JP7619008B2 - Positive electrodes for lithium-ion secondary batteries - Google Patents

Description

The present invention relates to a positive electrode for a lithium-ion secondary battery and a lithium-ion secondary battery using the same.

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. Layered rock salt transition metal lithium oxide positive electrode active materials, which are often used in large batteries for electric vehicles and other applications, are positive electrode active materials that exhibit particularly high energy density. However, they have safety issues, such as accelerated deterioration due to changes in crystal structure accompanying the release of oxygen during charging and discharging, which can lead to the risk of smoke and fire depending on the usage conditions.

On the other hand, lithium iron phosphate positive electrode active material, which is often used in stationary batteries, does not release oxygen easily because oxygen is covalently bonded to phosphorus, and is a relatively stable and safe positive electrode material even under high temperature conditions. However, its energy density is known to be lower than that of layered rock salt transition metal lithium oxide positive electrode active materials.

A lithium manganese iron phosphate positive electrode active material in which some of the iron atoms in lithium iron phosphate are replaced with manganese atoms has the same theoretical discharge capacity (mAh/g) as lithium iron phosphate, but the discharge voltage is improved by replacing the iron atoms with manganese atoms, so that a higher energy density is achieved than with a lithium iron phosphate positive electrode active material. Even if some of the iron atoms are replaced with manganese atoms, the state in which oxygen is covalently bonded to phosphorus is still maintained, so that the lithium manganese iron phosphate positive electrode active material has the potential to achieve both high energy density and high safety. Therefore, it is being considered as a positive electrode active material to replace the lithium iron phosphate positive electrode active material. Regarding the lithium manganese _iron phosphate positive electrode active material, for example, there is an electrode active material (see, for example, Patent _Document 1 ₎ characterized in that it contains sulfur or a sulfur compound in a compound represented by the formula Li _x A y B z PO 4 (wherein A is one or more elements selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr, B is one or more elements selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, Y, and rare earth elements, and 0 _< x< ₂ , 0<y<1.5, and 0≦z _< 1.5), (0<a≦1.2, 0.9≦x≦1.1, M is a transition metal including either Fe or Mn), and the peak intensity ratio (I(020)/I(101)) of (020) to (101) in X-ray diffraction measurement using Cu-Kα radiation of the positive electrode is. and a particulate cathode material comprising an electroactive material having an empirical formula Li _a Mn _b Fe _c D _d PO ₄ , in which a is a number between 0.85 and 1.15, b is between 0.65 and 0.95, c is between 0.049 and 0.349, d is between 0.001 and 0.1, and 2.75≦(a+2b+2c+dV)≦3.10, in which V is the valence of D and D is a specific metal ion, and further in which at least a portion of the electroactive material has an olivine structure (see, for example, Patent Document 3).

JP 2010-161038 A JP 2011-76820 A JP 2018-60811 A

Lithium manganese iron phosphate positive electrode active material has a higher energy density than lithium iron phosphate and is less likely to release oxygen than layered oxide positive electrode active material, and is therefore known as a high-performance and safe positive electrode active material. However, lithium manganese iron phosphate, especially a positive electrode active material containing a high proportion of manganese (active material with manganese content/(total manganese and iron content) ≧ 0.5), has a higher electrical resistivity than lithium iron phosphate by more than three orders of magnitude, making it difficult to achieve a capacity close to the theoretical value expected from the crystal structure, and therefore has insufficient energy density. According to Patent Documents 1 to 3, in an olivine positive electrode active material, the electron and ion conductivity of the positive electrode active material is improved by doping with a different element or optimizing the particle size, and the discharge energy is improved when used as a lithium ion secondary battery. However, the electrodes disclosed in Patent Documents 1 to 3 have a problem of insufficient discharge energy, especially under conditions of large charge and discharge currents.

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 excellent energy characteristics even under conditions of large charge/discharge currents.

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 an olivine-type lithium manganese iron phosphate containing elemental sulfur as an active material,
A positive electrode for a lithium ion secondary battery, wherein the crystallite size of the olivine-type lithium manganese iron phosphate is less than 60 nm, the molar fraction of the manganese content to the total content of manganese and iron (Mn content/(Mn+Fe) content) is 0.5 or more and less than 1.0, the molar ratio of the lithium content to the phosphorus content (Li content/P content) is 1.05 or more and 1.10 or less, and the molar fraction of sulfur in the olivine-type lithium manganese iron phosphate (S content/lithium manganese iron phosphate content) is 0.03 or more and 0.05 or less.

By using the positive electrode for a lithium-ion secondary battery of the present invention, a lithium-ion secondary battery with high discharge energy can be obtained even under conditions of large charge/discharge currents.

The positive electrode for a lithium-ion secondary battery of the present invention (hereinafter, sometimes simply referred to as the "positive electrode") contains lithium manganese iron phosphate (hereinafter, sometimes abbreviated as LMFP). As described above, LMFP is safe and can achieve a higher voltage than lithium iron phosphate (hereinafter, sometimes abbreviated as LFP), which is a highly safe material currently in use. LMFP particles can be doped with elements other than phosphorus, manganese, and iron, and the particle size and stoichiometry of each element can be changed by changing the manufacturing conditions. However, the inventors have found that, in LMFP, the capacity characteristics are improved and the discharge energy is increased even under conditions of large charge and discharge currents by setting the crystallite size to less than 60 nm, the molar fraction of the manganese content relative to the total content of manganese and iron (Mn content/(Mn+Fe) content) to 0.5 or more and less than 1.0, the molar ratio of the lithium content relative to the phosphorus content (Li content/P content) to 1.05 or more and 1.10 or less, and the molar fraction of sulfur in LMFP (S content/lithium manganese iron phosphate content) to 0.03 or more and 0.05 or less.

LMFP generally refers to a compound that satisfies 0<a≦1.1 _, 0<b≦1.1, and 0.9≦a+b≦1.1 in _{LiαMnαFebPO4} , and in the _present invention, 1.05≦α≦1.10. However, even if a _metal element other than the above is added as a doping element in the range of 0.1% by weight to 10% by weight, it is also included in the LMFP of the present invention.

The crystallite size of the LMFP in the present invention is less than 60 nm. LMFP with a high manganese ratio has low electronic conductivity and ionic conductivity compared to LFP, so that the overvoltage of the battery is likely to increase, and the actual capacity and discharge voltage when discharged within a certain voltage range are small, and the discharge energy is likely to decrease. When the crystallite size of LMFP is 60 nm or more, the distance that electrons and lithium ions diffuse in the active material becomes long, so that the capacity decreases significantly relative to the theoretical capacity, and the discharge energy decreases. On the other hand, particles with a crystallite size of less than ₆₀ nm have a short diffusion distance of electrons and lithium ions in the primary particles, while it is difficult to obtain a discharge capacity close to the theoretical capacity, especially in the case of stoichiometric LMFP represented by the general formula Li 1.00 Mn 1.00- _x Fe _x (PO ₄ ) _1.00 . This is presumed to be because LMFP with a small crystallite size has a large specific surface area, so that the crystal structure is easily disturbed near the crystal surface. The present inventors have found that in such LMFP having a small crystallite size, by setting the phosphorus content and sulfur content within the above-mentioned ranges, the capacity characteristics are improved and the discharge energy is improved.

The crystallite size of LMFP can be calculated by performing powder X-ray diffraction measurement on a powder sample of LMFP at a diffraction angle 2θ in the range of 10 degrees to 70 degrees, and performing Rietveld analysis on the obtained diffraction pattern. The powder sample of LMFP can be obtained by grinding a positive electrode mixture peeled off from a positive electrode for a lithium ion secondary battery. Alternatively, the LMFP powder, which is a raw material for the positive electrode for a lithium ion secondary battery, may be measured directly.

In the present invention, the molar fraction (a/(a+b)) of the manganese content (a) relative to the total manganese and iron content (a+b) of the LMFP is 0.5 or more and less than 1.0. In the LMFP, the larger the manganese ratio a and the smaller the iron ratio b, the larger the proportion of the redox pair Mn ^3+/2+ in the discharge reaction of the battery. Since the Fe ^3+/2+ redox pair shows a potential of about 3.4 V with respect to the Li ⁺ /Li redox pair, while the Mn ^3+/2+ redox pair shows a potential of about 4.1 V, the larger the manganese ratio a, the higher the average discharge voltage of the battery. On the other hand, the theoretical capacity is about 170 mAh/g. Therefore, the larger the manganese ratio a, the higher the theoretical discharge energy.

In the present invention, the molar ratio α (Li content/P content) of the lithium content to the phosphorus content of the LMFP particles is 1.05 or more and 1.10 or less. As mentioned above, it has been difficult to obtain a discharge capacity close to the theoretical capacity for particles with a crystallite size of less than 60 nm. Therefore, in the present invention, by containing an excess amount of lithium atoms relative to phosphorus, a capacity closer to the theoretical capacity can be expressed and the discharge energy can be increased. The lithium atoms contained in an excess amount may be present at the interstitial positions of the LMFP, or may be present as a Li ₃ PO ₄ coating layer on the outer periphery of the LMFP primary particles , or may be present as Li ₃ PO ₄ in the gaps between the LMFP primary particles. In any case, it is believed that the capacity decrease is suppressed by suppressing the disturbance of the crystal structure of the LMFP. If the molar fraction α of lithium to phosphorus is less than 1.05, such an effect cannot be obtained and the discharge energy decreases. On the other hand, when the molar fraction α of lithium to phosphorus exceeds 1.10, the excess lithium inhibits the charge and discharge in the positive electrode active material, resulting in a decrease in discharge energy.

In the present invention, the LMFP contains sulfur as a doping element, and the molar fraction α of sulfur in the LMFP (S content/lithium manganese iron phosphate content) is 0.03 or more and 0.05 or less. As described above, LMFP with a high manganese ratio has lower electronic conductivity and ionic conductivity compared to LFP. Therefore, in addition to making the crystallite size less than 60 nm , it is effective to dope with a different element. It is assumed that the doped sulfur exists in the crystal of LMFP by replacing (PO ₄ ) ^3- with (SO ₄ ) ^2- . In the replacement with ( SO ₄ ) ^2- , conductive carriers are supplied due to the difference in charge with (PO ₄ ) ^3- , which contributes to improving the electronic conductivity and ionic conductivity of the LMFP primary particles, and therefore the capacity is improved under conditions of large charge/discharge currents, and the discharge energy can be increased. If the molar fraction α of sulfur is less than 0.03, the effect is not obtained, and the discharge energy decreases under conditions of large charge/discharge current. The molar fraction α of sulfur is preferably 0.035 or more. On the other hand, if the molar fraction α of sulfur exceeds 0.05, the ratio of the component that can actually be charged and discharged contained in the positive electrode active material decreases, and the discharge energy decreases. The molar fraction α of sulfur is preferably 0.040 or less.

The sulfur content in LMFP can be easily adjusted to within the above range, for example, by adding a certain amount of a sulfide source such as hydrogen sulfide, carbon disulfide, or lithium sulfide as an additive to the raw material in the LMFP manufacturing method described below.

In addition, the molar ratio of the total content of polyvalent metal ions to the phosphorus content of the LMFP particles (total amount of polyvalent metal ions/P content) is preferably 0.97 or more and 1.03 or less. Here, the total content of polyvalent metal ions means the total content of manganese, iron, and other transition metal ions and alkaline earth metal ions contained as doping sources (hereinafter, these may be collectively referred to as polyvalent metal ions). By setting the molar ratio of the total content of polyvalent metal ions to 0.97 or more, valence matching can be almost completely achieved with the transfer of electrons due to charging and discharging, so that the discharge capacity can be further improved and the discharge energy can be further increased. On the other hand, by setting the molar ratio to 1.03 or less, the generation of impurity phases other than LMFP can be suppressed, so that the discharge capacity can be further improved and the discharge energy can be increased.

Here, the composition of LMFP can be determined by atomic absorption spectrometry for lithium, and ICP atomic emission spectrometry for manganese, iron, phosphorus, and sulfur. The content of each element is measured to three decimal places and rounded off to the second decimal place. In addition, if the raw material feed ratio during LMFP production is known, the composition can be determined from that feed ratio.

The LMFP primary 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, it is possible to improve electronic conductivity uniformly over the entire positive electrode active material made of LMFP. The content of the carbon layer in the LMFP primary particles is preferably 1 wt % or more from the viewpoint of further improving electronic conductivity and increasing discharge energy. On the other hand, the content of the carbon layer in the LMFP primary particles is preferably 6 wt % or less from the viewpoint of suppressing side reactions between the carbon layer and LMFP and increasing discharge energy.

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 volume resistivity ρ (Ω·cm) of the secondary particles of the LMFP in the present invention is preferably 10 ² ≦ρ≦10 ^6. As described above, LMFP has a stable olivine structure, making it a highly safe material, but in order to further improve the safety of the positive electrode, it is preferable to reduce the current flowing when an abnormality occurs by making the resistance of the positive electrode secondary particles a certain value or more. Therefore, the volume resistivity of the secondary particles of the LMFP 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 secondary particles of the LMFP is 10 ⁶ (Ω·cm) or less, the discharge energy of the secondary particles of the LMFP can be maintained higher. Here, the volume resistivity of the secondary particles of the LMFP can be measured under 25 MPa conditions for the LMFP secondary particles in the positive electrode, for example, using a powder resistance measurement system MCP-PD51 (manufactured by Mitsubishi Chemical Analytech Co., Ltd.). As a method for extracting the LMFP secondary particles from the positive electrode, for example, a method of peeling off the mixture layer from the current collector of the positive electrode, dissolving the binder with an organic solvent such as N-methylpyrrolidinone, and drying the remaining LFMP secondary particles can be mentioned. Furthermore, when a conductive assistant such as carbon black is contained, a method of removing the carbon black having a small specific gravity from the remaining solid after dissolving the binder by centrifugation, and drying the obtained LFMP secondary particles can be mentioned. In addition, the volume resistivity of the LMFP secondary particles as a raw material in manufacturing the positive electrode may be measured in the same manner.

The volume resistivity of the secondary particles of LMFP can be adjusted to a desired range by the ratio of manganese to iron, the primary or secondary particle size, the sulfur content, and, in the case of LMFP with a carbon layer, the carbon layer content, the sintering temperature, and other carbon coating conditions.

The secondary particle diameter of LMFP 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 secondary particle diameter of LMFP is preferably 40 μm or less from the relationship with the thickness of the mixture layer described later. The secondary particle diameter of LMFP refers to the arithmetic mean value of the particle diameter of the secondary particles, 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 secondary particle diameter of 100 randomly selected secondary particles is measured. The secondary particle diameter of LMFP can be obtained by calculating the number average value. In addition, the secondary particle diameter of LMFP as a raw material in manufacturing the positive electrode may be measured using a scanning electron microscope in the same manner.

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

The crystallinity and particle shape of the LMFP primary particles can be evaluated by the ratio I ₂₀ /I ₂₉ of the peak intensity at 20 ° to the peak intensity at 29 °, and the ratio I ₃₅ /I ₂₉ of the peak intensity at 35 ° to the peak intensity at 29 ° obtained by X-ray diffraction. When I _{20 /} I ₂₉ is 0.88 or more and 1.05 or less, it means that the LMFP primary particles are not extremely oriented in the b-axis direction, and the particle shape is closer to a sphere than a plate shape. By the particles approaching a spherical shape, the distortion of the crystal lattice due to the deintercalation and insertion of lithium ions during charging and discharging can be alleviated, and the discharge energy can be further improved even under conditions of large charging and discharging currents. In addition, when I ₃₅ /I ₂₉ is 1.05 or more and 1.20 or less, the crystal orientation of the LMFP primary particles is further reduced, resulting in a more homogeneous crystal orientation, and the particle shape is further closer to a sphere. This alleviates the distortion of the crystal lattice caused by the desorption and insertion of carrier ions during charging and discharging, and further improves the discharge energy under conditions of large charging and discharging currents.

The peak intensity ratio can be determined by performing powder X-ray diffraction measurements on a powder sample of LMFP at a diffraction angle 2θ in the range of 10 degrees to 70 degrees, removing the background, reading the peak intensities, and calculating the ratio.

As a method for adjusting I _{20 /} I ₂₉ and I ₃₅ /I ₂₉ to be within the above ranges, for example, a method for obtaining LMFP by a preferable production method described later can be mentioned.

The positive electrode of the present invention preferably has a mixture layer containing additives such as a binder and a conductive assistant together with LMFP on a current collector such as aluminum foil. It is preferable that the LMFP is contained as secondary particles.

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 shape of the coating film can be easily maintained when the coating film is formed due to the binding effect of the binder. On the other hand, by making the binder content 10% by weight or less, an increase in resistance in the positive electrode can be suppressed.

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 mixture layer contains as high a proportion of positive electrode active material as possible, and the LMFP secondary particle content in the positive electrode mixture layer is preferably 80% by weight or more, and more preferably 85% by weight or more.

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.

The LMFP 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 and 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 in the liquid phase 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. The ratio of manganese and iron contained in the LMFP can be adjusted to a desired range by the charging ratio of the raw materials. In addition, in the synthesis of LMFP by a liquid phase method, it is preferable to add a solution containing the remaining raw materials to a solution containing a portion of the raw materials while stirring the solution at high speed, and then heat the mixture to the synthesis temperature without applying pressure while maintaining the high speed stirring state, and the peak intensity ratios _I20 / _I29 and _I35 / _I29 obtained by X-ray diffraction of LMFP can be adjusted to the aforementioned preferred ranges.

When LMFP primary particles are obtained by the liquid phase method, the average primary particle size can be adjusted to a desired range by, for example, conditions such as the mixing 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.

A preferred carbon coating method for forming a carbon layer on LMFP obtained by the liquid phase method is to mix LMFP primary particles or secondary particles with sugars and then bake them in an inert gas atmosphere. Glucose and sucrose are preferred as sugars 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 present invention, it is preferable to use a spray dryer as a method for producing LMFP secondary particles 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 secondary 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 secondary particles, and further additives such as a conductive assistant as necessary, 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 each material may be added and mixed in order while repeatedly kneading in order to uniformly disperse each material in the paste. 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] Crystallite size and X-ray diffraction peak intensity ratio For the powder samples made of LMFP secondary particles used in each Example and Comparative Example, powder X-ray diffraction measurement was performed using an X-ray diffractometer D8 ADVANCE manufactured by Bruker AXS Co., Ltd., with a diffraction angle 2θ in the range of 10 degrees to 70 degrees. Based on the obtained diffraction pattern, Rietveld analysis was performed using analysis software TOPAS manufactured by Bruker AXS Co., Ltd., to calculate the crystallite size of LMFP. The peak intensity ratio was obtained by removing the background (coefficient 1.77) using powder X-ray diffraction analysis software DIFFRAC.EVA manufactured by Bruker AXS Co., Ltd., reading the peak intensity, and calculating the ratio. The values obtained by dividing the peak intensity at 20° and 35° by the peak intensity at 29° were I ₂₀ /I ₂₉ and I ₃₅ /I ₂₉ , respectively.

[Measurement B] Chemical composition analysis 15 mg of powder sample made of LMFP secondary particles used in each Example and Comparative Example was decomposed by heating using perchloric acid and nitric acid, and the volume was adjusted to 100 mL using ultrapure water. For this solution, Li was measured by atomic absorption spectrometry, and Mn, Fe, P, and S were measured by ICP atomic emission spectrometry, and the respective contents in the sample were calculated and converted into atomic ratios.

[Measurement C] Discharge energy 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, cutting a lithium foil 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 _LiPF6 at a concentration of 1 M as an electrolyte.

The prepared 2032-type coin battery was charged and discharged twice at a 0.1C rate, with a cutoff voltage of 2.5V and a maximum charging voltage of 4.3V, and then charged and discharged twice at a 3C rate. For each charge and discharge rate, the discharge energy was measured from the second discharge, and the energy density (Wh/kg) per unit weight of LMFP was calculated.

[Example 1]
(Step 1: Preparation of LMFP secondary particles)
200 g of dimethyl sulfoxide and 15 g of carbon disulfide were added to 150 g of pure water, and 390 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 was obtained as a precipitate. The obtained lithium manganese iron phosphate was washed with pure water, and 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 50% 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 secondary particles.

(Step 2: 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 LMFP secondary particles obtained by the above method were added and kneaded in a mortar. At that time, the weight ratio of each material contained was LMFP secondary particles: acetylene black: binder was 90:5:5. Then, 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 prepared in the same manner as in Example 1, except that in step 1, the amount of carbon disulfide added was 20 g, the amount of lithium hydroxide monohydrate added was 400 mmol, the amount of manganese (II) sulfate monohydrate added was 104 mmol, and the amount of iron (II) sulfate heptahydrate added was 26 mmol.

[Example 3]
An electrode plate was prepared in the same manner as in Example 1, except that the amount of manganese (II) sulfate monohydrate added in step 1 was 88 mmol, and the amount of iron (II) sulfate heptahydrate added was 22 mmol.

[Example 4]
An electrode plate was prepared in the same manner as in Example 1, except that the amount of carbon disulfide added in step 1 was 10 g.

[Example 5]
An electrode plate was prepared in the same manner as in Example 1, except that the amount of carbon disulfide added in step 1 was 25 g.

[Example 6]
An electrode plate was prepared in the same manner as in Example 1, except that the amount of carbon disulfide added in step 1 was 25 g, the amount of lithium hydroxide monohydrate added was 400 mmol, the amount of manganese (II) sulfate monohydrate added was 84 mmol, the amount of iron (II) sulfate heptahydrate added was 36 mmol, and the amount of glucose added to the LMFP dispersion was the same as 10% by weight of lithium manganese iron phosphate.

[Example 7]
(Step 1: Preparation of LMFP secondary particles)
After dissolving 65 mmol of lithium hydroxide monohydrate and 5 mmol of lithium sulfide in 16 g of pure water, 104 g of diethylene glycol was added to prepare a lithium hydroxide-lithium sulfide/diethylene glycol aqueous solution. Meanwhile, 20 mmol of phosphoric acid (85% aqueous solution), 16 mmol of manganese (II) sulfate monohydrate, and 4 mmol of iron (II) sulfate heptahydrate were dissolved in 10 g of pure water to prepare a manganese sulfate-iron sulfate/phosphoric acid aqueous solution. The lithium hydroxide-lithium sulfide/diethylene glycol aqueous solution was stirred at 2000 rpm using a homodisper (Homodisper 2.5 type manufactured by Primix Corporation), and the manganese sulfate-iron sulfate/phosphoric acid aqueous solution was added to obtain a LMFP precursor solution. The obtained LMFP precursor solution was heated to 110 ° C. and held for 2 hours to obtain LMFP particles. Pure water was added to the obtained particles, and the particles were washed by repeatedly removing the solvent using a centrifuge. The same procedure was repeated to obtain a dispersion containing 10 g of washed LMFP particles. Then, glucose was added to the dispersion in an amount equal to 15% by weight of LMFP, and then pure water was added and stirred to adjust the solid content of the dispersion to 50% by weight, thereby obtaining a 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 secondary particles.

The obtained carbon-coated LMFP secondary particles were used to prepare an electrode plate in the same manner as in Example 1 (step 2).

[Comparative Example 1]
An electrode plate was prepared in the same manner as in Example 1, except that the amount of water added in step 1 was 120 g and the amount of carbon disulfide added was 20 g.

[Comparative Example 2]
An electrode plate was prepared in the same manner as in Example 1, except that the amount of lithium hydroxide monohydrate added in step 1 was 400 mmol, the amount of manganese (II) sulfate monohydrate added was 36 mmol, and the amount of iron (II) sulfate heptahydrate added was 84 mmol.

[Comparative Example 3]
An electrode plate was prepared in the same manner as in Example 1, except that the amount of carbon disulfide added in step 1 was 20 g and the amount of lithium hydroxide monohydrate added was 360 mmol.

[Comparative Example 4]
An electrode plate was prepared in the same manner as in Example 1, except that the amount of lithium hydroxide monohydrate added in step 1 was 420 mmol.

[Comparative Example 5]
An electrode plate was prepared in the same manner as in Example 1, except that carbon disulfide was not added in step 1.

[Comparative Example 6]
An electrode plate was prepared in the same manner as in Example 1, except that the amount of carbon disulfide added in step 1 was 40 g.

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

Claims (6)

A positive electrode for a lithium ion secondary battery containing, as an active material, olivine-type lithium manganese iron phosphate particles containing elemental sulfur inside the particles ,
A positive electrode for a lithium ion secondary battery, wherein the crystallite size of the olivine-type lithium manganese iron phosphate is less than 60 nm, the molar fraction of the manganese content to the total content of manganese and iron (Mn content/(Mn+Fe) content) is 0.5 or more and less than 1.0, the molar ratio of the lithium content to the phosphorus content (Li content/P content) is 1.05 or more and 1.10 or less, and the molar fraction of sulfur in the olivine-type lithium manganese iron phosphate (S content/lithium manganese iron phosphate content) is 0.03 or more and 0.05 or less. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the molar ratio of the total content of polyvalent metal ions to the phosphorus content of the olivine-type lithium manganese iron phosphate (total amount of polyvalent metal ions/P content) is 0.97 or more and 1.03 or less. The positive electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the molar fraction of sulfur in the olivine-type lithium manganese iron phosphate is 0.035 or more and 0.040 or less. 4. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the secondary particles of the olivine-type lithium manganese iron phosphate have a volume resistivity of 10 ² Ω·cm or more and 10 ⁶ Ω·cm or less. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the ratio I ₂₀ /I ₂₉ of the peak intensity at 20° to the peak intensity at 29° obtained by X-ray diffraction of the olivine-type lithium manganese iron phosphate is 0.88 or more and 1.05 or less, and the ratio I ₃₅ /I ₂₉ of the peak intensity at 35° to the peak intensity at 29° obtained by X-ray diffraction of the lithium manganese iron phosphate is 1.05 or more and 1.20 or less. A lithium ion secondary battery using a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5.