A test method for measuring current distribution in mixed cathode lithium batteries and its application

Abstract

The present invention discloses a test method and application for measuring current distribution in a hybrid cathode lithium battery, relating to the technical field of lithium batteries. The test method comprises: independently preparing independent battery cells according to the surface loading of each material of the lithium battery hybrid cathode in the hybrid cathode, each independent battery cell being provided with a surface electrode on the negative electrode sheet for forming a pure electronic circuit with the negative electrode tab; measuring the equivalent resistance of each independent battery cell, collecting the voltage of each independent battery cell under preset power-on conditions after connecting all the independent battery cells in parallel, and calculating the current passing through each soft pack based on the equivalent resistance, which is the current corresponding to each material in the hybrid cathode under preset power-on conditions. The test method of the present invention designs more reasonable working conditions for evaluating the charge and discharge performance of the battery cells of the hybrid system, ensuring the rate performance and long cycle performance of the hybrid system, and can also be applied to other batteries such as sodium batteries.

Description

Technical Field

[0001] This invention relates to lithium batteries, specifically to a test method and application for measuring current distribution in a hybrid cathode lithium battery. Background Technology

[0002] Lithium-ion batteries are widely used in electric vehicles, requiring power batteries to excel in energy density, cycle life, fast charging, low-temperature performance, safety, and cost. The goal is to develop power batteries that are superior in all six dimensions. Currently, the main cathode materials on the power battery market are ternary lithium-ion batteries and lithium iron phosphate (LFP) cathodes. Ternary lithium-ion batteries are characterized by high capacity, high operating voltage platform, good low-temperature performance, and good rate performance, while LFP cathodes are characterized by good safety and long cycle life. To ensure the advantages of low-temperature performance, energy density, and safety in power batteries, a hybrid cathode combining LFP and ternary lithium-ion materials is an important development direction for power batteries. When the two materials are mixed, the charging voltage range of LFP is 2-3.8V, while that of ternary lithium-ion batteries is 2.5-4.2V. Therefore, the voltage range of the hybrid material generally covers the entire voltage range. However, at high voltages, the capacity may primarily be contributed by the ternary lithium-ion material, and the actual rate capability corresponding to the ternary lithium-ion material may be higher. Therefore, the charging current of the hybrid cathode system at different voltage levels needs to be considered from multiple perspectives. Currently, the capacity performance of hybrid cathode materials during charge and discharge is strongly correlated with the charging and discharging conditions. Therefore, it is necessary to measure the current distribution in the hybrid cathode material to better determine the charging and discharging conditions for the hybrid cathode.

[0003] Existing technology includes a method and apparatus for measuring the current distribution of a large-area planar solid oxide fuel cell, comprising: fabricating the cell under test into multiple smaller cells, testing the charge or current density of each of the smaller cells, and determining the current distribution of the cell under test based on the measured current and / or current density of the smaller cells. This approach primarily utilizes a parallel design to divide the current distribution of a large surface into several smaller surfaces for measurement, but it does not address how to effectively connect the cathodes of two mixed materials in a hybrid system in parallel. Furthermore, this patent only describes measuring the current magnitude of multiple paths separately, but does not specify how the measurements are performed.

[0004] Prior art discloses a battery system for a secondary battery including a mixed cathode material, and an apparatus and method for managing a secondary battery with a mixed cathode material. This approach primarily describes voltage relaxation that occurs within a specific voltage range of the mixed cathode material, but does not allow for real-time measurement of the relaxation current between the two materials.

[0005] Therefore, it is necessary to develop an accurate test method to measure the current distribution of each material in a hybrid cathode lithium battery, so as to design more reasonable operating conditions for evaluating the charging and discharging performance of the hybrid system cells and ensure the rate performance and long-cycle performance of the hybrid system. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the above-mentioned background technology and provide a test method for accurately measuring the current distribution in hybrid cathode lithium batteries, so as to design more reasonable operating conditions for evaluating the charging and discharging performance of hybrid system cells and ensure the rate performance and long-cycle performance of hybrid systems.

[0007] The technical solution of this invention is: a test method for measuring current distribution in a hybrid cathode lithium battery, comprising:

[0008] In a lithium battery hybrid cathode, each material is used to prepare an independent cell according to its areal loading. Each independent cell has a surface electrode on its negative electrode to form a pure electronic circuit with the negative electrode tab.

[0009] Measure the equivalent resistance of each individual cell;

[0010] After connecting all the independent cells in parallel, the voltage of each independent cell is collected under preset energizing conditions. The current passing through each independent cell is calculated based on the equivalent resistance, which is the current corresponding to each material in the hybrid cathode under preset energizing conditions.

[0011] This invention employs a hybrid system equivalent design approach, dividing the hybrid battery into multiple individual cells and then designing a parallel structure. Within each individual cell, a surface electrode is used to treat the entire cell as a pure ohmic resistor (a resistor whose resistance does not change with charging SOC and charging current). This allows for the testing of the real-time reaction currents of different materials based on the surface electrode and the negative electrode voltage side, enabling the design of more reasonable operating conditions for evaluating the charging and discharging performance of the hybrid battery cells, thus ensuring the rate performance and long-cycle performance of the hybrid system.

[0012] Preferably, the surface load of each material is calculated as follows: if the overall surface load of the hybrid cathode is A, and the mass percentage of each material in the hybrid cathode is a, then the surface load of each material = A·a.

[0013] Preferably, the independent battery cell is a rigid-shell battery cell or a pouch battery cell. The surface electrode material of each independent battery cell is the same as the negative electrode current collector material, and each surface electrode is elongated and extends beyond the encapsulation material of the independent battery cell. Pouch batteries are preferred, while rigid-shell batteries are easier to manufacture. The consistency of the surface electrode material and the negative electrode current collector material facilitates smooth current transmission in the circuit. Further, the surface electrode material is a porous copper foil or copper mesh. In this invention, a porous copper foil or copper mesh structure is used on the graphite negative electrode surface, constructing a purely electronic circuit between the surface electrode and the negative electrode tab. This circuit possesses ohmic resistance properties, therefore, under different currents, the voltage between the surface electrode and the negative electrode side satisfies Ohm's law linearly.

[0014] Preferably, the equivalent resistance of each individual cell is measured, specifically including: collecting the voltage of each individual cell under multiple current conditions, and obtaining the equivalent resistance value based on the correspondence between the collected voltage and current. Further, the voltage of each individual cell is collected under multiple current conditions, and a linear fit is performed between the collected voltage and current; the equivalent resistance value is obtained based on the slope of the linear fit. The equivalent resistance test should ideally be conducted at a rate within 2C, typically using 5 rate points, which can be selected from commonly used rate points (e.g., 0.1C, 0.2C, 0.33C, 0.5C, 1C, 1.5C, 2C, etc.).

[0015] Preferably, the voltage of each individual cell is obtained by collecting the voltage between the surface electrode and the negative electrode tab.

[0016] Preferably, the mixed cathode material is divided into two parts: lithium iron phosphate material and ternary material, or original cathode material and lithium replenishment agent. When the mixed cathode material is divided into lithium iron phosphate material and ternary material, the charging strategy with a preset rate can be determined using the testing method of the present invention; when the mixed cathode material is divided into original cathode material and lithium replenishment agent, the optimal formation step for lithium replenishment agent decomposition can be determined using the testing method of the present invention.

[0017] Preferably, the lithium replenishing agent is lithium ferrite or lithium nickelate, and the lithium replenishing agent accounts for ≤5% of the mass percentage of the mixed cathode material.

[0018] The present invention also provides an application of the test method for measuring current distribution in a hybrid cathode lithium battery as shown above, for determining the optimal charging strategy at a preset rate when a hybrid cathode is formed by lithium iron phosphate material and ternary material; or for determining the optimal formation step for decomposing the lithium replenishing agent when a lithium replenishing agent is added to the original cathode material.

[0019] Ternary materials are characterized by high capacity, high operating voltage platform, good low-temperature performance, and good rate performance, while lithium iron phosphate materials are characterized by good safety performance and long cycle performance. In order to ensure the advantages of low-temperature performance, energy density, and safety of power batteries, cathodes composed of lithium iron phosphate cathodes and ternary materials are also an important development direction for power batteries. Since the current contributed by lithium iron phosphate and ternary materials is constantly changing at preset rates and different voltage ranges, it is necessary to use different currents for charging and discharging at different voltage ranges to ensure that lithium iron phosphate and ternary materials maintain a preset charging rate throughout the entire charging and discharging process.

[0020] To ensure sufficient decomposition of lithium additives, a relatively low charging rate is generally required, typically 0.05C or lower. However, the formation current of lithium iron phosphate can reach 0.2C. Therefore, the charging and discharging strategies need to be adjusted within different voltage ranges to maximize the decomposition capacity.

[0021] Preferably, determining the optimal charging strategy at a preset charging rate specifically includes:

[0022] Under the condition of charging at a preset rate, the current obtained by each material in the mixed cathode within the charging voltage range is measured, the current abrupt change point of each material in the mixed cathode is determined, and the entire charging voltage range is divided into multiple segments using the charging voltage corresponding to the abrupt change point.

[0023] Adjust the charging current for each voltage range, measure the current distributed among each material in the hybrid cathode and calculate the true charging rate of each material. The charging current that ensures the true charging rate of all materials is as close as possible to or equal to the preset rate is taken as the optimal current. The optimal charging strategy can be obtained based on each voltage range and the corresponding optimal current.

[0024] Furthermore, the actual charging rate of each material is calculated as follows: Based on the mass m of the lithium battery hybrid cathode, the specific capacity of each material is B, and the mass percentage a in the hybrid cathode, the standard current of each material at a 1C rate is calculated as: m*a*B / 1h (mass of hybrid cathode * mass percentage * specific capacity of each material / 1 hour). Therefore, the actual charging rate of each material = the current obtained / standard current. The unit of B is mAh / g.

[0025] Preferably, the optimal formation step for the decomposition of the lithium replenishing agent is determined, specifically including:

[0026] The battery is formed according to each preset formation step, the current obtained by the lithium replenishment agent is measured, and the obtained current is integrated over time to obtain the lithium replenishment agent decomposition capacity.

[0027] The optimal formation step is the one with the highest lithium replenishment decomposition capacity among the preset formation steps.

[0028] The beneficial effects of the present invention include:

[0029] 1. This invention employs an equivalent design approach for a hybrid system, disassembling the hybrid battery into multiple independent cells. Each independent cell system is designed to closely resemble a primary battery. Within each cell, surface electrodes are used to represent the entire cell as a single ohmic resistor. Connecting all independent cells in parallel yields the equivalent hybrid system. Real-time reaction currents of different materials within the original hybrid system are obtained by testing the surface electrodes and negative electrode voltage of each independent cell. This allows for the design of more reasonable operating conditions for evaluating the charging and discharging performance of the hybrid system cells, ensuring its rate performance and long-cycle performance. This equivalent hybrid system design approach can also be applied to other batteries such as sodium batteries.

[0030] 2. When the hybrid system is obtained by mixing ternary and lithium iron phosphate materials, it is disassembled into independent ternary and lithium iron phosphate cells according to their areal load. After parallel connection, the individual cells are tested to obtain the current contributed by the electrochemical reactions of ternary and lithium iron phosphate under different operating conditions. Different currents are used for charging and discharging at different voltage ranges to ensure that both lithium iron phosphate and ternary materials maintain a preset charging rate throughout the entire charging and discharging process, thereby avoiding damage to the material structure caused by high-rate charging and affecting the cycle performance of the entire hybrid system.

[0031] 3. When the mixed system is obtained by mixing the original cathode material and the lithium replenisher, it is decomposed into independent cells for the original cathode material and independent cells for the lithium replenisher according to the areal load. After parallel connection, the current contributed by the electrochemical reaction of the original cathode material and the lithium replenisher under different operating conditions can be obtained by testing the independent cells. Among the many formation steps to be screened, the decomposition capacity of the lithium replenisher can be obtained by integrating the contribution current of the independent cells for the lithium replenisher over time. Selecting the formation step with the highest decomposition capacity can quickly obtain the optimal formation step that fully decomposes the lithium replenisher. Attached Figure Description

[0032] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0033] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of the morphology of a porous surface electrode;

[0035] Figure 2 A schematic diagram of constructing a pure electronic circuit between the surface electrode and the negative electrode tab in an independent battery cell;

[0036] Figure 3 A schematic diagram of a parallel circuit for disassembling a hybrid cathode system into two independent cells;

[0037] Figure 4 A schematic diagram of parallel connection of lithium iron phosphate cells and ternary lithium batteries;

[0038] Figure 5 A schematic diagram showing the parallel connection of the original cathode material independent cell and the lithium replenishment agent independent cell;

[0039] Figure 6 The current-voltage linear curve of the lithium iron phosphate independent cell in Example 1;

[0040] Figure 7 The current-voltage line of the ternary independent battery cell in Example 1;

[0041] Figure 8 The current-voltage linear curve of the independent cell of the original cathode material in Example 3;

[0042] Figure 9 This is the current-voltage line of the lithium-replenishing independent cell in Example 3. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0044] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0045] In addition to the development of conventional lithium iron phosphate and ternary materials, battery system development also includes the design and development of hybrid cathodes. The charge and discharge strategies for hybrid systems usually need to consider the materials that actually participate in the reaction in different voltage ranges. Therefore, it is necessary to accurately measure the reaction current of each material in the hybrid system.

[0046] Lithium replenishment agents, also known as "pre-lithiation" or "pre-intercalation," are used to add lithium ions to a lithium battery before it begins operation, replenishing lithium ions to offset irreversible lithium loss and thus increasing the battery's total capacity and energy. The mixed system formed between the existing lithium replenishment agent and the original cathode material (which can be one or more combinations of lithium iron phosphate, ternary materials, lithium iron manganese, lithium cobalt oxide, and lithium-rich manganese-based materials) also involves current distribution. However, because different cathode materials correspond to different voltage ranges, the current received by the lithium replenishment agent is difficult to measure. Generally, charging and decomposition are performed according to empirical steps, making it impossible to select the formation step that maximizes decomposition capacity based on actual conditions. Therefore, it is necessary to accurately measure the reaction current between the lithium replenishment agent and the original cathode material in the mixed system to ensure complete decomposition of the lithium replenishment agent.

[0047] To address the limitation of current measurement difficulties in current hybrid systems due to the inability to measure the current distribution of multiple materials, this invention employs an equivalent design approach for hybrid systems. The hybrid battery is divided into multiple individual cells, which are then connected in parallel. Within each individual cell, a surface electrode is used to treat the entire cell as a single ohmic resistor. This allows for real-time measurement of the reaction currents of different materials based on the surface electrode and the negative electrode voltage side. This enables the design of more reasonable operating conditions for evaluating the charging and discharging performance of the hybrid battery cells, ensuring the rate performance and long-cycle performance of the hybrid system.

[0048] This invention aims to provide a test method for measuring current distribution in a hybrid cathode lithium battery, comprising the following steps:

[0049] (1) Prepare independent cells by using the materials in the mixed cathode of lithium battery according to the areal loading. Each independent cell has a surface electrode on the negative electrode to form a pure electronic circuit with the negative electrode tab.

[0050] (2) Measure the equivalent resistance of each individual cell;

[0051] (3) After connecting all the independent cells in parallel, the voltage of each independent cell is collected under the preset power-on conditions. The current passing through each independent cell is calculated based on the equivalent resistance, which is the current obtained by each material in the hybrid cathode under the preset power-on conditions.

[0052] In step (1) above, the areal load of each material is calculated as follows: the overall areal load of the mixed cathode is A, and the mass percentage of each material in the mixed cathode is a, then the areal load of each material = A·a. When preparing independent cells, except for the cathode which is a pure material, the electrolyte, separator, negative electrode, etc. should be as consistent as possible with the original mixed system to avoid introducing new variables. The surface electrode material of each independent cell is consistent with the negative electrode current collector material of each independent cell. The consistency of the two materials facilitates the smooth transmission of circuit current. Generally, copper foil is used as the current collector for the negative electrode of lithium batteries. Therefore, porous copper foil or copper mesh can be used as the surface electrode (e.g., Figure 1(As shown). Each surface electrode is elongated and extends beyond the encapsulation material of the individual battery cell, facilitating connection with the negative electrode tab to form a circuit.

[0053] The key to this invention is that after each material is fabricated into an independent cell, it is equivalent to a resistor—a resistor that does not change with the charging state of charge (SOC) and charging current. Common resistors used in lithium batteries include DC internal resistance and AC internal resistance, both of which vary with SOC and testing conditions, thus they cannot be accurately represented as equivalent resistors. In this invention, the independent cell employs a porous surface electrode structure on the negative electrode surface, creating a purely electronic circuit between the surface electrode and the negative electrode tab, exhibiting ohmic resistance properties. Therefore, under different currents, the voltage between the surface electrode and the negative electrode side of the independent cell satisfies Ohm's law linearly. A voltmeter can be placed between the surface electrode and the negative electrode tab of this invention's independent cell for subsequent voltage measurements of the independent cell under various operating conditions (e.g., ...). Figure 2 As shown, the orange area at the top represents the surface electrode, and a voltmeter is installed between the surface electrode and the negative electrode tab.

[0054] In step (2) above, the equivalent resistance of each independent cell is determined as follows: the voltage of each independent cell is collected under multiple current conditions (at least 5 points at intervals), and the collected voltage and current are fitted with a straight line. The equivalent resistance value is obtained based on the slope of the straight line. On the current-voltage straight line with current as the abscissa and voltage as the ordinate, the slope of the straight line is the equivalent resistance value.

[0055] When the hybrid system consists of two materials A and B, the individual cells prepared from each material A are connected in parallel as follows: Figure 3 As shown, the voltage of each independent cell is collected under preset energizing conditions, and the current passing through each independent cell is calculated based on the equivalent resistance. This is the current that materials A and B receive under the preset energizing conditions.

[0056] By employing an equivalent design approach for hybrid systems, the hybrid battery is divided into two separate cells, which are then connected in parallel. Within each individual cell, surface electrodes are used to treat the entire cell as a single ohmic resistor. This allows for real-time testing of the reaction currents of different materials based on the surface electrodes and the negative electrode voltage side. This enables the design of more reasonable charging strategies for evaluating the charging and discharging performance of the hybrid battery cells, ensuring the rate performance and long-cycle performance of the hybrid system.

[0057] Based on a general inventive concept, this application also provides a specific application of the above-mentioned test method for measuring current distribution in a hybrid cathode lithium battery, used to determine the optimal charging strategy at a preset rate when a hybrid cathode is formed from multiple materials; or used to evaluate the optimal formation step for the decomposition of the lithium replenishing agent when a lithium iron phosphate cathode material is added.

[0058] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0059] Example 1

[0060] This embodiment provides a test method for measuring the current distribution in a hybrid cathode of ternary and lithium iron phosphate materials, the steps of which are as follows:

[0061] (1) Determine the areal loading of ternary and lithium iron phosphate materials in the hybrid cathode of the lithium battery. The bifacial loading of the hybrid cathode system is A = 400 g / cm³. 2 Given that the mass ratios (a) of lithium iron phosphate (LFP) and ternary lithium are 80% and 20% respectively, the load factor (A·a) of the LFP material is calculated to be 320 g / cm³. 2 The ternary material has a double-sided load capacity of 80 g / cm³. 2 Lithium iron phosphate and ternary materials are used to prepare soft-pack independent cells according to their respective areal loads. Each independent cell has a porous copper foil surface electrode material on its graphite negative electrode. The surface electrode is long and extends out of the packaging material of the soft-pack independent cell. A voltmeter is connected between the surface electrode and the negative electrode tab of each independent cell.

[0062] (2) The two independent cells were charged with currents of 10, 30, 60, 90, 120 and 150mA respectively, and the corresponding voltages were collected. The specific values ​​are shown in Table 1.

[0063] Table 1. Individual Cell Current and Corresponding Data Collection Voltage

[0064]

[0065]

[0066] like Figure 6-7 As shown, the current-voltage line obtained by fitting the independent lithium iron phosphate cell is y = 0.0106x + 0.0015, and the equivalent resistance of lithium iron phosphate is 10.6mΩ based on the slope; the current-voltage line obtained by fitting the independent ternary lithium battery cell is y = 0.0043x + 0.0002, and the equivalent resistance of ternary lithium is 4.3mΩ based on the slope.

[0067] (3) Figure 4 As shown, the lithium iron phosphate battery cell and the ternary lithium battery cell are connected in parallel (the positive terminal of the lithium iron phosphate battery cell is connected to the positive terminal of the ternary lithium battery cell to form the total positive terminal, and the negative terminal of the lithium iron phosphate battery cell is connected to the negative terminal of the ternary lithium battery cell to form the total negative terminal, thus achieving parallel connection).

[0068] For example, under the preset power-on condition of 60mA charging, the parallel system was charged with a current of 60mA, and the values ​​of the voltmeters of each independent cell were recorded at 3.1V, 3.3V, and 4V, as shown in Table 2. Then, based on the cell voltmeter values / equivalent resistance, the current values ​​of the independent lithium iron phosphate cells and ternary independent cells were calculated, which are the currents contributed by the electrochemical reactions of lithium iron phosphate materials and ternary materials to the mixed cathode system under the same 60mA charging condition.

[0069] Table 2 shows the voltage and current of each individual cell.

[0070] Charging voltage (V) at a current of 60mA 3.1V 3.3V 4V Lithium iron phosphate (LiFePO4) V value (mV) 0.5512 0.48972 0.0212 A ternary V-meter represents the negative value (mV). 0.0344 0.05934 0.2494 Lithium iron phosphate current (mA) 52 46.2 2 Ternary current (mA) 8 13.8 58 Lithium iron phosphate current percentage 86.7％ 77.5％ 3.3％ Three-dimensional current percentage 13.3％ 22.5％ 96.7％

[0071] This battery design allows us to determine the current contributed by the electrochemical reactions of the two cathode materials under different operating conditions in different hybrid systems. The percentage of current received by the material in the hybrid cathode under the same operating conditions is calculated as: (Current of each individual cell / Sum of currents of all individual cells) * 100%.

[0072] Example 2

[0073] This embodiment applies the test method for current distribution in the hybrid cathode of ternary and lithium iron phosphate materials from Example 1 to determine the optimal charging strategy for a preset charging rate, as follows:

[0074] For a system where the mass ratio of lithium iron phosphate (LFP) to ternary lithium is 80:20, assuming the mass of the mixed cathode in the battery is m = 0.8g, and the specific capacity of LFP is B = 146mAh / g with a = 80%, and the specific capacity of ternary lithium is B = 170mAh / g with a = 20%, then the total capacity of the battery is 120mAh. The specific calculation process is as follows:

[0075] 0.8g*170mAh / g*20%+0.8g*146mAh / g*80%=120mAh,

[0076] The standard current for lithium iron phosphate batteries at 1C rate is 0.8g * 146mAh / g * 80% / 1h = 93mA.

[0077] The standard current for a ternary 1C rate is 0.8g * 170mAh / g * 20% / 1h = 27mA.

[0078] If the battery cell is to be charged and discharged at a preset rate of 2C, the charging current is 240mA. The parallel system is charged at a charging current of 240mA. The current obtained by each independent battery cell within the charging voltage range (2-4.25V) is measured, which corresponds to the current obtained by each material in the mixed cathode with voltage. The current abrupt change points of each material in the mixed cathode are determined. Using the charging voltage corresponding to the abrupt change points (3.1V and 3.55V in this embodiment), the entire charging voltage range is divided into 2.0V≤charging voltage≤3.1V, 3.1V<charging voltage≤3.55V, and 3.55V<charging voltage≤4.25V.

[0079] The charging current for each voltage band is adjusted, the current allocated to each material in the hybrid cathode is measured, and the true charging rate of each material is calculated. The charging current that ensures the true charging rate of all materials is as close as possible to or equal to the preset rate is taken as the optimal current. Based on each voltage band and the corresponding optimal current, the optimal charging strategy can be obtained. The optimal 2C charging strategy determined in this embodiment is shown in Table 3.

[0080] Table 3. Equivalent 2C charging strategy for a system with a lithium iron phosphate and ternary lithium battery mixture ratio of 80:20.

[0081]

[0082]

[0083] Example 3

[0084] This embodiment provides a test method for measuring the current distribution in the original cathode material and the lithium replenishing agent after lithium replenishment, as detailed below:

[0085] (1) The original cathode material (all lithium iron phosphate) weighs 84g, and the lithium iron phosphate supplement material weighs 2.6g. The mass ratios (a) of lithium iron phosphate and lithium iron phosphate supplement material are 97% and 3%, respectively. The total areal load (double-sided load) after lithium supplementation is A = 400g / m². 2 The areal loading of lithium iron phosphate material is 388 g / m². 2 The lithium supplementation surface loading is 12 g / m². 2 Lithium iron phosphate material and lithium replenishing agent are used to prepare soft-pack independent cells according to their respective areal loads. Each independent cell has a porous copper foil surface electrode material on its graphite negative electrode. The surface electrode is long and extends out of the packaging material of the soft-pack independent cell. A voltmeter is connected between the surface electrode and the negative electrode tab of each independent cell.

[0086] (2) Two independent cells, one with a capacity of 11600mAh and the other with a capacity of 1300mAh, were charged with the currents shown in the table below and the corresponding voltages were collected. The specific values ​​are shown in Table 4.

[0087] Table 4. Individual Cell Current and Corresponding Data Acquisition Voltage

[0088] Lithium iron phosphate test current (mA) Lithium iron phosphate (LiFePO4) V value (mV) Lithium replenishment test current (mA) Lithium replenishment V meter negative (mV) 2400 27.35 130 0.442 4000 44.98 260 0.893 6000 67.76 325 1.135 12000 135.6 390 1.361 18000 202.1 650 2.278 Equivalent resistance (mΩ) 11.2 Equivalent resistance (mΩ) 3.5

[0089] like Figure 8-9 As shown, based on the current-voltage line obtained from fitting the independent lithium iron phosphate cell, y = 0.0112x + 0.3814, the equivalent resistance of lithium iron phosphate is 11.2 mΩ; based on the current-voltage line obtained from fitting the independent lithium supplement cell, y = 0.0035x - 0.019, the equivalent resistance of lithium supplement is 3.5 mΩ.

[0090] (3) Figure 5 As shown, the lithium iron phosphate (LFP) battery cell and the lithium replenisher battery cell are connected in parallel (by connecting the positive terminal of the LFP battery cell to the positive terminal of the lithium replenisher battery cell to form the total positive terminal, and connecting the negative terminal of the LFP battery cell to the negative terminal of the lithium replenisher battery cell to form the total negative terminal, the two can be connected in parallel). Figure 6 As shown in Table 5, the voltage of each individual cell was collected under preset charging conditions (i.e., charging to 3.65V at 2500mA, charging to 4.25V at 625mA, and constant voltage charging to a current less than 62.5mA). The current passing through each individual cell was calculated based on the equivalent resistance. This is the current obtained by the lithium iron phosphate material and the lithium replenishing agent in the mixed cathode under the same energizing conditions, as shown in Table 5 below. (Since the voltage is accurate to two decimal places, 3.51-3.65V below indicates the case where 3.50V < charging voltage ≤ 3.65V, 3.66-4.0V below indicates the case where 3.65V < charging voltage ≤ 4.0V, and 4.01-4.25V below indicates the case where 4.0V < charging voltage ≤ 4.25V.)

[0091] Table 5. Voltage and current of each individual cell

[0092] Charging voltage 2.5-3.5V 3.51-3.65V 3.66-4.0V 4.01-4.25V Lithium iron phosphate (LiFePO4) V value (mV) 28.22 27.320 0.843 0 Lithium replenishment V meter negative (mV) 0 0.280 1.911 2.255 Lithium iron phosphate current (mA) 2520 2440 75 0 Lithium replenishment current (mA) 0 80 550 625 Lithium iron phosphate current percentage 100％ 96.8％ 12％ 0 Lithium replenishment current percentage 0 3.2％ 88％ 100％

[0093] Example 4

[0094] This embodiment applies the test method for the current distribution of the original cathode material and the lithium replenishing agent from Example 3 to determine the optimal formation step for lithium replenishing agent decomposition, as follows:

[0095] Multiple formation steps are preset, and the parallel independent cell system is formed separately according to each formation step. The current obtained by the lithium replenishing agent cell is measured and the obtained current is integrated over time to obtain the actual decomposition capacity of the lithium replenishing agent. The capacity that lithium ferrite can contribute to the lithium replenishment is 650 mAh / g. When the actual decomposition capacity per gram of lithium ferrite is greater than 585 mAh / g (that is, it has reached 90%), the formation condition is better.

[0096] Formation step 1: Charge at 2500mA to 3.65V, charge at 625mA to 4.25V, and charge at constant voltage until the current is less than 62.5mA.

[0097] Formation step 2: Charge at 2500mA to 3.65V, charge at 310mA to 4.25V, and charge at constant voltage until the current is less than 31mA.

[0098] Formation step 3: Charge at 2500mA to 3.65V, charge at 160mA to 4.25V, and charge at constant voltage until the current is less than 16mA.

[0099] The results of the current integration over time for the three formation steps are shown in Table 6 below.

[0100] Table 6. Capacity and specific capacity contribution of lithium ferrite from different formation steps.

[0101] Transform into steps Lithium ferrite capacity (mAh) Actual decomposition capacity per gram (mAh / g) Transformation Step 1 943.8 363 Transformation Step 2 1183 455 Transformation into step 3 1606.8 618

[0102] As shown in Table 6, the actual decomposition capacity of lithium ferrite in formation step 3 is the largest, reaching 618 mAh / g, which is about 95% of the total capacity. Therefore, formation step 3 is the optimal formation step.

Claims

1. A test method for measuring current distribution in a hybrid cathode lithium battery, characterized in that, include: In a lithium battery hybrid cathode, each material is used to prepare an independent cell according to its areal loading. Each independent cell has a surface electrode on its negative electrode to form a pure electronic circuit with the negative electrode tab. Measure the equivalent resistance of each individual cell; After connecting all the independent cells in parallel, the voltage of each independent cell is collected under preset energizing conditions. The current passing through each independent cell is calculated based on the equivalent resistance, which is the current corresponding to each material in the hybrid cathode under preset energizing conditions.

2. The test method for measuring current distribution in a hybrid cathode lithium battery as described in claim 1, characterized in that, The surface load of each material is calculated as follows: the overall surface load of the hybrid cathode is A, and the mass percentage of each material in the hybrid cathode is a. Then the surface load of each material = A·a.

3. The test method for measuring current distribution in a hybrid cathode lithium battery as described in claim 1, characterized in that, The independent battery cell is either a hard-shell battery cell or a soft-pack battery cell. The surface electrode material of each independent battery cell is the same as the negative electrode current collector material. Each surface electrode is long and extends out of the packaging material of the independent battery cell.

4. The test method for measuring current distribution in a hybrid cathode lithium battery as described in claim 1, characterized in that, The equivalent resistance of each individual cell is determined by: collecting the voltage of each individual cell under multiple current conditions, performing linear fitting between the collected voltage and current, and obtaining the equivalent resistance value based on the slope of the line.

5. The test method for measuring current distribution in a hybrid cathode lithium battery as described in claim 1 or 4, characterized in that, The voltage of each individual cell is obtained by collecting the voltage between the surface electrode and the negative electrode tab.

6. The test method for measuring current distribution in a hybrid cathode lithium battery as described in claim 1, characterized in that, The hybrid cathode material is divided into two parts: lithium iron phosphate material and ternary material; or original cathode material and lithium replenishment agent.

7. The test method for measuring current distribution in a hybrid cathode lithium battery as described in claim 6, characterized in that, The lithium replenishing agent is lithium ferrite or lithium nickel oxide, and the lithium replenishing agent accounts for ≤5% of the mass percentage of the mixed cathode material.

8. An application of a test method for measuring current distribution in a hybrid cathode lithium battery as described in any one of claims 1 to 7, characterized in that, Used to determine the optimal charging strategy at a preset rate when a mixed cathode is formed from lithium iron phosphate and ternary materials; or used to evaluate the optimal formation step for the decomposition of the lithium replenishing agent when a lithium replenishing agent is added to the original cathode material.

9. The application of the test method for measuring current distribution in a hybrid cathode lithium battery as described in claim 8, characterized in that, Determining the optimal charging strategy at a preset charging rate includes: Under the condition of charging at a preset rate, the current obtained by each material in the mixed cathode within the charging voltage range is measured, the current abrupt change point of each material in the mixed cathode is determined, and the entire charging voltage range is divided into multiple segments using the charging voltage corresponding to the abrupt change point. Adjust the charging current for each voltage range, measure the current distributed among each material in the hybrid cathode and calculate the true charging rate of each material. The charging current that ensures the true charging rate of all materials is as close as possible to or equal to the preset rate is taken as the optimal current. The optimal charging strategy can be obtained based on each voltage range and the corresponding optimal current.

10. The application of the test method for measuring current distribution in a hybrid cathode lithium battery as described in claim 8, characterized in that, The optimal formation steps for evaluating the decomposition of lithium supplementers are specifically included; The battery is formed according to each preset formation step, the current obtained by the lithium replenishment agent is measured, and the obtained current is integrated over time to obtain the lithium replenishment agent decomposition capacity. The optimal formation step is the one with the highest lithium replenishment decomposition capacity among the preset formation steps.

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