# Patent

## Title
Pyrophosphate compound and production process thereof

## Abstract

To provide a positive electrode active material containing a pyrophosphate compound, ensuring that mixing of impurities is easily prevented to facilitate the synthesis and a high capacity battery is obtained, and a lithium ion battery using the positive electrode material. That is, the present invention relates to a pyrophosphate compound represented by the formula: Li2M1-xFexP2O7 (wherein M represents one or more elements selected from Mn, Zr, Mg, Co, Ni, V and Cu, and 0.3≦x≦0.9).

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a National Phase Patent Application and claims the priority of International Application Number PCT/JP2010/072183 filed on Dec. 3, 2010, which claims priority of Japanese Patent Application Number 2009-276513 filed on Dec. 4, 2009.
TECHNICAL FIELD
The present invention relates to a pyrophosphate compound, a production process thereof, and a positive electrode active material using the compound.
BACKGROUND ART
Heretofore, a rechargeable secondary battery has been continuously studied as a battery that is usable economically for a prolonged period of time. In particular, a lithium battery has advantages such as high output and high energy density and is usually composed of positive and negative electrodes each having an active material capable of reversible extraction/insertion of a lithium ion, and a nonaqueous electrolyte.
As the positive electrode active material of such a lithium ion battery, for example, a metal oxide, a metal sulfide or a polymer is used, and known examples include lithium-free compounds such as TiS2, MoS2, NbSe2 and V2O5, and a lithium composite oxide such as LiMO2 (M=Co, Ni, Mn, Fe or the like) and LiMn2O4. Among these, a battery fabricated using LiMn2O4 has a high battery capacity but in consideration of, for example, capacity deterioration during high-temperature storage and dissolution of Mn into the electrolytic solution, there remains a problem that the stability or cycle characteristics are not sufficient.
Also, LiFePO4 has been proposed but does not have sufficiently large capacity and has a problem that the discharge voltage is not high.
In this connection, it has been proposed to use LiMnPO4 based on Mn that is an element higher in oxidation-reduction potential than Fe, for the positive electrode of a lithium ion battery. However, a phosphoric acid compound having the basic composition of LiMnPO4 cannot easily realize redox generation of Mn. According to Journal of the Electrochemical Society, 144, 1188 (1997), among Mn-based phosphoric acid compounds, only LiMnxFe1-xPO4 having a structure formed by substituting Fe for a part of Mn is an example where redox generation of Mn is possible.
Furthermore, an electrode active material based on an oligophosphate represented by the following formula has been also proposed (Kohyo (Japanese Unexamined Patent Publication) No. 2006-523930):

AaMbXcO(3c+1) 

(wherein (a) A is at least one alkali metal and 0<a≦6; (b) M is at least one redox active element and 1≦b≦4; (c) X is selected from the group consisting of P, As, Sb, Si, Ge, V, S and a combination thereof; (d) 2≦c≦5; and A, M, X, a, b and c are selected so as to keep electrical neutrality of the compound).

As specific examples, Li2FeP2O7, Na2CoP2O7, Na2NiP2O7, Na2MnP2O7, Li2Co0.5Ni0.5P2O7, Li2MnP2O7, Li2CoP2O7, Li2NiP2O7, Na2CuP2O7, Li2VP2O7, and Li0.5Na0.5FeP2O7 are disclosed.
According to the present inventors, when the amount of Fe is increased in Li2FeP2O7, a high battery capacity may be obtained, but it is difficult to synthesize the compound without impurities.
RELATED ART
Patent Document


    Patent Document 1: Kohyo No. 2006-523930


Non-Patent Document


    Non-Patent Document 1: Journal of the Electrochemical Society, 144, 1188 (1997)


SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
An object of the present invention is to solve these problems and provide a positive electrode active material containing a pyrophosphate compound, ensuring that mixing of impurities is easily prevented to facilitate the synthesis and a high battery capacity is obtained, and a lithium ion battery using the positive electrode material.
Means to Solve the Problems
In order to solve the problems above, the present invention provides the following inventions:
(1) a pyrophosphate compound represented by the formula: Li2M1-xFexP2O7 (wherein M represents one or more elements selected from Mn, Zr, Mg, Co, Ni, V and Cu, and 0.3≦x≦0.9);
(2) the pyrophosphate compound as described in (1) above, wherein x is 0.7≦x≦0.9;
(3) the pyrophosphate compound as described in (1) or (2) above, wherein M is Mn;
(4) the pyrophosphate compound as described in any one of (1) to (3), having a low-temperature phase crystal structure or a high-temperature phase crystal structure;
(5) a production process for a pyrophosphate compound, comprising mixing starting materials of the pyrophosphate compound described in any one of (1) to (4) above, and firing the obtained raw material mixture at a temperature of 350 to 900° C.;
(6) the production process for a pyrophosphate compound as described in (5) above, wherein firing is performed at a temperature of 400 to 700° C.;
(7) a positive electrode active material containing the pyrophosphate compound described in any one of (1) to (4) above; and
(8) a lithium ion battery using a positive electrode containing the positive electrode active material described in (7) above.
Effects of the Invention
According to the present invention, a positive electrode active material containing a pyrophosphate compound, ensuring that mixing of impurities is easily prevented to facilitate the synthesis and a high battery capacity is obtained, and a lithium ion battery using the positive electrode material, can be provided.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the charge/discharge test results of Li2M1-xFexP2O7 obtained in Examples and Comparative Examples.
FIG. 2 shows the charge/discharge test results of Li2M1-xFexP2O7 obtained in Examples and Comparative Examples.
FIG. 3 shows the powder X-ray diffraction diagrams of Li2M0.5Fe0.5P2O7 obtained in Examples and Comparative Examples.
FIG. 4 shows the charge/discharge test results of Li2Mn0.3Fe0.7P2O7 obtained in Examples and Comparative Examples.



MODE FOR CARRYING OUT THE INVENTION
The pyrophosphate compound of the present invention is represented by the formula: Li2M1-xFexP2O7, wherein M represents one or more elements selected from Mn, Zr, Mg, Co, Ni, V and Cu, and 0.7≦x≦0.9. That is, specific examples of the compound represented by Li2M1-xFexP2O7 include Li2Mn1-xFexP2O7, Li2(Mn,Zr)1-xFexP2O7, Li2Mg1-xFexP2O7, Li2Zr1-xFexP2O7, Li2Ni1-xFexP2O7, Li2Co1-xFexP2O7, Li2V1-xFexP2O7, Li2Cu1-xFexP2O7, Li2(Mn,Co)1-xFexP2O7, Li2(Co,Ni)1-xFexP2O7, and Li2(Mn,Zr,Ni)1-xFexP2O7. In view of stability, discharge capacity and the like, Li2Mn1-xFexP2O7, Li2Ni1-xFexP2O7 and Li2Co1-xFexP2O7 are preferred, and Li2Mn1-xFexP2O7 is most preferred. In these pyrophosphate compounds, in order to ensure that mixing of impurities is easily prevented to facilitate synthesis and a high battery capacity is obtained, x must 0.3≦x≦0.9 and is preferably 0.5≦x≦0.9, most preferably 0.7≦x≦0.9. If x is less than 0.3, a high capacity battery is not obtained, whereas if x exceeds 0.9, a high capacity battery may be obtained, but the synthesis is difficult.
The BET specific surface area of the pyrophosphate compound is suitably 0.5 m2/g or more so that when the compound is used as a positive electrode active material, lithium can be smoothly diffused and a sufficient capacity can be obtained even under a large current. From this standpoint, the particle diameter is adjusted to be relatively small.
The pyrophosphate compound of the present invention is obtained by mixing starting materials of the pyrophosphate compound and firing the obtained raw material mixture at a temperature of 350 to 900° C., preferably from 400 to 700° C. If the firing temperature is less than 350° C., the starting material is not decomposed and the target compound cannot be obtained, whereas if the firing temperature exceeds 900° C., excessive grain growth may be caused.
Firing at a temperature of less than about 500° C. turns the later-described low-temperature phase into a crystal structure, and firing at about 500° C. or more turns a pyrophosphate compound having the later-described high-temperature phase into a crystal structure.
The starting material is not particularly limited, but suitable examples of the Li source include lithium carbonate (Li2CO3), suitable examples of the M source include manganese carbonate (MnCO3), titania (TiO2), magnesium oxalate (MgC2O4.2H2O), zinc oxide (ZnO) and cobalt oxalate (CoC2O4.2H2O), suitable examples of the Fe source include iron oxalate (FeC2O4.2H2O) and iron acetate (Fe(CH3COO)2), and suitable examples of the P2O7 source include ammonium dihydrogenphosphate (NH4H2PO4).
For example, a compound containing a metal element M such as Mn, manganese carbonate (MnCO3), ammonium dihydrogenphosphate (NH4H2PO4) and lithium carbonate (Li2CO3 are uniformly mixed as starting materials in a predetermined ratio, and the mixture is fired by heating at a predetermined temperature in an inert gas atmosphere such as argon and nitrogen, whereby the pyrophosphate compound of the present invention is obtained.
The pyrophosphate compound of the present invention is suitable as a positive electrode active material, and a positive electrode containing this positive electrode active material is preferably used particularly for a lithium ion battery. The lithium ion battery is composed of positive and negative electrodes each containing an active material capable of reversible extraction/insertion of a lithium ion, and a nonaqueous electrolyte. For example, such a lithium ion battery comprises a negative electrode, a negative electrode can housing the negative electrode, a positive electrode, a positive electrode can housing the positive electrode, and a separator and an insulating gasket which are disposed between the positive electrode and the negative electrode, and a nonaqueous electrolytic solution is charged into the negative electrode can and the positive electrode can. The negative electrode is obtained by forming a negative electrode active material layer containing a negative electrode active material on a negative electrode current collector, and as the negative electrode current collector, for example, nickel foil or copper foil is used. As the negative electrode active material, a material capable of doping/dedoping lithium is used and specifically, for example, lithium metal, a lithium alloy, or an electrically conductive high-molecular or layered compound doped with lithium, is used. As the binder contained in the negative electrode active material layer, a resin material or the like known in this field can be used. The positive electrode is obtained by forming a positive electrode active material layer containing a positive electrode active material that is the pyrophosphate compound of the present invention, on a positive electrode current collector such as aluminum foil. As the binder contained in the positive electrode active material layer, a resin material or the like can be used.
The separator is used to separate the positive electrode from the negative electrode and, for example, a polymer film such as polypropylene is used. The thickness of the separator is preferably 50 μm or less.
The insulating gasket is integrally incorporated into the negative electrode can and is used to prevent leakage of the nonaqueous electrolytic solution charged into the negative electrode can and the positive electrode can.
As the nonaqueous electrolytic solution, a solution obtained by dissolving an electrolyte in an aprotic nonaqueous solvent is used. Examples of the nonaqueous solvent which can be used include propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyllactone, sulfolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 2-methyltetrahydrofuran, 3-methyl 1,3-dioxolane, methyl propionate, methyl butyrate, dimethyl carbonate, diethyl carbonate, and dipropyl carbonate. Above all, in view of voltage stability, cyclic carbonates such as propylene carbonate and vinylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate and dipropyl carbonate are preferably used. As a nonaqueous solvent, one kind may be used alone, or two or more kinds may be mixed and used.
Examples of the electrolyte dissolved in the nonaqueous solvent, which can be used, include a lithium salt such as LiPF6, LiClO4, LiAsF6, LiBF4, LiCF3SO3 and LiN(CF3SO2)2. Among these lithium salts, LiPF6 and LiBF4 are preferably used.
Assembling a lithium ion battery may be performed according to the conventional method, and the shape of the battery is also not limited and may be, for example, a cylinder, square, coin or button shape.
EXAMPLES
Examples 1 to 4 and Comparative Examples 1 to 3
Lithium carbonate (Li2CO3), manganese oxalate hydrate (MnC2O4.0.5H2O), iron oxalate hydrate (FeC2O4.2H2O) and ammonium hydrogenphosphate [(NH4)2HPO4] were weighed to give Li2Mn1-xFexP2O7 (0≦x≦1) in the stoichiometric ratio shown in Table 1 and wet-pulverized/mixed using acetone as the solvent in a ball mill at 240 rpm for 30 minutes and at 480 rpm for 2 hours. The slurry after mixing was dried under reduced pressure at room temperature to obtain a precursor, and in an Ar atmosphere, this precursor was fired at 600° C. for 6 hours to obtain high-temperature phase HT-Li2Mn1-xFexP2O7 (0≦x≦1) and fired at 450° C. for 24 hours to obtain low-temperature phase LT-Li2Mn1-xFexP2O7 (0≦x≦1).












TABLE 1







Li2CO3
MnC2O4•0.5H2O
FeC2O4•2H2O
(NH4)2HPO4





















Comparative
Li2MnP2O7
1.5219 g
3.1300 g
—
5.4400 g


Example 1


Comparative
Li2Mn0.9Fe0.1P2O7
1.5213 g
2.8159 g
0.3704 g
5.4379 g


Example 2


Example 1
Li2Mn0.7Fe0.3P2O7
1.5202 g
2.1885 g
1.1103 g
5.4339 g


Example 2
Li2Mn0.5Fe0.5P2O7
1.5191 g
1.5621 g
1.8492 g
5.4298 g


Example 3
Li2Mn0.3Fe0.7P2O7
1.5179 g
0.9365 g
2.5869 g
5.4257 g


Example 4
Li2Mn0.1Fe0.9P2O7
1.5168 g
0.3119 g
3.3236 g
5.4217 g


Comparative
Li2FeP2O7
1.5162 g
—
3.6915 g
5.4197 g


Example 3









(Structural Analysis)

As the radiation source of the powder X-ray diffraction measurement, CuKα was used. Structural analysis of product, identification of impurity, and confirmation of the presence or absence of decomposition were performed by the Rietveld/Pawley analysis.
(Electrochemical Measurement)
An electrode using HT-Li2Mn1-xFexP2O7 (0≦x≦1) as the active material was produced by the following procedure.
Materials according to the formulation of active material:acetylene black (electrically conductive additive):polyvinylidene fluoride (binding material)=80:20:2 (by weight) were thoroughly mixed in a mortar, and the mixture was coated on an Al foil and dried under reduced pressure by heating at 120° C. for 30 minutes. This HT-Li2Mn1-xFexP2O7 was used as the electrode, and the charge-discharge test was performed in a 2032-type coil cell using the electrode as the positive electrode and using Li metal as the counter electrode. The electrolyte was 1 M LiPF6/EC+DEC, and porous CellGuard was used for the separator. Cell assembling was all performed in a glove box in an Ar atmosphere. In the charge/discharge test, the cutoff was set at 2.0 to 4.5 V vs. Li/Li+, and charging in CCCV mode and discharging in CC mode were performed. At this time, the rate was C/20 for both charging and discharging, and the measurement temperature was 25° C.
(Results)
FIG. 1 shows the obtained powder X-ray diffraction results. It is seen that in the high-temperature phase, a single phase is formed irrespective of the composition. This well agrees with the structure recently reported by Adam et al. [J. Solid State Chem., 181 (11), 3110 (2008)].
The XRD pattern of the low-temperature phase was found to be a novel pattern different from that of the high-temperature phase. Details thereof are shown in Table 2.













TABLE 2





Atom
Wyckoff
x
y
z
Occupancy
Biso/Å2
























Mn1
2a
0.8898
(11)
−0.193
(3)
0.5251
(17)
1.0
1.00 (5)


Mn2
2a
0.3909
(11)
0.049
(3)
0.0170
(17)
1.0
=Mn1


P1
2a
0.0272
(3)
0.157
(3)
0.986
(3)
1.0
0.71 (6)


P2
2a
0.2934
(16)
−0.130
(3)
0.5784
(19)
1.0
=P1


P3
2a
0.4722
(17)
0.214
(3)
0.512
(3)
1.0
=P1


P4
2a
0.7904
(14)
−0.011
(3)
1.0467
(19)
1.0
=P1


01
2a
0.232
(3)
0.415
(3)
0.184
(5)
1.0
0.55 (7)


02
2a
0.836
(2)
0.101
(3)
0.978
(4)
1.0
=01


03
2a
0.128
(3)
−0.151
(3)
0.697
(5)
1.0
=01


04
2a
0.365
(3)
0.282
(3)
0.633
(4)
1.0
=01


05
2a
0.925
(3)
−0.049
(3)
0.286
(4)
1.0
=01


06
2a
0.144
(3)
0.072
(3)
0.844
(4)
1.0
=01


07
2a
0.425
(3)
−0.095
(4)
0.755
(4)
1.0
=01


08
2a
0.493
(3)
0.110
(3)
0.661
(6)
1.0
=01


09
2a
0.367
(4)
0.497
(3)
0.826
(5)
1.0
=01


010
2a
0.068
(3)
0.161
(3)
0.271
(4)
1.0
=01


011
2a
0.281
(3)
−0.062
(3)
0.312
(5)
1.0
=01


012
2a
0.008
(4)
0.254
(3)
0.838
(6)
1.0
=01


013
2a
0.571
(3)
0.693
(3)
0.800
(4)
1.0
=01


014
2a
0.359
(2)
0.759
(3)
0.437
(4)
1.0
=01


Li1
2a
0.389

0.312

0.014

1.0
1.0


Li2
2a
0.917

0.387

0.922

1.0
1.0


Li3
2a
0.934

0.528

0.513

1.0
1.0


Li4
2a
0.389

0.484

0.472

1.0
1.0





Space group:


P21, a = 8.25641(12) Å, b = 12.9582(2) Å, c = 4.98361(8) Å, β = 92.8443(12)°


Rwp = 7.763%, Rp = 5.456%, gof = Rwp/Rexp = 1.132, RBragg = 1.215%






Both Mn1 and Mn2 have a six-coordinated octahedral structure, and both Li and P have a four-coordinated tetrahedral structure. Two P sites form P2O74− by sharing the apex of the tetrahedron. The impurities contained in the low-temperature phase are Li4P2O7 (P21/n) and Mn2P2O7 (C2/m).
FIG. 2 shows the results of respective charge/discharge tests for different formulations with respect to Mn and Fe. As the Mn amount decreases and the Fe amount increases, a flat potential region is observed and the discharge capacity is also enhanced. While the theoretical capacity is 220 mAh/g, the discharge capacity actually obtained is: when x=0, initial discharge capacity: 19.2 mAh/g and discharge capacity at fifth cycle: 16.7 mAh/g; when x=0.1, initial discharge capacity: 13.7 mAh/g and discharge capacity at fifth cycle: 12.5 mAh/g; when x=0.3, initial discharge capacity: 40.0 mAh/g and discharge capacity at firth cycle: 36.0 mAh/g; when x=0.5, initial discharge capacity: 58.6 mAh/g and discharge capacity at fifth cycle: 54.2 mAh/g; when x=0.7, initial discharge capacity: 63.4 mAh/g and discharge capacity at fifth cycle: 62.1 mAh/g; and when x=0.9, initial discharge capacity: 90.2 mAh/g and discharge capacity at fifth cycle: 88.2 mAh/g.
Furthermore, in the electrochemical measurement above, the charge/discharge test was performed for HT-Li2Mn0.5Fe0.5P2O7 and HT-Li2Mn0.3Fe0.7P2O7 by changing the measurement temperature to 60° C. from 25° C. FIGS. 3 and 4 show the results. It is seen that in both cases, remarkably enhanced charge/discharge characteristics are exhibited.
According to the present invention, when 0.3≦x≦0.9, preferably 0.7≦x≦0.9, a pyrophosphate compound ensuring easy synthesis and a high battery capacity can be obtained, and this pyrophosphate compound exhibits excellent repetitive charge/discharge characteristics thanks to an electron-accepting reaction utilizing oxidation and reduction of Fe.
INDUSTRIAL APPLICABILITY
According to the present invention, a positive electrode active material containing a pyrophosphate compound, ensuring that mixing of impurities is easily prevented to facilitate the synthesis and a high battery capacity is obtained, and a lithium ion battery using the positive electrode material, can be provided.

## Claims

1. A pyrophosphate compound represented by the formula:


Li2M1-xFexP2O7 


(wherein M represents one or more elements selected from Mn, Zr, Mg, Co, Ni, V and Cu, and 0.3≦x≦0.9).

2. The pyrophosphate compound as claimed in claim 1, wherein x is 0.7≦x≦0.9.

3. The pyrophosphate compound as claimed in claim 1, wherein M is Mn.

4. The pyrophosphate compound as claimed in claim 1, having a low-temperature phase crystal structure or a high-temperature phase crystal structure.

5. A production process for a pyrophosphate compound, comprising mixing starting materials of the pyrophosphate compound claimed in claim 1, and firing the obtained raw material mixture at a temperature of 350 to 900° C.

6. The production process for a pyrophosphate compound as claimed in claim 5, wherein firing is performed at a temperature of 400 to 700° C.

7. A positive electrode active material containing the pyrophosphate compound claimed in claim 1.

8. A lithium ion battery using a positive electrode containing the positive electrode active material claimed in claim 7.