JP2006127923A - Cathode active material for lithium secondary battery and lithium secondary battery - Google Patents

Abstract

Provided is a positive electrode active material capable of high-rate discharge in an extremely low temperature environment and capable of suppressing a decrease in capacity.
A lithium transition metal composite oxide as a positive electrode active material was obtained by firing raw materials each containing Li, Mn, Ni, and Co in the atmosphere. The obtained lithium transition metal composite oxide has a layered crystal structure represented by the chemical formula Li _a Mn _x Ni _y Co _z O ₂ . In the lithium transition metal composite oxide, the particle diameter at a cumulative frequency of 90% of the secondary particles was 20 μm or less. The primary particle distribution ratio R in the secondary particles satisfies the relationship 0.8 ≦ R ≦ 1.2. The tap density when this lithium transition metal composite oxide was tapped 200 times was set to a range of 1.5 to 2.5 g / cm ³ . The diffusibility of lithium ions between lithium transition metal composite oxide particles is ensured even in an extremely low temperature environment.
[Selection] Figure 8

Description

  The present invention relates to a positive electrode active material for a lithium secondary battery and a lithium secondary battery, and in particular, a positive electrode active material for a lithium secondary battery using a lithium transition metal composite oxide and a lithium secondary battery using the positive electrode active material. Next battery.

  High power and high energy density batteries are required for power sources for electric vehicles and hybrid electric vehicles that can efficiently use energy in consideration of the environment. A lithium secondary battery using a non-aqueous electrolyte has a high battery voltage and a high energy density, and thus is promising as a power source for electric vehicles. Until now, lithium secondary batteries have been widely used for consumer use, but when used as power sources for electric vehicles, long life performance, excellent input / output performance and stable operation over a wide temperature range are required. It is done. In order to achieve a long battery life, the positive electrode active material is required to have a stable crystal structure even after charge / discharge cycles or long-term storage. In addition, in order to start an engine that requires a high rate discharge in winter, it is essential to ensure battery performance in a low temperature environment. However, in a low temperature environment up to −30 ° C. (hereinafter, referred to as “very low temperature environment”), the viscosity of the non-aqueous electrolyte used in the lithium secondary battery increases, so the diffusion rate of lithium ions decreases. Furthermore, since the crystal structure of the positive electrode active material changes, the diffusion rate of lithium ions in the positive electrode active material is lowered, and the output performance is lowered.

  In order to improve output performance under such a cryogenic environment, non-aqueous electrolytes and positive electrode active materials have been widely studied. For example, the particle shape of the positive electrode active material is closely related to an increase in battery output. Increasing the specific surface area by reducing the particle size of the positive electrode active material and increasing the interface between the non-aqueous electrolyte and the positive electrode active material reduces the reaction resistance at the non-aqueous electrolyte / positive electrode active material interface. Thus, a technique for achieving high output is disclosed (for example, see Patent Document 1). However, when the particle size of the positive electrode active material is reduced, the filling amount of the positive electrode active material into the electrode is reduced, which causes a problem that the battery capacity is reduced. Moreover, the technique which enables high rate discharge and suppresses the fall of battery capacity by determining the tap density of a positive electrode active material is disclosed (for example, refer patent document 2).

JP-A-6-325791 JP 2000-323142 A

  However, in a cryogenic environment, the diffusion rate and electronic conductivity of lithium ions decrease between positive electrode active materials and within the positive electrode active material due to an increase in the viscosity of the non-aqueous electrolyte and a change in the crystal structure of the positive electrode active material. Discharge capacity is greatly reduced. In the technique of Patent Document 1 described above, the detailed relationship between the specific surface area, the positive electrode active material filling amount, and the battery output is unknown, and in particular, the suppression of the decrease in the discharge capacity is insufficient in a cryogenic environment. In the technique of Patent Document 2, even if the tap density is determined, the crystal structure changes in an extremely low temperature environment, so that the output decreases. Furthermore, if the tap density is too high, the gap between the positive electrode active materials becomes small, so that the non-aqueous electrolyte does not penetrate and the output is reduced. In order to solve these problems, it is important to ensure lithium ion diffusibility and electronic conductivity between the positive electrode active materials and in the positive electrode active material. Since the positive electrode active material is composed of secondary particles in which primary particles are aggregated, if the bondability between the primary particles can be increased, lithium ion diffusibility, electrons between adjacent primary particles in the secondary particles The conductivity can be improved, and an improvement in output and capacity can be expected.

  In view of the above circumstances, the present invention has an object to provide a positive electrode active material capable of high-rate discharge in a cryogenic environment and capable of suppressing a decrease in capacity, and a lithium secondary battery using the positive electrode active material. To do.

In order to solve the above-described problem, a first aspect of the present invention is a positive electrode active material for a lithium secondary battery using a lithium transition metal composite oxide, wherein the lithium transition metal composite oxide has the chemical formula Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44, 0.1 ≦ z ≦ 0.6, x + y + z = 1) The tap density is 1.5 g / cm ³ or more and 2.5 g / cm ³ or less.

In the positive electrode active material of the first aspect, the lithium transition metal composite oxide has the chemical formula Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44, 0.1 ≦ z ≦ 0.6, and x + y + z = 1), the change of the crystal structure is stable during charging and discharging of the lithium secondary battery, and is stable. Since the diffusibility of lithium ions in the positive electrode active material is ensured, it is possible to suppress a decrease in battery output even in an extremely low temperature environment. When the tap density is too high, the gap between the positive electrode active materials is reduced and the permeability of the non-aqueous electrolyte is lowered. Therefore, the tap density is 1.5 g / cm ^3. with more than 2.5 g / cm ³ or less, the positive Diffusion of lithium ions between the active materials, the permeability of the nonaqueous electrolytic solution is ensured, it is possible to suppress the battery capacity, decrease in battery output. In this case, the lithium transition metal composite oxide may have a particle size of 20 μm or less at a cumulative frequency of 90%.

The second aspect of the present invention is a positive electrode active material for a lithium secondary battery using a lithium transition metal composite oxide, wherein the lithium transition metal composite oxide has the chemical formula Li _a Mn _x Ni _y Co _z O. ₂ (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44, 0.1 ≦ z ≦ 0.6, x + y + z = 1) It has a structure, and the particle size at a cumulative frequency of 90% of the secondary particles in which the primary particles are aggregated is 20 μm or less, and is present in 20% of the total cross-sectional area at the center of the cross section of the secondary particles. The distribution of primary particles in the secondary particles represented by the ratio of the average particle size of the primary particles present in 20% of the total cross-sectional area at the outer periphery of the cross-section of the secondary particles with respect to the average particle size of the primary particles The ratio R satisfies the relationship of 0.8 ≦ R ≦ 1.2.

In the positive electrode active material of the second aspect, the lithium transition metal composite oxide has the chemical formula Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44, 0.1 ≦ z ≦ 0.6, and x + y + z = 1), the change of the crystal structure is stable during charging and discharging of the lithium secondary battery, and is stable. Since the diffusibility of lithium ions in the positive electrode active material is ensured, it is possible to suppress a decrease in battery output even in an extremely low temperature environment, and at the same time 90% cumulative particles of secondary particles in which primary particles are aggregated Since the diameter is 20 μm or less, the specific surface area can be increased to improve the battery output, and the average particle diameter of the primary particles present in 20% of the total cross-sectional area at the center of the cross-section of the secondary particles, One that exists in 20% of the total cross-sectional area at the outer periphery of the cross-section of the secondary particles When the primary particle distribution ratio R in the secondary particles expressed by the ratio of the average particle diameter of the particles satisfies the relationship of 0.8 ≦ R ≦ 1.2, the primary particles are distributed at the center portion and the outer peripheral portion of the secondary particles. Since there is no great difference in the average particle diameter, the bondability between the primary particles is ensured, and the electron conductivity and the diffusibility of lithium ions are ensured. Therefore, it is possible to suppress the decrease in battery capacity and battery output.

First, in a second aspect, the tap density of the lithium transition metal composite oxide 1.75 g / cm ³ or more 1.85 g / cm ³ may be less. In addition, if the specific surface area of the lithium transition metal composite oxide is 0.6 m ² / g or more and 1.4 m ² / g or less, the contact area with the non-aqueous electrolyte is ensured, so the output can be increased. Can do. At this time, the specific surface area may be 0.95 m ² / g or more and 1.05 m ² / g or less. Moreover, if the average particle diameter of the primary particles of the lithium transition metal composite oxide is 0.1 μm or more and 5 μm or less, the bondability between the primary particles is secured, and the electron conductivity and the diffusibility of lithium ions are secured. A decrease in battery capacity and battery output can be suppressed. At this time, the average particle diameter of the primary particles may be 0.5 μm or more and 3 μm or less. If the tap density of the lithium transition metal composite oxide is the filling density after tapping 200 times in the measurement container, the particles are brought into contact with each other by tapping 200 times and cannot be filled any further. Gaps between particles and in secondary particles can be accurately reflected. In addition, such a positive electrode active material includes a positive electrode using a lithium transition metal composite oxide, a counter electrode and a reference electrode using lithium metal, and a volume ratio of 1: 1: 1 of ethylene carbonate, diethyl carbonate, and dimethyl carbonate. When measured with a test battery having a nonaqueous electrolytic solution using a mixed solvent, the discharge capacity in an atmosphere at a temperature of −30 ° C. is preferably 8 mAh / g or more. Alternatively, the lithium transition metal composite oxide may be fired in an air atmosphere.

In order to solve the above-described problem, a third aspect of the present invention is a lithium secondary battery including a positive electrode including a positive electrode active material using a lithium transition metal composite oxide, a negative electrode, and a non-aqueous electrolyte. The lithium transition metal composite oxide has chemical formulas Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44, 0. It has a layered crystal structure represented by 1 ≦ z ≦ 0.6, x + y + z = 1), and its tap density is 1.5 g / cm ³ or more and 2.5 g / cm ³ or less. .

In the third aspect, the particle size of the secondary particles in which the primary particles of the lithium transition metal composite oxide are aggregated is set to 20 μm or less at a cumulative frequency of 90%, and is 20% of the total cross-sectional area at the center of the cross section of the secondary particles. Primary particle distribution ratio in secondary particles expressed by the ratio of the average particle size of primary particles existing in 20% of the total cross-sectional area at the outer peripheral portion of the cross-section of secondary particles to the average particle size of primary particles existing in R may satisfy the relationship of 0.8 ≦ R ≦ 1.2. The output energy density of the lithium secondary battery in a state where the discharge depth is 50% in an atmosphere at a temperature of −30 ° C. may be 200 W / Kg or more and 500 W / Kg or less. The positive electrode preferably has an electrode density of 2.5 g / cm ² or more and 2.7 g / cm ² or less. If the negative electrode contains amorphous carbon, the open circuit voltage when the charged state is 50% can be set to 3.6 V or more and 3.9 V or less.

According to the present invention, the lithium transition metal composite oxide has the chemical formula Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0. .44, 0.1 ≦ z ≦ 0.6, x + y + z = 1), the change of the crystal structure is stable even in an extremely low temperature environment, and the decrease in battery output is suppressed. In addition, by setting the tap density to 1.5 g / cm ³ or more and 2.5 g / cm ³ or less, diffusibility of lithium ions between the positive electrode materials and permeability of the nonaqueous electrolytic solution are ensured. Therefore, the effect that the fall of battery capacity and a battery output can be suppressed can be acquired.

  Embodiments of a golf cart to which the present invention is applicable will be described below with reference to the drawings.

  As shown in FIG. 1, the golf cart 30 of this embodiment includes a chassis 31 serving as a base. A battery box 36 containing 76 lithium ion secondary batteries 20 to be described later connected in series is fixed at a substantially central portion of the chassis 31. A cushion 35 is disposed on the battery case 36, and the battery case 36 and the cushion 35 constitute a front seat.

  In front of the chassis 31, a power transmission mechanism that transmits the rotational driving force of the drive shaft and the drive shaft to the wheels using the lithium ion secondary battery 20 as a power source is fixed to the chassis 31. It is designed to rotate. A handle connected to the wheel is disposed in front of the chest of the driver seated in the front seat. A key switch 32 for starting the golf cart 30 is disposed on the right side of the handle, and a shift switch 33 for switching forward / backward movement of the golf cart 30 is disposed on the left side. An acceleration pedal 37 for adjusting the speed of the golf cart 30 is disposed at the position of the driver's feet, and a brake pedal 38 for braking the golf cart 30 is disposed on the left side of the acceleration pedal. .

As shown in FIG. 2, the golf cart 30 is an IGBT (Insulated) using an induction motor as a control method.
A drive system 40 employing a gate bipolar transistor) vector control inverter system is provided. The drive system 40 includes a drive control device 42. The drive control device 42 is connected to a drive unit 43 coupled to wheels and a cooling device capable of circulating cooling water. The drive control device 42 is connected to a control circuit 45 that controls the drive system 40 in accordance with the depression amount of the acceleration pedal 37 and the brake pedal 38 and the switching of the shift switch 33. Connected to the control circuit 45 are an accelerator unit having a variable resistor linked to the depression amount of the acceleration pedal 37, a brake unit having a variable resistor linked to the depression amount of the brake pedal 38, and the shift switch 33. Yes. The control circuit 45 is connected to the DC / DC converter 47 and the auxiliary battery 48 via the key switch 32. The DC / DC converter 47 is connected to the main power supply 41 using the lithium ion secondary battery 20. The main power supply 41 is connected to the drive control device 42 via a conductor controlled by a protection circuit 46.

  The golf cart 30 is supplied with power from the main power supply 41 when the driver turns on the key switch 32. The drive control device 42 controls the torque or rotation of the drive machine 43 according to the depression amount of the acceleration pedal 37. By switching the shift switch 33, the golf cart 30 is switched between forward and reverse, and the gear ratio is always constant. When the acceleration pedal 37 is returned, the regenerative brake corresponding to the engine brake of the gasoline vehicle is operated, and when the brake pedal 38 is depressed, the regenerative braking force is increased. The main power supply 41 has a power supply voltage of 320 V from the IGBT withstand voltage, an output of 45 KW from the power performance (acceleration / hill climbing performance) of the golf cart 30 and a maximum torque of 176 N · m, and a rated output of 30 kW from the maximum speed specification. Is set. The drive system 40 performs not only forward control, reverse control and regenerative control of the car as main control items, but also fail-safe control (operates in a safer manner even if a failure or malfunction occurs in a part of the device). Can do.

The cylindrical lithium ion secondary battery 20 accommodated in the battery box 36 is manufactured as follows. As shown in FIG. 3, the lithium ion secondary battery 20 has a bottomed cylindrical battery can 6 made of SUS. The battery can 6 accommodates a group of electrodes wound with a separator 5 disposed therebetween so that the positive electrode plate 3 and the negative electrode plate 4 are not in direct contact with each other. In this example, the separator is a microporous polypropylene film having a thickness of 25 μm. Insulating plates 11 are disposed on the upper and lower sides of the electrode group, respectively. The insulating plate 11 is formed with openings for passing lead pieces. The positive electrode lead piece 9 led out from the positive electrode plate 4 is welded to the hermetic lid portion 8 also serving as a positive electrode current terminal through the opening of the insulating plate 11. The sealing lid 8 is provided with a cleavage valve that cleaves when the pressure inside the battery rises and releases the pressure inside the battery. The negative electrode lead piece 7 led out from the negative electrode plate 4 is welded to the inner bottom portion of the battery can 6 through the opening of the insulating plate 11. The positive electrode lead piece 9 and the negative electrode lead piece 7 are arranged on both end surfaces on the opposite sides of the electrode group. The battery can 6 is filled with a non-aqueous electrolyte. In this example, the non-aqueous electrolyte includes 1.0 mol / liter LiPF ₆ in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) in a volume ratio of 1: 1: 1. What was dissolved is used. The sealing lid 8 is caulked and fixed to the battery can 6 via the packing 10. For this reason, the lithium ion secondary battery 20 is sealed. In addition, the dimension of the lithium ion secondary battery 20 is set to 40 mm in diameter and 108 mm in length in this example.

  The negative electrode plate 4 constituting the electrode group has a rolled copper foil having a thickness of 10 μm as a negative electrode current collector. A negative electrode mixture containing a pseudo-isotropic carbon material (hereinafter abbreviated as PIC) which is amorphous carbon as a negative electrode active material is applied to both surfaces of the rolled copper foil substantially uniformly. A binder is blended in the negative electrode mixture. In this example, the blending ratio of the PIC material and the binder is set to 92: 8 in terms of dry weight. At the time of applying the negative electrode mixture, N-methyl-pyrrolidinone (hereinafter abbreviated as NMP) is used as a viscosity adjusting solvent. The rolled copper foil coated with the negative electrode mixture is cut after being compressed and shaped by a roll press. A negative electrode lead piece 7 made of copper foil is welded to the rolled copper foil.

On the other hand, the positive electrode plate 3 has an aluminum foil having a thickness of 20 μm as a positive electrode current collector. On both surfaces of the aluminum foil, a positive electrode mixture containing a lithium transition metal composite oxide described later as a positive electrode active material is applied substantially uniformly. A carbon-based conductive material and a binder are blended in the positive electrode mixture. In this example, the mixing ratio of the lithium transition metal composite oxide, the conductive material, and the binder is set to 85: 10.7: 4.3 by weight. At the time of application of the positive electrode mixture, NMP is used as a viscosity adjusting solvent. The aluminum foil coated with the positive electrode mixture is dried at 100 ° C., pressed with a press at 1.5 ton / cm ² , and then cut. The positive electrode mixture applied to the aluminum foil is formed to a thickness of about 40 μm. A positive electrode lead piece 9 made of aluminum foil is welded to the aluminum foil.

The lithium transition metal composite oxide of the positive electrode active material is prepared as follows. As raw materials, oxides, hydroxides, carbonates and the like containing lithium, manganese, nickel, and cobalt, respectively, were used. Each raw material is mixed at a predetermined ratio and pulverized by wet so that the particle size becomes submicron, and then dried to obtain particles of 5 to 100 μm. The mixing ratio of each raw material is such that the obtained lithium transition metal composite oxide has the chemical formula Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44, 0.1 ≦ z ≦ 0.6, and x + y + z = 1). After the particles are fired in the atmosphere, coarse particles having a particle size of 50 μm or more are removed to prepare a lithium transition metal composite oxide having a layered crystal structure.

In the obtained lithium transition metal composite oxide, as shown in FIG. 4, primary particles 1 aggregate to form secondary particles 2, and adjacent primary particles 1 are bonded to each other. The average particle diameter of the primary particles 1 is in the range of 0.1 to 5 μm, and the average particle diameter of the secondary particles 2 is in the range of 3 to 40 μm. In the particle size distribution of the secondary particles 2, the particle size at a cumulative frequency of 90% is set to 20 μm or less. For this reason, the lithium transition metal composite oxide has a specific surface area in the range of 0.6 to 1.4 m ² / g. Further, the average particle diameter D ₅₀ of the primary particles 1 existing in the range of 20% of the cross-sectional area at the center of the cross section of the secondary particle 2 (hereinafter, the particle diameter occupying 50% of the number of particles is abbreviated as the average particle diameter D _50). ), and secondary particles 2 of cross-sectional outer peripheral portion at a value obtained by dividing the average particle diameter D ₅₀ of the primary particles present in the 20% range of the cross-sectional area 1 intragranular (the secondary particles) and the primary particle distribution ratio R When defined, the relationship 0.8 ≦ R ≦ 1.2 is satisfied. When this lithium transition metal composite oxide is tapped 200 times, the tap density is in the range of 1.5 to 2.5 g / cm ³ .

Moreover, in the positive electrode plate 3 which apply | coated the prepared positive electrode mixture containing the lithium transition metal complex oxide, the density of a positive electrode mixture shows the range of 2.5-2.7 g / cm < ² >. In the test battery (described later in detail) produced using the positive electrode plate 3, the discharge capacity under a low temperature environment is 8 mAh / g or more.

  Hereinafter, examples of the lithium transition metal composite oxide, the lithium ion secondary battery 20 and the golf cart 30 manufactured according to the present embodiment will be described. A comparative example prepared for comparison is also shown.

Example 1
In Example 1, a lithium transition metal composite oxide was prepared using manganese dioxide, cobalt oxide, nickel oxide and lithium carbonate as raw materials. Each raw material was weighed so that the Ni: Mn: Co ratio was 1: 1: 1 and the Li: (Ni + Mn + Co) ratio was 1.03: 1. Pure water was added to the raw material and wet pulverized and mixed for 5 to 100 hours by a ball mill using a resin pot and zirconia balls to make the particle size submicron. Further, a polyvinyl alcohol (hereinafter abbreviated as PVA) solution is added to the mixed solution in an amount of 2 wt% in terms of the solid content ratio, mixed for 1 hour, granulated and dried with a spray dryer, and particles of 5 to 100 μm. Got. The obtained particles were fired at 1000 ° C. for 3 to 10 hours in the atmosphere. After firing, the mixture was crushed and coarse particles having a particle size of 50 μm or more were removed by classification to obtain a positive electrode material (1) having a layered crystal structure. At this time, the (primary) particle size of the raw material powder was controlled by changing the wet pulverization time to prepare positive electrode materials (1) a to (1) f. Each of the obtained positive electrode materials has a chemical formula Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44 by composition analysis. 0.1 ≦ z ≦ 0.6, x + y + z = 1).

  In order to evaluate the characteristics of the prepared lithium transition metal composite oxide, a test battery was prepared. As the positive electrode plate of the test battery, the positive electrode plate 3 produced in the same manner as in the above-described embodiment was used by punching to a diameter of 15 mm. A test battery was prepared using a lithium electrode as a counter electrode.

  Further, in order to evaluate the life performance and safety of the cylindrical battery using the prepared lithium transition metal composite oxide, a battery for evaluation smaller than the embodiment was manufactured. In the battery for evaluation, the positive electrode plate has a coating width of 5.4 cm and a coating length of 50 cm, the negative electrode plate has a coating width of 5.6 cm and a coating length of 54 cm, the separator has a width of 5.8 cm, Except for the cylindrical battery having a length of 18 mm and a length of 65 mm, it was the same as the above-described embodiment. The produced evaluation battery was designated as battery (1).

(Example 2)
In Example 2, a positive electrode material (2) and a positive electrode material (3) were prepared in the same manner as in Example 1 except that the Ni: Mn: Co ratio was changed. The Ni: Mn: Co ratio was 4: 3: 3 for the positive electrode material (2) and 3: 4: 3 for the positive electrode material (3). In addition, a test battery and an evaluation battery using the positive electrode material (2) and the positive electrode material (3) were produced in the same manner as in Example 1. The batteries for evaluation using the positive electrode material (2) and the positive electrode material (3) were referred to as a battery (2) and a battery (3), respectively. In addition, in the positive electrode material (2) and the positive electrode material (3), several types of batteries were prepared in the same manner as in Example 1, and a test battery and an evaluation battery were prepared for each of them. Only the results will be described (the same applies to Comparative Example 2).

(Example 3)
In Example 3, the discharge capacity of the lithium ion secondary battery 20 was measured. The charging conditions were constant current and constant voltage charging in which charging was performed at a constant current of 4.2 V at a charging current of 200 mA and a constant current charging to a charging end voltage of 4.2 V for a total of 8 hours. The discharge was a constant current discharge with a discharge current of 200 mA and a final discharge voltage of 2.5V. The battery internal resistance at −30 ° C. was measured when the state of charge of the battery capacity was 50% and the open circuit voltage was 3.5 to 4.1 V. Further, a lithium ion secondary battery was prepared by adjusting the amount of the positive electrode active material of the positive electrode plate 3 and the amount of the negative electrode active material of the negative electrode plate 4, and the battery internal resistance when the state of charge was 90% was measured in the same manner. . Further, the output energy density of the lithium ion secondary battery 20 was obtained. After charging at a constant current / constant voltage to 4.2 V with a charge rate of 0.25 C, cool to -30 ° C., discharge after 5 hours at a discharge rate of 0.5 C, measure the internal resistance of the battery, and output energy The density was determined.

Example 4
In Example 4, the golf cart 30 was subjected to a running test in a low temperature environment of −30 ° C., and the operating conditions such as acceleration performance and climbing performance were evaluated.

(Comparative Example 1)
In Comparative Example 1, when the lithium transition metal composite oxide was prepared, the amount of the PVA solution added to the mixed solution was converted to a solid content ratio of 0.2 wt%, and 20 to 50 at 1000 ° C. in an air atmosphere. A positive electrode material (1) g to a positive electrode material (1) m were produced in the same manner as in Example 1 except that baking was performed for a time.

(Comparative Example 2)
In Comparative Example 2, a comparative positive electrode material (1) and a comparative positive electrode material (2) were prepared in the same manner as in Example 1 except that the Ni: Mn: Co ratio was changed. The Ni: Mn: Co ratio was 45:45:10 for the comparative positive electrode material (1) and 80:10:10 for the comparative positive electrode material (2). Further, a test battery and an evaluation battery using the comparative positive electrode material (1) and the comparative positive electrode material (2), respectively, were produced in the same manner as in Example 1. The evaluation batteries using the comparative positive electrode material (1) and the comparative positive electrode material (2) were referred to as a comparative battery (1) and a comparative battery (2), respectively.

(Examination, evaluation)
The produced lithium transition metal composite oxide, test battery, and evaluation battery were subjected to the following tests and evaluations, respectively.

<Particle size, intraparticle primary particle distribution ratio R>
The particle size and intragranular primary particle distribution ratio R of the lithium transition metal double oxide were measured by cross-sectional observation using an electron microscope. That is, a measurement sample was prepared by cutting the approximate center of the secondary particles of each positive electrode material with a focused ion beam or filling the secondary particles in a resin and cutting and polishing the vicinity of the center. This measurement sample was observed with an electron microscope at a magnification of 5000 times, and a particle structure image was recorded. The particle size distribution of the secondary particles and the primary particles was measured by image processing of the particle structure image, and the intraparticle primary particle distribution ratio R was determined.

<Specific surface area>
The specific surface area of the lithium transition metal double oxide was measured by the BET method. That is, after each positive electrode material was put into the measurement cell, a mixed gas of nitrogen and helium was allowed to flow, the measurement cell was cooled to the liquid nitrogen temperature, and nitrogen gas was adsorbed on the surface of each positive electrode material. Nitrogen gas desorbed when the cell was returned to room temperature was measured, and the number of nitrogen gas molecules (monomolecular adsorption amount) necessary to form a monomolecular layer on the surface of each positive electrode material was calculated. The specific surface area was determined by multiplying this monomolecular adsorption amount by the occupied area and avocado number per adsorbed nitrogen molecule.

<Tap density>
Furthermore, the tap density of the lithium transition metal double oxide was measured using a powder tester (manufactured by Hosokawa Micron Corporation, PT-R type). After each positive electrode material is put on a sieve having a mesh opening of 710 μm, the sieve is vibrated to deposit each positive electrode material on a 100 cm ³ (diameter 50 mm) cup. After tapping 200 times to fill each positive electrode material, the powder on the top of the cup was scraped off. The mass of each positive electrode material in the cup was weighed and divided by the cup volume to calculate the tap density.

<Internal resistance, discharge capacity>
About each battery for a test, internal resistance and discharge capacity were measured in the following procedures. The battery was charged at a constant current / constant voltage up to 4.2 V at a charge rate of 0.25 C, cooled to −30 ° C., and discharged at a discharge rate of 0.5 C after 5 hours to determine internal resistance and discharge capacity. .

<Cycle characteristics, safety>
For each battery for evaluation, charge / discharge cycle characteristics and safety were measured by the following procedure. In the charge / discharge cycle characteristic test, charge and discharge were repeated at 0.5 C with a charge end voltage of 4.2 V and a discharge end voltage of 3.0 V. The discharge capacity after 200 cycles was measured, and the ratio with respect to the initial discharge capacity was obtained as a percentage to obtain the capacity maintenance rate. As a safety test, an internal short circuit test (nailing test) was performed. In the safety test, the charging condition was constant current constant voltage charging in which charging was performed at a charging current of 200 mA to a charging end voltage of 4.25 V and then charging was performed at a constant voltage of 4.25 V for 8 hours. After charging 10 batteries for each evaluation battery, a nail with a diameter of 5 mm was penetrated from the side of the battery at a penetration speed of 10 cm / min, and the presence or absence of abnormalities such as the operation of the cleavage valve, ignition or rupture was observed. The number of recognized batteries was counted.

(Test and evaluation results)
<Average particle size, intraparticle primary particle distribution ratio R>
As a result of image processing of the particle structure image, the positive electrode material (1) a to the positive electrode material (1) f of Example 1 and the positive electrode material (2) to the positive electrode material (3) of Example 2 are all secondary particles. The average particle diameter was 3 to 40 μm, and the average particle diameter of the primary particles was 0.1 μm to 5 μm. Further, the particle size at a cumulative frequency of 90% of the secondary particles was 20 μm or less. From this, it was found that the primary particles in the secondary particles were substantially homogeneous, and the intragranular primary particle distribution ratio R was in the range of 0.8 ≦ R ≦ 1.2 (see FIG. 4). ). On the other hand, in the positive electrode material (1) g to the positive electrode material (1) m of Comparative Example 1 and the comparative positive electrode material (1) to the comparative positive electrode material (2) of Comparative Example 2, the average particle diameter of the secondary particles is The average particle diameter of primary particles was 5 μm to 10 μm. The intragranular primary particle distribution ratio R was 1.3 or more. As shown in FIG. 5, in Comparative Examples 1 and 2, since the amount of the binder (PVA solution) added to the lithium transition metal composite oxide is small, there is no aggregation of primary particles during the preparation of the lithium transition metal composite oxide. Sufficient and voids were formed between the primary particles. Moreover, as a result of firing for a long time in order to sinter the particles with incomplete granulation, coarse primary particles having abnormally grown grains were formed in the secondary particles.

<Specific surface area>
As shown in Table 1 below, in the positive electrode material (1) a to the positive electrode material (1) f of Example 1, the specific surface area was in the range of 0.5 to 1.5 m ² / g. Moreover, the tap density showed the range of 1.4-2.6 g / cm < ³ >. On the other hand, in the positive electrode material (1) g to the positive electrode material (1) m of Comparative Example 1, the specific surface area is in the range of 0.5 to 1.5 m ² / g, and the tap density is 1.4 to 2. A range of 0.1 g / cm ³ was shown. In Comparative Example 1, unlike Example 1, since many voids were formed between the primary particles, it was difficult to increase the tap density to 2.1 g / cm ³ or more. Moreover, as a result of measuring the density of the positive electrode plate at the time of producing the test battery, the positive electrode plate using each positive electrode material of Example 1 had a density of 2.5 to 2.7 g / cm ² . On the other hand, in the positive electrode plate using each positive electrode material of Comparative Example 1 having a tap density lower than that of Example 1, the density was 2.3 to 2.5 g / cm ² .

<Internal resistance, discharge capacity>
As shown in Table 2 below, the positive electrode material (1) to the positive electrode material (3) prepared by changing the composition of the lithium transition metal composite oxide were measured in a low temperature environment of −30 ° C. using a test battery. The internal resistance was 78 to 120Ω, and the discharge capacity was 14 to 22 mAh / g. On the other hand, in the comparative positive electrode material (1) to the comparative positive electrode material (2), the internal resistance was 130 to 132Ω and the discharge capacity was 9 to 10 mAh / g.

As shown in FIG. 6, when the internal resistance in a low-temperature environment is compared with the specific surface area of the positive electrode material (1) for each test battery, the specific surface area of the positive electrode material of Example 1 is 0.6 to 1. The internal resistance at −30 ° C. in the range of 0.4 m ² / g is 100Ω or less. On the other hand, in the positive electrode material of Comparative Example 1, the specific surface area where the internal resistance was 100Ω or less was 1.1 to 1.4 m ² / g.

  Further, the relationship between the intragranular primary particle distribution ratio R and the internal resistance measured in a low temperature environment using a test battery was examined. As shown in FIG. 7, in the test battery using the positive electrode material (1) of Example 1, the distribution of primary particles in the secondary particles is substantially uniform, and the intragranular primary particle distribution ratio R is 0.8. Since the relationship of ≦ R ≦ 1.2 was satisfied, it was found that the internal resistance was 100Ω or less. On the other hand, in the test battery using the positive electrode material (1) of Comparative Example 1, the distribution of primary particles in the secondary particles is non-uniform, and the intragranular primary particle distribution ratio R is 1.3 or more. Therefore, the internal resistance was 100Ω or more.

Furthermore, the relationship between the tap density of each positive electrode material (1) and the discharge capacity measured in a low temperature environment using a test battery was examined. As shown in FIG. 8, in the test battery using the positive electrode material (1) a to the positive electrode material (1) f of Example 1, the tap density was in the range of 1.5 to 2.5 g / cm ^3. The capacity was 8 mAh / g or more. Among them, in the test battery using the positive electrode material (1) having a tap density in the range of 1.75 to 1.85 g / cm ³ , the discharge capacity was significantly improved to show 13 to 22 mAh / g. The specific surface area of the positive electrode material (1) having this tap density was 0.95 to 1.05 m ² / g (see Table 1), and the internal resistance of the test battery using this was 78 to 85Ω (see FIG. 6). In contrast, in the test battery using the positive electrode material (1) g to the positive electrode material (1) m of Comparative Example 1, the tap density was 1.5 g / cm ³ or more and the discharge capacity was 8 mAh / g or more. Indicated. However, in the positive electrode material (1) of Comparative Example 1, the specific surface area decreased and the low temperature internal resistance increased as the tap density increased (see Table 1 and FIG. 6). From this, the positive electrode material (1) of Example 1 can achieve both high discharge capacity and low internal resistance in a low temperature environment, whereas the positive electrode material (1) of Comparative Example 1 has a low temperature environment. It was found that high discharge capacity and low internal resistance cannot be achieved at the same time.

<Cycle characteristics, safety>
As shown in Table 3 below, as a result of measuring the capacity retention rate and the number of abnormal batteries for each evaluation battery, in the batteries (1) to (3) using the positive electrode material (1) to the positive electrode material (3), The capacity retention rate was as good as 84 to 87%. In the internal short circuit test, no abnormality such as the operation of the cleavage valve, ignition, or rupture was observed, and the number of abnormal batteries was zero. On the other hand, in the comparative battery (1) to the comparative battery (2) using the comparative positive electrode material (1) to the comparative positive electrode material (2), the capacity retention rate is as low as 56 to 72%, and the cycle life is deteriorated. Was recognized. Further, in the internal short circuit test of the comparative battery (2), abnormalities such as the operation of the cleavage valve, ignition and rupture were observed. From this, it has been clarified that when the Ni composition in the transition metal constituting the lithium transition metal composite oxide is 44% or less, the capacity maintenance ratio is high, and when the Ni composition is 45% or more, the capacity maintenance ratio is lowered. .

<Open circuit voltage>
As shown in FIG. 9, the internal resistance measured in a low temperature environment of −30 ° C. when the state of charge of the lithium ion secondary battery 20 is 50% and the open circuit voltage is 3.5 V to 4.1 V is The internal resistance was about 800Ω or less in the open circuit voltage range of 3.6 to 3.9V. On the other hand, when the state of charge was 90%, the internal resistance decreased when the open circuit voltage was 3.8 to 4.1V. Moreover, as a result of obtaining the internal resistance by discharging the lithium ion secondary battery 20 at a discharge rate of 0.5 C, the output energy density at a discharge depth of 50% showed 200 to 500 W / Kg.

<Running test>
As a result of running tests on the golf cart 30 in a low temperature environment of −30 ° C., the lithium ion secondary battery 20 excellent in output performance and capacity maintenance rate in a low temperature environment is installed, so acceleration performance, It showed excellent running performance including climbing performance and sufficient battery life.

(Action etc.)
Next, the operation and the like of this embodiment will be described focusing on the lithium transition metal composite oxide and the lithium ion secondary battery 20.

  Conventionally, lithium transition metal composite oxides containing Li, Mn, Ni, and Co have been used as positive electrode active materials for lithium ion secondary batteries. The positive electrode active material is composed of secondary particles having a particle structure in which a plurality of primary particles are aggregated and bonded. By making the primary particles small, the positive electrode active material has a high specific surface area, the reaction area between the non-aqueous electrolyte and the positive electrode active material is increased, and high rate discharge characteristics can be improved. However, in a low temperature environment of −30 ° C. (hereinafter referred to as “very low temperature environment”), the viscosity of the non-aqueous electrolyte increases, so the diffusion rate of lithium ions decreases and the discharge capacity decreases. For this reason, in the positive electrode, the diffusion of lithium ions in the solid (positive electrode active material particles) is dominant, and therefore it is important to form a conductive network by good bonding between secondary particles and between conductive materials. In order to construct such a conductive network, not only the bonding properties between the secondary particles but also the bonding properties between the primary particles constituting the secondary particles are important. Therefore, the particle structure of the positive electrode active material is important in order to improve the high rate discharge and suppress the decrease in the discharge capacity in a cryogenic environment.

  By joining individual primary particles with many other particles within the secondary particles, it becomes possible to conduct electrons or diffuse lithium ions to adjacent particles, thus forming a good conductive network inside the secondary particles. In addition, the diffusion rate of lithium ions is improved. If individual particles are bonded to a large number of particles, electron conduction and lithium ion diffusion can be achieved by bonding with other particles even if some particles are not bonded. For this reason, it is preferable to constitute a secondary particle structure in which primary particles having a uniform particle diameter are joined with many other particles.

  Further, in a general granulation process of secondary particles, the aggregation state between the particles changes depending on the binder concentration or the like, so that voids may be formed in the secondary particles. The primary particles facing the voids and the primary particles joined to the other primary particles have different reaction processes in the firing step, and tend to be secondary particles composed of heterogeneous primary particles. When the primary particles constituting the secondary particles grow locally to become coarse particles, a particle structure is formed in which voids are formed in the secondary particles. In such a particle structure, the bondability between the primary particles is lowered, so that the probability of electron conduction or diffusion of lithium ions between adjacent primary particles is lowered. Therefore, for electron conduction or lithium ion diffusion, the microparticle structure of the positive electrode active material, that is, there are few voids, good bondability between the primary particles, and a large number of other primary particles adjacent to each other are uniform. It is important to construct a particle structure joined with the.

  Specific examples of the powder physical properties of the positive electrode active material powder having a microscopic particle structure suitable for improving battery output characteristics in a cryogenic environment include specific surface area and powder packing density. The powder packing density includes a bulk density, a true density, a tap density, and the like. A tap density is appropriate as a powder packing density that can reflect a micro particle structure. The tap density is a density when the container is tapped with powder per unit volume, and as the number of tapping increases, voids between secondary particles decrease and the packing density increases. When the number of tappings exceeds 200, the positive electrode active material particles are bonded to each other and cannot be filled any more, and the tap density is saturated. For this reason, voids within and between secondary particles are reflected in the tap density measured with 200 tappings. As the tap density increases, the voids in the secondary particles and between the secondary particles are reduced, and the bondability between the particles is improved. By using a positive electrode active material with a high tap density, it is possible to increase the lithium ion that can contribute to discharge even in an extremely low temperature environment because of the excellent electron conductivity or lithium ion diffusibility of the entire electrode, resulting in a high discharge capacity. it can. When the tap density is too high, the permeability of the non-aqueous electrolyte solution is lowered, and the discharge capacity is reduced instead.

  In addition, the gap between the secondary particles when the powder is tapped is affected by the particle size of the secondary particles due to the relationship between the particle's own weight and the interparticle adhesion force. When the particle size of the secondary particles is greater than or equal to the critical particle size (particle size at a cumulative frequency of 90%), the voids in the powder packed bed are constant. The space between the secondary particles increases. For this reason, when the particle size of the secondary particles is reduced in order to obtain a high specific surface area suitable for high-rate discharge, voids between the secondary particles increase and the tap density decreases. As the tap density decreases, the filling rate of the positive electrode material in the electrode decreases, so the battery capacity decreases.

  From the above viewpoints, the present inventors have clarified the powder physical properties of the positive electrode active material powder having a micro particle structure suitable for improving the battery output characteristics in a cryogenic environment. That is, in order to make the positive electrode active material have a high specific surface area, it is desirable to reduce the primary particle size. Further, in the particle structure of the secondary particles composed of the primary particles having a reduced particle size, it is preferable that the bonding properties between the primary particles are good and the voids in the secondary particles are reduced. Furthermore, in order to reduce the voids between secondary particles, it is preferable to increase the tap density. By satisfying these conditions, it is possible to achieve high rate discharge and high discharge capacity at the same time even in a cryogenic environment. For this reason, the specific surface area, tap density, and primary particle size of the positive electrode active material were optimized.

In the lithium transition metal composite oxide of the present embodiment, the chemical formula Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0. 44, 0.1 ≦ z ≦ 0.6, x + y + z = 1). Lithium transition metal composite oxides that contain Ni, Mn, and Co as transition metals and have a layered crystal structure have a high discharge capacity. Therefore, it is necessary to completely desorb lithium ions from the crystals during charging for battery design. There is no. For this reason, there is little desorption of lithium ions at the time of charging, and the change in crystal structure is small, so that it has a stable crystal structure. Therefore, even when a charge / discharge cycle or long-term storage is passed when used in a lithium ion secondary battery, the crystal structure is stable, and thus a long life can be achieved.

In the lithium transition metal composite oxide of the present embodiment, the particle diameter at a cumulative frequency of 90% of the secondary particles is 20 μm or less. For this reason, since the coarse secondary particles are excluded, the specific surface area can be increased and the battery capacity can be improved. Moreover, the average particle diameter of the primary particles is set in the range of 0.1 μm to 5 μm. For this reason, the specific surface area can be increased as a whole of the lithium transition metal composite oxide. By setting the average particle size of the primary particles to 0.5 to 3 μm, the distribution of the primary particles in the secondary particles is homogenized, so that the bondability between the primary particles can be improved. Further, the tap density of the lithium transition metal composite oxide is in the range of 1.5 to 2.5 g / cm ³ when measured by tapping 200 times. By determining the tap density that reflects the voids in and between the secondary particles, the voids in and between the secondary particles are reduced and the bonding between the particles is excellent. And diffusibility of lithium ions are improved, and a high discharge capacity can be obtained even in a cryogenic environment. By setting the tap density in the range of 1.75 to 1.85 g / cm ³ , the discharge capacity in a cryogenic environment can be further improved (see FIG. 8).

Furthermore, in the lithium transition metal composite oxide of the present embodiment, the specific surface area is in the range of 0.6 to 1.4 m ² / g. For this reason, since the reaction area with a non-aqueous electrolyte is ensured by using for the lithium ion secondary battery 20, internal resistance can be reduced and high output can be achieved. By setting the specific surface area in the range of 0.95 to 1.05 m ² / g, the output in a cryogenic environment can be further improved (see FIG. 6).

  Furthermore, in the lithium transition metal composite oxide of this embodiment, the intragranular primary particle distribution ratio R satisfies the relationship 0.8 ≦ R ≦ 1.2. For this reason, since the primary particles have a substantially uniformly distributed structure in the secondary particles, the voids between the primary particles are reduced, so that the bondability between the primary particles can be improved and the internal resistance can be reduced. Yes (see FIG. 7).

Furthermore, since the lithium transition metal composite oxide of the present embodiment has the above-mentioned powder characteristics, when the test battery is prepared and measured in a cryogenic environment, the discharge capacity is as high as 8 mAh / g or more. Capacity can be obtained. By using such a lithium transition metal complex oxide, the electrode density of the positive electrode can be in the range of 2.5 to 2.7 g / cm ² . For this reason, since the packing density of the lithium transition metal composite oxide is ensured, it is possible to suppress a decrease in output and capacity even in a cryogenic environment.

  Moreover, in the lithium ion secondary battery 20 of this embodiment, since an increase in internal resistance is suppressed even in an extremely low temperature environment, an output energy density in the range of 200 to 500 W / Kg is obtained with a discharge depth of 50%. Obtainable. Moreover, in the lithium ion secondary battery 20, as shown in FIG. 9, when the internal resistance is measured in a cryogenic environment with the state of charge being 50%, the open circuit voltage is in the range of 3.6 to 3.9V. The internal resistance can be reduced.

In the present embodiment, an example of firing at 1000 ° C. in the atmosphere at the time of preparation of the lithium transition metal composite oxide is shown, but the present invention is not limited to this, and the chemical formula Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44, 0.1 ≦ z ≦ 0.6, x + y + z = 1) Any conditions may be used as long as the lithium transition metal composite oxide having a layered crystal structure can be obtained. In addition, the cost of the lithium ion secondary battery can be reduced by reducing the high-cost cobalt content of the raw material.

  In the present embodiment, the bottomed cylindrical lithium ion secondary battery 20 is illustrated, but the present invention is not limited to the shape and structure of the battery. For example, the battery shape may be a square shape or a polygonal shape, and the battery structure other than the present embodiment may have a structure in which positive and negative external terminals are pressed together in a battery can. The dimensions of the battery are not particularly limited.

Further, in the present embodiment, amorphous carbon is used as the negative electrode active material, and LiPF ₆ is dissolved in a mixed solvent having a volume ratio of 1: 1: 1 of EC, DEC, and DMC as the non-aqueous electrolyte. However, the present invention is not limited to these, and is not particularly limited as long as it is a material usually used for a lithium ion secondary battery.

  Furthermore, in this embodiment, although the golf cart 30 which mounted the lithium ion secondary battery 20 was illustrated, the use of the lithium ion secondary battery of this invention is not limited to this, For various industrial equipment uses It can be suitably used as a medium / high capacity power source. For example, it is suitable for an electric vehicle, a light vehicle, a hybrid vehicle or a train using both a power source driven by various power engines and a power source by an electric motor. Further, it may be used for various consumer medium-capacity household electric appliances.

  The present invention provides a positive electrode active material capable of high-rate discharge in a cryogenic environment and capable of suppressing a reduction in capacity, and a lithium secondary battery using the positive electrode active material. Since it contributes to sales, it has industrial applicability.

It is a side view which shows the golf cart of embodiment which can apply this invention. It is a block diagram which shows the drive system structure of a golf cart. It is sectional drawing which shows the cylindrical lithium ion secondary battery mounted in the golf cart. It is sectional drawing which shows typically the cross-section of the positive electrode active material particle of an Example. It is sectional drawing which shows typically the cross-section of the positive electrode active material particle of a comparative example. It is a graph which shows the relationship between the specific surface area of a positive electrode active material, and battery internal resistance in a low temperature environment. It is a graph which shows the relationship between the primary particle distribution ratio in a secondary particle, and battery internal resistance in a low temperature environment. It is a graph which shows the relationship between the tap density of a positive electrode active material, and the discharge capacity in a low temperature environment. It is a graph which shows the relationship between the open circuit voltage and internal resistance of a lithium ion secondary battery in a low temperature environment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Primary particle 2 Secondary particle 3 Positive electrode plate 20 Cylindrical type lithium ion secondary battery (lithium secondary battery)

Claims (16)

In the positive electrode active material for a lithium secondary battery using a lithium transition metal composite oxide, the lithium transition metal composite oxide has a chemical formula Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44, 0.1 ≦ z ≦ 0.6, x + y + z = 1), and the tap density is A positive electrode active material, which is 1.5 g / cm ³ or more and 2.5 g / cm ³ or less.   2. The positive electrode active material according to claim 1, wherein the lithium transition metal composite oxide has a particle size of 20 μm or less at a cumulative frequency of 90%. In the positive electrode active material for a lithium secondary battery using a lithium transition metal composite oxide, the lithium transition metal composite oxide has a chemical formula Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44, 0.1 ≦ z ≦ 0.6, x + y + z = 1), and the primary particles are The particle size at a cumulative frequency of 90% of the aggregated secondary particles is 20 μm or less, and the average particle size of the primary particles existing in 20% of the total cross-sectional area at the center of the cross-section of the secondary particles, The primary particle distribution ratio R in the secondary particles represented by the ratio of the average particle diameter of the primary particles existing in 20% of the total cross-sectional area at the outer peripheral portion of the cross section of the secondary particles is 0.8 ≦ R ≦ 1. The positive electrode active material characterized by satisfying the relationship of. The positive electrode active material according to any one of claims 1 to 3, wherein the tap density of the lithium transition metal composite oxide is less than 1.75 g / cm ³ or more 1.85 g / cm ³ . 4. The positive electrode active material according to claim 1, wherein the lithium transition metal composite oxide has a specific surface area of 0.6 m ² / g or more and 1.4 m ² / g or less. . The positive electrode active material according to claim 5, wherein a specific surface area of the lithium transition metal composite oxide is 0.95 m ² / g or more and 1.05 m ² / g or less.   6. The positive electrode active material according to claim 1, wherein the primary particles of the lithium transition metal composite oxide have an average particle size of 0.1 μm to 5 μm.   The positive electrode active material according to claim 7, wherein an average particle diameter of the primary particles is 0.5 μm or more and 3 μm or less.   The positive electrode active material according to claim 1, wherein the tap density is a packing density after tapping 200 times in a measurement container.   The positive electrode active material includes a positive electrode using the lithium transition metal composite oxide, a counter electrode and a reference electrode using lithium metal, and a mixed solvent of ethylene carbonate, diethyl carbonate and dimethyl carbonate in a volume ratio of 1: 1: 1. 10. The test battery according to claim 1, wherein a discharge capacity in an atmosphere at a temperature of −30 ° C. is 8 mAh / g or more in a test battery having the non-aqueous electrolyte used. Positive electrode active material.   The positive electrode active material according to any one of claims 1 to 10, wherein the lithium transition metal composite oxide is fired in an atmosphere in the air. In a lithium secondary battery including a positive electrode including a positive electrode active material using a lithium transition metal composite oxide, a negative electrode, and a non-aqueous electrolyte, the lithium transition metal composite oxide has the chemical formula Li _a Mn _x Ni _y Co _z O _2. (0 <a ≦ 1.2, 0.1 ≦ x ≦ 0.8, 0.1 ≦ y ≦ 0.44, 0.1 ≦ z ≦ 0.6, x + y + z = 1) The lithium secondary battery is characterized by having a tap density of 1.5 g / cm ³ or more and 2.5 g / cm ³ or less.   The lithium transition metal composite oxide has a particle size of 20 μm or less at a cumulative frequency of 90% of secondary particles in which primary particles are aggregated, and is within 20% of the total cross-sectional area at the center of the cross section of the secondary particles. Secondary particle primary primary expressed by the ratio of the average primary particle size of the primary particles present in 20% of the total cross-sectional area at the outer peripheral portion of the cross-section of the secondary particles to the average particle size of the primary particles present in The lithium secondary battery according to claim 12, wherein the particle distribution ratio R satisfies a relationship of 0.8 ≦ R ≦ 1.2.   The lithium secondary battery has an output energy density of 200 W / Kg or more and 500 W / Kg or less in a state where the discharge depth is 50% in an atmosphere at a temperature of -30 ° C. The lithium secondary battery as described. The lithium secondary battery according to claim 12, wherein the positive electrode has an electrode density of 2.5 g / cm ² or more and 2.7 g / cm ² or less.   14. The lithium secondary battery according to claim 12, wherein the negative electrode contains amorphous carbon, and an open circuit voltage when the charged state is 50% is 3.6 V or more and 3.9 V or less. .

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