Lithium-ion rechargeable battery

Abstract

This invention provides a lithium-ion secondary battery containing allyl succinic anhydride as an electrolyte additive, which exhibits excellent initial charge efficiency and output characteristics. [Solution] The lithium-ion secondary battery comprises a positive electrode containing a positive electrode active material capable of intercepting and releasing lithium ions, a negative electrode containing a negative electrode active material capable of intercepting and releasing lithium ions, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte containing a lithium salt, a non-aqueous solvent, and an electrolyte additive. The negative electrode active material contains natural graphite and artificial graphite, the non-aqueous solvent contains ethylene carbonate, diethyl carbonate, ethyl propionate, and propyl propionate, and the electrolyte additive contains allyl succinic anhydride and dinitrile, with the electrolyte additive containing dinitrile in a proportion of 1.5% by mass or more and 3.2% by mass or less.

Description

[Technical Field]

[0001] This invention relates to a lithium-ion secondary battery. [Background technology]

[0002] In recent years, lithium-ion rechargeable batteries have come to be used as power sources for electronic devices in a wide range of applications, including portable devices, electric vehicles, homes, drones, robots, and storage batteries for commercial facilities. With the widespread use of lithium-ion rechargeable batteries, there is a growing demand for batteries with superior initial charge efficiency, high output, and excellent cycle performance. However, especially under high-temperature conditions, the degradation of lithium-ion secondary batteries is accelerated, which may significantly reduce their storage and cycle performance, potentially preventing them from meeting the required characteristics.

[0003] Therefore, in order to improve the rapid degradation under high-temperature conditions, the addition of additives to the non-aqueous electrolyte is being considered. For example, Patent Document 1 states that by adding allyl succinic anhydride to the electrolyte, lithium secondary batteries with good cycle characteristics and high-temperature storage characteristics can be supplied. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 6453611 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, while the addition of allyl succinic anhydride does improve high-temperature characteristics, our research has revealed that the addition of allyl succinic anhydride forms a highly resistive film on the negative electrode surface, significantly reducing the initial charging efficiency and output characteristics.

[0006] The object of the present invention is to provide a lithium-ion secondary battery containing allyl succinic anhydride as an electrolyte additive that exhibits excellent initial charge efficiency and output characteristics. [Means for solving the problem]

[0007] To solve the above problems, one aspect of the present invention provides a lithium-ion secondary battery having the following configurations (1) to (5).

[0008] (1) A positive electrode comprising a positive electrode active material capable of intercepting and releasing lithium ions, a negative electrode comprising a negative electrode active material capable of intercepting and releasing lithium ions, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte comprising a lithium salt, a non-aqueous solvent, and an electrolyte additive. (2) The negative electrode active material includes natural graphite and artificial graphite. (3) The non-aqueous solvent includes ethylene carbonate, diethyl carbonate, ethyl propionate, and propyl propionate. (4) The electrolyte additive includes allyl succinic anhydride and dinitrile. (5) The electrolyte additive contains dinitrile in an amount of 1.5% by mass or more and 3.2% by mass or less. [Effects of the Invention]

[0009] According to the present invention, it is expected that a lithium-ion secondary battery with excellent initial charge efficiency and output characteristics can be provided, while containing allyl succinic anhydride as an electrolyte additive. [Brief explanation of the drawing]

[0010] [Figure 1] The Nyquist plots for Examples 1-5 and Comparative Examples 1 and 2 were obtained by electrochemical impedance measurements. [Figure 2] The Nyquist plots for Examples 6-10 and Comparative Examples 1 and 3 were obtained by electrochemical impedance measurements. [Modes for carrying out the invention]

[0011] The embodiments of the present invention will be described below, but the present invention is not limited to the embodiments shown below. The embodiments shown below have technically preferred limitations for carrying out the present invention, but these limitations are not essential requirements of the present invention.

[0012] This embodiment is a stacked lithium-ion secondary battery comprising an outer casing, an electrode group housed inside the outer casing, and positive and negative electrode terminals extending outside the outer casing. The electrode group consists of multiple positive and negative electrode plates stacked alternately with separators in between. A non-aqueous electrolyte containing a lithium salt, a non-aqueous solvent, and an electrolyte additive is injected into the inside of the outer casing. Each component of the stacked lithium-ion secondary battery of this embodiment is described below.

[0013] [Positive electrode] (Cathode active material layer) The positive electrode active material layer contains a positive electrode active material capable of storing and releasing lithium ions, and other components include, for example, a conductive agent and a binder. Examples of positive electrode active materials include LiCoO2, LiNiO2, LiMn2O4, LiFePO4, LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, LiCo 0.15 Ni 0.8 Al 0.05 O2, LiLiLi 0.5 Mn 1.5 Lithium-containing transition metal oxides such as O4 can be used.

[0014] (Positive electrode current collector) The positive electrode current collector is not particularly limited and can be any known or commercially available material, such as metal foils like aluminum foil, copper foil, or stainless steel foil, or porous metals like porous aluminum. (Conductive additive for positive electrode) The conductive agent is not particularly limited, and known or commercially available ones can be used. For example, carbon blacks such as acetylene black and ketjen black, carbon nanotubes, carbon fibers, activated carbon, graphite, etc. can be used. These can be used alone or in combination of multiple types.

[0015] (Binder for positive electrode) The binder is not particularly limited, and known or commercially available ones can be used. For example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyvinyl pyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene, ethylene - propylene copolymer, styrene - butadiene rubber (SBR), acrylic resin, etc. can be used. (Solvent for positive electrode) As the solvent for organic slurries, N - methyl - 2 - pyrrolidone (NMP) can be used. Also, as the solvent for aqueous slurries, water can be used.

[0016] [Negative electrode] (Negative electrode active material layer) The negative electrode active material layer contains a negative electrode active material capable of storing and releasing lithium ions, and contains, for example, a conductive agent and a binder as components other than the active material. As the negative electrode active material, a mixture of natural graphite and artificial graphite is used. The mixing ratio of natural graphite and artificial graphite is, for example, a mass ratio of natural graphite: artificial graphite = 9:1 to 1:9. Also, it is preferable that the ratio of natural graphite is higher than the ratio of artificial graphite, and it is more preferable that natural graphite: artificial graphite = 7:3.

[0017] (Binder for negative electrode) The binder is not particularly limited, and known or commercially available ones can be used. For example, PTFE, PVdF, fluorine - based rubber, SBR, carboxymethyl cellulose (CMC), core - shell binder, polyvinyl alcohol, polyimide, imide - based resins such as polyamideimide, etc. can be used. (Conductive agent for negative electrode) The conductive agent may or may not be added. When added, the same ones as those used for the positive electrode can be used.

[0018] (Solvent for negative electrode) For organic slurries, N-methyl-2-pyrrolidone (NMP) can be used as the solvent. For aqueous slurries, water can be used. (Negative electrode current collector) The negative electrode current collector is not particularly limited and can be any known or commercially available material. For example, metal foils such as aluminum foil, copper foil, or stainless steel foil, or porous metals such as porous aluminum can be used.

[0019] [Separator] The separator is not particularly limited and can be any known or commercially available material. For example, nonwoven fabrics made of synthetic resins such as polyethylene (PE), polyolefin resins such as polypropylene, or PTFE, or porous sheets can be used.

[0020] [Nonaqueous electrolyte] The non-aqueous electrolyte contains a lithium salt, a non-aqueous solvent, and electrolyte additives. (Non-aqueous solvent) Non-aqueous solvents mainly consist of cyclic carbonates, linear carbonates, and linear esters. Examples of cyclic carbonates include cyclic esters such as ethylene carbonate (EC), propylene carbonate, butylene carbonate, γ-butyrolactone, 2-methyl-γ-butyrolactone, acetyl-γ-butyrolactone, and γ-valerolactone. Cyclic carbonates can be used individually or as mixtures of several of these. Examples of linear carbonates include dimethyl carbonate, diethyl carbonate (DEC), and ethyl methyl carbonate. These linear carbonates can be used individually or as mixtures of several of them.

[0021] Examples of linear esters include alkyl propionates such as methyl propionate, ethyl propionate (EP), and propyl propionate (PP), as well as dialkyl malonates and alkyl acetates. These linear esters can be used individually or as mixtures of several of them. Furthermore, it is more preferable that the total amount of ethyl propionate and propyl propionate is 30% by volume or more and 60% by volume or less relative to the non-aqueous solvent of the non-aqueous electrolyte.

[0022] Ethyl propionate has low viscosity and excellent power characteristics, but it cannot suppress side reactions at each electrode. On the other hand, propyl propionate is excellent at suppressing side reactions at each electrode, but its viscosity is higher than that of ethyl propionate, resulting in reduced power characteristics. Therefore, by including both ethyl propionate and propyl propionate, it is possible to achieve a balance between power characteristics and cycle characteristics. Furthermore, by increasing the volume percentage of ethyl propionate above the volume percentage of propyl propionate, the decrease in power characteristics can be further suppressed.

[0023] The ethyl propionate content is usually about 5-80% by volume relative to the non-aqueous electrolyte's non-aqueous solvent, but is preferably 10-60% by volume from the viewpoint of its effect on output characteristics and its impact on the cycle. Similarly, the propyl propionate content is usually about 5-80% by volume relative to the non-aqueous electrolyte's non-aqueous solvent, but is preferably 10-60% by volume from the viewpoint of its effect on output characteristics and its impact on the cycle.

[0024] Furthermore, by including both ethyl propionate and propyl propionate, and setting their total amount to 30-60% by volume, it is possible to achieve a balance between the effects of power output characteristics and cycle characteristics, and to further improve them. A lithium-ion secondary battery according to one embodiment of the present invention comprises ethylene carbonate, diethyl carbonate, ethyl propionate, and propyl propionate as a non-aqueous solvent.

[0025] (Lithium salt) The lithium salt is not particularly limited, and lithium salts commonly used in lithium-ion secondary batteries can be used. For example, inorganic lithium salts such as LiBF4, LiPF6, LiAsF6, LiClO4, LiPO3F, and LiBOB, or organic lithium salts such as LiCF3CFO, LiSO3CF3, LiCF2CF2SO3, LiN(COCF2CF3)2, and LiN(SO2CF3)2 can be used. LiPF6 is preferred as the electrolyte due to its high conductivity. The electrolyte can be used as one of these or as a mixture of several. Furthermore, it is preferable that this electrolyte dissolves in a non-aqueous solvent at a concentration of 0.3 to 3.0 mol / L, preferably 0.5 to 1.5 mol / L.

[0026] (Electrolyte additive) The electrolyte additive (hereinafter sometimes simply referred to as "additive") contains allylsuccinic anhydride (ASAh). The amount of allylsuccinic anhydride added is preferably 0.25% by mass or more and 1.0% by mass or less. From the viewpoint of ensuring a good effect on cycle characteristics and storage characteristics, the amount of allylsuccinic anhydride added is preferably 0.25% by mass or more. Furthermore, since excessively high allylsuccinic anhydride content can lead to increased resistance, the content of allylsuccinic anhydride is preferably 1.0% by mass or less.

[0027] Furthermore, the electrolyte additive may also contain additives other than allyl succinic anhydride. Nitrile-based additives such as succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, and suberonitrile, as well as vinylene carbonate (VC), fluoroethylene carbonate, 1,3-dioxane (DO), lithium bisoxalate borate, lithium tetrafluoroborate (LiBF4), propanesultone, succinonitrile, acetonitrile, 1,3,6-hexanetricarbonate, and 1,2,3-tris(2-cyanoethoxy)propane can be used individually or in combination of two or more of these.

[0028] Furthermore, it is preferable to include, as an electrolyte additive other than allyl succinic anhydride, one or more dinitriles selected from succinonitrile, adiponitrile, pimelonitrile, and suberonitrile in a total proportion of 1.5% by mass or more and 3.2% by mass or less. If the amount of dinitrile added is excessive, it can accelerate cycle degradation and degradation during high-temperature storage; therefore, it is preferable to keep the amount to 3.2% by mass or less. Dinitrile mainly adsorbs onto the surface of the positive electrode and can suppress side reactions with the electrolyte, thereby suppressing side reactions with the electrolyte during continuous charging in a high-temperature environment. This suppresses swelling during continuous charging. If the amount of dinitrile added is less than 1.5% by mass, the above effects are not substantially obtained.

[0029] Furthermore, as an electrolyte additive other than allyl succinic anhydride, it is preferable to include vinylene carbonate (VC) in a proportion of 0.1% to 10.0% by mass, together with the above-mentioned dinitrile.

[0030] (Other additives) The non-aqueous electrolyte may contain other additives such as carbonates and their derivatives, vinylethylene carbonate, unsaturated carboxylic acid esters, phosphate esters, boric acid esters, or alcohols.

[0031] [Exterior laminating film] The laminate film constituting the exterior body includes, for example, a protective layer, a metal layer, and a heat-sealable resin layer. The protective layer, metal layer, and heat-sealable resin layer are formed from one or more layers, each made of, for example, materials described later. An adhesive layer may be formed between each layer. The protective layer is not particularly limited and can be made of known or commercially available materials, such as nylon or polyethylene terephthalate (PET). The metal layer is not particularly limited and can be a known or commercially available material, and is formed from a metal foil containing at least one metal selected from the group consisting of iron, nickel, copper, tin, and aluminum. The metal layer is preferably formed from aluminum foil or stainless steel foil. The heat-sealable resin layer is not particularly limited and can be any known or commercially available material, and is formed from, for example, a polyolefin resin such as polypropylene or PE.

[0032] [Insulating tape for electrode groups] Insulating tape is applied to two locations on each of the two sides of the electrode group where there are no positive or negative electrode leads, with the aim of preventing misalignment of the positive electrode plate, negative electrode plate, and separator in the electrode group. An insulating tape comprises, for example, a base material and an adhesive layer. The base material may be made of a resin, from the viewpoint of being readily available and low-cost. The type of resin is not particularly limited as long as it has appropriate elasticity, flexibility, and insulating properties. Examples of resins include polyimide, polyamide (such as aromatic polyamide), polyamide-imide, polyolefin (such as polypropylene), polyester (such as polyethylene naphthalate), polyphenylsulfone (PPS), and polyphenylene sulfide. These resins may be used individually or in combination. The adhesive layer contains an adhesive, and resin materials other than silicone can be used as the adhesive. Examples include acrylic resin, natural rubber, synthetic rubber (such as butyl rubber), epoxy resin, melamine resin, and phenolic resin. These may be used individually or in combination of multiple types.

[0033] [Manufacturing method for lithium-ion secondary batteries] An example of a manufacturing method for a lithium-ion secondary battery according to this embodiment will be described. To fabricate the positive electrode plate, first, a positive electrode slurry is prepared by mixing the positive electrode active material, conductive agent, binder, and solvent. Next, this positive electrode slurry is applied to both sides of the positive electrode current collector. After drying the solvent and compressing the material, the positive electrode plate is fabricated by cutting the rectangular positive electrode current collector, which has a positive electrode layer formed on one side, so that a strip-shaped positive electrode current collector lead extends from it. The negative electrode plate is first prepared by mixing the negative electrode active material, binder, and solvent to create a negative electrode slurry. Next, this negative electrode slurry is applied to both sides of the negative electrode current collector. After drying the solvent and compressing the material, the negative electrode plate is fabricated by cutting it so that a strip-shaped negative electrode current collector lead extends from one side of the rectangular negative electrode current collector on which the negative electrode layer has formed.

[0034] Next, the fabricated positive electrode plate, the fabricated negative electrode plate, and the separator are stacked alternately so that the separator is between the positive and negative electrode plates to create an electrode group. At this time, the positive electrode current collector lead and the negative electrode current collector lead are stacked so that they extend from one side of the electrode group. Then, each current collector lead extending from the electrode group is brought together towards the center along the stacking direction of the electrode group, and the ends of each gathered current collector lead are placed on the upper surface of each terminal and joined to each other by ultrasonic welding.

[0035] Next, two laminate films are prepared: one with a bottomed, hollow rectangular plate-shaped receiving recess and a flat portion surrounding the receiving recess, and the other with a rectangular plate-shaped laminate film. Two laminate films are placed with their heat-sealable resin layers facing each other, and the laminate films are overlapped so that the electrode group and a portion of each current collector lead are housed within the recessed area. However, it is also possible to fold a single laminate film in half and overlap it. In this case, the portion where the heat-sealable resin part of each terminal is formed passes between the periphery of the laminate films, and the electrode group is arranged so that a portion of each terminal is exposed to the outside. Then, a non-aqueous electrolyte is injected through one side of the outer casing that is not heat-sealed. Next, the remaining side of the outer casing is heat-sealed under reduced pressure to obtain a lithium-ion secondary battery.

[0036] [Chemical conversion process] It is preferable that lithium-ion secondary batteries undergo pretreatment such as initial charging and gas exhaust before being used as batteries. For example, as part of the initial charging, after injecting a non-aqueous electrolyte, constant current charging is performed at a small current value until the battery reaches 10% of its rated capacity as a preliminary charge. After this initial charge, the gas generated inside the casing during the initial charge is vented. This gas venting is performed, for example, by opening a portion of the sealing part of the casing and placing it inside a pressure-reducing chamber. Gas venting can be accelerated, for example, by applying external pressure to the casing during gas venting. By performing the initial charge with a small current value in this way, the negative electrode potential remains at a range of 1.2 to 0.8 V vs. Li / Li+, which is conducive to the formation of the SEI film, for a longer period of time. As a result, a denser and stronger SEI film can be formed on the surface of the negative electrode active material.

[0037] Furthermore, during the initial charging phase, there is a high possibility that the non-aqueous electrolyte is insufficiently impregnated into the constituent materials of the lithium-ion secondary battery, increasing the risk of lithium deposition due to overvoltage. Therefore, performing the initial charging with such a small current value can suppress the occurrence of overvoltage and thus suppress lithium deposition. Furthermore, performing the initial charge with a small current value promotes gas generation during the initial charge, and the gas can be efficiently discharged through a portion of the sealing part of the casing that is opened. Sufficient gas exhaust can suppress the expansion of the casing caused by gas generation when using lithium-ion secondary batteries.

[0038] After exhausting the gas, the battery is charged to the manufacturer's predetermined charging capacity to reach a fully charged state. In this fully charged state, the battery is left to stand at a temperature higher than room temperature (25°C) for a predetermined time to promote the decomposition of the electrolyte and strengthen the SEI coating on the surface of the negative electrode active material. After the standing process, the battery is charged to the manufacturer's predetermined upper voltage limit to reach a fully charged state. [Examples]

[0039] The present invention will be further described below with reference to specific examples. [Example 1] <Fabrication of the positive electrode> 93.7 parts by mass of lithium cobalt oxide (LiCoO₂) as the positive electrode active material, 2 parts by mass of polyvinylidene fluoride (PVDF) as the binder, 4 parts by mass of carbon black as the conductive agent, and 0.3 parts by mass of polyvinylpyrrolidone (PVP) as the dispersant were dispersed in N-methyl-2-pyrrolidone (NMP) as the solvent to prepare a positive electrode slurry. Next, the positive electrode slurry was coated on both sides of an aluminum foil (thickness 12 μm) serving as the positive electrode current collector with a single-sided coating amount of 144 g / m 2 by means of a coater so as to achieve this value, and dried at 80 to 130 °C. Thereafter, it was press-processed until the electrode density reached 3.5 g / cc to fabricate a positive electrode.

[0040] <Fabrication of Negative Electrode> As the negative electrode active material, a mixture of 88.2 parts by mass of natural graphite and 9.8 parts by mass of artificial graphite was used. The ratio of natural graphite to artificial graphite in the negative electrode active material is a mass ratio of natural graphite:artificial graphite = 9:1. First, this mixture and 1 part by mass each of styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as binders were dispersed in ion-exchanged water as the solvent to prepare a negative electrode slurry. Next, the obtained negative electrode slurry was coated on both sides of a copper foil (thickness 6 μm) serving as the negative electrode current collector with a single-sided coating amount of 76 g / m 2 by means of a coater so as to achieve this value, and dried at 80 to 110 °C. Thereafter, it was press-processed until the electrode density reached 1.6 g / cc to fabricate a negative electrode.

[0041] <Separator> As the separator, a separator having a coating layer made of AlOOH with a thickness of 3 μm on one side of a microporous membrane made of polyethylene (PE) resin with a thickness of 9 μm was used.

[0042] <Outer Package> As the outer casing, a first laminate film with a rectangular outer shape and a housing recess for housing the electrode group and a flat portion surrounding the housing recess was prepared, and a second laminate film with a rectangular outer shape and a flat portion was prepared. Each laminate film has a structure in which a heat-sealable resin layer made of polyolefin, a metal layer made of aluminum foil, and a protective layer made of polyamide film are laminated in this order.

[0043] <Preparation of non-aqueous electrolyte> The non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.3 mol / L in a solvent made by mixing ethylene carbonate (EC), diethyl carbonate (DEC), ethyl propionate (EP), and propyl propionate (PP) in a volume ratio of 30:10:45:15 (=EC:DEC:EP:PP). In addition, as electrolyte additives, vinylene carbonate (VC) was added at a concentration of 2.25% by mass, allyl succinic anhydride (ASAh) at a concentration of 1.0% by mass, adiponitrile (ADN) at a concentration of 1.25% by mass, and suberonitrile (SBN) at a concentration of 1.25% by mass relative to the total mass of the electrolyte.

[0044] <Assembly of prototype battery> An electrode group was fabricated by alternately stacking separators, eight positive electrode plates, and nine negative electrode plates, with separators interposed between the positive and negative electrode plates. The current collector leads were stacked so that they extended from the sides of the electrode group. Negative electrode plates were positioned at the outermost layers in the stacking direction of the electrode group. Furthermore, separators were placed on both end faces of the electrode group, and insulating tape was applied to secure the electrode group.

[0045] Each positive lead and each negative lead were bundled at their tips and joined together by ultrasonic welding. An aluminum tab was attached to the joint of the positive leads as a positive terminal by ultrasonic welding. A copper tab was attached to the joint of the negative leads as a negative terminal by ultrasonic welding. An electrode group with welded positive and negative terminals is placed in a recess in the first laminate film. A flat second laminate film is then placed over the flat edge surrounding the recess in the first laminate film so that parts of the positive and negative terminals extend outwards. Three of the four overlapping sides of the first and second laminate films are heat-sealed together, and the electrode group is housed within an outer casing made of the first and second laminate films.

[0046] Next, 7.4 g of non-aqueous electrolyte was injected through the remaining side of the outer casing. Then, the side was heat-sealed under reduced pressure of 1 hPa to 100 hPa to obtain the prototype battery after the injection of the non-aqueous electrolyte. The theoretical capacity of this prototype battery is 2.2 Ah.

[0047] <Chemical conversion process> After injecting the non-aqueous electrolyte, the prototype battery was left to stand for 12 hours to allow the non-aqueous electrolyte to permeate the constituent materials of the electrode group. Next, the positive and negative terminals of the prototype battery were connected to a power supply (charge / discharge device), and the prototype battery was initially charged to 10% of its rated capacity at a current of 0.25 ItA. After that, the prototype battery was placed in a depressurized chamber with a portion of the sealing part of the outer casing open to exhaust any gas generated inside the battery. Next, the sealing portion of the outer casing was sealed again, and the prototype battery was charged a second time at a current of 0.5 ItA until it reached 10-100% of its rated capacity. After that, the prototype battery was left to stand in a high-temperature (45°C) environment for 3 hours, then constant current charging was performed at a current of 0.5 ItA until it reached 4.40V, followed by constant voltage charging to 0.02 ItA. After that, it was discharged to 2.5V at a current of 0.2 ItA, and the initial discharge capacity was measured to obtain the prototype battery according to Example 1.

[0048] [Example 2] A lithium-ion secondary battery of Example 2 was obtained in the same manner as in Example 1, except that a mixture of 68.6 parts by mass of natural graphite and 29.4 parts by mass of artificial graphite was used as the negative electrode active material. The ratio of natural graphite to artificial graphite in the negative electrode active material was natural graphite:artificial graphite = 7:3 by mass. [Example 3] A lithium-ion secondary battery of Example 3 was obtained in the same manner as in Example 1, except that a mixture of 49.0 parts by mass of natural graphite and 49.0 parts by mass of artificial graphite was used as the negative electrode active material. The ratio of natural graphite to artificial graphite in the negative electrode active material was natural graphite:artificial graphite = 5:5 by mass.

[0049] [Example 4] A lithium-ion secondary battery of Example 4 was obtained in the same manner as in Example 1, except that a mixture of 29.4 parts by mass of natural graphite and 68.6 parts by mass of artificial graphite was used as the negative electrode active material. The ratio of natural graphite to artificial graphite in the negative electrode active material was natural graphite:artificial graphite = 3:7 by mass. [Example 5] A lithium-ion secondary battery of Example 5 was obtained in the same manner as in Example 1, except that a mixture of 9.8 parts by mass of natural graphite and 88.2 parts by mass of artificial graphite was used as the negative electrode active material. The ratio of natural graphite to artificial graphite in the negative electrode active material was natural graphite:artificial graphite = 1:9 by mass.

[0050] [Comparative Example 1] A lithium-ion secondary battery of Comparative Example 1 was obtained in the same manner as in Example 1, except that 98.0 parts by mass of artificial graphite were used as the negative electrode active material instead of natural graphite. The ratio of natural graphite to artificial graphite in the negative electrode active material was natural graphite:artificial graphite = 0:10 by mass ratio. [Comparative Example 2] A lithium-ion secondary battery of Comparative Example 1 was obtained in the same manner as in Example 1, except that 98.0 parts by mass of natural graphite were used as the negative electrode active material instead of artificial graphite. The ratio of natural graphite to artificial graphite in the negative electrode active material was natural graphite:artificial graphite = 10:0 by mass ratio.

[0051] [Example 6] As electrolyte additives, vinylene carbonate (VC) was added in an amount of 2.25% by mass, allyl succinic anhydride (ASAh) in an amount of 0.10% by mass, succinonitrile (SN) in an amount of 1.25% by mass, and adiponitrile (ADN) in an amount of 1.25% by mass relative to the total mass of the electrolyte. Suberonitrile (SBN) was not added. Otherwise, the lithium-ion secondary battery of Example 6 was obtained in the same manner as in Example 2. [Example 7] A lithium-ion secondary battery of Example 7 was obtained in the same manner as in Example 6, except that the amount of allyl succinic anhydride (ASAh) added to the electrolyte additive was set to 0.25% by mass.

[0052] [Example 8] A lithium-ion secondary battery of Example 8 was obtained in the same manner as in Example 6, except that the amount of allyl succinic anhydride (ASAh) added to the electrolyte additive was set to 0.50% by mass. [Example 9] A lithium-ion secondary battery of Example 9 was obtained in the same manner as in Example 6, except that the amount of allyl succinic anhydride (ASAh) added to the electrolyte additive was set to 0.75% by mass.

[0053] [Example 10] A lithium-ion secondary battery of Example 10 was obtained in the same manner as in Example 6, except that the amount of allyl succinic anhydride (ASAh) added to the electrolyte additive was set to 1.00% by mass. [Comparative Example 3] A lithium-ion secondary battery of Comparative Example 3 was obtained in the same manner as in Example 6, except that allyl succinic anhydride (ASAh) was not added as an electrolyte additive.

[0054] [Example 11] As electrolyte additives, vinylene carbonate (VC) was added in an amount of 2.25% by mass, allyl succinic anhydride (ASAh) in an amount of 0.25% by mass, and adiponitrile (ADN) in an amount of 1.50% by mass relative to the total mass of the electrolyte. Suberonitrile (SBN) was not added. Otherwise, the lithium-ion secondary battery of Example 11 was obtained in the same manner as in Example 2. The solvent ratio was EC:DEC:EP:PP = 30:10:45:15. [Example 12] A lithium-ion secondary battery of Example 12 was obtained in the same manner as in Example 11, except that the solvent ratio was set to EC:DEC:EP:PP = 30:10:30:30.

[0055] [Comparative Example 4] A lithium-ion secondary battery of Comparative Example 4 was obtained in the same manner as in Example 11, except that propyl propionate (PP) was not mixed into the solvent and the solvent ratio was set to EC:DEC:EP = 30:10:60. [Comparative Example 5] A lithium-ion secondary battery of Comparative Example 5 was obtained in the same manner as in Example 11, except that ethyl propionate (EP) was not mixed into the solvent and the solvent ratio was EC:DEC:PP = 30:10:60.

[0056] [Example 13] As electrolyte additives, vinylene carbonate (VC) was added at a concentration of 2.25% by mass, allyl succinic anhydride (ASAh) at a concentration of 0.25% by mass, adiponitrile (ADN) at a concentration of 1.50% by mass, and suberonitrile (SBN) at a concentration of 1.50% by mass relative to the total mass of the electrolyte. A lithium-ion secondary battery of Example 13 was obtained in the same manner as in Example 2. [Example 14] As electrolyte additives, vinylene carbonate (VC) was added at a concentration of 2.25% by mass, allyl succinic anhydride (ASAh) at 0.25% by mass, adiponitrile (ADN) at 1.60% by mass, and pimeronitrile (PMN) at 1.60% by mass relative to the total mass of the electrolyte. A lithium-ion secondary battery of Example 14 was obtained in the same manner as in Example 2.

[0057] [Example 15] As electrolyte additives, vinylene carbonate (VC) was added in an amount of 2.25% by mass, allyl succinic anhydride (ASAh) in an amount of 0.25% by mass, adiponitrile (ADN) in an amount of 1.25% by mass, and suberonitrile (SBN) in an amount of 1.25% by mass, relative to the total mass of the electrolyte. A lithium-ion secondary battery of Example 15 was obtained in the same manner as in Example 2.

[0058] [Performance Test] <Initial charge-discharge efficiency test and electrochemical impedance measurement> For each prototype battery in Examples 1-15 and Comparative Examples 1-5, the initial discharge capacity in the chemical conversion process was divided by the total chemical conversion charge capacity, and then multiplied by 100 to calculate the initial charge efficiency (%). Next, electrochemical impedance measurements were performed on each prototype battery in a fully charged state for Examples 1-10 and Comparative Examples 1-3, and Nyquist plots were obtained.

[0059] <High-temperature storage test> High-temperature storage tests were conducted on each of the prototype batteries in Examples 1-15 and Comparative Examples 1-5. The prototype batteries were discharged to 2.5V at 0.5ItA, then charged with a constant current of 0.5ItA to 4.45V, and then charged with a constant voltage of 0.02ItA to reach a fully charged state. Finally, they were discharged again to 2.5V at 0.5ItA to obtain the pre-storage capacity. Each prototype battery was discharged to 2.5V at 0.5ItA, then charged with a constant current of 0.5ItA to 4.45V, followed by constant voltage charging to 0.02ItA to reach full charge. After that, the batteries were left undisturbed in a 60°C environment for 10 days. Once it was confirmed that the battery temperature had returned to room temperature (25°C), they were discharged to 2.5V at 0.5ItA to obtain the post-storage capacity. Each prototype battery was charged with a constant current of 0.5 ItA up to 4.40V, then charged with a constant voltage of 0.02 ItA to reach a fully charged state, and finally discharged again at 0.5 ItA to 2.5V to obtain the recovered capacity. The percentage of the volume after storage, with the pre-storage volume set to 100%, was calculated as the residual rate after high-temperature storage, and the percentage of the recovered volume, with the pre-storage volume set to 100%, was calculated as the recovery rate.

[0060] <Output Characteristics Test> Output characteristic tests were conducted on each of the prototype batteries in Examples 7, 11-15 and Comparative Examples 4 and 5. Specifically, each prototype battery was discharged to 2.5V at 0.5ItA, then charged with a constant current of 0.5ItA up to 4.40V, then charged with a constant voltage of 0.02ItA to reach a fully charged state, and finally discharged to 2.5V at 0.2ItA to obtain a capacity of 0.2ItA. Furthermore, each prototype battery was discharged to 2.5V at 0.5ItA, then charged with a constant current of 0.5ItA up to 4.40V, and then charged with a constant voltage of 0.02ItA to reach a fully charged state. Finally, it was discharged to 2.5V at 7.0ItA to obtain a capacity of 7.0ItA. Using the obtained values, the percentage of 7.0ItA capacity, with 0.2ItA capacity set to 100%, was calculated as the 7.0ItA maintenance rate [%]. 7.0ItA maintenance rate [%] = (7.0ItA capacity / 0.2ItA capacity) × 100 7.0If the ItA maintenance rate was 90% or higher, the output characteristics were judged to be good (○), and if it was less than 90%, the output characteristics were judged to be insufficient (×).

[0061] <25°C Cycle Test> Charge-discharge cycle tests were performed on each of the prototype batteries in Examples 11 and 12 and Comparative Examples 4 and 5 under conditions of 25°C. Specifically, first, before the charge-discharge cycle test, the discharge capacity of each prototype battery was measured according to charge-discharge condition 1 below. Then, each prototype battery was left to stand in a constant temperature bath at 25°C for 1 to 3 hours, and then 200 charge-discharge cycles were performed, with each cycle consisting of one charge-discharge cycle according to charge-discharge condition 2 below. Next, after the charge-discharge cycle test, the discharge capacity of each prototype battery was measured according to charge-discharge condition 2 below. Then, the discharge capacity retention rate was calculated from the measured discharge capacity before and after the charge-discharge cycle test using the following formula. Cycle capacity retention rate [%] = (Discharge capacity after cycle test / Discharge capacity before cycle test) × 100

[0062] ≪Charge / Discharge Condition 1 (Measurement of Discharge Capacity)≫ Environmental temperature: 25℃ Charging: 1.0 ItA until 4.40V (continues until charging current drops to 0.1 ItA) Discharge: 0.2ItA up to 2.5V Pause time between charging and discharging: 10 minutes ≪Charge / Discharge Condition 2 (Charge / Discharge Cycle Test under 25°C Environment)≫ Environmental temperature: 25℃ Charging: 1.0 ItA until 4.40V (continues until charging current drops to 0.1 ItA) Discharge: 3.0 ItA up to 2.5V Pause time between charging and discharging: 10 minutes

[0063] [Performance comparison based on differences in the mixing ratio of natural graphite and artificial graphite in negative electrode active materials] The test results for Examples 1-5 and Comparative Examples 1 and 2, along with their compositions, are shown in Table 1. The Nyquist diagrams for Examples 1-5 and Comparative Examples 1 and 2 are shown in Figure 1.

[0064] [Table 1]

[0065] As shown in Table 1, using a negative electrode active material that is a mixture of natural graphite and artificial graphite improves the initial charging efficiency and enhances high-temperature storage performance. Furthermore, as shown in Figure 1, electrochemical impedance measurements revealed that the lithium-ion secondary battery of Example 5, with a natural graphite:artificial graphite ratio of 1:9, had significantly lower internal resistance compared to the lithium-ion secondary battery of Comparative Example 1, which used only artificial graphite as the negative electrode active material. This indicates that mixing natural graphite and artificial graphite can reduce internal resistance. Additionally, Figure 1 shows that the internal resistance increases as the proportion of artificial graphite increases. From the above, it can be seen that by using a negative electrode active material that is a mixture of natural graphite and artificial graphite, a lithium-ion secondary battery with low resistance while maintaining high-temperature storage characteristics can be obtained.

[0066] [Performance comparison based on different amounts of allyl succinic anhydride added] The test results for Examples 6-10 and Comparative Example 3, along with their compositions, are shown in Table 2. The Nyquist diagrams for Examples 6-10 and Comparative Example 3 are shown in Figure 2. Note that the Nyquist diagram for Comparative Example 1 is also included in Figure 2.

[0067] [Table 2]

[0068] As shown in Table 2, the lithium-ion secondary batteries of Examples 6-10, which have allyl succinic anhydride added as an electrolyte additive, exhibit higher high-temperature storage performance than the lithium-ion secondary battery of Comparative Example 3, which does not have allyl succinic anhydride added. Furthermore, it can be seen that the higher the amount of allyl succinic anhydride added, the higher the high-temperature storage performance.

[0069] Furthermore, as shown in Figure 2, electrochemical impedance measurements revealed that the internal resistance of lithium-ion secondary batteries in Examples 6-10 and Comparative Example 3, where the negative electrode active material was natural graphite:artificial graphite = 7:3 and the addition rate of allyl succinic anhydride was 0.10-1.00% by mass, was significantly lower than that of the lithium-ion secondary battery in Comparative Example 1, where the negative electrode active material was pure artificial graphite and the addition rate of allyl succinic anhydride was 1.00% by mass. Furthermore, the results in Figure 2 show that in the lithium-ion secondary batteries of Examples 6-10 and Comparative Example 3, which differ only in the addition ratio of allyl succinic anhydride, the internal resistance increases as the addition ratio of allyl succinic anhydride increases.

[0070] [Performance comparison based on differences in non-aqueous solvent composition] The test results for Examples 11 and 12, and Comparative Examples 4 and 5, along with their compositions, are shown in Table 3.

[0071] [Table 3]

[0072] As shown in Table 3, the lithium-ion secondary batteries of Examples 11 and 12, which used non-aqueous solvents containing all four types of EC, DEC, EP, and PP, exhibited better cycle capacity retention than the lithium-ion secondary battery of Comparative Example 4, which used a non-aqueous solvent that did not contain PP. Furthermore, the lithium-ion secondary batteries of Examples 11 and 12 exhibited better output characteristics than the lithium-ion secondary battery of Comparative Example 5, which used a non-aqueous solvent that did not contain EP.

[0073] [Performance comparison based on differences in the composition of electrolyte additives] The test results for Examples 7, 13-15, along with their configurations, are shown in Table 4.

[0074] [Table 4]

[0075] As shown in Table 4, the lithium-ion secondary batteries of Examples 7 and 13-15 contain one or more dinitriles selected from succinonitrile, adiponitrile, pimeronitrile, and suberonitrile as an electrolyte additive, in a total proportion of 1.5% by mass or more and 3.2% by mass or less. The results in Table 4 show that differences in the type of dinitrile added as an electrolyte additive and the amount of each additive (when the total amount is within the above range) do not affect the initial charge efficiency, high-temperature storage performance, or output characteristics.

Claims

1. The device comprises a positive electrode containing a positive electrode active material capable of intercepting and releasing lithium ions, a negative electrode containing a negative electrode active material capable of intercepting and releasing lithium ions, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte containing a lithium salt, a non-aqueous solvent, and an electrolyte additive. The negative electrode active material includes natural graphite and artificial graphite. The non-aqueous solvent includes ethylene carbonate, diethyl carbonate, ethyl propionate, and propyl propionate. The electrolyte additive includes allyl succinic anhydride and dinitrile. The aforementioned electrolyte additive is a lithium-ion secondary battery containing dinitrile in a proportion of 1.5% by mass or more and 3.2% by mass or less.

2. The lithium-ion secondary battery according to claim 1, wherein the electrolyte additive contains allyl succinic anhydride in a proportion of 0.25% by mass or more and 1.0% by mass or less.

3. The lithium-ion secondary battery according to claim 1, wherein the negative electrode active material contains natural graphite and artificial graphite in a mass ratio of natural graphite:artificial graphite = 9:1 to 1:

9.

4. The lithium-ion secondary battery according to claim 1, wherein the dinitrile contained in the electrolyte additive is one or more selected from succinonitrile, adiponitrile, pimeronitrile, and suberonitrile.

5. The lithium-ion secondary battery according to claim 4, further comprising vinylene carbonate as the electrolyte additive.

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