JP5272756B2 - Positive electrode active material for lithium secondary battery, positive electrode for lithium secondary battery and lithium secondary battery, and production method thereof - Google Patents

Description

  The present invention relates to a manganese phosphate-based positive electrode active material for a lithium secondary battery, a positive electrode for a lithium secondary battery, a lithium secondary battery, and a method for producing the same.

In recent years, non-aqueous electrolyte secondary batteries represented by lithium secondary batteries with high energy density, low self-discharge and good cycle characteristics as power sources for portable devices such as mobile phones and notebook computers and electric vehicles Is attracting attention. The current mainstream of lithium secondary batteries is for consumer use, mainly for mobile phones of 2 Ah or less. Many positive electrode active materials for lithium secondary batteries have been proposed. The most commonly known positive electrode active materials are lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide whose operating voltage is around 4V. object is a (LiNiO _2), or lithium manganese oxide having a spinel structure lithium-containing transition metal oxide to basic configuration (LiMn ₂ O ₄₎ or the like. Among these, lithium cobalt oxide is widely adopted as a positive electrode active material for small-capacity lithium secondary batteries up to a battery capacity of 2 Ah because of its excellent charge / discharge characteristics and energy density.

  However, when considering the development of non-aqueous electrolyte batteries for future medium-sized and large-sized, especially industrial applications where large demand is expected, safety is very important, so the specifications for current small batteries are not always sufficient. It cannot be said. One of the factors is the thermal instability of the positive electrode active material, and various countermeasures have been taken, but it is still not sufficient. In industrial applications, it is necessary to assume that the battery is used in a high-temperature environment that is not used in small consumer products. In such a high temperature environment, not only conventional non-aqueous electrolyte secondary batteries, but also nickel-cadmium batteries and lead batteries have a very short life, and there is no conventional battery that satisfies user requirements. Further, although the capacitor can only be used in this temperature range, it has a low energy density and does not satisfy the user's requirements in this respect, and a battery having a high temperature and a long life and a high energy density is required.

Therefore, recently, lithium iron phosphate (LiFePO ₄ ) having an olivine structure with excellent thermal stability has attracted attention. Since LiFePO ₄ having this olivine structure is covalently bonded to phosphorus and oxygen, it does not release oxygen even at high temperatures, and can be used as a battery active material to dramatically improve battery safety. It is assumed that it can be done. Furthermore, since the insertion / extraction of Li ions is performed at around 3.4 V, the amount of side reaction that occurs when used for the positive electrode of the battery can be suppressed, so that the life of the battery can be expected to be extended.

However, this lithium iron phosphate (LiFePO ₄ ) has an energy density lower than that of the 4V positive electrode active material because the insertion and release of Li ions are performed near 3.4V.

On the other hand, manganese phosphate (LiMnPO ₄ ) absorbs and releases Li ions at around 4.0 V, and has a larger theoretical capacity than LiFePO ₄ . However, even if the conventional LiMnPO ₄ is actually used as the positive electrode active material, a battery having a sufficient discharge capacity could not be obtained.

In Patent Document 1, “Example 1: Synthesis of LiMnPO ₄ by spray drying method”, “Lithium source, manganese source and phosphoric acid source were weighed in separate containers, respectively, and dissolved in distilled water. After mixing these three types of solutions, they were introduced into a spray drying apparatus and spray-dried to prepare raw material powders, which were pyrolyzed at 350 ° C. for 1 hour in an inert atmosphere and then inactivated. It was baked for 8 hours at 550 ° C. in an active atmosphere ”(paragraph 0027). In addition, “Example 3: Synthesis of LiMnPO ₄ / C complex”, “Lithium source, manganese source, phosphoric acid source and malic acid (carbon source) were weighed in separate containers respectively and dissolved in distilled water. Aqueous ammonia was added to the malic acid solution to adjust the pH to 5 to 6. After mixing these four kinds of solutions, they were introduced into a spray dryer and spray-dried to prepare a raw material powder. The obtained raw material powder was thermally decomposed at 350 ° C. for 2 hours under an inert atmosphere and then fired at 500 ° C. for 6 hours under an inert atmosphere. ”(Paragraph 0033).

Patent Document 2 states that “a precipitation step of obtaining a precipitate of manganese hydroxide (Mn (OH) x) by adding a precipitant to a Mn source solution in which a Mn source is dissolved; A reduction step of dispersing in a solvent to obtain a reduced dispersion solution; an addition step of adding an Li source solution and a P source solution to the reduced dispersion solution to obtain an added dispersion solution; and a pH of the added dispersion solution of 3 to 6. A method for producing LiMnPO ₄ characterized by comprising a pH adjusting step for obtaining a pH adjusted dispersion solution prepared within a range, and a synthesis step for reacting the pH adjusted dispersion solution under heating and pressurizing conditions ”(claim) 1) is described, and in the examples, the pH of the aqueous solution containing the raw material is 5.3 (Example 1), 5.2 (Example 5), 5.0 (Example 6) or 5.3 ( After adjusting to Example 7), the reaction is performed and the resulting precipitate is dried. It is described that give the LiMnPO ₄ with Rukoto.

  However, neither Patent Documents 1 and 2 describe nor suggest that a precipitate is obtained from an alkaline solution.

Non-Patent Document 1, entitled “Optimization of hydrothermal synthesis conditions and evaluation of electrochemical properties of olivine-type LiFePO ₄ ”, in the case of synthesizing a sample from a mixed solution containing an Fe source, a Li source and a phosphorus source, It was described that the experiment was performed by changing the pH of the solution. The smaller the pH, the greater the relative intensity of the diffraction line based on the (200) plane, and the initial discharge capacity of the sample synthesized from the solution with pH = 5.15 is It is described that the initial discharge capacity of the sample synthesized from the solution with pH = 8.99 was only about 120 mAh · g ⁻¹ , whereas it was 163 mAh · g ⁻¹ . Moreover, although the sample synthesized from the solution with pH = 3.47 has the highest relative intensity of the diffraction line based on the (200) plane, the initial discharge capacity, on the contrary, was obtained only about 120 mAh · g ^−1. Has been. Non-Patent Document 1 shows an X-ray diffraction diagram of LiFePO ₄ obtained under each condition, and this is cited as FIG.

As described later, the present invention uses a precipitate obtained from an aqueous solution containing at least manganese ions, phosphate ions and lithium ions as a precursor, and when producing a lithium manganese phosphate-based active material, the aqueous solution is made alkaline. However, non-patent document 1 describes the synthesis of a lithium manganese phosphate-based active material. In the case of synthesizing LiFePO ₄ in the case of synthesizing LiFePO ₄ , when the mixed solution is alkaline, the relative intensity of the (200) plane diffraction line is reduced, and the discharge capacity is also reduced. It is described that the discharge capacity is reduced when the relative intensity of the diffraction line on the (200) plane is too high, which is the opposite of the above feature of the present invention. Therefore, it should be said that Non-Patent Document 1 relating to LiFePO ₄ has an obvious inhibiting factor in leading to the present invention relating to a lithium manganese phosphate-based active material.

  Furthermore, the present invention relates to a diffraction line intensity (2θ = 16.9 ± 0.5 ° with respect to a diffraction line intensity (a) of 2θ = 29.2 ± 0.5 ° in a powder X-ray diffraction diagram using CuKα rays ( The characteristic feature is that the ratio (b / a) of b) is b / a ≧ 1.0. The diffraction line of 2θ = 29.2 ± 0.5 ° corresponds to the (020) plane, and 2θ = The diffraction line of 16.9 ± 0.5 ° corresponds to the (200) plane. In consideration of this point and taking into account the X-ray diffraction diagram described in Non-Patent Document 1 (see FIG. 3), in the sample (b) synthesized from a solution of pH = 5.15 and having a large discharge capacity, (020) In the sample (a) in which the peak intensity of the (200) plane was clearly smaller than the peak intensity of the plane and was synthesized from a solution of pH = 3.47 and only a small discharge capacity was obtained, the peak intensity of the (020) plane On the other hand, since the peak intensity of the (200) plane is clearly large, which is the opposite of the above-described feature of the present invention, non-patent document 1 also discloses phosphoric acid. It should be apparent that there are obvious obstacles to the present invention regarding manganese lithium-based active materials. It should be noted that the sample (c) synthesized from a solution having a pH of 8.99 and having only a small discharge capacity has a smaller peak intensity on the (200) plane than the peak intensity on the (020) plane. Features are not appearing.

FIG. 10 includes charging and discharging of a battery using LiFe _0.75 Mn _0.25 PO ₄ , LiFe _0.5 Mn _0.5 PO ₄ , LiFe _0.25 Mn _0.75 PO ₄ and LiMnPO ₄ as a positive electrode active material. A curve is described, and the discharge capacity increases as the Fe ratio increases.

JP 2007-48612 A JP 2007-119304 A

48th Battery Symposium (November 2007) Abstract 2A09 (p.66) A.K.Padhi, K.S.Nanjundaswamy and J.B.Goodenough, J.Electrochem.Soc., 1997, Vol.144, No.4, page.1188-1194.

  The present invention has been made in view of the above problems, and an object thereof is to provide a manganese phosphate-based positive electrode active material for lithium secondary batteries having a large discharge capacity and a lithium secondary battery using the same. .

  The configuration and effects of the present invention are as follows. However, the action mechanism described in this specification includes estimation, and its correctness does not limit the present invention.

The present invention has the general formula _{LiMn (1-x-y)} Fe x M y PO 4 (0 ≦ x ≦ 0.5,0 ≦ y ≦ 0.1, M = Mg, Co, Cr, Ti, Y, Mo Or in a powder X-ray diffraction diagram represented by Nb) and using CuKα rays, the diffraction line intensity of 2θ = 16.9 ± 0.5 ° is higher than that of 2θ = 29.2 ± 0.5 °. This is a positive electrode active material for a lithium secondary battery, characterized in that it is larger.

Further, the present invention has the general formula _{LiMn (1-x-y)} Fe x M y PO 4 (0 ≦ x ≦ 0.5,0 ≦ y ≦ 0.1, M = Mg, Co, Cr, Ti, Y In a powder X-ray diffraction diagram using a positive electrode active material represented by Mo, Nb) and using CuKα rays, 2θ = 16.9 ± 0 than the diffraction line intensity of 2θ = 29.2 ± 0.5 °. A positive electrode for a lithium secondary battery characterized by having a diffraction line intensity of .5 ° is larger.

  Moreover, this invention is a lithium secondary battery provided with the positive electrode containing the said positive electrode active material or the said positive electrode, the negative electrode, and the nonaqueous electrolyte.

With such a configuration, the mechanism of action that can increase the discharge capacity of the manganese phosphate-based positive electrode active material is not necessarily clear, but is orthorhombic (space group Pnma) lithium manganese phosphate (LiMnPO). ₄ ) is a powder X-ray diffraction line using CuKα rays. The peak at 2θ = 16.9 ± 0.5 ° is the (200) plane, and 2θ = 29.2 ± 0.5 ° is the (020) plane. Represents each. Here, the diffraction intensity of 2θ = 16.9 ± 0.5 ° corresponding to the (200) plane is larger than the diffraction intensity of 2θ = 29.2 ± 0.5 ° corresponding to the (020) plane. This means that the crystallites of LiMnPO ₄ have orientation in the (200) plane direction rather than the (020) plane, that is, the crystal grows more in the (200) plane direction than in the (020) plane direction. It is considered that the characteristics of the present invention have an effect on the relationship between the lithium ion diffusion path direction and the lithium ion diffusion distance in the lithium manganese phosphate-based active material. The inventors speculate.

Moreover, the present invention uses a precipitate obtained from an alkaline aqueous solution containing at least manganese ions, phosphate ions and lithium ions as a precursor, and calcinates the precursor to form a general formula LiMn _(1-xy) Fe _x M. _{For a} lithium secondary battery that obtains a positive electrode active material represented by yPO ₄ (0 ≦ x ≦ 0.5, 0 ≦ y ≦ 0.1, M = Mg, Co, Cr, Ti, Y, Mo, or Nb) It is a manufacturing method of a positive electrode active material.

  The lithium manganese phosphate according to the present invention can be obtained by preparing a precursor in an alkaline aqueous solution. Examples of the alkali source include hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide, and ammonia water. Among these, ammonia water is preferable. When ammonia water is used, alkali metal components other than lithium do not remain after thermal decomposition during firing. The pH of the alkaline aqueous solution at the time of preparing the precursor is preferably 7 to 14. Especially, if pH is the range of 8.5-12, since the effect of this invention appears most effectively, it is especially preferable.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode active material for lithium secondary batteries with a large discharge capacity and a lithium secondary battery using the same can be provided.

  The synthesis process of lithium manganese phosphate used in the present invention includes a precursor preparation step obtained by reacting a precursor for use in the firing step in an aqueous solution, and a firing step for firing the precursor. The raw materials used for the aqueous solution in the precursor preparation step are not limited, but include water-soluble metal organic acid salts (divalent and trivalent) as a metal source and phosphoric acid or water-soluble phosphate as a phosphoric acid source. As the lithium source, when the water-soluble lithium salt contains a substitution element, it is preferable to use a water-soluble organic acid salt of the element. In the precursor preparation step, an alkali source is added to the mixed aqueous solution of each raw material or an aqueous solution containing the reaction precipitate to maintain the pH of the solution alkaline, and moisture is dried by a technique such as heat drying or vacuum drying. A precursor powder can be obtained. Further, in the raw material mixing process, the pH is adjusted to alkali by adding an alkali source to a solution in which raw materials other than the phosphoric acid source are mixed, and the phosphoric acid source and the alkali source are dropped simultaneously to maintain the pH. A precipitate may be formed. As the alkali source, it is possible to use the reagent as described above, but it is particularly preferable to use aqueous ammonia because the particle size of the synthesized lithium manganese phosphate tends to be small. The firing step is preferably performed in an inert atmosphere in order to prevent the formation of metal oxide, and the firing temperature is preferably 400 to 900 ° C.

  The synthesis method called the so-called baking method has been described above. As a synthesis method of lithium manganese phosphate, there is a synthesis method called a so-called hydrothermal method. The hydrothermal method is a method in which a raw material aqueous solution is placed in a sealed container and heated, and the synthesis reaction proceeds under high temperature and pressure. Although the production method of the present invention may be applied to a hydrothermal synthesis method, the effect of the present invention is apparent due to the influence of another factor that relatively small active material particles are easily synthesized in the hydrothermal synthesis method. May be observed relaxed.

It is preferable that lithium manganese phosphate is used as a positive electrode for a lithium secondary battery as a powder having an average particle size of 100 μm or less. In particular, a smaller particle size is preferable, the average particle size of the secondary particles is 0.5 to 20 μm, and the particle size of the primary particles is more preferably 1 to 500 nm. The specific surface area of the powder particles is preferably large in order to improve the high rate performance of the positive electrode, and is preferably ¹ to 100 m ² · g ⁻¹ . More preferably, it is 5-100 m < ² > * g- ¹ . In order to obtain the powder in a predetermined shape, a pulverizer or a classifier can be used. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill, a sieve, or the like can be used. At the time of pulverization, wet pulverization in which an organic solvent such as water or alcohol or hexane coexists may be used. The classification method is not particularly limited, and a sieve, an air classifier, or the like can be used dry or wet as necessary.

  The effect of the present invention can be effectively exhibited even when carbon is adhered to the surface of lithium manganese phosphate particles mechanically or by thermal decomposition of an organic substance for the purpose of supplementing the electron conductivity.

  Further, impurities may be intentionally coexisted in the synthesized lithium manganese phosphate for the purpose of improving the performance, and in such a case, the effect of the present invention is not lost.

  As the conductive agent and the binder, well-known ones can be used in a well-known prescription.

  The amount of water contained in the positive electrode containing the positive electrode active material of the present invention is preferably as small as possible, specifically less than 500 ppm.

  Moreover, the thickness of the electrode mixture layer to which the present invention is applied is preferably 20 to 500 μm in view of the balance with the energy density of the battery.

The negative electrode of the battery of the present invention is not limited in any way, but lithium metal, lithium alloy (lithium metal such as lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium-gallium, and wood alloy) Alloys), alloys capable of inserting and extracting lithium, carbon materials (eg, graphite, hard carbon, low-temperature fired carbon, amorphous carbon, etc.), metal oxides, lithium metal oxides (Li ₄ Ti ₅ O ₁₂₎ Etc.), polyphosphoric acid compounds and the like. Among these, graphite is preferable as a negative electrode material because it has an operating potential very close to that of metallic lithium and can realize charge and discharge at a high operating voltage. For example, artificial graphite and natural graphite are preferable. In particular, graphite in which the surface of the negative electrode active material particles is modified with amorphous carbon or the like is desirable because it generates less gas during charging.

  In general, the form of a non-aqueous electrolyte battery is composed of a positive electrode, a negative electrode, and a non-aqueous electrolyte containing an electrolyte salt in a non-aqueous solvent. Is provided.

  Non-aqueous solvents include cyclic carbonates such as propylene carbonate and ethylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate Chain esters such as methyl formate, methyl acetate and methyl butyrate; tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, methyl jig Examples include ethers such as lime; nitriles such as acetonitrile and benzonitrile; dioxolane or a derivative thereof; ethylene sulfide, sulfolane, sultone or a derivative thereof alone or a mixture of two or more thereof. Is limited to There is no.

Examples of the electrolyte salt include ionic compounds such as LiBF ₄ and LiPF ₆ , and these ionic compounds can be used alone or in admixture of two or more. The concentration of the electrolyte salt in the nonaqueous electrolyte, in order to ensure the non-aqueous electrolyte battery having high battery ^{characteristics,} preferably ^{0.5mol · dm -3 ~5mol · dm -3} , more preferably, 1 mol · dm ^{−3 to} 2.5 mol · dm ⁻³ .

  The present invention is particularly limited to a non-aqueous electrolyte battery among lithium secondary batteries, but the effect of the present invention is effectively exhibited even in an aqueous solution system.

  Hereinafter, the method for producing a lithium secondary battery of the present invention will be exemplified, but the present invention is not limited to the following embodiment.

Example 1
(Preparation of LiMn _0.875 Fe _0.125 PO ₄ )
Manganese acetate tetrahydrate (Mn (CH ₃ COO) ₂ .4H ₂ O), iron acetate (Fe (CH ₃ COO) ₂ ), phosphoric acid (H ₃ PO ₄ ) and lithium hydroxide monohydrate ( LiOH.H ₂ O) was measured so that the molar ratio was 0.875: 0.125: 1: 1.02. First, manganese acetate tetrahydrate and iron acetate were dissolved in purified water to 0.5 mol · dm ⁻³ . Next, an aqueous phosphoric acid solution diluted to 2.0 mol · dm ⁻³ was added dropwise while stirring the mixed solution. After the dropwise addition of phosphoric acid, lithium hydroxide monohydrate powder was added. While stirring these mixed solutions in a 60 ° C. water bath, ammonia water was continuously added as an alkali source until the pH of the solution reached 9.5. Thereafter, the temperature of the water bath was changed to 80 ° C., and stirring was continued for 2 hours. The obtained precipitation solution was vacuum-dried and then crushed in a mortar to prepare a precursor powder. The obtained precursor powder was put in an alumina sagger (outside dimension 90 × 90 × 50 mm), and nitrogen gas was circulated using an atmosphere substitution type firing furnace (a table vacuum gas substitution furnace KDF-75 manufactured by Denken). Temporary calcination was performed at a lower flow rate (1.0 L · min ⁻¹ ). The pre-baking temperature was 300 ° C., and the pre-baking time (time for maintaining the baking temperature) was 2 hours. In addition, the temperature increase rate was 5 degreeC * min < ^-1 > and the temperature-fall was natural cooling. In this way, LiMn _0.875 Fe _0.125 PO ₄ was synthesized. The obtained calcined powder was weighed with polyvinyl alcohol (degree of polymerization of about 1500) so that the mass ratio was 1: 1, and then dry-mixed with a ball mill. LiMn _0.875 Fe _0.125 PO ₄ coated with carbon was synthesized by firing at 700 ° C. for 1 hour under a nitrogen flow (1.0 L · min ⁻¹ ) in a firing furnace. This was used as a positive electrode active material. A powder X-ray diffraction diagram using CuKα rays of the synthesized LiMn _0.875 Fe _0.125 PO ₄ powder is shown in FIG. As can be seen from FIG. 1, the relationship between the diffraction line intensity (a) of 2θ = 29.2 ± 0.5 ° and the diffraction line intensity (b) of 2θ = 16.9 ± 0.5 ° is expressed as a <b The diffraction line intensity at 2θ = 16.9 ± 0.5 ° was larger than the diffraction line intensity at 2θ = 29.2 ± 0.5 °.

(Preparation of positive electrode)
It contains the positive electrode active material, acetylene black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 80: 8: 12, and N-methyl-2-pyrrolidone (NMP) as a solvent. A positive electrode paste was prepared. The positive electrode paste was applied to one side of an aluminum foil current collector having a thickness of 20 μm, dried, and then pressed to obtain a positive electrode. An aluminum positive electrode terminal was connected to the positive electrode by ultrasonic welding. In addition, the result of having performed the X-ray-diffraction measurement (XRD) using a CuK alpha ray, maintaining the surface shape of the positive electrode of Example 1 is shown in FIG. As can be seen from FIG. 4, even when X-ray diffraction measurement was performed in the state of the positive electrode plate, the diffraction line intensity (a) of 2θ = 29.2 ± 0.5 ° and 2θ = 16.9 ± 0.5 ° The relationship with the diffraction line intensity (b) is a <b, and the diffraction line intensity of 2θ = 16.9 ± 0.5 ° is more than the diffraction line intensity of 2θ = 29.2 ± 0.5 °. It was big.

(Preparation of negative electrode)
A negative electrode was prepared by pasting a lithium metal foil having a thickness of 100 μm onto a nickel foil current collector having a thickness of 10 μm. A negative electrode terminal made of nickel was connected to the negative electrode by resistance welding.

(Preparation of electrolyte)
In a mixed solvent in which ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate are mixed at a volume ratio of 6: 7: 7, LiPF ₆ which is a fluorine-containing electrolyte salt is dissolved at a concentration of 1 mol · dm ⁻³ to obtain a non-aqueous electrolyte. Was made. The amount of water in the non-aqueous electrolyte was less than 50 ppm.

(Battery assembly)
A nonaqueous electrolyte battery was assembled in a dry atmosphere with a dew point of −40 ° C. or lower. One positive electrode and one negative electrode are opposed to each other via a polypropylene separator having a thickness of 20 μm. A metal resin composite film made of polyethylene terephthalate (15 μm) / aluminum foil (50 μm) / metal adhesive polypropylene film (50 μm) was used as the outer package, and this electrode group was used as the open end of the positive electrode terminal and the negative electrode terminal. Was hermetically sealed except for the portion that would be the liquid injection hole so as to be exposed to the outside. After injecting a certain amount of nonaqueous electrolyte from the injection hole, the injection hole part was heat sealed in a reduced pressure state, and the battery according to Example 1 was assembled.

(Comparative Example 1)
(Preparation of LiMn _0.875 Fe _0.125 PO ₄ )
Manganese acetate tetrahydrate (Mn (CH ₃ COO) ₂ .4H ₂ O), iron acetate (Fe (CH ₃ COO) ₂ ), phosphoric acid (H ₃ PO ₄ ) and lithium hydroxide monohydrate ( LiOH.H ₂ O) was measured so that the molar ratio was 0.875: 0.125: 1: 1.02. First, manganese acetate tetrahydrate and iron acetate were dissolved in purified water to 0.5 mol · dm ⁻³ . Next, an aqueous phosphoric acid solution diluted to 2.0 mol · dm ⁻³ was added dropwise while stirring the mixed solution. After the dropwise addition of phosphoric acid, lithium hydroxide monohydrate powder was added. The pH at this time was 4.4. These mixed solutions were continuously stirred in an 80 ° C. water bath for 2 hours. The obtained precipitation solution was vacuum-dried and then crushed in a mortar to prepare a precursor powder. The obtained precursor powder was put in an alumina sagger (outside dimension 90 × 90 × 50 mm), and nitrogen gas was circulated using an atmosphere substitution type firing furnace (a table vacuum gas substitution furnace KDF-75 manufactured by Denken). Temporary calcination was performed at a lower flow rate (1.0 L · min ⁻¹ ). The pre-baking temperature was 300 ° C., and the pre-baking time (time for maintaining the baking temperature) was 2 hours. In addition, the temperature increase rate was 5 degreeC * min < ^-1 > and the temperature-fall was natural cooling. In this manner, a lithium iron phosphate compound LiMn _0.875 Fe _0.125 PO ₄ was synthesized. The obtained calcined powder was weighed with polyvinyl alcohol (degree of polymerization of about 1500) so that the mass ratio was 1: 1, and then dry-mixed with a ball mill. LiMn _0.875 Fe _0.125 PO ₄ coated with carbon was synthesized by firing at 700 ° C. for 1 hour under a nitrogen flow (1.0 L · min ⁻¹ ) in a firing furnace. FIG. 2 shows a powder X-ray diffraction diagram using CuKα rays of the synthesized LiMn _0.875 Fe _0.125 PO ₄ powder. As can be seen from FIG. 2, in contrast to FIG. 1, the diffraction line intensity (a) of 2θ = 29.2 ± 0.5 ° and the diffraction line intensity (b) of 2θ = 16.9 ± 0.5 ° The relationship of a> b was satisfied, and the diffraction line intensity at 2θ = 29.2 ± 0.5 ° was larger than the diffraction line intensity at 2θ = 16.9 ± 0.5 °.

  A lithium secondary battery according to Comparative Example 1 was assembled in the same manner as in Example 1 except that the above positive electrode active material was used. In addition, the result of having performed the X-ray-diffraction measurement (XRD) using a CuK alpha ray, maintaining the surface shape of the positive electrode of the comparative example 1 is shown in FIG. As can be seen from FIG. 5, even when X-ray diffraction measurement was performed in the state of the positive electrode plate, the diffraction line intensity (a) of 2θ = 29.2 ± 0.5 ° and 2θ = 16.9 ± 0.5 ° The relationship with the diffraction line intensity (b) is a> b, and the diffraction line intensity of 2θ = 29.2 ± 0.5 ° is greater than the diffraction line intensity of 2θ = 16.9 ± 0.5 °. It was big.

(Charge / discharge test)
The batteries according to Example 1 and Comparative Example 1 were subjected to a charge / discharge test in which charge / discharge of 5 cycles was performed at a temperature of 20 ° C. Here, the charging conditions are a current of 0.1 ItmA (about 10 hours rate), a voltage of 4.55 V, and a constant current and constant voltage charging of 15 hours, and the discharging conditions are a current of 0.1 ItmA (about 10 hours rate) and an end voltage. 2. A constant current discharge of 50 V was used. In this way, a nonaqueous electrolyte battery was produced. The discharge capacity obtained at the fifth cycle is shown in Table 1.

(Example 2)
(Preparation of LiMn _0.875 Fe _0.125 PO ₄ )
Manganese acetate tetrahydrate (Mn (CH ₃ COO) ₂ .4H ₂ O), iron lactate trihydrate (Fe (CH ₃ H ₅ O ₃ ) ₂ .3H ₂ O) and phosphoric acid (H ₃ PO ₄ ), Lithium hydroxide monohydrate (LiOH.H ₂ O) and ascorbic acid (C ₆ H ₈ O ₇ ) in a molar ratio of 0.875: 0.125: 1: 1.02: 0.074 We measured as follows. First, manganese acetate tetrahydrate and iron lactate trihydrate were dissolved in purified water so as to be 0.5 mol · dm ⁻³ . After ascorbic acid was added and dissolved therein, an aqueous phosphoric acid solution diluted to 2.0 mol · dm ⁻³ was added dropwise while stirring the mixed solution. After dropwise addition of phosphoric acid, a precursor solution was prepared by adding lithium hydroxide monohydrate powder. While stirring this solution in a 60 ° C. water bath, ammonia water was continuously added as an alkali source until the pH of the precursor solution reached 11.6. Thereafter, the temperature of the water bath was changed to 80 ° C., and stirring was continued for 2 hours. The obtained precipitation solution was vacuum-dried and then crushed in a mortar to prepare a precursor powder. The obtained precursor powder was put in an alumina sagger (outside dimension 90 × 90 × 50 mm), and nitrogen gas was circulated using an atmosphere substitution type firing furnace (a table vacuum gas substitution furnace KDF-75 manufactured by Denken). Temporary calcination was performed at a lower flow rate (1.0 L · min ⁻¹ ). The pre-baking temperature was 300 ° C., and the pre-baking time (time for maintaining the baking temperature) was 2 hours. In addition, the temperature increase rate was 5 degreeC * min < ^-1 > and the temperature-fall was natural cooling. In this manner, lithium manganese iron phosphate compound LiMn _0.875 Fe _0.125 PO ₄ was synthesized. The obtained calcined powder was weighed with polyvinyl alcohol (degree of polymerization of about 1500) so that the mass ratio was 1: 1, and then dry-mixed with a ball mill. LiMn _0.875 Fe _0.125 PO ₄ coated with carbon was synthesized by firing at 700 ° C. for 1 hour under a nitrogen flow (1.0 L · min ⁻¹ ) in a firing furnace. This was used as a positive electrode active material.

(Example 3)
LiMn _0.875 Fe ₀ carbon-coated in the same manner as in Example 2 except that ammonia water as an alkali source was continuously added to the precursor solution until the pH of the precursor solution reached 9.5. ₁₂₅ PO ₄ was obtained.

Example 4
LiMn _0.875 Fe ₀ coated with carbon in the same manner as in Example 2 except that ammonia water as an alkali source was continuously added to the precursor solution until the pH of the precursor solution reached 7.0. ₁₂₅ PO ₄ was obtained.

(Comparative Example 2)
LiMn _0.875 Fe _0.125 PO ₄ coated with carbon was obtained in the same manner as in Example 2 except that no alkali source was added to the precursor solution.

(Example 5)
( _{Preparation of} LiMn _0.825 Fe _0.125 Mg _0.05 PO ₄ )
Manganese acetate tetrahydrate (Mn (CH ₃ COO) ₂ .4H ₂ O), iron lactate trihydrate (Fe (CH ₃ H ₅ O ₃ ) ₂ · 3H ₂ O) and magnesium acetate tetrahydrate ( Mg (CH ₃ COO) ₂ .4H ₂ O), phosphoric acid (H ₃ PO ₄ ), lithium acetate (LiCH ₃ COO) and ascorbic acid (C ₆ H ₈ O ₇ ) in a molar ratio of 0.825: 0. It measured so that it might be set to 125: 0.05: 1: 1.02: 0.074. First, manganese acetate tetrahydrate and iron lactate trihydrate were dissolved in purified water so as to be 0.5 mol · dm ⁻³ . After ascorbic acid was added and dissolved therein, an aqueous phosphoric acid solution diluted to 2.0 mol · dm ⁻³ was added dropwise while stirring the mixed solution. After dropwise addition of phosphoric acid, a precursor solution was prepared by adding lithium acetate powder. While stirring this solution in a 60 ° C. water bath, ammonia water was continuously added as an alkali source until the pH of the precursor solution reached 9.5. Thereafter, the temperature of the water bath was changed to 80 ° C., and stirring was continued for 2 hours. The obtained precipitation solution was vacuum-dried and then crushed in a mortar to prepare a precursor powder. The obtained precursor powder was put in an alumina sagger (outside dimension 90 × 90 × 50 mm), and nitrogen gas was circulated using an atmosphere substitution type firing furnace (a table vacuum gas substitution furnace KDF-75 manufactured by Denken). Temporary calcination was performed at a lower flow rate (1.0 L · min ⁻¹ ). The pre-baking temperature was 300 ° C., and the pre-baking time (time for maintaining the baking temperature) was 2 hours. In addition, the temperature increase rate was 5 degreeC * min < ^-1 > and the temperature-fall was natural cooling. In this manner, lithium manganese iron phosphate compound LiMn _0.875 Fe _0.125 PO ₄ was synthesized. The obtained calcined powder was weighed with polyvinyl alcohol (degree of polymerization of about 1500) so that the mass ratio was 1: 1, and then dry-mixed with a ball mill. LiMn _0.825 Fe _0.125 Mg _0.05 PO ₄ coated with carbon was obtained by firing at 700 ° C. for 1 hour under a nitrogen flow (1.0 L · min ⁻¹ ) in a firing furnace.

(Comparative Example 3)
LiMn _0.825 Fe _0.125 Mg _0.05 PO ₄ coated with carbon was obtained in the same manner as in Example 5 except that no alkali source was added to the precursor solution.

(Example 6)
( _{Preparation of} LiMn _0.825 Fe _0.175 PO ₄ )
Manganese acetate tetrahydrate (Mn (CH ₃ COO) ₂ .4H ₂ O), iron lactate trihydrate (Fe (CH ₃ H ₅ O ₃ ) ₂ .3H ₂ O) and phosphoric acid (H ₃ PO ₄ ), Lithium hydroxide monohydrate (LiOH.H ₂ O) and ascorbic acid (C ₆ H ₈ O ₇ ) in a molar ratio of 0.825: 0.175: 1: 1.02: 0.074 We measured as follows. First, manganese acetate tetrahydrate and iron lactate trihydrate were dissolved in purified water so as to be 0.5 mol · dm ⁻³ . After ascorbic acid was added and dissolved therein, an aqueous phosphoric acid solution diluted to 2.0 mol · dm ⁻³ was added dropwise while stirring the mixed solution. After dropwise addition of phosphoric acid, a precursor solution was prepared by adding lithium hydroxide monohydrate powder. While stirring this solution in a 60 ° C. water bath, ammonia water was continuously added as an alkali source until the pH of the precursor solution reached 9.7. Thereafter, the temperature of the water bath was changed to 80 ° C., and stirring was continued for 2 hours. The obtained precipitation solution was vacuum-dried and then crushed in a mortar to prepare a precursor powder. The obtained precursor powder was put in an alumina sagger (outside dimension 90 × 90 × 50 mm), and nitrogen gas was circulated using an atmosphere substitution type firing furnace (a table vacuum gas substitution furnace KDF-75 manufactured by Denken). Temporary calcination was performed at a lower flow rate (1.0 L · min ⁻¹ ). The pre-baking temperature was 300 ° C., and the pre-baking time (time for maintaining the baking temperature) was 2 hours. In addition, the temperature increase rate was 5 degreeC * min < ^-1 > and the temperature-fall was natural cooling. In this manner, lithium manganese iron phosphate compound LiMn _0.825 Fe _0.175 PO ₄ was synthesized. The obtained calcined powder was weighed with polyvinyl alcohol (degree of polymerization of about 1500) so that the mass ratio was 1: 1, and then dry-mixed with a ball mill. LiMn _0.825 Fe _0.175 PO ₄ coated with carbon was obtained by firing for 1 hour at 700 ° C. under nitrogen flow (1.0 L · min ⁻¹ ) in a firing furnace.

(Comparative Example 4)
LiMn _0.825 Fe _0.175 PO ₄ coated with carbon was obtained in the same manner as in Example 6 except that no alkali source was added to the precursor solution.

(Charge / discharge test)
The batteries according to Examples 3 to 6 and Comparative Examples 2 to 4 were subjected to a charge / discharge test in which charge / discharge of 5 cycles was performed at a temperature of 20 ° C. Here, the charging conditions are a current of 0.1 ItmA (about 10 hours rate), a voltage of 4.55 V, and a constant current and constant voltage charging of 15 hours, and the discharging conditions are a current of 0.1 ItmA (about 10 hours rate) and an end voltage. The constant current discharge was 2.0V. In this way, a nonaqueous electrolyte battery was produced. Tables 2 to 4 show the discharge capacities obtained in the fifth cycle.

  Moreover, about the positive electrode active material which concerns on Examples 3-6 and Comparative Examples 2-4, the powder X-ray-diffraction measurement using a CuK (alpha) ray was performed, and 2 (theta) = 29.2 +/- 0.5 (degree) from the obtained X-ray diffraction diagram. The ratio (b / a) of the diffraction line intensity (b) of 2θ = 16.9 ± 0.5 ° with respect to the diffraction line intensity (a) of FIG. As an example, an X-ray diffraction diagram corresponding to Example 3 is shown in FIG. 6, and an X-ray diffraction diagram corresponding to Comparative Example 2 is shown in FIG.

  As can be seen by comparing FIG. 6 according to the example and FIG. 7 according to the comparative example, the peak intensity of each peak is different. In particular, the difference of the peak at 2θ = 16.9 ± 0.5 ° is remarkable, and this peak is strongest in FIG. 6 according to the example, and 2θ = 29.2 ± 0. The peak intensity ratio with 5 ° is reversed. This is because the (200) plane in the direction orthogonal to the direction of the lithium ion diffusion path of lithium manganese phosphate was selectively grown, and the growth of the (020) plane in the lithium ion diffusion path direction was suppressed. It is thought that.

As can be seen from Table 2, a precipitate obtained from an aqueous solution containing at least manganese ions, phosphate ions and lithium ions is used as a precursor, and the precursor is calcined and expressed as LiMn _0.875 Fe _0.125 PO _4. When a positive electrode active material is obtained, by adding ammonia water to the aqueous solution and adjusting the pH to 7 or higher, lithium secondary battery can be obtained with a higher discharge capacity than when the acidic aqueous solution is kept without performing the adjustment. It turned out that the positive electrode active material for secondary batteries is obtained. Moreover, it is found that the pH is preferably 7.0 or more, and more preferably 7.0. It can also be seen that the pH is preferably 11.6 or less.

  And as for the positive electrode active material which concerns on Examples 2-4, all are 2theta = 16.9 rather than the diffraction line intensity | strength of 2 (theta) = 29.2 +/- 0.5 degree in the powder X-ray diffraction diagram which uses CuK (alpha) ray. It can be seen that it has a feature that the diffraction line intensity of ± 0.5 ° is larger.

From Table 3, it can be seen that the same applies to the case of obtaining a positive electrode active material represented by LiMn _0.825 Fe _0.125 Mg _0.05 PO ₄ .

From Table 4, LiMn _0.825 Fe _0.175 PO ₄ has a higher ratio of Fe than LiMn _0.875 Fe _0.125 PO ₄ , so that the discharge capacity of Comparative Example 4 is comparative example 2 or comparative. Although not as low as Example 3, even in such a composition, 2θ = 29.2 ± 0 in the powder X-ray diffraction diagram using CuKα rays by adding aqueous ammonia to the aqueous solution to make it alkaline. A positive electrode active material having a feature that the diffraction line intensity of 2θ = 16.9 ± 0.5 ° is larger than the diffraction line intensity of .5 ° is obtained, and the acid aqueous solution is left as it is without making the above adjustment. It can be seen that a positive electrode active material for a lithium secondary battery with a high discharge capacity can be obtained compared to the case.

As can be seen from these findings, the general formula _{LiMn (1-x-y)} Fe x M y PO 4 (0 ≦ x ≦ 0.5,0 ≦ y ≦ 0.1, M = Mg, Co, Cr, Ti , Y, Mo, or Nb), the higher the Mn composition ratio with respect to the Fe composition ratio, the more theoretically the discharge capacity near 4 V can be obtained. Although it becomes a substance, there is a problem that a substantial capacity cannot be obtained sufficiently. However, the present invention can remarkably improve such a problem when the composition ratio of Mn is relatively high. it can.

  According to the present invention, since it is possible to provide a positive electrode active material for a lithium secondary battery of a manganese phosphate system having a large discharge capacity and a lithium secondary battery using the positive active material, medium- and large-sized batteries expected to be developed in the future, particularly It is suitable for application to industrial batteries, and its industrial applicability is extremely large.

It is an X-ray diffraction pattern of the positive electrode active material for lithium secondary battery which concerns on an Example. It is an X-ray-diffraction figure of the positive electrode active material for lithium secondary battery which concerns on a comparative example. _{2 is} an X-ray diffraction diagram of LiFePO ₄ described in Non-Patent Document 1. FIG. It is an X-ray diffraction pattern of the positive electrode for lithium secondary battery which concerns on an Example. It is an X-ray diffraction pattern of the positive electrode for lithium secondary battery obtaining which concerns on a comparative example. It is an X-ray diffraction pattern of the positive electrode active material for lithium secondary battery which concerns on an Example. It is an X-ray-diffraction figure of the positive electrode active material for lithium secondary battery which concerns on a comparative example.

Claims (4)

Formula _{LiMn (1-x-y)} Fe x M y PO 4 at (0 ≦ x ≦ 0.5,0 ≦ y ≦ 0.1, M = Mg, Co, Cr, Ti, Y, Mo or Nb) In the powder X-ray diffraction diagram expressed using CuKα rays, the diffraction line intensity at 2θ = 16.9 ± 0.5 ° is larger than the diffraction line intensity at 2θ = 29.2 ± 0.5 °. A positive electrode active material for a lithium secondary battery. Formula _{LiMn (1-x-y)} Fe x M y PO 4 at (0 ≦ x ≦ 0.5,0 ≦ y ≦ 0.1, M = Mg, Co, Cr, Ti, Y, Mo or Nb) In the powder X-ray diffraction diagram including the positive electrode active material represented and using CuKα rays, the diffraction line of 2θ = 16.9 ± 0.5 ° is higher than the diffraction line intensity of 2θ = 29.2 ± 0.5 °. A positive electrode for a lithium secondary battery, characterized by having a higher strength. A lithium secondary battery comprising a positive electrode comprising the positive electrode active material according to claim 1 or a positive electrode according to claim 2, a negative electrode, and a nonaqueous electrolyte. Manganese ions, a precipitate of phosphate ions and lithium ions were obtained from the alkaline aqueous solution containing at least a precursor, generally by firing a precursor formula _{LiMn (1-x-y)} Fe x M y PO 4 (0 ≦ x ≦ 0.5, 0 ≦ y ≦ 0.1, M = Mg, Co, Cr, Ti, Y, Mo or Nb) A method for producing a positive electrode active material for a lithium secondary battery to obtain a positive electrode active material .