JP2012009401A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery which can suppress the reaction between an positive electrode active material and moisture sufficiently.SOLUTION: The lithium secondary battery has a positive electrode including positive electrode active material particles 10. The positive electrode active material particles 10 are coated with a water-repellent resin 12. If the average particle diameter (median diameter: d50) of the positive electrode active material particles 10 coated with the water-repellent resin 12 is represented by A[μm], the mass thereof is represented by B[g], the mass of coatings of the water-repellent resin 12 applied to the positive electrode active material particles 10 is represented by C[g], and the degree of crystallization of the water-repellent resin is represented by D[%], the following relational expression is satisfied: 3.0≤A×(C/B)×D≤10.0.

Description

  The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery of a type including a positive electrode having a structure in which a positive electrode active material layer including positive electrode active material particles is held on a positive electrode current collector.

  In recent years, lithium-ion batteries, nickel-metal hydride batteries, and other secondary batteries have become increasingly important as power sources for vehicles or as power sources for personal computers and portable terminals. In particular, a lithium ion battery that is lightweight and obtains a high energy density is expected to be preferably used as a high-output power source mounted on a vehicle.

  In one typical configuration of this type of lithium secondary battery, an electrode having a configuration in which a material (electrode active material) capable of reversibly occluding and releasing lithium ions is held in a conductive member (electrode current collector) Is provided. For example, as one of the electrode active materials (positive electrode active materials) used for the positive electrode, an oxide (lithium transition metal composite oxide) containing lithium and one or more transition metal elements as constituent metal elements can be given. In particular, a lithium-nickel composite oxide containing lithium and nickel as constituent metal elements is preferably used as a positive electrode active material that can increase energy density and also have load followability because of its relatively large capacity. However, a positive electrode active material such as a lithium nickel composite oxide reacts with moisture in the air on the surface thereof to generate LiOH, and an aluminum foil used as an electrode current collector is corroded. When the aluminum foil is corroded, resistance is increased and input / output characteristics are remarkably deteriorated. In addition, since the amount of reaction with moisture varies according to the variation in environmental conditions, it may be a factor that causes variations in battery performance.

  For the purpose of suppressing the reaction between the positive electrode active material and moisture, it has been studied to coat the surface of the positive electrode active material with a corrosion-resistant material. For example, in Patent Document 1, in a lithium ion secondary battery including a positive electrode including a positive electrode active material (positive electrode material) made of lithium-containing composite oxide fine particles, polyvinylidene fluoride is formed on the surface of the lithium-containing composite oxide fine particles. Forming a film of a water-repellent substance such as As another conventional technique regarding the coating material on the surface of this type of positive electrode active material, Patent Document 2 can be cited.

Japanese Patent Laid-Open No. 11-224664 JP 2007-265668 A

  However, as in Patent Document 1, in a lithium ion secondary battery including a positive electrode including a positive electrode active material made of lithium-containing composite oxide fine particles, water repellency such as polyvinylidene fluoride is formed on the surface of the lithium-containing composite oxide fine particles. When a coating of the material is formed, the water-repellent material such as polyvinylidene fluoride itself acts as a resistance component for lithium ion conduction, so that the reactivity of the positive electrode active material (the activity of lithium ion insertion / release reaction) is greatly increased. It will decline. That is, by providing a coating of a water repellent material such as polyvinylidene fluoride, it is possible to suppress the reaction between the positive electrode active material and moisture, but on the contrary, the presence of the coating inhibits the lithium ion insertion and desorption reaction. Therefore, there is a problem that the reaction resistance of the lithium secondary battery constructed using the positive electrode active material is increased, making it difficult to apply to a high-power battery.

  The present invention has been made in view of such points, and its main object is to provide a lithium secondary battery that can sufficiently suppress the reaction between the positive electrode active material and moisture without increasing the reaction resistance of the battery. It is to be.

  The battery provided by the present invention is a lithium secondary battery including a positive electrode including positive electrode active material particles. The surface of the positive electrode active material particles is coated with a water-repellent resin. Here, the average particle diameter (median diameter: d50) of the positive electrode active material particles coated with the water-repellent resin is A [μm], the mass is B [g], and the positive electrode active material particles are coated. When the mass of the water-repellent resin coating is C [g] and the crystallinity of the water-repellent resin is D [%], 3.0 ≦ A × (C / B) × D ≦ 10.0 It is characterized by satisfying the relational expression.

Here, “A × (C / B)” in the above formula is a parameter corresponding to the thickness of the water-repellent resin film covering the positive electrode active material particles, and the larger the value, the thicker the water-repellent resin film. Moisture and CO ₂ are difficult to permeate, but at the same time the lithium ion permeability of the coating is also reduced. “D” in the above formula is a parameter representing the crystallinity of the water-repellent resin, and the larger the value, the better the crystallinity of the water-repellent resin film, making it difficult for moisture and CO ₂ to pass through. At the same time, the lithium ion permeability of the coating also decreases.

According to the configuration of the present invention, since the surfaces of the positive electrode active material particles are coated with the water-repellent resin, the reaction between the positive electrode active material, moisture, and CO ₂ can be preferably suppressed. At that time, if the water-repellent resin film is too thin or the crystallinity of the water-repellent resin is too small (typically when the value of “A × (C / B) × D” is less than 3.0), Since moisture and CO ₂ permeate through the water-repellent resin film, the reaction between the positive electrode active material, moisture and CO ₂ cannot be sufficiently suppressed. On the other hand, if the water-repellent resin coating is too thick or the crystallinity of the water-repellent resin is too large (typically when the value of “A × (C / B) × D” exceeds 10.0), And the permeation of CO ₂ can be suppressed, but at the same time, the lithium ion permeability is also lowered, so that the insertion / extraction reaction of lithium ions is inhibited. Any of these events can increase the reaction resistance of the battery and cause performance degradation.

On the other hand, according to the lithium secondary battery of the present invention, the relational expression of 3.0 ≦ A × (C / B) × D ≦ 10.0 is satisfied, and the thickness of the water repellent resin film and the water repellent resin are satisfied. Therefore, the lithium ion permeability is good, and the permeation of moisture and CO ₂ can be preferably suppressed. Therefore, the reaction between the positive electrode active material, water, and CO ₂ can be reliably suppressed without inhibiting the lithium ion insertion / release reaction due to the presence of the water-repellent resin coating. As a result, the reaction resistance of the battery can be lowered as compared with the conventional case, and the performance deterioration can be improved.

In a preferred embodiment of the lithium secondary battery disclosed herein, the relational expression 0.05 ≦ A × (C / B) ≦ 0.20 is established between the average particle diameter A and the masses B and C. To establish. If it is smaller than this range, the reaction between the positive electrode active material and moisture may not be sufficiently suppressed by the water repellent resin film. If it is larger than this range, lithium ion insertion / desorption reaction may occur due to the presence of the water repellent resin film. May be disturbed. In the preferred technology disclosed herein, the average particle diameter A of the positive electrode active material particles is 4 μm to 8 μm, and the mass ratio (C / B) between the water-repellent resin film and the positive electrode active material particles is 0.01. ~ 0.04.
Preferably, the crystallinity D of the water repellent resin is 40% to 70%. Below this range, the reaction between the positive electrode active material, moisture, and CO ₂ may not be sufficiently suppressed, and above this range, the lithium ion insertion / desorption reaction may be inhibited due to the presence of the water-repellent resin coating. There is.

As the material constituting the positive electrode active material particles, one or two or more materials (typically particulate) having the same composition as the materials conventionally used in lithium secondary batteries are used without particular limitation. can do. As a preferred example, lithium and a transition metal element such as lithium nickel composite oxide (LiNiO ₂ ), lithium manganese composite oxide (LiMn ₂ O ₄ ), and lithium cobalt composite oxide (LiCoO ₂ ) are included as constituent metal elements. A positive electrode active material mainly containing an oxide (lithium transition metal oxide) can be given.

Among them, the following general formula:
_{Li 1 + x (Ni y Co} z Mn 1-y-z-γ M γ) O 2
(However, x, y, z and γ in the above formula are 0 ≦ x ≦ 0.2, 0.3 ≦ y ≦ 1.0, 0 ≦ z ≦ 0.5, 0 ≦ γ ≦ 0.2, 0.3 ≦ y + z + γ ≦ 1 is satisfied.
M is one or two or more elements selected from the group consisting of F, B, Al, W, Mo, Cr, Ta, Nb, V, Zr, Ti, and Y which do not exist or are young. )
Application to a lithium nickel composite oxide represented by

  In particular, y in the above general formula is preferably 0.5 ≦ y ≦ 1.0 and a number satisfying 0.5 ≦ y + z + γ ≦ 1.

The positive electrode active material mainly composed of a lithium nickel composite oxide having such a high Ni composition ratio is likely to cause a reaction between the positive electrode active material, moisture, and CO ₂ , so that the present invention is particularly useful. It is. Note that, for example, when LiNiO ₂ is used as the positive electrode active material, the reaction between the positive electrode active material, moisture, and CO ₂ is considered to occur in two stages. First, LiOH is generated by the reaction of LiNiO ₂ + H ₂ O → NiOOH + LiOH in the first stage, and this corrodes the aluminum foil used as the positive electrode current collector, causing an increase in resistance. Next, Li ₂ CO ₃ is generated by the reaction of 2LiOH + CO ₂ → Li ₂ CO ₃ + H ₂ O in the second stage, and this covers the surface of the positive electrode active material, thereby inhibiting the lithium ion insertion / desorption reaction. Causes further resistance increase. By applying the present invention, an increase in resistance due to the reaction between the positive electrode active material, moisture, and CO ₂ as described above can be reliably suppressed, and a high-performance lithium secondary battery can be constructed.
Note that in the chemical formula showing the lithium nickel composite oxide in this specification, the composition ratio of O (oxygen) is shown as 2 for convenience, but it is not strict, and some compositional variation (typically 1.95). Included in the range of 2.05 or less).

  In a preferred embodiment of the lithium secondary battery disclosed herein, the water repellent resin is a polyvinylidene fluoride resin. Polyvinylidene fluoride-based resins are particularly suitable because lithium ion conductivity is relatively high, and the presence of the coating hardly inhibits insertion / extraction of lithium ions.

  As described above, any of the lithium secondary batteries (for example, lithium ion batteries) disclosed herein can lower the reaction resistance of the battery as compared with the conventional battery, and therefore, particularly for vehicles that require high-rate charge / discharge. It has performance suitable as a battery to be installed. Therefore, according to the present invention, there is provided a vehicle including any of the lithium secondary batteries disclosed herein (which may be in the form of an assembled battery in which a plurality of lithium secondary batteries are connected). In particular, a vehicle (for example, an automobile) including the lithium secondary battery as a power source (typically, a power source of a hybrid vehicle or an electric vehicle) is provided.

It is a figure which shows typically the positive electrode active material particle with a film concerning one embodiment of the present invention. It is a figure which shows typically the lithium secondary battery which concerns on one Embodiment of this invention. It is a graph which shows an example of the thermal-mass-analysis result of the positive electrode active material powder with a PVDF film manufactured in one test example. It is a graph which shows an example of the differential thermal scanning calorimetry result of the positive electrode active material powder with a PVDF film manufactured in one test example. Is a graph showing the relationship between X = A × a PVDF film with the positive electrode active material powder produced (C / B) × values and Li ₂ CO ₃ amount and reaction resistance ratio of D in one test example. It is a side view which shows typically the laminate cell (lithium secondary battery for a test) manufactured in one test example. It is a side view showing typically a vehicle provided with a lithium secondary battery concerning one embodiment of the present invention.

  Embodiments according to the present invention will be described below with reference to the drawings. In the following drawings, members / parts having the same action are described with the same reference numerals. Note that the dimensional relationship (length, width, thickness, etc.) in each drawing does not reflect the actual dimensional relationship. Further, matters other than the matters specifically mentioned in the present specification and matters necessary for carrying out the present invention (for example, the configuration and manufacturing method of an electrode body including a positive electrode and a negative electrode, the configuration and manufacturing method of a separator and an electrolyte, The general technology related to the construction of the lithium secondary battery, etc.) can be grasped as a design matter of those skilled in the art based on the prior art in the field.

As shown in FIG. 1, the lithium secondary battery disclosed herein is a lithium secondary battery including a positive electrode including positive electrode active material particles 10. The surfaces of the positive electrode active material particles 10 are covered with a water repellent resin 12. Here, the average particle diameter (median diameter: d50) of the positive electrode active material particles 10 coated with the water-repellent resin 12 is A [μm] and the mass thereof is B [g], and the positive electrode active material particles are coated. When the mass of the water-repellent resin film is C [g] and the crystallinity of the water-repellent resin is D [%], a relationship of 3.0 ≦ A × (C / B) × D ≦ 10.0 It is characterized by satisfying the formula. Here, “A × (C / B)” in the above formula is a parameter corresponding to the thickness of the water-repellent resin coating 12, and the larger the value is, the thicker the water-repellent resin coating is, and moisture and CO ₂ are transmitted. At the same time, the lithium ion permeability decreases. “D” in the above formula is a parameter representing the crystallinity of the water-repellent resin, and the larger the value, the better the crystallinity of the water-repellent resin film, making it difficult for moisture and CO ₂ to pass through. At the same time, the lithium ion permeability decreases.

Thus, the relational expression of 3.0 ≦ A × (C / B) × D ≦ 10.0 is satisfied, and the thickness of the water repellent resin film 12 and the crystallinity of the water repellent resin are adjusted with an appropriate balance. In the lithium secondary battery, the water-repellent resin coating 12 has good lithium ion permeability and the permeation of moisture and CO ₂ is sufficiently suppressed. The reaction between the positive electrode active material 10, water, and CO ₂ can be reliably suppressed without inhibiting the reaction. As a result, the reaction resistance of the battery can be reduced as compared with the conventional case, and the performance deterioration of the battery can be improved.

For example, the positive electrode active material particles with a water-repellent resin coating provided by this configuration preferably satisfy 3.0 ≦ A × (C / B) × D ≦ 10.0, 4.0 ≦ A × Those satisfying (C / B) × D ≦ 9.0 are more preferable, and those satisfying 5.0 ≦ A × (C / B) × D ≦ 8.0 are particularly preferable. On the other hand, if the "A × (C / B) × D " exceeds 10.0, the crystallinity of the water-repellent resin or too thick water-repellent resin film is too large, water and CO ₂ Can be suppressed, but at the same time, lithium ion permeability is lowered. Therefore, the insertion / extraction reaction of lithium ions is hindered, which can be a factor that increases the reaction resistance of the battery.
When “A × (C / B) × D” is less than 3.0, the water-repellent resin film is too thin or the crystallinity of the water-repellent resin is too small. ₂ is easily transmitted. Therefore, the reaction between the positive electrode active material, moisture, and CO ₂ cannot be sufficiently suppressed. In addition, since the amount of reaction with moisture and CO ₂ varies according to the variation in environmental conditions, it can be a factor that causes variations in battery performance. Therefore, from the viewpoint of realizing a lithium secondary battery with low resistance and no performance variation, “A × (C / B) × D” is suitably 3.0 or more, preferably 4.0 or more, Most preferably, it is 5.0 or more.

In a preferred embodiment disclosed herein, there is a relational expression 0.05 ≦ between the average particle diameter A of the positive electrode active material particles and the mass ratio (C / B) of the water repellent resin coating to the positive electrode active material particles. A × (C / B) ≦ 0.20 is established. If it is below this range, the reaction between the positive electrode active material and water may not be sufficiently suppressed, and if it exceeds this range, the lithium ion permeability of the water-repellent resin film may be too low. Preferably, the average particle diameter A of the positive electrode active material particles is in the range of 4 μm to 8 μm, and preferably the mass ratio (C / B) of the water repellent resin coating to the positive electrode active material particles is in the range of 0.01 to 0.04. It is. Also preferably, the crystallinity D of the water repellent resin is 40% to 70% (more preferably 40% to 65%). Below this range, the reaction between the positive electrode active material, water, and CO ₂ may not be sufficiently suppressed by the water-repellent resin coating. When this range is exceeded, lithium ion insertion and desorption reactions may occur due to the presence of the water-repellent resin coating. It may be obstructed.

The material constituting the positive electrode active material particles disclosed herein is particularly limited to one or more materials (typically in particulate form) having the same composition as the materials conventionally used in lithium secondary batteries. Can be used without. As a preferred example, lithium and a transition metal element such as lithium nickel composite oxide (LiNiO ₂ ), lithium manganese composite oxide (LiMn ₂ O ₄ ), and lithium cobalt composite oxide (LiCoO ₂ ) are included as constituent metal elements. A positive electrode active material mainly containing an oxide (lithium transition metal oxide) can be given. Positive electrode active material (typically substantially composed of lithium nickel cobalt manganese composite oxide) mainly composed of lithium nickel cobalt manganese composite oxide (for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) It can be preferably applied to a positive electrode active material.

Among these, application to a lithium nickel composite oxide represented by the general formula Li _{1 + x} (Ni _y Co _z Mn _1-yz-γ M _γ ) O ₂ is preferable. Here, the value of x in the above formula is 0 ≦ x ≦ 0.2, the value of y is 0.3 ≦ y ≦ 1.0, and the value of z is 0 ≦ z ≦ 0.5. , Γ is 0 ≦ γ ≦ 0.2, and 0.3 ≦ y + z + γ ≦ 1. In particular, y in the above general formula is preferably 0.5 ≦ y ≦ 1.0 and a number satisfying 0.5 ≦ y + z + γ ≦ 1. Since the positive electrode active material mainly composed of a lithium nickel composite oxide having a high Ni composition ratio is likely to cause a reaction between the positive electrode active material, moisture, and CO ₂ , it is particularly beneficial to apply this configuration. It is. Note that, for example, when LiNiO ₂ is used as the positive electrode active material, the reaction between the positive electrode active material, moisture, and CO ₂ is considered to occur in two stages. First, LiOH is generated by the reaction of LiNiO ₂ + H ₂ O → NiOOH + LiOH in the first stage, and this corrodes the aluminum foil used as the positive electrode current collector, causing an increase in resistance. Next, Li ₂ CO ₃ is generated by the reaction of 2LiOH + CO ₂ → Li ₂ CO ₃ + H ₂ O in the second stage, and this covers the surface of the positive electrode active material, thereby inhibiting the lithium ion insertion / desorption reaction. Causes further resistance increase. When this configuration is applied, an increase in resistance due to the reaction between the positive electrode active material, moisture, and CO ₂ as described above can be reliably suppressed, and a high-performance lithium secondary battery can be constructed.

  Note that the lithium nickel composite oxide here is an oxide having Li and Ni as constituent metal elements, as shown by the above general formula, and at least one other metal element in addition to Li and Ni (that is, , Oxides containing transition metal elements and / or typical metal elements other than Li and Ni). Such metallic elements are typically Co and Mn. In addition to Co and Mn, the metal element is one or more selected from the group consisting of F, B, Al, W, Mo, Cr, Ta, Nb, V, Zr, Ti, and Y. Can be an element.

  The positive electrode active material particles containing such a lithium transition metal oxide as a main component are particles constituting a conventionally known lithium transition metal oxide powder, except that the positive electrode active material particles are coated with a water-repellent resin as described above. It can be the same property (outer shape). The positive electrode active material particles disclosed herein are, for example, secondary particles (a number of fine particles made of a lithium transition metal oxide are aggregated) having an average particle size in the range of about 2 μm to 10 μm (particularly preferably in the range of 4 μm to 8 μm). Positive electrode active material particles substantially composed of a lithium transition metal oxide substantially composed of a granular powder formed as described above.

  The material constituting the water-repellent resin film disclosed herein is not particularly limited and can be widely used. Among them, a material having relatively high lithium ion conductivity is preferable. A preferred example is a polyvinylidene fluoride resin. As the polyvinylidene fluoride resin, a homopolymer (PVDF) obtained by polymerizing one kind of vinylidene fluoride monomer is preferably used. The polyvinylidene fluoride resin may be a copolymer of a vinyl monomer copolymerizable with vinylidene fluoride. Examples of vinyl monomers copolymerizable with vinylidene fluoride include hexafluoropropylene, tetrafluoroethylene, and ethylene trichloride fluoride. Further, a mixture of two or more of the above homopolymers and copolymers may be used. Instead of the polyvinylidene fluoride resin, resin materials such as polyacrylonitrile and polyamideimide can be used.

  Next, a method for coating the positive electrode active material particles with a water repellent resin will be described. The positive electrode active material particles with a water-repellent resin coating are prepared as a slurry-like mixture (including paste or ink, the same applies hereinafter) in which positive electrode active material particles and water-repellent resin are dispersed and mixed in an appropriate solvent. This can be obtained by drying at an appropriate temperature. The kneading of the slurry mixture can be performed using, for example, a planetary mixer.

  Examples of the solvent used in the slurry mixture include organic solvents such as N-methylpyrrolidone (NMP), pyrrolidone, methyl ethyl ketone, methyl isobutyl ketone, ixahexanone, toluene, dimethylformamide, and dimethylacetamide, or two or more thereof. The combination of is mentioned. Alternatively, water or a mixed solvent mainly composed of water may be used. As a solvent other than water constituting such a mixed solvent, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. Although the content rate of the solvent in a slurry-like mixture is not specifically limited, About 30-80 mass% of the whole slurry is preferable.

  Here, as described above, the positive electrode active material particles with a water-repellent resin coating disclosed herein have an average particle diameter of the positive electrode active material particles of A [μm] and a mass of B [g]. When the mass is C [g] and the crystallinity of the water-repellent resin is D [%], the relational expression of 3.0 ≦ A × (C / B) × D ≦ 10.0 is satisfied. obtain.

  Among the above-described relational expressions, the average particle diameter (A) of the positive electrode active material particles can be adjusted by changing the average particle diameter of the positive electrode active material particles added to the slurry mixture. That is, the average particle diameter (A) of the positive electrode active material particles coated with the water repellent resin is appropriately disclosed by appropriately selecting the average particle diameter of the positive electrode active material particles added to the slurry mixture. Can be adjusted within a wide range. Preferably, a positive electrode active material powder in the form of fine particles having an average particle diameter of about 4 μm to 8 μm is added to prepare a slurry mixture. Here, the average particle diameter refers to the median diameter (d50), and can be easily measured by a particle size distribution measuring apparatus based on various commercially available laser diffraction / scattering methods.

  In the relational expression described above, the mass ratio (C / B) between the water-repellent resin coating and the positive electrode active material particles is the blending amount of the water-repellent resin (water repellent resin / positive electrode active material particles) added to the slurry mixture. It can be adjusted by changing the blending ratio. Generally, as the blending amount of the water repellent resin added to the slurry mixture (mixing ratio of the water repellent resin / positive electrode active material particles) increases, the mass ratio (C / B) between the water repellent resin coating and the positive electrode active material particles increases. To do. Therefore, by appropriately selecting the blending amount of the water-repellent resin (mixing ratio of the water-repellent resin and the positive electrode active material particles) to be added to the slurry mixture, the mass ratio of the water-repellent resin film and the positive electrode active material particles ( C / B) can be adjusted to the preferred range disclosed herein. Preferably, the blended amount of water repellent resin (water repellent resin / positive electrode active material particles) so that the mass ratio (C / B) between the water repellent resin coating and the positive electrode active material particles is 0.01 to 0.04. A slurry-like mixture may be prepared by appropriately selecting a blending ratio). The mass ratio (C / B) between the water-repellent resin coating and the positive electrode active material particles can be easily grasped from, for example, the mass reduction rate based on thermogravimetric analysis (TGA) when the water-repellent resin coating is thermally decomposed. be able to.

Of the above-described relational expressions, the crystallinity (D) of the water-repellent resin can be adjusted by changing the drying conditions (drying temperature, drying time, etc.) when the slurry-like mixture is dried. That is, the drying conditions (drying temperature, drying time, etc.) of the slurry mixture are one important factor from the viewpoint of controlling the crystallinity (D) of the water-repellent resin. By appropriately selecting the drying conditions (drying temperature, drying time, etc.) of the slurry-like mixture, the crystallinity (D) of the water-repellent resin can be adjusted to a suitable range disclosed herein. Preferably, the drying temperature of the slurry-like mixture may be determined within the range of 110 ° C to 160 ° C (more preferably 140 ° C to 160 ° C). Thereby, when the slurry-like mixture is dried, positive electrode active material particles with a water-repellent resin film are formed so that the crystallinity (D) of the water-repellent resin is 40% to 70% (more preferably 40% to 65%). can do. The crystallinity (D) of the water repellent resin can be easily grasped by, for example, measurement of heat of fusion using a commercially available differential thermal scanning calorimetry (DSC) apparatus.
The dried agglomerates obtained by the drying treatment as described above are preferably cooled and then lightly pulverized in a mortar or the like to obtain positive electrode active material particles with a water-repellent resin coating.

In addition, according to the technique disclosed here, the positive electrode active material particles with a water-repellent resin coating prepared so as to satisfy the relational expression of 3.0 ≦ A × (C / B) × D ≦ 10.0 are included. A method for manufacturing a lithium secondary battery including a positive electrode may be provided.
The manufacturing method includes forming positive electrode active material particles with a water-repellent resin coating prepared so as to satisfy the relational expression of 3.0 ≦ A × (C / B) × D ≦ 10.0; and
Constructing a lithium secondary battery using a positive electrode containing the positive electrode active material particles with the water-repellent resin coating;
Is included.

  Here, the positive electrode active material particles with a water-repellent resin film prepared so as to satisfy the relational expression of 3.0 ≦ A × (C / B) × D ≦ 10.0 are positive electrode active materials added to the slurry mixture. Average particle diameter (median diameter: d50) of material particles, blending amount of water repellent resin added to slurry mixture (mixing ratio of water repellent resin / positive electrode active material particles), drying conditions for drying slurry mixture ( At least one of the drying temperature and the like is set so that the appropriate range is realized, and the positive electrode active material particles with a water-repellent resin coating are formed according to the set conditions.

  Accordingly, the matters disclosed herein include positive electrode active material particles with a water-repellent resin coating prepared so as to satisfy the relational expression of 3.0 ≦ A × (C / B) × D ≦ 10.0. A method for producing a lithium secondary battery including a positive electrode, comprising: an average particle diameter (median diameter: d50) of positive electrode active material particles added to a slurry mixture; and a blending amount of a water repellent resin added to the slurry mixture (repellency). Setting ratio of aqueous resin / positive electrode active material particles) and / or drying conditions (drying temperature, etc.) for drying the slurry mixture so that the appropriate range is realized, Forming a positive electrode active material particle with a water-repellent resin coating in accordance with the set conditions.

The positive electrode active material particles with a water-repellent resin film disclosed herein satisfy the relational expression 3.0 ≦ A × (C / B) × D ≦ 10.0, and the thickness of the water-repellent resin film and the water-repellent resin Therefore, the lithium ion permeability is good, and the permeation of moisture and CO ₂ can be preferably suppressed. From this, the positive electrode active material particle with a water-repellent resin film disclosed here can be suitably used as a positive electrode active material of a lithium secondary battery (typically a lithium ion battery).

  A lithium secondary battery can be constructed by adopting the same materials and processes as in the prior art except that the positive electrode active material particles with a water-repellent resin coating disclosed herein are used.

  For example, carbon black such as acetylene black or ketjen black or other (graphite or the like) powdered carbon material may be mixed as a conductive material with the powder composed of the positive electrode active material particles with a water-repellent resin coating disclosed herein. it can. In addition to the positive electrode active material and the conductive material, a binder (binder) such as polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC) is added. be able to. By dispersing these in a suitable dispersion medium and kneading, the composition for forming a positive electrode active material layer (hereinafter referred to as “positive electrode active material layer forming paste”) is in the form of a paste (including slurry or ink. The same shall apply hereinafter). Can be prepared.). An appropriate amount of this paste is preferably applied onto a positive electrode current collector made of aluminum or an alloy containing aluminum as a main component, and further dried and pressed, whereby a positive electrode for a lithium secondary battery can be produced.

  On the other hand, the negative electrode for a lithium secondary battery serving as a counter electrode can be produced by a method similar to the conventional one. For example, the negative electrode active material may be any material that can occlude and release lithium ions. A typical example is a powdery carbon material made of graphite or the like. In particular, graphite particles can be a negative electrode active material that is more suitable for rapid charge / discharge (for example, high-power discharge) because of its small particle size and large surface area per unit volume.

  As in the case of the positive electrode, the powdery material is dispersed in a suitable dispersion medium together with a suitable binder (binder) and kneaded to obtain a paste-like composition for forming a negative electrode active material layer (hereinafter referred to as “negative electrode active material”). May be referred to as “layer forming paste”). An appropriate amount of this paste is preferably applied onto a negative electrode current collector composed of copper, nickel, or an alloy thereof, and further dried and pressed, whereby a negative electrode for a lithium secondary battery can be produced.

  In the lithium secondary battery using the positive electrode active material particles with the water-repellent resin film of this configuration, the same separator as the conventional one can be used. For example, a porous sheet (porous film) made of a polyolefin resin can be used.

Further, as the electrolyte, the same electrolyte as a non-aqueous electrolyte (typically, an electrolytic solution) conventionally used for a lithium secondary battery can be used without any particular limitation. Typically, the composition includes a supporting salt in a suitable nonaqueous solvent. Examples of the non-aqueous solvent include one or two selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and the like. More than seeds can be used. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ). ₃ , 1 type, or 2 or more types of lithium compounds (lithium salt) selected from LiI etc. can be used.

  Moreover, as long as the lithium nickel cobalt manganese composite oxide disclosed here is adopted as the positive electrode active material, the shape (outer shape and size) of the lithium secondary battery to be constructed is not particularly limited. The outer package may be a thin sheet type constituted by a laminate film or the like, and the battery outer case may be a cylindrical or cuboid battery, or may be a small button shape.

  Hereinafter, the usage mode of the positive electrode active material particles with a water-repellent resin film disclosed herein will be described with reference to a schematic diagram shown in FIG. 3, using a lithium secondary battery including a wound electrode body as an example. It is not intended that the invention be limited to such embodiments.

  As shown in the drawing, the lithium secondary battery 100 according to the present embodiment includes a case 82 made of metal (a resin or a laminate film is also suitable). The case (outer container) 82 includes a flat cuboid case main body 84 having an open upper end, and a lid 86 that closes the opening. On the upper surface of the case 82 (that is, the lid body 86), a positive electrode terminal 72 that is electrically connected to the positive electrode 70 of the electrode body 80 and a negative electrode terminal 74 that is electrically connected to the negative electrode 50 of the electrode body are provided. In the case 82, for example, a long sheet-like positive electrode (positive electrode sheet) 70 and a long sheet-like negative electrode (negative electrode sheet) 50 are laminated together with a total of two long sheet-like separators (separator sheets) 76. A flat wound electrode body 80 produced by winding and then crushing the resulting wound body from the side direction and kidnapping is housed.

  The negative electrode sheet 50 has a configuration in which a negative electrode active material layer mainly composed of a negative electrode active material is provided on both surfaces of a long sheet-like negative electrode current collector. Similarly to the negative electrode sheet, the positive electrode sheet 70 has a configuration in which a positive electrode mixture layer mainly composed of a positive electrode active material is provided on both surfaces of a long sheet-like positive electrode current collector. At one end of these electrode sheets 50 and 70 in the width direction, an electrode mixture layer non-formed portion where the electrode mixture layer is not provided on any surface is formed.

  In the above lamination, the positive electrode sheet 70 and the negative electrode mixture layer non-formed portion of the positive electrode sheet 70 and the negative electrode mixture layer non-formed portion of the negative electrode sheet 50 protrude from both sides of the separator sheet 76 in the width direction. The negative electrode sheet 50 is overlaid with a slight shift in the width direction. As a result, in the lateral direction with respect to the winding direction of the wound electrode body 80, the electrode mixture layer non-formed portions of the positive electrode sheet 70 and the negative electrode sheet 50 are respectively wound core portions (that is, the positive electrode mixture layer forming portion of the positive electrode sheet 70). And a portion where the negative electrode active material layer forming portion of the negative electrode sheet 50 and the two separator sheets 76 are wound tightly). A positive electrode lead terminal 78 and a negative electrode lead terminal 79 are respectively attached to the protruding portion (that is, the non-forming portion of the positive electrode mixture layer) 70A and the protruding portion (that is, the non-forming portion of the negative electrode active material layer) 50A. Are electrically connected to the positive terminal 72 and the negative terminal 74 described above.

  Then, the wound electrode body 80 is accommodated in the main body 84 from the upper end opening of the case main body 84 and an electrolytic solution containing an appropriate electrolyte is disposed (injected) in the case main body 84. Thereafter, the opening is sealed by welding or the like with the lid 86, and the assembly of the lithium secondary battery 100 according to the present embodiment is completed. The sealing process of the case 82 and the process of placing (injecting) the electrolyte may be the same as those used in the production of a conventional lithium secondary battery, and do not characterize the present invention. In this way, the construction of the lithium secondary battery 100 according to this embodiment is completed.

  Hereinafter, although the test example regarding this invention is demonstrated, it is not intending to limit this invention to what is shown to the following test examples.

<Test Example 1: Production of positive electrode active material particles with PVDF coating>
Li _1.05 Ni _0.8 Co _0.15 Al _0.05 O ₂ (hereinafter abbreviated as LNO) as a positive electrode active material and polyvinylidene fluoride (PVDF) as a water repellent resin are N- A slurry mixture (solid content concentration of about 10% by mass) was prepared by mixing in methylpyrrolidone (NMP) with a planetary mixer. The slurry mixture was dried at a predetermined temperature in a reduced pressure atmosphere. After the drying, the dried aggregate was lightly pulverized in a mortar to obtain a positive electrode active material powder with a PVDF coating.

  In this test, a positive electrode active material powder with a PVDF coating was prepared by the above method using three types of LNO powders having an average particle diameter (A) based on the laser diffraction / scattering method of 4 μm, 6 μm, or 8 μm. . Moreover, the positive electrode with a PVDF film by the said method, varying the compounding quantity of PVDF powder so that the compounding ratio of PVDF powder: LNO powder may be either 1: 100, 1:50, 3: 100, 1:25. An active material powder was prepared. Furthermore, the drying temperature of the slurry-like mixture was set to any one of a temperature range of 110 ° C. to 160 ° C., and a positive electrode active material powder with a PVDF coating was produced by the above method.

<Test Example 2: Calculation of mass ratio (C / B) and crystallinity (D)>
Thermogravimetric analysis (TGA) was performed on the various positive electrode active material powders with PVDF coating prepared in Test Example 1 above, and the mass ratio (C / B) between the PVDF coating and the positive electrode active material powder was examined. Specifically, as shown in FIG. 3, the actual PVDF film and the positive electrode active material powder were obtained from the mass reduction rate when the PVDF film-coated positive electrode active material powder was heated from room temperature to 500 ° C. to decompose and remove the PVDF film. The mass ratio (C / B) was calculated.

Moreover, the differential thermal scanning calorimetry (DSC) was performed about the various positive electrode active material powder with a PVDF film produced in the said Experiment 1, and the crystallinity (D) of PVDF was investigated. Specifically, as shown in FIG. 4, the heat of fusion ΔH is determined from the melting endothermic peak (shaded portion in the figure) near 160 ° C. when the PVDF-coated positive electrode active material powder is heated, and the heat of fusion ΔH, The degree of crystallinity D = [ΔH / ΔH ₁ ] × 100 was calculated from the theoretically calculated heat of fusion ΔH ₁ of the complete PVDF crystal. The results are shown in Tables 1 to 3. Here, A: average particle diameter (μm) of positive electrode active material powder, C / B: mass ratio of PVDF coating to positive electrode active material powder, Y = A × (C / B) value, T: drying temperature ( C), D: PVDF crystallinity (%), X = A × (C / B) × D.

<Test Example 3: Measurement of Li ₂ CO ₃ generation amount>
Various positive electrode active material powders with PVDF coating produced in Test Example 1 were immersed in water, and the amount of Li ₂ CO ₃ generated by the reaction of the positive electrode active material powder, water, and CO ₂ was measured. Specifically, an amount of 1 g of the positive electrode active material powder excluding the PVDF film of the positive electrode active material powder with a PVDF film was added to 100 g of water. After stirring for 10 minutes, filtered to remove the positive electrode active material powder, to precipitate _Li 2 CO ₃ was added to BaCl ₂ to the filtrate obtained as BaCO _3. Thereafter, neutralization titration (BaCO ₃ + 2HCl → BaCl ₂ + H ₂ O + CO ₂ ) was performed with 1M hydrochloric acid, and the amount of Li ₂ CO ₃ generated by the reaction between the positive electrode active material powder and water was measured. The results are shown in FIG. 5 and Tables 1 to 3. The horizontal axis of the graph of FIG. 5 is a value of X = A × (C / B) × D of the tested positive electrode active material powder with a PVDF coating, and the right vertical axis is the unit mass (gram) of the positive electrode active material powder. It is a ratio (wt%) of the amount of generated Li ₂ CO ₃ per unit. It can be said that the smaller the proportion (wt%) of Li ₂ CO ₃ generated, the more the reaction between the positive electrode active material powder, water and CO ₂ is suppressed.

As apparent from the graph shown in FIG. 5, when the value of X = A × (C / B) × D was less than 3.0, the ratio of the amount of Li ₂ CO ₃ generated significantly increased. On the other hand, when the value of X was 3.0 or more, the ratio of the amount of Li ₂ CO ₃ generated was suppressed to 0.22 wt% or less.

<Test Example 4: Measurement of reaction resistance (Rct)>
Using the various positive electrode active material powders with PVDF coating produced in Test Example 1, test lithium secondary batteries were constructed. And the alternating current impedance measurement was performed about each battery for a test, and the reaction resistance of those batteries was evaluated. The test lithium ion battery was constructed as follows.

  First, a positive electrode active material powder with a PVDF coating, acetylene black as a conductive material, and PVDF powder as a binder are N-methylpyrrolidone (96: 3: 1) so that the mass ratio of these materials is 96: 3: 1. NMP) to prepare a positive electrode active material layer forming paste. This paste is applied to a positive electrode current collector (aluminum foil having a thickness of about 20 μm) and pressed to volatilize NMP, so that a positive electrode active material layer having a thickness of about 100 μm is formed on both surfaces of the positive electrode current collector Was made.

  On the other hand, flaky graphite as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethylcellulose (CMC) as a thickener have a mass ratio of 98: 1: 1. Thus, a negative electrode active material layer forming paste was prepared by dispersing in water. The paste is applied to a negative electrode current collector (copper foil having a thickness of about 15 μm) and pressed to volatilize water, and a negative electrode sheet in which a negative electrode active material layer having a thickness of about 100 μm is formed on one surface of the negative electrode current collector Was made.

Next, the positive electrode active material layer of the positive electrode sheet was punched out to 3 cm × 4 cm to produce a positive electrode. Moreover, the negative electrode active material layer of the negative electrode sheet was punched out to 3 cm × 4 cm to produce a negative electrode. An aluminum lead is attached to the positive electrode, a nickel lead is attached to the negative electrode, they are arranged facing each other through a separator (a porous polyethylene sheet is used), and inserted into a laminate bag together with a non-aqueous electrolyte, as shown in FIG. A laminate cell 60 was constructed. In FIG. 6, reference numeral 61 indicates a positive electrode, reference numeral 62 indicates a negative electrode, reference numeral 63 indicates a separator impregnated with an electrolytic solution, and reference numeral 64 indicates a laminate bag. In addition, as a non-aqueous electrolytic solution, LiPF ₆ as a supporting salt is approximately mixed with a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3: 4: 3. The one contained at a concentration of 1 mol / liter was used. For reference, a lithium secondary battery was constructed using a positive electrode active material powder not coated with PVDF (a positive electrode active material powder without a PVDF film). A lithium secondary battery was constructed in the same manner as described above except that a positive electrode active material powder without a PVDF coating was used.

  The alternating current impedance of the lithium secondary batteries thus produced was measured at −15 ° C., and their reaction resistance (Rct) was evaluated. The AC impedance measurement conditions were an AC applied voltage of 10 mV and a frequency range of 0.001 Hz to 100,000 Hz. The results are shown in the graph of FIG. The horizontal axis of the graph of FIG. 5 is a value of X = A × (C / B) × D of the tested positive electrode active material powder with a PVDF coating, and the left vertical axis is the reaction resistance (Rct) ratio. This reaction resistance (Rct) ratio is a ratio when the reaction resistance value of a lithium secondary battery constructed using a positive electrode active material powder without a PVDF film is 1.

  As is clear from FIG. 5 and Tables 1 to 3, the reaction resistance ratio of the lithium secondary battery having a value of X = A × (C / B) × D exceeding 10.0 was greatly increased. In addition, the reaction resistance ratio of the lithium secondary battery having an X value of less than 3.0 was greatly increased.

  On the other hand, the reaction resistance ratio of the lithium secondary battery satisfying the relational expression of 3.0 ≦ X ≦ 10.0 was suppressed to 1.1 or lower. Particularly in a lithium secondary battery that satisfies 3.0 ≦ X ≦ 10.0 and satisfies 0.05 ≦ Y ≦ 0.20, an extremely low reaction resistance ratio of 1.08 or less can be realized, and the battery performance is favorable. It was confirmed that it improved.

  As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible.

  Since any of the lithium secondary batteries disclosed herein can reduce the reaction resistance of the battery as described above, the lithium secondary battery has a performance suitable as a battery mounted on a vehicle that requires high-rate charge / discharge. . Therefore, according to the present invention, as shown in FIG. 7, a vehicle 1 provided with any of the lithium secondary batteries 100 disclosed herein (which may be in the form of an assembled battery to which a plurality of lithium secondary batteries are connected). Is provided. In particular, a vehicle (for example, an automobile) including the lithium secondary battery as a power source (typically, a power source of a hybrid vehicle or an electric vehicle) is provided.

DESCRIPTION OF SYMBOLS 1 Vehicle 10 Positive electrode active material particle 12 Water repellent resin film 50 Negative electrode 50 Negative electrode sheet 60 Laminating cell 70 Positive electrode sheet 72 Positive electrode terminal 74 Negative electrode terminal 76 Separator sheet 78 Positive electrode lead terminal 79 Negative electrode lead terminal 80 Winding electrode body 82 Case 84 Case main body 86 Lid 100 Lithium Secondary Battery

Claims (9)

A lithium secondary battery including a positive electrode including positive electrode active material particles,
The positive electrode active material particles have a surface coated with a water-repellent resin,
Here, the average particle diameter (median diameter: d50) of the positive electrode active material particles coated with the water repellent resin is A [μm], the mass thereof is B [g], and
When the mass of the water repellent resin film covering the positive electrode active material particles is C [g] and the crystallinity of the water repellent resin is D [%], 3.0 ≦ A × (C / B) A lithium secondary battery satisfying a relational expression of × D ≦ 10.0.   2. The lithium secondary battery according to claim 1, wherein a relational expression of 0.05 ≦ A × (C / B) ≦ 0.20 is established between the average particle diameter A and the masses B and C. 3. The positive electrode active material particles have a general formula Li _{1 + x} (Ni _y Co _z Mn _1-yz-γ M _γ ) O ₂ (where 0 ≦ x ≦ 0.2, 0.3 ≦ y ≦ 1.0, 0 ≦ z ≦ 0.5, 0 ≦ γ ≦ 0.2, 0.3 ≦ y + z + γ ≦ 1, M is from F, B, Al, W, Mo, Cr, Ta, Nb, V, Zr, Ti, Y 3. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is a lithium nickel composite oxide.   The lithium secondary battery according to claim 3, wherein y is 0.5 ≦ y ≦ 1.0 and 0.5 ≦ y + z + γ ≦ 1 in the general formula.   The lithium secondary battery according to claim 1, wherein the water repellent resin is a polyvinylidene fluoride resin.   The lithium secondary battery according to any one of claims 1 to 5, wherein an average particle diameter A of the positive electrode active material particles is 4 µm to 8 µm.   7. The lithium secondary battery according to claim 1, wherein a mass ratio (C / B) between the water-repellent resin film and the positive electrode active material particles is 0.01 to 0.04.   The lithium secondary battery according to any one of claims 1 to 7, wherein a crystallinity D of the water repellent resin is 40% to 70%.   A vehicle comprising the lithium secondary battery according to claim 1.

Images