JP7494647B2 - Positive electrodes for lithium-ion secondary batteries - Google Patents

Description

The present invention relates to a positive electrode for a lithium-ion secondary battery.

Lithium-ion secondary batteries have high energy density and excellent output characteristics, but if a malfunction occurs, the stored energy is released in a short time, and there is a risk that the battery may catch fire or burst into flames. Therefore, improving safety, along with improving output characteristics, is an important issue for lithium-ion secondary batteries.

It is well known that the safety of lithium-ion secondary batteries is largely determined by the positive electrode active material. Spinel-type lithium manganese oxide positive electrode active material, which is often used in large batteries for electric vehicles and other applications, is relatively inexpensive and has excellent output characteristics. However, it has safety issues, such as manganese leaching under high temperature conditions, accelerated deterioration due to changes in crystal structure accompanied by oxygen release due to overcharging, and the risk of smoke and fire depending on the degree of overcharging.

On the other hand, olivine-based positive electrode active materials such as lithium iron phosphate, which are often used in stationary batteries, do not release oxygen easily because oxygen is covalently bonded to phosphorus, and are known to be relatively stable and safe even under high temperature conditions.

Therefore, studies are being conducted to combine these positive electrode active materials to achieve both high output characteristics and safety. For example, there is an electrode for a lithium ion secondary battery including a positive electrode including a positive electrode current collector and a positive electrode active material formed on the positive electrode current collector, and a negative electrode including a negative electrode current collector and a negative electrode active material formed on the negative electrode current collector, the negative electrode active material being graphite, the positive electrode active material being a composite lithium manganese oxide and an olivine-type composite lithium iron phosphate, and the content of the composite lithium manganese oxide is 30 mass % or less with respect to the total amount of the olivine-type composite lithium iron phosphate and the composite lithium manganese oxide (see, for example, Patent Document 1); a positive electrode active material including a lithium-containing phosphate compound having an olivine structure and a lithium manganese oxide having a spinel structure (see, for example, Patent Document 2); and a secondary battery having a positive electrode and a negative electrode that are capable of inserting and removing lithium ions and are immersed in an electrolyte that mediates the transport of the lithium ions, the positive electrode including a first positive electrode material and a second positive electrode material, the first positive electrode material being lithium manganese oxide (LiMn _{2- XMxO4 ,} M = Li, Fe, Co, Ni, Al _, Mg) and the second positive electrode material is LiMn1 _{- XMxPO4} (M = Fe, Co, Ni, 0 < X < 1) (see, for example, Patent Document 3).

JP 2009-93924 A International Publication No. 2012/090804 JP 2005-123024 A

It is known that olivine-based positive electrode active materials such as lithium iron phosphate, lithium manganese iron phosphate, and lithium manganese phosphate have a stronger resistance to overcharging than spinel-based positive electrode active materials. As described in Patent Documents 1 to 3, by mixing an olivine-based positive electrode active material having a large electrical resistance with a spinel-based positive electrode active material, it is possible to reduce the current flowing through the spinel-based positive electrode active material in a state in which a significantly larger external voltage than normal is applied to the lithium-ion secondary battery, i.e., in an overcharged state. This is explained by the fact that, when Ohm's law (I=V/R) is directly applied to the voltage V _overcharge applied between the positive and negative electrodes of a lithium-ion secondary battery during overcharging, the magnitude of the current flowing during overcharging is inversely proportional to the internal resistance. However, the electrodes disclosed in Patent Documents 1 and 2 have a problem of insufficient output characteristics, and the electrode disclosed in Patent Document 3 has a problem of insufficient overcharge resistance.

In view of these problems, the object of the present invention is to provide a positive electrode for a lithium-ion secondary battery that has high output characteristics and excellent overcharge resistance.

In order to solve the above problems, the present invention mainly has the following configuration.
A positive electrode for a lithium ion secondary battery containing spinel-based lithium metal oxide particles and lithium manganese iron phosphate particles,
a ratio of a particle size of the spinel-based lithium metal oxide particles to a particle size of the lithium manganese iron phosphate particles (particle size of the spinel-based lithium metal oxide particles/particle size of the lithium manganese iron phosphate particles) is 2.0 or more and 10.0 or less;
A positive electrode for a lithium ion secondary battery, in which a weight ratio of a content of spinel-based lithium metal oxide particles to a content of lithium manganese iron phosphate particles (content of spinel-based lithium metal oxide particles/content of lithium manganese iron phosphate particles) is 1.0 or more and 4.0 or less.

By using the positive electrode for a lithium-ion secondary battery of the present invention, a lithium-ion secondary battery with high output characteristics and excellent overcharge resistance can be obtained.

The positive electrode for lithium ion secondary battery of the present invention (hereinafter sometimes simply referred to as "positive electrode") contains spinel-based lithium metal oxide particles represented by _LiMn2O4 and _{lithium manganese} iron phosphate particles represented by _{LiαMnαFebPO4} (0.9≦α _{≦ 1.1} , 0<a≦1, 0<b≦1, 0.9≦a+b≦1.1). As described above, the spinel-based lithium metal oxide (hereinafter sometimes abbreviated as LMO) particles have excellent output characteristics, and the lithium manganese iron phosphate (hereinafter sometimes abbreviated as LMFP) particles have excellent overcharge resistance. The particle size of the spinel-based lithium metal oxide particles and the lithium manganese iron phosphate particles can be changed depending on the production conditions, but the inventors have found through their investigations that when the ratio of the particle size of the spinel-based lithium metal oxide particles to the particle size of the lithium manganese iron phosphate particles (particle size of spinel-based lithium metal oxide particles/particle size of lithium manganese iron phosphate particles) is 2.0 or more and 10.0 or less, and the weight ratio of the content of the spinel-based lithium metal oxide particles to the content of the lithium manganese iron phosphate particles (content of spinel-based lithium metal oxide particles/content of lithium manganese iron phosphate particles) is 1.0 or more and 4.0 or less, output characteristics and overcharge resistance can be achieved simultaneously.

The positive electrode of the present invention may contain other active material particles in addition to the LMO particles and LMFP particles.

In this _{specification} , LMFP refers to a compound that satisfies 0.9≦α≦1.1 _, 0<a≦1, ₀ <b≦1, and 0.9≦a+b≦1.1 in _{LiαMnαFebPO4} . However, even if an element other than the above is added as a doping element in a range of 0.1% by weight to 10% by weight, it is also included in the LMFP of the present invention.

In LMFP, the greater the manganese ratio a and the smaller the iron ratio b, the higher the energy density theoretically becomes, while the electronic conductivity and ionic conductivity decrease. In the present invention, the value of a is preferably 0.6 or more, and more preferably 0.75 or more, from the viewpoint of increasing the volume resistivity of the LMFP particles and further improving the overcharge resistance. On the other hand, the value of a is preferably 0.9 or less, and more preferably 0.85 or less, from the viewpoint of improving the cycle resistance.

The composition of LMFP can be determined by atomic absorption spectrometry for lithium, and ICP emission spectrometry for manganese, iron, and phosphorus. The formulas α, a, and b are measured to three decimal places and rounded off to the second decimal place. However, if the second decimal place is 0, the value is expressed to the first decimal place. In addition, if the raw material charge ratio during LMFP production is known, the composition can be determined from that charge ratio.

The LMFP particles preferably have a carbon layer on the surface, and more preferably have a carbon layer uniformly over the entire surface. By having a carbon layer, the electronic conductivity of the LMFP particles can be improved uniformly over the entire surface. The content of the carbon layer in the LMFP particles is preferably 1 wt % or more from the viewpoint of further improving the electronic conductivity and further improving the output characteristics. On the other hand, the content of the carbon layer in the LMFP particles is preferably 6 wt % or less from the viewpoint of suppressing side reactions between the carbon layer and LMFP and further improving the output characteristics.

Here, the carbon layer content of the LMFP particles can be measured, for example, using a carbon/sulfur simultaneous quantitative analyzer EMIA-920V (manufactured by Horiba, Ltd.).

The particle diameter of the LMFP particles is preferably 3 μm or more from the viewpoint of the handleability of the paste in the manufacturing method of the positive electrode described later. On the other hand, the particle diameter of the LMFP particles is preferably 40 μm or less from the viewpoint of the thickness of the mixture layer described later. The particle diameter of the LMFP particles refers to the arithmetic mean value of the particle diameter of the secondary particles when the particles present in the positive electrode are granules, and the arithmetic mean value of the particle diameter of the primary particles when the particles are not granules.

The volume resistivity ρ (Ω·cm) of the LMFP particles in the present invention is preferably 10 ² ≦ρ≦10 ^6. In order to improve the safety of the positive electrode, there is a method of suppressing Joule heat generated from current in an abnormal state such as a short circuit by reducing the resistance of the positive electrode, and a method of reducing the current value itself in an abnormal state by increasing the resistance of the positive electrode. The latter is more effective in improving the overcharge resistance of the LMO particles, and it is preferable that the resistance of the LMFP particles is high. Therefore, the volume resistivity of the LMFP particles in the present invention is preferably 10 ² (Ω·cm) or more, more preferably 10 ³ (Ω·cm) or more. On the other hand, if the volume resistivity of the LMFP particles is 10 ⁶ (Ω·cm) or less, the output characteristics of the LMFP particles can be maintained at a higher level. Here, the volume resistivity of the LMFP can be measured for the LMFP particles in the positive electrode under a condition of 25 MPa, for example, using a powder resistance measurement system MCP-PD51 (manufactured by Mitsubishi Chemical Analytech Co., Ltd.). As a method for extracting LMFP particles from a positive electrode, for example, after peeling off the mixture layer from the current collector of the positive electrode, the binder is dissolved with an organic solvent such as N-methylpyrrolidinone, and the LMO particles are dissolved with dilute hydrochloric acid, and the remaining LFMP particles are dried. In addition, when a conductive assistant such as carbon black is contained, a method of removing carbon black having a small specific gravity from the remaining solid after dissolving the binder and LMO particles by centrifugation, and drying the obtained LFMP particles, etc. may be mentioned. In addition, the volume resistivity of the LMFP particles as a raw material in the production of the positive electrode may be measured in the same manner.

The volume resistivity of the LMFP can be adjusted to a desired range by adjusting the ratio of manganese to iron, the primary particle size, and, in the case of an LMFP having a carbon layer, the carbon layer content, the firing temperature, and other carbon coating conditions.

In this specification, LMO refers to a compound represented by LiMn ₂ O _4. However, even when an element other than the above is added as a doping element in a range of 0.1 wt % to 10 wt %, it is also included in the LMO of the present invention.

Here, the composition of LMO can be determined by atomic absorption spectrometry for lithium and ICP optical emission spectrometry for manganese.

The particle diameter of the LMO particles is preferably 3 μm or more from the viewpoint of the ease of handling of the paste in the manufacturing method of the positive electrode described later. On the other hand, the particle diameter of the LMO particles is preferably 40 μm or less from the viewpoint of the thickness of the mixture layer described later. The particle diameter of the LMO particles refers to the arithmetic mean value of the particle diameter of the secondary particles when the particles present in the positive electrode are granules, and the arithmetic mean value of the particle diameter of the primary particles when the particles are not granules.

In the present invention, the ratio of the particle diameter of the LMO particles to the particle diameter of the LMFP particles (particle diameter of the LMO particles/particle diameter of the LMFP particles) is 2.0 or more and 10.0 or less. As described above, the LMO particles have excellent output characteristics but low overcharge resistance, and the LMFP particles have excellent overcharge resistance but low output characteristics. In the present invention, the voids between the LMO particles are filled with LMFP particles having high electrical resistance, thereby suppressing the current flowing through the LMO particles in an overcharged state and improving the overcharge resistance. If the particle diameter ratio is less than 2.0, the voids between the LMO particles in the positive electrode are not sufficiently filled with LMFP particles, and the overcharge resistance is deteriorated due to contact between the LMO particles. On the other hand, when the particle size ratio is greater than 10.0, the particle size of the LMFP particles is small, so the specific surface area of the LMFP becomes large, and irreversible chemical reactions between the electrolyte and the LMFP particles in an overcharged state are promoted, resulting in reduced overcharge resistance and cycle resistance.

Here, the particle size of the LMFP particles and the LMO particles refers to the arithmetic mean value of the particle size of each particle present in the positive electrode, that is, the secondary particles in the case of granules, and the primary particles in the case of non-granules, and can be measured using a scanning electron microscope. Specifically, the positive electrode is observed at a magnification of 3,000 times using a scanning electron microscope, and the particle size of randomly selected particles is measured. At this time, if the particle is not spherical, the average value of the major axis and minor axis that can be measured in the two-dimensional image is taken as the particle size. In addition, the particle is subjected to composition analysis using energy dispersive X-ray spectroscopy, and classified into LMFP particles and LMO particles. The same operation is repeated to measure the particle size of 100 LMFP particles and LMO particles, and the number average value of each is calculated, thereby obtaining the particle size of each particle. In addition, the particle size of LMFP particles and LMO particles as raw materials during the production of the positive electrode may be measured in the same manner using a scanning electron microscope.

The particle size of the LMFP particles and LMO particles can be easily adjusted to the desired range, for example, by using a spray dryer to change the weight concentration of the raw material LMFP aqueous dispersion and LMO aqueous dispersion in the manufacturing method of LMFP particles and LMO particles described below.

In the present invention, the weight ratio of the LMO particle content to the LMFP particle content (LMO particle content/LMFP particle content) is 1.0 or more and 4.0 or less. As described above, LMO particles have excellent output characteristics but low overcharge resistance, and LMFP particles have excellent overcharge resistance but low output characteristics, so when the weight ratio is less than 1.0, the LMO particles become relatively scarce and the output characteristics deteriorate. On the other hand, when the weight ratio exceeds 4.0, the overcharge resistance deteriorates.

Here, the content ratio of LMFP particles and LMO particles in the positive electrode can be determined by peeling the positive electrode of the lithium ion battery from the current collector, performing powder X-ray diffraction measurement on the LMFP particles and LMO particles, and applying Rietveld analysis to the obtained results. In addition, if the charge ratio of LMFP particles and LMO particles as raw materials in manufacturing the positive electrode is known, the content ratio can be determined from the charge ratio.

When the positive electrode of the present invention contains other active material particles in addition to LMO particles and LMFP particles, from the viewpoint of further improving the effects of the LMFP particles and LMO particles, the content of the other active material particles is preferably 30% by weight or less of the total active material particles (the sum of the LMO particles, LMFP particles, and other active material particles).

Here, the content of other active material particles in the positive electrode can be determined in the same manner as the content of LMFP particles and LMO particles.

The positive electrode of the present invention preferably has a mixture layer containing LMFP particles and LMO particles as well as additives such as a binder and a conductive assistant on a current collector such as aluminum foil.

Examples of binders include polyvinylidene fluoride and styrene-butadiene rubber. Two or more of these may be included.

The binder content in the positive electrode mixture layer is preferably 0.3% by weight or more and 10% by weight or less. By making the binder content 0.3% by weight or more, the binding effect of the binder makes it easy to maintain the shape of the coating film when it is formed. On the other hand, by making the binder content 10% by weight or less, it is possible to suppress an increase in resistance in the positive electrode.

Examples of conductive additives include acetylene black, ketjen black, carbon fiber, and carbon nanotubes. Two or more of these may be included.

The content of the conductive assistant in the mixture layer is preferably 0.3% by weight or more and 10% by weight or less. By making the content of the conductive assistant 0.3% by weight or more, the conductivity of the positive electrode can be improved and the electronic resistance can be reduced. On the other hand, by making the content of the conductive assistant 10% by weight or less, the inhibition of the movement of lithium ions by the conductive assistant can be suppressed, and the decrease in ion conductivity can be suppressed.

In order to increase the energy density of a lithium-ion secondary battery, it is preferable that the positive electrode active material is contained in the positive electrode mixture layer at as high a ratio as possible, and the total content of the active material particles in the positive electrode mixture layer is preferably 80% by weight or more, and more preferably 85% by weight or more. Here, the total content of the active material particles refers to the sum of the contents of the LMO particles, the LMFP particles, and other active material particles as necessary.

The thickness of the positive electrode mixture layer is preferably 10 μm or more and 200 μm or less. By making the thickness of the mixture layer 10 μm or more, the proportion of the current collector in the positive electrode can be reduced, and the energy density can be further improved. On the other hand, by making the thickness of the mixture layer 200 μm or less, the charge/discharge reaction can be made to proceed quickly throughout the entire mixture layer, and high-speed charge/discharge characteristics can be improved.

The lithium ion secondary battery of the present invention preferably has a negative electrode, a separator, and an electrolyte in addition to the positive electrode. The shape of the battery can be, for example, a square type, a wound type, a laminate type, etc., and can be appropriately selected depending on the purpose of use. Materials constituting the negative electrode can be, for example, graphite, lithium titanate, silicon oxide, etc. The separator and electrolyte can also be selected from any suitable materials.

Next, we will explain the method for manufacturing the positive electrode of the present invention.

First, the LMO particles of the present invention can be obtained by a solid-phase method or a liquid-phase method according to a known method, or a commercially available lithium manganate powder can be used. The LMFP particles of the present invention can be obtained by any method such as a solid-phase method or a liquid-phase method, but the liquid-phase method is preferable in that the ratio of manganese to iron is within the range of the present invention and particles having an average particle size in the above-mentioned preferred range can be obtained more easily. In addition to water, it is also preferable to use an organic solvent for the liquid phase in order to refine the particle size to nanoparticles, and the solvent type is preferably, for example, an alcohol solvent such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 2-propanol, 1,3-propanediol, or 1,4-butanediol, or dimethyl sulfoxide. In the synthesis process, pressure may be applied to increase the crystallinity of the particles. In addition, the ratio of manganese to iron contained in the LMFP particles can be adjusted to a desired range by the charging ratio of the raw materials.

When LMFP particles are obtained by a liquid phase method, the average primary particle size can be adjusted to a desired range by adjusting conditions such as the mixture ratio of water and organic solvent in the solvent, the concentration of the synthesis solution, and the synthesis temperature. Typically, in order to increase the average particle size, it is effective to increase the proportion of water in the solvent, increase the concentration of the synthesis solution, and increase the synthesis temperature.

As a carbon coating method for forming a carbon layer on the LMFP particles obtained by the liquid phase method, a method in which the LMFP particles are mixed with sugars and then baked under an inert gas atmosphere is preferably used. As the sugars, glucose and sucrose are preferred because they have a low ash content after baking. The baking temperature is preferably 500°C or higher and 800°C or lower.

In the method of producing LMFP particles and LMO particles in the present invention, it is preferable to use a spray dryer in order to narrow the particle size distribution of the resulting secondary particles as much as possible.

The positive electrode of the present invention can be obtained, for example, by applying a paste in which the above-mentioned LMFP particles and LMO particles are dispersed in a dispersion medium onto a current collector, drying, and pressing to form a mixture layer. As a method for producing the paste, it is preferable to mix and knead the above-mentioned LMFP particles and LMO particles, and further, if necessary, additives such as a conductive assistant, a binder, and a dispersion medium such as N-methylpyrrolidinone, and add water or a dispersion medium such as N-methylpyrrolidinone to adjust the viscosity. The solid content concentration of the paste can be appropriately selected depending on the application method. From the viewpoint of making the coating film thickness uniform, it is preferable to be 30% by weight or more and 80% by weight or less. Each material of the paste may be mixed at once, or in order to uniformly disperse each material in the paste, it may be added and mixed in order while repeating kneading. As a kneading device for the slurry, a planetary mixer or a thin film swirl type high-speed mixer is preferable in terms of being able to knead uniformly.

The lithium-ion secondary battery of the present invention can be obtained, for example, by stacking the above-mentioned positive electrode with a negative electrode via a separator in a dry environment with a dew point of -50°C or less, and adding an electrolyte.

The present invention will be specifically explained below using examples, but the present invention is not limited to these examples. First, the evaluation methods used in the examples will be explained.

[Measurement A] Particle diameter The LMFP particles and LMO particles used in each Example and Comparative Example were observed at a magnification of 3,000 times using a scanning electron microscope S-5500 (manufactured by Hitachi High-Technologies Corporation), and the particle diameter of each of 100 randomly selected secondary particles was measured, and the particle diameter was calculated by calculating the number average value. However, when the particles were not spherical, the average value of the major axis and minor axis that could be measured in the two-dimensional image was used as the particle diameter.

[Measurement B] Output characteristics of positive electrode A 2032-type coin battery was produced by cutting the electrode plate produced in each Example and Comparative Example to a diameter of 15.9 mm to form a positive electrode, using a lithium foil cut to a diameter of 16.1 mm and a thickness of 0.2 mm to form a negative electrode, using "Setira" (registered trademark) as a separator, and using ethylene carbonate:diethyl carbonate = 3:7 (volume ratio) containing _{1 M} LiPF6 as an electrolyte.

The 2032-type coin battery thus fabricated was charged and discharged twice at a 0.1C rate, followed by two cycles at a 5C rate, with a cutoff voltage of 2.5V and a maximum charging voltage of 4.3V. For each charge and discharge rate, the energy density (Wh/kg) was measured from the second discharge, and the ratio of the energy density at the 5C rate and the 0.1C rate was calculated to determine the output characteristics. The closer the energy density ratio is to 1, the better the output characteristics are.

[Measurement C] Overcharge resistance of positive electrode A 1 Ah cell was prepared using the electrode plate prepared in each Example and Comparative Example, a commercially available carbon-based negative electrode (negative electrode active material: artificial graphite MAG manufactured by Hitachi Chemical Co., Ltd.) as a negative electrode, "Setira" (registered trademark) as a separator, and ethylene carbonate:diethyl carbonate = 3:7 (volume ratio) containing 1M LiPF ₆ as an electrolyte. The number of layers was 7 layers for the positive electrode (size: 70 mm x 40 mm) and 8 layers for the negative electrode (size: 74 mm x 44 mm), and the capacity ratio (NP ratio) of the opposing positive and negative electrodes was 1.05.

The laminated cell thus produced was charged (overcharged) at a rate of 1C in a 25°C environment until it reached 250% (2.5 Ah) of the rated capacity or the charging voltage reached 10 V, and the maximum temperature of the cell was measured. The lower the maximum temperature, the better the overcharge resistance.

[Measurement D] Cycle resistance of positive electrode A laminated cell prepared in the same manner as in measurement C was charged and discharged three times at a 0.1C rate in a 25°C environment, and then a cycle test was performed in which charging and discharging were repeated at a 1C rate in a 55°C environment. The energy density in the first discharge test in a 55°C environment was taken as 100%, and the number of cycles until the energy density fell below 80% was measured and evaluated as cycle resistance. The greater the number of cycles, the better the cycle resistance.

[Example 1]
(Step 1: Preparation of LMO particles)
An LMO precursor solution was prepared by dissolving 50 mmol of lithium nitrate and 100 mmol of manganese (II) nitrate hexahydrate in 150 g of pure water. This LMO precursor solution was introduced into an ultrasonic atomizer (Omron Corporation, ultrasonic nebulizer NE-U12) and atomized, and then introduced into an electric furnace heated to 800° C. for drying and pyrolysis to obtain LMO particles, which are secondary particles.

(Step 2: Preparation of LMFP particles)
200 g of dimethyl sulfoxide was added to 150 g of pure water, and 360 mmol of lithium hydroxide monohydrate was added. 120 mmol of phosphoric acid was further added to the obtained solution using an 85 wt% aqueous phosphoric acid solution, and 96 mmol of manganese (II) sulfate monohydrate and 24 mmol of iron (II) sulfate heptahydrate were further added. The obtained solution was transferred to an autoclave and heated and held for 4 hours so that the temperature inside the container was maintained at 150 ° C. After heating, the supernatant of the solution was discarded, and lithium manganese iron phosphate LiMn _0.8 Fe _0.2 PO ₄ was obtained as a precipitate. After washing the obtained lithium manganese iron phosphate with pure water, the operation of removing the supernatant by centrifugation was repeated five times, and finally pure water was added again to obtain a dispersion. Subsequently, glucose was added to the dispersion in an amount equal to 15% by weight of the lithium manganese iron phosphate in the dispersion and dissolved, and then pure water was added to adjust the solid concentration of the dispersion to 20% by weight, to obtain an LMFP dispersion.

The resulting LMFP dispersion was dried with hot air at 200°C using a spray dryer (MDL-050B, manufactured by Fujisaki Electric Co., Ltd.) to obtain secondary particles. The resulting secondary particles were heated at 700°C for 4 hours in a nitrogen atmosphere using a rotary kiln (Desktop Rotary Kiln, manufactured by Takasago Industrial Co., Ltd.) to obtain carbon-coated LMFP particles.

(Step 3: Preparation of electrode plate)
After mixing acetylene black (Li-400 manufactured by Denka Co., Ltd.) and binder (L#9305 manufactured by Kureha KF Polymer Co., Ltd.), the LMO particles and LMFP particles obtained by the above method were added in a weight ratio of 2:1, and kneaded in a mortar. At that time, the weight ratio of each material contained was granules (total of LMO particles and carbon-coated LMFP particles): acetylene black: binder was 90:5:5. Thereafter, N-methylpyrrolidinone was added to adjust the solid content to 48% by weight, and a slurry-like electrode paste was obtained. N-methylpyrrolidinone was added until the obtained paste had fluidity, and the paste was treated for 30 seconds under stirring conditions of 40 m/s using a thin film swirling high-speed mixer ("Filmix" (registered trademark) 40-L type manufactured by Primix Corporation).

The obtained electrode paste was applied to aluminum foil (thickness 18 μm) using a doctor blade (300 μm), dried at 80°C for 30 minutes, and then pressed to produce an electrode plate.

[Example 2]
An electrode plate was produced in the same manner as in Example 1, except that in step 3, the weight ratio of the LMO particles to the LMFP particles was 3:1.

[Example 3]
An electrode plate was produced in the same manner as in Example 1, except that the solid content concentration of the LMFP dispersion in step 2 was 40% by weight.

[Example 4]
An electrode plate was prepared in the same manner as in Example 3, except that in step 2, the amount of manganese (II) sulfate monohydrate added was 84 mmol, the amount of iron (II) sulfate heptahydrate added was 36 mmol, and the heating temperature was 250° C.

[Example 5]
An electrode plate was prepared in the same manner as in Example 3, except that the amount of pure water added initially in step 2 was 100 g and the amount of dimethyl sulfoxide added was 120 g.

[Example 6]
An electrode plate was prepared in the same manner as in Example 3, except that the amount of manganese (II) sulfate monohydrate added in step 2 was 108 mmol, and the amount of iron (II) sulfate heptahydrate added was 12 mmol.

[Example 7]
An electrode plate was prepared in the same manner as in Example 3, except that the amount of glucose added in step 2 was set to be equal to 7% by weight of the lithium iron manganese phosphate.

[Example 8]
An electrode plate was prepared in the same manner as in Example 3, except that the amount of glucose added in step 2 was set to be equal to 25% by weight of the lithium iron manganese phosphate.

[Example 9]
An electrode plate was prepared in the same manner as in Example 8, except that the amount of manganese (II) sulfate monohydrate added in step 2 was 60 mmol, and the amount of iron (II) sulfate heptahydrate added was 60 mmol.

[Example 10]
In step 3, in addition to the LMO particles and LMFP particles _, layered oxide active material particles (granules _{of LiNi0.5Co0.2Mn0.3O2} with an average particle size of 13 μm manufactured by _Umicore ) were added so that the ratio of LMO particles:LMFP particles:layered oxide active material particles was 60:30:10 (weight ratio), and the weight ratio of each material contained was granules (total of LMO particles, carbon-coated LMFP particles, and layered oxide active material particles):acetylene black:binder was 90:5:5. An electrode plate was produced in the same manner as in Example 1.

[Example 11]
An electrode plate was prepared in the same manner as in Example 1, except that the amount of manganese (II) nitrate hexahydrate added in step 1 was 95 mmol, and further, 5 mmol of zinc nitrate hexahydrate was added to the LMO precursor solution.

[Comparative Example 1]
An electrode plate was produced in the same manner as in Example 1, except that the solid content concentration in the LMFP dispersion in step 2 was 60% by weight.

[Comparative Example 2]
An electrode plate was produced in the same manner as in Example 1, except that the solid content concentration in the LMFP dispersion in step 2 was 8% by weight.

[Comparative Example 3]
An electrode plate was produced in the same manner as in Example 3, except that in step 3, the weight ratio of the LMO particles to the LMFP particles was 5:1.

[Comparative Example 4]
An electrode plate was produced in the same manner as in Example 3, except that in step 3, the weight ratio of the LMO particles to the LMFP particles was 0.5:1.

The evaluation results for each example and comparative example are shown in Table 1.

Claims (5)

A positive electrode for a lithium ion secondary battery containing spinel-based lithium metal oxide particles and lithium manganese iron phosphate particles,
a ratio of a particle size of the spinel-based lithium metal oxide particles to a particle size of the lithium manganese iron phosphate particles (particle size of the spinel-based lithium metal oxide particles/particle size of the lithium manganese iron phosphate particles) is 2.0 or more and 10.0 or less;
A positive electrode for a lithium ion secondary battery, in which a weight ratio of a content of spinel-based lithium metal oxide particles to a content of lithium manganese iron phosphate particles (content of spinel-based lithium metal oxide particles/content of lithium manganese iron phosphate particles) is 1.0 or more and 4.0 or less. 2. The positive electrode for a lithium ion secondary battery in accordance with claim 1, wherein the volume resistivity of the lithium manganese iron phosphate particles is 10 ² Ω·cm or more and 10 ⁶ Ω·cm or less. The positive electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the molar ratio of the manganese content to the total content of manganese and iron contained in the lithium manganese iron phosphate particles (manganese content/total content of manganese and iron) is 0.6 or more and 0.9 or less. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the lithium manganese iron phosphate particles have a carbon layer on the surface thereof, and the weight ratio of the carbon layer in the lithium manganese iron phosphate particles is 1% or more and 6% or less. A lithium ion secondary battery using the positive electrode for lithium ion secondary batteries according to any one of claims 1 to 4.