Layered metal oxide positive electrode active material for alkali ion secondary batteries and method for producing the positive electrode active material

Abstract

To provide a positive electrode active material which can stably maintain an original layered structure without excessively compromising the symmetry of an electronic structure even after a charge and a discharge are repeated, to satisfy the required performance of a secondary battery, the capacity of the positive electrode active material being larger than the effective capacity of lithium cobalt oxide.SOLUTION: A positive electrode active material for an alkaline ion secondary battery is made of ByMO2 when the material is manufactured or is fully charged and is specifically made of an oxide crystal of an alkaline metal B, a transition metal M, and an oxygen atom O as the basic constituent elements of a layered structure (1: movable ions A, 2: an alkaline metal B existing between layers, 3: an octahedron as an oxygen 6-orientation made of the transition metal M and the oxygen O), and the Shannon ions radius is so set that movable ions A is smaller than movable ions B. The existence of B between layers suppresses change in the distance between layers if A is inserted between the layers or is separated from between layers (charge and discharge). The relation A>B in the ion diffusion coefficients makes the layered structure harder to break, and increase of the capacity at a high-rate charge / discharge can be thus attained.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention aims to achieve high capacity Electrolyte aqueous solution system The present invention relates to a layered metal oxide positive electrode active material for alkali ion secondary batteries and a method for producing the positive electrode active material. [Background technology]

[0002] In recent years, with the increasing multi-functionality and sophistication of portable electronic devices such as mobile phones, smartphones, and laptops, the practical application of electric vehicles and stationary power energy conditioning systems, and the trial introduction of smart grids that include these, there is a need for small, lightweight, and high-capacity alkaline-ion secondary batteries as the main and secondary power sources for these devices.

[0003] Currently, lithium-ion secondary batteries, which use lithium cobalt oxide (LiCoO2) with a layered rock salt structure as the positive electrode active material and carbon-based materials such as graphite as the negative electrode active material, are promising solutions to meet these demands. Lithium-ion secondary batteries are being installed as main and secondary power sources for the devices and systems in question, and commercialization is underway.

[0004] However, when an alkali ion secondary battery using lithium cobalt oxide as a positive electrode active material is fully charged, its crystal structure undergoes a phase transition from hexagonal to monoclinic (Non-Patent Document 1), and even when fully discharged, the crystal structure does not return to the original hexagonal, i.e., layered rock salt structure, resulting in a problem that the effective capacity is in the range of 120 to 140 mAh / g, which is only about half of the theoretical capacity of 274 mAh / g. 1-a There is a limit to the amount of lithium that can be extracted from CoO2, and in practice, charging and discharging are carried out within the range of 0 ≦ a < 0.5, which allows the layered rock-salt structure to be maintained, resulting in a problem where the effective capacity is about half of the theoretical capacity.

[0005] Li 1-a The phase transition of CoO2 (0.5 < a) occurs when the lithium ion desorption reaction progresses during charging, increasing the amount of tetravalent cobalt in the crystal structure, and the t 2gThis is because the proportion of unpaired electrons in the orbital increases, causing the electronic structure to become asymmetric (Non-Patent Document 2).

[0006] Furthermore, in the range of approximately 0≦a<0.5, where a phase transition is not considered to occur, the interlayer distance increases due to the desorption of lithium ions from between the layers during charging, and returns to its original state due to the insertion of lithium ions between the layers during discharging. However, due to the instability of the electronic structure in the 3d orbital of cobalt, the interlayer distance changes significantly during charging and discharging, making the layered structure prone to collapse, which has been a problem in that reversible long-term charging and discharging is difficult (Non-Patent Documents 2 and 3).

[0007] Therefore, in order to achieve the required secondary battery performance, it is desirable to develop a positive electrode active material that does not excessively disrupt the symmetry of the electronic structure even after repeated charge and discharge, stably maintains the original layered structure, and has an effective capacity that exceeds that of lithium cobalt oxide.Several attempts have been made to achieve this.

[0008] As a first attempt, t 2g LiNi, in which some or all of the cobalt in LiCoO2 is replaced with nickel in order to maintain the symmetry of the electronic structure without having unpaired electrons in the orbitals. 1-b Co b O2 (0 ≦ b < 1) was proposed. However, Li 1-a Ni 1-b Co b In LiCoO2, the covalent bond between Ni-O in the octahedrons that make up the layered structure is weaker than that of Co-O. Therefore, during charging and discharging, some of the nickel is replaced with lithium, hindering the progress of the electrochemical reaction. In addition, the layered structure cannot be maintained as in LiCoO2. As a result, the amount of lithium ion desorption is limited to 0 < a < 0.6, and the effective capacity is 140 to 170 mAh / g (Non-Patent Document 4). In addition, Li 1-a Ni 1-b Co b When producing O2, trivalent nickel is more easily reduced than trivalent cobalt, and the lithium source in the raw material is easily evaporated or insufficient, resulting in a non-stoichiometric composition. In the case of a non-stoichiometric composition, Li1-a Ni 1-b Co b It has been reported that electrochemically inactive rock-salt structure NiO is localized in the crystal structure of O2, resulting in a decrease in electrochemical reactivity (Non-Patent Document 5).

[0009] In a second attempt to exceed the effective capacity of lithium cobalt oxide, a zigzag layered structure, LiMnO2, was attempted, in which manganese was substituted for cobalt and nickel. However, LiCoO2 and LiNi 1-b Co b As with O2, local distortion of the crystal structure occurs depending on the manganese valence during charge and discharge. That is, when lithium ions are completely removed from LiMnO2 after the first full charge, the manganese valence is 4, and t 2g There are no unpaired electrons in the orbital, and the electronic structure is highly symmetric. However, as lithium ions are inserted during discharge after the initial charge, the increase in trivalent manganese leads to an increase in e g The increase in unpaired electrons in the orbitals leads to an increase in the length of the Mn-O covalent bonds in the octahedra that make up the layered structure, causing distortion of the crystal structure (Non-Patent Document 6). As a result, the optimal interlayer distance cannot be maintained, and the crystal structure undergoes a phase transition from a layered rock-salt structure to a spinel structure, resulting in a problem of a significant decrease in effective capacity from the second time onwards compared to the first time (Non-Patent Documents 7 and 8).

[0010] As mentioned above, the distortion of the Me-O bonds that constitute the layered structure of LiMeO2 (Me = one or more transition metals, such as Co, Ni, and Mn) during deep charge and discharge is the cause of the phase transition. However, research has been conducted to delay and uniformize the change in valence of Me and maintain the layered structure by reducing the surface charge density of the mobile ions present between the layers and slowing the diffusion rate of the mobile ions. For example, it has been reported that when a layered rock-salt structure in which monovalent lithium ions are replaced with monovalent sodium ions or potassium ions as mobile ions is repeatedly charged and discharged, the original layered structure is maintained (Non-Patent Documents 9 and 10). However, due to the slow diffusion rate of the mobile ions, the effective capacity was only about 60 mAh / g, which was insufficient.

[0011] As a third attempt to prevent the phase transition of the layered rock salt structure and obtain sufficient effective capacity, DMeO2 using divalent ions D as the mobile ions was attempted (Patent Document 1). By replacing monovalent ions with divalent ions as the mobile ions, the amount of reactive electrons doubles. In other words, the amount of valence change is doubled compared to monovalent ions, so there is a possibility of achieving a high capacity. For example, in the case of layered rock salt CaCoO2 where the mobile ion D = Ca and the transition metal Me = Co, it was theoretically expected that a capacity about twice that of LiCoO2 could be obtained. Experimentally produced layered rock salt Ca 0.5 In CoO2, a reversible charge-discharge reaction was confirmed (Non-Patent Document 11). However, in this reported case, the calcium composition corresponds to the trivalent / tetravalent change of cobalt, and Ca 0.5-a The desorption and insertion of calcium ions from CoO2 is possible only in the range of 0 < a ≦0.2. However, in this range, the interlayer distance changes significantly, suggesting a decrease in the effective capacity during long-term charge and discharge. 0.5-a When too many calcium ions were desorbed from CoO2, i.e., when a was 0.2 < a, a phase transition similar to that observed in LiCoO2 was observed. Calcium ions have a slower ion diffusion rate than lithium ions, so in order to obtain a high effective capacity, charging and discharging was limited to low currents. [Prior art documents] [Patent documents]

[0012] Patent Document 1: JP 2017-004770 [Non-patent literature]

[0013] Non-patent document 1: T. Ohzuku and A. Ueda, J. Electrochem. Soc. 141 (1994) 2972. Non-patent document 2: F. Xiong, et al., Int. J. Electrochem. Sci. 7 (2012) 9390. Non-licensed reference 3: Y. Takahashi, et al., J. Solid State Chem. 180 (2007) 313. Non-licensed reference 4: I. Saadoune and C. Delmas, J. Mater. Chem. 6 (1996) 193. Non-licensed reference 5: H. Arai, et al., Solid State Ionics 80 (1995) 261. Non-licensed Reference 6: SK Mishra and G. Ceder, Phys. Rev. B 59 (1999) 6120 Non-licensed reference 7: JN Reimers, EW Fuller, E. Rossen, and JR Dahn, J. Electrochem. Soc. 140 (1993) 3396. Non-licensed reference 8: G. Vitins and K. West, J. Electrochem. Soc. 144 (1997) 2587. Non-licensed reference 9: FR Beck, et al., J. Electrochem. Soc. 161 (2014) A961. Non-licensed reference 10: C. Vaalma, et al., J. Electrochem. Soc. 163 (2016) A1295. Non-licensed reference 11: M. Cabello, et al., Electrochem. Commun. 67 (2016) 59. Non-licensed reference 12: RK Lowry, et al., Contrib. Mineral. Petrol. 80 (1982) 254. Non-licensed reference 13: HS Harned and FM Polestra, J. Am. Chem. Soc. 75 (1953) 4168. Non-licensed reference 14: NH Kwon, et al., Electrochim. Acta 134 (2014) 215. Non-patent document 15: S. Gu, et al., J. Solid State Electrochem. 23 (2019) 1399. Non-patent document 16: C.-H. Wang, et al., J. Power Sources 274 (2015) 1016. Non-patent document 17: J.-G. Lee, et al., Mater. Res. Bull. 42 (2007) 1201. Summary of the Invention [Problem to be solved by the invention]

[0014] As described above, in the prior art, lithium ion secondary batteries using a positive electrode active material with a layered rock salt structure have been put to practical use and exhibit small size, light weight, high capacity, and high voltage, but have not been able to obtain sufficient effective capacity and have the following problems.

[0015] The first problem is that during charging and discharging, the number of unpaired electrons in the 3d orbitals of the transition metals that make up the layered structure increases, causing the electronic structure to become asymmetric, resulting in a significant change in the interlayer distance and an irreversible phase transition.

[0016] The second challenge is that although replacing mobile ions with monovalent, large-diameter ions and reducing the surface charge density of the mobile ions can suppress distortion of the layered structure and changes in interlayer distance, the slow diffusion rate of the mobile ions results in low effective capacity.

[0017] The present invention has been made to solve such problems, and its objects are to: In aqueous electrolyte alkaline ion secondary batteries To provide a positive electrode active material that does not excessively disrupt the symmetry of the electronic structure even after repeated charge and discharge, stably maintains the original layered structure, and has an effective capacity exceeding that of lithium cobalt oxide, in order to satisfy the required secondary battery performance. [Means for solving the problem]

[0018] In order to solve the above problems, the present invention Electrolyte aqueous solution systemThe layered metal oxide positive electrode active material for an alkali ion secondary battery and the method for producing the positive electrode active material are provided as follows.

[0019] (1) It is composed of an oxide crystal of alkali metal B, transition metal M, and oxygen atom O, which are the basic constituent elements of a layered structure, and is characterized by the Shannon ionic radius of the mobile ion A < B, B already existing between the layers, and the ion diffusion coefficient A > B, and is composed of ByMO2 when manufactured or fully charged. Electrolyte aqueous solution system Positive electrode active material for alkaline ion secondary batteries.

[0020] According to the present invention, a positive electrode active material can be provided that does not excessively disrupt the symmetry of its electronic structure even after repeated charge and discharge, stably maintains its original layered structure, and has an effective capacity exceeding that of lithium cobalt oxide. Specifically, the positive electrode active material is composed of an oxide crystal of an alkali metal B, a transition metal M, and oxygen O, which are the basic constituent elements of the layered structure. The Shannon ionic radius of the mobile ion A is less than B, and the presence of B between the layers suppresses changes in the interlayer distance even when A is inserted or extracted between the layers (charge and discharge). Furthermore, the ionic diffusion coefficient is greater than B, which makes the layered structure less disrupted, thereby enabling high capacity during high-speed charge and discharge. The positive electrode active material is composed of ByMO2 during manufacture or when fully charged. Electrolyte aqueous solution system A positive electrode active material for an alkali ion secondary battery can be provided.

[0021] (2) The composition formula: AxByMO2 (0 < x ≦ 1.0 - y, 0 < y < 0.4, A is an alkali metal or alkaline earth metal inserted between layers during discharge, M is one or more transition metal elements) as described in (1). Electrolyte aqueous solution system Positive electrode active material for alkaline ion secondary batteries.

[0022] According to the present invention, it is possible to provide a positive electrode active material that does not excessively disrupt the symmetry of its electronic structure even after repeated charge and discharge, stably maintains its original layered structure, and has an effective capacity exceeding that of lithium cobalt oxide.

[0023] (3) The method according to (1), wherein A is any one of Li, Na, Mg, and Ca. Electrolyte aqueous solution system Positive electrode active material for alkaline ion secondary batteries.

[0024] According to the present invention, it is possible to provide a positive electrode active material that does not excessively disrupt the symmetry of its electronic structure even after repeated charge and discharge, stably maintains its original layered structure, and has an effective capacity exceeding that of lithium cobalt oxide.

[0025] (4) The composition according to (1), wherein A is Ca, B is K, M is Mn, and 0.24≦y≦0.32. Electrolyte aqueous solution system Positive electrode active material for alkaline ion secondary batteries.

[0026] According to the present invention, it is possible to provide a positive electrode active material that does not excessively disrupt the symmetry of its electronic structure even after repeated charge and discharge, stably maintains its original layered structure, and has an effective capacity exceeding that of lithium cobalt oxide.

[0027] (5) Potassium permanganate is heated in the atmosphere at a temperature range of 300°C to 900°C for a time range of 5 to 24 hours to obtain a precursor compound containing KyMnO2, and then the compound is stirred in an aqueous solution such as distilled water at 300 to 400 revolutions per minute for 10 to 30 minutes, followed by repeated vacuum filtration and washing with distilled water to obtain only KyMnO2. Electrolyte aqueous solution system A method for producing layered metal oxide positive electrode active material KyMnO2 (0.24 ≦ y ≦ 0.32) for alkali ion secondary batteries.

[0028] According to the present invention, it is possible to provide a method for producing a positive electrode active material that does not excessively disrupt the symmetry of its electronic structure even after repeated charge and discharge, stably maintains its original layered structure, and has an effective capacity that exceeds that of lithium cobalt oxide. [Effects of the Invention]

[0029] A of the present invention x B yIn the crystal structure of MO2, if the Shannon ionic radius is A < B, the interlayer distance expands due to the effect of the alkali metal B already present between the layers, and significant changes in the interlayer distance due to the insertion and desorption of the mobile ion A are suppressed.

[0030] In addition to the above, if the ion diffusion coefficient is A > B, even if mobile ions A are inserted into the wide gaps between the layers during discharge and then all mobile ions A are desorbed from the gaps between the layers during charge, the diffusion rate of A is fast, while the diffusion rate of alkali metal B present between the layers is slow, so B remains between the layers, i.e., the layered structure is maintained, and as a result, the amount of insertion and desorption of A increases, i.e., the effective capacity increases.

[0031] Due to these effects, the present invention does not excessively destroy the symmetry of the electronic structure even after repeated charge and discharge, stably maintains the original layered structure, and has an effective capacity exceeding that of lithium cobalt oxide. For aqueous electrolyte alkaline ion secondary batteries A positive electrode active material and a method for producing the same can be provided. [Brief explanation of the drawings]

[0032] [Figure 1] 1A shows a schematic diagram of the crystal structure of the positive electrode active material according to the present invention, and FIG. 1B shows a schematic diagram of the crystal structure after discharge (after mobile ions are inserted between the layers). In the figure, 1 indicates a mobile ion A, 2 indicates an alkali metal B that is already present between the layers, and 3 indicates an octahedron (oxygen six-coordinated) composed of a transition metal M and oxygen O. [Figure 2] This is a cyclic voltammogram of a three-electrode cell using 1 mol / L CaCl2 / H2O as the electrolyte and prototype 4 as the positive electrode active material. [Figure 3] This is a cyclic voltammogram of a three-electrode cell using 1 mol / L MgCl2 / H2O as the electrolyte and prototype 4 as the positive electrode active material. [Figure 4] This shows the dependence of the reduction / oxidation peak current value on the square root of the sweep rate obtained from the cyclic voltammogram of a three-electrode cell using 1 mol / L CaCl2 / H2O as the electrolyte and prototype 4 as the positive electrode active material. [Figure 5]This shows the dependence of the reduction / oxidation peak current value on the square root of the sweep rate obtained from the cyclic voltammogram of a three-electrode cell using 1 mol / L MgCl2 / H2O as the electrolyte and prototype 4 as the positive electrode active material. [Figure 6] This is the constant current charge / discharge curve of a three-electrode cell using 1 mol / L CaCl2 / H2O as the electrolyte and prototype 4 as the positive electrode active material. [Figure 7] 1 shows scanning electron microscope images of raw materials and pulverized raw materials used in the production of the present active material, and of prototypes 1 to 6 obtained according to the present production procedure. [Figure 8] 1 shows the X-ray diffraction patterns of prototypes 1 and 4 obtained according to this manufacturing procedure. [Figure 9] These are the X-ray diffraction patterns of prototypes 2 and 5 obtained according to this manufacturing procedure. [Figure 10] These are the X-ray diffraction patterns of prototypes 3 and 6 obtained according to this manufacturing procedure. DETAILED DESCRIPTION OF THE INVENTION

[0033] The following describes specific embodiments of the present invention according to claim 1. Note that the present invention is not limited to the following embodiments, and the contents can be appropriately modified and implemented in accordance with the object of the present invention.

[0034] <Layered metal oxide cathode active material for alkali-ion secondary batteries> According to this embodiment Electrolyte aqueous solution system The layered metal oxide positive electrode active material for alkali ion secondary batteries has a chemical composition represented by the following formula (1) when produced or fully charged, and is two-dimensional planar particles with a layered structure.

[0035]

number

[0036] In the formula (1), B is an alkali metal for widening the gap between layers and maintaining the layered structure.

[0037] In the above formula (1), when inserting or desorbing the mobile ion A, in order to suppress the change in the interlayer distance, it is necessary to satisfy the condition that the ionic radius of Shannon is A < B.

[0038] In addition, when desorbing the mobile ion A from the interlayer, in order not to desorb B, it is necessary to utilize the difference in the ion diffusion coefficient and satisfy the condition that the ion diffusion coefficient is A > B.

[0039] In the above formula (1), M is one or more transition metal elements. As the cathode active material required for the above-mentioned applications, in order to obtain a high charge-discharge potential, those containing Co, Mn, and Ni are preferable. Based on the Clark number, in order to have a rich reserve and reduce costs, those containing Mn and Fe are preferable.

[0040] In the above formula (1), when the valence of M changes due to the insertion or desorption of the mobile ion A, since B in the interlayer承担部分电荷补偿, the symmetry of the electronic structure is not excessively disrupted, and the layered structure can be maintained during charge and discharge.

[0041] In lithium cobaltate having a layered rock salt-type structure, etc., due to phase transition, at most about 0.5 mol of mobile ions can be utilized in the charge-discharge reaction, and the effective capacity is about half of the theoretical capacity. However, for the layered metal oxide cathode active material for an alkali ion secondary battery according to this embodiment, since the layered structure is not excessively disrupted, depending on the abundance of B and the charge-discharge reaction amount of the mobile ion A, a charge-discharge reaction of mobile ions exceeding 0.5 mol can be utilized, and it is possible to exceed the effective capacity of lithium cobaltate having a layered rock salt-type structure, etc.

Example

[0042] From the perspective of increasing the capacity, the layered metal oxide cathode active material K y Using Prototype 4 in the range of 0.24 ≦ y ≦ 0.28 of MnO2, the following will explain this example together with the drawings, but the present invention is not limited to this example.

[0043] Also, in this example, A x K0.26±0.02 In the composition formula of MnO₂, calcium ions are adopted as A that satisfies the conditions of Shannon ionic radius A B (Non-Patent Documents 12 and 13), and can achieve a high theoretical capacity and high charge-discharge potential. Ca x K 0.26±0.02 An example of evaluating the electrochemical properties of MnO₂ will be described.

[0044] <Evaluation of Electrochemical Properties> In evaluating the charge-discharge performance of the positive electrode active material, a three-electrode cell consisting of a positive electrode (working electrode), a negative electrode (counter electrode), and a reference electrode was used to accurately measure the reaction current and reaction potential in the positive electrode active material. As the evaluation method of the charge-discharge performance, cyclic voltammetry measurement and constant current charge-discharge test were adopted.

[0045] <Preparation of Composite Positive Electrode> The composite positive electrode contains an active material, a conductive assistant, and a binder, and the ratio is 80:10:10% by weight, respectively. In this example, acetylene black (AB) was adopted as the conductive assistant, and polytetrafluoroethylene (PTFE) was adopted as the binder. The composite was mixed in an alumina mortar for 30 minutes at the above ratio, and the obtained positive electrode sheet was pressed at 1,000 kgf with a press machine to perform press rolling. The thickness of the rolled sheet is in the range of 20 μm to 30 μm. The rolled positive electrode sheet was punched with a punch of 3 mm in diameter, and this was used as the positive electrode (working electrode).

[0046] As the carbon material used for the conductive assistant, ketjen black (KB), vapor-grown carbon fiber (VGCF), carbon nanotube (CNT), graphene, carbon nanofiber (CNF), carbon black (CB), coke, graphite, etc. may be selected.

[0047] Also, as the polymer used for the binder, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), acrylonitrile-butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVdF), etc. may be selected.

[0048] <Selection of negative electrode> A platinum wire (0.5 mm diameter × 50 mm length) was used as the negative electrode (counter electrode).

[0049] The negative electrode (counter electrode) may be formed into an electrode shape by mixing a conductive additive and a binder with a negative electrode active material, as in the case of the positive electrode (working electrode). There are no limitations on the shape of the electrode as long as it is large enough to support the current generated at the positive electrode.

[0050] <Selection of reference electrode> The reference electrode was selected to be suitable for the type of solvent in the electrolyte. In other words, since Prototype 4 used an aqueous electrolyte, a silver-silver chloride electrode was used as the corresponding reference electrode.

[0051] Any material and shape of the reference electrode may be used as long as it is compatible with the electrolyte solution used.

[0052] <Selection of electrolyte> 1 mol / L CaCl2 / H2O was used as an electrolyte that is easy to handle in air and does not undergo reductive or oxidative decomposition due to the charge / discharge reaction potential of this active material. In addition, to confirm whether this active material functions by changing the mobile ion, 1 mol / L MgCl2 / H2O was used, in which calcium ions were replaced with magnesium ions.

[0053] <Cyclic voltammetry measurement> The conditions for measuring the reaction current-potential curve (cyclic voltammogram) in Prototype 4 are as follows: Measurement model: Hokuto Denko Potentio Galvanostat HA-303 Hokuto Denko Function Generator HB-305 Measurement method: Cyclic voltammetry ·Potential sweep speed: 0.1 mV / s, 1 mV / s, 3 mV / s, 5 mV / s Measurement count: 3 times per potential sweep rate (measurement diagram shows the results of the 3rd measurement) Measurement potential range: -450 to 350 mV vs. Ag / AgCl Electrolyte used: 1 mol / L CaCl2 / H2O or 1 mol / L MgCl2 / H2O ·Measurement temperature: 25℃ As shown in Figure 2, Prototype 4, at each sweep rate, a reduction peak can be observed at negative potential and an oxidation peak at positive potential. The reduction peak is due to the Ca supplied from the electrolyte. 2+ The oxidation peak is due to the insertion (intercalation) of Ca ions into the crystalline structure of the positive electrode active material. 2+ Measurements were performed successively at 0.1 mV / s, 1 mV / s, 3 mV / s, and 5 mV / s. 2+ The observed reversible insertion and deintercalation reactions indicate that the positive electrode active material maintains its original crystal structure. As shown in Figure 3, when the mobile calcium ion was replaced with a small-diameter magnesium ion, the Mg 2+ The reduction peak indicates that Mg is inserted into the layer structure. 2+ The oxidation peaks indicating that Mg was released from the layered structure were observed reversibly. Therefore, the positive electrode active material was found to have a mobile ion of Mg 2+ This indicates that the layered structure is maintained even during charging and discharging.

[0054] <Calculation of ion diffusion coefficient> The reduction and oxidation peak current values ​​in the cyclic voltammograms of Figures 2 and 3 are I p By reading the slope of the graph of the square root of the sweep rate: v^(1 / 2) and using the following Cottrell equation (Randle-Sevcik equation), the diffusion coefficients of calcium ions and magnesium ions during the reversible reaction can be calculated.

[0055]

number

[0056] The dependence characteristics of the reduction / oxidation peak current values ​​and the square root of the sweep rate corresponding to Figures 2 and 3 are shown in Figures 4 and 5, respectively. A linear approximation using the least squares method was applied to the measurement points, and the R value (the closer R = 1, the better the linearity) that indicates the degree of agreement between the measurement points and the approximation line is also shown in the figures. The diffusion coefficient (average value of insertion and desorption) calculated from the above formula (2) is 1.86 × 10 for the Ca-based electrolyte in Figure 4. -6 cm 2 / s, and 1.18 × 10 for the Mg-based electrolyte in Figure 5. -6 cm 2 / s, and by arranging B ions between the layers in advance, the lithium ion diffusion coefficient of the conventional lithium ion battery positive electrode active material LiCoO2: 1 × 10 -8 ~ 0.5 × 10 -6 cm 2 / s (Non-Patent Document 14) and magnesium ion battery positive electrode active material Mg 0.5 Magnesium ion diffusion coefficient of CoO2: 10 -12 cm 2 This indicates that ion diffusion occurs at a rate faster than 1 / s (Non-Patent Document 15). In addition, the excellent linearity of the straight-line approximations in Figures 4 and 5 indicates that the insertion and desorption reactions of mobile ions occur reversibly. From the above results, it is clear that as long as the conditions of claim 1 are met, reversible electrochemical reactions are possible even when mobile ions other than calcium ions and magnesium ions are used.

[0057] <Constant current charge / discharge test> The measurement conditions for the constant current charge / discharge curve of Prototype 4 are as follows: However, a three-electrode cell after cyclic voltammetry measurement was used, and the positive electrode active material was not replaced. Measurement model: Kikusui Electronics Charge / Discharge Battery Test System PFX2011 ·Measurement method: Constant current (CC) Pause time when switching between discharge and charge: 10 minutes Charge / discharge current: 50 μA (Current density: 0.71 mA / cm 2, 11 mA / g) Measurement potential range: -0.4 to 0.2 V vs. Ag / AgCl ·Electrolyte used: 1 mol / L CaCl2 / H2O ·Measurement temperature: 25℃ As shown in Figure 6, sample 4, a potential plateau was observed at the potential where the reduction and oxidation peaks in Figure 2 occurred. That is, around -0.3 V vs. Ag / AgCl, Ca was absorbed into the layered structure. 2+ The intercalation reaction of Ca from the layer structure occurs at around 0.1 V vs. Ag / AgCl. 2+ A deintercalation reaction of the cations occurred (Claim 1). Furthermore, no stepwise potential change associated with the phase transition was observed, and the layered structure was maintained (Claim 1). At the final potential, the discharge capacity was 59.1 mAh / g, the charge capacity was 59.2 mAh / g, and the charge / discharge efficiency (Coulomb efficiency) was 99.8%. Since a Coulomb efficiency of nearly 100% was obtained, Prototype 4 has high reaction reversibility and is suitable as a positive electrode active material for calcium secondary batteries (Claim 1). When the interlayer voids are completely filled with calcium, i.e., when the battery is fully discharged, the chemical composition becomes Ca 0.74 K 0.26 The theoretical capacity is 312.9 mAh / g. If we consider the reaction of desorption and insertion of only calcium ions from MnO2, the effective capacity during charge and discharge obtained in Figure 6 is 59.1 mAh / g. If we estimate the actual chemical composition, we find that 0.14 K 0.26 The result is MnO2. It is known that the mobile ion concentration on the surface of active material particles becomes very high during high-current discharge, limiting the amount of mobile ions intercalated between layers. Naturally, the effective capacity is lower than the theoretical capacity. However, due to limitations of the measurement equipment, the charge / discharge test was performed using a charge / discharge current density at which the discharge and charge reactions were completed in approximately one hour. Despite the high-current charge / discharge, the effective capacity was higher than that of a positive electrode active material without B ions present between layers (Non-Patent Documents 15, 16). Therefore, this material is promising as a positive electrode active material for calcium-ion secondary batteries. In this embodiment, since the cyclic voltammetry measurement history (12 calcium ion insertions / extractions) is carried forward, the constant current charge / discharge test in Fig. 6 corresponds to the 13th calcium ion insertion / extraction cycle. Considering this, in a lithium-ion battery using lithium cobaltate with a layered rock salt structure having a particle size equivalent to the primary particle size of the positive electrode active material of the present invention: approximately 40 μm as the positive electrode active material, even when compared with the case where the discharge reaction and the charge reaction are carried out in about 1 hour, it was confirmed in this embodiment that a high effective capacity can be obtained (Non-Patent Document 17). Therefore, it can be estimated that the effective capacity can be increased even when the discharge reaction and the charge reaction each exceed 1 hour and the charge / discharge reaction is carried out at a low speed, compared with the said positive electrode active material. However, since the discharge overvoltage is large and the potential rapidly decreases to around -0.3 V vs. Ag / AgCl at the initial stage of the reaction, it is necessary to improve the ionic conductivity and the electronic conductivity by optimizing the particle size adjustment of the active material, the mixing conditions of the active material and the conductive assistant, and the electrode formation method, etc. By satisfying such requirements, a further improvement in the effective capacity can be expected.

[0058] In this embodiment, for the sake of convenience, an aqueous electrolyte is used, but it can be inferred that the same effect can be obtained with an organic electrolyte.

[0059] In the above formula (1), by satisfying the conditions that the Shannon ionic radius A B, as shown in Example 1, the insertion of mobile ions into the interlayer and the desorption of mobile ions from the interlayer occur reversibly, and also, a rapid charge / discharge potential change associated with a phase transition does not occur, and the effect of the present invention according to Claim 1 could be confirmed. Further, when a constant current charge / discharge test was carried out with a large current value at which discharge or charge is completed in about 1 hour by adopting calcium ions for A, it was confirmed that a high effective capacity exceeding the effective capacity of lithium cobaltate having a layered rock salt structure was shown.

[0060] The following describes specific embodiments of the present invention according to claim 2. Note that the present invention is not limited to the following embodiments, and the contents can be appropriately modified and implemented in accordance with the object of the present invention.

[0061] In the above formula (1), when mobile ions A are inserted into the gaps between the layers, that is, when discharge is performed, the chemical composition is expressed by the following formula (3), and the particles are two-dimensional planar particles having a layered structure.

[0062]

number

[0063] If the crystal structure of the above formula (3) corresponds to an α-NaFeO2 type (space group R3(-)m), i.e., a layered rock salt type structure, then in order for all interlayer spaces in the above formula (3) to be occupied by A and B, x + y = 1 is required.

[0064] In lithium cobalt oxide with a layered rock-salt structure in which B does not exist between the layers, x = 1 and y = 0 in formula (3) above, i.e., it corresponds to the composition formula AMO2, where A is Li and M is Co. When the effective capacity of LiCoO2 is maximized within the range in which no phase transition occurs, only about 0.5 mol of lithium can be used for the charge-discharge reaction, and the chemical composition of the active material during charge-discharge is expressed by formula (4) below.

[0065]

number

[0066] In the formula (3), in order to obtain a capacity exceeding that of the formula (4), it is sufficient that the mobile ions A exceed 0.5 mol and are available for charge / discharge reactions.

[0067] That is, in the above formula (4), the amount of lithium that cannot be extracted due to the phase transition: Li 0.5Taking this into consideration, in order to exceed the theoretical capacity of lithium cobaltate, in the above formula (3), it is necessary to satisfy the conditions of 0 < x ≦ 1.0 - y and 0 < y < 0.4. To increase x and obtain the maximum capacity, it is preferable that y is closer to 0 and x is closer to 1.

[0068] Hereinafter, specific embodiments of the present invention according to claim 3 will be described. It should be noted that the present invention is not limited to the following embodiments, and the content can be appropriately changed and implemented in accordance with the object of the present invention.

[0069] In the above formula (3), in order to suppress the change in the interlayer distance due to the presence of B, it is necessary to satisfy the condition that the Shannon ionic radius A < B. Combinations that satisfy this condition include, for example, Li < Na, Li < K, Na < K, Mg < Na, Mg < K, Ca < K, etc.

[0070] In addition, in order to hardly desorb B from the interlayer and maintain the layered structure due to the presence of B when A is inserted into the interlayer and then desorbed, the difference in ion diffusion rate is utilized, that is, it is necessary to satisfy the condition that the ion diffusion coefficient A > B. Combinations that satisfy this condition include, for example, Li > Na, Li > K, Na > K, Mg > K, Ca > K, etc.

[0071] Hereinafter, specific embodiments of the present invention according to claim 4 will be described. It should be noted that the present invention is not limited to the following embodiments, and the content can be appropriately changed and implemented in accordance with the object of the present invention.

[0072] Among the combinations that satisfy the conditions of Shannon ionic radius A B, A is preferably Ca which exhibits a high theoretical capacity, and B is preferably K which has a rich embedding amount.

[0073] In addition, M is preferably Mn which has a high charge-discharge potential and a rich embedding amount.

[0074] In the above formula (3), the chemical compositions of prototypes 4 to 6 are in the range of 0.24 ≦ y ≦ 0.32, and in order to insert a large amount of calcium ions into the interlayer voids and obtain a high effective capacity, it is particularly preferable that y = 0.24.

[0075] A specific embodiment of the present invention according to claim 5 will be described below. <Method of manufacturing layered metal oxide positive electrode active material for alkali ion secondary batteries> To produce the positive electrode active material according to this embodiment, for example, a pertransition metal salt, specifically KMnO4, NaMnO4, K3CoO4, Na3CoO4, K2NiO4, Na2NiO4, K2NiO5, or Na2NiO5, is used as a raw material. By subjecting this raw material to a heat treatment, the raw material undergoes thermal decomposition, yielding a precursor containing the desired layered structure and impurity structure. Because the impurity structure contained in the precursor is soluble in water, repeated washing with distilled water or the like and filtration can yield a positive electrode active material containing only the desired layered structure. However, the raw material may be prepared by appropriately selecting it from commercially available products, or by preparing a compound containing the desired element. In addition, the positive electrode active material may be prepared by mixing raw materials containing the desired element, even if it is not a pertransition metal salt. Furthermore, the method for producing the positive electrode active material of the present invention is not limited to a solid-phase method, a liquid-phase method, a molten salt method, or the like.

[0076] The transition metal salts given as examples of raw materials can be used without adjusting the particle size, but it is preferable to pulverize the raw material powder particles in order to promote thermal decomposition during the heating process.

[0077] The pertransition metal salt, which is an example of a raw material, can be pulverized, for example, in a dry manner using a mortar, a ball mill, or the like.

[0078] Here, the crushed sample is sieved through a sieve of 280 mesh or less, and the powder that falls through the sieve can be used as the raw material. It is preferable that the particle size is such that the minor axis diameter is 40 μm or less, the major axis diameter is 50 μm or less, and the thickness is 50 μm or less.

[0079] The heat treatment of the transition metal salt, which is an example of a raw material, involves, for example, raising the temperature from room temperature to a predetermined temperature, performing heat treatment at the predetermined temperature for a certain period of time, and then naturally cooling to room temperature.

[0080] The predetermined temperature is, for example, 300°C or higher and 900°C or lower, and is preferably 300°C.

[0081] The constant heating time at a predetermined temperature is, for example, 5 hours or more and 24 hours or less, and is preferably 5 hours.

[0082] The rate of temperature rise to the predetermined temperature is 5° C. / min or more and 50° C. / min or less, and PID control or the like is preferred so as not to overshoot or undershoot the predetermined temperature.

[0083] The heat treatment may be performed in air or in an oxygen atmosphere.

[0084] The heat treatment can be carried out using, for example, a box-type muffle furnace or a horizontal tubular furnace.

[0085] Regarding the method for washing the precursor, it is preferable to use water from which mineral components have been removed, such as ion-exchanged water or distilled water, and in addition, distilled water from which organic components have been removed is also suitable.

[0086] As a washing method, it is sufficient to place the precursor powder and water from which mineral components have been removed into a beaker, a measuring flask, or the like, and stir the mixture with a rotor made of polytetrafluoroethylene or the like at a speed of 300 to 400 revolutions per minute for 10 to 30 minutes.

[0087] Next, the filtration method is not limited, but vacuum filtration, which has a high filtrate removal capacity, is preferred, and it is preferable to use a polytetrafluoroethylene membrane having a pore size smaller than the particle size.

[0088] Repeating the washing and filtering steps three times is sufficient.

[0089] After the washing and filtering steps, the powder can be dried overnight in an air atmosphere at about 80°C to obtain a dry powder.

[0090] <Prototype example> In this prototype example, for convenience, B y K, which uses potassium for B and manganese for M as inexpensive elements in the constituent elements of MO2 y This section describes the manufacturing method of MnO2. y The production of MnO2 is carried out as follows:

[0091] Five grams of commercially available potassium permanganate powder (raw material) with equivalent diameters of 0.2 mm or less in minor axis, 0.5 mm or less in major axis, and 0.3 mm or less in thickness was ground in an alumina mortar for 10 minutes. The powder was passed through a 280-mesh sieve to collect the under-sieve powder. The primary particles contained in the under-sieve powder had equivalent diameters of 40 μm or less in minor axis, 50 μm or less in major axis, and 50 μm or less in thickness (ground raw material). Two grams of the under-sieve powder was placed in a 25 mL alumina crucible and heated in air for five hours. After natural cooling, the sample was removed from the electric furnace. Three heating temperatures were used: 300°C, 600°C, and 900°C. These samples were designated Prototype 1, Prototype 2, and Prototype 3, respectively.

[0092] The samples of Prototypes 1 to 3 were subjected to the following cleaning treatment, and were designated as Prototypes 4 to 6, respectively. 1 g of sample was added to 500 mL of distilled water and stirred at 400 rpm for 30 minutes, after which vacuum filtration was carried out using a polytetrafluoroethylene membrane with a pore size of 1 μm. Next, 100 mL of distilled water was added and vacuum filtration was carried out. This process was repeated three times to remove water-soluble impurities. The powder deposited on the polytetrafluoroethylene membrane was dried in an air atmosphere at 80°C for one day and night. This resulted in a single-phase Birnessite-type manganese oxide (K y MnO2) was obtained.

[0093] <Scanning electron microscope image observation and surface element analysis> The observation conditions of the scanning electron microscope images and the elemental analysis conditions of the particle surfaces in the raw materials, crushed raw materials, and Prototypes 1 to 6 are as follows. · Measuring instrument: JEOL JSM-IT100 · Electron gun source: Tungsten wire · Observed image: Secondary electron image · Acceleration voltage: 15 kV · Measurable elements: 6 B~ 92 U [[ID=…]] · Energy resolution: 129 keV By crushing the raw material in Fig. 7, the presence of fine particles was confirmed in the crushed raw material. Although flat surfaces were observed on the particle surfaces of the raw material and the crushed raw material, no flat surfaces were observed on the particle surfaces of Prototypes 1 to 6 due to the thermal decomposition of the raw material, and many irregularities were observed. Also, in Prototypes 1 to 6, the higher the heating temperature, the more the adhesion of primary particles was observed. In the chemical composition ratios of Prototypes 1 to 6 in Table 1, excess potassium was removed in Prototypes 4 to 6 where washing and filtration were performed, and for K of Prototypes 4 to 6 y As the y composition range of KMnO2, in Prototype 4, 0.24 ≤ y ≤ 0.28, in Prototype 5, 0.26 ≤ y ≤ 0.32, and in Prototype 6, 0.28 ≤ y ≤ 0.32.

[0094]

Table 1

[0095] <X-ray diffraction pattern measurement> The measurement information of the X-ray diffraction patterns in Prototypes 1 to 6 is as follows. · Measuring instrument: Rigaku Smartlab · Irradiated X-ray source: Cu-Kα ray · Measurement mode: Step scan · Scan conditions: At 0.02° intervals for 2θ · Measurement range: From 10° to 90° for 2θ · Tube voltage: 40 kV · Tube current: 30 mA In Figures 8 to 10, the X-ray diffraction patterns of Samples 1 to 3 include those of Birnessite-type manganese oxide, as described in JCPDS (Joint Committee Powder Diffraction Standards) card No. 01-073-4155, and those of manganese oxide containing excess potassium, as described in JCPDS (Joint Committee Powder Diffraction Standards) card No. 01-078-3423 or No. 01-073-0459. Together, these patterns indicate the presence of a potassium-excess phase on the particle surface, as shown in Table 1. The potassium-excess phase was removed by vacuum filtration and washing, and Samples 4 to 6 corresponded to the single-phase X-ray diffraction patterns of Birnessite-type manganese oxide, as described in No. 01-073-4155. In particular, the 00l plane (l = 2, 4, 6) derived from the layered structure was observed in all samples. From this, we can infer that the crystal structure of Samples 4–6 is one in which the potassium-rich phase has been removed, the edge- and vertex-shared chains of MnO6 octahedra are aligned parallel to the c-axis of the crystal, and potassium is aligned between the parallel layers. Furthermore, the interlayer distance estimated from the diffraction peak of the 002 plane of Samples 4–6 is approximately 7.2 A, which is larger than the interlayer distance of LiCoO2, which is approximately 4 A. Based on the Shannon ionic radius, assuming that the ionic diameter of the lithium ions present between the layers is approximately 1.8 A and that of the potassium ions is approximately 3 A, if all the lithium in LiCoO2 is replaced with potassium, and the interlayer distance is enlarged by a factor of 1 to match the ratio of the ionic diameters, the interlayer distance is estimated to be approximately 6.7 A, which is roughly consistent. In samples 4 to 6, the higher the heating temperature, the smaller the peak half-width and the greater the peak intensity, suggesting improved crystallinity. Additionally, as shown in Table 1, the amount of potassium present between the layers increased in order to align the layered structure. [Industrial Applicability]

[0096] The positive electrode active material of the present invention can provide an effective capacity exceeding that of commercially available positive electrode active materials for secondary batteries, and has excellent output characteristics during large current charging and discharging. Therefore, it can be used as a main or secondary power source in small, medium, and large devices such as portable electronic devices, electric vehicles, and stationary power energy adjustment systems. Electrolyte aqueous solution systemIt is possible to provide a layered metal oxide positive electrode active material for an alkali ion secondary battery. The active material using a pertransition metal salt is manufactured using a simple method that involves heat treatment in air and washing with water that has been demineralized. Furthermore, because inexpensive elements can be used as the constituent elements of the positive electrode active material of the present invention, the production cost is lower than that of the conventional positive electrode active material LiCoO2, and therefore it can be easily used industrially. [Explanation of symbols]

[0097] 1···Mobile ion A, 2···Alkali metal B already present between the layers, 3···Octahedron (oxygen six-coordinated) composed of transition metal M and oxygen O.

Claims

1. ByMO is a lithium-ion battery that is manufactured or fully charged and is composed of an oxide crystal of an alkali metal B, a transition metal M, and oxygen O, which are basic constituent elements of a layered structure, and has a Shannon ion radius of mobile ions A<B in order to stably maintain the layered structure, and B is present between the layers in advance, and also has an ion diffusion coefficient A>B in order to maintain an effective capacity exceeding that of lithium cobalt oxide. ２ A positive electrode active material for an aqueous electrolyte alkaline ion secondary battery comprising:

2. Compositional formula: AxByMO ２ 2. The positive electrode active material for an aqueous electrolyte solution alkali ion secondary battery according to claim 1, having a basic composition of: (0<x≦1.0−y, 0<y<0.4, A is an alkali metal or alkaline earth metal that is inserted between the layers during discharge, and M is one or more transition metal elements).

3. 2. The positive electrode active material for an aqueous electrolyte alkaline ion secondary battery according to claim 1, wherein A is any one of Li, Na, K, Mg, and Ca.

4. 2. The positive electrode active material for an aqueous electrolyte alkaline ion secondary battery according to claim 1, wherein A is Ca, B is K, M is Mn, and 0.24≦y≦0.

32.

5. Potassium permanganate is heated in the air at a temperature range of 300 to 900°C for a time range of 5 to 24 hours to obtain KyMnO ２ Then, the compound is stirred in an aqueous solution such as distilled water at 300 to 400 rpm for 10 to 30 minutes, and then vacuum filtration and washing with distilled water are repeatedly carried out to obtain a precursor compound containing KyMnO ２ KyMnO, a layered metal oxide positive electrode active material for aqueous electrolyte alkaline ion secondary batteries. ２ (0.24≦y≦0.32)

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