Active material recovery and processing method

Abstract

Provided is an active material recovery and processing method with which it is possible to ameliorate reduced activity of an active material. The active material recovery and processing method is a processing method for ameliorating the reduced activity of an active material provided to a lithium ion battery, the method comprising: a first heat treatment step for heat-treating, at a temperature lower than the thermal decomposition temperature of a binder included in the lithium ion battery, an object to be processed in which a lithium compound is mixed with the active material or an electrode mixture that contains the active material; and a second heat treatment step for heat-treating, at a temperature of 600-1000°C, the object to be processed that has been heat-treated in the first heat treatment step.

Description

Method for restoring active material

[0001] The present invention relates to a method for restoring active material.

[0002] The active material in lithium-ion batteries experiences a decrease in activity as the lithium ions within the active material decrease due to the formation of SEI (Surface Electrolyte Interphase) during use. Furthermore, the active material's activity also decreases as the valence of the transition metal ions decreases during use. For example, the valence of nickel ions in the active material decreases from tetravalent to divalent during lithium-ion battery use; that is, nickel(IV) oxide changes to nickel(II) oxide.

[0003] The activity of an active material whose activity has decreased can be improved by subjecting it to a recovery treatment. For example, Patent Document 1 discloses a technique for improving the activity of a decreased positive electrode active material by mixing a lithium compound with an electrode mixture containing a positive electrode active material and a binder, and then heat-treating it at a temperature above the melting point of the lithium compound (for example, 750°C). In the technique disclosed in Patent Document 1, the lithium compound is molten, so lithium ions are inserted into the positive electrode active material by a reaction at the solid-liquid interface between the positive electrode active material and the lithium compound. In addition, the transition metal ions are oxidized by the heat treatment at high temperature, so the decreased valence is returned to its original valence. As a result, the activity of the positive electrode active material is improved.

[0004] International Publication No. 2021 / 177362

[0005] In the technology disclosed in Patent Document 1, the binder is heated to a high temperature along with the positive electrode active material and the lithium compound. Polyvinylidene fluoride (PVDF) is often used as the binder, but when polyvinylidene fluoride is thermally decomposed at high temperatures, hydrogen fluoride (HF) is generated, and the active material reacts with hydrogen fluoride to produce lithium fluoride (LiF). As a result, the amount of lithium in the active material decreases, which degrades the performance of the active material and may reduce the charge and discharge capacity of the lithium-ion battery.

[0006] If the heat treatment temperature is set to a low temperature that makes it difficult for the binder to decompose, the generation of hydrogen fluoride can be suppressed, thereby reducing the amount of lithium in the active material. However, with low-temperature heat treatment, oxidation of transition metal ions is less likely to occur, so there was a risk that the decreased valence of the ions would not fully return to its original valence. As a result, it was sometimes not possible to sufficiently improve the activity of the decreased active material. The object of the present invention is to provide an active material recovery treatment method that can improve the activity of the decreased active material.

[0007] A method for restoring active material according to one aspect of the present invention is a method for improving the reduced activity of an active material in a lithium-ion battery, comprising: a first heat treatment step of heating a workpiece, which is an active material or an electrode mixture containing an active material mixed with a lithium compound, at a temperature below the thermal decomposition temperature of the binder in the lithium-ion battery; and a second heat treatment step of heating the workpiece that has been heated in the first heat treatment step at a temperature of 600°C to 1000°C.

[0008] According to the present invention, the reduced activity of the active material can be improved.

[0009] This figure shows the results of X-ray diffraction analysis performed on the positive electrode active material. This figure shows the results of a test conducted to confirm the effect of the cleaning process on the recovered product of the example.

[0010] One embodiment of the present invention is described below. This embodiment is merely an example of the present invention, and the present invention is not limited to this embodiment. Furthermore, various modifications or improvements can be made to this embodiment, and such modified or improved forms may also be included in the present invention.

[0011] The active material recovery treatment method according to this embodiment is a treatment method for improving the reduced activity of the active material of a lithium-ion battery, and comprises: a first heat treatment step of heating a workpiece, which is an active material or an electrode mixture containing an active material mixed with a lithium compound, at a temperature below the thermal decomposition temperature of the binder of the lithium-ion battery; and a second heat treatment step of heating the workpiece that has been heated in the first heat treatment step at a temperature of 600°C to 1000°C.

[0012] According to the active material recovery treatment method of this embodiment, the reduced activity of the active material can be improved. The effects of the active material recovery treatment method of this embodiment will be described in more detail. The first heat treatment step in the active material recovery treatment method of this embodiment is a step of heat treatment at a temperature below the thermal decomposition temperature of the binder (for example, below 350°C), so even when a workpiece to be treated in which a lithium compound is mixed with an electrode mixture is heat-treated, thermal decomposition of the binder contained in the electrode mixture is unlikely to occur, and hydrogen fluoride is unlikely to be generated.

[0013] Therefore, the formation of lithium fluoride through the reaction between the active material and hydrogen fluoride is suppressed, thus preventing a decrease in the amount of lithium in the active material. In other words, when the lithium in the active material is converted into lithium fluoride, the amount of lithium constituting the active material decreases, and the amount of active lithium inside the active material necessary for charging and discharging decreases, but such a reaction is suppressed. As a result, the deterioration of the performance of the active material during the first heat treatment process is suppressed, and the reduced activity of the active material can be sufficiently improved.

[0014] Furthermore, in the recovery treatment method for active material according to this embodiment, the second heat treatment step is a step in which the material that was heat-treated in the first heat treatment step is heat-treated at a high temperature of 600°C to 1000°C, so that the transition metal ions contained in the active material are oxidized. In other words, the valence of the transition metal ions that has decreased due to the use of lithium-ion batteries is restored to its original valence.

[0015] From these facts, according to the method for recovering an active material according to the present embodiment, while suppressing the generation of hydrogen fluoride due to thermal decomposition of the binder, the valence of the decreased transition metal ions can be returned to the original valence. Therefore, the activity of the active material that has decreased with the use of the lithium-ion battery can be sufficiently improved. In the present invention, the electrode binder means a mixture containing an active material, a conductive assistant, a binder, and the like.

[0016] Here, taking the case where lithium transition metal composite oxide LiNi 0.5 Mn 0.3 Co 0.2 O2 is used as an example, the method for recovering an active material according to the present embodiment will be described. In an unused lithium-ion secondary battery, the lithium transition metal composite oxide as the positive electrode active material can be represented by the chemical formula LiNi 0.5 Mn 0.3 Co 0.2 O2. With the use of the battery, lithium ions are desorbed from the positive electrode active material, and the valence of each ion of nickel (Ni), manganese (Mn), and cobalt (Co), which are transition metals contained in the positive electrode active material, decreases. When the positive electrode active material with decreased activity (degraded product) is represented by a chemical formula in the same manner as above, it becomes Li 1-x Ni 0.5 Mn 0.3 Co 0.2 O 2-y Here, x in the chemical formula is a number greater than 0 and less than or equal to 1, and y is a number greater than 0 and less than or equal to 2.

[0017] When the conventional method for recovering an active material is applied to the positive electrode active material with decreased activity, hydrogen fluoride is generated by thermal decomposition of the binder, and the positive electrode active material reacts with hydrogen fluoride to generate lithium fluoride. Therefore, the reaction in which lithium ions are inserted into the positive electrode active material hardly occurs. As a result, the value of x in the chemical formula does not become very small.

[0018] On the other hand, when the first heat treatment step of the active material recovery treatment method according to this embodiment is performed on a positive electrode active material whose activity has decreased, although the valence of the decreased transition metal ions does not increase, a reaction occurs in which lithium ions are inserted into the positive electrode active material. The result can be expressed by the chemical formula LiNi 0.5 Mn 0.3 Co 0.2 O 2-y This is the result. Furthermore, the valence of the transition metal ions in the positive electrode active material that has undergone the first heat treatment process will be lower than the valence of the transition metal ions in the positive electrode active material whose activity has decreased due to charge compensation.

[0019] Next, when the positive electrode active material that has undergone the first heat treatment process is subjected to the second heat treatment process, the decreased valence of the transition metal ions is restored to its original valence, becoming close to or the same valence as the transition metal ions in the positive electrode active material (unused product) of an unused lithium-ion secondary battery. The result can be expressed by the chemical formula LiNi 0.5 Mn 0.3 Co 0.2 This results in O2. Since lithium ions are inserted into the positive electrode active material in the first heat treatment step, there is no problem even if the binder decomposes thermally in the second heat treatment step, generating hydrogen fluoride and then lithium fluoride.

[0020] The amount of lithium in the active material is reflected in the crystal structure of the active material. Therefore, the amount of lithium in the active material and the level of activity of the active material can be evaluated by the c-axis lattice constant calculated from the X-ray diffraction pattern obtained by X-ray diffraction analysis (XRD). For example, when an unused lithium-ion battery is used, the amount of lithium in the active material decreases due to SEI generation, and the c-axis lattice constant calculated from the X-ray diffraction pattern increases. Therefore, when an active material with a reduced amount of lithium is subjected to a recovery treatment according to the active material recovery treatment method of this embodiment, the amount of lithium in the active material is restored, and the c-axis lattice constant calculated from the X-ray diffraction pattern decreases.

[0021] Figure 1 shows the results of X-ray diffraction analysis performed on the positive electrode active material. Compared to the positive electrode active material of an unused lithium-ion secondary battery (unused product), the positive electrode active material whose activity has decreased due to use of the lithium-ion secondary battery (degraded product) shows that the peak representing the (003) plane in the XRD pattern is shifted to the lower angle side. In other words, the c-axis lattice constant has increased. From this result, it can be seen that the amount of lithium inside the active material has decreased due to use of the lithium-ion secondary battery.

[0022] Furthermore, compared to positive electrode active material (degraded product) whose activity has decreased due to use in lithium-ion secondary batteries, the positive electrode active material (recovered product) that has been treated by the active material recovery treatment method according to this embodiment shows that the peak representing the (003) plane of the XRD pattern is shifted to the higher angle side, and is in almost the same position as the peak of positive electrode active material (unused product) found in unused lithium-ion secondary batteries. From this result, it can be seen that lithium ions are inserted into the positive electrode active material and its activity is restored by the active material recovery treatment method according to this embodiment.

[0023] The recovery treatment method for active materials according to this embodiment will be described in more detail below. [Lithium-ion batteries] The active materials to which the recovery treatment method for active materials according to this embodiment can be applied are the active materials of lithium-ion batteries, but the type of lithium-ion battery is not particularly limited, and examples include lithium-ion secondary batteries. Examples of lithium-ion secondary batteries include cobalt-based lithium-ion secondary batteries, nickel-based lithium-ion secondary batteries, NAC (nickel-cobalt-aluminum)-based lithium-ion secondary batteries, manganese-based lithium-ion secondary batteries, iron phosphate-based lithium-ion secondary batteries, ternary (nickel-manganese-cobalt)-based lithium-ion secondary batteries, titanate-based lithium-ion secondary batteries, lithium polymer-based lithium-ion secondary batteries, and the like.

[0024] [Active Material] The type of active material to which the active material recovery method according to this embodiment can be applied is not particularly limited as long as it is an active material used in a lithium-ion battery. For example, the following compounds used as the positive electrode active material can be mentioned. That is, examples of the active material include composite compounds containing lithium as a constituent element, and examples of the composite compounds include lithium transition metal composite oxides. The composite compound may be used alone or in combination of two or more.

[0025] Examples of the lithium transition metal composite oxide include LiCoO2, LiNiO2, Li(Ni,Co)O2, Li(Ni,Co,Al)O2, Li(Ni,Mn)O2, Li(Ni,Mn,Co)O2, LiMn2O4, Li(Mn,Fe)2O4, Li2MnO3, Li2NiO3, Li2(Ni,Mn)O3, LiFePO4, LiMnPO4. Examples of Li(Ni,Mn,Co)O2 include LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, LiNi 0.5 Mn 0.3 Co 0.2 O2.

[0026] [Binder] The type of electrode binder to which the active material recovery method according to this embodiment can be applied is not particularly limited as long as it is an electrode binder used in a lithium-ion battery. The electrode binder contains a binder together with the active material, but the type of the binder is not particularly limited, and for example, a thermoplastic resin can be mentioned. The binder may be used alone or in combination of two or more.

[0027] Examples of the thermoplastic resin include fluororesins such as polyvinylidene fluoride, polytetrafluoroethylene (PTFE), ethylene tetrafluoride - hexafluoropropylene - vinylidene fluoride copolymer, hexafluoropropylene - vinylidene fluoride copolymer, ethylene tetrafluoride - perfluorovinyl ether copolymer, polyolefin resins such as polyethylene and polypropylene, and styrene-butadiene copolymers. That is, the binder may contain polyvinylidene fluoride.

[0028] [Lithium Compound] The lithium compound that can be used in the method for recovering an active material according to this embodiment is not particularly limited as long as it is a compound containing lithium as a constituent element. Examples thereof include lithium hydroxide (LiOH), lithium carbonate (Li2CO3), lithium oxide (Li2O), lithium sulfate (Li2SO4), lithium nitrate (LiNO3), and lithium salts of organic acids. Among these lithium compounds, at least one of lithium hydroxide and lithium carbonate is preferable.

[0029] The property of the lithium compound to be mixed with the active material or the electrode binder containing the active material is not particularly limited, and it may be a powder, a lump, or a liquid. Further, a liquid containing a lithium compound (for example, a liquid in which a lithium compound in the form of a powder or a lump is dissolved or dispersed in a liquid medium) may be mixed with the active material or the electrode binder containing the active material.

[0030] In the method for recovering an active material according to this embodiment, the mixing amount of the lithium compound to be mixed with the active material or the electrode binder containing the active material is not particularly limited. However, the larger the mixing amount of the lithium compound, the more interfaces where the reaction between the active material and the lithium compound occurs in the first heat treatment step. Therefore, it is preferable to increase the amount of the lithium compound used.

[0031] Particularly, in the active material with reduced performance, since the amount of lithium is less than the theoretical amount, in order to increase the amount of lithium to the theoretical amount by inserting lithium into the active material by the recovery treatment, it is preferable to mix the active material or the electrode binder containing the active material with a lithium compound having the same molar amount as the shortage of the lithium amount or a molar amount in excess of the shortage. However, even if the mixing amount of the lithium compound is less than the same molar amount as the shortage of the lithium amount, the activity of the active material can be improved according to the mixing amount.

[0032] [First Heat Treatment Step] In the recovery treatment method for active material according to this embodiment, the first heat treatment step is a step of heat-treating a workpiece, which is an active material or an electrode mixture containing an active material mixed with a lithium compound, at a temperature below the thermal decomposition temperature of the binder in a lithium-ion battery. The interface state between the active material and the lithium compound in the first heat treatment step is a solid-solid interface or a solid-liquid interface. When a solid lithium compound is used, the interface state between the active material and the lithium compound is a solid-solid interface. When a liquid lithium compound or a liquid containing a lithium compound is used, the interface state between the active material and the lithium compound is a solid-liquid interface.

[0033] The heating temperature in the first heating step is not particularly limited as long as it is below the thermal decomposition temperature of the binder in the lithium-ion battery, but it may be less than 350°C or 250°C or lower. The lower limit of the heating temperature in the first heating step is not particularly limited as long as a reaction occurs in which lithium ions are inserted into the active material, but it may be 100°C or higher or 200°C or higher. Furthermore, the heating temperature in the first heating step may be, for example, 200°C or higher and less than 350°C, or 200°C or higher and 250°C or lower. The heating time in the first heating step is not particularly limited, but it may be 6 hours or longer. If the heating time in the first heating step is 6 hours or longer, a sufficient amount of lithium ions are more likely to be inserted into the active material.

[0034] The atmosphere used when heating the active material in the first heat treatment step may be an inert gas such as nitrogen gas or argon, or a gas containing an oxidizing gas such as air or oxygen gas. The concentration of the oxidizing gas in the gas containing the oxidizing gas is not particularly limited, but the oxygen partial pressure of the atmosphere used for heating in the first heat treatment step may be 0.2 atm or higher, 1.0 atm or higher, or 1.0 atm (atmospheric pressure). The recovery treatment method for the active material according to this embodiment can be carried out even in air, making it a low-cost method.

[0035] [Second Heat Treatment Step] In the recovery treatment method for active material according to this embodiment, the second heat treatment step is a step in which the material to be treated, which was heat-treated in the first heat treatment step, is heat-treated at a temperature of 600°C to 1000°C. In the second heat treatment step, the lithium compound is melted at such a high temperature, so the interface state between the active material and the lithium compound in the second heat treatment step is a solid-liquid interface.

[0036] The heat treatment temperature in the second heat treatment step must be between 600°C and 1000°C, but it may also be between 600°C and 900°C, between 600°C and 750°C, or between 650°C and 750°C. The lower limit of the heat treatment temperature in the second heat treatment step may be 600°C or higher, or 650°C or higher, and the upper limit may be 1000°C or lower, 900°C or lower, 750°C or lower, or less than 650°C. The heat treatment time in the second heat treatment step is not particularly limited, but it may be 6 hours or longer. If the heat treatment time in the second heat treatment step is 6 hours or longer, the valence of the transition metal ions that have decreased is more likely to return to their original valence.

[0037] The atmosphere used when heating the active material in the second heat treatment step may be a gas containing an oxidizing gas such as air or oxygen gas. The concentration of the oxidizing gas in the gas containing the oxidizing gas is not particularly limited, but considering the need to restore the decreased valence of the transition metal ions to their original valence, the oxygen partial pressure of the atmosphere used for heating in the second heat treatment step may be 0.2 atm or higher, 1.0 atm or higher, or 1.0 atm (atmospheric pressure). The recovery treatment method for the active material according to this embodiment can be carried out even in air, making it a low-cost method.

[0038] As the activity of the active material decreases, the valence of the transition metal (especially nickel) in the active material decreases, causing the surface structure of the active material to change from a layered rock salt structure (international symbol for the space group representing the crystal structure is R-3m) to a rock salt structure (international symbol for the space group representing the crystal structure is Fm-3m). Performing the second heat treatment step in the presence of oxygen gas promotes the oxidation of the transition metal, causing the surface structure of the active material to revert from a rock salt structure to a layered rock salt structure, thus facilitating the insertion reaction of lithium ions into the active material. The change in surface structure from a rock salt structure to a layered rock salt structure can occur in air, but is more likely to occur in oxygen gas.

[0039] [Cleaning Process] If an excess amount of lithium compound is used in the first heat treatment process to compensate for the lithium deficiency, the active material after the second heat treatment process will contain excess lithium compound. The presence of lithium compound in the active material can lead to an increase in the resistance of the lithium-ion battery, and may also cause the binder, such as polyvinylidene fluoride, newly added during electrode coating to gel. Therefore, any remaining lithium compound may be removed after the first or second heat treatment process. In other words, a cleaning process to remove lithium compound may be performed after the first or second heat treatment process.

[0040] The method for removing lithium compounds is not particularly limited, but if the lithium compound is water-soluble, a method of removing the lithium compound by dissolving it in water can be used. However, if the active material comes into contact with water during the washing process, there is a risk that lithium ions may leach out from the active material. If the water used in the washing process is alkaline, the leaching of lithium ions from the active material is suppressed, so it is preferable that the aqueous solution of the lithium compound is alkaline. That is, when the remaining lithium compound after washing dissolves in the water used in the washing process, the water becomes alkaline, and the leaching of lithium ions from the active material is suppressed.

[0041] The present invention will be described in more detail below with reference to examples and comparative examples. [Example 1] An unused lithium-ion secondary battery was prepared. This lithium-ion secondary battery comprises an electrode mixture having a positive electrode active material, a conductive additive, and a binder. The positive electrode active material of this lithium-ion secondary battery is a lithium transition metal composite oxide LiNi 0.5 Mn 0.3 Co 0.2 The active material is O2 (NMC532), and the binder is polyvinylidene fluoride. The thermal decomposition temperature of polyvinylidene fluoride is 350°C. The proportion of the binder in the electrode mixture is 1.5% by mass of the positive electrode active material.

[0042] This unused lithium-ion secondary battery was repeatedly charged and discharged, degrading the performance of the positive electrode active material. After use, the electrode mixture containing the positive electrode active material and binder was extracted from the lithium-ion secondary battery. Due to degradation from use, lithium ions were detached from the positive electrode active material, and the valence of the transition metal ions in the positive electrode active material decreased, resulting in a lithium transition metal composite oxide whose chemical formula is Li 1-x Ni 0.5 Mn 0.3 Co 0.2 O 2-y This is the case. Here, in the above chemical formula, x is a number greater than 0 and less than 1, and y is a number greater than 0 and less than or equal to 2.

[0043] The lithium content and nickel ion valence were measured for positive electrode active material extracted from unused lithium-ion secondary batteries (hereinafter sometimes referred to as "unused material") and positive electrode active material extracted from used lithium-ion secondary batteries (hereinafter sometimes referred to as "degraded material"). The lithium content was evaluated by the c-axis lattice constant calculated from the X-ray diffraction pattern obtained by X-ray diffraction analysis. These results are shown in Table 1.

[0044] As shown in Table 1, the c-axis lattice constant of the unused product was 14.25 Å, while the c-axis lattice constant of the degraded product was 14.33 Å. In other words, the lithium ion content of the positive electrode active material decreased and the crystal structure changed due to the use of lithium-ion secondary batteries. Expressed as a chemical formula, the unused product was Li 1.03 Ni0.5 Mn 0.3 Co 0.2 It is O2, and the degraded product is Li 0.81 Ni 0.5 Mn 0.3 Co 0.2 The concentration was O2. Furthermore, the valency of nickel ions in the unused product was 2.4, while the valency of nickel ions in the degraded product was 2.6.

[0045]

[0046] If the c-axis lattice constant is 14.25 Å or less, it can be said that the positive electrode active material contains a sufficient amount of lithium ions. Furthermore, regarding the valence of nickel ions, the closer it is to 2.4, the closer the valence of nickel ions in the positive electrode active material is to that of an unused product, and a value of 2.3 or higher is preferable.

[0047] The lithium ion content of the positive electrode active material was measured as follows: X-ray diffraction analysis of the positive electrode active material was performed using a Rigaku Corporation SmartLab 9kW X-ray diffractometer, and the c-axis lattice constant was calculated from the obtained X-ray diffraction pattern. In calculating the c-axis lattice constant, the WPPF method was used, and the value was calculated after correction with standard Si (NIST640d). The measurement conditions were as follows: Measurement range: 2θ = 15° to 100° X-ray source: CuKα Voltage: 45kV Current: 200mA

[0048] Furthermore, the method for measuring the valence of nickel ions is as follows: The positive electrode active material was analyzed by the X-ray Absorption Fine Structure (XAFS) method, and the valence of nickel ions was calculated. The measurement conditions were as follows: Measurement method: Inverse partial fluorescence yield (IPFY) Measurement absorption edge: Ni-L absorption edge (854 eV) Measurement environment: Vacuum

[0049] The Ni-L3 peak splits into two orbitals, t2g and eg, and its intensity ratio (eg / t2g) correlates with the valency of the nickel ion. The valency of the nickel ion in nickel oxide (NiO) is +2, and the valency of the nickel ion in unused material is +2.4. Therefore, by utilizing the tendency for the intensity ratio eg / t2g to increase as the valency of the nickel ion increases, the valency of the nickel ion in the degraded material was calculated from the intensity ratio of nickel oxide (NiO) and the intensity ratio of unused material.

[0050] A mixture was obtained by adding powdered lithium hydroxide as a lithium compound to the electrode mixture extracted from a used lithium-ion secondary battery and mixing it using a mortar and pestle. This mixture was then used as the material to be treated. The amount of lithium compound mixed was as follows: the total molar amount of lithium ions in the positive electrode active material contained in the electrode mixture and lithium ions in the lithium compound was 1.1 times the total molar amount of transition metals (nickel, manganese, cobalt) in the positive electrode active material contained in the electrode mixture. Hereafter, this numerical value of the amount of lithium compound mixed may be referred to as the "lithium ratio."

[0051] Next, the material to be treated was placed in an alumina crucible and heated in oxygen gas at 250°C for 12 hours (first heat treatment step) to insert lithium ions into the positive electrode active material. In this first heat treatment step, the heating rate when raising the temperature of the material to be treated from room temperature to 250°C was set to 2°C / min.

[0052] Next, the workpiece that had undergone the first heat treatment process was subjected to a second heat treatment process at 650°C in oxygen gas for 6 hours to oxidize the transition metal ions and restore the decreased ion valence to its original value. In this second heat treatment process, the heating rate when raising the temperature of the workpiece from 250°C to 650°C was set to 2°C / min. After the second heat treatment process was completed, the workpiece was cooled. In this cooling process, the temperature was lowered from 650°C to 250°C at a rate of 2°C / min, and then cooled from 250°C to room temperature by natural cooling.

[0053] Next, the treated material (hereinafter sometimes referred to as "recovered product") that had undergone the heat treatment in the second heat treatment step and cooled was washed (washing step). That is, the recovered product was washed with an alkaline aqueous solution to remove any unreacted lithium compounds remaining as lithium carbonate. The alkaline aqueous solution used was a lithium hydroxide aqueous solution with a pH of 11 to 12. The amount of alkaline aqueous solution used was 100 mL per gram of positive electrode active material. The alkaline aqueous solution used for washing and the recovered product were separated by filtration. The separated recovered product was then heated to 100°C under atmospheric pressure and dried overnight, and then dried for 10 hours in a vacuum oven at 130°C.

[0054] For the dried and recovered samples, the lithium content and nickel ion valency were measured in the same manner as described above. As shown in Table 1, the c-axis lattice constant was 14.23 Å ​​and the nickel ion valency was 2.4. As can be seen from these results, the c-axis lattice constant of the positive electrode active material increases with use in lithium-ion secondary batteries, but the recovery treatment reduces the c-axis lattice constant of the positive electrode active material to a level similar to that of unused samples. Similarly, the nickel ion valency changes with use in lithium-ion secondary batteries, but the recovery treatment restores it to its original valency, becoming similar to that of unused samples. From these results, it can be seen that the activity of the deteriorated samples is improved by the recovery treatment, and they are restored to an activity level close to that of unused samples.

[0055] X-ray diffraction analysis was performed on the recovered product of Example 1 to confirm that lithium compounds were removed by the cleaning process. The results are shown in Figure 2. In Figure 2, the peaks marked with triangles are the peaks of the positive electrode active material, and the peaks marked with circles are the peaks of lithium carbonate. As shown in Figure 2, in the case of the recovered product of Example 1 that underwent the cleaning process (cleaned product), the lithium carbonate peak was not observed, but in the case of the recovered product of Example 1 that did not undergo the cleaning process (uncleaned product), the lithium carbonate peak was observed.

[0056] Furthermore, the recovered product of Example 1 was washed using an alkaline aqueous solution in the washing process, but a comparison was made with the case where washing was performed using water. For the recovered product of Example 1 washed with an alkaline aqueous solution, the total molar amount [M] of transition metals (nickel, manganese, cobalt) contained in the positive electrode active material was measured, as was the molar amount [Li] of lithium contained in the positive electrode active material. Then, the ratio of the molar amount of lithium to the total molar amount of transition metals [Li] / [M] was calculated. Similarly, the ratio of the molar amount of lithium to the total molar amount of transition metals [Li] / [M] was also calculated for the case where washing was performed using water.

[0057] As a result, the ratio [Li] / [M] was 0.945 for the recovered product of Example 1, which was washed with an alkaline aqueous solution, compared to 0.925 for the recovered product of Example 1, which was washed with water. From these results, it can be seen that when washing is performed with an alkaline aqueous solution, the surface of the positive electrode active material is kept alkaline during washing, thereby suppressing the elution of lithium from the positive electrode active material due to washing.

[0058] [Example 2] As shown in Table 1, the degraded product was subjected to a recovery treatment in the same manner as in Example 1, except that the lithium content ratio was different. The lithium content and nickel ion valence of the recovered product were measured in the same manner as above. As a result, as shown in Table 1, the c-axis lattice constant was 14.23 Å ​​and the nickel ion valence was 2.4. From these results, it can be seen that the activity of the degraded product was improved by the recovery treatment and recovered to an activity close to that of an unused product.

[0059] [Example 3] As shown in Table 1, the degraded product was subjected to a recovery treatment in the same manner as in Example 1, except that the lithium content ratio and the heat treatment temperature of the second heat treatment step were different. The lithium content and nickel ion valence of the recovered product were measured in the same manner as above. As a result, as shown in Table 1, the c-axis lattice constant was 14.21 Å and the nickel ion valence was 2.6. From these results, it can be seen that the activity of the degraded product was improved by the recovery treatment and recovered to an activity close to that of an unused product.

[0060] [Example 4] As shown in Table 1, the deteriorated product was subjected to a recovery treatment in the same manner as in Example 1, except that the atmosphere of the heat treatment in the first and second heat treatment steps and the heat treatment temperature in the second heat treatment step were different.

[0061] For the recovered samples, the lithium content and nickel ion valency were measured in the same manner as described above. As shown in Table 1, the c-axis lattice constant was 14.23 Å ​​and the nickel ion valency was 2.4. From these results, it can be seen that if the oxygen partial pressure of the heat treatment atmosphere in the second heat treatment step is 0.2 atm, the activity of the deteriorated product will improve by applying the recovery treatment, and it will recover to an activity close to that of an unused product.

[0062] [Example 5] As shown in Table 1, the degraded product was subjected to a recovery treatment in the same manner as in Example 1, except that the type of lithium compound was different. The lithium content and nickel ion valence of the recovered product were measured in the same manner as above. As a result, as shown in Table 1, the c-axis lattice constant was 14.24 Å and the nickel ion valence was 2.3. From these results, it can be seen that the activity of the degraded product was improved by the recovery treatment and recovered to an activity close to that of the unused product.

[0063] X-ray diffraction analysis was performed on the recovered product of Example 5 to confirm that lithium carbonate was removed by the washing process. As shown in Figure 2, no lithium carbonate peak was observed in the recovered product of Example 5 (washed product) that underwent the washing process, but a lithium carbonate peak was observed in the recovered product of Example 5 (unwashed product) that did not undergo the washing process.

[0064] [Example 6] As shown in Table 1, the degraded product was subjected to a recovery treatment in the same manner as in Example 1, except that the lithium content ratio and the heat treatment temperature of the second heat treatment step were different. The lithium content and nickel ion valence of the recovered product were measured in the same manner as above. As a result, as shown in Table 1, the c-axis lattice constant was 14.19 Å and the nickel ion valence was 2.8. From these results, it can be seen that the activity of the degraded product was improved by the recovery treatment and recovered to an activity close to that of an unused product.

[0065] [Comparative Example 1] As shown in Table 1, the deteriorated product was subjected to a recovery treatment in the same manner as in Example 2, except that the heat treatment time in the first heat treatment step was different and the second heat treatment step was omitted. The lithium content and nickel ion valence of the recovered product were measured in the same manner as above. As shown in Table 1, the c-axis lattice constant was 14.23 Å ​​and the nickel ion valence was 2.2. From these results, it can be seen that because the second heat treatment step was omitted, the decreased nickel ion valence did not fully return to its original valence. Therefore, the recovery of activity was insufficient and did not recover to an activity close to that of an unused product.

[0066] [Comparative Example 2] As shown in Table 1, the deteriorated product was subjected to a recovery treatment in the same manner as in Example 1, except that the atmosphere of the heat treatment in the first and second heat treatment steps and the heat treatment temperature in the first heat treatment step were different.

[0067] The lithium content of the recovered product was measured in the same manner as described above. As a result, as shown in Table 1, the c-axis lattice constant was 14.26 Å. From this result, it can be seen that because the heat treatment temperature in the first heat treatment step was the thermal decomposition temperature of the binder, hydrogen fluoride was generated, and the amount of lithium in the positive electrode active material decreased. Therefore, it can be seen that the recovery of activity was insufficient and did not recover to an activity level close to that of an unused product.

[0068] [Comparative Example 3] As shown in Table 1, the deteriorated product was subjected to a recovery treatment in the same manner as in Example 5, except that the lithium content ratio, the atmosphere of the heat treatment in the first and second heat treatment steps were different, the conditions of the first heat treatment step were changed, and the second heat treatment step was omitted. The lithium content of the recovered product was measured in the same manner as above. As a result, as shown in Table 1, the c-axis lattice constant was 14.26 Å. From this result, it can be seen that the activity recovery was not sufficient because the heat treatment temperature in the first heat treatment step was above the thermal decomposition temperature of the binder and the second heat treatment step was omitted, and the activity did not recover to a level close to that of an unused product.

Claims

1. A method for improving the reduced activity of an active material in a lithium-ion battery, comprising: a first heat treatment step of heating a workpiece, which is an active material or an electrode mixture containing the active material mixed with a lithium compound, at a temperature below the thermal decomposition temperature of the binder in the lithium-ion battery; and a second heat treatment step of heating the workpiece that has been heated in the first heat treatment step at a temperature of 600°C to 1000°C.

2. The method for restoring an active material according to claim 1, wherein the lithium compound is in powder form.

3. The method for restoring an active material according to claim 1 or 2, wherein the lithium compound is water-soluble.

4. The method for restoring an active material according to claim 1 or 2, wherein the lithium compound has an aqueous solution that is alkaline.

5. The method for restoring an active material according to claim 1 or claim 2, wherein the lithium compound is at least one of lithium hydroxide and lithium carbonate.

6. The method for restoring an active material according to claim 1 or claim 2, wherein the heating time in the first heating step and the second heating step is 6 hours or more.

7. The method for restoring an active material according to claim 1 or claim 2, wherein the binder contains polyvinylidene fluoride.

8. The method for restoring an active material according to claim 1 or claim 2, wherein the heat treatment temperature in the second heat treatment step is 600°C or more and 900°C or less.

9. The method for restoring an active material according to claim 1 or claim 2, wherein the partial pressure of oxygen in the heat treatment atmosphere in the first heat treatment step and the second heat treatment step is 0.2 atm or higher in both cases.

10. The method for restoring an active material according to claim 1 or claim 2, wherein the partial pressure of oxygen in the heat treatment atmosphere in the first heat treatment step and the second heat treatment step is 1.0 atm or higher in both cases.

Images