Method for producing lithium manganese iron phosphate

Abstract

The purpose of the present invention is to provide a method for producing lithium iron manganese phosphate, which is capable of suppressing waste at low cost. A method for producing lithium manganese iron phosphate, the method comprising: a step in which a metal powder containing iron and manganese is mixed with a phosphoric acid compound, stirred and pulverized to obtain first precursor particles; a step in which the first precursor particles and a lithium source are mixed, stirred and pulverized to obtain second precursor particles; and a step for obtaining lithium manganese iron phosphate by firing the second precursor particles.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for manufacturing lithium manganese iron phosphate. BACKGROUND

[0002] Japanese Patent Application Publication No. 2014-65641 discloses a method for manufacturing lithium manganese iron phosphate by reacting iron and phosphorus to produce a precipitate, and washing the precipitate to produce iron phosphate. Further, it is disclosed that the obtained iron phosphate is reacted with a lithium compound to produce lithium manganese iron phosphate.

[0003] Japanese Patent Application Publication No. 2016-81866 discloses a method for manufacturing lithium manganese iron phosphate by a hydrothermal reaction. SUMMARY

[0004] As a positive electrode active material, lithium iron phosphate and lithium manganese iron phosphate are used. As disclosed in Japanese Patent Application Publication No. 2014-65641, as a suitable precursor for lithium iron phosphate, there is iron phosphate. On the other hand, as for lithium manganese iron phosphate, there is no suitable precursor, and a method that can be manufactured at low cost has not been established.

[0005] Further, as disclosed in Japanese Patent Application Publication No. 2016-81866, lithium manganese iron phosphate can be manufactured by a hydrothermal reaction. On the other hand, in such a manufacturing method, it is necessary to use a sulfate or the like, and the sulfate or the like after the reaction has to be disposed of.

[0006] An object of the present disclosure is to provide a method for manufacturing lithium manganese iron phosphate that can suppress waste at low cost.

[0007] [1] A method for manufacturing lithium manganese iron phosphate, comprising:

[0008] a step of mixing, stirring, and pulverizing a metal powder containing iron and manganese and a phosphoric acid compound to obtain first precursor particles;

[0009] a step of mixing, stirring, and pulverizing the first precursor particles and a lithium source to obtain second precursor particles; and

[0010] a step of firing the second precursor particles to obtain lithium manganese iron phosphate.

[0011] A metal powder containing iron and manganese is used as a raw material, mixed, stirred, and pulverized with a phosphoric acid compound, whereby lithium manganese iron phosphate is manufactured at low cost via a phosphate (precursor). Further, it is not necessary to use a sulfate or the like during the manufacturing process, and waste can be suppressed. Furthermore, it is expected that low energy can be achieved through the entire process.

[0012] [2] The method for manufacturing lithium manganese iron phosphate according to [1], wherein the molar ratio of iron and manganese to phosphorus in the second precursor particles is 5:4.

[0013] [3] The method for producing lithium manganese iron phosphate according to [1] or [2], wherein the metal powder is manganese iron.

[0014] [4] The method for producing lithium manganese iron phosphate according to any one of [1] to [3], wherein, in the process of obtaining the second precursor particles, a carbon source is further added, and stirring and pulverization are performed.

[0015] The above and other objects, features, aspects and advantages of the present application will become more apparent from the following detailed description of the application when taken in conjunction with the accompanying drawings, in which: BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a schematic flowchart of the method for producing lithium manganese iron phosphate of the present embodiment.

[0017] Figure 2 is an explanatory diagram that explains the processes in the method for producing lithium manganese iron phosphate of the present embodiment.

[0018] Figure 3 is an example of an SEM image of the lithium manganese iron phosphate of No. 1.

[0019] Figure 4 is an example of an SEM image of the lithium manganese iron phosphate of No. 2. DETAILED DESCRIPTION

[0020] Hereinafter, an embodiment of the present disclosure (which can be hereinafter simply referred to as "the present embodiment") and an example of the present disclosure (which can be hereinafter simply referred to as "the present example") will be described. However, the present embodiment and the present example do not limit the technical scope of the present disclosure.

[0021] The lithium manganese iron phosphate produced in the present embodiment is suitably used, for example, as an electrode active material for a secondary battery, and particularly as a positive electrode active material for a nonaqueous electrolyte secondary battery.

[0022] <Method for producing lithium manganese iron phosphate>

[0023] Figure 1 is a schematic flowchart of the method for producing lithium manganese iron phosphate of the present embodiment. Hereinafter, the "method for producing lithium manganese iron phosphate of the present embodiment" can be simply referred to as "the present production method". The present production method at least includes (a) a first precursor particle production process, (b) a second precursor particle production process, and (c) a firing process. Hereinafter, the processes will be described with reference to Figure 2 The processes will be described.

[0024] ((a) First precursor particle production process)

[0025] In the first precursor particle manufacturing process, metal powder containing iron and manganese is mixed, stirred and pulverized with a phosphate compound to obtain the first precursor particles.

[0026] Examples of metal powders containing iron and manganese include powders formed by mixing iron and manganese powders, and iron-manganese alloy powders. In metal powders containing iron and manganese, when the molar number of iron is 'a' and the molar number of manganese is 'b', it is preferable that 'a' is greater than 0 and less than 0.5, and 'b' is greater than 0.5 and less than 1. Lithium manganese iron phosphate manufactured using such metal powders is expected to contribute to improved characteristics of secondary batteries.

[0027] Ferromanganese is preferred as an iron-manganese alloy powder. Ferromanganese is an iron-manganese alloy containing a high capacity of manganese, and is classified according to carbon content into high-carbon ferromanganese, medium-carbon ferromanganese, and low-carbon ferromanganese, etc. (refer to JIS standard G2301). Ferromanganese is mass-produced worldwide and can be obtained at low cost. It has the advantage of having an iron and manganese composition close to that of lithium iron phosphate (iron:manganese = 20-30:70-80 (molar ratio)) currently used as a positive electrode active material.

[0028] Examples of phosphoric acid compounds include phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. Among these, from the viewpoint of improving battery performance, phosphoric acid is preferred, and it is especially preferred to use an aqueous solution of phosphoric acid with a content of 70% to 90% by mass or more.

[0029] In this process, metal powder containing iron and manganese is mixed, stirred and pulverized with a phosphate compound.

[0030] Stirring can also be performed by adding metal powder containing iron and manganese to water to make a slurry, followed by the dropwise addition of a phosphoric acid compound. By adding the phosphoric acid compound dropwise to the slurry, a layer of metal powder containing iron and manganese (layer 11) and a layer of phosphoric acid compound (layer 22) are first formed. Figure 2 (a)). Next, by stirring, a coating of manganese iron phosphate caused by phosphorylation of layer 11 is formed between layer 11 and layer 22 (layer 3, 13). Figure 2 (b) The stirring speed during the dropwise addition of the phosphoric acid compound can be, for example, 100 rpm or more and 1000 rpm or less. There is no particular limitation on the stirring time, as hydrogen is produced when manganese ferric is added to water. Therefore, it is preferable to continue stirring until hydrogen production is complete.

[0031] After stirring, the resulting slurry is pulverized. By pulverizing the resulting slurry, the stripping of layer 2 (12) and the phosphorylation of layer 11 occur repeatedly. Figure 2(c)). Thereby, the first precursor particles are formed in the slurry. The pulverization can be performed, for example, also with a ball mill, a bead mill, a planetary mill, a jet mill, a planetary mixer, a homogenizer, or the like. The pulverization time is not particularly limited, and can be, for example, 5 minutes or more and 60 minutes or less. After the pulverization, the slurry can also be filtered.

[0032] (b) Second precursor particle manufacturing step

[0033] In the second precursor particle manufacturing step, the first precursor particles and a lithium source are mixed, stirred, and pulverized to obtain the second precursor particles.

[0034] As the lithium source, for example, lithium carbonate, lithium hydroxide, lithium nitrate, or the like can be listed.

[0035] In this step, the first precursor particles and the lithium source are mixed, stirred, and pulverized.

[0036] The stirring can be performed, for example, while adding the lithium source to the slurry containing the first precursor particles obtained in the above first precursor particle manufacturing step. The lithium source is added to the slurry while stirring, whereby a lithium phosphate layer (second layer 14) is formed. Figure 2 (d)). Here, the stirring speed at the time of adding the lithium source can be, for example, 100 rpm or more and 500 rpm or less. On the other hand, in this step, it is considered that heat generation based on a neutralization reaction and foaming based on the use of the lithium source are accompanied. Therefore, in order to suppress the heat generation and the foaming, it is preferable to slow down the stirring speed. The stirring time is not particularly limited, and it is preferable to continue the stirring until the reaction ends. Further, from the viewpoint of avoiding a sharp rise in the pH value, the addition of the lithium source is preferably performed for 15 minutes or more, more preferably for 30 minutes or more.

[0037] After the stirring, the slurry is pulverized. By pulverizing the slurry, peeling of the second layer 14 and phosphatization of the first layer 11 are repeatedly performed. Thereby, the second precursor particles are formed in the slurry. Figure 2 (e)). The pulverization can be performed, for example, also with a ball mill, a bead mill, a planetary mill, a jet mill, a planetary mixer, a homogenizer, or the like. The pulverization time is not particularly limited, and can be, for example, 30 minutes or more and 120 minutes or less. After the pulverization, the slurry can also be filtered.

[0038] The molar ratio of iron and manganese in the second layer 14 in the second precursor particles to phosphorus (phosphate) (iron + manganese: phosphorus) is, for example, 5:4, 3:2, 1:1, and is preferably 5:4. In the case where the molar ratio is 5:4, it is expected that a lithium manganese iron phosphate having a small particle diameter and a large density is obtained.

[0039] In the present process, a carbon source can also be further added and stirred and pulverized. The lithium manganese iron phosphate produced in the present embodiment is expected to become a positive active material for a nonaqueous electrolyte secondary battery that is high in safety and suitable for obtaining a high output. On the other hand, there is room for improvement in terms of conductivity and diffusivity of lithium ions. From such a viewpoint, it is preferable to coat the surface of the lithium manganese iron phosphate with carbon. Thereby, further improvement in battery characteristics is expected.

[0040] As the carbon source, for example, sugars such as glucose, fructose, starch, cellulose, and the like can be listed. The amount of the carbon source added can be, for example, 0.5 parts by mass or more and 15 parts by mass or less with respect to 100 parts by mass of the metal powder containing iron and manganese.

[0041] ((c) Firing Process)

[0042] In the firing process, the second precursor particles are fired to obtain lithium manganese iron phosphate 20 Figure 2 (f)).

[0043] The firing temperature can be, for example, 500°C or higher and 1000°C or lower. In the case where the firing temperature is too low, unreacted second precursor particles can remain, and the crystallinity of the lithium manganese iron phosphate obtained can become insufficient. In the case where the firing temperature is too high, amorphous lithium manganese iron phosphate can increase.

[0044] The temperature increase rate can be, for example, 1°C / minute or higher and 10°C / minute or lower. In the case where the temperature increase rate is within the above range, the reaction proceeds uniformly, and the crystallinity of the lithium manganese iron phosphate obtained is stable.

[0045] The firing time can be, for example, 1 hour or more and 12 hours or less. In the case where the firing time is too short, unreacted second precursor particles can remain, and the crystallinity of the lithium manganese iron phosphate obtained can become insufficient. In the case where the firing time is too long, amorphous lithium manganese iron phosphate can increase. Furthermore, the firing time in the present disclosure indicates the time during which heating is performed at the maximum temperature after reaching the maximum temperature after the start of heating, and the time before reaching the maximum temperature (temperature increase time) and the time until the completion of cooling to room temperature (30°C) after heating at the maximum temperature are excluded.

[0046] The firing atmosphere can be, for example, an inactive atmosphere containing argon, nitrogen, or the like.

[0047] (Other Processes)

[0048] The present production method can also include a drying process after the second precursor particle production process. In the present production method, any drying method can be used. For example, the slurry can be dried using a spray dryer, hot air, or the like.

[0049] [Examples]

[0050] <No. 1>

[0051] To 400 g of water, 35.2 g of ferromanganese was added. Thereafter, while stirring at 200 rpm, 73 g of phosphoric acid of 85 mass% was added dropwise, and a slurry was obtained. The stirring was performed for 1 hour until the generation of hydrogen ended.

[0052] The obtained slurry was put into a planetary mill, and pulverized for 15 minutes using a 5 mm bead. The pulverized slurry was filtered using a filter having a mesh size of 75 μm.

[0053] While adding 23.5 g of lithium carbonate to the filtered slurry over 30 minutes, the stirring was performed at 200 rpm. Thereafter, 9 g of fructose was added, and dissolved by stirring for 30 minutes, and thereby a slurry was obtained.

[0054] The obtained slurry was put into a bead mill (METAL & MACHINERY Co., Ltd., Ultra Apex Mill bead mill), and pulverized for 90 minutes using a 0.1 mm bead. The pulverized slurry was filtered using a filter having a mesh size of 75 μm.

[0055] The filtered slurry was dried using a spray dryer.

[0056] The dried second precursor particles were fired for 3 hours under an argon atmosphere at a temperature increase rate of 5°C / minute and a firing temperature of 680°C. The fired particles were sieved using a filter having a mesh size of 75 μm, and thereby a lithium ferromanganese phosphate (positive electrode active material) of No. 1 was obtained.

[0057] <No. 2>

[0058] A lithium ferromanganese phosphate (positive electrode active material) of No. 2 was obtained using the same materials and method as No. 1, except that 23.5 g of lithium carbonate was added in total at one time.

[0059] <Analysis>

[0060] In each of the second precursor particles of No. 1 and No. 2 after drying using a spray dryer, the molar ratio of iron and manganese to phosphorus (iron + manganese: phosphorus) in the second precursor particles was confirmed by irradiating X-rays using an X-ray Diffraction (XRD) device. In No. 1, the iron + manganese: phosphorus was 5:4, and in No. 2, the iron + manganese: phosphorus was 3:2.

[0061] <Observation>

[0062] The lithium manganese iron phosphate of No. 1 and No. 2 was observed by SEM (Scanning Electron Microscope) (magnification: 10000 times). The SEM images of each No. are shown in Figure 3 and Figure 4 .

[0063] As shown in Figure 3 and Figure 4 , the lithium manganese iron phosphate of No. 1 is a fine particle having a smaller particle size than the lithium manganese iron phosphate of No. 2. It is generally considered that a positive electrode active material having a small particle size is helpful to improve the characteristics of a secondary battery as compared with a positive electrode active material having a large particle size. That is, it is expected that the lithium manganese iron phosphate of No. 1 is helpful to improve the characteristics of a secondary battery as compared with the lithium manganese iron phosphate of No. 2.

[0064] In addition, the lithium manganese iron phosphate obtained as described above is obtained via a phosphate (precursor), and it is expected to be manufactured at a low cost. Furthermore, a sulfate or the like is not used in the manufacturing process, and waste can be suppressed.

[0065] The embodiments of the present application were described, but it should be considered that the embodiments disclosed this time are illustrative in all respects and are not restrictive. The scope of the present application is shown by the claims, and intended to include all modifications within the meaning and range of equivalency of the claims.

Claims

1. A method for manufacturing lithium manganese iron phosphate, comprising: The process of mixing, stirring and pulverizing metal powder containing iron and manganese with a phosphate compound to obtain the first precursor particles; The process of mixing, stirring, and pulverizing the first precursor particles and the lithium source to obtain the second precursor particles; and The process of sintering the second precursor particles to obtain lithium manganese iron phosphate.

2. In the method for manufacturing lithium manganese iron phosphate according to claim 1, the molar ratio of iron to manganese relative to phosphorus in the second precursor particles is 5:

4.

3. The method for manufacturing lithium manganese iron phosphate according to claim 1 or 2, wherein the metal powder is ferromanganese.

4. In the method for manufacturing lithium manganese iron phosphate according to claim 1 or 2, in the process of obtaining the second precursor particles, a carbon source is further added for stirring and pulverizing.

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