JP5289735B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium secondary battery that has excellent lithium ion conductivity and can achieve high output.

  With the miniaturization of personal computers, video cameras, mobile phones, etc., in the fields of information-related equipment and communication equipment, lithium secondary batteries have been put into practical use because of their high energy density as the power source used for these equipment. It has become widespread. On the other hand, in the field of automobiles, the development of electric vehicles has been urgently caused by environmental problems and resource problems, and lithium secondary batteries have been studied as power sources for the electric vehicles. In particular, when a lithium secondary battery is used as a power source for an electric vehicle, the output of the current lithium secondary battery is low. Therefore, it is necessary to increase the output of the lithium secondary battery.

  By the way, Patent Document 1 discloses a lithium secondary battery in which mesoporous silica having nano-sized pores is included in at least one of a positive electrode, a negative electrode, an electrolytic solution, and a separator. This technique uses mesoporous silica to improve charge / discharge characteristics at a low temperature particularly in an aqueous lithium secondary battery, and is not intended to increase the output of the lithium secondary battery.

  Patent Document 2 discloses a lithium secondary battery in which an inorganic porous particle powder capable of holding an electrolytic solution in a hole is added to and mixed with a carbon material that is a lithium ion storage material for a negative electrode. . In this technique, by adding inorganic porous particle powder, the electrolytic solution contained in the negative electrode is uniformly held, and the charge / discharge cycle characteristics are improved. Patent Document 3 discloses a lithium secondary battery using an ion conductor having silica gel having a pore volume and the like within a specific range as a solid electrolyte membrane (claim 13 of Patent Document 3). ).

Patent Document 4 discloses a solid electrolyte membrane in which an ion conductor is supported on a porous thin film in which directionality is imparted to pores. Patent Document 5 discloses a non-aqueous electrolyte secondary battery using a foamed metal or a fibrous metal containing Ni as a main component as a negative electrode current collector. Patent Document 6 discloses a technique for improving the energy density and the output density by increasing the content of the conductive material on the collector side and decreasing it on the separator side. Patent Document 7 discloses a technique for improving the output density by increasing the solid content concentration of the electrode material on the collector side and lowering it on the separator side.
JP 2005-243342 A JP-A-10-188957 Japanese Patent Laid-Open No. 2004-2114 Japanese Patent Laid-Open No. 2002-42549 JP-A-9-213307 Japanese Patent No. 37477981 JP 2005-50755 A

  The present invention has been made in view of the above circumstances, and has as its main object to provide a lithium secondary battery that is excellent in lithium ion conductivity and can achieve high output.

  In order to solve the above problems, in the present invention, a positive electrode current collector, a positive electrode layer formed on the positive electrode current collector and containing a positive electrode active material, a negative electrode current collector, and the negative electrode current collector A lithium secondary battery comprising a negative electrode layer formed thereon and containing a negative electrode active material, a separator disposed between the positive electrode layer and the negative electrode layer, and a non-aqueous electrolyte containing a lithium salt. Then, the positive electrode layer contains a porous lithium ion conductivity-improving porous member that improves lithium ion conductivity, and a lithium secondary battery is provided.

  According to the present invention, by using the lithium ion conductivity improving porous member for the positive electrode layer having low lithium ion conductivity, the lithium ion conductivity can be improved, the internal resistance of the lithium secondary battery is reduced, High output can be achieved.

  In the said invention, it is preferable that the said lithium ion conductivity improvement porous member is a mesoporous silica. This is because the surface acidity is high and the lithium ion conductivity is excellent.

  In the above invention, the positive electrode current collector is preferably a foam metal current collector. This is because the pore diameter is uniform, the surface area is large, the contact area between the positive electrode current collector and the positive electrode layer can be increased, and the electron conductivity can be improved.

  In the above invention, the content of the lithium ion conductivity improving porous member contained in the positive electrode layer increases stepwise or continuously from the positive electrode current collector side surface toward the separator side surface. Preferably it is. This is because the electron conductivity on the surface of the positive electrode current collector can be improved by reducing the content of the porous member for improving lithium ion conductivity on the surface of the positive electrode current collector.

  In this invention, there exists an effect that internal resistance is small and a high output lithium secondary battery can be obtained.

  Hereinafter, the lithium secondary battery of the present invention will be described in detail.

  The lithium secondary battery of the present invention is formed on the positive electrode current collector, the positive electrode current collector, the positive electrode layer containing the positive electrode active material, the negative electrode current collector, and the negative electrode current collector. A lithium secondary battery comprising: a negative electrode layer containing a negative electrode active material; a separator disposed between the positive electrode layer and the negative electrode layer; and a non-aqueous electrolyte containing a lithium salt. The layer contains a porous lithium ion conductivity-improving porous member that improves lithium ion conductivity.

  According to the present invention, by using the lithium ion conductivity improving porous member for the positive electrode layer having low lithium ion conductivity, the lithium ion conductivity can be improved, the internal resistance of the lithium secondary battery is reduced, High output can be achieved. Further, as will be described later, the lithium ion conductivity-improving porous member is preferably made of a material having a high surface acidity, but using such a material excellent in lithium ion conductivity, By using the material as the porous member, lithium ions can be effectively conducted on the porous surface, and the internal resistance of the lithium secondary battery can be reduced and the output can be increased.

  FIG. 1 is a schematic cross-sectional view showing an example of the lithium secondary battery of the present invention. A lithium secondary battery shown in FIG. 1 includes a positive electrode current collector 1, a positive electrode layer 3 containing a positive electrode active material 2, a negative electrode current collector 4, a negative electrode layer 6 containing a negative electrode active material 5, and a positive electrode The separator 7 disposed between the layer 3 and the negative electrode layer 6 and a non-aqueous electrolyte (not shown) containing a lithium salt, and the positive electrode layer 3 is a porous material that improves lithium ion conductivity. The porous member 8 containing improved lithium ion conductivity is contained. In the present invention, the lithium ion conductivity improving porous member 8 is usually contained in the positive electrode layer 3. As will be described later, in the present invention, at least one of the positive electrode current collector 1 and the negative electrode current collector 4 is preferably a current collector using a porous metal such as a foam metal.

FIG. 2 is a schematic cross-sectional view showing another example of the lithium secondary battery of the present invention. In the lithium secondary battery shown in FIG. 2, the content of the lithium ion conductivity improving porous member 8 contained in the positive electrode layer 3 is increased from the surface on the positive electrode current collector 1 side to the surface on the separator 7 side. It is continuously increasing. As a result, the internal resistance can be further reduced and the output can be increased.
Hereinafter, the lithium secondary battery of this invention is demonstrated for every structure.

1. Lithium ion conductivity improving porous member The lithium ion conductivity improving porous member used in the present invention will be described. The lithium ion conductivity improving porous member used in the present invention is a porous member that improves lithium ion conductivity.

  Although the said lithium ion conductivity improvement porous member will not be specifically limited if a lithium ion conductivity is improved, Especially, it is preferable that it is what uses a material with high surface acidity. A material having a high surface acidity has a high degree of dissociation of anions and cations, so that lithium ion conductivity can be improved. The surface acidity of the lithium ion conductivity-improving porous member is not particularly limited. However, when expressed by the Hammett acidity function (Ho), for example, within a range of -3.0 to -6.0, Especially, it is preferable to be in the range of -3.0 to -4.0.

  In the present invention, by using a porous member, lithium ions can be effectively conducted on the porous surface. Although it does not specifically limit as an average pore diameter of a lithium ion conductivity improvement porous member, For example, it exists in the range of 1 nm-10 nm.

  The shape of the porous member for improving lithium ion conductivity is not particularly limited, and examples thereof include a spherical shape and an elliptical sphere. As an average particle diameter of the said lithium ion conductivity improvement porous member, it is preferable to exist in the range of 0.01 micrometer-20 micrometers, for example.

The material for the lithium ion conductivity-improving porous member is not particularly limited as long as it can improve lithium ion conductivity, and examples thereof include a silica-based material. Furthermore, specific examples of the silica-based material include mesoporous silica. In the present invention, mesoporous silica having a radial structure described in JP-A-2005-243342 can also be used as the lithium ion conductivity improving porous member. Moreover, ZrO ₂ and WO ₃ can also be used as the lithium ion conductivity improving porous member.

  As content of the lithium ion conductivity improvement porous member contained in a positive electrode layer, it is preferable to exist in the range of 2 weight%-60 weight%, for example. If the content of the porous member with improved lithium ion conductivity is too high, the electron conductivity may be lowered. If the content of the porous member with improved lithium ion conductivity is too low, the lithium ion conductivity is improved. This is because there is a possibility that it cannot be sufficiently achieved.

  In the present invention, the content of the lithium ion conductivity improving porous member contained in the positive electrode layer is preferably increased stepwise or continuously from the positive electrode current collector side surface to the separator side surface. . This is because the electron conductivity on the surface of the positive electrode current collector can be improved by reducing the content of the porous member for improving lithium ion conductivity on the surface of the positive electrode current collector.

Let the average content rate of the lithium ion conductivity improvement porous member contained in the area | region from the separator side surface to the 5 micrometer inner side be a separator side surface area average content rate. Although it does not specifically limit as a separator side surface region average content rate, For example, it is preferable to exist in the range of 5 weight%-60 weight%.
Let the average content rate of the lithium ion conductivity improvement porous member contained in the area | region from the positive electrode collector side surface to the inside of 5 micrometers be a positive electrode collector side surface area average content rate. Although it does not specifically limit as a positive electrode collector side surface area average content rate, For example, it is preferable to exist in the range of 2 weight%-5 weight%.
The difference between the separator-side surface region average content and the positive electrode current collector-side surface region average content is preferably, for example, in the range of 5 wt% to 50 wt%.

  The method for imparting a gradient of the content of the lithium ion conductivity improving porous member to the positive electrode layer is not particularly limited as long as it is a method capable of imparting a desired gradient. Specifically, , A method utilizing the difference in specific gravity between the active material and the lithium ion conductivity-improving porous member, a method using a plurality of pastes in which the content of the lithium ion conductivity-improving porous member is changed, and the magnetism of the active material The method of utilization etc. can be mentioned.

In the method using the difference in specific gravity between the active material and the lithium ion conductivity-improved porous member, a lithium ion conductivity-improved porous member and an active material having a higher specific gravity than that are usually used. Thereby, since the active material with a heavy specific gravity tends to settle more, as a result, inclination arises in content of a lithium ion conductivity improvement porous member. The difference between the active material and lithium ion conductivity enhancing porous member and the specific gravity, but not desired tilt is particularly limited so Shojire, for example in a range from 1.0g ^/ cm ³ ~5.0g / cm 3 among them, preferably in the range of inter alia ^{2.0g / cm 3 ~4.0g / cm 3} .

  For example, a paste containing an active material, a lithium ion conductivity improving porous member, a binder, a conductive material and a solvent is prepared and applied to a current collector. Then, before drying, the paste applied on the current collector is allowed to stand for several hours to several tens of hours, whereby the content of the lithium ion conductivity improving porous member is inclined. By the method using the difference in specific gravity, an electrode layer (positive electrode layer or negative electrode layer) in which the content of the porous member having improved lithium ion conductivity is continuously changed can be obtained.

  The method of using a plurality of pastes in which the content of the porous member improved in lithium ion conductivity is changed is that the plurality of pastes are sequentially applied to the current collector to thereby contain the porous member improved in lithium ion conductivity. It is a method of tilting the amount. By a method using a plurality of pastes, an electrode layer (positive electrode layer or negative electrode layer) in which the content of the lithium ion conductivity improving porous member is changed stepwise can be obtained. In the present invention, usually 2 to 4 types of pastes are used, and 2 to 3 types of pastes are preferably used. Examples of the paste application method include a general application method such as a doctor blade method.

  The method using the magnetism of the active material is a method of inclining the content of the porous member with improved lithium ion conductivity by applying a magnetic field to the active material. By applying a magnetic field and increasing the content of the active material on the current collector side surface, the content of the lithium ion conductivity-improving porous member is inclined. In the method using the magnetism of the active material, a lithium ion conductivity improving porous member which is not affected by magnetism is usually used. Although it does not specifically limit as a magnetic flux density to apply, For example, it is preferable to exist in the range of 3000 gauss-7000 gauss. In this method, the content of the lithium ion conductivity-improving porous member can be changed stepwise or continuously by a magnetic field application method.

  In the present invention, an electrode layer in which the content of the lithium ion conductivity-improving porous member is changed stepwise or continuously can be obtained by the above-described gradient imparting method. That is, in the present invention, a paste containing an active material and a lithium ion conductivity-improving porous member is applied to a current collector, and the lithium ion conductivity-improving porous material from the current collector side surface toward the opposite surface. It is possible to provide a method for manufacturing a lithium secondary battery, which includes an electrode layer forming step of performing an inclination imparting method for obtaining an electrode layer in which the content of a member is increased stepwise or continuously.

2. Next, the positive electrode used in the present invention will be described. The positive electrode used in the present invention usually has a positive electrode current collector and a positive electrode layer formed on the positive electrode current collector and containing a positive electrode active material.

  The material of the positive electrode current collector is not particularly limited as long as it has conductivity, and examples thereof include aluminum, SUS, nickel, iron and titanium. Among them, aluminum and SUS are preferable. . Further, the positive electrode current collector may be a dense metal current collector or a porous metal current collector. In the present invention, the positive electrode current collector is a porous metal current collector. It is preferable that it is a body. This is because the contact area between the positive electrode current collector and the positive electrode layer is increased, and the electron conductivity can be improved. As described above, in the present invention, lithium ion conductivity can be improved by using the lithium ion conductivity improving porous member. In order to increase the output of the lithium secondary battery, it is preferable to improve the electron conductivity as well as the lithium ion conductivity. Therefore, by using the porous metal current collector, the electron conductivity can be improved, and as a result, both the electron conductivity and the lithium ion conductivity can be improved, and a high-power lithium secondary battery is obtained. It can be done.

  The porous metal current collector is not particularly limited as long as it has a three-dimensional network structure. For example, a current collector using foam metal, a current collector using fiber metal, etc. Among them, a current collector using a foam metal is preferable. This is because the pore diameter is uniform and the surface area is large. The porous metal current collector may have a surface plated. This is because the range of selection of a porous metal current collector that can be used can be widened by plating the surface. The metal used for the plating treatment is not particularly limited as long as it is an electrochemically stable metal, and examples thereof include platinum and gold. Among these, platinum is preferable. In particular, in the present invention, the positive electrode current collector is preferably foamed SUS subjected to Pt plating.

  The average pore diameter of the porous metal current collector is not particularly limited. For example, it is preferably in the range of 50 μm to 600 μm, and more preferably in the range of 50 μm to 150 μm. The thickness of the porous metal current collector is not particularly limited, but is preferably in the range of 100 μm to 500 μm, and more preferably in the range of 100 μm to 250 μm.

Next, the positive electrode layer formed on the positive electrode current collector will be described. The positive electrode layer used in the present invention contains at least a positive electrode active material, and usually further contains a conductive material and a binder. The positive electrode active material is not particularly limited as long as it can occlude / release lithium ions. Examples thereof include LiCoO ₂ , LiCoO ₄ , LiMn ₂ O ₄ , LiNiO ₂ , and LiFePO _4. Among these, LiNiO ₂ is preferable. Examples of the conductive material include carbon black and acetylene black. Furthermore, examples of the binder include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

3. Next, the negative electrode used in the present invention will be described. The negative electrode used in the present invention usually has a negative electrode current collector and a negative electrode layer formed on the negative electrode current collector and containing a negative electrode active material.

  The material for the negative electrode current collector is not particularly limited as long as it has electrical conductivity, and examples thereof include copper, stainless steel, nickel, and the like, among which copper is preferable. Furthermore, the negative electrode current collector may be a dense metal current collector or a porous metal current collector. In the present invention, the negative electrode current collector is a porous metal current collector. It is preferable that it is a body. The porous metal current collector is the same as that described in “2. Cathode” above, and will not be described here. In the present invention, the battery case may have a negative electrode function.

  The negative electrode layer used in the present invention contains at least a negative electrode active material, and may contain a conductive material and a binder as necessary. The negative electrode active material is not particularly limited as long as it can occlude / release lithium ions. For example, metal lithium, lithium alloy, metal oxide, metal sulfide, metal nitride, and Examples thereof include carbon materials such as graphite, among which graphite is preferable. The negative electrode active material may be in the form of a powder or a thin film. In addition, about the electrically conductive material and binder used for a negative electrode layer, the thing similar to the said positive electrode layer can be used.

4). Other Members The lithium secondary battery of the present invention has a separator disposed between the positive electrode layer and the negative electrode layer. The separator used in the present invention is not particularly limited as long as it has a function of holding a nonaqueous electrolytic solution. For example, a porous film such as polyethylene and polypropylene, a nonwoven fabric such as a resin nonwoven fabric, and a glass fiber nonwoven fabric. Etc.

The nonaqueous electrolytic solution used in the present invention usually has a lithium salt and a nonaqueous solvent. The lithium salt is not particularly limited as long as it is a lithium salt used in a general lithium secondary battery. For example, LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ and LiClO ₄ . On the other hand, the non-aqueous solvent is not particularly limited as long as it can dissolve the lithium salt. For example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxy Ethane, 1,2-diethoxyethane, acetonitrile, propionitrile, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, γ-butyrolactone, etc. Can be mentioned. In the present invention, these non-aqueous solvents may be used alone or in combination of two or more. Moreover, room temperature molten salt can also be used as said non-aqueous electrolyte.

  The shape of the battery case used in the present invention is not particularly limited as long as it can accommodate the above-described positive electrode, negative electrode, separator, and the like. Specifically, a cylindrical shape, a square shape, a coin shape, A laminating type etc. can be mentioned. Moreover, the lithium secondary battery of this invention has an electrode body comprised from a positive electrode layer, a separator, and a negative electrode layer. The shape of the electrode body is not particularly limited, and specific examples include a flat plate type and a wound type.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

  Hereinafter, the present invention will be described in more detail with reference to examples.

[Example 1-1]
5 parts by weight of polyvinylidene fluoride (PVDF) as a binder was added to 5 parts by weight of the n-methylpyrrolidone solution, and stirred for 3 minutes. Next, 8 parts by weight of acetylene black as a conductive material was added and stirred for 10 minutes. Next, 2 parts by weight of mesoporous silica (average particle size 0.64 μm, specific surface area 1093 m ² / g) was added and stirred for 10 minutes. Next, 85 parts by weight of LiNiO ₂ powder as a positive electrode active material was added and stirred for 10 minutes. Thus, a positive electrode layer forming paste was obtained.

The obtained paste for positive electrode layer formation was applied onto a 15 μm thick Al current collector with a doctor blade at a basis weight of 20.5 mg / cm ² and dried at 80 ° C. for 30 minutes. Next, it pressed with the roll press machine and made the electrode density 2.3g / cm < ³ >. Finally, punching was performed so that the electrode area was 2 cm ² to obtain a positive electrode.

Using the obtained positive electrode and Li metal prepared as the negative electrode, a 2032 type coin cell was obtained. The separator was a microporous film made of polyethylene (PE) having a thickness of 25 μm. In the electrolytic solution, lithium hexafluorophosphate (LiPF ₆ ) was dissolved as a supporting salt at a concentration of 1 mol / L in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7. A thing was used.

[Example 1-2 and Comparative Example 1-1]
A coin cell was obtained in the same manner as in Example 1-1 except that the composition and basis weight of the positive electrode layer forming paste and the electrode density after pressing were adjusted as shown in Table 1.

[Evaluation]
The IV resistance of the coin cell obtained in Example 1-1, Example 1-2, and Comparative Example 1-1 was evaluated. First, the following SOC (state of charge) adjustment was performed, and then the IV resistance was measured.
(1) SOC adjustment Conditions for SOC adjustment are shown below.
(A) Charging: current 500 μA, voltage 4.3 V, CC mode (b) Pause: 5 minutes (c) Discharge: current 500 μA, voltage 3.0 V, CC mode (d) Pause: 5 minutes These (a) to ( d) was performed for 3 cycles, and then the following charge was performed.
(E) Charging: current 500 μA, voltage 3.72 V, CCCV mode, 24 hours

(2) IV resistance measurement Conditions for IV resistance measurement are shown below.
(F) Discharge: current 1.5 mA, voltage 4.3 V, time 10 seconds (g) pause: 5 minutes (h) charge: current 1.5 mA, voltage 3.0 V, time 10 seconds (i) pause: 5 minutes Measured by changing (f) to (i) to one cycle and changing the current value to 5 mA and 15 mA, IV resistance (Ω) was calculated from the slope of the IV characteristics. The obtained results are shown in Table 1.

  When Example 1-1 and Example 1-2 were compared with Comparative Example 1-1, it was confirmed that the IV resistance was greatly increased by adding mesoporous silica.

[Example 2-1]
5 parts by weight of polyvinylidene fluoride (PVDF) as a binder was added to 5 parts by weight of the n-methylpyrrolidone solution, and stirred for 3 minutes. Next, 2 parts by weight of acetylene black (AB) as a conductive material was added and stirred for 10 minutes. Next, 8 parts by weight of mesoporous silica (average particle size 0.64 μm, specific surface area 1093 m ² / g) was added and stirred for 10 minutes. Next, 85 parts by weight of LiNiO ₂ powder as a positive electrode active material was added and stirred for 10 minutes. Thus, a positive electrode layer forming paste was obtained.

The obtained paste for forming a positive electrode layer was applied onto a foamed SUS current collector (thickness 0.5 mm) subjected to Pt plating with a basis weight of 105 mg / cm ² by a doctor blade, and dried at 80 ° C. for 30 minutes. . Next, it pressed with the flat plate press and made the electrode density 2.3g / cm < ³ >. Finally, punching was performed so that the electrode area was 2 cm ² to obtain a positive electrode.

A measurement cell was obtained using the obtained positive electrode, Li metal prepared as the negative electrode, and a commercially available cell (manufactured by Nippon Tom Cell). The separator was a microporous film made of polyethylene (PE) having a thickness of 25 μm. In the electrolytic solution, lithium hexafluorophosphate (LiPF ₆ ) was dissolved as a supporting salt at a concentration of 1 mol / L in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7. A thing was used.

[Comparative Example 2-1 and Comparative Example 2-2]
A measurement cell was obtained in the same manner as in Example 2-1, except that the composition and basis weight of the positive electrode layer forming paste and the electrode density after pressing were adjusted as shown in Table 2. In Comparative Example 2-2, an Al current collector having a thickness of 15 μm was used as the current collector foil.

[Evaluation]
The energy density and capacity retention rate of the measurement cells obtained in Example 2-1, Comparative Example 2-1, and Comparative Example 2-2 were evaluated. First, the following SOC (state of charge) adjustment was performed, and then the energy density and capacity retention rate were measured.
(1) SOC adjustment Conditions for SOC adjustment are shown below.
(A) Charging: Current (15 mA for 1 g of active material), voltage 4.3 V, CC mode (b) Pause: 5 minutes (c) Discharge: Current (15 mA for 1 g of active material), voltage 3.0 V CC mode (d) Pause: 5 minutes These (a) to (d) were performed for 3 cycles. Then, in order to evaluate an energy density and a capacity | capacitance maintenance factor, charging / discharging (cycle 1 and cycle 2) was performed on the following conditions, respectively.

(2) Energy density Cycle 1
(E) Charging: Current (15 mA for 1 g of active material), voltage 4.3 V, CCCV mode, 24 hours (f) Pause: 2 hours (g) Discharge: Current (15 mA for 1 g of active material), voltage 3.0 V, CC mode (h) Pause: 5 minutes The amount of energy was calculated from the charge / discharge curve of this cycle 1, and the energy density (mWh / cc) was determined by the following formula.

(3) Capacity maintenance rate Cycle 2
(E) Charging: current (15 mA for 1 g of active material), voltage 4.3 V, CCCV mode, 24 hours (f) pause: 2 hours (g) Discharge: current (1500 mA for 1 g of active material), voltage 3.0 V, CC mode (h) Pause: 5 minutes Using the discharge capacity of cycle 2 and the discharge capacity of cycle 1 described above, the capacity retention rate (%) was determined by the following equation. The obtained results are shown in Table 2.

  In Example 2-1, it was confirmed that the capacity retention rate was improved as compared with Comparative Example 2-1. Similarly, it was confirmed that the energy density of Example 2-1 was improved as compared with Comparative Example 2-2.

[Example 3-1]
(Preparation of positive electrode)
8 parts by weight of mesoporous silica (average particle size 0.64 μm, specific surface area 1093 m ² / g) is mixed with 85 parts by weight of the positive electrode active material LiNiO _{2, 5} parts by weight of PVDF as a binder, and carbon black 2 as a conductive material. A positive electrode layer forming paste was prepared by mixing parts by weight and using n-methylpyrrolidone as a solvent. The obtained positive electrode layer forming paste was applied onto a 15 μm thick Al current collector by a doctor blade so as to have a thickness of 45 μm. Immediately after applying the paste, drying, pressing, and punching (φ16 mm) were performed to obtain a positive electrode.

(Preparation of negative electrode)
Li metal was punched into φ19 mm to obtain a negative electrode.

(Production of coin cell)
A CR2030 coin cell was obtained using the obtained positive electrode and negative electrode. Incidentally, the electrolyte, a volume of ethylene carbonate (EC) and dimethyl carbonate (DMC) Ratio 3: a mixed solvent obtained by mixing 7, dissolved lithium hexafluorophosphate as a supporting salt (LiPF ₆₎ at a concentration of 1 mol / L What was done was used.

(Evaluation of coin cell)
Current density 1 mA / cm ² from changing stepwise until 20 mA / cm ² conducted discharge (upper and lower limit voltage 4.1V~3.0V), to measure the internal resistance of the battery from the voltage change at that time. The results are shown in Table 3.

[Example 3-2]
The positive electrode layer forming paste used in Example 3-1 was applied on a 15 μm thick Al current collector by a doctor blade so as to have a thickness of 45 μm. Next, in order to form a gradient distribution of mesoporous silica content in the electrode thickness direction by utilizing the difference in specific gravity of mesoporous silica (2.2 g / cm ³ ) and positive electrode active material (4.8 g / cm ³ ), The coated paste was allowed to stand for 20 hours. At this time, the positive electrode active material having a heavy specific gravity of 4.8 g / cm ³ settled on the Al current collector side, and the ratio of the positive electrode active material on the Al current collector side increased. As a result, the ratio of mesoporous silica on the separator side surface in the positive electrode layer was increased. After inclining the distribution of mesoporous silica in the electrode, drying, pressing, and punching (φ16 mm) were performed to obtain a positive electrode.
A coin cell was obtained in the same manner as in Example 3-1, except that the obtained positive electrode was used. Table 3 shows the results of battery internal resistance of the obtained coin cell.

[Example 3-3]
Three types of positive electrode layer forming pastes were obtained in the same manner as in Example 1 except that the amount of mesoporous silica used was 1 part by weight, 4 parts by weight, and 8 parts by weight (Paste A, Paste B, Paste, respectively) C). Next, these three types of pastes were applied onto a 15 μm thick Al current collector with a doctor blade. The pastes A, B, and C were applied in the order of 15 μm in thickness. After inclining the distribution of mesoporous silica in the electrode, drying, pressing, and punching (φ16 mm) were performed to obtain a positive electrode.
A coin cell was obtained in the same manner as in Example 3-1, except that the obtained positive electrode was used. Table 3 shows the results of battery internal resistance of the obtained coin cell.

[Example 3-4]
The positive electrode layer forming paste used in Example 3-1 was applied on a 15 μm thick Al current collector by a doctor blade so as to have a thickness of 45 μm. Next, using the magnetism of the positive electrode active material, a magnetic field (magnetic flux density of 5000 gauss) was applied from the bottom of the Al current collector to increase the ratio of the positive electrode active material on the Al current collector side. As a result, the ratio of mesoporous silica on the separator side surface in the positive electrode layer was increased. After inclining the distribution of mesoporous silica in the electrode, drying, pressing, and punching (φ16 mm) were performed to obtain a positive electrode.
A coin cell was obtained in the same manner as in Example 3-1, except that the obtained positive electrode was used. Table 3 shows the results of battery internal resistance of the obtained coin cell.

  When Example 3-1 was compared with Examples 3-2 to 3-4, it was confirmed that the internal resistance was decreased by forming a gradient of mesoporous silica in the positive electrode layer.

It is a schematic sectional drawing which shows an example of the lithium secondary battery of this invention. It is a schematic sectional drawing which shows the other example of the lithium secondary battery of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Positive electrode collector 2 ... Positive electrode active material 3 ... Positive electrode layer 4 ... Negative electrode collector 5 ... Negative electrode active material 6 ... Negative electrode layer 7 ... Separator 8 ... Porous member which improves lithium ion conductivity

Claims (4)

A positive electrode current collector, a positive electrode layer formed on the positive electrode current collector and containing a positive electrode active material, a negative electrode current collector, and a negative electrode layer formed on the negative electrode current collector and containing a negative electrode active material A lithium secondary battery comprising: a separator disposed between the positive electrode layer and the negative electrode layer; and a non-aqueous electrolyte containing a lithium salt,
The positive electrode layer contains a porous lithium ion conductivity improving porous member that improves lithium ion conductivity,
The content of the lithium ion conductivity-improving porous member contained in the positive electrode layer is increased stepwise or continuously from the positive electrode current collector side surface toward the separator side surface. Rechargeable lithium battery.   The lithium secondary battery according to claim 1, wherein the lithium ion conductivity improving porous member is mesoporous silica.   The lithium secondary battery according to claim 1, wherein the positive electrode current collector is a foam metal current collector.   4. The lithium secondary battery according to claim 1, wherein an average pore diameter of the lithium ion conductivity improving porous member is in a range of 1 nm to 10 nm.

Images