JP7404502B2 - Sodium transition metal phosphate and its uses - Google Patents

Description

The present disclosure relates to sodium transition metal phosphates and uses thereof.

Sodium secondary batteries that do not use the rare metal lithium not only have advantages in terms of resources and cost, but also have the potential to use new materials and have excellent high-rate charge/discharge characteristics.
So far, sodium secondary batteries have been considered that have the same electrolyte as current lithium secondary batteries, mainly non-aqueous electrolytes.
Recently, it has been reported that the narrow potential window of 1.23V derived from water decomposition, which was the biggest weak point of water-based batteries, exceeds 2V due to high concentration of electrolyte (Non-Patent Document 1). Expectations are rising for the practical application of sodium secondary batteries.
In particular, for larger batteries, a water-based sodium secondary battery using an aqueous electrolyte is preferable from the viewpoint of cost and safety.
Regarding the negative electrode material, which is another issue in the practical application of aqueous sodium secondary batteries, Nasicon type NaTi ₂ (PO ₄ ) ₃ has been found to be functional, but its cycle stability is low and its improvement is an issue. (Non-patent Document 2).

Ruben-Simon et al, ACS Energy Letters, 2, 2005 (2017) Sun Il Park et al, Journal of The Electrochemical Society, 158(10) A1067-A1070 (2011)

The present disclosure provides at least one sodium transition metal phosphate, a negative electrode active material that provides a sodium secondary battery with excellent cycle stability compared to conventional aqueous sodium secondary batteries, and a sodium secondary battery including the same. In particular, the present invention aims to provide an aqueous sodium secondary battery with higher cycle stability than conventional aqueous sodium secondary batteries equipped with Nasicon-type NaTi ₂ (PO ₄ ) ₃ as a negative electrode active material. Another purpose is to provide a negative electrode active material.

In the present disclosure, a sodium transition metal phosphate composition coated with a sodium-containing phosphate has superior cycle stability compared to conventionally known sodium secondary battery negative electrode materials, and can be used as a negative electrode. We have discovered that a sodium secondary battery with excellent cycle stability can be constructed.
That is, the present invention is as described in the claims, and the gist of the present disclosure is as follows.

[1] A sodium transition metal phosphate characterized by having a coating layer containing sodium phosphate.
[2] The sodium transition metal phosphate according to the above [1], wherein the sodium transition metal phosphate is a sodium titanium phosphate represented by the general formula Na _1+x Ti ₂ (PO ₄ ) ₃ (0≦x≦2). Sodium transition metal phosphate.
[3] The sodium transition metal phosphate according to [1] or [2] above, wherein the sodium transition metal phosphate has a Nasicon crystal structure.
[4] The sodium transition according to any one of [1] to [3] above, wherein the coating layer containing sodium phosphate is more than 0% by mass and less than 5% by mass based on the sodium transition metal phosphate. Metal phosphate.
[5] The sodium transition metal phosphate according to any one of [1] to [4] above, wherein the coating layer has a thickness of more than 0 nm and less than 50 nm.
[6] The method for producing a sodium transition metal phosphate according to any one of [1] to [5] above, comprising the step of firing a mixture of a sodium transition metal phosphate and a precursor of the coating layer. .
[7] The manufacturing method according to the above [6], wherein the precursor is sodium carbonate and diammonium hydrogen phosphate.
[8] A negative electrode active material containing the sodium transition metal phosphate according to any one of [1] to [5] above.
[9] A sodium secondary battery comprising a negative electrode containing the sodium transition metal phosphate according to any one of [1] to [5] above, a positive electrode, and an electrolyte.
[10] The sodium secondary battery according to the above [9], wherein the electrolyte is an aqueous electrolyte.

The present disclosure provides at least a sodium transition metal phosphate, a negative electrode active material that provides a sodium secondary battery with excellent cycle stability compared to conventional aqueous sodium secondary batteries, and a sodium secondary battery including the same. can provide either. Furthermore, it is possible to provide a negative electrode active material that provides an aqueous sodium secondary battery with higher cycle stability than conventional aqueous sodium secondary batteries that include Nasicon type NaTi ₂ (PO ₄ ) ₃ as a negative electrode active material. .

Figure 1 shows the XRD patterns of sodium transition metal phosphates of Example 1, Comparative Examples 1 and 2 (a: Comparative Example 1, b: Example 1, c: Comparative Example 2, d: PDF pattern (ICDD: 33 -1296 NaTi ₂ (PO ₄ ) ₃ ). FIG. 2 is a diagram showing a TEM observation diagram (a: overall view, b: enlarged view) of NaTi ₂ (PO ₄ ) ₃ of Example 1. FIG. 3 is a diagram showing charge-discharge curves of Example 1 and Comparative Examples 1 and 2 (a: Comparative Example 1, b: Example 1, c: Comparative Example 2). FIG. 4 is a graph showing the cycle maintenance rates of Example 1 and Comparative Examples 1 and 2 (◯: Comparative Example 1, △: Example 1, ▽: Comparative Example 2).

Hereinafter, the sodium transition metal phosphate of the present disclosure will be described by showing an example of an embodiment.

<Sodium transition metal phosphate>
This embodiment is a sodium transition metal phosphate characterized by having a coating layer containing sodium phosphate. Having a coating layer containing sodium phosphate facilitates the diffusion of sodium ions (Na ⁺ ) on the surface of the sodium transition metal phosphate (hereinafter also referred to as "NaMP salt").
The NaMP salt in this embodiment is a phosphoric acid compound containing sodium and a transition metal, more preferably sodium titanium phosphate, and has the general formula Na _1+x Ti ₂ (PO ₄ ) ₃ (0≦x≦ It is more preferable that it is a sodium titanium phosphate represented by 2), and that it is NaTi ₂ (PO ₄ ) ₃ (that is, x=0 in the general formula Na _1+x Ti ₂ (PO ₄ )). More preferred. In the present embodiment, the composition of the NaMP salt can be determined by a known composition analysis method, such as inductively coupled plasma emission spectrometry, atomic absorption spectrometry, TEM-EDS, etc.
The NaMP salt of this embodiment preferably has a Nasicon type crystal structure (a so-called Nasicon type NaMP salt). The Nasicon type crystal structure can be identified by comparing the XRD pattern of the NaMP salt of this embodiment with the PDF pattern (ICDD: 33-1296 NaTi ₂ (PO ₄ ) ₃ ).

The NaMP salt of this embodiment has a coating layer (hereinafter also simply referred to as "coating layer") containing sodium phosphate. As a result, it exhibits high cycle stability that is not exhibited by a mixture obtained by physically mixing sodium phosphate and NaMP salt. The coating layer may be present on at least a portion of the NaMP salt without inhibiting the diffusion of sodium, and is preferably present on at least a portion of the surface of the NaMP salt. The form in which the coating layer is present on at least a portion of the NaMP salt is that at least both the NaMP salt and sodium phosphate exist through an interface, that is, the coating layer forms an interface with the NaMP salt. This can be mentioned. As a result of the presence of an interface between the NaMP salt and sodium phosphate, it is thought that the NaMP salt exhibits a sodium diffusion coefficient equivalent to that of the NaMP salt without a coating layer. The sodium diffusion coefficient of the NaMP salt of this embodiment is, for example, a value measured by the method shown in the measurement example described later, where the sodium diffusion coefficient during the oxidation reaction and the reduction reaction is 3.0 × 10 ^-10 cm. ² /sec or more.
The covering layer does not need to cover the entire surface of the NaMP salt, and may have a sea-island shape. That is, the coating layer may be at least one of sodium phosphate and a sodium phosphate-containing composition, preferably sodium phosphate, present on at least a portion of the surface of the NaMP salt.
The sodium phosphate contained in the coating layer is preferably sodium phosphate represented by the general formula Na ₃ PO ₄ . The coating layer only needs to contain sodium phosphate, preferably does not contain a transition metal element, and more preferably contains the sodium phosphate. The NaMP salt of this embodiment only needs to have a coating layer on all or at least a part of its surface, and it is sufficient that it has a coating layer on at least a part of its surface. Moreover, it is sufficient that the NaMP salt particles have sodium phosphate on all or at least a portion of the surface, preferably on at least a portion of the surface. The properties of the sodium phosphate contained in the coating layer are not particularly limited, and examples include at least one of crystalline and amorphous, and further, at least one of dense and porous. Furthermore, it is preferable that the coating layer contains sodium phosphate in a state generated by a reaction of a sodium source and a phosphorus source on the surface of the NaMP salt. This makes it easier to suppress polarization (hysteresis) during charge/discharge cycles. The presence of the coating film can be confirmed by TEM observation (transmission electron microscopy). As shown in FIG. 2, in TEM observation, the coating layer is observed as a visual field image different from that of the NaMP salt, and the coating layer is observed as a visual field image with a darker tone than the NaMP salt.

Since sodium ions (Na ⁺ ) tend to diffuse easily, the ratio of the coating layer (hereinafter also referred to as "coating amount") in this embodiment is more than 0% by mass and 5% by mass based on the NaMP salt. It is preferably less than 0.01% by mass and 3% by mass or less, and further preferably 0.1% by mass and less than 2.5% by mass. The coating amount can be measured by any method, but examples include a method of calculating from the difference between before and after the formation of the coating layer by compositional analysis, and a quantitative analysis by TEM-EDS. When the coating amount is 5% by mass or more, a coating layer that does not inhibit the diffusion of sodium is not formed, and the diffusion coefficient of sodium ions is significantly reduced.
The thickness of the coating layer is arbitrary, but it may be as long as it does not inhibit the diffusion of sodium. The thickness of the coating layer may be, for example, more than 0 nm and less than 50 nm, more preferably more than 5 nm and less than 40 nm, or more than 0.1 nm and less than 5 nm. The thickness of the coating layer can be determined by TEM observation, and the thickness of the coating layer observed in the TEM observation diagram (i.e., the thickness of the coating layer can be confirmed in the TEM observation diagram, and the thickness of the coating layer observed in the TEM observation diagram) The maximum thickness) is more than 0 nm or 0.1 nm or more, and is 50 nm or less, 40 nm or less, 5 nm or less, or 3 nm or less.
The NaMP salt of this embodiment exhibits high cycle stability when used in a sodium secondary battery equipped as a negative electrode active material. For example, in a charge/discharge cycle test using a sodium secondary battery, the ratio (%) of the discharge capacity at the 50th cycle to the discharge capacity at the first cycle in the charge/discharge cycle test exceeds 80% or becomes 85%. can be mentioned. The configuration of the sodium secondary battery and the conditions of the charge/discharge cycle test are shown below.
(Sodium secondary battery)
Electrolyte: 17 mol/kg NaClO ₄ aqueous solution Positive electrode: 70% by mass of NaMP salt, 25% by mass of acetylene black, and
Positive electrode consisting of 5% by mass of polytetrafluoroethylene Negative electrode: 70% by mass of Na ₂ Ni [Fe(CN) ₆ ], 25% by mass of acetylene black, and
Negative electrode consisting of 5% by mass of polytetrafluoroethylene
(Charge/discharge cycle test conditions)
Current density: 2mA/ ^cm2
Voltage: -0.9V to -0.3V (vs Ag/AgCl reference electrode)
Charge/discharge temperature: 25℃

<Formation of coating layer>
Although the method for producing the NaMP salt of this embodiment is arbitrary, a method including a step of firing a mixture of the NaMP salt and the precursor of the coating layer can be mentioned. Thereby, a NaMP salt coating layer can be formed.
The precursor of the coating layer (hereinafter also simply referred to as "precursor") may be one that serves as a source of sodium and a source of phosphoric acid, or one that serves as a source of at least one of a sodium source and a phosphoric acid source. The sodium source may be any compound containing sodium (Na), including at least one selected from the group of sodium carbonate, sodium hydroxide, and sodium chloride, and at least one of sodium carbonate and sodium hydroxide, or even carbonate. Preferably it is sodium. Further, the phosphoric acid source may be any compound containing phosphoric acid (PO ₄ ), and may be one or more selected from the group of pyrophosphoric acid, polyphosphoric acid, and ammonium dihydrogen phosphate, and more preferably ammonium dihydrogen phosphate. preferable. Particularly preferred precursors include sodium carbonate and diammonium hydrogen phosphate.
The mixture can be obtained by mixing the NaMP salt and the precursor using any method. Examples of the mixing method include at least one of wet mixing and dry mixing, and further wet mixing. A particularly preferred mixing method includes a method in which a mixed solution containing the NaMP salt, the precursor, and the solvent is dried after or while being stirred. Examples of the solvent include at least one of water and alcohol, and further water. Stirring may be carried out under any condition as long as the NaMP salt and each precursor are mixed uniformly, and an example is stirring the mixed solution at a stirring speed of 100 to 500 rpm, and the larger the amount of the mixed solution, the higher the stirring speed. Bye. Drying may be carried out under any conditions as long as the solvent can be removed from the mixed solution, such as in the air at temperatures of 60°C or higher, 70°C or higher, and 150°C or lower, 120°C or lower, or 100°C or lower. This results in a mixture in which the precursors consisting of a sodium source and a phosphorus source are uniformly arranged on the NaMP salt.
The firing method is not particularly limited as long as the conditions are such that a coating layer is formed on the NaMP salt, but any conditions are sufficient as long as the conditions are such that sodium phosphate is generated from the sodium source and the phosphorus source. Examples of the firing method include treatment in an oxidizing atmosphere, preferably in the air, at a temperature of 400°C or higher or 500°C or higher and 800°C or lower or 750°C or lower. Compared to sodium phosphate mixed with NaMP salt and heat-treated, the sodium phosphate generated by the reaction of the sodium source and phosphorus source on the surface of the NaMP salt is considered to have a stronger interaction with the NaMP salt. It is thought that cycle stability will be higher when a NaMP salt having a coating layer containing such a form of sodium phosphate is used as a negative electrode active material.

<Negative electrode active material>
Next, an example embodiment of the negative electrode active material containing the NaMP salt of the present disclosure will be described.
In the present embodiment, the "negative electrode active material" refers to an electrode active material of a low potential electrode among the electrodes constituting an electrochemical device, and particularly an electrode active material of a negative electrode of a sodium secondary battery.
The negative electrode active material of this embodiment includes the NaMP salt of this embodiment (i.e., a NaMP salt having a coating layer containing sodium phosphate), and only the NaMP salt of this embodiment (i.e., a coating layer containing sodium phosphate). may consist of only NaMP salt having . On the other hand, the negative electrode active material of this embodiment may contain an active material other than the NaMP salt of this embodiment, for example, an active material such as a sodium transition metal compound.
In addition to the carbon layer, the negative electrode active material of the present embodiment may have a coating layer, preferably a conductive coating layer (ie, a conductive layer), on part or all of the surface of the negative electrode active material. In this case, the coating layer may be provided on a coating layer containing sodium phosphate. However, a coating layer may also be present on the surface of the NaMP salt.

<Sodium secondary battery>
Next, a sodium secondary battery characterized by comprising a negative electrode containing the NaMP salt of the present disclosure, a positive electrode, and an electrolyte will be described by showing an example of an embodiment.
In this embodiment, a "sodium secondary battery" is an electrochemical device in which charging and discharging occur due to the insertion and desorption of sodium ions (Na ⁺ ), and includes a sodium secondary battery, a sodium ion secondary battery, a sodium ion battery, and a sodium ion secondary battery. It is synonymous with storage battery, Na secondary battery, Na ion battery, Na storage battery, etc.
A "non-aqueous electrolytic solution" is an electrolytic solution containing a non-aqueous solvent as a solvent, and an "aqueous electrolytic solution" is an electrolytic solution containing an aqueous solvent as a solvent.
A "nonaqueous sodium secondary battery" is a sodium secondary battery that includes a nonaqueous electrolyte as an electrolyte, and a "aqueous sodium secondary battery" is a sodium secondary battery that includes an aqueous electrolyte as an electrolyte.

<Negative electrode>
The negative electrode may include a negative electrode mixture containing a negative electrode active material containing the NaMP salt of the present embodiment, and a current collector.
The negative electrode mixture contains a negative electrode active material, a binder, a conductive material, and, if necessary, additives. Known binders, conductive materials, and additives can be used.
The binder is one or more selected from the group of fluororesin, polyethylene, polypropylene, SBR material, and imide material, and further from the group of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and ethylenetetrafluoroethylene (ETFE). One or more selected ones can be exemplified.
Examples of the conductive material include one or more selected from carbon materials, conductive fibers such as metal fibers, metal powders such as copper, silver, nickel, and aluminum, and organic conductive materials such as porphenylene derivatives. Preferred carbon materials include graphite, soft carbon, hard carbon, carbon black, Ketjenblack, acetylene black, graphite, activated carbon, carbon nanotubes, carbon fiber, and mesoporous carbon.
The negative electrode mixture may be manufactured by a known method, and the negative electrode active material, binder, and conductive material may be mixed in a desired ratio.

<Positive electrode>
The positive electrode may include a positive electrode mixture containing a positive electrode active material, a current collector, and, if necessary, additives.
The positive electrode mixture includes a positive electrode active material, a binder, a conductive material, and, if necessary, additives.
The positive electrode active material only needs to contain a material that does not prevent insertion and desorption of sodium ions from the negative electrode active material, and examples include at least one of a sodium-containing transition metal oxide, a sodium-containing polyanion compound, and a carbon-based material.
The binder and conductive material may be any known one, and may be the same as the binder and conductive material that can be used in the negative electrode mixture described above.
The positive electrode mixture may be manufactured by a known method, and the positive electrode active material, binder, and conductive material may be mixed in a desired ratio.

<Electrolyte>
The electrolyte is either a nonaqueous electrolyte or an aqueous electrolyte, and preferably an aqueous electrolyte.
The electrolyte is a sodium salt, preferably a soluble sodium salt. Preferred electrolytes include one or more selected from the group of NaCl, Na ₂ SO ₄ , NaNO ₃ , NaClO ₄ , NaOH and Na ₂ S. In view of ease of handling, the electrolyte is preferably one or more selected from the group of NaCl, Na ₂ SO ₄ , NaNO ₃ and NaClO ₄ , and more preferably at least one of NaCl and NaClO ₄ .
The electrolyte concentration in the electrolytic solution is not particularly limited, but from the viewpoint of increasing the energy density as a sodium secondary battery, the electrolyte concentration (sodium salt concentration) in the electrolytic solution is preferably high, and the sodium salt concentration is 1 mol/kg. (1 m) or more and less than the saturation solubility.
The electrolyte may contain additives. Additives include, but are not limited to, succinic acid, glutamic acid, maleic acid, citraconic acid, gluconic acid, itaconic acid, diglycol, cyclohexanedicarboxylic acid, cyclopentanetetracarboxylic acid, 1,3-propane sultone, 1,4- Examples include one or more selected from the group of butane sultone, methyl methanesulfonate, sulfolane, dimethylsulfone, and N,N-dimethylmethanesulfonamide. An example of the content of the additive is 0.01% by mass or more and 10% by mass or less as a mass ratio of the additive to the mass of the electrolytic solution.

<Other configurations>
As other components such as current collectors and separators for the positive and negative electrodes, known components used in sodium secondary batteries and lithium secondary batteries can be used.
The sodium secondary battery equipped with the negative electrode containing the NaMP salt of this embodiment is a conventional sodium secondary battery, particularly a conventional aqueous sodium secondary battery equipped with Nasicon type NaTi ₂ (PO ₄ ) ₃ as the negative electrode active material, It shows high cycle stability compared to

Next, the present disclosure will be explained by examples. However, the present disclosure is not to be construed as limited to these examples.

<Evaluation of crystallinity>
The crystal structure of the NaMP salt was identified using a desktop X-ray analyzer (device name: MiniFlex600/ASC-8, manufactured by Rigaku Corporation) under the following conditions.
Target: Cu
Output: 0.6kW (15mA-40kV)
Step scan: 0.02° (2θ/θ)
Measurement time: 0.05 seconds

<Composition analysis>
The composition of the NaMP salt was determined by dissolving 50 mg of the sample in an aqueous solution containing 10 mL of 35% hydrochloric acid and 1 mL of 35% hydrogen peroxide, and performing ICP analysis of the solution. Furthermore, the composition of the coating layer was determined by the difference in the composition of NaMP salt before and after coating.

<Observation of coating layer>
The measurements were performed using a transmission electron microscope (equipment name: JEM-2100, manufactured by JEOL Ltd.) and a thermal field emission scanning electron microscope (equipment name: JSM-7600F, manufactured by JEOL Ltd.).

<Sodium secondary battery>
(Preparation of positive electrode)
Nickel hexacyanoferrate Na ₂ Ni ([Fe(CN) ₆ ]) with a cubic crystal structure was used as the positive electrode active material. Na ₂ Ni ([Fe(CN) ₆ ] was synthesized by the following method. That is, 10 mmol of 2.69 g of Ni(OCOCH ₃ ) _{2.4H 2 O} was dissolved in 175 mL of H ₂ O and 25 mL of DMF (N,N-dimethylformamide.HCON(CH ₃ ) ₂ ) to obtain a first solution. On the other hand, 10 mmol A second solution was obtained by dissolving 4.84 g of Na ₄ [Fe(CN) ₆ ].10H ₂ O and 7 g of NaCl in 175 mL of H ₂ O.
The first solution was slowly added to the second solution, and the mixture was stirred and reacted at room temperature for 72 hours to obtain a precipitate. The resulting precipitate was collected by centrifugation, washed with methanol, and then dried in air to obtain nickel hexacyanoferrate (Na ₂ Ni [Fe(CN) ₆ ]) with a cubic crystal structure. .
The obtained Na ₂ Ni ([Fe(CN) ₆ ], acetylene black (hereinafter also referred to as "AB") and polytetrafluoroethylene (hereinafter also referred to as "PTFE") were mixed in a mass ratio of 70:25: 5, and then molded into a pellet with a diameter of 3 mm, which was used as a positive electrode (positive electrode mixture).

(Preparation of negative electrode)
The negative electrode active material and AB were mixed at a mass ratio of 70:30 using a planetary ball mill at 400 rpm for 1 hour in an Ar atmosphere to obtain a mixture. Thereafter, the mixture was subjected to carbothermal treatment at 800°C for 1 hour under an Ar flow, and then AB and PTFE were mixed at a mass ratio of 70:25:5, formed into a pellet with a diameter of 2 mm, and this was used as a negative electrode. (negative electrode mixture).

(Preparation of water-based sodium secondary battery)
The working electrode (corresponding to the positive electrode) has a positive electrode mixture, the counter electrode (the electrode corresponding to the negative electrode) has a negative electrode mixture, the reference electrode has a silver chloride electrode (Ag/AgCl), and the electrolyte has an electrolyte concentration (NaClO ₄ concentration). ) A water-based sodium secondary battery equipped with 17 m of NaClO ₄ aqueous solution was fabricated.

(Synthesis of NaMP salt)
<Example 1>
To 40 mL of an ethanol solution in which Ti(OCH ₂ CH ₂ CH ₂ CH ₃ ) ₄ was dissolved, Na ₂ CO _{3 ,} NH ₄ H ₂ PO ₄ and citric acid in an amount twice the molar amount of Ti were added. It was evaporated to dryness at 60°C for 30 minutes with stirring at 500 rpm, followed by 1-2 hours at 80°C. Thereafter, it was heated in the air at 350° C. for 5 hours to obtain NaTi ₂ (PO ₄ ) ₃ .
A mixed aqueous solution of 2 g of the obtained NaTi ₂ (PO ₄ ) ₃ powder, 39 mg of sodium carbonate, 87 mg of diammonium hydrogen phosphate, and 2 g of distilled water was heated at 120° C. and evaporated to dryness while stirring at 300 rpm. . The mixture after _evaporation to dryness was calcined in the air at 700° C. _{for 12 hours to obtain NaTi 2 (PO 4 ) 3 (} NaTi ₂ (PO ₄ ) ₃ ) was obtained.

<Example 2>
NaTi ₂ (PO ₄ ) ₃ having a Nasicon-type structure having a coating layer consisting of 2% by mass of Na ₃ PO ₄ was obtained by the same method as in Example 1 except that the temperature of evaporation to dryness was 100° C. .

<Example 3>
NaTi ₂ (PO ₄ ) ₃ having a Nasicon-type structure having a coating layer consisting of 2% by mass of Na ₃ PO ₄ was obtained by the same method as in Example 1 except that the temperature of evaporation to dryness was 80° C. .

<Comparative example 1>
NaTi ₂ (PO ₄ ) ₃ was obtained in the same manner as in Example 1 and calcined in the air at 700° C. for 12 hours to form NaTi ₂ (PO ₄ ) ₃ with a Nasicon-type structure, that is, from Na ₃ PO ₄ A NaMP salt without a coating layer was obtained.

<Comparative example 2>
5% by mass in the same manner as in Example 1 except that a mixed aqueous solution of 2g of the obtained NaTi ₂ (PO ₄ ) ₃ powder, 96mg of sodium carbonate, 218mg of diammonium hydrogen phosphate, and 2g of distilled water was used. NaTi ₂ (PO ₄ ) ₃ having a Nasicon-type structure having a coating layer made of _Na 3 PO ₄ was obtained.

<Comparative example 3>
A mixed solution obtained by mixing 3.9 g of sodium carbonate, 8.7 g of diammonium hydrogen phosphate, and 20 g of distilled water was heated at 120° C. while stirring at 300 rpm to evaporate to dryness to obtain Na ₃ PO ₄ I got it. 0.02 g of the obtained Na ₃ PO ₄ , 0.98 g of NaTi ₂ (PO ₄ ) ₃ obtained in the same manner as in Example 1 (NaTi ₂ (PO ₄ ) ₃ without a coating layer), 2 g of distilled water The mixed solution obtained by mixing was heated at 120° C. while stirring at 300 rpm to evaporate to dryness. This was calcined in the air at 700° C. for 12 hours to obtain a mixture consisting of NaTi ₂ (PO ₄ ) ₃ having a Nasicon-type structure and 2% by mass Na ₃ PO ₄ .

FIG. 1 shows the XRD patterns of NaTi ₂ (PO ₄ ) ₃ obtained in Examples and Comparative Examples. In both cases, an XRD peak of NaTi ₂ (PO ₄ ) ₃ was confirmed. Further, FIG. 2 shows a TEM observation diagram of NaTi ₂ (PO ₄ ) ₃ of Example 1. Through TEM observation, a coating layer observed in black can be confirmed on the surface of the NaTi ₂ (PO ₄ ) ₃ particles, and NaTi ₂ (PO ₄ ) ₃ has a coating layer of 0.5 to 1 nm on its surface. This can be confirmed.

<Measurement example 1> (charge/discharge cycle test)
Using a water-based sodium secondary battery equipped with the negative electrode active material of the example or comparative example, charging and discharging were repeated under the following conditions, and the discharge capacity of each charge and discharge cycle was measured. The ratio (%) of the discharge capacity at the 50th cycle to the discharge capacity at the 1st cycle was defined as the cycle maintenance rate, and the cycle stability was evaluated based on this ratio.
Current density: 2mA/ ^cm2
Voltage: -0.9V to -0.3V (vs Ag/AgCl reference electrode)
Charge/discharge temperature: 25℃
FIG. 3 shows the charge/discharge curves for cycles 1 to 3, and the results of the charge/discharge cycle test are shown in the table below and FIG. 4.

表１より、実施例のＮａＴｉ_２（ＰＯ_４）_３は、サイクル維持率が高いことが確認できる。また、図３より、実施例１のＮａＴｉ_２（ＰＯ_４）_３は被覆層を有しているにも関わらず、被覆層を有していない比較例１と同等の充放電曲線の形状であり、被覆層の形成による分極がほとんどないことが分かる。

From Table 1, it can be confirmed that NaTi ₂ (PO ₄ ) ₃ of the example has a high cycle retention rate. Moreover, from FIG. 3, although NaTi ₂ (PO ₄ ) ₃ of Example 1 has a coating layer, the shape of the charge-discharge curve is the same as that of Comparative Example 1, which does not have a coating layer. , it can be seen that there is almost no polarization due to the formation of the coating layer.

<Measurement example 2> (diffusion coefficient of sodium ions)
Using an aqueous sodium secondary battery equipped with the negative electrode active material of Example 1, Comparative Example 1, or Comparative Example 3, cyclic voltammetry measurements were performed under the following conditions.
Negative electrode potential: -0.9V to 0.0V (vs Ag/AgCl reference electrode)
Potential scanning speed: 0.05, 0.1, 0.5, 1 mV/sec Charge/discharge temperature: 25°C
From the potential scanning speed and the obtained maximum current value (peak current value), the diffusion coefficient of Na ⁺ ions in the negative electrode during charging and discharging was determined using the following formula.
Ip=2.69×10 ⁵ n ^3/2 AD ^1/2 CV ^1/2
Here, Ip is the peak current value (unit: A), A is the area of the negative electrode (unit: cm ² ), D is the diffusion coefficient of sodium ions (unit: cm ² /sec), and C is the electrolyte concentration (unit: cm 2 ). mol/cm ³ ), and V is the potential scanning speed (unit: V/sec).
The results are shown in the table below.

これらの結果より、実施例１及び比較例１のナトリウムイオンの拡散速度はほぼ同等であり、被覆層の形成によるナトリウムイオンの拡散性への影響がないことが分かる。被覆層を有しているにも関わらず、ナトリウムイオンの拡散係数が同等であることから、被覆層とＮａＭＰ塩とが単に物理的に接触している状態とは異なり、界面を形成していると考えられる。
本出願は、２０２０年３月５日に出願された日本特許出願である特願２０２０－０３７４２４号に基づく優先権を主張し、当該日本特許出願のすべての記載内容を援用する。

From these results, it can be seen that the diffusion rates of sodium ions in Example 1 and Comparative Example 1 are almost the same, and that the formation of the coating layer has no influence on the diffusivity of sodium ions. Despite having a coating layer, the diffusion coefficients of sodium ions are the same, so unlike the state where the coating layer and NaMP salt are simply in physical contact, they form an interface. it is conceivable that.
This application claims priority based on Japanese Patent Application No. 2020-037424, which is a Japanese patent application filed on March 5, 2020, and incorporates all the contents of the Japanese patent application.

Claims (9)

Sodium titanium phosphate,
a coating layer containing sodium phosphate that covers the sodium titanium phosphate ;
A sodium titanium phosphate composition having
A sodium titanium phosphate composition , wherein the coating layer containing sodium phosphate is more than 0% by mass and less than 5% by mass based on the sodium titanium phosphate. The sodium titanium phosphate composition according to claim 1, wherein the sodium titanium phosphate is represented by the general formula Na _1+x Ti ₂ (PO ₄ ) ₃ (0≦x≦2). The sodium titanium phosphate composition according to claim 1 or 2, wherein the crystal structure of the sodium titanium phosphate is a Nasicon type crystal structure. The sodium titanium phosphate composition according to any one of claims 1 to 3, wherein the coating layer has a thickness of more than 0 nm and less than 50 nm. The method for producing a sodium titanium phosphate composition according to any one of claims 1 to 4 , comprising the step of firing a mixture of sodium titanium phosphate and a precursor of the coating layer. The manufacturing method according to claim 5, wherein the precursors are sodium carbonate and diammonium hydrogen phosphate. A negative electrode active material comprising the sodium titanium phosphate composition according to any one of claims 1 to 4 . A sodium secondary battery comprising a negative electrode comprising the sodium titanium phosphate composition according to any one of claims 1 to 4 , a positive electrode, and an electrolyte. The sodium secondary battery according to claim 8, wherein the electrolyte is an aqueous electrolyte.

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