active balancing mechanism in advance, suppress the voltage fluctuation amplitude, and ensure stable interaction between the energy storage system and the grid.

[0057] Figure 4This is a schematic diagram of a single battery cell with different LFP ratios in the positive electrode of a battery pack provided in at least one embodiment of this disclosure. Figure 1 Based on the existing solution, in order to improve the safety and durability of the battery pack, such as... Figure 4 As shown, the lithium iron phosphate (LFP) material content of multiple individual cells 1 decreases along their series connection direction, with LFP contents of 40%, 30%, 20%, and 10% from left to right. The cells with lower LFP contents contain NCM materials with high voltage plateau characteristics. This gradient cathode ratio design along the series connection direction allows the voltage change trends of each individual cell to complement each other during discharge. When the battery pack enters the final stage of discharge, the left-hand cell with a higher LFP content maintains basic output due to its stable voltage plateau, while the right-hand cell with a lower LFP content utilizes the electrochemical characteristics of ternary materials to smooth the overall voltage drop rate, effectively avoiding sudden voltage changes in the entire battery pack caused by sudden voltage drops in some cells. This gradient design at the material level, in conjunction with the dynamic balancing strategy mentioned earlier, optimizes the electrochemical performance of the individual cells themselves, providing dual protection for the stable operation of the battery pack at low charge levels. It also lays the theoretical foundation for the precise ratio of cathode materials in subsequent preparation methods.

[0058] Figure 5 This is a schematic diagram of the discharge curve of a battery pack provided in at least one embodiment of this disclosure. Figure 5 As shown, by connecting multiple individual cells with different LFP ratios in series, the voltage fluctuations in the battery pack are broken down into several short voltage plateaus. The number of voltage plateaus is determined by the LFP ratio, and the height of each voltage plateau is 0.3V. According to the schematic diagram of the discharge process, this disclosure, through the connection of individual cells with different LFP ratios in series, compared to… Figure 1 and Figure 2 The voltage curve of a battery pack composed of individual cells with the same LFP ratio connected in series becomes smoother.

[0059] In some embodiments, Figure 3 and Figure 4Based on this scheme, the mass ratio of lithium iron phosphate (LFP) material in each of the multiple individual cells 1 is different. The more types of individual cells connected in series, the smaller the voltage fluctuation amplitude and the more voltage steps in the resulting battery pack. When the mass ratio of LFP material in multiple individual cells exhibits a monotonically changing trend, this differentiated ratio configuration allows the individual cells to complement each other during discharge. When the voltage of a cell with a low LFP ratio begins to drop, the cell with a high LFP ratio can still maintain a stable voltage output, thus ensuring a smooth voltage transition for the entire battery pack. This improves discharge stability at low charge levels and provides clear parameter basis for precise control of the material ratio of different cells in subsequent manufacturing methods. Furthermore, this gradient ratio design can be adapted to the battery pack's thermal management system. By adjusting the material ratio of each individual cell, heat distribution can be optimized, reducing the risk of localized overheating and further ensuring the overall safety performance and cycle life of the battery pack.

[0060] In some embodiments, Figure 3 and Figure 4 Based on this design, each of the multiple individual cells 1 has the same capacity. This combination of identical capacity but differentiated material proportions ensures that each individual cell works collaboratively based on the same capacity benchmark during charging and discharging, avoiding inconsistent charging and discharging rhythms caused by capacity differences. Furthermore, the gradient design of material proportions maintains the smooth voltage output transition effect described earlier. In battery packs connected in series, individual cells of the same capacity ensure a balanced total capacity within each string, reducing imbalances within the pack caused by differences in capacity decay rates during cycling. Simultaneously, this design exhibits stronger compatibility with the battery management system. The heat dissipation characteristics of individual cells of the same capacity are more predictable, allowing the battery management system to pre-set heat dissipation strategies based on the gradient distribution of material proportions, effectively suppressing localized overheating and further ensuring the safety performance and cycle life of the battery pack.

[0061] If the capacities of multiple individual cells connected in series are inconsistent, during the charging process of the battery pack, some individual cells will be fully charged while others are not, or some individual cells will be overcharged while others are not. This inconsistency in capacity will affect the performance of the battery pack.

[0062] Furthermore, if the capacity of a single battery cell is significantly higher than that of other cells, its utilization rate of active materials during charging and discharging is lower. In this case, it is necessary to calculate the actual utilization rate of LFP material participating in the electrochemical reaction in that single cell. Under extreme conditions, i.e., when the LFP material utilization rate reaches 100% or approaches 0%, the voltage plateau characteristics of that single cell will not affect the identification of the voltage steps of other single cells. However, in non-extreme cases, the actual utilization rate of LFP material in that single cell may be similar to that in other cells, resulting in overlapping voltage plateau characteristics that are difficult to identify.

[0063] Cells with different LFP (Lithium Iron Phosphate) ratios exhibit varying cycle aging rates. Cells with a higher LFP ratio age more slowly and have a longer lifespan, while cells with a higher NCM (Neural Cell Molding Compound) ratio have a shorter lifespan. Theoretically, the plateau voltage of LFP materials is 3.2V, while that of NCM materials is 3.5V. During cycle aging, the LFP plateau voltage remains at 3.2V, while the 3.5V plateau of ternary materials becomes less pronounced due to structural collapse. This is reflected in composite cathode cells, where the voltage step becomes increasingly less significant. Because the step becomes less significant, the impact of voltage abrupt changes at the step on the battery management system decreases.

[0064] Therefore, ensuring that the cell capacity of multiple cells connected in series is consistent is the core prerequisite for ensuring that the aging rate of each cell in the battery pack is synchronized during long-term cycle use. This can effectively prevent some cells from becoming the bottleneck of the entire battery pack due to excessively rapid capacity decay, while maintaining the independence of the voltage platform characteristics of each cell and ensuring the reliable operation of the battery pack in vehicle application scenarios.

[0065] In some embodiments, Figure 3 and Figure 4 Based on this scheme, to ensure consistent heat generation power of each individual cell under the same current, the internal resistance of each of the multiple individual cells 1 is the same. This combination of consistent internal resistance and differentiated material proportions ensures uniform current distribution during charging and discharging of each individual cell, and also maintains the smooth voltage output transition effect described earlier through the gradient design of material proportions. Individual cells with the same internal resistance exhibit more consistent heat generation power under the same current.

[0066] In some embodiments, Figure 3 and Figure 4Based on the proposed solution, to improve the battery pack's lifespan, each of the multiple individual cells 1 uses the same molar ratio of nickel, cobalt, and manganese in its lithium nickel cobalt manganese oxide material, and all cells are made of the same lithium nickel cobalt manganese oxide material. This material consistency design significantly improves the matching of electrochemical performance among the multiple individual cells, making the voltage response, internal resistance changes, and capacity decay rates of each cell more synchronized during charging and discharging, effectively reducing the complexity and energy consumption of intra-pack equalization control. Simultaneously, the unified selection of lithium nickel cobalt manganese oxide material helps simplify the battery pack's manufacturing process, reduces the risk of quality fluctuations caused by batch differences in different materials, and improves the efficiency and stability of large-scale production. Furthermore, in vehicle applications, a group of individual cells with consistent materials can better adapt to the complex and varied charging and discharging conditions during vehicle operation, maintaining stable battery pack output performance and extending its effective service life throughout the vehicle's lifespan.

[0067] In some embodiments, Figure 3 and Figure 4 Based on the proposed design, to improve the thermal stability and safety performance of the battery pack, the number of different types of individual cells is m, where m is greater than 2. Furthermore, the mass percentage of lithium iron phosphate (LFP) material in each of the m types of individual cells is uniformly distributed within a set range. This uniform distribution design effectively optimizes the balance between the energy density and power density of the battery pack. Individual cells with a high LFP percentage possess higher energy storage capacity, supporting the continuous discharge requirements of the battery pack in long-range driving scenarios; while individual cells with a low LFP percentage exhibit superior power output characteristics, enabling rapid response to instantaneous high-power conditions such as vehicle acceleration and hill climbing. Simultaneously, the uniform distribution of material percentages in each individual cell helps to disperse the current load during charging and discharging, reducing overheating caused by overcurrent in localized cells and improving the thermal stability and safety performance of the battery pack.

[0068] As an exemplary implementation, assume the total voltage of the battery pack V If the rated voltage of each individual battery cell is 3.5V, then this battery pack needs to connect N individual battery cells in series. The formula for calculating N is as follows: N=[ V / 3.5] In the formula, [ ] represents the rounding up operation.

[0069] Given that the voltage fluctuation of a single battery cell is 0.3V, assume that the battery management system can withstand a single voltage fluctuation (i.e., the voltage fluctuation that the battery management system can withstand) is: U The mass percentage of lithium iron phosphate material needs to have m variations, and the formula for calculating m is as follows: m=[ N / [ U / 0.3]] Assuming the mixing ratio of lithium iron phosphate is within the range of [a, b], and following the principle of uniform voltage variation, a~b needs to be divided into m equal parts, meaning the specific mass percentage of lithium iron phosphate material is... a , a +( b - a ) / (m-1), a +2( b - a ) / (m-1), a +3( b - a ) / (m-1), ... b .

[0070] In some embodiments, Figure 3 and Figure 4 Based on this solution, to accurately determine the number of different cell types and ensure smooth handling of voltage fluctuations, the number of different cell types is greater than or equal to the ratio of the total number of cells in the battery pack to the voltage step parameter. The voltage step parameter is the ratio of the voltage fluctuation amplitude that the battery management system can withstand in a single event to the voltage fluctuation amplitude of a single cell. This design ensures that the battery management system remains within a safe operating range when dealing with cell voltage fluctuations. When some cells experience voltage decay due to charge-discharge cycles, the battery management system can flexibly allocate different types of cells to participate in energy output or storage based on the voltage step parameter. This avoids false triggering of system protection mechanisms due to excessive voltage fluctuations, and also reduces the accumulation of voltage differences between cells, slowing down the overall performance degradation rate of the battery pack.

[0071] To demonstrate the effectiveness of the disclosed scheme, the following tests and control group experiments were conducted.

[0072] Test Group 1: Following the liquid-state pouch battery manufacturing process, a series of individual cells with different lithium iron phosphate (LFP) material mass percentages were prepared, namely 1%, 2%, 3%, ..., 60%. The capacity of each individual cell was uniformly 50 Ah. The battery pack consisted of 60 individual cells connected in series. The individual cells with different LFP percentages were arranged from low to high, i.e., 1% (1 cell) → 2% (1 cell) → 3% (1 cell) → ..., 60% (1 cell). Following the pouch battery pack manufacturing process, the individual cells were connected in series to form the battery pack.

[0073] Test Group 2: Following the liquid-state pouch battery manufacturing process, a series of individual cells with different mass percentages of lithium iron phosphate (LFP) were prepared, with LFP material mass percentages of 1%, 3%, 5%, ..., 59%. The capacity of each individual cell was uniformly 50 Ah. The battery pack consisted of 60 individual cells connected in series. The individual cells with different LFP material mass percentages were arranged from low to high, i.e., 1% (2 cells) → 3% (2 cells) → 5% (2 cells) → ..., 59% (2 cells). Following the pouch battery pack assembly process, the individual cells were connected in series to form the battery pack.

[0074] Test Group 3: Following the liquid-state pouch battery manufacturing process, a series of individual cells with different mass percentages of lithium iron phosphate (LFP) were prepared, with LFP material mass percentages of 5%, 10%, 15%, ..., 60%. The capacity of each individual cell was uniformly 50 Ah. The battery pack consisted of 60 individual cells connected in series. The individual cells with different LFP material mass percentages were arranged from low to high: 1% (12 cells) → 3% (12 cells) → 5% (12 cells) → ..., 59% (12 cells). Following the pouch battery pack assembly process, the individual cells were connected in series to form the battery pack.

[0075] Control group: Following the liquid-state pouch battery manufacturing process, individual cells with the same mass percentage of lithium iron phosphate (LFP) material were prepared, with LFP material accounting for 30% of the mass. The capacity of each individual cell was uniformly 50 Ah. The battery pack consisted of 60 individual cells connected in series. Following the pouch battery pack manufacturing process, the individual cells were connected in series to form the battery pack.

[0076] Implementation Results: Under 25℃ conditions, using 1C charge-discharge, with a cutoff voltage of 3.0~4.3V, the voltage fluctuation amplitude (also known as maximum voltage drop) and the number of voltage steps of the test group battery pack and the control group battery pack were measured. The test results are shown in Table 1.

[0077] Table 1

[0078] As shown in Table 1, the more types of individual battery cells connected in series, the smaller the actual voltage fluctuation amplitude and the more voltage steps in the resulting battery pack. This indicates that increasing the variety of individual battery cells connected in series can effectively optimize the voltage output characteristics of the battery pack, reduce the risk of voltage fluctuations during charging and discharging, and improve the operational stability and reliability of the battery pack. For vehicle applications, this optimization helps reduce power output fluctuations, improves the driving experience, and may extend the overall lifespan of the battery system. Furthermore, this principle provides an important reference for battery pack design and fabrication: while meeting basic performance requirements such as battery pack capacity, the electrochemical performance of the battery pack can be further improved by rationally combining various types of individual battery cells connected in series.

[0079] Figure 6 A flowchart illustrating a method for manufacturing a battery pack according to at least one embodiment of this disclosure. Figure 6 As shown, the method includes the following steps S10-S30.

[0080] Step S10: Determine the number of cell types with different mass ratios of lithium iron phosphate materials in the battery pack based on the single voltage fluctuation amplitude that the battery management system can withstand. The number of cell types is negatively correlated with the single voltage fluctuation amplitude that the battery management system can withstand, and the number of cell types is ≥2.

[0081] Step S20: Prepare single cells with different mass ratios of lithium iron phosphate material based on the type and quantity of single cells. Each single cell contains lithium iron phosphate material and lithium nickel cobalt manganese oxide material in its positive electrode total active material.

[0082] Step S30: Connect all individual cells in series according to the pre-set mass ratio trend of lithium iron phosphate material to assemble the required battery pack. The number of different types of individual cells affects the number of voltage steps formed at voltage abrupt changes during battery pack operation, and the number of different types of individual cells is positively correlated with the number of voltage steps.

[0083] It should be noted that the trend of the mass proportion of lithium iron phosphate materials can be an increasing trend, a decreasing trend, or other trends.

[0084] The coordinated implementation of steps S10-S30 ensures more stable voltage changes in each individual cell during charging and discharging of the assembled battery pack. This effectively reduces the probability of the battery management system triggering protection mechanisms due to excessive voltage fluctuations, thereby improving the charging and discharging efficiency and cycle life of the battery pack. Step S10 precisely determines the type and quantity of cells based on the core parameters of the battery management system, ensuring that the system's adjustment capability matches the cell configuration. The hybrid cathode material cells prepared in step S20 fully leverage the high safety of lithium iron phosphate and the high energy density of lithium nickel cobalt manganese oxide. Step S30, by connecting the cells in series according to a preset proportion trend, makes the overall voltage output curve of the battery pack more closely match the smooth characteristics of vehicle power requirements, reducing performance losses caused by voltage step changes. This preparation method has a clear process and strong operability, enabling large-scale production, reducing the manufacturing cost of the battery pack, and allowing the prepared battery pack to be widely used in various electric vehicles, providing strong support for the vehicle's range and safety performance.

[0085] In some embodiments, Figure 6Based on the proposed scheme, step S20 employs a liquid-state pouch battery fabrication process to prepare a series of single-cell batteries with different mass ratios of lithium iron phosphate (LFP) material. To ensure that single-cell batteries with different LFP material mass ratios have the same capacity, internal resistance, and cycle stability, the fabrication process of the single-cell batteries must be controlled. Key process parameters that need to be adjusted for different LFP material mass ratios include solid content, coating thickness, compaction density, number of winding layers, and electrolyte injection volume.

[0086] Figure 7 A flowchart illustrating another method for preparing a battery pack according to at least one embodiment of this disclosure. Figure 6 Based on the existing plan, in order to further optimize the manufacturing process and ensure the quality of the manufactured battery pack, such as... Figure 7 As shown, the method further includes the following steps S40-S60.

[0087] Step S40: Obtain the actual discharge voltage curve of the battery pack.

[0088] Step S50: Extract the actual discharge features contained in the actual discharge voltage curve, wherein the actual discharge features include the number of voltage steps, the voltage step height, and the voltage step spacing.

[0089] Step S60: Based on the comparison between the actual discharge characteristics and the baseline characteristics under the health state of the battery pack, identify whether the battery pack is qualified, and if the battery pack is unqualified, generate a corresponding process improvement plan based on the current actual discharge characteristics.

[0090] Steps S40-S60 can be placed after step S30. The process improvement plan includes adjusting key process parameters such as solid content, coating thickness, compaction density, number of winding layers, and electrolyte injection volume. Steps S40-S60 enable dynamic quality feedback and closed-loop process optimization in the manufacturing process. On one hand, by extracting features from the actual discharge voltage curve and comparing it with a benchmark, it is possible to accurately determine whether the battery pack meets the performance indicators under healthy conditions, preventing unqualified products from entering the next stage. On the other hand, the process improvement plan generated for unqualified battery packs can directly guide parameter adjustments in the manufacturing process. For example, when the voltage step spacing deviates from the benchmark value, priority should be given to checking whether the number of winding layers meets the design requirements or whether the compaction density is uniform, thereby quickly correcting the process deviation. This method of deeply combining performance testing and process optimization not only improves the quality control efficiency of battery pack manufacturing but also continuously accumulates correlation data between process parameters and discharge characteristics, providing a solid basis for the intelligent upgrading of subsequent manufacturing processes, ultimately ensuring the performance stability and reliability of the battery pack.

[0091] Those skilled in the art will understand that all or part of the processes in the above method embodiments can be implemented by a computer program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, it can include the processes described in the above method embodiments. The storage medium can be a magnetic disk, optical disk, read-only memory (ROM), random access memory (RAM), flash memory (FM), hard disk drive (HDD), or solid-state drive (SSD), etc.; the storage medium can also include combinations of the above types of memory.

[0092] Figure 8 A structural block diagram of a vehicle provided for at least one embodiment of this disclosure. (See diagram below.) Figure 8 As shown, vehicle 10 includes battery management system 12 and battery pack 11 as described in the above embodiment.

[0093] Although embodiments of the present disclosure have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the present disclosure, and such modifications and variations all fall within the scope defined by the appended claims.

[0094] Although embodiments of the present disclosure have been shown and described above, it is to be understood that the above embodiments are exemplary and should not be construed as limiting the present disclosure. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present disclosure.

Claims

1. A battery pack, characterized in that, include: Multiple individual battery cells (1) connected in series, wherein the total active material of the positive electrode of each of the multiple individual battery cells (1) contains lithium iron phosphate material and lithium nickel cobalt manganese oxide material; Among them, at least two types of single cells (1) have different mass ratios of lithium iron phosphate material. The number of single cell types with different mass ratios of lithium iron phosphate material is determined based on the single voltage change amplitude that the battery management system of the battery pack can withstand, and its influence is on the number of voltage steps formed at the voltage change point of the battery pack during operation. The number of single cell types is negatively correlated with the single voltage change amplitude that the battery management system can withstand and positively correlated with the number of voltage steps.

2. The battery pack according to claim 1, characterized in that, The mass ratio of lithium iron phosphate material in the multiple individual cells (1) generally decreases along their series connection direction.

3. The battery pack according to claim 1 or 2, characterized in that, The mass ratio of lithium iron phosphate material in each of the multiple individual cells (1) is different.

4. The battery pack according to claim 1 or 2, characterized in that, Each of the plurality of individual cells (1) has the same cell capacity.

5. The battery pack according to claim 1 or 2, characterized in that, The nickel, cobalt and manganese elements in the lithium nickel cobalt manganese oxide material of each of the multiple individual cells (1) are in the same molar ratio and are the same lithium nickel cobalt manganese oxide material.

6. The battery pack according to claim 1 or 2, characterized in that, The number of different types of battery cells is m, where m is greater than 2, and the mass percentage of lithium iron phosphate material in each of the m types of battery cells (1) is uniformly distributed within a set range.

7. The battery pack according to claim 1 or 2, characterized in that, Each of the plurality of individual battery cells (1) has the same internal resistance; and, The number of different types of individual cells is greater than or equal to the ratio of the total number of individual cells in the battery pack to the voltage step parameter, wherein the voltage step parameter is the ratio of the voltage fluctuation amplitude that the battery management system can withstand in a single voltage change to the voltage fluctuation amplitude of the individual cell (1).

8. A method for preparing a battery pack, characterized in that, include: The number of different types of individual cells in the battery pack with different mass proportions of lithium iron phosphate materials is determined based on the single voltage surge amplitude that the battery management system can withstand. The number of different types of individual cells is negatively correlated with the single voltage surge amplitude that the battery management system can withstand, and the number of different types of individual cells is ≥2. Based on the types and quantities of the aforementioned individual cells, individual cells (1) with different mass ratios of lithium iron phosphate material are prepared, wherein the total active material of the positive electrode of each individual cell (1) includes both lithium iron phosphate material and lithium nickel cobalt manganese oxide material; and, All the individual cells (1) are connected in series according to a pre-set trend of lithium iron phosphate material mass ratio to assemble the required battery pack. The number of individual cell types affects the number of voltage steps formed at voltage change points during the operation of the battery pack, and the number of individual cell types is positively correlated with the number of voltage steps.

9. The method for preparing a battery pack according to claim 8, characterized in that, Also includes: Obtain the actual discharge voltage curve of the battery pack; Extract the actual discharge features contained in the actual discharge voltage curve, wherein the actual discharge features include the number of voltage steps, the voltage step height, and the voltage step spacing; as well as, The battery pack is identified as qualified based on the comparison between the actual discharge characteristics and the baseline characteristics under the health state of the battery pack, and if the battery pack is unqualified, a corresponding process improvement plan is generated based on the current actual discharge characteristics.

10. A vehicle, characterized in that, Includes a battery management system and a battery pack as described in any one of claims 1 to 7.

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