JP4876414B2 - Method for producing active material and lithium secondary battery - Google Patents

Description

  The present invention relates to a method for producing an active material comprising a lithium iron phosphorus-based composite oxide and a lithium secondary battery using the active material.

  Lithium secondary batteries that use the insertion and extraction of lithium ions are high voltage and have high energy density, so their practical use has advanced in the field of information equipment and communication equipment mainly for personal information terminals such as personal computers and mobile phones. It has become widely popular. In other fields, the use of lithium secondary batteries as a power source for electric vehicles is being studied while the development of electric vehicles is urgent due to environmental issues and resource issues.

A lithium secondary battery includes, as main components, a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an electrolyte solution that moves lithium ions between the positive electrode and the negative electrode.
As active materials, such as a positive electrode active material and a negative electrode active material, compounds, such as lithium cobaltate and lithium nickelate, are generally used, for example. However, since such an active material is mainly composed of Co or Ni having a large environmental load, it is not preferable in terms of environmental problems. In addition, in order to cope with environmental problems, active materials made of compounds such as lithium cobaltate and lithium nickelate have been recovered and reused by recycling. However, there is a problem that costs increase due to recycling. It was. In recent years, development of active materials to replace these has been desired due to the growing awareness of environmental problems.

In recent years, the use of olivine-structured LiFePO ₄ as an active material mainly used for positive electrodes has been studied. LiFePO ₄ has a merit that it is safe for the environment because it contains Fe with a low environmental load as a main component. LiFePO ₄ generates a high potential of 3.4 V and has a large theoretical capacity of 170 mAh / g.

However, LiFePO ₄ has a low electronic conductivity. Therefore, when a lithium secondary battery is configured using LiFePO ₄ as an active material, the internal resistance of the battery is increased, and as a result, the battery capacity may be reduced.
Further, Fe in LiFePO ₄ is in a divalent state and is very easily oxidized. Therefore, it is necessary to heat the raw material in an inert gas at the time of synthesis. However, for example, even when heated in an inert gas, a portion of divalent Fe that is easily oxidized is oxidized, and trivalent Fe is partially generated. When this trivalent Fe is generated, LiFePO ₄ cannot sufficiently exhibit its original excellent capacity, and there is a problem that the capacity is reduced.

Therefore, when synthesizing a positive electrode active material made of LiFePO ₄ , a method is disclosed in which a carbon material is added to a synthesis raw material to cause a heat reaction, and the temperature when exposed to the atmosphere is 305 ° C. or less (Patent Document 1). reference).
However, as described above, the method of adding a carbon material to a synthetic raw material has a problem that it is difficult to uniformly mix the synthetic raw material and the carbon material. That is, the mechanical strength differs between the synthetic raw material and the carbon material. Therefore, it has been difficult to mix the two sufficiently uniformly so as to prevent oxidation of Fe. Therefore, the reaction does not occur uniformly, the Fe of LiFePO ₄ is partially oxidized, and when used as an active material of a battery, the capacity decreases or the capacity deteriorates by repeated charge and discharge. There was a problem.
Moreover, in the said conventional manufacturing method, since it was necessary to mix a carbon material as uniformly as possible, there existed a problem that the number of manufacturing processes increased and it became complicated.

JP 2002-110164 A

  The present invention has been made in view of such conventional problems, and comprises a lithium iron-phosphorus-based composite oxide that can exhibit high charge / discharge capacity and that does not substantially decrease charge / discharge capacity even when repeated charge / discharge is performed. An object of the present invention is to provide a method for producing an active material and a lithium secondary battery.

The first invention is a method of manufacturing an active material for a lithium secondary battery comprising a lithium-iron-phosphorus compound oxide represented by the basic formula LiFePO _4,
A raw material preparation step of preparing a synthetic raw material containing a Li source, an Fe source, and a P source;
A reaction step of synthesizing the lithium iron phosphorus composite oxide by heating and reacting the synthetic raw material,
The Li source, the Fe source, and the P source are the remainder obtained by further subtracting an element that becomes a gas or vapor at the temperature of the reaction step from the remaining element obtained by subtracting Li, Fe, and PO ₄ from the constituent elements. elements have use of the raw materials, such as is the C of,
As the Fe source, iron fumarate is used,
In the method for producing an active material, the heating temperature in the reaction step is set to 550 ° C. to 850 ° C. (Claim 1).

The most notable point in the first invention is that the Li source, the Fe source, and the P source are obtained by subtracting Li, Fe, and PO ₄ from constituent elements, and the temperature of the reaction step from the remaining elements. The raw material is to use carbon (C) as the remaining element after further subtracting the element that becomes gas or vapor.
Therefore, in the reaction step, carbon (C) that is not used for the synthesis of LiFePO ₄ and is not released as gas or vapor remains. Therefore, the reaction atmosphere at the time of synthesis becomes a reducing atmosphere, and oxidation of Fe can be prevented. Therefore, after the reaction step, an active material composed of a lithium iron-phosphorus composite oxide containing almost no trivalent Fe can be obtained. Therefore, the active material can exhibit, for example, a high charge / discharge capacity close to the theoretical capacity, and a reduction in charge / discharge capacity that may occur by repeated charge / discharge is suppressed, and excellent charge / discharge cycle characteristics. Can be shown.

  Further, as described above, since the synthetic raw material itself contains C generated in the reaction step, it is not necessary to add a carbon material to the synthetic raw material as in the past. Therefore, it is not necessary to add a process of newly adding a carbon material for preventing reduction of Fe, and the active material can be easily manufactured.

  As described above, according to the first aspect of the invention, the production of an active material comprising a lithium iron-phosphorus composite oxide that can exhibit a high charge / discharge capacity and whose charge / discharge capacity hardly decreases even when charge / discharge is repeated. A method can be provided.

According to a second aspect of the present invention, there is provided a lithium secondary battery characterized in that the active material obtained by the production method of the first aspect of the invention is contained in a positive electrode or a negative electrode (claim 2 ).

  The lithium secondary battery of the second invention contains, as described above, the active material obtained by the manufacturing method of the first invention in the positive electrode or the negative electrode. Therefore, the lithium secondary battery can exhibit a high charge / discharge capacity by taking advantage of the excellent characteristics of the active material of the first invention, and the charge / discharge capacity hardly decreases even when charge / discharge is repeated.

Next, an embodiment of the present invention will be described.
In the present invention, the active material is composed of a lithium iron phosphorus-based composite oxide represented by the basic formula LiFePO ₄ .
Here, in the lithium iron phosphorus-based composite oxide represented by the above basic formula, for example, at least part of Fe is Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, It is a concept including a composite oxide substituted with one or more metal elements selected from B and Nb. Further, in the basic formula LiFePO ₄ , the molar ratio of Li to Fe (the total of the substitution element and Fe when there is a substitution element) may be other than 1.

Therefore, as the lithium iron phosphorus-based composite oxide, in addition to LiFePO ₄ , for example, the general formula Li _x Fe _{1- y My} PO ₄ (M is Mn, Cr, Co, Cu, Ni, V, Mo, Ti , Zn, Al, Ga, Mg, B, Nb, and the like, and compounds having an olivine structure represented by 0.05 ≦ x ≦ 1.2 and 0 ≦ y <1.0). Thus, the substitution type lithium iron phosphorus-based composite oxide has a more excellent discharge capacity and can exhibit charge / discharge cycle characteristics.

In the raw material preparation step, a synthetic raw material containing a Li source, an Fe source, and a P source is prepared.
In the synthetic raw material, separate compounds can be used as the Li source, the Fe source, and the P source, respectively, but a compound containing two or more of Li, Fe, and P can also be used. . For example, a compound that can be both a Li source and a P source includes lithium dihydrogen phosphate (LiH ₂ PO ₄ ).
Further, as described above, when preparing the compound represented by the general formula Li _x Fe _{1- y My} PO ₄ as the lithium iron phosphorus-based composite oxide, as described above, in the raw material preparation step, In addition to the Fe source and the P source, a synthetic raw material containing an M source is prepared.

The Li source, the Fe source, and the P source are further subtracted from the remaining elements obtained by subtracting Li, Fe, and PO ₄ from the constituent elements from the elements that become gas or vapor at the temperature of the reaction step. A raw material is used in which the remaining element is C.
In general, elements that become gas or vapor include H, O, C, and the like. In the present invention, Li, Fe, and PO constituting the lithium iron phosphorus-based composite oxide represented by the basic formula LiFePO ₄ after the reaction step from the constituent elements of the Li source, the Fe source, and the P source. The Li source, the Fe source, and the P source can be selected so that the element remaining after subtracting ₄ and further subtracting the element that becomes gas or vapor in the reaction step becomes C.

As said Fe source, organic acid iron etc. can be used, for example. Among them, it is preferable to use iron fumarate and / or iron oxalate as the Fe source .
The iron fumarate and iron oxalate are generally easily available as raw materials.

Moreover, in the said reaction process, the said synthetic raw material is heated and made to react, and the said lithium iron phosphorus type complex oxide is synthesize | combined.
The heating temperature in the above reaction step is preferably 500 ° C. to 900 ° C..
When the heating temperature is less than 500 ° C., the reaction of the synthesis raw material becomes difficult to proceed in the reaction step, and the lithium iron phosphorus composite oxide may not be sufficiently synthesized. As a result, the discharge capacity of the active material obtained after the reaction step may be reduced. On the other hand, when it exceeds 900 ° C., there is a possibility that grain growth proceeds excessively in the reaction step. As a result, the charge / discharge capacity and charge / discharge cycle characteristics of the active material obtained after the reaction step may be deteriorated. More preferably, the heating temperature in the said reaction process is 550-850 degreeC, More preferably, 600-800 degreeC is good.

The active material obtained by the production method of the first invention can be used as a positive electrode active material or a negative electrode active material. That is, the above active material can be used as either the positive electrode side or negative electrode side active material by appropriately selecting the other active material. Specifically, for example, when Li metal or a carbon material is used as the other active material, the active material obtained by the manufacturing method of the first invention is an active material on the positive electrode side (positive electrode active material). Can be used as When LiNiPO ₄ or the like is used as the other active material, the active material obtained by the manufacturing method of the first invention can be used as an active material on the negative electrode side (negative electrode active material).

  Next, in the second invention, the lithium secondary battery includes a positive electrode and a negative electrode, a separator that is sandwiched between the positive electrode and the negative electrode, and an electrolysis that moves lithium between the positive electrode and the negative electrode. A liquid etc. can be comprised as a main component.

For example, a positive electrode active material is mixed with a conductive material and a binder, and an appropriate solvent is added to form a paste-like positive electrode mixture on the surface of a metal foil current collector such as aluminum or stainless steel. It can be formed by applying and drying, and compressing to increase the electrode density as necessary. Moreover, as another form of a positive electrode, what mixed the electrically conductive material and the binder with the said positive electrode active material and made it into the positive electrode compound material, for example can be used for a pellet.
As the positive electrode active material, for example, an active material made of a lithium iron phosphorus composite oxide obtained by the production method of the first invention can be used.

  Further, the conductive material is for ensuring the electrical conductivity of the positive electrode. For example, one or more carbon powder materials such as carbon black, acetylene black, natural graphite, artificial graphite, and cokes are used. Can be used.

The binder serves to bind the active material particles and the conductive material particles. For example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. Etc. can be used. In addition, an aqueous dispersion of a cellulose-based or styrene-butadiene rubber that is an aqueous binder can also be used.
As a solvent for dispersing these active material, conductive material, and binder, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

  Next, the negative electrode is obtained by mixing a negative electrode active material such as an active material obtained by the manufacturing method of the first invention, a carbon material or the like with a binder, and adding a suitable solvent to form a paste, It can apply | coat and dry on the surface of metal foil collectors, such as copper, and can form by a press after that. Similarly to the positive electrode, a fluorine-containing resin such as polyvinylidene fluoride can be used as a binder to be mixed with the negative electrode active material, and an organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent.

  Examples of carbon materials that can be used as the negative electrode active material include natural or artificial graphite, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers and mixtures thereof, vapor-grown carbonized fibers, and phenol resins. Examples include organic compound fired bodies, cokes, carbon black, pyrolytic carbons, and carbon fibers. These carbon materials can be used alone or in combination of two or more.

Further, as another form of the negative electrode, for example, metallic lithium, which is a negative electrode active material, is formed into a sheet shape, or formed into a sheet shape by pressure bonding to a current collector network such as nickel or stainless steel. Can do. As the negative electrode active material in this case, it is also possible to use a lithium compound such as lithium alloy or Li ₄ TiO ₁₂ instead of metallic lithium.

  In addition, the separator to be narrowly attached to the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds the electrolytic solution. For example, a thin microporous film such as polyethylene or polypropylene can be used.

Next, as an electrolytic solution, an electrolyte dissolved in an organic solvent or water can be used. Examples of the electrolyte include lithium salts such as LiBF ₄ , LiPF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , and LiN (C ₂ F ₅ SO ₂ ). These lithium salts may be used alone or in combination of two or more thereof.

  As the organic solvent for dissolving the electrolyte, an aprotic organic solvent can be used. As such an organic solvent, for example, a mixed solvent composed of one or more selected from cyclic carbonate, chain carbonate, cyclic ester, cyclic ether, chain ether, and the like can be used.

  Here, examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Examples of the cyclic ester carbonate include gamma butyrolactone and gamma valerolactone. Examples of the cyclic ether include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of the chain ether include dimethoxyethane and ethylene glycol dimethyl ether. As the organic solvent, any one of these can be used alone, or two or more kinds can be mixed and used.

  Examples of the shape of the lithium secondary battery include a coin shape, a cylindrical shape, and a square shape. As a battery case that accommodates the positive electrode, the negative electrode, the separator, the electrolytic solution, and the like, those corresponding to these shapes can be used.

Example 1
Next, an embodiment of the present invention will be described with reference to FIGS.
In this example, an active material for a lithium secondary battery made of a lithium iron phosphorus composite oxide represented by the basic formula LiFePO ₄ is prepared, and a lithium secondary battery is manufactured using the active material.

In the manufacturing method of the active material of the lithium secondary battery of this example, a raw material preparation process and a reaction process are performed. In the raw material preparation step, a synthetic raw material containing a Li source, an Fe source, and a P source is prepared. Moreover, in the said reaction process, the said synthetic raw material is heated and made to react, and the said lithium iron phosphorus type complex oxide is synthesize | combined. The Li source, the Fe source, and the P source are the remainder obtained by further subtracting an element that becomes a gas or vapor at the temperature of the reaction step from the remaining element obtained by subtracting Li, Fe, and PO ₄ from the constituent elements. A raw material in which the element is C is used. Specifically, in this example, lithium dihydrogen phosphate (LiH ₂ PO ₄ ) containing Li and P is used as the Li and P sources, and iron fumarate (FeC ₄ H ₂ ) is used as the Fe source. O ₄₎ is used.

Moreover, as shown in FIG. 1, the lithium secondary battery 1 of this example has the positive electrode 2, the negative electrode 3, and electrolyte solution. The positive electrode 2 contains, as a positive electrode active material, an active material made of LiFePO ₄ obtained by performing the raw material preparation step and the reaction step. The negative electrode 3 contains lithium metal as a negative electrode active material. In addition, as the electrolytic solution of this example, a nonaqueous electrolytic solution obtained by dissolving an electrolyte in an organic solvent was used.

  As shown in the figure, the lithium secondary battery 1 of the present example has a positive electrode 2 and a negative electrode 3 in a battery case 11 of a 2016 coin cell, and a separator 4 sandwiched between them. The end of the battery case 11 has a gasket 5, and the battery case 11 is sealed with a sealing plate 12.

Next, an active material made of LiFePO ₄ and a method for manufacturing a lithium secondary battery of this example will be described in detail.
First, lithium dihydrogen phosphate (Li source, P source) and iron fumarate (Fe source) were prepared, weighed at a molar ratio of 1: 1, dispersed in ethanol, and mixed for 10 hours by a ball mill. . After mixing, ethanol was evaporated in a nitrogen atmosphere to obtain a powdery synthetic raw material.
Subsequently, the synthetic raw material was formed into pellets and fired at a temperature of 700 ° C. for 10 hours in a firing furnace filled with an inert atmosphere to obtain an active material made of LiFePO ₄ . This is designated as Sample E1.

In this example, an active material was prepared using iron sulfate heptahydrate as an Fe source for comparison with Sample E1. This is designated as Sample C1.
In preparing the sample C1, lithium dihydrogen phosphate (Li source, P source) and iron sulfate heptahydrate (Fe source) were prepared and weighed at a molar ratio of 1: 1. Similarly, it was dispersed in ethanol and mixed with a ball mill for 10 hours. After mixing, in the same manner as in sample E1, ethanol was evaporated, formed into a pellet, and fired at 700 ° C. for 10 hours to obtain an active material (sample C1) made of LiFePO ₄ .

Next, an X-ray diffraction (XRD) pattern was measured for the active materials of Sample E1 and Sample C1. The result is shown in FIG.
As can be seen from FIG. 2, in the XRD patterns of sample E1 and sample C1, only the peak attributed to LiFePO ₄ is observed, and in sample E1 and sample C1, LiFePO ₄ is synthesized in a single phase. Can be judged.

Although not clearly shown in this example, it has been confirmed that when the firing temperature is set to 500 ° C. or lower in the manufacturing methods of Sample E1 and Sample C1, a peak of unreacted material is observed. That is, it is considered that the reaction does not proceed sufficiently at a firing temperature of 500 ° C. or lower. On the other hand, at the firing temperature of 550 ° C. or higher, only the peak attributed to LiFePO ₄ was observed, confirming that the reaction was sufficiently completed.
Moreover, although not clearly shown in this example, when the particle diameter was confirmed by SEM observation about the active material of the sample E1 and the sample C1, the particle diameter was 1-2 micrometers. And in the sample E1 and the sample C1, the primary particle of 1-2 micrometers of particle sizes aggregated weakly, and comprised the secondary particle. On the other hand, it has been confirmed that when the firing temperature exceeds 850 ° C., the proportion of primary particles having a particle diameter of 3 μm or more increases.

Next, the lithium secondary battery 1 is manufactured using the two types of active materials (sample E1 and sample C1) manufactured as described above (see FIG. 1).
That is, first, a sample E1 (positive electrode active material), carbon black as a conductive material (conductive aid), and polyvinylidene fluoride as a binder (binder) in a weight ratio of 85: 10: 5 are N- Dispersed in methyl-2-pyrrolidone. Next, this dispersion was applied to both sides of an aluminum foil current collector with a doctor blade and dried. Then, it was densified with a roll press and punched to φ15 mm to produce a disc-shaped positive electrode 2.
Next, lithium metal punched to φ16 mm was prepared as the negative electrode 3.

  Next, the positive electrode 2 and the negative electrode 3 were disposed in a battery case 11 for a CR2016 type coin cell in a state in which a polypropylene separator 4 was sandwiched. Further, a gasket 5 is disposed at the end of the battery case 11.

Next, a solution obtained by dissolving LiPF ₆ as an electrolyte to a concentration of 1M in a mixed organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7 is used as an electrolytic solution. Prepared as. An appropriate amount of this electrolytic solution was injected into the battery case 11 and impregnated. After that, the sealing plate 12 was disposed in the upper opening of the battery case 11, and the battery case 11 was sealed by caulking the end of the battery case 11, whereby the lithium secondary battery 1 was produced. This is referred to as a battery E1.

  Further, a lithium secondary battery containing the sample C1 as a positive electrode active material was produced in the same manner as the battery E1 except that the sample C1 was used as the positive electrode active material instead of the sample E1. This is designated as battery C1.

Next, a charge / discharge cycle test was performed on the two types of batteries (battery E1 and battery C1) produced as described above.
In the charge / discharge cycle test, each battery was charged at a constant current with a current density of 0.5 C to an upper limit voltage of 4.2 V under the condition of a temperature of 20 ° C., and then the lower limit voltage 3 with a constant current of a current density of 0.5 C Charging / discharging to discharge to 0.0 V was taken as one cycle, and this cycle was repeated 100 times. In each charge / discharge cycle, a charge pause time and a discharge pause time were provided for 1 minute each.

At this time, the discharge capacity maintenance rate (%) of each battery in each charge / discharge cycle of 1 to 100 times was calculated. The result is shown in FIG.
The discharge capacity retention rate is a value obtained by measuring the discharge capacity before the charge / discharge cycle test and the discharge capacity in each charge / discharge cycle of 1 to 100 times, and dividing the discharge capacity in each charge / discharge cycle by the discharge capacity before the cycle test. Is expressed as a percentage (%).
Further, the discharge capacity is obtained by measuring the discharge current value (mA) and multiplying this discharge current value by the time (hr) required for discharge by the weight (g) of the positive electrode active material in the battery. It was calculated by dividing.
In FIG. 3, the horizontal axis represents the number of cycles (times), and the vertical axis represents the discharge capacity retention rate (%). In the drawing, a lithium secondary battery configured using the sample E1 as a positive electrode active material is shown as a battery E1, and a lithium secondary battery configured using the sample C1 as a positive electrode active material is shown as a battery C1.

  Moreover, in the said charging / discharging cycle test, the relationship between the battery voltage (V) of each battery and charging / discharging capacity | capacitance (mAh / g) was investigated at the time of charging / discharging of the 2nd cycle. The result is shown in FIG. In the figure, the horizontal axis indicates the charge / discharge capacity, and the vertical axis indicates the battery voltage.

As known from FIG. 3, the battery E1 containing an active material (sample E1, LiFePO ₄ ) produced using iron fumarate as a synthetic raw material Fe source as a positive electrode active material was subjected to a charge / discharge cycle test of 100 cycles. Also showed a high discharge capacity maintenance rate exceeding 90%. On the other hand, in the battery C1 containing an active material (sample C1, LiFePO ₄ ) produced using iron sulfate as the Fe source as the positive electrode active material, the discharge capacity was greatly reduced to about 65% after 100 cycles. .

Further, as is known from FIG. 4, both the battery E1 and the battery C1 showed a charge / discharge curve peculiar to LiFePO ₄ having a plateau region at a battery voltage of about 3.4V, both of which have a large charge / discharge capacity. It was different. That is, the discharge capacity of the battery E1 was 167 mAh / g (based on LiFePO ₄ ), whereas the discharge capacity of the battery C1 was 116 mAh / g. Since the theoretical capacity of LiFePO ₄ is 170 mAh / g, it can be seen that the battery E1 containing the sample E1 as an active material can exhibit a large capacity substantially close to the theoretical capacity.

As described above, the reason why the sample E1 can exhibit a capacity substantially equal to the theoretical capacity of LiFePO ₄ is considered as follows.
That is, when the sample E1 was produced, the gas (CO ₂ ) or vapor (H ₂ O) was used at the temperature of the reaction step from the remaining elements obtained by subtracting Li, Fe, and PO ₄ from the constituent elements of the synthetic raw material. A raw material (lithium dihydrogen phosphate and iron fumarate) in which the remaining element obtained by further subtracting the element to be C is used. Therefore, carbon (C) remains at the time of firing, and this C can suppress the firing atmosphere from a reducing atmosphere and prevent Fe from being oxidized from 2+ to 3+. In addition, since elements other than C are released from the reaction system to the outside as gas or vapor, nothing remains that adversely affects the composite. For this reason, it is considered that the sample E1 can exhibit a high capacity close to the theoretical capacity.

On the other hand, in the synthesis of the sample C1, since iron sulfide is used as the Fe source, no carbon remains during firing. As a result, even if it is a LiFePO ₄ single phase as an XRD pattern, it is considered that the surface of the composite LiFePO ₄ is slightly oxidized, and the capacity is reduced by the trivalent Fe generated by this oxidation.

  As described above, the sample E1 and the battery E1 of this example can exhibit a high charge / discharge capacity and have excellent charge / discharge cycle characteristics in which the charge / discharge capacity hardly decreases even when charge / discharge is repeated. Recognize.

(Example 2)
In this example, an active material composed of a plurality of LiFePO ₄ was produced by changing the firing temperature, and the discharge capacity was compared.
That is, in this example, as in the sample E1 of Example 1, a synthetic raw material was prepared using iron fumarate as an Fe source, and the firing temperature of the synthetic raw material was changed within a range of 500 ° C to 900 ° C. Then, active materials (sample E2 to sample E7) made of six types of LiFePO ₄ were prepared. Similarly to sample C1, a synthetic raw material is prepared using iron sulfate as an Fe source, and the firing temperature of the synthetic raw material is changed within a range of 500 ° C. to 900 ° C., and an active material composed of six types of LiFePO ₄ (Sample C2 to Sample C7) were prepared.

Specifically, Samples E2 to E7 were prepared by weighing lithium dihydrogen phosphate (Li source, P source) and iron fumarate at a molar ratio of 1: 1 and performing the same as Sample E1 of Example 1. Synthetic raw materials are prepared, and the synthetic raw materials are heated to 500 ° C. (sample E2), 550 ° C. (sample E3), 600 ° C. (sample E4), 800 ° C. (sample E5), 850 ° C. (sample E6), and 900 ° C. ( Sample E7) was fired for 10 hours.
Samples C2 to C7 were prepared by weighing lithium dihydrogen phosphate (Li source, P source) and iron sulfate at a molar ratio of 1: 1, and producing synthetic raw materials in the same manner as Sample C1 of Example 1. The synthetic raw materials were heated at 500 ° C. (sample C2), 550 ° C. (sample C3), 600 ° C. (sample C4), 800 ° C. (sample C5), 850 ° C. (sample C6), and 900 ° C. (sample C7), respectively. It was produced by firing for 10 hours.

Next, using each sample (sample E2 to sample E7 and sample C2 to sample C7), a lithium secondary battery was produced in the same manner as in Example 1, and the discharge capacity at the second cycle in the charge / discharge cycle test was measured as Example Measurement was performed in the same manner as in 1. The result is shown in FIG.
In FIG. 5, the horizontal axis indicates the firing temperature (° C.), and the vertical axis indicates the discharge capacity (mAh / g). In FIG. 5, the results of the sample E1 and the sample C1 produced at the firing temperature of 700 ° C. in Example 1 are also shown.

  As is known from FIG. 5, when samples E1 to E7 and samples C1 to C7 are compared with each other at the same temperature, samples E1 to E1 prepared using iron fumarate as the Fe source in any case. It can be seen that Sample E7 exhibits a higher discharge capacity than Samples C1 to C7 produced using iron sulfate as the Fe source.

In Samples C1 to C7, Sample C1 produced by firing at a temperature of 700 ° C. showed the highest discharge capacity, but the value was about 116 mAh / g.
On the other hand, Sample E1 and Sample 3 to Sample E6 produced by firing at a temperature of 550 ° C. to 850 ° C. exhibited a higher discharge capacity than Sample C1 showing the maximum discharge capacity among Samples C1 to C7. Therefore, it can be seen that the firing temperature is more preferably 550 ° C. to 850 ° C.

Samples E1, E4, and E5 produced by firing at a temperature of 600 ° C. to 800 ° C. exhibited a very high discharge capacity close to the theoretical capacity of 170 mAh / g of LiFePO ₄ . Therefore, it can be seen that the baking temperature is more preferably 600 ° C. to 800 ° C.

BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which shows the structure of the lithium secondary battery concerning Example 1. FIG. The diagram which shows the X-ray-diffraction pattern of the active material (sample E1 and sample C1) concerning Example 1. FIG. The diagram which shows the charging / discharging cycling characteristics of the lithium secondary battery (battery E1 and battery C1) concerning Example 1. FIG. The diagram which shows the relationship between the charging / discharging capacity | capacitance of a lithium secondary battery (battery E1 and battery C1) concerning Example 1, and a battery battery. The diagram which shows the relationship between the calcination temperature of an active material and discharge capacity concerning Example 1. FIG.

Explanation of symbols

1 Lithium secondary battery 2 Positive electrode 3 Negative electrode 4 Separator

Claims (2)

A method for producing an active material for a lithium secondary battery comprising a lithium iron-phosphorus composite oxide represented by the basic formula LiFePO ₄ ,
A raw material preparation step of preparing a synthetic raw material containing a Li source, an Fe source, and a P source;
A reaction step of synthesizing the lithium iron phosphorus composite oxide by heating and reacting the synthetic raw material,
The Li source, the Fe source, and the P source are the remainder obtained by further subtracting an element that becomes a gas or vapor at the temperature of the reaction step from the remaining element obtained by subtracting Li, Fe, and PO ₄ from the constituent elements. elements have use of the raw materials, such as is the C of,
As the Fe source, iron fumarate is used,
The manufacturing method of the active material characterized by making heating temperature in the said reaction process into 550 to 850 degreeC . An active material obtained by the manufacturing method according to claim 1 is contained in a positive electrode or a negative electrode, and a lithium secondary battery.

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