Method for judging whether NP ratio of lithium ion battery is good or not and application of method

Abstract

The invention relates to the technical field of battery detection, and discloses a method for judging whether the NP ratio of a lithium ion battery is good or not and application of the method. The fresh lithium battery is charged to 100% SOC with constant current; the negative electrode potential Vneg when the battery is 100% SOC is measured, whether the Vneg meets the formula Vneg belongs to [k + a (CIR)] or not is calculated, in the formula, IR is the shelving internal resistance of the battery in the 100% state, the unit is omega, C is the current during charging, the unit is A, a is a measurement error parameter, the range is 0.9-1.1, and k is the equilibrium potential when the negative electrode is completely embedded with lithium; if yes, the NP ratio of the battery is good, and if not, the NP ratio of the battery is poor. The method is simple and convenient to operate, can be used for judging whether the design of the battery is reasonable or not and whether the quality of the battery is better or not, and has important significance on design and manufacturing of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery testing technology, and more specifically, to a method for determining whether the NP ratio of a lithium-ion battery is optimal and its application. Background Technology

[0002] The NP ratio of a lithium-ion battery refers to the ratio of the actual capacity of the negative electrode to the actual capacity of the positive electrode. It is a core parameter in battery design, and its value is usually greater than 1. Essentially, it balances the lithium loss caused by the film formation reaction during the first charge and discharge process in the formation stage by "excess capacity of the negative electrode", while avoiding the formation of lithium dendrites on the negative electrode.

[0003] If the NP ratio is too small, the negative electrode capacity is insufficient. During charging, the negative electrode cannot embed all the lithium ions released from the positive electrode, causing excess lithium ions to precipitate on the surface of the negative electrode and form lithium dendrites. This not only leads to a continuous decrease in active lithium and a significant reduction in cycle life, but the continuous growth of lithium dendrites can also pierce the separator, causing direct contact between the positive and negative electrodes, resulting in a short circuit inside the battery, a risk of thermal runaway, and safety hazards.

[0004] If the NP ratio is too high, the negative electrode capacity is redundant. Although there is no safety hazard, too much active material in the negative electrode will thicken the negative electrode sheet, increasing the overall thickness and weight of the battery, which will directly lead to a decrease in the volumetric energy density and gravimetric energy density of the battery. An excessively thick negative electrode sheet will increase the lithium-ion transport path, leading to an increase in the battery's internal resistance, which may affect the battery's high-rate performance. In addition, too much active material in the negative electrode will also lead to a decrease in battery manufacturing efficiency and an increase in cost.

[0005] Therefore, an NP ratio that is too high or too low brings a series of problems, and how to determine whether the NP ratio of a battery is appropriate is an important issue.

[0006] Therefore, this application is hereby submitted. Summary of the Invention

[0007] The purpose of this invention is to provide a method for determining whether the NP ratio of a lithium-ion battery is optimal and its application. This method links the degree of lithium intercalation in the negative electrode with the negative electrode potential to determine the optimal NP ratio of the lithium-ion battery, and the operation is simple.

[0008] This invention is implemented as follows: In a first aspect, embodiments of the present invention provide a method for determining whether the NP ratio of a lithium-ion battery is optimal, comprising: Charge the fresh lithium battery to 100% SOC using a constant current. Measure the negative electrode potential V of the battery at 100% SOC neg Calculate V neg Does it satisfy formula V? neg ∈[k+a (C IR)], where IR is the internal resistance of the battery at 100% capacity, in Ω, C is the charging current, in A, a is the measurement error parameter, ranging from 0.9 to 1.1, and k is the equilibrium potential when the negative electrode is fully lithium-intercalated. If the conditions are met, the NP ratio of this battery is good; if not, the NP ratio of this battery is poor.

[0009] In an optional embodiment, the negative electrode active material of the fresh lithium battery is graphite or hard carbon, and k is the equilibrium potential when the graphite or hard carbon negative electrode is fully lithium-intercalated, with a value of 0.03~0.05V.

[0010] In an optional implementation, the fresh lithium battery is further placed in an environment of 23–27°C for 2–3 hours before being charged to 100% SOC.

[0011] In an optional implementation, a fresh lithium battery is charged to 100% SOC at a constant current of 0.2 to 0.5C.

[0012] In an optional embodiment, the positive electrode active material of the lithium battery is at least one of lithium iron phosphate, lithium manganese oxide, and lithium cobalt oxide.

[0013] In an optional implementation, the negative electrode potential V is tested. neg The method for manufacturing the reference electrode of the three electrodes used in this process is as follows: an unoxidized copper wire is led out from the separator inside the battery as the reference electrode, the copper wire is soldered onto a polished nickel sheet, and then a lithium layer is plated on the copper wire.

[0014] In an optional embodiment, the method of plating a lithium layer on a copper wire includes: connecting the negative electrode of the battery to the positive clamp of the charge / discharge cabinet, connecting the reference electrode to the negative clamp of the charge / discharge cabinet, and charging with a constant current of 0.01~0.02mA for 2~4 hours.

[0015] Secondly, embodiments of the present invention provide the application of the above-described method in the design and manufacture of batteries.

[0016] The beneficial effects of this invention are: The method for determining whether the NP ratio of a lithium-ion battery is optimal, provided in the embodiments of the invention, studies the dynamic potential V of the negative electrode active material when lithium ions are intercalated into it. neg The relationship between static potential and polarization potential reveals that the negative electrode potential V when the battery is fully charged is... neg When the relationship between the negative electrode equilibrium potential and the polarization potential satisfies the formula provided in this invention, the battery's NP (NP ratio) is optimal, and the battery's cycle performance is best. This method can be used to determine whether the battery design is reasonable and whether the battery quality is good, which is of great significance for battery design and manufacturing. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a comparison chart of the cycle curves of the five groups of batteries in Example 1; Figure 2 This is a comparison chart of the cycle curves of the five groups of batteries in Example 2; Figure 3 This is a comparison chart of the cycle curves of the five groups of batteries in Example 3. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0020] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0021] This invention provides a method for determining whether the NP ratio of a lithium-ion battery is optimal, comprising: Charge the fresh lithium battery to 100% SOC using a constant current. Measure the negative electrode potential V of the battery at 100% SOC neg Calculate V neg Does it satisfy formula V? neg ∈[k+a (C IR)], where IR is the internal resistance of the battery at 100% capacity, in Ω, C is the charging current, in A, a is the measurement error parameter, ranging from 0.9 to 1.1, and k is the equilibrium potential when the negative electrode is fully lithium-intercalated. If the conditions are met, the NP ratio of this battery is good; if not, the NP ratio of this battery is poor.

[0022] When lithium ions are intercalated into the negative electrode active material, the internal structure of the negative electrode active material changes, and its dynamic potential V neg The potential is determined by both the static potential and the polarization potential. The static potential, k, is the potential when the negative electrode active material is fully embedded, stationary, and without current. This is thermodynamically determined and will not change; it can be obtained through numerous experiments. The polarization potential, on the other hand, is directly determined by the current and internal resistance, and shows a linear correlation. Here, it is represented as C. IR and C represent current, with IR being the internal resistance (DC internal resistance), replacing the ohmic resistance, activation resistance, and concentration impedance (AC internal resistance) inside the battery. Studies have found that when the battery's NP ratio is optimal, the negative electrode potential V at full charge... neg The relationship between the negative electrode equilibrium potential and the polarization potential satisfies the formula provided in this invention. Therefore, the NP ratio of the battery can be tested using the formula provided in this invention. This method can be used to determine whether the battery design is reasonable and whether the battery quality is good, which is of great significance for battery design and manufacturing.

[0023] Specifically, the judgment methods include: S1. Prepare the battery to be tested. Freshly made lithium batteries are placed in an environment of 23-27°C (in some embodiments, the temperature can be 23°C, 25°C or 27°C) for 2-3 hours (in some embodiments, the time can be, for example, 2 hours, 2.5 hours or 3 hours) to stabilize the temperature and battery potential.

[0024] Then, the battery is fully charged to 100% SOC using a constant current C.

[0025] Optionally, C is 0.2 to 0.5C (in some embodiments, the rate can be 0.2C, 0.3C, or 0.5C). At this rate, a stable and reliable negative electrode potential response is obtained while ensuring controllable polarization and avoiding lithium plating.

[0026] Optionally, the positive electrode active material of the battery is at least one of lithium iron phosphate, lithium manganese oxide, and lithium cobalt oxide.

[0027] S2, Measure the negative electrode potential and the internal resistance of the resting electrode. The resting internal resistance IR of the battery under test was measured using a handheld internal resistance meter. An unoxidized copper wire was drawn from between the internal separators of the battery under test and soldered onto a polished nickel sheet. A lithium layer was then plated onto the copper wire. The aim was to construct a miniature "lithium reference electrode" using the copper wire, providing a stable Li / Li ratio. + Potential reference; at the same time, the copper wire surface is coated with a lithium layer to avoid the side reaction between copper and electrolyte that would cause potential drift, thereby achieving accurate and stable monitoring of the negative electrode potential inside the cell.

[0028] The method for depositing a lithium layer on a copper wire includes: connecting the negative electrode of the battery to the positive clamp of the charge / discharge cabinet, connecting the reference electrode to the negative clamp of the charge / discharge cabinet, and charging with a constant current of 0.01~0.02mA (in some embodiments, the charging current is, for example, 0.01mA, 0.015mA or 0.02mA) for 2~4 hours (in some embodiments, the charging time is, for example, 2 hours, 3 hours or 4 hours) to achieve the lithium layer deposition.

[0029] Then measure the negative electrode potential V.neg .

[0030] It should be noted that the method for testing negative electrode potential is existing technology, and the specific method is not limited. The above content is only a partial implementation method of the embodiments provided by the present invention.

[0031] S3, Substituting Formulas for Comparison Substitute the shelving internal resistance IR measured in step S2 into the formula [k+a] (C IR)] and V neg Compare and determine whether V is satisfied. neg ∈[k+a (C If the IR ratio is satisfied, the NP ratio of this battery is good; if it is not satisfied, the NP ratio of this battery is poor.

[0032] Where k is the static potential, which is the potential when the negative electrode active material is fully embedded, static, and without current. It is a range value, and the corresponding value is different for different materials.

[0033] Optionally, the negative electrode active material of the battery provided by the present invention is graphite or hard carbon. When lithium ions are intercalated into the graphite / hard carbon negative electrode, the internal structure of the negative electrode is LiC. 24 Transformation into LiC 12 The process of ultimately transforming graphite / hard carbon into LiC6 involves the dynamic potential V when graphite / hard carbon is completely transformed into LiC6. neg It is determined by both the static potential and the polarization potential. The static potential is k, which is the potential when the graphite is fully embedded, stationary, and without current. The inventors found that when the negative electrode active material is graphite or hard carbon, k is 0.03 to 0.05 V.

[0034] The present invention will be further described below with reference to specific embodiments.

[0035] Example 1 Five groups of fresh lithium batteries that had been left at 25°C for 2 hours were taken. Each group had 3 identical batteries, with the only difference between the groups being the NP ratio, which was 1.10, 1.14, 1.16, 1.18, and 1.22, respectively.

[0036] The positive electrode active layer of the battery consists of: 97% lithium iron phosphate, 1.5% conductive carbon black, and 1.5% PVDF. The composition of the negative electrode active layer of the battery is: 96% graphite, 1% conductive agent, 2% SBR, and 1% CMC; Battery separator: PP separator; The electrolyte formulation is: EC:DMC:EMC = 1:1:1 + 1.0 M LiPF6 + 2% VC film-forming additive; The battery's nominal capacity is 20Ah.

[0037] When these five battery groups were charged to 100% SOC at a constant current rate of 0.5C, the average negative electrode equilibrium potential V of each group at 100% SOC was measured. neg The values ​​are 0.042V, 0.070V, 0.085V, 0.112V, and 0.126V, respectively.

[0038] The internal resistance of the batteries when fully charged is measured to be 5.4 mΩ, therefore C = 0.5. 20 = 10A, IR = 5.4mΩ, then k + a (C) The IR range is 0.0786V to 0.1094V.

[0039] Comparing the negative electrode equilibrium potentials of the five battery groups above, when the NP ratio is 1.16, the formula V is satisfied. neg ∈[k+a (C IR).

[0040] To verify the effectiveness of the method provided in this embodiment, five more groups of the same batteries were taken, with three batteries in each group. The batteries were cycled at 25°C -0.5°C, and the average value of the three batteries in each group was plotted. The cycle results are shown below. Figure 1 As shown, from Figure 1 It can be seen that the battery has the best cycle performance when the NP ratio is 1.16, indicating that the result obtained by the method in this embodiment is accurate.

[0041] Example 2 Five groups of fresh lithium batteries that had been left at 25°C for 2 hours were taken. Each group had 3 identical batteries, with the only difference between the groups being the NP ratio, which was 1.10, 1.14, 1.16, 1.18, and 1.22, respectively.

[0042] The composition of the positive electrode active layer of the battery is: 96% lithium manganese oxide, 2% conductive carbon black, and 2% PVDF; The composition of the negative electrode active layer of the battery is: 95% hard carbon, 2% conductive agent, 1.5% SBR, and 1.5% CMC; Battery separator: PP separator; The electrolyte formulation is: EC:DMC:EMC = 1:1:1 + 1.0 M LiPF6 + 2% VC film-forming additive; The battery's nominal capacity is 25Ah.

[0043] When these five battery groups were charged to 100% SOC at a constant current rate of 0.2C, the average negative electrode equilibrium potential V of each group at 100% SOC was measured. negThe values ​​are 0.0356V, 0.0675V, 0.0812V, 0.0985V, and 0.1452V, respectively.

[0044] The internal resistance of the batteries when fully charged is measured to be 5.2 mΩ, therefore C = 0.2. 25 = 5A, IR = 5.2mΩ, then k + a (C) The IR range is 0.0534V to 0.0786V.

[0045] Comparing the negative electrode equilibrium potentials of the five battery groups above, when the NP ratio is 1.14, the formula V is satisfied. neg ∈[k+a (C IR).

[0046] To verify the effectiveness of the method provided in this embodiment, five more groups of the same batteries were taken, with three batteries in each group. The batteries were cycled at 25°C -0.5°C, and the average value of the three batteries in each group was plotted. The cycle results are shown below. Figure 2 As shown, from Figure 2 It can be seen that the battery has the best cycle performance when the NP ratio is 1.14, indicating that the result obtained by the method in this embodiment is accurate.

[0047] Example 3 Five groups of fresh lithium batteries that had been left at 25°C for 2 hours were taken. Each group had 3 identical batteries, with the only difference between the groups being the NP ratio, which was 1.10, 1.14, 1.16, 1.18, and 1.22, respectively.

[0048] The positive electrode active layer of the battery consists of: 95% lithium cobalt oxide, 3% conductive carbon black, and 2% PVDF. The composition of the negative electrode active layer of the battery is: 96% graphite, 1% conductive agent, 2% SBR, and 1% CMC; Battery separator: PP separator; The electrolyte formulation is: EC:DMC:EMC = 1:1:1 + 1.0 M LiPF6 + 2% VC film-forming additive; The battery's nominal capacity is 30Ah.

[0049] When these five battery groups were charged to 100% SOC at a constant current rate of 0.3C, the average negative electrode equilibrium potential Vneg of each group at 100% SOC was measured to be 0.0251V, 0.0568V, 0.0652V, 0.0795V, and 0.1021V, respectively.

[0050] The internal resistance of the batteries when fully charged is measured to be 4.8 mΩ, therefore C = 0.3. 30 = 9A, IR = 4.8mΩ, then k + a (C) The IR range is 0.0689V to 0.0975V.

[0051] Comparing the negative electrode equilibrium potentials of the five battery groups above, when the NP ratio is 1.18, the formula V is satisfied. neg ∈[k+a (C IR).

[0052] To verify the effectiveness of the method provided in this embodiment, five more groups of the same batteries were taken, with three batteries in each group. The batteries were cycled at 25°C -0.5°C, and the average value of the three batteries in each group was plotted. The cycle results are shown below. Figure 3 As shown, from Figure 3 It can be seen that the battery has the best cycle performance when the NP ratio is 1.18, indicating that the result obtained by the method in this embodiment is accurate.

[0053] In summary, the method for determining whether the NP ratio of a lithium-ion battery is optimal, provided in this embodiment of the invention, studies the dynamic potential V of the negative electrode active material when lithium ions are intercalated into it. neg The relationship between static potential and polarization potential reveals that the negative electrode potential V when the battery is fully charged is... neg When the relationship between the negative electrode equilibrium potential and the polarization potential satisfies the formula provided in this invention, the battery's NP (NP ratio) is optimal, and the battery's cycle performance is best. This method can be used to determine whether the battery design is reasonable and whether the battery quality is good, which is of great significance for battery design and manufacturing.

[0054] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for determining whether the NP ratio of a lithium-ion battery is optimal, characterized in that, include: Charge the fresh lithium battery to 100% SOC using a constant current. Measure the negative electrode potential V of the battery at 100% SOC neg Calculate V neg Does it satisfy formula V? neg ∈[k+a (C IR)], where IR is the internal resistance of the battery at 100% capacity, in Ω, C is the charging current, in A, a is the measurement error parameter, ranging from 0.9 to 1.1, and k is the equilibrium potential when the negative electrode is fully lithium-intercalated. If the conditions are met, the NP ratio of this battery is good; if not, the NP ratio of this battery is poor.

2. The method according to claim 1, characterized in that, The negative electrode active material of the fresh lithium battery is graphite or hard carbon, and k is the equilibrium potential when the graphite or hard carbon negative electrode is fully lithium-intercalated, with a value of 0.03~0.05V.

3. The method according to claim 1, characterized in that, Before charging the fresh lithium battery to 100% SOC, it is also included to let it rest in an environment of 23-27°C for 2-3 hours.

4. The method according to claim 1, characterized in that, Charge the fresh lithium battery to 100% SOC at a constant current of 0.2-0.5C.

5. The method according to claim 1, characterized in that, The positive electrode active material of the lithium battery is at least one of lithium iron phosphate, lithium manganese oxide, and lithium cobalt oxide.

6. The method according to claim 1, characterized in that, Test negative electrode potential V neg The method for manufacturing the reference electrode of the three electrodes used in this process is as follows: an unoxidized copper wire is led out from the separator inside the battery as the reference electrode, the copper wire is soldered onto a polished nickel sheet, and then a lithium layer is plated on the copper wire.

7. The method according to claim 6, characterized in that, The method for depositing a lithium layer on a copper wire includes: connecting the negative electrode of the battery to the positive electrode clamp of the charge / discharge cabinet, connecting the reference electrode to the negative electrode clamp of the charge / discharge cabinet, and charging with a constant current of 0.01~0.02mA for 2~4 hours.

8. The application of the method as described in any one of claims 1 to 7 in the design and manufacture of batteries.

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