JP5189466B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium iron composite oxide for a positive electrode active material capable of constituting a lithium secondary battery utilizing a lithium occlusion / desorption phenomenon, a method for producing the same, and a lithium secondary constituted using the same as a positive electrode active material It relates to batteries.

  Lithium secondary batteries that use the lithium absorption / desorption phenomenon have a high energy density. With the downsizing of mobile phones, personal computers, etc., lithium secondary batteries are used in the field of communication equipment and information-related equipment. It has become widespread. Further, due to resource problems and environmental problems, there is an increasing demand for electric vehicles in the field of automobiles, and there is an urgent need to develop lithium secondary batteries that are inexpensive, have large capacity, and have good cycle characteristics.

At present, lithium transition metal composite oxides such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ are preferably used as positive electrode active materials for lithium secondary batteries as being capable of constituting a 4V class secondary battery. In particular, since LiCoO ₂ is easy to synthesize and relatively easy to handle, and has excellent charge / discharge cycle characteristics, secondary batteries using LiCoO ₂ as a positive electrode active material have become mainstream. .

However, cobalt and the like are small in the amount of resources, and secondary batteries using LiCoO ₂ or the like as the positive electrode active material are difficult to cope with future mass production and enlargement in view of batteries for electric vehicles, and are also expensive. It must be extremely expensive. Therefore, in place of cobalt or the like, an attempt has been made to employ lithium iron composite oxide, which is abundant as a resource and contains inexpensive iron as a constituent element, as a positive electrode active material.

As a lithium secondary battery using lithium iron composite oxide as a positive electrode active material, for example, in Japanese Patent Application Laid-Open No. 9-134725, a lithium secondary battery using LiFePO ₄ , LiFeVO ₄ or the like having an olivine structure as a positive electrode active material. A battery is shown.

  However, as a result of further trial by the present inventors, a lithium secondary battery using the olivine-structure lithium iron composite oxide as shown in the above-mentioned publication as a positive electrode active material could not obtain a sufficient capacity. . That is, when charged and discharged at a practical charge / discharge density, an active material discharge capacity of 100 mAh / g or more is required, but when the lithium iron composite oxide is used as a positive electrode active material, It was difficult to obtain the active material discharge capacity.

This is because, as shown in Japanese Patent Laid-Open No. 11-25983, the active material discharge capacity of LiCoPO ₄ having a olivine structure similar to that of LiFePO ₄ is 80 mAh / g, which is about half of the theoretical capacity. It is also inferred from that.

  As a result of various experiments and studies to solve the above problem, the present inventor said that the above problem is caused by a slow reaction of occlusion / desorption of lithium ions in lithium iron composite oxide particles. Obtained knowledge. In general, when lithium iron composite oxide is used as the positive electrode active material, the positive electrode is a powdered lithium iron composite oxide mixed with a conductive material and a binder to obtain a paste-like positive electrode mixture, It is formed by coating or the like on the current collector surface. A lithium secondary battery using a conventional lithium iron composite oxide as a positive electrode active material has a small area involved in the reaction of the positive electrode, that is, the area involved in the reaction per unit weight of the lithium iron composite oxide. In addition, it is considered that the lithium ion occlusion / desorption reaction is not sufficiently performed.

  The present invention has been made to solve the above problems, and by increasing the surface area involved in the lithium ion occlusion / desorption reaction in the lithium iron composite oxide and activating the reaction, the active material An object of the present invention is to provide a lithium iron composite oxide for a positive electrode active material for a lithium secondary battery that has a large discharge capacity and that can maintain the capacity even after cycling, and has little cycle deterioration.

  Another object of the present invention is to provide a method for easily producing the lithium iron composite oxide for a positive electrode active material of a lithium secondary battery, and the present invention further includes the above lithium iron composite oxide as a positive electrode active material. Therefore, it is an object of the present invention to provide a lithium secondary battery that is inexpensive, has a large discharge capacity, and is excellent in cycle characteristics with little decrease in capacity even after repeated cycles.

A preferred embodiment of the lithium secondary battery disclosed herein includes a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and lithium ions between the positive electrode and the negative electrode. In a lithium secondary battery comprising an electrolyte to be moved,
The positive electrode includes a positive electrode active material, a conductive material, and a binder, and the positive electrode active material is an olivine-structure lithium iron composite oxide having a basic composition of LiFePO _4, and includes a lithium source, an iron source, and a phosphorus source. An olivine-structured lithium iron composite oxide produced by crushing a fired body obtained by firing a mixture and having an average particle size of particles that are present almost alone is 0.2 μm or more and 0.8 μm or less. The active material discharge capacity at 60 ° C. when discharged at a current density of 0.25 mA / cm ² is 100 mAh / g or more.
Further, another preferred embodiment of the lithium secondary battery disclosed herein includes a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and between the positive electrode and the negative electrode. In a lithium secondary battery comprising an electrolyte that moves lithium ions at
The positive electrode includes a positive electrode active material, a conductive material, and a binder, and the positive electrode active material is an olivine-structure lithium iron composite oxide having a basic composition of LiFePO _4, and includes a lithium source, an iron source, and a phosphorus source. An olivine-structured lithium iron composite oxide produced by crushing a fired body obtained by firing a mixture and having an average particle size of particles that are present almost alone is 0.2 μm or more and 0.8 μm or less. The active material discharge capacity at the beginning of the cycle at 20 ° C. is 100 mAh / g or more when discharged at a current density of 0.25 mA / cm ² .

  By setting the average particle size of the lithium iron composite oxide particles to 1 μm or less, the surface area involved in the lithium ion occlusion / desorption reaction in the positive electrode is increased and the reaction is activated. Therefore, it is considered that the insertion / extraction of lithium ions is performed smoothly. As a result, the lithium iron composite oxide for the positive electrode active material of the lithium secondary battery of the present invention has a large active material discharge capacity and a lithium iron composite oxide with little cycle deterioration such that the capacity can be maintained even after cycling. Become.

The method for producing the lithium iron composite oxide of the present invention is not particularly limited, but can be more easily produced by the following method. That is, in one preferred embodiment of the method for producing a lithium iron composite oxide provided by the present invention, a lithium source , an iron source, and a phosphorus source (preferably a phosphorus-containing ammonium salt ) are mixed to obtain a mixture. Particles present substantially independently, comprising a raw material mixing step, a firing step of firing the mixture at a temperature of 600 ° C. to 700 ° C. , and a crushing step of crushing the fired body obtained by the firing the average value of the particle size Ru manufacturing method der a lithium secondary battery positive electrode active material for a lithium-iron composite oxide is 1μm or less.

By performing the firing at a relatively low temperature of 600 ° C. or more and 700 ° C. or less, the growth of the particles in the firing is suppressed, and the obtained fired body is crushed so that the particle size of the particles that exist substantially alone is reduced. A lithium iron composite oxide having an average value as small as 1 μm or less can be obtained.

  Therefore, the method for producing a lithium iron composite oxide of the present invention is a method for easily producing a lithium iron composite oxide having a large active material discharge capacity and little cycle deterioration.

  The lithium secondary battery of the present invention is characterized by using the lithium iron composite oxide of the present invention as a positive electrode active material. In addition to being inexpensive, the lithium secondary battery of the present invention can effectively exhibit the action of the lithium iron composite oxide, in which lithium ions are smoothly absorbed and desorbed and the reaction is activated. As a result, the lithium secondary battery has a large discharge capacity and excellent cycle characteristics with little decrease in capacity even after repeated cycles.

  Hereinafter, preferred embodiments of the lithium iron composite oxide for a positive electrode active material of a lithium secondary battery according to the present invention, a production method thereof, and a lithium secondary battery using the same will be described in detail.

<Lithium iron composite oxide>
The lithium iron composite oxide for a lithium secondary battery positive electrode active material of the present invention is an olivine-structure lithium iron composite oxide having a basic composition of LiFePO _4, and the average particle size of the lithium iron composite oxide is 1 μm or less. It is an oxide.

  The “basic composition is assumed to be” means not only the composition represented by the composition formula but also, for example, a part of the Fe site in the crystal structure with other elements such as Co, Ni, Mn, and Mg. It is meant to include substituted ones. Furthermore, it means that not only a stoichiometric composition but also a non-stoichiometric composition in which some elements are deficient or the like is included.

  The lithium iron composite oxide of the present invention has an orthorhombic olivine structure in its crystal structure, and its space group is represented by Pmnb. In other words, the olivine structure is based on the hexagonal close-packed filling of oxygen, in which phosphorus is located at the tetrahedral site and both lithium and iron are located at the octahedral site.

  The lithium iron composite oxide of the present invention has an average particle size of 1 μm or less. When the average particle diameter exceeds 1 μm, the surface area involved in the lithium ion occlusion / desorption reaction of the lithium iron composite oxide particles in the positive electrode is reduced, and the lithium ion occlusion / desorption reaction is not smoothly performed. This is because sufficient active material discharge capacity cannot be obtained when charging / discharging at a practical charge / discharge density. In particular, it is desirable that the average particle size be 0.2 μm or more and 0.8 μm or less, considering that the electrode can be easily manufactured and the rate characteristics are good.

  The lithium iron composite oxide is formed from particles that exist substantially independently, as will be shown later in the photograph. Therefore, the average particle diameter is an average value of the particle diameters of the particles present almost alone, and each particle diameter is measured using, for example, a scanning electron microscope (SEM) photograph of the lithium iron composite oxide. can do. That is, a SEM photograph of lithium iron composite oxide is taken, the longest diameter and the shortest diameter of the lithium iron composite oxide particles in the photograph are measured, and the average value of these two values is adopted as the particle diameter of the one particle. can do.

<Method for producing lithium iron composite oxide>
The method for producing the lithium iron composite oxide of the present invention is not particularly limited, but can be more easily produced by the following method. That is, the method for producing a lithium iron composite oxide of the present invention is a method comprising a raw material mixing step of mixing raw materials to obtain a mixture, and a firing step of firing the mixture at a predetermined temperature.

(1) Raw material mixing step The raw material mixing step in the method for producing a lithium iron composite oxide of the present invention is a step of obtaining a mixture by mixing a lithium compound, an iron compound, and a phosphorus-containing ammonium salt.

Li ₂ CO ₃ , Li (OH), Li (OH) · H ₂ O, LiNO ₃ or the like can be used as a lithium compound that serves as a lithium source. In particular, it is desirable to use Li ₂ CO ₃ because of its low hygroscopicity.

As the iron compound serving as the iron source, FeC ₂ O ₄ .2H ₂ O, FeCl _2, or the like can be used as a compound having a bivalent iron valence. In particular, it is desirable to use FeC ₂ O ₄ .2H ₂ O because the corrosiveness of the gas generated during firing is low.

NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ , P ₂ O _5, or the like can be used as the phosphorus-containing ammonium salt serving as the phosphorus source. In particular, it is desirable to use NH ₄ H ₂ PO ₄ because it has relatively low hygroscopicity and generates a small amount of corrosive gas.

  Any of the above-mentioned raw materials may be used in the form of powder, and mixing thereof may be performed by a method used for normal powder mixing. Specifically, it may be mixed using, for example, a ball mill, a mixer, a mortar or the like. In addition, what is necessary is just to let the mixing ratio of each raw material be a ratio according to the composition of the lithium iron complex oxide to be manufactured.

  Further, in order to obtain a lithium iron composite oxide having an average particle size of 1 μm or less, it is desirable to control the average particle size of the above raw materials, and in particular, each raw material should have an average particle size of 1 μm or less. Is desirable.

(2) Firing step The firing step is a step of firing the mixture obtained in the raw material mixing step at a temperature of 600 ° C or higher and 700 ° C or lower. Firing may be performed in an inert atmosphere or a reducing atmosphere, specifically, for example, in an argon stream or a nitrogen stream to prevent iron from being oxidized to trivalent.

  The firing temperature is 600 ° C. or higher and 700 ° C. or lower. When the firing temperature is less than 600 ° C., the reaction does not proceed sufficiently, a subphase other than the intended orthorhombic crystal is generated, and the crystallinity of the lithium iron composite oxide is deteriorated. On the other hand, when the temperature exceeds 700 ° C., the lithium iron composite oxide particles grow and the particle size becomes large, so that a lithium iron composite oxide having an average particle size of 1 μm or less cannot be obtained. In particular, considering the improvement of rate characteristics, it is desirable that the temperature be 620 ° C. or higher and 670 ° C. or lower. Note that the firing time may be a time sufficient to complete the firing, and is usually performed for about 12 hours.

<Lithium secondary battery>
The lithium secondary battery of the present invention is a lithium secondary battery using the lithium iron composite oxide of the present invention as a positive electrode active material, and other configurations excluding the positive electrode active material are not particularly limited, The configuration of a known lithium secondary battery may be followed. Further, the lithium iron composite oxide of the present invention includes various lithium iron composite oxides depending on the composition, particle diameter, and the like. In the lithium secondary battery of the present invention, one of them may be used as a positive electrode active material, or a mixture of two or more may be used. Furthermore, the lithium iron composite oxide of the present invention and an already known positive electrode active material can be mixed to form a positive electrode active material.

  Generally, a lithium secondary battery includes a positive electrode and a negative electrode that occlude and release lithium ions, a separator sandwiched between the positive electrode and the negative electrode, a non-aqueous electrolyte that moves lithium ions between the positive electrode and the negative electrode, and the like. Can be configured as main components.

  A positive electrode is a mixture of a conductive material and a binder mixed with a powder of the lithium iron composite oxide of the present invention, which becomes a positive electrode active material, and an appropriate solvent is added to form a paste-like positive electrode mixture. It can be formed by applying and drying on the surface of a current collector made of a metal foil, etc., and compressing it to increase the electrode density as necessary. The conductive material is for ensuring the electrical conductivity of the positive electrode, and a mixture of one or two or more carbon material powders such as carbon black, acetylene black, and graphite can be used. The binder plays a role of connecting the active material particles and the conductive material particles, and a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and a thermoplastic resin such as polypropylene and polyethylene can be used. . An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing these active material, conductive material, and binder.

  The negative electrode can be formed by forming metal lithium, which is a negative electrode active material, into a sheet shape or by pressing a sheet into a current collector network such as nickel or stainless steel. Instead of metallic lithium, a lithium alloy or a lithium compound can also be used for the negative electrode active material.

  As another form of the negative electrode, the negative electrode can be configured using a carbon material capable of inserting and extracting lithium ions in the negative electrode active material. Examples of the carbon material that can be used include natural or artificial graphite, a fired organic compound such as phenol resin, and a powder such as coke. In this case, the negative electrode mixture prepared by mixing the negative electrode active material with a binder and adding an appropriate solvent to the paste is applied to the surface of a metal foil current collector such as copper, dried, and then pressed. Can be formed. Application, drying, pressing, and the like in this case may follow a normal method. When a carbon material is used as the negative electrode active material, a fluorine-containing resin such as polyvinylidene fluoride can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent.

  The separator sandwiched between the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used.

The nonaqueous electrolytic solution is obtained by dissolving a lithium salt as an electrolyte in an organic solvent. The lithium salt is dissociated by dissolving in an organic solvent, and becomes lithium ions and exists in the electrolytic solution. Examples of the lithium salt that can be used include 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 ) ₂ . These lithium salts may be used alone, or two or more of these may be used in combination.

  As the organic solvent for dissolving the lithium salt, an aprotic organic solvent is used. For example, a mixed solvent composed of one or more of cyclic carbonate, chain carbonate, cyclic ester, cyclic ether, chain ether and the like can be used. Examples of cyclic carbonates are ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, etc., examples of chain carbonates are dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, etc., examples of cyclic esters are gamma butyrolactone, gamma valero. Examples of lactones include tetrahydrofuran, 2-methyltetrahydrofuran and the like as examples of cyclic ethers, and examples of chain ethers include dimethoxyethane and ethylene glycol dimethyl ether. Any one of these can be used alone, or two or more can be mixed and used.

In place of the separator and the non-aqueous electrolyte, a polymer solid electrolyte using a high molecular weight polymer such as polyethylene oxide and a lithium salt such as LiClO ₄ or LiN (CF ₃ SO ₂ ) ₂ can be used. Alternatively, a gel electrolyte obtained by trapping the non-aqueous electrolyte in a solid polymer matrix such as polyacrylonitrile can also be used.

  The lithium secondary battery of the present invention constituted by the above-described constituent elements can be various shapes such as a cylindrical type, a laminated type, and a coin type. Regardless of the shape, the above components are housed in a battery case, and a current collecting lead or the like is used between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal. The battery case is sealed and the battery system is separated from the outside to complete the lithium secondary battery.

<Other forms>
As mentioned above, although the lithium iron composite oxide for lithium secondary battery positive electrode active material of the present invention, the manufacturing method thereof, and the embodiment of the lithium secondary battery using the same have been described, the above-described embodiment is only one embodiment. First, the lithium iron composite oxide for a positive electrode active material of a lithium secondary battery according to the present invention, a method for producing the same, and a lithium secondary battery using the same are variously based on the knowledge of those skilled in the art including the above-described embodiments. It can be implemented in various forms with changes and improvements.

  Based on the said embodiment, the lithium iron complex oxide of this invention whose average particle diameter is 1 micrometer or less was manufactured as an Example. Moreover, in order to compare with this, the lithium iron complex oxide whose average particle diameter exceeds 1 micrometer was manufactured as a comparative example. Then, lithium secondary batteries using the lithium iron composite oxides of Examples and Comparative Examples as positive electrode active materials were produced respectively, and by comparing their active material discharge capacities, the lithium iron composite oxide of the present invention was excellent. The sex was confirmed. Hereinafter, these will be described.

<Lithium iron composite oxide of Example>
Li ₂ CO ₃ was used as the lithium source, FeC ₂ O ₄ .2H ₂ O was used as the iron source, NH ₄ H ₂ PO ₄ was used as the phosphorus source, and Li, Fe, and P had a molar ratio of 1: 1: 1. It mixed so that it might become. An automatic mortar was used for mixing. This mixture was baked in an argon stream at 620 ° C. for 12 hours. And the obtained lithium iron complex oxide was crushed and it was set as the powdery lithium iron complex oxide used as a positive electrode active material. This lithium iron composite oxide was used as the lithium iron composite oxide of the examples.

An X-ray diffraction pattern of the lithium iron composite oxide of this example is shown in FIG. From the pattern of FIG. 1, it was confirmed that the lithium iron composite oxide of this example was a lithium iron composite oxide whose crystal structure was an orthorhombic olivine structure. From the composition analysis, it was found that the composition of the lithium iron composite oxide of this example was represented by the composition formula LiFePO ₄ .

  Moreover, the photograph which image | photographed the lithium iron complex oxide of a present Example with the scanning electron microscope (SEM) in FIG. 2 is shown. From the photograph of FIG. 2, it was confirmed that the lithium iron composite oxide of this example was composed of substantially single particles, and the average particle size of the particles was about 0.8 μm.

<Lithium iron composite oxide of comparative example>
In the production of the lithium iron composite oxide of the above example, it was produced in the same manner as the lithium iron composite oxide of the example except that the firing temperature was changed to 870 ° C. The obtained lithium iron composite oxide was used as a comparative example of lithium iron composite oxide.

An X-ray diffraction pattern of the lithium iron composite oxide of this comparative example is shown in FIG. From the pattern of FIG. 3, it was confirmed that the lithium iron composite oxide of this comparative example was a lithium iron composite oxide whose crystal structure was an orthorhombic olivine structure. From the composition analysis, it was found that the composition of the lithium iron composite oxide of this comparative example is represented by the composition formula LiFePO ₄ .

  Moreover, the photograph which image | photographed the lithium iron complex oxide of this comparative example with the scanning electron microscope (SEM) in FIG. 4 is shown. From the photograph of FIG. 4, it was confirmed that the lithium iron composite oxide of this comparative example was composed of substantially single particles, and the average particle size of the particles was about 5 μm.

<Lithium secondary battery>
A lithium secondary battery was fabricated using the lithium iron composite oxides of the above examples and comparative examples as the positive electrode active material. In the positive electrode, first, 70 parts by weight of each lithium iron composite oxide serving as a positive electrode active material is mixed with 25 parts by weight of carbon black as a conductive material and 5 parts by weight of polyvinylidene fluoride as a binder. An appropriate amount of N-methyl-2-pyrrolidone was added to prepare a paste-like positive electrode mixture. Next, the paste-like positive electrode mixture was applied to an aluminum foil current collector and pressed to make the thickness of the positive electrode mixture 45 μm, and then punched into a disk shape having a diameter of 15 mmφ to obtain a positive electrode.

  The negative electrode made to oppose used metallic lithium as an active material. Metal lithium was formed into a sheet shape and pressed onto a nickel current collector network to a thickness of 100 μm, and then punched into a disk shape having a diameter of 17 mmφ to form a negative electrode.

The separator uses a polyethylene microporous membrane, and the nonaqueous electrolytic solution impregnated in the separator is obtained by dissolving LiPF ₆ at a concentration of 1M in a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 3: 7. Was used. The positive electrode and the negative electrode were opposed to each other through a separator, and an appropriate amount of the non-aqueous electrolyte was injected and impregnated, and then stored in a coin-type battery case to produce a lithium secondary battery.

  The lithium secondary battery using the lithium iron composite oxide of the example as the positive electrode active material was used as the lithium secondary battery of the example, and similarly, the lithium secondary battery using the lithium iron composite oxide of the comparative example as the positive electrode active material was used. The secondary battery was a lithium secondary battery of a comparative example.

<Charge / discharge cycle test>
A charge / discharge cycle test was performed on each of the lithium secondary batteries of the above Examples and Comparative Examples. The charge / discharge test was performed under a temperature condition of 20 ° C., changing the current density every 5 cycles, for a total of 30 cycles. The conditions of 1 to 6 charge / discharge tests will be described below with 5 cycles having the same current density as one charge / discharge test.

In the first charge / discharge test, charging was performed at a constant current with a current density of 0.25 mA / cm ^{2 up to} a charging upper limit voltage of 4.0 V, and then a lower limit discharge voltage of 3.0 V at a constant current of 0.25 mA / cm ^2. The charging / discharging for discharging until 1 cycle was assumed to be 1 cycle, and this cycle was repeated 5 cycles. And the discharge capacity was measured about each lithium secondary battery for every cycle.

Hereinafter, the second and sixth charge / discharge tests were performed in the same manner as the first test, except that the current density during charge / discharge in the first test was changed. Similarly, the discharge capacity of each lithium secondary battery was measured for each cycle. That is, in the second test, was charged at 0.25 mA / cm ^2, and discharged at 0.5 mA / cm ^2, the third test, was charged at 0.5 mA / cm ^2, 0.5 mA / cm ² In the fourth test, the battery was charged at 0.5 mA / cm ² , discharged at 1.0 mA / cm ² , and in the fifth test, charged at 1.0 mA / cm ² , and 1.0 mA / cm ² was charged. discharged at cm ^2, and in the sixth test, the same as the first test, was charged at 0.25 mA / cm ^2, and discharged at 0.25 mA / cm ^2.

  Next, for the lithium secondary battery of the example, the above six charge / discharge tests were performed by changing only the temperature conditions. That is, under the high temperature environment of 60 ° C., which is regarded as the upper limit of the actual use temperature range of the lithium secondary battery, the other conditions are the same as in the case of 20 ° C., and the charge / discharge test of 30 times in total is performed 6 times. Went. And discharge capacity was measured for every cycle.

<Evaluation of lithium secondary battery>
About each lithium secondary battery of an Example and a comparative example, the discharge capacity per 1g of positive electrode active materials, ie, active material discharge capacity, was calculated | required from the discharge capacity measured by the said charging / discharging test, and the result was shown in FIG. Shown in Here, FIG. 5 shows the active material discharge capacity for each cycle of the lithium secondary battery of the example. The ♦ marks indicate those at 20 ° C., and the ◯ marks indicate those at 60 ° C. Moreover, FIG. 6 shows the active material discharge capacity | capacitance for every cycle of the lithium secondary battery of the comparative example in 20 degreeC.

As is clear from FIG. 5, the lithium secondary batteries of the examples had an active material discharge capacity of 105 mAh / g when discharged at 20 ° C. and a current density of 0.25 mA / cm ² at the beginning of the cycle. A large active material discharge capacity exceeding 100 mAh / g was obtained. This value is about three times that of the lithium secondary battery of the comparative example shown in FIG. In addition, the active material discharge capacity value at 60 ° C. at the same current density was about 130 mAh / g, and a larger active material discharge capacity was obtained. It is considered that ions are absorbed and desorbed smoothly. Therefore, it was confirmed that a lithium secondary battery using a lithium iron composite oxide having an average particle size of 1 μm or less as a positive electrode active material has a large active material discharge capacity.

  In the lithium secondary battery of the example, the charge / discharge current density increases and the active material discharge capacity gradually decreases. This is also the case with the lithium secondary battery of the comparative example, but it is considered that when charging / discharging at a large current density, a large load is applied to the active material, and the internal resistance increases.

  However, in FIG. 5, the active material discharge capacity in the sixth charge / discharge test (26 cycles to 30 cycles) in the same manner as the first current density is the first charge / discharge test (1 cycle to 5 cycles). The value is almost equal to the active material discharge capacity at. That is, the large active material discharge capacity is maintained even when the cycle is repeated. This is not limited to the test at 20 ° C., and the same applies to the test at 60 ° C. Therefore, a lithium secondary battery using a lithium iron composite oxide having an average particle size of 1 μm or less as a positive electrode active material has little capacity reduction even when the cycle is repeated, and is particularly excellent in cycle characteristics even at high temperatures. It was confirmed that it would be a battery.

  Since the average particle size of the lithium iron composite oxide of the present invention is as small as 1 μm or less, the surface area involved in the lithium ion occlusion / desorption reaction in the positive electrode increases, and the lithium ion occlusion / desorption occurs. Therefore, the lithium iron composite oxide has a large active material discharge capacity and little cycle deterioration.

  Moreover, according to the manufacturing method of the lithium iron complex oxide of this invention, the said lithium iron complex oxide can be manufactured simply, and also by using the lithium iron complex oxide of this invention as a positive electrode active material. Therefore, the lithium secondary battery is inexpensive, has a large discharge capacity, and has excellent cycle characteristics.

It is a X-ray-diffraction pattern of the lithium iron complex oxide of an Example baked at 620 degreeC and having an average particle diameter of 1 micrometer or less. It is a SEM photograph of the lithium iron complex oxide of an Example. It is an X-ray-diffraction pattern of the lithium iron complex oxide of the comparative example which baked at 870 degreeC and whose average particle diameter exceeds 1 micrometer. It is a SEM photograph of the lithium iron complex oxide of a comparative example. It is a graph which shows the active material discharge capacity for every cycle of the lithium secondary battery of an Example. It is a graph which shows the active material discharge capacity for every cycle of the lithium secondary battery of a comparative example.

Claims (14)

In a lithium secondary battery comprising a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and an electrolyte that moves lithium ions between the positive electrode and the negative electrode,
The positive electrode includes a positive electrode active material, a conductive material, and a binder,
The positive electrode active material is an olivine-structure lithium iron composite oxide having a basic composition of LiFePO _4, and is produced by crushing a fired body obtained by firing a mixture of a lithium source, an iron source, and a phosphorus source. Made of an olivine-structure lithium iron composite oxide having an average value of the particle size of the particles present substantially independently of 0.2 μm or more and 0.8 μm or less ,
An active material discharge capacity at 60 ° C. when discharged at a current density of 0.25 mA / cm ² is 100 mAh / g or more. In a lithium secondary battery comprising a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and an electrolyte that moves lithium ions between the positive electrode and the negative electrode,
The positive electrode includes a positive electrode active material, a conductive material, and a binder,
The positive electrode active material is an olivine-structure lithium iron composite oxide having a basic composition of LiFePO _4, and is produced by crushing a fired body obtained by firing a mixture of a lithium source, an iron source, and a phosphorus source. Made of an olivine-structure lithium iron composite oxide having an average value of the particle size of the particles present substantially independently of 0.2 μm or more and 0.8 μm or less ,
A lithium secondary battery, wherein an active material discharge capacity at an initial cycle at 20 ° C. is 100 mAh / g or more when discharged at a current density of 0.25 mA / cm ² . The positive active material, a lithium secondary battery according to claim 1 or 2 which is orthorhombic system. The conductive material is a lithium secondary battery according to any one of claims 1 to 3, carbon black. The positive electrode, a lithium secondary battery according to any one of claims 1-4 in which pressurized after coated on the current collector. The positive active material, a lithium secondary battery according to any one of claims 1 to 5 mixture obtained by mixing a lithium source and an iron source and phosphorus source is obtained by firing at a firing temperature of 700 ° C. or less. The lithium secondary battery according to claim 6 , wherein the firing temperature is 670 ° C. or lower. The lithium secondary battery according to claim 6 or 7 , wherein the firing temperature is 600 ° C or higher. The lithium secondary battery according to claim 6 or 7 , wherein the firing temperature is 620 ° C or higher. The lithium secondary battery according to any one of claims 6 to 9 , wherein the positive electrode active material is fired in an inert atmosphere or a reducing atmosphere. The lithium source, lithium secondary battery according to any one of claims 1 to 10 is _Li 2 CO _3. Wherein the iron source _is a lithium secondary battery according to any one of claims 1 to 11 is _{FeC 2 O 4 · 2H 2 O} . The phosphorus source is a lithium secondary battery according to any one of claims 1 to 12 is a phosphorus-containing ammonium salt. The lithium secondary battery according to claim 13 , wherein the phosphorus-containing ammonium salt is NH ₄ H ₂ PO ₄ .