JP2014127235A - Lithium ion secondary battery cathode and manufacturing method thereof, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cathode for a lithium ion secondary battery capable of withstanding high-voltage drive.SOLUTION: A cathode active material layer is formed from cathode active material particles containing a Li compound or solid solution selected from LiNiCoMnO, LiCoMnO, LiNiMnO, LiNiCoO, and LiMnO(however, 0.5≤x≤1.5, 0.1≤a<1, 0.1≤b<1, 0.1≤c<1), a binding part for binding the cathode active material particles together and binding the cathode active material particles and a current collector, and an amorphous boron oxide coat layer for coating at least part of the surfaces of at least the cathode active material particles. The amorphous boron oxide coat layer helps suppress direct contact between the cathode active material particles and an electrolyte during high-voltage drive.

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. Lithium ion secondary batteries have active materials capable of inserting and extracting lithium (Li) in the positive electrode and the negative electrode, respectively. And it operates by moving lithium ions in the electrolyte provided between the two electrodes. In lithium ion secondary batteries, lithium-containing metal composite oxides such as lithium cobalt composite oxide are mainly used as the active material for the positive electrode, and carbon materials having a multilayer structure are mainly used as the active material for the negative electrode. Yes.

  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 for achieving this, although a higher positive electrode potential has been studied, there has been a major problem that battery characteristics after repeated charge / discharge are extremely deteriorated when driven at a high voltage. 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 is a resistance for lithium ion conduction. Also in the method of forming the coating layer, it has been performed by spray coating or dipping coating, and it has been difficult to obtain a uniform film thickness.

  Further, JP-A-2002-175801 discloses that the elution of lithium manganese oxide is suppressed by forming a coating layer using boron oxide and lithium cobalt composite oxide on the surface of the spinel structure lithium manganese oxide. Thus, a positive electrode for a lithium secondary battery having improved storage characteristics is described. However, the evaluation when charging at a high voltage of 4.3 V or higher was not described, and it was unclear whether it could withstand such a high voltage drive. Moreover, although it describes that a coating layer is formed by heat-treating the mixture at 400 ° C., it is not specified what structure and state the coating layer has. The structure of the coating layer can affect the lithium ion conductivity.

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.

The positive electrode for a lithium ion secondary battery of the present invention that solves the above problems includes a current collector and a positive electrode active material layer bound to the current collector, and the positive electrode active material layer is 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, The positive electrode active material particles containing a Li compound or solid solution selected from 0.1 ≦ b <1, 0.1 ≦ c <1) are bound to the positive electrode active material particles, and the positive electrode active material particles and the current collector are bound to each other. It consists of a binding part and an amorphous boron oxide coating layer covering at least a part of the surface of the positive electrode active material particles.

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; applying a boron oxide solution in which boron oxide is dissolved in a solvent to the positive electrode active material layer; Forming a quality boron oxide coating layer.

For a lithium ion secondary battery positive electrode 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 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) is selected on the surface of at least part of the positive electrode active material particles made of a Li compound or a solid solution. A crystalline boron oxide coating layer is formed. Since this boron oxide coating layer coats 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. Further, if the thickness of the boron oxide coating layer is on the order of nm to submicron, the resistance does not become lithium ion conductive. Therefore, it is possible to provide a lithium ion secondary battery that can suppress the decomposition of the electrolytic solution even when driven at a high voltage of 4.3 V or higher, has high capacity, and can maintain high battery characteristics even after repeated charge and discharge.

  Further, according to the manufacturing method of the present invention, since the amorphous boron oxide coating layer is formed using the boron oxide solution, a uniform boron oxide coating layer can be formed and the quality of the positive electrode is stabilized. Furthermore, if an amorphous boron oxide coating layer is formed by using a dipping method, a roll-to-roll process becomes 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. The reason for this is not clear, but it is considered that the current collector is prevented from eluting into the electrolyte at high temperatures. Examples of the conductor include carbon such as graphite, hard carbon, acetylene black, furnace black, and ketjen black, ITO (Indium-Tin-Oxide), tin (Sn), 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 an infinite number of positive electrode active material particles made of a positive electrode active material, a binder that binds the positive electrode active material particles to each other, and binds the positive electrode active material particles and the current collector, and at least a positive electrode active material. And a boron oxide coating layer covering at least a part of the surface of the substance particles. The positive electrode active materials are 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 (however, 0.5 ≦ x ≦ 1.5, 0.1 ≦ a ≦ 1, 0.1 ≦ b ≦ 1, 0.1 ≦ c ≦ 1). 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. An amorphous boron oxide coating layer may be formed on at least a part of the binding portion.

  The boron oxide coating layer is made of amorphous boron oxide. No effect can be obtained with crystalline boron oxide. In order to form the boron oxide coating layer, a CVD method, a PVD method, or the like can be used, but this is not preferable from the viewpoint of cost. Therefore, it is desirable to form by dissolving boron oxide in a solvent and applying it. In application, it may be applied by spray, roller, brush or the like, but in order to uniformly apply the surface of the positive electrode active material, it is desirable to apply by dipping.

  When applied by dipping, the gap between the positive electrode active material particles is impregnated with the boron oxide solution, so that a boron oxide 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 immersed in a boron oxide solution, pulled up and dried. If necessary, this is repeated to form a boron oxide coating layer having a predetermined thickness.

  As another method, the positive electrode active material powder is first mixed with a boron oxide solution and dried by freeze-drying or the like. If necessary, this is repeated to form a boron oxide coating layer having a predetermined thickness. Then, a positive electrode is formed using the positive electrode active material in which the boron oxide coating layer was formed. In this method, the former method is preferable because there is a possibility that the conductive path from the current collector to the positive electrode active material may be hindered due to insufficient conductivity of boron oxide.

  The thickness of the boron oxide coating layer is preferably in the range of 1 nm to 100 nm, and particularly preferably in the range of 1 nm to 10 nm. When the thickness of the boron oxide coating 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 boron oxide 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 boron oxide coat layer, the concentration of boron oxide in the above-mentioned dipping solution (boron oxide solution) is lowered and repeatedly applied to form a thin and uniform boron oxide coat layer. Can be formed.

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

  An alcohol solvent or water can be used as a solvent for dissolving boron oxide. Since alcohol solvent has higher solubility of boron oxide than water, it is desirable to use alcohol solvent. In order to facilitate drying, it is preferable to use a low boiling point alcohol such as ethyl alcohol, propyl alcohol, or isopropyl alcohol.

  The concentration of boron oxide in the boron oxide solution is preferably 0.001% by mass or more and less than 2.0% by mass, and 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.

  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 also preferable that the positive electrode active material layer contains a conductive additive. The conductive assistant is added to increase the conductivity of the electrode. Carbon black, graphite, acetylene black (AB), ketjen black (KB), vapor grown carbon fiber (VGCF), etc., which are carbonaceous fine particles, are used alone or in combination as a conductive additive. Can be added. 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.

  In the production method of the present invention, a slurry containing a positive electrode active material and a binder is applied to the surface of the current collector and dried to form a positive electrode active material layer, and a boron oxide solution in which boron oxide is dissolved in a solvent is added to the positive electrode. It is applied to the active material layer and dried to form a boron oxide coat layer. By this method, an amorphous boron oxide layer can be easily formed. Moreover, a coating film thickness can be easily controlled by performing an application | coating process in multiple times. Various application methods such as spraying, brushing, and roller can be adopted for the application step, but a dipping method that enables a roll-to-roll process is particularly preferable.

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. It is also preferable to use a silicon oxide represented by SiO _x (0.3 ≦ x ≦ 1.6). 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 . In order to combine the carbon material with SiO _x , 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 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 an 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 preferable to use at least fluoroethylene carbonate (FEC).

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.

  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, 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 (Mg doped) as the positive electrode active material, 3 parts by mass of acetylene black (AB) as the conductive additive, and polyvinylidene fluoride (PVdF) as the binder ) Was applied to the surface of an aluminum foil (current collector) using a doctor blade, and dried to prepare a positive electrode having a positive electrode active material layer having a thickness of about 15 μm.

  An ethanol solution in which boron oxide was dissolved to a concentration of 0.2% by mass was prepared. The positive electrode was immersed in this ethanol solution at 25 ° C. for 10 minutes, and then taken out and allowed to air dry. This treatment was repeated three times to form a boron oxide coat layer. As a result of X-ray diffraction analysis, it was confirmed that the boron oxide coating layer was in an amorphous state.

<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), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) are mixed at 4: 26: 30: 40 (volume%). A solution obtained by dissolving LiPF ₆ at a concentration of 1 mol was used.

  A microporous polypropylene / polyethylene / polypropylene laminate film having a thickness of 20 μm and having an alumina coat layer formed on both sides as a separator was sandwiched between the positive electrode and the negative electrode 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 was injected and impregnated into the electrode body.

[Comparative Example 1]
A positive electrode was produced in the same manner as in Example 1 except that the boron oxide coating layer was not formed, and a lithium ion secondary battery was produced in the same manner as in Example 1 except that this positive electrode was used.

<Test example>
Using the lithium ion secondary battery of Example 1 and Comparative Example 1, first, CCCV charging was performed at a temperature of 25 ° C. at 0.2 C / 4.5 V, after 10 minutes of rest, the initial stage before the cycle test at CC discharge 0.33 C / 3.0 V Each discharge capacity was measured. The results are shown in Table 1. After that, it is charged to 4.5V under the conditions of CCCV charge (constant current constant voltage charge) at 25 ° C and 1C, held at that voltage for 1 hour, paused for 10 minutes, and then 1C CC discharge (constant current discharge) In each cycle test, a cycle of discharging at 3.0 V and resting for 10 minutes was repeated 100 times.

  After the cycle test, CCCV charging was performed again at a temperature of 25 ° C. at 0.2 C / 4.5 V, and after 10 minutes of rest, the discharge capacity at CC discharge 0.33 C / 3.0 V was measured. The capacity retention ratio, which is the ratio of the discharge capacity after the cycle test to the discharge capacity before the cycle test at 25 ° C., was calculated, and the results are shown in Table 1.

<Evaluation>

  From Table 1, the capacity retention rate of the lithium ion secondary battery of Example 1 is improved compared to the lithium ion secondary battery of Comparative Example 1 despite the fact that a cycle test is performed with a high voltage of 4.5V. You can see that It is clear that this effect is due to the formation of an amorphous boron oxide coating layer.

  A positive electrode was produced in the same manner as in Example 1, and a lithium ion secondary battery was produced in the same manner.

[Comparative Example 2]
<Test example>
Using the lithium ion secondary battery of Example 2 and Comparative Example 2, first, CCCV charge was performed at a temperature of 25 ° C. at 0.2 C / 4.5 V, after 10 minutes of rest, the initial stage before the cycle test at CC discharge 0.33 C / 3.0 V Each discharge capacity was measured. The results are shown in Table 1. After that, it is charged to 4.5V under the condition of CCCV charge (constant current and constant voltage charge) at 60 ° C and 1C, held at that voltage for 1 hour, paused for 10 minutes, and then 1C CC discharge (constant current discharge) In each cycle test, a cycle of discharging at 3.0 V and resting for 10 minutes was repeated 100 times.

  After the cycle test, CCCV charging was performed again at a temperature of 25 ° C. at 0.2 C / 4.5 V, and after 10 minutes of rest, the discharge capacity at CC discharge 0.33 C / 3.0 V was measured. The capacity retention ratio, which is the ratio of the discharge capacity after the cycle test to the discharge capacity before the cycle test at 25 ° C., was calculated, and the results are shown in Table 2.

<Evaluation>

  From Table 2, the lithium ion secondary battery of Example 2 was subjected to a cycle test at a high temperature of 60 ° C. and was charged at a high voltage of 4.5 V, but the lithium ion secondary battery of Comparative Example 2 It can be seen that the capacity retention rate is improved as compared with. It is clear that this effect is due to the formation of an amorphous boron oxide coating layer.

  In order to elucidate the reason, the impedance characteristics of the lithium ion secondary batteries of Example 2 and Comparative Example 2 before and after the cycle test were evaluated. Specifically, the frequency was changed from 0.02 Hz to 1,000,000 Hz at a temperature of 25 ° C. and a voltage of 3.5 V, and the impedance value was / Z / value at 0.1 Hz. The results are shown in Table 3.

  Compared to Comparative Example 2, it can be seen that the lithium ion secondary battery of Example 2 has a low resistance of 0.1 Hz even after the cycle test. Therefore, it is considered that the decrease in resistance increase contributes to the excellent 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 (8)

A current collector and a positive electrode active material layer bound to the current collector,
Positive electrode active material layer is _{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 The positive electrode active material particles containing 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) and the positive electrode active material particles are bound to each other. A binding portion that binds the positive electrode active material particles and the current collector, and an amorphous boron oxide coating layer that covers at least a part of the surface of the positive electrode active material particles. Positive electrode for lithium ion secondary battery.   2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the boron oxide coating layer has a thickness of 1 nm to 100 nm.   3. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the boron oxide coat layer is also formed on at least a part of the surface of the binding part. 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 of claims 1 to 3. A method for producing a positive electrode for a secondary battery according to any one of claims 1 to 4, wherein 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; and applying a boron oxide solution in which boron oxide is dissolved in a solvent to the positive electrode active material layer And drying to form an amorphous boron oxide coating layer. A method for producing a positive electrode for a lithium ion secondary battery.   6. The method for producing a positive electrode for a secondary battery according to claim 5, wherein the step of applying the boron oxide solution to the positive electrode active material layer is performed by a dipping method.   A lithium ion secondary battery comprising the positive electrode according to claim 1.   8. The lithium ion secondary battery according to claim 7, wherein the lithium ion secondary battery is driven at a high voltage of 4.3 V or more.