JP2014186945A - Positive electrode for lithium ion secondary battery, process of manufacturing the same, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for a lithium ion secondary battery that may resist to high temperature and high voltage driving.SOLUTION: At least part of surfaces of positive electrode active material particles are coated with a polymer coat layer that includes an amino group and a phosphate group. Since the polymer coat layer contains a phosphoric acid based polymer, capacity drop during a cycle test is suppressed.

Description

  The present invention relates to a positive electrode used in a lithium ion secondary battery, a manufacturing method thereof, and a lithium ion secondary battery using the positive electrode.

  A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of high output. Currently, it is mainly used as a power source for portable electronic devices, and further expected as a power source for electric vehicles that are expected to be widely used in the future. A lithium ion secondary battery has an active material capable of inserting and extracting lithium (Li) in each of a positive electrode and a negative electrode, and operates by moving lithium ions in an electrolytic solution provided between both electrodes. . A lithium-containing metal composite oxide such as lithium cobalt composite oxide is mainly used as the positive electrode active material, and a carbon material having a multilayer structure is mainly used as the negative electrode active material.

  However, the current capacity of lithium ion secondary batteries cannot be said to be satisfactory, and a further increase in capacity is required. As an approach to achieve this, although a higher positive electrode potential has been studied, there is a major problem that battery characteristics after repeated charge and discharge are extremely deteriorated when driven at a high voltage, particularly at a high temperature and high voltage. there were. It is thought that this is because the electrolytic solution and electrolyte are oxidatively decomposed near the positive electrode during charging.

  That is, since lithium ions are consumed by oxidative decomposition of the electrolyte in the vicinity of the positive electrode, the capacity is reduced, and the decomposition product of the electrolytic solution is deposited on the electrode surface and in the voids of the separator, and becomes a resistor against lithium ion conduction. It is believed that the output will decrease. Therefore, in order to solve such a problem, it is necessary to suppress decomposition of the electrolytic solution and the electrolyte.

  In view of this, Japanese Patent Application Laid-Open No. 11-097027, Japanese Patent Publication No. 2007-510267, and the like propose non-aqueous electrolyte secondary batteries in which a coating layer made of an ion conductive polymer or the like is formed on the positive electrode surface. By forming the coating layer, it is said that deterioration such as elution and decomposition of the positive electrode active material can be suppressed.

  However, these publications do not describe evaluation when charging is performed at a high voltage of 4.3 V or higher, and it is unclear whether it can withstand such high voltage driving. Further, the thickness of the coating layer is substantially on the order of μm, which greatly impedes lithium ion conduction.

JP 11-097027 A Special Table 2007-510267 Publication

  This invention is made | formed in view of the said situation, and makes it the problem which should be solved to provide the positive electrode for lithium ion secondary batteries which can endure high voltage drive.

  A feature of the positive electrode for a lithium ion secondary battery of the present invention that solves the above problems is that at least a part of the surface of the positive electrode active material particles is covered with a polymer coat layer, and the polymer coat layer contains an amino group and a phosphate group. There is.

The feature of the method for producing a cathode for a lithium ion secondary battery of the present _{invention, Li x Ni a Co b Mn} c O 2, Li x Co b Mn c O 2, Li x Ni a Mn c O 2, Li x Ni positive electrode active material particles containing a Li compound or a solid solution selected from _a Co _b O ₂ and Li ₂ MnO ₃ (where 0.5 ≦ x ≦ 1.5, 0.1 ≦ a <1, 0.1 ≦ b <1, 0.1 ≦ c <1); Applying a slurry containing a binder to the surface of the current collector and drying to form a positive electrode active material layer;
A cationic polymer solution in which a cationic polymer having a positive zeta potential in a neutral condition is dissolved in a solvent is applied to the positive electrode active material layer and dried to form a cationic polymer coat layer, and then the zeta potential in a neutral condition. Is to apply a phosphoric acid polymer solution in which a phosphoric acid polymer having a phosphoric acid group in the side chain is dissolved in a solvent and then dry to form a phosphoric acid polymer coat layer.

  In the positive electrode for a lithium ion secondary battery of the present invention, a polymer coat layer is formed on at least a part of the surface of the positive electrode active material particles. Since the polymer coat layer covers the positive electrode active material particles, direct contact between the positive electrode active material particles and the electrolytic solution can be suppressed during high voltage driving, and decomposition of the electrolytic solution and the electrolytic salt can be suppressed.

  The polymer coat layer contains an amino group and a phosphate group. When the polymer coat layer of the positive electrode contains an amino group and a phosphate group, the effect of suppressing a decrease in capacity of the lithium ion secondary battery during the cycle test is great. In addition, since the thickness of the polymer coat layer is on the order of nm, it does not become a lithium ion conductive resistance. Therefore, it is possible to provide a lithium ion secondary battery that can suppress the decomposition of the electrolytic solution even by high voltage driving, has high capacity, and can maintain high battery characteristics even after repeated charge and discharge.

  In the production method of the present invention, a cationic polymer solution in which a cationic polymer having a positive zeta potential in a neutral condition is dissolved in a solvent is applied to the positive electrode active material layer and dried to form a cationic polymer coat layer. Next, a phosphoric acid polymer solution in which a zeta potential under a neutral condition is negative and a phosphoric acid polymer having a phosphoric acid group in the side chain is dissolved in a solvent is applied and dried to form a phosphoric acid polymer coating layer. Yes. Since the zeta potential of the positive electrode active material particles according to the present invention is negative, the positive electrode active material particles and the cationic polymer coat layer are firmly bonded by Coulomb force. In addition, since the zeta potential of the phosphoric acid polymer is negative, the cationic polymer coat layer and the phosphoric acid polymer coat layer are firmly bonded by the Coulomb force. Therefore, both the cationic polymer coat layer and the phosphoric acid polymer coat layer can be formed into a thin film, and the total thickness of the polymer coat layer can be set to the nm order, so that a thin and uniform polymer coat layer can be formed. Can do.

  Furthermore, since the polymer coat layer can be formed by using the dipping method, a roll-to-roll process is possible and productivity is improved.

  The positive electrode for a lithium ion secondary battery of the present invention includes a current collector and a positive electrode active material layer bound to the current collector. What is necessary is just to use what is generally used for the positive electrode for lithium ion secondary batteries etc. as a collector. For example, aluminum foil, aluminum mesh, punched aluminum sheet, aluminum expanded sheet, stainless steel foil, stainless steel mesh, punched stainless steel sheet, stainless steel expanded sheet, nickel foam, nickel non-woven fabric, copper foil, copper mesh, punched copper sheet, Examples include a copper expanded sheet, a titanium foil, a titanium mesh, a carbon nonwoven fabric, and a carbon woven fabric.

When the current collector contains aluminum, it is desirable to form a conductive layer made of a conductor on the surface of the current collector and form a positive electrode active material layer on the surface of the conductive layer. By doing in this way, the cycling characteristics of a lithium ion secondary battery further improve. This is because elution into the electrolyte is suppressed. Examples of the conductor include carbon such as graphite, hard carbon, acetylene black, and furnace black, ITO (Indium-Tin-Oxide), tin oxide (SnO ₂ ), and the like. From these conductors, a conductive layer can be formed by a PVD method or a CVD method.

  The thickness of the conductive layer is not particularly limited, but is preferably 5 nm or more. If it is thinner than this, it will be difficult to achieve the effect of improving the cycle characteristics.

  The positive electrode active material layer includes positive electrode active material particles made of a positive electrode active material, a binding portion that binds the positive electrode active material particles to each other and binds the positive electrode active material particles and the current collector, and positive electrode active material particles. And a polymer coat layer covering at least a part of the surface.

As the positive electrode active material, metallic lithium, LiCoO ₂ , Li ₂ MnO ₃ , sulfur and the like can be used, but Li _x Ni _a Co _b Mn _c O ₂ , Li _x Co _b Mn _c O ₂ , Li _x Ni _a Mn _{c O 2, Li x Ni a} Co b O 2 and Li ₂ MnO ₃ (where 0.5 ≦ x ≦ 1.5,0.1 ≦ a < 1,0.1 ≦ b <1,0.1 ≦ c <1) Li compound or solid solution selected from It is preferable to contain. One of these may be used, or a plurality of types may be mixed. In the case of multiple types, a solid solution may be formed. In the case of a ternary positive electrode active material containing all of Ni, Co and Mn, it is desirable that a + b + c ≦ 1. Of these, Li _x Ni _a Co _b Mn _c O ₂ is particularly preferable. A part of the surface of these Li compounds or solid solutions may be modified, or a part of the surface may be covered with an inorganic substance. In this case, the modified surface and the coated inorganic material are referred to as positive electrode active material particles.

  Further, these positive electrode active materials may be doped with a different element in the crystal structure. Although the element and amount to be doped are not limited, Mg, Zn, Ti, V, Al, Cr, Zr, Sn, Ge, B, As and Si are preferable as the element, and the amount is preferably 0.01 to 5%.

  The binding portion is a portion formed by drying the binder, and binds the positive electrode active material particles or the positive electrode active material particles and the current collector. It is desirable that the polymer coat layer is also formed on at least a part of the binding portion. By doing so, the binding portion is protected and the binding strength is further increased, so that the positive electrode active material layer can be prevented from cracking or peeling even after a severe cycle test of high temperature and high voltage.

  The positive electrode active material layer generally contains a conductive additive, but it is desirable that the polymer coat layer be formed on at least a part of the conductive additive. By doing in this way, electrolyte solution can be protected from the conductive support agent used as the high electric potential.

  The polymer coat layer includes an amino group and a phosphate group. The amino group and the phosphate group may be contained in the same polymer, but it is preferable to include a plurality of types of polymers including a polymer having an amino group and a polymer having a phosphate group. Examples of the amino polymer having an amino group include polyethyleneimine, polyallylamine, polyvinylamine, polyaniline, polydiallyldimethylammonium chloride and the like. As the phosphoric acid polymer having a phosphoric acid group, for example, a polymer represented by the general formula shown in Chemical Formula 1 can be used.

In the chemical formula 1, R ₁ , R ₂ and R ₄ are arbitrary functional groups, and are preferably any of H, CH ₃ , COOH and OH. R ₃ includes at least a phosphate group, and may include an alkyl group, an aryl group, a glycol group, and the like in addition to the phosphate group.

  The phosphate polymer preferably has a number average molecular weight in the range of 100 to 2,000,000, more preferably in the range of 1,000 to 500,000. If the molecular weight is smaller than this range, the adhesion may be lowered, and if the molecular weight is larger than this range, uneven film formation may occur.

  The ratio of the amino-based polymer and the phosphoric acid-based polymer in the polymer coat layer is preferably in the range of 1 / 1000-1000 / 1 by weight ratio of amino-based polymer / phosphoric acid-based polymer, A range is further preferred. If the ratio of the phosphoric acid polymer is out of this range, it may be difficult to achieve the effect.

  Although a polymer coat layer can be formed from a mixture of an amino polymer and a phosphoric acid polymer, there is a problem that bonding with the positive electrode active material due to Coulomb force becomes weak. Therefore, first, a cationic amino-based polymer is formed on a positive electrode active material having a negative zeta potential by electrostatic interaction, and then a phosphate-based polymer coat layer is formed from the anionic phosphate-based polymer. It is desirable. Also in this case, the ratio of the amino polymer coat layer to the phosphoric acid polymer coat layer is preferably in the above-mentioned range by weight ratio.

  The polymer coat layer preferably further contains at least one selected metal selected from alkali metals, alkaline earth metals and rare earth elements. Li is particularly desirable as the alkali metal, Mg is particularly desirable as the alkaline earth metal, and La is particularly desirable as the rare earth element. The content of the selected metal in the polymer coat layer is preferably in the range of 0.1 to 90% by mass, and particularly preferably in the range of 1 to 50% by mass. When the content of the selected metal is less than 0.1% by mass, the effect due to the inclusion is not expressed, and when it exceeds 90% by mass, it may be difficult to form a uniform polymer coat layer.

  In order to form a polymer coat layer containing a selective metal, a CVD method, a PVD method, or the like can be used. However, it is not preferable in terms of cost, and it is not easy to include a selective metal. Therefore, it is preferable to form a polymer coat layer by applying a mixed solution in which an amino polymer or phosphoric acid polymer and a compound of a selected metal are dissolved in a solvent and drying the mixture. The compound of the selected metal is not particularly limited as long as it is soluble in a solvent, and nitrates, acetates and the like can be used.

  As a solvent for the polymer solution, an organic solvent or water capable of dissolving an amino polymer or a phosphoric acid polymer and a compound of a selected metal as required can be used. There is no restriction | limiting in particular in an organic solvent, The mixture of a some solvent may be sufficient. For example, alcohols such as methanol, ethanol and propanol, ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone, esters such as ethyl acetate and butyl acetate, aromatic hydrocarbons such as benzene and toluene, DMF, N-methyl- Mixed solvent of 2-pyrrolidone, N-methyl-2-pyrrolidone and ester solvents (ethyl acetate, n-butyl acetate, butyl cellosolve acetate, butyl carbitol acetate, etc.) or glyme solvents (diglyme, triglyme, tetraglyme, etc.) Can be used. Those having a low boiling point that can be easily removed from the polymer coat layer are desirable.

  The polymer solution may be applied by spraying, rollers, brushes, or the like, but it is desirable to apply by a dipping method in order to uniformly apply the surface of the positive electrode active material. When applied by dipping, the gap between the positive electrode active material particles is impregnated with the polymer solution, so that a polymer coat layer can be formed on almost the entire surface of the positive electrode active material particles. Therefore, direct contact between the positive electrode active material and the electrolytic solution can be reliably prevented.

  There are two methods for applying by dipping. First, a slurry containing at least a positive electrode active material and a binder is bound to a current collector to form a positive electrode, and the positive electrode is dipped in a polymer solution, pulled up and dried. This is repeated if necessary to form a polymer coat layer having a predetermined thickness.

  As another method, the positive electrode active material powder is first mixed with the polymer solution and dried by freeze-drying or the like. This is repeated if necessary to form a polymer coat layer having a predetermined thickness. Then, a positive electrode is formed using the positive electrode active material in which the polymer coat layer was formed.

  The thickness of the polymer coat layer is preferably in the range of 0.1 nm to 100 nm, more preferably in the range of 0.1 nm to 10 nm, and particularly preferably in the range of 0.1 nm to 5 nm. If the thickness of the polymer coat layer is too thin, the positive electrode active material may be in direct contact with the electrolytic solution. On the other hand, when the thickness of the polymer coat layer is on the order of μm or more, when a secondary battery is formed, resistance increases and ion conductivity decreases. In order to form such a thin polymer coat layer, a thin and uniform polymer coat layer can be formed by keeping the concentration of the polymer in the dipping solution (polymer solution) low.

  The polymer coat layer may cover at least a part of the surface of the positive electrode active material particles, but in order to prevent direct contact with the electrolytic solution, it is preferable to cover almost the entire surface of the positive electrode active material particles.

  The concentration of the polymer in the polymer solution is preferably 0.001% by mass or more and less than 2.0% by mass, and is preferably in the range of 0.1% by mass to 0.5% by mass. If the concentration is too low, the probability of contact with the positive electrode active material is low and the coating takes a long time. If the concentration is too high, the electrochemical reaction on the positive electrode may be inhibited.

  It is also preferable that the polymer coat layer further contains a lithium compound having an oxidation reaction potential higher than that of the carbonate-based electrolytic solution. To further include a lithium compound having an oxidation reaction potential higher than that of the carbonate-based electrolytic solution means to include a lithium compound having an oxidation reaction potential higher than that of the carbonate-based electrolytic solution. By further including such a lithium compound, the voltage resistance is further improved, and the cycle characteristics of the lithium ion secondary battery are improved. Here, the oxidation reaction potential means a potential at which the oxidation reaction starts, that is, a decomposition start voltage. Such oxidation reaction potential has different values depending on the type of organic solvent of the electrolytic solution used in the lithium ion secondary battery. In the present invention, the oxidation reaction potential is measured using a carbonate solvent as the organic solvent of the electrolytic solution. Means the value that appears when

Lithium compounds with higher oxidation reaction potential than carbonate electrolyte include lithium bis (pentafluoroethylsulfonyl) imide (LiBETI), lithium bis (trifluoromethanesulfonyl) imide (LiTFSI), LiBF ₄ , LiCF ₃ SO ₃ and so on. Illustrated. The content of the lithium compound in the polymer coat layer is preferably in the range of 10 to 80% by mass, and particularly preferably in the range of 40 to 60% by mass. If the content of the lithium compound is less than 10% by mass, the effect due to the inclusion is not expressed, and if it exceeds 80% by mass, it may be difficult to form a coating layer that encapsulates the lithium compound.

  Including the lithium compound in the polymer coat layer can be easily performed by, for example, immersing the electrode on which the polymer coat layer is formed in a solution in which the lithium compound is dissolved in a solvent, and lifting and drying the electrode.

The zeta potential referred to in the present invention is measured by a microscopic electrophoresis method, a rotating diffraction grating method, a laser Doppler electrophoresis method, an ultrasonic vibration potential (UVP) method, and an electrokinetic acoustic (ESA) method. . Particularly preferably, it is measured by laser Doppler electrophoresis. (Specific measurement conditions are described below, but are not limited thereto. First, a solution (suspension) having a solid content concentration of 0.1 wt% was prepared using DMF, acetone, and water as solvents. (Measured 3 times at ℃, and calculated the average value, and the pH was neutral.)
It is also preferred that the polymer in the polymer coat layer is crosslinked. By cross-linking the polymer, a high-density polymer coat layer can be formed. It does not specifically limit as a crosslinking agent, Crosslinking agents, such as an epoxy, isocyanate, acrylate, and a methacrylate, are illustrated.

  Since the polymer coat layer formed in this way has high bonding strength with the positive electrode active material, direct contact between the positive electrode active material and the electrolytic solution can be suppressed during high voltage driving. If the total thickness of the polymer coat layer is on the order of nm, it can be suppressed that the resistance becomes lithium ion conductive. Therefore, it is possible to provide a lithium ion secondary battery that can suppress the decomposition of the electrolytic solution even by high voltage driving, has high capacity, and can maintain high battery characteristics even after repeated charge and discharge.

  As binders constituting the binder contained in the positive electrode active material layer, polyvinylidene fluoride (PolyVinylidene DiFluoride: PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamide Imido (PAI), carboxymethylcellulose (CMC), polyvinyl chloride (PVC), methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene ( PP) etc. are exemplified. Curing agents such as epoxy resin, melamine resin, polyblock isocyanate, polyoxazoline, polycarbodiimide, ethylene glycol, glycerin, polyether polyol, polyester polyol, acrylic oligomer, phthalic acid, as long as the properties as a positive electrode binder are not impaired You may mix | blend various additives, such as ester, a dimer acid modified material, and a polybutadiene type compound, individually or in combination of 2 or more types.

  It is desirable that the amino polymer constituting the polymer coat layer has a good covering property on the binding part. Therefore, it is preferable to use a binder having a negative zeta potential. For example, since the zeta potential of polyvinylidene fluoride (PVdF) is negative, the coverage of the cationic amino polymer is improved.

  Moreover, the larger the potential difference between the binder and the amino polymer, the better. Therefore, when polyvinylidene fluoride (PVdF) is used as the binder, it is preferable to use polyethyleneimine (PEI) as the amino polymer and select a solvent so that the zeta potential is +20 mV or more.

  In addition, the positive electrode active material layer preferably contains a conductive additive. The conductive assistant is added to increase the conductivity of the electrode. Carbon black, graphite, acetylene black (AB), vapor grown carbon fiber (VGCF), etc., which are carbonaceous fine particles, can be added alone or in combination of two or more as conductive aids. The amount of the conductive aid used is not particularly limited, but can be, for example, about 2 to 100 parts by mass with respect to 100 parts by mass of the active material. If the amount of the conductive auxiliary is less than 2 parts by mass, an efficient conductive path cannot be formed, and if it exceeds 100 parts by mass, the moldability of the electrode deteriorates and the energy density decreases.

The lithium ion secondary battery of the present invention includes the positive electrode of the present invention. A well-known thing can be used for a negative electrode and electrolyte solution. The negative electrode includes a current collector and a negative electrode active material layer bound to the current collector. The negative electrode active material layer includes at least a negative electrode active material and a binder, and may include a conductive additive. As the negative electrode active material, known materials such as graphite, hard carbon, silicon, carbon fiber, tin (Sn), and silicon oxide can be used. A silicon oxide represented by SiO _x (0.3 ≦ x ≦ 1.6) can also be used. Each particle of the silicon oxide powder is composed of SiO _x decomposed into fine Si and SiO ₂ covering Si by a disproportionation reaction. When x is less than the lower limit, the Si ratio increases, so that the volume change during charge / discharge becomes too large, and the cycle characteristics deteriorate. When x exceeds the upper limit value, the Si ratio is lowered and the energy density is lowered. A range of 0.5 ≦ x ≦ 1.5 is preferable, and a range of 0.7 ≦ x ≦ 1.2 is more desirable.

In general, when oxygen is turned off, it is said that almost all SiO disproportionates and separates into two phases at 800 ° C. or higher. Specifically, the raw material silicon oxide powder containing amorphous SiO powder is subjected to heat treatment at 800 to 1200 ° C. for 1 to 5 hours in an inert atmosphere such as in a vacuum or an inert gas. A silicon oxide powder containing two phases of an amorphous SiO ₂ phase and a crystalline Si phase is obtained.

As silicon oxides, with respect to SiO _x may be used as complexed with from 1 to 50% by weight of carbon material. By combining carbon materials, cycle characteristics are improved. If the composite amount of the carbon material is less than 1% by mass, the effect of improving the conductivity cannot be obtained, and if it exceeds 50% by mass, the proportion of SiO _x is relatively decreased and the negative electrode capacity is decreased. The composite amount of the carbon material is preferably in the range of 5 to 30% by mass, more preferably in the range of 5 to 20% by mass with respect to SiO _x . A CVD method or the like can be used to combine a carbon material with SiO _x .

  The silicon oxide powder desirably has an average particle size in the range of 1 μm to 10 μm. When the average particle size is larger than 10 μm, the charge / discharge characteristics of the non-aqueous secondary battery are degraded.When the average particle size is less than 1 μm, the particles are aggregated and become coarse particles. May decrease.

  As the current collector, binder, and conductive additive in the negative electrode, the same materials as those used in the positive electrode active material layer can be used.

The lithium ion secondary battery of the present invention using the positive electrode and the negative electrode described above can use known electrolyte solutions and separators that are not particularly limited. The electrolytic solution is obtained by dissolving a lithium metal salt as an electrolyte in an organic solvent. The electrolytic solution is not particularly limited. As the organic solvent, an aprotic organic solvent such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) or the like is used. Can do. As the electrolyte to be dissolved, a lithium metal salt soluble in an organic solvent such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiI, LiClO ₄ , LiCF ₃ SO ₃ can be used.

For example, an organic solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, or dimethyl carbonate is mixed with a lithium metal salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 at} a concentration of about 0.5 mol / l to 1.7 mol / l. A dissolved solution can be used. Among them, it is desirable to use LiBF ₄ . By using a positive electrode with a polymer coat layer and including LiBF ₄ in the electrolyte, the effect of making the electrolyte difficult to decompose is obtained synergistically, so that even higher battery characteristics are maintained after repeated charging and discharging at high voltage drive can do.

  The separator separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used. In addition, these microporous films may be provided with a heat-resistant layer mainly composed of an inorganic substance, and the inorganic substance used is preferably aluminum oxide or titanium oxide.

  The lithium ion secondary battery of the present invention is not particularly limited in shape, and various shapes such as a cylindrical shape, a stacked shape, and a coin shape can be adopted. In any case, the separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the current collector is connected between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal. After being connected using a lead or the like, this electrode body is sealed in a battery case together with an electrolytic solution to form a battery.

  Hereinafter, embodiments of the present invention will be described in more detail with reference to examples.

<Preparation of positive electrode>
94 parts by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ as a positive electrode active material, 3 parts by mass of acetylene black (AB) as a conductive additive, 3 parts by mass of polyvinylidene fluoride (PVdF) as a binder, The mixed slurry containing was applied to the surface of an aluminum foil (current collector) using a doctor blade and dried to form a positive electrode active material layer having a thickness of about 40 μm.

  Polyethyleneimine (PEI) (molecular weight 1,800) was dissolved in ethanol to a concentration of 1% by mass to prepare an amino polymer solution. The positive electrode was immersed in this amino polymer solution at 25 ° C. for 10 minutes and then washed with ethanol to obtain a positive electrode having an amino polymer coating layer.

  Subsequently, a phosphoric acid polymer solution in which a phosphoric acid polymer having a number average molecular weight of 5,000 represented by the chemical formula 2 was dissolved in ethanol to a concentration of 0.01% by mass was prepared. A positive electrode having an amino polymer coat layer was immersed in this phosphate polymer solution at 25 ° C. for 10 minutes and then washed with ethanol to form a phosphate polymer coat layer.

  Then, it vacuum-dried at 120 degreeC for 12 hours, and obtained the positive electrode which has a polymer coat layer.

  Using a transmission electron microscope (H9000NAR manufactured by Hitachi High-Technologies Corporation), a polymer coat layer with a thickness of about 0.8 nm covering the positive electrode active material particles was observed when measured at an acceleration voltage of 200 kV and a magnification of 2,050,000 times. . The particle size of 2 nm ± 0.3 nm described above is the particle size of the polymer swollen with the solvent, and the coat layer of about 0.8 nm is the thickness after drying.

<Production of negative electrode>
First, SiO powder (manufactured by Sigma-Aldrich Japan, average particle size 5 μm) was heat-treated at 900 ° C. for 2 hours to prepare SiO _x powder having an average particle size of 5 μm. With this heat treatment, if silicon monoxide SiO is a homogeneous solid having a ratio of Si to O of approximately 1: 1, it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. The Si phase obtained by separation is very fine.

A slurry was prepared by mixing 32 parts by mass of this SiO _x powder, 50 parts by mass of natural graphite, 8 parts by mass of acetylene black (AB) as a conductive additive, and 10 parts by mass of polyamideimide as a binder. This slurry was applied to the surface of an electrolytic copper foil (current collector) having a thickness of 18 μm using a doctor blade, and a negative electrode having a negative electrode active material layer having a thickness of about 15 μm was produced on the copper foil.

<Production of lithium ion secondary battery>
The non-aqueous electrolyte is an organic solvent in which fluoroethylene carbonate (FEC), ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) are mixed at 4: 26: 30: 40 (volume%). 1 mol of LiPF ₆ and LiPF ₂ (C ₂ O ₄ ) ₂ dissolved at a concentration of 0.01 mol were used.

  A microporous polypropylene / polyethylene / polypropylene laminate film having a thickness of 20 μm was sandwiched between the positive electrode and the negative electrode as a separator to obtain an electrode body. This electrode body was wrapped with a polypropylene laminate film, and the periphery was thermally fused to produce a film-clad battery. Before the last side was heat-sealed and sealed, the non-aqueous electrolyte solution was injected and impregnated into the electrode body to produce a lithium ion secondary battery of this example.

  A positive electrode having a polymer coating layer in the same manner as in Example 1 except that a phosphoric acid polymer coating layer was formed using a phosphoric acid polymer solution in which a phosphoric acid polymer was dissolved in ethanol to a concentration of 0.05% by mass. A lithium ion secondary battery was fabricated in the same manner as in Example 1.

  A positive electrode was formed in the same manner as in Example 1 except that a phosphoric acid polymer coating layer was formed using a phosphoric acid polymer solution in which a phosphoric acid polymer was dissolved in ethanol to a concentration of 0.2% by mass. A lithium ion secondary battery was produced in the same manner as in Example 1.

  Using an amino polymer solution in which lanthanum nitrate is dissolved in ethanol to a concentration of 2.5 mmol / L and polyethyleneimine (PEI) is dissolved to a concentration of 1% by mass, an amino system containing lanthanum (La) is used. A polymer coat layer was formed. A phosphoric acid polymer coating layer was formed using a phosphoric acid polymer solution in which a phosphoric acid polymer was dissolved in ethanol to a concentration of 0.2% by mass, and the polymer coating layer was formed in the same manner as in Example 1. A positive electrode was formed, and a lithium ion secondary battery was produced in the same manner as in Example 1.

  When the amino polymer solution was prepared, lanthanum nitrate was first dissolved in ethanol and then polyethyleneimine (PEI) was dissolved, and the solution became transparent after becoming cloudy for a moment. When this mixed solution was analyzed using a particle size distribution analyzer ("NANO PARTIVLE ANALYZER SZ-100", manufactured by HORIBA), it was found that there were fine particles with a particle size in a very narrow range of 2nm ± 0.3nm. . These fine particles are presumed to be formed by coordination of lanthanum ions to polyethyleneimine.

[Comparative Example 1]
A lithium ion secondary battery was fabricated in the same manner as in Example 1 except that the positive electrode similar to that in Example 1 was used except that the polymer coat layer was not formed.

[Comparative Example 2]
After forming the amino polymer coat layer in the same manner as in Example 1, Example 1 except that the polyacrylic acid coat layer was formed using polyacrylic acid having a number average molecular weight of 25,000 instead of the phosphoric acid polymer. A positive electrode having a polymer coat layer was produced in the same manner as in Example 1, and a lithium ion secondary battery was produced in the same manner as in Example 1 except that this positive electrode was used.

[Comparative Example 3]
After forming an amino polymer coating layer containing lanthanum (La) in the same manner as in Example 4, a polymer coating layer was formed in the same manner as in Example 1 except that a polyacrylic acid coating layer was formed in the same manner as in Comparative Example 2. A lithium ion secondary battery was produced in the same manner as in Example 1 except that this positive electrode was used.

<Test Example 1>
Using the lithium ion secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 3, charging to 4.32 V under the conditions of 60 ° C. and 1 C CCCV charging, respectively, and resting for 10 minutes, followed by 1 C CC discharge A cycle test was performed in which a cycle of discharging at 3.26 V and resting for 10 minutes was repeated 200 times.

  When the battery before and after the cycle test was charged to 4.5V under the condition of 1C CCCV charge at a temperature of 25 ° C, paused for 10 minutes, discharged at 2.5V with a CC discharge of 0.33C, and paused for 10 minutes The load characteristics of each were measured.

  The capacity retention ratios at the 100th cycle and the 200th cycle were measured, and the results are shown in Table 1. The capacity maintenance ratio is a value obtained by a percentage of a value obtained by dividing the discharge capacity at the Nth cycle by the initial discharge capacity ((discharge capacity at the Nth cycle) / (initial discharge capacity) × 100).

  Further, SOC 20% discharge resistance at 25 ° C. was measured at the first, 100th and 200th cycles, respectively. The SOC 20% discharge resistance was obtained by charging the SOC to 20% and then discharging it at 3C for 10 seconds. The results are shown in Table 1.

10-second resistance = Voltage drop in 10 seconds / 3C current value The resistance increase rate is the percentage of the value obtained by subtracting the initial 10-second resistance from the 10-second resistance in the Nth cycle and dividing it by the initial 10-second resistance. ((Nth cycle discharge resistance−initial 10 second resistance) / (initial 10 second resistance) × 100).

  Compared to the lithium ion secondary batteries according to Comparative Examples 1 and 2, the lithium ion secondary battery according to Example 1 has a higher capacity retention rate after the cycle test and an extremely low resistance increase rate. That is, the lithium ion secondary battery according to Example 1 shows excellent cycle characteristics even in a severe test of high voltage driving. This is the effect of using a phosphoric acid polymer instead of polyacrylic acid.

  Moreover, although the capacity retention rate increases as the film thickness of the phosphoric acid polymer coat layer increases, it is recognized from the comparison of Examples 1 to 3 that the rate of increase in resistance increases.

  Furthermore, from the comparison of Examples 3 and 4 and Comparative Examples 2 and 3, it is clear that the addition of lanthanum (La) to the amino polymer coat layer increases the capacity retention rate and decreases the resistance increase rate. It can be seen that the addition of lanthanum is extremely effective in improving the cycle characteristics. It is clear that the lithium ion secondary battery according to Example 4 has a very low resistance increase rate even after 200 cycles and is extremely excellent in cycle characteristics.

<Test Example 2>
For the lithium ion secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 3, the battery was charged to a battery voltage of 4.32 V under the condition of CCCV 2.5 h (including CCCV) at 1 C at a temperature of 25 ° C., and placed in a furnace at 60 ° C. For 6 days and 12 days.

  For the batteries after 6 days storage and 12 days storage, the capacity retention rate was measured, and the results are shown in Table 2. The capacity retention rate is a value determined by a percentage ((discharge capacity after storage for N days) / (initial discharge capacity) × 100) obtained by dividing the discharge capacity after storage for N days by the initial discharge capacity.

  Further, the SOC 20% discharge resistance at 25 ° C. was measured for each of the battery before storage and the battery after storage for 6 days and after storage for 12 days. The SOC 20% discharge resistance was obtained by charging the SOC to 20% and then discharging at 3C for 10 seconds. The 10-second resistance was determined by the following calculation formula, and the resistance increase rate was calculated. The results are shown in Table 2.

10-second resistance = voltage drop in 10 seconds / 3C current value The resistance increase rate is the value obtained by subtracting the initial 10-second resistance from the 10-second resistance after storage for N days and dividing it by the initial 10-second resistance. It is a value obtained by percentage ((discharge resistance after storage for N days−initial 10 second resistance) / (initial 10 second resistance) × 100).

  Compared to the lithium ion secondary batteries according to Comparative Examples 1 and 2, the lithium ion secondary battery according to Example 1 has a high capacity retention rate after a storage test and an extremely low resistance increase rate. That is, the lithium ion secondary battery according to Example 1 is excellent in storage characteristics even when driven at high temperature and high voltage. This is the effect of using a phosphoric acid polymer instead of polyacrylic acid.

  In addition, from the comparison of Examples 1 to 3, the film thickness of the phosphoric acid-based polymer coat layer has little influence on the capacity maintenance rate, but the resistance increase rate is particularly low in the lithium ion secondary battery according to Example 2. This indicates that there is an optimum range for the film thickness of the phosphoric acid polymer coat layer.

  Furthermore, in Comparative Examples 2 and 3, the addition of lanthanum (La) to the amino polymer coat layer increases the capacity retention rate and decreases the resistance increase rate. However, from the comparison of Examples 3 and 4, even when lanthanum (La) is added to the phosphoric acid polymer coating layer, there is almost no effect on the capacity retention rate and the resistance increase rate.

  2 parts by mass of binder consisting of 97 parts by mass of graphite, 1 part by mass of acetylene black (AB) powder as a conductive additive, and a mixture (mass ratio 1: 1) of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) Mix to prepare a slurry. This slurry is applied to the surface of an electrolytic copper foil (current collector) having a thickness of 18 μm using a doctor blade, and a negative electrode active material layer having a thickness of about 15 μm is formed on the copper foil to obtain a negative electrode precursor. It was.

  Using this negative electrode precursor, a polymer coat layer was formed in the same manner as in Example 3 to form a negative electrode, and a lithium ion secondary battery was produced in the same manner as in Example 1 except that this negative electrode was used.

[Comparative Example 4]
A positive electrode was produced in the same manner as in Example 1 except that the amino polymer coat layer was formed in the same manner as in Example 1 and then the phosphate polymer was not coated. A lithium ion secondary battery was produced in the same manner as in Example 5 except that this positive electrode was used.

<Test Example 3>
About the lithium ion secondary battery of Example 5 and Comparative Example 4, it was charged to a battery voltage of 4.5 V under a condition of CCCV 2.5 h (including CCCV) of 0.33 C at a temperature of 25 ° C., and 6 days in a furnace at 60 ° C. And held for 12 days.

  For the batteries after 6 days storage and 12 days storage, the capacity retention rate was measured, respectively, and the results are shown in Table 3.

  From Table 3, it is clear that the capacity retention rate is improved by coating the phosphoric acid polymer even in the case of the negative electrode using graphite as an active material.

  Each negative electrode after the above test was analyzed by EDS, and the results are also shown in Table 3. A difference was found in the fluorine concentration per unit area between Example 5 and Comparative Example 4, and the fluorine concentration was lower in Example 5. That is, it is considered that the coating of the phosphoric acid polymer on the positive electrode suppresses the generation of fluorine-based deposits on the negative electrode and improves the capacity retention rate.

  The positive electrode for a lithium ion secondary battery of the present invention is useful as a positive electrode for a lithium ion secondary battery used in motor driving of electric vehicles and hybrid vehicles, personal computers, portable communication devices, home appliances, office equipment, industrial equipment, etc. In particular, the lithium ion secondary battery can be optimally used for driving a motor of an electric vehicle or a hybrid vehicle that requires a large capacity and a large output.

Claims (11)

  A positive electrode for a lithium ion secondary battery, wherein at least a part of the surface of the positive electrode active material particles is covered with a polymer coat layer, and the polymer coat layer contains an amino group and a phosphate group.   2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the polymer coat layer is a composite of an amino polymer having an amino group and a phosphate polymer having a phosphate group in a side chain. The active material _{particles, Li x Ni a Co b Mn} c O 2, Li x Co b Mn c O 2, Li x Ni a Mn c O 2, Li x Ni a Co b O 2 and Li ₂ MnO ₃ (where 3. The positive electrode for a lithium ion secondary battery according to claim 1, comprising a Li compound or a solid solution selected from 0.5 ≦ x ≦ 1.5, 0.1 ≦ a <1, 0.1 ≦ b <1, 0.1 ≦ c <1). .   4. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the polymer coat layer has a thickness of 10 nm or less. The positive active material particles _{Li x Ni a Co b Mn c} O 2 positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4.   6. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the amino polymer having an amino group is polyethyleneimine. A secondary battery positive electrode manufacturing method according to any one of claims _{1~6, Li x Ni a Co b} Mn c O 2, Li x Co b Mn c O 2, Li x Ni a Mn c O ₂ , Li _x Ni _a Co _b O ₂ and Li ₂ MnO ₃ (where 0.5 ≦ x ≦ 1.5, 0.1 ≦ a <1, 0.1 ≦ b <1, 0.1 ≦ c <1) are included. Applying a slurry containing positive electrode active material particles and a binder to the surface of the current collector and drying to form a positive electrode active material layer;
A cationic polymer solution in which a cationic polymer having a positive zeta potential in a neutral condition is dissolved in a solvent is applied to the positive electrode active material layer and dried to form a cationic polymer coat layer, and then the zeta in a neutral condition. Lithium ion dioxide characterized by forming a phosphate polymer coat layer by applying a phosphate polymer solution in which a phosphate polymer having a negative potential and having a phosphate group in the side chain is dissolved in a solvent and drying. A method for producing a positive electrode for a secondary battery.   8. The method for producing a positive electrode for a secondary battery according to claim 7, wherein the step of applying the cationic polymer solution is performed by a dipping method.   9. The method for producing a positive electrode for a secondary battery according to claim 7, wherein the step of applying the phosphoric acid polymer solution is performed by a dipping method.   A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 6.   11. The lithium ion secondary battery according to claim 10, wherein the battery voltage at the time of charging is 4.3 V or more.