JP2008277242A - Electrode for lithium secondary battery and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a lithium secondary battery capable of improving discharge rate characteristics and to provide a lithium secondary battery using it. <P>SOLUTION: The electrode 5 for the lithium secondary battery has an active material layer 1 formed on a current collector, a slit-like or a grid-like groove 3 is formed on the surface of the active material layer 1, wherin the groove 3 is formed so that the end in the face direction of the groove 3 does not reach the peripheral part 1a of the active material layer 1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery using the same.

  A current lithium secondary battery is formed by packing a group of electrodes, each composed of a positive electrode, a negative electrode, and a separator, into a battery can together with an electrolyte solution in a spiral or stack. In each electrode, an active material layer is laminated on a current collector. Therefore, when the active material layer is thickened and the number of stacked electrode groups is reduced, the volume of the active material layer in the battery can increases and the battery capacity can be increased.

  However, when the active material layer is thickened, the current density per electrode surface area increases under the same charge / discharge current time ratio. In addition, since the active material near the current collector has a long distance to the electrode surface, lithium ions cannot be smoothly diffused from the reaction site of the active material to the electrolyte phase during charging and discharging. For this reason, the utilization factor of an active material falls in the charge / discharge voltage range practically effective for the operation | movement of a lithium secondary battery, As a result, the merit of the capacity increase by thickening an active material layer cannot be acquired.

  In order to solve the above problem, it is conceivable to make the electrolyte easily penetrate into the active material layer. For example, Patent Document 1 proposes a positive electrode provided with a slit-like uncoated part, and can improve the liquid-containing property of the electrode.

  However, when such a method is applied to an electrode having a thick active material layer, a problem of peeling of the active material layer occurs. That is, an electrode with a thick active material layer is easily peeled off from the current collector due to the specifications of the electrode, and if there is a boundary between the uncoated part and the coated part on the outer edge of the electrode, this is the starting point of peeling. Cheap. In such an electrode, if a number of uncoated portions reaching the outer edge of the electrode are provided, the active material layer is likely to be peeled when the electrode is rolled or the electrode group is overlapped. It is likely to occur.

Further, the above-described conventional technique is not effective in terms of reduction in current density per surface area of the electrode and reduction in distance to the electrolyte phase of the active material in the vicinity of the current collector. Therefore, even if the above-described conventional technology is used, lithium ions are not diffused smoothly, and it is difficult to increase the utilization rate of the active material. Moreover, a lithium secondary battery with high discharge rate characteristics could not be obtained.
JP 2001-35484 A

  The objective of this invention is providing the electrode for lithium secondary batteries which can improve a discharge rate characteristic, and a lithium secondary battery using the same.

  The present invention is an electrode for a lithium secondary battery in which an active material layer is provided on a current collector, wherein slits or lattice-like grooves are formed on the surface of the active material layer, and the surface direction of the grooves A groove is formed so that the end portion of the active material layer does not reach the peripheral edge portion of the active material layer.

  In the present invention, since a slit-like or lattice-like groove is formed on the surface of the active material layer, the contact area between the active material and the electrolyte can be increased, and lithium ions can be diffused more smoothly. Can do. For this reason, the discharge rate characteristic can be enhanced. Further, since the groove is formed so that the end portion in the surface direction of the groove does not reach the peripheral portion of the active material layer, it is possible to prevent the active material layer from peeling from the current collector.

  In the present invention, it is preferable that no groove is formed in a region of 1 mm inward from the peripheral edge of the active material layer. By not forming a groove in such a region, it is possible to more effectively prevent the active material layer from being separated from the current collector.

  Moreover, in this invention, it is preferable that the groove | channel is formed in the depth which does not reach the electrical power collector surface. By forming the groove at a depth that does not reach the current collector surface, it is possible to more effectively prevent the active material layer from being separated from the current collector due to the groove formation.

  The volume of the groove is preferably 3% or more, more preferably 4% or more with respect to the total volume of the groove and the active material layer. As a result, the current density in the groove portion is sufficiently lowered, and lithium diffusion can be further smoothed. Therefore, the discharge rate characteristics can be further improved. Although the discharge rate characteristics can be improved by increasing the ratio of the volume of the groove to the total volume of the groove and the active material layer (groove porosity), the capacity tends to decrease. Is preferably 30% or less, and more preferably 20% or less.

  Moreover, it is preferable that the groove | channel is formed in the depth used as 1/2 or more of the thickness of an active material layer. As a result, the electrolyte solution can be disposed up to the vicinity of the current collector of the active material layer, lithium ions can be diffused more smoothly, the utilization rate of the active material can be increased, and the discharge rate characteristics can be improved. Can do.

  In the present invention, the depth of the groove is more preferably in the range of 50 to 99.5% of the thickness of the active material layer.

  In the present invention, the groove is preferably formed so as to extend in a direction substantially perpendicular to the current collector surface. Thereby, since the distance to the active material surface in a groove | channel and a counter electrode becomes the shortest, the spreading | diffusion of lithium ion becomes smoother.

  In the present invention, the grooves are preferably formed at intervals such that the distance between adjacent grooves is 500 μm or less. The interval in this case is the distance between the centers in the groove surface direction. The interval between the grooves is more preferably in the range of 100 to 400 μm.

  By setting it as such a range, the surface area of an active material layer increases, the current density per surface area can be reduced, and lithium ion can be spread | diffused smoothly.

  In the present invention, the width in the surface direction of the groove is not particularly limited, but the current per surface area can be formed so as to have a width in the range of 10 to 100 μm without significantly reducing the capacity. This is preferable because the density can be reduced.

  It is preferable that the groove | channel in this invention is arrange | positioned uniformly in the groove | channel formation area | region in an active material layer. Accordingly, lithium ions are more uniformly diffused in the electrode, and the active material layer is less likely to be damaged due to deformation of the electrode.

  In the present invention, the method of forming the groove is not particularly limited, and examples thereof include a method of irradiating a laser to melt the active material layer to form the groove. Also, when forming the active material layer or rolling the electrode, mechanically remove the active material layer to form a groove by physically removing the active material layer or apply the particles to the surface of the active material layer at high speed. Examples thereof include a method of forming a groove using stress. Further, the groove may be formed by chemical etching.

  It is also possible to form grooves in the stage of forming the active material layer. For example, a method of forming an active material layer by patterning so as to form a groove by using various printers such as screen printing and inkjet may be used. In addition, the current collector is masked in advance in a slit shape or a lattice shape, and a slurry is applied thereon, or an active material is physically or chemically attached by sputtering, vapor deposition, electrodeposition, etc. And a method of removing grooves and forming grooves.

  In the present invention, the thicker the active material layer, the greater the effect of the present invention. Therefore, in this invention, it is preferable that the thickness of an active material layer is 100 micrometers or more, More preferably, it is the range of 100-1000 micrometers.

The active material layer in the present invention may be a positive electrode active material layer or a negative electrode active material layer. Examples of the positive electrode active material include layered transition metal oxides such as LiCoO ₂ and LiNiO ₂ , spinel type transition metal oxides such as LiMn ₂ O ₄ and Li ₄ Ti ₅ O ₁₂ , and olivine types such as LiFePO ₄ and LiCoPO _4. A phosphoric acid compound etc. are mentioned. Examples of the negative electrode active material include spinel transition metal oxides such as Li ₄ Ti ₅ O ₁₂ and carbon materials such as graphite and coke. Since the positive electrode active material layer is generally formed thick, the effect of the lithium secondary battery electrode of the present invention is particularly significant when used as a positive electrode.

  The lithium secondary battery of the present invention is characterized by comprising a positive electrode comprising the electrode of the present invention, a negative electrode, and a nonaqueous electrolyte.

  Since the lithium secondary battery of the present invention uses the electrode of the present invention, the battery can have a high active material utilization rate and excellent discharge rate characteristics.

The electrolyte salt used for the non-aqueous electrolyte is not particularly limited, but LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , LiAsF ₆ , LiClO ₄ , Li ₂ B ₁₀ Cl ₁₀ , Li ₂ Examples include B ₁₂ Cl ₁₂ and mixtures thereof.

  In the present invention, the effect of the present invention is further exhibited particularly when the concentration of the electrolyte salt in the nonaqueous electrolyte is 1.5 mol / liter or more. When the concentration of the electrolyte salt is 1.5 mol / liter or more, the viscosity of the nonaqueous electrolyte increases and lithium ions are difficult to diffuse. According to the present invention, by using an electrode in which slit-like or lattice-like grooves are formed in the active material layer, the contact area between the nonaqueous electrolyte and the electrode surface can be increased, and lithium ions can be further diffused. Therefore, the effect of the present invention is further exhibited.

  The solvent used for the non-aqueous electrolyte is not particularly limited, but a cyclic carbonate or a chain carbonate is preferable. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate and the like. Among these, ethylene carbonate is particularly preferably used. Examples of the chain carbonate include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate and the like. Further, the solvent is preferably a mixed solvent obtained by mixing two or more solvents. In particular, a mixed solvent containing a cyclic carbonate and a chain carbonate is preferable.

  A mixed solvent of the cyclic carbonate and an ether solvent such as 1,2-dimethoxyethane or 1,2-diethoxyethane is also preferably used.

  The current collector used in the present invention is not particularly limited, and a current collector that has been conventionally used as a current collector for an electrode of a lithium secondary battery can be used. In the case of the positive electrode current collector, a metal foil such as an aluminum foil can be used, and in the case of the negative electrode current collector, a metal foil such as a copper foil can be used.

  ADVANTAGE OF THE INVENTION According to this invention, the utilization factor of an active material can be raised and a discharge rate characteristic can be improved.

  Hereinafter, the present invention will be described with reference to specific examples. However, the present invention is not limited to the following examples, and can be appropriately modified and implemented without departing from the scope of the present invention. is there.

<Experiment 1>
[Production of electrodes]
LiCoO ₂ as an active material, a carbon material as a conductive agent, and polyvinylidene fluoride as a binder are N-methyl-pyrrolidone so that the weight ratio of active material: conductive agent: binder is 96: 2: 2. To prepare an electrode mixture slurry. This electrode mixture slurry was applied on one surface of an aluminum foil as a current collector, then dried and rolled, and then a current collecting tab was attached to the opposite surface where the active material layer was not applied.

  In the electrode active material layer produced as described above, slit-like or lattice-like grooves were formed as follows.

[Groove formation on the surface of the active material layer]
The surface of the active material layer of the electrode produced as described above was irradiated with a laser to form slit-like or lattice-like grooves. FIG. 1 is a plan view showing an electrode in which a groove is formed, and FIG. 2 is an enlarged sectional view showing the groove. 1 and 2 show an electrode in which a slit-like groove is formed.

  As shown in FIG. 1, a plurality of slit-like grooves 3 are formed at a predetermined interval on the surface of the active material layer 1. The groove 3 is not formed in the region from the peripheral edge 1a of the active material layer 1 to the distance L inside. The active material layer 1 is formed in a 20 mm × 20 mm region, and the distance L is 2 mm. Therefore, the groove 3 is formed within a range of 16 mm × 16 mm in the center of the active material layer 1.

  As shown in FIG. 2, an active material layer 1 is formed on the current collector 2, and grooves 3 are formed in the active material layer 1 so as to have a depth d and a width W. The groove 3 is formed with a depth d such that the bottom 3 a does not reach the surface 2 a of the current collector 2. Accordingly, the depth d of the groove 3 is formed to be smaller than the thickness t of the active material layer 1. Moreover, the groove | channel 3 is formed so that it may become the predetermined space | interval D. FIG.

  Moreover, as shown in FIG. 1, the electrode 5 is produced by attaching the aluminum current collection tab 4 to a collector.

  FIG. 1 shows slit-like grooves formed in the horizontal direction. However, by forming slit-like grooves in the vertical direction in FIG. 1, lattice-like grooves can be obtained.

[Preparation of electrolyte]
An electrolyte solution was prepared by dissolving LiPF ₆ in a mixed solvent in which ethylene carbonate (EC) and methyl ethyl carbonate (MEC) were mixed at a volume ratio of 3: 7 so as to be 1.0 mol / liter. .

[Fabrication of monopolar cell]
A monopolar cell was produced using an electrode in which grooves having the groove shape, groove interval D, groove width W, and groove depth d shown in Table 1 were formed. Specifically, the above electrode as a working electrode, a counter electrode made of lithium metal, and a reference electrode were immersed in a container containing the above electrolytic solution to produce a single electrode cell.

[Charge / discharge test]
The single electrode cell produced as described above was subjected to a charge / discharge test to evaluate the discharge rate characteristics. Charging / discharging conditions were 25 degreeC and performed charging / discharging in the range of 4.3-2.75V (vs.Li/Li ^<+> ). Here, charging means desorption of lithium from the working electrode, and discharging means insertion of lithium into this electrode. The charging was performed at a constant current of 3 mA, and the discharging was performed at a constant current of 3 mA or 15 mA. Table 1 shows the discharge capacity at this time.

  The groove depth d was calculated using the following formulas (1) and (2).

(Groove depth (μm)) = {(groove porosity (%)) ÷ 100} × {(20000 (μm) × 20000 (μm) × (film thickness t (μm)) ÷ (16000 (μm)) × groove Width (μm) x number of grooves)) ... formula (1)
(Groove porosity (%)) = (1− (Membrane weight (mg)) ÷ (Membrane weight of electrode not forming grooves (197.8 mg))) × 100 (2)
In the above formula (1), 20000 μm is the electrode width, and 16000 μm is the groove length.

  The groove depth d can also be calculated using the following formula (3). Even in that case, the same result as that obtained by calculation using the above equation (1) was obtained.

(Groove depth (μm)) = (1− (discharge capacity of electrode in which groove is formed) ÷ (discharge capacity of electrode in which groove is not formed)) × (20000 (μm) × 20000 (μm) × (film Thickness t (μm)) ÷ (16000 (μm) × (groove width W (μm)) × (number of grooves)) Equation (3)
The thickness t of the active material layer is all 140 μm in Experiment 1.

  The discharge capacity is calculated using the value at the time when the current density is the lowest (capacity at 3 mA in Experiments 1 to 3 and 2.6 mA in Experiment 4 and 2.3 mA in Experiment 5). The unit of the groove depth d, the film thickness t, and the groove width W is μm.

  As is clear from the results shown in Table 1, it can be seen that the discharge rate characteristics are improved by forming grooves in the active material layer according to the present invention. In particular, a remarkable effect is obtained when the groove interval is 400 μm or less.

<Experiment 2>
Electrodes were produced in the same manner as in Experiment 1 except that the groove interval D was 200 μm, the groove width W was 30 μm and 20 μm, and the groove depth d was 139 μm and 70 μm, and the charge / discharge characteristics were evaluated. The measurement results are shown in Table 2. The results when the groove width W is 30 μm and the groove depth d is 139 μm are those of Experiment 1. In Experiment 2, all the active material layer thicknesses t are 140 μm.

  As is apparent from the results shown in Table 2, it can be seen that good discharge rate characteristics are obtained even when the groove depth d is ½ of the thickness t of the active material layer.

<Experiment 3>
Under the conditions of Experiment 1 in which the groove interval D was 200 μm, the concentration of the electrolyte salt in the electrolytic solution was 1.5 mol / liter and 1.0 mol / liter, and the discharge rate characteristics were evaluated. The evaluation results are shown in Table 3. The result when the concentration of the electrolyte salt is 1.0 mol / liter is that of Experiment 1. In Experiment 3, the active material layer thickness t is all 140 μm.

  As shown in Table 3, it can be seen that the effect of improving the discharge rate characteristics can be obtained more significantly by setting the concentration of the electrolyte salt to 1.5 mol / liter.

<Experiment 4>
The electrodes were prepared in the same manner as in Experiment 1 except that the thickness t of the active material layer was all 120 μm, and the groove interval D, groove width W, and groove depth d were as shown in Table 4, and the charge / discharge current was changed. The discharge rate characteristics were evaluated in the same manner as in Experiment 1 except that the constant current was 2.6 mA during charging and discharging and the constant current was 2.6 mA or 13 mA during discharging. The evaluation results are shown in Table 4.

  As is clear from the results shown in Table 4, the effect of improving the discharge rate characteristics is obtained even when the thickness t of the active material layer is 120 μm.

<Experiment 5>
An electrode was prepared in the same manner as in Experiment 1 except that the thickness t of the active material layer was 100 μm and the groove interval D, groove width W, and groove depth d were values shown in Table 5, and the charge / discharge current was changed. The discharge rate characteristics were evaluated in the same manner as in Experiment 1 except that the constant current was 2.3 mA during charging and discharging and the constant current was 2.3 mA or 23 mA during discharging. The evaluation results are shown in Table 5.

  As is clear from the results shown in Table 5, the effect of improving the discharge rate characteristics is obtained even when the thickness t of the active material layer is 100 μm.

<Experiment 6>
In the same manner as in Experiment 1, an electrode having a plurality of slit-like grooves 3 formed on the surface of the active material layer 1 at a predetermined interval was produced, and the discharge rate characteristics were evaluated in the same manner as in Experiment 1. The evaluation results are shown in Table 6. Table 6 includes the data shown in Table 1.

  In Experiment 6, the groove porosity (the ratio of the volume of the groove to the total volume of the groove and the active material layer) was calculated using the following formula (2).

  (Groove porosity (%)) = (1− (Membrane weight (mg)) ÷ (Membrane weight of electrode not forming grooves (197.8 mg))) × 100 (2)

  As is apparent from the results shown in Table 6, it can be seen that by setting the groove porosity to 3% or more, the capacity at 15 mA can be further increased, and the discharge capacity characteristics can be further improved. In particular, a remarkable effect is obtained when the groove porosity is 4% or more.

  As is apparent from the above, according to the present invention, by forming slit-like or lattice-like grooves on the surface of the active material layer, the utilization factor of the active material can be increased and the discharge rate characteristics can be improved. it can.

The top view which shows the electrode in the Example according to this invention. Sectional drawing which expands and shows the part of the groove | channel of the electrode in the Example according to this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Active material layer 1a ... Peripheral part of active material layer 2 ... Current collector 3 ... Groove 3a ... Bottom of groove 4 ... Current collecting tab 5 ... Electrode

Claims (11)

  An electrode for a lithium secondary battery in which an active material layer is provided on a current collector, wherein a slit-like or lattice-like groove is formed on a surface of the active material layer, and an end portion in the surface direction of the groove Wherein the groove is formed so as not to reach the peripheral edge of the active material layer.   2. The electrode for a lithium secondary battery according to claim 1, wherein the groove is not formed in a region from the periphery of the active material layer to 1 mm inward.   The electrode for a lithium secondary battery according to claim 1, wherein the groove is formed at a depth that does not reach the surface of the current collector.   2. The electrode for a lithium secondary battery according to claim 1, wherein a volume of the groove is 3% or more with respect to a total volume of the groove and the active material layer.   2. The electrode for a lithium secondary battery according to claim 1, wherein a volume of the groove is 4% or more with respect to a total volume of the groove and the active material layer.   2. The electrode for a lithium secondary battery according to claim 1, wherein the groove is formed with a depth that is ½ or more of the thickness of the active material layer.   2. The electrode for a lithium secondary battery according to claim 1, wherein the groove is formed to extend in a direction substantially perpendicular to the surface of the current collector.   2. The electrode for a lithium secondary battery according to claim 1, wherein the grooves are formed at intervals of 400 μm or less.   It is a positive electrode, The electrode for lithium secondary batteries of Claim 1 characterized by the above-mentioned.   A lithium secondary battery comprising a positive electrode comprising the electrode according to claim 1, a negative electrode, and a nonaqueous electrolyte. The lithium secondary battery according to claim 10, wherein the concentration of the electrolyte salt in the non-aqueous electrolyte is 1.5 mol / liter or more.

Images