JP6578777B2 - Positive electrode for lithium ion secondary battery, method for producing the same, and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a positive electrode for a lithium ion secondary battery, a method for producing the same, and a lithium ion secondary battery.

A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of high output. Currently, lithium ion secondary batteries are mainly used as power sources for portable electronic devices and electric vehicles. The positive electrode and the negative electrode of the lithium ion secondary battery have an electrode active material capable of inserting and extracting lithium. A lithium-containing metal composite oxide such as LiCoO ₂ is mainly used as the positive electrode active material, and a carbon material or a Si-based material is used as the negative electrode active material.

In recent years, olivine-type lithium phosphate composite oxides such as LiFePO ₄ have been proposed as positive electrode active materials. Since the olivine-type lithium phosphate complex oxide has little change in crystal structure accompanying charge / discharge, the cycle characteristics are good. On the other hand, since the olivine-type lithium phosphate complex oxide has low conductivity, the surface is often covered with a carbon film made of a carbon material (Patent Documents 1 and 2).

   However, since the carbon film has good electron conductivity, an electron and an electrolytic solution react with each other in the vicinity of the particle surface of the olivine type lithium phosphate complex oxide to form an SEI film. The formation of the SEI film consumed Li ions, which hindered improvement of cycle characteristics.

  On the other hand, Patent Document 3 discloses forming an organic-inorganic coating layer having one or more selected metals selected from alkali metals, alkaline earth metals, and rare earth elements and a polymer on the positive electrode surface. .

  However, the surface of the positive electrode active material disclosed in Patent Document 3 is not covered with a carbon film. It was unclear whether the cycle characteristics were improved by forming an organic / inorganic coating layer for the positive electrode active material whose surface was coated with a carbon film.

JP 2011-113783 A JP 2011-238490 A JP 2014-160635 A

  This invention is made | formed in view of this situation, and makes it a subject to provide the positive electrode for lithium ion secondary batteries excellent in cycling characteristics, its manufacturing method, and a lithium ion secondary battery.

The positive electrode for a lithium ion secondary battery of the present invention includes a current collector and a positive electrode active material layer that covers the current collector,
The positive electrode active material layer is
First active material particles having a first positive electrode active material capable of inserting and extracting lithium ions, and a carbon coating formed on the surface of the first positive electrode active material, and on the surface of the first active material particles Formed organic inorganic coating layer,
Have
The organic / inorganic coating layer includes a selection polymer having at least one selected from an amino group, an amide group, an imino group, an imide group, a maleimide group, a carboxyl group, and an ether group, an alkali metal, an alkaline earth metal, and a rare earth element. And at least one selected metal selected from the group consisting of:

The manufacturing method of the positive electrode for lithium ion secondary batteries of this invention has the 1st positive electrode active material which can occlude and discharge | release lithium ion, and the carbon coating part formed in the surface of the said 1st positive electrode active material. Applying a slurry containing active material particles to the surface of the current collector and drying to form a positive electrode active material layer;
A selection polymer having at least one selected from an amino group, an amide group, an imino group, an imide group, a maleimide group, a carboxyl group and an ether group, and at least one selection selected from an alkali metal, an alkaline earth metal and a rare earth element Applying a mixed solution in which a metal is dissolved in a solvent to the positive electrode active material layer and drying to form an organic-inorganic coating layer on the surfaces of the first active material particles.

  The lithium ion secondary battery of the present invention includes the positive electrode for a lithium ion secondary battery described above or the positive electrode for a lithium ion secondary battery manufactured by the method for manufacturing a positive electrode for lithium ion secondary battery described above. It is characterized by.

  Since this invention comprises the said structure, it can provide the positive electrode for lithium ion secondary batteries excellent in cycling characteristics, its manufacturing method, and a lithium ion secondary battery.

FIG. 3 is a diagram showing the electronic resistance of the positive electrode before and after the cycle test of Example 1 and the positive electrode electronic resistance after the sacule test of Comparative Example 1. It is a figure which shows the reaction resistance of the positive electrode after the cycle test of Example 1 and Comparative Example 1. It is a figure which shows the interfacial resistance between the electrical power collector of Example 1 and Comparative Example 1, and a positive electrode active material layer, and various other resistance.

  A positive electrode for a lithium ion secondary battery, a manufacturing method thereof, and a lithium ion secondary battery according to an embodiment of the present invention will be described in detail.

(Positive electrode for lithium ion secondary battery)
In the positive electrode for a lithium ion secondary battery according to the present embodiment, the positive electrode active material layer includes a first positive electrode active material capable of inserting and extracting lithium ions, and a carbon coating portion formed on the surface of the first positive electrode active material. The first active material particles have, and the organic / inorganic coating layer formed on the surface of the first active material particles.

  The lithium ion secondary battery using the positive electrode for a lithium ion secondary battery of the present embodiment has a high capacity retention rate and a small increase in positive electrode electronic resistance after a cycle test, as shown in the examples described later. For this reason, cycle characteristics are improved. The reason is considered as follows.

  In general, the carbon covering portion of the first active material particles has good conductivity. For this reason, the vicinity of the carbon coating portion is in a highly oxidized state, and the electrolytic solution and the first positive electrode active material react with each other, so that an SEI film (non-conductive film) is easily formed. However, in this embodiment, the 1st active material particle is coat | covered with the organic inorganic coating layer. The carbon covering portion of the first active material particle is covered with an organic / inorganic coating layer. For this reason, it is prevented that the SEI film is formed on the surface of the carbon coating portion. The organic / inorganic coating layer does not hinder the movement of Li ions. Therefore, even if charging / discharging is repeated, a high capacity can be maintained.

  The organic / inorganic coating layer includes a selection polymer having at least one selected from an amino group, an amide group, an imino group, an imide group, a maleimide group, a carboxyl group, and an ether group, an alkali metal, an alkaline earth metal, and a rare earth. And at least one selected metal selected from elements. Since a selective polymer containing nitrogen or a carboxyl group has an unshared electron pair, the selected metal ion is likely to coordinate. Moreover, the selective polymer containing an ether group is likely to form a complex with an alkali metal or the like. Therefore, the organic / inorganic coating layer is a uniform and dense film, and the electrolyte solution is less likely to be in direct contact with the carbon coating portion, so that electrons passing through the carbon coating portion are not easily consumed by oxidative decomposition of the electrolyte solution. For this reason, the raise of the electronic resistance of a positive electrode can be suppressed significantly.

  Further, excessive formation of the SEI film can be suppressed and gas generation can be suppressed. Abrupt reaction during overcharging is suppressed, and excessive heat generation of the battery is also suppressed.

  The positive electrode active material layer has a first active material particle and an organic-inorganic coating layer formed on the surface of the first active material particle.

  The first active material particles have a first positive electrode active material capable of inserting and extracting lithium ions, and a carbon coating portion formed on the surface of the first positive electrode active material. The first positive electrode active material is preferably made of an olivine type lithium phosphate complex oxide. In general, since the olivine-type lithium phosphate composite oxide has a low average discharge potential, it is decomposed when exposed to a high potential exceeding 3.5 V (for example, 4.2 V or more). Elution or decomposition of the electrolyte near the positive electrode is likely to occur. In this embodiment, the carbon coating part which coat | covers an olivine type lithium phosphate complex oxide is coat | covered with the organic inorganic coating layer. This prevents direct contact between the olivine-type lithium phosphate composite oxide and the electrolyte, and effectively decomposes and dissolves the olivine-type lithium phosphate composite oxide and decomposes the electrolyte even when exposed to a high potential. Can be suppressed.

The olivine-type lithium phosphate composite oxide used as the first positive electrode active material is represented by a chemical formula: LiMPO ₄ (wherein M is selected from at least one of Co, Ni, Mn, and Fe). . Specific examples include LiFePO ₄ , LiCoPO ₄ , LiNiPO ₄ , and LiMnPO ₄ . Of these, LiFePO ₄ is preferred.

As the first positive electrode active material, a layered rock salt type lithium composite oxide or a spinel type lithium composite oxide can be used in addition to the olivine type lithium phosphate composite oxide. Moreover, it is also possible to use polyanionic compounds other than the olivine type lithium phosphate complex oxide as the first positive electrode active material. Examples of polyanionic compounds other than the olivine-type lithium phosphate complex oxide include LiMVO ₄ or Li ₂ MSiO ₄ (wherein M is selected from at least one of Co, Ni, Mn, and Fe). Mention may be made of the compounds represented.

  A carbon coating portion is formed on the surface of the first positive electrode active material. A carbon coating | cover part improves the electroconductivity between 1st positive electrode active materials. The carbon coating portion has a carbon material. The carbon material is preferably conductive, and for example, acetylene black (AB), ketjen black (KB, registered trademark), carbon nanotube, graphene, carbon fiber, graphite, or the like can be used. Of these, acetylene black (AB) and ketjen black (KB) are preferable.

  The mass ratio of the carbon coating part when the total mass of the first positive electrode active material and the carbon coating part is 100% by mass is preferably 1% by mass to 50% by mass, and more preferably 5% by mass to 30% by mass. % Or less is preferable. In this case, the conductivity between the first positive electrode active materials can be increased while maintaining a high capacity of the positive electrode.

  By applying mechanical energy to the first positive electrode active material and the carbon material, first active material particles having the first positive electrode active material and the carbon coating portion are formed. Mechanical energy may be imparted to the first positive electrode active material and the carbon material by mechanical milling. Thereby, mechanical energy can be uniformly given to the first positive electrode active material and the carbon material. The mechanical milling method is not particularly limited, but ball milling that introduces mechanical energy by moving the container with an external force in a state where a hard ball is housed in the container together with the sample is suitable. As the ball milling device, any of a planetary type that gives energy to the sample by rotation and revolution and a vibration type that gives energy to the sample by vibration in the horizontal direction or the vertical direction can be adopted.

  When applying energy, it is preferable to mix the first positive electrode active material and the carbon material in an inert gas atmosphere (argon gas, nitrogen gas) or in an air atmosphere.

  After the energy application, it is preferable to perform heat treatment on the first active material particles to which mechanical energy is applied. By the heat treatment, the first positive electrode active material is recrystallized and sintered, so that the particles adhere to each other. Thereby, the electroconductivity of 1st active material particle improves.

  The heat treatment temperature is preferably 500 to 800 ° C. If the heat treatment temperature is too low, it is difficult to deposit carbon uniformly around the first positive electrode active material. On the other hand, if the heat treatment temperature is too high, decomposition of the first positive electrode active material and lithium deficiency may occur. In addition, the charge / discharge capacity decreases, which is not preferable. Moreover, what is necessary is just to make heat processing time into 1 to 10 hours normally.

  The thickness of the carbon coating part is preferably about several nm. When the thickness of the carbon coating portion is too small, the effect of improving the conductivity of the first active material particles may be reduced. When the thickness of the carbon coating portion is excessive, the mass of the first positive electrode active material in the positive electrode active material layer becomes relatively small, and the capacity may be reduced.

  The organic / inorganic coating layer includes a selective polymer having at least one of an amino group, an amide group, an imino group, an imide group, a maleimide group, a carboxyl group, and an ether group. As this selective polymer, polyethyleneimine, polyallylamine, polyvinylamine, polyaniline, polydiallyldimethylammonium chloride, polyacrylic acid, polyethylene oxide, polyallylamine, polylysine, polyacrylimide, bismaleimide triazine resin, carboxymethylated polyethyleneimine, Examples thereof include phosphate ester polymers. The selection polymer only needs to contain at least one kind of group, and may contain plural kinds of groups.

  Examples of the selective polymer contained in the organic / inorganic coating layer include polyacetylene imine.

  When the organic / inorganic coating layer is 100% by mass, the content of the selected polymer is preferably 10 to 99.9% by mass, and more preferably 50 to 99% by mass.

  The organic / inorganic coating layer contains at least one selected metal selected from alkali metals, alkaline earth metals, and rare earth elements in addition to the selected polymer. 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 organic / inorganic coating 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 too small, the effect of having the selected metal contained in the organic / inorganic coating layer does not appear, and when the content of the selected metal is excessive, it is difficult to form a uniform coating layer. There is a case.

  In order to form the organic / inorganic coating layer, 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, in the manufacturing method of this embodiment, a mixed solution in which a selected polymer and a compound of a selected metal are dissolved in a solvent is applied to the positive electrode active material layer and dried to form an organic inorganic coating layer. 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.

  Moreover, as a solvent of a mixed solution, the organic solvent or water which can melt | dissolve both the selection polymer and the compound of a selection metal 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, dimethylformamide (DMF), N-methyl-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.) A mixed solvent or the like can be used. A low boiling point that can be easily removed from the organic / inorganic coating layer is desirable.

  When the mixed solution is applied to the surface of the positive electrode active material layer, it may be applied by spraying, rollers, brushes, etc., but it is desirable to apply the surface of the positive electrode active material layer by the dipping method in order to apply uniformly. . When the coating is applied by the dipping method, the mixed solution is impregnated in the gap between the first active material particles, so that the organic / inorganic coating layer can be formed on almost the entire surface of the first active material particles. Therefore, direct contact between the first active material particles and the electrolytic solution can be reliably prevented.

  There are two methods for applying the mixed solution to the surface of the positive electrode active material layer by dipping. First, a positive electrode precursor is formed by binding a slurry containing at least first active material particles and a binder to a current collector, and the positive electrode precursor is dipped in a mixed solution, and then pulled up and dried. This is repeated if necessary to form an organic / inorganic coating layer having a predetermined thickness.

  When using this dipping method, it is preferable to immerse the positive electrode precursor in the mixed solution for 2 minutes or more. It is also preferable to immerse in a reduced pressure atmosphere. By doing so, the mixed solution is sufficiently impregnated in the positive electrode active material layer, and the organic-inorganic coating layer can be more reliably formed on the surface of the first active material particles.

  As another method, the powder of the first active material particles is first mixed into the mixed solution and dried by freeze-drying or the like. This is repeated if necessary to form an organic / inorganic coating layer having a predetermined thickness. Then, a positive electrode is formed using the 1st active material particle in which the organic inorganic coating layer was formed.

  By forming the organic / inorganic coating layer by the dipping method described above, a roll-to-roll process becomes possible.

  After dipping, the organic / inorganic coating layer is preferably washed with a suitable solvent. If the cleaning is insufficient, the residue at the time of coating is generated on the surface of the positive electrode, so that the initial resistance is increased, or the residue flows out into the electrolytic solution. The organic / inorganic coating layer formed by the dipping method is preferably subjected to heat treatment. The heat treatment temperature can be 80 to 140 ° C., and the heat treatment time can be 10 minutes to 3 days. The heat treatment atmosphere is preferably a vacuum atmosphere or a non-oxidizing gas atmosphere.

  The thickness of the organic / inorganic coating 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 organic / inorganic coating layer is too thin, the first active material particles may come into direct contact with the electrolytic solution. On the other hand, when the thickness of the organic / inorganic coating layer is on the order of μm or more, when a secondary battery is formed, the resistance increases and the ionic conductivity decreases. In order to form such a thin organic / inorganic coating layer, the concentration of the selected polymer and the selected metal in the above dipping solution (mixed solution) is kept low and repeatedly applied to form a thin and uniform organic / inorganic coating layer. A layer can be formed.

  The organic / inorganic coating layer may cover at least a part of the surface of the first 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 first active material particles. The organic / inorganic coating layer desirably covers at least the surface of the carbon coating portion.

  The concentration of the selected polymer in the mixed solution is preferably 0.001% by mass or more and less than 5.0% by mass, and preferably in the range of 0.1% by mass to 1.0% by mass. Within this range, the higher the concentration, the lower the capacity retention rate after cycling and the positive electrode resistance rise after cycling. Moreover, if a mixed solution is apply | coated in this range, the thickness of an organic inorganic coating layer will be the range of 0.2 nm-4 nm. If the concentration is too low outside this range, the probability of contact with the first active material particles 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 organic inorganic coating 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 an oxidation reaction potential has a different value depending on the type of the organic solvent of the electrolytic solution used in the lithium ion secondary battery. In this embodiment, the oxidation reaction potential is set using a carbonate solvent as the organic solvent of the electrolytic solution. The value that appears when measured.

Examples of the lithium compound having a higher oxidation reaction potential than the carbonate-based electrolyte include lithium bis (pentafluoroethylsulfonyl) imide (LiBETI), lithium bis (trifluoromethanesulfonyl) imide (LiTFSI), LiBF ₄ , LiCF ₃ SO _{3 and the} like. Illustrated. The content of the lithium compound in the organic / inorganic coating 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 coat layer containing the lithium compound.

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

  It is also preferable to form an organic coat layer on the surface of the organic inorganic coat layer. The organic coat layer can further suppress direct contact between the first active material particles and the electrolyte during high-voltage driving. However, when the total thickness of the organic / inorganic coating layer and the organic coating layer increases, the resistance of lithium ion conductivity increases. Therefore, as the polymer contained in the organic coat layer, it is preferable to use a polymer having a zeta potential whose polarity is opposite to the zeta potential of the polymer constituting the lower organic inorganic coat layer. In this way, the lower organic / inorganic coating layer and the organic coating layer are firmly bonded by Coulomb force, so that both the lower organic / inorganic coating layer and the upper organic coating layer can be formed into a thin film. The total thickness of the coating layer composed of the organic inorganic coating layer and the organic coating layer can be set to the nm order.

  When the organic coat layer is formed, the total thickness of the coat layer composed of the organic inorganic coat layer and the organic coat layer is preferably in the range of 0.1 nm to 100 nm, and preferably in the range of 0.1 nm to 10 nm. It is more preferable that the thickness is in the range of 0.1 nm to 5 nm. Further, the organic coat layer may contain a lithium compound having an oxidation reaction potential higher than that of the selected metal and / or carbonate-based electrolytic solution described above. In this case, the selected metal and / or lithium compound contained in the organic coat layer may be the same as or different from the selected metal and / or lithium compound contained in the organic / inorganic coat layer. The amount of the selected metal and / or lithium compound added is the same as in the case of the organic / inorganic coating layer.

  It has been found that the first active material particles used in the positive electrode of the present embodiment are negative when the zeta potential is measured when dispersed in water or an organic solvent in which no electrolyte is added. From this phenomenon, it is preferable to use a cationic polymer having a positive zeta potential such as polyethyleneimine for the organic / inorganic coating layer. By doing so, the first active material particles and the selected polymer are firmly bonded by the Coulomb force to form the organic / inorganic coating layer. It is preferable to form the organic coat layer on the surface of the organic inorganic coat layer using an anionic polymer having a negative zeta potential such as polyacrylic acid.

  The zeta potential referred to in this embodiment is measured by a microscopic electrophoresis method, a rotating diffraction grating method, a laser Doppler electrophoresis method, an ultrasonic vibration potential (UVP) method, or an electrokinetic acoustic (ESA) method. is there. Particularly preferably, it is measured by laser Doppler electrophoresis. Specific measurement conditions will be described below, but are not limited thereto. First, a solution (suspension) having a solid concentration of 0.1% by mass was prepared using DMF, acetone, and water as solvents. The measurement was carried out three times at a temperature of 25 ° C., and the average value was calculated. The pH was neutral.

  Since the organic / inorganic coating layer thus formed has high bonding strength with the first active material particles, direct contact between the first active material particles and the electrolytic solution can be suppressed during high voltage driving. Moreover, if the total thickness of the coating layer which consists of an organic inorganic coating layer and an organic coating layer is nm order, it can also suppress that it becomes resistance of lithium ion conductivity. 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.

The positive electrode active material layer may further include second active material particles having a second positive electrode active material capable of inserting and extracting lithium ions. The second positive electrode active material may be composed of a layered rock salt type lithium metal composite oxide and / or a spinel type lithium metal composite oxide. Lithium-metal composite oxide having a spinel structure represented by the general _{formula: Li x (A y Mn 2} -y) is O ₄ (A, transition metal elements, Ca, Mg, S, Si , Na, K, Al, P, It may be represented by at least one element selected from Ga and Ge, 0 <x ≦ 2.2, 0 ≦ y ≦ 1). The transition metal element that can constitute A in the general formula is, for example, at least one element selected from Fe, Cr, Cu, Zn, Zr, Ti, V, Mo, Nb, W, La, Ni, and Co. There should be. A specific example of the lithium metal composite oxide having a spinel structure is preferably at least one selected from the group consisting of LiMn ₂ O ₄ and LiNi _0.5 Mn _1.5 O ₄ .

A lithium metal composite oxide having a layered rock salt structure is also referred to as a layered compound. The lithium metal composite oxide having a layered rock salt structure has a general formula: Li _a Ni _b Co _c Mn _{d De} O _f (0.2 ≦ a ≦ 1.2, b + c + d + e = 1, 0 ≦ e <1, D is At least one element selected from Fe, Cr, Cu, Zn, Ca, Mg, S, Si, Na, K, Al, Zr, Ti, P, Ga, Ge, V, Mo, Nb, W, and La; .7 ≦ f ≦ 2.1) and Li ₂ MnO ₃ .

The lithium metal composite oxide may contain a solid solution composed of a mixture of a layered rock salt structure and a spinel such as LiMn ₂ O ₄ or LiNi _0.5 Mn _1.5 O ₄ .

Further, as the second positive electrode active material, a polyanion represented by LiMPO ₄ , LiMVO ₄ or Li ₂ MSiO ₄ (wherein M is selected from at least one of Co, Ni, Mn, and Fe) A compound may be used.

  The organic / inorganic coating layer may be formed on the surface of the second positive electrode active material. Furthermore, the organic inorganic coating layer on the surface of the second positive electrode active material may be coated with an organic coating layer.

  The positive electrode active material layer preferably has a binding portion. The binding portion is a portion formed by drying the binder, and the first active material particles may be bound to each other or the first active material particles and the current collector are bound. It is desirable that the organic / inorganic coating 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.

  As a binder constituting the binding part included in the positive electrode active material layer, polyvinylidene fluoride (polyvinylidene, difluoride: PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI) , Polyamideimide (PAI), carboxymethylcellulose (CMC), polyvinyl chloride (PVC), methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), Examples include polypropylene (PP). 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.

  The selective polymer constituting the organic / inorganic coating layer is preferably one having good coverage with respect to the binding portion. Therefore, it is preferable to use a selective polymer having a zeta potential opposite to the zeta potential of the binder. For example, when polyvinylidene fluoride (PVDF) is used as the binder, the zeta potential of polyvinylidene fluoride (PVDF) is negative, so that it is preferable to use a cationic selection polymer.

  Further, the larger the potential difference between the binder and the selected polymer, the better. Therefore, when polyvinylidene fluoride (PVDF) is used as the binder, it is preferable to select a solvent so that the zeta potential becomes +20 mV or more using polyethyleneimine (PEI) which is easily cationized for the organic / inorganic coating layer.

  The positive electrode active material layer generally contains a conductive additive, but it is desirable that the organic / inorganic coating layer is also formed on at least a part of the conductive additive. By doing in this way, a conductive support agent can be protected.

  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. As the conductive auxiliary agent, 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. The content of the conductive additive in the positive electrode active material layer is not particularly limited. For example, when the total mass of the first active material particles and the second active material particles is 100 parts by mass, The content can be about 2 to 100 parts by mass. 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 is deteriorated and the energy density is lowered.

  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 considered to be because the current collector is prevented from eluting into the electrolyte at a high temperature. Examples of the conductor include carbon such as graphite, hard carbon, acetylene black, and furnace black, ITO (Indium-Tin-Oxide), tin, and the like. A conductive layer can be formed from these conductors 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.

(Lithium ion secondary battery)
The lithium ion secondary battery of this invention is equipped with the positive electrode for lithium ion secondary batteries of this embodiment. 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 that covers 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, and silicon oxide can be used. In particular, when a negative electrode active material made of carbon such as graphite or hard carbon is used, a unique effect is exhibited in that the resistance after cycling is greatly reduced and the output is improved after cycling.

Moreover, the silicon oxide represented by SiOx (0.3 <= x <= 1.6) can also be used as a negative electrode active material. Each particle of the silicon oxide powder is composed of SiOx 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. The range of 0.5 ≦ x ≦ 1.5 is preferable, and the range of 0.7 ≦ x ≦ 1.22 is more desirable.

In general, when oxygen is turned off, it is said that almost all SiO is disproportionated and separated 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.

  Moreover, what compounded the carbon material with 1-50 mass% with respect to silicon oxide SiOx as a negative electrode active material can also be used. By combining silicon oxide and a carbon material, cycle characteristics are improved. If the composite amount of the carbon material is less than 1% by mass, the effect of improving the electrical conductivity cannot be obtained, and if it exceeds 50% by mass, the proportion of SiOx 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 SiOx. In order to combine a carbon material with SiOx, a CVD method or the like can be used.

  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 deteriorated. When the average particle diameter is smaller than 1 μm, the particles are aggregated into coarse particles. May decrease.

A silicon material can also be used as the negative electrode active material. The silicon compound includes a reaction step of reacting CaSi ₂ and an acid, and a step of heating the layered silicon compound at 300 ° C. or higher (hereinafter also referred to as a silicon material manufacturing step). CaSi ₂ generally has a structure in which a Ca layer and a Si layer are laminated. CaSi ₂ may be synthesized by a known production method, or a commercially available one may be adopted. CaSi ₂ used in the method for producing a layered silicon compound is preferably pulverized in advance.

  Acids include hydrogen fluoride, hydrogen chloride, hydrogen bromide, hydrogen iodide, sulfuric acid, nitric acid, phosphoric acid, formic acid, acetic acid, methanesulfonic acid, tetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroarsenic acid, fluoro Examples include antimonic acid, hexafluorosilicic acid, hexafluorogermanic acid, hexafluorotin (IV) acid, trifluoroacetic acid, hexafluorotitanic acid, hexafluorozirconic acid, trifluoromethanesulfonic acid, and fluorosulfonic acid. These acids may be used alone or in combination.

  In addition, the acid is preferably used as an aqueous solution from the viewpoints of workability and safety, and removal of by-products.

Acid used in the reaction step, may be used in an amount capable of providing 2 or more equivalents of protons relative CaSi _2. Therefore, if it is a monovalent acid, it may be used in an amount of 2 mol or more per 1 mol of CaSi ₂ .

  The reaction conditions of the reaction step are preferably reduced pressure conditions such as vacuum or an inert gas atmosphere, and are preferably temperature conditions of room temperature or lower such as an ice bath. What is necessary is just to set the reaction time of the same process suitably.

In the reaction step, the reaction formula when hydrogen chloride is used as the acid is as follows.
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl ₂

Si ₆ H ₆ which is polysilane corresponds to an ideal layered silicon compound. It can be considered that this reaction forms Si—H bonds while Ca in the layered CaSi ₂ is substituted with 2H. CaSi ₂ has a structure in which a Ca layer and a Si layer are laminated. The layered silicon compound is layered because the basic skeleton of the Si layer in the raw material CaSi ₂ is maintained.

In the reaction step, the acid is preferably used as an aqueous solution as described above. Here, since Si ₆ H ₆ can react with water, usually, the layered silicon compound is hardly obtained with Si ₆ H ₆ alone, and contains an element derived from oxygen or an acid.

  After the reaction step, a filtration step for filtering the layered silicon compound, a washing step for washing the layered silicon compound, a drying step for drying the layered silicon compound, and a step for pulverizing or classifying the layered silicon compound are performed as necessary. It is preferable to do this.

  Alternatively, the layered silicon compound may be heated to release hydrogen or the like to form a silicon material.

  The silicon material manufacturing process includes a process of heating the layered silicon compound at 300 ° C. or higher.

The silicon material manufacturing process is represented by an ideal reaction formula as follows.
Si ₆ H ₆ → 6Si + 3H ₂ ↑

  However, the layered silicon compound actually used in the silicon material manufacturing process contains elements derived from oxygen and acid, and also contains inevitable impurities, so the silicon material actually obtained also contains elements derived from oxygen and acid, Further, inevitable impurities are also contained. In the silicon material of this embodiment, when the molar amount of silicon is 100, the molar amount of oxygen element is preferably 50 or less, and particularly preferably 40 or less. Further, when the molar amount of silicon is 100, the molar amount of the acid-derived element is preferably 8 or less, and particularly preferably 5 or less.

  The silicon material manufacturing process is preferably performed in a non-oxidizing atmosphere having a lower oxygen content than in normal air. Examples of the non-oxidizing atmosphere include a reduced pressure atmosphere including a vacuum and an inert gas atmosphere. The heating temperature is preferably in the range of 350 ° C to 1200 ° C, more preferably in the range of 400 ° C to 1200 ° C. If the heating temperature is too low, hydrogen may not be released sufficiently. On the other hand, if the heating temperature is too high, energy is wasted. What is necessary is just to set a heating time suitably according to heating temperature, and it is also preferable to determine a heating time, measuring the quantity of hydrogen etc. which escapes out of a reaction system. By appropriately selecting the heating temperature and the heating time, the ratio of amorphous silicon and silicon crystallites contained in the silicon material to be manufactured, and the size of the silicon crystallites can also be adjusted, and further manufactured. The shape and size of a nano-level layer containing amorphous silicon and silicon crystallites contained in the silicon material can also be prepared.

  The size of the silicon crystallite is preferably nano-sized. Specifically, the silicon crystallite size is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, still more preferably in the range of 1 nm to 50 nm, and particularly in the range of 1 nm to 10 nm. preferable. The silicon crystallite size is calculated from Scherrer's equation using X-ray diffraction measurement (XRD measurement) on the silicon material and using the half width of the diffraction peak of the Si (111) plane of the obtained XRD chart.

  By the silicon material manufacturing process, a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction can be obtained. This structure can be confirmed by observation with a scanning electron microscope or the like. In consideration of using a silicon material as an active material of a lithium ion secondary battery, the plate-like silicon body has a thickness in the range of 10 nm to 100 nm for efficient insertion and desorption reaction of lithium ions. A thing in the range of 20 nm-50 nm is more preferable. The length of the plate-like silicon body in the major axis direction is preferably in the range of 0.1 μm to 50 μm. The plate-like silicon body preferably has a (length in the major axis direction) / (thickness) range of 2 to 1000.

  The silicon material may be pulverized or classified to form particles having a certain particle size distribution. As a preferable particle size distribution of the silicon material, D50 can be exemplified within a range of 1 to 30 μm when measured with a general laser diffraction particle size distribution measuring apparatus.

  The silicon material can be used as a negative electrode active material for a secondary battery such as a lithium ion secondary battery. In that case, it is preferable to use a silicon material coated with carbon.

  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 embodiment using the above-described positive electrode and negative electrode can use a known electrolyte solution and separator 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, from aprotic organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), etc. One or more selected can be used. It is desirable to include at least fluoroethylene carbonate (FEC). As the electrolyte to be dissolved, a lithium metal salt that is 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, a lithium metal salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , and LiCF ₃ SO _{3 is} added to an organic solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, and dimethyl carbonate at about 0.5 mol / L to 1.7 mol / L. A solution dissolved in a concentration can be used. Of these, LiPF ₆ is preferably used. By using a positive electrode having an organic / inorganic coating layer and including LiPF ₆ in the electrolyte solution, the effect of making the electrolyte difficult to decompose is obtained synergistically, so that even higher battery characteristics can be obtained even after repeated charge and discharge in high voltage driving. Can be maintained.

   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 embodiment is not particularly limited in shape, and various shapes such as a cylindrical shape, a stacked shape, and a coin shape can be employed. 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.

  In addition, it is preferable that the lithium ion secondary battery of this embodiment performs an initial charging / discharging process (conditioning process) under high temperature. The conditioning treatment is preferably performed at 35 to 90 ° C. In the conditioning process, charging / discharging is repeated once or several times for an unused lithium ion secondary battery. Thereby, deterioration of electrolyte solution is suppressed and a capacity maintenance rate improves further.

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

Example 1
A lithium ion secondary battery of Example 1 was produced by the following method.

<Preparation of positive electrode>
In order to produce the first active material particles, olivine-type LiFePO ₄ powder as the first positive electrode active material and acetylene black (AB) were mixed at a mass ratio of 5: 4, and a mechanical milling device (Fritsch Japan) (Made by Co., Ltd.) was subjected to mechanical milling for 5 hours at 450 rpm in an air atmosphere. Next, the treated powder was subjected to heat treatment by heating at 700 ° C. for 2 hours in a mixed gas atmosphere of carbon dioxide and hydrogen at a volume ratio of 100: 3. Thereby, the 1st active material particle formed by forming a carbon coating part on the surface of the 1st positive electrode active material was obtained. The average particle diameter D50 of the 1st positive electrode active material in the obtained 1st active material particle was 2-5 micrometers, and the thickness of the carbon coating part was 2 nm.

A powder of LiNi _0.5 Co _0.3 Mn _0.2 O ₂ as second active material particles was prepared. The average particle diameter D50 of the second active material particles was 5 μm.

  First active material particles, second active material particles, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder, first active material particles: second active material particles: AB: PVDF was mixed at a composition ratio of 69: 25: 3: 3. The mixture is mixed with N-methyl-2-pyrrolidone as a solvent to form a slurry, which is applied to the surface of a 15 μm thick aluminum foil (current collector) using a doctor blade and dried to a thickness of about 40 μm. A positive electrode active material layer was formed.

  Lanthanum nitrate was dissolved in ethyl alcohol to 2.5 mmol / L, and polyethyleneimine (PEI) was dissolved to a concentration of 1% by mass to prepare a mixed solution. The positive electrode active material layer was immersed in this mixed solution at 25 ° C. for 1 minute and then washed with ethanol, and then immersed in an ethanol solution in which 0.2% by mass of polyacrylic acid was dissolved at 25 ° C. for 1 minute. Then, after washing with ethanol, vacuum drying was performed at 120 ° C. for 12 hours. When the positive electrode active material layer of this positive electrode was observed with a TEM, an organic inorganic coat layer and an organic coat layer were formed. Hereinafter, the whole of the organic / inorganic coating layer and the organic coating layer may be simply referred to as a coating layer. The coat layer was formed on the surfaces of the first active material particles and the second active material particles of the positive electrode active material layer. The coating layers formed on the surfaces of the first active material particles and the second active material particles of the positive electrode active material layer both had a thickness of about 1 nm.

  When preparing the above mixed solution, lanthanum nitrate was first dissolved in ethyl alcohol, 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 fine particles having a particle size in an extremely narrow range of 2 nm ± 0.3 nm existed. It was. The inorganic fine particles do not have such a sharp particle size distribution, and it is assumed that the fine particles are obtained by coordination of lanthanum ions to polyethyleneimine.

<Production of negative electrode>
A silicon material as a negative electrode active material was prepared by the following method.

Reaction step under an argon atmosphere, to a concentration of 35 wt% aqueous HCl 500g which was 10 ° C., a CaSi ₂ of 50g was added and stirred. After confirming that foaming disappeared from the reaction solution, the reaction solution was further stirred for 4 hours under the same conditions. Then, it heated up to room temperature and filtered. The residue was washed with 300 mL of distilled water three times, then with 300 mL of ethanol, and dried under reduced pressure to obtain 39.4 g of a solid. The solid was used as a layered silicon compound.

-Silicon material manufacturing process The layered silicon compound was heated at 900 ° C. for 1 hour in an argon atmosphere containing O ₂ in an amount of 1% by volume or less to obtain a silicon material.

-Lithium ion secondary battery manufacturing process 58 parts by mass of the above silicon material as a negative electrode active material, 24.5 parts by mass of natural graphite as a negative electrode active material, 6.5 parts by mass of acetylene black as a conductive additive, and polyamideimide as a binder 11 parts by mass and N-methyl-2-pyrrolidone as a solvent were mixed to prepare a slurry. The slurry was applied to the surface of an electrolytic copper foil having a thickness of about 20 μm as a current collector using a doctor blade and dried to form a negative electrode active material layer on the copper foil. Thereafter, the current collector and the negative electrode active material layer were firmly and closely joined by a roll press. This was dried under reduced pressure at 200 ° C. for 2 hours to obtain a negative electrode having a negative electrode active material layer thickness of 23 μm.

A separator was interposed between the positive electrode and the negative electrode to obtain an electrode body. The separator is a microporous membrane made of polyethylene. This electrode body was accommodated in a battery case. In a battery case, fluoroethylene carbonate (FEC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) are mixed in a volume ratio of FEC: EC: EMC: DMC = 0.4: 2.6. : 3: 4 mixed. A non-aqueous electrolyte solution in which LiPF ₆ was dissolved at a concentration of 1 M was injected into the mixed solvent, and the battery case was sealed to obtain a lithium ion secondary battery of Example 1.

(Comparative Example 1)
The lithium ion secondary battery of Comparative Example 1 is the same as Example 1 except that the organic inorganic coating layer and the organic coating layer are not formed on the positive electrode.

<Capacity maintenance rate>
A cycle test was performed on the lithium ion secondary batteries of Example 1 and Comparative Example 1, and the capacity retention rate was measured. The conditions of the cycle test are 60 ° C., up to the third cycle, charging is performed at a constant current of 1C and a constant voltage of 4.5V (1C-CCCV, 4.5V charging), and discharging is performed at a constant current of 1C. The voltage was dropped to 6V (1C-CC, 3.6V discharge). For the fourth and subsequent cycles, the battery is charged to 4.4V with a constant current of 1C (1C-CC, 4.4V charge), rested for 1 minute, and discharged to 3.6V with a constant current of 1C (1C-CC, 3V .6V discharge) and rest for 1 minute was repeated 400 cycles. For each lithium ion secondary battery, the charge / discharge efficiency at the third cycle and the capacity retention rate after the 400 cycle test were calculated. The charge / discharge efficiency at the third cycle was calculated by the following equation.
Charge / discharge efficiency (%) = 100 × (discharge capacity at the third cycle) / (charge capacity at the third cycle)

The capacity retention rate after the 400 cycle test was calculated by the following formula.
Capacity maintenance rate (%) = 100 × (charge capacity at 400th cycle) / (initial charge capacity)
The measurement results are shown in Table 1.

 As shown in Table 1, the capacity retention rate of the lithium ion secondary battery of Example 1 was 42.0%, and the capacity retention rate of the lithium ion secondary battery of Comparative Example 1 was 32.2%. When the upper limit voltage was as high as 4.4 V, the capacity retention rate was improved by forming a coat layer on the positive electrode.

<Electronic resistance of positive electrode>
The alternating current impedance of the cell was measured before and after performing the cycle test on the lithium ion secondary batteries of Example 1 and Comparative Example 1. The resistance component was separated using a triode cell and metallic lithium as a reference electrode. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ in EC / EMC / DMC = 30/30/40 (volume%) as an organic solvent so as to be 1 mol / L was used. The applied voltage was 3.6V.

Two unused positive electrodes of Example 1 were stacked to assemble a bipolar symmetric cell. The electrolytic solution used in the bipolar symmetric cell was obtained by dissolving LiPF ₆ so as to be 1 mol / L in EC / EMC / DMC = 30/30/40 (volume%) as an organic solvent. With respect to the bipolar symmetric cell, the AC impedance was measured under the same conditions as the above-mentioned tripolar cell. According to the analysis method of the equivalent circuit model using the measured AC impedance, liquid resistance (Rsol), Rion (Li ion resistance), electric double layer capacitance (Cdl), Re (electronic resistance), Ce 2, Rct (reaction resistance) ) Was calculated. Based on each of these resistances and capacities, the AC impedance of the triode cell was analyzed, and the electronic resistance (Re) and reaction resistance (Rct) of the positive electrode of the triode cell were calculated. FIG. 1 shows the electronic resistance before and after the cycle test of Example 1 and the electronic resistance after the cycle test of Comparative Example 1. In FIG. 2, the reaction resistance after the cycle test of Example 1 and Comparative Example 1 is shown.

  As shown in FIG. 1, the electronic resistance of the positive electrode of Example 1 before the cycle test was a value close to zero. The electronic resistance of the positive electrode of Example 1 after the cycle test was higher than the electronic resistance before the cycle test, but was significantly smaller than the electronic resistance of the positive electrode of Comparative Example 1 after the cycle test.

  As shown in FIG. 2, the reaction resistances of the positive electrodes of Example 1 and Comparative Example 1 after the cycle test were almost the same. From this, it was found that the coat layer formed on the positive electrode active material layer of Example 1 has a function of suppressing an increase in electronic resistance of the positive electrode.

  The electronic resistance of the positive electrode of Example 1 and Comparative Example 1 was divided into an interfacial resistance between the current collector and the positive electrode active material layer and other resistances by a method according to the above equivalent circuit model. The results are shown in FIG. In Example 1 and Comparative Example 1, the interfacial resistance was almost the same, but for other resistances, Example 1 was much lower than Comparative Example 1. The ratio of the interfacial resistance and the other resistance in the positive electrode of Comparative Example 1 was interfacial resistance: various resistances = 15%: 85%, whereas in the positive electrode of Example 1, the interfacial resistance: various resistances. = 40%: 60%.

  The resistances other than the interfacial resistance of the positive electrode of Example 1 were significantly reduced as compared with Comparative Example 1. By forming a coat layer on the surface of the carbon coating part, the electrolyte directly contacted the carbon coating part. This makes it difficult for electrons passing through the carbon coating portion to be consumed by the oxidative decomposition of the electrolytic solution, and it is considered that the increase in the electronic resistance of the positive electrode after the cycle could be greatly suppressed.

  In addition, it is thought that the coat layer has coat | covered the 2nd active material surface besides a carbon coating part. It is considered that the coating layer covers the surface of the second active material particles, and thus has a good influence on the capacity retention rate and the electronic resistance. However, unlike the second active material particles, the first active material particles have a carbon coating portion and are in a strong oxidation state with many electrons gathering. Moreover, the first active material particles are composed of an olivine-type lithium phosphate complex oxide having an average discharge potential as low as 3.4V. For this reason, when the positive electrode is exposed to a high voltage of 4.4 V during charging, there is a concern about the destruction or elution of the first active material particles. The olivine-type lithium phosphate composite oxide constituting the first active material particles is more easily destroyed or eluted than the second active material particles made of a lithium metal composite oxide having a layered rock salt structure that is stable at a high potential. For this reason, by covering the surface of the first active material particles, and further the surface of the carbon coating portion with the coat layer, it is possible to effectively destroy or dissolve the particles compared to the case where the surface of the second active material particles is coated with the coat layer. In addition, it is estimated that the deterioration of the electrolyte near each particle can be effectively suppressed.

Claims (11)

A current collector, and a positive electrode active material layer covering the current collector,
The positive electrode active material layer is
First active material particles having a first positive electrode active material capable of inserting and extracting lithium ions, and a carbon coating formed on the surface of the first positive electrode active material, and on the surface of the first active material particles Formed organic inorganic coating layer,
Have
The organic / inorganic coating layer includes a selection polymer having at least one selected from an amino group, an amide group, an imino group, an imide group, a maleimide group, a carboxyl group, and an ether group, an alkali metal, an alkaline earth metal, and a rare earth element. only containing at least and one of the selected metals, selected from the group consisting of,
The carbon coating portion of the first active material particles is covered with the organic / inorganic coating layer,
The positive electrode for a lithium ion secondary battery, wherein the organic / inorganic coating layer is formed of fine particles in which ions of the selected metal are coordinated with the selected polymer .
  The positive electrode for a lithium ion secondary battery according to claim 1, wherein the first positive electrode active material is made of an olivine type lithium phosphate complex oxide.   The positive electrode for a lithium ion secondary battery according to claim 1, wherein the positive electrode active material layer further includes second active material particles having a second positive electrode active material capable of inserting and extracting lithium ions.   4. The positive electrode for a lithium ion secondary battery according to claim 3, wherein the second positive electrode active material comprises a layered rock salt type lithium metal composite oxide and / or a spinel type lithium metal composite oxide. 5.   The positive electrode for a lithium ion secondary battery according to claim 1, wherein the selective polymer is polyethyleneimine.   The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the organic / inorganic coating layer further contains a lithium compound having an oxidation reaction potential higher than that of the carbonate-based electrolytic solution.   The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 6, further comprising an organic coat layer on a surface of the organic / inorganic coat layer. A slurry containing first active material particles having a first positive electrode active material capable of inserting and extracting lithium ions and a carbon coating portion formed on the surface of the first positive electrode active material is applied to the surface of the current collector and dried. And forming a positive electrode active material layer,
A selection polymer having at least one selected from an amino group, an amide group, an imino group, an imide group, a maleimide group, a carboxyl group and an ether group, and at least one selection selected from an alkali metal, an alkaline earth metal and a rare earth element And a step of applying a mixed solution in which a metal is dissolved in a solvent to the positive electrode active material layer and drying to form an organic / inorganic coating layer on the surface of the first active material particles. A method for producing a positive electrode for a secondary battery.   The method for producing a positive electrode for a lithium ion secondary battery according to claim 8, wherein the mixed solution is applied to the positive electrode active material layer by a dipping method.   A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to claim 1. The lithium ion secondary battery according to claim 10, wherein the battery voltage during charging is 4.3 V or more.

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