Non-aqueous alkali metal energy storage element

Abstract

To raise an energy density by increasing a volume density of an active material in a sodium carbonate-containing cathode active material layer, to increase a Na ion pre-doping amount to an anode by increasing the efficiency of decomposition of sodium carbonate in a cathode active material layer, to increase an energy density of a sodium ion battery having an anode larger, in capacity, than a cathode, and to provide a sodium ion secondary battery superior in charge / discharge cycle endurance.SOLUTION: Provided is a precursor of a sodium ion secondary battery including a cathode precursor, an anode, and a separator, and a Na ion-containing non-aqueous electrolyte. The cathode precursor has a cathode active material layer containing a cathode active material and a sodium carbonate. A weight ratio C1 (mass%) of the cathode active material in the cathode active material layer, a weight ratio C2 (mass%) of the sodium carbonate, an initial charge capacity Q1 (mAh / g) of the cathode active material, a weight A (g) per one face of the cathode active material layer, and an anode half cell initial charge capacity B (mAh) satisfy: C1 / 100×Q1×A<B; and 5.0≤C2≤10.0.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sodium ion secondary battery precursor and a sodium ion secondary battery. [Background technology]

[0002] In recent years, from the perspective of protecting the global environment and effectively utilizing energy to conserve resources, attention has been drawn to wind power generation power smoothing systems or late-night power storage systems, distributed home energy storage systems based on solar power generation technology, and energy storage systems for electric vehicles.

[0003] A requirement for the batteries used in these energy storage systems is a high energy density. Lithium-ion secondary batteries are being actively developed as a promising candidate for a high-energy-density battery that can meet this requirement. Because lithium-ion secondary batteries have an energy density exceeding 100 Wh / L and excellent durability, such as cycle life, they are currently in widespread use as the most suitable energy storage element for electric vehicles and mobile devices, which require high capacity and high durability.

[0004] However, the crucial element, lithium, has an average concentration of only about 20 ppm in the Earth's crust, and its mining areas are unevenly distributed, meaning that if lithium-ion secondary batteries become more widespread in the future, the Earth's lithium resources will be depleted. Therefore, it will be necessary to use more ubiquitous elements as a substitute for lithium, and research into using alkali metals such as sodium or potassium in energy storage devices is being actively conducted.

[0005] For example, Patent Document 1 describes a method for manufacturing a cathode using Na x Me yIt has a composition represented by the general formula of O2 (where Me is at least one selected from the group consisting of Fe, Mn, and Ni). In the above formula, y satisfies 0.95 ≦ y < 1.05, x satisfies 0.8 < x ≦ 1.0, and the composite metal oxide composed of the P2 structure has been shown to increase the initial charge capacity in addition to a large reversible charge-discharge capacity as a positive electrode active material.

[0006] Patent Document 2 discloses a technique of pre-doping by oxidatively decomposing sodium carbonate contained in the positive electrode active material layer and reducing the generated sodium ions at the negative electrode. Therefore, Patent Document 2 shows that even if the sodium ions contained in the positive electrode active material are insufficient, the sodium ions can be supplemented by pre-doping, so that it can be charged and discharged as a sodium ion secondary battery.

[0007] Patent Document 3 discloses a sodium ion secondary battery in which the energy density is increased by increasing the potential difference between the positive electrode and the negative electrode.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] The present invention has been made in view of the above-mentioned current situation. Therefore, the problems to be solved by the present invention are to increase the energy density by increasing the bulk density of the active material in a positive electrode active material layer containing sodium carbonate, to increase the amount of sodium ions pre-doped into the negative electrode by increasing the decomposition efficiency of sodium carbonate contained in the positive electrode active material layer, to increase the energy density of a sodium ion secondary battery having a negative electrode with a larger capacity than the positive electrode, and to provide a sodium ion secondary battery with excellent charge-discharge cycle durability. [Means for solving the problem]

[0010] The present inventors have conducted extensive research and experiments to solve the above problems. As a result, the present inventors have found that charge-discharge cycle durability can be improved by using a negative electrode with a larger capacity than the positive electrode and pre-doping the negative electrode with sodium ions by decomposing sodium carbonate contained in the positive electrode. They have also found that optimizing the content of sodium carbonate contained in the positive electrode active material layer can simultaneously promote the decomposition of sodium carbonate at the positive electrode, increase the amount pre-doped into the negative electrode, and increase the bulk density of the active material in the positive electrode active material layer containing sodium carbonate, thereby achieving a high energy density. The present invention has been made based on this finding, and is as follows: [1] A sodium ion secondary battery precursor comprising a positive electrode precursor, a negative electrode, a separator, and a non-aqueous electrolyte solution containing sodium ions, the positive electrode precursor includes a positive electrode current collector and a positive electrode active material layer provided on one or both surfaces of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material and sodium carbonate, the positive electrode active material including a transition metal oxide capable of absorbing and releasing sodium ions; the negative electrode has a negative electrode current collector and a negative electrode active material layer including a negative electrode active material provided on one or both surfaces of the negative electrode current collector, When the weight ratio of the positive electrode active material contained in the positive electrode active material layer is C1 (wt%), the weight ratio of sodium carbonate is C2 (wt%), the initial charge capacity of the positive electrode active material is Q1 (mAh / g), the weight per side of the positive electrode active material layer is A (g), and the initial charge capacity of the negative electrode half cell formed from the negative electrode is B (mAh), a sodium ion secondary battery precursor in which C1 / 100×Q1×A < B and 5.0 ≤ C2 ≤ 10.0. [2] The positive electrode active material is Na x M1 y M2 z O2 (where M1 and M2 are each one selected from Fe, Mn, Co, Ni, Ti, or Cu, and satisfy 0.5 ≤ x ≤ 1.0, 0.1 < y < 0.9, 0.1 < z < 0.9, and 0.95 < y + z < 1.05), the sodium ion secondary battery precursor according to [1]. [3] The M1 and the M2 are each Fe, Mn, or Co, the sodium ion secondary battery precursor according to [2]. [4] For the sodium ion secondary battery precursor according to any one of [1] to [3], a method for manufacturing a sodium ion secondary battery, characterized in that a voltage of 3.5 V or more and 4.4 V or less is applied at an environmental temperature of 20°C or more and 50°C or less to decompose the sodium carbonate. [5] After reaching a predetermined voltage by constant current charging, constant voltage charging is continued at that voltage, the method for manufacturing a sodium ion secondary battery according to [4]. [6] The current density in the constant current charging is 0.01 mA / cm 2 or more and 0.50 mA / cm 2 or less, the method for manufacturing a sodium ion secondary battery according to [5]. [7] A sodium ion secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte containing sodium ions, The positive electrode has a positive electrode current collector and a positive electrode active material layer provided on one or both sides of the positive electrode current collector and containing a positive electrode active material. The positive electrode active material is Na x M1 y M2 z O2 (where M1 and M2 are each one selected from Fe, Mn, Co, Ni, Ti, or Cu, and satisfy 0.5 ≦ x ≦ 1.0, 0.1 < y < 0.9, 0.1 < z < 0.9, and 0.95 < y + z < 1.05), and contains a transition metal oxide represented by The negative electrode has a negative electrode current collector and a negative electrode active material layer provided on one or both sides of the negative electrode current collector and containing a negative electrode active material. A sodium ion secondary battery in which, in the XRD (X-ray diffraction) measurement of the positive electrode active material when the voltage of the sodium ion secondary battery is 1.0 V, the peak of the 002 diffraction line is at 15.70° or more and 16.00° or less. [8] The sodium ion secondary battery according to [7], wherein M1 and M2 are each Fe, Mn, or Co. [9] When the weight ratio of the positive electrode active material contained in the positive electrode active material layer is C1 (wt%), the initial charge capacity of the positive electrode active material is Q1 (mAh / g), the weight per side of the positive electrode active material layer is A (g), and the initial charge capacity of the negative electrode half-cell formed from the negative electrode is B (mAh), C1 / 100 × Q1 × A < B. The sodium ion secondary battery according to [7] or [8]. [Advantages of the Invention]

[0011] According to the present invention, it is possible to provide a sodium ion secondary battery precursor having a high energy density, excellent charge / discharge cycle characteristics, and excellent decomposition efficiency of sodium carbonate contained in the positive electrode active material layer, and a sodium ion secondary battery using the same. [Brief Description of the Drawings]

[0012] [Figure 1]1 is a chart showing the relationship between the state of a positive electrode active material and the peak of the 002 diffraction line in an XRD (X-ray diffraction) measurement of a positive electrode included in a sodium ion secondary battery. [Figure 2] 10 is a graph showing the influence of temperatures of 25° C. (Example 7) and 45° C. (Example 8) on cyclic voltammetry measurement of a sodium ion secondary battery before pre-doping. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail.

[0014] In the sodium ion secondary battery according to this embodiment, sodium ions are selected as the metal ions that are inserted into and removed from the positive and negative electrode active materials during charge and discharge. Sodium ion secondary batteries generally have a positive electrode, a negative electrode, a separator, an electrolyte, and an exterior body as their main components. The electrolyte used is an organic solvent in which a sodium salt has been dissolved (hereinafter referred to as a non-aqueous electrolyte).

[0015] In this specification, the state of the positive electrode before the pre-doping step is defined as a "positive electrode precursor," and the state of the sodium ion secondary battery before the pre-doping step is defined as a "sodium ion secondary battery precursor."

[0016] <Positive electrode> The positive electrode in this embodiment has a positive electrode current collector and a positive electrode active material layer present on one or both sides of the positive electrode current collector.

[0017] Furthermore, the positive electrode in this embodiment preferably contains a positive electrode active material and sodium carbonate as a positive electrode precursor before assembly of the energy storage device.

[0018] As described below, in this embodiment, it is preferable to pre-dope sodium ions into the negative electrode during the process of assembling the energy storage element. As the pre-doping method, after assembling the energy storage element using the positive electrode precursor containing sodium carbonate, the negative electrode, the separator, the exterior body, and the non-aqueous electrolyte, it is preferable to apply a voltage between the positive electrode precursor and the negative electrode. The sodium carbonate is preferably contained in the positive electrode active material layer formed on the positive electrode current collector of the positive electrode precursor.

[0019] [Positive electrode active material layer] The positive electrode active material layer preferably contains a positive electrode active material containing a transition metal oxide, and in addition, may contain optional components such as a conductive filler, a binder, and a dispersion stabilizer as required.

[0020] Also, the positive electrode active material layer of the positive electrode precursor preferably contains a positive electrode active material and sodium carbonate.

[0021] -Positive electrode active material- The positive electrode active material preferably contains a transition metal oxide capable of occluding and releasing sodium ions. There is no particular limitation on the transition metal oxide used as the positive electrode active material. Examples of the transition metal oxide include oxides containing one or more elements selected from the group consisting of cobalt, nickel, manganese, iron, copper, and chromium.

[0022] Specific examples of the positive electrode active material include Na x M1 y M2 z O2 (where M1 and M2 are each one selected from Fe, Mn, Co, Ni, Ti, or Cu, and satisfy 0.5 ≦ x ≦ 1.0, 0.1 < y < 0.9, 0.1 < z < 0.9, and 0.95 < y + z < 1.05), Na x M3O2 (where M3 is one selected from Fe, Mn, Co, Ni, Ti, or Cu, and satisfies 0.5 ≦ x ≦ 1.0), etc.

[0023] Specific examples of sodium-containing transition metal oxides include Na x Fe 0.5 Mn 0.5 O2, Na x Fe 0.5 Co 0.5 O2, Na x Fe 0.5 Ni 0.5 O2, Na x Ni 0.5 Mn 0.5 O2, and Na x FeO2 (wherein x each independently satisfies 0≦x≦1), and the like.

[0024] The average particle diameter of the positive electrode active material is preferably 1 to 20 μm. If the average particle diameter is 1 μm or more, the density of the active material layer is high, and therefore the capacity per electrode volume tends to be high. Here, a small average particle diameter may result in the drawback of low durability, but if the average particle diameter is 1 μm or more, such a drawback is unlikely to occur. On the other hand, if the average particle diameter is 20 μm or less, it tends to be more suitable for high-speed charging and discharging. The average particle diameter is more preferably 1 to 15 μm, and even more preferably 1 to 10 μm. The upper and lower limits of the above range of average particle diameter can be combined arbitrarily.

[0025] The content ratio of the positive electrode active material in the positive electrode active material layer (also referred to as C1) is preferably 80% by weight or more and 95% by weight or less, based on the total weight of the positive electrode active material layer in the positive electrode precursor. The lower limit of the content ratio of the positive electrode active material is more preferably 83% by weight or more, and even more preferably 85% by weight or more. On the other hand, the upper limit of the content ratio of the positive electrode active material is preferably 95% by weight or less. By maintaining a content ratio within this range, favorable charge / discharge characteristics are exhibited. The upper and lower limits of this range can be combined as desired.

[0026] (sodium carbonate) Sodium carbonate can be decomposed in the positive electrode or its precursor in the pre-doping step described later, and release sodium ions. Sodium carbonate decomposes upon application of a voltage, functions as a dopant source for pre-doping the negative electrode, and forms pores in the positive electrode active material layer, so that a positive electrode having excellent electrolyte retention and ion conductivity can be formed.

[0027] The sodium carbonate content C2 in the positive electrode active material layer is preferably 5.0 wt % or more and 10.0 wt % or less, more preferably 6.0 wt % or more and 9.0 wt % or less, and even more preferably 6.5 wt % or more and 8.5 wt % or less, based on the total mass of the positive electrode active material layer in the positive electrode precursor. A content C2 of 5.0 wt % or more is preferable because it allows sufficient sodium ions to be pre-doped into the negative electrode. A content C2 of 10.0 wt % or less is preferable because it allows the positive electrode density after the sodium carbonate reaction to be increased, thereby increasing the energy density. The upper and lower limits of the above content ranges can be combined arbitrarily.

[0028] -Optional ingredients- The positive electrode active material layer in this embodiment may contain optional components such as a conductive filler, a binder, a dispersion stabilizer, etc., in addition to the positive electrode active material and sodium carbonate, as needed.

[0029] The conductive filler is not particularly limited, and examples thereof include acetylene black, ketjen black, vapor-grown carbon fiber, graphite, carbon nanotubes, and mixtures thereof. The amount of conductive filler used is preferably 0 to 30 parts by weight, more preferably 0.01 to 25 parts by weight, and even more preferably 1 to 20 parts by weight, relative to 100 parts by weight of the positive electrode active material. If the amount of conductive filler used is more than 30 parts by weight, the content of the positive electrode active material in the positive electrode active material layer decreases, which is undesirable because it reduces the energy density per volume of the positive electrode active material layer.

[0030] The binder is not particularly limited, but examples thereof include PVdF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), polyimide, latex, styrene-butadiene copolymer, fluororubber, and acrylic copolymer. The amount of binder used is preferably 1 to 30 parts by weight, more preferably 1 to 15 parts by weight, and even more preferably 1 to 10 parts by weight, per 100 parts by weight of the positive electrode active material. If the amount of binder used is 1% by weight or more, sufficient electrode strength is achieved. On the other hand, if the amount of binder used is 30% by weight or less, the ingress and egress of ions into and diffusion from the positive electrode active material are not hindered, and high input / output characteristics are achieved.

[0031] The dispersion stabilizer is not particularly limited, and examples thereof include PVP (polyvinylpyrrolidone), PVA (polyvinyl alcohol), and cellulose derivatives. The amount of the dispersion stabilizer used is preferably 0 or 0.1 to 10 parts by weight per 100 parts by weight of the positive electrode active material. When the amount of the dispersion stabilizer used is 10 parts by weight or less, the ingress and egress of ions into and diffusion from the positive electrode active material are not inhibited, and high input / output characteristics are exhibited.

[0032] [Positive electrode current collector] The material for the positive electrode current collector in this embodiment is not particularly limited as long as it has high electronic conductivity and is not susceptible to degradation due to elution in the electrolytic solution or reaction with the electrolyte or ions, etc., but metal foil is preferred. Aluminum foil is particularly preferred as the positive electrode current collector in the sodium ion secondary battery or its precursor according to this embodiment.

[0033] The metal foil may be a normal metal foil without irregularities or through holes, or may be a metal foil with irregularities that has been subjected to embossing, chemical etching, electrolytic deposition, blasting, or the like, or may be a metal foil with through holes such as expanded metal, punched metal, or etched foil.

[0034] The thickness of the positive electrode current collector is not particularly limited as long as it can sufficiently maintain the shape and strength of the positive electrode, but it is preferably, for example, 1 to 100 μm.

[0035] <Negative electrode> The negative electrode of this embodiment contains a substance involved in a faradaic reaction or a non-faradaic reaction, and preferably has a negative electrode current collector and a negative electrode active material layer present on one or both sides of the negative electrode current collector.

[0036] [Negative electrode active material layer] The negative electrode active material layer contains a negative electrode active material capable of absorbing and releasing sodium ions, and may further contain optional components such as a conductive filler, a binder, and a dispersion stabilizer, as necessary.

[0037] -Negative electrode active material- The negative electrode active material may be a material capable of absorbing and releasing sodium ions. Specifically, carbon materials, sodium metal, and alloys containing sodium are preferably used. The carbon material content relative to the total amount of the negative electrode active material is preferably 50% by weight or more, and more preferably 70% by weight or more. While the carbon material content can be 100% by weight, from the viewpoint of obtaining the desired effect of the combined use of other materials, it is preferably 98% by weight or less, and may be 95% by weight or less. The upper and lower limits of the range of the carbon material content can be arbitrarily combined.

[0038] Examples of carbon materials include non-graphitizable carbon materials, carbon black, carbon nanoparticles, amorphous carbonaceous materials such as polyacene-based substances, carbonaceous materials obtained by heat-treating carbonaceous material precursors such as petroleum-based pitch, coal-based pitch, mesocarbon microbeads, coke, and synthetic resins (e.g., phenolic resins), pyrolysates of furfuryl alcohol resins or novolac resins, fullerenes, carbon nanophones, and composite carbon materials thereof. Among these, non-graphitizable carbon materials are preferably used in sodium ion secondary batteries because they can achieve a good doped state or exhibit high capacity.

[0039] The average particle size of the negative electrode active material is preferably 1 μm or more and 30 μm or less. The lower limit is more preferably 2 μm or more, and even more preferably 2.5 μm or more. The upper limit is more preferably 10 μm or less, and even more preferably 5 μm or less. When the average particle size is 1 μm or more and 30 μm or less, good durability is maintained. The upper and lower limits of the range of the average particle size of the negative electrode active material can be combined arbitrarily.

[0040] -Optional ingredients- The negative electrode active material layer in this embodiment may contain optional components such as a conductive filler, a binder, and a dispersion stabilizer, in addition to the negative electrode active material, as needed.

[0041] The type of conductive filler is not particularly limited, and examples thereof include acetylene black, ketjen black, vapor-grown carbon fiber, etc. The amount of the conductive filler used is preferably 0 to 30 parts by weight, more preferably 0.01 to 20 parts by weight, and even more preferably 0.1 to 15 parts by weight, relative to 100 parts by weight of the negative electrode active material.

[0042] The binder is not particularly limited, but examples thereof include PVdF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), polyimide, latex, styrene-butadiene copolymer, fluororubber, acrylic copolymer, polyethylene oxide, and carboxymethyl cellulose. The amount of binder used is preferably 1 to 30 parts by weight, more preferably 2 to 27 parts by weight, and even more preferably 3 to 25 parts by weight, per 100 parts by weight of the negative electrode active material. When the amount of binder used is 1% by weight or more, sufficient electrode strength is achieved. On the other hand, when the amount of binder used is 30% by weight or less, the movement of sodium ions into and out of the negative electrode active material is not inhibited, and high input / output characteristics are achieved.

[0043] The dispersion stabilizer is not particularly limited, and examples thereof include PVP (polyvinylpyrrolidone), PVA (polyvinyl alcohol), and cellulose derivatives. The amount of the dispersion stabilizer used is preferably 0 or 0.1 to 10 parts by weight per 100 parts by weight of the negative electrode active material. When the amount of the dispersion stabilizer used is 10 parts by weight or less, the movement of sodium ions into and out of the negative electrode active material is not inhibited, and high input / output characteristics are achieved.

[0044] [Negative electrode current collector] The material constituting the negative electrode current collector in this embodiment is preferably a metal foil that has high electronic conductivity and is not susceptible to degradation due to elution in a non-aqueous electrolyte solution or reaction with an electrolyte or ions. There are no particular limitations on such metal foil, and examples include aluminum foil, copper foil, nickel foil, and stainless steel foil. Aluminum foil is preferred as the negative electrode current collector in the sodium ion secondary battery according to this embodiment.

[0045] The metal foil may be a normal metal foil without irregularities or through holes, or may be a metal foil with irregularities that has been subjected to embossing, chemical etching, electrolytic deposition, blasting, or the like, or may be a metal foil with through holes such as expanded metal, punched metal, or etched foil.

[0046] The thickness of the negative electrode current collector is not particularly limited as long as it can sufficiently maintain the shape and strength of the negative electrode, but it is preferably, for example, 1 to 100 μm.

[0047] [Production of positive electrode precursor and negative electrode] The positive electrode precursor and the negative electrode have a positive electrode active material layer on one or both sides of a positive electrode current collector and a negative electrode active material layer on one or both sides of a negative electrode current collector. In a typical embodiment, the positive electrode active material layer is adhered to the positive electrode current collector, and the negative electrode active material layer is adhered to the negative electrode current collector.

[0048] The positive electrode precursor and the negative electrode can be manufactured using known electrode manufacturing techniques for sodium ion secondary batteries, electric double layer capacitors, and the like. For example, various materials including a positive electrode active material or a negative electrode active material are dispersed or dissolved in water or an organic solvent to prepare a slurry coating liquid, and this coating liquid is applied to one or both sides of a positive electrode current collector or a negative electrode current collector to form a coating film, which is then dried to obtain a positive electrode precursor or a negative electrode. Furthermore, the obtained positive electrode precursor or negative electrode may be pressed to adjust the film thickness or bulk density of the positive electrode active material layer or the negative electrode active material layer. Alternatively, various materials including a positive electrode active material or a negative electrode active material may be dry-mixed without using a solvent, the resulting mixture is press-molded, and then attached to the positive electrode current collector or the negative electrode current collector using a conductive adhesive.

[0049] The coating liquid may be prepared by dry-blending some or all of various material powders including the positive electrode active material or the negative electrode active material, and then adding water or an organic solvent and / or a liquid or slurry substance in which a binder or a dispersion stabilizer is dissolved or dispersed therein. Alternatively, the coating liquid may be prepared by adding various material powders including the positive electrode active material or the negative electrode active material to a liquid or slurry substance in which a binder or a dispersion stabilizer is dissolved or dispersed in water or an organic solvent.

[0050] As a method for dry-blending some or all of the various material powders including the positive electrode active material, for example, a ball mill or the like may be used to premix the positive electrode active material, sodium carbonate, and optionally a conductive filler, and then premix the sodium carbonate, which has low conductivity, with the conductive material. This facilitates decomposition of the sodium carbonate in the positive electrode precursor in the pre-doping step described below. When water is used as the solvent for the coating liquid, the addition of sodium carbonate may make the coating liquid alkaline, so a pH adjuster may be added as needed.

[0051] The coating liquid can be prepared using any suitable dispersing machine, including, but not limited to, a homodisper or multi-axis disperser, a planetary mixer, or a thin film rotary high-speed mixer. To obtain a coating liquid in a well-dispersed state, dispersion is preferably carried out at a peripheral speed of 1 m / s or more and 50 m / s or less. A peripheral speed of 1 m / s or more is preferred because various materials are well dissolved or dispersed. Furthermore, a peripheral speed of 50 m / s or less is preferred because the various materials are not destroyed by heat or shear force due to dispersion, and re-aggregation does not occur.

[0052] The degree of dispersion of the coating liquid is preferably 0.1 μm or more and 100 μm or less as measured with a particle gauge. The upper limit of the degree of dispersion is more preferably 80 μm or less, and even more preferably 50 μm or less. A particle size of less than 0.1 μm is not preferable because it is smaller than the particle size of the powders of various materials, including the positive electrode active material, and the materials are crushed during preparation of the coating liquid. Furthermore, a particle size of 100 μm or less prevents clogging during discharge of the coating liquid or the occurrence of streaks in the coating film, allowing for stable coating.

[0053] The coating film may be formed using any suitable coating machine, including, but not limited to, a die coater, comma coater, knife coater, or gravure coater. The coating film may be formed by single-layer coating or multi-layer coating. In the case of multi-layer coating of the positive electrode precursor, the coating liquid composition may be adjusted so that the sodium carbonate content in each layer of the coating film varies. The coating speed is preferably 0.1 m / min to 100 m / min, more preferably 0.5 m / min to 70 m / min, and even more preferably 1 m / min to 50 m / min. A coating speed of 0.1 m / min or higher ensures stable coating. On the other hand, a coating speed of 100 m / min or lower ensures sufficient coating accuracy.

[0054] The drying method for the coating film is not particularly limited, but preferably includes hot air drying, infrared (IR) drying, and the like. The coating film may be dried at a single temperature or at multiple temperatures. Alternatively, the coating film may be dried using a combination of several drying methods. The drying temperature is preferably 25°C or higher and 200°C or lower, more preferably 40°C or higher and 180°C or lower, and even more preferably 50°C or higher and 160°C or lower. A drying temperature of 25°C or higher allows the solvent in the coating film to be sufficiently evaporated. On the other hand, a drying temperature of 200°C or lower can prevent cracking of the coating film due to rapid solvent evaporation, uneven distribution of the binder due to migration, oxidation of the positive electrode current collector or negative electrode current collector, and oxidation of the positive electrode active material layer or negative electrode active material layer.

[0055] The method for pressing the positive electrode precursor and the negative electrode is not particularly limited, but a press such as a hydraulic press, a vacuum press, etc. The film thickness, bulk density, and electrode strength of the positive electrode active material layer and the negative electrode active material layer can be adjusted by the pressing pressure, gap, and surface temperature of the pressing part.

[0056] The thickness of the positive electrode active material layer is preferably 20 μm to 200 μm per side of the current collector. The thickness of the positive electrode active material layer is more preferably 25 μm to 100 μm per side, and even more preferably 30 μm to 80 μm per side. A thickness of 20 μm or more can achieve sufficient charge / discharge capacity. On the other hand, a thickness of 200 μm or less can maintain low ion diffusion resistance within the electrode. Therefore, sufficient output characteristics can be obtained, and the cell volume can be reduced, thereby increasing the energy density. The upper and lower limits of the thickness range of the positive electrode active material layer can be arbitrarily combined. Note that, when the current collector has through-holes and / or irregularities, the thickness of the positive electrode active material layer refers to the average thickness per side of the current collector in the portion that does not have through-holes or irregularities.

[0057] The thickness of the negative electrode active material layer is preferably 5 μm or more and 100 μm or less per side of the current collector. The lower limit of the thickness of the negative electrode active material layer is more preferably 7 μm or more, more preferably 10 μm or more. The upper limit of the thickness of the negative electrode active material layer is more preferably 80 μm or less, even more preferably 60 μm or less. If the thickness is 5 μm or more, no streaks or the like are generated when the negative electrode active material layer is applied, resulting in excellent coatability. On the other hand, if the thickness is 100 μm or less, a high energy density can be achieved by reducing the cell volume. The upper and lower limits of the thickness range of the negative electrode active material layer can be arbitrarily combined. Note that, when the current collector has through-holes and / or irregularities, the thickness of the negative electrode active material layer refers to the average thickness per side of the current collector in a portion that does not have through-holes or irregularities.

[0058] The bulk density of the negative electrode active material layer is preferably 0.30 g / cm 3 More than 1.8g / cm 3 or less, more preferably 0.40 g / cm 3 More than 1.5g / cm 3 or less, more preferably 0.45 g / cm 3 More than 1.3g / cm 3 Bulk density is 0.30 g / cm or less. 3 If the bulk density is 1.8 g / cm or more, sufficient strength can be maintained and sufficient conductivity between the negative electrode active materials can be exhibited. 3 If the content is equal to or less than this, pores that allow ions to diffuse sufficiently can be secured within the negative electrode active material layer.

[0059] [Separator] The positive electrode precursor and the negative electrode are stacked or wound with a separator interposed therebetween to form an electrode stack or an electrode wound body having the positive electrode precursor, the negative electrode, and the separator.

[0060] The separator may be a polyethylene or polypropylene microporous membrane used in sodium ion secondary batteries, or a cellulose nonwoven paper used in electric double layer capacitors. A membrane made of organic or inorganic particles may be laminated on one or both sides of the separator. The separator may also contain organic or inorganic particles.

[0061] The thickness of the separator is preferably 5 μm or more and 35 μm or less. A thickness of 5 μm or more is preferred because it tends to reduce self-discharge due to internal micro-short circuits. On the other hand, a thickness of 35 μm or less is preferred because it tends to improve the output characteristics of the battery.

[0062] The thickness of the film made of organic or inorganic fine particles is preferably 1 μm or more and 10 μm or less. A thickness of 1 μm or more is preferred because it tends to reduce self-discharge due to internal micro-short circuits. On the other hand, a thickness of 10 μm or less is preferred because it tends to improve the output characteristics of the battery.

[0063] [assembly] The electrode laminate obtained in the cell assembly process is a laminate formed by stacking a positive electrode precursor and a negative electrode cut into sheets with a separator interposed therebetween, to which a positive electrode terminal and a negative electrode terminal are connected. The electrode wound body is a wound body formed by winding a positive electrode precursor and a negative electrode with a separator interposed therebetween, to which a positive electrode terminal and a negative electrode terminal are connected. The electrode wound body may be cylindrical or flat in shape. The method for connecting the positive electrode terminal and the negative electrode terminal is not particularly limited, but can be resistance welding, ultrasonic welding, or the like.

[0064] [Exterior body] The exterior body can be a metal can, a laminated packaging material, or the like. The metal can is preferably made of aluminum. The laminated packaging material is preferably a film formed by laminating a metal foil and a resin film, and an example thereof is a three-layer structure consisting of an outer layer resin film / metal foil / inner layer resin film. The outer layer resin film is intended to prevent damage to the metal foil due to contact or the like, and resins such as nylon or polyester can be suitably used. The metal foil is intended to prevent permeation of moisture and gas, and foils such as copper, aluminum, and stainless steel can be suitably used. The inner layer resin film protects the metal foil from the electrolyte solution stored inside and serves to melt-seal the exterior body when it is heat-sealed, and polyolefins, acid-modified polyolefins, and the like can be suitably used.

[0065] [Storage in exterior body] The dried electrode laminate or electrode wound body is preferably housed in an exterior package such as a metal can or a laminate packaging material, and sealed with only one opening remaining. There are no particular restrictions on the method for sealing the exterior package, but when a laminate packaging material is used, methods such as heat sealing or impulse sealing are used.

[0066] [Drying] The electrode laminate or electrode wound body housed in the outer casing is preferably dried to remove the remaining solvent. There are no limitations on the drying method, but it can be dried by vacuum drying or the like. The remaining solvent is preferably 1.5 wt % or less based on the weight of the positive electrode active material layer or the negative electrode active material layer. If the remaining solvent is more than 1.5 wt %, the solvent remains in the system, which deteriorates the self-discharge characteristics or cycle characteristics, and is therefore undesirable.

[0067] The above electrode laminate or electrode wound body can be used as a precursor of a sodium-ion secondary battery, and a sodium-ion secondary battery can be manufactured through the steps of injecting an electrolyte solution and a pre-doping process, which will be described later. Regarding the positive electrode precursor included in the sodium-ion secondary battery precursor, the weight ratio of the positive electrode active material included in the positive electrode active material layer is C1 (wt%), the weight ratio of sodium carbonate is C2 (wt%), the initial charge capacity of the positive electrode active material is Q1 (mAh / g), and the weight per side of the positive electrode active material layer (for example, with respect to the current collector) is A (g). Regarding the negative electrode included in the sodium-ion secondary battery precursor, when the initial charge capacity of the negative electrode half-cell formed from the negative electrode is B (mAh) as described later, it is preferable that C1 / 100 × Q1 × A < B, and / or it is preferable that 5.0 ≦ C2 ≦ 10.0.

[0068] Regarding the charge-discharge cycle durability of a sodium-ion secondary battery, sodium ions are deactivated by side reactions, and the amount of sodium ions available for charge-discharge decreases, causing a decrease in capacity. Therefore, by previously containing an excessive amount of sodium ions in the battery or the battery precursor, it is possible to improve the charge-discharge cycle characteristics. However, as a negative electrode for occluding the excessively present sodium ions, it is necessary to design its capacity to be larger than that of the positive electrode. That is, by setting C1 / 100×Q1×A < B, the charge-discharge cycle durability can be improved, and (C1 / 100×Q1×A) / B is preferably 0.40 or more and 0.95 or less, more preferably 0.45 or more and 0.90 or less, and particularly preferably 0.50 or more and 0.85 or less. However, simply making the capacity of the negative electrode larger than that of the positive electrode results in a shortage of sodium ions available for charge-discharge, and the energy density decreases. Therefore, its effect can be exerted by pre-filling sodium ions into the sodium-ion secondary battery by pre-doping. The method of pre-doping sodium ions is not particularly limited, but a method using sodium carbonate, which can be stably handled even in air, as a doping source is preferred. Sodium carbonate is an insulator, and in order to decompose it, it is necessary to form an electron conduction path with the positive electrode active material or the conductive material. Therefore, when C2 > 10.0, the decomposition efficiency of sodium carbonate decreases, so the effect of improving the charge-discharge cycle durability decreases. Furthermore, since the positive electrode active material density in the positive electrode active material layer decreases, the energy density of the sodium-ion secondary battery decreases. On the other hand, when C2 < 5.0, the amount of sodium ions pre-doped into the negative electrode is insufficient, and the effect of improving the charge-discharge cycle durability becomes small.

[0069] [Electrolyte solution] The electrolyte solution in this embodiment is a non-aqueous electrolyte solution. That is, this electrolyte solution contains a non-aqueous solvent described later. The non-aqueous electrolyte solution contains a sodium salt of 0.5 mol / L or more based on the total amount of the non-aqueous electrolyte solution. That is, the non-aqueous electrolyte solution contains sodium ions as an electrolyte.

[0070] The nonaqueous electrolyte in this embodiment uses a sodium salt as the electrolyte. For example, Na(SO2CF3)2, NaN(SO2F)2, NaN(C2F5SO2)2, NaCF3SO3, NaC(CF3SO2)3, NaPF6, NaBF4, NaClO4, NaAsF6, NaAlCl4, etc. can be used alone or in combination of two or more. For example, it is preferable to include NaClO4, NaPF6, and / or NaN(SO2CF3)2, as these can exhibit high conductivity.

[0071] The sodium salt concentration in the non-aqueous electrolyte is preferably 0.5 mol / L or higher, more preferably in the range of 0.5 to 2.0 mol / L. If the sodium salt concentration is 0.5 mol / L or higher, sufficient anions are present, allowing the battery capacity to be sufficiently high. Furthermore, if the sodium salt concentration is 2.0 mol / L or lower, this is preferable because it prevents undissolved sodium salt from precipitating in the non-aqueous electrolyte and the viscosity of the electrolyte from becoming too high, preventing a decrease in conductivity and a decrease in output characteristics.

[0072] The non-aqueous electrolyte solution in this embodiment preferably contains a cyclic carbonate and a chain carbonate as a non-aqueous solvent. The non-aqueous electrolyte solution containing a cyclic carbonate and a chain carbonate is advantageous in terms of dissolving sodium salt at a desired concentration and exhibiting high ionic conductivity. Examples of cyclic carbonates include alkylene carbonate compounds such as ethylene carbonate, propylene carbonate, and butylene carbonate. The alkylene carbonate compound is typically unsubstituted. Examples of chain carbonates include dialkyl carbonate compounds such as dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, and dibutyl carbonate. The dialkyl carbonate compound is typically unsubstituted.

[0073] The total content of the cyclic carbonate and the chain carbonate is preferably 50% by weight or more, more preferably 65% by weight or more, and preferably 95% by weight or less, more preferably 90% by weight or less, based on the total amount of the non-aqueous electrolyte. If the total content is 50% by weight or more, it is possible to dissolve the sodium salt at a desired concentration, and high ionic conductivity can be achieved. If the total concentration is 95% by weight or less, the electrolyte can further contain the additives described below. The upper and lower limits of the range of the total concentration can be combined arbitrarily.

[0074] [Injection, impregnation, sealing process] After the assembly process is completed, a non-aqueous electrolyte solution is injected into the electrode stack housed in the outer casing. After the injection process is completed, it is desirable to further perform impregnation, thoroughly immersing the positive electrode, negative electrode, and separator in the non-aqueous electrolyte solution. If at least a portion of the positive electrode, negative electrode, and separator is not immersed in the electrolyte solution, the doping of sodium ions will proceed unevenly in the pre-doping process described below, resulting in increased resistance and reduced durability of the resulting sodium-ion secondary battery. The impregnation method is not particularly limited, but for example, after injection, the outer casing can be placed in a decompression chamber with the opening, and the chamber can be decompressed using a vacuum pump and then returned to atmospheric pressure. After the impregnation process is completed, the outer casing can be sealed while decompressing the pressure while remaining open.

[0075] [Pre-doping process] In this embodiment, the positive electrode active material containing sodium ions and sodium carbonate function as a dopant source for the negative electrode active material. In the pre-doping step, a voltage of 3.5 V or more and 4.4 V or less is preferably applied between the positive electrode precursor and the negative electrode to decompose the sodium carbonate. The sodium carbonate in the positive electrode precursor is decomposed to release sodium ions, and the sodium ions are then reduced at the negative electrode, thereby pre-doping the sodium ions into the negative electrode active material layer. When the applied voltage is 3.5 V or more, the decomposition of sodium carbonate is accelerated, allowing the pre-doping step to be completed in a short time. When the applied voltage is 4.4 V or less, degradation of the positive electrode active material is suppressed, and a high energy density can be achieved.

[0076] Although the method of voltage application is not particularly limited, a preferred method is to reach a predetermined voltage through constant-current charging and then continue constant-voltage charging at that voltage. By continuing charging while maintaining a constant voltage, the decomposition of sodium carbonate in the positive electrode precursor proceeds uniformly, thereby suppressing gas generation due to residual sodium carbonate in the sodium-ion secondary battery. Furthermore, when charging at a high current density, a set voltage value is reached before the sodium carbonate is completely decomposed due to a reaction overvoltage. However, by continuing constant-voltage charging, the decomposition reaction of sodium carbonate can be continued, thereby suppressing residual sodium carbonate. Voltage application can be performed in the ambient environment of the sodium-ion secondary battery, preferably at an ambient temperature of 20°C or higher and 50°C or lower.

[0077] The current density during constant current charging was 0.01 mA / cm 2 More than 0.50mA / cm 2 It is preferable that the current density is 0.01 mA / cm or less. 2 If the current density is 0.50 mA / cm or more, pre-doping can be performed in a short time. 2 If the temperature is equal to or lower than this, heat generation due to the decomposition of sodium carbonate can be suppressed, and deterioration of the positive electrode active material can be suppressed.

[0078] In this pre-doping step, gases such as CO2 may be generated due to the oxidative decomposition of sodium carbonate in the positive electrode precursor. Therefore, when applying a voltage, it is preferable to take measures to release the generated gas to the outside of the exterior body. Examples of such measures include: A method in which a voltage is applied with a part of the exterior body opened; A method in which a voltage is applied in a state in which an appropriate gas release means such as a gas vent valve or a gas permeable film is installed in advance on a part of the exterior body; The following can be mentioned:

[0079] [Aging process] After the pre-doping step, it is preferable to perform aging on the sodium ion secondary battery. In the aging step, the solvent in the electrolyte decomposes at the negative electrode, and a sodium ion-permeable solid polymer coating is formed on the negative electrode surface. The aging method is not particularly limited, but for example, a method of reacting the solvent in the electrolyte in a high-temperature environment, such as above 30°C or above 50°C, can be used.

[0080] [Gas removal process] After the aging step is completed, it is preferable to further degas the battery to completely remove any remaining gas in the electrolyte, the positive electrode, and the negative electrode. If gas remains in at least a portion of the electrolyte, the positive electrode, and the negative electrode, ion conduction is inhibited, resulting in an increase in the resistance of the resulting sodium-ion secondary battery. The degassing method is not particularly limited, and may involve, for example, placing the sodium-ion secondary battery in a reduced-pressure chamber with the exterior body open and using a vacuum pump to reduce the pressure inside the chamber.

[0081] <Sodium-ion secondary battery> By the above method, a sodium ion secondary battery can be manufactured as an energy storage element. a positive electrode having a porous positive electrode active material layer having pores formed by the decomposition and dissipation of sodium carbonate contained in a positive electrode precursor; A negative electrode having a negative electrode active material layer doped with the above sodium carbonate as a dopant source, is provided.

[0082] In the XRD (X-ray diffraction) measurement of the positive electrode active material when the voltage of the sodium ion secondary battery according to this embodiment is 1.0 V, it is preferable that the peak of the 002 diffraction line is at 15.7° or more and 16.0° or less. When the peak of the 002 diffraction line is present at 15.7° or more and 16.0° or less, it suggests that an excess amount of sodium ions is present in the sodium ion secondary battery, and it has excellent charge-discharge cycle durability. When the peak of the 002 diffraction line is present above 16.0°, it suggests that a large excess amount of sodium ions is present in the sodium ion secondary battery, and metallic sodium is likely to precipitate on the negative electrode, resulting in a decrease in charge-discharge cycle characteristics. Also, it is preferable that the sodium ion secondary battery obtained by the method described above using the sodium ion secondary battery also satisfies C1 / 100×Q1×A < B.

[0083] <Quantification method of sodium carbonate> The method for quantifying sodium carbonate contained in the positive electrode precursor is described below. The positive electrode precursor can be washed with distilled water, and sodium carbonate can be quantified from the weight change of the positive electrode or the positive electrode precursor before and after washing with distilled water. The area of the positive electrode precursor to be measured is not particularly limited, but from the viewpoint of reducing measurement variations, it is preferably 1 cm 2 or more and 100 cm 2 or less, and more preferably 2 cm 2 or more and 50 cm 2 or less. If the area is 1 cm 2 or more, the reproducibility of the measurement is ensured. If the area is 100 cm 2 or less, the sample handling property is excellent. The upper and lower limits of this area range can be arbitrarily combined.

[0084] The weight of the cut positive electrode precursor is designated as M0 (g). The positive electrode precursor is thoroughly immersed in distilled water with a volume 100 times the weight of the positive electrode precursor (100M0 (g)) for at least one hour. The positive electrode precursor is then removed from the distilled water and vacuum dried. The weight of the positive electrode precursor at this point is designated as M1 (g). Next, to measure the weight of the current collector, the positive electrode active material layer on the current collector is removed using a spatula, brush, or paintbrush. If the weight of the resulting positive electrode current collector is designated as M2 (g), the weight ratio C2 (wt%) of sodium carbonate contained in the positive electrode precursor and the weight A (g) per side of the positive electrode active material layer can be calculated using the following formulas (I) and (II), respectively. C2=100×[1-(M1-M2) / (M0-M2)] Formula (I) A=M0-M2 formula (II) [Example]

[0085] The features of the present invention will be further clarified by the following examples and comparative examples, but the present invention is not limited to the following examples.

[0086] Example 1 [Production of positive electrode active material] MnCO3 was heated in air at a rate of 1°C / min and calcined at 700°C for 5 hours to synthesize Mn2O3. Next, Mn2O3, Fe2O3, and Na2CO3 were weighed so that the molar ratio of Mn, Fe, and Na was 1:1:0.67, and mixed in a ball mill at 600 rpm for 24 hours before being pelletized. The resulting pellets were heated at a rate of 5°C / min, calcined at 900°C for 12 hours, and then crushed to produce P2-type Na, the positive electrode active material. 0.67 Fe 0.5 Mn 0.5 Got O2.

[0087] [Measurement of positive electrode capacity Q1] The obtained Na 0.67 Fe 0.5 Mn 0.5A positive electrode slurry was prepared by mixing 80.0 parts by weight of O2 powder, 10.0 parts by weight of acetylene black, 10.0 parts by weight of PVdF (polyvinylidene fluoride), and NMP (N-methylpyrrolidone). The positive electrode slurry was applied to one side of a 15 μm thick aluminum foil, dried, and pressed to obtain positive electrode 1.

[0088] Positive electrode 1 was vacuum-dried at 80°C for 12 hours and then punched out into a 10 mm diameter circle to form the working electrode. Metallic sodium was used as the counter electrode, and a glass filter was used as the separator. The working electrode, separator, and counter electrode were stacked in this order to obtain an electrode stack. Next, a coin cell was fabricated using nonaqueous electrolyte solution 1, obtained by dissolving NaPF6 at a concentration of 1.0 mol / L in a 1:1 volumetric mixture of ethylene carbonate (EC) and propylene carbonate (PC).

[0089] The coin cell was fabricated at a current density of 0.02 mA / cm under a 25°C environment. 2 The battery was charged at a constant current up to 4.3 V. The initial charge capacity Q1 was 139 mAh / g.

[0090] [Production of positive electrode precursor] The obtained Na 0.67 Fe 0.5 Mn 0.5 A positive electrode slurry was prepared by mixing 70.0 parts by weight of O2 powder, 10.0 parts by weight of sodium carbonate, 10.0 parts by weight of acetylene black (AB), 10.0 parts by weight of PVdF (polyvinylidene fluoride), and N-methylpyrrolidone (NMP). The positive electrode slurry was applied to one side of a 15 μm thick aluminum foil and dried to obtain positive electrode precursor 1. The thickness of the positive electrode active material layer on one side of positive electrode precursor 1 was 28.0 μm, and the basis weight was 60.0 g / m. 2 It was.

[0091] [Preparation of negative electrode] A negative electrode slurry was prepared by mixing 85.0 parts by weight of commercially available hard carbon (Kureha Corporation), 10.0 parts by weight of acetylene black, 5.0 parts by weight of sodium polyacrylate (PANa), and ion-exchanged water. The resulting negative electrode slurry was applied to one side of a 15 μm-thick aluminum foil that served as a negative electrode current collector, and then dried to obtain negative electrode 1. The negative electrode active material layer on the resulting negative electrode 1 had a thickness of 40 μm per side and a coating weight of 35.0 g / m. 2 It was.

[0092] [Measurement of initial charge capacity B of negative electrode half cell] Negative electrode 1 was vacuum-dried at 80°C for 12 hours and punched out into a 10 mm diameter circle to form the working electrode. Metallic sodium was used as the counter electrode, and a glass filter was used as the separator. The working electrode, separator, and counter electrode were stacked in this order to obtain an electrode stack. Subsequently, nonaqueous electrolyte solution 1 was used as the nonaqueous electrolyte solution, and a coin cell was fabricated.

[0093] The coin cell was fabricated at a current density of 0.02 mA / cm under a 25°C environment. 2 The battery was discharged at a constant current of 0.002 V. The discharge capacity at this time, that is, the initial charge capacity of sodium ions to the negative electrode, was 320 mAh / g, and the initial charge capacity B was 0.75 mAh.

[0094] [Assembly of sodium-ion secondary battery precursors] The obtained positive electrode precursor 1 and negative electrode 1 were punched into circles with a diameter of 10 mm, and a glass filter with a thickness of 200 μm was used as a separator. The positive electrode precursor 1, separator, and negative electrode 1 were laminated in this order to obtain a sodium ion secondary battery precursor, which is an electrode laminate.

[0095] [Injection, impregnation, sealing process] The sodium ion secondary battery precursor was inserted into an R2032 type coin cell, and after the non-aqueous electrolyte solution 1 was injected, the exterior was sealed to form a sodium ion secondary battery (precursor) before pre-doping.

[0096] [Pre-doping process] The resulting sodium ion secondary battery (precursor) was charged and discharged at a current density of 0.02 mA / cm in a 25°C environment using a charge-discharge device (TOSCAT-3100U) manufactured by Toyo Systems Co., Ltd. 2 The battery was charged at a constant current until a voltage of 4.3 V was reached, and then initially charged at a constant voltage of 4.3 V for 3 hours, and sodium ions were pre-doped into the negative electrode to produce two sodium-ion secondary batteries.

[0097] [Discharge capacity measurement] One of the obtained sodium ion secondary batteries was placed in a thermostatic chamber set at 25°C and charged at 0.02 mA / cm 2 The discharge capacity Q when constant current discharge was performed at a current value of 0.05 to 1.0 V is summarized in Table 2. The energy density of the sodium ion secondary battery was calculated from the obtained discharge capacity and the volume of the sodium ion secondary battery precursor, and the results are also summarized in Table 2.

[0098] [Charge / discharge cycle durability] The sodium ion secondary battery obtained in the above process was placed in a thermostatic chamber set at 25°C and charged to 0.02 mA / cm 2 The battery was charged at a constant current of 0.02 mA / cm until it reached 4.3 V, and then at a constant current of 0.02 mA / cm 2 This charge-discharge test was repeated 50 times, in which constant-current discharge was performed at a current value of 0.05 V down to 1.0 V. Table 2 shows the discharge capacity after 50 cycles and the discharge capacity retention rate at the 50th cycle relative to the 1st cycle.

[0099] The remaining sodium ion secondary battery obtained was placed in a thermostatic chamber set at 25°C and charged to 0.02 mA / cm 2 A constant current discharge was performed at a current value of 0.05 V until the voltage reached 1.0 V. The cell was then disassembled and the positive electrode was removed. The resulting positive electrode was immersed in diethyl carbonate to wash off the electrolyte adhering to the positive electrode. The diethyl carbonate adhering to the positive electrode was then vacuum dried in a 25°C environment, and XRD (X-ray diffraction) was measured under the conditions shown below. The measurement results are shown in Figure 1. X-ray diffraction source: Cu-Kα1 radiation (λ1=1.54059Å) Tube voltage: 40kV Tube current: 45mA Scanning range (2θ): 10° to 70° Scanning conditions: step scanning Step width: 0.02° Scanning speed: 2° / min

[0100] <Examples 2 to 4, Comparative Examples 1 to 5> Sodium ion secondary batteries were fabricated and evaluated under the same conditions as in Example 1, except that the positive electrode precursors and negative electrodes shown in Tables 1 and 2 were used.

[0101] [Table 1]

[0102] [Table 2-1]

[0103] [Table 2-2]

[0104] [Current density during pre-doping] <Examples 5 and 6, Comparative Example 6> A sodium ion secondary battery before pre-doping was produced in the same manner as in Example 1, and pre-doping was carried out under the conditions shown in Table 3.

[0105] [Table 3]

[0106] Increasing the current density during pre-doping reduces the charge capacity during constant current charging, but by continuing constant voltage charging after constant current charging, the charge capacity can be increased, and the discharge capacity as a sodium ion secondary battery can be increased.

[0107] [Applied voltage during pre-doping] <Examples 7 and 8> 70.0 parts by weight of the obtained Co3O4 powder, 10.0 parts by weight of sodium carbonate, 10.0 parts by weight of acetylene black, 10.0 parts by weight of PVdF (polyvinylidene fluoride), and NMP (N-methylpyrrolidone) were mixed to obtain a positive electrode slurry. The positive electrode slurry was applied to one side of a 15 μm thick aluminum foil, dried, and pressed to obtain a positive electrode precursor 8.

[0108] [Assembly of sodium-ion secondary battery precursors] The obtained positive electrode precursor 8 was punched into a circle with a diameter of 10 mm, and metallic sodium was used as a working electrode, a counter electrode, and a reference electrode. A glass filter was used as a separator, and the working electrode, separator, and counter electrode were stacked in this order to produce a sodium ion secondary battery precursor.

[0109] [Injection, impregnation, sealing process] The sodium ion secondary battery precursor was inserted into a bipolar coin cell, and after the non-aqueous electrolyte solution 1 was poured into it, the exterior was sealed to form a sodium ion secondary battery before pre-doping.

[0110] [Decomposition voltage of sodium carbonate] The sodium ion secondary battery thus produced before pre-doping was subjected to a test at a temperature of 25°C (Example 7) or 45°C (Example 8) in a voltage range of 1.5 to 4.5V (vs. Na + Cyclic voltammetry was measured in the range of 0.1% to 1.2% (Na / Na). The measurement results are shown in Figure 2.

[0111] As shown in Figure 2, in a 25°C environment, the current due to the oxidation reaction of sodium carbonate increases at voltages of 3.8 V or higher, and the peak current value at 4.3 V is 0.217 mAh / cm 2 In a 45°C environment, the current due to the oxidation reaction of sodium carbonate increased at voltages of 3.5 V or higher, and the peak current value at 4.1 V was 0.316 mAh / cm 2 showed. [Industrial Applicability]

[0112] The sodium ion secondary battery and its precursor of the present invention can be suitably used, for example, as an electricity storage element in hybrid drive systems for automobiles, assisting in instantaneous power peaks, and the like.

Claims

1. A sodium ion secondary battery precursor comprising a positive electrode precursor, a negative electrode, a separator, and a non-aqueous electrolyte solution containing sodium ions, the positive electrode precursor includes a positive electrode current collector and a positive electrode active material layer provided on one or both surfaces of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material and sodium carbonate, the positive electrode active material including a transition metal oxide capable of absorbing and releasing sodium ions; the negative electrode has a negative electrode current collector and a negative electrode active material layer provided on one or both surfaces of the negative electrode current collector and containing a negative electrode active material, A sodium ion secondary battery precursor, wherein C1 / 100×Q1×A<B and 5.0≦C2≦10.0 are satisfied, where C1 (wt %) is the weight ratio of the positive electrode active material contained in the positive electrode active material layer, C2 (wt %) is the weight ratio of the sodium carbonate, Q1 (mAh / g) is the initial charge capacity of the positive electrode active material, A (g) is the weight per one side of the positive electrode active material layer, and B (mAh) is the initial charge capacity of a negative electrode half cell formed from the negative electrode.

2. The positive electrode active material is Na ｘ M1 ｙ M2 ｚ O ２ (wherein M1 and M2 are each one selected from Fe, Mn, Co, Ni, Ti, or Cu, and satisfy 0.5≦x≦1.0, 0.1<y<0.9, 0.1<z<0.9, and 0.95<y+z<1.05).

3. The sodium ion secondary battery precursor according to claim 2 , wherein M1 and M2 are Fe, Mn, or Co, respectively.

4. A method for producing a sodium ion secondary battery, comprising applying a voltage of 3.5 V or more and 4.4 V or less to the sodium ion secondary battery precursor according to any one of claims 1 to 3 at an environmental temperature of 20 ° C. or more and 50 ° C. or less to decompose the sodium carbonate.

5. 5. The method for producing a sodium ion secondary battery according to claim 4, wherein after a predetermined voltage is reached by constant current charging, constant voltage charging is continued at that voltage.

6. The current density in the constant current charging is 0.01 mA / cm ２ 0.50mA / cm or more ２ The method for producing a sodium ion secondary battery according to claim 5, wherein:

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