Method for determining abnormality in lithium-ion secondary batteries

Abstract

To accurately determine abnormality in lithium deposition resistance while reflecting electrode conditions, even when there is variation in manufacturing of a lithium ion secondary battery.SOLUTION: An abnormality determination method for a lithium ion secondary battery comprises: a reference correlative relationship acquisition step (S1) of acquiring a reference correlative relationship that serves as a reference between a deposition current value and a reaction resistance under a preset electrode condition; a normal range definition step (S2) of correcting the reference correlative relationship to a corrected correlative relationship based on an electrode condition to be determined, and defining a normal range based on the corrected correlative relationship; a reaction resistance acquisition step (S5) of acquiring a reaction resistance to be determined; a deposition current value acquisition step (S6) of acquiring a deposition current value to be determined; and an abnormality determination step (S7) of determining that a lithium ion secondary battery to be determined is abnormal when the relationship between the reaction resistance and the deposition current value is out of the normal range.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for determining an abnormality in a lithium ion secondary battery, and more particularly to a method for determining an abnormality in a lithium ion secondary battery that reflects variations in electrode conditions. [Background technology]

[0002] Lithium ion secondary batteries are widely used, for example, as driving power sources for electric vehicles, etc. Since such lithium ion secondary batteries are used under severe conditions, it is desirable to accurately determine the quality of manufactured lithium ion secondary batteries before shipping.

[0003] In particular, since the deposition of metallic lithium in the electrodes has a large effect on battery life, it is desirable to accurately determine whether the lithium deposition resistance is good or not before shipping. Therefore, in the invention described in Patent Document 1, the resistance value is estimated using a correlation between the resistance value and the capacity prepared in advance and a capacity estimated based on the temperature frequency distribution of the target secondary battery. If there is a deviation between the estimated resistance value and a threshold value, it is determined that an abnormality has occurred.

[0004] In addition, in the invention described in Patent Document 2, the reaction current is calculated using the overpotential and exchange current density in the precipitation reaction and dissolution reaction, and the amount of lithium precipitation is estimated. At that time, the amount of lithium precipitation dependent on the reaction resistance is calculated by using the exchange current density corrected by the reaction resistance change rate.

[0005] According to the invention described in Patent Document 1, it is possible to estimate a resistance value, which is important for battery performance. Furthermore, according to the invention described in Patent Document 2, it is possible to estimate the amount of lithium deposition, which depends on the reaction resistance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2020-139913 A [Patent Document 2] JP 2016-4726 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, the invention described in Patent Document 1 does not take into account changes in the correlation between resistance and capacitance due to manufacturing variations, which may lead to erroneous determination of an abnormality. In addition, the invention does not take into account the relationship between resistance and lithium precipitation resistance.

[0008] In addition, the invention described in Patent Document 2 can also estimate the amount of lithium precipitation, but does not take into account changes in the correlation between the reaction current and the resistance value due to manufacturing variations, and there is a possibility that the accuracy of estimating the amount of lithium precipitation is reduced.

[0009] The problem to be solved by the method for determining an abnormality in a lithium ion secondary battery of the present invention is to accurately determine an abnormality in lithium precipitation resistance by reflecting electrode conditions, even when there is variation in the manufacture of lithium ion secondary batteries. [Means for solving the problem]

[0010] In order to solve the above problems, the method for determining an abnormality in a lithium-ion secondary battery of the present invention includes a reference correlation acquisition step for acquiring a reference correlation that serves as a standard for the deposition current value and reaction resistance of a lithium-ion secondary battery under set electrode conditions, a reaction resistance acquisition step for acquiring the reaction resistance of the lithium-ion secondary battery to be determined, a deposition current value acquisition step for acquiring the deposition current value of the lithium-ion secondary battery to be determined, a normal range determination step for correcting the reference correlation to acquire a corrected correlation based on the electrode conditions of the lithium-ion secondary battery to be determined, and determining that a normal range is based on the corrected correlation, and an abnormality determination step for determining that the lithium-ion secondary battery to be determined is abnormal if the relationship between the reaction resistance acquired in the reaction resistance acquisition step and the deposition current value acquired in the deposition current value acquisition step is outside the normal range.

[0011] The electrode conditions may include at least one of an electrode opposing portion area, a negative electrode density, a positive electrode basis weight, and a negative electrode basis weight. The reference correlation may be obtained as a linear function of deposition current value against reaction resistance, and in the step of defining the normal range, the corrected correlation may be obtained by adjusting the intercept while keeping the slope of the linear function unchanged based on the electrode conditions to be determined.

[0012] The intercept can also be adjusted by multiplying the value of each electrode condition by a constant set for each electrode condition and integrating the result to calculate the correction amount for the intercept. The object to be judged may be selected in advance from among non-defective products that fall within a tolerance range of the design of the electrode conditions.

[0013] The deposition current value can be obtained by a low-temperature cycle test. The reaction resistance can be obtained by AC impedance measurement. Effect of the Invention

[0014] The method of determining an abnormality in a lithium ion secondary battery of the present invention can accurately determine an abnormality in lithium precipitation resistance by reflecting electrode conditions even if there is variation in the manufacture of lithium ion secondary batteries. [Brief description of the drawings]

[0015] [Figure 1] 1 is a graph of a "reference correlation line L0" showing the correlation between the reaction resistance [mΩ] and the deposition current [A] for the design median value of the electrode conditions. [Diagram 2] 1 is a graph of "corrected correlation lines L1 to L3" showing that the correlation between the deposition current [A] and the reaction resistance [mΩ] changes depending on the electrode conditions. [Diagram 3] 1 is a graph showing a "normal range SZ" defined based on a "corrected correlation line L1" that matches the electrode conditions of a lithium ion secondary battery to be manufactured. [Figure 4] 3 is a flowchart showing the steps of a method for determining an abnormality in a lithium ion secondary battery according to the present embodiment. [Diagram 5] FIG. 1 is a circuit diagram illustrating an equivalent circuit of a lithium ion secondary battery. [Figure 6] 1 is a graph showing a relationship between a Nyquist plot by an AC impedance method and each resistance component. [Figure 7] 1 is a graph showing the relationship between deterioration of a lithium ion secondary battery and a Nyquist plot. [Figure 8] 13 is a graph showing the relationship between the electrode opposing area reduction rate and the change in the intercept b0 of the reference correlation line L0. [Figure 9] 1 is a graph showing the relationship between the negative electrode density and the change in the intercept b0 of the reference correlation line L0. [Figure 10] 13 is a graph showing the relationship between the positive electrode weight and the change in the intercept b0 of the baseline correlation line L0. [Figure 11] 13 is a graph showing the relationship between the negative electrode basis weight and changes in the intercept b0 of the reference correlation line L0. [Figure 12] 13 is a graph of corrected correlation lines L1 to L2 obtained by correcting the intercept b0 of the reference correlation line L0 to reflect the electrode conditions to be determined. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] (Configuration of this embodiment) Hereinafter, one embodiment of the method for determining an abnormality in a lithium ion secondary battery of the present invention will be described with reference to FIGS.

[0017] <Principle of this embodiment> In lithium-ion secondary batteries, the deposition of metallic lithium (hereinafter sometimes abbreviated as "Li") can cause short circuits between the plates, so batteries with low resistance to lithium deposition (metallic Li is easily deposited at low currents) are undesirable. Therefore, in lithium-ion secondary batteries, the electrode facing area [cm 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 Then, only those batteries that are within a certain tolerance range from the design value are deemed to be good products with the specified lithium precipitation resistance, and all other batteries are deemed defective, and only the good products are shipped as products.

[0018] FIG. 1 is a graph of the "reference correlation line L0" showing the correlation between the reaction resistance [mΩ] and the deposition current [A] (the minimum current value at which metallic lithium is deposited) for the design median of the electrode conditions. Here, the straight line of the one-time function of the slope a of the graph showing the reference correlation shown in FIG. 1 is called the "reference correlation line L0". The intercept is b0. In this way, in the case of only a good lithium-ion secondary battery that is the design median of the manufactured electrode conditions, the correlation between the reaction resistance [mΩ] and the deposition current [A] shows a constant relationship. Therefore, if the correlation between the reaction resistance [mΩ] and the deposition current [A] of the manufactured lithium-ion secondary battery is as shown in FIG. 1, it can be determined that it is a normal good lithium-ion secondary battery.

[0019] However, in reality, there is manufacturing variation from the median design value of the electrode conditions, and the criteria for determining whether a product is good or bad vary depending on the variation. FIG. 2 is a graph of "corrected correlation lines L1 to L3" showing that the correlation between the deposition current [A] and the reaction resistance [mΩ] changes depending on the electrode conditions.

[0020] When the electrode conditions are different, the correlation between the reaction resistance [mΩ] and the deposition current [A] for the design median of the electrode conditions changes. For example, the reference correlation line L0, which is a graph showing the correlation of the design median of the electrode conditions shown by a circle in the graph of Figure 2, changes to, for example, the corrected correlation line L1, which is a graph shown by a square. At this time, the slope a of the reference correlation line L0 does not change, but moves downward in parallel. Furthermore, when the electrode conditions change significantly, it changes to the corrected correlation line L2, which is a graph shown by a triangle, and when the electrode conditions change further, it changes to the corrected correlation line L3, which is a graph shown by a diamond. In this way, when the electrode conditions change from the design median, the correlation between the reaction resistance [mΩ] and the deposition current [A] changes. In other words, the center, which is the standard for abnormality judgment, is shifted.

[0021] This can result in a battery that is actually normal being erroneously determined to be abnormal, or a battery that is actually defective being erroneously determined to be normal, resulting in a problem of reduced accuracy in determination.

[0022] Therefore, in the present invention, the electrode facing area [cm 2 ], which is closely related to the lithium precipitation resistance, is determined in advance. 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 ] is used to obtain the correlation between the deposition current value [A] and reaction resistance [mΩ] of the deposition resistance, taking into account manufacturing variations.

[0023] Here, the electrode facing area [cm 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 If any of these values ​​is outside the tolerance, the lithium-ion secondary battery cannot be shipped as it is deemed a defective product that does not meet the design values.

[0024] On the other hand, the normal electrode facing area [cm 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 A lithium-ion secondary battery with this function is a good product. Random inspections are performed from such a lot of good products. Then, the deposition current value (the minimum current value at which metallic lithium deposits) [A] and reaction resistance [mΩ] of the multiple product samples that are sampled are measured.

[0025] The correlation between the deposition current value [A] and the reaction resistance [mΩ] at the median design value as shown in Figure 1 is called the "reference correlation," and this graph is called the reference correlation line L0. Based on this "reference correlation line L0," a "corrected correlation" is calculated in advance, which is the correlation between the deposition current value [A] and the reaction resistance [mΩ] that has been corrected in advance to take into account the manufacturing variations in the electrode conditions.

[0026] Fig. 3 is a graph showing a "normal range SZ" defined based on the "corrected correlation line L1" that matches the electrode conditions of the lithium ion secondary battery to be manufactured. As shown in Fig. 3, the allowable tolerance range is defined as the "normal range" based on the "corrected correlation line L1" that matches the electrode conditions of the lithium ion secondary battery to be manufactured.

[0027] Then, the reaction resistance value [mΩ] and the deposition current value [A] of metallic lithium of the removed lithium ion secondary battery are determined. The removed lithium ion secondary battery is compared with the normal range of lithium ion secondary batteries shown in Figure 3. This comparison determines whether the removed lithium ion secondary battery is normal or abnormal depending on whether the reaction resistance value [mΩ] is within the normal range of the normal deposition current value [A].

[0028] By carrying out such a judgment, it is possible to provide a method for judging an abnormality in a lithium ion secondary battery with higher accuracy even if the electrode conditions differ due to variations in electrode manufacturing. The method for determining an abnormality in a lithium ion secondary battery according to this embodiment will be described in detail below.

[0029] <Overview of the method for determining abnormality in a lithium-ion secondary battery according to the present embodiment> 4 is a flow chart showing the procedure of the method for determining an abnormality in a lithium ion secondary battery according to the present embodiment. First, an outline of the procedure of the method for determining an abnormality in a lithium ion secondary battery according to the present embodiment will be described with reference to FIG.

[0030] In the method for determining an anomaly in a lithium ion secondary battery according to the present embodiment, first, as a preparation step, a step (S1) of acquiring a reference correlation is performed. The "reference correlation" is a reference correlation between the deposition current value [A] and the reaction resistance [mΩ] of a lithium ion secondary battery under electrode conditions set at the design median value.

[0031] Next, in the step (S2) of defining a normal range, the reference correlation acquired in the step (S1) of acquiring a reference correlation is corrected based on the electrode conditions of a lithium ion secondary battery to be manufactured in advance to acquire a corrected correlation. The "corrected correlation" is a correlation obtained by correcting the reference correlation acquired in the step (S1) of acquiring a reference correlation so as to conform to the electrode conditions of the lithium ion secondary battery to be evaluated. The electrode conditions of the lithium ion secondary battery to be evaluated are measured at the time of the source process and the manufacture of the electrode body. That is, the electrode facing area [cm 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 Based on these electrode conditions, the correction correlation shown in Figure 3 is obtained.

[0032] As shown in FIG. 3, once the corrected correlation is obtained, a normal range is defined based on the corrected correlation. Next, a step (S3) of manufacturing the lithium ion secondary battery is performed. Here, the lithium ion secondary battery to be inspected is manufactured in lots by the usual method. In the same lot, the electrode conditions of the lithium ion secondary battery, i.e., the electrode facing area [cm 2 ], negative electrode density [mg / cm 3], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 ] is uniform within the range of the allowed tolerance. If the electrode conditions are outside the tolerance, the lithium ion secondary battery of that lot is treated as a defective product.

[0033] Next, in a step (S4) of sampling good products, lithium ion secondary batteries to be judged are sampled from the lithium ion secondary batteries in the same production lot. The reason for sampling inspection is that if the step (S6) of obtaining the deposition current value by the low-temperature cycle test is performed, the lithium ion secondary batteries will deteriorate, and it is not possible to perform 100% inspection.

[0034] The reaction resistance [mΩ] of the removed lithium ion secondary battery is measured in a reaction resistance acquisition step (S5). Thereafter, in a deposition current value acquisition step (S6), the deposition current value [A] of the extracted lithium ion secondary battery is acquired.

[0035] In the step of determining anomaly (S7), the relationship between the reaction resistance [mΩ] obtained in the step of obtaining the reaction resistance (S5) and the deposition current value [A] obtained in the step of obtaining the deposition current value (S6) is determined. Here, it is determined whether the value is within the normal range SZ defined in the step of defining the normal range (S2) or outside the normal range SZ.

[0036] In step S8, it is determined whether the relationship between the reaction resistance [mΩ] obtained in the reaction resistance acquisition step (S5) and the deposition current value [A] obtained in the deposition current value acquisition step (S6) is within the normal range SZ specified in the normal range definition step (S2). If it is determined to be within the normal range SZ (S8: YES), it is determined to be normal (S9), and all lithium ion secondary batteries in that lot are determined to be normal, non-defective products and shipped as products.

[0037] On the other hand, the relationship between the reaction resistance [mΩ] acquired in the reaction resistance acquisition step (S5) and the deposition current value [A] acquired in the deposition current value acquisition step (S6) is determined. If it is determined that the relationship between the reaction resistance [mΩ] and the deposition current value [A] is outside the normal range SZ defined in the normal range definition step (S2) (S8: NO), it is determined that there is an abnormality (S10). Then, the entire lot of lithium ion secondary batteries is determined to be defective and shipment as products is suspended.

[0038] This completes the procedure for the method for determining an abnormality in a lithium ion secondary battery according to the present embodiment. Each step will now be described in detail. <Step of obtaining reference correlation (S1)> In the method for determining an abnormality in a lithium ion secondary battery according to the present embodiment, first, as a preparation step, a step (S1) of acquiring a reference correlation is performed. Here, a reference correlation serving as a reference between the deposition current value [A] and the reaction resistance [mΩ] of the lithium ion secondary battery under the set electrode conditions is acquired.

[0039] Here, the reaction resistance [mΩ] and the deposition current value [A] at that time are actually measured for a lithium-ion secondary battery with electrode conditions in the middle of the design values. Several lithium-ion secondary batteries with different reaction resistances [mΩ] are prepared, and the results of the deposition current values ​​[A] are plotted to obtain a straight line using the mean square method or the like. It is also possible to correct the theoretical values ​​for the actual measurements in response to errors in the electrode conditions.

[0040] <Measurement of reaction resistance [mΩ]> Here, a method for determining the reaction resistance [mΩ] in this embodiment will be described. The reaction resistance [mΩ] is measured by AC impedance measurement.

[0041] <Equivalent circuit of lithium-ion secondary battery 1> FIG. 5 is a circuit diagram that shows a schematic equivalent circuit of a lithium-ion secondary battery. A lithium-ion secondary battery can be expressed by an equivalent circuit as shown in FIG. 5. That is, it can be expressed as a parallel circuit of a solution resistance Rsol, a charge transfer resistance Rct, and an electric double layer Cdl connected in series thereto. A configuration having an electrolyte between electrodes is considered as an electric double layer Cdl, and functions as a capacitor. For this reason, the AC resistance of the electric double layer Cdl becomes a resistance component equivalent to a dielectric loss due to a delay in the polarization of the dielectric in the low frequency region, and becomes a resistance component equivalent to the skin effect and proximity effect of the electrodes in the high frequency region. In this way, the impedance changes depending on the frequency. Theoretically, the electric double layer Cdl does not conduct electricity in direct current, and the resistance value becomes zero as the frequency of the AC voltage increases. Therefore, at high frequencies (for example, 100 Hz or higher), the combined resistance of the circuit becomes equal to the solution resistance Rsol. As the frequency increases (100 Hz to 100 mHz), in addition to the solution resistance Rsol, the combined resistance becomes the charge transfer resistance Rct and the electric double layer Cdl. And at low frequencies (less than 100 mHz), the total resistance of the circuit is the sum of the solution resistance Rsol and the charge transfer resistance Rct.

[0042] In other words, the current flowing between the positive and negative electrodes, which is closely related to the deposition of metallic lithium, depends on this reaction resistance Rct [mΩ]. <AC impedance method> Because of the above-mentioned properties of lithium-ion secondary batteries, their characteristics are measured by the AC impedance method, which is a method for observing the impedance spectrum by applying and sweeping a voltage or current with a small amplitude and stepwise changing frequency to the electrode system of the lithium-ion secondary battery.

[0043] Here, "alternating current (AC)" refers to a current (alternating current) that periodically changes between positive and negative with time, and is an abbreviation of "alternating current." In this embodiment, the AC is a sine wave, but it also includes waveforms other than sine waves as long as the magnitude and direction change periodically. AC other than a sine wave is called non-sinusoidal alternating current, and includes square wave AC, triangular wave AC, sawtooth wave, and the like. Furthermore, instead of AC, measurement may be performed while changing the frequency using a DC pulse wave such as a square wave, triangular wave, or sawtooth wave having a constant period.

[0044] <Nyquist plot> FIG. 6 is a graph showing the relationship between the Nyquist plot by the AC impedance method and each resistance component.

[0045] The analysis results of lithium-ion secondary batteries using the AC impedance method are output as Nyquist plots, for example. From the impedance and phase difference, Nyquist plots allow the measurement of characteristics such as the electrode reaction rate of the secondary battery, the electrical conductivity of the electrolyte, and the electric double layer capacity.

[0046] In a Nyquist plot, the vertical axis is the imaginary value of resistance Zimg [Ω] and the horizontal axis is Zreal [Ω]. A voltage or current is applied to the electrode system of a lithium-ion secondary battery with a stepwise change in frequency from high frequency to minute amplitude. This causes an arc-shaped graph with its center on the horizontal axis to extend upward from the zero cross Px, which is a position shifted to the right along the horizontal axis from the origin Po at the bottom left. Then, a linear graph extends radially outward from a specified point at the top right.

[0047] <Electron transfer resistance Rs> In this embodiment, the electron transfer resistance Rs is expressed as the distance from the origin Po to the zero cross Px. In other words, it is expressed as the real value Zreal [Ω] of the resistance at the zero cross Px, and it is possible to analyze the resistance when electrons move in the electrolyte, the pole, the current collector, etc. at high frequencies of, for example, 100 Hz or more.

[0048] <Reaction resistance Rct> In the example of this embodiment, at intermediate frequencies (100 Hz to 0.1 Hz), the "reaction resistance Rct" which is the resistance when charge (ion) transfer occurs in a chemical reaction at the electrode can be analyzed. It is expressed as an arc-shaped graph with its center on the horizontal axis from the zero crossing Px, and the arc becomes larger in radius when the electrode performance deteriorates.

[0049] <Diffusion resistance Zw> At low frequencies below 0.1 Hz, it is possible to observe the "diffusion resistance Zw," which is the resistance experienced when charges diffuse within the active material.

[0050] <Analysis using Nyquist plot> FIG. 7 is a graph showing the relationship between the degradation of a lithium-ion secondary battery and the Nyquist plot. The solid line graph shows the Nyquist plot of a lithium-ion secondary battery in an initial state without degradation. When the resistance of the electrolyte, poles, current collectors, etc. when electrons move increases, the zero cross Px moves to the right along the horizontal axis to the zero cross Px'. Accordingly, the arc portion also shifts to the right and to the position shown by the dashed line. In other words, the real value Zreal of the resistance of the electron transfer resistance Rs increases.

[0051] When the electrolyte performance deteriorates, the radius of the arc portion of the Nyquist plot of a lithium ion secondary battery in an initial state that is not deteriorated increases, resulting in a graph as shown by the dashed dotted line.

[0052] As described above, in this embodiment, the reaction resistance Rct [mΩ] can be measured by the AC impedance method. By accurately measuring the reaction resistance Rct [mΩ], it is possible to clarify the relationship with the current that flows between the positive electrode and the negative electrode and is closely related to the deposition of metallic lithium.

[0053] <Measurement of deposition current value [A]> The low-temperature cycle test is a test in which a current of several tens to several hundreds of A is applied for several to several tens of seconds at a temperature of -6.7 to -40°C, and then a current of about 1 / 10 of that is applied in the opposite direction with the same capacity, and this test is repeated. By repeating this process, the test is performed to reproduce an environment in which metallic lithium is likely to precipitate due to overcharging and overdischarging at low temperatures.

[0054] In this embodiment, for example, a plurality of lithium ion secondary batteries were charged at a battery temperature of -6.7° C. for 5 seconds at a current of 100 to 200 A for each sample, and then discharged at 1 / 10 of that current for 50 seconds. This cycle was repeated 1000 times.

[0055] The presence or absence of metallic lithium deposition in the electrodes of each lithium-ion secondary battery with different currents after 1000 cycles was confirmed.Then, the boundary between the current value where deposition occurred and the current value where deposition did not occur was obtained, and this was designated as the metallic lithium deposition current value [A] of that lithium-ion secondary battery.

[0056] <Getting baseline correlation> The deposition current value [A] versus reaction resistance [mΩ] measured as described above is plotted on a multi-point graph, and a straight line passing through these values ​​is obtained using the least squares method, etc. A reference correlation is obtained between the deposition current value [A] and reaction resistance [mΩ] of the lithium-ion secondary battery under electrode conditions set at the design median value.

[0057] In this case, the line that represents the standard correlation, "standard correlation line L0", is "Deposition current value [A] = a × reaction resistance [mΩ] + b0 [A]" This can be expressed by a linear function.

[0058] <Step of defining normal range (S2)> In the step (S2) of defining the normal range, the reference correlation obtained in the step (S1) of obtaining the reference correlation is corrected based on the electrode conditions of the lithium ion secondary battery to be manufactured to obtain a corrected correlation. The electrode conditions of the lithium ion secondary battery to be judged are measured at the time of the source process and the manufacture of the electrode body. That is, the electrode facing area [cm 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 Based on these electrode conditions, the correction correlation shown in Figure 3 is obtained.

[0059] Once the corrected correlation is obtained, the normal range is defined based on this corrected correlation. Here, it is known that the "reference correlation line L0" shown in Figure 1 changes depending on the electrode conditions of the lithium-ion secondary battery. This change occurs as a parallel shift while keeping the slope a of the reference correlation line L0 unchanged, as shown in the graph of the "corrected correlation lines L1 to L3" in Figure 2, and only the intercept b0 changes. The change in this intercept is proportional to the area [cm2] of the electrode facing portion. 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 The corrections due to the respective electrode conditions will be explained below separately.

[0060] <Relationship between electrode facing area reduction rate [%] and intercept b0 of reference correlation line L0> 8 is a graph showing the relationship between the electrode facing area reduction rate [%] and the change in the intercept b0 of the reference correlation line L0. 2 ] becomes larger than the design median value, the current density [A / cm 2 ] is reduced, improving resistance to lithium deposition. 2 ] becomes smaller, the current density [A / cm 2 ] becomes larger, and the lithium precipitation resistance deteriorates. That is, as shown in FIG. 8, the design median electrode facing area [cm 2] is reduced in the electrode opposing area reduction rate [%], the intercept b0 of the reference correlation line L0 shifts downward.

[0061] <Negative electrode density [mg / cm 3 ] and the intercept b0 of the reference correlation line L0> Figure 9 shows the negative electrode density [mg / cm 3 ] and the change in the intercept b0 of the reference correlation line L0. 3 When ] becomes larger than the design median, Li + The diffusibility decreases and the resistance to deposition deteriorates. That is, the negative electrode density [mg / cm 3 ] becomes larger than the design median, the intercept b0 of the reference correlation line L0 shifts downward.

[0062] <Positive electrode weight [mg / cm 2 ] and the intercept b0 of the reference correlation line L0> Figure 10 shows the positive electrode weight [mg / cm 2 ] and the change in the intercept b0 of the baseline correlation line L0. 2 ] becomes larger than the design median value, the current density [A / cm 2 ] becomes larger and the precipitation resistance deteriorates. 2 When ] becomes larger than the design median, the intercept b0 of the reference correlation line L0 shifts downward.

[0063] <Negative electrode weight [mg / cm 2 ] and the intercept b0 of the reference correlation line L0> FIG. 11 shows the negative electrode weight [mg / cm 2 ] and the change in the intercept b0 of the reference correlation line L0. 2 When ] becomes larger than the design median value, Li per unit area + In other words, in this case, the negative electrode weight [mg / cm 2 ] becomes larger than the design median, the intercept b0 of the reference correlation line L0 shifts upward instead.

[0064] <Relationship between electrode conditions and intercept b0 of reference correlation line L0> FIG. 12 is a graph of corrected correlation lines L1 to L2 obtained by correcting the intercept b0 of the reference correlation line L0 to reflect the electrode conditions to be determined.

[0065] As explained above, the electrode facing area [cm 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 ] varies from the design median value, the intercept b0 of the reference correlation line L0 is changed accordingly. The change in intercept b0 for each change varies. Therefore, the intercept b0 is adjusted by multiplying each electrode condition change value by a constant set for each change and converting it into the intercept b0. The amount of correction for the intercept b0 can then be calculated by accumulating the shift amounts Δb[A] of these intercepts.

[0066] Here, the area of ​​the electrode facing part [cm 2 ] constant is "i", negative electrode density [mg / cm 3 ] is the constant "j", the positive electrode weight [mg / cm 2 ] is the constant "k", and the negative electrode weight [mg / cm 2 ] is expressed as a constant "l", then the downward shift amount Δb[A] relative to the intercept b0 obtained by integrating these is derived by the following formula.

[0067] “Shift amount Δb[A]=i×electrode facing area area reduction rate [%]+j×negative electrode density [mg / cm 3 ] + k × positive electrode weight [mg / cm 2 ]+l×Negative electrode weight [mg / cm 2 ]" In this case, since the "constant l" is a shift in the opposite direction (upward), the constant l is a negative number.

[0068] After the shift amount Δb[A] is calculated as described above, the intercept b0 of the reference correlation line L0 is shifted by the shift amount Δb, and translated downward to become the intercept b2, resulting in a corrected correlation line L1. <Definition of normal range> After obtaining the corrected correlation line L1, the area within the tolerance range, which is the allowable range for the tolerance of the lithium-ion secondary battery to be manufactured, is defined as the "normal range SZ" as shown in Figure 3. The area outside this normal range SZ is defined as the "abnormal range FZ."

[0069] <Step (S3) of manufacturing lithium-ion secondary batteries> Next, a step (S3) of manufacturing the lithium ion secondary battery is performed. Here, the lithium ion secondary batteries to be inspected are manufactured in lots. In the same lot, the electrode conditions of the lithium ion secondary batteries, i.e., the electrode facing area [cm 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 ] is uniform within the range of the allowed tolerance. If the electrode conditions are outside the tolerance, the lithium ion secondary battery of that lot is treated as a defective product.

[0070] <Step of removing non-defective products (S4)> Next, in a step (S4) of sampling good products, lithium ion secondary batteries to be judged are sampled from lithium ion secondary batteries in the same production lot. Since all lithium ion secondary batteries in the same production lot are good products within the tolerance range of the design of the same electrode conditions, this is a random sampling inspection. Sampling inspection is performed on multiple samples. This is because, in order to measure the deposition current value [A] at which metallic lithium is precipitated, it is necessary to measure and compare the deposition current values ​​[A] of multiple samples by inspection using a low-temperature cycle test.

[0071] The reason why a random inspection was performed is that when the step (S6) of obtaining the deposition current value by the low-temperature cycle test is performed, the lithium-ion secondary battery deteriorates and a destructive inspection is required to confirm the deposition of metallic lithium, so that a full inspection cannot be performed.

[0072] <Step of obtaining reaction resistance (S5)> The reaction resistance [mΩ] of the removed lithium ion secondary battery is measured in the reaction resistance acquisition step (S5) by the same method as the AC impedance measurement in the reference correlation acquisition step (S1).

[0073] <Step of acquiring deposition current value (S6)> In the step of acquiring the reaction resistance (S5), the object to be evaluated whose reaction resistance [mΩ] has been measured has a deposition current value [A] acquired in the step of acquiring the deposition current value (S6). The deposition current value [A] is measured in the same manner as in the low-temperature cycle test in the step of acquiring the reference correlation (S1).

[0074] <Abnormality determination step (S7)> In the step (S7) of determining an abnormality, it is determined whether the relationship between the reaction resistance [mΩ] obtained in the step (S5) of obtaining the reaction resistance and the deposition current value [A] obtained in the step (S6) of obtaining the deposition current value is within the normal range SZ specified in the step (S6) of specifying the normal range, or whether an abnormality outside the normal range is in the range FZ.

[0075] In step S8, if it is determined that the relationship between the reaction resistance [mΩ] obtained in the reaction resistance acquisition step (S5) and the deposition current value [A] obtained in the deposition current value acquisition step (S6) is within the normal range SZ specified in the normal range definition step (S2) (S8: YES), it is determined to be normal (S9), and the lithium-ion secondary batteries in that lot are determined to be normal, non-defective products, and shipped as products.

[0076] On the other hand, if it is determined that the relationship between the reaction resistance [mΩ] obtained in the reaction resistance acquisition step (S5) and the deposition current value [A] obtained in the deposition current value acquisition step (S6) is within the abnormal range FZ defined in the normal range definition step (S2) (S8: NO), it is determined to be abnormal (S10), and the entire lot of lithium-ion secondary batteries is determined to be defective and shipment of the products is suspended.

[0077] (Action of this embodiment) The method for determining an anomaly in a lithium ion secondary battery of the present invention corrects the determination criteria to match the electrode conditions even if there is variation in the manufacturing process of the lithium ion secondary battery, and therefore accurately determines an anomaly in lithium precipitation resistance by reflecting the electrode conditions.

[0078] (Effects of this embodiment) (1) The method for determining an abnormality in a lithium ion secondary battery of the present embodiment can accurately determine an abnormality in lithium precipitation resistance by reflecting the electrode conditions, even if there is variation in the manufacturing conditions of the electrode of the lithium ion secondary battery.

[0079] (2) As electrode conditions, the electrode facing area [cm 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 This reflects the electrode conditions in detail, allowing for a more accurate assessment of the lithium precipitation resistance of lithium-ion secondary batteries.

[0080] (3) The reference correlation is obtained as a linear function of the deposition current value against the reaction resistance, and in the step of defining the normal range, the corrected correlation is obtained by adjusting the intercept while keeping the slope of the linear function unchanged based on the electrode conditions to be evaluated, thereby simplifying the processing.

[0081] (4) The intercept b0 is adjusted by multiplying the values ​​of each electrode condition by a constant that is set for each electrode condition and then accumulating the results to calculate the intercept correction amount, thereby enabling an accurate judgment to be made by comprehensively taking into account various influences.

[0082] (5) Products to be judged are selected from non-defective products after removing products that are outside the design tolerance range of the electrode conditions in advance, so that judgment can be made efficiently without making unnecessary judgments. (6) The deposition current value [A] is obtained by a low-temperature cycle test, which allows reproduction of high-rate charge and discharge at low temperatures and accurate determination of lithium deposition resistance.

[0083] (7) Since the reaction resistance [mΩ] is obtained by AC impedance measurement, the reaction resistance [mΩ] between the electrodes can be accurately measured, and the lithium deposition resistance can be accurately determined.

[0084] (Another example) The numerical values ​​in the present embodiment are merely examples, and the present invention is not limited to these numerical values.

[0085] In this embodiment, the electrode conditions are: electrode facing area [cm 2 ], negative electrode density [mg / cm 3 ], positive electrode weight [mg / cm 2 ], negative electrode weight [mg / cm 2 [ ] are shown as examples, but are not limited to these. For example, the determination may be made based on any one to three of these. In addition, various parameters that bring about a change in the reference correlation may be referenced, without being limited to these four parameters.

[0086] In this embodiment, the deposition current value [A] is obtained by the "low temperature cycle test", but the method is not limited as long as the deposition current value [A] can be obtained. It may be a theoretically estimated value as well as an actual measurement value by other methods.

[0087] In this embodiment, the reaction resistance value [mΩ] is obtained by "AC impedance measurement", but the method is not limited as long as the reaction resistance value [mΩ] can be obtained. It may be an actual measurement value by another method, or a theoretical estimate.

[0088] In this embodiment, the reference correlation is obtained as a baseline quasi-correlation line L0, which is a linear function of the deposition current value against the reaction resistance. For convenience of explanation, in the step (S6) of defining the normal range, the slope of the baseline quasi-correlation line L0 is kept unchanged based on the electrode conditions to be determined, and the intercept b0 is adjusted to obtain the corrected correlation, but this is not limited to this embodiment. For example, the corrected correlation may be obtained by recording the actual measured values ​​when the electrode conditions are actually changed, without using a graph.

[0089] In the present embodiment, a thin-plate lithium ion secondary battery for vehicle use is illustrated, but the present invention can also be applied to cylindrical batteries, etc. In addition to being applied to vehicle use, the present invention can also be applied to batteries for ships, aircraft, and even stationary use.

[0090] The flowchart shown in FIG. 4 is an example and a person skilled in the art can add, delete, or modify the steps, or change the order of steps. For example, after the step of manufacturing the lithium ion secondary battery (S3), the step of determining the normal range (S6) may be performed before the step of determining an abnormality (S7).

[0091] It goes without saying that those skilled in the art may add, delete, or modify the components of the present invention, and change the order of the components, without departing from the scope of the claims. [Explanation of symbols]

[0092] L0: Reference correlation line L1, L2, L3...Correlation line b0, b1, b2...intercept

Claims

1. A step of acquiring a reference correlation, which is a linear function obtained from the relationship between a change in reaction resistance, which is charge transfer resistance, and a corresponding deposition current value, which is the minimum current value at which metallic lithium is deposited, the change being determined by measuring the AC impedance of a lithium ion secondary battery selected from non-defective products that fall within the design tolerance range of electrode conditions including at least one of the electrode opposing area, negative electrode density, positive electrode basis weight, and negative electrode basis weight; A reaction resistance acquisition step of acquiring a reaction resistance of the lithium ion secondary battery to be determined by AC impedance measurement; A deposition current value acquisition step of acquiring a deposition current value which is a minimum current value at which metallic lithium is deposited in the lithium ion secondary battery to be determined by a low-temperature cycle test; a step of correcting the reference correlation to obtain a corrected correlation based on a difference between the electrode condition of the lithium ion secondary battery to be determined and the electrode condition of the reference correlation obtained in advance, and defining a normal range that is permissible based on the corrected correlation; A method for determining an abnormality in a lithium ion secondary battery, comprising: an abnormality determination step of determining that the lithium ion secondary battery being determined to be abnormal if the relationship between the reaction resistance obtained in the reaction resistance acquisition step and the deposition current value obtained in the deposition current value acquisition step is outside the normal range.

2. The method for determining an abnormality in a lithium ion secondary battery as described in claim 1, characterized in that in the step of specifying a normal range, the corrected correlation is obtained by adjusting the intercept while keeping the slope of the linear function unchanged based on the electrode condition to be determined.

3. 3. The method for determining an abnormality in a lithium ion secondary battery according to claim 2, wherein the intercept is adjusted by multiplying the value of each electrode condition by a constant set for each electrode condition and integrating the results to calculate a correction amount for the intercept.

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