Method of diagnosing deterioration of electrode active material for lithium secondary battery

Abstract

The present invention provides a method of diagnosing deterioration of an electrode active material for a secondary battery, the method including: preparing a lithium secondary battery including a positive electrode and a negative electrode, the positive electrode including a positive electrode active material of a lithium transition metal oxide containing nickel (Ni) at 60 mol% or more based on the total moles of transition metals in addition to lithium, the negative electrode facing the positive electrode; obtaining a first differential curve (dQ / dV) by differentiating an initial charge / discharge curve obtained by performing a first charge and a first discharge of the lithium secondary battery in a voltage range of 2.5 V to 4.2 V; and obtaining a second differential curve (dQ / dV) by differentiating a charge / discharge curve obtained by performing a second charge and a second discharge of the lithium secondary battery in a voltage range of 2.5 V to 4.2 V, wherein when a maximum discharge peak of the second differential curve appears at an interval peak spaced apart by 0.01 V to 0.1 V from a maximum discharge peak of the first differential curve at 4 V or more, it is diagnosed that a β phase of the positive electrode active material has been formed.

Description

TECHNICAL FIELD

[0001] This application claims priority to Korean Patent Application No. 10-2020-0007495, filed on January 20, 2020, the disclosure of which is incorporated herein by reference.

[0002] The present application relates to a method of diagnosing deterioration of an electrode active material for a lithium secondary battery. BACKGROUND

[0003] With the development of technology and an increase in demand for mobile devices, the demand for secondary batteries as an energy source has significantly increased. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharge rate have been commercialized and widely used. In particular, in recent years, as electric vehicles are rapidly spreading, the development of high-energy batteries that can be used as power sources for medium- to large-sized devices has become increasingly important.

[0004] Lithium transition metal composite oxides have been used as positive electrode active materials for lithium secondary batteries, and among these oxides, lithium cobalt composite metal oxides such as LiCoO2 having high operating voltage and excellent capacity characteristics are mainly used. However, LiCoO2 has very poor thermal properties due to unstable crystal structure caused by delithiation. Also, since LiCoO2 is expensive, there is a limitation in using a large amount of LiCoO2 as a power source for applications such as electric vehicles.

[0005] Lithium manganese composite metal oxides (LiMnO2 or LiMn2O4), lithium iron phosphate compounds (LiFePO4 or the like), or lithium nickel composite metal oxides (LiNiO2 or the like) have been developed as materials for replacing LiCoO2. Among these materials, research and development of lithium nickel composite metal oxides have been more actively conducted, in which, due to high reversible capacity of about 200 mAh / g, a large-capacity battery can be easily implemented. However, LiNiO2 has a limitation in that the thermal stability of LiNiO2 is poorer than that of LiCoO2, and when internal short-circuiting occurs in a charged state due to external pressure, the positive electrode active material itself is decomposed to cause rupture and fire of the battery. Therefore, as a method of improving low thermal stability while maintaining excellent reversible capacity of LiNiO2, a lithium transition metal oxide in which a part of nickel (Ni) is replaced with cobalt (Co), manganese (Mn), or aluminum (Al) has been developed.

[0006] However, regarding lithium transition metal oxides in which a part of Ni is replaced with Co, Mn, or Al, in the case where the amount of nickel is increased to 60 mol% or more in order to achieve high energy by improving the capacity characteristics, there is a problem in that a new phase is formed when lithium present in the positive electrode active material is deintercalated at a high potential, and the structural stability of the positive electrode active material is reduced due to this phase change.

[0007] In general, in order to diagnose the change in the structural stability of the positive electrode active material due to the phase change, a method of disassembling the battery after charging and discharging and observing the disassembled battery has been used. In this case, diagnosing the phase change of the positive electrode active material takes a long time and a high cost.

[0008] Therefore, there is a need for a diagnosis method capable of diagnosing the deterioration of the battery due to the phase change of the structure of the positive electrode active material without disassembling the secondary battery. SUMMARY

[0009] TECHNICAL PROBLEM

[0010] An aspect of the present application provides a method of diagnosing the deterioration of an electrode active material for a secondary battery, which can diagnose the deterioration of the positive electrode active material due to the phase change without disassembling the battery after charging and discharging.

[0011] TECHNICAL SOLUTION

[0012] According to an aspect of the present application, there is provided a method of diagnosing the deterioration of an electrode active material for a secondary battery, the method including: preparing a lithium secondary battery including a positive electrode and a negative electrode, the positive electrode including a positive electrode active material of a lithium transition metal oxide containing nickel (Ni) of 60 mol% or more based on the total moles of transition metals, in addition to lithium, the negative electrode facing the positive electrode; obtaining a first differential curve (dQ / dV) by differentiating an initial charge / discharge curve obtained by performing a first charge and a first discharge of the lithium secondary battery in a voltage range of 2.5 V to 4.2 V; and obtaining a second differential curve (dQ / dV) by differentiating a charge / discharge curve obtained by performing a second charge and a second discharge of the lithium secondary battery in a voltage range of 2.5 V to 4.2 V, wherein when a maximum discharge peak value of the second differential curve appears at a peak interval of 0.01 V to 0.1 V apart from the maximum discharge peak value of the first differential curve at 4 V or more, it is diagnosed that a β phase of the positive electrode active material has been formed.

[0013] ADVANTAGEOUS EFFECTS

[0014] According to the present application, since the differential capacity curve is measured without disassembling the battery when diagnosing the deterioration of the positive electrode, the occurrence of the phase change of the positive electrode active material structure can be predicted by using the differential capacity curve, and the deterioration of the positive electrode and the secondary battery resulting therefrom can be predicted. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 are charge and discharge curves of a coin cell including a positive electrode and a negative electrode of Example 1, respectively;

[0016] Figure 2 is a differential capacity curve of a secondary battery prepared in Example 1; and

[0017] Figure 3 is a differential capacity curve of a secondary battery prepared in Example 2. DETAILED DESCRIPTION

[0018] Hereinafter, the present application will be described in more detail.

[0019] In the present specification, regarding the "β phase" of the positive electrode active material, as the tendency that nickel contained in a lithium transition metal oxide having a high nickel content is maintained as Ni 2+ with the charging and discharging of the lithium secondary battery, the phase change of the lithium transition metal oxide in which lithium (Li) is deintercalated, where, in this case, the phase immediately after the preparation of the positive electrode active material is referred to as an "α phase", and the phase of the positive electrode active material in which the phase change occurs after the charging and discharging is referred to as a "β phase".

[0020] Method of diagnosing deterioration of electrode active material for lithium secondary battery

[0021] The method of diagnosing the deterioration of the electrode active material for the secondary battery according to the present application includes preparing a lithium secondary battery including a positive electrode and a negative electrode, the positive electrode including a positive electrode active material of a lithium transition metal oxide containing nickel (Ni) of 60 mol% or more based on the total moles of transition metals in addition to lithium, the negative electrode facing the positive electrode; obtaining a first differential curve (dQ / dV) by differentiating an initial charge / discharge curve obtained by performing a first charge and a first discharge of the lithium secondary battery in a voltage range of 2.5 V to 4.2 V; and obtaining a second differential curve (dQ / dV) by differentiating a charge / discharge curve obtained by performing a second charge and a second discharge of the lithium secondary battery in a voltage range of 2.5 V to 4.2 V, wherein when a maximum discharge peak value of the second differential curve appears at an interval peak spaced apart by 0.01 V to 0.1 V from the maximum discharge peak value of the first differential curve at 4 V or more, it is diagnosed that the β phase of the positive electrode active material has been formed.

[0022] Hereinafter, the method will be described in more detail.

[0023] First, regarding a lithium secondary battery according to the present application, a secondary battery is prepared, the secondary battery including: a positive electrode including a positive electrode active material including a lithium transition metal oxide containing 60 mol% or more of Ni based on the total moles of transition metals; a negative electrode facing the positive electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte.

[0024] The positive electrode according to the present application can be prepared by coating a composition for forming a positive electrode, the composition including the above-described positive electrode active material, a binder, a conductive agent, and a solvent, on a positive electrode current collector.

[0025] For example, in order to achieve a high energy density of a secondary battery, the positive electrode can contain 60 mol% or more of nickel based on the total moles of transition metals in addition to lithium, and preferably, the positive electrode active material can be represented by the following Molecular Formula 1.

[0026] [Molecular Formula 1]

[0027] Li 1+a Ni 1-x-y Co x Ml y O2

[0028] In Molecular Formula 1, 0≤a≤0.3, 0≤x≤0.2, 0≤y≤0.2, and 0≤x+y≤0.4.

[0029] Specifically, Ml in Molecular Formula 1 is an element substituted at a transition metal site in the oxide represented by Molecular Formula 1, and Ml can include at least one selected from manganese (Mn) and aluminum (Al).

[0030] 1+a represents a molar ratio of lithium in the oxide represented by Molecular Formula 1, wherein a can satisfy 0≤a≤0.3, and preferably 0≤a≤0.2.

[0031] x represents a molar ratio of cobalt (Co) in the oxide represented by Molecular Formula 1, wherein x can satisfy 0≤x≤0.2, and preferably 0

[0032] y represents a molar ratio of Ml in the oxide represented by Molecular Formula 1, wherein y can satisfy 0≤y≤0.2, and preferably 0

[0033] 1-x-y represents a molar ratio of Ni in the oxide represented by Molecular Formula 1, wherein 1-x-y can satisfy 0.6≤1-x-y≤1.0, and preferably 0.8≤1-x-y≤1.0.

[0034] In the case where the content of nickel other than lithium in the lithium transition metal oxide is 60 mol% or more, particularly 80 mol% or more, based on the total moles of transition metals, as in the present application, a high capacity of the secondary battery can be achieved when the lithium transition metal oxide is used, but the nickel in the lithium transition metal oxide remains as Ni 2+ When lithium (Li) present in the lithium transition metal oxide is deintercalated due to the tendency of the nickel in the lithium transition metal oxide to be Ni

[0035] In this case, the positive electrode active material can be included in an amount of 80 parts by weight to 99 parts by weight, for example, 85 parts by weight to 98 parts by weight, based on the total weight of the solid content of the composition for forming the positive electrode. When the content of the positive electrode active material is within the above range, excellent capacity characteristics can be obtained.

[0036] The positive electrode current collector is not particularly limited as long as it has conductivity and does not cause an adverse chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, or the like can be used.

[0037] The binder is a component that contributes to the binding between the active material and the conductive agent and the binding with the current collector, and generally, the binder can be added in an amount of 1 wt% to 30 wt%, based on the total weight of the solid content of the composition for forming the positive electrode. Examples of the binder can be polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluororubber, or various copolymers thereof.

[0038] The conductive agent can generally be added in an amount of 1 wt% to 30 wt%, based on the total weight of the solid content of the composition for forming the positive electrode.

[0039] The conductive agent is not particularly limited as long as it has conductivity and does not cause an adverse chemical change in the battery, and for example, a conductive material such as graphite; a carbon-based material such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; a conductive fiber such as a carbon fiber or a metal fiber; a metal powder such as a fluorocarbon powder, an aluminum powder, and a nickel powder; a conductive whisker such as a zinc oxide whisker and a potassium titanate whisker; a conductive metal oxide such as titanium oxide; or a polyphenylene derivative can be used. Specific examples of commercially available conductive agents can be acetylene black-based products (Chevron Chemical Company, Denka black (Denka Singapore Private Limited), or Gulf Oil Company products), ketjen black, ethylene carbonate (EC)-based products (Armak Company products), Vulcan XC-72 (Cabot Corporation product), and Super P (Timcal Graphite and Carbon Products).

[0040] Further, if necessary, the positive electrode active material layer can selectively further include a dispersing agent.

[0041] The dispersing agent can be used without particular limitation as long as it functions as a dispersing agent for the positive electrode, and for example, an aqueous dispersing agent or an organic dispersing agent can be selectively used as necessary. Preferably, the dispersing agent can include a cellulose-based compound, a polyalkylene oxide, a polyvinyl alcohol, a polyvinylpyrrolidone, a polyvinyl acetal, a polyvinyl ether, a polyvinyl sulfonic acid, a polyvinyl chloride (PVC), a polyvinylidene fluoride, chitosan, starch, amylose, polyacrylamide, poly-N-isopropyl acrylamide, poly-N,N-dimethyl acrylamide, polyethyleneimine, polyoxyethylene, poly(2-methoxyethoxyethylene), poly(acrylamide-co-diallyldimethylammonium chloride), an acrylonitrile / butadiene / styrene (ABS) polymer, an acrylonitrile / styrene / acrylate (ASA) polymer, a mixture of acrylonitrile / styrene / acrylate (ASA) polymer and propylene carbonate, a styrene / acrylonitrile (SAN) copolymer, a methyl methacrylate / acrylonitrile / butadiene / styrene (MABS) polymer, a butadiene styrene rubber, a nitrile rubber, or a fluorine rubber, and any one thereof or a mixture of two or more thereof can be used. A hydrogenated nitrile rubber (H-NBR) can be used. In the case where the positive electrode active material layer further includes a dispersing agent, dispersibility of the components of the positive electrode active material layer, particularly the conductive material, can be increased, but the present application is not limited thereto.

[0042] Further, the solvent can be a solvent generally used in the art. The solvent can include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl pyrrolidone (NMP), acetone, or water, and any one thereof or a mixture of two or more thereof can be used. The amount of the solvent used can be sufficient in that the solvent can dissolve or disperse the positive active material, the conductive agent, and the binder, and can allow to have a viscosity that can provide excellent thickness uniformity during a subsequent coating for preparing the positive electrode, in consideration of a coating thickness of the slurry and a manufacturing yield.

[0043] The negative electrode according to the present application can be prepared by coating a composition for forming a negative electrode on a negative current collector, the composition including a negative active material, a binder, a conductive agent, and a solvent. Further, if necessary, the composition for forming a negative electrode can selectively further include a dispersant.

[0044] A compound capable of reversibly intercalating and deintercalating lithium can be used as the negative active material. Preferably, the negative electrode can include a silicon-based negative active material exhibiting high capacity characteristics.

[0045] Further, the negative active material can further include a carbon-based negative active material as well as the silicon-based negative active material. For example, in the case where the negative active material includes the silicon-based negative active material and the carbon-based negative active material, the irreversible capacity can be reduced while having high capacity characteristics, compared to the case where only the silicon-based negative active material is included.

[0046] The silicon-based negative active material and the carbon-based negative active material can be used by being mixed at a weight ratio of 1:99 to 20:80.

[0047] The negative current collector is not particularly limited as long as it has high conductivity and does not cause an adverse chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., and an aluminum-cadmium alloy can be used. Further, the negative current collector can generally have a thickness of 3 μm to 500 μm, and similar to the positive current collector, a micro-irregularity can be formed on the surface of the current collector to improve the adhesion of the negative active material. For example, the negative current collector can be used in various shapes such as a film, a sheet, a foil, a mesh, a porous body, a foam body, a non-woven fabric body, etc.

[0048] The conductive agent, the binder, the solvent, or the dispersant included in the composition for forming a negative electrode can be used without particular limitation as long as it can be typically used in a composition for forming an electrode, and for example, the conductive material, the binder, the solvent, or the dispersant described in the above composition for forming a positive electrode can be used.

[0049] In the lithium secondary battery, a separator separates a negative electrode and a positive electrode and provides a movement path of lithium ions, wherein any separator can be used as the separator without particular limitation as long as it is generally used in lithium secondary batteries, and particularly, a separator having a high moisture retaining ability to an electrolyte and a low resistance to ion transfer of the electrolyte can be used. Specifically, a porous polymer film, for example, a porous polymer film prepared from a polyolefin-based polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butylene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure having two or more layers thereof can be used. Also, a typical porous nonwoven fabric, for example, a nonwoven fabric formed from high-melting point glass fibers or polyethylene terephthalate fibers can be used. In addition, a coated separator including a ceramic component or a polymer material can be used to secure heat resistance or mechanical strength, and a separator having a single layer or a multi-layer structure can be selectively used.

[0050] Also, the electrolyte used in the present application can include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte, which can be used to prepare a lithium secondary battery, but the present application is not limited thereto.

[0051] Specifically, the electrolyte can include an organic solvent and a lithium salt.

[0052] Any organic solvent can be used as the organic solvent without particular limitation as long as it can be used as a medium through which ions participating in a battery electrochemical reaction can move. Specifically, the following can be used as the organic solvent: an ester-based solvent, such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; an ether-based solvent, such as dibutyl ether or tetrahydrofuran; a ketone-based solvent, such as cyclohexanone; an aromatic hydrocarbon-based solvent, such as benzene and fluorobenzene; or a carbonate-based solvent, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); an alcohol-based solvent, such as ethanol and isopropyl alcohol; a nitrile, such as R-CN (wherein R is a linear, branched, or cyclic C2-C20 hydrocarbon group, and can include a double-bonded aromatic ring or an ether bond); an amide, such as dimethylformamide; a dioxolane, such as 1,3-dioxolane; or a sulfolane. Among these solvents, a carbonate-based solvent can be used, and for example, a mixture of a cyclic carbonate (for example, ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can improve the charge / discharge performance of a battery, and a low-viscosity linear carbonate-based compound (for example, methyl ethyl carbonate, dimethyl carbonate, or diethyl carbonate) can be used. In this case, when the cyclic carbonate and the linear carbonate are mixed at a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte solution can be excellent.

[0053] The lithium salt can be used without particular limitation, as long as it is a compound capable of providing lithium ions for a lithium secondary battery. Specifically, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2) can be used as the lithium salt. The lithium salt can be used in a concentration range of 0.1 M to 2.0 M. In the case where the concentration of the lithium salt is included in the above range, since the electrolyte can have appropriate conductivity and viscosity, excellent performance of the electrolyte can be obtained and lithium ions can move efficiently.

[0054] To improve the life characteristics of the battery, suppress the decrease in the capacity of the battery, and improve the discharge capacity of the battery, at least one additive, such as a halogenated alkylene carbonate compound, such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, a cyclic ether, ethylenediamine, n-glyme, triamide hexaphosphate, a nitrobenzene derivative, sulfur, a quinonimine dye, an N-substituted oxazolidinone, an N,N-substituted imidazolidine, an ethylene glycol dialkyl ether, an ammonium salt, a pyrrole, 2-methoxyethanol, or aluminum trichloride, can be added to the electrolyte in addition to the electrolyte components. In this case, the content of the additive can be 0.1 parts by weight to 5 parts by weight based on 100 parts by weight of the total weight of the electrolyte.

[0055] Subsequently, a first differential curve (dQ / dV) is obtained by differentiating an initial charge / discharge curve obtained by performing first charging and first discharging on the above-prepared lithium secondary battery in a voltage range of 2.5 V to 4.2 V (first step).

[0056] In this case, the first charging and the first discharging indicate the first charging and the first discharging performed immediately after the preparation of the lithium secondary battery.

[0057] In the present application, the charging and the discharging of the lithium secondary battery can be performed at a low rate of 0.01 C to 0.1 C, for example, 0.02 C to 0.05 C. When the charging and the discharging are performed at a low rate in the above range, the effect of excluding the resistance element of the profile can be achieved.

[0058] The differential capacity curve (dq / dV curve) can be obtained by differentiating the initial charge / discharge curve obtained above.

[0059] Subsequently, a second differential curve (dQ / dV) is obtained by differentiating a charge / discharge curve obtained by performing a second charge and a second discharge on the lithium secondary battery in a voltage range of 2.5 V to 4.2 V (second step).

[0060] In this case, the second charge and the second discharge mean a charge and a discharge after the charge and the discharge of the secondary battery subjected to the first charge and the first discharge are repeated 1 to 10 times, for example, 1 to 5 times.

[0061] For example, the charge and the discharge of the secondary battery subjected to the first charge and the first discharge are repeated 1 to 10 times, and thereafter, a second differential curve (dQ / dV) can be obtained by differentiating a charge / discharge curve obtained by performing a charge and a discharge again on the secondary battery subjected to the repeated charge and discharge in a voltage range of 2.5 V to 4.2 V.

[0062] Also, the second charge and the second discharge mean a charge and a discharge after the secondary battery subjected to the first charge and the first discharge is stored at 60°C for 2 weeks to 10 weeks.

[0063] For example, the secondary battery subjected to the first charge and the first discharge is stored at a high temperature, and a second differential curve (dQ / dV) can be obtained by differentiating a charge / discharge curve obtained by performing a charge and a discharge again on the secondary battery after the storage in a voltage range of 2.5 V to 4.2 V.

[0064] According to the method of diagnosing deterioration of the electrode according to the present application, in the case where the charge and the discharge of the lithium secondary battery are repeatedly performed in a working voltage range of the battery, or in the case where the lithium secondary battery is stored for a long time after the initial charge and the discharge, a differential curve peak in a second differential capacity curve obtained after the repeated charge and discharge or the storage can be spaced apart at about 4 V or more, compared to a first differential capacity curve obtained after the lithium secondary battery is prepared and the initial charge and the discharge are performed.

[0065] For example, a maximum discharge peak value of the second differential curve can appear at 4 V or more with a peak spaced apart at 0.01 V to 0.1 V, compared to a maximum discharge peak value of the first differential capacity curve. When a peak with a spacing less than the above range appears, it means that deterioration of the positive active material hardly occurs.

[0066] The spacing of the differential curve peak is a shift of the negative electrode curve caused by a reversible capacity loss of lithium due to the repeated charge and discharge of the secondary battery.

[0067] For example, when a lithium secondary battery is repeatedly charged and discharged within the operating voltage range of the battery, i.e., within a voltage range of 2.5 V to 4.2 V, a solid electrolyte interphase (SEI) is formed on the surface of the negative electrode while the electrolyte is decomposed. Thus, irreversible capacity loss occurs, and as a result, the negative electrode curve can shift due to the loss of available lithium.

[0068] Also, the above-described interval of the differential curve peak is a shift of the negative electrode curve due to the loss of reversible capacity of lithium caused by storage of the secondary battery subjected to initial charging and discharging.

[0069] For example, in the case where the secondary battery subjected to initial charging and discharging is stored at 60°C for 2 to 10 weeks, irreversible capacity loss occurs due to a reduction side reaction and deterioration of the electrode active material, and thus, the negative electrode curve can shift due to the loss of available lithium.

[0070] Since the potential of the positive electrode capable of reacting with the negative electrode also increases due to the shift of the negative electrode, i.e., the change in the operating voltage of the negative electrode, the lower end potential of the positive electrode can not be used.

[0071] Thus, as Figure 1 indicated, since the positive electrode curve also shifts due to the shift of the above-described negative electrode curve, the positive electrode curve shifts in the direction in which the lower end voltage is not used.

[0072] Thus, in the case where the lower end potential of the positive electrode is not used, since the reaction occurs relatively only at the upper potential of the positive electrode, complete shrinkage and expansion of the positive electrode active material can not occur, and thus, it can be predicted that the structure of the positive electrode active material will further change.

[0073] This structural change of the positive electrode active material forms a β phase in the positive electrode active material.

[0074] Specifically, as described above, in the case where the charging / discharging curve shifted in the direction in which the lower end voltage is not used is differentiated, due to the shift of the positive electrode curve and the negative electrode curve, an interval peak appears at 4 V or more in the differential.

[0075] In contrast, when an interval peak appears at 4 V or more in the second differential curve compared to the first differential curve obtained during initial charging and discharging, even without observing the structure of the positive electrode active material by disassembling the battery after charging and discharging of the secondary battery, it can be considered that the following has occurred: the shift of the negative electrode curve due to the reduction in the amount of available lithium, the formation of the β phase of the positive electrode active material, and the resulting deterioration of the positive electrode.

[0076] Hereinafter, the present application will be described in detail according to specific examples.

[0077] Example 1

[0078] LiNi 0.8 Co 0.1 Mn 0.1 O2, carbon black conductive agent, dispersant, and polyvinylidene fluoride binder were mixed in a weight ratio of 97.5:1:0.15:1.35, and the mixture was mixed in an N-methylpyrrolidone solvent to prepare a composition for forming a positive electrode. An aluminum foil was coated with the composition for forming a positive electrode, dried, and then roll-pressed to prepare a positive electrode.

[0079] A mixture used as a negative electrode active material in which graphite and SiO were mixed in a weight ratio of 90:10, carbon black, carbon nanotube conductive agent, and polyvinylidene fluoride binder were mixed in a weight ratio of 96:0.5:0.1:3.4, and the mixture was added to an N-methylpyrrolidone solvent to prepare a composition for forming a negative electrode. A copper current collector having a thickness of 6 μm was coated with the composition for forming a negative electrode, dried, and then roll-pressed to prepare a negative electrode.

[0080] After stacking the above-prepared positive and negative electrodes with a safety reinforcement separator (SRS) to prepare an electrode assembly, the electrode assembly was put into a battery case, and an electrolyte in which 1.2 M of LiPF6 electrolyte salt was dissolved in a solvent in which ethylene carbonate (EC), propylene carbonate (PC), and methyl ethyl carbonate (EMC) were mixed in a volume ratio of 25:5:70 was injected thereinto to prepare a lithium secondary battery.

[0081] The lithium secondary battery prepared as described above was first charged at a constant current of 0.05 C to 4.2 V, and the charge was cut off at 0.025 C. Thereafter, the lithium secondary battery was first discharged at a constant current of 0.05 C to 2.5 V. A first differential capacity curve (dq / dV) was obtained by differentiating the initial charge / discharge curve measured after the charge and discharge.

[0082] Subsequently, the secondary battery after the initial charge and discharge was stored at 60°C for 8 weeks.

[0083] After the storage for 8 weeks, the secondary battery was second charged at a constant current of 0.05 C to 4.2 V and the charge was cut off at 0.025 C, and was second discharged at a constant current of 0.05 C to 2.5 V to obtain a charge / discharge curve, a second differential capacity curve was obtained by differentiating the charge / discharge curve, and the second differential capacity curve was shown as Figure 2

[0084] As Figure 2 ​As shown, since the secondary battery including the negative electrode of Example 1 contains silicon oxide having a relatively large irreversible capacity as the negative electrode active material, the loss of available lithium during charging and discharging is large, and thus, it can be confirmed that the interval between the interval peaks at 4.0 V or more is large, about 0.05 V.

[0085] Example 2

[0086] A lithium secondary battery was prepared in the same manner as Example 1 except that only graphite was used as the negative electrode active material, first and second differential capacity curves were obtained, and these curves are shown as Figure 3

[0087] Since the secondary battery including the negative electrode of Example 2 above does not contain a material having a large irreversible capacity as the negative electrode material, the loss of available lithium is small even if charging and discharging are performed, and thus, it can be confirmed that interval peaks are observed at 4.0 V or more, but the interval between the corresponding peaks is small, about 0.02 V.​

Claims

1. A method of diagnosing deterioration of an electrode active material for a secondary battery, comprising: preparing a lithium secondary battery including a positive electrode and a negative electrode, the positive electrode including a positive electrode active material of a lithium transition metal oxide containing nickel (Ni) at 60 mol% or more based on the total moles of transition metals other than lithium, the negative electrode facing the positive electrode; obtaining a first differential curve (dQ / dV) by differentiating an initial charge / discharge curve obtained by performing a first charge and a first discharge on the lithium secondary battery in a voltage range of 2.5 V to 4.2 V; and obtaining a second differential curve (dQ / dV) by differentiating a charge / discharge curve obtained by performing a second charge and a second discharge on the lithium secondary battery in which the first charge and the first discharge have been performed in a voltage range of 2.5 V to 4.2 V, wherein when a maximum discharge peak of the second differential curve appears at an interval peak spaced apart by 0.01 V to 0.1 V from a maximum discharge peak of the first differential curve at 4 V or more, it is diagnosed that a β phase of the positive electrode active material has been formed.

2. The method of claim 1, wherein, The second charge and the second discharge are a charge and a discharge after a charge and a discharge of a secondary battery subjected to the first charge and the first discharge are repeated 1 to 10 times.

3. The method of claim 1, wherein, The second charge and the second discharge are a charge and a discharge after a secondary battery subjected to the first charge and the first discharge is stored at 60°C for 2 weeks to 10 weeks.

4. The method of claim 2 or 3, wherein, A reversible capacity of lithium is decreased by repeated charging and discharging of a secondary battery subjected to the first charge and the first discharge or storage of a secondary battery subjected to the first charge and the first discharge.

5. The method of claim 4, wherein, An operating voltage of the negative electrode is changed by the decrease in the reversible capacity of lithium.

6. The method of claim 5, wherein, An operating voltage of the negative electrode is increased by the decrease in the reversible capacity of lithium.

7. The method of claim 5, wherein, An operating voltage of the negative electrode is changed by the decrease in the reversible capacity of lithium to increase a potential of the positive electrode.

8. The method of claim 7, wherein, A structure of the positive electrode active material is changed by the increase in the potential of the positive electrode.

9. The method of claim 1, wherein, The positive electrode active material is represented by Formula 1: [Formula 1] Li 1+a Ni 1-x-y Co x Ml y O2 wherein, in Formula 1, 0 ≤ a ≤ 0.3, 0 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.2, and 0 ≤ x + y ≤ 0.4, and Ml is at least one selected from manganese (Mn) and aluminum (Al).

10. The method of claim 1, wherein, The negative electrode includes a silicon-based negative electrode active material.

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