JP2006156021A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having a good charge/discharge cycle lifetime. <P>SOLUTION: This nonaqueous electrolyte secondary battery is provided with a positive electrode 3 containing a positive electrode active material, a negative electrode 4, and a nonaqueous electrolyte. The positive electrode active material includes secondary grains formed by aggregation of lithium cobalt containing composite oxide primary grains. When a mean maximum grain size of the secondary grain is represented by L<SB>2max</SB>, a mean maximum grain size L<SB>1max</SB>of the primary grain is within a range 0.1×L<SB>2max</SB>≤L<SB>1max</SB>≤0.5×L<SB>2max</SB>, and sulfur exists on the surface of the positive electrode 3. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

  In recent years, electronic devices such as mobile communication devices, notebook computers, palmtop computers, integrated video cameras, portable CD (MD) players, cordless telephones, etc. have been reduced in size and weight. As a power source, a battery having a small size and a large capacity is required.

  Examples of batteries that are widely used as power sources for these electronic devices include primary batteries such as alkaline manganese batteries, and secondary batteries such as nickel cadmium batteries and lead storage batteries. Among them, non-aqueous electrolyte secondary batteries that use a lithium composite oxide for the positive electrode and a carbonaceous material or graphite material that can occlude and release lithium ions for the negative electrode are small and light, have a high single cell voltage, It is attracting attention because of its energy density.

  In Patent Document 1, in order to increase the capacity of a non-aqueous electrolyte secondary battery and improve rapid charge / discharge characteristics, sulfur atoms are present at least on the surface of the positive electrode in contact with the electrolytic solution, and 20 to It is disclosed to use a positive electrode for a non-aqueous electrolyte secondary battery containing a sulfur atom in a range of 350 μmol / g.

However, when a secondary aggregate of LiCoO ₂ is used as an active material in the positive electrode described in Patent Document 1 in order to satisfy the recent demands for thinning and high capacity of secondary batteries, the conductivity and lithium occlusion of the positive electrode are increased. The discharge amount is reduced, and the charge / discharge cycle life is reduced.
JP 2002-170564 A

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery having an improved charge / discharge cycle life.

A nonaqueous electrolyte secondary battery according to the present invention is a nonaqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode, and a nonaqueous electrolyte,
The positive electrode active material includes secondary particles in which primary particles of lithium cobalt-containing composite oxide are aggregated, and when the average maximum particle size of the secondary particles is L _2max , the average maximum particle size L _{1max of the} primary particles is 0.1 × L _2max ≦ L _1max ≦ 0.5 × L _2max
Sulfur is present on the surface of the positive electrode.

The nonaqueous electrolyte secondary battery according to the present invention is a nonaqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode, and a nonaqueous electrolyte,
The positive electrode active material includes secondary particles in which primary particles of lithium cobalt-containing composite oxide are aggregated, 50% cumulative frequency particle size D50 is 7 μm or more and 13 μm or less, 90% cumulative frequency particle size D90 is 15 μm or more, 40 μm. The specific surface area by the BET method is 0.2 m ² / g or more and 0.4 m ² / g or less,
Sulfur is present on the surface of the positive electrode.

  According to the present invention, it is possible to provide a nonaqueous electrolyte secondary battery having a high discharge capacity and an excellent charge / discharge cycle life.

  A positive electrode active material containing a lithium cobalt-containing composite oxide has a feature of excellent thermal stability. The secondary agglomerates of lithium-cobalt-containing composite oxides have a greater degree of freedom of deformation during pressing than single particles, so it is easy to take a close-packed structure, and between the primary particles constituting the secondary agglomerates Since it is bonded by intermolecular force, the required amount of binder can be reduced compared to single particles, so the ratio of active material per unit volume can be increased compared to the case of using single particles, and high capacity can be obtained. Is possible. On the other hand, if a protective film containing sulfur element is formed on the surface of the positive electrode in order to suppress the reaction between the positive electrode active material and the nonaqueous electrolyte, the conductivity of the positive electrode and the amount of occluded lithium are significantly reduced.

  According to the research by the present inventors, in the secondary aggregate of the lithium cobalt-containing composite oxide, the formation reaction of the protective film is concentrated on the primary particles having a small particle size, and as a result, the crystal structure of the primary particles having a small particle size is obtained. It has been found that the conductivity of the positive electrode and the lithium occlusion / desorption amount decrease because of the collapse or increase in resistance, which causes poor conduction between primary particles and a decrease in lithium sites.

Furthermore, the present inventors have found that the average maximum particle diameter L _1max is 0.1 × L _2max ≦ L _1max of the primary particles an average maximum particle size of the secondary aggregate of the lithium-cobalt-containing complex oxide upon the L _2max If it is in the range of ≦ 0.5 × L _2max , the protective film formation reaction occurs uniformly without concentrating on the specific primary particles, and therefore avoids a decrease in the positive electrode conductivity and a decrease in the amount of occluded lithium. At the same time, the inventors have found that high-density filling of the positive electrode active material is possible. As a result, a non-aqueous electrolyte secondary battery having a high capacity and an excellent charge / discharge cycle life could be realized.

  Hereinafter, a positive electrode, a negative electrode, and a nonaqueous electrolyte of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention will be described.

1) Positive electrode The positive electrode includes a current collector and a positive electrode layer that is supported on one or both surfaces of the current collector and includes a positive electrode active material. Sulfur element is present on the surface of the positive electrode layer.

In the lithium cobalt-containing complex oxide contained in the positive electrode active material, the molar ratio of lithium (X _Li) and cobalt _{(X Co) (X Li /} X Co) is in the range of 0.95 to 1.1 preferable. When the molar ratio (X _Li / X _Co ) is less than 0.95, the decrease in conductivity due to the formation of the protective film is small, and therefore, according to the definition of 0.1 × L _2max ≦ L _1max ≦ 0.5 × L _2max There is a possibility that sufficient effect cannot be obtained. Moreover, since the crystal structure becomes unstable due to overcharge and the reactivity with the nonaqueous electrolyte increases, there is a risk that safety during overcharge may be insufficient. On the other hand, when the molar ratio (X _Li / X _Co ) is larger than 1.1, there is a high possibility that the alkali fusion does not proceed completely during the firing for synthesizing the positive electrode active material and Li remains. If this residual alkali content is large, the safety during overcharging may be insufficient. A more preferable range of the molar ratio (X _Li / X _Co ) is 0.97 to 1.08.

  The lithium cobalt-containing composite oxide may contain elements other than Li, Co, and O. Examples of such elements include Ni, Mn, Al, Sn, Fe, Cu, Cr, Zr, Mg, Si, P, F, Cl, and B. There may be one kind of additive element or two or more kinds.

  As a composition of lithium cobalt containing complex oxide, what is represented by a following (1) formula can be mentioned, for example.

_{Li a Co b M1 c O 2} (1)
However, M1 is one or more elements selected from the group consisting of Ni, Mn, B, Al and Sn, and the molar ratios a, b and c are 0.95 ≦ a ≦ 1. 05, 0.95 ≦ b ≦ 1.05, 0 ≦ c ≦ 0.05, 0.95 ≦ b + c ≦ 1.05. More preferable ranges of the molar ratios a, b, and c are 0.97 ≦ a ≦ 1.03, 0.97 ≦ b ≦ 1.03, and 0 ≦ c ≦ 0.03, respectively.

  The positive electrode active material may be composed of a lithium cobalt-containing composite oxide or may contain other types of active materials. When other types of active materials are included, the proportion of the lithium cobalt-containing composite oxide in the positive electrode active material is desirably 50% by weight or more. Other types of active materials include, for example, manganese dioxide, lithium manganese composite oxide, lithium nickel composite oxide, lithium nickel cobalt composite oxide, lithium-containing iron oxide, lithium-containing vanadium oxide, and titanium disulfide. And chalcogen compounds such as molybdenum disulfide. Among these, it is desirable to use a lithium nickel cobalt composite oxide having a composition represented by the following formula (2). In this case, the proportion of the lithium cobalt-containing composite oxide in the positive electrode active material is 50 wt% or more, 80 It is desirable to make it not more than wt%.

_{LiNi 1-xy Co x M y} O 2 (2)
However, said M contains 1 or more types of elements selected from the group which consists of Mn, B, and Al, and said molar ratio x and y are 0 <x <= 0.5, 0 <= y <= 0. 1. More preferable ranges of the molar ratios x and y are 0.1 ≦ x ≦ 0.25 and 0 ≦ y <0.06, respectively.

The reason why the average maximum particle size L _1max of the primary particles is defined within the above range will be described in detail. When L _1max is smaller than 0.1 × L _2max , the formation of the protective film containing the sulfur element is biased to some particles, so that the conductivity of the positive electrode and the amount of occluded lithium are reduced. On the other hand, when L _1max is larger than 0.5 × L _2max , when the press pressure is increased to increase the density of the positive electrode, the agglomeration structure of secondary particles tends to collapse, and secondary batteries such as discharge capacity are destroyed. The various characteristics of are reduced. A more preferable range is 0.2 × L _2max ≦ L _1max ≦ 0.4 × L _2max .

The positive electrode active material has a 50% cumulative frequency particle size D50 of 7 μm or more and 13 μm or less, a 90% cumulative frequency particle size D90 of 15 μm or more (more preferably 20 μm or more, more preferably 25 μm or more), or 40 μm or less (more preferably). in 35μm or less, more desirably at 30μm or less), and a specific surface area by BET method of 0.2 m ² / g or more (more preferably 0.27 m ² / g or more), 0.4 m ² / g or less (more preferably Is preferably 0.34 m ² / g or less). Thereby, since the primary particle number and primary particle size which comprise the secondary aggregate of lithium cobalt containing complex oxide can be optimized, a discharge rate characteristic can further be improved.

The positive electrode active material may be composed only of the lithium cobalt-containing composite oxide, but may include other types of active materials. In this case, the content of the lithium cobalt-containing composite oxide in the positive electrode active material is desirably 50% by weight or more. Examples of other positive electrode active materials include lithium nickel oxides such as LiNiO ₂ and lithium manganese composite oxides such as LiMn ₂ O ₄ and LiMnO ₂ .

A positive electrode active material is produced by the method demonstrated below, for example. First, metallic cobalt is dissolved in an aqueous nitric acid solution, and then an aqueous sodium hydroxide solution is added thereto to obtain an aggregate composed of flaky Co (OH) ₂ primary particles. By firing this aggregate, Co (OH) ₂ is oxidized to Co ₃ O ₄ . Even after firing, the aggregate structure of the flaky primary particles is maintained. A positive electrode active material is obtained by baking lithium salt and an aggregate of Co ₃ O _{4 in} an air atmosphere or an oxygen atmosphere.

  In the positive electrode, when the number of Co atoms on the positive electrode surface measured by X-ray photoelectron spectroscopy in the discharge state is 100 atomic%, the atomic percentage of sulfur is 5 atomic% or more and 100 atomic% or less. desirable.

  X-ray photoelectron spectroscopy (XPS) is effective for quantitatively measuring the amount of sulfur present on the surface of the positive electrode. However, when measuring the ratio of elements present on the surface of the positive electrode by XPS, if the measurement is performed using the positive electrode taken out from the charged battery, an error is likely to occur due to the influence of impurities or the like adhering to the surface of the positive electrode during operation. For this reason, the inventors made two non-aqueous electrolyte secondary batteries having exactly the same structure, took one of the non-aqueous electrolyte secondary batteries into a discharged state, took out the positive electrode, and performed XPS measurement. A method of obtaining the relationship between the state of the positive electrode surface in a discharged state and the battery characteristics by evaluating the battery characteristics using one of the nonaqueous electrolyte secondary batteries was adopted.

  Most of the number of Co atoms on the positive electrode surface measured by XPS is considered to be derived from the positive electrode active material. If the sulfur atom number ratio x when the Co atom number is 100 atom% is less than 5 atom%, the sulfur distribution to the active material existing on the surface of the positive electrode becomes non-uniform. There is a possibility that the reaction with the water electrolyte proceeds and the discharge capacity and the charge / discharge cycle characteristics deteriorate. Since the function of the protective coating containing sulfur formed on the surface of the positive electrode active material can be made sufficient by setting the sulfur atom number ratio x to 5 atomic percent or more, the positive electrode active material and the nonaqueous electrolyte Reaction with can be suppressed. However, since the protective film containing sulfur covering the surface of the positive electrode active material is not necessarily high in lithium ion permeability, when the amount of sulfur present on the surface of the positive electrode is increased and exceeds 100 atomic%, There is a possibility that the lithium ion permeability is lowered and the discharge capacity of the battery is lowered. A more preferable range of the number of sulfur atoms is 8 atom% or more and 50 atom% or less. The sulfur component present on the surface of the positive electrode is preferably derived from a cyclic sulfonic acid ester having an unsaturated hydrocarbon group described later.

  The positive electrode layer may contain a conductive agent. Examples of the conductive agent include acetylene black, carbon black, and graphite.

  The positive electrode layer may contain a binder. As the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyether sulfone, ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR) or the like is used. be able to.

  The mixing ratio of the positive electrode active material, the conductive agent and the binder is preferably in the range of 85 to 98% by weight of the positive electrode active material, 1 to 10% by weight of the conductive agent, and 1 to 5% by weight of the binder.

  As the current collector, a conductive substrate having a porous structure or a nonporous conductive substrate can be used. These conductive substrates can be formed from, for example, aluminum, stainless steel, or nickel.

  The positive electrode is produced, for example, by suspending a conductive agent and a binder in an appropriate solvent in a positive electrode active material, and applying the suspension to a current collector and drying to form a thin plate.

The density of the positive electrode is desirably 3.5 g / cm ³ or more and 3.9 g / cm ³ or less. A more preferred range, 3.55 g / cm ³ or more and 3.7 g / cm ³ or less.

2) Negative electrode The negative electrode includes a current collector and a negative electrode layer carried on one or both sides of the current collector.

  The negative electrode layer includes a negative electrode active material that absorbs and releases lithium ions and a binder.

Examples of the negative electrode active material include graphite materials, carbonaceous materials such as graphite, coke, carbon fiber, spherical carbon, pyrolytic vapor phase carbonaceous material, and resin fired body; thermosetting resin, isotropic pitch, mesophase pitch Graphite material obtained by performing heat treatment at 500 to 3000 ° C. on carbon-based carbon, mesophase pitch-based carbon fiber, mesophase microspheres, etc. (especially, mesophase pitch-based carbon fiber is preferable because of high capacity and charge / discharge cycle characteristics) or Carbonaceous materials; chalcogen compounds such as titanium disulfide, molybdenum disulfide, and niobium selenide; light metals such as aluminum, aluminum alloy, magnesium alloy, lithium, and lithium alloy; Among them, it is preferable to use a graphite material having a graphite crystal having a (002) plane spacing _d002 of 0.34 nm or less. A nonaqueous electrolyte secondary battery provided with a negative electrode containing such a graphite material as a negative electrode active material can greatly improve battery capacity and large current discharge characteristics. More preferably, the surface interval _d002 is 0.337 nm or less.

  In addition, when a graphite material is used as the negative electrode active material, propylene carbonate (PC) is easily reduced and decomposed on the surface of the graphite material, so that the surface of the graphite material is made of amorphous carbon, low crystalline carbon, organic It is also possible to suppress reductive decomposition of PC by coating with a polymer compound, an inorganic compound, or the like.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like. Can be used.

  The blending ratio of the negative electrode active material and the binder is preferably in the range of 90 to 98% by weight of the carbonaceous material and 2 to 10% by weight of the binder.

  As the current collector, a conductive substrate having a porous structure or a nonporous conductive substrate can be used. These conductive substrates can be formed from, for example, copper, stainless steel, or nickel.

  For example, the negative electrode is prepared by kneading a negative electrode active material and a binder in the presence of a solvent, applying the obtained suspension to a current collector, drying, and then pressing once at a desired pressure or 2 to 2. It is produced by performing multistage pressing 5 times.

  The positive electrode and the negative electrode form an electrode group with a separator interposed therebetween. As this separator, a microporous film, a woven fabric, a non-woven fabric, a laminate of the same material or different materials among these can be used. In particular, the microporous membrane causes a so-called shutdown phenomenon in which the resin constituting the separator plastically deforms and closes the fine pores when the temperature of the electrode group rises abnormally due to heat generated by overcharging, etc., and the flow of lithium ions Is preferable because the overcharge state can be safely terminated. Examples of the material for forming the separator include polyethylene, polypropylene, ethylene-propylene copolymer, and ethylene-butene copolymer. As a material for forming the separator, one type or two or more types selected from the types described above can be used.

  For example, (i) the positive electrode and the negative electrode are wound in a flat shape or a spiral shape with a separator interposed therebetween, or (ii) the positive electrode and the negative electrode are wound in a spiral shape with a separator interposed therebetween. Rotating and then compressing in the radial direction, (iii) bending the positive electrode and the negative electrode one or more times with a separator interposed therebetween, or (iv) laminating the positive electrode and the negative electrode with a separator interposed therebetween It is produced by.

  The electrode group need not be pressed, but may be pressed to increase the integrated strength of the positive electrode, the negative electrode, and the separator. It is also possible to heat at the time of pressing.

  The electrode group can contain an adhesive polymer in order to increase the integrated strength of the positive electrode, the negative electrode, and the separator. It is desirable that the polymer having adhesiveness is one that can maintain high adhesiveness while holding a nonaqueous electrolytic solution. Furthermore, it is more preferable that such a polymer has high lithium ion conductivity. Specific examples include polyacrylonitrile (PAN), polyacrylate (PMMA), polyvinylidene fluoride (PVdF), polyvinyl chloride (PVC), and polyethylene oxide (PEO).

3) Nonaqueous electrolyte A nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte (for example, lithium salt) dissolved in the nonaqueous solvent. The form of the non-aqueous electrolyte can be liquid (non-aqueous electrolyte) or gel. Each substance constituting the nonaqueous electrolyte will be described.

a. Nonaqueous solvent As the nonaqueous solvent, cyclic carbonate, chain carbonate, γ-butyrolactone (GBL) and the like can be used in terms of ion conductivity and redox stability.

  As the cyclic carbonate, ethylene carbonate (EC) or propylene carbonate (PC) is preferably used.

  As the chain carbonate, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are preferably used.

  These non-aqueous solvent components are desirably used in a mixture of two or more.

  In particular, from the viewpoint of improving the capacity retention rate during high-temperature storage, a combination of a cyclic carbonate and a chain carbonate and a combination of two or more types of cyclic carbonates are preferred. From the viewpoint of suppressing gas generation during high temperature storage, a combination of cyclic carbonate and GBL and a combination of two or more types of cyclic carbonate are preferable.

  A combination of two or more types of cyclic carbonates is preferable from the viewpoint of achieving both high temperature storage characteristics and safety. Among these, PC has the advantage that it is inexpensive and has high ionic conductivity even in a low temperature state. However, as described above, when a graphite material is used for the negative electrode, it also has the disadvantage that it is easily reduced and decomposed on the surface of the graphite material. By mixing EC with PC, it becomes possible to reduce reductive decomposition of PC.

  In the above preferred composition, the ratio of PC to the total weight of the non-aqueous solvent is desirably in the range of 30 to 80% by weight. If the ratio of PC to the total amount of non-aqueous solvent is less than 30%, problems such as low ionic conductivity at a low temperature and reduced discharge capacity occur. On the other hand, if the ratio of PC exceeds 80% by weight, there is a possibility that reductive decomposition of PC cannot be suppressed even when EC is mixed, and the discharge capacity is reduced due to the increase in the viscosity of the electrolyte. A more preferable range of the ratio of PC to the total weight of the non-aqueous solvent is 35 to 70% by weight.

b. Electrolyte Examples of the electrolyte dissolved in the non-aqueous solvent include lithium perchlorate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), and lithium arsenic hexafluoride. (LiAsF ₆ ), lithium trifluorometasulfonate (LiCF ₃ SO ₃ ), lithium bistrifluoromethylsulfonylimide (LiN (CF ₃ SO ₂ ) ₂ ), lithium bispentafluoroethylsulfonylimide (LiN (C ₂ F ₅ SO ₂ ) Lithium salts such as ₂ ) can be mentioned. The type of electrolyte used can be one type or two or more types. Among these, LiPF ₆ has high conductivity when dissolved in a non-aqueous electrolyte, and discharge capacity can be improved. Therefore, it is desirable to mainly use LiPF ₆ as the electrolyte. Here, “mainly LiPF ₆ ” means that the ratio of the weight of LiPF ₆ to the total weight of the electrolyte is approximately 70% or more.

  The amount of electrolyte dissolved in the non-aqueous solvent is preferably 0.5 to 2.5 mol / L. A more preferable range is 0.7 to 2 mol / L.

c. Subcomponent In the nonaqueous electrolyte, the nonaqueous solvent and a substance other than the electrolyte can be contained as a subcomponent.

  As an auxiliary component, for example, vinylene carbonate, vinyl ethylene carbonate, divinyl ethylene carbonate, phenyl ethylene carbonate, γ-valerolactone, methyl propionate, ethyl propionate, ethyl acetate, acetonitrile, 2-methylfuran, furan, catechol carbonate, 12-crown-4, tetraethylene glycol dimethyl ether, succinic anhydride, diglycolic anhydride, tris (trioctyl) phosphate, dibutyl carbonate, benzene derivative, halogenated benzene, halogenated benzene derivative, naphthalene derivative, halogenated naphthalene, halogenated And naphthalene derivatives.

  Among these, subcomponents including vinylene carbonate, vinyl ethylene carbonate, phenyl ethylene carbonate, succinic anhydride, diglycolic anhydride, etc., generate a dense protective film on the negative electrode surface, so the reactivity between the negative electrode and the non-aqueous electrolyte Can be further reduced, reductive decomposition of PC can be suppressed, and stability during high-temperature storage can be improved. Moreover, the accessory component containing a tris (trioctyl) phosphate, dinormal butyl carbonate, etc. can improve the permeability | transmittance to the positive electrode of a nonaqueous electrolyte, a negative electrode, and a separator.

  Auxiliary components including benzene derivatives, halogenated benzenes, halogenated benzene derivatives, naphthalene derivatives, halogenated naphthalene, halogenated naphthalene derivatives, etc. can be detected on the surface of the positive electrode active material when the potential of the positive electrode rises abnormally due to overcharging. The overcharged state can be safely terminated by oxidative polymerization and increasing the resistance of the positive electrode active material.

  The weight ratio of the total amount of subcomponents in the non-aqueous solvent is preferably within the range of 10% by weight or less. This is because if the weight ratio of the subcomponent is more than 10% by weight, the lithium ion permeability of the protective film on the negative electrode surface is lowered and the low-temperature discharge characteristics may be greatly impaired. A more preferable range of the weight ratio of the subcomponent is 0.01 to 5% by weight, and a further preferable range is 0.1 to 3% by weight.

  The amount of the non-aqueous electrolyte is preferably 2 to 6 g per battery unit capacity 1 Ah. A more preferable range of the nonaqueous electrolytic mass is 2.5 to 5.5 g / 1 Ah.

  By the way, a sulfone compound such as 3-hydroxy-1-propenesulfonic acid-γ-sultone is added to a non-aqueous electrolyte, a tripolar cell is constructed using this non-aqueous electrolyte, and the potential is controlled by cyclic voltammetry. The case of sweeping in the direction of and the case of sweeping in the base direction are compared. When a sweep is made in the base direction, a large reduction peak appears around 1.0 V with respect to the Li potential, but when it is swept in the noble direction, clear oxidation is possible even if the potential is raised to about 3.0 to 5.0 V. No peak appears. Therefore, a sulfone compound such as 3-hydroxy-1-propenesulfonic acid-γ-sultone is more likely to undergo a reduction reaction than an oxidation reaction, and the film formation reaction is exclusively performed during the initial charge in an actual cell. This is thought to have occurred.

By examining the positive electrode active material, the standing condition before the initial charge, the initial charge condition, the aging condition performed after the initial charge, the kind of the sulfur-containing compound added to the non-aqueous electrolyte, etc., 0.1% In a positive electrode including a lithium cobalt-containing composite oxide satisfying × L _2max ≦ L _1max ≦ 0.5 × L _2max , a protective film is formed so that sulfur is uniformly distributed in the composite oxide exposed on the surface, and discharge In this state, the number of sulfur atoms when the number of Co atoms on the positive electrode surface measured by X-ray photoelectron spectroscopy was set to 100 atomic% was set to 5 atomic% or more and 100 atomic% or less.

A positive electrode active material containing a lithium cobalt-containing composite oxide that satisfies 0.1 × L _2max ≦ L _1max ≦ 0.5 × L _2max can increase the reactivity with the sulfur-containing compound. By using this positive electrode active material and setting the standing time from assembly to initial charge to 12 to 72 hours, the familiarity between the nonaqueous electrolyte and the positive electrode active material can be improved. Then, after the initial charge at a current of 1 C or less from 3.8 to 4.4 V with respect to the positive electrode potential is 3.8 to 4.4 V at a temperature of 0 ° C. or more and 50 ° C. or less, 1 By leaving for more than an hour (aging), the reductive polymerization of the sulfur-containing compound occurs exclusively in the initial charge, but the non-aqueous electrolyte is uniformly dispersed in the highly reactive positive electrode active material. The sulfur-containing compound in the reaction can be reacted uniformly with the high potential positive electrode without excess or deficiency. Since this film formation reaction is based on an electrochemical reaction between the positive electrode active material and the sulfur-containing compound, the film formation reaction is selectively performed on the active material on the positive electrode surface by causing it to occur uniformly with respect to the active material on the positive electrode surface. It becomes possible to adsorb sulfur. As a result, sulfur can be uniformly present in the active material on the positive electrode surface. 1C is a current value necessary for discharging the nominal capacity (Ah) in 1 hour.

  Examples of sulfur-containing compounds include sulfone compounds, sulfonic acids, and esters of sulfonic acids. The kind of sulfur-containing compound to be added can be one kind or two or more kinds. By using these sulfur-containing compounds, a strong protective film can be formed on the surface of the positive electrode, and cycle performance can be improved.

  Examples of the sulfone compounds include sulfone compounds having saturated hydrocarbon groups such as dimethyl sulfone, diethyl sulfone, and ethyl methyl sulfone, and unsaturated hydrocarbon groups such as divinyl sulfone, methyl vinyl sulfone, ethyl vinyl sulfone, diphenyl sulfone, and phenyl vinyl sulfone. And a sulfone compound such as sulfolane.

  Examples of sulfonic acids and esters thereof include sulfonic acids having linear saturated hydrocarbon groups such as methanesulfonic acid and ethanesulfonic acid, sulfones having linear saturated hydrocarbon groups such as methyl methanesulfonate and methyl ethanesulfonate. Acid ester, sulfonic acid having a linear unsaturated hydrocarbon group such as vinyl sulfonic acid, sulfonic acid ester having a linear unsaturated hydrocarbon group such as methyl vinyl sulfonate, ethyl vinyl sulfonate, benzene sulfonic acid, Aromatic sulfonic acids such as toluenesulfonic acid, sulfobenzoic acid and anhydrous sulfobenzoic acid, aromatic sulfonic acid esters such as methyl benzenesulfonate and ethyl benzenesulfonate, saturated hydrocarbon groups such as ethane sultone and 1,5-pentanthruton Cyclic sulfonic acid ester having ethylene , Unsaturated hydrocarbon groups such as 3-hydroxy-1-propenesulfonic acid-γ-sultone, 4-hydroxy-1-butylenesulfonic acid-γ-sultone, 5-hydroxy-1-pentenesulfonic acid-γ-sultone And cyclic sulfonic acid esters having the following.

  Among them, sulfone compounds having an unsaturated hydrocarbon group such as divinyl sulfone, 3-hydroxy-1-propenesulfonic acid-γ-sultone, 4-hydroxy-1-butylenesulfonic acid-γ-sultone, sulfonic acid, and sulfonic acid. The ester not only oxidatively polymerizes on the surface of the positive electrode when a non-aqueous electrolyte secondary battery is charged, but also produces a dense protective film on the surface of the negative electrode by a reduction reaction on the surface of the negative electrode. Reductive decomposition can be suppressed.

  The concentration of the sulfur-containing compound contained in the non-aqueous electrolyte of the secondary battery after assembly is preferably within the range of 0.1 to 10% by weight with respect to the total weight of the non-aqueous electrolyte. This is because if the concentration of the sulfur-containing compound is more than 10% by weight, the lithium ion permeability of the protective film on the surface of the positive electrode and the negative electrode may be reduced, and the discharge capacity may be reduced. A more preferable range of the weight ratio of the sulfur-containing compound is 0.1 to 8% by weight, and a further preferable range is 0.5 to 5% by weight.

  The present invention can be applied to various forms of nonaqueous electrolyte secondary batteries such as thin, rectangular, cylindrical, or coin-type. An example of the thin non-aqueous electrolyte secondary battery will be described with reference to FIGS.

  As shown in FIG. 1, an electrode group 2 is accommodated in a container body 1 having a rectangular cup shape. The electrode group 2 has a structure in which a laminate including a positive electrode 3, a negative electrode 4, and a separator 5 disposed between the positive electrode 3 and the negative electrode 4 is wound into a flat shape. The nonaqueous electrolyte is held in the electrode group 2. The lid 6 is integrated with the container body 1. The container body 1 and the cover plate 6 are each composed of a laminate film. This laminate film includes a resin layer 7 as an external protective layer, a thermoplastic resin layer 8 as an internal protective layer, and a metal layer 9 disposed between the resin layer 7 and the thermoplastic resin layer 8. A lid 6 is fixed to the container main body 1 by heat sealing using a thermoplastic resin layer 8, whereby the electrode group 2 is sealed in the container. A positive electrode tab 10 is connected to the positive electrode 3, and a negative electrode tab 11 is connected to the negative electrode 4, and each is pulled out of the container and serves as a positive electrode terminal and a negative electrode terminal.

[Example]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Example 1
<Preparation of positive electrode>
After dissolving metallic cobalt in an aqueous nitric acid solution, an aqueous sodium hydroxide solution was gradually added to the aqueous solution while stirring to cause precipitation of cobalt hydroxide, thereby obtaining an aggregate of flaky particles. The precipitate was collected by filtration, and washed repeatedly with water to dry when the pH was stable, thereby obtaining secondary aggregated particles having an average particle size of 10 μm consisting of flaky Co (OH) ₂ primary particles. . The agglomerated particles were fired at 600 ° C. in an air atmosphere to oxidize Co (OH) ₂ to Co ₃ O ₄ . The obtained oxide and lithium carbonate powder are mixed at a ratio of 1: 1 of Co: Li and fired at 850 ° C. in an air atmosphere, whereby secondary aggregated particles made of LiCoO ₂ primary particles are converted into a positive electrode active material. Got as. Tables 1 and 2 below show the 50% cumulative frequency particle size D50, the 90% cumulative frequency particle size D90, the specific surface area by the BET method, and the average maximum particle size L _1max of the primary particles of this positive electrode active material.

  A slurry was prepared by adding 5% by weight of acetylene black and 5% by weight of polyvinylidene fluoride (PVdF) in N-methyl-2-pyrrolidone (NMP) to 90% by weight of the positive electrode active material. The slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm, then dried and pressed to produce a positive electrode having a structure in which the positive electrode layer was supported on both sides of the current collector. The thickness of the positive electrode layer was 60 μm per side.

<Production of negative electrode>
95% by weight of a graphite material powder whose surface is coated with amorphous carbon by chemical vapor deposition at 1000 ° C. in a benzene / N ₂ stream under a benzene / N ₂ gas stream and 5% by weight of polyvinylidene fluoride (PVdF) dimethyl A slurry was prepared by mixing with a formamide (DMF) solution. The slurry was applied to both surfaces of a current collector made of a copper foil having a thickness of 12 μm, dried, and pressed to prepare a negative electrode having a structure in which the negative electrode layer was supported on the current collector. The thickness of the negative electrode layer was 55 μm per side.

The surface spacing d ₀₀₂ of (002) plane of the graphite material was determined by the half-width mid-point method by the X-ray diffraction was measured under the same conditions (XRD), was 0.3356 nm. During this measurement, scattering correction such as Lorentz scattering was not performed.

<Separator>
A separator made of a microporous polyethylene film having a thickness of 25 μm and a porosity of 45% was prepared.

<Production of electrode group>
A positive electrode lead made of a strip-shaped aluminum foil (thickness: 100 μm) is ultrasonically welded to the positive electrode current collector, and a negative electrode lead made of a strip-shaped nickel foil (thickness: 100 μm) is ultrasonically welded to the negative electrode current collector. And the negative electrode was spirally wound through the separator in the meantime, and the electrode group was produced. The electrode group was formed into a flat shape by applying pressure with a press while heating.

  A laminate film having a thickness of 100 μm in which both surfaces of an aluminum foil were covered with polyethylene was formed into a rectangular cup shape by a press, and the electrode group was housed in the obtained container.

  Next, the electrode group in the container was vacuum dried at 80 ° C. for 12 hours to remove moisture contained in the electrode group and the laminate film.

<Preparation of liquid nonaqueous electrolyte>
A nonaqueous solvent was prepared by mixing ethylene carbonate (EC) and methyl ethyl carbonate (MEC) so that the volume ratio (EC: MEC) was 33.3: 66.7. 3-hydroxy-1-propenesulfonic acid-γ-sultone (PRS) is added to the obtained non-aqueous solvent, and the concentration of lithium hexafluorophosphate (LiPF ₆ ) is 1.0 mol / L. Thus, a liquid non-aqueous electrolyte (non-aqueous electrolyte) was prepared. The PRS content (% by weight) in the obtained nonaqueous electrolyte is shown in Table 1 below.

  A non-aqueous electrolyte is injected into the electrode group in the container so that the amount per battery capacity of 1 Ah is 4.5 g, sealed by heat sealing, and has the structure shown in FIGS. Was a non-aqueous electrolyte secondary battery having a nominal capacity of 0.7 Ah.

  This non-aqueous electrolyte secondary battery was left for 24 hours and then charged at a constant current / constant voltage up to 4.2 V at 0.2 C at room temperature (20 ° C.) for 15 hours as an initial charge. Subsequently, after aging treatment was performed at 45 ° C. for 4 hours in a charged state, the battery was discharged to 3.0 V at 0.2 C at room temperature.

  When the element abundance ratio of the surface of the positive electrode in the discharged state was measured by X-ray photoelectron spectroscopy (XPS) for the non-aqueous electrolyte secondary battery after the first discharge, the abundance ratio of sulfur to cobalt (atomic%) was 23. The number of atoms was%.

Here, a specific method for measuring the element abundance ratio is as follows. First, in a state where the nonaqueous electrolyte secondary battery is discharged at room temperature to 0.2 V at 0.2 C, it is placed in an argon glove box having a dew point of about −70 ° C., and the nonaqueous electrolyte secondary battery is disassembled to form a positive electrode. A part (about 1 cm square) is taken out. The taken out positive electrode is washed with ethyl methyl carbonate and dried under reduced pressure to remove the adhering non-aqueous electrolyte (non-aqueous solvent, subcomponent, electrolyte). With this positive electrode sealed in an argon atmosphere, it was placed in an XPS measuring apparatus (ESCA300 manufactured by SCIENTA), and an X-ray photoelectron spectrum was obtained with a photoelectron detection angle of 90 degrees by X-rays using monochromatic AlKα rays as a radiation source. In this X-ray photoelectron spectrum, the abundance ratio of sulfur and cobalt was determined from the ratio of the peak attributed to S _2p appearing near a binding energy of 170 eV and the peak area attributed to Co _2p appearing near a binding energy of 780 eV. In addition, when calculating | requiring the abundance ratio of each element from peak area ratio, the relative sensitivity coefficient of each peak is correct | amended and the difference in the detection depth by an element is not considered.

(Examples 2 to 53)
The composition of the nonaqueous electrolyte, the positive electrode active material (composition, D50, D90, specific surface area by BET method, and average maximum particle size L _{1max of} primary particles), and the abundance ratio of sulfur to cobalt on the positive electrode surface by XPS are shown in Tables 1 to 3 below. A nonaqueous electrolyte secondary battery having the same configuration as that described in Example 1 was manufactured except that the conditions shown in Table 6 were used. In Table 1, GBL is γ-butyrolactone, PC is propylene carbonate, DEC is diethyl carbonate, DMC is dimethyl carbonate, BTS is 4-hydroxy-1-butylenesulfonic acid-γ-sultone, PS is propane sultone, and VC is Vinylene carbonate.

  The positive electrode active materials of Examples 46 to 47 were the same as those in Example 1 described above except that cobalt and nickel were dissolved in an aqueous nitric acid solution at a molar ratio of 0.95: 0.05 instead of dissolving metallic cobalt in the aqueous nitric acid solution. It was synthesized in the same manner as described in.

The positive electrode active materials of Examples ₄₈ to ₅₃ include LiCoO ₂ particles having the same specific surface area by D50, D90, BET method and average maximum particle size L _1max of primary particles as described in Example 1 above, and LiNiO It is a mixture in which ₂ particles are mixed at a mixing ratio (% by weight) shown in Table 6.

(Comparative Examples 1-8)
The composition of the nonaqueous electrolyte, the positive electrode active material (composition, D50, D90, specific surface area by BET method, and average maximum particle size L _{1max of} primary particles), and the abundance ratio of sulfur to cobalt on the positive electrode surface by XPS are shown in Tables 5 to 5 below. A non-aqueous electrolyte secondary battery having the same configuration as that described in Example 1 was manufactured except as shown in Table 6.

Note that the positive electrode active material of Comparative Example 5 and 6 were used single particles composed of LiCoO ₂ primary particles.

  About the obtained nonaqueous electrolyte secondary battery of Examples 1-53 and Comparative Examples 1-8, 0.2C discharge capacity and the discharge capacity maintenance factor after 500 cycles were evaluated.

  A constant current / constant voltage charge is performed for 3 hours at room temperature with a charge current of 1C to 4.2V, and then a charge / discharge cycle of discharging at 0.2C to 3.0V at room temperature is performed for a 0.2C discharge capacity. . Thereafter, constant current / constant voltage charging is performed for 3 hours at a charging current of 1 C to 4.2 V at room temperature, and then a charging and discharging cycle for discharging to 1.0 V at 1.0 C at room temperature is repeated 500 cycles, and the 500th cycle discharging. The capacity was measured. The capacity maintenance rate at the 500th cycle was calculated with the discharge capacity at the first cycle of the 500 cycles as 100%, and the results are shown in Tables 1 to 6 below.

  The method for measuring the 50% cumulative frequency particle size D50, 90% cumulative frequency particle size D90, specific surface area, and average maximum particle size of primary particles and secondary particles of the positive electrode active material is as follows.

<Measurement of median diameter of positive electrode active material>
After stirring 0.5 g of the positive electrode active material in 100 ml of water, ultrasonic dispersion was performed under the condition of 100 W-3 min. Then, 50% cumulative frequency particle size D50 and 90% cumulative frequency particle size D90 were measured using LEEDS & NORTHRUP MICROTRAC II PATICLE-ANALYZER TYPE 7997-10. Here, the 50% cumulative frequency particle size D50 indicates the particle size of particles that have reached 50% by measuring the particle size distribution by the microtrack method and integrating the volume from particles having a small particle size. The particle diameter of the particles whose accumulated volume reaches 90% is 90% cumulative frequency particle diameter D90.

<Measurement of specific surface area of positive electrode active material by BET method>
Using a specific surface area meter, Kantasorb QS-20, manufactured by Kantachrome Co., Ltd., 3 g of the positive electrode active material was filled in the measurement cell, vacuum degassed at 120 ° C. for 20 minutes, and then the specific surface area was measured by the BET 1-point method.

<Measurement of average maximum particle size of primary particles and secondary particles>
A scanning electron micrograph of an arbitrary cross section of the positive electrode was photographed at 3 magnifications at a magnification of 2000 times. Ten secondary particle images (two-dimensional images of secondary particles) in which the entire contour is observable were randomly selected for each of the three fields of scanning electron micrographs. The average maximum particle size L ₂ max was calculated by measuring the maximum particle size of a total of 30 selected secondary particles and calculating the average value of these. Further, the maximum particle size of the primary particles full contour are observable was determined for each secondary particles was calculated an average maximum particle diameter L ₁ max by calculating the average of these values. The obtained L ₁ max was expressed as L ₂ max.

  As is clear from Tables 1 to 6, it can be seen that the secondary batteries of Examples 1 to 53 are excellent in both the 0.2C discharge capacity and the 1C discharge capacity retention rate after 500 cycles.

  As for the non-aqueous solvent composition, the discharge capacity maintenance rate of Example 3 using the non-aqueous solvent containing EC and PC is the highest by comparison of Examples 1 to 5, and the use of the non-aqueous solvent containing EC and PC is the highest. It turns out that it is advantageous in improving the charge / discharge cycle life.

  Regarding the sulfur-containing compound, from the comparison between Examples 1 and 3 and Examples 21 and 22, the secondary batteries of Examples 1 and 3 using the cyclic sulfonic acid ester having an unsaturated hydrocarbon group are saturated hydrocarbon groups. It can be seen that the discharge capacity retention rate is higher than in Examples 21 and 22 using cyclic sulfonic acid esters having the above. Moreover, when Examples 1-3 are compared with Examples 6-8, the secondary battery of Examples 1-3 using PRS is a discharge capacity maintenance factor rather than the secondary battery of Examples 6-8 using BTS. It can be seen that PRS is preferable among the cyclic sulfonic acid esters having an unsaturated hydrocarbon group.

Regarding the average maximum particle size L _1max of the primary particles, from the comparison of Examples 1, 9, 13, 15, and 17, the discharge of the secondary batteries of Examples 1, 13, and 15 of 0.2 or more and 0.4 or less was performed. It can be seen that the capacity retention rate is higher than Example 9 of less than 0.2 and Example 17 of greater than 0.4. A similar trend was obtained from the comparison of Examples 3, 10, 14, 16, and 18. Therefore, in order to improve the charge / discharge cycle life, it is desirable to set L _1max to 0.2 or more and 0.4 or less.

  Regarding D90 of the positive electrode active material, the lower limit of D90 is 15 μm (Example 29), 20 μm (Example 31), and 25 μm (Example 33) by comparing Examples 29, 31, 33, 35, 37, and 39. As the discharge capacity maintenance ratio increases, the upper limit value of D90 increases to 40 μm (Example 39), 35 μm (Example 37), and 30 μm (Example 35). It can be understood that the rate is high. A similar tendency could be confirmed from the comparison of Examples 30, 32, 34, 36, 38 and 40.

  Regarding the abundance ratio of sulfur to cobalt on the positive electrode surface by XPS, the secondary batteries of Examples 1, 42 to 44, which are in the range of 8 to 50 atomic%, are 8 atoms in comparison with Examples 1 and 41 to 45. Compared with the secondary battery of Example 41 less than% and Example 45 exceeding 50 atomic%, a high discharge capacity retention rate was obtained, and it was confirmed that the range of 8 to 50 atomic% was preferable.

As for the composition of the positive electrode active material, a comparison of Examples 1, 46 and 47 shows that a part of Co is substituted with another element as in the above-described formula (1) (Li _a Co _b M1 _c O ₂ ). It can be seen that a high discharge capacity retention ratio can be obtained even in the above system.

  In the mixed system of the lithium cobalt-containing composite oxide and other positive electrode active materials, when Examples 48, 50, and 52 are compared, Example 48, in which the mixing ratio of the lithium cobalt-containing composite oxide is 50 to 80% by weight. It can be understood that the discharge capacity retention rate of 50 secondary batteries is higher than that of Example 52 in which the mixing ratio is less than 50% by weight. A similar tendency can be seen from the comparison of Examples 49, 51 and 53. From these results, the mixing ratio is preferably in the range of 50 to 80% by weight.

On the other hand, although there is sulfur on the positive electrode surface, L _1max is out of the range of 0.1 to 0.5, D50 is out of the range of 7 to 13 μm, and D90 is out of the range of 15 to 40 μm. In the secondary batteries of Examples 1 to 8, both the 0.2C discharge capacity and the discharge capacity retention rate were smaller than those of Examples 1 to 53.

  In addition, about the secondary battery of the said Example, after carrying out 2 to 3 cycle charging / discharging after an initial discharge, it carries out similarly to having demonstrated the element abundance ratio by XPS on the surface of a positive electrode similarly to Example 1 mentioned above. As a result, the same results as those shown in Tables 1 to 6 were obtained.

  The present invention is not limited to the above-described embodiments, and can be similarly applied to other types of combinations of positive electrode, negative electrode, separator, and container. In addition to the non-aqueous electrolyte secondary battery in which the container is formed from the laminated film as in the above-described embodiment, the present invention can be applied to a secondary battery having a cylindrical or rectangular container.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

  The nonaqueous electrolyte secondary battery according to the present invention can be used for safety of electronic devices such as mobile communication devices, notebook computers, palmtop computers, integrated video cameras, portable CD (MD) players, and cordless phones. Available as a usable power source.

The perspective view which shows the thin nonaqueous electrolyte secondary battery which is one Embodiment of the nonaqueous electrolyte secondary battery concerning this invention. The fragmentary sectional view which cut | disconnected the thin nonaqueous electrolyte secondary battery of FIG. 1 along the II-II line.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Container body, 2 ... Electrode group, 3 ... Positive electrode, 4 ... Negative electrode, 5 ... Separator, 6 ... Cover plate, 7 ... External protective layer, 8 ... Internal protective layer, 9 ... Metal layer, 10 ... Positive electrode tab, 11 ... negative electrode tab.

Claims (4)

A non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode, and a non-aqueous electrolyte,
The positive electrode active material includes secondary particles in which primary particles of lithium cobalt-containing composite oxide are aggregated, and when the average maximum particle size of the secondary particles is L _2max , the average maximum particle size L _{1max of the} primary particles is 0.1 × L _2max ≦ L _1max ≦ 0.5 × L _2max
A non-aqueous electrolyte secondary battery, wherein sulfur is present on the surface of the positive electrode. A non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode, and a non-aqueous electrolyte,
The positive electrode active material includes secondary particles in which primary particles of lithium cobalt-containing composite oxide are aggregated, 50% cumulative frequency particle size D50 is 7 μm or more and 13 μm or less, 90% cumulative frequency particle size D90 is 15 μm or more, 40 μm. The specific surface area by the BET method is 0.2 m ² / g or more and 0.4 m ² / g or less,
A non-aqueous electrolyte secondary battery, wherein sulfur is present on the surface of the positive electrode.   In the positive electrode, the atomic% of sulfur is 5 atomic% or more and 100 atomic% or less when the number of cobalt atoms on the positive electrode surface measured by X-ray photoelectron spectroscopy in the discharged state is 100 atomic%. The nonaqueous electrolyte secondary battery according to claim 1 or 2.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte includes a cyclic sulfonic acid ester having an unsaturated hydrocarbon group.

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