Positive electrode active material for lithium-ion secondary batteries

Abstract

Improved cycle characteristics. [Solution] The positive electrode active material for lithium-ion secondary batteries contains a lithium transition metal composite oxide. The lithium transition metal composite oxide has a crystal structure that belongs to the space group R-3m. The lithium transition metal composite oxide is Ni 4+ Includes.

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode active material for lithium-ion secondary batteries. [Background technology]

[0002] Japanese Patent Publication No. 2008-257992 (Reference 1) discloses a lithium nickel cobalt manganese composite oxide having a layered structure. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2008-257992 [Overview of the project] [Problems that the invention aims to solve]

[0004] Lithium transition metal composite oxides may have a layered structure. The layered structure includes 3a sites and 3b sites. The 3a sites and 3b sites are alternately stacked in the c-axis direction. The 3a sites contain lithium ions (Li + ) contains. The 3b site contains a transition metal (TM) ion. If the 3b site contains a nickel (Ni) ion, the Ni ion is usually trivalent (Ni 3+ It is thought that this is likely to occur.

[0005] Reference 1 discloses that the valence of Ni ions can be fixed to divalent by having the occupancy rate of metal ions other than Ni, cobalt (Co), and manganese (Mn) at the 3b site be 5% or less. In a battery, lithium transition metal composite oxide is charged. In a highly charged state, the valence of Ni is divalent (Ni 2+ ) to tetravalent (Ni 4+) changes. The ionic radii differ between divalent and tetravalent. Therefore, in the highly charged state, the 3b site can contract rapidly in the c-axis direction. Due to the rapid contraction of the crystal structure, an irreversible structural change (such as a change to a rock-salt structure, etc.) may occur. The irreversible structural change is accompanied by a decrease in capacity. Therefore, the desired cycle characteristics may not be obtained.

[0006] An object of the present disclosure is to improve cycle characteristics.

Means for Solving the Problems

[0007] 1. The positive electrode active material for a lithium-ion secondary battery contains a lithium transition metal composite oxide. The lithium transition metal composite oxide has a crystal structure belonging to the space group R-3m. The lithium transition metal composite oxide contains Ni 4+ .

[0008] The layered structure belongs to the space group R-3m. In the present disclosure, tetravalent Ni ions are introduced into the crystal structure in advance before the first charge in the battery. Even if a valence change to tetravalent Ni occurs during charging, since tetravalent Ni ions originally exist, it is considered that the contraction in the c-axis direction is alleviated. By reducing the irreversible structural change in the highly charged state, an improvement in cycle characteristics is expected. Hereinafter, the "positive electrode active material for a lithium-ion secondary battery" may be abbreviated as the "positive electrode active material".

[0009] 2. The positive electrode active material for a lithium-ion secondary battery described in the above "1" may, for example, include the following configuration. The crystal structure includes a 3a site and a 3b site. The 3a site contains Li + and Ni 2+ . The 3b site contains Ni 4+ and Ni 3+ .

[0010] For example, divalent Ni ions (Ni 2+The ion may be transferred to the 3a site. The phenomenon of Ni ions transferring to the 3a site is also called "cation mixing (CM)". The 3b site may contain tetravalent and trivalent Ni ions.

[0011] 3. The positive electrode active material for lithium-ion secondary batteries described in "2" above may include, for example, the following components: Ni 3+ Ni 4+ The ratio of the amounts of substance is between 0.09 and 0.35.

[0012] Below, Ni 3+ Ni 4+ The ratio of the amounts of substance is the molar ratio "Ni 4+ / Ni 3+ It is also written as "Ni 4+ / Ni 3+ When the value is between 0.09 and 0.35, an improvement in cycle characteristics can be expected.

[0013] 4. The positive electrode active material for lithium-ion secondary batteries described in "2" or "3" above may include, for example, the following components: Ni 2+ Ni 4+ The ratio of the amounts of substance is between 2.7 and 7.2.

[0014] Below, Ni 2+ Ni 4+ The ratio of the amounts of substance is the molar ratio "Ni 4+ / Ni 2+ It is also written as "Ni 4+ / Ni 2+ When the value is between 2.7 and 7.2, an improvement in cycle characteristics can be expected.

[0015] 5. The positive electrode active material for lithium-ion secondary batteries described in any one of items "1" to "4" above may include, for example, the following components: The lithium transition metal composite oxide has the general formula "Li z Ni 1-c-a Co c Mn a O dIt has the composition represented by the formula. In the general formula, "z, a, c, d" satisfy the following relationships: "0.1 ≤ z ≤ 1.5", "0.5 ≤ 1 - ca ≤ 1.0", "0 ≤ c ≤ 0.3, 0 ≤ a ≤ 0.3", and "1.5 ≤ d ≤ 2.1".

[0016] Hereinafter, one embodiment of the present disclosure (which may be abbreviated as "this embodiment") and one example of the present disclosure (which may be abbreviated as "this example") will be described. However, this embodiment and this example will not limit the technical scope of the present disclosure. This embodiment and this example are illustrative in all respects. This embodiment and this example are not restrictive. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the claims. For example, it is intended from the outset that any configuration may be extracted from this embodiment and combined in any way. [Brief explanation of the drawing]

[0017] [Figure 1] This graph shows an example of fitting results in Rietveld analysis. [Figure 2] This is an example of an XAFS spectrum. [Figure 3] This is a table showing the experimental results. [Figure 4] This graph shows the relationship between the molar ratio "Ni4+ / Ni3+" and the volume retention rate. [Figure 5] This graph shows the relationship between the molar ratio "Ni4+ / Ni2+" and the volume retention rate. [Modes for carrying out the invention]

[0018] Terms, phrases "Equipped with," "includes," "possesses," and variations thereof are open-ended expressions. Configurations expressed in an open-ended manner may or may not include additional elements in addition to the essential elements. The statement "consists of" is a closed expression. However, even configurations expressed in a closed manner may include additional elements that are usually incidental impurities or irrelevant to the subject technology. The statement "substantially consists of..." is a semi-closed expression. In configurations expressed in a semi-closed manner, the addition of elements that do not substantially affect the basic and novel characteristics of the subject technology is permitted.

[0019] Numerical values ​​may be displayed with significant figures. Unless otherwise specified, measured values ​​may be the average of multiple measurements. The number of measurements may be three or more, five or more, or ten or more. Generally, the reliability of the average is expected to improve with a larger number of measurements. Measured values ​​may be rounded to the nearest significant figure. Measured values ​​may include errors, such as those associated with the detection limits of the measuring device.

[0020] The devices, software, etc., used for measuring various values ​​are merely examples. Equivalent devices may be used. If equivalent devices are used, the measurement conditions may be adjusted to suit the device.

[0021] The space group to which the crystal structure belongs is determined by the XRD (X-Ray Diffraction) pattern. Measurement conditions may be, for example, as follows: Analysis method: Wide-angle method Measurement device: Smart Lab II (manufactured by Rigaku Corporation) Measurement angle: 10° to 120° Tube:CuKα Tube voltage: 45kV Tube current: 200mA Measurement method: Continuous method Step: 0.02 Speed: 2° / min IS:1 / 2 RS: 20mm Detection mode: 1D

[0022] If the crystal structure is assigned to the space group R-3m, then the following procedure can be used to determine the various Ni ions (Ni) in lithium transition metal composite oxides. 2+ Ni 3+ Ni 4+ The composition ratio of ) is determined. As an example, the case in which a lithium transition metal composite oxide has a composition represented by the following general formula is explained.

[0023] Li z Ni 1-c-a Co c Mn a O d The Co at the 3b site is trivalent (Co 3+ It is thought that the Mn at the 3b site is tetravalent (Mn 4+ It is thought that the Ni that has been transferred to the 3a site is divalent (Ni 2+ It is thought that the Ni ion at the 3b site is trivalent (Ni 3+ ) or tetravalent (Ni 4+ It is thought to be trivalent (Ni 3+ When the composition ratio of ) is "y", the above general formula can be rewritten as follows, taking into account the composition of each site.

[0024] [Li + z Ni 2+ b ] 3a site [Ni 3+ y Ni 4+ (1-y-c-b-a) Co c Mn a ] 3b site [O d ] 6c site Assume that the average valency of Ni in a lithium transition metal composite oxide is "n". The relationship "n(1-ca)=2b+3y+4(1-ycba)" holds. Therefore, "y=(4-n)(1-ca)-2b" can be found. 3+ Depending on the composition ratio "y", Ni 4+The composition ratio "1-ycba" can also be identified. "a" and "c" are determined by compositional analysis. "b" is determined by Rietveld analysis of the XRD pattern. "n" is determined by XAFS (X-ray Absorption Fine Structure) measurement in the hard X-ray region.

[0025] The composition of the positive electrode active material can be measured by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy). A sample solution is prepared by dissolving 0.1 g of the sample (positive electrode active material) in a mixed acid (10 ml) of hydrochloric acid and sulfuric acid. The sample solution is diluted to an appropriate concentration in a volumetric flask. After dilution, compositional analysis is performed using an ICP-AES instrument. For example, a product such as "PS3520 UVDD II (manufactured by Hitachi High-Tech Science Corporation)" may be used.

[0026] Rietveld analysis is performed on the XRD pattern using the software "GSAS-II". The structural model is the space group R-3m (ITA No. 166). Background processing and structural refinement are performed on the XRD pattern. Ni at site 3a 2+ Optimization is performed by setting the composition ratio "b" as a variable. Generally, it can be set to "0 ≤ b ≤ 0.1". From this, "Rwp", one of the indicators of refinement, is calculated. A quadratic function "αx" is applied to the values ​​of Rwp and b. 2 The curve "+βx+γ" is fitted. Figure 1 is a graph showing an example of the fitting results in Rietveld analysis. Within the refined range of the obtained approximation curve, x corresponding to the minimum value of Rwp is Ni 2+ This is considered to be the composition ratio "b". Note that "b / (z+b)" is also called the CM ratio.

[0027] XAFS measurements may be performed, for example, at the beamline "BL5S1" of the "Aichi Synchrotron Radiation Center." The sample is prepared by coating the positive electrode active material onto a substrate. The sample may be a part of the positive electrode plate of a battery. A Quick measurement is performed using the transmission method in 3 minutes to obtain the XAFS spectrum of the K absorption edge of Ni. The XAFS spectrum is normalized using the software "Athena." Figure 2 shows an example of an XAFS spectrum. In the normalized XAFS spectrum, the energy at which the absorption is 0.5 is considered the absorption edge energy. Ni 2+ Ni 3+ Ni 4+ The average valence state "n" of Ni in the positive electrode active material is calculated by linear fitting based on the absorption edge energies of standard samples (e.g., NiO, Ni2O3, NiO2, etc.).

[0028] The stoichiometric composition formula shows a representative example of a compound. The compound may have a non-stoichiometric composition. For example, "Al2O3" is not limited to compounds with a molar ratio of "Al / O = 2 / 3". Unless otherwise specified, "Al2O3" refers to a compound containing Al and O in any molar ratio. For example, the compound may be doped with trace elements. Some of the Al and O may be substituted with other elements.

[0029] "Primary particles" refer to the smallest unit of particles, solid particles that are recognized as being unable to be divided any further. Primary particles appear to have no grain boundaries in SEM (Scanning Electron Microscope) images. The number of primary particles contained in single-crystal or polycrystalline particles is counted in the SEM image of the powder. The image magnification is, for example, 10,000x. Primary particles existing alone, or aggregates of 2 to 10 primary particles, are considered single-crystal particles. Aggregates of more than 10 primary particles are considered polycrystalline particles. In an SEM image, the percentage of single-crystal particles can be determined from 100 randomly selected particles.

[0030] "D50" indicates the particle size at which the cumulative frequency reaches 50% in the volume-based particle size distribution (cumulative distribution). D50 can be measured, for example, by laser diffraction. Similarly, the particle size at which the cumulative frequency reaches 10% is also written as "D10," and the particle size at which the cumulative frequency reaches 90% is also written as "D90."

[0031] positive electrode active material The positive electrode active material is for a battery. The battery may be a liquid-based battery or an all-solid-state battery. The battery may have any structure. The battery may have, for example, a wound or stacked power generation element. The battery may have, for example, a unipolar or bipolar structure.

[0032] The positive electrode active material contains a lithium transition metal composite oxide. The lithium transition metal composite oxide is a compound containing Li, a transition metal (TM), and oxygen (O). The lithium transition metal composite oxide has a crystal structure assigned to space group R-3m. A crystal structure assigned to space group R-3m is also referred to as a "layered structure."

[0033] The layered structure includes the 3a site (Li site), the 3b site (TM site), and the 6c site (O site). The 3a site is Li + It contains. The 3b site contains Ni ions. Exchange (CM) may occur between some of the Li ions at the 3a site and some of the Ni ions at the 3b site. The Ni ions that have moved to the 3a site are divalent (Ni 2+ ) may also be acceptable. + The ionic radius of is 0.76 Å. (Trivalent Ni ion (Ni) 3+ The ionic radius of the divalent Ni ion (Ni) is 0.56 Å to 0.60 Å. 2+ The ionic radius of ) is 0.69 Å. Ni 3+ Compared to Ni 2+ Li + Because it is close to the ionic radius of Ni, within the 3a site, 2+ It is considered stable. Therefore, the 3a site is Li + and Ni 2+may also be included.

[0034] The 3b site contains Ni 4+ That is, the lithium transition metal composite oxide contains Ni 4+ Including. The presence of Ni in the crystal structure 4+ is expected to improve the cycle characteristics. Ni 4+ is considered not to be introduced into the crystal structure by the conventional synthesis method. According to the new findings of the present disclosure, by charging an excessive amount of Li source during synthesis and performing appropriate water washing after firing, Ni 4+ can be introduced into the 3b site.

[0035] The 3b site may further contain Ni 4+ in addition to Ni 3+ The molar ratio "Ni 4+ / Ni 3+ " may be, for example, greater than 0, 0.02 or more, 0.04 or more, 0.06 or more, 0.08 or more, 0.09 or more, 0.10 or more, 0.12 or more, 0.14 or more, 0.16 or more, 0.18 or more, 0.20 or more, 0.21 or more, 0.22 or more, 0.24 or more, 0.26 or more, 0.28 or more, 0.30 or more, 0.32 or more, 0.34 or more, 0.35 or more, 0.36 or more, 0.38 or more, 0.40 or more, 0.42 or more, 0.44 or more, 0.46 or more, 0.48 or more, or 0.50 or more. The molar ratio "Ni 4+ / Ni 3+ " may be, for example, 5 or less, 4 or less, 3 or less, 2 or less, 1 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.48 or less, 0.46 or less, 0.44 or less, 0.42 or less, 0.40 or less, 0.38 or less, 0.36 or less, 0.35 or less, 0.34 or less, 0.32 or less, 0.30 or less, 0.28 or less, 0.26 or less, 0.24 or less, 0.22 or less, 0.21 or less, 0.20 or less, 0,18 or less, 0.16 or less, 0.14 or less, 0.12 or less, 0.10 or less, 0.09 or less, 0.08 or less, with 0.06 or less, 0.04 or less, or 0.02 or less. For example, the molar ratio "Ni 4+ / Ni 3+When " " is from 0.09 to 0.35, improvement in cycle characteristics is expected.

[0036] The molar ratio "Ni 4+ / Ni 2+ " may be, for example, more than 0, 1 or more, 2 or more, 2.4 or more, 2.7 or more, 2.8 or more, 3.2 or more, 3.6 or more, 3.8 or more, 4.0 or more, 4.4 or more, 4.8 or more, 5.2 or more, 5.6 or more, 6.0 or more, 6.4 or more, 6.7 or more, 6.8 or more, 7.2 or more, 7.6 or more, 8.0 or more, 9.0 or more, 10.0 or more, 12.0 or more, 14.0 or more, 16.0 or more, 16.8 or more, 18.0 or more, or 20.0 or more. The molar ratio "Ni 4+ / Ni 2+ " may be, for example, 40.0 or less, 30.0 or less, 20.0 or less, 18.0 or less, 16.8 or less, 16.0 or less, 14.0 or less, 12.0 or less, 10.0 or less, 9.0 or less, 8.0 or less, 7.6 or less, 7.2 or less, 6.8 or less, 6.7 or less, 6.4 or less, 6.0 or less, 5.6 or less, 5.2 or less, 4.8 or less, 4.4 or less, 4.0 or less, 3.8 or less, 3.6 or less, 3.2 or less, 2.8 or less, 2.7 or less, 2.4 or less, 2 or less, or 1 or less. For example, when the molar ratio "Ni 4+ / Ni 2+ " is from 2.7 to 7.2, improvement in cycle characteristics is expected.

[0037] The 3b site may further contain Co, Mn, etc. in addition to Ni. The lithium transition metal composite oxide may have, for example, a composition represented by the general formula "Li z Ni 1-c-a Co c Mn a O d ". The Li composition ratio "z" may satisfy, for example, the relationship of "0.1 ≦ z ≦ 1.5". The Li composition ratio "z" may be, for example, 0.4 or more, 0.6 or more, 0.8 or more, 1.0 or more, 1.2 or more, or 1.4 or more. The Li composition ratio "z" may be, for example, 1.4 or less, or 1.2 or less.

[0038] In the general formula above, the O composition ratio "d" may satisfy, for example, the relationship "1.5 ≤ d ≤ 2.1". The O composition ratio "d" may be, for example, 1.6 or more, 1.7 or more, 1.8 or more, 1.9 or more, or 2.0 or more. The O composition ratio "d" may be, for example, 2.0 or less, 1.9 or less, 1.8 or less, 1.7 or less, or 1.6 or less.

[0039] In the general formula above, the Ni composition ratio "1-ca" may satisfy, for example, the relationship "0.5 ≤ 1-ca ≤ 1.0". The Ni composition ratio "1-ca" may be, for example, 0.6 or more, 0.7 or more, 0.8 or more, or 0.9 or more. The Ni composition ratio "1-ca" may be, for example, 0.9 or less, 0.8 or less, 0.7 or less, or 0.6 or less. When the Ni composition ratio "1-ca" is 0.5 or more, an increase in discharge capacity can be expected.

[0040] In the above general formula, the Co composition ratio "c" may satisfy, for example, the relationship "0 ≤ c ≤ 0.3". The Co composition ratio "c" may be, for example, 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, 0.10 or more, 0.15 or more, 0.20 or more, or 0.25 or more. The Co composition ratio "c" may be, for example, 0.25 or less, 0.20 or less, 0.15 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less.

[0041] In the general formula above, the Mn composition ratio "a" may satisfy, for example, the relationship "0 ≤ a ≤ 0.3". The Mn composition ratio "a" may be, for example, 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, 0.10 or more, 0.15 or more, 0.20 or more, or 0.25 or more. The Mn composition ratio "a" may be, for example, 0.25 or less, 0.20 or less, 0.15 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less.

[0042] In addition, in the above general formula, all or part of Mn may be substituted with Al, etc. That is, lithium transition metal composite oxides are, for example, those with the general formula "Li z Ni 1-c-a Co c Al a O d It may have a composition represented by ". The range of Al composition ratio "a" is the same as that of Mn composition ratio "a" above.

[0043] Lithium transition metal composite oxides may contain any dopant. The dopant represents an element other than Li, Ni, Co, Mn, and O. For example, the dopant may contain at least one selected from the group consisting of Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, and Ag. The composition ratio of the dopant may be, for example, 0.005 or more, 0.01 or more, 0.02 or more, 0.03 or more, or 0.04 or more. The composition ratio of the dopant may be, for example, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less.

[0044] The positive electrode active material may be in powder form. The D50 of the positive electrode active material may be, for example, 1 μm or more, 3 μm or more, 5 μm or more, 10 μm or more, or 15 μm or more. The D50 of the powder may be, for example, 30 μm or less, 20 μm or less, or 10 μm or less.

[0045] Lithium transition metal composite oxides may form single-crystal particles. Lithium transition metal composite oxides may form polycrystalline particles. That is, the positive electrode active material may contain both single-crystal particles and polycrystalline particles. Polycrystalline particles may have substantially the same crystal structure and composition as single-crystal particles. The positive electrode active material (powder) may consist, for example, of 50% or more single-crystal particles and the remainder being polycrystalline particles. The percentage of single-crystal particles may be, for example, 60% or more, 70% or more, 80% or more, or 90% or more. The percentage of single-crystal particles may be, for example, 100% or less, 90% or less, or 80% or less. For example, an improvement in cycle characteristics can be expected when the percentage of single-crystal particles is 70% or more. This is thought to be because single-crystal particles have fewer grain boundaries compared to polycrystalline particles, making them less prone to cracking during charging and discharging (volume change).

[0046] It is expected that the fewer the number of primary particles constituting a single crystal particle, the less likely cracks are to occur. The number of primary particles constituting a single crystal particle may be, for example, 9 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4 or less, 3 or less, or 2 or less. The number of primary particles constituting a polycrystalline particle may be, for example, 15 or more, 20 or more, 25 or more, or 50 or more. The number of primary particles constituting a polycrystalline particle may be, for example, 100 or less, 50 or less, 25 or less, or 20 or less. For example, if a single crystal particle is redefined to contain 5 or fewer primary particles, the above particle count percentages shall represent the particle count percentage of the single crystal particle after the redefinition.

[0047] The positive electrode active material (powder) may be a monodisperse system. Improved cycle characteristics are expected when the powder is mainly composed of single-crystal particles and is a monodisperse system. The powder may have a span of, for example, 1 or less. "Span" is a value obtained by the calculation formula "(D90-D10) / D50". A smaller span indicates a sharper particle size distribution. The powder span may be, for example, 0.9 or less, 0.8 or less, 0.7 or less, 0.6 or less, or 0.5 or less. The powder span may also be, for example, 0.1 or greater, 0.2 or greater, 0.3 or greater, 0.4 or greater, or 0.5 or greater. [Examples]

[0048] Sample preparation Figure 3 is a table showing the experimental results. Samples No. 1 to No. 8 were prepared according to the following procedure.

[0049] Preparation of the precursor The raw material solution is prepared by dissolving NiSO4 in deionized water. The molar concentration of the raw material solution is 0.2%. For example, if the target substance is "LiNi 0.90 Co 0.05 Mn 0.05 In the case of "O2", the raw material solution is prepared by dissolving NiSO4, CoSO4, and MnSO4 in deionized water. The mixing ratio of NiSO4, CoSO4, and MnSO4 is adjusted to "Ni / Co / Mn = 90 / 5 / 5 (mole ratio)".

[0050] A predetermined amount of NH3 aqueous solution is placed in the reaction vessel. The reaction vessel is stirred with a stirrer while being purged with nitrogen. NaOH is added to the reaction vessel to adjust the pH of the aqueous solution to alkaline. The raw material solution is added dropwise while the temperature and pH of the aqueous solution are adjusted to maintain a constant range (effectively constant value), forming a precipitate of TM hydroxide (Ni hydroxide). The precipitate is dehydrated and calcined. The calcination temperature is 120°C to 220°C. The calcination time is 4 to 10 hours. The calcination pressure is 0.2 MPa to 1.0 MPa.

[0051] After calcination, the precipitate is washed with water. The residue (TM hydroxide) is recovered by filtration. The TM hydroxide is dried at 110°C for 12 hours to remove the water. Thus, the precursor (TM hydroxide) is prepared.

[0052] Adding Li sources In an agate mortar, the precursor (TM hydroxide) and the lithium compound (LiOH) are mixed to prepare the mixture. The lithium compound may be, for example, Li2CO3. The ratio of the amount of Li to the total amount of TM(Ni), "Li / TM," is shown in the "Li / TM" column of Figure 3. For example, if "Li / TM" is greater than 1, a molten salt may be formed during calcination, promoting the single crystallization of the lithium transition metal composite oxide. That is, it is expected that the percentage of single crystal particles in the powder will be 70% or more.

[0053] firing In a firing furnace, the mixture is heat-treated to produce the positive electrode active material. The firing temperature is between 650°C and 1100°C. The firing time is between 5 and 15 hours.

[0054] After firing, the positive electrode active material is crushed in an agate mortar until the particle size is 0.2 mm or less. The positive electrode active material is dispersed in 500 mL of pure water to form a slurry. The slurry is vigorously stirred for a predetermined time, that is, the positive electrode active material is washed with water. The washing time for each sample is shown in the "Washing Time" section of Figure 3. Washing was not performed in No. 7.

[0055] After washing with water, the slurry is filtered through filter paper and a Buchner funnel. The residue is rinsed with 500 mL of pure water to form a cake. The cake is vacuum-dried at 90°C. After drying, the cake is crushed in an agate mortar to adjust it to a predetermined particle size. For example, the cake may be crushed using a lab mill or other mill. From the above, the positive electrode active material (LiNiO2) is produced. The crystal structure of each sample is determined by Rietveld analysis, and Ni 2+ Ni 3+ and Ni 4+ The composition ratios are determined. As an example, the XAFS spectra of No. 1 and No. 7 are shown in Figure 2.

[0056] evaluation A laminated cell is prepared. A laminated cell is a cell in which the power generation elements are housed in a pouch made of aluminum laminate film. The configuration of the power generation elements is as follows: Positive electrode: Positive electrode active material, conductive material (acetylene black) Negative electrode: Negative electrode active material (natural graphite) Electrolytes: LiPF6 (1 ml / L), EC / DMC / EMC = 3 / 4 / 3 (volume ratio)

[0057] The positive and negative electrodes are manufactured by coating the surface of a substrate (metal foil) with a slurry. For example, a film applicator (with film thickness adjustment function) manufactured by Allgood is used as the coating equipment. After coating with the slurry, the coating is dried at 80°C for 5 minutes.

[0058] A cycle test will be performed on the laminated cell under the following conditions. Ambient temperature: 60℃ Number of cycles: 100 Current rate: 0.3C Voltage range: 4.25V to 2.5V

[0059] At a current rate of 1C, the rated capacity of the cell is supplied in one hour. 0.3C is 0.3 times 1C. The capacity retention rate can be determined by dividing the discharge capacity at 100 cycles by the discharge capacity at 1 cycle. A higher capacity retention rate indicates better cycle characteristics.

[0060] Experimental results In Figure 3, for example, "Ni 4+ / Ni total The statement "Ni 2+ Ni 3+ and Ni 4+ Ni 4+ This shows the ratio of the amounts of substance.

[0061] As shown in Figure 3, Ni is present in the crystal structure. 4+ The presence of this element tends to improve cycle characteristics.

[0062] Figure 4 shows the molar ratio "Ni 4+ / Ni 3+ This is a graph showing the relationship with the volume retention rate. Molecular ratio "Ni 4+ / Ni 3+ When the molar ratio "Ni" is between 0.09 and 0.35, there is a tendency for the cycle characteristics to improve. 4+ / Ni 3+ In regions where the value exceeds 0.35, it is expected that an inert region is formed in part of the crystal structure due to excessive Li deficiency.

[0063] Figure 5 shows the molar ratio "Ni 4+ / Ni 2+ This is a graph showing the relationship with the volume retention rate. Molecular ratio "Ni 4+ / Ni 2+ When the value is between 2.7 and 7.2, there is a tendency for the cycle characteristics to improve. [Explanation of symbols]

[0064] none

Claims

1. Contains lithium transition metal composite oxide, The lithium transition metal composite oxide is It has a crystal structure that belongs to the space group R-3m, and, Ni 4+ including, Positive electrode active material for lithium-ion secondary batteries.

2. The aforementioned crystal structure includes a 3a site and a 3b site, The site 3a is Li + and Ni 2+ including, and, The aforementioned site 3b is Ni 4+ and Ni 3+ including, The positive electrode active material for a lithium-ion secondary battery according to claim 1.

3. Ni 3+ Ni 4+ The ratio of the amounts of substance is between 0.09 and 0.

35. The positive electrode active material for a lithium-ion secondary battery according to claim 2.

4. Ni 2+ The ratio of the amount of substance of Ni 4+ to the amount of substance of Ni is from 2.7 to 7.2, The positive electrode active material for a lithium-ion secondary battery according to claim 2 or claim 3.

5. The lithium transition metal composite oxide has the general formula: Li z Ni 1-c-a Co c Mn a O d Having a composition represented by, In the above general formula, z, a, c, and d satisfy the following relationships: 0.1 ≤ z ≤ 1.5, 0.5 ≤ 1 - c - a ≤ 1.0, 0 ≤ c ≤ 0.3, 0 ≤ a ≤ 0.3, and 1.5 ≤ d ≤ 2.

1. The positive electrode active material for a lithium-ion secondary battery according to claim 2 or claim 3.

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