JP2016066461A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the rise in resistance accompanying high-rate charge/discharge.SOLUTION: A lithium ion secondary battery comprises: a negative electrode mixture layer 22; a separator 40; a positive electrode mixture layer 12 opposed to the negative electrode mixture layer 22 through the separator; and an electrolytic solution. The negative electrode mixture layer 22 includes hollow silica particles 4 by 3-7 mass%. The hollow silica particles 4 have an average particle diameter of 50-110 nm. The negative electrode mixture layer 22 has an average pore diameter which is 1.0-1.83 times that of the positive electrode mixture layer 12.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium ion secondary battery.

  Japanese Patent Application Laid-Open No. 7-212343 (Patent Document 1) discloses a sealed lead-acid battery in which amorphous silica particles are filled in a gap between a positive electrode plate and a negative electrode plate as an electrolyte solution holding agent and the electrolyte solution is held in the gap. Is disclosed.

JP-A-7-212343

  Lithium ion secondary batteries are used under a wide variety of conditions depending on the application or user preferences, and the progress of deterioration may be accelerated depending on the use conditions. In the research of the present inventors, in a high-rate charge / discharge, that is, in a usage pattern in which charging is performed at a high rate (large current) and discharging is performed at a low rate (small current), or charging is performed at a low rate and discharging is performed at a high rate, It has been found that the resistance increase of the battery is large.

  Therefore, an object of the present invention is to suppress an increase in resistance accompanying high-rate charge / discharge.

  [1] A lithium ion secondary battery of the present invention includes a negative electrode mixture layer, a positive electrode mixture layer facing the negative electrode mixture layer with a separator interposed therebetween, and an electrolytic solution. The negative electrode mixture layer contains 3% by mass or more and 7% by mass or less of hollow silica particles. The average particle diameter of the hollow silica particles is 50 nm or more and 110 nm or less. The average pore diameter of the negative electrode mixture layer is 1.0 to 1.83 times the average pore diameter of the positive electrode mixture layer.

  According to the inventor's research, it is considered that the resistance increase accompanying high-rate charge / discharge proceeds as follows. This will be described below with reference to the drawings.

  The battery 100 shown in FIG. 4 includes an electrode body 80 and a surplus electrolyte solution SE inside the exterior body 50. As shown in FIG. 5, the electrode body 80 is formed by winding the positive electrode 10 and the negative electrode 20 with the separator 40 interposed therebetween. In the electrode body 80, an electrolytic solution is held in the pores in each electrode and the gap between the electrodes. The electrolytic solution contains a supporting salt (Li salt) at a predetermined concentration and is responsible for ionic conduction between the electrodes.

  In the battery 100 having such a configuration, when high-rate charging or discharging is performed, the battery element generates heat, which causes the electrolyte to be heated and expand. The expanded electrolyte solution overflows from the originally held space (pores and the like), and the overflowed electrolyte solution flows out of the electrode body 80, for example, through a path indicated by an arrow A1 in FIG. To do. The electrolyte flowing out from the electrode body 80 is once integrated with the excess electrolyte solution SE, and when the temperature of the electrolyte solution decreases and the volume of the electrolyte solution decreases, for example, in the electrode body 80 along the path shown by the arrow A2. Will return. If this series of electrolytic solution outflow and reinflow is repeated, the salt concentration (Li salt concentration) in the electrolytic solution, which was initially uniform, varies. As a result, the electrode body 80 has local variations. Resistance rises.

  According to the inventor's research, the outflow amount and reinflow amount of the electrolyte solution in the positive electrode 10 and the negative electrode 20 are different from each other in the above-described series of processes, and this causes variations in the salt concentration between the positive electrode and the negative electrode. It is conducive. Thus, the positive electrode 10 and the negative electrode 20 are different in the outflow amount and reinflow amount of the electrolyte, as shown in FIG. 1, in the positive electrode mixture layer 12 and the negative electrode mixture layer 22 in the size of the pore p. It is thought that this is because of the difference in the permeation rate of the electrolyte solution.

  Therefore, in the above [1], as shown in FIG. 1, by filling the pores p of the negative electrode mixture layer 22 with the hollow silica particles 4 smaller than the pores p, the average fineness of the negative electrode mixture layer 22 is increased. The pore diameter is close to the average pore diameter of the positive electrode mixture layer 12. Specifically, by mixing the hollow silica particles 4 with the negative electrode mixture, the average pore diameter of the negative electrode mixture layer 22 is 1.0 to 1.83 times the average pore diameter of the positive electrode mixture layer 12. It is controlled to become. Thereby, the difference between the penetration rate of the electrolyte solution in the positive electrode mixture layer 12 and the penetration rate of the electrolyte solution in the negative electrode mixture layer 22 can be reduced. Moreover, at this time, since the hollow silica particle 4 has a cavity inside and holds the electrolytic solution therein, the liquid retaining property of the negative electrode mixture layer 22 can also be secured. Therefore, the salt concentration does not easily vary, and an increase in resistance due to high-rate charge / discharge is suppressed.

  Here, in the above [1], the average particle diameter of the hollow silica particles is set to 50 nm to 110 nm. When the average particle diameter of the hollow silica particles is less than 50 nm, the pore volume in the negative electrode mixture layer 22 may decrease, and the liquid retention of the negative electrode mixture layer 22 may be reduced. This is because the pore distribution in the material layer 22 becomes broad and it is difficult to adjust the permeation rate of the electrolytic solution in the negative electrode mixture layer 22.

  Moreover, in said [1], content of the hollow silica particle contained in a negative electrode compound-material layer is 3 mass% or more and 7 mass% or less. When the hollow silica particles are less than 3% by mass, the average pore diameter of the negative electrode mixture layer may not be sufficiently small, and when it exceeds 7% by mass, the pore diameter becomes excessively small, resulting in a decrease in capacity or an increase in resistance. Because there is.

  [2] In the above [1], the average pore diameter of the positive electrode mixture layer is preferably 0.30 μm, and the average pore diameter of the negative electrode mixture layer is preferably from 0.30 μm to 0.55 μm. This is to more reliably suppress an increase in resistance associated with high rate charge / discharge.

  FIG. 2 is a graph in which the average particle diameter of the hollow silica particles is the horizontal axis, and the content of the hollow silica particles contained in the negative electrode mixture layer is the vertical axis. A straight line L1 in FIG. 2 indicates a set of points where the average pore diameter of the negative electrode mixture layer becomes 0.30 μm when the average particle diameter and content of the hollow silica particles are changed. Similarly, the straight line L2 indicates a set of points at which the average pore diameter of the negative electrode mixture layer 22 becomes 0.55 μm when the average particle diameter and content of the hollow silica particles are similarly changed. Therefore, in the region R2 sandwiched between the straight line L1 and the straight line L2, the average pore diameter of the negative electrode mixture layer 22 is 0.30 μm or more and 0.55 μm or less.

  According to the study of the present inventor, when the average pore diameter of the positive electrode mixture layer 12 is 0.30 μm and the average particle diameter and content of the hollow silica particles are included in the region R2, high rate charge / discharge is performed. The accompanying increase in resistance is significantly reduced.

  2 is a region where the average particle diameter of the hollow silica particles exceeds 110 nm. As described above, the pore distribution of the negative electrode mixture layer 22 becomes broad in this region. Further, the region R3 is a region where the average particle diameter of the hollow silica particles is less than 50 nm. In this region, even if the average pore diameter of the negative electrode mixture layer 22 is 0.30 to 0.55 μm, the liquid retaining property is improved. Problems may arise.

  Here, the “average particle size” of the hollow silica particles indicates the particle size (so-called D50) at an integrated value of 50% in the particle size distribution obtained by the laser diffraction / scattering method. The average particle size can be measured, for example, by a particle size distribution measuring device “Microtrack MT3000II” manufactured by Nikkiso Co., Ltd.

  The “average pore diameter” of the negative electrode mixture layer and the positive electrode mixture layer indicates the pore diameter at an integrated value of 50% in the pore distribution obtained by the mercury intrusion method. The average pore diameter can be measured by, for example, a pore distribution measuring device “Autopore IV9500” manufactured by Shimadzu Corporation.

  According to the above, it is possible to suppress an increase in resistance associated with high rate charge / discharge.

It is a schematic sectional drawing which shows the principal part of the lithium ion secondary battery which concerns on one Embodiment of this invention. It is a graph which shows an example of the relationship between the average particle diameter of a hollow silica particle, and content of the hollow silica particle contained in a negative electrode compound material layer. It is the schematic which shows an example of a structure of the lithium ion secondary battery which concerns on one Embodiment of this invention. It is a schematic sectional drawing in the IV-IV line of FIG. It is the schematic which shows an example of a structure of the electrode body which concerns on one Embodiment of this invention. It is the schematic which shows an example of a structure of the negative electrode which concerns on one Embodiment of this invention. It is the schematic which shows an example of a structure of the positive electrode which concerns on one Embodiment of this invention.

  Hereinafter, an embodiment of the present invention (hereinafter referred to as “the present embodiment”) will be described in detail, but the present embodiment is not limited thereto.

[Lithium ion secondary battery]
FIG. 3 is a schematic diagram illustrating an example of the configuration of the lithium ion secondary battery according to the present embodiment. As shown in FIG. 3, the battery 100 is a sealed battery and includes a rectangular outer package 50. The exterior body 50 includes a bottomed square casing 52 and a lid body 54. The material of the exterior body 50 is, for example, an Al alloy. The casing 52 and the lid 54 are joined by, for example, laser welding. The lid 54 is provided with a positive terminal 70 and a negative terminal 72. The safety valve 55 is opened when the internal pressure in the battery 100 exceeds a predetermined pressure.

(Electrode body)
FIG. 4 is a schematic cross-sectional view taken along line IV-IV in FIG. As shown in FIG. 4, the battery 100 includes an electrode body 80 and a surplus electrolyte solution SE. The electrode body 80 has an exposed portion EP in which the positive electrode current collector 11 and the negative electrode current collector 21 are exposed, and the exposed portion EP passes through the positive electrode current collector plate 74 and the negative electrode current collector plate 76 to be in the positive electrode. The terminal 70 and the negative terminal 72 are electrically connected.

  FIG. 5 is a schematic diagram illustrating an example of the configuration of the electrode body 80. As shown in FIG. 5, the electrode body 80 is a wound electrode body. The positive electrode 10, the negative electrode 20, and the separator 40 constituting the electrode body 80 are all long belt-like sheet members. The electrode body 80 is formed by winding the positive electrode 10 and the negative electrode 20 opposite to each other with the separator 40 interposed therebetween, and winding them along the longitudinal direction of each member.

  FIG. 1 is a schematic cross-sectional view taken along the line II of FIG. As shown in FIG. 1, the battery 100 includes a negative electrode mixture layer 22 and a positive electrode mixture layer 12 that faces the negative electrode mixture layer 22 with the separator 40 interposed therebetween. The negative electrode mixture layer 22 includes a negative electrode active material 2, hollow silica particles 4, a binder (not shown), and the like, and the positive electrode mixture layer 12 includes a positive electrode active material 1, a conductive material, and a binder (not shown). Etc.).

  A large number of pores p are formed in each of the negative electrode mixture layer 22 and the positive electrode mixture layer 12. At this time, normally, the pore diameter of the negative electrode mixture layer 22 tends to be larger than the pore diameter of the positive electrode mixture layer 12. However, in the negative electrode mixture layer 22 according to this embodiment, the hollow silica particles 4 are filled in the pores p, and the average pore diameter of the negative electrode mixture layer 22 is 1. It is adjusted to be 0 times or more and 1.83 times or less. Therefore, in the electrode body 80, variations in salt concentration are unlikely to occur, and an increase in resistance due to high rate charge / discharge is suppressed.

Hereinafter, each member constituting the battery 100 will be described.
[Negative electrode]
FIG. 6 is a schematic diagram illustrating an example of the configuration of the negative electrode 20. As shown in FIG. 6, the negative electrode 20 includes a long strip-shaped negative electrode current collector 21 (for example, Cu foil) and a negative electrode mixture layer 22 formed on both main surfaces thereof. The negative electrode 20 can be produced by a conventionally known method. For example, a negative electrode paste formed by dispersing a negative electrode mixture and hollow silica particles in a predetermined solvent is applied to both main surfaces of the negative electrode current collector 21 and dried to produce the negative electrode 20. . For example, water or the like can be used as a solvent for the paste. After drying, the negative electrode mixture layer 22 may be compressed to adjust its thickness and mixture density.

  The negative electrode mixture layer 22 includes a negative electrode active material, hollow silica particles, a thickener, and a binder. In the present embodiment, the proportion of the hollow silica particles in the negative electrode mixture is 3% by mass or more and 7% by mass or less. The average particle diameter of the hollow silica particles needs to be 50 nm or more and 110 nm or less. Since the hollow silica particles have pores (cavities) inside, they exhibit high insulating properties and an action of improving liquid retention while narrowing the pores of the negative electrode mixture layer 22. The porosity of the hollow silica particles is, for example, about 30 to 90%.

  The negative electrode active material is not particularly limited as long as it can function as the negative electrode active material of the lithium ion secondary battery. For example, carbon-based negative electrode active materials such as graphite and coke, or alloy-based negative electrode active materials such as Si and Sn can be used. The proportion of the negative electrode active material in the remainder of the negative electrode mixture excluding the hollow silica particles is, for example, about 90 to 99% by mass.

  The thickener and the binder are not particularly limited. For example, carboxymethyl cellulose (CMC) can be used as the thickening material. As the binder, for example, styrene butadiene rubber (SBR) or the like can be used. The proportion of the thickener and the binder in the remainder of the negative electrode mixture excluding the hollow silica particles is, for example, about 1 to 10% by mass.

[Positive electrode]
FIG. 7 is a schematic diagram illustrating an example of the configuration of the positive electrode 10. As shown in FIG. 7, the positive electrode 10 includes a long belt-like positive electrode current collector 11 (for example, an Al foil) and a positive electrode mixture layer 12 formed on both main surfaces thereof. The positive electrode 10 can be produced by a conventionally known method. For example, a positive electrode paste formed by dispersing a positive electrode mixture in a predetermined solvent is applied on both main surfaces of the positive electrode current collector 11 and dried to produce the positive electrode 10. For example, N-methyl-2-pyrrolidone (NMP) can be used as a solvent for the paste. After drying, the positive electrode mixture layer 12 may be compressed to adjust the thickness and the mixture density.

The positive electrode mixture layer 12 includes a positive electrode active material, a conductive material, and a binder. Examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _a Co _b Mn _c O ₂ (a + b + c = 1, 0 <a <1, 0 <b <1, 0 <c < 1) LiFePO ₄ or the like can be used. The ratio of the positive electrode active material in the positive electrode mixture is, for example, about 80 to 98% by mass. For example, acetylene black (AB) or the like can be used as the conductive material, and polyvinylidene fluoride (PVDF) or the like can be used as the binder.

[Separator]
The separator 40 prevents electrical contact between the positive electrode 10 and the negative electrode 20 while allowing Li ions to pass therethrough. The separator 40 is preferably a microporous film made of a polyolefin-based material from the viewpoint of mechanical strength and chemical stability. For example, a microporous film such as polyethylene (PE) or polypropylene (PP) is suitable.

  Moreover, the separator 40 may be a laminate of a plurality of microporous membranes, or may have a heat resistant layer containing an inorganic filler (for example, alumina particles) formed on the surface thereof. The thickness of the separator 40 is, for example, about 5 to 40 μm. What is necessary is just to adjust suitably the hole diameter and porosity of the separator 40 so that air permeability may become a desired value.

[Electrolyte]
The electrolytic solution is obtained by dissolving a Li salt in an aprotic solvent. Examples of the aprotic solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (γBL) and vinylene carbonate (VC), and dimethyl carbonate (DMC). Further, chain carbonates such as ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) can be used. These aprotic solvents are desirably used in combination of two or more from the viewpoint of electrical conductivity and electrochemical stability. In particular, it is desirable to use a mixture of a cyclic carbonate and a chain carbonate. In this case, the volume ratio of the cyclic carbonate to the chain carbonate is preferably about 1: 9 to 5: 5.

For example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li (CF ₃ SO ₂ ) ₂ N, LiCF ₃ SO _{3 and the} like can be used as the Li salt. Two or more of these Li salts may be used in combination. Although the density | concentration of Li salt in electrolyte solution is not specifically limited, About 0.5-2.0 mol / L is preferable.

  As described above, the present embodiment has been described taking a square battery as an example. However, the present embodiment is not limited to this, and can be applied to a cylindrical battery, a laminated battery, and the like. The electrode body is not limited to a wound type, and may be a stacked type (also referred to as a “stacked type”).

  Hereinafter, although this embodiment is described further in detail using an example, this embodiment is not limited to these.

[Sample preparation]
A positive electrode 10 including the positive electrode mixture layer 12 was prepared as a common member (see FIG. 7). Next, as shown in Table 1, the average particle diameter of the hollow silica particles and the content of the hollow silica particles contained in the negative electrode mixture layer 22 were changed to change the No. 1-No. 8 was produced (see FIG. 6). In each negative electrode, the thickness of the negative electrode mixture layer 22 after compression was the same.

  The average pore diameter of the positive electrode and each negative electrode was measured using a pore distribution measuring apparatus “Autopore IV9500” manufactured by Shimadzu Corporation. Furthermore, “Pa / Pc” was calculated by dividing “Pa” by “Pc”, where “Pa” was the average pore diameter of the negative electrode mixture layer and “Pc” was the average pore diameter of the positive electrode mixture layer. The results are shown in Table 1.

[Evaluation]
Using the above positive electrode and each negative electrode, no. 1-No. The lithium ion secondary battery which concerns on 8 was produced. Subsequently, the resistance increase of each battery was evaluated by a high rate charge / discharge test. Specifically, the charge increase / decrease cycle [1] is obtained by dividing the battery resistance after the test by the battery resistance before the test by performing 2500 charge / discharge cycles with one cycle of charging and discharging under the following conditions. Calculated. The results are shown in Table 1. Here, the unit [C] of the current value in the charge / discharge condition indicates the current value at which the rated capacity of the battery can be discharged in one hour. Charging: 30 C × 10 seconds Discharging: 30 C × 10 seconds.

〔Results and discussion〕
As is clear from Table 1, the negative electrode mixture layer contains 3% by mass or more and 7% by mass or less of hollow silica particles, the average particle size of the hollow silica particles is 50 nm or more and 110 nm or less, and the average pore size of the negative electrode mixture layer Are 1.0 to 1.83 times the average pore diameter of the positive electrode composite material layer, the lithium ion secondary batteries (No. 5 to No. 8) according to the examples are comparative examples that do not satisfy such conditions. Compared to lithium ion secondary batteries (No. 1 to No. 4), the resistance increase rate is low and good.

  Although one embodiment and example of the present invention have been described above, the embodiment and example disclosed this time are illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 Positive electrode active material, 2 Negative electrode active material, 4 Hollow silica particle, 10 Positive electrode, 11 Positive electrode collector, 12 Positive electrode composite material layer, 20 Negative electrode, 21 Negative electrode current collector, 22 Negative electrode composite material layer, 40 Separator, 50 Exterior Body, 52 housing, 54 lid body, 55 safety valve, 70 positive electrode terminal, 72 negative electrode terminal, 74 positive electrode current collector plate, 76 negative electrode current collector plate, 80 electrode body, A1, A2 arrow, EP exposed portion, p pore, SE Excess electrolyte, R1, R2, R3 region, L1, L2 straight line.

Claims (1)

A negative electrode mixture layer;
A positive electrode mixture layer facing the negative electrode mixture layer with a separator interposed therebetween;
An electrolyte solution,
The negative electrode mixture layer contains 3% by mass or more and 7% by mass or less of hollow silica particles,
The hollow silica particles have an average particle size of 50 nm to 110 nm,
The average pore diameter of the negative electrode mixture layer is a lithium ion secondary battery that is 1.0 to 1.83 times the average pore diameter of the positive electrode mixture layer.

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