JP2010238603A - Lithium iron fluorophosphate solid solution positive electrode active material powder, manufacturing method therefor, and lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium iron fluorophosphate solid solution powder manufactured inexpensively and easily and having a high discharge capacity in a high current load as a secondary battery, and to provide the secondary battery using the same. <P>SOLUTION: A carbon content is 0.5-5 wt.%, and a primary particle size is 50-200 nm, in this lithium iron fluorophosphate solid solution powder Na<SB>2-a</SB>Li<SB>a</SB>Fe<SB>1-b-c</SB>Mg<SB>b</SB>Co<SB>c</SB>P<SB>1-d</SB>B<SB>d</SB>O<SB>4</SB>F (1.5<a≤2, 0.025≤b+c≤0.1, 0≤d≤0.1) positive electrode active material containing carbon. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  Low-cost, easily manufactured and high-capacity discharge capacity at high current load when used as a secondary battery, carbon-containing lithium iron fluorophosphate solid solution positive electrode active material powder, manufacturing method, and lithium ion secondary Provide the next battery.

  In recent years, electronic devices such as AV devices and personal computers, and power tools such as electric tools have been rapidly becoming portable and cordless, and secondary batteries having high output and high energy density have been used as power sources for driving these devices. The demand is high. In recent years, in consideration of the global environment, hybrid vehicles and electric vehicles have been developed and put into practical use, and there is an increasing demand for lithium ion secondary batteries having excellent charge / discharge repetition characteristics as large-scale applications. Under such circumstances, a lithium ion secondary battery having advantages such as a large charge / discharge capacity and high safety has attracted attention.

Recently, as a positive electrode active material useful for a high energy type lithium ion secondary battery having a voltage of 3.5 V or more, orthorhombic A ₂ MPO ₄ F (A is at least one of Li or Na, M Has been reported as a battery having a high charge / discharge capacity. However, these materials have few reported examples and a sufficient capacity cannot be taken out, so that improvement in characteristics is required (Non-Patent Documents 1 and 2).

That is, orthorhombic Li ₂ MPO ₄ F is composed of fluorine ions, a strong phosphoric acid tetrahedral skeleton, an oxygen octahedron centered on a divalent transition metal that contributes to redox, and lithium ions that are current carriers. Composed. Because of this crystal structure, the crystal structure is stable even by repeating the charge / discharge reaction, and the characteristics are less likely to deteriorate compared to other lithium ion cathode materials even if charge / discharge is repeated. In addition, the movement path of lithium ions is two-dimensional, and high load current characteristics are expected to be improved. On the other hand, there is a drawback that it has a high electric resistance due to a small number of free electrons. In order to solve this problem, orthorhombic lithium iron fluorophosphate Li ₂ FePO ₄ F is coated with carbon (Non-patent Document 2).

The orthorhombic lithium iron fluoride phosphate Li ₂ FePO ₄ F, like LiFePO ₄ having an olivine structure, improves the charge / discharge characteristics at a higher current load as the primary particle size of the powder is smaller, In addition, it is expected that the charge / discharge characteristics are improved by increasing the sites where electrode reaction can occur by sufficiently reducing the electric resistance of the active material surface, and the charge / discharge characteristics are improved by substituting different elements.

That is, as a positive electrode active material powder for a non-aqueous electrolyte secondary battery, an orthorhombic dissimilar element substitution type lithium iron phosphate solid solution Li ₂ FePO ₄ F having a high capacity and a small electric resistance is used as an environmental load. Are required to be produced in a small industrial way.

Conventionally, various improvements have been made to improve various properties of Li ₂ MPO ₄ F (M is a transition metal). For example, Li ₂ MPO ₄ F (Patent Document 1, Non-Patent Document 1) using Fe, Mn, Ni, Cu, Ti, Co as the transition metal M is melted at a rapid temperature rise of 1000 ° C./hr and Li _2. A method of producing MPO ₄ F (Patent Document 2), a method of synthesizing Na ₂ FePO ₄ F by solid phase reaction, and ion-exchanging Na with Li in a solution (Patent Document 3, Non-Patent Document 2), etc. are known. ing.

JP 2003-229126 A JP 2007-73360 A US Patent Application Publication No. 2008/0153002

S. Okada et al. Power. Sources, 2005, Vol. 146, p. 565-569. B. L. Ellis et al., Nature Materials, 2007, Vol. 148, p. 749-753.

  However, as a positive electrode active material for a non-aqueous electrolyte secondary battery, there is very little impurity crystal phase, and an inexpensive and low environmental load manufacturing method for obtaining orthorhombic lithium iron phosphate phosphate solid solution powder with low electrical resistance. It is the most demanded now, but it has not been established yet.

That is, the techniques described in Patent Documents 1 to 3 and Non-Patent Documents 1 and 2 are composed of an orthorhombic lithium iron fluoride phosphate solid solution Li ₂ FePO ₄ F having excellent high current load characteristics. The positive electrode active material powder is not industrially obtained.

In addition, the techniques described in Patent Documents 1 to 3 and Non-Patent Documents 1 and 2 do not mention the influence of the addition of different elements to Li ₂ FePO ₄ F on the crystal structure.

Accordingly, the present invention is a lithium iron phosphate containing carbon solid solution _{Na 2-a Li a Fe 1} -b-c Mg b Co c P 1-d B d O 4 F (1.5 <a ≦ 2,0. 025 ≦ b + c ≦ 0.1, 0 ≦ d ≦ 0.1), and an industrial product with a small environmental load for obtaining the positive electrode active material powder. It is a technical problem to establish a method and to provide a lithium ion secondary battery excellent in high current load characteristics.

  The technical problem can be achieved by the present invention as follows.

That is, the present invention, lithium fluoride iron phosphate containing carbon solid solution _{Na 2-a Li a Fe 1} -b-c Mg b Co c P 1-d B d O 4 F (1.5 <a ≦ 2, 0.025.ltoreq.b + c.ltoreq.0.1, 0.ltoreq.d.ltoreq.0.1) The positive electrode active material powder has a carbon content of 0.5 to 5% by weight and a primary particle size of 50 to 200 nm. It is the positive electrode active material powder to perform (Invention 1).

Further, the present invention is the positive electrode active material powder according to the first aspect of the present invention, wherein the electric resistivity of the compression-molded body is 10 ³ Ω · cm or less (the second aspect of the present invention).

In addition, the present invention also relates to a sodium fluorophosphate solid solution Na ₂ Fe _1-bc Mg _b Co _c P _1-d B _d O ₄ F containing carbon (0.025 ≦ b + c ≦ 0.1, 0 ≦ d ≦ 0.1) After the preparation, the sodium iron phosphate solid solution is dispersed in a solution, and then Na in the sodium iron fluoride solid solution is subjected to an ion exchange reaction with Li. It is a manufacturing method of the positive electrode active material powder of description (invention 3).

  Further, the present invention is a lithium ion secondary battery of a non-aqueous solvent electrolyte containing the positive electrode active material powder according to the first or second aspect of the present invention (Invention 4).

  Since the positive electrode active material powder according to the present invention has a defect structure due to substitution of different elements, the movement of electrons and Li ions is facilitated. Moreover, the positive electrode active material powder according to the present invention can improve the output density and energy density per volume. Therefore, the secondary battery using the positive electrode active material powder according to the present invention is excellent in high current load characteristics. In addition, the method for producing a positive electrode active material according to the present invention is suitable for industrial production because it can be produced at low cost and with low environmental impact.

It is a flowchart of the manufacturing method of the lithium iron phosphate solid solution powder in this invention. It is a secondary electron image by a scanning electron microscope _{Na 2 Fe 0.95 Mg 0.05 PO 4} F containing carbon obtained in Example 1. A _{Na 2 Fe 0.95 Mg 0.05 PO 4} F Rietveld analysis results of X-ray diffraction pattern of including carbon obtained in Example 1. 2 is a secondary electron image of the positive electrode active material powder obtained in Example 1 by a scanning electron microscope. In an Example and a comparative example, it is a lattice constant ratio of the sodium fluorophosphate solid solution containing carbon obtained by the solid-phase reaction. 3 shows the discharge characteristics of Example 3.

  The configuration of the present invention will be described in more detail as follows.

  First, the positive electrode active material powder according to the present invention will be described.

The composition of the positive electrode active material powder according to the present _{invention, Na 2-a Li a Fe} 1-b-c Mg b Co c P 1-d B d O 4 F (1.5 <a ≦ 2,0.025 ≦ b + c ≦ 0.1, 0 ≦ d ≦ 0.1), and it is sufficient that at least one of Mg and Co is included. When crystals forming the orthorhombic system are greatly deviated from the stoichiometric ratio, a heterogeneous phase is likely to be formed. In some cases, grain growth is promoted during firing, and high performance battery characteristics cannot be obtained.
The value of a is 1.5 <a ≦ 2, preferably 1.65 ≦ a ≦ 2, and more preferably 1.8 ≦ a ≦ 1.95. This is because when the amount is 1.5 or less, the amount of Na movement increases, which may deteriorate battery characteristics.
The value of b + c is 0.025 ≦ b + c ≦ 0.1, and preferably 0.03 ≦ b + c ≦ 0.08. This is because the effect of substituting different elements hardly appears in the battery characteristics when it is less than 0.025, and deteriorates the battery characteristics when it exceeds 0.1.
Further, in the positive electrode active material powder according to the present invention, it is considered that electrons are hopped and conducted through Fe sites, and by replacing P sites with B, electrons can also conduct hopping conduction at P sites, and battery characteristics are also improved. I think it can be improved. If the value of d exceeds 0.1, a heterogeneous phase is likely to be generated, and the battery characteristics are deteriorated.

  The amount of carbon contained in the positive electrode active material powder according to the present invention is 0.5 to 5% by weight. If it is less than 0.5% by weight, the electrical resistance of the powder cannot be improved, and if it exceeds 5% by weight, the electrical resistance cannot be effectively reduced. Preferably it is 1.5 to 3 weight%.

  The primary particle diameter of the positive electrode active material powder according to the present invention is 50 to 200 nm. If it is less than 50 nm, it is difficult to coat the carbon, the density of the molded body also decreases, and the energy density per volume of the secondary battery also decreases. On the other hand, when it exceeds 200 nm, it is difficult to obtain the capacity of the secondary battery at a high load current because it makes it difficult to move Li in the particles. More preferably, it is 70-120 nm.

The electrical resistivity of the compression molded body of the positive electrode active material powder according to the present invention is preferably 10 ³ Ω · cm or less. When it exceeds 10 ³ Ω · cm, a load is applied to the mixing with the conductive agent when producing the positive electrode sheet. More preferably, it is 10 ² Ω · cm or less.

  Next, a method for producing the positive electrode active material according to the present invention will be described.

The positive electrode active material powder according to the present invention comprises carbon-containing sodium iron fluoride phosphate solid solution Na ₂ Fe _1-bc Mg _b Co _c P _1-d B _d O ₄ F (0.025 ≦ b + c ≦ 0.1,0 ≦ d ≦ 0.1) After the preparation, the sodium fluorophosphate solid solution is dispersed in a solution, and then Na in the sodium fluorophosphate solid solution is ion-exchanged with Li. .

  The sodium fluorophosphate solid solution in the present invention can be produced by a solid phase reaction.

As a raw material of the sodium fluorophosphate solid solution in the present invention, Na source is NaOH, NaHCO ₃ , Na ₂ CO ₃ , Fe raw material is FeC ₂ O ₄ .nH ₂ O, Fe ₃ O ₄ , α-FeOOH, As P raw materials, H ₃ PO ₄ , (NH ₄ ) H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ , NaH ₂ PO ₄ , Na ₂ HPO ₄ , F sources as HF, NaF, (NH ₄ ) F, etc. Is used. Also, as a carbon source, sugars that can be pyrolyzed at low temperature such as sucrose, fatty acids such as stearic acid that easily hydrophobize the material to be crushed, hydrophilic polymers such as PVA that are easily adsorbed on the particle surface of the calcined precursor, Hydrophobic polymers such as polyethylene that easily leave voids in the aggregated active material particles are used.

In order to perform low-temperature firing in the solid phase reaction, carbon black having high conductivity can be mixed as a carbon source. Examples of usable carbon black include acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) and ketjen black (manufactured by Lion Corporation). Thereby, the compression molded body of the obtained positive electrode active material powder satisfies the electrical resistivity of 10 ³ Ω · cm or less and exhibits high performance secondary battery characteristics even at low temperature firing of 400 to 500 ° C.

Further, as the different element additive, Mg (OH) ₂ , MgCO ₃ , Mg source, CoC ₂ O ₄ .nH ₂ O, Co ₃ O ₄ , CoOOH as Co source, H ₃ BO ₃ as B source, B ₂ O ₃ or the like is used.

  The particle size of the raw material to be used is desirably a submicron size, and it is preferable to adjust the particle size by wet synthesis or pulverization.

  Examples of the raw material mixing and pulverizing apparatus include a Henschel mixer, a rakai machine, a high speed mixer, a ball mill, a vibration mill, and a medium stirring mill.

  As firing conditions for the solid phase reaction, it is desirable to perform firing for 1 to 10 hours at a temperature of 250 to 750 ° C. in an inert gas atmosphere having an oxygen concentration of 0.1% or less or a reducing gas atmosphere.

  It is also desirable to perform pulverization after the solid-phase reaction by calcination, add a carbon source, and perform calcination again under the above conditions.

Examples of the firing apparatus include a gas flow type box muffle furnace, a gas flow type rotary furnace, and a fluidized heat treatment furnace. As the inert gas, N ₂ , Ar, H ₂ O, CO ₂ or a mixed gas thereof is used. As the reducing gas, H ₂ , CO, or a mixed gas of these gases and the inert gas is used.

In order to produce the sodium fluorophosphate solid solution containing carbon in the present invention, since Fe ³⁺ contained in the transition metal Fe raw material is changed to Fe ²⁺ by an added carbon source or a reducing gas, the oxygen concentration is 0. It is necessary to perform firing in an atmosphere of 1% or less. In order to complete the solid-phase reaction and form a graphite phase with high electron conductivity from a carbon source, it is preferable to perform heat treatment at 400 to 800 ° C. for several hours.

In the present invention, during the heat treatment, the local gas concentration distribution may affect the quality due to the generation of water vapor and the generation of oxidizing gas accompanying the precursor Fe ³⁺ reduction. Therefore, it is also possible to perform the heat treatment (main firing) again by performing the so-called calcination, again adding the carbon source-containing material, pulverizing and mixing. At that time, the calcining temperature is preferably as low as about 250 to 500 ° C., and the main baking temperature is preferably as high as 400 to 750 ° C. The order of carbon source content addition, pulverization / mixing, and operation during calcination and main calcination is not particularly limited.

  The elemental composition ratio hardly changes before and after the heat treatment except for carbon C, and the obtained sodium iron fluoride phosphate solid solution has the same composition ratio as before firing. Carbon C may be reduced to less than 50% by reduction heat treatment of the transition metal. The amount of carbon C remaining under each firing condition is measured in advance, and the amount of carbon contained in the positive electrode active material powder after firing is 0. It is necessary to adjust so that it may become 5 to 5 weight%.

  The solid solution of sodium iron phosphate in the present invention has a lattice constant ratio (b / a) between the a-axis and b-axis of 2.647 to 2.652, and a lattice constant ratio (c / a) between the a-axis and c-axis. Is preferably 2.250 to 2.254.

  Then, Na of the sodium fluorophosphate solid solution containing carbon synthesized by the solid phase reaction is subjected to an ion exchange reaction with Li to obtain a positive electrode active material powder of lithium fluorophosphate solid solution. A solvent capable of dissolving a Li salt such as acetonitrile, ethanol, or pure water is used as a solvent, and the solid solution of sodium iron phosphate is dispersed in a solvent in which a Li raw material such as LiBr, LiCl, or LiOH is dissolved. The reaction is carried out at a temperature of 50 to 150 ° C. for several hours, and the Na salt precipitated in the solvent is removed by washing and drying.

  Next, a positive electrode sheet and a nonaqueous electrolyte secondary battery using the positive electrode active material powder according to the present invention will be described.

When a positive electrode sheet is produced using the positive electrode active material according to the present invention, a conductive agent and a binder are added and mixed according to a conventional method. As the conductive agent, acetylene black, carbon black, graphite and the like are preferable, and as the binder, polytetrafluoroethylene, polyvinylidene fluoride and the like are preferable. As the solvent, for example, N-methyl-pyrrolidone is used, and the positive electrode active material sieved to 75 μm or less and the slurry containing the additive are kneaded until they become honey. The obtained slurry is applied onto the current collector with a doctor blade having a groove of 25 μm to 500 μm. The coating speed is about 60 cm / sec, and an Al foil of about 20 μm is usually used as a current collector. In order to remove the solvent and soften the binder, drying is performed at 80 to 120 ° C. in a non-oxidizing atmosphere of Fe ²⁺ . The sheet is subjected to a calender roll treatment so as to have a pressure of 1 to 3 t / cm ² . In the sheet forming step, there is a possibility that an oxidation reaction of Fe ²⁺ to Fe ³⁺ may occur at room temperature. Therefore, it is desirable to carry out in a non-oxidizing atmosphere as much as possible.

The density of the positive electrode made of the positive electrode active material, carbon, and binder on the current collector of the obtained positive electrode sheet is 1.8 g / cc or more. In the positive electrode sheet according to the present invention, the density of the compression molded body of the positive electrode active material is as high as 2.0 g / cc or more, and the electrical resistivity of the compression molded body of the positive electrode active material is as low as 10 ³ Ω · cm or less. Therefore, the amount of carbon added during sheet preparation can be suppressed, and as a result, a positive electrode sheet having a high density can be obtained.

  As the negative electrode active material, lithium metal, lithium / aluminum alloy, lithium / tin alloy, graphite or the like can be used. In the case of a metal or alloy, a rolled sheet is used. The negative electrode sheet is produced by the doctor blade method.

  In addition to the combination of ethylene carbonate and diethyl carbonate, an organic solvent containing at least one of carbonates such as propylene carbonate and dimethyl carbonate and ethers such as dimethoxyethane can be used as the solvent for the electrolytic solution.

  Further, as the electrolyte, in addition to lithium hexafluorophosphate, at least one lithium salt such as lithium perchlorate and lithium tetrafluoroborate can be dissolved in the above solvent and used.

<Action>
Since the positive electrode active material powder according to the present invention has a defect structure due to substitution of different elements, the movement of electrons and Li ions is facilitated. Moreover, the positive electrode active material powder according to the present invention can improve the output density and energy density per volume. Further, since the positive electrode active material powder according to the present invention controls the substitution of different elements, the impurity crystal phase can be extremely reduced. Therefore, the present inventor presumes that the positive electrode active material powder according to the present invention is a positive electrode active material powder that is excellent in high current load characteristics and has little decrease in capacity even after repeated charge and discharge. In addition, since a solid phase reaction by firing and an ion exchange reaction in a solution are used, it can be manufactured at low cost and with a small environmental load.

  A typical embodiment of the present invention is as follows.

  A scanning electron microscope SEM is used for evaluating the primary particles of the positive electrode active material powder according to the present invention, and a transmission electron microscope TEM is used for observing the coating state of carbon.

  In order to identify the crystal phase of the sodium fluorophosphate solid solution in the present invention, an X-ray diffractometer RINT-2500 [manufactured by Rigaku Corporation] was used, and the tube Cu-Kα, voltage 40 kV, current 300 mA, 2θ range 10 Measured at a scan speed such that the main peak intensity is 8000 to 12000 counts. The lattice constant ratio as 60 was quantified by the Rietveld method. The analysis was performed under the assumption that there was no anisotropic expansion of the crystallites using Rietan 2000 as the analysis software, using the TCH pseudo-void function as the profile function, and using a method such as Finger for asymmetry of the function.

  The amount of remaining carbon was quantified by burning it in an oxygen stream in a combustion furnace using EMIA-820 [manufactured by Horiba Ltd.].

  Elements added in addition to Li, Fe, P main elements and carbon were measured using an emission plasma analyzer ICAP-6500 [manufactured by Thermo Fisher Scientific Co., Ltd.].

The compression-molded body density was compressed to 1.5 t / cm ² with a jig of 13 mmφ, the powder electrical resistance was measured by the two-terminal method, and the electrical resistivity was calculated from the weight and volume.

Using the positive electrode active material according to the present invention, the secondary battery characteristics of a CR2032-type coin cell were evaluated.
The carbon used as the positive electrode conductive auxiliary agent was acetylene black, the binder was polyvinylidene fluoride having a weight average molecular weight of about 630,000 (manufactured by Kureha Corporation), and the positive electrode active material: carbon: binder = 88: 4. : Weighed to 8 (wt%) and dissolved in N-methylpyrrolidone (manufactured by Kanto Chemical Co., Inc.).

Positive electrode sheet pressed to 1 t / cm ² , punched to 2 cm ² , 0.15 mm Li negative electrode punched to 17 mmφ, separator (Celgard # 2400) to 19 mmφ, ethylene carbonate EC and carbonic acid dissolved in 1 mol / l LiPF ₆ A CR22032 type coin cell (manufactured by Hosen Co., Ltd.) was prepared using an electrolytic solution (manufactured by Kishida Chemical Co., Ltd.) mixed with diethyl DEC (volume ratio 3: 7).

  The battery characteristics of the secondary battery manufactured using the positive electrode active material according to the present invention are measured at room temperature, fully charged, and then evaluated by changing the discharge rate.

C / 20 corresponding to the discharge rate is a current value fixed so that a current of a theoretical capacity of 146 mAh / g of an orthorhombic lithium iron phosphate solid solution Li ₂ FePO ₄ F positive electrode active material flows in 20 hours. Further, 5C is a current value fixed so that a current having a theoretical capacity of the positive electrode active material flows in 1/5 hour. A higher C coefficient means higher current load characteristics.

  Although the current value at the time of charging is not particularly limited, in the present invention, charging was sufficiently performed using a constant current value of C / 10. Moreover, the voltage range at the time of charge and discharge is not particularly limited, but in the present invention, the voltage range was 2.0 to 4.5V.

[Example 1]
NaHCO ₃ , FeC ₂ O ₄ .2H ₂ O, Mg (OH) ₂ , (NH ₄ ) H ₂ PO ₄ and NaF are weighed to 10 g of Na ₂ Fe _0.95 Mg _0.05 PO ₄ F and calcined. Next, 0.45 g of PVA powder was weighed so that the amount of residual carbon was about 1 wt%, and mixed and pulverized in a ethanol solvent in a ball mill for 3 days. The obtained raw material slurry was dried in a container, pulverized in an agate mortar, and sieved to 75 μm or less. After calcining in N ₂ at 400 ° C. for 2 hours, 5% by weight of polyethylene powder was added to the calcined product, and then pulverized again with a ball mill for 1 day. The pulverized product was calcined in N ₂ at 700 ° C. for 2 hours, cooled, and sieved to 75 μm or less. FIG. 2 shows an SEM photograph of the obtained Na ₂ Fe _0.95 Mg _0.05 PO ₄ F powder containing carbon, and FIG. 3 shows the Rietveld analysis result of the powder X-ray diffraction pattern. The Rietveld analysis was a reliable result with a low reliability factor Rwp = 7.41% and S = 1.66.

The obtained carbon-containing Na ₂ Fe _0.95 Mg _0.05 PO ₄ F powder was dispersed in an acetonitrile solution containing LiBr, subjected to an ion exchange reaction at 120 ° C. for 50 minutes, washed with a methanol solution, dried at 60 ° C., An orthorhombic Na _0.2 Li _1.8 Fe _0.95 Mg _0.05 PO ₄ F positive electrode active material powder containing carbon was obtained (FIG. 4 shows an SEM photograph).

  Further, it was confirmed by TEM observation that carbon sufficiently covered the particle surface.

Next, using the obtained Na _0.2 Li _1.8 Fe _0.95 Mg _0.05 PO ₄ F positive electrode active material powder containing carbon, electrode slurry was applied onto the Al foil current collector with a doctor blade having a gap of 250 μm. After the sheet drying, punched 1t / cm ² pressurized to 2 cm ^2, and a positive electrode.

  Table 1 shows experimental conditions, powder characteristics, and battery characteristics of the above and the following examples and comparative examples.

[Examples 2 to 5]
Using lithium powder as a carbon source and changing the kind of different elements to be added and the charge ratio of each element, the same method as in Example 1 was used to obtain a lithium iron fluorophosphate solid solution positive electrode active material containing carbon. It was.

[Comparative Examples 1-2]
A lithium fluoride fluorophosphate solid solution positive electrode active material containing carbon was obtained in the same manner as in Example 1 except that polyethylene powder was used as the carbon source and no different element was added.

[Comparative Examples 3 to 7]
Using lithium powder as a carbon source and changing the kind of different elements to be added and the charge ratio of each element, the same method as in Example 1 was used to obtain a lithium iron fluorophosphate solid solution positive electrode active material containing carbon. It was.

  In the battery characteristics of Examples shown in Table 1, it was found that the positive electrode active material according to the present invention satisfied high battery characteristics at a high load current of 1 C or more.

  In the battery characteristics of the comparative example described in Table 1, the discharge capacity at 5C was particularly low because the heterogeneous phases were precipitated and the carbon coating was insufficient.

  FIG. 5 shows the lattice constant ratio of orthorhombic sodium iron fluoride phosphate solid solution containing carbon obtained by solid phase reaction in the examples and comparative examples shown in Table 1. As a trend, it has been found that excellent battery characteristics are exhibited in a region where the lattice constant does not change much even when different elements are substituted.

  Finally, the secondary battery discharge characteristics of Example 3 shown in Table 1 are shown in FIG. In the measurement, C / 10, C1C,..., 10C were discharged in this order. It was confirmed that the voltage drop was small even at a high rate and the high load current characteristics were good.

  From the above results, the lithium iron phosphate solid solution positive electrode active material powder substituted with a different element according to the present invention is obtained by a low cost and low environmental impact manufacturing method, and the secondary battery using the positive electrode active material powder is It was confirmed that the high current load characteristics were excellent.

  The present invention uses a lithium iron phosphate solid solution powder produced by a low-cost and low environmental load production method as a secondary battery positive electrode active material, so that the energy density per volume is high and high current load characteristics are also achieved. A nonaqueous solvent secondary battery capable of obtaining a high capacity can be obtained.

Claims (4)

Lithium fluoride iron phosphate solid solution containing carbon _{Na 2-a Li a Fe 1} -b-c Mg b Co c P 1-d B d O 4 F (1.5 <a ≦ 2,0.025 ≦ b + c ≦ 0.1, 0 ≦ d ≦ 0.1) A positive electrode active material powder having a carbon content of 0.5 to 5% by weight and a primary particle size of 50 to 200 nm. The positive electrode active material powder according to claim 1, wherein the compression-molded body has an electrical resistivity of 10 ³ Ω · cm or less. Carbon-containing sodium iron fluorophosphate solid solution Na ₂ Fe _1- bc Mg _b Co _c P _1-d B _d O ₄ F (0.025 ≦ b + c ≦ 0.1, 0 ≦ d ≦ 0. 1) The positive electrode active material powder according to claim 1 or 2, wherein after the production, the solid solution of sodium iron phosphate is dispersed in a solution, and then Na in the solid solution of sodium iron phosphate is subjected to an ion exchange reaction with Li. Manufacturing method. The lithium ion secondary battery of the nonaqueous solvent electrolyte containing the positive electrode active material powder of Claim 1 or 2.