Concentration gradient-type porous silicon-based anode active material and sodium ion secondary battery comprising same

Abstract

The present disclosure provides a concentration gradient-type porous silicon-based anode active material, a method for manufacturing same, and a sodium ion secondary battery comprising an anode to which the concentration gradient-type porous silicon-based anode active material is applied, the concentration gradient-type porous silicon-based anode active material including silicon oxide (SiO2) having an oxidation number of 2+, which is distributed mainly inside particles (or in central regions), and silicon oxide in a reduction state or elemental silicon (SiOx(0≤x<2)), which is distributed mainly on the surfaces (or surface regions) of the particles.

Description

Concentration gradient type porous silicon-based negative electrode active material and sodium ion secondary battery including the same

[0001] The present disclosure relates to a concentration gradient porous silicon-based negative electrode active material and a sodium-ion secondary battery comprising the same. More specifically, the present disclosure describes a silicon oxide (SiO2) having an oxidation state of 2+ distributed mainly within the interior (or central region) of a particle, while silicon oxide in a reduced state and / or elemental silicon (SiO2) is mainly distributed on the surface (or surface region) of the particle. x The present invention relates to a concentration gradient porous silicon-based negative electrode active material with a distribution of (0≤x<2)), a method for manufacturing the same, and a sodium ion secondary battery comprising an anode to which the concentration gradient porous silicon-based negative electrode active material is applied.

[0002] Due to the recent explosive increase in demand for mobile devices, electric vehicles, and energy storage systems, the secondary battery market, particularly lithium-ion batteries, is on a continuous expansion trend.

[0003] Currently, lithium-ion batteries are the most widely used due to their advantages, such as high energy density, long cycle life, and good charge / discharge efficiency. However, as the need for large-scale applications increases over the long term, there is a demand for alternatives to replace them. In particular, as lithium sources are unevenly distributed globally, prices are continuously rising. Furthermore, as applications expand into high-capacity systems such as electric vehicles and energy storage systems, it is becoming difficult to meet the growing demand for energy storage using lithium-ion batteries alone.

[0004] As an alternative to this, research is underway on sodium-based secondary batteries that have a mechanism similar to that of lithium-ion batteries, are abundant globally, and are available at a low price (Chemistry, 2023 Jan 12;29(3):e202202380).

[0005] Sodium and lithium are both elements belonging to the alkali metals and have similar physicochemical properties. In particular, since sodium sources are abundant worldwide, utilizing them can provide a solution to the problem of lithium source shortages.

[0006] Sodium-ion secondary batteries utilize sodium ions, which are larger in size than lithium, as charge storage materials. Similar to lithium-ion batteries, carbonaceous materials (e.g., hard carbon) with good capacity and relatively long-term stability are used as anodes. Recently, research on anode materials that can replace carbon-based anodes and provide various improvements is also actively underway. In particular, there is increasing interest in silicon-based anodes, which can achieve higher capacity and higher capacity retention rates than carbon-based materials, with the goal of realizing high capacity and high capacity retention rates.

[0007] However, when an anode containing a silicon-based negative electrode active material is applied to a conventional sodium-ion secondary battery, the sodium ion diffusion path to the silicon inside the negative electrode particle matrix is ​​long during the charging process. Consequently, sodium cannot effectively diffuse and be stored inside the particles, resulting in low charge / discharge capacity and reduced ion conductivity, which in turn degrades rate capability. Furthermore, silicon metal is located in the deep matrix where ion diffusion does not reach, which can increase the cost of particles per unit of energy. Even if ion diffusion proceeds, excessive volume expansion caused by alloying with sodium leads to particle collapse, and additional irreversible reactions occur, resulting in a decrease in Coulomb efficiency. Additionally, battery degradation and short-circuit problems cannot be avoided due to electrode collapse.

[0008] Therefore, a solution is required to address the problems associated with applying an anode containing a silicon-based negative electrode active material to a sodium-ion secondary battery.

[0009] In one embodiment of the present disclosure, we aim to provide a silicon-based negative electrode active material suitable as an anode for a sodium-ion secondary battery and having properties that are further improved compared to the prior art, and a method for manufacturing the same.

[0010] In another embodiment of the present disclosure, an anode based on an improved silicon-based negative electrode active material and a sodium-ion secondary battery including the same are provided.

[0011] According to the first aspect of the present disclosure,

[0012] An internal region of the particle where silicon oxide (SiO2) is predominantly distributed; and

[0013] Reduced silicon oxide and / or elemental silicon ((SiO₂) x Particle surface region where (0≤x<2)) is predominantly distributed;

[0014] A porous silicon-based negative electrode active material with a concentration gradient including is provided.

[0015] According to an exemplary embodiment, the particle diameter of the porous silicon-based negative electrode active material may be in the range of 0.01 to 100 μm.

[0016] According to an exemplary embodiment, the specific surface area (BET) and pore volume of the porous silicon-based negative electrode active material may be at least 100 m² / g and at least 0.1 cm³ / g, respectively, when measured by the nitrogen adsorption method.

[0017] According to an exemplary embodiment, the pore diameter of the porous silicon-based negative electrode active material may be in the range of 0.1 to 5000 nm.

[0018] According to an exemplary embodiment, the tap density of the porous silicon-based negative electrode active material may be 0.5 to 2.7 g / cm³.

[0019] According to an exemplary embodiment, the ratio of the internal region of the silicon oxide (SiO2) state particles in the porous silicon-based negative electrode active material may be in the range of 1 to 70 volume%.

[0020]

[0021] According to the second aspect of the present disclosure,

[0022] A method for manufacturing a concentration gradient type porous silicon-based negative electrode active material,

[0023] a) by performing a reduction heat treatment on a reaction mixture comprising silicon oxide (SiO2) particles and metal powder as a reducing agent under an inert gas, reducing gas, a mixture thereof, or a reduced pressure atmosphere, thereby (i) an internal region of the particles in which silicon oxide (SiO2) is predominantly distributed and (ii) reduced silicon oxide and / or elemental silicon ((SiO2) x A step of forming a silicon-based particle of a concentration gradient including a particle surface region in which (0≤x<2)) is predominantly distributed; and

[0024] b) a step of performing acid treatment on the above-mentioned concentration gradient silicon-based particles to form concentration gradient porous silicon-based particles with increased porosity;

[0025] A method for manufacturing a porous silicon-based negative electrode active material with a concentration gradient including is provided.

[0026] According to an exemplary embodiment, step a) above is,

[0027] a1) a step of providing a metal powder as a silicon oxide (SiO2) precursor and a reducing agent, respectively; and

[0028] a2) A step of providing a reaction mixture by applying mechanical energy to the silicon oxide (SiO2) precursor and metal powder to grind them;

[0029] It may include.

[0030] According to an exemplary embodiment, the metal powder is at least one selected from the group consisting of magnesium, aluminum, iron, zinc, tin, potassium, calcium, titanium, nickel, and chromium, and the weight ratio of the silicon oxide (SiO2) precursor to the metal powder can be controlled in the range of 1:0.3 to 2.

[0031] According to an exemplary embodiment, in step a2), at least one selected from the group consisting of sodium chloride, potassium chloride, magnesium chloride, barium chloride, and calcium chloride is further added as a heat absorbent, ground, and mixed, and the weight ratio of the silicon oxide (SiO2) precursor to the heat absorbent can be controlled in the range of 1:0.5 to 3.

[0032] According to an exemplary embodiment, the mechanical energy may be provided using at least one grinding device selected from the group consisting of a ball mill, a jet mill, a cutter mill, a turbomill, and a disc mill.

[0033] According to an exemplary embodiment, the reduction heat treatment can be performed at a temperature of 300 to 1500 ℃ by heating at a heating rate of 1 to 100 ℃ / min.

[0034] According to an exemplary embodiment, the inert gas during the reduction heat treatment may be at least one selected from the group consisting of nitrogen, helium, neon, argon, krypton, and xenon.

[0035] According to an exemplary embodiment, the reducing gas for the reducing heat treatment may be at least one selected from the group consisting of hydrogen, carbon monoxide, methane, ammonia, formic acid, and methanol.

[0036] According to an exemplary embodiment, the reduction heat treatment is at least 10 2 to 10 -8 It can be carried out in a reduced pressure atmosphere of Pa.

[0037] According to an exemplary embodiment, the acid treatment of step b) may include the step of adding acid to a dispersion of silicon-based particles of a concentration gradient and maintaining it at 10 to 100°C.

[0038] According to an exemplary embodiment, the acid used in the acid treatment may be at least one selected from the group consisting of hydrochloric acid, nitric acid, hydrofluoric acid, sulfuric acid, phosphoric acid, chloric acid, acetic acid, and iodic acid.

[0039]

[0040] According to the third aspect of the present disclosure,

[0041] Anode whole; and

[0042] A layer containing the aforementioned porous silicon-based negative electrode active material formed on the above-mentioned anode current collector;

[0043] An anode for a sodium-ion secondary battery comprising is provided.

[0044] According to an exemplary embodiment, the capacity of the anode may be in the range of at least 50 mAh / g.

[0045]

[0046] According to the fourth aspect of the present disclosure,

[0047] Cathode;

[0048] Anode; and

[0049] An electrolyte layer disposed between the above cathode and anode;

[0050] Includes,

[0051] At this time, the anode is provided as a sodium ion secondary battery, which is the anode described above.

[0052] A porous silicon-based negative electrode active material according to a specific embodiment of the present disclosure introduces a concentration gradient characteristic that changes the oxidation state within silicon oxide particles and simultaneously maximizes the specific surface area of ​​the particles, thereby effectively overcoming the technical limitations associated with the application of conventional silicon-based negative electrode active materials when applied to the anode of a sodium-ion secondary battery.

[0053] Specifically, the concentration gradient porous silicon-based anode active material can secure structural stability by predominantly arranging silicon, which has sodium ion storage capacity, outside the matrix and increasing the specific surface area of ​​the particles to maximize the sodium ion storage capacity that is mainly expressed on the surface of the silicon-based anode active material due to low ion conductivity, and predominantly arranging silica, an electrically and chemically inert material, inside the matrix where sodium ion conduction is difficult.

[0054] In particular, it is possible to improve charge / discharge capacity and capacity retention rate compared to conventional silicon-based anode active materials through efficient diffusion of sodium ions, and since the region inside the silicon-based anode material where sodium ion diffusion is difficult is composed of relatively inexpensive silica, it offers the advantage of reducing the cost per unit of energy.

[0055] FIG. 1 is a flowchart showing a series of processes for manufacturing a concentration gradient porous silicon-based negative electrode active material according to an exemplary embodiment;

[0056] FIG. 2 is a diagram schematically showing the distribution characteristics of a silicon component in a concentration gradient silicon-based negative electrode active material according to an exemplary embodiment; and

[0057] FIGS. 3a to 3c are scanning electron microscope images of silicon-based negative electrode active materials prepared according to Example 1, Example 2, and Comparative Example 1, respectively;

[0058] FIG. 4 is a graph showing the electrochemical test results of silicon-based negative electrode active materials prepared according to Example 1, Example 2 and Comparative Example 1, respectively (current density: 500 mA / g).

[0059] The present invention can be fully achieved by the following description. The following description should be understood as describing preferred embodiments of the present invention, but the present invention is not necessarily limited thereto. Furthermore, the attached drawings are for illustrative purposes only and do not limit the present invention; details regarding individual components can be appropriately understood in accordance with the specific intent of the relevant descriptions provided below.

[0060] "Cathode (positive electrode)" and "anode (negative electrode)" respectively refer to the electrodes of a battery; in the charging cycle of a sodium-ion secondary battery, Na ions (Na + ) separates from the cathode (positive electrode) and moves to the anode (negative electrode), while electrons move from the positive electrode to the negative electrode through the external circuit. On the other hand, in a discharge cycle, Na ions move from the anode to the cathode, while electrons detach from the anode and move to the cathode through the external circuit.

[0061] "Prussian blue analogue (PBA)" is a type of metal-organic framework (MOF) with the unit molecular formula A x M P [M R (CN)6]·wH2O (A: alkali metal cation, M P , M R : transition metal cation, H2O: crystal water). The lattice structure of PBA can be represented by two types of transition metal cations (M P , M R Each of the 6 cyanide ligands (CN - It is formed by coordinate bonding with ) in an octahedral structure, and a cyanide ligand acts as a bridge to form two transition metal cations (M P , M RIt exhibits a face-centered cubic structure connecting the elements. Therefore, PBA has an open framework, and as a result, the interstitial sites and ion diffusion channels within the crystal are wider compared to other crystalline inorganic materials.

[0062] "Loading" or "coating" can be understood as a concept that includes not only cases where it is formed in the form of an externally distinct layer or film on the surface of a specific structure, but also cases where it is formed in a form that penetrates into pores or voids formed in the surface area of ​​the structure.

[0063] "Electrolyte" can refer to a substance that acts as an ion-conducting medium, enabling the movement of electrons and cations within electrochemical devices, such as secondary batteries.

[0064] The term “predominant” may mean that a particular component exceeds, for example, at least about 50 weight%, specifically at least about 60 weight%, at least about 70 weight%, at least about 80 weight%, at least about 90 weight%, at least about 95 weight%, at least about 97 weight%, at least about 99 weight%, or substantially about 100 weight%.

[0065] "Nitrogen adsorption method" or "BET (Brunauer-Emmett-Teller) analysis" may refer to an analytical method that measures the specific surface area and pore size distribution of a solid sample by adsorbing and desorbing nitrogen onto the surface of the solid sample and measuring the amount of adsorption at different partial pressures.

[0066] "Microscale" can be understood as referring to particles having a diameter (or grain size) from about 500 nm to about 5 µm, specifically from about 800 nm to about 3 µm, from the nanometer level to the micrometer level.

[0067] "Capacity" refers to a measured value of the total charge that can be delivered by a battery; it determines the run time that the battery can provide under given load conditions and can be calculated by multiplying the delivered current by the delivery time. Additionally, specific capacity can be defined as the total capacity per unit weight or volume of the battery and is typically expressed in Ah / g or Ah / cm³.

[0068] "Capacity retention rate" may refer to the ratio of the reversible capacity to the initial capacity available from the battery under specific discharge conditions after being cycled or stored for a certain period.

[0069] In a narrow sense, "reduction" may refer to any conversion reaction that removes oxygen bound within a specific compound or substance, for example, a thermochemical reaction.

[0070] When any range of numbers is mentioned in this invention, all numbers within the range are specifically included in this invention.

[0071] In a narrow sense, "contact" means direct contact between two objects, but in a broad sense, it can be understood as allowing any additional component to be involved.

[0072] Where a numerical range is specified as a lower limit and / or an upper limit in this specification, it may be understood that any sub-combination within said numerical range is also disclosed. For example, if "1 to 5" is written, it may include 1, 2, 3, 4 and 5, as well as any sub-combination between them.

[0073] The expressions "on" and "above" can be understood as being used to refer to concepts of relative position. Therefore, this includes not only cases where other components or layers exist directly on the mentioned layer, but also cases where other layers (intermediate layers) or components are interposed or exist between them, or forms formed by partially overlapping in the space between the relevant members. Similarly, expressions such as "below," "in the lower part," and "below," as well as the expression "between," can also be understood as concepts of relative position. Furthermore, the expression "sequentially" can also be understood as a concept of relative position.

[0074] In this specification, when a component is described as "comprising," it means that, unless otherwise noted, it may further include other components and / or steps.

[0075]

[0076] Concentration gradient type porous silicon-based negative electrode active material and preparation thereof

[0077] According to the present disclosure, a silicon-based negative electrode active material applied as a negative electrode active material of an anode of a sodium-ion secondary battery is provided. A flowchart of a series of processes for manufacturing a concentration gradient porous silicon-based negative electrode active material according to an exemplary embodiment is shown in FIG. 1.

[0078] Referring to the drawings above, a reaction mixture comprising silicon oxide (SiO2) particles as a precursor (or starting material) and metal powder as a reducing agent is first provided.

[0079] At this time, silicon oxide having an oxidation state of 2+, i.e., silica (SiO2), can be used as a silicon-containing precursor, and such silica can typically be in the form of amorphous particles. The silica precursor can be prepared according to a separate procedure, but commercially available types may be used.

[0080] In this regard, the properties of silica as a precursor, particularly its porosity, can be measured by the nitrogen adsorption method.

[0081] As an example, the specific surface area (BET) of the silica precursor may be in the range of, for example, about 10 to 700 m² / g, specifically about 50 to 500 m² / g, more specifically about 100 to 350 m² / g. In addition, the pore volume of the silica precursor may be in the range of, for example, about 0.1 to 0.8 cm³ / g, specifically about 0.3 to 0.6 cm³ / g, more specifically about 0.4 to 0.46 cm³ / g. Furthermore, the pore diameter of the silica precursor may be in the range of, for example, about 0.1 to 2000 nm, specifically about 0.5 to 1000 nm, more specifically about 2 to 500 nm, as it can be measured from an image obtained from, for example, a high-resolution transmission electron microscope (HR-TEM).

[0082] According to an exemplary embodiment, a metal may be used as a reducing agent to reduce the surface area of ​​a silica precursor, and such metal may be any type capable of reducing silica by heating, that is, removing at least some of the oxygen in the silica to convert it to an oxidation state of less than 2+ (a type having a higher chemical affinity for oxygen than silica).

[0083] According to exemplary embodiments, the metal used as a reducing agent may be in powder form, for example, in a range of about 10 to 200 μm, specifically about 20 to 150 μm, more specifically about 30 to 80 μm, but this should be understood as exemplary.

[0084] According to an exemplary embodiment, the metal material used as the reducing agent may be at least one selected from, for example, magnesium, aluminum, iron, zinc, tin, potassium, calcium, titanium, nickel, chromium, etc., and more specifically, magnesium may be used. In this regard, using magnesium as the reducing agent may be advantageous because effective reduction can be performed at a relatively low temperature (e.g., about 500 to 700 °C) below the melting point of silicon (1,414 °C), thereby preserving the microstructure within the silica precursor.

[0085] According to exemplary embodiments, the weight ratio of silicon oxide (SiO2) precursor to metal reducing agent in the reaction mixture can be determined by taking into account the target degree of reduction, for example, in the range of 1: about 0.3 to 2, specifically 1: about 0.5 to 1.5, more specifically 1: about 0.6 to 1.2, and particularly specifically 1: about 0.7 to 1.1, but this should be understood as being for illustrative purposes.

[0086] According to an exemplary embodiment, since overheating may occur due to an exothermic reaction during the thermal reduction process, a heat absorber may be further added to the reaction mixture along with the aforementioned silicon oxide (SiO2) precursor and metal reducing agent. This heat absorber may be selected from a type of compound with thermal behavior that limits the exothermic reaction of the thermal reduction process, for example, at least one selected from sodium chloride, potassium chloride, magnesium chloride, barium chloride, calcium chloride, etc. At this time, the ratio of silicon oxide (SiO2) precursor to heat absorber may be determined by considering the target degree of reduction and degree of crystallinity, and may be in the range of, for example, 1: about 0.5 to 3, specifically 1: about 1 to 2.7, more specifically 1: about 1.5 to 2.5, and particularly specifically 1: about 1.8 to 2.2 on a weight basis, but this should be understood as being for illustrative purposes.

[0087] According to an exemplary embodiment, when the aforementioned reaction mixture components are prepared, a reaction mixture is prepared therefrom. At this time, the reaction mixture can be prepared by applying mechanical energy (e.g., milling, grinding, etc.) to grind the components, which is due to the maximization and homogenization of the reaction surface area. The means for applying the aforementioned mechanical energy, specifically the grinding means, is known in the art and may be, for example, a ball mill (specifically a planetary ball mill), a jet mill, a cutter mill, a turbo mill, a disc mill, etc., and as a result, a mixture for reduction heat treatment in which the individual components are uniformly mixed can be prepared.

[0088] According to exemplary embodiments, the grinding process during the preparation of the mixture may be performed for, for example, about 30 to 600 minutes, specifically about 60 to 300 minutes, more specifically about 120 to 180 minutes, but is not limited thereto.

[0089] Referring again to Fig. 1, once the reaction mixture is prepared, a reduction heat treatment is performed to produce a concentration gradient silicon-based particle.

[0090] In this embodiment, through reduction heat treatment, the central region of the silica precursor particles is predominantly maintained in a silicon oxide (SiO2) state with oxidation state 2+, while the reduced state gradually increases toward the surface, and in particular, in the surface region, silicon oxide in a reduced state and / or elemental silicon may be predominantly distributed (concentration gradient silicon-based particles are formed).

[0091] According to an exemplary embodiment, when a reduction heat treatment is performed using a metal (or metal powder) as a reducing agent as described above, the surface region of the silica particles is reduced silicon oxide and / or elemental silicon (SiO₂). x It can be converted to (0≤x<2)) (when x is 0, it is elemental silicon, and 0 <x<2인 경우에는 산화수 2+ 미만의 실리콘 산화물로 전환됨).

[0092] In this regard, when magnesium is used as a reducing agent, the mechanism by which silica is converted into an elemental form by reduction can be carried out by a simple reduction reaction between silica (SiO2) and magnesium as shown in Reaction Scheme 1 below.

[0093]

[0094] [Reaction Equation 1]

[0095] SiO2 + 2Mg → 2MgO + Si

[0096]

[0097] In addition, silica adjacent to the magnesium reducing agent can be converted into a magnesium silicide (Mg2Si) byproduct as shown in reaction scheme 2 below.

[0098]

[0099] [Reaction Equation 2]

[0100] Si + 2Mg → Mg2Si

[0101]

[0102] According to exemplary embodiments, the above-described reaction may occur spontaneously upon heating, wherein the reduction heat treatment temperature may be determined by considering the reducing agent reaction temperature, control of byproduct formation, and the target degree of reduction. As an example, the heat treatment temperature may be controlled in the range of, for example, about 300 to 1500 °C, specifically about 450 to 1200 °C, and more specifically about 600 to 800 °C. In addition, the heating rate for heating may be controlled in the range of, for example, about 1 to 100 °C / min, specifically about 3 to 50 °C / min, and more specifically about 5 to 20 °C / min. The above-described heating conditions may be understood as exemplary.

[0103] Meanwhile, the reduction heat treatment may be performed under an inert gas, a reducing gas, a mixture thereof, or a reduced pressure atmosphere; examples of such inert gases may include nitrogen, helium, neon, argon, krypton, xenon, etc., and examples of reducing gases may include hydrogen, carbon monoxide, methane, ammonia, formic acid, methanol, etc., and one of the types listed above may be applied alone or in a combination of two or more. The reduced pressure atmosphere is, for example, about 10 2 to 10 -8 Pa, specifically 10 to 10 -6 Pa, more specifically 10 -1 to 10 -5 It can be controlled within the range. According to a specific embodiment, the atmosphere-forming gas may be argon.

[0104] According to exemplary embodiments, the heat treatment time is not particularly limited and can be adjusted according to the desired degree of reduction on the surface of the silica particles. As an example, the heat treatment time can be adjusted in the range of at least about 0.5 hours, specifically about 1 to 12 hours, more specifically about 2 to 8 hours, and particularly specifically about 3 to 4 hours, but this should be understood as exemplary.

[0105] According to exemplary embodiments, when magnesium is used as a reducing agent, problems such as a decrease in porosity and specific surface area can be mitigated, as the reduction method of silica using carbon requires high-temperature treatment, which affects the physical properties of silicon oxide, resulting in the melting or sintering of silicon oxide. In particular, when magnesium reduction is applied, it may be advantageous in that the reduction product (by-product) can be effectively removed by acid during the subsequent acid treatment step.

[0106] Concentration gradient characteristics can be imparted to the silica precursor through the aforementioned reduction heat treatment.

[0107] Referring to FIG. 1, silicon oxide particles whose surface oxidation characteristics have been altered through a reduction heat treatment step can have their porosity increased through subsequent acid treatment (forming porous silicon-based particles with a concentration gradient).

[0108] According to an exemplary embodiment, the acid used for acid treatment may be selected from a type having good solubility for the reduction product (oxide), and may be at least one selected from hydrochloric acid, nitric acid, hydrofluoric acid, sulfuric acid, phosphoric acid, chloric acid, acetic acid, iodic acid, etc. According to a specific embodiment, hydrochloric acid may be used as the acid.

[0109] At this time, for acid treatment, the reduction heat treatment product may be introduced into a solvent to form a dispersion under stirring or non-stirring conditions and / or sonication conditions. At this time, the solvent that can be used to prepare the dispersion may be, for example, water (specifically deionized water), alcohol (as an aliphatic alcohol, for example, having 1 to 5 carbon atoms, specifically having 2 to 4 carbon atoms, particularly specifically ethanol), or a mixture thereof. When preparing the dispersion, for example, about 5 to 100 times, specifically about 8 to 70 times, more specifically about 10 to 30 times the amount of solvent may be used relative to the reduction-treated particles.

[0110] According to an exemplary embodiment, acid treatment can be performed by adding acid to a dispersion (which may contain by-products such as MgO and Mg2Si). For example, depending on the addition of acid, the concentration of acid in the total dispersion may be in the range of, for example, about 0.5 to 5 M, specifically about 1 to 4 M, more specifically about 2 to 3 M.

[0111] According to exemplary embodiments, conditions during acid treatment can be adjusted considering the desired level of porosity increase, and the acid treatment temperature can be set, for example, in the range of about 10 to 100°C, specifically about 30 to 90°C, more specifically about 50 to 80°C, and particularly specifically about 60 to 70°C, but this should be understood as exemplary. In addition, the acid treatment time can be adjusted, for example, in the range of about 0.1 to 12 hours, specifically about 0.5 to 8 hours, more specifically about 1 to 5 hours, and particularly specifically about 1.5 to 4 hours. However, the aforementioned acid treatment conditions should be understood as exemplary and may be changed depending on the acid used, the acid concentration in the dispersion, etc.

[0112] Meanwhile, by-products generated during the reduction heat treatment can be effectively detached or removed from silicon-based particles during the acid treatment process and can be removed externally through a subsequent washing or cleaning process. Such washing may be, for example, water washing and may be performed two or more times if necessary; since post-treatment processes such as washing are known in the art, a detailed description will be omitted.

[0113] In addition, a normal drying step may be performed after the washing is completed. This drying is not limited to a specific method, and various methods such as high-temperature drying, hot air drying, and reduced-pressure drying may be applied. For example, drying may be performed under elevated temperature conditions, at which time the drying temperature can be controlled in the range of, for example, about 40 to 100°C, specifically about 50 to 80°C, and more specifically about 55 to 70°C.

[0114] Through the process described above, as illustrated in FIG. 2, the oxidized state of the starting material precursor, i.e., silicon oxide (SiO2), is predominantly distributed within the particle, while the proportion of the reduced state increases toward the particle surface, so that at the particle surface, instead of silicon oxide (or silica) with an oxidation state of 2+, reduced silicon oxide and / or elemental silicon (SiO2) x A silicon-based negative electrode active material in which (0≤x<2)) is predominantly distributed can be obtained.

[0115] According to an exemplary embodiment, the internal region of the porous silicon-based negative electrode active material particle is occupied by silicon oxide (SiO2), and based on the total particle, the proportion of the internal region composed of silicon oxide (SiO2) may be, for example, about 1 to 70 volume%, specifically about 3 to 40 volume%, more specifically about 5 to 20 volume%, but the present disclosure is not limited thereto.

[0116] According to exemplary embodiments, the particle diameter of the concentration gradient type porous silicon-based negative electrode active material can vary depending on the degree of grinding, etc., during the preparation of the reaction mixture; for example, it may be in the range of about 0.01 to 100 μm, specifically about 0.1 to 50 μm, and more specifically about 1 to 8 μm. If the particle diameter of the negative electrode active material is smaller than 0.01 μm, the particles are prone to detaching from the electrode, and the total surface area of ​​the particles becomes excessively large, causing excessive viscosity, which reduces the workability of the electrode slurry and may cause a degradation in battery performance due to passing through the separator. If the particle diameter is larger than 100 μm, uniformity is reduced during electrode slurry coating, and tap density is lowered, which reduces electrode density and lowers the energy density of the battery. Since the particle diameter of the negative electrode active material can affect electrode workability and initial charge / discharge efficiency, it may be controlled within the aforementioned range, but the present disclosure is not limited thereto.

[0117] In addition, according to an exemplary embodiment, the porous silicon-based negative electrode active material may exhibit significantly increased porosity as described above, and the specific surface area (BET) may be in the range of, for example, at least about 100 m² / g, specifically about 200 to 900 m² / g, more specifically about 400 to 800 m² / g when measured by the nitrogen adsorption method. In addition, the pore volume may be in the range of, for example, at least about 0.1 cm³ / g, specifically about 0.3 to 0.9 cm³ / g, more specifically about 0.5 to 0.8 cm³ / g when measured by the nitrogen adsorption method. If the specific surface area of ​​the porous silicon-based negative electrode active material is less than 100 m² / g or the pore volume is less than 0.1 cm³ / g, the sodium ion storage capacity is weak, and the energy density per unit mass and unit volume of the battery decreases.

[0118] In addition, the pore diameter of the porous silicon-based negative electrode active material may be increased through reduction heat treatment and acid treatment, for example, in the range of about 0.1 to 5000 nm, specifically about 0.5 to 1000 nm, and more specifically about 1 to 100 nm. If the pore diameter of the porous silicon-based negative electrode active material is smaller than 0.1 nm, it is difficult for the electrolyte to sufficiently penetrate between the particles, thereby lowering the rate capability and reducing the sodium ion storage capacity. If the pore diameter is larger than 5000 nm, the tap density of the particles decreases, which lowers the electrode density and reduces the energy density of the battery.

[0119] Meanwhile, the porous silicon-based negative electrode active material according to the present embodiment changes from a silica precursor to silicon oxide in a reduced state and / or silicon in an elemental state, and as porosity is secured, the tap density also changes, for example, in the range of about 0.5 to 2.7 g / cm³, specifically about 0.6 to 2.1 g / cm³, and more specifically about 0.7 to 1.5 g / cm³. At this level of tap density, as described below, the thickness of the negative electrode active material structure formed during anode manufacturing can be minimized to shorten the transport path of sodium ions, and sodium ions can be efficiently inserted into the negative electrode active material layer. In addition, the energy density of the battery can be increased by increasing the electrode density during electrode fabrication.

[0120] In addition, for electrode workability and yield of good products, it may be advantageous for the negative electrode active material particles to exhibit a uniform size distribution; thus, the porous silicon-based negative electrode active material according to the present embodiment d 50 The range may be, for example, about 0.1 to 80 μm, specifically about 1 to 50 μm, more specifically about 3 to 20 μm, but this can be understood as being for illustrative purposes.

[0121] In the case of the silicon-based negative electrode active material according to the present embodiment, through reduction heat treatment and acid treatment, the porosity of the particles (e.g., specific surface area, etc.) is significantly increased compared to the starting material, and by contributing to the diffusion of sodium ions and the increase in the reaction area, the capacity of the sodium-ion secondary battery can be increased, and the advantages of excellent output characteristics and rate capability characteristics can be provided. Considering that when the negative electrode active material according to the present embodiment is applied to a lithium secondary battery, the aforementioned advantages are not derived due to excessive volume expansion resulting from the expansion of the reaction area, the unique advantages achieved when applied to a sodium-ion secondary battery are noteworthy.

[0122] For example, the capacity of the silicon-based negative electrode active material according to the present embodiment may be in the range of, for example, about 50 to 900 mAh / g, specifically about 100 to 600 mAh / g, more specifically about 150 to 300 mAh / g, which may be approximately 1.5 to 3 times higher than that of a previously known silicon-based negative electrode active material.

[0123]

[0124] Manufacturing of anodes

[0125] According to another embodiment of the present disclosure, an anode for a sodium-ion secondary battery is provided by applying the aforementioned concentration gradient porous silicon-based particles as a negative electrode active material. In this case, the negative electrode active material may be loaded onto a negative electrode current collector. A process for manufacturing an anode using the negative electrode active material is known in the art, and, for example, coating using a slurry, deposition, spray coating, casting, etc., may be applied. An exemplary description thereof is as follows.

[0126] First, a concentration gradient porous silicon-based negative electrode active material, a conductive material, and a binder are introduced into a solvent and mixed to prepare a negative electrode active material slurry (or composition). Subsequently, the slurry is loaded or coated onto the surface of a negative electrode current collector, and an anode can be manufactured through a pressing / drying (or drying) process. Alternatively, the anode may be manufactured by casting the negative electrode active material slurry (or composition) onto a separate support and then laminating the film obtained by peeling it off from the support onto a current collector.

[0127] According to an exemplary embodiment, the slurry coating can be performed by applying it onto a current collector using a coating device known in the art (e.g., a slurry tank, a slurry pump, etc.). In addition, the pressing process can be performed, for example, by a roll pressing method, and the thickness of the electrode layer and the loading amount can be controlled by adjusting the gap with the roll.

[0128] According to exemplary embodiments, the negative electrode current collector may be selected from types that are conductive without causing chemical changes in the sodium-ion secondary battery, such as gold, stainless steel, nickel, aluminum, titanium, copper, and alloys thereof. According to specific embodiments, the material of the negative electrode current collector may be aluminum or an aluminum alloy. According to other embodiments, a material in which the surface of aluminum or stainless steel is surface-treated with carbon, nickel, titanium, copper, silver, etc. may be used. Additionally, fine irregularities may be formed on the surface to increase the bonding strength of the negative electrode active material, and it may be used in various forms such as films, sheets, foils, foams, and nonwoven fabrics.

[0129] According to an exemplary embodiment, the thickness of the negative current collector is not particularly limited, but may be in the range of, for example, about 2 to 700 μm, specifically about 5 to 500 μm, more specifically about 10 to 300 μm.

[0130] According to an exemplary embodiment, the binder usable when preparing a cathode active material slurry may be of a type known in the art, for example, a polymeric binder. For example, a fluorine-containing resin may be used, examples of which may include polytetrafluoroethylene (PTFE), polyvinylene difluoride (PVdF), poly(vinylene difluoride-hexafluoropropylene) copolymer (PVdF-HFP), etc. According to an exemplary embodiment, a thermoplastic resin may be used as a binder, for example, styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), nitrile-butylene rubber (NBR), polybutadiene, polyisoprene, etc. According to exemplary embodiments, acrylic resins may be used, for example, polyacrylic acid, polymethyl (meth)acrylate, polyethyl (meth)acrylate, polyisopropyl (meth)acrylate, polyisobutyl (meth)acrylate, polybutyl (meth)acrylate, etc. In addition, polycondensation polymers may be used, for example, polyurea, polyamide, polyimide, polyester, etc., and one or more of the binders listed above may be used in combination. According to exemplary embodiments, the content of the binder in the slurry may be controlled in the range of, for example, about 1 to 50 parts by weight, specifically about 3 to 30 parts by weight, more specifically about 5 to 20 parts by weight, based on 100 parts by weight of the concentration gradient porous silicon-based negative electrode active material, but is not limited thereto.

[0131] According to exemplary embodiments, the solvent (dispersion medium) is not limited to a specific type as long as it can effectively dissolve (or disperse) the aforementioned electrode active material, conductive material, binder, etc. In this regard, exemplary solvents (dispersion mediums) may be water (e.g., deionized water), alcohol-based solvents, ether-based solvents, amide-based solvents, ketone-based solvents, aromatic-based solvents, ester-based solvents, etc. As an example, in the case of alcohol-based solvents, methanol, ethanol, 1-propanol, 2-propanol, 2-butanol, t-butanol, ethylene glycol, propylene glycol, 1,6-hexanediol, cyclohexanediol, sorbitol, xylitol, 2-methyl-2,4-pentanediol, 1,3-butanediol, 1,4-butanediol, etc. may be used. Examples of ether-based solvents include alkylene glycol alkyl ethers (e.g., ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol, dipropylene glycol, propylene glycol monomethyl ether, propylene glycol dimethyl ether, etc.), dimethyl ether, diethyl ether, diisopropyl ether, dibutyl ether, cyclohexylmethyl ether, t-butylmethyl ether, tetrahydrofuran, dioxane, etc. In the case of amide-based solvents, examples include N,N-dimethylformamide, 1-methyl-2-pyrrolidone, 2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, ε-caprolactam, formamide, N-methylformamide, acetamide, N-methylacetamide, N,N-dimethylacetamide, N-methylpropanamide, etc. In addition, as ketone-based solvents, examples such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, etc. may be used. Examples of aromatic-based solvents may include benzene, toluene, xylene, etc. Furthermore, as ester-based solvents, ethyl acetate, propyl acetate, butyl acetate, ethyl formate, etc. may be used. One or more combinations of the solvents (dispersion media) listed above may be used, but the present disclosure is not limited thereto.

[0132] According to exemplary embodiments, the conductive material may be any type known in the art without special limitations, such as graphite types including natural graphite and artificial graphite; carbon black types including acetylene black; amorphous carbon such as needle coke; carbon fiber types such as vapor-grown carbon fiber or carbon nanotubes (CNT); and carbonaceous materials such as graphene and fullerene. Alternatively, metal powders or metal fibers such as copper, nickel, tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3; and conductive polymers such as polyaniline, polypyrrole, polythiophene, polyacetylene, and polyphenylene derivatives may also be used. One of the types listed above may be used, or two or more may be used in combination.

[0133] According to an exemplary embodiment, a dispersant may be further included in the cathode active material slurry. Such a dispersant may be selected from a type capable of effectively dispersing the porous silicon-based cathode active material in the slurry, for example, it may be a dispersant having a hydrophile-lipophile balance (HLB) of about 10 to 30, specifically about 12 to 25, more specifically about 15 to 20. As an example, the dispersant may be an oligomer or copolymer containing at least about 30 (e.g., at least about 35, more specifically at least about 40) ethylene oxide units. In addition, in addition to ethylene oxide units, it may include at least one unit selected from units derived from alkyl ethers, units derived from alkylaryl ethers, phenylene oxide units, etc.

[0134] According to an exemplary embodiment, after applying a cathode active material slurry onto a current collector, a drying process may be performed to remove a solvent or the like contained in the slurry layer. At this time, the drying method may be heat drying, reduced pressure drying, or a combination thereof. As an example, the drying temperature may be controlled in the range of, for example, about 40 to 200°C, specifically about 50 to 150°C, more specifically about 60 to 100°C. In addition, the drying time may be controlled in the range of, for example, about 3 to 24 hours, specifically about 5 to 18 hours, more specifically about 8 to 12 hours.

[0135] According to an exemplary embodiment, the capacity of the anode manufactured as described above may be, for example, in the range of at least about 50 mAh / g, specifically about 100 to 600 mAh / g, more specifically about 150 to 300 mAh / g, but this should be understood as exemplary.

[0136]

[0137] Fabrication of sodium ion secondary batteries

[0138] According to another embodiment of the present disclosure, a sodium-ion secondary battery can be manufactured using an anode to which a concentration gradient type porous silicon-based negative electrode active material is applied.

[0139] The basic structure of a sodium-ion secondary battery is known in the art and includes, for example, a cathode (positive electrode), an anode (negative electrode), and an electrolyte or electrolyte layer disposed between the cathode and the anode, and if necessary, a separator may be further provided between the cathode and the anode as in a lithium secondary battery.

[0140] At this time, the cathode (positive electrode) may include a sodium-based positive electrode active material, for example, a positive electrode active material in powder form may be prepared in a slurry form and the cathode may be manufactured by coating it onto a metal substrate or a current collector (for example, a metal foil).

[0141] According to exemplary embodiments, a Prussian blue analogue (PBA) can be used as a sodium-based cathode active material. However, the present disclosure is not limited to a specific cathode active material, and, for example, layered materials, NASICON-type materials, phosphate-based polyanion materials, etc. may be applied.

[0142] According to an exemplary embodiment, a separator may be disposed between mutually opposing cathodes and anodes, wherein the cathodes, anodes, and separator are in contact with an electrolyte (a medium that transfers sodium ions between the cathodes and anodes), and sodium ions may pass through the separator and move toward the cathode or anode.

[0143] According to an exemplary embodiment, the material of the separator may be a polyolefin-based polymer of a type known in the art, such as ethylene homopolymer, propylene homopolymer, ethylene / butylene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer, etc., and may be in the form of a porous polymer film prepared therefrom. In addition, the separator may include a nonwoven fabric formed of high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0144] According to an exemplary embodiment, the electrolyte layer functions as a medium for transferring sodium ions between the cathode and the anode, so that sodium ions within the electrolyte layer can pass through a separator and move toward the cathode or anode side. In this regard, the electrolyte layer may include an ether-based solvent, and may be at least one selected from, for example, dimethyl ether, diethyl ether, dibutyl ether, dimethoxyethane, diethoxyethane, ethylene glycol dimethyl ether, diethylene glycol, triethylene glycol, 1,3-dioxolane, 4-methyldioxolane, 1,4-dioxane, 3,5-dimethyl isoxazole, 2,5-dimethylfuran, furan, 2-methyl furan, tetrahydrofuran, 2-methyltetrahydrofuran, etc.

[0145] As the basic structure of sodium-ion secondary batteries and the like are known in the industry, detailed explanations other than those described above will be omitted.

[0146] According to another embodiment of the present disclosure, a sodium-ion secondary battery to which the aforementioned anode is applied, and a battery module or battery pack in which a plurality of such secondary batteries are assembled are provided. The sodium-ion secondary battery, battery module, or battery pack according to the present embodiment can be applied to various mobile devices (e.g., smartphones, tablets, laptops, etc.), electric vehicles (pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), energy storage systems (ESS), etc.

[0147] The present invention can be more clearly understood by the following examples, which are merely for illustrative purposes and are not intended to limit the scope of the invention.

[0148]

[0149] Examples

[0150]

[0151] Example 1

[0152] Preparation of concentration gradient porous silicon-based cathode active material

[0153]

[0154] - Preparation of precursor silica (SiO2)

[0155] 100 mL of sodium silicate (Na2SiO3; Samchun Chemical Co., LTd.) solution and 400 mL of deionized water (DIW) were stirred using an overhead stirrer (stirring speed: 600 rpm) for 30 minutes, and the temperature of the solution was raised to 80 ℃. To the sufficiently mixed solution, a 10 wt% sulfuric acid solution (H2SO4; Samchun Chemical Co., LTd.) was added at a rate of 1 mL / min until the pH of the solution reached 8.7. The solution, having reached a pH of 8.7, was stirred for another hour at a temperature of 80 ℃ (stirring speed: 600 rpm). Then, the solution was cooled to room temperature, and added at a rate of 1 mL / min until the pH reached 6.

[0156] Meanwhile, a 20 wt% sodium chloride solution was prepared by mixing sodium chloride (NaCl; Samchun Chemical Co., LTd.) with deionized water (DIW) in a mass ratio of 1:4.

[0157] Afterwards, 20 g of a 20 wt% sodium chloride solution was added to the solution at which the pH had reached 6 for 20 minutes.

[0158] The precipitated silica was filtered and washed with 200 g of deionized water, and this process was repeated four times. Afterward, the filtered silica was dried in a convection oven at 60°C for 24 hours to obtain precursor silica.

[0159]

[0160] - Reduction heat treatment

[0161] 10 g of the previously prepared silica precursor (SiO2), 9 g of magnesium metal powder (Samchun Chemical Co., LTd.), and 20 g of sodium chloride (NaCl; Samchun Chemical Co., LTd.) as a heat absorbent were ground and mixed using a planetary ball mill.

[0162] The prepared reaction mixture is introduced into an electric furnace and heat-treated under an inert gas (Ar) atmosphere by heating at a heating rate of 5 ℃ / min until the internal temperature of the furnace reaches 660 ℃, and then maintaining it for 2 hours, thereby reducing the silica in the reaction mixture to elemental silicon and / or reduced silicon oxide (SiO₂). x It was made to transition to , 0≤x<2) (generation of concentration gradient silicon-based particles).

[0163] 39 g of concentration gradient silicon-based particles prepared through reduction heat treatment were added to 400 g of water and dispersed for 30 minutes under conditions of magnetic stirring (400 rpm) and ultrasonic treatment.

[0164] Next, a 35 wt% aqueous HCl solution was added to the dispersion solution and mixed (final HCl concentration in the dispersion: 2.0 M), and subsequently stirred at 80 °C for 1 hour to completely remove MgO and Mg2Si by-products generated during the reduction heat treatment process. Afterward, the acid-treated silicon-based particles were washed with deionized water at least three times to completely remove residual hydrochloric acid solution and reaction by-products (creation of concentration gradient porous silicon-based particles).

[0165] The washed silicon-based particles were dried in a convection oven at 60°C for 12 hours, and finally, a concentration gradient porous silicon-based negative electrode active material was obtained.

[0166]

[0167] Example 2

[0168] Preparation of concentration gradient porous silicon-based cathode active material

[0169]

[0170] - Preparation of precursor silica (SiO2)

[0171] A solution was prepared by mixing a 20 wt% sodium hydroxide solution (NaOH; Samchun Chemical Co., LTd.) and a 2 wt% ammonium hydroxide solution (NH4OH; Samchun Chemical Co., LTd.) in a mass ratio of 1:1. Subsequently, the prepared solution was added to 400 mL of deionized water (DIW) at a rate of 0.5 mL / min and stirred (stirring speed: 400 rpm) until the pH of the solution became 11.5. Then, the temperature of the solution was raised to 60 ℃, and 10 g of silicon (Silicon powder; Sigma-Aldrich) was added and stirred for 1 hour.

[0172] Separately, 20 g of a 20 wt% sodium hydroxide solution was mixed with 180 g of deionized water (DIW) to prepare a 2 wt% sodium hydroxide solution. Then, 40 g of silicon powder was added and stirred, and the 2 wt% sodium hydroxide solution was added at a rate of 0.8 mL / min.

[0173] After all 2 wt% sodium hydroxide solutions were added, the mixture was stirred for 2 hours (stirring speed: 400 rpm). The solution was filtered to remove residual silicon powder.

[0174] 2 g of a 20 wt% aqueous magnesium sulfate (MgSO4; Samchun Chemical Co., LTd.) solution was added to the obtained solution and stirred (stirring speed: 200 rpm) to gel the silica. The gelled silica was filtered, washed once with 200 g of deionized water (DIW), and dried in a convection oven at 60 ℃ for 24 hours to obtain precursor silica.

[0175]

[0176] - A concentration gradient porous silicon-based negative electrode active material was obtained according to the same reduction heat treatment and subsequent acid treatment procedures as in Example 1.

[0177]

[0178] Comparative Example 1

[0179] Preparation of uniformly distributed non-porous silicon-based cathode active material

[0180]

[0181] High-purity quartz (SiO2; Sigma-Aldrich), a carbon source (coke; Sigma-Aldrich), and an electric furnace capable of high-temperature heat treatment were prepared.

[0182] 10g of high-purity quartz and the total carbon content of the carbon source were adjusted to 4g, and then ground and mixed using a planetary ball mill.

[0183] The reaction mixture was introduced into an arc discharge furnace and heat treatment was performed by raising the temperature to 2000 to 2500 ℃. Afterward, the molten silicon remaining was cooled and mechanically ground using a planetary ball mill, resulting in the acquisition of a silicon-based negative electrode active material that does not have porosity.

[0184]

[0185] Characteristic evaluation

[0186] The negative electrode active materials prepared according to Examples 1 and 2 and Comparative Example 1, respectively, were analyzed using a Field Emission Scanning Electron Microscopy (FE-SEM, TESCAN S8000), and the results are shown in Figures 3a to 3c.

[0187] Referring to FIG. 3a and FIG. 3b, it can be seen that the silicon-based negative electrode active material prepared according to the example has pores developed by the aggregation of fine particles of uniform size. Referring to FIG. 3c, it can be seen that the silicon-based negative electrode active material according to Comparative Example 1 consists of non-uniform fine particles, and the degree of pore development is weak compared to the example.

[0188] In addition, the porosity of the cathode active materials prepared according to Examples 1 and 2 and Comparative Example 1 was evaluated through nitrogen adsorption-desorption experiments (Adsorption-Desorption Isotherms, Micrometrics ASAP2000), and the results are shown in Table 1 below.

[0189]

[0190] Classification Specific Surface Area Pore Volume Pore Diameter (Size) Particle Diameter (D50) (m 2 / g)(cm 3 / g)(nm)(㎛) Example 16 76.10.72 17.985 Example 26 23.10.69 98.545 Comparative Example 1 10.20.08 175.858

[0191]

[0192] Referring to the table above, it was confirmed that Examples 1 and 2 had significantly higher specific surface area and pore volume compared to Comparative Example 1, and in particular, the specific surface area (BET) and pore volume of the silicon-based negative electrode active material according to Example 1 were the highest, while the pore diameter was the lowest. In addition, the particle diameter of the silicon-based negative electrode active materials prepared in Examples 1 and 2, respectively, was lower than that of Comparative Example 1.

[0193]

[0194] Performance evaluation

[0195] - An anode was fabricated using the silicon-based negative electrode active material prepared according to Example 1 and Comparative Example 1, respectively, and a sodium-ion secondary battery was fabricated and tested.

[0196] Specifically, the negative electrode active material prepared according to Example 1 and Comparative Example 1, Super-P (Imerys Graphite & Carbon) as a conductive material, and PAA (Sigma-Aldrich) as a binder were mixed in a weight ratio of 7:1:2, and 1 g of the mixture was added to 1.1 mL of deionized water (DIW) to prepare a negative electrode active material slurry.

[0197] The manufactured cathode active material slurry was applied to the surface of an aluminum foil with a thickness of 16 μm, vacuum dried at a temperature of 100 ℃ for 8 hours, and a roll press process was performed to fabricate an anode with a 40% void.

[0198]

[0199] - Charge / Discharge Evaluation

[0200] Half-cells of coin-cell type sodium-ion secondary batteries were fabricated using anodes prepared using the negative electrode active materials according to Examples 1 and 2 and Comparative Example 1, respectively, and charging and discharging were performed at room temperature (25℃) and under current density conditions of 500 mA / g (Measurement equipment: Battery Test System, BTS, WonATech, WBCS3000Ls32)

[0201] The results of the charge / discharge test are shown in Figure 4.

[0202] Referring to the drawings above, when an anode with a negative electrode active material according to Examples 1 and 2, respectively, is used, the initial capacity at a current density of 25 mA / g was measured to be 258 mAh / g in Example 1 and 235 mAh / g in Example 2. In addition, the initial capacity in Comparative Example 1 was measured to be 187 mAh / g. Furthermore, excellent capacity retention characteristics of the silicon-based negative electrode active materials according to Examples 1 and 2 were observed as the cycle progressed.

[0203] As seen above, when an anode using a concentration gradient porous silicon-based negative electrode active material obtained through reduction heat treatment and acid treatment according to the example was applied to a sodium-ion battery, a charge / discharge capacity was obtained that was significantly improved compared to conventional silicon-based negative electrode active materials, and in particular, the capacity retention rate was maintained at a high level even when cycles were repeated.

[0204] All simple variations or modifications of the present invention fall within the scope of the present invention, and the specific scope of protection of the present invention will be clarified by the appended claims.

Claims

1. Internal regions of the particle where silicon oxide (SiO2) is predominantly distributed; and Reduced silicon oxide and / or elemental silicon (SiO₂) x Particle surface region where (0≤x<2)) is predominantly distributed; A porous silicon-based negative electrode active material with a concentration gradient including 2. A concentration gradient type porous silicon-based negative electrode active material according to claim 1, characterized in that the specific surface area (BET) and pore volume of the porous silicon-based negative electrode active material are at least 100 m² / g and at least 0.1 cm³ / g, respectively, when measured by the nitrogen adsorption method.

3. A concentration gradient type porous silicon-based negative electrode active material according to claim 1, characterized in that the particle diameter of the porous silicon-based negative electrode active material is in the range of 0.01 to 100 μm.

4. A concentration gradient type porous silicon-based negative electrode active material according to claim 1, characterized in that the pore diameter of the porous silicon-based negative electrode active material is in the range of 0.1 to 5000 nm.

5. A concentration gradient type porous silicon-based negative electrode active material according to claim 1, characterized in that the tap density of the porous silicon-based negative electrode active material is in the range of 0.5 to 2.7 g / cm³.

6. A concentration gradient type porous silicon-based negative electrode active material according to claim 1, characterized in that the ratio of the internal region of the silicon oxide (SiO2) state particles among the porous silicon-based negative electrode active material is in the range of 1 to 70 volume%. 7.a) By performing a reduction heat treatment on a reaction mixture comprising silicon oxide (SiO2) particles and a metal powder as a reducing agent under an inert gas, a reducing gas, a mixture thereof, or a reduced pressure atmosphere, (i) an internal region of the particles in which silicon oxide (SiO2) is predominantly distributed and (ii) reduced silicon oxide and / or elemental silicon ((SiO2) x A step of forming a silicon-based particle of a concentration gradient including a particle surface region in which (0≤x<2)) is predominantly distributed; and b) a step of performing acid treatment on the above-mentioned concentration gradient silicon-based particles to form concentration gradient porous silicon-based particles with increased porosity; A method for manufacturing a porous silicon-based negative electrode active material with a concentration gradient including 8. In paragraph 7, the above step a) is, a1) a step of providing a metal powder as a silicon oxide (SiO2) precursor and a reducing agent, respectively; and a2) A step of providing a reaction mixture by applying mechanical energy to the silicon oxide (SiO2) precursor and metal powder to grind them; A method for manufacturing a porous silicon-based negative electrode active material characterized by including 9. A method for manufacturing a porous silicon-based negative electrode active material according to claim 7, wherein the metal powder is at least one selected from the group consisting of magnesium, aluminum, iron, zinc, tin, potassium, calcium, titanium, nickel, and chromium, and the weight ratio of the silicon oxide (SiO2) precursor to the metal powder is controlled in the range of 1:0.3 to 2.

10. A method for manufacturing a porous silicon-based negative electrode active material according to claim 9, wherein in step a2), at least one selected from the group consisting of sodium chloride, potassium chloride, magnesium chloride, barium chloride, and calcium chloride is further added as a heat absorbent, ground, and mixed, and wherein the weight ratio of the silicon oxide (SiO2) precursor to the heat absorbent is controlled within the range of 1:0.5 to 3.

11. Anode entire house; and A layer containing a porous silicon-based negative electrode active material according to any one of claims 1 to 6 formed on the anode current collector; An anode for a sodium-ion secondary battery comprising 12. Cathode; Anode; and An electrolyte layer disposed between the above cathode and anode; Includes, At this time, the above anode is a sodium ion secondary battery that is an anode according to claim 11.

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