Sodium titanium phosphate and its uses

Abstract

This invention provides at least one of sodium titanium phosphate, a negative electrode active material for a sodium secondary battery with output characteristics superior to conventional aqueous sodium secondary batteries, and a sodium secondary battery incorporating the negative electrode active material. In particular, it provides a negative electrode active material that yields an aqueous sodium secondary battery with output characteristics superior to conventional aqueous sodium secondary batteries using NASICON-type NaTi2(PO4)3 as the negative electrode active material. 1+ x The sodium titanium phosphate salt represented by Ti2(PO4)3 (where 0≤x≤2) is characterized by having a surface layer comprising Na5Ti(PO4)3.

Description

Technical Field

[0001] This disclosure relates to sodium titanium phosphate and its uses. Background Technology

[0002] Sodium-ion batteries, which do not use lithium, a rare metal, not only have advantages in terms of resources and cost, but also have the potential to excel in the application of new materials and high-rate charge-discharge characteristics.

[0003] Among sodium secondary batteries to date, batteries with the same electrolyte as existing lithium secondary batteries, primarily non-aqueous electrolytes, have been studied.

[0004] Recently, the biggest weakness of aqueous batteries, namely the narrow potential window of 1.23V due to the decomposition of water, has been reported, which exceeds 2V by increasing the concentration of electrolyte (Non-Patent Document 1), raising expectations for the practical application of aqueous sodium secondary batteries.

[0005] Especially for the increasing size of batteries, from the perspectives of cost and ensuring safety, it can be said that aqueous sodium secondary batteries using aqueous electrolytes are suitable.

[0006] Regarding another topic concerning the practical application of sodium-based secondary batteries, namely the anode material, it has been found that NASICON-type NaTi2(PO4)3 functions well, but has low cycle stability, and its improvement has become a research topic (Non-Patent Literature 2).

[0007] Existing technical documents

[0008] Non-patent literature

[0009] Non-patent literature 1: Ruben-Simon et al, ACS Energy Letters, 2, 2005 (2017).

[0010] Non-patent literature 2: Sun II Park et al, Journal of The Electrochemical Society, 158(10), A1067-A1070 (2011) Summary of the Invention

[0011] The technical problem that the invention aims to solve

[0012] The purpose of this disclosure is to provide at least one of sodium titanium phosphate, a negative electrode active material for a sodium secondary battery that provides output characteristics superior to conventional aqueous sodium secondary batteries, and a sodium secondary battery having the negative electrode active material. In particular, the purpose is to provide a negative electrode active material that provides an aqueous sodium secondary battery with output characteristics superior to conventional aqueous sodium secondary batteries having NASICON-type NaTi2(PO4)3 as the negative electrode active material.

[0013] Technical solutions for solving technical problems

[0014] In this disclosure, it was discovered that sodium titanium phosphate salt compositions having surface layers comprising sodium titanium phosphate salts of different compositions exhibit superior output characteristics compared to previously known sodium secondary battery anode materials. By using them as anodes, sodium secondary batteries with excellent output characteristics can be constructed.

[0015] That is, as described in the claims, the essence of this disclosure is as follows.

[0016] [1] A type of general formula Na 1+x A sodium titanium phosphate salt represented by Ti2(PO4)3 (where 0≤x≤2) is characterized by having a surface layer comprising Na5Ti(PO4)3.

[0017] [2] According to the sodium titanium phosphate salt described in [1], wherein the crystal structure of the sodium titanium phosphate salt is a NASICON type crystal structure.

[0018] [3] According to [1] or [2] sodium titanium phosphate, wherein the surface layer is relative to the sodium titanium phosphate of the general formula Na 1+x Ti2(PO4)3 (where 0≤x≤2) represents sodium titanium phosphate salts greater than 0% by mass and less than 5% by mass.

[0019] [4] The sodium titanium phosphate salt according to any one of [1] to [3], wherein the thickness of the surface layer is more than 0 nm and less than 50 nm.

[0020] [5] The sodium titanium phosphate salt according to any one of [1] to [4], wherein the crystallite diameter is 10 nm or more and 100 nm or less.

[0021] [6] The method for manufacturing sodium titanium phosphate according to any one of [1] to [5] includes the following steps: calcining a mixture containing a sodium source, a phosphate source and a titanium source at a temperature of 150°C or higher and 400°C or lower to obtain a first-calcined product, pulverizing the first-calcined product and calcining it at a temperature of 400°C or higher and 900°C or lower.

[0022] [7] A negative electrode active material comprising any one of [1] to [5] sodium titanium phosphate.

[0023] [8] A sodium secondary battery comprising a positive electrode, an electrolyte and a negative electrode comprising any one of [1] to [5] sodium titanium phosphate salt.

[0024] [9] According to the sodium secondary battery of [8], wherein the electrolyte is an aqueous electrolyte.

[0025] Invention Effects

[0026] According to this disclosure, at least one of the following can be provided: sodium titanium phosphate salt, a negative electrode active material for a sodium secondary battery with output characteristics superior to conventional aqueous sodium secondary batteries, and a sodium secondary battery incorporating the negative electrode active material. Furthermore, a negative electrode active material can be provided that yields an aqueous sodium secondary battery with output characteristics superior to conventional aqueous sodium secondary batteries using NASICON-type NaTi2(PO4)3 as the negative electrode active material. Attached Figure Description

[0027] Figure 1 The image shows the XRD patterns of sodium transition metal phosphates of Examples 1-3 and Comparative Examples (a: Comparative Example 1, b: Example 1, c: Example 2, d: Example 3, e: PDF pattern (ICDD: 33-1296 NaTi2(PO4)3).

[0028] Figure 2 The diagram shows the XRD patterns of NaTi2(PO4)3 in Examples 5 and 6 (f: Example 5, g: Example 6, h: PDF pattern (Na5Ti(PO4)3)).

[0029] Figure 3A This is a cross-sectional TEM image showing the sodium transition metal phosphate of Example 2.

[0030] Figure 3B This is a cross-sectional TEM image showing the sodium transition metal phosphate of Comparative Example 1.

[0031] Figure 4 The figure shows the results of charge-discharge cycle tests of Examples 1 to 3 and Comparative Example 1 (a (▲): Example 1, b (■): Example 2, c (◆): Example 3, d (●): Comparative Example 1). Detailed Implementation

[0032] Hereinafter, an example of an embodiment of the sodium titanium phosphate salt of this disclosure will be described.

[0033] <Sodium Titanium Phosphate>

[0034] This implementation method is based on the general formula Na. 1+xSodium titanium phosphate salt represented by Ti2(PO4)3 (where 0≤x≤2) is characterized by having a surface layer containing Na5Ti(PO4)3. Through the surface layer containing Na5Ti(PO4)3, sodium ions (Na+) on the surface of the sodium titanium phosphate salt (hereinafter also referred to as "NaTP salt")... + It spreads easily.

[0035] In this embodiment, the NaTP salt is a phosphate compound containing sodium and titanium, specifically composed of the general formula Na... 1+x Sodium titanium phosphate represented by Ti2(PO4)3 (where 0≤x≤2), more preferably NaTi2(PO4)3 (i.e., general formula Na 1+x In Ti2(PO4) (x=0), the composition of the NaTP salt can be determined by known compositional analysis methods, such as inductively coupled plasma light emission analysis (ICP analysis), atomic absorption spectrometry, TEM-EDS, etc., and can be determined by ICP analysis using a conventional ICP light emission analysis device (ICPS-8100, manufactured by Shimadzu Corporation).

[0036] The preferred crystal structure of the NaTP salt in this embodiment is a NASICON-type crystal structure (so-called NASICON-type NaTP salt). The identification of the NASICON-type crystal structure can be performed by comparing the XRD pattern of the NaTP salt in this embodiment with the PDF pattern (ICDD:33-1296 NaTi2(PO4)3).

[0037] The NaTP salt of this embodiment has a surface layer (hereinafter also simply referred to as the "surface layer") comprising Na5Ti(PO4)3. This is achieved by having a surface layer with the general formula Na... 1+x The surface layer of the sodium titanium phosphate shown in Ti2(PO4)3 (where 0≤x≤2) has an excess of sodium compared to that of the sodium phosphate. Therefore, this surface layer functions as a sodium buffer layer during high-output discharge, and is considered to exhibit high-output characteristics.

[0038] The surface layer only needs to be located on at least a portion of the NaTP salt without hindering sodium diffusion, preferably on at least a portion of the surface of the NaTP salt. Examples of a surface layer located on at least a portion of the NaTP salt include the presence of an interface between the NaTP salt and Na5Ti(PO4)3, i.e., the surface layer forming an interface with the NaTP salt. It is believed that the presence of an interface between the NaTP salt and Na5Ti(PO4)3 results in a sodium diffusion coefficient equivalent to that of NaTP salt without a surface layer.

[0039] The surface layer does not need to cover the entire surface of the NaTP salt and can be in the form of an island. That is, the surface layer can be at least one of a composition containing Na5Ti(PO4)3 and Na5Ti(PO4)3 present in at least a portion of the surface of the NaTP salt, preferably Na5Ti(PO4)3.

[0040] The surface layer only needs to contain Na5Ti(PO4)3, or it can contain TiO2 in addition to Na5Ti(PO4)3, but it is preferred to be composed of Na5Ti(PO4)3, and more preferably composed of Na5Ti(PO4)3 with a NASICON-type crystal structure.

[0041] In this embodiment, the NaTP salt only needs to have a surface layer on all or at least a portion of its surface, and it is sufficient that it has a surface layer on at least a portion of its surface. Furthermore, it is sufficient that all or at least a portion, preferably at least a portion, of the surface of the NaTP salt particles contains Na5Ti(PO4)3. The properties of the Na5Ti(PO4)3 contained in the surface layer are not particularly limited, and examples include at least one of crystalline and amorphous substances, and more specifically, at least one of dense and porous substances. Furthermore, the surface layer preferably contains Na5Ti(PO4)3 in a state formed by the reaction of a sodium source, a phosphorus source, and a titanium source on the surface of the NaTP salt. This facilitates the suppression of polarization (hysteresis) during charge-discharge cycles. The presence of the surface layer as a coating film can be confirmed by TEM observation (transmission electron microscopy). In TEM observation, the surface layer is observed as a different field of view than the NaTP salt, and the surface layer is observed as a field of view with a hue darker than that of the NaTP salt.

[0042] Due to the presence of sodium ions (Na) + Due to the tendency for sodium to diffuse easily, the lower limit of the surface layer ratio (hereinafter also referred to as "coating amount") in this embodiment is preferably more than 0% by mass relative to NaTP salt, more preferably 0.01% by mass or more, and even more preferably 0.1% by mass or more. The upper limit is preferably 20% by mass or less relative to NaTP salt, more preferably 15% by mass or less, and even more preferably 5% by mass or less. These upper and lower limits can be any combination. Therefore, the surface layer ratio in this embodiment is preferably, for example, more than 0% by mass and less than 20% by mass relative to NaTP salt, more preferably 0.01% by mass and less than 15% by mass, and even more preferably 0.1% by mass and less than 5% by mass. The method for determining the coating amount is arbitrary; for example, quantitative analysis using TEM-EDS can be used. If the coating amount exceeds 20% by mass, a surface layer that does not hinder the diffusion of sodium cannot be formed, and the diffusion coefficient of sodium ions is significantly reduced.

[0043] The thickness of the surface layer is arbitrary, as long as it does not impede the diffusion of sodium.

[0044] The lower limit of the surface layer thickness is preferably greater than 0 nm, and more preferably greater than 0.1 nm. The upper limit is preferably less than 50 nm, more preferably less than 40 nm, and even more preferably less than 5 nm. These upper and lower limits can be any combination. Therefore, in this embodiment, the surface layer thickness is preferably greater than 0 nm and less than 50 nm, and more preferably greater than 0.1 nm and less than 5 nm. The surface layer thickness can be determined by TEM observation. Examples of surface layer thicknesses observed in a TEM image (i.e., the surface layer that can be confirmed in a TEM image, and the maximum thickness of the surface layer observed in that TEM image) are greater than 0 nm or greater than 0.1 nm, and less than 50 nm, less than 40 nm, less than 5 nm, or less than 3 nm.

[0045] The lower limit of the crystallite diameter of the NaTP salt in this embodiment is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more. The upper limit is preferably 100 nm or less, more preferably 50 nm or less, and even more preferably 40 nm or less. These upper and lower limits can be any combination. Therefore, the crystallite diameter of the NaTP salt in this embodiment is preferably, for example, 10 nm or more and 100 nm or less, more preferably 20 nm or more and 50 nm or less, more preferably 20 nm or more and 40 nm or less, and even more preferably 30 nm or more and 40 nm or less. By setting the crystallite diameter to this range, the NaTP salt of this embodiment can more effectively insert and de-insert sodium, and can be expected to have high output characteristics as a negative electrode active material in sodium secondary batteries.

[0046] In this embodiment, "crystal diameter" is based on the general formula Na. 1+x The diameters (hereinafter referred to as "WH diameters") of two or more XRD peaks in the XRD pattern of sodium titanium phosphate, represented by Ti2(PO4)3 (where 0≤x≤2), determined by the Williamson-Hall method. Specifically, for those belonging to the general formula Na... 1+x Two or more XRD peaks of Ti2(PO4)3 (where 0≤x≤2), preferably belonging to the general formula Na 1+x All XRD peaks of Ti2(PO4)3 (where 0≤x≤2) were plotted as follows. The following first-order approximation was obtained by the least squares method from the obtained plotted points, where the reciprocal of the y-intercept of the first-order approximation is the crystallite diameter.

[0047] <Plotting Points>

[0048] Y=(β·sinθ) / λ

[0049] X=sinθ / λ

[0050] <First-order approximation>

[0051] Y = 2η·X + (1 / ε) ... (Equation 1)

[0052] In these formulas, β is the half-width at half maximum (°), θ is the diffraction angle (°), λ is the wavelength of the X-ray source (nm), η is the non-uniform strain, and ε is the crystallite diameter (Å), with 1 / ε in the first approximation being the y-intercept. It should be noted that there are no particular limitations on the XRD peaks used when calculating the WH diameter in this embodiment; for example, all XRD peaks belonging to NaTP can be used.

[0053] The NaTP salt of this embodiment exhibits high output characteristics when used in a sodium secondary battery with it as the negative electrode active material. The configuration of the sodium secondary battery and the conditions for charge-discharge tests are shown below.

[0054] (Sodium secondary battery)

[0055] Electrolyte: 17 mol / kg NaClO4 aqueous solution

[0056] Negative electrode: composed of 70% by mass of NaTP salt, 25% by mass of acetylene black, and...

[0057] Anode composed of 5% by mass of polytetrafluoroethylene

[0058] Positive electrode: composed of 70% by mass of Na2Ni[Fe(CN)6], 25% by mass of acetylene black, and...

[0059] Positive electrode composed of 5% by mass of polytetrafluoroethylene

[0060] (Charge and discharge test conditions)

[0061] Current density: (1-5 cycles) 2 mA / cm² 2

[0062] (6-10 cycles) 5mA / cm 2

[0063] (11-15 cycles) 10 mA / cm 2

[0064] (16-20 cycles) 15 mA / cm 2

[0065] (21-25 cycles) 20 mA / cm 2

[0066] (25-30 cycles) 25mA / cm 2

[0067] (31-35 cycles) 2mA / cm 2

[0068] Voltage: -0.9V to -0.3V (vs Ag / AgCl reference electrode)

[0069] Charge / discharge temperature: 25℃

[0070] <Method for manufacturing NaTP salt with a surface layer>

[0071] The method for manufacturing the NaTP salt in this embodiment is arbitrary. An example is a method comprising the following steps: calcining a mixture containing a sodium source, a phosphate source, and a titanium source at 150°C or higher and 400°C or lower to obtain a first-calcined product; pulverizing the first-calcined product; and then calcining it at 400°C or higher and 900°C or lower (hereinafter also referred to as the "calcination process"). This yields a NaTP salt having a surface layer containing Na₅Ti(PO₄)₃.

[0072] The mixture supplied for the calcination process contains a sodium source, a phosphate source, and a titanium source (hereinafter, they are also collectively referred to as "precursors"), and is a composition containing precursors.

[0073] The sodium source can be any compound containing sodium (Na), preferably one or more selected from the group consisting of sodium carbonate, sodium hydroxide and sodium chloride, more preferably at least one of sodium carbonate and sodium hydroxide, and even more preferably sodium carbonate.

[0074] The phosphoric acid source can be any compound containing phosphoric acid (PO4), preferably one or more selected from the group consisting of pyrophosphoric acid, polyphosphoric acid and ammonium dihydrogen phosphate, and more preferably ammonium dihydrogen phosphate.

[0075] The titanium source can be any compound containing titanium (Ti), and examples include at least one of titanium oxides and organotitanium compounds. Preferred titanium oxides include titanium dioxide (TiO2). Preferred organotitanium compounds include one or more selected from the group consisting of titanium-containing (alkane)alkoxides, acylates, and chelates, such as titanium-containing (alkane)alkoxides or tetrabutyl titanate (Ti(OCH2CH2CH2CH3)4). Tetrabutyl titanate is a preferred titanium source.

[0076] Sodium carbonate, diammonium hydrogen phosphate, and tetrabutyl titanate are examples of particularly preferred precursors.

[0077] The composition of the above mixture is preferably an excess of sodium relative to NaTP salt, and particularly preferably an excess of sodium relative to general formula Na. 1+xThe composition of sodium titanium phosphate salt as shown in Ti2(PO4)3 (where 0≤x≤2) is excessive. For example, examples can be given of mixed compositions in which the ratio of Na [mol] to Ti [mol] is greater than 1.0 or 1.1, and also less than 3, less than 2, or less than 1.5. It is believed that by having such a composition, a surface layer is formed on the surface of the NaTP salt particles after pre-firing and formal firing as described later.

[0078] Such a mixture only needs to be relative to the target general formula Na 1+x Sodium titanium phosphate salts represented by Ti2(PO4)3 (where 0≤x≤2) can be obtained by mixing precursors in a manner that results in an excess of sodium. For example, in the case of manufacturing NaTP salts represented by NaTi2(PO4)3, sodium and titanium sources are mixed in a ratio of Na[mol] to Ti[mol] of more than 0.5 or more than 0.55, and in a manner that is less than 1.5, less than 1, or less than 0.75; in the case of manufacturing NaTP salts represented by Na2Ti2(PO4)3, sodium and titanium sources are mixed in a ratio of Na[mol] to Ti[mol] of more than 1.0 or more than 1.1, and in a manner that is less than 3, less than 2, or less than 1.5.

[0079] The above mixture is obtained by uniformly mixing the precursors. Examples of mixing methods include at least one of wet mixing and dry mixing, with wet mixing being a further example. A particularly preferred mixing method is to stir the mixed solution containing the precursor and solvent, or to dry it while stirring. Examples of solvents include at least one of water and alcohol, with water being a further example. Stirring is only required to ensure uniform mixing of the precursors; examples include stirring the mixed solution at 100 rpm to 500 rpm, with more stirring required for a larger volume of mixed solution. Drying is only required to remove the solvent from the mixed solution; examples include temperatures above 60°C or 70°C in atmospheric conditions, and also below 150°C, 120°C, or 100°C. Thus, a mixture in which the precursors are uniformly mixed is obtained.

[0080] In the manufacturing method of this embodiment, the mixture is fired at a temperature of 150°C or higher and 400°C or lower. Examples of firing methods for obtaining a single-fired product (hereinafter also referred to as "pre-firing") include firing in an oxidizing atmosphere, preferably atmospheric, at a temperature of 150°C or higher, 250°C or higher, 300°C or higher and 400°C or lower, less than 400°C or 370°C or lower. The pre-firing time is arbitrary and can be exemplified as 30 minutes or more and 10 hours or less. Through pre-firing, the sodium concentration becomes a gradient composition between the surface and interior of the particles of the mixture, with the sodium concentration at the particle surface being higher than that inside the particles.

[0081] After pre-firing, the resulting primary fired material is pulverized. The pulverization method can be any method that removes the slow agglomeration of the primary fired material, such as dry pulverization using a mortar and pestle.

[0082] In the manufacturing method of this embodiment, the pulverized first-fired material is fired at 400°C or higher and 900°C or lower (hereinafter also referred to as "formal firing"). Examples of formal firing methods include firing in an oxidizing atmosphere, preferably atmospheric, at 400°C or higher, exceeding 400°C, or 600°C or higher and 900°C or lower, 800°C or lower, or 750°C or lower. The formal firing time is arbitrary and can be exemplified as 30 minutes or more and 10 hours or less. Through formal firing, a NaTP salt with a surface layer is generated from a mixture having a sodium concentration gradient.

[0083] <Negative Electrode Active Material>

[0084] Next, an example of an embodiment of a negative electrode active material comprising the NaTP salt of this disclosure will be described.

[0085] In this embodiment, the "negative electrode active material" is the electrode active material of the electrode with the low potential in the electrode of the electrochemical device, especially the electrode active material of the negative electrode of a sodium secondary battery.

[0086] The negative electrode active material of this embodiment includes the NaTP salt of this embodiment (i.e., the NaTP salt having a surface layer), or may contain only the NaTP salt of this embodiment (i.e., only the NaTP salt having a surface layer). On the other hand, the negative electrode active material of this embodiment may contain active materials other than the NaTP salt of this embodiment, such as active materials such as sodium transition metal compounds.

[0087] In this embodiment, the negative electrode active material, in addition to the carbon layer, may have a coating layer on part or all of its surface, preferably a conductive coating layer (i.e., a conductive layer). In this case, the coating layer may exist on a surface layer containing Na5Ti(PO4)3. However, the coating layer may also exist on the surface of NaTP salt.

[0088] <Sodium Secondary Battery>

[0089] Next, an example of an embodiment of a sodium secondary battery will be described, characterized by having a positive electrode, an electrolyte, and a negative electrode containing the NaTP salt disclosed herein.

[0090] In this embodiment, a "sodium secondary battery" refers to a battery that uses sodium ions (Na+) to generate sodium secondary ions. +Electrochemical devices that generate charge and discharge through the insertion and detachment of sodium ions, and have the same meaning as sodium secondary batteries, sodium-ion secondary batteries, sodium-ion batteries, sodium storage batteries, Na secondary batteries, Na-ion batteries, or Na storage batteries.

[0091] "Non-aqueous electrolyte" is an electrolyte that contains a non-aqueous solvent as a solvent, while "aqueous electrolyte" is an electrolyte that contains an aqueous solvent as a solvent.

[0092] A "non-aqueous sodium secondary battery" is a sodium secondary battery that uses a non-aqueous electrolyte as its electrolyte, while an "aqueous sodium secondary battery" is a sodium secondary battery that uses an aqueous electrolyte as its electrolyte.

[0093] <Negative electrode>

[0094] The negative electrode only needs to have a negative electrode mixture containing a negative electrode active material and a current collector, wherein the negative electrode active material includes the NaTP salt of this embodiment.

[0095] The negative electrode mixture comprises a negative electrode active material, a binder and a conductive material, and additives as needed. The binder, conductive material and additives can each be made from known materials.

[0096] The adhesive may be exemplified by one or more selected from the group consisting of fluoropolymers, polyethylene, polypropylene, SBR materials and imide materials, and may further be exemplified by one or more selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE) and ethylene-tetrafluoroethylene copolymer (ETFE).

[0097] The conductive material may be selected from one or more of the following: carbon materials, conductive fibers such as metal fibers, metal powders such as copper, silver, nickel, and aluminum, and organic conductive materials such as polyphenylene derivatives. Preferred carbon materials may include graphite, soft carbon, hard carbon, carbon black, Ketjen black, acetylene black, activated carbon, carbon nanotubes, carbon fibers, and mesoporous carbon.

[0098] The negative electrode mixture can be manufactured using known methods by mixing the negative electrode active material, binder, and conductive material in the desired ratio.

[0099] <Positive electrode>

[0100] The positive electrode only needs to have a positive electrode mixture containing positive electrode active material, a current collector, and additives as needed.

[0101] The positive electrode mixture contains positive electrode active material, binder and conductive material, and additives as needed.

[0102] The positive electrode active material can be any material that does not impede the insertion and removal of sodium ions in the negative electrode active material, and examples include at least one of sodium-containing transition metal oxides, sodium-containing polyanionic compounds, and carbon-based materials.

[0103] The adhesive and conductive material can be any known type, and can be the same as the adhesive and conductive material that can be used in the negative electrode compound mentioned above.

[0104] The positive electrode mixture can be manufactured using known methods by mixing the positive electrode active material, binder, and conductive material in the desired ratio.

[0105] Electrolyte

[0106] The electrolyte can be either a non-aqueous electrolyte or an aqueous electrolyte, preferably an aqueous electrolyte.

[0107] The electrolyte is a sodium salt, preferably a soluble sodium salt. Examples of preferred electrolytes include one or more selected from the group consisting of NaCl, Na₂SO₄, NaNO₃, NaClO₄, NaOH, and Na₂S. For ease of processing, the electrolyte is preferably one or more selected from the group consisting of NaCl, Na₂SO₄, NaNO₃, and NaClO₄, and more preferably at least one of NaCl and NaClO₄.

[0108] There is no particular limitation on the concentration of electrolyte in the electrolyte. From the viewpoint of improving the energy density of sodium secondary batteries, it is preferable to have a high concentration of electrolyte (sodium salt concentration) in the electrolyte. For example, a sodium salt concentration of 1 mol / kg (1 m) or higher and below saturation solubility can be used.

[0109] The electrolyte may contain additives. Additives are not particularly limited, but examples include one or more selected from the group consisting of succinic acid, glutamic acid, maleic acid, citraconic acid, gluconic acid, itaconic acid, diethylene glycol, cyclohexanedicarboxylic acid, cyclopentanetetracarboxylic acid, 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, methyl methanesulfonate, sulfolane, dimethyl sulfone, and N,N-dimethylmethanesulfonamide. The content of the additives, expressed as a mass ratio of the additives to the electrolyte, may be 0.01% by mass or more and 10% by mass or less.

[0110] <Other Components>

[0111] Other components such as current collectors and separators for the positive and negative electrodes can use known components used in sodium secondary batteries and lithium secondary batteries.

[0112] Compared with conventional sodium secondary batteries, especially conventional aqueous sodium secondary batteries that use NASICON-type NaTi2(PO4)3 as the negative electrode active material, sodium secondary batteries with a negative electrode containing the NaTP salt of this embodiment exhibit high cycle stability. Example

[0113] The present disclosure will now be described through examples. However, the present disclosure is not limited to these examples.

[0114] <Evaluation of Crystallinity>

[0115] The crystal structure of NaTP salt was determined under the following conditions using a benchtop X-ray analysis system (MiniFlex600 / ASC-8, manufactured by Rigaku Corporation).

[0116] Target: Cu

[0117] Output: 0.6kW (15mA-40kV)

[0118] Step scan: 0.02° (2θ / θ)

[0119] Measurement time: 0.05 seconds

[0120] <Composition Analysis>

[0121] Regarding the composition of NaTP salt, 50 mg of the sample was dissolved in an aqueous solution containing 10 mL of 35% hydrochloric acid and 1 mL of 35% hydrogen peroxide, and ICP analysis was performed to determine the composition. The ICP analysis was conducted using an ICP luminescence analyzer (instrument name: ICPS-8100, manufactured by Shimadzu Corporation).

[0122] <Observations on the Surface Layer>

[0123] The measurements were performed using a transmission electron microscope (JEM-2100, manufactured by NEC Corporation) and a thermal field emission scanning electron microscope (JSM-7600F, manufactured by NEC Corporation).

[0124] <Sodium Secondary Battery>

[0125] (The production of the positive electrode)

[0126] Nickel ferrocyanate (Na₂Ni[Fe(CN)₆)) with a cubic crystal structure was used as the positive electrode active material. Na₂Ni[Fe(CN)₆] was synthesized by the following method: 10 mmol of 2.69 g of Ni(OCOCH₃)₂·4H₂O was dissolved in 175 mL of H₂O and 25 mL of DMF (N,N-dimethylformamide·HCON(CH₃)₂) to obtain a first solution. Conversely, 10 mmol of 4.84 g of Na₄[Fe(CN)₆]·10H₂O and 7 g of NaCl were dissolved in 175 mL of H₂O to obtain a second solution.

[0127] The first solution was slowly added to the second solution, and the mixture was stirred at room temperature for 72 hours to allow the reaction to proceed, resulting in a precipitate. The precipitate was centrifuged and recovered, washed with methanol, and dried in air to obtain nickel hexacyanoferrate (Na2Ni[Fe(CN)6]) with a cubic crystal structure.

[0128] The obtained Na2Ni[Fe(CN)6], acetylene black (hereinafter also referred to as "AB") and polytetrafluoroethylene (hereinafter also referred to as "PTFE") are mixed in a mass ratio of 70:25:5 and then shaped into granules with a diameter of 3 mm, which are used as positive electrodes (positive electrode mixtures).

[0129] (Making the negative electrode)

[0130] Using a planetary ball mill, the negative electrode active material and AB were mixed at a mass ratio of 84:16 at 400 rpm under an Ar atmosphere for 1 hour to obtain a mixture. Then, the mixture was carbotherm-treated at 800°C under an Ar gas flow for 1 hour, and then PTFE was mixed in at a mass ratio of 95:5 and shaped into granules with a diameter of 2 mm, which were used as the negative electrode (negative electrode mixture).

[0131] (Fabrication of an aqueous sodium secondary battery)

[0132] An aqueous sodium secondary battery is fabricated with a positive electrode mixture at the working electrode (equivalent to the positive electrode), a negative electrode mixture at the counter electrode (equivalent to the negative electrode), a silver chloride electrode (Ag / AgCl) at the reference electrode, and an aqueous NaClO4 solution with an electrolyte concentration (NaClO4 concentration) of 17m in the electrolyte.

[0133] (Synthesis of NaTP salt)

[0134] <Example 1>

[0135] In 40 mL of an ethanol solution containing dissolved Ti(OCH2CH2CH2CH3)4, citric acid and hydrogen peroxide (H2O2) in a molar ratio of 2 relative to Ti(OCH2CH2CH2CH3)4 were mixed to obtain a mixed solution. Relative to the Ti(OCH2CH2CH2CH3)4 in the obtained mixed solution, 1.05 molar amounts of Na2CO3 (Na to Ti molar ratio of 0.525) and 1 molar amount of NH4H2PO4 were added. The mixture was then evaporated and dried at 60°C for 30 minutes while stirring at 500 rpm, followed by evaporation and drying at 80°C for 1–2 hours. The solid obtained by heating at 350°C for 5 hours in air was then pulverized and mixed, and heated at 700°C for 12 hours in air to obtain NaTi2(PO4)3 with a surface layer containing Na5Ti(PO4)2, which was used as the NaTP salt of this embodiment. The crystallite diameter of the NaTP salt in this embodiment was 36 nm, and the surface layer had a mass ratio of 1% by mass.

[0136] Using a planetary ball mill, the NaTP salt and AB were mixed at 400 rpm for 1 hour under an argon atmosphere with a mass ratio of 84:16. The resulting mixture was then calcined at 800°C for 1 hour in an argon stream to obtain a negative electrode active material containing a NASICON-type structure of NaTi2(PO4)3 (NASICON-type NaTi2(PO4)3) with a surface layer.

[0137] <Example 2>

[0138] Except that the molar amount of Na2CO3 was 1.1 times (the molar ratio of Na to Ti was 0.55), NaTi2(PO4)3 with a NASICON-type structure and a negative electrode active material with a surface layer were obtained by the same method as in Example 1. The crystallite diameter of the NaTP salt in this example was 38 nm, and the mass ratio of the surface layer was 2 by mass.

[0139] <Example 3>

[0140] Except that the molar amount of Na2CO3 was 1.15 times that of Ti (the molar ratio of Na to Ti was 0.575), NaTi2(PO4)3 with a NASICON-type structure and a negative electrode active material with a surface layer were obtained by the same method as in Example 1. The crystallite diameter of the NaTP salt in this example was 41 nm.

[0141] <Example 4>

[0142] Except that the molar amount of Na2CO3 was 1.2 times that of Ti (the molar ratio of Na to Ti was 0.6), NaTi2(PO4)3 with a NASICON-type structure and a negative electrode active material with a surface layer were obtained by the same method as in Example 1. The crystallite diameter of the NaTP salt in this example was 49 nm.

[0143] <Example 5>

[0144] Except that the molar amount of Na2CO3 was 1.5 times that of Ti (the molar ratio of Na to Ti was 0.75), NaTi2(PO4)3 with a NASICON-type structure and a negative electrode active material with a surface layer were obtained by the same method as in Example 1. The crystallite diameter of the NaTP salt in this example was 50 nm.

[0145] <Example 6>

[0146] Except for using twice the molar amount of Na₂CO₃ (the molar ratio of Na to Ti is 1), NaTi₂(PO₄)₃ with a NASICON-type structure and a negative electrode active material with a surface layer were obtained by the same method as in Example 1. The crystallite diameter of the NaTP salt in this example is 50 nm.

[0147] <Comparative Example 1>

[0148] Except that the molar amount of Na₂CO₃ was 1 (the molar ratio of Na to Ti was 0.5), NaTi₂(PO₄)₃ and the negative electrode active material without a NASICON-type structure containing a surface layer of Na₅Ti(PO₄)₃ were obtained by the same method as in Example 1. The crystallite diameter of the NaTP salt in this comparative example was 34 nm.

[0149] The XRD patterns of the negative electrode active materials obtained in the examples and comparative examples are shown in... Figure 1 and Figure 2 XRD peaks of NaTi2(PO4)3 were confirmed in all cases. In Examples 5 and 6, XRD peaks belonging to Na5Ti(PO4)3 were also confirmed, indicating that NaMP salts with a surface layer (coating layer) containing Na5Ti(PO4)3 were obtained in the examples.

[0150] Figure 3A The image shows a cross-sectional TEM image of NaTi2(PO4)3 from Example 2. Additionally, Figure 3B The image shows a cross-sectional TEM image of NaTi2(PO4)3 from Comparative Example 1. In NaTi2(PO4)3 from Example 2, it was confirmed that NaTi2(PO4)3 was white, the surface layer containing Na5Ti(PO4)3 was gray, and the outer surface of the particles (background) was black (see reference). Figure 3AThis confirms that there is a surface layer of 2 nm to 7 nm, and further, 5 nm, between NaTi2(PO4)3 and the particles. In contrast, it confirms that in Comparative Example 1, the NaTi2(PO4)3 is gray, and the outer surface of the particles (background) is black (see reference). Figure 3B ).Depend on Figure 3A and Figure 3B It can be confirmed that the surface structures of the NaTi2(PO4)3 particles in Example 2 and Comparative Example 1 are different, and that the NaTi2(PO4)3 in Example 2 has a surface layer on its particle surface.

[0151] <Example 1 of the test> (charge-discharge cycle test)

[0152] Using an aqueous sodium secondary battery with the negative electrode active material of the examples or comparative examples, the discharge capacity of each charge-discharge cycle was measured by repeated charge-discharge cycles under the following conditions. The ratio (%) of the discharge capacity of the 30th cycle to the discharge capacity of the 1st cycle was used as the capacity retention rate to evaluate the output characteristics.

[0153] Current density: (1-5 cycles) 2 mA / cm² 2

[0154] (6-10 cycles) 5mA / cm 2

[0155] (11-15 cycles) 10 mA / cm 2

[0156] (16-20 cycles) 15 mA / cm 2

[0157] (21-25 cycles) 20 mA / cm 2

[0158] (26-30 cycles) 25 mA / cm 2

[0159] (31-35 cycles) 2mA / cm 2

[0160] Voltage: -0.9V to -0.3V (vs Ag / AgCl reference electrode)

[0161] Charge / discharge temperature: 25℃

[0162] Figure 4 The discharge capacity for 1 to 35 cycles is shown. Additionally, the capacity retention rate is shown in Table 1 below.

[0163] [Table 1]

[0164]

[0165] according to Figure 4 At over 20 mA / cm 2 In high-output discharge, NaTP salts with a surface layer (coating layer) have a large charge-discharge capacity, and this tendency is more pronounced in embodiments where the amount of Na2CO3 that is considered to affect the composition of the surface layer is small.

[0166] It should be noted that after 30 cycles, the discharge current density was set to 2 mA / cm². 2 Charging and discharging were performed, and the results showed that the discharge capacity was the same in both the examples and the comparative examples. This confirms that the surface layer has little impact on the cycling characteristics and suppresses the capacitance reduction during high-output discharge, i.e., charging and discharging at high current densities.

[0167] This application claims priority based on Japanese Patent Application No. 2021-39890, filed on March 12, 2021, and incorporates all the contents of that Japanese Patent Application.

Claims

1. A type of compound with the general formula Na 1+x Ti2(PO4)3 represents sodium titanium phosphate, 0≤x≤2, characterized in that, It has a surface layer comprising Na5Ti(PO4)3, said surface layer being relative to the general formula Na 1+x Ti2(PO4)3 represents sodium titanium phosphate salts that are greater than 0% by mass and less than 5% by mass.

2. The sodium titanium phosphate salt according to claim 1, wherein, The sodium titanium phosphate salt has a NASICON-type crystal structure.

3. The sodium titanium phosphate salt according to claim 1, wherein, The surface layer is relative to the general formula Na 1+x Ti2(PO4)3 represents sodium titanium phosphate at a concentration of 0.1% by mass or more and less than 5% by mass.

4. The sodium titanium phosphate salt according to claim 1, wherein, The thickness of the surface layer exceeds 0 nm and is less than 50 nm.

5. The sodium titanium phosphate salt according to claim 1, wherein, The diameter of the microcrystals is greater than 10 nm and less than 100 nm.

6. A method for manufacturing sodium titanium phosphate salt according to any one of claims 1 to 5, comprising the following steps: calcining a mixture comprising a sodium source, a phosphate source, and a titanium source at a temperature of 150°C or higher and 400°C or lower to obtain a first-calcined product; pulverizing the first-calcined product and calcining it at a temperature of 400°C or higher and 900°C or lower. The molar ratio of Na to Ti in the sodium source and the titanium source exceeds 0.

5.

7. A negative electrode active material comprising sodium titanium phosphate salt according to any one of claims 1 to 5.

8. A sodium secondary battery comprising a positive electrode, an electrolyte, and a negative electrode comprising sodium titanium phosphate salt according to any one of claims 1 to 5.

9. The sodium secondary battery according to claim 8, wherein, The electrolyte is an aqueous electrolyte.

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