Title of Invention: Irreversible additive contained in cathode material for secondary battery, cathode material including same, and secondary battery including cathode material technical field [One] Cross-Citation with Related Application(s) [2] This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0093872 dated July 28, 2020 and Korean Patent Application No. 10-2021-0094000 dated July 19, 2021, and All content disclosed in the literature is incorporated as a part of this specification. [3] The present invention relates to an irreversible additive contained in a cathode material for a secondary battery, a cathode material comprising the same, and a secondary battery including the cathode material. background [4] Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy and clean energy is increasing. [5] Currently, a secondary battery is a representative example of an electrochemical device using such electrochemical energy, and its use area is gradually expanding. [6] Recently, as technology development and demand for portable devices such as portable computers, portable telephones, and cameras increase, the demand for secondary batteries as an energy source is rapidly increasing. Among such secondary batteries, many studies have been conducted on lithium secondary batteries that exhibit high energy density, have a long cycle life, and have a low self-discharge rate, and are also commercialized and widely used. [7] In addition, as interest in environmental problems grows, research on electric vehicles and hybrid electric vehicles that can replace vehicles using fossil fuels such as gasoline vehicles and diesel vehicles, which are one of the main causes of air pollution, is being conducted. have. Although nickel-metal hydride secondary batteries are mainly used as power sources for such electric vehicles and hybrid electric vehicles, research using lithium secondary batteries with high energy density is being actively conducted, and some are in the commercialization stage. [8] A carbon material is mainly used as an anode active material of such a lithium secondary battery, and a lithium transition metal composite oxide is used as a cathode active material of a lithium secondary battery. Among them, various lithium transition metal oxides such as LiNiO 2 , LiMnO 2 , LiMn 2O 4 or LiFePO 4 have been developed in addition to lithium cobalt composite metal oxides such as LiCoO 2 having high operating voltage and excellent capacity characteristics. [9] Meanwhile, due to the consumption of Li ions during initial charging and discharging, a solid electrolyte interphase (SEI) layer is formed and irreversibility of the positive and negative electrodes occurs. Due to this, the energy density is reduced, and there is a problem in that the theoretical amount that can be designed cannot be sufficiently used. [10] To solve this problem, lithium ions may be supplemented by adding an irreversible additive to the cathode material. However, Li 2NiO 2 , which is an irreversible additive conventionally used, has an orthorhombic crystal structure and belongs to a space group of Immm. However, there is a problem in that the material undergoes three-step structural change in the operating voltage range after initial charging of the secondary battery, causing impurities or gas to be generated. [11] Specifically, the material maintains an orthorhombic crystal structure in the range of 3.0 to 3.5 V, but according to the detachment of Li, trigonal at 3.5 to 4.0 V, and monoclinic at 3.5 to 4.25 V ( monoclinic) and undergoes three crystal structure changes. In particular, when an irreversible additive (Li 2NiO 2) having an orthorhombic crystal structure changes to an orthorhombic crystal structure, unpredictable by-products and excessive gas generation occur. Moreover, since it undergoes a change in the crystal structure, there is also a problem of lowering structural stability. [12] DETAILED DESCRIPTION OF THE INVENTION technical challenge [13] An object of the present invention is to provide an irreversible additive that sufficiently expresses Li ions during initial charging, minimizes the generation of impurities or gases in the operating voltage range of a secondary battery, and has high structural stability. [14] Another object of the present invention is to provide a cathode material for a secondary battery including the irreversible additive, and a secondary battery having excellent electrochemical properties including the same. means of solving the problem [15] The irreversible additive according to an embodiment of the present invention is an irreversible additive included in a cathode material for a secondary battery, wherein the irreversible additive is an oxide represented by the following Chemical Formula 1, and the oxide has a trigonal crystal structure. It is an additive. [16] Li 2+aNi 1-bTi bO 2+c (1) [17] In the above formula, -0.2≤a≤0.2, 0 [92] After mixing Li 2 O 22.9 g and NiO 30 g (molar ratio 1:1), heat treatment was performed at 685 degrees Celsius for 18 hours under N 2 atmosphere, and then the resulting reactant was cooled to obtain irreversible additive particles Li 2NiO 2 . [93] [94] [95] LiNiO 2 and Li+benzophenone- exceeding 1.5M were reacted under THF (tetrahydrofuran) in an inert atmosphere. [96] Specifically, the mixture of the above substances was stirred for one day, and the mixed powders were filtered. The obtained mixed powder was washed with dry THF and dried under vacuum to obtain a preliminary powder in which a small amount of trigonal Li 2NiO 2 and LiNiO 2 were mixed. [97] Thereafter, the preliminary powder was heat-treated at 225° C. for 14 hours under a flow of dry helium to obtain Li 2NiO 2 powders having a trigonal structure with improved crystallinity. [98] [99] [100] After mixing Li 2 O 22.9 g, NiO 30 g, and TiO 2 2.39 g (molar ratio 1:1:0.03), heat treatment was performed at 685 degrees Celsius for 18 hours under N 2 atmosphere, and then the resulting reactant was cooled to obtain irreversible additive particles LiNi 0.97Ti 0.03O 2 was obtained. [101] LiNi 0.97Ti 0.03O 2 and more than 1.5M Li +benzophenone - were reacted under THF (tetrahydrofuran) in an inert atmosphere. [102] Specifically, the mixture of the above substances was stirred for one day, and the mixed powders were filtered. The obtained mixed powder was washed with dry THF and dried under vacuum to obtain a preliminary powder in which a small amount of trigonal Li 2Ni 0.97Ti 0.03O 2 and LiNi 0.97Ti 0.03O 2 were mixed. [103] Thereafter, the preliminary powder was heat-treated at 225° C. for 14 hours under a flow of dry helium to obtain Li 2Ni 0.97Ti 0.03O 2 powders having a trigonal structure with improved crystallinity. [104] [105] [106] 2 g of the irreversible additive particles prepared in Comparative Examples 1 and 2 and Example 1 were sampled, respectively, and XRD analysis was performed thereon, and the results are shown in FIGS. 1 to 3 . [107] XRD analysis was measured with a Bruker XRD D4 device, a Cu source target was used, and the experiment was performed from 10 degrees to 80 degrees in 0.02 steps. [108] 1 to 3 , it can be seen that irreversible additives having different structures are formed according to Comparative Examples 1 and 2 and Example 1. Specifically, it can be seen that Comparative Example 1 is formed in an orthorhombic structure, and Comparative Examples 2 and 1 are formed in a trigonal structure. [109] [110] [111] A positive electrode and a lithium secondary battery were manufactured in the following manner using the irreversible additive prepared in Comparative Examples 2 and 1 above. [112] Specifically, the irreversible additive prepared in Comparative Examples 2 and 1, LiNi 0.4Mn 0.3Co 0.3O 2 as a positive electrode active material, a carbon black conductive material, and a PVdF binder in a weight ratio of 4.6:87.9:3.5:4 A positive electrode slurry was prepared by mixing in N-methylpyrrolidone solvent, which was applied to an aluminum current collector, and then dried and rolled to prepare a positive electrode. [113] In addition, MCMB (mesocarbon microbead), a carbon black conductive material, and PVdF binder, which is artificial graphite mixed with SiO in an amount of 10% by weight as an anode active material, are mixed in an N-methylpyrrolidone solvent in a weight ratio of 90:5:5. A negative electrode was prepared by preparing a composition for forming an anode, and applying it to a copper current collector. [114] An electrode assembly was prepared by interposing a separator of porous polyethylene between the positive electrode and the negative electrode prepared as described above, the electrode assembly was placed inside the case, and the electrolyte was injected into the case to prepare a lithium secondary battery. At this time, the electrolyte is prepared by dissolving lithium hexafluorophosphate (LiPF 6) at a concentration of 1.15M in an organic solvent consisting of ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate (mixed volume ratio of EC/DMC/EMC = 3/4/3). did [115] [116] [117] 2 g of the irreversible additive particles prepared in Comparative Examples 1, 2 and 1 were sampled, respectively, and their oxygen formation energy was measured, and the results are shown in Table 1 below. [118] Specifically, the calculation of oxygen formation energy is DFT (density functional theory) calculation, PBE functional PAW_PBE pseudopotential, Cut-off energy = 520 eV, calculation model: supercell with Li 48(Ni -) 24O 48 atoms- 1 Ni to Ti substituted with (ratio~4.17at%), Oxygen vacancy(VO) concentration = 1/48 (~2.1 at.%) The calculated value for O 2 gas was used as the standard (O-rich environment). [119] [Table 1] Oxygen (V0) formation energy (eV) Comparative Example 1 3.77 Comparative Example 2 4.21 Example 1 4.69 [120] Referring to Table 1, it is confirmed that the energy of the orthorhombic irreversible additive of Comparative Examples 2 and 1 is higher than that of the orthorhombic irreversible additive of Comparative Example 1. This is because the orthorhombic irreversible additive undergoes a structural change in the intercalation of Li ions through a trigonal system to a monoclinic system during charging and discharging, whereas the trigonal irreversible additive progresses to a monoclinic system. It is expected that the orthorhombic system will not cause side reactions than the orthorhombic system. [121] In addition, considering that the energy of the trigonal irreversible additive of Example 1 is higher than that of the trigonal system of Comparative Example 2, it will form a stronger structure than the trigonal system without substitution in the intercalation of Li ions during charging and discharging. It is estimated. Therefore, it is expected that a side reaction will not occur rather than a trigonal irreversible additive in which Ti is not substituted. [122] Although the preferred embodiment of the present invention has been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements by those skilled in the art using the basic concept of the present invention as defined in the following claims are also presented. It belongs to the scope of the right of the invention. [123] Claims [Claim 1] An irreversible additive included in a cathode material for a secondary battery, wherein the irreversible additive is an oxide represented by the following Chemical Formula 1, and the oxide has a trigonal crystal structure: Li 2+aNi 1-bTi bO 2 +c (1) In the above formula, -0.2≤a≤0.2, 0