Positive-electrode active material for secondary battery, positive electrode for secondary battery, and secondary battery

Abstract

This secondary battery comprises a positive electrode including a positive-electrode active material, a negative electrode, and an electrolytic solution. The positive-electrode active material comprises a core and a coating which covers the surface of the core. The core comprises a first lithium composite oxide having a lamellar rock salt type crystal structure. The coating comprises a second lithium composite oxide having an orthorhombic-system crystal structure represented by space group lmmm and containing nickel as a constituent element.

Description

Positive electrode active material for secondary battery, positive electrode for secondary battery, and secondary battery

[0001] The present technology relates to a positive electrode active material for a secondary battery, a positive electrode for a secondary battery, and a secondary battery.

[0002] Due to the widespread use of various electronic devices such as mobile phones, secondary batteries have been developed as small, lightweight power sources that can provide high energy density. These secondary batteries include a positive electrode (a positive electrode active material for secondary batteries and a positive electrode for secondary batteries), a negative electrode, and an electrolyte solution, and various studies have been conducted on the configuration of such secondary batteries.

[0003] Specifically, the composite positive electrode active material includes secondary particles and a coating film, the secondary particles include a lithium transition metal oxide having a layered crystal structure, and the coating film includes a lithium cobalt composite oxide having a spinel crystal structure (see, for example, Patent Document 1). The positive electrode active material layer includes a positive electrode active material and a coating layer, and the coating layer has a specific resistance within a predetermined range (see, for example, Patent Document 2).

[0004] The sintered body of the positive electrode active material includes a powder body and a coating layer, the powder body includes a lithium composite oxide, and the coating layer includes an amorphous lithium transition metal oxide (see, for example, Patent Document 3).The electrode active material includes a lithium-nickel composite oxide and a lithium-transition metal M composite oxide, and the surface of the lithium-nickel composite oxide is coated with a lithium-transition metal M composite oxide (see, for example, Patent Document 4).

[0005] JP 2019-046795 A JP 2014-137848 A JP 2010-177042 A JP 09-050810 A

[0006] Although various studies have been conducted on the configuration of secondary batteries, the battery characteristics of the secondary batteries are still insufficient and there is room for improvement.

[0007] There is a demand for a positive electrode active material for a secondary battery, a positive electrode for a secondary battery, and a secondary battery that are capable of providing excellent battery characteristics.

[0008] A positive electrode active material for a secondary battery according to one embodiment of the present disclosure includes a positive electrode active material. The positive electrode active material includes a core and a coating portion covering the surface of the core. The core includes a first lithium composite oxide having a layered rock-salt crystal structure. The coating portion includes a second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immm and containing nickel as a constituent element.

[0009] Moreover, a positive electrode for a secondary battery according to an embodiment of the present technology includes a positive electrode active material, and the positive electrode active material has a configuration similar to that of the positive electrode active material for a secondary battery according to the embodiment of the present technology described above.

[0010] Furthermore, the secondary battery according to one embodiment of the present technology includes a positive electrode, a negative electrode, and an electrolyte solution, and the positive electrode has a configuration similar to the configuration of the positive electrode for the secondary battery according to the embodiment of the present technology described above.

[0011] According to the positive electrode active material for a secondary battery, the positive electrode for a secondary battery, or the secondary battery of one embodiment of the present technology, the positive electrode active material for a secondary battery includes a core and a coating portion, the core includes a first lithium composite oxide having a layered rock-salt crystal structure, and the coating portion includes a second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immm and containing nickel as a constituent element, thereby enabling excellent battery characteristics to be obtained.

[0012] Note that the effects of the present technology are not necessarily limited to the effects described here, but may be any of a series of effects related to the present technology described below.

[0013] Fig. 1 is a cross-sectional view showing the configuration of a positive electrode active material for a secondary battery according to an embodiment of the present technology. Fig. 2 is a perspective view showing the configuration of a secondary battery according to an embodiment of the present technology. Fig. 3 is an enlarged cross-sectional view showing the configuration of the battery element shown in Fig. 2. Fig. 4 is a block diagram showing the configuration of an application example of a secondary battery. Fig. 5 is a cross-sectional view showing the configuration of a test secondary battery.

[0014] Hereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The description will be made in the following order: 1. Cathode active material for secondary battery 1-1. Configuration 1-2. Manufacturing method 1-3. Action and effect 2. Secondary battery (cathode for secondary battery) 2-1. Configuration 2-2. Operation 2-3. Manufacturing method 2-4. Action and effect 3. Modification 4. Use of secondary battery

[0015] 1. Cathode Active Material for Secondary Battery First, a cathode active material for secondary battery (hereinafter simply referred to as "cathode active material") according to one embodiment of the present technology will be described.

[0016] The positive electrode active material described here is used in a secondary battery, which is an electrochemical device. However, the positive electrode active material may also be used in electrochemical devices other than secondary batteries. Specific examples of other electrochemical devices include primary batteries and capacitors.

[0017] <1-1. Structure> Fig. 1 shows a cross-sectional structure of a positive electrode active material 100, which is an example of a positive electrode active material. This positive electrode active material 100 is a plurality of particulate substances that absorb and release lithium, and as shown in Fig. 1, includes a central portion 110 and a coating portion 120. However, Fig. 1 shows only one positive electrode active material 100.

[0018] [Center Portion] The center portion 110 is a portion that essentially absorbs and releases lithium, and contains one or more of the first lithium composite oxides.

[0019] The first lithium composite oxide is an oxide containing lithium and one or more transition metal elements as constituent elements, and has a layered rock-salt crystal structure. The transition metal elements are not particularly limited, but specific examples include nickel, cobalt, and manganese.

[0020] The first lithium composite oxide may further contain, as a constituent element, one or more metal elements other than transition metal elements (hereinafter referred to as "first additional metal elements." The type of the first additional metal element is not particularly limited, but specific examples include aluminum and magnesium.

[0021] A specific example of the first lithium composite oxide is LiNiO ２　 , LiCoO ２　 , Li １．０２ Ni ０．９０ Co ０．０５ Al ０．０５ O ２ 　 , LiCo ０．９８ Al ０．０１ Mg ０．０１ O ２　 , LiNi ０．５　 Co ０．２　 Mn ０．３　 O ２　 and LiMn ２　 O ４　 And so on.

[0022] The reason why the central portion 110 contains the first lithium composite oxide is that a sufficient amount of lithium is absorbed and released in the central portion 110, and thus a high battery capacity can be obtained in a secondary battery using the positive electrode active material 100.

[0023] The crystal structure of the first lithium composite oxide can be identified by analyzing the first lithium composite oxide using an analytical method such as powder X-ray diffraction. In this case, copper is used as the X-ray target, and the temperature of the analysis environment is room temperature (23°C), the measurement range (2θ) is 10° to 80°, and the step operation is 0.01° / sec. In addition, a material database is referenced to analyze the measurement chart, and a crystal structure is searched based on the interplanar spacing d calculated from the measurement results (2θ value), thereby identifying a crystal structure that matches the conditions such as peak interplanar spacing, peak intensity ratio, crystal system, and lattice constant (database: Li ２　 NiO ２　 ICSD No. 25000).

[0024] The composition of the first lithium composite oxide can be identified by analyzing the first lithium composite oxide using an analytical method such as inductively coupled plasma (ICP) atomic emission spectroscopy. In this analysis, lithium, transition metal elements, and the first additional metal element are quantified, thereby identifying the composition of the first lithium composite oxide. In this case, the first lithium composite oxide is dissolved using a microwave decomposition method, and then the first lithium composite oxide is analyzed.

[0025] (Covering Portion) The covering portion 120 is a portion that covers the central portion 110, and contains one or more types of the second lithium composite oxides.

[0026] The covering portion 120 may cover the entire surface of the central portion 110, or may cover only a portion of the surface of the central portion 110. In the latter case, a plurality of covering portions 120 spaced apart from one another may cover the surface of the central portion 110.

[0027] The second lithium composite oxide is an oxide containing lithium and nickel as constituent elements. Unlike the first lithium composite oxide described above, the second lithium composite oxide has an orthorhombic crystal structure represented by the space group Immm.

[0028] The second lithium composite oxide may further contain one or more metal elements other than nickel (hereinafter referred to as "second additional metal elements") as constituent elements. The second additional metal elements may be transition metal elements or metal elements other than transition metal elements. The type of the second additional metal element is not particularly limited, but specific examples include titanium, platinum, copper, and tungsten.

[0029] More specifically, the second lithium composite oxide contains one or more of the compounds represented by formula (1).

[0030] Li ２　 Ni １－ｘ　 M ｘ　 O ２　...(1) (M is at least one of Ti, Pt, Cu, and W. x satisfies 0≦x≦0.25.)

[0031] As shown in formula (1), the second lithium composite oxide is an oxide containing lithium and nickel as constituent elements. As is clear from the range of x, the second lithium composite oxide may or may not contain the second metal element M as a constituent element.

[0032] Specific examples of the second lithium composite oxide include Li ２　 NiO ２　 , Li ２　 Ni ０．９９ Ti ０．０１ O ２　 , Li ２　 Ni ０．７５ Ti ０．２５ O ２　 , Li ２　 Ni ０．７５ Pt ０．２５ O ２　 , Li ２　 Ni ０．７５ Cu ０．２５ O ２　 and Li ２　 Ni ０．７５ W ０．２５ O ２　 And so on.

[0033] The reason why the covering portion 120 contains the second lithium composite oxide is that the highly reactive surface of the core portion 110 is electrochemically protected by the covering portion 120. This suppresses the decomposition reaction of the electrolyte on the surface of the core portion 110 in a secondary battery using the positive electrode active material 100. Therefore, even when the secondary battery is repeatedly charged and discharged, an increase in electrical resistance and a decrease in discharge capacity are suppressed.

[0034] In particular, it is preferable that the second additional metal element M contains one or both of Cu and W as constituent elements, because this further suppresses an increase in electrical resistance when the secondary battery is repeatedly charged and discharged.

[0035] Alternatively, the second additional metal element M preferably contains one or more of Ti, Pt, and Cu as constituent elements, because this further suppresses the decrease in discharge capacity when the secondary battery is repeatedly charged and discharged.

[0036] The average coating amount of the coating portion 120 is not particularly limited, but is preferably 0.01 mmol / m ２　 ~0.05mmol / m ２　 This is because the state of coverage of the surface of the central portion 110 by the covering portion 120 is optimized, and the surface of the central portion 110 is sufficiently electrochemically protected by the covering portion 120. This sufficiently suppresses the decomposition reaction of the electrolyte, thereby sufficiently suppressing an increase in electrical resistance and a decrease in discharge capacity.

[0037] The method for forming the covering portion 120, i.e., the method for covering the surface of the core portion 110 with the material for forming the covering portion 120 (hereinafter referred to as the "covering material"), is not particularly limited and can be selected arbitrarily.

[0038] As an example, the coating portion 120 is formed using a dry coating method. In this dry coating method, the coating material is decomposed by using shear force and adheres to the surface of the central portion 110. This fixes the coating material to the surface of the central portion 110, thereby forming the coating portion 120. Note that when the coating material melts in response to thermal energy and undergoes plastic deformation in response to mechanical energy, the coating material becomes a thin film that coats the surface of the central portion 110.

[0039] The crystal structure of the second lithium composite oxide can be identified by analyzing the second lithium composite oxide using an analytical method such as X-ray absorption spectroscopy (XAS). When XAS is used as the analytical method, the crystal structure of the second lithium composite oxide is analyzed based on the X-ray absorption near edge structure (XANES) using a first-principles calculation program for X-ray absorption spectroscopy (FEFF), thereby identifying the crystal structure. This XANES is a fine structure detected near the absorption edge in the X-ray absorption spectrum (a fine structure detected within a range of energy E of about ±50 eV based on the position of the absorption edge).

[0040] The procedure for determining the composition of the second lithium composite oxide is similar to the procedure for determining the composition of the first lithium composite oxide, except that in the procedure for determining the composition of the second lithium composite oxide, the second additional metal element is quantified instead of the first additional metal element.

[0041] Furthermore, the procedure for calculating the average coating amount of the coating portion 120 is as described below. Below, a case where the second lithium composite oxide contains the second additional metal element as a constituent element will be described.

[0042] First, the coating portion 120 was analyzed using ICP atomic emission spectroscopy to determine the content (g / m) of the second additional metal element contained as a constituent element in the second lithium composite oxide. ２　 ) is measured. Next, the atomic weight of the second additional metal element is determined. For example, if the second additional metal element is titanium (atomic number 22), the atomic weight of the second additional metal element is 47.867. Next, the coating amount (mmol / m ２　 Finally, the process of calculating the coating amount while changing the analysis position of the coating portion 120 is repeated 10 times to calculate 10 coating amounts, and then the average of the 10 coating amounts is calculated, and this average value is used as the average coating amount.

[0043] <1-2. Manufacturing Method> The positive electrode active material 100 is manufactured by the following example procedure.

[0044] First, a raw material for forming the coating material is prepared. This raw material is a compound containing nickel as a constituent element, and may further contain a second additional metal element as a constituent element. The type of raw material is not particularly limited, but specific examples include carbonates and hydroxides.

[0045] The raw materials are then fired to form a precursor for forming the coating material. This precursor is an oxide containing nickel as a constituent element, and as described above, may further contain a second additional metal element as a constituent element. Note that firing conditions such as firing temperature and firing time can be set as desired.

[0046] Finally, a solid-state reaction is used to form the coating material.

[0047] Specifically, a precursor and a lithium compound are mixed together to obtain a mixture, and then the mixture is fired to obtain a fired product. This lithium compound is a compound containing lithium as a constituent element. The type of lithium compound is not particularly limited, but specific examples include oxides and hydroxides. Note that firing conditions such as firing temperature and firing time can be set as desired.

[0048] The fired product is then pulverized to obtain a pulverized product, and coarse particles are removed from the pulverized product using a sieve. In this case, a pulverizing tool such as a mortar is used. The mesh size (μm) of the sieve is not particularly limited and can be set as desired.

[0049] This produces a powdery coating material, which contains the second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immm.

[0050] In this case, the crystalline structure of the coating material (second lithium composite oxide) is adjusted so as to have an orthorhombic crystalline structure represented by the space group Immmm by adjusting the firing conditions when firing the mixture of the precursor and the lithium compound.

[0051] Next, a plurality of (powdered) cores 110 are prepared. The cores 110 contain a first lithium composite oxide having a layered rock salt type crystal structure.

[0052] Finally, the plurality of cores 110 and the coating material are granulated. In this case, a granulating device such as a centrifugal fluid granulating device is used. The granulation conditions, such as the granulation temperature and granulation time, can be set as desired.

[0053] As a result, the coating material is fixed to the surface of the central portion 110, thereby forming the coating portion 120. Thus, a plurality of positive electrode active materials 100 including the central portion 110 and the coating portion 120 are completed.

[0054] In this case, the average coating amount of the coating portion 120 can be adjusted by adjusting the amount of coating material added when granulating using a plurality of cores 110 and the coating material.

[0055] <1-3. Actions and Effects> This positive electrode active material 100 includes a core 110 and a coating portion 120. The core 110 includes a first lithium composite oxide having a layered rock-salt crystal structure. The coating portion 120 includes a second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immm and containing nickel as a constituent element.

[0056] In this case, since the central portion 110 contains the first lithium composite oxide, as described above, a sufficient amount of lithium is absorbed and released in the central portion 110. This allows a secondary battery using the positive electrode active material 100 to obtain a high battery capacity.

[0057] Moreover, because coating portion 120 contains the second lithium composite oxide, as described above, the highly reactive surface of core portion 110 is electrochemically protected by coating portion 120. This suppresses the decomposition reaction of the electrolyte on the surface of core portion 110 in a secondary battery using positive electrode active material 100, and therefore suppresses an increase in electrical resistance and a decrease in discharge capacity even when the secondary battery is repeatedly charged and discharged.

[0058] For these reasons, a secondary battery using the positive electrode active material 100 can obtain a high battery capacity, and an increase in electrical resistance and a decrease in discharge capacity are suppressed even when charge and discharge are repeated. Therefore, a secondary battery having excellent battery characteristics can be realized by using the positive electrode active material 100.

[0059] In particular, if the second lithium composite oxide contains the compound shown in formula (1), the surface of the core 110 is sufficiently electrochemically protected by the covering 120, and the decomposition reaction of the electrolyte is sufficiently suppressed. Therefore, an increase in electrical resistance and a decrease in discharge capacity are sufficiently suppressed, and therefore a greater effect can be obtained.

[0060] In this case, if the second additional metal element M contains one or both of Cu and W as a constituent element, an increase in electrical resistance is further suppressed, thereby achieving an even greater effect. Alternatively, if the second additional metal element M contains one or more of Ti, Pt, and Cu as a constituent element, a decrease in discharge capacity is further suppressed, thereby achieving an even greater effect.

[0061] The average coating amount of the coating portion 120 is 0.01 mmol / m ２　 ~0.05mmol / m ２　 In this case, the surface of the core 110 is sufficiently electrochemically protected by the covering 120, and the decomposition reaction of the electrolyte is sufficiently suppressed. Therefore, an increase in electrical resistance is sufficiently suppressed, and a decrease in discharge capacity is sufficiently suppressed, resulting in a greater effect.

[0062] 2. Secondary Battery (Positive Electrode for Secondary Battery) Next, a secondary battery according to an embodiment of the present technology to which the positive electrode active material 100 is applied will be described.

[0063] Note that, since the positive electrode for a secondary battery according to one embodiment of the present technology (hereinafter simply referred to as the "positive electrode") is one component of the secondary battery described herein, the positive electrode will also be described below.

[0064] This secondary battery is a battery that obtains battery capacity by absorbing and releasing an electrode reactant, and includes a positive electrode, a negative electrode, and an electrolyte. More specifically, a secondary battery that obtains battery capacity by absorbing and releasing lithium, which is an electrode reactant, is a so-called lithium-ion secondary battery. In this lithium-ion secondary battery, lithium is absorbed and released in an ionic state.

[0065] The charge capacity of the negative electrode is preferably larger than the discharge capacity of the positive electrode. That is, the electrochemical capacity per unit area of ​​the negative electrode is preferably larger than the electrochemical capacity per unit area of ​​the positive electrode. This is to prevent lithium from being deposited on the surface of the negative electrode during charging.

[0066] <2-1. Configuration> Fig. 2 shows a perspective configuration of a secondary battery, and Fig. 3 shows an enlarged cross-sectional configuration of the battery element 20 shown in Fig. 2. However, Fig. 2 shows a state in which the exterior film 10 and the battery element 20 are separated from each other, and the cross section of the battery element 20 along the XZ plane is shown by a dashed line.

[0067] 2 and 3, this secondary battery includes an exterior film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and 42. The secondary battery described here is a laminate film type secondary battery that uses a flexible or pliable exterior film 10.

[0068] 2, the exterior film 10 is an exterior member that houses the battery element 20, and has a bag-like structure that is sealed when the battery element 20 is housed inside. As a result, the exterior film 10 houses an electrolyte solution together with a positive electrode 21 and a negative electrode 22, which will be described later.

[0069] Here, the exterior film 10 is a single film-like member that is folded in a folding direction F. The exterior film 10 is provided with a recessed portion 10U (a so-called deep drawn portion) for accommodating the battery element 20.

[0070] Specifically, the exterior film 10 is a three-layer laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order from the inside out, and when the exterior film 10 is folded, the outer peripheral edges of the opposing fusion layers are fused to each other. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

[0071] However, the configuration (number of layers) of the exterior film 10, which is a laminate film, is not particularly limited, and may be one layer, two layers, or four or more layers.

[0072] [Battery Element] As shown in Figures 2 and 3, the battery element 20 is a power generation element including a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown), and is housed inside the exterior film 10.

[0073] This battery element 20 is a so-called wound electrode body. That is, the positive electrode 21 and the negative electrode 22 are wound around a winding axis P while facing each other with a separator 23 interposed therebetween. This winding axis P is an imaginary axis extending in the Y-axis direction.

[0074] The three-dimensional shape of battery element 20 is not particularly limited. Here, battery element 20 is flat, and therefore the shape of a cross section (cross section along the XZ plane) of battery element 20 intersecting winding axis P is a flat shape defined by a major axis J1 and a minor axis J2. This major axis J1 is an imaginary axis that extends in the X-axis direction and has a length greater than that of minor axis J2, and minor axis J2 is an imaginary axis that extends in the Z-axis direction intersecting with the X-axis direction and has a length smaller than that of major axis J1. Here, the three-dimensional shape of battery element 20 is a flat cylindrical shape, and therefore the shape of the cross section of battery element 20 is a flat, approximately elliptical shape.

[0075] (Positive Electrode) As shown in FIG. 3, the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B.

[0076] The positive electrode current collector 21A has a pair of surfaces on which the positive electrode active material layer 21B is provided. The positive electrode current collector 21A contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum.

[0077] The positive electrode active material layer 21B contains one or more types of positive electrode active materials that absorb and release lithium. However, the positive electrode active material layer 21B may further contain one or more types of other materials such as a positive electrode binder and a positive electrode conductive agent. The method for forming the positive electrode active material layer 21B is not particularly limited, but specifically includes a coating method.

[0078] Here, the positive electrode active material layer 21B is provided on both sides of the positive electrode current collector 21A. However, the positive electrode active material layer 21B may be provided on only one side of the positive electrode current collector 21A, on the side where the positive electrode 21 faces the negative electrode 22.

[0079] The positive electrode active material has the same configuration as the positive electrode active material 100. Details regarding the configuration of the positive electrode active material 100 are as described above.

[0080] The positive electrode binder contains one or more of materials such as synthetic rubber and polymer compounds. Specific examples of synthetic rubber include styrene-butadiene rubber, fluorine-containing rubber, and ethylene-propylene-diene. Specific examples of polymer compounds include polyvinylidene fluoride, polyimide, and carboxymethyl cellulose.

[0081] The positive electrode conductive agent contains one or more conductive materials such as a carbon material, a metal material, and a conductive polymer compound, and specific examples of the carbon material include graphite, carbon black, acetylene black, and ketjen black.

[0082] (Negative Electrode) As shown in FIG. 3, the negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B.

[0083] The negative electrode current collector 22A has a pair of surfaces on which the negative electrode active material layer 22B is provided. The negative electrode current collector 22A contains a conductive material such as a metal material, and a specific example of the conductive material is copper.

[0084] The negative electrode active material layer 22B includes one or more types of negative electrode active materials that absorb and release lithium. However, the negative electrode active material layer 22B may further include one or more types of other materials, such as a negative electrode binder and a negative electrode conductive agent. The method for forming the negative electrode active material layer 22B is not particularly limited, and specifically includes one or more types of a coating method, a vapor phase method, a liquid phase method, a thermal spraying method, and a firing method (sintering method).

[0085] Here, the anode active material layer 22B is provided on both sides of the anode current collector 22A. However, the anode active material layer 22B may be provided on only one side of the anode current collector 22A on the side where the anode 22 faces the cathode 21.

[0086] The type of negative electrode active material is not particularly limited, but specific examples include carbon materials and metal-based materials, because high energy density can be obtained.

[0087] Specific examples of carbon materials include graphitizable carbon, non-graphitizable carbon, and graphite (natural graphite and artificial graphite).

[0088] The metallic material is a material containing, as a constituent element, one or more of metallic elements and semi-metallic elements that can form an alloy with lithium, and specific examples of the metallic element and semi-metallic element include silicon and tin. The metallic material may be a simple substance, an alloy, a compound, a mixture of two or more of these, or a material containing two or more of these phases. Specific examples of metallic materials include TiSi ２　 and SiO ｘ　 (0<x≦2 or 0.2<x<1.4), etc.

[0089] The details regarding the negative electrode binder are the same as those regarding the positive electrode binder, and the details regarding the negative electrode conductive agent are the same as those regarding the positive electrode conductive agent.

[0090] 3, the separator 23 is an insulating porous film interposed between the positive electrode 21 and the negative electrode 22, and allows lithium ions to pass through while preventing contact (short circuit) between the positive electrode 21 and the negative electrode 22. The separator 23 contains a polymer compound such as polyethylene.

[0091] (Electrolyte) The electrolyte is a liquid electrolyte that is impregnated into the positive electrode 21, the negative electrode 22, and the separator 23, and contains a solvent and an electrolyte salt.

[0092] Here, the solvent contains one or more types of non-aqueous solvents (organic solvents), and the electrolyte solution containing the non-aqueous solvent is a so-called non-aqueous electrolyte solution. The non-aqueous solvents include esters and ethers, more specifically, carbonate ester compounds, carboxylic acid ester compounds, and lactone compounds. This is because the dissociation of the electrolyte salt is improved and the mobility of ions is also improved.

[0093] The carbonate ester compounds include cyclic carbonate esters and chain carbonate esters. Specific examples of the cyclic carbonate esters include ethylene carbonate and propylene carbonate, and specific examples of the chain carbonate esters include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0094] The carboxylic acid ester compound is a chain carboxylic acid ester, and specific examples of the chain carboxylic acid ester include ethyl acetate, ethyl propionate, propyl propionate, and ethyl trimethylacetate. The lactone compound is a lactone, and specific examples of the lactone include γ-butyrolactone and γ-valerolactone. The ether may be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, or 1,4-dioxane.

[0095] The electrolyte salt contains one or more light metal salts such as lithium salts. Specific examples of lithium salts include lithium hexafluorophosphate (LiPF ６　 ), lithium tetrafluoroborate (LiBF ４　 ), lithium trifluoromethanesulfonate (LiCF ３　 SO ３　 ), lithium bis(fluorosulfonyl)imide (LiN(FSO ２　 ) ２　 ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF ３　 SO ２　 ) ２　 ), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF ３　 SO ２　 ) ３　), lithium bis(oxalato)borate (LiB(C ２　 O ４　 ) ２　 ), lithium monofluorophosphate (Li ２　 PFO ３　 ) and lithium difluorophosphate (LiPF ２　 O ２　 ) etc. This is because a high battery capacity can be obtained.

[0096] The content of the electrolyte salt is not particularly limited, but specifically, it is 0.3 mol / kg to 3.0 mol / kg relative to the solvent, because high ionic conductivity can be obtained.

[0097] The electrolyte may further contain one or more additives, which improves the electrochemical stability of the electrolyte. The additives are not particularly limited, but specific examples include unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonates, phosphates, acid anhydrides, nitrile compounds, and isocyanate compounds.

[0098] Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonates include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of sulfonic acid esters include propane sultone and propene sultone. Specific examples of phosphate esters include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride. Specific examples of nitrile compounds include succinonitrile. Specific examples of isocyanate compounds include hexamethylene diisocyanate.

[0099] 2 and 3 , the positive electrode lead 31 is a positive electrode terminal connected to the positive electrode current collector 21A of the positive electrode 21, and is led out of the exterior film 10. The positive electrode lead 31 contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum. The shape of the positive electrode lead 31 is not particularly limited, but is specifically either a thin plate shape or a mesh shape.

[0100] 2 and 3 , the negative electrode lead 32 is a negative electrode terminal connected to the negative electrode current collector 22A of the negative electrode 22, and is led out of the exterior film 10. This negative electrode lead 32 contains a conductive material such as a metal material, and a specific example of the conductive material is copper. Here, the details regarding the lead-out direction and shape of the negative electrode lead 32 are the same as the details regarding the lead-out direction and shape of the positive electrode lead 31.

[0101] [Sealing Film] The sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and the sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32. However, one or both of the sealing films 41 and 42 may be omitted.

[0102] The sealing film 41 is a sealing member that prevents outside air and the like from entering the interior of the exterior film 10. Specifically, the sealing film 41 contains a polymer compound such as polyolefin that has adhesiveness to the positive electrode lead 31, and a specific example of the polyolefin is polypropylene.

[0103] The configuration of the sealing film 42 is the same as the configuration of the sealing film 41, except that the sealing film 42 is a sealing member that has adhesiveness to the negative electrode lead 32. That is, the sealing film 42 contains a polymer compound such as polyolefin that has adhesiveness to the negative electrode lead 32.

[0104] <2-2. Operation> The secondary battery operates as follows during charging and discharging.

[0105] During charging, lithium is released from the positive electrode 21 of the battery element 20 and is absorbed into the negative electrode 22 via the electrolyte. During discharging, lithium is released from the negative electrode 22 of the battery element 20 and is absorbed into the positive electrode 21 via the electrolyte. During charging and discharging, lithium is absorbed and released in an ionic state.

[0106] <2-3. Manufacturing Method> When manufacturing a secondary battery, the positive electrode 21 and the negative electrode 22 are fabricated and an electrolytic solution is prepared according to the procedure described below as an example. Thereafter, the positive electrode 21, the negative electrode 22, and the electrolytic solution are used to assemble a secondary battery, and a stabilization process is performed on the assembled secondary battery.

[0107] [Fabrication of Positive Electrode] First, a paste-like positive electrode mixture slurry is prepared by adding a mixture (positive electrode mixture) of a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent to a solvent. This solvent may be an aqueous solvent or an organic solvent. Next, the positive electrode mixture slurry is applied to both surfaces of the positive electrode current collector 21A to form the positive electrode active material layer 21B. Finally, the positive electrode active material layer 21B may be compression-molded using a roll press or the like. In this case, the positive electrode active material layer 21B may be heated, or the compression molding may be repeated multiple times. As a result, the positive electrode active material layer 21B is formed on both surfaces of the positive electrode current collector 21A, thereby fabricating the positive electrode 21.

[0108] [Fabrication of Negative Electrode] The negative electrode 22 is fabricated by a procedure substantially similar to that of fabricating the positive electrode 21 described above. Specifically, a paste-like negative electrode mixture slurry is prepared by adding a mixture (negative electrode mixture) of a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent to a solvent, and then the negative electrode mixture slurry is applied to both surfaces of the negative electrode current collector 22A to form the negative electrode active material layer 22B. The negative electrode active material layer 22B may then be compression-molded using a roll press or the like. As a result, the negative electrode active material layer 22B is formed on both surfaces of the negative electrode current collector 22A, thereby fabricating the negative electrode 22.

[0109] [Preparation of Electrolyte Solution] An electrolyte salt is added to a solvent, whereby the electrolyte salt is dispersed or dissolved in the solvent, thereby preparing an electrolyte solution.

[0110] [Assembly of Secondary Battery] First, the positive electrode lead 31 is connected to the positive electrode current collector 21A of the positive electrode 21 using a joining method such as welding, and the negative electrode lead 32 is connected to the negative electrode current collector 22A of the negative electrode 22 using a joining method such as welding.

[0111] Next, the positive electrode 21 and the negative electrode 22 are stacked one on top of the other with the separator 23 interposed therebetween, and then the positive electrode 21, the negative electrode 22, and the separator 23 are wound together to produce a wound body (not shown). Next, the wound body is pressed using a press or the like to form a flat shape. The wound body after this formation has the same configuration as the battery element 20, except that the positive electrode 21, the negative electrode 22, and the separator 23 are not impregnated with an electrolyte solution.

[0112] Next, after the roll is housed inside the recess 10U, the exterior film 10 (adhesive layer / metal layer / surface protection layer) is folded to face each other. Next, the outer peripheral edges of two sides of the opposing adhesive layers are joined together using an adhesive method such as heat fusion, thereby housing the roll inside the bag-shaped exterior film 10.

[0113] Finally, after injecting an electrolyte solution into the bag-shaped exterior film 10, the outer peripheral edges of the remaining sides of the opposing fusion layers are joined together using an adhesive method such as heat fusion. In this case, a sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32.

[0114] As a result, the wound body is impregnated with the electrolyte, producing the battery element 20 which is a wound electrode body, and the battery element 20 is sealed inside the bag-shaped exterior film 10, thereby assembling a secondary battery.

[0115] [Stabilization of Secondary Battery] The assembled secondary battery is charged and discharged. Conditions such as the ambient temperature, the number of charge / discharge cycles (number of cycles), and the charge / discharge conditions can be set arbitrarily. This forms a coating on the surface of the positive electrode 21 and the surface of the negative electrode 22, electrochemically stabilizing the state of the battery element 20. This completes the secondary battery.

[0116] <2-4. Actions and Effects> In this secondary battery, the positive electrode 21 contains a positive electrode active material, and the positive electrode active material has a configuration similar to that of the positive electrode active material 100. Therefore, for the reasons described above, a high battery capacity can be obtained, and an increase in electrical resistance and a decrease in discharge capacity are suppressed even when charging and discharging are repeated, thereby obtaining excellent battery characteristics.

[0117] In particular, if the secondary battery is a lithium ion secondary battery, a sufficient battery capacity can be stably obtained by utilizing the absorption and release of lithium, and therefore a greater effect can be obtained.

[0118] Other functions and effects of this secondary battery are similar to those of the positive electrode active material 100 .

[0119] 3. Modifications Next, modifications of the above-described secondary battery will be described.

[0120] The configuration of the secondary battery can be modified as appropriate, as described below, although the series of modifications described below may be combined with each other.

[0121] [Modification 1] A porous film separator 23 is used. However, although not specifically shown here, a laminated separator including a polymer compound layer may also be used.

[0122] Specifically, the laminated separator includes a porous membrane having a pair of surfaces and a polymer compound layer provided on one or both surfaces of the porous membrane. This is because the separator improves adhesion to each of the positive electrode 21 and the negative electrode 22, thereby suppressing miswinding of the battery element 20. This prevents swelling of the secondary battery even if a decomposition reaction of the electrolyte occurs. The polymer compound layer includes a polymer compound such as polyvinylidene fluoride. Polyvinylidene fluoride has excellent physical strength and is electrochemically stable.

[0123] One or both of the porous film and the polymer compound layer may contain a plurality of insulating particles. This is because the plurality of insulating particles promotes heat dissipation when the secondary battery generates heat, thereby improving the safety (heat resistance) of the secondary battery. The insulating particles contain one or more of inorganic materials and resin materials. Specific examples of inorganic materials include aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. Specific examples of resin materials include acrylic resin and styrene resin.

[0124] When fabricating a laminated separator, a precursor solution containing a polymer compound and a solvent is prepared, and then the precursor solution is applied to one or both sides of a porous membrane. In this case, instead of applying the precursor solution to the porous membrane, the porous membrane may be immersed in the precursor solution. In addition, multiple insulating particles may be added to the precursor solution.

[0125] Even when this laminated separator is used, the same effect can be obtained because lithium becomes movable between the positive electrode 21 and the negative electrode 22. In this case, as described above, the safety of the secondary battery is particularly improved, and therefore, a greater effect can be obtained.

[0126] [Modification 2] An electrolytic solution that is a liquid electrolyte is used. However, although not specifically shown here, an electrolyte layer that is a gel electrolyte may also be used.

[0127] In the battery element 20 including the electrolyte layer, the positive electrode 21 and the negative electrode 22 are stacked with the separator 23 and the electrolyte layer interposed therebetween, and the positive electrode 21, the negative electrode 22, the separator 23, and the electrolyte layer are wound together. The electrolyte layer is interposed between the positive electrode 21 and the separator 23, and also between the negative electrode 22 and the separator 23. However, the electrolyte layer may be interposed only between the positive electrode 21 and the separator 23, or only between the negative electrode 22 and the separator 23.

[0128] This electrolyte layer contains a polymer compound together with an electrolytic solution, and the electrolytic solution is held by the polymer compound. This is because leakage of the electrolytic solution is prevented. The composition of the electrolytic solution is as described above. The polymer compound includes polyvinylidene fluoride, etc. When forming the electrolyte layer, a precursor solution containing the electrolytic solution, the polymer compound, and a solvent is prepared, and then the precursor solution is applied to one or both surfaces of each of the positive electrode 21 and the negative electrode 22.

[0129] Even when this electrolyte layer is used, the same effect can be obtained because lithium can move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer. In this case, particularly, as described above, leakage of the electrolyte solution is prevented, so that a greater effect can be obtained.

[0130] 4. Uses of Secondary Batteries Finally, uses (application examples) of secondary batteries will be described.

[0131] The use of the secondary battery is not particularly limited. The secondary battery used as a power source may be a main power source for electronic devices, electric vehicles, etc., or an auxiliary power source. A main power source is a power source that is used preferentially regardless of the presence or absence of other power sources. An auxiliary power source may be a power source used in place of the main power source, or a power source that can be switched from the main power source.

[0132] Specific examples of uses for secondary batteries are as follows: Electronic devices such as video cameras, digital still cameras, mobile phones, laptop computers, headphone stereos, portable radios, and portable information terminals. Storage devices such as backup power supplies and memory cards. Power tools such as power drills and power saws. Battery packs installed in electronic devices. Medical electronic devices such as pacemakers and hearing aids. Electric vehicles such as electric cars (including hybrid cars). Power storage systems such as home or industrial battery systems that store power in preparation for emergencies. In these uses, one secondary battery may be used, or multiple secondary batteries may be used.

[0133] The battery pack may include a single cell or a battery pack. The electric vehicle is a vehicle that runs on a secondary battery as a driving power source, and may be a hybrid vehicle that also includes a driving source other than the secondary battery. In a home power storage system, household electrical appliances can be powered by using the power stored in the secondary battery, which is a power storage source.

[0134] Here, an example of the use of the secondary battery will be specifically described. The configuration described below is merely an example and can be modified as appropriate.

[0135] Figure 4 shows the block diagram of a battery pack, which is an example of an application of a secondary battery. The battery pack described here is a battery pack (a so-called soft pack) that uses a single secondary battery, and is installed in electronic devices such as smartphones.

[0136] 4, the battery pack includes a power supply 51 and a circuit board 52. The circuit board 52 is connected to the power supply 51 and includes a positive terminal 53, a negative terminal 54, and a temperature detection terminal 55.

[0137] The power source 51 includes one secondary battery. The positive electrode lead of this secondary battery is connected to a positive electrode terminal 53, and the negative electrode lead is connected to a negative electrode terminal 54. The power source 51 is connected to an external power source via the positive electrode terminal 53 and the negative electrode terminal 54, and is therefore capable of being charged and discharged. The circuit board 52 includes a control unit 56, a switch 57, a thermosensitive resistor (PTC element) 58, and a temperature detection unit 59. However, the PTC element 58 may be omitted.

[0138] The control unit 56 includes a central processing unit (CPU) and memory, and controls the operation of the battery pack. The control unit 56 detects and controls the usage state of the power source 51 as necessary.

[0139] When the voltage of power supply 51 (secondary battery) reaches the overcharge detection voltage or the overdischarge detection voltage, control unit 56 turns off switch 57 to prevent charging current from flowing through the current path of power supply 51. The overcharge detection voltage is not particularly limited, but specifically, it is 4.20 V±0.05 V, and the overdischarge detection voltage is not particularly limited, but specifically, it is 2.40 V±0.10 V.

[0140] Switch 57 includes a charge control switch, a discharge control switch, a charge diode, a discharge diode, etc., and switches between the connection and disconnection of power supply 51 and an external device in response to instructions from control unit 56. Switch 57 includes a metal oxide semiconductor field effect transistor (MOSFET), and the charge / discharge current is detected based on the ON resistance of switch 57.

[0141] Temperature detection unit 59 includes a temperature detection element such as a thermistor. Temperature detection unit 59 measures the temperature of power supply 51 using temperature detection terminal 55 and outputs the temperature measurement result to control unit 56. The temperature measurement result measured by temperature detection unit 59 is used when control unit 56 controls charging and discharging in the event of abnormal heat generation, and when control unit 56 performs correction processing when calculating the remaining capacity.

[0142] An embodiment of the present technology will be described.

[0143] Experimental Examples 1 to 8 and Comparative Examples 1 to 5 After secondary batteries were fabricated, the characteristics of the secondary batteries were evaluated.

[0144] [Fabrication of Secondary Battery] Fig. 5 shows a cross-sectional structure of a test secondary battery. This test secondary battery is a so-called coin-type secondary battery (lithium ion secondary battery).

[0145] As shown in FIG. 5, this secondary battery includes a test electrode 61, a counter electrode 62, a separator 63, an exterior cup 64, an exterior can 65, a gasket 66, and an electrolyte (not shown).

[0146] The test electrode 61 is housed in an exterior cup 64, and the counter electrode 62 is housed in an exterior can 65. The test electrode 61 and the counter electrode 62 are stacked together with a separator 63 interposed therebetween, and the test electrode 61, the counter electrode 62, and the separator 63 are impregnated with an electrolyte. The exterior cup 64 and the exterior can 65 are crimped together with a gasket 66, so that the test electrode 61, the counter electrode 62, and the separator 63 are sealed by the exterior cup 64 and the exterior can 65.

[0147] Here, in order to simply evaluate the battery characteristics, a coin-type secondary battery shown in FIG. 5 was fabricated according to the procedure described below.

[0148] (Preparation of Test Electrode) First, raw materials were prepared.

[0149] When forming a coating material containing titanium as a constituent element as the second additional metal element, the raw material is Ni having a purity of 99%. １．９８ Ti ０．０２ CO ３ 　 and Ni with purity = 99% １．５０ Ti ０．５０ CO ３ 　 When forming a coating material containing platinum as a constituent element as the second additional metal element, Ni having a purity of 99% was used as the raw material. １．５０ Pt ０．５０ CO ３ 　When forming a coating material containing copper as a constituent element as the second additional metal element, Ni having a purity of 99% was used as the raw material. １．５０ Cu ０．５０ CO ３ 　 When forming a coating material containing tungsten as a constituent element as the second additional metal element, Ni having a purity of 99% was used as the raw material. １．５０ W ０．５０ CO ３ 　 When forming a coating material that does not contain the second additional metal element as a constituent element, Ni having a purity of 99% was used as the raw material. ２　 CO ３ 　 was used.

[0150] The raw materials were then calcined (calcination temperature=400° C. and calcination time=10 hours) to form a precursor.

[0151] Ni as raw material １．９８ Ti ０．０２ CO ３ 　 When using Ni as a precursor, ０．９９ Ti ０．０１ O was formed. Ni was used as the raw material. １．５０ Ti ０．５０ CO ３ 　 When using Ni as a precursor, ０．７５ Ti ０．２５ O was formed. Ni was used as the raw material. １．５０ Pt ０．５０ CO ３ 　 When using Ni as a precursor, ０．７５ Pt ０．２５ O was formed. Ni was used as the raw material. １．５０ Cu ０．５０ CO ３ 　 When using Ni as a precursor, ０．７５ Cu ０．２５ O was formed. Ni was used as the raw material. １．５０ W ０．５０ CO ３ 　 When using Ni as a precursor,０．７５ W ０．２５ O was formed. Ni was used as the raw material. ２　 CO ３ 　 When used as a precursor, NiO was formed.

[0152] Next, the precursor and a lithium compound (lithium oxide (Li) with a purity of 99.5%) were mixed. ２　 O)) were mixed with each other to obtain a mixture, and then the mixture was fired (firing time = 650°C and firing time = 24 hours) to obtain a fired product.

[0153] Ni as precursor ０．９９ Ti ０．０１ When O is used, Li is used as the fired material. ２　 Ni ０．９９ Ti ０．０１ O ２　 was formed. Ni was used as a precursor. ０．７５ Ti ０．２５ When O is used, Li is used as the fired material. ２　 Ni ０．７５ Ti ０．２５ O ２　 was formed. Ni was used as a precursor. ０．７５ Pt ０．２５ When O is used, Li is used as the fired material. ２　 Ni ０．７５ Pt ０．２５ O ２　 was formed. Ni was used as a precursor. ０．７５ Cu ０．２５ When O is used, Li is used as the fired material. ２　 Cu ０．７５ Pt ０．２５ O ２　 was formed. Ni was used as a precursor. ０．７５ W ０．２５ When O is used, Li is used as the fired material. ２　 Ni ０．７５ W ０．２５ O ２　 When NiO was used as the precursor, Li was formed as the fired product. ２　 NiO ２　 was formed.

[0154] Subsequently, the fired product was pulverized in a mortar to obtain a pulverized product, and then coarse particles were removed from the pulverized product using a sieve (mesh size = 90 μm), thereby obtaining a powdery coating material.

[0155] Next, a plurality of (powder-like) central portions 110 (first lithium composite oxides having a layered rock salt crystal structure) were prepared. １．０２ Ni ０．９０ Co ０．０５ Al ０．０５ O ２ 　 (NCA) was used.

[0156] Next, the coating material was charged into a centrifugal fluidized bed granulator together with the plurality of cores 110, and then granulation was performed using the centrifugal fluidized bed granulator. In this case, the amount of the cores 110 charged was 1 kg, the amount of the coating material charged was 45 g, and the air volume was 0.1 m ３　 / min, intake air temperature = 60°C, exhaust air temperature = 40°C, rotation speed = 150 min, coating time = 20 min.

[0157] As a result, the coating material was fixed to the surface of the central portion 110, thereby forming the coating portion 120 (a second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immmm). Thus, the powdered positive electrode active material 100, i.e., a plurality of positive electrode active materials 100 including the central portion 110 and the coating portion 120, was obtained.

[0158] The completed positive electrode active material 100 was analyzed to examine the composition and crystalline structure of the core portion 110 and the composition and crystalline structure of the coating portion 120, and the results are shown in Table 1. The details of the analysis procedure for the positive electrode active material 100 are as described above.

[0159] When manufacturing this positive electrode active material 100, the average coating amount (mmol / m ２　 The average coating amount was examined after the completion of the positive electrode active material 100, and the results are shown in Table 1. The details of the procedure for calculating the average coating amount are as described above.

[0160] Next, the positive electrode active material 100, the positive electrode binder (polyvinylidene fluoride), and the positive electrode conductor (graphite) were mixed together to prepare a positive electrode mixture, with the mixing ratio (weight ratio) being positive electrode active material 100:positive electrode binder:positive electrode conductor=96:2:2.

[0161] Next, the positive electrode mixture was added to a solvent (N-methyl-2-pyrrolidone, an organic solvent), and the solvent was stirred to prepare a paste-like positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to one side of a positive electrode current collector 21A (aluminum foil with a thickness of 15 μm) using a coating device, and then the positive electrode mixture slurry was dried to form a positive electrode active material layer 21B.

[0162] Subsequently, the positive electrode active material layer 21B was compression-molded using a roll press. In this case, the area density of the positive electrode active material layer 21B was 18 mg / cm. ２　 Finally, the positive electrode current collector 21A on which the positive electrode active material layer 21B was formed was cut into a disk shape (diameter = 16 mm), thereby producing a test electrode 61 including the positive electrode current collector 21A and the positive electrode active material layer 21B.

[0163] For comparison, a test electrode 61 was fabricated in the same manner except that the covering portion 120 was not formed.

[0164] For comparison, a test electrode 61 was prepared in the same manner except that another compound was used instead of the second lithium composite oxide. As shown in Table 1, the other compound was Li ２　 CoO ２　 and Li having an orthorhombic crystal structure represented by the space group Immmm. ２　 CoO ２　 and Li having an orthorhombic crystal structure represented by the space group C2 / m. ２　 NiO ３　 and NiO having a cubic crystal structure represented by the space group Fm-3m.

[0165] (Preparation of Counter Electrode) A disk-shaped lithium metal plate (thickness: 0.24 mm, diameter: 17 mm) was used as the counter electrode 62 .

[0166] (Preparation of Electrolyte Solution) A solvent (ethylene carbonate, which is a cyclic carbonate ester, and ethyl methyl carbonate, which is a chain ethylene carbonate) was dissolved in an electrolyte salt (lithium salt, lithium hexafluorophosphate (LiPF ６　 In this case, the mixing ratio (mass ratio) of the solvents was ethylene carbonate:ethyl methyl carbonate=30:70, and the content of the electrolyte salt was 1 mol / L (=1 mol / dm ３　 ) Thus, the electrolyte solution was prepared.

[0167] (Assembly of Secondary Battery) First, the test electrode 61 was housed in an exterior cup 64 (SUS304, thickness = 200 μm), and the counter electrode 62 was housed in an exterior can 65 (SUS304, thickness = 200 μm). Next, the test electrode 61 housed in the exterior cup 64 and the counter electrode 62 housed in the exterior can 65 were stacked together via a disc-shaped separator 63 (a microporous polyethylene film, thickness = 15 μm and diameter = 17.5 mm) impregnated with an electrolyte. In this case, the positive electrode active material layer 21B and the negative electrode active material layer 22B faced each other via the separator 63. Finally, with the test electrode 61 and the counter electrode 62 stacked together via the separator 63, the exterior cup 64 and the exterior can 65 were crimped together via a gasket 66 (a polypropylene film, thickness = 0.3 mm). As a result, the test electrode 61 and the counter electrode 62 were sealed inside the exterior cup 64 and the exterior can 65, and thus a secondary battery was assembled.

[0168] (Stabilization of Secondary Battery) First, the secondary battery was charged in a room temperature environment (temperature = 25°C). During charging, the battery was charged at a constant current of 0.1 C until the voltage reached 4.25 V, and then at the same voltage of 4.25 V, the battery was charged at a constant voltage of 0.005 C until the current reached 0.005 C. 0.1 C is the current value at which the battery capacity (theoretical capacity) is fully discharged in 10 hours, and 0.005 C is the current value at which the battery capacity is fully discharged in 20 hours.

[0169] Subsequently, the charged secondary battery was left standing in the same environment (standing time=10 minutes).

[0170] Finally, the secondary battery was discharged in the same environment at a constant current of 0.1 C until the voltage reached 2.0 V.

[0171] As a result, a coating was formed on the surface of each of the test electrode 61 and the counter electrode 62, and the state of the secondary battery was electrochemically stabilized. Thus, the secondary battery was completed.

[0172] [Evaluation of Battery Characteristics] The cycle characteristics and electrical resistance characteristics of the battery were evaluated according to the procedures described below, and the results shown in Table 1 were obtained.

[0173] (Cycle Characteristics) First, the discharge capacity (discharge capacity at the first cycle) was measured by charging and discharging the secondary battery in a constant temperature bath (temperature = 60°C). In this case, a secondary battery that had not been charged or discharged for 3 hours or more before charging and discharging was used. Next, the discharge capacity (discharge capacity at the 100th cycle) was measured by repeatedly charging and discharging the secondary battery in the same environment until the number of cycles reached 100. Finally, the capacity retention rate, which is an index for evaluating cycle characteristics, was calculated based on the formula: capacity retention rate (%) = (discharge capacity at the 100th cycle / discharge capacity at the first cycle) × 100.

[0174] The charge-discharge conditions for one cycle are as described below. That is, when evaluating the cycle characteristics, the process of charging and discharging the secondary battery based on the charge-discharge conditions described below was repeated 100 times.

[0175] First, the secondary battery was charged. In this case, it was charged at a constant current of 1 C until the voltage reached 4.25 V, and then it was charged at a constant voltage of 4.25 V until the current reached 0.01 C. 1 C is the current value at which the battery capacity is fully discharged in 1 hour, and 0.01 C is the current value at which the battery capacity is fully discharged in 100 hours.

[0176] Subsequently, the secondary battery after charging was left to stand (standing time=1 minute) to suspend charging of the secondary battery.

[0177] Subsequently, the charged secondary battery was discharged at a constant current of 5 C until the voltage reached 2.5 V. 5 C is a current value at which the battery capacity is fully discharged in 0.2 hours.

[0178] Finally, the discharged secondary battery was left standing (standing time=5 minutes) to stop discharging the secondary battery.

[0179] The values ​​of the capacity maintenance ratios shown in Table 1 are normalized with the value of the capacity maintenance ratio in the case where the covering portion 120 was not formed (Comparative Example 1) set to 100.

[0180] (Electrical Resistance Characteristics) After the secondary battery was charged and discharged 100 cycles in a thermostatic chamber (temperature = 25°C), the electrochemical impedance (EIS) of the test electrode 61 was measured using an AC impedance method. Based on the EIS measurement results, the electrical resistance (Ω) of the test electrode 61, which is an index for evaluating the electrical resistance characteristics, was calculated. In this case, the semicircular component with a frequency in the range of 500 Hz to 1 Hz was taken as the electrical resistance of the test electrode 61. The charge and discharge conditions were the same as those used to evaluate the cycle characteristics described above.

[0181] The EIS measurement device used was a multi-channel potentiostat VMP-3 manufactured by Bio-Logic Science Instruments, Inc. The measurement conditions were a frequency range of 1 MHz to 0.1 Hz and an AC amplitude of 10 mV.

[0182] The electrical resistance values ​​shown in Table 1 are normalized with the electrical resistance value in the case where the covering portion 120 was not formed (Comparative Example 1) set to 100.

[0183]

[0184] [Discussion] As shown in Table 1, the capacity retention rate and electrical resistance varied significantly depending on the configuration of the positive electrode active material 100. In the following, the capacity retention rate and electrical resistance in the case where the covering portion 120 was not formed (Comparative Example 1) will be used as a comparison standard.

[0185] Specifically, the positive electrode active material 100 includes a core portion 110 and a coating portion 120, and when the coating portion 120 contains other compounds (Comparative Examples 2 to 5), the capacity retention rate increased slightly and, in some cases, the electrical resistance also decreased slightly.

[0186] In contrast, when the positive electrode active material 100 includes the core portion 110 and the coating portion 120, and the coating portion 120 includes the second lithium composite oxide (Examples 1 to 8), the capacity retention rate increased significantly and the electrical resistance also decreased significantly.

[0187] In particular, when the coating portion 120 contained the second lithium composite oxide (Examples 1 to 8), the following tendency was observed.

[0188] First, when the core portion 110 contained the first lithium composite oxide, the capacity retention rate was sufficiently increased and the electrical resistance was sufficiently reduced.

[0189] Second, the average coating amount of the coating portion 120 is 0.01 mmol / m ２　 ~0.05mmol / m ２　 When the capacity retention rate was sufficiently increased, the electrical resistance was sufficiently reduced.

[0190] [Summary] From the results shown in Table 1, it can be seen that when positive electrode active material 100 includes core 110 and coating portion 120, core 110 includes a first lithium composite oxide having a layered rock-salt crystal structure, and coating portion 120 has an orthorhombic crystal structure represented by space group Immm and includes nickel as a constituent element, the capacity retention rate significantly increases and the electrical resistance significantly decreases. Therefore, the cycle characteristics and electrical resistance characteristics are improved, and a secondary battery with excellent battery characteristics is obtained.

[0191] The present technology has been described above with reference to one embodiment and one example, but the configuration of the present technology is not limited to the configuration described in the one embodiment and one example, and can be modified in various ways.

[0192] Specifically, the battery structure of the secondary battery has been described as being of a laminate film type and a coin type. However, the battery structure of the secondary battery is not particularly limited, and may be of a cylindrical type, a square type, a button type, or the like.

[0193] The battery element has been described as having a wound structure. However, the structure of the battery element is not particularly limited, and may be a stacked structure or a zigzag structure. In the stacked structure, the positive and negative electrodes are stacked on top of each other, and in the zigzag structure, the positive and negative electrodes are folded in a zigzag pattern.

[0194] Furthermore, although the electrode reactant is lithium in the above description, the type of the electrode reactant is not particularly limited. Specifically, as described above, the electrode reactant may be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium, and calcium. In addition, the electrode reactant may be other light metals such as aluminum.

[0195] The effects described in this specification are merely examples, and the effects of the present technology are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to the present technology.

[0196] The present technology may also be configured as follows. <1> A secondary battery comprising: a positive electrode containing a positive electrode active material; a negative electrode; and an electrolyte; wherein the positive electrode active material includes a core; and a coating portion coating a surface of the core; the core includes a first lithium composite oxide having a layered rock-salt crystal structure; and the coating portion includes a second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immmm and containing nickel as a constituent element. <2> The secondary battery according to <1>, wherein the second lithium composite oxide includes a compound represented by formula (1). Li ２　 Ni １－ｘ　 M ｘ　 O ２　...(1) (M is at least one of Ti, Pt, Cu, and W, and x satisfies 0≦x≦0.25.) <3> The secondary battery according to <2>, wherein M includes at least one of Cu and W, or includes at least one of Ti, Pt, and Cu. <4> The average coating amount of the coating portion is 0.01 mmol / m ２　 0.05 mmol / m or more ２　 The secondary battery according to any one of <1> to <3>, which is as follows: <5> The secondary battery according to any one of <1> to <4>, which is a lithium-ion secondary battery. <6> A positive electrode for a secondary battery, comprising a positive electrode active material, the positive electrode active material including: a core; and a coating portion coating a surface of the core, wherein the core includes a first lithium composite oxide having a layered rock-salt crystal structure, and the coating portion includes a second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immm and containing nickel as a constituent element. <7> A positive electrode active material for a secondary battery, comprising: a core; and a coating portion coating a surface of the core, wherein the core includes a first lithium composite oxide having a layered rock-salt crystal structure, and the coating portion includes a second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immm and containing nickel as a constituent element.

Claims

1. a positive electrode including a positive electrode active material; a negative electrode; Electrolyte and Equipped with The positive electrode active material is The center and a covering portion that covers the surface of the central portion; Including, the core portion includes a first lithium composite oxide having a layered rock salt crystal structure, the coating portion includes a second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immm and containing nickel as a constituent element; Secondary battery.

2. The second lithium composite oxide contains a compound represented by formula (1): The secondary battery according to claim 1 . Li ２ Ni １－ｘ M ｘ O ２ ・・・(1) (M is at least one of Ti, Pt, Cu, and W. x satisfies 0≦x≦0.25.)

3. The M is Contains at least one of Cu and W, Or, containing at least one of Ti, Pt, and Cu, The secondary battery according to claim 2 .

4. The average coating amount of the coating portion is 0.01 mmol / m ２ 0.05 mmol / m or more ２ Below is the The secondary battery according to any one of claims 1 to 3.

5. It is a lithium-ion secondary battery. The secondary battery according to any one of claims 1 to 3.

6. a positive electrode active material, The positive electrode active material is The center and a covering portion that covers the surface of the central portion; Including, the core portion includes a first lithium composite oxide having a layered rock salt crystal structure, the coating portion includes a second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immm and containing nickel as a constituent element; Positive electrode for secondary batteries.

7. The center and a covering portion that covers the surface of the central portion; Including, the core portion includes a first lithium composite oxide having a layered rock salt crystal structure, the coating portion includes a second lithium composite oxide having an orthorhombic crystal structure represented by the space group Immm and containing nickel as a constituent element; Positive electrode active material for secondary batteries.