TECHNICAL 5 AL FIELD [0001] The present invention relates to a porous siliconbased anode active material, and more particularly, to a porous silicon-based anode active material including porous SiOx particles having surfaces coated with an oxide layer, a 10 method for preparing the same, and a lithium secondary battery including the porous silicon-based anode active material. BACKGROUND ART [0002] Recently, in line with miniaturization, lightweight, 15 thin profile, and portable trends in electronic devices according to the development of information and telecommunications industry, the need for high energy density batteries used as power sources of such electronic devices has increased. Currently, research into lithium secondary 20 batteries, as batteries that may best satisfy the above need, has actively conducted. [0003] Various types of carbon-based materials including artificial graphite, natural graphite, or hard carbon, which are capable of intercalating/deintercalating lithium, have 25 been used as anode active materials of lithium secondary 3 batteries. Among the carbon-based materials, since graphite provides advantages in terms of energy density of a lithium secondary battery and also guarantees long lifespan of the lithium secondary battery due to excellent reversibility, graphite 5 e has been most widely used. [0004] However, since graphite may have a low capacity in terms of energy density per unit volume of an electrode and may facilitate side reactions with an organic electrolyte at a high discharge voltage, there is a risk of fire or 10 explosion due to malfunction and overcharge of the battery. [0005] Thus, metal-based anode active materials, such as silicon (Si), have been studied. It is known that a siliconbased anode active material exhibits high capacity. However, the silicon-based anode active material may cause a maximum 15 volume change of 300% or more before and after the reaction with lithium, i.e., during charge and discharge. As a result, conductive networks in the electrode may be damaged and contact resistance between particles may be increased to degrade lifetime characteristics of the battery. 20 [0006] In addition, a thick non-conductive side reaction product layer may be formed on the surface of the siliconbased anode active material during charge and discharge due to the continuous reaction with an electrolyte solution. As a result, the silicon-based anode active material may be 25 electrically short-circuited in the electrode to degrade the 4 lifetime characteristics. [0007] Therefore, there is a need to develop an anode active material which may replace a typical anode active material and may improve the lifetime characteristics and effect of reducing volume expansion of a lithium secondary battery 5 ry due to less reaction with the electrolyte solution when used in the lithium secondary battery. DISCLOSURE OF THE INVENTION TECHNICAL PROBLEM 10 [0008] The present invention is provided to solve technical problems of the related art. [0009] The present invention provides a porous silicon-based anode active material which may reduce the occurrence of an electrical short circuit in an electrode and volume expansion 15 rate by reducing a side reaction product layer that is formed on the surfaces of porous silicon-based particles due to the reaction between the particles and an electrolyte solution. [0010] The present invention also provides a method of easily preparing an anode active material which may improve 20 the lifetime characteristics and effect of reducing volume expansion of a lithium secondary battery. [0011] The present invention also provides an anode and a lithium secondary battery including the anode active material. TECHNICAL SOLUTION 25 [0012] According to an aspect of the present invention, 5 there is provided an anode active material including porous SiOx particles (0≤x<2), wherein the porous SiOx particles include an oxide layer coated on surfaces thereof. [0013] According to another aspect of the present invention, there is provided a method of preparing an anode 5 active material including: preparing porous SiOx particles (0≤x<2) by forming pores on surfaces or the surfaces and inside of SiOx particles; and heat treating the porous SiOx particles in air or an oxygen atmosphere to prepare porous SiOx 10 particles having surfaces coated with an oxide layer. [0014] According to another aspect of the present invention, there is provided an anode including the anode active material. [0015] According to another aspect of the present invention, 15 there is provided a lithium secondary battery including the anode. ADVANTAGEOUS EFFECTS [0016] Since an anode active material according to an embodiment of the present invention includes porous SiOx 20 particles (0≤x<2) having surfaces coated with an oxide layer, a reactivity between the anode active material and an electrolyte solution may be reduced and, as a result, an electrical short circuit in an electrode may be minimized. [0017] Also, since a plurality of pores is included in 25 surfaces or the surfaces and inside of the SiOx particles, a 6 thickness change rate of the electrode generated during charge and discharge of a secondary battery may be reduced and lifetime characteristics may be improved. [0018] Furthermore, a method of preparing an anode active material according to an embodiment of 5 f the present invention, as a simple method, may easily prepare an anode active material which improves the lifetime characteristics and effect of reducing volume expansion of the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS 10 [0019] 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 15 therefore the present invention should not be interpreted only with matters in such drawings. [0020] FIG. 1 is a cross-sectional structural view schematically illustrating a structure of porous SiOx particles (0≤x<2) having surfaces coated with an oxide layer 20 in an anode active material according to an embodiment of the present invention. [0021] [Description of the Symbols] [0022] 1 Porous SiOx particle (0≤x<2) [0023] 2 Oxide layer 25 [0024] 1a Pores 7 [0025] 10 Porous SiOx particle (0≤x<2) having surface coated with oxide layer MODE FOR CARRYING OUT THE INVENTION [0026] Hereinafter, the present invention will be described in more detail to allow for a clearer 5 understanding of the present invention. [0027] 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 10 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 15 terms to best explain the invention. [0028] An anode active material according to an embodiment of the present invention includes porous SiOx particles (0≤x<2), wherein the porous SiOx particles include an oxide layer coated on surfaces thereof. 20 [0029] With respect to a silicon-based anode active material, a thick non-conductive side reaction product layer may be formed on the surface of the silicon-based anode active material during charge and discharge due to the continuous reaction with an electrolyte solution. As a result, the 25 anode active material may be electrically short-circuited in 8 an electrode to degrade lifetime characteristics and the volume expansion of the electrode may be further increased due to the side reaction product layer. In the present invention, a reactivity between the anode active material and the electrolyte solution is reduced by 5 y forming an oxide layer on porous SiOx particles (0≤x<2), i.e., silicon-based particles, and thus, the formation of the side reaction product layer, which may be formed on the surface of the anode active material, may be minimized. Also, since a 10 plurality of pores is included in surfaces or the surfaces and inside of the SiOx particles, a thickness change rate of the electrode generated during charge and discharge of a secondary battery may be reduced and lifetime characteristics may be further improved. 15 [0030] FIG. 1 is a cross-sectional structural view schematically illustrating a structure of porous SiOx particles (0≤x<2) having surfaces coated with an oxide layer (hereinafter simply referred to as “porous silicon-based anode active material”) in an anode active material according 20 to an embodiment of the present invention. FIG. 1 is only an example for describing the present invention, and the present invention is not limited thereto. Hereinafter, the present invention will be described with reference to FIG. 1. [0031] In the anode active material according to the 25 embodiment of the present invention, porous silicon-based 9 anode active material 10 includes porous SiOx particles (0≤x<2) 1 and an oxide layer 2 formed on the porous SiOx particles, and the porous SiOx particles 1 include a plurality of pores 1a in the surfaces and inside thereof. [0032] Also, in the porous silicon-based 5 ased anode active material 10, the oxide layer 2 may specifically include a silicon oxide. Furthermore, the silicon oxide may specifically include SiOy (0 [0080] Preparation Example 1 25 [0081] 28 [0082] Step i) Electrodepositing Ag on Surfaces of Si Particles [0083] A 300 mℓ solution containing 10% hydrogen fluoride (HF) and a 300 mℓ solution containing 10 mM silver nitrate (AgNO3) were mixed for 10 minutes to prepare a m5 ixture solution. 2 g of Si was added to the mixture solution, in which the hydrogen fluoride and the silver nitrate were mixed, and mixed for 5 minutes. Then, Si electrodeposited with Ag was prepared by filtering, washing, and drying the mixture. 10 [0084] Step ii) Etching [0085] A 200 mℓ solution containing 5% hydrogen fluoride and a 100 mℓ solution containing 1.5 wt% hydrogen peroxide (H2O2) were mixed for 10 minutes to prepare an etching solution. Si electrodeposited with Ag was introduced into the etching 15 solution, in which the hydrogen fluoride and the hydrogen peroxide were mixed, and mixed for 30 minutes. Then, porous SiOx (x=0) (hereinafter simply referred to as “porous Si”) was prepared by filtering, washing, and drying the mixture. [0086] Step iii) Removing Ag 20 [0087] 100 mℓ of 60% nitric acid (HNO3) was heated to 50°C, and the porous Si was then added thereto and mixed for 2 hours to prepare porous Si having Ag removed therefrom by filtering, washing, and drying the mixture. In this case, an average particle diameter (D50) of the porous Si was 5 μm and 25 a specific surface area (BET-SSA) was 18 m2/g. 29 [0088] [0089] The porous Si particles prepared in step iii) and having an average particle diameter (D50) of 5 μm and 5 a specific surface area (BET-SSA) of 18 m2/g were heat treated at about 800°C for 2 hours in air to prepare porous Si particles including an oxide layer of 20 nm thick SiOy (y=2) on the surface of the porous Si. 10 [0090] In this case, the amount of oxygen, for example, may be analyzed by secondary ion mass spectroscopy (SIMS) or high-frequency inductively coupled plasma (ICP). Also, the thickness of the oxide layer may be analyzed by a transmission electron microscope (TEM) or X-ray photoelectron 15 spectroscopy (XPS). [0091] Preparation Example 2 [0092] Porous Si particles including an oxide layer were prepared in the same manner as in Preparation Example 1 20 except that Si particles including an oxide layer of 40 nm thick SiOy (y=2) on the surface of the porous Si were obtained by heat treating porous Si particles at about 900°C for 2 hours in air in the of Preparation Example 1. 25 30 [0093] Preparation Example 3 [0094] Graphite and the porous Si particles including an oxide layer prepared in Preparation Example 1 were mixed at a weight ratio of 95:5 to be used as an anode active material. 5 [0095] Preparation Example 4 [0096] Graphite and the porous Si particles including an oxide layer prepared in Preparation Example 2 were mixed at a weight ratio of 95:5 to be used as an anode active material. 10 [0097] Comparative Example 1-1 [0098] The porous Si particles having an average particle diameter (D50) of 5 μm and a specific surface area (BET-SSA) of 18 m2/g, which were prepared in step iii) of Preparation 15 Example 1, were used as an anode active material. [0099] Comparative Example 1-2 [00100] The porous Si particles having an average particle diameter (D50) of 5 μm and a specific surface area (BET-SSA) 20 of 18 m2/g, which were prepared in step iii) of Preparation Example 1, were surface coated with carbon in an amount of 5 wt% based on a total weight of the anode active material by chemical vapor deposition and then used as an anode active material. 25 31 [00101] Comparative Example 1-3 [00102] Graphite and the porous Si particles having an average particle diameter (D50) of 5 μm and a specific surface area (BET-SSA) of 18 m2/g, which were prepared in step iii) of Preparation Example 1, were mixed at 5 a weight ratio of 95:5 to be used as an anode active material. [00103] Comparative Example 1-4 [00104] Non-porous Si particles and graphite were mixed at a 10 weight ratio of 5:95 to be used as an anode active material. [00105] Comparative Example 1-5 [00106] Si particles including an oxide layer of 20 nm thick SiOy (y=2) on the surface of Si were obtained by heat 15 treating non-porous Si particles at about 800°C for 2 hours in air. [00107] Graphite and the Si particles including an oxide layer were mixed at a weight ratio of 95:5 to be used as an anode active material. 20 [00108] Comparative Example 1-6 [00109] Graphite and the porous silicon particles having a carbon coating layer prepared in Comparative Example 1-2 were mixed at a weight ratio of 95:5 to be used as an anode active 25 material. 32 [00110] [00111] Example 1-1 [00112] The anode active material prepared in Preparation Example 1, an acetylene black conductive material, and 5 a polyvinylidene fluoride binder were mixed at a weight ratio of 80:10:10 in an N-methyl-2-pyrrolidone solvent to prepare a slurry. One surface of a copper current collector was coated with the prepared slurry to a thickness of 30 μm, dried and 10 rolled. Then, an anode was prepared by punching into a predetermined size. [00113] A non-aqueous electrolyte solution was prepared by adding 10 wt% of fluoroethylene carbonate based on a total amount of the electrolyte solution to a mixed solvent 15 including 1.0 M LiPF6 and an organic solvent which was prepared by mixing ethylene carbonate and diethyl carbonate at a weight ratio of 30:70. [00114] A lithium foil was used as a counter electrode, a polyolefin separator was disposed between both electrodes, 20 and a coin-type half cell was then prepared by injecting the electrolyte solution. [00115] Example 1-2 [00116] A coin-type half cell was prepared in the same manner 25 as in Example 1-1 except that the anode active material 33 prepared in Preparation Example 2 was used as an anode active material instead of using the anode active material prepared in Preparation Example 1. [00117] Comparative 5 ive Example 2-1 [00118] A coin-type half cell was prepared in the same manner as in Example 1-1 except that the anode active material prepared in Comparative Example 1-1 was used as an anode active material instead of using the anode active material 10 prepared in Preparation Example 1. [00119] Comparative Example 2-2 [00120] A coin-type half cell was prepared in the same manner as in Example 1-1 except that the anode active material 15 prepared in Comparative Example 1-2 was used as an anode active material instead of using the anode active material prepared in Preparation Example 1. [00121] Experimental Example 1: Capacity Characteristics, 20 Lifetime Characteristics, and Thickness Expansion Rate Analysis [00122] Capacity characteristics and lifetime characteristics of the coin-type half cells prepared in Examples 1-1 and 1-2 and Comparative Examples 2-1 and 2-2 according to charge and 25 discharge cycles were evaluated. 34 [00123] Specifically, the coin-type half cells prepared in Examples 1-1 and 1-2 and Comparative Examples 2-1 and 2-2 were charged at 0.1 C to a voltage of 0.005 V and a current of 0.005 C under constant current/constant voltage (CC/CV) conditions at 23°C, and then discharged at 0.1 C to 5 a voltage of 1.5 V under a constant current (CC) condition to measure capacities. Thereafter, the coin-type half cells were charged at 0.5 C to a voltage of 5 mV and a current of 0.005 C under constant current/constant voltage (CC/CV) conditions, 10 and then discharged at 0.5 C to a voltage of 1.0 V under a constant current (CC) condition. This charge and discharge cycle was repeated 1 to 50 times. The results thereof are presented in Table 1 below. [00124] Also, each coin-type half cell was disassembled in a 15 charge state of a 50th cycle and a thickness of an electrode was measured. Then, a thickness change rate was obtained by comparing the above thickness with a thickness of the electrode before the first cycle. The results thereof are presented in Table 1 below. 20 [00125] [Table 1] Examples Capacity (mAh/g) Lifetime characteristics (%) Thickness expansion rate (%) Example 1-1 2795 75.6 155 Example 1-2 2550 78.5 140 35 Comparative Example 2-1 3059 50.4 170 Comparative Example 2-2 2863 62.6 160 [00126] - Lifetime characteristics = (discharge capacity in a 50th cycle/ discharge capacity in the first cycle) x 100 [00127] - Thickness expansion rate = [(electrode thickness in a charge state of a 50th cycle – electrode thickness before 5 a first cycle)/ electrode thickness before the first cycle] x 100 [00128] As illustrated in Table 1, the cells including a porous silicon-based anode active material, in which an oxide 10 layer of SiOy (y=2) was formed on the surfaces of porous Si particles, as in Examples 1-1 and 1-2 had a slightly decreasing effect on the capacity characteristics, but had a significantly increasing effect on the lifetime characteristics and thickness expansion rate in comparison to 15 Comparative Example 2-1 including porous silicon particles, on which an oxide layer was not formed, as an anode active material and Comparative Example 2-2 including porous silicon particles having a carbon coating layer on the surfaces thereof as an anode active material. 20 [00129] Specifically, the lifetime characteristics of the cells of Examples 1-1 and 1-2 including a porous siliconbased anode active material, in which an oxide layer of SiOy 36 (y=2) was formed on the surfaces of porous Si particles, were increased by 50% or more in comparison to that of Comparative Example 2-1 including porous silicon particles, on which an oxide layer was not formed, as an anode active material. Also, the electrode thickness expansion 5 on rates of Examples 1-1 and 1-2 were decreased by 10% or more in comparison to that of Comparative Example 2-1. The above result was due to the fact that, in the case that an oxide layer was formed on the surfaces of porous Si particles as in Examples 1-1 and 1-2, 10 the lifetime characteristics were improved and the electrode thickness expansion rate was decreased due to a reduction in side reactions with the electrolyte solution. [00130] Also, the lifetime characteristics of the cells of Examples 1-1 and 1-2 were increased by about 20% or more in 15 comparison to that of Comparative Example 2-2 including porous silicon particles having a carbon coating layer on the surfaces thereof as an anode active material, and the electrode thickness expansion rates of Examples 1-1 and 1-2 were decreased by about 3% or more in comparison to that of 20 Comparative Example 2-2. From these results, it may be understood that the case of forming an oxide layer on the surface of porous Si had better effects of improving the lifetime characteristics and decreasing the electrode thickness expansion rate due to the reduction in side 25 reactions with the electrolyte solution than the case of 37 forming a carbon coating layer as in Comparative Examples 2-2. [00131] [00132] Example 2-1 [00133] The anode active material 5 erial prepared in Preparation Example 3, an acetylene black conductive material, a carboxymethyl cellulose thickener, and a styrene-butadiene rubber binder were mixed at a weight ratio of 97:1:1:1 in an N-methyl-2-pyrrolidone solvent to prepare slurry. One 10 surface of a copper current collector was coated with the prepared slurry to a thickness of 50 μm, dried and rolled. Then, an anode was prepared by punching into a predetermined size. [00134] A non-aqueous electrolyte solution was prepared by 15 adding 10 wt% of fluoroethylene carbonate based on a total amount of the electrolyte solution to a mixed solvent including 1.0 M LiPF6 and an organic solvent which was prepared by mixing ethylene carbonate and diethyl carbonate at a weight ratio of 30:70. 20 [00135] A lithium foil was used as a counter electrode, a polyolefin separator was disposed between both electrodes, and a coin-type half cell was then prepared by injecting the electrolyte solution. 25 [00136] Example 2-2 38 [00137] A coin-type half cell was prepared in the same manner as in Example 2-1 except that the anode active material prepared in Preparation Example 4 was used as an anode active material instead of using the anode active material prepared in 5 Preparation Example 3. [00138] Comparative Example 2-3 [00139] A coin-type half cell was prepared in the same manner as in Example 2-1 except that the anode active material 10 prepared in Comparative Example 1-3 was used as an anode active material instead of using the anode active material prepared in Preparation Example 3. [00140] Comparative Example 2-4 15 [00141] A coin-type half cell was prepared in the same manner as in Example 2-1 except that the anode active material prepared in Comparative Example 1-4 was used as an anode active material instead of using the anode active material prepared in Preparation Example 3. 20 [00142] Comparative Example 2-5 [00143] A coin-type half cell was prepared in the same manner as in Example 2-1 except that the anode active material prepared in Comparative Example 1-5 was used as an anode 25 active material instead of using the anode active material 39 prepared in Preparation Example 3. [00144] Comparative Example 2-6 [00145] A coin-type half cell was prepared in the same manner as in Example 2-1 except that the anode 5 node active material prepared in Comparative Example 1-6 was used as an anode active material instead of using the anode active material prepared in Preparation Example 3. 10 [00146] Experimental Example 2: Capacity Characteristics, Lifetime Characteristics, and Thickness Expansion Rate Analysis [00147] Capacity characteristics and lifetime characteristics of the coin-type half cells prepared in Examples 2-1 and 2-2 15 and Comparative Examples 2-3 to 2-6 according to charge and discharge cycles were evaluated in the same manner as in Experimental Example 1. [00148] The results thereof are presented in Table 2 below. [00149] [Table 2] Examples Capacity (mAh/g) Lifetime characteristics (%) Thickness expansion rate (%) Example 2-1 484.8 83.5 93.9 Example 2-2 483.2 82.2 92.1 Comparative Example 2-3 485.7 77.4 103.5 40 Comparative Example 2-4 480.5 70.1 140.5 Comparative Example 2-5 484.8 78.1 97.6 Comparative Example 2-6 478.5 80.1 101.5 [00150] As confirmed in Table 2, Examples 2-1 and 2-2 including a porous silicon-based anode active material, in which an oxide layer of SiOy (y=2) was formed on the surfaces of porous Si particles, with a carbon-based anode 5 ode active material had a significantly improving effect on the lifetime characteristics and electrode thickness expansion rate in comparison to Comparative Examples 2-3 to 2-6 as well as excellent capacity characteristics the same as or better than 10 those of Comparative Examples 2-3 to 2-6. With respect to Examples 2-1 and 2-2, capacity tended to decrease in comparison to Comparative Example 2-3, but the decreased value was within the range of error and was in a range that did not affect the capacity characteristics when used in the 15 secondary battery. [00151] Specifically, under the condition of including a carbon-based anode active material by mixing, the thickness expansion rate of the anode including porous Si particles having an oxide layer of SiOy (y=2) on the surfaces thereof 20 as in Examples 2-1 and 2-2 was decreased by about 7% to about 10% in comparison to that of Comparative Example 2-3 which 41 included the anode including porous Si particles on which an oxide layer was not formed, and was decreased by about 30% in comparison to that of Comparative Example 2-4 which included the anode including non-porous Si particles on which an oxide layer 5 was not formed. [00152] Also, with respect to the lifetime characteristics, the lifetime characteristics of Examples 2-1 and 2-2 including an oxide layer were improved by about 7% to about 11% in comparison to that of Comparative Example 2-3 which 10 included the anode including porous Si particles on which an oxide layer was not formed, and were improved by about 17% to about 19% in comparison to that of Comparative Example 2-4 which included the anode including non-porous Si particles on which an oxide layer was not formed. 15 [00153] Thus, it may be understood that since the side reactions with the electrolyte were reduced by forming an oxide layer on porous Si particles as in Examples 2-1 and 2-2, the lifetime characteristics were improved and the electrode expansion rate was decreased. 20 [00154] Also, under the same condition of including a carbonbased anode active material by mixing, in the case of including a porous silicon-based anode active material including an oxide layer of SiOy (y=2) on the surfaces of porous Si particles as in Examples 2-1 and 2-2, the lifetime 25 characteristics was increased by about 7% and the electrode 42 thickness expansion rate was decreased by about 4% or more in comparison to Comparative Example 2-5 including an anode active material in which an oxide layer was formed on nonporous Si particles. Also, the larger the thickness of the oxide layer was, the greater 5 the effect of improving the lifetime characteristics and reducing the thickness expansion rate was. From these results, it may understood that the lifetime characteristics and thickness expansion rate may vary depending on the presence of porosity of Si particles 10 coated even if an oxide layer was formed on the surfaces of the Si particles, and the effect of improving the lifetime characteristics and reducing the thickness expansion rate may be further increased when the coated Si particles was porous. [00155] Furthermore, under the same condition of including a 15 carbon-based anode active material by mixing, in the case of including a porous silicon-based anode active material including an oxide layer of SiOy (y=2) on the surfaces of porous Si particles as in Examples 2-1 and 2-2, the lifetime characteristics was increased by about 2% or more and the 20 electrode thickness expansion rate was decreased by about 3% or more in comparison to Comparative Example 2-6 including an anode active material in which a carbon coating layer was formed on porous Si particles. From these results, it may be understood that, as a coating layer on the porous Si 25 particles, the oxide layer had better effects of improving 43 the lifetime characteristics and decreasing the electrode thickness expansion rate than the carbon coating layer. [00156] Thus, from the results of Table 2, it may be confirmed that, with respect to Examples 2-1 and 2-2, the lifetime characteristics and electrode thickness expansio5 n rate were significantly improved while maintaining the capacity characteristics. INDUSTRIAL APPLICABILITY [00157] Since an anode active material according to an 10 embodiment of the present invention includes porous SiOx particles (0≤x<2) having surfaces coated with an oxide layer, a reactivity between the anode active material and an electrolyte solution may be reduced and, as a result, an electrical short circuit in an electrode may be minimized. 15 Also, since a plurality of pores is included in surfaces or the surfaces and inside of the SiOx particles, a thickness change rate of the electrode generated during charge and discharge of a secondary battery may be reduced and lifetime characteristics may be improved. Accordingly, the anode 20 active material may be used in an anode for a lithium secondary battery and a lithium secondary battery including the anode, and the lithium secondary battery may not only be used in a battery cell that is used as a power source of a small device, but may also be used as a unit cell in a medium 25 and large sized battery module including a plurality of 44 battery cells. I/We Claim: 1. An anode active material comprising porous SiOx particles (0≤x<2), wherein the porous SiOx particles comprise an oxid5 e layer coated on surfaces thereof. 2. The anode active material of claim 1, wherein the oxide layer comprises SiOy (0