JP5944810B2 - Active material and secondary battery using the same - Google Patents

Description

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

  In recent years, secondary batteries have been used not only for mobile phones and notebook PCs but also as batteries for electric vehicles, and are expected to be used for large-scale applications such as voltage stabilization for wind power and solar power generation. .

  A secondary battery is generally composed of a positive electrode, a negative electrode, and an electrolyte (quality), and the positive electrode and the negative electrode contain an active material capable of inserting or adsorbing and desorbing ions that conduct electric conduction. Yes. In the development of a secondary battery, it is particularly important to develop an active material capable of producing an electrode having a large capacity and a long life depending on charge / discharge conditions and use conditions.

Conventionally, carbon-based materials and alloy materials such as graphite and hard carbon are used as the active material for the negative electrode, and ions (Li, Na, etc.) that are responsible for electrical conduction and Co, Ni, etc. are used as the active material for the positive electrode Oxides have been investigated and utilized. In recent years, active materials mainly composed of low-cost Fe, Mn, Ti, P, and the like have been actively studied due to an increase in the resource risk of rare metals. Among them, TiP ₂ O ₇ has been reported to be useful as an active material even though it does not contain ions responsible for electrical conduction as constituent elements.

For example, in Patent Document 1, Li _Y Fe _X M _1-X P ₂ O ₇ (wherein M represents a transition metal that is stably present even in a tetravalent state, 0 ≦ X <1, Y is 0 ≦ Y ≦ 1), an electrode active material for a nonaqueous electrolyte secondary battery is proposed, and an example using TiP ₂ O ₇ as a positive electrode active material is shown.

On the other hand, Patent Document 2 and Non-Patent Document 1 propose using TiP ₂ O ₇ as an active material for a negative electrode of a lithium secondary battery using an aqueous electrolyte.

Japanese Patent Application Laid-Open No. 2002-246025 JP 2008-282665 A

Haibo Wang, Kelong Huang, Yuqun Zeng, Sai Yang, Liquan Chen, Electrochimica Acta, Volume 52, Issue 9, 15 February 2007, Pages 3280-3285

However, the positive electrode containing TiP ₂ O ₇ described in Patent Document 1 has a problem that the energy density is low because the potential is low. Further, as described in Patent Document 2 and Non-Patent Document 1, when TiP ₂ O ₇ is used as the negative electrode active material, there is a problem in that the capacity decreases due to repeated charge and discharge.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an active material having a small reduction in discharge capacity due to repeated charge and discharge and excellent cycle characteristics, and a secondary battery using the active material. .

The active material of the present invention is an active material for a negative electrode of a secondary battery, both of which are an active material including a first crystal phase containing Ti and P and a second crystal phase, The crystal phase is cubic TiP ₂ O ₇ , and the second crystal phase is Ti ₅ P ₄ O ₂₀ , Ti ₅ P ₆ O _25, and TiP having a different crystal structure from the first crystal phase. Ri least 1 Tanedea of ₂ O ₇
In the X-ray diffraction pattern of the active material, the maximum is the X-ray diffraction peak derived from the second crystal phase and present independently from the X-ray diffraction peak derived from the first crystal phase. The intensity of the diffraction peak having the above intensity is 1 to 10% with respect to the intensity of the diffraction peak showing the (600) plane of the first crystal phase .

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

  ADVANTAGE OF THE INVENTION According to this invention, deterioration of the discharge capacity by repetition of charging / discharging is small, and the active material excellent in cycling characteristics, and a secondary battery using the same can be provided.

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. Sample No. of Example 12 is an X-ray diffraction chart of an active material used for 12 negative electrodes.

  A secondary battery according to an embodiment of the present invention will be described with reference to FIGS. 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 battery case forms such as a laminate type, a tube type, and a coin type, but any form may be used. 6N and 6P shown in FIG. 2 are a negative electrode terminal and a positive electrode terminal, respectively, for electrically connecting the external circuit to the negative electrode and the positive electrode, and 7 is a laminate film.

For the negative electrode 3, an active material containing a first crystal phase and a second crystal phase both containing Ti and P is used. The first crystal phase is cubic TiP ₂ O ₇ (JCPDS No. 00-038).
-1468), and the second crystal phase is at least one of Ti ₅ P ₄ O ₂₀ , Ti ₅ P ₆ O ₂₅ and TiP ₂ O ₇ having a different crystal structure from the first crystal phase. is there. When the crystal phase constituting the active material is only one type of cubic TiP ₂ O ₇ , a discharge capacity of about 90 mAh / g can be obtained in terms of capacity, but sufficient characteristics regarding charge / discharge cycle characteristics are obtained. Can't get. One of the causes is considered to be deterioration of the electrode due to the volume change of the crystal lattice of the active material due to charge / discharge.

On the other hand, a crystal phase containing Ti and P other than cubic TiP ₂ O ₇ , for example, Ti ₅ P ₄ O ₂₀ , Ti ₅ P ₆ O ₂₅ has low activity as an active material or is inactive. By using an active material containing these crystalline phases as the second crystalline phase together with cubic TiP ₂ O ₇ that is the first crystal, the second crystalline phase is changed when the volume of the first crystalline phase is changed by charge / discharge. The crystal phase becomes a buffer material and the cycle characteristics are improved. Such crystal phase may be a TiP ₂ O ₇ having a crystal structure different from the first crystal phase other than the above, examples of such TiP ₂ O _7, or pseudo-hexagonal, first Some have a cubic crystal (JCPDS No. 00-052-1470) having a crystal structure different from that of the crystal phase. By setting the second crystal phase to such TiP ₂ O ₇ , the composition and structure of the second crystal phase can be close to those of the first crystal phase. Since a highly consistent interface can be formed with the crystal phase, the effect of improving the cycle characteristics becomes particularly remarkable. Note that the second crystal phase may be a single crystal phase or may include a plurality of crystal phases.

  The presence of the second crystal phase in this embodiment can be confirmed by the X-ray diffraction pattern of the active material, but the X-ray diffraction peak derived from the second crystal phase is derived from the X-ray diffraction peak derived from the first crystal phase. And the presence or absence of an X-ray diffraction peak that exists independently of the X-ray diffraction peak of the first crystal phase (hereinafter referred to as the X-ray diffraction peak of the second crystal phase alone), It is determined whether or not a second crystal phase is present. Hereinafter, the X-ray diffraction peak may be simply referred to as a diffraction peak.

  As the abundance ratio of the second crystal phase to the first crystal phase, in the X-ray diffraction pattern of the active material, the ratio of the second crystal phase to the intensity of the X-ray diffraction peak indicating the (600) plane that is the main peak of the first crystal phase. The intensity of the X-ray diffraction peak having the maximum intensity among the diffraction peaks of the single crystal phase of 2 (hereinafter also referred to as the peak intensity ratio of the second crystal phase) is preferably 1 to 10%.

  By setting the peak intensity ratio of the second crystal phase to 1% or more, the second crystal phase can sufficiently obtain an effect of relaxing the volume change of the first crystal phase, and the cycle characteristics are improved. Further, by setting the peak intensity ratio of the second crystal phase to 10% or less, an amount of the first crystal phase necessary for obtaining a sufficient capacity can be secured in the active material.

  In the present embodiment, the negative electrode 3 may include an active material other than the active material including the first crystal phase and the second crystal phase. Moreover, you may contain Al, Zr, Mg, Mn, Ti, Fe, V, Co, Cr etc. in the ratio of 0.5 mass% or less as an inevitable impurity on a process.

Such an active material can be produced by various known methods such as a solid phase reaction method and a hydrothermal synthesis method. For example, a mixed raw material powder containing Ti and P having a predetermined composition and a precursor containing Ti and P obtained by a coprecipitation method are heated at a temperature in the range of 700 to 1150 ° C. in the atmosphere. Thus, an active material including cubic TiP ₂ O ₇ as the first crystal phase and Ti ₅ P ₄ O ₂₀ and Ti ₅ P ₆ O ₂₅ as the second crystal phase can be synthesized. In this case, in order to adjust the ratio of the second crystal phase, the ratio of Ti and P contained in the raw material powder or the precursor may be adjusted as appropriate.

Further, before heating to the above temperature range, TiP ₂ O ₇ having a crystal structure different from that of the first crystal phase is included as a pretreatment by holding at a temperature in the range of 300 to 600 ° C. for 1 hour or more. An active material can be synthesized.

  The obtained active material may be adjusted in particle size 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 powder of the active material may be adjusted to an appropriate range according to the use condition of the secondary battery using the active material and the method for producing the electrode, for example, in the range of 0.1 to 50 μm. It is sufficient to select and adjust an appropriate range according to the purpose. The average particle size of the powder can be confirmed by, for example, particle size distribution measurement by a diffraction scattering method.

  An electrode is produced using the obtained active material. For example, 80% by mass of the obtained active material, 10% by mass of acetylene black as a conductive assistant, 10% by mass of polyvinylidene fluoride as a binder, and 15% by mass of NMP (N-methylpyrrolidone) as a solvent are added. To prepare a slurry. The prepared slurry is applied to a metal foil such as Al, stainless steel, Ni, or Cu by a well-known sheet forming method such as a doctor blade method and the solvent is dried. An electrode including an active material including a crystal phase and a second crystal phase, a conductive additive, and a binder can be manufactured.

  In addition to polyvinylidene fluoride, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride, fluorine rubber, styrene butadiene rubber, polyimide resin (PI), polyamide resin, polyamideimide resin, etc. Can be used. In addition, the conductive auxiliary agent is ketjen black, carbon nanotube, graphite, hard carbon, metal (aluminum, 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.

  The obtained active material powder may be used to form an electrode by forming a green compact by a molding method such as extrusion molding or a roll compaction method. Alternatively, the active material powder may be fired and used as a sintered body that does not contain a conductive additive or a binder.

  The average particle size of the active material particles in the electrode can be adjusted by selecting an appropriate range from a range of 0.1 to 50 μm, for example, depending on the use conditions such as the voltage range and temperature of the secondary battery using the active material. Good. For example, when used in a secondary battery application that requires high output, the average particle size of the active material particles is preferably in a relatively small range of 0.5 to 1.0 μm.

  The average particle size of the active material particles in the electrode can be controlled by adjusting the particle size of the active material powder when the electrode is formed by sheet molding or green compact. When forming an electrode, it can be performed by adjusting the particle size of the active material powder and adjusting the firing temperature. 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.

  Examples of the active material used for the positive electrode 1 include lithium cobalt composite oxide, lithium manganese composite oxide, manganese dioxide, lithium nickel composite oxide, lithium nickel cobalt composite oxide, lithium vanadium composite oxide, vanadium oxide, and sodium. Examples include cobalt composite oxide, sodium manganese composite oxide, manganese dioxide, sodium nickel composite oxide, sodium nickel iron composite oxide, sodium iron composite oxide, sodium chromium composite oxide, and the like. The positive electrode 1 is obtained by producing an electrode by the above-described manufacturing method using these active material particles.

  As the electrolyte layer 2, any one of an aqueous electrolyte, a non-aqueous electrolyte such as an organic electrolyte or an ionic liquid impregnated in a separator, a polymer solid electrolyte, an inorganic solid electrolyte, or a molten salt may be used. it can.

  The separator impregnated with the aqueous electrolyte solution or the non-aqueous electrolyte solution is required to pass ions and prevent the positive and negative electrodes from being short-circuited. 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, for example, an aqueous solution of 1 to 2 mol / L lithium sulfate, lithium nitrate, sodium sulfate, sodium nitrate, or the like can be used. Such an aqueous electrolyte solution can change the electrolysis potential of water by adjusting the pH, and thus can change the charging potential of the secondary battery.

The organic electrolyte is composed of an organic solvent and an electrolyte salt, and an additive for the purpose of forming a film on the electrode surface, preventing overcharge, imparting flame retardancy, or the like may be added as necessary. As the organic solvent, those having a high dielectric constant, low viscosity and low vapor pressure are preferably used. Examples of such materials include ethylene carbonate (EC), propylene carbonate, butylene carbonate, and γ-butyrolactone. , Sulfolane, 1,2-dimethoxyethane, 1,3-dimethoxypropane, dimethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, or a mixed solvent of two or more. It is done. As the electrolyte salt, for example LiBF4, LiPF6, LiClO4, LiCF3SO3, LiAsF6, LiN (CF3SO2) 2, and lithium salts such as LiN (C2F5SO2), sodium perchlorate (NaClO _4), tetrafluoride sodium borate (NaBF ₄ ), Sodium hexafluorophosphate (NaPF ₆ ), NaN (FSO ₂ ) ₂ , NaN (CF ₃ SO ₂ ) ₂ , NaN (C ₂ F ₅ SO ₂ ) _{2 and the} like. Among these, NaN (SO ₂ F) ₂ , NaN (CF ₃ SO ₂ ) _2, and NaN (C ₂ F ₅ SO ₂ ) ₂ are mixed with other alkali metal salts and used in an environment at a certain temperature or higher. Therefore, it can also be used as a molten salt.

  When a polymer solid electrolyte or an inorganic solid electrolyte is used as the electrolyte layer 2, the thickness can be reduced to, for example, 10 μm or less, and further 3 μm or less, and more active materials can be used compared to a secondary battery having the same volume. Since packing can be performed, the capacity can be increased, and as a result, the energy density can be improved. However, in order to prevent a short circuit, the solid electrolyte needs to have a necessary minimum thickness that does not cause a dielectric breakdown or a short circuit due to a pinhole.

  In addition, since the active material in this embodiment contains the 2nd crystal phase with little contribution to charging / discharging, since the stability in water of an active material improves, especially the secondary battery using aqueous electrolyte solution It is suitable as an active material for the negative electrode.

  Moreover, the active material in this embodiment has a remarkable effect of improving the cycle characteristics, particularly when used for the negative electrode 3 of a secondary battery that is charged and discharged by exchanging sodium ions. The ionic radius of sodium is larger than that of lithium, and the volume change of the crystal lattice due to charge / discharge is also large. Therefore, the sodium secondary battery generally has a greater deterioration in cycle characteristics than the lithium secondary battery, but the sodium secondary battery using the active material of the present embodiment for the negative electrode 3 has a small volume change due to charge / discharge. By the presence of the phase, the volume change of the negative electrode 3 due to charge / discharge is alleviated, so that deterioration of cycle characteristics can be particularly effectively suppressed.

  A material having corrosion resistance that does not cause a reaction such as dissolution at the potential of the positive electrode may be used for the positive current collecting layer 5P. Examples of such materials include metal materials and alloys including aluminum, tantalum, niobium, titanium, gold, platinum, etc., carbonaceous materials such as graphite, hard carbon, and glassy carbon, inorganic materials such as ITO glass and tin oxide. A conductive oxide material or the like can be used. Among these, aluminum, gold, and platinum are preferable because they are excellent in corrosion resistance and easily available. Aluminum is particularly preferable because it is passivated by forming an oxide film on the surface and excellent in corrosion resistance even at a high potential.

  A material that does not cause side reactions such as alloying with Li or Na at the potential of the negative electrode may be used for the negative current collecting layer 5N. Examples of such materials include metal materials and alloys including copper, nickel, brass, zinc, aluminum, stainless steel, tungsten, gold, platinum, carbonaceous materials such as graphite, hard carbon, and glassy carbon, ITO glass. An inorganic conductive oxide material such as tin oxide can be used. In particular, it is preferable to use aluminum or nickel from the viewpoint of high conductivity and relatively low cost. Aluminum, in particular, has high conductivity and is relatively inexpensive, like copper and nickel, and cannot be used because it forms an alloy with lithium, but is inactive with sodium. Also, it can be used as a current collector.

  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. Moreover, what vapor-deposited metals, such as platinum, aluminum, and titanium, on the electrode material surface may be used.

  In addition, when using metal foil or a mesh, it is preferable that the thickness shall be 5-20 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, an active material of the present invention and a secondary battery using the active material will be described in detail based on examples.

First, anatase type TiO ₂ (manufactured by Toho Titanium) and NH ₄ H ₂ PO ₄ (ammonium dihydrogen phosphate) as raw materials for the active material are blended so that the ratio of Ti and P is the ratio shown in Table 1. Then, isopropyl alcohol (IPA) was slurried as a solvent and mixed in a ball mill using ZrO ₂ balls for 20 hours. After drying the mixed slurry, heat treatment was performed in the air to synthesize an active material. The heat treatment was performed under the conditions shown in Table 1.

The synthesized active material was subjected to X-ray diffraction (XRD) measurement, and the diffraction pattern was analyzed to identify the crystal phase contained in the active material. Of the crystal phases contained in the active material, JCPDS No. 00-0
It is the main peak of the first crystal phase with the crystal phase of TiP ₂ O ₇ identified by 38-1468 as the first crystal phase and the other crystal phase containing Ti and P as the second crystal phase. The ratio I2 / I1 of the peak intensity I2 having the maximum intensity among the peaks of the second crystal phase alone with respect to the peak intensity I1 indicating the (600) plane was calculated. Table 1 shows the type of the crystal phase identified as the second crystal phase, the diffraction peak 2θ used as I2 and the ratio of the diffraction peak intensities I2 / I1. X-ray diffraction measurement was performed using CuKα rays.

  A negative electrode was produced using the synthesized active material. 80% by mass of the synthesized active material powder, 10% by mass of acetylene black as a conductive assistant, 10% by mass of polyvinylidene fluoride as a binder, and 15% by mass of NMP (N-methylpyrrolidone) as a solvent were mixed to prepare a slurry. Produced. This slurry was applied onto a metal foil serving as a negative electrode side current collecting layer by a doctor blade method, and the solvent was dried to form a negative electrode having a thickness of 50 μm.

As the positive electrode, a commercially available LiMn ₂ O ₄ powder (manufactured by Toda Kogyo) or a synthesized Na _0.7 MnO _2.05 powder was used as an active material. A slurry was prepared by mixing 80% by mass of these active material powders, 10% by mass of acetylene black as a conductive additive, 10% by mass of polyvinylidene fluoride as a binder, and 15% by mass of NMP (N-methylpyrrolidone) as a solvent. And it apply | coated with the doctor blade method on the metal foil used as a positive electrode side current collection layer, and the positive electrode with a thickness of 50 micrometers was formed by drying a solvent.

  The positive electrode and the negative electrode obtained were cut into a 10 × 10 cm square shape together with the metal foil as the current collecting layer, and the same as the end of the surface on the side where the electrode of the metal foil as the current collecting layer was not formed A metal foil made of the above material was attached as a positive electrode terminal or a negative electrode terminal by spot welding.

Between the produced positive electrode and negative electrode, a separator made of polypropylene / polyethylene containing an electrolytic solution was placed, housed in a bag-like aluminum laminate film as an exterior body, and the electrolytic solution was injected. As the electrolyte, LiClO ₄ or NaN (CF ₃ SO ₂ ) ₂ (also referred to as NaTFSI) dissolved in 1 mol / L in propylene carbonate (PC), which is an organic solvent, or ₂ mol of LiNO ₃ or Na ₂ SO ₄ is used. / L dissolved aqueous solution was used. Table 2 shows the active material used for the positive electrode, the materials of the positive current collector layer and the negative current collector layer, and the type of electrolyte.

  After injecting the electrolytic solution, the opening of the outer package was sealed by thermal welding in a state where the ends of the positive electrode terminal and the negative electrode terminal were exposed from the opening of the outer package, thereby obtaining a secondary battery.

  The charge / discharge characteristics of the produced secondary battery were evaluated under the condition of 0.2C. Table 2 shows the initial discharge capacity and the discharge capacity retention rate after a 100-cycle charge / discharge test with one discharge-charge cycle.

As shown in Tables 1 and 2, Sample No. Secondary batteries 2 to 4, 6 to 8, 10 to 13, 15, 16, 18, and 19 include, in addition to cubic TiP ₂ O ₇ that is the first crystal phase, Ti as the second crystal phase. ₅ viewed contains at least one of _TiP 2 _{O 7} having a different crystal structure from the _{P 4 O 20, Ti 5 P} 6 O 25 and the first crystal phase, the second crystal phase, a predetermined X-ray Since the negative electrode is formed using the active material which has a diffraction peak intensity , cycling characteristics are improved significantly and it turns out that the capacity | capacitance maintenance factor after a 100-cycle charging / discharging test exceeds 80%.

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 ... Laminate film

Claims (5)

An active material for a negative electrode of a secondary battery,
Each includes a first crystal phase and a second crystal phase containing Ti and P,
The first crystalline phase is cubic TiP ₂ O ₇ ;
The second crystal _phase, Ri least 1 Tanedea of _TiP 2 _{O 7} having a different crystal structure from _{Ti 5 P 4 O 20, Ti} 5 P 6 O 25 and the first crystal phase,
In the X-ray diffraction pattern of the active material,
Among the X-ray diffraction peaks that are derived from the second crystal phase and exist independently from the X-ray diffraction peak derived from the first crystal phase, the intensity of the diffraction peak having the maximum intensity is
1 to 10% of the intensity of the diffraction peak showing the (600) plane of the first crystal phase . 2. The active material according to claim 1, wherein the TiP ₂ O ₇ which is the second crystal phase is a pseudo hexagonal crystal or a cubic crystal having a crystal structure different from that of the first crystal phase. A positive electrode, a negative electrode, and an electrolyte, a secondary battery comprising the active material according the negative electrode to claim 1 or 2. The secondary battery according to claim 3 , wherein the electrolyte is an aqueous electrolyte solution. The secondary battery according to claim 3 or 4, characterized in that charging and discharging by exchanging sodium ions between the positive electrode and the negative electrode.

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