Patent Description:
It has been studied to determine a practical limit of a positive electrode active material that has been used conventionally, and expand a practical range to just before the limit to increase the capacity and energy density of the battery. For example, in recent years, development has been underway to raise the upper limit of a use potential of lithium cobalt oxide (LiCoO<NUM>) (that is, to increase a battery charging pressure) to remove and insert more lithium, and thus to increase a charge/discharge capacity.

Since a crystal structure on a surface of positive electrode active material particles has a great influence on a battery reaction, various studies have been conducted on the crystal structure on the surface of the particles. Patent Document <NUM> has studied a technique for providing lithium cobalt oxide having a certain performance by analyzing a structural change of lithium cobalt oxide by Raman spectrum and predicting cycle life characteristics and charge/discharge characteristics.

<CIT> describes a positive electrode acitve material comprising core particles including a lithium cobalt composite oxide and a layer that is provided on surfaces of the core particles and includes a lithium cobalt composite oxide. The lithium cobalt composite oxide included in the core particles and the lithium cobalt composite oxide included in the layer have (almost) the same composition.

In recent years, simultaneous achievement of charge/discharge cycle characteristics and storage characteristics has been desired. In particular, when charging and discharging are repeated by raising the upper limit of the use potential, reactivity at an interface between a positive electrode active material and an electrolytic solution tends to increase as an amount of Li drawn increases, the positive electrode active material and the electrolytic solution tend to deteriorate during charging, and there has been a problem that the charge/discharge cycle characteristics and the storage characteristics significantly deteriorate.

Even with the technique described in Patent Document <NUM>, it is difficult to achieve both the charge/discharge cycle characteristics and the storage characteristics, and it is desired to achieve both of these characteristics.

An object of the present invention is to provide a positive electrode active material and a battery capable of achieving both charge/discharge cycle characteristics and storage characteristics.

In order to solve the above-mentioned problems, the first invention is a positive electrode active material according to claim <NUM>.

In the first invention, the "surface of the positive electrode active material particles" refers to a region having a depth of <NUM> or less from an outermost surface of the positive electrode active material particles.

The second invention is a battery according to claim <NUM>.

According to the present invention, both charge/discharge cycle characteristics and storage characteristics can be achieved.

Embodiments of the present invention will be described in the following order.

According to the findings of the present inventors, in positive electrode active material particles containing a composite oxide having a hexagonal crystal structure, a surface state of the positive electrode active material particles greatly affects charge/discharge cycle characteristics (hereinafter simply referred to as "cycle characteristics") and storage characteristics. Based on the above findings, in order to achieve both cycle characteristics and storage characteristics, the present inventors have intensively studied positive electrode active material particles containing a composite oxide having a hexagonal crystal structure, and the composite oxide including Li, Co, and at least one element M1 selected from the group consisting of Mg, Al and Ti.

As a result, it has been found that the content and crystal structure of at least one element M1 on the surface of the positive electrode active material particles are defined.

Specifically, it has been found that the content and crystal structure are defined as follows. (A) When an atomic ratio (total amount of at least one element M1/Co amount) of an amount of Co on the surface of the positive electrode active material particles and a total amount of at least one element M1 is <NUM> or more and <NUM> or less, the content of the at least one element M1 on the surface of the positive electrode active material particles is defined. (B) When a half-value width of a peak of an A<NUM> vibration mode of the hexagonal crystal structure in a Raman spectrum is <NUM>-<NUM> or more and <NUM>-<NUM> or less, and a peak intensity ratio IA<NUM>-H/IEg of a peak intensity IA<NUM>-H of the A<NUM> vibration mode of the hexagonal crystal structure in the Raman spectrum and a peak intensity IEg of an Eg vibration mode of the hexagonal crystal structure is <NUM> or more and <NUM> or less, the crystal structure on the surface of the positive electrode active material particles is defined.

<FIG> shows an example of a configuration of the positive electrode active material according to the first embodiment of the present invention. The positive electrode active material according to the first embodiment is a positive electrode active material for a so-called non-aqueous electrolyte secondary battery, and contains a powder of surface-coated positive electrode active material particles <NUM>. The surface-coated positive electrode active material particle <NUM> includes a core particle <NUM> and a spinel phase <NUM> present on at least a portion of the surface of the core particle <NUM>. As described above, the presence of the spinel phase <NUM> on at least a portion of the surface of the core particle <NUM> can suppress decomposition of the positive electrode active material in a high temperature environment. In the present specification, "decomposition of the positive electrode active material" means a reaction or the like in which oxygen or the like is released due to elution of transition metal ions such as cobalt and a part of the crystal structure is rendered defective.

A positive electrode potential (vsLi/Li+) in a fully charged state of the battery to which the positive electrode active material according to the first embodiment is applied preferably exceeds <NUM> V, more preferably is <NUM> V or more, and still more preferably <NUM> V or more, particularly preferably <NUM> V or more, and most preferably <NUM> V or more. When the positive electrode active material according to the first embodiment is applied to a battery having a positive electrode potential exceeding <NUM> V in a fully charged state, the effect of achieving both cycle characteristics and storage characteristics becomes particularly remarkable. The upper limit of the positive electrode potential (vsLi/Li+) in the fully charged state is not particularly limited, but is preferably <NUM> V or less, more preferably <NUM> V or less.

Here, a configuration in which the positive electrode active material particle <NUM> includes the spinel phase <NUM> on the surface of the core particle <NUM> will be described. However, the spinel phase <NUM> may not be included. However, from the viewpoint of suppressing the decomposition of the positive electrode active material in a high temperature environment, it is preferable that the positive electrode active material particle <NUM> contain the spinel phase <NUM> on the surface of the core particle <NUM>.

The core particle <NUM> is capable of inserting and extracting lithium, which is an electrode reactant, and contains a composite oxide having a hexagonal-crystal layered rock salt type structure. The composite oxide is a so-called lithium transition metal composite oxide and specifically includes Li, Co, and at least one element M1 selected from the group consisting of Mg, Al and Ti. A lithium transition metal oxide preferably contains Co as a main component. Here, "containing Co as a main component" means that an atomic ratio of Co to a total amount of metal elements contained in the composite oxide is <NUM>% or more.

It is preferable that the composite oxide have an average composition represented by the following formula (<NUM>): LixCoyM1<NUM>-yO<NUM>. (<NUM>) (in the formula (<NUM>), M1 includes at least one element selected from the group consisting of Mg, Al and Ti. x and y satisfy <NUM> ≤ x ≤ <NUM> and <NUM> < y < <NUM>.

Crystallinity on the surface of the core particle <NUM> is preferably lower than crystallinity inside the core particle <NUM>. More specifically, it is preferable that the crystallinity of the positive electrode active material particle <NUM> decrease from the surface of the positive electrode active material particle <NUM> toward the inside. In this case, the crystallinity may gradually change from the surface of the positive electrode active material particle <NUM> toward the inside, or may change rapidly in a stepped manner or the like. When the crystallinity on the surface of the core particle <NUM> is low as described above, a strain of the lattice and crystallite in the core particle <NUM> caused by Li insertion/elimination reaction accompanied by phase transition can be relaxed, so that the cycle characteristics can be further improved. In the present specification, the "surface of the core particle <NUM>" refers to a region having a depth of <NUM> or less from an outermost surface of the core particle <NUM>, and the "inside of the core particle <NUM>" refers to a region having a depth exceeding <NUM> from the outermost surface of the core particle <NUM>.

When the spinel phase <NUM> is present on a portion of the surface of the core particle <NUM> in an island-like or mottled shape, a portion of the surface of the core particle <NUM> may be exposed from the spinel phase <NUM>. When a portion of the surface of the core particle <NUM> is thus exposed, lithium ions can move between the core particle <NUM> and an electrolytic solution without being hindered by the spinel phase <NUM> through this exposed portion. Therefore, an increase in resistance can be suppressed, and the cycle characteristics can be further improved.

The spinel phase <NUM> contains an oxide containing Li and at least one element M1 described above. From the viewpoint of improving the cycle characteristics and the storage characteristics, this oxide includes at least one selected from the group consisting of Mg, Al and Ti among the elements M1. All of the elements M1 contained in the core particle <NUM> and the spinel phase <NUM> may be common, some of the elements M1 contained in the core particle <NUM> and the spinel phase <NUM> may be common, or all of the elements M1 contained in the core particle <NUM> and the spinel phase <NUM> do not have to be common.

It is preferable that the spinel phase <NUM> have an average composition represented by the following formula (<NUM>) : CoxM2<NUM>-xO<NUM>. (<NUM>) (provided that in the formula (<NUM>), M2 includes at least one element selected from the group consisting of Mg, Al, and Ti. x satisfies <NUM> < X < <NUM>.

The crystal structure of the positive electrode active material particles <NUM> may gradually change from the spinel phase (spinel type crystal structure) <NUM> to the hexagonal-crystal layered rock salt type structure from the surface of the positive electrode active material particles <NUM> toward the inside, or may change rapidly in a stepped manner or the like.

The positive electrode active material particles <NUM> preferably further contain a compound containing at least one element M2 selected from the group consisting of S, P and F. When the positive electrode active material particles <NUM> thus further contain the compound containing at least one element M2, it is possible to improve performance deterioration and long-term cycle characteristics particularly during long-term storage.

Since the compound containing at least one element M2 functions outside a crystal system of the composite oxide, it is preferable that the compound be present on at least one of the surface of the positive electrode active material particles <NUM> and a crystal grain boundary of the positive electrode active material particles <NUM>. However, the compound of the element M2 may be present on the surface of the positive electrode active material particles <NUM> and at a portion other than the crystal grain boundary of the positive electrode active material particles <NUM>. When the compound of the element M2 is present on the surface of the positive electrode active material particles <NUM>, the compound of the element M2 may be dotted on the surface of the positive electrode active material particles <NUM>, or the surface of the positive electrode active material particles <NUM> may be covered with the compound of the element M2. Here, in the covering, the surface of the positive electrode active material particles <NUM> may be partially covered, or the entire surface of the positive electrode active material particles <NUM> may be covered.

At least one element M1 is present on the surface of the positive electrode active material particles <NUM> in a specified ratio. Specifically, the atomic ratio (total amount of at least one element M1/Co amount) of the amount of Co on the surface of the positive electrode active material particles <NUM> and the total amount of at least one element M1 is <NUM> or more and <NUM> or less, and preferably <NUM> or more and <NUM> or less. If the atomic ratio (M1/Co) is less than <NUM>, the amount of the element M1 present on the surface of the positive electrode active material particles <NUM> is too small, and a function of stabilizing the crystal structure is deteriorated. Therefore, there is a risk that the cycle characteristics and storage characteristics cannot be achieved simultaneously. On the other hand, when the atomic ratio (M1/Co) exceeds <NUM>, the amount of the element M1 present on the surface of the positive electrode active material particles <NUM> is too large, transition metals such as cobalt are relatively reduced, and conductivity of the particles is lowered. Since a resistance component increases, the cycle characteristics may deteriorate. In the present specification, the "surface of the positive electrode active material particles <NUM>" refers to a region having a depth of <NUM> or less from an outermost surface of the positive electrode active material particles <NUM>. The "inside of the positive electrode active material particles <NUM>" refers to a region having a depth exceeding <NUM> from the outermost surface of the positive electrode active material particles <NUM>.

Since the element M1 functions on the surface side of the positive electrode active material particles <NUM>, the concentration of the element M1 on the surface of the positive electrode active material particles <NUM> is preferably higher than the concentration of the element M1 inside the positive electrode active material particles <NUM>. More specifically, it is preferable that the concentration of the element M1 in the positive electrode active material particle <NUM> decrease from the surface of the positive electrode active material particle <NUM> toward the inside. In this case, the concentration of the element M1 may gradually change from the surface of the positive electrode active material particle <NUM> toward the inside, or may change rapidly in a stepped manner or the like. The composition in the core particle <NUM> is preferably uniform. This is because if the composition in the core particle <NUM> changes, an effect of stabilizing the crystal structure by adding the element M1 varies between a center portion and an outer surface portion of the core particle <NUM>, which may adversely affect the cycle characteristics.

A region (hereinafter referred to as the "high concentration region of the element M1") containing a high concentration of the element M1 on the surface of the positive electrode active material particles <NUM> functions to suppress decomposition of the positive electrode active material in a high temperature environment. However, since the decomposition proceeds near the surface of the particles, it is preferable that the high concentration region of the element M1 be present on the entire surface of the positive electrode active material particles <NUM>. However, even if the high concentration regions of the element M1 are dotted on the surface of the positive electrode active material particles <NUM>, the above-mentioned function of suppressing the decomposition of the positive electrode active material can be sufficiently exerted. Here, the "high concentration region of the element M1" refers to a region in which the atomic ratio (total amount of at least one element M1/Co amount) of the amount of Co and the total amount of at least one element M1 is <NUM> or more and <NUM> or less.

<FIG> shows an example of a Raman spectrum of the positive electrode active material according to the first embodiment of the present invention. In the Raman spectrum of the positive electrode active material, a peak of an Eg vibration mode of a hexagonal crystal structure is observed near <NUM>-<NUM>, and a peak of an A<NUM> vibration mode of the hexagonal crystal structure is observed near <NUM>-<NUM>. The Eg vibration mode belongs to O-Co-O bending, and the A<NUM> vibration mode belongs to Co-O stretching. The peak of the A<NUM> vibration mode of the spinel phase <NUM> is observed in a range of <NUM>-<NUM> or more and <NUM>-<NUM> or less.

A half-value width of the peak of the A<NUM> vibration mode of the hexagonal crystal structure in the Raman spectrum is <NUM>-<NUM> or more and <NUM>-<NUM> or less, preferably <NUM>-<NUM> or more and <NUM>-<NUM> or less. Here, the half-value width means a full width at half maximum (FWHM). If the half-value width is less than <NUM>-<NUM>, the crystallinity of the surface of the positive electrode active material particles <NUM> is high, so that good storage characteristics can be obtained; however, the cycle characteristics may deteriorate. On the other hand, when the half-value width exceeds <NUM>-<NUM>, the crystallinity of the surface of the positive electrode active material particles <NUM> is low, so that good cycle characteristics can be obtained; however, the storage characteristics may deteriorate.

A peak intensity ratio IA<NUM>-H/IEg of a peak intensity IA<NUM>-H of the A<NUM> vibration mode of the hexagonal crystal structure in the Raman spectrum and a peak intensity IEg of the Eg vibration mode of the hexagonal crystal structure is <NUM> or more and <NUM> or less, preferably <NUM> or more and <NUM> or less. If the peak intensity ratio IA<NUM>-H/IEg is less than <NUM>, the crystallinity of the surface of the positive electrode active material particles <NUM> is low, so that good cycle characteristics can be obtained; however, the storage characteristics may deteriorate. On the other hand, when the peak intensity ratio IA<NUM>-H/IEg exceeds <NUM>, the crystallinity of the surface of the positive electrode active material particles <NUM> is high, so that good storage characteristics can be obtained; however, the cycle characteristics may deteriorate.

A peak intensity ratio IA<NUM>-S/IA<NUM>-H of a peak intensity IA<NUM>-S of the A<NUM> vibration mode of the spinel phase <NUM> in the Raman spectrum and a peak intensity IA<NUM>-H of the A<NUM> vibration mode of the hexagonal crystal structure is preferably <NUM> or less. The spinel phase is generated when the amount of Li element on the surface of the positive electrode active material particles <NUM> is small in synthesis of composite oxide particles. However, the positive electrode active material particles <NUM> synthesized in such a state are preferable because a distribution of the added element M1 tends to be particularly uneven near the surface. Therefore, it is preferable that the spinel phase be present to an extent that it can be observed when locally observed by, for example, a transmission electron microscope (TEM). However, in the Raman spectrum, when the peak intensity ratio IAg-S/IAg-H exceeds <NUM>, the amount of the spinel phase <NUM> present on the surface of the core particle <NUM> is too large, so that the increase in resistance may deteriorate the cycle characteristics and the storage characteristics.

The positive electrode active material according to the first embodiment of the present invention is prepared, for example, as follows. First, a powder of composite oxide particles containing Li, Co, and at least one element M1 and having the hexagonal-crystal layered rock salt type structure is prepared. Subsequently, a mixture is obtained by mixing the prepared composite oxide particle powder, lithium carbonate, cobalt oxide, and at least one compound selected from the group consisting of Mg-containing compound, Al-containing compound and Ti-containing compound.

At this time, the compounding ratio of each material is adjusted so that the atomic ratio (total amount of at least one element M1/Co amount) of the amount of Co on the surface of the positive electrode active material particles <NUM> to be obtained finally and the total amount of at least one element M1 is <NUM> or more and <NUM> or less. Instead of lithium carbonate, at least one of lithium phosphate, lithium fluoride and lithium sulfide may be used. Next, the mixture is stirred at high speed, heat-treated under an air stream, for example, and then finely pulverized. As a result, the positive electrode active material according to the first embodiment is obtained.

The processing time for high-speed stirring is preferably <NUM> hours or more and <NUM> hours or less. If the processing time for high-speed stirring is less than <NUM> hours, the peak intensity ratio IA<NUM>-H/IEg may be less than <NUM>. On the other hand, if the processing time for high-speed stirring exceeds <NUM> hours, the peak intensity ratio IA<NUM>-H/IEg may exceed <NUM>.

The heat treatment temperature is preferably <NUM> or higher and <NUM> or lower. If a heat treatment temperature is less than <NUM>, the half-value width of the peak of the A<NUM> vibration mode of the hexagonal crystal structure may be less than <NUM>-<NUM>. On the other hand, if the heat treatment temperature exceeds <NUM>, the half-value width of the peak of the A<NUM> vibration mode of the hexagonal crystal structure may exceed <NUM>-<NUM>.

The positive electrode active material according to the first embodiment contains a powder of the surface-coated positive electrode active material particles <NUM>. The positive electrode active material particle <NUM> includes the core particle <NUM> and the spinel phase <NUM> present on at least a portion of the surface of the core particle <NUM>. The core particles <NUM> include the positive electrode active material particles <NUM> containing a composite oxide having a hexagonal crystal structure. The above composite oxide includes Li, Co, and at least one element M1 selected from the group consisting of Mg, Al and Ti.

The spinel phase <NUM> contains an oxide containing Li and at least one element M1. The atomic ratio (total amount of at least one element M1/Co amount) of the amount of Co on the particle surface and the total amount of at least one element M1 is <NUM> or more and <NUM> or less, the half-value width of the peak of the A<NUM> vibration mode of the hexagonal crystal structure in the Raman spectrum is <NUM>-<NUM> or more and <NUM>-<NUM> or less, and the peak intensity ratio IA<NUM>-H/IEg of the peak intensity IA<NUM>-H of the A<NUM> vibration mode of the hexagonal crystal structure in the Raman spectrum and the peak intensity IEg of the Eg vibration mode of the hexagonal crystal structure is <NUM> or more and <NUM> or less. As a result, both the cycle characteristics and the storage characteristics can be achieved.

Using the positive electrode active material according to the first embodiment of the present invention, it is possible to produce non-aqueous electrolyte secondary batteries (hereinafter, simply referred to as the "battery") of various shapes and sizes. An example of a battery using the positive electrode active material according to the first embodiment of the present invention will be described below.

<FIG> shows an example of a battery configuration according to a second embodiment of the present invention. The battery according to the second embodiment is a so-called laminated battery. In this battery, an electrode body <NUM> having a positive electrode lead <NUM> and a negative electrode lead <NUM> installed therein is housed in a film-shaped outer package material <NUM>, and the battery can be smaller, lighter, and thinner.

Each of the positive electrode lead <NUM> and the negative electrode lead <NUM> goes from an inside of the outer package material <NUM> to an outside of the outer package material <NUM>, and for example, is led out in the same direction. The positive electrode lead <NUM> and the negative electrode lead <NUM> are each made of a metal material such as Al, Cu, Ni, or stainless steel, and have a thin plate shape or a network shape.

The outer package material <NUM> is made of, for example, a rectangular aluminum laminated film obtained by sticking a nylon film, an aluminum foil and a polyethylene film in this order. For example, the outer package material <NUM> is disposed such that a side of the polyethylene film faces the electrode body <NUM>, and outer peripheral portions thereof are in close contact with each other by fusion or an adhesive. An adhesive film <NUM> is inserted between the outer package material <NUM> and each of the positive electrode lead <NUM> and the negative electrode lead <NUM> in order to prevent entrance of the outside air. The adhesive film <NUM> is made of a material having adhesion to each of the positive electrode lead <NUM> and the negative electrode lead <NUM>, for example, a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene.

The outer package material <NUM> may be made of a laminated film having another structure, a polymer film such as polypropylene, or a metal film in place of the above-described aluminum laminated film. Alternatively, the outer package material <NUM> may be made of a laminated film having a polymer film stacked on one surface or each of both surfaces of an aluminum film as a core material.

<FIG> is a cross-sectional view of the electrode body <NUM> shown in <FIG> taken along line IV-IV. The electrode body <NUM> is of a winding type, and has a structure in which a positive electrode <NUM> and a negative electrode <NUM> both having an elongated shape are stacked via a separator <NUM> having an elongated shape and wound in a flat and spiral shape. An outermost periphery of the electrode body <NUM> is protected by a protective tape <NUM>. An electrolytic solution as an electrolyte is injected into the outer package material <NUM> and impregnated in the positive electrode <NUM>, the negative electrode <NUM>, and the separator <NUM>.

Hereinafter, the positive electrode <NUM>, the negative electrode <NUM>, the separator <NUM>, and the electrolytic solution constituting the battery will be sequentially described.

The positive electrode <NUM> includes, for example, a positive electrode current collector 21A and a positive electrode active material layer 21B provided on both surfaces of the positive electrode current collector 21A. The positive electrode current collector 21A is made of, for example, a metal foil such as an aluminum foil, a nickel foil, or a stainless steel foil. The positive electrode active material layer 21B contains a positive electrode active material. The positive electrode active material layer 21B may further contain at least one of a binder and a conductive agent, if necessary.

The positive electrode active material is the positive electrode active material according to the first embodiment described above.

As a binder, for example, at least one selected from the group consisting of resin materials such as polyvinylidene fluoride, polytetrafluoroethylene, polyacrylonitrile, styrene-butadiene rubber, and carboxymethyl cellulose, copolymers mainly containing these resin materials, and the like is used.

As the conductive agent, for example, at least one carbon material selected from the group consisting of graphite, carbon fiber, carbon black, Ketjen black, carbon nanotube, and the like is used. The conductive agent may be a material having conductivity, and is not limited to the carbon material. For example, a metal material, a conductive polymer material, or the like may be used as the conductive agent.

The negative electrode <NUM> includes, for example, a negative electrode current collector 22A and a negative electrode active material layer 22B provided on both surfaces of the negative electrode current collector 22A. The negative electrode current collector 22A is made of, for example, a metal foil such as a copper foil, a nickel foil, or a stainless steel foil.

The negative electrode active material layer 22B contains one or two or more kinds of negative electrode active materials capable of inserting and extracting lithium. The negative electrode active material layer 22B may further contain at least one of a binder and a conductive agent, if necessary.

In this battery, the electrochemical equivalent of the negative electrode <NUM> or the negative electrode active material is greater than the electrochemical equivalent of the positive electrode <NUM>, and it is theoretically preferable that lithium metal do not precipitate on the negative electrode <NUM> during charging.

Examples of the negative electrode active material include carbon materials such as hardly graphitizable carbon, easily graphitizable carbon, graphite, pyrocarbons, cokes, glassy carbons, organic polymer compound burnt body, carbon fiber, and activated carbon. Among these, examples of the cokes include pitch coke, needle coke, petroleum coke and the like. The organic polymer compound burnt body refers to a carbonized material obtained by baking a polymer material such as a phenol resin or a furan resin at an appropriate temperature. Some of such carbonized materials are classified as hardly graphitizable carbon or easily graphitizable carbon. These carbon materials are preferred because they exhibit very little change in their crystal structures during charging and discharging, and provide a high charge/discharge capacity and excellent cycle characteristics. Graphite is especially preferred, as it has a large electrochemical equivalent and provides a high energy density. Further, hardly graphitizable carbon is preferable because it provides excellent cycle characteristics. Furthermore, it is preferable to use a carbon material having a low charge/discharge potential, specifically, a carbon material having a charge/discharge potential that is close to that of lithium metal, because the higher energy density can be easily realized for the battery.

Examples of other negative electrode active materials capable of increasing the capacity include materials containing at least one of a metal element and a metalloid element as a constituent element (for example, an alloy, a compound or a mixture). This is because a high energy density can be obtained by using such a material. In particular, it is more preferred to use such a negative electrode active material together with a carbon material because this enables a high energy density as well as excellent cycle characteristics to be obtained. In the present invention, the alloy includes, in addition to materials made of two or more kinds of metal elements, materials containing one or more kinds of metal elements and one or more kinds of metalloid elements. Further, the alloy may contain a non-metal element. The compositional structure of the alloy includes a solid solution, a eutectic (eutectic mixture), an intermetallic compound, and a material in which two or more kinds of these coexist.

Examples of such a negative electrode active material include a metallic element and a metalloid element capable of forming an alloy with lithium. Specific examples include Mg, B, Al, Ti, Ga, In, Si, Ge, Sn, Pb, Bi, Cd, Ag, Zn, Hf, Zr, Y, Pd and Pt. These materials may be crystalline or amorphous.

The negative electrode active material preferably contains, as a constituent element, a metal element or a metalloid element of 4B group in the short periodical table. The negative electrode active material more preferably contains at least one of Si and Sn as a constituent element. This is because Si and Sn each have a high capability of inserting and extracting lithium, so that a high energy density can be obtained. Examples of such a negative electrode active material include an elemental substance, alloy and compound of Si, an elemental substance, alloy and compound of Sn, and a material partially having one kind or two or more kinds of these.

Examples of Si alloys include an alloy that includes at least one kind selected from the group consisting of Sn, Ni, Cu, Fe, Co, Mn, Zn, In, Ag, Ti, Ge, Bi, Sb, Nb, Mo, Al, P, Ga, and Cr as a second constituent element other than Si. Examples of Sn alloys include an alloy that includes at least one kind selected from the group consisting of Si, Ni, Cu, Fe, Co, Mn, Zn, In, Ag, Ti, Ge, Bi, Sb, Nb, Mo, Al, P, Ga, and Cr as a second constituent element other than Sn.

Examples of Sn compounds or Si compounds include those containing O or C as a constituent element. These compounds may contain the second constituent element described above.

Among them, the Sn-based negative electrode active material preferably contains Co, Sn, and C as constituent elements and has a low crystalline structure or an amorphous structure.

Examples of the other negative electrode active materials include metal oxides and polymer compounds capable of inserting and extracting lithium. Examples of the metal oxides include lithium titanium oxide containing Li and Ti such as lithium titanate (Li<NUM>Ti<NUM>O<NUM>), iron oxide, ruthenium oxide, and molybdenum oxide. Examples of the polymer compounds include polyacetylene, polyaniline, and polypyrrole.

As a binder, the binder as that for the positive electrode active material layer 21B can be used.

As a conductive agent, the conductive agent as that for the positive electrode active material layer 21B can be used.

The separator <NUM> isolates the positive electrode <NUM> and the negative electrode <NUM> from each other to prevent short circuit of a current due to contact between both the electrodes, and allows a lithium ion to pass therethrough. The separator <NUM> is formed of, for example, a porous film made of polytetrafluoroethylene, polyolefin resin (such as polypropylene (PP) or polyethylene (PE)), acrylic resin, styrene resin, polyester resin or nylon resin, or a resin obtained by blending these resins, and may be a laminate of porous films formed of two or more of these resin materials.

Among these, the porous film made of polyolefin is preferable, since such a film has a superior short circuit preventive effect and can improve safety of the battery by shutdown effect. In particular, polyethylene is preferable as a material for constituting the separator <NUM>, since polyethylene can provide shutdown effect in a range of <NUM> or higher and <NUM> or lower and has superior electrochemical stability. Among them, low density polyethylene, high density polyethylene, and linear polyethylene are preferably used because they have an appropriate melting temperature and are easily available. In addition, it is possible to use a material obtained by copolymerizing or blending a resin having chemical stability with polyethylene or polypropylene. Alternatively, the porous film may have a structure of three or more layers in which a polypropylene layer, a polyethylene layer, and a polypropylene layer are sequentially stacked. For example, it is desirable to have a three-layer structure of PP/PE/PP and set a mass ratio [wt%] of PP to PE to PP : PE = <NUM> : <NUM> to <NUM> : <NUM>. Alternatively, from the viewpoint of cost, a single-layer substrate having <NUM> wt% PP or <NUM> wt% PE may be used. A method for producing the separator <NUM> may be wet or dry.

A nonwoven fabric may be used as the separator <NUM>. As the fibers constituting the nonwoven fabric, aramid fibers, glass fibers, polyolefin fibers, polyethylene terephthalate (PET) fibers, nylon fibers, or the like can be used. Alternatively, these two or more kinds of fibers may be mixed to form a nonwoven fabric.

The separator <NUM> may have a configuration including a substrate and a surface layer provided on one or both surfaces of the substrate. The surface layer includes inorganic grains having electrical insulating properties and a resin material that binds the inorganic grains to the surface of the substrate and binds the inorganic grains to each other. The resin material may have, for example, a three-dimensional network structure in which the material is fibrillated and the plurality of fibrils are connected to each other. The inorganic grains are supported on the resin material having the three-dimensional network structure. Further, the surface of the substrate and the inorganic grains may be bound to each other without the resin material being fibrillated. In this case, higher binding properties can be obtained. By providing a surface layer on one surface or both surfaces of the substrate as described above, oxidation resistance, heat resistance and mechanical strength of the separator <NUM> can be enhanced.

The substrate is a porous film through which lithium ions permeate and which is formed of an insulating film having a predetermined mechanical strength. Since an electrolytic solution is held in pores of the substrate, the substrate preferably has such characteristic properties as high resistance to the electrolytic solution, a low reactivity, and low expansibility.

As the material constituting the substrate, the resin material or the nonwoven fabric constituting the separator <NUM> described above can be used.

The inorganic grains include at least one selected from the group consisting of metal oxides, metal nitrides, metal carbides, metal sulfides and the like. As the metal oxides, aluminum oxide (alumina, Al<NUM>O<NUM>), boehmite (hydrated aluminum oxide), magnesium oxide (magnesia, MgO), titanium oxide (titania, TiO<NUM>), zirconium oxide (zirconia, ZrO<NUM>), silicon oxide (silica, SiO<NUM>), yttrium oxide (itria, Y<NUM>O<NUM>) or the like can be preferably used. As the metal nitride, silicon nitride (Si<NUM>N<NUM>), aluminum nitride (AlN), boron nitride (BN), titanium nitride (TiN) and the like can be preferably used. As the metal carbide, silicon carbide (SiC), boron carbide (B<NUM>C) or the like can be preferably used. As the metal sulfide, barium sulfate (BaSO<NUM>) or the like can be preferably used. Among the above-mentioned metal oxides, it is preferable to use alumina, titania (particularly those having a rutile structure), silica or magnesia, and it is more preferable to use alumina.

The inorganic grains may contain minerals such as porous aluminosilicates such as zeolite (M<NUM>/nO·Al<NUM>O<NUM>·xSiO<NUM>·yH<NUM>O, where M is a metallic element, x ≥ <NUM>, and y ≥ <NUM>), laminar silicates, barium titanate (BaTiO<NUM>), or strontium titanate (SrTiO<NUM>). The inorganic grains have oxidation resistance and heat resistance, and a surface layer of a side surface facing the positive electrode containing the inorganic grains has strong resistance to an oxidizing environment in the vicinity of the positive electrode during charging. The shape of the inorganic grains is not particularly limited, and any of a spherical shape, a plate shape, a fiber shape, a cubic shape, a random shape, and the like can be used.

The grain size of the inorganic grains is preferably in a range of <NUM> or more and <NUM> or less. This is because inorganic grains having a grain size less than <NUM> are difficult to obtain, if the grain size is more than <NUM>, a distance between the electrodes becomes large, and the active material cannot be filled in a limited space in a sufficient amount, so that the battery capacity is reduced.

Examples of the resin material constituting the surface layer include fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, fluorine-containing rubbers such as a vinylidene fluoride-tetrafluoroethylene copolymer and an ethylene-tetrafluoroethylene copolymer, rubbers such as a styrene-butadiene copolymer or a hydride thereof, an acrylonitrile-butadiene copolymer or a hydride thereof, an acrylonitrile-butadiene-styrene copolymer or a hydride thereof, a methacrylic acid ester-acrylic acid ester copolymer, a styrene-acrylic acid ester copolymer, an acrylonitrile-acrylic acid ester copolymer, an ethylene propylene rubber, polyvinyl alcohol, and polyvinyl acetate, cellulose derivatives such as ethylcellulose, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose, polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyether imide, polyimide, polyamide such as wholly aromatic polyamide (aramid), polyamideimide, polyacrylonitrile, polyvinyl alcohol, polyether, and a high heat resistant resin having at least one of a melting point and a glass transition temperature of <NUM> or higher, such as an acrylic acid resin or polyester. These resin materials may be used alone or in combination of two or more. Among them, a fluorine-based resin such as polyvinylidene fluoride is preferable from the viewpoint of oxidation resistance and flexibility, and it is preferable to contain aramid or polyamideimide from the viewpoint of heat resistance.

A method of forming the surface layer may include, for example, applying a slurry composed of a matrix resin, a solvent, and inorganic grains onto a substrate (porous membrane), and allowing the coated substrate to pass through a poor solvent of the matrix resin and a bath compatible with the above-described solvent, thereby causing phase separation, and then drying the resulting substrate.

The above-described inorganic grains may be contained as a substrate in the porous film. Further, the surface layer may not include inorganic grains and may be made only of a resin material.

The electrolytic solution is a so-called non-aqueous electrolytic solution, and contains an organic solvent (non-aqueous solvent) and an electrolyte salt dissolved in the organic solvent. In order to improve the battery characteristics, the electrolytic solution may contain known additives. Instead of the electrolytic solution, an electrolyte layer containing an electrolytic solution and a polymer compound serving as a holding material for holding the electrolytic solution therein may be used. In this case, the electrolyte layer may be in the form of a gel.

As the organic solvent, a cyclic carbonate such as ethylene carbonate or propylene carbonate can be used. It is preferable to use one of ethylene carbonate and propylene carbonate, and it is particularly preferable to mix both of these for use. This is because the cycle characteristics can be further improved.

In addition, as the organic solvent, it is preferable to mix a chain carbonate such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, or methyl propyl carbonate in addition to these cyclic carbonates. This is because high ionic conductivity can be obtained.

Further, the organic solvent preferably contains <NUM>,<NUM>-difluoroanisole or vinylene carbonate. This is because <NUM>,<NUM>-difluoroanisole can further improve the discharge capacity, and vinylene carbonate can further improve the cycle characteristics. Thus, use of these compounds in mixture is preferable because the discharge capacity and the cycle characteristics can be further improved.

In addition to these compounds, examples of the organic solvent include butylene carbonate, γ-butyrolactone, γ-valerolactone, <NUM>,<NUM>-dimethoxyethane, tetrahydrofuran, <NUM>-methyltetrahydrofuran, <NUM>,<NUM>-dioxolane, <NUM>-methyl-<NUM>,<NUM>-dioxolane, methyl acetate, methyl propionate, acetonitrile, glutaronitrile, adiponitrile, methoxy acetonitrile, <NUM>-methoxy propylonitrile, N,N-dimethylformamide, N-methylpyrrolidinone, N-methyloxazolidinone, N,N-dimethylimidazolidinone, nitromethane, nitroethane, sulfolane, dimethyl sulfoxide, and trimethyl phosphate.

A compound obtained by replacing at least some hydrogen atoms in these organic solvents with fluorine atoms may be preferable because such a compound may improve reversibility of electrode reaction depending on the types of electrodes to be combined.

Examples of the electrolyte salt include lithium salts, and the lithium salts can be used singly or in mixture of two or more kinds thereof. Examples of the lithium salt include LiPF<NUM>, LiBF<NUM>, LiAsF<NUM>, LiClO<NUM>, LiB(C<NUM>H<NUM>)<NUM>, LiCH<NUM>SO<NUM>, LiCF<NUM>SO<NUM>, LiN(SO<NUM>CF<NUM>)<NUM>, LiC(SO<NUM>CF<NUM>)<NUM>, LiAlCl<NUM>, LiSiF<NUM>, LiCl, lithium difluoro[oxolato-<NUM>,<NUM>']borate, lithium bis(oxalate)borate, and LiBr. Among these lithium salts, LiPF<NUM> is preferable because LiPF<NUM> can provide high ionic conductivity and can further improve cycle characteristics.

In the battery having the above-described configuration, when charging is performed, for example, lithium ions are extracted from the positive electrode active material layer 21B and inserted into the negative electrode active material layer 22B through the electrolytic solution. On the other hand, when discharging is performed, for example, lithium ions are extracted from the negative electrode active material layer 22B and inserted into the positive electrode active material layer 21B through the electrolytic solution.

Next, an example of a method of manufacturing a battery according to the second embodiment of the present invention will be described.

The positive electrode <NUM> is prepared as follows. First, for example, the positive electrode active material, a conductive agent and a binder are mixed to prepare a positive electrode mixture, and the positive electrode mixture is dispersed in a solvent such as N-methyl-<NUM>-pyrrolidone (NMP), thus producing a paste-like positive electrode mixture slurry. Then, the positive electrode mixture slurry is applied on the positive electrode current collector 21A, the solvent is dried, and the dried mixture is compression molded with a rolling press machine or the like, so that the positive electrode active material layer 21B is formed, and the positive electrode <NUM> is obtained.

The negative electrode <NUM> is prepared as follows. First, for example, a negative electrode active material and a binder are mixed to prepare a negative electrode mixture, and the negative electrode mixture is dispersed in a solvent such as N-methyl-<NUM>-pyrrolidone, thus producing a paste-like negative electrode mixture slurry. Then, the negative electrode mixture slurry is applied on the negative electrode current collector 22A, the solvent is dried, and the dried mixture is compression molded with a rolling press machine or the like, so that the negative electrode active material layer 22B is formed, and the negative electrode <NUM> is obtained.

The winding type electrode body <NUM> is produced as follows. First, the positive electrode lead <NUM> is attached to one end of the positive electrode current collector 21A by welding, and the negative electrode lead <NUM> is attached to one end of the negative electrode current collector 22A by welding. Next, the positive electrode <NUM> and the negative electrode <NUM> are wound around a flat winding core via the separator <NUM>, wound many times in a longitudinal direction, and then the protective tape <NUM> is adhered to the outermost periphery to obtain the electrode body <NUM>.

The electrode body <NUM> is sealed with the outer package material <NUM> as follows. Then, the electrode body <NUM> is sandwiched between the outer package materials <NUM>, and the outer peripheral edges excluding one side are subjected to heat fusion to form a bag shape, and then the electrode body <NUM> is housed in the outer package material <NUM>. At this time, the adhesive film <NUM> is inserted between the outer package material <NUM> and each of the positive electrode lead <NUM> and the negative electrode lead <NUM>. The adhesive film <NUM> may be attached to the positive electrode lead <NUM> and the negative electrode lead <NUM> in advance. Next, the electrolytic solution is injected into the outer package material <NUM> from one unfused side, and then the unfused side is subjected to heat fusion in a vacuum atmosphere to be sealed. Accordingly, the battery shown in <FIG> and <FIG> is obtained.

In the battery according to the second embodiment, since the positive electrode active material layer 21B contains the positive electrode active material according to the first embodiment, both the cycle characteristics and the storage characteristics can be achieved. In particular, both the cycle characteristics and the storage characteristics can be achieved in a high temperature environment.

When the positive electrode potential (vsLi/Li +) of the battery in the fully charged state exceeds <NUM> V, in addition to the above-mentioned effects, both the cycle characteristics and the storage characteristics can be achieved.

An electronic device including the battery according to the second embodiment described above will be described.

<FIG> shows an example of a configuration of an electronic device <NUM>.

The electronic device <NUM> includes an electronic circuit <NUM> of an electronic device body and a battery pack <NUM>. The battery pack <NUM> is electrically connected to the electronic circuit <NUM> via a positive electrode terminal 331a and a negative electrode terminal 331b. The electronic device <NUM> may have a configuration in which the battery pack <NUM> is detachable.

Examples of the electronic device <NUM> include a notebook personal computer, a tablet computer, a mobile phone (for example, a smart phone), a personal digital assistant (PDA), a display device (for example, an LCD (Liquid Crystal Display), an EL (Electro Luminescence) display, or electronic paper), an imaging device (for example, a digital still camera or a digital video camera), an audio device (for example, a portable audio player), a game device, a cordless handset phone machine, an electronic book, an electronic dictionary, a radio, a headphone, a navigation system, a memory card, a pacemaker, a hearing aid, an electric tool, an electric shaver, a refrigerator, an air conditioner, a television, a stereo, a water heater, a microwave oven, a dishwasher, a washing machine, a dryer, a lighting device, a toy, a medical device, a robot, a load conditioner, and a traffic signal. However, the electronic device <NUM> is not limited thereto.

For example, the electronic circuit <NUM> includes a CPU (Central Processing Unit), a peripheral logic unit, an interface unit, and a storage unit, and controls the entire electronic device <NUM>.

The battery pack <NUM> includes an assembled battery <NUM> and a charge-discharge circuit <NUM>. The battery pack <NUM> may further include an outer package material (not shown) that houses the assembled battery <NUM> and the charge-discharge circuit <NUM>, if necessary.

The assembled battery <NUM> is formed by connecting a plurality of secondary batteries 301a to each other in series and/or in parallel. For example, the plurality of secondary batteries 301a are connected to each other in n parallel m series (each of n and m is a positive integer). <FIG> shows an example in which six secondary batteries 301a are connected to each other in <NUM> parallel <NUM> series (2P3S). As the secondary battery 301a, the battery according to the second embodiment described above is used.

The case where the battery pack <NUM> includes the assembled battery <NUM> formed by the plurality of secondary batteries 301a will be described. However, the battery pack <NUM> may include the single secondary battery 301a instead of the assembled battery <NUM>.

The charge-discharge circuit <NUM> is a controller that controls charging and discharging of the assembled battery <NUM>. Specifically, during charging, the charge-discharge circuit <NUM> controls charging to the assembled battery <NUM>. On the other hand, during discharging (that is, during use of the electronic device <NUM>), the charge-discharge circuit <NUM> controls discharging to the electronic device <NUM>.

As the outer package material, for example, a case made of a metal, a polymer resin, a composite material thereof, or the like can be used. Examples of the composite material include a laminate in which a metal layer and a polymer resin layer are stacked.

Hereinafter, the present invention will be described specifically with examples, but the present invention is not limited only to the examples.

First, commercially available lithium carbonate, cobalt oxide, aluminum hydroxide, and magnesium carbonate were mixed with each other in such a manner that a molar ratio of Li, Co, Mg and Al was <NUM> : <NUM> : <NUM> : <NUM>, and thus a mixture was obtained. Next, this mixture was fired in air at <NUM> for <NUM> hours and slow-cooled to obtain a powder of Mg and Al-containing LiCoO<NUM> particles having an average particle size of <NUM> and a specific surface area of <NUM><NUM>/g.

First, commercially available lithium carbonate, cobalt oxide, magnesium carbonate, aluminum hydroxide, and titanium oxide were mixed with each other in such a manner that a molar ratio of Li, Co, Mg, Al and Ti was <NUM> : <NUM> : <NUM> : <NUM> : <NUM>, and thus a mixture was obtained. Next, <NUM>% by mass of this mixture was added to <NUM>% by mass of the powder of LCO<NUM> particles (base material) obtained in the process (<NUM>), treated with a high-speed stirrer for <NUM> hours, then fired at <NUM> under an air stream for <NUM> hours, and finely pulverized with a ball mill. As a result, a powder of positive electrode active material particles in which Mg, Al and Ti were present on the surface at high concentrations was obtained.

A powder of positive electrode active material particles in which Mg, Al, and Ti were present on the surface at high concentrations was obtained in the same manner as in Example <NUM> except that the treatment time with the high-speed stirrer in the process (<NUM>) was <NUM> hours.

A powder of positive electrode active material particles in which Mg, Al, and Ti were present on the surface at high concentrations was obtained in the same manner as in Example <NUM> except that the firing temperature in the process (<NUM>) was <NUM>.

A powder of positive electrode active material particles in which Mg, Al, and Ti were present on the surface at high concentrations was obtained in the same manner as in Example <NUM> except that the molar ratio of Li, Co, Mg, Al, and Ti in the process (<NUM>) was <NUM> : <NUM> : <NUM>.

A powder of positive electrode active material particles in which Al and Ti were present on the surface at high concentrations was obtained in the same manner as in Example <NUM> except that in the process (<NUM>), commercially available lithium carbonate, cobalt oxide, aluminum hydroxide, and titanium oxide were mixed with each other in such a manner that a molar ratio of Li, Co, Al and Ti was <NUM> : <NUM> : <NUM> : <NUM>, and thus a mixture was obtained.

A powder of positive electrode active material particles in which Mg and Al were present on the surface at high concentrations was obtained in the same manner as in Example <NUM> except that in the process (<NUM>), commercially available lithium carbonate, cobalt oxide, magnesium carbonate, and aluminum hydroxide were mixed with each other in such a manner that a molar ratio of Li, Co, Mg and Al was <NUM> : <NUM> : <NUM> : <NUM>, and thus a mixture was obtained.

A powder of positive electrode active material particles in which Mg and Ti were present on the surface at high concentrations was obtained in the same manner as in Example <NUM> except that in the process (<NUM>), commercially available lithium carbonate, cobalt oxide, magnesium carbonate, and titanium oxide were mixed with each other in such a manner that a molar ratio of Li, Co, Mg and Ti was <NUM> : <NUM> : <NUM> : <NUM>, and thus a mixture was obtained.

A powder of positive electrode active material particles in which Al was present on the surface at a high concentration was obtained in the same manner as in Example <NUM> except that in the process (<NUM>), commercially available lithium carbonate, cobalt oxide, and aluminum hydroxide were mixed with each other in such a manner that a molar ratio of Li, Co, and Al was <NUM> : <NUM> : <NUM>, and thus a mixture was obtained.

A powder of positive electrode active material particles in which Ti was present on the surface at a high concentration was obtained in the same manner as in Example <NUM> except that in the process (<NUM>), commercially available lithium carbonate, cobalt oxide, and titanium oxide were mixed with each other in such a manner that a molar ratio of Li, Co, and Ti was <NUM> : <NUM> : <NUM>, and thus a mixture was obtained.

A powder of positive electrode active material particles in which Mg was present on the surface at a high concentration was obtained in the same manner as in Example <NUM> except that in the process (<NUM>), commercially available lithium carbonate, cobalt oxide, and magnesium carbonate were mixed with each other in such a manner that a molar ratio of Li, Co, and Mg was <NUM> : <NUM> : <NUM>, and thus a mixture was obtained.

A powder of positive electrode active material particles in which Mg, Al, and Ti were present on the surface at high concentrations and P was also present on the surface was obtained in the same manner as in Example <NUM> except that lithium phosphate was used instead of lithium carbonate in the process (<NUM>).

A powder of positive electrode active material particles in which Mg, Al, and Ti were present on the surface at high concentrations and F was also present on the surface was obtained in the same manner as in Example <NUM> except that lithium fluoride was used instead of lithium carbonate in the process (<NUM>).

A powder of positive electrode active material particles in which Mg, Al, and Ti were present on the surface at high concentrations and S was also present on the surface was obtained in the same manner as in Example <NUM> except that lithium sulfide was used instead of lithium carbonate in the process (<NUM>).

A powder of positive electrode active material particles in which Mg, Al, and Ti were present on the surface at high concentrations was obtained in the same manner as in Example <NUM> except that in the process (<NUM>), <NUM> ppm of LiAlMg-containing cobalt oxide prepared by the method shown below was mixed during the treatment with a high-speed stirrer.

Amounts corresponding to <NUM> parts by mass of lithium carbonate, <NUM> part by mass of magnesium oxide, and <NUM> part by mass of aluminum oxide based on <NUM> parts by mass of cobalt carbonate were mixed, then fired at <NUM> under an air stream for <NUM> hours, and finely pulverized with a ball mill, and thus LiMgAl-containing cobalt oxide was synthesized. When the obtained fine powder was analyzed by XRD (X-ray diffraction), it was confirmed that the fine powder had a spinel phase.

In the process (<NUM>), commercially available lithium carbonate and cobalt oxide were mixed with each other in such a manner that a molar ratio of Li and Co was <NUM> : <NUM>, and thus a mixture was obtained. In the process (<NUM>), commercially available lithium carbonate and cobalt oxide were mixed with each other in such a manner that the molar ratio of Li and Co was <NUM> : <NUM>, and thus a mixture was obtained. A powder of positive electrode active material particles was obtained in the same manner as in Example <NUM> except for the above.

A powder of positive electrode active material particles in which Mg, Al, and Ti were present on the surface at high concentrations was obtained in the same manner as in Example <NUM> except that the treatment time with the high-speed stirrer in the process (<NUM>) was <NUM> hour.

A powder of positive electrode active material particles in which Mg, Al, and Ti were present on the surface at high concentrations was obtained in the same manner as in Example <NUM> except that the molar ratio of Li, Co, Mg, Al, and Ti in the process (<NUM>) was <NUM> : <NUM> : <NUM> : <NUM> : <NUM>.

The powder of the positive electrode active material particles obtained as described above was evaluated as follows.

First, a scanning X-ray photoelectron spectrometer (Quantera SXM) manufactured by ULVAC-PHI, INCORPORATED, was used as a measuring device, and measurement was performed under the following measurement conditions. X-ray source: Monochromatized Al-Kα (<NUM> eV).

Next, from peak areas of all detected elements, a surface atomic concentration was calculated using a relative sensitivity factor provided by ULVAC-PHI, INCORPORATED. , and an atomic ratio (total amount of M1/Co amount) of a total amount of the element M1 (at least one element M1 of Mg, Al and Ti) to the amount of Co was calculated.

First, RAMAN-<NUM> manufactured by Nanophoton Corporation was used as a measuring device, and the Raman spectrum was measured under the following measurement conditions.

Next, in the measured Raman spectrum (see <FIG>), the half-value width (FWHM) of the peak of the A<NUM> vibration mode of the hexagonal crystal structure was calculated by Gaussian fitting. Further, the peak intensity ratio IA<NUM>-H/IEg of the peak intensity IA<NUM>-H of the A<NUM> vibration mode of the hexagonal crystal structure and the peak intensity IEg of the Eg vibration mode of the hexagonal crystal structure was calculated. Furthermore, the peak intensity ratio IA<NUM>-S/IA<NUM>-H of the peak intensity IA<NUM>-S of the A<NUM> vibration mode of the spinel phase in the Raman spectrum and the peak intensity IA<NUM>-H of the A<NUM> vibration mode of the hexagonal crystal structure was calculated.

Using the powder of the positive electrode active material particles obtained as described above, a battery was produced as follows.

The positive electrode was prepared as follows. First, a positive electrode mixture was prepared by mixing <NUM>% by weight of positive electrode active material (powder of positive electrode active material particles), <NUM>% by weight of amorphous carbon powder (Ketjen black), and <NUM>% by weight of polyvinylidene fluoride (PVdF). Subsequently, this positive electrode mixture was dispersed in N-methyl-<NUM>-pyrrolidone (NMP) to prepare a positive electrode mixture slurry, and then the positive electrode mixture slurry was uniformly applied to a positive electrode current collector made of a band-shaped aluminum foil to form a coating layer. Next, the coating layer was dried with warm air, then punched to ϕ15 mm and subjected to compression molding with a hydraulic press machine, whereby a positive electrode was prepared.

The negative electrode was prepared as follows. First, <NUM>% by weight of graphite powder and <NUM>% by weight of PVdF were mixed to prepare a negative electrode mixture. Next, this negative electrode mixture was dispersed in N-methyl-<NUM>-pyrrolidone to prepare a negative electrode mixture slurry, and then the negative electrode mixture slurry was uniformly applied to a negative electrode current collector made of a band-shaped copper foil to form a coating layer. Next, the coating layer was dried with warm air, then punched to ϕ16 mm and subjected to compression molding with a hydraulic press machine, whereby a negative electrode was prepared.

Using the positive electrode and the negative electrode prepared as described above, a battery was produced as follows. First, an electrode body was prepared by stacking a positive electrode and a negative electrode with a porous polyolefin film interposed therebetween. Subsequently, ethylene carbonate and propylene carbonate were mixed such that a volume mixing ratio was <NUM> : <NUM> to prepare a mixed solution. Next, LiPF<NUM> was dissolved in this mixed solution to a concentration of <NUM> mol/dm<NUM> to prepare a non-aqueous electrolytic solution. Finally, a CR2032 coin-type battery was produced using the above-mentioned electrode body and electrolytic solution.

A cycle retention (cycle characteristics) of the battery produced as described above was determined as follows. First, the battery was charged under conditions of an environmental temperature of <NUM>, a charging voltage of <NUM> V, a charging current of <NUM> mA, and a charging time of <NUM> hours, and then discharged under conditions of a discharge current of <NUM> mA and a final voltage of <NUM> V, and an initial discharge capacity was measured. Next, charging and discharging were repeated under the same charging and discharging conditions as when the initial discharge capacity was determined. The discharge capacity at the 500th cycle was measured, and the cycle retention with respect to the initial discharge capacity was calculated by the following formula: "cycle retention" (%) = ("500th cycle discharge capacity"/"initial discharge capacity") × <NUM> (%).

First, a battery was produced as in the evaluation of the cycle characteristics evaluation described above. Subsequently, the produced battery was charged under the same charging conditions as when the initial discharge capacity was determined in the above-mentioned cycle characteristics evaluation, and stored at an environmental temperature of <NUM> for <NUM> days. After storage, the battery was discharged under the same discharging conditions as when the initial discharge capacity was determined in the above-mentioned cycle characteristics evaluation. The discharge capacity after storage at <NUM> for <NUM> days was measured, and a storage retention for the initial discharge capacity was calculated by the following formula. The initial discharge capacity was determined as in the cycle characteristics evaluation described above. "Storage retention" (%) = ("discharge capacity after storage at <NUM> for <NUM> days"/"initial discharge capacity") × <NUM> (%).

Next, the battery after storage was disassembled, and the negative electrode was taken out. Subsequently, the taken-out negative electrode was boiled in <NUM> of <NUM> hydrochloric acid for <NUM> minutes, the solution was filtered, and the concentration of Co contained in the solution was measured by a sequential ICP emission spectrophotometer (SPS3100, manufactured by Hitachi High-Tech Science Corporation). Then, the amount of Co eluted during storage was measured from the following formula: Co elution amount = (Co concentration)/(weight of active material contained in positive electrode).

Next, the measured Co elution amount of each Example and each Comparative Example was converted into a relative value with the Co elution amount of Example <NUM> defined as <NUM>.

Table <NUM> shows the configurations and evaluation results of the positive electrode active materials of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM>-<NUM>.

In Table <NUM>, in the column of the evaluation result of the peak intensity ratio IA<NUM>-S/IA<NUM>-H, the notation of "< <NUM>" means "IA<NUM>-S/IA<NUM>-H < <NUM>".

The following can be seen from the above evaluation results. Since the positive electrode active materials of Examples <NUM> to <NUM> have the following configurations (<NUM>) to (<NUM>), a high cycle retention, a high storage retention, and a reduction in Co elution can be achieved in a high temperature environment. That is, both the cycle characteristics and the storage characteristics can be achieved in a high temperature environment. (<NUM>) The lithium transition metal composite oxide includes Li, Co, and at least one element M1 selected from the group consisting of Mg, Al, and Ti, and the at least one element M1 is present on the surface of the positive electrode active material particles. (<NUM>) The atomic ratio (total amount of at least one element M1 described above/Co amount) of the amount of Co on the surface of the particles and the total amount of at least one element M1 described above is <NUM> or more and <NUM> or less. (<NUM>) The half-value width of the peak of the A<NUM> vibration mode of the hexagonal crystal structure in the Raman spectrum is <NUM>-<NUM> or more and <NUM>-<NUM> or less. (<NUM>) The peak intensity ratio IA<NUM>-H/IEg of the peak intensity IA<NUM>-H of the A<NUM> vibration mode of the hexagonal crystal structure in the Raman spectrum and the peak intensity IEg of the Eg vibration mode of the hexagonal crystal structure is <NUM> or more and <NUM> or less.

Since the positive electrode active materials of Examples <NUM> to <NUM> further have the following configuration (<NUM>) in addition to the above configurations (<NUM>) to (<NUM>), the cycle characteristics and the storage characteristics in a high temperature environment can be further improved. (<NUM>) The peak intensity ratio IA<NUM>-S/IA<NUM>-H of the peak intensity IA<NUM>-S of the A<NUM> vibration mode of the spinel phase in the Raman spectrum and the peak intensity IA<NUM>-H of the A<NUM> vibration mode of the hexagonal crystal structure is <NUM> or less.

On the other hand, since the positive electrode active materials of Comparative Examples <NUM> to <NUM>-<NUM> do not have at least one of the above configurations (<NUM>) to (<NUM>), the cycle characteristics and the storage characteristics cannot be simultaneously achieved in a high temperature environment.

The first and second embodiments of the present invention have been specifically described above; however, the present invention is not limited to the above-described first and second embodiments. Various modifications of the present invention can be made.

Claim 1:
A positive electrode active material obtainable by a method of preparing the positive electrode active material, the positive electrode active material comprising a positive electrode active material particle containing a composite oxide having a hexagonal crystal structure,
wherein the composite oxide has an average composition represented by the following formula (<NUM>):

        LixCoyM1<NUM>-yO<NUM> ...     (<NUM>)

and includes Li, Co, and at least one element M1 selected from the group consisting of Mg, Al, and Ti, wherein x and y satisfy <NUM> ≤ x ≤ <NUM> and <NUM> < y < <NUM>,
the at least one element M1 is present on a surface of the positive electrode active material particle,
an atomic ratio (total amount of the at least one element M1/Co amount) of an amount of Co on the surface of the positive electrode active material particle and a total amount of the at least one element M1 is <NUM> or more and <NUM> or less,
a half-value width of a peak of an A<NUM> vibration mode of the hexagonal crystal structure in a Raman spectrum is <NUM>-<NUM> or more and <NUM>-<NUM> or less, wherein the half-value width is a full width at half maximum, and
a peak intensity ratio IA<NUM>-H/IEg of a peak intensity IA<NUM>-H of the A<NUM> vibration mode of the hexagonal crystal structure in the Raman spectrum and a peak intensity IEg of an Eg vibration mode of the hexagonal crystal structure is <NUM> or more and <NUM> or less,
wherein the Raman spectrum is measurable as indicated in the description,
the method of preparing the positive electrode active material comprising:
preparing a powder of composite oxide particles containing Li, Co, and the at least one element M1 and having the hexagonal crystal structure,
obtaining a mixture by mixing the prepared composite oxide particle powder, a lithium salt, cobalt oxide, and at least one compound selected from the group consisting of a Mg-containing compound, an Al-containing compound, and a Ti-containing compound and adjusting, at this time, a compounding ratio of each material so that the atomic ratio of the amount of Co on the surface of the positive electrode active material particles and the total amount of at least one element M1 is <NUM> or more and <NUM> or less, wherein the lithium salt is lithium carbonate or at least one of lithium phosphate, lithium fluoride and lithium sulfide,
stirring the mixture at high speed, and
heat-treating and finely pulverizing the mixture to obtain the positive electrode active material.