Battery active material

Abstract

This invention provides a composite active material with excellent battery properties, having conductive paths within the active material particles and between the active material particles. [Solution] A battery active material comprising primary particles of a battery active material and secondary particles of a composite structure of a conductive material, wherein the conductive material constitutes external protrusions at least two locations on the secondary particles, forming a continuum in which the external protrusions are electrically conductive, and furthermore, the conductive material has portions longer than the particle diameter of the secondary particles and is fibrous carbon.

Description

Technical Field

[0001] The present invention relates to a battery active material, and more specifically, relates to an active material that imparts conductivity to a battery active material (hereinafter also simply referred to as "active material") and can secure conductive paths even when formed into an aggregate composed of a large number of active materials.

Background Art

[0002] As conventional techniques, a technique of securing a conductive path from an active material to a current collector by arranging a conductive substance (granular or fibrous) on or around the surface of the active material (for example, see Patent Document 1), and a technique of improving the conductivity of active material particles by compositing a conductive substance (granular or fibrous) with active material particles (for example, see Patent Document 2) are widely known.

[0003] However, in such conventional techniques, there are problems that aggregation of active material particles in the electrode generates active material particles that have no conductive path to the current collector, or that the presence of an active material located at a position away from the conductive substance inside the active material particles composited with the conductive substance reduces the electron conductivity of the active material particles.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of Invention

Problems to be Solved by the Invention

[0005] The present invention has been made to correct the above-described conventional problems, and provides a composite active material that has excellent battery characteristics and is provided with conductive paths inside active material particles and between active materials.

Means for Solving the Problems

[0006] In other words, according to one aspect of the present invention for solving the above problems, the first aspect of the present invention is a battery active material comprising primary particles of a battery active material and secondary particles of a composite structure of a conductive material, wherein the conductive material constitutes external protrusions at least two locations on the secondary particles, and the space between the external protrusions constitutes a continuum in which electrical conductivity is maintained.

[0007] A second aspect of the present invention is a battery active material characterized in that the conductive material in the first aspect comprises a portion longer than the particle diameter of the secondary particles.

[0008] A third aspect of the present invention is a battery active material characterized in that the conductive material in the first or second aspect is fibrous carbon. [Effects of the Invention]

[0009] According to the present invention, by having a conductive material with a length greater than or equal to the particle diameter of the secondary particles of the active material penetrate the secondary particles of the active material, it is possible to prevent a decrease in the conductivity of the electrode due to aggregation of the secondary particles of the active material. Furthermore, even when the electronic conductivity of the active material is extremely low, the electrical resistance of the conductive path of the primary particles within the secondary particles to the current collector can be reduced. [Brief explanation of the drawing]

[0010] [Figure 1] This figure shows an active material through which a conductive material according to the present invention has penetrated, where (a) is a cross-sectional view and (b) is a front view showing the protruding portion 10 of the conductive material. [Figure 2] (a) is a diagram showing the formation of a conductive path by an active material through which the conductive material according to the present invention has penetrated, and (b) is a diagram showing the formation of a conductive path by an active material in which the conductive material has not penetrated, according to a comparative example. [Modes for carrying out the invention]

[0011] Figure 1 shows a cross-sectional diagram of the active material through which the conductive material according to the present invention has penetrated. In Figure 1(a), 1 is an active material through which the conductive material according to the present invention has penetrated, and it shows secondary particles formed by the aggregation of primary particles 3. 2 is the conductive material, which is in contact with the surface of the primary particles of the active material (conductive state), and is incorporated during the formation of secondary particles, penetrating the secondary particles and acting as a component that brings about the conductivity of the active material. In Figure 2, 4 is a conductive additive, which is responsible for imparting conductivity to the electrode, which is almost entirely formed by secondary particles, and carbon particles are typically used. 5a indicates a region where the conductive additive does not contribute to the formation of conductive pathways between primary particles of the active material, but conductive pathways are formed by the active material through which the conductive material according to the present invention has penetrated.

[0012] The present invention relates to a battery active material comprising primary particles of a battery active material and secondary particles having a composite structure of a conductive material, characterized in that the conductive material forms external protrusions 10, 10 at at least two locations on the secondary particle 1 as shown in Figure 1(b), and the space between the external protrusions forms a continuum in which electrical conductivity is maintained.

[0013] Furthermore, the present invention is characterized by the length of the conductive material that penetrates the active material (secondary particles). While conventional techniques composite active material particles with conductive material (granular or fibers with a length less than or equal to the secondary particle size of the active material), the present invention uses fibrous or rod-shaped conductive material that is longer than the particle diameter (diameter of the secondary particles) of the active material (secondary particles), and composites them in a manner that appears as if it penetrates the secondary particles.

[0014] In the active material according to the present invention, which has these characteristics, when secondary particles constituting the electrode gather together to form an aggregated portion 5a with a small amount of conductive additive as shown in Figure 2(a), the conductive material 2 penetrates the secondary particles 1 which cooperate to form the aggregated portion 5a, so that the conductivity is not impaired. Furthermore, as shown in Figure 1(a), the long fibrous or rod-shaped conductive material 2 penetrates the secondary particle 1, causing many primary particles 3 within the secondary particle 1 to come into contact with the conductive material. As a result, the active material according to the present invention contributes to increasing battery capacity and improving charge-discharge cycle characteristics. On the other hand, as shown in Figure 2(b) relating to the conventional technology, in the aggregated portion 5b, which is mainly composed of secondary particles 1a with few conductive additives and in which the conductive material does not penetrate, electrical conductivity between the secondary particles cannot be obtained. Furthermore, in the conventional technology, since the active material particles are compounded with a conductive material consisting of granular or fibers with a length less than or equal to the secondary particle size of the active material, the proportion of primary particles that do not come into contact with the conductive material within the secondary particles becomes high. As a result of the above, improvements in battery capacity and cycle characteristics cannot be obtained with the active material of the conventional technology. [Examples]

[0015] The following describes in detail an embodiment of the present invention, namely a "battery active material," with reference to examples. However, the present invention is not limited to these examples. [Examples]

[0016] 50 g of ferric phosphate, 13 g of lithium carbonate (as a lithium source), 12 g of sucrose (as a carbon source), and 75 ml of deionized water were mixed and milled using a ball mill to adjust the particle size so that the median diameter D50 was 200 nm, thereby obtaining slurry X. 200 g of deionized water and 0.5 g of carbon fibers (VGCF, manufactured by Resonaq) with an average diameter of 150 nm and an average length of 4 μm were added to slurry X and mixed with an agitator to obtain slurry Y. Slurry Y was spray-dried using a spray dryer (PL-2 / CPL-2, manufactured by Okawara Chemical Machinery Co., Ltd.) at an inlet temperature of 200 °C to obtain a precursor powder with a median diameter D50 of 15 μm. The obtained precursor powder was sintered at 650 °C in a nitrogen atmosphere for 10 hours. After natural cooling, it was pulverized using a jet mill, and then carbon fibers that were not compounded with lithium iron phosphate were removed by air classification to obtain a battery active material consisting of lithium iron phosphate and carbon fibers. In this embodiment, the battery active material had a median diameter D50 of 2 μm in a roughly spherical portion made of lithium iron phosphate, and carbon fibers penetrated the roughly spherical portion.

[0017] (Comparative example) A battery active material composed of lithium iron phosphate was obtained through the same steps as the production method of Example 1, except that the above slurry X was spray-dried without adding carbon fibers and air flow classification was not performed. The obtained battery active material according to the comparative example had a median diameter D50 of 2 μm for the approximately spherical portion composed of lithium iron phosphate.

[0018] [Evaluation] Battery electrodes were produced and evaluated using the battery active materials produced by the production methods according to Example 1 and the Comparative Example, respectively. The battery electrode was produced by mixing the battery active material and a conductive auxiliary agent (carbon black, "Denka Black" manufactured by Denka) in a mortar, then adding an NMP (N-methyl-2-pyrrolidone) solution of PVDF (polyvinylidene fluoride) ("L#7208" manufactured by Kureha) as a binder, kneading and rolling the mixture in a mortar, and molding it into a disk shape with a diameter of 1 cm. A battery was fabricated using the battery electrode dried for 1 hour in a 100°C vacuum electric furnace as the positive electrode, a negative electrode made of a circular lithium foil larger than the battery electrode, and a separator made of a circular polyethylene microporous membrane larger than the negative electrode. As the electrolytic solution, 1 M LiPF6 / (EC (ethylene carbonate)-EMC (ethyl methyl carbonate)) (EC:EMC = 3:7 (volume ratio)) was used.

[0019] The following battery performances were evaluated using the batteries fabricated using the respective battery electrodes of Example 1 and Comparative Example, and the results are shown in Table 1. First, after charging to 3 V at a charge rate of 1 C, the battery was left in an open circuit state for 1 second, and then discharged to 1 V at 1 C to measure the initial discharge capacity. The direct current resistance of the battery was determined by dividing the difference between the battery voltage before the start of discharge and the battery voltage 5 seconds after the start of discharge by the discharge current value. After the initial discharge capacity measurement, 100 cycles of charge and discharge were performed under the same conditions, and the capacity retention rate was determined by dividing the discharge capacity at the 100th cycle by the initial discharge capacity.

[0020] The DC resistance of the battery using the battery active material of Example 1 is lower than that of the battery using the battery active material of the Comparative Example. This is thought to be because, in the battery active material manufactured by the manufacturing method of the Comparative Example, carbon fibers were not added, resulting in a region (reference numeral 5b in Figure 2) within the electrode where there is no conductive material between secondary particles and the presence of fewer conductive additives, thus increasing the electrical resistance. Furthermore, the battery using the battery active material of Example 1 was superior to the battery using the battery active material of the Comparative Example in terms of initial discharge capacity and capacity retention rate after 100 charge-discharge cycles. This is thought to be because the battery using the battery active material of Example 1 had a higher proportion of primary particles that ensured a conductive path to the current collector compared to the battery using the battery active material of the Comparative Example.

[0021] [Table 1] [Explanation of Symbols]

[0022] 1. Active material according to the present invention (penetrating conductive active material / secondary particles) 1a Active material (non-penetrating conductive active material / secondary particles) 2 Conductive substances 3 Primary particles 4 Conductive additives 5a Aggregation of the active material according to the present invention when the conductive additive is small 5b Aggregation of active material according to the conventional technology when the conductive additive is small. 10 External protrusion

Claims

1. A battery active material comprising primary particles of a battery active material and secondary particles of a composite structure of a conductive material, The battery active material is characterized in that the conductive material has external protrusions at least two locations on the secondary particles, and the area between the external protrusions is electrically conductive, forming a continuum.

2. The battery active material according to claim 1, characterized in that the conductive material has portions longer than the particle diameter of the secondary particles.

3. The battery active material according to claim 1 or 2, characterized in that the conductive material is fibrous carbon.

Images