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Negative electrode active material , nonaqueous electrolyte battery, battery pack and vehicle

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Negative electrode active material , nonaqueous electrolyte battery, battery pack and vehicle


A negative electrode active material includes lithium-titanium composite oxide porous particles having an average pore size of 50 to 500 Å.
Related Terms: Electrode Electrolyte Lithium Battery Pack Titanium



USPTO Applicaton #: #20130029228 - Class: 4292311 (USPTO) - 01/31/13 - Class 429 
Inventors: Hiroki Inagaki, Norio Takami

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The Patent Description & Claims data below is from USPTO Patent Application 20130029228, Negative electrode active material , nonaqueous electrolyte battery, battery pack and vehicle.

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CROSS-REFERENCE TO RELATED APPLICATIONS

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This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-199457, filed Jul. 7, 2005, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

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1. Field of the Invention

The present invention relates to a negative electrode active material, a nonaqueous electrolyte battery using the negative electrode active material, a battery pack using the nonaqueous electrolyte battery, and a vehicle having the battery pack mounted thereto.

2. Description of the Related Art

Vigorous research is being conducted on a nonaqueous electrolyte battery in which the battery is charged and discharged by the migration of lithium ions between the negative electrode and the positive electrode in an attempt to develop a high energy density battery.

The nonaqueous electrolyte battery is required to satisfy various characteristics depending on the use of the battery. For example, it is desirable for the nonaqueous electrolyte battery used as a power source of a digital camera to achieve the discharge not lower than about 3 C, and for the nonaqueous electrolyte battery mounted to a vehicle such as a hybrid automobile to achieve the discharge not lower than about 10 C. Such being the situation, the nonaqueous electrolyte battery used in the fields exemplified above is required to exhibit an excellent charge-discharge cycle life when the charge-discharge is repeated under a large current.

The nonaqueous electrolyte battery available on the market nowadays comprises a positive electrode in which a lithium-transition metal composite oxide is used as the positive electrode active material and a negative electrode in which a carbonaceous material is used as the negative electrode active material. In general, Co, Mn, Ni, etc. are used as the transition metals contained in the lithium-transition metal composite oxide used as the positive electrode active material.

In recent years, a nonaqueous electrolyte battery in which lithium-titanium oxide having a high Li ion insertion potential, compared with the carbonaceous material, is used as a negative electrode active material has been put to the practical use. The lithium-titanium oxide is small in change of volume accompanying the charge-discharge operation of the secondary battery, and, thus, permits the nonaqueous electrolyte battery using the lithium-titanium oxide as the negative electrode active material to be excellent in the charge-discharge cycle characteristics, compared with the nonaqueous electrolyte battery using the carbonaceous material as the negative electrode active material. Particularly, it is desirable to use lithium titanate having a spinel structure as the negative electrode active material.

For example, Japanese Patent Disclosure (Kokai) No. 09-199179 discloses a nonaqueous electrolyte battery comprising lithium titanate, which is small in change of volume during the charge-discharge operation of the secondary battery, as the negative electrode active material. It is taught that the nonaqueous electrolyte battery is small in change of volume, and that the short circuiting and the decrease of the battery capacity accompanying the swelling of the electrode are unlikely to take place.

On the other hand, Japanese Patent Disclosure No. 09-309727 refers to secondary particles of lithium titanate having a laminate structure constructed such that a plurality of plate-like or flake-like lithium titanate primary particles are superposed one upon the other. It is taught that pores, each sized about 4 nm (40 Å), are formed among the primary particles to increase the specific surface area of the secondary particles of lithium titanate.

BRIEF

SUMMARY

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OF THE INVENTION

An object of the present invention is to provide a negative electrode active material excellent in the large current characteristics and in the charge-discharge cycle characteristics, a nonaqueous electrolyte battery using the negative electrode active material, a battery pack using the nonaqueous electrolyte battery, and a vehicle using the battery pack.

According to a first aspect of the present invention, there is provided a nonaqueous electrolyte battery, comprising:

a positive electrode;

a negative electrode containing lithium-titanium composite oxide porous particles having an average pore size of 50 to 500 Å; and

a nonaqueous electrolyte.

According to a second aspect of the present invention, there is provided a battery pack, comprising nonaqueous electrolyte batteries, each comprising:

a positive electrode;

a negative electrode containing lithium-titanium composite oxide porous particles having an average pore size of 50 to 500 Å; and

a nonaqueous electrolyte.

According to a third aspect of the present invention, there is provided a negative electrode active material comprising lithium-titanium composite oxide porous particles having an average pore size of 50 to 500 Å.

Further, according to a fourth aspect of the present invention, there is provided a vehicle comprising a battery pack including nonaqueous electrolyte batteries, each comprising:

a positive electrode;

a negative electrode containing lithium-titanium composite oxide porous particles having an average pore size of 50 to 500 Å; and

a nonaqueous electrolyte.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross sectional view schematically showing the construction of a flat type nonaqueous electrolyte battery according to a first embodiment of the present invention;

FIG. 2 is a cross sectional view showing in detail in a magnified fashion the construction of the circular region A of the nonaqueous electrolyte battery shown in FIG. 1;

FIG. 3 is an oblique view, partly broken away, schematically showing the construction of another nonaqueous electrolyte battery according to the first embodiment of the present invention;

FIG. 4 is a cross sectional view showing in a magnified fashion the construction of region B of the nonaqueous electrolyte battery shown in FIG. 3;

FIG. 5 is an oblique view showing in a dismantled fashion the construction of the battery pack according to a second embodiment of the present invention;

FIG. 6 is a block diagram showing the electric circuit of the battery pack shown in FIG. 5;

FIG. 7 is a graph showing the log differential pore volume distribution of the negative electrode active material measured by the gas adsorption method (BHJ analytical result (desorption side));

FIG. 8 is a graph showing the particle diameter distribution, as determined by the laser diffraction, of the negative electrode active material for Example 1;

FIG. 9 is a photo by a scanning electron microscope (SEM) showing the lithium titanate having the spinel structure for Example 2; and

FIG. 10 shows the X-ray diffraction pattern of the lithium titanate having the spinel structure for Example 2.

DETAILED DESCRIPTION

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OF THE INVENTION

A lithium-titanium composite oxide is small in change of volume accompanying the charge-discharge operation of the battery, i.e., accompanying the absorption-release of lithium ions. The electrode containing the composite oxide as the active material is unlikely to be swollen. On the other hand, the volume of the negative electrode available on the market, which contains a carbonaceous material such as graphite as the negative electrode active material, is expanded or shrunk by several percent in accordance with the charge-discharge operation of the battery. As a result, where, for example, graphite is used as the negative electrode active material, the nonaqueous electrolyte is diffused in accordance with the expansion and shrinkage of the electrode. As a result, the impregnation of the negative electrode with the nonaqueous electrolyte tends to be made uniform. Alternatively, the concentration of the lithium salt tends to be made uniform. It has been found, however, that the electrode containing a lithium-titanium composite oxide and, thus, small in change of volume, is markedly poor in the impregnation capability with the nonaqueous electrolyte. Particularly, in the case of manufacturing a large battery mounted to, for example, a vehicle, the poor impregnation capability of the electrode with the nonaqueous electrolyte lowers not only the productivity but also the battery performance, in particular, the large current performance and the charge-discharge cycle characteristics.

Under the circumstances, the present inventors have strongly pulverized the lithium-titanium composite oxide powder containing the lithium-titanium oxide having a spinel structure as the main phase, followed by baking again the pulverized material under an appropriate heat treating condition to succeed in the synthesis of lithium-titanium composite oxide porous particles having an average pore size of 50 to 500 Å. It has been found that, by synthesizing the lithium-titanium composite oxide porous particles having an average pore size of 50 to 500 Å, it is possible to markedly improve the impregnation capability of the negative electrode with the nonaqueous electrolyte to improve not only the productivity but also the large current characteristics and the charge-discharge cycle life of the battery. Incidentally, the impregnation capability of the negative electrode with the nonaqueous electrolyte can be further improved, if the specific pore volume of the lithium-titanium composite oxide porous particles is not smaller than 0.01 mL/g.

It should also be noted that, if the specific volume of pores having a size not larger than 10 Å, i.e., so-called micro pores, is not smaller than 0.001 mL/g in the lithium-titanium composite oxide porous particles having the average pore size noted above, it is possible to permit the lithium ions to be migrated to reach the region that was not involved in the reaction in the past. As a result, it is possible to realize the lithium absorption capability close to the theoretical capacity of the lithium-titanium oxide to increase the energy density of the battery.

Some embodiments of the present invention will now be described with reference to the accompanying drawings. Incidentally, the common constituents of the invention are denoted by the same reference numerals in the accompanying drawings to omit the overlapping description. Also, the accompanying drawings are schematic drawings that are simply intended to facilitate the description and understanding of the invention. It is possible for the shape, the size, the ratio, etc. shown in the drawing to differ from those of the actual battery. Of course, the design relating to the size, shape, etc. can be changed appropriately in view of the description given below and the known technology.

First Embodiment

An example of the construction of the unit cell, i.e., nonaqueous electrolyte battery, according to the first embodiment of the present invention will now be described with reference to FIGS. 1 and 2.

Specifically, FIG. 1 is a cross sectional view schematically showing the construction of a flat type nonaqueous electrolyte battery according to a first embodiment of the present invention, and FIG. 2 is a cross sectional view schematically showing in detail in a magnified fashion the construction of the circular region A of the nonaqueous electrolyte battery shown in FIG. 1.

As shown in FIG. 1, a flat type wound electrode group 6 is housed in a case 7. The wound electrode group 6 is formed of a laminate structure comprising a positive electrode 3, a negative electrode 4, and a separator 5 interposed between the positive electrode 3 and the negative electrode 4. The electrode group 6 is obtained by spirally winding the laminate structure noted above. Further, a nonaqueous electrolyte is retained by the wound electrode group 6.

As shown in FIG. 2, the negative electrode 4 is positioned to constitute the outermost circumferential region of the wound electrode group 6. Also, the positive electrode 3 and the negative electrode 4 are alternately laminated one upon the other with the separator 5 interposed therebetween. For example, the separator 5, the positive electrode 3, the separator 5, the negative electrode 4, the separator 5, the positive electrode 3 and the separator 5 are laminated one upon the other in the order mentioned. The negative electrode 4 comprises a negative electrode current collector 4a and a negative electrode active material-containing layer 4b supported by the negative electrode current collector 4a. In that region of the negative electrode 4 which constitutes the outermost circumferential region, the negative electrode active material-containing layer 4b is formed on one surface of the negative electrode current collector 4a. On the other hand, the positive electrode 3 comprises a positive electrode current collector 3a and a positive electrode active material-containing layer 3b supported by the positive electrode current collector 3a.

As shown in FIG. 1, a band-like positive electrode terminal 1 is electrically connected to the positive electrode current collector 3a in the vicinity of the outer circumferential region of the wound electrode group 6. On the other hand, a band-like negative electrode terminal 2 is electrically connected to the negative electrode current collector 4a in the vicinity of the outer circumferential region of the wound electrode group 6. Further, the tip portions of the positive electrode terminal 1 and the negative electrode terminal 2 are withdrawn to the outside of the case 7 via the same side of the case 7.

The negative electrode, the nonaqueous electrolyte, the positive electrode, the separator, the case, the positive electrode terminal and the negative electrode terminal will now be described in detail.

1) Negative Electrode

The negative electrode comprises a negative electrode current collector and a negative electrode layer supported on one surface or both surfaces of the negative electrode current collector and containing a negative electrode active material, a negative electrode conductive agent and a binder.

The negative electrode active material includes porous particles of lithium-titanium composite oxide which have an average pore size not smaller than 50 Å. It is desirable for the pore to be an open cell formed inside a porous material and extending to reach the surface of the porous material, as defined in “Iwanami\'s Dictionary of Physics and Chemistry 5th Edition CD-ROM”. Also, it is desirable for the lithium-titanium composite oxide to contain any of the lithium-titanium oxide phase and a lithium/titanium-containing oxide phase obtained by selectively substituting a foreign element for the constituting elements of the lithium-titanium oxide. In order to obtain excellent large current characteristics and excellent charge-discharge cycle characteristics, it is desirable for the lithium-titanium composite oxide to contain the lithium-titanium oxide phase as the main phase. The term “main phase” noted above denotes the phase having the highest presence ratio among the lithium-titanium composite oxide.

The presence ratio of the constituting phase can be confirmed by the method described in the following.

Specifically, an X-ray diffraction measurement is applied to the porous particles of lithium-titanium composite oxide, and the constituting phase of the lithium-titanium composite oxide is identified from the resultant X-ray diffraction pattern. It is possible to specify the main phase of the lithium-titanium composite oxide by comparing the intensity ratios of the main peaks of the identified constituting phases.

For example, it is possible for the lithium-titanium composite oxide having the spinel structure and the composition of Li4+xTi5O12 (0≦x≦3) to contain the anatase type TiO2, the rutile type TiO2 and Li2TiO3, etc. as impurity phases. If the X-ray diffractometry measurement using Cu—Kα is applied to the lithium-titanium composite oxide noted above, the main peak of Li4+xTi5O12 (0≦x≦3) appears at 4.83 Å (2θ:18°), and the main peaks of anatase type TiO2, rutile type TiO2 and Li2TiO3 appear at 3.51 Å (2θ:25°), 3.25 Å (2θ:27°) and 2.07 Å (2θ:43°), respectively. It is possible to specify the main phase by comparing these intensities.

Incidentally, where the lithium-titanium composite oxide having the spinel structure, which forms the main phase, it is desirable for the main peak intensity of each of the rutile type TiO2, the anatase type TiO2 and Li2TiO3 to be not higher than 7, more desirably not higher than 3, on the basis that the main peak intensity of lithium titanate having the spinel structure as determined by the X-ray diffractometry is set at 100. It should be noted that the diffusion rate of the lithium ions is improved and the ionic conductivity and the large current characteristics are improved with decrease in the amount of these impurity phases.

The lithium-titanium oxide includes, for example, the lithium-titanium oxide having the spinel structure, e.g., Li4+xTi5O12 (0≦x≦3), and the lithium-titanium oxide having the ramsdellite structure, e.g., Li2+yTi3O7 (0≦y≦3). It is desirable to use the lithium-titanium oxide having the spinel structure because it is possible to obtain excellent charge-discharge cycle characteristics of the battery.

It is acceptable for the lithium-titanium composite oxide to contain constituting phases other than the lithium-titanium oxide phase and the lithium/titanium-containing oxide phase. For example, it is possible for the lithium-titanium composite oxide to contain the TiO2 phase and the Li2TiO3 phase.

The average pore size of the lithium-titanium composite oxide porous particles falls within a range of 50 to 500 Å as described previously. Where the average pore size falls within the range noted above, it is possible to improve the wettability of the nonaqueous electrolyte with the surface of the lithium-titanium composite oxide porous particles to improve the impregnation capability of the negative electrode with the nonaqueous electrolyte and, thus, to improve the charge-discharge cycle characteristics of the battery. It should be noted that the reaction of the negative electrode with the nonaqueous electrolyte causes a by-product (organic or inorganic material) to be deposited on the surface of the lithium-titanium composite oxide porous particles, though the deposition amount is small. If the average pore size of the lithium-titanium composite oxide porous particles is smaller than 50 Å, the pores are closed in accordance with growth of the by-product, with the result that the liquid retaining capability, i.e., the capability of retaining the nonaqueous electrolyte, of the negative electrode is lowered to lower the large current characteristics. Such being the situation, it is desirable for the average pore size of the lithium-titanium composite oxide porous particles to be not smaller than 50 Å, more desirably not smaller than 100 Å. On the other hand, if the pore size is large, the strength of the powdery material is lowered, with the result that it is possible for the lithium-titanium composite oxide porous particles, which is a powdery material, to be collapsed when the density of the electrode is increased. Under the circumstances, it is desirable for the upper limit of the average pore size of the lithium-titanium composite oxide porous particles to be set at 500 Å in order to increase the density of the electrode, i.e., in order to increase the volume energy density of the electrode.

It is desirable for the pores formed in the lithium-titanium composite oxide porous particles to have micro pores having a pore size not larger than 10 Å. If the lithium-titanium composite oxide porous particles have the micro pores, the wettability of the negative electrode with the nonaqueous electrolyte is improved to increase the impregnation capability of the negative electrode with the nonaqueous electrolyte. It is also possible to decrease that region of the lithium-titanium composite oxide porous particles which does not contribute to the charge-discharge reaction to improve the charge-discharge capacity of the battery. In order to obtain a sufficient effect produced by the presence of the micro pores, it is desirable for the specific volume of the pores having a pore size not larger than 10 Å to be not smaller than 0.001 mL/g. The upper limit of the specific volume of the pores is not particularly specified. However, it is desirable for the upper limit of the specific volume of the pores to be set at 0.01 mL/g in view of the volume energy density of the negative electrode. It is more desirable for the specific volume of the pores to fall within a range of 0.0015 to 0.003 mL/g.

If the specific pore volume of the lithium-titanium composite oxide porous particles is not smaller than 0.01 mL/g, the negative electrode is impregnated smoothly with the nonaqueous electrolyte to improve the wettability of the lithium-titanium composite oxide porous particles with the nonaqueous electrolyte, with the result that the reaction of the lithium-titanium composite oxide proceeds smoothly when the battery is charged and discharged. It follows that an over-voltage is not applied locally to improve the charge-discharge cycle life of the battery. It should also be noted that the negative electrode exhibits an improved capability of retaining the nonaqueous electrolyte, with the result that the depletion of the nonaqueous electrolyte is unlikely to take place to further improve the charge-discharge cycle life of the battery. It is more desirable for the specific pore volume of the lithium-titanium composite oxide porous particles to be not smaller than 0.02 mL/g, furthermore desirably to be not smaller than 0.1 mL/g. The upper limit of the specific pore volume noted above is not particularly specified. However, it is desirable for the specific pore volume to be not larger than 1 mL/g in view of the volume energy density of the negative electrode.

It is desirable for the lithium-titanium composite oxide porous particles to have an average particle diameter not larger than 1 μm. If the average particle diameter exceeds 1 μm, it is difficult to expect the sufficient impregnation capability of the negative electrode with the nonaqueous electrolyte even if the average pore size is set to fall within the range specified in the first embodiment. It should be noted, however, that, if the average particle diameter of the lithium-titanium composite oxide porous particles is excessively small, the distribution of the nonaqueous electrolyte is inclined on the negative electrode to bring about possibly the depletion of the nonaqueous electrolyte on the positive electrode. Such being the situation, it is desirable for the lower limit in the average particle diameter of the lithium-titanium composite oxide porous particles to be set at 0.001 μm. It is more desirable for the lithium-titanium composite oxide porous particles to have an average particle diameter not larger than 1 μm and to have a BET specific surface area as determined by the N2 adsorption falling within a range of 5 to 50 m2/g.

A method of manufacturing the lithium-titanium composite oxide porous particles will now be exemplified.

In the first step, a lithium salt such as lithium hydroxide, lithium oxide or lithium carbonate is prepared as the lithium source. Then, a prescribed amount of the lithium salt is dissolved in a pure water, followed by adding titanium oxide to the resultant solution such that lithium and titanium have a prescribed atomic ratio. In the case of synthesizing the lithium-titanium oxide having the composition of Li4Ti5O12 and the spinel structure, Li and Ti are mixed in a manner to have an atomic ratio of 4:5.

In the next step, the mixture is dried while stirring the resultant solution to obtain a baking precursor. The drying method employed in this stage includes, for example, a spray drying method, a granulating drying method, a freeze drying method, and a combination thereof. The baking precursor thus obtained is baked to obtain a lithium-titanium composite oxide. It suffices to carry out the baking under the air atmosphere. It is also possible to carry out the baking under an oxygen gas atmosphere or an inert gas atmosphere such as an argon gas atmosphere.

It suffices to carry out the baking at 680 to 1,000° C. for 1 to 24 hours. Preferably, the baking should be carried out at 720 to 800° C. for 5 to 10 hours.

If the baking temperature is lower than 680° C., the reaction between titanium oxide and the lithium compound is not carried out sufficiently to increase the impurity phases such as the anatase type TiO2, the rutile type TiO2 and Li2TiO3, with the result that the electric capacity of the negative electrode is lowered. On the other hand, if the baking temperature exceeds 1,000° C., the crystallite diameter is rendered excessively large in accordance with progress of the baking in the case of the lithium titanate having the spinel structure to lower the large current performance.

It is possible to control the pore volume and the average pore size of the primary particles by pulverizing and re-baking the lithium-titanium composite oxide porous particles obtained by the baking treatment described above. The pulverizing treatment and the re-baking treatment noted above are carried out under the conditions described in the following. In the pulverizing method, it is possible to use, 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 whirling air stream type jet mill or a sieve. In the pulverizing stage, it is possible to employ a wet pulverization that is carried out in the presence of a known liquid pulverizing aid such as water, ethanol, ethylene glycol, benzene or hexane. It is effective to use the pulverizing aid for improving the pulverizing efficiency and for increasing the formed amount of the fine powdery material. It is more desirable to use a ball mill utilizing zirconia balls as the pulverizing medium to carry out the wet pulverization with the liquid pulverizing aid added thereto. Further, in order to improve the pulverizing efficiency, it is possible to add an organic material such as polyol as a pulverizing aid. The polyol used as the pulverizing aid is not particularly limited. However, it is possible to use pentaerythritol, triethylol ethane, and trimethylol propane singly or in combination.

It also suffices to carry out the re-baking under the air atmosphere. It is also possible to carry out the re-baking under an oxygen atmosphere or an inert gas atmosphere such as an argon gas atmosphere. It suffices to carry out the re-baking at 250 to 900° C. for about one minute to 10 hours. If the re-baking temperature is higher than 900° C., the re-baking of the pulverized powder proceeds excessively to collapse the pores even if the heat treatment is carried out for a short time, resulting in failure to obtain the pore size distribution specified in the first embodiment of the present invention. On the other hand, if the re-baking temperature is lower than it is impossible to remove the impurities (organic material) attached to the pulverized powder in the wet pulverizing stage to lower the battery performance. It is more desirable for the re-baking to be carried out at 400 to 700° C. for 10 minutes to 3 hours.

It is desirable for the pH value of the lithium-titanium composite oxide porous particles having an average pore size of 50 to 500 Å to fall within a range of 10 to 11.2. In the baking process of the lithium-titanium composite oxide such as lithium titanate, lithium carbonate or lithium hydroxide is formed as a by-product, which is derived from the unreacted Li component that is not taken into the lithium titanate. It is possible to improve the battery performance, particularly, the high temperature charge-discharge cycle performance and the output performance, by decreasing the amount of the unreacted Li component to 11.2 or less in terms of the pH value.

It should be noted that the unreacted Li component such as lithium carbonate or lithium hydroxide remaining on the surface of the active material reacts with the nonaqueous electrolyte to generate a carbon dioxide gas or a hydrocarbon gas. Also, an organic film acting as a resistance component is formed on the surface of the active material by the side reaction noted above.

However, where the lithium titanate powder is mechanically pulverized under the conditions given previously, the unreacted Li component tends to be exposed to the surface to cause the pH value to be increased to exceed 11.2, thereby lowering the battery performance. Such being the situation, the re-baking process is carried out after the pulverizing process. By the re-baking process, the unreacted Li component exposed to the surface is taken into the inner region of the active material to decrease the unreacted Li component remaining on the surface. Such being the situation, the pH value can be controlled to 11.2 or less by applying the re-baking process after the pulverizing process.

It is possible to decrease the amount of the excessive Li such as lithium carbonate formed as a by-product by lowering the ratio of the Li source in the stage of carrying out the reaction between the Li source such as lithium hydroxide or lithium carbonate, which is used as the raw material of lithium titanate, and titanium oxide, e.g., anatase type TiO2 or rutile type TiO2. However, if the ratio of the Li source is lowered, the ratio of lithium contained in the obtained active material is lowered, with the result that the electric capacity of lithium titanate is lowered. Such being the situation, in order to maintain a high electric capacity, it is desirable for the pH value of the resultant active material to be maintained at 10 or more without decreasing the amount of the Li source.

Also, it is more desirable for the pH value to fall within a range of 10.3 to 11 in order to maintain the high electric capacity and in view of the aspect of suppressing the side reaction.

Incidentally, the pH value of the lithium-titanium composite oxide porous particles can be measured as follows. Specifically, 2 g of the lithium-titanium composite oxide porous particles used as the negative electrode active material are dispersed in 100 mL of a pure water (25° C.), and the suspension is stirred for about 10 minutes, followed by filtering the active material to obtain a filtrate. The pH value of the filtrate is used as the pH value of the lithium-titanium composite oxide porous particles.

It is desirable for the negative electrode current collector to be formed of an aluminum foil or an aluminum alloy foil. In the case of using an aluminum foil or an aluminum alloy foil as the negative electrode current collector, it is possible to prevent the negative electrode current collector from being dissolved in and corroded by the nonaqueous electrolyte during the over-discharge cycle.

It is desirable for the negative electrode current collector to have an average crystal grain size not larger than 50 μm. In this case, the mechanical strength of the current collector can be drastically increased to make it possible to increase the density of the negative electrode by applying the pressing under a high pressure to the negative electrode. As a result, the battery capacity can be increased. Also, since it is possible to prevent the dissolution and corrosion deterioration of the negative electrode current collector over a long over-discharge cycle under an environment of a high temperature not lower than, for example, 40° C., it is possible to suppress the elevation in the impedance of the negative electrode. Further, it is possible to improve the high-rate characteristics, the rapid charging properties, and the charge-discharge cycle characteristics of the battery. It is more desirable for the average crystal grain size of the negative electrode current collector to be not larger than 30 μm, furthermore desirably, not larger than 5 μm.

The average crystal grain size can be obtained as follows. Specifically, the texture of the current collector surface is observed with an electron microscope to obtain the number n of crystal grains present within an area of 1 mm×1 mm. Then, the average crystal grain area S is obtained from the formula “S=1×106/n (μm2)”, where n denotes the number of crystal grains noted above. Further, the average crystal grain size d (μm) is calculated from the area S by formula (I) given below:


d=2(S/π)1/2  (I)

The aluminum foil or the aluminum alloy foil having the average crystal grain size not larger than 50 μm can be complicatedly affected by many factors such as the composition of the material, the impurities, the process conditions, the history of the heat treatments and the heating conditions such as the annealing conditions, and the crystal grain size can be adjusted by an appropriate combination of the factors noted above during the manufacturing process.

It is desirable for the aluminum foil or the aluminum alloy foil to have a thickness not larger than 20 μm, more desirably not larger than 15 μm. Also, it is desirable for the aluminum foil to have a purity not lower than 99%. It is desirable for the aluminum alloy to contain another element such as magnesium, zinc or silicon. On the other hand, it is desirable for the amount of the transition metal such as iron, copper, nickel and chromium contained in the aluminum alloy to be not larger than 1%.

It is possible for the negative electrode active material-containing layer to contain a conductive agent. The conductive agent includes, for example, a carbon material, a metal powder such as an aluminum powder, and a conductive ceramic material such as TiO. The carbon material used as the conductive agent includes, for example, acetylene black, carbon black, coke, a carbon fiber and graphite. It is more desirable for the carbon material to include, for example, coke subjected to a heat treatment at 800 to 2,000° C. and having an average particle diameter not larger than 10 μm, graphite, a TiO powder, and a carbon fiber having an average particle diameter not larger than 1 μm. It is desirable for the carbon material to have at least 10 m2/g of the BET specific surface area as determined by the N2 adsorption.

It is also possible for the negative electrode active material-containing layer to contain a binder. The binder includes, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), a fluorinated rubber, styrene-butadiene rubber and a core shell binder.

Concerning the mixing ratio of the negative electrode active material, the negative electrode conductive agent, and the binder, it is desirable for the negative electrode active material to be used in an amount not smaller than 70% by weight and not larger than 96% by weight, for the negative electrode conductive agent to be used in an amount not smaller than 2% by weight and not larger than 28% by weight, and for the binder to be used in an amount not smaller than 2% by weight and not larger than 28% by weight. If the mixing amount of the negative electrode conductive agent is smaller than 2% by weight, the current collecting performance of the negative electrode layer may be lowered so as to possibly lower the large current characteristics of the nonaqueous electrolyte battery. Also, if the mixing amount of the binder is smaller than 2% by weight, the bonding between the negative electrode layer and negative electrode current collector may be lowered so as to possibly lower the charge-discharge cycle characteristics of the nonaqueous electrolyte battery. On the other hand, it is desirable for the mixing amount of each of the negative electrode conductive agent and the binder to be not larger than 28% by weight in view of the improvement in the capacity of the nonaqueous electrolyte battery.

The negative electrode can be manufactured by suspending the negative electrode active material, the conductive agent and the binder described above in a suitable solvent, followed by coating the current collector with the resultant suspension and subsequently drying, pressing the current collector coated with the suspension.

In the case of evaluating the properties of the negative electrode active material such as the pore size distribution and the particle diameter by taking the negative electrode active material out of the nonaqueous electrolyte battery, the negative electrode is taken out by dismantling the nonaqueous electrolyte battery under an argon gas atmosphere, followed by peeling off the negative electrode active material-containing layer from the negative electrode current collector. If the negative electrode active material-containing layer is dispersed in acetone, the conductive agent and the binder attached to the negative electrode active material is dissolved in acetone to make it possible to extract the negative electrode active material. After it has been confirmed by, for example, the X-ray diffraction that the conductive agent and the binder have been removed from the negative electrode active material, the required evaluation of the characteristics is carried out.

2) Nonaqueous Electrolyte

The nonaqueous electrolyte includes a liquid nonaqueous electrolyte that is prepared by dissolving an electrolyte in an organic solvent and a gel-like nonaqueous electrolyte that is prepared by using a composite material containing a liquid nonaqueous electrolyte and a polymer material.




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stats Patent Info
Application #
US 20130029228 A1
Publish Date
01/31/2013
Document #
13644500
File Date
10/04/2012
USPTO Class
4292311
Other USPTO Classes
423598, 241 23, 241 17
International Class
/
Drawings
9


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Chemistry: Electrical Current Producing Apparatus, Product, And Process   Current Producing Cell, Elements, Subcombinations And Compositions For Use Therewith And Adjuncts   Electrode   Chemically Specified Inorganic Electrochemically Active Material Containing   Alkalated Transition Metal Chalcogenide Component Is Active Material  

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