Method for determining recycling of electrodes of secondary batteries and program for determining recycling of electrodes of secondary batteries

Abstract

Provided is a recycle determination method for an electrode of a secondary battery, the method enabling determination of a deterioration degree of an electrode to be recycled. This recycle determination method for an electrode of a secondary battery comprises: a step for estimating a characteristic parameter at the time of determination of the secondary battery subject to the determination (step S1); a step for obtaining the relationship between an electrolyte diffusion coefficient and discharge capacity (step S2); a step for defining a threshold Dth for the electrolyte diffusion coefficient on the basis of the relationship between the electrolyte diffusion coefficient and the discharge capacity (step S3); and a step for comparing the threshold Dth with a threshold Dth0 for an electrolyte diffusion coefficient prior to deterioration that has been obtained in advance for the secondary battery subject to the determination or for a secondary battery of the same type as the secondary battery subject to the determination, to classify the secondary battery subject to the determination on the basis of a difference between the threshold Dth and the threshold Dth0, that is ∆Dth = Dth - Dth0 (step S4).

Description

[Technical Field]

[0001] The present invention relates to a method for determining whether electrodes of a secondary battery are recycled and a program for determining whether electrodes of a secondary battery are recycled. [Background technology]

[0002] The EU is currently working to establish legal regulations for the recycling of secondary batteries such as lithium-ion batteries. Specifically, legal regulations are being established that require that a specified percentage or more of the materials used in secondary batteries be reused in other secondary batteries.

[0003] In this specification, the term "recycling" of secondary batteries is used to refer to a concept that includes all of the following: (a) dismantling a secondary battery and recovering reusable parts, (b) subjecting the parts obtained by dismantling a secondary battery to a predetermined process to make them reusable, and (c) recovering materials (valuable metals, etc.) from the parts that make up a secondary battery. In this specification, "recycling" of secondary batteries is a different concept from "reusing" secondary batteries (reusing the secondary battery itself rather than parts or materials). Generally, secondary batteries that are deemed unsuitable for reuse are subject to recycling.

[0004] Japanese Patent Application Laid-Open Publication No. 2010-34021 describes a method for recovering oxide-containing battery materials from waste battery materials. This recovery method includes the steps of immersing a substrate to which an oxide-containing battery material is attached in a solvent in which the oxide is substantially insoluble to remove the battery material from the substrate, and separating the removed battery material from the substrate. The publication describes solvents such as N-methyl-2-pyrrolidone, water, dimethyl carbonate, diethyl carbonate, and chloroform. The publication also describes calcining the recovered oxide at a temperature of 600°C to 1100°C.

[0005] Japanese Patent Publication No. 5141970 describes a method for recovering positive electrode active material from the positive electrode of a lithium battery. This recovery method targets lithium batteries that have a positive electrode active material layer formed by applying a material containing a positive electrode active material dispersed in an aqueous solvent, a conductive material, and a binder to the surface of a positive electrode current collector. This recovery method includes the steps of immersing the positive electrode in an alkaline aqueous solution to peel the positive electrode active material layer from the positive electrode current collector, adding an organic solvent to the peeled positive electrode active material layer to extract the binder from the peeled material, and separating a supernatant containing the conductive material from a sediment containing the positive electrode active material from the extracted material after the extraction step.

[0006] JP 2017-97997 A describes a characteristic analysis method for a secondary battery that uses a model equation with the characteristic values of the components constituting the battery as parameters and estimates the characteristic values of the components by fitting the battery voltage value expressed by the model equation to actual measurement data. This characteristic analysis method uses the actual measurement data obtained by applying to the battery under analysis a charge / discharge pattern including an operating period consisting of either a constant current discharge period or a constant current charge period, and a rest period provided following the operating period. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-34021 [Patent Document 2] Patent No. 5141970 [Patent Document 3] Japanese Patent Application Publication No. 2017-97997 Summary of the Invention [Problem to be solved by the invention]

[0008] There are three main methods for recycling secondary battery electrodes: (1) Horizontal recycling: Secondary batteries are dismantled, the electrode mixture is cleaned, and the batteries are reused as electrodes. (2) Wet recycling: Secondary batteries are dismantled, and the active material is recovered from the electrode mixture. If necessary, lithium or other substances are added to regenerate the battery, allowing the active material to be reused. (3) Dry recycling: Secondary batteries are dismantled or burned as is, and the materials used are refined and recovered as metal materials.

[0009] Of the above methods, dry recycling is the most costly, so horizontal or wet recycling is preferable if possible. Whether horizontal or wet recycling is possible depends on the degree of electrode deterioration. Therefore, it is possible to evaluate the degree of electrode deterioration individually and change the recycling method depending on the degree of deterioration.

[0010] An object of the present invention is to provide a method for determining the recycling of electrodes of secondary batteries, which can determine the degree of deterioration of electrodes to be recycled, and to provide a program for determining the recycling of electrodes of secondary batteries. [Means for solving the problem]

[0011] a step of determining a relationship between the electrolyte diffusion coefficient and the discharge capacity by using a predetermined model formula based on data obtained by measuring the secondary battery; a step of determining a discharge capacity when the electrolyte diffusion coefficient is changed based on the model formula and the estimated characteristic parameters, and determining a relationship between the electrolyte diffusion coefficient and the discharge capacity; a step of determining a threshold value Dth for the electrolyte diffusion coefficient based on the relationship between the electrolyte diffusion coefficient and the discharge capacity; and a step of comparing the threshold value Dth with a threshold value Dth0 for the electrolyte diffusion coefficient before deterioration that has been obtained in advance for the secondary battery or a secondary battery of the same type as the secondary battery being determined, and classifying the secondary battery to be determined based on a difference ΔDth=Dth−Dth0 between the threshold value Dth and the threshold value Dth0.

[0012] a step of determining a relationship between the electrolyte diffusion coefficient and the discharge capacity by using a predetermined model formula based on data obtained by measuring the secondary battery; a step of determining a relationship between the electrolyte diffusion coefficient and the discharge capacity by calculating a discharge capacity when the electrolyte diffusion coefficient is changed based on the model formula and the estimated characteristic parameters; a step of determining a threshold value Dth for the electrolyte diffusion coefficient based on the relationship between the electrolyte diffusion coefficient and the discharge capacity; and a step of comparing the threshold value Dth with a threshold value Dth0 for the electrolyte diffusion coefficient before deterioration that has been obtained in advance for the secondary battery or a secondary battery of the same type as the secondary battery being determined, and classifying the secondary battery to be determined based on the difference ΔDth=Dth−Dth0 between the threshold value Dth and the threshold value Dth0. [Effects of the Invention]

[0013] According to the present invention, it is possible to determine the degree of deterioration of an electrode to be recycled. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a flow chart of a diagnostic method for a secondary battery. [Figure 2] FIG. 2 is a flowchart showing an example of a more specific procedure for the step of estimating the characteristic parameters. [Figure 3] FIG. 3 is a graph showing an example of the relationship between the electrolyte diffusion coefficient and the discharge capacity. [Figure 4] FIG. 4 is a diagram showing an example of how the threshold value Dth is determined. [Figure 5] FIG. 5 is a flowchart of the method for determining whether an electrode of a secondary battery is recycled according to the first embodiment of the present invention. [Figure 6]FIG. 6 is a flowchart of a method for determining whether an electrode of a secondary battery is recycled according to a second embodiment of the present invention. [Figure 7] FIG. 7 shows an example of a method for classifying batteries to be judged according to this embodiment. [Figure 8] FIG. 8 shows another example of a method for classifying batteries to be judged according to this embodiment. [Figure 9] FIG. 9 is a flowchart of a method for determining whether an electrode of a secondary battery is recycled according to the third embodiment of the present invention. [Figure 10] FIG. 10 is an SEM photograph of the positive electrodes (LCO) of Judgment Examples 1 to 3 in their initial states. [Figure 11] FIG. 11 is an SEM photograph of the positive electrode of Judgment Example 1 after deterioration. [Figure 12] FIG. 12 is an SEM photograph of the positive electrode of Judgment Example 2 after deterioration. [Figure 13] FIG. 13 is an SEM photograph of the positive electrode of Judgment Example 3 after deterioration. [Figure 14] FIG. 14 is an SEM photograph of the positive electrodes (NCM) of Judgment Examples 4 and 5 in their initial states. [Figure 15] FIG. 15 is an SEM photograph of the positive electrode of Judgment Example 4 after deterioration. [Figure 16] FIG. 16 is an SEM photograph of the positive electrode of Evaluation Example 5 after deterioration. DETAILED DESCRIPTION OF THE INVENTION

[0015] The degree of electrode deterioration is judged mainly from two points: discharge capacity per weight and discharge characteristics. Even if the discharge capacity per weight is maintained, if the discharge characteristics have deteriorated, it is difficult to use the electrode for horizontal recycling.

[0016] The present inventors previously developed a diagnostic method for secondary batteries that focuses on the electrolyte diffusion coefficient (PCT / JP2023 / 003516). In this diagnostic method, characteristic parameters (e.g., electrolyte diffusion coefficient Dn, electrolyte conductivity) of the secondary battery at the time of diagnosis are estimated based on data obtained by measuring the load characteristics of the secondary battery. Next, the relationship between the electrolyte diffusion coefficient and discharge capacity is determined based on a model formula and the estimated parameters. Based on this "relationship between the electrolyte diffusion coefficient and discharge capacity," the value (threshold value) of the electrolyte diffusion coefficient Dth at which the secondary battery becomes unsuitable for reuse is determined. The larger the difference ΔD = Dn - Dth between the electrolyte diffusion coefficient Dn at the time of diagnosis and the threshold value Dth, the more likely it is that the secondary battery to be diagnosed will be usable for a long period of time (i.e., will have a long remaining life).

[0017] The inventors focused on the threshold value Dth obtained in the process of carrying out the above diagnostic method. The threshold value Dth is a value that changes depending on the current rate. Therefore, the threshold value Dth may indirectly reflect the discharge characteristics of the electrode to be recycled.

[0018] The above diagnostic method allows the remaining life to be evaluated using a single parameter (electrolyte diffusion coefficient) by rounding off changes in characteristic parameters other than the electrolyte diffusion coefficient into changes in the electrolyte diffusion coefficient. It has been found that this method can also evaluate the remaining life of a secondary battery with a certain degree of accuracy.

[0019] However, in an actual secondary battery, characteristic parameters other than the electrolyte diffusion coefficient (e.g., electrolyte conductivity) also change with cycle deterioration. Therefore, if the latest characteristic parameters are estimated by measuring the load characteristics again after the time of diagnosis and the threshold value Dth is determined from these latest characteristic parameters using the same method as above, the threshold value Dth reflecting the latest characteristic parameters may differ from the threshold value Dth determined from the characteristic parameters at the time of diagnosis.

[0020] That is, strictly speaking, the threshold value Dth obtained in the course of carrying out the above-described diagnostic method changes depending on the timing at which the diagnosis is carried out, or in other words, the threshold value Dth itself changes with the cycle deterioration of the secondary battery.

[0021] The present inventors investigated the relationship between the deterioration of electrodes in secondary batteries and the threshold Dth. As a result, they found a new relationship in which, in secondary batteries in which the threshold Dth increases with cycle deterioration, either (i) the porosity of the electrode mixture layer is significantly reduced, obstructing the ion pathway in the electrolyte, or (ii) structural deterioration of the electrode occurs.

[0022] Regarding (i), the electrode mixture layer generally tends to expand with use, and if there is little precipitate due to the electrolyte reaction, the porosity increases with cycling. However, if there is a large amount of precipitate due to the electrolyte reaction, the precipitate may clog the pores, causing a decrease in the porosity.

[0023] However, in actual secondary batteries, the significant decrease in porosity due to cycle degradation as in (i) does not occur in most cases. Therefore, when the threshold Dth increases with cycle degradation, it can be concluded that the structural deterioration of the electrode as in (ii) has occurred.

[0024] That is, by comparing the threshold value Dth0 obtained by applying the above diagnostic method to a secondary battery before deterioration (for example, in its initial state) with the threshold value Dth obtained by applying the above diagnostic method to a secondary battery after deterioration, if the difference between the two, ΔDth=Dth-Dth0, is greater than a predetermined value, it can be determined that there is a high possibility that structural deterioration has occurred in the electrodes.

[0025] Here, "structural deterioration" of an electrode refers to a state in which the ion diffusion within the solid phase of the active material particles themselves is reduced, for example, due to cracks occurring in the active material particles in the electrode or a phase change caused by chemical degradation. Electrodes that have undergone structural deterioration are considered unsuitable not only for horizontal recycling but also for wet recycling. The regeneration process in wet recycling is, for example, a process in which an active material whose lithium content has decreased due to degradation is mixed with a lithium raw material and re-fired to restore the lithium content to its initial amount. If cracks occur due to cracking of the active material itself, it is difficult to restore this through regeneration.

[0026] On the other hand, if the difference ΔDth is less than a predetermined value, it can be determined that (ii) structural deterioration has not occurred, or even if it has occurred to some extent, the porosity of the electrode mixture layer has increased due to the expansion of the mixture, ensuring a wider ion diffusion path, and that the discharge characteristics are likely to be maintained.

[0027] The present invention has been completed based on the above findings. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0028] [Embodiment Mode] The method for determining whether electrodes of a secondary battery are recyclable according to one embodiment of the present invention is a method for classifying secondary batteries by determining the degree of deterioration of the electrodes of the secondary battery based on the "threshold value Dth of the electrolyte diffusion coefficient" described below. First, to explain the "threshold value Dth of the electrolyte diffusion coefficient," a "method for diagnosing a secondary battery" using this "threshold value Dth of the electrolyte diffusion coefficient" will be described.

[0029] The "secondary battery diagnostic method" described below is different from the "secondary battery electrode recycling determination method" and is a method used primarily to determine whether a secondary battery can be reused, and more specifically, a method for evaluating the remaining life of a secondary battery. The applicant has also filed a patent application for this method (PCT / JP2023 / 003516).

[0030] (Diagnostic method for secondary batteries) 1 is a flow diagram of a method for diagnosing a secondary battery. This method includes the steps of estimating characteristic parameters of a secondary battery to be diagnosed (hereinafter referred to as the "target battery") at the time of diagnosis (step SA1), determining the relationship between the electrolyte diffusion coefficient and the discharge capacity (step SA2), determining a threshold value Dth for the electrolyte diffusion coefficient (step SA3), and determining the difference ΔD between the threshold value Dth and the electrolyte diffusion coefficient Dn at the time of diagnosis (step SA4). Each step will be described in detail below.

[0031] [Process for estimating characteristic parameters] The characteristic parameters of the target battery at the time of diagnosis are estimated (step SA1). More specifically, the characteristic parameters of the target battery at the time of diagnosis, including the electrolyte diffusion coefficient Dn of the target battery at the time of diagnosis, are estimated using a predetermined model formula based on data obtained by measuring the load characteristics of the target battery.

[0032] In this process, the characteristic parameters of the target battery at the time of diagnosis are estimated by fitting data obtained by measuring the load characteristics of the target battery using a predetermined model formula. This analysis (simulation) can be performed using a computer program capable of fluid analysis, such as the software Battery Design Studio manufactured by Siemens.

[0033] The model formula may be one well known in the art, such as the one described in Marc Doyle et al., "Modeling of Galvanostatic Charge and Discharge of the Lithium / Polymer / Insertion Cell," J. Electrochem. Soc., Vol. 140, No. 6, June (1993).

[0034] The target battery is, for example, a lithium ion battery.

[0035] The data obtained by measuring the load characteristics of the target battery may be, for example, a discharge curve obtained by measuring the target battery at multiple discharge rates. This data preferably includes a discharge curve measured at a very low discharge rate (e.g., 0.02 C). This data also preferably includes a discharge curve measured at a discharge rate of 1 C or higher. This data preferably includes discharge curves measured at three or more levels of discharge rate, and more preferably includes discharge curves measured at four or more levels of discharge rate. The data obtained by measuring the load characteristics of the target battery may also be a charge curve obtained by measuring the target battery at multiple charge rates.

[0036] The characteristic parameters estimated in this step (characteristic parameters at the time of diagnosis of the target battery) include at least the electrolyte diffusion coefficient Dn at the time of diagnosis of the target battery. The characteristic parameters may also include, for example, the solid phase diffusion coefficient of the positive and negative electrode active materials, the electrolyte conductivity, etc. Other specific examples of the characteristic parameters will be described later.

[0037] 2 is a flow diagram showing an example of a more specific procedure for the step of estimating the characteristic parameters (step SA1). In this example, the step of estimating the characteristic parameters (step SA1) includes a step of inputting the basic specifications of the target battery (step SA1-1), a step of inputting data obtained by measuring the load characteristics of the target battery (step SA1-2), a step of estimating the static parameters of the target battery (step SA1-3), and a step of estimating the dynamic parameters of the target battery (step SA1-4).

[0038] The basic specifications of the secondary battery to be diagnosed are input into the analysis software (step SA1-1). The basic specifications to be input are not limited to these, but may include, for example, the following: Positive and negative electrode composition (component materials, content, particle size, etc.) · Thickness, density, and bending rate of the positive and negative electrodes (= about 1.5 in most cases) ·Material, thickness, and electrical conductivity of positive and negative electrode current collector foils Separator thickness and porosity Electrolyte composition (component materials, content) -Thermal conductivity and heat capacity of the above constituent materials (basic physical properties specific to the material) ·Electrode area

[0039] Since the diagnosis is basically non-destructive, accurate information on the composition of the electrolyte at the time of diagnosis is not available. Therefore, general information on the secondary battery to be diagnosed (or specification information for a new battery) is obtained and input as a parameter. Although some values must be input when actually performing a simulation, the composition of the electrolyte itself does not have a significant impact on the results of the simulation. In the diagnosis method of this embodiment, the composition information on the electrolyte is only used as a reference.

[0040] The density of the positive and negative electrodes is also expected to have changed due to expansion from the initial state, but the exact value cannot be measured at the time of diagnosis. Therefore, the initial value (standard value, etc.) or a value predicted from the initial value is entered. If it is completely unknown, a general value may be entered. If necessary, fine adjustments may be made in steps SA1-4.

[0041] The data obtained by measuring the load characteristics of the target battery is input into the analysis software (step SA1-2). As described above, the data obtained by measuring the load characteristics of the target battery is a discharge curve obtained by measuring the target battery at multiple discharge rates. Hereinafter, "data obtained by measuring the load characteristics of the target battery" may be referred to as "actual measurement data."

[0042] The static parameters of the target battery are estimated from the actual measurement data and the model formula (step SA1-3). For example, the static parameters of the target battery are adjusted to fit the shape of the discharge curve measured at a very low discharge rate. The discharge curve measured at a very low discharge rate (e.g., 0.02 C) can be considered to roughly match the voltage curve (OCV curve) when no load is connected. The static parameters can include, but are not limited to, the following: Capacity per unit weight of positive and negative electrode active material (discharge capacity of a battery decreases after use) Utilization rate of each positive and negative electrode active material (not all of the active materials are used) - Maximum and minimum voltages for the target battery range

[0043] The dynamic parameters of the target battery are estimated from the actual measurement data and the model formula (step SA1-4). For example, a simulation is performed in which the target battery is discharged at a current value equivalent to the measurement conditions of the actual measurement data, and the results of this simulation are compared with the actual measurement data, and the dynamic parameters are adjusted so that the two match. This simulation can be performed, for example, using the discharge curve prediction function of Battery Design Studio mentioned above. The dynamic parameters can include, but are not limited to, the following: Electrolyte conductivity Electrolyte diffusion coefficient Diffusion coefficient in the solid phase of positive and negative electrode active materials -Heat capacity of the target battery

[0044] It is preferable to set the environmental temperature during the simulation to match the environmental temperature when the actual measurement data was obtained. For medium-sized or larger product batteries, especially those expected to be used at high rates, it is preferable to consider the effects of heat generation. To do so, it is preferable to perform measurements at least at 1C and perform fitting with the actual measurement data affected by heat generation. On the other hand, if the target battery is a small cell for desktop testing, it is not necessary to consider the effects of heat generation.

[0045] Through the above steps, it is possible to estimate the characteristic parameters of the target battery at the time of diagnosis, including the electrolyte diffusion coefficient Dn of the target battery at the time of diagnosis.

[0046] [Step of determining the relationship between electrolyte diffusion coefficient and discharge capacity] Based on the model formula used in step SA1 and the characteristic parameters estimated in step SA1, the relationship between the electrolyte diffusion coefficient and the discharge capacity is determined (step SA2). More specifically, among the characteristic parameters estimated in step SA1, a discharge simulation is performed by varying only the electrolyte diffusion coefficient while keeping the other characteristic parameters constant, and the discharge capacity is determined. The discharge rate and environmental temperature during the discharge simulation are preferably set according to the intended reuse application. For example, if the intended application is one in which the battery will be used at an average rate of about 1 C, the discharge rate used to determine the relationship between the electrolyte diffusion coefficient and the discharge capacity is also 1 C. Since it is difficult to precisely match all environments, the simulation may be performed using average values.

[0047] 3 is a graph showing an example of the relationship between the electrolyte diffusion coefficient and the discharge capacity. In this example, the discharge capacity when the environmental temperature is 45°C and the discharge rate is 0.5 C is calculated by dividing the discharge capacity by the electrolyte diffusion coefficient of 4.8×10 -6 , 4.0×10 -6 , 2.95×10 -6 , 1.85×10 -6 , 1.48×10 -6 , and 1.1 × 10 -6 cm 2 / sec.

[0048] As shown in this example, the smaller the electrolyte diffusion coefficient, the smaller the discharge capacity. Furthermore, the relationship between the electrolyte diffusion coefficient and the discharge capacity is not linear, but tends to show a curve in which the smaller the electrolyte diffusion coefficient, the greater the decrease in discharge capacity.

[0049] [Step of determining the threshold value Dth of the electrolyte diffusion coefficient] Based on the relationship between the electrolyte diffusion coefficient and the discharge capacity obtained in step SA2, a threshold value Dth for the electrolyte diffusion coefficient is determined (step SA3). More specifically, the value of the electrolyte diffusion coefficient at which the target battery becomes unsuitable for reuse is determined as the threshold value Dth, with reference to the relationship between the electrolyte diffusion coefficient and the discharge capacity obtained in step SA2. The circumstances under which a battery is determined to be "unsuitable for reuse" vary depending on the reuse application of the target battery. Therefore, a criterion for determining that a battery is "unsuitable for reuse" is set according to the application.

[0050] For example, when the discharge capacity falls below a predetermined allowable value, the battery may be determined to be unsuitable for reuse. In this case, the electrolyte diffusion coefficient when the discharge capacity falls below the predetermined allowable value is determined as the threshold value Dth. For example, in the example of FIG. 3, if the allowable value of the discharge capacity is 36.02 mAh, the threshold value Dth is 1.40×10 -6 cm 2 / sec.

[0051] Alternatively, the battery may be determined to be unsuitable for reuse when the discharge capacity begins to drop sharply. In this case, the electrolyte diffusion coefficient when the discharge capacity begins to drop sharply is determined as the threshold value Dth. For example, the electrolyte diffusion coefficient when the slope of the discharge capacity becomes equal to or greater than a predetermined magnitude may be set as the threshold value Dth. Furthermore, as shown in FIG. 4, the point where the tangents to the curves before and after the start of the drop in discharge capacity intersect may be set as the threshold value Dth.

[0052] [Step of calculating the difference ΔD between the threshold value Dth and the electrolyte diffusion coefficient Dn at the time of diagnosis] The difference ΔD between the threshold value Dth determined in step SA3 and the electrolyte diffusion coefficient Dn estimated in step SA1 at the time of diagnosis is calculated (step SA4). For example, Dn=2.22×10 -6 cm 2 / sec, Dth=1.40×10 -6 cm 2 / sec, ΔD=Dn-Dth=0.82×10 -6 cm 2 / sec.

[0053] This ΔD can be used as an indicator of the remaining lifespan of the target battery. In other words, the larger ΔD, the higher the possibility that the target battery can be used for a long period of time, and the smaller ΔD, the lower the possibility that the target battery can be used for a long period of time. Even if the discharge capacity at the time of diagnosis is approximately the same, ΔD may differ. By using ΔD, the remaining lifespan of the target battery can be evaluated more accurately than the conventional method of evaluating the remaining lifespan based on the magnitude of the discharge capacity at the time of diagnosis.

[0054] (Method for determining whether electrodes in secondary batteries can be recycled) The above has described the "method for diagnosing a secondary battery" using the "threshold value Dth of the electrolyte diffusion coefficient." Next, the "method for determining whether or not to recycle electrodes of a secondary battery" using this "threshold value Dth of the electrolyte diffusion coefficient" will be described.

[0055] A method for determining whether electrodes of a secondary battery can be recycled according to one embodiment of the present invention is a method for determining the degree of deterioration of electrodes included in a secondary battery to be determined (hereinafter referred to as a "target battery"). The recycling determination method according to this embodiment, for example, evaluates the degree of deterioration of the electrodes of the target battery and classifies the target battery into a plurality of categories (e.g., "horizontal recyclable," "wet recyclable," "unsuitable for either horizontal recycling or wet recycling," etc.).

[0056] The battery to be evaluated is, for example, a lithium ion battery. The electrodes of the battery to be evaluated include, but are not limited to, a current collector (typically a metal foil) and an electrode mixture layer formed on the current collector. The electrode mixture layer includes, for example, an electrode active material, a conductive additive, a binder, etc. The electrode to be evaluated is preferably a positive electrode.

[0057] The battery to be judged is not limited to this, but for example, it is a secondary battery that has been collected because its discharge capacity has decreased due to repeated charging and discharging and it is no longer suitable for reuse.

[0058] [First embodiment] 5 is a flow chart of a method for determining the recycle status of electrodes of a secondary battery according to a first embodiment of the present invention. This method includes the steps of: (1) estimating characteristic parameters of the battery at the time of determination, including the electrolyte diffusion coefficient of the battery at the time of determination, using a predetermined model formula based on data obtained by measuring the battery; (2) determining the discharge capacity when the electrolyte diffusion coefficient is changed based on the model formula and the estimated characteristic parameters, and determining the relationship between the electrolyte diffusion coefficient and the discharge capacity; (3) determining a threshold value Dth of the electrolyte diffusion coefficient based on the relationship between the electrolyte diffusion coefficient and the discharge capacity; and (4) comparing the threshold value Dth with a threshold value Dth0 of the electrolyte diffusion coefficient before deterioration that has been previously obtained for the battery or a secondary battery of the same type as the battery, and classifying the battery based on the difference ΔDth=Dth−Dth0 between the threshold value Dth and the threshold value Dth0.

[0059] In this embodiment, first, as in the case of the secondary battery diagnostic method, characteristic parameters at the time of the evaluation are estimated from actual measurement data of the battery to be evaluated, and a threshold value Dth of the electrolyte diffusion coefficient is determined from these characteristic parameters (steps S1 to S3). Steps S1 to S3 can be performed in the same way as steps SA1 to SA3 of the secondary battery diagnostic method (FIG. 1). This threshold value Dth is compared with a previously acquired threshold value Dth0 of the secondary battery before deterioration, and the battery to be evaluated is classified based on the difference ΔDth=Dth-Dth0 between the threshold value Dth and the threshold value Dth0 (step S4).

[0060] The threshold value Dth0 is a threshold value of the electrolyte diffusion coefficient that is determined in advance by carrying out steps similar to steps S1 to S3 on the battery to be evaluated or a secondary battery of the same type as the battery to be evaluated before deterioration. "Before deterioration" preferably refers to the state before use, i.e., a new state.

[0061] A "secondary battery of the same type as the battery to be evaluated" refers to a secondary battery whose positive and negative electrode materials and shapes, and the type and amount of electrolyte, etc., are equivalent to those of the battery to be evaluated. For example, if the battery to be evaluated is a product battery, a "secondary battery of the same type as the battery to be evaluated" refers to a secondary battery having the same model number. When determining the threshold value Dth0 using a "secondary battery of the same type as the battery to be evaluated," it is preferable to determine the threshold value Dth0 using multiple secondary batteries and calculate the average value.

[0062] When steps S1 to S3 are performed, the model formula, discharge simulation conditions, and criteria for determining "unsuitable for reuse" are used in common when determining the threshold Dth and when determining the threshold Dth0. In particular, the magnitude of the current used in the discharge simulation is set to be the same when determining the threshold Dth and when determining the threshold Dth0. For example, if the discharge rate when determining the threshold Dth0 is 1C, the magnitude of the current used when determining the threshold Dth is set to a magnitude equivalent to 1C based on the cell capacity when determining the threshold Dth0. The thresholds Dth and Dth0 vary depending on the magnitude of the current during use. Therefore, in step SA2 (FIG. 1), it is preferable to perform the discharge simulation taking into account the normal range of usage rates for the battery.

[0063] On the other hand, in the process of estimating the characteristic parameters at the time of determination, it is not necessary to update all of the characteristic parameters of the battery to be determined. For example, for characteristic parameters that are thought to remain almost unchanged before and after degradation, the characteristic parameters estimated when determining the threshold Dth0 may be used as they are. Therefore, the data used to estimate the characteristic parameters at the time of determination only needs to be data necessary to estimate the characteristic parameters to be updated, and does not necessarily need to be the same type of data as the actual measurement data used when determining the threshold Dth0. For example, the data used to estimate the characteristic parameters at the time of determination may be only a discharge curve at a single discharge rate. However, if discharge curves at multiple discharge rates are available, it is preferable to use them.

[0064] When estimating the characteristic parameters at the time of judgment, it is preferable to change only the discharge capacity, the electrolyte diffusion coefficient, and the electrolyte conductivity from the characteristic parameters estimated when determining the threshold Dth0, and to fix the other characteristic parameters to the characteristic parameters estimated when determining the threshold Dth0. When estimating the characteristic parameters at the time of judgment, fitting may be performed by changing all the characteristic parameters, but if performed by someone other than a skilled technician, the large number of parameters may result in inaccurate fitting.

[0065] In this embodiment, the battery to be evaluated is classified based on the difference ΔDth=Dth-Dth0 between the threshold value Dth and the threshold value Dth0. For example, if the difference ΔDth is equal to or less than a predetermined boundary value, the battery to be evaluated is classified into the first classification, and if the difference ΔDth is greater than the predetermined boundary value, the battery to be evaluated is classified into the second classification.

[0066] It has been experimentally found that there is a high possibility that structural deterioration has occurred in the electrodes of a secondary battery when the threshold value Dth increases with cycle deterioration, i.e., when ΔDth > 0. Therefore, for example, the "predetermined boundary value" may be set to 0, and the electrodes of a secondary battery with ΔDth greater than 0 (secondary batteries classified as the second category) may be determined to be unsuitable for either horizontal recycling or wet recycling.

[0067] The analytical error in ΔDth is 0.01×10 -6 cm 2 / sec. Therefore, the above "predetermined boundary value" (the boundary value that separates the first and second categories) is -0.01 × 10 -6 ~0.01×10 -6 cm 2 It is preferable to set the "predetermined boundary value" within the range of 0.01 × 10 -6 cm 2 / sec, the difference ΔDth is 0.01×10 -6 cm 2 / sec or less, the battery is classified into the first category, and the difference ΔDth is 0.01×10 -6 cm 2 If the value is greater than / sec, the battery is classified into the second category.

[0068] The classified secondary batteries may be subjected to further measurement and inspection to determine a recycling method. For example, the classification in step S4 may be performed as a primary screening, and additional measurement and inspection may be performed only on secondary batteries classified into the first classification or only on secondary batteries classified into the second classification. Alternatively, the content of the additional measurement and inspection may be different between the first classification and the second classification.

[0069] The above has described the method for determining whether or not to recycle electrodes of a secondary battery according to the first embodiment of the present invention. According to this embodiment, it is possible to determine the degree of deterioration of electrodes to be recycled.

[0070] [Second embodiment] Fig. 6 is a flow diagram of a method for determining the recycling of electrodes of a secondary battery according to a second embodiment of the present invention. In addition to the steps (Fig. 5) included in the recycling determination method according to the first embodiment, this method further includes a step (step S5) of classifying the target batteries based on the rate of decrease in discharge capacity of the target batteries (hereinafter referred to as "capacity decrease rate"). In Fig. 6, the step (step S5) of classifying the target batteries based on the capacity decrease rate is illustrated as being performed after the step (step S4) of classifying the target batteries based on the difference ΔDth, but the order of steps S4 and S5 may be reversed.

[0071] The capacity loss rate is calculated by subtracting the discharge capacity of the secondary battery at the time of judgment from the discharge capacity of the secondary battery before use (when new), and dividing the result by the discharge capacity of the secondary battery before use (when new). Because the discharge capacity of a secondary battery decreases for various reasons, even if the capacity loss rate of a secondary battery (cell) is high, it does not necessarily mean that the electrodes have deteriorated to the same extent. However, even if this is the case, if the capacity loss rate of a secondary battery is high, it is highly likely that the electrodes have also deteriorated, and it can be determined that the battery is at least not suitable for horizontal recycling.

[0072] In the step of classifying the target batteries based on the capacity loss rate (step S5), the batteries are classified into those whose capacity loss rate is less than a predetermined threshold and those whose capacity loss rate is equal to or greater than a predetermined threshold. The threshold for the capacity loss rate here varies depending on the intended use after recycling, but can be set to, for example, 20 to 40%.

[0073] 7 shows an example of a method for classifying batteries to be evaluated according to this embodiment. In this example, batteries are first classified by their capacity loss rate. Specifically, batteries with a capacity loss rate equal to or greater than a predetermined threshold are classified as eligible for wet recycling or dry recycling. Batteries with a capacity loss rate less than the predetermined threshold are further classified by the difference ΔDth. Specifically, batteries with a difference ΔDth equal to or less than a predetermined boundary value are classified as eligible for horizontal recycling or wet recycling, and batteries with a difference ΔDth greater than the predetermined boundary value are classified as eligible for dry recycling.

[0074] 8 shows another example of a method for classifying target batteries according to this embodiment. In this example, first, classification is performed based on the difference ΔDth. Specifically, batteries with a difference ΔDth greater than a predetermined boundary value are classified as eligible for dry recycling. For batteries with a difference ΔDth equal to or less than the predetermined boundary value, further classification is performed based on the capacity loss rate. Specifically, batteries with a capacity loss rate equal to or greater than a predetermined threshold value are classified as eligible for wet recycling, and batteries with a capacity loss rate less than the predetermined threshold value are classified as eligible for horizontal recycling or wet recycling.

[0075] 7 and 8 are merely examples, and the recycling determination method according to this embodiment is not limited to these. Also, as in the first embodiment, further measurements and inspections may be performed to determine the recycling method.

[0076] [Third embodiment] 9 is a flow diagram of a method for determining the recycle status of electrodes of a secondary battery according to a third embodiment of the present invention. In addition to the steps (FIG. 5) included in the method for determining the recycle status of the first embodiment, this method further includes a step (step S6) of classifying the target batteries based on the difference ΔD between the threshold value Dth and the electrolyte diffusion coefficient at the time of determination. The step (step S6) of classifying the target batteries based on the difference ΔD is preferably performed before the step (step S4) of classifying the target batteries based on the difference ΔDth.

[0077] The recycling determination method according to this embodiment is a combination of the secondary battery diagnostic method described in FIG. 1 and the secondary battery electrode recycling determination method. That is, in this embodiment, it is determined whether the battery under evaluation is reusable (reusable). Specifically, for example, if the difference ΔD is equal to or greater than a predetermined threshold, the battery under evaluation is classified as a reusable secondary battery. If the difference ΔD is less than the predetermined threshold, the battery under evaluation is classified as a secondary battery unsuitable for reuse, and then classification (determination of the electrode deterioration level) is performed based on ΔDth.

[0078] The method for diagnosing a secondary battery and the method for determining whether an electrode of a secondary battery is recycled share steps S1 to S3, so by combining these methods, it is possible to reduce the cost of work and calculations.

[0079] In this embodiment, a step (step S5) of classifying the target batteries based on the capacity decrease rate described in the second embodiment may be further performed.

[0080] (Recycling assessment program for secondary battery electrodes, etc.) The above-described method for determining the recyclability of secondary battery electrodes can also be implemented as a computer program. A program for determining the recyclability of secondary battery electrodes according to one embodiment of the present invention instructs a computer to execute the following steps: estimating characteristic parameters of the target battery at the time of determination, including the electrolyte diffusion coefficient at the time of determination, using a predetermined model formula based on data obtained by measuring the target battery; calculating the discharge capacity when the electrolyte diffusion coefficient is changed based on the model formula and the estimated characteristic parameters, thereby determining the relationship between the electrolyte diffusion coefficient and the discharge capacity; determining a threshold value Dth for the electrolyte diffusion coefficient based on the relationship between the electrolyte diffusion coefficient and the discharge capacity; and comparing the threshold value Dth with a threshold value Dth0 for the electrolyte diffusion coefficient before deterioration that has been previously obtained for the target battery or a secondary battery of the same type as the target battery, and classifying the target battery based on the difference ΔDth = Dth - Dth0 between the threshold value Dth and the threshold value Dth0. This embodiment also enables the degree of deterioration of electrodes to be recycled to be determined.

[0081] The above-described method for determining whether an electrode of a secondary battery is recycled can also be realized as a computer-readable recording medium having the above-described computer program recorded thereon.

[0082] The above-described method for determining the recyclability of secondary battery electrodes can also be realized as a computer system. A computer system for determining the recyclability of secondary battery electrodes according to one embodiment of the present invention includes a memory and a processor, and the processor, in accordance with a program stored in the memory, executes the following steps: estimating characteristic parameters of the battery at the time of determination, including the electrolyte diffusion coefficient at the time of determination, using a predetermined model formula based on data obtained by measuring the battery, calculating the discharge capacity when the electrolyte diffusion coefficient is changed based on the model formula and the estimated characteristic parameters, and determining the relationship between the electrolyte diffusion coefficient and the discharge capacity, determining a threshold value Dth for the electrolyte diffusion coefficient based on the relationship between the electrolyte diffusion coefficient and the discharge capacity, and comparing the threshold value Dth with a threshold value Dth0 for the electrolyte diffusion coefficient before deterioration that has been obtained in advance for the battery or a secondary battery of the same type as the battery, and classifying the battery at the time of determination based on the difference ΔDth = Dth - Dth0 between the threshold value Dth and the threshold value Dth0. [Example]

[0083] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0084] [Secondary battery production] We fabricated a number of medium-sized laminated cells with rated capacities of 13 Ah, 5 Ah, and 2.7 Ah.

[0085] [5Ah medium-sized laminated cell] <Preparation of positive electrode> A positive electrode mixture-containing slurry was prepared by uniformly mixing 93 parts by weight of LiCoO2 (positive electrode active material), 3 parts by weight of carbon black (conductive additive), and 4 parts by weight of PVDF (binder) using NMP (solvent). This positive electrode mixture-containing slurry was applied to both sides of a positive electrode current collector made of aluminum foil with a thickness of 15 μm, dried, and then pressure-molded using a roller press. A positive electrode was fabricated by punching out a portion of the positive electrode current collector not coated with the positive electrode mixture-containing slurry to form a tab.

[0086] <Preparation of negative electrode> A negative electrode mixture slurry was prepared by mixing 97.5 parts by mass of graphite (a negative electrode active material), 1.5 parts by mass of carboxymethyl cellulose (a binder), and 1 part by mass of styrene-butadiene rubber, adding an appropriate amount of water and thoroughly mixing. This negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil with a thickness of 10 μm, dried, and then pressure-molded using a roller press. A negative electrode was fabricated by punching out a portion of the negative electrode current collector not coated with the negative electrode mixture slurry to form a tab.

[0087] <Battery construction> The seven positive electrodes and the eight negative electrodes were alternately stacked with an 18 μm-thick polyolefin microporous film separator having a three-layer structure with a polyethylene layer as the middle layer and two polypropylene layers as the outer layers interposed therebetween to form a laminated electrode body.

[0088] Next, the positive electrode tabs of the laminated electrode body were welded together, and the negative electrode tabs were welded together, and leads were connected to each of them. After that, LiPF was dissolved at a concentration of 1 mol / L in a solution obtained by mixing ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate in a volume ratio of 1:1:1, and then vinylene carbonate was further dissolved in an amount to make 1 mass % to prepare a non-aqueous electrolyte solution. This was then sealed in an exterior body made of an aluminum laminate film, together with the non-aqueous electrolyte solution, to produce a non-aqueous electrolyte secondary battery with a rated capacity of 5 Ah.

[0089] [2.7Ah medium-sized laminated cell] <Preparation of positive electrode> A positive electrode mixture-containing slurry was prepared by uniformly mixing 93 parts by weight of LiCoO2 (positive electrode active material), 3 parts by weight of carbon black (conductive additive), and 4 parts by weight of PVDF (binder) using NMP (solvent). This positive electrode mixture-containing slurry was applied to both sides of a positive electrode current collector made of aluminum foil with a thickness of 15 μm, dried, and then pressure-molded using a roller press. A positive electrode was fabricated by punching out a portion of the positive electrode current collector not coated with the positive electrode mixture-containing slurry to form a tab.

[0090] <Preparation of negative electrode> A negative electrode mixture slurry was prepared by mixing 94.5 parts by mass of graphite (negative electrode active material), 3 parts by mass of carbon-coated SiO particles (D50: 5.0 μm), 1.5 parts by mass of carboxymethyl cellulose (binder), and 1 part by mass of styrene-butadiene rubber, and adding an appropriate amount of water and thoroughly mixing. This negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil with a thickness of 10 μm, dried, and then pressure-molded using a roller press. A negative electrode was fabricated by punching out a portion of the negative electrode current collector not coated with the negative electrode mixture slurry to form a tab.

[0091] <Battery construction> The seven positive electrodes and the eight negative electrodes were alternately stacked with an 18 μm-thick polyolefin microporous film separator having a three-layer structure with a polyethylene layer as the middle layer and two polypropylene layers as the outer layers interposed therebetween to form a laminated electrode body.

[0092] Next, the positive electrode tabs of the laminated electrode body were welded together, and the negative electrode tabs were welded together, and leads were connected to each of them. After that, LiPF was dissolved at a concentration of 1 mol / L in a solution obtained by mixing ethylene carbonate and diethyl carbonate in a volume ratio of 3:7, and then vinylene carbonate was further dissolved in an amount to make 1 mass % to prepare a non-aqueous electrolyte solution. This was then sealed in an exterior body made of aluminum laminate film, and a non-aqueous electrolyte secondary battery with a rated capacity of 2.7 Ah was produced.

[0093] [13Ah medium-sized laminated cell] LiNi instead of LiCoO2 0.5 Co 0.2 Mn 0.3 A medium-sized laminated cell with a rated capacity of 13 Ah was manufactured in the same manner as the 5 Ah medium-sized laminated cell, except that O2 was used as the positive electrode active material and the areas of the positive electrode, negative electrode, and separator, as well as the amount of electrolyte, were changed to suit the desired rated capacity.

[0094] [Preparation of degradation cell] Charge-discharge cycle tests were conducted on medium-sized laminated cells with a rated capacity of 5 Ah under different conditions, such as charge-discharge rate and ambient temperature, to produce degraded cells with discharge capacities reduced to 4.5 Ah and 3.7 Ah. Similarly, degraded cells were produced from medium-sized laminated cells with a rated capacity of 2.7 Ah by reducing the discharge capacity to 2.4 Ah, and degraded cells were produced from medium-sized laminated cells with a rated capacity of 13 Ah by reducing the discharge capacity to 11 Ah and 12.5 Ah. Note that the "discharge capacity" after degradation here is the discharge capacity measured at a current value of 0.2 C, based on the "rated capacity" of each battery (hereinafter referred to as "cell capacity").

[0095] [Recycling diagnosis of secondary battery electrodes] The load characteristics of these cells were measured before and after degradation. Specifically, discharge curves were measured at discharge rates of 0.02C, 0.2C, 0.5C, and 1C, based on the initial rated capacity of each cell.

[0096] Next, the threshold value Dth0 of the electrolyte diffusion coefficient was determined using the method described in the embodiment for the cell before degradation. The analysis (simulation) was performed using the software Battery Design Studio manufactured by Siemens. The basic specifications, such as the solvent ratio and salt concentration, were input as they were at the time of fabrication.

[0097] After estimating the characteristic parameters of the cell before degradation, the discharge capacity was calculated at an ambient temperature of 45°C and a discharge rate of 0.5C while changing the electrolyte diffusion coefficient based on the estimated characteristic parameters, and the relationship between the electrolyte diffusion coefficient and discharge capacity was obtained. The point where the tangents to each curve before and after the discharge capacity began to drop sharply was determined as the threshold value Dth0.

[0098] Next, the threshold value Dth of the electrolyte diffusion coefficient for the deteriorated cell was determined. The threshold value Dth of the electrolyte diffusion coefficient for the deteriorated cell was estimated by fitting the characteristic parameters estimated when determining the threshold value Dth0 of the electrolyte diffusion coefficient before deterioration, changing only the cell capacity, electrolyte diffusion coefficient, and electrolyte conductivity, while fixing the other characteristic parameters to the characteristic parameters estimated when determining the threshold value Dth0 of the electrolyte diffusion coefficient before deterioration. The magnitude of the current during the discharge simulation was set to the same magnitude as the current used to determine the threshold value Dth0. In other words, it was set to a magnitude equivalent to 0.5 C, based on the capacity (rated capacity) of each battery before deterioration.

[0099] The results are shown in Table 1. In Table 1, "LCO" (Judgment Examples 1 to 3) represents lithium cobalt oxide, "SiO·graphite" (Judgment Example 3) represents a mixture of graphite and SiO particles whose surfaces are coated with carbon, and "NCM" (Judgment Examples 4 and 5) represents nickel·cobalt·lithium manganese oxide.

[0100] [Table 1]

[0101] In Examples 1, 3, and 5, the capacity loss rate of the secondary battery (cell) was 10% or less, and therefore the capacity loss rate of the electrode was also expected to be 10% or less. Furthermore, since the difference ΔDth in the threshold value of the electrolyte diffusion coefficient before and after degradation was a negative value, it can be inferred that the electrode structure was not degraded. Therefore, it can be determined that the electrodes of these secondary batteries can be recycled horizontally or wet.

[0102] On the other hand, in Evaluation Example 2, the capacity loss rate of the secondary battery (cell) was large, at 25% or more. It is highly likely that the discharge capacity of the electrodes in this secondary battery also significantly decreased from its initial value. Furthermore, since the difference ΔDth in the threshold value of the electrolyte diffusion coefficient before and after degradation was a positive value, it can be inferred that the structural deterioration of the electrodes is highly likely to have occurred. Therefore, it can be determined that the electrodes of this secondary battery are unsuitable for either horizontal recycling or wet recycling.

[0103] In Evaluation Example 4, the rate of capacity loss of the secondary battery (cell) was suppressed, but the difference ΔDth in the threshold of the electrolyte diffusion coefficient before and after degradation was a positive value, suggesting a high possibility of structural deterioration of the electrodes. Therefore, it can be determined that the electrodes of this secondary battery are not suitable for either horizontal recycling or wet recycling.

[0104] Next, the positive and negative electrodes were removed from each secondary battery, and the capacity loss rate of the positive electrode, the capacity loss rate of the negative electrode, the expansion rate of the positive electrode mixture, and the expansion rate of the negative electrode mixture were measured. Scanning electron microscope (SEM) photographs of the positive electrodes of each secondary battery were also taken to check for cracks in the active material particles. The results are shown in Table 2.

[0105] [Table 2]

[0106] Fig. 10 is an SEM photograph of the initial state of the positive electrodes (LCO) of Judgment Examples 1 to 3. Fig. 11 is an SEM photograph of the positive electrode of Judgment Example 1 after deterioration, Fig. 12 is an SEM photograph of the positive electrode of Judgment Example 2 after deterioration, and Fig. 13 is an SEM photograph of the positive electrode of Judgment Example 3 after deterioration. No cracks are observed in Judgment Example 1 (Fig. 11) and Judgment Example 3 (Fig. 13), whereas it can be seen that many cracks have occurred within individual particles in Judgment Example 2 (Fig. 12).

[0107] Figure 14 is an SEM photograph of the initial state of the positive electrodes (NCM) of Evaluation Examples 4 and 5. The particles of the NCM are composed of secondary particles made of sintered primary particles. Figure 15 is an SEM photograph of the positive electrode of Evaluation Example 4 after deterioration, and Figure 16 is an SEM photograph of the positive electrode of Evaluation Example 5 after deterioration. In Evaluation Example 4 (Figure 15), the sintered parts between the primary particles have broken, causing numerous cracks. On the other hand, in Evaluation Example 5 (Figure 16), there are very few cracks.

[0108] As shown in Table 2 and FIGS. 10 to 16, it was confirmed that the judgment results and the deterioration state of the electrodes were in good agreement.

[0109] Furthermore, the unipolar discharge characteristics of the horizontally recycled positive electrodes were measured for Evaluation Examples 1 and 4. As a result, in Evaluation Example 1, the initial discharge rate was 0.5C / 96%, but after recycling it decreased to 0.5C / 92%, maintaining a high level. On the other hand, in Evaluation Example 4, the initial discharge rate was 0.5C / 94%, but after recycling it decreased to 0.5C / 72%, a significant decrease.

[0110] The percentages for the initial electrodes (0.5C / 96% and 0.5C / 94%) represent the ratio of the discharge capacity at 0.5C to the discharge capacity (cell capacity) at 0.2C before degradation. The percentages for the recycled electrodes (0.5C / 92% and 0.5C / 72%) represent the ratio of the discharge capacity at 0.5C to the discharge capacity at 0.2C, measured in the same manner as for the initial electrodes, with the cell capacity after degradation listed in Table 1 as the "rated capacity" of the recycled battery.

[0111] For example, in the case of Evaluation Example 1, the initial unipolar discharge characteristic "0.5C / 96%" is the ratio of the discharge capacity measured at 0.5C (i.e., 2.5A) based on the rated capacity of 5Ah to the discharge capacity measured at 0.2C (i.e., 1A) based on the rated capacity of 5Ah. In contrast, the unipolar discharge characteristic "0.5C / 92%" of the horizontally recycled positive electrode is the ratio of the discharge capacity measured at 0.5C (i.e., 2.25A) based on the aged cell capacity of 4.5Ah to the discharge capacity measured at 0.2C (i.e., 0.9A) based on the aged cell capacity of 4.5Ah.

[0112] The reason why the presence or absence of structural deterioration of an electrode can be determined from the difference ΔDth in the threshold of the electrolyte diffusion coefficient before and after deterioration has not been fully elucidated, but is presumed to be as follows.

[0113] The electrolyte diffusion coefficient threshold Dth may depend on the internal resistance of the secondary battery. The electrolyte conductivity calculated in this analysis reflects not only the ohmic resistance and the actual resistance of the electrolyte, but also the interfacial resistance during the positive and negative electrode reactions. Therefore, structural deterioration of the electrode (when cracks occur in the active material, an electrically discontinuous interface is created, causing a significant increase in resistance) is reflected in the electrolyte conductivity. This suggests that structural deterioration of the electrode can be indirectly evaluated by examining the electrolyte diffusion coefficient threshold Dth.

[0114] The results of this assessment primarily reflect the degree of deterioration of the positive electrode. If the negative electrode also deteriorates, the interfacial resistance increases, but the increase is often smaller than that of the positive electrode. Deterioration of the negative electrode is mainly due to deactivation due to isolation of the active material and potential deviation caused by the consumption of Li in SEI formation, and the probability of structural deterioration such as cracking occurring is lower than that of the positive electrode.

[0115] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the invention.

Claims

1. A method for determining the degree of deterioration of electrodes included in a secondary battery to be determined, comprising: estimating characteristic parameters of the secondary battery to be evaluated at the time of evaluation, including an electrolyte diffusion coefficient of the secondary battery to be evaluated at the time of evaluation, using a predetermined model formula based on data obtained by measuring the secondary battery to be evaluated; determining a discharge capacity when the electrolyte diffusion coefficient is changed based on the model formula and the estimated characteristic parameters, and determining a relationship between the electrolyte diffusion coefficient and the discharge capacity; determining a threshold Dth of the electrolyte diffusion coefficient based on the relationship between the electrolyte diffusion coefficient and the discharge capacity; a step of comparing the threshold value Dth with a threshold value Dth0 of the electrolyte diffusion coefficient before deterioration that has been obtained in advance for the secondary battery to be judged or a secondary battery of the same type as the secondary battery to be judged, and classifying the secondary battery to be judged based on the difference ΔDth = Dth - Dth0 between the threshold value Dth and the threshold value Dth0.

2. 2. The method for determining whether an electrode of a secondary battery is recycled according to claim 1, The method for determining whether an electrode of a secondary battery is recycled further comprises the step of classifying the secondary battery to be determined based on a rate of decrease in discharge capacity of the secondary battery to be determined.

3. 2. The method for determining whether an electrode of a secondary battery is recycled according to claim 1, The method for determining whether an electrode of a secondary battery is recycled further comprises a step of classifying the secondary battery to be determined based on a difference ΔD between the threshold value Dth and the electrolyte diffusion coefficient at the time of determination.

4. 2. The method for determining whether an electrode of a secondary battery is recycled according to claim 1, A method for determining recycling of electrodes of a secondary battery, wherein the characteristic parameters of the secondary battery to be determined at the time of determination further include electrolyte conductivity of the secondary battery to be determined at the time of determination.

5. 2. The method for determining whether an electrode of a secondary battery is recycled according to claim 1, The step of classifying the secondary battery to be determined based on the difference ΔDth=Dth−Dth0 includes: classifying the secondary battery to be determined into a first category when the difference ΔDth is equal to or less than a predetermined boundary value, and classifying the secondary battery to be determined into a second category when the difference ΔDth is greater than the predetermined boundary value; The predetermined boundary value is −0.01×10 －６ ~0.01 x 10 －６ cm ２ / sec range.

6. 2. The method for determining whether an electrode of a secondary battery is recycled according to claim 1, The method for determining whether an electrode of a secondary battery is recycled, wherein the electrode is a positive electrode.

7. 2. The method for determining whether an electrode of a secondary battery is recycled according to claim 1, The method for determining recycling of electrodes of a secondary battery, wherein the secondary battery to be determined is a lithium ion battery.

8. A secondary battery electrode recycling determination program for determining a degree of deterioration of an electrode included in a secondary battery to be determined, estimating characteristic parameters of the secondary battery to be evaluated at the time of evaluation, including an electrolyte diffusion coefficient of the secondary battery to be evaluated at the time of evaluation, using a predetermined model formula based on data obtained by measuring the secondary battery to be evaluated; determining a discharge capacity when the electrolyte diffusion coefficient is changed based on the model formula and the estimated characteristic parameters, and determining a relationship between the electrolyte diffusion coefficient and the discharge capacity; determining a threshold Dth of the electrolyte diffusion coefficient based on the relationship between the electrolyte diffusion coefficient and the discharge capacity; comparing the threshold value Dth with a threshold value Dth0 of the electrolyte diffusion coefficient before deterioration that has been obtained in advance for the secondary battery to be judged or a secondary battery of the same type as the secondary battery to be judged, and classifying the secondary battery to be judged based on the difference ΔDth = Dth - Dth0 between the threshold value Dth and the threshold value Dth0.

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