JP5043290B2 - Cathode active material for nonaqueous electrolyte secondary battery, method for producing the same, and nonaqueous electrolyte secondary battery using the same - Google Patents

Description

  The present invention relates to a high-capacity non-aqueous electrolyte secondary battery.

A lithium ion battery used as a main power source for mobile communication devices and portable electronic devices has a high electromotive force and a high energy density. Examples of the positive electrode active material used here include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), manganese spinel (LiMn ₂ O ₄ ), and mixtures thereof. These active materials have a voltage of 4 V or more with respect to lithium.

In addition to the above oxides, an active material made of phosphate has been proposed. For example, Patent Document 1 proposes LiMePO ₄ . Patent Document 2 proposes LiMePO ₄ F. Further, Patent Document 3 and Non-Patent Document 1 disclose Li ₂ MePO ₄ F.

Although the active material which introduce | transduced the nitrogen atom is described in patent document 4, all are related to an oxide, and it does not disclose or suggest about a phosphate compound. Patent Document 4 discloses that a compound containing at least one atom of silicon, germanium, and tin, an oxygen atom, and a nitrogen atom is used as a negative electrode material. This negative electrode material has a general formula: M _x N _y O _z (M is at least one atom of Si, Ge and Sn, and x, y and z are 1.4 <x <2.1 and 1.4 <, respectively. y <2.1 and 0.9 <z <1.6).

  An object of the present invention is to produce a high-capacity negative electrode material. In the negative electrode material described in Patent Document 4, a pseudo-plane composed of a chair-type six-membered ring made of silicon and nitrogen spreads in a matrix, and a silicon-oxygen-silicon bond is bridged between the planes (interlayers). Exist to form a one-dimensional tunnel-like portion. This tunnel-like portion becomes a lithium doping / undoping site, and the obtained negative electrode material exhibits a large capacity.

Therefore, the negative electrode material shown in the example of Patent Document 4 has a crystal structure as a matrix made of Si ₂ N ₂ O, Ge ₂ N ₂ O, or Sn ₂ N ₂ O, which is an oxygen nitride containing N. is important. In Patent Document 4, there is no disclosure or suggestion regarding the electronic conductivity of the negative electrode material, and in the examples, a carbon material is simply mixed as a conductive additive.

US Pat. No. 6,514,640 US Pat. No. 6,387,568 JP 2003-229126 A JP-A-11-102705 J. et al. Solid State Chem. 142, 1 (1999)

However, the phosphate compound, which is a positive electrode active material for lithium ion batteries represented by LiFePO ₄ , produces less oxygen during high-temperature heating and has higher safety and reliability than conventional oxide-based materials. While having the characteristics, as is apparent from the composition, it is a simple inorganic salt, and the material required as a battery active material has extremely low electronic conductivity. Therefore, as in the case of using an oxide such as LiCoO ₂ in a battery, it is conceivable to obtain an electrode by mixing a highly conductive material such as acetylene black or graphite as a conductive auxiliary agent. Since the compound's electronic conductivity is extremely low, it is not sufficient to simply mix the conductive aid, and it is possible to refine the particles to increase the specific surface area or to cover the active material surface with a graphite material. Is made.

However, as a result of these contrivances, new problems such as a decrease in capacity due to a decrease in the filling amount of the active material and an increase in cost due to an increase in the number of processes (pulverization and surface coating) occur.
Accordingly, an object of the present invention is to propose a highly conductive active material and to provide a high capacity non-aqueous electrolyte secondary battery in order to solve such problems.

In order to solve the above problems, the present invention provides a composition formula: Li _x MePO ₄ F _z N _y (0 ≦ x ≦ 2, 0 <y ≦ 1.2, 0 <z ≦ 1, Me represents Fe, Co, propose at least one atom) the positive electrode active material for a nonaqueous electrolyte secondary battery represented by is selected from the group consisting of Ni and Mn. By introducing nitrogen to phosphate, oxygen arrangement phosphate surface is changed, carrier occurs resistivity of the positive electrode active material involved in conducting decreases. For this reason, the conductive agent can be added by the same simple process as in the case of utilizing an oxide such as LiCoO ₂ .

The present invention also relates to a method for producing the positive electrode active material for a nonaqueous electrolyte secondary battery. This manufacturing method, after heating the phosphate compound in a reducing atmosphere, by reacting with ammonia gas, to synthesize a non-aqueous electrolyte cathode active material for a secondary battery comprising a phosphate containing a nitrogen atom includes the step, the non-aqueous electrolyte cathode active material for a secondary battery, the composition _{formula: Li x MePO 4 F z N} y (0 ≦ x ≦ 2,0 <y ≦ 1.2,0 <z ≦ 1, Me Is represented by at least one atom selected from the group consisting of Fe, Co, Ni and Mn.
It is effective that the reducing atmosphere contains at least one gas selected from the group consisting of argon, nitrogen, carbon monoxide and hydrogen, and the heating temperature is 300 to 1500 ° C.

The present invention further provides a non-aqueous electrolyte secondary battery including a positive electrode active material for a nonaqueous electrolyte secondary battery positive electrode.

  According to the active material for a non-aqueous electrolyte secondary battery according to the present invention, it is possible to avoid a problem that the conventional phosphate active material has, that is, a decrease in capacity and an increase in cost caused by an original measure for improving conductivity. Can do.

2 is a diagram showing a charge / discharge curve of LiFePO ₄ -z N _y used in the reference test battery 1. FIG. Is a diagram showing charge-discharge curves of LiCoPO ₄ F _z N _y used in the test cells 1 of the present invention. 4 is a schematic longitudinal sectional view of a cylindrical battery produced in Reference Example 1. FIG.

Hereinafter, the best mode for carrying out the present invention will be described.
The active material for a non-aqueous electrolyte secondary battery according to the present invention comprises a nitrogen atom-containing phosphate, and in order to obtain this, it is necessary to introduce nitrogen atoms into the phosphate. This is because, as described above, introduction of nitrogen atoms is indispensable in order to change the oxygen arrangement on the phosphate surface and avoid problems (high resistance) peculiar to phosphate.

The composition of the nitrogen atom-containing phosphate in the present invention, the set formed _{formula: Li x MePO 4 F z N} y (0 ≦ x ≦ 2,0 <y ≦ 1.2,0 <z ≦ 1, Me is Fe And at least one atom selected from the group consisting of Co, Ni and Mn.
Of course, elements other than those described above, such as inevitable impurities, may be included as long as the effects of the present invention are not impaired.

Here, various kinds of phosphate compounds (oxides and lithium-containing oxides) used in the synthesis of the nitrogen atom-containing phosphate in the present invention and the reaction conditions are changed, and the ratio of Me, F, and N is analyzed. As a result, it was found that 0 <y ≦ 1.2 and 0 <z ≦ 1 are preferable. Further, regarding lithium atoms, although depending on the amount contained in the raw material, it is considered conventionally that 0 ≦ x <2 is appropriate from LiMePO ₄ , Li ₂ MePO ₄ F and the like.
Here, the reason why the upper limit of y, which is the atomic ratio of N, is set to 1.2 is that when it exceeds 1.2, the discharge capacity of the obtained active material tends to decrease.

Moreover, as a manufacturing method of said nitrogen atom containing phosphate compound, after heating a phosphate compound in a reducing atmosphere, it is necessary to make it react with ammonia gas. In addition, as gas of the reducing atmosphere at this time, it is preferable to contain 1 or more types chosen from argon, nitrogen, carbon monoxide, and hydrogen, and it is preferable that heating temperature is 300-1500 degreeC. If it is less than 300 ° C., the reaction time is long. On the other hand, if it exceeds 1500 ° C., the temperature becomes higher than necessary, and in either case, the cost increases. From the cost and the amount of nitrogen to be introduced, it is preferably 350 to 1000 ° C.
Further, a phosphate compound containing a lithium atom may be reacted with ammonia gas after the surface oxide is reduced in a hydrogen gas atmosphere.

However, when the obtained conductive material is sintered by the above-described heating, it may be difficult to use the sintered body as an active material as it is for a secondary battery.
In this case, for example, if the sintered body is mechanically pulverized and dried in the presence of a solvent or the like, it can be easily used as an active material.

  Furthermore, the active material according to the present invention is heat-treated at 400 ° C. or lower in the presence of an organic reducing agent such as alcohol such as methanol or butanol, or in an atmosphere such as ammonia gas, and surplus oxygen and ammonia on the particle surface. Adsorbed species such as nitrogen and ammonia derived from gas can be removed. Thereby, the resistivity of the obtained powdery active material falls. In this case, as the organic reducing agent, in addition to alcohols, ketones, esters, amines and the like may be used.

  By introducing nitrogen atoms from the surface of the phosphate compound by the method as described above, the oxygen sequence on the surface of the phosphate compound is changed, and the resistivity is increased by generating carriers involved in conduction. It is thought that it can be reduced. In addition, when the amount of introduced nitrogen atoms is large, it is also considered that the entire matrix is changed to change the crystal structure.

  By including the active material in at least one of the positive electrode and the negative electrode, a high-capacity non-aqueous electrolyte secondary battery can be realized while maintaining high safety and battery characteristics. At that time, a positive electrode mixture, a conductive agent, a binder and a current collector in the positive electrode, a negative electrode mixture, a conductive agent, a binder and a current collector in the negative electrode, a separator, and a non-aqueous electrolyte are conventionally known. Can be used as appropriate. Further, the shape of the battery and the manufacturing method of each component are not particularly limited.

FIG. 3 shows a schematic longitudinal sectional view of the cylindrical battery.
An electrode plate group 4 in which a positive electrode plate and a negative electrode plate are wound in a spiral shape a plurality of times via a separator is housed in the battery case 1. A positive electrode lead 5 is drawn from the positive electrode plate and connected to the sealing plate 2, and a negative electrode lead 6 is drawn from the negative electrode plate and connected to the bottom of the battery case 1. For the battery case and the lead plate, a metal or alloy having an organic electrolyte resistance and an electron conductivity can be used. For example, a metal such as iron, nickel, titanium, chromium, molybdenum, copper, aluminum, or an alloy thereof is used. In particular, the battery case is most preferably a stainless steel plate or an Al—Mn alloy plate processed, the positive electrode lead is aluminum, and the negative electrode lead is most preferably nickel. In addition, various engineering plastics and a combination of these and a metal can be used for the battery case in order to reduce the weight.

  Insulating rings 7 are respectively provided on the upper and lower portions of the electrode plate group 4. And electrolyte solution is inject | poured and a battery case is sealed using a sealing board. At this time, a safety valve can be provided on the sealing plate. In addition to the safety valve, various conventionally known safety elements may be provided. For example, a fuse, bimetal, PTC element, or the like is used as the overcurrent prevention element. In addition to the safety valve, as a countermeasure against an increase in the internal pressure of the battery case, a method of cutting the battery case, a method of cracking the gasket, a method of cracking the sealing plate, or a method of cutting the lead plate can be used. Further, the charger may be provided with a protection circuit incorporating a countermeasure against overcharge or overdischarge, or may be connected independently.

In addition, as a measure against overcharging, a method of cutting off current by increasing the battery internal pressure can be provided. At this time, a compound for increasing the internal pressure can be contained in the mixture or the electrolyte. Examples of the compound raising the internal pressure and carbonates such as _{Li 2 CO 3, LiHCO 3,} Na 2 CO 3, NaHCO 3, CaCO 3 and MgCO ₃ and the like. As a method for welding the cap, the battery case, the sheet, and the lead plate, a known method (eg, direct current or alternating current electric welding, laser welding, ultrasonic welding, or the like) can be used. As the sealing agent for sealing, a conventionally known compound or mixture such as asphalt can be used.
By combining these elements, the nonaqueous electrolyte secondary battery of the present invention can be configured.
Examples of the present invention are shown below.

<< Experiment 1 >>
(1) LiFePO _4-z N _y
LiFePO ₄ which is a phosphate compound was used as a raw material. Based on the technology described in Japanese Patent Application Laid-Open No. 2000-294238, pulverized ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), iron oxalate dihydrate (FeC ₂ O ₂ .H ₂ O), and lithium carbonate (Li ₂ CO ₃ ) were mixed at a molar ratio of 2: 2: 1 and baked at 600 ° C. for 24 hours in a nitrogen atmosphere to obtain powdered LiFePO ₄ .

Next, LiFePO ₄ powder was put in a quartz reaction tube and heated to 700 ° C. in a nitrogen gas atmosphere. Thereafter, flow of ammonia gas, by reacting 10 hours, the composition formula: was obtained active material represented by LiFePO _4-z N _y. In addition, although z was not calculated | required correctly by analysis, some O was substituted by N and was about 0.2. Moreover, y was about 0.2.

30 parts by weight of acetylene black (AB) as a conductive auxiliary agent and 10 parts by weight of PVdF (polyvinylidene fluoride) as a binder are mixed with 60 parts by weight of the active material obtained as described above. The obtained mixture was diluted with NMP (N-methyl-2-pyrrolidone) and coated on an aluminum foil current collector. Thereafter, it was dried at 60 ° C. under reduced pressure for 30 minutes, cut into 15 × 20 mm ² , and further dried at 110 ° C. under reduced pressure for 14 hours to obtain a reference test electrode 1 having a thickness of 190 μm.

In contrast, the mixing amount of PVdF was constant of 10 parts by weight, 10 parts by weight of AB, 5 parts by weight or parts by weight, 80 parts by weight of active material, except for using 85 parts by weight or 88 parts by weight, Reference Reference test electrodes 2 to 4 were obtained under exactly the same conditions as test electrode 1.
In addition, Reference Test Electrode 5 was obtained under exactly the same conditions as Reference Test Electrode 1 except that methanol was added to the active material prepared in Experimental Example 1 and reacted at 300 ° C. for 10 minutes in a nitrogen gas atmosphere.

(2) LiCoPO ₄ F _z N _y
Further, based on the technology described in JP 2003-229126, Co ₃ O ₄ and P ₂ O ₅ and a LiF, molar ratio 2: 3: mixed with 6, the resultant mixture at 850 ° C. in vacuo LiCoPO ₄ F was obtained by baking for 48 hours.
Using this LiCoPO ₄ F as a raw material, LiCoPO ₄ F _z N _y was obtained by the same method as described above, and a test electrode 1 was obtained under exactly the same conditions as the reference test electrode 1 except that this was used as an active material. In addition, although z was not calculated | required correctly by analysis, y was about 0.2.

(3) Li ₂ CoPO ₄ F
Furthermore, based on the technique described in Japanese Patent Application Laid-Open No. 2003-229126, Li ₂ Co ₃ , CoO, and P ₂ O ₅ are mixed at a molar ratio of 1: 2: 1 and pre-baked at 500 ° C. in the atmosphere for 12 hours. Then, this was baked at 780 ° C. for 48 hours to prepare LiCoPO ₄ .
Thereafter, LiCoPO ₄ and a stoichiometric ratio of LiF were mixed, and the resulting mixture was baked at 780 ° C. in vacuum for 72 hours to obtain Li ₂ CoPO ₄ F.
The Li give Li ₂ CoPO ₄ F _z N _y a ₂ CoPO ₄ F in the same manner as described above as a raw material, except for using this as an active material, a test electrode 2 in exactly the same conditions as in Reference test electrode 1 Obtained. In addition, although z was not calculated | required correctly by analysis, y was about 0.2.
(4) Other active materials Further, as comparative examples, LiFePO ₄ and LiCoPO ₄ F or Li ₂ CoPO ₄ F as raw materials were used as active materials, and Comparative Test Electrodes 1 to 3 were prepared in the same manner as Reference Test Electrode 1. Obtained.

[Evaluation]
For these reference test electrodes 1 to 5, test electrodes 1 and 2, and comparative test electrodes 1 to 3, lithium metal crimped on stainless steel was used as the counter electrode. The separator was a polyethylene porous film, and the electrolyte was a solution prepared by dissolving 1.0 M LiPF ₆ in a solvent in which EC (ethylene carbonate) and DMC (dimethyl carbonate) were mixed at a ratio of 3 to 7.
A charge / discharge test was performed using the reference test batteries 1 to 5, the test electrodes 1 and 2, and the comparative test batteries 1 to 3 assembled in the above procedure.
Charging / discharging was repeated between predetermined voltage areas at a current density of 0.17 mA / cm ² . Here, FIG. 1 shows the electrochemical characteristics of the active material obtained in the reference test battery 1. Electrochemical measurement was performed by the method described above, and lithium metal was used as the counter electrode. From FIG. 1, it was found that this active material can be charged / discharged at substantially the same potential as the behavior of LiFePO ₄ before introducing nitrogen atoms. Further, the active materials used in the reference test batteries 2 to 5 were almost the same as the results shown in FIG.

FIG. 2 shows the electrochemical characteristics of the active material obtained in the test battery 1 . Electrochemical measurement was performed by the method described above, and lithium metal was used as the counter electrode. From FIG. 2, the active material was found to be capable of substantially the same potential, that lifting one charging and discharging the flat portion in the vicinity of 4.8V and behavior of previous LiCoPO ₄ F for introducing the nitrogen atoms. The active material used in the test battery 2 was almost the same as the result shown in FIG.
The comparative test battery 1 has a discharge voltage flat portion around 3.4 V, and the comparative test batteries 2 and 3 have a flat discharge voltage around 4.8 V (the flat potential did not change significantly due to nitriding). With respect to volume, larger comparative test cell 1 as compared with Comparative Test Cells 2 and 3, was almost the same when compared with the comparative test cell 2 and 3.

<< Experiment 2 >>
Next, when the active material according to the present invention is used, the required amount of the conductive auxiliary agent can be expected by improving the conductivity, but in order to evaluate this, the active material obtained above is used ( Reference test electrodes 6 to 10, test electrodes 3 and 4, and comparative test electrode in the same manner as the reference test electrode 1 except that the amount of acetylene black (AB), which is a conductive auxiliary agent, was changed. 4 to 9 , reference test batteries 6 to 10, test batteries 3 and 4, and comparative test batteries 4 to 9 were produced, and the capacities of the conductive auxiliary agents were compared.
The reference test batteries 6 to 10 are the same as the reference test batteries 1 to 5, and the comparative test battery 4 is the same as the comparative test battery 1.

  Table 1 shows the results. Using the above-described method for evaluating electrochemical characteristics, the amount of acetylene black, which is a conductive auxiliary agent to be added, was reduced and evaluated. The discharge capacity was shown as a comparative value when the capacity when 10% of the conductive additive was added was taken as 100.

  From Table 1, it was found that the active material according to the present invention can maintain the capacity even when the amount of the conductive aid is drastically reduced. As a result, when this active material is applied to a non-aqueous electrolyte secondary battery, the capacity per volume can be increased by reducing the conductive additive.

<< Reference Example 1 >>
Here, a cylindrical battery having the structure shown in FIG. 3 was produced.
The positive electrode plate was produced as follows . The positive electrode active material powder _{(LiFePO 4-z N y,} y = 0.2) to 100 parts by weight, were mixed polyvinylidene fluoride resin 3 parts by weight of carbon powder 2 parts by weight of a binder of the conductive agent. These were dispersed in dehydrated N-methylpyrrolidinone to obtain a slurry, which was applied onto a positive electrode current collector made of aluminum foil, dried and rolled, and then cut into a predetermined size.

Graphite was used as the negative electrode active material. A graphite / styrene butadiene rubber-based binder mixed at a weight ratio of 100: 5 was applied to both sides of a copper foil, dried, rolled, and then cut into a predetermined size. As the separator, a polypropylene nonwoven fabric was used. As the organic electrolyte, a solution obtained by dissolving 1.0 mol / liter of LiPF ₆ in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1: 3 was used. The produced cylindrical battery had a diameter of 14.1 mm and a height of 50.0 mm.

The cylindrical battery thus produced ( reference test battery 11 ) was charged at a constant voltage of 4.0V and discharged at a constant current of 100 mA up to 2.5V. The discharge capacity obtained at this time was 400 mAh.
Further, in the case of the comparative test battery 10 produced in the same manner as the reference test battery 11 except that LiFePO ₄ was used as the positive electrode active material, the discharge capacity was as low as 120 mAh.

<< Experiment 3 >>
In this experimental example, the amount of nitrogen atoms in the phosphate containing nitrogen atoms according to the present invention was examined.
Using the raw materials shown in Table 2, nitrogen atoms were introduced under various synthesis conditions to synthesize active materials according to the present invention. In Table 2, the case where LiFePO ₄ is used as a raw material is a reference example. Table 2 shows the synthesis conditions and the introduction amount (y value) of nitrogen atoms. The absolute value of resistivity indicating the conductivity of the active material tended to cause a large error depending on the physical properties of the powder. Therefore, in each experimental example and reference example , the resistivity of untreated raw materials was set to 100, and the resistivity was obtained as a relative value.

  From Table 2, it can be seen that the resistivity of the active material according to the present invention is reduced by about two orders of magnitude. In particular, it can be seen that y is preferably 0.11 to 1.1.

Furthermore, the surface elemental analysis of the active material in the said experiment example and reference example was performed using ESCA. As a result, it was confirmed that the composition varied gradually in the following manner.
That is, the surface layer was mainly nitride, and while the nitrogen content gradually decreased from the surface layer to the inside, an acid nitride in which the oxygen content increased was observed. In addition, some of the surface and the inside were composed of acid nitride, and some of the nitrogen was not observed and the oxygen content was reduced from untreated raw materials. The active materials in the experimental examples and the reference examples were these alone or a mixture thereof.

  The nonaqueous electrolyte secondary battery according to the present invention is suitable for a main power source of a mobile communication device or a portable electronic device.

DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Insulation packing 4 Electrode plate group 5 Positive electrode lead 6 Negative electrode lead 7 Insulation ring

Claims (5)

Composition formula: Li _x MePO ₄ F _z N _y (0 ≦ x ≦ 2, 0 <y ≦ 1.2, 0 <z ≦ 1, Me is at least 1 selected from the group consisting of Fe, Co, Ni and Mn for a non-aqueous electrolyte secondary battery positive electrode active material represented by the species of atoms). After heating the phosphate compound in a reducing atmosphere, by reacting with ammonia gas, comprising the step of combining the non-aqueous electrolyte cathode active material for a secondary battery comprising a phosphate containing nitrogen atom,
The non-aqueous electrolyte cathode active material for a secondary battery, the composition _{formula: Li x MePO 4 F z N} y (0 ≦ x ≦ 2,0 <y ≦ 1.2,0 <z ≦ 1, Me is Fe, Co the method of the positive electrode active material for a non-aqueous electrolyte secondary battery characterized by being represented by at least one atom) selected from the group consisting of Ni and Mn. The reducing atmosphere, argon, nitrogen, a manufacturing method of at least one non-aqueous electrolyte secondary battery positive electrode active material according to claim 2, further comprising a gas selected from the group consisting of carbon monoxide and hydrogen. Method of manufacturing a nonaqueous electrolyte secondary battery positive electrode active material according to claim 2, wherein said heating temperature is 300 to 1,500 ° C.. Non-aqueous electrolyte for a secondary battery positive electrode active material, a non-aqueous electrolyte secondary battery comprising the positive electrode according to claim 1, wherein.

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