JP2013201124A - Method of manufacturing electrode for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the degradation characteristics (cycle characteristics) of capacity incident to repeated charging and discharging, in a lithium secondary battery.SOLUTION: The electrode of a lithium secondary battery is manufactured by forming a joining layer by coating the surface of a collector with a conductive paste produced by mixing conductive particles, a binder resin and a solvent, forming a joined body by pressing a collector and multiple active material particles arranged two-dimensionally with the joining layer interposed therebetween, with a predetermined pressure, in such a state that the volume ratio of solvent in the joining layer is within a predetermined range, and then performing uniaxial pressurization of the joined body from the active material particles side with a predetermined pressure.

Description

  The present invention relates to a method for producing an electrode for a lithium secondary battery.

  For example, as a positive electrode of a lithium secondary battery, a positive electrode current collector, a plurality of positive electrode active material particles arranged two-dimensionally along the surface of the positive electrode current collector, the positive electrode current collector and the positive electrode active material A conductive bonding layer provided so as to be interposed between substance particles is known (see, for example, Patent Document 1).

JP 2012-009193 A

  In a lithium secondary battery, it is required to improve capacity reduction characteristics (cycle characteristics) due to repeated charge and discharge. The present invention has been made to cope with such a problem.

  The production method which is an object of the present invention includes a current collector, a plurality of (typically plate-like) active material particles arranged two-dimensionally along the surface of the current collector, and the current collector. And a bonding layer provided so as to be interposed between the active material particles and the active material particles.

The manufacturing method of the present invention is characterized in that the manufacturing method includes the following steps.
-Pressurization in which a bonded body obtained by bonding the current collector and a plurality of the active material particles two-dimensionally arranged through the bonding layer is uniaxially pressed from the active material particle side Process.

  The pressurizing step is, for example, a step of pressurizing the joined body with a pressurizing member whose surface facing the active material particles has a flat or block shape. In this case, “uniaxial pressing” may also be referred to as planar pressing.

  In the electrode for a lithium secondary battery manufactured by the manufacturing method of the present invention, a decrease in capacity is suppressed as much as possible even when charging and discharging of the lithium secondary battery are repeated. That is, by using the electrode for a lithium secondary battery manufactured by the manufacturing method of the present invention, excellent cycle characteristics in the lithium secondary battery are realized. The reason for this is considered to be that the occurrence of delamination at the interface between the current collector and the bonding layer and the interface between the bonding layer and the active material particles is suppressed. Hereinafter, the reason will be described in detail.

  Dimensional changes occur in the active material particles due to the entry and exit of lithium ions accompanying charging and discharging. For this reason, when charging / discharging of the lithium secondary battery is repeated, usually, peeling occurs gradually at each of the above-described interfaces, resulting in a decrease in capacity (hereinafter referred to as “interface peeling”). In this respect, in the electrode for a lithium secondary battery manufactured by the manufacturing method of the present invention, the occurrence of interface peeling as described above is suppressed through the pressurizing step, and in the unlikely event that the interface peeling occurs. Even when it occurs, it is considered that the electrical connection in the peeled portion is maintained well.

FIG. 1 is a cross-sectional view showing an example of a schematic configuration of a lithium secondary battery. FIG. 2 is an enlarged cross-sectional view of an example of the positive electrode plate shown in FIG. FIG. 3 is a schematic explanatory view of an example of a method for manufacturing the positive electrode plate shown in FIG.

  Hereinafter, preferred embodiments of the present invention will be described using examples and comparative examples. It should be noted that the following description of the embodiment is merely an embodiment of the present invention considered to be the best at the time of filing in order to satisfy the description requirements (description requirements, feasibility requirements, etc.) of the specification required by law. An example is merely what is specifically described to the extent possible.

  Therefore, as will be described later, it is quite natural that the present invention is not limited to the specific configurations of the embodiments and examples described below. Since illustrations of various modifications (modifications) that can be made to the present embodiment and examples are inserted during the description of the embodiment, it is difficult to understand the description of the consistent embodiment. It is listed at the end as much as possible.

1. Schematic Configuration of Lithium Secondary Battery FIG. 1 is a cross-sectional view illustrating an example of a schematic configuration of a lithium secondary battery 1. Referring to FIG. 1, the lithium secondary battery 1 is a so-called liquid type, and includes a positive electrode plate 2, a negative electrode plate 3, a separator 4, a positive electrode tab 5, and a negative electrode tab 6. Yes.

  A separator 4 is provided between the positive electrode plate 2 and the negative electrode plate 3. That is, the positive electrode plate 2, the separator 4, and the negative electrode plate 3 are laminated in this order. A positive electrode tab 5 is electrically connected to the positive electrode plate 2. Similarly, the negative electrode tab 6 is electrically connected to the negative electrode plate 3.

  A lithium secondary battery 1 shown in FIG. 1 includes a laminate of a positive electrode plate 2, a separator 4, and a negative electrode plate 3, and an electrolyte containing a lithium compound as an electrolyte, in a predetermined battery case (not shown). It is configured by being sealed in a liquid-tight manner.

2. Configuration of Positive Electrode FIG. 2 is an enlarged cross-sectional view of an example of the positive electrode plate 2 shown in FIG. Referring to FIG. 2, the positive electrode plate 2 includes a positive electrode current collector 21, a large number of positive electrode active material particles 22, and a bonding layer 23.

  The positive electrode current collector 21 is a plate-like or film-like member having good conductivity, and is formed of, for example, an aluminum foil.

The positive electrode active material particles 22 are particles made of a lithium composite oxide (lithium transition metal oxide) and are formed in a plate shape. A large number of positive electrode active material particles 22 are two-dimensionally arranged along the surface (upper surface in the drawing) of the positive electrode current collector 21. Specifically, a large number of positive electrode active material particles 22 are arranged so that 85 to 98% of the above-described surface of the positive electrode current collector 21 is covered with the positive electrode active material particles 22. The “lithium composite oxide” means Li _x MO ₂ (0.05 <x <1.10, M is at least one kind of transition metal: typically, M is one of Co, Ni, and Mn. Including oxides).

By the way, among various lithium composite oxides, for example, lithium cobaltate expands in volume during charging (when lithium ions are released), whereas lithium nickelate expands in volume during discharge (when lithium ions enter). To do. For this reason, it is possible to make the volume expansion / contraction apparently zero during charge / discharge by adjusting the composition ratio as appropriate. However, even in this case, the length of the grating changes. Specifically, Li (Co _0.5 Ni _0.5 ) O ₂ extends in the c-axis direction but contracts in the a-axis direction.

Therefore, the present invention provides a lithium composite oxide having a layered rock salt structure (for example, lithium cobaltate Li _p CoO ₂ [general formula 1 ≦ p ≦ 1.1], lithium nickelate LiNiO ₂ , lithium manganate Li ₂ MnO _3, the lithium nickel manganese oxide _{Li p (Ni 0.5, Mn 0.5} ) O 2, the general formula _{Li p (Co x, Ni y} , Mn z) O 2 [ in the formula 0.97 ≦ p ≦ 1. 07, x + y + z = 1] these solid solution represented _{by, Li p (Co x, Ni} y, Al z) O 2 [ general formula 0.97 ≦ p ≦ 1.07, x + y + z = 1,0 <x ≦ 0.25, 0.6 ≦ y ≦ 0.9, 0 <z ≦ 0.1], a solid solution of Li ₂ MnO ₃ and LiMO ₂ (M is a transition metal such as Co or Ni), etc. Very effective for the plate-like particles having the composition It is effective. As long as the above general formula is satisfied, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, One or more elements such as Te, Ba, Bi and the like may be included.

The composition expressing greater volumetric expansion and contraction _{(e.g., Li p (Co x, Ni} y, Mn z) When the molar ratio of nickel in the _{O 2} is 0.75 or more, or the molar ratio of cobalt less than 0.9 If it is, and _Li p relative to _{(Co x, Ni y, Al} z) when the molar ratio of nickel in the _{O 2} is 0.7 or higher), the application of the present invention is particularly effective.

  The conductive bonding layer 23 is provided so as to be interposed between the positive electrode current collector 21 and the positive electrode active material particles 22. That is, the positive electrode plate 2 is formed in a state in which the positive electrode current collector 21 and a large number of positive electrode active material layers 22 are bonded (laminated) to each other via the conductive bonding layer 23.

  The bonding layer 23 is formed by applying a conductive paste on the above-described surface (upper surface in the drawing) of the positive electrode current collector 21. This conductive paste is a paste obtained by mixing conductive particles, a binder resin, and a solvent. As the conductive particles, carbon, gold, platinum, nickel, aluminum, or the like can be used, and carbon is preferable. As the binder resin, an acrylic resin, a fluororesin, or the like can be used, and a fluororesin (particularly, polyvinylidene fluoride) is preferable.

3. 2. Outline of Manufacturing Method Hereinafter, an outline of a manufacturing method of the positive electrode plate 2 shown in FIG. 2 will be described with reference to FIG. The manufacturing method of this embodiment is performed as follows.

(A) Bonding layer paste application process (see the top row in FIG. 3)
The above-mentioned conductive paste constituting the bonding layer 23 is uniformly applied to the surface (upper surface in the drawing) of the positive electrode current collector 21. As a result, a conductive paste coating layer 23 ′ is formed on the surface of the positive electrode current collector 21.

(B) Joining process (refer to the middle stage in FIG. 3)
The two-dimensionally arrayed positive electrode active material particles 22 are joined to the conductive paste coating layer 23 '.

  Specifically, the joining step is performed as follows: First, a large number of positive electrode active material particles 22 are two-dimensionally arranged on the surface of a platen having high flatness. Next, the positive electrode current collector 21 on which the conductive paste coating layer 23 ′ is formed is placed on the large number of positive electrode active material particles 22 arranged on the surface of the surface plate, and the conductive paste coating layer 23 ′ is formed on the positive electrode active material. It is placed so as to face the particle 22 side, lightly pressed and joined. Thereafter, the conductive paste coating layer 23 ′ is sufficiently dried by heating at 80 ° C.

At this time, in order to obtain a joined body in which the positive electrode active material particles are arranged at high density, the pressure during pressing is desirably 50 g / cm ² or more and 500 g / cm ² or less. When the pressure is less than 50 g / cm ² , the two-dimensionally arranged positive electrode active material particles and the conductive paste coating layer are not sufficiently adhered, and good cycle characteristics cannot be obtained. On the other hand, if the pressure is greater than 500 g / cm ² , even if the volume ratio of the solvent (for example, NMP) in the conductive paste coating layer is adjusted to an appropriate range (details will be described later), for example, the conductive paste When the positive electrode plate is peeled off from the surface plate in the manufacturing process of the positive electrode plate by reaching the interface between the positive electrode active material particles and the surface plate, the arrangement of the positive electrode active material particles is undesirably increased.

(C) Pressurization step (see the bottom row in FIG. 3)
The joined body obtained by the joining process described above is uniaxially pressurized (planar pressure) by the pressing member 70 from the positive electrode active material particle 22 side. Here, the pressure member 70 is a member having a flat surface facing the positive electrode active material particles 22. That is, the pressure member 70 is a plate-like or block-like member having a planar surface.

When the pressurizing member 70 is a plate-like or block-like member having a flat surface, the entire joined body is uniformly pressurized almost simultaneously. At this time, in order to obtain a bonded body in which the positive electrode active material particles are arranged at a high density and are in close contact, the pressure during pressurization is 100 kg / cm ² or more and 2000 kg / cm ² or less. desirable. When the pressure is less than 100 kg / cm ² , the adhesion between the positive electrode current collector constituting the positive electrode plate and the layer containing the positive electrode active material particles (active material layer) and the bonding layer becomes insufficient, and the cycle characteristics deteriorate. So undesirable. On the other hand, when the pressure is greater than 2000 kg / cm ² , cracks occur frequently in the active material layer, making it difficult for Li ions in the active material to move, and as a result, the lithium secondary using the positive electrode plate This is not desirable because the initial capacity (capacity of the first cycle) of the battery decreases.

  By the way, when it pressurizes with a roller-shaped member (roll pressurization), the above-mentioned joined object is sequentially pressed by a pressure roller, being conveyed in a predetermined direction. In this case, the pressure part in the above-mentioned joined body is substantially linear. Therefore, in this case, a defect (such as interfacial peeling or generation of cracks in the positive electrode active material particles 22) may occur due to an uneven force applied to the above-described joined body.

  However, in order to obtain a joined body in which the positive electrode active material particles are arranged at high density, the volume ratio of the solvent (for example, NMP) in the conductive paste coating layer at the time of pressing in the joining process is 10% or more, and It is desirable that it is 30% or less. If the volume ratio is less than 10%, adhesion between the two-dimensionally arranged positive electrode active material particles and the conductive paste coating layer becomes insufficient, and it is not desirable because good cycle characteristics cannot be obtained. On the other hand, when the volume ratio is larger than 30%, for example, when the conductive paste reaches the interface between the positive electrode active material particles and the surface plate, the positive electrode plate is peeled off from the surface plate in the manufacturing process of the positive electrode plate. This is not desirable because the possibility of disturbing the arrangement of the positive electrode active material particles is increased.

  In the bonding step, the positive electrode current collector and a plurality of two-dimensionally arranged active material particles are bonded through the conductive paste coating layer (bonding layer), and then the bonding layer is dried to perform the next addition. It is desirable that the volume ratio of the solvent (for example, NMP) in the bonding layer subjected to the pressing step is 5% or less, more preferably 1% or less. When the volume ratio is larger than 5%, in the pressurizing step, for example, the conductive paste reaches the interface between the positive electrode active material particles and the surface plate, so that the positive electrode plate is removed from the surface plate in the positive electrode plate manufacturing step. This is not desirable because the possibility that the arrangement of the positive electrode active material particles is disturbed during peeling is increased.

4). Example Hereinafter, a specific example (Example) of the manufacturing method of the positive electrode plate 2 shown in FIG. 2 and an evaluation result thereof will be described.

4-1. Manufacturing Method (1) Positive Electrode Active Material Particle Manufacturing Process First, positive electrode active material particles 22 were obtained as follows.

(1-1) Slurry preparation 81.6 parts by weight of Ni (OH) ₂ powder (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and 15.0 parts by weight of Co (OH) ₂ powder (manufactured by Wako Pure Chemical Industries, Ltd.) And 3.4 parts by weight of Al ₂ O ₃ .H ₂ O powder (manufactured by SASOL) were weighed. Next, 97.3 parts by weight of pure water, 0.4 part by weight of a dispersant (manufactured by NOF Corporation: product number AKM-0521), and 1-octanol (manufactured by Katayama Chemical Co., Ltd.) as an antifoaming agent A vehicle consisting of 2 parts by weight and 2.0 parts by weight of a binder (manufactured by Nippon Vinegar Poval Co., Ltd .: product number PV3) was prepared.

  Subsequently, a slurry was prepared by mixing and pulverizing the vehicle and the raw material powder (the above-mentioned weighed product) in a wet manner. Wet mixing and pulverization were carried out by treating with a ball mill using zirconia balls having a diameter of 2 mm for 24 hours and then treating with a bead mill using zirconia beads having a diameter of 0.1 mm for 40 minutes.

(1-2) Granulation A granulated body was formed by introducing the slurry described above into a two-fluid nozzle spray dryer. By appropriately adjusting parameters such as the spray pressure of the spray dryer, the nozzle diameter, the circulating air volume, etc., it is possible to form granules of various sizes.

(1-3) Heat treatment (temporary firing)
The above-mentioned granulated body is heat-treated at 1100 ° C. for 3 hours (atmosphere), so that a composite oxide of nickel, cobalt, and aluminum ((Ni _0.8 , Co _0.15 , Al _0.05 ) O) The positive electrode active material precursor particle | grains which are this particle | grains were obtained.

(1-4) Molding 100 parts by weight of the obtained positive electrode active material precursor particle powder, 50 parts by weight of a dispersion medium (xylene: butanol = 1: 1), and polyvinyl butyral (made by Sekisui Chemical Co., Ltd.) as a binder: No. BM-2, 10 parts by weight, 4.5 parts by weight of DOP (dioctyl phthalate: manufactured by Kurokin Kasei Co., Ltd.) as a plasticizer, and a dispersant (product name “Reodol SPO-30” manufactured by Kao Corporation) ) Weighed 3 parts by weight, pre-kneaded in a mortar, and then kneaded using tri-roll to prepare a molding slurry having a viscosity of 2000 to 3000 cP (viscosity was measured using a Brookfield LVT viscometer). Measured).

  A sheet having a thickness of 50 μm was formed by the doctor blade method using the obtained molding slurry. By punching the dried sheet, a 1 mm square green sheet molded body was obtained.

(1-5) Firing (Lithium introduction)
The 1 mm square green sheet molded body obtained as described above was heat-treated at 900 ° C. in an air atmosphere, whereby the molded body was degreased and pre-fired. The temperature of the green body preliminary firing is lower than the above-described heat treatment (granulated body preliminary firing) temperature. This is because lithium is uniformly diffused and reacted during the subsequent main firing by suppressing the progress of sintering between the internal particles during the temporary firing of the molded body.

A mixture of the obtained calcined body and powdered lithium hydroxide monohydrate (LiOH.H ₂ O) in a mortar is subjected to heat treatment at 750 ° C. for 6 hours (oxygen atmosphere), whereby Li (Ni Plate-like particles of a positive electrode active material having a composition of _0.8 , Co _0.15 , Al _0.05 ) O ₂ were prepared.

(2) Bonding layer paste application step A conductive paste was prepared by mixing acetylene black, polyvinylidene fluoride (PVDF), and NMP (N-methylpyrrolidone) at 1: 1: 100 parts by weight. This conductive paste was applied to the surface of an aluminum foil having a thickness of 20 μm to be the positive electrode current collector 21 so that the thickness after application was 80 μm using a coating machine. Then, the application layer of the conductive paste was dried by heating at 80 ° C. in an air atmosphere. At this time, the heating time was adjusted so that the volume ratio of the solvent (NMP) in the coating layer of the conductive paste was 20%.

(3) Joining Step Next, the positive electrode active material particles 22 obtained as described above were two-dimensionally arranged on the surface of the surface plate so that a gap between them was not generated as much as possible. Subsequently, as described above, the positive electrode current collector 21 on which the conductive paste layer after drying is formed is applied on the positive electrode active material particles 22 arranged on the surface of the surface plate, and the conductive paste coating layer is applied. Was placed so as to face the positive electrode active material particle 22 side, and pressed at a pressure of 100 g / mm ² .

(4) Pressurization process By heating the joined body obtained by the joining process described above at 80 ° C. for 1 hour, the volume ratio of the solvent (NMP) in the conductive paste coating layer becomes 1% or less. As described above, the coating layer of the conductive paste was sufficiently dried. Then, it pressurized with the predetermined press pressure (refer the following Table 1) from the positive electrode active material particle 22 side using the uniaxial press. During pressurization, the press pressure was maintained for 60 seconds.

4-2. Evaluation Method About the positive electrode manufactured by the manufacturing method of the above-described specific example, the cycle characteristics (capacity maintenance ratio) were evaluated as follows.

The obtained positive electrode, negative electrode plate made of a lithium metal plate, stainless steel negative electrode current collector plate, and separator, with the aluminum foil side of the positive electrode facing outward (opposite side of the separator), positive electrode-separator-negative electrode-negative electrode collector A coin cell was produced by arranging the electrodes in this order and filling the integrated body with an electrolyte. The electrolytic solution was prepared by dissolving LiPF ₆ in an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at an equal volume ratio to a concentration of 1 mol / L.

  About the produced battery, the current value is reduced to 1/20 under the current condition that the battery voltage is 4.3 V at a current value of 0.2 C rate until the battery voltage becomes 4.3 V, and then the battery voltage is maintained at 4.3 V. Charging and discharging operation of 10 minutes after charging at a constant voltage until charging, and then stopping at a constant current discharge until the battery voltage reaches 3.0 V at a current value of 0.2 C rate, and 1 cycle, A total of 20 cycles was repeated under the condition of 25 ° C., and the value obtained by dividing the measured value of the discharge capacity at the 20th cycle by the measured value of the discharge capacity at the 1st cycle was shown as “capacity maintenance ratio”.

4-3. Evaluation results The press pressure in the above-described examples was changed (Comparative Example 1, Examples 1 to 4 and Comparative Example 2), the pressurization process was not performed (Comparative Example 3), and uniaxial pressurization ( Table 1 shows the evaluation results of those using cold isostatic pressing (CIP: Cold Isostatic Press) (Comparative Example 4) instead of flat pressing. It should be noted that the press pressure shown in Table 1 is much larger than the applied pressure that the positive electrode continuously receives in the assembled battery.

As is clear from the results in Table 1, an appropriate pressure (100 kg / cm ² or more) among Comparative Example 1, Examples 1 to 4, and Comparative Example 2 that have undergone a pressing process by uniaxial pressing (planar pressing). In Examples 1 to 4 which were pressed at a pressing pressure of 2000 kg / cm ² or less, good cycle characteristics (capacity retention ratio) were obtained. On the other hand, in Comparative Example 1 in which the press pressure was lower than the desired range and in Comparative Example 3 in which the pressing step was not performed, good cycle characteristics were not obtained. On the contrary, in Comparative Example 2 in which the press pressure was excessively higher than the desired range, the active material layer was frequently cracked and the capacity at the first cycle was reduced. Also, in Comparative Example 4 using hydrostatic pressure, good cycle characteristics could not be obtained. These reasons are considered as follows.

  Dimensional changes occur in the positive electrode active material particles 22 due to the entry and exit of lithium ions accompanying charging and discharging. For this reason, when charging / discharging of the lithium secondary battery 1 is repeated, the interface between the positive electrode current collector 21 and the bonding layer 23 or the interface between the bonding layer 23 and the positive electrode active material particles 22 is usually used. As a result, interfacial peeling occurs gradually. Thereby, a capacity | capacitance fall arises. In particular, when the lithium secondary battery 1 is a so-called liquid type (a type using a liquid electrolyte), the bonding layer 23 is expanded by the liquid electrolyte, thereby promoting the occurrence of the above-described interface peeling.

  In this regard, in the positive electrode plate 2 of the embodiment, the thickness is made uniform through a pressurizing process by uniaxial pressurization with an appropriate pressure (pressing pressure). Then, the applied state of the pressing force to the positive electrode becomes uniform inside the assembled battery. Thereby, the occurrence of the interfacial delamination as described above is suppressed uniformly, and the electrical connection in the exfoliation portion is well maintained even if the interfacial delamination occurs.

  On the other hand, in Comparative Example 1 in which the pressing pressure is lower than the desired range, Comparative Example 3 in which the pressing pressure was not applied, and Comparative Example 4 in which the hydrostatic pressure was applied, as shown in the middle stage in FIG. Due to the variation in the thickness of the positive electrode active material particles 22, the thickness of the positive electrode varies. As a result, the applied state of the pressing force to the positive electrode also varies within the assembled battery.

  For this reason, in the portion where the thickness of the positive electrode is small, the effect of suppressing the occurrence of interfacial delamination as described above is reduced, or the electrical connection retention effect of the exfoliated portion in the event that interfacial delamination occurs should be sufficiently obtained. It becomes impossible.

As described above, in a lithium secondary battery, a capacity-decreasing characteristic (cycle characteristic) associated with repeated charging / discharging can be improved, and a bonded body in which positive electrode active material particles are arranged at high density and is in close contact is obtained. Therefore, it has been confirmed that the pressure when the bonded body is pressed in the pressing step is preferably 100 kg / cm ² or more and 2000 kg / cm ² or less.

  When roller pressurization was used instead of uniaxial pressurization (plane pressurization) (linear pressure and feed rate were adjusted as appropriate so that the thickness after pressurization was the same as in Example 3), the positive electrode active Many cracks were generated in the material particles, and interfacial delamination occurred.

5. Effect of the solvent content of the conductive paste coating layer on the cycle characteristics at the time of bonding The content ratio of the solvent (NMP) in the conductive paste coating layer at the time of pressing in the bonding process is a predetermined volume ratio (see Table 2 below). Comparative Example 5 was performed under the same conditions as in “4. Examples” described above, except that the positive electrode plate was uniaxially pressed at a press pressure of 1000 kg / cm ² for all positive electrode plates. And 6, and positive electrode plates according to Examples 5 and 6 were produced, and various lithium ion batteries (coin cells) were produced using these various positive electrode plates. The results of evaluating the cycle characteristics of these batteries in the same manner as in “4. Examples” are listed in Table 2 below.

  As is apparent from the results in Table 2, among Comparative Examples 5 and 6 and Examples 5 and 6, the volume ratio of the solvent (NMP) in the conductive paste coating layer at the time of pressing in the joining step is in an appropriate range (10 In Examples 5 and 6 that are at least% and not more than 30%), good cycle characteristics (capacity retention ratio) were obtained. In contrast, in Comparative Example 5 in which the volume ratio of the solvent (NMP) was lower than the desired range, good cycle characteristics could not be obtained. This is because the plate-like particles of the positive electrode active material used in this example are formed and fired from a granulated body produced by a spray dryer, and the plate-like particles have a large number of voids (open pores). In Comparative Example 5 where the residual amount of the solvent (NMP) in the conductive paste coating layer is insufficient, the conductive paste may permeate into the voids of the plate-like particles during pressing in the joining step. Example 5 in which an appropriate amount of solvent (NMP) was left in the conductive paste coating layer while the adhesiveness between the conductive paste coating layer and the plate-like particles was insufficient. And 6, the conductive paste can penetrate into the voids of the plate-like particles relatively easily during pressing in the joining step, and as a result, the adhesion between the conductive paste coating layer and the plate-like particles is improved. Erareru. In Comparative Example 6 in which the volume ratio of the solvent (NMP) is higher than the desired range, the conductive paste reaches the interface between the positive electrode active material particles and the surface plate, and the positive electrode plate is defined in the positive electrode plate manufacturing process. When peeling from the board, the arrangement of the positive electrode active material particles was disturbed, and the positive electrode plate could not be produced.

  As described above, in a lithium secondary battery, a capacity-decreasing characteristic (cycle characteristic) associated with repeated charging / discharging can be improved, and a bonded body in which positive electrode active material particles are arranged at high density and is in close contact is obtained. Therefore, it was confirmed that the volume ratio of the solvent (for example, NMP etc.) in the conductive paste coating layer at the time of pressing in the joining process is preferably 10% or more and 30% or less.

6). Effect of pressure at the time of pressing in the joining process on cycle characteristics The pressure at the time of pressing in the joining process is set to a predetermined pressure (see Table 3 below), and all positive electrodes are uniaxial at a press pressure of 1000 kg / cm ^2. A positive electrode plate according to Comparative Examples 7 and 8 and Examples 7 and 8 was manufactured under the same conditions as in “4. Various lithium ion batteries (coin cells) were produced using the various positive electrode plates. The results of evaluating the cycle characteristics of these batteries in the same manner as in “4. Examples” are listed in Table 3 below.

As is clear from the results in Table 3, in Comparative Examples 7 and 8, and Examples 7 and 8, the pressure during pressing in the joining step is in an appropriate range (50 g / cm ² or more and 500 g / cm ² or less. In Examples 7 and 8 in (), good cycle characteristics (capacity retention ratio) were obtained. On the other hand, in Comparative Example 7 where the pressure during pressing was lower than the desired range, good cycle characteristics were not obtained. This is because the plate-like particles of the positive electrode active material used in this example are also formed and fired from a granulated body produced by a spray dryer, and the plate-like particles have a large number of voids (open pores). Therefore, in this example, the residual amount of the solvent (NMP) in the conductive paste coating layer was in a suitable range for any positive electrode plate, but the pressure at the time of pressing in the joining process was not sufficient. In sufficient comparative example 7, it is relatively difficult for the conductive paste to penetrate into the voids of the plate-like particles during pressing in the joining step, and as a result, the adhesion between the conductive paste coating layer and the plate-like particles is low. In Examples 7 and 8, in which the pressure during pressing in the joining step was in a suitable range, whereas the conductive paste became less than the voids of the plate-like particles during pressing in the joining step, it was insufficient. Manner easily can penetrate, so that the conductive paste coating layer and adhesion of the plate-like particles is considered to be due to improved. In Comparative Example 8 where the pressure during pressing is higher than the desired range, the volume ratio of the solvent (NMP) in the conductive paste coating layer is adjusted to an appropriate range (specifically 20%). Regardless, the conductive paste reaches the interface between the positive electrode active material particles and the surface plate, and when the positive electrode plate is peeled off from the surface plate in the manufacturing process of the positive electrode plate, the arrangement of the positive electrode active material particles is disturbed. Could not be produced.

As described above, in a lithium secondary battery, a capacity-decreasing characteristic (cycle characteristic) associated with repeated charging / discharging can be improved, and a bonded body in which positive electrode active material particles are arranged at high density and is in close contact is obtained. Therefore, it was confirmed that the pressure during pressing in the joining process is preferably 50 g / cm ² or more and 500 g / cm ² or less.

7). The above-described embodiments and specific examples are merely examples of realization of the present invention that the applicant considered to be the best at the time of filing of the present application as described above. Therefore, the present invention should not be limited at all by the above-described embodiments and specific examples. Therefore, it goes without saying that various modifications can be made to the above-described embodiments and specific examples without departing from the essential part of the present invention.

  Hereinafter, some modifications will be exemplified. In the following description of the modification, components having the same configurations and functions as the components in the above-described embodiment are given the same name and the same reference numerals in this modification. And about description of the said component, description in the above-mentioned embodiment shall be used suitably in the range which is not inconsistent.

  However, it goes without saying that the modified examples are not limited to the following. Limiting the present invention based on the description of the above-described embodiment and the following modifications unfairly harms the applicant's interests, but improperly imitators and is not allowed (particularly the application). This is especially true under the first-to-file principle.

  It goes without saying that the configuration of the above-described embodiment and the configuration described in each of the following modifications can be combined in an appropriate manner within a technically consistent range.

  The configuration of the lithium secondary battery 1 to which the present invention is applied is not limited to the configuration described above. For example, the present invention is not limited to a so-called liquid battery configuration. That is, for example, a gel electrolyte or a polymer electrolyte can be used as the electrolyte. Moreover, the positive electrode active material that can be used in the present invention is not limited to the composition and the manufacturing method shown in the above-described specific examples.

  The present invention is not limited to the specific examples described above. That is, the present invention can be applied to a method for producing a negative electrode as long as the negative electrode active material is in the form of particles (typically when the negative electrode active material is a plate-like ceramic).

  The pressurization method in the uniaxial pressurization may be a method in which the thickness unevenness of the positive electrode plate 2 due to the thickness unevenness of the arranged active material particles is eliminated (the surface height of the positive electrode plate 2 is uniform). For example, instead of or in addition to controlling the pressing force in uniaxial pressing, the target height of the surface of the positive electrode plate 2 may be fixed (that is, the stroke amount of the pressing member is controlled). In this case, each particle may be pressurized one by one, or a plurality of particles may be pressurized every predetermined range (for example, one row).

  In addition, when pressurizing a plurality of particles for each predetermined range (for example, one row), it is necessary that the entire positive electrode active material particles 22 to be pressurized are uniformly pressurized almost simultaneously in one pressurization. Become. In this respect, the roller pressurization applies a non-uniform force, which may cause the above-described problems.

  Other modifications not specifically mentioned are naturally included in the technical scope of the present invention without departing from the essential part of the present invention.

  In addition, in each element constituting the means for solving the problems of the present invention, elements expressed functionally and functionally include the specific structures disclosed in the above-described embodiments and modifications, It includes any structure that can realize this action / function. Furthermore, the contents of the prior application and each publication (including the specification and the drawings) cited in the present specification may be incorporated as appropriate as part of the present specification.

Claims (3)

A current collector, a plurality of active material particles arranged two-dimensionally along the surface of the current collector, and a bonding layer provided so as to be interposed between the current collector and the active material particles A method for producing an electrode for a lithium secondary battery, comprising:
A bonding layer application step of forming a bonding layer by applying a conductive paste formed by mixing conductive particles, a binder resin, and a solvent into a paste on the surface of the current collector;
In the state where the volume ratio of the solvent in the bonding layer is 10% or more and 30% or less, the current collector and the plurality of active material particles two-dimensionally arranged are interposed through the bonding layer. After pressing and joining at a pressure of 50 g / cm ² or more and 500 g / cm ² or less, the joining layer is dried, and the joined body obtained by the joining process is 100 kg / cm ^2. A pressurizing step of uniaxially pressurizing from the active material particle side at a pressure of 2000 kg / cm ² or less.
It is characterized by having
A method for producing an electrode for a lithium secondary battery. It is a manufacturing method of the electrode for lithium secondary batteries according to claim 1,
The pressurizing step is a step of planarly pressing the joined body,
A method for producing an electrode for a lithium secondary battery. It is a manufacturing method of the electrode for lithium secondary batteries according to claim 1,
The active material particles are plate-like ceramic particles,
A method for producing an electrode for a lithium secondary battery.

Images