JP2018006289A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery improved in resistance against lithium precipitation.SOLUTION: A negative electrode active material layer has: a center region opposed to a positive electrode active material layer; and end regions not opposed to the positive electrode active material layer. The negative electrode active material layer includes first and second negative electrode active materials. The first negative electrode active material includes graphite, and has a first charge/discharge potential region (p1). The second negative electrode active material has a second charge/discharge potential region (p2) higher than the first charge/discharge potential region (p1). The end regions are higher than the center region in the content of the second negative electrode active material.SELECTED DRAWING: Figure 9

Description

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

  Japanese Patent Laying-Open No. 2015-170449 (Patent Document 1) has a configuration in which a negative electrode active material layer includes a central region that faces the positive electrode active material layer and an end region that does not face the positive electrode active material layer. Disclosure.

JP2015-170449A JP 2014-143152 A

  Nonaqueous electrolyte secondary batteries using graphite as a negative electrode active material and lithium metal composite oxide as a positive electrode active material have been put into practical use. In this nonaqueous electrolyte secondary battery, lithium (Li) ions that are charge carriers are initially stored in the positive electrode active material.

  In general, the active material of a nonaqueous electrolyte secondary battery is particles. The active material is applied to the current collector foil together with a binder or the like to form an active material layer. In the battery, the negative electrode active material layer and the positive electrode active material layer are arranged to face each other.

  At the time of charging, all lithium (Li) ions released from the positive electrode active material layer need to be occluded by the negative electrode active material layer. This is because Li ions that are not occluded in the negative electrode active material layer may be precipitated as metallic Li. When metal Li precipitates, for example, charge / discharge efficiency will fall.

  Since all the Li ions released from the positive electrode active material layer are occluded in the negative electrode active material layer, the positive electrode active material layer needs to face the negative electrode active material layer over the entire area. When the area of the negative electrode active material layer is equal to the area of the positive electrode active material layer, if the positions of both layers are slightly shifted, a region that does not face the negative electrode active material layer is generated in the positive electrode active material layer. Therefore, the negative electrode active material layer is configured to have a larger area than the positive electrode active material layer. In this electrode configuration, the negative electrode active material layer includes a central region facing the positive electrode active material layer and an end region not facing the positive electrode active material layer.

  This time, a new problem has been found in the above electrode configuration. That is, it has been found that metal Li is likely to precipitate locally near the boundary between the central region and the end region of the negative electrode active material layer. This tendency is more prominent when the battery is stored at a high temperature. It is estimated that this local Li precipitation occurs by the following mechanism.

  FIG. 3 is a diagram for explaining the mechanism of Li precipitation. FIG. 3 shows a cross section of the XZ plane. The X-axis direction in FIG. 3 corresponds to the width direction of the negative electrode active material layer 12 and the positive electrode active material layer 22. The Z-axis direction in FIG. 3 corresponds to the stacking direction of the negative electrode active material layer 12 and the positive electrode active material layer 22. The end region 2 of the negative electrode active material layer 12 does not face the positive electrode active material layer 22. Therefore, the end region 2 does not receive Li ions directly from the positive electrode active material layer 22 (that is, the end region 2 is not directly charged). However, Li ions occluded in the central region 1 may diffuse into the end region 2. The end region 2 contains an electrolytic solution as in the central region 1. Therefore, the end region 2 and the central region 1 are in a liquid junction state. When the central region 1 receives Li ions from the positive electrode active material layer 22 (that is, when charged), a potential difference is generated between the uncharged end region 2 and the central region 1.

During high temperature storage, Li ions in the central region 1 diffuse into the end region 2 using heat and potential difference as driving forces. A part of the Li ions diffused to the end region 2 further diffuses to the end region 2 on the back side. Li ions diffused to the end region 2 on the back side react with oxygen (O ₂ ) present in the battery case or with the electrolytic solution. In this manner, the central region 1 is substantially discharged due to the loss of Li ions. Thereby, in the area | region adjacent to the edge part area | region 2 among the center part area | regions 1, an electric potential rises locally. However, at this time, the potential difference (that is, battery voltage) between the entire negative electrode active material layer 12 and the entire positive electrode active material layer 22 is kept substantially constant. Therefore, as the potential of the negative electrode active material layer 12 is locally increased, the potential of the positive electrode active material layer 22 opposed thereto is also locally increased.

  In FIG. 3, the positive electrode active material layer 22 is illustrated away from the separator 30. However, actually, the positive electrode active material layer 22 is in contact with the separator 30. When the potential of the positive electrode active material layer 22 increases, the separator 30 is oxidized by the positive electrode active material layer 22. A clogging 41 occurs in a portion where the separator 30 is oxidized. The clogging 41 inhibits the permeation of Li ions.

Further, when the potential of the positive electrode active material layer 22 is increased, the metal (M) is eluted from the lithium metal composite oxide that is the positive electrode active material. The eluted metal ions (M ⁺ ) move to the negative electrode active material layer 12 by electrophoresis and are deposited on the surface of the negative electrode active material layer 12. Thereby, the metal film 42 is formed.

  When the battery is discharged in this state, Li ions around the end region 2 are not discharged because they are blocked by the metal film 42 and the clogging 41. That is, this Li ion cannot return to the positive electrode side. Thereby, the potential increase in the end region 2 is promoted.

  When the battery is charged, Li ions moving from the positive electrode active material layer 22 are blocked by the metal film 42 on the surface of the negative electrode active material layer 12. Therefore, Li ions cannot reach the negative electrode active material layer 12. Li ions blocked by the metal film 42 are deposited as metal Li 43 on the surface of the metal film 42.

  Due to the above mechanism, it is presumed that after the battery is stored at a high temperature, metal Li is likely to precipitate in the vicinity of the boundary between the central region 1 and the end region 2.

  Here, when no oxygen is present in the battery case, it is considered that the reaction between oxygen and Li ions does not occur, so that Li precipitation is suppressed. However, in order for the inside of the battery case to have an oxygen-free atmosphere, the inside of the battery case needs to be replaced with an inert gas (for example, argon (Ar) gas). This necessitates a significant increase in manufacturing costs.

  Further, as described above, it may be possible to remove the end region 2 that does not face the positive electrode active material layer 22, but it is not practical in view of the alignment of the electrodes.

  Therefore, an object of the present disclosure is to provide a nonaqueous electrolyte secondary battery with improved Li deposition resistance.

The nonaqueous electrolyte secondary battery of the present disclosure includes a negative electrode active material layer and a positive electrode active material layer.
The negative electrode active material layer has a larger area than the positive electrode active material layer. The positive electrode active material layer faces the negative electrode active material layer over the entire area. The negative electrode active material layer includes a central region that faces the positive electrode active material layer and an end region that does not face the positive electrode active material layer.
The positive electrode active material layer contains a lithium metal composite oxide. The negative electrode active material layer contains a first negative electrode active material and a second negative electrode active material. The first negative electrode active material includes graphite and has a first charge / discharge potential region. The second negative electrode active material has a second charge / discharge potential region that is higher than the first charge / discharge potential region. The end region has a higher content of the second negative electrode active material than the central region.

The negative electrode active material layer of the present disclosure contains a first negative electrode active material (graphite) and a second negative electrode active material (high potential active material). FIG. 8 is a graph showing a charging profile of graphite alone. The curve in the graph is a capacity-potential curve of graphite alone. At the time of discharge, it may be considered that the potential increases along the arrow in the graph. Graphite has a region where the potential is flat, that is, a first charge / discharge potential region (p1) below 0.2 V (vs. Li ^/ Li ⁺ ). The first charge / discharge potential region (p1) extends over a wide range. However, at the end of discharge, the potential increases rapidly. Therefore, when the negative electrode active material layer contains graphite alone, when Li ions diffuse from the central region to the end region, the potential rapidly increases near the boundary between the central region and the end region. It will be.

  FIG. 9 is a graph showing a charging profile of a mixed negative electrode of graphite and a high potential active material. The curve in the graph is a capacity-potential curve of the mixed negative electrode. The high potential active material has a second charge / discharge potential region (p2) higher than the first charge / discharge potential region (p1) of graphite. Therefore, the curve has a charge / discharge potential region (p2) derived from the high potential active material at the end of discharge. When the negative electrode active material layer contains this mixed negative electrode, the second charge / discharge potential region (p2) serves as a buffer when Li ions diffuse from the central region to the end region, so that the potential increase is mitigated. Is done. Thereby, a series of reactions resulting from the potential increase of the negative electrode, and thus Li deposition can be suppressed. That is, Li precipitation resistance is improved.

  However, the second charge / discharge potential region (p2) of the high-potential active material may deviate from the potential region used for charging / discharging the battery. In that case, the irreversible capacity of the negative electrode increases and the battery capacity decreases. Therefore, in the nonaqueous electrolyte secondary battery of the present disclosure, the content of the high potential active material in the end region is set higher than the content of the high potential active material in the central region. The end region does not face the positive electrode active material layer. Therefore, even if the irreversible capacity of the end region increases, the substantial influence on the battery capacity is small. Therefore, when the content rate of the high potential active material (second negative electrode active material) in the end region is higher than the content rate of the second negative electrode active material in the center region, a decrease in battery capacity is suppressed.

It is a schematic sectional drawing showing an example of composition of a nonaqueous electrolyte secondary battery concerning an embodiment of this indication. It is the schematic which shows an example of a structure of an electrode group. It is a figure explaining the mechanism of Li precipitation. It is the schematic which shows an example of a structure of a negative electrode plate. It is an enlarged view of the V area | region of FIG. It is a figure explaining the preparation methods of the negative electrode plate of an Example. It is the schematic which shows an example of a structure of a positive electrode plate. It is a graph which shows the charge profile of graphite alone. It is a graph which shows the charge profile of a mixed negative electrode.

  Hereinafter, an embodiment of the present disclosure (hereinafter referred to as “the present embodiment”) will be described. However, the following description does not limit the invention of the present disclosure. In the following description, the nonaqueous electrolyte secondary battery may be abbreviated as “battery”.

<Nonaqueous electrolyte secondary battery>
The nonaqueous electrolyte secondary battery of the present embodiment is typically a lithium ion secondary battery.
FIG. 1 is a schematic cross-sectional view showing an example of the configuration of the nonaqueous electrolyte secondary battery of the present embodiment. The battery 100 is a vehicle-mounted battery, for example. When the battery 100 is for in-vehicle use, the current regulation during regenerative charging can be relaxed, for example, by improving Li deposition resistance. Thereby, in a vehicle equipped with the battery 100, an improvement in fuel efficiency is expected. However, the use of the nonaqueous electrolyte secondary battery of the present embodiment is not limited to in-vehicle use. The battery 100 is applicable to all uses.

  The battery 100 includes a battery case 90. The battery case 90 has a rectangular (flat rectangular parallelepiped) outer shape. However, the battery case of the present embodiment may be cylindrical. The battery case 90 is made of, for example, an aluminum (Al) alloy. However, the battery case of this embodiment may be a bag made of an aluminum laminate film or the like.

  The battery case 90 is provided with a negative electrode terminal 91 and a positive electrode terminal 92. In the battery case 90, the electrode group 50 and the electrolytic solution 60 are accommodated. The electrolytic solution 60 is impregnated in the electrode group 50. The electrode group 50 is electrically connected to the negative terminal 91 and the positive terminal 92. Although not illustrated, the battery case 90 may be provided with a liquid injection port, a safety valve, a current interruption mechanism (CID), and the like.

  In the present embodiment, the atmosphere in the battery case 90 may contain oxygen. That is, the injection operation of the electrolytic solution 60 may be performed in an atmosphere containing oxygen. Thereby, reduction of manufacturing cost is expected.

  FIG. 2 is a schematic diagram illustrating an example of the configuration of the electrode group 50. The electrode group 50 is a wound electrode group. However, the electrode group of this embodiment may be a stacked electrode group. The electrode group 50 includes a negative electrode plate 10, a positive electrode plate 20, and a separator 30. The electrode group 50 is configured such that the negative electrode plate 10 and the positive electrode plate 20 face each other with the separator 30 interposed therebetween, and these are further wound.

<Negative electrode plate>
FIG. 4 is a schematic diagram illustrating an example of the configuration of the negative electrode plate. The negative electrode plate 10 is a strip-shaped sheet member. However, this is not the case when the electrode group is a laminated type. The negative electrode plate 10 includes a negative electrode current collector foil 11 and a negative electrode active material layer 12. The negative electrode active material layer 12 is formed on the surface of the negative electrode current collector foil 11. Although not shown, the negative electrode active material layer 12 is formed on both surfaces (front and back) of the negative electrode current collector foil 11. The negative electrode current collector foil 11 is, for example, a copper (Cu) foil. The negative electrode current collector foil 11 has a thickness of about 5 to 25 μm, for example.

  In FIG. 4, the X-axis direction and the Y-axis direction are orthogonal to each other. The X-axis direction corresponds to the width direction of the electrode plate. The Y-axis direction corresponds to the longitudinal direction of the electrode plate. A region where the negative electrode current collector foil 11 is exposed from the negative electrode active material layer 12 is formed at the end in the X-axis direction. This region is used for current collection and connection with the negative electrode terminal 91.

  The negative electrode active material layer 12 contains a negative electrode active material. The negative electrode active material layer 12 may contain a binder or the like in addition to the negative electrode active material. The negative electrode active material layer 12 contains, for example, 95 to 99% by mass (mass%) of the negative electrode active material and 1 to 5% by mass of a binder. The binder is not particularly limited. The binder may be, for example, carboxymethylcellulose (CMC), styrene butadiene rubber (SBR), polyacrylic acid (PAA), and the like.

  In the present embodiment, the negative electrode active material layer 12 contains at least two types of negative electrode active materials. That is, the negative electrode active material layer 12 contains a first negative electrode active material and a second negative electrode active material. The first negative electrode active material includes graphite. The graphite may be natural graphite or artificial graphite. The particle shape of graphite is not particularly limited. The graphite may be, for example, scaly graphite or spheroidized graphite. The first negative electrode active material may be a composite material of graphite and non-graphitic carbon. For example, the first negative electrode active material may include graphite and amorphous carbon attached to the surface of the graphite.

The charge / discharge potential region of the negative electrode active material can be confirmed, for example, by a charge / discharge test in a single electrode cell. In the monopolar cell, the negative electrode active material is the working electrode, and the metal Li is the counter electrode and the reference electrode. The first negative electrode active material has a first charge / discharge potential region (p1). The first charge / discharge potential region (p1) is, for example, higher than 0 V (vs. Li ^/ Li ⁺ ) and lower than 0.2 V (vs. Li ^/ Li ⁺ ).

  The second negative electrode active material is a high potential active material. The second negative electrode active material has a second charge / discharge potential region (p2). The second charge / discharge potential region (p2) is higher than the first charge / discharge potential region (p1). When the negative electrode active material layer 12 contains the second negative electrode active material in addition to the first negative electrode active material, a rapid increase in potential of the negative electrode active material layer 12 and Li precipitation resulting therefrom are suppressed.

The second charge / discharge potential region (p2) is preferably 0.8 V (vs. Li ^/ Li ⁺ ) or more and 1.8 V (vs. Li ^/ Li ⁺ ) or less. Examples of the active material having such a charge / discharge potential region include lithium titanate (for example, Li ₄ Ti ₅ O ₁₂ ), titanium oxide (TiO ₂ ), hydrogen titanium oxide (for example, H ₂ Ti ₁₂ O). ₂₅ ) and the like. A 2nd negative electrode active material may be used individually by 1 type, and 2 or more types may be used in combination. That is, the second negative electrode active material may be at least one selected from the group consisting of lithium titanate, titanium oxide, and hydrogen titanium oxide.

  In the present embodiment, the composition of the negative electrode active material layer 12 is not uniform. FIG. 5 is an enlarged view of a region V in FIG. The negative electrode active material layer 12 includes a central region 1 that faces the positive electrode active material layer 22 and an end region 2 that does not face the positive electrode active material layer 22. The end region 2 has a higher content of the second negative electrode active material than the central region 1. Thereby, the fall of battery capacity is suppressed.

  The end region 2 preferably contains 1% by mass or more and 10% by mass or less of the second negative electrode active material. The end region 2 more preferably contains 5% by mass or more and 10% by mass or less of the second negative electrode active material.

  The central region 1 preferably does not substantially contain the second negative electrode active material. However, from the viewpoint of productivity, the central region 1 may contain a second negative electrode active material that is higher than 0% by mass and lower than 1% by mass. For example, when the region to be the end region 2 is formed by applying a paste, the paste containing the second negative electrode active material may protrude from the region to be the central region 1.

  As long as it contains the first negative electrode active material and the second negative electrode active material, the negative electrode active material layer 12 may contain a third negative electrode active material. Examples of the third negative electrode active material include non-graphitic carbon such as soft carbon and hard carbon.

《Positive electrode plate》
FIG. 7 is a schematic diagram illustrating an example of the configuration of the positive electrode plate. The positive electrode plate 20 is a belt-like sheet member. However, this is not the case when the electrode group is a laminated type. The positive electrode plate 20 includes a positive electrode current collector foil 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 is formed on the surface of the positive electrode current collector foil 21. Although not shown, the positive electrode active material layer 22 is formed on both surfaces (front and back) of the positive electrode current collector foil 21. The positive electrode current collector foil 21 is, for example, an Al foil. The positive electrode current collector foil 21 has a thickness of about 5 to 25 μm, for example. In FIG. 7, a region where the positive electrode current collector foil 21 is exposed from the positive electrode active material layer 22 is formed at the end in the X-axis direction. This area is used for current collection and connection with the positive terminal 92.

  The positive electrode active material layer 22 contains a lithium metal composite oxide as a positive electrode active material. The positive electrode active material layer 22 may contain a conductive material, a binder, and the like in addition to the positive electrode active material. The positive electrode active material layer 22 contains, for example, 80 to 98 mass% lithium metal composite oxide, 1 to 15 mass% conductive material, and 1 to 5 mass% binder.

The lithium metal composite oxide is a metal oxide that can electrochemically store and release Li ions. The lithium metal composite oxide has, for example, a layered rock salt type, a spinel type, an olivine type or the like crystal structure. Examples of the lithium metal composite oxide include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _a Co _b Mn _c O ₂ (a + b + c = 1, 0 <a <1, 0 <b <1, 0 < c <1) and the like. The lithium metal composite oxide of this embodiment includes a compound containing an element other than metal and oxygen. Examples of such a compound include LiFePO ₄ . This embodiment is particularly effective when the lithium metal composite oxide contains manganese (Mn). This is because in the lithium metal composite oxide containing Mn, when the potential is increased, Mn is likely to be eluted. The positive electrode active material layer 22 may contain two or more positive electrode active materials.

  The conductive material is not particularly limited. For example, the conductive material may be acetylene black, thermal black, furnace black, or the like. The binder is not particularly limited. For example, the binder may be polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or the like.

<< Separator >>
The separator 30 is, for example, a resin porous film. The separator 30 may be, for example, a polyethylene (PE) porous film, a polypropylene (PP) porous film, or the like. The separator 30 may be a single layer. The separator 30 may include a plurality of layers. For example, the separator 30 may be configured by laminating a porous film made of PE and a porous film made of PP. The separator 30 may include an inorganic particle layer on the surface thereof. The inorganic particle layer contains, for example, inorganic particles such as alumina.

<Electrolyte>
The electrolytic solution 60 is a liquid electrolyte containing an aprotic solvent and a supporting electrolyte. However, the nonaqueous electrolyte of the present embodiment is not limited to a liquid electrolyte, and may be a gel electrolyte, a solid electrolyte, or the like.

  The aprotic solvent may be, for example, a mixed solvent of a cyclic carbonate and a chain carbonate. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, and the like. Examples of the chain carbonate include ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). The mixing ratio of the cyclic carbonate and the chain carbonate may be, for example, about “cyclic carbonate: chain carbonate = 1: 9 to 5: 5” in volume ratio.

The supporting electrolyte may be a Li salt such as LiPF ₆ , LiBF ₄ , Li [N (FSO ₂ ) ₂ ] (LiFSI). The electrolytic solution 60 may contain a supporting electrolyte of 0.5 to 2 mol / L, for example. The electrolytic solution 60 may further contain various additives in addition to the aprotic solvent and the supporting electrolyte. For example, the electrolytic solution 60 may contain cyclohexylbenzene (CHB), lithium bis (oxalate) borate (LiBOB), or the like.

  Examples will be described below. However, the following examples do not limit the invention of the present disclosure.

<Manufacture of non-aqueous electrolyte secondary batteries>
The nonaqueous electrolyte secondary battery according to the comparative example and the example was manufactured as follows.

<< Comparative Example 1 >>
1. Preparation of positive electrode The following materials were prepared.

Cathode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂
Conductive material: Acetylene black Binder: PVdF
Solvent: NMP
Positive electrode current collector foil: Al foil

  The positive electrode active material, the conductive material, and the binder were weighed so that “positive electrode active material: conductive material: binder = 90: 8: 2” by mass ratio. The positive electrode active material, the conductive material, and the binder were dispersed in the solvent. Thereby, a positive electrode paste was prepared.

The positive electrode paste was applied to one surface of the positive electrode current collector foil and dried. Thereby, the positive electrode active material layer was formed. The basis weight of the positive electrode active material layer (mass per unit area after drying) was adjusted to 6 mg / cm ² . Similarly, a positive electrode active material layer was also formed on the other surface of the positive electrode current collector foil. That is, the basis weight of the positive electrode active material layer is 12 mg / cm ^{2 on} both sides.

  The positive electrode active material layer and the positive electrode current collector foil were rolled so that the positive electrode active material layer had a predetermined thickness. The positive electrode active material layer and the positive electrode current collector foil were cut so that the positive electrode plate had a predetermined planar dimension. From the above, a positive electrode plate was produced.

2. Production of Negative Electrode Plate The following materials were prepared.

First negative electrode active material: spheroidized graphite Binder: CMC and SBR
Solvent: Water Negative electrode current collector foil: Cu foil

  The first negative electrode active material and the binder were weighed so as to be “first negative electrode active material: CMC: SBR = 98: 1: 1” by mass ratio. The negative electrode active material and the binder were dispersed in the solvent. Thereby, a negative electrode paste was prepared.

The negative electrode paste was applied to one surface of the negative electrode current collector foil and dried. Thereby, the negative electrode active material layer was formed. The basis weight of the negative electrode active material layer was adjusted to be 3.00 mg / cm ² . Similarly, a negative electrode active material layer was formed on the other surface of the negative electrode current collector foil. That is, the basis weight of the negative electrode active material layer is 6.00 mg / cm ^{2 on} both sides.

  The negative electrode active material layer and the negative electrode current collector foil were rolled so that the negative electrode active material layer had a predetermined thickness. The negative electrode active material layer and the negative electrode current collector foil were cut so that the negative electrode plate had a predetermined planar dimension. From the above, a negative electrode plate was produced.

3. Assembly A porous membrane made of PE was prepared as a separator. It arrange | positioned so that a negative electrode plate and a positive electrode plate may oppose on both sides of a separator. Furthermore, these were wound to produce a wound electrode group. A square battery case was prepared. The electrode group was accommodated in the battery case. The electrode group was electrically connected to a negative electrode terminal and a positive electrode terminal provided in the battery case.

4). Injection of electrolyte and sealing of battery case An electrolyte having the following composition was prepared.
LiPF ₆ (1 mol / L), EC: EMC: DMC = 3: 3: 4 (volume ratio)

  In a dry room having a dew point of −45 ° C., a predetermined amount of electrolyte was injected into the battery case. The battery case was sealed in the dry room. The oxygen concentration in this dry room may be regarded as 20% by volume (vol%), as in the atmosphere. Therefore, the oxygen concentration in the battery case after sealing may be regarded as 20% by volume.

5. Initial charging and aging After the electrolyte solution was sufficiently impregnated in the electrode group, initial charging was performed. After the initial charging, the SOC (State Of Charge) of the battery was adjusted to 100%. The battery was allowed to stand for 24 hours in a thermostat set to 60 ° C. From the above, the nonaqueous electrolyte secondary battery according to Comparative Example 1 was manufactured.

<< Comparative Example 2 >>
In Comparative Example 2, a part of the negative electrode active material layer was peeled off and removed. Specifically, in the electrode group, the negative electrode active material layer located on the outermost periphery and not facing the positive electrode mixture layer was removed. The removed negative electrode active material layer corresponds to the outermost peripheral region 3 in FIG. Except for this, a non-aqueous electrolyte secondary battery according to Comparative Example 2 was manufactured by the same procedure as Comparative Example 1.

<< Comparative Example 3 >>
In Comparative Example 3, the electrolyte solution was injected into the battery case in the glove box replaced with Ar gas, and the battery case was sealed in the glove box. The oxygen concentration in the glove box may be regarded as substantially 0% by volume. Therefore, the oxygen concentration in the battery case may be regarded as substantially 0% by volume. Except for this, a non-aqueous electrolyte secondary battery according to Comparative Example 3 was manufactured by the same procedure as Comparative Example 1.

<< Comparative Example 4 >>
Lithium titanate (Li ₄ Ti ₅ O ₁₂ ) was prepared as the second negative electrode active material. The first negative electrode active material (spheroidized graphite), the second negative electrode active material, and the binder are “first negative electrode active material: second negative electrode active material: CMC: SBR = 98: 5: 1: 1” by mass ratio. Weighed as follows. The first negative electrode active material, the second negative electrode active material, and the binder were dispersed in the solvent. Thereby, a negative electrode paste was prepared.

The negative electrode paste was applied to one surface of the negative electrode current collector foil and dried. Thereby, the negative electrode active material layer was formed. The basis weight of the negative electrode active material layer was adjusted to 3.15 mg / cm ² . Similarly, a negative electrode active material layer was formed on the other surface of the negative electrode current collector foil. That is, the basis weight of the negative electrode active material layer is 6.30 mg / cm ^{2 on} both sides. Except for these, a non-aqueous electrolyte secondary battery according to Comparative Example 4 was manufactured by the same procedure as Comparative Example 3.

<< Comparative Example 5 >>
The first negative electrode active material, the second negative electrode active material, and the binder were weighed so that the mass ratio was “first negative electrode active material: second negative electrode active material: CMC: SBR = 98: 10: 1: 1”. . The first negative electrode active material, the second negative electrode active material, and the binder were dispersed in the solvent. Thereby, a negative electrode paste was prepared.

The negative electrode paste was applied to one surface of the negative electrode current collector foil and dried. Thereby, the negative electrode active material layer was formed. The basis weight of the negative electrode active material layer was adjusted to be 3.30 mg / cm ² . Similarly, a negative electrode active material layer was formed on the other surface of the negative electrode current collector foil. That is, the basis weight of the negative electrode active material layer is 6.60 mg / cm ^{2 on} both sides. Except for these, a non-aqueous electrolyte secondary battery according to Comparative Example 5 was manufactured by the same procedure as Comparative Example 4.

<< Comparative Example 6 >>
A non-aqueous electrolyte secondary battery according to Comparative Example 6 is manufactured by the same procedure as Comparative Example 4 except that the electrolyte solution is injected and the battery case is sealed in a dry room having a dew point of −45 ° C. It was done.

<< Comparative Example 7 >>
A non-aqueous electrolyte secondary battery according to Comparative Example 7 is manufactured by the same procedure as Comparative Example 5 except that the electrolyte solution is injected and the battery case is sealed in a dry room having a dew point of −45 ° C. It was done.

Example 1
The first negative electrode active material (spheroidized graphite) and the binder were weighed so as to be “first negative electrode active material: CMC: SBR = 98: 1: 1” by mass ratio. The first negative electrode active material and the binder were dispersed in the solvent. Thereby, the 1st negative electrode paste was prepared.

  The first negative electrode active material (spheroidized graphite), the second negative electrode active material (lithium titanate), and the binder are expressed as “first negative electrode active material: second negative electrode active material: CMC: SBR = 98: 5: 1”. : 1 ". The first negative electrode active material, the second negative electrode active material, and the binder were dispersed in the solvent. Thereby, the second negative electrode paste was prepared.

FIG. 6 is a diagram illustrating a method for producing the negative electrode plate of the example. First, the first negative electrode paste was applied to a predetermined position of the negative electrode current collector foil 11 and dried. Thereby, the center region 1 was formed. The basis weight of the central region 1 was adjusted to 3.00 mg / cm ² . Next, the second negative electrode paste was applied to the end of the central region 1 in the X-axis direction and dried. Thereby, the edge part area | region 2 was formed. The basis weight of the end region 2 was adjusted to 3.15 mg / cm ² . Similarly, the central region 1 and the end region 2 were also formed on the other surface of the negative electrode current collector foil 11. That is, the basis weight of the central region 1 is 6.00 mg / cm ^{2 on} both sides, and the basis weight of the end region 2 is 6.30 mg / cm ^{2 on} both sides. Thereby, the negative electrode active material layer 12 was formed.

  The negative electrode active material layer 12 and the negative electrode current collector foil 11 were rolled so that the negative electrode active material layer 12 had a predetermined thickness. The end region 2 and the negative electrode current collector foil 11 were cut along the cutting line (CL) in FIG. 6, whereby a part of the end region 2 and the negative electrode current collector foil 11 was removed. Thereby, the negative electrode plate 10 according to Example 1 was manufactured.

  Except for these, the nonaqueous electrolyte secondary battery according to Example 1 was manufactured by the same procedure as in Comparative Example 4.

Example 2
The first negative electrode active material (spheroidized graphite), the second negative electrode active material (lithium titanate), and the binder are expressed as “first negative electrode active material: second negative electrode active material: CMC: SBR = 98: 10: 1”. : 1 ". The first negative electrode active material, the second negative electrode active material, and the binder were dispersed in the solvent. Thereby, the second negative electrode paste was prepared.

The second negative electrode paste was applied to a predetermined position of the negative electrode current collector foil and dried. Thereby, the edge part area | region 2 was formed. Basis weight of the end regions 2, was adjusted to 6.60mg / cm ² in both (3.3 mg / cm ² on one side).

  Except for these, the nonaqueous electrolyte secondary battery according to Example 2 was manufactured by the same procedure as in Example 1.

Example 3
A non-aqueous electrolyte secondary battery according to Example 3 is manufactured by the same procedure as Example 1 except that the electrolyte solution is injected and the battery case is sealed in a dry room having a dew point of −45 ° C. It was done.

Example 4
A non-aqueous electrolyte secondary battery according to Example 4 is manufactured by the same procedure as Example 2 except that the electrolyte solution is injected and the battery case is sealed in a dry room having a dew point of −45 ° C. It was done.

<Evaluation>
Each battery was evaluated as follows. In the following description, “CC” indicates a constant current method, “CV” indicates a constant voltage method, and “CC-CV” indicates a constant current-constant voltage method. The magnitude of the current value is represented by “C”. Here, “1C” is defined as a current value at which the SOC changes from 0% to 100% after charging for one hour.

1. Measurement of battery capacity The battery was charged and discharged under the following conditions. Thereby, the capacity (initial capacity) of the CC discharge was measured. The measurement results are shown in Table 1 above. Table 1 shows the relative values when the battery capacity of Comparative Example 1 is 100%.

CC-CV charge: CC current = 1C, CV voltage = 4.1V, cut current = 0.1C
CC discharge: CC current = 1 / 3C, cut voltage = 3.0V

2. Measurement of initial limit current value The battery was placed in a thermostatic chamber set to −10 ° C. A predetermined current value was set. Charging / discharging was repeated 1000 times at the same current value, with “5 seconds charging → 10 minutes rest → 5 seconds discharging → 10 minutes discharging” as one cycle. After 1000 times, the battery was disassembled. The presence or absence of Li precipitation on the surface of the negative electrode active material layer was confirmed by visual observation. When there was no Li precipitation, the current value was increased and charging / discharging was repeated 1000 times again. In this way, a current value at which metal Li began to precipitate was confirmed. Of the current values at which the metal Li does not precipitate, the maximum current value was set as the limit current value. The measurement results are shown in Table 1 above. In Table 1 above, relative values when the limit current value of Comparative Example 1 is set as a reference (100%) are shown. The larger the value, the better the initial Li precipitation resistance.

3. Measurement of limit current value after storage at high temperature The SOC of the battery was adjusted to 80%. The battery was placed in a thermostat set at 75 ° C. In the same environment, the battery was stored for 60 days. After 60 days, the limit current value after high-temperature storage was measured by the same procedure as in “2. Measurement of limit current value (initial)”. The measurement results are shown in Table 1 above. Table 1 shows the relative values when the limit current value of Comparative Example 1 is 100%. The larger the value, the better the Li precipitation resistance after high temperature storage.

<Results and discussion>
In Table 1 above, from the results of Comparative Examples 1 and 2, by removing a region (outermost peripheral region 3 in FIG. 5) of the negative electrode active material layer that does not face the positive electrode active material layer, Li precipitation resistance Can be seen to improve. However, this aspect is not realistic considering productivity.

  From the results of Comparative Example 3, it can be seen that if the inside of the battery case is an oxygen-free atmosphere, the Li precipitation resistance is improved. However, the gas replacement in the battery case is expected to significantly increase the manufacturing cost.

  In Comparative Examples 4 to 7, the initial capacity is reduced. It is considered that the irreversible capacity of the negative electrode is increased because the second negative electrode active material (lithium titanate) having a high charge / discharge potential region is distributed throughout the negative electrode active material layer.

  In Examples 1 to 4, the initial capacity was not reduced. This is probably because the second negative electrode active material is not substantially contained in the central region that occupies most of the negative electrode active material layer. From the results of Examples 1 to 4, it was proved that according to the configuration of the example, the Li precipitation resistance was improved even when oxygen was present in the battery case.

  The embodiments and examples disclosed herein are illustrative in all aspects and should not be construed as being restrictive. The scope of the invention of the present disclosure is shown not by the above description but by the scope of the claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of the claims.

  DESCRIPTION OF SYMBOLS 1 Central part area | region, 2 End area | region, 3 Outermost peripheral area | region, 10 Negative electrode plate, 11 Negative electrode current collector foil, 12 Negative electrode active material layer, 20 Positive electrode plate, 21 Positive electrode current collector foil, 22 Positive electrode active material layer, 30 Separator 41 clogging, 42 metal film, 43 metal Li, 50 electrode group, 60 electrolyte, 90 battery case, 91 negative electrode terminal, 92 positive electrode terminal, 100 battery.

Claims (1)

A negative electrode active material layer and a positive electrode active material layer;
The negative electrode active material layer has a larger area than the positive electrode active material layer,
The positive electrode active material layer is opposed to the negative electrode active material layer over the entire area,
The negative electrode active material layer includes a central region facing the positive electrode active material layer, and an end region not facing the positive electrode active material layer,
The positive electrode active material layer contains a lithium metal composite oxide,
The negative electrode active material layer contains a first negative electrode active material and a second negative electrode active material,
The first negative electrode active material includes graphite and has a first charge / discharge potential region,
The second negative electrode active material has a second charge / discharge potential region higher than the first charge / discharge potential region,
The end region has a higher content of the second negative electrode active material than the central region.
Non-aqueous electrolyte secondary battery.

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