JP2018147772A - Lithium ion secondary battery negative electrode material and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery negative electrode material having high capacity and capable of maintaining high charge/discharge efficiency even when charge/discharge is repeated.SOLUTION: The lithium ion secondary battery negative electrode material including silicon particles covered with nano graphene and a carbon material has a structure that silicon particles are embedded in the carbon material and is granular as a whole. Concerning a differential value dQ/dV in a relationship between a voltage V of a negative electrode to lithium metal used as a reference and a charge capacity Q of a lithium ion secondary battery, when defined that the first local maximum being the local maximum of differential value dQ/dV in a voltage V of 270-330 mV is dQ/dV, and the second local maximum being the local maximum of differential value dQ/dV in a voltage V of 420-480 mV is dQ/dV, a ratio (dQ/dV)/(dQ/dV) is 2.5 or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode material for a lithium ion secondary battery and a lithium ion secondary battery.

Graphite-based carbon materials are widely used as negative electrode active materials for lithium ion secondary batteries. The stoichiometric composition when graphite is filled with lithium ions is LiC ₆ and its theoretical capacity can be calculated as 372 mAh / g.

On the other hand, the stoichiometric composition when silicon is filled with lithium ions is Li ₁₅ Si ₄ or Li ₂₂ Si ₅ , and the theoretical capacity can be calculated as 3577 mAh / g or 4197 mAh / g. Thus, silicon is an attractive material that can store 9.6 times or 11.3 times as much lithium as graphite. However, when the silicon particles are filled with lithium ions, the volume expands to about 2.7 times or 3.1 times, so that the silicon particles are mechanically destroyed while repeatedly filling and releasing lithium ions. When the silicon particles are broken, the broken fine silicon particles are electrically isolated, and a new electrochemical coating layer is formed on the broken surface, whereby the irreversible capacity is increased and the charge / discharge cycle characteristics are remarkably lowered.

By making silicon particles nano-sized as a negative electrode active material of a lithium ion secondary battery, physical destruction associated with filling and releasing of lithium ions can be prevented. However, there has been a problem that due to the volume change accompanying the filling and releasing of lithium ions, some of the silicon nanoparticles are electrically isolated and the life characteristics are greatly deteriorated due to this. In addition, since nanoparticles have essentially a large specific surface area of several tens of m ² / g or more, there is a problem that the amount of electrochemical film produced is proportionally large and the Coulomb efficiency is low.

  Patent Document 1 includes, as a constituent element of a negative electrode of a power storage device such as a lithium ion secondary battery, an Si / C composite in which at least a part of nanosilicon agglomerated particles is covered with a carbon layer, and an aqueous binder. A negative electrode active material layer is described.

  Patent Document 2 uses a three-electrode cell using a working electrode having a silicon-containing material as an active material, a reference electrode made of metallic lithium, a counter electrode made of metallic lithium, and a lithium ion conductive electrolyte. By charging / discharging, the charge / discharge capacity Q is measured, and the differential value dQ / dV is calculated by differentiating with the potential V of the working electrode with reference to the reference electrode, and between 260 mV and 320 mV at the time of discharging A silicon-containing material is described in which the ratio B / A between the maximum value A of the differential value dQ / dV and the maximum value B of the differential value dQ / dV between 420 mV and 520 mV is 2 or less. In patent document 2, it aims at improving a capacity | capacitance maintenance factor (cycle property) with high capacity | capacitance.

JP-A-2015-179593 Japanese Patent Laying-Open No. 2015-2036

  In Patent Document 1, as the carbon constituting the carbon layer covering the Si / C composite, only amorphous or crystalline carbon, or a mixture of amorphous carbon and crystalline carbon is used. Are listed. In Patent Document 1, the formation of a carbon layer is performed by reacting nanosilicon aggregated particles with a substance considered to be a liquid such as an aromatic heterocyclic compound in a mixed state. Accordingly, there is no description of an example of using a vapor deposition method when forming a carbon layer, and no description of graphene as a constituent element. When a liquid is used as the carbon source, it is difficult to form a carbon layer densely. When carbon is not dense, the electrolyte solution easily passes through the carbon layer, so that it easily reacts with the electrolyte solution on the surface of the nanosilicon particles, and the charge / discharge efficiency may be reduced.

  The silicon-containing material described in Patent Document 2 is intended to improve the capacity retention rate (cycleability) with a high capacity, and therefore it is desirable that the ratio B / A regarding the maximum value of dQ / dV is low. The conclusion has been reached. Patent Document 2 does not focus on maintaining charge / discharge efficiency. In order to maintain charge / discharge efficiency, it is important to suppress reactions in which lithium is partially released from an amorphous silicon alloy containing lithium. In this case, the maximum value A of the differential value dQ / dV between 260 mV and 320 mV described in Patent Document 2 is considered to be small. In that case, since A is small, the ratio B / A is high. Therefore, the feature described in Patent Document 2 that the ratio B / A is low is undesirable from the viewpoint of maintaining the charge / discharge efficiency.

  An object of this invention is to provide the negative electrode material for lithium ion secondary batteries which is high capacity | capacitance and can maintain high charging / discharging efficiency even if charging / discharging is repeated.

The negative electrode material for a lithium ion secondary battery of the present invention includes a silicon particle coated with nanographene and a carbon material, and has a structure in which the silicon particle is embedded in the carbon material, and is in the form of particles as a whole. The differential value dQ / dV in the relationship between the negative electrode voltage V with respect to the reference lithium metal and the charge capacity Q of the lithium ion secondary battery is the maximum value of the differential value dQ / dV in the voltage V range of 270 mV to 330 mV. first dQ / dV ₁ the maximum value, when defining a second maximum value dQ / dV ₂ which is the maximum value of the differential value dQ / dV in a range 420mV~480mV voltage V, the ratio (dQ / dV ₂ ) / (DQ / dV ₁ ) is 2.5 or more.

  According to the present invention, a negative electrode material having a high capacity and capable of maintaining high charge / discharge efficiency even after repeated charge / discharge can be obtained, and a high capacity and long life lithium ion secondary battery is provided. can do. Problems, configurations, and effects other than those described above will become apparent from the following description of embodiments.

4 is a graph showing a differential value dQ / dV of a charge capacity Q with respect to a voltage V. 1 is a schematic diagram showing a negative electrode material for a lithium ion secondary battery of Example 1. FIG. It is an expanded sectional view which shows the nano graphene covering scale-like silicon particle of FIG. 2A. It is a graph which shows the relationship between the cycle number of the negative electrode material for lithium ion secondary batteries, and Coulomb efficiency. It is a graph which shows the pore distribution of the negative electrode material for lithium ion secondary batteries. Is a graph showing the charge-discharge efficiency with respect to _{(dQ / dV 2) / (} dQ / dV 1). It is a fragmentary longitudinal cross-section which shows the example of the lithium ion secondary battery of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions, but by those skilled in the art within the scope of the technical idea disclosed in this specification. Various changes and modifications are possible.

  In the present specification, the numerical range “a to b”, that is, “a to b” is synonymous with “from a to b”. This is also synonymous with “a or more and b or less”.

  In the present invention, the surface of silicon particles as a negative electrode active material is coated with nanographene, and the coated silicon particles are embedded in amorphous carbon, thereby suppressing reaction (surface reaction) on the surface of silicon. Characteristic voltages corresponding to the phenomenon in which lithium is desorbed from silicon during the charging process are around 0.3 V (around 300 mV) and around 0.45 V (around 450 mV). Here, the voltage is the voltage (potential difference) of the negative electrode with respect to the reference lithium metal.

  Regarding two peak values (maximum values) of the differential value dQ / dV of the charge capacity Q corresponding to these voltages V, the maximum value of dQ / dV near 0.45V and the maximum value of dQ / dV near 0.3V. It is desirable that the ratio with the value is large.

  As will be described later, the first maximum value (dQ / dV maximum value near 300 mV) and the second maximum value (dQ / dV maximum value near 450 mV) are defined, but in this specification, The vicinity of 300 mV means the range of 270 mV to 330 mV, and the vicinity of 450 mV means the range of 420 mV to 480 mV.

  Here, graphene and nanographene are defined.

  Graphene refers to a single-layer assembly (molecule) in which carbon atoms are densely packed in a two-dimensional honeycomb lattice. And nano graphene means the graphene which has a dimension of a nanometer order. In the present specification, the nano graphene more specifically refers to those having a diameter of the disc of 100 nm or less, which is a representative dimension when the molecules constituting the graphene are assumed to be disc-like.

  FIG. 1 is a graph showing a differential value dQ / dV of the charge capacity Q with respect to the voltage V. The horizontal axis represents the negative electrode voltage V (potential difference) relative to the reference lithium metal, and the vertical axis represents dQ / dV when the charge capacity of the lithium ion secondary battery is Q. On the vertical axis, the positive region is the discharge side and the negative region is the charge side. Examples 1 and 2 described later and a comparative example are shown.

  As shown in this figure, on the discharge side, the peak is present at the voltage V of about 300 mV, about 450 mV, and about 500 mV, which is particularly noticeable in the comparative example. In each of these three peaks, the following reaction occurs. In the following description, “about” may be omitted.

i) In the vicinity of the 300 mV peak, amorphous Li _x Si (hereinafter referred to as “a-Li _x Si”, where “a” means amorphous) changed to a-Li _{x ′} Si (x> x ′). To do.

ii) In the vicinity of the 450 mV peak, Li _{x ′} Si changes to a-Si.

iii) near the peak of 500 mV, a-Li _{x ′} Si changes to a-Si.

  Of these peaks, the peak at 450 mV is the highest.

In Examples 1 and 2, the 300 mV and 500 mV peaks are not clear. That is, at 300 mV and 500 mV, the differential value (secondary differential value) d ² Q / dV ² of the differential value dQ / dV with respect to the voltage V may not be zero. This is because, in Examples 1 and 2, the reaction i) is suppressed by the structure in which the silicon particles coated with nanographene are embedded in amorphous carbon or nanographene.

  On the other hand, in the comparative example, since the silicon particles coated with nanographene are not embedded in the carbon material, the reaction i) is likely to proceed. Moreover, in the case of a comparative example, the particle | grains of negative electrode material are easy to contact with electrolyte solution, and it is easy to produce SEI (Solid Electrolyte Interface).

  Thus, in the case of Examples 1 and 2, since the reaction i) is suppressed and the generation of SEI is also suppressed, the capacity is high and high charge / discharge efficiency is maintained even after repeated charge / discharge. can do.

In Examples 1 and 2, the value of dQ / dV at 300 mV is 2 kAh · (kg · V) ⁻¹ or less. That is, it is 2000 Ah · (kg · V) ⁻¹ or less. This is a result of the suppression of the reaction i).

  In Examples 1 and 2, since the peak is not clear at 300 mV, for convenience, the value of dQ / dV at 300 mV is regarded as the first maximum value. In the case of the comparative example, since the peak is clear, the value of dQ / dV in the vicinity of 300 mV (range of 270 mV to 330 mV) is set as the first maximum value.

  On the other hand, for the local maximum value in the vicinity of 450 mV, the actual local maximum value of dQ / dV is used, and this is used as the second local maximum value. When the peak is clear at any voltage, the actual dQ / dV value at the peak is adopted as the maximum value.

Hereinafter, the first maximum value is dQ / dV ₁ and the second maximum value is dQ / dV ₂ . Therefore, the first maximum value and the second maximum value are obtained in order from the lowest voltage V.

As a result of repeated studies, the present inventor determined that the ratio of dQ / dV ₂ to dQ / dV ₁ (dQ / dV ₂ ) / (dQ / dV ₁ ) is Q ′, and the value of Q ′ is high. It has been found that the cycle characteristics are improved.

  When the value of Q ′ near 0.30 V is high, the amount of surface reaction is large and the amount of film formation is large. In this case, the battery life tends to be short.

  The value of Q ′ is preferably 2.5 or more. Further, in order to achieve a life similar to that of a carbon material, Q ′ is desirably 3.5 or more.

  The average particle size of the negative electrode material is desirably 5 μm or more and 30 μm or less. By making the average particle diameter 5 μm or more, the production cost can be suppressed. Further, when the average particle size is 30 μm or less, electrode formation by slurry application is facilitated.

  The pore distribution of silicon, which is a negative electrode active material, desirably has many pores in a pore diameter range of 100 nm to 1 μm. The pores in this range alleviate expansion and contraction in the charge / discharge cycle, and can suppress the destruction of the silicon surface due to the cycle. Thereby, cycle deterioration due to generation of a new surface can be suppressed.

  By setting the pore diameter to 100 nm or more, expansion and contraction of silicon can be relaxed. On the other hand, by setting the pore diameter to 1 μm or less, the silicon particles and the electrolytic solution can be prevented from coming into contact with each other, the generation of the surface coating can be suppressed, and the deterioration of silicon can be prevented. Hereinafter, the pore diameter is also referred to as “pore diameter”.

  The content of silicon contained in the negative electrode material is desirably 30% by mass or more and 80% by mass or less. Moreover, it is more desirable that it is 40 mass% or more and 70 mass% or less.

  In addition, as a silicon material to be used, a scaly thing is desirable. By adopting this shape, destruction due to expansion during lithium occlusion can be suppressed, and cycle characteristics can be improved.

  Details of the examples and comparative examples will be described below.

  FIG. 2A schematically shows the negative electrode material for a lithium ion secondary battery of this example.

  The particle 100 of the negative electrode material shown in this drawing has a structure in which nanographene-coated scale-like silicon particles 101 are embedded in amorphous carbon 102. In other words, the nanographene-coated scale-like silicon particles 101 are dispersed in amorphous carbon 102 (carbon material) to form the particles 100 as a whole.

  The major axis of the nanographene-coated scale-like silicon particles 101 is 10 nm or more and 1 μm or less. By setting the major axis to 10 nm or more, aggregation of silicon particles can be suppressed, and a negative electrode material having a desired structure can be manufactured. In addition, by setting the major axis to 1 μm or less, it is possible to suppress breakage due to physical expansion during lithium storage.

  The thickness of the nanographene-coated scale-like silicon particles 101 is 5 nm or more and 100 nm or less. By setting the thickness to 5 nm or more, it is possible to increase physical strength and prevent pulverization when a battery is manufactured. Further, by setting the thickness to 100 nm or less, it is possible to prevent destruction due to physical expansion during lithium storage.

  FIG. 2B is an enlarged cross-sectional view showing the silicon particles of FIG. 2A.

  As shown in FIG. 2B, the nanographene-coated scale-like silicon particles 101 have a structure in which the surface of the scale-like silicon particles 103 is covered with a nanographene layer 104 (graphite thin film). The nano graphene layer 104 is preferably formed so as to be in direct contact with the surface of the scaly silicon particles 103.

  The thickness of the nanographene layer 104 is preferably 2 nm or more and 10 nm or less. By setting the thickness to 2 nm or more, physical strength can be ensured. In addition, by setting the thickness to 10 nm or less, the nano graphene layer 104 can be arranged along the surface of silicon, and electrical conductivity can be improved.

  For the formation of the nano graphene layer 104, a chemical vapor deposition apparatus (not shown) equipped with a reaction furnace was used, and a vapor phase growth method in which propylene gas was supplied to the scaly silicon particles and heated was used. The film thickness (the amount of nanographene to be stacked) can be controlled by setting the growth time and the like.

  Hereinafter, an example of the formation process of the nano graphene layer by the vapor deposition method will be described.

  In the chemical vapor deposition apparatus, scale-like silicon particles as a base material are put into a reaction furnace, hydrogen gas at a predetermined flow rate is supplied to the reaction furnace, and the temperature is raised from room temperature to 1000 ° C. at a rate of 10 ° C./min. Hold at 1000 ° C. for 1 hour. By this heat treatment step, the natural oxide film formed on the surface of the scaly silicon particles can be reduced.

  Thereafter, the supply of hydrogen gas was stopped, a predetermined flow rate of argon gas was supplied, and the temperature was lowered at a rate of 10 ° C./min, and the temperature was lowered to 800 ° C. When the temperature reached 800 ° C., a propylene gas having a predetermined flow rate was supplied, and at the same time, the flow rate of the argon gas was adjusted to grow a nanographene layer.

  Thereafter, the supply of propylene gas was stopped, and only argon gas was flowed to cool naturally.

The nano graphene layer manufactured by the above process has a structure in which nano graphene is stacked in multiple layers. Since the oxide film has been removed in the above process, there is almost no oxide film on the surface of the scaly silicon particles, and the nanographene layer is in direct contact with silicon. The electrical conductivity of the nanographene-coated scaly silicon particles is 1000 S / m or more.
Next, a method for manufacturing the negative electrode material particle 100 illustrated in FIG. 2A will be described.

  A structure in which nanographene-coated scale-like silicon particles 101 were embedded in amorphous carbon 102 was produced by the following method.

  Carbon raw materials such as petroleum pitch, naphthalene, anthracene, phenanthrolen, coal tar, phenol resin, and polyvinyl alcohol and nanographene-coated scaly silicon particles were mixed at a predetermined mass ratio and subjected to high temperature treatment. That is, the high-temperature treatment is performed by putting the above mixture into a heat treatment furnace, raising the temperature to 900 ° C. at a temperature increase rate of 10 ° C./hr while flowing an inert gas such as nitrogen and argon, and holding the mixture for a certain time. It was. The carbon raw material was carbonized by high temperature treatment.

  Then, it cooled naturally and it was set as the particulate negative electrode material. By pulverizing and classifying the negative electrode material, a negative electrode material having an arbitrary average particle size and particle size distribution can be obtained.

  The average particle size is desirably 5 μm or more and 30 μm or less. By setting the thickness to 5 μm or more, the manufacturing cost can be suppressed. Moreover, the electrode formation by slurry application | coating can be made easy by setting it as 30 micrometers or less.

  In this example, nanographene-coated scale-like silicon particles were produced by the same method as in Example 1.

  Then, the nanographene-coated scaly silicon particles were embedded in nanographene (carbon material) instead of being embedded in amorphous carbon. Thereby, instead of the amorphous carbon 102 shown in FIG. 2A, particles of a negative electrode material having a structure embedded in nanographene were produced.

  The process of embedding in nanographene was performed using the above-described vapor phase growth method. Then, the treated particulate material was pulverized and classified to obtain a negative electrode material having the same average particle diameter and particle size distribution as in Example 1.

(Comparative example)
The negative electrode material of the comparative example was used as it was without producing nano-graphene-coated scale-like silicon particles by the same method as in Example 1 and embedding in amorphous carbon or the like. The average particle diameter is equivalent to the nano graphene-coated scale-like silicon particles of Example 1.

  The pore distribution was measured by the same method as in Example 1, electrodes were prepared, and negative electrode capacity and lifetime were evaluated.

  Hereinafter, the measurement method and the like will be described.

(Measurement of pore distribution)
The pore distribution was measured by a mercury intrusion method. The measurement range of the pore diameter is 0.01 to 10 μm.

(Electrode fabrication method)
The negative electrode was produced by the following procedure.

  The negative electrode material produced by the above-described method, styrene-butadiene rubber (SBR), and carboxymethylcellulose (CMC) were mixed at a mass ratio of 90: 5: 5. Here, SBR and CMC are binders.

  A negative electrode mixture slurry was prepared from this mixture using water as a dispersion medium.

  This negative electrode mixture slurry was applied to a copper foil having a thickness of 10 μm and dried in a vacuum.

This was molded by a roll press so as to have a density of 1.3 g / cm ³ to produce a negative electrode.

(Negative electrode capacity and life evaluation method)
A battery having a Li metal counter electrode was produced.

  The battery was discharged to 0.01 V at a constant current of 200 mA / g. Then, it charged to 1.5V at 200mA / g.

  This discharging and charging was repeated 10 times.

  The negative electrode voltage V when charging and discharging, the battery discharge capacity, and the battery charge capacity Q were measured.

  Based on this measurement result, dQ / dV was calculated and the relationship with the voltage V was evaluated.

The ratio (dQ / dV ₂ ) / (dQ / dV ₁ ) (= Q ′) was calculated and evaluated.

  Hereinafter, measurement results and the like will be described.

  FIG. 3 is a graph showing the relationship between the number of cycles and the coulomb efficiency. Here, the Coulomb efficiency (%) represents the ratio between the discharge capacity and the charge capacity in the same cycle as a percentage. Coulomb efficiency is called charge / discharge efficiency.

  In this figure, the result of Example 1 and a comparative example is shown.

  From this figure, it can be seen that Example 1 has higher Coulomb efficiency. This is particularly noticeable in the initial stage.

  FIG. 4 is a graph showing the pore distribution of the negative electrode material particles.

  In this figure, the result of Example 1 and a comparative example is shown.

  In the case of Example 1, there is one peak around the pore diameter of 0.2 μm, that is, 200 nm. In addition, in the range where the pore diameter is 1 μm or less, the pore volume is concentrated in the range of 0.1 μm to 0.8 μm (100 nm to 800 nm). In particular, it is concentrated in the range of 0.1 μm to 0.5 μm (100 nm to 500 nm).

  The pore distribution of Example 1 is such that in the negative electrode material particles having a structure in which silicon particles coated with nanographene are embedded in amorphous carbon, the amorphous carbon absorbs the volume change of the silicon particles, It is thought that it contributes to suppressing particle destruction.

  On the other hand, in the case of the comparative example, there are two peaks in the pore diameters of 0.04 μm and 0.12 μm, that is, around 40 nm and 120 nm. For this reason, in the case of the comparative example, when the pore diameter is 1 μm or less, there are many pores in the range of less than 0.1 μm.

  From the pore distribution of Example 1, the negative electrode material particles are within the pore diameter range of 0.01 to 1 μm and within the pore diameter range of 0.1 to 1 μm (100 nm to 1 μm) on the pore volume basis. It is desirable that 80% or more of the pores are distributed. Further, the range of the pore diameter in which 80% or more of the pores are distributed based on the above criteria is more preferably 100 nm to 800 nm. A particularly desirable range is 100 nm to 500 nm.

  In addition, although the pore distribution is not illustrated for the particles of the negative electrode material of Example 2, there are few pores in the range of 100 nm to 800 nm as in Example 1, and the volume change of the silicon particles is absorbed. Compared with Example 1, the effect of suppressing the destruction of the particles of the material is small. This effect appears as a temporary decrease in Coulomb efficiency when several tens of charge / discharge cycles are performed. However, this decrease is not so problematic as to be practical.

FIG. 5 is a graph showing the charge / discharge efficiency (Coulomb efficiency) at the 10th cycle with respect to (dQ / dV ₂ ) / (dQ / dV ₁ ). The broken line in the figure clearly shows the position where (dQ / dV ₂ ) / (dQ / dV ₁ ) is 2.5 and the charge / discharge efficiency is 98.5%. The oblique solid line in the figure represents an approximate expression calculated by taking into account data of an example (not shown) using a negative electrode material obtained by subjecting nanographene-coated scaly silicon particles to electroless copper plating. It is a thing. The negative electrode material subjected to the electroless copper plating treatment is in the form of particles having the same average particle size and particle size distribution as in Example 1.

In this figure, Examples 1 and 2 belong to the upper right region (desired region) where both (dQ / dV ₂ ) / (dQ / dV ₁ ) and charge / discharge efficiency are high. In contrast, the comparative example belongs to the lower left region where both (dQ / dV ₂ ) / (dQ / dV ₁ ) and the charge / discharge efficiency are low. Although not shown, the negative electrode material subjected to electroless copper plating also belongs to the lower left region.

The value of (dQ / dV ₂ ) / (dQ / dV ₁ ) at which the slanted solid line shown in FIG. Therefore, the upper limit value of (dQ / dV ₂ ) / (dQ / dV ₁ ) is considered to be 4.5.

  Table 1 summarizes the data of the examples and comparative examples.

  This table shows the Coulomb efficiency (charge / discharge efficiency) and Q 'after 10 cycles.

In addition, as shown in FIG. 1, in Example 1 and 2, the peak of 300 mV is not clear. In Examples 1 and 2, a-Li _x Si is changed to a-Li _{x ′} Si (x> x ′) due to the structure in which silicon particles covered with nano graphene are embedded in amorphous carbon or nano graphene. It is because the reaction to suppress is suppressed.

  In the comparative example, since the silicon particles covered with nanographene are not embedded in the carbon material, the particles of the negative electrode material are likely to come into contact with the electrolytic solution, and SEI is likely to occur.

  Hereinafter, the whole structure of the lithium ion secondary battery of this invention is demonstrated.

  FIG. 6 is a partial cross-sectional view showing an example of a lithium ion secondary battery.

  In this figure, each code | symbol has shown the following.

  601 is a positive electrode, 602 is a separator, 603 is a negative electrode, 604 is a battery can, 605 is a positive electrode current collecting tab, 606 is a negative electrode current collecting tab, 607 is an inner lid, 608 is an internal pressure release valve, 609 is a gasket, 610 is a positive temperature A coefficient resistance element (also referred to as a “PTC resistance element”. PTC is an abbreviation for Positive Temperature Coefficient), 611 is a battery lid.

  The battery lid 611 is an integrated part including an inner lid 607, an internal pressure release valve 608, a gasket 609, and a positive temperature coefficient resistance element 610.

  The positive electrode 601 can be manufactured, for example, by the following procedure.

LiMn ₂ O ₄ is used as the positive electrode active material. 7.0% by mass and 2.0% by mass of graphite powder and acetylene black are added as conductive materials to 85.0% by mass of the positive electrode active material, respectively. Further, a solution in which 6.0% by mass of polyvinylidene fluoride (hereinafter abbreviated as PVDF) was dissolved in 1-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) as a binder was added, and planetary was added. Mix with a mixer. Furthermore, bubbles in the slurry are removed under vacuum to prepare a homogeneous positive electrode mixture slurry.

This slurry is uniformly and evenly applied to both surfaces of an aluminum foil having a thickness of 20 μm using an applicator. After the application, compression molding is performed by a roll press so that the electrode density is 2.55 g / cm ³ . This is cut with a cutting machine to produce a positive electrode 601 having a thickness of 100 μm, a length of 900 mm, and a width of 54 mm.

  On the other hand, the negative electrode 603 can be produced, for example, by the following procedure.

  As a binder, 5.0% by mass of PVDF (solution dissolved in NMP) is added to 95.0% by mass of the negative electrode material of Example 1 or 2. It is mixed with a planetary mixer, and bubbles in the slurry are removed under vacuum to prepare a homogeneous negative electrode mixture slurry.

This slurry is uniformly and evenly applied to both surfaces of a rolled copper foil having a thickness of 10 μm with an applicator. After application, the electrode is compression-molded by a roll press to make the electrode density 1.3 g / cm ³ . This is cut with a cutting machine to produce a negative electrode 603 having a thickness of 110 μm, a length of 950 mm, and a width of 56 mm.

  The positive electrode current collecting tab 605 and the negative electrode current collecting tab 606 are ultrasonically welded to the uncoated portions (current collector plate exposed surfaces) of the positive electrode 601 and the negative electrode 603 manufactured as described above. An aluminum lead piece can be used for the positive electrode current collecting tab 605, and a nickel lead piece can be used for the negative electrode current collecting tab 606.

  Thereafter, a separator 602 made of a porous polyethylene film having a thickness of 30 μm is sandwiched between the positive electrode 601 and the negative electrode 603, and the positive electrode 601, the separator 602, and the negative electrode 603 are wound. The wound body is accommodated in the battery can 604, and the negative electrode current collecting tab 606 is connected to the bottom of the battery can 604 by a resistance welder. The positive electrode current collecting tab 605 is connected to the bottom surface of the inner lid 607 by ultrasonic welding.

Before attaching the upper battery lid 611 to the battery can 604, a non-aqueous electrolyte is injected. As a solvent for the electrolytic solution, for example, a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) is used. The volume ratio of the solvent components is, for example, 1: 1: 1. For example, LiPF ₆ is used as the solute electrolyte. The concentration is, for example, 1 mol / L (about 0.8 mol / kg). Such an electrolytic solution is dropped from above the wound body. Then, the battery lid 611 can be caulked and sealed to the battery can 604 to obtain a lithium ion secondary battery.

  100: particles of negative electrode material, 101: nanographene-coated scale-like silicon particles, 102: amorphous carbon, 103: scale-like silicon particles, 104: nanographene layer, 601: positive electrode, 602: separator, 603: negative electrode, 604: battery can, 605: Positive current collecting tab, 606: Negative current collecting tab, 607: Inner lid, 608: Internal pressure release valve, 609: Gasket, 610: Positive temperature coefficient resistance element, 611: Battery lid.

Claims (15)

Silicon particles coated with nanographene;
Carbon material, and
The silicon particles have a structure embedded in the carbon material, and are in the form of particles as a whole,
About the differential value dQ / dV in the relationship between the voltage V of the negative electrode with respect to the reference lithium metal and the charge capacity Q of the lithium ion secondary battery, the maximum value of the differential value dQ / dV in the voltage V range of 270 mV to 330 mV. When the first maximum value is defined as dQ / dV ₁ and the second maximum value that is the maximum value of the differential value dQ / dV in the voltage V range of 420 mV to 480 mV is defined as dQ / dV ₂ ,
The negative electrode material for a lithium ion secondary battery having a ratio (dQ / dV ₂ ) / (dQ / dV ₁ ) of 2.5 or more. 2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the first maximum value dQ / dV ₁ is 2000 Ah · (kg · V) ⁻¹ or less. 3. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein a secondary differential value d ² Q / dV ^{2 at the} voltage V corresponding to the first maximum value dQ / dV ₁ is not 0. 4.   The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the carbon material includes amorphous carbon or nano graphene.   The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the silicon particles have a configuration in which a nano graphene layer is in direct contact with silicon.   The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein an average particle size as a whole is 5 µm or more and 30 µm or less.   7. The method according to claim 1, wherein the pore diameter ranges from 0.01 to 1 μm, and the pore diameter ranges from 100 nm to 800 nm, and the pore volume is 80% or more on a pore volume basis. The negative electrode material for lithium ion secondary batteries as described.   The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the silicon content is 30% by mass or more and 80% by mass or less.   The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 8, wherein the silicon particles have a structure dispersed in the carbon material. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 9, wherein the ratio (dQ / dV ₂ ) / (dQ / dV ₁ ) is 4.5 or less. A positive electrode, a negative electrode, and a separator sandwiched between them,
The negative electrode includes a negative electrode material for a lithium ion secondary battery,
The negative electrode material for a lithium ion secondary battery includes silicon particles coated with nanographene and a carbon material, and has a structure in which the silicon particles are embedded in the carbon material. The differential value dQ / dV in the relationship between the negative electrode voltage V and the charge capacity Q with respect to the reference lithium metal is the first maximum value of the differential value dQ / dV in the voltage V range of 270 mV to 330 mV. dQ / dV ₁ the maximum value, when the second maximum value the a maximum value of the differential values dQ / dV in a range 420mV~480mV of the voltage V was defined as dQ / dV _2, the ratio (dQ / _dV 2) / (dQ / dV ₁₎ is 2.5 or more, the lithium ion secondary battery. 12. The lithium ion secondary battery according to claim 11, wherein the first maximum value dQ / dV ₁ is 2000 Ah · (kg · V) ⁻¹ or less. The lithium ion secondary battery according to claim 11 or 12, wherein a secondary differential value d ² Q / dV ^{2 at the} voltage V corresponding to the first maximum value dQ / dV ₁ is not 0.   The lithium ion secondary battery according to any one of claims 11 to 13, wherein the carbon material includes amorphous carbon or nanographene.   The lithium ion secondary battery according to any one of claims 11 to 14, wherein the silicon particles have a configuration in which a nano graphene layer is in direct contact with silicon.

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