JP2016058129A - Lithium ion battery and lithium ion battery separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion battery suppressed in the formation of a hole in a separator after charge/discharge cycles and furthermore excellent in initial charge/discharge efficiency, rate characteristics and cycle characteristics, and a lithium ion battery separator used in a lithium ion battery.SOLUTION: In a separator 18 used in a lithium ion battery 10 of one embodiment of the present invention, a uniform lithium metal film is provided on one surface of a separator base material mainly composed of polyolefin. A porous heat resistant layer may be formed on the other surface of the separator base material, and the lithium metal film is arranged on a side facing a negative electrode.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a lithium ion battery and a lithium ion battery separator for use therein.

  Metal materials that can be alloyed with lithium such as silicon, germanium, tin and zinc instead of carbonaceous materials such as graphite as negative electrode active materials, and these metals for higher energy density and higher output of lithium ion batteries The use of these oxides is being studied.

For example, if the negative electrode active material made of a metal material alloyed with lithium or an oxide of these metals is silicon, lithium can be inserted up to the composition of Li _4.4 Si, so lithium can be inserted only up to the composition of LiC _6. Has a larger theoretical capacity than graphite-based carbonaceous materials. Even if any negative electrode active material is used, lithium from the positive electrode active material is taken into the negative electrode active material at the time of the first charge, but not all of this lithium can be taken out at the time of discharge. The unspecified amount is fixed in the negative electrode active material, resulting in an irreversible capacity. Since the irreversible capacity of the metal material alloyed with lithium and the negative electrode active material made of an oxide of these metals is larger than the irreversible capacity of the carbonaceous material, there is a problem that the battery capacity does not reach a desired value.

  Patent Document 1 below discloses a lithium ion battery separator in which a lithium powder having a surface treatment with an average particle size of 20 μm is attached to a separator mainly composed of polyolefin. When the lithium ion battery separator disclosed in Patent Document 1 is used, the lithium capacity can be supplemented to the irreversible capacity of the negative electrode by the stabilized lithium powder, so that the battery capacity is improved. In addition, since lithium is not mixed in the negative electrode mixture layer, the deactivation of lithium does not proceed, and it is not necessary to provide an environment in which lithium does not react before the battery group manufacturing process.

JP 2008-84842 A

  The lithium ion battery separator disclosed in Patent Document 1 uses a lithium powder that has been subjected to stabilization treatment on the surface, and thus the particle size of the lithium powder varies and is not uniform. Therefore, when a lithium ion battery is manufactured using the lithium ion battery separator disclosed in Patent Document 1, a hole larger than the diameter of a fine hole that the separator itself originally has in the separator after charge and discharge. May open. Since such a hole causes an internal short circuit between the positive electrode and the negative electrode, it is desired to suppress as much as possible.

  In the lithium ion battery manufactured using the lithium ion battery separator disclosed in Patent Document 1, the components formed on the surface of the lithium powder by the stabilization treatment remain on the surface of the separator after the first charge. Therefore, it becomes a resistance component and causes deterioration of battery characteristics.

  A lithium ion battery according to one aspect of the present invention includes a positive electrode plate including a positive electrode active material layer capable of reversibly occluding and releasing lithium, and a negative electrode plate including a negative electrode active material layer capable of reversibly occluding and releasing lithium. A separator that separates the positive electrode plate and the negative electrode plate, and a non-aqueous electrolyte solution containing a non-aqueous solvent and an electrolyte salt, wherein the separator is a separator base material mainly composed of polyolefin, and the separator base material And a homogeneous lithium metal film formed on an unopposed portion of the positive electrode active material layer and the negative electrode active material layer, wherein the lithium metal film is formed on the separator base material. It is provided on the negative electrode plate side.

  The separator used in the lithium ion battery of one aspect of the present invention is provided with a lithium metal film on one surface of the separator base material, and this lithium metal film is homogeneous and has an extra component. Not. According to the lithium ion battery of one aspect of the present invention, lithium from the lithium metal film is taken into the negative electrode active material during the first charge, but a hole may be formed in the separator base material or internal resistance may increase. A lithium ion battery that is suppressed, has high initial charge / discharge efficiency, and excellent cycle characteristics can be obtained.

It is a perspective view of a flat electrode body. 2A is a schematic front view of a flat lithium ion battery according to one aspect of the present invention, and FIG. 2B is a cross-sectional view taken along line IIB-IIB in FIG. 2A. 3A to 3D are schematic cross-sectional views of the separators of Experimental Examples 1 to 4, respectively. 6 is a scanning electron microscope (SEM) photograph of the separator surface of Experimental Example 4. 5A is an enlarged SEM photograph of the VA portion of FIG. 4, and FIG. 5B is an enlarged SEM photograph of the VB portion of FIG.

  Hereinafter, a lithium ion battery according to one aspect of the present invention and a lithium ion battery separator for use in the lithium ion battery will be described in detail using various experimental examples. However, the following experimental examples are illustrated for explaining an example of a lithium ion battery for embodying the technical idea of the present invention and a separator for a lithium ion battery for use in the present invention. Is not intended to be limited to any of these experimental examples. The present invention can be equally applied to those in which various modifications are made to those shown in these experimental examples without departing from the technical idea shown in the claims.

[Experimental Example 1]
The lithium ion battery of Experimental Example 1 was produced as follows.

(Preparation of positive electrode)
Lithium cobalt composite oxide (LiCoO ₂ ) 100 parts by mass, acetylene black 1.5 parts by mass, polyvinylidene fluoride 1.5 parts by mass, and an appropriate amount of N-methylpyrrolidone (NMP) are mixed in a mixer, and the positive electrode A mixture slurry was prepared. This positive electrode mixture slurry was applied to both sides of a positive electrode current collector sheet made of an Al foil having a thickness of 15 μm, dried, and after rolling, cut into a size corresponding to a battery case made of a predetermined laminate material. The positive electrode used with a lithium ion battery was obtained. The charge capacity of this positive electrode was 3.6 mAh / cm ² .

(Preparation of negative electrode)
And an average particle diameter _(D 50) SiO particles 10 parts by mass of 6 [mu] m, and an average particle diameter _(D 50) graphite particles 90 parts by weight of 25 [mu] m, and carboxymethyl cellulose (CMC) 1 part by weight of a thickening agent, a binder 1 part by mass of styrene butadiene rubber (SBR) was mixed with an appropriate amount of water with a mixer to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to both surfaces of a negative electrode current collector sheet made of a copper foil having a thickness of 10 μm, dried, cut into a size corresponding to a battery case made of a predetermined laminate material after rolling, and Experimental Example 1 The negative electrode used with a lithium ion battery was obtained. The charge capacity of this negative electrode was 5.0 mAh / cm ² .

(Heat-resistant layer formation on the separator)
Using a microporous membrane made of polyethylene having a thickness of 20 μm as a base material, a ceramic layer made of inorganic particles is formed on one surface of the microporous membrane base material as follows, and homogeneous lithium is formed on the other surface. A metal film was formed. As the inorganic particles, alumina powder having an average particle diameter (D ₅₀ ) of 0.7 μm manufactured by Sumitomo Chemical Co., Ltd. was used. 100 parts by weight of this alumina powder was mixed with 3 parts by weight of polyvinylidene fluoride (PVdF) as a binder and an appropriate amount of NMP to prepare a slurry which is a precursor of the porous heat-resistant layer. This slurry was applied to one side of a microporous membrane substrate using a doctor blade and dried in a drier maintained at 60 ° C. to form a porous heat-resistant layer having a thickness of 4 μm on the microporous membrane substrate.

(Lithium metal film formation on the separator)
Next, the surface of the microporous membrane substrate opposite to the surface on which the porous heat-resistant layer is formed is homogenized by vacuum evaporation (lithium source is evaporated by resistance heating) so that the thickness becomes 4.0 μm. A separator used for the lithium ion battery of Experimental Example 1 was obtained.

(Production of lithium ion battery)
A specific manufacturing process of the lithium ion battery 10 of Experimental Example 1 will be described with reference to FIGS. The positive electrode tab 11 was welded to the end of the positive electrode current collector of the positive electrode 16 produced as described above, and the negative electrode tab 12 was also welded to the end of the negative electrode current collector of the negative electrode 17. Next, the positive electrode 16 and the negative electrode 17 are placed in a state where the positive electrode 16 and the negative electrode 17 are insulated from each other through the two separators 18 manufactured as described above, and the positive electrode tab 11 and the negative electrode 17 Both the tabs 12 were wound in a spiral shape so as to be on the outermost peripheral side. At this time, the lithium metal film was made to face the negative electrode. Thereafter, an insulating anti-winding tape was attached to the winding end portion and pressed to obtain a flat wound electrode group 13 as shown in FIG.

  As the outer package 14, as shown in FIGS. 2A and 2B, an aluminum laminate film material was previously molded into a container so as to accommodate the flat wound electrode group 13 produced as described above. A thing was used. Then, the flat wound electrode group 13 and the non-aqueous electrolyte prepared as described above are inserted into the outer package 14 in a carbon dioxide atmosphere at 25 ° C. and 1 atm, and the end of the aluminum laminate material is inserted. The closed part 35 was formed by heat-sealing the parts, and the flat lithium secondary battery 10 according to Experimental Example 1 having the structure shown in FIGS. 2A and 2B was produced.

[Experiment 2]
The lithium ion battery of Experimental Example 2 is an experiment except that only a porous heat-resistant layer is formed on one surface of the microporous membrane substrate as a separator, but a lithium metal membrane is not formed. A device having the same configuration as in Example 1 was produced.

[Experiment 3]
As the lithium ion battery of Experimental Example 3, a porous heat-resistant layer is formed on one surface of a microporous membrane substrate as a separator, and a lithium metal film formed on the porous heat-resistant layer is used. Were manufactured in the same configuration as in Experimental Example 1.

[Experimental Example 4]
In the lithium ion battery of Experimental Example 4, as a separator, a porous heat-resistant layer is formed on one surface of a microporous membrane substrate, and the other surface is stabilized with an average particle size of 20 μm manufactured by FMC. The thing of the structure similar to the case of the experiment example 1 was produced except having formed the film | membrane consisting of the lithium metal particle which did. The separator used in the lithium ion battery of Experimental Example 4 corresponds to the separator disclosed in Patent Document 1 above.

“Measurement of battery characteristics”
About each lithium ion battery of Experimental Examples 1-4 produced as mentioned above, the initial stage charge / discharge efficiency, the rate characteristic, and the capacity maintenance rate were measured as follows. In addition, all the following measurements were performed in a 25 degreeC environment.

(Measurement of initial charge / discharge efficiency)
The lithium ion batteries immediately after assembling in Experimental Examples 1 to 4 were charged with a constant current of 0.5 It until the battery voltage reached 4.3 V, and after the battery voltage reached 4.3 V, 4.3 V The battery was charged until the charge current became 0.05 It at a constant voltage of. The amount of electricity that flowed at this time was determined as the initial charge capacity. Next, the battery was discharged at a constant current of 0.2 It until the battery voltage reached 3.0 V, and the amount of electricity that flowed at this time was determined as the initial discharge capacity. And the initial stage charge / discharge efficiency was calculated | required based on the following formulas. The results are summarized in Table 1.
Initial charge / discharge efficiency (%) = (initial discharge capacity / initial charge capacity) × 100

(Rate characteristics)
About each lithium ion battery which measured the said initial stage charge / discharge efficiency, it charges until a battery voltage will be 4.3V with a constant current of 0.5 It again, and after a battery voltage reaches 4.3V, it will be 4.3V The battery was charged until the charge current became 0.05 It at a constant voltage of. Thereafter, discharging was performed at a constant current of 1.0 It until the battery voltage reached 3.0 V, and the amount of electricity flowing at this time was determined as a 1 It discharge capacity. Next, discharging was performed at a constant current of 0.2 It until the battery voltage reached 3.0 V, and the amount of electricity flowing at this time was determined as a 0.2 It discharge capacity. And the rate characteristic was calculated | required based on the following formulas. The results are summarized in Table 1.
Rate characteristic (%) = (1 It discharge capacity / 0.2 It discharge capacity) × 100

(Measurement of capacity maintenance rate)
The lithium ion batteries immediately after assembling in Experimental Examples 1 to 4 were charged with a constant current of 0.5 It until the battery voltage reached 4.3 V, and after the battery voltage reached 4.3 V, 4.3 V The battery was charged until the charge current became 0.05 It at a constant voltage of. Next, the battery was discharged at a constant current of 1.0 It until the battery voltage reached 3.0 V, and the amount of electricity flowing at this time was determined as the discharge capacity of the first cycle. This charge / discharge was repeated 100 times, and the amount of electricity that flowed during the 100th discharge was determined as the discharge capacity of the 100th cycle. Then, the capacity maintenance rate at the 100th cycle was determined based on the following calculation formula. The results are summarized in Table 1.
Capacity maintenance rate (%)
= (Discharge capacity at the 100th cycle / discharge capacity at the first cycle) × 100

  The schematic cross-sectional views of the separators of Experimental Examples 1 to 4 are shown in FIGS. 3A to 3D. In the separator 18A of Experimental Example 1, the porous heat-resistant layer 18b is formed on one surface of the separator substrate 18a, and the homogeneous lithium metal layer 18c is formed on the other surface. In the separator 18B of Experimental Example 2, only the porous heat-resistant layer 18b is formed on one surface of the separator substrate 18a, and no lithium metal layer is formed. In the separator 18C of Experimental Example 3, the porous heat-resistant layer 18b is formed on one surface of the separator substrate 18a, and the lithium metal layer 18d is further formed on the surface thereof. In the separator 18D of Experimental Example 4, the porous heat-resistant layer 18b is formed on one surface of the separator substrate 18a, and the lithium metal particle layer 18e whose surface is stabilized similarly is formed on the other surface.

  From the results shown in Table 1, the following can be understood. The difference in configuration between the batteries of Experimental Examples 1, 3 and 4 and the battery of Experimental Example 2 is that a lithium layer is formed on the separator substrate (Experimental Examples 1, 3 and 4) or not (Experimental Example). 2), the battery of Experimental Example 2 has a large negative electrode irreversible capacity, and thus the initial charge / discharge efficiency is considered to have decreased.

  The batteries of Experimental Examples 1, 3, and 4 differ in the microstructure of the lithium metal layer based on the difference in the method of forming the lithium metal layer formed on the separator substrate. That is, since the lithium metal layer in the battery of Experimental Example 1 is formed on the separator substrate by a vacuum deposition method, the lithium metal layer is formed to have a uniform thickness. On the other hand, the lithium metal layer in the battery of Experimental Example 3 reacts with the porous heat-resistant layer to consume lithium. Therefore, it is considered that the charge / discharge efficiency of the battery of Experimental Example 3 is lower than that of the battery of Experimental Example 1.

  Since the lithium metal layer in the battery of Experimental Example 4 is provided with lithium metal particles whose surface is stabilized on the separator substrate, the lithium metal layer has an uneven structure corresponding to the shape of the lithium metal particles used. Yes. In addition, during the first charge, the portion of the lithium metal layer facing the positive electrode and the negative electrode disappears by moving to the negative electrode, but the components used for the lithium stabilization treatment remain on the surface of the negative electrode. ing. Therefore, in the battery of Experimental Example 4, the internal resistance increases due to the components used for the stabilization treatment of lithium, and thus it is considered that the initial charge / discharge efficiency is lower than that of the battery of Experimental Example 1.

  In the batteries of Experimental Example 1, Experimental Example 3 and Experimental Example 4, when the charge / discharge cycle is repeated, the lithium metal film in the portion where each separator faces the positive electrode and the negative electrode disappears as lithium is supplemented to the negative electrode. To do. However, in a portion where each separator does not face the positive electrode and the negative electrode, lithium is not supplemented to the negative electrode, so that the lithium metal film remains even after repeated charge / discharge cycles.

  Regarding the rate characteristics, the same results were obtained for both the batteries of Experimental Example 1 and Experimental Example 2, but the batteries of Experimental Example 3 and Experimental Example 4 were both inferior to the results of Experimental Example 1 and Experimental Example 2. It was. This means that the reaction product between the lithium metal and the porous heat-resistant layer such as alumina is the resistance component in the battery of Experimental Example 3, and the component used for the lithium stabilization treatment in the battery of Experimental Example 4 is the resistance component. This is thought to have led to a decrease in rate characteristics.

  As for the capacity maintenance ratio, almost the same results were obtained for both the batteries of Experimental Example 1 and Experimental Example 3, but the battery of Experimental Example 3 was inferior to the battery of Experimental Example 1, and the battery of Experimental Example 2 was the most. It was inferior. In the battery of Experimental Example 2, since the irreversible capacity of the negative electrode is large, it is considered that the capacity retention rate was reduced due to the consumption of lithium in the irreversible capacity supplement during the charge / discharge cycle. In the battery of Experimental Example 3, the reaction product between the lithium metal and the porous heat-resistant layer such as alumina becomes a resistance component, and in the battery of Experimental Example 4, the component used for the lithium stabilization treatment becomes a resistance component. This is thought to have led to a decrease in the maintenance rate.

  FIG. 4 shows an SEM photograph (50 times) of the surface of the separator of Experimental Example 4 after two charge / discharge cycles, and FIG. 5A shows enlarged photographs (2000 times) of the VA portion and VB portion of FIG. And shown in FIG. 5B. As shown in FIGS. 4 and 5, the separator of Experimental Example 4 was confirmed to have a hole in the separator after a charge / discharge cycle. Around this hole, the fibrous structure of the separator substrate has disappeared, and there is concern about the possibility of thermal damage associated with the charge / discharge cycle. On the other hand, such a hole was not formed in the separator used in the batteries of Experimental Examples 1 to 3 even after the charge / discharge cycle.

  The cause of the formation of holes in the separator used in the battery of Experimental Example 4 is not clear at present, and it is necessary to wait for future research results. However, when comparing the results of the batteries of Experimental Example 3 and Experimental Example 4, the battery of Experimental Example 4 is inferior to the battery of Experimental Example 3 in rate characteristics and capacity retention. Therefore, in order to improve the rate characteristics and capacity retention rate, it is suggested that at least the lithium metal film directly formed on the surface of the separator substrate needs to be a homogeneous film without unevenness.

  In Experimental Example 1, the thickness of the lithium metal film was 4 μm, but the thickness of the lithium metal film is not particularly limited. However, the appropriate thickness of the lithium metal film varies depending on the irreversible capacity of the negative electrode active material layer to be used, and needs to be appropriately adjusted. If the thickness of the lithium metal film is too small, the irreversible capacity in the negative electrode active material layer may not be sufficiently compensated, and the initial efficiency and cycle characteristics may not be sufficiently improved. If the thickness of the lithium metal film is too large, lithium is likely to precipitate on the negative electrode, which may reduce safety.

When SiO _x (x = 0.5 to 1.5) having a large irreversible capacity is used as the negative electrode active material, the mass ratio of SiO _x to the carbon-based active material (such as graphite) is 5:95 to 100: 0. In this case, when the negative electrode mixture coating amount is 100 to 300 g / m ² , the preferable thickness of the lithium metal film is 1 to 40 μm.

  A negative electrode for a lithium ion battery according to one aspect of the present invention and a lithium ion battery using the negative electrode are, for example, a driving power source for a mobile information terminal such as a mobile phone, a notebook personal computer, and a PDA, and applications that require a particularly high energy density. Can be applied to. In addition, it can be expected to be used for high output applications such as electric vehicles (EV), hybrid electric vehicles (HEV, PHEV) and electric tools.

DESCRIPTION OF SYMBOLS 10 ... Lithium ion battery 11 ... Positive electrode tab 12 ... Negative electrode tab 13 ... Winding electrode group 14 ... Exterior body 15 ... Closure part 16 ... Positive electrode 17 ... Negative electrode 18, 18A-18D ... Separator 18a ... Separator base | substrate 18b ... Porous heat-resistant layer 18c, 18d ... lithium metal layer 18e ... lithium metal particle layer with stabilized surface

Claims (6)

A positive electrode plate including a positive electrode active material layer capable of reversibly inserting and extracting lithium;
A negative electrode plate including a negative electrode active material layer capable of reversibly inserting and extracting lithium;
A separator that separates the positive electrode plate and the negative electrode plate;
A non-aqueous electrolyte containing a non-aqueous solvent and an electrolyte salt, and
The separator is
A separator base material based on polyolefin;
A homogeneous lithium metal film formed on one surface of the separator base material and on the non-opposing portion of the positive electrode active material layer and the negative electrode active material layer,
The lithium metal film is a lithium ion battery provided on the negative electrode plate side of the separator substrate.   The lithium ion battery according to claim 2, wherein the separator has a porous heat-resistant layer formed on the other surface on the separator substrate.   The lithium ion battery according to claim 1 or 2, wherein the lithium metal film has a thickness of 1 to 40 µm.   A separator for a lithium ion battery, in which a homogeneous lithium metal film is provided on only one surface of a separator base material mainly composed of polyolefin.   The separator for lithium ion batteries according to claim 4, wherein a porous heat-resistant layer is formed on the other surface of the separator substrate.   The lithium ion battery separator according to claim 4 or 5, wherein the lithium metal film has a thickness of 1 to 40 µm.