CN1922753A - Lithium ion secondary cell - Google Patents

Abstract

A lithium ion secondary cell, which uses an olivine type LiFePO4 as the active material of its positive electrode, and gamma-butyrolactone as a non-aqueous electrolyte. The olivine type LiFePO4 achieves, in combination, high potential and high energy density, and, high safety and stability, which are contrary to each other, and also contains iron as a main component and thus is reduced in the load to the environment. gamma-butyrolactone exhibits high dielectric constant and low viscosity in combination, and further is excellent in the resistance to oxidation, and is advantageous in that it has a high boiling point, a low vapor pressure and a high flash point. As a result, the above cell can be advantageously used for providing a lithium ion secondary cell which is large-sized and excellent in safety, as compared to such a conventional cell used as an electricity source for a portable device.

Description

Lithium-ion secondary battery

technical field

The invention relates to a rechargeable secondary battery, in particular to a large lithium ion secondary battery.

Background technique

Lithium-ion secondary batteries that use metal oxides for the positive electrode, organic electrolytes for the electrolyte, and carbon materials such as graphite for the negative electrode have been miniaturized and lightweight since they were first commercialized in 1991 due to their high energy density. It has been rapidly popularized in portable electronic devices such as video cameras, mobile phones, notebook computers, and small disks.

Lithium-ion secondary batteries, which are the power sources of these portable electronic devices, are mostly formed by coating or extruding a material containing an electrode active material on a current collector made of a perforated metal plate or metal foil to form a 200-300 μm thick battery. For a film-shaped electrode, the film-shaped electrode and the separator are wound or laminated together, and the wound or laminated film-shaped electrode is sealed in a cylindrical or angular-shaped outer casing. The electrodes of this battery are thinner, so the electrode area can be enlarged, and high-rate charge and discharge can be performed.

And, as the electrolyte of lithium ion secondary battery, use the following organic solvent or the non-aqueous electrolytic solution that dissolved Li salt in the organic solvent that mixed two or more thereof: diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate Chain carbonates such as esters, cyclic carbonates such as propylene carbonate and ethylene carbonate, or cyclic esters such as γ-butyrolactone and γ-valerolactone. In particular, in order to smoothly move lithium ions in a low-temperature environment, chain carbonates are generally used in combination with cyclic carbonates.

Among such lithium-ion secondary batteries, a rectangular battery can efficiently utilize space when mounted on a machine compared with a cylindrical battery, and it can be easily used as a battery pack for a power source. And in the angular battery, when it is necessary to increase the filling rate of the electrode group relative to the internal volume of the battery, the stacked structure of the rectangular electrodes is more advantageous than the wound structure.

In addition, in order to increase the capacity of the stacked rectangular battery, it is necessary to accurately align and stack a plurality of positive electrodes and negative electrodes. In contrast, when the thickness of the electrode is increased, there is no need to align the positions of multiple positive and negative electrodes, but new problems such as electrode material falling off during charging and discharging, and battery capacity decrease. Therefore, for these problems, the prior art has proposed a technique in which a porous sheet made of aluminum fibers is used as a positive electrode core material (refer to Patent Document 1), and an electrode technology in which a negative electrode active material is held on a metal porous body (refer to Patent Document 1). Document 2).

In addition, in lithium cobaltate (LiCoO ₂ ) containing cobalt (Co) used as a positive electrode so far, the storage amount of Co is smaller than that of iron (Fe) and Mn (manganese), and there is a problem in continuous use. In response to this problem, in recent years, olivine-type LiFePO ₄ containing iron as a main component has attracted attention as a low-environment load and ultra-low-cost positive electrode material. For example, the following technology has been proposed in the prior art: a conductive material with a high redox potential is mixed with the active material of olivine-type LiFePO ₄ to improve the conductivity of the positive electrode (see Patent Document 3); a part of olivine-type LiFePO ₄ Substitution with fluorine lowers the resistance of the positive electrode active material and improves conductivity (see Patent Document 4).

Also, as described above, the lithium ion secondary battery uses an organic electrolytic solution as an electrolyte. Therefore, several measures are taken in terms of safety, and accidents such as rupture and fire will not occur even under severe operating conditions. For example, by adding a protection circuit, accidents caused by overcharging and overdischarging are avoided, and as a safety measure when the battery temperature rises, the resistance becomes infinite when the temperature exceeds a certain value on a part of the conductive path from the terminal to the inside of the battery. Large PTC (Positive Temperature Coefficient, positive temperature coefficient) element.

Even if such safety measures are taken, due to external factors (such as piercing nails, etc.), internal short-circuit, etc., the current will flow in concentratedly at the short-circuit point, and heat will be generated by the resistance. The heat will cause the active material and electrolyte in the battery to The chemical reaction of the chemical reaction produces the so-called "heat explosion" and eventually leads to rupture and fire. As one of the countermeasures, there is a so-called "shutdown function" as follows: In a small lithium-ion secondary battery, when the battery temperature rises, the separator melts, the pores of the separator are blocked, and it becomes an insulating film. , no current flows. In addition, a safety valve or the like is provided so that the battery does not rupture when the internal pressure of the lithium-ion secondary battery rises abnormally.

In addition, lithium-ion secondary batteries have higher energy efficiency (power efficiency) than lead-acid batteries and nickel metal hydride during charge and discharge, so they are expected to be used in electric vehicles and power storage equipment, and the development of medium-sized and large-scale batteries is being actively promoted. In addition, medium-sized lithium-ion secondary batteries have already been partially put into practical use in bicycles and the like with electric auxiliary equipment. The development of medium-sized to large-sized batteries is promoted by inheriting the battery structure obtained from the development of existing small-sized batteries.

Patent Document 1: Japanese Unexamined Patent Publication No. 6-196170

Patent Document 2: Japanese Unexamined Patent Publication No. 7-22021

Patent Document 3: JP-A-2001-110414

Patent Document 4: JP-A-2003-187799

Contents of the invention

The problem to be solved by the invention

A lithium-ion secondary battery is manufactured using the prior art, and a charge-discharge test is carried out on the manufactured lithium-ion secondary battery. First, the manufactured lithium ion secondary battery will be described below. Lithium cobaltate (LiCoO ₂ ) is used as the positive electrode active material, 20 parts by weight of acetylene black is added as a conductive material, 10 parts by weight of polyvinylidene fluoride (hereinafter referred to as PVdF) is added as a binding material (binder), and a solvent is used N-methyl-2-pyrrolidone (hereinafter referred to as NMP) was used to produce a positive electrode paste. The obtained paste was filled into foamed aluminum (size: 10cm×20cm, thickness 4mm, porosity 92%), and after being fully dried, punched with a hydraulic punch to obtain an electrode with a thickness of 3.0mm. The active material mass per unit area of the obtained electrode was 210 mg/cm ² , and the porosity of the positive electrode was 55%. In addition, in this specification, "weight part" is the value which expressed the weight ratio with respect to the positive electrode active material weight in %.

In the negative electrode active material, artificial graphite powder (average particle diameter 12 μm, d002=0.3365nm, BET specific surface area 7m ² /g) was used, 12 parts by weight of PVdF was added as a binder, and NMP was used as a solvent to prepare negative electrode paste. Fill the obtained paste into a copper fiber non-woven fabric (size: 10.2cm×20.2cm, thickness 2.5mm, porosity 88%), and after being fully dried, punch it with a hydraulic punch to obtain an electrode with a thickness of 1.5mm . The active material mass per unit area of the obtained electrode was 95 mg/cm ² , and the porosity of the negative electrode was 50%. In addition, in this specification, "d002" is an adjacent layer and the interval between layers (interval between (002) planes) in graphite which has a layered crystal structure. In addition, "BET specific surface area" refers to the area per unit weight of pores capable of adsorbing nitrogen at 77K obtained by the BET formula extending the monolayer adsorption theory to multimolecular layers.

In addition, as a separator for preventing a short circuit between the positive electrode and the negative electrode in direct contact and separating the two poles, two microporous membranes made of polyethylene with a thickness of 25 μm were used, and the obtained electrodes were placed in such a manner that one positive electrode and one negative electrode faced each other. For lamination, insert into bag-shaped aluminum laminations.

In addition, as a non-aqueous electrolytic solution, lithium tetrafluoroborate (LiBF ₄ ) dissolved in a solvent mixed with dimethyl carbonate (hereinafter referred to as DMC) and ethylene carbonate (hereinafter referred to as EC) at a volume ratio of 7:3 was used. , so that the concentration of 1.5mol/l electrolyte. The non-aqueous electrolytic solution was poured into an aluminum laminated bag in which the electrode laminate was inserted, and then sealed by thermal fusion to manufacture a lithium ion secondary battery with a design capacity of 5 Ah.

The battery obtained by the above method was subjected to a charge and discharge test under the following conditions. When charging, the charging current is 1.5A, and the charging is performed until the voltage reaches 4.2V. Then, when the voltage reaches 4.2V, 15 hours have elapsed, or when the charging current reaches 0.1A, the charging is terminated. And when discharging, the discharge current is 1.5A, and the discharge voltage is 2.75V. After repeated charging and discharging 100 times under these conditions, it was charged again to the maximum capacity. The battery capacity of the lithium-ion secondary battery in a fully charged state was measured, and a nail penetration test in which a 2.5 mmφ nail was pierced was performed in a state where the battery was placed on its side. As a result, as shown in FIG. 5 , not only did the battery capacity decrease after repeated charging and discharging 100 times, but also white smoke was generated as a result of the nail puncture test, which was in a dangerous state, and there was a safety problem.

Therefore, in a large lithium ion secondary battery manufactured merely to increase in size following the conventional small battery structure, safety measures applicable to the above-mentioned small lithium ion secondary battery are not sufficient.

In addition, when a chain carbonate such as DMC is used in an electrolytic solution, the vapor pressure of the solvent becomes high, and a large amount of gas is generated in a high-temperature environment. Therefore, in a lithium ion secondary battery containing a large amount of chain carbonate, problems such as swelling and deformation of the exterior body arise. Furthermore, when a chain carbonate is contained in the electrolytic solution, the amount of heat generated during the battery overcharge test and heat sensitivity test during manufacture is large. Therefore, in order to ensure sufficient safety, in addition to providing a protection circuit for preventing overcharging, it is necessary to use multiple protection mechanisms such as a safety valve, a current blocking valve, and a PTC element at the same time. The manufacturing process of the battery becomes complicated, and the battery issues such as the reduction of energy density. This problem is particularly conspicuous in a large lithium ion secondary battery.

The present invention was made in view of the above problems, and its object is to provide a large lithium-ion secondary battery with a battery capacity of 5 Ah or more and a capacity of 10 mAh or more per ¹ cm of the positive electrode and the negative electrode, which is excellent in safety, A lithium-ion secondary battery with good battery performance.

means of solving problems

In order to achieve the above object, the lithium ion secondary battery of the present invention has: a positive electrode, equipped with a current collector with a positive active material; a negative electrode, equipped with a current collector with a negative active material; Contact and short circuit; non-aqueous electrolyte solution containing lithium salt, the lithium ion secondary battery is characterized in that the battery capacity is 5 Ah or more, and the electric capacity per ¹ cm of the above-mentioned positive electrode and the above-mentioned negative electrode is 10 mAh or more, and the above-mentioned positive electrode The active material is olivine-type LiFePO ₄ , and the non-aqueous electrolytic solution contains at least γ-butyrolactone (hereinafter referred to as GBL).

Furthermore, the lithium ion secondary battery of the present invention is characterized in that the GBL content contained in the non-aqueous electrolytic solution is 50% or more and 80% or less in volume percentage.

In such a lithium ion secondary battery, it is possible to prevent a decrease in the safety of a large lithium ion secondary battery due to the content of GBL in the nonaqueous electrolytic solution being less than 50% by volume. And, can prevent the above-mentioned non-aqueous electrolytic solution counter electrode that causes because the content rate of GBL is higher than 80%, and constitute the other parts of lithium-ion secondary battery such as the permeability reduction of above-mentioned diaphragm, thereby cause the performance of lithium-ion secondary battery drop problem. In addition, a gel electrolyte can also be used as the above-mentioned non-aqueous electrolytic solution.

In addition, the lithium ion secondary battery of the present invention is characterized in that at least one of the positive electrode and the negative electrode has a thickness of 1 mm or more and less than 10 mm.

In such a lithium ion secondary battery, the following problem can be prevented: when the thickness of at least one of the positive electrode and the negative electrode is 10 mm or more, the non-aqueous electrolyte solution cannot sufficiently penetrate into the positive electrode and the negative electrode, and performance maintenance is difficult. In addition, there is no problem that the internal porosity of the electrode becomes low due to the thickness of the electrode being less than 1 mm, the weight of the positive electrode active material and the negative electrode active material decreases, and the number of laminated objects does not increase.

In addition, the lithium ion secondary battery of the present invention has a plurality of the negative electrodes and the separator, and the negative electrodes are arranged on both sides of the positive electrodes so that the positive electrodes are sandwiched by the separators.

In such a lithium ion secondary battery, by thickening the positive electrode and arranging the negative electrode on both sides of the positive electrode, the polarization of the negative electrode can be reduced, so that lithium deposition can be avoided. The structure in this case is such that a thick positive electrode is sandwiched between the above-mentioned negative electrode having about half the capacity of the above-mentioned positive electrode.

In addition, the lithium ion secondary battery of the present invention is characterized in that the separator has a porosity of 30% to 90% and a thickness of the separator of 5 μm to 100 μm.

In such a lithium ion secondary battery, it is possible to prevent the problem that the internal resistance of the lithium ion secondary battery increases due to a decrease in the content of the nonaqueous electrolyte solution due to the porosity of the separator being less than 30%. Also, it is possible to prevent the problems of physical contact between positive and negative electrodes, internal short circuit of lithium ion secondary battery due to the porosity of the separator above 90%. Further, it is possible to prevent the problem that the mechanical strength of the separator is insufficient due to the thickness of the separator being less than 5 μm, which in turn leads to an internal short circuit of the lithium ion secondary battery. In addition, it is possible to prevent the problem that the distance between the positive electrode and the negative electrode increases when the thickness of the separator exceeds 100 μm, and the internal resistance of the lithium ion secondary battery increases. In addition, the separator can be selected from nonwoven fabrics and microporous films made of polyethylene, polypropylene, polyester, etc., and when the separator is a nonwoven fabric made of polyester, the nonwoven fabric and the microporous film Compared with porous membranes, the permeability to the above-mentioned non-aqueous electrolytic solution containing GBL is higher, so it is preferable.

Furthermore, the lithium ion secondary battery of the present invention is characterized in that the salt concentration of the lithium salt dissolved in the non-aqueous electrolytic solution is not less than 0.5 mol/l and not more than 3 mol/l.

In such a lithium ion secondary battery, it is possible to prevent the problem that the carrier concentration in the electrolyte solution decreases when the salt concentration of the nonaqueous electrolyte solution is 0.5 mol/l or less, and the resistance of the nonaqueous electrolyte solution becomes high. In addition, the following problems can be prevented: the degree of dissociation of the salt itself decreases due to the salt concentration of the non-aqueous electrolytic solution exceeding 3 mol/l, and the carrier concentration in the non-aqueous electrolytic solution cannot be increased.

Furthermore, the lithium ion secondary battery of the present invention is characterized in that the current collector is a three-dimensional metal porous body having a plurality of pores.

In such a lithium ion secondary battery, when the above-mentioned metal porous body having a three-dimensional structure having a plurality of pores is used as the above-mentioned current collector constituting the above-mentioned electrode, in the inside of the lithium-ion secondary electrode, a metal having good thermal conductivity is contained. The electrodes are uniformly present throughout the entirety of the electrodes, thereby improving heat dissipation within the electrodes. Security can thereby be further improved.

Furthermore, the lithium ion secondary battery of the present invention is characterized in that the size of pores in the porous metal body constituting the current collector is 1 mm or less, and the porosity of the current collector is 50% to 98%.

In such a lithium ion secondary battery, the following problem can be prevented: because the size of the pores of the above-mentioned metal porous body constituting the above-mentioned current collector is greater than 1 mm, the gap between the above-mentioned positive electrode active material and the negative electrode active material to the above-mentioned current collector will be prevented. The distance to the inner wall of the pore increases, resulting in an increase in electrical resistance. In addition, when the pore size of the porous metal body is 1 mm or less, the positive electrode active material or the negative electrode active material located in the pores is less likely to drop out of the pores of the porous metal body, which is preferable. Further, regarding the porosity of the current collector, if the porosity is within the above range, it is possible to prevent the energy density of the lithium ion secondary battery from being reduced due to insufficient filling of the active material due to too low porosity. On the other hand, it is possible to prevent that the above-mentioned electrode strength is weakened due to too high porosity, and a sufficient heat dissipation effect cannot be obtained.

Furthermore, the lithium ion secondary battery of the present invention is characterized in that the porosity of the negative electrode is not less than 30% and not more than 90%.

In such a lithium ion secondary battery, the negative electrode active material and binder are mixed to form a paste and filled into the three-dimensional metal porous body, and then the metal porous body is punched to form the negative electrode. When the negative electrode uses a graphite material as the negative active material, when GBL is contained in the non-aqueous electrolytic solution, the permeability of the non-aqueous electrolytic solution to the negative electrode is reduced, and the discharge characteristics and charging characteristics of the lithium-ion secondary battery are reduced. Discharge cycle characteristics and low-temperature characteristics deteriorate. Therefore, in order to suppress the above-mentioned problems, it is preferable that voids exist within the above-mentioned negative electrode with a porosity within the above-mentioned range.

Furthermore, the lithium ion secondary battery of the present invention is characterized in that the negative electrode active material is a mixture of graphite powder and carbon fiber powder.

When GBL is used as the solvent for the non-aqueous electrolyte, in order to further improve the low permeability of the non-aqueous electrolyte to the negative electrode, the negative active material is a mixture of graphite powder and carbon fiber powder. In particular, in the case of natural graphite powder, it has a scale shape, and there is a problem that the natural graphite powder is arranged inside the electrode and the voids are reduced. In this lithium ion secondary battery, by mixing the above-mentioned carbon fiber powder having a different shape from the above-mentioned natural graphite, the reduction of the above-mentioned voids in the above-mentioned negative electrode can be suppressed, and the above-mentioned carbon fiber powder can also function as the above-mentioned negative electrode active material, Capacity loss can be suppressed.

In addition, the lithium ion secondary battery of the present invention is characterized in that the negative electrode active material includes graphite with good crystallinity.

As negative electrode active materials for lithium-ion secondary batteries, carbonaceous materials have been widely used conventionally, but graphite materials with high crystallinity are preferably used in view of the flatness of charge and discharge potential, the level of charge and discharge efficiency, and the like. As the above-mentioned graphite material having high crystallinity, for example, natural graphite can also be used. Such natural graphite not only has a large amount of reserves in the world, but also has higher crystallinity than artificial graphite, so it is preferably used as the above-mentioned negative electrode active material of the present invention.

Furthermore, the lithium ion secondary battery of the present invention is characterized in that the negative electrode active material has vapor-phase grown carbon fibers.

The carbon fiber used as the above-mentioned negative electrode active material is Vapor Grown Carbon Fibers (hereinafter referred to as VGCF). VGCF has the shape of carbon fiber and has the same high crystallinity as natural graphite. Discharge potential flatness, the effect of high discharge efficiency, and the effect of suppressing the reduction of negative electrode voids are preferable as the above-mentioned negative electrode active material of the present invention.

Invention effect

Through the structure of the present invention, the performance of the large-scale lithium ion secondary battery can be excellent, and the safety can be improved.

According to the present invention, olivine-type LiFePO ₄ is used as the positive electrode active material. The above-mentioned olivine-type LiFePO ₄ can not only take into account the two opposite elements of high potential/high energy density and high safety/stability, but also has iron as the main component. , is a material that can realize low environmental load. And GBL is used for non-aqueous electrolyte. The above-mentioned GBL has the properties of high dielectric constant and low viscosity, has good oxidation resistance, and has the advantages of high boiling point, low vapor pressure, high ignition point, etc., so that it can be stored at high temperature , Overcharging generates less heat and produces less gas. Therefore, safety can be improved compared with conventional lithium-ion secondary batteries used as power sources for portable devices.

Description of drawings

FIG. 1 is a schematic cross-sectional view showing the configuration of the lithium ion secondary battery of the present invention.

2 is a schematic cross-sectional view showing the configuration of a modified example of the lithium ion secondary battery of the present invention.

3 is a schematic cross-sectional view showing the configuration of a modified example of the lithium ion secondary battery of the present invention.

FIG. 4 is a graph showing test results of lithium ion secondary batteries in Example 1, Example 2, and Example 3 of the present invention.

FIG. 5 is a diagram showing test results of a conventional lithium ion secondary battery.

Explanation of reference signs

1 positive active material

2 Negative active material

3a Collector

3b Current collector

4 diaphragm (separator)

4a Diaphragm

4b Diaphragm

5 exterior materials

6 non-aqueous electrolyte

Detailed ways

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery according to the present embodiment. This lithium-ion secondary battery has: a positive electrode active material 1 and a negative electrode active material 2 that are mixed with a binder, a conductive material, etc. (not shown) to form a paste; Body 3a, 3b; Separator 4 disposed between current collectors 3a, 3b so that the positive and negative electrodes do not directly contact and cause short circuit; Covering current collectors 3a, 3b and separator 4; and dissolving lithium tetrafluoroborate (LiBF ₄ ) or other electrolyte salt (not shown) non-aqueous electrolytic solution 6 .

In such a lithium ion secondary battery, the positive electrode active material 1 is applied to the current collector 3 a to constitute the positive electrode, and the negative electrode active material 2 is applied to the current collector 3 b to constitute the negative electrode. The separator 4 is disposed between the current collectors 3a, 3b to prevent a short circuit caused by direct contact between the positive electrode and the negative electrode. Lithium ions detach from the positive electrode side and move to the negative electrode during charging, and conversely, lithium ions detach from the negative electrode side and return to the positive electrode side during discharge. That is, charge and discharge operations are performed by movement of lithium ions between the positive electrode and the negative electrode.

The lithium ion secondary battery configured as shown in FIG. 1 will be described in detail below. First, when such a lithium ion secondary battery is used in a system that requires a large capacity such as a distributed power supply for household use or a power storage system of a solar power generation system, a battery pack is required to obtain a large capacity. However, when a small lithium-ion secondary battery with a small charge-discharge capacity is used as a single cell, hundreds to thousands of single cells are required, and the maintenance of the power storage system becomes very complicated. Therefore, it is preferable that the lithium ion secondary battery is a medium or large battery with a large charge and discharge capacity, and the charge and discharge capacity as a single cell is preferably 5 Ah or more.

And, at this time, in the positive electrode composed of the positive electrode active material 1 and the current collector 3a, and the negative electrode composed of the negative electrode active material 2 and the current collector 3b, when the electric capacity per ^1cm2 is less than 10mAh, each single cell The number of layers to be stacked becomes tens to dozens, and the manufacturing operation of the single cell becomes complicated. Therefore, it is preferable that the capacitance per 1 cm ² of the positive electrode and the negative electrode is 10 mAh or more. The configuration of a lithium ion secondary battery having such a capacity value is described below.

(Positive and Negative)

First, when the thickness of the positive electrode and the negative electrode is 10 mm or more, the electrolytic solution does not penetrate sufficiently, making it difficult to maintain performance. In addition, when the thickness of the electrode is less than 1 mm, the porosity inside the electrode decreases, the weight of the electrode active material decreases, and the number of layers increases. Therefore, in this embodiment, the thickness of the positive electrode and the negative electrode is preferably 1 mm to 10 mm, although it also depends on the density of the active material, the type of binder and conductive material mixed, the pressing pressure of the electrode, and the like.

In addition, as for the thickness of the positive electrode and the negative electrode used in this embodiment, when one of the electrodes is a thick electrode, it is preferable to make the positive electrode thicker. This is because, in a lithium ion secondary battery, the negative electrode is charged and discharged at a potential close to that of lithium metal, and therefore lithium may be deposited when the polarization of the negative electrode increases. In addition, as described below, by making the positive electrode thicker, the structure is such that the negative electrodes are arranged on both sides of the positive electrode, and the polarization of the negative electrode can be reduced, so that the deposition of lithium can be avoided. In this case, the structure is such that a thick positive electrode is sandwiched between a negative electrode having about half the capacity of the positive electrode (see FIG. 3 ).

In addition, anode materials such as LiCoO ₂ , which have been commonly used in cathodes so far, release oxygen as the temperature rises, and the electrolyte burns and heats up violently. In addition, in LiCoO ₂ containing cobalt (Co), the storage amount of Co is smaller than that of iron (Fe) and manganese (Mn), and continuous use causes problems. In response to this problem, in recent years, olivine-type LiFePO ₄ containing iron as a main component has attracted attention as a low-environment load/ultra-low-cost cathode material. This LiFePO ₄ not only achieves both high potential/high energy density and high safety/stability, but also contains iron as a main component and has a small environmental load. Moreover, all the oxygen in LiFePO ₄ is combined with phosphorus through a strong covalent bond, so there is no heat generation like other positive electrode materials such as LiCoO ₂ mentioned above, and it is difficult to release oxygen due to temperature rise, and the safety is good . In addition, because it contains phosphorus, it can also play a cooling role when the positive electrode generates heat and the electrolyte leaks. Therefore, in the positive electrode of this embodiment, olivine-type LiFePO ₄ is used as the positive electrode active material 1 .

This lithium-ion secondary battery using olivine-type LiFePO ₄ as the positive electrode active material 1 has a charging voltage of about 3.5V, and the charging is basically completed at 3.8V, so it is about 4.5V away from the voltage that causes the electrolyte to decompose, There is still plenty. In addition, when a positive electrode material having a charging voltage of 4.0 V or higher is used as the positive electrode active material 1, it is not preferable because the electrolyte solution is likely to be decomposed when the charging voltage is further increased.

In addition, the safety of a large battery is largely affected by its heating action and the speed at which it dissipates heat. In large lithium-ion secondary batteries, heat tends to be trapped inside due to the large size of the battery. However, when a metal three-dimensional structure is used for the current collectors 3a and 3b constituting the electrodes, in the lithium ion secondary electrode, a metal with good thermal conductivity is uniformly present throughout the entire electrode, thereby improving heat dissipation in the electrode. Therefore, in the present embodiment, the current collectors 3a and 3b use a three-dimensionally connected porous metal, thereby further improving safety.

In addition, in the pores of the three-dimensionally connected porous metal used in the current collectors 3a, 3b, when the size exceeds 1mm, the distance from the active material to the inner wall of the pores of the current collectors 3a, 3b becomes large. , the result is a larger resistance. Furthermore, when the pore size of the porous metal body is 1 mm or less, the possibility of detachment of the positive electrode active material 1 or the negative electrode active material 2 in the pores from the pores of the porous metal body is reduced, which is preferable. In the present embodiment, the pore size of the three-dimensionally connected metal porous bodies used for the current collectors 3 a and 3 b is preferably 1 mm or less, more preferably 0.5 mm or less.

Furthermore, regarding the porosity of the above-mentioned porous metal body, when the porosity is low, the active material cannot be sufficiently filled, and thus the energy density of the lithium ion secondary battery decreases. In addition, when the porosity is high, the electrode strength becomes weak, and a sufficient heat dissipation effect cannot be obtained. Therefore, in the present embodiment, the porosity of the porous metal body is preferably not less than 50% and not more than 98%, more preferably not less than 75% and not more than 98%.

Using this three-dimensionally connected metal porous body as the electrodes of the current collectors 3a and 3b is different from conventional electrodes coated on metal foils, in that ions can penetrate from the back to the surface. Therefore, since counter electrodes are arranged on both sides and ions are supplied from both sides, there is an effect of improving cycle characteristics.

After mixing the negative electrode active material 2 and the binder to form a paste and filling the three-dimensionally connected porous metal body, the porous metal body is punched to form the negative electrode. This negative pole electrode, when using graphite material as negative pole active material, if contain GBL in the non-aqueous electrolytic solution, then the permeability of non-aqueous electrolytic solution 6 to negative pole reduces, and the discharge characteristic of lithium-ion secondary battery, charge-discharge cycle characteristic , Low temperature characteristics deteriorate. Therefore, in order to suppress the above-mentioned problems, it is preferable that voids exist inside the negative electrode. In this embodiment, the porosity of the negative electrode is not less than 30% and not more than 90%.

The above-mentioned three-dimensional structure is used as the electrodes of the current collectors 3a and 3b, and the metal with high thermal conductivity is three-dimensionally arranged, so the temperature in the electrodes can be kept uniform, and it is possible to suppress the problem of large-scale lithium-ion secondary batteries. Circulation deterioration due to localized temperature rise. In addition, in the electrode used in this embodiment, metal fibers may be dispersed in the positive electrode active material 1 or the negative electrode active material 2 in order to improve electrical conductivity between active materials and improve heat dissipation. The length of the metal fibers is preferably equivalent to the size of voids in the three-dimensional structures used as current collectors 3a, 3b.

In addition, "three-dimensionally connected metal porous body" refers to an object obtained by sintering a sponge-like metal structure, a non-woven fabric made of metal fibers, and a gold-containing powder, and an object formed by molding a metal foil into a honeycomb structure. . In addition, the material of the metal porous body used for each of the current collectors 3a and 3b is not particularly limited, but among the materials used for the current collector 3a for the positive electrode, aluminum, titanium, stainless steel, etc. have high oxidation resistance and are preferred. Among the materials used for the current collector 3b for the negative electrode, copper, nickel, iron, stainless steel, etc. are not easily alloyed with lithium and have high conductivity, so they are preferably used.

(non-aqueous electrolyte)

GBL has properties of both high dielectric constant and low viscosity, and has advantages such as excellent oxidation resistance, high boiling point, low vapor pressure, and high ignition point. Therefore, when used in a non-aqueous electrolyte, there is less calorific value when stored at a high temperature or when overcharged, and the amount of gas generated is also small. Compared with the existing small lithium-ion secondary batteries, very high safety requirements are required. It is very suitable as a solvent for the electrolyte of a large-scale lithium-ion secondary battery. Therefore, GBL is contained in the non-aqueous electrolytic solution 6 used in this embodiment.

And, in this non-aqueous electrolytic solution 6, as the solvent that can be mixed with GBL, include: cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, dimethyl carbonate Ester (DMC), diethyl carbonate (DEC), ethyl methyl carbonate, dipropyl carbonate and other chain carbonates, γ-valerolactone and other lactones, tetrahydrofuran, 2-methyltetrahydrofuran and other furans, Diethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, dioxane and other ethers, dimethyl sulfoxide, sulfolane, methyl Sulfolane, acetonitrile, methyl formate, methyl acetate, etc. may be used in combination of one or more of the above. In particular, cyclic carbonates such as PC, EC, and butylene carbonate are high-boiling-point solvents, and therefore preferred.

Furthermore, regarding the GBL content in the non-aqueous electrolytic solution 6, when the GBL content is less than 50% by volume, the safety of the large lithium-ion secondary battery decreases. And, when the content of GBL exceeds 80%, the electrolyte solution not only reduces the permeability of the electrode, but also reduces the permeability of other components constituting the lithium ion secondary battery, such as the separator, and the performance of the lithium ion secondary battery decreases. Therefore, in the non-aqueous electrolytic solution 6 of the lithium ion secondary battery of this embodiment, the content of GBL in the solvent for non-aqueous electrolytic solution is 50% or more and 80% or less in volume percentage. In addition, as the non-aqueous electrolytic solution 6, a gel electrolyte or the like in which an electrolytic solution composed of the above-mentioned solvent is held in a polymer matrix (polymer matrix) can also be used.

In addition, as the electrolyte salt dissolved in the non-aqueous electrolytic solution 6, lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium trifluoroacetate Lithium salts such as (LiCF ₃ COO ) and lithium bis(trifluoromethanesulfo)imide (LiN(CF ₃ SO ₂ ) ₂ ) may be used in combination of one or more of them.

When the salt concentration of the above-mentioned nonaqueous electrolytic solution 6 is 0.5 mol/l or less, the carrier concentration in the electrolytic solution decreases, so the resistance of the nonaqueous electrolytic solution 6 increases. Furthermore, when the salt concentration of the nonaqueous electrolytic solution 6 exceeds 3 mol/l, the degree of dissociation of the salt itself decreases, and the carrier concentration in the nonaqueous electrolytic solution 6 does not increase. Therefore, the salt concentration of the nonaqueous electrolytic solution 6 in this embodiment is 0.5 to 3 mol/l.

(diaphragm)

The separator 4 used in this embodiment can be selected from non-woven fabrics and microporous films made of polyethylene, polypropylene, polyester, etc., and when the separator 4 is a non-woven fabric made of polyester, the non-woven The non-aqueous electrolytic solution 6 containing GBL has higher permeability than the microporous membrane, so it is more preferable.

And, with regard to the above separator 4, when the porosity is lower than 30%, the content of the non-aqueous electrolyte 6 decreases, and the internal resistance of the lithium-ion secondary battery becomes large, and when it is higher than 90%, the positive electrode and the negative electrode are in physical contact. , causing an internal short circuit in the lithium ion secondary battery. In addition, when the thickness of the separator 4 is less than 5 μm, the mechanical strength of the separator 4 is insufficient, which becomes the cause of the internal short circuit of the lithium ion secondary battery, and when it exceeds 100 μm, the distance between the positive and negative electrodes becomes long, and the The internal resistance becomes larger. Therefore, in the present embodiment, the porosity of the separator 4 is not less than 30% and not more than 90%, and the thickness of the separator 4 is not less than 5 μm and not more than 100 μm.

(exterior material)

In addition, as the exterior material 5 of the lithium ion secondary battery used in this embodiment, a metal can, for example, a can made of iron, stainless steel, aluminum, or the like is used. In addition, a film-like bag obtained by laminating extremely thin aluminum with resin can also be used. The shape of the exterior material 5 can be cylindrical, angular, thin, etc., and large lithium-ion secondary batteries are mostly used as battery packs, so it is preferably angular or thin.

In addition, the lithium ion secondary battery according to the embodiment of the present invention has the structure shown in FIG. 1 , but separators 4 a and 4 b may be provided between current collectors 3 a and 3 b as shown in FIG. 2 . In addition, as shown in FIG. 3 , two negative electrodes including the negative electrode active material 2 and the current collector 3 b and the separator 4 may be provided respectively.

Examples 1 to 3 of the lithium ion secondary battery manufactured in the present embodiment and evaluation results of the respective examples will be described below. In addition, Embodiment 1 has the structure shown in FIG. 1 , Embodiment 2 has the structure shown in FIG. 2 , and Embodiment 3 has the structure shown in FIG. 3 .

Example 1

The positive electrode active material 1 used olivine-type LiFePO ₄ , added 20 parts by weight of acetylene black as a conductive material, added 10 parts by weight of PVdF as a binder, and used NMP as a solvent to prepare a positive electrode paste. Fill the obtained paste into the foamed aluminum (size: 10cm×20cm, thickness 4mm, porosity 92%) used as the current collector 3a, and after sufficient drying, punch it with a hydraulic punch to obtain a 3.0mm thick electrode. The active material mass per unit area of the obtained electrode was 210 mg/cm ² , and the porosity of the positive electrode was 55%.

In the negative active material 2, natural graphite powder (average particle diameter 15 μm, d002=0.3357nm, BET specific surface area 3m ² /g) and VGCF powder (average particle diameter 15 μm, d002=0.3359nm, BET specific surface area) produced in China were used. 2m ² /g) at a weight ratio of 50:50, adding 12 parts by weight of PVdF as a binder, and using NMP as a solvent to prepare a negative electrode paste. Fill the obtained paste into a non-woven fabric made of copper fibers (size: 10.2cm×20.2cm, thickness 2.5mm, porosity 88%) used as the current collector 3b, and after sufficiently drying, use oil pressure Punching is carried out by a punching machine to obtain an electrode with a thickness of 1.5 mm. The mass of the active material per unit area of the electrode thus obtained was 95 mg/cm ² , and the porosity of the negative electrode was 50%.

In addition, as the separator 4 , a polyester nonwoven fabric with a thickness of 50 μm was used, and the electrodes obtained by the above method were laminated so that one positive electrode and one negative electrode faced each other, and inserted into a bag-shaped aluminum laminate.

In addition, as the non-aqueous electrolyte solution 6, an electrolyte solution in which LiBF ₄ was dissolved in a solvent mixed with GBL and EC at a volume ratio of 7:3 to a concentration of 1.5 mol/l was used, and the electrode laminate was inserted into the aluminum laminate. After injecting the electrolytic solution into the pouch, it was heat-fused and sealed to manufacture a lithium ion secondary battery with a design capacity of 5 Ah in this embodiment. The lithium ion secondary battery thus produced was tested. In addition, this test and test results will be described later.

Example 2

The positive electrode is the same as in Example 1, so detailed description thereof is omitted, and Example 1 can be referred to.

In the negative electrode active material 2, natural graphite powder (average particle diameter 15 μm, d002=0.3357nm, BET specific surface area 3m ² /g) and PAN (polyacrylonitrile) carbon fiber powder (average particle diameter 15 μm) produced in China are used. , d002=0.3400nm, BET specific surface area 3m ² /g) mixed with a weight ratio of 80:20, adding 6 parts by weight of latex (latex) rubber as a binder, adding 6 parts by weight of CMC (carboxylate Methyl cellulose)-NH ₄ was used as a thickener, and was dissolved and dispersed in water to make a paste for the negative electrode. Fill the obtained paste into foamed nickel (size: 10.2cm×20.2cm, thickness 2.5mm, porosity 90%) used as the current collector 3b, and after sufficient drying, punch it with a hydraulic punch to obtain a thick 1.8mm electrode. The mass of the active material per unit area of the electrode thus obtained was 99 mg/cm ² , and the porosity of the negative electrode was 60%.

In addition, as the separators 4a and 4b (see FIG. 2 ), two polyethylene non-woven fabrics with a thickness of 25 μm were used, and the obtained electrodes were laminated in such a manner that one positive electrode and one negative electrode faced each other, and inserted into a bag-shaped aluminum laminate. in the film.

In addition, as the non-aqueous electrolyte solution 6, an electrolyte solution in which LiBF ₄ was dissolved in a solvent mixed with GBL and EC at a volume ratio of 5:5 to have a concentration of 1.7 mol/l was used, and the electrode laminate was inserted into the aluminum laminate. After injecting the electrolytic solution into the pouch, it was heat-fused and sealed to manufacture a lithium ion secondary battery with a design capacity of 5 Ah in this embodiment. The same test as in Example 1 was carried out on the lithium ion secondary battery manufactured in this way. In addition, this test and test results will be described later as in Example 1.

Example 3

The evaluation results of the lithium ion secondary battery manufactured in this embodiment will be described below. The positive electrode is the same as in Example 1, so detailed description thereof is omitted, and Example 1 can be referred to.

In negative electrode active material 2, use artificial graphite powder (average particle diameter 12 μm, d002=0.3365nm, BET specific surface area 7m ² /g), add the latex type rubber of 4 weight parts as binding agent, add the CMC- of 4 weight parts NH ₄ acts as a thickener and is dissolved and dispersed in water to make a paste for the negative electrode. The obtained paste was filled into foamed nickel (size: 10.2 cm × 20.2 cm, thickness 1.4 mm, porosity 95%) used as the current collector 3b, and after sufficient drying, it was punched with a hydraulic punch to obtain two An electrode with a thickness of 1.1 mm. The mass of the active material per unit area of the electrode thus obtained was 45 mg/cm ² , and the porosity of the negative electrode was 70%.

In addition, as the separator 4, two non-woven fabrics made of polypropylene coated with a surfactant on the surface were used as the separator 4. As shown in FIG. Bag-shaped aluminum laminations.

In addition, as the non-aqueous electrolyte solution 6, an electrolyte solution in which LiBF ₄ was dissolved in a solvent mixed with GBL and DEC at a volume ratio of 5:5 to have a concentration of 1.2 mol/l was used, and the electrode laminate was inserted into the aluminum laminate. After injecting the electrolytic solution into the pouch, it was heat-fused and sealed to manufacture a lithium ion secondary battery with a design capacity of 5 Ah in this embodiment. The same test as in Example 1 was carried out on the lithium ion secondary battery manufactured in this way. In addition, this test and test results will be described later as in Example 1 and Example 2.

For each of the lithium ion secondary batteries obtained in Example 1 to Example 3, charge and discharge tests were performed under the following conditions. When charging, the charging current is 1.5A, and the charging is performed until the voltage reaches 4.2V. Then, when the voltage reaches 4.2V, 15 hours have elapsed, or when the charging current reaches 0.1A, the charging is terminated. When discharging, the discharge current is 1.5A, and the discharge voltage is 2.75V. After repeated charge and discharge 100 times under this condition, recharge to the maximum capacity, measure the battery capacity of the lithium-ion secondary battery in a fully charged state, and implement a 2.5mmφ nail penetration test. The result is shown in Figure 4.

The present invention is compared with the prior art by Fig. 4 and Fig. 5, positive active material 1 has used olivine type LiFePO ₄ , non-aqueous electrolytic solution 6 has used the lithium ion secondary battery of the present invention of GBL, even if carry out nail test , no white smoke will be produced, no abnormal heating will occur inside, and compared with the lithium-ion secondary battery in the prior art, the safety is improved. Furthermore, comparing the evaluation results of Example 1 and Example 2, it can be seen that in Example 1 in which the negative active material 2 is mixed with VGCF, the decrease in battery capacity is small and has excellent cycle characteristics. Similarly, comparing Example 1 and Example 3, it can be seen that Example 1 in which VGCF powder is mixed in the negative active material 2 has excellent cycle characteristics, and in addition, Example 1 and Example 2 in which natural graphite is used in the negative active material 2 Compared with Example 3 using artificial graphite, it is superior in cycle characteristics. Further, comparing Example 3 with Example 1 or 2, it can be seen that the maximum surface temperature of Example 3 is lower. From this, it can be seen that Example 3, which has a structure in which two negative electrodes sandwich a positive electrode, has a higher thermal diffusivity than Examples 1 and 2, which have a structure in which electrodes are laminated such that one positive electrode and one negative electrode face each other.

Industrial availability

The lithium ion secondary battery according to the present invention can be used not only as a battery for portable electronic devices such as video cameras, mobile phones, notebook computers, and compact disks, but also as a battery for electric vehicles and power storage.

claims

(Amended in accordance with Article 19 of the Treaty)

1. A lithium ion secondary battery has: a positive pole, which is equipped with a current collector with a positive active material; a negative pole, which is equipped with a current collector with a negative active material; a separator; a non-aqueous electrolyte containing a lithium salt, the lithium ion The secondary battery is characterized in that,

The battery capacity is 5 Ah or more, and the electric capacity per 1 cm ² of the above-mentioned positive electrode and the above-mentioned negative electrode is 10 mAh or more, and,

The above-mentioned positive electrode active material is olivine-type LiFePO ₄ ,

The non-aqueous electrolytic solution contains at least γ-butyrolactone.

2. The lithium ion secondary battery according to claim 1, wherein the content of the γ-butyrolactone contained in the non-aqueous electrolytic solution is not less than 50% and not more than 80% by volume.

3. The lithium ion secondary battery according to claim 1 or 2, wherein at least one of the positive electrode and the negative electrode has a thickness of not less than 1 mm and less than 10 mm.

4. The lithium ion secondary battery according to any one of claims 1 to 3, characterized in that,

Having a plurality of the above-mentioned negative electrodes and the above-mentioned separators, and,

The above-mentioned negative electrodes are arranged on both sides of the above-mentioned positive electrodes so that the above-mentioned positive electrodes are sandwiched between the separators.

5 . The lithium ion secondary battery according to claim 1 , wherein the separator has a porosity of not less than 30% and not more than 90%, and a thickness of the separator of not less than 5 μm and not more than 100 μm.

6. The lithium ion secondary battery according to any one of claims 1 to 5, wherein the salt concentration of the lithium salt dissolved in the non-aqueous electrolytic solution is not less than 0.5 mol/l and not more than 3 mol/l.

7. The lithium ion secondary battery according to any one of claims 1 to 6, wherein the current collector is a metal three-dimensional structure.

8. The lithium ion secondary battery according to claim 7, wherein the current collector is a three-dimensional metal porous body having a plurality of pores.

9. The lithium ion secondary battery according to claim 8, wherein the size of pores in the porous metal body constituting the current collector is 1 mm or less, and the porosity of the current collector is 50%. Above 98% below.

10 . The lithium ion secondary battery according to claim 1 , wherein the negative electrode has a porosity of not less than 30% and not more than 90%. 11 .

11. The lithium ion secondary battery according to any one of claims 1 to 10, characterized in that the negative electrode active material is a mixture of graphite powder and carbon fiber powder.

12. The lithium ion secondary battery according to any one of claims 1 to 11, wherein the negative electrode active material comprises graphite with good crystallinity.

13. The lithium ion secondary battery according to any one of claims 1 to 12, wherein the negative electrode active material has vapor-phase grown carbon fibers.

Claims (12)

1. lithium rechargeable battery, have: positive pole possesses the collector body with positive active material; Negative pole possesses the collector body with negative electrode active material; Barrier film; The nonaqueous electrolytic solution that contains lithium salts, this lithium rechargeable battery be characterised in that,

Battery capacity is more than the 5Ah, and every 1cm of above-mentioned positive pole and above-mentioned negative pole ²Capacitance be more than the 10mAh, and,

Above-mentioned positive active material is olivine-type LiFePO ₄,

At least contain gamma-butyrolacton in the above-mentioned nonaqueous electrolytic solution.

2. lithium rechargeable battery according to claim 1 is characterized in that, the containing ratio of the above-mentioned gamma-butyrolacton that contains in the above-mentioned nonaqueous electrolytic solution is more than 50% below 80% under percentage by volume.

3. lithium rechargeable battery according to claim 1 and 2 is characterized in that, the thickness of at least one is more than the 1mm and less than 10mm in above-mentioned positive pole and the above-mentioned negative pole.

4. according to any described lithium rechargeable battery of claim 1 to 3, it is characterized in that,

Have a plurality of above-mentioned negative poles and above-mentioned barrier film, and,

Dispose above-mentioned negative pole in above-mentioned anodal both sides, to clip above-mentioned positive pole by barrier film.

5. according to any described lithium rechargeable battery of claim 1 to 4, it is characterized in that the voidage of above-mentioned barrier film is more than 30% below 90%, and the thickness of above-mentioned barrier film is below the above 100 μ m of 5 μ m.

6. according to any described lithium rechargeable battery of claim 1 to 5, it is characterized in that the salinity of the lithium salts that dissolves in the above-mentioned nonaqueous electrolytic solution is below the above 3mol/l of 0.5mol/l.

7. according to any described lithium rechargeable battery of claim 1 to 6, it is characterized in that above-mentioned collector body is the metal porous body with three-dimensional structure of a plurality of emptying apertures.

8. according to the described lithium rechargeable battery of claim 7, it is characterized in that the size of emptying aperture that constitutes the above-mentioned metal porous body of above-mentioned collector body is below the 1mm, and the voidage of above-mentioned collector body is more than 50% below 98%.

9. according to any described lithium rechargeable battery of claim 1 to 8, it is characterized in that the voidage of above-mentioned negative pole is more than 30% below 90%.

10. according to any described lithium rechargeable battery of claim 1 to 9, it is characterized in that above-mentioned negative electrode active material is the mixture of powdered graphite and carbon fibre powder.

11. any described lithium rechargeable battery according to claim 1 to 10 is characterized in that above-mentioned negative electrode active material has the good graphite of crystallinity.

12. any described lithium rechargeable battery according to claim 1 to 11 is characterized in that above-mentioned negative electrode active material has the vapor phase growth carbon fibre.

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