JP4887671B2 - Lithium secondary battery, positive electrode provided in the battery, and manufacturing method thereof - Google Patents

Description

  The present invention relates to a manufacturing technology of a lithium ion secondary battery, and more particularly to a positive electrode provided in a lithium secondary battery and a manufacturing method thereof.

A lithium secondary battery includes a positive electrode having a material (active material) capable of inserting and extracting lithium ions, and lithium ions travel back and forth between an electrolyte (typically a non-aqueous electrolyte) between the positive electrode and the negative electrode. It is the secondary battery which charges / discharges by.
In general, a positive electrode equipped in a lithium secondary battery is composed of a conductive substrate (hereinafter referred to as “current collector”) made of aluminum or the like and an active material layer formed on the current collector. . The active material layer includes a positive electrode active material such as LiNiO ₂ and LiCoO ₂ , a binder, and a conductive material used as necessary, and is formed into a paste (or slurry) composition for forming an active material layer (Ie, active material layer forming paste) is applied to a current collector.

As a paste for forming an active material layer, conventionally, a solvent-based paste (Patent Documents 3 and 4) containing an organic solvent as a solvent and a solvent-based binder such as polyvinylidene fluoride (PVDF), water as a solvent and polytetra An aqueous paste (Patent Documents 1 and 2) containing an aqueous binder such as fluoroethylene (PTFE) is used, but the use of these pastes has advantages and disadvantages depending on the content of the active material.
For example, in the case of a lithium ion secondary battery employing a non-aqueous electrolyte, the use of a solvent-based paste (that is, a solvent-based binder) has a relatively low initial internal resistance and can achieve a high initial output. On the other hand, there is a disadvantage that the binding force is reduced by the swelling or dissolution of the binder due to absorption of the non-aqueous electrolyte, that is, the adhesion between the active material and the current collector is easily lowered. Such a decrease in adhesion is not preferable because there is a risk that the cycle characteristics of the battery will deteriorate (low life).
On the other hand, the use of an aqueous paste (that is, an aqueous binder) is superior in binding performance of the active material since the aqueous binder is insoluble in the non-aqueous electrolyte, and has good cycle characteristics (long life) of the battery. It can be realized. On the other hand, depending on the content of the positive electrode active material (for example, a lithium transition metal composite oxide such as LiNi _x Co _1-x O ₂ (0 ≦ x ≦ 1)), There is a disadvantage that a compound (for example, oxide or hydroxide) having high electrical resistance is easily generated on the surface of the active material, and the initial internal resistance of the battery is likely to increase. An increase in the initial internal resistance is not preferable because it causes a decrease in discharge characteristics.

  In addition, in a lithium secondary battery, oxygen may be generated inside the battery due to the influence of the usage method, environmental change (for example, overcharge, high temperature exposure), and the like. If such oxygen generation is left unattended, problems such as oxidation of the solvent constituting the electrolyte and overheating of the battery may occur. Patent Document 5 describes a lithium secondary battery including a positive electrode having a composite material of a substance capable of absorbing oxygen generated from a positive electrode active material and a conductive agent. It would be beneficial to provide a lithium secondary battery that can more appropriately suppress the generation of oxygen.

JP 2002-42817 A JP 2000-182606 A JP-A-11-260411 JP-A-8-106897 Japanese Patent Laid-Open No. 11-144734

  The present invention was created in order to solve the above-described problems in the case of forming an active material layer on the positive electrode of a lithium secondary battery using the active material layer forming paste. An object of the present invention is to provide a higher-performance positive electrode that has advantages and disadvantages when using a paste, and to provide a lithium secondary battery including such a positive electrode. The present invention also provides a positive electrode that has the advantages of both the case of using the aqueous paste and the case of using the solvent-based paste, eliminates the disadvantage, and can further suppress the release of oxygen at high temperatures. It aims at providing a lithium secondary battery provided with a positive electrode.

The present invention provides a method for producing a positive electrode used in a lithium secondary battery (typically a lithium ion secondary battery). That is, one positive electrode manufacturing method disclosed herein is a method for manufacturing a positive electrode used in a lithium secondary battery, and is (1) an organic solvent and a positive electrode active material which is substantially free from an organic solvent. An active material whose surface is coated with a coating material containing at least one binder that is insoluble, and at least soluble in an organic solvent (including properties that can be highly dispersed in the organic solvent; the same shall apply hereinafter). A step of preparing a composition for forming an active material layer having a kind of binder, and (2) forming the cathode active material layer on the current collector by applying the composition to the surface of the cathode current collector. Including the step of: The coating material contains an oxygen storage material mainly composed of cerium composite oxide (described later).

  In the positive electrode manufacturing method disclosed herein, the surface is coated with at least one binder that is substantially insoluble in an organic solvent (typically a polymer that can be used as an aqueous binder in the aqueous paste). In addition to using a positive electrode active material, an organic solvent (that is, a non-aqueous solvent) and a solvent-based binder are included as materials for forming the active material layer by applying the coated active material on the positive electrode current collector An active material layer forming composition (typically prepared in a paste form) is used. According to the positive electrode manufacturing method having such a configuration, the advantages of both the case where the active material layer is formed using the solvent-based binder and the case where the active material layer is formed using the water-based binder are combined. A positive electrode for a lithium secondary battery (typically a lithium ion secondary battery) can be produced.

Thus, the positive electrode for a Brighter lithium secondary battery provided by the present invention, a current collector and a positive electrode comprising an active material layer formed on the current collector, wherein the active material layer in an organic solvent At least a coating layer containing at least one binder that is substantially insoluble and containing an oxygen storage material mainly composed of a cerium composite oxide is soluble in the positive electrode active material and the organic solvent formed on the surface. It has a kind of binder. Moreover, this invention provides the lithium secondary battery (typically lithium ion secondary battery) provided with such a positive electrode.
In the lithium secondary battery including the positive electrode disclosed herein, the active material layer is formed using the solvent-based composition for forming an active material layer, and as a result, the internal resistance is kept low and high initial output is realized. Can do. In addition, even when a nonaqueous electrolytic solution is employed as a result of the active material being coated and bound with a binder that is substantially insoluble in the organic solvent, the active material due to the presence of the electrolytic solution is used. Of adhesion (typically, adhesion between active material secondary particles and current collector, adhesion between active materials (between secondary particles or primary particles)) can be prevented. . As a result, good cycle characteristics (long life) of the battery can be realized.

A preferable positive electrode manufacturing method disclosed herein is a method in which the coating material used includes a conductive material, and a preferable positive electrode disclosed herein includes the conductive layer in the coating layer. It is a positive electrode. It is particularly preferable that the content of the conductive material is 5 to 20% by mass when the total amount of the binder and the conductive material is 100% by mass.
In a lithium secondary battery (typically a lithium ion secondary battery) having such a positive electrode, it is possible to further reduce the initial internal resistance while realizing good cycle characteristics (long life).

Further, another preferable positive electrode manufacturing method disclosed herein is a method using a positive electrode active material substantially composed of secondary particles having an average particle diameter in the range of 5 μm to 25 μm, and is disclosed herein. One preferable positive electrode is a positive electrode having an active material substantially composed of secondary particles having an average particle diameter in the range of 5 μm to 25 μm.
In a lithium secondary battery (typically a lithium ion secondary battery) including a positive electrode having such an active material, the positive electrode active material has high adhesiveness, and high output is easily obtained. However, the embodiment of the present invention is not limited to the average particle size exemplified above. Embodiments of the present invention can include a method of using a positive electrode active material substantially composed of secondary particles having an average particle size of less than 5 μm, and a positive electrode having the active material.

The particularly preferable method for producing a positive electrode disclosed herein is that the thickness of the coating material deposit (that is, the deposit constituting the coating layer) existing on the surface of the active material is substantially 0.5 μm or less (average thickness). Is less than 0.5 μm, and the presence of a thickness portion exceeding this value can be allowed. The same shall apply hereinafter.) One particularly preferred positive electrode disclosed herein is the surface of the active material. The thickness of the coating layer present in is substantially 0.5 μm or less.
A lithium secondary battery (typically a lithium ion secondary battery) having a positive electrode having an active material with a coating layer of such a thickness has a high initial output while realizing good cycle characteristics (long life). Can be obtained.

Further, another particularly preferable positive electrode manufacturing method disclosed herein is a method in which at least one lithium transition metal composite oxide is used as a positive electrode active material, and one particularly preferable positive electrode disclosed herein is an active material. Is a positive electrode characterized by being at least one lithium transition metal complex oxide. Here, the “lithium transition metal composite oxide” refers to a metal composite oxide having lithium and one or more transition metals that can function as an active material as constituent elements, and other constituent elements (for example, Al). It may be included.
In a lithium secondary battery (typically a lithium ion secondary battery) having a positive electrode using a composite oxide of this type as a positive electrode active material, while achieving good cycle characteristics (long life) and a reduction in initial internal resistance, The energy density (discharge capacity) can be improved.

The positive electrode manufacturing method according to the present invention, the coating material to be the used is a method which comprises an oxygen storage material composed mainly of cerium complex oxide, the one according to the present invention disclosed herein A positive electrode is a positive electrode in which the said coating layer contains the oxygen storage material which has a cerium complex oxide as a main component. As the oxygen storage material, a material mainly composed of a cerium-zirconium composite oxide is particularly preferable.
As described above, a lithium secondary battery including a positive electrode containing an oxygen storage material has any other material (for example, positive electrode active) that constitutes the positive electrode due to the influence of usage, environmental change (for example, overcharge, high temperature exposure), or the like. When oxygen is generated from the substance, the oxygen can be stored (absorbed) in the oxygen storage material. As a result, an event in which the oxygen is released from the positive electrode can be suppressed. Since the oxygen storage material is disposed on the surface of the positive electrode active material, this oxygen storage (oxygen release suppression) function can be efficiently exhibited. Therefore, in the lithium secondary battery including such a positive electrode, it is possible to reduce the initial internal resistance while realizing good cycle characteristics (long life), and to suppress oxygen release from the positive electrode.

Further, another positive electrode manufacturing method disclosed herein is a method in which the coating material used includes the oxygen storage material and a conductive material, and one preferable positive electrode disclosed herein is: The coating layer is a positive electrode including the oxygen storage material and a conductive material.
In the lithium secondary battery including such a positive electrode, it is possible to further reduce the initial internal resistance while achieving good cycle characteristics (long life), and to suppress oxygen release from the positive electrode.

A particularly preferable method for producing a positive electrode disclosed herein is a method using a coating material containing the oxygen storage material, and deposits of the coating material existing on the surface of the active material (deposits constituting the coating layer). The thickness is substantially 0.5 μm or less. One particularly preferable positive electrode disclosed herein is a positive electrode in which the coating layer contains an oxygen storage material, and the thickness of the coating layer present on the surface of the active material is substantially 0.5 μm or less. It is characterized by. For example, a method in which the thickness of the coating material deposit is substantially in the range of 0.1 to 0.5 μm is preferable, and the thickness of the coating layer is substantially in the range of 0.1 to 0.5 μm. Is preferred.
In a lithium secondary battery (typically a lithium ion secondary battery) including a positive electrode having an active material on which a coating layer having such a thickness is formed, good cycle characteristics (long life) and high initial output are obtained. In addition, oxygen release from the positive electrode can be effectively suppressed.

  Hereinafter, preferred embodiments of the present invention will be described in detail. It should be noted that technical matters other than the contents particularly mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters for those skilled in the art based on the prior art. The present invention can be implemented based on the technical contents disclosed in the present specification and the common general technical knowledge in the field.

As described above, the positive electrode for a lithium secondary battery disclosed herein uses a positive electrode active material on which a coating layer containing at least one binder that is substantially insoluble in an organic solvent is formed. As long as it is a positive electrode obtained by forming an active material layer on the surface of a predetermined current collector using a solvent-based composition containing an active material, there is no particular limitation on the composition and shape of the current collector that is a constituent element thereof. Absent. A conductive member made of a metal having good conductivity can be used as a current collector, but a positive electrode current collector for a lithium ion secondary battery is particularly preferably made of aluminum (Al).
The shape of the current collector can vary depending on the shape of the positive electrode and the battery, and is not particularly limited. For example, a preferred embodiment of a lithium ion secondary battery constructed using any positive electrode disclosed herein includes a battery including a wound electrode body. In this embodiment, a current collector made of a foil metal such as aluminum foil is used. That is, in a battery equipped with a wound electrode body, a positive electrode sheet obtained by laminating a conductive layer and an active material layer on the surface of a positive electrode foil-shaped current collector made of Al or the like, and a metal (for example, copper) or carbon A negative electrode sheet obtained by adhering an appropriate negative electrode active material to the surface of a foil-like current collector for negative electrode made of, for example, is overlapped via a separator and wound to produce a wound electrode body. As the separator, for example, a porous polyolefin (polyethylene, polypropylene, etc.) sheet can be used. And this lithium ion secondary battery is constructed | assembled by accommodating this winding type electrode body in a suitable container with electrolyte solution (typically nonaqueous electrolyte solution mentioned later). The current collectors having various shapes may be produced by any conventionally known method in the field of manufacturing secondary batteries, and do not characterize the present invention.

The positive electrode active material used in the positive electrode for a lithium secondary battery disclosed herein is not limited to a specific type of active material as long as it is conventionally used in this type of secondary battery. An oxide-based positive electrode active material having a layered structure or an oxide-based positive electrode active material having a spinel structure used for a general lithium secondary battery can be preferably used. For example, when the desired battery is a lithium ion secondary battery, it is preferable to use various lithium transition metal composite oxides. For example, lithium cobalt complex oxide (typically LiCoO ₂ ), lithium nickel complex oxide (typically LiNiO ₂ ), lithium manganese complex oxide (LiMn ₂ O ₄ ) or the like is used as the positive electrode active material. Can be used. A composite oxide containing two or more transition metal elements (for example, a composite oxide represented by the general formula: LiNi _x Co _1-x O ₂ , where x is a positive real number satisfying 0 <x <1). May be. Alternatively, an element other than Ni and Co may be used as a constituent element. For example, a composite oxide represented by the general formula: Li _x (Ni _1-y Co _y ) _1-z M _z O ₂ (where x, y, and z are preferably 0.9 ≦ x ≦ 1.1, 0 ≦ y ≦ 1, 0.01 ≦ z ≦ 0.2, and M is at least one selected from the group consisting of Al, Mn, Ti, Mg, Cr, Fe, V, Zn, and B It is preferable that it is an element, and the thing containing Al is especially preferable. Alternatively, a lithium composite oxide containing three elements of Ni, Co, and Mn (for example, a composite oxide represented by LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) may be used. In addition, a lithium composite oxide containing one or more other transition metal elements (that is, a four-element or more lithium composite oxide) may be used.
Various lithium transition metal composite oxides (active materials) whose surfaces are coated with the above-mentioned coating materials, at least one kind that is substantially soluble in organic solvents and organic solvents (can be highly dispersible) By using a composition for forming an active material layer containing a binder (typically prepared in a paste form), a high-performance positive electrode for a lithium secondary battery and a lithium secondary equipped with the positive electrode A battery can be provided.

The coating material disclosed herein includes at least one binder that is substantially insoluble in an organic solvent, and may have other contents as long as the binder can coat the surface of a predetermined positive electrode active material. There are no restrictions on the components. For example, a conventional aqueous paste for forming an active material layer can be used as the coating material according to the present invention.
“At least one binder that is substantially insoluble in an organic solvent (typically an organic solvent that can be used as a non-aqueous electrolyte in a lithium ion secondary battery, etc.)” has various properties having such properties. Conventionally, various polymers that have been suitably used as an aqueous binder for forming an active material layer of a positive electrode of a lithium secondary battery are suitable. For example, various cellulose derivatives such as carboxymethylcellulose (CMC), methylcellulose (MC), cellulose acetate phthalate (CAP), hydroxypropylmethylcellulose (HPMC), and hydroxypropylmethylcellulose phthalate (HPMCP) which are hydrophilic (water-soluble) polymers Is a preferred example. Of these, the use of CMC is preferred. Alternatively, water-dispersible polymers such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetra Rubbers such as fluororesin such as fluoroethylene copolymer (ETFE), vinyl acetate copolymer, styrene butadiene block copolymer (SBR), acrylic acid modified SBR resin (SBR latex), gum arabic, etc. It is mentioned as a suitable example. Of these, fluorine resins such as PTFE are preferred.
A composition obtained by dissolving or dispersing these polymers (binders) in, for example, water is preferable as the coating material according to the present invention.

  Moreover, as a suitable binder contained in the composition for forming an active material layer (typically prepared in a paste form), conventionally, an active material layer for a positive electrode of a lithium secondary battery is formed. And various organic solvent-soluble (dispersible) polymers used as solvent-based binders. Examples of this type of polymer include polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene oxide-propylene oxide copolymer (PEO-PPO), and the like. The PVDF, PVDC and the like are preferable.

  Various conductive materials can be added to the composition for forming an active material layer and / or the coating material as necessary to improve conductivity (electron conductivity). The conductive material used is not limited to a specific conductive material as long as it is conventionally used in this type of secondary battery. For example, carbon (carbon) powder such as carbon black (acetylene black or the like), conductive metal powder such as nickel powder, or the like can be used.

  Next, the positive electrode manufacturing method will be described. The method disclosed here is substantially different from an appropriate organic solvent and a positive electrode active material that is an organic solvent (typically an organic solvent that can be used as a non-aqueous electrolyte in a lithium ion secondary battery or the like). An active material layer-forming composition comprising an active material whose surface is coated with a coating material containing at least one binder that is insoluble in water and at least one binder material that is soluble in an organic solvent is prepared ( Typically prepared) and applied (typically applied) to the surface of a suitable positive electrode current collector to form a positive electrode active material layer on the current collector. Other processes required for positive electrode manufacturing (for example, preparation of positive electrode active materials (primary particles and secondary particles) having a predetermined particle diameter and current collector molding) are the same as those of conventional positive electrode manufacturing methods. It is sufficient.

The coating material may be prepared in the same manner as the aqueous active material layer forming paste conventionally used in the production of positive electrodes for lithium secondary batteries, and does not require any special operation. For example, a paste (or slurry) -like coating material is prepared by adding at least one binder (CMC, PTFE, etc.) that is substantially insoluble in an organic solvent to water at an appropriate mass ratio and mixing. be able to. The coating material preferably further contains a conductive material in order to improve the conductivity (electron conductivity) in the coating layer made of the coating material (mainly binder) formed on the surface of the positive electrode active material.
Although it does not specifically limit, For example, it is suitable to mix | blend these so that 1-30 mass% when the total amount of the said binder and electrically conductive material shall be 100 mass% becomes the content rate of an electrically conductive material, Such content The rate is preferably 5 to 20% by mass. In addition, the compounding ratio of the element which comprises these coating layers, and an aqueous solvent is not specifically limited. An appropriate blending ratio may be used so that suitable fluidity (viscosity) and binder density are given according to the type of binder used. For example, it can be prepared so that approximately 80 to 95% by mass of the entire coating material becomes an aqueous solvent.

The surface of the target positive electrode active material is coated using the coating material prepared as described above. As the coating means, various conventionally known methods that can be employed for coating powders with any substance can be used. For example, it is preferable to employ a method of immersing the positive electrode active material powder in a coating material, or a method of applying or spraying a liquid or mist-like coating material to the positive electrode active material (typically powder). After application of the coating material, the coating layer having a desired thickness can be formed on the active material surface by drying the positive electrode active material under appropriate conditions.
The thickness of the coating layer may vary depending on the composition of the coating material used and the properties of the positive electrode active material, but the thickness of the deposit is substantially 1 μm or less (for example, 0.005 μm to 1 μm, typically 0.005 μm). It is appropriate to coat so that it becomes 01 μm to 1 μm), and it is coated so as to be substantially 0.5 μm or less (for example, 0.01 μm to 0.5 μm, particularly preferably 0.05 μm to 0.5 μm). It is preferable. Alternatively, the size corresponding to approximately 0.5 to 5% (typically 1 to 3%) of the average particle diameter (for example, 5 to 25 μm) of the positive electrode active material (secondary particles) to be coated is set to be the coating layer. You may set to thickness.
When the thickness is within such a range, an active material layer (positive electrode) excellent in reduction of internal resistance and good conductivity (electron conductivity) in the active material layer can be formed. When an appropriate amount of conductive material is contained in the coating layer, the conductivity (electron conductivity) is improved thereby, so that the coating layer is formed relatively thicker than when no conductive material is contained. be able to. For example, when a conductive material is included, the preferable range is substantially 1.5 μm or less (for example, 0.01 μm to 1.5 μm, more preferably 0.05 μm to 1 μm, particularly preferably 0.1 μm to 1 μm, for example, 0.1 μm). 1 μm to 0.5 μm).

  In addition, what is necessary is just to use the positive electrode active material to be used as it is prepared and provided by a conventionally well-known method, and does not require a special process. For example, an active material composed of a lithium transition metal composite oxide can be prepared by mixing several raw material compounds appropriately selected according to the atomic composition at a predetermined molar ratio and firing by an appropriate means. . Also, secondary particles having a desired average particle size and / or particle size distribution by pulverizing, granulating and classifying the fired product by appropriate means, for example, secondary particles having an average particle size in the range of 5 μm to 25 μm. Thus, a positive electrode active material powder substantially constituted can be obtained. Such a method of preparing the positive electrode active material powder itself does not characterize the present invention at all, and thus a detailed description thereof is omitted.

Using the positive electrode active material with the coating layer thus obtained, an active material layer is formed on the current collector. Typically, an active material layer comprising the positive electrode active material with a coating layer, at least one binder material that is substantially soluble (including highly dispersed) in an organic solvent, and a suitable organic solvent. A forming composition is prepared, and a positive electrode active material layer is formed on the positive electrode current collector using the composition.
Such a solvent-based composition is preferably prepared in a paste like the conventional solvent-based composition (that is, the solvent-based paste), and does not require any special treatment. That is, a paste (or slurry) composition is easily prepared by adding a positive electrode active material with a coating layer and a solvent-based binder (for example, PVDF) to an appropriate organic solvent at an appropriate mass ratio and mixing them. be able to. For example, when the total amount of the active material and the binder is 100% by mass, the active material content is 85 to 99% by mass, and the binder content is 1 to 15% by mass. Thus, it is preferably added to an applicable organic solvent. As the organic solvent (non-aqueous solvent), an organic solvent used for preparing a conventional solvent-based paste can be suitably used. For example, N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone, toluene and the like are exemplified. The use of NMP is preferred. The compounding ratio of the elements constituting the active material layer and the organic solvent is not particularly limited. For example, it can be prepared such that approximately 40 to 90% by mass of the entire composition for forming an active material layer (paste) is an organic solvent (for example, NMP).

  Moreover, in order to ensure a sufficient conductive path (conductive path) between the positive electrode active material and the current collector in the active material layer to be formed, it is preferable to further include a conductive material. Although the suitable example of the electrically conductive material to be used is as above-mentioned, it may be the same as what is contained in a coating material, and may differ. Although not particularly limited, the content of the active material is 80 to 95% by mass, the content of the conductive material is 2 to 15% by mass, and the binder is 100% by mass when the total amount of the active material, the conductive material and the binder is 100% by mass. These are preferably blended so that the content of the dressing is 1 to 15% by mass.

  A conductive layer is formed on the current collector by applying the active material layer forming paste prepared as described above to the surface of the current collector. Typically, the active material layer forming paste can be applied in a layer with a predetermined thickness on the surface of the current collector using an appropriate application device (coater). The thickness to be applied is not particularly limited, and may vary depending on the shape and application of the positive electrode and the battery. For example, the conductive layer forming paste is applied to the surface of a foil-like current collector (for example, Al foil) having a thickness of about 10 to 30 μm so as to have a thickness of about 5 to 100 μm (after drying). After the application, the active material layer having a predetermined thickness can be formed on the surface of the current collector by drying the applied product using an appropriate dryer. By pressing the positive electrode thus obtained as desired, a positive electrode sheet having a desired thickness can be obtained.

According to the present invention, a lithium secondary battery (typically a lithium ion secondary battery) including a positive electrode obtained by the manufacturing method disclosed herein can be manufactured. Therefore, this invention provides the manufacturing method of the lithium secondary battery characterized by producing or preparing the positive electrode disclosed here as another aspect.
In addition, the means for producing a lithium secondary battery (for example, a lithium ion secondary battery) is based on a conventional method for producing a lithium secondary battery, except that the positive electrode is manufactured (or prepared) and the positive electrode is used. Well, it does not require special processing that requires special explanation.

For example, lithium having a wound type or other sheet structure electrode as described above based on a conventionally known method using a positive electrode sheet (see the above description) according to the present invention and an appropriate negative electrode sheet and separator. An ion secondary battery can be constructed.
Specifically, a negative electrode sheet for a lithium ion secondary battery having a carbon material (that is, a negative electrode active material) having a graphite structure (layered structure) capable of inserting and removing lithium ions (Li ⁺ ) and the separator described above are prepared in advance. To do. Then, the positive electrode sheet and the negative electrode sheet are overlapped (stacked) via a separator. This laminate is housed in a suitable battery container. In a preferred embodiment, the laminate is wound to form a wound electrode structure, which is accommodated in a battery container. Alternatively, a stacked electrode structure in which a plurality of positive electrode sheets and a plurality of negative electrode sheets are alternately stacked with separators interposed therebetween may be configured and accommodated in a battery container.

A positive electrode, a negative electrode, and a separator are impregnated with an electrolytic solution by supplying (injecting) an electrolytic solution prepared in advance to a battery container containing such an electrode structure. Thereby, the desired lithium ion secondary battery can be constructed.
Typically, the electrolytic solution for a lithium ion secondary battery is a nonaqueous electrolytic solution containing a nonaqueous solvent and a lithium salt (supporting salt) added to and dissolved in the solvent.
As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be used. Propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate, ethyl methyl carbonate (EMC), 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, Generally, non-aqueous batteries (lithium ion secondary batteries) such as dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, γ-butyrolactone, etc. 1 type or 2 types or more selected from the non-aqueous solvent known as what can be used for electrolyte solution etc. of a battery etc. can be used.

As the supporting salt to be contained in the electrolytic solution, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , one or more lithium compounds (lithium salts) selected from LiI and the like can be used.
In addition, the density | concentration of the supporting salt in electrolyte solution may be the same as that of the electrolyte solution used with the conventional lithium ion secondary battery, and there is no restriction | limiting in particular. An electrolytic solution containing an appropriate lithium compound (supporting salt) at a concentration of about 0.1 to 5 mol / L can be used.

The positive electrode disclosed here includes a positive electrode active material having the coating layer formed on the surface thereof. By using this coating layer, a further function can be imparted to the positive electrode and a lithium secondary battery using the positive electrode. That is, according to the present invention, it comprises a current collector and an active material layer formed on the current collector, wherein the active material layer is at least one binder that is substantially insoluble in an organic solvent. And a positive electrode active material having a coating layer formed on the surface thereof, and an oxygen storage material mainly containing cerium composite oxide (typically, an oxygen storage material substantially consisting of the cerium composite oxide), and A positive electrode is provided that contains at least one binder that is soluble in an organic solvent. Such a positive electrode has the advantages of both when a solvent-based binder is used to form an active material layer and when a water-based binder is used to form an active material layer, and at high temperatures. Or the like, in which the release of oxygen is suppressed.
In the positive electrode having the above configuration, the oxygen storage material is disposed on the surface of the positive electrode active material as a component of the coating layer, so that oxygen generated from the positive electrode active material can be effectively absorbed by the oxygen storage material. Therefore, it is possible to construct a lithium secondary battery that achieves both good cycle characteristics (long life) and a reduction in initial internal resistance, and in which oxygen release from the positive electrode is effectively suppressed. In addition, an oxygen storage material mainly composed of a cerium composite oxide can store oxygen more efficiently than other metal oxides conventionally known as oxygen storage materials (for example, iron oxide and copper oxide). . For example, the phenomenon in which oxygen is released from the positive electrode can be suppressed even under more severe conditions (eg, high temperature). Moreover, desired performance (performance which suppresses discharge | release of oxygen) can be exhibited by use of a smaller amount. This means that an excessive addition of an oxygen storage material is unnecessary, and a lithium secondary battery in which the performance of suppressing the release of oxygen and other battery performance (for example, internal resistance value) is more highly balanced can be constructed. To preferred.

  The cerium composite oxide is preferably a composite oxide capable of storing (absorbing) oxygen under conditions where oxygen can be generated from the positive electrode active material (eg, generally about 200 ° C. or higher for lithium transition metal oxides). For example, a cerium composite oxide containing cerium (Ce) and at least one selected from zirconium (Zr) and yttrium (Y) as a constituent metal element can be preferably selected. A cerium composite oxide containing cerium and zirconium as constituent metal elements (that is, a cerium-zirconium composite oxide, hereinafter sometimes referred to as “Ce—Zr composite oxide”) is particularly preferable. Examples of the binder contained in such a coating layer include the same polymers as the positive electrode described above (hydrophilic or water-soluble polymers, water-dispersible polymers, etc.). Moreover, as a binder contained in an active material layer, the polymer (organic solvent soluble polymer etc.) similar to the positive electrode mentioned above can be illustrated.

The Ce-Zr composite oxide constituting the oxygen storage material may be a material in which zirconium oxide is dissolved in cerium oxide (sometimes referred to as ceria-zirconia solid solution). For example, a Ce—Zr composite oxide having a ratio of the number of atoms of cerium and zirconium (Ce / Zr) in the range of about 1/10 to 10/1 can be preferably used. A Ce-Zr composite oxide having the atomic ratio (Ce / Zr) in the range of about 1/1 to 5/1 is more preferable.
As such an oxygen storage material, a cerium composite oxide prepared and provided by a conventionally known method can be used as it is. For example, an oxygen storage material composed of a Ce-Zr composite oxide can be prepared by mixing a compound containing cerium and a compound containing zirconium in a predetermined molar ratio according to the atomic composition, and firing by a suitable means. it can. Further, by pulverizing, granulating and classifying the fired product by an appropriate means, an oxygen storage material powder having a desired average particle size and / or particle size distribution can be obtained. For example, an oxygen storage material powder substantially composed of particles (typically secondary particles) having an average particle diameter in the range of 0.05 μm to 10 μm can be preferably used. The oxygen storage material having such properties is suitable for efficiently absorbing oxygen generated from the positive electrode active material or the like.

  In a preferred embodiment of the positive electrode disclosed herein, the oxygen storage material is, for example, about 0.05 to 30 parts by mass (preferably about 0.05 to 25 parts by mass, and more preferably about 100 parts by mass of the positive electrode active material). 0.1 to 20 parts by mass). For example, when a positive electrode active material made of a lithium transition metal composite oxide is employed, the metal contained in the oxygen storage material with respect to the total number of atoms 100 of metal elements other than lithium contained in the lithium transition metal composite oxide The total number of atoms of elements (for example, the total number of atoms of cerium and zirconium in the case of an oxygen storage material consisting essentially of Ce—Zr composite oxide) is in the range of about 0.1 to 20 (more preferably about 1 to 10). It can be set as the positive electrode which has this oxygen storage material in the ratio which becomes. The positive electrode having the positive electrode active material and the oxygen storage material in the above ratio is in a balance between the performance of suppressing the release of oxygen and other performance (for example, the internal resistance value when a battery is constructed using the positive electrode). It can be excellent.

In a preferred embodiment of the positive electrode disclosed herein, the coating layer further contains a conductive material in addition to the binder and the oxygen storage material. The material that can be preferably used as the conductive material and the preferable content thereof are the same as those of the positive electrode described above. For example, it is appropriate to blend these so that the content of the conductive material is 1 to 30% by mass when the total amount of the binder and the conductive material is 100% by mass, and the content is 5 to 20%. It is preferable that it is mass%. In relation to the positive electrode active material, it is usually appropriate that the content of the conductive material is about 3 to 20 parts by mass (preferably 5 to 15 parts by mass) with respect to 100 parts by mass of the positive electrode active material.
The substantial thickness (average thickness) of the coating layer containing the oxygen storage material (or oxygen storage material and conductive material) is, for example, 0.5 μm or less (typically 0.01 μm to 0.5 μm, preferably 0. 05 μm to 0.5 μm), particularly preferably in the range of 0.1 μm to 0.5 μm. By setting the thickness to this level, good cycle characteristics (long life), high initial output, and performance of suppressing oxygen release from the positive electrode can be realized at a high level.

Thus, the positive electrode which has an active material layer containing an oxygen storage material, for example, includes the following steps:
An oxygen storage material (for example, a Ce-Zr composite oxide substantially comprising an organic solvent, a positive electrode active material and at least one binder that is substantially insoluble in the organic solvent and a cerium composite oxide) And a step of preparing an active material layer forming composition having an active material whose surface is coated with a coating material containing an oxygen storage material) and at least one binder material that is soluble in an organic solvent. And applying the composition to the surface of a positive electrode current collector to form a positive electrode active material layer on the current collector;
It can manufacture preferably by the positive electrode manufacturing method including this. Such a production method can be carried out in the same manner as the positive electrode production method described above, except that a coating material containing an oxygen storage material is used. In one preferred embodiment, the oxygen storage material powder substantially composed of particles (typically secondary particles) having an average particle size in the range of 0.05 μm to 5 μm as the oxygen storage material included in the coating material Use the body.

  The positive electrode active material whose surface is coated with a coating material containing at least one binder that is substantially insoluble in an organic solvent and an oxygen storage material mainly composed of a cerium composite oxide includes, for example, an aqueous solvent, A binder dissolved or dispersed in the aqueous solvent and having at least one binder substantially insoluble in an organic solvent, a positive electrode active material powder, and a cerium composite oxide as a main component (typically, Preparing a coating composition comprising an oxygen storage material powder (consisting essentially of a Ce-Zr composite oxide) and removing the aqueous solvent from the composition (ie, drying the composition) Can be obtained by: A coating comprising at least one binder that is substantially insoluble in an organic solvent, an oxygen storage material mainly composed of a cerium composite oxide, and a conductive material by further including a conductive material in the coating composition. A positive electrode active material whose surface is coated with a material can be suitably obtained.

  Hereinafter, experimental examples relating to the present invention will be described. However, the present invention is not intended to be limited to the specific examples.

< Reference Examples 1 to 6: Production of lithium ion secondary battery (1)>
A cylindrical standard type 18650 type lithium ion secondary battery was manufactured as follows. Here, a lithium transition metal composite oxide powder represented by the formula: LiNi _0.8 Co _0.15 Al _0.05 O ₂ was used as the positive electrode active material. This composite oxide (secondary particles) had an average particle diameter of about 10 μm, and was a positive electrode active material rich in secondary particles having a particle diameter of 5 μm to 25 μm.

Carboxymethylcellulose (CMC) was dissolved in water to prepare a coating material having a CMC concentration of about 1.0% by mass. Using this coating material, a coating layer having a predetermined thickness was formed on the surface of the composite oxide. That is, the composite oxide powder was immersed in the coating material and then dried in air. By repeating this dipping treatment, the active material having an average thickness of the coating layer of about 0.01 μm ( Reference Example 1), the active material of about 0.05 μm ( Reference Example 2), and the active material of about 0.1 μm. A material ( Reference Example 3), an active material of about 0.2 μm ( Reference Example 4), an active material of about 0.5 μm ( Reference Example 5) and an active material of about 1 μm ( Reference Example 6) were obtained. A total of six types of active material powders ( Reference Examples 1 to 6) having different coating layer thicknesses were used, and a total of six types of lithium ion secondary batteries were manufactured as shown below. Hereinafter, a total of six types of lithium ion secondary batteries will be referred to as Reference Examples 1, 2, 3, 4, 5, and 6, respectively, corresponding to the types of active materials used ( Reference Examples 1 to 6).

The active material with a coating layer obtained as described above, acetylene black as a conductive material, and PVDF as a binder are mixed with an organic solvent (NMP) to prepare an active material layer forming paste according to each reference example. did. The approximate mass ratio of each material (other than NMP) contained in this paste is active material: 85% by mass, acetylene black: 10% by mass, and PVDF: 5% by mass.
This paste was applied (attached) to both sides of a long aluminum foil having a thickness of about 15 μm as a positive electrode current collector and dried to form a positive electrode active material layer having a thickness of about 20 μm on both surfaces of the current collector. Subsequently, it pressed so that the whole thickness might be set to about 40 micrometers. A total of six types of positive electrode sheets ( Reference Examples 1 to 6) were prepared for each active material powder used in this manner.

On the other hand, a graphite powder (Kishida Chemical Co., Ltd. product) is used as the carbon material for the negative electrode active material, and CMC and a styrene butadiene block copolymer (SBR) are used as the binder. Was prepared. That is, the negative electrode active material and the binder were mixed with ion exchange water to prepare a negative electrode active material layer forming paste. The approximate mass ratio of each material (other than water) contained in the paste is carbon material: 98 mass%, CMC: 1 mass%, and SBR: 1 mass%.
Then, the paste is applied (attached) to both sides of a long copper foil having a thickness of about 15 μm as a negative electrode current collector and dried to form a negative electrode active material layer having a thickness of about 20 μm on both sides of the current collector. did. Subsequently, it pressed so that the whole thickness might be set to about 40 micrometers. In this way, a negative electrode sheet was produced.

The positive electrode sheet and the negative electrode sheet thus obtained were laminated together with two separators (here, a porous polyethylene sheet was used), and this laminated sheet was wound to produce a wound electrode structure. A total of six types of this electrode structure were prepared for each positive electrode sheet obtained by accommodating a cylindrical lithium ion secondary battery having a diameter of 18 mm and a height of 65 mm (that is, 18650 type) by accommodating the electrode structure together with an electrolytic solution ( Reference Example 1). ~ 6). As the electrolytic solution, an electrolytic solution used in a conventional lithium ion secondary battery can be used without any particular limitation. Here, a 3: 7 (volume ratio) mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) is used. A nonaqueous electrolytic solution having a composition in which 1 mol / L LiPF ₆ was dissolved was used.

< Reference Examples 7 to 12: Production of lithium ion secondary battery (2)>
A 18650 type lithium ion secondary battery was manufactured by the same material and procedure as those of Reference Examples 1 to 6 described above except that a conductive material containing a conductive material was used.
That is, CMC and conductive material (acetylene black powder) were dissolved in water at a mass ratio of 10% by mass when the total content of CMC and conductive material was 100% by mass, and the CMC concentration was A coating material having about 1.0% by mass and a conductive material concentration of about 10% by mass was produced. Using this coating material, an active material ( Reference Example 7) having an average thickness of the coating layer of about 0.01 μm and an active material (about 0.05 μm) by the same process as in Reference Examples 1 to 6 above ( Reference Example 8), an active material of about 0.1 μm ( Reference Example 9), an active material of about 0.2 μm ( Reference Example 10), an active material of about 0.5 μm ( Reference Example 11) and about 1 μm An active material ( Reference Example 12) was obtained.
A total of 6 types of active material powders ( Reference Examples 7 to 12) having different coating layer thicknesses were used to prepare a total of 6 types of active material layer forming pastes. Positive electrode sheets ( Reference Examples 7 to 12) were prepared. Except for using these positive electrode sheets, a total of six types of lithium ion secondary batteries were manufactured using the same materials (negative electrode sheets, separators, etc.) and processes as in Reference Examples 1 to 6 described above. Hereinafter, the six types of lithium ion secondary batteries are referred to as Reference Examples 7, 8, 9, 10, 11, and 12, respectively, corresponding to the types of active materials used ( Reference Examples 7 to 12).

<Comparative Example 1: Production of lithium ion secondary battery (3)>
In this comparative example, the composite oxide powder not subjected to the treatment with the coating material is used as a positive electrode active material (that is, a positive electrode active material without a coating layer), and other materials and conditions are the same as those in Reference Examples 1 to 6. An active material layer forming paste (solvent type) according to this comparative example was prepared. Thereafter, a positive electrode sheet was produced from the paste by the same material and process as in Reference Examples 1 to 6 described above, and then an 18650 type lithium ion secondary battery according to this comparative example was produced using the positive electrode sheet.

<Comparative example 2: Production of lithium ion secondary battery (4)>
In this comparative example, the composite oxide powder not subjected to the treatment with the coating material was used as a positive electrode active material (that is, a positive electrode active material having no coating layer) to prepare an aqueous active material layer forming paste.
That is, a positive electrode active material without a coating layer, CMC, and acetylene black powder were mixed with ion-exchanged water to prepare an active material layer forming paste. The approximate mass ratio of each material (other than water) contained in this aqueous paste is active material: 87% by mass, acetylene black: 10% by mass, and CMC: 3% by mass. Thereafter, a positive electrode sheet was produced from the paste by the same material and process as in Reference Examples 1 to 6 described above, and then an 18650 type lithium ion secondary battery according to this comparative example was produced using the positive electrode sheet.

<Test Example 1: Initial internal resistance and durability evaluation>
About the lithium ion secondary battery obtained by each reference example and the comparative example, the initial internal resistance value (henceforth "initial resistance value") was measured with the resistance measuring method shown below. In addition, after performing a charge / discharge cycle test under the following conditions on these secondary batteries, the value of the internal resistance after repeated charge / discharge (hereinafter referred to as “resistance value after durability”) was measured by the same measurement method.

[Measurement of initial resistance]
After charging to 3.75 V at a constant current of 1000 mA / cm ^{2 at} an environmental temperature of 25 ° C., constant potential charging was performed at 3.75 V, and charging was continued until the total charging time was 1.5 hours. The state of charge (SOC) at this time is about 60% of full charge. Thereafter, charge and discharge are performed in the order of the following (a) to (f), and the primary value of the current (I) -voltage (V) plot value with the voltage after each charge / discharge as the vertical axis and the charge / discharge current as the horizontal axis. The initial resistance value was determined from the slope of the approximate line. The results are shown in Table 1.
(a) Discharge at 300 mA / cm ² for 10 seconds.
(b) Charge at 300 mA / cm ² for 10 seconds.
(c) Discharge at 900 mA / cm ² for 10 seconds.
(d) Charge at 900 mA / cm ² for 10 seconds.
(e) Discharge at 2700 mA / cm ² for 10 seconds.
(f) Charge at 2700 mA / cm ² for 10 seconds.

[Endurance resistance charge / discharge cycle test]
At an environmental temperature of 60 ° C., the battery was charged to 4.1 V at a constant current of 2 C, and then discharged to 3.0 V at the same current density. This cycle (a cycle of charging to 4.1 V at a constant current of 2 C and discharging to 3.0 V) was repeated 500 times. The symbol “C” represents the discharge time rate. Therefore, the current density 2C means a current density (A) that can supply an amount of electricity corresponding to the battery capacity (Ah) of the battery in 0.5 hours. Then, similarly to the initial resistance value, the resistance value after 500 cycles of charge / discharge (resistance value after endurance) was determined from the slope of the primary approximate line of the IV plot value after charge / discharge. The results are shown in Table 1.

As is clear from the results shown in Table 1, in the lithium ion secondary battery (each reference example ) employing the positive electrode provided by the present invention, a positive electrode active material layer is formed using a solvent-based active material layer forming paste. As a result, it was confirmed that the initial internal resistance value was lower than that of the lithium ion secondary battery of Comparative Example 2 constructed using a conventional aqueous active material layer forming paste. This result shows that the lithium ion secondary battery provided by the present invention has a low initial internal resistance and can realize a high output. Furthermore, it was confirmed that the initial internal resistance value can be more effectively reduced by including a conductive material in the coating layer.
At the same time, in the lithium ion secondary batteries of each reference example , as a result of using the positive electrode active material having the coating layer of the present invention, comparative examples constructed using conventional solvent-based active material layer forming pastes It was confirmed that the resistance value after endurance was significantly lower than that of No. 1 lithium ion secondary battery. This result shows that the lithium ion secondary battery provided by the present invention is excellent in charge / discharge cycle characteristics and can realize high durability, that is, long life.

<Example 13: Production of lithium ion secondary battery (5)>
Reference Examples 1 to 6 described above, except that a coating material containing ceria-zirconia composite oxide powder (oxygen storage material powder, average particle size of about 1 μm) with Ce / Zr being an atomic ratio of about 1/1 is used. A 18650 type lithium ion secondary battery was manufactured by the same material and procedure.
That is, lithium transition metal composite oxide powder (positive electrode active material powder), ceria-zirconia composite oxide powder (oxygen storage material powder), acetylene black and CMC as a conductive material are mixed with water (ion exchange water) and coated. A material was prepared. In this coating material, the positive electrode active material and the oxygen storage material are composed of the metal element constituting the oxygen storage material with respect to the total number of atoms 100 of metal elements (Ni, Co and Al) other than lithium constituting the positive electrode active material. (Ce and Zr) are contained at a ratio of approximately 5 atoms. That is, it contains about 5 atom% oxygen storage material with respect to the positive electrode active material. By gently heating this coating material to evaporate water, an active material having a coating layer containing CMC and an oxygen storage material and having an average thickness of about 0.2 μm was obtained.

This active material with a coating layer, acetylene black as a conductive material, and PVDF as a binder were mixed with an organic solvent (NMP) to prepare an active material layer forming paste according to Example 13. The approximate mass ratio of each material (other than NMP) contained in this paste is active material: 85% by mass, acetylene black: 10% by mass, and PVDF: 5% by mass. This paste was applied (attached) to both sides of a long aluminum foil having a thickness of about 15 μm as a positive electrode current collector and dried to form a positive electrode active material layer having a thickness of about 20 μm on both surfaces of the current collector. Subsequently, it pressed so that the whole thickness might be set to about 40 micrometers. In this way, a positive electrode sheet was produced. A lithium ion secondary battery was manufactured by the same material (negative electrode sheet, separator, etc.) and process as those in Reference Examples 1 to 6 except that the positive electrode sheet was used.

<Example 14: Production of lithium ion secondary battery (6)>
A lithium ion secondary battery was produced by the same material and procedure as in Example 13 except that a conductive material containing a conductive material was used.
That is, the positive electrode active material powder, the oxygen storage material powder, the conductive material (acetylene black powder), and CMC were mixed with water. As a result, a coating material containing an oxygen storage material at a rate of about 5 atom% with respect to the positive electrode active material and containing a conductive material at a rate of approximately 10 parts by mass with respect to 100 parts by mass of the positive electrode active material was prepared. . Using this coating material, an active material in which a coating layer having an average thickness of about 0.2 μm including CMC, an oxygen storage material and a conductive material was formed on the surface by the same process as in Example 13 was obtained. An active material layer forming paste was prepared using this active material with a coating layer, and a positive electrode sheet was prepared using the paste. A lithium ion secondary battery was manufactured by the same material and process as in Example 13 except that the positive electrode sheet was used.

<Comparative Examples 3 and 4: Production of Lithium Ion Secondary Battery (7)>
A lithium ion secondary battery was manufactured by the same material and procedure as in Examples 13 and 14, except that a positive electrode sheet was prepared using an aqueous active material layer forming paste.
That is, the active material layer-forming paste according to Comparative Example 3 was prepared by mixing the active material with a coating layer according to Example 13, acetylene black as a conductive material, and CMC as a binder with water. The approximate mass ratio of each material (other than water) contained in this aqueous paste is active material: 85% by mass, acetylene black: 10% by mass, and CMC: 5% by mass. A lithium ion secondary battery according to Comparative Example 3 was manufactured using the same materials and processes as in Example 13 except that the paste was used.
Moreover, it replaced with the active material with a coating layer which concerns on Example 13, and was using the active material with a coating layer which concerns on Example 14 by the same material and process as the comparative example 3, and the lithium ion secondary which concerns on the comparative example 4 A battery was manufactured.

<Comparative Example 5: Production of lithium ion secondary battery (8)>
In this comparative example, the composite oxide powder not subjected to the treatment with the coating material was used as a positive electrode active material (that is, a positive electrode active material having no coating layer) to prepare an aqueous active material layer forming paste.
That is, a positive electrode active material without a coating layer, CMC, and acetylene black powder were mixed with ion-exchanged water to prepare an active material layer forming paste. The approximate mass ratio of each material (other than water) contained in this aqueous paste is active material: 85% by mass, acetylene black: 10% by mass, and CMC: 5% by mass. Then, the positive electrode sheet was produced from this paste with the material and process similar to the above-mentioned Example 13, and the lithium ion secondary battery was manufactured using this positive electrode sheet.

<Comparative Examples 6 and 7: Production of lithium ion secondary battery (9)>
A lithium ion secondary battery was produced by the same material and procedure as in Examples 13 and 14 except that a coating material containing an organic solvent-soluble binder and an organic solvent was used.
That is, positive electrode active material powder, oxygen storage material powder, and PVDF were mixed with an organic solvent (NMP). This prepared the coating material which contains oxygen storage material powder in the ratio of about 5 atom% with respect to a positive electrode active material. Using this coating material, an active material (Comparative Example 6) having an average thickness of the coating layer of about 0.2 μm was obtained by the same process as in Example 13. A lithium ion secondary battery according to Comparative Example 6 was manufactured using the same materials and processes as in Example 13 except that this active material with a coating layer was used.
Moreover, positive electrode active material powder, oxygen storage material powder, conductive material (acetylene black powder) and PVDF were mixed with an organic solvent (NMP). As a result, a coating material containing the oxygen storage material powder at a rate of about 5 atom% with respect to the positive electrode active material and containing a conductive material at a rate of about 10 parts by mass with respect to 100 parts by mass of the positive electrode active material is prepared. did. Using this coating material, an active material (Comparative Example 7) having an average thickness of the coating layer of about 0.2 μm was obtained by the same process as in Example 13. A lithium ion secondary battery according to Comparative Example 7 was manufactured using the same materials and processes as in Example 13 except that this active material with a coating layer was used.

<Test Example 2: Initial internal resistance and durability evaluation and temperature rise test>
For the lithium ion secondary batteries obtained in Examples 13 to 14 and Comparative Examples 3 to 7, the initial internal resistance value (initial resistance value) and the internal resistance value after repeated charge and discharge were measured by the same measurement method as in Test Example 1. The value of (resistance value after endurance) was measured.
Each lithium ion secondary battery was charged to 100% SOC, disassembled in this state, and the positive electrode sheet was taken out. The positive electrode sheet and the electrolytic solution having the above composition were placed in an aluminum sample pan, and DSC (Differential Scanning Calorimetry) measurement was performed using a differential scanning calorimeter at a heating rate of 10 ° C./min. The oxygen desorption temperature (oxygen desorption peak temperature) was read from the peak position of the DSC curve thus obtained.
The obtained results are shown in Table 2. This table also shows the results of performing a temperature increase test on the lithium ion secondary battery according to Comparative Example 1 in the same manner.

As is clear from this table, the positive electrodes of Examples 13 and 14 using the positive electrode active material having a coating layer containing an oxygen storage material both have an oxygen desorption peak temperature of 350 ° C. or higher, and the coating layer has oxygen Compared to the positive electrodes of Reference Examples 4 and 10 that did not contain a storage material, the peak temperature increased significantly. In addition, the lithium ion secondary batteries of Examples 13 and 14 exhibited low initial resistance values and post-endurance resistance values, respectively, similarly to the lithium ion secondary batteries of Reference Example 4 and Reference Example 10. This result shows that in the lithium ion secondary batteries of Examples 13 and 14, the release of oxygen from the positive electrode was further suppressed while maintaining the features of Reference Examples 4 and 10. In Example 14 in which the coating layer contains a conductive material, a better result was obtained than in Example 13 in which the conductive material was not included.
On the other hand, in the positive electrodes of Comparative Examples 3 and 4 including the oxygen storage material, the oxygen desorption peak was significantly increased as compared with the positive electrode of Comparative Example 5 not including the oxygen storage material. In addition, in the positive electrodes of Comparative Examples 6 and 7 including the oxygen storage material, the oxygen desorption peak was significantly increased as compared with the positive electrode of Comparative Example 5 not including the oxygen storage material. However, since the lithium ion secondary batteries of Comparative Examples 3 and 4 use the aqueous active material layer forming paste, the initial internal resistance tends to be high in the same manner as the lithium ion secondary battery of Comparative Example 5. Indicated. Moreover, since the lithium ion secondary batteries of Comparative Examples 6 and 7 use PVDF (organic solvent-soluble binder) as a binder constituting the coating layer, lithium ion secondary batteries using CMC as the binder. The resistance value after endurance was likely to increase compared to.

<Examples 15 to 20: Production of lithium ion secondary battery (10)>
A lithium ion secondary battery was manufactured by the same material and procedure as in Example 13 except that the thickness of the coating layer was varied.
That is, a total of six types of coating materials (coating materials containing CMC at a concentration corresponding to the target coating layer thickness) having different CMC content ratios relative to the positive electrode active material were prepared. Each of these coating materials contains about 5 atom% oxygen storage material with respect to the positive electrode active material. By gently heating these coating materials to evaporate the water, an active material having an average thickness of the coating layer of about 0.01 μm (Example 15) and an active material having an average thickness of about 0.05 μm (Example 16). ), An active material that is about 0.1 μm (Example 17), an active material that is about 0.3 μm (Example 18), an active material that is about 0.5 μm (Example 19), and an active material that is about 1 μm (Example 20) was obtained. Lithium ions according to Examples 15 to 20 were produced in the same manner as in Example 13 except that a total of six types of active material powders (Examples 15 to 20) having different coating layer thicknesses were used. A secondary battery was manufactured.

<Test Example 3: Initial internal resistance and durability evaluation and temperature rise test>
With respect to the lithium ion secondary batteries obtained in Examples 15 to 20, the initial resistance value and the resistance value after durability) were measured by the same measurement method as in Test Example 1. Further, a temperature increase test was performed in the same manner as in Test Example 2, and the oxygen desorption peak temperature was read from the peak position of the DSC curve. The obtained results are shown in Table 3.

  As is clear from this table, according to the positive electrodes of Examples 15 to 20 containing about 5 atom% of oxygen storage material with respect to the positive electrode active material, a high oxygen desorption peak temperature of 350 ° C. or higher was realized. . Further, the lithium ion secondary batteries of Examples 17 and 18 having a coating layer thickness in the range of 0.1 to 0.3 μm have high oxygen desorption peak temperatures, low initial resistance values, and good cycle characteristics (low durability). The post-resistance value) is realized in a particularly well-balanced manner at a high level.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. For example, the composition for forming an active material layer and the coating material can contain additional constituent elements (subcomponents) other than the main elements described above as long as the object of the present invention is not impaired.
The technical elements described in this specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Moreover, the technique illustrated in this specification or the drawings achieves a plurality of objects at the same time, and has technical usefulness by achieving one of the objects.

Claims (19)

A positive electrode used in a lithium secondary battery,
A current collector,
An active material layer formed on the current collector, the cathode active material having a coating layer containing at least one binder that is substantially insoluble in an organic solvent formed on the surface, and an organic solvent An active material layer having at least one binder that is soluble with respect to,
Equipped with a,
The said coating layer is a positive electrode containing the oxygen storage material which has a cerium complex oxide as a main component .   The positive electrode according to claim 1, wherein the coating layer includes a conductive material.   The positive electrode according to claim 2, wherein the content of the conductive material is 5 to 20% by mass when the total amount of the binder and the conductive material is 100% by mass.   The positive electrode according to claim 1, wherein the active material is substantially composed of secondary particles having an average particle diameter in the range of 5 μm to 25 μm.   The positive electrode according to claim 1, wherein the thickness of the coating layer present on the surface of the active material is substantially 0.5 μm or less.   The positive electrode according to claim 1, wherein the active material is at least one lithium transition metal composite oxide. The cerium complex oxide of cerium - zirconium composite oxide, the positive electrode according to claim 1. Wherein the coating layer comprises a conductive material, a positive electrode according to claim 1. The thickness of the coating layer on the surface of the active material is substantially less than 0.5 [mu] m, the positive electrode according to claim 1. A method for producing a positive electrode used in a lithium secondary battery, comprising the following steps:
The surface is coated with a coating material containing an organic solvent and a positive electrode active material and at least one binder that is substantially insoluble in the organic solvent and containing an oxygen storage material mainly composed of cerium composite oxide . Providing a composition for forming an active material layer having an active material and at least one binder soluble in an organic solvent; and applying the composition to the surface of a positive electrode current collector Forming a positive electrode active material layer on the current collector;
A method for producing a positive electrode comprising: The method of claim 10 , wherein the coating material comprises a conductive material. The method according to claim 11 , wherein the content of the conductive material is 5 to 20% by mass when the total amount of the binder and the conductive material is 100% by mass. The method according to any one of claims 10 to 12 , wherein a positive electrode active material substantially composed of secondary particles having an average particle diameter in the range of 5 µm to 25 µm is used. The method according to any one of claims 10 to 13 , wherein the thickness of the deposit of the coating material existing on the surface of the active material is substantially 0.5 µm or less. The method according to claim 10 , wherein at least one lithium transition metal composite oxide is used as the active material. The method according to claim 10, wherein the cerium composite oxide is a cerium-zirconium composite oxide. The method according to claim 10 , wherein the coating material includes a conductive material. The method according to any one of claims 10 to 17 , wherein a thickness of the coating material deposit on the surface of the active material is substantially 0.5 µm or less. Characterized by comprising a positive electrode according to any one of claims 1 to 9 lithium secondary battery.