JP7471992B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

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

In recent years, non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries have been used in a wide range of applications, including as storage batteries for portable devices, electric vehicles, and residential or business facilities. For this reason, lithium-ion secondary batteries may be used in low-temperature or high-temperature environments, depending on the application. Lithium-ion secondary batteries are required to have charge/discharge characteristics that do not deteriorate even when repeatedly charged and discharged in high-temperature or low-temperature environments.

In general, lithium ion secondary batteries are charged and discharged by transferring lithium ions between a positive electrode and a negative electrode. The positive electrode contains a positive electrode active material capable of absorbing and releasing lithium ions. The negative electrode contains a negative electrode active material capable of absorbing and releasing lithium ions. A separator is interposed between the positive electrode and the negative electrode to prevent internal short circuits. The separator is impregnated with a nonaqueous electrolyte.

In recent years, as a battery that can be used in a wide range of temperature environments, a battery using a lithium titanium composite oxide (hereinafter also referred to as LTO) such as Li ₄ Ti ₅ O ₁₂ as a negative electrode active material has been attracting attention. Since LTO has a spinel structure, it has excellent thermal stability and has the advantage of small volume change due to expansion and contraction accompanying charge and discharge. In addition, in LTO, the absorption potential of lithium ions is 0.5V (vs. Li/Li ⁺ ) or more, and the operating voltage is high. This has the advantage of being able to suppress the reductive decomposition reaction of the non-aqueous solvent at the negative electrode interface.
As a positive electrode active material to be used in combination with LTO, lithium nickel cobalt manganese composite oxide (hereinafter, also referred to as LNCM) has attracted attention from the viewpoint of achieving high capacity.

However, in batteries that use LTO as the negative electrode active material, there is a risk of expansion due to gas generation during charging and discharging in a high-temperature environment. Gas generation increases the distance between the electrodes, increasing the internal resistance of the battery and reducing life performance. In addition, in batteries that use LNCM as the positive electrode active material, the positive electrode active material expands and contracts significantly during charging and discharging in a high-temperature environment, which may lead to defects in the positive electrode layer, such as cracks. Such defects lead to deterioration of charge and discharge characteristics in a high-temperature environment and deterioration of life performance.

Patent Document 1 discloses a method for producing a lithium ion secondary battery that uses a non-aqueous electrolyte containing an overcharge additive and a difluorophosphate to ensure excellent durability (cycle characteristics, etc.) and high reliability (safety, etc.).

Patent Document 2 discloses a positive electrode active material in which titanium, a dissimilar metal, is present on the surface of a layered lithium transition metal composite oxide in order to improve gas generation and charge/discharge characteristics in harsh usage environments (such as high-temperature environments). Patent Document 2 discloses LNCM as an example of a lithium transition metal composite oxide.

JP 2015-191853 A JP 2005-123111 A

However, as disclosed in Patent Document 1, even when a non-aqueous electrolyte containing an overcharge additive and a difluorophosphate was used, the suppression of gas generation in a high-temperature environment was insufficient. Also, as disclosed in Patent Document 2, even when LNCM containing a different metal was used as the positive electrode active material, the improvement of the charge/discharge characteristics and life performance in a high-temperature environment was insufficient. Furthermore, these batteries also had problems in improving the charge/discharge characteristics in a low-temperature environment.

The present invention aims to solve the above problems and provide a non-aqueous electrolyte secondary battery that can suppress gas generation in high-temperature environments and has excellent charge/discharge characteristics in both high-temperature and low-temperature environments.

In order to solve the above problems, according to one embodiment, there is provided a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte, in which the positive electrode includes a positive electrode layer including a lithium nickel cobalt manganese-based composite oxide as a positive electrode active material, and the negative electrode includes a negative electrode layer including a lithium titanium-based composite oxide as a negative electrode active material, and the nonaqueous electrolyte includes a nonaqueous solvent including propylene carbonate, dimethyl carbonate, and methyl ethyl carbonate in a volume ratio of 1.6 to 2.4:4.0 to 4.4:3.6 to 4.0; an electrolyte including _LiPF6 or _LiBF4 and having a concentration of 1.0 mol/L or more and 1.3 mol/L or less relative to the _nonaqueous solvent; and an additive including _LiPO2F2 . When the concentration of the additive is 0.5% by weight or more and 1.0% by weight or less with respect to the total amount of the nonaqueous solvent and the electrolyte, the capacity ratio (Y/X) of the capacity (X) of the positive electrode to the capacity (Y) of the negative electrode at the time of initial charging is 0.80 to 0.85.

The present invention provides a nonaqueous electrolyte secondary battery that can suppress gas generation in high-temperature environments and has excellent charge/discharge characteristics in both high-temperature and low-temperature environments.

1 is a plan view illustrating an example of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. FIG. 2 is a plan view showing the electrode group of FIG. 1 . FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2 .

The nonaqueous electrolyte secondary battery (lithium ion secondary battery) according to the embodiment will be described in detail below. The lithium ion secondary battery according to the embodiment includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The lithium ion secondary battery may further include a separator impregnated with the nonaqueous electrolyte and an exterior body that houses them.

<Non-aqueous electrolyte>
The non-aqueous electrolyte includes a non-aqueous solvent, an electrolyte, and an additive.
As the non-aqueous electrolyte, a liquid non-aqueous electrolytic solution prepared by dissolving an electrolyte and an additive in a non-aqueous solvent can be used.

The non-aqueous solvent contains propylene carbonate (PC), dimethyl carbonate (DMC), and methyl ethyl carbonate (MEC) in a volume ratio of 1.6-2.4:4.0-4.4:3.6-4.0. The non-aqueous solvent preferably consists of PC, DMC, and MEC in the above volume ratio.

PC is a cyclic carbonate. PC has the characteristics of high dielectric constant and high electrical conductivity. Compared to other cyclic carbonates, this characteristic of PC is more pronounced in low temperature environments. PC's high dielectric properties can improve the solubility of electrolytes and additives, and its high electrical conductivity can improve the electrical conductivity of non-aqueous electrolytes.

DMC and MEC are chain carbonates. DMC and MEC have low viscosity properties and can improve the electrical conductivity of non-aqueous electrolytes, especially in low-temperature environments. MEC can impart antioxidant properties to non-aqueous solvents.

By containing PC, DMC, and MEC in the above volume ratio, the nonaqueous solvent has a good balance of the above-mentioned properties, and can have high electrical conductivity, low viscosity, and high dielectric constant. This nonaqueous solvent configuration allows the nonaqueous electrolyte to have high electrical conductivity even in a low-temperature environment, reducing the internal resistance of the battery and improving the charge/discharge characteristics in a low-temperature environment. Furthermore, this nonaqueous solvent configuration can suppress decomposition of the nonaqueous electrolyte in a high-temperature environment, thereby suppressing gas generation.

If the volume ratio of PC is less than 1.6, the dielectric constant of the non-aqueous solvent decreases, which is undesirable as it may increase the risk of gas generation in a high-temperature environment. It is also undesirable as the conductivity in a low-temperature environment decreases, which may degrade the discharge characteristics of the battery in a low-temperature environment. On the other hand, if the volume ratio of PC is more than 2.4, the content of DMC and MEC tends to decrease relatively, which increases the viscosity of the non-aqueous solvent. As a result, the conductivity in a low-temperature environment decreases, which may degrade the discharge characteristics of the battery in a low-temperature environment, which is undesirable.

Here, in a typical lithium ion secondary battery, graphite is used as the negative electrode active material. Conventionally, when the non-aqueous solvent contains PC in the above volume ratio and the negative electrode active material is graphite, there is a risk that PC will be reductively decomposed at the interface of the negative electrode layer during charging. This reductive decomposition leads to gas generation at the interface of the negative electrode, leading to defects such as peeling of the negative electrode layer. In this embodiment, as described later, a lithium titanium composite oxide (LTO) having a high operating voltage is used as the negative electrode active material. This allows the battery to be charged and discharged at a potential at which the reductive decomposition of the PC does not occur. In this embodiment, the combination of LTO and the non-aqueous solvent can improve the charge and discharge characteristics, especially in low temperature environments, without causing the disadvantages caused by the reductive decomposition of PC described above.

The non-aqueous solvent may contain a non-aqueous solvent that is a minor component other than PC, DMC, and MEC.
Examples of the minor component of the non-aqueous solvent include cyclic carbonates such as ethyl methyl carbonate (EMC) and vinylene carbonate (VC), chain carbonates such as diethyl carbonate (DEC), cyclic ethers such as tetrahydrofuran (THF) and 2-methyltetrahydrofuran (2MeTHF), chain ethers such as dimethoxyethane (DME), acetonitrile (AN), sulfolane (SL), etc. The minor component of the non-aqueous solvent can be used alone or as a mixture of a plurality of these.

The electrolyte contains lithium hexafluorophosphate (LiPF ₆ ) or lithium tetrafluoroborate (LiBF ₄ ). The electrolyte is contained in a concentration of 1.0 mol/L or more and 1.3 mol/L or less in the non-aqueous solvent. By containing the electrolyte in the above concentration in the non-aqueous electrolyte, the amount of lithium ions contributing to the reaction at the electrode interface is appropriate, so that the conductivity of the non-aqueous electrolyte can be increased. As a result, polarization during charging and discharging is suppressed, particularly in a low temperature environment, and the charging and discharging characteristics of the battery can be improved.

If the electrolyte concentration is less than 1.0 mol/L relative to the non-aqueous solvent, the amount of lithium ions that contribute to the reaction at the electrode interface decreases, which is undesirable since it may result in a decrease in discharge capacity, particularly in a low-temperature environment. On the other hand, if the electrolyte concentration is more than 1.3 mol/L relative to the non-aqueous solvent, the viscosity of the non-aqueous electrolyte increases, which increases polarization at the electrode interface, which is undesirable since it may result in a decrease in discharge capacity, particularly in a low-temperature environment.

The electrolyte may contain an electrolyte other than LiPF ₆ or LiBF ₄ as a secondary component, but preferably does not contain any. Examples of the secondary component of the electrolyte include lithium arsenic hexafluoride (LiAsF ₆ ), lithium perchlorate (LiClO ₄ ), and lithium trifluoromethasulfonate (LiCF ₃ SO ₃ ). The secondary component of the electrolyte can be used alone or as a mixture of multiple components.

The additive consists of lithium difluorophosphate (LiPO ₂ F ₂ ), which dissolves in a non-aqueous solvent and releases the PO ₂ F ₂ ^- anion.

The additive is contained at a concentration of 0.5% by weight or more and 1.0% by weight or less based on the total amount of the non-aqueous solvent and electrolyte. When the non-aqueous electrolyte contains the additive at the above concentration, the internal resistance of the positive electrode layer of the battery can be reduced and lithium deposition in a low-temperature environment can be suppressed. As a result, gas generation during charging and discharging in a high-temperature environment can be suppressed, and the charging and discharging characteristics in a low-temperature environment can be improved. This can improve the life performance and reliability of the battery.

If the additive concentration is less than 0.5% by weight relative to the combined weight of the nonaqueous solvent and electrolyte, this is not preferred because it may cause deterioration in charge/discharge characteristics, particularly in low-temperature environments, and increase gas generation during charge/discharge in high-temperature environments. On the other hand, if the additive concentration exceeds 1.0% by weight relative to the combined weight of the nonaqueous solvent and electrolyte, the viscosity of the nonaqueous electrolyte increases, increasing polarization at the electrode interface during charge/discharge, which is not preferred because it may cause a decrease in discharge capacity, particularly in low-temperature environments.

<Positive electrode>
The positive electrode includes, for example, a positive electrode current collector and a positive electrode layer formed on one or both surfaces of the positive electrode current collector and including a positive electrode active material, a conductive assistant, and a binder.
The positive electrode current collector may be, for example, an aluminum foil or an aluminum alloy foil.

The positive _electrode active material includes a lithium _{nickel cobalt manganese} based composite oxide (LNCM). The LNCM is, for example, _{Li1.05Ni0.50Co0.20Mn0.30O2} . Compared with other positive electrode active materials such as lithium manganese based composite oxides (e.g., _LiMnO2 ), the LNCM has good charge/discharge characteristics, particularly in low temperature environments. The positive electrode active material is preferably made of LNCM.

LNCM is preferably _{a compound represented} by the general formula _{LiαNixCoyMnzM (1-x-y-z) O2} (wherein 1.05≦α≦1.13, 1≧x+y+z, and M is at least one substitution element selected from Mg, Al, Ti, Fe, Cu, Zn, Ga, Nb, Sn, Zr, and Ta), and M is preferably Al.

In the LNCM of the above general formula, a portion of the Ni or Mn element is replaced with a different metal element. When the LNCM contains a different metal replacement element, the thermal stability of the positive electrode active material is improved. This is because the different metal replacement element strengthens the covalent bond between oxygen and the metal element, suppressing oxygen release from the positive electrode active material during charging and discharging in a high-temperature environment. As a result, the expansion and contraction of the positive electrode active material during charging and discharging in a high-temperature environment can be suppressed, and defects such as cracks in the positive electrode layer can be suppressed. As a result, by improving the thermal stability of the positive electrode layer, it is possible to improve the charge and discharge characteristics, especially in a high-temperature environment. In particular, by combining a non-aqueous electrolyte containing the additive at the above specific concentration, the charge and discharge characteristics in a high-temperature environment can be further improved.

The conductive assistant is not particularly limited, and any known or commercially available one can be used. The conductive assistant helps the electrons generated by the lithium ion absorption/release reaction to be conducted within the positive electrode layer and reach the positive electrode current collector. Examples of the conductive assistant include graphite and acetylene black. The conductive assistant can be used alone or as a mixture of multiple conductive assistants.

The binder is not particularly limited, and any known or commercially available binder may be used. The binder binds the positive electrode active material and the conductive assistant, and the positive electrode layer and the positive electrode current collector. It is preferable that the binder does not deteriorate due to contact with the non-aqueous electrolyte.
Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorine-based rubber, styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC). The binder can be used alone or as a mixture of multiple binders.

The positive electrode layer preferably contains 85 to 95% by weight of positive electrode active material, 4 to 8% by weight of conductive additive, and 3 to 5% by weight of binder.

The positive electrode can be prepared, for example, by the following method. First, the positive electrode active material, conductive additive, and binder are dispersed in a solvent to prepare a positive electrode slurry. The positive electrode slurry is then applied to one or both surfaces of a positive electrode current collector, and then dried to form a positive electrode layer, thereby preparing a positive electrode. A thickener or dispersant may be further added to the positive electrode slurry. Examples of the solvent used to prepare the positive electrode slurry include N-methyl-2-pyrrolidone (NMP) and water.

The moisture content of the cathode layer after drying per sheet is preferably 110 ppm or less, more preferably 100 ppm or less. If the moisture content is 110 ppm or less, gas generation during charging and discharging in a high-temperature environment can be further suppressed. The moisture content of the cathode layer can be measured by the Karl Fischer method or the like. The moisture content can be measured by a method conforming to JIS K0068:2001, for example. When drying the cathode layer, vacuum drying is preferably performed for 40 hours or more in a high-temperature environment of 120°C to 130°C.

<Negative Electrode>
The negative electrode includes, for example, a negative electrode current collector and a negative electrode layer formed on one or both surfaces of the negative electrode current collector and including a negative electrode active material, a conductive material, and a binder.
The negative electrode current collector may be, for example, an aluminum foil or an aluminum alloy foil.

The negative electrode active material includes a lithium titanium based composite oxide (LTO). The LTO preferably has a spinel structure. The negative electrode active material is preferably made of LTO.
LTO is preferably a compound represented by the general formula _LiαTixO12 (wherein 3.0<α<5.0 _{, 4.0} <x<6.0). LTO preferably has a lithium ion absorption potential of 0.5 V (vs. Li/Li ⁺ ) or more, more preferably 0.5 V or more and 1.0 V (vs. Li/Li ⁺ ) or less.

The average particle size of LTO is preferably in the range of 0.6 μm to 0.7 μm. By using an average particle size in this range, the specific surface area can be increased, resulting in good discharge characteristics.

The specific surface area of the LTO is preferably in the range of 5.0 m ² /g to 6.5 ^{m 2} /g. When a specific surface area in this range is used, the surface area that effectively contributes to the electrode reaction is sufficient, so that good discharge characteristics can be obtained and gas generation on the negative electrode surface during charge and discharge can be effectively suppressed.

The conductive assistant is not particularly limited, and any known or commercially available one can be used. The conductive assistant helps the electrons generated by the lithium ion absorption/release reaction to be conducted within the negative electrode layer and reach the negative electrode current collector. Examples of the conductive assistant include graphite, acetylene black, and carbon black. The conductive assistant can be used alone or as a mixture of multiple conductive assistants.

The binder is not particularly limited, and any known or commercially available binder can be used. The binder binds the negative electrode active material and the conductive assistant, and the negative electrode layer and the negative electrode current collector. It is preferable that the binder does not deteriorate due to contact with the non-aqueous electrolyte. For example, the same binder as that for the positive electrode can be used as the binder.

The negative electrode layer preferably contains 93 to 97% by weight of negative electrode active material, 1.5 to 3% by weight of conductive additive, and 2 to 4% by weight of binder.

The negative electrode can be prepared, for example, by the following method. First, the negative electrode active material, conductive additive, and binder are dispersed in a solvent to prepare a negative electrode slurry. The negative electrode slurry is then applied to one or both surfaces of a negative electrode current collector, and then dried to form a negative electrode layer, thereby preparing a negative electrode. A thickener or dispersant may be further added to the negative electrode slurry. Examples of the solvent used to prepare the negative electrode slurry include N-methyl-2-pyrrolidone (NMP) or water.

The moisture content of the negative electrode layer after drying per negative electrode is preferably 220 ppm or less, more preferably 200 ppm or less. If the moisture content is 220 ppm or less, gas generation during charging and discharging in a high-temperature environment can be further suppressed. The moisture content of the negative electrode layer can be measured, for example, by a method similar to that for measuring the moisture content of the positive electrode layer described above. When drying the negative electrode layer, vacuum drying is preferably performed for 40 hours or more in a high-temperature environment of 100°C to 130°C.

<Capacity ratio of positive and negative electrodes>
The capacity of the positive electrode active material contained in the positive electrode at the time of the first discharge is X, and the capacity of the negative electrode active material contained in the negative electrode at the time of the first charge is Y. When the concentration of the additive (LiPO ₂ F ₂ ) is 0.5% by weight or more and 1.0% by weight or less with respect to the total amount of the nonaqueous solvent and the electrolyte, the capacity ratio (Y/X) of the capacity of the positive electrode (X) to the capacity of the negative electrode (Y) at the time of the first charge is 0.80 to 0.85. The capacity ratio is preferably 0.80 to 0.83.

As a result of careful investigation, the inventors found that there is a correlation between the concentration of the additive and the above capacity ratio, and that there is an appropriate value for suppressing deterioration of battery performance.

By setting the capacity ratio to 0.80 or more and less than 0.85, the positive electrode potential during charging is kept relatively low, and deterioration of the positive electrode layer due to charging and discharging in a high-temperature environment can be suppressed. This improves the charge and discharge characteristics of the battery in a high-temperature environment and suppresses gas generation.

If the capacity ratio is less than 0.80, the negative electrode current collector (aluminum foil, etc.) may dissolve during charging, and may form an alloy with Li metal. The deterioration of the negative electrode layer may lead to a deterioration of the charge-discharge cycle characteristics. If the capacity ratio is 0.85 or more, the positive electrode potential may become high, and the deterioration of the positive electrode layer may progress due to charging and discharging in a high-temperature environment.

When the concentration of the additive is 0.5 wt%, the volume ratio is preferably 0.80 to 0.81, and when the concentration of the additive is 1.0 wt%, the volume ratio is preferably 0.80 to 0.82.
When the concentration of the additive and the capacity ratio satisfy the above relationship, the potential on the positive electrode side during charging can be kept relatively low, thereby suppressing deterioration of the positive electrode layer due to charging and discharging in a high-temperature environment.

The capacity X and capacity Y are the design capacity per unit area of the positive electrode and the negative electrode, respectively. The capacity X of the positive electrode is a design value that can be calculated from the theoretical capacity [mAh/g] of the positive electrode active material, the electrode density [g/cc], and the proportion [wt%] of the positive electrode active material in the positive electrode layer. Similarly, it is a design value that can be calculated from the theoretical capacity [mAh/g] of the negative electrode active material, the electrode density [g/cc], and the proportion [wt%] of the negative electrode active material in the negative electrode layer. The capacity X and the capacity Y are represented by the following formula: Capacity X = Theoretical capacity of positive electrode active material [mAh/g] × Electrode density [g/cc] × Volume of positive electrode coated surface [cc] × Proportion of positive electrode active material in the positive electrode layer [% by weight], and the capacity Y is calculated by the following formula: Capacity Y = Theoretical capacity of negative electrode active material [mAh/g] × Electrode density [g/cc] × Volume of negative electrode coated surface [cc] × Proportion of negative electrode active material in the negative electrode layer [% by weight].
The capacity ratio is the value at the time of "initial charging."

<Separator>
The separator is not particularly limited, and a known or commercially available one can be used. The separator is made of an insulating material and prevents the positive electrode and the negative electrode from coming into electrical contact. The separator is configured to allow the electrolyte to move between the positive electrode and the negative electrode. Examples of the separator include a porous film containing polyethylene, polypropylene, cellulose, or polyvinylidene fluoride (PVDF), and a synthetic resin nonwoven fabric. The separator is preferably a porous film made of polyethylene or polypropylene. This porous film is preferable from the viewpoint of improving the safety of the battery because it can be dissolved at a certain temperature or higher to cut off the current.

<Exterior body>
The exterior body is not particularly limited, and any publicly known or commercially available exterior body can be used. Examples of the exterior body include a cylindrical can, a rectangular can, and a laminate film.

From the viewpoint of light weight and compact size, a laminate film is preferably used for the exterior body. The laminate film has a laminated film structure with a metal layer and a heat-sealable resin layer covering the metal layer. The metal layer improves the strength of the entire sheet while preventing the intrusion of moisture from the outside. For the metal layer, aluminum foil or aluminum alloy foil is preferably used for weight reduction. For the heat-sealable resin layer, polyethylene or polypropylene is preferably used because of the temperature range in which heat fusion is possible and the blocking ability of non-aqueous electrolytes. In addition, a protective layer may be provided on the surface of the metal layer opposite the heat-sealable resin layer to protect and insulate the metal layer. For example, polyethylene terephthalate or nylon is used as the protective layer.

In one example, the metal layer has a thickness of 36 to 44 μm, the heat-sealable resin layer has a thickness of 40 to 100 μm, and the protective layer has a thickness of 10 to 30 μm. Adhesive layers may be provided between each layer to improve adhesion between them. The laminate film may be a composite film with a 3 to 5 layer structure.

Hereinafter, the structure of an example of a non-aqueous electrolyte secondary battery according to an embodiment will be described with reference to the drawings. In one example, the non-aqueous electrolyte secondary battery is a stacked type lithium ion secondary battery.
Fig. 1 is a plan view showing an example of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention, Fig. 2 is a plan view showing an electrode group of Fig. 1, Fig. 3 is a cross-sectional view taken along line III-III of Fig. 2.

As shown in FIG. 1, the nonaqueous electrolyte secondary battery 1 has an exterior body 2 made of two laminate films. The exterior body 2 is formed into a bag shape by stacking two laminate sheets with their heat-sealable resin layers facing each other and heat-sealing the sealing portions 21 of each.

The exterior body 2 is made up of a rectangular first laminate film having a recess 22 and a flat edge around the recess 22, and a flat second laminate film having a rectangular shape. Each of these laminate films has a structure in which a heat-sealable resin layer (inner layer), a metal layer, and a protective layer (outer layer) are laminated. Inside the exterior body 2, an electrode group 3 is housed at a position corresponding to the recess 22. The electrode group 3 is sealed by the exterior body 2.

As shown in FIG. 3, the electrode group 3 has a structure in which a positive electrode 4, a negative electrode 5, and a separator 6 arranged between them are stacked. The positive electrode 4, the negative electrode 5, and the separator 6 are stacked so that the negative electrode 5 is located in the outermost layer. In this embodiment, the nonaqueous electrolyte described above is held in the separator.

The positive electrode 4 is composed of a positive electrode collector 42 and positive electrode layers 41, 41 formed on both sides of the collector 42. The negative electrode 5 is composed of a negative electrode collector 52 and negative electrode layers 51, 51 formed on both sides or one side of the collector 52.

As shown in FIG. 2, the positive electrode 4 has a positive electrode current collector 42 connected to a positive electrode lead 43 extending from, for example, the right side of the upper surface F1 of the electrode group 3. The positive electrode leads 43 are bundled at their tips inside the exterior body 2 and welded to both sides of the positive electrode terminal 7 at two joints 44. One end of the positive electrode terminal 7 is joined to the joint 44 of the positive electrode lead 43. The other end of the positive electrode terminal 7 extends to the outside through the sealing portion 21 of the exterior body 2 (see FIG. 1).

The surface of the portion of the positive electrode terminal 7 that passes through the sealing portion 21 is covered with an adhesive portion (sealant) 9. The adhesive portion 9 is heat-sealed to the heat-sealable resin layer (inner layer) located in the sealing portion 21 of the two laminate films. Therefore, the adhesive portion 9 can prevent the non-aqueous electrolyte from leaking to the outside of the exterior body 2.

As shown in FIG. 2, the negative electrode 5 has a negative electrode current collector 52 connected to a negative electrode lead 53 extending from, for example, the left side of the upper surface F1 of the electrode group 3. Each negative electrode lead 53 is bundled at the tip side inside the exterior body 2 and welded to both sides of the negative electrode terminal 8 at two joints 54. One end of the negative electrode terminal 8 is joined to the joint of the negative electrode lead 53.

The other end of the negative electrode terminal 8 extends to the outside through the sealing portion 21 of the exterior body 2 (see FIG. 1). The surface of the portion of the negative electrode terminal 8 that passes through the sealing portion 21 is covered with an adhesive portion (sealant) 9, similar to the positive electrode terminal 7.

The shape of the nonaqueous electrolyte secondary battery according to the embodiment has been described as a stacked lithium ion secondary battery, but is not limited to this. For example, the battery may be a coin type, button type, sheet type, cylindrical type, flat type, square type, or other shape.

<Examples and Comparative Examples>
The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

A method for producing the nonaqueous electrolyte secondary battery of Example 1 will be described below.
[Preparation of Positive Electrode]
The positive electrode active material was _a lithium _nickel cobalt manganese composite oxide represented by the general formula _{Li1.05Ni0.50Co0.20Mn0.29Al0.01O2} . _The conductive assistant was a mixture of carbon black and graphite. The binder was polyvinylidene fluoride (PVDF) dispersed in an N-methyl- ₂ - _pyrrolidone (NMP) solution. 90% by weight of the positive electrode active material, 6% by weight of the conductive assistant, and 4% by weight (solid content) of the binder were added to the solvent NMP, mixed and dispersed to prepare a positive electrode slurry.

Next, the prepared positive electrode slurry was applied to both sides of an aluminum foil (thickness 20 μm) serving as a positive electrode current collector using a coater, and then dried. The drying was performed in a vacuum for 40 hours or more in a high-temperature environment of 120° C. to 130° C. so that the moisture content of the positive electrode layer after drying per sheet was 110 ppm or less. The coating amount was adjusted to 74 g/m ² and the capacity ratio of the positive electrode to the negative electrode was 0.8.
Next, the sheet was pressed, slit, and punched to prepare a positive electrode having a predetermined electrode area and an electrode density of 2.5 g/cm ³ .

[Preparation of negative electrode]
The negative electrode active material was a lithium titanium composite oxide represented by the general formula Li ₄ Ti ₅ O _12. The conductive assistant was carbon black. The binder was PVDF dispersed in an NMP solution. 95% by weight of the negative electrode active material, 2% by weight of the conductive assistant, and 3% by weight (solid content) of the binder were added to the solvent NMP, mixed and dispersed to prepare a negative electrode slurry.

Next, the prepared negative electrode slurry was applied to both sides of an aluminum foil (thickness 20 μm) serving as a negative electrode current collector using a coater, and then dried. The negative electrode was dried in a vacuum for 40 hours or more in a high-temperature environment at 100° C. to 130° C. so that the moisture content of the negative electrode layer after drying per sheet was 220 ppm or less. The coating amount was adjusted to 68 g/m ² and the capacity ratio of the positive electrode active material to the negative electrode active material was 0.8.
Next, the sheet was pressed, slit, and punched to prepare a negative electrode having a predetermined electrode area and an electrode density of 1.8 g/cm ³ .

[Preparation of non-aqueous electrolyte]
The non-aqueous solvent used was a mixture of propylene carbonate (PC), dimethyl carbonate (DMC), and methyl ethyl carbonate (MEC) in a volume ratio of 2:4:4. The electrolyte used was lithium hexafluorophosphate (LiPF ₆ ). The additive used was lithium difluorophosphate (LiPO ₂ F ₂ ). The electrolyte was dissolved in the non-aqueous solvent at a concentration of 1.0 mol/L, and the additive was dissolved in a concentration of 0.5 wt % relative to the total amount of the non-aqueous solvent and electrolyte to prepare a non-aqueous electrolyte.

[Assembly of prototype battery]
The prototype battery has the same structure as the lithium ion secondary battery shown in FIGS. 1 to 3, except that the numbers of positive and negative electrodes are changed.

The positive and negative electrodes were alternately stacked with a separator between them to produce an electrode group. The stacking was performed so that the negative electrode was located in the outermost layer. A 16 μm thick porous polypropylene film was used as the separator.

The positive and negative electrode current collector leads were bundled at the tip and joined together by ultrasonic welding. An aluminum tab was joined to the joint of the positive electrode current collector lead by ultrasonic welding as a positive electrode terminal. An aluminum tab was joined to the joint of the negative electrode current collector lead by ultrasonic welding as a negative electrode terminal.

The electrode group with the positive and negative terminals welded to it was placed in the recess of the first laminate film, and the flat second laminate film was placed over the flat edge of the recess of the first laminate film so that the positive and negative terminals partially extended outward. Three of the four overlapping sides of the first and second laminate films were heat-sealed to each other, and the electrode group was housed inside the exterior body made of the first and second laminate films.

The left and lower sides were heat-sealed at a temperature of 180°C and a pressure of 0.45 MPa for 15 seconds. Meanwhile, the upper side from which the terminals extend was heat-sealed at a temperature of 220°C and a pressure of 0.45 MPa for 30 seconds. The battery housed inside the exterior was then vacuum-dried for 12 hours at a temperature of 70°C and a vacuum degree of -99 kPa. Then, 85 g of non-aqueous electrolyte was injected from the remaining side of the exterior in an environment at room temperature with a dew point temperature of -70 to -60°C. The side (the right side) was then degassed and sealed to obtain a prototype battery after the non-aqueous electrolyte was injected. The prototype battery weighed 436 g and had a design capacity of 10 Ah.

The prototype batteries according to Examples 2 to 3 and Comparative Examples 1 to 15 were fabricated in the same manner as the prototype battery according to Example 1, except that the type of positive electrode active material, the capacity ratio between the positive electrode active material and the negative electrode active material, and the composition of the non-aqueous electrolyte (nonaqueous solvent, electrolyte concentration [mol/L], additive (LiPO ₂ F ₂ ) concentration [wt %]) were changed as shown in the following Table 1. The capacity ratio was changed by adjusting the coating amount so as to change the electrode density of the positive electrode layer while keeping the electrode density of the negative electrode layer constant.

The prototype batteries of Examples 4 to 10 were fabricated in the same manner as the prototype battery of Example 1, except that the capacity ratio of the positive electrode active material to the negative electrode active material and the composition of the non-aqueous electrolyte (non-aqueous solvent, additive (LiPO ₂ F ₂ ) concentration [wt %]) were changed as shown in Table 2 below.

Table 1 shows the results of measurements of the prototype batteries of Examples 1 to 3 and Comparative Examples 1 to 15, including the discharge capacity retention rate [%] and gas generation amount [ml] in a high-temperature environment in Evaluation 1, the discharge characteristics [C] in a low-temperature environment in Evaluation 2, and the charge characteristics [%] in a low-temperature environment in Evaluation 3, which will be described later.

Table 2 shows the measurement results of the discharge capacity retention rate [%] in a high-temperature environment for the prototype batteries of Examples 4 to 10 in Evaluation 1, which will be described later.

<Evaluation 1: Charge-discharge cycle test under high temperature (60° C.) environment>
A charge-discharge cycle test was performed in a high temperature (60° C.) environment for each prototype battery of Examples 1 to 10 and Comparative Examples 1 to 15. Before the charge-discharge cycle test, the discharge capacity before the cycle test was measured in a room temperature (25° C.) environment under the conditions of 0.5 C charging to 2.8 V and 0.5 C discharging to 1.5 V. Then, each prototype battery was left to stand in a constant temperature bath at 60° C. for 8 hours.

Next, each prototype battery was charged and discharged 1,000 times in a high temperature (60°C) environment, with one cycle consisting of charging at 3.0 C to 2.8 V and discharging at 3.0 C to 1.5 V. The discharge capacity of each prototype battery was measured every 100 charge and discharge cycles. This discharge capacity was measured in the same manner as the discharge capacity before the cycle test described above. From the measurement results of the discharge capacity before and after the charge and discharge cycle test, the discharge capacity retention rate was calculated using the following formula (1).

Discharge capacity retention rate [%]
= (discharge capacity after cycle test/discharge capacity before cycle test) × 100 (1)

After the charge/discharge cycle test, the amount of gas generated from the prototype battery was measured using the Archimedes method.

<Evaluation 2: Discharge characteristic test in an extremely low temperature (-30°C) environment>
A discharge characteristic test was performed on each prototype battery of Examples 1 to 3 and Comparative Examples 1 to 15 in an extremely low temperature (-30°C) environment. This test was performed as a cold cranking test. First, each prototype battery was left in a thermostatic chamber at -30°C for 8 hours in a fully charged state. Next, each prototype battery was discharged for 30 seconds from a fully charged state in an extremely low temperature (-30°C) environment at discharge rates of 1.0C, 2.0C, 5.0C, and 10.0C. The voltage during discharge at each discharge rate was measured and plotted, and the discharge rate (C) was calculated when extrapolating to a lower limit voltage of 1.44V. Note that a discharge rate of about 20C or more is one indicator of good discharge characteristics in an extremely low temperature (-30°C) environment.

<Evaluation 3: Charging characteristics test under extremely low temperature (-30°C) environment>
A charging characteristic test in an extremely low temperature (−30° C.) environment was carried out on each of the prototype batteries of Examples 1 to 3 and Comparative Examples 1 to 15. First, each prototype battery was left to stand in a thermostatic chamber at −30° C. for 8 hours.
Next, each prototype battery was discharged at 1.0 C in an extremely low temperature (-30°C) environment to a lower limit voltage of 1.5 V. Next, the battery was charged to full charge at a high rate of 5.0 C, and the constant current/constant voltage charge capacity was measured. After that, the charging characteristics in an extremely low temperature (-30°C) environment were calculated using the following formula (2).

Charging characteristics [%]
= (constant current charging capacity / constant current constant voltage charging capacity) × 100 (2)

From the results shown in Tables 1 and 2, it can be seen that, by having the configuration of the present invention, the prototype batteries of Examples 1 to 10 achieved a good discharge capacity retention rate of 87% or more in the charge/discharge cycle test in a high-temperature environment. Also, from the results shown in Table 1, the prototype batteries of Examples 1 to 3 achieved a good gas generation amount of less than 1.13 mL. Furthermore, good results were obtained for the discharge characteristics and charge characteristics in a low-temperature environment.

In the prototype batteries of Examples 1 to 10, when the concentration of the additive was 0.5 to 1.0 wt % or less relative to the total weight of the nonaqueous solvent and electrolyte, the capacity ratio of the positive electrode to the negative electrode was 0.80 to 0.85, and the positive electrode potential during charging was kept relatively low, thereby suppressing deterioration of the positive electrode layer due to charging and discharging in a high-temperature environment.
In addition, by including a specific concentration of an additive (LiPO ₂ F ₂ ), the internal resistance of the positive electrode layer of the battery can be reduced, lithium deposition in a low-temperature environment can be suppressed, and gas generation during charging and discharging in a high-temperature environment can be suppressed, thereby improving the charging and discharging characteristics in a low-temperature environment.
In addition, since the positive electrode contains LNCM as the positive electrode active material, and the LNCM further contains Al as a dissimilar metal, expansion and contraction of the positive electrode active material during charge and discharge in a high-temperature environment can be suppressed, and the charge and discharge characteristics in a high-temperature environment can be improved.

In addition, by making the concentration of the electrolyte 1.0 to 1.3 mol/L LiPF ₆ relative to the non-aqueous solvent, the viscosity of the non-aqueous electrolyte is appropriate, particularly in a low-temperature environment, so that the conductivity can be increased, polarization during charging and discharging is suppressed, and the charging and discharging characteristics of the battery can be improved.
In addition, since the negative electrode contains LTO as the negative electrode active material and the non-aqueous solvent contains PC, DMC, and MEC in a specific blend ratio, the charge/discharge characteristics can be improved, particularly in a low-temperature environment, without causing any disadvantages due to the reductive decomposition of PC.
From the above results, it is evident that a battery having the configuration of the present invention can suppress gas generation in a high-temperature environment, and can provide a nonaqueous electrolyte secondary battery having excellent charge/discharge characteristics in both high-temperature and low-temperature environments.

In contrast, the prototype batteries of Comparative Examples 1 to 5, in which the non-aqueous electrolyte did not contain an additive and the capacity ratio was not within a specific range, had a low discharge capacity retention rate in a high-temperature environment, did not obtain good low-temperature characteristics, and were unable to sufficiently suppress gas generation in a high-temperature environment. Also, the battery of Comparative Example 6, in which the non-aqueous electrolyte did not contain an additive, did not obtain good low-temperature characteristics, and was unable to sufficiently suppress gas generation in a high-temperature environment.

In the prototype batteries of Comparative Examples 7 to 14, which do not have a capacity ratio within a specific range, the discharge capacity retention rate in a high-temperature environment was low, good low-temperature characteristics were not obtained, and gas generation in a high-temperature environment could not be sufficiently suppressed. In addition, a comparison between the prototype batteries of Comparative Examples 7 to 10 and Comparative Examples 11 to 14 shows that the high-temperature characteristics and low-temperature characteristics are improved by using LNCM containing a different metal as the positive electrode active material.
In the prototype battery of Comparative Example 15 in which the non-aqueous electrolyte contained 1.5 mol/L, good results could not be obtained in terms of discharge characteristics and charge characteristics, particularly in a low temperature environment.

The results shown in Table 2 also show the relationship between the amount of additive added and the capacity ratio of the positive electrode to the negative electrode during the initial charge.
In Examples 5 and 6, when the additive concentration was 0.5 wt%, the capacity ratio was 0.8 or more and 0.81 or less, and therefore a particularly good capacity retention rate of 90% or more was obtained compared to Example 4. In Examples 8 to 10, when the additive concentration was 1.0 wt%, the capacity ratio was 0.8 or more and 0.82 or less, and therefore a particularly good capacity retention rate of 90% or more was obtained compared to Example 4.

Although the embodiments of the present invention have been specifically described, the present invention is not limited to these embodiments and examples, and various modifications based on the technical concept of the present invention are possible.

1...non-aqueous electrolyte secondary battery, 2...exterior body, 21...sealing portion, 22...recess, 3...electrode group, 4...positive electrode, 5...negative electrode, 41...positive electrode layer, 42...positive electrode current collector, 43...positive electrode lead, 44...joint, 51...negative electrode layer, 52...negative electrode current collector, 53...negative electrode lead, 54...joint, 6...separator, 7...positive electrode terminal, 8...negative electrode terminal, 9...adhesive portion

Claims (5)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte,
The positive electrode has a positive electrode layer containing a lithium nickel cobalt manganese based composite oxide as a positive electrode active material,
The negative electrode includes a negative electrode layer containing a lithium-titanium composite oxide as a negative electrode active material,
The non-aqueous electrolyte is
a non-aqueous solvent comprising propylene carbonate, dimethyl carbonate, and methyl ethyl carbonate in a volume ratio of 1.6-2.4:4.0-4.4:3.6-4.0;
An electrolyte containing _LiPF6 or _LiBF4 and having a concentration of 1.0 mol/L or more and 1.3 mol/L or less in the non-aqueous solvent;
an additive consisting _{of LiPO2F2} ;
Including,
When a concentration of the additive is 0.5% by weight or more and 1.0% by weight or less with respect to a total amount of the nonaqueous solvent and the electrolyte, a capacity ratio (Y/X) of a capacity (X) of the positive electrode to a capacity (Y) of the negative electrode during an initial charge is 0.80 to 0.85. The lithium _{nickel cobalt manganese} composite oxide is represented by the general formula _{LiαNixCoyMnzM (1-x-y-z) O2} (wherein 1.05≦α≦1.13, 1≧x+y+z, and M is at least one substitution element selected from Mg, Al, Ti, Fe, Cu, Zn, Ga, Nb, Sn, Zr, and Ta),
The nonaqueous electrolyte secondary battery according to claim 1, wherein _{the lithium} titanium-based composite oxide is represented by a general formula _LiαTixO12 (wherein 3.0<α<5.0, 4.0<x<6.0). The nonaqueous electrolyte secondary battery according to claim 1 or 2, characterized in that when the concentration of the additive is 0.5 wt % relative to the total amount of the nonaqueous solvent and electrolyte, the capacity ratio is 0.80 or more and 0.81 or less. The nonaqueous electrolyte secondary battery according to claim 1 or 2, characterized in that when the concentration of the additive is 1.0 wt % relative to the total amount of the nonaqueous solvent and electrolyte, the capacity ratio is 0.80 or more and 0.82 or less. The positive electrode has a moisture content of 110 ppm or less per sheet of the positive electrode layer after drying,
5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode has a moisture content of 220 ppm or less per sheet of the negative electrode layer after drying.

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