JP2014167900A - Negative electrode active material for electricity storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material for an electricity storage device excellent in cycle characteristic.SOLUTION: A negative electrode active material for an electricity storage device contains 40 to 70% SnO, 5 to 25% PO, and 5 to 22% SiO, and 2 to 20% AlOin mol% expressed in terms of oxides.

Description

  The present invention relates to a negative electrode active material for power storage devices used for portable electronic devices, electric vehicles, electric tools, backup emergency power supplies, and the like.

  In recent years, with the spread of portable personal computers and mobile phones, there is an increasing demand for higher capacity and smaller size of power storage devices such as lithium ion secondary batteries. If the capacity of the electricity storage device is increased, it will be easy to reduce the size of the battery. Therefore, there is an urgent need to develop the capacity of the electricity storage device.

  Generally, carbon materials such as graphitic carbon material, pitch coke, fibrous carbon, and high-capacity soft carbon are used as the negative electrode active material for lithium ion secondary batteries. However, since the theoretical capacity of the carbon material is about 372 mAh / g, there is a problem that it is difficult to increase the capacity of the battery.

  A negative electrode active material containing Si and Sn has been proposed as a negative electrode active material that can occlude and release lithium ions and has a higher capacity density than a negative electrode active material made of a carbon material (for example, a patent) Reference 1). However, although the negative electrode active material containing Si or Sn is excellent in the initial charge / discharge efficiency (ratio of the discharge capacity to the initial charge capacity), the volume change due to the lithium ion occlusion and release reaction during charge / discharge is remarkable. Since it is large, the negative electrode active material is structurally deteriorated and repeatedly cracked when repeatedly charged and discharged. As cracks progress, in some cases, cavities are formed in the negative electrode active material and may be pulverized. When a crack occurs in the negative electrode active material, the electron conduction network is divided, so that there is a problem that the discharge capacity (cycle characteristics) after repeated charge and discharge is lowered.

Furthermore, a glass material made of SnO and P ₂ O ₅ has been proposed as a negative electrode active material excellent in both high capacity density and cycle characteristics (see, for example, Patent Document 2).

JP 2000-36323 A JP 2011-187434 A

An aqueous paste for an electrode used for manufacturing a negative electrode uses the glass material composed of SnO and P ₂ O ₅ as a negative electrode active material, and carboxymethyl cellulose (hereinafter, CMC) as a binder for the negative electrode active material. And an aqueous binder such as polyvinyl alcohol (hereinafter referred to as PVA). When this paste is stored for a long period of time, unwanted heterogeneous crystals (for example, SnHPO ₄ etc.) are generated in the glass material due to moisture in the paste, so that the P ₂ O ₅ network in the glass material is cut and lithium ions or There is a problem that the volume change of SnO during occlusion and release of sodium ions cannot be mitigated, resulting in a decrease in cycle characteristics.

Further, when the glass material is exposed to a high temperature and high humidity atmosphere for a long period of time, an undesirable heterogeneous crystal is generated in the glass material, so that the P ₂ O ₅ network in the glass material is cut, and lithium ions or sodium ions There is a problem that the volume change of SnO during occlusion and release cannot be mitigated, and as a result, cycle characteristics deteriorate.

  The present invention has been made in view of the above-described state of the art, and a main object of the present invention is to provide a negative electrode active material for an electricity storage device having excellent cycle characteristics.

Negative electrode active material for an electricity storage device of the present invention is the mole percent oxide _{equivalent, SnO 40~70%, P 2 O} 5 5~25%, SiO 2 5~22%, the _Al 2 O 3 2 to 20% It is characterized by containing.

The negative electrode active material for an electricity storage device of the present invention, it is preferable that the value of SnO _/ P 2 _{O 5} is in the range of 3.0 to 5.0 in a molar ratio of oxide equivalent.

  The negative electrode active material for an electricity storage device of the present invention is preferably substantially amorphous.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the negative electrode active material for electrical storage devices excellent in cycling characteristics.

Negative electrode active material for an electricity storage device of the present invention is the mole percent oxide _{equivalent, SnO 40~70%, P 2 O} 5 5~25%, SiO 2 5~22%, the _Al 2 O 3 2 to 20% It is characterized by containing. The reason why each component is limited in this way will be described below.

SnO is an active material component that becomes a site for occluding and releasing lithium ions or sodium ions, and its content is 40 to 70%. The SnO content is preferably 45 to 68%, more preferably 48 to 65%, and still more preferably 50 to 63%. When there is too little content of SnO, the discharge capacity per unit mass of a negative electrode active material will become small, and there exists a tendency for the charge / discharge efficiency at the time of first time charge / discharge to fall. On the other hand, if the content of SnO is too large, the amorphous component in the negative electrode active material is relatively reduced, and the volume change associated with insertion and extraction of lithium ions or sodium ions during charge / discharge cannot be mitigated. , The cycle characteristics tend to deteriorate. In the present invention, the content of SnO is oxidized tin component other than SnO (SnO _2, etc.) also refers to that summed in terms of SnO.

P ₂ O ₅ is a network-forming oxide that surrounds lithium ion or sodium ion storage and release sites in SnO and has an effect of improving cycle characteristics, and its content is 5 to 25%. The content of P ₂ O ₅ is preferably 5.5 to 24%, and more preferably 6 to 23%. If the content of P ₂ O ₅ is too small, the volume change of SnO associated with insertion and extraction of lithium ions or sodium ions during charge / discharge cannot be alleviated, resulting in structural deterioration, and cycle characteristics are likely to deteriorate. On the other hand, if the content of P ₂ O ₅ is too large, the water resistance tends to decrease, and when a water-based electrode paste is produced, unwanted heterogeneous crystals are produced in the glass material. The P ₂ O ₅ network is cut, and the volume change of SnO when occlusion and release of lithium ions or sodium ions cannot be mitigated, and as a result, the cycle characteristics are likely to deteriorate.

SiO _{2 is} also a network-forming oxide, has an effect of surrounding the lithium ion or sodium ion occlusion and release sites in SnO and improving cycle characteristics, and its content is 5 to 22%. The content of SiO ₂ is preferably 5.5 to 21%, and more preferably 6 to 20%. If the content of SiO ₂ is too small, the volume change of SnO accompanying the occlusion and release of lithium ions or sodium ions during charge / discharge cannot be relaxed, resulting in structural deterioration, so that the cycle characteristics are likely to deteriorate. On the other hand, when the content of SiO ₂ is too large, crystals such as SiO ₂ and SiP ₂ O ₇ tend to precipitate, and the cycle characteristics tend to be lowered.

Al ₂ O ₃ has an effect of improving water resistance, and its content is 2 to 20%. The content of Al ₂ O ₃ is preferably 3 to 17%, more preferably 4 to 15%. When Al ₂ content of O ₃ is too small, it water resistance tends to decrease, the cycle characteristics are easily lowered. When the content of Al ₂ O ₃ is too large, _Al 2 _{O 3} or AlPO ₄ becomes crystals are likely to deposit, such as the cycle characteristics tend to decrease.

SnO / P ₂ O ₅ (molar ratio) is preferably 3.0 to 5.0, more preferably 3.2 to 4.8, and more preferably 3.3 to 4.5. Further preferred. When SnO / P ₂ O ₅ is too small, SnO ₂ crystal, SnP ₂ O ₇ crystal, SiO ₂ crystal, and Al ₂ O ₃ crystal are likely to precipitate, and it becomes difficult to obtain a homogeneous glass, and the cycle characteristics tend to be lowered. There is. On the other hand, if SnO / P ₂ O ₅ is too large, a disproportionation reaction of SnO occurs, SnO ₂ crystals and metal Sn tend to precipitate, and the cycle characteristics tend to deteriorate.

Further, various components can be added in addition to the above components within the range not impairing the effects of the present invention. Examples of such components include CuO, ZnO, B ₂ O ₃ , MgO, CaO, R ₂ O (R represents Li, Na, K, or Cs). The total content of the above components is preferably 0 to 20%, more preferably 0 to 15%, and further preferably 0.1 to 13%.

  The degree of crystallinity of the negative electrode active material for an electricity storage device of the present invention is preferably 95% or less, more preferably 80% or less, and more preferably 70% or less in mass% before the charge / discharge reaction. Preferably, it is 50% or less, more preferably 40% or less, and particularly preferably 10% or less. The negative electrode active material is preferably substantially amorphous. Here, “consisting essentially of amorphous” means that the crystallinity is substantially 0% (specifically, less than 0.1%), and will be described later by powder X-ray diffraction measurement. In which no crystalline diffraction line is detected. The smaller the degree of crystallinity (the greater the proportion of amorphous material), the more the change in volume during repeated charge / discharge can be alleviated, and the cycle characteristics tend to improve.

The crystallinity is obtained by separating a diffraction line profile of 10 to 60 ° with a 2θ value obtained by powder X-ray diffraction measurement using CuKα ray into a crystalline diffraction line and an amorphous halo. It is done. Specifically, the integrated intensity obtained by peak-separating a broad diffraction line (amorphous halo) at 10 to 40 ° from the total scattering curve obtained by subtracting the background from the diffraction line profile is Ia, 10 When the total integrated intensity obtained by peak separation of each crystalline diffraction line detected at ˜60 ° is Ic, the crystallinity Xc can be obtained from the following equation.
Xc = [Ic / (Ic + Ia)] × 100 (%)

  When the negative electrode active material is in a powder form, the average particle size is preferably 0.1 to 20 μm, more preferably 0.2 to 15 μm, still more preferably 0.3 to 10 μm, and 0 It is particularly preferably 5 to 5 μm. The maximum particle size is preferably 150 μm or less, more preferably 100 μm or less, further preferably 75 μm or less, and particularly preferably 55 μm or less. If the average particle size or the maximum particle size is too large, the volume change of the negative electrode active material due to the insertion and extraction of lithium ions or sodium ions during charge / discharge cannot be mitigated, and it becomes easy to peel off from the current collector, resulting in cycle characteristics. Tends to decrease significantly. On the other hand, if the average particle size is too small, the powder is in a poorly dispersed state when formed into a paste, and it tends to be difficult to produce a uniform electrode. In addition, since the specific surface area becomes too large and the negative electrode active material powder is difficult to disperse when producing a paste for forming an electrode, a large amount of binder and solvent are required. Furthermore, it is inferior to the applicability of the electrode forming paste, and it becomes difficult to form a negative electrode having a uniform thickness.

  Here, the average particle size and the maximum particle size are D50 (50% volume cumulative diameter) and D90 (90% volume cumulative diameter), respectively, in terms of the median diameter of primary particles, and were measured by a laser diffraction particle size distribution analyzer. Value.

  In order to obtain a powder of a predetermined size in the negative electrode active material for an electricity storage device of the present invention, a general pulverizer or classifier is used. For example, a mortar, a ball mill, a vibrating ball mill, a satellite ball mill, a planetary ball mill, a jet mill, a sieve, a centrifugal separator, an air classification, or the like is used.

The negative electrode active material for an electricity storage device of the present invention is produced, for example, by heating and melting raw material powder to vitrify it. In the oxide material containing Sn, the oxidation state of Sn atoms easily changes depending on the melting conditions. When melted in the atmosphere, unwanted crystals such as SnO ₂ and SnP ₂ O ₇ are formed on the surface of the glass melt or in the glass melt. It is easy to be formed. As a result, the initial charge / discharge efficiency and cycle characteristics of the negative electrode active material are likely to deteriorate. Therefore, by performing melting in a reducing atmosphere or an inert atmosphere, it is possible to suppress an increase in the valence of Sn ions in the oxide material, to suppress formation of unwanted crystals, and to be excellent in initial charge / discharge efficiency and cycle characteristics. It is possible to obtain an electricity storage device.

In order to melt in a reducing atmosphere, it is preferable to supply a reducing gas into the melting tank. The reducing gas, by _volume%, it is preferable to use a mixed gas containing N 2 90 to 99.5% and _{H 2 0.5~10%, N 2 92~99} % and _H 2 1 to 8 It is more preferable to use a mixed gas containing%.

  When melting in an inert atmosphere, it is preferable to supply an inert gas into the melting tank. As the inert gas, it is preferable to use any of nitrogen, argon, and helium.

  The reducing gas or the inert gas may be supplied to the upper atmosphere of the molten glass in the melting tank, may be supplied directly from the bubbling nozzle into the molten glass, or both methods may be performed simultaneously.

Moreover, in the manufacturing method of the said negative electrode active material, it becomes easy to obtain the negative electrode active material with few devitrification foreign materials and excellent uniformity by using composite oxide for starting raw material powder. If the negative electrode active material is used, an electricity storage device having a stable discharge capacity can be easily obtained. Examples of such complex oxides include stannous pyrophosphate (Sn ₂ P ₂ O ₇ ).

  As mentioned above, although the negative electrode active material for lithium ion secondary batteries and sodium ion secondary batteries has been mainly described, the negative electrode active material of the present invention is not limited to this, other non-aqueous secondary batteries, Furthermore, the present invention can be applied to a hybrid capacitor in which a negative electrode active material for a lithium ion secondary battery and a sodium ion secondary battery and a positive electrode material for a non-aqueous electric double layer capacitor are combined.

  Lithium ion capacitors and sodium ion capacitors, which are hybrid capacitors, are one type of asymmetric capacitors that have different charge / discharge principles for the positive and negative electrodes. The lithium ion capacitor and the sodium ion capacitor have a structure in which a negative electrode for a lithium ion secondary battery and a sodium ion secondary battery and a positive electrode for an electric double layer capacitor are combined. Here, the positive electrode forms an electric double layer on the surface and is charged and discharged by utilizing a physical action (electrostatic action), whereas the negative electrode is a lithium ion secondary battery and a sodium ion secondary battery described above. Similarly, charging / discharging is performed by a chemical reaction (storage and release) of lithium ions or sodium ions.

  A positive electrode active material made of a carbonaceous powder having a high specific surface area such as activated carbon, polyacene, or mesophase carbon is used for the positive electrode of the lithium ion capacitor and the sodium ion capacitor. On the other hand, the negative electrode active material of the present invention can be used for the negative electrode.

  In addition, after charging / discharging the electrical storage device using the negative electrode active material for electrical storage devices of this invention, the said negative electrode active material for electrical storage devices is lithium oxide, sodium oxide, Sn-Li alloy, Sn-Na alloy, or metal May contain tin.

  Examples of the negative electrode active material for an electricity storage device of the present invention will be described below with reference to examples applied to the use of a nonaqueous secondary battery, but the present invention is not limited to these examples.

(1) Preparation of negative electrode active material Tin and phosphorus composite oxide (stannous pyrophosphate: Sn ₂ P) as the main raw material so as to have the compositions of Examples 1 to 5 and Comparative Examples 1 to 4 shown in Table 1 ₂ O ₇ ), raw material powders were prepared using various oxides, phosphate raw materials, carbonate raw materials, metal powder raw materials or carbon raw materials as a reducing agent. The raw material powder was put into a quartz crucible and melted at 950 ° C. for 40 minutes in a nitrogen atmosphere using an electric furnace to be vitrified.

  Next, the molten glass was poured out between a pair of rotating rollers and molded while being rapidly cooled to obtain a film-like glass having a thickness of 0.1 to 2 mm. This film-like glass was pulverized at 100 rpm for 3 hours using a ball mill containing zirconia balls having a diameter of 20 to 30 mm, and then passed through a resin sieve having an opening of 120 μm to obtain a glass coarse powder having an average particle size of 3 to 15 μm. . Subsequently, this coarse powder glass was air classified to obtain a glass powder having an average particle size of 3 μm and a maximum particle size of 28 μm.

  The structure of the obtained glass powder was identified by powder X-ray diffraction measurement. The glass powders of Examples 1 to 5 had a crystallinity of 2.1% or less. In particular, the glass powders of Examples 1 and 3 to 5 were amorphous, and no crystals were detected. The samples of Comparative Examples 1 to 4 had a crystallinity of 11.3% or more and a structure containing many crystals.

(2) Production of Negative Electrode for Lithium Ion Secondary Battery For glass powder, carboxymethyl cellulose as a binder, substance name SuperC65 (manufactured by Timcal) as a conductive aid, glass powder: binder: conductive aid = 83 : Weighed so as to be 12: 5 (mass ratio) and dispersed them in pure water, and then sufficiently stirred with a rotation / revolution mixer to form a slurry. Next, using a doctor blade with a gap of 75 μm, the obtained slurry was coated on a copper foil having a thickness of 20 μm as a negative electrode current collector, dried under reduced pressure at 70 ° C. with a dryer, and then a pair of rotating rollers An electrode sheet was obtained by pressing in between. The electrode sheet was punched to a diameter of 11 mm with an electrode punching machine and dried at 160 ° C. for 8 hours while reducing the pressure to obtain a circular negative electrode.

(3) Production of test battery (lithium ion secondary battery) Next, the obtained negative electrode was placed on the lower lid of the coin cell with the copper foil surface facing down, and dried under reduced pressure at 70 ° C. for 8 hours. A test battery was manufactured by laminating a separator made of a polypropylene porous membrane having a diameter of 16 mm (Celguard # 2400 manufactured by Hoechst Celanese) and metallic lithium as a counter electrode. As the electrolytic solution, 1M LiPF ₆ solution / EC: DEC = 1: 1 (EC = ethylene carbonate, DEC = diethyl carbonate) was used. The test battery was assembled in an environment with a dew point temperature of −40 ° C. or lower.

(4) Charging / discharging test The test battery was charged at a unit mass of the negative electrode active material by performing CC (constant current) charge (lithium ion occlusion to the negative electrode active material) from 1 V to 0 V at 30 ° C. The amount (charge capacity) was determined. Next, CC discharge (lithium ion release from the negative electrode active material) was performed from 0 V to 1 V, and the amount of electricity (discharge capacity) discharged in the unit mass of the negative electrode active material was determined. The C rate was 0.5C. Table 1 shows the results of the charge / discharge characteristics. The discharge capacity retention rate is the ratio of the discharge capacity at the 100th cycle to the initial discharge capacity.

  In Examples 1 to 5, the initial discharge capacity was 458 mAh / g or more, the discharge capacity retention rate was 81.0% or more, the capacity was high, and the cycle performance was also good. On the other hand, Comparative Examples 1 to 4 had a high capacity of 479 mAh / g or more, but the discharge capacity retention rate was 59.9% or less, both of which were low.

(5) Production of negative electrode for sodium ion secondary battery For the glass powder of Example 1 or 2 in Table 1, a thermosetting polyimide resin as a binder and a substance name SuperC65 (manufactured by Timcal) as a conductive auxiliary agent, Glass powder: Binder: Conductive aid = 83: 12: 5 (mass ratio) Weighed so that these were dispersed in N-methylpyrrolidone (NMP), and then sufficiently stirred with a rotation / revolution mixer To make a slurry. Next, using a doctor blade with a gap of 100 μm, the obtained slurry was coated on a 20 μm thick copper foil as a negative electrode current collector, dried under reduced pressure at 70 ° C. in a dryer, and then a pair of rotating rollers An electrode sheet was obtained by pressing in between. The electrode sheet was punched to a diameter of 11 mm with an electrode punching machine and dried at 300 ° C. for 3 hours while reducing the pressure to obtain a circular negative electrode.

(6) Production of test battery (sodium ion secondary battery) Next, the obtained negative electrode was placed on the bottom lid of the coin cell with the copper foil surface facing down, and dried under reduced pressure at 70 ° C. for 8 hours. A test battery was manufactured by laminating a separator made of a polypropylene porous film having a diameter of 16 mm and metallic sodium as a counter electrode. As the electrolytic solution, 1M NaPF ₆ solution / EC: DEC = 1: 1 was used. The test battery was assembled in an environment with a dew point temperature of −70 ° C. or lower.

(7) Charge / Discharge Test The above test battery was charged at a unit mass of the negative electrode active material by performing CC (constant current) charge (sodium ion occlusion in the negative electrode active material) from 1 V to 0 V at 30 ° C. The amount (charge capacity) was determined. Next, CC discharge (sodium ion release from the negative electrode active material) was performed from 0 V to 1 V, and the amount of electricity (discharge capacity) discharged in the unit mass of the negative electrode active material was determined. The C rate was 0.1C. Table 2 shows the results of the charge / discharge characteristics. The discharge capacity retention rate is the ratio of the discharge capacity at the 25th cycle to the initial discharge capacity.

  In Examples 1 and 2, the initial discharge capacity was 293 mAh / g or more, the discharge capacity retention rate was 91% or more, the capacity was high, and the cycle performance was also good.

The negative electrode active material for power storage devices of the present invention is suitable as a negative electrode active material for power storage devices used for portable electronic devices, electric vehicles, electric tools, backup emergency power supplies, and the like.

Claims (3)

In mole percent oxide _{equivalent, SnO 40~70%, P 2 O} 5 5~25%, SiO 2 5~22%, the negative electrode for an electricity storage device which is characterized by containing _Al 2 O 3 2 to 20% Active material. Negative electrode active material for an electricity storage device according to claim 1 in which the value of SnO _/ P 2 _{O 5,} characterized in that the in the range of 3.0 to 5.0 in a molar ratio of oxide equivalent. The negative electrode active material for an electricity storage device according to claim 1, wherein the negative electrode active material is substantially amorphous.