This application claims priority to and the benefit of Korean Patent 5 Application No. 10-2016-0068956, filed on June 2, 2016, Korean Patent Application No. 10-2016-0068940, filed on June 2, 2016, and Korean Patent Application No. 10- 2017-0068856, filed on June 2, 2017 the disclosure of which is incorporated herein by reference in their entirety. 􀳿Technical Field􀴀 10 The present invention relates to a negative electrode active material, a negative electrode including the same and a lithium secondary battery including the same. 􀳿Background Art􀴀 With a recent trend of miniaturization and weight lightening of electronic 15 devices, miniaturization and weight lightening of batteries used therein as a power supply also have been also required. Lithium secondary batteries are commercialized as batteries that are small, light, chargeable and dischargeable with high capacity, and used in portable electronic devices such as small video cameras, mobile phones and laptops, communication devices, etc. 20 Generally, a lithium secondary battery is formed with a positive electrode, a negative electrode, a separator and an electrolyte, and charge and discharge are possible due to lithium ions perform a role of transferring energy while travelling back and forth between both electrodes, for example lithium ions coming out of a positive electrode active material and being intercalated into a negative electrode 3 active material, that is, carbon particles, by first charge, and deintercalated again during discharge. Further, with the development of portable electronic devices, high capacity batteries have been continuously required, and research has been actively conducted 5 on high capacity negative electrode materials such as tin, silicon or the like having significantly higher capacity per unit weight compared to carbon used currently as negative electrode material. Among them, a negative electrode material using silicon has about 10 times higher capacity than a negative electrode material using carbon. 10 As a result, research has been conducted on a negative electrode material with high capacity using silicon in which there is no damage to the electrode even when lithium is intercalated and deintercalated repeatedly. [Prior Art Literature] [Patent Literature] 15 (Patent Literature 1) KR2005-0090218A 􀳿Disclosure􀴀 􀳿Technical Problem􀴀 The present invention provides a negative electrode active material which can prevent a negative electrode from expanding and contracting due to an 20 electrochemical reaction between lithium ions, which are discharged from a positive electrode during charging and discharging of lithium secondary batteries, and silicon, which are included in a negative electrode. The present invention provides a negative electrode active material having many paths through which lithium ions can move. 4 The present invention provides a lithium secondary battery having high capacity and high output characteristics. The present invention provides a lithium secondary battery which can increase initial efficiency and has improved rate capability. 5 􀳿Technical Solution􀴀 According to an embodiment of the present invention, there is provided a negative electrode active material which includes a secondary particle including a first particle which is a primary particle, wherein the first particle includes a first core, and a first surface layer which is disposed on a surface of the first core and contains 10 carbon, and the first core includes one or more of silicon and a silicon compound; and a metal compound which includes one or more of a metal oxide and a metal silicate. According to another embodiment of the present invention, there is provided a negative electrode including the negative electrode active material. 15 According to still another embodiment of the present invention, there is provided a lithium secondary battery including the negative electrode. 􀳿Advantageous Effects􀴀 The negative electrode active material according to the present invention includes secondary particles including first particles which are primary particles, and 20 thus paths through which lithium ions can move are allowed to increase such that output characteristics of a lithium secondary battery can be improved, an initial efficiency of the lithium secondary battery is high, and rate capability (charge and discharge characteristics) can be improved. 5 Further, according to the present invention, due to pores between the primary particles, damage to the electrode can be minimized even when intercalation and deintercalation of lithium ions are repeated and cores contract and expand repeatedly. Further, according to the present invention, the initial efficiency of the 5 battery can be further enhanced because the first core is doped with a metal compound. Moreover, the first particle including the first core doped with the metal compound and the second particle including the second particle undoped with the metal compound is mixed at a suitable weight ratio, and thereby a battery having 10 high capacity and excellent initial efficiency can be provided. 􁇻Description of Drawings􁇼 FIG. 1 is a schematic view showing a cross section of a negative electrode active material according to an embodiment of the present invention. FIG. 2 is a schematic view showing a cross section of a negative electrode 15 active material according to another embodiment of the present invention. FIG. 3 is a schematic view showing a cross section of a negative electrode active material according to still another embodiment of the present invention. FIG. 4 is a schematic view showing a cross section of a negative electrode active material according to yet another embodiment of the present invention. 20 FIG. 5 is a schematic view showing a cross section of a negative electrode active material according to yet another embodiment of the present invention. FIG. 6 is a schematic view showing a cross section of a negative electrode active material according to yet another embodiment of the present invention. FIG. 7 is a graph showing normalized capacity of examples of the present 25 invention and comparative examples. 6 􀳿Modes of the Invention􀴀 The terms and words used in this specification and claims should not be interpreted as limited to commonly used meanings or meanings in dictionaries and should be interpreted with meanings and concepts which are consistent with the 5 technological scope of the invention based on the principle that the inventors have appropriately defined concepts of terms in order to describe the invention in the best way. While the invention has been described with reference to exemplary embodiments illustrated in accompanying drawings, these should be considered in a 10 descriptive sense only, and it will be understood by those skilled in the art that various alterations and equivalent other embodiment may be made. Therefore, the scope of the invention is defined by the appended claims. The negative electrode active material according to an embodiment of the present invention may include a secondary particle including a first particle which is 15 a primary particle, where the first particle may include a first core, and a first surface layer which is disposed on a surface of the first core and contains carbon, and the first core may include one or more of silicon and a silicon compound; and a metal compound which includes one or more of a metal oxide and a metal silicate. FIG. 1 is a schematic view showing a cross section of a negative electrode 20 active material according to an embodiment of the present invention. Referring to FIG. 1, the negative electrode active material includes a secondary particle 200 including first particles 110 which are primary particles. Here, the term “secondary particle” refers to a particle formed by aggregation of primary particles. 7 The first particle 110 may include a first core 111 and a first surface layer 112. The first core 111 may include one or more of silicon and a silicon compound; and a metal compound 113. 5 Since the silicon has a theoretical capacity of about 3,600 mAh/g, the silicon has a very high capacity compared to existing negative electrode active material including graphite, and thus the capacity of a lithium secondary battery including the silicon can be improved. The silicon compound refers to a compound containing silicon, and may be a 10 silicon oxide (SiOx, 0 Silica oxides (SiOx, 0 20 10 g of the core and 0.5 g of sucrose were added to 30 g of isopropanol to prepare a solution. The mixture was pulverized for 12 hours at a bead rotation rate of 1,200 rpm using beads formed of zirconia (average particle size: 0.3 mm). Subsequently, the mixture was dried in an oven at 120 ºC for 2 hours. The dried mixture was pulverized again in a mortar and classified to form silicon particles 25 mixed with sucrose. The heat treatment was performed at 800 ºC under a nitrogen 33 atmosphere to carbonize the sucrose to form a surface layer having a thickness of 2 nm to prepare preliminary first particles. The content of the surface layer was 2.1 wt% based on the total weight of the core. 5 8 g of the preliminary first particles and 0.9 g of magnesium powder were mixed in an argon gas atmosphere to prepare a mixed powder. The mixed powder was placed in a tube furnace and heated to 1030 ºC at a rate of 5ºC/min in an argon gas atmosphere, followed by heating for 2 hours. Thereafter, the temperature of the 10 reaction furnace was lowered to room temperature, and the heat-treated mixed powder was taken out and washed with 1M HCl for 1 hour while stirring. The washed mixed powder was washed with distilled water while filtering, and then dried in an oven at 60 ºC for 8 hours to prepare first particles. As a result of analyzing the prepared first particles, the content of magnesium silicate and magnesium oxide 15 formed by oxidation of magnesium in the first particles was 15 wt% based on the total weight of the first particles, which was measured by quantitative analysis using X-ray diffraction (XRD). 20 The solution containing the first particles and ethanol/water (volume ratio=1:9) in a volume ratio of 1:10 was stirred with a mechanical homogenizer at 10,000 rpm for 30 minutes to prepare a dispersion solution for spray drying. The dispersion solution was spray-dried under the conditions of an inlet temperature of 180 ºC, an aspirator of 95%, and a feeding rate of 12 of a mini spray-dryer 25 (manufactured by Buchi Co., Ltd., model: B-290 Mini Spray-Dryer) to prepare 34 preliminary secondary particles, which were then transferred to an alumina boat. The temperature of a tube furnace equipped with a quartz tube having a length of 80 cm and an inner diameter of 4.8 cm was raised to 600 ºC at a rate of 10 ºC/min, and then calcined while maintaining the temperature for 2 hours to prepare secondary particles. The prepared 5 secondary particles had a porosity of 1% and an average particle size D50 of 5 μm. The porosity was measured by a mercury porosimeter method. Example 2: Preparation of negative electrode active material 10 The core and the preliminary first particle were prepared in the same manner as in Example 1. 15 8 g of the preliminary first particles and 10 g of magnesium powder were mixed in an argon gas atmosphere to prepare a mixed powder. The mixed powder was placed in a tube furnace and heated to 1030 ºC at a rate of 5ºC/min in an argon gas atmosphere, followed by heating for 2 hours. Thereafter, the temperature of the reaction furnace was lowered to room temperature, and the heat-treated mixed 20 powder was taken out and washed with 1M HCl for 1 hour while stirring. The washed mixed powder was washed with distilled water while filtering, and then dried in an oven at 60 ºC for 8 hours to prepare first particles. As a result of analyzing the prepared first particles, the content of magnesium silicate and magnesium oxide formed by oxidation of magnesium in the first particles was 51 wt% based on the 35 total weight of the first particles, which was measured by quantitative analysis using X-ray diffraction (XRD). Secondary 5 particles of Example 2 were prepared using the first particles by the same method as a method of preparing the secondary particles of Example 1. The prepared secondary particles had a porosity of 1% and an average particle size D50 of 4 μm. The porosity was measured by a mercury porosimeter method. 10 Example 3: Preparation of negative electrode active material Silica oxides (SiOx, 0 Preliminary first particles on which surface layers having a thickness of 2 nm were formed were prepared using the core through the same method as a method of 20 preparing the preliminary first particles of Example 1. The content of the surface layer was 2.1 wt% based on the total weight of the core. First particles were prepared using the preliminary first particles through the 25 same method as a method of preparing the first particles of Example 1. As a result 36 of analyzing the prepared first particles, the content of magnesium silicate and magnesium oxide formed by oxidation of magnesium in the first particles was 15 wt% based on the total weight of the first particles, which was measured by quantitative analysis using X-ray diffraction (XRD). 5 Secondary particles of Example 3 were prepared using the first particles by the same method as a method of preparing the secondary particles of Example 1. The prepared secondary particles had a porosity of 1% and an average particle size D50 10 of 2 μm. The porosity was measured by a mercury porosimeter method. Example 4: Preparation of negative electrode active material Silica oxides (SiOx, 0 20 Preliminary first particles on which surface layers having a thickness of 2 nm were formed were prepared using the core through the same method as a method of preparing the preliminary first particles of Example 1. The content of the surface layer was 2.1 wt% based on the total weight of the core. 25 37 8 g of the preliminary first particles and 5 g of magnesium powder were mixed in an argon gas atmosphere to prepare a mixed powder. The mixed powder was placed in a tube furnace and heated to 1030 ºC at a rate of 5ºC/min in an argon gas atmosphere, followed by heating for 2 hours. Thereafter, the temperature of the 5 reaction furnace was lowered to room temperature, and the heat-treated mixed powder was taken out and washed with 1M HCl for 1 hour while stirring. The washed mixed powder was washed with distilled water while filtering, and then dried in an oven at 60 ºC for 8 hours to prepare first particles. As a result of analyzing the prepared first particles, the content of magnesium silicate and magnesium oxide 10 formed by oxidation of magnesium in the first particles was 55 wt% based on the total weight of the first particles, which was measured by quantitative analysis using X-ray diffraction (XRD). 15 Secondary particles of Example 4 were prepared using the first particles by the same method as a method of preparing the secondary particles of Example 1. The prepared secondary particles had a porosity of 1% and an average particle size D50 of 3 μm. The porosity was measured by a mercury porosimeter method. 20 Example 5: Preparation of negative electrode active material Preliminary first particles on which surface layers having a thickness of 2 nm were formed were prepared through the same method as a method of preparing the core and the preliminary first particles of Example 1. The content of the surface 25 layer was 2.1 wt% based on the total weight of the core. 38 First particles were prepared using the preliminary first particles through the method of preparing the first particles of Example 1. As a result of analyzing the 5 prepared first particles, the content of magnesium silicate and magnesium oxide formed by oxidation of magnesium in the first particles was 15 wt% based on the total weight of the first particles, which was measured by quantitative analysis using X-ray diffraction (XRD). 10 Secondary particles were prepared, through the first particles and the second particles, by using the preliminary first particles as the second particles. Specifically, after the first particles and the second particles were mixed in a weight ratio of 6:4, the solution containing the mixture and ethanol/water (volume ratio=1:9) 15 in a volume ratio of 1:10 was stirred with a mechanical homogenizer at 10,000 rpm for 30 minutes to prepare a dispersion solution for spray drying. The dispersion solution was spray-dried under the conditions of an inlet temperature of 180 ºC, an aspirator of 95%, and a feeding rate of 12 of a mini spray-dryer (manufactured by Buchi Co., Ltd., model:B-290 Mini Spray-Dryer) to prepare preliminary secondary 20 particles, which were then transferred to an alumina boat. The temperature of a tube furnace equipped with a quartz tube having a length of 80 cm and an inner diameter of 4.8 cm was raised to 600 ºC at a rate of 10 ºC/min, and then calcined while maintaining the temperature for 2 hours to prepare secondary particles. The prepared secondary particles had a porosity of 1% and an average particle size D50 of 25 5 μm. The porosity was measured by a mercury porosimeter method 39 Example 6: Preparation of negative electrode active material The core, the first particle and the second particle (preliminary first particle) 5 were prepared in the same manner as in Example 5. The secondary particles were prepared in the same manner as in Example 6 except that the first particles and second particles were mixed in a weight ratio of 10 1.5:8.5. The prepared secondary particles had a porosity of 1% and an average particle size D50 of 5 μm. The porosity was measured by a mercury porosimeter method. Comparative Example 1: Preparation of negative electrode active material 15 Preliminary first particles on which surface layers having a thickness of 2 nm were formed were prepared through the same method as a method of preparing the core and the preliminary first particles of Example 1. The content of the surface layer was 2.1 wt% based on the total weight of the core. 20 Secondary particles were prepared by the same method as the method of preparing the secondary particles of Example 1 except that the first particles of Example 1 were not used and the preliminary first particles were used. The 40 prepared secondary particles had a porosity of 1% and an average particle size D50 of 5 μm. The porosity was measured by a mercury porosimeter method. Comparative Example 2: Preparation of negative electrode active material 5 8 g of the preliminary first particles which were prepared in Example 1 and 0.9 g of magnesium powder were mixed in an argon gas atmosphere to prepare a mixed powder. The mixed powder was placed in a tube furnace and heated to 1030 ºC at a rate of 5ºC/min in an argon gas atmosphere, followed by heating for 2 hours. Thereafter, the temperature of the reaction furnace was lowered to room temperature, 10 and the heat-treated mixed powder was taken out and washed with 1M HCl for 1 hour while stirring. The washed mixed powder was washed with distilled water while filtering, and then dried in an oven at 60 ºC for 8 hours to prepare a negative electrode active material in the form of a single particle. As a result of analyzing the prepared negative electrode active material, the content of magnesium silicate 15 and magnesium oxide formed by oxidation of magnesium in the negative electrode active material was 15 wt% based on the total weight of the negative electrode active material, which was measured by quantitative analysis using X-ray diffraction (XRD). 20 Examples 7 to 12 and Comparative Examples 3 and 4: Preparation of battery Each of the negative electrode active materials prepared in Examples 1 to 6 and Comparative Examples 1 and 2, fine graphite as a conductive material, and polyacrylonitrile as a binder were mixed in a weight ratio of 7:2:1 to prepare 0.2 g of 25 a mixture. 3.1 g of N-methyl-2-pyrrolidone (NMP) as a solvent was added to the 41 mixture to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied onto a copper (Cu) metal thin film as a negative electrode current collector having a thickness of 20 μm and dried. Here, the temperature of the circulating air was 80 ºC. Subsequently, the resultant was roll-pressed, dried in 5 a vacuum oven at 130 ºC for 12 hours, and then punched to a circular shape of 1.4875 cm2 to prepare each of negative electrodes of Examples 7 to 12. Each of the negative electrodes thus prepared was cut into a circular shape of 10 1.4875 cm2, which was used as a negative electrode, and a lithium metal thin film cut into a circle of 1.4875 cm2 was used as a positive electrode. A porous polyethylene separator was interposed between the positive electrode and the negative electrode, 0.5 wt% of vinylene carbonate was dissolved in a mixed solution in which ethyl methyl carbonate (EMC) and ethylene carbonate (EC) were mixed in a mixing 15 volume ratio of 7:3, and then an electrolyte in which 1M LiPF6 was dissolved was injected thereto to prepare a lithium coin half-cell. Experimental example 1: Evaluation of discharge capacity, initial efficiency, capacity retention ratio and electrode thickness change ratio 20 The batteries of Examples 7 to 12 and Comparative Examples 3 and 4 were charged and discharged to evaluate a discharge capacity, an initial efficiency, a capacity retention ratio and an electrode thickness change ratio, and the results are listed in the following Table 1. Further, during the first and second cycles, charging and discharging was 25 performed at 0.1 C, and during the 3rd through 49th cycles, charging and discharging 42 was performed at 0.5 C. At the 50th cycle, charging and discharging was terminated in a charging state (lithium ions were put in the negative electrode), and after disassembling the battery, a thickness was measured and an electrode thickness change ratio was calculated. 5 Charging condition: CC(constant current)/CV(constant voltage)(5mV/0.005C current cut-off) Discharging condition: CC(constant current) condition 1.5V The discharge capacity (mAh/g) and initial efficiency (%) were derived from the result after charging and discharging once. Specifically, the initial efficiency 10 (%) was derived by the following calculation. Initial efficiency (%) = (discharge capacity after one discharge/one charge capacity)×100 Each of the capacity retention ratio and the electrode thickness change ratio was derived by the following calculation. 15 Capacity retention ratio(%) = (49 times discharge capacity/one discharge capacity)×100 Electrode thickness change ratio (%) = (final electrode thickness change amount/initial electrode thickness)×100 􀳿Table 1􀴀 Active material Discharge capacity (mAh/g) Initial efficiency (%) Capacity retention ratio (%) Electrode thickness change ratio (%) Example 7 Example 1 1420 82.2 87.5 107 Example 8 Example 2 1400 84.2 87 108 Example 9 Example 3 1350 81.5 87.3 105 Example 10 Example 4 1300 83.5 87.2 109 Example 11 Example 5 1508 80.08 88 105 43 Example 12 Example 6 1520 75.5 87.0 115 Comparative Example 3 Comparativ e Example 1 1550 74.0 86.5 123 Comparative Example 4 Comparativ e Example 2 1320 80.1 80 110 Referring to Table 1, it can be confirmed that Examples 7 to 12 in which the active material according to the present invention was used were superior in terms of the initial efficiency, capacity retention rate, and electrode thickness change ratio as 5 compared to Comparative Example 3. It can be seen that this was an effect obtained due to the core of the first particle containing a metal compound. Further, it can be seen that Example 7 in which a negative electrode active material suitably containing 15 wt% of a metal compound in the core was used had a higher discharge capacity and a higher capacity retention rate compared to Example 10 8 in which a negative active material containing a metal compound at a high content of 51 wt% was used. In the case of Example 8, the metal doping amount for forming the metal compound was excessively high, and thus the crystal size of Si in the negative electrode active material was too large, and a part of the metal acted as an impurity, thereby adversely affecting battery life and lowering the capacity 15 retention ratio. Further, Example 7 in which the negative electrode active material of Example 1 having a core with a suitable size of 1 μm was used had higher discharge capacity, initial efficiency, and capacity retention ratio compared to Example 9 in which the negative electrode active material of Example 3 having an excessively small core with a size of 0.4 μm was used. 20 This is because the irreversible reaction is increased due to an increase in the specific surface area when a small-sized core is used. 44 Examples 13 to 17 and Comparative Examples 5 and 6: Preparation of battery A mixed negative electrode active material prepared by mixing each of the 5 negative electrode active materials prepared in Examples 1 to 5 and Comparative Examples 1 and 2 with graphite (natural graphite) at a weight ratio of 1:9, carbon black as a conductive material, carboxylmethyl cellulose (CMC), and a styrene butadiene rubber (SBR) were mixed at a weight ratio of 95.8:1:1.7:1.5 to prepare 5 g of a mixture. 28.9 g of distilled water was added to the mixture to prepare a 10 negative electrode mixture slurry. The negative electrode mixture slurry was applied onto a copper (Cu) metal thin film as a negative electrode current collector having a thickness of 20 μm and dried. Here, the temperature of the circulating air was 60 ºC. Subsequently, the resultant was roll-pressed, dried in a vacuum oven at 130 ºC for 12 hours, and then punched to a circular shape of 1.4875 cm2 to prepare 15 each of negative electrodes of Examples 13 to 17 and Comparative Examples 5 and 6. Each of the negative electrodes thus prepared was cut into a circular shape of 1.4875 cm2, which was used as a negative electrode, and a lithium metal thin film cut 20 into a circle of 1.4875 cm2 was used as a positive electrode. A porous polyethylene separator was interposed between the positive electrode and the negative electrode, 0.5 wt% of vinylene carbonate was dissolved in a mixed solution in which ethyl methyl carbonate (EMC) and ethylene carbonate (EC) were mixed in a mixing ratio of 7:3, and then an electrolyte in which 1M LiPF6 was dissolved was injected thereto 25 to prepare a lithium coin half-cell. 45 Experimental example 2: Evaluation of initial efficiency, capacity retention ratio and electrode thickness change ratio The batteries of Examples 13 to 17 and Comparative Examples 5 and 6 were charged and dischar 5 ged to evaluate an initial efficiency, a capacity retention ratio, and an electrode thickness change ratio, and the results are listed in the following I/We Claim: 􀳿Claim 1􀴀 A negative electrode active material, comprising a secondary particle including a first particle which is a primary particle, 5 wherein the first particle includes a first core, and a first surface layer which is disposed on a surface of the first core and contains carbon, and the first core includes: one or more of silicon and a silicon compound; and a metal compound which includes one or more of a metal oxide and a metal 10 silicate. 􀳿Claim 2􀴀 The negative electrode active material according to claim 1, wherein the metal compound is doped in an amount of 1 to 50 wt% based on the total weight of the first particle. 15 􀳿Claim 3􀴀 The negative electrode active material according to claim 1, wherein the secondary particle further includes a second particle which is a primary particle, the second particle includes a second core and a second surface layer which is disposed on a surface of the second core and contains carbon, and the second core includes 20 one or more of silicon and a silicon compound. 􀳿Claim 4􀴀 The negative electrode active material according to claim 3, wherein a weight ratio of the first particle and the second particle is in a range of 1:0.25 to 1:4. 49 􀳿Claim 5􀴀 The negative electrode active material according to claim 3, wherein an average particle size D50 of each of the first core and the second core is in a range of 0.5 to 20 μm. 5 􀳿Claim 6􀴀 The negative electrode active material according to claim 3, wherein the silicon included in each of the first core and the second core includes one or more of an amorphous silicon and a crystalline silicon having a crystal size of more than 0 to 30 nm or less. 10 􀳿Claim 7􀴀 The negative electrode active material according to claim 3, wherein the silicon compound included in each of the first core and the second core is a silicon oxide (SiOx, 0