Positive electrode active material for lithium-ion secondary batteries, positive electrode, lithium-ion secondary battery, and method for manufacturing the positive electrode active material for lithium-ion secondary batteries.

Abstract

Provided is a positive-electrode active material for lithium-ion secondary batteries which can achieve an even higher capacity, improved cycle and discharge characteristics, and a low cost.　A positive-electrode active material 11 for lithium-ion secondary batteries has a fluoride layer 11b which covers at least part of a core particle 11a and is constituted by fluoride of the lithium metal composite oxide above. The lithium metal composite oxide of the core particle 11a is represented by LiNikColMnmO2(k+l+m=1, k≧0.6). The fluoride of the lithium metal composite oxide, in the fluoride layer 11b, is represented by Li1-zNikColMnmO2-xFx(k+l+m=1, k≧0.6, z≦0.62, 0<x≦1).

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for lithium-ion secondary batteries, a positive electrode, a lithium-ion secondary battery, and a method for producing a positive electrode active material for lithium-ion secondary batteries. This application claims priority based on Japanese Patent Application No. 2021-044282, filed in Japan on March 18, 2021, and the contents of that application are incorporated herein by reference. [Background technology]

[0002] In recent years, secondary batteries with non-aqueous electrolytes, such as lithium-ion batteries, have been proposed and put into practical use as batteries that are expected to be smaller, lighter, and have higher capacity. These lithium-ion batteries consist of a positive electrode and a negative electrode that have the property of being able to reversibly insert and remove lithium ions, and a non-aqueous electrolyte. Lithium metal composite oxides are used as positive electrode active materials in lithium-ion batteries. Lithium-ion batteries are already in practical use as small power sources for mobile phones and laptop computers. Furthermore, their application is being explored in medium and large power sources for automotive and energy storage applications. With this expansion of applications, extending the lifespan of lithium-ion batteries is a crucial challenge.

[0003] Lithium metal composite oxides used as positive electrode active materials for lithium-ion secondary batteries include NCM-type composite oxides containing lithium, nickel, cobalt, manganese, and oxygen, for example. In particular, NCM-type lithium composite oxides with a high nickel content are being increasingly applied to large batteries due to their high capacity, high thermal stability, and low cost, and attempts are being made to further increase capacity and improve cycle characteristics and discharge characteristics.

[0004] Generally, lithium composite oxides used as positive electrode active materials include not only NCM type but also LNMO type composite oxides containing, for example, lithium, nickel, manganese, and oxygen. While LNMO type composite oxides have the advantage of being usable at high potentials, they have the problem that metal elements dissolve into the electrolyte during high-potential operation, causing battery degradation. Therefore, electrodes equipped with a water-repellent coating layer on the surface of the electrode active material have been proposed (Patent Document 1).

[0005] Furthermore, as a modification of the electrode active material surface, positive electrode active materials have been proposed in which the surface of lithium cobalt composite oxide is covered with Y2O3 or Li2YO3 to achieve high operating voltage, excellent charge / discharge characteristics, and storage characteristics (Patent Document 2), and positive electrode active materials have been proposed in which hydroxides or oxyhydroxides of rare earth elements such as Y are fixed to the surface of lithium transition metal composite oxides such as lithium cobalt composite oxide to suppress capacity reduction (Patent Document 3). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2017-174692 [Patent Document 2] Japanese Patent Application Publication No. 05-06780 [Patent Document 3] Japanese Patent Publication No. 2011-141989 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the above-mentioned patent documents do not disclose any modification of the surface of high-nickel NCM-type composite oxides, nor do they disclose the specific structure of the coating formed on the surface of the composite oxide. Therefore, there is room for improvement in order to enhance the characteristics of lithium-ion secondary batteries using NCM-type composite oxides.

[0008] The object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery, a positive electrode, a lithium ion secondary battery, and a method for manufacturing a positive electrode active material for a lithium ion secondary battery, which can achieve further higher capacity, improve cycle characteristics and discharge characteristics, and further realize low cost.

Means for Solving the Problems

[0009] As a result of intensive research, the inventors of the present invention found that by forming a fluoride of the above composite oxide on the surface or near the surface of a positive electrode active material for a lithium ion secondary battery composed of a high-nickel-based NCM-type composite oxide, the solid electrolyte interface layer (hereinafter also referred to as the CEI layer) formed during charge and discharge cycles can be thinned. Further, by thinning the CEI layer, the high resistance to lithium ion transport at the electrolyte interface and electron conduction between positive electrode active materials is suppressed, and by achieving both the contradictory problems of surface stabilization by the CEI layer and suppression of high resistance, it was found that further higher capacity and improvement of cycle characteristics and discharge characteristics can be realized.

[0010] That is, the present invention provides the following configurations. [1] Core particles composed of a lithium metal composite oxide, and a fluoride layer covering at least a part of the core particles and composed of a fluoride of the lithium metal composite oxide. The lithium metal composite oxide is represented by LiNi k Co l Mn m O2 (k + l + m = 1, k ≥ 0.6). [[ID=`23]]The fluoride of the lithium metal composite oxide is represented by Li 1-z Ni k Co l Mn m O 2-x F x (k + l + m = 1, k ≥ 0.6, z ≤ 0.62, 0 < x ≤ 1), a positive electrode active material for a lithium ion secondary battery.

[0011] [2] The positive electrode active material for a lithium ion secondary battery according to [1] above, wherein the fluoride layer contains lithium fluoride.

[0012] [3] The fluoride layer has a concentration gradient in which the atomic concentration ratio of fluorine atoms to oxygen atoms in the fluoride layer decreases from the outer surface to the inside of the positive electrode active material for the lithium-ion secondary battery, and the positive electrode active material for the lithium-ion secondary battery according to [1] or [2] above.

[0013] [4] The fluoride of the lithium metal composite oxide has a layered rock salt structure, and fluorine atoms are coordinated between adjacent transition metal layers of the layered rock salt structure, and the positive electrode active material for the lithium-ion secondary battery according to [1] or [2] above.

[0014] [5] The atomic concentration ratio of fluorine atoms to oxygen atoms in the fluoride layer is 0.1% or more in the range where the depth from the outer surface of the positive electrode active material for the lithium-ion secondary battery is 0 to 50 nm, and the positive electrode active material for the lithium-ion secondary battery according to any one of [1] to [3] above.

[0015] [6] The atomic concentration ratio of fluorine atoms to oxygen atoms in the fluoride layer is 0.01 or more and 0.08 or less in the range where the depth from the outer surface of the positive electrode active material for the lithium-ion secondary battery is 50 nm to 100 nm, and the positive electrode active material for the lithium-ion secondary battery according to [5] above.

[0016] [7] The lithium metal composite oxide is represented by LiNi 0.8 Co 0.1 Mn 0.1 O2, and the fluoride of the lithium metal composite oxide is Li 1-z Ni 0.8 Co 0.1 Mn 0.1 O 2-x F x (z ≤ 0.62, 0 < x ≤ 1), and the positive electrode active material for the lithium-ion secondary battery according to any one of [1] to [4] above.

[0017] [8] A positive electrode for a lithium-ion secondary battery, comprising a positive electrode current collector and a positive electrode active material containing the positive electrode active material for a lithium-ion secondary battery described in any of [1] to [7] above, provided on the positive electrode current collector.

[0018] [9] A lithium-ion secondary battery comprising a positive electrode for a lithium-ion secondary battery as described in [8] above, a negative electrode, and an electrolyte.

[0019]

[10] A method for producing a positive electrode active material for a lithium-ion secondary battery, comprising the steps of placing a noble gas fluoride and a lithium metal composite oxide in a sealed space and forming a fluoride layer on the lithium metal composite oxide composed of the fluoride of the lithium metal composite oxide.

[0020]

[11] A method for producing a positive electrode active material for a lithium-ion secondary battery according to

[10] , comprising the step of heat-treating the lithium metal composite oxide on which the fluoride layer has been formed after the step of forming the fluoride layer.

[0021]

[12] A method for producing a positive electrode active material for a lithium-ion secondary battery according to

[10] , wherein the lithium metal composite oxide on which the fluoride layer is formed is heat-treated at 300°C to 1000°C for 3 to 10 hours. [Effects of the Invention]

[0022] According to the present invention, it is possible to provide a positive electrode active material for lithium-ion secondary batteries, a positive electrode, a lithium-ion secondary battery, and a method for manufacturing a positive electrode active material for lithium-ion secondary batteries, which can achieve further high capacity, improve cycle characteristics and discharge characteristics, and reduce costs. [Brief explanation of the drawing]

[0023] [Figure 1] Figure 1(a) is an electron microscope image showing the appearance of the positive electrode active material for a lithium-ion secondary battery according to this embodiment, and Figure 1(b) is a schematic diagram showing the structure of the positive electrode active material for a lithium-ion secondary battery. [Figure 2]Figures 2(a) and 2(b) are schematic diagrams showing the layered rock salt structure of NCM811 fluoride, as an example of a lithium metal composite oxide fluoride. [Figure 3] Figure 3 is a schematic diagram conceptually illustrating the difference in lattice volume change between a conventional positive electrode active material for lithium-ion secondary batteries without a fluoride layer and a positive electrode active material for lithium-ion secondary batteries according to this embodiment. [Figure 4] Figure 4(a) is a graph showing the change in lattice volume during the lithium ion desorption reaction, and Figure 4(b) is a schematic diagram illustrating the mechanism by which the change in lattice volume is suppressed in lithium metal composite oxide fluorides. [Figure 5] Figure 5 shows an example of a specific configuration of a lithium-ion secondary battery according to an embodiment of the present invention. [Figure 6] Figures 6(a) to 6(d) are electron microscope images showing the appearance of the positive electrode active material for lithium-ion secondary batteries in Examples 1 to 3 and Comparative Example 1. [Figure 7] Figure 7 shows the XRD patterns of the positive electrode active materials for lithium-ion secondary batteries in Examples 1-3 and Comparative Example 1. [Figure 8] Figure 8(a) shows the XPS spectra of fluorine atoms (1s) in the positive electrode active material for lithium-ion secondary batteries in Examples 1-3 and Comparative Example 1, and Figure 8(b) shows the peaks attributed to the transition metal-fluorine bond and the peaks attributed to lithium fluoride in Examples 1-3. [Figure 9] Figures 9(a) and 9(b) show the XPS spectra of nickel element (2p) in the positive electrode active material for lithium-ion secondary batteries in Examples 1-3 and Comparative Example 1. [Figure 10] Figures 10(a) and 10(b) show the XPS spectra of manganese element (2p) in the positive electrode active material for lithium-ion secondary batteries in Examples 1-3 and Comparative Example 1. [Figure 11] Figure 11 shows the XPS spectra of cobalt (2p) in the positive electrode active material for lithium-ion secondary batteries in Examples 1-3 and Comparative Example 1. [Figure 12] Figure 12 shows the initial charge-discharge curves of the cycle tests in Examples 1-3 and Comparative Example 1. [Figure 13] Figure 13(a) shows the change in discharge capacity during the cycle test in Examples 1-3 and Comparative Example 1, and Figure 13(b) shows the change in discharge capacity retention rate. [Figure 14] Figure 14(a) is a schematic diagram showing the solution resistance, CEI layer resistance, and charge transfer resistance at the positive electrode in Examples 1-3 and Comparative Example 1, and Figure 14(b) is a graph showing the impedance after one cycle of the cycle test. [Figure 15] Figure 15 is a graph showing the impedance after 200 cycles of the cycle test in Examples 1-3 and Comparative Example 1. [Figure 16] Figure 16(a) shows the change in discharge capacity during the rate characteristic test in Examples 1 to 3 and Comparative Example 1, and Figure 16(b) shows the change in discharge capacity retention rate. [Figure 17] Figure 17 is a graph showing the UPS analysis results for Examples 1-3 and Comparative Example 1. [Figure 18] Figures 18(a) to 18(c) are graphs showing the changes in the content of each atom in the direction from the outer surface to the interior of the positive electrode active material in Examples 1 to 3, and Figure 18(d) is a graph showing the change in the fluorine atom concentration relative to oxygen atoms in the above direction in Example 1. [Figure 19] Figure 19(a) shows the cyclic voltammetry (CV) measurement results in Comparative Example 1, and Figure 19(b) shows the cyclic voltammetry (CV) measurement results in Example 1. [Figure 20] Figure 20(a) is a comparison of cyclic voltammetry (CV) in Example 1 and Comparative Example 1, Figure 20(b) shows the relationship between sweep speed and peak current in Comparative Example 1, and Figure 20(c) shows the relationship between sweep speed and peak current in Example 1. [Figure 21]FIG. 21(a) is a diagram showing the change in discharge capacity in the cycle test under high rate conditions in Example 1 and Comparative Example 1, and FIG. 21(b) is a diagram showing the change in discharge capacity retention rate. [Figure 22] FIG. 22(a) is a diagram showing the change in discharge capacity in the cycle test under high potential conditions in Examples 1 to 3 and Comparative Example 1, and FIG. 22(b) is a diagram showing the change in discharge capacity retention rate. [Figure 23] FIG. 23(a) is a diagram showing the change in discharge capacity in the rate performance test under high potential conditions in Example 1 and Comparative Example 1, and FIG. 23(b) is a diagram showing the change in discharge capacity retention rate. [Figure 24] FIG. 24(a) is a diagram showing the measurement results of the first cycle cyclic voltammetry (CV) under high potential conditions in Example 1 and Comparative Example 1, and FIG. 2X(b) is a diagram showing the measurement results of the fifth cycle cyclic voltammetry (CV). [Figure 25] FIG. 25(a) is a diagram showing the lithium ion diffusion coefficient (DLi) with respect to the open circuit voltage obtained from GITT measurement, and FIG. 25(b) is a diagram showing the lithium ion diffusion coefficient with respect to the lithium composition (Li1-zNi0.8Co0.1Mn0.1O2-xFx: 0 < z < 0.8, x = 0.8). [Figure 26] FIG. 26(a) is a diagram showing the c-axis length with respect to the lithium composition obtained from Ex-situ XRD measurement, and FIG. 26(b) is a diagram showing the lithium ion diffusion coefficient with respect to the lithium composition (Li1-zNi0.8Co0.1Mn0.1O2-xFx: 0 < z < 0.8, x = 0.8) obtained from GITT measurement. [Figure 27] FIG. 27 is a diagram showing the results of XPS analysis of the positive electrode active material in Examples 4 to 5 and Comparative Example 2 and examining the change in the valence band spectrum before and after fluoride ion substitution. [Figure 28] FIG. 28 is a diagram showing the results of UPS measurement on the surface of the positive electrode active material in Examples 4 to 5 and Comparative Example X and evaluating the work function of the positive electrode active material. [Figure 29]Figure 29 shows the results of evaluating the cycle characteristics over 200 cycles in Examples 4-5 and Comparative Example 2. [Figure 30] Figure 30(a) shows the equivalent circuit models used for frequency response analysis in Examples 4-5 and Comparative Example 2, and Figure 30(b) shows the Nyquist plot obtained from the full coin cell before the cycle test (open circuit voltage). [Figure 31] Figure 31(a) shows the equivalent circuit models used for frequency response analysis in Examples 4-5 and Comparative Example 2, and Figure 31(b) shows the Nyquist plot obtained from a full coin cell after 100 cycles. [Figure 32] Figure 32(a) shows the XPS-C1s core-level spectra of Examples 4 and 5 and Comparative Example 2 before waveform separation, while Figures 32(b) to 32(d) show the XPS-C1s core-level spectra of Comparative Example 2, Example 4, and Example 5 after waveform separation, respectively. [Figure 33] Figure 33 shows the PS-P2p core-level spectra of Examples 4-5 and Comparative Example 2. [Figure 34] Figures 34(a) to 34(c) are electron microscope images showing the appearance of the positive electrode active material for lithium-ion secondary batteries in Examples 6 to 8. [Figure 35] Figure 35(a) shows the lithium-ion diffusion coefficient (DLi) during charging with respect to the open-circuit voltage obtained from GITT measurements, and Figure 35(b) shows the lithium-ion diffusion coefficient (DLi) during discharge. [Figure 36] Figure 36 is a graph showing the change in fluorine atom concentration relative to oxygen atoms in the positive electrode active material in Example 7, with respect to the direction from the outer surface to the interior. [Modes for carrying out the invention]

[0024] Embodiments of the present invention will be described in detail below with reference to the drawings.

[0025] [Composition of positive electrode active material for lithium-ion secondary batteries] Fig. 1(a) is an electron microscope image showing the appearance of the positive electrode active material for a lithium ion secondary battery according to the present embodiment, and Fig. 1(b) is a schematic diagram showing the structure of the positive electrode active material for a lithium ion secondary battery. As shown in the figure, the positive electrode active material 11 for a lithium ion secondary battery includes core particles 11a composed of a lithium metal composite oxide, and a fluoride layer 11b that covers at least a part of the core particles 11a and is composed of a fluoride of the lithium metal composite oxide.

[0026] The lithium metal composite oxide of the core particles 11a is LiNi k Co l Mn m O2 (k + l + m = 1, k ≥ 0.6). Also, the fluoride of the lithium metal composite oxide of the fluoride layer 11b is Li 1-z Ni k Co l Mn m O 2-x F x (k + l + m = 1, k ≥ 0.6, z ≤ 0.62, 0 < x ≤ 1). [[ID=2ff]] The lithium metal composite oxide of the core particles 11a is preferably represented by LiNi[[ID=ff]] 0.8 Co[[ID=ff]] 0.1 Mn[[ID=ff]] 0.1 O2 (hereinafter also referred to as NCM811), and the fluoride of the lithium metal composite oxide of the fluoride layer 11b is preferably represented by Li[[ID=ff]] 1-z Ni[[ID=ff]] 0.8 Co[[ID=ff]] 0.1 Mn[[ID=ff]] 0.1 O[[ID=ff]] 2-x F[[ID=ff]] x (z ≤ 0.62, 0 < x ≤ 1).

[0027] Figures 2(a) and 2(b) are schematic diagrams showing the layered rock salt structure of the fluoride of NCM811 as an example of a lithium metal composite oxide fluoride. As shown in the figures, the fluoride of the lithium metal composite oxide in the fluoride layer 11b is similar to that of the core particles 11a in that it has the basic crystal structure of the lithium metal composite oxide, but differs in that some of the oxygen ions in that crystal structure are replaced with fluoride ions. In this way, a part of the crystal structure of the lithium metal composite oxide is modified, and the covalent bonds between the transition metal and oxygen in the crystal structure of the lithium metal composite oxide are changed to ionic bonds between the transition metal and fluorine, which lowers the charge band (VB) level. As a result, the fluoride layer 11b becomes thinner than the conventional coating, and the crystal structure itself is stabilized, enabling good lithium ion conduction while suppressing the decomposition of the electrolyte and the elution of metal elements such as Ni.

[0028] The fluoride layer 11b may constitute a coating layer that covers the surface of the core particles 11a, as shown in Figure 1(b). This coating layer can also be called a thinner CEI layer than a conventional solid electrolyte interface layer (CEI layer). The coating layer may cover a portion of the surface of the core particles 11a, or it may cover the entire surface.

[0029] From the viewpoint of achieving a good balance and sufficient performance of the above-mentioned effects, the thickness of the fluoride layer 11b is preferably, for example, 0.5 nm to 10,000 nm, more preferably 1 nm to 7,000 nm, and even more preferably 2 nm to 5,000 nm.

[0030] The fluoride layer 11b may be a portion formed by heat treatment of the lithium metal composite oxide on which the fluoride layer is formed in the manufacturing method described later, or it may be a portion formed without the above heat treatment. If the heat treatment process described later is not performed, the thickness of the fluoride layer 11b is preferably, for example, 0.5 nm or more and 100 nm or less, more preferably 1 nm or more and 50 nm or less, and even more preferably 2 nm or more and 20 nm or less. When performing the heat treatment process described later, the thickness of the fluoride layer 11b is preferably, for example, 0.5 nm or more and 10,000 nm or less, more preferably 1 nm or more and 7,000 nm or less, and even more preferably 2 nm or more and 5,000 nm or less. In this way, when performing the heat treatment process described later, the thickness of the fluoride layer 11b can be increased.

[0031] The fluoride layer 11b may further contain lithium fluoride (LiF). In this case, the amount of fluorine atoms contained in the fluoride layer 11b is the sum of the fluorine atoms constituting the ionic bonds between the transition metal and fluorine, and the fluorine atoms constituting the lithium fluoride. The atomic concentration ratio of fluorine atoms to oxygen atoms in the fluoride layer 11b decreases from the outer surface to the interior of the positive electrode active material 11 for lithium-ion secondary batteries. This suppresses electron emission and the occurrence of side reactions, and enables better lithium-ion conduction.

[0032] The fluoride layer 11b may have a concentration gradient in the depth direction from the outer surface of the positive electrode active material 11 for lithium-ion secondary batteries, indicating the decrease in the ratio of fluorine atom concentration to oxygen atom concentration. This concentration gradient can be determined, for example, as the slope of the tangent to an approximate curve obtained by the least squares method. Furthermore, it is preferable that the concentration gradient at a certain depth in the heat-treated fluoride layer 11b in the manufacturing method described later is smaller than the concentration gradient at the same depth in the unheat-treated fluoride layer 11b. In addition, in the case of the unheat-treated fluoride layer 11b, the above atomic concentration ratio becomes substantially 0 at a certain depth (e.g., 100 nm), but in the case of the heat-treated fluoride layer 11b, a constant atomic concentration ratio (e.g., 0.05) is maintained even at a certain depth (e.g., 100 nm).

[0033] From the viewpoint of the above-mentioned effects, the atomic concentration ratio of fluorine atoms to oxygen atoms in the fluoride layer 11b is preferably 0.03 to 0.5, and more preferably 0.05 to 0.25, in the range of 0 to 50 nm from the outer surface of the lithium-ion secondary battery positive electrode active material 11. Furthermore, in the range of 50 nm to 100 nm from the outer surface of the lithium-ion secondary battery positive electrode active material 11, it is preferably 0.01 to 0.08, and more preferably 0.04 to 0.06.

[0034] Figure 3 is a schematic diagram conceptually illustrating the difference in lattice volume change between a conventional positive electrode active material for lithium-ion secondary batteries without a fluoride layer and a positive electrode active material for lithium-ion secondary batteries according to this embodiment. For example, in NCM811 (hereinafter also simply referred to as NCM811-bare), which is an example of a conventional positive electrode active material for lithium-ion secondary batteries without a fluoride layer, when lithium ions are detached from the lithium metal composite oxide by a desorption reaction (deintercalation reaction) during discharge, the volume of the crystal lattice decreases. At this time, due to the stability of the crystal structure, the lithium elements constituting the lithium metal composite oxide transition to take on predetermined values ​​(1-z=1.0→0.5→0.3→0), so the lattice volume of the lithium metal composite oxide decreases stepwise, and a mismatch in lattice volume occurs in the transition before and after the stable state, leading to deterioration of the crystal structure.

[0035] On the other hand, in NCM811 (hereinafter also simply referred to as NCM811-F), which is an example of a positive electrode active material for lithium-ion secondary batteries in this embodiment, the presence of fluorine atoms in the lithium metal composite oxide in the fluoride layer stabilizes the intermediate composition between each stable state, and the lattice volume decreases continuously. Therefore, it is presumed that degradation of the crystal structure is suppressed by reducing lattice strain, and as a result, battery characteristics such as cycle characteristics under high potential conditions are improved.

[0036] Figure 4(a) is a graph showing the change in lattice volume during the lithium ion desorption reaction, and Figure 4(b) is a schematic diagram illustrating the mechanism by which the change in lattice volume is suppressed in lithium metal composite oxide fluorides. As shown in Figure 4(a), in the conventional NCM811-bare, the rate of change in lattice volume during the desorption reaction process during discharge is large, and in particular in the range z≦0.7, the rate of change in lattice volume increases as the number of lithium elements in the lithium metal composite oxide decreases, showing a maximum value of -3.303% (z=1.0).

[0037] On the other hand, in the NCM811-F of this embodiment, the rate of change in lattice volume during the desorption reaction process during discharge is small, showing a maximum value of -1.069% (z=1.0) in the range z≦0.7, and the maximum value of the rate of change in lattice volume is significantly smaller than in the conventional configuration. Considering the reason for this, for example as shown in Figure 4(b), by introducing fluoride elements into the lithium metal composite oxide, fluoride elements are coordinated between adjacent transition metal layers in the layered rock salt type structure. Therefore, the Coulomb force generated between the fluorine atoms of one transition metal layer and the oxygen atoms of the other transition metal layer with respect to the c-axis direction of the transition metal layers is greater than the Coulomb force generated between oxygen atoms, suppressing the decrease in distance between transition metal layers, and as a result, the rate of change in lattice volume is suppressed, which is thought to improve the battery characteristics under high potential conditions.

[0038] [Configuration of the positive electrode for lithium-ion secondary batteries] The positive electrode for a lithium-ion secondary battery according to this embodiment comprises a positive electrode current collector and an electrode active material-containing layer provided on the positive electrode current collector.

[0039] <Positive electrode current collector> The positive electrode current collector is made of, for example, metal foil. Metal foil is suitable for use in batteries of various shapes, such as cylindrical, prismatic, and laminated types. To further improve the adhesion between the positive electrode active material and the positive electrode current collector, carbon may be deposited on the surface of the positive electrode current collector.

[0040] For example, aluminum foil can be used as the positive electrode current collector. It is preferable that the positive electrode current collector is hydrophilic through surface treatment. By making the surface of the positive electrode current collector hydrophilic, hydrogen bonds are more easily formed when the slurry for forming the positive electrode dries, and an electrode with high adhesive strength can be obtained. Examples of hydrophilic treatment of the surface of the positive electrode current collector include irradiation with ultraviolet (UV) light under an ozone (O3) atmosphere (UV / O3 treatment).

[0041] <Cathode active material containing layer> The positive electrode active material-containing layer contains the positive electrode active material for a lithium-ion secondary battery according to this embodiment. By using the lithium salt of the ternary transition metal oxide having a layered rock salt-type structure as the positive electrode active material, a lithium-ion secondary battery with excellent energy density and thermal stability can be obtained. Furthermore, lithium salt particles of ternary transition metal oxides such as NCM have a smaller particle size and a larger specific surface area (approximately 10 times) than particles such as LCO. This allows for a larger contact area between the active material particles and the electrolyte. As a result, the fluoride layer significantly suppresses the reaction between the electrode active material and the electrolyte, improving lithium ion conductivity between the active material particles and the electrolyte compared to a configuration without this setting, thereby increasing the power output of the lithium-ion secondary battery.

[0042] Furthermore, by using a lithium salt of the ternary transition metal oxide having a layered rock salt-type structure as the positive electrode active material, the positive electrode active material will contain Ni as a constituent element. In this case, the capacity density of the lithium-ion secondary battery increases, and the leaching of metal elements in the charged state tends to decrease. In particular, high-nickel LiNi k Co l Mn m The above effect becomes particularly pronounced when a lithium metal composite oxide represented by O2(k+l+m=1, k≧0.6) is used. This improves the long-term reliability of the lithium-ion secondary battery in the charged state and enhances its cycle characteristics compared to cases where this configuration is not adopted.

[0043] The positive electrode active material may have first positive electrode active material particles and second positive electrode active material particles having a larger particle size than the first positive electrode active material particles. The first positive electrode active material particles are, for example, primary particles. The second positive electrode active material particles may be primary or secondary particles. This allows the small-particle first positive electrode active material particles to fill the gaps between the large-particle second positive electrode active material particles, creating densification and increasing the electrode density, thereby increasing the energy density. Furthermore, by using first positive electrode active material particles with a smaller particle size than the second positive electrode active material particles, the destruction of the positive electrode active material due to volume expansion and contraction can be mitigated.

[0044] The particle size of the first positive electrode active material particles is preferably 0.1 μm or more and 4 μm or less, and more preferably 0.7 μm or more and 2 μm or less. The particle size of the second positive electrode active material particles is preferably 5 μm or more and 20 μm or less, and more preferably 6 μm or more and 15 μm or less. The material constituting the first positive electrode active material particles may be the same as or different from the material constituting the second positive electrode active material particles. Furthermore, the shape of the first positive electrode active material particles may be the same as or different from the shape of the second positive electrode active material particles.

[0045] [Method for manufacturing positive electrode active material for lithium-ion secondary batteries] The method for producing a positive electrode active material for a lithium-ion secondary battery according to this embodiment includes the step of placing a rare gas fluoride and a lithium metal composite oxide in a sealed space, and forming a fluoride layer on the lithium metal composite oxide composed of the fluoride of the lithium metal composite oxide. Specifically, for example, a noble gas fluoride (solid) and a lithium metal composite oxide are placed in a sealed container and left to stand for a predetermined time at room temperature or while being heated as needed. As the noble gas fluoride, for example, xenon fluoride can be used. At this time, the XeF2 (solid) left to stand in the container vaporizes to produce XeF2 (gas), and further decomposition of XeF2 (gas) produces fluoride ions (F). -This process generates fluorine atoms, which replace oxygen atoms on or near the surface of the lithium metal composite oxide. This allows for the easy formation of a fluoride layer on or near the surface of the lithium metal composite oxide.

[0046] In the process of forming the fluoride layer described above, the time for placing the noble gas fluoride and lithium metal composite oxide in a sealed space is preferably 12 to 180 minutes, more preferably 15 to 90 minutes, and even more preferably 20 to 30 minutes.

[0047] From the viewpoint of ease of evaporation, the xenon fluoride is preferably composed of xenon difluoride (XeF2), but it is not limited to this, and in addition to xenon difluoride (XeF2), xenon tetrafluoride (XeF4) and / or xenon hexafluoride (XeF6) may also be included. Furthermore, from the viewpoint of generating a larger amount of fluoride ions, the container may be heated to a predetermined temperature when the noble gas fluoride and lithium metal composite oxide are left to stand in the container.

[0048] A method for producing a positive electrode active material for lithium-ion secondary batteries may further include a step of heat-treating the lithium metal composite oxide on which the fluoride layer has been formed, after the step of forming the fluoride layer described above. This heat treatment causes the fluoride ions in the fluoride layer to diffuse further into the interior, and oxygen atoms inside the lithium metal composite oxide are replaced with fluorine atoms. As a result, a thicker fluoride layer can be formed on the surface of the lithium metal composite oxide. Furthermore, this heat treatment can increase the atomic concentration ratio of fluorine atoms to oxygen atoms in the fluoride layer 11b compared to the case where the heat treatment is not performed.

[0049] In the above process, the lithium metal composite oxide on which the fluoride layer is formed is heated, for example, at 300°C to 1000°C for 3 to 10 hours. The temperature range may be 500°C or below, or 470°C or below, or 450°C or below. Alternatively, the lithium metal composite oxide on which the fluoride layer is formed may be heat-treated at 330°C or above, or 350°C or above.

[0050] [Composition of a lithium-ion secondary battery] The lithium-ion secondary battery according to this embodiment comprises a positive electrode for lithium-ion secondary batteries, a negative electrode, and an electrolyte. This secondary battery can have the same configuration as conventional or known secondary batteries, except for having the electrodes described above.

[0051] Figure 5 is a cross-sectional view showing an example of a specific configuration of a lithium-ion secondary battery according to this embodiment. As shown in Figure 5, the lithium-ion secondary battery 10 is a coin-type secondary battery and comprises a positive electrode 1, a negative electrode 2, and an electrolyte 3. The positive electrode 1 comprises a positive electrode current collector 1a and a positive electrode active material containing layer 1b provided on the positive electrode current collector 1a. The negative electrode 2 comprises a negative electrode current collector 2a and a negative electrode active material containing layer 2b provided on the negative electrode current collector 2a. The electrolyte 3 is, for example, an electrolyte solution. Furthermore, the lithium-ion secondary battery 10 may include a separator 7 provided between the positive electrode 1 and the negative electrode 2, a stainless steel positive electrode case 4 and a negative electrode case 5 that cooperate with each other to house the positive electrode 1, the negative electrode 2, and the electrolyte 3, and a polypropylene gasket 6 interposed between the positive electrode case 4 and the negative electrode case 5 and on their outer periphery.

[0052] (positive electrode) The positive electrode 1 is not particularly limited, except that it has a positive electrode current collector 1a and a positive electrode active material containing layer 1b as described above. The positive electrode 1 can be manufactured, for example, by preparing a positive electrode mixture containing the lithium metal composite oxide, conductive material and binder.

[0053] (Conductive material) Carbon materials can be used as the conductive material for the positive electrode. Examples of carbon materials include graphite powder, carbon black (e.g., acetylene black), and fibrous carbon materials. Because carbon black is fine and has a large surface area, adding a small amount to the positive electrode mixture can increase the conductivity inside the positive electrode, improving charge / discharge efficiency and output characteristics. However, adding too much will reduce the binding force between the positive electrode mixture and the positive electrode current collector due to the binder, as well as the binding force inside the positive electrode mixture, which can actually increase internal resistance.

[0054] (binder) A thermoplastic resin can be used as the binder for the positive electrode. Examples of such thermoplastic resins include fluororesins such as polyvinylidene fluoride (hereinafter also referred to as PVdF), polytetrafluoroethylene (hereinafter also referred to as PTFE), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers; and polyolefin resins such as polyethylene and polypropylene.

[0055] When forming a paste from the positive electrode mixture, suitable organic solvents include amine solvents such as N,N-dimethylaminopropylamine and diethylenetriamine; ether solvents such as tetrahydrofuran; ketone solvents such as methyl ethyl ketone; ester solvents such as methyl acetate; and amide solvents such as dimethylacetamide and N-methyl-2-pyrrolidone (hereinafter sometimes referred to as NMP).

[0056] (Negative electrode) The negative electrode active material-containing layer 2b of the negative electrode 2 contains at least the negative electrode active material. As the negative electrode active material, compounds capable of intercalating and releasing lithium ions can be used alone or in combination. Examples of compounds capable of intercalating and releasing lithium ions include metallic materials such as lithium, alloy materials containing titanium, silicon, tin, etc., graphite, coke, calcined organic polymer compounds, or carbon materials such as amorphous carbon. These negative electrode active materials can be used individually or in combination of multiple types. Among these materials, titanium-containing oxides (for example, titanium oxide with a bronze structure, TiO2(B), lithium titanate, Li4Ti5O) can be used as negative electrode active materials. 12 ), silicon oxide, natural graphite, artificial graphite, hard carbon, soft carbon, silicon and silicon-containing alloys (for example, Si 80 Ti 20 It is preferable to use materials such as ) or tin. For example, when lithium foil is used as the negative electrode active material, it can be formed by pressing the lithium foil onto the surface of a negative electrode current collector made of a metal such as copper.

[0057] Furthermore, when using alloy materials or carbon materials as the negative electrode active material, the negative electrode can be formed by mixing the negative electrode active material with a binder, conductive additive, etc., in a solvent such as water or N-methylpyrrolidone, and then coating the mixture onto a negative electrode current collector made of a metal such as copper. The binder is preferably made from a polymer material and is preferably a material that is chemically and physically stable in the atmosphere inside a lithium secondary battery.

[0058] Examples of binders include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), and fluororubber. Examples of conductive additives include Ketjenblack, acetylene black, carbon black, graphite, carbon nanotubes, amorphous carbon, etc. Furthermore, conductive polymers such as polyaniline, polypyrrole, polythiophene, polyacetylene, and polyacene are also examples.

[0059] (electrolyte) The electrolyte is a medium for transporting charged carriers such as ions between the positive and negative electrodes. While not particularly limited, it is desirable that it be physically, chemically, and electrically stable in the atmosphere in which lithium-ion secondary batteries are used.

[0060] For example, as the electrolyte, it is preferable to use one or more of the following as the supporting electrolyte, dissolved in an organic solvent.

[0061] Examples of organic solvents include propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, and mixtures thereof. Among these, electrolytes containing carbonate-based solvents are preferred due to their high stability at high temperatures. In addition, solid polymer electrolytes containing the above electrolytes in solid polymers such as polyethylene oxide, and solid electrolytes such as ceramics and glass that have lithium ion conductivity can also be used.

[0062] It is desirable to interpose a separator between the positive and negative electrodes, which is a material that provides both electrical insulation and ion conduction. If the electrolyte is liquid, the separator also plays a role in holding the liquid electrolyte. Examples of separators include porous synthetic resin membranes, particularly porous membranes made of polyolefin polymers (polyethylene, polypropylene) or glass fibers, and nonwoven fabrics. Furthermore, it is preferable that the separator be larger than the positive and negative electrodes in order to ensure insulation between them.

[0063] The positive electrode, negative electrode, electrolyte, separator, etc., are generally housed in a case composed of the positive electrode case 4 and negative electrode case 5 described above. The case is not particularly limited and can be made of known materials and in known forms. In other words, the lithium secondary battery of the present invention is not particularly limited in shape and can be used as a battery of various shapes such as coin type, cylindrical type, or prismatic type. Furthermore, the case of the lithium secondary battery of this embodiment is not limited and can be used as a battery of various forms such as a metal or resin case that can maintain its external shape, or a soft case such as a laminate pack. [Examples]

[0064] The following describes embodiments of the present invention. The present invention is not limited to the following embodiments.

[0065] (Example 1) <Manufacturing of positive electrodes for lithium-ion secondary batteries> 100 parts by mass of NCM811 (manufactured by Hosen Co., Ltd., Ni: 80% by mass, Co: 10% by mass, Mn: 10% by mass) and 12 parts by mass of XeF2 (manufactured by Wako Pure Chemical Industries, Ltd.) were sealed in a PTFE container in a glove box and left to stand at room temperature for 20 minutes. The molar ratio of NCM811 to XeF2 at the time of preparation was 14.5:1. After 20 minutes, the mixture was removed from the container to obtain a positive electrode active material (NCM811-F_20min) composed of lithium metal composite oxide fluoride. The positive electrode active material (NCM811-F_20min) and Denka Black (registered trademark) (manufactured by Denka Co., Ltd., conductive additive) were weighed and mixed in a mortar, then placed in a dedicated container and kneaded for 2 minutes with a mixer (manufactured by Shinky Co., Ltd., product name "Awatori Rentaro"). After confirming that it was thoroughly mixed, the binder (PVdF / NMP: 10% by mass) was weighed and added dropwise, and kneaded with the mixer for 2 minutes. Next, 40 μL of NMP was added dropwise, kneaded with the mixer for 3 minutes, degassed for 30 seconds, and then another 50 μL of NMP was added dropwise, kneaded with the mixer for 5 minutes, and degassed for 30 seconds.

[0066] The obtained slurry was applied to aluminum foil and dried at atmospheric pressure and 100°C. Next, it was punched out using a punching machine and vacuum dried at 120°C for 24 hours. After vacuum drying, it was press-formed using a roll press to obtain the positive electrode. At this time, the mass ratio of positive electrode active material:conductive additive:binder in the positive electrode was 90:5:5.

[0067] <Making a coin half-cell> A coin half-cell was fabricated by stacking a positive electrode can (positive electrode side case), the lithium-ion secondary battery positive electrode obtained above, a separator (Cellguard®, #2400), a gasket, a negative electrode (lithium metal), a spacer, a spring, and a negative electrode can (negative electrode side case) in this order. 140 μL of electrolyte (1M LiPF6EC / DMC(1:2)) was placed inside the positive electrode can, and 70 μL of the same electrolyte was placed inside the negative electrode can. Lithium foil was used for the negative electrode.

[0068] (Example 2) The positive electrode active material (NCM811-F_30min) was manufactured in the same manner as in Example 1, except that the standing time of NCM811 and XeF2 in the PTFE container was changed from 20 minutes to 30 minutes, and a coin half cell was fabricated.

[0069] (Example 3) The positive electrode active material (NCM811-F_90min) was manufactured in the same manner as in Example 1, except that the standing time of NCM811 and XeF2 in the PTFE container was changed from 20 minutes to 90 minutes, and a coin half cell was fabricated.

[0070] (Comparative Example 1) A coin half-cell was fabricated using the positive electrode active material (NCM811-bare) in the same manner as in Example 1, except that a fluoride layer was not formed on the surface of NCM811 (manufactured by Hosen Co., Ltd., Ni: 80% by mass, Co: 10% by mass, Mn: 10% by mass).

[0071] Examples 1-3 and Comparative Example 1 were measured and evaluated using the following methods.

[0072] [Appearance of the positive electrode active material] The positive electrode active material was observed using a field emission scanning electron microscope (FE-SEM). As a result, in Examples 1 and 2 (Figures 6(b) to 6(c)), no change was observed in the surface appearance of the positive electrode active material, and it had the same appearance as Comparative Example 1 (Figure 6(a)). In Example 3 (Figure 6(c)), the deposition of fine-grained deposits on the surface of the positive electrode active material was confirmed.

[0073] [XRD measurement] The positive electrode active material (NCM811-F) used in Examples 1-3 and the positive electrode active material (NCM811-bare) used in Comparative Example 1 were identified using powder X-ray diffraction (XRD) with an XRD apparatus (Rigaku Corporation, "Smart Lab®"). As a result, as shown in Figure 7, the spectral peaks of Examples 1-3, which were doped with fluorine atoms, showed no change compared to Comparative Example 1, which was not doped with fluorine atoms. This confirmed that the layered structure of the raw material NCM811 was almost maintained in the positive electrode active materials of Examples 1-3.

[0074] [XPS analysis (1)] XPS analysis was performed on the surface of the positive electrode active material under the following conditions to obtain narrow-scan spectra of each element on the surface of the positive electrode active material. Measurement method: X-ray photoelectron spectroscopy (XPS) X-ray source: MgKα ray (1486.6eV) X-ray spot diameter: 400 μm Neutralization conditions: Neutralization electron gun (acceleration voltage 0.2V, current 100μA)

[0075] First, the XPS spectra of fluorine atoms (1s) in the positive electrode active material in Examples 1-3 and Comparative Example 1 are shown in Figures 8(a) and 8(b). As a result, in Examples 1-3, a peak attributable to the transition metal-fluorine bond (685.7 eV) and a peak attributable to lithium fluoride (LiF) (684.9 eV) were detected.

[0076] Next, the XPS spectra of the nickel element (2p) in the positive electrode active material in Examples 1-3 and Comparative Example 1 are shown in Figures 9(a) and 9(b). As a result, it was confirmed that in Examples 1-3, the peak attributed to nickel was shifted to the higher energy side due to the nickel-fluorine bond. In addition, in Examples 1-3, due to charge compensation for substitution with fluorine atoms, Ni 2+ / Ni 3+ It was confirmed that the ratio had increased compared to Comparative Example 1.

[0077] Furthermore, the XPS spectra of the manganese element (2p) in the positive electrode active material in Examples 1-3 and Comparative Example 1 are shown in Figures 10(a) and 10(b). As a result, it was confirmed that in Examples 1-3, similar to nickel, the peak attributed to manganese shifted to the higher energy side due to the manganese-fluorine bond. In addition, since there was no change in the valence (4+) of manganese in Examples 1-3, it was confirmed that the valence of nickel changed.

[0078] Furthermore, Figure 11 shows the XPS spectra of the cobalt element (2p) in the positive electrode active material in Examples 1-3 and Comparative Example 1. As a result, it was confirmed that in Examples 1-3, the peaks attributed to cobalt were not shifted, and there was no change in the valence state of cobalt.

[0079] [Cycle Characteristics Evaluation] The coin half-cells prepared in Examples 1-3 and Comparative Example 1 were set in a charge / discharge device (Hokuto Denko Co., Ltd., product name "HJ1001SD8"), and Li insertion and removal into the positive electrode was performed in one cycle by charge and discharge. The cycle characteristics of the positive electrode in one cycle were evaluated. The charge / discharge cycle was performed at a current density of 0.2C. The charge / discharge capacity and Coulomb efficiency of this charge / discharge were measured. The results are shown in Table 1 and Figure 12.

[0080] [Table 1]

[0081] From the results in Table 1 and Figure 12, the initial charge-discharge curves of Examples 1-3 were similar to those of Comparative Example 1, and the charging capacity, discharging capacity, and Coulomb efficiency were equivalent to those of Comparative Example 1. Therefore, it was found that under the above charge-discharge conditions, the charge-discharge characteristics in one cycle of Examples 1-3 were equivalent to those of Comparative Example 1.

[0082] Furthermore, the cycle characteristics of the coin half-cell were evaluated over 200 cycles. The 200-cycle charge-discharge was performed with a cutoff voltage of 4.3V to 2.8V, CCCV-CC mode, and 0.5C / 1C. The discharge capacity and capacity retention rate of this charge-discharge were measured. The results are shown in Figures 13(a) and 13(b). As a result, the discharge capacity of Examples 1 to 3 decreased with increasing cycle count, following a similar trend to Comparative Example 1. Similarly, the discharge capacity retention rate of Examples 1 to 3 also decreased with increasing cycle count, following a similar trend to Comparative Example 1. Therefore, under the above charge-discharge conditions, the cycle characteristics of Examples 1 to 3 over 200 cycles were found to be equivalent to those of Comparative Example 1.

[0083] [Impedance Analysis] The lithium insertion and removal from the positive electrode was performed in one cycle by charging and discharging, and the impedance of the coin half-cell was measured after one cycle. As shown in Figure 14(a), the resistance of the electrolyte was set to R. sol The resistance of the CEI layer (coating layer) is R CEI The charge transfer resistance between the CEI layer and the core particle is R. ct The values ​​were then calculated. The results are shown in Table 2 and Figure 14(b). As a result, the CEI layer resistance R of Examples 1-3 CEI This is the CEI layer resistance R of Comparative Example 1. CEI It was found to be smaller than [the specified value]. Also, the charge transfer resistance R of Examples 1-2 ct This is the charge transfer resistance R of Comparative Example 1. ct It was found to be smaller than that.

[0084] [Table 2]

[0085] Next, the total resistance of the cells is R tol (=R sol +R CEI +R ct The impedance of the coin half-cell was measured after 1 cycle and after 200 cycles. The results are shown in Table 3, Figure 15(a), and Figure 15(b). As a result, the total cell resistance R after 1 cycle for Examples 1-3 was tol The total cell resistance R of Comparative Example 1 is shown.tol It is smaller than, and the total cell resistance R after 200 cycles tol Also, the total cell resistance R of Comparative Example 1 tol It became smaller than that. Also, the total cell resistance R after 1 cycle and 200 cycles tol The resistance difference was significantly larger than that of Comparative Example 1, indicating that the formation of the oxyfluoride layer suppressed the increase in the total cell resistance after the cycle. Furthermore, the total cell resistance R for one cycle in Examples 1 and 2 tol The total cell resistance R in Example 3 is shown. tol Smaller than the total cell resistance R after 200 cycles. tol Also, the total cell resistance R of Example 3 tol It became smaller than in Example 3. From this, it can be inferred that in Examples 1 and 2, a thinner CEI layer was formed compared to Example 3, suppressing the decomposition of electrolytes such as LiPF6 and EC, and the elution of nickel, while improving lithium ion conductivity.

[0086] [Table 3]

[0087] [Output Characteristics Evaluation] The output characteristics of the coin half-cell were evaluated by performing Li insertion and deinsertion into the positive electrode through charging and discharging for 40 cycles. The 40-cycle charging and discharging was performed at a cutoff voltage of 4.3V to 2.8V, CCCV-CC mode, 0.2C to 10C, and at room temperature. The results are shown in Figures 16(a) and 16(b). As a result, the discharge capacity of Examples 1 to 3 was greater than that of Comparative Example 1 after 20 cycles, and the discharge capacity retention rate of Examples 1 to 3 was also greater than that of Comparative Example 1 after 20 cycles. Therefore, it was found that the output characteristics of Examples 1 to 3 after 40 cycles were superior to those of Comparative Example 1 due to the formation of the acid fluoride layer.

[0088] [UPS measurement] The UPS measurement of the surface of the positive electrode active material was performed under the following conditions, and the work function of the positive electrode active material was evaluated. The work function was calculated using the following formula. The results are shown in Figure 17 and Table 4. Measurement method: Ultraviolet photoelectron spectroscopy (UPS) UV light source: Deuterium lamp Measured light intensity: 50nW Set light output: 50nW Starting energy: 4.2 eV Ending energy: 6.2 eV Step: 0.10eV

[0089] [Table 4]

[0090] From the results in Figure 17 and Table 4, the work functions of Examples 1-3 were greater than those of Comparative Example 1, and the energy difference Δ between Examples 1-3 and Comparative Example 1 was +0.31 to +0.42 eV. Therefore, it was found that the formation of the acid fluoride layer suppressed electron emission on the surface of the positive electrode active material more effectively than in Comparative Example 1.

[0091] [Measurement of composition ratio in the thickness direction of the fluoride layer] Using the above XPS, the changes in the concentration of each element in the positive electrode active material in the direction from the outer surface to the interior were measured. In each example, the oxygen atom concentration in the transition metal-oxygen atom bond, the fluorine atom concentration in the transition metal-fluorine bond, and the fluorine atom concentration of lithium fluoride (LiF) were measured. The results are shown in Figures 18(a) to 18(c). Note that the "depth from the surface" on the horizontal axis in Figures 18(a) to 18(d) is the value converted to SiO2. From these results, it was found that in all of Examples 1 to 3, the fluorine atom concentration of lithium fluoride (LiF) decreased as the depth from the surface increased. In addition, in Examples 2 to 3, it was found that the fluorine atom concentration in the transition metal-fluorine bond decreased slightly as the depth from the surface increased. Furthermore, in Example 1, as shown in Figure 18(d), it was found that the ratio of fluorine atom concentration to oxygen atom concentration (F / O ratio) in the fluoride layer decreased as the depth from the surface increased.

[0092] [Cyclic voltammetry measurement] The coin half-cells prepared in Example 1 and Comparative Example 1 were measured using cyclic voltammetry (CV method). The results are shown in Figures 19(a) and 19(b). In the figures, the horizontal axis represents the voltage applied to the solution (V, vs Li). + The graph shows the current density (mA) on the vertical axis, and the vertical axis shows the output current density (mA). As a result, in Comparative Example 1, D Li_Ox = 5.0 × 10 -7 cm 2 ·s -1 , D Li_Red = 2.3 × 10 -7 cm 2 ·s -1 In contrast, in Example 1, D Li_Ox = 5.8 × 10 -7 cm 2 ·s -1 , D Li_Red = 2.6 × 10 -7 cm 2 s -1 Thus, in Example 1, it was found that the diffusion coefficient of lithium ions increased due to the formation of the oxyfluoride layer.

[0093] Furthermore, comparing the cyclic voltammetry (CV) results in Example 1 and Comparative Example 1, Example 1 maintained its redox peak shape even at a fast voltage sweep rate. Moreover, as shown in Figure 20(b), the reaction rate constants for charge and discharge in Comparative Example 1 were k=14.8 and -10.1, while in Example 1 they were k=16.0 and -10.7, indicating that charge transfer by lithium ions was faster in Example 1 compared to Comparative Example 1.

[0094] [Evaluating cycle characteristics under high-rate conditions] Under high-rate conditions, Li insertion and deinsertion into the positive electrode was performed for 200 cycles by charging and discharging, and the cycle characteristics of the coin half-cell were evaluated at 200 cycles. The 200 cycles of charging and discharging were performed with a cutoff voltage of 4.3V to 2.8V, CCCV-CC mode, and 0.5C / 4C. The discharge capacity and capacity retention rate of this charge and discharge were measured. The results are shown in Figures 21(a) and 21(b). As a result, it was found that in Example 1, the decrease in discharge capacity with increasing cycle count was smaller than in Comparative Example 1, and also that the decrease in discharge capacity retention rate with increasing cycle count was smaller than in Comparative Example 1. In particular, the discharge capacity retention rate after 200 cycles in Comparative Example 1 was 68%, while the discharge capacity retention rate after 200 cycles in Example 1 was 88%. Therefore, under high-rate conditions, it was found that the cycle characteristics of Example 1 were significantly superior to those of Comparative Example 1 due to the formation of the acid fluoride layer.

[0095] [Evaluation of cycle characteristics under high potential conditions] Under high-potential conditions, Li insertion and deinsertion into the positive electrode was performed for 200 cycles by charging and discharging, and the cycle characteristics of the coin half-cell were evaluated. The 200 cycles of charging and discharging were performed at a cutoff voltage of 4.8V to 2.8V, CCCV-CC mode, 0.5C to 4C, and at room temperature. The results are shown in Figures 22(a) and 22(b). As a result, the discharge capacity of Examples 1 to 3 was greater than that of Comparative Example 1 after approximately 70 cycles, and the discharge capacity retention rate of Examples 1 to 3 was also greater than that of Comparative Example 1 after approximately 70 cycles. In particular, the discharge capacity retention rate after 200 cycles in Comparative Example 1 was approximately 17%, while the discharge capacity retention rate after 200 cycles in Examples 1 to 3 was approximately 50%. Therefore, under high-potential conditions, the cycle characteristics of Examples 1 to 3 were found to be significantly superior to those of Comparative Example 1 due to the formation of the acid fluoride layer.

[0096] [Evaluation of output characteristics under high potential conditions] Under high potential conditions, Li insertion and deinsertion into the positive electrode was performed for 40 cycles by charging and discharging, and the output characteristics of the coin half-cell were evaluated. The 40-cycle charge-discharge was performed at a cutoff voltage of 4.8V to 2.8V, CCCV-CC mode, 0.2C to 10C, and at room temperature. The results are shown in Figures 23(a) and 23(b). As a result, the discharge capacity of Examples 1 to 3 was greater than that of Comparative Example 1 after 30 cycles, and the discharge capacity retention rate of Examples 1 to 3 was also greater than that of Comparative Example 1 after 30 cycles. In particular, the discharge capacity retention rate in Comparative Example 1 was 55% from 30 to 35 cycles, while that of Example 1 was 61% from 30 to 35 cycles. Therefore, under high potential conditions, the output characteristics of Example 1 were found to be superior to those of Comparative Example 1 due to the formation of the acid fluoride layer.

[0097] [Cyclic voltammetry measurement under high potential conditions] The coin half-cells prepared in Example 1 and Comparative Example 1 were subjected to cycle tests of 1 cycle and 5 cycles under the above high potential conditions, and measurements were performed using the cyclic voltammetry (CV) method. The results are shown in Figures 24(a) and 24(b). In the figures, the horizontal axis is the voltage applied to the solution (V, vs Li). + The graph shows the current density (mA) on the vertical axis, with the vertical axis representing the output current density. The results showed that in Example 1, the peak current value increased compared to Comparative Example 1 in both cycle 1 and cycle 5 due to the formation of the acid fluoride layer. Furthermore, in Example 1, the current peak on the oxidation side shifted to a higher potential compared to Comparative Example 1, while the current peak on the reduction side shifted to a lower potential compared to Comparative Example 1.

[0098] [Measurement of ion diffusion coefficient] To investigate the effect of fluoride ion substitution on the change in the lithium ion diffusion coefficient in the active material during the charging process, constant current intermittent titration (GITT) measurements were performed. The charging rate was set to 0.2C, and constant current pulses were applied and released repeatedly for 10 minutes each, charging from 2.8V to 4.8V. The lithium ion diffusion coefficient (D) as a function of the open-circuit voltage obtained from the GITT measurements was measured. Li ) is shown in Figure 25(a), Lithium composition (Li1-z Ni 0.8 Co 0.1 Mn 0.1 O 2-x F x The lithium-ion diffusion coefficients for (0 < z < 0.8, x = 0.8) are shown in Fig. 25(b), respectively.

[0099] As shown in Fig. 25(a), the lithium-ion diffusion coefficient of the cathode active material with fluoride ion substitution (hereinafter also referred to as NCM811-F_20min) gradually increases from 3.6 V (z = 0.1), and the difference from the lithium-ion diffusion coefficient of the cathode active material without fluoride ion substitution (hereinafter also referred to as NCM811-bare) reaches the maximum at about 4.0 V (z = 0.5). Further charging, the lithium-ion diffusion coefficient of NCM811-F_20min rapidly decreases at about 4.2 V (z = 0.6) and becomes equal to the lithium-ion diffusion coefficient of NCM811-bare at 4.4 V (z = 0.7). Also, as shown in Fig. 25(b), the lithium-ion diffusion coefficient of NCM811-F gradually increases from y = 0.2, and the difference from the lithium-ion diffusion coefficient of NCM811-bare reaches the maximum at y = 4.5. Also, the lithium-ion diffusion coefficient of NCM811-F_20min rapidly decreases at y = 0.67 and becomes equal to the lithium-ion diffusion coefficient of NCM811-bare at y = 0.7.

[0100] The change in the lithium-ion diffusion coefficient during the charging process is due to the diffusion barrier that changes according to the lithium vacancy concentration in the cathode active material and the charge balance of transition metal cations. It was confirmed that the lithium-ion diffusion coefficient increases in the region of 3.6 V to 4.4 V and the region of lithium composition 0.2 < y < 0.7 due to fluoride ion substitution.

[0101] Next, the correlation between the phase transition of the crystal structure from the H1 phase (O3) to the H2 phase (O3) and the H3 phase (O1) and the lithium-ion diffusion coefficient during the charging process of NCM811-bare and NCM811-F_20min was analyzed. The c-axis length with respect to the lithium composition obtained from ex-situ XRD measurement is shown in Fig. 26(a), and the lithium composition (Li1-z Ni 0.8 Co 0.1 Mn 0.1 O 2-x F x The lithium-ion diffusion coefficients for (0 < z < 0.8, x = 0.8) are shown in Fig. 26(b), respectively. As shown in Figs. 26(a) and 26(b), in the region of 0 < y < 0.5 where the phase transition from the H1 phase to the H2 phase occurs, the lithium-ion diffusion coefficient increases with the increase in the c-axis length indicating the lattice volume change rate, and the lithium-ion diffusion coefficient also reaches the maximum at z = 0.5 where the c-axis length is the maximum. In the region of z > 0.5 where the phase transition from the H2 phase to the H3 phase occurs, the lithium-ion diffusion coefficient decreases with the decrease in the c-axis length. From this, it became clear that there is a correlation between the phase transition of the crystal structure (change in the c-axis length) and the lithium-ion diffusion coefficient in the positive electrode active material.

[0102] Also, in the region of z > 0.6 where the phase transition from the H2 phase to the H3 phase (contraction of the c-axis length) was suppressed by fluoride ion substitution, in the region of 0.6 < z < 0.7, the lithium-ion diffusion coefficient of NCM811-F_20min was larger than that of NCM811-bare. However, in the region of z > 0.7, no difference was observed in the lithium-ion diffusion coefficients between NCM811-bare and NCM811-F_20min. Therefore, the correlation between the suppression of the contraction of the c-axis length (Li-O layer) in the high voltage region by fluoride ion substitution and the lithium-ion diffusion coefficient was not confirmed.

[0103] (Example 4) <Fabrication of Coin Full Cell> A coin full cell was fabricated by laminating a positive electrode can (positive electrode side case), a positive electrode for a lithium-ion secondary battery containing the positive electrode active material (NCM811-F_20min) obtained in Example 1, a separator (Celgard (registered trademark), #2400), a gasket, a negative electrode (graphite), a spacer, a spring, and a negative electrode can (negative electrode side case) in this order. An electrolytic solution (1M LiPF6 EC / DMC (1:2), 140 μL of VC (vinylene carbonate 1.0 mass%)) was accommodated inside the positive electrode can, and 70 μL of the same electrolytic solution was accommodated inside the negative electrode can. Graphite was used for the negative electrode.

[0104] (Example 5) A coin-full cell was fabricated in the same manner as in Example 4, except that a positive electrode for a lithium-ion secondary battery containing the positive electrode active material (NCM811-F_30min) obtained in Example 2 was used, and the coin-half cell was changed to a coin-full cell. (Comparative Example 2) A coin-full cell was fabricated in the same manner as in Example 4, except that a positive electrode for a lithium-ion secondary battery containing the positive electrode active material (NCM811-bare) obtained in Comparative Example 1 was used, and the coin-half cell was changed to a coin-full cell.

[0105] Examples 4-5 and Comparative Example 2 were measured and evaluated using the following methods.

[0106] [XPS analysis (2)] Under the same conditions as in the coin half-cell case, XPS analysis was performed on the positive electrode active material, and the changes in the valence band spectrum before and after fluoride ion substitution were investigated. The results are shown in Figure 27. As a result, in Examples 4-5 of the coin-full cell, the position of the spectral rise was shifted to a higher energy side by approximately 0.4 eV compared to Comparative Example 2, due to the nickel-fluorine bond. This result indicates an increase in the binding energy of the outermost electrons due to fluoride ion substitution, suggesting a decrease in the energy level of the valence band due to the fluoride ion substitution.

[0107] [UPS measurement] Under the same conditions as in the case of the coin half-cell, UPS measurements were performed on the surface of the positive electrode active material, and the work function of the positive electrode active material was evaluated. The results are shown in Figure 28. As a result, in Examples 4 and 5 of the coin-full cell, the position of the spectral rise was shifted to the higher energy side compared to Comparative Example 2. Furthermore, while the work function of the Comparative Example was 5.16 eV, the work functions of Examples 4 and 5 were 5.49 eV (Δ=0.33 eV) and 5.57 eV (Δ=0.41 eV), respectively. Therefore, an increase in work function due to fluoride ion substitution was observed, suggesting a decrease in the Fermi level due to this fluoride ion substitution.

[0108] [Cycle Characteristics Evaluation] The cycle characteristics over 200 cycles were evaluated under the same conditions as for the coin half-cell. The results are shown in Figure 29. From these results, the initial discharge capacities of Examples 4 and 5 were 172.7 mAh / g and 171.0 mAh / g, respectively, while the initial discharge capacity of Comparative Example 2 was 165.8 mAh / g. The reason why the initial discharge capacities of Examples 4 and 5 were larger than those of Comparative Example 2 is thought to be due to a decrease in lithium ion consumption due to CEI generation during aging. Furthermore, the initial discharge capacity of Example 5 was smaller than that of Example 4. This is due to the large amount of LiF precipitated as a by-product when lithium ions in the active material are consumed during surface fluoride ion substitution. The discharge capacities after 100 cycles for Examples 4 and 5 were 160.7 mAh / g and 160.0 mAh / g, respectively, while the discharge capacity after 100 cycles for Comparative Example 2 was 127.0 mAh / g. Furthermore, the discharge capacity retention rates after 100 cycles for Examples 4 and 5 were 93.0% and 93.6%, respectively, while the discharge capacity retention rate after 100 cycles for Comparative Example 2 was 76.6%. These results demonstrate that fluoride ion substitution improves the cycle characteristics of NCM811.

[0109] On the other hand, the significant decrease in discharge capacity in Comparative Example 2 is thought to be due to the thick CEI formed on the positive electrode surface. Since the thick CEI acts as a resistive layer, it inhibits lithium ion diffusion and reduces the discharge capacity. Alternatively, it is thought that the CEI formed on the positive electrode surface of Comparative Example 2 was unstable, and lithium ions were consumed in the continuous decomposition of the electrolyte, resulting in a decrease in discharge capacity.

[0110] [Impedance Analysis] To investigate the effect of fluoride ion substitution on the resistance of the electrode / electrolyte interface, AC impedance measurements were carried out. Under the same conditions as in the coin half-cell case, Li insertion and extraction into the positive electrode by charge and discharge were performed in one cycle, and the impedance of the coin full cell after one cycle was measured. The equivalent circuit model used for the frequency response analysis is shown in Fig. 30(a), and the Nyquist plot obtained from the full coin cell before the cycle test (open circuit voltage) is shown in Fig. 30(b). As a result, an arc corresponding to one impedance component was confirmed in the frequency range of 200 kHz to 850 Hz. This arc can be attributed to the insertion and extraction (charge transfer) resistance of lithium ions between the electrolyte and the positive (negative) electrode active material particles. Note that the frequency response regions of the resistance components of the positive and negative electrodes are close, and separation of the resistance components was impossible.

[0111] Also, the resistance values of each resistance component (R sol、 R ct ) are shown in Table 5. The charge transfer resistances of Examples 4 and 5 showed values equivalent to those of Comparative Example 2, and no fluoride ion substitution effect on the charge transfer resistance before the cycle test was observed.

[0112]

Table 5

[0113] Next, under the same conditions as in the coin half-cell case, the impedance of the coin full cell after 100 cycles was measured. The equivalent circuit model used for the frequency response analysis is shown in Fig. 31(a), and the Nyquist plot obtained from the full coin cell after 100 cycles is shown in Fig. 31(b). As a result, arcs corresponding to two impedance components were identified in the frequency ranges of 200kHz to 121Hz and 120Hz to 0.35Hz. The arc on the higher frequency side originates from the resistance of lithium ion diffusion within the CEI layer formed on the positive and negative electrode surfaces. The arc on the lower frequency side originates from the resistance of lithium ion deinsertion (charge transfer) between the CEI layer and the positive (negative electrode) active material particles. Furthermore, the frequency response ranges of the resistance components of the positive and negative electrodes were close, making separation of the resistance components impossible.

[0114] Also, each resistance component (R sol、 R CEI , R ct Table 6 shows the resistance values ​​of the two. The CEI resistances of Examples 4 and 5 were lower than those of Comparative Example 2. This result suggests thinning of the CEI layer due to fluoride ion substitution and improvement of lithium ion conductivity within the CEI layer. Therefore, it is considered that the decomposition of the electrolyte was suppressed on the positive electrode surface of Examples 4 and 5. In addition, the CEI resistance of Example 5 was higher than that of Example 4. This is due to the large amount of LiF precipitated as a by-product due to the solid solubility limit during surface fluoride ion substitution. The LiF on the particle surface acts as a resistive layer that inhibits lithium ion diffusion.

[0115] The charge transfer resistance increased in the order of Comparative Example 2 > Example 4 > Example 5. A higher charge transfer resistance suggests that the degradation of the electrode surface, such as a decrease in lithium sites within the active material or a decrease in lithium ion diffusion pathways, is more severe. Therefore, in Examples 4 and 5, it is considered that the structure of the NCM811 crystal surface was stabilized by fluoride ion substitution, and the degradation of the positive electrode active material was suppressed.

[0116] [Table 6]

[0117] [XPS analysis (3)] The chemical state of the cathode surface in Examples 4 and 5 and Comparative Example 2 after the cycle test was analyzed by X-ray photoelectron spectroscopy (XPS). The XPS-C1s core-level spectra are shown in Figures 32(a) to (d). Figure 32(a) shows the spectra of Examples 4 and 5 and Comparative Example 2 before waveform separation, and Figures 32(b) to 32(d) show the spectra of Comparative Example 2, Example 4, and Example 5 after waveform separation, respectively. Waveform separation of the spectrum revealed the following classifications: CC (284.6 eV), CH (285.0 eV), CO (285.8 eV), OCO (286.4 eV), C=O (287.4 eV), -CH2-OCO2Li (287.8 eV), Li2CO3 (289.2 eV), and CH2-CF2 (290.8 eV). Table 7 shows the area ratio of each peak to the total area of ​​the spectrum. These peaks originate from the decomposition products of organic solvent molecules (EC and DMC).

[0118] Of these, the 286.0 eV peaks attributed to CO and OCO originate from polymer degradation products such as polyethylene oxide. Polymer degradation products are generated by chain reactions of organic solvent molecules (EC and DMC). In Examples 4 and 5, the area ratios of the peaks attributed to CO and OCO were 9.8% and 9.0%, respectively, while the area ratio of the same peaks in Comparative Example 2 was 15.4%. Therefore, this suggests that fluoride ion substitution suppresses the degradation of polymer degradation products.

[0119] Furthermore, the 287.6 eV peaks attributed to C=O, -CH2-OCO2Li, and Li2CO3 originate from carbonate-based decomposition products such as inorganic carbonates. In Examples 4 and 5, the area ratios of the peaks attributed to C=O, -CH2-OCO2Li, and Li2CO3 were 8.9% and 9.3%, respectively, while the area ratio of the same peaks in Comparative Example 2 was 11.4%. Therefore, the suppression of decomposition of carbonate-based decomposition products by fluoride ion substitution was suggested. It has been reported that carbonate-based decomposition products act as a resistive layer for lithium ion diffusion, causing a decrease in battery capacity. In fact, the CEI resistance after the cycle test increased in the order of Example 4, Example 5, and Comparative Example 2 (Example 4 < Example 5 < Comparative Example 2), and the discharge capacity decreased in the order of Example 4, Example 5, and Comparative Example 2 (Example 4 > Example 5 > Comparative Example 2). From the above analysis results, the suppression of decomposition of organic solvent molecules by fluoride ion substitution was confirmed in Examples 4 and 5.

[0120] Furthermore, when comparing the peak area ratios of CO and C=O ((CO):(C=O)), with the sum of the peak area ratios of CO and C=O set to 100%, the ratios were 52.7%:47.3% in Example 4, 48.7%:51.3% in Example 5, and 57.5%:42.5% in Comparative Example 2. From these results, it was confirmed that Examples 4 and 5 promoted the formation of carbonate-based decomposition products more than polymer-based decomposition products. This is thought to be because the reactivity with the electrolyte changed due to a decrease in the electron-donating ability of the electrode surface caused by fluoride ion substitution.

[0121] [Table 7]

[0122] Furthermore, the XPS-P2p core-level spectrum is shown in Figure 33. This spectrum is PO(134.4eV),Li x PO y F z (135.4 eV), Li x PF yIt is attributed to (136.0 eV) and originates from the decomposition products of the lithium salt LiPF6. However, the detection intensity was low, making waveform separation impossible. The area ratios of the spectra in Examples 4 and 5 were 0.46% and 0.44%, respectively, while the area ratio of the same spectrum in Comparative Example 2 was 1.63%. These results suggest that fluoride ion substitution suppresses the decomposition of LiPF6.

[0123] (Example 6) <Manufacturing of positive electrodes for lithium-ion secondary batteries> LiNi 0.8 Co 0.1 Mn 0.1 Oxide ions on the surface of an O2 crystal (manufactured by Hosensha) were replaced with fluoride ions. LiNi 0.8 Co 0.1 Mn 0.1 1.2 g of O2 powder and 0.096 g of xenon difluoride (molar ratio 1:0.046) were weighed out. LiNi 0.8 Co 0.1 Mn 0.1 O2 powder was packed into a 2 mL alumina sample container (Nagano Keiki Co., Ltd.), and xenon difluoride was packed into a φ5 × 2.5 mm alumina sample container (Rigaku Corporation). The two alumina sample containers filled with samples were placed in a sealed container and left to stand in a glove box. The standing time was 90 minutes. The solid xenon difluoride vaporized, generating gaseous xenon and fluorine radicals, and the generated fluorine radicals and LiNi 0.8 Co 0.1 Mn 0.1 When O2 particles react, the surface of LiNi is replaced with fluoride ions. 0.8 Co 0.1 Mn 0.1 O 2-x F x A particle was obtained. Next, in order to extend the fluoride ion replacement layer on the particle surface into the particle interior, LiNi was fluorinated for 90 minutes using an ultra-precise miniature electric furnace (Furutech FT-01X). 0.8 Co 0.1 Mn 0.1 O 2-x F xThe powder was heat-treated at 300°C for 17 minutes. Other heat treatment conditions are as shown in Table 8. Using the obtained positive electrode active material (NCM811-F_90min_300℃), a positive electrode for a lithium-ion secondary battery was obtained in the same manner as in Example 1.

[0124] <Making a coin half-cell> A positive electrode for a lithium-ion secondary battery was fabricated in the same manner as in Example 1, except that a positive electrode active material (NCM811-F_90min_300℃) was used, and a coin half-cell was constructed.

[0125] (Example 7) Obtained LiNi 0.8 Co 0.1 Mn 0.1 O 2-x F x A positive electrode for a lithium-ion secondary battery containing the positive electrode active material (NCM811-F_90min_500℃) was prepared in the same manner as in Example 1, except that the powder was heat-treated at 500℃ for 15 minutes, and a coin half-cell was fabricated.

[0126] (Example 8) Obtained LiNi 0.8 Co 0.1 Mn 0.1 O 2-x F x A positive electrode for a lithium-ion secondary battery containing the positive electrode active material (NCM811-F_90min_1000℃) was prepared in the same manner as in Example 1, except that the powder was heat-treated at 1000℃ for 10 minutes, and a coin half-cell was fabricated.

[0127] [Table 8]

[0128] The obtained Examples 6-8 were measured and evaluated using the following methods.

[0129] [Appearance of the positive electrode active material] The positive electrode active material was observed using a field emission scanning electron microscope (FE-SEM). As a result, in Example 6 (Figure 34(a)), the fine-grained deposits that had precipitated on the surface of the positive electrode active material (see Figure 6(c)) disappeared, and in Example 7 (Figure 34(b)), a striped uneven structure was observed on the surface of the positive electrode active material. This striped uneven structure is thought to have been formed by the fluoride ion replacement layer. Furthermore, in Example 8 (Figure 34(c)), the striped uneven structure that had formed on the surface of the positive electrode active material disappeared. The striped uneven structure is thought to have disappeared because the fluoride ion concentration on the outermost surface decreased, and the surface energy of the fluoride ion replacement layer became close to the surface energy of the lithium metal composite oxide.

[0130] [Impedance Analysis] Similar to the coin half-cell, the insertion and removal of Li into the positive electrode of a coin full cell was performed in one cycle by charging and discharging, and the impedance of the coin full cell after one cycle was measured. Using the equivalent circuit model used in the frequency response analysis, the resistance of the electrolyte was set to R. sol The resistance of the CEI layer (coating layer) is R CEI The charge transfer resistance between the CEI layer and the core particle is R. ct The values ​​were then calculated (see Figure 14(a)). The results are shown in Table 9. Based on these results, the CEI layer resistance R when using fluoride ion-substituted positive electrode active material (NCM811-F_20min, NCM811-_90min_500℃) was obtained. CEI This is the CEI layer resistance R when using a positive electrode active material (NCM811-bare) that has not undergone fluoride ion substitution (Comparative Example 2). CEI It was found to be smaller than [the specified value]. Also, the charge transfer resistance R when using positive electrode active material (NCM811-F_20min, NCM811-F_90min_500℃) ct This is the charge transfer resistance R when using the positive electrode active material (NCM811-bare) (Comparative Example 2). ct It was found to be smaller than that.

[0131] [Table 9]

[0132] Furthermore, using the equivalent circuit model used in the frequency response analysis, the electrolyte resistance was defined as R0, the first component of the CEI resistance as R1, the second component as R2, the desolvation resistance of the charge transfer resistance as R3, and the deinsertion resistance as R4, and each value was determined. The results are shown in Table 10. As a result, it was found that the sum of the first and second components R1 and R2 of the CEI resistance when using a positive electrode active material that has undergone fluoride ion substitution (NCM811-F_20min, NCM811-_90min_500℃) is smaller than the sum of the first and second components R1 and R2 of the CEI resistance when using a positive electrode active material that has not undergone fluoride ion substitution (NCM811-bare) (Comparative Example 2). Furthermore, it was found that the sum of the desolvation resistance R3 and the deinsertion resistance R4 when using the positive electrode active material (NCM811-F_20min, NCM811-_90min_500℃) was smaller than the sum of the desolvation resistance R3 and the deinsertion resistance R4 when using the positive electrode active material (NCM811-bare) (Comparative Example 2).

[0133] [Table 10]

[0134] [Measurement of ion diffusion coefficient] Similar to the coin half-cell measurement, constant current intermittent titration (GITT) was performed on the coin full cell. The charge rate was set to 0.2C, and constant current pulses were applied and released repeatedly for 10 minutes each, charging from 3.6V to 4.2V. Similarly, the discharge rate was set to 0.2C, and constant current pulses were applied and released repeatedly for 10 minutes each, discharging from 4.2V to 3.6V. The lithium-ion diffusion coefficient (D) during charging relative to the open-circuit voltage obtained from the GITT measurement was calculated. Li Figure 35(a) shows the lithium ion diffusion coefficient (D) during discharge. Li These are shown in Figure 35(b).

[0135] As shown in Figure 35(a), the lithium ion diffusion coefficient of the positive electrode active material that underwent fluoride ion substitution (NCM811-F_20min, NCM811-F_90min_500℃) was greater than that of the positive electrode active material that did not undergo fluoride ion substitution (NCM811-bare) in the region from 3.7V to 4.2V, and especially in the region from 3.8V to 4.2V.

[0136] Changes in the lithium-ion diffusion coefficient during the charging process are due to the diffusion barrier, which changes depending on factors such as the lithium vacancy concentration in the positive electrode active material and the charge balance of transition metal cations. It was confirmed that the lithium-ion diffusion coefficient increased in the 3.8V to 4.2V range after heat treatment following fluoride ion substitution.

[0137] Furthermore, as shown in Figure 35(b), the lithium ion diffusion coefficient of the positive electrode active material that underwent fluoride ion substitution (NCM811-F_20min, NCM811-F_90min_500℃) was greater than that of the positive electrode active material that did not undergo fluoride ion substitution (NCM811-bare) in the region from 4.2V to 3.6V, and especially in the region from 4.2V to 3.8V.

[0138] The change in the lithium ion diffusion coefficient during the discharge process is due to a change in the lithium ion desorption mechanism. Lithium ion desorption reactions include solid solution reactions (where the lithium composition changes continuously) and two-phase coexistence reactions (where two states with different lithium compositions are generated). The former is preferable from the viewpoint of lithium ion diffusivity and suppression of lattice distortion. In this example, it is presumed that the ion diffusion coefficient increased due to the expansion of the solid solution reaction region by fluoride ion substitution. Heat treatment after fluoride ion substitution confirmed an increase in the lithium ion diffusion coefficient in the 4.2V to 3.6V range.

[0139] [Measurement of composition ratio in the thickness direction of the fluoride layer] Similar to the coin half-cell measurement, the change in fluorine atom concentration relative to oxygen atoms in the positive electrode active material of Example 7, in the direction from the outer surface toward the interior, was measured using the above XPS. The results are shown in Figure 36. Note that the "depth from the surface" on the horizontal axis in Figure 36 is the value converted to SiO2. The measurement results for a coin half-cell manufactured using the positive electrode active material (NCM811-F_90min) used in Example 3 are also shown.

[0140] As shown in Figure 36, when a fluoride ion-substituted positive electrode active material (NCM811-F_90min_500℃) was used (Example 7), the ratio of fluorine atom concentration to oxygen atom concentration in the fluoride layer (F / O ratio) was found to decrease in the range of 0 to 60 nm compared to when an unheat-treated positive electrode active material (NCM811-F_90min) was used. On the other hand, in the range above 60 nm, the F / O ratio was found to be higher when using the positive electrode active material (NCM811-F_90min_500℃) than when using the positive electrode active material (NCM811-F_90min). Furthermore, when using the positive electrode active material (NCM811-F_90min_500℃), the F / O ratio was between 0.04 and 0.06 in the range of 50 nm to 100 nm, while when using the positive electrode active material (NCM811-F_20min), the F / O ratio was between 0.01 and 0.04 in the same range. Also, when using the positive electrode active material (NCM811-F_90min), the F / O ratio was between 0.02 and 0.07 in the same range. These results indicate that the heat treatment after fluoride ion substitution causes fluorine atoms near the surface of the positive electrode active material to diffuse further into the interior. Furthermore, since the decrease in the F / O ratio in the depth direction is suppressed by the heat treatment after fluoride ion substitution, it was found that the concentration gradient of the F / O ratio can be controlled by whether or not the above heat treatment is performed. [Explanation of symbols]

[0141] 10 Lithium-ion rechargeable batteries 1 positive electrode 1a Positive electrode current collector 1b Positive electrode active material containing layer 2 negative electrode 2a Negative electrode current collector 2b Negative electrode active material containing layer 3 Electrolytes 4. Positive side case 5. Negative electrode case 6 Gasket 7 Separator 11. Positive electrode active material for lithium-ion secondary batteries 11a core particle 11b Fluoride layer

Claims

1. It has core particles composed of a lithium metal composite oxide and a fluoride layer that coats at least a part of the core particles and is composed of a fluoride of the lithium metal composite oxide, and the lithium metal composite oxide is LiNi ｋ Co ｌ Mn ｍ O ２ represented by (k + l + m = 1, k ≥ 0.6), and the fluoride of the lithium metal composite oxide is Li １－ｚ Ni ｋ Co ｌ Mn ｍ O ２－ｘ F ｘ represented by (k + l + m = 1, k ≥ 0.6, z ≤ 0.62, 0 < x ≤ 1), A positive electrode active material for a lithium-ion secondary battery, wherein the atomic concentration ratio of fluorine atoms to oxygen atoms in the fluoride layer is 0.03 or more and 0.5 or less in the range of 0 to 50 nm from the outer surface of the positive electrode active material for a lithium-ion secondary battery.

2. The positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the fluoride layer contains lithium fluoride.

3. The positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2, wherein the ratio of fluorine atom concentration to oxygen atom concentration in the fluoride layer decreases from the outer surface to the interior of the positive electrode active material for a lithium-ion secondary battery.

4. The positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2, wherein the fluoride of the lithium metal composite oxide has a layered rock salt type structure, and fluorine atoms are coordinated between adjacent transition metal layers of the layered rock salt type structure.

5. The positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the atomic concentration ratio of fluorine atoms to oxygen atoms in the fluoride layer is 0.01 or more and 0.08 or less at a depth of 50 nm to 100 nm from the outer surface of the positive electrode active material for a lithium-ion secondary battery.

6. The lithium metal composite oxide is LiNi ０．８ Co ０．１ Mn ０．１ O ２ It is represented as, and the fluoride of the lithium metal composite oxide is Li １－ｚ Ni ０．８ Co ０．１ Mn ０． １ O ２－ｘ F ｘ A positive electrode active material for a lithium-ion secondary battery according to any one of claims 1 to 4, represented by (z ≤ 0.62, 0 < x ≤ 1).

7. A positive electrode for a lithium-ion secondary battery, comprising a positive electrode current collector and a positive electrode active material containing the positive electrode active material for a lithium-ion secondary battery described in any one of claims 1 to 6, provided on the positive electrode current collector.

8. A lithium-ion secondary battery comprising a positive electrode for a lithium-ion secondary battery, a negative electrode, and an electrolyte as described in claim 7.

9. A method for producing a positive electrode active material for a lithium-ion secondary battery, comprising the step of standing a solid noble gas fluoride and a lithium metal composite oxide in a sealed space at room temperature for 12 to 180 minutes, thereby forming a fluoride layer on the lithium metal composite oxide composed of the fluoride of the lithium metal composite oxide.

10. A method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 9, further comprising the step of heat-treating the lithium metal composite oxide on which the fluoride layer is formed after the step of forming the fluoride layer.

11. A method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 9, wherein the lithium metal composite oxide on which the fluoride layer is formed is heat-treated at 300°C to 1000°C for 3 to 10 hours.

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