JP6270056B2 - Positive electrode active material and secondary battery using the same - Google Patents

Description

  The present invention belongs to the technical field of secondary batteries, and particularly relates to a novel positive electrode active material constituting an aqueous sodium ion secondary battery and an aqueous sodium ion secondary battery using the same.

  Lithium ion secondary batteries are actively studied as secondary batteries that can achieve high energy density at high voltage. The electrolyte solution of the lithium ion secondary battery mainly uses a non-aqueous solvent using an organic solvent. However, it is expensive and easily flammable, and thus has a problem in terms of safety and cost. That is, the current lithium ion battery uses a non-aqueous solvent with a low flash point and high viscosity at high cost, and there are problems in terms of safety, economy, and rate characteristics. In order to solve these three problems, an aqueous electrolyte solution having no flash point, low cost and high conductivity has been studied, and there is an advantage that it is inexpensive and does not ignite within the constraints of a potential window that does not cause water electrolysis. A secondary battery using an aqueous electrolyte solution has been proposed (see, for example, Patent Document 1).

However, since the lithium ion battery has a limitation of an aqueous potential window of about 1.2 V, the characteristics of lithium having a high potential cannot be utilized. A water-based secondary battery in which lithium is replaced with sodium can be expected as a low-cost and highly safe next-generation secondary battery and can eliminate the disadvantages of sodium ion batteries that have difficulty in rate characteristics in non-aqueous systems. If a secondary battery in which lithium ions responsible for electrical conduction are replaced with sodium ions in the electrolyte of a lithium ion secondary battery among water-based secondary batteries can be realized, it can be a secondary battery with even lower cost and higher safety. At present, several sodium ion secondary batteries that use non-aqueous electrolytes have been proposed (see, for example, Patent Document 2, Patent Document 3, and Non-Patent Document 1). Regarding the secondary battery, only a full cell characteristic of a combination of Na _0.44 MnO ₂ as a positive electrode, NaTi ₂ (PO ₄ ) ₃ as a negative electrode, and 2M Na ₂ SO ₄ aqueous solution as an electrolyte has been reported (Non-Patent Document). 2).

JP 2009-094034 A JP 2005-317511 A Japanese Patent Application No. 2011-227413 P. Barpanda et.al.; ”Sodium iron pyrophosphate: A novel 3.0V iron-based cathode for sodium-ion batteries,” Electrochemistry Communications 24 (2012) 116-119 S. Okada, E. Kobayashi, and K. Nakamoto, IUMRS-ICEM Meeting Abstract, A-6, Yokohama (2012).

However, so far, the full cell characteristics have been reported in sodium ion secondary batteries (aqueous sodium ion secondary batteries) using aqueous electrolytes, only in the electrode combination of Non-Patent Document 2, I haven't found anything in practical use yet. This is because by using an aqueous electrolyte, the operating potential region of the secondary battery is limited to a potential range in which no electrolysis reaction of water occurs, and options for the positive electrode and the negative electrode are limited. In particular, in the case of a combination of a Na _0.44 MnO ₂ positive electrode and a NaTi ₂ (PO ₄ ) ₃ negative electrode, there is a problem that the capacity of this battery system is restricted by _0.44 Na contained in the positive electrode.

  The object of the present invention has been proposed to solve the above problems, and is a new type of aqueous sodium ion that can be stably charged and discharged even when an aqueous electrolyte is used and can be manufactured at low cost. The object is to provide a positive electrode active material for a secondary battery.

  As a result of diligent research, the present inventors have developed a positive electrode active material suitable for an aqueous sodium ion secondary battery that can be used in an aqueous electrolyte and that has a large capacity by extracting a large amount of sodium. Newly found. Furthermore, it has been found that an aqueous sodium ion secondary battery with high operational stability can be constructed by combining this positive electrode active material with an appropriate negative electrode active material and an aqueous electrolyte.

  Thus, according to the present invention, there is provided a positive electrode active material for an aqueous sodium ion secondary battery characterized by comprising a sodium-containing phosphate having a specific structural formula. According to the present invention, there is also provided an aqueous sodium ion secondary battery comprising the above positive electrode active material and negative electrode active material and using an aqueous electrolyte.

It is the schematic of a coin cell. FIG. 2 is an X-ray diffraction pattern of a positive electrode active material Na ₂ FeP ₂ O ₇ synthesized in Example 1 of the present invention, a carbon coated product thereof, and an annealed product. FIG. 2 is an X-ray diffraction pattern of a negative electrode active material NaTi ₂ (PO ₄ ) ₃ synthesized in Example 1 of the present invention, a carbon coated product thereof, and an annealed product. Is a schematic diagram of the synthesized positive electrode active material Na ₂ FeP ₂ O ₇ unipolar evaluation beaker-type half cell in Example 1 of the present invention. It is a charging / discharging curve of the non-aqueous type and the aqueous type half cell in the current density of 0.2 mA / cm ² of the positive electrode active material Na ₂ FeP ₂ O ₇ single electrode half cell synthesized in Example 1 of the present invention. It is a charging / discharging curve of the non-aqueous type and the aqueous type half cell of the positive electrode active material Na ₂ FeP ₂ O ₇ single electrode half cell synthesized in Example 1 of the present invention at a current density of 2 mA / cm ² . The positive electrode active material Na ₂ FeP ₂ O ₇ rate characteristics unipolar half-cell synthesized in Example 1 of the present invention is a graph showing. It is a graph showing the cycle characteristics at high rate condition 2 mA / cm ² of the positive electrode active material Na ₂ FeP ₂ O ₇ monopolar half cell synthesized in Example 1 of the present invention. Na ₂ FeP ₂ O ₇ on the positive electrode prepared in Example 1 of the present invention, a charge-discharge curve of the non-aqueous and aqueous coin type sodium ion battery using NaTi ₂ (PO _{4) 3} in the negative electrode. It is a X-ray diffraction pattern of positive electrode active material Na ₂ MnP ₂ O ₇ synthesized in Example 2 of the present invention. A charge-discharge curve of the non-aqueous half-cell of Example cathode active material Na ₂ synthesized in 2 MnP ₂ O ₇ unipolar half-cell of the current density 0.2 mA / cm ² of the present invention. A charge-discharge curve of aqueous half-cell in the synthesized positive electrode active material Na ₂ MnP ₂ O ₇ unipolar half-cell of the current density 0.2 mA / cm ² in Example 2 of the present invention.

The positive electrode active material for an aqueous sodium ion secondary battery according to the present invention comprises a sodium-containing phosphate of the general formula Na ₂ MP ₂ O ₇ (M is a transition metal element). Here, various transition metal elements can be applied to M in principle, and among these, Fe or Mn is particularly preferable from the viewpoint of potential and ease of handling, and Fe is particularly preferable.

The sodium-containing phosphate Na ₂ MP ₂ O ₇ which is a positive electrode active material according to the present invention can be produced using a known means, for example, synthesized by a conventionally known solid phase method. it can. For example, as an example of the production method of the present invention, powdery NaH ₂ PO ₄ and Fe (COO) ₂ .2H ₂ O are mixed in a stoichiometric ratio of 2: 1 as a starting material, and a planetary ball mill is used. Sodium-containing phosphate Na ₂ FeP ₂ O ₇ can be obtained by firing in an argon gas atmosphere by mixing in an acetone solvent and vacuum drying.

The positive electrode active material may be used as it is as the positive electrode of the aqueous sodium ion secondary battery, but a composite with a known conductive material may be formed in order to improve the rate characteristics of the electrode.
That is, according to the present invention, from the viewpoint of improving rate characteristics, the sodium-containing phosphate Na ₂ MP ₂ O ₇ that is the positive electrode active material obtained above is pulverized together with carbon fine particles under an inert gas atmosphere. By mixing, carbon coating can be performed. As the carbon fine particles, furnace black, channel black, acetylene black, thermal black, and the like can be used, but acetylene black is preferred because of its high conductivity when used as an electrode. As the inert gas, nitrogen gas, argon gas, or the like can be used. For example, argon gas can be used.

  Specific means applied to the pulverization / mixing at the time of carbon coating are not particularly limited, and various means conventionally used for the purpose of pulverization / mixing of solid substances can be applied, A ball mill is preferable, and among these, a planetary ball mill is preferably used because the raw materials can be sufficiently pulverized and mixed.

  The positive electrode active material of the present invention is preferably subjected to an annealing treatment after the carbon coating. By performing the annealing treatment, it is presumed that the contact state between the carbon atoms constituting the carbon coat and the positive electrode active material is improved, the electron conductivity is improved, and the battery characteristics of the positive electrode active material are improved. In addition, although the conditions of annealing treatment are not specifically limited, For example, it can implement at 600 degreeC for 10 hours.

According to the present invention, the positive electrode active material Na ₂ MP ₂ O ₇ obtained as described above, a sodium ion aqueous secondary battery positive electrode containing the positive electrode active material, and an aqueous sodium ion secondary battery combining the positive electrode Is provided.

  When the positive electrode according to the present invention is manufactured, a known electrode manufacturing method may be followed except that the positive electrode active material described above is used. For example, the active material powder is mixed with a known binder such as polyethylene, if necessary, and a known conductive material such as acetylene black, if necessary, and the resulting mixed powder is made of stainless steel or the like. It can be pressure-molded on the support or filled into a metal container.

  In addition, for example, the positive electrode of the present invention can also be produced by a method such as applying a slurry obtained by mixing the mixed powder with an organic solvent such as toluene onto a metal substrate such as aluminum, nickel, stainless steel, or copper. Can do.

As the negative electrode for the sodium-containing phosphate Na ₂ MP ₂ O ₇ positive electrode, various materials having predetermined potential characteristics can be used when combined with the positive electrode using an aqueous electrolyte, and are not particularly limited. Absent. For example, the general formula NaM _{'2 (PO 4)} include sodium-containing phosphate salt represented by _3. Here, various transition metal elements can be considered as M ′, and among these, sodium-containing titanium phosphate NaTi ₂ (PO ₄ ) ₃ is particularly preferable.

In the aqueous sodium ion secondary battery of the present invention, the electrolytic solution is not particularly limited as long as it is an aqueous electrolytic solution containing a sodium salt as a main electrolyte. Examples of the sodium salt serving as the main electrolyte include Na ₂ SO ₄ , NaNO ₃ , NaClO ₄ , NaOH, Na ₂ S, and the like. These sodium salts can be used alone or in combination of two or more. A particularly preferable electrolyte used for the electrolytic solution of the aqueous sodium ion secondary battery of the present invention is Na ₂ SO ₄ from the viewpoint of easy handling in consideration of battery characteristics and safety.

From the above, the water-based sodium ion secondary battery of the present invention has, as a preferred embodiment, a positive electrode made of a positive electrode active material Na ₂ FeP ₂ O ₇ , a negative electrode made of NaTi ₂ (PO ₄ ) ₃ , an electrolyte Na those containing an electrolyte consisting of ₂ SO ₄ and the like. It has been confirmed that the aqueous sodium ion secondary battery according to the present invention exhibits charge / discharge characteristics superior to those of conventional non-aqueous sodium ion secondary batteries (see Example 1 described later).

  As another component, what is used for a well-known sodium ion secondary battery can be used as a component.

  Here, FIG. 1 is a cross-sectional view schematically showing a coin-type test battery as an example. In this figure, an aqueous sodium ion battery 1 is roughly composed of a negative electrode case member 2 that functions as an external negative electrode of the battery, a positive electrode case member 3 that functions as an external positive electrode of the battery, and a negative electrode current collector between both members. 4, a negative electrode active material layer 5, a separator 8, a positive electrode active material layer 7, and a positive electrode current collector 6 are arranged in this order, and a coin 10 is caulked with a leak-proof gasket 10 while securing an electrolyte space 9. A battery is constructed. In the aqueous sodium ion secondary battery according to the present invention, various conventionally known materials can be used for elements such as a separator, a battery case, and other structural materials.

  Examples will be described below in order to more specifically illustrate the features of the present invention. However, the present invention is not limited to the following examples, and various other types such as a cylindrical shape and a rectangular shape are available in addition to the coin shape. The shape and size can be implemented in variously modified and improved forms based on the knowledge of those skilled in the art. Moreover, the use of the secondary battery of the present invention is not particularly limited.

Example 1
Synthesis positive electrode active material Na ₂ FeP ₂ O ₇ of the positive electrode active material Na ₂ FeP ₂ O ₇ were synthesized by solid phase method. NaH ₂ PO ₄ and Fe (COO) ₂ · 2H ₂ O are mixed in a stoichiometric ratio of 2: 1 to the starting materials, mixed at 400 rpm for 10 hours in an acetone solvent using a planetary ball mill, and vacuum dried. The obtained powder was calcined at 600 ° C. for 10 hours in an Ar atmosphere to obtain Na ₂ FeP ₂ O ₇ (FIG. 2—Item No. 1). From the XRD pattern of the figure, Na ₃ PO ₄ , NaPO ₃ , and Fe ₂ P ₂ O ₇ were recognized as impurities in a very small part, but most of the main phase was identified as trigonal Na ₂ FeP ₂ O _7. It was. The synthesized positive electrode active material and acetylene black (AB) were mixed at a weight ratio of 70:25, and subjected to carbon coating treatment at 300 rpm for 10 hours using a planetary ball mill (FIG. 2—No. 2). The obtained powder (positive electrode active material and AB) was heat-treated in an Ar atmosphere at 600 ° C. for 10 hours (FIG. 2—Item No. 3). The obtained powder (positive electrode active material / C) and PTFE were mixed at a weight ratio of 95: 5 and molded into pellets to make a positive electrode.

The anode active material _{NaTi 2 (PO 4) NaTi 2} (PO 4) to be synthesized negative electrode _{3 3} were synthesized by Pechini method. 40 mL of a solution of Ti (OCH ₂ CH ₂ CH ₂ CH ₃ ) ₄ in 30% hydrogen peroxide solution, 15 mL of 28% aqueous ammonia and 2 times the molar amount of citric acid are added, and Na ₂ CO ₃ is added. dissolved nitrate solution 10mL and NH ₄ H ₂ PO ₄ aqueous solution 10mL, after evaporation to dryness at 1 to 2 hours at 80 ° C. by adding ethylene glycol, brown and heated further for 1-2 hours at 140 ° C. gel Get. This was fired in air at 350 ° C. and 800 ° C., respectively, to obtain NaTi ₂ (PO ₄ ) ₃ (FIG. 3—Item No. 1). From the XRD pattern of the figure, the main phase coincided with ICDD # 33-1296, and it was identified as a NASICON type NaTi ₂ (PO ₄ ) ₃ single phase. The synthesized negative electrode active material and acetylene black (AB) were mixed at a weight ratio of 70:25, and carbon coating treatment was performed for 1 hour at 400 rpm using a planetary ball mill (FIG. 3—No. 2). The obtained powder (negative electrode active material and AB) was heat-treated in a nitrogen atmosphere at 800 ° C. for 1 hour (FIG. 3—Item No. 3). The obtained powder (negative electrode active material / C) and PTFE were mixed at a weight ratio of 95: 5 and molded into pellets to make a negative electrode.

Preparation of non-aqueous and aqueous sodium half-cells A positive electrode pellet was prepared for each of the Na ₂ FeP ₂ O ₇ ball mill product obtained in Example 1 and the annealed annealed product, and this was used as a working electrode. The counter electrode was a Zn plate and the reference electrode was Ag / AgCl. As the solution electrolyte, three kinds of sodium salts of Na ₂ SO ₄ , NaNO ₃ , and NaClO ₄ were selected, and the oxygen concentration after Ar bubbling treatment in an Ar glove box was 10 ^−6. A beaker-type half-cell was prepared using 3 types of aqueous electrolytes, 2M Na ₂ SO ₄ , 4M NaNO ₃ , 4M NaClO ₄ , prepared using ultrapure water of about M (FIG. 4).
For comparison, a non-aqueous half-cell was prepared using a Na ₂ FeP ₂ O ₇ positive electrode pellet as a working electrode, a counter electrode using sodium metal, and a 1 M NaClO ₄ / PC as a non-aqueous electrolyte.

Non-aqueous and aqueous sodium half cell charge-discharge profile view 5 1 obtained above at a current density of 0.2 mA / cm ² to M NaClO ₄ / PC nonaqueous and 2M Na ₂ SO ₄ aqueous electrolyte sodium half cell single of Polar characteristics are shown. In the water-based half cell, a charge / discharge test is performed in the voltage range of −0.6 to 0.7 V with respect to the Ag / AgCl reference electrode, and in the non-aqueous sodium half cell, in the voltage range of 2.3 to 3.6 V with respect to the sodium counter electrode. A charge / discharge test was conducted. A capacity equivalent to that of the non-aqueous system can be obtained with the aqueous system, and the effect of the annealing treatment is remarkable regardless of the non-aqueous system and the aqueous system, as compared with the ball mill product.

FIG. 6 shows sodium half-cell monopolar characteristics when the current density is increased to ² mA / cm ² . Although the charge / discharge overvoltage of the non-aqueous sodium half-cell is large under the high current charge / discharge conditions of ² mA / cm ² , all the aqueous sodium ion batteries have a low viscosity of the aqueous electrolyte, a high sodium ion concentration, and a high ion conductivity. Sex was successful and maintained a smaller charge / discharge overvoltage than non-aqueous.

  FIG. 7 shows the rate characteristics of these batteries. It is apparent that all three types of aqueous sodium ion batteries have rate characteristics that surpass those of non-aqueous sodium ion batteries.

Further, as shown in FIG. 8, the aqueous sodium ion battery exhibited characteristics superior to the nonaqueous sodium ion battery even in the cycle characteristics at a high rate of ² mA / cm ² .

Production of Non-aqueous and Aqueous Sodium Ion Battery Full Cells The positive and negative electrode active materials obtained in Example 1 were formed into pellets such that the positive electrode: negative electrode = 1: 1.5 by weight ratio were used as positive and negative electrodes, respectively. . In an Ar dry box, a coin cell was prepared using Na ₂ FeP ₂ O _{7 for} the positive electrode, NaTi ₂ (PO ₄ ) _{3 for} the negative electrode, and 1 M NaClO ₄ / PC for the non-aqueous electrolyte. Further, as an aqueous electrolyte, as in the half cell, three kinds of sodium salts of Na ₂ SO ₄ , NaNO ₃ and NaClO ₄ were subjected to Ar bubbling treatment in an Ar glove box, and the oxygen concentration was 10 ^−6. Coin cells were prepared using ₂ M Na ₂ SO ₄ , 4 M NaNO ₃ , 4 M NaClO ₄ , and three types of aqueous electrolytes prepared using about M ultrapure water.

Charge / Discharge Profiles of Non-aqueous and Aqueous Sodium Ion Battery Full Cells FIG. 9 shows the current density in the case of using non-aqueous and aqueous electrolytes with the positive electrode Na ₂ FeP ₂ O ₇ and the negative electrode NaTi ₂ (PO ₄ ) _3. The charging / discharging profile in the voltage range 0-1.4V of the full cell in cm < ² > is shown. With the exception of 4M NaNO ₃ , all other aqueous sodium ion batteries showed charge / discharge profiles equivalent to 1M NaClO ₄ / PC non-aqueous sodium ion batteries. Even in the case of a full cell, the low viscosity, high sodium ion concentration, and high ion conductivity of the aqueous electrolyte solution are effective, as with the half cell described above, and the aqueous sodium ion battery is better in terms of charge / discharge overvoltage than the non-aqueous one. It was confirmed that there was.
From the above results, the positive electrode active material Na ₂ FeP ₂ O _{7 according} to the present invention can actually insert and desorb sodium in an aqueous electrolyte without causing electrolysis of water. Clearly, it has been shown that by using Na ₂ FeP ₂ O ₇ as an insertion guest in an aqueous secondary battery, an aqueous sodium ion battery with good characteristics that has never been obtained can be realized.

(Example 2)
Only the changed portion is described instead of the first embodiment. The positive electrode active material Na ₂ MnP ₂ O ₇ was synthesized by a solid phase method. Na ₂ CO ₃ , Mn ₃ O ₄ , (NH ₄ ) ₂ HPO ₄ were mixed in a stoichiometric ratio of 3: 1: 6 to the starting materials, and mixed at 200 rpm for 2 hours using a planetary ball mill. The obtained powder was calcined at 600 ° C. for 3 hours in the air, further calcined at 650 ° C. for 15 hours in the air, gradually cooled at 5 ° C./h for 20 hours, and then cooled to room temperature at 100 ° C./h. To obtain Na ₂ MnP ₂ O ₇ (FIG. 10). From the XRD pattern of the figure, most of the main phase was identified as trigonal Na ₂ MnP ₂ O ₇ , but many impurities were observed. The synthesized positive electrode active material and acetylene black (AB) were mixed at a weight ratio of 70:25, and carbon coating treatment was performed using a planetary ball mill for 200 prm-24 hours. The obtained powders (positive electrode active material and AB) were each heat-treated at 500 ° C. for 1 hour. The obtained powder (positive electrode active material / C) and PTFE were mixed at a weight ratio of 95: 5 and molded into pellets to make a positive electrode.

Preparation of non-aqueous and aqueous sodium half-cells The Na ₂ MnP ₂ O ₇ annealed product obtained in Example 2 was prepared as a positive electrode pellet, which was used as a working electrode, and the counter electrode was a Zn plate and the reference electrode was Ag. / AgCl was used. Na ₂ SO ₄ was selected as the solution electrolyte, and a beaker-type half cell was prepared using a 2M Na ₂ SO ₄ aqueous electrolyte (FIG. 4).
For comparison, a non-aqueous half-cell was manufactured using a Na ₂ MnP ₂ O ₇ positive electrode pellet as a working electrode, a counter electrode using sodium metal, and a non-aqueous electrolyte using 1 M NaClO ₄ / PC.

Non-aqueous and sodium half cell charge-discharge profile 11 to a current density of 0.2 mA / cm 1M NaClO obtained above in ² ₄ / PC nonaqueous and 12 to 2M Na ₂ SO ₄ aqueous electrolyte solution of aqueous sodium half cell Shows monopolar characteristics. In the water-based half cell of FIG. 12, a charge / discharge test is performed in a voltage range of −0.6 to 0.7 V with respect to the Ag / AgCl reference electrode, whereas in the non-aqueous sodium half-cell of FIG. A charge / discharge test was conducted in a voltage range of 7 to 4.5V. It was also found that the positive electrode active material Na ₂ MnP ₂ O _{7 according} to the present invention can be reversibly charged and discharged in an aqueous solution.

Claims (7)

A positive electrode active material for an aqueous sodium ion secondary battery, characterized by comprising sodium-containing phosphate Na ₂ MP ₂ O ₇ (M is a transition metal element).   The positive electrode active material for an aqueous sodium ion secondary battery according to claim 1, wherein M is either Fe or Mn.   The positive electrode active material for an aqueous sodium ion secondary battery according to claim 1 or 2, which has been carbon-coated.   The positive electrode active material for an aqueous sodium ion secondary battery according to any one of claims 1 to 3, which has been annealed. 5. An aqueous sodium comprising: a positive electrode comprising the positive electrode active material according to claim 1; and an electrolytic solution containing one or more of Na ₂ SO ₄ , NaNO _{3, and} NaClO ₄ as an electrolyte. Ion secondary battery. The aqueous sodium ion secondary battery according to claim 5, wherein the negative electrode is made of NaTi ₂ (PO ₄ ) ₃ . An aqueous sodium ion secondary battery comprising a positive electrode made of a positive electrode active material Na ₂ FeP ₂ O ₇ , a negative electrode made of NaTi ₂ (PO ₄ ) _3, and an electrolyte solution made of an electrolyte Na ₂ SO ₄ .

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