Method for estimating the degradation of lithium-ion secondary batteries

Abstract

This technology provides a method for estimating degradation caused by the oxidation of silicon contained in the negative electrode active material. [Solution] The method for estimating the degradation of a lithium-ion secondary battery comprises: an oxidation progression calculation step, which is a step of calculating the degree of oxidation based on an elemental map of the negative electrode active material for each different charge-discharge cycle, wherein the negative electrode active material has composite particles containing silicon and carbon, the elemental map is an image of the composite particles in which regions containing oxygen can be identified, and the degree of oxidation is the proportion of the composite particle image in which oxygen is contained; a correlation formula creation step, which is a step of creating a correlation formula that shows the correlation between the degree of oxidation and the capacity retention rate of the lithium-ion secondary battery; and a degradation estimation step, which is a step of estimating the degradation of the lithium-ion secondary battery based on the correlation formula.

Description

Technical Field

[0001] The technology disclosed in this specification relates to a method for estimating the deterioration of a lithium-ion secondary battery.

Background Art

[0002] Patent Document 1 discloses a technique for measuring an open-circuit voltage for each different charge-discharge cycle and estimating the life of a lithium-ion secondary battery based on the size of an arc passing through the plotted points of the measured open-circuit voltage.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, silicon contained in the negative electrode active material is known to have a large theoretical capacity. Therefore, in order to adopt silicon as a material for the negative electrode active material, the development of composite particles containing silicon and carbon has been promoted. However, it is known that silicon contained in such a negative electrode active material is oxidized by an oxidizing agent such as oxygen or water, and the deterioration progresses. In the technology of Patent Document 1, since the positive and negative electrodes cannot be separated to evaluate the deterioration, it is impossible to estimate the deterioration caused by the oxidation of silicon contained in the negative electrode active material. This specification provides a technology capable of estimating the deterioration caused by the oxidation of silicon contained in the negative electrode active material in a lithium-ion secondary battery in which composite particles containing silicon and carbon are adopted as the negative electrode active material.

Means for Solving the Problems

[0005] A method for estimating the degradation of a lithium-ion secondary battery disclosed herein may comprise: an oxidation progression calculation step, which is a step of calculating the degree of oxidation based on an elemental map of a negative electrode active material for each different charge-discharge cycle, wherein the negative electrode active material includes composite particles containing silicon and carbon, the elemental map is an image of the composite particles in which regions containing oxygen can be identified, and the degree of oxidation represents the proportion of the composite particle image in which oxygen is contained; a correlation formula creation step, which creates a correlation formula showing the correlation between the degree of oxidation and the capacity retention rate of the lithium-ion secondary battery; and a degradation estimation step, which estimates the degradation of the lithium-ion secondary battery based on the correlation formula.

[0006] The above degradation estimation method calculates the degree of oxidation based on the elemental map of the negative electrode active material and uses this degree of oxidation to estimate the degradation of the lithium-ion secondary battery. Therefore, this degradation estimation method can estimate degradation caused by the oxidation of silicon contained in the negative electrode active material. [Brief explanation of the drawing]

[0007] [Figure 1] This shows a flowchart for estimating the degradation of lithium-ion secondary batteries. [Figure 2] This diagram illustrates how to calculate the degree of oxidation based on an elemental map. [Figure 3] (A) An example of a regression line showing the correlation between the degree of oxidation and the capacity retention rate is shown. (B) An example of a regression line showing the correlation between the number of charge / discharge cycles and the degree of oxidation is shown. [Modes for carrying out the invention]

[0008] As described below, the degradation estimation method disclosed herein estimates the degradation of a lithium-ion secondary battery in which a composite particle containing silicon and carbon is used as the negative electrode active material. In particular, the degradation estimation method disclosed herein estimates the degradation of a lithium-ion secondary battery from the degree of oxidation of the negative electrode active material. For this reason, the material of the positive electrode active material is not particularly limited and may be, for example, a ternary positive electrode material (NMC) containing nickel, manganese, and cobalt.

[0009] The negative electrode active material for a lithium-ion secondary battery is not particularly limited, but may be prepared by, for example, the following procedure. First, a mixture is prepared by mixing silicon, which has been pulverized to about 100 nm to 1 μm using a planetary ball mill, and graphite, which has been pulverized to about 100 nm to 1 μm using a planetary ball mill. This mixture is mixed with the same weight of a carbon precursor (e.g., coal tar pitch) and fired at about 800°C. Next, the fired product is pulverized using a hammer mill or jet mill, etc., to an average particle size of about 10 μm. Through these steps, a material is produced in which composite particles containing silicon and carbon (hereinafter referred to as "Si / C particles") and carbon particles are mixed. In this embodiment, the negative electrode active material for the lithium-ion secondary battery is the mixed material prepared by the above procedure.

[0010] The following describes a method for estimating the degradation of a lithium-ion secondary battery having a negative electrode active material prepared according to the procedure described above. This degradation estimation method is not particularly limited, but may be performed, for example, as a pre-shipment sampling inspection to determine whether the life cycle number of a lithium-ion secondary battery meets a standard value. This degradation estimation method may be performed by a degradation estimation device connected via a network to a scanning electron microscope-energy dispersive X-ray analyzer (SEM-EDX device) and a charge retention rate measuring device that measures the charge retention rate of a lithium-ion secondary battery. The degradation estimation device consists of a computer and can perform the processes described below.

[0011] As shown in Figure 1, the degradation estimation method is carried out as a cycle test. This cycle test is not particularly limited, but may be performed under conditions such as a test temperature of 25°C, a SOC of 5-95%, and a C rate of 1 / 3C. The charge / discharge control method may be CCCV charging with a 4.2V cut, a 30-minute pause, and CCCV discharge with a 2.5V cut.

[0012] First, the degradation estimation method performs a step (step S1) to determine whether the number of charge-discharge cycles is a predetermined number. The predetermined number of cycles includes a set of predetermined cycle counts. The predetermined number of cycles is not particularly limited, but may be three or more cycle counts at certain intervals (for example, 10, 20, and 30 cycles). If the number of charge-discharge cycles is a predetermined number, the degradation estimation method proceeds to the next step.

[0013] Next, in the degradation estimation method, a step is performed to acquire an elemental map of Si / C particles using a SEM-EDX instrument (step S2). Specifically, after processing a cross-section of the negative electrode active material using ion milling, the elemental map is acquired by observing the cross-section using an FE-SEM and performing EDX analysis. The observation magnification is set to a magnification in which several Si / C particles are present in the field of view. The elemental map is created using EDX map data for two elements, silicon and oxygen.

[0014] Next, the degradation estimation method performs a step of calculating the degree of oxidation based on the acquired elemental map (step S3). Referring to Figure 2, the method for calculating the degree of oxidation will be explained. The elemental map is a two-dimensional image that can identify the region where the target element exists. In this embodiment, the elemental map includes a Si elemental map showing the region where silicon exists and an O elemental map showing the region where oxygen exists. In the degradation estimation method, the Si elemental map and the O elemental map are superimposed to create a composite map. The region with the symbol "2" in the composite map is the region where silicon exists in the Si elemental map, which is inside the region where oxygen exists in the O elemental map. The region with the symbol "4" in the composite map is the region where the region where silicon exists in the Si elemental map and the region where oxygen exists in the O elemental map overlap, and is a region where both silicon and oxygen exist. The region with the symbol "6" in the composite map is the region where oxygen exists in the O elemental map, which is outside the region where silicon exists in the Si elemental map. The degree of oxidation is calculated by the following formula.

number

[0015] The combined area of ​​the three regions 2, 4, and 6 represents the area of ​​the Si / C particle. The combined area of ​​the two regions 4 and 6 represents the area of ​​the Si / C particle where oxidation has progressed. The degree of oxidation is calculated as the average value of several Si / C particles that are fully visible in the field of view. The area is calculated by the number of pixels.

[0016] Next, the degradation estimation method performs a step (step S4) to calculate the capacity retention rate of the lithium-ion secondary battery from the discharge capacity measured using a charge retention rate measuring device. Specifically, the capacity retention rate is calculated as the ratio of the measured discharge capacity to the initial discharge capacity, with the discharge capacity measured after aging defined as 100% of the initial discharge capacity. Note that step S4 may be performed before step S2, or it may be performed in parallel with steps S2 and S3.

[0017] Next, the degradation estimation method performs a step to determine whether the number of charge-discharge cycles is the predetermined maximum number of cycles (step S5). That is, in step S1, the degradation estimation method determines whether the number of cycles is the maximum among a plurality of predetermined number of cycles. If the degradation estimation method determines that it is not the maximum number of cycles, it returns to step S1 and repeats steps S1 to S4. If the degradation estimation method determines that it is the maximum number of cycles, it proceeds to the next step.

[0018] Next, the degradation estimation method performs a step (step S6) to create a correlation equation that shows the correlation between the degree of oxidation and the capacity retention rate. Figure 3(A) shows an example of the created correlation equation. In this example, the correlation equation is created as a simple regression equation that shows the correlation between the degree of oxidation and the capacity retention rate calculated for each predetermined number of cycles.

[0019] Next, in the degradation estimation method, a step of calculating a correlation formula showing the correlation between the degree of oxidation progress and the number of cycles is executed (step S7). FIG. 3(B) shows an example of the created correlation formula. The correlation formula in this example is created as a simple regression formula showing the correlation between the degree of oxidation progress and the number of cycles calculated for each predetermined number of cycles. Note that this step S7 may be executed before step S6 or may be executed in parallel with step S6.

[0020] Next, in the degradation estimation method, a step of estimating the number of life cycles based on the degradation of the capacity retention rate is executed (step S8). A lower limit reference value is defined for the capacity retention rate. In the degradation estimation method, the lower limit reference value of the capacity retention rate is substituted into the simple regression formula shown in FIG. 3(A), and the degree of oxidation progress when the capacity retention rate reaches the lower limit reference value is estimated. Next, in the degradation estimation method, the estimated degree of oxidation progress is substituted into the simple regression formula shown in FIG. 3(B), and the number of cycles when the capacity retention rate reaches the lower limit reference value, that is, the number of life cycles is estimated.

[0021] As described above, the degradation estimation method disclosed in this specification calculates the degree of oxidation progress based on the elemental map of the negative electrode active material, and estimates the degradation of the capacity retention rate of the lithium-ion secondary battery based on the degree of oxidation progress. Therefore, this degradation estimation method can estimate the degradation of the lithium-ion secondary battery caused by the oxidation of silicon contained in the negative electrode active material. In addition, this degradation estimation method can also estimate the number of life cycles based on the degradation of the capacity retention rate of the lithium-ion secondary battery. This degradation estimation method can estimate the number of life cycles without performing a long-term cycle test.

[0022] Although several specific examples have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. The technical elements described in this specification or the drawings exhibit technical utility alone or in combination.

Claims

[Claim 1] A method for estimating the degradation of lithium-ion secondary batteries, A step for calculating the degree of oxidation progression based on an elemental map of a negative electrode active material for each different charge-discharge cycle, wherein the negative electrode active material has composite particles containing silicon and carbon, the elemental map is an image of the composite particles in which regions containing oxygen can be identified, and the degree of oxidation progression is the proportion of the composite particles in the image in which oxygen is contained, A correlation formula creation step to create a correlation formula that shows the correlation between the degree of oxidation and the capacity retention rate of the lithium-ion secondary battery, A degradation estimation method comprising a degradation estimation step of estimating the degradation of the lithium-ion secondary battery based on the correlation formula.

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