JP2006344395A - Cathode for lithium secondary battery and utilization and manufacturing method of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cathode equipped to a lithium secondary battery realizing excellent cycle properties and discharge capacity, and utilization and a manufacturing method of the same. <P>SOLUTION: The manufacturing method of the cathode for lithium secondary battery comprises a process of preparing a powdered main activator mainly containing lithium nickelate and a powdered auxiliary activator capable of storing and releasing lithium within a range of 1.5 to 3.0 V at lithium metal standard voltage, a process of preparing a main activator layer forming slurries prepared by mixing the main activator and the auxiliary activator in a dispersed state, and a process of forming the activator layer on a current collector by applying it on a surface of a cathode current collector. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a positive electrode manufacturing method for a lithium secondary battery and a positive electrode obtained by the manufacturing method. Moreover, this invention relates to the lithium secondary battery provided with such a positive electrode.

A lithium secondary battery includes a positive electrode and a negative electrode having a material (active material) capable of inserting and extracting lithium ions, and lithium ions travel back and forth between the electrolyte (typically a nonaqueous electrolyte) between the two electrodes. It is a secondary battery that is charged and discharged by the above, and has become increasingly important as a vehicle-mounted battery or a power source for personal computers and portable terminals. In particular, it is desired that a relatively inexpensive lithium secondary battery using lithium nickelate (LiNiO ₂ ) as a positive electrode active material instead of the conventionally used expensive lithium cobalt oxide (LiCoO ₂ ) is desired.
By the way, since the lithium secondary battery is repeatedly charged and discharged and used for a long period of time, further improvement in cycle characteristics is desired. As a method for improving the cycle characteristics, a method of adding various elements (auxiliary components) to the electrode active material has been proposed. For example, Patent Document 1 proposes a lithium secondary battery using a LiNiO ₂ particle surface coated with vanadium oxide (V ₂ O ₅ ) as a positive electrode active material. However, according to the technique described in Patent Document 1, since the process of coating V ₂ O ₅ is complicated, problems such as an increase in cost and time for manufacturing the positive electrode occur.
Patent Document 2 includes a positive electrode in which LiCoO ₂ is used as a main active material and another lithium-containing oxide is added as a secondary material, and a negative electrode in which a carbon material previously containing lithium is used as an active material. Lithium secondary batteries have been proposed. However, according to the technique described in Patent Document 2, when LiCoO ₂ used as the positive electrode active material is an expensive cobalt compound and is applied to a large battery that requires a large amount of active material, the cause of battery cost increase End up.

Japanese Patent Laid-Open No. 9-293508 Japanese Patent Laid-Open No. 5-151995

The present inventor has found that the cause of the deterioration of the cycle characteristics of the lithium secondary battery is the deterioration of the positive electrode active material during overdischarge. In the case of a lithium secondary battery using lithium nickelate (LiNiO ₂ ) as a positive electrode active material, the electrode potential of the positive electrode rapidly decreases as the discharge proceeds, and Li ₂ O is generated by the positive electrode reaction in such a low potential state. It may be possible. Since the electrode reaction in which Li ₂ O is generated is an irreversible reaction, as a result, the cathode active material is deteriorated and the effective amount of lithium is reduced. And the electrical capacity of a battery falls by such deterioration of a positive electrode active material and the loss of lithium amount, and cycling characteristics may deteriorate. In particular, when a battery using a positive electrode having a relatively large surface area is charged / discharged with a large current, a potential bias is likely to occur locally on the electrode surface, and the active material may deteriorate from a portion biased to a low potential.
In view of this, the present invention aims to improve the performance of a lithium secondary battery using lithium nickelate as a positive electrode active material, to prevent a rapid decrease in the positive electrode potential during the progress of discharge and to suppress the generation of Li ₂ O. One object is to provide a method for producing a positive electrode. Another object of the present invention is to provide a positive electrode manufactured by such a method. Moreover, another one of this invention is providing the lithium secondary battery provided with this positive electrode.

  The present invention provides a method for producing a positive electrode for a lithium secondary battery using lithium nickelate as an active material. That is, the positive electrode manufacturing method disclosed here is based on a powdery main active material mainly composed of lithium nickelate and a lithium metal standard potential (that is, a standard potential when lithium metal is used as a counter electrode; the same applies hereinafter). A step of preparing a powdery side active material capable of occluding and releasing lithium in a range of 1.5 to 3 V, for forming an active material layer in which the main active material and the side active material are mixed in a dispersed state A step of preparing the material, and a step of applying the material to the surface of the positive electrode current collector and forming an active material layer on the current collector.

In the positive electrode manufacturing method disclosed herein, in addition to lithium nickelate as a main component of the positive electrode active material, a secondary active material that can participate in the positive electrode reaction at the end of discharge (for example, when the battery voltage is 1.5 to 3 V) is included. By adding, it is possible to obtain a positive electrode in which a rapid decrease in the electrode potential of the positive electrode is suppressed. Further, when the positive electrode manufacturing method disclosed herein is applied to the manufacture of a sheet-shaped positive electrode having a large area, a rapid potential drop during discharge is suppressed or local potential bias on the electrode surface is alleviated. A positive electrode is obtained. That is, in the positive electrode manufactured by such a method, the generation reaction of Li ₂ O, which is an irreversible reaction, is delayed, and deterioration of the positive electrode and a decrease in the amount of lithium in the battery are prevented.

In a preferred embodiment of the positive electrode manufacturing method disclosed herein, the total amount of the main active material and the sub active material contained in the active material layer forming material (typically slurry) is 100% by mass. The content rate of a by-active material is 2-25 mass%, It is characterized by the above-mentioned. The content is preferably 2 to 10% by mass.
The positive electrode obtained using the material for forming an active material layer (typically slurry) containing the secondary active material at such a content rate is suppressed from a rapid decrease in the positive electrode potential during overdischarge. Li ₂ O production reaction (ie, irreversible reaction) is delayed. And according to the lithium secondary battery provided with such a positive electrode, the cycle characteristics are improved and the discharge capacity is maintained at a high level.

In a particularly preferable aspect of the positive electrode manufacturing method disclosed herein, the secondary active material is at least one selected from the group consisting of titanium sulfide, manganese oxide, and vanadium oxide.
The positive electrode obtained by using such a substance as a secondary active material further suppresses a rapid decrease in the positive electrode potential during overdischarge, further delaying the Li ₂ O generation reaction (ie, irreversible reaction) in a low potential state. The And according to the lithium secondary battery provided with such a positive electrode, the cycle characteristics are further improved, and the discharge capacity is maintained at a higher level.

As understood from the above description, as another aspect, the present invention provides a positive electrode for a lithium secondary battery having the following configuration.
That is, a positive electrode for a lithium secondary battery disclosed herein is a positive electrode current collector, an active material layer formed on the current collector, and a main active material mainly composed of lithium nickelate and lithium. An active material layer including a secondary active material capable of occluding and releasing lithium in a range of 1.5 to 3.0 V at a metal standard potential. And in the said active material layer, the said powdery main active material and the said powdery subactive material are mixed and mixed, It is characterized by the above-mentioned.
Preferably, the content of the secondary active material is 2 to 25% by mass when the total amount of the primary active material and the secondary active material contained in the active material layer is 100% by mass. The content is preferably 2 to 10% by mass. In addition, it is particularly preferable that the secondary active material is at least one selected from the group consisting of titanium sulfide, manganese oxide, and vanadium oxide.
Moreover, this invention provides the lithium secondary battery provided with one of the positive electrodes disclosed here as another aspect.

  Hereinafter, preferred embodiments of the present invention will be described. In addition, matters other than matters specifically mentioned in the present specification (for example, a method for preparing a material for forming a positive electrode active material layer) and matters necessary for the implementation of the present invention (for example, a method for constructing a lithium secondary battery) The kind of the electrolyte solution) can be grasped as a design matter of those skilled in the art based on the prior art in the field. The present invention can be implemented based on the contents disclosed in the present specification and common general technical knowledge in the field.

As described above, the positive electrode for a lithium secondary battery disclosed herein is a positive electrode obtained by forming an active material layer on the surface of the current collector, and has a composition and shape of the current collector that is a component thereof. There is no particular limitation. A conductive member made of a metal with good conductivity can be used as a current collector, but an aluminum-made positive electrode current collector for a lithium secondary battery is particularly preferable.
The shape of the current collector is not particularly limited because it can vary depending on the shape of the positive electrode and the battery, and may be various shapes such as a rod shape, a plate shape, a sheet shape, or a foil shape. For example, a battery including a wound electrode body unit can be cited as a preferred embodiment of a lithium secondary battery constructed using any positive electrode disclosed herein. In this embodiment, a current collector made of a foil metal such as an aluminum foil is used. That is, in a battery including a wound electrode body unit, a positive electrode sheet obtained by forming a positive electrode active material layer on the surface of a positive electrode foil-shaped current collector made of aluminum or the like, and a metal (for example, copper) or carbon A negative electrode sheet obtained by forming an appropriate negative electrode active material layer on the surface of the negative electrode foil-shaped current collector is overlapped via a separator and wound to produce a wound electrode body unit. As the separator, for example, a porous polyolefin (polyethylene, polypropylene, etc.) sheet can be used. A lithium secondary battery is constructed by housing this wound electrode body unit in an appropriate container together with an electrolyte (typically a non-aqueous electrolyte as described later). The current collector itself may be produced by a conventionally known method in the field of manufacturing a secondary battery, and does not characterize the present invention.

The active material layer may include the same elements as those of the active material layer in the conventional lithium secondary battery positive electrode, in addition to the main active material and the sub-active material that characterize the present invention. Typically, a conductive material, a binder, and the like are included as well as a composite material constituting a conventional active material layer.
As the main active material provided in the positive electrode disclosed here, a material mainly composed of lithium nickelate (LiNiO ₂ ) is used. The “lithium nickelate” referred to here is a composite in which a part of nickel is substituted with at least one metal element selected from Co, Al, Mg, Mn, Ti, etc. in addition to general LiNiO _2. This concept includes lithium nickelate. Such LiNiO ₂ is relatively inexpensive and can realize a high battery voltage. Moreover, since a theoretical discharge capacity is large, it is preferable as a positive electrode active material of a lithium secondary battery.

The secondary active material provided in the positive electrode disclosed herein is preferably a material that occludes and releases lithium ions within a range of 1.5 to 3.0 V at a lithium metal standard potential. In the case of a positive electrode using lithium nickelate (typically LiNiO ₂ ) as an active material, lithium ions are occluded and released at a high potential (for example, a lithium metal standard potential of 3.0 to 4.3 V). For this reason, as the electrode reaction, that is, the discharge proceeds, the electrode potential rapidly decreases. Further, when the potential is lowered to 1.5 V or less at the lithium metal standard potential, an irreversible reaction for generating Li ₂ O occurs, the positive electrode active material is deteriorated, and the cycle characteristics are deteriorated. However, when the secondary active material having the above-described characteristics is added as the positive electrode active material, the secondary active material can occlude and release lithium at a relatively low potential, so that a rapid decrease in the electrode potential during the progress of discharge hardly occurs. As a result, the Li ₂ O production reaction that occurs at a low potential (typically 1.5 V or less at the lithium metal standard potential) is delayed, and battery performance such as cycle characteristics is easily maintained.

  The content of the secondary active material in the positive electrode active material layer is 2 to 25 masses when the total amount of the positive electrode active materials contained in the positive electrode active material layer (that is, the total amount of the main active material and the secondary active material) is 100 mass%. % (For example, 2 to 23% by mass) is preferable, and 2 to 12% by mass (particularly 2 to 10% by mass) is more preferable. If the content of the secondary active material is too high, the discharge capacity (that is, the electric capacity of the lithium secondary battery) is reduced, which is not preferable. Moreover, when the content rate of a by-active material is too low, since the objective objective of this invention will become difficult, it is unpreferable.

The secondary active material can be used without particular limitation as long as it is a material that occludes and releases lithium ions within a range of 1.5 to 3 V at a lithium metal standard potential. Preferred materials for the _secondary active material include titanium sulfide (typically TiS ₂ ), manganese oxide (typically MnO ₂ ), and vanadium oxide (typically V ₂ O ₅ ).
Among these, TiS ₂ which is a titanium sulfide is particularly preferable as a _secondary active material, and since lithium is well stored and released at a low potential, a rapid decrease in the positive electrode potential is well suppressed. Therefore, formation reaction of Li 2 _O is an irreversible reaction is delayed, the maintenance of the battery performance such as preventing and cycle characteristics of the positive electrode deterioration is realized.

  The conductive material used for the positive electrode for a lithium secondary battery disclosed herein is not limited to a specific conductive material as long as it is conventionally used in this type of secondary battery. For example, carbon (carbon) powder such as carbon black (acetylene black or the like), conductive metal powder such as nickel powder, or the like can be used.

Moreover, the binder used for the positive electrode for lithium secondary batteries disclosed here should just be conventionally used with this kind of secondary battery, and is not limited to a specific binder. For example, when a non-aqueous solvent is used when preparing a positive electrode active material layer forming slurry described later, a polymer that is soluble in an organic solvent can be preferably used. Preferable examples include polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene oxide-propylene oxide copolymer (PEO-PPO), and the like. Of these, the use of PVDF, PVDC and the like is preferable.
Or when preparing the slurry for positive electrode active material layer formation using an aqueous solvent, the hydrophilic polymer melt | dissolved in water and / or the polymer disperse | distributed in water can be used preferably. Preferable examples of such hydrophilic polymer include various cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose (MC), cellulose acetate phthalate (CAP), hydroxypropyl methylcellulose (HPMC), hydroxypropyl methylcellulose phthalate (HPMCP) and the like. It is done. Of these, the use of CMC is preferred. Suitable water-dispersed polymers include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene- Fluorine resins such as tetrafluoroethylene copolymer (ETFE), vinyl acetate copolymer, styrene butadiene block copolymer (SBR), acrylic acid-modified SBR resin (SBR latex), rubbers such as gum arabic It is done. A fluorine resin such as PTFE is particularly preferable.

Next, the positive electrode manufacturing method of the present invention will be described. In the method disclosed herein, a positive electrode active material layer forming material (typically slurry) is applied to the surface of the current collector (which may be either or both surfaces of the current collector depending on the shape and application). To form an active material layer.
The material for forming the active material layer is obtained by mixing and dispersing a main active material made of lithium nickelate (LiNiO ₂ ), at least one _secondary active material having the above characteristics, a binder, and a conductive material in an appropriate solvent. Can be prepared. The positive electrode active material (main active material and sub-active material), binder, and conductive material to be used are preferably in powder form.
As the solvent (dispersion medium), an organic solvent (non-aqueous solvent) or an aqueous solvent (typically water) can be used. A suitable binder may be employed depending on the nature of the solvent (aqueous or non-aqueous). When a non-aqueous solvent is used, a polymer (binder) that is soluble in an organic solvent can be preferably used. Alternatively, when an aqueous solvent is used, it is preferable to use a hydrophilic polymer that dissolves in water and / or a polymer that disperses in water.

  Thus, the positive electrode active material layer forming slurry obtained by appropriately mixing the materials as described above is applied to the surface of an appropriate current collector using an appropriate application device, thereby providing a positive electrode active material layer. A positive electrode can be constructed. For example, a sheet-like positive electrode can be constructed by applying a positive electrode mixture on the surface of a metal foil such as an aluminum foil.

According to the present invention, a lithium secondary battery having a positive electrode obtained by the method disclosed herein can be manufactured. Therefore, this invention provides the manufacturing method of the lithium secondary battery characterized by producing or preparing the positive electrode disclosed here as another aspect.
In addition, the means for producing the lithium secondary battery may be a lithium secondary battery manufactured according to a conventional method except that the positive electrode is manufactured and the positive electrode is used, and a special process requiring a particular description is required. do not do.

For example, by using a sheet-like positive electrode according to the present invention and an appropriate negative electrode and separator having a sheet shape corresponding thereto, based on a conventionally known method, a wound type or other structure as described above (for example, a sheet-like positive electrode) A laminated secondary battery in which a large number of positive electrodes, negative electrodes, and separators are stacked can be constructed.
Specifically, a negative electrode sheet for a lithium secondary battery having a carbon material (that is, a negative electrode active material) having a graphite structure (layered structure) capable of inserting and removing lithium ions (Li ⁺ ) and the separator described above are prepared, The positive electrode sheet according to the present invention and the prepared negative electrode sheet are overlapped (laminated) via a separator. This laminate is housed in a suitable battery container. In a preferred embodiment, the laminate is wound to form a wound electrode body unit, which is accommodated in a battery container. Alternatively, a stacked electrode body unit in which a plurality of positive electrode sheets and a plurality of negative electrode sheets are alternately stacked with separators interposed therebetween may be configured and accommodated in a battery container.

By supplying (injecting) an electrolyte prepared in advance into a battery container containing such an electrode body unit, the electrode body unit composed of a positive electrode, a negative electrode, and a separator is impregnated with the electrolyte. Thereby, the desired lithium secondary battery can be constructed.
A typical electrolyte for a lithium secondary battery is a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt (supporting salt) added to and dissolved in the solvent.

  As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be used. Propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate, ethyl methyl carbonate (EMC), 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, Generally non-aqueous batteries (lithium secondary batteries) such as dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, and γ-butyrolactone 1) or two or more selected from non-aqueous solvents known to be usable for the electrolytic solution etc.).

As the supporting salt to be contained in the electrolytic solution, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , 1 type, or 2 or more types of lithium compounds (lithium salt) selected from LiI etc. can be used.

  In addition, the density | concentration of the supporting salt in electrolyte solution may be the same as that of the electrolyte solution used with the conventional lithium secondary battery, and there is no restriction | limiting in particular. An electrolytic solution containing an appropriate lithium compound (supporting salt) at a concentration of about 0.1 to 5 mol / L can be used.

  Hereinafter, experimental examples relating to the present invention will be described. However, the present invention is not intended to be limited to the specific examples.

<Example 1: Production of lithium secondary battery (1)>
In this example, the 18650 type lithium secondary battery 10 which is a cylindrical standard type was manufactured.
FIG. 1 shows a schematic diagram of a typical 18650 type lithium secondary battery 10. In FIG. 1, in order to show the wound state of the electrode body unit 18, it is shown in a partially disassembled form.
A typical 18650 type lithium secondary battery 10 has a positive electrode current collector terminal (tab) 26 and a negative electrode current collector terminal (tab) 16 attached to a positive electrode sheet 12 and a negative electrode sheet 14, respectively. 20 (here, a porous polypropylene sheet is used), and the laminated sheet is wound around the winding core 36 to produce a wound electrode body unit 18.
Next, the electrode body unit 18 is accommodated in the negative electrode can 34 and the bottom of the negative electrode can 34 and the negative electrode current collecting tab 16 are welded. The non-aqueous electrolyte is impregnated into the wound electrode body unit 18 accommodated by injecting the non-aqueous electrolyte into the negative electrode can 34 and reducing the pressure inside the negative electrode can 34.
Then, the positive electrode lid 32 on which components such as the gasket 24 and the safety valve 30 are set and the positive electrode current collecting tab 26 are welded. A positive electrode lid 32 is attached to the negative electrode can 34 and sealed.
The general cylindrical lithium secondary battery 10 having a diameter of 18 mm and a height of 65 mm (that is, 18650 type) is manufactured by the above procedure.
The lithium secondary battery according to this example was also manufactured by the same method as that described above. Hereinafter, each component of the battery will be described.

First, lithium nickel oxide (LiNiO ₂ ) powder as a positive electrode main active material, titanium sulfide (TiS ₂ ) powder as a secondary active material, acetylene black as a current collector, carboxymethyl cellulose (CMC) as a binder, and polytetra Fluoroethylene (PTFE) was mixed with ion-exchanged water to prepare a positive electrode active material layer forming slurry (positive electrode slurry). The approximate content of each component (other than water) contained in the slurry is as follows: LiNiO ₂ is 88 mass%, TiS ₂ is 2 mass%, acetylene black is 8 mass%, CMC is 1 mass%, and PTFE is 1 mass%. It is. Table 1 shows the content of each component.
This positive electrode slurry was applied (attached) to both sides of a long aluminum foil having a thickness of about 15 μm as a positive electrode current collector and dried to form a positive electrode active material layer having a thickness of 120 μm on both surfaces of the aluminum foil current collector. . Subsequently, it pressed so that the whole thickness might be set to 85 micrometers. In this way, a positive electrode sheet was produced.

On the other hand, a graphite powder was used as a carbon material for the negative electrode active material, and a slurry for forming a negative electrode active material layer was prepared using CMC and a styrene butadiene block copolymer (SBR) as a binder. That is, the negative electrode active material and the binder were mixed with ion-exchanged water to prepare a negative electrode active material layer forming slurry (negative electrode slurry). The approximate content of each component (other than water) contained in the slurry is 98% by mass for the carbon material, 1% by mass for CMC, and 1% by mass for SBR.
The negative electrode slurry was applied (attached) to both sides of a long copper foil having a thickness of about 15 μm as a negative electrode current collector and dried to form a negative electrode active material layer having a thickness of 120 μm on both sides of the copper foil current collector. . Subsequently, it pressed so that the whole thickness might be set to 85 micrometers. In this way, a negative electrode sheet was produced.

The produced positive electrode sheet and negative electrode sheet were laminated together with two separators (here, a porous polyethylene sheet was used), and this laminated sheet was wound to produce a wound electrode body unit. This electrode body unit was housed in a container together with an electrolytic solution to produce a cylindrical lithium secondary battery having a diameter of 18 mm and a height of 65 mm (ie, 18650 type). As an electrolytic solution, an electrolytic solution used in a conventional lithium secondary battery can be used without any particular limitation. Here, a 3: 7 (volume ratio) mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) is used. An electrolytic solution having a composition in which 1 mol / L LiPF ₆ was dissolved was used.

<Example 2: Production of lithium secondary battery (2)>
The positive electrode slurry of this example was prepared by mixing LiNiO ₂ powder, TiS ₂ powder, acetylene black, and CMC and PTFE as binders. The approximate content of each component (other than water) contained in the material is as follows: LiNiO ₂ is 80% by mass, TiS ₂ is 10% by mass, acetylene black is 8% by mass, CMC is 1% by mass, and PTFE is 1% by mass. It is. Table 1 shows the content of each component.
In addition, the cylindrical lithium secondary battery of the same shape was produced with the material and process similar to Example 1 except using the positive electrode slurry of the aspect mentioned above.

<Example 3: Production of lithium secondary battery (3)>
The positive electrode slurry of this example was prepared by mixing LiNiO ₂ powder, TiS ₂ powder, acetylene black, and CMC and PTFE as binders. The approximate content of each component (other than water) contained in the material is as follows: LiNiO ₂ is 70% by mass, TiS ₂ is 20% by mass, acetylene black is 8% by mass, CMC is 1% by mass, and PTFE is 1% by mass. It is. Table 1 shows the content of each component.
In addition, the cylindrical lithium secondary battery of the same shape was produced with the material and process similar to Example 1 except using the positive electrode slurry of the aspect mentioned above.

<Comparative Example 1: Production of lithium secondary battery (4)>
The positive electrode slurry of this comparative example was prepared by mixing LiNiO ₂ powder, acetylene black, and CMC and PTFE as binders. The approximate content of each component (other than water) contained in the slurry is 90% by mass for LiNiO ₂ , 8% by mass for acetylene black, 1% by mass for CMC, and 1% by mass for PTFE. Table 1 shows the content of each component.
In addition, the cylindrical lithium secondary battery of the same shape was produced with the material and process similar to Example 1 except using the positive electrode slurry of the aspect mentioned above.

<Comparative Example 2: Production of lithium secondary battery (5)>
The positive electrode slurry of this comparative example was prepared by mixing LiNiO ₂ powder, TiS ₂ powder, carbon black, and CMC and PTFE as binders. The approximate content of each component (other than water) contained in such a slurry is as follows: LiNiO ₂ is 89 mass%, TiS ₂ is 1 mass%, acetylene black is 8 mass%, CMC is 1 mass%, and PTFE is 1 mass%. It is. Table 1 shows the content of each component.
A similar cylindrical lithium secondary battery was produced using the same material and process as in Example 1 except that the positive electrode slurry having the above-described embodiment was used.

<Comparative Example 3: Production of lithium secondary battery (6)>
In this comparative example, LiNiO ₂ powder, TiS ₂ powder, acetylene black, and binders CMC and PTFE were mixed to prepare a positive electrode slurry. The approximate content of each component (other than water) contained in the material is as follows: LiNiO ₂ is 65 mass%, TiS ₂ is 25 mass%, acetylene black is 8 mass%, CMC is 1 mass%, and PTFE is 1 mass%. It is. Table 1 shows the content of each component.
In addition, the cylindrical lithium secondary battery of the same shape was produced with the material and process similar to Example 1 except using the slurry for positive electrode active material layer formation of the aspect mentioned above.

<Test Example 1: Measurement of initial discharge capacity>
The initial discharge capacity was measured for each of the lithium secondary batteries according to each example and comparative example. That is, at an environmental temperature of 25 ° C., charging was performed at a constant current of 800 mA until the battery voltage reached 4.1 V, and then constant potential charging (CCCV charging) was performed at a battery voltage of 4.1 V. Thereafter, the battery was discharged (CC discharge) at a constant current of 800 mA until the battery voltage reached 3.0 V, and the discharge capacity (mAh) at this time was determined. The results are shown in Table 1.

<Test Example 2: Capacity maintenance rate measurement by charge / discharge cycle test>
At an environmental temperature of 60 ° C., the battery was charged until the battery voltage reached 4.1V, and then discharged until the battery voltage reached 2.0V. This cycle (charge and discharge) was repeated 300 times, and the discharge capacity after the first charge / discharge cycle and the discharge capacity after 300 cycle charge / discharge cycles were obtained. The ratio of the first discharge capacity after 1 cycle charge / discharge and the discharge capacity after 300 cycle charge / discharge (discharge capacity after 300 cycle charge / discharge / discharge capacity after 1 cycle charge / discharge) is multiplied by 100 to maintain the capacity (%). Was calculated. The results are shown in Table 1. In addition, the charging / discharging cycle test was implemented with the battery of Examples 1-3 and the comparative examples 2 and 3 by the constant current of 800 mA, and the battery of the comparative example 1 was implemented by the constant electricity of 1C.

As is clear from the results shown in Table 1, in Test Example 1, the batteries according to Examples 1 to 3 had a high initial discharge capacity of 650 mAh or more. In particular, the batteries according to Examples 1 and 2 exhibited excellent initial discharge capacity. On the other hand, in Comparative Example 3 where the content of the secondary active material was large, the initial discharge capacity was as low as 600 mAh.
In Test Example 2, the batteries according to Examples 1 to 3 had a result that the discharge capacity retention rate after the 300 cycle durability test was 70% or more. On the other hand, the batteries according to Comparative Example 1 that did not contain the sub-active material and Comparative Example 2 with a small amount of the sub-active material of 1% by mass had low discharge capacity retention rates of 40% and 44%.

<Example 4: Production of monopolar cell for discharge curve measurement (1)>
In this example, a monopolar cell used to measure the discharge curve of the positive electrode of the battery according to Example 1 was produced.
As the test electrode of the single electrode cell according to the present example, the same electrode as the positive electrode used in Example 1 was used. That is, in this example, the approximate content of each component (other than water) is as follows: LiNiO ₂ is 88% by mass, TiS ₂ is 2% by mass, acetylene black is 8% by mass, CMC is 1% by mass, and PTFE is 1%. The slurry which is a mass% was prepared, and this slurry was produced by providing to aluminum foil.
On the other hand, a lithium metal electrode was used as the counter electrode. In addition, as in Example 1, an electrolyte having a composition in which 1 mol / L LiPF ₆ was dissolved in a 3: 7 (volume ratio) mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the electrolyte. It was.

<Comparative Example 4: Production of a monopolar cell for discharge curve measurement (2)>
In this example, a monopolar cell used for measuring the discharge curve of the positive electrode of the battery according to Comparative Example 1 was produced.
As the test electrode of the single electrode cell according to this comparative example, the same one as the positive electrode used in Comparative Example 1 was used. That is, in this example, the approximate content of each component (other than water) is such that 90% by mass of LiNiO ₂ , 8% by mass of acetylene black, 1% by mass of CMC, and 1% by mass of PTFE are prepared. The slurry was prepared by applying the slurry to an aluminum foil.
On the other hand, a lithium metal electrode was used as the counter electrode. In addition, as in Comparative Example 1, as the electrolyte, an electrolytic solution having a composition in which 1 mol / L LiPF ₆ was dissolved in a 3: 7 (volume ratio) mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) was used. It was.

<Test Example 3: Discharge curve measurement>
For the monopolar cells of Example 4 and Comparative Example 4, the rate of change in electrode potential with respect to the discharge capacity (that is, the discharge curve) was measured. A constant current discharge was performed at a rate of 0.2 C in a potential range of 4.8 to 1.4 V at a lithium metal standard (reference) potential, and the transition of the electrode potential (V) with respect to the discharge capacity (mAh) was measured. . The results are shown in FIG. The solid line in FIG. 2 shows the discharge curve for the electrode potential of the test electrode according to Example 4, and the broken line shows the discharge curve for the electrode potential of the test electrode according to Comparative Example 4.
As is apparent from the illustrated discharge curve, the electrode potential of the test electrode according to Comparative Example 4 rapidly decreased at the end of discharge. On the other hand, the electrode potential of the test electrode according to Example 4 gradually decreased from the vicinity of 780 mAh to the end of the discharge, and further decreased gradually near 800 mAh (around 2.1 V in terms of electrode potential) to show a shoulder-like curve. That is, when the test electrode according to Example 4 is used for the positive electrode, the rapid potential drop at the end of discharge is preferably mitigated.

As is clear from Table 1 showing the results of Test Examples 1 to 3 and FIG. 2, the lithium secondary battery provided by the present invention can realize excellent cycle characteristics while maintaining a high initial discharge capacity. .
In the lithium secondary battery provided by the present invention, since the positive electrode potential gradually decreases at the end of discharge, the positive electrode potential does not rapidly decrease, and the Li ₂ O generation reaction, which is an irreversible reaction, hardly occurs. Therefore, even if charging / discharging is repeated, deterioration of the positive electrode is suppressed, and as a result, cycle characteristics are improved.
In addition, since the positive electrode potential does not rapidly decrease, even when a large battery having a large-area electrode is charged / discharged with a large current, partial deterioration of the positive electrode due to potential deviation in the electrode is suppressed. be able to.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.
Further, the technical elements described in the present specification exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in the present specification achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.

It is the schematic diagram which showed the structure of the battery which concerns on one Example of this invention. FIG. 4 is a graph showing the change (discharge curve) of the potential of the test electrode with respect to the discharge capacity for the single electrode cells of Example 4 and Comparative Example 4, the horizontal axis shows the discharge capacity (mAh) of the test electrode, and the vertical axis shows lithium. The electrode potential (V) for the metal (as a Li reference) is shown.

Explanation of symbols

10 Cylindrical lithium secondary battery 12 Positive electrode sheet 14 Negative electrode sheet

Claims (9)

A method for producing a positive electrode used in a lithium secondary battery, comprising the following steps:
A step of preparing a powdery main active material mainly composed of lithium nickelate and a powdery side active material capable of inserting and extracting lithium in a range of 1.5 to 3 V at a lithium metal standard potential;
A step of preparing an active material layer forming material in which the main active material and the sub active material are mixed in a dispersed state; and the material is applied to the surface of the positive electrode current collector, and on the current collector Forming an active material layer;
And a method for producing a positive electrode for a lithium secondary battery.   The content rate of the said subactive material is 2-25 mass% when the total amount of the said main active material and subactive material contained in the said active material layer forming material is 100 mass%. the method of.   The content rate of the said subactive material is 2-10 mass% when the total amount of the said main active material and subactive material contained in the said active material layer forming material is 100 mass%. the method of.   The method according to claim 1, wherein the secondary active material is at least one selected from the group consisting of titanium sulfide, manganese oxide, and vanadium oxide. A positive electrode used in a lithium secondary battery,
A current collector for the positive electrode;
An active material layer formed on the current collector, which is a main active material mainly composed of lithium nickelate and a secondary active material capable of inserting and extracting lithium at a lithium metal standard potential in a range of 1.5 to 3 V An active material layer containing
With
The positive electrode for a lithium secondary battery in which the powdery main active material and the powdery subactive material are mixed in the active material layer.   The positive electrode according to claim 5, wherein a content of the secondary active material is 2 to 25 mass% when a total amount of the main active material and the secondary active material included in the active material layer is 100 mass%.   The positive electrode according to claim 5, wherein a content of the secondary active material is 2 to 10% by mass when a total amount of the main active material and the secondary active material contained in the active material layer is 100% by mass.   The positive electrode according to claim 5, comprising at least one selected from the group consisting of titanium sulfide, manganese oxide, and vanadium oxide as the secondary active material.   A lithium secondary battery comprising the positive electrode according to claim 5.

Images