Non-aqueous electrolyte secondary battery

This application is a Divisional of U.S. application Ser. No. 13/969,104 filed on Aug. 16, 2013, which is a Continuation Application of PCT Application No. PCT/JP2012/053071, filed Feb. 10, 2012, and based upon and claiming the benefit of priority from Japanese Patent Application No. 2011-033448, filed Feb. 18, 2011, the entire contents of which are all incorporated herein by reference.

Embodiments described herein relate generally to a non-aqueous electrolyte secondary battery.

A lithium ion secondary battery using lithium titanate such as Li 4 Ti 5 O 12 as a negative electrode active material has been recently developed. In such a lithium ion secondary battery, it is known that carbon dioxide is adsorbed as an impurity at the negative electrode. Carbon dioxide may react with the negative electrode active material and thereby, produce a large amount of gas. Such a reaction is remarkable particularly when the battery was stored at a temperature equal to or higher than room temperature. If a large amount of gas is generated, the battery internal pressure increases. Further, in such a case, plastic deformation of the battery may occur.

In addition, it is known that an SEI (Solid Electrolyte Interface) coating (hereinafter, referred to as a coating) is formed on the surface of a negative electrode active material of a lithium ion secondary battery. This coating is produced mainly due to the reductive decomposition of a non-aqueous electrolyte. Reaction between an active material and the non-aqueous electrolyte can be suppressed by this coating. However, when the coating is too thick, diffusion resistance of the lithium ion becomes larger. As a result, the large current performance is reduced. On the other hand, when the coating is too thin, the reaction of the non-aqueous electrolyte with the active material is less likely to be suppressed. As a result, self-discharge during storage of the battery is increased.

Patent Literature

Patent Literature 1: Jpn. Pat. Appln. KOKAI Publication No. 2002-216843

Patent Literature 2: Jpn. Pat. Appln. KOKAI Publication No. 11-339856

FIG. 1 is a schematic cross-sectional view of a non-aqueous electrolyte secondary battery according to the embodiment;

FIG. 2 is an enlarged sectional view of section A shown in FIG. 1 ;

FIG. 3 is a measurement chart of the negative electrodes according to Examples and Comparative Example by pyrolysis GC/MS; and

FIG. 4 is a measurement chart of the positive electrode according to Example 1 by pyrolysis GC/MS.

There is provided a non-aqueous electrolyte secondary battery which shows a suppression of gas generation during its storage as well as shows a suppression of self-discharge reaction.

In general, according to one embodiment, there is provided a non-aqueous electrolyte secondary battery comprising a positive electrode including a positive electrode active material layer; a negative electrode including a negative electrode active material layer; and a non-aqueous electrolyte. At least one of the positive electrode active material layer and the negative electrode active material layer contains carbon dioxide and releases the carbon dioxide in the range of 0.1 ml to 10 ml per 1 g when heated at 350° C. for 1 minute.

Hereinafter, embodiments will be explained with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of a flat type non-aqueous electrolyte secondary battery. FIG. 2 is an enlarged sectional view of section A in FIG. 1 . A battery 1 includes a wound electrode group 2 . The wound electrode group 2 is accommodated in a container 3 . A non-aqueous electrolyte is also filled in the container 3 (not shown).

The wound electrode group 2 , as shown in FIG. 2 , is a laminate. In the laminate, a separator 6 is interposed between a positive electrode 4 and a negative electrode 5 . A flat type wound electrode group can be obtained by winding the laminate spirally and pressing it.

As shown in FIG. 1 , in the vicinity of the circumferential edge of the wound electrode group 2 , a positive electrode terminal 7 is connected to the positive electrode 4 , and a negative electrode terminal 8 is connected to the negative electrode 5 . An exterior bag made of a laminated film is used for the container 3 . The wound electrode group 2 and a non-aqueous electrolyte are sealed by heat-sealing an opening of the exterior bag made of a laminated film, in the state where the positive electrode terminal 7 and the negative electrode terminal 8 are extended through the opening. The container 3 is not limited to the exterior bag made of a laminated film, and, for example, it is also possible to use a metal can.

The positive electrode 4 includes a positive electrode current collector 4 a and a positive electrode active material layer 4 b. The positive electrode active material layer 4 b includes a positive electrode active material and optionally a conductive agent and a binder. The positive electrode active material layer 4 b is formed on one or both surfaces of the positive electrode current collector 4 a.

At least one oxide selected from the group consisting of lithium manganese composite oxide, lithium nickel composite oxide, and lithium composite phosphate compound is preferably used as the positive electrode active material.

Examples of the lithium manganese composite oxide include an oxide such as LiMn 2 O 4 , and another oxide such as Li(Mn x Al y ) 2 O 4 (x+y=1) in which a part of Mn is substituted by a hetero element.

Examples of the lithium nickel composite oxide include an oxide such as LiNiO 2 , and another oxide such as Li(Ni x Mn y Co z )O 2 and Li(Ni x Co y Al z )O 2 (x+y+z=1) in which a part of Ni is substituted by a hetero element.

Examples of the lithium composite phosphate compound include a phosphate such as LiFePO 4 , and another phosphate such as Li(Fe x Mn y )PO 4 (x+y=1) in which a part of Fe is substituted by a hetero element.

Examples of the conductive agent include acetylene black, carbon black, and graphite.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-based rubber, ethylene-butadiene rubber (SBR), polypropylene (PP), polyethylene (PE), and carboxymethyl cellulose (CMC).

The mixing ratio of the positive electrode active material, the conductive agent and the binder is preferably 80 to 95% by mass of the positive electrode active material, 3 to 18% by mass of the conductive agent, and 2 to 7% by mass of the binder.

The positive electrode current collector is preferably an aluminum foil or an aluminum alloy foil containing one or more elements selected from the group consisting of Mg, Ti, Zn, Mn, Fe, Cu and Si.

The negative electrode 5 includes a negative electrode current collector 5 a and a negative electrode active material layer 5 b. The negative electrode active material layer 5 b includes a negative electrode active material and optionally a conductive agent and a binder. The negative electrode active material layer 5 b is formed on one or both surfaces of the negative electrode current collector 5 a.

Lithium titanium composite oxide is preferably used as the negative electrode active material. It is more preferable to use a lithium titanium composite oxide having a lithium ion absorption potential of 0.4 V or more (vs. Li/Li + ) as the negative electrode active material. Examples of such lithium titanium composite oxide include lithium titanate (Li 4+x Ti 5 O 12 ) having a spinel structure, and lithium titanate (Li 2+x Ti 3 O 7 ) having a ramsdellite structure. These lithium titanium oxides may be used alone or as a mixture of two or more thereof. Further, titanium oxide (e.g., TiO 2 ) that becomes lithium titanium oxide by charge and discharge may be used as the negative electrode active material.

The average primary particle diameter of the lithium titanium composite oxide is preferably 5 μm or less. If the average primary particle diameter is 5 μm or less, superior large current discharge performance can be obtained because an effective area to contribute to the electrode reaction is sufficient.

In addition, the specific surface area of the lithium titanium composite oxide is preferably 1 to 10 m 2 /g. If the specific surface area is 1 m 2 /g or more, it is possible to obtain superior large current discharge performance because an effective area to contribute to the electrode reaction is sufficient. On the other hand, if the specific surface area is 10 m 2 /g or less, the reaction with the non-aqueous electrolyte is suppressed, and thus it is possible to suppress decrease in charge and discharge efficiency and gas generation.

Examples of the conductive agent include a carbonaceous material, such as acetylene black, carbon black, and graphite. It is preferable to use a carbonaceous material having a high absorption of alkali metal and a high conductivity.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-based rubber, styrene-butadiene rubber (SBR), polypropylene (PP), polyethylene (PE), and carboxymethyl cellulose (CMC).

The mixing ratio of the negative electrode active material, the conductive agent and the binder is preferably 70 to 95% by mass of the negative electrode active material, 0 to 25% by mass of the conductive agent, and 2 to 10% by mass of the binder.

The negative electrode current collector is preferably an aluminum foil, an aluminum alloy foil containing one or more elements selected from the group consisting of Mg, Ti, Zn, Mn, Fe, Cu and Si, or a copper foil.

The non-aqueous electrolyte is prepared by dissolving an electrolyte in a non-aqueous solvent. The concentration of the electrolyte in the non-aqueous solvent is preferably 0.5 to 2 mol/L.

Known non-aqueous solvents used in lithium batteries can be used as the non-aqueous solvent. Examples of the non-aqueous solvent include a cyclic carbonate such as ethylene carbonate (EC) and propylene carbonate (PC), and a mixed solvent of a cyclic carbonate and a non-aqueous solvent having a lower viscosity than the cyclic carbonate (hereinafter referred to as “second solvent”).

Examples of the second solvent include a linear carbonate (e.g., dimethyl carbonate, methyl ethyl carbonate or diethyl carbonate); γ-butyrolactone, acetonitrile, methyl propionate, ethyl propionate; a cyclic ether (e.g., tetrahydrofuran or 2-methyltetrahydrofuran); and a linear ether (e.g., dimethoxyethane or diethoxyethane).

As the electrolyte, an alkali salt can be used. Preferably, a lithium salt is used. Examples of the lithium salt include lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium hexafluoroarsenate (LiAsF 6 ), lithium perchlorate (LiClO 4 ), and lithium trifluoromethanesulfonate (LiCF 3 SO 3 ). In particular, lithium hexafluorophosphate (LiPF 6 ) and lithium tetrafluoroborate (LiBF 4 ) are preferable.

A separator 6 prevents physical and electrical contact of the positive electrode 4 with the negative electrode 5 . The separator 6 is comprised of an insulating material, and has a shape enabling an electrolyte to permeate through it. The separator 6 may be formed from, for example, a non-woven fabric made of synthetic resin, a porous polyethylene film, a porous polypropylene film, or a cellulose-based separator.

The positive electrode active material layer and the negative electrode active material layer may contain carbon dioxide as an impurity. According to the embodiment, at least one of the positive electrode active material layer and the negative electrode active material layer contains carbon dioxide in an amount capable of releasing the carbon dioxide in the range of 0.1 ml to 10 ml per 1 g when heated at 350° C. for 1 minute.

The carbon dioxide is contained in a state of being adsorbed on an active material, or as a compound such as lithium carbonate. The carbon dioxide contained in the negative electrode may be reduction by charge and discharge on the surface of the active material and thereby released as CO gas. On the other hand, it is considered that carbon dioxide contained in the positive electrode may move to the negative electrode and be reduced, then released as CO gas.

In addition to release as a gas, carbon dioxide reacts with a lithium ion in the electrolyte, or reacts with the negative electrode active material such as a lithium titanium composite oxide, thereby producing Li 2 CO 3 (lithium carbonate). The lithium carbonate is considered to form an inorganic coating and to have an effect of promoting the generation of an organic polymer. Thus, if the carbon dioxide is contained in the active material layer, coating formation is promoted and self-discharge is suppressed. On the other hand, if the amount of carbon dioxide is too much, a large amount of gas generates and the coating becomes thicker, resulting in the decrease of the rate performance.

However, suppression of gas generation as well as formation of suitable coatings can be attained by containing carbon dioxide in an amount within the above range in the active material layer of at least one of the positive electrode and the negative electrode. If the content of carbon dioxide is too little, a stable coating is hardly produced. On the other hand, if the content of carbon dioxide is too much, gas generation is remarkable when batteries are stored at a high temperature.

The amount of carbon dioxide contained in the active material layer can be measured by gas chromatography (GC). In the embodiment, the amount of carbon dioxide generated when the active material layer was maintained at 350° C. for 1 minute is measured. In the case where heating is carried out at a temperature exceeding 350° C. or heating time is more than 1 minute, the conductive agent and the binder may undergo thermal decomposition or combustion, which may cause the generation of carbon dioxide. Therefore, in this case, carbon dioxide other than the carbon dioxide contained as an impurity may also be measured.

GC measurement can be carried out as follows. Using a spatula or the like, several mg of the active material layer is scraped out from the electrode, and placed in a measurement container. This operation is carried out in a glove box under an inert gas atmosphere. As for such inert gas, an inert gas such as nitrogen and argon other than carbon dioxide is used. Then, the measurement container is introduced into a device while maintaining the inert gas atmosphere, then, the amount of carbon dioxide generated when heating is carried out at 350° C. for 1 minute is measured. The measurement is performed under an inert gas atmosphere so that the active material layer will not adsorb carbon dioxide and moisture.

Measurement of the active material layer in a non-aqueous electrolyte secondary battery is performed as follows. At first a battery is dismantled in an inert gas atmosphere to take out an electrode. This electrode is washed with methyl ethyl carbonate (MEC) for 10 minutes. Then, the electrode is dried under a reduced pressure atmosphere of −80 kPa for 1 hour at room temperature so that the electrode is not exposed to atmosphere. Then, in the same manner as described above, the active material layer is collected and measured.

The above-mentioned GC measurement is carried out for a battery or an electrode before the initial charge. In addition, the active material layer is taken from an electrode that is in a state having a potential within a range of 2.5 V to 3.5 V relative to metallic lithium. When the electrode active material layer collected from an electrode having a potential outside the above range, for example, such as an electrode in a state of charge, is measured, the resulting peaks will vary.

The existence form of carbon dioxide contained in the active material layer can be confirmed by the pyrolysis-gas chromatography/mass spectrometry (pyrolysis GC/MS). The pyrolysis GC/MS can be performed by using a device including gas chromatography (GC) equipped with a pyrolyzer and mass spectrometry (MS) directly connected to the GC. In the case where carbon dioxide is contained in the active material layer or in the case where a substance that causes the generation of carbon dioxide is present, a peak appears in a peak chart obtained by the pyrolysis GC/MS measurement. The pyrolysis GC/MS measurement can be carried out in the same manner as described for the GC measurement.

When the active material layer is subjected to the pyrolysis GC/MS measurement, two peaks appear in the range of between 120° C. and 350° C. These two peaks show that carbon dioxide exists in at least two different forms in the active material layer. Out of these two peaks, the peak at the low-temperature side is referred to as peak A, and the peak at the high-temperature side is referred to as peak B. The pyrolysis GC/MS was measured under a temperature rising condition of 5° C./minute.

When the active material layer was exposed to the dry atmosphere of carbon dioxide for a long time and then it was measured, the intensity of only peak A was increased. From this, it is thought that peak A of the low-temperature side indicates the state where carbon dioxide has been loosely, physically adsorbed onto the active material layer.

The physically adsorbed carbon dioxide in such a way influences greatly on gas generation. Therefore, it is preferred that such a carbon dioxide is present in a small amount. However, the physically adsorbed carbon dioxide reacts with moisture in the battery and lithium ion in the electrolyte, thereby to produce lithium carbonate. Therefore it is thought that it gives a dehydration effect. In addition, the resulting lithium carbonate promotes the formation of coatings. Accordingly, a small amount of carbon dioxide in a state of being physically adsorbed is preferred.

On the other hand, as a result of investigation by XRD or the like, it is considered that the peak B at the high temperature side is derived from the existence of another compound which is comprised of mainly lithium carbonate (for example, Li 2 CO 3 ) rather than carbon dioxide. It should be noted, in this case, that lithium carbonate is intended to include lithium hydrogen carbonate ((LiH) 2 CO 3 ) wherein a part of the lithium is substituted by H.

Carbon dioxide existing as such a compound may gasify by react with an acid component in the electrolyte. Therefore, it is preferable that such carbon dioxide exists in an amount as small as possible. However, carbon dioxide existing as a compound produces an inorganic coating. In addition, it is thought that such carbon dioxide promotes the production of an organic polymer, resulting in the formation of an organic-based coating. Thus, it is preferable that a small amount of carbon dioxide exists as the compound.

Based upon the foregoing, it is preferable that carbon dioxide contained in the active material layer includes both of the physically adsorbed carbon dioxide that is represented by the peak A and the carbon dioxide that exists as the compound represented by the peak B. In this case, when the peak intensity of the peak A is expressed as a and the peak intensity of the peak B is expressed as b, it is preferable to satisfy an expression a≤b. The peak intensity ratio (a/b) is preferably 1 or less.

The carbon dioxide existing as the compound (that appears as peak B) produces an organic-based polymer coating. The organic-based polymer coating makes the reduction reaction of the physically adsorbed carbon dioxide (that appears as peak A) slow. Thus, the carbon dioxide existing as the compound decreases the amount of gas generated during storage. The relatively more organic-based polymer coating can effectively reduce the amount of gas generation. Therefore, if the peak intensity ratio (a/b) is 1 or less, gas generation is significantly suppressed. In addition, since a moderate coating is produced, self-discharge reaction is suppressed while maintaining a good rate performance. The peak intensity ratio (a/b) is typically 0.3 or more. In addition, it is more preferable that the peak intensity ratio (a/b) is 0.95 or less, and further preferable that the peak intensity ratio (a/b) is 0.9 or less.

In the case of the negative electrode active material layer, it is preferable that the peak A at the low-temperature side appears in the range of 120° C. or more and less than 200° C., and the peak B at the high-temperature side appears in the range of 200° C. to 350° C. Here, the peak A is the highest intensity peak appearing in the range of 120° C. or more and less than 200° C. The peak B is the highest intensity peak appearing in the range of 200° C. to 350° C.

In addition, it is preferable that the negative electrode active material layer contains carbon dioxide in an amount that releases the carbon dioxide in the range of 0.1 ml to 5 ml per 1 g when heated at 350° C. for 1 minute.

When the two peaks are in the above range or the content of carbon dioxide is in the above range, gas generation during storage of batteries as well as self-discharge reaction can be effectively suppressed.

In the case of the positive electrode active material layer, it is preferable that the peak A at the low-temperature side appears in the range of 120° C. or more and less than 250° C., and the peak B at the high-temperature side appears in the range of 250° C. to 350° C. Here, the peak A is the highest intensity peak appearing in the range of 120° C. or more and less than 250° C. The peak B is the highest intensity peak appearing in the range of 250° C. to 350° C.

In addition, it is preferable that the positive electrode active material layer contains carbon dioxide in an amount that releases the carbon dioxide in the range of 0.5 ml to 10 ml per 1 g when heated at 350° C. for 1 minute.

When the two peaks are in the above range or the content of carbon dioxide is in the above range, gas generation during storage of batteries as well as self-discharge reaction can be effectively suppressed.

Since the electric potential of the positive electrode is different from that of the negative electrode, reduction speed of carbon dioxide in the vicinity of the positive electrode is slower compared to the negative electrode. It is also thought that a larger amount of lithium carbonate is required in order to produce a coating comprising an organic-base polymer at the positive electrode than at the negative electrode. Accordingly, in order to form a stable coating on the surface of the positive electrode and suppress gas generation effectively, it is preferable that the amount of carbon dioxide contained in the positive electrode is greater than that contained in the negative electrode.

A positive electrode and a negative electrode can be produced as follows. First, an active material, a conductive agent and a binder are suspended in a solvent to prepare a slurry. As the solvent, for example, N-methylethylpyrrolidone can be used. Next, the slurry is applied to a current collector and dried to form an active material layer. Then, the active material layer is rolled to produce an electrode.

The amount of carbon dioxide contained in the active material layer can be controlled by adjusting either of carbon dioxide concentration and moisture content in the atmosphere, and stirring time and dispersion strength of the slurry, in the step of preparing the slurry.

The amount of carbon dioxide that is physically adsorbed can be reduced and the intensity a of the peak A can be decreased particularly by reducing the concentration of carbon dioxide in the atmosphere. In addition, by reducing the amount of moisture in the atmosphere, it is possible to decrease the amount of carbon dioxide existing as a compound and then, the intensity b of the peak B can be reduced.

The carbon dioxide concentration in the atmosphere is preferably in the range of 0.003% to 0.03%. Further, the amount of moisture in the atmosphere is preferably within a range of a dew point equal to or less than −10° C. It should be noted that the dew point refers to a temperature at which a partial water vapor pressure in the air equals a saturation pressure, and as used herein, it is defined as a temperature at which a partial water vapor pressure in the air equals a saturation pressure when the temperature is lowered under a constant pressure.

If the concentration of carbon dioxide and the amount of moisture in the atmosphere are too high, carbon dioxide adsorbed on the surface of the negative electrode active material may react with lithium in the active material. As a result, lithium carbonate may be increased. If the moisture content in the atmosphere is too low, the intensity of the peak B is significantly reduced. As a result, a may become larger than b (i.e., a>b). Therefore, it is preferable to contain a moderate amount of moisture in the atmosphere. Typically, the dew point is preferably −20° C. or more.

In addition, ease of adsorption of carbon dioxide varies depending on the type of active materials. Thus, even by using a combination of different active materials, it is possible to adjust the content of carbon dioxide.

According to the above embodiments, a non-aqueous electrolyte secondary battery which shows a suppression of gas generation during its storage as well as a suppression of self-discharge reaction can be provided.