Method for manufacturing positive electrode active material and method for manufacturing secondary battery

Abstract

The present invention provides a method for manufacturing a positive electrode active material and a method for manufacturing a secondary battery. The material and battery have a higher maximum capacity and a higher activation speed to reach the maximum capacity than conventional materials and batteries.　In each method, the manufacturing is carried out by adding an additive containing the element boron to a lithium composite oxide or a precursor thereof, and heating and sintering the lithium composite oxide or precursor thereof. The lithium composite oxide has a layered rock-salt structure and contains at least lithium, manganese, nickel, and a divalent element. The divalent element is calcium or magnesium.

Description

Method for manufacturing positive electrode active material and method for manufacturing secondary battery

[0001] The present invention relates to a method for producing a positive electrode active material and a method for producing a secondary battery.

[0002] In recent years, there has been an increasing demand for lithium-ion secondary batteries for use in electric vehicles, large-scale energy storage systems, and other devices requiring high energy density. Therefore, lithium-excess nickel-manganese oxides with a layered structure that can achieve high capacity have attracted attention (see, for example, Patent Document 1).

[0003] JP 2011-129269 A

[0004] However, the lithium-excess nickel-manganese oxide of Patent Document 1 has a small initial charge capacity and needs to be activated by repeating charge and discharge for several tens of cycles in order to achieve the desired high capacity, leaving room for further improvement.

[0005] Therefore, an object of the present invention is to provide a method for producing a positive electrode active material and a method for producing a secondary battery that have a larger maximum capacity and a faster activation rate until the maximum capacity is reached than conventional methods.

[0006] One aspect of the present invention for solving the above-described problems is a method for producing a positive electrode active material, which comprises adding an additive containing boron element to a lithium composite oxide or a precursor of the lithium composite oxide, and heating and sintering the resulting material, wherein the lithium composite oxide has a layered rock salt structure and contains at least lithium, manganese, nickel, and a divalent element, and the divalent element is calcium or magnesium.

[0007] The term "lithium composite oxide precursor" used herein refers to a substance at a stage prior to the production of a lithium composite oxide.

[0008] According to this aspect, an additive containing boron element is added to a lithium composite oxide or its precursor, which has a layered rock salt structure and contains at least lithium, manganese, nickel, and a divalent element, and therefore the maximum capacity is larger than conventional ones and the activation speed until the maximum capacity is reached can be increased.

[0009] In a preferred aspect, the amount of boron in the additive is more than 0.00075 equivalents and not more than 0.2 equivalents per equivalent of the total of manganese (Mn) and nickel (Ni) in the lithium composite oxide.

[0010] In a preferred aspect, the amount of boron in the additive is 0.005 equivalents or more and 0.1 equivalents or less per equivalent of the total of Mn and Ni in the lithium composite oxide.

[0011] A preferred embodiment is sintered by heating at a temperature of 850°C or higher.

[0012] In a preferred aspect, the additive includes at least one selected from the group consisting of lithium tetraborate, boric acid, lithium borate, and lithium metaborate.

[0013] In a preferred aspect, a boron compound is attached to the lithium composite oxide.

[0014] One aspect of the present invention is a secondary battery having a positive electrode portion, a negative electrode portion, and an electrolyte, the positive electrode portion including the above-described positive electrode active material.

[0015] According to this aspect, the activation speed is faster than conventional methods.

[0016] One aspect of the present invention is a method for producing a secondary battery, the method including: forming a positive electrode active material using the method for producing a positive electrode active material described above; and an initial charging step of terminating charging when a predetermined battery capacity is reached at a current density of 10 mA / g or less, wherein the predetermined battery capacity is set based on the capacity of the positive electrode active material and corresponds to a capacity of 180 mAh / g or more and 300 mAh / g or less when the battery is initially charged using only the positive electrode active material.

[0017] One aspect of the present invention is a method for producing the above-mentioned secondary battery, which includes an initial charging step of terminating charging when a predetermined battery capacity is reached at a current density of 10 mA / g or less, wherein the predetermined battery capacity is set based on the capacity of the positive electrode active material and corresponds to a capacity of 180 mAh / g or more and 300 mAh / g or less when the battery is initially charged using only the positive electrode active material.

[0018] According to these aspects, the initial irreversible capacity, which is the difference between the initial charge capacity and the initial discharge capacity, can be reduced.

[0019] The above aspects may be made dependent on each other, or some of the configurations may be quoted or substituted for each other, as long as they are included in the technical scope of the present invention.

[0020] According to the present invention, the maximum capacity is larger than that of the prior art, and the activation speed until the maximum capacity is reached can be increased.

[0021] 1 is a cross-sectional view conceptually showing a lithium-ion secondary battery according to a first embodiment of the present invention.

[0022] Hereinafter, embodiments of the present invention will be described in detail.

[0023] As shown in FIG. 1 , the lithium ion secondary battery 1 of the first embodiment of the present invention includes a positive electrode portion 2, a negative electrode portion 3, an electrolyte 5, and a separator 6, and an external load 7 can be connected to the positive electrode portion 2 and the negative electrode portion 3.

[0024] 1, the positive electrode part 2 is an intercalation electrode in which a positive electrode active material layer 11 is laminated on a positive electrode current collector 10. The positive electrode active material layer 11 contains a positive electrode active material 20, a conductive additive, and a binder.

[0025] As shown in the enlarged view of FIG. 1, the positive electrode active material 20 is a coated positive electrode active material in which a coating layer 31 is coated on the surface of an oxide active material 30 .

[0026] The negative electrode part 3 is an intercalation electrode in which a negative electrode active material layer 13 is laminated on a negative electrode current collector 12. The negative electrode active material layer 13 contains a negative electrode active material 21, a conductive additive, and a binder.

[0027] <Oxide active material 30> The oxide active material 30 has a layered rock salt crystal structure and contains at least lithium, manganese, nickel, and a divalent element, the divalent element being calcium or magnesium. The oxide active material 30 preferably contains lithium, manganese, nickel, and magnesium, and more preferably contains lithium, manganese, nickel, calcium, and magnesium.

[0028] The oxide active material 30 preferably has a layered rock salt type crystal structure represented by the following formula (1) and has a basic skeleton of a lithium-excess type lithium composite oxide: Li ｗ Mn ｘ Ni ｙ M ｚ O ３ ... (1) (wherein, in formula (1), 1.8≦w≦2.0, 0<x<1, 0<y<1, 0<z≦0.1, x+y+z≦1.2, and M is a divalent element.)

[0029] The oxide active material 30 preferably has a layered rock salt type crystal structure represented by the following formula (2) and has a basic skeleton of a lithium-excess type lithium composite oxide. ２ Mn ｘ Ni ｙ M ｚ O ３ ... (2) (wherein, in formula (2), 0<x<1, 0<y<1, 0<z≦0.1, x+y+z=1, and M is a divalent element.)

[0030] In the above formulas (1) and (2), M is preferably composed of some or more of the main group elements, more preferably composed of alkaline earth metal elements, and even more preferably at least one element selected from Ca and Mg.

[0031] The oxide active material 30 is preferably a lithium composite oxide represented by the following formula (3): Li ２ Mn ｘ Ni ｙ Ca ａ Mg ｂ O ３... (3) (where, in formula (3), 0<x<1, 0<y<1, 0≦a≦0.1, 0≦b≦0.1, 0<1-x-y, 0<a+b≦0.1)

[0032] In addition, the oxide active material 30 may be a layered rock salt type lithium composite oxide in which some of the sites are substituted with boron.

[0033] <Coating layer 31> The coating layer 31 contains at least boron element and is preferably composed of a boron compound, specifically, a boron oxide or a lithium boron compound. The coating layer 31 may have a boron compound attached to its surface.

[0034] <Negative Electrode Active Material 21> The negative electrode active material 21 is not particularly limited, and for example, an oxide-based material, an alloy-based material, a phosphorus-based material, a silicon-based material, a carbon-based material, etc. can be used. More specifically, lithium titanate, niobium titanium oxide, and carbon-based materials such as silicon and carbon can be used.

[0035] <Conductive Aid> The conductive aid used in the active material layers 11 and 13 is not particularly limited, but a carbon material is preferred. The carbon material is preferably at least one selected from natural graphite, artificial graphite, vapor-grown carbon fiber, carbon nanotubes, acetylene black, ketjen black, and furnace black. The amount of conductive aid contained in the active material layers 11 and 13 is preferably 1 part by weight to 30 parts by weight per 100 parts by weight of the active material 20 and 21. Within this range, the conductivity of the active material layers 11 and 13 is ensured while maintaining adhesion to the binder and achieving sufficient adhesion to the current collectors 10 and 12. The amounts of conductive aid contained in the active material layers 11 and 13 may be the same or different.

[0036] <Binder> The binder used in the active material layers 11, 13 is not particularly limited, but may be at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyimide, and derivatives thereof. The amount of binder contained in the active material layers 11, 13 is preferably 1 part by weight to 30 parts by weight per 100 parts by weight of the active material 20, 21. Within this range, adhesion between the active material 20, 21 and the conductive additive is maintained, and sufficient adhesion with the current collectors 10, 12 can be obtained. The amounts of binder contained in the active material layers 11, 13 may be the same or different.

[0037] <Current Collectors 10, 12> The current collectors 10, 12 are not particularly limited, but are preferably made of aluminum or an aluminum alloy because they are stable in an electrode reaction atmosphere. The current collectors 10, 12 may also be made of a metal other than aluminum (copper, SUS, nickel, titanium, or an alloy thereof) coated with a metal that does not react with the potential of the positive electrode part 2 and the negative electrode part 3.

[0038] <Electrolyte 5> The electrolyte 5 is not particularly limited as long as it has lithium ion conductivity, but examples thereof include a non-aqueous electrolyte solution in which a solute is dissolved in a non-aqueous solvent, a gel electrolyte in which a polymer is impregnated with a non-aqueous electrolyte solution in which a solute is dissolved in a non-aqueous solvent, etc. The electrolyte 5 may be in a solid state or may be a solid electrolyte.

[0039] The electrolyte 5 may be contained in the positive electrode part 2, the negative electrode part 3, and the separator 6 in advance, or may be added after the separator 6 is disposed between the positive electrode part 2 side and the negative electrode part 3 side and then wound or laminated.

[0040] <Separator 6> The separator 6 may be disposed between the positive electrode part 2 and the negative electrode part 3 and may have any structure as long as it is insulating and can contain the electrolyte 5. Examples of the separator 6 include woven fabrics, nonwoven fabrics, and microporous membranes made of nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, and composites of two or more of these materials.

[0041] The separator 6 may contain various plasticizers, antioxidants, and flame retardants, and may be coated with metal oxides or the like.

[0042] Next, a method for manufacturing the lithium ion secondary battery 1 of this embodiment will be described.

[0043] The method for manufacturing the lithium ion secondary battery 1 of this embodiment is mainly composed of a cathode active material forming process of forming the cathode active material 20, a cathode forming process of forming the cathode portion 2 using the cathode active material 20, an anode active material forming process of forming the anode active material 21, an anode forming process of forming the anode portion 3 using the anode active material 21, and a secondary battery assembling process of assembling the cathode portion 2, the anode portion 3, the electrolyte 5, and the separator 6. The anode active material forming process, the anode forming process, and the secondary battery assembling process are the same as conventional processes, and therefore will not be described here.

[0044] <Positive Electrode Active Material Formation Step> The positive electrode active material formation step is mainly comprised of an intermediate synthesis step, a mixing step, and a firing step.

[0045] (Intermediate Synthesis Step) The intermediate synthesis step is a step of synthesizing an intermediate containing nickel element, manganese element, and calcium element and / or magnesium element that constitutes a precursor of a lithium composite oxide using a coprecipitation method.

[0046] Specifically, the intermediate synthesis process is mainly comprised of a mixed solution preparation process, a coprecipitate extraction process, and a drying process.

[0047] The mixed solution preparation step is a step of mixing a nickel salt, a manganese salt, and a calcium salt and / or a magnesium salt to prepare a mixed solution containing nickel ions, manganese ions, and calcium ions and / or magnesium ions.

[0048] The nickel salt is not particularly limited as long as it contains nickel element, and examples thereof include nickel acetate, nickel oxide, and nickel sulfate. The manganese salt is not particularly limited as long as it contains manganese element, and examples thereof include manganese acetate, manganese oxide, manganese carbonate, and manganese sulfate. The calcium salt is not particularly limited as long as it contains calcium element, and examples thereof include calcium acetate, calcium oxide, calcium carbonate, and calcium sulfate. The magnesium salt is not particularly limited as long as it contains magnesium element, and examples thereof include magnesium acetate, magnesium oxide, magnesium carbonate, and magnesium sulfate. The amounts of the calcium salt and magnesium salt added in the mixed solution preparation step are preferably such that the total amount of calcium in the calcium salt and the total amount of magnesium in the magnesium salt is 0.005 equivalents (equivalent to 0.5 mol%) or more and 0.1 equivalents (equivalent to 10 mol%) or less, more preferably 0.01 equivalents (equivalent to 1.0 mol%) or more and 0.05 equivalents (equivalent to 5 mol%) or less, and even more preferably 0.01 equivalents (equivalent to 1.0 mol%) or more and 0.043 equivalents (equivalent to 4.3 mol%) or less, relative to 1 equivalent of the total amount of manganese in the manganese salt and the nickel in the nickel salt.

[0049] The coprecipitate extraction step is a step in which the mixed solution obtained in the mixed solution preparation step is dropped into an alkaline buffer solution to extract a coprecipitate containing elemental nickel, elemental manganese, and elemental calcium and / or elemental magnesium as a precipitate. The buffer solution is not particularly limited, and examples of the buffer solution that can be used include ammonia, ammonium acetate, ammonium sulfate, and sodium hydroxide.

[0050] The drying step is a step in which the coprecipitate extracted in the coprecipitate extraction step is filtered, washed with water, and dried to produce an intermediate. The drying temperature is not particularly limited, but is preferably 100°C or higher from the viewpoint of efficiently removing water from the intermediate. The drying time is not particularly limited, but is preferably 2 hours or longer from the viewpoint of sufficiently removing water from the intermediate.

[0051] (Mixing Step) The mixing step is a step of mixing the intermediate (precursor of the lithium composite oxide) synthesized in the intermediate synthesis step with a lithium salt (precursor of the lithium composite oxide) and an additive containing elemental boron to form a mixture.

[0052] The lithium salt used in the mixing step is not particularly limited as long as it contains lithium element, and for example, lithium carbonate or lithium hydroxide can be used. The additive used in the mixing step is not particularly limited as long as it contains boron element. For example, the additive used in the mixing step is lithium tetraborate (Li ２ B ４ O ７ ), boric acid (H ３ BO ３ ), lithium borate (Li ３ BO ３ ) (also known as lithium orthoborate), lithium metaborate (LiBO ２ The amount of the additive added in the mixing step is such that the amount of boron in the additive is more than 0.00075 equivalents and not more than 0.2 equivalents relative to 1 equivalent of the total of Mn and Ni in the lithium composite oxide, preferably 0.0001 equivalents or more and 0.1 equivalents or less, more preferably 0.005 equivalents or more and 0.1 equivalents or less, and more preferably 0.005 equivalents or more and 0.05 equivalents or less.

[0053] (Heating Step) The heating step is a step of heating the mixture obtained in the mixing step in an oxygen atmosphere to form the positive electrode active material 20 .

[0054] The heating temperature in the heating step is the temperature at which the mixture is sintered, and is a temperature of 850° C. or higher. The heating temperature in the heating step can be appropriately set in accordance with the heating time, but is preferably 1200° C. or lower.

[0055] The heating time in the heating step can be appropriately set according to the heating temperature, but is preferably 6 hours or more, more preferably 10 hours or more, and is preferably 48 hours or less, more preferably 36 hours or less, and even more preferably 24 hours or less.

[0056] <Positive Electrode Forming Step> The positive electrode forming step is composed of a positive electrode applying step and a positive electrode drying step.

[0057] (Positive Electrode Application Step) The positive electrode application step is a step of mixing the positive electrode active material 20 obtained in the positive electrode active material formation step with a conductive additive and a binder to prepare a positive electrode mixture, and applying the positive electrode mixture to the positive electrode current collector 10.

[0058] The binder used in the positive electrode coating step is preferably dissolved or dispersed in a non-aqueous solvent or water, for ease of preparation of the positive electrode part 2. The non-aqueous solvent is not particularly limited, but examples thereof include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, ethyl acetate, and tetrahydrofuran. A dispersant and a thickener may be added to these.

[0059] (Positive Electrode Drying Step) The positive electrode drying step is a step of forming the positive electrode portion 2 by drying the positive electrode current collector 10 to which the positive electrode mixture has been applied in the positive electrode application step.

[0060] The positive electrode part 2 formed in the above-described positive electrode forming step is assembled with the negative electrode part 3 formed in the negative electrode forming step and the electrolyte 5 in the same manner as in the conventional method, thereby completing the lithium ion secondary battery 1 .

[0061] Next, a recommended charging method for the lithium ion secondary battery 1 of this embodiment will be described.

[0062] The charging method for the lithium-ion secondary battery 1 of this embodiment involves alternating the following charging and discharging steps at 25°C under an inert gas atmosphere: constant current charging at a current density of 10 mA / g until the battery reaches 4.8 V relative to lithium metal, followed by constant voltage charging at 1 mA / g while maintaining 4.8 V relative to lithium metal until the current density decays to 1 mA / g; and constant current discharging at a current density of 10 mA / g until the battery reaches 2.0 V relative to lithium metal. The charging and discharging steps are then repeated until the maximum discharge capacity is reached, followed by a charging operation. During the initial charging operation (initial charging step), if a predetermined charge capacity (battery capacity) is reached during charging, the charging operation is terminated and the next discharge operation is initiated. The current density is a value per gram of positive electrode active material 20. That is, the charging method of this embodiment forcibly terminates the initial charging operation when the predetermined charge capacity is reached, regardless of the voltage or current. Even if 4.8 V has not yet been reached and constant current charging is in progress, the charging operation is terminated when the predetermined charge capacity is reached. The charging method of this embodiment starts with a charging operation, and one charging operation and one discharging operation together constitute one cycle.

[0063] The predetermined charge capacity is a capacity smaller than the theoretical capacity and is set appropriately depending on the combination with the negative electrode active material 21 and the type of the positive electrode active material 20. For example, it is a capacity set based on the capacity of the positive electrode active material 20 (a previously measured actual capacity or a theoretical capacity), and is preferably a capacity equivalent to a capacity of 180 mAh / g or more and 300 mAh / g or less when converted to the capacity of the positive electrode active material 20 alone (capacity per 1 g of the positive electrode active material 20). The predetermined charge capacity is more preferably a capacity equivalent to a capacity of 190 mAh / g or more, even more preferably a capacity equivalent to a capacity of 200 mAh / g or more, and particularly preferably a capacity equivalent to a capacity of 250 mAh / g or more.

[0064] In the first discharge operation when charged by the above charging method, the positive electrode part 2 preferably has a discharge capacity of 125 mAh / g or more, more preferably 160 mAh / g or more, and even more preferably 180 mAh / g or more per gram of the positive electrode active material 20. In the first discharge operation when charged by the above charging method, the positive electrode part 2 preferably has an initial irreversible capacity (the difference between the initial charge capacity and the initial discharge capacity) of 120 mAh / g or less.

[0065] The positive electrode part 2 preferably has a maximum charge capacity of 220 mAh / g or more, more preferably 240 mAh / g or more, when charged by the above charging method. The positive electrode part 2 preferably has a maximum discharge capacity of 220 mAh / g or more, more preferably 240 mAh / g or more, when charged by the above charging method. The positive electrode part 2 preferably undergoes 9 or fewer cycles, more preferably 5 or fewer cycles, and even more preferably 3 or fewer cycles, until it reaches its maximum discharge capacity. Note that these capacities of the positive electrode part 2 are all converted capacities obtained by converting the capacity of the lithium-ion secondary battery 1 to the capacity of the positive electrode part 2 alone.

[0066] Next, a case will be described in which the lithium-ion secondary battery 1 of this embodiment is charged and discharged by alternately repeating a charging operation at 25° C. in an inert gas atmosphere, in which the battery is charged at a constant current density of 10 mA / g until the voltage reaches 4.8 V relative to lithium metal, and then charged at a constant voltage until the current density attenuates to 1 mA / g while maintaining 4.8 V relative to lithium metal, and a discharging operation in which the battery is discharged at a constant current density of 10 mA / g until the voltage reaches 2.0 V relative to lithium metal. That is, this case differs from the case of charging by the above-mentioned charging method in that the charging operation is not terminated but continues even when a predetermined charge capacity is reached in the initial charging operation.

[0067] In the initial charging operation, the positive electrode portion 2 preferably has a charge capacity per gram of the positive electrode active material 20 of 180 mAh / g or more, more preferably 190 mAh / g or more, even more preferably 200 mAh / g or more, even more preferably 250 mAh / g or more, and particularly preferably 320 mAh / g or more. In the initial charging operation, the positive electrode portion 2 preferably has a charge capacity per gram of the positive electrode active material 20 of 400 mAh / g or less, preferably 370 mAh / g or less. In the initial discharging operation, the positive electrode portion 2 preferably has a discharge capacity per gram of the positive electrode active material 20 of 120 mAh / g or more, more preferably 150 mAh / g or more, and even more preferably 180 mAh / g or more. The positive electrode part 2 preferably has a discharge capacity per gram of the positive electrode active material 20 of 300 mAh / g or less during the initial discharge operation. The positive electrode part 2 preferably has a maximum charge capacity per gram of the positive electrode active material 20 of 220 mAh / g or more during the second or subsequent cycle of charge operation. The positive electrode part 2 preferably has a maximum charge capacity per gram of the positive electrode active material 20 of 300 mAh / g or less during the second or subsequent cycle of charge operation. The positive electrode part 2 preferably has a maximum discharge capacity per gram of the positive electrode active material 20 of 220 mAh / g or more during the second or subsequent cycle of discharge operation. The positive electrode part 2 preferably has a maximum discharge capacity per gram of the positive electrode active material 20 of 300 mAh / g or less during the second or subsequent cycle of discharge operation. The positive electrode part 2 preferably undergoes 9 or fewer cycles, more preferably 5 or fewer, until it reaches its maximum discharge capacity during the second or subsequent cycle of charge / discharge operation. The capacities of the positive electrode part 2 are all converted capacities obtained by converting the capacity of the lithium ion secondary battery 1 into the capacity of the positive electrode part 2 .

[0068] According to the manufacturing method of the cathode active material 20 of this embodiment, an additive containing boron is added to the oxide active material 30 (lithium composite oxide) or a precursor of the oxide active material 30, and the resulting mixture is heated and sintered to form the oxide active material 30. The oxide active material 30 has a layered rock salt structure and contains at least lithium, manganese, nickel, and a divalent element, the divalent element being calcium or magnesium. Therefore, the maximum capacity is larger than that of conventional oxide active materials, and the activation speed until the maximum capacity is reached can be increased.

[0069] According to the positive electrode active material 20 of this embodiment, a portion of the lithium sites (A sites) or a portion of the manganese nickel sites (B sites) of the lithium-excess nickel-manganese oxide are substituted with a divalent element, and an additive containing boron element is added, so that the activation speed until the maximum capacity is reached can be increased.

[0070] In the above-described embodiment, the cathode active material 20 has the coating layer 31 coated on the surface of the oxide active material 30, but the present invention is not limited to this. The cathode active material 20 does not necessarily have to have the coating layer 31 coated on the surface of the oxide active material 30.

[0071] In the above-described embodiment, the separator 6 is interposed between the positive electrode part 2 and the negative electrode part 3, but the present invention is not limited to this. In cases where the electrolyte 5 is a solid electrolyte, the separator 6 does not need to be provided.

[0072] In the above-described embodiment, the additive containing boron is mixed with the precursor of the lithium composite oxide in the mixing step, but the present invention is not limited to this. The additive containing boron may be mixed with the lithium composite oxide in the mixing step.

[0073] In the above-described embodiments, each component can be freely substituted or added between the respective embodiments as long as it falls within the technical scope of the present invention.

[0074] The present invention will be specifically described below with reference to examples. Note that the present invention is not limited to the following examples and can be practiced with appropriate modifications within the scope of the present invention.

[0075] Examples 1 and 2 First, a mixed solution of nickel acetate tetrahydrate (5.34 g, 21.45 mmol), manganese acetate tetrahydrate (15.77 g, 64.34 mmol), and calcium acetate monohydrate (0.91 g, 1.17 mmol) was prepared, and the mixed solution was added dropwise to an ammonium acetate buffer solution adjusted to pH 13 with an aqueous sodium hydroxide solution to generate a coprecipitate by coprecipitation. The coprecipitate was filtered, washed with water, and dried at 100°C for about 2 to 3 hours, thereby synthesizing an intermediate containing Ni, Mn, and Ca elements.

[0076] Here, in order to calculate the respective component ratios of Ni, Mn, and Ca, this intermediate was subjected to ICP-AES analysis to quantify the component elements. Specifically, 0.1 g of the synthesized intermediate was precisely weighed into a TFM (modified polytetrafluoroethylene) decomposition vessel, nitric acid and hydrochloric acid were added, and the mixture was subjected to acid decomposition under pressure using a microwave decomposition device, and the decomposition solution was adjusted to a constant volume of 50 mL. This was further diluted to a predetermined ratio (200 times for Ni and Mn, 20 times for Ca) and subjected to ICP-AES analysis to quantify Ni, Mn, and Ca. From the obtained analytical data (ICP measurement concentration (mg / L), blank concentration (mg / L)), the concentration in the sample (wt%) was calculated using the following formula, and the concentration (wt%) of each element in the sample was divided by its atomic weight to convert it to the amount of substance, and the element ratio was calculated. As a result, the intermediate contained 0.019 equivalents of Ca (corresponding to 1.9 mol%) relative to 1 equivalent of the total of Mn and Ni.

[0077] Concentration in sample (wt%) = [ICP measurement concentration (mg / L) x dilution rate - BLANK concentration (mg / L)] x liquid volume (mL) / (sample volume (g) x 10000)

[0078] (ICP-AES measurement conditions) Apparatus: Shimadzu Corporation ICPE-9820 High frequency power: 1.2 kW Plasma gas: 14 L / min Auxiliary gas: 1.2 L / min Carrier gas: 0.70 L / min

[0079] Quantitative mode calibration curve: multi-point calibration curve method Observation: axial direction Measurement elements: Ni, Mn, Ca Internal standard element: Y

[0080] Next, the synthesized intermediate (1.5 g), lithium carbonate (1.07 g, 14.07 mmol), and lithium tetraborate (0.0068 g, 0.04 mmol) were weighed and mixed in an automatic mortar for 60 minutes to form a mixed powder, in which the boron amount was 0.01 equivalent (corresponding to 1 mol%) and the Ca amount was 0.019 equivalent (corresponding to 1.9 mol%) relative to 1 equivalent of the total of Mn and Ni.

[0081] The mixed powder was then filled into an alumina firing container and heated at 900°C for 24 hours in an air atmosphere using a muffle furnace (FP411, manufactured by Yamato Scientific Co., Ltd.). Thereafter, the mixture was allowed to cool naturally in the furnace to obtain a positive electrode active material of lithium composite oxide.

[0082] Next, a mixture containing the obtained positive electrode active material, acetylene black as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder in solid concentrations of 88 parts by weight, 6 parts by weight, and 6 parts by weight, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry. Note that the binder was prepared as an N-methyl-2-pyrrolidone (NMP) solution with a solid concentration of 5% by weight, and NMP was further added to adjust the viscosity to facilitate coating, as described below.

[0083] The slurry was applied to a 15 μm thick aluminum foil, dried in an oven at 120° C., and then further dried in a vacuum at 170° C. to prepare a positive electrode, which was designated as Examples 1 and 2.

[0084] Example 3 was the same as in Example 1, except that the mixed powder was formed so that the amount of Ca was 0.038 equivalents (corresponding to 3.8 mol%) per equivalent of the total of Mn and Ni.

[0085] (Example 4) In Example 3, the synthesized intermediate (1.5 g), lithium carbonate (1.07 g, 14.00 mmol), and boric acid (0.0099 g, 0.16 mmol) were weighed and mixed in an automatic mortar for 60 minutes to form a mixed powder. That is, the same procedure as in Example 3 was carried out except that boric acid was used instead of lithium tetraborate. This was designated Example 4.

[0086] Example 5 In Example 3, the synthesized intermediate (1.5 g), lithium carbonate (1.07 g, 14.00 mmol), and lithium borate (0.0127 g, 0.16 mmol) were weighed and mixed in an automatic mortar for 60 minutes to form a mixed powder. That is, in Example 3, lithium borate (Li ３ BO ３ This was designated as Example 5.

[0087] Example 6 In Example 3, the synthesized intermediate (1.5 g), lithium carbonate (1.07 g, 14.00 mmol), and lithium metaborate (0.0080 g, 0.16 mmol) were weighed and mixed in an automatic mortar for 60 minutes to form a mixed powder. That is, in Example 3, lithium metaborate (LiBO) was used instead of lithium tetraborate. ２ This was designated as Example 6.

[0088] Example 7 was the same as in Example 1, except that the mixed powder was formed so that the amount of boron was 0.005 equivalents (equivalent to 0.5 mol%) and the amount of Ca was 0.043 equivalents (equivalent to 4.3 mol%) relative to 1 equivalent of the total of Mn and Ni.

[0089] Examples 8 and 9 were prepared in the same manner as in Example 1, except that the mixed powder was prepared so that the amount of Ca was 0.043 equivalents (corresponding to 4.3 mol%) relative to 1 equivalent of the total of Mn and Ni.

[0090] Examples 10 to 12 were the same as in Example 1, except that Mg was used instead of Ca, and the mixed powder was formed so that the amount of boron was 0.01 equivalent (corresponding to 1 mol%) and the amount of Mg was 0.020 equivalent (corresponding to 2.0 mol%) relative to 1 equivalent of the total of Mn and Ni. These were designated Examples 10 to 12.

[0091] Examples 13 to 15 were the same as in Example 10, except that the mixed powder was formed so that the amount of boron was 0.01 equivalent (corresponding to 1 mol%) and the amount of Mg was 0.040 equivalent (corresponding to 4.0 mol%) relative to 1 equivalent of the total of Mn and Ni. These were designated Examples 13 to 15.

[0092] Example 16 was the same as in Example 10, except that the mixed powder was formed so that the amount of boron was 0.01 equivalents (corresponding to 1 mol%) and the amount of Mg was 0.060 equivalents (corresponding to 6.0 mol%) relative to 1 equivalent of the total of Mn and Ni.

[0093] Examples 17 to 19 were the same as in Example 1, except that Mg was also used and the mixed powder was formed so that the amount of boron was 0.01 equivalents (corresponding to 1 mol%), the amount of Ca was 0.022 equivalents (corresponding to 2.2 mol%), and the amount of Mg was 0.020 equivalents (corresponding to 2.0 mol%) relative to 1 equivalent of the total of Mn and Ni. These were designated Examples 17 to 19.

[0094] Comparative Example 1 In Example 1, a mixed solution not containing calcium acetate monohydrate was used to synthesize an intermediate not containing calcium. The synthesized intermediate (1.5 g) and lithium carbonate (1.12 g, 14.66 mmol) were weighed and mixed in an automatic mortar for 60 minutes to form a mixed powder not containing boron. The subsequent steps were the same as in Example 1. This was designated Comparative Example 1.

[0095] Comparative Example 2 was prepared in the same manner as in Comparative Example 1, except that the mixed powder was prepared so that the amount of boron was 0.01 equivalent (corresponding to 1 mol%) relative to 1 equivalent of the total of Mn and Ni.

[0096] Comparative Example 3 The same procedure as in Example 1 was carried out except that the mixed powder was formed so that the amount of Ca was 0.019 equivalents (corresponding to 1.9 mol%) relative to 1 equivalent of the total of Mn and Ni. This was designated Comparative Example 3. That is, Comparative Example 3 differs from Example 1 in that the mixed powder did not contain lithium tetraborate.

[0097] Comparative Example 4 was the same as Comparative Example 3, except that the mixed powder was formed so that the amount of Ca was 0.038 equivalents (corresponding to 3.8 mol%) per equivalent of the total of Mn and Ni.

[0098] Comparative Example 5 was prepared in the same manner as in Comparative Example 3, except that the mixed powder was prepared so that the amount of Ca was 0.043 equivalents (corresponding to 4.3 mol%) relative to 1 equivalent of the total of Mn and Ni.

[0099] Comparative Example 6 was the same as Comparative Example 3, except that Mg was used instead of Ca, and the mixed powder was formed so that the amount of Mg was 0.020 equivalents (corresponding to 2.0 mol%) per equivalent of the total of Mn and Ni.

[0100] Comparative Example 7 was the same as Comparative Example 6, except that the mixed powder was formed so that the amount of Mg was 0.040 equivalents (corresponding to 4.0 mol%) per equivalent of the total of Mn and Ni.

[0101] Comparative Example 8 was the same as Comparative Example 6, except that the mixed powder was formed so that the amount of Mg was 0.060 equivalents (corresponding to 6.0 mol%) per equivalent of the total of Mn and Ni.

[0102] Comparative Example 9 was the same as Comparative Example 3, except that Mg was also used and the mixed powder was formed so that the amount of Ca was 0.022 equivalents (corresponding to 2.2 mol%) and the amount of Mg was 0.020 equivalents (corresponding to 2.0 mol%) relative to 1 equivalent of the total of Mn and Ni.

[0103] (Charge-Discharge Characteristics Evaluation) In the evaluation of the charge-discharge characteristics of Examples 1, 3 to 8, 10, 13, 16, and 17 and Comparative Examples 1 to 9, first, in a glove box, the positive electrodes of Examples 1, 3 to 8, 10, 13, 16, and 17 and Comparative Examples 1 to 9 were used as working electrodes, and lithium metal was used as a counter electrode. These electrodes were stacked in the order of positive electrode / separator / lithium metal and placed in a test cell. A nonaqueous electrolyte (ethylene carbonate / propylene carbonate / ethyl methyl carbonate = 15 / 15 / 70 (volume ratio), lithium hexafluorophosphate LiPF６ A half-cell was fabricated by placing a solution of 1 mol / L of ethanol (NaCl) in a test cell. The fabricated half-cell was connected to a charge / discharge device (HJ1005SD8, manufactured by Hokuto Denko Corporation) and subjected to a charge / discharge cycle. Under a 25°C environment, constant-current charging was performed at a current density of 10 mA / g until the battery voltage reached an upper limit voltage of 4.8 V. This was followed by a constant-voltage charging operation at 4.8 V while maintaining the current density until the current density attenuated to 1 mA / g. This was followed by a constant-current discharging operation at a current density of 10 mA / g, which was terminated when the battery voltage reached a lower limit voltage of 2.0 V. The charging / discharging operation began with a charging operation, with one charging operation and one discharging operation constituting one cycle.

[0104] In the evaluation of the charge-discharge characteristics of Examples 2, 9, 11, and 18, the initial charging operation was terminated when the charge capacity reached 280 mAh / g at a current density of 10 mA / g in an environment of 25° C., and the discharge operation was started. The other conditions were the same as those of Examples 1, 3 to 8, 10, 13, 16, and 17 and Comparative Examples 1 to 9.

[0105] In the evaluation of the charge-discharge characteristics of Examples 12 and 19, the initial charging operation was terminated when the charge capacity reached 260 mAh / g at a current density of 10 mA / g in an environment of 25° C., and the discharge operation was started. The other conditions were the same as those of Examples 1, 3 to 8, 10, 13, 16, and 17 and Comparative Examples 1 to 9.

[0106] In the evaluation of the charge-discharge characteristics of Example 14, the initial charging operation was terminated when the charge capacity reached 200 mAh / g at a current density of 10 mA / g in an environment of 25° C., and the discharge operation was started. The other conditions were the same as in Examples 1, 3 to 8, 10, 13, 16, and 17 and Comparative Examples 1 to 9.

[0107] In the evaluation of the charge-discharge characteristics of Example 15, the first charge operation was terminated when the charge capacity reached 220 mAh / g at a current density of 10 mA / g in an environment of 25°C, and the discharge operation was initiated. The second charge operation was terminated when the charge capacity reached 240 mAh / g at a current density of 10 mA / g in an environment of 25°C, and the discharge operation was initiated. The third charge operation was terminated when the charge capacity reached 260 mAh / g at a current density of 10 mA / g in an environment of 25°C, and the discharge operation was initiated. The rest of the procedure was the same as in Examples 1, 3 to 8, 10, 13, 16, and 17 and Comparative Examples 1 to 9.

[0108] (Evaluation of Cycle Characteristics) In the evaluation of charge / discharge characteristics, charge / discharge cycles were repeated even after the maximum capacity was reached, and the capacity retention rate was calculated by dividing the discharge capacities after 15 and 25 cycles after the maximum capacity was reached by the maximum discharge capacity. That is, the capacity retention rate (%) can be calculated by 100 × (discharge capacity after n cycles after the maximum capacity is reached) / (maximum capacity), where n is a natural number.

[0109] The evaluation results of Examples 1 to 19 and Comparative Examples 1 to 9 are shown in Tables 1 and 2.

[0110]

[0111]

[0112] The activation rate is expressed as the number of charge / discharge cycles required from the start of charge / discharge until the maximum discharge capacity is reached, and the lower the number, the higher the activation rate.

[0113] It has a layered rock salt type crystal structure containing divalent elements and contains Li as an additive. ２ B ４ O ７ In Examples 1 to 19 in which the compound was added, the maximum charge capacity and maximum discharge capacity were all larger than those of Comparative Example 1, and the activation speed up to the maximum discharge capacity was improved.

[0114] In Comparative Examples 1, 3, 4, and 5, the initial irreversible capacity decreased as the Ca content increased. In Comparative Examples 1 and 6 to 8, the initial charge capacity and initial discharge capacity decreased as the Mg content increased, and the initial irreversible capacity also decreased. This indicates that the initial irreversible capacity can be suppressed by adding or introducing a divalent element such as Ca or Mg into the crystal structure.

[0115] In Comparative Examples 1 and 2, Li ２ B ４ O ７ By adding Li, the initial charge capacity, the initial discharge capacity, and the activation speed are improved, but the initial irreversible capacity increases, and the maximum charge capacity and maximum discharge capacity decrease. ２ B ４ O ７ It was suggested that the addition of α-pyridine improves the initial charge-discharge capacity and activation rate, but increases the initial irreversible capacity and decreases the maximum charge-discharge capacity.

[0116] Li ２ B ４ O ７ In Examples 7 and 8, which contain Ca in the structure, Li ２ B ４ O ７ Although the initial irreversible capacity increased compared to Comparative Example 5 in which no Li was added, the activation rate improved and the maximum charge capacity and maximum discharge capacity also increased. ２ B ４ O ７ In Examples 10, 13, and 16, in which Mg is added in the structure, Li ２ B ４ O ７ Although the initial irreversible capacity increased compared to Comparative Examples 6, 7, and 8 in which no Li was added, the activation rate improved and the maximum charge capacity and maximum discharge capacity also increased. ２ B ４ O ７ In Example 17, which contains Ca and Mg in the structure, Li ２ B ４ O ７Compared to Comparative Example 9, in which no H was added, the initial irreversible capacity was comparable, the activation rate was improved, and the maximum charge capacity and maximum discharge capacity were increased. ３ BO ３ In Example 4, which contains Ca in the structure, ３ BO ３ Although the initial irreversible capacity increased compared to Comparative Example 4 in which no Li was added, the activation rate improved and the maximum charge capacity and maximum discharge capacity also increased. ３ BO ３ In Example 5, which contains Ca in the structure, Li ３ BO ３ Although the initial irreversible capacity increased compared to Comparative Example 4 in which no LiBO was added, the activation rate improved and the maximum charge capacity and maximum discharge capacity also increased. ２ In Example 6, which contains Ca in the structure, LiBO ２ Although the initial irreversible capacity increased compared to Comparative Example 4 in which no additive was added, the activation rate improved and the maximum charge capacity and maximum discharge capacity also increased. From these results, it was found that the activation rate can be improved and the maximum charge capacity and maximum discharge capacity increase by adding an additive containing boron element to a lithium composite oxide in which a divalent element (alkaline earth metal element) such as Ca or Mg is added or introduced into the crystal structure.

[0117] In Examples 2, 9, 11, and 18, in which the capacity was limited to 280 mAh / g during the initial charging operation, the initial irreversible capacity was reduced, and the maximum charge capacity and maximum discharge capacity were comparable or larger, and the activation rate was also comparable, compared to Examples 1, 8, 10, and 17, in which no capacity limit was imposed. In Examples 12 and 19, in which the capacity was limited to 260 mAh / g during the initial charging operation, and in Example 14, in which the capacity was limited to 200 mAh / g during the initial charging operation, the initial irreversible capacity was reduced, and the maximum charge capacity and maximum discharge capacity were larger, and the activation rate was also comparable, compared to Examples 10, 17, and 13, in which no capacity limit was imposed. Furthermore, in Example 15, in which capacity limit was imposed during each of the first to third charging operations, the initial irreversible capacity was significantly reduced, and the maximum charge capacity and maximum discharge capacity were comparable, and the activation rate was slightly improved, compared to Example 13, in which no capacity limit was imposed. From these results, it was found that the initial irreversible capacity can be suppressed by limiting the initial charging operation to a predetermined charge capacity.

[0118] As shown in Table 2, in Examples 1, 2, 7 to 15, and 17 to 19, the capacity retention rate after 15 cycles and / or 25 cycles after reaching the maximum capacity was higher than that in Comparative Examples 1 and 2, and the capacity retention rate was improved.

[0119] The above results demonstrate that adding an additive containing boron to a lithium composite oxide or a lithium composite oxide precursor with a layered rock-salt structure containing at least lithium, manganese, nickel, and a divalent element, followed by heating and sintering, can achieve a higher charge-discharge capacity and a shorter number of cycles to reach maximum capacity than a lithium nickel manganese oxide with a layered rock-salt structure. It was also found that the initial irreversible capacity can be suppressed by limiting the charge capacity during the initial charging operation to a predetermined value smaller than the theoretical capacity.

[0120] REFERENCE SIGNS LIST 1 Lithium ion secondary battery 2 Positive electrode part 3 Negative electrode part 5 Electrolyte 20 Positive electrode active material 30 Oxide active material 31 Coating layer

Claims

1. A method for producing a positive electrode active material, comprising adding an additive containing boron element to a lithium composite oxide or a precursor of the lithium composite oxide, and heating and sintering the resulting mixture, wherein the lithium composite oxide has a layered rock salt structure and contains at least lithium, manganese, nickel, and a divalent element, and the divalent element is calcium or magnesium.

2. The method for producing a positive electrode active material according to claim 1, wherein the amount of boron in the additive is more than 0.00075 equivalents and not more than 0.2 equivalents per equivalent of the total of manganese and nickel in the lithium composite oxide.

3. The method for producing a positive electrode active material according to claim 2, wherein the amount of boron in the additive is 0.005 equivalents or more and 0.1 equivalents or less per equivalent of the total of manganese and nickel in the lithium composite oxide.

4. The method for producing a positive electrode active material according to any one of claims 1 to 3, wherein sintering is carried out by heating at a temperature of 850°C or higher.

5. The method for producing a positive electrode active material according to any one of claims 1 to 3, wherein the additive comprises at least one selected from the group consisting of lithium tetraborate, boric acid, lithium borate, and lithium metaborate.

6. A method for manufacturing a secondary battery, comprising: a step of forming a positive electrode active material using the method for manufacturing a positive electrode active material according to any one of claims 1 to 3; and an initial charging step of terminating charging when a predetermined battery capacity is reached at a current density of 10 mA / g or less, wherein the predetermined battery capacity is set based on the capacity of the positive electrode active material and corresponds to a capacity of 180 mAh / g or more and 300 mAh / g or less when the battery is initially charged using only the positive electrode active material.

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