technical field [One] This application claims the benefit of the filing date of Korean Patent Application No. 10-2020-0007495 filed with the Korean Intellectual Property Office on January 20, 2020, the entire contents of which are incorporated herein by reference. [2] The present invention relates to a method for diagnosing deterioration of an electrode active material for a lithium secondary battery. background [3] As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, a lithium secondary battery having a high energy density and voltage, a long cycle life, and a low self-discharge rate has been commercialized and widely used. In particular, as the popularization of electric vehicles is rapidly progressing in recent years, the development of high-energy batteries that can be used as power sources for mid- to large-sized devices is becoming more important. [4] As a cathode active material of the lithium secondary battery, a lithium transition metal composite oxide is used, and among them, a lithium cobalt composite metal oxide such as LiCoO 2 having a high operating voltage and excellent capacity characteristics is mainly used. However, LiCoO 2 has very poor thermal properties due to the destabilization of the crystal structure due to delithiation. In addition, since the LiCoO 2 is expensive, there is a limit to its mass use as a power source in fields such as electric vehicles. [5] As a material for replacing the LiCoO 2 , a lithium manganese composite metal oxide (LiMnO 2 or LiMn 2 O 4 , etc.), a lithium iron phosphate compound (LiFePO 4 etc.) or a lithium nickel composite metal oxide (LiNiO 2 etc.) has been developed. . Among them, research and development on lithium-nickel composite metal oxide, which has a high reversible capacity of about 200 mAh/g, and is easy to implement in a large-capacity battery, is being studied more actively. However, the LiNiO 2 has inferior thermal stability compared to LiCoO 2 , and when an internal short circuit occurs due to external pressure in a charged state, the positive active material itself is decomposed, resulting in rupture and ignition of the battery. Accordingly , as a method for improving low thermal stability while maintaining excellent reversible capacity of the LiNiO 2 , a lithium transition metal oxide in which a part of Ni is substituted with Co, Mn or Al has been developed. [6] However, in the case of lithium transition metal oxide in which a part of Ni is substituted with Co, Mn or Al, when the content of nickel is increased to 60 mol% or more to achieve high energy by increasing capacity characteristics, it is present in the positive electrode active material at high potential As lithium is released, a new phase is formed, and there is a problem in that the structural stability of the positive electrode active material is deteriorated by this phase change. [7] Conventionally, in order to diagnose the structural stability change of the positive electrode active material due to such a phase change, a method of separating the battery after charging and discharging and observing the separated battery has been used. In this case, it took a long time and high cost to diagnose the phase change of the positive electrode active material. [8] Accordingly, there is a need for a diagnostic method capable of diagnosing deterioration of a battery due to a phase change in the structure of a positive electrode active material without disassembling the secondary battery. DETAILED DESCRIPTION OF THE INVENTION technical challenge [9] In order to solve the above problems, the first technical object of the present invention is to provide a method for diagnosing the deterioration of an electrode active material for a secondary battery, which can diagnose the deterioration of a cathode active material during a phase change without disassembling the battery after charging and discharging. . means of solving the problem [10] The present invention manufactures a lithium secondary battery comprising a positive electrode comprising a positive electrode active material of a lithium transition metal oxide in which Ni is contained in an amount of 60 mol% or more with respect to the total number of moles of transition metals excluding lithium, and a negative electrode present opposite to the positive electrode to do; obtaining a first differential curve (dQ/dV) by differentiating an initial charge/discharge curve obtained by performing first charging and first discharging of the lithium secondary battery in a voltage range of 2.5V to 4.2V; And obtaining a second differential curve (dQ / dV) by differentiating the charge-discharge curve obtained by performing a second charge and a second discharge of the lithium secondary battery in a voltage range of 2.5V to 4.2V; and , Compared with the maximum discharge peak value of the first differential curve, when the maximum discharge peak value of the second differential curve is 0.01V to 0.1V at 4V or more, when a spaced peak occurs, the beta phase of the positive electrode active material ( To provide a method for diagnosing degradation of electrode active materials for secondary batteries, which is to diagnose that beta phase) has been generated. Effects of the Invention [11] According to the present invention, it is possible to predict that a phase change in the structure of the positive electrode active material has occurred by measuring the differential capacity curve without disassembling the battery when diagnosing the deterioration of the positive electrode and predicting the deterioration of the positive electrode and the secondary battery due to this. . Brief description of the drawing [12] 1 shows the charging and discharging curves of the coin cell including the positive electrode and the negative electrode of Example 1, respectively. [13] 2 is a differential capacity curve of the secondary battery prepared in Example 1. FIG. [14] 3 is a differential capacity curve of the secondary battery prepared in Example 2. Best mode for carrying out the invention [15] Hereinafter, the present invention will be described in more detail. [16] [17] In the present specification, the 'beta phase' of the positive active material is due to the tendency that the nickel contained in the lithium transition metal oxide containing a high content of nickel is maintained as Ni 2+ according to the charging and discharging of the lithium secondary battery, As Li present in the lithium transition metal oxide escapes, the phase of the lithium transition metal oxide is changed. At this time, the phase immediately after the preparation of the positive active material is called the 'alpha phase', and charging and discharging. Thereafter, the phase of the positive electrode active material in which the phase change has occurred is referred to as a 'beta phase'. [18] [19] Method for diagnosing deterioration of electrode active material for lithium secondary batteries [20] The method for diagnosing deterioration of an electrode active material for a secondary battery according to the present invention is a positive electrode comprising a positive electrode active material of a lithium transition metal oxide in which Ni is contained in an amount of 60 mol% or more with respect to the total number of moles of transition metals excluding lithium, and the positive electrode is present opposite to the positive electrode manufacturing a lithium secondary battery including an anode; obtaining a first differential curve (dQ/dV) by differentiating an initial charge/discharge curve obtained by performing first charging and first discharging of the lithium secondary battery in a voltage range of 2.5V to 4.2V; and obtaining a second differential curve (dQ/dV) by differentiating the charge/discharge curve obtained by performing second charging and second discharging of the lithium secondary battery in a voltage range of 2.5V to 4.2V, Compared with the maximum discharge peak value of the first differential curve, when the maximum discharge peak value of the second differential curve is 0.01V to 0.1V at 4 V or more, when a spaced peak occurs, the beta phase (beta) of the positive active material phase) has been established. [21] [22] Hereinafter, this will be described in more detail. [23] [24] First, the lithium secondary battery according to the present invention includes a positive electrode including a positive active material of a lithium transition metal oxide containing 60 mol% or more of Ni with respect to the total number of moles of transition metal, a negative electrode facing the positive electrode, and the positive electrode and the negative electrode A secondary battery including a separator and an electrolyte interposed therebetween is prepared. [25] [26] The positive electrode according to the present invention may be prepared by coating the positive electrode active material, the binder, the conductive material and the solvent for forming a positive electrode including the above-described positive electrode current collector on the positive electrode current collector. [27] [28] For example, in order to achieve a high energy density of the secondary battery, the positive electrode may contain 60 mol% or more of nickel based on the total number of moles of transition metal oxides excluding lithium, and preferably, the positive active material is represented by the following Chemical Formula 1 may be displayed. [29] [Formula 1] [30] Li 1+a Ni 1-xy Co x M1 y O 2 [31] In Formula 1, 0≤a≤0.3, 0≤x≤0.2, 0≤y≤0.2, and 0≤x+y≤0.4. [32] Specifically, in Formula 1, M1 is an element substituted for a transition metal site in the oxide represented by Formula 1, and M1 may include at least one of Mn and Al. [33] 1+a represents the molar ratio of lithium in the oxide represented by Formula 1, and may be 0≤a≤0.3, preferably 0≤a≤0.2. [34] The x represents the molar ratio of Co in the oxide represented by Formula 1, and may be 0≤x≤0.2, preferably 0