JP4868381B2 - Electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  In recent years, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have attracted attention as batteries having high energy density. The lithium ion secondary battery is configured as follows, for example. That is, the electrode body is accommodated in the sealed container, and a pair of positive and negative electrode terminal mechanisms are attached to the lid body of the sealed container, and the electric power generated by the electrode body is taken out from the electrode terminal mechanism. It is possible. Each lid is provided with a pressure open / close type gas discharge valve.

  The electrode body is configured by stacking a belt-like positive electrode and a negative electrode via a separator and winding them. The positive electrode is composed of a current collector made of aluminum foil or the like and a positive electrode mixture layer containing a positive electrode active material present on the surface of the current collector, and the negative electrode is made of a current collector made of copper foil or the like, It is comprised with the negative mix layer containing the negative electrode active material which exists in this collector surface. The positive electrode mixture layer faces the negative electrode mixture layer with a separator interposed therebetween. The positive electrode active material is, for example, a lithium transition metal composite oxide, and the negative electrode active material is a material that can occlude and release lithium ions, such as graphite.

  In the charge / discharge reaction of the lithium ion secondary battery, lithium ions move between the positive electrode mixture layer and the negative electrode mixture layer facing each other through the electrolytic solution.

Examples of the lithium transition metal composite oxide used as the positive electrode active material include lithium / cobalt composite oxide (LiCoO ₂ ), lithium / nickel composite oxide (LiNiO ₂ ), and lithium / manganese composite oxide (LiMn ₂ O ₄ ). It has been known. Among these, especially LiCoO ₂ is frequently used because a battery having high voltage, high energy density, and excellent cycle characteristics can be formed.

By the way, in the conventional non-aqueous electrolyte secondary battery using LiCoO ₂ as the positive electrode active material, the maximum potential of the positive electrode during charging is limited to about 4.3 V with respect to metallic lithium, and the discharge capacity of LiCoO _{2 in} the battery Has been limited to 140 mAh / g or less.

A non-aqueous electrolyte secondary battery using LiCoO ₂ as a positive electrode active material theoretically has all the LiCoO ₂ Li released when charged to about 4.8 V on the basis of metallic lithium, and the theoretical capacity is about It reaches up to 274 mAh / g. However, when the charging potential is increased to increase the capacity, the heavy load characteristic and the cycle characteristic are remarkably deteriorated, and the safety is lowered.

Therefore, as a positive electrode active material, a general formula Li _x Co _1-y M _y O ₂ (M represents one or more elements including at least a group IIIB element, and x and y are 0.9 ≦ x ≦ 1.1, respectively. , 0 <y ≦ 0.2), and when the atomic ratio of M to Co on the particle surface is a, the value of a is the average value of the atomic ratio of M to Co of the entire particle [ Studies have been made to improve heavy load characteristics by using a lithium-containing composite oxide that is 1.5 times or more of y / (1-y)] (Patent Document 1).

JP 2002-75356 A

  By the way, non-aqueous electrolyte secondary batteries used as driving power sources for mobile phones and the like are required to have higher capacity, improved storage characteristics and cycle characteristics, as well as improved safety. It is necessary to be satisfied.

  However, in the technique of the above-mentioned Patent Document 1, although high capacity and high load characteristics are improved particularly by high-voltage charging, studies on improvement of storage characteristics, cycle characteristics, and safety are sufficiently performed. There was still room for improvement in this regard.

  The present invention has been made in view of the above circumstances, and its purpose is to improve storage characteristics and cycle characteristics while achieving high capacity by high voltage charging, and also to ensure excellent safety. An object of the present invention is to provide an electrode that can constitute a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery having the electrode.

The electrode for a non-aqueous electrolyte secondary battery of the present invention that has achieved the above object has the following active material, that is, the composition formula Li _{(1 + δ)} Mn _x Ni _y Co _(1-xy) O ₂ (− 0.15 <δ <0.15, 0.1 <x ≦ 0.5, 0.5 <x + y ≦ 1.0), the average particle size is 5 to 25 μm, and the specific surface area is 0.1 to 0. Having a positive electrode mixture layer containing an active material of .6 m ² / g, and when the Co amount relative to the total amount of Mn, Ni and Co on the surface of the positive electrode mixture layer is a by atomic ratio, The value of a is 1.5 times or more of the average value [(1-xy) / 1] in the atomic ratio of Co amount to the total amount of Mn, Ni and Co in the whole positive electrode mixture layer. It is characterized by.

  Moreover, the nonaqueous electrolyte secondary battery which has the electrode of this invention as a positive electrode is also included by this invention.

  According to the present invention, it is possible to provide a nonaqueous electrolyte secondary battery having a high capacity and excellent safety, and also having excellent cycle characteristics and storage characteristics during high voltage charging.

In the electrode of the present invention, as the active material, the composition formula Li _{(1 + δ)} Mn _x Ni _y Co _(1-xy) O ₂ (−0.15 <δ <0.15, 0.1 <x ≦ 0. 5, 0.5 <x + y ≦ 1.0) is used. This active material is a layered type Li-containing composite oxide. By using the active material, in a battery having the above electrode as a positive electrode, for example, a high voltage of about 4.3 to 4.6 V on the basis of Li ⁺ Even when charging is performed, the positive electrode during charging can be kept stable and heat generation can be suppressed, so that there is no risk of ignition and the safety is excellent. Further, in a battery using the electrode containing the layered Li-containing composite oxide as a positive electrode, a high capacity can be achieved. That is, the above-mentioned layered Li-containing composite oxide is superior in that it has a capacity equivalent to that of LiCoO ₂ conventionally used as a positive electrode active material of a non-aqueous electrolyte secondary battery and has increased stability during charging. It is considered that a structure showing high safety is formed.

The layered Li-containing composite oxide has an average particle size of 5 μm or more, preferably 7 μm or more, 25 μm or less, preferably 20 μm or less, and a specific surface area of 0.1 m ² / g or more. , Preferably 0.2 m ² / g or more, 0.6 m ² / g or less, preferably 0.5 m ² / g or less. By using the layered Li-containing composite oxide having such a form, in addition to being able to achieve high capacity and high safety in a battery using the electrode having the composite oxide as a positive electrode, storage characteristics and cycle The characteristics can also be improved. That is, in the layered type Li-containing composite oxide having the above form, since the specific surface area is relatively small, the reaction area with the electrolytic solution can be reduced, thereby improving the safety of the battery, Since the amount of a conductive additive and a binder (described later) used for the electrode can be reduced, it is possible to achieve a high density of the electrode, and thus to increase the capacity of the battery. In addition, since the layered Li-containing composite oxide has the above-described form and has a relatively small specific surface area, a positive electrode mixture for forming a positive electrode mixture layer having the composite oxide in the above electrode is prepared. In the battery having the electrode as the positive electrode, storage characteristics and cycle characteristics can be improved because the composite oxide can be prevented from collapsing at the time of formation of the positive electrode mixture layer. In addition, the average particle diameter and specific surface area of said layered type Li containing complex oxide as used in the field of this invention are the values measured by the method employ | adopted in the postscript Example.

  Furthermore, the electrode of the present invention has a positive electrode mixture layer containing the layered Li-containing composite oxide, but the amount of Co relative to the total amount of Mn, Ni and Co on the surface of the positive electrode mixture layer is When the atomic ratio is a, the value of a is the average value [(1-xy) / 1] of the atomic ratio of Co amount to the total amount of Mn, Ni and Co in the whole positive electrode mixture layer. It is 1.5 times or more, preferably 1.7 times or more.

  In the layered Li-containing composite oxide, simply securing the composition and form described above cannot sufficiently suppress elution of Mn during high-voltage charging in a battery having an electrode containing such an active material as a positive electrode. In particular, it is difficult to sufficiently improve the storage characteristics. However, in the positive electrode mixture layer containing the layered Li-containing composite oxide, the atomic ratio of the Co amount to the total amount of Mn, Ni, and Co in the entire positive electrode mixture layer, and the Mn on the surface of the positive electrode mixture layer When the relationship between the atomic ratio of Co and the total amount of Ni and Co satisfies the above lower limit value, Co exhibiting good behavior with respect to storage characteristics at a high charge voltage is the surface of the composite oxide particles. It is considered that this can suppress elution of Mn and improve the storage characteristics and cycle characteristics of the battery. That is, if the value of a is too small relative to the value of [(1-xy) / 1], the ratio of Co on the particle surface of the layered Li-containing composite oxide becomes too small, resulting in high Mn elution during voltage charging cannot be reduced, and it becomes difficult to produce a battery having excellent cycle characteristics and storage characteristics.

  The ratio of the value a to the value [(1-x−y) / 1] is preferably 4 times or less, and more preferably 3 times or less. If it is 4 times or more, the Co ratio inside the particles of the layered Li-containing composite oxide becomes too small, and the conductivity and ion conductivity inside the particles become small, which adversely affects the load characteristics and low temperature characteristics. Because. The value of a and the value of [(1-xy) / 1] are obtained by the method described in Examples described later.

  For the positive electrode material mixture layer in which the value of a and the value of [(1-xy) / 1] are in the above relationship, the layered Li-containing composite oxide having a high Co concentration on the particle surface is used. The composite oxide can be manufactured by the following method, for example. A nickel-containing salt (such as a sulfate), a manganese-containing salt (such as a nitrate), and a cobalt-containing salt (such as a sulfate) are mixed so that the Co ratio is lower than a predetermined composition ratio for each metal element. The mixture is dissolved in water to obtain a mixed aqueous solution (aqueous solution A). Aqueous ammonia is prepared as the aqueous solution B. Furthermore, aqueous ammonia having a pH adjusted to 12 is prepared, and the aqueous solution A and aqueous solution B are dropped into the stirring aqueous ammonia to obtain a coprecipitated hydroxide of Ni, Mn, and Co. Thereafter, the coprecipitated hydroxide is sufficiently washed with water and dried. Li hydroxide and Co salt (such as hydroxide) are added to the coprecipitated hydroxide after drying so that each metal element has a predetermined composition ratio, and mixed sufficiently. The layered Li-containing composite oxide is obtained by firing this mixture in an oxidizing atmosphere. In addition, as an atmosphere at the time of baking, it is preferable that oxygen partial pressure shall be 0.19-1 atmosphere, for example, baking temperature shall be 600-1000 degreeC, for example, and baking time will be 6-48, for example. Time is desirable. In addition to this, a raw material of Co (such as a hydroxide of Co) is mixed with a layered Li-containing composite oxide having a Co ratio lower than a predetermined composition ratio, and calcined under the same conditions as described above. A layered Li-containing composite oxide according to the present invention having a high concentration can also be obtained.

  The electrode of the present invention can be produced, for example, by forming a positive electrode mixture layer containing the above active material (the above-mentioned layered Li-containing composite oxide) on the surface of a current collector. In addition to the active material, the positive electrode mixture layer contains a conductive additive and a binder. For example, in a mixture (positive electrode mixture) containing the active material, the conductive additive and the binder, A positive electrode mixture paste, a positive electrode mixture slurry, or the like obtained by adding an appropriate solvent and sufficiently kneaded can be applied to the surface of the current collector and dried to be formed while controlling the desired thickness and density. it can.

  The conductive auxiliary agent is not particularly limited as long as it is a substantially chemically stable electron conductive material in the battery using the electrode of the present invention. For example, graphites such as natural graphite (flaky graphite, etc.) and artificial graphite; acetylene black; ketjen black; carbon blacks such as channel black, furnace black, lamp black, and thermal black; carbon fibers; Other conductive fibers such as metal fibers; carbon fluoride; metal powders such as aluminum; zinc oxide; conductive whiskers such as potassium titanate; conductive metal oxides such as titanium oxide; organics such as polyphenylene derivatives These may be used alone, or two or more of them may be used simultaneously. Among these, carbon materials such as acetylene black, ketjen black, and carbon black are particularly preferable.

The binder (binder) plays a role of binding the active material and the conductive additive in the positive electrode mixture layer. The binder according to the electrode of the present invention may be either a thermoplastic resin or a thermosetting resin. Specifically, for example, polyolefins such as polyethylene and polypropylene; polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer (ETFE resin), polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chloro Fluoropolymers such as trifluoroethylene copolymer (ECTFE), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer; styrene butadiene rubber (SBR) ); Ethylene-acrylic acid copolymer or Na ⁺ ion crosslinked product of the copolymer; ethylene-methacrylic acid copolymer or Na ⁺ ion crosslinked product of the copolymer; ethylene-methyl acrylate copolymer or the copolymer Copolymer Na ⁺ ion cross-linked product; ethylene-methyl methacrylate copolymer or Na ⁺ ion cross-linked product of the copolymer; and the like, and these materials may be used alone or in combination. The above may be used simultaneously. Among these materials, PVDF and PTFE are particularly preferable.

  In the positive electrode mixture layer according to the electrode of the present invention, the specific surface area of the active material (the layered Li-containing composite oxide) is small as described above. In the positive electrode mixture layer, it is easy to transfer and immobilize the charge between the particles just by adding a small amount of conductive aid such as carbon black, acetylene black, ketjen black and the above binder. It is considered that the amount of the conductive auxiliary and the amount of the binder can be reduced, and it is effective in improving the density of the electrode (positive electrode mixture layer) and, in turn, increasing the capacity of the battery.

  That is, at least one selected from the group consisting of carbon black, acetylene black, and ketjen black in the total amount of the positive electrode mixture containing the active material (the layered Li-containing composite oxide), a conductive additive, and a binder. The content of the carbon material is 0.5% by mass or more, more preferably 0.7% by mass or more, and is preferably 3% by mass or less, more preferably 2% by mass or less. When the content of the carbon material is too large, the load characteristics of the battery can be improved, but the filling property of the active material is lowered. On the other hand, if the content of the carbon material is too small, it is difficult to take electrical conductivity in the positive electrode. In the case where a conductive auxiliary other than at least one carbon material selected from the group consisting of carbon black, acetylene black, and ketjen black is also used, the total conductive auxiliary content in the total positive electrode mixture is 0.5 It is desirable that the content is not less than mass%, more preferably not less than 1 mass%, and not more than 6 mass%, more preferably not more than 5 mass%. When there is too much content of a conductive support agent, although the load characteristic at the time of setting it as a battery can be aimed at, the filling property of the said active material will fall. On the other hand, when there is too little content of a conductive support agent, it will become difficult to take the electroconductivity in a positive electrode.

  The content of the binder in the total amount of the positive electrode mixture is 0.5% by mass or more, more preferably 1% by mass or more, and 3% by mass or less, more preferably 2.5% by mass or less. It is desirable to be. When the binder content is too high, the strength of the electrode is improved, but the filling properties of the active material and the load characteristics when used as a battery are lowered. On the other hand, when there is too little binder content, the binding property of a positive mix layer will fall.

  Incidentally, the content of the active material in the total amount of the positive electrode mixture is 94% by mass or more, more preferably 95% by mass or more, and 99% by mass or less, more preferably 98% by mass or less. . When there is too much said active material content, the amount of the conductive support agent and binder amount in a positive mix layer may fall, and said malfunction may arise. Moreover, when there is too little content of the said active material, the capacity | capacitance at the time of setting it as a battery may become small.

  In addition, as a solvent which can be used for the positive mix paste and positive mix slurry for forming a positive mix layer, N-methyl-2-pyrrolidone (NMP) is mentioned, for example.

The thickness of the positive electrode mixture layer is preferably 50 to 100 μm, for example. Moreover, it is recommended that the electrode density (density of the positive electrode mixture layer) is, for example, 3.2 to 3.8 g / cm ³ .

  The current collector used for the positive electrode is not particularly limited as long as it is a substantially chemically stable electronic conductor in the battery using the electrode of the present invention. Examples of the material constituting the current collector include aluminum and its alloys, stainless steel, nickel and its alloys, titanium and its alloys, carbon, conductive resin, etc., as well as carbon or aluminum on the surface of aluminum or stainless steel. A material obtained by treating titanium is used. Of these, aluminum and aluminum alloys are particularly preferable. These materials can also be used after oxidizing the surface. Moreover, it is preferable to give unevenness | corrugation to the collector surface by surface treatment. Examples of the shape of the current collector include films, sheets, nets, punched materials, lath bodies, porous bodies, foamed bodies, and molded bodies of fiber groups, in addition to foils. Although the thickness of a collector is not specifically limited, For example, it is preferable that it is 1-500 micrometers.

  The nonaqueous electrolyte secondary battery of the present invention has the positive electrode of the present invention, and there are no particular restrictions on the other components, and each component employed in a conventionally known nonaqueous electrolyte secondary battery Can be applied.

  As a negative electrode of the nonaqueous electrolyte secondary battery of the present invention, for example, a negative electrode mixture layer containing a negative electrode active material is formed on the current collector surface. The negative electrode mixture layer contains, in addition to the negative electrode active material, a binder and a conductive aid (if necessary). For example, the negative electrode active material and the binder (and further a conductive aid) A negative electrode mixture slurry obtained by adding a suitable solvent to the mixture (negative electrode mixture) and kneading it sufficiently can be applied to the surface of the current collector and dried to form a desired thickness. it can.

  Examples of the negative electrode active material include graphite materials such as natural graphite (flaky graphite), artificial graphite, and expanded graphite; graphitizable carbonaceous materials such as coke obtained by calcining pitch; furfuryl alcohol resin ( Carbon materials such as non-graphitizable carbonaceous materials such as amorphous carbon obtained by low-temperature firing of PFA), polyparaphenylene (PPP), and phenol resins. In addition to the carbon material, lithium or a lithium-containing compound can also be used as the negative electrode active material. Examples of the lithium-containing compound include lithium alloys such as Li—Al, and alloys containing elements that can be alloyed with lithium such as Si and Sn. Furthermore, oxide-based materials such as Sn oxide and Si oxide can also be used. The negative electrode active material content in the total amount of the negative electrode mixture is preferably 90 to 98% by mass, for example.

  The conductive aid is not particularly limited as long as it is an electron conductive material, and may not be used. Specific examples of conductive aids include acetylene black; ketjen black; carbon blacks such as channel black, furnace black, lamp black, and thermal black; carbon materials such as carbon fibers; and conductive fibers such as metal fibers. Carbon fluoride, metal powders such as copper and nickel, organic conductive materials such as polyphenylene derivatives, and the like. These may be used alone or in combination of two or more. Absent. Among these, acetylene black and carbon fiber are particularly preferable. In the case of using a conductive additive for the negative electrode, the conductive auxiliary agent content in the total amount of the negative electrode mixture is preferably 10% by mass or less.

The binder related to the negative electrode mixture layer plays a role of binding the negative electrode active material, the conductive auxiliary agent and the like in the negative electrode mixture layer. Such a binder may be either a thermoplastic resin or a thermosetting resin. Specifically, for example, the same materials as the binder according to the electrode of the present invention can be used, and these materials may be used alone or in combination of two or more. Among them, PVDF, SBR, ethylene-acrylic acid copolymer or Na ⁺ ion crosslinked product of the copolymer, ethylene-methacrylic acid copolymer or Na ⁺ ion crosslinked product of the copolymer, ethylene-methyl acrylate A copolymer or a Na ⁺ ion crosslinked product of the copolymer, an ethylene-methyl methacrylate copolymer or a Na ⁺ ion crosslinked product of the copolymer is particularly preferred. The binder content in the total amount of the negative electrode mixture is preferably 2 to 10% by mass, for example.

  The current collector used for the negative electrode is not particularly limited as long as it is a substantially chemically stable electronic conductor in the nonaqueous electrolyte secondary battery of the present invention. Examples of the material constituting the current collector include stainless steel, nickel or an alloy thereof, copper or an alloy thereof, titanium or an alloy thereof, carbon, conductive resin, carbon, or the like on the surface of copper or stainless steel. A material obtained by treating titanium is used. Among these, copper and copper alloys are particularly preferable. These materials can also be used after oxidizing the surface. Moreover, it is preferable to give unevenness | corrugation to the collector surface by surface treatment. Examples of the shape of the current collector include films, sheets, nets, punched materials, lath bodies, porous bodies, foamed bodies, and molded bodies of fiber groups, in addition to foils. Although the thickness of a collector is not specifically limited, For example, it is preferable that it is 1-500 micrometers.

  In addition, as a solvent which can be used for the negative mix slurry for forming a negative mix layer, water, NMP, etc. are mentioned, for example.

  The thickness of the negative electrode mixture layer is preferably 30 to 100 μm, for example.

  As the nonaqueous electrolyte according to the nonaqueous electrolyte secondary battery of the present invention, for example, a solution (nonaqueous electrolyte) prepared by dissolving the following inorganic ion salt in the following nonaqueous solvent can be used.

  Examples of the solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate (MEC), γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, 3-methyl- Aprotic organics such as 2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, diethyl ether, 1,3-propane sultone, etc. Can be used medium as alone in a mixed solvent or a mixture of two or more.

The inorganic ion salt according to the non-aqueous electrolyte, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) 2, From lithium salts such as LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group] There may be mentioned at least one selected. The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 2 mol / l, and more preferably 0.8 to 1.5 mol / l.

  In the non-aqueous electrolyte secondary battery of the present invention, a separator containing the non-aqueous electrolyte is disposed between the positive electrode (the electrode of the present invention) and the negative electrode. As the separator, an insulating microporous thin film having a large ion permeability and a predetermined mechanical strength is used. Moreover, as a separator, what has a function which a hole obstruct | occludes by fusion | melting of a constituent material above a fixed temperature (for example, 100-140 degreeC), and raises resistance (namely, what has a shutdown function) is preferable. Specific examples of the separator include a polyolefin polymer (polyethylene, polypropylene, etc.) having resistance to organic solvents and hydrophobicity, or a sheet (porous sheet) made of a material such as glass fiber, a nonwoven fabric or a woven fabric; the polyolefin And a porous body in which fine particles of a polymer are fixed with an adhesive. The pore diameter of the separator is preferably such that the active material of the positive and negative electrodes, the conductive auxiliary agent, the binder and the like detached from the positive and negative electrodes do not pass through, and is preferably 0.01 to 1 μm, for example. The thickness of the separator is recommended to be 10 to 300 μm, for example. Further, the porosity of the separator is determined according to the constituent material and thickness, but is generally 30 to 80%.

  The positive electrode and the negative electrode can be used, for example, as a laminated electrode body laminated via a separator, or after the positive electrode and the negative electrode are laminated via a separator and wound.

  As a form of the nonaqueous electrolyte secondary battery of the present invention, for example, a tubular shape (such as a rectangular tube shape or a cylindrical shape) using a steel can, an aluminum can, or the like as an outer can is cited. Moreover, it can also be set as the soft package battery which used the laminated film which vapor-deposited the metal as an exterior body.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention. In addition, each evaluation method employ | adopted by the present Example is as follows.

(1) Average particle size of layered Li-containing composite oxide (positive electrode active material) A sample is dispersed in pure water using a laser-type particle size distribution measuring device ("HRA (9320-X100)" manufactured by Microtrac). The particle size (D ₅₀ ) was 50% in total under the conditions of light absorption mode (laser light is absorbed without being reflected or scattered), aqueous solvent, and room temperature (20 ° C.).

(2) Specific surface area of layered Li-containing composite oxide (positive electrode active material) As a pretreatment, using a one-point BET measuring device (“Macsorb HM-1201” manufactured by Mounttech) using N ₂ gas adsorption The BET specific surface area after maintaining for 1 hour in an environment of 150 ° C. in an N ₂ gas flow was determined.

(3) Battery capacity About the battery produced by the Example and the comparative example, constant current charge was performed to 4.4V on the conditions of 0.2A, and then constant voltage charge was performed until the electric current value was set to 0.02A. Next, constant current discharge was performed at 0.2 A to 3.0 V to obtain the battery capacity.

(4) Battery cycle characteristics The batteries produced in the examples and comparative examples were charged at a constant current to 4.4 V at 1 A, and then charged at a constant voltage until the current value reached 0.1 A, followed by 1 A. The operation of performing a constant current discharge up to 3.0 V was defined as one cycle, and this was repeated up to 300 cycles. The maintenance rate (%) of the discharge capacity at the 300th cycle relative to the discharge capacity at the first cycle was determined, and thereby the cycle characteristics were evaluated.

(5) Storage characteristics About the battery produced by the Example and the comparative example, the constant current charge was performed to 4.4V on condition of 0.2A, and the constant voltage charge was then performed until the electric current value was set to 0.02A. The battery after charging is stored at 60 ° C. for 30 days, then discharged at a constant current of 0.2 A to 3.0 V to determine the battery capacity, and stored according to the maintenance rate (%) with respect to the battery capacity determined in (3) above. Characteristics were evaluated.

(6) Safety The battery was overcharged under the conditions of 1.0 A and 12 V. The case where there was no problem was evaluated as ◯, and the case where it ignited was evaluated as ×.

(7) Analysis of the surface ratio of the positive electrode active material layer and the overall Co abundance ratio The ICP (Inductive Coupled Plasma) method was used as follows. First, the positive electrode mixture layer was peeled off from the electrode after measuring the battery capacity in (3) above, washed with dimethyl carbonate, and 0.2 g was collected therefrom and placed in a 100 mL container. Thereafter, 5 mL of pure water, 2 mL of aqua regia, and 10 mL of pure water were sequentially added and dissolved by heating. After cooling, the solution was further diluted 25 times and analyzed by ICP (“ICP-757” manufactured by JARREL ASH) ( Calibration curve method). From the obtained results, the composition of the positive electrode, that is, the [(1-xy) / 1] value was derived. On the other hand, the Co abundance ratio (a value) on the surface of the positive electrode mixture layer was determined by washing the electrode after the battery capacity measurement of (3) with dimethyl carbonate and directly using XPS (X-ray Photoelectron Spectroscopy) (ULVAC Analysis was performed under the conditions of aperture diameter: 800 μm, X-ray source: MgKα 400 W, and photoelectron emission angle: 45 ° by “ESCA5500MC” manufactured by the company.

(8) The density of the positive electrode mixture layer and the negative electrode mixture layer is first cut positive and negative electrodes to a size of 1 cm × 1 cm, thickness with a micrometer, respectively (l _1), measuring the mass (m ₁₎ a precision balance did. Next, the positive electrode mixture layer of the positive electrode, and scrape a negative electrode mixture layer of the negative electrode is taken out only the collector, measuring the thickness of each current collector (l _c) and mass (m _c) in the same manner as the positive electrode did. From the obtained thickness and mass, the positive electrode mixture layer density and the negative electrode mixture layer density (d _ca ) were determined by the following formulas (in addition, the unit of thickness is cm and the unit of mass is g).

Example 1
In Example 1, an active material (layered Li-containing composite oxide) was synthesized by changing the production method and composition, an electrode was produced using the active material, and a non-aqueous electrolyte 2 was produced using the electrode as a positive electrode. A secondary battery was produced.

Development batteries 1-3
<Preparation of positive electrode>
A layered Li-containing composite oxide (positive electrode active material) represented by Li _{(1 + δ)} Mn _x Ni _y Co _(1-xy) O ₂ having a high Co concentration on the particle surface was synthesized. Nickel sulfate, manganese sulfate, and cobalt sulfate are made so that the Co ratio is lower than the predetermined composition ratio for each metal element (specifically, the Co ratio of post-addition shown in Table 1) The resulting mixture was mixed in water (approximately 1000 g of water with respect to 1 mol of each sulfate) to obtain an aqueous solution A. As aqueous solution B, aqueous ammonia (concentration: 25% by mass) was prepared. While stirring aqueous ammonia (concentration: 2% by mass) adjusted to pH 12 with NaOH, aqueous solution A was added dropwise at 46 ml / min, and aqueous solution B was added dropwise at 3.3 ml / min. Co-precipitated water of Mn, Ni, and Co An oxide was obtained, taken out, sufficiently washed with water and dried. To this coprecipitated hydroxide, Li hydroxide and Co carbonate were added and mixed well so that each metal element had the composition ratio shown in Table 1. This mixture was baked in an oxidizing atmosphere to obtain a layered Li-containing composite oxide having the composition shown in Table 1. As the atmosphere during firing, the oxygen partial pressure is 0.19 to 1 atmosphere, the firing temperature is 600 to 1000 ° C., and the firing time is 6 to 48 hours. Conditions suitable for the synthesis of the layered Li-containing composite oxide were selected. Each layered type Li-containing composite oxide thus obtained had an average particle diameter (diameter) of 10 μm and a BET specific surface area of 0.25 m ² / g.

  Positive electrode active material powder: 96% by mass, artificial graphite powder: 1% by mass, acetylene black (AB): 1% by mass, and an NMP solution containing 8% by mass of PVDF are mixed, and a positive electrode mixture slurry Was prepared. The amount of PVDF in the slurry was 2% by mass in the total solid content. This slurry was applied to both sides of an aluminum foil having a thickness of 15 μm, a length of 420 mm and a width of 170 mm, vacuum-dried at 120 ° C. for 12 hours, and a roll-type (width 30 cm, linear pressure 40 kg) press. After pressing, it was cut into a predetermined width to obtain a positive electrode having a positive electrode mixture layer having a thickness of 55 μm on both sides of the current collector.

<Production of negative electrode>
Natural graphite: 97.5% by mass, SBR: 1.5% by mass, and carboxymethyl cellulose (CMC) as a thickener: 1% by mass were mixed with water to prepare a negative electrode mixture slurry. This slurry was applied to both sides of a copper foil having a thickness of 8 μm, a length of 420 mm, and a width of 170 mm, vacuum-dried at 120 ° C. for 12 hours, and a roll type (30 cm width, linear pressure 40 kg) press. After pressing, the negative electrode was cut into a predetermined width to produce a negative electrode in which a negative electrode mixture layer having a thickness of 60 μm was formed on both sides of the current collector. The negative electrode mixture layer density of the negative electrode was 1.65 g / cm ³ .

<Production of electrode body>
The positive electrode and the negative electrode were overlapped via a separator, and these were wound up to produce a wound electrode body. The separator was a polyethylene porous film having a thickness of 18 μm.

<Non-aqueous electrolyte>
A non-aqueous electrolyte solution in which 1 mol / l LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate having a volume ratio of 1: 1: 1 was used.

<Battery assembly>
A square nonaqueous electrolyte secondary battery (lithium ion secondary battery) was assembled using the electrode body and the nonaqueous electrolyte. In assembly, first, a current collector plate was joined to each end face of the electrode body by welding. Next, the lead portion of the current collector plate was connected to an electrode terminal mechanism attached to an aluminum alloy lid. Then, the electrode body was accommodated in the positive electrode can made of aluminum alloy, and the lid body was welded and fixed to the periphery of the opening of the positive electrode can. Finally, the electrolytic solution is injected from the liquid injection hole into a sealed container (that is, a positive electrode can and a lid welded), and has the structure shown in FIG. 1, having a thickness of 4 mm, a width of 34 mm, A square non-aqueous electrolyte secondary battery (developed batteries 1 to 3) having a height of 50 mm was produced.

  Here, the battery 1 shown in FIG. 1 will be described. The positive electrode 6 and the negative electrode 7 are formed in the rectangular battery case 2 as the wound electrode body 9 wound in a spiral shape through the separator 8 as described above. It is housed with a non-aqueous electrolyte. However, in FIG. 1, in order to avoid complication, the collectors of the positive electrode 6 and the negative electrode 7, the non-aqueous electrolyte, and the like are not shown.

  The battery case 2 constitutes a main part of the outer packaging material of the battery 1, and the battery case 2 also serves as a positive electrode terminal. An insulator 10 made of a polytetrafluoroethylene sheet is disposed at the bottom of the battery case 2, and the wound electrode body 9 made up of the positive electrode 6, the negative electrode 7 and the separator 8 is connected to one end of each of the positive electrode 6 and the negative electrode 7. The connected positive electrode lead body 11 and negative electrode lead body 12 are drawn out. Further, a stainless steel terminal 5 is attached to the lid plate 3 that seals the opening of the battery case 2 via an insulating packing 4 made of polypropylene, and a stainless steel lead is connected to the terminal 5 via an insulator 13. A plate 14 is attached.

  And this cover plate 3 is inserted in the opening part of the said battery case 2, the opening part of the battery case 2 is sealed by welding the junction part of both, and the inside of the battery 1 is sealed.

  In the battery 1 of Example 1, the battery case 2 and the lid plate 3 function as positive terminals by directly welding the positive electrode lead body 11 to the lid plate 3, and the negative electrode lead body 12 is welded to the lead plate 14, The terminal 5 functions as a negative electrode terminal by connecting the negative electrode lead body 8 and the terminal 11 through the lead plate 13. However, depending on the material of the battery case 2, the sign is reversed. In some cases.

Comparative Example 1 (Comparative batteries 1 and 2)
Comparative batteries 1 and 2 were produced in the same manner as in Example 1 except that the positive electrode active material having the composition shown in Table 1 was synthesized by a normal method (a method in which the Co concentration on the particle surface was not increased). That is, the normal method is the following method. Nickel sulfate, manganese sulfate, and cobalt sulfate (only the cobalt sulfate for forming the positive electrode active material for Comparative Battery 2) mixed in such a manner that each metal element has a predetermined composition is added to water. An aqueous solution A was obtained by dissolution. As aqueous solution B, aqueous ammonia (concentration: 25% by mass) was prepared. While stirring aqueous ammonia (concentration: 2% by mass) adjusted to pH 12 with NaOH, aqueous solution A was added dropwise at 46 ml / min, and aqueous solution B was added dropwise at 3.3 ml / min. Co-precipitated water of Mn, Ni, and Co An oxide was obtained, taken out, sufficiently washed with water and dried. Li hydroxide is added to the coprecipitated hydroxide so that each metal element has a predetermined composition ratio, and after sufficient mixing, suitable conditions are selected from the firing conditions described in Example 1. Firing was performed to obtain a composite oxide.

  Table 1 shows the evaluation results for each positive electrode active material, each electrode, and each battery obtained in Example 1 and Comparative Example 1.

  In Table 1, “Co abundance ratio” means the ratio between the a value and the [(1-xy) / 1] value in the positive electrode mixture layer, that is, “a value / [(1-x− y) / 1] value ”(the same applies to Tables 2 and 3 below).

As is clear from the results shown in Table 1, a layered Li-containing composite oxide represented by Li _{(1 + δ)} Mn _x Ni _y Co _(1-xy) O ₂ (with high Co concentration on the surface was used. The batteries (development batteries 1 to 3) of Example 1 showed good characteristics, while the comparative battery 1 using a normal positive electrode having a uniform composition had problems in cycle characteristics and storage characteristics. Since the positive electrode active material in the vicinity of the interface with the electrolyte is exposed to the highest potential, elution of elements due to the reaction with the electrolyte (liquid electrolyte) is likely to proceed, but the surface has a high concentration of Co. The presence of dissolved particles on the positive electrode surface (positive electrode mixture layer surface) reduces the elution of Mn under high voltage, thereby enabling non-aqueous electrolyte secondary that has excellent cycle characteristics and storage characteristics under high voltage It seems that the battery has come to be obtained In Comparative battery 2 using the material in the LiCoO ₂ based on the active material, there is a safety problem, the overcharging test could not be cleared. This is because the amount of stable Mn during charging is too low For this reason, it is considered that a battery mainly using a LiCoO ₂ system cannot be used for a battery that is used at a high charging voltage.

Example 2 (developed batteries 4-6) and Comparative Example 2 (comparative batteries 3-4)
In Example 2 and Comparative Example 2, a positive electrode was produced by changing the average particle diameter and BET specific surface area of the positive electrode active material, and a nonaqueous electrolyte secondary battery was produced using the positive electrode. That is, except that the above layered Li-containing composite oxide (having the same composition as the positive electrode active material according to the developed battery 1) having the average particle size and BET specific surface area shown in Table 2 was used as the positive electrode active material. A positive electrode was produced in the same manner as in Example 1, and a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the positive electrode was used. Table 2 shows the evaluation results for each positive electrode active material, each electrode, and each battery.

As is clear from the results shown in Table 2, the batteries of Example 2 (developed batteries 4 to 6) in which the average diameter and specific surface area of the layered Li-containing composite oxide particles are suitable values show good characteristics. ing. On the other hand, the battery of Comparative Example 2 (Comparative batteries 3 to 4) is inferior in capacity and cycle characteristics. In addition, the comparative battery 3 has low storage characteristics. This is because the surface area of the positive electrode active material is too large, and the amount of alkali components derived from unreacted or residual raw material components (Li ₂ CO ₃ , sulfate, etc.) increases or Mn is eluted. This is probably because gas was generated in the battery.

Example 3 (development batteries 7 , 9, 10) and reference example (reference battery 8)
In Example 3 and the reference example , the positive electrode active material having the same composition as the positive electrode active material according to the developed battery 1 was used, and the positive electrode active material content was kept at 96% by mass for the composition of the positive electrode mixture layer. A positive electrode was prepared in the same manner as in Example 1 except that the amount of conductive auxiliary agent and the amount of binder were changed as shown in Table 3, and the non-aqueous solution was prepared in the same manner as in Example 1 except that the positive electrode was used. An electrolyte secondary battery was produced. Table 3 shows the evaluation results for each electrode and each battery.

  In Table 3, “(KB)” in the “AB” column of the developed battery 10 means that ketjen black (KB) was used instead of acetylene black (AB).

As is clear from the results shown in Table 3, a specific carbon material (carbon black, acetylene black, ketjen black) is used as the conductive auxiliary agent for the positive electrode mixture layer, and the conductive auxiliary agent in the positive electrode mixture layer. The batteries of Example 3 (development batteries 7 , 9, 10) in which the content and the binder content are suitable values show good characteristics.

It is a longitudinal cross-sectional view which shows an example of the nonaqueous electrolyte secondary battery of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Battery 2 Battery case 3 Cover plate 4 Insulation packing 5 Terminal 6 Positive electrode 7 Negative electrode 8 Separator 9 Winding electrode body 10 Insulator 11 Positive electrode lead body 12 Negative electrode lead body 13 Insulator 14 Lead plate

Claims (4)

Compositional formula Li _{(1 + δ)} Mn _x Ni _y Co _(1-xy) O ₂ (−0.15 <δ <0.15, 0.1 <x ≦ 0.5, 0.5 <x + y < 1. 0), and has a positive electrode mixture layer containing an active material having a mean particle size of 5 to 25 μm and a specific surface area of 0.1 to 0.6 m ² / g, a conductive additive and a binder. And
When the Co amount relative to the total amount of Mn, Ni, and Co on the surface of the positive electrode mixture layer is defined as a, the value of a is Co relative to the total amount of Mn, Ni, and Co in the entire positive electrode mixture layer. der 1.5 times or more the average value of an amount of atomic ratio [(1-x-y) / 1 ] is,
The conductive auxiliary agent contains at least one carbon material selected from the group consisting of carbon black, acetylene black, and ketjen black, and the content of the carbon material in the total positive electrode mixture is 0.5-3. is the mass%, and as the conductive auxiliary agent, further including also graphite, a non-aqueous electrolyte content of Zenshirubeden aids in the positive electrode mixture total amount, characterized in 1-6% by mass Rukoto two Secondary battery electrode. The content of the binder in the positive electrode mix total amount, the electrode according to claim 1 which is 0.5 to 3 wt%.   A nonaqueous electrolyte secondary battery comprising the electrode according to claim 1 as a positive electrode. The potential of the positive electrode during charging is Li ⁺ The nonaqueous electrolyte secondary battery according to claim 3, which has a reference voltage of 4.3 to 4.6 V.

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