Lithium Ion Secondary Battery and Method for Manufacturing the Same

Abstract

To provide a lithium ion secondary battery having both high capacity and high durability.SOLUTION: The present invention provides a lithium ion secondary battery including a positive electrode, a negative electrode, and an electrolyte. The positive electrode contains, as a positive electrode active material, a lithium transition metal composite oxide, an initial charge / discharge efficiency being 80% or more. The negative electrode contains, as a negative electrode active material, graphite and a Si-containing material, the content of the Si-containing material in the negative electrode active material being 4 mass% or more and 15 mass% or less. A ratio (CA / CC) of an initial charge capacity CA of the negative electrode to an initial charge capacity CC of the positive electrode is 1 or more and 1.25 or less. The ratio (IC / IA) of an initial irreversible capacity IC of the positive electrode to an initial irreversible capacity IA of the negative electrode is 1 or more and 2 or less.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a lithium ion secondary battery and a method for manufacturing the same.

Background Art

[0002] In recent years, for the purpose of increasing the capacity, etc., a lithium ion secondary battery containing a lithium-excess transition metal composite oxide in the positive electrode and a Si (silicon)-containing material in the negative electrode is known (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As a result of intensive studies by the present inventors, in the configuration of Patent Document 1, for example, the expansion and contraction of the Si-containing material may become large or Li deposition may occur on the negative electrode, and there may be insufficient durability in consideration of long-term use. That is, in a lithium ion secondary battery containing a lithium-excess transition metal composite oxide in the positive electrode and a Si-containing material in the negative electrode, there is still a problem in achieving both high capacity and high durability.

[0005] The present invention has been made in view of the above circumstances, and its main object is to provide a lithium ion secondary battery that includes a lithium-excess transition metal composite oxide in the positive electrode, a Si-containing material in the negative electrode, and has both high capacity and high durability.

Means for Solving the Problems

[0006] The present invention provides a lithium-ion secondary battery including a positive electrode, a negative electrode, and an electrolyte. The positive electrode contains, as a positive electrode active material, a lithium transition metal composite oxide in which the molar ratio (Li / Me) of lithium element (Li) to transition metal element (Me) is 1.3 or more, and the initial charge-discharge efficiency when charged and discharged with metallic Li as the counter electrode is 80% or more. The negative electrode contains graphite and an Si-containing material as a negative electrode active material, and the content of the Si-containing material in the negative electrode active material is 4% by mass or more and 15% by mass or less. The ratio (C C of the initial charge capacity C A of the negative electrode to the initial charge capacity C A of the positive electrode (C C / C A ) is 1 or more and 1.25 or less. The ratio (I C of the initial irreversible capacity I C of the negative electrode to the initial irreversible capacity I A of the positive electrode (I

[0007] / I ) is 1 or more and 2 or less.

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

[0009] ​​​​Hereinafter, with reference to the drawings, some preferred embodiments of the technology disclosed herein will be described. Matters other than those specifically mentioned in this specification and necessary for the implementation of the present invention (for example, the general configuration and manufacturing process of a lithium-ion secondary battery that does not characterize the present invention) can be grasped as matters of design by those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and the common general knowledge in the relevant field. In this specification, the notation "A to B" indicating a range includes the meaning of "greater than A" and "less than B" in addition to the meaning of "A or more and B or less".

[0010] <Battery 100> FIG. 1 is a perspective view of a lithium-ion secondary battery (hereinafter, also simply referred to as a battery) 100. FIG. 2 is a schematic longitudinal sectional view taken along line II-II of FIG. 1. In the following description, the reference signs L, R, F, Rr, U, and D in the drawings represent left, right, front, rear, top, and bottom, respectively, and the reference signs X, Y, and Z in the drawings represent the short-side direction, the long-side direction orthogonal to the short-side direction, and the up-down direction orthogonal to the short-side direction and the long-side direction of the battery 100, respectively. However, these are merely directions for convenience of explanation and do not limit the installation form of the battery 100 in any way.

[0011] As shown in FIG. 2, the battery 100 includes a battery case 10, an electrode body 20, a positive electrode terminal 30, a negative electrode terminal 40, a positive electrode current collector 50, a negative electrode current collector 60, and an electrolyte (not shown).

[0012] The battery case 10 is a housing that houses the electrode body 20 and the electrolyte. As shown in FIG. 1, the battery case 10 here has an outer shape that is flat, bottomed, and rectangular parallelepiped (angular). However, in other embodiments, the battery case 10 may be, for example, in the shape of a bag made of a laminated film. As shown in FIG. 2, the battery case 10 here includes an exterior body 12 having an opening 12h and a sealing plate (lid body) 14 that closes the opening 12h. The sealing plate 14 is attached to the exterior body 12 so as to close the opening 12h of the exterior body 12. The battery case 10 is integrated by joining (for example, welding) the sealing plate 14 to the peripheral edge of the opening 12h of the exterior body 12. The battery case 10 is hermetically sealed.

[0013] As shown in FIG. 2, the sealing plate 14 is provided with two terminal lead-out holes 18 and 19. The terminal lead-out holes 18 and 19 are respectively formed at both ends in the long side direction Y of the sealing plate 14 (the left end and the right end in FIG. 2). The terminal lead-out holes 18 and 19 penetrate the sealing plate 14 in the thickness direction (vertical direction Z).

[0014] As shown in FIG. 2, the positive electrode terminal 30 and the negative electrode terminal 40 extend from the inside to the outside of the sealing plate 14 through the terminal lead-out holes 18 and 19. Here, the positive electrode terminal 30 and the negative electrode terminal 40 are caulked to the peripheral edge portion surrounding the terminal lead-out holes 18 and 19 of the sealing plate 14 by caulking. Caulking portions 30c and 40c are formed at the ends of the positive electrode terminal 30 and the negative electrode terminal 40 on the side of the exterior body 12 (the lower end in FIG. 2). The positive electrode terminal 30 is electrically connected to the positive electrode tab group 23 of the electrode body 20 via the positive electrode current collector 50 inside the battery case 10. The positive electrode terminal 30 is insulated from the sealing plate 14 by the positive electrode insulating member 70 and the gasket 90. The negative electrode terminal 40 is electrically connected to the negative electrode tab group 25 of the electrode body 20 via the negative electrode current collector 60 inside the battery case 10. The negative electrode terminal 40 is insulated from the sealing plate 14 by the negative electrode insulating member 80 and the gasket 90.

[0015] As shown in FIG. 2, the electrode body 20 is housed inside the battery case 10 (specifically, inside the exterior body 12). The number of electrode bodies 20 disposed inside one battery case 10 is not particularly limited, and may be one, or may be two or more (a plurality).

[0016] FIG. 3 is a schematic diagram showing the configuration of the electrode body 20. As shown in FIG. 3, the electrode body 20 includes a positive electrode 22, a negative electrode 24, and a separator 26. Here, the electrode body 20 is a flat wound electrode body. The electrode body 20 is configured such that a strip-shaped positive electrode 22 and a strip-shaped negative electrode 24 are laminated via a strip-shaped separator 26 and wound around a winding axis WL. However, the electrode body 20 may be a laminated electrode body in which a rectangular (typically rectangular) positive electrode and a rectangular (typically rectangular) negative electrode are stacked in an insulated state.

[0017] The positive electrode 22 has a positive electrode current collector 22c, a positive electrode active material layer 22a and a positive electrode protective layer 22p fixed on at least one surface of the positive electrode current collector 22c. However, the positive electrode protective layer 22p is not essential and can be omitted in other embodiments. The positive electrode current collector 22c is strip-shaped. The positive electrode current collector 22c is made of a conductive metal such as aluminum, an aluminum alloy, nickel, or stainless steel. Here, the positive electrode current collector 22c is a metal foil, specifically, an aluminum foil.

[0018] A plurality of positive electrode tabs 22t are provided at one end (the left end in FIG. 3) in the long side direction Y of the positive electrode current collector 22c. Here, the positive electrode tabs 22t are a part of the positive electrode current collector 22c and are made of a metal foil (aluminum foil). The plurality of positive electrode tabs 22t are laminated at one end (the left end in FIG. 3) in the long side direction Y to form a positive electrode tab group 23 as shown in FIG. 2. A positive electrode current collecting portion 50 is attached (specifically, joined) to the positive electrode tab group 23. The positive electrode tab group 23 is electrically connected to the positive electrode terminal 30 via the positive electrode current collecting portion 50.

[0019] The positive electrode active material layer 22a is provided in a strip shape along the longitudinal direction of the strip-shaped positive electrode current collector 22c. The positive electrode active material layer 22a contains a positive electrode active material capable of reversibly occluding and releasing charge carriers. In the present embodiment, the positive electrode active material essentially contains a lithium-excess transition metal composite oxide. In this specification, the "lithium-excess transition metal composite oxide" is a lithium transition metal composite oxide in which the molar ratio (Li / Me) of the lithium element (Li) to the transition metal element (Me) is 1.3 or more. Although not particularly limited, in some embodiments, the above molar ratio (Li / Me) is preferably 2 or less, and more preferably 1.5 or less. Thereby, the structural stability of the lithium-excess transition metal composite oxide is improved, and the effects of the technology disclosed herein can be exerted at a high level.

[0020] As the lithium-excess transition metal composite oxide, those conventionally known to be usable in a lithium-ion secondary battery can be used alone or in combination of two or more. The lithium-excess transition metal composite oxide preferably contains at least one of Ni, Co, and Mn as the transition metal element, more preferably contains Mn, and particularly preferably contains all of Ni, Co, and Mn. Thereby, high capacity and high durability can be achieved at a higher level. The transition metal element of the lithium-excess transition metal composite oxide may consist of Ni, Co, and Mn. The lithium-excess transition metal composite oxide preferably has a composite model of a layered crystal structure, specifically, a composite model of a layered rock salt-type crystal structure of space groups R-3m and C2 / m.

[0021] Preferable examples of the lithium-excess transition metal composite oxide include the following formula (I): Li 1+α Ni x Co y Mn z O2 Formula (I) (However, α, x, y, z satisfy 0.13 ≦ α ≦ 0.33, 0 ≦ x ≦ 0.3, 0 ≦ y ≦ 0.2, 0.47 ≦ z ≦ 0.67, and α + x + y + z = 1.); and the compounds represented thereby can be mentioned.

[0022] In formula (I), a is preferably 0.15 ≦ a ≦ 0.25, more preferably 0.17 ≦ a ≦ 0.2. x is preferably 0 < x, more preferably 0.10 ≦ x, and even more preferably 0.13 ≦ x ≦ 0.28. y is preferably 0 < y, more preferably 0.01 ≦ y ≦ 0.15, and even more preferably 0.03 ≦ y ≦ 0.13. z is preferably 0 < z, more preferably 0.3 ≦ z ≦ 0.6, and preferably 0.5 ≦ z ≦ 0.55.

[0023] Although not particularly limited, when the total amount of the positive electrode active material is 100% by mass, the content ratio of the lithium-excess transition metal composite oxide is preferably 50% by mass or more, more preferably 80% by mass or more, preferably 90% by mass or more, and even more preferably 95% by mass or more. The positive electrode active material may be composed of a lithium-excess transition metal composite oxide. Further, the positive electrode active material may further contain one or more positive electrode active materials other than the lithium-excess transition metal composite oxide, for example, a lithium transition metal composite oxide having a molar ratio (Li / Me) of less than 1.3.

[0024] The positive electrode active material layer 22a may contain optional components other than the positive electrode active material, such as a binder, a conductive material, and various additive components. As the binder, for example, polyvinylidene fluoride (PVdF) can be preferably used. As the conductive material, for example, a carbon material such as acetylene black (AB) can be preferably used. Although not particularly limited, the ratio of the binder in the positive electrode active material layer 22a is preferably 0.1 to 10 parts by mass, more preferably 1 to 5 parts by mass, when the positive electrode active material is 100 parts by mass. The ratio of the conductive material in the positive electrode active material layer 22a is preferably 0.1 to 10 parts by mass, more preferably 1 to 5 parts by mass, when the positive electrode active material is 100 parts by mass.

[0025] In this embodiment, when the positive electrode 22 is charged and discharged using metallic Li as the counter electrode, the initial charge-discharge efficiency is 80% or more. The initial charge-discharge efficiency of the positive electrode 22 is preferably 82% or more, and more preferably 85% or more. By setting the initial charge-discharge efficiency of the positive electrode 22 to a predetermined value or more, the amount of oxygen released from the positive electrode active material (for example, lithium-excess transition metal composite oxide) can be suppressed, and the stability of the positive electrode active material can be improved. Therefore, the durability of the battery 100 can be improved. In some aspects, the initial charge-discharge efficiency of the positive electrode 22 is preferably less than 95%, and more preferably less than 90%. The method for measuring the "initial charge-discharge efficiency" will be described in detail in the examples described later.

[0026] The positive electrode protective layer 22p is provided between the positive electrode current collector 22c and the positive electrode active material layer 22a in the long-side direction Y. The positive electrode protective layer 22p is provided in a strip shape along the positive electrode active material layer 22a. The positive electrode protective layer 22p contains an inorganic filler (for example, alumina). The positive electrode protective layer 22p may contain optional components other than the inorganic filler, such as a conductive material, a binder, and various additive components. The conductive material and the binder may be the same as those exemplified as being capable of being contained in the positive electrode active material layer 22a.

[0027] The negative electrode 24 includes a negative electrode current collector 24c and a negative electrode active material layer 24a fixed on at least one surface of the negative electrode current collector 24c. The negative electrode current collector 24c is strip-shaped. The negative electrode current collector 24c is made of a conductive metal such as copper, a copper alloy, nickel, or stainless steel. Here, the negative electrode current collector 24c is a metal foil, specifically a copper foil.

[0028] A plurality of negative electrode tabs 24t are provided at one end (the right end in FIG. 3) of the negative electrode current collector 24c in the long-side direction Y. Here, the negative electrode tabs 24t are a part of the negative electrode current collector 24c and are made of a metal foil (copper foil). The plurality of negative electrode tabs 24t are laminated at one end (the right end in FIG. 3) in the long-side direction Y to form a negative electrode tab group 25 as shown in FIG. 2. A negative electrode current collecting portion 60 is attached (specifically, joined) to the negative electrode tab group 25. The negative electrode tab group 25 is electrically connected to the negative electrode terminal 40 via the negative electrode current collecting portion 60.

[0029] The negative electrode active material layer 24a is provided in a strip shape along the longitudinal direction of the strip-shaped negative electrode current collector 24c. The negative electrode active material layer 24a contains a negative electrode active material capable of reversibly occluding and releasing charge carriers. In the present embodiment, the negative electrode active material essentially contains graphite and a Si-containing material. Examples of the graphite include artificial graphite and natural graphite. As the Si-containing material, one or more materials known to be conventionally usable in a lithium ion secondary battery can be used. Preferable examples of the Si-containing material include Si, silicon oxide (SiOa, where 0 < a < 2), SiC-containing materials (including silicon carbide and SiC composites in which silicon is dispersed inside carbon particles), SiN-containing materials (silicon nitride), and those in which nano-Si particles are dispersed in porous particles. Among them, the SiC-containing material is preferable.

[0030] The negative electrode active material may be composed of graphite and a Si-containing material, or may further contain one or more negative electrode active material materials other than graphite and the Si-containing material. Specific examples of such negative electrode active material materials include carbon materials such as hard carbon, soft carbon, and amorphous carbon.

[0031] When the total amount of the negative electrode active material is 100% by mass, the content ratio of the Si-containing material is 4 to 15% by mass. By setting the content ratio of the Si-containing material to a predetermined value or more, a high capacity can be achieved. By setting the content ratio of the Si-containing material to a predetermined value or less, capacity deterioration can be suppressed and high durability can be achieved. Therefore, high capacity and high durability can be achieved at a higher level. From the viewpoint of increasing the capacity, the content ratio of the Si-containing material is preferably 5% by mass or more.

[0032] The negative electrode active material layer 24a may contain optional components other than the negative electrode active material, such as a binder, a dispersant, a conductive material, various additive components, etc. As the binder, for example, rubbers such as styrene-butadiene rubber (SBR) and acrylic resins such as polyacrylic acid (PAA) can be preferably used. As the dispersant, for example, celluloses such as carboxymethyl cellulose (CMC) can be preferably used. As the conductive material, for example, carbon materials such as carbon fibers and carbon nanotubes can be preferably used. Although not particularly limited, when the amount of the negative electrode active material is 100 parts by mass, the proportion of the binder in the negative electrode active material layer 24a is preferably 0.1 to 10 parts by mass, and more preferably 1 to 5 parts by mass. When the amount of the negative electrode active material is 100 parts by mass, the proportion of the dispersant in the negative electrode active material layer 24a is preferably 0.1 to 10 parts by mass, and more preferably 1 to 5 parts by mass.

[0033] Although not particularly limited, it is preferable that the negative electrode 24 has a higher initial charge-discharge efficiency than the positive electrode 22 when charged and discharged with metallic Li as the counter electrode. The initial charge-discharge efficiency of the negative electrode 24 is preferably 85% or more, and more preferably 90% or more.

[0034] The separator 26 is a member that insulates the positive electrode active material layer 22a of the positive electrode 22 and the negative electrode active material layer 24a of the negative electrode 24. The separator 26 may be the same as the conventional one and is not particularly limited. As the separator 26, for example, a porous sheet made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP) is preferable. The separator 26 may have a heat resistance layer (HRL) or an adhesive layer on the surface of the base material portion made of a porous sheet made of resin. The configuration of the heat resistance layer and the adhesive layer may be the same as the conventional one.

[0035] In this embodiment, the initial charge capacity C of the positive electrode 22 C to the initial charge capacity C of the negative electrode 24 A ratio (C A / C C ) is 1 to 1.25. The above ratio (C A / C CBy setting the above ratio (C A / C C ) to a predetermined value or more, Li deposition at the negative electrode can be suppressed, and high durability can be achieved. By setting the above ratio (C A / C C ) to a predetermined value or less, high capacity can be achieved. Therefore, high capacity and high durability can be compatible at a higher level. From the viewpoint of high durability, the above ratio (C A / C C ) is preferably 1.1 or more. From the viewpoint of increasing the capacity, it is preferably 1.2 or less. The measurement method of "the above ratio (C A / C C )" will be described in detail in the examples described later.

[0036] In this embodiment, the ratio (I A of the initial irreversible capacity I of the positive electrode 22 to the initial irreversible capacity I C of the negative electrode 24 C / I A ) is 1 to 2. By setting the above ratio (I C / I A ) to a predetermined value or more, a decrease in capacity due to deterioration of the Si-containing material can be suppressed. By setting the above ratio (I C / I A ) to a predetermined value or less, a decrease in capacity due to deterioration of the lithium-excess transition metal composite oxide can be suppressed. Therefore, the durability of the battery 100 can be improved. The above ratio (I C / I A ) is preferably 1.1 or more, more preferably 1.15 or more, and particularly preferably 1.19 or more. The above ratio (I C / I A ) is preferably less than 2, more preferably 1.9 or less, and particularly preferably 1.85 or less. Thereby, the effects of the technology disclosed herein can be exhibited at a particularly high level. The measurement method of "the above ratio (I C / I A )" will be described in detail in the examples described later.

[0037] The electrolyte may be the same as the conventional one and is not particularly limited. The electrolyte may be liquid, gel-like, or solid. Typically, the electrolyte is a non-aqueous electrolyte containing a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, one or more of those known to be usable in a lithium-ion secondary battery conventionally can be used. As a preferred example of the non-aqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, lactones, etc. can be mentioned. The non-aqueous solvent preferably contains carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), vinylene carbonate (VC), propylene carbonate (PC), etc.

[0038] The supporting salt is not particularly limited as long as it contains a charge carrier, and one or more of those known to be usable in a lithium-ion secondary battery conventionally can be used. As a preferred example of the supporting salt, fluorine-containing lithium salts such as LiPF6 and LiBF4 can be mentioned. The supporting salt preferably contains LiPF6. The electrolyte may further contain additional components (additives).

[0039] Although not particularly limited, the battery 100 preferably has a volumetric energy density of generally 800 Wh / L or more, more preferably 810 Wh / L or more, 820 Wh / L or more, and 850 Wh / L or more.

[0040] <Method for manufacturing battery 100> The battery 100 can be manufactured by a manufacturing method including, for example, a preparation step (step S1), an electrode body production step (step S2), and a construction step (step S3) in this order. Also, at any stage, other steps may be further included.

[0041] In the preparation process (step S1), the positive electrode 22 and the negative electrode 24 are prepared. As described above, the positive electrode 22 contains a lithium-excess transition metal composite oxide with a molar ratio (Li / Me) of 1.3 or more as the positive electrode active material, and has an initial charge-discharge efficiency of 80% or more when charged and discharged with metallic Li as the counter electrode. The positive electrode 22 can be produced, for example, by mixing a positive electrode active material, a conductive material, a binder, and a dispersion solvent to prepare a positive electrode composite material slurry, and applying the prepared positive electrode composite material slurry onto the positive electrode current collector 22c by a conventionally known method, drying it, and appropriately pressing it.

[0042] As described above, the negative electrode 24 contains graphite and an Si-containing material as the negative electrode active material, and the content of the Si-containing material in the negative electrode active material is 4 to 15% by mass. The negative electrode 24 can be produced, for example, by mixing a negative electrode active material, a binder, a dispersant, and a dispersion solvent to prepare a negative electrode composite material slurry, and applying the prepared negative electrode composite material slurry onto the negative electrode current collector 24c by a conventionally known method, drying it, and appropriately pressing it.

[0043] In the electrode body manufacturing process (step S2), the positive electrode 22 and the negative electrode 24 manufactured above are opposed to each other in an insulated state via the separator 26 to manufacture the electrode body 20. At this time, the electrode body 20 is such that the ratio (C C of the initial charge capacity C of the negative electrode 24 to the initial charge capacity C of the positive electrode 22 A (C A / C C ) is 1 to 1.25, and the ratio (I A of the initial irreversible capacity I of the positive electrode 22 to the initial irreversible capacity I of the negative electrode 24 C (I C / I A ) is 1 to 2 or less, and it is manufactured to satisfy both.

[0044] In the construction process (step S3), the battery 100 is constructed by housing the electrode body 20 and the electrolyte in the battery case 10. For example, first, the electrode body 20 is housed inside the exterior body 12 through the opening 12h of the exterior body 12 (housing process). Next, the sealing plate 14 is fitted to the opening 12h of the exterior body 12, and the sealing plate 14 is welded to the periphery of the opening 12h to integrate the exterior body 12 and the sealing plate 14 (case integration process). Next, after injecting a non-aqueous electrolyte through the liquid injection hole of the exterior body 12, the liquid injection hole is sealed (sealing process). The battery 100 can be preferably manufactured as described above.

[0045] <Use of the battery 100> The battery 100 can be used for various applications. For example, since it has both high capacity and high durability, it can be preferably used as a power source (driving power source) for motors mounted on vehicles such as passenger cars and trucks. The type of vehicle is not particularly limited, and examples include plug-in hybrid electric vehicles (PHEV), hybrid electric vehicles (HEV), battery electric vehicles (BEV), etc.

[0046] Hereinafter, several embodiments of the present invention will be described, but the present invention is not intended to be limited to such embodiments.

[0047] ≪Examples 1 to 6, Comparative Examples 1 to 7≫ <Preparation of the positive electrode> First, a positive electrode active material (lithium transition metal composite oxide) having a molar ratio (Li / Me) and a composition formula shown in Table 1, AB as a conductive material, and PVdF as a binder are mixed at a mass ratio of positive electrode active material:AB:PVdF = 100:1:1, and the fluidity is adjusted with a dispersion solvent (NMP) to prepare a positive electrode composite slurry. Next, the prepared positive electrode composite slurry is coated on an Al foil as a positive electrode current collector, dried, and pressed to a predetermined thickness. Then, it is cut out to a predetermined size to produce a positive electrode.

[0048] Next, the initial characteristics of the positive electrode were evaluated. That is, first, a Li foil as a counter electrode was cut into a predetermined size, and the positive electrode fabricated above and the Li foil were opposed to each other with a separator (a porous sheet with a two-layer structure in which a heat-resistant layer was formed on the surface of PP), and an electrode body was fabricated. At this time, the separator was arranged such that the side of the heat-resistant layer faced the positive electrode. Next, the Al foil of the positive electrode current collector was welded to an Al plate for external current collection, and the Li foil was welded to a Cu plate for external current collection. Next, the electrode body was housed together with the electrolytic solution in a bag-shaped battery case made of an aluminum laminate film and sealed to construct a test cell. As the electrolytic solution, a solution in which LiPF6 (1.15 mol / L) as a supporting salt was dissolved in EC + EMC + DMC (volume ratio 3:3:4) as a non-aqueous solvent was prepared. After the injection of the solution, the test cell was left standing at 25 °C for 12 hours.

[0049] Then, in an environment at 25 °C, after constant current charging up to 4.7 V (vs. Li / Li+) at a charging rate of 0.1 C, constant current discharging was performed at a discharging rate of 0.1 C down to 2.5 V (vs. Li / Li+). From the initial charge capacity and the initial discharge capacity at this time, the initial charge-discharge efficiency of the positive electrode (= (initial discharge capacity / initial charge capacity) × 100) was calculated. The results are shown in Table 1. Also, FIG. 4 shows, as an example, the charge-discharge curve of the positive electrode of Example 1.

[0050] <Preparation of Negative Electrode> First, graphite and SiC as negative electrode active materials were mixed so that SiC had the mass ratio shown in Table 1 (in Examples 1 to 4, the mass ratio of graphite:SiC was 95:5) to obtain a mixture. Next, SBR as a binder and CMC as a dispersant were added to the obtained mixture (100 parts by mass) at a ratio of 1 part by mass each, and the fluidity was adjusted with a dispersion solvent (water) to prepare a negative electrode composite slurry. Next, the prepared negative electrode composite slurry was coated on a Cu foil as a negative electrode current collector, and after drying, it was pressed to a predetermined thickness. Then, it was cut into a predetermined size to fabricate a negative electrode.

[0051] Next, in accordance with the above-described positive electrode, the initial characteristics of the negative electrode were evaluated. That is, first, a Li foil as a counter electrode was cut into a predetermined size, and the negative electrode prepared above and the Li foil were opposed to each other with a separator (a porous sheet having a two-layer structure in which a heat-resistant layer was formed on the surface of PP) interposed therebetween to prepare an electrode body. At this time, the separator was arranged such that the side of the heat-resistant layer faced the negative electrode. Next, the Cu foil of the negative electrode current collector was welded to a Cu plate for external current collection, and the Li foil was welded to the Cu plate for external current collection. Next, the electrode body was housed together with the above electrolytic solution in a bag-shaped battery case made of an aluminum laminate film and sealed to construct a test cell. Further, after injecting the electrolyte solution, the test cell was left standing at 25 °C for 12 hours.

[0052] Then, in an environment at 25 °C, after performing constant-current charging up to 1.5 V (vs. Li / Li+) at a charging rate of 0.1C, constant-current discharging was performed at a discharging rate of 0.1C down to 0 V (vs. Li / Li+). From the initial charge capacity and the initial discharge capacity at this time, the initial charge-discharge efficiency of the negative electrode (= (initial discharge capacity / initial charge capacity) × 100) was calculated. The results are shown in Table 1. Further, FIG. 4 shows the charge-discharge curve of the negative electrode of Example 1 as an example.

[0053] <Capacity ratio (C A / C C ) and irreversible capacity ratio (I C / I A ) calculation> The capacity ratio (C C of the initial charge capacity of the negative electrode with respect to the initial charge capacity C A of the positive electrode (C A / C C ) was calculated by the following formula: initial charge capacity of the negative electrode / initial charge capacity of the positive electrode;. The results are shown in Table 1. The irreversible capacity ratio (I A of the initial irreversible capacity of the positive electrode with respect to the initial irreversible capacity I C of the negative electrode (I C / I A ) was calculated by the following formula: (initial charge capacity of the positive electrode - initial discharge capacity of the positive electrode) / (initial charge capacity of the negative electrode - initial discharge capacity of the negative electrode);. The results are shown in Table 1.

[0054] <Construction of a lithium-ion secondary battery> The positive electrode and the negative electrode fabricated above were opposed to each other with the separator interposed therebetween to fabricate an electrode body. At this time, the separator was arranged such that the heat-resistant layer side faced the positive electrode. Next, the Al foil of the positive electrode current collector was welded to the Al plate for external current collection, and the Cu foil of the negative electrode current collector was welded to the Cu plate for external current collection. Next, the electrode body was housed together with the above electrolytic solution in a bag-shaped battery case made of an aluminum laminate film and sealed to construct an evaluation battery.

[0055] <Evaluation of Durability (Cycle Characteristics)> First, under an environment of 25°C, after performing constant current charging up to 4.65 V at a charging rate of 0.1C, constant current discharging was performed up to 2.5 V at a discharging rate of 0.1C, and the discharging capacity at this time was defined as the discharging capacity before the cycle test. Next, under an environment of 25°C, after performing constant current charging up to 4.65 V at a charging rate of 0.5C, charging and discharging in which constant current discharging was performed up to 2.5 V at a discharging rate of 0.5C was taken as one cycle, and high-rate charging and discharging of 200 cycles was repeated. Then, in the same manner as the discharging capacity before the cycle test, the discharging capacity after the cycle test was measured, and the following formula: Capacity retention rate = (Discharging capacity after cycle test / Discharging capacity before cycle test) × 100; was used to calculate the capacity retention rate (%). The results are shown in Table 1. In Table 1, together with the value of the capacity retention rate, when the capacity retention rate is 95% or more, "〇" is shown, and when the capacity retention rate is less than 95%, "×" is shown.

[0056] <Evaluation of Volume Energy Density> The volume energy density was calculated by the following formula: Volume energy density = Energy capacity of battery / Volume of battery = {(Initial discharging capacity of positive electrode) × (3.7 V)} / (Internal volume of test cell); Here, 3.7 V is the nominal voltage, which is a value defined as a guideline for the voltage between terminals obtained when the battery is used in a normal state. The results are shown in Table 1. In Table 1, together with the value of the volume energy density, when the volume energy density is 800 Wh / L or more, "〇" is shown, and when the volume energy density is less than 800 Wh / L, "×" is shown.

[0057] ≪Comparative Example 8≫ In Comparative Example 8, a compound with a molar ratio (Li / Me) of 1 was used as the positive electrode active material. When measuring the initial charge-discharge efficiency of the positive electrode, the voltage range was set to 4.3~2.5V (vs. Li / Li+), and when evaluating the durability of the battery, the voltage range was set to 4.14~2.5V. Except for this, the evaluation was carried out in the same manner as in Examples 1~6 and Comparative Examples 1~7. The results are shown in Table 1.

[0058]

Table 1

[0059] <Ratio of irreversible capacity (I C / I A )> As shown in Table 1, Comparative Example 1, in which the ratio of irreversible capacity (I C / I A ) was as small as 0.93 (less than 1), had relatively low durability (cycle characteristics). As a reason for this, deterioration of the Si-containing material was considered. Specifically, although not intended to be construed in a particularly limited manner, in Comparative Example 1, during charge and discharge, the potential of the negative electrode became relatively high, for example, 0.65V (vs. Li / Li+) or more, and as a result, the amount of expansion and contraction of the Si-containing material increased, and it was considered that cracks occurred in the Si-containing particles. Also, Comparative Example 2, in which the ratio of irreversible capacity (I C / I A ) was as large as 2.77 (exceeding 2), also had relatively low durability (cycle characteristics). As a reason for this, deterioration of the lithium-excess transition metal composite oxide was considered. Specifically, although not intended to be construed in a particularly limited manner, in Comparative Example 2, during charge and discharge, the potential of the positive electrode became relatively low, for example, 2.9V (vs. Li / Li+) or less, and as a result, a phase transition from a layered structure to a spinel structure occurred in the lithium-excess transition metal composite oxide, and it was considered that Li deintercalation / insertion during charge and discharge was inhibited. Therefore, it was considered that by setting the ratio of irreversible capacity (I C / I A ) to 1~2, it was possible to suppress the expansion and contraction of the Si-containing material and also suppress the inhibition of Li movement in the lithium-excess transition metal composite oxide, improving the cycle durability.

[0060] <Initial charge-discharge efficiency of the positive electrode> In Comparative Example 3 where the initial charge-discharge efficiency of the positive electrode was as low as 79% (less than 80%), the durability (cycle characteristics) was relatively low. For this reason, in Comparative Example 3, the amount of oxygen released from the positive electrode active material increased, resulting in (1) thickening of the SEI film on the positive electrode due to oxidation of the electrolyte and an increase in electrical resistance; (2) diffusion of transition metal ions into the Li layer and inhibition of Li diffusion during charge and discharge; etc. were considered. Therefore, it was considered that the cycle durability would be improved by setting the initial charge-discharge efficiency of the positive electrode to 80% or more.

[0061] <Capacity ratio (C A / C C )> In Comparative Example 4 where the capacity ratio was 0.9 (less than 1), the durability (cycle characteristics) was relatively low. For this reason, it was confirmed that Li deposition occurred in Comparative Example 4. Also, in Comparative Example 5 where the capacity ratio was 1.3 (exceeding 1.25), although the durability (cycle characteristics) was excellent, the negative electrode portion not used for charge and discharge became large and the volume energy density was insufficient. Therefore, it was considered that by setting the capacity ratio (C A / C C ) to 1 to 1.25, it would be possible to have both high capacity and high durability.

[0062] <Content of Si-containing material> In Comparative Example 6 where the SiC blending ratio was 3% by mass (less than 5% by mass), the volume energy density was insufficient. Also, in Comparative Example 7 where the SiC blending ratio was 20% by mass (exceeding 15% by mass), the durability (cycle characteristics) was relatively low. For this reason, in Comparative Example 7, it was considered that the isolation of Si-containing particles was accelerated along with the expansion and contraction of Si. Therefore, it was considered that by setting the proportion of the silicon content in the negative electrode active material to 5 to 15% by mass, it would be possible to have both high capacity and high durability.

[0063] <Molar ratio of positive electrode active material (Li / Me)> In Comparative Example 8, which used a positive electrode active material with a molar ratio (Li / Me) of 1 (less than 1.3), the volume energy density was significantly low, and furthermore, the durability (cycle characteristics) was also low. For this reason, in Comparative Example 8, it was considered that the initial irreversible capacity of the positive electrode became small, and even if the composition of NiCoMn was changed, the high potential region of the negative electrode was used. Therefore, it was considered that by using a lithium-excess transition metal composite oxide with a molar ratio (Li / Me) of 1.3 or more, high capacity and high durability can be achieved simultaneously. The above results indicate the significance of the technology disclosed herein.

[0064] As described above, some embodiments of the present invention have been explained, but the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the content disclosed herein and the common general knowledge in the art. The technology described in the claims includes various modifications and changes of the above-exemplified embodiments. For example, it is possible to replace a part of the above-described embodiments with other modified examples, and it is also possible to add other modified examples to the above-described embodiments. Also, if the technical features are not described as essential, they can be appropriately deleted.

[0065] In the embodiment of FIG. 1 described above, the outer shape of the battery 100 was a rectangular parallelepiped shape (angular). However, it is not limited to this. In other embodiments, the battery 100 may be, for example, a sheet shape, a cylindrical shape, a button shape, a coin shape, or the like.

[0066] As described above, specific aspects of the technology disclosed herein include those described in the following items. Item 1: A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains, as a positive electrode active material, a lithium transition metal composite oxide in which the molar ratio (Li / Me) of lithium element (Li) to transition metal element (Me) is 1.3 or more, and the initial charge-discharge efficiency when charged and discharged with metallic Li as the counter electrode is 80% or more; the negative electrode contains graphite and an Si-containing material as negative electrode active materials, the content of the Si-containing material in the negative electrode active material is 4% by mass or more and 15% by mass or less; the ratio (C C of the initial charge capacity C A of the negative electrode to the initial charge capacity C A of the positive electrode (C C / C A ) is 1 or more and 1.25 or less; and the ratio (I C of the initial irreversible capacity I C of the negative electrode to the initial irreversible capacity I A ) of the positive electrode is 1 or more and 2 or less. A lithium-ion secondary battery. Item 2: The lithium-ion secondary battery according to Item 1, wherein the lithium-excess transition metal composite oxide contains manganese as the transition metal element. Item 3: The lithium-ion secondary battery according to Item 1 or Item 2, wherein the lithium-excess transition metal composite oxide is a compound represented by the following formula (I): Li 1+α Ni x Co y Mn z O2 (where α, x, y, z satisfy 0.13 ≤ α ≤ 0.33, 0 ≤ x ≤ 0.3, 0 ≤ y ≤ 0.2, 0.47 ≤ z ≤ 0.67, and α + x + y + z = 1). Item 4: The lithium-ion secondary battery according to any one of Items 1 to 3, wherein the molar ratio (Li / Me) is 1.5 or less. Item 5: The lithium-ion secondary battery according to any one of Items 1 to 4, wherein the Si-containing material is at least one of Si, silicon oxide, and SiC-containing materials. Item 6: A method for producing a lithium ion secondary battery, comprising: a preparation step of preparing a positive electrode and a negative electrode; an electrode assembly preparation step of preparing an electrode assembly by placing the positive electrode and the negative electrode in an insulated state with a separator interposed therebetween; and an accommodation step of accommodating the electrode assembly and an electrolyte in a battery case, wherein the positive electrode contains a lithium transition metal composite oxide having a molar ratio (Li / Me) of lithium element (Li) to transition metal element (Me) of 1.3 or more as a positive electrode active material, and has an initial charge / discharge efficiency of 80% or more when charged and discharged using metallic Li as a counter electrode; the negative electrode contains graphite and a Si-containing material as a negative electrode active material, and the content of the Si-containing material in the negative electrode active material is 4% by mass or more and 15% by mass or less; and the initial charge capacity C C The initial charge capacity C of the negative electrode A Ratio of (C A / C C ) is 1 or more and 1.25 or less, and the initial irreversible capacity I A The initial irreversible capacity I of the positive electrode C The ratio (I C / I A ) is a method for manufacturing a lithium-ion secondary battery, which is 1 or more and 2 or less. [Explanation of symbols]

[0067] 10 Battery case 20 Electrode body 22 Positive electrode 22a Cathode active material layer 24 Negative electrode 24a Negative electrode active material layer 100 batteries

Claims

1. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains, as a positive electrode active material, a lithium transition metal composite oxide in which the molar ratio (Li / Me) of lithium element (Li) to transition metal element (Me) is 1.3 or more, and the initial charge-discharge efficiency is 80% or more when charged and discharged with metallic Li as the counter electrode, wherein the negative electrode contains graphite and an Si-containing material as a negative electrode active material, and the content of the Si-containing material in the negative electrode active material is 4% by mass or more and 15% by mass or less, The initial charge capacity C of the positive electrode Ｃ with respect to the initial charge capacity C of the negative electrode Ａ ratio (C Ａ / C Ｃ ) is 1 or more and 1.25 or less, The initial irreversible capacity I of the negative electrode Ａ to the initial irreversible capacity I of the positive electrode Ｃ ratio (I Ｃ / I Ａ ) is 1 or more and 2 or less, A lithium-ion secondary battery.

2. The lithium transition metal composite oxide contains manganese as the transition metal element, The lithium-ion secondary battery according to Claim 1.

3. The lithium transition metal composite oxide is represented by the following formula (I): Li １＋α Ni ｘ Co ｙ Mn ｚ O ２ Formula (I) (However, α, x, y, z satisfy 0.13 ≤ α ≤ 0.33, 0 ≤ x ≤ 0.3, 0 ≤ y ≤ 0.2, 0.47 ≤ z ≤ 0.67, and α + x + y + z = 1.); It is a compound represented by The lithium-ion secondary battery according to Claim 1 or 2.

4. The molar ratio (Li / Me) is 1.5 or less, The lithium-ion secondary battery according to Claim 1 or 2.

5. The Si-containing material is at least one of Si, silicon oxide, and an SiC-containing material, The lithium-ion secondary battery according to Claim 1 or 2.

6. A method for manufacturing a lithium-ion secondary battery, comprising a preparation step of preparing a positive electrode and a negative electrode, an electrode body manufacturing step of opposing the positive electrode and the negative electrode in an insulated state via a separator to produce an electrode body, and a housing step of housing the electrode body and an electrolyte in a battery case, wherein the positive electrode contains, as a positive electrode active material, a lithium transition metal composite oxide in which the molar ratio (Li / Me) of lithium element (Li) to transition metal element (Me) is 1.3 or more, and the initial charge-discharge efficiency is 80% or more when charged and discharged with metallic Li as the counter electrode, wherein the negative electrode contains graphite and an Si-containing material as a negative electrode active material, and the content of the Si-containing material in the negative electrode active material is 4% by mass or more and 15% by mass or less, The initial charge capacity C of the positive electrode Ｃ with respect to the initial charge capacity C of the negative electrode Ａ The ratio (C Ａ / C Ｃ ) is 1 or more and 1.25 or less, The initial irreversible capacity I of the negative electrode Ａ to the initial irreversible capacity I of the positive electrode Ｃ ratio (I Ｃ / I Ａ ) is 1 or more and 2 or less, A method for manufacturing a lithium-ion secondary battery.

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