JP7646334B2 - Active material particles - Google Patents

Description

The present invention relates to active material particles, positive electrodes, secondary batteries, and methods for manufacturing active material particles.

In general, secondary batteries are composed of electrodes (positive and negative electrodes) and an electrolyte, and are charged and discharged by the movement of ions between the electrodes via the electrolyte. Such secondary batteries are used in a wide range of applications, from small devices such as mobile phones to large devices such as electric vehicles. For this reason, there is a demand for further improvements in the performance of secondary batteries. In order to improve the charge and discharge characteristics of secondary batteries, it is generally important to increase the interface between the active material in the electrodes and the electrolyte. Here, active material refers to a substance that is involved in the reaction that generates electricity.

As a specific measure to improve charge/discharge characteristics, a method is known in which an active material with fine protrusions is used in the positive electrode of a solid secondary battery. Patent Document 1 discloses a technology in which a pattern of lithium cobalt oxide with an increased specific surface area of 1.1 to 2 is formed on a current collector by a flux method in which a plating layer containing cobalt is brought into contact with a raw active material containing lithium and heated. Patent Document 1 discloses that the gaps between the active materials formed by the protrusions make it possible to secure a transport path for the active material ions that allow the active material to enter the positive electrode.

JP 2015-220080 A

The method of obtaining an active material layer with an increased specific surface area described in Patent Document 1, which uses a flux method, has a single-layer structure supported by a metal layer on the current collector side, which is structurally limited, and limits the design and improvement of characteristics of secondary batteries. In addition, the method of obtaining an active material layer with an increased specific surface area described in Patent Document 1, which uses a flux method, requires heating the contact area between the metal in the plating layer and the Li-containing active material in the temperature range of 500 to 1000°C.

For this reason, in the process of manufacturing secondary batteries, there is a demand for heat resistance in the other elements that make up the secondary battery, and there is hope for an alternative method to the flux method in terms of speeding up the manufacturing process and reducing energy consumption.

The present invention aims to provide active material particles that can be used in positive electrodes with high ionic conductivity and that can be adapted to low-temperature battery manufacturing processes. It also aims to provide a secondary battery with excellent charge/discharge characteristics in a low-temperature manufacturing process by using active material particles that do not require excessively high heat resistance.

The active material particles according to an embodiment of the present invention are applied to a positive electrode containing lithium cobalt oxide, and are characterized in that they have a particle portion and a protruding portion protruding in multiple directions from the particle portion and having a region with a crystallite size of 1 nm or more and 20 nm or less, and exhibit a bimodal diffraction angle peak as determined by 2θ method, including a diffraction angle peak at 18.9 degrees or more and 19.1 degrees or less and a diffraction angle peak at 19.2 degrees or more and 19.7 degrees or less .

Moreover, the active material particles according to an embodiment of the present invention are characterized in that they have a particle portion having a discontinuous texture in cross section and a protrusion portion protruding in multiple directions from the particle portion, and exhibit a bimodal diffraction angle peak as measured by the 2θ method, including a diffraction angle peak at 18.9 degrees or more and 19.1 degrees or less, and a diffraction angle peak at 19.2 degrees or more and 19.7 degrees or less .

According to the present invention, it is possible to provide active material particles that can be used in a positive electrode having high ionic conductivity and that can be used in a battery manufacturing process at low temperatures. In addition, according to the present invention, it is possible to provide a secondary battery with excellent charge/discharge characteristics in a low-temperature manufacturing process by using active material particles that do not require excessively high heat resistance.

1A is a schematic cross-sectional view of an active material particle according to a first embodiment, and FIG. 1B is an X-ray diffraction profile of the active material particle; and FIG. 1C is a schematic cross-sectional view of an active material particle according to a reference embodiment, and FIG. 1A is a SEM image showing the appearance of an active material particle according to a first embodiment, and FIG. 1B is a cross-sectional TEM image showing the appearance of an active material particle according to a first embodiment, and FIG. 1A is a schematic cross-sectional view of a secondary battery according to a first embodiment, and FIG. 1B is a schematic cross-sectional view of a positive electrode. 4 is a flowchart showing a method for manufacturing a secondary battery according to the first embodiment. 1A is a flow chart showing a manufacturing process of active material particles according to a first embodiment, and FIG. 1B is an example of a temperature profile. 1 shows a predicted change in the cross-sectional structure of a fine particle corresponding to the manufacturing process of the active material particle according to the first embodiment. FIG. 2C is a diagram illustrating the results of a thermogravimetric differential thermal analysis of the resin (a) and the laminate (b) according to the first embodiment, and a diagram illustrating the thermal decomposition temperatures based on the thermogravimetric analysis. 13 is a SEM image showing the results of investigating the dependence of active material particles and resin on the heating atmosphere in the air (a) to (e). 1A and 1B are diagrams illustrating a preparation step (a), a placement step (b), and modifications of the preparation step (c) and (d) according to the first embodiment. 1A is a flowchart showing a method for producing a positive electrode according to a second embodiment, and FIG. 1B is a schematic cross-sectional view thereof; FIG. 1C is a flowchart showing a modified example thereof, and FIG.

Below, preferred embodiments of the present invention are described in detail with reference to the drawings. The dimensions, materials, shapes, and relative positions of the components described in these embodiments are not intended to limit the scope of the present invention.

First Embodiment
<Microscopic structure of active material particles>
As a result of the inventors' investigations, they have found that the charge/discharge and characteristics of secondary batteries, which have been restricted by the high barrier for the transfer of active material ions between active material particles and an electrolyte, can be improved by using metastable lithium cobalt oxide as a specific electrolyte particle. That is, they have found that in order to increase the conductivity of active material ions, it is preferable to use metastable lithium cobalt oxide, whose crystal structure of the active material particles is different from that of stable lithium cobalt oxide.

Next, active material particle 22 according to the first embodiment, which contains metastable lithium cobalt oxide of this embodiment, will be described with reference to FIG. 1(a). Also, to compare with active material particle 22, active material particle 21, which contains stable lithium cobalt oxide of a reference form, will be described with reference to FIG. 1(c). FIG. 1 shows a schematic cross-sectional view (a) and an X-ray diffraction profile (b) of active material particle 22 according to the first embodiment, and a schematic cross-sectional view (c) and an X-ray diffraction profile (d) of active material particle 21 according to the reference form.

As shown in FIG. 1(a), the active material particle 22 has a particle portion 22b and a protrusion portion 22p that protrudes radially in multiple directions from the outer surface of the particle portion 22b. Also, as shown in FIG. 1(a), the inside of the active material particle 22 has a core portion 22c, a plurality of layered gaps 20g, a plurality of shell portions 22s, and radial gaps 20r. The particle portion 22b has an increased specific surface area and is porous both inside and outside the particle, and is different from the active material particle 21 containing stable lithium cobalt oxide in that it has a plain cross-sectional structure as shown in FIG. 1(c). The active material particle 21 containing stable lithium cobalt oxide has not undergone the first and second heating steps described below, and in other words, the particle cross-section has a homogeneous and continuous texture.

<Crystal structure of active material particles>
FIG. 1(b) is a diffraction angle profile of the 2θ method measured by X-ray diffraction method (hereinafter, sometimes referred to as XRD method) of a powder sample prepared by collecting a plurality of active material particles 22 shown in FIG. 1(a). Such a powder sample can be prepared by disassembling the positive electrode 30 shown in FIG. 3(b) described later. From the obtained X-ray diffraction angle profile, as shown in FIG. 1(b), a bimodal XRD profile is shown having at least two diffraction angle peaks at 18.9 degrees or more and 19.1 degrees or less and 19.2 degrees or more and 19.7 degrees or less at X-ray diffraction angles of 18 degrees to 20 degrees. On the other hand, it is known that stable lithium cobalt oxide exhibits a single-peak XRD profile having one diffraction angle peak at 18.9 degrees to 19.1 degrees.

The low-angle diffraction angle peak of the active material particles 22 observed at a diffraction angle of 18.9 degrees or more and 19.1 degrees or less is specifically a result of overlapping of the diffraction angle peaks of 18.99 degrees and 19.03 degrees, but for simplicity, the highest intensity of 19.03 degrees is represented as the diffraction angle peak on the low-angle side. The half-width corresponding to the diffraction angle peak of 19.03 degrees of the active material particles 22 was 0.28 degrees. In addition, the high-angle diffraction angle peak of the active material particles 22 observed at a diffraction angle of 19.2 degrees or more and 19.7 degrees or less is a result of overlapping of multiple diffraction angle peaks, but for simplicity, the highest intensity of 19.25 degrees is represented as the diffraction angle peak on the high-angle side. The high-angle diffraction angle peak of the active material particles 22 is specifically a result of overlapping of multiple diffraction angle peaks of 19.17 degrees, 19.21 degrees, 19.25 degrees, and 19.29 degrees. The half width corresponding to the diffraction angle peak of 19.25 degrees of the active material particles 22 was 0.26 degrees. The crystallite sizes φgc of the crystal structures corresponding to the diffraction angle peaks of 19.03 degrees and 19.25 degrees of the active material particles 22 were 28.8 nm and 31.0 nm, respectively, according to the Schuller's formula of general formula (1).
Scherrer's formula: τ = Kλ/(β cos θ) Equation (1)

The parameters in formula (1) are: τ: crystallite size, K: shape factor (0.9), λ: X-ray wavelength, β: half-width of the diffraction angle peak, and θ: Bragg angle.

As a reference embodiment, FIG. 1(d) shows an XRD profile including a diffraction angle 2θ of 18-20 degrees for lithium cobalt oxide sold as a commercial product. The active material particles 21 containing stable LCO according to the reference embodiment are virgin commercial products (manufactured by Nippon Chemical Industry Co., Ltd., registered trademark CELLSEED) that have not undergone the first and second heating processes described below. It can be seen that the active material particles 21 containing stable LCO have a single peak at 18.95 degrees, which is slightly lower than 19 degrees. The crystallite size φgc of the crystal structure corresponding to the diffraction angle peak of 18.95 degrees for the active material particles 21 was 89.6 nm according to the Schuller formula in the following formula (1). The diffraction angle peak observed near the diffraction angle 2θ = 19° when the X-ray diffraction angle is from 18 degrees to 20 degrees corresponds to the (003) plane of the lithium cobalt oxide crystal.

The active material particles 22 of this embodiment have a unique broad high-angle diffraction angle peak at 19.2 degrees or more and 19.7 degrees or less, which is not observed in the active material particles 21 containing stable lithium cobalt oxide. In other words, the active material particles 22 of this embodiment exhibit multiple diffraction angle peaks at X-ray diffraction angles of 19.2 degrees or more and 19.7 degrees or less by the 2θ method. In other words, the active material particles 22 of this embodiment have a high-angle diffraction angle peak at X-ray diffraction angles of 19.2 degrees or more and 19.7 degrees or less by the 2θ method, and a low-angle diffraction angle peak at 18.9 degrees or more and 19.1 degrees or less.

The active material particles 22 of this embodiment have multiple unique peaks (19.17 degrees, 19.21 degrees, 19.25 degrees, 19.29 degrees) split on the high-angle side compared to stable lithium cobalt oxide. From these multiple diffraction angle peaks, it can be read that the active material particles 22 have multiple crystal structures with a distribution in lattice spacing and crystallite size. Also, from these multiple diffraction angle peaks, it can be read that the active material particles 22 have multiple crystal structures with smaller lattice spacing and crystallite size compared to the stable active material particles 21. In other words, multiple crystal structures with smaller lattice spacing and crystallite size compared to the stable active material particles 21 are mixed in the active material particles 22.

Therefore, it can be seen that the crystallite size of the active material particles 22 contained in the positive electrode 30 of this embodiment is finer than that of the stable active material particles 21. Considering the distribution of the diffraction angle peaks with diffraction angles of 19.2 degrees or more and 19.7 degrees or less, it is believed that the active material particles 22 have multiple crystallites with different crystallite sizes ranging from 10 nm to 50 nm.

Next, the lattice constant of the crystal structure of the active material particle 22 of this embodiment will be described. Using the angle 2θ of the diffraction angle peak and the Bragg equation of the following formula (2), the lattice constant c of the active material particle 22 at the diffraction angle peaks of 19.03 degrees and 19.25 degrees was obtained. The lattice constants c of the active material particle 22 at the diffraction angle peaks of 19.03 degrees and 19.25 degrees were 1.40 nm and 1.38 nm, respectively.

On the other hand, the lattice constant c corresponding to the diffraction angle peak of 18.95 degrees of the active material particle 21 containing stable lithium cobalt oxide was 1.40 nm. Therefore, it can be seen that the interplanar spacing of the lattice planes of the active material particle 22 of this embodiment has a portion slightly narrower than the interplanar spacing of the lattice planes of the active material particle 21 of the stable system.
Bragg's equation: ^c2 = ^λ2 ( ^h2 + ^k2 + ^l2 ) / (4 ^sin2θ ) Equation (2)

In formula (2), θ is the Bragg angle, λ is the X-ray wavelength, and h, k, and l (integers) are Miller indices.

Next, the microscopic structure of the active material particles 22 contained in the positive electrode 30 of this embodiment will be described with reference to the microscope images shown in FIGS.
2(a), (b), and (c) are, respectively, an SEM image (a) showing the appearance of an active material particle according to the first embodiment, a cross-sectional TEM image (b), and a cross-sectional TEM image (c) of an active material particle of a reference embodiment.

The active material particles 22 in Figs. 2(a) and (b) correspond to the active material particles 22 in Figs. 1(a) and (b). As shown in Figs. 2(a) and (b), the active material particles 22 according to this embodiment have particle portions 22b and protrusions 22p that protrude radially in multiple directions from the outer surface of the particle portions 22b. The active material particles 22 seen at the bottom in Fig. 2(a) are considered to be the ones in which a part of the protrusions 22p on the right side of the active material particles 22 in the figure has fallen off, exposing the surface of the particle portions 22b, during the process of preparing a sample for SEM from a sintered body of the active material particles 22 produced by the method described below.

As shown in FIG. 2(b), the active material particle 22 has a core portion 22c, multiple layer gaps 20g, multiple shell portions 22s, and radial gaps 20r inside the particle portion 22b, and exhibits a core-shell-like, discontinuous texture. The active material particle 22 differs from the active material particle 21 containing stable lithium cobalt oxide and having a plain cross-sectional structure shown in FIG. 2(c) in that the specific surface area is increased and the active material particle is porous both inside and outside the particle.

The SEM image in Figure 2(a) is a backscattered electron image taken at an accelerating voltage of 6 kV and a magnification of 12k, and the cross-sectional TEM image in Figure 2(b) was taken at an accelerating voltage of 300 kV for a sample with a slice thickness in the range of 100 to 150 μm.

The active material particles 22 according to this embodiment have a surface with an increased specific surface area and protrusions 22p that protrude in multiple directions, which is thought to increase the probability of contact between the active material particles 22 and the electrolyte, making it easier for active material ions to be exchanged between the active material particles 22 and the electrolyte.

In addition, a cross-sectional TEM image of the active material particle 22 of this embodiment was obtained as a lattice image (not shown). Such a cross-sectional TEM lattice image was obtained by photographing a sample with a slice thickness in the range of 100 to 150 μm at an acceleration voltage of 200 kV or 300 kV. The sliced sample was prepared using an ion milling device (manufactured by Leica) capable of FIB processing. From the obtained cross-sectional TEM image, it was possible to see that multiple protrusions protrude from the particle portion, as in the SEM image of FIG. 2(a). Furthermore, a striped pattern corresponding to the c-axis orientation of the crystal structure of the protrusions was observed. It was possible to read that the observed striped pattern is a distribution of multiple crystallites whose c-axes are inclined at a predetermined angle with respect to the axial direction along which the protrusions protrude. The sizes of the multiple crystallites were distributed in the range of 1 nm to 20 nm. The size of the crystallite was identified by identifying the area of the striped pattern arranged in a specific direction of the protrusions and fitting the boundary area to a circle. A crystallite can also be referred to as a single crystal domain.

On the other hand, the crystallite size obtained from the cross-sectional TEM lattice image of the active material particle 21, which is a virgin commercial material that has not undergone the first heating step and the second heating step described below, was 90 nm, which was larger than that of the active material particle 22. As described above, the results of the X-ray diffraction method and the cross-sectional TEM lattice image were consistent with each other regarding the crystallinity of the active material particle 22 of this embodiment and the stable active material particle 21 of the reference form. Both the results of the X-ray diffraction method and the cross-sectional TEM lattice image showed that the active material particle 22 had a finer crystallite size than the active material particle 21. In addition, both the results of the X-ray diffraction method and the cross-sectional TEM lattice image showed that the active material particle 22 had a more variable crystallite size than the active material particle 21. It is presumed that the active material particle 22 of this embodiment is metastable lithium cobalt oxide in a metastable state.

The diffusion coefficient of Li ions in the active material particles is thought to depend on the size of the lithium cobalt oxide crystallites, the distribution of crystallite orientation, and the specific surface area corresponding to the effective reaction area of the active material particles.

As shown in Fig. 1(a), Fig. 2(b) and (c), the active material particle 22 having radial protrusions 22p and made porous is considered to have a large effective reaction area for transferring Li ions between the active material particle 22 and the surrounding elements, and thus an efficient transfer effect can be obtained. On the other hand, the stable active material particle 21 has a smooth surface as shown in Fig. 8(a), and compared to the active material particle 22, it is presumed that the effective reaction area for transferring Li ions between the surrounding elements is not large, and therefore an efficient transfer effect cannot be obtained. The active material particle 22 of this embodiment is considered to have such unique morphological characteristics, thereby exhibiting high transportability (mobility) of the active material particle.

As shown in the X-ray diffraction profile of FIG. 1(b), the active material particles 22 exhibit a broad diffraction angle peak shifted to the high angle side compared to the stable active material particles 21, and have a finer crystallite size and a more dispersed orientation. It is believed that Li ions are transported along the crystallites inside the active material particles, and it is known that the smaller the crystallite size is, the greater the diffusion length of Li ions inside the active material particles. Therefore, it is believed that the active material particles 22 of this embodiment are more effective at transporting Li ions exchanged between the surrounding elements to the center inside the active material particles than the stable active material particles 21. In other words, the active material particles 22 of this embodiment are positive electrode active materials with guaranteed ion conductivity at the raw material stage before the positive electrode 30 and the positive electrode active material layer 20 are formed.

Therefore, it is believed that by applying the active material particles 22 of this embodiment to the positive electrode 30 shown in FIG. 3(a), a secondary battery 100 (FIG. 3(b)) with improved charge/discharge characteristics can be provided.

<Secondary battery, positive electrode structure>
The positive electrode 30 having the active material particles 22 and the secondary battery 100 according to the first embodiment will be described with reference to FIGS.

3A is a schematic cross-sectional view of a secondary battery 100 including a positive electrode 30 to which the active material particles 22 of this embodiment are applied. The secondary battery 100 includes an electrolyte layer 40 on the side opposite to the side of the positive electrode collector layer 10 that contacts the positive electrode active material layer 20. The secondary battery 100 includes an anode 70 on the side opposite to the side where the electrolyte layer 40 contacts the positive electrode active material layer 20. The anode 70 includes an anode active material layer 50 on the side opposite to the side where the electrolyte layer 40 contacts the positive electrode active material layer 20. The anode 70 includes an anode collector layer 60 on the side opposite to the side where the anode active material layer 50 contacts the electrolyte layer 40. In other words, the secondary battery 100 includes an anode 70, an electrolyte layer 40, and a positive electrode 30 in the stacking direction 200.

The positive electrode 30 to which the active material particles 22 of this embodiment are applied has a positive electrode current collector layer 10 and a positive electrode active material layer 20 containing the active material particles 22 and the positive electrode electrolyte 24, as shown in FIG. 3(b). In this specification, the structure in which active material ions are exchanged with the electrolyte layer 40 is referred to as a positive electrode, so the positive electrode active material layer 20 obtained by removing the positive electrode current collector layer 10 from the positive electrode 30 in FIG. 1(a) may be referred to as the positive electrode 20. In addition, since the positive electrode active material layer 20 of this embodiment contains the positive electrode electrolyte 24, it may be referred to as a composite positive electrode active material layer 20.

The current collector layer 10 is a conductor that conducts electrons between an external circuit (not shown) and the active material layer. The current collector layer 10 is formed in the form of a free-standing film of a metal such as SUS or aluminum, a metal foil, or a laminated form with a resin base.

The positive electrode active material layer 20 includes positive electrode active material layers 20a, 20b, and 20c as sublayers. The positive electrode active material layers 20a, 20b, and 20c are distinguished by the units of stacking in the layer thickness direction 200 before the active material particles 22 and the electrolyte in the positive electrode 24 are sintered. The positive electrode active material layers 20a, 20b, and 20c may have a distribution in the layer thickness direction in the volume fraction of the active material particles 22 and the electrolyte in the positive electrode 24, the conductive additive (not shown), the porosity, and the like. The layer thickness direction 200 is parallel or antiparallel to the stacking direction in which each layer is stacked, so it may be referred to as the stacking direction 200.

(Negative electrode)
A known method can be applied to the manufacturing method of the negative electrode. As in the modified example of the fourth embodiment of the present application, the manufacturing method of the positive electrode 30 of the first embodiment may be applied to the production of the negative electrode. As in the case of the positive electrode 30, the negative electrode may be formed from particles containing a negative electrode active material, or a metal such as metallic Li or In-Li may be formed as a film.

[Electrolytes]
Examples of electrolytes that can be applied to the electrolyte layer 40 include solid electrolytes and liquid electrolytes. In the case of a solid-state battery using a solid electrolyte, the electrolyte may be produced by the same manufacturing method as the positive electrode, or may be produced by a known method. As with the negative electrode, known methods include a coating process, a powder pressurization process, a vacuum process, and the like, but are not particularly limited thereto. In addition, the electrolyte may be produced alone, or may be produced as a laminate of two with the positive electrode and the negative electrode, or as a laminate of three with the positive electrode and the negative electrode, all at once. In addition, when a liquid electrolyte or a polymer electrolyte produced by a manufacturing method different from that of the electrodes is used, the manufacturing method is not particularly limited.

[Solid electrolyte]
Examples of solid electrolytes that can be used for the electrolyte layer 40 include oxide-based solid electrolytes, sulfide-based solid electrolytes, and complex _hydride -based solid electrolytes. Examples of oxide _- based solid electrolytes include Nasicon-type compounds such as _{Li1.5Al0.5Ge1.5} ( _PO4 ) _{3 and Li1.3Al0.3Ti1.7} ( _PO4 ) _{3 ,} and _garnet -type compounds such _{as Li6.25La3Zr2Al0.25O12 .} Examples of oxide- _based solid electrolytes include perovskite-type _{compounds such} as _{Li0.33Li0.55TiO3} . Examples of oxide-based solid electrolytes include lithicon-type compounds such as Li ₁₄ Zn(GeO ₄ ) ₄ , and acid compounds such as Li ₃ PO ₄ , Li ₄ SiO ₄ , and Li ₃ BO _3. Specific examples of sulfide-based solid electrolytes include Li ₂ S—SiS ₂ , LiI-Li ₂ S—SiS ₂ , LiI-Li ₂ S—P ₂ S ₅ , LiI-Li ₂ S—P ₂ O ₅ , LiI-Li ₃ PO ₄ —P ₂ S ₅ , and Li ₂ S—P ₂ S ₅ . The solid electrolyte may be crystalline or amorphous, or may be glass ceramics. In addition, the description Li ₂ S-P _{2 S5} and the like means a sulfide-based solid electrolyte formed using raw materials containing Li ₂ S and P _{2 S5} .

[Liquid electrolyte]
Examples of liquid electrolytes that can be used for the electrolyte layer 40 include non-aqueous electrolytes. The non-aqueous electrolyte is a liquid in which about 1 mole of lithium salt is dissolved in a non-aqueous solvent. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of the lithium salt include LiPF ₆ , LiBF ₄ , and LiClO _4. Alternatively, an aqueous electrolyte using an aqueous solvent may be used.

[Negative electrode active material]
Examples of the negative electrode active material include metals, metal fibers, carbon materials, oxides, nitrides, silicon, silicon compounds, tin, tin compounds, and various alloy materials. Among them, from the viewpoint of capacity density, metals, oxides, carbon materials, silicon, silicon compounds, tin, tin compounds, and the like are preferable. Examples of the metal include metal Li and In-Li, and examples of the oxide include Li ₄ Ti ₅ O ₁₂ (LTO: lithium titanate). Examples of the carbon material include various natural graphites (graphite), coke, carbon in the process of graphitization, carbon fibers, spherical carbon, various artificial graphites, and amorphous carbon. Examples of the silicon compound include silicon-containing alloys, silicon-containing inorganic compounds, silicon-containing organic compounds, and solid solutions. Examples of the tin compound include SnO _b (0<b<2), SnO ₂ , SnSiO ₃ , Ni ₂ Sn ₄ , and Mg ₂ Sn. The negative electrode material may also contain a conductive assistant. Examples of the conductive assistant include graphite such as natural graphite and artificial graphite, and carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black. Examples of the conductive assistant include conductive fibers such as carbon fibers, carbon nanotubes, and metal fibers, metal powders such as carbon fluoride and aluminum, conductive whiskers such as zinc oxide, conductive metal oxides such as titanium oxide, and organic conductive materials such as phenylene dielectrics.

<Secondary Battery Manufacturing Method>
The secondary battery can be manufactured by using known cell forming techniques such as laminate cell type, coin cell type, pressurized cell type, etc. A typical laminate cell type will be described as an example.

・Assembly of Laminated Cell The assembly of laminated cells will be described using an all-solid-state battery or a polymer battery as an example. The positive electrode, electrolyte, and negative electrode produced by the above manufacturing method are laminated and placed between the positive electrode collector and the negative electrode collector. The collector has an electrode tab for drawing out welded at its end. The laminate in which the collector, positive electrode, electrolyte, and negative electrode are laminated is set on an Al laminate film, the laminate is wrapped in the Al laminate film, and sealed while evacuating with a vacuum packaging machine. At this time, the electrode tab is pulled out of the laminate film, but the tab and the Al laminate film are bonded by thermocompression bonding, so that the sealing is maintained. After sealing, if necessary, pressure may be applied using an isostatic pressure pressurizing device or the like. The electrolyte may be a solid electrolyte or a polymer electrolyte, but both may be used for lamination. In addition to the laminate, an elastic material or a resin material may be laminated in the Al laminate film for the purpose of strength, molding, etc. In addition, a bipolar type (series/parallel) in which a plurality of the laminates are stacked may be used. In the case of a conventional lithium ion battery using a liquid electrolyte, a polyethylene separator is stacked instead of the electrolyte. The liquid electrolyte is injected and sealed before sealing with a vacuum packaging machine.

Next, a general method for manufacturing a secondary battery S4000 to which the active material particles 22 of this embodiment can be applied will be described with reference to FIG. 4. FIG. 4 is a flowchart showing an example of a method for manufacturing a secondary battery 100 as an all-solid-state battery having a solid electrolyte layer 40.

When manufacturing an all-solid-state battery (secondary battery 100), first, the raw materials constituting the positive electrode 30, the negative electrode 30, and the electrolyte layer 40 are prepared. The manufacturing method of the secondary battery 100 of this embodiment has a process for manufacturing the positive electrode 30, including a process S400 for preparing the positive electrode collector layer 10 and a process S5000 for manufacturing the positive electrode active material layer 20. The manufacturing method S5000 for the positive electrode active material layer 20 will be described later. Similarly, the manufacturing method of the secondary battery 100 of this embodiment includes a process for manufacturing the negative electrode 70, including a process S420 for preparing the negative electrode collector layer 60, a process S460 for disposing the negative electrode active material layer 50, and a process S440 for preparing the electrolyte layer 40. In the manufacturing method S4000 of the secondary battery of this embodiment shown in FIG. 4, the manufacturing steps for manufacturing the positive electrode 30, the electrolyte 40, and the negative electrode 70 are performed in parallel, but they may be performed in series, or the order of the steps for the positive electrode collector layer 10 and the positive electrode active material layer 20 may be changed.

Next, the positive electrode 30, electrolyte 40, and negative electrode 70 are assembled in an assembly process S470 so that the positive electrode collector layer 10, the positive electrode active material layer, the solid electrolyte layer 40, the negative electrode active material layer 50, and the negative electrode collector 10 are stacked in this order. In the assembly process S470, the positive electrode 30, electrolyte 40, and negative electrode 70 may be assembled with a sealing member such as a sealing film, a heat-sealing seal material, or a pressure-sensitive seal material (not shown).

Next, the laminated body that is the precursor of the assembled secondary battery is degassed in a degassing process S480 and compressed in the stacking direction in a compression process S490. The degassing process S480 and the compression process S490 may be performed simultaneously, or the order of the start and end times of the processes may be changed. The degassing process S480 and the compression process S490 may include a process of sealing the sealing member described above. The degassing process S480 may be omitted when the compression process S490 is performed under a reduced pressure atmosphere, a dry atmosphere, or an inert gas atmosphere. The degassing process S480 may be referred to as a drying process S480 or an exhaust process S480. The assembly process S470, the degassing process S480, and the compression process S490 may be referred to as a cell assembly process.

The components (or precursors) of the secondary battery 100 are in contact with other components (or precursors) adjacent in the layer direction 200 during the cell fabrication process. In addition, the positive electrode active material layer 20 may adopt a form in which the active material particles 22 and the positive electrode electrolyte 24 are in contact with each other within the layer, as shown in FIG. 2(b).

The active material particles 22 of this embodiment are particulate active material that already has guaranteed ionic conductivity, so there is a high degree of freedom in terms of arrangement to ensure contact with the in-layer electrolyte 24, conductive assistant, etc. In addition, since the active material particles 22 of this embodiment already have guaranteed ionic conductivity, there is no need to heat the positive electrode precursor pattern to 500 to 1000°C as in the conventional technology in any of the manufacturing steps of the secondary battery manufacturing method S4000.

Therefore, the positive electrode active material layer 20 and the positive electrode 30 in which the active material particles 22 are arranged do not require any further heat treatment to improve the transportability of the active material ions, and it is possible to lower the process temperature of the manufacturing method S4000 for the secondary battery.

In addition, in the degassing step S480 and the compression step S490, the laminate of the secondary battery 100 is subjected to high temperature and high pressure. In addition, the degassing step S480 and the compression step S490 involve an increase in the temperature of the laminate, but the degassing or compression action may be accelerated by applying heat from the outside.

Thus, although aluminum (melting point 660°C) has traditionally been desirable from the standpoint of electrical conductivity and workability, its use as a material for the current collector layer 10 may have been overlooked due to limitations in its heat resistance during the cell fabrication process. However, the use of the active material particles 22 of this embodiment makes it possible to use aluminum.

Similarly, carbon black powder (spontaneous ignition temperature 500°C), which is desirable for reasons such as material cost and sulfur resistance but may not be used as a conductive additive due to its heat resistance in the cell formation process, can be used by using active material particles 22.

Similarly, although the use of Nasicon-based solid electrolytes is desirable for reasons such as ion transportability, their use as electrolytes in the positive electrode may have been overlooked in terms of heat resistance during the cell fabrication process, but this can be achieved by using the active material particles 22 of this embodiment. Such Nasicon-based solid electrolytes include LAGP/LATP/LICGC, etc., and may form a reaction layer with the active material particles 22 at around 600° C., resulting in elution. If the Nasicon-based solid electrolyte and the active material particles 22 form a reaction layer, there is concern that the interface structure between the positive electrode 30 and the solid electrolyte layer 40 will be damaged, resulting in a decrease in ion conductivity. The structural formulas of LAGP/LATP/LICGC are Li1 _{+ xAlxGe2 -x(PO4)3, Li1+xAlxTi2-x ( PO4 ) 3 ,} and Li1+ _{x + yAlxTi2 - xSiyP3 - yO12 ,} respectively.

Similarly, in a manufacturing process of a secondary battery including a sulfide-based solid electrolyte, which generally has lower heat resistance than oxide-based solid electrolytes such as LBO and LATP, the use of the active material particles 22 of this embodiment makes it possible to lower the temperature of the cell fabrication process. Examples of sulfide-based solid electrolytes include Li ₂ S—SiS ₂ , LiI-Li ₂ S—SiS ₂ , LiI-Li ₂ S-P ₂ S ₅ , LiI-Li ₂ S-P ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , Li ₂ S-P ₂ S _{5 ,} and the like.

Similarly, in the manufacturing process of a secondary battery including a liquid electrolyte (electrolytic solution) that generally has lower heat resistance than solid electrolytes such as LBO and LATP, the active material particles 22 of this embodiment can be used to lower the temperature of the cell assembly process. Examples of such liquid electrolytes include non-aqueous electrolytes. The non-aqueous electrolyte is a liquid in which about 1 mol of lithium salt is dissolved in a non-aqueous solvent. Examples of non-aqueous solvents include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of lithium salts include LiPF ₆ , LiBF ₄ , and LiClO _4. In addition, such liquid electrolytes may be aqueous electrolytes using an aqueous solvent.

<Mechanism of modification of active material particles during manufacturing process>
Next, a method for producing the active material particles 22 of this embodiment and a mechanism by which the active material particles 22 are modified will be described with reference to FIGS.

Figure 5 shows a flowchart (a) of the manufacturing method S5000 of active material particles 22 according to the first embodiment, and an example of the temperature profile of each step (b). Figure 6 shows an estimated mechanism of denaturation of active material particles 22 according to the first embodiment. Figure 9 (a) is a schematic diagram showing the setting inside the furnace for the heating preparation step S520 in the manufacturing method S5000.

The manufacturing method S5000 of the active material particles 22 according to this embodiment will be described with reference to Figures 5(a) and 5(b), 6, and 9(a). The manufacturing method S5000 of the active material particles 22 includes a step S500 of preparing stable active material particles 21, a heating preparation step S520 of placing the resin and the active material particles 21 in the furnace of a heating furnace, a first heating step S540, a second heating step S560, and a temperature lowering step S580.

(Step S500 of preparing stable active material particles 21)
This step is a step of preparing active material particles 21 containing stable lithium cobalt oxide as shown in Figures 1 (c) and (d). The active material particles 21 containing stable lithium cobalt oxide are available as a commercial product. From the viewpoints of the yield and reaction rate of the method for producing the active material particles 22 of this embodiment, it is preferable that the active material as the starting material, which is the raw material of the active material particles 22, is also particulate. The particle size of the active material as the starting material is adjusted by classification.

The prepared stable active material particles 21 may be sintered with each other in the first heating step S540 and the second heating step S560 of S5000. For this reason, in order to manufacture independent active material particles 22 with guaranteed freedom of arrangement, the active material particles 21 are arranged on a ceramic plate or the like in this step S500 while being spaced apart from each other. In this step S500, it is also possible to prepare the active material particles 21 as a powder aggregate without spacing them apart, but a post-step is required to separate the obtained active material particles 22 from the sintered body, i.e., to divide the sintered body. In such a post-step, the microstructures such as the layered gaps 22g and the radial protrusions 22p within the grains may fall off or become defective, so it is preferable to separate the active material particles 21 in this step S500.

Incidentally, arranging the active material particles 21 at a distance in this step S500 is expected to have the effect of promoting the gas phase reaction described below in the first heating step S540 and the second heating step S560, and to have the effect of making the gas phase reaction proceed uniformly.

In this process S500, as shown in Figures 6 and 9(a), stable active material particles 21 are placed on an alumina plate 84 with island-like recesses 84d. The active material particles 21 are placed at a distance from each other in the first heating process S540 and the second heating process S540, which are subsequent processes of this process S520, in order to prevent the active material particles 21 from sintering together, and to promote gas-phase reactions in the two heating processes. The material constituting the plate 84 can be replaced with other ceramics, heat-resistant glass, or metal.

The active material particles 21 can be made of a particle material of lithium cobalt oxide, which is a stable commercial material. The active material particles 21 correspond to the precursor or starting material of the active material particles 22 of this embodiment.

In this step S520, the active material particles 21 can be placed in a form in which a plate 84 is placed on the resin 25 that releases reducing gas when heated, or in a form in which the active material particles 21 are placed directly on the resin 25, as shown in Figures 9(c) and (d). The form shown in Figure 9(a) is an arrangement in which reducing gas can be supplied from outside the heating furnace, while the form shown in Figures 9(c) and (d) is a modified version of the first embodiment, which differs in that reducing gas is supplied from inside the furnace by thermal decomposition of the resin 25 that is co-fired. The resin 25 may be in a bulk form, as shown in Figures 9(b) and (c), or in the form of powder or chips. The resin 25 and the active material particles 21 correspond to the precursors of the active material particles 22 from this step S500 onwards until the second heating step S560.

In the modified form shown in FIG. 9(b)(c), the resin 25 is selected from materials that can be thermally decomposed in the first heating step S540 until the solid content becomes zero. That is, a material having a transformation point temperature such as a thermal decomposition temperature or a combustion temperature according to the atmosphere and heating profile of the first heating step S540 is selected. When the resin 25 is a polyethylene terephthalate (PET) resin, as shown in FIG. 7(a)(b) described later, the resin is thermally decomposed while releasing a gas having a specific equivalent atomic weight for each temperature range. When the PET resin shown in FIG. 7(a)(b) is used as the resin 25, the resin 25 can be burned at a heating temperature of 450°C or higher in an oxygen-containing atmosphere and the solid content can be reduced to zero.

The resin 25 is a source that supplies a reducing gas that reduces the cobalt contained in the stable active material particles 21 in the first heating step S540, and can be said to be an atmosphere adjusting material that provides the conditions for transitioning from the first heating step S540 to the second heating step S560.

This step S500 is carried out in an air atmosphere at room temperature RT (15-25°C). When using a patterning device or clean bench, it may be carried out in a specific temperature range or in an inert atmosphere purged with inert gas. If it is desired to reduce the effects of adsorbed water, the atmosphere may be set to 50°C or higher, or the plate 84 may be heated.

(Arrangement step S520 of arranging stable active material particles in the heating furnace)
In step S520, as shown in FIG. 9B, active material particles 21, which are precursors of active material particles 22, are placed in a furnace interior 82 of a heating furnace.

The heating furnace may be of a batch type, continuous type, or sheet type, but in order to create a predetermined atmosphere in the space in which the active material particles 21 are heated, a type in which a casing is provided to cover the heated area in which the active material particles 21 are placed is adopted. The heating furnace can set the interior of the furnace to a predetermined atmosphere and a predetermined temperature. In order to create a predetermined atmosphere in the space in which the active material particles 21 are heated, at least, a type in which the gas conductance between the inside and outside of the heating furnace, or between the inside and outside of an inner container such as a crucible is adopted. This makes it possible to efficiently contact the active material particles 21 with a reactive gas in the first heating step S540 and the second heating step S560 described later. For reactive gases that are lighter than the main component of the atmosphere or the atmospheric permeation atomic weight 29, a casing that covers the center of the upper part of the heating furnace is effective.

In the case where the heating furnace is not completely sealed, the atmospheric pressure (total pressure) in the furnace in the first heating step S540 and the second heating step S560 is considered to be in an equal pressure relationship with the surroundings. For safety, the heating furnace may be placed in a room or work bench that is evacuated to a weak negative pressure (0.8-0.95 atm). When the surroundings of the heating furnace are the atmosphere, the inside of the furnace is maintained at atmospheric pressure to a weak negative pressure of atmospheric pressure in the heating step, and it is considered that the atmosphere is composed of stable and inert nitrogen _N2 up to a predetermined temperature range.

This process can be carried out at room temperature (RT) (15-25°C) or in an air atmosphere, just like preparation process S500. When using a patterning device or clean bench, it may be carried out in a specific temperature range or in an inert atmosphere purged with an inert gas. If it is desired to reduce the effects of adsorbed water, the atmosphere may be set to 50°C or higher, or the stage on which the resin 25 is placed may be heated.

In the first embodiment, the preparation step S500 and the arrangement step S520 are both performed at room temperature of 20°C in an air atmosphere. Therefore, in the preparation step S500 and the arrangement step S520, the resin 25 and the active material particles 21 are performed in an air atmosphere containing nitrogen, oxygen, and carbon dioxide.

(First heating step S540)
This step S540 is a step of contacting active material particles 21 containing stable lithium cobalt oxide with a reducing gas while heating, thereby thermally reducing the cobalt contained in active material particles 21. In other words, this step S540 is a step of contacting active material particles 21 containing stable lithium cobalt oxide with a reducing gas while heating, thereby generating active material particles 21r containing reduced cobalt.

In this step S540, the reducing gas supplied from the intake port 86 is brought into contact with the active material particles 21 in the furnace interior 82 of the heating furnace. The reducing gas supplied from the intake port 86 includes H ₂ diluted with an inert gas (Ar/H ₂ ), carbon monoxide CO, carbon monoxide CO diluted with nitrogen N ₂ , and the like. The supply amount of the reducing gas supplied from the intake port 86 is controlled by using a regulator, a pressure gauge, a flow meter, and the like (not shown). In addition, in order to adjust the partial pressure of the gas components carbon dioxide CO ₂ and water H ₂ O consumed and generated in the reduction reaction, and the gas component oxygen O ₂ contained in the furnace interior 82 from the arrangement step S520, the intake valve 87 and the exhaust valve 89 may be adjusted, respectively. By adjusting the exhaust valve, at least a part of the gas generated by combustion, the pyrolyzed gas, and the like is exhausted from the exhaust port 88 connected to an exhaust device (not shown). An atmosphere in which a reducing gas is the main component among active gas components, such as in the first heating step S540, may be referred to as a reducing atmosphere.

In the modified embodiment shown in Fig. 9(c)(d), a step of contacting the reducing gas released by thermally decomposing the resin 25 with the active material particles 21 is included. In other words, the heating step S540 in the modified embodiment shown in Fig. 9(c)(d) is performed until the atmosphere in the furnace 82 of the heating furnace is reduced in the reducing gas derived from the resin contained in the resin 25 and the partial pressure of the oxidizing gas containing oxygen exceeds the partial pressure of the reducing gas. The first heating step S540 of this embodiment is started under an oxygen-containing atmosphere containing oxygen _O2 . The heating temperature in the first heating step S540 of this embodiment can be performed at 300 ° C or more and 690 ° C or less.

The present inventors presume that in the first heating step S540, the active material particles 21 undergo a thermal reduction reaction with carbon monoxide (CO) derived from the resin 25, reducing the cobalt (Co) and making the microstructure within the particles porous.

The heating furnace 85 is capable of heating the atmosphere inside the furnace 82, the active material particles 21, and the plate 84 with a heater (not shown). The heating temperature is monitored by a thermocouple, an infrared sensor, etc.

In this embodiment, in FIG. 9B, carbon monoxide CO diluted with nitrogen N ₂ (N ₂ /CO) is supplied into the furnace 82 as a reducing gas.

(Second heating step S560)
In the second heating step S560, the active material particles 21r in which at least a part of the cobalt has been reduced are oxidized by oxygen _O2 remaining in the atmosphere in place of the reducing gas (carbon monoxide CO) whose supply has been stopped, and the reduced cobalt returns to lithium cobalt oxide. The inventors presume that the LCO obtained by reoxidation has a microstructure and crystal structure different from that of the stable LCO. The heating temperature in the second heating step S560 can be set to 400°C or higher and 690°C or lower.

The reasons for this will be explained using Figures 5(a) and (b), 6, 7(a) to (c), and 8(a) to (d).

7(a) and (b) are the results of differential thermal analysis. FIG. 7(a) is a differential thermal analysis (DTA) profile of a sheet-like PET resin used for the resin 25. In the figure, the solid line is the DTA curve of the equivalent atomic weight 28 (left axis), the dashed line is the DTA curve of the equivalent atomic weight 32 (right axis), and the dotted line is the DTA curve of the equivalent atomic weight 44 (right axis). The equivalent atomic weight 28 includes nitrogen _N2 and monoxide, but from the profile of the DTA curve that increases from room temperature to 520°C and decreases above 520°C, the composition of the PET resin, and the analysis environment, nitrogen gas is not considered, and the solid line profile is considered to be carbon monoxide CO. The dashed and dotted line profiles are considered to be oxygen _O2 and carbon dioxide _CO2 , respectively, for the same reason.

7(a) shows that the PET resin is gradually thermally decomposed by heating from room temperature, and releases carbon monoxide CO, peaking at around 520°C. Also, because oxygen and carbon dioxide qualitatively show opposite tendencies of increase and decrease, it can be seen that part of the carbon monoxide CO, or part of the carbon that constitutes the PET resin, consumes oxygen in the atmosphere and becomes carbon dioxide _CO2 . Carbon dioxide _CO2 peaks at around 590°C and begins to increase mainly at higher temperatures than carbon monoxide CO.

On the other hand, the thermal decomposition temperature of the PET resin used in resin 25 was approximately 400°C, which was determined by the temperature at which the solid content decreased by 50% in the thermogravimetric analysis TG profile shown in Figure 6 (c).

Therefore, the first heating step S540 can be considered to include a step of bringing the reducing gas supplied from outside the furnace or inside the furnace 82 into contact with the stable active material particles 21.

Next, FIG. 6(b) shows a differential thermal analysis (DTA) profile of a form in which a sheet-shaped PET resin corresponding to FIG. 6(a) is co-fired with a plurality of stable active material particles 21.

6(b), the carbon monoxide CO, which increases in the case of PET resin alone due to heating in the temperature range below 520°C, starts to decrease at 350°C or higher, so it is estimated that at 350°C or higher, part of the carbon monoxide derived from the PET resin is consumed in the thermal reduction reaction of LCO. That is, from FIG. 6(b), it is estimated that stable lithium cobalt oxide is thermally decomposed by carbon monoxide CO at 350°C or higher. It is also estimated that when the PET resin is baked together with the active material particles 21 containing LCO, at least part of the released carbon monoxide CO is immediately consumed in the thermal reduction reaction of LCO at 350°C or higher, and is directly oxidized by the surrounding oxygen to carbon dioxide CO at 510°C or higher. That is, in the first heating step S540, it is estimated that at least part of the carbon monoxide CO derived from the resin 25 is immediately consumed in the thermal reduction reaction of LCO at 350°C or higher, and is consumed in the production of carbon dioxide CO at 510°C or higher.

The inventors used a small container equipped with a crucible 80 and a lid 81 as shown in FIG. 9(c) to investigate the dependence of the heating atmosphere of active material particles 21 and PET-containing resin 25 in the air.

Fig. 8(a) is an SEM image showing the appearance of active material particles that have been subjected to a first heating test step in which only active material particles 21 are placed in a crucible 80 without resin 25, a lid 81 is placed, and a heating test step in which a heating temperature of 400°C is performed for 1 hour in an air atmosphere, and a second heating test step in which a heating temperature of 510°C is performed for 1 hour. The active material particles in Fig. 8(a) have been subjected to a heating test in a state in which there is no source for generating reducing gas CO, so that oxygen _O2 is affected by the active gas, but they have the appearance of stable lithium cobalt oxide without radial protrusions.

FIG. 8(b) is an SEM image showing the appearance of active material particles that have been subjected to a first heating test step in which the resin 25 and active material particles 21 are placed in a crucible 80 and heated at a temperature of 400° C. for 1 hour in an air atmosphere without a lid 81, and a second heating test step in which the active material particles are heated at 510° C. for 1 hour. The active material particles in FIG. 8(b) have been subjected to a heating test in which carbon monoxide CO, a reducing gas generated from the resin 25, diffuses outside the crucible 80 and does not remain in the crucible 80, so it is presumed that they are affected by both active gases, oxygen O ₂ and carbon monoxide CO. The active material particles in FIG. 8(b) had the appearance of a stable lithium cobalt oxide without radial protrusions. The results of this test showed that the appearance of a stable lithium cobalt oxide without radial protrusions was exhibited only in the second heating test step, regardless of whether the crucible 80 was closed with the lid 81 or not.

Figure 8(c) is an SEM image showing the appearance of active material particles that have been subjected to a first heating test process in which resin 25 and active material particles 21 are placed in a crucible 80, the lid 81 is closed, and the first heating test process is performed in an air atmosphere at a heating temperature of 400°C for 1 hour, and a second heating test process is performed without the lid 81 and at 510°C for 1 hour. The active material particles in Figure 8(c) have been subjected to a heating test in a state in which they stay in the crucible 80, where carbon monoxide CO, a reducing gas generated from the resin 25, is generated, and therefore are presumed to be affected by the reducing active gas containing carbon monoxide CO. The active material particles in Figure 8(c) have radial protrusions and have an appearance similar to that of the active material particles 22 of this embodiment.

FIG. 8(d) is an SEM image showing the appearance of active material particles that have been subjected to a first heating test step in which the resin 25 and active material particles 21 are placed in a crucible 80, the lid 81 is closed, and the first heating test step is performed in an air atmosphere at a heating temperature of 400 ° C. for 1 hour, followed by a second heating test step at 510 ° C. for 1 hour. The active material particles in FIG. 8(d) have undergone the first heating test step in a state in which the resin 25 stays in the crucible 80 where carbon monoxide CO, a reducing gas, is generated, and therefore are presumed to have been fired in a reducing atmosphere containing carbon monoxide CO as an active gas. Subsequently, the active material particles in FIG. 8(d) have undergone the second heating test step after the supply of carbon monoxide CO, a reducing gas, is stopped as the thermal decomposition of the resin 25 progresses, and therefore are presumed to have been fired in an oxidizing atmosphere containing oxygen O ₂ as an active gas. The active material particles in FIG. 8(d) have radial protrusions and have an appearance similar to that of the active material particles 22 of this embodiment. Fig. 8(e) is a cross-sectional SEM image of the active material particle in Fig. 8(d). In the cross section of the active material particle in Fig. 8(d), radial protrusions were observed on the outside of the particle, and a porous fine structure such as layered gaps was observed within the particle.

The active material particles for which the heating atmosphere dependence was examined were measured by X-ray diffraction, and as a _result , in addition to LCO, cobalt oxide (CoO) and lithium carbonate ( _Li2CO3 ) were detected only in the active material particles after the first heating test step corresponding to Figures 8(c) and (d). That is, cobalt with an oxidation number of III and II was detected only in the active material particles after the first heating test step corresponding to Figures 8(c) and (d).

This is a corresponding SEM image of the cross section of an LCO particle after step 2 in (d) above, and gaps were observed inside the LCO particle that were not present in the cross section before heating shown in Figure 4(a). However, no precipitation of protrusions was observed on the LCO particle surface at this time. The cross section of the LCO particle after step 3 is as shown in Figure 4(b), and in addition to the gaps inside the LCO particle, precipitation of protrusions was observed on the LCO particle surface after step 3.

On the other hand, when a sample that had been subjected to the second heating test process (500°C firing conditions) corresponding to FIG. 8(d) was reheated at 700°C for 10 minutes, the active material particles of the sample had a homogeneous structure inside the particles, and exhibited the morphology of a stable LCO with no protrusions on the particle surface (not shown). Since the porous structure including the protrusions 22p and layered gaps 22g was burned away by the heating test at 700°C, it is believed that the oxidation reaction and melting reaction proceeded too far at 700°C or higher, resulting in a stable LCO with neither a fine structure nor a specific crystal structure.

Therefore, in the second heating step, which reoxidizes the active material particles 21r containing the cobalt reduced in the first heating step, the heating temperature is set to 690°C or lower, so that oxidation and melting do not progress to the stable LCO.

Based on the analysis results of Figures 7 and 8, the inventors of the present application have drawn up an image of each process shown in Figures 5(a), (b) and 6. Figures 5(a), (b) and 6 show the estimated mechanisms corresponding to each of steps S500 to S580.

At the preparation step S500 and arrangement step S520, there is no significant structural change in the active material particles 21 (precursors of the active material particles 22).

In the first heating step S540, the active material particles 21 are heated to 500° C. In the early stage of the first heating step S540, the cobalt in the active material particles 21 that comes into contact with the supplied reducing positive gas, carbon monoxide CO, is reduced from a valence of II to a valence of III, and at least a part of the lithium cobalt oxide is transformed into cobalt oxide (CoO/Co ₃ O ₄ ). Furthermore, in the early stage of the first heating step S540, the cobalt in the active material particles 21 that comes into contact with the supplied reducing gas, carbon monoxide CO, is reduced from a valence of II to a valence of III, and is transformed into reduced active material particles 21r having a microstructure in which the inside of the particles is made porous. In the early stage of the first heating step S540, the supplied carbon monoxide CO dominates as an active gas in the heating atmosphere and is consumed in the transformation of LCO. After the supply of carbon monoxide CO is cut off in the latter part of the first heating step S540, the carbon monoxide CO is oxidized by oxygen _O2 in the atmosphere and replaced with inert carbon dioxide _CO2 , and the atmosphere inside the furnace 82 shifts from a reducing atmosphere to an inert atmosphere.

Furthermore, in the second heating step S540, when the partial pressure of carbon monoxide CO becomes substantially zero, the partial pressure of carbon dioxide _CO2 decreases and oxygen _O2 is no longer consumed, so that oxygen _O2 , which is active at high temperature, reoxidizes the active material particles 21r in which part of the cobalt has been reduced. That is, the atmosphere in the second heating step S560 becomes dominated by oxygen _O2 at high temperature, and shifts from inactive to oxidizing.

In the second heating step S560, the oxidation number of at least a portion of the cobalt (Co) in the active material particles changes from II or II2/3 to III. In the second heating step S560, the oxidation reaction does not proceed completely within the particles, so it is believed that the layered gaps 22g formed in the first heating step and the protrusions 22p formed in the first half of the second heating step remain even after the temperature-reducing step S580. "The oxidation reaction does not proceed completely" can be rephrased as "the oxidation reaction proceeds incompletely" or "the oxidation reaction proceeds locally."

In addition, in the modified embodiment in which the setting of FIG. 9(c) was performed, a test was performed to examine the temperature rise rate dependency by changing only the level of the temperature rise rate in the first heating step S540. The result of the test to examine the temperature rise rate dependency showed that when the temperature rise rate was 10° C./min or less, active material particles having a microstructure and a crystal structure common to the active material particles 22 of the first embodiment were obtained. When the temperature rise rate exceeded 10° C./min, the obtained active material particles did not have a microstructure and a crystal structure common to the active material particles 22 of the first embodiment. It is considered that such a temperature rise rate dependency requires that the active material particles 21 stay in the temperature range of 300° C. or more and 500° C. or less, where carbon monoxide CO is generated from the resin 25, for 20 minutes or more in the first heating step. If the temperature rise rate exceeds 10°C/min and the residence time of the active material particles 21 in the temperature range of 300°C to 500°C is less than 20 minutes, it is estimated that the PET resin is rapidly and completely combusted, and inactive carbon dioxide _CO2 is supplied from the beginning of the heating process, resulting in a shortage of carbon monoxide CO. The second heating process S560 can be performed at 400°C to 690°C for 10 minutes to 90 minutes. The residence time of the active material particles 21 in the temperature range of 300°C to 500°C may be expressed in other words as the heating time of the active material particles 21 in the temperature range of 300°C to 500°C.

(Temperature lowering step S580)
In this step, the temperature of the active material particles 22 having the reduced and reoxidized cobalt is lowered to form modified active material particles 22. After the second heating step S560, which is a localized oxidation reaction, the fine structure within the active material particles formed in steps S540 to S560 remains in this step S560, as shown in FIG.

Second Embodiment
The positive electrode 32 according to the second embodiment will be described with reference to Figs. 10(a) and (b). Figs. 10(a) and (b) show a flow chart showing a manufacturing method S10000 of the positive electrode 32 according to the second embodiment and a schematic cross-sectional view. The positive electrode 32 according to this embodiment differs from the positive electrode 30 according to the first embodiment shown in Fig. 3(b) in that the positive electrode active material 20 is composed of active material particles 22 without containing an electrolyte in the positive electrode.

As shown in FIG. 10(a), the positive electrode 32 of this embodiment starts with preparing the active material particles 22 by the manufacturing method S5000 of the active material particles 22 according to the first embodiment.

Next, step S900 is performed to place the active material particles 22 on the positive electrode collector layer 10 using a known particle deposition technique. In other words, step S900 of placing the active material particles 22 on the positive electrode collector layer 10 may include a step of placing a plurality of active material particles 22 on a predetermined surface. As the particle deposition technique, an inkjet method, a spin coating method, a screen printing method, a chemical vapor deposition method (CVD), a vapor deposition method, an electrophotographic method, or the like is appropriately adopted.

Next, step S920 is performed to fix the deposited active material particles 22 onto the positive electrode current collector 10. In this step S920, energy is applied by heating, light irradiation, etc. This step S920 includes a step of thermally decomposing the binder matrix component applied onto the positive electrode current collector layer 10 in the previous step S900 and vaporizing the solvent component by applying energy. This step S920 includes a step of bonding active material particles 22 that have weak bonding strength to each other by applying energy.

The heating temperature in step S920 is preferably less than 700°C, for example 690°C or less, at which point the cobalt contained in the active material particles 22 is completely oxidized to form stable lithium cobalt oxide.

Next, the positive electrode 34, which is a modified version of the second embodiment, will be described with reference to Figures 10(c) and (d). Figures 10(c) and (d) show a flow chart showing a manufacturing method S10200 for the positive electrode 34 according to this modified version, and a schematic cross-sectional view.

The positive electrode 34 differs from the positive electrode 30 of the first embodiment in that the positive electrode active material 20 and the positive electrode electrolyte 24 have a sea-island pattern in each of the positive electrode active material layers 20a, 20b, and 20c, as shown in FIG. 10(d), and that the positive electrode active material layer 20 is deposited on the electrolyte layer 40. The positive electrode 34 also differs from the positive electrode 30 of the first embodiment in that the sea-island pattern is aligned between each of the positive electrode active material layers 20a, 20b, and 20c.

As in the second embodiment, the positive electrode 30 of this embodiment starts with preparing the active material particles 22 by the manufacturing method S5000 of the active material particles 22 according to the first embodiment, as shown in FIG. 10(c).

Next, step S940 is performed to pattern the active material particles 22 and the positive electrode electrolyte 24 on the active material layer 40 using a known particle deposition technique.

Next, step S960 is performed to fix the patterned active material particles 22 and the positive electrode electrolyte 24 pattern onto the active material layer 40.

REFERENCE SIGNS LIST 100 secondary battery 30 positive electrode 20 positive electrode active material layer 22 active material particles S520 arrangement step S540 first heating step S560 second heating step

Claims (5)

1. An active material particle used in a positive electrode containing lithium cobalt oxide, the active material particle having a particle portion and a protruding portion protruding in multiple directions from the particle portion and having a region with a crystallite size of 1 nm or more and 20 nm or less, and exhibiting a bimodal diffraction angle peak of a diffraction angle peak at 18.9 degrees or more and 19.1 degrees or less and a diffraction angle peak at 19.2 degrees or more and 19.7 degrees or less, as determined by an X-ray diffraction angle 2θ method. An active material particle which is applied to a positive electrode containing lithium cobalt oxide, the active material particle having a particle portion having a discontinuous texture in cross section and protrusions protruding in multiple directions from the particle portion, and which exhibits a bimodal diffraction angle peak, as determined by 2θ method, having a diffraction angle peak at 18.9 degrees or more and 19.1 degrees or less and a diffraction angle peak at 19.2 degrees or more and 19.7 degrees or less. 3. The active material particle according to claim 1 , wherein a plurality of diffraction angle peaks are observed in the X-ray diffraction angle range of 19.2 degrees or more and 19.7 degrees or less. 2. The active material particle according to claim 1 , wherein the particle portion has a discontinuous texture in cross section. The active material particle according to claim 1 , wherein the particle portion has a core portion and a shell portion.

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