JP2021120931A - Electrode active material, production method thereof, and solid-state battery - Google Patents

Abstract

To provide an electrode active material with further improved voltage characteristics, a production method thereof, and a solid-state battery.SOLUTION: An electrode active material includes a compound represented by Na3Cr2(PO4)2F3. A solid-state battery comprises the electrode active material including a compound represented by Na3Cr2(PO4)2F3. A production method of the electrode active material includes a mixing step for mixing sodium fluoride and chromium phosphate in a solid phase, and a calcination step for calcining, preferably in the temperature range of 750-775°C, the mixture obtained in the mixing step.SELECTED DRAWING: Figure 5

Description

The present invention relates to an electrode active material and a method for producing the same, and also relates to an all-solid-state battery provided with the electrode active material.

Lithium-ion batteries have a higher energy density than other secondary batteries and are used in a wide range of fields such as mobile devices and mobility. In the future, there are increasing demands for long-term use of mobile devices, high power consumption, and an increase in the cruising range of mobility. Therefore, there is a demand for both high energy density and safety of secondary batteries, and expectations are rising for next-generation batteries that will replace lithium-ion batteries. One of these candidates is an all-solid-state battery.

In order to improve the energy density, it is necessary to increase the capacity of the battery per weight or increase the voltage of the battery. Various materials are being studied as high-potential positive electrode materials for improving the voltage of batteries. However, the conventional lithium ion battery has problems such as decomposition of the electrolytic solution at its high potential. In all-solid-state batteries, not only the safety advantage of replacing the electrolyte (flammable) in the lithium-ion battery with the solid electrolyte (non-flammable), but also the fact that desolvation reaction is not required and high lithium-ion conductivity Therefore, there are merits in terms of input and output. Further, many solid electrolytes are stable even at a high potential, and since the above-mentioned high-potential positive electrode material can be used, high energy density can be expected.

The all-solid-state battery is mainly composed of laminating positive and negative electrode materials and a solid electrolyte. As the electrode material, a conventional electrode material for a liquid lithium ion battery is often used. Further, as the negative electrode, an oxide such as lithium titanate, a metal such as silicon or tin, or an oxide or alloy containing the same may be used. Oxide-based and sulfide-based electrolytes are mainly used as the solid electrolyte, and in recent years, solid electrolytes having a sulfide-based electrolyte that exceeds the ionic conductivity of the electrolytic solution have been developed.

Adoption of a high-voltage positive electrode material is one method for increasing the energy density of a battery. As a positive electrode material showing the highest potential in a sodium ion battery, Na ₄ Ni ₃ (PO ₄ ) ₂ P ₂ O ₇ having an average operating voltage of about 4.8 V is known (Patent Document 1). However, Ni is a relatively expensive transition metal, and a search for a new material system has been required. _{On the other hand, Na 3} M ₂ (PO ₄ ) ₂ F ₃ (M = Ti, V, Fe) is also known as a high-voltage positive electrode material for sodium ion batteries (Patent Document 2). _{However, even when Na 3} V ₂ (PO ₄ ) ₂ F ₃ showing the highest voltage among these was used, the average operating voltage was only about 4 V. Further, in recent years, _{it has been reported that Na 3} Cr ₂ (PO ₄ ) ₃ exhibits a high potential of 4.5 V (vs. Na) (Non-Patent Document 1), but an electrode material having a higher potential is still desired. ing.

Special Table 2019-505954 Japanese Unexamined Patent Publication No. 2013-08391

K. Kawai et al., “High-Voltage Cr4 + / Cr3 + Redox Couple in Polyanion Compounds”, ACS Appl. Energy Mater., 2018, 1, 3, 928-931

Therefore, in view of the above problems, it is an object of the present invention to provide an electrode active material having further improved voltage characteristics, a method for producing the same, and an all-solid-state battery.

In order to achieve the above object, one embodiment of the present invention comprises an electrode active material, which is a compound represented by _{Na 3} Cr ₂ (PO ₄ ) ₂ F _3.

In another aspect, the present invention comprises an all-solid-state battery comprising an electrode active material containing a compound represented by _{Na 3} Cr ₂ (PO ₄ ) ₂ F _3.

Another aspect of the present invention is a method for producing an electrode active material, which comprises a mixing step of mixing sodium fluoride and chromium phosphate in a solid phase, and firing of the mixture obtained in the mixing step. It includes steps.

As described above, according to the present invention _{, by using the compound represented by Na 3} Cr ₂ (PO ₄ ) ₂ F ₃ as the electrode active material, the electrode active material having further improved voltage characteristics, the production method thereof, and the like. And all-solid-state batteries can be provided.

It is a flow chart which shows one Embodiment of the method for manufacturing the electrode active material which concerns on this invention. It is a schematic diagram which shows one Embodiment of the all-solid-state battery which concerns on this invention. It is a schematic diagram which shows another embodiment of the all-solid-state battery which concerns on this invention. It is an exploded view which shows typically the evaluation coin cell used in an Example. It is a graph which shows the diffraction pattern of the XRD measurement result of an Example, and the standard pattern from ICSD and ICDD. It is a graph which shows the charge / discharge test result of an Example together with the reference value of a conventional example. It is a graph which shows the cycle characteristic test result of an Example.

Hereinafter, the electrode active material according to the present invention, a method for producing the same, and each embodiment of the all-solid-state battery will be described with reference to the accompanying drawings.

First, an embodiment of a method for producing an electrode active material according to the present invention will be described. _{As shown in FIG. 1, in the production method of the present embodiment, Na 3} Cr is obtained from a first flow 10 for obtaining chromium phosphate from a chromium source and a phosphoric acid source, and from the chromium phosphate, a sodium source, and a fluorine source. ₂ (PO ₄ ) ₂ Includes a second flow 20 for obtaining a compound (abbreviation: NCPF) represented by F _3.

In the first flow 10, as shown in FIG. 1, first, in the stirring step 11, chromium nitrate (Cr (NO ₃ ) _{3 ) as a chromium source and diammonium hydrogen phosphate ((NH 4} )) as a phosphoric acid source. ) ₂ HPO ₄ ) is added to water, mixed and stirred to obtain a mixed solution. The ratio of the chromium source and the phosphoric acid source is preferably 1: 2 in stoichiometric ratio. The chromium source and the phosphoric acid source are not limited to the above compounds. For example, as the chromium source, _{a chromium-based material such as Cr 2} O ₃ or Cr ₂ (SO ₄ ) ₃ can be used, and phosphoric acid can be used. As the source, a phosphoric acid source-based material such _{as H 3} PO ₄ or NH ₄ H ₂ PO _{4 can be used.} The stirring step 11 is preferably performed at a temperature in the range of, for example, 70 ° C. to 90 ° C., and the stirring time is preferably in the range of, for example, 60 minutes to 2 hours.

Next, in the evaporation-drying step 12, the mixed solution obtained in the stirring step 11 is heated and evaporated to dryness to obtain a solid substance. The heating temperature is preferably in the range of, for example, 100 ° C to 120 ° C. In the tentative firing step 13, the solid matter obtained in the evaporation-drying step 12 is tentatively fired in an air atmosphere. The temperature of the tentative firing is preferably in the range of, for example, 400 ° C. to 500 ° C., and the time of the tentative firing is preferably in the range of, for example, 2 hours to 4 hours. In the pulverization step 14, the solid matter thus obtained is pulverized to obtain a powder. Then, in the main firing step 15, the powder obtained in the crushing step 14 is main fired in an air atmosphere to obtain powdered chromium phosphate (CrPO ₄ ). The temperature of the main firing is preferably in the range of 1000 ° C. to 1200 ° C., and the time of the main firing is preferably in the range of 12 hours to 24 hours, for example.

In the second flow 20, the powdered chromium phosphate obtained in the first flow 10 in the hand mill mixing step 21 and the powdered sodium fluoride (NaF) as a sodium source and a fluorine source are subjected to an air atmosphere. , Mix with a hand mill to obtain a powdery mixture. The ratio of chromium phosphate to sodium fluoride is preferably 2: 3 in stoichiometric ratio. The powdery mixture is molded into pellets. Then, in the firing step 22, the pellets are fired in an inert atmosphere. The firing temperature is preferably in the range of 750 ° C. to 950 ° C., and more preferably in the range of 750 ° C. to 775 ° C. in order to obtain a single-phase NCPF. As the inert atmosphere, for example, argon gas, nitrogen gas, or the like is preferably used. The firing time is preferably in the range of, for example, 2 hours to 6 hours. In the pulverization step 23, the fired product thus obtained is pulverized to obtain a powdered NCPF.

Since the NCPF thus obtained has a high potential of 4.7 V (vs. Na), it can exhibit excellent performance as an electrode active material. _{It is known that Na 3} Cr ₂ (PO ₄ ) ₃ (abbreviation: NCP) having a similar composition has a high potential of 4.5 V (vs. Na), but NCPF is simply as described above. Since the firing temperature range is very narrow to obtain in phase and it is difficult to synthesize in a single phase, it has not been electrochemically evaluated as a battery material. Further, NCPF has a high potential and a smaller molecular weight than NCP, so that it is considered that the theoretical capacity is also high.

Therefore, one embodiment of the electrode active material according to the present invention includes NCPF. As described above, NCPF can exhibit excellent performance as an electrode active material. Further, NCPF can improve the cell voltage by using it as an electrode active material for the positive electrode of the cell, for example. The cell may be, for example, an all-solid-state battery or a liquid-based sodium-ion secondary battery.

Next, an embodiment of the all-solid-state battery according to the present invention will be described. As shown in FIG. 2, the all-solid-state battery 30 of the first embodiment includes a positive electrode layer 31, a negative electrode layer 32, and a solid electrolyte layer 33 located between the positive electrode layer 31 and the negative electrode layer 32. The positive electrode layer 31 contains the above-mentioned NCPF as an electrode active material. Further, the positive electrode layer 31 may contain, for example, a conductive auxiliary agent, a binder, and a solid electrolyte in addition to NCPF. Examples of the conductive auxiliary agent include highly conductive carbon black such as acetylene black and Ketjen black, graphite and coke, and metal powder such as Ni powder, Cu powder and Ag powder. Examples of the binder include a fluorine-based binder such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), and a rubber-based binder such as styrene-butadiene rubber (SBR). The content of NCPF, which is the positive electrode active material, in the positive electrode layer 31 is preferably, for example, 70% by weight to 95% by weight.

The solid electrolyte layer 33 contains a sulfide-based sodium-containing solid electrolyte material and an oxide-based sodium-containing solid electrolyte material. Examples of the sulfide-based sodium-containing solid electrolyte material include Na ₃ PS ₄ , Na ₃ PS _{4- Na 4} SiS ₄ , Na ₂ SP ₂ S ₅ , Na ₂ S-SiS ₂ , and Na ₂ S-GeS _2. and so on. Examples of the oxide-based sodium-containing solid electrolyte material include NASICON type Na ₃ Zr ₂ Si ₂ PO ₁₂ , Na ₃ Zr _1.88 Y _0.12 Si ₂ PO _11.94 , and Na _{1 + x} Zr _2. There are SixP _3-x O ₁₂ (2 ≦ x ≦ 2.5) and the like. If necessary, these sulfide-based or oxide-based sodium-containing solid electrolyte materials and other solid electrolyte materials may be contained in the positive electrode layer 31.

The negative electrode layer 32 contains a negative electrode active material capable of occluding and releasing Na ions, which are conduction ions. Examples of such a negative electrode active material include a carbon-based active material and a titanium oxide-based active material. Examples of carbon-based active materials include hard carbon and graphene. Other active materials include Si-based active materials such as titanium oxide-based active material TiO ₂ , Sn-based active materials, and others, Na ₂ Ti ₃ O ₇ , NaTi ₂ (PO ₄ ) ₃ , Na ₃ V ₂ (PO ₄ ) ₃ , Na ₁₅ Sn _{4, and the} like can be mentioned. Further, the negative electrode layer 32 may contain the above-mentioned conductive auxiliary agent, a sulfide-based or oxide-based sodium-containing solid electrolyte material, and other solid electrolyte materials, if necessary.

The all-solid-state battery 30 may include a positive electrode current collector (not shown) in the positive electrode layer 31 and a negative electrode current collector (not shown) in the negative electrode layer 32, if necessary. As the material of the positive electrode current collector and the negative electrode current collector, for example, platinum, copper, stainless steel, nickel, titanium, aluminum and the like can be used.

Further, as shown in FIG. 3, the all-solid-state battery 40 of the second embodiment is a single-phase all-solid-state battery located between two current collectors 41a and b and these two current collectors 41a and b. It includes a battery layer 42. In the single-phase all-solid-state battery layer 42, the positive electrode, the negative electrode, and the solid electrolyte are made of the same material. As a result, the interfacial resistance between the electrode and the solid electrolyte is eliminated, and excellent battery characteristics can be obtained. The single-phase all-solid-state battery layer 42 includes NCPF. In addition to NCPF, the single-phase all-solid-state battery layer 42 may contain a conductive auxiliary agent, a solid electrolyte material, or the like as described above.

As the material of the current collector 41, for example, platinum, copper, stainless steel, nickel, titanium, aluminum and the like can be used as in the all-solid-state battery 30 of the first embodiment.

[1. Synthesis of Na ₃ Cr ₂ (PO ₄ ) ₂ F ₃ (abbreviation: NCPF)]
NCPF was synthesized through a two-step flow to obtain a single-phase NCPF. In the first flow, 40% aqueous chromium nitrate solution [Cr ₂ (NO ₃ ) ₃ aq, purity: 40% or more, manufactured by Junsei Chemical Co., Ltd.] and diammonium hydrogen phosphate [(NH ₄ ) ₂ HPO ₄ , purity: 99. .0% or more, manufactured by Wako] was added to water at a ratio of chemical ratio of 1: 2, and the mixture was stirred at a temperature of about 80 ° C. for about 2 hours to obtain a mixed solution. This mixed solution was heated to about 120 ° C., evaporated to dryness, and then calcined at a temperature of about 400 ° C. for about 3 hours in an air atmosphere. The solid matter thus obtained was crushed and subjected to main firing at a temperature of about 1000 ° C. for about 12 hours in an air atmosphere to obtain a powder of _{CrPO 4.}

Next, in the second flow, powdered sodium fluoride [NaF, purity: 99.0% or more, manufactured by FUJIFILM Wako) was added to this in a stoichiometric ratio of 2: 3 by a hand mill in an air atmosphere. Mixed. The resulting mixture was molded into pellets. Then, this pellet was calcined in an argon atmosphere for about 2 hours. The firing temperature was 700 ° C., 750 ° C., 775 ° C., 800 ° C., 850 ° C., 950 ° C., and 6 different temperatures. The obtained calcined products were pulverized into powders and used as samples for the following measurements.

[2. X-ray diffraction (XRD) measurement]
For the samples obtained at each firing temperature, their diffraction patterns were obtained using an X-ray diffraction measuring device (manufactured by Rigaku, product number: MiniFlex 600). The results are shown in FIG. _{As a reference, Na 3} Cr ₂ (PO ₄ ) ₂ F ₃ and Na ₃ Cr ₂ (PO ₄ ) ₃ and Na ₅ Cr registered in the Inorganic Crystal Structure Database (ICSD) or the International Center for Diffraction Data (ICDD). (PO ₄ ) ₂ F ₂ , Na ₃ (CrF ₆ ), Cr ₂ (CrO ₄ ) ₂ (Cr ₄ O ₁₃ ), CrPO ₄ , Cr ₂ O ₃ diffraction patterns are shown at the same time.

As shown in FIG. 5, the diffraction pattern of the sample obtained at the firing temperatures of 750 ° C. and 775 ° C. is the diffraction pattern of Na ₃ Cr ₂ (PO ₄ ) ₂ F ₃ (abbreviation: NCPF) registered in ICSD. The pattern and peak match, indicating that NCPF was obtained in a single phase. On the other hand, at a firing temperature of 700 ° C., 2θ has moderate intensity peaks at about 21 °, 26 °, and 37 °, and it is considered that a plurality of compounds are present in the sample. Further, at the firing temperature of 800 ° C. to 950 ° C., 2θ has a plurality of weak peaks in the range of about 25 ° to 27 ° and 37 °, and it is considered that a plurality of compounds are present in the sample.

[3. Charge / discharge measurement]
In order to test the voltage characteristics of the sample (Example) obtained at a firing temperature of 750 ° C. as an electrode active material, acetylene black (AB) and polytetrafluoroethylene (PTFE) were added to the sample (NCPF) as binders by weight. %, NCPF: AB: PTFE = 70: 25: 5 was added to prepare electrode pellets (diameter 10 mm). Then, as shown in FIG. 4, an evaluation coin cell 50 using this as a positive electrode and a sodium metal as a negative electrode was produced. The evaluation coin cell 50 is formed by stacking an upper lid 51, a spacer 52, a positive electrode 53, a titanium mesh 54, a separator 55, a negative electrode 56, a spacer 57, a gasket 58, and a lower lid 59 in this order, and enclosing an electrolytic solution (not shown). be.

The upper lid 51, the lower lid 59, and the two spacers 52 and 58 were all made of stainless steel. A titanium mesh was laminated on the spacer 52 on the upper lid side, and a nickel mesh was laminated on the spacer 57 on the lower lid side. A separator (diameter 19 mm, thickness 24 μm) manufactured by Celgard was used as the separator 55, and polypropylene (PP) was used as the gasket 58. A sodium metal foil (diameter 10 mm) was used for the negative electrode 56. As the electrolytic solution, 1M NaPF ₆ / ethylene carbonate (EC) -dimethyl carbonate (DMC) (EC: DEC = 1: 1 in volume ratio) was used. The voltage characteristics of the evaluation coin cell 50 were measured.

Charging / discharging of the evaluation coin cell 50 using a charging / discharging test device (manufactured by Nagano Co., Ltd., product number: BTS2004H) with potential regulation: 4.9-2.5V and charging capacity regulation: 127.6mAh / g (theoretical capacity). The test was conducted. The results are shown in FIGS. 6 and 7. The x-axis and y-axis of FIG. 6 indicate the potential seen from the negative electrode of sodium metal and the discharge capacity of the positive electrode. The discharge capacity is a value per metal weight. _{Note that FIG. 6 shows Na 3} Ti ₂ (PO ₄ ) ₂ F ₃ and Na ₃ V ₂ (PO ₄ ) ₂ F ₃ and Na ₃ Fe ₂ (PO ₄ ) in the same test described in Patent Document 2. ) The initial discharge curves of ₂ F _{3 are shown together.}

As shown by the first and second (second cycle) discharge curves of the example of FIG. 6, the NCPF was 4.7 V (vs. Na), which was higher than that of the conventional example described above. The voltage characteristics of Examples and Conventional Examples are also summarized in Table 1 together with _{Na 3} Cr ₂ (PO ₄ ) _{3 (abbreviation: NCP) described in Non-Patent Document 1.} Since the NCPF of the example has a smaller molecular weight than that of the conventional NCPF, it is considered that the theoretical capacity is also high.

Further, as shown in FIG. 7, the NCPF of the example had a low initial Coulomb efficiency, but the Coulomb efficiency increased to about 55% by repeating the number of cycles. On the other hand, the discharge capacity decreased as the number of cycles increased. However, it is considered that the theoretical capacity of NCPF is large, assuming that sodium of up to 2 electrons is released as compared with NCP, which is also a high-voltage material. It is considered that the reason why the measured value of the discharge capacity in the above test is low is that since NCPF is a high-voltage material, decomposition proceeds at that high voltage in the conventional electrolytic solution.

10 First flow 20 Second flow 30, 40 All-solid-state battery 31 Positive electrode layer 32 Negative electrode layer 33 Solid electrolyte layer 41 Current collector 42 Single-phase all-solid-state battery layer 50 Evaluation coin cell 51 Top lid 52, 57 Spacer 53 Positive electrode 54 Titanium mesh 55 Separator 56 Negative electrode 58 Gasket 59 Lower lid

Claims (5)

An electrode active material containing a compound represented by _{Na 3} Cr ₂ (PO ₄ ) ₂ F _3. The electrode active material according to claim 1, wherein the electrode active material is an electrode active material for a positive electrode. An all-solid-state battery comprising the electrode active material according to claim 1 or 2. It is a method of manufacturing electrode active material.
A mixing step of mixing sodium fluoride and chromium phosphate in a solid phase,
A method including a firing step of firing a mixture obtained in the mixing step. The method according to claim 4, wherein the firing temperature in the firing step is in the range of 750 to 775 ° C.

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