JP2021132020A - Negative electrode for lithium secondary battery, and lithium secondary battery - Google Patents

Abstract

To provide a negative electrode for a lithium secondary battery, high in capacity and further improved in rate characteristics.SOLUTION: A negative electrode for a lithium secondary battery includes a current collector and a negative electrode mixture layer on the current collector. The negative electrode mixture layer has: a negative electrode material including a silicon particle 201, a first conductive material 202 and a second conductive material 203; and a binder. The first conductive material 202 is a material with a porous structure covering a surface of the silicon particle 201, and the second conductive material 203 is a fibrous material arranged to connect the first conductive materials 202 covering the surfaces of different silicon particles 201. The ratio of weight of the first conductive materials 202 with respect to the weight of the silicon particles 201 is 0.053-0.333, the ratio of weight of the second conductive materials 203 with respect to the total weight of the silicon particles 201 and the first conductive materials 202 is 0.020-0.176, and the density of the negative electrode mixture layer is 0.5-1.3 g/cm3.SELECTED DRAWING: Figure 2

Description

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

Lithium secondary batteries have a high energy density and are attracting attention as batteries for electric vehicles and power storage. Electric vehicles include zero-emission electric vehicles without an engine, hybrid electric vehicles with both an engine and a secondary battery, and plug-in electric vehicles that are charged from a grid power source, especially zero-mission electric vehicles and plugs. In-electric vehicles are required to have high energy density type lithium secondary batteries.

In order to increase the capacity of lithium secondary batteries, it is necessary to develop high-capacity electrode active materials. In particular, looking at the negative electrode, a silicon negative electrode having a theoretical capacity eight times or more that of the current graphite negative electrode is expected as a next-generation high-capacity negative electrode to replace graphite. However, since the volume change of the silicon negative electrode due to charging and discharging is large, improvement of the cycle life is an issue. The following prior arts are available to improve cycle life.

Patent Document 1 discloses a nanosilicon carbon composite material in which silica is electrochemically reduced by a molten salt electrolysis method. By using this, it is possible to overcome the defect that the silicon and the carbon material are separated due to the huge volume change of the silicon material in the process of occlusion and release of lithium and the cycle stability is deteriorated, and good cycle stability can be obtained.

Patent Document 2 discloses a negative electrode structure including silicon particles and an amorphous surface layer formed on the surface of the silicon particles. Since the oxygen content of the silicon particles is low and the aggregation of the silicon particles is suppressed, it is said that the performance of high energy density, high output density, and longer charge / discharge cycle life of the lithium secondary battery can be obtained.

Patent Document 3 discloses a multi-element composite negative electrode material having a multi-shell layer core-shell structure, and by using the multi-shell composite negative electrode material, the cycle characteristics of the lithium secondary battery and the initial charge / discharge efficiency are significantly improved.

Patent Document 4 is a porous silicon composite cluster containing a porous core containing secondary particles of the porous silicon composite and a shell containing a second graphene arranged on the upper part of the core, which is a porous silicon composite. The body secondary particles include an agglomerate of two or more silicon composite primary particles, and the silicon composite primary particles are silicon, silicon oxide (SiO _x ) (O <x <2) arranged on silicon, and the like. And a porous silicon composite cluster containing the first graphene arranged on the silicon oxide, a carbon composite using the same, an electrode containing the composite or the carbon composite, and the electrode containing the same. A lithium battery and a biosensor, an electric field emitting element, a thermoelectric element, and a semiconductor element containing the composite or a carbon composite are disclosed. When used, the formation of a silicon particle network can reduce electrode plate swelling during charging and discharging, improving initial efficiency and volumetric energy density, as well as providing a highly conductive and durable silicon protective layer. Can be formed to improve charge / discharge durability.

Special Table 2015-503185 Special Table 2016-509739 Special Table 2017-526118 JP-A-2018-48070

An object of the present invention is to provide a negative electrode for a lithium secondary battery having a high capacity and further improved rate characteristics, and a lithium secondary battery using the same.

In order to increase the capacity and rate of the silicon negative electrode, a conductive network capable of following the volume change of the silicon particles during charging and discharging is required. The conductive agent added with the silicon particles forms its conductive network.

Since the silicon particles, the conductive agent, and the binder are mixed during the production of the negative electrode, the conductive agent adheres to the surface of the silicon particles before charging. Since there is no change in the volume of the silicon particles before charging, electrons are distributed throughout the negative electrode via the conductive agent.

However, as charging progresses, the volume of the silicon particles gradually changes, and when the charging capacity exceeds 3000 mAh / g, the volume of the silicon particles expands to three times or more that before charging. In this process, if the conductive network is disconnected, electrons may not be supplied to some of the silicon particles, and subsequent charging may not proceed.

In the discharge process, electrons are taken from silicon and lithium ions are released from silicon. As the discharge capacity increases, the size of the silicon particles gradually decreases and returns to the size before charging. At this time, if the conductive agent is peeled off from the silicon surface and the silicon particles escape from the conductive network, electrons are not transmitted from the silicon to the conductive network during the discharge, and even charging cannot be performed again.

In this way, it is important for the conductive network to follow the volume change of the silicon particles in the charge / discharge process of the silicon particles, and if this does not work sufficiently, the charge / discharge of the silicon particles will occur in the middle of the charge / discharge cycle. This becomes impossible, the initial capacitance of the negative electrode is lowered, and the rate characteristics and the life of the charge / discharge cycle are also deteriorated.

Further, since the silicon surface is covered with an ultrathin oxide layer, the electron resistance of the surface is high. Surface modification is also required to impart electron conductivity to this high resistance layer. If the electron resistance on the surface is high, it becomes difficult to charge and discharge a large current, and the input / output density of the lithium secondary battery decreases.

Therefore, as a result of diligent studies to solve the above-mentioned problems, the present inventors have devised a negative electrode material composed of a first conductive material and a second conductive material that adhere to silicon particles. The first conductive material is a material having a porous structure that covers the surface of silicon particles. Since this is coated with silicon in a layered manner, it can follow the volume change of silicon and maintain electron conductivity. In addition, since it has a porous structure, lithium ions can be inserted and removed. Since the second conductive material is a fibrous material arranged so as to connect the first conductive materials that coat different silicon particles, the silicon particles change in volume and their positions shift. Also allows the transfer of electrons between particles. Furthermore, by setting the density of the negative electrode composed of the negative electrode material and the binder to 0.5 to 1.3 g / cm ³ , a space that can accommodate the body integral when the silicon particles expand is secured inside the negative electrode, and inside the negative electrode. No abnormal stress is generated, and the negative electrode is not destroyed during the charge / discharge cycle.

Based on such an invention concept, it has become possible to provide a negative electrode having a high capacity and excellent rate characteristics. That is, the present invention is a negative electrode for a lithium secondary battery including a current collector and a negative electrode mixture layer on the current collector, wherein the negative electrode mixture layer is a silicon particle, a first conductive material. The first conductive material has a negative electrode material including the second conductive material and a second conductive material, and the first conductive material is a material having a porous structure that covers the surface of the silicon particles, and the second conductive material. Is a fibrous material arranged so as to connect the first conductive materials that coat the surfaces of different silicon particles, and the ratio of the weight of the first conductive material to the weight of the silicon particles is The ratio of the weight of the second conductive material to the total weight of the silicon particles and the first conductive material is 0.020 to 0.176, which is 0.053 to 0.333, and the negative electrode combination. The agent layer has a density of 0.5 to 1.3 g / cm ³ .

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to increase the capacity of a lithium secondary battery and improve the rate characteristics. Issues, configurations and effects other than those described above will be clarified by the description of the following embodiments.

It is sectional drawing which shows one Embodiment of the lithium secondary battery which concerns on this invention. It is a figure for schematically explaining the structure of the negative electrode material in this invention.

Hereinafter, the present invention will be described in detail based on the embodiments.
FIG. 1 is a cross-sectional view showing an embodiment of a lithium secondary battery according to the present invention, and schematically shows the internal structure of the lithium secondary battery. The lithium secondary battery 101 is an electrochemical device that stores or makes available electrical energy by storing and releasing lithium ions to electrodes in a non-aqueous electrolyte. This is also called by another name such as a lithium ion battery, a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery, etc., but any battery is the subject of the present invention.

The lithium secondary battery 101 has a configuration in which an electrode group including a positive electrode 107, a negative electrode 108, and a separator 109 is housed in a battery container 102 in a sealed state. As the electrode group, various configurations can be adopted, such as a configuration in which strip-shaped electrodes are laminated, a configuration in which strip-shaped electrodes are wound and formed into a cylindrical or flat shape. The battery container 102 can be selected from any shape such as a cylindrical shape, a flat oval shape, and a square shape, depending on the shape of the electrode group. After accommodating the electrode group from the opening provided in the upper part of the battery container 102, the opening is closed by the lid 103 and sealed.

The outer edge of the lid 103 is joined to the opening of the battery container 102 by welding, caulking, adhesion, or the like over the entire circumference, and the battery container 102 is sealed in a sealed state. The lid 103 has a liquid injection port for injecting the electrolytic solution L into the battery container 102 after sealing the opening of the battery container 102. The liquid injection port is sealed by a liquid injection plug 106 after the electrolytic solution L is injected into the battery container 102. It is also possible to provide a safety mechanism to the liquid injection plug 106. As the safety mechanism, a pressure valve for releasing the pressure inside the battery container 102 may be provided. The lid 103 may be removed from the battery container 102 without providing the liquid injection port, and the electrolytic solution L may be added directly to the electrode group.

The positive electrode external terminal 104 and the negative electrode external terminal 105 are fixed to the lid 103 via the insulating seal member 112, and the short circuit between the positive electrode external terminal 104 and the negative electrode external terminal 105 is prevented by the insulating seal member 112. The positive electrode external terminal 104 is connected to the positive electrode 107 via the positive electrode lead wire 110, and the negative electrode external terminal 105 is connected to the negative electrode 108 via the negative electrode lead wire 111. The material of the insulating seal member 112 can be selected from fluororesin, thermosetting resin, glass hermetic seal, etc., and any insulating material that does not react with the electrolytic solution L and has excellent airtightness can be used. can.

The electrode group is formed by laminating a positive electrode 107 and a negative electrode 108 via a separator 109. The separator 109 is arranged between the positive electrode 107 and the negative electrode 108 to prevent short circuits between them, and is also inserted between the electrode group and the battery container 102 so that the positive electrode 107 and the negative electrode 108 are short-circuited through the battery container 102. It is configured not to. The electrolytic solution L is held on the surfaces of the separator 109, the positive electrode 107, and the negative electrode 108 and inside the pores. As the separator 109, a polyolefin-based polymer sheet made of polyethylene, polypropylene, or the like, or a multi-layered separator in which a polyolefin-based polymer and a fluorine-based polymer sheet typified by tetrafluoride polyethylene are welded is used. Can be done. For example, a mixture of ceramics and a binder may be formed in a thin layer on the surface of the separator 109 so that the separator 109 does not shrink when the battery temperature rises. Since these separators 109 need to allow lithium ions to permeate during charging and discharging of the lithium secondary battery, they must have a porous structure having a pore diameter of 0.01 to 10 μm and a void ratio of 20 to 90%. preferable.

The battery container 102 can have any shape, and can be appropriately selected from shapes such as a square shape and a cylindrical shape. As the material of the battery container 102, any material can be selected as long as it is not corroded by the electrolytic solution L or dissolved in the electrolytic solution L. For example, a laminate of stainless steel, galvanized steel sheet, plastic, or aluminum. Sheets and the like can be used.

(Manufacturing of positive electrode)
The positive electrode 107 is composed of a positive electrode active material, a conductive agent, a binder, and a current collector. As the positive electrode active material, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _4, or the like can be used. In addition, LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , Li ₄ Mn ₅ O ₁₂ , LiMn _2-x M _x O ₂ (where M is selected from the group consisting of Co, Ni, Fe, Cr, Zn and Ta). 1 or more, x = 0.01 to 0.2), Li ₂ Mn ₃ MO ₈ (where M is one or more selected from the group consisting of Fe, Co, Ni, Cu and Zn. ), Li _1-x A _x Mn ₂ O ₄ (where A is one or more selected from the group consisting of Mg, B, Al, Fe, Co, Ni, Cr, Zn and Ca, and x = 0.01 to 0.1), LiNi _1-x M _x O ₂ (where M is one or more selected from the group consisting of Co, Fe and Ga, and x = 0.01 to 0. .2), LiFeO ₂ , Fe ₂ (SO ₄ ) ₃ , LiCo _1-x M _x O ₂ (where M is one or more selected from the group consisting of Ni, Fe and Mn, and x = 0.01 to 0.2), LiNi _1-x M _x O ₂ (where M is one or more selected from the group consisting of Mn, Fe, Co, Al, Ga, Ca and Mg. (x = 0.01 to 0.2), Fe (MoO ₄ ) ₃ , FeF ₃ , LiFePO ₄ , LiMnPO ₄ , Li ₂ MnO _3- LiMnO _2- based solid solution and the like can be mentioned.

The average particle size (D ₅₀ ) of the positive electrode active material is specified to be equal to or less than the thickness of the mixture layer. If the positive electrode active material powder contains coarse particles having a size equal to or larger than the thickness of the mixture layer, the coarse particles are removed in advance by sieving, air flow classification, or the like to prepare particles having a thickness equal to or less than the thickness of the mixture layer. The average particle size can be measured using a particle size distribution meter using laser scattering.

In order to prepare the positive electrode 107, it is necessary to prepare a positive electrode slurry. The solvent used for preparing the slurry may be any solvent as long as it dissolves the binder, and can be appropriately selected depending on the type of binder. For example, when PVDF is used as the binder, it is preferable to use 1-methyl-2-pyrrolidone as the solvent. A known kneader or disperser can be used for the dispersion treatment of the positive electrode material such as the positive electrode active material.

After adhering the positive electrode slurry, which is a mixture of the positive electrode active material, the conductive agent, the binder and the organic solvent, to the current collector by the doctor blade method, the dipping method, the spray method, etc., the organic solvent is dried, and pressure molding is performed by a roll press. By doing so, a positive electrode can be produced. It is also possible to stack a plurality of mixture layers on the current collector by performing the process from application to drying a plurality of times.

As the current collector, an aluminum foil having a thickness of 10 to 100 μm, an aluminum perforated foil having a thickness of 10 to 100 μm and a hole diameter of 0.11 to 10 mm, an expanded metal, a foamed metal plate, or the like is used. In addition to aluminum, stainless steel, titanium, etc. can also be applied as the material. Any material can be used for the current collector without being limited by the material, shape, manufacturing method, etc., as long as the battery does not undergo changes such as dissolution and oxidation during use.

The thickness of the positive electrode mixture layer is preferably equal to or larger than the average particle size of the positive electrode active material. This is because if the thickness is less than the average particle size, the electron conductivity between adjacent particles deteriorates. The average particle size (D ₅₀ ) of the positive electrode active material is measured by a laser scattering method, and the range thereof is preferably 1 to 4 μm. It is desirable to use this positive electrode active material and make the mixture thickness 10 μm or more. Further, it is desirable that the upper limit of the thickness of the positive electrode mixture layer is 30 μm. This is because if the thickness is higher than that, unless the amount of the conductive agent is increased, the charge level of the positive electrode active material near the surface of the positive electrode mixture layer and the surface of the current collector varies, and uneven charging / discharging occurs. Increasing the amount of the conductive agent increases the volume of the positive electrode and reduces the energy density of the battery.

(Manufacturing of negative electrode)
The negative electrode 108 is composed of a current collector and a negative electrode mixture layer containing a negative electrode material and a binder provided on the current collector. In the present embodiment, the negative electrode material includes silicon particles, a first conductive material, and a second conductive material.

Silicon particles are crystalline or amorphous metal particles. A trace amount of silicon oxide may be present on the surface, a trace amount of oxygen or carbon may be contained inside the particle, and the particle as a whole can be applied as long as silicon is the main component. Further, the shape of the silicon particles may be any shape such as spherical, lump-shaped, and plate-shaped. Since the lithium secondary battery becomes smaller as the density of the negative electrode mixture layer increases, it is desirable that the silicon particles are spherical or agglomerated.

Due to the volume change during charging and discharging, a large stress is generated in the silicon particles, and the particles may collapse. This is so-called micronization of silicon particles, and as this progresses, the conductivity deteriorates and the number of chargeable and dischargeable silicon particles decreases. In order to avoid this, the stress can be reduced and the micronization can be suppressed by reducing the particle size of the silicon particles in advance. In order to suppress micronization during the initial charge / discharge cycle and increase the capacitance density of the negative electrode, it is preferable that the average particle size of the silicon particles is 5 nm or more and 500 nm or less. In order to improve the rate characteristics and cycle life, the average particle size is preferably 100 nm or less, and more preferably 70 nm or less. Further, if the silicon particles are too fine, the filling property in the negative electrode is deteriorated, and the capacity density per volume of the lithium secondary battery is lowered. Further, when the silicon particles become finer, the surface area thereof increases, so that the amount of side reactions other than charging and discharging increases, and the initial capacity of the lithium secondary battery decreases. Therefore, the average particle size of the silicon particles is preferably 10 nm or more, and more preferably 20 nm or more.

Next, with reference to FIG. 2, the structure of the negative electrode material in the present embodiment will be described with a focus on one silicon particle 201. As shown on the left side of FIG. 2, the silicon particles 201 in this embodiment have a first conductive material 202 that covers the surface of the particles.

The first conductive material 202 is preferably an agglomerate of fine particles or primary particles, and has a porous structure. Since the pores are present, lithium ions permeate, and the silicon particles 201 can occlude and release lithium ions. In the first conductive material 202, if pores remain, the layer having a porous structure may be densified by heat treatment.

As the material of the first conductive material 202, carbon black, furnace black, graphite, amorphous carbon and the like can be used. Their primary particle size is preferably 1/10 or less of that of the silicon particles 201, whereby the surface of the silicon particles 201 can be coated.

The weight ratio of the silicon particles 201 to the first conductive material 202 is in the range of 95: 5 to 75:25, in other words, the ratio of the weight of the first conductive material 202 to the weight of the silicon particles 201 is. It needs to be in the range of 0.053 to 0.333. If the ratio of the weight of the first conductive material 202 to the weight of the silicon particles 201 is smaller than 0.053, the conductive material is insufficient and a highly resistant surface remains on the silicon particles 201. On the contrary, when it exceeds 0.333, the ratio of the silicon particles 201 decreases, and the capacitance density of the negative electrode 108 decreases.

The coating thickness of the first conductive material 202 is preferably 1 nm or more. Further, the coverage of the silicon particles 201 on the surface is preferably 70% or more and 90% or less. When the surface exposed on the surface of the silicon particles 201 is set to 10% or more and 30% or less, lithium ions permeate and are easily occluded and released into the silicon particles 201, so that the capacity of the negative electrode can be increased.

The first conductive material 202 may contain, as a material other than carbon, a material that does not occlude, release, or dissolve lithium ions when the negative electrode is charged or discharged. Examples of such a material include titanium, iron, nickel, alumina, silica, lithium titanate, barium titanate, alkali metal oxide, alkaline earth metal oxide and the like.

Examples of the method of adhering the first conductive material 202 to the surface of the silicon particles 201 include a mechanical coating method using a ball mill, mechanical fusion, or the like, a vapor phase coating using a gas pyrolysis reaction, and the like. In the ball mill method, silicon powder and carbon powder are mixed, and the surface of the silicon particles 201 can be coated with carbon by the impact of high-speed rotation. Further, in the vapor phase coating method, silicon powder 201 and hydrocarbons (acetylene, propylene, etc.) are heated to a high temperature of 800 ° C. or higher, thereby decomposing the hydrocarbons and coating the surface of the silicon particles 201 with carbon. can.

The porous structure of the first conductive material 202 can be controlled by the amount of the first conductive material 202 added to the surface area of the silicon particles 201. The average surface area per silicon particle can be estimated based on the measured values obtained by measuring the particle size of the silicon particles 201, the specific surface area of the silicon particles 201, and the like. The amount of the first conductive material 202 added can be determined from the change in weight of the negative electrode material. By changing the coating amount and the coating thickness of the first conductive material 202, the average coating amount per silicon particle and the average pore size can be controlled.

The presence of the first conductive material 202 on the surface of the silicon particles 201 can be confirmed from the peaks of silicon and carbon by photoelectron spectroscopy. As a simple method, the coating thickness can be measured by scraping the surface by ion sputtering while performing photoelectron spectroscopy measurement. As a more rigorous method, the coating thickness of the first conductive material 202 can be measured from a transmission electron micrograph. The porous structure can be observed with a scanning electron microscope.

The coverage (θ) of the first conductive material 202 is the total specific surface area (S) of the silicon particles 201, and the thickness (t), weight (w), and density (ρ) of the first conductive material 202. ) Can be calculated. The density can be determined from the weight and thickness of the first conductive material adhered to the flat plate by the same method, but if it is a known material, the literature value may be used. The formula for calculating the coverage (θ) is shown below.
θ = w / (t ・ ρ ・ S)

The second conductive material 203 comes into contact with the first conductive material 202 and connects the first conductive materials 202 that coat the plurality of silicon particles 201 to each other. The second conductive material 203 functions as a conductive network of the entire negative electrode material.

If the length of the second conductive material 203 is ½ or more of the average particle size of the silicon particles 201, the silicon particles 201 can be connected to each other. Further, since the second conductive material 203 does not contribute to charging / discharging, it is desirable that the volume is small. When the diameter is 100 nm or less, the volume is small, which is preferable. The diameter may be 1/10 or less of the average particle size of the silicon particles 201, and is preferably 50 nm or less. When it is 10 nm or less, the volume ratio of the second conductive material 203 in the negative electrode material can be made extremely small.

Examples of the material that can be used as the second conductive material 203 include carbon fibers and carbon nanotubes. In particular, carbon nanotubes are suitable because they can impart electron conductivity to the negative electrode material with a small amount of addition.

The ratio of the total weight of the silicon particles 201 and the first conductive material 202 to the weight of the second conductive material 202 is in the range of 85:15 to 98: 2, in other words, the silicon particles 201 and the first. The ratio of the weight of the second conductive material 203 to the total weight of the conductive material 202 of the above must be in the range of 0.020 to 0.176. Within this range, the composition of the first conductive material 202 and the second conductive material 203 is optimized, and the conductive network is strengthened even if there is a volume change during charging / discharging.

FIG. 2 shows a state in which the negative electrode material is changed from the state before charging (discharged state) on the left side to the charged state on the right side. When the ratio of the weight of the second conductive material 203 to the total weight of the silicon particles 201 and the first conductive material 202 is in the range of 0.020 to 0.176, the state changes from the discharged state to the charged state. However, the conductive network is maintained, and it is possible to return to the discharged state as it is.

The composite of the silicon particles 201 and the first conductive material 202 and the second conductive material 203 is composed of a unit of primary particles and a unit of secondary particles. The primary particles are composed of the first conductive material 202 that covers one silicon particle, and are the smallest units obtained by mechanically crushing the composite. Secondary particles are defined as agglomerate particles in which a plurality of primary particles are electrically connected by a second conductive material 203 and aggregated in the form of particles. Inside the negative electrode, the plurality of primary particles are electrically connected by the second conductive material 203, so that all the silicon particles 201 can be charged and discharged.

The composite of silicon particles (secondary particles) is composed of a plurality of primary particles and a second conductive material 203. The average particle size (D ₅₀ ) of the secondary particles is specified to be equal to or less than the thickness of the negative electrode mixture layer. When there are coarse particles in the silicon particles having a size equal to or larger than the thickness of the mixture layer, the coarse particles are removed in advance by sieving classification, wind flow classification, etc. to prepare secondary particles having a size equal to or less than the thickness of the mixture layer. The average particle size can be measured using a particle size distribution meter using laser scattering.

The negative electrode 108 in this embodiment can further contain graphite in the negative electrode mixture layer. Since graphite itself has conductivity, when it comes into contact with the first conductive material 202 or the second conductive material 203, it is possible to impart conductivity to the entire negative electrode material. Further, since graphite has a theoretical capacity of 372 Ah / kg, lithium ions can be occluded and released.

If the amount of graphite added is controlled to be within the following range, a high-capacity negative electrode material can be obtained. That is, the total weight of carbon contained in the first conductive material 202, the second conductive material 203, and graphite is defined as the total carbon amount, and the ratio of the total carbon amount to the weight of the silicon particles 201 is defined as the total carbon amount. Is preferably 0.5 or more and 1.5 or less. In the absence of graphite, the total carbon content is defined as the total weight of carbon contained in the two materials, the first conductive material 202 and the second conductive material 203, again in this case the silicon particles 201. The ratio of the total carbon content to the weight of the above is preferably 0.5 or more and 1.5 or less.

The negative electrode material is mixed with a binder to form a layer on the negative electrode current collector to obtain the negative electrode 108. A mixed layer composed of a negative electrode material and a binder is called a negative electrode mixture layer. The density of the negative electrode mixture layer is in the range of ^{0.5 to 1.3 g / cm 3.}

When the density of the negative electrode mixture layer is within the above range, a space is secured in the negative electrode mixture layer, and even when the silicon particles 201 expand as shown in FIG. 2, the space remains in the negative electrode mixture layer and the electrolytic solution. Channel is retained. Therefore, even in the charged state, lithium ions can be diffused inside the negative electrode, and lithium ions can be occlusioned. Further, even in the case of discharging on the contrary, the necessary electrolytic solution is supplied in the vicinity of the silicon particles 201, and lithium ions can be discharged to the electrolytic solution.

The negative electrode 108 can be produced by using the same method as that of the positive electrode 107. That is, a negative electrode material containing silicon particles 201, the first conductive material 202 and the second conductive material 203, a binder and an organic solvent were mixed to prepare a negative electrode slurry, and the negative electrode slurry was attached to the current collector. After that, the negative electrode 108 can be obtained by drying the organic solvent and press-molding it by a roll press. It is also possible to stack a plurality of negative electrode mixture layers on the current collector by performing the process from application of the negative electrode slurry to drying a plurality of times.

As the binder, PVDF, a copolymer of vinylidene fluoride and hexafluoropropylene (PVDF-HFP), styrene butadiene rubber, carboxymethyl cellulose and the like can be used. In addition, fluororubber, ethylene / propylene rubber, polyacrylic acid, polymethylmethacrylate, chitosan, alginic acid, polyimide, polyamide and the like are also applicable and are not particularly limited. Any binder that does not decompose on the surface of the negative electrode 108 and does not dissolve in the electrolytic solution L can be used as appropriate. The compounding ratio of the negative electrode material and the binder in the negative electrode mixture layer can be appropriately set. The negative electrode material: binder = 98: 2 to 80:20 (weight ratio) is preferable, and the negative electrode material: binder = 95: 5 to 85:15 (weight ratio) is more preferable. ..

It is desirable that the binder is made of a material that covers at least a part of the surface of the silicon particles 201 and is permeable to lithium ions. As a result, at the time of charging, the lithium ions in the electrolytic solution L permeate through the binder and reach the surface of the silicon particles 201 to occlude the lithium ions. On the contrary, at the time of discharge, lithium ions can be discharged from the surface of the silicon particles 201 to the electrolytic solution L via the binder. By covering the surface of the silicon particles 201 with the binder, the reduction reaction of the electrolytic solution L is suppressed, so that the initial charge / discharge efficiency (Coulomb efficiency) can be increased and the cycle life can be improved. Since the binder functions as a lithium ion conductor and the first conductive material 202 functions as an electron conductor, even if there is a volume change of the silicon particles 201 due to charging / discharging, the binder or the like follows the volume change and ions. It is thought that the conductivity of electrons is less likely to deteriorate.

The binder may be integrally formed with the first conductive material 202 and formed as a composite layer on at least a part of the surface of the silicon particles 201. The composite layer can transfer both electrons and ions.

It is desirable that the thickness of the negative electrode mixture layer is equal to or larger than the average particle size of the negative electrode material. This is because if the thickness is less than the average particle size, the electron conductivity between the adjacent silicon particles 201 deteriorates. Further, the thickness of the negative electrode mixture layer is preferably 10 μm or more, more preferably 15 μm or more. The upper limit of the thickness of the negative electrode mixture layer is preferably 50 μm. This is because if the thickness is higher than that, the charging level of the negative electrode active material varies between the surface of the mixture layer and the vicinity of the surface of the current collector unless the amount of the conductive agent is increased, and uneven charging / discharging occurs. When the amount of the conductive agent is increased, the volume of the negative electrode becomes bulky and the energy density of the battery decreases.

As the current collector, a copper foil having a thickness of 10 to 100 μm, a copper perforated foil having a thickness of 10 to 100 μm and a pore diameter of 0.1 to 10 mm, an expanded metal, a foamed metal plate, or the like is used. As the material of the current collector, stainless steel, titanium, nickel and the like can be applied in addition to copper. In this embodiment, any current collector can be used without being limited by the material, shape, manufacturing method, and the like.

(Electrolyte)
_{As a typical example of the electrolytic solution L that can be used in the present embodiment, lithium hexafluorophosphate (LiPF 6} ) or lithium hexafluorophosphate (LiPF 6) as an electrolyte is used in a solvent obtained by mixing ethylene carbonate with dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, or the like. Examples thereof include a solution in which lithium borofluoride (LiBF ₄ ) or the like is dissolved. Electrolytes having other compositions can be used without being limited by the type of solvent and electrolyte, and the mixing ratio of the solvent. It is also possible to use the electrolyte in a state of being contained in an ionic conductive polymer such as polyvinylidene fluoride or polyethylene oxide. In this case, the separator 109 becomes unnecessary.

In addition to the above typical examples, the solvents that can be used in the electrolytic solution L include propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, chloroethylene carbonate, chloropropylene carbonate, and γ-. Dioxolanes such as butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, methyl propionate, ethyl propionate, phosphorus There are non-aqueous solvents such as acid triester, trimethoxymethane, diethyl ether, sulfolane and 3-methyl-2-oxazolidinone. A solvent other than this may be used as long as it is a solvent that does not decompose on the positive electrode or the negative electrode built in the lithium secondary battery.

As the _{electrolyte, LiPF 6, LiBF 4, LiClO} 4, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6 or a variety of such imide lithium salts represented by lithium trifluoromethane sulfonimide, Lithium salt can be mentioned. A non-aqueous electrolytic solution obtained by dissolving these electrolytes in the above-mentioned solvent can be used as the electrolytic solution L. Other electrolytes may be used as long as they do not decompose on the positive electrode or the negative electrode built in the lithium secondary battery.

When a solid polymer electrolyte (polymer electrolyte) is used, an ionic conductive polymer such as ethylene oxide, acrylonitrile, polyvinylidene fluoride, methyl methacrylate, or hexafluoropropylene can be used as the electrolyte. When these solid polymer electrolytes are used, there is an advantage that the separator 109 can be omitted.

In addition, ionic liquids can be used. For example, a mixed complex of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF ₄ ), lithium salt LiN (SO ₂ CF ₃ ) ₂ (LiTFSI) and triglime, tetraglime and the like can be mentioned. .. Cyclic quaternary ammonium cations (eg N-methyl-N-propylpyrrolidinium) and imide anions (eg bis (fluorosulfonyl) imide) do not decompose at the positive and negative electrodes A combination can be selected and used as the ionic liquid in the lithium secondary battery according to the present embodiment.

Next, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

(Examples 1 to 11)
The composition of the negative electrode mixture layer was adjusted as shown in Table 1 to prepare the lithium secondary batteries of Examples 1 to 11. Specifically, a negative electrode material composed of silicon particles (Si), carbon black (CB), carbon nanotubes (CNT), and graphite (only in Example 6), a binder, and a solvent are mixed in the composition shown in Table 1. A negative electrode slurry was prepared. Polyamideimide was used as the binder, and 1-methyl-2-pyrrolidone (NMP) was used as the solvent. This negative electrode slurry was applied once on a rolled copper foil having a thickness of 10 μm by the doctor blade method, dried at 100 ° C., and then heat-treated at 300 ° C. in vacuum to cure the binder to produce a negative electrode. .. The "total carbon composition" in Table 1 is the total weight of carbon contained in the first conductive material (CB), the second conductive material (CNT), and graphite (Example 7 only). It refers to the ratio of the total carbon content to the weight of silicon particles (Si) when defined as the carbon content. Further, "negative electrode density" means the density of the negative electrode mixture layer.

The positive electrode was prepared as follows. Using LiNi _1/3 Mn _1/3 Co _1/3 O ₂ as the positive electrode active material, 7 parts by weight of polyvinylidene fluoride (PVDF) as a binder was mixed with a total of 93 parts by weight of the positive electrode active material and the conductive agent. A positive electrode slurry was prepared, and the positive electrode slurry was applied to a current collector and dried to produce a positive electrode.

As the electrolytic solution, an electrolytic solution in which 1 molar concentration (1M = 1 mol / dm ³ ) of LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was used. The mixing ratio of EC and EMC was 1: 2 by volume. Further, 1% by weight of vinylene carbonate was added to the electrolytic solution.

As the separator, a polyethylene single-layer separator having a thickness of 25 μm and a porosity of 45% was used.

The rated capacity (calculated value) of the lithium secondary battery manufactured as an example is 3 Ah. When the electrode coating amount was changed, the area and the number of electrodes were changed, and the electrode size was adjusted so that the rated capacity could be obtained.

As shown in Table 1, Examples 1 and 2 have changed the composition of the second conductive material (CNT). In Examples 1 and 3 and Examples 2 and 4, the composition of the first conductive material (CB) is changed, respectively. In Examples 5 and 6, the weight ratio of the negative electrode material and the binder was changed with respect to Examples 1 to 4. Example 7 is an example in which graphite is mixed in the negative electrode mixture layer.

Example 8 is an example of a composition having the least amount of the first conductive material. Further, Example 9 is an example of a composition having the least amount of the second conductive material.

Further, Example 10 is an example having the lowest negative electrode density, and Example 11 is an example having the highest negative electrode density.

Table 3 shows the evaluation results of the lithium secondary batteries of Examples 1 to 11. The lithium secondary batteries of each example were subjected to initial aging treatment. First, charging was started from the open circuit state. At 0.2 CA current, the voltage was maintained when 4.2 V was reached, and charging was continued until the current became 1/10. After that, a rest period of 30 minutes was provided, and discharge was started at 0.2 CA. When the battery voltage reached 2.8 V, the discharge was stopped and the battery was paused for 30 minutes. In the same manner, charging and discharging were repeated 5 times to complete the initial aging process of the lithium secondary battery.

Next, charging was set as the condition at the time of initialization, and discharge tests of 0.5 CA and 1 CA were carried out. The measurement results of the discharge capacity are shown in Table 3.

As shown in Table 3, in each of the examples, the initial charge / discharge efficiency exceeded 85%, and the initial capacity matched the design value. In addition, the discharge capacity ratio of 0.5CA and 1CA was also high. With the configuration of the present invention, it was possible to obtain a negative electrode for a lithium secondary battery having a high capacity and excellent rate characteristics.

(Comparative Examples 1 to 7)
The lithium secondary batteries of Comparative Examples 1 to 7 were produced in the same manner as in Examples except that the composition of the negative electrode mixture layer was changed as shown in Table 2. Comparative Examples 1 and 2 relate to a negative electrode that does not use the first conductive material (CB) or the second conductive material (CNT). As shown in Table 4, when one of the first conductive material and the second conductive material is missing, the initial charge / discharge efficiency is lowered and the discharge capacity ratio is also lowered. The discharge capacity of 0.2CA in Comparative Example 1 was the same as that of Examples 1 and 2, but the rate characteristics of 0.5CA and 1CA were lowered. In the negative electrodes of Comparative Examples 3 and 4, the ratio of the weight of the first conductive material to the weight of the silicon particles (CB / Si) is outside the scope of the present invention. In the negative electrode of Comparative Example 5, the ratio of the weight of the second conductive material to the total weight of the silicon particles and the first conductive material (CNT / (Si + CB)) is outside the scope of the present invention. In the negative electrodes of Comparative Examples 6 and 7, the density of the negative electrode mixture layer is outside the range of the present invention. In both cases, the initial charge / discharge efficiency deteriorated and the discharge capacity also decreased significantly.

Although the embodiments of the present invention have been described in detail above, the specific configuration is not limited to this embodiment, and even if there are design changes and the like within a range that does not deviate from the gist of the present invention, they are not limited to this embodiment. It is included in the present invention.

101 Lithium secondary battery 102 Battery container 103 Lid 104 Positive electrode external terminal 105 Negative electrode external terminal 106 Injection plug 107 Positive electrode 108 Negative electrode 109 Separator 110 Positive electrode lead wire 111 Negative electrode lead wire 112 Insulation seal member 201 Silicon particles 202 First conductive material 203 Second conductive material L Electrolyte

Claims (8)

A negative electrode for a lithium secondary battery including a current collector and a negative electrode mixture layer on the current collector.
The negative electrode mixture layer has a negative electrode material containing silicon particles, a first conductive material and a second conductive material, and a binder.
The first conductive material is a material having a porous structure that covers the surface of the silicon particles.
The second conductive material is a fibrous material arranged so as to connect the first conductive materials that cover the surfaces of different silicon particles.
The ratio of the weight of the first conductive material to the weight of the silicon particles is 0.053 to 0.333.
The ratio of the weight of the second conductive material to the total weight of the silicon particles and the first conductive material is 0.020 to 0.176.
The negative electrode for a lithium secondary battery, wherein the density of the negative electrode mixture layer is 0.5 to 1.3 g / cm ^3. The negative electrode for a lithium secondary battery according to claim 1, wherein the first conductive material is carbon black. The negative electrode for a lithium secondary battery according to claim 1, wherein the coating thickness of the first conductive material is 1 nm or more, and the coating ratio on the surface of the silicon particles is 70% or more and 90% or less. The negative electrode for a lithium secondary battery according to claim 1, wherein the second conductive material is carbon nanotubes. The negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode mixture layer further contains graphite. The total weight of carbon contained in the first conductive material and the second conductive material is defined as the total carbon content, and the ratio of the total carbon content to the weight of the silicon particles is 0.5 or more and 1.5. The negative electrode for a lithium secondary battery according to claim 1, which is as follows. The total weight of carbon contained in the first conductive material, the second conductive material, and graphite is defined as the total carbon content, and the ratio of the total carbon content to the weight of the silicon particles is 0.5 or more. The negative electrode for a lithium secondary battery according to claim 5, which is 1.5 or less. The lithium secondary battery including the negative electrode for a lithium secondary battery according to claim 1, the positive electrode, and an electrolyte.

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