JP2008270086A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery excelling in a cycle characteristic and a storage characteristic while achieving increase of capacity by high-voltage charging. <P>SOLUTION: A positive electrode of this nonaqueous electrolyte secondary battery contains a first positive electrode active material Li<SB>(1+a)</SB>Mn<SB>x</SB>Ni<SB>y</SB>Co<SB>(1-x-y-z)</SB>M<SB>z</SB>O<SB>2</SB>or/and a second positive electrode active material Li<SB>(1-s-b)</SB>Mg<SB>s</SB>Co<SB>(1-t-u)</SB>Al<SB>t</SB>M'<SB>u</SB>O<SB>2</SB>; a compound having a bond represented by -SO<SB>n</SB>- (1≤n≤4) is present on a surface of the positive electrode; the content of sulfur present on the surface of the positive electrode as the bond represented by the -SO<SB>n</SB>- (1≤n≤4) is 0.2-1.5 atomic% when analyzed by an X-ray photoelectron spectroscopic method. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

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

  In recent years, nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries have attracted attention as batteries having a high energy density. A lithium ion secondary battery is configured, for example, by accommodating an electrode body inside a sealed container having a lid. A pair of positive and negative electrode terminal mechanisms are attached to the lid, and the electric power generated by the electrode bodies can be taken out from the electrode terminal mechanism. In addition, a pressure open / close type gas discharge valve is attached to the lid to ensure safety.

  Each of the electrode bodies is configured by laminating a belt-like positive electrode and a negative electrode via a separator and winding them. The positive electrode is configured by applying a positive electrode active material to a current collector made of aluminum foil, the negative electrode is configured by applying a negative electrode active material to a current collector made of copper foil, and the positive electrode active material is a separator It faces the negative electrode active material via The positive electrode active material is made of a lithium transition metal composite oxide or the like, and the negative electrode active material is made of a carbon material such as graphite that can occlude and release lithium ions.

  In the charge / discharge reaction of the lithium ion secondary battery, lithium ions move between the positive electrode active material and the negative electrode active material facing each other through the electrolytic solution.

As the positive electrode active material, lithium transition metal composite oxides such as lithium cobalt composite oxide (LiCoO ₂ ), lithium / nickel composite oxide (LiNiO ₂ ), and lithium / manganese composite oxide (LiMn ₂ O ₄ ) are usually used. Used. Among these, batteries using LiCoO ₂ are particularly frequently used. The reason is that a lithium ion secondary battery using LiCoO ₂ as a positive electrode active material has a high battery voltage, a high energy density, and excellent cycle characteristics.

On the other hand, a conventional lithium ion secondary battery using LiCoO ₂ as a positive electrode active material theoretically discharges all Li of LiCoO ₂ by charging up to about 4.8 V with respect to metallic lithium. The capacity reaches up to about 274 mAh / g. However, when the charging potential is increased to increase the capacity, the heavy load characteristic and the cycle characteristic are remarkably deteriorated.

For this reason, in the conventional lithium ion secondary battery using LiCoO ₂ as the positive electrode active material, the maximum potential of the positive electrode during charging is limited to about 4.3 V with respect to metallic lithium, and the discharge capacity of LiCoO _{2 in} the battery Has been limited to 140 mAh / g or less.

Therefore, as the positive electrode active material, the general formula _{Li x Co 1-y M y} O 2 (M represents one or more elements including at least IIIB group elements, x, y are respectively 0.9 ≦ x ≦ 1. 1 and 0 <y ≦ 0.2), and when the atomic ratio of M to Co on the particle surface is a, the value of a is the average value of the atomic ratio of M to Co in the entire particle A non-aqueous secondary battery having high heavy load characteristics under high voltage has been proposed by using a lithium-containing composite oxide that is 1.5 times or more [y / (1-y)] ( (See Patent Document 1).

Note that there are Patent Documents 2 to 4 as prior art documents related to the present invention.
JP 2002-75356 A JP 2002-170564 A JP 2003-22805 A JP-A-6-111818

However, the non-aqueous secondary battery of Patent Document 1 has a problem that the cycle characteristics and the storage characteristics are not sufficient, although the capacity is increased by high voltage charging and the heavy load characteristics are improved. This is because, in the positive electrode surface and the positive electrode active material reaction interface, the reaction between oxygen in the positive electrode active material and the carbonate organic solvent in the electrolytic solution, further lithium in the positive electrode active material or lithium salt in the electrolytic solution, and In order to form lithium carbonate (Li ₂ CO ₃ ) by reaction with an organic solvent, it is considered that the positive electrode surface during high-voltage charging cannot be stabilized and cycle characteristics and storage characteristics are deteriorated.

  The present invention solves the above-mentioned conventional problems, and provides a nonaqueous electrolyte secondary battery that is excellent in cycle characteristics and storage characteristics while realizing high capacity by high voltage charging.

The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, and the positive electrode is at least one selected from a first positive electrode active material and a second positive electrode active material. The first positive electrode active material includes a general composition formula Li _{(1 + a)} Mn _x Ni _y Co _(1-xyz) M _z O ₂ (where M is Ti, Zr, Nb) , Mo, W, Al, Si, Ga, Ge and Sn, at least one element selected from the group consisting of -0.15 <a <0.15, 0.1 <x ≦ 0.5, 0 .6 <x + y + z <1.0, 0 ≦ z ≦ 0.1), and the second positive electrode active material has a general composition formula of Li _(1-sb) Mg _s Co _(1-tu) Al _t M ' _{uO 2} (wherein M is at least one element selected from the group consisting of Ti, Zr and Ge, and 0.01 ≦ s <0.1, 0 <u <0.1, 0. 01 <t + u <0.1, −0.06 ≦ b <0.05), and a compound having a bond represented by —SO _n — (1 ≦ n ≦ 4) is formed on the surface of the positive electrode. The content of sulfur present as a bond represented by the —SO _n — (1 ≦ n ≦ 4) on the surface of the positive electrode is 0.2 atomic% when analyzed by X-ray photoelectron spectroscopy. It is characterized by being 1.5 atomic% or less.

  According to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery that is further excellent in cycle characteristics and storage characteristics while realizing high capacity by high voltage charging.

  The nonaqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte. The positive electrode includes at least one positive electrode active material selected from a first positive electrode active material and a second positive electrode active material.

The first positive electrode active materials, the general composition formula _{Li (1 + a) Mn x} Ni y Co (1-xyz) M z O 2 ( where, M is, Ti, Zr, Nb, Mo , W, Al, It is at least one element selected from the group consisting of Si, Ga, Ge, and Sn, and −0.15 <a <0.15, 0.1 <x ≦ 0.5, 0.6 <x + y + z <1. 0, 0 ≦ z ≦ 0.1).

In addition, the second positive electrode active material has a general composition formula Li _(1-sb) Mg _s Co _(1-tu) Al _t M ′ _u O ₂ (where M ′ is a group consisting of Ti, Zr and Ge). At least one element selected from the group consisting of 0.01 ≦ s <0.1, 0 <u <0.1, 0.01 <t + u <0.1, −0.06 ≦ b <0.05) It is represented by

The non-aqueous electrolyte secondary battery of the present invention can maintain the stability of the positive electrode during charging even when the capacity is increased by high-voltage charging by using the positive electrode active material, and is safe by suppressing heat generation. Sex can be secured. Further, by using a layered Li-containing composite oxide similar to LiCoO ₂ as in the above general composition formula, it is easier to achieve both high capacity and ensuring safety. The layered Li-containing composite oxide is considered to be able to exhibit excellent safety because it has a capacity equivalent to that of LiCoO ₂ that has been used in the past, and stability during charging increases.

In the nonaqueous electrolyte secondary battery of the present invention, a compound having a bond represented by —SO _n — (1 ≦ n ≦ 4) is present on the surface of the positive electrode. Thereby, the cycle characteristics and storage characteristics of the nonaqueous electrolyte secondary battery can be improved. This is because oxygen and sulfur in the positive electrode active material at the positive electrode surface and the positive electrode active material reaction interface are combined to allow a large amount of electrochemically stable elements to be present on the positive electrode surface. It is considered that the formation of lithium carbonate at the material reaction interface is suppressed, the stability of the positive electrode during charging is further increased, and the cycle characteristics and storage characteristics are improved.

The sulfur content present as a bond represented by —SO _n — (1 ≦ n ≦ 4) on the surface of the positive electrode is 0.2 atoms when analyzed by X-ray photoelectron spectroscopy (XPS). % To 1.5 atomic% is preferable, and 0.5 atomic% to 1.5 atomic% is more preferable. This is because, within this range, the cycle characteristics and storage characteristics of the nonaqueous electrolyte secondary battery can be reliably improved.

The sulfur content is obtained by disassembling the battery in an argon inert gas and taking out the positive electrode, and analyzing the surface of the positive electrode taken out without being exposed to the atmosphere by XPS. Specifically, MgKα is used as the X-ray source, the photoelectron extraction angle is 45 degrees, the aperture diameter is 800 μm, and the analysis depth is about 3 to 5 μm. Then, -SO _n - 166.5eV attributed to S2p in the binding (1 ≦ n ≦ 2), from the intensity of 168.3eV (2 ≦ n ≦ 3) , 169.2eV (3 ≦ n ≦ 4) Calculate the atomic percent of sulfur.

The method for allowing the compound having a bond represented by —SO _n — (1 ≦ n ≦ 4) to be present on the surface of the positive electrode is not particularly limited. However, the nonaqueous electrolyte is preliminarily set to 4.1 V or more and 4.3 V or less. By containing a sulfur compound that decomposes at the charging voltage and then charging the nonaqueous electrolyte secondary battery to 4.4 V or higher, it is expressed as -SO _n- (1 ≦ n ≦ 4) on the surface of the positive electrode. A compound having a bond can be present.

  Examples of the sulfur compound include dimethyl sulfate, diethyl sulfate, ethyl methyl sulfate, methyl propyl sulfate, ethyl propyl sulfate, methyl phenyl sulfate, ethyl phenyl sulfate, phenyl propyl sulfate, benzyl methyl sulfate, benzyl ethyl sulfate, and diphenyl sulfate. Chain sulfate ester: cyclic sulfonate such as 1,3-propane sultone, 1,4-butane sultone, 3-phenyl-1,3-propane sultone, 4-phenyl-1,4-butane sultone, propane cyclic sulfate However, it is not limited to these.

The method of synthesizing the first positive electrode active material _{(Li (1 + a) Mn} x Ni y Co (1-xyz) M z O 2) is not limited, for example, can be synthesized as follows. First, nickel hydroxide, manganese and cobalt sulfate mixed at a predetermined composition ratio are precipitated by adding alkali hydroxide, and then sufficiently washed with water and dried to obtain a coprecipitated hydroxide. Next, a compound containing lithium hydroxide and additive element M is added to the coprecipitated hydroxide and mixed sufficiently, followed by firing at a predetermined temperature in an oxidizing atmosphere. The oxygen partial pressure in the oxidizing atmosphere during firing may be 0.19 to 1 atm, the firing temperature may be 600 to 1000 ° C., and the firing time may be 6 to 48 hours.

Further, the second positive electrode active material _{(Li (1-sb) Mg} s Co (1-tu) Al t M 'u O 2) is not also limited synthetic methods, for example, cobalt hydroxide, lithium carbonate, water After mixing a compound of aluminum oxide, magnesium oxide and additive element M ′ at a predetermined composition ratio, it can be synthesized by firing in an air atmosphere at 600 to 1000 ° C. for 6 to 48 hours.

The positive electrode is prepared by applying a positive electrode mixture paste obtained by sufficiently adding a solvent to a mixture containing the positive electrode active material, the positive electrode conductive agent, the positive electrode binder, and the like, and drying the mixture. After that, the positive electrode mixture layer can be formed by controlling to a predetermined thickness and a predetermined electrode density.

The average particle diameter of the positive electrode active material measured by the laser diffraction / scattering method is preferably 5 to 25 μm, and the specific surface area measured by the BET method is preferably 0.1 to 0.6 m ² / g. Thereby, in addition to the increase in capacity, cycle characteristics and storage characteristics can be further improved. This is because, by reducing the specific surface area of the positive electrode active material, the reaction area with the electrolyte can be reduced, and the amount of conductive agent and binder used can be reduced, thereby increasing the electrode density and thus the battery. This is because the capacity can be increased. In addition, by reducing the specific surface area by advancing the sintering reaction at the time of firing the positive electrode active material, it is possible to suppress particle collapse during paint preparation and electrode preparation, and to improve cycle characteristics and storage characteristics. it can.

  The conductive agent for positive electrode is not limited as long as it is an electron conductive material that is substantially chemically stable in the constructed battery. For example, graphite such as natural graphite (flaky graphite, etc.), artificial graphite, etc .; acetylene Carbon blacks such as black, ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fiber and metal fiber; carbon fluoride; metal powders such as aluminum; zinc oxide and titanium Conductive whiskers made of potassium acid or the like; conductive metal oxides such as titanium oxide; or organic conductive materials such as polyphenylene derivatives can be used. Although these electrically conductive agents for positive electrodes can be used independently, you may mix and use 2 or more types. Among these conductive agents for positive electrodes, acetylene black, ketjen black, and carbon black are particularly preferable. The addition amount of the positive electrode conductive agent may be 0.5 to 3% by weight with respect to the total weight of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder.

The positive electrode binder can be either a thermoplastic resin or a thermosetting resin. For example, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber, tetrafluoroethylene-hexa. Fluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride- Chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer (ETFE resin), polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymer Propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer Polymer, ethylene-acrylic acid copolymer or its Na ⁺ ion crosslinked product, ethylene-methacrylic acid copolymer or its Na ⁺ ion crosslinked product, ethylene-methyl acrylate copolymer or its Na ⁺ ion crosslinked product, ethylene -A methyl methacrylate copolymer or its Na ⁺ ion crosslinked body etc. are mentioned, These materials can be used individually or in mixture. Among these materials, PVDF and PTFE are particularly preferable. The addition amount of the positive electrode binder may be 0.5 to 3% by weight with respect to the total weight of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder.

  The positive electrode current collector is not particularly limited as long as it is an electron conductor that is substantially chemically stable in the constituted battery. Examples of the material for the positive electrode current collector include aluminum, stainless steel, nickel, titanium, carbon, conductive resin, and the like, as well as aluminum or stainless steel with carbon applied on the surface. Among these, aluminum or an aluminum alloy is particularly preferable. When a metal material is used, the surface can be oxidized and used. Moreover, it is preferable to give an unevenness | corrugation to the collector surface by surface treatment. This is because the active material can be prevented from falling off. As the shape of the positive electrode current collector, for example, a film, a sheet, a net, a punched thing, a lath body, a porous body, a foamed body, a molded body of a fiber group, and the like are used in addition to a foil. The thickness of the positive electrode current collector is not particularly limited, and may be, for example, 1 to 500 μm.

  The negative electrode includes a first negative electrode active material and a second negative electrode active material, the first negative electrode active material is made of a carbon material, and the second negative electrode active material is more conductive than the first negative electrode active material. It is preferably made of a carbon material having a high specific surface area. The specific surface area of the second negative electrode active material is preferably not less than 10 times the specific surface area of the first negative electrode active material, and the ratio of the second negative electrode active material is 0% by weight of the whole negative electrode active material. It is preferably 5% by weight or more and 3% by weight or less. When the negative electrode active material contains a carbon material having high conductivity in the above-described weight ratio, cycle characteristics and storage characteristics can be further improved under high voltage charging. If the ratio of the second negative electrode active material is less than 0.5% by weight, there is no effect, and the second negative electrode active material, which is a highly conductive carbon material, has a larger specific surface area and bulk than the first negative electrode active material. If it exceeds 3% by weight, the density of the negative electrode decreases and the battery capacity also decreases.

  As described above, when the non-aqueous electrolyte secondary battery is placed in a high temperature environment, oxygen in the positive electrode active material reacts with the carbonate organic solvent in the electrolyte solution at the positive electrode surface and the positive electrode active material reaction interface. Constituent metal elements such as cobalt, nickel, and manganese are eluted as metal ions from the surface of the positive electrode active material. For this reason, these metal ions are reduced on the negative electrode and deposited on the surface of the negative electrode active material, thereby reducing the activity of the negative electrode and reducing the battery capacity. Since elution of metal elements from the positive electrode active material is further promoted in a high voltage state, it has been difficult to improve cycle characteristics and storage characteristics under high voltage charging from the viewpoint of the negative electrode. Therefore, as described above, by adding a carbon material (second negative electrode active material) having higher conductivity than the first negative electrode active material to the negative electrode active material, the second negative electrode active material is preferentially used. It is considered that metal ions eluted from the positive electrode active material can be deposited as metal, and deterioration of the first negative electrode active material as the main component can be reduced. Thereby, it is considered that the cycle characteristics and the storage characteristics can be further improved under high voltage charging.

  As said 1st negative electrode active material, carbon materials, such as artificial graphite, such as natural graphite or block graphite, scale-like graphite, and earth-like graphite, are used, for example. The average particle diameter of the carbon material used as the first negative electrode active material is preferably 3 μm or more and 15 μm or less, more preferably 10 μm or more and 13 μm or less, and the purity thereof is preferably 99.9% or more. This is because it is easy to obtain within this range.

  Further, the first negative electrode active material can be used by mixing the above carbon material with a metal that can be alloyed with lithium such as Si and Sn, or an alloy thereof.

  As said 2nd negative electrode active material, fibrous carbon materials, such as carbon fiber and a carbon nanotube, are used, for example. Specifically, for example, graphite whisker, filamentous carbon, graphite fiber, ultrafine carbon tube, carbon tube, carbon fibril, carbon microtube, carbon nanofiber, and the like can be used. Moreover, what is necessary is just to use these fibrous carbon materials whose fiber diameter is 0.5 nm-100 nm and whose fiber length is 0.01 micrometer-100 micrometers, for example.

  The negative electrode is obtained by applying a negative electrode mixture paste obtained by sufficiently adding a solvent to a mixture containing the negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and the like, and drying the mixture. After that, the negative electrode mixture paste can be formed by controlling to a predetermined thickness and a predetermined electrode density.

  The negative electrode conductive agent is not limited as long as it is an electron conductive material, but the negative electrode conductive agent may not be added. In the case of adding a negative electrode conductive agent, for example, graphite such as natural graphite (flaky graphite, etc.), artificial graphite, expanded graphite, etc .; acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, etc. Carbon blacks, conductive fibers such as carbon fibers and metal fibers, metal powders such as copper and nickel, or organic conductive materials such as polyphenylene derivatives can be used. Although these electrically conductive agents for negative electrodes can be used independently, you may mix and use 2 or more types. Among these conductive agents for negative electrodes, artificial graphite, acetylene black, and carbon fibers are particularly preferable. The addition amount of the negative electrode conductive agent may be 10% by weight or less based on the total weight of the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder.

As the negative electrode binder, the same material as that of the positive electrode binder described above can be used alone or in combination. Among these materials, styrene-butadiene rubber, polyvinylidene fluoride, ethylene-acrylic acid copolymer or its Na ⁺ ion crosslinked product, ethylene-methacrylic acid copolymer or its Na ⁺ ion crosslinked product, ethylene-methyl acrylate copolymer A polymer or a Na ⁺ ion crosslinked product thereof, an ethylene-methyl methacrylate copolymer or a Na ⁺ ion crosslinked product thereof is particularly preferred. The addition amount of the negative electrode binder may be 0.5 to 3% by weight with respect to the total weight of the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder.

  The negative electrode current collector is not particularly limited as long as it is an electron conductor that is substantially chemically stable in the battery. Examples of the material for the positive electrode current collector include copper, stainless steel, nickel, titanium, carbon, conductive resin, etc., as well as copper or stainless steel coated with carbon. Among these, copper or a copper alloy is particularly preferable. When a metal material is used, the surface can be oxidized and used. Moreover, it is preferable to give an unevenness | corrugation to the collector surface by surface treatment. This is because the active material can be prevented from falling off. As the shape of the negative electrode current collector, for example, a film, a sheet, a net, a punched one, a lath body, a porous body, a foam, a molded body of a fiber group, and the like are used in addition to a foil. The thickness of the negative electrode current collector is not particularly limited, and may be, for example, 1 to 500 μm.

Examples of the electrolyte include vinylene carbonate (VC), propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate (MEC), γ-butyrolactone, 1, 2 -Dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivative, 3-methyl-2 -A solution in which one or more aprotic organic solvents such as oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, and diethyl ether are mixed. To, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) 2, LiN (CF 3 SO 2) 2, At least one lithium salt selected from LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{(2n + 1)} SO ₃ (2 ≦ n ≦ 6), LiN (RfOSO ₂ ) ₂ (Rf is a fluoroalkyl group), etc. An electrolytic solution in which is dissolved may be used. The concentration of Li ions in the electrolytic solution is preferably 0.5 to 1.5 mol / L, and more preferably 0.9 to 1.25 mol / L.

  A separator impregnated with the non-aqueous electrolyte is disposed between the positive electrode and the negative electrode. As the separator, an insulating microporous thin film having a large ion permeability and a predetermined mechanical strength is used. Moreover, what has the function to block | close a micropore and to raise resistance above a fixed temperature (100-140 degreeC) is preferable from the point of the safety | security improvement of a battery. Specifically, as the separator, a sheet, a nonwoven fabric, a woven fabric, or an olefin-based particle made of an olefin-based polymer or glass fiber such as polypropylene and polyethylene having organic solvent resistance and hydrophobicity is fixed with an adhesive. A porous body layer or the like is used.

The pore diameter of the separator is preferably in a range that does not allow the active material, conductive agent, binder, and the like detached from the positive electrode or negative electrode to pass through, and is preferably 0.01 to 1 μm, for example. Further, assuming that the thickness of the separator is T, the porosity is P, and the air permeability is R, T is preferably 10 μm to 20 μm, P is preferably 40% to 55%, and R is 50 seconds / It is preferably 100 cm ³ or more and 200 seconds / 100 cm ³ or less.

  The nonaqueous electrolyte secondary battery of the present invention is a rolling type in which a positive electrode, a negative electrode, and a separator are stacked in a strip shape, but is rolled after laminating a long positive electrode, a negative electrode, and a separator. May be.

  Hereinafter, an embodiment of a lithium ion secondary battery which is an example of a nonaqueous electrolyte secondary battery of the present invention will be described with reference to the drawings. In the following description, since the same thing as what was mentioned above can be used about a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte, the detailed description may be abbreviate | omitted.

  FIG. 1 is a perspective view showing an example of the lithium ion secondary battery of the present embodiment, and FIG. 2 is a cross-sectional view taken along the line II of FIG.

  In FIG. 1, a lithium ion secondary battery 1 of this embodiment includes a rectangular (flat) battery case 2 and a lid plate 3. In FIG. 2, the insulator 10, the nonaqueous electrolyte (not shown), and the electrode body 9 are accommodated in a sealed container including the battery case 2 and the lid plate 3. However, in FIG. 2, the inner peripheral portion of the electrode body 9 is not cross-sectional.

  The battery case 2 is formed of a metal such as an aluminum alloy and serves as a battery exterior material.

  The cover plate 3 is formed of a metal such as an aluminum alloy, and a terminal 5 formed of a metal such as stainless steel is provided via an insulating packing 4 formed of a synthetic resin such as polypropylene. Moreover, the cover plate 3 is inserted in the opening part of the battery case 2, and the opening part of the battery case 2 is sealed and the inside of the battery is sealed by welding the joint part of both.

  The insulator 10 is formed of a synthetic resin sheet such as a polytetrafluoroethylene sheet and is disposed at the bottom of the battery case 2.

  The electrode body 9 has a flat wound structure in which a long positive electrode 6 and a negative electrode 7 are laminated via a separator 8, wound in a spiral shape, and then pressed to form a flat shape. . However, the structure of the electrode body 9 is not limited to this wound structure as long as it includes the positive electrode 6, the negative electrode 7, and the separator 8 disposed between the positive electrode 6 and the negative electrode 7. 6, a structure in which the negative electrode 7 and the separator 8 are laminated may be used.

  One end of the positive electrode 6 and the cover plate 3 are electrically connected by a positive electrode current collecting tab 11 formed of a metal such as nickel, and the cover plate 3 and the battery case 2 function as a positive electrode terminal.

  Also, one end of the negative electrode 7 and the terminal 5 are electrically connected by a negative electrode current collecting tab 12 formed of a metal such as nickel and a metal such as stainless steel and a current collector plate 14, and the terminal 5 serves as a negative electrode It is supposed to function. At this time, the current collector plate 14 and the lid plate 3 are insulated because they are interposed via an insulator 13 formed of a synthetic resin such as polypropylene.

  In FIG. 2, by directly welding the positive electrode current collecting tab 11 to the lid plate 3, the battery case 2 and the lid plate 3 function as positive electrode terminals, and the negative electrode current collecting tab 12 is welded to the current collector plate 14. By connecting the negative electrode current collecting tab 12 and the terminal 5 through the current collector plate 14, the terminal 5 functions as a negative electrode terminal. In some cases, the reverse is also true.

  As the battery case 2, a metal square case is used. However, the shape is not particularly limited, and a metal cylindrical or button-shaped case, a laminate case formed of a laminate film, or the like may be used. Good.

  Although the manufacturing method of the lithium secondary battery 1 is not particularly limited, the battery case 2 and the cover plate 3 are completely connected after the positive electrode 6, the negative electrode 7, the separator 8 and the nonaqueous electrolyte are stored in the battery case 2. It is preferable to charge before sealing. Thereby, the gas generated at the initial stage of charging and the residual moisture in the battery can be removed outside the battery.

  Next, the present invention will be described more specifically with reference to examples. In addition, this invention is not limited to the following Example.

Example 1
A lithium ion secondary battery having the same structure as the prismatic lithium secondary battery shown in FIG. 1 was produced as follows.

<Preparation of positive electrode>
First, a positive electrode active material having a composition of Li _1.02 Mn _0.33 Ni _0.33 Co _0.33 Zr _0.01 O ₂ was synthesized as follows.

  First, a sodium hydroxide aqueous solution was added to nickel, manganese and cobalt sulfates mixed so as to have the above composition ratio to cause precipitation, and then sufficiently washed with water and dried to obtain a coprecipitated hydroxide. Next, lithium hydroxide and zirconium oxide were added to the coprecipitated hydroxide and mixed well, followed by firing in an air atmosphere at a temperature of 900 ° C. for 12 hours to obtain a composite oxide.

  Next, N-methyl containing 96 parts by weight of the positive electrode active material, 1 part by weight of artificial graphite (conductive agent), and 1 part by weight of acetylene black (conductive agent) at a ratio of 8% by weight of PVDF (binder). In addition to the -2-pyrrolidone (NMP) solution, the mixture was mixed so that the total PVDF weight ratio was 2% by weight to prepare a positive electrode mixture paste.

  Subsequently, a predetermined amount of the positive electrode mixture paste was applied to both surfaces of an aluminum foil (thickness 15 μm) serving as a current collector, and then vacuum drying was performed at 120 ° C. for 12 hours to produce a positive electrode. Next, calendar treatment was performed to adjust the thickness of the positive electrode active material layer so that the thickness of the entire positive electrode was 87 μm (thickness of the positive electrode active material layer on one side: 36 μm). Next, it cut | disconnected so that a width | variety might be set to 43 mm, and produced the strip | belt-shaped positive electrode. Moreover, the positive electrode current collection tab which consists of nickel was attached to the exposed part of aluminum foil.

<Production of negative electrode>
To 97.5 parts by weight of massive natural graphite (negative electrode active material), 1.5 parts by weight of styrene butadiene rubber (binder), and 1 part by weight of carboxymethyl cellulose (thickener), water is added and mixed to mix the negative electrode. An agent paste was prepared.

Next, a predetermined amount of the above-mentioned negative electrode mixture paste was applied to both surfaces of a copper foil (thickness 8 μm) serving as a current collector, and then vacuum-dried at 120 ° C. for 12 hours to produce a negative electrode. Next, calendar treatment was performed to adjust the thickness of the negative electrode active material layer so that the total thickness of the negative electrode was 78 μm (the thickness of the negative electrode active material layer on one side: 35 μm). The density of the negative electrode active material layer was 1.65 g / cm ³ . Next, it cut | disconnected so that a width | variety might be set to 45 mm, and produced the strip | belt-shaped negative electrode. Moreover, the negative electrode current collection tab which consists of nickel was attached to the exposed part of copper foil.

<Battery assembly>
Separator A composed of a microporous polyethylene film (thickness: 18 μm, pore diameter: 0.5-1 μm, porosity: 49%, air permeability: 90 seconds / 100 cm ³ ) between the positive electrode and the negative electrode Was wound in a spiral shape, and then pressed to form a flat shape to produce an electrode body.

  Next, the electrode body was filled into a bottomed cylindrical outer can made of aluminum (bottom surface 34 mm × 4.0 mm, height 50.0 mm). The positive electrode was welded to the lid of the outer can (positive electrode terminal) via the positive electrode current collecting tab, and the negative electrode was welded to the terminal (negative electrode terminal) provided on the lid of the outer can via the negative electrode current collecting tab.

Subsequently, 1.0 mol of LiPF ₆ in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC) (EC: MEC: DEC mixed volume ratio is 1: 1: 3). An electrolytic solution (electrolyte) in which / L was dissolved was prepared. Further, 2% by weight of 1,3-propane sultone (a sulfur compound that decomposes at a charging voltage of 4.1 V or more and 4.3 V or less) is added to the electrolytic solution in a weight ratio of the whole electrolytic solution. The electrolyte solution was as follows.

  Finally, this electrolyte solution was poured into the outer can, and the electrolyte solution was sufficiently permeated into the electrode body, which was then sealed to produce a lithium ion secondary battery of this example.

(Example 2)
A lithium ion secondary battery was fabricated in the same manner as in Example 1 except that the composition of the positive electrode active material was Li _1.05 Mn _0.33 Ni _0.33 Co _0.33 Ti _0.01 O ₂ .

(Example 3)
Lithium was obtained in the same manner as in Example 1 except that the composition of the positive electrode active material was Li _0.99 Mn _0.33 Ni _0.33 Co _0.33 Ge _0.01 O ₂ and the amount of 1,3-propane sultone added in the electrolyte was 3% by weight. An ion secondary battery was produced.

Example 4
Example 1 except that the composition of the positive electrode active material was Li _1.02 Mn _0.33 Ni _0.33 Co _0.33 Sn _0.01 O ₂ and 2% by weight of propane cyclic sulfate was added to the electrolyte instead of 1,3-propane sultone. In the same manner, a lithium ion secondary battery was produced.

(Example 5)
First, a positive electrode active material having a composition of Li _1.05 Mg _0.01 Co _0.985 Al _0.01 Ti _0.005 O ₂ was synthesized as follows.

  First, cobalt hydroxide, lithium carbonate, aluminum hydroxide, magnesium oxide and anatase-type titanium dioxide are sufficiently mixed so as to have the above composition ratio, and then calcined in an air atmosphere at a temperature of 900 ° C. for 12 hours. Thus, a composite oxide was obtained.

  A lithium ion secondary battery was prepared in the same manner as in Example 1 except that the composite oxide was used as a positive electrode active material and 2% by weight of propane cyclic sulfate was added to the electrolyte instead of 1,3-propane sultone. Was made.

(Example 6)
A lithium ion secondary battery was produced in the same manner as in Example 5 except that the composition of the positive electrode active material was Li _0.99 Mg _0.01 Co _0.985 Al _0.01 Ge _0.005 O ₂ .

(Example 7)
A lithium ion secondary battery was fabricated in the same manner as in Example 5 except that the composition of the positive electrode active material was Li _0.99 Mg _0.01 Co _0.985 Al _0.01 Zr _0.005 O ₂ .

(Comparative Example 1)
Lithium was obtained in the same manner as in Example 1 except that the composition of the positive electrode active material was Li _1.02 Mn _0.34 Ni _0.34 Co _0.32 O ₂ and the amount of 1,3-propane sultone added in the electrolyte was 0.5 wt%. An ion secondary battery was produced.

(Comparative Example 2)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the composition of the positive electrode active material was LiCoO ₂ and 1,3-propane sultone was not added to the electrolytic solution.

(Comparative Example 3)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the composition of the positive electrode active material was LiCoO ₂ .

(Comparative Example 4)
The composition of the positive electrode active material was Li _0.99 Mg _0.01 Co _0.985 Al _0.01 Zr _0.005 O ₂ and the amount of 1,3-propane sultone added in the electrolyte was 4 wt%. An ion secondary battery was produced.

(Comparative Example 5)
A lithium ion secondary battery was fabricated in the same manner as in Example 1 except that the composition of the positive electrode active material was LiCo _0.97 Al _0.03 O ₂ and 1,3-propane sultone was not added to the electrolytic solution.

  Next, the cycle characteristics and the storage characteristics of the lithium ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 5 were evaluated. Moreover, the sulfur content of the positive electrode surface of each battery was measured. For the following evaluation and measurement, batteries immediately after production were used.

<Cycle characteristics>
The cycle characteristics were evaluated by the capacity retention rate after 200 cycles. The capacity retention rate was expressed as a ratio (%) of the discharge capacity at 200 cycles to the initial capacity.

  First, constant current charging was performed up to 4.4 V at 0.2 A for each battery, and then constant voltage charging was performed until the current value reached 0.02 A. Next, constant current discharge was performed to 0.2V at 0.2 A, and the initial capacity was obtained.

  The charge / discharge cycle was performed as follows. Charging was performed at a constant current of up to 4.4 V at 0.5 A, and then at a constant voltage until the current value reached 0.02 A. The discharge was a constant current discharge up to 3.0V at 1A. This charging / discharging was made into 1 cycle, and it repeated to 20 cycles at 20 degreeC.

<Storage characteristics>
The storage characteristics were evaluated by the residual capacity ratio after storage at 60 ° C. for 20 days. The remaining capacity ratio was expressed as a ratio (%) of the remaining capacity after storage to the initial capacity.

  First, the initial capacity of each battery was determined in the same manner as described above. Next, constant current charging was performed up to 4.4 V at 0.2 A, and then constant voltage charging was performed until the current value reached 0.02 A. Thereafter, each battery was stored at 60 ° C. for 20 days. Each battery after storage was discharged at a constant current of 0.2 A to 3.0 V to determine the remaining capacity.

<Measurement of sulfur content on positive electrode surface>
The content (atomic%) of sulfur existing as a bond represented by —SO _n — (1 ≦ n ≦ 4) on the surface of the positive electrode was analyzed by X-ray photoelectron spectroscopy (XPS).

First, constant current charging was performed up to 4.4 V at 0.2 A for each battery, and then constant voltage charging was performed until the current value reached 0.02 A. Then, each battery was decomposed | disassembled in argon atmosphere, the positive electrode was pick_out | removed, and it measured using the X-ray photoelectron diffraction apparatus "5500MC" by ULVAC-PHI Co., Ltd. under argon atmosphere. The measurement conditions were an X-ray source of MgKα, an output of 400 W (15 kV), a photoelectron extraction angle of 45 degrees, an aperture diameter of 800 μm, and an analysis depth of about 3 to 5 μm. 166.5 eV (1 ≦ n ≦ 2), 168.3 eV (2 ≦ n ≦ 3), 169. attributable to S2p in the compound having a bond represented by —SO _n — (1 ≦ n ≦ 4). The atomic% of sulfur was calculated from the intensity of 2 eV (3 ≦ n ≦ 4).

Incidentally, -SO _n present on the surface of the positive electrode - Examples of the compound having the bond represented by (1 ≦ n ≦ 4), for _{example, C 2 H 5 -SO 3 -R} , C 3 H 5 -SO 3 -R , C ₃ H ₅ SO ₄ —R, C ₃ H ₅ OH—O—CH ₂ —CH ₂ —SO ₃ —R, and the like are considered to be applicable.

  The results are shown in Table 1. In addition, the positive electrode active material composition of Table 1 shows a preparation value.

<Confirmation of positive electrode active material composition>
The composition of the positive electrode active material was confirmed using ICP spectroscopy for the positive electrode mixture layer of the produced battery. First, constant current charging was performed up to 4.4 V at 0.2 A for each battery, and then constant voltage charging was performed until the current value reached 0.02 A. Next, constant current discharge was performed to 0.2V at 0.2A. Then, each battery was disassembled, the positive electrode was taken out, the positive electrode mixture was peeled off from the positive electrode, and 0.2 g from the positive electrode mixture was collected in a 100 mL container. Thereafter, 5 mL of pure water, 2 mL of aqua regia, and 10 mL of pure water were added in this order and dissolved by heating. After cooling, the mixture was further diluted 25 times and analyzed by ICP spectroscopy. The obtained results almost coincided with the charged values shown in Table 1.

(Example 8)
As a positive electrode active material, a first positive electrode active material having a composition of Li _1.02 Mn _0.33 Ni _0.33 Co _0.33 Ti _0.01 O _2, a second positive electrode active material having a composition of Li _0.99 Mg _0.01 Co _0.985 Al _0.01 Ti _0.005 O ₂ , and A lithium ion secondary battery was fabricated in the same manner as in Example 1 except that the weight mixing ratio of the first positive electrode active material and the second positive electrode active material was 50:50.

Example 9
A lithium ion secondary battery was produced in the same manner as in Example 8, except that the weight mixing ratio of the first positive electrode active material and the second positive electrode active material was 20:80.

(Example 10)
A lithium ion secondary battery was produced in the same manner as in Example 8 except that the weight mixing ratio of the first positive electrode active material and the second positive electrode active material was 10:90.

(Comparative Example 6)
Example except that the weight mixing ratio of the first positive electrode active material and the second positive electrode active material was 90:10, and the addition amount of 1,3-propane sultone in the electrolytic solution was 0.5% by weight. In the same manner as in Example 8, a lithium ion secondary battery was produced.

(Comparative Example 7)
Lithium ions were obtained in the same manner as in Example 8, except that the weight mixing ratio of the first positive electrode active material and the second positive electrode active material was 70:30, and 1,3-propane sultone was not added to the electrolytic solution. A secondary battery was produced.

  Next, for the lithium ion secondary batteries of Examples 8 to 10 and Comparative Examples 6 and 7, evaluation of cycle characteristics and storage characteristics and measurement of the sulfur content on the positive electrode surface of each battery were performed in the same manner as described above. It was. The results are shown in Table 2.

  From Table 1, it can be seen that Examples 1 to 7 are more excellent in cycle characteristics (capacity maintenance ratio) and storage characteristics (residual capacity ratio) than Comparative Examples 1 to 5. Further, from Table 2, Comparative Examples 6 and 7 using the same positive electrode active material also in Examples 8 to 10 using the positive electrode active material obtained by mixing the first positive electrode active material and the second positive electrode active material. It can be seen that the cycle characteristics (capacity retention ratio) and the storage characteristics (remaining capacity ratio) are superior to Furthermore, it can be seen from Tables 1 and 2 that storage characteristics (remaining capacity ratio) are better when the mixed positive electrode active material is used than when the positive electrode active material is used alone.

(Example 11)
A negative electrode active material composed of 0.5% by weight of vapor-phase-generated carbon fibers (average fiber diameter: 50 nm, average fiber length: about 2 μm) and 99.5% by weight of massive natural graphite in the total weight ratio of the negative electrode active material Except for using 97.5 parts by weight, 1.5 parts by weight of styrene butadiene rubber (binder), and 1 part by weight of carboxymethyl cellulose (thickener) and using a negative electrode mixture paste prepared by adding water and mixing. Produced a negative electrode in the same manner as in Example 1.

Further, as the separator, a separator C made of a microporous polyethylene film (thickness: 18 μm, pore diameter: 0.5 to 1 μm, porosity: 50%, air permeability: 100 seconds / 100 cm ³ ) was used.

In addition, as an electrolytic solution, LiPF ₆ was mixed in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC) (EC: MEC: DEC mixed volume ratio is 1: 1: 3). An electrolytic solution in which 1.0 mol / L was dissolved was prepared. Further, the electrolyte solution was mixed with 1% by weight of 1,3-propane sultone (a sulfur compound decomposed at a charging voltage of 4.1 V or more and 4.3 V or less) and 3% by weight of vinylene carbonate (by weight of the whole electrolyte solution). Solvent) was added.

  A lithium ion secondary battery was produced in the same manner as in Example 1 except for the above.

(Example 12)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Example 2 was used and the weight ratio of the vapor-grown carbon fiber in the negative electrode active material was 1% by weight.

(Example 13)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Example 5 was used.

(Example 14)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Example 6 was used.

(Example 15)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Example 7 was used.

(Example 16)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Comparative Example 1 was used and no gas-phase-generated carbon fiber was added to the negative electrode active material.

(Example 17)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Example 7 was used and the weight ratio of the vapor-generated carbon fiber in the negative electrode active material was changed to 4% by weight.

  The negative electrode calendering conditions in Examples 11 to 17 were all performed under the same conditions as in Example 16 on the basis of the conditions in Example 16.

  Next, the cycle characteristics and the storage characteristics of the lithium ion secondary batteries of Examples 11 to 17 were evaluated in the same manner as described above. Moreover, the density of the negative electrode active material layer of each battery was measured. The results are shown in Table 3.

  From Table 3, it can be seen that Examples 11 to 17 have excellent cycle characteristics (capacity retention ratio) and storage characteristics (residual capacity ratio). Table 3 also shows that the density of the negative electrode active material decreases when the weight ratio of the vapor-generated carbon fiber in the negative electrode active material exceeds 3% by weight. Moreover, when the sulfur content on the positive electrode surface of each battery was measured in the same manner as described above, all were within the range of 0.2 to 1.5 atomic%.

(Example 18)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Example 8 was used.

(Example 19)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Example 9 was used.

(Example 20)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Example 10 was used.

(Example 21)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Example 8 was used and no gas-phase-generated carbon fiber was added to the negative electrode active material.

(Example 22)
A lithium ion secondary battery was produced in the same manner as in Example 11 except that the positive electrode active material used in Example 9 was used and no vapor-phase-generated carbon fiber was added to the negative electrode active material.

  Next, the cycle characteristics and the storage characteristics of the lithium ion secondary batteries of Examples 18 to 22 were evaluated in the same manner as described above. Moreover, the density of the negative electrode active material layer of each battery was measured. The results are shown in Table 4.

  From Table 4, also in Examples 18 to 22 using the positive electrode active material obtained by mixing the first positive electrode active material and the second positive electrode active material, cycle characteristics (capacity retention ratio) and storage characteristics (remaining capacity ratio) It turns out that is excellent. However, it can be seen from Table 4 that when the weight ratio of the vapor-generated carbon fiber in the negative electrode active material is less than 0.5% by weight, the cycle characteristics and the storage characteristics are somewhat deteriorated. Moreover, when the sulfur content on the positive electrode surface of each battery was measured in the same manner as described above, all were within the range of 0.2 to 1.5 atomic%.

  As described above, the present invention can provide a non-aqueous electrolyte secondary battery that is superior in cycle characteristics and storage characteristics while realizing high capacity by high voltage charging. This non-aqueous electrolyte secondary battery can be widely used not only as a power source for mobile devices but also as a power source for various devices.

It is a perspective view which shows an example of the lithium ion secondary battery of this invention. It is sectional drawing of the II line | wire of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 2 Battery case 3 Cover plate 4 Insulation packing 5 Terminal 6 Positive electrode 7 Negative electrode 8 Separator 9 Electrode body 10 Insulator 11 Positive electrode current collection tab 12 Negative electrode current collection tab 13 Insulator 14 Current collection plate

Claims (3)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte,
The positive electrode includes at least one positive electrode active material selected from a first positive electrode active material and a second positive electrode active material,
The first positive electrode active materials, the general composition formula _{Li (1 + a) Mn x} Ni y Co (1-xyz) M z O 2 ( where, M is, Ti, Zr, Nb, Mo , W, Al, It is at least one element selected from the group consisting of Si, Ga, Ge, and Sn, and −0.15 <a <0.15, 0.1 <x ≦ 0.5, 0.6 <x + y + z <1. 0, 0 ≦ z ≦ 0.1),
The second cathode active material, the general composition formula _{Li (1-sb) Mg s} Co (1-tu) Al t M 'u O 2 ( where, M' is selected Ti, from the group consisting of Zr and Ge At least one element, represented by 0.01 ≦ s <0.1, 0 <u <0.1, 0.01 <t + u <0.1, −0.06 ≦ b <0.05). And
On the surface of the positive electrode, there is a compound having a bond represented by -SO _n- (1 ≦ n ≦ 4),
Wherein -SO _n to the positive pole of the surface - the content of sulfur present as bond represented by (1 ≦ n ≦ 4), when analyzed by X-ray photoelectron spectroscopy, or 0.2 atomic% 1. A nonaqueous electrolyte secondary battery characterized by being 5 atomic% or less. The negative electrode includes a first negative electrode active material and a second negative electrode active material,
The first negative electrode active material is made of a carbon material,
The second negative electrode active material is made of a carbon material having higher conductivity than the first negative electrode active material,
2. The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of the second negative electrode active material is a total weight ratio of the negative electrode active material and is 0.5 wt% or more and 3 wt% or less.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous electrolyte contains a sulfur compound that decomposes at a charging voltage of 4.1 V or more and 4.3 V or less.

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