Secondary battery

Abstract

An embodiment of the present invention provides a secondary battery that comprises a positive electrode active material composite including a positive electrode active material containing an oxide containing Li and Co, and two or more coating layers at least partially covering the positive electrode active material, wherein the two or more coating layers include a first coating layer and a second coating layer from the positive electrode active material toward the outside, one of the first coating layer and the second coating layer contains the element Al, and the other of the first coating layer and the second coating layer contains at least one element selected from the group consisting of F, Mg, Mn, and Ti.

Description

secondary battery

[0001] The present invention relates to a secondary battery.

[0002] Secondary batteries that can be repeatedly charged and discharged have been used for various purposes. For example, secondary batteries are used as power sources for electronic devices such as smartphones and laptop computers.

[0003] A secondary battery has a structure in which an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes, and an electrolyte are housed in an exterior body. The positive electrode has a positive electrode material layer containing a positive electrode active material, and the negative electrode has a negative electrode material layer containing a negative electrode active material.

[0004] International Publication No. 2019 / 159563 Patent Publication No. 2017-531911 Patent Publication No. 2002-279991

[0005] In some secondary batteries, a first coating is applied to the surface of the cathode active material, a component of the battery, and a second coating is applied to the first coating to suppress side reactions with the electrolyte. In this case, the presence of these coatings can hinder the migration of lithium ions during charging and discharging of the battery, increasing the internal resistance of the battery. Therefore, it is necessary to more effectively achieve both the suppression of side reactions with the electrolyte and the ease of migration of lithium ions during charging and discharging of the battery.

[0006] The present invention has been devised in view of the above circumstances. Specifically, an object of the present invention is to provide a secondary battery that can more suitably achieve both suppression of side reactions with an electrolyte and ease of lithium ion migration during charging and discharging of the battery.

[0007] In order to achieve the above object, one embodiment of the present invention provides a secondary battery comprising a cathode active material composite including a cathode active material containing an oxide containing Li and Co, and two or more coating layers at least partially covering the cathode active material, wherein the two or more coating layers comprise, from the cathode active material side toward the outside, a first coating layer and a second coating layer, one of the first coating layer and the second coating layer containing Al element, and the other of the first coating layer and the second coating layer containing at least one element selected from the group consisting of F, Mg, Mn, and Ti.

[0008] According to the secondary battery according to one embodiment of the present invention, it is possible to more suitably achieve both suppression of side reactions with the electrolyte and ease of movement of lithium ions during charging and discharging of the battery.

[0009] 1 is a cross-sectional view schematically showing the basic structure of an electrode-constituting layer, and FIG. 2 is a cross-sectional view schematically showing a positive electrode active material composite of a secondary battery according to one embodiment of the present invention.

[0010] Hereinafter, a secondary battery according to an embodiment of the present invention will be described with reference to the drawings. The various elements in the drawings are merely shown schematically and exemplarily to facilitate understanding of the present invention, and the appearance, dimensional ratio, etc. may differ from the actual ones.

[0011] The term "secondary battery" as used herein refers to a battery capable of repeated charging and discharging. The term "secondary battery" is not limited to its name and may also encompass, for example, "electricity storage device." The term "cross-sectional view" as used herein refers to a state when viewed from a direction substantially perpendicular to the thickness direction based on the stacking direction of the electrode materials constituting the secondary battery. The terms "vertical direction" and "horizontal direction" used directly or indirectly in this specification correspond to the vertical direction and horizontal direction in the drawings, respectively. In a preferred embodiment, the vertical downward direction (i.e., the direction in which gravity acts) can be considered to correspond to the "downward direction," and the opposite direction can be considered to correspond to the "upward direction."

[0012] The various numerical ranges mentioned in this specification are intended to include both the lower and upper limits. For example, a numerical range such as 1 to 10 can be interpreted as including the lower limit of 1 and the upper limit of 10.

[0013] [Basic Structure of Secondary Battery] First, the basic structure of a secondary battery will be described. A secondary battery has a structure in which an electrode assembly and an electrolyte are housed and sealed inside an exterior housing. The electrode assembly may include a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The electrode assembly may be a stacked electrode assembly or a wound (jelly roll) electrode assembly. A stacked electrode assembly is formed by stacking multiple electrode constituent layers, each including a positive electrode, a negative electrode, and a separator. A wound electrode assembly is formed by winding an electrode constituent layer, each including a positive electrode, a negative electrode, and a separator. Furthermore, for example, the electrode assembly may have a so-called stack-and-fold structure in which a positive electrode, a separator, and a negative electrode are stacked on a long film and then folded.

[0014] The positive electrode 10A is composed of at least a positive electrode current collector 11A and a positive electrode material layer 12A (see FIG. 1 ), with the positive electrode material layer 12A provided on at least one side of the positive electrode current collector 11A. A positive electrode side lead tab is located at a portion of the positive electrode current collector 11A where the positive electrode material layer 12A is not provided, i.e., at an end of the positive electrode current collector 11A. The positive electrode material layer 12A contains a positive electrode active material as an electrode active material. The negative electrode 10B is composed of at least a negative electrode current collector 11B and a negative electrode material layer 12B (see FIG. 1 ), with the negative electrode material layer 12B provided on at least one side of the negative electrode current collector 11B. A negative electrode side lead tab is located at a portion of the negative electrode current collector 11B where the negative electrode material layer 12B is not provided, i.e., at an end of the negative electrode current collector 11B. The negative electrode material layer 12B contains a negative electrode active material as an electrode active material.

[0015] The positive electrode active material contained in the positive electrode layer 12A and the negative electrode active material contained in the negative electrode layer 12B are materials directly involved in the transfer of electrons in the secondary battery and are the main materials of the positive and negative electrodes responsible for charge and discharge, i.e., the battery reaction. More specifically, the "positive electrode active material contained in the positive electrode layer 12A" and the "negative electrode active material contained in the negative electrode layer 12B" provide ions to the electrolyte, and these ions move between the positive electrode 10A and the negative electrode 10B, transferring electrons and causing charge and discharge. The positive electrode layer 12A and the negative electrode layer 12B are preferably layers capable of absorbing and releasing lithium ions. In other words, a secondary battery in which lithium ions move between the positive electrode 10A and the negative electrode 10B via the electrolyte to charge and discharge the battery is preferred. When lithium ions are involved in charge and discharge, the secondary battery corresponds to a so-called "lithium ion battery."

[0016] The positive electrode active material of the positive electrode layer 12A is, for example, made of granules, and preferably contains a binder to ensure sufficient contact between the particles and maintain their shape. Furthermore, the positive electrode layer 12A may contain a conductive additive to facilitate the transfer of electrons that drive the battery reaction. Similarly, the negative electrode active material of the negative electrode layer 12B is, for example, made of granules, and preferably contains a binder to ensure sufficient contact between the particles and maintain their shape. The negative electrode layer 12B may contain a conductive additive to facilitate the transfer of electrons that drive the battery reaction. Because they contain multiple components, the positive electrode layer 12A and the negative electrode layer 12B may also be referred to as a "positive electrode composite layer" and a "negative electrode composite layer," respectively.

[0017] The positive electrode active material is preferably a material that contributes to the absorption and desorption of lithium ions. From this perspective, the positive electrode active material is preferably, for example, a lithium-containing composite oxide. More specifically, the positive electrode active material is preferably a lithium-transition metal composite oxide containing lithium and at least one transition metal selected from the group consisting of cobalt, nickel, manganese, and iron. In other words, such a lithium-transition metal composite oxide is preferably contained as the positive electrode active material in the positive electrode layer 12A of the secondary battery. For example, the positive electrode active material may be lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium iron phosphate, or a material in which part of the transition metal is replaced with another metal. Such positive electrode active materials may be contained alone or in combination of two or more types. Note that, in the following description of the features of the present invention and subsequent sections, it is assumed that the positive electrode active material contained in the positive electrode layer 12A is lithium cobalt oxide.

[0018] The binder that can be contained in the positive electrode layer 12A is not particularly limited, but can include at least one selected from the group consisting of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, etc. The conductive additive that can be contained in the positive electrode layer 12A is not particularly limited, but can include at least one selected from the group consisting of carbon black such as thermal black, furnace black, channel black, ketjen black, and acetylene black; carbon fibers such as graphite, carbon nanotubes, and vapor-grown carbon fibers; metal powders such as copper, nickel, aluminum, and silver; and polyphenylene derivatives. For example, the binder of the positive electrode layer 12A can be polyvinylidene fluoride. By way of example only, the conductive additive of the positive electrode layer 12A is carbon black. Furthermore, the binder and conductive additive of the positive electrode layer 12A may be a combination of polyvinylidene fluoride and carbon black.

[0019] The negative electrode active material is preferably a material that contributes to the absorption and desorption of lithium ions, and from this perspective, the negative electrode active material is preferably, for example, any of various carbon materials, oxides, or lithium alloys.

[0020] Examples of various carbon materials for the negative electrode active material include graphite (natural graphite, artificial graphite), soft carbon, hard carbon, and diamond-like carbon. Graphite is particularly preferred due to its high electronic conductivity and excellent adhesion to the negative electrode current collector 11B. Examples of oxides for the negative electrode active material include at least one selected from the group consisting of silicon oxide, tin oxide, indium oxide, zinc oxide, and lithium oxide. The lithium alloy for the negative electrode active material may be any metal capable of forming an alloy with lithium, such as a binary, ternary, or higher alloy of lithium with a metal such as Al, Si, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn, or La. Such oxides preferably have an amorphous structure, as this reduces degradation due to inhomogeneities such as grain boundaries or defects. By way of example only, the negative electrode active material for the negative electrode material layer 12B may be artificial graphite.

[0021] The binder that can be contained in the negative electrode material layer 12B is not particularly limited, but can include at least one selected from the group consisting of styrene-butadiene rubber, polyacrylic acid, polyvinylidene fluoride, polyimide resin, and polyamide-imide resin. For example, the binder contained in the negative electrode material layer 12B can be styrene-butadiene rubber. The conductive additive that can be contained in the negative electrode material layer 12B is not particularly limited, but can include at least one selected from carbon black such as thermal black, furnace black, channel black, ketjen black, and acetylene black; carbon fiber such as graphite, carbon nanotubes, and vapor-grown carbon fiber; metal powder such as copper, nickel, aluminum, and silver; and polyphenylene derivatives. The negative electrode material layer 12B may also include a component derived from a thickener component (e.g., carboxymethyl cellulose) used during battery manufacture.

[0022] By way of example only, the negative electrode active material and binder in the negative electrode material layer 12B may be a combination of artificial graphite and styrene-butadiene rubber.

[0023] The positive electrode current collector 11A and the negative electrode current collector 11B used in the positive electrode 10A and the negative electrode 10B are components that contribute to collecting and supplying electrons generated in the active material due to the battery reaction. Such current collectors may be sheet-like metal members and may be porous or perforated. For example, the current collectors may be metal foil, punched metal, mesh, expanded metal, or the like. The positive electrode current collector 11A used in the positive electrode 10A is preferably made of a metal foil containing at least one selected from the group consisting of aluminum, stainless steel, nickel, and the like, and may be, for example, aluminum foil. On the other hand, the negative electrode current collector 11B used in the negative electrode 10B is preferably made of a metal foil containing at least one selected from the group consisting of copper, stainless steel, nickel, and the like, and may be, for example, copper foil.

[0024] The separator is a component provided from the viewpoint of preventing short circuits due to contact between the positive and negative electrodes and maintaining electrolyte retention. In other words, the separator is a component that allows ions to pass through while preventing electronic contact between the positive electrode 10A and the negative electrode 10B. Preferably, the separator is a porous or microporous insulating component, and has a membrane form due to its small thickness. Although merely exemplary, a microporous membrane made of polyolefin may be used as the separator. In this regard, the microporous membrane used as the separator may contain, for example, only polyethylene (PE) or only polypropylene (PP) as the polyolefin. Furthermore, the separator may be a laminate composed of a "PE microporous membrane" and a "PP microporous membrane." The surface of the separator may be covered with an inorganic particle coating layer and / or an adhesive layer, etc. The surface of the separator may have adhesive properties.

[0025] The separator should not be limited to a specific name, and may be a solid electrolyte, a gel electrolyte, insulating inorganic particles, or the like, which have similar functions. From the perspective of further improving the ease of handling of the electrodes, it is preferable that the separator and the electrode (positive electrode 10A / negative electrode 10B) are bonded together. Bonding between the separator and the electrode can be achieved by using an adhesive separator, applying an adhesive binder to the electrode material layer (positive electrode material layer 12A / negative electrode material layer 12B) and / or by thermocompression bonding. Examples of adhesive binder materials that provide adhesiveness to the separator or electrode material layer include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene polymer, and acrylic resin. The thickness of the adhesive layer formed by applying an adhesive binder or the like may be 0.5 μm or more and 5 μm or less.

[0026] When the positive electrode 10A and the negative electrode 10B have layers capable of absorbing and releasing lithium ions, the electrolyte is preferably a "non-aqueous" electrolyte such as an organic electrolyte and / or an organic solvent (i.e., the electrolyte is preferably a non-aqueous electrolyte). The electrolyte contains metal ions released from the electrodes (positive electrode 10A and negative electrode 10B), and therefore the electrolyte assists the migration of metal ions in the battery reaction.

[0027] The nonaqueous electrolyte is an electrolyte containing a solvent and a solute. A specific solvent for the nonaqueous electrolyte preferably contains at least a carbonate. Such a carbonate may be a cyclic carbonate and / or a chain carbonate. While not particularly limited, the cyclic carbonate may include at least one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and vinylene carbonate (VC). The chain carbonate may include at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dipropyl carbonate (DPC). By way of example only, a combination of a cyclic carbonate and a chain carbonate may be used as the nonaqueous electrolyte, such as a mixture of ethylene carbonate and diethyl carbonate. As a specific solute of the non-aqueous electrolyte, a Li salt such as LiPF6, LiBF4, etc. is preferably used. As a specific solute of the non-aqueous electrolyte, a Li salt such as LiPF6 and / or LiBF4 is preferably used.

[0028] Any current collecting leads used in the field of secondary batteries can be used as the positive electrode current collecting lead and the negative electrode current collecting lead. Such current collecting leads may be made of any material that allows electron transfer, such as conductive materials such as aluminum, nickel, iron, copper, and stainless steel. The positive electrode current collecting lead is preferably made of aluminum, and the negative electrode current collecting lead is preferably made of nickel. The shape of the positive electrode current collecting lead and the negative electrode current collecting lead is not particularly limited and may be, for example, wire-like or plate-like.

[0029] Any external terminal used in the field of secondary batteries can be used as the external terminal. Such external terminals may be made of any material that allows electron transfer, typically a conductive material such as aluminum, nickel, iron, copper, or stainless steel. The external terminals 5 may be electrically connected directly to the substrate, or electrically connected indirectly to the substrate via another device. However, this is not limited thereto. A positive electrode current collecting lead connected to each of the multiple positive electrodes may function as a positive electrode external terminal, and a negative electrode current collecting lead connected to each of the multiple negative electrodes may function as a negative electrode external terminal.

[0030] The exterior body may take the form of a conductive hard case or a flexible case (e.g., a pouch). When the exterior body is in the form of a flexible case (e.g., a pouch), each of the multiple positive electrodes is connected to a positive electrode external terminal via a positive electrode current collecting lead. The positive electrode external terminal is fixed to the exterior body by a seal portion, which prevents leakage of the electrolyte. Similarly, each of the multiple negative electrodes is connected to a negative electrode external terminal via a negative electrode current collecting lead. The negative electrode external terminal is fixed to the exterior body by a seal portion, which prevents leakage of the electrolyte. However, this is not limited thereto, and the positive electrode current collecting lead connected to each of the multiple positive electrodes may have the function of a positive electrode external terminal, and the negative electrode current collecting lead connected to each of the multiple negative electrodes may have the function of a negative electrode external terminal. When the exterior body is in the form of a conductive hard case, each of the multiple positive electrodes is connected to a positive electrode external terminal via a positive electrode current collecting lead. The positive electrode external terminal is fixed to the exterior body by a seal portion, and the seal portion prevents leakage of the electrolyte.

[0031] The conductive hard case consists of a main body and a lid. The main body consists of a bottom and side sections that form the bottom surface of the exterior body. The main body and lid are sealed together after housing the electrode assembly, electrolyte, current collecting leads, and external terminals. The sealing method is not particularly limited, and examples include laser irradiation. Any material capable of forming a hard case-type exterior body in the field of secondary batteries can be used as the material for the main body and lid. Such materials may be any material capable of achieving electron transfer, including conductive materials such as aluminum, nickel, iron, copper, and stainless steel. The dimensions of the main body and lid are determined primarily based on the dimensions of the electrode assembly. For example, it is preferable that the dimensions are large enough to prevent the electrode assembly from moving (shifting) within the exterior body when the electrode assembly is housed. Preventing electrode assembly movement prevents damage to the electrode assembly and improves the safety of the secondary battery.

[0032] The flexible case is composed of a soft sheet. The soft sheet is sufficient to allow the sealing portion to be folded, and is preferably a plastic sheet. The plastic sheet is a sheet that maintains its deformation due to an external force after the external force is applied and then removed, and a so-called laminated film can be used, for example. A flexible pouch made of a laminated film can be produced, for example, by overlapping two laminated films and heat-sealing their periphery. A typical laminated film is a film made by laminating a metal foil and a polymer film, and a specific example is a three-layer structure consisting of an outer layer polymer film / metal foil / inner layer polymer film. The outer layer polymer film is intended to prevent moisture permeation and damage to the metal foil due to contact, and polymers such as polyamide and polyester are suitable. The metal foil is intended to prevent moisture and gas permeation, and foils such as copper, aluminum, and stainless steel are suitable. The inner layer polymer film protects the metal foil from the electrolyte contained inside and serves to melt and seal the opening during heat sealing, and polyolefin or acid-modified polyolefin can be suitably used.

[0033] [Characteristics of the Present Invention] The characteristic features of the present invention will be described below. The characteristic features of the present invention relate to a cathode active material, which is a component of the secondary battery. Compared to conventional cathode active materials, the present invention is characterized in that it includes a cathode active material composite 5 including a cathode active material 1 and two or more coating layers that at least partially cover the cathode active material 1 (see FIG. 2). FIG. 2 is a cross-sectional view schematically showing a cathode active material composite of a secondary battery according to one embodiment of the present invention. This cathode active material composite can also be referred to as a cathode active material with a coating layer.

[0034] In the positive electrode active material composite 5, two or more coating layers at least partially covering the positive electrode active material 1 include, from the positive electrode active material 1 side toward the outside, a first coating layer 2 and a second coating layer 3. In this specification, the two or more coating layers may at least partially cover the surface of the positive electrode active material 1, and preferably cover the entire surface. In the present invention, one of the first coating layer 2 and the second coating layer 3 contains Al element, and the other of the first coating layer 2 and the second coating layer 3 contains at least one element selected from the group consisting of F, Mg, Mn, and Ti. Preferably, one of the first coating layer 2 and the second coating layer 3 is made of Al element. The positive electrode active material 1 and the two or more coating layers may both be sintered bodies.

[0035] According to this configuration, the presence of the coating layer can suppress contact between the cathode active material (core) located inside and the electrolyte, compared to when no coating layer is present, thereby improving the voltage drop after the battery is stored in a charged state.

[0036] Furthermore, by selecting specific elements for the constituent elements of at least the first coating layer 2 and the second coating layer 3, even when the coating layer is composed of two or more layers, it is possible to suppress the impediment of lithium ion migration during battery charge and discharge, and ensure the ease of lithium ion migration, compared to when the number of coating layers is one. This allows for improved discharge rate characteristics compared to when the number of coating layers is one. Furthermore, it is possible to reduce the pre-cycling resistance and the rate of increase in battery resistance. As a result, it is possible to improve battery characteristics overall.

[0037] As used herein, the term "pre-cycle resistance" refers to the battery resistance before cycling. The term "rate of resistance increase" refers to the rate of resistance increase before and after charging and discharging. The term "discharge rate" refers to the relative ratio of the discharge current to the battery capacity.

[0038] In addition, compared to when one of the first coating layer 2 and the second coating layer 3 is composed of Al element and the other of the first coating layer 2 and the second coating layer 3 is composed of one element selected from the group consisting of F, Mg, Mn, and Ti, when the other of the first coating layer 2 and the second coating layer 3 contains two or more of the above elements, it is possible to improve the discharge rate characteristics and reduce the pre-cycle resistance and the rate of increase in resistance of the battery.

[0039] When the first coating layer 2 contains the elements F, Mg, and Mn and the second coating layer 3 contains the element Al, the discharge rate characteristics can be improved compared to when the first coating layer 2 contains the element Al and the second coating layer 3 contains the elements F, Mg, and Mn.

[0040] When the second coating layer 3 contains the elements Ti, Mg, and Mn, it is possible to improve the discharge rate characteristics and reduce the pre-cycling resistance and the rate of increase in the resistance of the battery compared to when it contains the elements F, Mg, and Mn. In other words, it can be said that these effects are more easily achieved when the second coating layer 3 contains more Ti than F. At least one of the first coating layer 2 and the second coating layer 3 may have the oxide form of the above-mentioned constituent elements.

[0041] In the present invention, the cathode active material 1 preferably contains trivalent Co ions (Co3+). However, the present invention is not limited thereto, and the cathode active material 1 may also contain divalent or tetravalent Co ions. By controlling the valence of the Co ions contained in the cathode active material 1 to be trivalent, charge imbalance may occur on the surface of the cathode active material 1. This charge imbalance allows two or more coating layers (which may also be referred to as composite coating layers) to adhere favorably to the surface of the cathode active material. This improves overall electronic conductivity, and reduces the reaction resistance of the battery during battery charge and discharge, even when the coating layer has two or more layers.

[0042] Furthermore, when the above-described positive electrode active material 1 contains trivalent Co ions (Co3+), it is preferable that the positive electrode active material 1 contains both Co3+ in a low spin (LS) state and Co3+ in an intermediate spin (IS) state. In this case, the ratio of Co3+ in the low spin (LS) state to Co3+ in the intermediate spin (IS) state is preferably 5% or more from the viewpoint of favorable charge bias on the surface of the positive electrode active material 1, and may be 1% or less from the viewpoint of stability of the crystal structure.

[0043] Spin determines the magnetic properties of electrons and is a factor in the 3d electron orbital. In this 3d electron orbital, Co electrons can have two types of orbitals: a t2g orbital that extends in the direction away from oxygen, and an eg orbital that extends in the direction where oxygen is present. Because the t2g orbital extends in the direction away from oxygen, it is energetically more stable than the eg orbital. The t2g orbital has three orbitals, and each orbital can accommodate a total of two electrons, consisting of an up spin and a down spin. The t2g orbital can accommodate a total of six electrons. On the other hand, the eg orbital has two orbitals, and each orbital can accommodate a total of two electrons, consisting of an up spin and a down spin. The eg orbital can accommodate a total of four electrons.

[0044] Based on this configuration, if the six 3d electrons of a trivalent Co ion (Co3+) are placed in a stable t2g orbital, exactly six electrons will be placed in the t2g orbital, with three electrons pointing up and three electrons pointing down, which cancel each other out, resulting in a total spin of S = 0. In this specification, this state may be referred to as a low spin state.

[0045] On the other hand, in Co atoms, it may be possible to choose to align the spin direction rather than aligning the spin direction in a stable orbital. In this case, the spin-up electrons enter the t2g orbital and the eg orbital in order to align the spin direction. In this state, the spins of five electrons can be arranged with an upward orientation. Meanwhile, the remaining electron can enter the t2g orbital, which has stable energy, with a downward spin. As a result, a high spin state of S=2 can be achieved.

[0046] The intermediate spin (IS) state referred to in this specification refers to a state between the low spin state and the high spin state. Specifically, of the four electron spins, three electrons may be arranged with their spins pointing up into the t2g orbital and one electron may be arranged with their spins pointing up into the eg orbital. Meanwhile, the remaining two electrons may be arranged with their spins pointing down into the t2g orbital, which has stable energy. As a result, an intermediate spin state of S=1 may be achieved.

[0047] The inclusion of Co3+ in such an intermediate spin state in the positive electrode active material can facilitate charge bias, thereby facilitating the diffusion of lithium ions from the inner positive electrode active material 1 to the outer coating composite layer, thereby improving discharge rate characteristics and further reducing the pre-cycling resistance and the rate of increase in resistance of the battery.

[0048] Furthermore, from the viewpoint of alleviating stress due to expansion / contraction of the positive electrode active material during charge / discharge, the thickness of the first coating layer 2 may be 0.1 μm or more. Furthermore, from the viewpoint of not interfering with the movement of lithium ions during charge / discharge, the thickness of the first coating layer 2 may be 2 μm or less. Furthermore, from the viewpoint of suppressing contact between the positive electrode active material 1 and the first coating layer and the electrolyte, the thickness of the second coating layer 3 may be 0.1 μm or more. For example, the thickness of the second coating layer 3 may be smaller than the average particle size of the particles of the positive electrode active material 1, for example, 100 nm or less. In this case, the particle size of the positive electrode active material itself may be 3 μm or more and 30 μm or less in terms of D50 (corresponding to the median diameter).

[0049] [Method for Producing Secondary Battery] Hereinafter, a method for producing a secondary battery according to one embodiment of the present invention, which includes the step of producing the above-described positive electrode active material composite 5 (positive electrode active material + two or more coating layers), will be outlined.

[0050] First, a cobalt-containing material is mixed with a lithium carbonate solution so that the ratio of Co to Li (Li / Co) is 1.05. This mixture is subjected to primary firing in an air atmosphere. Specifically, the mixture is pre-fired at a temperature of 500°C to 700°C for a predetermined time (e.g., 4 hours to 6 hours), and then fired at a temperature of 800°C to 900°C for a predetermined time (e.g., 12 hours to 20 hours). This allows the composite oxide cathode active material 1 to be obtained.

[0051] The resulting positive electrode active material 1 is then immersed in either an Al-containing solution or a solution containing at least one element selected from the group consisting of F, Mg, Mn, and Ti, and then dried at room temperature (about 25° C.), thereby forming a precursor of the first coating layer.

[0052] After forming the precursor of the first coating layer, the precursor is immersed in the other of the Al-containing solution and the solution containing at least one element selected from the group consisting of F, Mg, Mn, and Ti, and then dried at room temperature (about 25° C.), thereby forming a precursor of the second coating layer.

[0053] After forming the precursor of the second coating layer, secondary baking is performed with the precursors of the two coating layers attached to the positive electrode active material. The secondary baking is performed at a baking temperature of 600°C to 1100°C at a temperature increase rate of 80°C to 120°C per minute (°C / min). The holding time can be 5 minutes to 250 minutes. This process allows the positive electrode active material composite to be obtained.

[0054] After obtaining the positive electrode active material composite, elemental mapping of this positive electrode active material composite is performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer). Specifically, the positive electrode active material composite is irradiated with X-rays using the EDX analyzer, and the energy (or wavelength) and intensity of the generated fluorescent X-rays are analyzed, thereby allowing the type and content of elements constituting each coating layer to be analyzed.

[0055] After the positive electrode active material composite is formed, the positive electrode active material composite, the binder described above, and the conductive additive are weighed out in predetermined proportions and mixed with a solvent to form a slurry for the positive electrode material layer. Similarly, the negative electrode active material, the binder, and the conductive additive are weighed out in predetermined proportions and mixed with a solvent to form a slurry for the negative electrode material layer. The positive electrode material layer slurry is then applied to a positive electrode current collector to form a positive electrode precursor. The negative electrode material layer slurry is also applied to a negative electrode current collector to form a negative electrode precursor. Each electrode precursor is then dried and pressed to finally form the desired positive and negative electrodes.

[0056] Thereafter, the formed positive electrodes and negative electrodes are alternately stacked with separators interposed therebetween to form an electrode assembly, and the formed electrode assembly is housed in an outer casing and filled with the above-described electrolytic solution, thereby finally producing a secondary battery according to one embodiment of the present invention.

[0057] Examples of the present invention will be described below.

[0058] Example 1 [Step of preparing a positive electrode active material] Cobalt (II) sulfate heptahydrate (FUJIFILM Wako Pure Chemical Industries, Ltd.) was dissolved in distilled water to a molar concentration of 1.5 mol / L to obtain a cobalt sulfate aqueous solution. Ammonium sulfate (FUJIFILM Wako Pure Chemical Industries, Ltd.) was dissolved in distilled water to a molar concentration of 1.5 mol / L to obtain an ammonium sulfate aqueous solution.

[0059] Next, distilled water was placed in a 2 L reaction vessel and heated to 50°C using a heater. The sulfate aqueous solution and ammonium sulfate aqueous solution were added while stirring the solution in the reaction vessel with a paddle-type stirring blade. The sulfate aqueous solution was added at a rate of 5.0 g / min. The ammonium sulfate aqueous solution was added so as to maintain the pH at 8.0. Each solution was added over 24 hours, resulting in the precipitation of a coprecipitate.

[0060] The resulting coprecipitate was washed by repeated pressure filtration and dispersion in distilled water to remove impurity ions. The washing was completed when the electrical conductivity of the filtrate became less than 20 mS / m. The washed coprecipitate was heated at 100°C for 10 hours and dried.

[0061] Next, the obtained coprecipitate and lithium carbonate were mixed so that the molar ratio of Li to Co (Li / Co) was 1.05. This mixture was subjected to primary firing in an air atmosphere. Specifically, the mixture was pre-fired at 600°C for 5 hours, and then fired at 845°C for 16 hours. This resulted in a positive electrode active material made of a composite oxide containing Co and Li. The primary firing was performed using a firing apparatus.

[0062] [Positive Electrode Active Material Coating Treatment Step] The obtained positive electrode active material was then immersed in an Al(OH)3 solution (concentration: 10%) for 1 hour and then dried at room temperature (about 25°C), thereby forming a precursor of the first coating layer.

[0063] After forming the precursor of the first coating layer, the precursor was immersed in an MgF2 solution (concentration: 10%) for 1 hour and then dried at room temperature (about 25°C), thereby forming a precursor of the second coating layer.

[0064] After forming the precursor of the second coating layer, secondary baking was performed with the precursors of the two coating layers attached to the positive electrode active material. The secondary baking was performed at a baking temperature of 1050°C, with a temperature increase rate of 100°C per minute (°C / min). The holding time was 0.5 hours. The secondary baking was performed using a baking device. As a result of the above, a positive electrode active material composite was obtained.

[0065] [Step of elemental analysis of coating layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al element. In addition, it was found that the second coating layer attached to the first coating layer was composed of F element.

[0066] [Cell Fabrication Process] After forming the positive electrode active material composite, the positive electrode active material composite, a binder (polyvinylidene fluoride), and a conductive additive (carbon black) were weighed out in predetermined proportions and mixed with NMP (N-methyl-2-pyrrolidone) solvent to form a positive electrode material layer slurry. Similarly, a negative electrode active material (graphite), a binder (polyvinylidene fluoride), and a conductive additive (carbon black) were weighed out in predetermined proportions and mixed with NMP solvent to form a negative electrode material layer slurry. The positive electrode material layer slurry was then applied to a positive electrode current collector (Al metal foil) using a coating machine to form a positive electrode precursor. Specifically, the positive electrode precursor was coated so that the area density was 18 mg / cm2. The positive electrode precursor was then dried at a predetermined temperature and pressed to finally form a positive electrode.

[0067] The pressed electrode sheet was then punched out to a diameter of 16.5 mm to form an electrode for evaluation, metallic lithium as the counter electrode, an organic electrolyte solution prepared by dissolving 1 mol / L of lithium hexafluorophosphate (LiPF6) in a solvent with a weight ratio of EC:EMC of 1:3 as the electrolyte, and a polyethylene porous film as the separator, to prepare a coin cell having a diameter of 20 mm and a thickness of 1.6 mm.

[0068] [Evaluation of Battery Characteristics] The obtained coin cell was then initially charged and discharged in a thermostatic chamber at 25°C with a voltage range of 4.55 to 3.0 V and a current value equivalent to 0.1 C. Subsequently, it was charged at 0.1 C up to a voltage range of 4.55 V, and then discharged at 0.1 C. Charge and discharge at 1 C and 3 C were also repeated. Thereafter, a 1 C-1 C charge and discharge cycle was repeated 100 times. In addition, a low-rate cycle at 0.05 C was performed before and after the cycle test and at any cycle point. The degradation rate was calculated based on the results of the 0.05 C cycle.

[0069] Furthermore, the magnetic susceptibility of the Co electrons was measured using a magnetic susceptibility measurement device, revealing an intermediate spin state of S = 1. For this measurement, a powder sample of the positive electrode active material was enclosed in a diamagnetic gelatin capsule. The sample holder inside the capsule was then attached to a rod for moving the sample up and down. The sample space was filled with helium gas at subatmospheric pressure. When the positive electrode active material was placed in a superconducting wire detection coil under a constant magnetic field generated by a superconducting magnet, an output voltage dependent on the sample position within the detection coil was obtained. The magnetic moment value was then calculated by comparing it with the sample position dependence of the output voltage during measurement of a magnetic moment standard sample. The magnetic susceptibility was then measured from the sum of the magnetic moments. The number of atoms with unpaired electrons in the sample, n, can be calculated using the following measurement equation: n = 3kB C Curie (μ0μeff2H)-1, where kB and μ0 are the Boltzmann constant and the magnetic permeability of vacuum. CCurie is the gradient of the magnetic moment m measured by SQUID and the temperature T. The effective magnetic moment meff = gJ[J(J+1)]1 / 2mB, where gJ is the Lande g-factor and J is the resultant angular momentum. Both can be theoretically determined from the electron configuration based on the Pauli exclusion principle and Hund's rule.

[0070] [Measurement Results] In Example 1, after charge and discharge, the discharge rate was 25%, the resistance before cycling was 10 ohms, and the battery resistance increase rate was 17%, as shown in Table 1. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.05, it was possible to ensure 5% (molar ratio) of Co in an intermediate spin state.

[0071] Each embodiment will be described below, focusing on the differences from embodiment 1. Descriptions of similar steps will be omitted to avoid duplication.

[0072] In Example 2, the [Cathode Active Material Preparation Step] was carried out in the same manner as in Example 1. Then, in the [Cathode Active Material Coating Treatment Step], the obtained cathode active material was immersed in an Al(OH)3 solution (concentration: 10%) for 1 hour and then dried at room temperature (approximately 25°C), in the same manner as in Example 1. This formed a precursor of the first coating layer.

[0073] After forming the precursor of the first coating layer, the precursor was immersed in a Mg(OH) solution (concentration: 10%) for 1 hour and then dried at room temperature (about 25° C.), thereby forming a precursor of the second coating layer.

[0074] After forming the precursor of the second coating layer, secondary baking was performed with the precursors of the two coating layers attached to the positive electrode active material. The secondary baking was performed at a baking temperature of 1050°C, with a temperature increase rate of 100°C per minute (°C / min). The holding time was 0.5 hours. The secondary baking treatment was performed using a baking device. A positive electrode active material composite was obtained in this manner.

[0075] [Step of elemental analysis of coating layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al element. In addition, it was found that the second coating layer attached to the first coating layer was composed of Mg element.

[0076] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charge and discharge in Example 2 were as follows: discharge rate: 27%, pre-cycle resistance: 10 ohms, and battery resistance increase rate: 18%, as shown in Table 1 below. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.05, it was possible to ensure 5% (molar ratio) of Co in an intermediate spin state.

[0077] In Example 3, the [Cathode Active Material Preparation Step] was carried out in the same manner as in Example 1. Then, in the [Cathode Active Material Coating Treatment Step], the obtained cathode active material was immersed in an Al(OH)3 solution (concentration: 10%) for 1 hour and then dried at room temperature (approximately 25°C), in the same manner as in Example 1. This formed a precursor of the first coating layer.

[0078] After forming the precursor of the first coating layer, the precursor was immersed in a Mn(CO) solution (concentration: 10%) for 1 hour and then dried at room temperature (about 25° C.), thereby forming a precursor of the second coating layer.

[0079] After forming the precursor of the second coating layer, secondary baking was performed with the precursors of the two coating layers attached to the positive electrode active material. The secondary baking was performed at a baking temperature of 1050°C, with a temperature increase rate of 100°C per minute (°C / min). The holding time was 0.5 hours. The secondary baking treatment was performed using a baking device. A positive electrode active material composite was obtained in this manner.

[0080] [Step of elemental analysis of coating layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al element. In addition, it was found that the second coating layer attached to the first coating layer was composed of Mn element.

[0081] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charge and discharge in Example 3 were as follows: discharge rate: 25%, pre-cycle resistance: 10 ohms, and battery resistance increase rate: 17%, as shown in Table 1 below. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.05, it was possible to ensure 5% (molar ratio) of Co in an intermediate spin state.

[0082] In Example 4, unlike Example 1, the obtained coprecipitate and lithium carbonate were mixed in the [preparation step of the positive electrode active material] so that the molar ratio of Li to Co (Li / Co) was 1.00. This mixed solution was subjected to primary firing in an air atmosphere.

[0083] Thereafter, in the [coating treatment step of the positive electrode active material], the obtained positive electrode active material was immersed in an Al(OH)3 solution (concentration: 10%) for 1 hour, and then dried at room temperature (about 25°C), in the same manner as in Example 1. In this way, a precursor of the first coating layer was formed.

[0084] After forming the precursor of the first coating layer, it was immersed for 1 hour in a solution (concentration: 10%) in which MgF2 and Mn(CO)3 were mixed in a weight ratio of 1:1, and then dried at room temperature (about 25°C), thereby forming a precursor of the second coating layer.

[0085] After forming the precursor of the second coating layer, secondary baking was performed with the precursors of the two coating layers attached to the positive electrode active material. The secondary baking was performed at a baking temperature of 1050°C, with a temperature increase rate of 100°C per minute (°C / min). The holding time was 0.5 hours. The secondary baking was performed using a baking device. A positive electrode active material composite was obtained as described above.

[0086] [Step of elemental analysis of coating layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al elements. In addition, it was found that the second coating layer attached to the first coating layer was composed of F, Mg, and Mn elements.

[0087] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charge and discharge in Example 4 were as follows: discharge rate: 29%, pre-cycle resistance: 15 ohms, and battery resistance increase rate: 23%, as shown in Table 1 below. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.00, the amount of Co in the intermediate spin state was 0% (molar ratio).

[0088] In Example 5, the [Cathode Active Material Preparation Step] was carried out in the same manner as in Example 1. Then, in the [Cathode Active Material Coating Treatment Step], the obtained cathode active material was immersed for 1 hour in a solution (concentration: 10%) in which MgF2 and Mn(CO)3 were mixed in a 1:1 weight ratio, and then dried at room temperature (approximately 25°C). This formed a precursor of the first coating layer.

[0089] After forming the precursor of the first coating layer, the precursor was immersed in an Al(OH) solution (concentration: 10%) for 1 hour and then dried at room temperature (about 25° C.), thereby forming a precursor of the second coating layer.

[0090] After forming the precursor of the second coating layer, secondary baking was performed with the precursors of the two coating layers attached to the positive electrode active material. The secondary baking was performed at a baking temperature of 1050°C, with a temperature increase rate of 100°C per minute (°C / min). The holding time was 0.5 hours. The secondary baking treatment was performed using a baking device. A positive electrode active material composite was obtained in this manner.

[0091] [Step of elemental analysis of coating layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of F, Mg, and Mn elements. In addition, it was found that the second coating layer attached to the first coating layer was composed of Al element.

[0092] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charge and discharge for Example 5 were as follows: discharge rate: 32%, pre-cycle resistance: 9 ohms, and battery resistance increase rate: 15%, as shown in Table 1 below. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.05, it was possible to ensure 5% (molar ratio) of Co in an intermediate spin state.

[0093] In Example 4, unlike Example 1, the obtained coprecipitate and lithium carbonate were mixed in the [preparation step of the positive electrode active material] so that the molar ratio of Li to Co (Li / Co) was 1.00. This mixed solution was subjected to primary firing in an air atmosphere.

[0094] Thereafter, in the [coating treatment step of the positive electrode active material], the obtained positive electrode active material was immersed in an Al(OH)3 solution (concentration: 10%) for 1 hour, and then dried at room temperature (about 25°C), in the same manner as in Example 1. In this way, a precursor of the first coating layer was formed.

[0095] After forming the precursor of the first coating layer, the sample was immersed in a solution (concentration: 10%) in which TiO2, Mg(OH)2, and Mn(CO)3 were mixed in a weight ratio of 1:1:1 for 1 hour, and then dried at room temperature (approximately 25°C), thereby forming a precursor of the second coating layer.

[0096] After forming the precursor of the second coating layer, secondary baking was performed with the precursors of the two coating layers attached to the positive electrode active material. The secondary baking was performed at a baking temperature of 1050°C, with a temperature increase rate of 100°C per minute (°C / min). The holding time was 4 hours. The secondary baking treatment was performed using a baking device. A positive electrode active material composite was obtained in this manner.

[0097] [Step of Elemental Analysis of Coating Layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al elements. In addition, it was found that the second coating layer attached to the first coating layer was composed of Ti, Mg, and Mn elements.

[0098] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charge and discharge for Example 6 were as follows: discharge rate: 29%, pre-cycle resistance: 15 ohms, and battery resistance increase rate: 20%, as shown in Table 1 below. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.00, the amount of Co in the intermediate spin state was 0% (molar ratio).

[0099] In Example 7, unlike Example 4, the obtained coprecipitate and lithium carbonate were mixed in the [preparation step of a positive electrode active material] so that the molar ratio of Li to Co (Li / Co) was 1.05. This mixed solution was subjected to primary firing in an air atmosphere.

[0100] Thereafter, in the [coating treatment step of the positive electrode active material], the obtained positive electrode active material was immersed in an Al(OH)3 solution (concentration: 10%) for 1 hour, and then dried at room temperature (about 25°C), in the same manner as in Example 1. In this way, a precursor of the first coating layer was formed.

[0101] After forming the precursor of the first coating layer, it was immersed for 1 hour in a solution (concentration: 10%) in which MgF2 and Mn(CO)3 were mixed in a weight ratio of 1:1, and then dried at room temperature (about 25°C), thereby forming a precursor of the second coating layer.

[0102] After forming the precursor of the second coating layer, secondary baking was performed with the precursors of the two coating layers attached to the positive electrode active material. The secondary baking was performed at a baking temperature of 1050°C, with a temperature increase rate of 100°C per minute (°C / min). The holding time was 0.5 hours. The secondary baking treatment was performed using a baking device. A positive electrode active material composite was obtained in this manner.

[0103] [Step of Elemental Analysis of Coating Layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, similar to Example 4, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al element. In addition, it was found that the second coating layer attached to the first coating layer was composed of F, Mg, and Mn element.

[0104] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charge and discharge for Example 7 were as follows: discharge rate: 36%, pre-cycle resistance: 9 ohms, and battery resistance increase rate: 15%, as shown in Table 1 below. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.05, it was possible to ensure 5% (molar ratio) of Co in an intermediate spin state.

[0105] In Example 8, unlike Example 6, the obtained coprecipitate and lithium carbonate were mixed in the [preparation step of a positive electrode active material] so that the molar ratio of Li to Co (Li / Co) was 1.05. This mixed solution was subjected to primary firing in an air atmosphere.

[0106] Thereafter, in the [coating treatment step of the positive electrode active material], the obtained positive electrode active material was immersed in an Al(OH)3 solution (concentration: 10%) for 1 hour, and then dried at room temperature (about 25°C), in the same manner as in Example 1. In this way, a precursor of the first coating layer was formed.

[0107] After forming the precursor of the first coating layer, the sample was immersed in a solution (concentration: 10%) in which TiO2, Mg(OH)2, and Mn(CO)3 were mixed in a weight ratio of 1:1:1 for 1 hour, and then dried at room temperature (approximately 25°C), thereby forming a precursor of the second coating layer.

[0108] After forming the precursor of the second coating layer, secondary baking was performed with the precursors of the two coating layers attached to the positive electrode active material. The secondary baking was performed at a baking temperature of 850°C, with a temperature increase rate of 100°C per minute (°C / min). The holding time was 240 minutes. The secondary baking was performed using a baking device. As a result, a positive electrode active material composite was obtained.

[0109] [Step of Elemental Analysis of Coating Layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al elements. In addition, it was found that the second coating layer attached to the first coating layer was composed of Ti, Mg, and Mn elements.

[0110] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charging and discharging in Example 8 were as follows: discharge rate: 38%, pre-cycle resistance: 8 ohms, and battery resistance increase rate: 13%, as shown in Table 1 below. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.05, it was possible to ensure 5% (molar ratio) of Co in an intermediate spin state.

[0111] In Example 9, unlike Example 1, the obtained coprecipitate and lithium carbonate were mixed in the [preparation step of the positive electrode active material] so that the molar ratio of Li to Co (Li / Co) was 1.00. This mixture was subjected to primary firing in an air atmosphere. Thereafter, the [coating treatment step of the positive electrode active material] was carried out in the same manner as in Example 1.

[0112] [Step of elemental analysis of coating layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al element. In addition, it was found that the second coating layer attached to the first coating layer was composed of F element.

[0113] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charge and discharge for Example 9 were as follows: discharge rate: 24%, pre-cycle resistance: 12 ohms, and battery resistance increase rate: 18%, as shown in Table 1 below. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.00, the amount of Co in the intermediate spin state was 0% (molar ratio).

[0114] Example 10 In Example 10, unlike Example 2, the obtained coprecipitate and lithium carbonate were mixed in the [preparation step of the positive electrode active material] so that the molar ratio of Li to Co (Li / Co) was 1.00. This mixture was subjected to primary baking in an air atmosphere. Thereafter, the [coating treatment step of the positive electrode active material] was carried out in the same manner as in Example 2.

[0115] [Step of elemental analysis of coating layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al element. In addition, it was found that the second coating layer attached to the first coating layer was composed of Mg element.

[0116] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charge and discharge for Example 10 were as follows: discharge rate: 25%, pre-cycle resistance: 11 ohms, and battery resistance increase rate: 18%, as shown in Table 1 below. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.00, the amount of Co in the intermediate spin state was 0% (molar ratio).

[0117] Example 11 In Example 11, unlike Example 3, the obtained coprecipitate and lithium carbonate were mixed in the [preparation step of the positive electrode active material] so that the molar ratio of Li to Co (Li / Co) was 1.00. This mixture was subjected to primary firing in an air atmosphere. Thereafter, the [coating treatment step of the positive electrode active material] was carried out in the same manner as in Example 3.

[0118] [Step of elemental analysis of coating layer] Thereafter, elemental analysis of the two coating layers located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al element. In addition, it was found that the second coating layer attached to the first coating layer was composed of Mn element.

[0119] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charge and discharge for Example 11 were as follows: discharge rate: 24%, pre-cycle resistance: 11 ohms, and battery resistance increase rate: 19%, as shown in Table 1 below. It was found that by adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.00, the amount of Co in the intermediate spin state was 0% (molar ratio).

[0120] In Comparative Example 1, unlike Example 1, the obtained coprecipitate and lithium carbonate were mixed in the [preparation step of the positive electrode active material] so that the molar ratio of Li to Co (Li / Co) was 1.00. This mixed solution was subjected to primary firing in an air atmosphere.

[0121] Thereafter, in the [coating treatment step for the cathode active material], the obtained cathode active material was immersed in an Al(OH)3 solution (concentration: 10%) for 1 hour and then dried at room temperature (about 25°C), as compared with Example 1. This formed a precursor for the first coating layer. However, in Comparative Example 1, the formation of the precursor for the second coating layer in the above examples was not carried out.

[0122] After forming only the precursor of the first coating layer, secondary firing was performed with one coating layer precursor attached to the positive electrode active material. The secondary firing was performed at a firing temperature of 1050°C, with a temperature increase rate of 100°C per minute (°C / min). The holding time was 1 hour. The secondary firing was performed using a firing device. A positive electrode active material composite was obtained as described above.

[0123] [Step of elemental analysis of coating layer] Thereafter, elemental analysis of one coating layer located on the surface of the positive electrode active material was performed using an EDX analyzer (energy dispersive X-ray fluorescence analyzer) (FE-TEM / EDX (JEOL JEM-F200 / Noran system 7). As a result, it was found that the first coating layer attached to the surface of the positive electrode active material was composed of Al element. No second coating layer was observed.

[0124] The same cell fabrication process and battery characteristic evaluation were carried out as in Example 1. The measurement results obtained after charging and discharging in Comparative Example 1 were as follows: discharge rate: 20%, pre-cycle resistance: 45 ohms, and battery resistance increase rate: 33%, as shown in Table 1 below. By adjusting the molar ratio of Li to Co (Li / Co) in the positive electrode active material to 1.00, the amount of Co in the intermediate spin state was 0% (molar ratio).

[0125] [Table 1] Coating layer configurations and measurement results for Examples 1 to 11 and Comparative Example 1 (*) Those with a pre-cycle resistance value of 30 ohms or more and a resistance increase rate of 25% or more were treated as comparative examples.

[0126] [Discussion] The measurement results of Examples 1 to 11 and Comparative Example 1 revealed the following: ● When one of the first coating layer and the second coating layer is composed of Al element and the other of the first coating layer and the second coating layer is composed of one element selected from F, Mg, and Mn (see Examples 1 to 3 and Examples 9 to 11), it was found that, compared to Comparative Example 1, it was possible to improve the discharge rate characteristics and reduce the pre-cycle resistance and the rate of increase in the battery resistance. ● Comparing Examples 9 to 11 with Examples 1 to 3, it was found that when Co in the intermediate spin state was 5% (molar ratio), it was possible to further improve the discharge rate characteristics and reduce the pre-cycle resistance and the rate of increase in the battery resistance. ● Comparing Examples 4 and 6 with Examples 9 to 11, it was found that when the second coating layer was composed of three elements (F, Mg, Mn or Ti, Mg, Mn), it was possible to further improve the discharge rate characteristics and reduce the pre-cycle resistance and the rate of increase in the battery resistance. Comparing Example 5 with Example 7, it was found that the discharge rate characteristics were improved when the first coating layer was composed of Al and the second coating layer was composed of three elements (F, Mg, Mn) compared to when the first coating layer was composed of three elements (F, Mg, Mn) and the second coating layer was composed of Al. Comparing Examples 7 and 8 with Examples 4 and 6, it was found that when the second coating layer was composed of three elements (F, Mg, Mn or Ti, Mg, Mn) and contained 5% (molar ratio) of Co in the intermediate spin state, it was possible to further improve the discharge rate characteristics and reduce the pre-cycle resistance and the battery resistance increase rate. Comparing Example 7 with Example 8, it was found that when the second coating layer contained Ti, Mg, and Mn, it was possible to further improve the discharge rate characteristics and reduce the pre-cycle resistance and the battery resistance increase rate compared to when the second coating layer contained F, Mg, and Mn. In Example 8, the second coating layer was composed of the elements Ti, Mg, and Mn, and contained 5% (molar ratio) of Co in an intermediate spin state. This was found to be the most effective way to improve discharge rate characteristics and reduce pre-cycle resistance and the rate of increase in battery resistance compared to the other Examples.

[0127] The present invention may include the following aspects. <1> A secondary battery comprising a cathode active material composite including a cathode active material containing an oxide containing Li and Co, and two or more coating layers at least partially covering the cathode active material, wherein the two or more coating layers comprise, in order from the cathode active material side to the outside, a first coating layer and a second coating layer, one of the first coating layer and the second coating layer containing elemental Al, and the other of the first coating layer and the second coating layer containing at least one element selected from the group consisting of F, Mg, Mn, and Ti. <2> The secondary battery according to <1>, wherein one of the first coating layer and the second coating layer contains elemental Al. <3> The secondary battery according to <1> or <2>, wherein the cathode active material and the two or more coating layers are sintered bodies. <4> The secondary battery according to any one of <1> to <3>, wherein the cathode active material contains trivalent Co ions (Co3+). <5> The secondary battery according to <4>, wherein the positive electrode active material contains Co3+ in an intermediate spin (IS) state. <6> The secondary battery according to <5>, wherein the positive electrode active material further contains Co3+ in a low spin (LS) state, and wherein the ratio of the Co3+ in the intermediate spin (IS) state to the Co3+ in the low spin (LS) state is 5% or more. <7> The secondary battery according to any one of <1> to <4>, wherein the other of the first coating layer and the second coating layer contains elements of F, Mg, and Mn. <8> The secondary battery according to <7>, wherein the first coating layer contains elements of F, Mg, and Mn, and the second coating layer contains elements of Al. <9> The secondary battery according to any one of <1> to <6>, wherein the second coating layer contains elements of Ti, Mg, and Mn. <10> The secondary battery according to any one of <1> to <9>, wherein at least one of the first coating layer and the second coating layer has an oxide form of a constituent element. <11> The secondary battery according to any one of <1> to <10>, further comprising an electrolytic solution.

[0128] The secondary battery according to one embodiment of the present invention can be used in various fields where power storage is expected. By way of example only, the secondary battery according to one embodiment of the present invention, particularly the nonaqueous electrolyte secondary battery, can be used in the electrical, information, and communications fields where mobile devices are used (e.g., mobile devices such as mobile phones, smartphones, laptops, digital cameras, activity monitors, arm computers, and electronic paper), household and small industrial applications (e.g., power tools, golf carts, and household, nursing care, and industrial robots), large industrial applications (e.g., forklifts, elevators, and port cranes), transportation systems (e.g., hybrid cars, electric cars, buses, trains, electrically assisted bicycles, and electric motorcycles), power system applications (e.g., various power generation systems, road conditioners, smart grids, and general household power storage systems), medical applications (e.g., medical devices such as earphones and hearing aids), medical applications (e.g., medication management systems), as well as the IoT field and space and deep-sea applications (e.g., space probes, submersible research vessels, and the like).

[0129] 1 Positive electrode active material 2 First coating layer 3 Second coating layer 5 Positive electrode active material composite 10A Positive electrode 10B Negative electrode 11A Positive electrode current collector 11B Negative electrode current collector 12A Positive electrode material layer 12B Negative electrode material layer

Claims

1. A secondary battery comprising: a cathode active material composite including a cathode active material containing an oxide containing Li and Co; and two or more coating layers at least partially covering the cathode active material, wherein the two or more coating layers include, from the cathode active material side toward the outside, a first coating layer and a second coating layer, one of the first coating layer and the second coating layer containing Al element, and the other of the first coating layer and the second coating layer containing at least one element selected from the group consisting of F, Mg, Mn, and Ti.

2. The secondary battery according to claim 1, wherein one of the first coating layer and the second coating layer is made of elemental Al.

3. The secondary battery according to claim 1 or 2, wherein the positive electrode active material and the two or more coating layers are sintered.

4. The secondary battery according to any one of claims 1 to 3, wherein the positive electrode active material contains trivalent Co ions (Co3+).

5. The secondary battery of claim 4, wherein the positive electrode active material comprises Co3+ in an intermediate spin (IS) state.

6. The secondary battery according to claim 5, wherein the positive electrode active material further contains Co3+ in a low spin (LS) state, and a ratio of Co3+ in the low spin (LS) state to Co3+ in the intermediate spin (IS) state is 5% or more.

7. The secondary battery according to any one of claims 1 to 4, wherein the other of the first coating layer and the second coating layer contains the elements F, Mg, and Mn.

8. The secondary battery according to claim 7, wherein the first coating layer contains the elements F, Mg, and Mn, and the second coating layer contains the element Al.

9. The secondary battery according to any one of claims 1 to 6, wherein the second coating layer contains the elements Ti, Mg, and Mn.

10. The secondary battery according to any one of claims 1 to 9, wherein at least one of the first coating layer and the second coating layer has an oxide form of a constituent element.

11. The secondary battery according to any one of claims 1 to 10, further comprising an electrolyte solution.

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