JP2012079560A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having excellent life performance in which generation of gas is reduced during storage.SOLUTION: The nonaqueous electrolyte secondary battery 1 includes a positive electrode 3, a negative electrode 4 having a negative electrode layer, and a nonaqueous electrolyte. The negative electrode layer contains a lithium titanium oxide. The negative electrode layer has a first region of low lithium concentration, and a second region of high lithium concentration existing around the first region, and satisfies a following formula (I). T<T, where (I)Tis the ratio of tetravalent titanium among titanium atoms in the lithium titanium oxide in the first region, and Tis the ratio of tetravalent titanium among titanium atoms in the lithium titanium oxide in the second region.

Description

  Embodiments described herein relate generally to a non-aqueous electrolyte secondary battery.

In recent years, high energy density non-aqueous electrolyte secondary batteries using a lithium titanium oxide such as Li ₄ Ti ₅ O ₁₂ as a negative electrode active material have been developed. Lithium titanium oxide has very little change in the thickness of the battery because there is almost no volume change associated with charge / discharge. In addition, since the potential of the negative electrode in the normal charge / discharge operating range is 0.4 V (vs. Li / Li ⁺ ) or more, film formation due to the decomposition reaction of the nonaqueous electrolyte hardly occurs. Therefore, a battery using lithium titanium oxide as a negative electrode active material has excellent battery characteristics.

  However, a battery using lithium titanium oxidation as a negative electrode active material has a problem that gas is easily generated when stored in a high temperature environment in a low SOC state of 50% or less. When a large amount of gas is generated during storage, there is a risk that the internal pressure of the battery will increase and the battery will be plastically deformed, and the internal resistance of the battery will increase and the life performance will decrease.

JP 2004-355977 A JP 2005-26145 A

M.Q.Snyder et al., Appl.Surface Science, 253 (2007) 9336

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery that suppresses gas generation during high-temperature storage and has excellent life performance.

  According to the embodiment, a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode including a negative electrode layer, and a nonaqueous electrolyte is provided. In the non-aqueous electrolyte secondary battery, the negative electrode layer includes lithium titanium oxide, and includes a first region having a low lithium concentration and a second region having a high lithium concentration present around the first region, The following formula (I) is satisfied.

T ₂ <T ₁ (I)
T ₁ is a ratio of tetravalent titanium among titanium atoms in the lithium titanium oxide in the first region, and T ₂ is four of titanium atoms in the lithium titanium oxide in the second region. Is the proportion of titanium of valence.

FIG. 2 is a cutaway perspective view schematically showing the nonaqueous electrolyte secondary battery of the embodiment. The schematic diagram of the negative electrode surface in one embodiment. The schematic diagram of the negative electrode surface in other embodiment. The schematic diagram which shows an example of the manufacturing process of the nonaqueous electrolyte secondary battery of embodiment. The schematic diagram which shows the other example of the manufacturing process of the nonaqueous electrolyte secondary battery of embodiment. FIG. 3 is an XPS measurement diagram of the negative electrode surface of Example 1.

  Hereinafter, a nonaqueous electrolyte secondary battery according to an embodiment will be described in detail with reference to the drawings.

  FIG. 1 is a partially cutaway perspective view of a thin nonaqueous electrolyte secondary battery 1. The flat electrode group 2 includes a positive electrode 3, a negative electrode 4, and a separator 5. The positive electrode 3 and the negative electrode 4 are stacked with a separator 5 interposed therebetween. The electrode group 2 is formed by winding this laminated body into a flat shape. A belt-like positive electrode terminal 6 is electrically connected to the positive electrode 3. A strip-like negative electrode terminal 7 is electrically connected to the negative electrode 4. The electrode group 2 is accommodated in an outer bag 8 made of a laminate film with the ends of the positive electrode terminal 6 and the negative electrode terminal 7 extended from the outer bag 8. A non-aqueous electrolyte solution (not shown) is accommodated in the laminate film outer bag 8. The electrode group 2 and the non-aqueous electrolyte are sealed by heat-sealing the opening of the laminated film-made exterior bag 8 with the positive electrode terminal 6 and the negative electrode terminal 7 extended.

  Hereinafter, the negative electrode, the positive electrode, the non-aqueous electrolyte, the separator, and the exterior member will be described in detail.

1) Negative electrode The negative electrode includes a current collector and a negative electrode layer. The negative electrode layer is formed on one side or both sides of the current collector. The negative electrode layer includes a negative electrode active material, a conductive agent, and a binder. As the negative electrode active material, lithium titanium oxide is used.

  The partial schematic diagram of the negative electrode layer in this embodiment is shown in FIG. The negative electrode layer includes a first region 21 having a low lithium concentration and a second region 22 having a high lithium concentration present around the first region 21.

Here, the lithium concentration in the negative electrode layer will be described. In the negative electrode during charging, the lithium titanium oxide exists in a state where two phases of Li ₇ Ti ₅ O ₁₂ and Li ₄ Ti ₅ O ₁₂ are mixed. Of charge; if (State of Charge SOC) are the same, the Li ₇ Ti ₅ O ₁₂ with respect to the total amount of the Li ₇ Ti ₅ O ₁₂ and Li ₄ Ti ₅ O ₁₂ amount (hereinafter, the "Li ₇ Ti ₅ O ₁₂ The abundance ratio ”is the same at any location of the negative electrode. For example, when the SOC is 50%, the ratio of Li ₇ Ti ₅ O ₁₂ and Li ₄ Ti ₅ O ₁₂ is 50:50, and the existing ratio of Li ₇ Ti ₅ O ₁₂ is 50%.

However, the negative electrode layer included in the battery in the present embodiment has two or more regions with different abundance ratios of Li ₇ Ti ₅ O ₁₂ . For example, in any given region, the ratio of Li ₇ Ti ₅ O ₁₂ to Li ₄ Ti ₅ O ₁₂ is 60:40 and the abundance ratio of Li ₇ Ti ₅ O ₁₂ is 60%, In other regions, the ratio between Li ₇ Ti ₅ O ₁₂ and Li ₄ Ti ₅ O ₁₂ is 40:60, and the existing ratio of Li ₇ Ti ₅ O ₁₂ is 40%.

Here, a region where the abundance ratio of Li ₇ Ti ₅ O ₁₂ is high is referred to as a “region with a high lithium concentration”, and a region where the abundance ratio of Li ₇ Ti ₅ O ₁₂ is low is referred to as a “region with a low lithium concentration”.

The valence of Ti in Li ₄ Ti ₅ O ₁₂ is +4, and the Ti valence in Li ₇ Ti ₅ O ₁₂ is +3.4 (Ti ⁴⁺ : Ti ³⁺ = 40: 60). Therefore, determine the abundance ratio of Li ₇ Ti ₅ O ₁₂ by measuring the valence of Ti in any region of the negative electrode layer surface and observing whether Ti ⁴⁺ is more or less than other regions. Thus, the level of lithium concentration can be determined.

That is, in an arbitrary region on the surface of the negative electrode layer, when Ti ⁴⁺ is larger than other regions, the region is rich in Li ₄ Ti ₅ O ₁₂ . Such a region is a “region where the concentration of Li ₇ Ti ₅ O ₁₂ is low and the concentration of lithium is low”. On the other hand, when Ti ⁴⁺ is less than other regions, it is a region with less Li ₄ Ti ₅ O ₁₂ . Such a region is a “region where the concentration of Li ₇ Ti ₅ O ₁₂ is high and the lithium concentration is high”.

In order to compare the amount of Ti ⁴⁺ , the ratio (T) of tetravalent titanium among titanium atoms in lithium titanium oxide represented by the following formula can be used. It will be referred to as “abundance ratio of valence titanium”. Further, the abundance ratio of tetravalent titanium in the first region is represented as T _1, and the abundance ratio of tetravalent titanium in the second region is represented as T ₂ .
T = (Ti ⁴⁺ / Ti) × 100
Ti shown here is the total amount of titanium, and the valence of titanium contained therein may be any valence such as Ti ³⁺ and Ti ⁴⁺ .

  From the above, in the negative electrode layer of the battery of this embodiment, the first region 21 having a low lithium concentration and the second region 22 having a high lithium concentration have a relationship satisfying the following formula (I).

T ₂ <T ₁ (I)
The valence of Ti in the negative electrode layer can be measured using a spectroscopic method such as Auger electron spectroscopy (AES) or X-ray spectroscopy (XPS).

  According to the present embodiment, the negative electrode layer containing lithium titanium oxide has the first region 21 having a low lithium concentration and the second region 22 having a high lithium concentration, so that the charging depth (SOC) of the battery is particularly low. It is possible to greatly reduce the gas generation amount.

  It has been confirmed by the present inventors that gas generation during storage of the battery continues until the negative electrode is at least 95% discharged. Therefore, it can be said that gas generation continues until the discharge is almost completed. Since the discharge capacity of the region having a low lithium concentration is lower than that of the region having a high lithium concentration, the discharge reaction of the first region 21 is completed before the second region 22. Therefore, the amount of gas generated from the first region 21 is less than that of the second region 22, and the presence of the first region 21 in the negative electrode can reduce the amount of gas generated as a whole of the negative electrode. It is done. Furthermore, since the region where the lithium concentration is low has a high diffusion rate of lithium ions, the presence of the first region 21 can improve the large current characteristics.

  It is also conceivable to reduce the amount of gas generated by reducing the amount of the negative electrode active material. However, since gas generation at the negative electrode occurs in the vicinity of the electrode surface, the amount of gas generation does not change when the electrode surface area, that is, the area that actually faces the positive electrode is the same. Therefore, even if the amount of the negative electrode active material is decreased, the effect of suppressing gas generation cannot be obtained. Although it is possible to reduce the amount of gas generated while maintaining the capacity by reducing the electrode surface area without changing the amount of the negative electrode active material, large current characteristics such as discharge rate characteristics and cycle characteristics are remarkably high. Getting worse.

  In the present embodiment, it is sufficient that at least one first region 21 exists in one negative electrode layer. That is, when the negative electrode layer is formed on both surfaces of the negative electrode current collector, it is sufficient that at least one first region 21 exists in each negative electrode layer.

  When there are a plurality of first regions 21, it is preferably dispersed in the negative electrode layer. When the region having a high lithium concentration is “sea” and the region having a low lithium concentration is “island”, a so-called sea-island structure is used. Is preferred.

  The second region 22 having a high lithium concentration has a slower diffusion of lithium ions than the first region 21. Therefore, if the first region 21 is localized without being dispersed, the diffusion of lithium ions is also likely to be localized. As a result, the charge / discharge reaction region of the opposing positive electrode is likely to be localized, which may deteriorate the cycle characteristics. In addition, localization of lithium ion diffusion adversely affects the discharge characteristics particularly at high currents. Therefore, the first region 21 is preferably dispersed in the negative electrode layer, and more preferably uniformly dispersed. Since the first region 21 is uniformly dispersed, current concentration can be suppressed, and therefore, it is possible to maintain good cycle characteristics as well as the effect of suppressing the amount of gas generation.

  Furthermore, the negative electrode layer preferably satisfies the following formula (II).

3 ≦ T ₁ −T ₂ ≦ 30 (II)
When the difference in the abundance ratio of tetravalent titanium (T ₁ -T ₂ ) is 3% or more, the influence of lithium ion diffusion from the second region 22 to the first region 21 is small, and the difference in lithium concentration is maintained. Is done. On the other hand, the decrease in negative electrode capacity can be suppressed by setting the difference in the abundance ratio of tetravalent titanium (T ₁ -T ₂ ) to 30% or less.

The first region 21 is completely discharged when the battery is discharged. On the other hand, when the battery is charged, the battery is not fully charged and charging of the battery is completed. On the contrary, the second region 22 is not completely discharged when the battery is discharged, and the discharge of the battery is completed. On the other hand, when the battery is charged, a fully charged state is reached. Therefore, since the first region 21 is not completely charged, the negative electrode capacity decreases as the difference in lithium concentration from the second region 22 increases. If charging is forcibly advanced to compensate for the decrease in negative electrode capacity, the positive electrode potential inside the battery shifts in a noble direction. As a result, a side reaction between the positive electrode and the electrolytic solution is induced. Alternatively, the crystal structure of the positive electrode active material may change, resulting in a decrease in stability and a decrease in battery safety. Therefore, it is preferable that the difference (T ₁ -T ₂ ) in the ratio of the tetravalent titanium in the first region 21 and the second region 22 is 30% or less.

When the battery having the configuration of the present embodiment was allowed to stand for 1 month in an environment of 25 ° C., the battery was then disassembled and the concentration distribution of the negative electrode was examined, and no change was found in the concentration distribution. Therefore, in the battery of this embodiment, the distribution of the first region 21 in the negative electrode and the difference in the abundance ratio of tetravalent titanium in the first region 21 and the second region 22 (T ₁ -T ₂ ) are maintained. It was confirmed.

If there is a lithium concentration distribution, lithium ions may diffuse and the concentration distribution may decrease. However, in both the first region and the second region, two phases of Li ₇ Ti ₅ O ₁₂ and Li ₄ Ti ₅ O ₁₂ exist in a mixed state, such as Li ₅ Ti ₅ O ₁₂ . There is no intermediate phase state. Therefore, in order to reduce the concentration distribution, it is necessary that lithium ions diffuse between two crystals of Li ₇ Ti ₅ O ₁₂ and Li ₄ Ti ₅ O ₁₂ and be accompanied by a phase transition reaction. However, the lithium diffusion rate in the crystal is not so fast, and the phase reaction between Li ₄ Ti ₅ O ₁₂ and Li ₇ Ti ₅ O ₁₂ is not so fast. Therefore, it is not easy to cause a phase transition from Li ₇ Ti ₅ O ₁₂ to Li ₄ Ti ₅ O ₁₂ only by diffusion of lithium ions due to the concentration distribution.

Therefore, it is considered that a lithium concentration distribution having a concentration difference of a certain level or more is maintained even after a charge / discharge cycle or after long-term storage. In other words, the difference (T ₁ -T ₂ ) in the ratio of the tetravalent titanium in the first region 21 and the second region 22 is maintained. Therefore, the difference in the abundance ratio of tetravalent titanium (T ₁ -T ₂ ) can be measured with a battery in any SOC.

When the density difference is very small, the density distribution becomes smaller under the influence of slow diffusion, and the density difference is considered to eventually disappear. Therefore, two regions where the difference in the abundance ratio of tetravalent titanium (T ₁ -T ₂ ) is less than 3% are defined as the same continuous region.

In the present embodiment, the first region 21 preferably has an area of 0.1 mm ² or more and 20 cm ² or less. When the area of the first region 21 is 0.1 mm ² or more, an effect of reducing the gas generation amount can be obtained. On the other hand, when the area of the first region 21 is 20 cm ² or less, current can be prevented from being concentrated in the first region 21 or the second region 22 when the battery is rapidly charged and discharged. More preferably, the first region 21 has an area of 1 mm ² or more and 15 cm ² or less.

Further, it is preferable that one or more first regions 21 exist per 25 cm ² of the negative electrode layer surface. The presence of one or more first regions 21 per 25 cm ^{2 of the} negative electrode layer surface suppresses the influence of a decrease in lithium concentration difference between the first region 21 and the second region 22 due to lithium ion diffusion even when left for a long time. Is done.

  Further, in the present embodiment, as shown in FIG. 3, in the negative electrode layer, the third region 23 exists around the first region 21, and the third region 23 is located between the first region and the second region. It is preferable. The third region 23 is a region having a lithium concentration higher than the lithium concentration in the first region 21 and lower than the lithium concentration in the second region 22. Although not limited thereto, the third region 23 preferably has a lithium concentration gradient that continuously decreases from the lithium concentration of the second region 22 to the lithium concentration of the first region.

  Since the third region 23 acts as a diffusion path for lithium ions, localization of lithium ions is less likely to occur particularly during charge / discharge with a large current, and the charge / discharge characteristics with a large current can be further improved. Can do. Further, since the area of the lithium ion diffusion path is increased, cycle deterioration due to current concentration can be suppressed, and good cycle characteristics can be maintained.

The lithium titanium oxide contained in the negative electrode active material preferably has a lithium ion storage potential of 0.4 V (vs. Li / Li ⁺ ) or more. Examples of active materials having a lithium ion storage potential of 0.4 V (vs. Li / Li ⁺ ) or higher include spinel-type lithium titanate (Li _{4 + x} Ti ₅ O ₁₂ ) and ramsdellite-type lithium titanate. (Li _{2 + x} Ti ₃ O ₇ ) is included. Said lithium titanium oxide may be used independently, but 2 or more types may be mixed and used for it. Further, titanium oxide having a lithium-titanium oxide (e.g., TiO ₂₎ may be used as an active material by charging and discharging. The upper limit of the lithium ion storage potential of the lithium titanium oxide is not limited to this, but is preferably 2 V or less.

  The lithium titanium oxide preferably has an average primary particle size of 5 μm or less. When the average primary particle diameter is 5 μm or less, the effective area contributing to the electrode reaction is sufficient, and good large current discharge characteristics can be obtained.

The lithium titanium oxide preferably has a specific surface area of 1 to 10 m ² / g. When the specific surface area is 1 m ² / g or more, the effective area contributing to the electrode reaction is sufficient, and good large current discharge characteristics can be obtained. On the other hand, when the specific surface area is 10 m ² / g or less, the reaction with the nonaqueous electrolyte is suppressed, and the decrease in charge / discharge efficiency and the generation of gas during storage can be suppressed.

  A carbon material can be used for the conductive agent contained in the negative electrode layer. The carbon material preferably has a high alkali metal occlusion property and conductivity. Examples of the carbon material include acetylene black and carbon black.

  Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-based rubber, styrene-butadiene rubber (SBR), polypropylene (PP), polyethylene (PE), and carboxymethyl cellulose (CMC). ) Is included.

  The mixing ratio of the active material, the conductive agent and the binder is preferably 70 to 95% by weight of the negative electrode active material, 0 to 25% by weight of the conductive agent, and 2 to 10% by weight of the binder.

2) Positive electrode The positive electrode includes a current collector and a positive electrode layer. The positive electrode layer is formed on one side or both sides of the current collector. The positive electrode layer includes a positive electrode active material, a conductive agent, and a binder.

Examples of the positive electrode active material include lithium manganese composite oxide (for example, LiMn ₂ O ₄ or LiMnO ₂ ), lithium nickel composite oxide (for example, LiNiO ₂ ), lithium cobalt composite oxide (LiCoO ₂ ), and lithium nickel cobalt composite oxide. (For example, LiNi _1-x Co _x O ₂ , 0 <x ≦ 1), lithium manganese cobalt composite oxide (for example, LiMn _x Co _1-x O _2, 0 <x ≦ 1), lithium iron phosphate (LiFePO ₄ ) And lithium complex phosphate compounds (for example, LiMn _x Fe _1-x PO _4, 0 <x ≦ 1).

  Examples of the conductive agent include acetylene black, carbon black, and graphite.

  Examples of binders are polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-based rubber, ethylene-butadiene rubber (SBR), polypropylene (PP), polyethylene (PE), and carboxymethylcellulose (CMC) including.

  The blending ratio of the active material, the conductive agent and the binder is preferably 80 to 95% by weight of the active material, 3 to 20% by weight of the conductive agent, and 2 to 7% by weight of the binder.

3) Nonaqueous electrolyte A nonaqueous electrolyte is prepared by dissolving an electrolyte in a nonaqueous solvent.
As the non-aqueous solvent, a non-aqueous solvent known to be used for lithium batteries can be used.

  Examples of non-aqueous solvents include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC); mixed solvents of cyclic carbonate and a non-aqueous solvent having a viscosity lower than that of the cyclic carbonate (hereinafter referred to as second solvent). .

  Examples of the second solvent are linear carbonates such as dimethyl carbonate, methyl ethyl carbonate or diethyl carbonate; γ-butyrolactone, acetonitrile, methyl propionate, ethyl propionate; cyclic ethers such as tetrahydrofuran or 2-methyltetrahydrofuran; Includes chain ethers such as dimethoxyethane or diethoxyethane.

An alkaline salt can be used as the electrolyte. Preferably lithium salt is used. Examples of lithium salts are lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenide (LiAsF ₆ ), lithium perchlorate (LiClO ₄ ), and trifluorometa Includes lithium sulfonate (LiCF ₃ SO ₃ ). In particular, lithium hexafluorophosphate (LiPF ₆ ) and lithium tetrafluoroborate (LiBF ₄ ) are preferable. The concentration of the electrolyte in the nonaqueous solvent is preferably 0.5 to 2 mol / L.

4) Separator The separator is disposed between the positive electrode and the negative electrode, and prevents the positive electrode and the negative electrode from contacting each other. The separator is made of an insulating material. The separator has a shape in which the electrolyte can move between the positive electrode and the negative electrode.

  Examples of the separator include a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film, and a cellulose-based separator.

5) Exterior member A laminate film can be used as the exterior member. A metal container is plastically deformed when the container swells due to gas generation, but a laminate film is preferably used because it does not plastically deform.

  As the laminate film, a multilayer film made of a metal foil covered with a resin film is used. As the resin forming the resin film, polymers such as polypropylene (PP), polyethylene (PE), nylon, and polyethylene terephthalate (PET) can be used. The inner surface of the laminate film exterior member is formed of a thermoplastic resin such as PP and PE.

  The thickness of the laminate film is preferably 0.2 mm or less.

  The battery according to the above embodiment can be manufactured by the following method. The method includes: housing the positive electrode, the negative electrode, and the nonaqueous electrolyte in an exterior member; and temporarily sealing an opening of the exterior member to obtain a temporarily sealed secondary battery; and Adjusting the depth of charge (SOC) to 100% or less (not including 0%), holding the adjusted temporarily sealed secondary battery in an atmosphere of 35 ° C. or higher and 90 ° C. or lower; Opening the sealed secondary battery, discharging the internal gas, and fully sealing the exterior member. The process of storing the SOC adjusted to a desired value in this way is called an aging process.

By performing an aging treatment in the battery manufacturing process, gas can be intentionally generated from the negative electrode. At this time, in a state where no potential is applied to the negative electrode, for example, in a state where the lithium occlusion / release potential of 1.55 V (vs. Li / Li ⁺ ) of lithium titanium oxide is not reached, a reaction of generating gas in the negative electrode occurs. Does not progress. Therefore, the SOC is adjusted to a value exceeding 0%.

  The reaction in which the gas is generated does not proceed in the same manner in the entire region of the negative electrode and occurs nonuniformly. This is presumably because when a reaction occurs at any one place, it tends to occur intensively in the vicinity of the periphery. In particular, since a normal negative electrode has micro unevenness, the reaction hardly proceeds uniformly on the negative electrode surface.

  Since the supply of electrons from the negative electrode is essential for the gas generation reaction, the self-discharge reaction of the negative electrode proceeds with gas generation. Therefore, in the region where the gas generation reaction has occurred, the lithium concentration decreases, thereby forming the first region.

  The formation of the first region can be controlled by adjusting the temperature of the aging treatment, the SOC during storage, and the storage time.

  Below, an example of the manufacturing method of the battery which concerns on this embodiment is demonstrated concretely.

(First step)
In the first step, a temporarily sealed secondary battery as shown in FIG. 4 is produced. First, an electrode group is accommodated in the exterior member 8 made of a laminate film.

  The electrode group includes a positive electrode, a negative electrode, and a separator. As shown in FIG. 4, the flat electrode group 2 is formed by laminating the positive electrode 3, the separator 5, the negative electrode 4, and the separator 5 and winding the laminated body into a flat shape.

  The positive electrode 3 can be manufactured by preparing a slurry by suspending a positive electrode active material, a conductive agent, and a binder in an appropriate solvent, applying the slurry to one or both sides of a current collector, and drying the slurry. it can.

  The negative electrode 4 can be prepared by suspending a negative electrode active material, a conductive agent, and a binder in a suitable solvent to prepare a slurry, applying the slurry to one or both sides of a current collector, and drying the slurry. it can.

  As the separator 5, the above can be used.

  A belt-like positive electrode terminal 6 is electrically connected to the positive electrode 3. A strip-like negative electrode terminal 7 is electrically connected to the negative electrode 4. The positive and negative electrode terminals may be formed integrally with the positive and negative electrode current collectors, respectively. Alternatively, a terminal formed separately from the current collector may be connected to the current collector. The positive and negative electrode terminals may be connected to each of the positive and negative electrodes before winding the laminate. Or you may connect, after winding a laminated body.

  The exterior member 8 is formed by extending or deep drawing the laminate film from the thermoplastic resin film side to form a cup-shaped electrode group housing portion, and then folding the laminate film 180 ° with the thermoplastic resin film side inward to cover the exterior member 8 It is formed by making a body.

  The electrode group 2 is arrange | positioned in the electrode group accommodating part of the exterior member 8, and a positive / negative electrode terminal is extended to the container exterior. Next, the upper end portion of the exterior member 8 where the positive and negative electrode terminals extend and one of the end portions orthogonal to the upper end portion are heat sealed to form the sealing portions 10b and 10c. Thereby, the exterior member 8 in a state where one side is opened as the opening 9 is formed.

  Next, a non-aqueous electrolyte is injected from the opening 9 and the electrode group is impregnated with the non-aqueous electrolyte. As the non-aqueous electrolyte, those described above can be used. Here, in order to promote the impregnation of the electrolytic solution, the battery may be stored under pressure in the thickness direction.

  Thereafter, the temporary sealing secondary battery in which the electrode group and the nonaqueous electrolyte impregnated in the electrode group are sealed is obtained by heat-sealing the opening 9 to form the temporary sealing part 10a.

(Second step)
Next, the second step is performed. A current is passed between the positive electrode terminal 6 and the negative electrode terminal 7 of the temporarily sealed secondary battery, and the initial charge is performed so that the SOC is less than 100% (not including 0%). Even if the SOC temporarily exceeds a desired value, the SOC may be adjusted by discharging after that. Charging / discharging may be performed at room temperature.

  Water, carbon dioxide, or the like is adsorbed on the surface of the lithium titanium oxide that is the negative electrode active material. These impurities are more easily released as gas as the negative electrode is in a lower potential state. Therefore, in order to promote the gas generation reaction, it is preferable to set the SOC to a relatively low state. In addition, when the SOC is high and the positive electrode potential is high, the gas released from the negative electrode is easily oxidized and absorbed again by the positive electrode, but by lowering the SOC, the oxidation reaction at the positive electrode is suppressed and the gas is released. Can be promoted. When the SOC of the negative electrode is 0%, no gas is released from the negative electrode even when stored at a high temperature. The SOC is more preferably in the range of 0.5% or more and less than 20%.

  In the present embodiment, the charging depth (SOC) is a ratio of the charging capacity of the unit cell to the full charging capacity. The nominal capacity of the unit cell is used for the full charge capacity.

(Third step)
Next, the temporarily sealed secondary battery adjusted to less than 100% SOC in the second step is held in an atmosphere at a temperature of 35 ° C. or higher and 90 ° C. or lower. At this time, mainly water or carbon dioxide adsorbed on the negative electrode layer is released as a gas from the electrode group and stays in the exterior member 8.

  When the atmospheric temperature is less than 35 ° C., gas is not sufficiently released from the electrode group. Further, when the atmospheric temperature exceeds 90 ° C., the reaction of the nonaqueous electrolyte on the surface of the positive electrode or the negative electrode is likely to occur, and a film is formed to increase the impedance, or the discharge capacity of the secondary battery is decreased. A more preferable range of the atmospheric temperature is 45 to 80 ° C.

  The time for holding the temporarily sealed secondary battery in an atmosphere at a temperature of 35 ° C. or higher and 90 ° C. or lower may be a time for sufficiently releasing the gas from the negative electrode. Although it is not limited to this, For example, it can be set as 5 to 50 hours, Preferably it can be set as 10 to 40 hours.

(Fourth process)
Next, a part of the exterior member 8 is cut or a hole is formed, and the gas retained in the exterior member 8 in the second step is discharged to the outside. For example, the exterior member 8 can be opened by cutting the laminate film at any position of the opening portion 11 that is the inside of the temporary sealing portion 10a and is not heat-sealed. The opening is preferably performed under reduced pressure, in an inert atmosphere or in dry air.

  After opening the exterior member 8, the nonaqueous electrolyte secondary battery may be in a reduced pressure atmosphere using a decompression chamber or the like, or gas may be sucked from the opening or hole of the exterior member 8 using a suction nozzle. Good. According to these methods, the gas inside the exterior member 8 can be discharged more reliably.

  After the gas is discharged, the outer sealing member 8 is heat-sealed inside the cut portion of the opening portion 11 to form the main sealing portion 12, and the electrode group and the nonaqueous electrolyte are sealed again. Further, the opening part 11 is cut outside the main sealing part 12. Thereby, a nonaqueous electrolyte secondary battery is obtained. At this time, it is preferable to seal under reduced pressure. Or you may seal by sticking an adhesive tape etc. in the location which opened the hole of the exterior member 8. FIG. The obtained nonaqueous electrolyte secondary battery may be optionally charged and discharged one or more times.

  Through the above steps, a non-aqueous electrolyte secondary battery as shown in FIG. 1 is manufactured.

  In the above description, the temporarily sealed secondary battery for manufacturing the nonaqueous electrolyte secondary battery shown in FIG. 1 is used as an example, but the positive electrode terminal and the negative electrode terminal are respectively extended from separate ends. Can be manufactured by the method according to the present embodiment, for example, by using a temporarily sealed secondary battery as shown in FIG.

  For example, in the first step, the electrode group is accommodated in a state where the positive electrode terminal 13 and the negative electrode terminal 14 are extended from opposite ends of the exterior member 15. The end portions where the positive and negative electrode terminals 13 and 14 extend are heat-sealed to form the sealing portions 17c and 17b. Next, after injecting the nonaqueous electrolyte and impregnating the electrode group, heat sealing is performed in the temporary sealing portion 17a, whereby the electrode group and the temporary sealing secondary in which the nonaqueous electrolyte impregnated in the electrode group is sealed are sealed. Get a battery.

  Further, in the fourth step, the exterior member 15 is opened by cutting the laminate film at any position of the opening part 18 that is the inside of the temporary sealing part 17a and is not heat-sealed. .

  After exhausting the internal gas, the exterior sealing member 15 is heat-sealed inside the cut portion of the opening portion 18 to form the main sealing portion 19, and the electrode group and the nonaqueous electrolyte are sealed again. Then, the opening part 18 is cut | disconnected by the outer side of this sealing part 19. FIG. Thereby, a nonaqueous electrolyte secondary battery is obtained.

  According to the above embodiment, it is possible to provide a non-aqueous electrolyte secondary battery that suppresses gas generation during high-temperature storage and has excellent life performance.

Example 1
<Preparation of positive electrode>
As a positive electrode active material, 91% by weight of lithium nickel cobalt oxide (LiNi _0.8 Co _0.2 O ₂ ) powder, 2.5% by weight of acetylene black, 3% by weight of graphite, 3.5% by weight of polyvinylidene fluoride (PVdF), Was added to N-methylpyrrolidone and mixed to prepare a slurry. This slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm, dried and pressed to prepare a positive electrode having a positive electrode layer with a density of 3.0 g / cm ³ .

<Production of negative electrode>
As a negative electrode active material, 85% by weight of spinel type lithium titanium oxide (Li ₄ Ti ₅ O ₁₂ ) powder having a lithium occlusion potential of 1.55 V (vs. Li / Li ⁺ ), 5% by weight of graphite, and acetylene black 3 % By weight and 7% by weight of PVdF were added to NMP and mixed to prepare a slurry. This slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 11 μm, dried and pressed to prepare a negative electrode having a negative electrode layer with a density of 2.0 g / cm ³ .

<Production of electrode group>
After stacking the positive electrode prepared above, a separator made of a polyethylene porous film having a thickness of 20 μm, the negative electrode prepared above, and the separator in this order, a spiral shape is formed so that the negative electrode is positioned on the outermost periphery. The electrode group was produced by winding the film. This was heated and pressed at 90 ° C. to produce a flat electrode group having a width of 58 mm, a height of 95 mm, and a thickness of 3.0 mm. The obtained electrode group is housed in an exterior member made of a laminate film having a thickness of 0.1 mm, which is composed of an aluminum foil having a thickness of 40 μm and a polypropylene layer formed on both surfaces of the aluminum foil. Vacuum-dried at 24 ° C. for 24 hours.

<Preparation of non-aqueous electrolyte>
Ethylene carbonate (EC) and methyl ethyl carbonate (MEC) were mixed at a volume ratio of 1: 2 to prepare a mixed solvent. In this mixed solvent, lithium hexafluorophosphate (LiPF ₆ ) was dissolved at a concentration of 1.0 mol / L to prepare a nonaqueous electrolytic solution.

<Production of battery>
A temporary sealed secondary battery having a structure as shown in FIG. 4 was produced using the electrode group and the non-aqueous electrolyte. This temporarily sealed secondary battery was charged at 2.8 V in a 0.2 C rate and 25 ° C. environment, and then discharged at a 0.2 C rate until 1.5 V was reached. Then, it charged at 1C rate so that SOC might be 50%, and stored for 12 hours in a 70 degreeC environment (aging process). Then, the exterior member was opened in the opening part, and the gas in a battery was discharged | emitted. Then, after heat-sealing the second sealing portion, the outside was cut, and a nonaqueous electrolyte secondary battery having a capacity of 3.0 Ah was produced at the design value stage.

(Comparative Examples 2 to 10)
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the aging treatment was performed as described in Table 1.

(Comparative Example 1)
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the aging treatment was not performed.

<Capacity measurement>
Each of the batteries of Examples 1 to 10 and Comparative Example 1 was charged and discharged once at a 1C rate in a 25 ° C. environment, and the initial discharge capacity was measured. The results are shown in Table 1.

<Observation of negative electrode surface>
The secondary battery of Example 1 was adjusted to a state of 2.4 V, decomposed in an inert atmosphere, and the negative electrode was taken out. Ten samples were randomly cut from the taken-out negative electrode at 10 locations, 5 cm × 5 cm square (25 cm ² ).

Using an arbitrary sample (25 cm ² ), the abundance ratio of tetravalent titanium was measured by AES, and the area with the lowest abundance ratio of tetravalent titanium was determined as the second area. Further, the region where the difference in the abundance ratio between the second region and the tetravalent titanium was 3% or more was determined as the first region. As a result, it was confirmed that there was at least one first region in any sample. For example, it has been observed that six first regions are scattered in the second region in the sample.

The first region had an area of 12 cm ^{2 at} the maximum and 5 mm ^{2 at} the minimum.

  XPS measurement was performed using 10 samples. As a result, in any sample, a peak derived from Ti appeared in the energy band of 450 to 475 eV.

FIG. 6 shows an example of an XPS measurement result of a site derived from the Ti orbit. FIG. 6 shows a typical high lithium concentration region (second region) and a low region (first region) in the sample. The X axis represents the binding energy, and the peak appearing near 456 eV is a peak attributed to Ti ^3+, and the peak near 458 eV is a peak attributed to Ti ⁴⁺ . The Y axis was normalized and expressed with reference to the peak intensity attributed to Ti4 +. From this result, it is possible to calculate the ratio of Ti ⁴⁺ and Ti ³⁺ by performing peak division.

In the peak diagram of FIG. 6, the abundance ratio T ₁ of tetravalent titanium in the first region having a low lithium concentration is 68%, and the abundance ratio T ₂ of tetravalent titanium in the second region having a high lithium concentration is 55%. Met. Therefore, the difference in the abundance ratio of tetravalent titanium (T ₁ -T ₂ ) in this case was 13%.

In each sample, the value of the region with the lowest lithium concentration was used as the abundance ratio (T ₁ ) of tetravalent titanium in the lithium titanium oxide in the first region. Further, the value of the region having the highest lithium concentration was used as the abundance ratio (T ₂ ) of tetravalent titanium in the lithium titanium oxide in the second region. Among the 10 samples of Example 1, (T ₁ -T ₂ ) in the sample having the largest difference in the abundance ratio of tetravalent titanium was 21%. Examples 2 to 10 also showed (T ₁ -T ₂ ) values in the samples having the largest difference in the abundance ratio of tetravalent titanium.

  When the first region does not exist in the negative electrode layer, there is almost no region where the abundance ratio of tetravalent titanium is different in the negative electrode layer. However, it was confirmed that by performing the aging treatment as in the present embodiment, regions having different abundance ratios of tetravalent titanium are generated in the negative electrode layer. That is, it was confirmed that the lithium concentration distribution was generated by performing the aging treatment.

  Further, by AES measurement, the valence of Ti was measured while shifting the measurement position by 1 mm, and the abundance ratio of tetravalent titanium was measured. As a result, it was observed that the abundance ratio of tetravalent titanium was continuously changed. Therefore, it was confirmed that a third region having a lithium concentration gradient that continuously decreases from the lithium concentration in the second region to the lithium concentration in the first region exists between the second region and the first region.

  The surfaces of Examples 2 to 10 and Comparative Example 1 were similarly observed. As a result, in Examples 2 to 10, it was confirmed that the first region and the third region were scattered in the second region as in Example 1. On the other hand, in Comparative Example 1, it was not confirmed that a lithium concentration distribution was present in the negative electrode layer. The results are shown in Table 1.

<Storage test>
About the secondary battery of Examples 1-10 and Comparative Example 1, the thickness of the battery in a state of SOC 50% was measured. Thereafter, the SOC was adjusted to 30% at a 1C rate, and stored at 55 ° C. for 1 month.

  The battery after storage was adjusted to a SOC of 50% at a 1C rate in an environment of 25 ° C., and the thickness was measured. The change in thickness at 50% SOC before and after storage was calculated. The results are shown in Table 1.

<Cycle test>
Batteries similar to those in Examples 1 to 10 and Comparative Example 1 were separately prepared, and each battery was subjected to a cycle test at a 1 C rate in a charge / discharge range of 2.8 V to 1.5 V in a 50 ° C. environment. The capacity after 500 cycles was measured, and the capacity retention rate after 500 cycles was calculated with the initial capacity as 100%. The results are shown in Table 1.

<Result>

  It was shown that the batteries of Examples 1 to 10 had a smaller thickness change after storage than the battery of Comparative Example 1, and the amount of gas generated was small. In addition, the batteries of Examples 1 to 10 had a higher capacity retention rate after 500 cycles than the battery of Comparative Example 1. Therefore, it was shown that the battery having the configuration in the present embodiment has an excellent lifetime performance while suppressing gas generation during high-temperature storage.

In addition, the batteries of Examples 1 to 10 hardly changed in discharge capacity from the battery of Comparative Example 1. Therefore, it was shown that the discharge capacity hardly decreased even with the battery having the first region. However, when the maximum value of (T ₁ -T ₂ ) is large as in Example 10, the discharge capacity tends to decrease.

  In the battery of Comparative Example 1, the thickness change after storage was large, and the capacity retention rate after 500 cycles was also low. This is considered to be because a large amount of gas was generated from the negative electrode during the cycle test, and the gas was accumulated in the battery.

  As described above, it was shown that by performing an appropriate aging treatment, it is possible to provide a nonaqueous electrolyte secondary battery that suppresses gas generation during storage and has excellent life performance. .

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... Nonaqueous electrolyte secondary battery, 2 ... Electrode group, 3 ... Positive electrode, 4 ... Negative electrode, 5 ... Separator, 6 ... Positive electrode terminal, 7 ... Negative electrode terminal, 8 ... Exterior bag, 9 ... Opening part, 10 ... 1st Sealing part, 11 ... opening part, 12 ... second sealing part, 21 ... first region, 22 ... second region, 23 ... third region.

Claims (5)

A positive electrode;
A negative electrode including a negative electrode layer;
A non-aqueous electrolyte,
Including
The negative electrode layer includes a lithium titanium oxide, has a first region having a low lithium concentration, and a second region having a high lithium concentration present around the first region, and satisfies the following formula (I): Features non-aqueous electrolyte secondary battery:
T ₂ <T ₁ (I)
here,
T ₁ is a ratio of tetravalent titanium among titanium atoms in the lithium titanium oxide in the first region;
T ₂ is a ratio of tetravalent titanium among titanium atoms in the lithium titanium oxide in the second region. The nonaqueous electrolyte secondary battery according to claim 1, wherein the following formula (II) is satisfied:
3 ≦ T ₁ −T ₂ ≦ 30 (II) The non-aqueous electrolyte secondary battery according to claim 1, wherein the first region has an area of 0.1 mm ² or more and 20 cm ² or less. The non-aqueous electrolyte secondary battery according to claim 1, wherein at least one first region is present per 25 cm ^{2 of the} negative electrode layer. The negative electrode layer further includes a third region located between the first region and the second region;
The third region has a lithium concentration higher than the lithium concentration in the first region and lower than the lithium concentration in the second region, according to any one of claims 1 to 4, Non-aqueous electrolyte secondary battery.

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