Source: http://www.google.com/patents/US7592100?dq=4182933
Timestamp: 2016-08-28 07:27:59
Document Index: 705720730

Matched Legal Cases: ['Application No. 2002', 'Application No. 2003', 'Application No. 2002', 'Application No. 2003', 'Application No. 10', 'application No. 2006', 'Application No. 2000', 'application No. 2002']

Patent US7592100 - Positive-electrode active material and nonaqueous-electrolyte secondary ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsThe present invention provides a high-capacity and low-cost non-aqueous electrolyte secondary battery, comprising: a negative electrode containing, as a negative electrode active material, a ssubstance capable of absorbing/desorbing lithium ions and/or metal lithium; a separator; a positive electrode;...http://www.google.com/patents/US7592100?utm_source=gb-gplus-sharePatent US7592100 - Positive-electrode active material and nonaqueous-electrolyte secondary battery containing the sameAdvanced Patent SearchPublication numberUS7592100 B2Publication typeGrantApplication numberUS 10/333,269PCT numberPCT/JP2001/009756Publication dateSep 22, 2009Filing dateNov 7, 2001Priority dateMar 22, 2001Fee statusPaidAlso published asCN1287474C, CN1430795A, EP1296391A1, EP1296391A4, US7682747, US7718318, US20030170540, US20080096111, US20080193844, WO2002078105A1Publication number10333269, 333269, PCT/2001/9756, PCT/JP/1/009756, PCT/JP/1/09756, PCT/JP/2001/009756, PCT/JP/2001/09756, PCT/JP1/009756, PCT/JP1/09756, PCT/JP1009756, PCT/JP109756, PCT/JP2001/009756, PCT/JP2001/09756, PCT/JP2001009756, PCT/JP200109756, US 7592100 B2, US 7592100B2, US-B2-7592100, US7592100 B2, US7592100B2InventorsTsutomu Ohzuku, Hiroshi Yoshizawa, Masatoshi NagayamaOriginal AssigneePanasonic Corporation, Osaka City UniversityExport CitationBiBTeX, EndNote, RefManPatent Citations (98), Non-Patent Citations (51), Classifications (30), Legal Events (6) External Links: USPTO, USPTO Assignment, EspacenetPositive-electrode active material and nonaqueous-electrolyte secondary battery containing the same
US 7592100 B2Abstract
The present invention provides a high-capacity and low-cost non-aqueous electrolyte secondary battery, comprising: a negative electrode containing, as a negative electrode active material, a ssubstance capable of absorbing/desorbing lithium ions and/or metal lithium; a separator; a positive electrode; and an electrolyte, wherein the positive electrode active material contained in the positive electrode is composed of crystalline particles of an oxide containing two kinds of transition metal elements, the crystalline particles having a layered crystal structure, and oxygen atoms constituting the oxide forming a cubic closest packing structure.
1. A positive electrode active material for a non-aqueous electrolyte battery, comprising crystalline particles of a lithium-containing oxide containing two kinds of transition metal elements,
said crystalline particles having a layered crystal structure, and
oxygen atoms constituting said lithium-containing oxide forming a cubic closest packing structure, and
said two kinds of transition metal elements are nickel element and manganese element,
wherein said lithium-containing oxide contains said two kinds of transition metal elements in substantially the same proportion, and
said lithium-containing oxide is expressed by formula (3):
Li[Lix(AyByCp)1-x]O2 where A and B are said two kinds of transition metal elements, C is at least one kind of an added element different from A and B,
0≦x≦0.3, 0<2y+p≦1, and ⅓≦y≦0.5 and 0<p≦⅓,
a mixture of crystalline particles of said lithium-containing oxide having a particle size of 0.1 to 2 μm and secondary particles of said crystalline particles having a particle size of 2 to 20 μm, and
said transition metals are uniformly dispersed in said crystalline particles.
2. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 1, wherein the integral intensity ratio I003/I104 of the X-ray diffraction peak attributed to Miller indices (003) to that attributed to Miller indices (104) satisfies
I003/I104<1 in said crystal structure of said crystalline particles.
3. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 1, wherein the powder X-ray diffraction peaks attributed to Miller indices (108) and (110) are observed as two split peaks in said crystal structure of the crystalline particles.
4. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 1, wherein said crystalline particles are spherical in shape.
5. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 1, wherein the volume of unit cells of said crystalline particles decreases by oxidation.
6. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 1, wherein the error of the ratio of the nickel element to the manganese element is within 10 atomic %.
7. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 1, wherein said lithium-containing oxide is obtained by using a hydroxide or an oxide containing two or more kinds of transition metals as a precursor, in which the half-width of a peak observed in the range of 15 to 20� in X-ray diffraction peaks measured with Ku ray of copper is 3� or less.
8. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 7, wherein, in the X-ray diffraction peaks, the peak height H1 observed in the range of 15 to 20� and the peak height H2 observed in the range of 30 to 40� satisfy the relation:
H 1≧2�H 2. 9. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 8, wherein said lithium-containing oxide is obtained by mixing said precursor with a lithium compound such as lithium carbonate and/or lithium hydroxide and sintering the mixture.
10. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 9, wherein said sintering is performed at a temperature of 900� or higher.
11. A non-aqueous electrolyte secondary battery comprising:
a negative electrode containing, as a negative electrode active material, a substance capable of absorbing/desorbing lithium ions and/or metal lithium; a separator; a positive electrode containing the positive electrode active material in accordance with claim 8; and an electrolyte.
12. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 1, wherein said added element C is at least one kind selected from the group consisting of aluminum, magnesium, calcium, strontium, yttrium, ytterbium, iron, and cobalt.
13. The positive electrode active material for a non-aqueous electrolyte battery in accordance with claim 1, wherein the ratio of the amount of said added element C to the total amount of said transition metal elements A and B and said added element C is 5 to 35 mol%.
14. A positive electrode active material for a non-aqueous electrolyte battery, comprising crystalline particles of a lithium-containing oxide containing two kinds of transition metal elements,
a mixture of crystalline particles of said lithium-containing oxide having a particle size of 0.1 to 2 μm and secondary particles of said crystalline particles having a particle size of 2 to 20 μm,
said transition metals are uniformly dispersed in said crystalline particles, and
said secondary particles are substantially spherical sintered particles.
The present invention relates to a positive electrode active material, particularly to a positive electrode active material for a non-aqueous electrolyte battery. The present invention further relates to a high-capacity and low-cost non-aqueous electrolyte secondary battery having a positive electrode containing a specific positive electrode active material.
Japanese Laid-Open Patent Publication No. Hei 8-171910 proposes a method in which manganese and nickel are coprecipitated by adding an alkaline solution into an aqueous solution mixture of manganese and nickel, then lithium hydroxide is added and the resulting mixture is baked, to obtain an active material represented by the formula LiNixMn1−xO2 wherein 0.7≦x≦0.95.
Further, Japanese Laid-Open Patent Publication No. Hei 10-69910 proposes an active material synthesized by a coprecipitation synthesis method, represented by the formula Liy−x1Ni1−x2MxO2 wherein M is Co, Al, Mg, Fe, Mg or Mn, 0<x2≦0.5, 0≦x1<0.2, x=x1+x2, and 0.9≦y≦1.3. This patent publication describes that the discharge capacity is inherently small if M is Mn, and the essential function of the positive electrode active material for a lithium secondary battery intended to achieve a high capacity is dismissed if x2 is more than 0.5. LiNi0.6Mn0.4O2 is exemplified as a material having the highest proportion of Mn.
Based on the finding that a nickel-manganese composite oxide exhibiting a new function was obtained by dispersing a nickel compound and a manganese compound uniformly in the atomic level to form a solid solution, the present inventors have made further vigorous examinations on oxides containing various transition metals, together with the compositions, crystal structures, functions and the like thereof.
The present invention relates to a positive electrode active material for a non-aqueous electrolyte battery, comprising crystalline particles of a lithium-containing oxide containing two kinds of transition metal elements, the crystalline particles having a layered crystal structure and oxygen atoms constituting the lithium-containing oxide forming a cubic closest packing structure.
Li[Lix(AyB1−y)1−x]O2
wherein A and B are different transition metal elements, 0≦x≦0.3 and 0<y<1). That is, the lithium-containing oxide expressed by formula (1) contains two kinds of transition metal elements.
Li[Lix(Ni1/2Mn1/2)1−x]O2
wherein 0≦x≦0.3.
Li[Lix(AyByCy)1-x]O2
wherein A and B are different transition metal elements, C is at least one kind of an added element different from A and B, 0≦x≦0.3 and 0<2y+p≦1.
FIG. 1 is a schematic view of an experimental apparatus used for producing a positive electrode active material by coprecipitation method according to the present invention.
FIG. 7 show X-ray diffraction images of lithium-containing nickel-manganese oxides having various compositions.
FIG. 8 is a view showing charge/discharge curves of Li[Lix(Ni1/2Mn1/2)1−x]O2 wherein x is 0.1, 0.2 and 0.3, respectively.
As described above, a positive electrode active material composed of a nickel-manganese composite oxide exhibiting a new function is conventionally obtained by forming a solid solution by dispersing a nickel compound and a manganese compound uniformly in the atomic level. Based on this prior art, the present inventors have found that a further new function can be obtained by mixing two kinds of any transition metals if a specific structure is further provided. In particular, the inventors have found that it is important from the viewpoint of the composition that the two kinds of transition metals should be roughly equal in quantity to each other, and also important from the viewpoint of the crystal structure that a layered structure should be established and oxygen atoms should form a cubic closest packing structure.
U.S. Pat. No. 5,264,201 discloses an active material having a composition of formula LixNi2−x−yMyO2 wherein 0.8≦x≦1.0 and y≦0.2; if M is Co, y<0.5. As the added element M, Co, Fe, Ti, Mn, Cr and V are disclosed. As is found from this prior art, many added elements M are proposed for Ni as the reference, and the added amount is a trace. Therefore, this prior art neither discloses nor suggests the idea of performing the potential-related control with the combination of an added element only by adding the added element while maintaining the potential-related feature of Ni. This prior art only describes that the added amount is large when it is Co, and it is considered that this combination has been examined due to the well-known facts that Co has a high potential and the potential of Co is roughly equal to the potential of Ni.
The above phenomenon cannot be expected from the active material having a layered structure including Co, Ni or Mn alone, because the potentials of these active materials are higher in the order of Co>Ni>Mn. That is, from the prior art and the potentials of the single materials, it is expected that the voltages of the active materials are higher in the order of Ni—Co>Ni>Ni—Mn and, however, they are higher in the order of Ni—Mn>Ni>Ni—Co in actual. That is, the opposite phenomenon occurs. This indicates that development of a new function is possible by mixing two kinds of transition metals in the same proportion to synthesize an active material having a layered structure. Thus, the present invention also includes the obtainment of a new function by using, not only the combination of Ni and Mn in the same proportion, but also a combination of other transition metal elements in the same proportion. For example, the use of Ni, Mn, Fe, Co and Ti is suggested.
Recent research has revealed that wet coprecipitation method can provide good results when employed for production of a nickel composite oxide. For example, a nickel-manganese coprecipitation method is disclosed in Japanese Laid-Open Patent Publication No. 8-171910. The coprecipitation method is a technique of precipitating two elements simultaneously in an aqueous solution by use of neutralizing reaction to obtain a composite hydroxide as a precursor. So far, a normal coprecipitation method is sufficient for the conventional use of replacing a part of nickels with a small amount of another element. However, the conventional method is useless for the purpose of the present invention because a higher-level technology is required to incorporate both the nickel element and the manganese element in substantially the same amount in the atomic level. Moreover, when the hydroxide as the precursor obtained by the coprecipitation method is made to react with lithium to obtain the target lithium-containing composite oxide, the electrochemical properties of the resultant battery largely vary with the particle shape of the composite oxide. The conventional method finds difficulty in controlling the variation. In addition, the sintering temperature should be appropriately selected because it largely affects the electrochemical properties.
To attain a layered structure for the crystal structure of crystalline particles of an oxide containing two kinds of transition metal elements and also attain a cubic closest packing structure for oxygen atoms constituting the oxide, in an oxidation atmosphere, for example, the oxide is sequentially subjected to primary sintering (400 to 650� C. when lithium salt is lithium hydroxide, 600 to 650� C. when lithium salt is lithium carbonate), pulverization as required, secondary sintering (950 to 1000� C.) and tertiary sintering (700 to 800� C).
If dissolved oxygen exists in the aqueous solution, manganese is very easily oxidized changing from divalent to trivalent. Therefore, when it is intended to obtain β type Ni1−xMn(OH)2, such dissolved oxygen must be purged from the reaction bath by taking measures such as bubbling inert gas such as nitrogen or argon or adding some reducing agent, to thereby suppress oxidation of manganese.
On the contrary, when it causes no problem to obtain, or rather it is intended to obtain, α type Ni1−xMnx(OH)2.xSO4 2−.yH2O, the dissolved oxygen in the solution may be effectively used.
FIG. 5 shows typical SEM photographs of particles obtained by the method described above. FIGS. 5( a), (b) and (c) are SEM photographs of 1000�, 2000� and 20000�, respectively. It is found that a large spherical particle filled with crystallites at high density is formed, which is a little different from the porous particle described above. Although the crystalline particles may be left in the state of hydroxide, they may be dried/sintered at a low temperature to be changed an oxide if change with time during preservation may cause a problem.
Thereafter, the resultant hydroxide or oxide as the precursor is mixed with a lithium source such as lithium hydroxide, and the mixture is sintered, to thereby obtain LiyNi1−xMnxO2 as the target positive electrode active material for lithium secondary batteries.
It is desirable to supply lithium uniformly to reach the inside of the spherical nickel-manganese hydroxide (precursor). The use of lithium hydroxide is idealistic in this respect, because lithium hydroxide is melted first at a relatively low temperature, lithium is supplied into the inside of the particles of the nickel-manganese hydroxide, and then, oxidation gradually occurs from outside of the particles with increase of the temperature.
The nickel-manganese composite hydroxide as the precursor and lithium hydroxide are mixed sufficiently in the dry state. It is ideal to mix lithium hydroxide and the nickel-manganese hydroxide so that the atomic ratio of Li to Ni and Mn satisfies Li/(Ni+Mn)=1. However, for control of the sintering temperature and the particle shape, the amount of one element may be somewhat increased or decreased. For example, when the sintering temperature is high, or when it is desired to increase the size of primary particles after sintering, the amount of lithium to be mixed may be somewhat increased. An increase/decrease by about 3% is preferable.
H 1≧2�H 2
The difference described above indicates that in (a) and (b) the crystallinity has already developed to some extent in the precursor. This is easily recognized by comparing (a) and (b) with (i) and (j). In (i) and (j), although there is no evident Mn2O3 peak, the peak intensity ratio and the half-width are clearly different from those in (a) and (b).
To obtain the positive electrode active material having a specific structure, it is ideal to satisfy Li/(Ni+Mn)=1, and it is also possible to increase this ratio for various purposes to be described later. That is, a lithium-containing oxide expressed by formula (2):
wherein 0≦x≦0.3. If the atomic ratio of lithium in the lithium-containing oxide is further increased, the electric capacity as the active material decreases and also synthesis of the target layered-structure active material tends to fail. Therefore, it is preferable to satisfy 0≦x≦0.3, and particularly preferable to satisfy 0.03≦x≦0.25. The atmosphere for sintering may be an oxidation atmosphere. In this examination, the ordinary atmosphere was used.
As an example, three kinds of lithium-containing oxides expressed by formula (2) in which x was 0.1, 0.2 and 0.3, respectively were synthesized. In the synthesis, the amount of lithium hydroxide was adjusted to attain each of the above ratios when a nickel-manganese composite hydroxide as the precursor prepared by the coprecipitation method and lithium hydroxide were mixed sufficiently in the dry state. The resultant oxide was subjected to primary sintering at 500� C. for 8 hours, pulverization with Masscolloider, secondary sintering at 950� C. for 10 hours, and tertiary sintering at 700� C. for 5 hours, to have the specific crystal structure described above. It is possible to confirm that the crystal structure of crystalline particles of the oxide is a layered structure and that oxygen atoms constituting the oxide form a cubic closest packing structure, by analyzing the pattern of the powder X-ray diffraction image with the Rietveld method.
FIG. 8 shows charge/discharge curves, in the overlap state, of Li[Lix(Ni1/2Mn1/2)1−x]O2 wherein x is 0.1, 0.2 and 0.3 as the positive electrode active materials having the specific structure of the present invention. From this figure, it is found that the charge/discharge potentials of these three materials are the same. Also, only the charge/discharge capacity varies in proportion of the total amount of the transition metals. From this, in addition to the X-ray diffraction patterns in FIG. 7, it is found that these three materials have the same structure. Moreover, using this feature, the charge/discharge capacity can be controlled by controlling the quantity of the lithium element in the material freely within this range while maintaining the charge/discharge potential constant. Thus, the present invention, which enables non-electrochemical synthesis of the material having such charge/discharge behavior, presents a guideline for novel material design.
FIG. 9 shows charge/discharge curves of Li[Li0.2(Ni1/2Mn1/2)0.8]O2 in the range 5 to 2.5 V. This material is normally controlled to a potential up to 4.3 V with respect to the lithium metal for application to a battery. This also applies to generally available LiCoO2. However, in the event of failure of this control, the material is overcharged, that is, charged up to near 5 V. Once in such an overcharged state, the crystal structure of LiCoO2 becomes very unstable. This will be described later in Example 4 for the case of LiNiO2, which also applies to the case of LiCoO2. However, Li[Li0.2(Ni1/2Mn1/2)0.8]O2 largely changes it crystal structure by the first charge, to have a thermally stable structure. As is found from the charge/discharge curves shown in FIG. 9, a clear difference exists between the first charge behavior and the subsequent charge/discharge behavior. Entirely different behavior is also shown for the charge/discharge curve up to 4.3 V shown in FIG. 8.
The same results were obtained for the active material expressed by Li[Li0.1(Ni1/2Mn1/2)0.9]O2 and the active material expressed by Li[Li0.3(Ni1/2Mn1/2)0.7]O2. Therefore, from the results for Li[Lix(Ni1/2Mn1/2)1−x]O2 (X=0.1˜0.3) containing lithium excessively as described above, there is found a merit that the thermal stability of the active material at an overcharge can be improved by the mechanism described above. No prior art discloses or suggests this idea, and thus the present invention presents a guideline for entirely novel material design.
To state more specifically, it is expected from the potentials of the prior art active material and active materials containing transition metal elements singly that the voltages of oxides containing Ni—Co, Ni and Ni—Mn will be higher in the order of Ni—Co>Ni>Ni—Mn. Actually, however, the voltages are higher in the order of Ni—Mn>Ni>Ni—Co, which is the reverse of the expected order. FIG. 10 shows an example of this phenomenon. Taking LiNiO2 as the reference, the potential decreases when Co is added, while the potential increases when Mn is added, contrary to the expectation that it will decrease. From this result, also, it is clear that a new function can be developed by mixing two kinds of transition metals in the same proportion to synthesize an active material having a layered structure.
The research related to the present invention has been conducted focusing on LiNi1−xMnxO2. And, it has been clarified that a new function is developed when nickel and manganese are incorporated in each other in substantially the same proportion. It is easily predictable that added values will be obtained by further adding a further new element to the material.
For example, consider a material expressed by formula LiNi1−xMnxAzO2. It is expected that by adding aluminum, magnesium, calcium, strontium, yttrium, ytterbium or the like as A in an adequate amount, the resultant material will improve in thermal stability. It is also expected that by adding another transition metal as A, the cycle life and the polarization will be improved. Further, by combining these elements, it is expected that these improvements are obtained simultaneously.
As the lithium alloys, there are exemplified Li—Al based alloys, Li—Al—Mn based alloys, Li—Al—Mg based alloys, Li—Al—Sn based alloys, Li—Al—In based alloys, Li—Al—Cd based alloys, Li—Al—Te based alloys, Li—Ga based alloys, Li—Cd based alloys, Li—In based alloys, Li—Pb based alloys, Li—Bi based alloys, Li—Mg based alloys and the like. In this case, the lithium content is preferably 10% by weight or higher.
Lithium salts dissolved in these solvents include, for example, LiC1O4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LISCN, LiCF3SO3, LiCF3CO2, Li(CF3SO2)2, LiAsF6, LiN(CF3SO2)2, LiB10Cl10, lithium lower aliphatic carboxylate, chloroborane lithium, lithium tetraphenyl borate, and imides such as LiN(CF3SO2)(C2F5SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2 and LiN(CF3SO2)(C4F9SO2). These salts can be used in the electrolyte alone or in any combination thereof within the scope that does not impair the effect of the present invention. Among them, it is particularly preferable to add LiPF6.
As the inorganic solid electrolyte, nitrides of Li, halides of Li, and oxysalt of Li are well known. Among them, Li4SiO4, Li4SiO4-LiI—LiOH, xLi3PO4-(1−x)Li4SiO4, Li2SiS3, Li3PO4-Li2S—SiS2 and phosphorus sulfide compounds are effectively used.
FIG. 12 is a schematic vertical cross-sectional view of a cylindrical battery produced in this example.
A battery case 11 houses an electrode plate group 14 composed of a positive electrode plate and a negative electrode plate wound in a helical shape with a separator therebetween forming a plurality of windings. A positive electrode lead 15 is drawn out from the positive electrode plate and connected to a sealing plate 12, while a negative electrode lead 16 is drawn out from the negative electrode plate and connected to the bottom of the battery case 11. The battery case and the lead plates may be made of a metal or an alloy that is resistant to an organic electrolyte and has electron conductivity. Examples of such a metal and alloy include metals such as iron, nickel, titanium, chromium, molybdenum, copper and aluminum and alloys of these metals. In particular, one machined from a stainless steel plate or an Al—Mn alloy plate is most suitable for the battery case, aluminum for the positive electrode lead, and nickel for the negative electrode lead. Also, for the battery case, various engineering plastics and these in combination with metals may be used for reduction in weight.
The positive electrode plate was produced in the following manner. Ten parts by weight of carbon powder as the conductive material and 5 parts by weight of a polyvinylidene fluoride resin as the binder were mixed with 85 parts by weight of the positive electrode active material powder of the present invention. The resultant mixture was dispersed in dehydrated N-methyl pyrrolidinone to obtain slurry, and the slurry was applied to a positive electrode current collector made of aluminum foil, which was then dried, rolled, and cut to a predetermined size. The negative electrode plate was produced in the following manner. A carbonaceous material as the main material was mixed with a styrene-butadiene rubber binder at a weight ratio of 100:5 and, the resultant mixture was applied to both surfaces of copper foil, which was then dried, rolled, and cut to a predetermined size. As the separator, a polyethylene microporous film was used. As the organic electrolyte, that obtained by dissolving 1.5 mol/liter of LiPF6 in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:1 was used. The resultant cylindrical battery was 18 mm in diameter and 650 mm in height.
As the positive electrode active material, used were four kinds of positive electrode active materials expressed by formula (2) Li[Lix(Ni1/2Mn11/2)1−x]O2 wherein x was 0, 0.1, 0.2 and 0.3.
The conditions for the check of the capacity are as follows. As the charge, 4.2 V constant voltage charge was performed with a maximum current of 1 A. The charge was terminated when the current value reached 50 mA. As the discharge, 300 mA constant current discharge was performed down to 2.5 V. The discharge capacity obtained at this time was determined as the discharge capacity of the battery. The charge/discharge was conducted in an atmosphere of 25� C. The high-rate discharge ratio was obtained in the following manner; regarding the battery capacity is 1C, the discharge capacity at a current value in the 5 hour rate discharge (0.2C) and the discharge capacity at a current value in the 0.5 hour rate discharge (2C) were measured, and the capacity ratio 0.2C/2C was calculated. The low-temperature discharge ratio was obtained by measuring the discharge capacities obtained when discharged at the 1C current at 20� C. and at −10� C. and calculating the discharge capacity ratio (−10� C./20� C.). The cycle life was obtained by calculating the ratio of the capacity after 100 cycles to the initial capacity.
x in Li[Lix High-rate
(Ni1/2Mn1,2)
1−x]O2 (mAh)
0 1588
When the oxide expressed by Li[Lix(Ni1/2Mn1/2)1−x]O2 was used as the positive electrode active material, charge/discharge was repeated at substantially the same discharge voltage as described above. Also, as is found from Table 2, the battery capacity decreases with increase of the value of x and, therefore, the charge/discharge capacities can be controlled by controlling the quantity of the lithium element freely within the above range. Thus, the present invention capable of non-electrochemically synthesizing the positive electrode active material with such charge/discharge behavior, presents a guideline for novel material design.
Reduction in Polarization
LiNiO2 and LiMnO2 are not so good in electron conductivity. Therefore, large polarization occurs in the final stage of discharge, causing decrease in capacity particularly during high-rate discharge. The nickel element and the manganese element have different electron structures. When these elements are incorporate together in the atomic level, one electron structure interacts with the electron structure of the neighboring different element.
When the composition of the oxide is LiNi1/2Mn1/2O2, polarization can clearly be reduced compared with the cases of LiNiO2 and LiMnO2. Further, polarization can be reduced by adding another transition metal element while maintaining the 1:1 nickel-manganese ratio. In this example, an oxide expressed by formula LiCo1/3Ni1/3Mn1/3O2 was synthesized. The high-rate discharge rate measurement shown in Table 3 was also performed in this example by producing the battery shown in FIG. 12.
When Li is removed from LiNiO2 by charging, LiNiO2 becomes very unstable and is reduced to NiO releasing oxygen at a comparatively low temperature. This is fatal when LiNiO2 is used as the positive electrode active material of a battery, and the battery may possibly be led to thermal runaway, that is, ignition or explosion due to oxygen generated.
x in Li[Lix(Ni1/2Mn1/2)
1−x]O2 (� C.) of DSC measurement
From Table 4, it is found that the exothermal temperature rose compared with the comparative example. Also, the exothermal temperature roses as the value of x of formula Li[Lix(Ni1/2Mn1/2)1−x]O2 increased. The reason is regarded as follows.
As shown in FIG. 9, the crystal structure of Li[Lix(Ni1/2Mn1/2)1−x]O2 greately changes by overcharge and this brings thermal stability. It is therefore considered that the thermal stability of Li[Lix(Ni1/2Mn1/2)1−x]O2 (x=0.1 to 0.3) with excessively added Li improves by overcharge due to the mechanism described above. Moreover, it is found that, with addition of aluminum, the exothermal temperature further rose and thus the thermal stability significantly increased. The added amount of aluminum was examined and it was found that the range of 5 to 35 mol % with respect to the total amount of aluminum and the transition metals exhibited preferable results. When the added amount was less than 5 mol %, no sufficient effect was obtained and, when it exceeded 35 mol %, the capacity decreased.
EXAMPLES 4 AND 5 AND COMPARATIVE EXAMPLES 2 TO 9
Peak of Precursor
A mixed solution of 1.2 mol/liter of an aqueous nickel sulfate solution, 1.2 mol/liter of an aqueous manganese sulfate solution, and 1.2 mol/liter of an aqueous cobalt sulfate solution, as well as 4.8 mol/liter of an aqueous NaOH solution and 4.8 mol/liter of a NH3 solution, were fed into the reaction bath 6 of the apparatus shown in FIG. 4 at a rate of 0.5 milliliter/minute, to obtain a nickel-manganese-cobalt composite hydroxide as a precursor “a” of the present invention. Dissolved oxygen in the reaction bath was purged by bubbling argon gas. Also, hydrazine was added under adjustment to prevent a magnetic substance such as excessively reduced CoO from being included in the precursor as the reactant. The X-ray diffraction pattern of the precursor “a” is shown in (a) of FIG. 6.
The precursor “a” and lithium hydroxide were mixed so that the atomic ratio of Li to Ni, Mn and Co satisfies Li/(Ni+Mn+Co)=1, and the mixture was heated to 1000� C. at one rise and sintered at this temperature for 10 hours. After the sintering, the temperature was first lowered to 700� C., at which annealing was performed for 5 hours, and then gradually lowered, thereby to obtain the positive electrode active material “a” (LiNi1/3Mn1/3Co1/3O2) of the present invention (Example 4).
A nickel-manganese oxide (nickel:manganese=1:1) as a precursor “b” of the present invention was also obtained in the same manner as that described above except that cobalt sulfate was not used. The X-ray diffraction pattern of the precursor “b” is shown in (b) of FIG. 6.
The precursor “b” and lithium hydroxide were mixed so that the atomic ratio of Li to Ni and Mn satisfied Li/(Ni+Mn)=1, and the resultant mixture was heated to 1000� C. at one rise and sintered at this temperature for 10 hours. After the sintering, the temperature was first lowered to 700� C., at which annealing was performed for 5 hours, and then gradually lowered, thereby to obtain the positive electrode active material “b” (LiNi1/2Mn1/2O2) of the present invention (Example 5).
Nickel-manganese hydroxides “c” to “j” (nickel:manganese=1:1) were obtained in the same manner as that described above except that neither bubbling of argon gas nor addition of hydrazine was performed. The X-ray diffraction patterns of these hydroxides are shown in (c) to (j) of FIG. 6. Using the hydroxides c to j and lithium hydroxide, the positive electrode active materials “c” to “j” were obtained in the manner described above (Comparative Examples 2 to 9).
The coin-shaped batteries were produced in the following manner. The positive electrode active materials “a” to “j” obtained at various sintering temperatures, acetylene black as the conductive material, and a polyvinylidene fluoride resin (PVDF) as the binder were mixed at a weight ratio of 80:10:10, to obtain a sheet-shaped molded article. The molded article was stamped into a disk shape and dried under vacuum at 80� C. for about 15 hours, to obtain a positive electrode. Also, a sheet-shaped lithium metal was stamped into a disk shape, to obtain a negative electrode. A polyethylene microporous film was used as a separator. One mol of LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in 1:3 (volume ratio) to prepare an electrolyte.
TABLE 5 Discharge capacity Cycle Precursor Composition (mAh/g) life a LiNi1/3Mn1/3Co1/3O2 165 100 b LiNi1/2Mn1/2O2 155 102 c LiNi1/2Mn1/2O2 142 69 d LiNi1/2Mn1/2O2 139 68 e LiNi1/2Mn1/2O2 138 65 f LiNi1/2Mn1/2O2 140 68 g LiNi1/2Mn1/2O2 136 65 h L1Ni1/2Mn1/2O2 145 72 i LiNi1/2Mn1/2O2 146 75 j LiNi1/2Mn1/2O2 144 73 Industrial Applicability
According to the present invention, inexpensive nickel-manganese composite oxide can be effectively used as the positive electrode active material, and a good non-aqueous electrolyte battery with high capacity and high charge/discharge efficiency can be provided.
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