WO2024204686A1 - Electrode recovery method and electrode for power storage device - Google Patents

Abstract

This electrode recovery method is a method for recovering an electrode from a used power storage device, the recovery method comprising: a first step (S1) for evaluating the state of the electrode; and a second step (S2) for compressing the electrode in the thickness direction of the electrode.

Description

Electrode recovery method and electrode for power storage device

The present invention relates to a method for restoring an electrode and an electrode for an electricity storage device.
This application claims priority based on Japanese Patent Application No. 2023-057930, filed on March 31, 2023, the contents of which are incorporated herein by reference.

In recent years, research and development has been conducted into the reuse of secondary batteries, which contribute to energy efficiency, in order to ensure that more people have access to affordable, reliable, sustainable, and advanced energy (see, for example, Patent Document 1).

Japanese Patent Application Publication No. 2012-022969

In the technology for reusing secondary batteries such as that described in Patent Document 1, although it is possible to remove deteriorated active material surfaces, there is an issue in that it is not possible to recover the increased resistance caused by the reduced adhesion of particles.

The present application aims to solve the above problem by recovering the increase in resistance caused by the decrease in particle adhesion. This will ultimately contribute to improving energy efficiency.

In order to solve the above problems, the present invention proposes the following means.
<1> The method for recovering an electrode according to aspect 1 of the present invention comprises the steps of:
A method for recovering electrodes of a used electricity storage device, comprising the steps of:
a first step of evaluating the condition of the electrode;
a second step of compressing the electrode in a thickness direction of the electrode;
may include:
<2> Aspect 2 of the present invention is the electrode recovery method of aspect 1,
The first step further includes the step of measuring a resistance component of the electrode.
<3> Aspect 3 of the present invention is the electrode recovery method according to aspect 1 or 2, wherein the resistance component may be a composite layer resistance or an interface resistance of the electrode.
<4> Aspect 4 of the present invention is a method for recovering an electrode according to any one of aspects 1 to 3,
In the second step, compression may be performed while heating.
<5> Aspect 5 of the present invention is the electrode recovery method according to any one of aspects 1 to 4, wherein the second step further includes a calculation step of measuring a thickness of the electrode and calculating a difference between the thickness and an estimated thickness before use,
In the second step, the electrode may be compressed in the thickness direction so that a thickness of the electrode is reduced by the difference acquired in the calculation step.
<6> A sixth aspect of the present invention is the electrode recovery method according to any one of the first to fifth aspects, wherein the second step further includes a coating step of coating a conductive agent on a surface of the electrode.
<7> A seventh aspect of the present invention is the method for recovering an electrode according to any one of the first to sixth aspects, wherein the conductive agent is carbon fiber.
<8> Aspect 8 of the present invention may be the electrode recovery method according to any one of Aspects 1 to 7, wherein the coating step is a step of coating a dispersion liquid in which the conductive agent is dispersed, and drying the coating liquid.
<9> A ninth aspect of the present invention is the electrode recovery method according to any one of the first to eighth aspects, wherein ultrasonic waves may be applied to the electrode in the coating step.
<10> An electrode for a power storage device according to a tenth aspect of the present invention is obtained by the electrode recovery method according to any one of the first to ninth aspects.

The above aspects of the present invention make it possible to recover from the increase in resistance caused by the decrease in particle adhesion, which in turn contributes to improved energy efficiency.

2 is a flowchart of a method for recovering an electrode according to a first embodiment of the present invention. FIG. 2 is a schematic diagram showing the configuration of an electrode. FIG. 2 is a diagram for explaining an example of a dV/dQ curve for each of a positive electrode and a negative electrode in an initial state. 11 is a diagram illustrating an example of a curve fitted to an actual measured dV/dQ curve of a power storage device. FIG. FIG. 13 is a diagram for explaining a comparison between a dV/dQ curve in an initial state and a diagnostic dV/dQ curve. 5 is a flowchart of a method for recovering an electrode according to a second embodiment of the present invention.

The electrode recovery method according to one embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a flowchart of the electrode recovery method according to one embodiment of the present invention. The electrode recovery method disclosed herein is a method for recovering electrodes of a used energy storage device, and includes a first step S1 of evaluating the state of the electrode, and a second step S2 of compressing the electrode in the thickness direction of the electrode. Each step will be described below.

(First step S1)
In the first step S1, the state of the electrodes of the used energy storage device is evaluated. In the first step S1, the state of the electrodes of the used energy storage device may be evaluated based on physical quantity data obtained when the used energy storage device is charged and discharged. The physical quantity data is, for example, a voltage value and a current value during charging.

(Used Electricity Storage Device)
The used electricity storage device has electrodes including an active material, a binder, and a current collector. The electricity storage device is not particularly limited as long as it can store electricity and has electrodes including an active material, a binder, and a current collector. The electricity storage device is, for example, a lithium ion secondary battery. FIG. 2 is a schematic diagram showing the configuration of the electrodes. The electrodes targeted by the active material separation method of the present disclosure are a positive electrode 10 and a negative electrode 20. The configuration of the electrodes will be described below.

"Positive electrode"
The positive electrode 10, which is an electrode, includes a positive electrode active material 11, a positive electrode conductive assistant 12, a positive electrode binder 13, and a positive electrode current collector 14. A layer consisting of the positive electrode active material 11, the positive electrode conductive assistant 12, and the positive electrode binder 13 is a positive electrode mixture layer. The positive electrode mixture layer may be formed on one side or both sides of the positive electrode current collector 14. Note that, as long as the positive electrode active material 11 is conductive, the positive electrode mixture layer does not need to include the positive electrode conductive assistant 12.

The positive electrode active material 11 used in the positive electrode is not particularly limited as long as it is capable of absorbing and releasing Li ions. Examples of the positive electrode active material 11 include lithium nickel oxide (e.g., LiNiO ₂ ), lithium cobalt oxide (e.g., LiCoO ₂ ), lithium nickel cobalt oxide, lithium nickel cobalt manganese oxide, LiFePO ₄ , LiMn _1-x Fe _x PO ₄ , LiMnPO ₄ , LiCoPO ₄ , and LiNiPO _4. The positive electrode active material 11 preferably contains one or more elements selected from the group consisting of manganese, nickel, and cobalt.

The positive electrode conductive assistant 12, which is a conductive assistant used in the positive electrode 10, assists in the formation of a conductive path between the positive electrode active material 11 and the positive electrode current collector 14. There are no particular limitations on the positive electrode conductive assistant 12 as long as it has conductivity, and examples of the positive electrode conductive assistant 12 include carbon black such as acetylene black, carbon nanotubes, and graphite such as artificial graphite.

The positive electrode binder 13, which is a binder for the positive electrode active material 11, binds the positive electrode active material 11, the positive electrode conductive assistant 12, and the positive electrode current collector 14 together. Examples of the positive electrode binder 13 include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyacrylic acid and its copolymers, polyamideimide (PAI), polybenzimidazole, polyethersulfone (PES), maleic anhydride-modified polypropylene, and mixtures thereof. The positive electrode binder 13 preferably contains a crystalline polymer having a melting point. The positive electrode binder 13 is preferably a polymer containing fluorine. Examples of the polymer containing fluorine include PVDF and PTFE.

The positive electrode collector 14 may be, for example, a metal foil such as aluminum foil, stainless steel foil, or nickel foil. A carbon coating layer may be formed on the positive electrode collector 14. The positive electrode collector 14 may also be processed into a mesh shape.

"Negative electrode"
The negative electrode 20, which is an electrode, includes a negative electrode active material 21, a negative electrode conductive assistant 22, a negative electrode binder 23, and a negative electrode current collector 24. A layer consisting of the negative electrode active material 21, the negative electrode conductive assistant 22, and the negative electrode binder 23 is defined as a negative electrode mixture layer. The negative electrode mixture layer may be formed on one side or both sides of the negative electrode current collector 24. Note that, as long as the negative electrode active material 21 is conductive, the negative electrode mixture layer does not need to include the negative electrode conductive assistant 22.

The negative electrode active material 21, which is the active material of the negative electrode 20, is not particularly limited as long as it is capable of absorbing and releasing Li ions. Examples of the negative electrode active material 21 include graphite (artificial graphite, natural graphite), amorphous carbon (hard carbon), mesocarbon microbeads, carbon fiber, and Si materials (silicon, Si alloys, Si oxides).

The negative electrode conductive assistant 22, which is a conductive assistant for the negative electrode 20, assists in the formation of a conductive path between the negative electrode active material 21 and the negative electrode current collector 24. There are no particular limitations on the negative electrode conductive assistant 22 as long as it has conductivity, and examples of the negative electrode conductive assistant 22 include carbon black such as acetylene black, carbon nanotubes, and graphite such as artificial graphite.

The negative electrode binder 23, which is a binder for the negative electrode 20, binds the negative electrode active material 21, the negative electrode conductive assistant 22, and the negative electrode current collector 24 together. Examples of the negative electrode binder 23 include carboxymethyl cellulose, polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, fluororubber, and diene rubber such as styrene butadiene rubber. The negative electrode binder 23 preferably contains a crystalline polymer having a melting point. The negative electrode binder 23 is preferably a polymer containing fluorine. Examples of the polymer containing fluorine include PVDF, PTFE, and fluororubber.

The negative electrode current collector 24, which is a current collector for the negative electrode 20, may be, for example, a metal foil such as copper foil, stainless steel foil, or nickel foil. A carbon coating layer may be formed on the negative electrode current collector 24. The negative electrode current collector 24 may also be processed into a mesh shape.

(Advance preparation)
More specifically, as a preparation for the first step S1, for example, first, a storage device with the same specifications as the target storage device is disassembled, and single-pole data in coin cell units, that is, dV/dQ curves for each single pole of the positive and negative electrodes in the initial state, as exemplified in FIG. 3, are obtained. Furthermore, these single-pole dV/dQ curves are added together and fitted to the actual measurement curve of the storage device (curve drawn by a dashed line in FIG. 4) as shown in FIG. 4. In FIG. 4, the fitted curve is shown by a solid line. This allows each extreme value of the actual measurement curve to be assigned to the positive and negative electrodes. In this way, the dV/dQ curve in the initial state of the used storage device is obtained in advance. Then, the positions of each peak based on the positive and negative electrodes in the dV/dQ curve are determined, and the peak widths in the initial state of each of the positive and negative electrodes are determined.

In Figures 3 and 4, the horizontal axis indicates the cell capacity (Ah), and the vertical axis indicates the amount of change in voltage relative to the change in reference capacity (dV/dQ).

Here, assuming a fully charged state to be 100%, if a peak position can be detected between 5% and 95% of the charge rate or state of charge (SOC: State of Charge), differential capacity analysis becomes possible. Specifically, in the dV/dQ curve in the initial state, two peaks based on the positive electrode are identified, and the width between these peaks is determined. In Figures 3 and 4, the two peaks based on the positive electrode are located at the intersections with the dashed lines. Also, in the dV/dQ curve in the initial state, two peaks based on the negative electrode are identified, and the width between these peaks is determined. In Figures 3 and 4, the two peaks based on the negative electrode are located at the intersections with the dashed lines.

For example, as a guideline, the two peaks between 0 and 30% on the low SOC side may be determined as the peaks of the positive electrode, the two peaks between 30 and 60% as the peaks of the negative electrode (graphite in this embodiment) (intersections with the dashed line Gr), and the two peaks between 60 and 100% on the high SOC side as the peaks of silicon oxide (SiO) in the negative electrode (intersections with the dashed line SiO). It can also be determined that the positive electrode peak is convex on the negative side of dV/dQ, and the negative electrode peak is convex on the positive side of dV/dQ. Note that although peaks originating from the material that constitutes the electrodes are generated, if the negative electrode does not contain SiO, for example, no peaks originating from SiO are generated.

(After battery use begins)
After the battery is first used, the voltage and current are continuously measured by charging at a low current. It is important that the current is relatively low. The capacity is obtained by integrating the current value with respect to time, and a curve of the voltage value versus the capacity is obtained from this. Furthermore, a differential capacity curve (dV/dQ curve) versus the capacity can be obtained by differentiating the voltage value with respect to the capacity.

Analysis is possible even with a typical charging rate of 0.2 to 0.5C. However, a lower rate of, for example, 0.02 to 0.07C, allows for more accurate analysis. Note that once charging has progressed to a certain extent and the voltage has stabilized, charging can be continued with the current gradually reduced.

As described above, the dV/dQ curve is obtained by calculating the derivative of the voltage in the charge/discharge curve of the storage device at the reference capacity. This method makes it possible to accurately recognize the fluctuation characteristics of the voltage with respect to the reference capacity of the storage device to be subjected to degradation judgment, and by comparing the generated dV/dQ curve for degradation judgment with the dV/dQ curve obtained in the initial state of the storage device, it becomes possible to evaluate and determine the degree to which the battery at that time has deteriorated from the initial state.

The dV/dQ curve obtained for a used energy storage device is called a diagnostic dV/dQ curve, which indicates the amount of change in voltage relative to the change in the reference capacity of the used energy storage device.

Next, based on the obtained diagnostic dV/dQ curve and the dV/dQ curve in the initial state, the first capacity loss rate due to deterioration of the positive electrode, the second capacity loss rate due to deterioration of the negative electrode, and the amount of capacity loss due to other factors are evaluated.

Specifically, for the cells of a used energy storage device, two peaks originating from each electrode are identified, and the change in the width between these two peaks (peak width) between the initial state and the state after the battery has started to be used is evaluated. In other words, the peak widths of the positive and negative electrodes in the state after the battery has started to be used are determined, and the change in the peak widths due to use of the energy storage device is evaluated by comparing them with the peak widths in the initial state described above.

(First Capacity Decay Rate and Second Capacity Decay Rate)
The dV/dQ curve in the initial state is compared with the diagnostic dV/dQ curve, and a rate of change in the width between two peaks based on the positive electrode and the negative electrode is calculated. The rate of change in the width between two peaks based on the positive electrode of the diagnostic dV/dQ curve relative to the dV/dQ curve in the initial state is defined as the first capacity decrease rate, and the rate of change in the width between two peaks based on the negative electrode of the diagnostic dV/dQ curve relative to the dV/dQ curve in the initial state is defined as the second capacity decrease rate.

Figure 5 shows a comparison between the dV/dQ curve in the initial state and the diagnostic dV/dQ curve. In Figure 5, the solid line shows the dV/dQ curve in the initial state, and the dashed line shows the diagnostic dV/dQ curve. The diagnostic dV/dQ curve is shifted along the vertical axis to avoid overlap. As shown in Figure 5, when comparing the dV/dQ curve in the initial state with the diagnostic dV/dQ curve, it can be seen that the width between the two peaks based on the positive and negative poles has changed.

(Capacity loss due to other factors)
The amount of shift of the diagnostic dV/dQ curve from the dV/dQ curve in the initial state is defined as the amount of capacity loss due to factors (other factors) other than the deterioration of the positive or negative electrode. The amount of shift of the diagnostic dV/dQ curve from the dV/dQ curve in the initial state is determined to be mainly due to the decrease in the amount of Li involved in charging and discharging due to the deposition of the negative electrode film containing Li caused by the swelling and shrinkage of the active material. For example, the amount of shift of the midpoint of the two peaks based on the negative electrode is defined as the amount of capacity loss due to other factors.

It is determined whether the obtained first capacity decline rate and second capacity decline rate are both below a predetermined threshold (e.g., 5%) and the capacity decline amount exceeds a predetermined threshold. The threshold can be set for each storage device.

If it is confirmed that the first capacity loss rate and the second capacity loss rate are equal to or less than the threshold value, it is preferable to then analyze the state of the electrode in more detail. Specifically, the resistance component of the electrode is measured. That is, in the electrode recovery method, it is preferable that the first step S1 further includes a step of measuring the resistance component of the electrode. The resistance component of the electrode can be measured using an AC impedance method or a multi-point probe method. Note that, for example, when refilling the power storage device with Li, the electrode may be removed as described below and the second step S2 may be carried out without performing the measurements using the AC impedance method or the multi-point probe method.

When measuring the resistance components using the AC impedance method, for example, the following method can be used. For example, the AC impedance is measured in the state of a storage device (cell) using a commercially available AC impedance measuring device. For example, an AC voltage with an amplitude of 10 mV superimposed on the OCV (open circuit voltage) is applied from 1 MHz to 1 mHz, and the internal resistance can be obtained from the response current. The obtained results are plotted as a cole-cole plot. The resistance components of the electrodes to be measured include, for example, composite layer resistance and interface resistance. Each resistance component can be separated based on the time constant. The measurement conditions are set appropriately depending on the storage device. The separation of each resistance component based on the time constant can be performed by, for example, taking the measurement results from 1 MHz to 1 kHz as the high-frequency side resistance, judging them as metal resistance and resistance due to deterioration of the coating at the battery interface and electrolyte components, judging 1 kHz to 1 Hz as ion conduction resistance between active materials, and judging 1 Hz or less as solid ion diffusion within the particles.

When determining the state of the electrodes using the AC impedance method, for example, a reference curve obtained from the AC impedance of a storage device immediately after manufacture is compared with the curve of a used storage device. For example, in a cole-cole plot of a used storage device, if the region where the impedance component increases in a roughly linear manner (right-shouldering region) with increasing frequency is larger than that of the initial storage device, it is determined that the conductive resistance has increased, and the second step S2 is carried out.

In the case of the multi-point probe measurement method, the electrodes removed from the electricity storage layer are evaluated. There are no particular limitations on the method for removing the electrodes, the positive electrode 10 and the negative electrode 20, from the electricity storage device, and any known method can be used. The electrode to be evaluated is preferably the positive electrode 10. It is dangerous to dismantle the electricity storage device while it is charged, so it is preferable to fully discharge the electricity storage device. After discharging, the exterior material of the electricity storage device is cut, and the positive electrode 10 and the negative electrode 20 are removed from the electricity storage device.

In the multi-point probe measurement method, a fine probe is placed on the electrode surface, a constant current is passed, and the potential at multiple points is measured. A virtual electrode is also assumed, and modeling is performed to calculate the potential generated on the surface. Then, using the resistance of the composite layer and the interfacial resistance as variables, measurements are repeated until the actual potential matches the measured potential. If it is determined that the resistance of the composite layer and the interfacial resistance obtained in this way are higher than a predetermined threshold, the second step S2 is performed. For example, if the resistance of the composite layer of the electrode before use increases by 10% or more, it is determined that the resistance of the composite layer has increased, and the second step S2 is performed. As an example of a multi-point probe measurement system, the electrode resistance measurement system RM2610 manufactured by HIOKI Corporation can be used.

(Second step S2)
In the second step S2, the electrode is compressed in the thickness direction of the electrode. Compression can be performed by a known means such as a roll press. The electrode may be either the positive electrode 10 or the negative electrode 20, but the positive electrode 10 is preferred. In the second step S2, compression while heating is preferably performed. By pressing while heating, the binders 13, 23 can be softened, making it easier to regenerate the adhesive force. The heating temperature is, for example, from the melting point of the binder to 200°C.

It is preferable that the second step S2 further includes a calculation step of measuring the thickness of the electrodes 10, 20 and calculating the difference from the estimated pre-use thickness. In the second step S2, it is preferable to compress the electrodes 10, 20 in the thickness direction so that the thickness of the electrodes 10, 20 is reduced by the difference obtained in the calculation step. By compressing in this manner, it is possible to restore inter-particle contact that has been reduced due to the expansion and contraction of the active material associated with charging and discharging. If there is information on the thickness of the electrodes at the time of manufacture of the energy storage device, this information is used for the estimated pre-use thickness. If there is no information on the thickness at the time of initial manufacture, for example, the thickness obtained by excluding voids in the electrode portion from the thickness of the used electrode may be used as the estimated pre-use thickness.

The electrode recovery method according to the first embodiment has been described above. The electrode recovery method according to this embodiment can recover the increase in resistance caused by the decrease in particle adhesion. A recovered electrode for a power storage device can be obtained by the electrode recovery method according to this embodiment.

Next, a method for restoring an electrode according to a second embodiment of the present invention will be described. FIG. 6 is a flowchart of the method for restoring an electrode according to the second embodiment of the present invention. The electrode restoration method disclosed herein is a method for restoring an electrode of a used energy storage device, and includes a first step S1 of evaluating the state of the electrode, and a second step S2A of compressing the electrode in the thickness direction of the electrode. In the following description, the same reference numerals are used for configurations that are the same as those in the first embodiment, and descriptions thereof may be omitted. Each step will be described below.

(Second step S2)
The second step S2A preferably further includes a coating step of coating the surfaces of the electrodes 10, 20 with a conductive agent. The electrode may be either the positive electrode 10 or the negative electrode 20, but the positive electrode 10 is preferred. The conductive agent is not particularly limited, but may be, for example, a carbonaceous material such as acetylene black or carbon nanotubes. Carbon fiber is preferred as the conductive agent. By coating the conductive agent, the addition of the conductive agent can compensate for the conductivity, and the composite layer resistance and the interface resistance can be reduced.

In the coating process, the method of coating the conductive agent is not particularly limited. For example, in the coating process, a dispersion liquid in which the conductive agent is dispersed may be applied and dried. That is, the coating process may be a process in which a dispersion liquid in which the conductive agent is dispersed is applied and dried. In addition, it is preferable to apply ultrasonic waves in the coating process. By applying ultrasonic waves, the conductive agent can penetrate into the gaps in the electrodes 10 and 20, and the resistance can be further reduced. This can further improve and restore the condition of the electrodes.

In the second step S2A, the electrodes 10, 20 after the application step are compressed in the thickness direction of the electrodes 10, 20. Compression can be performed by a known means such as a roll press. In the second step S2A, it is preferable to compress the electrodes 10, 20 while heating them. By pressing while heating, the binders 13, 23 can be softened, making it easier to restore the adhesive force. The heating temperature is, for example, from the melting point of the binder to 200°C.

The second step S2A preferably further includes a calculation step of measuring the thickness of the electrodes 10, 20 and calculating the difference from the estimated thickness before use. In the second step S2A, it is preferable to compress the electrodes 10, 20 in the thickness direction so that the thickness of the electrodes 10, 20 is reduced by the difference obtained in the calculation step. By compressing in this manner, it is possible to restore interparticle contact and the like that has been reduced due to the expansion and contraction of the active material associated with charging and discharging.

As described above, the active material separation method according to the second embodiment makes it possible to recover from the increase in resistance caused by a decrease in particle adhesion.

The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. In addition, the components in the above-described embodiment can be replaced with well-known components as appropriate without departing from the spirit of the present invention.

The recovery method disclosed herein makes it possible to recover from the increase in resistance caused by a decrease in particle adhesion, and is therefore highly applicable industrially.

10 Positive electrode, 11 Positive electrode active material, 12 Positive electrode conductive additive, 13 Positive electrode binder, 14 Positive electrode current collector, 20 Negative electrode, 21 Negative electrode active material, 22 Negative electrode conductive additive, 23 Negative electrode binder, 24 Negative electrode current collector, S1 First step, S2 Second step

Claims (10)

A method for recovering electrodes of a used electricity storage device, comprising the steps of:
a first step of evaluating the condition of the electrode;
a second step of compressing the electrode in a thickness direction of the electrode;
A method for recovering an electrode, comprising: The electrode recovery method according to claim 1, wherein the first step further includes a step of measuring the resistance component of the electrode. The electrode recovery method according to claim 2, wherein the resistance component is a composite layer resistance or an interface resistance of the electrode. The electrode recovery method according to claim 1, in which the second step involves compressing while heating. The second step further includes a calculation step of measuring the thickness of the electrode and calculating a difference between the thickness and an estimated thickness before use,
The electrode recovery method according to claim 1 , wherein in the second step, the electrode is compressed in the thickness direction so that the thickness of the electrode is reduced by the difference acquired in the calculation step. The electrode recovery method according to claim 1, wherein the second step further includes a coating step of coating a conductive agent on the surface of the electrode. The method for recovering an electrode according to claim 6, wherein the conductive agent is carbon fiber. The electrode recovery method according to claim 6, wherein the coating step is a step of coating a dispersion liquid in which the conductive agent is dispersed, and drying the dispersion liquid. The electrode recovery method according to claim 8, wherein ultrasonic waves are applied to the electrode during the application process. An electrode for a power storage device obtained by the electrode recovery method described in any one of claims 1 to 9.

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