JP7671673B2 - Method for determining abnormality in lithium-ion secondary batteries - Google Patents

Description

The present invention relates to a method for determining an abnormality in a lithium-ion secondary battery, and more specifically, to a method for determining an abnormality in a lithium-ion secondary battery that reflects variations in electrode conditions.

Lithium ion secondary batteries are widely used, for example, as power sources for electric vehicles and the like. Because such lithium ion secondary batteries are used under harsh conditions, it is desirable to accurately determine the quality of manufactured lithium ion secondary batteries before shipping.

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.
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.

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. In this case, the amount of lithium precipitation that depends on the reaction resistance is calculated by using the exchange current density corrected by the rate of change of reaction resistance.

The invention described in Patent Document 1 makes it possible to estimate the resistance value, which is important for battery performance. Furthermore, the invention described in Patent Document 2 makes it possible to estimate the amount of lithium deposition that depends on the reaction resistance.

JP 2020-139913 A JP 2016-4726 A

However, the invention described in Patent Document 1 does not take into account changes in the correlation between resistance value and capacity due to manufacturing variability, which may lead to erroneous determination of an abnormality. In addition, the relationship between resistance value and lithium precipitation resistance is not taken into account.

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 this may result in reduced accuracy in estimating the amount of lithium precipitation.

The problem that the lithium ion secondary battery anomaly determination method of the present invention aims to solve is to accurately determine anomalies in lithium precipitation resistance by reflecting electrode conditions, even when there is variation in the manufacturing of lithium ion secondary batteries.

In order to solve the above problem, the method for determining an abnormality of a lithium-ion secondary battery of the present invention includes a step of acquiring a reference correlation that is a reference for the deposition current value and reaction resistance of a lithium-ion secondary battery under set electrode conditions, a step of acquiring a reaction resistance of the lithium-ion secondary battery to be determined, a step of acquiring a deposition current value of the lithium-ion secondary battery to be determined, a step of correcting the reference correlation to obtain a corrected correlation based on the electrode conditions of the lithium-ion secondary battery to be determined, and a step of determining a normal range based on the corrected correlation, and an abnormality determination step of determining that the lithium-ion secondary battery to be determined is abnormal if the relationship between the reaction resistance obtained in the step of acquiring the reaction resistance and the deposition current value obtained in the step of acquiring the deposition current value falls outside the normal range.

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.

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.

The deposition current value can be obtained by a low-temperature cycle test.
The reaction resistance can be obtained by AC impedance measurement.

The method for determining anomalies in lithium ion secondary batteries of the present invention can accurately determine anomalies in lithium precipitation resistance by reflecting electrode conditions, even when there is variation in the manufacturing of lithium ion secondary batteries.

1 is a graph of a "reference correlation line L ₀ " showing the correlation between the deposition current [A] and the reaction resistance [mΩ] for the design median value of the electrode conditions. 1 is a graph of "corrected correlation lines L ₁ to L ₃ " showing that the correlation between the deposition current [A] and the reaction resistance [mΩ] changes depending on the electrode conditions. 1 is a graph showing a "normal range SZ" defined based on a "corrected correlation line L ₁ " that matches the electrode conditions of a lithium ion secondary battery to be manufactured. 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. FIG. 1 is a circuit diagram illustrating an equivalent circuit of a lithium ion secondary battery. 1 is a graph showing a relationship between a Nyquist plot by an AC impedance method and each resistance component. 1 is a graph showing the relationship between deterioration of a lithium ion secondary battery and a Nyquist plot. 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 . 1 is a graph showing the relationship between negative electrode density and changes in intercept _b0 of reference correlation line _L0 . 13 is a graph showing the relationship between the positive electrode basis weight and the change in the intercept _b0 of the baseline correlation line _L0 . 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 . 13 is a graph of corrected correlation lines L ₁ to L ₂ obtained by correcting the intercept b ₀ of the reference correlation line L ₀ to reflect the electrode conditions to be determined.

(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.

<Principle of this embodiment>
In lithium ion secondary batteries, the deposition of metallic lithium (hereinafter sometimes abbreviated as "Li") can cause short circuits between plates, and so-called batteries with low lithium deposition resistance (metallic Li is easily deposited at low current) are undesirable. Therefore, in lithium ion secondary batteries, electrode conditions such as the electrode facing area [ ^cm2 ], negative electrode density [mg/ ^cm3 ], positive electrode weight [mg/ ^cm2 ], and negative electrode weight [mg/ ^cm2 ] are inspected. Then, only those batteries that are within a certain tolerance range from the design value are deemed to be good products having a specified lithium deposition resistance, and other batteries are deemed to be defective products, and only the good products are shipped as products.

FIG. 1 is a graph of the "reference correlation line L ₀ " 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 L ₀ ". The intercept is b _0. 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.

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 L ₁ to L ₃ " which show that the correlation between the deposition current [A] and the reaction resistance [mΩ] changes depending on the electrode conditions.

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 FIG. 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.

This can lead to 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. This can lead to a problem of reduced accuracy in the determination.

Therefore, in the present invention, the correlation between the deposition current value [ ^A ] and the reaction resistance [mΩ] of the deposition resistance is obtained in advance, taking into account manufacturing variations in the electrode opposing area [cm2], negative electrode density [mg/ ^cm3 ], positive electrode basis weight [mg/ ^cm2 ], and negative electrode basis weight [mg/ ^cm2 ], which are closely related to lithium deposition resistance.

If the electrode opposing area [ ^cm2 ], negative electrode density [mg/ ^cm3 ], positive electrode basis weight [mg/ ^cm2 ], or negative electrode basis weight [mg/ ^cm2 ] is outside the tolerances, the lithium-ion secondary battery is deemed a defective product that does not meet the design values and cannot be shipped.

On the other hand, lithium ion secondary batteries having normal electrode facing area [ ^cm2 ], negative electrode density [mg/ ^cm3 ], positive electrode weight [mg/ ^cm2 ], and negative electrode weight [mg/ ^cm2 ] within a certain range of tolerance are considered to be good products. Random inspection is performed from such a lot of good products. Then, the deposition current value (the minimum current value at which metallic lithium is deposited) [A] and reaction resistance [mΩ] of the multiple product samples taken are measured.

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 L _0. Based on this "reference correlation line L ₀ ", a "corrected correlation" is calculated in advance, which is the correlation between the deposition current value [A] and the reaction resistance [mΩ] corrected in advance to take into account the manufacturing variations in the electrode conditions.

3 is a graph showing a "normal range SZ" defined based on the "corrected correlation line L ₁ " that matches the electrode conditions of the lithium ion secondary battery to be manufactured. As shown in FIG. 3, an allowable tolerance range is defined as a "normal range" based on the "corrected correlation line L ₁ " that matches the electrode conditions of the lithium ion secondary battery to be manufactured.

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 reaction resistance value [mΩ] is within the normal range of the normal deposition current value [A], and whether the removed lithium ion secondary battery is normal or abnormal.

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.

<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.

In the method for determining an anomaly in a lithium ion secondary battery according to this embodiment, first, as a preparation step, a step (S1) of acquiring a reference correlation is carried out. The "reference correlation" is a reference correlation that is a standard 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.

Next, in the step (S2) of defining the normal range, the reference correlation acquired in the step (S1) of acquiring the reference correlation is corrected based on the electrode conditions of the 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 the reference correlation so as to conform to the electrode conditions of the lithium ion secondary battery to be judged. 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 ], the negative electrode density [mg/cm ³ ], the positive electrode basis weight [mg/cm ² ], and the negative electrode basis weight [mg/cm ² ] are measured. Based on these electrode conditions, the corrected correlation as shown in FIG. 3 is obtained.

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 a lot unit by a normal method. In the same lot, the electrode conditions of the lithium ion secondary battery, i.e., the electrode facing area [cm ² ], the negative electrode density [mg/cm ³ ], the positive electrode basis weight [mg/cm ² ], and the negative electrode basis weight [mg/cm ² ], are uniform within the range of the allowable tolerance. If the electrode conditions are out of the tolerance, the lithium ion secondary battery of that lot is treated as a defective product.

Next, in the step of sampling good products (S4), lithium ion secondary batteries to be judged are sampled from the lithium ion secondary batteries in the same production lot. Sampling inspection is performed because the lithium ion secondary batteries will deteriorate if the step of obtaining the deposition current value by low-temperature cycle testing (S6) is performed, and 100% inspection cannot be performed.

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.

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.

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 are shipped as products.

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 halted.

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 acquiring 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. In this step, 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.

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 similar. Note that it is also possible to correct the theoretical values for the actual measured values in response to errors in the electrode conditions.

<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.

<Equivalent circuit of lithium ion secondary battery 1>
FIG. 5 is a circuit diagram showing a schematic equivalent circuit of a lithium ion secondary battery. A lithium ion secondary battery can be represented by an equivalent circuit as shown in FIG. 5. That is, it can be represented 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. 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.

In other words, the current flowing between the positive electrode and the negative electrode, 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.

The term "alternating current (AC)" refers to a current whose plus and minus values change periodically over time (alternating current), 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, measurements may be made while changing the frequency using a DC pulse wave such as a square wave, triangular wave, or sawtooth wave with a fixed period.

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

The results of the analysis 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.

In a Nyquist plot, the vertical axis represents the imaginary value of resistance Zimg [Ω] and the horizontal axis represents Zreal [Ω], and voltage or current is applied to the electrode system of a lithium-ion secondary battery at a stepwise changing frequency, from high frequency to minute amplitude. This results in an arc-shaped graph with its center on the horizontal axis extending upward from the zero cross Px, which is 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.

<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 a high frequency of, for example, 100 Hz or more.

<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 cross Px, and the arc becomes larger in radius when the electrode performance deteriorates.

<Diffusion resistance Zw>
At low frequencies below 0.1 Hz, it is possible to observe the "diffusion resistance Zw," which is the resistance when the charge in the active material diffuses.

<Analysis using Nyquist plot>
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, the poles, the current collector, etc. when electrons move increases, the zero cross Px moves to a zero cross Px', which is a position shifted to the right along the horizontal axis. Accordingly, the arc portion also shifts to the right and shifts 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.

When the electrolyte performance deteriorates, the radius of the Nyquist plot arc of a lithium-ion secondary battery in its initial, undegraded state increases, resulting in a graph like that shown by the dashed dotted line.

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.

<Measurement of deposition current value [A]>
The low-temperature cycle test involves applying a current of several tens to several hundreds of A for several to several tens of seconds at temperatures between -6.7 and -40°C, and then repeatedly applying a current of about 1/10 of that amount in the opposite direction with the same capacity. This test is carried out by repeating this process to recreate an environment in which metallic lithium is likely to precipitate due to overcharging and overdischarging at low temperatures.

In this embodiment, for example, multiple lithium-ion secondary batteries were charged at a battery temperature of -6.7°C for 5 seconds at 100 to 200 A, with the current changed for each sample, and then discharged for 50 seconds at 1/10 of that current. This cycle was repeated 1,000 times.

Then, 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. 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.

Obtaining baseline correlations
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 a lithium-ion secondary battery under electrode conditions set at the design median value.

In this case, the straight line "reference correlation line L ₀ " representing the reference correlation is
"Deposition current value [A] = a x reaction resistance [mΩ] + b ₀ [A]"
This can be expressed by a linear function.

<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 [ ^cm2 ], the negative electrode density [mg/ ^cm3 ], the positive electrode basis weight [mg/ ^cm2 ], and the negative electrode basis weight [mg/ ^cm2 ] are measured. Based on these electrode conditions, the corrected correlation as shown in FIG. 3 is obtained.

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 L ₀ " shown in FIG. 1 changes depending on the electrode conditions of the lithium ion secondary battery. The change is a parallel shift of the slope a of the reference correlation line L ₀ as shown in the graph of the "corrected correlation lines L ₁ to L ₃ " shown in FIG. 2, and only the intercept b ₀ changes. The change of this intercept depends on each of the electrode conditions, namely the electrode facing area [cm ² ], the negative electrode density [mg/cm ³ ], the positive electrode basis weight [mg/cm ² ], and the negative electrode basis weight [mg/cm ² ]. Hereinafter, the correction according to each electrode condition will be explained separately.

<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 opposing area reduction rate [%] and the change in the intercept _b0 of the reference correlation line _L0 . When the electrode opposing area [ ^cm2 ] is larger than the design median, the current density [A/ ^cm2 ] is reduced, and the lithium precipitation resistance is improved. When the electrode opposing area [ ^cm2 ] is reduced, the current density [A/ ^cm2 ] is increased, and the lithium precipitation resistance is deteriorated. That is, as shown in FIG. 8, the intercept _b0 of the reference correlation line _L0 shifts downward as the design median electrode opposing area [ ^cm2 ] decreases in accordance with the decrease in the electrode opposing area reduction rate [%].

<Relationship between negative electrode density [mg/cm ³ ] and intercept b ₀ of reference correlation line L ₀ >
9 is a graph showing the relationship between the negative electrode density [mg/cm ³ ] and the change in the intercept b ₀ of the reference correlation line L _0. When the negative electrode density [mg/cm ³ ] is greater than the design median, the Li ⁺ diffusivity decreases and the precipitation resistance deteriorates. That is, when the negative electrode density [mg/cm ³ ] is greater than the design median, the intercept b ₀ of the reference correlation line L ₀ shifts downward.

<Relationship between positive electrode weight [mg/cm ² ] and intercept b ₀ of reference correlation line L ₀ >
10 is a graph showing the relationship between the positive electrode basis weight [mg/ ^cm2 ] and the change in the intercept _b0 of the baseline correlation line _L0 . When the positive electrode basis weight [mg/ ^cm2 ] is greater than the design median, the current density [A/ ^cm2 ] increases and the precipitation resistance deteriorates. In other words, when the positive electrode basis weight [mg/ ^cm2 ] is greater than the design median, the intercept _b0 of the baseline correlation line _L0 shifts downward.

<Relationship between negative electrode weight [mg/cm ² ] and intercept b ₀ of reference correlation line L ₀ >
11 is a graph showing the relationship between the negative electrode basis weight [mg/ ^cm2 ] and the change in the intercept _b0 of the reference correlation line _L0 . When the negative electrode basis weight [mg/ ^cm2 ] is greater than the design median, the Li ⁺ acceptability per unit area improves and the precipitation resistance improves. That is, in this case, when the negative electrode basis weight [mg/ ^cm2 ] is greater than the design median, the intercept _b0 of the reference correlation line _L0 shifts upward instead.

<Relationship between electrode conditions and intercept _b0 of reference correlation line _L0 >
FIG. 12 is a graph of corrected correlation lines L _{1 to} L ₂ obtained by correcting the intercept b ₀ of the reference correlation line L 0 to reflect the electrode conditions to be determined.

As described above, when the electrode opposing area [cm ² ], negative electrode density [mg/cm ³ ], positive electrode basis weight [mg/cm ² ], and negative electrode basis weight [mg/cm ² ] change from their design median values, the intercept b ₀ of the reference correlation line L ₀ is changed. The change in intercept b ₀ for each change varies. Therefore, the intercept b ₀ is adjusted by multiplying the value of each electrode condition change by a constant set for each change to convert it into intercept b _0. Then, the correction amount of intercept b ₀ can be calculated by integrating the shift amount Δb [A] of these intercepts.

Here, if the electrode opposing area [ ^cm2 ] is a constant "i", the negative electrode density [mg/ ^cm3 ] is a constant "j", the positive electrode basis weight [mg/ ^cm2 ] is a constant "k", and the negative electrode basis weight [mg/ ^cm2 ] is a constant "l", the downward shift Δb [ _A ] relative to the intercept b0 obtained by integrating these is derived by the following formula.

"Shift amount Δb [A] = i × electrode opposing area reduction rate [%] + j × negative electrode density [mg/cm ³ ] + k × positive electrode area weight [mg/cm ² ] + l × negative electrode area 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.

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 the corrected correlation line _L1 is obtained, the region within the tolerance range, which is the range permitted as the tolerance of the lithium ion secondary battery to be manufactured, is defined as the "normal range SZ" as shown in Fig. 3. The region outside this normal range SZ is defined as the "abnormal range FZ".

<Step of manufacturing lithium ion secondary battery (S3)>
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. In the same lot, the electrode conditions of the lithium ion secondary battery, i.e., the electrode facing area [cm ² ], the negative electrode density [mg/cm ³ ], the positive electrode basis weight [mg/cm ² ], and the negative electrode basis weight [mg/cm ² ], are uniform within the range of the allowable tolerance. If the electrode conditions are out of the tolerance, the lithium ion secondary battery of that lot is treated as a defective product.

<Step of removing non-defective products (S4)>
Next, in a step of sampling good products (S4), lithium ion secondary batteries to be judged are sampled from lithium ion secondary batteries of the same production lot. Since all lithium ion secondary batteries of 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. A plurality of samples are sampled for sampling inspection. 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 the plurality of samples by inspection using a low-temperature cycle test.

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 a full inspection cannot be performed.

<Reaction resistance acquisition step (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).

<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 undergoes the step of acquiring the deposition current value (S6) to acquire the deposition current value [A]. 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).

<Abnormality determination step (S7)>
In the abnormality determination step (S7), 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 (S6), or whether an abnormality outside the normal range is in the range FZ.

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 defined 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 are shipped as products.

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 product is suspended.

(Operation 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.

(Effects of this embodiment)
(1) The method for determining an abnormality in a lithium ion secondary battery according to 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.

(2) The electrode conditions are divided into the electrode facing area [cm ² ], negative electrode density [mg/cm ³ ], positive electrode basis weight [mg/cm ² ], and negative electrode basis weight [mg/cm ² ], which are closely related to lithium precipitation resistance. As a result, the electrode conditions are reflected in detail, and the lithium precipitation resistance of lithium ion secondary batteries can be determined more accurately.

(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, thus simplifying the process.

(4) The adjustment of the intercept _b0 calculates the correction amount of the intercept by multiplying the value of each electrode condition by a constant that is set for each electrode condition and integrating the results, so that an accurate judgment can be made by comprehensively taking into account various influences.

(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 the judgment can be made efficiently without making unnecessary judgments.
(6) The deposition current value [A] is obtained by a low-temperature cycle test, which reproduces high-rate charge and discharge at low temperatures and allows accurate determination of lithium deposition resistance.

(7) The reaction resistance [mΩ] is obtained by AC impedance measurement, so the reaction resistance [mΩ] between the electrodes can be accurately measured, enabling accurate assessment of lithium precipitation resistance.

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

In this embodiment, the electrode conditions are exemplified by four items, namely, the electrode facing area [cm ² ], the negative electrode density [mg/cm ³ ], the positive electrode basis weight [mg/cm ² ], and the negative electrode basis weight [mg/cm ² ], but are not limited to these. For example, the electrode conditions may be determined based on any one to three of these. In addition, various parameters that change the reference correlation may be referenced, without being limited to these four parameters.

In this embodiment, the deposition current value [A] is obtained by a "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 obtained by another method.

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 a theoretically estimated value, or an actual measurement value obtained by another method.

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 intercept _b0 is adjusted while keeping the slope of the baseline quasi-correlation line _L0 unchanged based on the electrode conditions to be determined to obtain a 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.

In this embodiment, a thin-plate lithium-ion secondary battery for vehicle use is shown as an example, but the battery can also be applied to cylindrical batteries. The battery can also be applied not only to vehicle use, but also to marine, aircraft, and stationary batteries.

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).

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

L ₀ : Reference correlation line L ₁ , L ₂ , L ₃ : Corrected correlation lines b ₀ , b ₁ , b ₂ : Intercepts

Claims (3)

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 a minimum current value at which metallic lithium is deposited, the change being obtained by measuring the AC impedance of a lithium ion secondary battery selected from non-defective products within a tolerance range of a design of electrode conditions including at least one of an electrode opposing portion area, a negative electrode density, a positive electrode basis weight, and a 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. The method for determining an abnormality in a lithium ion secondary battery according to claim 1, characterized in that 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 condition to be determined. 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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