JP7731941B2 - Precursor of positive electrode active material and manufacturing method - Google Patents

Description

The present invention relates to a precursor for a positive electrode active material and a method for producing the same.

Patent Document 1 discloses a method for producing a precursor of a positive electrode active material for a non-aqueous electrolyte secondary battery. One of the features of this precursor is that it has a layered structure consisting of a central portion formed by the aggregation of plate-like primary particles, a low-density portion formed by the aggregation of fine primary particles outside the central portion, and a high-density portion formed by the aggregation of the plate-like primary particles. Patent Document 2 also discloses a precursor for producing a porous positive electrode active material. For example, a positive electrode active material can be produced by mixing the precursor with a Li compound and firing the mixture.

JP 2016-154143 A Japanese Patent Application Laid-Open No. 2019-021610

However, when producing a positive electrode active material by firing a precursor of the positive electrode active material, the reaction with Li may be insufficient, resulting in insufficient output characteristics.

Therefore, the main objective of this disclosure is to provide technology that can realize lithium-ion secondary batteries with excellent output characteristics.

One aspect of the technology disclosed herein is a precursor for a positive electrode active material used in a lithium-ion secondary battery. This precursor contains Ni-containing transition metal composite hydroxide particles, and the transition metal composite hydroxide particles have an outer shell portion formed by aggregation of primary particles having an average aspect ratio (major axis length/minor axis length) of 2 or more based on SEM observation, and a hollow portion surrounded by the outer shell portion. 50% or more of the primary particles are oriented radially from the center of the transition metal composite hydroxide particle toward the surface, and 90% or more of the primary particles contained in the entire transition metal composite hydroxide particle are present in the outer shell portion.

The transition metal composite hydroxide particles (also referred to as precursor particles) contained in the precursor have 90% or more of the primary particles present in the outer shell, and have hollow sections with few or no primary particles. This increases the reactive surface area of the precursor particles, improving the reaction with Li during firing and suppressing unreacted Li. Furthermore, because 50% or more of the primary particles in the outer shell are radially oriented, Li can diffuse more easily, improving output characteristics.

FIG. 1 is a schematic diagram of a cross-sectional structure of a precursor particle of a positive electrode active material according to one embodiment. FIG. 2 is a flow diagram showing an outline of a method for producing a positive electrode active material used in a lithium ion secondary battery in one embodiment. FIG. 3 is a schematic diagram showing the general configuration of a crystallizer. FIG. 4 is a schematic cross-sectional view taken along line IV-IV shown in FIG. FIG. 5 is a cross-sectional view that schematically shows the configuration of a lithium ion secondary battery that uses the fired product of precursor particles according to one embodiment. FIG. 6 is a schematic exploded view showing the configuration of the wound electrode body 20. FIG. 7 is a cross-sectional SEM image of the precursor particles of Example 1.

Embodiments of the technology disclosed herein are described below with reference to the drawings. Matters necessary for implementing the technology other than those specifically mentioned in this specification (for example, the general configuration and manufacturing process of lithium-ion secondary batteries that do not characterize the technology) can be understood as design matters for those skilled in the art based on conventional technology in the relevant field. The technology can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. Note that the drawings are schematic, and dimensional relationships (length, width, thickness, etc.) do not necessarily reflect actual dimensional relationships. Furthermore, in the drawings described below, components and parts that perform the same function are designated by the same reference numerals, and redundant descriptions may be omitted or simplified.

In this specification, when a numerical range is stated as "A to B (where A and B are arbitrary numbers)," it means "greater than A and less than B," and also encompasses the meanings of "greater than A and less than B," "greater than A and less than B," and "greater than A and less than B."

In this specification, the term "lithium ion secondary battery" refers to all secondary batteries that use lithium ions as charge carriers and are charged and discharged by the transfer of charge associated with the lithium ions between positive and negative electrodes, and is a term that also encompasses lithium ion capacitors, etc.

The present disclosure provides a precursor material for a positive electrode active material used in a lithium-ion secondary battery. The precursor material is, for example, in powder form. The precursor material includes a plurality of precursor particles. Figure 1 is a schematic diagram of the cross-sectional structure of a precursor particle 200 for a positive electrode active material according to one embodiment. The precursor particle 200 has a hollow structure including an outer shell portion 210 and a hollow portion 220.

The precursor material contains precursor particles 200 as a main component. The precursor particles 200 account for, for example, 80 wt% or more, 90 wt% or more, 95 wt% or more, or 100 wt% of the precursor material.

The precursor particles 200 are particles containing a transition metal composite hydroxide (also referred to as "transition metal composite hydroxide particles 200"). The precursor particles 200 contain a transition metal, and preferably contain at least nickel (Ni). The precursor particles 200 are converted into a lithium transition metal composite oxide by firing in the presence of lithium element (Li element). This lithium transition metal composite oxide can be used as a positive electrode active material.

In some embodiments, precursor particles 200 have the following general formula (i):
Ni _(1-x-y-z) Co _x Mn _y A _z (OH) _2+a (i)
It has a composition represented by the following formula (i): where, in general formula (i), 0≦x<0.5, 0≦y<0.5, 0≦z≦0.1, 0≦a≦0.5, and x+y+z<1 are satisfied. In general formula (i), A represents at least one element selected from the group consisting of Mg, Ca, V, Cr, Al, Ti, Zr, Nb, Mo, and W.

In some embodiments, the proportion of Ni is higher than the combined proportion of Co and Mn. That is, in general formula (i), x + y + z < 0.5.

In some embodiments, precursor particles 200 include Ni, Co, and Mn. That is, in general formula (i), 0 < x < 0.5 and 0 < y < 0.5 may be satisfied. In some embodiments, 0 < x < 0.3 and 0 < y < 0.3 may be satisfied.

In some embodiments, the precursor particles 200 are generally spherical. In other embodiments, the precursor particles 200 may be irregularly shaped, etc.
As used herein, "substantially spherical" refers to a shape that can be considered generally spherical overall, and the average aspect ratio based on a cross-sectional image observed under an electron microscope is approximately 1 to 2, preferably 1 to 1.5, and more preferably 1 to 1.2. Furthermore, as used herein, "aspect ratio" refers to the value obtained by dividing the long side of the rectangle with the smallest area among rectangles circumscribing the particle outline by the short side. Furthermore, "average aspect ratio" refers to the arithmetic mean of the aspect ratios of at least 100 randomly selected particles.

The average particle size of the precursor particles 200 is, for example, 3.0 μm or more, preferably 4.0 μm or more, and more preferably 4.5 μm or more. The average particle size of the precursor particles 200 is, for example, 9.0 μm or less, preferably 7.0 μm or less, and more preferably 5.5 μm or less.
In this specification, the term "average particle size" refers to the median diameter ( _D50 particle size), which is the particle size corresponding to a cumulative frequency of 50% by volume from the side of fine particles with small particle sizes in a volume-based particle size distribution based on a laser diffraction/scattering method.

The outer shell 210 includes a plurality of primary particles 212. In this embodiment, the outer shell 210 is formed by agglomerating a plurality of primary particles 212 through physical or chemical bonding forces. In this specification, the term "primary particle" refers to the smallest unit of particles that make up the precursor particle 200, and more specifically, the smallest unit determined from the apparent geometric shape.

The outer shell 210 may have a single-layer structure or a multi-layer structure with multiple layers stacked on top of each other.

The average thickness of the outer shell 210 may be, for example, 1/5 or less, and preferably 1/6 or less, of the average particle size of the precursor particles 200. The average thickness of the outer shell 210 may be, for example, 1/10 or more, or 1/8 or more, of the average particle size of the precursor particles 200. By making the average thickness of the outer shell 210 thinner than the average particle size of the precursor particles 200, the diffusion resistance of Li ions is reduced, and the output characteristics are improved.
In this specification, the thickness of the outer shell 210 refers to the thickness measured in a cross-sectional observation image of the precursor particle 200 taken with an electron microscope, and the average thickness of the outer shell 210 refers to the arithmetic mean of the thicknesses of the outer shell 210 measured for at least 100 precursor particles 100.

The shape of the primary particles 212 may be, for example, needle-like, plate-like, elliptical, or the like. Furthermore, the average aspect ratio (length of major axis/length of minor axis) of the precursor particles 200 based on a cross-sectional electron microscope image is, for example, 2 or more, preferably 3 or more, and more preferably 5 or more. The longer the average aspect ratio of the primary particles 212, the lower the diffusion resistance of Li ions. The average aspect ratio of the primary particles 212 may be, for example, 50 or less, 20 or less, or 10 or less.

The average length of the major axes of the primary particles 212 is, for example, 0.05 μm or more, 0.1 μm or more, 0.2 μm or more, or 0.3 μm or more. Furthermore, the average length of the major axes of the primary particles 212 is, for example, 1.2 μm or less, 1 μm or less, or 0.8 μm or less.

The average length of the minor axes of the primary particles 212 is, for example, 0.01 μm or more, 0.02 μm or more, 0.05 μm or more, or 0.1 μm or more. Furthermore, the average length of the minor axes of the primary particles 212 is, for example, 0.6 μm or less, 0.4 μm or less, or 0.2 μm or less.

The average lengths of the major and minor axes of the primary particles 212 are measured based on cross-sectional observation images of the precursor particles 200 taken with an electron microscope. In this specification, the average lengths of the major and minor axes of the primary particles 212 refer to the arithmetic mean of the major and minor axes of at least 100 primary particles 212.

At least 50% of the primary particles 212 contained in the outer shell 210 are radially oriented from the center of the precursor particle 200 toward the surface of the precursor particle 200. In a preferred embodiment, the oriented primary particles 212 account for at least 60%, more preferably at least 65%, and even more preferably at least 70%. Radially oriented primary particles 212 in a larger number can reduce the diffusion resistance of Li ions and improve output characteristics. The percentage of the primary particles 212 mentioned above indicates a percentage based on the number, and indicates the percentage of radially oriented primary particles 212 out of the total number (at least 100) of measured primary particles 212.
In this specification, "primary particles are radially oriented" means that the angle formed by the long axis direction of the primary particle 212 with respect to a reference line connecting the center of the primary particle 212 and the center of the precursor particle 200 is 30° or less. Note that this measurement can be performed by processing a cross section of the precursor particle 200 with an ion beam and observing the cross section with an electron microscope. Note that as another method, the orientation can also be confirmed by electron backscatter diffraction (EBSD).

The hollow portion 220 is a space surrounded by the outer shell portion 210. The hollow portion 220 does not have to be completely surrounded by the outer shell portion 210, and includes embodiments in which a portion of the outer shell portion 210 is missing, etc. The number of primary particles in the hollow portion 220 is fewer than that in the outer shell portion 210, or there are no primary particles in the hollow portion 220. That is, the proportion of primary particles 212 contained in the outer shell portion 210 among the primary particles contained in the entire precursor particle 200 can be, for example, 90% or more, 95% or more, 98% or more, or 100%. This makes it possible to obtain an active material that reacts well with Li during firing.
In this specification, the proportion of such primary particles is measured by the area ratio in a cross-sectional SEM.

Figure 2 is a flow diagram showing an overview of one embodiment of a method for producing a positive electrode active material for use in a lithium-ion secondary battery. This production method includes a raw material preparation step S10, a crystallization step S20, a mixing step S30, and a firing step S40. The raw material preparation step S10 and the crystallization step are included in the production process for the precursor particles 200 described above. Note that the production method disclosed herein may further include other steps at any stage.

The raw material preparation step S10 is a step of preparing, for example, raw materials for the precursor material. Examples of precursor materials include a metal compound containing the metal element contained in the precursor particle 200 and an ammonium ion donor. The metal compound containing the metal element contained in the precursor particle 200 may be, for example, a water-soluble salt. Such water-soluble salts may include, for example, sulfates, nitrates, oxalates, etc., as appropriate. These water-soluble salts may be used alone or in combination of two or more. The metal element contained in the precursor particle 200 is, for example, a metal element represented by the above general formula (1) and includes at least Ni.

In the raw material preparation step S10, for example, the metal compound is dissolved in water to prepare a raw metal aqueous solution containing the metal elements contained in the precursor particles 200. From the perspective of preventing the incorporation of impurities, ion-exchanged water, distilled water, ultrafiltered water, reverse osmosis water, etc. can be suitably used as the water.

In the raw material preparation step S10, an ammonium ion donor is prepared. The ammonium ion donor is, for example, an aqueous solution containing ammonia ions. Such an aqueous solution may be purchased commercially (ammonia water), or may be prepared by dissolving a compound that supplies ammonium ions in a solvent containing water. Examples of such compounds that can be used include ammonium sulfate, ammonium nitrate, ammonium hydroxide, aqueous ammonium halide, and ammonia. These compounds may be used alone or in combination. Ammonium ions can promote the formation or growth of nuclei and increase the adhesiveness or adhesion between primary particles in the crystallization step S20, described below. While not limited to these, the aqueous solution containing ammonium ions contains, for example, 0.1 wt% to 5 wt%, preferably 0.5 wt% to 2 wt%, of ammonium ions.

In the crystallization step S20, precursor particles are prepared using a crystallizer. The crystallizer is preferably a device capable of generating a Taylor vortex flow in the reaction field. Devices that generate Taylor vortex flow may be commercially available. Figure 3 is a schematic diagram showing the general configuration of a crystallizer. Figure 4 is a schematic diagram of a cross section taken along line IV-IV shown in Figure 3. L and R in Figure 3 indicate left and right, respectively, but this does not limit the installation mode of the crystallizer.

The crystallization apparatus 300 shown in Figure 3 comprises an outer cylinder 310, an inner cylinder 320, and a motor 322. The crystallization apparatus 300 has a reaction chamber 330 between the outer cylinder 310 and the inner cylinder 320. The reaction chamber 330 is the region where the crystallization reaction takes place and where Taylor vortices are generated. Note that Taylor vortices refer to the flow of fluid that generates Taylor vortices. Taylor vortices refer to the form of vortices that flow in a circular pattern.

In this embodiment, the outer cylinder 310 is cylindrical. The central axis of the outer cylinder 310 extends in the left-right direction in Figure 3. Here, the outer cylinder 310 is fixed so that it does not rotate around its central axis.

In this embodiment, the outer cylinder 310 is provided with a first supply port 312, a second supply port 314, a third supply port 316, and an exhaust port 318. All of these are connected to the reaction chamber 330.

In this embodiment, the first supply port 312 is a portion that supplies the raw metal aqueous solution to the reaction chamber 330. The second supply port 314 is a portion that supplies the pH adjuster to the reaction chamber 330. The third supply port 316 is a portion that supplies the ammonium ion supplier to the reaction chamber 330. Note that in this embodiment, three supply ports are provided, but the number is not particularly limited and may be one, two, or four or more. The type of supply may be different for each supply port, or two or more types of supply may be supplied from a single supply port.

The outlet 318 is provided for discharging the fluid within the reaction chamber 330. The fluid discharged from the outlet 318 contains the precursor particles 200 obtained by the crystallization reaction. This allows the precursor particles 200 to be recovered from the reaction chamber 330.

In this embodiment, the inner cylinder 320 is cylindrical. However, the inner cylinder 320 may also be cylindrical. The diameter of the inner cylinder 320 is designed to be smaller than the diameter of the outer cylinder 310. The space between the inner diameter of the outer cylinder 310 and the outer diameter of the inner cylinder 320 forms the reaction chamber 330. The diameter of the inner cylinder 320 is designed to be large enough to generate a Taylor vortex flow in the reaction chamber 330. The central axis of the inner cylinder 320 extends in the left-right direction in Figure 3. In this embodiment, the central axis of the inner cylinder 320 is positioned so as to overlap with the central axis of the outer cylinder 310.

The inner cylinder 320 is connected to a motor 322 and is designed so that the motor 322 can rotate the inner cylinder 320 clockwise or counterclockwise around its central axis. By rotating the inner cylinder 320, Taylor vortices can be generated in the fluid (e.g., liquid phase) in the reaction chamber 330.

The rotation speed of the inner cylinder 320 around its central axis is not particularly limited as long as it can generate a Taylor vortex flow, but is, for example, 300 rpm or more, 500 rpm or more, 1000 rpm or more, 1500 rpm or more, or 2000 rpm or more. Furthermore, the rotation speed of the inner cylinder 320 around its central axis is, for example, 3000 rpm or less, 2800 rpm or less, or 2600 rpm or less.

To prepare precursor particles 200 using the crystallizer 300, for example, a reaction solution containing a metal compound containing the metal element contained in the precursor particles and an ammonium ion donor is prepared in the reaction chamber 330. This reaction solution is prepared, for example, by supplying a raw metal aqueous solution and an ammonium ion donor to the reaction chamber 330. Alternatively, a mixture of the raw metal aqueous solution and the ammonium ion donor may be supplied to the reaction chamber 330.

The crystallization step S20 is preferably carried out by adjusting the oxygen concentration in the reaction field. The oxygen concentration in the reaction chamber 330 is, for example, 20 vol% or higher, and preferably 22 vol% or higher. The oxygen concentration in the reaction chamber 330 is, for example, 40 vol% or lower, preferably 35 vol% or lower, more preferably 30 vol% or lower, and even more preferably 25 vol% or lower. The greater the supply of oxygen in the reaction chamber 330, the higher the proportion of hollow portions in the precursor particles tends to be. In other words, the greater the supply of oxygen in the reaction chamber 330, the smaller the thickness of the outer shell relative to the average particle size of the precursor particles can be.

The pH of the reaction solution at 25°C is, for example, 9 or higher, preferably 10.5 or higher, more preferably 10.8 or higher, even more preferably 11 or higher, and particularly preferably 11.4 or higher. Furthermore, although not particularly limited, the pH of the reaction solution at 25°C is, for example, 13.5 or lower, preferably 13 or lower, more preferably 12.5 or lower, and even more preferably 12 or lower. In some preferred embodiments, the pH of the reaction solution is maintained constant from the start to the end of the crystallization reaction. Under these conditions, the pH does not change during the crystallization reaction, improving the production efficiency of the precursor particles 200. Note that a constant pH does not have to be exactly the same pH value; for example, a pH change of ±0.3, preferably ±0.2, from the pH at the start of the crystallization reaction is acceptable.

To adjust the pH of the reaction solution, for example, a pH adjuster can be used. Examples of pH adjusters that can be used include carbonates, hydroxides, and sulfates of alkali metals or alkaline earth metals. These pH adjusters may be used alone or in combination of two or more. Among these, hydroxides of alkali metals or alkaline earth metals are preferred, with sodium hydroxide being particularly preferred.

The reaction solution contained in the reaction chamber 330 is agitated by rotating the inner tube 320 around the central axis. Rotating the inner tube 320 generates Taylor vortices in the reaction solution. The Taylor vortices are generated in at least a portion of the reaction solution in the reaction chamber 330. They may also be generated throughout the entire reaction solution. For example, a pair of a first Taylor vortex 332 and an adjacent second Taylor vortex 334 are generated. The first Taylor vortex 332 is generated along the circumference of the inner tube 320 and has a clockwise vortex in the circumferential direction. The second Taylor vortex 334 is generated along the circumference of the inner tube 320 and has a vortex that rotates in the opposite direction (counterclockwise) to the first Taylor vortex in the circumferential direction. The Taylor vortices themselves are generated in a ring shape along the circumference of the inner tube 320, but the fluid forming the Taylor vortices moves to the adjacent Taylor vortex (moving from left to right in Figure 3). Therefore, the reaction liquid stirred by the Taylor vortex moves from the supply ports 312, 314, and 316 toward the outlet 318, and crystallization progresses, allowing the precursor particles 200 to be recovered from the outlet 318.

The precursor particles 200 recovered from the reaction chamber 330 can be separated, for example, by a solid-liquid separation method such as filtration, washed with water, etc., and dried to recover them.

Although not particularly limited, the reaction time of the crystallization reaction is, for example, 0.1 to 5 hours, preferably 0.3 to 2 hours, and more preferably 0.5 to 1 hour. Here, the reaction time of the crystallization reaction can be measured, for example, as the time from when the raw metal aqueous solution is supplied to the reaction chamber 330 through the first supply port 312 with the ammonium ion donor contained in the reaction chamber 330 to when the precursor particles 200 are first recovered through the discharge port 318.

In some preferred embodiments, an ammonium ion supplier is supplied to the reaction chamber 330 before the aqueous raw metal solution is supplied to the reaction chamber 330. At this time, the ammonium ion supplier is preferably adjusted to achieve the oxygen concentration and pH of the reaction solution in the reaction chamber 330 described above. This allows the crystallization reaction to begin when the aqueous raw metal solution is supplied to the reaction chamber 330 from the first supply port 312, making it possible to control the time from the start of the crystallization reaction to the recovery of the precursor particles 200 from the outlet 318, thereby achieving a more uniform particle size distribution of the precursor particles.

In some preferred embodiments, before supplying the raw metal aqueous solution to the reaction chamber 330, it is preferable to rotate the inner cylinder 320 inside the reaction chamber 330 to generate Taylor vortices in the ammonium ion supplier. This ensures consistent stirring conditions in the reaction chamber 330 from the start of the crystallization reaction until the precursor particles are recovered, thereby making the quality of the precursor particles more uniform.

In some preferred embodiments, the rotation speed of the inner cylinder 320 is kept constant from the start to the end of the crystallization reaction. This reduces variation in the quality of the precursor particles 200, eliminates the need for complicated operations, and improves production efficiency.

In some preferred embodiments, the raw metal aqueous solution is continuously supplied to the reaction solution in the reaction chamber 330. This allows the precursor particles 200 obtained by the crystallization reaction to be continuously collected from the outlet 318, improving the production efficiency of the precursor particles. Also, preferably, an ammonium ion donor is continuously supplied to the reaction solution in the reaction chamber 330. This allows the concentration balance between the metal compounds and ammonium ions in the reaction solution in the reaction chamber 330 to be maintained.

In the mixing step S30, the obtained precursor particles 200 are mixed with a lithium compound. The mixing method is not particularly limited, and for example, a known mixing device (e.g., a shaker mixer, V blender, ribbon mixer, Julia mixer, Lödige mixer, etc.) can be used to mix and obtain a mixture. As the lithium compound, a compound that is converted to an oxide by firing can be used, and for example, lithium carbonate, lithium nitrate, lithium hydroxide, etc. can be used as appropriate.

In the calcination step S40, the resulting mixture is calcined to obtain a positive electrode active material that is a lithium transition metal composite oxide. Calcination can be carried out in a conventionally known calcination furnace. While not particularly limited, the calcination temperature is preferably about 600 to 1000°C. The calcination temperature refers to the temperature inside the calcination furnace. The calcination time is generally 1 hour or more, for example, 1 to 48 hours, and preferably 5 to 24 hours. To enhance the crystallinity of the resulting oxide, the calcination atmosphere is preferably an oxygen-containing atmosphere, such as an oxygen atmosphere or air atmosphere. The oxygen concentration of the oxygen-containing atmosphere is preferably 10% by volume or more, and more preferably 18 to 100% by volume.

Lithium-ion secondary batteries using the lithium transition metal composite oxide obtained by firing have excellent output characteristics and cycle characteristics. Specific configuration examples of lithium-ion secondary batteries are described below with reference to the drawings.

Figure 5 is a cross-sectional view schematically illustrating the configuration of a lithium-ion secondary battery 100 using a fired product of precursor particles 200 according to one embodiment. The lithium-ion secondary battery 100 in Figure 5 is a sealed battery comprising a flat wound electrode assembly 20 and a nonaqueous electrolyte (not shown) housed in a flat, rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, as well as a thin-walled safety valve 36 designed to release internal pressure if the internal pressure of the battery case 30 rises above a predetermined level. The positive and negative electrode terminals 42, 44 are electrically connected to positive and negative electrode current collector plates 42a, 44a, respectively. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

Figure 6 is a schematic exploded view showing the configuration of the wound electrode assembly 20. As shown in Figures 5 and 6, the wound electrode assembly 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are stacked with two long separators 70 interposed between them and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long negative electrode current collector 62. The positive electrode current collector exposed portion 52a (i.e., the portion where the positive electrode current collector 52 is exposed because the positive electrode active material layer 54 is not formed) and the negative electrode current collector exposed portion 62a (i.e., the portion where the negative electrode current collector 62 is exposed because the negative electrode active material layer 64 is not formed) are formed to protrude outward from both ends of the wound electrode body 20 in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode current collector exposed portion 52a and the negative electrode current collector exposed portion 62a are joined to the positive electrode current collector plate 42a and the negative electrode current collector plate 44a, respectively.

The positive electrode current collector 52 constituting the positive electrode sheet 50 can be, for example, aluminum foil. The positive electrode active material layer 54 contains, as the positive electrode active material, at least a lithium transition metal composite oxide, which is a fired product of the precursor particles according to the present embodiment described above. The positive electrode active material layer 54 can further contain a conductive material, a binder, etc. Suitable conductive materials include, for example, carbon black such as acetylene black (AB) and other carbon materials (such as graphite). Suitable binders include, for example, polyvinylidene fluoride (PVDF).

The negative electrode current collector 62 constituting the negative electrode sheet 60 may be, for example, copper foil. The negative electrode active material layer 64 contains a negative electrode active material. Examples of the negative electrode active material that can be used include carbon materials such as graphite, hard carbon, and soft carbon. The negative electrode active material layer 64 may further contain a binder, a thickener, and the like. Examples of the binder that can be used include styrene butadiene rubber (SBR). Examples of the thickener that can be used include carboxymethyl cellulose (CMC).

Separator 70 can be made of various porous sheets similar to those conventionally used in lithium-ion secondary batteries, including porous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP). Such porous resin sheets may have a single-layer structure or a multi-layer structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). Separator 70 may also have a heat-resistant layer (HRL).

The nonaqueous electrolyte can be the same as that used in conventional lithium-ion secondary batteries, and typically, a nonaqueous solvent containing a supporting salt can be used. Nonaqueous solvents that can be used include aprotic solvents such as carbonates, esters, and ethers. Among these, carbonates are preferred because they are particularly effective in reducing low-temperature resistance due to the positive electrode material. Examples of carbonates include ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). These nonaqueous solvents can be used alone or in appropriate combinations of two or more. As the supporting salt, for example, lithium salts such as LiPF ₆ , LiBF ₄ , lithium bis(fluorosulfonyl)imide (LiFSI) can be suitably used. The concentration of the supporting salt is preferably 0.7 mol/L or more and 1.3 mol/L or less.

The non-aqueous electrolyte may contain components other than the non-aqueous solvent and supporting salt described above, such as various additives such as gas generating agents, film-forming agents, dispersants, and thickeners, as long as the effects of the present invention are not significantly impaired.

Such a lithium-ion secondary battery 100 can be manufactured by a manufacturing method that includes a positive electrode manufacturing process in which a positive electrode sheet 50 is made using the positive electrode active material (lithium transition metal composite oxide) manufactured by the above manufacturing method.

The foregoing describes a preferred example of a prismatic lithium-ion secondary battery equipped with a flat wound electrode assembly. However, the lithium transition metal composite oxide, which is a product of calcining the precursor particles according to this embodiment, can also be used in other types of lithium-ion secondary batteries using known methods. For example, a lithium-ion secondary battery equipped with a stacked electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked) can be constructed using the lithium transition metal composite oxide, which is a product of calcining the precursor particles according to this embodiment. Furthermore, a cylindrical lithium-ion secondary battery, a coin-type lithium-ion secondary battery, a laminated lithium-ion secondary battery, etc. can also be constructed using the lithium transition metal composite oxide, which is a product of calcining the precursor particles according to this embodiment. Furthermore, an all-solid-state secondary battery using a solid electrolyte can also be constructed.

The lithium-ion secondary battery 100 can be used for a variety of purposes. Suitable applications include a driving power source mounted on vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium-ion secondary battery 100 can also be used as a storage battery for small power storage devices. The lithium-ion secondary battery 100 can also be used in the form of a battery pack in which multiple batteries are connected in series and/or parallel.

As described above, specific aspects of the technology disclosed herein include those described in the following sections.
Item 1:
A precursor of a positive electrode active material used in a lithium ion secondary battery,
The composite hydroxide particles contain Ni.
The transition metal composite hydroxide particles have an outer shell portion formed by aggregation of primary particles having an average aspect ratio (length of major axis/length of minor axis) of 2 or more based on SEM observation, and a hollow portion surrounded by the outer shell portion,
50% or more of the primary particles are oriented radially from the center toward the surface of the transition metal composite hydroxide particles,
A precursor in which 90% or more of the primary particles contained in the entire transition metal composite hydroxide particle are present in the outer shell portion.
Item 2: The precursor according to Item 1, wherein the average thickness of the outer shell portion is one-sixth or less of the average particle size of the transition metal composite hydroxide particles.
Item 3: The transition metal composite hydroxide particles are represented by the general formula:
Ni _(1-x-y-z) Co _x Mn _y A _z (OH) _2+a
(wherein 0≦x<0.5, 0≦y<0.5, 0≦z≦0.1, 0≦a≦0.5, and x+y+z<1; A is at least one element selected from the group consisting of Mg, Ca, V, Cr, Al, Ti, Zr, Nb, Mo, and W).
Item 3. The precursor according to item 1 or 2, which is a compound represented by the formula:
Item 4: The precursor according to any one of Items 1 to 3, wherein the average length of the major axis of the primary particles is 0.05 μm or more and 1.2 μm or less.
Item 5:
A method for producing a precursor of a positive electrode active material used in a lithium ion secondary battery, comprising:
Preparing a metal compound containing Ni and an ammonium ion donor;
preparing an apparatus including an outer cylinder, an inner cylinder disposed inside the outer cylinder, and a reaction chamber disposed between the outer cylinder and the inner cylinder, and rotating the inner cylinder around the axial direction of the inner cylinder to generate Taylor vortices in a reaction solution that contains the metal compound and the ammonium ion donor and is contained in the reaction chamber, thereby crystallizing a transition metal composite hydroxide;
Including,
The pH of the reaction solution at 25°C is 9 or higher,
The oxygen concentration in the reaction chamber is 20 vol% or more and 50 vol% or less,
The rotation speed of the inner cylinder is 300 rpm or more and 3000 rpm or less.
Manufacturing method.
Item 6: The method according to item 5, wherein the pH of the reaction solution at 25°C is 10.5 or more and 13 or less.
Item 7:
A method for producing a positive electrode active material for use in a lithium ion secondary battery, comprising:
Item 7. Mixing the precursor obtained by the production method according to Item 5 or 6 with a lithium compound; and calcining the mixed mixture;
The manufacturing method includes the steps of:
Item 8: A method for producing a lithium ion secondary battery, comprising producing a positive electrode containing the positive electrode active material obtained by the production method according to Item 7.

Test examples related to this technology are described below. However, this technology is not limited to the test examples shown below.

Example 1
[Precursor crystallization]
A crystallizer capable of generating Taylor vortices in the reaction chamber was used. The general configuration of the crystallizer was similar to that of the crystallizer 300 described above.
The oxygen concentration in a 0.1 L reaction chamber was adjusted to 25 vol%, 1 wt% aqueous ammonia was added, and the inner cylinder of the apparatus was rotated at 2000 rpm to generate a Taylor vortex in the aqueous ammonia in the reaction chamber. Next, a 30 wt% aqueous sodium hydroxide solution was supplied to the reaction chamber, and the pH of the mixture in the reaction chamber was adjusted to 12 at a liquid temperature of 25°C.
Nickel sulfate, cobalt sulfate, and manganese sulfate were dissolved in water to prepare a 1 mol/L aqueous solution of raw metals, with a molar ratio of nickel sulfate:cobalt sulfate:manganese sulfate of 60:20:20.
The raw metal aqueous solution, 30 wt % sodium hydroxide aqueous solution, and 1 wt % ammonia aqueous solution were supplied to the mixed solution in which Taylor vortices were generated in the reaction chamber in a volume ratio of 1:1:1, and crystallization was carried out for 30 minutes under a condition of pH 12. Thereafter, the crystallized product was washed with water, filtered, and dried to obtain a powdery composite hydroxide (precursor).
The composite hydroxide obtained above was confirmed to be Ni _0.6 Co _0.2 Mn _0.2 (OH) ₂ by ICP emission spectroscopy.

[Active material firing]
A lithium compound was mixed with the composite hydroxide so that the molar ratio (Li:M) of lithium (Li) to a metal element other than lithium (M) was 1.1:1. This mixture was calcined in an oxidizing atmosphere in a calcination furnace at 700°C for 5 hours. This produced a positive electrode active material.

[Preparation of positive electrode plate]
The obtained positive electrode active material, conductive material, and binder were mixed with N-methyl-2-pyrrolidone (NMP) in a ratio of 100:1:1 (by weight) to prepare a positive electrode mixture paste. Acetylene black was used as the conductive material. PVDF was used as the binder. The prepared positive electrode mixture paste was applied to a 15 μm aluminum foil and dried, and then pressed to a predetermined thickness and processed to a predetermined size to produce a positive electrode plate.

[Preparation of negative electrode plate]
The graphite, SBR, and CMC were mixed with water in a ratio of 100:1:1 (by weight) to prepare a negative electrode mixture paste. The prepared negative electrode mixture paste was applied to a copper foil, dried, pressed to a predetermined thickness, and cut to a predetermined size to prepare a negative electrode plate.

[Fabrication of Non-Aqueous Electrolyte Secondary Battery]
The prepared positive and negative electrodes were stacked on top of each other with a separator interposed therebetween to prepare a laminated electrode assembly. Leads were attached to each of the positive and negative electrodes. The prepared laminated electrode assembly was then housed in an exterior housing made of an aluminum laminate sheet, after which a nonaqueous electrolyte was poured in and the opening of the exterior housing was sealed to prepare a test cell. The nonaqueous electrolyte used contained a solvent consisting of EC:FEC:EMC:DMC = 15:5:40:40 (by volume) and _1M LiPF6 as a Li salt.

<SEM Observation of Precursor>
The cross-sections of the composite hydroxide particles (precursor particles) obtained above were processed with an ion beam, and then the cross-sections of the particles were observed with an SEM. The structure of the precursor particles was evaluated by SEM observation. Figure 7 shows a cross-sectional SEM image of the precursor particles obtained in Example 1.

The orientation of the primary particles contained in the outer shell was also confirmed by SEM observation of the cross section of the composite hydroxide particle. Specifically, particles were evaluated as being radially oriented if the angle between the long axis of the primary particle and the reference line connecting the center of the primary particle and the center of the composite hydroxide particle was 30° or less. More than 100 primary particles contained in the outer shell were randomly observed, and the percentage of radially oriented primary particles was determined. The results are shown in Table 1.

The thickness of the outer shell was also measured by SEM observation of the cross section of the composite hydroxide particles. The thickness of the outer shell of 100 or more randomly selected particles was measured, and the arithmetic average was calculated. The arithmetic average of the outer shell thicknesses was divided by the average particle size of the composite hydroxide particles shown below, and the value obtained is shown in Table 1 as the "shell thickness relative to the average particle size."

<Analysis of the average particle size of precursor>
The average particle size of the composite hydroxide particles was defined as the particle size (D50) corresponding to 50% cumulative particle size in the volume-based particle size distribution measured by a laser diffraction scattering particle size analyzer.

<IV resistance measurement>
The test cell was charged to a state of charge (SOC) of 50% in a 25°C environment and left to stand for 1 hour. Then, it was discharged for 10 seconds at a current of 5 C. The OCV voltage immediately before discharge was V0, and the voltage at the time of 10 seconds of discharge was V1.
The IV resistance value was calculated using the formula: IV resistance = (V0 - V1)/current value of 5 C. Table 1 shows the results as a ratio based on Comparative Example 1 described below.

<Measurement of cycle capacity retention rate>
In an environment of 25° C., one cycle consisted of CCCV charging at 0.4 C, 4.2 V, and 0.1 C cut, followed by CC discharging at 0.4 C, 2.5 V cut, and this cycle was repeated 250 times.
Formula: Volume at 250th cycle/Volume at 1st cycle×100(%)
The capacity retention rate was calculated by the following equation. The results are shown in Table 1.

Example 2
The oxygen concentration in the reaction chamber was changed to 30 vol% from the precursor crystallization conditions of Example 1. The rotation speed of the inner cylinder of the crystallizer was also changed to 700 rpm. A 30 wt% aqueous sodium hydroxide solution was supplied to the reaction chamber so that the mixed liquid in the reaction chamber had a pH of 10.8 at a liquid temperature of 25°C. A powdery composite hydroxide was obtained in the same manner as in Example 1, except for these points.
The subsequent procedures up to the preparation of the test cell, and the evaluation of the precursor and the test cell were the same as in Example 1 (the same applies to the following examples).

Example 3
The oxygen concentration in the reaction chamber was changed from the precursor crystallization conditions of Example 1 to 22 vol%. The rotation speed of the inner cylinder of the crystallizer was also changed to 2600 rpm. A 30 wt% aqueous sodium hydroxide solution was supplied to the reaction chamber so that the mixed solution in the reaction chamber had a pH of 11.4 at a liquid temperature of 25°C. A 1 mol/L aqueous solution of raw metals was prepared so that the molar ratio of nickel sulfate, cobalt sulfate, manganese sulfate, and zirconium sulfate was 59.9:19.9:19.9:0.3. A powdered composite hydroxide was obtained in the same _manner as in Example 1, except for these points. The obtained composite hydroxide was confirmed _to be _{Ni0.599Co0.199Mn0.199Zr0.003} (OH) ₂ using an ICP emission spectrometer _.

Example 4
The oxygen concentration in the reaction chamber was changed to 22 vol% from the conditions for precursor crystallization in Example 1. In addition, a 30 wt% aqueous sodium hydroxide solution was supplied to the reaction chamber so that the mixed liquid in the reaction chamber had a pH of 13.5 at a liquid temperature of 25° C. A powdery composite hydroxide was obtained in the same manner as in Example 1, except for these points.

Example 5
Based on the conditions for precursor crystallization in Example 1, a 30 wt % aqueous solution of sodium hydroxide was supplied to the reaction chamber so that the mixed liquid in the reaction chamber had a pH of 9 at a liquid temperature of 25° C. A powdery composite hydroxide was obtained in the same manner as in Example 1 except for this.

(Comparative Example 1)
The oxygen concentration in the reaction chamber was changed to 10 vol% from the precursor crystallization conditions of Example 1. The rotation speed of the inner cylinder of the crystallizer was also changed to 1000 rpm. A 30 wt% aqueous sodium hydroxide solution was supplied to the reaction chamber so that the mixed solution in the reaction chamber had a pH of 11.8 at a liquid temperature of 25°C. A powdery composite hydroxide was obtained in the same manner as in Example 1, except for these points.

(Comparative Example 2)
The rotation speed of the inner cylinder of the crystallizer was changed to 3500 rpm from the conditions for precursor crystallization in Example 1. Except for this, a powdery composite hydroxide was obtained in the same manner as in Example 1. In addition, a 30 wt % aqueous sodium hydroxide solution was supplied to the reaction chamber so that the mixed liquid in the reaction chamber had a pH of 12.6 at a liquid temperature of 25°C. Except for this, a powdery composite hydroxide was obtained in the same manner as in Example 1.

(Comparative Example 3)
The oxygen concentration in the reaction chamber was changed to 22 vol% from the precursor crystallization conditions in Example 1. The rotation speed of the inner cylinder of the crystallizer was also changed to 200 rpm. A 30 wt% aqueous sodium hydroxide solution was supplied to the reaction chamber so that the mixed liquid in the reaction chamber had a pH of 10.8 at a liquid temperature of 25°C. A powdery composite hydroxide was obtained in the same manner as in Example 1, except for these points.

As shown in Table 1 and Figure 7, precursor particles with a hollow structure can be produced by performing a crystallization reaction accompanied by stirring using Taylor vortex currents in an oxidizing atmosphere (e.g., an oxygen concentration of 20 vol% or higher). Furthermore, it can be seen that the primary particles that make up the outer shell of precursor particles produced by this method tend to be radially oriented. Furthermore, it can be seen that lithium-ion secondary batteries equipped with a positive electrode active material produced using precursor particles with such a structure have reduced resistance and improved output characteristics.

Specific examples of the present technology have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet 52 Positive electrode current collector 52a Positive electrode current collector exposed portion 54 Positive electrode active material layer 60 Negative electrode sheet 62 Negative electrode current collector 62a Negative electrode current collector exposed portion 64 Negative electrode active material layer 70 Separator 100 Lithium ion secondary battery 200 Precursor particles (transition metal composite hydroxide particles)
210 Outer shell 212 Primary particles 220 Hollow portion 300 Crystallizer 310 Outer cylinder 320 Inner cylinder 330 Reaction chamber

Claims (8)

A precursor of a positive electrode active material used in a lithium ion secondary battery,
The composite hydroxide particles contain Ni.
the transition metal composite hydroxide particles have an outer shell portion formed by aggregation of primary particles having an average aspect ratio (length of major axis/length of minor axis) of 2 or more based on SEM observation, and a hollow portion surrounded by the outer shell portion;
50% or more of the primary particles are oriented radially from the center toward the surface of the transition metal composite hydroxide particle,
90% or more of the primary particles contained in the entire transition metal composite hydroxide particles are present in the outer shell portion.
Precursor. The precursor described in claim 1, wherein the average thickness of the outer shell is one-sixth or less of the average particle size of the transition metal composite hydroxide particles. The transition metal composite hydroxide particles have the general formula:
Ni _(1-x-y-z) Co _x Mn _y A _z (OH) _2+a
(wherein 0≦x<0.5, 0≦y<0.5, 0≦z≦0.1, 0≦a≦0.5, and x+y+z<1; A is at least one element selected from the group consisting of Mg, Ca, V, Cr, Al, Ti, Zr, Nb, Mo, and W).
The precursor according to claim 1, which is a compound represented by the formula: The precursor described in any one of claims 1 to 3, wherein the average length of the major axes of the primary particles is 0.05 μm or more and 1.2 μm or less. A method for producing a precursor of a positive electrode active material used in a lithium ion secondary battery, comprising:
Preparing a metal compound containing Ni and an ammonium ion donor;
an apparatus including an outer cylinder, an inner cylinder disposed inside the outer cylinder, and a reaction chamber disposed between the outer cylinder and the inner cylinder is prepared, and the inner cylinder is rotated around the axial direction of the inner cylinder to generate Taylor vortices in a reaction solution containing the metal compound and the ammonium ion donor contained in the reaction chamber, thereby crystallizing a transition metal composite hydroxide;
Including,
the pH of the reaction solution at 25°C is 9 or higher,
the oxygen concentration in the reaction chamber is 20 vol% or more and 50 vol% or less,
The rotation speed of the inner cylinder is 300 rpm or more and 3000 rpm or less.
Manufacturing method. The manufacturing method described in claim 5, wherein the pH of the reaction solution at 25°C is 10.5 or higher and 13 or lower. A method for producing a positive electrode active material for use in a lithium ion secondary battery, comprising:
mixing the precursor obtained by the method according to claim 5 or 6 with a lithium compound; and calcining the mixed mixture;
The manufacturing method includes the steps of: A method for manufacturing a lithium ion secondary battery, comprising producing a positive electrode containing a positive electrode active material obtained by the manufacturing method described in claim 7.