JP7743983B2 - Lithium-ion secondary battery - Google Patents

Description

The present invention relates to a lithium-ion secondary battery.

In recent years, lithium-ion batteries equipped with anodes that use titanium oxide as the anode active material, which has higher output than conventional carbon-based anodes, have been put to practical use. However, titanium oxide has a higher potential relative to metallic lithium than carbon-based materials, and titanium oxide also has the disadvantage of having a low capacity per mass and a low energy density.

For this reason, monoclinic Nb-Ti composite oxides (hereinafter sometimes referred to as "TNO" or "niobium titanium oxide") such as TiNb ₂ O ₇ have been investigated as negative electrode active materials. A negative electrode using this TNO as the negative electrode active material in the composite layer (hereinafter sometimes referred to as "TNO negative electrode") has a theoretical capacity of 387 mAh/g, which is equivalent to that of conventional carbon-based negative electrodes, and the use of this material may potentially solve the problem of low energy density, which has been a drawback of using titanium oxides to date (see Patent Document 1).

Patent No. 6636758

However, this TNO anode has different electrochemical behavior from anodes that use conventional titanium oxide. Therefore, simply replacing titanium oxide with TNO does not allow for the construction of a lithium-ion secondary battery that achieves both high output and energy density.

In particular, TNO anodes exhibit behavior different from that of conventional titanium oxides, in that a solid electrolyte interface (hereinafter sometimes referred to as an "SEI film") forms during charging. This SEI film forms on the surface of the anode, leading to an increase in internal resistance. As a result, the high output that is a major feature of lithium-ion secondary batteries using titanium oxide as the anode active material cannot be achieved, resulting in a decline in output characteristics, which is a major factor hindering practical application.

Although the details are unclear, it is thought that the niobium (Nb) in the TNO negative electrode acts as a catalyst when forming the SEI film, and unless this issue is resolved, lithium-ion secondary batteries using TNO negative electrodes cannot be put into practical use. Furthermore, the formation of the SEI film is related to the material composition of the negative electrode active material and the composition of the electrolyte, and applying the electrolyte used in lithium-ion secondary batteries that use titanium oxide to TNO negative electrodes as is will result in a decline in output characteristics.

The present invention was therefore made in response to the above-mentioned problems, and aims to provide a lithium-ion secondary battery that achieves both output characteristics and energy density.

To solve the above-mentioned problems, the inventors investigated the electrolyte composition when using a TNO negative electrode. As a result of extensive research, they discovered that when a TNO negative electrode is used and a non-aqueous electrolyte containing a specific lithium alkoxysulfonate as an electrolyte additive, the effects unique to this combination are generated, making it possible to increase the energy density by approximately 1.7 times compared to when lithium titanate is used, without significantly reducing the output characteristics of the lithium-ion secondary battery, and thus completed the present invention.

In order to solve the above problems, the lithium ion secondary battery of the present invention comprises a negative electrode containing niobium titanium oxide as a negative electrode active material in a composite layer, and a nonaqueous electrolyte containing lithium alkoxysulfonate represented by RSOOO ^- Li ⁺ (where R is at least one substance selected from the group consisting of _{OCH3 and OC2H5} ).

The content of the lithium alkoxysulfonate in the non-aqueous electrolyte is preferably 0.5 to 2.0% by mass.

The present invention makes it possible to provide a lithium-ion secondary battery that combines output characteristics and energy density.

FIG. 1 is a schematic cross-sectional view of a coin-type battery, which is an example of the lithium ion secondary battery of the present invention. FIG. 2 is a schematic cross-sectional view of a stacked battery shown as an example of the lithium ion secondary battery of the present invention.

The following are examples of the forms and configurations of the present invention, but the present invention is not limited to these. All aspects that are in line with the intent of the claims, the means for solving the problems, the effects of the invention, etc. are included in the present invention.

The present invention relates to a lithium-ion secondary battery using a TNO negative electrode, and has the following two features:

The first point is that the negative electrode uses a composite layer containing niobium titanium oxide as the negative electrode active material. While the TNO negative electrode has been described above, the present invention attempts to solve the problem by utilizing the unique effects of the TNO negative electrode and electrolyte additive. Therefore, the effects of the present invention cannot be obtained with carbon materials, which are typical negative electrode active materials for lithium ion secondary batteries, or even with lithium ion secondary batteries that use lithium titanate as the negative electrode active material.

The second point is the use of lithium alkoxysulfonate (see Chemical Formula 1) as an electrolyte additive. In the present invention, by using the lithium alkoxysulfonate shown in Chemical Formula 1 as an electrolyte additive, the additive penetrates into the SEI coating, lowering the coating density and thereby reducing internal resistance. Therefore, the decomposition of the lithium alkoxysulfonate itself is not intended by the present invention.

Furthermore, in the present invention, it is preferable that the content of lithium alkoxysulfonate in the non-aqueous electrolyte be 0.5 to 2.0% by mass. As described above, lithium alkoxysulfonate exerts its effect by penetrating into the SEI coating. Therefore, if the content is less than 0.5% by mass, the number of additive molecules penetrating into the SEI coating will also be reduced. As a result, the coating density will not decrease, and the effect of reducing internal resistance may not be achieved. On the other hand, if the content is more than 2.0% by mass, due to the solubility of lithium alkoxysulfonate in the non-aqueous electrolyte, some of the lithium alkoxysulfonate may remain undissolved depending on the temperature of the non-aqueous electrolyte, making it impossible to prepare a non-aqueous electrolyte with a concentration greater than 2.0% by mass.

More preferably, if the content of lithium alkoxysulfonate in the non-aqueous electrolyte is 1.0 to 2.0 mass%, the effect of reducing internal resistance can be sufficiently achieved, and solubility in the non-aqueous electrolyte can be sufficiently ensured.

(Method for quantifying lithium alkoxysulfonate in non-aqueous electrolyte)
The content of lithium alkoxysulfonate in the non-aqueous electrolyte can be determined by preparing a calibration curve as follows, and measuring the non-aqueous electrolyte as a sample by ion chromatography.

<Preparation of calibration curve>
A calibration curve was prepared by appropriately changing the amount of lithium alkoxysulfonate.

<Quantitative method for lithium alkoxysulfonate>
If the non-aqueous electrolyte solution to be quantified, whose lithium alkoxysulfonate content is unknown, is measured by ion chromatography, and a peak appears at the same retention time as the solution for which the calibration curve was prepared, the electrolyte solution contains lithium alkoxysulfonate. The amount of addition can be determined from the peak area measured by ion chromatography and the calibration curve.

[Lithium-ion secondary battery]
The lithium ion secondary battery of the present invention can be composed of, for example, a positive electrode, a negative electrode, a separator, a non-aqueous electrolyte, terminals, and an exterior body, all of which are described below.

<Positive electrode>
The positive electrode is composed of, for example, a positive electrode active material composite electrode layer and a current collector.

(Cathode active material composite electrode layer)
The positive electrode active material composite electrode layer can be composed of, for example, a positive electrode active material, a conductive material, a binder, and a solvent. Examples of the positive electrode active material include compounds such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiFePO ₄ , LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiCo _0.15 Ni _0.8 Al _0.05 O ₂ , and LiNi _0.5 Mn _1.5 O _4. In a more preferred embodiment, the positive electrode active material may contain a lithium composite oxide having an atomic ratio of Ni of 50% or more. Examples of such oxides include oxides in which the atomic ratio of nickel is 50% or more among the cation species excluding lithium contained as the positive electrode active material in the positive electrode active material composite electrode layer. Specific examples include _{LiNi0.5Co0.2Mn0.3O2} , _{LiNi0.6Co0.2Mn0.2O2 , LiNi0.8Co0.1Mn0.1O2 , and LiCo0.15Ni0.80Al0.05O2} . The _present invention mentions _high energy density as _an issue, and for that purpose _, a high energy density can be achieved by increasing _the battery capacity, but a material with a nickel _atomic ratio _{of 50} % or more is characterized by a large unit capacity, and by using the _above oxides, the amount of the positive electrode active material composite electrode layer in the positive electrode can be reduced and the electrode thickness can be made thinner, thereby achieving a high energy density.

For example, when graphite is used as the negative electrode active material and a nickel-containing oxide is used as the positive electrode active material, it has been found that the proportion of nickel in the positive electrode active material does not have a significant effect on energy density. This is because graphite containing lithium has a reduction potential roughly equal to that of lithium, making it the most base potential of all substances. On the other hand, the oxidation potential, which varies with the amount of nickel in the positive electrode active material, is only about 0.1 V. For this reason, in the case of a graphite negative electrode, the average discharge voltage only varies within a range of approximately 4.3 ± 0.05 V, whereas in the case of a TNO negative electrode, it varies within a range of approximately 2.8 ± 0.05 V. Therefore, the impact of the nickel proportion in the positive electrode active material on energy density cannot be ignored.

Furthermore, the preferred nickel atomic ratio is 50% or more because a nickel atomic ratio below 50% not only makes it difficult to achieve high capacity on the positive electrode side, but also takes into consideration the early loss of the benefits of the present invention, which is thought to be due to the disintegration of active material particles due to the elution of manganese and aluminum contained in the positive electrode active material. In the present invention, TNO is used as the negative electrode active material, allowing the use of esters and ethers, which are not suitable for use in general lithium-ion batteries, as electrolyte solvents. However, these components also have a high affinity for manganese ions and aluminum ions, which is thought to make them more susceptible to elution than general electrolytes containing only carbonate. While the details are unclear, the relationship between the positive charge of manganese or aluminum and their electronegativity with carbon causes the electrons used in the covalent bond to lean toward the oxygen, resulting in the coordination of the δ ^- charged oxygen of the ester or ether, forming a complex, which is thought to enhance the stability of manganese and aluminum in the electrolyte.

In addition, there are no particular limitations on the conductive material used in the present invention, and known or commercially available materials such as carbon blacks such as acetylene black and ketjen black, carbon nanotubes, carbon fibers, activated carbon, and graphite can be used.

There are no particular restrictions on the binder, and known or commercially available binders can be used. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene-butadiene rubber (SBR), acrylic resin, etc. can be used.

The solvent used in the present invention is not particularly limited, and various solvents can be selected depending on the positive electrode active material or binder used. Specifically, when PVDF is used as the binder, it is preferable to use N-methyl-2-pyrrolidone as the solvent. On the other hand, when a rubber-based binder such as styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinyl alcohol, or carboxymethyl cellulose (CMC) is used, water is the preferred solvent.

(Current collector)
The current collector is not particularly limited in the present invention, but aluminum foil is generally used as a current collector for the positive electrode of a lithium ion battery, and porous aluminum current collectors may also be used depending on the application.

A positive electrode can be produced, for example, by the following method. First, a positive electrode active material, a conductive additive, a binder, etc. are dispersed in a solvent to prepare a positive electrode slurry. Next, the positive electrode slurry is applied to one or both surfaces of a positive electrode current collector, and then dried, for example, at 80°C under vacuum to form a positive electrode active material composite electrode layer. A positive electrode can be produced through these steps.

The above description also describes a positive electrode composed of a positive electrode active material composite electrode layer and a current collector. However, the present invention is not limited to this, and when a half cell is used instead of a full cell, metallic lithium, for example, may be used as a counter electrode instead of the positive electrode.

<Negative electrode>
The negative electrode is composed of a negative electrode active material composite electrode layer and a current collector.

(Negative active material composite electrode layer)
The negative electrode active material composite electrode layer (sometimes referred to as the "composite layer" in this specification) is composed of a negative electrode active material, a conductive material, a binder, and a solvent. In the present invention, only niobium titanium oxide, which is a monoclinic Nb-Ti composite oxide, is applicable as the negative electrode active material, and TiNb ₂ O ₇ and Ti ₂ Nb ₁₀ O ₂₉ are particularly effective.

The content of niobium titanium oxide in the negative electrode active material composite electrode layer is preferably 80% to 95% by mass. Furthermore, the average secondary particle diameter of the niobium titanium oxide used is preferably 5 μm to 20 μm. Furthermore, the average secondary particle diameter in this specification refers to the particle size at 50% of the cumulative value in the particle size distribution determined by laser diffraction/scattering.

The conductive material used in the present invention is not particularly limited, and known or commercially available materials such as carbon blacks such as acetylene black and ketjen black, carbon nanotubes, carbon fiber, activated carbon, and graphite can be used.

There are no particular restrictions on the binder, and known or commercially available binders can be used. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene-butadiene rubber (SBR), acrylic resin, etc. can be used.

The solvent used for the binder is not particularly limited in the present invention, and various solvents can be selected depending on the active material or binder used. Specifically, when PVDF is used as the binder, it is preferable to use N-methyl-2-pyrrolidone as the solvent. On the other hand, when a rubber-based binder such as styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinyl alcohol, or carboxymethyl cellulose (CMC) is used, water is the preferred solvent.

(Current collector)
The current collector is not particularly limited in the present invention, but aluminum foil is suitable as the current collector for the TNO negative electrode, and porous aluminum current collectors may also be used depending on the application.

The negative electrode can be produced, for example, by the following method. First, niobium titanium oxide, a conductive additive, a binder, etc. are dispersed in a solvent to prepare a negative electrode slurry. Next, the negative electrode slurry is applied to one or both surfaces of the negative electrode current collector, and then dried, for example, at 80°C under vacuum to form a negative electrode active material composite electrode layer. The negative electrode can be produced through these steps.

<Separator>
The separator interposed between the positive electrode and the negative electrode can be a commonly used nonwoven fabric or porous sheet made of a synthetic resin such as a polyolefin resin such as PE (polyethylene) or PP (polypropylene), or PTFE (polytetrafluoroethylene). The nonwoven fabric or porous sheet may have a single layer or a multilayer structure.

<Non-aqueous electrolyte>
The non-aqueous electrolyte solution contains an electrolyte, an electrolyte solvent, and the above-mentioned lithium alkoxysulfonate as an electrolyte additive, and is a non-aqueous electrolyte solution that can be used in the lithium ion secondary battery of the present invention.

The electrolyte is not particularly limited, but is preferably an electrolyte salt containing lithium ions that is commonly used in lithium ion secondary batteries. For example, lithium salts such as LiBF ₄ and LiPF ₆ can be used. These electrolytes may be used alone or in combination of two or more. The concentration of the electrolyte salt is not particularly limited, and may be, for example, about 0.3 M (mol/L) to 3 M relative to the electrolyte solvent.

More specifically, an electrolyte containing, for example, 0.5M to 3M LiPF ₆ and/or LiBF ₄ but not containing LiFSI, LiBOB, or LiPO ₂ F ₂ can be used.

If the content of _LiPF6 and/or _LiBF4 is less than 0.5 M, the amount of ions in the electrolyte may be insufficient, making it difficult to achieve high output. Furthermore, if the content is greater than 3 M, the viscosity of the electrolyte may become too high, resulting in increased penetration into the separator and increased resistance of the electrolyte, making it difficult to achieve high output. Therefore, taking into account high output and the resistance of the electrolyte, the content is preferably set to 0.5 M to 3 M.

Furthermore, in the present invention, it is preferable not to include LiFSI, LiBOB, or _LiPO2F2 . This is because these electrolytes all undergo reductive decomposition on the negative _electrode side during charging, forming excessive SEI, making it impossible to achieve the high output that is the effect of the present invention. Although the details are unclear, the niobium in TNO, the negative electrode active material in the present invention, acts as a catalyst and may specifically react and decompose with carbonate, a component of the electrolyte solution, to produce hydrofluoric acid.

In the present invention, high-power characteristics are mentioned as a problem, and the ionic conductivity of the electrolyte is an important parameter in solving this problem. The ionic conductivity of the electrolyte will be discussed later, but this is an approach from the non-aqueous solvent side, and the parameters that have the greatest impact on the ionic conductivity of the electrolyte are the type and amount of electrolyte. In particular, _LiPF6 and/or _LiBF4 have large anions, which allow for good ion dissociation, making it possible to construct a high-power lithium-ion battery. Furthermore, they are suitable for the present invention because they are less likely to adversely affect TNO, which is used as the negative electrode active material in the present invention.

The nonaqueous electrolyte used in liquid electrolyte lithium-ion secondary batteries can be used as the electrolyte solvent.

The non-aqueous electrolyte preferably contains a cyclic solvent and/or a chain solvent as the non-aqueous solvent. The cyclic solvent preferably contains a cyclic carbonate and/or a cyclic ester, and the chain solvent preferably contains a chain carbonate and/or a chain ether. The cyclic carbonate is related to the degree of dissociation of the electrolyte components, and the chain carbonate is related to the viscosity of the electrolyte.

Cyclic carbonates have a high dielectric constant, which contributes to improving the lithium ion conductivity of electrolytes. The Born equation, which shows the solvation energy of ionic species in electrolytes, includes the dielectric constant, and as the dielectric constant increases, the Gibbs free energy becomes more negative. A negative Gibbs free energy means that the reaction will proceed spontaneously, and the larger its absolute value, the more likely the reaction will proceed. For this reason, when cyclic carbonates are included in non-aqueous solvents, the dissociation of the cation and anion components of the electrolyte is promoted, increasing the proportion of the electrolyte present as ions in the non-aqueous solvent, leading to improved lithium ion conductivity.

Furthermore, because lithium ion conductivity is a parameter directly linked to the rate characteristics of a battery, it is preferable to include a cyclic carbonate in the non-aqueous solvent. Furthermore, chain carbonates are related to the viscosity of the non-aqueous solvent. While cyclic carbonates have a high dielectric constant, they often also have a high viscosity. The viscosity of the non-aqueous solvent is directly linked to the fluidity of the electrolyte and affects the permeability of the electrolyte into the separator. If areas of the separator are created where the electrolyte does not permeate, no current will flow through those areas, preventing the battery from achieving its full potential and resulting in a decline in overall battery performance. Chain carbonates can be used to prevent this, and while they do not have a high dielectric constant, they are characterized by their low viscosity. Because cyclic and chain carbonates thus play different roles, including both as non-aqueous solvent components is preferable for building better lithium-ion batteries. In the present invention, it is preferable to include at least one of these components.

γ-butyrolactone, a cyclic ester, is a compound that possesses the properties of both a cyclic carbonate and an ester. While γ-butyrolactone alone could be used in lithium-ion batteries as an electrolyte solvent component, it has not been used until now because graphite is often used as the negative electrode material, and in this case, γ-butyrolactone molecules may enter between the graphite layers, potentially promoting graphite decomposition. The present invention requires the use of TNO as the negative electrode active material, which eliminates the problem of γ-butyrolactone entering between the active material layers, thereby allowing γ-butyrolactone to fully demonstrate its inherent performance.

The cyclic carbonate is preferably ethylene carbonate and/or propylene carbonate, and does not include vinylene carbonate, fluoroethylene carbonate, or vinylethylene carbonate.

Among cyclic carbonates, the effects of the present invention are enhanced when ethylene carbonate and propylene carbonate are used. Ethylene carbonate has an extremely high dielectric constant among cyclic carbonates, making it the ideal material for maintaining high-rate performance. Propylene carbonate also has properties similar to ethylene carbonate, but like gamma-butyrolactone, propylene carbonate has not been used until now because, when the negative electrode active material is graphite, its molecules may enter between the layers and promote decomposition. However, the present invention requires the use of TNO instead of graphite as the negative electrode active material, which eliminates the problem of molecules entering between the active material layers, allowing propylene carbonate to fully demonstrate its inherent performance.

In addition, in the present invention, it is preferable not to include vinylene carbonate, fluoroethylene carbonate, and vinylethylene carbonate. The reason for this is that these carbonates are related to the SEI formation specific to TNO negative electrodes, and if these carbonates are included, there is a risk of excessive SEI formation to the extent that it inhibits current flow, resulting in a decrease in battery capacity and rate characteristics.

Furthermore, it is preferable that the chain carbonate is at least one selected from dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

As mentioned above, chain carbonates are used to reduce the viscosity of the electrolyte. Chain carbonates containing methyl or ethyl groups have extremely low viscosity among chain carbonates, so by using such chain carbonates, the high-rate characteristics that are an effect of the present invention can be fully achieved.

The non-aqueous electrolyte may also contain 0.1% to 5% by mass of at least one cyclic ether selected from 1,3-dioxane, adiponitrile, and succinonitrile.

1,3-dioxane has advantageous characteristics as an electrolyte for high-power batteries, such as a lower viscosity than chain carbonates and a high dielectric constant. However, it has a fatal drawback: its oxidative decomposition potential is approximately 2.2 V, which is approximately 1 V lower than chain carbonates. This means that it is easily oxidatively decomposed on the positive electrode side, and so it has not been widely used in lithium-ion battery electrolytes until now. However, in this invention, it has been discovered that a concentration in the range of 0.1% to 5% by mass does not adversely affect battery performance. By including this compound in an appropriate amount as an electrolyte component, the compound's characteristics of a lower viscosity than chain carbonates and a high dielectric constant can be utilized, resulting in effective results in achieving both high energy density and high power characteristics, the effects of this invention.

Adiponitrile and succinonitrile are compounds known as additives for improving safety or reliability, and it is preferable for these compounds to be included in the present invention within the above-mentioned mass % range. These compounds coordinate with metal ions eluted from the current collector and active material to form complexes, which are thought to deactivate highly active metal ion species and suppress leakage current and gas generation, leading to long-term maintenance of the effects of the present invention.

In particular, in the present invention, as described above, the positive electrode active material composite electrode layer can contain a lithium composite oxide with an atomic ratio of Ni of 50% or more as the positive electrode active material. Due to its ionization tendency, nickel tends to be the most susceptible to leaching out of the cationic species contained in the positive electrode active material. In the present invention, by including such dinitriles in the electrolyte, nickel ions, which are generated in large quantities during overcharging, are captured in the form of a complex, preventing battery degradation and enabling the effects of the present invention, namely, high energy density and high output characteristics, to be achieved for a long period of time.

However, because 1,3-dioxane, adiponitrile, and succinonitrile have strong coordinating power, if their content in the non-aqueous electrolyte exceeds 5% by mass, a high proportion of them will capture ions necessary for charging and discharging, resulting in a decrease in cycle characteristics. Therefore, as stated above, it is recommended that they be used in the range of 0.1% to 5% by mass.

In addition to the cyclic carbonates and cyclic esters mentioned above, sulfolane can also be used as the electrolyte solvent. Furthermore, dimethoxyethane, diethoxyethane, various glymes, and other chain ethers can be used as the chain solvent by mixing them with the main solvent.

The effects of including lithium alkoxysulfonate as an electrolyte additive are as described above. Examples of lithium alkoxysulfonate include lithium methoxysulfonate and lithium ethoxysulfonate. The electrolyte additive contains at least one of these. Specific combinations include lithium methoxysulfonate alone, lithium ethoxysulfonate alone, and a combination containing both lithium methoxysulfonate and lithium ethoxysulfonate. The total content of all lithium alkoxysulfonates in the non-aqueous electrolyte is 0.5% to 2.0% by mass, preferably 1.0% to 2.0% by mass. Furthermore, the saturated concentration of lithium methoxysulfonate is 2.0% by mass. If the total additive concentration is greater than 2.0% by mass, it is mixed with lithium ethoxysulfonate. Furthermore, in RSOOO ^- Li ⁺ , the substance where R is _OCH3 is lithium methoxysulfonate, and _the substance where R is _OC2H5 is lithium ethoxysulfonate.

Terminal
Metal is generally used for the terminals. There are no restrictions on the material or shape, but in the present invention, aluminum and copper are suitable as materials, and a shape that will not be deformed by wiring or the like is suitable.

<Exterior body>
The exterior body can be made of, for example, a can or a laminate. There are no restrictions on the material or shape, but stainless steel is a suitable can material, and a laminate material made of aluminum foil coated with a plastic film is a suitable material. The shape can be changed depending on the cell capacity, and generally, the larger the cell capacity, the larger the shape.

The shape of the lithium ion secondary battery of the present invention is not particularly limited, but examples include coin type, button type, sheet type, laminated type, cylindrical type, prismatic type, and flat type.

The structure of the lithium-ion secondary battery of the present invention will be described below with reference to the drawings, using a coin-type lithium-ion secondary battery as an example. Figure 1 is a schematic cross-sectional view of a coin-type battery, shown as an example of the lithium-ion secondary battery of the present invention.

The coin-type lithium-ion secondary battery 1 comprises a positive electrode 2, a negative electrode 3, and a separator 4 disposed between the positive electrode 2 and the negative electrode 3. The positive electrode 2, negative electrode 3, and separator 4 are housed between a first external terminal 5 located on the lower side and a second external terminal 6 located on the upper side. The contact areas between the first external terminal 5 and the second external terminal 6 are insulated by a gasket 7.

The positive electrode 2 is composed of a positive electrode current collector 21 located on the inner surface of the first external terminal 5 and connected thereto, and a positive electrode active material composite electrode layer 22 provided on the surface of the positive electrode current collector 21 facing the separator 4. The negative electrode 3 is composed of a negative electrode current collector 31 located on the inner surface of the second external terminal 6 and connected thereto, and a composite material layer 32 provided on the surface of the negative electrode current collector 31 facing the separator 4. The separator 4 is impregnated with, for example, a nonaqueous electrolyte. The second external terminal 6 is inserted into the first external terminal 5 with its lower end and both side surfaces wrapped in gaskets 7. The lower open end of the first external terminal 5 is bent toward the gasket 7 to crimp and fix the second external terminal 6 to the first external terminal 5, and the abutting portions of the first external terminal 5 and second external terminal 6 are insulated by the gasket 7.

Next, with reference to Figure 2, we will explain a stacked battery, which is an example of a lithium-ion secondary battery of the present invention.

The laminated lithium secondary battery 100 includes a bag-shaped exterior body 200 made of laminate film. A laminated electrode group 300 is housed within the exterior body 200. The laminate film has a structure in which, for example, multiple (e.g., two) plastic films are stacked with a metal foil such as aluminum foil sandwiched between them. One of the two plastic films is a heat-sealable resin film. The exterior body 200 is made by stacking two laminate films with the heat-sealable resin films facing each other, and housing the electrode group 300 between these laminate films. The two laminate film portions around the electrode group 300 are heat-sealed to each other to seal the electrode group 300, thereby hermetically housing the electrode group 300.

The electrode group 300 has a structure in which a positive electrode 400, a negative electrode 500, and a separator 600 sandwiched between the positive electrode 400 and the negative electrode 500 are stacked in multiple layers, with the negative electrode 500 positioned as the outermost layer and the separator 600 positioned between the negative electrode 500 and the inner surface of the outer casing 200. The positive electrode 400 is composed of a positive electrode current collector 410 and a positive electrode active material composite electrode layer 420 formed on both sides of the current collector 410. The negative electrode 500 is composed of a negative electrode current collector 510 and a composite layer 520 formed on both sides of the current collector 510.

The positive electrode 400 has a positive electrode lead 700 in which the positive electrode current collector 410 extends from, for example, the left side of the positive electrode active material composite electrode layer 420. The positive electrode leads 700 are bundled at their tip ends within the exterior case 200 and joined to each other. One end of the positive electrode tab 800 is joined to the joint of the positive electrode lead 700, and the other end extends to the outside through the sealing portion of the exterior case 200. The negative electrode 500 has a negative electrode lead 900 in which the negative electrode current collector 510 extends from, for example, the right side of the composite layer 520. The negative electrode leads 900 are bundled at their tip ends within the exterior case 200 and joined to each other. One end of the negative electrode tab 1000 is joined to the joint of the negative electrode lead 900, and the other end extends to the outside through the sealing portion of the exterior case 200. A nonaqueous electrolyte is injected into the exterior case 200. The injection site of the exterior body 200 is sealed after the non-aqueous electrolyte is injected.

The present invention will be explained in more detail below using examples. Note that the present invention is not limited to the following examples.

<Example 1-1>
(1) Preparation of Electrolyte Solution LiPF _{6 was dissolved in a solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 3:7 so that the concentration of LiPF 6 was} 1.0 M, and 0.5% by weight of lithium methoxysulfonate was further added to the solution, followed by stirring with a magnetic stirrer for 1 hour to prepare the electrolyte solution of the present invention.

(2) Preparation of test cell TiNb ₂ O ₇ particles obtained by hydrothermal synthesis were used as the active material, and the active material, conductive material, and binder were kneaded with a solvent to prepare a slurry. Acetylene black manufactured by Denka Corporation was used as the conductive material. KF polymer manufactured by Kureha Corporation was used as the binder. The mass ratio of the active material, conductive material, and binder was 85:10:5. N-methylpyrrolidone manufactured by Kanto Chemical Co., Ltd. was used as the solvent. The obtained slurry was coated on one side of 20 μm thick aluminum foil at a basis weight of 50 g/m ^2. After coating, it was pressed, punched out to Φ13, and dried to evaporate the solvent, obtaining a negative electrode.

The resulting negative electrode, a Φ14 lithium metal counter electrode, and a Φ15 glass separator were placed inside the exterior of a 2032-type coin cell, with the coated surface of the negative electrode facing the lithium metal through the separator, and the prepared electrolyte was then added. After placing the lid, the periphery was crimped using a crimping machine to create a test cell.

(3) Charge/Discharge Test A charge/discharge test was performed using a charge/discharge tester (TOSCAT-3000) manufactured by Toyo Systems Co., Ltd. To measure the discharge capacity, the prepared test cell was first charged at a constant current/constant voltage to 1.5 V at 0.1 ItA and left for 15 minutes. Thereafter, a constant current discharge was performed at 0.1 ItA to 2.5 V, resulting in a capacity of 252 mAh/g-active material. Furthermore, a constant current/constant voltage charge was performed at 0.1 ItA to 1.5 V, left for 15 minutes, and a discharge test was further performed at 5.0 ItA, resulting in a capacity ratio of 89% vs. 0.1 ItA. Here, "89% vs. 0.1 ItA" refers to the discharge capacity ratio at 5.0 ItA to 0.1 ItA, and the same applies below. In the table, this result is shown in the "5.0 ItA Discharge Capacity Ratio" section.

<Example 1-2>
A test cell was prepared and tested in the same manner as in Example 1-1, except that no lithium methoxysulfonate was added to the electrolyte and the amount of lithium ethoxysulfonate added was 0.5 wt %, and a discharge capacity of 250 mAh/g-active material and a capacity ratio of 87% vs. 0.1 ItA were obtained.

<Example 1-3>
A test cell was prepared and tested in the same manner as in Example 1-1, except that no lithium methoxysulfonate was added to the electrolyte and the amount of lithium ethoxysulfonate added was 1.0 wt %. A discharge capacity of 250 mAh/g-active material and a capacity ratio of 87% vs. 0.1 ItA were obtained.

<Example 1-4>
A test cell was prepared and tested in the same manner as in Example 1-1, except that no lithium methoxysulfonate was added to the electrolyte and the amount of lithium ethoxysulfonate added was 1.5 wt %. A discharge capacity of 249 mAh/g-active material and a capacity ratio of 87% vs. 0.1 ItA were obtained.

<Example 1-5>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the amount of lithium methoxysulfonate added to the electrolyte was 0.5 wt % and the amount of lithium ethoxysulfonate added was 0.5 wt %, and a discharge capacity of 251 mAh/g-active material and a capacity ratio of 85% vs. 0.1 ItA were obtained.

<Example 1-6>
A test cell was prepared and tested in the same manner as in Examples 1-5, except that the amount of lithium ethoxysulfonate added to the electrolyte was 1.0 wt %. A discharge capacity of 249 mAh/g-active material and a capacity ratio of 91% vs. 0.1 ItA were obtained.

<Example 1-7>
A test cell was prepared and tested in the same manner as in Examples 1-5, except that the amount of lithium methoxysulfonate added to the electrolyte was 0.2 wt % and the amount of lithium ethoxysulfonate added was 0.3 wt %, and a discharge capacity of 248 mAh/g-active material and a capacity ratio of 88% vs. 0.1 ItA were obtained.

<Example 1-8>
A test cell was prepared and tested in the same manner as in Examples 1-5, except that the amount of lithium ethoxysulfonate added to the electrolyte was 1.5 wt %, and a discharge capacity of 245 mAh/g-active material and a capacity ratio of 90% vs. 0.1 ItA were obtained.

<Example 1-9>
A test cell was prepared and tested in the same manner as in Examples 1-5, except that the amount of lithium methoxysulfonate added to the electrolyte was 0.4 wt %. A discharge capacity of 250 mAh/g-active material and a capacity ratio of 78% vs. 0.1 ItA were obtained.

<Example 1-10>
A test cell was prepared and tested in the same manner as in Example 1-5, except that no lithium methoxysulfonate was added to the electrolyte and the amount of lithium ethoxysulfonate added was 0.4 wt %. A discharge capacity of 249 mAh/g-active material and a capacity ratio of 79% vs. 0.1 ItA were obtained.

<Comparative Example 1>
A test cell was prepared and tested in the same manner as in Example 1-1, except that an electrolyte solution containing no lithium methoxysulfonate was used. A discharge capacity of 247 mAh/g-active material and a capacity ratio of 70% vs. 0.1 ItA were obtained.

<Conventional Example 1>
A test cell was prepared and tested in the same manner as in Example 1-1, except that lithium titanate was used as the negative electrode active material particles and an electrolyte solution to which lithium methoxysulfonate was not added was used. A discharge capacity of 150 mAh/g-active material and a capacity ratio of 99% vs. 0.1 ItA were obtained.

The results of charge/discharge tests for Examples 1-1 to 1-10, Comparative Example 1, and the Conventional Example are shown in Table 1. The discharge capacity at 0.1 ItA was approximately 1.7 times greater in the Examples and Comparative Example than in the Conventional Example, demonstrating that the TNO active material used in the Examples and Comparative Example is advantageous for achieving high energy density in lithium-ion batteries. In the overall evaluation section of Table 1, a 0.1 ItA discharge capacity greater than 240 mAh/g and a ratio of 0.1 ItA discharge capacity to 5.0 ItA discharge capacity of 80 or greater was evaluated as ◎; a 0.1 ItA discharge capacity greater than 200 mAh/g and a ratio of 0.1 ItA discharge capacity to 5.0 ItA discharge capacity of 75 or greater, although inferior to ◎, was evaluated as ○; and all other cases were evaluated as ×. The same evaluation was conducted for the overall evaluation section of the other tables below unless otherwise noted.

Examples 1-1 to 1-8 satisfy claims 1 and 2 of the present invention. Compared to Conventional Example 1, the 0.1 ItA discharge capacities of Examples 1-1 to 1-8 are at least 95 mAh/g greater than the 0.1 ItA discharge capacity of Conventional Example 1. Compared to Comparative Example 1, the 0.1 ItA discharge capacities of Examples 1-1 to 1-8 are similar to that of Comparative Example 1, but the 5.0 ItA discharge capacity ratio is at least 15% greater than the 5.0 ItA discharge capacity ratio of Comparative Example 1. This result enables the achievement of both output characteristics and energy density, which are the effects of the present invention. Furthermore, Examples 1-9 and 1-10 are included in claim 1 but not claim 2. Although the 0.1 ItA discharge capacity of Examples 1-9 and 1-10 was similar to that of Comparative Example 1, the 5.0 ItA discharge capacity ratio of Examples 1-9 and 1-10 was improved by at least 8% compared to Comparative Example 1, and although inferior to Examples 1-1 to 1-8, the effect was confirmed.

From the above, it was found that in all of the examples, both the output characteristics and energy density, which are the effects of the invention, were achieved, but the effect of the invention was greater in the range included in claim 2.
It should be noted that lithium methoxysulfonate does not dissolve when added alone up to 2.0 M without the addition of lithium ethoxysulfonate, so experiments were conducted up to the saturated concentration of 0.5 M. Lithium ethoxysulfonate does not dissolve when added alone up to 2.0 M without the addition of lithium methoxysulfonate, so experiments were conducted up to the saturated concentration of 1.5 M.

<Example 2-1>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the ethylene carbonate in the electrolyte was replaced with propylene carbonate (PC). A discharge capacity of 252 mAh/g-active material and a capacity ratio of 86% vs. 0.1 ItA were obtained.

<Example 2-2>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the solvent component in the electrolyte was γ-butyrolactone (GBL). A discharge capacity of 247 mAh/g-active material and a capacity ratio of 90% vs. 0.1 ItA were obtained.

<Example 2-3>
Test cells were prepared and tested in the same manner as in Example 1-1, except that ethylene carbonate in the electrolyte was replaced with γ-butyrolactone, and a discharge capacity of 255 mAh/g-active material and a capacity ratio of 88% vs. 0.1 ItA were obtained. Table 2 shows the results of Examples 2-1 to 2-3.

<Example 2-4>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the ethylene carbonate in the electrolyte was replaced with propylene carbonate and the dimethyl carbonate in the electrolyte was replaced with γ-butyrolactone, resulting in a discharge capacity of 250 mAh/g-active material and a capacity ratio of 85% vs. 0.1 ItA. Table 2 shows the results of Examples 2-1 to 2-4.

At all levels, Examples 2-1 to 2-4 demonstrated the effects of the present invention, which are a combination of output characteristics and energy density. Because these electrolyte solvents are liquid at room temperature, and GBL and DMC have low viscosity, it is believed that the above configuration did not adversely affect performance.

<Example 3-1>
Test cells were prepared and tested in the same manner as in Example 1-1, except that propylene carbonate was added to the electrolyte and the volume ratio of ethylene carbonate, dimethyl carbonate, and propylene carbonate was 1.5:7:1.5. A discharge capacity of 245 mAh/g-active material and a capacity ratio of 88% vs. 0.1 ItA were obtained. Table 3 shows the results of Examples 3-1, 1-1, and 2-1.

Example 3-1 is a system in which two cyclic carbonates, EC and PC, were added to the electrolyte, and the results demonstrated the effects of the present invention. EC is a solid at room temperature and becomes a highly viscous liquid when melted at around 40°C. However, this electrolyte system also contains a low-viscosity chain carbonate, and it is believed that the results were satisfactory within the scope of the electrolyte's composition.

<Example 4-1>
A test cell was prepared and tested in the same manner as in Example 1-1, except that ethyl methyl carbonate (EMC) was used as the chain carbonate in the electrolyte solution. A discharge capacity of 248 mAh/g-active material and a capacity ratio of 90% vs. 0.1 ItA were obtained.

<Example 4-2>
A test cell was prepared and tested in the same manner as in Example 1-1, except that diethyl carbonate (DEC) was used as the chain carbonate in the electrolyte, and a discharge capacity of 248 mAh/g-active material and a capacity ratio of 88% vs. 0.1 ItA were obtained.

<Example 4-3>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the same amounts of dimethyl carbonate and ethyl methyl carbonate were used as the chain carbonate in the electrolyte solution. A discharge capacity of 253 mAh/g-active material and a capacity ratio of 90% vs. 0.1 ItA were obtained.

<Example 4-4>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the same amounts of dimethyl carbonate and diethyl carbonate were used as the chain carbonate in the electrolyte solution. A discharge capacity of 251 mAh/g-active material and a capacity ratio of 87% vs. 0.1 ItA were obtained.

<Example 4-5>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the same amounts of ethyl methyl carbonate and diethyl carbonate were used as the chain carbonate in the electrolyte solution, and a discharge capacity of 247 mAh/g-active material and a capacity ratio of 85% vs. 0.1 ItA were obtained. Table 4 shows the results of Examples 4-1 to 4-5 and 1-1.

At all levels, Examples 4-1 to 4-5 demonstrated that the effects of the present invention, namely, output characteristics and energy density, could be achieved simultaneously. Since these electrolyte solvents, with the exception of EC, are liquid at room temperature and have low viscosity, it is presumed that the above configuration produced satisfactory results.

<Example 5-1>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 0.1 wt % of 1,3-dioxane (1,3-DO) was added to 100 wt % of the electrolyte solution. A discharge capacity of 252 mAh/g-active material and a capacity ratio of 90% vs. 0.1 ItA were obtained.

<Example 5-2>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 2.5 wt % of 1,3-dioxane was added to 100 wt % of the electrolyte solution. A discharge capacity of 250 mAh/g-active material and a capacity ratio of 92% vs. 0.1 ItA were obtained.

<Example 5-3>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 5.0 wt % of 1,3-dioxane was added to 100 wt % of the electrolyte solution. A discharge capacity of 246 mAh/g-active material and a capacity ratio of 92% vs. 0.1 ItA were obtained.

<Example 5-4>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 0.1 wt % of adiponitrile (AZN) was added to 100 wt % of the electrolyte solution. A discharge capacity of 254 mAh/g-active material and a capacity ratio of 90% vs. 0.1 ItA were obtained.

<Example 5-5>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 2.5 wt % of adiponitrile was added to 100 wt % of the electrolyte solution. A discharge capacity of 249 mAh/g-active material and a capacity ratio of 88% vs. 0.1 ItA were obtained.

<Examples 5-6>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 5.0 wt % of adiponitrile was added to 100 wt % of the electrolyte solution. A discharge capacity of 242 mAh/g-active material and a capacity ratio of 87% vs. 0.1 ItA were obtained.

<Examples 5-7>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 0.1 wt % of succinonitrile (SCN) was added to 100 wt % of the electrolyte solution. A discharge capacity of 252 mAh/g-active material and a capacity ratio of 90% vs. 0.1 ItA were obtained.

<Examples 5-8>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 2.5 wt % of succinonitrile was added to 100 wt % of the electrolyte solution. A discharge capacity of 250 mAh/g-active material and a capacity ratio of 88% vs. 0.1 ItA were obtained.

<Examples 5-9>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 5.0 wt % of succinonitrile was added to 100 wt % of the electrolyte solution. A discharge capacity of 243 mAh/g-active material and a capacity ratio of 90% vs. 0.1 ItA were obtained.

<Examples 5-10>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 6.0 wt % of 1,3-dioxane was added to 100 wt % of the electrolyte solution. A discharge capacity of 215 mAh/g-active material and a capacity ratio of 92% vs. 0.1 ItA were obtained.

<Example 5-11>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 6.0 wt % of adiponitrile was added to 100 wt % of the electrolyte solution. A discharge capacity of 213 mAh/g-active material and a capacity ratio of 92% vs. 0.1 ItA were obtained.

<Example 5-12>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 6.0 wt % of succinonitrile was added to 100 wt % of the electrolyte solution. A discharge capacity of 210 mAh/g-active material and a capacity ratio of 92% vs. 0.1 ItA were obtained.

<Example 5-13>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 0.08 wt % of 1,3-dioxane was added to 100 wt % of the electrolyte solution. A discharge capacity of 239 mAh/g-active material and a capacity ratio of 85% vs. 0.1 ItA were obtained.

<Example 5-14>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 1.0 wt % of 1,3-dioxane and 1.0 wt % of adiponitrile were added to 100 wt % of the electrolyte solution. A discharge capacity of 247 mAh/g-active material and a capacity ratio of 86% vs. 0.1 ItA were obtained.

<Example 5-15>
A test cell was prepared and tested in the same manner as in Example 1-1, except that 0.08 wt % of 1,3-dioxane and 1.0 wt % of succinonitrile were added to 100 wt % of the electrolyte solution. A discharge capacity of 250 mAh/g-active material and a capacity ratio of 87% vs. 0.1 ItA were obtained.

<Example 5-16>
Test cells were prepared and tested in the same manner as in Example 1-1, except that 1.0 wt% adiponitrile and 1.0 wt% succinonitrile were added to 100 wt% of the electrolyte solution, and a discharge capacity of 248 mAh/g-active material and a capacity ratio of 86% vs. 0.1 ItA were obtained. Table 5 shows the results of Examples 5-1 to 5-16.

In all levels of Examples 5-1 to 5-9 and Examples 5-14 to 5-16, results demonstrated that the effects of the present invention, namely, output characteristics and energy density, were compatible. It is believed that 1,3-dioxane exhibited a high dielectric constant, while adiponitrile and succinonitrile contributed to improved safety and reliability. Furthermore, in Examples 5-10 to 5-13, in which the amounts added were greater than 5.0 wt% or less than 0.1 wt%, respectively, the 0.1 ItA discharge capacity was higher than 200 mAh/g, and the ratio of the 0.1 ItA discharge capacity to the 5.0 ItA discharge capacity was 75 or greater. These results demonstrate that adding 0.1 to 5 mass% of 1,3-dioxane, adiponitrile, and succinonitrile, respectively, resulted in favorable battery characteristics.

<Comparative Example 6-1>
A test cell was prepared and tested in the same manner as in Example 1-1, except that no electrolyte was added to the electrolytic solution, and a discharge capacity of 0 mAh/g-active material and a capacity ratio of 0% vs. 0.1 ItA were obtained.

<Example 6-1>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the electrolyte concentration was 0.5 M, and a discharge capacity of 250 mAh/g-active material and a capacity ratio of 82% vs. 0.1 ItA were obtained.

<Example 6-2>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the electrolyte concentration was 1.5 M. A discharge capacity of 252 mAh/g-active material and a capacity ratio of 86% vs. 0.1 ItA were obtained.

<Example 6-3>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the electrolyte concentration was 3.0 M, and a discharge capacity of 253 mAh/g-active material and a capacity ratio of 83% vs. 0.1 ItA were obtained.

<Example 6-4>
A test cell was prepared and tested in the same manner as in Example 1-1, except that the electrolyte concentration was 4.0 M, and a discharge capacity of 246 mAh/g-active material and a capacity ratio of 75% vs. 0.1 ItA were obtained.

<Example 6-5>
A test cell was prepared and tested in the same manner as in Example 1-1, except that lithium tetrafluoroborate was used as the electrolyte and the concentration was 0.5 M. A discharge capacity of 245 mAh/g-active material and a capacity ratio of 82% vs. 0.1 ItA were obtained.

<Example 6-6>
A test cell was prepared and tested in the same manner as in Example 1-1, except that lithium tetrafluoroborate was used as the electrolyte and the concentration was 1.5 M. A discharge capacity of 248 mAh/g-active material and a capacity ratio of 86% vs. 0.1 ItA were obtained.

<Examples 6-7>
A test cell was prepared and tested in the same manner as in Example 1-1, except that lithium tetrafluoroborate was used as the electrolyte and the concentration was 3.0 M. A discharge capacity of 249 mAh/g-active material and a capacity ratio of 83% vs. 0.1 ItA were obtained.

<Examples 6 to 8>
A test cell was prepared and tested in the same manner as in Example 1-1, except that lithium tetrafluoroborate was used as the electrolyte and the concentration was 4.0 M. A discharge capacity of 242 mAh/g-active material and a capacity ratio of 75% vs. 0.1 ItA were obtained.

<Examples 6 to 9>
A test cell was fabricated and tested in the same manner as in Example 1-1, except that LiFSI was used as the electrolyte and the concentration was 1.5 M. A discharge capacity of 246 mAh/g-active material and a capacity ratio of 75% vs. 0.1 ItA were obtained.

<Examples 6-10>
A test cell was prepared and tested in the same manner as in Example 1-1, except that LiBOB was used as the electrolyte and the concentration was 1.5 M. A discharge capacity of 248 mAh/g-active material and a capacity ratio of 75% vs. 0.1 ItA were obtained.

<Example 6-11>
A test cell was prepared and tested in the same manner as in Example 1-1, except that LiPF ₂ O ₂ was used as the electrolyte at a concentration of 1.5 M. A discharge capacity of 246 mAh/g-active material and a capacity ratio of 75% vs. 0.1 ItA were obtained. Table 6 shows the results of Examples 6-1 to 6-11 and Comparative Example 6-1.

At any level in Examples 6-1 to 6-3, and Examples 6-5 to 6-7, results were shown that the effects of the present invention, namely, output characteristics and energy density, can be achieved simultaneously. However, when more than 3M of each is added, as in Examples 6-4 and 6-8, the resistance of the electrolyte increases, and the 5.0 ItA discharge capacity ratio is slightly deteriorated. Therefore, if the amount of electrolyte added is too large, the viscosity of the electrolyte increases, which has a negative effect on the discharge rate characteristics, so an appropriate concentration is required. Furthermore, when LiFSI, LiBOB, or LFO is added, the effect is not as good as that of _LiPF6 or _LiBF4 , and the 5.0 ItA discharge capacity ratio is slightly deteriorated. Note that Comparative Example 6-1 does not contain an electrolyte and therefore does not function as a battery.
From the above, it was found that adding 0.5 to 3.0M of _LiPF6 or _LiBF4 showed good battery characteristics.

<Example 7-1>
The metallic lithium counter electrode was changed to a positive _electrode using _{LiNi0.5Co0.2Mn0.3O2} ( _NCM ), and _the coin cell was further changed to a 1 Ah laminate cell. Specifically, an electrode element was produced by alternately stacking a positive electrode having a positive electrode current collector lead and a negative electrode having a negative electrode current collector lead on a separator connected in a zigzag pattern. The separator used was a polyethylene substrate layer with a polypropylene surface layer on both sides (PE/PP/PE). The separator thickness was 20 μm. Next, the positive electrode lead and the negative electrode lead were each bundled, and a positive electrode terminal was connected to the bundled positive electrode lead by ultrasonic welding, and a negative electrode terminal was connected to the bundled negative electrode lead by ultrasonic welding. The produced electrode element had a thickness of 4.8 mm and a rated capacity of 1 Ah. The term "rated capacity" used herein refers to the discharge capacity when a constant current-constant voltage charge (cutoff current: 0.05 C) is performed at an upper limit voltage of 4.2 V and a current value of 0.5 C, followed by a constant current discharge at a lower limit voltage of 2.7 V and a current value of 0.2 C.

Next, two laminate films were prepared as exterior bodies. Each laminate film had a structure consisting of a heat-sealed resin layer made of polyolefin, a metal layer made of aluminum foil, and a protective layer made of nylon resin and polyester resin, laminated in that order. The heat-sealed resin layers of the two laminate films were positioned facing each other, and the laminate films were overlapped with their adhesive surfaces facing each other so that the electrode groups would be housed in the two housing recesses. The electrode groups were positioned so that the portions of the heat-sealed resin portions of each terminal passed between the edges of the two laminate films, leaving a portion of each terminal exposed to the outside. In this state, the heat-sealed resin layers of the laminate films were heat-sealed to each other along three edges, including the two edges from which the tabs of the laminate films extended. The electrolyte prepared above was then injected into the one edge of the exterior body that was not heat-sealed. Next, the remaining edge of the exterior body was heat-sealed in a reduced-pressure environment to create a test cell.

Tests were conducted in the same manner as in Example 1-1, resulting in a discharge capacity of 1012 mAh/g, an average discharge voltage of 2.35 V, and a capacity ratio of 89% vs. 0.1 ItA.

<Example 7-2>
A test cell was prepared and tested in the same manner as in Example _7-1 , except that _the positive _electrode was _{LiNi0.6Co0.2Mn0.2O2} . A discharge capacity of 1020 mAh/g-active material, a discharge average voltage of 2.34 V, and a capacity ratio of 92% vs. 0.1 ItA were obtained.

<Example 7-3>
A test cell was prepared and tested in the same manner as in Example _7-1 , except that _the positive _electrode was _{LiNi0.8Co0.1Mn0.1O2} . A discharge capacity of 999 mAh/g-active material, a discharge average voltage of 2.38 V, and a capacity ratio of 89% vs. 0.1 ItA were obtained.

<Example 7-4>
A test cell was prepared and tested in the same manner as in Example 7-1, except that the positive electrode was LiNi _0.5 Mn _0.5 O ₂ , and a discharge capacity of 1010 mAh/g-active material, a discharge average voltage of 3.00 V, and a capacity ratio of 80% vs. 0.1 ItA were obtained.

<Example 7-5>
A test cell was prepared and tested in the same manner as in Example 7-1, except that the positive electrode was LiNiO ₂ , and a discharge capacity of 996 mAh/g-active material, a discharge average voltage of 2.44 V, and a capacity ratio of 85% vs. 0.1 ItA were obtained.

<Example 7-6>
A test cell was prepared and tested in the same manner as in Example 7-1, except that _the positive _electrode was _{LiNi0.33Co0.33Mn0.33O2 .} A discharge capacity of 950 mAh/g of active material, an average discharge voltage of 2.37 V, and a capacity ratio of 92% vs. 0.1 ItA were obtained. Table 7 shows the results of Examples 7-1 to 7-6. In the overall evaluation section of Table 7, the ratio of the 0.1 ItA discharge capacity to the 5.0 ItA discharge capacity was higher than 75, and the 0.1 ItA discharge capacity was higher than 950 mAh/g. A 0.1 ItA discharge capacity higher than 900 mAh/g, although inferior to A, was evaluated as ◯. A 0.1 ItA discharge capacity lower than 900 mAh/g was evaluated as ×.

In Examples 7-1 to 7-6, results showed that the effects of the present invention, which are the compatibility of output characteristics and energy density, were achieved at all levels. However, if the nickel ratio in the positive electrode active material were to fall to 33.3%, the discharge capacity would decrease and it would no longer be possible to ensure energy density, so the nickel ratio must be greater than 33.3%.

The objective of this invention was to achieve both output characteristics and energy density in a lithium-ion battery. Both of these performance characteristics were achieved only when TNO was used as the negative electrode active material and 0.5 to 2.0 wt.% of lithium methoxysulfonate and lithium ethoxysulfonate were added as electrolyte additives. Using TNO alone or these additives alone would not have been effective, demonstrating that the invention is attributable to the unique effect of the combination. While the output characteristics are inferior to those achieved when lithium titanate is used as the negative electrode active material, the difference is only about 10%, and considering that the use of TNO increases discharge capacity by approximately 1.7 times, it goes without saying which battery configuration is superior.

As described above, by using lithium niobium titanate as the negative electrode active material and lithium methoxysulfonate and lithium ethoxysulfonate as electrolyte additives in lithium-ion batteries, it is possible to create batteries that combine good output characteristics and energy density. This battery is expected to be used in a wide range of fields, including mobility applications such as electric vehicles and drones, as well as natural energy storage applications.
The inventions described in the original claims of this application are set forth below.
[1]
a negative electrode containing niobium titanium oxide as a negative electrode active material in a mixture layer;
and a non-aqueous electrolyte solution containing lithium alkoxysulfonate represented by RSOOO ⁻ Li ⁺ (wherein R is at least one substance selected from the group consisting of OCH ₃ and OC ₂ H ₅ ).
[2]
The lithium ion secondary battery according to [1], wherein the content of the lithium alkoxysulfonate in the non-aqueous electrolyte solution is 0.5 to 2.0 mass %.
[3]
The lithium ion secondary battery according to [1] or [2], wherein the non-aqueous electrolyte solution contains at least one non-aqueous solvent selected from the group consisting of cyclic carbonates, chain carbonates, and γ-butyrolactone.
[4]
The lithium ion secondary battery according to any one of [1] to [3], wherein the cyclic carbonate is ethylene carbonate and/or propylene carbonate, and does not include vinylene carbonate, fluoroethylene carbonate, or vinylethylene carbonate.
[5]
The lithium ion secondary battery according to any one of [1] to [4], wherein the chain carbonate is at least one selected from dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.
[6]
The lithium ion secondary battery according to any one of [1] to [5], wherein the non-aqueous electrolyte solution contains 0.1 wt % to 5 wt % of at least one selected from cyclic ethers: 1,3-dioxane, adiponitrile, and succinonitrile.
[7]
The lithium ion secondary battery according to any one of [1] to [6], which contains 0.5M to 3M LiPF6 and/or LiBF4 as an electrolyte, but does not contain LiFSi _, LiBOB _, or _{LiPO2F2 .}
[8]
The lithium ion secondary battery according to any one of [1] to [7], comprising a positive electrode containing a lithium composite oxide in which Ni is 50% or more in atomic ratio in a mixture layer.
[9]
The lithium ion secondary battery according to [8], wherein the lithium composite oxide is at least one selected from LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , and LiNi _0.8 Co _0.1 Mn _0.1 O ₂ .
[10]
A non-aqueous electrolyte solution for use in the lithium ion secondary battery according to any one of [1] to [9].

1...Lithium ion secondary battery, 2...Positive electrode, 3...Negative electrode, 4...Separator, 5...First external terminal, 6...Second external terminal, 7...Gasket, 21...Positive electrode current collector, 22...Positive electrode active material composite electrode layer, 31...Negative electrode current collector, 32...Composite layer, 100...Stacked lithium secondary battery, 200...Exterior body, 300...Electrode group, 400...Positive electrode, 500...Negative electrode, 600...Separator, 410...Positive electrode current collector, 420...Positive electrode active material composite electrode layer, 510...Negative electrode current collector, 520...Composite layer, 700...Positive electrode lead, 800...Positive electrode tab, 900...Negative electrode lead, 1000...Negative electrode tab.

Claims (9)

a negative electrode containing niobium titanium oxide as a negative electrode active material in a mixture layer;
a non-aqueous electrolyte solution containing a lithium alkoxysulfonate represented by RSOOO ^- Li ⁺ (wherein R is at least one selected from the group consisting _{of OCH3} and _OC2H5 );
A lithium-ion secondary battery comprising: The lithium-ion secondary battery according to claim 1, wherein the content of the lithium alkoxysulfonate in the nonaqueous electrolyte is 0.5 to 2.0 mass %. 3. The lithium ion secondary battery according to claim 1, wherein the non-aqueous electrolyte contains at least one non-aqueous solvent selected from the group consisting of cyclic carbonates, chain carbonates, and γ-butyrolactone. 4. The lithium ion secondary battery according to claim 3 , wherein the cyclic carbonate is ethylene carbonate and/or propylene carbonate, and does not include vinylene carbonate, fluoroethylene carbonate, or vinylethylene carbonate. 4. The lithium ion secondary battery according to claim 3 , wherein the chain carbonate is at least one selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. The lithium ion secondary battery according to any one of claims 1 to 5, wherein the non-aqueous electrolyte solution further contains at least one cyclic ether selected from the group consisting of 1,3 -dioxane, adiponitrile , and succinonitrile, and the content of the cyclic ether is 0.1 wt% to 5 wt%. The lithium ion secondary battery according to any one of claims 1 to 6, wherein the non-aqueous electrolyte solution contains an electrolyte, and the electrolyte contains 0.5M to 3M _LiPF6 and/or _LiBF4 , but does not contain LiFSi, LiBOB, or _{LiPO2F2 .} The lithium ion secondary battery according to any one of claims 1 to 7, further comprising a positive electrode containing a lithium composite oxide in which Ni is 50 atomic % or more in the mixture layer. 9. The lithium _ion secondary battery according to claim 8 _, wherein the _lithium composite oxide is at least one selected _{from the group consisting of LiNi0.5Co0.2Mn0.3O2 , LiNi0.6Co0.2Mn0.2O2 ,} and _{LiNi0.8Co0.1Mn0.1O2 .}