JP2014149943A - Active material and secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an active material capable of being used in an aqueous electrolyte, less in deterioration of discharge capacity due to repetition of charging/discharging, and excellent in cycle characteristics, and a secondary battery using the active material and an aqueous electrolyte.SOLUTION: A secondary battery having a sufficient charge/discharge function and excellent in cycle characteristics can be obtained by using: an active material that is a phosphate containing sodium and at least one element selected from transition metal elements and having a maricite-type crystal structure, as a positive electrode active material; and an aqueous electrolyte.

Description

  The present invention relates to an active material for a secondary battery and a secondary battery using the same.

  In recent years, secondary batteries have been used not only as mobile phones and notebook PCs, but also as electric vehicle batteries and residential / store storage batteries.

  A secondary battery is generally composed of a positive electrode, a negative electrode, and an electrolyte solution (quality). The positive electrode includes, for example, transition metal and alkali metal oxides, and the negative electrode includes, for example, carbon-based materials such as graphite and hard carbon. An alloy material, an oxide material, and an organic (non-aqueous) solvent or an aqueous solvent are used as a solvent in the electrolytic solution. Typical examples of such batteries include lithium ion batteries and lead batteries.

  Since lithium-ion batteries have a high energy density, they will not only supply storage batteries for electric vehicles and households, but also provide voltage stabilization and storage, such as wind power and solar power generation, and power stored at night during the day. It is also expected as a stationary large storage battery having a large capacity of several kW to several MW, which is used for the purpose of leveling the electric power load. However, in lithium ion batteries, rare metals such as Li, Co, and Ni are used as electrode materials, and the amount existing on the earth, the so-called Clark number, is small. If the demand for lithium ion batteries increases in the future, there is a concern that the unit price will rise further. Is done.

In recent years, therefore, sodium ion batteries that charge and discharge with Na ions have been researched and developed in place of lithium ion batteries. A feature of the sodium ion battery is that the ion radius of Na ions is larger than that of Li ions. Therefore, for example, when a composite oxide is used as the active material, in the case of lithium, there is an advantage that a manganese composite oxide (NaMnO ₂ ) having a layered structure that is unstable as a compound can be synthesized relatively easily. On the other hand, in the case of phosphate, the orthorhombic marisite structure is more stable than the olivine structure known from LiFePO ₄ or the like, but the marisite structure has almost no Na ion path and is electrochemical. It is generally known that it is inactive, and the active material has hardly been studied.

  Regarding sodium phosphate, for example, Patent Document 1 proposes a phosphate containing sodium and Fe or Mn as an active material of the positive electrode. In Patent Document 2, it has been proposed to use olivine-type sodium phosphate as the active material of the positive electrode.

Japanese Patent No. 2010-018472 JP 2011-034963 A

Kyu Tae Lee, L.F.Nazar, et al, Chem.mater. 2011.23.3593-3600

However, phosphate-based active materials such as the chemical formula NaMPO ₄ (where M is a transition metal element) are known to have low electrical conductivity. In particular, those having an orthorhombic marisite structure described in Patent Document 1 have low ionic conductivity, and are usually electrochemical as described in Non-Patent Document 1 and Patent Document 2. Almost inactive. This is a structural problem caused by the crystal structure, and even if the specific surface area of BET is increased as in Patent Document 1, it is difficult to obtain a sufficient capacity as an active material used for the electrode of the secondary battery.

  Therefore, Patent Document 2 proposes an Na-based phosphate having an olivine structure as an active material. However, since Na-based phosphates usually tend to have a thermodynamically stable marisite structure, when preparing Na-based phosphates having an olivine structure, the olivine structure is once thermodynamic. A Li-based phosphate that is stable, and then immersed in a solution containing Na ions or heated by contacting a Na sheet to replace Li in the Li-based phosphate with Na Na-phosphate having an olivine type structure can be obtained. However, such a manufacturing method requires many steps, time, and costs, and the Na-based phosphate having an olivine structure is originally a thermodynamically unstable crystal structure. However, there is a high possibility that the battery performance will deteriorate.

  Moreover, in alkaline ion batteries such as lithium ion batteries, non-aqueous electrolytes using ethylene carbonate or dimethyl carbonate as a solvent are used, but these solvents are flammable and have a low-temperature flash point for safety. There's a problem. On the other hand, lead batteries and nickel metal hydride batteries are generally known as batteries using aqueous electrolytes, but these use acidic or alkaline electrolytes, and lead batteries contain lead that is extremely harmful to the human body. Since it is used, there is still a problem with safety.

  The present invention has been made in view of the above problems, and an active material that can be used in an aqueous electrolyte, has little deterioration in discharge capacity due to repeated charge and discharge, and has excellent cycle characteristics, and an aqueous electrolyte. It aims at providing the used secondary battery.

  The active material of the present invention is a phosphate containing sodium and at least one element selected from transition metal elements, and has a marisite crystal structure.

  The secondary battery of the present invention includes a positive electrode, a negative electrode, and an aqueous electrolyte, and the positive electrode includes the active material described above.

  According to the present invention, it is possible to provide an active material that can be used in an aqueous electrolyte, has little deterioration in discharge capacity due to repeated charge and discharge, and has excellent cycle characteristics, and a secondary battery using the active material and the aqueous electrolyte.

It is a schematic sectional drawing of the secondary battery which is one Embodiment of this invention. It is a perspective view which shows the external appearance of a secondary battery.

A secondary battery according to an embodiment of the present invention will be described with reference to FIG. The secondary battery of this embodiment has an electrolyte layer 2 between the positive electrode 1 and the negative electrode 3, and these constitute a power generation element 4. Further, on the surface opposite to the side facing the electrolyte layer 2 of the positive electrode 1 and the negative electrode 3, a positive electrode side current collecting layer 5P and a negative electrode side current collecting layer 5N are provided, respectively. A secondary battery is formed by housing the power generation element 4 shown in FIG. 1 in a battery case as shown in FIG. There are various types of battery cases such as a laminate type, a cylindrical type, and a coin type, but any form may be used. 6P and 6N shown in FIG. 2 are a positive electrode terminal and a negative electrode terminal for electrically connecting the external circuit, the positive electrode 1 and the negative electrode 3, respectively, and 7 is a box type case used for a lead storage battery for automobiles and the like. is there. 8 is a safety valve for releasing hydrogen and oxygen generated by electrolysis during overcharge. As the electrolyte layer 2, a separator impregnated with an aqueous electrolyte is used.

  The positive electrode 1 is a phosphate containing sodium and at least one element selected from transition metal elements (hereinafter also referred to as Na-based phosphate), and an active material having a marisite crystal structure Is used. Phosphate is a highly safe material because it has a tetrahedral structure centered on oxygen atoms, with phosphorus atoms at the vertices, so the bonds between oxygen atoms and phosphorus atoms are strong and it is difficult to release oxygen. In the Na-based phosphate, since the marisite structure is thermodynamically stable, stable battery performance can be obtained. It should be noted that a material having a marisite crystal structure has a low electronic conductivity, so that it is usually difficult to use it as an active material. However, when an aqueous electrolyte having high ionic conductivity is used, the material is filled as an active material. Discharge can be performed.

  The transition metal element contained in the Na-based phosphate preferably contains at least manganese. In the case of a secondary battery using an aqueous electrolyte, the most important point is the electrolysis of water. From the Nernst equation, when the pH is 7, the potential at which oxygen is generated is +0.82 V with respect to the standard hydrogen electrode. This is 3.5V when converted to a potential difference of Na / Na + which is an oxidation-reduction potential of Na, and is about 3.9V even when overvoltage is taken into consideration. That is, the active material used for the sodium ion secondary battery using the aqueous electrolyte solution needs to be an active material that can be charged at a potential of 3.9 V or less.

In the Na-based phosphate represented by the chemical formula NaMPO ₄ (where M is a transition metal element), in order to set the charging voltage to 3.9 V or less, the transition metal element M that is a constituent element thereof is Mn or Fe. May be used. In particular, when Mn is used as M, the charging potential is close to the electrolysis voltage of water and a high voltage is obtained, which is preferable. For example, when Co, Ni, or the like is used as M, the chargeable potential is 4.2 V or more, which is too high with respect to the water electrolysis potential, that is, cannot be charged in a voltage range where water does not electrolyze. Therefore, it cannot be used as an active material used for a sodium ion battery using an aqueous electrolyte.

  In this embodiment, it is preferable to have a substance having electron conductivity on the surface of Na-based phosphate or in the vicinity of the surface. A material having a marisite crystal structure has low electronic conductivity, so it is usually difficult to use it as an active material. However, by using an aqueous electrolyte with high ionic conductivity, sufficient charge and discharge as an active material can be achieved. Demonstrate the function. Further, the presence of a substance having electron conductivity on the surface of Na-based phosphate having a marisite structure supplements the electron conductivity of Na-based phosphate as an active material, and a secondary battery using an aqueous electrolyte. It can be suitably used as a positive electrode active material. Examples of such a material having electronic conductivity include conductive carbon, metal, and metal compound.

These can be provided on the surface of the Na-based phosphate by, for example, CVD, sputtering, vapor deposition, or the like. In particular, when the metal compound containing Fe and P is a phosphate containing Fe, it is a constituent element. Based on the above, it is preferable because it can be easily formed without adding an additive or increasing the number of steps. Of metal compounds containing Fe and P, Fe ₂ P and Fe ₃ P are particularly preferable because they are electrochemically stable and have sufficient conductivity.

The content of Fe contained in the Na-based phosphate is preferably in the range of 1 to 20 atomic% with respect to the total amount of the transition metal element M. Fe and P sufficient to express the conductivity necessary for using Na-based phosphate having a marisite structure as an active material by setting the Fe content in the Na-based phosphate in such a range. The metal compound can be formed on the surface of the Na-based phosphate or in the vicinity of the surface, and the effect of lowering the potential due to the introduction of Fe can be reduced.

  In addition, a coating layer containing carbon may be formed on the surface of the Na-based phosphate having a marisite structure, and this coating layer is further provided on a metal compound of Fe and P. There may be. Thereby, electroconductivity can be improved further. The coating layer is preferably composed mainly of electrochemically stable carbon. In addition, the coating layer containing carbon does not need to cover the entire surface of the Na-based phosphate, and may cover, for example, 1% or more of the surface. By covering 1% or more of the surface of the phosphate, sufficient conductivity can be obtained between the active materials during charge and discharge when the positive electrode is formed using the phosphate as the active material.

  The average thickness of the coating layer containing carbon is preferably in the range of 1 to 100 nm. When the thickness is greater than 100 nm, ion conduction between the active material and the electrolyte becomes difficult, and the ionic resistance increases. When the thickness is less than 1 nm, sufficient conductivity may not be obtained between the active materials.

  In addition, it can confirm that an active material has the substance which has electronic conductivity on the surface by impedance measuring methods, such as a Nyquist plot. In addition, the type of substance having electron conductivity can be confirmed by EDS (energy dispersive X-ray spectroscopy). The carbon coating layer can be confirmed by a transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDS). Further, the thickness of the carbon coating layer may be confirmed using a transmission electron microscope (TEM). For example, a cross-sectional photograph of a phosphate is taken with a TEM, and the thickness of the carbon coating layer is determined by EDS. Five locations are measured, and the average may be the average thickness of the carbon coating layer.

  The composition in the active material can be confirmed by ICP emission spectral analysis or the like. The crystal structure of the active material can be confirmed by performing X-ray diffraction (XRD) measurement of the active material and identifying the crystal phase from the obtained X-ray profile.

  As an example of a method for producing such an active material, synthesis of a phosphate containing Na, Mn, and Fe by a solid phase method will be described. As the Na source, for example, sodium carbonate, sodium hydrogen carbonate, sodium hydroxide and the like can be used. As the Mn source, manganese trioxide (III), manganese dioxide (IV), manganese carbonate (II) and the like are used. As the Fe source, ferric oxide (III), ferrous oxide (II), and iron trioxide ( II, III) and the like can be used. As the P source, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and other phosphates can be used. A predetermined amount of these raw material powders are mixed, mixed in a solvent such as alcohol, dried, and then heat-treated in a non-oxidizing atmosphere at a temperature of 600 to 1100 ° C, preferably 700 to 1000 ° C. , Na, Mn, and Fe-containing phosphate (hereinafter, also simply referred to as phosphate) can be synthesized. In addition to the solid phase method, any known synthesis method such as a hydrothermal method or a sol-gel method may be used. The obtained phosphate may be subjected to particle size adjustment by pulverization by a technique such as a ball mill or a bead mill, if necessary. When adjusting the particle size, the average particle size of the phosphate powder is preferably set to 0.5 to 1.0 μm, for example, which can cope with high output and is preferable. The average particle size of the powder can be confirmed by, for example, particle size distribution measurement by a diffraction scattering method.

  In order to form a metal compound containing Fe and P on or near the surface of the Na-based phosphate, the obtained phosphate may be heat-treated in a non-oxidizing atmosphere. Further, heat treatment may be performed in a non-oxidizing atmosphere during the synthesis of the above-described phosphate, and a metal compound containing Fe and P can be formed more easily than in this case.

The carbon coating layer is obtained by mixing the obtained phosphate powder and a carbon source such as PVA or a phenol resin by a known method such as a ball mill and firing at a temperature of 500 to 900 ° C. in a non-oxidizing atmosphere. Can be formed.

  Next, an electrode is produced using the obtained phosphate. For example, 80% by mass of this phosphate as an active material, 15% by mass of acetylene black as a conductive assistant, 5% by mass of polytetrafluoroethylene (PTFE) as a binder, and water as a solvent are added to form a slurry. Make it. The prepared slurry is applied on a Ti foil, for example, by a known molding method such as a doctor blade method or a roll press, and the solvent is dried, so that the active material that is a phosphate containing Na, Mn, and Fe is electrically conductive. An electrode containing an auxiliary agent and a binder (binder) can be produced.

  In addition to polytetrafluoroethylene, the binder can be selected and used depending on the application, such as polyvinylidene fluoride, fluorine-based rubber, styrene butadiene rubber, polyimide resin (PI), polyamide resin, and polyamideimide resin. Also, the conductive auxiliary agent is ketjen black, carbon nanotube, graphite, hard carbon, metal (Ni, gold, platinum, etc.) powder instead of acetylene black, inorganic conductive oxide (indium tin oxide (ITO) glass, Any material may be used as long as it is chemically stable and exhibits conductivity in the operating voltage range, such as tin oxide.

  Alternatively, an electrode may be formed using the obtained composite oxide powder by forming a green compact by a molding method such as extrusion molding or a roll compaction method. Moreover, you may form as a sintered compact obtained by baking without including a conductive support material and a binder.

  The average particle size of the active material particles in the electrode may be adjusted by selecting an appropriate range according to the use conditions such as the voltage range and temperature of the secondary battery using the active material. Preferably there is. If the average particle size of the active material particles is too small, it can cope with high output, but the contact area with the electrolytic solution becomes large, so that deterioration may easily occur depending on use conditions. Also, the tape molding itself becomes difficult. On the other hand, if it is too large, there is a concern that protrusions may be formed in the tape molding or the output response of the battery will be reduced. The average particle diameter of the active material particles in the electrode is determined by, for example, distinguishing the active material particles in the cross section of the electrode using a scanning electron microscope (SEM) and wavelength dispersive X-ray analysis (WDS), and taking a photograph. It can be obtained by analyzing and calculating.

As the active material used for the negative electrode 3, activated carbon, NaTi ₂ (PO ₄ ) ₃ , LiTi ₂ (PO ₄ ) ₃ , Ti ₂ P ₂ O _7, or the like can be used. In particular, a high energy density can be obtained by using either NaTi ₂ (PO ₄ ) ₃ or LiTi ₂ (PO ₄ ) ₃ .

  As the electrolyte layer 2, a separator impregnated with an aqueous electrolyte is used. The separator impregnated with the aqueous electrolyte solution is required to pass ions and prevent a short circuit between the positive and negative electrodes. Specifically, a polyolefin fibrous nonwoven fabric, a polyolefin microporous film, a glass filter, a ceramic porous material, or the like can be used. Here, examples of the polyolefin include polyethylene and polypropylene, and a separator generally used for a secondary battery such as a lithium ion battery is applicable.

As the aqueous electrolyte, a 1-2 mol / L sodium sulfate aqueous solution can be used. Since the electrolysis potential of water can be changed by adjusting the pH, the charging potential can also be changed. As the electrolyte, sodium nitrate can be used in addition to sodium sulfate, and any sodium salt that dissolves in water may be used. By using the aqueous solution for the positive electrode of the water-based secondary battery using the electrolytic solution, it is possible to provide a secondary battery with high safety and little deterioration in discharge capacity due to repeated charge and discharge and excellent cycle characteristics.

  A material having corrosion resistance that does not generate dissolution or oxygen at the potential of the positive electrode may be used for the positive current collecting layer 5P. As such a material, for example, nickel, titanium, stainless steel, gold, platinum, graphite sheet or the like can be used. Among these, titanium and nickel are preferable because they are easily available and the cost is relatively easily available. In particular, titanium is preferable because it is excellent in corrosion resistance even at a high potential.

  A material that does not cause side reactions such as alloying with Na at the potential of the negative electrode may be used for the negative current collecting layer 5N. As such a material, for example, a metal material including nickel, titanium, stainless steel, an alloy, a carbonaceous material such as graphite, hard carbon, glassy carbon, or the like can be used. In particular, it is preferable to use titanium or nickel from the viewpoint of high conductivity and relatively low cost.

  The positive electrode side current collecting layer 5P and the negative electrode side current collecting layer 5N may use a metal foil or mesh made of these metal materials, or an inorganic conductive oxide such as a metal material, a carbonaceous material, ITO glass or tin oxide. A conductive ink or the like using a material material as a filler may be applied to the electrode material surface and dried.

  In addition, when using metal foil or a mesh, it is preferable that the thickness shall be 5-40 micrometers. Moreover, when using metal foil, you may use what roughened the surface of metal foil in order to improve the adhesive force with electrode material. In this case, the surface roughness of the metal foil is preferably 0.5 to 2 μm in terms of arithmetic average roughness (Ra). The surface roughness of the metal foil is measured using a surface roughness meter such as a stylus type or a light interference type, a laser microscope, an atomic force microscope (AFM), or the like. When using a stylus-type surface roughness meter that is generally used, based on JIS B0601, for example, on the condition that the stylus tip diameter is 2 μm, the measurement length is 4.8 mm, and the cutoff value is 0.8 mm Just measure.

  The secondary battery of the present embodiment has been described above, but the present invention is not limited to the present embodiment, and can be applied to various modifications without departing from the present invention.

  Hereinafter, the secondary battery of this invention is demonstrated in detail based on an Example. First, Na-based phosphate as a positive electrode active material was synthesized. As raw materials, sodium hydrogen carbonate powder (manufactured by Kanto Chemical), manganese trioxide (III) powder (manufactured by high purity chemical), iron (III) oxide powder (manufactured by Kanto Chemical), and diammonium hydrogen phosphate (manufactured by Taihei Chemical Sangyo) ) Was used.

These raw materials, composition formula _{NaMn 1-x Fe x (PO} 4) x and y are blended so that the value shown in Table 1 when expressed in _y, slurried with isopropyl alcohol (IPA) as a solvent And ZrO ₂ balls were mixed in a ball mill for 20 hours. After drying the mixed slurry, heat treatment was performed for 10 hours in a non-oxidizing atmosphere at the temperatures shown in Table 1 to synthesize Na-based phosphate. The synthesized Na phosphate was subjected to X-ray diffraction (XRD) measurement using CuKα rays, and the crystal phase contained in the Na phosphate was confirmed by analyzing the diffraction pattern. Further, the composition of the Na-based phosphate was confirmed by ICP emission spectroscopic analysis, and the Fe content relative to the total amount of transition metal elements detected was calculated. Table 1 shows the crystal phase and Fe content contained in the synthesized Na-based phosphate.

Furthermore, the following process was performed about the sample which forms a carbon coating layer. First, 10 mass% polyvinyl alcohol (PVA) was added with respect to 100 mass% of produced Na-type phosphate, and it mixed, heating. Polyvinyl alcohol was used as a 20% by mass aqueous solution. The obtained mixture of Na-based phosphate and polyvinyl alcohol was heat-treated at 800 ° C. for 10 minutes in a non-oxidizing atmosphere. About the sample obtained in this way, it was confirmed that the carbon coating layer was formed using the transmission electron microscope (TEM) and the energy dispersive X-ray spectroscopy (EDS). At the same time, it was confirmed whether or not the metal compound of Fe and P was present on or near the surface of all the samples. The results are shown in Table 1.

  Next, a positive electrode was produced using these produced Na-based phosphates as an active material. As a positive electrode active material, 80% by mass of the synthesized Na-based phosphate powder, 15% by mass of acetylene black as a conductive auxiliary agent, and 5% by mass of polytetrafluoroethylene as a binder were sufficiently mixed in a mortar. The polytetrafluoroethylene used was previously dispersed in water at a solid content concentration of 60% by mass. The obtained mixture was formed into a sheet having a thickness of 300 μm by a roll press, and further cut into a disk having a diameter of 2 cm to produce a positive electrode.

  The negative electrode active material shown in Table 2 was used. 80% by mass of the negative electrode active material, 15% by mass of acetylene black as a conductive additive, and 5% by mass of polytetrafluoroethylene as a binder were mixed in a mortar, passed through a mesh, and then subjected to press molding to have a diameter of 2 cm and a thickness of 3 mm. Disk-shaped negative electrodes of (activated carbon) and 300 μm (other negative electrode active materials) were prepared. The polytetrafluoroethylene used was the same as that for the positive electrode.

  A Ti mesh was pressure-bonded to the produced positive and negative electrodes to form a current collecting layer. Using the produced positive electrode and negative electrode, a positive electrode, a separator, and a negative electrode were arranged in this order inside the battery evaluation cell and assembled, and an electrolytic solution was injected. A polypropylene / polyethylene porous membrane was used as the separator, and the electrolyte was an aqueous electrolyte in which 1 mol / L of sodium sulfate was dissolved in pure water. An Ag / AgCl electrode was used as the reference electrode.

  The charge / discharge test of the manufactured cell was performed under the following conditions.

Charging / discharging voltage range: upper limit 1.45V, lower limit -0.5V (vs. Ag / AgCl)
Charge / discharge current value: 1.0 mA / cm ² (constant current charge / discharge)
Measurement temperature: 30 ° C
Table 2 shows the initial discharge capacity per mass of the positive electrode active material and the discharge capacity retention rate after a 50-cycle charge / discharge test with one discharge-charge cycle.

Sample No. 1 to 11 are composed of a marisite phosphate containing Na and a transition metal element as a positive electrode active material, and constitute a power generation element using an aqueous electrolyte, and thus sufficiently function as a secondary battery. And it had excellent cycle characteristics. In particular, Sample No. having Fe ₃ P and a carbon coating layer. 1 to 4 and 8 to 11 all showed a high initial discharge capacity of 100 mAh / g or more. Sample No. 2, the diffraction peak of the Fe ₃ P crystal phase could not be confirmed by X-ray diffraction (XRD) measurement, but the presence of Fe and P metal compounds on the active material surface could be confirmed by a transmission electron microscope (TEM). It was.

DESCRIPTION OF SYMBOLS 1 .... Positive electrode 2 .... Electrolyte layer 3 .... Negative electrode 4 .... Electric power generation element 5N ... Negative electrode side current collecting layer 5P ... Positive electrode side current collecting layer 6N ... Negative electrode terminal 6P ... Positive terminal 7 ... Resin case 8 ... Safety valve

Claims (10)

  An active material characterized in that it is a phosphate containing sodium and at least one element selected from transition metal elements, and has a marisite crystal structure.   The active material according to claim 1, wherein the phosphate contains manganese.   3. The active material according to claim 1, wherein the active material has a metal compound containing Fe and P on at least one of the surface of the phosphate and the vicinity of the surface. 4. The active material according to claim 3, wherein the metal compound containing Fe and P is at least one of Fe ₂ P and Fe ₃ P.   The active material according to claim 2, wherein the phosphate further contains iron.   The active material according to claim 5, wherein the iron content is in the range of 1 to 20 atomic% with respect to the total amount of the transition metal elements contained in the phosphate.   The active material according to claim 1, further comprising a coating layer containing carbon on at least a part of the surface of the phosphate.   The active material according to claim 7, wherein an average thickness of the coating layer is in a range of 1 to 100 nm.   A secondary battery comprising a positive electrode, a negative electrode, and an aqueous electrolyte, wherein the positive electrode includes the active material according to claim 1. The secondary battery according to claim 9, wherein the negative electrode includes at least one of NaTi ₂ (PO ₄ ) ₃ and LiTi ₂ (PO ₄ ) ₃ as an active material.

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