JP2017117579A - Lithium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion battery having a positive electrode with improved electron conductivity.SOLUTION: A lithium ion battery comprises a positive electrode including: lithium-containing metal oxide particles 5 and 10; and an osmium coating 30 which covers at least part of a surface of the lithium-containing metal oxide particles 5 and 10. A coating thickness of the osmium coating 30 ranges 0.02 to 45 nm.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a lithium ion battery.

  Japanese Patent Laying-Open No. 2015-170476 (Patent Document 1) discloses a positive electrode for a lithium ion battery, in which a slurry containing a positive electrode active material and a conductive additive is applied to the surface of a positive electrode current collector. .

Japanese Patent Laying-Open No. 2015-170476 International Publication No. 2013/065096 JP 2013-214355 A

As a positive electrode active material of a lithium ion battery, lithium-containing metal oxide particles such as LiCoO ₂ are used. Since such a positive electrode active material is an oxide, it has poor electronic conductivity.

  Conventionally, carbon fine particles (conductive aid) such as acetylene black have been used to supplement the electron conductivity of the positive electrode active material. For example, carbon fine particles can be attached to the surface of the positive electrode active material by mixing the positive electrode active material and the carbon fine particles. On the surface of the positive electrode active material, an electron conduction path through which electrons can move is formed in a portion where carbon fine particles are continuously present.

  However, since the carbon fine particles easily aggregate during mixing, it is difficult to continuously and uniformly adhere the carbon fine particles to the surface of the positive electrode active material. In other words, there is room for improvement in the electron conduction path on the surface of the positive electrode active material.

  The lithium ion battery of the present invention includes a positive electrode including lithium-containing metal oxide particles and an osmium coating that covers at least a part of the surface of the lithium-containing metal oxide particles. The film thickness of the osmium coating is 0.02 nm or more and 45 nm or less.

  The present invention will be described below. In the following description, the lithium-containing metal oxide particles may be abbreviated as “LiMeO particles” and the osmium film may be abbreviated as “Os film”.

  1 (A) and 1 (B) are conceptual diagrams showing the adhesion state of carbon fine particles on the surface of LiMeO particles. As shown in FIGS. 1A and 1B, LiMeO particles are generally secondary particles 10 in which primary particles 5 are aggregated, and have fine irregularities on the order of nano to micron on the surface. . The carbon fine particles 20 such as acetylene black have a particle size of about 10 to 1000 nm. However, with this particle size, it is difficult to fill the unevenness on the surface of the LiMeO particles without any gaps.

  Further, as shown in FIG. 1A, since the carbon fine particles 20 are likely to aggregate, it is difficult to form a continuous electron conduction path. When the carbon fine particles are aggregated, lithium (Li) ions in the LiMeO particles are difficult to move, and the diffusion resistance of Li ions increases. In particular, during high-rate charge / discharge, the positive electrode reaction does not proceed due to the slow movement of Li ions.

  In order to form an electron conduction path over a wide range, it is conceivable to increase the amount of carbon fine particles. However, when the amount of carbon fine particles is increased, a side reaction accompanied by gas generation is likely to occur during high temperature storage, and performance deterioration is promoted. Even if the carbon fine particles are ideally continuous, grain boundaries exist as long as they are particles, and the electron conduction path may be interrupted at the grain boundaries, so that it is difficult to form an electron conduction path over a wide range.

  2A and 2B are conceptual diagrams showing the positive electrode of the present invention. In the positive electrode of the present invention, an extremely thin Os film 30 is present on the surface of LiMeO particles (primary particles 5 and secondary particles 10). Since this Os film is a metal, it is excellent in electronic conductivity, and since it is a very thin film with a film thickness of sub-nano to nano-order, it can follow the irregularities on the surface of the LiMeO particles. That is, LiMeO particles can be uniformly coated.

  The ultra-thin Os film can be formed by, for example, plasma CVD (plasma-enhanced chemical vapor deposition). According to such a method, Os constituting the Os film becomes an amorphous metal. That is, there is substantially no grain boundary in the Os film, and an electron conduction path over a wide range is formed.

  The present inventor has succeeded in forming an Os film having a thickness of 0.02 nm to 45 nm on the surface of LiMeO particles by plasma CVD. Furthermore, in the lithium ion battery including the LiMeO particles and the Os film, an improvement in performance that confirms an improvement in electronic conductivity on the positive electrode side has been confirmed.

  According to the above, a lithium ion battery in which the electron conductivity of the positive electrode is improved is provided.

FIGS. 1A and 1B are conceptual diagrams showing the adhesion state of carbon fine particles on the surface of lithium-containing metal oxide particles. 2A and 2B are conceptual diagrams showing the positive electrode of the present invention.

  Hereinafter, an embodiment of the present invention (hereinafter referred to as “the present embodiment”) will be described. However, the present invention is not limited to the following description.

<Lithium ion battery>
The lithium ion battery includes at least a positive electrode, a negative electrode, and a nonaqueous electrolyte. The lithium ion battery may include a separator interposed between the positive electrode and the negative electrode. Hereinafter, each member constituting the lithium ion battery will be described.

《Positive electrode》
The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The thickness of the positive electrode current collector is, for example, about 5 to 25 μm. The thickness of the positive electrode mixture layer is, for example, about 10 to 150 μm.

  The positive electrode current collector is, for example, an aluminum (Al) foil. The positive electrode mixture layer contains LiMeO particles, an Os film that covers at least a part of the surface of the LiMeO particles, and a binder. That is, the positive electrode of the present embodiment includes LiMeO particles and an Os film that covers at least a part of the surface of the LiMeO particles. The binder may be, for example, polyvinylidene fluoride (PVdF). The positive electrode mixture layer may further contain carbon fine particles. The carbon fine particles may be acetylene black, thermal black, or the like, for example. The composition of the positive electrode mixture layer in the present embodiment is, for example, as follows. As described below, in the present embodiment, a configuration that substantially does not contain carbon fine particles may be employed.

(Composition of positive electrode mixture layer)
Positive electrode active material: about 80 to 99% by mass Carbon fine particles: about 0 to 10% by mass Binder: about 1 to 10% by mass.

(Lithium-containing metal oxide particles)
LiMeO particle | grains function as a positive electrode active material of a lithium ion battery. The LiMeO particles are not particularly limited. LiMeO particles include, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , general formula LiNi _a Co _b Mn _c O ₂ (where a + b + c = 1, 0 <a <1, 0 <b <1, 0 <c <1) may be used. In this embodiment, Li, as long as it contains a metal (Me) and oxygen (O), in for example be a phosphate such as LiFePO _4, and those belonging to LiMeO particles.

  The d50 of LiMeO particles (secondary particles) is, for example, about 1 to 20 μm. The d50 of the primary particles constituting the secondary particles is, for example, about 10 nm to 5 μm. Here, “d50” indicates the particle size at a cumulative 50% in the volume-based particle size distribution measured by the laser diffraction / scattering method.

(Osmium coating)
The Os film covers at least a part of the surface of the LiMeO particles. Here, the “surface of LiMeO particles” indicates the surface of primary particles and the surface of secondary particles. The Os coating preferably covers the entire surface of the LiMeO particles. However, the Os film only needs to cover at least part of the LiMeO particles, and the effect of the present invention is considered to be exhibited as long as at least part of the Os film is covered.

  In addition, the positive electrode mixture layer of the present embodiment may contain LiMeO particles that are not covered with the Os film as long as they contain LiMeO particles that are at least partially covered with the Os film. Of the LiMeO particles contained in the positive electrode mixture layer, the larger the proportion of LiMeO particles covered with the Os film, the greater the effect of improving the electron conductivity.

  The film thickness of the Os film is 0.02 nm or more and 45 nm or less. In such a range, it has been confirmed that the battery performance is improved by improving the electron conductivity. From the viewpoint of electron conductivity, the film thickness is preferably 1.8 nm or more. Furthermore, from the viewpoint of high-temperature storage characteristics and cycle characteristics, the film thickness is preferably 9.5 nm or less, and more preferably 4.3 nm or less.

  Os constituting the Os film is preferably an amorphous metal. However, a part of Os may be a metal crystal. Even if Os which is a metal crystal is included in the Os film, it is considered that the effect of the present invention is exhibited.

  The Os film is substantially composed only of Os. Here, “substantially composed only of Os” indicates that the Os film may contain unavoidable impurities inevitably mixed in the formation process in addition to Os. The constituent ratio of Os in the Os film is preferably 95 atomic% or more, more preferably 98 atomic% or more, and particularly preferably 99 atomic% or more.

(Method of forming osmium coating)
The Os film of this embodiment can be formed by the following method, for example. First, LiMeO particles and a binder are dispersed in a predetermined solvent (for example, N-methyl-2-pyrrolidone; NMP) to prepare a positive electrode slurry. A positive electrode slurry is applied on the positive electrode current collector and dried to form a positive electrode mixture layer. Thereby, a positive electrode is obtained.

  Next, an Os film is formed by vapor-depositing amorphous Os on at least a part of the surface of the LiMeO particles using a plasma CVD apparatus. A so-called “osmium plasma coater” (sometimes referred to as “osmium coater”) is suitable for the plasma CVD apparatus. Hereinafter, an example using an osmium plasma coater will be described.

The osmium plasma coater uses OsO ₄ , which is a raw material for the Os film, as a sublimation gas and converts it into plasma by direct current glow discharge. When OsO ₄ is turned into plasma, two regions of a negative glow phase region (cathode side) and a positive column region (anode side) are formed in the chamber. In the negative glow phase region, positively ionized Os gas molecules concentrate and collide violently. Therefore, by placing the sample in the negative glow phase region, a thin Os film with high purity is formed on the surface of the sample. It is formed. The Os film formed in this way is composed of an amorphous metal having substantially no grain boundary.

  In this method, since the raw material is gasified, the Os film can be uniformly formed on the surface of the LiMeO particles having severe irregularities. Further, since the positive electrode mixture layer is a particle layer, the voids between the particles are not completely filled and are porous. Therefore, the source gas penetrates deep into the positive electrode mixture layer, and an Os film can be formed on the surface of the LiMeO particles existing inside the positive electrode mixture layer. The film thickness of the Os film can be adjusted by the discharge current and the discharge time. If the relationship between the discharge time and the film thickness under a predetermined condition is measured in advance, the film thickness of the Os film formed on the surface of the LiMeO particles can be estimated from the discharge time under the same condition. .

<Negative electrode>
The negative electrode includes, for example, a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The thickness of the negative electrode current collector is, for example, about 5 to 25 μm. The thickness of the negative electrode mixture layer is, for example, about 10 to 150 μm.

  The negative electrode current collector is, for example, a copper (Cu) foil. The negative electrode mixture layer contains a negative electrode active material and a binder. The negative electrode active material may be a carbon-based negative electrode active material such as graphite, graphitizable carbon, and non-graphitizable carbon, or may be an alloy-based negative electrode active material containing silicon (Si), tin (Sn), or the like. The binder may be, for example, a mixture of carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR).

<< Separator >>
The separator is a porous film having insulating properties. The separator is made of a polyolefin material such as polyethylene (PE) or polypropylene (PP). The thickness of the separator is, for example, about 5 to 50 μm. The pore diameter and porosity of the separator can be appropriately adjusted so that the air permeability becomes a desired value.

《Nonaqueous electrolyte》
The non-aqueous electrolyte may be a liquid electrolyte (that is, an electrolytic solution), a solid electrolyte, or a gel electrolyte. For example, the electrolytic solution is a solution in which a Li salt is dissolved in an aprotic solvent.

  Examples of the aprotic solvent include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), and chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). It may be a mixture. The mixing ratio of the cyclic carbonate and the chain carbonate may be a volume ratio, for example, cyclic carbonate: chain carbonate = 1: 9 to 5: 5.

The Li salt may be, for example, LiPF ₆ , Li [(FSO ₂ ) ₂ N] (abbreviation “LiFSI”), or the like. The concentration of the Li salt in the electrolytic solution may be about 0.5 to 1.5 mol / liter, for example.

  Hereinafter, the present embodiment will be described using examples. However, the present embodiment is not limited to the following example.

<Manufacture of lithium ion batteries>
Various lithium ion batteries were produced as follows and their performance was evaluated.

Example 1
1. Production of positive electrode The following materials were prepared.

(Positive electrode material)
LiMeO particles: LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM)
Carbon fine particles: Acetylene black (AB)
Binder: PVdF
Solvent: NMP
Positive electrode current collector: Al foil (thickness = 15 μm).

  A positive electrode slurry was prepared by mixing NCM, AB, PVdF, and NMP in a predetermined composition using a planetary mixer. The positive electrode slurry was applied to the surface of the Al foil using a die coater and dried to form a positive electrode mixture layer. Furthermore, the positive electrode which is a strip | belt-shaped sheet member was obtained by processing into a predetermined dimension.

  The positive electrode was placed in the chamber (cathode side) of an osmium plasma coater (plasma CVD apparatus) to form an Os film covering at least part of the surface of the NCM. The conditions for forming the Os film are as follows.

(Os film formation conditions)
Degree of vacuum: 7Pa
Discharge current: 10 mA
Discharge time: 2 seconds.

2. Production of negative electrode The following materials were prepared.

(Material for negative electrode)
Negative electrode active material: Graphite Binder: CMC and SBR
Solvent: Water Negative electrode current collector: Copper foil.

  A negative electrode slurry was prepared by mixing graphite, CMC, SBR, and water in a predetermined composition using a planetary mixer. A negative electrode slurry was applied to the surface of the copper foil using a die coater and dried to form a negative electrode mixture layer. Furthermore, the negative electrode which is a strip | belt-shaped sheet member was obtained by processing into a predetermined dimension.

3. Assembly A separator composed of a polyolefin material was prepared. A positive electrode and a negative electrode were laminated with a separator interposed therebetween, and further wound to form a wound electrode group. A predetermined current collector plate was welded to the wound electrode group. Further, the wound electrode group was inserted into a predetermined battery case.

An electrolytic solution [EC: DMC: EMC = 3: 4: 3 (volume ratio), LiPF ₆ = 1 mol / l] was injected into the battery case, and then the battery case was sealed. From the above, the lithium ion battery (rated capacity = 500 mAh) according to Example 1 was manufactured.

<< Examples 2 to 5 >>
As shown in Table 1, the lithium according to Examples 2 to 5 is the same as Example 1 except that the film thickness of the Os film is changed by changing the discharge time in the osmium plasma coater. An ion battery was manufactured.

<< Comparative Example 1 >>
A lithium ion battery according to Comparative Example 1 was prepared in the same manner as in Example 1 except that the content of carbon fine particles (AB) in the positive electrode mixture layer was increased to 8% by mass and no Os film was formed. Manufactured.

<< Comparative Example 2 >>
In Comparative Example 2, a conductive polymer film was formed instead of the Os film. First, a polymer solution was prepared by dissolving pyrrole in acetonitrile (MeCN). A film of a conductive polymer (polypyrrole) was formed by immersing the positive electrode in the polymer solution for a predetermined time.

<< Comparative Example 3 >>
In Comparative Example 3, a gold (Au) film was formed by a sputter coater instead of the Os film formed by an osmium plasma coater. The film formation time was 5 seconds.

<Evaluation of battery performance>
The lithium ion battery was evaluated as follows. In the following description, the unit of current “C” indicates a current that can discharge the rated capacity of the battery in one hour.

1. Measurement of initial discharge capacity The battery was charged to 4.1 V with a current of 1C. Thereafter, the battery was discharged to 3.0 V with a current of 1/3 C. The initial discharge capacity per unit mass of the positive electrode active material was determined by dividing the discharge capacity (initial discharge capacity) at this time by the total mass of LiMeO particles contained in the lithium ion battery. The results are shown in Table 1.

2. Measurement of IV resistance The IV resistance of the battery was measured. The results are shown in Table 1. The lower the IV resistance, the better the electron conductivity.

3. Measurement of capacity retention rate after storage at high temperature The SOC of the battery was adjusted to 90%. The battery was placed in a thermostat set at 60 ° C. and stored in the same environment for 60 days. After 60 days, the discharge capacity after storage was measured in the same manner as in “1. Measurement of initial discharge capacity”. The capacity retention rate (percentage) after high-temperature storage was determined by dividing the discharge capacity after storage by the initial discharge capacity. The results are shown in Table 1.

4). Measurement of capacity retention rate after cycle A battery was placed in a thermostat set at 60 ° C. Charging / discharging was repeated 500 cycles in an SOC range of 0 to 100% at a current of 2C. After 500 cycles, the post-cycle discharge capacity was measured in the same manner as in “1. Measurement of initial capacity”. By dividing the post-cycle discharge capacity by the initial discharge capacity, the post-cycle capacity retention rate (percentage) was determined. The results are shown in Table 1.

<Results and discussion>
As shown in Table 1, the lithium ion battery according to the example including the positive electrode including the LiMeO particle and the Os film that covers at least a part of the surface of the LiMeO particle has low IV resistance. It is considered that an electron conduction path covering a wide range is formed on the surface of the LiMeO particle by the Os coating.

  Example 5 is slightly lower in capacity retention after storage and after cycling than Examples 1-4. Since the thickness of the Os film is thick (45 nm), the electron conductivity is excessively high, and it is considered that the gas generation reaction is likely to occur in a high temperature environment. Therefore, from the viewpoint of high-temperature storage characteristics, the thickness of the Os film is preferably 9.5 nm or less.

  In Comparative Example 1 in which the amount of carbon fine particles is increased, neither performance is sufficient. This is considered to be because the electron conduction path is easily interrupted with carbon fine particles.

  Comparative Example 2 in which a film made of a conductive polymer is formed has a high IV resistance. It is considered that the conductive polymer does not enter the grain boundaries of the primary particles and the inside of the secondary particles. The reason why the high-temperature storage characteristics and the cycle characteristics are low is considered to be that the electron conduction path is interrupted when the LiMeO particles are cracked.

  Comparative Example 3 in which the Au coating is formed by ion sputtering also has a high IV resistance. In sputtering, it is considered that a film is formed on the surface of the positive electrode mixture layer, and the film cannot be formed even on the surface of the LiMeO particles existing inside the positive electrode mixture layer.

  The embodiments and examples disclosed herein are illustrative in all aspects and should not be construed as being restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  5 Primary particles (LiMeO particles), 10 Secondary particles (LiMeO particles), 20 Carbon fine particles, 30 Coatings.

Claims (1)

Including a lithium-containing metal oxide particle, and an osmium coating that covers at least a portion of the surface of the lithium-containing metal oxide particle,
The lithium-ion battery has a thickness of the osmium film of 0.02 nm to 45 nm.

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