JP2010118179A - Lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium-ion secondary battery with an increase of battery resistance restrained and exhibiting superb battery characteristics even after repeated charge and discharge cycles. <P>SOLUTION: The lithium-ion secondary battery 10 includes a positive electrode sheet 13 having a positive electrode active material 12 containing transition metal formed on a collector 11, a negative electrode sheet 18 having a negative electrode active material 17 formed, and nonaqueous electrolyte solution 20 filling a space between the positive electrode and the negative electrode. The battery has a photoelectron spectrum of phosphorus by the X-ray photoelectron spectroscopy (XPS) on a positive electrode surface within a range of 133 eV to 137 eV, and a ratio of Ap/At is to be within 0.1 to 1.0, provided, a ratio of the atom number of phosphorus by the XPS on the positive electrode surface is Ap (at%), and a ratio of the sum of the atom number of transition metal elements is At (at%). With the battery, a compound containing phosphorus is considered to be formed on the surface of the positive electrode active material 12 in a film-like or an adsorbed state with a thickness less than 10 nm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery.

Conventionally, as a lithium ion secondary battery, a battery including a positive electrode active material in which a layer containing boron, phosphorus, or nitrogen is formed on the surface of each particle constituting the transition metal composite oxide has been proposed. (For example, see Patent Documents 1 to 3). In the batteries of Patent Documents 1 to 3, even when the battery temperature rises due to a short circuit or the like, the electrolytic solution is hardly decomposed.
JP-A-7-192720 JP-A-2005-190996 JP 2006-318815 A

  However, in the lithium ion secondary batteries described in Patent Documents 1 to 3, since the layer containing phosphorus or the like is formed relatively thick on the surface of the transition metal composite oxide, for example, the battery resistance is high, and the battery output There was a problem of adversely affecting itself.

  The present invention has been made in view of such problems, and it is a main object of the present invention to provide a lithium ion secondary battery that further suppresses an increase in battery resistance and exhibits good battery characteristics even after repeated charge and discharge cycles. Objective.

  As a result of diligent research to achieve the above-mentioned object, the present inventors have found that the photoelectron spectrum of phosphorous by X-ray photoelectron spectroscopy on the positive electrode surface capable of occluding and releasing lithium ions and containing a transition metal is 133 eV or more and 137 eV or less. When the ratio of the number of phosphorus atoms by X-ray photoelectron spectroscopy on the positive electrode surface is Ap (at%) and the ratio of the total number of atoms of the transition metal element is At (at%), Ap / Assuming that a positive electrode having an At value in the range of 0.1 or more and 1.0 or less is used, it is found that the increase in battery resistance is further suppressed and good battery characteristics are exhibited even after repeated charge and discharge cycles. The present invention has been completed.

That is, the lithium ion secondary battery of the present invention is
A negative electrode having a negative electrode active material capable of inserting and extracting lithium ions;
It has a positive electrode active material capable of occluding and releasing lithium ions and containing a transition metal, the photoelectron spectrum of phosphorus by X-ray photoelectron spectroscopy on the positive electrode surface is in the range of 133 eV to 137 eV, and the X-ray on the positive electrode surface When the ratio of the number of phosphorus atoms by photoelectron spectroscopy is Ap (at%) and the ratio of the total number of atoms of the transition metal element is At (at%), Ap / At is 0.1 or more and 1.0 or less. A positive electrode in range;
An ion conductive medium that is interposed between the positive electrode and the negative electrode and conducts lithium ions;
It is equipped with.

This lithium ion secondary battery further suppresses an increase in battery resistance and exhibits good battery characteristics even after repeated charge and discharge cycles. The reason why such an effect is obtained is not clear, but is presumed as follows. For example, the value of Ap / At is in the range of 0.1 to 1.0, and the presence of phosphorus on the surface of the positive electrode active material causes reaction of the positive electrode such as charge transfer reaction and desolvation reaction of the positive electrode. It can be considered that the rate-limiting elementary process can be performed smoothly. In addition, the presence of phosphorus-containing material on the surface of the positive electrode active material in a film state or adsorbed state causes the release of oxygen contained in the positive electrode active material during repeated charge and discharge, and the resulting reduction in the valence of the transition metal. It is presumed that good battery characteristics are exhibited even after charging and discharging cycles are repeated. At this time, for example, when a compound containing phosphorus is added to the positive electrode active material to form a thick layer on the surface of the positive electrode active material, battery resistance increases. In the lithium ion secondary battery of the present invention, it is considered that the thickness of the compound layer containing phosphorus can be made suitable to suppress an increase in battery resistance. Further, the photoelectron spectrum of phosphorus by X-ray photoelectron spectroscopy is in the range of 133 eV to 137 eV, that is, for example, a compound of phosphorus and oxygen and a compound of phosphorus, oxygen and fluorine (PO ₂ , PO ₃ , PO ₂ F ₂ , PO ₃ F, LiPO ₃ F), and the like, and the presence of phosphorus on the surface of the positive electrode active material is presumed to have the effect of the present invention.

  The lithium ion secondary battery of the present invention includes a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode having a negative electrode active material capable of occluding and releasing lithium ions, and interposed between the positive electrode and the negative electrode. And an ion conduction medium that conducts lithium ions.

The positive electrode of the lithium ion secondary battery of the present invention is prepared by mixing a positive electrode active material, a conductive material, and a binder, and adding a suitable solvent to form a paste-like positive electrode material on the surface of the current collector. It may be formed by coating and drying, and compressing to increase the electrode density as necessary. As the positive electrode active material, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, or the like can be used. Specifically, transition metal sulfides such as TiS ₂ , TiS ₃ , MoS ₃ , FeS ₂ , Li _(1-x) MnO ₂ (0 <x <1, etc., the same shall apply hereinafter), Li _(1-x) Mn Lithium manganese composite oxide such as ₂ O ₄ , lithium cobalt composite oxide such as Li _(1-x) CoO ₂ , lithium nickel composite oxide such as Li _(1-x) NiO ₂ , lithium such as LiV ₂ O ₃ Vanadium composite oxides, transition metal oxides such as V ₂ O _{5, and the} like can be used. Of these, lithium transition metal composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiV ₂ O ₃ are preferable. A lithium nickel composite oxide such as Li _(1-x) NiO ₂ to which Co, Al or Mg is added is also suitable. Of these, the positive electrode is more preferably basic composition formula is _{Li a Ni x Co y Me z} O b positive active compound represented by the material. In this basic composition formula, Me represents one or more selected from the group consisting of Mn, Al, Mg, Ti, V, Cu, Zn, Cr, Zr, Sr, and Si, and a, b, x, y, and z 0.9 ≦ a ≦ 1.1, 1.9 ≦ b ≦ 2.2, 0.3 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.5, 0.01 ≦ z ≦ 0.5 , X + y + z = 1. In this way, the battery performance can be further improved.

In the positive electrode of the lithium ion secondary battery of the present invention, the photoelectron spectrum of phosphorus by X-ray photoelectron spectroscopy on the positive electrode surface is in the range of 133 eV to 137 eV. Thus, phosphorus is present on the positive electrode surface as a compound of phosphorus and oxygen and a compound of phosphorus, oxygen and fluorine (PO ₂ , PO ₃ , PO ₂ F ₂ , PO ₃ F, LiPO ₃ F), etc. Inferred. Note that the electrolyte LiPF ₆ described later has a photoelectron spectrum in a range higher than the range of 133 eV to 137 eV (for example, a range of 137.1 eV to 141 eV). Further, the positive electrode of the lithium ion secondary battery of the present invention is a transition metal obtained from XPS on the positive electrode surface, where the ratio of the number of phosphorus atoms by the X-ray photoelectron spectroscopy (XPS) on the positive electrode surface is Ap (at%). Ap / At is in the range of 0.1 or more and 1.0 or less when the ratio of the sum of the number of atoms of the element is At (at%). The Ap / At value is more preferably in the range of 0.2 to 0.8, and still more preferably in the range of 0.3 to 0.6. When Ap / At is 0.1 or more, phosphorus is sufficiently present on the surface of the positive electrode active material, and when 1.0 or less, it is preferable that phosphorus is excessively present on the surface of the positive electrode active material. .

  The positive electrode of the lithium ion secondary battery of the present invention is measured for the amount of elements present in the positive electrode by high frequency plasma emission spectrometry (ICP), the weight of the positive electrode active material is Wt (g), and the weight of phosphorus is Wp. (G), the value of Wp / Wt × 100, which is the ratio of the weight of phosphorus (Wp) to the sum of the weights of transition metals in the positive electrode active material (Wt), is 0.1 wt% or more and 0.0. It is preferably 5% by weight or less, more preferably 0.1% by weight or more and 0.4% by weight or less, and further preferably 0.1% by weight or more and 0.25% by weight or less. When the weight of phosphorus with respect to the sum of the weights of transition metals in the positive electrode active material is in the range of 0.1 wt% or more and 0.5 wt% or less, battery resistance can be further reduced, which is preferable. In the positive electrode of the lithium ion secondary battery of the present invention, it is preferable that a layer of a compound containing phosphorus is formed on the surface of the positive electrode active material in a film state having a thickness of less than 10 nm or in an adsorbed state. In this way, the increase in battery resistance can be further suppressed.

  The conductive material contained in the positive electrode is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the positive electrode. For example, graphite such as natural graphite (scale-like graphite, scale-like graphite) or artificial graphite, acetylene black , Carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) or a mixture of two or more thereof can be used. Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability. The binder serves to bind the active material particles and the conductive material particles. For example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-containing resin such as fluororubber, polypropylene, Thermoplastic resins such as polyethylene, ethylene-propylene-dienemer (EPDM), sulfonated EPDM, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. Examples of the solvent for dispersing the positive electrode active material, the conductive material, and the binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, and N, N-dimethylaminopropyl. Organic solvents such as amine, ethylene oxide, and tetrahydrofuran can be used. Moreover, a dispersing agent, a thickener, etc. may be added to water, and an active material may be slurried with latex, such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, and aluminum, copper, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. For example, a collector having a thickness of 1 to 500 μm is used.

  The negative electrode of the lithium ion secondary battery of the present invention is prepared by mixing a negative electrode active material, a conductive material, and a binder, and adding a suitable solvent to form a paste-like negative electrode material on the surface of the current collector. It may be formed by coating and drying, and compressing to increase the electrode density as necessary. Examples of negative electrode active materials include inorganic compounds such as lithium, lithium alloys and tin compounds, carbonaceous materials capable of occluding and releasing lithium ions, and conductive polymers. Of these, carbonaceous materials are used in terms of stability. It is preferable to see. The carbonaceous material is not particularly limited, and examples thereof include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Of these, graphites such as artificial graphite and natural graphite have an operating potential close to that of metallic lithium, can be charged and discharged at a high operating voltage, and suppresses self-discharge when a lithium salt is used as an electrolyte salt. In addition, the irreversible capacity during charging can be reduced, which is preferable. In addition, as the conductive material, binder, solvent, and the like used for the negative electrode, those exemplified for the positive electrode can be used. The negative electrode current collector includes copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as improved adhesion, conductivity and reduction resistance. For the purpose, for example, a copper surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. The shape of the current collector can be the same as that of the positive electrode.

As the ion conduction medium of the lithium ion secondary battery of the present invention, a non-aqueous electrolyte solution or a non-aqueous gel electrolyte solution containing an electrolyte can be used. Examples of the solvent for the nonaqueous electrolytic solution include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t -Chain carbonates such as butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, cyclic esters such as γ-butyllactone and γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, Chain esters such as methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxyethane and diethoxyethane, nitriles such as acetonitrile and benzonitrile,
Examples include furans such as tetrahydrofuran and methyltetrahydrofuran, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, the combination of cyclic carbonates and chain carbonates is preferable. According to this combination, not only the cycle characteristics representing the battery characteristics in repeated charge and discharge are excellent, but also the viscosity of the electrolyte, the electric capacity of the obtained battery, the battery output, etc. should be balanced. it can. In addition, it is considered that cyclic carbonates have a relatively high relative dielectric constant and increase the conductivity of the electrolytic solution, and chain carbonates are considered to suppress the viscosity of the electrolytic solution.

Examples of the electrolyte contained in the ion conductive medium include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiSbF ₆ , LiSiF _6. , LiAlF ₄ , LiSCN, LiClO ₄ , LiCl, LiF, LiBr, LiI, LiAlCl _{4 and the} like. Of these, LiPF ₆ and LiBF ₄ are preferred. Moreover, you may use electrolyte combining 1 type or 2 or more types of salts chosen from the group mentioned above. This electrolyte salt preferably has a concentration in the ion conductive medium of 0.1 mol / L or more and 3 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. When the concentration of the electrolyte salt is 0.1 mol / L or more, a sufficient current density can be obtained, and when the concentration is 3 mol / L or less, the salt is not completely dissolved or the salt is prevented from being deposited due to charge / discharge. Can do.

In the positive electrode of the lithium ion secondary battery of the present invention, by adding an additive compound containing an anion compound represented by the general formula (1) to the ion conductive medium and charging the ion conductive medium, phosphorus is added to the surface of the positive electrode active material. Can exist. That is, the positive electrode of the lithium ion secondary battery of the present invention may have a photoelectron spectrum of phosphorus on the surface of the positive electrode that appears after the first charge rather than before the first charge. In the general formula (1), the valence b of the anion is 1 to 3, of which 1 is preferable. When the valence b is larger than 3, it is not preferable because the salt of the anion compound tends to be hardly dissolved in the mixed organic solvent. The constants m and n are values related to the number of ligands, m is an integer of 1 to 4, and n is an integer of 0 to 8. The constant q is 0 or 1. When q is 0, the chelate ring is a five-membered ring. R ¹ represents alkylene having 1 to 10 carbons, halogenated alkylene having 1 to 10 carbons, arylene having 6 to 20 carbons, or halogenated arylene having 6 to 20 carbons. These alkylene and arylene may have a substituent or a hetero atom in the structure. Specifically, instead of hydrogen on alkylene and arylene, halogen, chain or cyclic alkyl group, aryl group, alkenyl group, alkoxy group, aryloxy group, sulfonyl group, amino group, cyano group, carbonyl group, acyl It may have a group, an amide group or a hydroxyl group as a substituent, or may have a structure in which nitrogen, sulfur or oxygen is introduced instead of carbon on alkylene and arylene. When q is 1 and m is 2 to 4, m R ^{1 s} may be bonded to each other. Examples thereof include a ligand such as ethylenediaminetetraacetic acid. R ² is halogen, alkyl having 1 to 10 carbon atoms, alkyl halide having 1 to 10 carbon atoms, aryl having 6 to 20 carbon atoms, aryl halide having 6 to 20 carbon atoms, or —X ³ R ³ (X ³ , R ³ will be described later. Alkyl and aryl here may also have a substituent or a hetero atom in the structure in the same manner as R ^1, and when n is 2 to 8, n R ² are bonded to each other to form a ring. May be formed. R ² is preferably an electron-withdrawing group, particularly preferably a fluorine atom. This is because in the case of a fluorine atom, the solubility and dissociation degree of the salt of the anion compound are improved, and the ionic conductivity is improved accordingly. Moreover, it is because oxidation resistance improves and generation | occurrence | production of a side reaction can be suppressed by this. X ¹ , X ² and X ³ each independently represent O, S or NR ⁴ . That is, the ligand is bonded to phosphorus (P) through these heteroatoms. R ³ and R ⁴ each independently represent hydrogen, an alkyl having 1 to 10 carbon atoms, an alkyl halide having 1 to 10 carbon atoms, an aryl having 6 to 20 carbon atoms, or an aryl halide having 6 to 20 carbon atoms. . Similarly to R ¹ , these alkyls and aryls may have a substituent or a hetero atom in the structure. Further, when a plurality of R ³ or R ⁴ are present, they may be bonded to each other to form a ring.

  Examples of the cation paired with the anionic compound include lithium, sodium, potassium, magnesium, calcium, barium, cesium, rubidium, silver, zinc, copper, cobalt, iron, nickel, manganese, titanium, lead, chromium, vanadium, and ruthenium. In addition to cations such as yttrium, lanthanoid, and actinoid, tetraalkylammonium (alkyl is methyl, ethyl, butyl, etc.), ammonium cation such as triethylammonium, pyridinium, imidazolium, protons, and the like. Among these, a lithium cation, a sodium cation, or a potassium cation is preferable.

When such an anionic compound is charged at least once with a lithium ion secondary battery, all or part of the anionic compound is decomposed and coated on the surface of the positive electrode active material and / or the negative electrode active material to form a film. Conceivable. This coating can be detected by, for example, X-ray photoelectron spectroscopy (XPS). Such an anionic compound is preferably at least one of PTFO, PFO, and PO represented by formulas (2) to (4). The reason is that when the battery is charged in a state where any one of the anionic compounds represented by the formulas (2) to (4) is contained in the ion conductive medium, a film or adsorption layer of the compound containing phosphorus on the surface of the positive electrode active material. Is formed, the positive electrode active material is protected, and the film thickness thereof is thin (for example, less than 10 nm or less than 5 nm), thereby suppressing an increase in battery resistance. The concentration of the anionic compound in the ion conductive medium is preferably 0.01 mol% or more and 0.5 mol% or less, more preferably 0.02 mol% or more and 0.1 mol% or less, and 0.03 mol% or more. More preferably, it is 0.08 mol% or less. When this concentration is 0.01 mol% or more, a film containing phosphorus can be sufficiently formed on the surface of the active material, or a compound containing phosphorus can be sufficiently adsorbed, and when it is 0.5 mol% or less, it can be formed on the surface of the active material. It can suppress that the film | membrane made becomes too thick. As a method for synthesizing such anionic compounds, for example, in the case of PFO, LiPF ₆ and 4 times moles of lithium alkoxide are reacted in a non-aqueous solvent, and then oxalic acid is added to bind to phosphorus. There is a method of substituting alkoxide with oxalic acid. In these cases, a lithium salt of an anionic compound can be obtained.

  The lithium ion secondary battery may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it is a composition that can withstand the range of use of the lithium ion secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a thin olefin resin such as polyethylene or polypropylene is thin. A microporous membrane is mentioned. These may be used alone or in combination.

  The shape of the lithium ion secondary battery is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a rectangular type. In addition, for example, a plurality of lithium ion secondary batteries of the present invention may be connected in series to provide a large power supply for an electric vehicle used for an electric vehicle, a hybrid electric vehicle, or the like. In addition, the lithium ion secondary battery of the present invention can be used for portable terminals, portable electronic devices, small power storage devices, and the like. FIG. 1 is a schematic view showing an example of a lithium ion secondary battery 10 of the present invention. The lithium ion secondary battery 10 includes a positive electrode sheet 13 in which a positive electrode active material 12 is formed on a current collector 11, a negative electrode sheet 18 in which a negative electrode active material 17 is formed on the surface of the current collector 14, a positive electrode sheet 13 and a negative electrode The separator 19 provided between the sheet | seat 18 and the non-aqueous electrolyte solution 20 satisfy | filled between the positive electrode sheet | seat 13 and the negative electrode sheet | seat 18 are provided. In this lithium ion secondary battery 10, a separator 19 is sandwiched between a positive electrode sheet 13 and a negative electrode sheet 18, these are wound and inserted into a cylindrical case 22, and a positive electrode terminal 24 and a negative electrode sheet connected to the positive electrode sheet 13. And a negative electrode terminal 26 connected to each other. Here, the positive electrode sheet 13 contains a lithium-containing transition metal as the positive electrode active material 12, and a phosphorus-containing compound is formed on the surface of the positive electrode active material 12 in a film state having a thickness of less than 5 nm or in an adsorbed state. Has been.

  According to the lithium ion secondary battery 10 of this embodiment described in detail above, since the photoelectron spectrum of phosphorus by X-ray photoelectron spectroscopy is in the range of 133 eV to 137 eV, a suitable compound containing phosphorus is, for example, phosphorus and oxygen. And a compound of phosphorus, oxygen and fluorine. Moreover, since the value of Ap / At is in the range of 0.1 or more and 1.0 or less, the abundance of phosphorus on the surface of the positive electrode active material is suitable. Moreover, since the weight of phosphorus with respect to the sum of the weights of transition metals in the positive electrode active material by ICP is 0.1 wt% or more and 0.5 wt% or less, a suitable thickness (such as a film state or an adsorbed state) ( It was presumed that a compound layer containing phosphorus was formed on the surface of the positive electrode active material (probably less than 10 nm). Therefore, it is considered that the increase in battery resistance is further suppressed and good battery characteristics are exhibited even after repeated charge / discharge cycles.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  Below, the example which produced the lithium ion secondary battery concretely is demonstrated as an Example.

[Example 1]
LiNi _0.8 Co _0.15 Al _0.05 O ₂ was used as the positive electrode active material. 85% by weight of this positive electrode active material, 10% by weight of carbon black as a conductive material, 5% by weight of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl-2-pyrrolidone as a dispersant is added and dispersed. Thus, a slurry-like positive electrode mixture was obtained. This slurry-like positive electrode mixture was applied to both surfaces of an aluminum foil current collector with a thickness of 20 μm, dried, then densified with a roll press, cut into a predetermined shape, and a positive electrode sheet was produced. Further, artificial graphite was used as the negative electrode active material. 95% by weight of this negative electrode active material, 5% by weight of polyvinylidene fluoride as a binder were mixed, an appropriate amount of N-methyl-2-pyrrolidone was added as a dispersant, and dispersed to obtain a slurry-like negative electrode mixture. This slurry-like negative electrode mixture was applied to both sides of a 10 μm-thick copper foil current collector, dried, then densified with a roll press, cut into a predetermined shape, and a negative electrode sheet was produced. Next, the positive electrode sheet and the negative electrode sheet were wound with a 25 μm-thick polyethylene separator interposed therebetween, and a roll-shaped electrode body was produced. This electrode body was housed in a 18650 type cylindrical case, impregnated with a non-aqueous electrolyte as an ion conducting medium, and then sealed to produce a cylindrical lithium ion secondary battery of Example 1 (see FIG. 1). In the non-aqueous electrolyte, LiPF ₆ is added to a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7 so that the concentration becomes 1.0 mol / L. What dissolved the lithium salt containing the anion compound (PTFO) shown to the said Formula (1) so that it might become 0.05 mol /% as an additive was used. Additives to the positive electrode active material and ion conduction medium of the produced lithium ion secondary battery, the value of Ap / At described later, the weight percent of phosphorus per weight of the positive electrode active material, the internal resistance value (mΩ), repeated charge and discharge Table 1 summarizes the capacity maintenance ratio (%) of the above. In Table 1, numerical values of Examples 2 to 15 and Comparative Examples 1 to 4 are also shown. The manufactured battery was subjected to charge and discharge for 5 cycles with an upper limit of 4.1 V and a lower limit of 3.0 V at a current of 0.2 C (100 mA). This initial five charging / discharging operations will be referred to as conditioning.

[Examples 2 and 3]
The same steps as in Example 1 were performed except that the additive added to the non-aqueous electrolyte was a lithium salt containing the anion compound (PFO) shown in (2) above at a concentration of 0.05 mol / L. A lithium ion secondary battery was defined as Example 2. Further, the same steps as in Example 1 were carried out except that the additive added to the non-aqueous electrolyte was a lithium salt containing the anion compound (PO) shown in (3) at a concentration of 0.05 mol / L. The obtained lithium ion secondary battery was defined as Example 3.

[Examples 4 to 6]
The same steps as in Example 1 were carried out except that the additive to be added to the non-aqueous electrolyte was a sodium salt containing the anion compound (PTFO) shown in (1) above at a concentration of 0.05 mol / L. A lithium ion secondary battery was taken as Example 4. Further, the same steps as in Example 1 were carried out except that the additive added to the non-aqueous electrolyte was a sodium salt containing the anion compound (PFO) shown in (2) at a concentration of 0.05 mol / L. The obtained lithium ion secondary battery was named Example 5. Further, the same steps as in Example 1 were carried out except that the additive added to the non-aqueous electrolyte was a sodium salt containing the anion compound (PO) shown in (3) at a concentration of 0.05 mol / L. The obtained lithium ion secondary battery was referred to as Example 6.

[Examples 7 to 9]
The same steps as in Example 1 were carried out except that the additive added to the non-aqueous electrolyte was a potassium salt containing the anion compound (PTFO) shown in (1) above at a concentration of 0.05 mol / L. A lithium ion secondary battery was taken as Example 7. Further, the same steps as in Example 1 were carried out except that the additive added to the non-aqueous electrolyte was a potassium salt containing the anion compound (PFO) shown in (2) at a concentration of 0.05 mol / L. The obtained lithium ion secondary battery was designated as Example 8. Further, the same steps as in Example 1 were carried out except that the additive added to the non-aqueous electrolyte was a potassium salt containing the anion compound (PO) shown in (3) above at a concentration of 0.05 mol / L. The obtained lithium ion secondary battery was referred to as Example 9.

[Examples 10 to 12]
The same process as in Example 1 was performed except that LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ was used as the positive electrode active material. Further, except for using _{LiNi 0.75 Co 0.15 Al 0.05 Mg 0.05} O 2 as the positive electrode active material subjected to the same process as in Example 2, a lithium ion secondary battery obtained were as in Example 11. Further, the same process as in Example 3 was performed except that LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ was used as the positive electrode active material, and the obtained lithium ion secondary battery was defined as Example 12.

[Examples 13 to 15]
The same process as in Example 1 was performed except that LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was used as the positive electrode active material, and the obtained lithium ion secondary battery was taken as Example 13. Further, except for using LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as the positive electrode active material, the same procedure as in Example 2, a lithium ion secondary battery obtained were as in Example 14 . Further, except for using LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as the positive electrode active material, the same procedure as in Example 3, lithium ion secondary batteries obtained were as in Example 15 .

[Comparative Examples 1-4]
Except that the non-aqueous electrolyte was prepared without adding an additive containing phosphorus, the same steps as in Example 1 were performed, and the obtained lithium ion secondary battery was referred to as Comparative Example 1. Moreover, the same process as Example 10 was performed except having produced the non-aqueous electrolyte solution without adding the additive containing phosphorus, and the obtained lithium ion secondary battery was referred to as Comparative Example 2. Moreover, the same process as Example 13 was performed except having produced without adding the additive containing phosphorus to nonaqueous electrolyte solution, and the obtained lithium ion secondary battery was made into the comparative example 3. Further, the same steps as in Example 2 were carried out except that the additive added to the non-aqueous electrolyte was a lithium salt containing the anion compound (PFO) shown in (2) at a concentration of 0.5 mol / L. The obtained lithium ion secondary battery was referred to as Comparative Example 4.

[Surface analysis of positive electrode active material by X-ray photoelectron spectroscopy (XPS)]
The positive electrode surface analysis by XPS was performed about Examples 1-15 and Comparative Examples 1-4. First, the battery was disassembled in an Ar flow glove box, and the extracted electrode was sufficiently washed with diethyl carbonate (DEC). The washed sample was sufficiently dried in the glove box, fixed to the XPS sample stage, and introduced into the sample chamber of the XPS measurement apparatus (PHI-5500MC manufactured by ULVAC-PHI) without being exposed to the atmosphere using a dedicated transfer vessel. XPS measurement was performed using MgKα as an X-ray source. The photoelectron extraction angle was 45 °, and the analysis region was sufficiently wide with respect to the particle size of the positive electrode active material. For example, when the positive electrode active material has a particle size of 10 μm, the analysis region has a diameter of 800 μm. The obtained measurement data was corrected so that the peak of hydrocarbon or conductive additive (carbon) present on the sample surface appearing in the C1s spectrum was 285 eV.

[Elemental analysis by high frequency plasma emission spectroscopy (ICP)]
About Examples 1-15 and Comparative Examples 1-4, the elemental analysis by ICP was performed. First, the battery after conditioning was disassembled and the electrode was taken out. The taken out positive electrode was thoroughly washed with DEC and dried, and then the vicinity of the center of the electrode was cut into a 1 cm ² square. The cut out sample was pulverized, 30 mL of hydrochloric acid was added, and the mixture was dissolved by heating on a heater at 200 ° C. for 11 hours. Each component in the solution was quantified using an ICP analyzer (CIROS120EOP manufactured by Rigaku).

[Internal resistance measurement]
About Examples 1-15 and Comparative Examples 1-4, IV resistance, ie, the internal resistance of a battery, was measured. The internal resistance was measured by adjusting the battery capacity to 50% (SOC = 50%), and then flowing currents of 0.5A, 1A, 2A, 3A, and 5A, and measuring the battery voltage after 10 seconds. The applied current and voltage were linearly approximated, and the internal resistance was determined from the slope.

[High-temperature charge / discharge cycle measurement]
About Examples 1-15 and Comparative Examples 1-4, the charge / discharge cycle test at high temperature (60 degreeC) was done, and the capacity | capacitance maintenance factor showing the grade by which the discharge capacity in repeated charging / discharging was maintained was evaluated. In the charge / discharge cycle test, the upper limit charge voltage is charged to 4.1 V at a constant current of ² mA / cm ² under a temperature condition of 60 ° C., and then the lower limit discharge voltage is set at a constant current of ² mA / cm ^2. Charging / discharging to 3.0 is defined as one cycle, and this charging / discharging cycle is performed 500 times. When the discharge capacity in the first cycle of the charge / discharge cycle test is the initial discharge capacity CAP ₁ (mAh / g) and the discharge capacity in the _500th cycle is the post-cycle discharge capacity CAP ₅₀₀ (mAh / g), the capacity is maintained. It calculated using the formula of rate CAPma (%) = CAP ₅₀₀ / CAP ₁ × 100.

[Measurement result]
When Examples 1 to 15 and Comparative Examples 1 to 4 before conditioning were subjected to XPS measurement of the positive electrode, no spectrum attributable to phosphorus was obtained. When the batteries of Examples 1 to 15 were disassembled after conditioning and subjected to XPS measurement, it was found that the photoelectron spectrum of phosphorus was in the range of 133 eV to 137 eV. FIG. 2 is an XPS spectrum (P2p spectrum) after conditioning in Example 2 and Comparative Example 1. Thus, in the Example, it turned out that the photoelectron spectrum of phosphorus exists in the range of 133 eV or more and 137 eV or less after conditioning. On the other hand, in Comparative Examples 1 to 3 in which no additive was added, a spectrum attributable to phosphorus was not obtained even after conditioning. Therefore, it was found that the phosphorus-containing compound formed on the positive electrode surface was not caused by the electrolyte (LiPF ₆ ) but by the additives shown in the formulas (2) to (4). Using this measurement result, Ap / At value when the ratio of the number of phosphorus atoms by XPS on the positive electrode surface is Ap (at%) and the ratio of the total number of atoms of the transition metal element is At (at%). Calculated. In XPS measurement, it is presumed that this Ap / At value reflects the state of phosphorus existing on the sample surface because it is information in the vicinity of the sample surface (for example, a range of about 5 nm). In Comparative Examples 1 to 3, this Ap / At value was approximately 0, indicating that no compound containing phosphorus was present on the positive electrode surface. Moreover, in the comparative example 4 which added many additives, Ap / At value showed a big value with 1.9. In Examples 1-15, it turned out that Ap / At value exists in the range of 0.1 or more and 1.0 or less. When comparing the results of the internal resistance measurement with the same battery configuration, in the example in which phosphorus is appropriately present on the positive electrode active material, the resistance value is generally lower than that of the comparative example and the capacity retention rate is high. I understood it. FIG. 3 shows the relationship of internal resistance with respect to the weight ratio of phosphorus by ICP. From this result, the internal resistance is low when the weight of phosphorus is in the range of 0.1 wt% or more and 0.5 wt% or less, more preferably 0.1 wt% or more and 0.25 wt% or less. It turns out that there is a tendency to show a value. In Examples 1 to 15, the Ap / At value by XPS is in the range of 0.1 to 1.0, and the weight ratio of phosphorus by ICP is in the range of 0.1 to 0.5% by weight. Therefore, it was assumed that phosphorus was present in the vicinity of the positive electrode surface with a thickness of less than 5 nm. Further, since the photoelectron spectrum is in the range of 133 eV to 137 eV, phosphorus is, for example, a compound of phosphorus and oxygen and a compound of phosphorus, oxygen and fluorine (PO ₂ , PO ₃ , PO ₂ F ₂ , PO ₃ F, LiPO It was inferred that it was present on the positive electrode active material in a suitable state such as ₃ F).

The schematic diagram which shows an example of the lithium ion secondary battery 10 of this invention. The XPS spectrum after the conditioning of Example 2 and Comparative Example 1. The figure showing the relationship of the internal resistance with respect to the weight ratio of the phosphorus by ICP.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery, 11 Current collector, 12 Positive electrode active material, 13 Positive electrode sheet, 14 Current collector, 17 Negative electrode active material, 18 Negative electrode sheet, 19 Separator, 20 Nonaqueous electrolyte, 22 Cylindrical case, 24 Positive electrode Terminal, 26 Negative terminal

Claims (6)

A negative electrode having a negative electrode active material capable of inserting and extracting lithium ions;
It has a positive electrode active material capable of occluding and releasing lithium ions and containing a transition metal, the photoelectron spectrum of phosphorus by X-ray photoelectron spectroscopy on the positive electrode surface is in the range of 133 eV to 137 eV, and the X-ray on the positive electrode surface When the ratio of the number of phosphorus atoms by photoelectron spectroscopy is Ap (at%) and the ratio of the total number of atoms of the transition metal element is At (at%), Ap / At is 0.1 or more and 1.0 or less. A positive electrode in range;
An ion conductive medium that is interposed between the positive electrode and the negative electrode and conducts lithium ions;
Lithium ion secondary battery equipped with. The positive electrode, one basic formula is _{Li a Ni x Co y Me z} O b (Me is that Mn, Al, Mg, Ti, V, Cu, Zn, Cr, Zr, Sr, selected from the group consisting of Si A, b, x, y, z represent 0.9 ≦ a ≦ 1.1, 1.9 ≦ b ≦ 2.2, 0.3 ≦ x ≦ 0.8, 0.1 ≦ y The lithium ion secondary battery according to claim 1, wherein a compound represented by: ≦ 0.5, 0.01 ≦ z ≦ 0.5, satisfying x + y + z = 1) is used as the positive electrode active material.   When the abundance of the elements present in the positive electrode is measured by high-frequency plasma emission spectrometry, the weight of phosphorus with respect to the sum of the weights of transition metals in the positive electrode active material is 0.1 wt% or more and 0.5 wt% The lithium ion secondary battery according to claim 1 or 2, wherein:   4. The lithium according to claim 1, wherein a layer of a compound containing phosphorus is formed on the surface of the positive electrode active material in a film state having a thickness of less than 10 nm or adsorbed on the surface of the positive electrode active material. Ion secondary battery.   5. The lithium ion secondary battery according to claim 1, wherein the positive electrode has a photoelectron spectrum obtained by X-ray photoelectron spectroscopy of phosphorus on the surface of the positive electrode before and after the first charge. The lithium ion secondary battery according to claim 1, wherein the ion conductive medium includes at least one of LiPF _{6 and} LiBF ₄ as an electrolyte.

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