JP6286829B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  With the widespread use of mobile devices such as mobile phones and laptop computers, the role of secondary batteries as power sources is gaining importance. These secondary batteries are required to be small, light and have a high capacity, and to be less susceptible to deterioration of charge / discharge capacity even when charge / discharge is repeated. Currently, many lithium ion secondary batteries are used as secondary batteries satisfying such characteristics. In recent years, the market for motor-driven vehicles such as electric vehicles and hybrid vehicles has rapidly risen, and further development of high-performance lithium-ion secondary batteries has been promoted by accelerating the development of household and industrial power storage systems. ing.

  Carbon such as graphite and hard carbon is mainly used for the negative electrode of the lithium ion secondary battery. Although these carbons can repeat charge / discharge cycles satisfactorily, since capacity near the theoretical capacity has already been realized, it is not possible to expect significant increase in capacity in the future. On the other hand, since there is a strong demand for capacity enhancement of lithium ion secondary batteries, negative electrode materials having a higher capacity than carbon, that is, a higher energy density, have been studied.

As a material for increasing the energy density, it is considered to use, as a negative electrode active material, a Li storage material that forms an alloy with lithium represented by the composition formula Li _X A (A is an element such as aluminum) (x> 0). Has been. This negative electrode active material has a large capacity with a large amount of occlusion and release of lithium ions per unit volume. Non-Patent Document 1 describes the use of silicon as a negative electrode active material. It is described that a secondary battery having a high energy density can be obtained by using such a negative electrode active material.

  Patent Document 1 describes a non-aqueous electrolyte secondary battery using a silicon oxide or silicate containing lithium as a negative electrode active material. It is described that by using such a negative electrode active material, a secondary battery having high energy density, excellent charge / discharge characteristics, and excellent cycle life can be obtained.

  Patent Document 2 describes a negative electrode for a lithium ion secondary battery obtained by using silicon and / or a silicon alloy as a negative electrode active material, using polyimide as a negative electrode binder, and sintering in a non-oxidizing atmosphere. . It is described that a secondary battery having excellent cycle characteristics can be obtained by using such a negative electrode.

Japanese Patent Laid-Open No. 06-325765 JP 2004-022433 A

Li (Li) et al., “A High Capacity Nano-Si Composite Material for Lithium Rechargeable Batteries and Electro-Semiconductor Materials and Lithium Rechargeable Batteries” -State Letters), Vol. 2, No. 11, p547-549 (1999)

  However, as described in Non-Patent Document 1, a battery using silicon as a negative electrode active material has a large amount of lithium ion storage / release per unit volume and a high capacity, but lithium ion is stored / released. At this time, since the fine powdering due to the expansion and contraction of the electrode active material itself proceeds, there is a problem that the charge / discharge cycle life is short.

  Moreover, although the secondary battery described in Patent Document 1 tends to improve the energy density, the irreversible capacity in the first charge / discharge is large, so that the degree of improvement in the energy density is not sufficient. is there.

  The secondary battery described in Patent Document 2 has a problem that the charge / discharge cycle life is short because the electrode active material itself expands and contracts when lithium ions are occluded and released. . Although the charge / discharge cycle life can be improved to some extent by using polyimide as the negative electrode binder, the degree of improvement is not sufficient.

  An object of the present invention is to solve the above problems and to provide a lithium ion secondary battery having good cycle characteristics and a high energy density.

  The present invention is a lithium ion secondary battery having a structure in which a positive electrode, a separator, and a negative electrode are stacked in this order, and the negative electrode includes a negative electrode active material containing at least silicon oxide and carbon, and a binder having an imide group. In addition, the present invention relates to a lithium ion secondary battery, wherein lithium is pre-doped and the surface of the negative electrode contains 5% by mass or more of lithium carbonate.

  According to the embodiment of the present invention, it is possible to provide a lithium ion secondary battery having high energy density and good cycle characteristics.

It is typical sectional drawing for demonstrating an example of the structure of the lithium ion secondary battery by embodiment of this invention.

  Hereinafter, the present embodiment will be described with reference to the drawings.

  The lithium ion secondary battery according to the present embodiment has a structure in which a positive electrode, a separator, and a negative electrode are stacked in this order. For example, as shown in FIG. 1, the negative electrode current collector 2 such as a copper foil and the surface thereof are formed. And a positive electrode made of a positive electrode current collector 4 such as aluminum and a positive electrode active material layer 3 formed on the surface thereof. The negative electrode active material layer 1 and the positive electrode active material layer 3 are disposed to face each other with a separator 5 interposed therebetween. The portion where the separator 5 and the negative electrode active material layer 1 face each other and the portion where the separator 5 and the positive electrode active material layer 3 face each other are impregnated with an electrolytic solution. A negative electrode terminal 6 and a positive electrode terminal 7 are connected to the negative electrode current collector 2 and the positive electrode current collector 4 for taking out the electrodes, respectively. In the example shown in FIG. 1, a negative electrode active material layer is provided on the surface of the negative electrode current collector 2, and a positive electrode active material layer is disposed on each negative electrode active material via a separator. It is connected by welding etc. at the end. Further, a laminated structure including a negative electrode current collector layer on the negative electrode current collector, a positive electrode current collector layer on the positive electrode current collector, and a separator between the positive electrode current collector layer and the negative electrode current collector layer is laminated. May be. The positive electrode current collectors and the negative electrode current collectors are connected to each other at the end portions by welding or the like. The element configured as described above is sealed with an exterior body 8. In the present specification, the lithium ion secondary battery may be simply referred to as “secondary battery”.

  Here, the negative electrode active material layer is preferably disposed such that the surface Fa on the side of the positive electrode active material layer faces the surface Fc of the positive electrode active material layer facing the negative electrode active material layer. If the area of the face Fa of the negative electrode active material layer is too larger than the area of the positive electrode active material layer Fc, the difference in the degree of volume change of the negative electrode between the part facing the positive electrode and the part not so increases. In addition, wrinkles, cuts and the like are generated, which causes deterioration of battery characteristics.

(Negative electrode)
In the lithium ion secondary battery of the present embodiment, the negative electrode includes a negative electrode active material and a negative electrode binder. The negative electrode active material contains at least silicon oxide and carbon, and the negative electrode binder contains a compound having an imide group. Furthermore, the negative electrode of the lithium ion secondary battery according to the present embodiment is pre-doped with lithium, and the surface of the negative electrode contains 5% by mass or more of lithium carbonate.

  The negative electrode binder is formed by binding and integrating the negative electrode active material and the current collector, and the negative electrode active material. By including a compound having an imide group as the negative electrode binder, the negative electrode active material and the current collector are combined. The function to make it adhere | attach becomes higher, and the battery which has favorable charging / discharging cycling characteristics can be manufactured.

  In the present embodiment, examples of the compound having an imide group used as a negative electrode binder include polyimide and polyamideimide.

  The polyimide resin can be obtained by dehydrating and cyclizing the polyamic acid (or polyamic acid) produced by reacting tetracarboxylic anhydride and diamine with heating or the like. Polyamideimide resin can be obtained, for example, by reaction of trimellitic acid or trimellitic anhydride or acid chloride thereof with aromatic diamine or aromatic diisocyanate.

  In addition, the negative electrode binder may further include other compounds as long as the object of the present invention is not impaired, and may include, for example, polyamide, polyacrylic acid resin, polymethacrylic acid resin, and the like.

  The content of the compound having an imide group in the total mass of the negative electrode binder is preferably 50% by mass or more, and may be 100% by mass.

  The negative electrode binder in the present embodiment preferably has an elastic modulus of 1.5 GPa or more and 5.0 GPa or less. If the elastic modulus is too low, the initial charge / discharge characteristics may become insufficient due to insufficient strength of the resin. On the other hand, if the elastic modulus is too high, the elongation of the resin is lowered, which may lead to a rapid drop in capacity during repeated charge / discharge.

  The negative electrode active material layer of the secondary battery according to the present embodiment contains silicon oxide and carbon as the negative electrode active material. In the present embodiment, the negative electrode active material may further contain simple silicon. Silicon oxide particles have a role as a negative electrode active material and have an advantage that the volume change is smaller than that of silicon particles. Hereinafter, the simple silicon and / or silicon oxide may be simply referred to as “silicon-based compound”.

  The content of the silicon oxide contained in the negative electrode active material is not particularly limited, but is preferably 10% by weight or more and 98% by weight or less, and 30% by weight or more and 95% by weight with respect to the total weight of the negative electrode active material. The following is more preferable.

  The negative electrode active material layer of the secondary battery according to the present embodiment contains carbon as the negative electrode active material, thereby relaxing expansion and contraction of the negative electrode during repeated charge and discharge and ensuring conduction of the silicon compound as the negative electrode active material. Therefore, better cycle characteristics can be obtained. Examples of the method of incorporating carbon into the negative electrode active material include a method of simply mixing a silicon compound or the like and carbon particles, a method of combining carbon on the surface of the negative electrode active material, and the like. Specifically, for example, a method of introducing a silicon compound into a gas atmosphere of an organic compound in a high temperature non-oxygen atmosphere, or a method of mixing a silicon compound and a carbon precursor resin in a high temperature non-oxygen atmosphere Thus, a carbon coating layer can be formed around the core of the silicon-based compound. Thereby, the negative electrode volume expansion in charging / discharging is suppressed and the further improvement effect of cycling characteristics is acquired.

  In the present embodiment, the carbon preferably includes, for example, at least one selected from graphite, carbon black, acetylene black, and carbon fiber. Moreover, although content of carbon is not specifically limited, It is preferable that it is 1 to 20 weight% with respect to 100 weight% of negative electrode active materials. If the amount of carbon is too large, the electrode density decreases, and the merit of battery capacity improvement, which is a feature of the negative electrode active material containing silicon compounds, is reduced. Therefore, the carbon amount is set in consideration of the desired battery capacity and cycle specification. It is preferable to do.

  The negative electrode active material layer may contain a conductive agent such as carbon black or acetylene black as necessary in order to increase conductivity. The content of the conductive agent is preferably 5 parts by mass or less with respect to 100 parts by mass of the negative electrode active material.

  The negative electrode active material layer is prepared by, for example, dispersing and kneading the negative electrode active material formed by the above method and the negative electrode binder in a solvent, coating the obtained slurry on the negative electrode current collector, and drying in a high temperature atmosphere. Can be formed. The content of the negative electrode binder is preferably 5 to 20 parts by mass with respect to 100 parts by mass of the negative electrode active material. As the solvent, N-methyl-2-pyrrolidone (NMP) and the like are suitable. As the negative electrode current collector, copper, nickel, silver, or an alloy thereof is preferable in view of electrochemical stability. Examples of the shape of the negative electrode current collector include foil, flat plate, and mesh. If necessary, the electrode density can be increased by pressing the negative electrode active material layer at room temperature or high temperature.

The density of the negative electrode active material layer (electrode density) is preferably in the range of 1.0 g / cm ^{3 to} 2.0 g / cm ³ . If the electrode density is too low, the charge / discharge capacity tends to decrease, and if the electrode density is too high, it becomes difficult to impregnate the electrode including the negative electrode with the electrolyte, and the charge / discharge capacity also tends to decrease. is there.

In addition, when small particle diameter particles are included in the negative electrode active material layer, the cycle characteristics tend to deteriorate. On the other hand, if the particle size is too large, the electrical characteristics tend to deteriorate. For this reason, the average particle diameter D ₅₀ of the active material particles contained in the negative electrode active material layer is preferably 0.1 μm or more and 20 μm or less, and more preferably 0.5 μm or more and 10 μm or less.

In the present embodiment, the basis weight of the negative electrode (the mass per unit area of the negative electrode excluding the negative electrode current collector) is preferably 3.0 to 15.0 mg / cm ² . If the basis weight is too small, the proportion of members (current collector, separator, etc.) that are not directly involved in charge / discharge increases, and the mass energy density decreases. On the other hand, if the basis weight is too large, peeling from the current collector and increase in contact resistance in the vertical direction of the electrode (depth direction when the electrode is placed in a flat plate shape) become large.

  The surface of the negative electrode of the lithium ion secondary battery of the present embodiment contains lithium carbonate in an amount of 5 wt% or more, preferably 5.5 wt% or more, more preferably 6 wt% or more, preferably 95 wt% or less, more preferably 80 wt% or less. To do. By using a negative electrode containing 5 wt% or more of lithium carbonate on the surface, the energy density and cycle characteristics of the lithium ion secondary battery can be improved. Here, the “surface of the negative electrode” is a portion where the depth from the surface of the negative electrode active material layer of the negative electrode to the surface on the negative electrode current collector side is, for example, preferably up to 10 nm, more preferably up to 5 nm. is there. In the present embodiment, in the negative electrode, lithium carbonate may be present not only on the surface of the negative electrode but also on the entire negative electrode.

  As a method for adding 5 wt% or more of lithium carbonate to the surface of the negative electrode, as described later, a method of doping lithium into the negative electrode using a lithium source containing lithium carbonate is preferable.

  The content of lithium carbonate on the surface of the negative electrode can be calculated, for example, by analyzing the elemental composition and chemical state by X-ray photoelectron spectroscopy.

  In the lithium ion secondary battery according to the present embodiment, lithium is previously doped in the negative electrode active material in the negative electrode. The negative electrode active material containing silicon oxide causes an irreversible reaction with lithium due to the presence of oxygen present inside, but by previously doping lithium into the negative electrode, the irreversible capacity is reduced and the energy density is further improved. I can plan. In the present specification, “lithium dope” may be described as “lithium pre-dope”.

In the present embodiment, the lithium doping amount of the negative electrode (lithium amount per unit area of the negative electrode) is preferably 1.6 to 4.0 mg / cm ² . If the amount of lithium doping is too small, the lithium doping effect of reducing the large irreversible capacity of the negative electrode active material cannot be obtained sufficiently. Moreover, when there is too much lithium dope amount, the negative electrode deterioration at the time of discharge will become large, and cycling characteristics will fall. The lithium doping amount can be confirmed by preparing a cell using Li as the counter electrode and measuring the charge / discharge efficiency.

  In this embodiment, the negative electrode is doped with lithium before the battery is manufactured (preferably only in the negative electrode state, more preferably the negative electrode is in the electrode plate state). As a method for doping lithium, a lithium source, preferably carbonic acid is used. A lithium source containing lithium is heated in contact with the negative electrode active material. Here, the form of the lithium source containing lithium carbonate is desirably a foil or a foil formed on a base material such as copper.

  In the present embodiment, the lithium source brought into contact with the negative electrode is preferably made of a metal containing 80% by mass or more of lithium. The metal containing 80% by mass or more of lithium may be a lithium alloy, but the lithium content is preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably 95% by mass or more. Examples of metal elements other than lithium contained in the metal having lithium as a main component include sodium, magnesium, silicon, calcium, copper, and cesium.

  The form of the metal containing 80% by mass or more of lithium (hereinafter sometimes simply referred to as lithium metal) may be an arbitrary form such as a foil or piece, but preferably has a larger area that can be in contact with the negative electrode. preferable. The lithium metal foil can be manufactured by, for example, extrusion or rolling. It can also be produced by vapor deposition.

  The lithium metal foil may be in the form of a single foil, or may be a form in which the lithium metal foil is formed on a substrate. As the substrate, metal foil that does not alloy with lithium such as copper, polyester such as polyethylene terephthalate (PET), plastic film such as polypropylene (PP), and the like are used. It is preferable that the base material has flexibility capable of being in close contact with the electrode. On the other hand, if the base material has a strength that does not cause wrinkles or the like when a volume change due to expansion or contraction occurs due to lithium doping, the electrode This has the effect of relaxing the shape change. Therefore, for example, the copper foil preferably has a thickness of 10 μm to 40 μm.

  The surface layer composition of this lithium metal often varies greatly depending on the production method, use conditions, use environment, storage environment, and the like. Even when a lithium film is formed by vapor deposition, sputtering, or the like in a vacuum apparatus, the generation state of the surface layer varies depending on conditions such as the degree of vacuum, gas introduced, and temperature during vapor deposition. Further, the distribution in the depth direction and the lateral direction also varies depending on the use conditions and environment.

  In this embodiment, it is preferable that the lithium source used for lithium dope to a negative electrode contains lithium carbonate, Preferably lithium carbonate is 8 mass% or more, More preferably, it is 10 mass% or more, More preferably, it is 25 mass% or more ( (Including 100% by mass). A surface layer containing 5% by mass or more of lithium carbonate is formed on the negative electrode by performing lithium doping by bringing the surface layer containing lithium carbonate of the lithium source having a surface layer containing lithium carbonate into contact with the negative electrode active material layer. be able to.

  The lithium source is preferably a metal containing 80% by mass or more of lithium as already described, and the form thereof is an arbitrary form such as a foil or a piece, preferably a foil. For example, the lithium source is a single foil of lithium metal and has a surface layer containing lithium carbonate on both or one side of the lithium metal. Here, when the lithium source is a single foil of lithium metal, it is sufficient that at least one surface of the lithium source (surface to be brought into contact with the negative electrode) is a surface layer containing lithium carbonate. The surface layer may be included. Moreover, you may have a surface layer containing lithium carbonate in the surface (opposite side to a base material) of the lithium metal formed on the base material.

  In the lithium source, the thickness of the surface layer containing lithium carbonate is preferably 30% or less of the thickness of the lithium source, more preferably 20% or less, still more preferably 10% or less. More preferably, it is% or less. The thickness of the surface layer containing lithium carbonate in the lithium source is preferably 15 nm or less, more preferably 10 nm or less, more preferably 8 nm or less, and more preferably 5 nm or less. Preferably, it is 3 nm or less. Here, the “thickness” is a thickness in a direction perpendicular to the contact surface between the lithium source and the negative electrode. The thickness of the lithium source is not particularly limited, and may be 20 mm or less, for example, 5 mm or less, and preferably about 1 mm or less. In particular, when the lithium source is in the form of a foil, the thickness of the foil is more preferably 500 μm or less.

  Although the thickness of a lithium source is not specifically limited, It is preferable to adjust with the composition and thickness of a negative electrode. By using a lithium source having a surface layer thickness in the above range, non-uniformity at the contact interface between the electrode and the lithium source is improved. In particular, in the case of a thin surface layer of 0.5 μm or less, the surface layer is pierced by irregularities on the surface of the negative electrode, so that the ratio of contact between the lithium metal portion below the surface layer and the negative electrode is particularly preferable. Since the surface layer of the lithium source is an impurity layer formed by the reaction of lithium metal, it is originally preferred to be extremely thin, but it must be uniform on the surface of the lithium source, so at least 0.5 nm or more The thickness is preferably 1 nm or more.

  A method for forming a surface layer containing lithium carbonate on the surface of the lithium source is not particularly limited. For example, it can be formed by holding lithium metal in a mixed atmosphere of argon and carbon dioxide, or argon, carbon dioxide, and a small amount of moisture.

  In contacting the negative electrode with a lithium source having a surface layer containing lithium carbonate, the negative electrode active material layer of the negative electrode and the lithium source may be in close contact with each other, and the method is not particularly limited. For example, (1) A method in which a negative electrode and a lithium source are stacked and then the negative electrode and the lithium source are brought into close contact with each other by pressurization. (2) A gap between the electrode and the lithium source is removed by depressurization after the negative electrode and the lithium source are stacked. The method of doing is mentioned.

In the method of pressurizing the superimposed negative electrode and lithium source, it is necessary to apply a pressure within a range in which relatively soft lithium does not protrude beyond the electrode. Specifically, 0.5 kgf / cm ² to 30 kgf / cm A pressure of ² is preferred. In particular, when the thickness of the lithium source is 100 μm or less, pressurization of 15 kgf / cm ² or less is preferable. Further, in order to perform uniform lithium pre-doping in the electrode surface, it is preferable to apply pressure uniformly to the electrode surface.

  In the method in which the superimposed negative electrode and the lithium source are brought into contact with each other under reduced pressure, the electrode and the lithium source are arranged in a deformable decompression container such as a bag in a state where the contact can be maintained, and the decompression container of the atmospheric pressure received from the outside is received. While the electrode and the lithium source are pressed against each other due to the deformation, the pressure is reduced within a range where there is no excess gap at the interface. Alternatively, the electrode and lithium may be put in a sealable bag such as a laminate to reduce pressure, and sealed under reduced pressure. For example, when the negative electrode and the lithium foil are brought into contact with each other, the pressure to be reduced is preferably in close contact with a reduced pressure of -0.05 MPa to -0.1 MPa.

  Further, the methods (1) and (2) may be combined, and if the contact between the negative electrode and the lithium source is good, pressurization or decompression may not be performed. Diffusion (doping) from the lithium source to the negative electrode proceeds at the part where the lithium and the electrode are in contact. The lithium pre-doping tends to proceed uniformly.

  The temperature at which the negative electrode and the lithium source are heated needs to be within a temperature range in which lithium diffusion proceeds stably and does not exceed the melting point of metallic lithium (180.5 ° C.), for example, from room temperature (20 ° C.). A range of 180 ° C. is preferred. At a temperature exceeding the melting point of metallic lithium, the molten lithium protrudes beyond the electrode and cannot be doped efficiently. In addition, when good contact is obtained by the method of bringing the negative electrode and the lithium source into contact with each other, dope is likely to proceed, and room temperature (20 ° C.) to 130 ° C. is more preferable. In particular, when the negative electrode and the lithium source are in close contact with each other with no extra space, room temperature (20 ° C.) to 80 ° C. is particularly preferable. Although the heating time depends on the required dope amount and the heating temperature, pre-doping can be generally completed within the range of 1 hour to 100 hours, preferably 2 hours to 12 hours.

  Further, in the step of heating while the negative electrode and the lithium source are in contact with each other, the heating may be performed while applying pressure while maintaining the close contact between the negative electrode and the lithium source.

  After completion of the required amount of lithium pre-doping, if lithium remains on the electrode, it may be removed. When a lithium foil or the like formed on a substrate is used as a lithium source, it can be easily removed.

  As described above, metallic lithium is a highly reactive metal and reacts violently with moisture. Therefore, it is preferable to carry out the work related to lithium pre-doping in a low-humidity environment, and to suppress deterioration of the lithium source, argon It is preferable to work in a gas atmosphere inert to lithium.

(Positive electrode)
In the secondary battery according to the present embodiment, examples of the positive electrode active material included in the positive electrode active material layer include lithium manganate; lithium cobaltate; lithium nickelate; a mixture of two or more of these lithium compounds; In which the manganese, cobalt, and nickel portions are partially or entirely replaced with other metal elements such as aluminum, magnesium, titanium, and zinc; lithium iron phosphate and the like can be used.

In addition, lithium manganate; nickel-substituted lithium manganate in which a part of manganese of lithium manganate is substituted with at least nickel; lithium nickelate; cobalt substitution in which a part of nickel in lithium nickelate is substituted with at least cobalt Lithium nickelate; An active material in which the manganese or nickel portion of these lithium compounds is replaced with another metal element (for example, at least one of aluminum, magnesium, titanium, and zinc) can be used. For example, an active material represented by the following composition formula can be used.
Li _a Ni _b Co _c Al _d O ₂
(0.80 ≦ a ≦ 1.05, 0.50 ≦ b ≦ 0.95, 0.10 ≦ c ≦ 0.50, 0.01 ≦ d ≦ 0.15).

  The positive electrode active material layer can be formed by dispersing and kneading a positive electrode active material and a positive electrode binder in a solvent, applying the obtained slurry onto a positive electrode current collector, and drying in a high temperature atmosphere. Polyvinylidene fluoride and polytetrafluoroethylene are mainly used as the positive electrode binder. As the solvent, N-methyl-2-pyrrolidone (NMP) and the like are suitable. As the positive electrode current collector, aluminum or an alloy containing aluminum as a main component can be used because high corrosion resistance in an organic electrolyte is required.

  In the secondary battery according to the present embodiment, as the separator, a porous film made of polyolefin such as polypropylene or polyethylene, fluororesin, polyimide, polyamideimide, or the like can be used.

  In the secondary battery according to the present embodiment, as the electrolytic solution, a nonaqueous electrolytic solution in which a lithium salt is dissolved in one or more types of nonaqueous solvents can be used. Nonaqueous solvents include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), cyclic carbonates such as vinylene carbonate (VC); dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate ( EMC), chain carbonates such as dipropyl carbonate (DPC); aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate; γ-lactones such as γ-butyrolactone; 1,2-ethoxyethane (DEE) And chain ethers such as ethoxymethoxyethane (EME); cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran.

  Other non-aqueous solvents include dimethyl sulfoxide, 1,3-dioxolane, dioxolane derivatives, formamide, acetamide, dimethylformamide, acetonitrile, propionitrile, nitromethane, ethyl monoglyme, phosphate triester, trimethoxymethane, sulfolane, methyl Non-protons such as sulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone An organic solvent can also be used.

Examples of the lithium salt dissolved in the non-aqueous solvent include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiN ( CF ₃ SO ₂ ) ₂ , LiB ₁₀ Cl ₁₀ , lower aliphatic lithium carboxylate, chloroborane lithium, lithium tetraphenylborate, LiBr, LiI, LiSCN, LiCl, imides and the like. Further, a polymer electrolyte may be used instead of the non-aqueous electrolyte solution.

  In the secondary battery according to the embodiment of the present invention, a can case, an exterior film, or the like can be used as the exterior body. Stainless steel cans are often used as can cases. As the exterior film, a laminate film having a thermoplastic resin such as an ionomer resin in which polyethylene, polypropylene, ethylene-methacrylic acid copolymer or ethylene-acrylic acid copolymer as an adhesive layer is intermolecularly bonded with metal ions is used. It is done.

  Hereinafter, examples according to the present embodiment will be described. The present invention is not limited to the following examples.

In the following examples and comparative examples, the following formula (a):

で表されるポリアミドイミド、または下記式（ｂ）：

Or a polyamideimide represented by the following formula (b):

で表されるポリイミドを用いた。

The polyimide represented by this was used.

Example 1
As the negative electrode active material, silicon oxide particles (silicon / oxygen = 1.1.05 (atomic ratio)) adjusted so that the average particle diameter D ₅₀ measured by the laser diffraction / scattering method is 5 μm are prepared. In addition, 85 parts by weight of the silicon oxide particles, 50 parts by weight of a polyamideimide-NMP solution represented by the above formula (a) as a binder solution (corresponding to 10 parts by weight of the finally obtained polyamideimide), and an average particle diameter A slurry of a negative electrode material was prepared by mixing 5 parts by weight of natural graphite powder adjusted to have D ₅₀ of 5 μm, and further adding NMP as a solvent to dissolve and disperse it. This slurry was applied to both sides of a copper foil having a thickness of 10 μm in a 160 × 90 mm square shape, dried at 125 ° C. for 5 minutes in a drying furnace, then compression molded with a roll press, and again A drying treatment was performed at 300 ° C. for 10 minutes in a drying furnace to form a negative electrode active material layer on both surfaces of the negative electrode current collector.

  At that time, the weight of the formed negative electrode active material layer is a weight corresponding to 1.50 Ah of the active material capacity (the initial charge capacity of the negative electrode when the potential reaches 0.02 V with respect to metallic lithium; hereinafter the same applies to the negative electrode). It was.

  In this manner, a single negative electrode active material layer formed on both sides of the negative electrode current collector was produced, and punched into a 160 × 90 mm square shape.

[Lithium dope]
As a lithium source, a lithium foil having a thickness of 40 μm and having a surface layer (about 30% by mass of lithium carbonate) mainly made of lithium carbonate having a thickness of about 8 nm was prepared. The lithium foil was placed on the negative electrode in an aluminum laminate bag and sealed while reducing the pressure to -0.1 MPa. Next, the aluminum laminate bag was heat-treated at 100 ° C. for 24 hours in a thermostatic bath. After the heat treatment was completed, the electrode was taken out from the bag after the aluminum laminate bag was completely returned to room temperature (23 ° C.) or less, and lithium doping was completed.

  The obtained lithium-doped negative electrode surface was analyzed for elemental composition and chemical state by X-ray photoelectron spectroscopy, and the proportion of lithium carbonate contained on the surface was calculated from the obtained data. The proportion of lithium carbonate is shown in Table 1.

  On the other hand, 92 parts by weight of positive electrode active material particles made of lithium cobaltate are mixed with 4 parts by weight of polyvinylidene fluoride as a binder and 4 parts by weight of carbon powder (amorphous carbon powder) as a conductive agent, and further as a solvent. NMP was added and dissolved and dispersed to prepare a slurry of the positive electrode material. This slurry is applied to one side of a 20 μm thick aluminum foil in a 160 × 90 mm square shape, dried at 125 ° C. for 5 minutes in a drying furnace, and then subjected to compression molding with a roll press. A positive electrode active material layer was formed on one side of the positive electrode current collector.

  At that time, the weight of the formed positive electrode active material layer is the weight corresponding to 1.00 Ah of the active material capacity (the initial charge capacity of the positive electrode when the potential reaches 4.3 V with respect to metallic lithium; hereinafter the same applies to the positive electrode). It was.

  In this way, two sheets of positive electrode current collectors each having a positive electrode active material layer formed thereon were produced, and punched into a 165 × 95 mm square shape to obtain a positive electrode.

  Next, a 170 × 100 mm square separator made of a polypropylene porous film was prepared. And the laminated body which piled up in order of the positive electrode, the separator, the negative electrode, the separator, and the positive electrode was obtained from the bottom.

  Next, a negative electrode terminal made of nickel for drawing out the electrode was fused to the negative electrode current collector by ultrasonic bonding. Further, two positive electrode current collectors were superposed on the opposite side of the negative electrode terminal, and a positive electrode terminal made of aluminum for drawing out the electrode was fused thereto by ultrasonic bonding. Thus, the positive electrode terminal and the negative electrode terminal were arranged on the long side portions facing each other.

  After overlaying the exterior film so that the adhesive layer is on the laminated cell side from both sides of the resulting laminate, heat sealing (sealing) the three sides of the four sides where the outer periphery of the exterior film overlaps I stopped it. Then, the electrolyte solution was poured and the last side was heat-sealed under vacuum.

Here, as the electrolytic solution, a solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / L in a solvent in which EC, DEC, and EMC were mixed at a volume ratio of 3: 5: 2 was used. In the laminated battery thus obtained, the tips of the negative electrode terminal and the positive electrode terminal protrude outward from the exterior film in opposite directions. Five laminate type batteries were produced.

(Evaluation of initial discharge capacity, cycle characteristics, and cell thickness increase rate)
The discharge capacity when the five laminated batteries obtained were first charged to a rated voltage of 4.2 V in a constant temperature atmosphere of 20 ° C. and then discharged to 2.7 V. Was measured. This was defined as the initial discharge capacity, that is, charge / discharge capacity. Next, in a constant temperature atmosphere of 45 ° C., charging to 4.2 V and discharging to 2.7 V were repeated 100 times at a 1 C rate for each laminated battery, and the discharge capacity after 100 cycles was measured at 20 ° C. did. The 1C rate means a current value for charging / discharging the nominal capacity (mAh) in one hour. The ratio of the discharge capacity after 100 cycles to the initial discharge capacity was calculated and used as the cycle characteristics. Further, the thickness of each laminated battery was measured before the start of the cycle and after 100 cycles, and the cell thickness increase rate was calculated. Table 1 shows the average values of the initial discharge capacity, the cycle characteristics, and the cell thickness increase rate in the obtained five laminated batteries.

(Example 2)
As a binder solution, 50 parts by weight of a polyamic acid-NMP solution (10 parts by weight of polyimide represented by the above formula (b) finally obtained) was prepared. Other than that was carried out in the same manner as in Example 1. Table 1 shows the average values of the ratio of lithium carbonate in the negative electrode, the initial discharge capacity, the cycle characteristics, and the cell thickness increase rate in the obtained five laminated batteries.

(Example 3)
As a lithium source, a lithium foil having a thickness of about 100 μm and having a surface layer (about 50% by mass of lithium carbonate) mainly made of lithium carbonate having a thickness of about 5 nm was prepared. Other than that was carried out in the same manner as in Example 1. Table 1 shows the average values of the ratio of lithium carbonate in the negative electrode, the initial discharge capacity, the cycle characteristics, and the cell thickness increase rate in the obtained five laminated batteries.

Example 4
As a lithium source, a lithium foil having a thickness of 20 μm having a surface layer (about 10% by mass of lithium carbonate) mainly made of lithium carbonate having a thickness of about 10 nm was prepared. Other than that was carried out in the same manner as in Example 1. Table 1 shows the average values of the ratio of lithium carbonate in the negative electrode, the initial discharge capacity, the cycle characteristics, and the cell thickness increase rate in the obtained five laminated batteries.

(Example 5)
As a lithium source, a 10 μm thick lithium foil having a surface layer (about 60% by mass of lithium carbonate) mainly made of lithium carbonate having a thickness of about 20 nm was prepared. Other than that was carried out in the same manner as in Example 1. Table 1 shows the average values of the ratio of lithium carbonate in the negative electrode, the initial discharge capacity, the cycle characteristics, and the cell thickness increase rate in the obtained five laminated batteries.

(Comparative Example 1)
As a lithium source, a 50 μm thick lithium foil having a surface layer (lithium carbonate about 6 mass%) mainly made of lithium carbonate having a thickness of about 15 nm was prepared. Other than that was carried out in the same manner as in Example 1. Table 1 shows the average values of the ratio of lithium carbonate in the negative electrode, the initial discharge capacity, the cycle characteristics, and the cell thickness increase rate in the obtained five laminated batteries.

(Comparative Example 2)
As a negative electrode binder solution, 100 parts by weight of a polyvinylidene fluoride-NMP solution (10 parts by weight of finally obtained polyvinylidene fluoride) was prepared. Other than that was carried out in the same manner as in Example 1. Table 1 shows the average values of the ratio of lithium carbonate in the negative electrode, the initial discharge capacity, the cycle characteristics, and the cell thickness increase rate in the obtained five laminated batteries.

(Comparative Example 3)
The same operation as in Example 1 was performed except that silicon was used instead of silicon oxide as the negative electrode active material. Table 1 shows the average values of the ratio of lithium carbonate in the negative electrode, the initial discharge capacity, the cycle characteristics, and the cell thickness increase rate in the obtained five laminated batteries.

  Since Examples 1 to 5 had better cycle characteristics than Comparative Example 1, it was shown that the proportion of lithium carbonate in the negative electrode surface was preferably 5 wt% or more. Moreover, it was suggested that the lithium carbonate content in the surface layer of the lithium foil is preferably 8 wt% or more.

  In Examples 1 to 4, the initial discharge capacity was also particularly excellent, suggesting that the thickness of the surface layer containing lithium carbonate is preferably 10 nm or less in the lithium foil used as the lithium source.

  In Comparative Example 2, a compound having no imide bond was used for the negative electrode binder, but the cycle characteristics were clearly reduced as compared with Examples 1 and 2. This is presumably because, in Comparative Example 2, the binder strength was low, so that every time charging / discharging was performed, binding between particles was lost due to volume change of the negative electrode, and conductivity was lost.

  In Comparative Example 3, silicon oxide was not mixed in the negative electrode, but the cycle characteristics were clearly reduced as compared with Examples 1 and 2. This is because in Comparative Example 3, the merit of suppressing the volume change of the particles due to containing silicon oxide is not obtained, the volume change of the particles is increased, the particles are cracked every charge and discharge, and the conductivity is lost. It is thought that it was because of.

  Moreover, in Example 1, although the example which mixed the silicon oxide and natural graphite powder is shown, as preparation of a negative electrode, it is not limited to this. For example, similar characteristics can be obtained when silicon oxide particles and a phenol resin are mixed and fired in a nitrogen atmosphere, and when carbon is deposited on the surface of the silicon oxide particles. From this, it can be seen that good initial discharge capacity and cycle characteristics can be obtained by a negative electrode in which both silicon oxide and carbon are present.

  As described above, according to the present embodiment, excellent characteristics can be obtained in the initial characteristics of the lithium ion secondary battery, that is, the energy density, and, for example, in the cycle characteristics after 45 cycles at 45 ° C.

  The lithium ion secondary battery according to the present embodiment is a lithium ion secondary battery such as an energy regeneration application in an electric vehicle, an engine drive, a power storage application in combination with a solar battery, an emergency power supply for industrial equipment, and a consumer equipment drive. It can be used for applicable products.

DESCRIPTION OF SYMBOLS 1 Negative electrode active material layer 2 Negative electrode collector 3 Positive electrode active material layer 4 Positive electrode collector 5 Separator 6 Negative electrode terminal 7 Positive electrode terminal 8 Exterior body

Claims (7)

A lithium ion secondary battery having a structure in which a positive electrode, a separator, and a negative electrode are stacked in this order,
The negative electrode is
A negative electrode active material containing at least silicon oxide and carbon, a binder having an imide group, lithium is pre-doped, and
The lithium ion secondary battery front surface is characterized by containing a lithium carbonate or 5 wt% of the depth of the negative electrode 5 nm. ^2. The lithium ion secondary battery according to claim 1, wherein the weight per unit area of the negative electrode (mass per unit area excluding the negative electrode current collector) is 3.0 to 15.0 mg / cm ² . 3. The lithium ion secondary battery according to claim 1, wherein a lithium doping amount of the negative electrode is 1.6 to 4.0 mg / cm ² . Producing a negative electrode comprising a negative electrode active material containing silicon oxide and carbon, and a binder containing an imide group;
Producing a negative electrode doped with lithium by contacting and heating the negative electrode and a lithium source having a surface layer containing lithium carbonate;
Laminating a positive electrode, a separator, and a negative electrode doped with lithium, to produce an electrode element;
The manufacturing method of the lithium ion secondary battery containing this. The lithium source, have a surface layer containing lithium carbonate or 8 wt%, the thickness of the surface layer, in claim 4, wherein 30% or less der Rukoto the thickness of the lithium source The manufacturing method of the lithium ion secondary battery of description. Method for producing a lithium ion secondary battery according to claim 4 or 5, wherein the thickness of the surface layer containing lithium carbonate of the lithium source is 10nm or less. The method for producing a lithium ion secondary battery according to any one of claims 4 to 6 , wherein the lithium source having a surface layer containing lithium carbonate is a foil.

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