JP7660813B2 - Lithium-ion secondary battery - Google Patents

Description

The present invention relates to a lithium-ion secondary battery that is made up of a negative electrode that is mainly composed of silicon and can store and release lithium ions.

In recent years, it has been pointed out that the greenhouse effect, which is mainly caused by the increase in the amount of carbon dioxide gas in the atmosphere, may be causing global climate change. Air pollution, including carbon dioxide, nitrogen oxides, and hydrocarbons, emitted from automobiles used as a means of transportation, has also been pointed out as having an impact on health. In terms of environmental conservation, there are high expectations for hybrid and electric vehicles, which combine an engine with an electric motor that runs on electricity stored in a storage device, and smart grids, which are systems that store electricity from solar and wind power generation facilities and manage the network to optimize the balance of electricity demand. In the field of information and communications, information terminals such as smartphones are rapidly penetrating society because they can easily send and receive information. Under these circumstances, there are high expectations for the development of storage devices such as secondary batteries that combine high power density, high energy density, and long life in order to improve the performance of smartphones, hybrid and electric vehicles, smart grids, etc. and reduce production costs.

Among the above-mentioned energy storage devices currently on the market, the one with the highest energy density is the lithium-ion secondary battery (broadly called lithium secondary battery), which uses carbon such as graphite for the negative electrode and a compound of lithium and transition metals for the positive electrode. However, in this lithium-ion secondary battery, since the negative electrode is made of carbon material, theoretically only a maximum of 1/6 lithium atoms can be intercalated per carbon atom. For this reason, it is difficult to further increase the capacity, and new electrode materials for increasing the capacity are desired. In addition, the above-mentioned lithium-ion secondary battery is expected to be used as a power source for electric vehicles because of its high energy density, but in order to increase the driving distance per charge, it is necessary to load many lithium-ion secondary batteries on the vehicle body, and although the manufacturing cost of lithium-ion batteries has dropped significantly compared to the beginning, it still accounts for a large part of the vehicle price, so there is a demand for the development of a lithium-ion battery with a higher energy density and lower cost. In order to meet these demands, the application of silicon materials such as silicon and their alloys, which can store and release more lithium ions than graphite, is being researched. Silicon materials can electrochemically store a large amount of lithium ions, but the silicon material particles expand in volume by about four times when lithium is absorbed during charging, and the electrolyte held in the voids in the electrode is expelled into the voids in the battery housing. In addition, the volume expansion of the silicon material particles creates areas where the conductive paths between the silicon material particles or between the silicon material particles and the current collector are interrupted. This creates areas on the silicon material particle surface where there is an extremely small amount of electrolyte and isolated silicon material particles exist where electrochemical reactions are difficult to occur, resulting in a decrease in the charge and discharge performance of the battery. For the above reasons, lithium ion secondary batteries with a long charge and discharge cycle life and consisting of a negative electrode formed with silicon material particles as the main active material have not yet been put to practical use.

In order to improve the cycle life of a silicon electrode using a silicon material, Patent Document 1 proposes a method of gelling a mixture containing silica sol formed by hydrolysis of a silicic acid ester, fine carbon particles, and the silicon-containing particles, hydrothermally treating the mixture, drying and baking the mixture, and then pulverizing the mixture to obtain a composition for a negative electrode active material. However, the composition for a negative electrode active material obtained in Patent Document 1 is a composite particle of silica gel, fine carbon particles, and silicon-containing particles, and the composite step increases costs, and the silicon-containing particles have a relatively large diameter and a high silica content, so that the composition for a negative electrode active material is prone to collapse due to volume expansion during charging.
In addition, Patent Document 2 proposes a nanosilicon carbon composite material that includes a carbon substrate, nanosilicon dispersed on the carbon substrate, nanosilicon SiC dispersed at the interface between the carbon substrate and the nanosilicon, and silicon oxide SiO x (0<x≦2) that covers the surface of the nanosilicon except for the interface between the nanosilicon and the carbon, which is produced by electrochemically reducing silica using a molten salt electrolysis method using a porous electrode made of silica and carbon as raw materials. However, high-temperature processing is required for production, and the product contains SiC, which is not involved in charge and discharge, and SiO _{x ,} which reduces charge and discharge efficiency by irreversible reaction, resulting in high costs and a lower capacity density compared to Si.

JP 2016225207 A Patent Publication No. 2015503185

Silicon is a material that can electrochemically store and release a large amount of lithium, but when it absorbs lithium ions, it undergoes a large volume expansion. In addition, when it stores lithium ions, it expands to a maximum of about four times its volume, and when it releases lithium ions, it shrinks. However, when the silicon-containing particles (silicon-based particles including silicon alloy particles are collectively referred to as silicon-containing particles here) have a large particle size, they collapse and become finely divided due to repeated expansion and contraction. Even when finely divided silicon-containing particles are used from the beginning and do not become finely divided when lithium ions are absorbed, in an electrode formed at least from silicon-containing particles and a binder, the expansion and contraction of the silicon-containing particles pushes out the electrolyte that was present in the voids of the electrode layer, causing a shortage of electrolyte around the silicon-containing particles, resulting in isolated areas in the electrode reaction during charging, and the electrochemical reaction is concentrated in the silicon-containing particles where the electrolyte is present, causing uneven volume expansion of the entire electrode, and the silicon-containing particles that have become secondary particles collapse. When the electrolyte is solidified by the polymer gel, when the volume of the silicon-containing particles expands during charging, the electrolyte is trapped between the molecules of the polymer gel and is therefore less likely to be pushed out than a liquid electrolyte. However, the electrolyte is pushed out of the polymer gel by the pressure caused by the volume expansion of the silicon-containing particles. As a result, the electronic conductivity and lithium ion conductivity of the electrode decrease, leading to a decrease in the performance of the battery.

The present inventors have conducted intensive research to solve the above problems and have found a method for solving the above problems. In a negative electrode layer of a lithium-ion secondary battery made of silicon-containing particles (including silicon alloy particles), porous ceramic particles (an inorganic compound material that is a combination of metal elements and nonmetal elements) that have been hydrophobized and have a low bulk density are arranged, and it has been found that an electrolyte can be held between the ceramic particles or in the pores of the ceramic particles, and even if the silicon-containing particles are alloyed with lithium and expand in volume during charging, the electrolyte can be secured around the silicon-containing particles, and the decrease in ion conductivity in the electrode during charging and discharging can be suppressed, and the decrease in battery performance can be reduced. Since the ceramic particles have a strength that does not deform even under the pressure generated by the volume expansion of silicon, the electrolyte can be held in the pores of the ceramic particles, and the decrease in lithium ion conductivity in the electrode can be prevented. If ceramic particles that have not been hydrophobized are used instead of ceramic particles that have been hydrophobized by hydrophobization, the ceramic particle surfaces are hydrophilic, so a lot of moisture is adsorbed on the pore surfaces, which reduces the performance of the lithium-ion secondary battery.

Furthermore, they discovered that by adding monomers with multiple vinyl groups in the molecule, such as monovinyl monomers and divinyl monomers, trivinyl monomers, or tetravinyl monomers, to the electrolyte, the polymers produced by the polymerization and crosslinking reaction of the monomers during charging absorb the electrolyte and form a polymer gel, solidifying the electrolyte on the porous ceramic surface and inside the electrode, thereby suppressing the decomposition reaction of the electrolyte in a battery with a negative electrode made of silicon-containing particles and further preventing performance degradation.

The secondary battery of the present invention capable of absorbing and releasing lithium ions, which solves the above problems, is characterized in that it is composed of at least a negative electrode, an electrolyte solution which is a lithium ion conductor, and a positive electrode, and the negative electrode is composed of at least an inorganic electrode active material containing silicon element which stores lithium upon charging and releases lithium upon discharging, hydrophobically treated porous ceramic particles (referred to as hydrophobic porous ceramic particles or hydrophobic porous ceramic particles) which hold the electrolyte, a carbon material, and a binder made of an organic polymer.

The material of the porous ceramic particles is preferably an oxide containing at least one element selected from the group consisting of silicon, aluminum, magnesium, zirconium, and titanium. The ceramic of an oxide consisting of an element selected from the group consisting of silicon, aluminum, magnesium, zirconium, and titanium is chemically more stable and is easy to produce porous microparticles. The average particle size of the ceramic particles is preferably in the range of 0.1 to 4 μm. When the average particle size of the ceramic particles is in the range of 0.1 to 4 μm, the ceramic particles are more easily dispersed and arranged around the microparticulated silicon-containing particles, which are the negative electrode active material. If the particle size of the ceramic particles is too large, the ceramic particles are not arranged in the vicinity of the silicon-containing particles and cannot hold the electrolyte, and if the particle size of the ceramic particles is too small, the silicon-containing particles tend to flow from the vicinity of the silicon-containing particles when the volume of the silicon-containing particles expands during charging, making it difficult to hold the electrolyte in the vicinity of the silicon-containing particles. Furthermore, the bulk density of the ceramic particles is preferably in the range of 0.05 to 0.2 g/cm ³ , and a sufficiently small bulk density makes it possible to hold more electrolyte between the ceramic particle surface and the ceramic particles. The specific surface area of the ceramic particles is preferably in the range of 35 to 350 m ² /g. When the specific surface area of the ceramic particles is larger, the particle diameter becomes smaller, and when the silicon-containing particles expand in volume during charging, they tend to flow from the vicinity of the silicon-containing particles, making it difficult to hold the electrolyte near the silicon-containing particles. The mass percentage of the porous ceramic particles in the negative electrode is preferably in the range of 0.5 to 20%. By setting the mass percentage of the ceramic particles in the negative electrode to the range of 0.5 to 20%, it is possible to maintain an electrolyte containing sufficient lithium ions for the battery reaction around the negative electrode active material particles even when the silicon-containing particles, which are the negative electrode active material, expand in volume during charging, and as a result, it is possible to prevent a sudden performance deterioration that occurs during repeated charging and discharging. When the negative electrode contains more than 20 mass% of ceramic particles, the content of the active material particles in the negative electrode decreases, which reduces the storage capacity density and increases the electrical resistance of the electrode.

In addition, it is preferable that the oxide particles of the porous ceramic particles in the negative electrode have a lithium element. By containing the lithium element, the lithium ion conductivity near the surface of the ceramic particles is improved. It is preferable that the electrolyte contains both a monovinyl monomer and a monomer having multiple vinyl groups in the molecule, which undergo polymerization and crosslinking reactions during charging. It is also preferable that the electrolyte is gelled and held in the pores of the porous ceramic particles by a crosslinked polymer. By using an electrolyte containing both a monovinyl monomer and a monomer having multiple vinyl groups in the molecule for the secondary battery of the present invention, the above monomer undergoes an electrolytic polymerization reaction during charging in the electrolyte that has permeated into the inside of the electrode during assembly, and a polymer gel is formed by the polymerization and crosslinking reaction of the monovinyl monomer and the monomer having multiple vinyl groups in the molecule, and the electrolyte is solidified, so that a side reaction in which the electrolyte decomposes due to repeated charging is suppressed. Specific examples of the monovinyl monomer include vinylene carbonate, vinyl acetate, acrylonitrile, vinyl ethylene carbonate (4-vinyl-1,3-dioxolane-2-one), 1-vinyl-2-pyrrolidone, N-vinylphthalimide, vinyl propionate, and vinyl pivalate. Specific examples of the monomer having a plurality of vinyl groups include allyl ether, diallyl adipate, divinyl adipate, triallyl citrate, diethylene glycol divinyl ether, triethylene glycol divinyl ether, 3,9-divinyl spirobi(m-dioxane), tetraallyloxyethane, triallyl 1,3,5-benzenetricarboxylate, vinyl methacrylate, and vinyl crotonate.

The negative electrode of the lithium ion secondary battery of the present invention, which is made of an inorganic material (referred to as silicon-containing particles) containing silicon as an active material, is characterized in that it is a structure in which an electrode layer made of silicon-containing particles, a carbon material as a conductive assistant, hydrophobic porous ceramic particles, and a polymer as a binder is formed on a metal foil current collector such as copper foil. The silicon-containing particles are preferably silicon oxide, silicon, silicon alloys, and composites containing these silicon-based materials, and the average particle size is preferably in the range of 10 nm to 10 μm in order to make the volume expansion during the lithium insertion reaction during charging more uniform and suppress local volume expansion, and more preferably in the range of 200 nm to 2 μm in order to facilitate handling and extend the charge-discharge cycle life. Silicon alloys are more preferable because they can be manufactured more cheaply using ingots as a raw material, have the advantage of being able to reduce the crystallite size, have excellent electronic conductivity, and can control the volume expansion rate by changing the content of silicon elements. The content of silicon-containing particles in the electrode layer is preferably 10 to 60 mass % in order to maintain a high capacity density and extend the charge-discharge cycle life. If the content of the silicon-containing particles is less than 10% by mass, a capacity density significantly exceeding that of graphite cannot be obtained. If the content of the silicon-containing particles is more than 60% by mass, a higher capacity density can be obtained, but the charge/discharge cycle life is shortened. The content of the silicon-containing particles is preferably 20 to 50% by mass in order to maintain a high capacity and reduce the volume expansion rate. Furthermore, it is more preferable that the silicon-containing particles are a composite of silicon or a silicon alloy and an inorganic lithium ion conductor, and that amorphous or nanocrystalline silicon is dispersed in the silicon-containing composite particles in order to reduce the decomposition side reaction of the electrolyte during charging.

The inorganic lithium ion conductor is a compound that can be expressed as Li _x M _y A _z = Li _x (M1 _a M2 _b M3 _c M4 _d M5 _e M6 _f M7 _g ) (A1 _h A2 _i A3 _j ), in which M is a metal element and is one or more elements selected from Group 1 elements (M1), Group 2 elements (M2), Group 3 elements (M3), Group 4 elements (M4), Group 5 elements (M5), Group 13 elements (M6), and Group 14 elements (M7) of the periodic table of elements, A is a non-metallic element and is one or more elements selected from Group 15 elements (A1), Group 16 elements (A2), and Group 17 elements (A3), and x>0, y>0, z>0, a≧0, b≧0, c≧0, d≧0, e≧0, f≧0, g≧0, h≧0, i≧0, j≧0, (a + It is preferable that the compound satisfies (b+c+d+e+f+g)>0 and (h+i+j)>0. Furthermore, in the lithium ion conductor Li _x (M1 _a M2 _b M3 _c M4 _d M5 _e M6 _f M7 _g ) (A1 _h A2 _i A3 _j ), the Group 1 element (M1) is one or more elements selected from Na and K, the Group 2 element (M2) is one or more elements selected from Mg, Ca, Sr, Ba, the Group 3 element (M3) is one or more elements selected from Sc, Y, and La, the Group 4 element (M4) is one or more elements selected from Ti, Zr, and Hf, the Group 5 element (M5) is one or more elements selected from V, Nb, and Ta, the Group 13 element (M6) is one or more elements selected from B, Al, Ga, and In, the Group 14 element (M7) is one or more elements selected from Si, Ge, and Sn, the Group 15 element (A1) is one or more elements selected from N, P, and Bi, and the Group 16 element (A2) is O, The group 17 element (A3) is preferably one or more elements selected from S, and the group 17 element (A4) is preferably one or more elements selected from F, Cl, Br, and I. In order to obtain high ionic conductivity of the ionic conductor, it is more preferable that the ionic conductor is composed of three or more elements in the combination of metal elements and nonmetal elements. In order to obtain even higher ionic conductivity of the ionic conductor, it is preferable that the group 15 element (A1) contains P element. It is more preferable that the group 16 element (A2) is O element. An oxide in which the group 16 element (A2) is O element has a higher hardness than a sulfide in which the group 16 element (A2) is S element, and it is possible to easily make silicon or silicon alloy amorphous during the mechanical milling and mechanical alloying process at high acceleration in the manufacturing process of the composite particles as the negative electrode active material of the lithium ion secondary battery of the present invention, thereby making it possible to improve the charge and discharge cycle life of the negative electrode.

Furthermore, in the secondary battery of the present invention, it is preferable that the silicon-containing composite particles are composited with one or more types of carbon materials selected from the group consisting of graphite, amorphous carbon, carbon nanofibers, carbon nanotubes, and graphene. It is also preferable that the silicon-containing composite particles composited with an inorganic lithium ion conductor are composited with the above-mentioned carbon material. By being composited with a carbon material, the electronic conductivity of the silicon-containing composite particles is improved.

In the secondary battery (lithium ion secondary battery) of the present invention capable of absorbing and releasing lithium ions, the negative electrode made of a silicon material contains porous ceramic particles that hold an electrolyte. During charging, the silicon-containing particles absorb lithium ions, alloy with lithium, and expand in volume, preventing the electrolyte held in the pores of the porous ceramic particles from being expelled. This maintains lithium ion conduction in the negative electrode even after repeated charging and discharging, making it possible to suppress deterioration in secondary battery performance.

The present invention will be described in detail below.
The secondary battery capable of absorbing and releasing lithium ions of the present invention comprises a negative electrode, a lithium ion conductor, and a positive electrode. A liquid electrolyte is used as the ion conductor during assembly of the battery.

[Negative electrode for lithium ion secondary battery]
The negative electrode of the lithium ion secondary battery of the present invention is composed of at least a negative electrode active material of silicon-containing particles containing silicon element that stores lithium during charging and releases lithium during discharging, hydrophobic porous ceramic (inorganic compound material that is a combination of metal elements and nonmetal elements) particles that hold an electrolyte, a carbon material, and a binder made of an organic polymer. In the secondary battery of the present invention, when the silicon-containing particles, which are the active material in the negative electrode, are alloyed with lithium during charging and volume expansion occurs, the liquid electrolyte present in the negative electrode has fluidity and tends to be pushed out of the negative electrode, but the porous ceramic particles holding the electrolyte are not pushed out of the negative electrode, so the ionic conductivity in the negative electrode is maintained.

The negative electrode of the battery of the present invention is manufactured by the following method. First, a negative electrode active material of silicon-based inorganic material (silicon-containing particles), hydrophobic porous ceramic particles that hold the electrolyte, a carbon material, a binder made of an organic polymer, and its solvent are mixed and kneaded to prepare a slurry whose viscosity is adjusted to be suitable for coating. Next, the slurry is applied to a current collector, dried, and then pressed to adjust the density and thickness of the electrode layer to manufacture an electrode structure for the negative electrode.

[Negative electrode active material for lithium ion secondary batteries]
The negative electrode active material of the lithium ion secondary battery of the present invention includes silicon-containing particles containing silicon element, and specifically, as the negative electrode active material, one or more materials selected from the group consisting of silicon, silicon alloy, a composite of silicon and graphite, a composite of silicon alloy and graphite, a composite of silicon and lithium ion conductor, and a composite of silicon alloy and lithium ion conductor, and further a mixture of these materials and graphite are used. The silicon alloy is preferably an alloy composed of at least silicon and a transition metal element. The composite of silicon and lithium ion conductor and the composite of silicon alloy and lithium ion conductor are preferably particles in which amorphous or nanocrystalline silicon is dispersed in an inorganic lithium ion conductor, characterized in that the silicon in the particles is 20 to 60 mass %, and the amorphous or nanocrystalline silicon is dispersed in the inorganic lithium ion conductor. The composite of silicon and lithium ion conductor and the composite of silicon alloy and lithium ion conductor may be further composited with a carbon material.

The above-mentioned composite of silicon or silicon alloy fine particles and graphite can be formed by mixing graphite powder, a resin that acts as a binder, and silicon or silicon alloy fine particles at a temperature above the melting point of the resin, and firing the mixture in an inert gas atmosphere at a temperature above the carbonization temperature of the resin. The above-mentioned composite can also be formed by mixing graphite powder, a resin that acts as a binder, and silicon or silicon alloy fine particles with a resin solvent added, drying the mixture, and firing the mixture in an inert gas atmosphere at a temperature above the carbonization temperature of the resin that acts as a binder.

The crystallite size of silicon in the negative electrode active material is preferably 50 nm or less, and more preferably 20 nm or less. The smaller the silicon crystallite size, the more uniform the Li insertion and the less volume expansion. The crystallite size is calculated using the half-width of the X-ray diffraction peak and the Scherrer formula. The crystal size can also be observed from a transmission electron microscope image.

In the above-mentioned composite of silicon and a lithium ion conductor or the composite of a silicon alloy and a lithium ion conductor, the inorganic material of the lithium ion conductor is Li _x M _y A _z = Li _x (M1 _a M2 _b M3 _c M4 _d M5 _e M6 _f M7 _g )(A1 _h A2 _i A3 _j In the compound, M is a metallic element and is one or more elements selected from Group 1 elements (M1), Group 2 elements (M2), Group 3 elements (M3), Group 4 elements (M4), Group 5 elements (M5), Group 13 elements (M6), and Group 14 elements (M7) of the Periodic Table of the Elements; A is a non-metallic element and is one or more elements selected from Group 15 elements (A1), Group 16 elements (A2), and Group 17 elements (A3); and x>0, y>0, z>0; a≧0, b≧0, c≧0, d≧0, e≧0, f≧0, g≧0, h≧0, i≧0, j≧0, (a + b + c + d + e + f + g)>0, and (h + i + j)>0. In order to obtain high ionic conductivity, it is more preferable that the combined metal and nonmetal elements consist of three or more elements. In order for the ionic conductor to have even higher ionic conductivity, it is preferable that the Group 15 element (A1) contains P. It is more preferable that the Group 16 element (A2) is O. An oxide in which the Group 16 element (A2) is O has a higher hardness than a sulfide in which the Group 16 element (A2) is S, and facilitates the amorphization of silicon.

Representative examples of the inorganic _{lithium ion} conductor include various inorganic solid electrolytes such _{as Li7La3Zr2O12} , _Li10GeP2O12 , _{Li3BO3 - Li2SO4} , _{argyrodite ( Li6PS5Cl ) , and} glass ceramics _{Li2SP2S5 .} Examples of the above inorganic solid electrolytes include Li _0.34 La _0.51 TiO _2.94 , Li _1.07 Ti _1.46 Al _0.69 P ₃ O ₁₂ , Li _1.5 Ti _1.5 Al _0.5 P ₃ O ₁₂ , Li _1.5 Ti _1.7 Al _0.3 Si _0.2 P _2.8 O ₁₂ , Li _1.5 Al _0.5 Ge _1.5 P ₃ O ₁₂ , Li ₇ La ₃ Zr ₂ O ₁₂ , Li ₃ YCl ₆ , Li ₃ YBr ₆ , Li _9.54 Si _1.74 P _1.44 S _11.7 Cl _0.3 , Li ₁₀ GeP ₂ S ₁₂ , 57Li ₂ S-38SiS ₂ -5Li ₄ SiO ₄ , 75Li ₂ Examples include S-25P ₂ S ₅ .

The lithium ion conductor of the inorganic material is compounded with silicon or a silicon alloy by mixing the powdered lithium ion conductor with silicon or a silicon alloy using a high-acceleration mechanical grinding device such as a vibration mill, attritor, planetary ball mill, or other device using similar technology, and then mechanically milling and mechanically alloying the mixture to form a composite. Compounding is also possible by mechanically alloying the raw material of the lithium ion conductor of the inorganic material with a silicon material instead of the inorganic lithium ion conductor.

(Ceramic material powder)
The hydrophobic ceramic powder constituting the negative electrode of the present invention is preferably an oxide of at least one element selected from the group consisting of silicon, aluminum, magnesium, titanium, and zirconium, and more preferably an oxide of an element selected from silicon, aluminum, and titanium. The content of the ceramic powder in the negative electrode layer (mixture layer) is preferably in the range of 0.5 to 20%, and more preferably in the range of 0.5 to 15%. This allows the battery to maintain a high storage capacity and obtain a long charge/discharge cycle life.

The ceramic material powder preferably has an average particle size in the range of 0.1 to 4 μm. The ceramic material powder preferably has a specific surface area in the range of 35 ^m2 /g to 350 ^m2 /g. Furthermore, the ceramic material powder more preferably has a bulk density in the range of 0.05 to 0.2 g/ ^cm3 . A large specific surface area and a small bulk density mean that the volume of the space in the unevenness of the particle surface and the volume of the gaps between particles are large, and by including the ceramic material in an electrode, an electrolyte can be held in the gaps in an electricity storage device that uses an electrolyte.
In addition, it is preferable that the oxide particles of the ceramic particles contain lithium elements, since this contributes to lithium ion conduction.

Furthermore, it is preferable that the ceramic particles are hydrophobized to suppress the adsorption of moisture, and it is also preferable that the surface of the ceramic particles is carbon-coated or a carbon material is composited to improve the electronic conductivity in the negative electrode. The hydrophobization of the ceramic particles is achieved by treating the ceramic particles with an organosilane compound having an aliphatic hydrocarbon group or a fluorinated hydrocarbon group. Examples of the organosilane compound include polydimethylsiloxane, methylchlorosilane, hexamethyldisilazane, and alkyltrimethoxysilane.

By dispersing ceramic particles in the negative electrode of the present invention, the flame retardancy of the negative electrode is increased, and safety is also improved. Similarly, by including the ceramic particles in the positive electrode, it is possible to increase the flame retardancy of the positive electrode.

(Carbon materials)
The carbon material constituting the electrode structure for the negative electrode of the lithium ion secondary battery of the present invention is one or more carbon materials selected from the group consisting of graphite powder, amorphous carbon powder, carbon nanofibers, carbon nanotubes, and graphene, and plays a role in enhancing the electronic conductivity of the electrode. By adjusting the content of the silicon-containing particles and graphite, which are the negative electrode active materials in the electrode structure, when the electrode structure is incorporated as the negative electrode of the lithium ion secondary battery, the capacity of the battery and the volume expansion rate of the negative electrode can be adjusted.

(binder)
Specific examples of binders used in forming the electrode layer of the negative electrode structure of the battery of the present invention include sodium alginate, sodium carboxymethylcellulose, carboxymethylcellulose, sodium polyacrylate, polyacrylic acid, polyvinyl alcohol, polyvinyl alcohol copolymers, chitin, chitosan, polyamic acid (polyimide precursor), polyimide, polyamideimide, and epoxy resins.

(Current collector)
The material of the current collector of the electrode structure used in the negative electrode of the secondary battery of the present invention must be stable and not dissolved in the charge/discharge reaction of the power storage device, and specific examples of the material include copper, stainless steel, titanium, and nickel. The shape of the current collector is plate-like, but the thickness of this "plate-like" is not specified within the practical range, and includes a form called "foil" having a thickness of about 5 μm to 100 μm. In addition, a plate-like member having, for example, a mesh, sponge, or fiber shape, a punching metal, a metal foil having a three-dimensional uneven pattern formed on both sides, an expanded metal, etc. can also be used.

[Method of manufacturing a negative electrode for lithium ion secondary batteries]
The negative electrode for a lithium ion secondary battery of the present invention is prepared by mixing silicon-containing particles, graphite powder, a conductive assistant such as a carbon material, a hydrophobic porous ceramic powder, and a polymer as a binder, adding a solvent for the binder as appropriate, and kneading the mixture to prepare a slurry. Next, the prepared slurry is applied onto a current collector, dried, and the electrode layer density is adjusted with a roll press machine, and then the mixture is heated under reduced pressure to remove moisture, thereby preparing an electrode structure.

[Lithium-ion secondary battery]
The lithium ion secondary battery of the present invention is an electricity storage device that utilizes the reduction-oxidation reaction of lithium ions, and is configured by sequentially stacking at least the negative electrode of the present invention, a lithium ion conductor, and a positive electrode made of a lithium transition metal compound. Specific cell shapes of the battery include, for example, flat, cylindrical, rectangular, and sheet shapes. Also, cell structures include, for example, single-layer, multi-layer, and spiral types.

(Positive electrode)
The positive electrode has a positive electrode active material layer formed on a positive electrode current collector, the positive electrode active material layer being made of a lithium-transition metal compound, a binder, and a conductive auxiliary material such as carbon black.

As the lithium-transition metal compound, lithium-transition metal oxides and lithium-transition metal phosphate compounds are used. As the transition metal elements contained in the positive electrode active material, Ni, Co, Mn, Fe, Cr, V, etc. are more preferably used as the main elements. Furthermore, the surface of the positive electrode active material is preferably made of lithium transition metal compound fine particles whose surface layer is coated with a composite metal oxide composed of at least one metal element selected from Al, Zr, Mg, Ca, La, and Li. The composite metal oxide of the surface coating layer of the positive electrode material is preferably further composited with amorphous carbon.

The binder may be a fluororesin such as polyvinylidene fluoride, polyacrylate, polyamic acid (polyimide precursor), polyimide, polyamide-imide, epoxy resin, or styrene-butadiene copolymer-carboxymethylcellulose.

The material of the current collector is preferably one that has high electrical conductivity and is inactive in the battery reaction. Preferred materials include one or more metal materials selected from aluminum, nickel, iron, stainless steel, and titanium. A more preferred material is aluminum, which is inexpensive and has low electrical resistance. The shape of the current collector is plate-like, but the thickness of this "plate-like" is not specified within a practical range, and includes a form called "foil" having a thickness of about 5 μm to 100 μm. Plate-like members such as mesh-like, sponge-like, and fibrous members, punched metal, metal foil with three-dimensional uneven patterns formed on both sides, and expanded metal can also be used.

(Lithium ion conductor)
As the ionic conductor, lithium ion conductors such as a separator holding an electrolytic solution (an electrolytic solution prepared by dissolving an electrolyte in a solvent), a solid electrolyte, a solidified electrolyte obtained by gelling an electrolytic solution with a polymer gel or the like, a composite of a polymer gel and a solid electrolyte, and an ionic liquid can be used.

As the separator for preventing electrical short-circuiting between the negative and positive electrodes, a resin film having a micropore structure or a nonwoven fabric structure is used, and the resin material is preferably a polyolefin such as polyethylene or polypropylene, polyimide, polyamideimide, or cellulose. In order to increase heat resistance, the surface of the microporous resin film may be coated with a layer containing metal oxide particles such as alumina, zirconia, or titania that are permeable to lithium ions.

Examples of the electrolyte include salts consisting of lithium ions (Li ⁺ ) and Lewis acid ions (BF ₄ ^- , PF ₆ ^- , AsF ₆ ^- , ClO ₄ ^- , CF ₃ SO ₃ ^- , BPh ₄ ^- (Ph: phenyl group)), lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide and mixed salts thereof, and ionic liquids.

The salt is preferably heated under reduced pressure to sufficiently dehydrate and deoxidize it. Furthermore, an electrolyte prepared by dissolving the lithium salt in an ionic liquid can also be used. As the solvent for the electrolyte, for example, acetonitrile, benzonitrile, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethylformamide, tetrahydrofuran, nitrobenzene, dichloroethane, diethoxyethane, 1,2-dimethoxyethane, chlorobenzene, γ-butyrolactone, dioxolane, sulfolane, nitromethane, dimethyl sulfide, dimethyl sulfoxide, 3-methyl-2-oxodazolidinone, 2-methyltetrahydrofuran, 3-propylsydnone, sulfur dioxide, or a mixture thereof can be used. Solvents having a structure in which the hydrogen element of the above solvent is replaced with a fluorine element can also be used.
The above solvents are dehydrated, for example, with activated alumina, molecular sieves, phosphorus pentoxide, calcium chloride, etc., or, depending on the solvent, may be distilled in the presence of an alkali metal in an inert gas to remove impurities and to dehydrate the solvent.
In order to suppress the reaction between the electrode and the electrolyte, it is preferable to add an organic fluorine compound such as fluoroethylene carbonate or difluoroethylene carbonate, or a compound such as vinylene carbonate, which forms a stable SEI layer on the electrode surface.

As the solid electrolyte _, various _inorganic solid electrolytes such _{as Li7La3Zr2O12} , _{Li10GeP2O12 , Li3BO3 -Li2SO4 , argyrodite ( Li6PS5Cl )} , and glass _{ceramic Li2SP2S5} can be used, and are used by dispersing _them in a liquid electrolyte. Examples of the above inorganic solid electrolytes include Li _0.34 La _0.51 TiO _2.94 , Li _1.07 Ti _1.46 Al _0.69 P ₃ O ₁₂ , Li _1.5 Ti _1.5 Al _0.5 P ₃ O ₁₂ , Li _1.5 Ti _1.7 Al _0.3 Si _0.2 P _2.8 O ₁₂ , Li _1.5 Al _0.5 Ge _1.5 P ₃ O ₁₂ , Li ₇ La ₃ Zr ₂ O ₁₂ , Li ₃ YCl ₆ , Li ₃ YBr ₆ , Li _9.54 Si _1.74 P _1.44 S _11.7 Cl _0.3 , Li ₁₀ GeP ₂ S ₁₂ , 57Li ₂ S-38SiS ₂ -5Li ₄ SiO ₄ , 75Li ₂ S-25P ₂ S ₅ , and the like. Furthermore, it may be an amorphous solid electrolyte having a different stoichiometric ratio from the above.

The solid electrolyte is preferably one obtained by gelling the electrolytic solution with a gelling agent. As the gelling agent, it is preferable to use a polymer that absorbs the solvent of the electrolytic solution and swells, or a porous material with a large liquid absorption capacity, such as silica gel. As the polymer, polyethylene oxide, polyacrylonitrile, polymethyl methacrylate, vinylidene fluoride-hexafluoropropylene copolymer, polyethylene glycol, 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, etc. are used. Furthermore, it is more preferable that the polymer has a crosslinked structure.

The present invention will be explained in more detail below with reference to the examples.

[Preparation of silicon alloy composite as negative electrode active material for lithium ion secondary battery]
<Preparation of silicon alloy/C (carbon) composite>
First, metal silicon, metal tin, and metal copper were mixed and melted in a mass ratio of 65:30:5, and a water atomizer was used to form Si-Sn-Cu alloy powder. The resulting Si-Sn-Cu alloy and graphite powder were then mixed in a mass ratio of 95:5, and a small amount of lithium citrate, aluminum nitrate, and ethyl alcohol was added. The mixture was then ground for 10 hours in a vibration mill with a zirconia ball and pot to produce an amorphous Si-Sn-Cu alloy/C (carbon) composite powder formed of a composite coating of amorphous lithium aluminum oxide and carbon. The mixture was then crushed in a jet mill and classified into particles with a particle size of approximately 1 μm or less in a classification device to obtain a silicon alloy powder composited with graphite.

<Preparation of silicon alloy/oxide composite>
Metallic silicon and metallic nickel were mixed in a mass ratio of 65:35, and a flake-shaped Si-Ni alloy powder was formed using a single-roll liquid quenching and solidification apparatus. The resulting Si-Ni alloy, trilithium phosphate, aluminum hydroxide, germanium dioxide, and silicon dioxide were mixed in a mass ratio of 60:21.2:2.4:9.6:6.9, respectively, and ground for 8 hours in a planetary ball mill with silicon nitride balls and pots. After heat treatment at 350 °C, a composite powder of amorphous Si-Ni alloy and oxide was produced. The powder was then crushed in a jet mill and classified into particles with a particle size of about 1 μm or less using a classifier to obtain a silicon alloy/oxide composite powder. The above composite also has lithium ion conductivity due to the inclusion of lithium element.

[Preparation of electrode structure as negative electrode of lithium ion secondary battery]
In the following examples, electrode structures were formed that consisted of various silicon material particles (silicon-containing particles), carbon material particles including graphite, hydrophobic porous ceramic particles, and a binder.

(Example A1)
Silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, acetylene black, hydrophobically treated silica powder with an average particle size of 3.9 μm, a bulk density of 0.15 g/cm ³ and a specific surface area of 180 m ² /g as porous ceramic, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:17.5:0.5:7.5:2.5, ion-exchanged water was added, and the mixture was kneaded with a bead mill device to prepare a slurry. Next, the prepared slurry was applied to copper foil with a coater and dried at 110 ° C., and the electrode layer density was adjusted with a roll press machine, and then moisture was removed at 150 ° C. under reduced pressure to prepare an electrode structure. Next, the electrode structure was cut to a predetermined size, and then a nickel lead was welded to the copper foil tab of the current collector with a spot welding machine, and the lead terminal was taken out to prepare an electrode structure. In addition, a 10% by mass aqueous solution of PVA and a 2% by mass aqueous solution of CMC were used to prepare the slurry.

(Comparative Example A1)
In preparing the slurry of Example 1, no ceramic powder was mixed, but silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, acetylene black, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:18:7.5:2.5 to prepare an electrode structure.

(Example A2)
In preparing the slurry of Example 1, silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, acetylene black, hydrophobically treated silica powder with a particle size of 3.9 μm, a bulk density of 0.15 g/ ^cm3 , and a specific surface area of 180 ^m2 /g as porous ceramic, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:17:1:7.5:2.5 to produce an electrode structure.

(Example A3)
In preparing the slurry of Example 1, silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, acetylene black, hydrophobically treated silica powder with a particle size of 3.9 μm, a bulk density of 0.15 g/ ^cm3 , and a specific surface area of 180 ^m2 /g as porous ceramic, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:13:5:7.5:2.5 to produce an electrode structure.

(Example A4)
In preparing the slurry of Example 1, silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, acetylene black, hydrophobically treated silica powder with a particle size of 3.9 μm, a bulk density of 0.15 g/ ^cm3 , and a specific surface area of 180 ^m2 /g as porous ceramic, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:8:10:7.5:2.5 to produce an electrode structure.

(Example A5)
In preparing the slurry of Example 1, silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, acetylene black, hydrophobically treated silica powder with a particle size of 3.9 μm, a bulk density of 0.15 g/ ^cm3 , and a specific surface area of 180 ^m2 /g as porous ceramic, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:3:15:7.5:2.5 to produce an electrode structure.

(Example A6)
In preparing the slurry of Example 1, silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, hydrophobically treated silica powder with a particle size of 3.9 μm, a bulk density of 0.15 g/ ^cm3 , and a specific surface area of 180 ^m2 /g as porous ceramic, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:16:20:7.5:2.5 to produce an electrode structure.

(Example A7)
In preparing the slurry of Example 1, silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, acetylene black, carbon-coated conductive silica powder with a particle size of 3.0 μm, a bulk density of 0.13 g/ ^cm3 , and a specific surface area of 160 ^m2 /g as porous ceramic, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:3:15:7.5:2.5 to produce an electrode structure.

(Example A8)
In preparing the slurry of Example 1, silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, acetylene black, hydrophobically treated alumina powder with a particle size of 0.1 μm, a bulk density of 0.05 g/cm ³ and a specific surface area of 90 m ² /g as porous ceramic, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:17.5:0.5:7.5:2.5,
An electrode structure was fabricated.

(Example A9)
In preparing the slurry of Example 1, silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, acetylene black, hydrophobically treated titania powder as porous ceramic with a particle size of 0.1 μm, a bulk density of 0.1 g/cm3, and a specific ^surface area of 35 ^m2 /g, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:13:5:7.5:2.5 to produce an electrode structure.

(Example A10)
In preparing the slurry of Example 1, silicon powder with an average particle size of 3 μm, graphite powder with an average particle size of 5 μm, acetylene black, hydrophobically treated zeolite powder as porous ceramic with a particle size of 1.0 μm, a bulk density of 0.16 g/cm3, and a specific surface area of 320 m2/g, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:8:10:7.5:2.5 to prepare an electrode structure.

(Example A11)
In preparing the slurry of Example 1, the silicon alloy/C (carbon) composite powder having an average particle size of 1.6 μm obtained by the above-mentioned manufacturing method, graphite powder having an average particle size of 5 μm, acetylene black, hydrophobically treated silica powder having a particle size of 3.9 μm, a bulk density of 0.15 g/ ^cm3 and a specific surface area of 180 ^m2 /g as porous ceramic, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:13:5:7.5:2.5 to prepare an electrode structure.

(Comparative Example A2)
In preparing the slurry of Example 1, the silicon alloy/C (carbon) composite powder having an average particle size of 1.6 μm obtained by the above-mentioned manufacturing method, graphite powder having an average particle size of 5 μm, acetylene black, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:18:7.5:2.5 to prepare an electrode structure.

(Example A12)
In preparing the slurry of Example 1, the silicon alloy/oxide composite powder having an average particle size of 1.4 μm obtained by the above-mentioned manufacturing method, graphite powder having an average particle size of 5 μm, acetylene black, hydrophobically treated silica powder having a particle size of 3.9 μm, a bulk density of 0.15 g/ ^cm3 and a specific surface area of 180 ^m2 /g as a porous ceramic, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:13:5:7.5:2.5 to prepare an electrode structure.

(Comparative Example A3)
In preparing the slurry of Example 1, the silicon alloy/oxide composite powder having an average particle size of 1.4 μm obtained by the above-mentioned manufacturing method, graphite powder having an average particle size of 5 μm, acetylene black, polyvinyl alcohol (PVA), and carboxymethylcellulose sodium salt (CMC) were mixed in a mass ratio of 54:18:18:7.5:2 to prepare an electrode structure.

[Evaluation of Electrochemical Lithium Insertion Amount of Electrode Structure]
The electrochemical lithium insertion amount of the electrode structure for the negative electrode of the electricity storage device as a single electrode was evaluated by the following procedure.
Each of the electrodes of Examples A1 to A12 and Comparative Examples A1 to A3 was used as a working electrode, and a cell (half cell) was prepared by combining metallic lithium as a counter electrode, and the electrochemical amounts of lithium inserted and released were evaluated.

The lithium electrode was prepared by pressing a metallic lithium foil onto a nickel foil and punching it to a predetermined size. A pouch cell was used as the evaluation cell. The pouch cell evaluation cell was prepared by the following procedure. The pouch cell (laminate type cell) was prepared in a dry atmosphere with moisture controlled at a dew point of -60°C or less. An electrode group consisting of a working electrode, a separator, and a lithium electrode was inserted into a battery container made of an aluminum laminate film with a polyethylene/aluminum foil/nylon structure in the shape of a pocket, an electrolyte was injected, the electrode lead was removed, and the cell for evaluation was prepared by heat sealing. The outside of the aluminum laminate film was made of nylon film, and the inside of that was made of polyethylene film. A polyethylene film with a micropore structure was used as the separator.
The electrolyte was prepared by dissolving 1.2 M (mol/liter) of lithium hexafluorophosphate (LiPF6) in a solvent made by mixing ethylene carbonate and diethyl carbonate in a volume ratio of 3: ₇ , from which moisture had been thoroughly removed, and adding 3 mass% of vinylene carbonate (VC).

Charge and discharge were performed at a constant current of about 0.2C (1C: the current at which the battery capacity is charged and discharged in 1 hour), and the cell was discharged until the voltage reached 0.01 V, and then charged to 1.50 V for evaluation. The amount of electricity discharged was taken as the amount of electricity used to insert lithium, and the amount of electricity charged was taken as the amount of electricity used to release lithium.
The performance was evaluated by repeating charging and discharging, and the amount of Li released in the 50th charge was compared to the amount of Li released in the first charge (amount of electricity). The evaluation results were as follows:

(Comparative evaluation results of the electrodes of Examples A1 to A10 and Comparative Example A1, Example A11 and Comparative Example A2, and Example A12 and Comparative Example A3)
The ratios of the amount of Li released at the 50th time to the amount of Li released at the 1st time (amount of electricity) for the electrodes of Examples A1 to A10 were all greater than that for the electrode of Comparative Example A1. The ratios of the amount of Li released at the 50th time to the amount of Li released at the 1st time (amount of electricity) for the electrodes of Examples A11 and Comparative Example A2, and the electrodes of Example A12 and Comparative Example A3 were all greater for the electrodes of the examples.
The first electrode capacity of the electrodes of Examples A1 to A5, A7 to A10, and Comparative Example A1 was about 1700 mAh/g. The electrodes of Example A6, Example A11, and Comparative Example A2 had a capacity of about 1500 mAh/g, and the electrodes of Example A12 and Comparative Example A3 had a capacity of about 1000 mAh/g. It was found that if the content of ceramic powder in the electrode is increased, the content of conductive material must be reduced, and if the content of graphite to be mixed must also be reduced, the storage capacity of the electrode must be reduced and the electronic conductivity of the electrode also decreases.

[Preparation of Lithium-Ion Secondary Battery]
A lithium-ion secondary full cell was fabricated by combining the fabricated electrode with a positive electrode as a counter electrode, and the charge/discharge performance was evaluated. The battery was charged at a constant current and constant voltage up to an upper limit voltage of 4.2 V, and discharged at a constant current down to 2.5 V to evaluate the charge/discharge characteristics.

(Example F1)
<Preparation of Positive Electrode>
The positive electrode material _{LiNi0.8Co0.1Mn0.1O2 was} dispersed in an ethyl alcohol solution of _trilithium citrate tetrahydrate and _aluminum nitrate nonahydrate (mass ratio of trilithium citrate tetrahydrate:aluminum nitrate nonahydrate:ethyl alcohol is 1.25:5.0:100), spray-dried at an ambient temperature of approximately 200°C, and then heat-treated at 350°C in a nitrogen atmosphere to prepare _{LiNi0.8Co0.1Mn0.1O2} powder with a _lithium -aluminum _{composite oxide LixAlyO2} and a surface coating _of amorphous _carbon .

_LixAlyO2 and _{LiNi0.8Co0.1Mn0.1O2 powder} surface _- coated with _{amorphous carbon} , acetylene black, and polyvinylidene fluoride (PVdF) 12% _by mass in N-methyl-2-pyrrolidone (NMP) solution were mixed so that the mass ratio of the solid content was 97:1:2, and NMP was appropriately added and kneaded to prepare a slurry for forming an electrode active material layer. Next, the obtained slurry was applied to an aluminum foil using a coater, and then dried at 110 ° C for 1 hour, and the electrode surface was flattened and the thickness was adjusted with a roll press machine, and further dried at 150 ° C under reduced pressure to obtain an electrode structure with an electrode active material layer formed. The obtained electrode structure was punched out to a predetermined size, and an aluminum lead was welded to an aluminum current collector tab by ultrasonic welding to prepare a positive electrode.

<Preparation of Electrolyte Solution>
An electrolyte solution was prepared by dissolving 1.2 M (mol/liter) of lithium hexafluorophosphate (LiPF6) in a solvent made by mixing ethylene carbonate and diethyl carbonate in a volume ratio of 3: ₇ , from which moisture had been thoroughly removed, and adding 3 mass% of vinylene carbonate (VC).

<Production and performance evaluation of lithium-ion secondary batteries>
A pouch cell was prepared by the following procedure using the electrode structure of Example A1 as the negative electrode. The pouch cell (laminate type cell) was prepared in a dry atmosphere with moisture controlled at a dew point of -60°C or less. An electrode group consisting of a negative electrode, a separator, and a positive electrode was inserted into a battery case made of an aluminum laminate film with a polyethylene/aluminum foil/nylon structure in the shape of a pocket, an electrolyte was poured in, the electrode leads were removed, and the cell was heat sealed to prepare a cell for evaluation as a lithium ion secondary battery.
The weight per unit area of the active material layer of the positive electrode was adjusted so that the negative electrode capacity/positive electrode capacity ratio was 1.05, based on the charge storage capacities of various negative electrodes obtained from the results of the above-mentioned herb cell.
The pouch cell thus prepared was initially charged to 4.2 V at a constant current of 0.1 C, then charged at a constant voltage of 4.2 V, and then discharged at a constant current of 0.1 C to 2.5 V. The generated gas was released and the cell was sealed again.

(Examples F2 to F12)
An evaluation cell was produced in the same manner as in Example F1, except that in Example F1, each of the electrode structures of Examples A2 to A12 was used as the negative electrode.

(Comparative Examples F1 to F3)
An evaluation cell was produced in the same manner as in Example F1, except that in Example F1, each of the electrode structures of Comparative Examples A1 to A3 was used as the negative electrode.

(Example F13)
In Example F3, the electrolyte was changed to the following electrolyte to prepare an evaluation cell: The electrolyte used was prepared by dissolving 1.2 M (mol/liter) of lithium hexafluorophosphate (LiPF6) in a solvent in which ethylene carbonate and diethyl carbonate, from which moisture had been thoroughly removed, were mixed in a volume ratio of 3: ₇ , and adding 6 mass % of vinyl ethylene carbonate and 1 mass % of diethylene glycol divinyl ether.

[Performance evaluation of electricity storage devices]
The full cell thus fabricated was charged at a constant current of 0.2 C and a constant voltage of 4.2 V, then discharged at 0.2 C to 2.5 V, and then discharged under the same charging conditions at a discharge current of 1.0 C to 2.5 V to determine the ratio of the 1.0 C discharge capacity to the 0.2 C discharge capacity, and the rate performance was evaluated. Furthermore, the full cell was charged at a constant current of 0.5 C and a constant voltage of 4.2 V, then discharged at 0.5 C to 2.5 V for 100 cycles, and the ratio of the discharge amount at the 100th cycle to the initial discharge amount (capacity retention rate) was evaluated.

The ratio of the 1.0C discharge capacity to the 0.2C discharge capacity in Examples F5, F6, and F7 was Example F7 > Example F5 > Example F6. It was found that in the cells of the examples, when the amount of ceramic contained in the electrodes was as high as 20%, the rate performance decreased, and that the rate performance improved by adding a ceramic with electrical conductivity.
As for the capacity retention rate, similar to the evaluation results of the half cells described above, the cells formed from the negative electrodes containing the ceramic of the embodiment had a higher capacity retention rate during repeated charging and discharging than the cells of the comparative example.
In a comparison between the cells of Example F3 and Example F13, the capacity retention rate at the 100th cycle was higher in Example F13, and when the cell of Example F13 was disassembled and observed after charging and discharging, it was found that the electrolyte had gelled.
When the rate characteristics of the ratio of the discharge capacity at 1.0 C to the discharge capacity at 0.2 C were compared for Examples F1 to F6, the rate characteristics of Examples F1, F2, F3, and F4 were almost the same and high, while the rate characteristics of Example F5 were slightly lower and the rate characteristics of Example F6 were even lower. It was found that the rate characteristics tend to decrease when the ceramic powder content is high, and it was also found that the electrode design according to the intended use is necessary.

As explained above, the present invention can provide a lithium-ion secondary battery, which is an electricity storage device with high energy density and long rechargeable life.

Claims (3)

In a secondary battery (lithium ion secondary battery) capable of absorbing and releasing lithium ions, which is composed of a negative electrode, an electrolyte as a lithium ion conductor, and a positive electrode,
The negative electrode is at least composed of a negative electrode active material made of an inorganic material containing silicon element that stores lithium upon charging and releases lithium upon discharging, hydrophobically treated porous ceramic particles containing lithium element that hold an electrolyte, a carbon material, and a binder made of an organic polymer. 2. The secondary battery according to claim 1, wherein the electrolyte contains both a monovinyl monomer that undergoes polymerization and crosslinking reactions during charging and a monomer having multiple vinyl groups in the molecule when the battery is assembled. In a secondary battery (lithium ion secondary battery) capable of absorbing and releasing lithium ions, which is composed of a negative electrode, an electrolyte as a lithium ion conductor, and a positive electrode,
the negative electrode is composed of at least an inorganic negative electrode active material containing silicon element that stores lithium when charged and releases lithium when discharged, hydrophobically treated porous ceramic particles that hold an electrolyte, a carbon material, and a binder made of an organic polymer;
A secondary battery characterized in that, when the battery is assembled, the electrolyte contains both a monovinyl monomer that undergoes polymerization and crosslinking reactions during charging, and a monomer having a plurality of vinyl groups in the molecule.