JP4186507B2 - Carbon-containing lithium iron composite oxide for positive electrode active material of lithium secondary battery and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a carbon-containing lithium iron composite oxide for a positive electrode active material capable of constituting a lithium secondary battery utilizing a lithium occlusion / desorption phenomenon, and also relates to a method for producing the same.
[0002]
[Prior art]
With the miniaturization of personal computers, video cameras, mobile phones, etc., in the fields of information-related equipment and communication equipment, lithium secondary batteries have been put into practical use because of their high energy density as the power source used for these equipment. It has become widespread. On the other hand, in the field of automobiles, the development of electric vehicles has been urgently caused by environmental problems and resource problems, and lithium secondary batteries have been studied as power sources for the electric vehicles.
[0003]
At present, as a positive electrode active material of a lithium secondary battery, LiCoO can be used as a secondary battery of 4V class.₂, LiNiO₂, LiMn₂O_FourLithium transition metal composite oxides such as LiCoO are particularly preferred.₂In addition to being easy to synthesize and relatively easy to handle, LiCoO is excellent in charge / discharge cycle characteristics.₂Rechargeable batteries using as the positive electrode active material are the mainstream.
[0004]
However, cobalt and other resources are small, and LiCoO₂In the secondary battery using the positive electrode active material as a positive electrode active material, it is difficult to cope with future mass production and enlargement in view of the battery for electric vehicles, and the price 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.
[0005]
As one of the attempts, for example, JP-A-9-134725 discloses LiFePO having an olivine structure._FourLiFeVO_FourA lithium secondary battery using the above as a positive electrode active material is shown.
[0006]
[Problems to be solved by the invention]
However, as a result of a further trial by the present inventors, a lithium secondary battery using the olivine-structure lithium iron composite oxide as described in the above-mentioned publication as a positive electrode active material could not obtain a sufficient capacity. . That is, when charging / discharging at a practical charge / discharge density, it is considered that an active material discharge capacity of 90 mAh / g or more is necessary, 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. Further, the LiFePO_FourIt has been found that lithium secondary batteries using the above as the positive electrode active material have a large so-called cycle deterioration in which the active material discharge capacity is reduced by repeated charge and discharge.
[0007]
The present invention has been made in view of the above-described circumstances, and constitutes a lithium secondary battery having a good active cycle discharge capacity, which has a large active material discharge capacity and can maintain the capacity even after repeated charge and discharge. An object of the present invention is to provide a lithium iron composite oxide for a positive electrode active material that can be used.
[0008]
[Means for Solving the Problems]
  The carbon-containing lithium iron composite oxide for a positive electrode active material of a lithium secondary battery according to the present invention has a basic composition of LiFePO₄The olivine-structured lithium iron composite oxide particlesThe carbon material fine particles have an average particle size of 5 nm or more and 100 nm or less, the molar ratio between carbon atoms and lithium atoms of the carbon material fine particles is 0.02 to 0.2, and the lithium iron composite oxide has a composition Formula LiFe _1-x M _x PO ₄ (M is represented by at least one selected from Mn, Mg, Ni, Co, Cu, Zn, and Ge; 0.02 ≦ x ≦ 0.2)It is characterized by that.
[0009]
The lithium iron composite oxide serving as the base of the carbon-containing lithium iron composite oxide of the present invention has an orthorhombic olivine structure in crystal structure, and the space group is represented by Pmnb. The olivine structure is based on the hexagonal close-packed packing of oxygen, in which phosphorus is located at the tetrahedral site and both lithium and iron are located at the octahedral site. Then, by incorporating the carbon material fine particles into the lithium iron composite oxide particles, the lithium iron composite oxide and the carbon material fine particles are combined. Compounding is a state in which carbon material fine particles are dispersed in lithium iron composite oxide particles, and nanometer-order carbon material fine particles are dispersed in lithium iron composite oxide particles. A nanocomposite of a so-called lithium iron composite oxide and carbon material fine particles is realized. As described above, since the carbon fine particles are combined with the lithium iron composite oxide particles, more conductive paths are formed, and the internal resistance is reduced.
[0010]
As will be described in detail later, the carbon material fine particles are compounded by adding the carbon material fine particles to the raw material mixture when synthesizing the lithium iron composite oxide. By adding the carbon fine particles, the reducing atmosphere during the synthesis of the lithium iron composite oxide is maintained, and Fe²⁺To Fe³⁺Oxidation is suppressed, and grain growth and sintering of the lithium iron composite oxide are also suppressed.
[0011]
For example, if the basic composition is LiFePO_FourWhen the lithium iron composite oxide is used as the positive electrode active material, Fe²⁺To Fe³⁺Oxidation to is essential. Therefore, during the synthesis of lithium iron composite oxide, Fe²⁺Suppression of the oxidation of the secondary battery leads to an increase in the capacity of the secondary battery. Further, grain growth and sintering of the lithium iron composite oxide are suppressed, and the particle diameter of the synthesized carbon-containing lithium iron composite oxide particles becomes relatively small. As a result, the lithium ion diffusion distance is shortened, and the lithium ion storage / desorption reaction is activated, thereby increasing the capacity of the secondary battery.
[0012]
Therefore, the carbon-containing lithium iron composite oxide of the present invention can constitute a lithium secondary battery with a good active characteristic and a so-called cycle characteristic in which the capacity can be maintained even after repeated charge and discharge. It becomes a positive electrode active material.
[0013]
  Moreover, the carbon-containing lithium iron composite oxide of the present invention is not particularly limited in its production method, but can be more easily produced according to the production method of the present invention. The method for producing a carbon-containing lithium iron composite oxide of the present invention includes a raw material mixing step of mixing a lithium compound, an iron compound, a phosphorus-containing ammonium salt, and carbon material fine particles to obtain a mixture, And a baking step of baking at a temperature of 750 ° C. to 750 ° C.Is preferred.
[0014]
Since the carbon material fine particles are mixed and fired with each raw material, the carbon material fine particles are taken into the lithium iron composite oxide and dispersed substantially uniformly. Further, the carbon-containing lithium iron composite oxide can be obtained by an extremely simple process of mixing and firing the respective raw materials. Therefore, the method for producing a carbon-containing lithium iron composite oxide of the present invention is a method for easily producing the carbon-containing lithium iron composite oxide having a large active material capacity and little cycle deterioration.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Below, those embodiments are described in detail about the carbon content lithium iron complex oxide for lithium secondary battery positive electrode active materials of the present invention, and its manufacturing method. Moreover, the lithium secondary battery which is a utilization form of the carbon containing lithium iron complex oxide of this invention is also demonstrated.
[0016]
<Carbon-containing lithium iron composite oxide>
The carbon-containing lithium iron composite oxide for a positive electrode active material of a lithium secondary battery according to the present invention has a basic composition of LiFePO_FourThe olivine structure lithium iron composite oxide particles are combined with carbon material fine particles.
[0017]
“The basic composition is assumed to be” means not only the composition represented by the composition formula, but, for example, a part of the Fe site in the crystal structure is Co, Ni, Mn, Mg, Cu, Zn, Ge. It is meant to include those having compositions substituted with other elements such as. Furthermore, it means that not only the stoichiometric composition but also a non-stoichiometric composition in which some elements are deficient.
[0018]
For example, Mn, Mg, Ni, Co, Cu, Zn, and Ge have an ionic radius substantially the same as that of Fe, and are oxidized and reduced at a potential different from that of Fe. Therefore, the crystal structure of the lithium iron composite oxide can be stabilized by replacing a part of the Fe site with one or more of these elements. Accordingly, the lithium iron composite oxide has a composition formula LiFe in which part of the Fe site is replaced with another element M._1-xM_xPO_FourIt is desirable that M is represented by at least one selected from Mn, Mg, Ni, Co, Cu, Zn, and Ge. In particular, the substitution element M is desirably Mn because it is abundant in terms of resources and is inexpensive.
[0019]
When a part of the Fe site is substituted with at least one element selected from Mn, Mg, Ni, Co, Cu, Zn, and Ge, the substitution ratio, that is, the value of x in the above composition formula is 0. .02 ≦ x ≦ 0.2 is desirable. This is because when x is less than 0.02, the substitution effect is small and the crystal structure cannot be sufficiently stabilized, and when x exceeds 0.2, the substitution rate is large, so that the initial discharge. This is because sufficient capacity cannot be obtained. In consideration of constituting a battery having a larger capacity and good cycle characteristics, it is more desirable that the range is 0.05 ≦ x ≦ 0.15.
[0020]
Further, as described above, the lithium iron composite oxide that is the base of the carbon-containing lithium iron composite oxide of the present invention has an orthorhombic olivine structure as described above, and its space group is Pmnb. It is represented by That is, it is based on the hexagonal close-packed packing of oxygen, and has a structure in which phosphorus is located at the tetrahedral site and both lithium and iron are located at the octahedral site.
[0021]
The carbon material fine particles to be compounded with the lithium iron composite oxide are not particularly limited in the type of the carbon material. Examples thereof include carbonaceous materials such as natural graphite, spherical or fibrous artificial graphite and the like, graphitizable carbon such as coke, and non-graphitizable carbon such as a phenol resin fired body. These fine particles can be used alone or in admixture of two or more. Among these, it is desirable to use carbon black when considering the dispersibility in the lithium iron composite oxide and the effect of improving the conductivity. In this case, hydrocarbon gas may be burned to form fine particles.
[0022]
The average particle diameter of the carbon material fine particles is not particularly limited, but is preferably 5 nm or more and 100 nm or less from the viewpoint of being compounded with lithium iron composite oxide particles. This is because when the average particle size is less than 5 nm, the reactivity when synthesizing the lithium iron composite oxide is lower than that in the above range, and when it exceeds 100 nm, it is within the above range. This is because the dispersibility is lower than that of the material and the effect of improving the conductivity is small.
[0023]
In addition, the molar ratio between the carbon atom and the lithium atom in the carbon fine particles, that is, the molar ratio between the carbon atom contained in the carbon-containing lithium iron composite oxide and the lithium atom contained in the carbon-containing lithium iron composite oxide is 0.02-0.2 is desirable. When the amount is less than 0.02, the amount of carbon atoms is small, and therefore the above-described effect of the composite of the carbon fine particles is small as compared with those within the above range. This is because the reactivity when synthesizing the lithium iron composite oxide is reduced and the active material discharge capacity is reduced as compared with those within the above range.
[0024]
The average particle diameter of the particles of the carbon-containing lithium iron composite oxide of the present invention is not particularly limited. In particular, when taking into account that a sufficient active material discharge capacity is obtained when charging / discharging at a practical charge / discharge density is performed, the lithium ion occlusion / desorption reaction is performed smoothly. It is desirable to do. Further, from the viewpoint of facilitating the production of the electrode, it is desirable that the thickness is 0.2 μm or more.
[0025]
The average particle diameter of the carbon-containing lithium iron composite oxide particles is an average value of the particle diameters of the respective particles, and each particle diameter is measured using, for example, a scanning electron microscope (SEM). be able to. Specifically, the longest and shortest diameters of carbon-containing lithium iron composite oxide particles are measured using a scanning electron microscope (SEM), and the average value of these two values is used as the particle diameter of the one particle. Adopt it.
[0026]
<Method for producing carbon-containing lithium iron composite oxide>
The carbon-containing lithium iron composite oxide of the present invention is not particularly limited in its production method, but can be more easily produced according to the production method of the present invention. The method for producing a carbon-containing 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.
[0027]
(1) Raw material mixing process
The raw material mixing step in the method for producing a carbon-containing lithium iron composite oxide of the present invention is a step of obtaining a mixture by mixing a lithium compound, an iron compound, a phosphorus-containing ammonium salt, and carbon substance fine particles.
[0028]
Lithium compounds that serve as lithium sources include Li₂CO_Three, Li (OH), Li (OH) · H₂O, LiNO_ThreeEtc. can be used. In particular, because of its low hygroscopicity, Li₂CO_ThreeIt is desirable to use
[0029]
As an iron compound as an iron source, a compound in which the valence of iron is divalent is FeC.₂O_Four・ 2H₂O, FeCl₂Etc. can be used. In particular, FeC because of the low corrosiveness of the gas generated during firing.₂O_Four・ 2H₂It is desirable to use O.
[0030]
Examples of phosphorus-containing ammonium salts that serve as phosphorus sources include NH._FourH₂PO_Four, (NH_Four)₂HPO_FourEtc. can be used. In particular, NH is relatively low in hygroscopicity and has a low generation amount of corrosive gas._FourH₂PO_FourIt is desirable to use
[0031]
In addition, since it does not generate | occur | produce ammonia, it can also be set as a lithium source and a phosphorus source using the compound which does not contain an ammonia salt. In that case, LiH such that Li: P is contained 1: 1 instead of lithium compound and phosphorus-containing ammonium salt.₂PO_FourOr the like.
[0032]
As the carbon material fine particles, the carbon material fine particles described above may be used. In particular, when considering the dispersibility in the lithium iron composite oxide and the effect of improving the conductivity, it is desirable to use carbon black. When a part of the Fe site is replaced with another element such as Co, Ni, Mn, Mg, Cu, Zn, or Ge, a compound containing the replacement element may be mixed similarly to the above compound. As a compound containing a substitution element, for example, MnCO_ThreeMgO, NiO, CoO, CuO, ZnO, GeO₂Etc. can be used.
[0033]
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 carbon containing lithium iron complex oxide to be manufactured.
[0034]
(2) Firing process
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 750 ° 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.
[0035]
The firing temperature is 600 ° C. or higher and 750 ° 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, if the temperature exceeds 750 ° C., the lithium iron composite oxide particles grow and the particle diameter increases. In particular, in view of improving input / output characteristics and securing a high capacity, it is desirable to set the temperature to 620 ° C. or higher and 700 ° C. or lower. Note that the firing time may be a time sufficient to complete the firing, and is usually performed for about 6 hours.
[0036]
<Lithium secondary battery>
An embodiment of a lithium secondary battery that is a form of utilization of the carbon-containing lithium iron composite oxide of the present invention will be described. Generally, a lithium secondary battery includes a positive electrode and a negative electrode that occlude and release lithium ions, a separator that is 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, Consists of The secondary battery of this embodiment may follow this configuration. The following description will be given for each of these components.
[0037]
For the positive electrode, a conductive material and a binder are mixed with a positive electrode active material capable of inserting and extracting lithium ions, and an appropriate solvent is added as necessary to form a paste-like positive electrode mixture, such as a metal such as aluminum. It is formed by applying and drying on the surface of the current collector made of foil and then increasing the active material density by pressing.
[0038]
In this embodiment, the above carbon-containing lithium iron composite oxide is used as the positive electrode active material. The carbon-containing lithium iron composite oxide of the present invention includes various carbon-containing lithium iron composite oxides depending on the composition, particle diameter, type of carbon fine particles, and the like. Therefore, one of them may be used as the positive electrode active material, or a mixture of two or more may be used. Furthermore, it is also possible to employ a configuration in which the carbon-containing lithium iron composite oxide of the present invention and a known positive electrode active material are mixed to form a positive electrode active material.
[0039]
The conductive material used for the positive electrode is for ensuring the electrical conductivity of the positive electrode active material layer, and is a mixture of one or more carbon material powders such as carbon black, acetylene black, and graphite. Can be used. The binder plays a role of anchoring the active material particles, and a fluorine-containing resin 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.
[0040]
The negative electrode opposed to the positive electrode can be formed by pressing metal lithium, a lithium alloy, or the like into a sheet shape or a sheet-like shape to a current collector network such as nickel or stainless steel. However, in consideration of the precipitation of dendrites and the like, in order to obtain a lithium secondary battery excellent in safety, it is desirable to use a negative electrode using a carbon material capable of inserting and extracting lithium as an active material. Examples of the carbon material that can be used include natural or artificial graphite, a fired organic compound such as a phenol resin, and a powdery material such as coke. In this case, a binder is mixed with the negative electrode active material, and a negative electrode mixture made into a paste by adding an appropriate solvent is applied to the surface of a metal foil current collector such as copper and dried. When the carbon material is a negative electrode active material, as with the positive electrode, a fluorine-containing resin such as polyvinylidene fluoride is used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone is used as the solvent. it can.
[0041]
The separator sandwiched between the positive electrode and the negative electrode retains the electrolytic solution while isolating the positive electrode and the negative electrode and allows ions to pass through. A thin microporous film such as polyethylene or polypropylene can be used.
[0042]
The non-aqueous electrolyte is obtained by dissolving an electrolyte in an organic solvent. Examples of the organic solvent include aprotic organic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile, dimethoxyethane, and tetrahydrofuran. , Dioxolane, methylene chloride or the like, or a mixture of two or more thereof can be used. Further, as the electrolyte to be dissolved, LiI and LiClO that generate lithium ions when dissolved are used._Four, LiAsF₆, LiBF_Four, LiPF₆Etc. can be used.
[0043]
Instead of the separator and the non-aqueous electrolyte, a high molecular weight polymer such as polyethylene oxide and LiClO are used._FourAnd LiN (CF_ThreeSO₂)₂It is also possible to use a solid polymer electrolyte using a lithium salt such as a gel electrolyte, or a gel electrolyte obtained by trapping the non-aqueous electrolyte in a solid polymer matrix such as polyacrylonitrile.
[0044]
Although it is a lithium secondary battery comprised from the above, the shape can be made into various things, such as a coin type, a laminated type, and a cylindrical type. Regardless of which shape is adopted, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the electrode body is electrically connected between the positive electrode and the negative electrode and the positive electrode terminal and the negative electrode terminal. Can be sealed in a battery case together with a non-aqueous electrolyte to complete the battery.
[0045]
<Acceptance of other embodiments>
As mentioned above, although embodiment of the carbon containing lithium iron complex oxide of this invention and its manufacturing method was described, embodiment mentioned above is only one Embodiment, The carbon containing lithium iron complex oxide of this invention, and its manufacturing method In addition to the above-described embodiment, it can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art.
[0046]
【Example】
Based on the said embodiment, the various carbon containing lithium iron complex oxide from which the content rate of a carbon atom differs was manufactured. And the lithium secondary battery was produced using the produced various carbon containing lithium iron complex oxide as a positive electrode active material, and the battery characteristic was evaluated by measuring those active material charging / discharging capacities. Here, the active material charge / discharge capacity means the charge / discharge capacity per unit weight of the positive electrode active material excluding the carbon material fine particles. This will be described in detail below.
[0047]
<Production of carbon-containing lithium iron composite oxide>
Carbon-containing lithium iron composite oxides (LiFe with different carbon atom content ratios)_1-xMn_xPO_Four: C_y, X = 0.15, y = 0, 0.02, 0.05, 0.1). LiH as a lithium and phosphorus source₂PO_FourAs an iron source₂O_Four・ 2H₂O as a substitution element source MnCO_ThreeCarbon black was used as the carbon material fine particles. Carbon black having an average particle diameter of 24 nm was used. First, FeC₂O_Four・ 2H₂O and MnCO_ThreeWere mixed so that the molar ratio of Fe: Mn was 0.85: 0.15. This FeC₂O_Four・ 2H₂O and MnCO_ThreeLiH₂PO_FourAnd carbon black were mixed such that the molar ratio of Li: (Fe + Mn): C was 1: 1: 0, 0.02, 0.05, 0.1. An automatic mortar was used for mixing. Each of these mixtures was fired at 650 ° C. for 6 hours in an argon stream. And each obtained carbon containing lithium iron complex oxide was crushed and it was set as the powdery carbon containing lithium iron complex oxide used as a positive electrode active material. The average particle size of the carbon-containing lithium iron composite oxide was 1 μm.
[0048]
<Production of lithium secondary battery>
A lithium secondary battery was fabricated using each of the carbon-containing lithium iron composite oxides as a positive electrode active material. First, the positive electrode was prepared by mixing 15 parts by weight of carbon black as a conductive material and 8 parts by weight of polyvinylidene fluoride as a binder with 77 parts by weight of each carbon-containing lithium iron composite oxide serving as a positive electrode active material, An appropriate amount of N-methyl-2-pyrrolidone was added as a solvent to prepare a paste-like positive electrode mixture. Subsequently, this paste-like positive electrode mixture was applied to both surfaces of an aluminum foil current collector having a thickness of 20 μm, dried, and then compressed by a roll press to produce a sheet-like positive electrode. This sheet-like positive electrode was cut into a size of 54 mm × 450 mm and used.
[0049]
As the negative electrode to be opposed, graphitized mesocarbon microbeads (graphitized MCMB) were used as an active material. First, 92 parts by weight of graphitized MCMB as an active material was mixed with 8 parts by weight of polyvinylidene fluoride as a binder, an appropriate amount of N-methyl-2-pyrrolidone was added as a solvent, and a paste-like negative electrode composite was added. Then, the paste-like negative electrode mixture was applied to both sides of a 10 μm thick copper foil current collector, dried, and then compressed by a roll press to prepare a sheet-like negative electrode. This sheet-like negative electrode was cut into a size of 56 mm × 500 mm and used.
[0050]
Each of the positive electrode and the negative electrode was wound with a polyethylene separator having a thickness of 25 μm and a width of 58 mm interposed therebetween to form a roll-shaped electrode body. Then, the electrode body was inserted into a 18650 type cylindrical battery case (outer diameter 18 mmφ, length 65 mm), a non-aqueous electrolyte was injected, and the battery case was sealed to produce a cylindrical lithium secondary battery. . The non-aqueous electrolyte is LiPF in a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 3: 7.₆Was dissolved at a concentration of 1M.
[0051]
<Evaluation of battery characteristics>
(1) Measurement of charge / discharge capacity and calculation of charge / discharge efficiency
The active material charge capacity in each produced lithium secondary battery was measured. Under a temperature condition of 20 ° C., a current density of 0.2 mA / cm²The secondary battery was charged to a charge upper limit voltage of 4.0 V at a constant current of ≦ 5 to measure the charge capacity of each secondary battery. From the value of the charge capacity, the charge capacity per 1 g of the positive electrode active material excluding the carbon material fine particles, that is, the active material charge capacity was determined. Then, current density 0.2mA / cm²The secondary battery was discharged at a constant current of up to a discharge lower limit voltage of 2.6 V, and the discharge capacity of each secondary battery was measured. The active material discharge capacity was obtained from the value of the discharge capacity. The charge / discharge efficiency (%) is calculated from the values of the active material charge capacity and the active material discharge capacity in each secondary battery by the formula [charge / discharge efficiency (%) = active material discharge capacity / active material charge capacity × 100]. did. The values of the active material charge capacity and charge / discharge efficiency of each secondary battery are shown in FIG. FIG. 1 also shows the values of a lithium secondary battery using as a positive electrode active material a lithium iron composite oxide produced in the same manner as described above, with carbon fine particles not compounded and the substitution ratio with Mn being 0.1. Show.
[0052]
As can be seen from FIG. 1, the active material charge capacity increased as the composite ratio of the carbon substance fine particles, that is, the carbon content ratio increased. The charge / discharge efficiency also increased as the carbon content increased. This is because the carbon fine particles were mixed during the synthesis of the lithium iron composite oxide.²⁺Since the oxidation of iron was suppressed, Fe³⁺It is thought that the capacity at the time of charging, in which oxidation to benzene is essential, increased. In addition, the growth and sintering of lithium iron composite oxide was suppressed, and more conductive paths were formed due to the presence of fine carbon material particles. It is thought that the discharge capacity of the battery also increased and the charge / discharge efficiency was improved. In addition, although the one where the substitution ratio of the Fe site by Mn was large, both the charge capacity and the charge / discharge efficiency increased. This is considered to be because the crystal structure was stabilized by substituting with Fe. From the above, it was confirmed that the lithium secondary battery using the carbon-containing lithium iron composite oxide of the present invention as the positive electrode active material has a large active material capacity.
[0053]
(2) Charge / discharge cycle test and evaluation of cycle characteristics
Next, among the produced secondary batteries, the secondary battery using the carbon-containing lithium iron composite oxide of the present invention in which the composite ratio of the carbon substance fine particles is 0.02, lithium iron not composited with the carbon substance fine particles A charge / discharge cycle test was performed on the secondary battery using the composite oxide. The charge / discharge cycle test is performed at a current density of 1.0 mA / cm under a temperature condition of 60 ° C., which is regarded as the upper limit of the actual use temperature range of the battery.²The battery is charged to a charge upper limit voltage of 4.0 V with a constant current of 1.0 mA / cm²The charge / discharge for discharging to a discharge lower limit voltage of 2.6 V at a constant current of 1 cycle was defined as one cycle, and this cycle was performed for a total of 500 cycles. Then, for each cycle, the discharge capacity of each lithium secondary battery was measured to obtain the active material discharge capacity. FIG. 2 shows changes in the active material discharge capacity of each secondary battery in the charge / discharge cycle test. In addition, the change of the active material discharge capacity of the lithium secondary battery which used the lithium iron complex oxide whose substitution ratio by Mn is 0.1 as a positive electrode active material is also shown similarly.
[0054]
From FIG. 2, the active material discharge capacity of each secondary battery decreases as it goes through the cycle. However, the secondary battery using the carbon-containing lithium iron composite oxide of the present invention has an initial active material discharge capacity value as compared with the secondary battery using the lithium iron composite oxide not composited with carbon. As well as a large decrease in the active material discharge capacity. That is, the initial active material discharge capacity of the secondary battery using the carbon-containing lithium iron composite oxide of the present invention was 88 mAh / g, while the capacity retention after 500 cycles was a high value of about 84%, An initial active material discharge capacity of a secondary battery using a lithium iron composite oxide not compounded with carbon material fine particles was 80 mAh / g, and a capacity retention rate was about 77%. Further, the larger the substitution ratio of Fe sites with Mn, the larger the initial active material discharge capacity value, and the lower the active material discharge capacity was smaller. This is presumably because the crystal structure was stabilized by substitution with Fe as described above.
[0055]
Therefore, the lithium secondary battery using the carbon-containing lithium iron composite oxide of the present invention as the positive electrode active material has a large active material discharge capacity and cycle characteristics with little decrease in capacity even when the cycle is repeated at a high temperature. It was confirmed that the lithium secondary battery was excellent.
[0056]
【The invention's effect】
The carbon-containing lithium iron composite oxide of the present invention is obtained by combining carbon material fine particles with lithium iron composite oxide particles having an olivine structure. When a lithium secondary battery is constructed using the carbon-containing lithium iron composite oxide of the present invention as a positive electrode active material, lithium having a large active material capacity and excellent cycle characteristics with little decrease in capacity even after repeated cycles A secondary battery can be obtained. Moreover, according to the method for producing a carbon-containing lithium iron composite oxide of the present invention, the carbon-containing lithium iron composite oxide having a large active material capacity and little cycle deterioration can be easily produced.
[Brief description of the drawings]
FIG. 1 shows values of active material charge capacity and charge / discharge efficiency of a secondary battery using lithium iron composite oxides having different carbon content ratios as a positive electrode active material.
FIG. 2 shows a change in active material discharge capacity of each secondary battery in a charge / discharge cycle test.

Claims (3)

Ri the basic composition to the particles of the olivine-type lithium iron composite oxides to LiFePO ₄ carbon material particles greens and composite,
The carbon material fine particles have an average particle size of 5 nm to 100 nm,
The carbon material fine particles have a molar ratio of carbon atoms to lithium atoms of 0.02 to 0.2,
The lithium iron composite oxide has a composition formula LiFe _1-x M _x PO ₄ (M is at least one selected from Mn, Mg, Ni, Co, Cu, Zn, Ge; 0.02 ≦ x ≦ 0.2 A carbon-containing lithium iron composite oxide for a positive electrode active material of a lithium secondary battery, which is represented by : 2. The carbon-containing lithium iron composite oxide for a lithium secondary battery positive electrode active material according to claim 1, wherein the average particle size of the particles is 0.2 μm or more and 5 μm or less. A method of manufacturing a basic composition of LiFePO ₄ to olivine structure lithium-iron composite particles in the carbon material particles of oxide is complexed lithium secondary battery positive electrode active material for a carbon-containing lithium-iron composite oxide,
A raw material mixing step of mixing a lithium compound, an iron compound, a phosphorus-containing ammonium salt, and carbon material fine particles to obtain a mixture;
A firing step of firing the mixture at a temperature of 600 ° C. or higher and 750 ° C. or lower;
The manufacturing method of the carbon containing lithium iron complex oxide for lithium secondary battery positive electrode active materials of Claim 1 which comprises this.

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