JP2000260479A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive lithium ion secondary battery excellent in a cycle characteristic and a power characteristic without reducing the battery capacity by selecting suitable materials for a positive electrode active material and a negative electrode active material. SOLUTION: This lithium ion secondary battery is so composed as to include a positive electrode using a lithium-nickel composite oxide having a regularly arrayed layer halite structure expressed by the formula: LiNixM1yM2zO2 M1 is at least one kind selected from Co and Mn; M2 is at least one kind selected from Al, B, Fe, Cr and M2; x+y+z=1; 0.5<x<0.95; 0.01<y<0.4; 0.001<z<0.2) for a positive electrode active material, and a negative electrode using a graphitic material in a powder form having a crystal thickness in the c-axis direction of 1000 Å or more and the median diameter of the powder particles of 10-50 μm for a negative electrode active material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium ion secondary battery utilizing the phenomenon of insertion and extraction of lithium ions.

[0002]

2. Description of the Related Art With the miniaturization of personal computers, video cameras, mobile phones, and the like, in the fields of information-related equipment and communication equipment, lithium secondary batteries are used as power sources for these equipments because of their high energy density. Has been put to practical use and has spread widely. On the other hand, in the field of automobiles, the development of electric vehicles is urgent due to environmental problems and resource problems, and lithium secondary batteries are being studied as power sources for electric vehicles. With lithium secondary batteries,
Most commonly used are rocking chair type secondary batteries, which use lithium composite oxide for the positive electrode and carbon material for the negative electrode, and repeat the insertion and extraction of lithium ions to and from both electrodes as they are charged and discharged. Have been.

As a lithium composite oxide serving as a positive electrode active material of a lithium ion secondary battery, a layered rock salt structure LiCoO ₂ , a layered rock salt structure LiNiO ₂ , and a spinel structure LiMn ₂ O ₄ can be obtained so as to obtain an operating voltage of 4 V class. well known. Among these, LiCo is currently used for reasons such as easy synthesis and the highest operating voltage.
Secondary batteries using O ₂ as a positive electrode active material dominate.

[0004] However, cobalt, which is an element constituting LiCoO ₂ , is an extremely expensive element with a small amount of resources, and is a major factor that increases the cost of lithium ion secondary batteries. Therefore, for example, when a lithium ion secondary battery is used as a power source for an electric vehicle, a large capacity is required.
It is considered that practical use of a lithium ion secondary battery using O ₂ as a positive electrode active material is extremely difficult.

A promising alternative to this LiCoO ₂ is a layered rock salt structure LiNiO ₂ . Since nickel, which is cheaper than cobalt, is used as a constituent element, the cost is excellent, and the theoretical discharge capacity is LiCoO ₂
Although it is not much different from the above, it is expected that a battery with a large capacity can be constructed from the advantage that the effective capacity (capacity that can be actually taken out when the battery is constructed) is excellent.

However, since LiNiO ₂ absorbs and releases a lot of lithium during charging and discharging due to its large effective capacity, there is a disadvantage that the crystal structure is easily collapsed by repeating large expansion and contraction of LiNiO ₂ itself. . Therefore, when a battery is configured, there is a problem of so-called cycle deterioration in which the discharge capacity of the battery is reduced by repeated charging and discharging. In particular, in the case of an application such as a power source for an electric vehicle that may be left outdoors, the secondary battery also has a small cycle deterioration at a high temperature because the deterioration proceeds further at a high temperature at which the battery reaction is activated. It is one of the important characteristics required for

Conventionally, as means for solving the problem of cycle deterioration caused by a positive electrode using LiNiO ₂ as an active material,
P- and B-based compounds coated on the surface of LiNiO ₂ or added into the positive electrode (JP-A-4-231975, etc.) and those in which some of the constituent elements are replaced with other elements (JP-A-4-141954) Gazettes). However, these means cause a reduction in battery capacity, and it has been difficult to obtain a secondary battery having practically satisfactory cycle characteristics even with these means.

On the other hand, the negative electrode active material of the lithium ion secondary battery includes materials having low crystallinity (low graphitization degree) which are heat-treated at a relatively low temperature, such as coke and non-graphitizable amorphous carbon. Natural graphite and artificial graphite heat-treated at a high temperature near 3000 ° C. are used. Generally, graphitized natural graphite and artificial graphite are superior in conductivity to coke, non-graphitizable amorphous carbon, etc., and lithium ion secondary batteries using these as negative electrode active materials Has good rate characteristics and power characteristics, and is suitable as a battery for applications requiring rapid charging and discharging, such as a power supply for an electric vehicle. Also, LiNi to be used as a positive electrode active material
O ₂ has a large slope of the discharge curve and an average voltage of LiCo.
About 0.1 V lower than O ₂ . For this reason, a highly crystalline graphite material having flatter charge / discharge characteristics and a low potential is advantageous. However, the decrease in battery capacity due to repeated charging and discharging also occurs due to the graphite material, and even when this graphite material is used as a negative electrode active material, improvement in cycle characteristics has been desired.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of a lithium ion secondary battery, and by selecting appropriate materials for a positive electrode active material and a negative electrode active material, Without reducing battery capacity,
It is an object of the present invention to provide an inexpensive lithium ion secondary battery having excellent cycle characteristics and power characteristics.

[0010]

The lithium ion secondary battery of the present invention has a composition formula of LiNi _x M1 _y M2 _z O ₂ (M1 is C
at least one selected from o and Mn; M2 is Al;
At least one selected from B, Fe, Cr, and Mg; x
+ Y + z = 1; 0.5 <x <0.95; 0.01 <y <
0.4; 0.001 <z <0.2), a positive electrode using a lithium nickel composite oxide having a regularly arranged layered rock salt structure as a positive electrode active material, and a crystallite thickness Lc in the c-axis direction of 100.
0 ° or more, and a negative electrode using a powdery graphite material having a median diameter of powder particles of 10 μm or more and 50 μm or less as a negative electrode active material.

[0011] Positive electrode active material of a lithium ion secondary battery of the present invention, the layered rock salt structure LiNiO ₂ is less expensive than a layered rock-salt structure LiCoO ₂ as the base, the LiNiO ₂
In order to stabilize the crystal structure of
It has been replaced with atoms of at least one kind of a predetermined element. Therefore, this lithium nickel composite oxide can constitute a secondary battery which is inexpensive and has excellent cycle characteristics without reducing the battery capacity.

The negative electrode active material of the lithium ion secondary battery according to the present invention has a crystallite thickness Lc in the c-axis direction of 1000 ° or more, and a median diameter of powder particles of 10 μm to 50 μm.
The following powdery graphite material is used. This graphitic material has a considerably high crystallinity because the crystallite thickness Lc in the c-axis direction, which is a parameter indicating the crystallinity (degree of graphitization), is 1000 ° or more, and the graphitization proceeds. A carbon material with a high degree of graphitization has excellent conductivity and low electric resistance at the negative electrode, so it charges and discharges a large current (high load)
It becomes a negative electrode active material suitable for a secondary battery used for a purpose.

The particle size of the present graphite material is adjusted so that the particle diameter is 10 μm or more and 50 μm or less in terms of median diameter. Use of a powder having a median diameter of less than 10 μm requires an increase in the amount of a binder necessary for forming the negative electrode, which is one of the causes of an increase in electric resistance of the negative electrode. Furthermore, the specific surface area of the active material increases, the decomposition reaction of the electrolyte solution is activated, and the product generated by the decomposition adheres to the surface of the active material particles, thereby deteriorating the cycle characteristics of the secondary battery. Conversely, if the median diameter is 50
The use of a material having a thickness of more than μm may not be able to secure the smoothness of the negative electrode surface, and may cause problems in negative electrode preparation.In addition, lithium dendrite may be generated in protruding portions, which may cause an internal short circuit of the battery. There are also safety issues. By using a material having an appropriate median diameter of 10 μm or more and 50 μm or less, a secondary battery having good cycle characteristics can be configured.

Therefore, the lithium ion secondary battery of the present invention comprising the combination of the positive electrode active material and the negative electrode active material has the characteristics of being excellent in cycle characteristics and power characteristics and being inexpensive without reducing the battery capacity. For example, when the battery is used as a large-output battery or a large-capacity battery such as a power supply for an electric vehicle, the battery becomes a practical secondary battery.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of a lithium ion secondary battery of the present invention will be described in detail in the order of a positive electrode active material, a negative electrode active material, the configuration and manufacture of a battery. <Positive Electrode Active Material> As described above, the nonaqueous electrolyte secondary battery of the present invention uses the composition formula LiNi _x M1 _y M2 _z O ₂
(M1 is at least one selected from Co and Mn; M2 is at least one selected from Al, B, Fe, Cr and Mg)
Species; x + y + z = 1; 0.5 <x <0.95; 0.01
<Y <0.4; 0.001 <z <0.2) A lithium nickel composite oxide having an ordered layered rock salt structure represented by: This LiNi _x M1 _y M2 _z O ₂ is composed of M having different roles.
1, a part of the Ni site is substituted by two or more elements of M2. If the substitution ratio is defined by the ratio of leaving Ni without substitution, that is, the value of x in the composition formula,
0.5 <x <0.95. If x ≦ 0.5, not only a layered rock salt structure but also a second phase such as a spinel structure is generated. If x ≧ 0.95, the substitution effect is too small. This is because a battery having the desired good cycle characteristics cannot be formed. Note that 0.7 <
More preferably, x is in the range of 0.9.

The element M1 selected from Co and Mn mainly plays a role of stabilizing the crystal structure of the lithium nickel composite oxide. By stabilizing the crystal structure at M1, the cycle characteristics of the non-aqueous electrolyte secondary battery are kept good, and deterioration of the battery capacity due to charge / discharge at high temperature and storage at high temperature is particularly suppressed. In order to sufficiently exert the effect of improving the cycle characteristics, the substitution ratio of M1, that is, the value of y in the composition formula, is set to 0.01 <y <0.4. y
When ≦ 0.01, the crystal structure of the formed secondary battery is not sufficiently stabilized, so that the cycle characteristics are not good.
If ≧ 0.4, the crystallinity of the lithium-nickel composite oxide is undesirably reduced. It is more preferable that 0.1 <y <0.3. Further, the replacing element M1 is Co
Is more desirable. In Co, while suppressing the capacity reduction due to the element substitution, the obtained composite oxide Li
This is because (Co, Ni) O ₂ is an all-solid solution type and has an advantage of minimizing a decrease in crystallinity.

The element M2 selected from Al, B, Fe, Cr and Mg mainly serves to suppress the decomposition reaction of the active material due to the release of oxygen and to improve the thermal stability. Due to this role, the substitution ratio of M2, that is, the value of z in the composition formula, is set to 0.001 <z <0.2. z ≦
In the case of 0.001, a sufficient effect on safety cannot be obtained, and in the case of z ≧ 0.2, the capacity of the positive electrode decreases, which is not preferable. In addition, 0.01 <z <0.
It is more preferably set to 1. Further, the substituting element M2
It is more preferable to use Al. This is because Al has an advantage of minimizing a decrease in capacity while improving thermal stability.

For example, to manufacture a layered rock-salt lithium nickel composite oxide represented by the composition formula LiNi _x Co _y Al _z O ₂ , use LiOH.H ₂ O, Ni (O
This can be synthesized by mixing predetermined amounts of H) ₂ , Co ₃ O ₄ , and Al (OH) ₃ and baking them in an oxygen stream at a temperature of about 850 ° C. for about 20 hours.

<Negative Electrode Active Material> The negative electrode active material of the lithium ion secondary battery of the present invention has a crystallite thickness Lc in the c-axis direction of 1000 ° or more and a powder particle having a median diameter of 10 μm.
A powdery graphite material having a size of m to 50 μm is used. The graphite material may include natural graphite or graphitizable carbon whose carbon structure is easily changed by heat treatment.
Artificial graphite obtained by heat treatment at a temperature of 00 ° C. or more can be used. Among these graphite materials, it is closer to the crystal structure of graphite (higher crystallinity)
For this reason, the spacing d ₀₀₂ of the (002) plane is 3.4.
It is desirable to use less than Å. More preferably, it is less than 3.36 °.

When artificial graphite is used, the graphitizable carbon as a raw material includes carbon materials obtained by heat-treating tar pitch obtained from coal and petroleum, coke, mesophase microspheres (mesocarbon microbeads: MCMB), mesophase pitch. A system carbon fiber, a pyrolytic vapor growth carbon fiber, or the like can be used. Among these, it is more preferable to use MCMB. MCMB is an optically anisotropic small sphere obtained in the process of heating pitches at around 400 ° C.
The crystallites are arranged in a lamellar shape. Due to the spherical shape, the specific surface area is small and decomposition of the electrolytic solution can be suppressed to a minimum. Therefore, a lithium ion secondary battery having better cycle characteristics can be configured. Further, since the crystallite end faces are exposed on the particle surfaces, lithium ions can be smoothly absorbed and released during charge and discharge, and a battery having more excellent output characteristics can be constructed.

In the lithium ion secondary battery of the present invention, the above-mentioned natural graphite or artificial graphite obtained by pulverizing and adjusting the particle size is used as a negative electrode active material. The method of pulverization is not particularly limited, for example, mortar, ball mill, vibration mill,
What is necessary is just to perform using a jet mill, a rod mill, etc. The particle size to be adjusted is 10 μm in median diameter for the reason described above.
m and 50 μm or less. More preferably, 15 μm
From the viewpoint of the coating performance on the current collector and the binding property during cycle charge / discharge of the battery, it is desirable that 80% or more of the particles be present in the range of 15 μm to 30 μm.

<Structure and manufacture of lithium ion secondary battery>
The lithium ion secondary battery of the present invention has a positive electrode using the lithium nickel composite oxide as a positive electrode active material, and a negative electrode using the above graphite material as a negative electrode active material as main constituent elements. It can be configured by storing a separator, a non-aqueous electrolytic solution, and the like sandwiched between the negative electrode and the negative electrode in a battery case.

The positive electrode was prepared by mixing a powder of the above-mentioned lithium nickel composite oxide, which is a positive electrode active material, with a conductive material and a binder, and adding an appropriate solvent to form a paste-like positive electrode mixture. And the like, and then applied and dried on the surface of a current collector made of metal foil, and if necessary, compressed to increase the electrode density to form a sheet. The conductive material is for ensuring the electrical conductivity of the positive electrode, and may be a mixture of one or more of carbon material powders such as carbon black, acetylene black, and graphite. The binding agent plays a role of binding the active material particles and the conductive material particles, and may be a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. . An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent in which the active material, the conductive material, and the binder are dispersed.

The negative electrode is prepared by mixing a binder into a powdery material of the above-mentioned graphite material, which is a negative electrode active material, and adding a solvent to form a paste into a negative electrode mixture. , And then, if necessary, compression to increase the electrode density to form a sheet. Like the positive electrode, the binder may be a fluororesin such as polyvinylidene fluoride, and the active material may be an organic solvent such as N-methyl-2-pyrrolidone as a solvent for dispersing the binder. Can be.

The separator sandwiched between the positive electrode and the negative electrode configured as described above separates the positive electrode and the negative electrode and holds the electrolyte, and a thin microporous film of polyethylene, polypropylene or the like can be used. . The non-aqueous electrolyte is obtained by dissolving a lithium salt as an electrolyte in an organic solvent. The lithium salt is dissociated by dissolving in an organic solvent and forms lithium ions in the electrolyte. LiBF ₄ , Li
PF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiAsF ₆ ,
LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) _{2 and the} like. Each of these lithium salts may be used alone, or two or more of these lithium salts may be used in combination.

An aprotic organic solvent is used as the organic solvent for dissolving the lithium salt. For example, a solvent mixture of one or more of cyclic carbonate, chain carbonate, cyclic ester, cyclic ether or chain ether can be used. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate.Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.Examples of the cyclic ester include gamma butyl lactone and gamma. Examples of barrel lactone include cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, and examples of chain ether include dimethoxyethane and ethylene glycol dimethyl ether. Any one of these can be used alone, or two or more can be used as a mixture.

The non-aqueous electrolyte secondary battery having the above-mentioned components as constituent elements can be of various shapes such as a cylindrical type and a laminated type. In any case, a separator is sandwiched between the positive electrode and the negative electrode, and the positive electrode and the negative electrode are alternately laminated or wound into a roll to form an electrode body. The negative electrode current collector is connected to the positive electrode terminal and the negative electrode terminal that communicate with the outside using a current collecting lead, etc., and the electrode body is impregnated with a non-aqueous electrolyte solution, and sealed in a battery case to form a secondary battery. Is completed.

[0028]

EXAMPLES Based on the above embodiment, a lithium ion secondary battery of the present invention was manufactured as an example. A lithium ion secondary battery using a graphite material having a Lc of less than 1000 ° and a particle median diameter of less than 10 μm as a negative electrode active material was prepared as a comparative example, and the battery performance was compared with the secondary battery of the above example. did. This will be described below.

<Lithium ion secondary battery of Example 1> The positive electrode active material includes a composition formula of LiNi _0.85 Co _0.1 Al _0.05 O ₂
A lithium nickel composite oxide having a layered rock salt structure represented by The _{LiNi 0.85 Co 0.1 Al 0.0 5 O} 2 is, L
iOH.H ₂ O, Ni (OH) ₂ , Co ₃ O ₄ , Al (O
H) ₃ in molar ratio of 1: 0.85: 0.1: 0.0 respectively.
The mixture obtained by baking at a temperature of 800 ° C. was mixed at a ratio of 05. For the negative electrode active material, M
A graphite material obtained by heat-treating CMB at 2800 ° C. was used.
This graphite material has a (002) plane spacing d ₀₀₂ of 3.
36 °, and the crystallite thickness Lc in the c-axis direction is about 1200
Was Å. The particle size was adjusted to be 25 μm in median diameter by crushing using a crushing crusher.

The positive electrode of this secondary battery was prepared as follows.
Was. First, 85 parts by weight of the above LiNi _0.85Co_0.1Al
_0.05O_TwoAcetylene black as conductive material
5 parts by weight (HS-100 manufactured by Denki Kagaku Kogyo) bound
5 parts by weight of polyvinylidene fluoride (PVDF)
And further mixing an appropriate amount of N-methyl-2-pyrrolidone
(NMP) was added to obtain a paste-like positive electrode mixture.
Next, this positive electrode mixture was applied to an aluminum foil having a thickness of 20 μm.
Apply on both sides of current collector, dry and compress with roll press
It was formed into a sheet-shaped positive electrode.

The negative electrode was manufactured as follows. First, 95 parts by weight of the graphite material was mixed with 5 parts by weight of PVDF as a binder, and an appropriate amount of NMP was added to obtain a paste-like negative electrode mixture. Next, similarly to the positive electrode, this negative electrode mixture was applied to both surfaces of a copper foil current collector having a thickness of 10 μm, dried, and compression-molded by a roll press to obtain a sheet-shaped negative electrode.

The above-mentioned positive electrode and each negative electrode have a thickness of 25
It was wound through a μm polyethylene separator to form a roll-shaped electrode body. This electrode body is 18650
The battery was inserted into the battery case, and a non-aqueous electrolyte was injected to impregnate the electrode body. Then, the battery can was sealed to complete the assembly of the secondary battery. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1 M in a mixed solvent obtained by mixing ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 was used.

<Lithium ion secondary battery of Example 2> The same graphite material as that used in Example 1 above, but the graphite material was adjusted by pulverization so that the particle size became 13 μm in median diameter. This is a secondary battery using a material for a negative electrode active material. Except for this negative electrode active material, the other configuration was the same as that of Example 1.

<Lithium ion secondary battery of comparative example> Unlike the above embodiment, the crystallite thickness Lc in the c-axis direction is about 600.
A secondary battery using, as a negative electrode active material, a graphitic material having a median diameter of 6 μm by pulverization. Except for this negative electrode active material, the other configuration was the same as that of Example 1.

<Evaluation of Battery Characteristics> The secondary batteries of the above Examples and Comparative Examples were subjected to a charge / discharge cycle test to evaluate the cycle characteristics. The charge / discharge cycle test was performed at a high temperature of 60 ° C., which is generally assumed to be the upper limit of the operating temperature of the battery. Charge-discharge cycle test conditions, charged to 4.1V at a current density of 2.0 mA / cm ² as the charging and discharging of the relatively large current, then, 3.0 V at a current density of the same 2.0 mA / cm ² Discharge is one cycle, and this is 3 cycles.
It should be performed over 00 cycles. FIG. 1 shows the discharge capacity per unit weight of the positive electrode active material in each cycle of each battery obtained by the charge / discharge cycle test.

As is apparent from FIG. 1, Lc is 1000
Å and at least 10μ and 50μ in median diameter
Example 1 in which a graphite material having a particle size of less than m was used as a negative electrode active material.
Also, it can be seen that the secondary battery of Example 2 is superior in cycle characteristics to the secondary battery of Comparative Example using a graphite material having Lc of less than 1000 ° and a median diameter of less than 10 μm. In addition, the median diameter is 13 μm
The secondary battery of Example 2 using the graphite material is slightly inferior in cycle characteristics to the secondary battery of Example 1 using the graphite material having a median diameter in a more preferable range of 25 μm. This result clearly indicates that the smaller the particle diameter, the larger the specific surface area, and the decomposition reaction of the electrolyte solution is likely to proceed.

The secondary battery of the comparative example has a smaller initial discharge capacity than the secondary batteries of the two examples because the negative electrode active material used in the comparative example is the active material used in the two examples. This is probably because more capacity was lost due to the retention reaction due to lower conductivity and higher specific surface area.

[0038]

According to the present invention, a lithium ion secondary battery is provided.
A graphite material having a layered rock-salt structure lithium-nickel composite oxide in which the Ni site is replaced with another element to stabilize the crystal structure as a positive electrode active material, and having a high crystallinity and a controlled particle diameter. The material is configured to be a negative electrode active material. With such a configuration, the lithium ion secondary battery of the present invention is an inexpensive secondary battery, and has excellent cycle characteristics and power characteristics without reducing the battery capacity. From this, the present invention greatly advances the use of the lithium ion secondary battery as a large capacity battery.

[Brief description of the drawings]

FIG. 1 shows the discharge capacity of each cycle of the secondary batteries of Examples and Comparative Examples in a charge / discharge cycle test.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hideyuki Nakano 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Institute, Inc. (41) Inventor Yoshio Ukyo Yoshitou Ukyo 41, Nagachite-cho, Aichi-gun, Aichi Prefecture, Japan Toyota Toyota Central Research Laboratory Co., Ltd. 41 Toyota Chuo R & D Co., Ltd., No. 41, Chuchu-Yokomichi, Nagakute-cho, Aichi-gun, Japan (72) Inventor Tetsuro Kobayashi 1 Inventor Tatsuo Noritake 41 Toyota Chuo Research Institute, Inc. 1F, 41-Chome, Toyoda Central R & D Laboratories, Toyota Chuo R & D Co., Ltd. 5F003 AA07 BB01 BB05 BC01 BC06 BD00 BD01 BD02 5H014 AA02 BB01 EE08 EE10 HH08 5H029 AJ14 AK03 AL07 AM03 AM04 AM05 B07 BJ12 BJ14 CJ02 HJ02 HJ05 HJ13 HJ14

Claims (3)

[Claims] A composition formula LiNi _x M1 _y M2 _z O ₂ (M1 is C
at least one selected from o and Mn; M2 is Al;
At least one selected from B, Fe, Cr, and Mg; x
+ Y + z = 1; 0.5 <x <0.95; 0.01 <y <
0.4; 0.001 <z <0.2), a positive electrode using a lithium nickel composite oxide having a regularly arranged layered rock salt structure as a positive electrode active material, and a crystallite thickness Lc in the c-axis direction of 1000 ° or more. And a negative electrode using a powdery graphite material as a negative electrode active material, wherein the median diameter of the powder particles is 10 μm or more and 50 μm or less, and a lithium ion secondary battery comprising: 2. The lithium ion secondary battery according to claim 1, wherein M1 in the composition formula of the lithium nickel composite oxide is Co, and M2 is Al. 3. The lithium ion secondary battery according to claim 1, wherein the graphite material is obtained by heat treating mesophase small spheres at a temperature of 2000 ° C. or higher.