Lithium-iron composite oxide, cathode material, cathode and lithium-ion secondary battery

Abstract

This lithium iron composite oxide is represented by Li5FeO4, and has a quadrupolar splitting (QS) that is obtained by an analysis using 57Fe Mossbauer spectroscopy and that includes two peaks different from each other. Peak A, which is one of the two peaks, satisfies QS>0, and peak B, which is the other of the two peaks, satisfies QS=0.

Description

[Technical Field]

[0001] The present invention relates to a lithium-iron composite oxide, a cathode material, a cathode, and a lithium-ion secondary battery. This application claims priority to Japanese Patent Application Nos. 2021-002344, 2021-002345, and 2021-002346, filed on January 8, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] Lithium-ion secondary batteries are characterized by their small size and large capacity. They are not only used in electronic devices such as mobile phones and laptops, but also in mobile devices such as automobiles and drones in recent years. The applications of lithium-ion secondary batteries are expanding more and more.

[0003] As applications become more powerful, further improvements in the performance of lithium-ion secondary batteries are required. There is a particularly strong demand for improved energy density. In order to increase energy density, it is necessary to increase the capacity of both the positive and negative electrode active materials. Graphite is a typical material used as a negative electrode active material. Graphite has excellent lifespan and output characteristics, and is inexpensive, so it has been widely used as a negative electrode active material. However, the capacity of graphite has already been used up to its theoretical value, and the conversion from graphite to other materials is being considered to increase the capacity of the negative electrode.

[0004] As an alternative to graphite, alloy-based anode active materials, such as silicon and silicon oxide, are being researched. These anode active materials can store several to several dozen times more lithium than graphite, and have a significantly greater capacity than graphite. However, these anode active materials undergo an irreversible structural change during the first charge, trapping lithium ions within the structure. The trapped lithium ions are unable to contribute to subsequent charge-discharge reactions, preventing the battery from achieving sufficient cell capacity.

[0005] To prevent these lithium ions from being trapped in the negative electrode active material, a technique called lithium pre-doping has been proposed, in which lithium is absorbed into the negative electrode beforehand. Lithium pre-doping prevents the lithium released from the positive electrode from being consumed, enabling the alloy-based negative electrode active material to have a high capacity. Lithium pre-doping can be performed on either the positive electrode or the negative electrode. For example, Patent Document 1 reports an example in which a lithium-iron composite oxide having an inverse fluorite crystal structure, which has a large irreversible capacity during the initial charge / discharge, is used as the positive electrode. For example, Patent Document 2 reports that carbon coating by chemical vapor deposition is performed to improve the high resistance of the lithium-iron composite oxide, thereby improving its characteristics. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-207055 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-130359 Summary of the Invention [Problem to be solved by the invention]

[0007] One example of lithium-iron composite oxides is lithium-iron composite oxide. However, lithium-iron composite oxides have a problem in that they have high resistance and their irreversible capacity is not fully realized. In other words, lithium-iron composite oxides may not be able to fully exhibit their effect as lithium pre-doped materials.

[0008] The present disclosure has been made in view of the above problems, and aims to increase the irreversible capacity of a lithium-iron composite oxide. [Means for solving the problem]

[0009] (1) The lithium iron composite oxide according to the first aspect is represented by LiFeO, 57Analysis using Fe Mössbauer spectroscopy showed two peaks with different quadrupole splitting values ​​(QS), one of which, Peak A, satisfied QS > 0, and the other, Peak B, satisfied QS = 0.

[0010] (2) In the lithium-iron composite oxide according to the above aspect, the peak A and the peak B may each have an isomer shift value (IS) of 0.05 or more and 0.25 or less.

[0011] (3) In the lithium-iron composite oxide according to the above aspect, the area value of the peak A is S A , the area value of the peak B is S B When this is done, 0.01≦S B / (S A +S B )≦0.50 may be satisfied.

[0012] (4) A positive electrode material according to a second aspect includes the lithium-iron composite oxide according to the above aspect.

[0013] (5) A positive electrode material according to a third aspect includes the lithium-iron composite oxide according to the above aspect and a coating layer that coats the surface of the lithium-iron composite oxide, and the coating layer contains lithium silicate.

[0014] (6) In the positive electrode material according to the above aspect, the mass ratio of the coating layer may be 0.1 mass % or more and 3.0 mass % or less with respect to the total mass of the positive electrode material.

[0015] (7) A positive electrode material according to a fourth aspect includes the lithium-iron composite oxide according to the above aspect and a coating layer that coats the surface of the lithium-iron composite oxide, and the coating layer contains diamond-like carbon.

[0016] (8) In a Raman spectroscopy of the coating layer of the positive electrode material according to the above aspect, -1 G-band spectrum and 1350cm -1The spectrum of the G band may partially overlap with the spectrum of the D band, and the G / D ratio, which is the ratio of the peak intensities of the G band and the D band, may be 1.5 or less.

[0017] (9) A positive electrode according to a fifth aspect includes the positive electrode material according to the above aspect.

[0018] (10) A lithium ion secondary battery according to a sixth aspect includes the positive electrode according to the above aspect. [Effects of the Invention]

[0019] The lithium-iron composite oxide according to the above embodiment can increase the irreversible capacity. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. [Figure 2] 4 shows the results of Mössbauer spectroscopic analysis of an example of a positive electrode material of the lithium ion secondary battery according to the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, the embodiments will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present disclosure is not limited thereto. Appropriate changes can be made within the scope of the present disclosure. The components described below can also be combined as appropriate.

[0022] "Lithium-ion secondary battery" Fig. 1 is a schematic diagram of a lithium-ion secondary battery according to a first embodiment. The lithium-ion secondary battery 100 shown in Fig. 1 includes a laminate 30, an electrolyte solution containing lithium ions, a case 50, a lead 60, and a lead 62. The case 50 houses the laminate 30 and the electrolyte solution in a sealed state. One end of the lead 60 is electrically connected to the positive electrode 10, and the other end protrudes outside the case 50. One end of the lead 62 is electrically connected to the negative electrode 20, and the other end protrudes outside the case.

[0023] The laminate 30 includes a positive electrode 10, a negative electrode 20, and a separator 18. The positive electrode 10, the negative electrode 20, and the separator 18 are each, for example, a plate-shaped member. The separator 18 is located between the positive electrode 10 and the negative electrode 20. The laminate 30 may be replaced by a wound body formed by winding a structure in which the positive electrode 10, the separator 18, and the negative electrode 20 are stacked in this order. The positive electrode 10 includes, for example, a positive electrode current collector 12 and a positive electrode active material layer 14. The positive electrode active material layer 14 is in contact with at least one surface of the positive electrode current collector 12. The negative electrode 20 includes, for example, a negative electrode current collector 22 and a negative electrode active material layer 24. The negative electrode active material layer 24 is in contact with at least one surface of the negative electrode current collector 22. The separator 18 is located between the positive electrode active material layer 14 and the negative electrode active material layer 24.

[0024] <Positive electrode> The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 .

[0025] [Positive electrode current collector] The positive electrode current collector 12 is, for example, a conductive plate material. The positive electrode current collector 12 is, for example, a thin plate made of a metal or alloy such as aluminum, copper, nickel, titanium, or stainless steel. The positive electrode current collector 12 is, for example, a metal foil.

[0026] [Cathode active material layer] The positive electrode active material layer 14 includes, for example, a positive electrode active material, a positive electrode binder, a conductive additive, and an additive. The additive is an example of a positive electrode material. The additive functions, for example, as a pre-dope material.

[0027] (additives) The additive contains a lithium-iron composite oxide represented by Li5FeO4. Li5FeO4 is a representation of the stoichiometric composition, and the lithium-iron composite oxide may have element deficiencies or the like.

[0028] Lithium iron composite oxide is 57 The figure shows two peaks with different quadrupole splitting values ​​(QS) analyzed using Fe Mössbauer spectroscopy. Figure 2 shows the results of Mössbauer spectroscopy analysis of an example of an additive for the lithium-ion secondary battery according to the first embodiment. The solid line in Figure 2 represents the measured values, the dotted line represents one peak (hereinafter referred to as Peak A) obtained by analyzing the measured values ​​and separating them, and the dashed-dotted line represents one peak (hereinafter referred to as Peak B) obtained by analyzing the measured values ​​and separating them.

[0029] Peak A is a peak due to the crystalline component of the lithium-iron composite oxide. The quadrupole splitting value (QS) of Peak A is the interval between two local minima (the interval between split levels), and QS>0.

[0030] Peak B is a peak due to the amorphous component of the lithium-iron composite oxide. Peak B has one minimum value, and the quadrupole splitting value (QS) is QS = 0. In other words, the presence of Peak B indicates that the lithium-iron composite oxide contains X-ray amorphous matter.

[0031] Lithium iron composite oxide is 57 The quadrupole splitting value (QS) analyzed using Fe Mössbauer spectroscopy shows peaks A and B, suggesting that the material is microcrystalline. The lithium-iron composite oxide according to this embodiment readily releases lithium ions. A lithium-iron composite oxide with low crystallinity readily releases lithium ions and is more likely to reduce resistance than a lithium-iron composite oxide with high crystallinity. As a result, a lithium-ion secondary battery containing this material has a sufficiently large irreversible capacity at the time of initial charge.

[0032] The pre-dope material preferably has a large irreversible capacity. This is because the pre-dope material releases Li only during the initial charge and contributes to the reaction, and is not intended to cause a reversible reaction. Therefore, the lithium-iron composite oxide according to this embodiment functions favorably as a pre-dope material.

[0033] In the lithium-iron composite oxide according to this embodiment, the isomer shift value (IS) of each of the peak A and the peak B is preferably 0.05 or more and 0.25 or less. 57 It is one of the measurement parameters of Fe Mössbauer spectroscopy. 57 This is the numerical value of the deviation from the central relative velocity output from the measuring instrument that performs Fe Mössbauer spectroscopy.

[0034] The lithium iron composite oxide according to this embodiment is 57 In Fe Mössbauer spectroscopy, the IS of each of Peak A and Peak B is preferably 0.05 to 0.25, more preferably 0.11 to 0.22. If this value is within this range, it can be assumed that there is no change in the valence of Fe due to microcrystallization, and an increase in grain boundary resistance can be suppressed.

[0035] The lithium iron composite oxide according to this embodiment has an area value of the peak A of S A , the area value of the above peak B is S B When this is the case, 0.01≦S B / (S A +S B )≦0.50, and 0.01≦S B / (S A +S B )≦0.20 is more preferable. When this range is satisfied, structural deterioration of the microcrystalline lithium-iron composite oxide can be suppressed. The area values ​​of Peak A and Peak B can be calculated by expressing the respective observation data using two Lorentz functions, extracting the hyperfine parameter using the least squares method, and then using the half-width method.

[0036] The lithium-iron composite oxide according to this embodiment can be produced by a synthesis step and a microcrystallization step.

[0037] The synthesis process can be carried out by a known method and is not particularly limited. For example, an iron source and a lithium source are combined by mechanical milling using a ball mill or the like, and then calcined. The iron source can be, for example, Fe2O3, Fe3O4, or the like. The lithium source can be, for example, elemental lithium, Li2O, Li2CO3, LiOH, or the like.

[0038] In the microcrystallization process, the material synthesized in the synthesis process is subjected to mechanical milling. Mechanical milling can be performed, for example, using a ball mill. During mechanical milling, materials collide with each other, causing the powder to grow and break down repeatedly, resulting in the formation of a lamellar structure. As the processing time increases, the layer spacing of the lamellar structure gradually narrows and becomes random, resulting in a microcrystalline structure.

[0039] Lithium iron composite oxide 57 Analysis by Fe Mössbauer spectroscopy can be performed even if the material is not in powder form. For example, if the positive electrode does not contain other iron compounds, it can be performed in the positive electrode state. First, the positive electrode active material layer is separated from the positive electrode current collector using a spatula or similar tool, and the resulting positive electrode active material layer is analyzed.

[0040] Although the examples in which the lithium-iron composite oxide is used alone have been shown so far, a coating layer may be formed on the surface of the lithium-iron composite oxide. That is, the additive may include the lithium-iron composite oxide and a coating layer that coats the surface of the lithium-iron composite oxide.

[0041] The coating layer may contain, for example, lithium silicate. The lithium silicate protects the surface of the lithium-iron composite oxide and further increases the irreversible capacity of the additive. Although the reason for this is not clear, it is thought that silicide is formed between the lithium silicate and the lithium-iron composite oxide, improving the adhesion of the coating layer.

[0042] Lithium silicate prevents the lithium-iron composite oxide from reacting with H2O and CO2 in the atmosphere. When the lithium-iron composite oxide reacts with gases in the atmosphere, highly alkaline compounds such as LiOH and Li2CO3 are produced on the particle surface. These highly alkaline compounds react with the electrolyte during charging to form a highly resistive film, which inhibits the charging reaction of the lithium-iron composite oxide.

[0043] The mass ratio of the coating layer containing lithium silicate is preferably 0.1 mass% or more and 3.0 mass% or less with respect to the total mass of the additive. The total mass of the additive is, for example, the total mass of the lithium iron composite oxide and the coating layer. If the ratio of the coating layer is within this range, excess silicate is not formed, and adverse effects on battery characteristics are small. The mass ratio of the coating layer can be determined by optical emission spectroscopy (ICP).

[0044] A coating layer containing lithium silicate can be produced by several methods. For example, a lithium silicate-containing coating layer can be formed on the surface of a lithium iron composite oxide by mechanical milling, such as by ball milling, a lithium iron composite oxide and a silicate material. Alternatively, a lithium silicate-containing coating layer can be formed by forming a silicon oxide coating, such as SiO2, on the surface of a lithium iron composite oxide by a sol-gel method or the like, followed by firing. The coating layer can be analyzed by, for example, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM) observation, inductively coupled plasma (ICP) emission spectroscopy, etc.

[0045] The coating layer may include, for example, diamond-like carbon (DLC). DLC is a material in which the carbon in the film is sp 3 The DLC-containing coating layer prevents the lithium-iron composite oxide from reacting with H2O and CO2 in the atmosphere. When the lithium-iron composite oxide reacts with gases in the atmosphere, highly alkaline compounds such as LiOH and Li2CO3 are produced on the particle surface. These highly alkaline compounds react with the electrolyte during charging to form a highly resistive coating, which inhibits the charging reaction of the lithium-iron composite oxide.

[0046] DLC can be formed into a film at low temperatures, for example, below 500°C. For example, if a carbon film is formed at a high temperature exceeding 600°C, some transition metals in the lithium-iron composite oxide are reduced, which may adversely affect the battery characteristics. By using DLC, a carbon coating can be formed without reducing the lithium-iron composite oxide. The use of a coating material containing DLC ​​improves the cycle characteristics of lithium-ion secondary batteries.

[0047] The coating layer containing DLC ​​exhibited a Raman spectrum of 1580 cm -1 G-band spectrum and 1350cm -1 It is preferable that the spectra of the G and D bands overlap partially, and the G / D ratio, which is the ratio of the peak intensities of the G and D bands, is 1.5 or less.

[0048] Carbon coatings formed at high temperatures tend to develop a graphite structure and have a high G / D ratio. A G / D ratio of 1.5 or less indicates that the coating was formed at a low temperature. In other words, a coating layer that satisfies these conditions suppresses the reduction of the lithium-iron composite oxide, further improving the cycle characteristics of lithium-ion secondary batteries.

[0049] The thickness of the coating layer containing DLC ​​is, for example, 1 nm or more and 100 nm or less. If the thickness is 1 nm or less, the improvement effect of the carbon coating is small, and if it is more than 100 nm, it causes an increase in resistance. The thickness of the coating layer can be measured by observation with a transmission electron microscope (TEM).

[0050] Coating layers containing DLC ​​can be produced by physical vapor deposition (PVD), such as sputtering and arc ion plating. The physical properties of DLC can be controlled. For example, when depositing DLC ​​by sputtering, the G / D ratio and film thickness can be set as desired by controlling the power output, processing time, sample temperature, gas pressure, etc.

[0051] (Cathode active material) The positive electrode active material includes an electrode active material capable of reversibly undergoing lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or doping and dedoping of lithium ions and counter anions.

[0052] The positive electrode active material is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMnO2), lithium manganese spinel (LiMn2O4), and the general formula: LiNi x Co y Mn z M a O2 compound (where x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium vanadium compound (LiV2O5), olivine-type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li4Ti5O 12 )、LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). The positive electrode active material may also be an organic substance. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0053] The positive electrode active material may, for example, contain any one selected from the group consisting of nickel, cobalt, manganese, and aluminum. The positive electrode active material may be, for example, a ternary compound containing any one selected from the group consisting of nickel, cobalt, manganese, and aluminum. Lithium nickel cobalt manganate (NCM) and lithium nickel cobalt aluminate (NCA) are examples of ternary compounds. Ternary compounds can also be used at high potentials.

[0054] The positive electrode active material may be a lithium-free material. Examples of the lithium-free material include FeF3, conjugated polymers containing organic conductive materials, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, and niobium oxides. The lithium-free material may be any one of these materials alone or in combination. When the positive electrode active material is a lithium-free material, for example, discharge is first performed. Lithium is inserted into the positive electrode active material by discharging. Alternatively, lithium may be pre-doped chemically or electrochemically into the lithium-free positive electrode active material.

[0055] (Conductive additive) The conductive additive enhances the electronic conductivity between the positive electrode active materials. Examples of the conductive additive include carbon powder, carbon nanotubes, carbon materials, metal powder, a mixture of carbon materials and metal powder, and conductive oxides. Examples of the carbon powder include carbon black, acetylene black, and ketjen black. Examples of the metal powder include powders of copper, nickel, stainless steel, and iron.

[0056] There are no particular limitations on the content of the conductive additive in the positive electrode active material layer 14. For example, the content of the conductive additive relative to the total mass of the positive electrode active material, conductive additive, and binder is 0.5 mass% or more and 20 mass% or less, and preferably 1 mass% or more and 5 mass% or less.

[0057] (Positive electrode binder) The binder in the positive electrode active material layer 14 binds the positive electrode active material together. Known binders can be used. Examples of binders include fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, and polyamide-imide resin. The binder may also be an electronically conductive polymer or an ionically conductive polymer. Examples of electronically conductive polymers include polyacetylene, polythiophene, and polyaniline. Examples of ionically conductive polymers include a composite of a polyether polymer compound such as polyethylene oxide or polypropylene oxide with a lithium salt such as LiClO4, LiBF4, or LiPF6.

[0058] There are no particular limitations on the content of the binder in the positive electrode active material layer 14. For example, the content of the binder relative to the total mass of the positive electrode active material, conductive additive, and binder is 0.5 mass % or more and 5 mass % or less.

[0059] <Negative electrode> The negative electrode 20 includes, for example, a negative electrode current collector 22 and a negative electrode active material layer 24. The negative electrode active material layer 24 is formed on at least one surface of the negative electrode current collector 22.

[0060] [Negative electrode current collector] The negative electrode current collector 22 is, for example, a conductive plate material. The negative electrode current collector 22 may be the same as the positive electrode current collector 12. The negative electrode current collector 22 is, for example, a copper foil.

[0061] [Negative electrode active material layer] The negative electrode active material layer 24 contains, for example, a negative electrode active material. The negative electrode active material layer 24 may contain a conductive additive and a binder as necessary.

[0062] (Negative electrode active material) The negative electrode active material may be any compound capable of absorbing and releasing lithium ions, and may be any negative electrode active material used in known lithium ion secondary batteries. The negative electrode active material, for example, reversibly promotes the desorption and insertion of lithium ions.

[0063] Examples of negative electrode active materials include metallic lithium, lithium alloys, carbon materials, and substances that can be alloyed with lithium. Examples of carbon materials include graphite (natural graphite, artificial graphite) that can absorb and release ions, carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, and low-temperature calcined carbon. Examples of substances that can be alloyed with lithium include silicon, tin, zinc, lead, and antimony. The substances that can be alloyed with lithium may be, for example, these simple metals, or alloys or oxides containing these elements. Furthermore, the substances that can be alloyed with lithium may be composites in which at least a portion of the surface is coated with a conductive material (for example, a carbon material) or the like.

[0064] (Conductive additive, binder for negative electrode) The conductive additive and binder may be the same as those used in the positive electrode 10. The binder in the negative electrode 20 may be, in addition to those listed for the positive electrode 10, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, etc. The cellulose may be, for example, carboxymethyl cellulose (CMC).

[0065] <Separator> The separator 18 is sandwiched between the positive electrode 10 and the negative electrode 20. The separator 18 separates the positive electrode 10 from the negative electrode 20 and prevents short-circuiting between the positive electrode 10 and the negative electrode 20. The separator 18 extends in-plane along the positive electrode 10 and the negative electrode 20. Lithium ions can pass through the separator 18.

[0066] The separator 18 has, for example, an electrically insulating porous structure. The separator 18 is, for example, a monolayer or laminate of a polyolefin film. The separator 18 may be a stretched membrane of a mixture of polyethylene, polypropylene, or the like. The separator 18 may be a fibrous nonwoven fabric made of at least one material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 18 may be, for example, a solid electrolyte. Examples of the solid electrolyte include a polymer solid electrolyte, an oxide-based solid electrolyte, and a sulfide-based solid electrolyte. The separator 18 may also be an inorganic-coated separator. The inorganic-coated separator is formed by coating the surface of the above-mentioned film with a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0067] <Electrolyte> The electrolytic solution is sealed in the case 50 and impregnates the laminate 30. The electrolytic solution includes, for example, a solvent and an electrolyte.

[0068] (solvent) The solvent can be a mixture of solvents commonly used in lithium ion secondary batteries in any ratio. For example, cyclic carbonate compounds such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate, chain carbonate compounds such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), cyclic ester compounds such as γ-butyrolactone (GBL), and chain ester compounds such as propyl propionate (PrP), ethyl propionate (PrE), and ethyl acetate can be used as the solvent.

[0069] (electrolyte) The electrolyte is not particularly limited as long as it is a lithium salt that can be used as an electrolyte for lithium ion secondary batteries. For example, inorganic acid anion salts such as LiPF, LiBF, and lithium bis(oxalato)borate, and organic acid anion salts such as LiCFSO, (CFSO)NLi, and (FSO)NLi can be used as the electrolyte.

[0070] <Case> The case 50 seals the laminate 30 and the electrolyte solution inside. The case 50 prevents leakage of the electrolyte solution to the outside and prevents moisture and the like from entering the lithium ion secondary battery 100 from the outside.

[0071] 1, the case 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The case 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0072] For example, aluminum foil can be used as the metal foil 52. A polymer film such as polypropylene can be used as the resin layer 54. The materials constituting the inner and outer resin layers 54 may be different. For example, the outer material may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the inner polymer film may be made of polyethylene (PE), polypropylene (PP), or the like.

[0073] <Lead> The leads 60 and 62 are connected to the positive electrode 10 and the negative electrode 20, respectively. The lead 60 connected to the positive electrode 10 is a positive electrode terminal, and the lead 62 connected to the negative electrode 20 is a negative electrode terminal. The leads 60 and 62 are responsible for electrical connection to the outside. The leads 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screw fastening. The leads 60 and 62 are preferably protected with insulating tape to prevent short circuits.

[0074] The lithium ion secondary battery 100 can be produced by a known method. When producing the positive electrode active material layer 14, an additive containing the above-mentioned lithium iron composite oxide is added.

[0075] The lithium ion secondary battery 100 according to this embodiment has: 57 The lithium-iron composite oxide includes a microcrystalline lithium-iron composite oxide whose analysis results using Fe Mössbauer spectroscopy satisfy predetermined conditions. The microcrystalline lithium-iron composite oxide has a large reaction surface area and a sufficiently large irreversible capacity. Therefore, it functions as a pre-dope material in the lithium-ion secondary battery 100, and can increase the reversible capacity of the lithium-ion secondary battery 100.

[0076] Furthermore, by coating the lithium-iron composite oxide with a predetermined coating layer, the reduction of the lithium-iron composite oxide can be suppressed, which increases the irreversible capacity of the pre-dope material and the reversible capacity of the lithium-ion secondary battery 100.

[0077] The above describes the embodiments of the present invention in detail with reference to the drawings. However, each configuration and combination thereof in each embodiment is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope that does not deviate from the spirit of the present invention. [Example]

[0078] [Example 1] (Synthesis of Li5FeO4) A precursor was prepared by placing 0.805 mmol (9.63 g) of LiOH as a Li source and 0.0805 mmol (12.80 g) of Fe2O3 as a transition metal oxide in a stainless steel pot together with 100 g of 2 mm diameter ZrO2 beads at 100 rpm for 12 hours using a Makino pot mill turntable. The precursor was then heated in an Ar atmosphere at 600 °C for 24 hours to obtain Li5FeO4.

[0079] (microcrystallization) 5.0 g of the Li5FeO4 obtained above was placed in a stainless steel pot together with 100 g of 2 mmφ ZrO2 beads, and treated at 300 rpm for 3 minutes using a Fristch planetary ball mill to microcrystallize Li5FeO4.

[0080] (Mössbauer spectroscopy) The Li5FeO4 microcrystallized as described above was subjected to Mössbauer spectroscopy. When the obtained spectrum was fitted with a Lorentz function, the separation of Peak A and Peak B was confirmed. The spectroscopic results are shown in Table 1.

[0081] (Preparation of positive electrode) LiCoO2 was used as the positive electrode active material, microcrystallized lithium-iron composite oxide as an additive, carbon black as a conductive additive, and PVDF as a binder. They were mixed in a ratio of LiCoO2:lithium-iron composite oxide:carbon black:PVDF = 80:10:5:5 (parts by mass). This was dispersed in N-methyl-2-pyrrolidone (NMP) using a hybrid mixer to prepare a slurry for forming the positive electrode active material layer. This slurry was applied to a 20 μm-thick aluminum foil at a coating amount of 13.0 mg / cm. 2 The positive electrode active material layer was formed by applying the coating solution so that the thickness of the coating solution was 100° C. and drying the coating solution at 100° C. Thereafter, the positive electrode active material layer was pressure-molded using a roller press to prepare a positive electrode.

[0082] (Preparation of negative electrode) A 500 μm thick Li foil was used as the negative electrode active material. The Li foil was attached to a 20 μm thick copper foil, which was then pressure-molded using a roller press to prepare a negative electrode.

[0083] (Preparation of electrolyte) Ethylene carbonate (EC) and diethyl carbonate (DEC) were used as the solvent, and lithium hexafluorophosphate (LiPF6) was used as the supporting electrolyte. The EC and DEC were mixed in a ratio of 50:50 (by volume), and LiPF6 was dissolved in the mixture to a concentration of 1.0 mol / L to prepare the electrolyte.

[0084] (Fabrication of lithium-ion secondary batteries for evaluation) The positive and negative electrodes prepared above were stacked in order with a polyethylene separator interposed between them. After ultrasonic welding of a tab lead to this stack, the stack was packaged in an aluminum laminate pack. The electrolyte prepared above was then poured into the pack and vacuum sealed to prepare a lithium-ion secondary battery for evaluation.

[0085] (Measurement of irreversible capacity of Li5FeO4) The lithium-ion secondary battery for evaluation prepared above was placed in a thermostatic chamber set at 25°C and evaluated using a charge / discharge tester manufactured by Hokuto Denko Corporation. First, the battery was charged at a constant current of 0.1 C until the battery voltage reached 4.4 V. Subsequently, the battery was discharged at a constant current of 0.1 C until the battery voltage reached 3.0 V. Note that charging at a current of (X) C refers to the current value at which the battery can be charged in (1 / X) hours. The irreversible capacity was defined as the charge capacity minus the discharge capacity obtained above, and the obtained value is shown in Table 1. Because Li5FeO4 and LiCoO2 have different reaction potentials, only the irreversible capacity of Li5FeO4 could be extracted. The larger this value, the more effectively the additive acts as a lithium pre-doping material.

[0086] [Example 2] The difference from Example 1 is that the treatment conditions for microcrystallizing Li5FeO4 were changed. In Example 2, treatment at 500 rpm for 3 minutes was performed a total of three times. The other conditions were the same as in Example 1, and evaluation was performed.

[0087] [Example 3] The difference from Example 1 is that the treatment conditions for microcrystallizing Li5FeO4 were changed. In Example 3, treatment at 550 rpm for 3 minutes was performed a total of three times. The other conditions were the same as in Example 1, and evaluation was performed.

[0088] [Example 4] The difference from Example 1 is that the treatment conditions for microcrystallizing Li5FeO4 were changed. In Example 4, treatment at 600 rpm for 3 minutes was performed a total of three times. The other conditions were the same as in Example 1, and evaluation was performed.

[0089] [Example 5] The difference from Example 1 is that the treatment conditions for microcrystallizing Li5FeO4 were changed. In Example 5, treatment at 500 rpm for 1 minute was performed a total of 9 times. The other conditions were the same as in Example 1, and evaluation was performed.

[0090] [Example 6] The difference from Example 1 is that the treatment conditions for microcrystallizing Li5FeO4 were changed. In Example 5, treatment was performed once at 500 rpm for 9 minutes. The other conditions were the same as in Example 1, and evaluation was performed.

[0091] [Comparative Example 1] The difference from Example 1 is that no treatment for microcrystallizing Li5FeO4 was performed. The other conditions were the same as in Example 1, and the evaluation was performed.

[0092] For the evaluation lithium ion secondary batteries fabricated in Examples 2 to 6 and Comparative Example 1, the irreversible capacity of Li5FeO4 was measured in the same manner as in Example 1. The results are shown in Table 1.

[0093] In all of Examples 1 to 6, the irreversible capacity was increased compared to Comparative Example 1. It is believed that the reaction efficiency of Li5FeO4 was improved by microcrystallization.

[0094] Furthermore, the results of Examples 5 and 6 revealed that the IS value is preferably 0.05 or more and 0.25 or less.

[0095] [Table 1]

[0096] [Example 7] Example 7 differs from Example 1 in that a lithium silicate coating layer was formed on the surface of LiFeO. The other conditions were the same as in Example 1, and the evaluation was performed. The amount of slurry applied to the aluminum foil when preparing the positive electrode active material layer was 13.0 mg / cm. 2 It was decided.

[0097] (Formation of coating layer) The coating layer was formed by placing 10.0 g of Li5FeO4 and 12.0 mg of Li4SiO4 as a lithium silicate material in a polypropylene pot together with 100 g of 2 mm diameter ZrO2 beads and rotating the pot mill turntable manufactured by Makino Corporation at 100 rpm for 2 hours. Quantitative analysis of the coating layer was performed using inductively coupled plasma (ICP), and the amount of coating layer formed was shown in Table 2.

[0098] [Example 8] The difference from Example 7 is that when the coating layer was formed, 54.0 mg of Li4SiO4 was used as the lithium silicate material. The other conditions were the same as in Example 7, and the evaluation was carried out.

[0099] [Example 9] The difference from Example 7 is that when the coating layer was formed, 304 mg of Li4SiO4 was used as the lithium silicate material. The other conditions were the same as in Example 7, and the evaluation was carried out.

[0100] [Example 10] The difference from Example 7 is that when the coating layer was formed, 404 mg of Li4SiO4 was used as the lithium silicate material. The other conditions were the same as in Example 7, and the evaluation was carried out.

[0101] [Example 11] The difference from Example 7 is that when the coating layer was formed, 54.0 mg of Li2SiO3 was used as the lithium silicate material. The other conditions were the same as in Example 7, and the evaluation was carried out.

[0102] [Example 12] The difference from Example 7 is that when the coating layer was formed, 54.0 mg of Li8SiO6 was used as the lithium silicate material. The other conditions were the same as in Example 7, and the evaluation was carried out.

[0103] [Example 13] The difference from Example 7 is that when the coating layer was formed, 54.0 mg of Li2Si2O6 was used as the lithium silicate material. The other conditions were the same as in Example 7, and the evaluation was carried out.

[0104] [Example 14] This example differs from Example 7 in that 54.0 mg of carbon black was used instead of the lithium silicate material when forming the coating layer. The other conditions were the same as in Example 7, and the evaluation was carried out.

[0105] [Example 15] Example 15 differs from Example 7 in that the coating layer is made of diamond-like carbon (DLC). The other conditions were the same as in Example 7, and evaluation was carried out.

[0106] The DLC was formed using a barrel sputtering device under the following conditions: output voltage: 4.0 kV, gas pressure: 1 Pa, treatment time: 3 minutes, barrel rotation speed: 10 rpm.

[0107] The results of Examples 7 to 15 are summarized in Table 2.

[0108] [Table 2]

[0109] In Examples 7 to 13 and Example 15, the irreversible capacity of Li5FeO4 was larger than that of Example 14. That is, when a predetermined coating layer containing lithium silicate or diamond-like carbon is formed on the surface of Li5FeO4, the reaction efficiency as a pre-dope material is increased.

[0110] Furthermore, the irreversible capacity of Li5FeO4 in Examples 7 to 9 was greater than that in Example 10. In other words, it was confirmed that the coating amount of the coating layer was appropriate.

[0111] [Example 16] A full cell of a lithium ion secondary battery was produced using a positive electrode produced under the same conditions as in Example 1 and a negative electrode having the following configuration, and the cycle characteristics of the lithium ion secondary battery were measured.

[0112] (Preparation of negative electrode) Graphite was used as the negative electrode active material, carbon black as a conductive additive, and styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as binders. The graphite, carbon black, SBR, and CMC were mixed in a ratio of 90:4:3:3 (parts by mass) and dispersed in N-methyl-2-pyrrolidone (NMP) using a hybrid mixer to prepare a slurry for forming the negative electrode active material layer. This slurry was applied to a 20 μm-thick copper foil at a coating amount of 9.0 mg / cm. 2 The negative electrode active material layer was formed by applying the coating solution so that the coating solution was in the range of 100° C. and drying at 100° C. Thereafter, the negative electrode active material layer was pressure-molded using a roller press to prepare a negative electrode.

[0113] (Measurement of cycle characteristics) The full-cell lithium-ion secondary battery for evaluation prepared above was placed in a thermostatic chamber set to 25°C and evaluated using a charge / discharge tester manufactured by Hokuto Denko Corp. First, the battery was charged at a constant current of 0.1 C until the battery voltage reached 4.4 V, and then discharged at a constant current of 0.1 C until the battery voltage reached 3.0 V.

[0114] The battery was then charged at a constant current of 0.5 C until the battery voltage reached 4.4 V, and then discharged at a constant current of 0.5 C until the battery voltage reached 3.0 V. This cycle of charging and discharging was defined as one cycle, and 300 cycles of charging and discharging were repeated. The retention rate after 300 cycles (cycle characteristics) was defined as the 300th cycle discharge capacity / 1st cycle discharge capacity × 100 [%].

[0115] [Example 17] Example 17 differs from Example 16 in that a diamond-like carbon (DLC) coating layer was formed on the surface of Li5FeO4. The other conditions were the same as in Example 16, and evaluation was performed.

[0116] Diamond-like carbon (DLC) was prepared in the same manner as in Example 15. The DLC-coated lithium-iron composite oxide was analyzed using a microscopic laser Raman spectrometer manufactured by Aiser, with a laser beam having a wavelength of 532 nm. The G / D ratio and the presence or absence of peak overlap were determined from the obtained spectrum. Furthermore, the thickness of the coating layer was measured using a transmission electron microscope manufactured by Hitachi High-Technologies Corporation at an accelerating voltage of 200 kV.

[0117] [Example 18] The procedure was the same as in Example 17 except that the treatment time for forming DLC ​​was 15 minutes. The other conditions were the same as in Example 16, and evaluation was performed.

[0118] [Example 19] The same procedure as in Example 17 was carried out except that the treatment time for forming DLC ​​was 5 hours. The other conditions were the same as in Example 16, and evaluation was carried out.

[0119] [Example 20] The procedure was the same as in Example 18, except that the treatment time for forming DLC ​​was 5.5 hours. The other conditions were the same as in Example 16, and evaluation was performed.

[0120] [Example 21] The same procedures as in Example 18 were carried out except that the output voltage during DLC ​​formation was 3.0 kV. The other conditions were the same as in Example 16, and evaluation was carried out.

[0121] [Example 22] The same procedures as in Example 18 were carried out except that the output voltage during DLC ​​formation was 5.0 kV. The other conditions were the same as in Example 16, and evaluation was carried out.

[0122] [Example 23] Example 23 differs from Example 16 in that a carbon coating layer was formed on the surface of Li5FeO4 by thermal CVD. The other conditions were the same as in Example 17, and evaluation was performed.

[0123] The results of Examples 16 to 23 are summarized in Table 3.

[0124] [Table 3]

[0125] Examples 17 to 22, in which a DLC coating layer was formed, were superior to Examples 16 and 23 in terms of cycle characteristics. [Explanation of symbols]

[0126] 10 positive electrode 12 Positive electrode current collector 14 Cathode active material layer 18 Separator 20 negative electrode 22 Negative electrode current collector 24 Negative electrode active material layer 30 laminate 50 cases 52 Metal foil 54 Resin layer 60, 62 leads 100 Lithium-ion secondary battery

Claims

1. Li ５ FeO ４ is expressed as ５７ The quadrupole splitting values ​​(QS) analyzed using Fe Mössbauer spectroscopy show two different peaks, One of the two peaks, peak A, satisfies QS>0, A lithium iron composite oxide in which the other peak B of the two peaks satisfies QS=0.

2. 2. The lithium-iron composite oxide according to claim 1, wherein the peak A and the peak B each have an isomer shift value (IS) of 0.05 or more and 0.25 or less.

3. The area value of the peak A is S Ａ , the area value of the peak B is S Ｂ When this is the case, 0.01≦S Ｂ / (S Ａ +S Ｂ 3. The lithium iron composite oxide according to claim 1, wherein the ratio of the total mass of the lithium iron composite oxide to the total mass of the lithium iron composite oxide satisfies the following:

4. A positive electrode material comprising the lithium iron composite oxide according to any one of claims 1 to 3.

5. The lithium iron composite oxide according to any one of claims 1 to 3, a coating layer that coats the surface of the lithium-iron composite oxide, The coating layer comprises lithium silicate.

6. The positive electrode material according to claim 5 , wherein the mass ratio of the coating layer is 0.1 mass % or more and 3.0 mass % or less with respect to the entire positive electrode material.

7. The lithium iron composite oxide according to any one of claims 1 to 3, a coating layer that coats the surface of the lithium-iron composite oxide, The coating layer comprises diamond-like carbon.

8. In the Raman spectroscopy of the coating layer, －１ The G-band spectrum and the 1350 cm －１ The D-band spectrum of and partially overlap, 8. The positive electrode material according to claim 7, wherein a G / D ratio, which is a ratio of the peak intensities of the G band and the D band, is 1.5 or less.

9. A positive electrode comprising the positive electrode material according to any one of claims 4 to 8.

10. A lithium ion secondary battery comprising the positive electrode according to claim 9.

Images