WO2019212041A1

WO2019212041A1 - Lithium ion secondary battery

Info

Publication number: WO2019212041A1
Application number: PCT/JP2019/017865
Authority: WO
Inventors: 幸弘小松原; 巧美三尾; 健太郎飯塚; 崇文藤井; 西　幸二; 直輝大参; 雄輔木元
Original assignee: 株式会社ジェイテクト
Priority date: 2018-05-02
Filing date: 2019-04-26
Publication date: 2019-11-07
Also published as: JP7396271B2; JPWO2019212041A1

Abstract

The present disclosure provides a lithium ion secondary battery (1) that can operate at a high-temperature environment of 85°C. This lithium ion secondary battery (1) comprises a positive electrode active material that can occlude and release lithium ions Li+, a positive electrode binder that binds the positive electrode active material, a negative electrode active material that can occlude and release lithium ions Li+, a negative electrode binder that binds the negative electrode active material, and an electrolytic solution (40) containing an organic solvent and an imide lithium salt. The positive electrode binder has a Hansen solubility parameter-based RED value of more than 1 with respect to the electrolytic solution (40).

Description

Lithium ion secondary battery

This disclosure relates to a lithium ion secondary battery.

Lithium ion secondary batteries are widely used because they exhibit excellent characteristics such as excellent energy density. And since the use of a lithium ion secondary battery increases as the heat resistance increases, various techniques for improving the heat resistance of the lithium ion secondary battery have been proposed. For example, Japanese Patent Application Laid-Open No. 2014-160638 discloses a lithium ion secondary battery having heat resistance of about 60 ° C.

However, the heat resistance of the lithium ion secondary battery described in JP-A-2014-160638 is about 60 ° C., and a lithium ion secondary battery that can withstand higher temperatures is demanded. For example, in order to use a lithium ion secondary battery for an automobile, heat resistance of about 85 ° C. is required. Therefore, a more improved lithium ion secondary battery is required.

One feature of the present disclosure is that a positive electrode active material capable of occluding and releasing lithium ions, a positive electrode binder binding the positive electrode active material, a negative electrode active material capable of occluding and releasing lithium ions, and the negative electrode A negative electrode binder that binds the active material; and an electrolyte solution containing an organic solvent and an imide-based lithium salt, wherein the positive electrode binder has a RED value greater than 1 based on a Hansen solubility parameter for the electrolyte solution, Next battery.

According to the above characteristics, the lithium ion secondary battery can have a heat resistance of 85 ° C. In the present disclosure, that the lithium ion secondary battery has heat resistance means that the lithium ion secondary battery has a performance operable in a high temperature environment.

1 is a schematic exploded perspective view of a lithium ion secondary battery according to an embodiment. 1 is a perspective view of a lithium ion secondary battery according to an embodiment. FIG. 3 is a schematic diagram of a III-III cross section in the lithium ion secondary battery of FIG. 2. It is a figure explaining the example of the external appearance of the positive electrode plate shown in FIG. FIG. 5 is a VV sectional view of the positive electrode plate of FIG. 4. It is a figure explaining the example of the external appearance of the negative electrode plate shown in FIG. FIG. 7 is a VII-VII sectional view of the negative electrode plate of FIG. It is a figure explaining the positional relationship of the positive electrode plate of a positive electrode shown in FIG. 1, the negative electrode plate of a negative electrode, a separator, and electrolyte solution. It is a figure explaining the upper limit of the amount of pre dope of a negative electrode. In Test Example 1, it is a graph showing a change with time of internal resistance (mΩ) at 85 ° C. of a lithium ion secondary battery. In Test Example 1, it is a graph showing the change over time of the discharge capacity (mAh) at 85 ° C. of the lithium ion secondary battery. In Test Example 2, it is a graph showing the change over time of the internal resistance (mΩ) at 85 ° C. of the lithium ion secondary battery. In Experiment 2, it is a graph which shows a time-dependent change of the discharge capacity (mAh) in 85 degreeC of a lithium ion secondary battery. FIG. 6 is a graph showing changes over time in internal resistance of lithium ion secondary batteries at 85 ° C. in Test Examples 3 to 5. FIG. 6 is a graph showing changes with time in discharge capacity of a lithium ion secondary battery at 85 ° C. in Test Examples 3 to 5.

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. As shown in the exploded perspective view of FIG. 1, the lithium ion secondary battery 1 includes a plurality of plate-like positive electrode plates 11 and a plurality of plate-like negative electrode plates 21, which are alternately stacked. Yes. Each positive electrode plate 11 includes an electrode terminal connection portion 12b protruding in one direction. Each negative electrode plate 21 also includes an electrode terminal connection portion 22b protruding in the same direction as the direction in which the electrode terminal connection portion 12b of the positive electrode plate 11 protrudes. As shown in FIG. 1, the direction in which the electrode terminal connecting portion 12b of the positive electrode plate 11 protrudes is the X-axis direction, the stacked direction is the Z-axis direction, and the direction orthogonal to the X-axis and the Z-axis is the Y-axis. The direction. These X axis, Y axis, and Z axis are orthogonal to each other. In all of the drawings in which the X axis, the Y axis, and the Z axis are described, these axial directions indicate the same direction, and in the following description, descriptions regarding directions may be based on these axial directions. In the following description, illustration and detailed description of the incidental configuration are omitted.

<1. Overall structure of lithium ion secondary battery 1 (FIGS. 1 to 3)>
As shown in FIG. 1, the lithium ion secondary battery 1 includes a plurality of positive plates 11, a plurality of negative plates 21, a plurality of separators 30, an electrolytic solution 40, and a laminate member 50. Here, as shown in FIG. 1, the positive plates 11 and the negative plates 21 are alternately stacked, and the separators 30 are sandwiched between the positive plates 11 and the negative plates 21. The electrolyte solution 40 is wrapped and sealed in two laminate members 50 together with a part of the plurality of positive electrode plates 11, a part of the plurality of negative electrode plates 21, and the plurality of separators 30 laminated in this manner. Yes.

The electrode terminal connection portions 12 b of the plurality of positive electrode plates 11 protrude in the same direction and are electrically connected to the positive electrode terminal 14. Conductive members constituting the positive terminal side such as the positive terminal 14 and the plurality of positive plates 11 connected thereto can be collectively referred to as the positive electrode 10. Similarly, the electrode terminal connecting portions 22b of the plurality of negative electrode plates 21 and the negative electrode terminals 24 are electrically connected, and the negative electrode terminals such as the negative electrode terminals 24 and the plurality of negative electrode plates 21 connected thereto are configured. The conductor members can be collectively referred to as the negative electrode 20.

The lithium ion secondary battery 1 has the above-described configuration inside, and the appearance is shown in FIG. FIG. 3 schematically shows a III-III cross section of the lithium ion secondary battery 1 shown in FIG. In FIG. 3, in order to make it easy to understand, each member in the lithium ion secondary battery 1 is illustrated with an interval. However, in practice, the positive electrode plate 11, the negative electrode plate 21, and the separator 30 are stacked with almost no gap.

<2. Each part of the lithium ion secondary battery 1 (FIGS. 1, 3 to 7)>
<2-1. Regarding the positive electrode plate 11 (FIGS. 1, 3 to 5)>
The positive electrode plate 11 includes a thin plate-like positive electrode current collector 12 and a positive electrode active material layer 13 coated on the positive electrode current collector 12 (see FIGS. 3 to 5). The positive electrode active material layer 13 is provided on both surfaces of the positive electrode current collector 12, but may be provided on either side of the positive electrode current collector 12. Then, at the time of manufacture, the positive electrode active material layer 13 is coated on the positive electrode current collector 12 so that the lithium ion secondary battery 1 does not contain excessive moisture, and then the coated positive electrode active material layer 13 is sufficiently dried. It is necessary to let

The positive electrode current collector 12 is a metal foil having a plurality of holes 12c penetrating in the Z direction (see FIGS. 4 and 5), a rectangular current collector 12a (see FIG. 4), and a current collector 12a. Are integrally formed with an electrode terminal connecting portion 12b protruding outward from one end (the left end of the upper side in the example of FIG. 4). The width in the Y-axis direction of the electrode terminal connecting portion 12b shown in FIGS. 1 and 4 can be changed as appropriate, and may be the same width as the current collecting portion 12a, for example. The current collector 12a has a plurality of holes 12c (see FIGS. 4 and 5), but the electrode terminal connector 12b has a plurality of holes similar to the holes 12c of the current collector 12a. It may not be formed and may be formed. Here, since the current collector 12a has a plurality of holes 12c, cations and anions contained in the electrolytic solution 40 can pass through the current collector 12a. The current collector 12a may not have a plurality of holes 12c, and the electrode terminal connecting portion 12b may not have a plurality of holes similar to the holes 12c. For the positive electrode current collector 12, for example, a metal foil made of aluminum, stainless steel, copper, or nickel can be used.

The positive electrode active material layer 13 includes a positive electrode active material capable of occluding and releasing lithium ions, a positive electrode binder that binds the positive electrode active material, and the positive electrode active material and the current collector 12a of the positive electrode current collector 12. including. As described above, the positive electrode active material layer 13 includes the positive electrode active material, and is configured to be able to occlude and release lithium ions. The positive electrode active material layer 13 may further contain other components such as a conductive additive for increasing the electrical conductivity of the positive electrode active material layer 13 and a thickener for facilitating the creation of the positive electrode plate 11. . As the conductive auxiliary agent, for example, ketjen black, acetylene black, graphite fine particles, and graphite fine fibers can be used. As the thickener, for example, carboxymethyl cellulose [CMC] can be used.

As the positive electrode active material, a positive electrode active material capable of occluding and releasing lithium ions, which is used in a conventional lithium ion secondary battery, can be used. Examples of the positive electrode active material include manganese dioxide (MnO ₂ ), iron oxide, copper oxide, nickel oxide, lithium manganese composite oxide (for example, Li _x Mn ₂ O ₄ or Li _x MnO ₂ ), and lithium nickel composite oxide (for example, Li _x NiO ₂ ), lithium cobalt composite oxide (Li _x CoO ₂ ), lithium nickel cobalt composite oxide (eg, LiNi _1-y Co _y O ₂ ), lithium nickel cobalt manganese composite oxide (NMC, ternary system, _{_{_{LiNi x Co y Mn 1-y}}} -z O 2), spinel type lithium-manganese-nickel composite oxide _{_{_{(Li x Mn 2-y Ni}}} y O 4), lithium polyanion compound _{_{_{(LiFePO 4, LiCoPO 4, LiVOPO}}} 4, LiVPO 4 F, LiMnPO ₄ , LiMn _1-x Fe _x PO ₄ , LiNiVO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , LiFeP ₂ O ₇ , Li ₃ Fe ₂ (PO ₄ ) ₃ , Li ₂ CoSiO ₄ , Li ₂ MnSiO ₄ , Li ₂ FeSiO ₄ , LiTePO ₄ Etc.), iron sulfate (Fe ₂ (SO ₄ ) ₃ ), vanadium oxide (for example, V ₂ O ₅ ) and the like. In addition, conductive polymer materials such as polyaniline and polypyrrole, disulfide-based polymer materials, organic materials such as sulfur (S) and carbon fluoride, and inorganic materials are also included. These may be used alone or in combination of two or more.

The positive electrode active material is preferably a material whose upper limit of the operating potential based on Li is less than a predetermined value. Here, in this specification, the operating potential on the basis of Li is an operating potential with respect to the reference potential (Li / Li ⁺ ). Examples of the predetermined value of the operating potential on the basis of Li include 5.0V, 4.0V, 3.8V, and 3.6V. In the case where the predetermined value is 5.0 V, as a positive electrode active material whose upper limit of the operating potential on the basis of Li is less than 5.0 V, for example, spinel type lithium manganese nickel composite oxide (Li _x Mn _2-y Ni _y O ₄ ). Further, in the case where this predetermined value is 4.0 V, as the positive electrode active material whose upper limit of the operating potential based on Li is less than 4.0 V, for example, lithium manganese composite oxide (for example, Li _x Mn ₂ O ₄ or Li _x MnO ₂ ). In addition, when the predetermined value is 3.8 V, as a positive electrode active material whose upper limit of the operating potential on the basis of Li is less than 3.8 V, for example, lithium cobalt composite oxide (Li _x CoO ₂ ) or lithium nickel cobalt manganese _Examples thereof include composite oxides (NMC, ternary system, LiNi _x Co _y Mn _1-yz O ₂ ). Further, in the case where this predetermined value is 3.6 V, as the positive electrode active material whose upper limit of the operating potential on the basis of Li is less than 3.6 V, for example, LiFePO ₄ can be mentioned.

In particular, when the positive electrode current collector 12 is formed of aluminum, it is preferable to select a positive electrode active material in which the upper limit of the operating potential on the basis of Li is less than a predetermined value. If the operating potential of the positive electrode active material based on Li is 4.2 V or higher, the positive electrode current collector 12 formed of aluminum is relatively easily corroded in the charge / discharge process. In this case, for example, LiFePO ₄ having an upper limit of the operating potential based on Li of less than 3.6 V can be selected as the positive electrode active material.

As the positive electrode binder, among the positive and negative electrode binders used in the conventional lithium ion secondary battery, a binder having a RED value (described later) based on the Hansen solubility parameter with respect to the electrolytic solution 40 is larger than 1. Here, as a binder of positive and negative electrodes of a conventional lithium ion secondary battery, for example, polyvinylidene fluoride [PVdF], polytetrafluoroethylene [PTFE], polyvinylpyrrolidone [PVP], polyvinyl chloride [PVC], polyethylene [PE ], Polypropylene [PP], ethylene-propylene copolymer, styrene butadiene rubber [SBR], acrylic resin, and polyacrylic acid.

Moreover, since the RED value based on the Hansen solubility parameter (HSP) with respect to the electrolytic solution 40 is larger than 1, the positive electrode binder exhibits poor solubility in the electrolytic solution 40. The Hansen solubility parameter was published by Charles M Hansen and is known as a solubility index indicating how much a certain substance is dissolved in a certain substance. For example, water and oil generally do not melt together because the “properties” of water and oil are different. As the “property” of the substance relating to the solubility, in the Hansen solubility parameter, three items of the dispersion term D, the polar term P, and the hydrogen bond term H are expressed numerically for each substance. Here, the dispersion term D is a value representing the magnitude of van der Waals force, the polar term P is a value representing the magnitude of the dipole moment, and the hydrogen bond term H is a value representing the magnitude of the hydrogen bond. is there. The basic idea is explained below. For this reason, the description of the case where the hydrogen bond term H is divided into donor property and acceptor property will be omitted.

Hansen solubility parameters (D, P, H) are plotted in a three-dimensional orthogonal coordinate system (Hansen space, HSP space) in order to study solubility. For example, the Hansen solubility parameter for each of the solution A and the solid B can be plotted on two coordinates (coordinate A and coordinate B) corresponding to the solution A and the solid B, respectively, in the Hansen space. And, the shorter the distance Ra (HSP distance, Ra) between the coordinates A and B, the solutions A and the solids B have the above-mentioned “properties”, so the solid B is more likely to dissolve in the solution A. it can. On the contrary, it can be considered that the longer the distance Ra is, the more difficult the solution A and the solid B are dissolved in the solution A because the solution A and the solid B have “properties” that are not similar to each other.

Further, a distance Ra that becomes a boundary between a substance that dissolves and a substance that does not dissolve in the solution A is defined as an interaction radius R0. Therefore, for the solution A and the solid B, when the distance Ra is smaller than the interaction radius R0 (Ra <R0), it can be considered that the solid B is dissolved in the solution A. On the other hand, when this Ra is larger than the interaction radius R0 (R0 <Ra), it can be considered that the solid B does not dissolve in the solution A. Further, a value obtained by dividing the distance Ra by the interaction radius R0 is defined as a RED value (= Ra / R0, Relative Energy Difference). Then, when the RED value is smaller than 1 (RED = Ra / R0 <1), Ra <R0, and it can be considered that the solid B is dissolved in the solution A. On the other hand, when the RED value is greater than 1 (RED = Ra / R0> 1), R0 <Ra, and it can be considered that the solid B does not dissolve in the solution A. In this way, it can be determined whether or not the solid B is dissolved in the solution A based on the RED values relating to the solution A and the solid B.

The electrolytic solution 40 corresponds to the solution A here, and the positive electrode binder corresponds to the solid B. Since the positive electrode binder has a RED value based on the Hansen solubility parameter with respect to the electrolytic solution 40 larger than 1, the positive electrode binder is hardly soluble in the electrolytic solution 40. Conversely, if the RED value based on the Hansen solubility parameter with respect to the electrolytic solution 40 is a positive electrode binder that is hardly soluble in the electrolytic solution 40 to the extent that the RED value is greater than 1, the positive electrode binder also has a RED value based on the Hansen solubility parameter. Can be considered greater than 1.

The Hansen solubility parameter and the interaction radius R0 can be calculated using the chemical structure and composition ratio of the components and experimental results. In that case, it can be obtained using software HSPiP developed by Hansen et al. (Hansen Solubility Parameters in Practice: Windows [registered trademark] software for efficiently handling HSP). This software HSPiP is available as of May 2, 2018 from http://www.hansen-solubility.com/. Also, the Hansen solubility parameters (D, P, H) can be calculated for a mixed solvent in which a plurality of solvents are mixed.

<2-2. Regarding Negative Electrode Plate 21 (FIGS. 1, 3, 6, and 7)>
The negative electrode plate 21 roughly has the same configuration as the positive electrode plate 11 described above, and includes a thin plate-like negative electrode current collector 22 and a negative electrode active material layer 23 coated on the negative electrode current collector 22. I have. The negative electrode active material layer 23 is coated on both surfaces of the negative electrode current collector 22, but the coated surface may be either one surface. Then, at the time of manufacture, the negative electrode active material layer 23 is coated on the negative electrode current collector 22 so that the lithium ion secondary battery 1 does not contain excessive moisture, and then the coated negative electrode active material layer 23 is sufficiently dried. It is necessary to let Further, as will be described later, the negative electrode active material layer 23 can be occluded (so-called pre-doped) with lithium ions Li ⁺ during manufacturing. However, the negative electrode active material layer 23 may not be pre-doped.

The negative electrode current collector 22 is a metal foil in which a plurality of holes 22c penetrating in the Z direction are formed (see FIGS. 6 and 7), like the positive electrode current collector 12 of the positive electrode plate 11 described above. The current collector 22a and the electrode terminal connection 22b that protrudes outward from one end of the current collector 22a (the right end on the upper side in the example of FIG. 6) are integrally formed. The current collector 22a has a plurality of holes 22c (see FIGS. 6 and 7), but the electrode terminal connection 22b has a plurality of holes similar to the holes 22c of the current collector 22a. It may not be formed and may be formed. Here, since the current collector 22a has a plurality of holes 22c, cations and anions contained in the electrolytic solution 40 can pass through the current collector 12a. The current collector 22a may not have a plurality of holes 22c, and the electrode terminal connection portion 22b may not have a plurality of holes similar to the holes 22c.

Further, as shown in FIG. 1, the positive electrode terminal connection portion 12 b and the electrode terminal connection portion 22 b of the negative electrode plate 21 are provided at positions spaced apart from each other in the surface direction of the negative electrode plate so as not to overlap. Yes. In addition, the width | variety of the Y-axis direction of the electrode terminal connection part 22b shown in FIG. 1 and FIG. 6 can be changed suitably, for example, is good also as the same width as the current collection part 22a. As the negative electrode current collector 22, a metal foil made of, for example, aluminum, stainless steel, or copper can be used in the same manner as the positive electrode current collector 12 of the positive electrode plate 11.

Similar to the positive electrode active material layer 13 described above, the negative electrode active material layer 23 includes a negative electrode active material capable of occluding and releasing lithium ions, binding of the negative electrode active material, and collection of the negative electrode active material and the negative electrode current collector 22. A negative electrode binder that binds the electric part 22a. And the negative electrode active material layer 23 is comprised so that occlusion and discharge | release of lithium ion are possible by providing a negative electrode active material. The negative electrode active material layer 23 may further contain other components such as a conductive additive for enhancing the electrical conductivity of the negative electrode active material layer 23 and a thickener for facilitating the creation of the negative electrode plate 21. . As the conductive auxiliary agent and the thickener, the same materials as those of the positive electrode plate 11 described above can be used. That is, for example, ketjen black, acetylene black, graphite fine particles, and graphite fine fibers can be used as the conductive assistant. As the thickener, for example, carboxymethyl cellulose [CMC] can be used.

As the negative electrode active material, a negative electrode active material capable of occluding and releasing lithium ions, which is used in conventional lithium ion secondary batteries, can be used. That is, as a negative electrode active material, for example, carbonaceous materials such as graphite, metal oxides such as tin oxide and silicon oxide, and phosphorus and boron are added to these materials for the purpose of improving the negative electrode characteristics. What has been done can be used. In addition, as the negative electrode active material, lithium titanate represented by the chemical formula Li _{4 + x} Ti ₅ O ₁₂ (0 ≦ x ≦ 3) and having a spinel structure may be used. Here, a material in which a part of Ti is substituted with an element such as Al or Mg may be used. In addition, as the negative electrode active material, silicon-based materials such as silicon, silicon alloy, SiO, and silicon composite material may be used. These may be used alone or in combination of two or more.

As the negative electrode binder, positive and negative electrode binders used in conventional lithium ion secondary batteries can be used. That is, as a binder of a conventional lithium ion secondary battery, for example, polyvinylidene fluoride [PVdF], polytetrafluoroethylene [PTFE], polyvinylpyrrolidone [PVP], polyvinyl chloride [PVC], polyethylene [PE], polypropylene [ PP], ethylene-propylene copolymer, styrene butadiene rubber [SBR], acrylic resin, and polyacrylic acid. Such a binder can be used as the negative electrode binder of the lithium ion secondary battery 1.

During production, the negative electrode active material layer 23 can also occlude lithium ions Li ⁺ . This process is called pre-doping.

There are roughly two types of pre-doping methods. That is, in one method, as shown in FIG. 1, a plurality of positive plates 11, a plurality of negative plates 21, and a plurality of separators 30 are laminated, and these are put together with the electrolyte 40 inside the laminate member 50 (see FIG. 2). In this method, the pre-doping is performed after being housed in the laminate member 50. The other is a method of pre-doping outside the laminate member 50 in which lithium ion Li ⁺ is previously occluded in the negative electrode active material before the negative electrode plate 21 is formed.

More specifically, the method of pre-doping inside the laminate member 50 includes two methods, a chemical method and an electrochemical method. In the method of pre-doping inside the laminate member 50, the plurality of positive plates 11, the plurality of negative plates 21, and the plurality of separators 30 are accommodated in the laminate member 50 (see FIG. 2) together with the electrolytic solution 40 and then pre-doped. . The chemical method is a method in which lithium metal is dissolved in the electrolytic solution 40 to form lithium ion Li ⁺ and the lithium ion Li ⁺ is occluded in the negative electrode active material. On the other hand, in the electrochemical method, a voltage is applied to the lithium metal and the negative electrode plate 21 to convert the lithium metal to lithium ion Li ⁺ and to store the lithium ion Li ⁺ in the negative electrode active material.

In both of these chemical methods and electrochemical methods, the current collector portion of the positive electrode current collector 12 of the positive electrode plate 11 so that the lithium ions Li ⁺ are easily diffused in the electrolytic solution 40. It is desirable that lithium ions Li ⁺ can pass through 12a (see FIG. 5) and the current collector 22a (see FIG. 7) of the negative electrode current collector 22 of the negative electrode plate 21. Therefore, when pre-doping is performed by a chemical method or an electrochemical method, the current collecting portion 12a of the positive electrode plate 11 has a plurality of holes 12c, and the current collecting portion 22a of the negative electrode plate 21 (see FIG. 7). ) Is preferably formed with a plurality of holes 22c.

Meanwhile, in the method for pre-doping at the outside of the laminate member 50, before creating the negative electrode plate 21, for occluding pre lithium ion Li ⁺ in the anode active material, a lithium ion Li ⁺ in the electrolyte 40 to pre-dope It is not necessary to diffuse. For this reason, when the method of pre-doping outside the laminate member 50 is used, the current collecting portion 12a of the positive electrode plate 11 does not need to have a plurality of holes 12c, and the current collecting portion 22a of the negative electrode plate 21 (FIG. 7). A plurality of holes 22c may not be formed.

It should be noted that the method of pre-doping inside the laminate member 50 and the method of pre-doping outside the laminate member 50 may be appropriately combined. That is, in addition to the method of pre-doping outside the laminate member 50, the plurality of positive electrode plates 11, the plurality of negative electrode plates 21, and the plurality of separators 30 are accommodated inside the laminate member 50 (see FIG. 2) together with the electrolytic solution 40. Thereafter, pre-doping may be performed by a chemical method or an electrochemical method, which is a method of pre-doping inside the laminate member 50.

<2-3. About Separator 30 (FIGS. 1 and 3)>
As shown in FIG. 1, the separator 30 is made of a porous material that separates the positive electrode plate 11 and the negative electrode plate 21 and can transmit the cation and anion of the electrolytic solution 40, and is formed in a rectangular sheet shape. Has been. The vertical and horizontal lengths of the separator 30 (see FIGS. 1 and 3) are the length of the current collector 12a of the positive electrode current collector 12 of the positive electrode plate 11 (see FIG. 4) and the negative electrode current collector of the negative electrode plate 21. It is set to be longer than the length of 22 current collectors 22a (see FIG. 6). As the separator 30, a separator used in a conventional lithium ion secondary battery can be used. For example, papermaking such as viscose rayon or natural cellulose, or nonwoven fabric such as polyethylene or polypropylene can be used.

<2-4. About Electrolyte 40>
The electrolytic solution 40 includes an organic solvent (nonaqueous solvent) and an imide-based lithium salt as an electrolyte. You may add an additive to the electrolyte solution 40 suitably. As the additive, for example, an additive that promotes the formation of a SEI film (Solid Electrolyte Interface film) on the negative electrode, such as vinylene carbonate [VC], fluoroethylene carbonate [FEC], or ethylene sulfite [ES], is used. Can do.

As the organic solvent, an organic solvent having a heat resistance of 85 ° C. can be used. For example, carbonate organic solvent, nitrile organic solvent, lactone organic solvent, ether organic solvent, alcohol organic solvent, ester organic solvent, amide organic solvent, sulfone organic solvent, ketone organic solvent, aromatic An organic solvent can be illustrated. A solvent in which one or two or more of these organic solvents are mixed at an appropriate composition ratio can be used as the organic solvent. Here, as the carbonate organic solvent, cyclic carbonates such as ethylene carbonate [EC], propylene carbonate [PC] and fluoroethylene carbonate [FEC], ethyl methyl carbonate [EMC], diethyl carbonate [DEC], dimethyl carbonate [DMC] and the like The chain carbonate can be illustrated. Here, it is preferable that the organic solvent does not contain dimethyl carbonate [DMC] which is a kind of chain carbonate. Dimethyl carbonate [DMC] rarely causes heat resistance deterioration depending on the use environment.

Examples of nitrile organic solvents include acetonitrile, acrylonitrile, adiponitrile, valeronitrile, and isobutyronitrile. Examples of the lactone organic solvent include γ-butyrolactone and γ-valerolactone. Examples of ether organic solvents include cyclic ethers such as tetrahydrofuran and dioxane, and chain ethers such as 1,2-dimethoxyethane, dimethyl ether, and triglyme. Examples of the alcohol organic solvent include ethyl alcohol and ethylene glycol. Examples of the ester organic solvent include phosphate esters such as methyl acetate, propyl acetate and trimethyl phosphate, sulfate esters such as dimethyl sulfate, and sulfite esters such as dimethyl sulfite. Examples of the amide organic solvent include N-methyl-2-pyrrolidone and ethylenediamine. Examples of the sulfone-based organic solvent include chain sulfones such as dimethyl sulfone and cyclic sulfones such as 3-sulfolene. Examples of the ketone organic solvent include methyl ethyl ketone, and toluene as the aromatic organic solvent. The above-mentioned various organic solvents excluding the carbonate-based organic solvent are preferably used by mixing with cyclic carbonate, and in particular, mixed with ethylene carbonate [EC] capable of forming an SEI film (Solid Electrolyte Interface film) on the negative electrode. It is preferable to use it. In this case, the positive electrode binder and the negative electrode binder described above are preferably polyacrylic acid. The organic solvent preferably contains ethyl methyl carbonate [EMC] and diethyl carbonate [DEC].

As the electrolyte, an imide-based lithium salt (a lithium salt having —SO ₂ —N—SO ₂ — in a partial structure) can be used. Here, as the imide-based lithium salt, lithium bis (fluorosulfonyl) imide [LiN (FSO ₂ ) ₂ , LiFSI], lithium bis (trifluoromethanesulfonyl) imide [LiN (SO ₂ CF ₃ ) ₂ , LiTFSI], lithium bis (Pentafluoroethanesulfonyl) imide [LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiBETI] can be exemplified. As an electrolyte, these imide-based lithium salts may be used alone or in combination of two or more. These imide-based lithium salts have a heat resistance of 85 ° C. Even in the imide lithium salt described above, an imide lithium salt having no trifluoromethane group (—CF ₃ ), pentafluoroethane group (—CF ₂ CF ₃ ), or pentafluorophenyl group (—C ₆ F ₅ ) (for example, , Lithium bis (fluorosulfonyl) imide [LiN (FSO ₂ ) ₂ , LiFSI]) is preferable in the following points. That is, the positive electrode binder and the negative electrode binder tend to have a RED value greater than 1 based on the Hansen solubility parameter. Further, even at high and low temperatures, the ionic conductivity of the electrolytic solution 40 hardly decreases, and the electrolytic solution 40 is stabilized.

The concentration of the electrolyte in the electrolytic solution 40 is preferably 0.5 to 10.0 mol / L. From the viewpoint of an appropriate viscosity of the electrolytic solution 40 and ion conductivity, the concentration of the electrolyte in the electrolytic solution 40 is more preferably 0.5 to 2.0 mol / L. When the concentration of the electrolyte is less than 0.5 mol / L, it is not preferable because the ionic conductivity of the electrolytic solution 40 is too low due to a decrease in the concentration of ions from which the electrolyte is dissociated. Moreover, it is not preferable that the concentration of the electrolyte is higher than 10.0 mol / L because the ionic conductivity of the electrolytic solution 40 is too low due to an increase in the viscosity of the electrolytic solution 40. Moreover, when using the electrolyte solution 40 containing the above organic solvent and electrolyte, it is preferable that the positive electrode binder and negative electrode binder which were mentioned above are polyacrylic acid.

<2-5. Regarding Laminate Member 50 (FIGS. 1 and 3)>
As shown in FIG. 3, the laminate member 50 includes a core material sheet 51, an outer sheet 52, and an inner sheet 53. The outer sheet 52 is bonded to the outer surface of the core material sheet 51, and the inner sheet 53 is bonded to the inner surface of the core material sheet 51. For example, the core material sheet 51 can be an aluminum foil, the outer sheet 52 can be a resin sheet such as a nylon pet film, and the inner sheet 53 can be a resin sheet such as polypropylene.

<3. Regarding the charging / discharging process of the lithium ion secondary battery 1 (FIGS. 3, 8, and 9)>
FIG. 8 schematically shows the positional relationship (see FIG. 1) among the positive electrode plate 11 of the positive electrode 10, the negative electrode plate 21 of the negative electrode 20, the separator 30, and the electrolytic solution 40 of the lithium ion secondary battery 1. As shown in FIG. 8, the lithium ion secondary battery 1 has a configuration in which a positive electrode plate 11 and a negative electrode plate 21 face each other with a separator 30 interposed therebetween. As described above, both the positive electrode active material layer 13 and the negative electrode active material layer 23 are configured to be able to occlude and release lithium ions Li ⁺ .

In the lithium ion secondary battery 1 during charging, ⁺ lithium ion Li occluded in the positive electrode active material layer 13 are released into the electrolyte solution 40, and this lithium ions in the same amount of electrolyte solution 40 Li ⁺ Is occluded in the negative electrode active material layer 23 (see FIGS. 8 and 9). On the contrary, in the lithium ion secondary battery 1 at the time of discharge, lithium ion Li ⁺ occluded in the negative electrode active material layer 23 is released into the electrolytic solution 40 and the same amount of the electrolytic solution 40 in the electrolytic solution 40 is discharged. Lithium ions Li ⁺ are occluded in the positive electrode active material layer 13. As if charging and discharging, lithium ions Li ⁺ move between the positive electrode active material layer 13 and the negative electrode active material layer 23 via the electrolytic solution 40 (see FIGS. 8 and 9). That is, lithium ion Li ⁺ moves from the positive electrode active material layer 13 to the negative electrode active material layer 23 during charging, and moves from the negative electrode active material layer 23 to the positive electrode active material layer 13 during discharging via the electrolytic solution 40 ( FIG. 8 and FIG. 9). The maximum amount of lithium ions Li ⁺ occluded in the negative electrode active material layer 23 is during full charge during the charge / discharge process. More discharge process in lithium ion Li ⁺ As is by being insertion and extraction, the amount of occluded lithium ion Li ⁺ in the positive electrode active material layer 13 and the negative electrode active material layer 23 is increased or decreased. In this specification, the amount of lithium ion Li ⁺ may be a value proportional to the number of atoms of lithium ion Li ^+, for example, be a mol number.

<4. About pre-dope>
If the anode active material layer 23 is a lithium ion Li ⁺ is pre-doped, the amount of lithium ion Li ⁺ to the pre-doping can also be an upper limit value as described below.

When the amount Pt of all lithium ions Li ⁺ occluded in the positive electrode active material layer 13 at the time of full discharge is the same as the amount Pt of lithium ions Li ⁺ , Occluded (see FIG. 9). Here, the amount N of lithium ions Li ⁺ occluded in the negative electrode active material layer 23 at the time of full charge is the amount Pt of lithium ions Li ⁺ transferred from the positive electrode active material layer 13 to the negative electrode active material layer 23 and the pre-doped negative electrode This is the sum Np + Pt with the amount Np of lithium ions Li ⁺ occluded in the active material layer 23 (see FIG. 9). Here, Pt is the amount of all the lithium ion Li ⁺ that was stored in the positive electrode active material layer 13 at the time of full discharge, the charge-discharge process in lithium ion Li ⁺ is also changed to inactive compounds, It is supplemented by lithium ions Li ⁺ occluded in the negative electrode active material layer 23 by pre-doping. For this reason, Pt is the same amount as the amount of lithium ion Li ⁺ occluded by the positive electrode active material layer 13 before the initial charge (that is, the amount of lithium ion Li ⁺ occluded by the positive electrode active material before manufacture). become.

The amount of lithium ion Li ^{+ that} can be occluded by the negative electrode active material layer 23 before pre-doping is Nt (see FIG. 9). As described above, at the time of full charge, the amount N of lithium ions Li ⁺ occluded in the negative electrode active material layer 23 is Np + Pt (that is, lithium ion Li ⁺ occluded in the negative electrode active material layer 23 by pre-doping). This is the sum of the amount Np and the amount Pt of lithium ion Li ⁺ that moves from the positive electrode active material layer 13 to the negative electrode active material layer 23 from full discharge to full charge (see FIG. 9).

If the amount N (= Np + Pt) of lithium ions Li ⁺ stored in the negative electrode active material layer 23 at the time of this full charge is the amount Nt of lithium ions Li ^{+ that} can be stored in the negative electrode active material layer 23 before pre-doping. If it exceeds (that is, Np + Pt> Nt), the excess (that is, Np + Pt−Nt) cannot be fully occluded in the negative electrode active material layer 23, so that it may be deposited as lithium metal in the electrolytic solution 40. Therefore, an upper limit Npmax can be provided for the amount Np of lithium ions Li ⁺ stored in the negative electrode active material layer 23 by pre-doping, and Npmax = Nt−Pt. As a result, Np + Pt ≦ Nt is satisfied, and the negative electrode active material layer 23 can always occlude the lithium ion Li ⁺ released from the positive electrode active material layer 13, and the precipitation of lithium ion Li ⁺ can be suppressed. Note that Npmax, Nt, and Pt can be represented by the number of moles.

Here, as described above, Pt is the amount of lithium ion Li ⁺ occluded by the positive electrode active material layer 13 before the initial charge (that is, lithium ion Li ⁺ occluded by the positive electrode active material before production). Amount). Therefore, the upper limit value Npmax of the amount of lithium ion Li ⁺ occluded in the negative electrode active material layer 23 by pre-doping is determined from the amount Nt of lithium ion Li ⁺ occluded by the negative electrode active material layer 23 before pre-doping before the positive electrode active This is the amount obtained by subtracting the amount (Pt) of lithium ions Li ⁺ stored by the substance. Nt and Pt can be calculated from, for example, theoretical values of the positive electrode active material and the negative electrode active material. In addition, in the experiment, the amount of the negative electrode active material before pre-doping can occlude lithium ions Li ⁺ and the positive electrode active material can be calculated. It is also possible to measure the amount of lithium ion Li ⁺ occluded by the substance and calculate from the measured value.

As described above, the upper limit value Npmax of the amount Np of lithium ions Li ⁺ stored in the negative electrode active material layer 23 by pre-doping is Npmax = Nt−Pt. Therefore, Npmax varies depending on the value of Nt and the value of Pt (see FIG. 9). Roughly speaking, Npmax increases as the value of Nt increases, and Npmax decreases as the value of Pt increases (see FIG. 9). For example, when Nt is twice Pt (that is, Nt = 2 · Pt), Npmax is equal to Pt (see FIG. 9). For example, when Nt is 3 times Pt (that is, Nt = 3 · Pt), Npmax is equal to 2 times Pt (ie, 2 · Pt) (see FIG. 9). Thus, Npmax varies depending on the value of Nt and the value of Pt (see FIG. 9). That is, the upper limit of the lithium ion Li ⁺ in an amount Np to be occluded in the negative electrode active material layer 23 in the pre-doping Npmax, the pre-doped prior to the negative electrode active material layer 23 is storable lithium ion Li ⁺ in an amount Nt, and before charge and discharge It varies depending on the amount Pt of lithium ion Li ⁺ stored in the positive electrode active material layer 13.

In addition, as described above, the upper limit Npmax is set for the amount Np of lithium ions Li ⁺ to be occluded in the negative electrode active material layer 23 by pre-doping, and Npmax = Nt−Pt can be rephrased as follows. . The maximum amount of lithium ions Li ⁺ occluded in the negative electrode active material layer 23 is during full charge during the charge / discharge process. As described above, the amount N of lithium ions Li ⁺ stored in the negative electrode active material layer 23 at the time of full charge is the amount of all lithium ions Li ⁺ stored in the positive electrode active material layer 13 at the time of full discharge. It is the sum Np + Pt (that is, N = Np + Pt) of Pt and the amount Np of lithium ions Li ⁺ stored in the negative electrode active material layer 23 by pre-doping (see FIG. 9). When the amount Np of lithium ions Li ⁺ stored in the negative electrode active material layer 23 by pre-doping is the upper limit Npmax (Np = Npmax = Nt−Pt), the lithium ion Li ⁺ stored in the negative electrode active material layer 23 at full charge The quantity N (= Np + Pt) is N = Np + Pt = Nt−Pt + Pt = Nt.

Here, the amount N of lithium ion Li ⁺ occluded in the negative electrode active material layer 23 during full charge (see FIG. 9) is the amount Nt of lithium ion Li ⁺ occluded in the negative electrode active material layer 23 before pre-doping. When N is expressed as% when N is 100%, N is 100% when N = Nt. As described above, when the amount Np of lithium ions Li ⁺ stored in the negative electrode active material layer 23 by pre-doping is the upper limit Npmax (Np = Npmax = Nt−Pt), it is stored in the negative electrode active material layer 23 at full charge. The amount N (= Np + Pt) of lithium ions Li ⁺ is N = Nt, and therefore N = 100%. As described above, the maximum amount of lithium ions Li ⁺ stored in the negative electrode active material layer 23 is the amount N (= Np + Pt) when fully charged. Therefore, when the amount Np of lithium ions Li ⁺ stored in the negative electrode active material layer 23 by pre-doping is the upper limit Npmax (Np = Npmax = Nt−Pt), the amount of lithium ions Li ⁺ stored in the negative electrode active material layer 23 is N = 100% at the maximum, and does not exceed 100%. That is, the negative electrode active material layer is 23 to an upper limit Npmax (= Nt-Pt) in a lithium ion Li ⁺ in an amount Np occluding, the amount of lithium ion Li ⁺ to be occluded in the negative electrode active material layer 23 in the pre-doping, the charge During the discharge process, the negative electrode active material layer 23 before pre-doping is always adjusted to 100% or less of the amount Nt of lithium ion Li ^{+ that} can be occluded. In addition, the dope rate of the negative electrode active material in a negative electrode active material layer is represented as follows.
Doping rate (%) = N / Nt × 100
N: Amount of lithium ion (mol) stored in the negative electrode active material (negative electrode active material layer) at full charge
Nt: Amount of lithium ion (mol) that can be occluded by the negative electrode active material (negative electrode active material layer) before pre-doping

[Other embodiments]
As another embodiment, for example, the lithium ion secondary battery described above is a stacked lithium ion secondary battery in which the positive electrode plate 11, the negative electrode plate 21, and the separator 30 are stacked. A wound lithium ion secondary battery in which a long negative electrode and a long separator are wound can be obtained.

The lithium ion secondary battery may be a lithium polymer secondary battery.

<< About heat resistance of lithium ion secondary battery >>
With the configuration described above, the lithium ion secondary battery 1 has a heat resistance of 85 ° C.

In addition, when the conventional lithium ion secondary battery is kept at about 85 ° C., the lithium ion Li ⁺ gradually changes into an inactive compound, so that the amount of lithium ion Li ^{+ that} can participate in charging and discharging is reduced. The charge / discharge capacity decreases. Such a lithium ion secondary battery has a low charge / discharge capacity at high temperatures, that is, low temperature durability. In the present specification, the high temperature durability means that the charge / discharge capacity of the lithium ion secondary battery is maintained at a sufficient amount even when the lithium ion secondary battery is kept at a high temperature.

In contrast, the lithium-ion secondary battery 1, when the negative electrode active material a lithium ion Li ⁺ is pre-doped, the lithium ion Li ⁺ is occluded in the negative electrode active substance in. For this reason, even if lithium ion Li ⁺ required for charge / discharge changes to an inactive compound, the lithium ion Li ⁺ occluded in the negative electrode active material by pre-doping compensates for the change, so that the lithium ion secondary battery 1 The decrease in charge / discharge capacity can be suppressed. For this reason, the lithium ion secondary battery 1 not only has a heat resistance of 85 ° C. or higher, but also has a high temperature durability.

Also, when a lithium ion secondary battery is used in a high temperature environment for a long time, the discharge capacity is reduced and the internal resistance is increased. However, as the doping rate increases, the rate of decrease in discharge capacity and the rate of increase in internal resistance tend to decrease. Therefore, the doping rate is preferably 50% to 100%, more preferably 80% to 100%, and still more preferably 90% to 100%.

The lithium ion secondary battery of the present disclosure is not limited to the structure, configuration, appearance, shape, and the like described in the above embodiment, and various modifications and additions can be made by understanding the above embodiment. Can be deleted.

Hereinafter, the technology of the present disclosure will be described more specifically with reference to test examples, but the technology of the present disclosure is not limited to these ranges.

<Creation of lithium ion secondary battery>
[Creation of positive electrode]
First, 88 parts by mass of LiFePO ₄ as a positive electrode active material, 6 parts by mass of polyacrylic acid (sodium neutralized salt of polyacrylic acid) as a binder, 15 parts by mass of carbon black as a conductive additive, carboxymethylcellulose as a thickener A positive electrode slurry containing a positive electrode active material was prepared by using 0.3 part by mass of water and 217 parts by mass of water as a solvent.

The positive electrode slurry was prepared by the following procedure.
(1) A pre-slurry was prepared by mixing all materials and water with a mixer a (Shinky Co., Ltd. Awatori Nertaro ARE-310).
(2) The pre-slurry obtained in (1) was further mixed with a mixer b (Filmix 40-L manufactured by PRIMIX Co., Ltd.) to prepare an intermediate slurry.
(3) The intermediate slurry obtained in (2) was mixed again with the mixer a to prepare a positive electrode slurry.

Next, an aluminum foil (porous foil) having a thickness of 15 μm was used as the current collector foil, and each positive electrode slurry was applied to the current collector foil and dried to prepare a positive electrode. The coating amount of the positive electrode slurry was adjusted so that the mass of the activated carbon after drying was 3 mg / cm ² . A blade coater or a die coater was used for coating the positive electrode slurry on the current collector foil.

[Creation of negative electrode]
98 parts by mass of graphite as a negative electrode active material, 1.4 parts by mass of styrene butadiene rubber (SBR) as a binder, 0.7 parts by mass of carboxymethyl cellulose as a thickener, and 96 parts by mass of water as a solvent are mixed. A negative electrode slurry was prepared by the following procedure.
(1) A material excluding the binder and water were mixed in a mixer a to prepare a pre-slurry.
(2) The pre-slurry obtained in (1) was further mixed with a mixer b to prepare an intermediate slurry.
(3) A binder was added to the intermediate slurry obtained in (2) and mixed by a mixer a to prepare a negative electrode slurry.

Next, a copper foil (porous foil) having a thickness of 10 μm was used as the current collector foil, and the negative electrode slurry was applied to the current collector foil and dried to prepare a negative electrode. The coating amount of the negative electrode slurry was adjusted so that the mass of graphite after drying was 3 mg / cm ² . A blade coater was used for coating the negative electrode slurry on the current collector foil.

[Preparation of electrolyte]
In a mixed solvent containing 20.0 vol% of ethylene carbonate [EC], 10.0 vol% of propylene carbonate [PC], 46.7 vol% of ethyl methyl carbonate [EMC], and 23.3 vol% of diethyl carbonate [DEC], Lithium bis (fluorosulfonyl) imide [LiN (FSO ₂ ) ₂ , LiFSI], which is an imide-based lithium salt, was added as an electrolyte. The electrolytic solution contains 1.0 mol / L of LiFSI.

[Assembly of lithium ion secondary battery]
A lithium ion secondary battery was produced by the following procedure.
(1) The positive electrode and the negative electrode are each punched out into a rectangle of 60 mm × 40 mm, and the current collecting tab is formed by stripping off the 20 mm × 40 mm region of the coating on the long side, leaving the 40 mm × 40 mm coating film. Attached.
(2) A laminate was prepared by making the coating portions of the positive electrode and the negative electrode face each other with a cellulose separator having a thickness of 20 μm interposed therebetween.
(3) The laminate produced in (2) and a lithium metal foil for lithium pre-doping were encapsulated in an aluminum laminate foil, and an electrolyte was injected and sealed to produce a lithium ion secondary battery.
When the RED value with respect to the electrolyte solution of the positive electrode binder was calculated, it was confirmed that it was larger than 1.

This lithium ion secondary battery was pre-doped to prepare a lithium ion secondary battery of Test Example 1. The number of moles of pre-doped lithium ion Li ⁺ is, according to literature values, the amount of lithium ion Li ⁺ occluded in the positive electrode active material of the positive electrode active material layer is 0.0010 mol, and the lithium ion that can be occluded by the negative electrode active material layer The amount of Li ⁺ is 0.0030 mol. In the pre-doping, 0.0102 g of metallic lithium was dissolved in the electrolytic solution, so that 0.0015 mol of lithium ion Li ⁺ was occluded in the negative electrode active material layer. In addition, the lithium ion secondary battery of Test Example 2 is different from the lithium ion secondary battery of Test Example 1 only in that it is not pre-doped.

The following tests were performed using the lithium ion secondary batteries of Test Examples 1 and 2.

[Measurement of battery performance]
The internal resistance and discharge capacity of the lithium ion secondary battery were measured at room temperature (25 ° C.) with a cutoff voltage of 3.0 to 3.5 V, a measurement current of 5 mA, and 0.2 C. Here, the internal resistance was measured by measuring the internal resistance (mΩ) at 0 to 0.1 sec by the DC-IR method.

[Durability test (85 ° C float test)]
The lithium ion secondary battery in a state where the voltage was maintained at 3.5 by connecting an external power source was left in a constant temperature bath at 85 ° C. The standing time corresponds to 85 ° C. and 3.5 V float time. After the elapse of a predetermined time, the lithium ion secondary battery was taken out from the thermostat and returned to room temperature, and then the battery performance was measured.

[Measurement result]
The change with time of the internal resistance (mΩ) of the lithium ion secondary battery of Test Example 1 is shown in FIG. 10, and the change with time of the discharge capacity (mAh) is shown in FIG. Moreover, the time-dependent change of the internal resistance (mΩ) of the lithium ion secondary battery of Test Example 2 is shown in FIG. 12, and the time-dependent change of the discharge capacity (mAh) is shown in FIG.

As shown in FIG. 10, the internal resistance (mΩ) of the lithium ion secondary battery of Test Example 1 did not increase significantly even after 400 hours. In addition, as shown in FIG. 11, the discharge capacity (mAh) of the lithium ion secondary battery of Test Example 1 was not significantly reduced even after 400 hours. This confirmed that the lithium ion secondary battery of Test Example 1 had heat resistance at 85 ° C. and high temperature durability.

In the lithium ion secondary battery of Test Example 2, as shown in FIG. 12, the internal resistance (mΩ) did not increase significantly after 400 hours. However, as shown in FIG. 13, the discharge capacity (mAh) of the lithium ion secondary battery of Test Example 2 gradually decreased with time. Thereby, it became clear that the lithium ion secondary battery of Test Example 2 has heat resistance at 85 ° C. and some high temperature durability.

<Examination of influence by doping rate>
Next, the influence of the lithium ion doping rate was examined. The lithium ion secondary batteries of Test Examples 3 to 5 were prepared by the above-described preparation method, and the following tests were performed. However, the doping rate of Test Example 3 was adjusted to 80%, the doping rate of Test Example 4 was 90%, and the doping rate of Test Example 5 was adjusted to 100%.

[Float test]
The internal resistance and discharge capacity of the lithium ion secondary battery were measured at room temperature (25 ° C.) with a cutoff voltage of 3.0 to 3.5 V, a measurement current of 5 mA, and 0.2 C. The internal resistance was measured by the DC-IR method at 0 to 0.1 sec. Subsequently, the lithium ion secondary battery in a state where the voltage was maintained at 3.8 V by connecting an external power source was left in a constant temperature bath at 85 ° C. After the elapse of a predetermined time, the lithium ion secondary battery was taken out from the thermostat and returned to room temperature, and then the battery performance was measured. FIG. 14 shows the increase rate of the internal resistance in Test Examples 3 to 5. FIG. 15 shows changes in the discharge capacity of Test Examples 3 to 5.

As shown in FIG. 14, in the lithium ion secondary batteries of Test Examples 3 to 5, the rate of increase in internal resistance was less than 50% even after 1600 hours had elapsed. Further, as shown in FIG. 15, the capacity retention of the lithium ion secondary batteries of Test Examples 3 to 5 was 85% or more even after 1600 hours had elapsed. From these facts, it became clear that the lithium ion secondary batteries of Test Examples 3 to 5 have heat resistance at 85 ° C. and high temperature durability. Test Examples 4 and 5 were superior to Test Example 3 in the increase rate of internal resistance and the change in discharge capacity. From this, it became clear that the doping rate is preferably 90 to 100% rather than 80%.

<Examination of influence of RED value>
Next, the influence of the RED value was examined. In addition, since only the positive electrode and the electrolytic solution were changed from the above-described method in the production of the lithium ion secondary battery, only these changed points will be described below, and redundant description will be omitted.

[Creation of positive electrode]
LiFePO ₄ as a positive electrode active material, polyacrylic acid (sodium neutralized salt of polyacrylic acid) as a binder, acrylic acid ester or styrene-butadiene rubber [SBR], acetylene black as a conductive aid, carboxymethylcellulose [CMC as a thickener The positive electrode slurries A to C containing the positive electrode active material having the composition shown in Table 1 were prepared by the above-described method using water as a solvent. In Table 1, “part” represents part by mass, and “%” represents mass%.

[Electrolyte adjustment]
As a solvent, a mixed solvent of 30% by volume of ethylene carbonate (EC), 30% by volume of dimethyl carbonate (DMC) and 40% by volume of ethyl methyl carbonate (EMC) was used, and 1 mol / L of lithium bis (fluorosulfonylimide) (LiFSI) was used as the mixed solvent. The electrolyte solution I was adjusted by adding. Further, lithium hexafluorophosphate (LiPF ₆ ) was added to the mixed solvent to prepare an electrolytic solution P. As a solvent, a mixed solvent of ethylene carbonate (EC) 30 vol%, ethyl methyl carbonate (EMC) 46.7 vol%, diethyl carbonate (DEC) 23.3 vol%, and propylene carbonate (PC) 10 vol% was used. Electrolytic solution I2 was prepared by adding 1 mol / L of bis (fluorosulfonylimide) (LiFSI).

[Assembly of lithium ion secondary battery]
The lithium ion secondary batteries of Test Examples 6 to 10 were prepared with combinations of positive electrodes and electrolyte solutions shown in Table 2. Table 2 also shows the RED value in each combination.

[Measurement of initial performance]
The lithium ion secondary batteries of Test Examples 6 to 10 were charged / discharged and aged. Thereafter, the internal resistance and discharge capacity of each lithium ion secondary battery were measured at room temperature (25 ° C.) with a cut-off voltage of 2.2 to 3.8 V and a measurement current of 10 C, and the results were used as initial performance.

[Durability test (85 ° C float test)]
The lithium ion secondary batteries of Test Examples 6 to 10 were left in a constant temperature bath at 85 ° C. with an external power supply connected and the voltage maintained at 3.8V. The standing time corresponds to 85 ° C. and 3.8 V float time. After elapse of a predetermined time, the lithium ion secondary battery is taken out from the thermostatic chamber, returned to room temperature, and then measured for internal resistance and discharge capacity under the same conditions as the above initial performance measurement. The percentage of the discharge capacity at the time) and the internal resistance increase rate (internal resistance increase rate from the initial performance) were calculated. The results are shown in Table 3.

As shown in Table 3, when left in a high temperature environment of 85 ° C., in Example 10 using an electrolytic solution containing lithium fluorophosphate that is not an imide-based lithium salt as an electrolyte, the capacity retention rate is halved in a short time. On the other hand, in Test Examples 6 to 9 using an electrolyte containing an imide lithium salt as the electrolyte, the capacity retention rate was kept high for a long time. However, even when an electrolytic solution containing an imide-based lithium salt is used as the electrolyte, it has become clear that there is a difference in the rate of increase in internal resistance depending on the configuration of the binder of the positive electrode. Therefore, when comparing the RED value (see Table 2) with respect to the electrolyte of the polymer constituting the positive electrode binder, in Test Example 8 using an acrylate ester having a RED value of 1 or less and Test Example 9 using SBR, It was found that the rate of increase in internal resistance was high. On the other hand, in Test Examples 6 and 7, an electrolytic solution containing an imide lithium salt is used as an electrolyte, and polyacrylic acid having a RED value greater than 1 is used as a polymer constituting the positive electrode binder. . In this case, it is clear that the polymer constituting the positive electrode binder is not easily dissolved in the electrolyte, and the capacity retention rate is kept high even when left in a high temperature environment of 85 ° C., and the internal resistance increase rate can be kept small. became.

Claims

A lithium ion secondary battery,
A positive electrode active material capable of occluding and releasing lithium ions;
A positive electrode binder for binding the positive electrode active material;
A negative electrode active material capable of occluding and releasing lithium ions;
A negative electrode binder for binding the negative electrode active material;
An electrolyte solution containing an organic solvent and an imide-based lithium salt,
The positive electrode binder has a RED value greater than 1 based on a Hansen solubility parameter for the electrolyte;
Lithium ion secondary battery.
The lithium ion secondary battery according to claim 1,
In the positive electrode active material, the upper limit of the operating potential on the basis of Li is less than a predetermined value.
Lithium ion secondary battery.
The lithium ion secondary battery according to claim 1 or 2,
The organic solvent does not contain dimethyl carbonate,
Lithium ion secondary battery.
The lithium ion secondary battery according to any one of claims 1 to 3,
At least one of the positive electrode binder and the negative electrode binder is polyacrylic acid,
Lithium ion secondary battery.