JP4534559B2 - Lithium secondary battery and positive electrode material for lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium secondary battery and a positive electrode material for a lithium secondary battery, and in particular, a positive electrode in which a positive electrode mixture containing a lithium manganese nickel cobalt composite oxide having a layered crystal structure and a conductive material is applied to a positive electrode current collector. The present invention also relates to a lithium secondary battery in which a non-aqueous electrolyte is infiltrated with a negative electrode containing a carbon material capable of detaching and inserting lithium ions, and the positive electrode material for the lithium secondary battery.

  Conventionally, in the field of rechargeable secondary batteries, aqueous batteries such as lead batteries, nickel-cadmium batteries, and nickel-hydrogen batteries have been mainstream. In recent years, electric vehicles (EVs) and hybrid vehicles (HEVs) that assist a part of driving with electric motors have attracted attention due to problems of global warming and fuel depletion, and batteries serving as power sources have high capacity and high output. It has come to be required. As a battery that meets such requirements, a lithium secondary battery having a high voltage has attracted attention.

  A lithium transition metal composite oxide is used for the positive electrode material of a lithium secondary battery. Among them, lithium cobaltate is used from the balance of capacity, cycle characteristics, etc., but the amount of cobalt as a raw material is small. Due to the high cost, lithium transition metal composite oxides containing manganese such as lithium manganate are promising as positive electrode materials for EV and HEV and are being developed. When the lithium transition metal composite oxide has a spinel crystal structure, the lithium ion diffusion path is three-dimensional, whereas in the case of a layered crystal structure, it has a two-dimensional structure. Therefore, the lithium transition metal composite oxide with a layered crystal structure has high lithium ion diffusibility and excellent output characteristics under normal temperature environment, but the crystal shrinks and the lithium ion diffusivity decreases under low temperature environment. Incurs a decrease in output.

  On the other hand, the negative electrode material is generally an amorphous carbon material obtained by firing natural graphite, artificial graphite such as flakes or lumps, graphite carbon material such as mesophase pitch graphite, furan resin such as furfuryl alcohol, and the like. It is used. Since the amorphous carbon material has a theoretical capacity value higher than that of the graphite-based carbon material, a lithium secondary battery excellent in capacity and cycle characteristics can be obtained. In addition, since the voltage characteristics during charging and discharging have an inclination, it is possible to easily and accurately estimate the state of the battery by voltage measurement. However, since the irreversible capacity of the amorphous carbon material is larger than that of the graphite-based carbon material, it is difficult to increase the capacity in the battery, and the electronic conductivity between the negative electrode particles is inferior to that of the graphite-based carbon material. There is. On the other hand, the graphite-based carbon material has a low irreversible capacity and flat voltage characteristics, so that a high-capacity, high-power lithium secondary battery can be obtained, but the volume change of the crystal accompanying charge / discharge is large. Therefore, there is a problem that the electron conductivity between the negative electrode material particles cannot be maintained for a long period of time and the life is reached early, and the charge acceptability at a large current density is inferior to that of the amorphous carbon material.

  The above-described batteries for EV and HEV have a large current density in charge / discharge, and require a long life and high output characteristics. Therefore, a plurality of single cells are connected and used. Since the variation in the characteristics of the single cells greatly affects the life and safety, usually, the variation is suppressed by monitoring and controlling the voltage of the single cells in combination with a control system. However, since the voltage characteristics of the graphite-based carbon material are flat, it is difficult to accurately monitor the state of the battery from the voltage, and in order to do this, a highly accurate control system is required. Accordingly, it is desirable that the negative electrode material of the lithium secondary battery used for the power source for EV or HEV is mainly an amorphous carbon material, and the positive electrode material is made of a lithium transition metal composite oxide having a layered crystal structure. It is promising to improve the output.

  In order to increase the output of the lithium secondary battery, it is necessary to improve the electron conductivity and lithium ion diffusibility in the positive and negative electrode mixture layers. Various studies have been made. In order to improve the electron conductivity, various low resistance improvements such as adding a conductive material to the positive and negative electrode mixture and increasing the positive and negative electrode mixture density to secure an electron conduction network Has been made. On the other hand, in order to improve the diffusibility of lithium ions, a space through which the nonaqueous electrolyte solution permeates is required in the positive and negative electrode mixture layers. For example, when lithium manganate having a spinel crystal structure is used, a technique for setting the porosity of the positive electrode mixture layer according to the thickness of the positive electrode mixture layer is disclosed (see Patent Document 1). Further, under a low temperature environment, the mobility of lithium ions in the non-aqueous electrolyte is lowered, and the output characteristics are remarkably changed due to the distribution of the non-aqueous electrolyte in the positive and negative electrode mixture layers. In order to solve this, a technique for suppressing a decrease in lithium ion mobility in a low temperature environment by using a mixed organic solvent in a nonaqueous electrolytic solution is disclosed (for example, see Patent Document 2).

JP 2001-325948 A JP 2001-155766 A

  However, in the technique of Patent Document 1, in the case of lithium manganate having a spinel crystal structure, a space through which a non-aqueous electrolyte permeates is ensured. However, as described above, a layered crystal structure in which crystal shrinkage occurs in a low temperature environment. In the case of a lithium transition metal composite oxide, it cannot be said that the space through which the nonaqueous electrolyte permeates is sufficient. Moreover, although the technique of patent document 2 suppresses the fall of the lithium ion mobility in a non-aqueous electrolyte, it is difficult to improve the diffusibility of the lithium ion in a positive / negative electrode mixture layer. Furthermore, increasing the positive and negative electrode mixture density improves the electronic conductivity, but as the positive and negative electrode mixture density increases, the amount of non-aqueous electrolyte that permeates into the positive and negative electrode mixture decreases and the diffusion of lithium ions As a result, the electrode reaction is partially biased, which prevents the formation of an electron conduction network.

  In view of the above-described case, an object of the present invention is to provide a lithium secondary battery that can increase the output in a low temperature environment and can improve the life.

In order to solve the above-described problem, a first aspect of the present invention includes a positive electrode in which a positive electrode mixture including a lithium manganese nickel cobalt composite oxide having a layered crystal structure and a conductive material is applied to a positive electrode current collector, lithium In a lithium secondary battery in which a nonaqueous electrolyte is infiltrated with a negative electrode containing a carbon material capable of desorbing and inserting ions, the lithium manganese nickel cobalt composite oxide has a tapping density of 1.6 g / cm ³ or more and 2.2 g. / Cm ³ or less, and an angle of repose is 47.5 degrees or more and 50 degrees or less.

In the lithium secondary battery according to the first aspect, the repose angle of the lithium manganese nickel cobalt composite oxide is set to 47.5 degrees or more and 50 degrees or less, whereby dispersion when applying the positive electrode mixture to the positive electrode current collector is performed. Since the wettability of the lithium manganese nickel cobalt composite oxide to the medium is optimized, the lithium manganese nickel cobalt composite oxide can be applied almost uniformly without separation and settling, and the tapping density can be reduced. By making the amount 1.6 g / cm ³ or more and 2.2 g / cm ³ or less, the amount of the lithium manganese nickel cobalt composite oxide occupying the positive electrode mixture is secured, and the non-aqueous electrolyte is a lithium manganese nickel cobalt composite oxide. Therefore, it is possible to suppress variations in the electrode reaction even in a low temperature environment.

According to the first aspect, the tapping density of the lithium manganese nickel cobalt composite oxide is 1.6 g / cm ³ or more and 2.2 g / cm ³ or less, and the angle of repose is 47.5 degrees or more and 50 degrees or less. Thus, peeling of the positive electrode mixture applied substantially uniformly to the positive electrode current collector is prevented, and partial concentration of the electrode reaction is suppressed even in a low temperature environment, so the output and life of the lithium secondary battery Can be suppressed.

According to a second aspect of the present invention, in the positive electrode material for a lithium secondary battery using the lithium manganese nickel cobalt composite oxide having a layered crystal structure, the tapping density of the lithium manganese nickel cobalt composite oxide is 1.6 g / cm. ³ or more and 2.2 g / cm ³ or less, and the angle of repose is 47.5 degrees or more and 50 degrees or less. In the present embodiment, a lithium-manganese-nickel-cobalt composite oxide represented by the chemical formula _{Li X Ni Y Mn Z Co (} 1-Y-Z) O 2 (0 <X ≦ 1.2, Y + Z <1) so as to be represented by Also good. Moreover, the average particle diameter of the lithium manganese nickel cobalt composite oxide may be 20 μm or less.

According to the present invention, the tapping density of the lithium manganese nickel cobalt composite oxide is 1.6 g / cm ³ or more and 2.2 g / cm ³ or less, and the angle of repose is 47.5 degrees or more and 50 degrees or less. The positive electrode mixture applied almost evenly to the positive electrode current collector is prevented from being peeled off, and partial concentration of electrode reactions is suppressed even in a low temperature environment, so the output and life of the lithium secondary battery are reduced. The effect that it can suppress can be show | played.

  Embodiments in which the present invention is applied to a cylindrical lithium ion secondary battery will be described below with reference to the drawings.

(Constitution)
As shown in FIG. 1, the cylindrical lithium ion secondary battery 20 of the present embodiment is made of nickel-plated steel and bottomed cylindrical battery can 7 and a polypropylene resin and is cylindrical. Around the winding core 1, there is an electrode plate group 6 in which a belt-like positive electrode plate and a negative electrode plate are wound in a cross-sectional spiral shape via a separator.

  On the upper side of the electrode plate group 6, a ring-shaped positive electrode current collecting ring 4 for collecting the electric potential from the positive electrode plate is disposed. The positive electrode current collecting ring 4 is fixed to the upper end portion of the winding core 1 via a positive electrode current collecting ring support that supports the positive electrode current collecting ring 4. The edge of the positive electrode lead piece 2 extended from the positive electrode plate is ultrasonically welded to the periphery of the positive electrode current collecting ring 4. Above the positive electrode current collecting ring 4, a disk-shaped battery lid 13 having a central portion formed in a convex shape is disposed. One end of a ribbon-like positive electrode lead plate 9 made of aluminum is fixed to the upper part of the positive electrode current collecting ring 4. The other end of the positive electrode lead plate 9 is joined to the lower portion of the battery lid 13 by welding via the lid lead plate.

  On the other hand, a ring-shaped negative electrode current collecting ring 5 for collecting a potential from the negative electrode plate is disposed below the electrode plate group 6, and the negative electrode current collecting ring 5 supports the negative electrode current collecting ring 5. It is fixed to the lower end portion of the winding core 1 via a negative electrode current collecting ring support. The edge of the negative electrode lead piece 3 extending from the negative electrode plate is welded to the periphery of the negative electrode current collecting ring 5. A negative electrode lead plate 8 is welded to the lower part of the negative electrode current collecting ring 5, and the negative electrode lead plate 8 is welded to the inner bottom portion of the battery can 7. The battery can 7 has an outer diameter of 40 mm and an inner diameter of 39 mm.

The battery lid 13 is crimped and fixed to the upper part of the battery can 7 via an insulating and heat resistant resin gasket. For this reason, the inside of the lithium ion secondary battery 20 is sealed. Further, a predetermined amount of non-aqueous electrolyte (not shown) is injected into the battery can 7. For the non-aqueous electrolyte, for example, 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) can be used by dissolving in a mixed solvent of ethylene carbonate and dimethyl carbonate. The battery lid 13 is provided with a cleavage valve (internal pressure release mechanism) that cleaves when the internal pressure of the lithium ion secondary battery 20 increases, and the cleavage pressure is set to 9 × 10 ⁵ Pa. Further, the lithium ion secondary battery 20 is electrically operated in response to an increase in battery temperature, for example, a PTC element, or a current interruption in which a positive or negative electrical lead is disconnected in response to an increase in battery internal pressure. The mechanism is not arranged.

  In the electrode plate group 6, the positive electrode plate and the negative electrode plate are wound around the core 1 through, for example, a polyethylene separator having a width of 90 mm and a thickness of 40 μm so that the two electrode plates do not directly contact each other. The positive electrode lead piece 2 and the negative electrode lead piece 3 are respectively disposed on opposite end surfaces of the electrode plate group 6. An insulating coating is applied to the entire outer peripheral surfaces of the electrode plate group 6 and the positive electrode current collecting ring 4. For the insulating coating, for example, an adhesive tape in which a hexamethacrylate adhesive is applied to one side of a polyimide base material is used. The diameter of the electrode plate group 6 is set to 38 ± 0.1 mm by adjusting the lengths of the positive electrode plate, the negative electrode plate, and the separator.

The negative electrode plate constituting the electrode plate group 6 has a rolled copper foil having a thickness of 10 μm as a negative electrode current collector. On both surfaces of the rolled copper foil, a negative electrode mixture containing amorphous carbon powder having an average particle diameter of 5 to 20 μm capable of occluding and releasing lithium ions as a negative electrode active material is applied almost uniformly and uniformly. In the negative electrode mixture, polyvinylidene fluoride (PVDF) as a binder (binder) is blended, and the blend ratio of the negative electrode active material and the binder is set to, for example, a mass ratio of 100: 5. N-methylpyrrolidone (NMP) is used as a dispersion solvent when applying the negative electrode mixture to the rolled copper foil. An uncoated portion of a negative electrode mixture having a width of 30 mm is formed on the side edge on one side in the longitudinal direction of the rolled copper foil. The uncoated part is notched in a comb shape, and the negative electrode lead piece 3 is formed in the notch remaining part. The negative electrode plate is pressed by a heatable roll press so that the negative electrode mixture density is 1.0 g / cm ³ and is cut into a width of 85 mm.

On the other hand, the positive electrode plate has an aluminum foil having a thickness of 20 μm as a positive electrode current collector. A positive electrode mixture containing lithium manganese nickel cobalt composite oxide powder having a layered crystal structure (hereinafter simply referred to as composite oxide) as a positive electrode active material is applied to both surfaces of the aluminum foil almost uniformly and uniformly. . In the positive electrode mixture, flake graphite as a conductive material and PVDF as a binder are blended. The compounding ratio of the composite oxide, the conductive agent, and the binder is set to, for example, a mass ratio of 100: 10: 5. NMP is used as a dispersion solvent when the positive electrode mixture is applied to the aluminum foil. An uncoated portion of the positive electrode mixture is formed on the side edge on one side in the longitudinal direction of the aluminum foil in the same manner as the negative electrode plate, and the positive electrode lead piece 2 is formed. The positive electrode plate is pressed in the same manner as the negative electrode plate so as to have a positive electrode mixture density of 2.70 g / cm ³ and cut into a width of 80 mm.

(Preparation of positive electrode material)
The composite oxide described above can be represented by the following general formula (1), and can be obtained by mixing and firing raw materials containing lithium, nickel, manganese, and cobalt. In the general formula (1), X is a number that satisfies 0 <X ≦ 1.2, and Y and Z are numbers that satisfy (Y + Z) <1. When the proportion of manganese increases, it becomes difficult to synthesize a single-phase composite oxide. Therefore, Z ≦ 0.4 is desirable, and when the proportion of cobalt increases, the cost increases and the battery capacity significantly decreases. -Y-Z) <Y, Z are desirable. Note that the valence of oxygen in the general formula (1) is not limited to 2, and some oxygen deficiency may occur.

  Examples of the raw material include lithium carbonate, lithium nitrate, and lithium hydroxide. Examples of the nickel source include nickel carbonate, nickel sulfate, nickel hydroxide, nickel oxide, nickel peroxide and the like. Manganese sources include manganese dioxide, nitric oxide, trimanganese tetroxide, manganese carbonate, manganese nitrate, manganese sulfate and the like. Examples of the cobalt source include cobalt oxide, niobium trioxide, tricobalt tetroxide, cobalt hydroxide, cobalt nitrate, and cobalt sulfate. These nickel source, manganese source, and cobalt source are mixed in a wet manner, then co-precipitated in an alkaline atmosphere, dried and pulverized, and a fired raw material is obtained by a two-stage method in which a lithium source is mixed in a dry manner. The firing raw materials are a method of mixing and pulverizing a lithium source, nickel source, manganese source and cobalt source in a dry manner, a method of mixing raw materials in a wet manner, granulating and drying, and a co-precipitation after mixing the raw materials in a wet manner. It can also be obtained by a method of pulverizing and drying.

  In the wet mixing of each raw material, water or an organic solvent can be used as a dispersion medium, but water is preferably used. Representative examples of the mixer and the disperser include a bead mill, a ball mill, and a jet mill. The drying method is not particularly limited, and a spray drying method that can be easily dried at an appropriate drying temperature and spray amount can be employed. Air or an inert gas can be used as the atomizing gas, but air is usually used. When the drying temperature is low, it may cause poor drying or dew condensation in the furnace. By adjusting the spray speed, the drying temperature, the spray nozzle shape, the spray gas supply amount, and the like during spray drying, the particle diameter and packing density of the calcined raw material can be controlled.

  Next, the obtained firing raw material is left standing in an electric furnace and fired. Firing is usually performed at a temperature of 700 to 1100 ° C for 8 to 48 hours. The rate of temperature increase is not particularly limited, but it is necessary to maintain the firing temperature for at least 4 hours in order to form a single crystal layer. When the firing temperature is low, the firing time becomes long and the crystal growth is insufficient, so that the packing density of the primary particles in the secondary particles of the obtained composite oxide becomes small. On the other hand, when the firing temperature is high, another crystal phase is formed or defects are increased. Further, the lithium source may sublimate and compositional deviation may occur. In order to suppress the formation of another crystal phase and the formation of complex oxides with many defects even after cooling after firing, it is desirable to slowly cool by limiting the cooling rate, for example, a cooling rate of 300 ° C / h or less It is desirable to do in. As a baking apparatus, a gas furnace, a tunnel furnace, a rotary kiln, etc. can be used besides an electric furnace.

The calcined composite oxide has a tapping density of 1.6 g / cm ³ or more and 2.2 g / cm ³ or less and a repose angle of 47.5 degrees or more and 50 degrees or less. Measured with a powder tester PT-R type manufactured by Hosokawa Micron Corporation), it was confirmed by powder X-ray diffraction measurement that it was a layered crystal structure. Moreover, it confirmed that the average particle diameter was 20 micrometers or less with the laser type particle size measuring apparatus. In the measurement of the tapping density, a 100 cc tapping cell was used, the tapping height was set to 3 mm, and the tapping frequency was set to 180 times for 5 minutes. This tapping density can also be measured by the method described in Japanese Industrial Standard (JIS K 7370). The angle of repose was measured according to Japanese Industrial Standard (JIS R 9301-2-2).

  Next, an example of the lithium ion secondary battery 20 manufactured using the composite oxide in which the tapping density and the angle of repose are changed by changing the above-described spray drying conditions and firing conditions according to the present embodiment will be described. In addition, it describes together about the lithium ion secondary battery of the comparative example produced for the comparison.

Example 1
As shown in Table 1 below, in Example 1, the compound oxide was a rhombohedral crystal in which the molar ratio of each element was Li: Ni: Mn: Co = 1: 0.34: 0.33: 0.33. (Composition LiNi _0.34 Mn _0.33 Co _0.33 O ₂ ) was synthesized, and a composite oxide having a tapping density of 1.6 g / cm ³ and an angle of repose of 50 degrees was used. As raw materials, 89.4 g of nickel sulfate hexahydrate having an average particle diameter of 1 μm, 92.8 g of cobalt sulfate heptahydrate having an average particle diameter of 1 μm, and manganese sulfate hexahydrate having an average particle diameter of 1 μm 79.6 g was dissolved and dispersed in 1 liter of ion exchange water, and then the aggregate was removed through a 400 mesh filter. A 0.5 mol / liter aqueous sodium hydroxide solution was gradually added to the resulting solution to coprecipitate a nickel / cobalt / manganese mixed hydroxide. The coprecipitate is filtered with an accordion type filter press, dried with stirring at 120 ° C. for 1 hour, and then pulverized with a jet mill to have various average particle sizes. 74 g of lithium carbonate was added to the oxide and mixed with a jet mill to obtain a firing raw material. The firing raw material placed in a zirconia container was left in an electric furnace and heated to a firing temperature of 850 to 1050 ° C. at a heating rate of 5 ° C./min. After maintaining at the maximum temperature for 8 hours, it was gradually cooled at a cooling rate of 5 ° C / min.

(Example 2 to Example 5)
As shown in Table 1, Examples 2 to 5 were the same as Example 1 except that the tapping density and the angle of repose were changed. In Example 2, the tapping density was 1.8 g / cm ³ and the angle of repose was 49 degrees. In Example 3, the tapping density was 2.0 g / cm ³ and the angle of repose was 48 degrees. In Example 4, the tapping density was 2.0 g / cm ³ and the repose was. A composite oxide having an angle of 47.5 °, a tapping density of 1.8 g / cm ³ in Example 5, and an angle of repose of 50 ° was obtained.

(Comparative Examples 1 to 3)
As shown in Table 1, Comparative Examples 1 to 3 were the same as Example 1 except that the tapping density and the angle of repose were changed. In Comparative Example 1, the tapping density is 1.5 g / cm ³ and the angle of repose is 50 degrees. In Comparative Example 2, the tapping density is 2.3 g / cm ³ and the angle of repose is 46 degrees. In Comparative Example 3, the tapping density is 1.8 g / cm ³ and the repose is A composite oxide having an angle of 52 degrees was obtained.

(Evaluation of composite oxide)
Regarding the composite oxides obtained in each Example and Comparative Example, the stability of the coating solution in which the above-described positive electrode mixture was dispersed and dissolved in NMP and the state of the electrode surface when applied to the aluminum foil were visually observed. And evaluated. The evaluation results are shown in Table 1.

As shown in Table 1, in the coating solution using the composite oxide of Comparative Example 1 having a tapping density of less than 1.6 g / cm ³ and Comparative Example 3 having an angle of repose of more than 50 degrees, the solution was only in a low solid content concentration. In some cases, the release of NMP and the precipitation of the positive electrode mixture particles were gradually observed only by leaving it to stand. Moreover, when apply | coating to aluminum foil, complex oxide aggregated in the application | coating solution to stir and it became impossible to perform smooth application | coating, and the unevenness | corrugation was recognized partially on the electrode surface. On the other hand, in a coating solution using a complex oxide having a tapping density of 1.6 g / cm ³ or more and 2.2 g / cm ³ or less and an angle of repose of 47.5 degrees or more and 50 degrees or less, it is left as it is. Also, no release or sedimentation of NMP was observed and it was stable, and the electrode surface coated with NMP was smooth without any irregularities.

(Battery test)
Next, the following tests were performed on the batteries of the examples and comparative examples. First, a constant-current / constant-voltage charge (set voltage 4.1 V) is performed for 5 hours at a 3-hour rate (0.33 C) in a room temperature (25 ° C.) atmosphere, and then a discharge end voltage is set at a 1-hour rate (1 C). The battery was discharged to 2.7 V and charged again under the same conditions. Next, in accordance with Japanese Industrial Standard (JIS C 8711), discharge was performed at each current value of discharge current 1, 3, 6A, the voltage at the 5th second was measured, and the initial output was obtained from this current-voltage characteristic. The battery for which the initial output was measured was left in a low temperature (−25 ° C.) constant temperature bath for 24 hours to cool the entire battery to −25 ° C., which was the same as the measurement of the initial output at room temperature described above. Under the conditions, the output under a low temperature atmosphere was measured. Furthermore, in order to measure the decrease in output due to the charge / discharge cycle, the sample was allowed to stand at room temperature for 24 hours and then charged at a constant current and constant voltage (set voltage 4.1 V) at a rate of 3 hours (0.33 C) in an atmosphere of 25 ° C. ) Was performed for 5 hours, and charging and discharging were repeated at a rate of 1 hour (1C) until reaching a final discharge voltage of 2.7 V. After 100 cycles, the output of the battery was measured in the same manner as the measurement of the initial output, and the ratio of the output after 100 cycles to the initial output was obtained as a percentage to obtain the maintenance rate. Table 2 below shows the measurement results of the output and the maintenance factor in a room temperature and low temperature atmosphere.

As shown in Table 2, in the lithium ion secondary battery of Comparative Example 2 using a composite oxide having a tapping density of 2.2 g / cm ³ or less even at an angle of repose of 50 degrees or less, the output under a low temperature atmosphere is significantly reduced. Moreover, in the overcharge test, it was confirmed that the battery ruptured and the safety was significantly impaired. This is because the nonaqueous electrolyte distribution in the positive electrode mixture layer is non-uniform, so the electrode reaction is concentrated in the area where the nonaqueous electrolyte is present, and that part has reached an overcharged state from an early stage. it is conceivable that. Further, in the lithium ion secondary battery of Comparative Example 1 having a tapping density of less than 1.6 g / cm ^{3 and} the lithium ion secondary battery of Comparative Example 3 having an angle of repose exceeding 50 degrees, the initial output is low and maintained after 100 cycles. The decline in rate is also great. This is because the binder is selectively taken into the voids of the secondary particles of the composite oxide to inhibit the migration of lithium ions, and because of the volume change of the composite oxide due to repeated charge and discharge, the secondary particles This is thought to be due to a decrease in electron conductivity and non-uniformity of the electrode reaction.

In contrast, the tapping density is 1.6 g / cm ³ or more 2.2 g / cm ³ or less, and angle of repose were used following composite oxides 50 degrees 47.5 degrees Example 1 to Example 5 In the lithium ion secondary battery 20, an output of 300 W or more in a low temperature atmosphere is secured, and an output of 90% or more of the initial output is maintained even after 100 cycles. Among them, in Examples 1 to 3 and 5 in which the tapping density is 1.6 g / cm ³ or more and 2.0 g / cm ³ or less and the angle of repose is 48 degrees to 50 degrees, the output is 850 W in a room temperature environment. As described above, it was possible to secure an output of 330 W or more in a low temperature environment.

As described above, the tapping density is at 1.6 g / cm ³ or more 2.2 g / cm ³ or less, and, by using the angle of repose is not more than 50 degrees 47.5 degrees composite oxide in the positive electrode material It was found that a lithium ion secondary battery having high capacity, high output and excellent life performance can be obtained.

(Action etc.)
Next, the operation and the like of the lithium ion secondary battery 20 of the present embodiment will be described.

  In the composite oxide obtained by firing, secondary particles in which primary particles are aggregated are formed, and the void volume in the secondary particles varies depending on the drying conditions and firing conditions of the firing raw material. For this reason, the tapping density that represents the apparent density after applying vibration (tapping) to the composite oxide placed in the measurement vessel varies depending on the particle size distribution of the composite oxide and the influence of the primary particle density in the secondary particles. It also changes with the smoothness, surface charge and surface tension of the primary and secondary particle surfaces. For example, the tapping density is decreased by increasing the void volume in the secondary particles (decreasing the density of the primary particles), increasing the smoothness of the particle surface, decreasing the surface tension, and the like. In addition, the repose angle, which represents the angle of the peaks formed by dropping the composite oxide, is greatly affected by the smoothness, surface charge, and surface tension of the primary and secondary particle surfaces. For example, the angle of repose decreases due to an increase in the smoothness of the particle surface, a decrease in surface tension, and the like.

Tapping density is less than 1.6 g / cm ^3, wetting of the use of composite oxide angle of repose exceeds 50 °, the NMP for dilution dispersing medium during the production of the coating solution when applying a positive electrode mixture to the aluminum foil The stability (for example, separation, sedimentation, etc.) of the obtained coating solution also deteriorates while the properties deteriorate. As a result, the amount of the composite oxide in the positive electrode mixture is reduced, and peeling of the positive electrode mixture occurs due to non-uniform distribution of the conductive material and binder in the positive electrode mixture. Moreover, in order to suppress the output fall by increasing the positive electrode mixture density, a strong pressure is required in the press working, and both sides of the electrode end portion are remarkably extended, and the smoothness and linearity of the electrode surface are lost. Such a failure during electrode production causes battery characteristics and variations thereof, resulting in a decrease in capacity and output, and a decrease in life and safety. On the other hand, when a composite oxide having a tapping density of more than 2.2 g / cm ³ is used, since the voids in the secondary particles are small, the distribution of the conductive material is biased toward the surface of the secondary particles, and the dense conductive in the positive electrode mixture Since it becomes difficult to form a network and the penetration of the non-aqueous electrolyte solution is insufficient, the electrode reaction in the secondary particles varies and partially concentrates. For this reason, the composite oxide is deteriorated and the life is shortened. In particular, since the layered crystal of the composite oxide shrinks in a low temperature environment, the lithium ion diffusibility is lowered and the variation in electrode reaction becomes remarkable.

In the lithium ion secondary battery 20 of this embodiment, the tapping density of the composite oxide is 1.6 g / cm ³ or more and 2.2 g / cm ³ or less, and the angle of repose is 47.5 degrees or more and 50 degrees or less. For this reason, since the non-aqueous electrolyte permeates the composite oxide while securing the amount of the composite oxide in the positive electrode mixture, variations in the electrode reaction can be suppressed even in a low temperature environment. Moreover, since the wettability of the composite oxide with respect to NMP is optimized, the positive electrode mixture can be applied substantially evenly without the composite oxide being separated and settled in the coating solution. Accordingly, concentration of the electrode reaction is suppressed and peeling of the positive electrode mixture is prevented, so that it is possible to suppress a decrease in output and life of the lithium secondary battery.

  Moreover, in this embodiment, since the fluidity of the composite oxide is optimized by setting the tapping density and the angle of repose of the composite oxide within the above-described ranges, mixing and dispersion in NMP can be easily performed. At the same time, since the formation of aggregates can be suppressed, the workability during the production of the positive electrode can be improved.

In this embodiment, the composition LiNi _0.34 Mn _0.33 Co _0.33 O _{2 in} which the manganese / nickel / cobalt ratio of the lithium manganese nickel cobalt composite oxide is 1: 1: 1 is exemplified. This invention is not limited to this, What is necessary is just a ratio which has a layered crystal structure. Moreover, although the example which made lithium / (manganese + nickel + cobalt) ratio 1.0 was shown, it is good also as lithium excess. Furthermore, a material in which a part of lithium, manganese, nickel, cobalt, oxygen is substituted or doped with at least one element of, for example, Fe, Cu, Al, Cr, Mg, Zn, V, Ga, B, F Can also be used.

  Further, in the present embodiment, the cylindrical battery is illustrated in which the positive and negative electrodes are wound and accommodated in a bottomed cylindrical battery can. However, the present invention is not limited to the shape and structure of the battery. The present invention can also be applied to other polygonal batteries and stacked type batteries in which positive and negative electrodes are stacked. Moreover, as a battery structure to which the present invention can be applied, for example, a battery having a structure in which positive and negative external terminals pass through a battery lid and are pressed through a winding core in a battery container can be exemplified.

  Furthermore, in the present embodiment, amorphous carbon is exemplified as the negative electrode active material, but the present invention is not limited to this. For example, graphite-based carbon materials such as natural graphite, various artificial graphite materials, and coke may be used, and the particle shape is not particularly limited, such as scaly, spherical, fibrous, or massive.

  Furthermore, in this embodiment, PVDF is exemplified as the binder, but the present invention is not limited to this. For example, polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, butyl rubber, nitrile rubber, styrene / Polybutadiene rubber, polysulfide rubber, nitrocellulose, cyanoethyl cellulose, various latexes, acrylonitrile, vinyl fluoride, vinylidene fluoride, propylene fluoride, chloroprene fluoride, and mixtures thereof may be used.

  Furthermore, in the present embodiment, scaly graphite is exemplified as the positive electrode conductive material, but the present invention is not limited to this, and any graphite-based carbon material may be used. Examples of the positive electrode conductive material that can be used other than the present embodiment include natural graphite, various artificial graphite materials, coke, and the like, and also in the particle shape, particularly in the form of scaly, spherical, fibrous, massive, etc. It is not limited. Moreover, you may mix | blend a electrically conductive material with a negative electrode, for example, amorphous carbon, such as ketjen black and acetylene black, can be used.

In the present embodiment, the non-aqueous electrolytic solution is exemplified by dissolving lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and dimethyl carbonate, but the present invention is not limited to this, A general lithium salt can be used as an electrolyte, which is dissolved in an organic solvent. There are no particular limitations on the lithium salt or organic solvent used. For example, as the electrolyte, LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, or a mixture thereof may be used. Examples of the organic solvent include propylene carbonate, diethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3- Dioxolane, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propionitrile, or a mixed solvent of two or more of these may be used. The mixing ratio is not limited.

  The present invention provides a lithium secondary battery that can increase the output under a low temperature environment and can improve the life, contribute to manufacture and sales, and can be used industrially.

It is sectional drawing which shows the cylindrical lithium ion secondary battery of embodiment which can apply this invention.

Explanation of symbols

6 electrode plate group 20 cylindrical lithium ion secondary battery (lithium secondary battery)

Claims (4)

Nonaqueous electrolysis of a positive electrode in which a positive electrode mixture containing a lithium manganese nickel cobalt composite oxide having a layered crystal structure and a conductive material is applied to a positive electrode current collector, and a negative electrode containing a carbon material capable of desorbing and inserting lithium ions In the lithium secondary battery infiltrated in the liquid, the lithium manganese nickel cobalt composite oxide has a tapping density of 1.6 g / cm ³ or more and 2.2 g / cm ³ or less, and an angle of repose of 47.5 degrees or more. A lithium secondary battery, which is 50 degrees or less. In a positive electrode material for a lithium secondary battery using a lithium manganese nickel cobalt composite oxide having a layered crystal structure, a tapping density of the lithium manganese nickel cobalt composite oxide is 1.6 g / cm ³ or more and 2.2 g / cm ³ or less. And an angle of repose of 47.5 degrees or more and 50 degrees or less. The lithium-manganese-nickel-cobalt composite oxide has the formula _{Li X Ni Y Mn Z Co (} 1-Y-Z) O 2 (0 <X ≦ 1.2, Y + Z <1) claims, characterized by being represented by Item 3. The positive electrode material according to Item 2.   The positive electrode material according to claim 2, wherein the lithium manganese nickel cobalt composite oxide has an average particle size of 20 μm or less.