JP5499281B2 - Positive electrode active material, magnesium secondary battery, and method for producing positive electrode active material - Google Patents

Description

  The present invention relates to a positive electrode active material, a magnesium secondary battery, and a method for producing a positive electrode active material.

  Conventionally, a mixture of a metal oxide and sulfur is known as a positive electrode active material of a secondary battery (see, for example, Patent Documents 1 to 4). In the composite electrode described in Patent Document 1, an organic disulfide compound is dissolved in a 2-pyrrolidone derivative, and polyaniline is further added to make a uniform liquid. It is a uniform mixture of a compound and polyaniline.

The composite cathode described in U.S. Patent No. 6,057,031 includes an electroactive sulfur-containing cathode material that includes a polysulfide moiety of formula -Sm- in the oxidized state and an electroactive transition metal chalcogenide composition. The positive electrode for a lithium-sulfur battery described in Patent Document 3 includes a positive electrode active material made of inorganic sulfur (S ₈ ), a sulfur series compound, and a mixture thereof. The positive electrode material for a lithium battery described in Patent Document 4 has a transition metal oxide such as vanadium oxide and a chalcogenide composite compound or an adsorbate layer including the oxide disposed on the surface of the transition metal oxide. Both the metal oxide and the adsorbate layer are electrochemically active.

  On the other hand, batteries using magnesium or an alloy thereof as a negative electrode active material are known (see, for example, Patent Documents 5 and 6 and Non-Patent Documents 1 and 2). In these batteries, a positive electrode active material containing a magnesium compound or the like is used.

Japanese Patent Application Laid-Open No. 7-211310 JP 2000-511342 A JP 2004-179160 A International Publication No. 2003/103078 Pamphlet JP 2001-76720 A JP 2002-25555 A

P. Novak and J. Desilvestro, J. Electrochem. Soc. 1993, 140, 140. Y. Long, X. Zhang, J. Colloid Int. Sci. 2004, 278, 160.

  As described above, a mixture of a metal oxide and sulfur is used as the positive electrode active material of the secondary battery. However, the sulfide is easily dissolved in the electrolytic solution as a sulfur compound, and the structure is unstable. On the other hand, the structure of oxide is stable, but since the bond between cation and oxygen is strong, it is difficult to desorb the cation from the combined metal oxide and cation after discharge and it is difficult to charge. . If this is forcibly charged at a high voltage, the crystal structure may be destroyed. As a result of this action, the cycle characteristics of the secondary battery are lowered. In particular, in a magnesium secondary battery, when a mixture of a metal oxide and sulfur is used as a positive electrode active material, the Mg—S bond is strong and the sulfur compound is easily dissolved in the electrolytic solution. As a result, the cycle characteristics are significantly deteriorated.

  On the other hand, the metal oxide constituting the positive electrode active material is easily reduced, and sulfur is easily oxidized and volatilized. All of them lose electrochemical activity when reduced or oxidized. Therefore, in order to suppress the dissolution of sulfur, it is important that the metal oxide is not reduced and the sulfur is not oxidized, and firing is necessary. However, in the method of firing by controlling the oxygen concentration at a low temperature, sulfur is oxidized or the metal oxide is reduced. Thus, it is not easy to improve the cycle characteristics of the secondary battery.

  This invention is made | formed in view of such a situation, and it aims at providing the manufacturing method of the positive electrode active material which improves the cycling characteristics of a secondary battery, a magnesium secondary battery, and a positive electrode active material.

  (1) In order to achieve the above object, a positive electrode active material according to the present invention is a positive electrode active material for a secondary battery, and includes a particulate metal oxide and particulate sulfur distributed on the surface thereof. It is characterized by becoming. Thereby, sulfur distributed on the surface of the metal oxide prevents the cation from coming into contact with oxygen, and can inhibit the strong bond between the cation and oxygen during discharge. As a result, charging by detachment of cations can be facilitated, and the cycle characteristics of the secondary battery can be improved.

  (2) Further, the positive electrode active material according to the present invention is characterized in that both the metal oxide and sulfur distributed on the surface thereof are electrochemically active. Thereby, since a metal oxide and sulfur chemically react chemically, discharge and charge of a secondary battery become easy. Note that electrochemically active means a chemical reaction that involves the delivery of electrons.

  (3) Moreover, the positive electrode active material which concerns on this invention is produced | generated by adding water to the mixture of a metal oxide and sulfur, and baking it. As a result, the temperature does not rise too much during firing, and oxidation and reduction are controlled by the boiling state of water, so that a positive electrode active material in which sulfur is distributed on the surface of the metal oxide is formed.

  (4) A magnesium secondary battery according to the present invention is characterized by using the positive electrode active material. In particular, in a magnesium secondary battery, sulfur is easily bonded to magnesium, magnesium is easily detached, and the cycle characteristics are improved.

  (5) Moreover, the manufacturing method of the positive electrode active material which concerns on this invention is a manufacturing method of the positive electrode active material used for a secondary battery, Comprising: The step which mixes a metal oxide and sulfur, The said metal oxide, The method includes the steps of adding water to the mixture with sulfur and firing the mixture to which the water has been added. Thus, by adding water to the mixture of metal oxide and sulfur, the temperature does not rise too much during firing, and oxidation and reduction are controlled by the boiling state of water, so that the surface of the metal oxide A positive electrode active material in which sulfur is distributed is formed.

  (6) Moreover, the manufacturing method of the positive electrode active material which concerns on this invention is characterized by performing the said baking by heating water with a microwave. As described above, the particles can be uniformly heated by the internal heating by the microwave, and the positive electrode active material having excellent cycle characteristics can be easily formed in a short time.

  (7) Moreover, the manufacturing method of the positive electrode active material which concerns on this invention is characterized by performing the said baking by water plasma. Thus, since the mixture is fired with water plasma, sulfur oxidation and metal oxide reduction can be further suppressed, and a positive electrode active material having excellent cycle characteristics can be formed in a short time.

  (8) Moreover, the method for producing a positive electrode active material according to the present invention is characterized in that the water plasma is generated by microwave discharge of water held between carbon felt pieces under reduced pressure. Thereby, water molecules can be evenly distributed and uniform baking can be performed.

  According to the present invention, sulfur distributed on the surface of the metal oxide prevents cations from contacting oxygen and can inhibit the strong binding between cations and oxygen during discharge. As a result, charging by detachment of cations can be facilitated, and the cycle characteristics of the secondary battery can be improved.

It is a schematic diagram which shows the structure of the secondary battery of this invention. It is a perspective view which shows the apparatus for producing water plasma. It is a graph which shows the emission spectrum at the time of microwave irradiation. It is a XRD profile of the metal oxide before a process. It is an XRD profile of the produced positive electrode active material. It is a figure which shows the discharge curve of the secondary battery of a comparative example. It is a figure which shows the discharge curve of the secondary battery of an Example. It is a figure which shows the discharge curve of the secondary battery of a comparative example. It is a figure which shows the discharge curve of the secondary battery of an Example. It is a figure which shows the charge curve and discharge curve of the secondary battery of a comparative example. It is a figure which shows the charge curve and discharge curve of the secondary battery of an Example. It is a graph which shows the capacity | capacitance of a secondary battery with respect to the addition amount of sulfur. It is a figure which shows the charging / discharging curve of the secondary battery produced by microwave irradiation. It is a figure which shows the charging / discharging curve of the secondary battery produced by the heating of the water with an electric furnace. It is a figure which shows the discharge curve when not baking. It is a figure which shows the discharge curve in the case of baking at 200 degreeC.

  Next, embodiments of the present invention will be described with reference to the drawings.

(Configuration of secondary battery)
FIG. 1 is a schematic diagram showing a configuration of a secondary battery 100 of the present invention. As shown in FIG. 1, the secondary battery 100 of the present invention includes a positive electrode 110, a separator 120, and a negative electrode 130.
The positive electrode 110 includes a positive electrode current collector (not shown) and a positive electrode active material 115. The positive electrode current collector constitutes a positive electrode together with the positive electrode active material, and donates electrons to the positive electrode active material during discharge. The positive electrode active material 115 includes a particulate metal oxide and particulate sulfur distributed on the surface thereof.

  The metal oxide and sulfur constituting the positive electrode active material 115 are both electrochemically active. Thereby, since a metal oxide and sulfur chemically react chemically, discharge and charge of a secondary battery become easy. As a result, the positive electrode active material 115 is reduced by cations in the electrolyte during discharge. When the discharge is performed, the positive electrode active material 115 is reduced, and the cation in the electrolyte solution 125 becomes a combination of a metal oxide and a cation.

  At that time, the presence of sulfur on the surface of the positive electrode active material 115 prevents direct contact between the cation and oxygen, thereby inhibiting the binding between the cation and oxygen. Further, at the time of charging, cations can be detached from the positive electrode active material 115, and charging becomes easy. As a result, the cycle characteristics of the secondary battery can be improved. This is because the sulfur placed on the surface of the metal oxide prevents the cation from coming close to the surface of the metal oxide until the sulfur interferes with it and the cation cannot bind to oxygen to form an ordered crystal structure. It can be inferred that this is formed.

The metal oxide is preferably V ₂ O ₅ , MnO ₂ , MoO ₃ or the like. It is preferable that sulfur distributed on the surface of the metal oxide exists mainly in an electroactive state having an S—S bond such as S ₈ and that SO ₂ and SO ₄ are small. Therefore, it is necessary to suppress sulfur oxidation when the positive electrode active material 115 is manufactured.

  The ratio of the metal oxide and sulfur constituting the positive electrode active material 115 is preferably in the range of 5: 1 to 3: 2 in molar ratio. When the ratio of sulfur is smaller than this range, since the surface of the metal oxide cannot be covered with sulfur, the cycle characteristics deteriorate. On the other hand, when the ratio of sulfur is larger than this range, the sulfur becomes excessive and the electric resistance of the secondary battery increases. And since a secondary battery becomes electrochemically inactive, cycling characteristics fall.

  The separator 120 separates the positive electrode 110 and the negative electrode 130 and holds the electrolytic solution 125 to maintain ionic conductivity between the positive electrode 110 and the negative electrode 130. The separator 120 has a liquid holding ability and holds the electrolytic solution 125. The electrolytic solution 125 contains cations. Charging / discharging is possible as the oxidation-reduction reaction proceeds in the electrolytic solution. Examples of the cation include magnesium ion and lithium ion.

  As the electrolytic solution 125, a solution generally used in most aqueous batteries or non-aqueous batteries can be used. The negative electrode 130 causes an oxidation reaction during discharge. For the negative electrode 130, for example, magnesium or lithium can be used. The negative electrode 130 is not particularly limited as long as it does not interfere with the function of the positive electrode active material 115, but is preferably composed of magnesium. In particular, in a magnesium secondary battery, sulfur is easily bonded to magnesium, magnesium is easily detached, and cycle characteristics are improved.

(Method for manufacturing secondary battery)
Next, a method for manufacturing a secondary battery will be described. First, a positive electrode active material is prepared. Metal oxide and sulfur are mixed at a predetermined molar ratio in the range of 5: 1 to 3: 2, and water is added to the mixture. The amount of water to be added is not less than the amount that evaporates and burns out during the firing time, and is preferably such that the powder of the mixture is moist at the end of firing. This is because when water disappears completely, reduction of metal oxides and volatilization of sulfur occur. In order to produce 2 g of the positive electrode active material, a standard of 1 g is used.

  Next, the mixture to which water has been added is fired. Examples of the firing method include (1) a method of heating water by electricity or combustion, (2) a method of heating water by microwaves, and (3) a method of performing water plasma. By adding water, the temperature does not rise too much during firing, and oxidation and reduction are controlled by the boiling state of water, so that a positive electrode active material in which sulfur is distributed on the surface of the metal oxide is formed.

  (1) The method of heating water by electricity or combustion can be performed by normal firing in a furnace. By heating a mixture of a metal oxide and sulfur to which water has been added at 100 ° C. or more for 1 hour or more, the surface of the metal oxide is activated and sulfur is fired.

  (2) In the method of heating water with microwaves, the water is heated to 100 ° C. or higher with microwaves, and water is boiled for several minutes. Particles can be heated uniformly by internal heating using microwaves, and a positive electrode active material having excellent cycle characteristics can be easily formed in a short time. Thus, when heating water, it is preferable to set it as 100 degreeC or more under atmospheric pressure.

  (3) In the method using water plasma, for example, water held between carbon felt pieces under reduced pressure can be subjected to microwave discharge to generate water plasma. Although boiling of water is required, it can be performed at a low temperature by reducing the pressure. The treatment can be performed in a few minutes, and since the temperature is low, oxidation of sulfur and reduction of metal oxide can be suppressed.

  FIG. 2 is a perspective view showing an apparatus for generating water plasma between the carbon felt pieces 215. As shown in FIG. 2, a vacuum chamber 220 is provided in a microwave irradiation chamber 210, and a material in which a raw material 218 is sandwiched between two carbon felt pieces 215 is installed. Thereby, water molecules can be evenly distributed and uniform baking can be performed. The raw material 218 is obtained by mixing a metal oxide and sulfur and adding an appropriate amount of water. In FIG. 2, the microwave irradiation chamber 210 is drawn with a broken line and the vacuum chamber 220 is drawn with a solid line, and the carbon felt piece 215 and the raw material 218 therein are shown to be transparent.

  With such an apparatus configuration, the inside of the vacuum chamber 220 is depressurized to 0.01 MPa or less, and the raw material 218 is irradiated with microwaves to be discharged to generate water plasma, whereby the positive electrode active material 115 can be manufactured. . Thus, since it bakes with water plasma, the oxidation of sulfur and the reduction | restoration of a metal oxide can be suppressed further, and the positive electrode active material excellent in cycling characteristics can be formed in a short time.

  In this way, the positive electrode active material 115 obtained is brought into contact with the positive electrode current collector to produce the positive electrode 110. Next, the negative electrode 130 is prepared using Mg metal or the like, and a secondary battery is manufactured using an aqueous or non-aqueous solution as the electrolytic solution 125.

  Below, preparation of a positive electrode active material and the charge / discharge experiment of a secondary battery using the same will be described.

(Production of S-MnO ₂ )
MnO ₂ and S were mixed at a predetermined molar ratio in the range of 5: 1 to 3: 2, and water was added to the mixture for firing. Firing was performed in a 500 W microwave oven with the mixture sandwiched between carbon felt pieces. Performed with water plasma. Water plasma was generated by depressurizing the vacuum chamber to 0.001 MPa, and microwave-discharging the mixture held between the carbon felt pieces under the reduced pressure. Microwave irradiation was performed twice for 40 seconds.

FIG. 3 is a graph showing an emission spectrum during microwave irradiation. As shown in FIG. 3, OH, H ₂ O and H peaks appear, and it was found that water plasma was generated. The positive electrode active material thus produced was subjected to X-ray diffraction measurement (XRD) using copper Kα rays.

FIG. 4A is an XRD profile of the metal oxide before processing. FIG. 4B is an XRD profile of the produced positive electrode active material. Although the profile of MnO ₂ appears in both FIG. 4B and FIG. 4A, the peak of sulfur that does not exist in FIG. 4A appears in FIG. 4B, and the produced positive electrode active material contains sulfur. confirmed.

(Cycle characteristics of S-MnO ₂ )
A positive electrode was constructed using the positive electrode active material of S-MnO ₂ produced above, and a secondary battery having Mg as the negative electrode was produced as an example. Further, as a comparative example, a secondary battery having MnO ₂ as a positive electrode active material and Mg as a negative electrode was produced. And about each secondary battery 100 of a comparative example and an Example, charging / discharging was repeated and cycling characteristics were measured. FIG. 5A is a diagram showing a discharge curve of a secondary battery of a comparative example, and FIG. 5B is a diagram showing a discharge curve of the secondary battery of an example. A hollow arrow in the figure indicates a transition direction of an approximate charge / discharge curve with respect to an increase in the number of charge / discharge cycles (the same applies hereinafter).

As shown in FIG. 5A, in the comparative example, the electromotive force was maintained up to a capacity of 80 mAhg ⁻¹ at the time of the first discharge, but at the time of the fifth discharge, the voltage significantly decreased in the range where the capacity exceeded 20 mAhg ^−1. In the vicinity of 40 mAhg ⁻¹ , the capacity was almost 0V. Further, as shown in FIG. 5B, the capacity in the embodiment is maintained a voltage of approximately 1V at 140MAhg ^-1 lesser extent, particularly capacity is maintained approximately 1.6V at 70MAhg ^-1 lesser extent. It was also found that a high voltage was maintained even after repeated discharge. Thus, the sulfur discharge increased the discharge capacity and improved the cycle characteristics of the secondary battery. In the range where the voltage of about 1 V is maintained, sulfur mainly acts as an active material, and in the range where about 1.6 V is maintained, mainly MnO ₂ acts as an active material. It is thought that.

(Cycle characteristics of S-MoO ₃ )
Similarly to the positive electrode active materials of MnO ₂ and S—MnO ₂ described above, positive electrode active materials of MoO ₃ and S—MoO ₃ were prepared, and secondary batteries using Mg as a negative electrode were prepared as examples. Further, as a comparative example, a secondary battery having MoO ₃ as a positive electrode active material and Mg as a negative electrode was produced. And charge / discharge was repeated about each secondary battery of the comparative example and the Example, and cycling characteristics were measured. 6A is a diagram illustrating a discharge curve of a secondary battery of a comparative example, and FIG. 6B is a diagram illustrating a discharge curve of the secondary battery of an example.

As shown in FIG. 6A, in the comparative example, the electromotive force was maintained up to the capacity of 150 mAhg ⁻¹ at the first or second discharge, but the voltage from the capacity of 0 mAhg ⁻¹ at the fourth to sixth discharges. Was significantly reduced, and became approximately 0 V in the vicinity of 40 mAhg ⁻¹ . Further, as shown in FIG. 6B, the capacitance in the embodiment is maintained high voltage 100MAhg ^-1 lesser extent, than 1V was maintained, especially capacity 50MAhg ^-1 smaller range. It was also found that a high voltage was maintained even after repeated discharge. Thus, the doping capacity of sulfur increased the discharge capacity up to 300 mAhg ⁻¹ and improved the cycle characteristics of the secondary battery.

(Cycle characteristics of S-V ₂ O ₅ )
Similarly to each of the positive electrode active materials of MnO ₂ and S—MnO ₂ described above, a positive electrode active material of V ₂ O ₅ and S—V ₂ O ₅ was prepared, and a secondary battery using Mg as a negative electrode as an example was prepared. did. Further, to prepare a secondary battery with a negative electrode of the V ₂ O ₅ as a comparative example as the positive electrode active material Mg. And charge / discharge was repeated about each secondary battery of the comparative example and the Example, and cycling characteristics were measured. FIG. 7A is a diagram showing a charge curve and a discharge curve of a secondary battery of a comparative example, and FIG. 7B is a diagram showing a charge curve and a discharge curve of the secondary battery of an example.

As shown in FIG. 7A, in the comparative example, the electromotive force was maintained up to a capacity of 100 mAhg ⁻¹ at the first discharge, but the voltage was significantly reduced from the capacity of 50 mAhg ⁻¹ at the second to sixth discharges. In the vicinity of 70 mAhg ⁻¹ , the capacity was almost 0V. In addition, a sufficient capacity could not be charged during the second to sixth charging. On the other hand, as shown in FIG. 7B, in the example, a high voltage of about 1.3 V was maintained in a range where the capacity was smaller than 100 mAhg ⁻¹ . Moreover, even if it charged / discharged repeatedly, it turned out that charging / discharging of sufficient capacity | capacitance is possible, and the discharge by a high voltage is maintained. Thus, the dope of sulfur increased the discharge capacity up to 200 mAhg ⁻¹ and improved the cycle characteristics of the secondary battery.

(Relationship with sulfur addition)
Experiments were performed on the characteristics of the positive electrode active material of S—V ₂ O ₅ with respect to the amount of sulfur added. A secondary battery having S—V ₂ O ₅ with a different amount of sulfur added as a positive electrode active material was prepared, and the average capacity when charging and discharging were repeated five times was measured. FIG. 8 is a graph showing the capacity of the secondary battery with respect to the amount of sulfur added. As shown in FIG. 8, the average capacity when the amount of sulfur added to V ₂ O ₅ is repeatedly charged and discharged five times in the range of 20 mol% or more and 66 mol% or less exceeds 150 mAhg ⁻¹ , and the cycle characteristics are It was shown to be expensive. In addition, when not adding sulfur, an average capacity | capacitance became lower than 100 mAhg ^{-1 and} it turned out that cycling characteristics are falling. In this experiment, V ₂ O ₅ is used as the metal oxide, but the mechanism for inhibiting the binding between cation and oxygen is the same for other metal oxides. Effective in the range of.

(Differences due to the manufacturing method of the positive electrode active material)
The S—V ₂ O ₅ positive electrode active material used in the above experiment was prepared by firing the mixture with water plasma, but the same effect can be obtained even if the mixture is fired with microwave irradiation or an electric furnace. . FIG. 9A is a diagram showing a charge / discharge curve of a secondary battery using S—V ₂ O ₅ produced by microwave irradiation as a positive electrode active material. FIG. 9B is a diagram showing a charge / discharge curve of a secondary battery in which S—V ₂ O ₅ produced by heating water with an electric furnace is used as a positive electrode active material. As shown in FIGS. 9A and 9B, it has been demonstrated that the capacity does not change greatly even when the secondary battery using the positive electrode active material produced by any method is repeatedly charged and discharged.

(Differences due to the manufacturing method of the positive electrode active material)
In the above experiment, the influence on the cycle characteristics due to the difference in the firing method was tested, but the cycle characteristics also change depending on the firing conditions. Simply by mixing the sulfur and V ₂ O _3, the secondary battery using the positive electrode active material prepared without calcination was measured voltage against capacity. FIG. 10 is a diagram showing a discharge curve when not firing. As a result, it was confirmed that the voltage could be maintained with respect to the change in capacity during the first discharge, but the high voltage could not be maintained during the second and third discharges. Cycle characteristics could not be obtained.

Further, mixing the sulfur and V ₂ O _3, with the positive electrode active material prepared by baking at 200 ° C., it was measured voltage against capacity. FIG. 11 is a diagram showing a discharge curve when firing at 200 ° C. FIG. In this case, compared with the case of not firing as shown in FIG. 10, the generated voltage was kept relatively high, but the third to sixth voltages were reduced. As described above, it was proved that, when not calcinated or when calcinated at a high temperature of 200 ° C. or higher, the cycle characteristics are deteriorated even with a positive electrode active material mixed with sulfur.

100 Secondary battery 110 Positive electrode 115 Positive electrode active material 120 Separator 125 Electrolytic solution 130 Negative electrode 210 Microwave irradiation chamber 215 Carbon felt piece 218 Raw material 220 Vacuum chamber

Claims (6)

A positive electrode active material for a secondary battery,
Particles formed by mixing a metal oxide consisting of divanadium pentoxide, manganese dioxide, or molybdenum trioxide and sulfur, and adding and firing more than the amount of water that evaporates and burns off during the firing time. A positive electrode active material comprising a metal oxide and sulfur distributed on the surface thereof.   The positive electrode active material according to claim 1, wherein both the metal oxide and sulfur distributed on the surface thereof are electrochemically active. A magnesium secondary battery using the positive electrode active material according to claim 1 . A method for producing a positive electrode active material used in a secondary battery,
Mixing a metal oxide comprising divanadium pentoxide, manganese dioxide or molybdenum trioxide with sulfur;
Adding to the mixture of metal oxide and sulfur more water than the amount that evaporates and burns during the firing time ;
Firing the mixture to which the water has been added, and a method for producing a positive electrode active material. The said baking is performed by heating water with a microwave, The manufacturing method of the positive electrode active material of Claim 4 characterized by the above-mentioned. 5. The method for producing a positive electrode active material according to claim 4 , wherein the firing is performed by sandwiching the mixture to which water has been added between carbon felts and irradiating microwaves under reduced pressure .