JP2014235925A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which is arranged by use of a negative electrode active material including silicon particles, and arranged to reduce a mechanical strain strength when a lithium ion enters each silicon particle, thereby achieving good charge and discharge cycle characteristics.SOLUTION: A lithium ion secondary battery according to the present invention comprises: an electrolyte including a lithium salt; and a negative electrode having a negative electrode active material including silicon particles having an average radius Rbetween 0.5[nm] and 280a[nm] inclusive before charging, provided that the lithium ion secondary battery is fully charged in the time a [hours]. Alternatively, the lithium ion secondary battery comprises: an electrolyte including a lithium salt; and a negative electrode having a negative electrode active material including silicon particles. The silicon particles included by the negative electrode active material each have, at a center portion thereof, a region which no lithium ion enters at the time of charging; the average radius Rof the regions is no larger than 20% of the average radius Rof the silicon particles before charging.

Description

  The present invention relates to a lithium ion secondary battery.

As a negative electrode active material for a negative electrode of a lithium ion secondary battery, a graphite-based carbon material is widely used. When the lithium ion secondary battery is charged, lithium ions are inserted into the negative electrode active material graphite. Stoichiometric composition when the lithium ions are inserted into the graphite is LiC _6, the theoretical capacity can be calculated to be 372 mAh / g. On the other hand, the stoichiometric composition when lithium ions are inserted into silicon is Li ₁₅ Si ₄ , and its theoretical capacity can be calculated as 3571 mAh / g. Thus, silicon is an attractive material that can store 9.6 times as much lithium as graphite. For this reason, negative electrode active materials containing silicon particles are used in recent lithium ion secondary batteries.

  However, since the volume of silicon particles expands to about four times when lithium ions are inserted, the silicon particles are dynamically destroyed during repeated insertion and release of lithium ions. When the silicon particles are broken, the fine particles formed by the breaking are electrically isolated, and a new electrochemical coating layer is formed on the broken surface, thereby increasing the irreversible capacity. For this reason, the conventional lithium ion secondary battery using the negative electrode active material containing silicon particles has a problem that the charge / discharge cycle characteristics are remarkably deteriorated during repeated charging and discharging (insertion and release of lithium ions). .

  Patent Document 1 describes a non-aqueous secondary battery in which the expansion coefficient associated with lithium insertion in a negative electrode containing a compound containing a silicon atom is 1.05 or more and 3.0 or less in order to increase the cycle life. Yes.

JP 2010-108944 A

  As described above, in a conventional lithium ion secondary battery using a negative electrode active material containing silicon particles, when lithium ions are inserted into the silicon particles, the silicon particles expand and break down, so that the charge / discharge cycle characteristics are significantly reduced. There is a problem.

  The present invention relates to a lithium ion secondary battery using a negative electrode active material containing silicon particles, which reduces the mechanical strain strength when lithium ions are inserted into silicon particles and has good charge / discharge cycle characteristics. An object is to provide a secondary battery.

  A lithium ion secondary battery according to the present invention comprises an electrolyte containing a lithium salt and a negative electrode having a negative electrode active material containing silicon particles having an average radius before charging of 280 a [nm] or less and 0.5 [nm] or more. And fully charged in time a [time].

  Alternatively, a lithium ion secondary battery according to the present invention includes an electrolyte solution containing a lithium salt and a negative electrode having a negative electrode active material containing silicon particles, and the negative electrode active material is mainly in a region where lithium ions are not inserted during charging. And the silicon particles have an average radius of 20% or less of the average radius of the silicon particles before charging.

  According to the present invention, in a lithium ion secondary battery using a negative electrode active material containing silicon particles, the mechanical strain strength when lithium ions are inserted into the silicon particles can be reduced, so that the charge / discharge cycle characteristics can be improved. it can.

The figure which shows the calculation model used for calculating the magnitude | size of the stress (sigma) _t of the circumferential direction of a silicon particle in this invention. The figure which shows the type | formula used for calculation of stress (sigma) _t of the circumferential direction of a silicon particle in this invention. It is a figure which shows the density | concentration distribution model of lithium ion in the radial direction of the silicon particle used by this invention, and is a figure which shows concentration distribution when lithium ion is diffusing inside the silicon particle. Used in the present invention, is a diagram showing a model of a lithium ion concentration distribution in the radial direction of the silicon particles, it shows a concentration distribution when the lithium ion concentration at the position R _c reaches the saturation value C _s. Used in the present invention, is a diagram showing a model of a lithium ion concentration distribution in the radial direction of the silicon particles, are inserted into further lithium ions silicon particles, spreads range lithium ion concentration is a saturated value C _s The figure which shows density distribution at the time. Used in the present invention, it is a diagram showing a model of a lithium ion concentration distribution in the radial direction of the silicon particles, when the lithium ion concentration in the whole range from the position R _c to the position R ₀ is a saturated value C _s The figure which shows density | concentration distribution. FIG. 3C is a diagram showing a model of lithium ion concentration distribution in the radial direction of the silicon particles shown in FIG. 3C and an expression representing this concentration distribution. The figure which shows the formula showing the stress (sigma) _t of the circumferential direction in the surface of a silicon particle, and the graph showing the stress (sigma) _t of the circumferential direction calculated | required using this formula. The figure which shows the result of having calculated (sigma) _t ( _R0 ) / (sigma) _t ( _Rc ) as a function of _Rc / _R0 . The figure which shows the calculation model used for calculating the relationship between the charging / discharging speed | rate and the stress (sigma) _t of the circumferential direction of a silicon particle in this invention. The figure which shows the calculation process for calculating the relationship between charging / discharging speed | rate and the stress (sigma) _t of the circumferential direction of a silicon particle. The figure which shows the relationship between time a which takes to fully charge, and radius _R0 of a silicon particle. The figure which shows the structure of the negative electrode of the lithium ion secondary battery by Example 1 of this invention. The figure which shows the structure of the negative electrode of the lithium ion secondary battery by Example 2 of this invention. The figure which shows the structure of the negative electrode of the lithium ion secondary battery by Example 3 of this invention. The figure which shows the internal structure of the lithium ion secondary battery by Example 4 of this invention. The figure which shows the charging / discharging cycling characteristics of the lithium ion secondary battery in Example 5 of this invention.

In the lithium ion secondary battery according to the present invention, the negative electrode has a negative electrode active material containing silicon particles, and the silicon particles are destroyed by reducing the mechanical strain strength when lithium ions are inserted into the silicon particles. Prevent and improve charge / discharge cycle characteristics. In order to reduce the mechanical strain strength when lithium ions are inserted into silicon particles, the mechanical strain strength is calculated, and the mechanical strain strength is calculated with respect to the charge / discharge speed when charging the lithium ion secondary battery. The average radius of silicon particles that was as small as possible was determined. Hereinafter, how to determine the average radius of such silicon particles will be described. In the following description, assuming that the silicon particles are spherical, the stress σ _t in the circumferential direction of the silicon particles is obtained as the mechanical strain strength of the silicon particles. This is because whether or not silicon particles are destroyed is mainly determined by the stress σ _t in the circumferential direction. The average radius of the silicon particles is simply referred to as “radius”.

FIG. 1 is a diagram showing a calculation model used to calculate the magnitude of the stress σ _{t in} the circumferential direction of silicon particles in the present invention. The silicon particles were spheres with a radius R ₀ before charging, and the circumferential stress was σ _t . Further, the region where the lithium ions are not inserted in the central portion of the silicon particle is assumed to be a spherical region, the average radius of this spherical region is R _c , and the center is the center of the silicon particle. The average radius of the sphere region is simply referred to as “radius”.

It is known that when lithium ions are inserted into silicon particles during charging of a lithium ion secondary battery, a region where lithium ions are not inserted is formed at the center of the silicon particles. This phenomenon can be observed using a transmission electron microscope (TEM). Reference Document 1 describes an example of a silicon nanopillar, and Reference Document 2 describes an example of a silicon nanowire.
[Reference 1] Proceedings of National Academy of Sciences of the United State of America, 109, pp.4080-4085, 2012.
[Reference 2] Nano Letters, 11, pp.2251-2258, 2011.
Therefore, also in this calculation model, it is assumed that there is a region where lithium ions are not inserted in the center portion of the silicon particle, and this region is a spherical region whose radius is _{Rc and} whose center is the center of the silicon particle.

Furthermore, it was assumed that lithium ions were inserted into the silicon particles with a constant flux and instantly diffused into the silicon particles. The lithium ions are concentrated as they diffuse into the silicon particles, and the diffusion stops when the lithium ion concentration becomes equal to the lithium ion concentration of Li ₁₅ Si ₄ . At this time, it was assumed that a spherical region into which lithium ions are not inserted (radius is R _c and the center is the center of the silicon particle) remains in the center portion of the silicon particle. Accordingly, lithium ions are present in a region between the radius R _c and the radius R ₀ inside the silicon particle.

FIG. 2 is a diagram showing an equation (2-1) used in the calculation of the stress σ _{t in} the circumferential direction of the silicon particles in the present invention. Formula (2-1) is a formula obtained by applying a general formula for calculating the circumferential stress due to thermal expansion to a formula for calculating the circumferential stress from the concentration of lithium ions. . The method shown in Reference 3 was used to derive Equation (2-1).
[Reference 3] Journal of the Electrochemical Society, 154, pp. A910-916, 2007.
FIG. 2 also shows the explanation of symbols used in Expression (2-1).

  3A to 3D are diagrams showing models of lithium ion concentration distribution in the radial direction of silicon particles used in the present invention. 3A to 3D, the horizontal axis indicates the position in the radial direction from the center of the silicon particles, and the vertical axis indicates the concentration of lithium ions. Time elapses in the order of FIGS. 3A, 3B, 3C, and 3D.

As shown in FIG. 3A, as lithium ions are inserted into the silicon particles with a constant flux from the radial position _R0 and diffuse into the silicon particles, the lithium ion concentration becomes the diffusion distance (from the position _R0 ). It becomes higher at the ratio of the square of the distance.

As shown in FIG. 3B, when the lithium ion concentration at the radial position R _c reaches the saturation value C _s , the lithium ions are located inside the silicon particles (position r where r <R _c ) rather than the position R _c. Does not spread. In FIG. 3B, the concentration C ₀ is the lithium ion concentration at the position R ₀ when the lithium ion concentration is C _s at the position R _c . As shown in FIG. 3A, the lithium ion concentration increases at a ratio of the square of the diffusion distance (distance from position R ₀ ), so the lithium ion concentration at position R ₀ from the lithium ion concentration C _s at position R _c. C ₀ can be determined.

Further lithium ions are inserted into the silicon particles, as shown in FIG. 3C, the range lithium ion concentration is a saturated value C _s is spread toward the position R ₀ from the position R _c.

Further lithium ions are inserted into the silicon particles, as shown in FIG. 3D, the lithium ion concentration in the whole range from the position R _c to the position R ₀ is the saturation value C _s, to the silicon particles of the lithium-ion The insertion process ends.

FIG. 4 is a diagram showing a model of the lithium ion concentration distribution in the radial direction of the silicon particles shown in FIG. 3C and equations (4-1), (4-2), and (4-3) representing the concentration distribution. It is. The position R _{i in} the radial direction of the silicon particles is a boundary position between a range where the lithium ions reach the saturation value C _s and a range where the lithium ion does not reach the saturation value C _s . Expression (4-3) is obtained on the assumption that the lithium ion concentration c (r) is expressed by a quadratic expression at the position r in the range between the position R _i and the position R ₀ . When Expressions (4-1), (4-2), and (4-3) are substituted into Expression (2-1) in FIG. 2, the position r in the radial direction of the silicon particles, the stress σ _{t in the} circumferential direction, and Is required. When r = R ₀ , the stress σ _t in the circumferential direction on the surface of the silicon particle is obtained (when obtaining the stress σ _t at the position R ₀ , Equation (4-3) is changed to Equation (2-1). Use the formula obtained by substitution).

Figure 5 is a formula (5-1) representing the stress sigma _t of (position in _{R 0)} circumferentially at the surface of the silicon particles, the stress in the circumferential direction obtained using Equation (5-1) sigma _t It is a figure which shows the graph showing. The stress σ _t in the circumferential direction at the position R ₀ can be expressed by Expression (5-1) as a function of the position R _i . Graph of Figure 5, the horizontal axis represents the position _{R i,} the vertical axis at the position _{R 0,} indicates the circumferential direction of the stress sigma _{t normalized,} calculated R 0 = 100 _nm, as R c = 10 nm. The saturation concentration C _s of lithium ions is 3.75 as the molar ratio of lithium of Li ₁₅ Si ₄ to silicon, and the concentration C ₀ is determined as 0.0375 (= C _s * (R _c / R ₀ ) ² ). It was.

In the calculation example of FIG. 5, on the surface of the silicon particles, the value of R _i is consuming tensile stress between 46nm from 10 nm, the value of R _i is understood that the take compressive stress on the contrary more than 46nm. In the calculation example of FIG. 5, the tensile stress when R _i = R _c is larger than the compressive stress when R _i = R ₀ .

FIG. 6 shows that R _c / R ₀ is used as an evaluation parameter of the stress σ _{t in} the circumferential direction (at the position R ₀ ) on the surface of the silicon particle, and σ _t (R ₀ ) / σ _t (R _c ) is R _c. It is a figure which shows the result calculated as a function of / _R0 . In the graph shown in FIG. 6, the horizontal axis indicates R _c / R ₀ , and the vertical axis indicates σ _t (R ₀ ) / σ _t (R _c ) at the position R ₀ .

The _value of σ _t (R ₀ ) / σ _t (R _c ) at the position R ₀ is desirably 2.0 or less. This is because if the _value of σ _t (R ₀ ) / σ _t (R _c ) at the position R ₀ exceeds 2.0, it is considered that the silicon particles are broken by tensile stress. FIG. 6 shows that when R _c / R ₀ is 20% or less, the _value of σ _t (R ₀ ) / σ _t (R _c ) at position R ₀ is 2.0 or less. In order to more reliably prevent the silicon particles from being destroyed, it is more desirable that the value of σ _t (R ₀ ) / σ _t (R _c ) at the position R ₀ is 1.5 or less. This can be realized when R _c / R ₀ is 10% or less.

In addition, the value of _Rc / _R0 can be _calculated | required by taking out silicon particle from the negative electrode active material of a lithium ion secondary battery, and observing the taken-out silicon particle with a transmission electron microscope (TEM).

Next, the relationship between the charge / discharge rate and the stress σ _{t in} the circumferential direction of the silicon particles will be described.

FIG. 7 is a diagram showing a calculation model used to calculate the relationship between the charge / discharge rate and the stress σ _{t in} the circumferential direction of the silicon particles. This calculation model is basically the same as the calculation model shown in FIG. That is, the silicon particles are spheres having a radius R ₀ and the stress in the circumferential direction is σ _t . In addition, the region where the lithium ions are not inserted in the central portion of the silicon particle is assumed to be a spherical region, the radius of the spherical region is R _c , and the center is the center of the silicon particle.

FIG. 7 also shows an expression (7-1) that represents the definition of the parameter p used in the following description. The parameter p is R _c / R ₀ and is the horizontal axis of the graph shown in FIG. That is, the parameter p is an evaluation parameter of the stress σ _{t in} the circumferential direction (at the position R ₀ ) on the surface of the silicon particle. As described above, the value of the parameter p (= R _c / R ₀ ) can be obtained by observing silicon particles with a transmission electron microscope (TEM).

FIG. 8 is a diagram showing a calculation process for calculating the relationship between the charge / discharge rate and the stress σ _{t in} the circumferential direction of the silicon particles. FIG. 8 shows four formulas (8-1) to (8-4).

In the silicon particle, assuming that lithium ions are inserted in the range of the radial position between R _c and R ₀ and the composition of Li ₁₅ Si ₄ is formed (that is, fully charged), the charging is performed. The amount of lithium ions formed is given by equation (8-1).

  When the amount of lithium ions given by the equation (8-1) is charged at time a [hour], that is, when fully charged at time a [hour], lithium ion charged per second is charged. Is given by equation (8-2).

Radius atomic number of silicon at the surface of the sphere area of R _c is given by Equation (8-3).

If it is assumed that Li ₁₅ Si ₄ is formed on the surface of the spherical region having a radius R _c by charging at the speed of the equation (8-2), from the equations (8-2) and (8-3), The relationship of Formula (8-4) is obtained.

FIG. 9 is a diagram illustrating the relationship between the time a required for full charge and the radius R _{0 of the} silicon particles. FIG. 9 shows an equation (9-1) representing the relationship between the time a [time] and the radius R ₀ [nm]. Expression (9-1) can be derived from Expression (8-4) and Expression (7-1). The relationship of Expression (9-1) depends on the value of the parameter p (= R _c / R ₀ ) obtained by TEM. Further, FIG. 9 also shows the value of the radius R ₀ when p is 0.1 and when p is 0.2. From equation (9-1), when p = 0.1, R ₀ = 70a [nm], and when p = 0.2, R ₀ = 278a [nm].

As described with reference to FIG. 6, in order to minimize the stress σ _{t in the} circumferential direction (at the position R ₀ ) on the surface of the silicon particle and prevent the silicon particle from being destroyed, p (= R _c / R ₀ ) Is preferably 0.2 or less, and more preferably p is 0.1 or less. That is, in order to prevent destruction of silicon particles when fully charged at time a [time], the radius R _{0 of the} silicon particles is preferably 278 a [nm] or less, and more preferably 70 a [nm] or less. Is desirable.

Further, the radius R ₀ of the silicon particles for preventing the silicon particles from being destroyed when fully charged at time a [time] may vary by about several nm. Accordingly, in order to prevent the silicon particles from being destroyed when fully charged at time a [time], the radius R _{0 of the} silicon particles may be 280 a [nm] or less.

However, since the silicon particles of the radius _{R 0} of less than 0.5 [nm] are difficult to prepare, the radius _{R 0} of the silicon particles is desirably at 0.5 [nm] or more.

  The value of time a [hour] required to fully charge the lithium ion secondary battery varies depending on the specifications and use conditions of the lithium ion secondary battery, but is not less than 0.1 [hour] and not more than 10 [hour]. It is common to set a range.

Note that the value of the radius R ₀ of the silicon particles can be obtained by observing the silicon particles with a transmission electron microscope (TEM).

  From the above detailed calculation, it was possible to clarify the radius of the silicon particles necessary for minimizing the stress in the circumferential direction of the silicon particles when lithium ions were inserted into the silicon particles. In the lithium ion secondary battery according to the embodiment of the present invention, silicon particles having such a radius are used as the negative electrode active material. The specific value of the radius of the silicon particles can be obtained by determining in advance the time a [time] required for full charge, that is, the charge / discharge rate. If the charge / discharge rate is 1 / a [C], the time required for full charge is a [time]. Therefore, the desired value of the radius of the silicon particles can be obtained using the value of a.

  Hereinafter, the negative electrode structure of the lithium ion secondary battery according to the example of the present invention will be described in Examples 1 to 3. Example 4 describes the internal structure of a lithium ion secondary battery according to an example of the present invention, and Example 5 describes charge / discharge cycle characteristics of a lithium ion secondary battery according to an example of the present invention.

FIG. 10 is a diagram showing the structure of the negative electrode of the lithium ion secondary battery according to Example 1 of the present invention. The negative electrode of the lithium ion secondary battery of this example includes a negative electrode current collector 1003, a nano thin film graphite 1001 disposed on the negative electrode current collector 1003, and silicon particles 1002 supported on the surface of the nano thin film graphite 1001. With. The nano thin film graphite 1001 is a conductive material, and is a carbon material for supporting the silicon particles 1002. A composite material in which silicon particles 1002 are supported on the nano-thin graphite 1001 is the negative electrode active material in this example. As the negative electrode active material, when the charge / discharge rate is 1 / a [C], silicon particles 1002 having a radius R ₀ of 280 a [nm] or less are used, and more preferably, the radius R ₀ is 70 a [nm] or less. Some silicon particles 1002 are used.

  Nano-thin graphite 1001 is oxidized by a so-called Hummers method in which flake graphite having a diameter of 10 μm to 500 μm is immersed in concentrated sulfuric acid, sodium nitrate and potassium permanganate for several days and stirred, and further, at 600 ° C. in an argon atmosphere. For 1 hour. The film thickness in the C-axis direction of the nano thin film graphite 1001 is 1.0 nm to 50.0 nm, and can be adjusted to some extent by changing the oxidation conditions and the foaming treatment conditions. Alternatively, the nano-thin graphite 1001 is oxidized by mixing flake graphite and benzoyl peroxide powder and heat-treating at 110 ° C. for 10 minutes, and further performing foaming treatment at 600 ° C. for 1 hour in an argon atmosphere. It can also be produced.

Assuming that the untreated flaky graphite has a disk shape with a diameter of 100 μm and the thickness in the C-axis direction is 10 μm, the specific surface area of the flaky graphite is 0.11 m ² / g. However, the calculation was performed assuming that the true density of graphite was 2.26 g / cm ³ . On the other hand, assuming that the thickness of the nano thin film graphite 1001 in the C-axis direction is 10 nm, the specific surface area of the nano thin film graphite graphite is 1000 times 110 m ² / g.

  Next, a method for supporting silicon particles 1002 on the surface of the nano thin film graphite 1001 will be described. The nano thin film graphite 1001 is arranged inside the quartz furnace, hydrogen and argon are bubbled by using flow rates of 40 mL / min and 360 mL / min, respectively, and tetraethyl orthosilicate is introduced into the quartz furnace, and the nano thin film graphite 1001 is formed. Heated at 600 ° C. for 1 hour. Silicon particles 1002 were supported on the surface of the nano thin film graphite 1001 by such a thermal vapor deposition method. In this case, tetraethyl orthosilicate is a source of silicon material and hydrogen acts as a reducing agent. The average radius of the silicon particles 1002 is 20 nm when determined by observation with a transmission electron microscope (TEM). The weight ratio between the silicon particles 1002 and the nano thin film graphite 1001 is 10:90.

  In this thermal vapor deposition method, it is possible to use silane gas, silicon tetrachloride, or the like as a silicon material source in addition to tetraethyl orthosilicate. Since silicon tetrachloride is a liquid at room temperature, it needs to be introduced into the quartz furnace by bubbling, like tetraethyl orthosilicate.

  In addition to the method described above, the silicon particles 1002 are supported on the surface of the nano thin film graphite 1001. In addition to the above, after sodium ions in the sodium silicate aqueous solution are subjected to hydrogen ion exchange, this solution and the nano thin film graphite 1001 are mixed. A method of carrying by sonication is also possible. In this case, it is possible to use silicates such as potassium silicate and aluminum silicate in addition to sodium silicate.

Assuming that the silicon particles 1002 are hemispherical with a radius of 20 nm and the spherical cross section is in contact with the nano thin film graphite 1001, the total contact area of 0.1 g of silicon particles is 3.2 m ² . Further, since the specific surface area of 0.9 g of the nano thin film graphite 1001 is 99 m ² (= 110 × 0.9), the coverage of the surface of the nano thin film graphite 1001 with the silicon particles 1002 is 3.2 / 99 = 3. 2%. Since this coverage is a considerably small value, it is possible to carry more silicon particles 1002 on the nano-thin graphite 1001.

FIG. 11 is a diagram showing the structure of the negative electrode of the lithium ion secondary battery according to Example 2 of the present invention. 11, the same reference numerals as those in FIG. 10 denote the same or common components as those in FIG. 10, and the description of these components will be omitted. The difference of the negative electrode of the lithium ion secondary battery of this example from that of Example 1 is that carbon nanotubes 1101 were used instead of nano thin film graphite as a carbon material for supporting silicon particles 1002. . A composite material in which silicon particles 1002 are supported on carbon nanotubes 1101 as a conductive material is the negative electrode active material in this example. As described in Example 1, when the charge / discharge rate is 1 / a [C], silicon particles 1002 having a radius R ₀ of 280 a [nm] or less are used as the negative electrode active material. Silicon particles 1002 having R ₀ of 70 a [nm] or less are used.

  As the carbon nanotube 1101, at least one of a single wall carbon nanotube and a multi-wall carbon nanotube can be used. The single-walled carbon nanotube desirably has a diameter of 0.7 nm to 2.0 nm and a length of 1 μm or more. The multiwall carbon nanotubes preferably have a diameter of 1 nm to 30 nm and a length of 1 μm or more. As the carbon nanotubes 1101, those produced by an arbitrary method such as a thermal vapor deposition method or an arc discharge method can be used. As a method for supporting the silicon particles 1002 on the carbon nanotubes 1101, the same method as in the first embodiment can be used.

FIG. 12 is a diagram showing the structure of the negative electrode of the lithium ion secondary battery according to Example 3 of the present invention. 12, the same reference numerals as those in FIG. 10 indicate the same or common components as those in FIG. 10, and description of these components is omitted. The difference of the negative electrode of the lithium ion secondary battery of this example from that of Example 1 is that a graphene sheet 1201 was used in place of nano-thin graphite as a carbon material for supporting silicon particles 1002. . A composite material in which the silicon particles 1002 are supported on the graphene sheet 1201 which is a conductive material is the negative electrode active material in this example. As described in Example 1, when the charge / discharge rate is 1 / a [C], silicon particles 1002 having a radius R ₀ of 280 a [nm] or less are used as the negative electrode active material. Silicon particles 1002 having R ₀ of 70 a [nm] or less are used.

  The graphene sheet 1201 can be produced by using stronger oxidation conditions in the method for producing nano-thin graphite from flaky graphite described in Example 1. The graphene sheet 1201 can also be manufactured by a thermal vapor deposition method using alumina as a substrate. For example, an alumina substrate is placed inside a quartz furnace, propylene and argon are flown into the quartz furnace at flow rates of 50 mL / min and 250 mL / min, respectively, and growth is performed at 800 ° C. for 2 hours. In addition, the graphene sheet 1201 can be manufactured. As a method for supporting the silicon particles 1002 on the graphene sheet 1201, the same method as in Example 1 can be used.

  In Examples 1 to 3 described above, the case where nano-thin graphite, carbon nanotubes, and graphene sheets are used as materials for supporting silicon particles has been described, but a plurality of supporting materials are selected and mixed from these three supporting materials. It is also possible to use the above as a support material. It is also possible to use a conductive carbon material other than these three as the support material.

  FIG. 13 is a diagram illustrating an internal structure of a lithium ion secondary battery according to Example 4 of the present invention. The lithium ion secondary battery of this example includes a positive electrode 1310, a separator 1311, a negative electrode 1312, a battery can 1313, a positive electrode current collector tab 1314, a negative electrode current collector tab 1315, an inner lid 1316, an internal pressure release valve 1317, a gasket 1318, and a positive temperature. A coefficient (PTC: Positive temperature coefficient) resistance element 1319 and a battery lid 1320 are provided. The battery lid 1320 is an integrated part composed of an inner lid 1316, an internal pressure release valve 1317, a gasket 1318, and a positive temperature coefficient resistance element 1319.

The positive electrode 1310 was produced by the following procedure. LiMn ₂ O ₄ was used as the positive electrode active material. To the 85.0 wt% positive electrode active material, 7.0 wt% graphite powder and 2.0 wt% acetylene black were added as conductive materials. Further, as a binder, a solution of 6.0 wt% polyvinylidene fluoride (hereinafter referred to as “PVDF”) dissolved in 1-methyl-2-pyrrolidone (hereinafter referred to as “NMP”) was added. The slurry was prepared by mixing with a planetary mixer. Thereafter, bubbles in the slurry were removed under vacuum to prepare a homogeneous positive electrode mixture slurry. This positive electrode mixture slurry was applied uniformly and evenly on both surfaces of an aluminum foil having a thickness of 20 μm using an applicator. This aluminum foil is a positive electrode current collector. The aluminum foil coated with the positive electrode mixture slurry was compressed and molded by a roll press so that the electrode density was 2.55 g / cm ³ . The compression-molded aluminum foil was cut with a cutting machine to produce a positive electrode 1310 having a thickness of 100 μm, a length of 900 mm, and a width of 54 mm.

  Note that the positive electrode 1310 manufactured as described above has a portion (current collector exposed surface) where the positive electrode mixture slurry is not applied to the positive electrode current collector.

The negative electrode 1312 was produced by the following procedure. As the negative electrode active material, the composite material described in Example 1 in which silicon particles 1002 are supported on nano-thin graphite 1001 was used. A solution in which 5.0 wt% PVDF was dissolved in NMP as a binder was added to 95.0 wt% of the negative electrode active material, and mixed with a planetary mixer to prepare a slurry. Thereafter, bubbles in the slurry were removed under vacuum to prepare a homogeneous negative electrode mixture slurry. This negative electrode mixture slurry was applied uniformly and evenly on both surfaces of a rolled copper foil having a thickness of 10 μm using a coating machine. This rolled copper foil is a negative electrode current collector 1003. The rolled copper foil coated with the negative electrode mixture slurry was compressed and molded by a roll press so that the electrode density was 1.3 g / cm ³ . The compression-molded rolled copper foil was cut with a cutting machine to produce a negative electrode 1312 having a thickness of 110 μm, a length of 950 mm, and a width of 56 mm.

Two types of negative electrodes 1312 were produced. One silicon particle having an average radius R ₀ of 20 [nm] and the other silicon particle having an average radius R ₀ of 40 [nm] were used as silicon particles 1002 included in the negative electrode active material. This is the negative electrode 1312. When the charge / discharge rate is 3.5 [C], that is, a = 2/7 [hour], the desirable average radius of the silicon particles is 83 (= 280a) [nm] or less. Therefore, when the lithium ion secondary battery of this example is fully charged at a charge / discharge rate of 3.5 [C] (a = 2/7 [hour]), the average radius _R0 before charging is 280 a [nm] or less. That is, the negative electrode active material containing silicon particles is used for the negative electrode.

  Note that the negative electrode 1312 manufactured as described above has a portion where the negative electrode mixture slurry is not applied to the negative electrode current collector 1003 (current collector exposed surface).

  The positive electrode current collecting tab 1314 and the negative electrode current collecting tab 1315 were connected to the exposed surfaces of the current collector plates of the positive electrode 1310 and the negative electrode 1312 produced as described above by an ultrasonic welding machine, respectively. An aluminum lead piece was used for the positive electrode current collecting tab 1314, and a nickel lead piece was used for the negative electrode current collecting tab 1315. Thereafter, a separator 1311 made of a porous polyethylene film having a thickness of 30 μm was inserted between the positive electrode 1310 and the negative electrode 1312, and the positive electrode 1310, the separator 1311, and the negative electrode 1312 were wound to form a wound body. The wound body is stored in the battery can 1313, the negative electrode current collector tab 1315 is connected to the bottom of the battery can 1313 by a resistance welder, and the positive electrode current collector tab 1314 is connected to the bottom surface of the inner lid 1316 by an ultrasonic welder. did.

Before attaching the battery lid 1320 to the battery can 1313, a non-aqueous electrolyte was injected into the battery can 1313. The solvent of the electrolytic solution was composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), and the volume ratio was 1: 1: 1. The electrolyte is LiPF ₆ at a concentration of 1 mol / L (about 0.8 mol / kg). Such a non-aqueous electrolyte containing a lithium salt is dropped from above the electrode group (rolled body) and injected into the battery can 1313, and the battery lid 1320 is caulked and sealed in the battery can 1313, and the lithium ion secondary battery is sealed. Got.

Two types of lithium ion secondary batteries were produced. One is a lithium ion secondary battery (lithium ion secondary battery A) including a negative electrode manufactured using a negative electrode active material using silicon particles having an average radius R ₀ of 20 [nm]. Is a lithium ion secondary battery (lithium ion secondary battery B) including a negative electrode manufactured using a negative electrode active material using silicon particles having an average radius R ₀ of 40 [nm].

  As the negative electrode active material, the composite material described in Example 2 in which silicon nanotubes 1002 are supported on carbon nanotubes 1101 may be used, and the silicon particles 1002 are supported in graphene sheet 1201 described in Example 3. You may use the made composite material.

  FIG. 14 is a diagram showing charge / discharge cycle characteristics of the lithium ion secondary battery. The graph of FIG. 14 shows cycle characteristics when the time a required for full charge is 2/7 [hours], that is, when charge / discharge is performed at a charge / discharge rate of 3.5 [C]. The number of cycles is shown, and the vertical axis shows the capacity maintenance rate.

“●” in the graph represents the charge / discharge cycle characteristics of the lithium ion secondary battery A produced in Example 4. The average radius R ₀ of the silicon particles contained in the negative electrode active material of the lithium ion secondary battery A is 20 [nm]. The lithium ion secondary battery A was able to obtain a high capacity retention rate of 90% even after repeated charging and discharging 50 times.

“▲” in the graph represents the charge / discharge cycle characteristics of the lithium ion secondary battery B produced in Example 4. The average radius R ₀ of the silicon particles contained in the negative electrode active material of the lithium ion secondary battery B is 40 [nm]. The lithium ion secondary battery B was able to obtain a good capacity maintenance rate of 75% even when charging and discharging were repeated 50 times.

“■” in the graph is a charge / discharge cycle characteristic of a lithium ion secondary battery C produced as a comparative example. The lithium ion secondary battery C is a lithium ion secondary battery including a negative electrode manufactured using a negative electrode active material using silicon particles having an average radius R ₀ of 100 [nm], as described in Example 4. It was produced by the same method. In the lithium ion secondary battery B, when the charge and discharge were repeated 50 times, the capacity retention rate was significantly reduced to 20% or less.

  From FIG. 14, the lithium ion secondary batteries A and B of the present example have a higher capacity retention rate and significantly improved charge / discharge cycle characteristics as compared with the lithium ion secondary battery C produced as a comparative example. I understand that. As described above, the lithium ion secondary battery according to the present example uses the silicon particles whose average radius is determined in consideration of the charge / discharge speed as the negative electrode active material, so that lithium ions are inserted into the silicon particles. Since the stress in the circumferential direction of the silicon particles can be reduced, the charge / discharge cycle characteristics can be greatly improved.

  In addition, this invention is not limited to said Example, Various modifications are included. For example, the above-described embodiments are described in detail for easy understanding of the present invention, and the present invention is not necessarily limited to an aspect including all the configurations described.

  DESCRIPTION OF SYMBOLS 1001 ... Nano thin film graphite, 1002 ... Silicon particle, 1003 ... Negative electrode collector, 1101 ... Carbon nanotube, 1201 ... Graphene sheet, 1310 ... Positive electrode, 1311 ... Separator, 1312 ... Negative electrode, 1313 ... Battery can, 1314 ... Positive electrode current collector Tab, 1315 ... Negative current collector tab, 1316 ... Inner lid, 1317 ... Internal pressure release valve, 1318 ... Gasket, 1319 ... Positive temperature coefficient resistance element, 1320 ... Battery lid.

Claims (7)

An electrolyte containing a lithium salt;
A negative electrode having a negative electrode active material containing silicon particles having an average radius before charging of 280a [nm] or less and 0.5 [nm] or more,
Fully charged at time a [hour],
The lithium ion secondary battery characterized by the above-mentioned.   The lithium ion secondary battery according to claim 1, wherein the time a is 0.1 [hour] or more and 10 [hour] or less. An electrolyte containing a lithium salt;
Comprising a negative electrode having a negative electrode active material containing silicon particles,
The negative electrode active material includes the silicon particles having a region in which lithium ions are not inserted at the time of charging, the average radius of the region being 20% or less of the average radius of the silicon particles before charging,
The lithium ion secondary battery characterized by the above-mentioned.   4. The lithium ion secondary battery according to claim 1, wherein the negative electrode active material has the silicon particles supported on a conductive carbon material. 5.   The lithium ion secondary battery according to claim 4, wherein the carbon material is nano-thin graphite.   The lithium ion secondary battery according to claim 4, wherein the carbon material is a carbon nanotube.   The lithium ion secondary battery according to claim 4, wherein the carbon material is a graphene sheet.

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