FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION (See section 10, rule 13) “POSITIVE ELECTRODE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY INCLUDING THE SAME” LG CHEM, LTD., of 128, Yeoui-daero, Yeongdeungpo-gu, Seoul 07336, Republic of Korea The following specification particularly describes the invention and the manner in which it is to be performed. DESCRIPTION POSITIVE ELECTRODE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY INCLUDING THE SAME TECHNICAL FIELD Cross-reference to Related Applications [0001] This application claims the benefit of Korean Patent Application Nos. 2017-0081273, filed on June 27, 2017, and 2018-0074359, filed on June 27, 2018, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. Technical Field [0002] The present invention relates to a positive electrode, and a lithium secondary battery including the same. BACKGROUND ART [0003] Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased, and, among these secondary batteries, lithium secondary batteries having high energy density, high operating potential, long cycle life, and low self-discharging rate have been commercialized and widely used. [0004] A lithium secondary battery is a secondary battery which is generally composed of a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator, and an electrolyte and is charged and discharged by intercalation-deintercalation of lithium ions. Since the lithium secondary battery is advantageous in that it has large electromotive force as well as high energy density and may exhibit high capacity, the lithium secondary battery has been applied to various fields. [0005] Various methods have been studied to achieve higher capacity of the lithium secondary battery. Specifically, a method of achieving high capacity of a lithium secondary battery by using at least one material of LCO, LNMCO, and LMO, as a positive electrode active material included in a positive electrode for a lithium secondary battery, has been attempted. However, in order to actually increase the capacity of the lithium secondary battery, capacity of a negative electrode as well as capacity of the positive electrode must be increased, wherein, for this purpose, a method of using a high capacity silicon-based negative electrode active material as a negative electrode has also been attempted. However, with respect to the silicon-based negative electrode active material, since irreversible capacity is also high, there is a limitation in that charge and discharge efficiency is low. In order to address the limitation of irreversible capacity while using the silicon-based negative electrode active material, the silicon-based active material must be lithiated, but the lithiation may cost a lot. [0006] Accordingly, there is a need to develop a lithium secondary battery which may exhibit high capacity and excellent charge and discharge efficiency and may be prepared at low cost. DISCLOSURE OF THE INVENTION TECHNICAL PROBLEM [0007] An aspect of the present invention provides a positive electrode in which high capacity and excellent initial capacity of a secondary battery may be achieved by including an additive. [0008] Another aspect of the present invention provides a lithium secondary battery which has excellent charge and discharge efficiency as well as high capacity and may be prepared at low cost without a separate lithiation process by including the positive electrode. [0009] Another aspect of the present invention provides an additive for a positive electrode in which excellent initial capacity of the secondary battery may be achieved by including metal particles and lithium oxide. TECHNICAL SOLUTION [0010] According to an aspect of the present invention, there is provided a positive electrode including: a nickel-containing positive electrode active material; and an additive including metal particles and lithium oxide. [0011] According to another aspect of the present invention, there is provided a lithium secondary battery including the positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. [0012] According to another aspect of the present invention, there is provided an additive for a positive electrode which includes metal particles and lithium oxide. ADVANTAGEOUS EFFECTS [0013] According to the present invention, since an additive including metal particles and lithium oxide is included during the preparation of a positive electrode, the metal particles and the lithium oxide, which are included in the additive, are reacted at less than a driving voltage (2.5 V to 4.3 V) of a lithium secondary battery to form lithium ions and metal oxide, and, among them, the lithium ions move to a negative electrode to lithiate a negative electrode active material. Accordingly, since a separate lithiation process may not be further performed, a lithium secondary battery having excellent capacity may be prepared at low cost. [0014] Also, since the metal oxide formed by the reaction of the metal particles with the lithium oxide may adsorb gas such as CO or CO2, a decrease in stability due to CO or CO2 gas generated during charge and discharge of a cell may be prevented and swelling may also be reduced. BRIEF DESCRIPTION OF THE DRAWINGS [0015] The following drawings attached to the specification illustrate preferred examples of the present invention by example, and serve to enable technical concepts of the present invention to be further understood together with detailed description of the invention given below, and therefore the present invention should not be interpreted only with matters in such drawings. [0016] FIG. 1 illustrates charge capacities of test secondary batteries 1 to 3 respectively including Preparation Examples 1 and 2 and Comparative Preparation Example 1. MODE FOR CARRYING OUT THE INVENTION [0017] Hereinafter, the present invention will be described in more detail. [0018] It will be understood that words or terms used in the specification and claims shall not be interpreted as the meaning defined in commonly used dictionaries. It will be further understood that the words or terms should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the technical idea of the invention, based on the principle that an inventor may properly define the meaning of the words or terms to best explain the invention. [0019] Positive Electrode [0020] A positive electrode according to an embodiment of the present invention includes a nickel-containing positive electrode active material; and an additive including metal particles and lithium oxide. [0021] Specifically, with respect to the positive electrode, a composition for forming a positive electrode including a nickel-containing positive electrode active material; and an additive including metal particles and lithium oxide is formed on a positive electrode collector. [0022] The positive electrode collector is not particularly limited as long as it has conductivity without causing adverse chemical changes in a battery, and, for example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel that is surface-treated with one of carbon, nickel, titanium, silver, or the like may be used. [0023] The metal particles included in the additive may preferably include at least one selected from the group consisting of iron (Fe), cobalt (Co), chromium (Cr), manganese (Mn), and nickel (Ni). Particularly, in a case in which particles of the at least one metal selected therefrom are included, a nanosized composite having capacity about 4 times higher than that of a conventional lithium transition metal oxide may be formed, and, since the composite has a large charge/discharge voltage hysteresis curve, it may improve initial charge and discharge efficiency when being added as a positive electrode additive. [0024] Also, the lithium oxide included in the additive may include at least one selected from the group consisting of Li2O, Li2O2, and LiO2. [0025] Since the composition for forming a positive electrode includes the additive including the metal particles and the lithium oxide, the metal particles and the lithium oxide, which are included in the additive, may be electrochemically reacted in a driving voltage range of the additive to form lithium ions and metal oxide. In a case in which the additive forms the lithium ions and the metal oxide, when the additive is used in a secondary battery, the lithium ions move to a negative electrode according to charge and discharge of the secondary battery to further increase capacity of the battery, and the metal oxide may reduce a swelling phenomenon by adsorbing gas, such as CO or CO2, which may be generated during the charge and discharge of the secondary battery. [0026] The metal particles may have an average particle diameter (D50) of 5 µm or less, preferably 1 nm to 5 µm or 1 nm to 1 µm, and more preferably 10 nm to 50 nm. In a case in which the average particle diameter of the metal particles is greater than 5 µm, the reaction with the lithium oxide may hardly occur, and, since the reaction with the lithium oxide easily occurs as the average particle diameter of the particles is smaller, preferably, in a nm range, the initial capacity may be further improved. [0027] The average particle diameter (D50) of the metal particles may be defined as a particle diameter at a cumulative volume of 50% in a particle size distribution curve. For example, the average particle diameter (D50) of the metal particles may be measured by a laser diffraction method. The laser diffraction method may generally measure a particle diameter ranging from a submicron level to a few mm, and may obtain highly repeatable and high resolution results. For example, in the measurement method of the average particle diameter (D50) of the metal particles, the metal particles are introduced into a commercial laser diffraction particle size measurement instrument (e.g., Microtrac MT 3000) and irradiated with ultrasonic waves having a frequency of about 28 kHz and an output of 60 W, and the average particle diameter (D50) at 50% in a cumulative particle diameter distribution of the measurement instrument may then be calculated. [0028] For example, the additive may include the metal particles and the lithium oxide in a molar ratio of 1:0.1 to 1:4, preferably 1:0.3 to 1:4, more preferably 1:0.3 to 1:3, and most preferably 1:0.5 to 1:2. In a case in which the metal particles and the lithium oxide are included within the above range, since the metal particles and the lithium oxide, which are included in the additive, may be electrochemically reacted in the driving voltage range of the additive, lithium ions and metal oxide may be easily formed and the capacity of the secondary battery may be further improved when the additive is used in the battery. For example, in a case in which the metal particles and the lithium oxide are included in a molar ratio of less than 1:0.1, since an amount of the lithium oxide with which the metal may react is small, the lithium ions and the metal oxide may not be formed, and thus, capacity may be very low. Accordingly, the capacity may be smaller than that of lithium metal used as a conventional positive electrode. In contrast, in a case in which the metal particles and the lithium oxide are included in a molar ratio of greater than 1:4, since the amount of the lithium oxide reacted is small, the capacity may be very low. [0029] The nickel-containing positive electrode active material included in the composition for forming a positive electrode may include one selected from the group consisting of LiNiO2, Li1+w(Ni1-x-y-zCoxM1yM2z)O2 (where M1 and M2 are each independently any one selected from the group consisting of aluminum (Al), iron (Fe), manganese (Mn), vanadium (V), chromium (Cr), titanium (Ti), tungsten (W), tantalum (Ta), magnesium (Mg), and molybdenum (Mo), 0≤w≤1, 0≤x<1, 0≤y<1, 0≤z<1, and x+y+z<1), Li1+w1NiaCobM1cO2 (where M1 is any one selected from the group consisting of Al, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, 0≤w1≤1, a≥0.6, 0≤b<1, 0≤c<1, and a+b+c=1), and a combination thereof. Specifically, the nickel-containing positive electrode active material may preferably include nickel in a large amount of 60 mol% or more based on the total number of moles of transition metal oxide included in the nickel-containing positive electrode active material. For example, the nickel-containing positive electrode active material may be LiNi0.8Co0.1Mn0.1O2 or LiNi0.6Co0.2Mn0.2O2, but the present invention is not limited thereto. [0030] The nickel-containing positive electrode active material may be included in an amount of 1 part by weight to 99 parts by weight, preferably 30 parts by weight to 99 parts by weight, and more preferably 50 parts by weight to 99 parts by weight based on a total weight of the positive electrode. [0031] An amount of the additive included in the composition for forming a positive electrode may be controlled depending on irreversible capacity of the negative electrode. For example, the additive may be included in an amount of 0.1 part by weight to 100 parts by weight, preferably 1 part by weight to 50 parts by weight, more preferable 1 part by weight to 10 parts by weight, and most preferably 3 parts by weight to 7 parts by weight based on a total weight of the composition for forming a positive electrode. For example, in a case in which the additive is included in an amount of less than 0.1 part by weight based on the total weight of the composition for forming a positive electrode, it may be difficult to achieve the formation of lithium ions due to the addition of the additive and the resultant effect of increasing the capacity and initial capacity of the secondary battery. [0032] Also, the positive electrode may further include a conductive agent and a binder. [0033] The binder is a component that assists in the binding between the active material and the conductive agent and in the binding with the current collector, wherein the binder is typically added in an amount of 1 wt% to 30 wt% based on a total weight of solid content of the composition for forming a positive electrode. Examples of the binder may be polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene-propylene-diene terpolymer (EPDM), a sulfonated EPDM, a styrene-butadiene rubber, a fluoro rubber, various copolymers, and the like. [0034] The conductive agent may be typically added in an amount of 1 wt% to 30 wt% based on the total weight of the solid content of the composition for forming a positive electrode. [0035] The conductive agent is not particularly limited as long as it has conductivity without causing adverse chemical changes 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; conductive fibers such as carbon fibers or metal fibers; metal powder such as fluorocarbon powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxide such as titanium oxide; or polyphenylene derivatives may be used. Specific examples of a commercial conductive agent may be acetylene black-based products (Chevron Chemical Company, Denka black (Denka Singapore Private Limited), or Gulf Oil Company), Ketjen black, ethylene carbonate (EC)-based products (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal Graphite & Carbon). [0036] The positive electrode may be prepared according to a typical method of preparing a positive electrode except that the above-described composition for forming a positive electrode is used. Specifically, the above-described composition for forming a positive electrode is coated on the positive electrode collector, and the positive electrode may then be prepared by drying and rolling the coated positive electrode collector. [0037] Also, as another method, the positive electrode may be prepared by casting the composition for forming a positive electrode on a separate support and then laminating a film separated from the support on the positive electrode collector. [0038] Secondary Battery [0039] Furthermore, the present invention provides a lithium secondary battery including the above-described positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. [0040] The lithium secondary battery specifically includes a positive electrode, a negative electrode disposed to face the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte. Also, the lithium secondary battery may further selectively include a battery container accommodating an electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member sealing the battery container. [0041] Since the positive electrode is the same as described above, detailed descriptions thereof will be omitted, and the remaining configurations will be only described in detail below. [0042] The negative electrode, for example, may be prepared by coating a negative electrode collector with a composition for forming a negative electrode which includes a silicon-based negative electrode active material, a binder, a conductive agent, and a solvent. [0043] For example, the silicon-based negative electrode active material may include at least one selected from the group consisting of silicon (Si) and SiOx (0