Method for preparing negative electrode active material, negative electrode active material obtained by using the method, and lithium secondary battery

Abstract

The present invention relates to a method for preparing a negative electrode active material, a negative electrode active material prepared by using the same, and a lithium secondary battery, and more particularly to a method for preparing a negative electrode active material, the method comprising the steps of: (a) preparing a coating composition containing a precursor of a metal-phosphorus-nitrogen oxide; (b) forming a precursor layer on the negative electrode active material using the coating composition by a solution method; and (c) forming a metal-phosphorus-nitrogen oxide protective layer on the negative electrode active material by heat-treating the negative electrode active material on which the precursor layer is formed. The method for preparing a negative electrode active material according to the present invention uses a solution method, which is advantageous in simplifying the entire process and reducing costs, and also achieves high capacity, high stability, and long life by including the formed protective layer having excellent properties.

Description

Technical Field

[0001] This application claims the benefit of Korean Patent Application Nos. 10-2017-0112012, filed on September 1, 2017, and 10-2018-0099369, filed on August 24, 2018, with the Korean Intellectual Property Office, the entire disclosures of which are incorporated herein by reference.

[0002] The present invention relates to a method for preparing a negative electrode active material, a negative electrode active material obtained by the method, and a lithium secondary battery. Background Art

[0003] Recently, with the development of portable electronic devices, electric vehicles, and large-capacity energy storage systems, etc., the demand for large-capacity batteries as an energy source has increased, and in response to this demand, extensive research has been conducted on batteries. Among many secondary batteries, lithium secondary batteries, which have advantages such as high energy density, discharge voltage, and output stability, have attracted attention.

[0004] A lithium secondary battery has a structure obtained by laminating or winding an electrode assembly, and is formed by inserting the electrode assembly into a battery case and injecting a non-aqueous electrolyte therein. The electrode assembly includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. Herein, the capacity of a lithium secondary battery varies depending on the type of electrode active material, and commercialization has not been successful because a sufficient capacity as the theoretical capacity is not ensured during actual operation.

[0005] As a negative electrode active material of a lithium secondary battery, carbon-based materials such as crystalline carbon, including natural graphite or artificial graphite; or amorphous (low-crystalline) carbon having a pseudo-graphite structure or a turbostratic structure obtained by carbonizing hydrocarbons, polymers, etc. at a temperature of 1000 °C to 1500 °C are generally used. The advantage of such carbon-based materials is that they have a low standard redox potential of about -3V with respect to the standard hydrogen electrode (SHE) potential, and have excellent charge and discharge reversibility by using a layered structure that is very useful for lithium ion insertion and extraction. However, since graphite has a low theoretical capacity of 372 mAh / g, there are limitations in obtaining a high capacity.

[0006] In order to obtain a high-capacity lithium secondary battery, metal materials with high theoretical capacity such as lithium (3,860 mAh / g), silicon (4,200 mAh / g), or tin (990 mAh / g) have been used as the negative electrode active material. However, when using a metal such as lithium, silicon, or tin as the negative electrode active material, the volume greatly expands to about 4 times during the charging process of alloying with lithium and shrinks during discharging. Due to this significant change in the electrode volume that occurs repeatedly during charging and discharging, the active material slowly pulverizes and separates from the electrode, resulting in a rapid decline in capacity. As a result, commercialization has not been successful because it is difficult to ensure stability and reliability.

[0007] Meanwhile, the negative electrode active material reacts with the electrolyte to form a passivation layer (solid electrolyte interphase; SEI) on the surface, and this may lead to an irreversible reduction in capacity. In addition, the formed passivation layer causes local differences in current density, forming dendritic lithium dendrites on the surface of the negative electrode active material. Lithium dendrites cause internal short circuits and inactive lithium (dead lithium) in the battery, shortening the life of the lithium secondary battery, and have an adverse impact on the battery capacity, cycle performance, and life by increasing the physical and chemical instability of the lithium secondary battery. In addition, the passivation layer is thermally unstable and may be slowly damaged during the battery charging and discharging process, especially during high-temperature storage in a fully charged state, due to the increased electrochemical energy and thermal energy. Because of the destruction of this passivation layer, the exposed surface of the negative electrode active material directly reacts with the electrolyte, resulting in a continuous reaction of decomposing the electrolyte. As a result, the negative electrode resistance increases, and the charging and discharging efficiency of the battery decreases.

[0008] In view of the above situation, various methods have been studied to improve the life performance and electrochemical performance of the negative electrode active material.

[0009] As an example, Korean Patent Application Publication No. 2015-0077053 discloses that by forming a ceramic coating containing alumina, zirconia, etc. on the surface of a negative electrode active material core containing a silicon single phase and a silicon alloy phase, the life and performance of the negative electrode active material and the secondary battery containing it can be enhanced.

[0010] In addition, Korean Patent Application Publication No. 2016-0034183 discloses that by forming a protective layer using a polymer matrix on a negative electrode active layer containing lithium metal or a lithium alloy, loss of electrolyte and formation of dendrites can be prevented, and the polymer matrix can accumulate the electrolyte while protecting the negative electrode.

[0011] Prior art documents have to some extent improved the reaction stability of the negative electrode active material with respect to the electrolyte through a separate layer, however the effect is still insufficient. In addition, the formation of the layer requires a separate process, or deterioration such as hardening or swelling of the protective layer occurs during driving of the battery, which limits the application. Therefore, in order to improve the performance and lifespan of lithium secondary batteries, there is a greater need to develop a negative electrode active material that minimizes the initial irreversible capacity and has excellent stability.

[0012] Prior art documents

[0013] [Patent document]

[0014] Korean Patent Application Publication No. 2015-0077053 (July 7, 2015), Negative Electrode Active Material, Secondary Battery Comprising Same and Method for Manufacturing Negative Electrode Active Material

[0015] Korean Patent Application Publication No. 2016-0034183 (March 29, 2016), Negative Electrode for Lithium Secondary Battery and Lithium Secondary Battery Comprising Same Summary of the Invention

[0016] Technical Problem

[0017] As a result of extensive research in view of the above, the inventors of the present invention have confirmed that when forming a metal-phosphorus-nitrogen oxide protective layer on a negative electrode active material by a solution method by preparing a coating composition containing a precursor of metal-phosphorus-nitrogen oxide in a solution state, the corresponding protective layer is easily formed and the lifespan performance of the negative electrode active material is also improved by having excellent coating properties, thereby completing the present invention.

[0018] Accordingly, one aspect of the present invention provides a method for preparing a negative electrode active material, the method being capable of preparing a metal-phosphorus-nitrogen oxide protective layer having excellent properties on a negative electrode active material using a method simpler than conventional methods.

[0019] Another aspect of the present invention provides a negative electrode active material prepared according to the above preparation method and a lithium secondary battery comprising the negative electrode active material.

[0020] Technical solution

[0021] According to one aspect of the present invention, a method for preparing a negative electrode active material is provided. The method includes the following steps: (a) preparing a coating composition containing a precursor of a metal-phosphorus-nitrogen oxide; (b) forming a precursor layer on the negative electrode active material using the coating composition by a solution method; and (c) forming a metal-phosphorus-nitrogen oxide protective layer on the negative electrode active material by heat-treating the negative electrode active material on which the precursor layer is formed.

[0022] A precursor of a metal-phosphorus-nitrogen oxide can be prepared by reacting a compound containing a phosphorus-nitrogen bond with a metal salt compound.

[0023] The compound containing a phosphorus-nitrogen bond may include at least one selected from the compounds represented by the following Chemical Formulas 1 to 3:

[0024] [Chemical Formula 1]

[0025]

[0026] [Chemical Formula 2]

[0027]

[0028] [Chemical Formula 3]

[0029]

[0030] Wherein X1, X2, R1 to R7, and n follow the descriptions provided in the specification.

[0031] The metal salt compound may include at least one element selected from the group consisting of the following elements: lithium, sodium, magnesium, calcium, zinc, and aluminum.

[0032] The coating composition may include the compound containing a phosphorus-nitrogen bond, the metal salt compound, and an organic solvent.

[0033] Based on the total weight of the coating composition, the coating composition may include the compound containing a phosphorus-nitrogen bond in an amount of 0.002 wt% to 27 wt%, the metal salt compound in an amount of 0.0005 wt% to 12 wt%, and the organic solvent in an amount of 70 wt% to 99.99 wt%.

[0034] The coating composition may further include a chalcogen element compound.

[0035] The negative electrode active material may include at least one material selected from the group consisting of the following materials: lithium metal, lithium alloy, transition metal oxide, silicon-based material, tin-based material, and carbon-based material.

[0036] The negative electrode active material may have a spherical shape, an oval shape, a spindle shape, a flake shape, a plate shape, a fibrous shape, a rod shape, a core-shell shape, or an irregular shape.

[0037] In step (b), the solution method may use at least one of the following methods: spraying, spin coating, dip coating, inkjet printing, offset printing, reverse offset printing, gravure printing, and roll printing.

[0038] In step (c), the heat treatment may be carried out in a temperature range above 150°C and below 500°C.

[0039] Before step (c), it may further include removing the organic solvent contained in the precursor layer formed in step (b).

[0040] According to another aspect of the present invention, there is provided a negative electrode active material prepared by using the above preparation method.

[0041] According to still another aspect of the present invention, there is provided a lithium secondary battery including the negative electrode active material.

[0042] Advantageous Effects

[0043] The method for preparing a negative electrode active material according to the present invention can easily form a protective layer containing a metal-phosphorus-nitrogen oxide and / or its derivative on the negative electrode active material by a solution method using a liquid coating composition containing a precursor of a metal-phosphorus-nitrogen oxide. Therefore, the negative electrode active material can be prepared by a simple reaction under mild conditions. In addition, the preparation cost can be minimized and the preparation time can be shortened, making commercial application possible. In addition, by having excellent reaction stability with the electrolyte and having ion conductivity, the negative electrode active material with a protective layer prepared by the preparation method according to the present invention can provide a negative electrode active material with high capacity and long life, and can also enhance the capacity and life performance of the lithium secondary battery containing it. Brief Description of the Drawings

[0044] Figure 1 is a scanning electron microscope image of the negative electrode active material prepared in Comparative Example 1 of the present invention.

[0045] Figure 2 is a scanning electron microscope image of the negative electrode active material prepared in Example 1 of the present invention.

[0046] Figure 3 is an image showing the result of EDS elemental analysis of the negative electrode active material prepared in Example 1 of the present invention.

[0047] Figure 4It is an image showing the results of performing EDS elemental analysis on the negative electrode active material prepared in Example 1 of the present invention. Detailed implementation mode

[0048] Hereinafter, the present invention will be described in more detail.

[0049] The terms or words used in this specification and claims should not be construed as being limited to the ordinary or dictionary meanings, and should be interpreted as meanings and concepts corresponding to the technical idea of the present disclosure on the basis of the principle that the inventor can appropriately define the concept of the terms so as to describe the present invention in the best possible way.

[0050] The terms used in the present invention are only used to describe specific implementation modes and are not intended to limit the present invention. Unless otherwise clearly stated in the context, the singular forms used herein also include the plural forms. In the present invention, terms such as "comprising" or "having" are used to clearly indicate the existence of the features, numbers, steps, actions, components, parts or combinations thereof described in the specification, and need to be interpreted as not excluding the possibility of the existence or addition of one or more other features, numbers, steps, actions, components, parts or combinations thereof.

[0051] As the applications of lithium secondary batteries have expanded from mobile phones and wireless electronic devices to electric vehicles, there has been a continuous need to develop lithium secondary batteries that are smaller, lighter, thinner, portable, and have high performance, long life, and high reliability.

[0052] In response to such requirements, various materials are used and developed as negative electrode active materials. However, in actual operation, it is difficult to fully ensure the life performance because of insufficient cycle efficiency and stability and the inability to obtain the complete theoretical capacity and energy density. This is because of the following fact: as the battery cycles, due to the formation of a passivation layer on the negative electrode surface, the initial irreversible capacity (initial charge capacity - initial discharge capacity) and resistance increase, resulting in a decrease in output performance, and due to volume changes and lithium dendrite growth during charge and discharge, the stability of the negative electrode decreases.

[0053] In view of the above situation, methods of changing the composition of the negative electrode active material or introducing a protective layer on the surface have been proposed in the prior art. However, such methods cannot effectively improve the life performance of the negative electrode active material and are not suitable for commercial use in terms of productivity, process efficiency, and economic feasibility.

[0054] Therefore, the present invention provides a method for preparing a negative electrode active material, which uses a solution method to form a protective layer on the negative electrode active material, thereby uniformly forming a protective layer containing a specific compound on the surface of the negative electrode active material through a simple process and ensuring the improvement effect of the performance and life of the lithium secondary battery containing the negative electrode active material.

[0055] The method for preparing a negative electrode active material according to an embodiment of the present invention includes the following steps: (a) preparing a coating composition containing a precursor of a metal-phosphorus-nitrogen oxide; (b) forming a precursor layer on the negative electrode active material using the coating composition by a solution method; and (c) forming a metal-phosphorus-nitrogen oxide protective layer on the negative electrode active material by heat-treating the negative electrode active material on which the precursor layer is formed.

[0056] Hereinafter, the present invention will be described in more detail through each step.

[0057] First, in step (a), a coating composition containing a precursor of a metal-phosphorus-nitrogen oxide is prepared.

[0058] The coating composition may contain a compound containing a phosphorus (P)-nitrogen (N) bond, a metal salt compound, and an organic solvent.

[0059] The compound containing a phosphorus-nitrogen bond in the present invention is a skeletal compound containing a phosphorus-nitrogen bond and serves to provide a phosphorus-nitrogen-oxygen skeleton, which is the crystal lattice structure of the metal-phosphorus-nitrogen oxide finally produced by the following heat treatment. Herein, the phosphorus-nitrogen bond may be a single bond or a double bond.

[0060] The compound containing a phosphorus-nitrogen bond may be at least one compound selected from the compounds represented by the following Chemical Formulas 1 to 3:

[0061] [Chemical Formula 1]

[0062]

[0063] Wherein X1s are the same as or different from each other and are each independently OR1, F, Cl, Br, or I, and R1 is an alkyl group having 1 to 5 carbon atoms;

[0064] [Chemical Formula 2]

[0065]

[0066] Wherein R2 and R3 are the same as or different from each other and are each independently an alkyl group having 1 to 10 carbon atoms or an aryl group having 6 to 20 carbon atoms;

[0067] [Chemical Formula 3]

[0068]

[0069] wherein X2s are the same as or different from one another and are each independently R4, OR5, NR6R7, F, Cl, Br or I, where R4 to R7 are the same as or different from one another and are each independently an alkyl group having 1 to 10 carbon atoms or an aryl group having 6 to 20 carbon atoms, and

[0070] n is an integer from 100 to 100,000.

[0071] The term "alkyl" used in the present invention may be linear or branched, and although not particularly limited thereto, preferably has 1 to 10 carbon atoms. Specific examples thereof may include methyl, ethyl, propyl, isopropyl, butyl, tert-butyl, pentyl, hexyl, heptyl, etc., but are not limited thereto.

[0072] The term "aryl" used in the present invention refers to a single or multiple aromatic carbon rings having 6 to 20 carbon atoms. Examples of aryl may include phenyl, biphenyl, fluorenyl, etc., but are not limited thereto.

[0073] The phosphorus-nitrogen bond-containing compound may be a monomeric compound represented by Chemical Formula 1 or 2, a polymeric compound represented by Chemical Formula 3, or a mixture thereof.

[0074] For example, the phosphorus-nitrogen bond-containing compound may include at least one substance selected from the group consisting of: hexachlorophosphazene, hexafluorophosphazene, hexabromophosphazene, hexaiodophosphazene, hexamethylphosphoramide, diethyl aminophosphate, diphenyl aminophosphate, poly(dichlorophosphazene), poly(bis(ethoxy)phosphazene), and poly(bis(phenoxy)phosphazene). Preferably, the phosphorus-nitrogen bond-containing compound may include at least one substance selected from the group consisting of: hexachlorophosphazene, diphenyl aminophosphate, and poly(dichlorophosphazene).

[0075] Based on the total weight of the coating composition, the content of the compound containing a phosphorus-nitrogen bond can be 0.002% by weight to 27% by weight. Herein, when the solvent is excluded from the coating composition of the present invention, based on the total weight of the solids of the coating composition, the content of the compound containing a phosphorus-nitrogen bond can be 20% by weight to 90% by weight, preferably 50% by weight to 80% by weight. When the content of the compound containing a phosphorus-nitrogen bond included in the coating composition is less than the above range, the formation of the metal-phosphorus-nitrogen oxide (final product) may be uneven as a whole, and a content greater than the above range may cause unnecessary reactions, have an adverse effect on the composition of the protective layer or the battery performance, or cause a decrease in the battery performance due to the resistance caused by excessive formation. Therefore, it is preferred to determine an appropriate content within the above range. However, the specific optimal content of the compound containing a phosphorus-nitrogen bond can be set to vary according to the negative electrode active material to be provided and other properties and usage environments of the battery containing it, and such an application does not mean being limited by the above preferred range.

[0076] The metal salt compound of the present invention is not particularly limited as long as it can provide metal cations by dissociating into cations and anions in an organic solvent, and metal salt compounds commonly used in the art can be used without limitation.

[0077] The metal salt compound is a salt containing at least one metal selected from the group consisting of: monovalent metals such as lithium (Li) or sodium (Na); divalent metals such as magnesium (Mg), calcium (Ca) or zinc (Zn); and trivalent metals such as aluminum (Al), and can have various forms such as halides, hydroxides, acetates, alkoxides, etc.

[0078] For example, the metal salt compound can be a lithium salt containing lithium ions (Li + ) as monovalent metal cations, and the lithium salt can be CH3COOLi, LiX (here, X represents F, Cl, Br or I), LiNO3, LiOH, LiOR (here, R represents an alkyl group having 1 to 5 carbon atoms), etc. In addition, when the metal salt compound is a magnesium salt containing magnesium ions (Mg 2+ ) as divalent metal cations, the magnesium salt can be (CH3COO)2Mg, MgX2 (here, X represents F, Cl, Br or I), etc.

[0079] Based on the total weight of the coating composition, the content of the metal salt compound can be 0.005% by weight to 12% by weight. Herein, when the solvent is excluded from the coating composition of the present invention, the content of the metal salt compound can be 5% by weight to 40% by weight, preferably 10% by weight to 30% by weight. When the content of the metal salt compound is less than the above range, it is difficult to ensure the ion conductivity of the target protective layer of the present invention. On the contrary, when the content is greater than the above range, the formation of a non-uniform layer and the increase in resistance reduce the ion mobility, and it may not be possible to suppress side reactions on the surface of the negative electrode active material or the battery performance may deteriorate. Therefore, the content is appropriately controlled within the above range.

[0080] The organic solvent of the present invention provides metal cations by ionizing the above metal salt compound. The organic solvent can be a protic solvent or an aprotic polar solvent.

[0081] Examples of the organic solvent can include dimethyl sulfoxide (DMSO), N,N-dimethylformamide, N-methylformamide, methanol, ethanol, isopropanol, 2-methoxyethanol, water, etc. They can be used alone or as a mixture of two or more.

[0082] Based on the total weight of the coating composition, the content of the organic solvent can be 70% by weight to 99.99% by weight, preferably 80% by weight to 99.9% by weight. When the content of the organic solvent is less than the above range, the metal salt compound cannot be sufficiently dissolved, and the performance of the negative electrode active material may deteriorate due to side reactions, and it is difficult to form a layer with a uniform thickness. On the contrary, when the content is greater than the above range, excessive energy is consumed during the solvent removal process. As a result, the economic feasibility and productivity are reduced, and it is difficult to obtain the function of the target protective layer.

[0083] The coating composition containing the above components is uniformly mixed in an organic solvent to form a precursor of metal-phosphorus-nitrogen oxide for forming metal-phosphorus-nitrogen oxide. The precursor of metal-phosphorus-nitrogen oxide is prepared by the reaction of a compound containing a phosphorus-nitrogen bond with a metal salt compound.

[0084] Herein, after mixing the above components, the coating composition can be heated at a predetermined temperature. For example, the coating composition can be heated at a temperature of 40°C to 150°C and then used in a solution method for subsequent processes. This heating process is used for a pretreatment, and it can more easily form a precursor that partially replicates the lattice structure of metal-phosphorus-nitrogen oxide and is obtained by the reaction of the components constituting the above coating composition.

[0085] The coating composition may further contain a chalcogen compound (chalcogenide).

[0086] Chalcogenide compounds have excellent electrical conductivity, and when the coating composition further contains a chalcogenide compound, the chalcogen is incorporated into the lattice structure of the finally formed metal-phosphorus-nitrogen oxide, which further enhances the electrical conductivity.

[0087] For example, the chalcogenide compound may contain at least one substance selected from the group consisting of: TiS2, VS2, FeS2, MoS2, CoS3, TiSe2, VSe2, NbSe3, SeO2, TiTe2, VTe2, LiTiS2, and LiVS2.

[0088] Subsequently, in step (b), a precursor layer is formed on the negative electrode active material using a solution method with the coating composition prepared in the above step (a).

[0089] To form the precursor layer by coating the coating composition on the negative electrode active material, in the prior art, when forming a protective layer on the negative electrode active material, deposition processes such as sputtering or chemical vapor deposition have been used. Using a deposition process to form a protective layer results in excellent results in terms of quality. However, when the target is particles, it is difficult to apply such processes, and since the work needs to be carried out in a vacuum state and requires a separate apparatus, it is not commercially adaptable in terms of time, cost, and quantity. In contrast, in step (a) of the present invention, the coating composition is prepared in a liquid state, which is suitable for a solution method. Therefore, different from the existing deposition processes, such a process can be applied to various shapes. Additionally, by using the solution method in the present invention, the coating uniformity of the protective layer is very excellent, and advantages are also obtained in terms of simplifying the process and greatly reducing the process time and cost.

[0090] As the solution method, at least one method selected from spraying, spin coating, dip coating, inkjet printing, offset printing, reverse offset printing, gravure printing, and roll printing can be used.

[0091] Herein, as the negative electrode active material to be coated, all negative electrode active materials known in the art can be used as the negative electrode active material used in a lithium secondary battery.

[0092] The negative electrode active material is capable of reversibly intercalating or deintercalating lithium ions (Li +) or a material capable of reversibly forming a lithium-containing compound by reacting with lithium ions, and may include at least one material selected from the group consisting of: lithium metal, lithium alloy, transition metal composite oxide, silicon-based material, tin-based material, and carbon-based material. Examples of the negative electrode active material may include a material selected from the group consisting of: carbon-based materials selected from the group consisting of crystalline artificial graphite, crystalline natural graphite, amorphous hard carbon, low-crystalline soft carbon, carbon black, acetylene black, Ketjen black, Super-P, graphene, and fibrous carbon; transition metal composite oxides, such as silicon-based materials, Li x Fe2O3(0 ≤ x ≤ 1), Li x WO2(0 ≤ x ≤ 1) or Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, elements in Groups 1, 2, and 3 of the periodic table, halogens; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8); lithium metal; lithium alloy; silicon-based alloy; tin-based alloy; metal oxides, such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, or Bi2O5; conductive polymers, such as polyacetylene; Li-Co-Ni-based materials; titanium oxides; lithium titanium oxides, etc., but not limited thereto.

[0093] In addition, the negative electrode active material may have a spherical, oval, spindle-shaped, scaly, plate-shaped, fibrous, rod-shaped, core-shell, or irregular shape.

[0094] As an example, when the negative electrode active material has a spherical particle form, the particulate negative electrode active material is introduced into the coating composition and then stirred to coat the precursor layer on the surface of the negative electrode active material.

[0095] The solution method can be carried out under an inert atmosphere such as nitrogen or argon, or under dry air conditions with a relative humidity of 5% or less.

[0096] After forming the precursor layer on the negative electrode active material through step (b) of the present invention, before the heat treatment process for forming the metal-phosphorus-nitrogen oxide protective layer, an organic solvent removal process can be implemented. The organic solvent removal process is a heat treatment process implemented at a temperature lower than the heat treatment process for forming the protective layer, and the temperature of the organic solvent removal process can vary according to the type of organic solvent contained in the coating composition. The organic solvent removal process can be implemented at a temperature close to the boiling point of the organic solvent, such as at 40°C to 150°C. By implementing the organic solvent removal process, the mechanical stress of the protective layer caused by volume reduction after the following heat treatment process for forming the protective layer can be reduced. Therefore, a protective layer uniformly coated on all the negative electrode active materials can be formed. The organic solvent removal process can also be implemented in an inert atmosphere such as nitrogen or argon, or under dry air conditions with a relative humidity of 5% or less.

[0097] Subsequently, in step (c), the negative electrode active material formed with the precursor layer prepared in the above step (b) is heat-treated to form a metal-phosphorus-nitrogen oxide protective layer on the negative electrode active material.

[0098] The components forming the precursor layer undergo a thermal polymerization reaction through the heat treatment process in step (c) of the present invention to form a metal-phosphorus-nitrogen oxide or its derivative, and a protective layer containing the same is formed. In the thermal polymerization reaction, the components contained in the precursor layer undergo a ring-opening reaction, a condensation reaction, and a polymerization reaction to form a metal-phosphorus-nitrogen oxide in a mixed form of phosphorus and nitrogen. Here, a phosphorus-nitrogen-oxygen skeleton in which one nitrogen is bonded to two or three atoms is formed. During the thermal polymerization reaction occurring in the heat treatment process, impurities unnecessary for the reaction can be removed by heating. The impurities may include carbon, hydrogen, chlorine, etc. contained in the coating composition.

[0099] The metal-phosphorus-nitrogen oxide contained in the protective layer may contain chemical structures represented by the following Chemical Formulas 4 and 5, and these structures may include an amorphous phase of these metal-phosphorus-nitrogen oxides. Here, M in each of the following Chemical Formulas 4 and 5 n+ represents a monovalent, divalent, or trivalent metal cation. In other words, M represents the metal type, and n represents any integer from 1 to 3, and M n+ may represent Li + 、Mg 2+ 、Al 3+ etc.

[0100] [Chemical Formula 4]

[0101]

[0102] [Chemical Formula 5]

[0103]

[0104] The chemical structure of Chemical Formula 4 is a partial chemical structure containing single bonds of phosphorus and nitrogen, and for monovalent cations, it can be represented by M + [PO2O 1 / 2 N 1 / 3 - For divalent cations, it can be represented by M 2+ 2[PO2O 1 / 2 N 1 / 3 - For trivalent cations, it can be represented by M 3+ 3[PO2O 1 / 2 N 1 / 3 - The chemical structure represented by Chemical Formula 4 can be connected to the phosphorus of M 1 / n PO3 to further form two phosphorus-nitrogen bonds. As a result, a phosphorus-nitrogen-phosphorus bond can be formed. In addition, O - can bond with M n+ and O 1 / 2 can bond with other phosphorus to form a phosphorus-oxygen-phosphorus bond.

[0105] The chemical structure of Chemical Formula 5 is a chemical structure containing a double bond (P=N) of phosphorus and nitrogen. For monovalent cations, it can be represented by 2M + [PO2O 1 / 2 N 1 / 2 2- For divalent cations, it can be represented by M 2+ [PO2O 1 / 2 N 1 / 2 2- For trivalent cations, it can be represented by 2M 3+ 3[PO2O 1 / 2 N 1 / 3 2- The chemical structure represented by Chemical Formula 5 can be connected to the phosphorus of M2PO3 to further form a phosphorus-nitrogen bond, and a P=N-P bond can be formed. Similarly, in the chemical structure of Chemical Formula 5, O - can bond with M n + and O 1 / 2 can bond with other phosphorus to form a phosphorus-oxygen-phosphorus bond.

[0106] The heat treatment process can be carried out under an inert atmosphere such as nitrogen or argon, or under dry air conditions with a relative humidity of 5% or less. The temperature of the heat treatment process can be 150°C or higher and lower than 500°C. Compared with the conventional deposition process that requires a high temperature of 500°C or higher, by forming the precursor layer by the solution method in step (b), the process of the present invention can be carried out under milder conditions.​​​​​​

[0107] In addition, the heat treatment step is preferably carried out for at least 5 minutes or longer, and the heat treatment step can be carried out for, for example, 5 minutes to 1 hour.

[0108] The metal-phosphorus-nitrogen oxide protective layer formed in step (c) can be formed on at least a part of the above negative electrode active material. Specifically, the protective layer can be formed on a part of the surface of the negative electrode active material or on the entire surface. As an example, relative to the total surface area of the negative electrode active material, the protective layer can be formed at 50% or more, preferably 70% or more.

[0109] In addition, based on the total weight of the negative electrode active material, the content of the protective layer can be 0.01% by weight to 5% by weight, preferably 0.05% by weight to 3% by weight. The protective layer formed within the above range is preferred because it ensures ion conductivity and improves the stability of the active material.

[0110] In one embodiment of the present invention, the protective layer can have a thickness of 1 nm to 1000 nm, preferably 1 nm to 100 nm.

[0111] By continuously carrying out the above steps (a) to (c), a protective layer can be formed. After initially forming the protective layer, that is, in a state where the protective layer has been formed on the negative electrode active material, by carrying out steps (b) and (c) again, another protective layer can be formed, and by repeatedly carrying out steps (b) and (c) as above, the protective layers are laminated to form a protective layer having a multilayer structure. In other words, by repeating the process, the thickness and lamination structure of the protective layer can be easily controlled.

[0112] The method for preparing the above negative electrode active material can easily manufacture a protective layer containing metal-phosphorus-nitrogen oxide or its derivative in a manner having uniform quality by the metal-phosphorus-nitrogen oxide precursor solution method. In addition, the protective layer prepared by using the method has excellent uniformity and coating properties, and the negative electrode active material prepared according to the present invention has enhanced reaction stability with the electrolyte, and by utilizing the protective layer, a specific level of ion conductivity can be obtained, and an effective reduction in initial irreversibility can be achieved. Therefore, the lithium secondary battery containing the negative electrode active material of the present invention can have improved cycle performance, stability, and life performance.

[0113] In addition, the present invention provides a lithium secondary battery containing the above negative electrode active material.

[0114] The secondary battery includes: a positive electrode; a negative electrode; and a separator and an electrolyte disposed between the positive electrode and the negative electrode, wherein the negative electrode contains the negative electrode active material prepared according to the present invention.

[0115] The positive electrode and the negative electrode can be prepared by methods well-known in the art, and can be prepared as follows: The positive electrode active material and the negative electrode active material are each mixed with a binder, a conductive material, etc. to prepare an electrode paste, the prepared electrode paste is coated on a current collector, and then the obtained product is calendered and dried. Here, a small amount of conductive material and / or binder can be selectively added.

[0116] The positive electrode may include a positive electrode current collector and a positive electrode active material layer coated on one or two surfaces of the positive electrode current collector.

[0117] The positive electrode current collector is used to support the positive electrode active material layer and is not particularly limited as long as it has excellent conductivity and is electrochemically stable within the voltage range of the lithium secondary battery.

[0118] For example, the positive electrode current collector can be any metal selected from the group consisting of the following metals: copper, aluminum, stainless steel, titanium, silver, palladium, nickel, their alloys and their combinations, and the stainless steel can have a surface treated with carbon, nickel, titanium or silver. As an alloy, an aluminum-cadmium alloy can be preferably used. In addition, calcined carbon, a non-conductive polymer whose surface is treated with a conductive material, a conductive polymer, etc. can also be used.

[0119] The positive electrode current collector can enhance the bonding strength with the positive electrode active material by forming minute irregularities on its surface, and can be in various forms such as a film, a sheet, a foil, a mesh, a net, a porous body, a foam, and a non-woven fabric.

[0120] The positive electrode active material layer may selectively contain a conductive material, a binder, and a positive electrode active material.

[0121] The positive electrode active material may include: layered compounds such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or compounds obtained by substituting one or more transition metals; lithium manganese oxides such as the chemical formula Li 1+x Mn 2-x O4 (0 ≤ x ≤ 0.33), LiMnO3, LiMn2O3 or LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, Li3VO4, V2O5 or Cu2V2O7; Ni-site type lithium nickel oxides represented by the chemical formula LiNi 1-x M x O2 (M = Co, Mn, Al, Cu, Fe, Mg, B or Ga; 0.01 ≤ x ≤ 0.3); LiMn represented by the chemical formula 2-x M xLithium manganese composite oxides represented by O2 (M = Co, Ni, Fe, Cr, Zn or Ta; 0.01 ≤ x ≤ 0.1) or Li2Mn3MO8 (M = Fe, Co, Ni, Cu or Zn); lithium manganese composite oxides with a spinel structure represented by LiNi x Mn 2-x O4; LiCoPO4; LiFePO4; elemental sulfur (S8); sulfur compounds such as Li2S n (n ≥ 1), organic sulfur compounds or carbon-sulfur polymers ((C2S x ) n : x = 2.5 to 50, n ≥ 2), but not limited thereto.

[0122] A conductive material is a material that electrically connects the positive electrode active material and the electrolyte and thus serves as a path for electrons to migrate from the current collector to the active material, and can be used without limitation as long as it has porosity and conductivity and does not cause chemical changes in the formed battery.

[0123] For example, porous carbon-based materials can be used, and such carbon-based materials include: carbon black, graphite, graphene, activated carbon, carbon fiber, etc., and include: metal fibers such as metal meshes; metal powders such as copper, silver, nickel or aluminum; or organic conductive materials such as polyphenylene derivatives. The conductive material can be used alone or as a mixture. Commercially available products as the conductive material may include acetylene black series (products of Chevron Chemical Company or Gulf Oil Company, etc.), Ketjenblack EC series (products of Armak Company), Vulcan XC-72 (products of Cabot Company), Super-P (products of MMM), etc.

[0124] An adhesive is a material included to hold the slurry composition forming the positive electrode on the current collector, and a material that dissolves smoothly in a solvent and can stably form a conductive network having the above active material and conductive material is used. Unless otherwise specifically limited, all adhesives known in the art can be used. For example, as the adhesive, one or a mixture or copolymer of two or more selected from the group consisting of: fluororesin adhesives including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); rubber adhesives including styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber or styrene-isoprene rubber; cellulose adhesives including carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose or regenerated cellulose; polyol adhesives; polyolefin adhesives including polyethylene or polypropylene; polyimide adhesives; polyester adhesives; silane adhesives can be used.

[0125] Similar to the positive electrode, the negative electrode may include a negative electrode active material, a conductive material, and a binder, where the conductive material and the binder are the same as those described above.

[0126] The negative electrode active material follows the instructions provided above.

[0127] The separator is used to physically separate the two electrodes in the lithium secondary battery of the present invention and is not particularly limited as long as it is commonly used as a separator in a lithium secondary battery. In particular, a separator having a low resistance to ion migration of the electrolyte and excellent electrolyte moisturizing ability is preferred.

[0128] The separator may be formed of a porous substrate, and as the porous substrate, all porous substrates commonly used in electrochemical devices can be used. Examples thereof may include polyolefin porous membranes or non-woven fabrics, but are not limited thereto.

[0129] Examples of the polyolefin porous membrane may include a membrane formed of a polymer using only the following substances or a membrane formed of a polymer obtained by mixing the following substances: polyolefin polymers such as polyethylene (e.g., high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene), polypropylene, polybutene, and polyisopentene.

[0130] Examples of non-woven fabrics other than polyolefin non-woven fabrics may include non-woven fabrics formed of a polymer using only the following substances or non-woven fabrics formed of a polymer obtained by mixing the following substances: polyesters such as polyethylene terephthalate, polybutylene terephthalate, polycarbonate, polyacetal, polyamide, polyimide, polyether ether ketone, polyether sulfone, polyphenylene ether, polyphenylene sulfide, polyethylene naphthalate, etc. The non-woven fabric structure may be a spunbond non-woven fabric or a meltblown non-woven fabric formed of long fibers.

[0131] The thickness of the porous substrate is not particularly limited, but may be 1 μm to 100 μm, and preferably 5 μm to 50 μm.

[0132] The size and porosity of the pores present in the porous substrate are also not particularly limited, but the pore size and porosity may be 0.001 μm to 50 μm and 10% to 95%, respectively.

[0133] The electrolyte contains lithium ions and uses them as a medium to generate an electrochemical oxidation or reduction reaction in the positive electrode and the negative electrode.

[0134] The electrolyte may be a non-aqueous electrolyte or a solid electrolyte that does not react with lithium metal, but a non-aqueous electrolyte is preferred and contains an electrolyte salt and an organic solvent.

[0135] The electrolyte salt contained in the non-aqueous electrolyte is a lithium salt. The lithium salt can be used without limitation as long as it is commonly used in electrolytes for lithium secondary batteries. For example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate, lithium imide, etc.

[0136] As the organic solvent contained in the non-aqueous electrolyte, the organic solvents commonly used in electrolytes for lithium secondary batteries can be used without limitation. For example, ethers, esters, amides, linear carbonates, cyclic carbonates, etc. can be used alone or as a mixture of two or more. Among them, ether compounds are usually included.

[0137] Examples of the ether compound can include at least one substance selected from the group consisting of: dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methyl ethyl ether, but are not limited thereto.

[0138] As the ester in the organic solvent, any one or a mixture of two or more selected from the group consisting of: methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone can be used. However, the ester is not limited thereto.

[0139] Specific examples of the linear carbonate compound usually can include any one or a mixture of two or more selected from the group consisting of: dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, but are not limited thereto.

[0140] Specific examples of the cyclic carbonate compound may include any one selected from the group consisting of the following or a mixture of two or more thereof: ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, ethylene ethylene carbonate, and halides thereof. Examples of the halide may include fluoroethylene carbonate (FEC), etc., but are not limited thereto.

[0141] The electrolyte may include at least one electrolyte selected from the group consisting of the following electrolytes: liquid electrolyte, gel polymer electrolyte, and solid polymer electrolyte. A liquid electrolyte may be preferably included.

[0142] Depending on the manufacturing process and required performance of the final product, the non-aqueous electrolyte may be injected at an appropriate stage in the manufacturing process of the electrochemical device. In other words, the non-aqueous electrolyte may be injected at a stage before assembling the electrochemical device or at the final stage of assembling the electrochemical device.

[0143] In addition to winding as an ordinary process, the lithium secondary battery according to the present invention may also undergo a lamination (stacking) and folding process of the separator and the electrode.

[0144] The appearance of the secondary battery is not particularly limited, and various shapes such as cylindrical, laminated, or coin-shaped may be adopted.

[0145] Mode of Invention

[0146] Examples and Comparative Examples: Preparation of Anode Active Material

[0147] [Example 1]

[0148] To prepare a coating composition containing a precursor having an atomic ratio of lithium to phosphorus of 2:1, 0.0034 M of hexachlorophosphazene and 0.0204 M of lithium hydroxide hydrate were dissolved in 20 mL of a mixture of water and 2-methoxyethanol (1:19 (volume ratio)) by heating at 70 °C for 40 minutes, thereby preparing the coating composition.

[0149] Under non-active conditions, graphite having an average particle diameter of 15 μm was introduced into the coating composition such that the ratio of the active material to the coating composition was 0.5% by weight, and the resulting product was stirred at 50 °C for 1 hour and then vacuum dried to remove the organic solvent.

[0150] Subsequently, heat treatment was carried out at 500 °C under non-active nitrogen conditions to prepare an anode active material having a protective layer formed thereon.

[0151] [Example 2]

[0152] The negative electrode active material was prepared in the same manner as in Example 1, except that the ratio of the active material to the coating composition of the precursor having an atomic ratio of lithium and phosphorus of 2:1 was 0.1% by weight.

[0153] [Example 3]

[0154] The negative electrode active material was prepared in the same manner as in Example 1, except that the ratio of the active material to the coating composition of the precursor having an atomic ratio of lithium and phosphorus of 2:1 was 0.05% by weight.

[0155] [Comparative Example 1]

[0156] Graphite having an average particle size of 15 μm without forming a protective layer was used as the negative electrode active material.

[0157] Experimental Example 1. Scanning Electron Microscope Analysis

[0158] The negative electrode active materials prepared in Example 1 and Comparative Example 1 were analyzed using a scanning electron microscope (SEM) (model name: S-4800, manufactured by Hitachi, Ltd.) and an energy dispersive spectrometer (EDS) (model name: SNE-3000M, manufactured by Bruker Corporation). The results obtained here are shown in Figures 1 to 4 in.

[0159] When referring to Figure 1 , the surface of the negative electrode active material of Comparative Example 1 was exposed as it was, while in Figure 2 , it was confirmed that the negative electrode active material according to Example 1 had a protective layer uniformly formed on the surface.

[0160] Specifically, the elemental distribution and content of the protective layer could be identified by EDS elemental analysis. As Figure 3 shown, the phosphorus (P) element derived from the precursor of metal-phosphorus-nitrogen oxide was uniformly coated on the entire surface of the negative electrode active material, and by Figure 4 , it was confirmed that the content of the phosphorus (P) element was about 0.3% by weight.

[0161] Experimental Example 2. Battery Performance Evaluation

[0162] Negative electrodes were prepared using various negative electrode active materials prepared in the examples and comparative examples.

[0163] Specifically, 0.9 g of various negative electrode active materials prepared in the examples and comparative examples, 0.05 g of a binder (PVDFHFP), and 0.05 g of a conductive material (Super C-65) were mixed with N-methylpyrrolidone (NMP) to prepare a negative electrode slurry. The negative electrode slurry was coated on a copper current collector having a thickness of 20 μm, and the resulting product was dried to prepare a negative electrode having a thickness of 200 μm.

[0164] As the counter electrode, a lithium metal foil with a thickness of 150 μm was used.

[0165] The negative electrode and the counter electrode prepared above were placed facing each other, and after inserting a polyethylene separator therebetween, a mixed solvent (25:50:25 (volume ratio)) of ethylene carbonate, diethyl carbonate, and dimethyl carbonate dissolved with 1 M LiPF6 as the electrolyte was injected therein to fabricate a half cell.

[0166] For the battery fabricated using the above method, the initial negative electrode efficiency was evaluated using the constant current - constant voltage method. The results obtained here are shown in Table 1 below.

[0167] [Table 1]

[0168] Efficiency of the first time (%) Example 1 93.45 Example 2 92.9 Example 3 91.69 Comparative Example 1 91.43

[0169] When referring to Table 1, it was confirmed that the negative electrode containing the negative electrode active material according to the present invention had excellent initial efficiency characteristics compared with the comparative example.

[0170] Specifically, it was confirmed that Examples 1 to 3 in which a protective layer was formed had excellent initial efficiency compared with Comparative Example 1 in which no protective layer was formed by suppressing the initial irreversibility with the electrolyte.

Claims

1. A method for preparing a negative electrode active material having a protective layer, the method comprising the following steps: (a) Preparing a coating composition containing a precursor of a metal-phosphorus-nitrogen oxide; (b) Using a solution method to form a precursor layer on the negative electrode active material using the coating composition; and (c) Forming a metal-phosphorus-nitrogen oxide protective layer on the negative electrode active material by heat-treating the negative electrode active material having the precursor layer formed thereon, wherein in step (c), the heat treatment is carried out in a temperature range of 150 °C to 500 °C, wherein the precursor of the metal-phosphorus-nitrogen oxide is prepared by the reaction of a compound containing a phosphorus-nitrogen bond with a metal salt compound, wherein the compound containing a phosphorus-nitrogen bond contains at least one compound selected from the compounds represented by Chemical Formula 1 and 3 below: [Chemical Formula 1] wherein X1 are the same as or different from each other and are each independently OR1, F, Cl, Br or I, wherein R1 is an alkyl group having 1 to 5 carbon atoms; [Chemical Formula 3] wherein X2 are the same as or different from each other and are each independently R4, OR5, NR6R7, F, Cl, Br or I, wherein R4 to R7 are the same as or different from each other and are each independently an alkyl group having 1 to 10 carbon atoms or an aryl group having 6 to 20 carbon atoms, and n is an integer of 100 to 100,000.

2. The method for preparing a negative electrode active material having a protective layer according to claim 1, wherein the metal salt compound contains at least one element selected from the group consisting of the following elements: lithium, sodium, magnesium, calcium, zinc and aluminum.

3. The method for preparing a negative electrode active material having a protective layer according to claim 1, wherein the coating composition contains a compound containing a phosphorus-nitrogen bond, a metal salt compound and an organic solvent.

4. The method for preparing a negative electrode active material having a protective layer according to claim 3, wherein based on the total weight of the coating composition, the coating composition contains 0.002% to 27% by weight of the compound containing a phosphorus-nitrogen bond, 0.0005% to 12% by weight of the metal salt compound and 70% to 99.99% by weight of the organic solvent.

5. The method for preparing a negative electrode active material having a protective layer according to claim 3, wherein the coating composition further contains a chalcogen element compound.

6. The method for preparing a negative electrode active material having a protective layer according to claim 1, wherein the negative electrode active material contains at least one material selected from the group consisting of the following materials: lithium metal, lithium alloy, transition metal composite oxide, silicon-based material and carbon-based material.

7. The method for preparing a negative electrode active material having a protective layer according to claim 1, wherein the negative electrode active material has a spherical, elliptical, spindle-shaped, scaly, plate-shaped, fibrous, rod-shaped, core-shell-shaped or irregular shape.

8. The method for preparing a negative electrode active material with a protective layer according to claim 1, wherein in step (b), the solution method uses at least one of the following methods: spraying, spin coating, dip coating, inkjet printing, offset printing, reverse offset printing, gravure printing, and roll printing.

9. The method for preparing a negative electrode active material with a protective layer according to claim 1, wherein in step (c), the heat treatment is carried out in a temperature range above 150 °C and below 500 °C.

10. The method for preparing a negative electrode active material with a protective layer according to claim 1, wherein before step (c), there is further a step of removing the organic solvent contained in the precursor layer formed in step (b).

11. The method for preparing a negative electrode active material with a protective layer according to claim 1, wherein the metal-phosphorus-nitrogen oxide protective layer formed in step (c) contains amorphous metal-phosphorus-nitrogen oxide or its derivatives.

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