JP2017050195A - Negative electrode active material for power storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material for a power storage device, having good cycle characteristics.SOLUTION: A negative electrode active material for a power storage device is characterized to contain at least one selected from the group consisting of SiO, BO, and PO, and FeO, as a composition in terms of oxides.SELECTED DRAWING: None

Description

  The present invention relates to a negative electrode active material used for power storage devices such as lithium ion secondary batteries, sodium ion secondary batteries, and hybrid capacitors.

In recent years, with the widespread use of portable electronic devices and electric vehicles, development of power storage devices such as lithium ion secondary batteries has been promoted. As a negative electrode active material used for an electricity storage device, a material containing Si or Sn having a high theoretical capacity has been studied. In addition, the use of iron oxide (α-Fe ₂ O ₃ ) that has a high theoretical capacity, is rich in resources, and is inexpensive has been proposed (see, for example, Patent Document 1).

JP 2012-28248 A

  When the negative electrode active material described above is used for the negative electrode, the volume change due to the expansion and contraction of the negative electrode active material that occurs during the lithium ion or sodium ion insertion / desorption reaction is large. There is a problem that the cycle characteristics are severely deteriorated.

  Therefore, an object of this invention is to provide the negative electrode active material for electrical storage devices which has favorable cycling characteristics.

The negative electrode active material for an electricity storage device of the present invention contains at least one selected from the group of SiO ₂ , B ₂ O ₃ and P ₂ O ₅ as a composition represented by an oxide, and Fe ₂ O _3. Features.

In the negative electrode active material of the present invention, Fe ions exist trivalent before occlusion of alkali ions such as lithium ions and sodium ions, and change to divalent after occlusion of alkali ions accompanying charging. Further, when alkali ions are released again, Fe ions return to trivalent. Thus, Fe ions change in valence due to occlusion and release of alkali ions associated with charge and discharge, but this involves expansion and contraction of volume. In the negative electrode active material of the present invention, a structure in which Fe ions as active material components are dispersed in a matrix containing at least one selected from the group of SiO ₂ , B ₂ O ₃ and P ₂ O ₅ is formed. The Since the matrix plays a role of relaxing the volume change of Fe ions accompanying charge / discharge, it becomes a negative electrode active material having excellent cycle characteristics.

The negative electrode active material for an electricity storage device according to the present invention preferably contains 10 to 90% of Fe ₂ O _{3 and 5 to} 85% of SiO ₂ + B ₂ O ₃ + P ₂ O _{5 in} terms of mol% in terms of oxide. In the present invention, “◯ + ◯ +...” Means the total content of each component.

In the negative electrode active material for an electricity storage device of the present invention, the molar ratio of Fe ₂ O ₃ / (SiO ₂ + B ₂ O ₃ + P ₂ O ₅ ) is preferably 0.1 to 5.

The negative electrode active material for an electricity storage device of the present invention preferably contains 1 to 50% of Li ₂ O + Na ₂ O in mol% expressed as an oxide.

The negative electrode active material for an electricity storage device of the present invention preferably contains 1 to 50% of Na ₂ O in mol% in oxide notation.

  The negative electrode active material for an electricity storage device of the present invention preferably contains an amorphous phase. Since the amorphous phase has a function of relaxing the volume change of Fe ions accompanying charging / discharging, it becomes possible to improve cycle characteristics.

  The negative electrode active material for an electricity storage device of the present invention is suitable for a sodium ion secondary battery.

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

The negative electrode active material for an electricity storage device of the present invention contains at least one selected from the group of SiO ₂ , B ₂ O ₃ and P ₂ O ₅ as a composition represented by an oxide, and Fe ₂ O _3. Features. Specifically, the negative electrode active material for an electricity storage device of the present invention contains 10% to 90% of Fe ₂ O _{3 and 5} % to 85% of SiO ₂ + B ₂ O ₃ + P ₂ O _{5 in} terms of mol% of oxide. It is preferable. The reason for limiting the content of each component in this way will be described below. In the following description of the content of each component, “%” means “mol%” unless otherwise specified.

Fe ₂ O ₃ is an active material component that serves as a site for occluding and releasing alkali ions. The content of Fe ₂ O ₃ is preferably 10 to 90%, more preferably 15 to 85%, 20 to 80%, and particularly preferably 25 to 75%. When the content of Fe ₂ O ₃ is too small, the anode active discharge capacity per unit mass of the material is reduced, and tends to decrease the charge-discharge efficiency at initial charge and discharge. On the other hand, when the content of Fe ₂ O ₃ is too large, unable alleviate the volume change associated with insertion and extraction of the charging and discharging time of the alkali ions, the cycle characteristics tend to be lowered.

SiO ₂ , B ₂ O _3, and P ₂ O ₅ are network-forming oxides that surround the alkali ion occlusion and release sites in Fe ₂ O ₃ and have the effect of improving cycle characteristics. The content of SiO ₂ + B ₂ O ₃ + P ₂ O ₅ is preferably ₅ to 85%, more preferably 10 to 80%, and particularly preferably 15 to 75%. If the content of SiO ₂ + B ₂ O ₃ + P ₂ O ₅ is too small, the volume change of Fe ions that accompanies occlusion and release of alkali ions during charge / discharge cannot be mitigated, resulting in structural destruction, resulting in reduced cycle characteristics. It becomes easy. On the other _hand, when the content of _{SiO 2 + B 2 O 3 +} P 2 O 5 is too large, relatively Fe content decreases the ions tend to charge and discharge capacity per unit mass of the negative electrode active material is reduced.

SiO ₂ has a high effect of improving cycle characteristics. Moreover, since it is excellent in alkali ion conductivity, it also has the effect of improving rapid charge / discharge characteristics. The content of SiO ₂ is preferably 5 to 85%, more preferably 10 to 78%, 15 to 60%, and particularly preferably 18 to 50%. If the content of SiO ₂ is too small, the above effect is difficult to obtain. On the other hand, when the content of SiO ₂ is too large, heterogeneous crystals such as SiP ₂ O ₇ tend to precipitate, and the cycle characteristics tend to deteriorate.

P ₂ O ₅ is also highly effective in improving cycle characteristics. Moreover, since it is excellent in alkali ion conductivity, it also has the effect of improving rapid charge / discharge characteristics. The content of P ₂ O ₅ is preferably 0 to 55%, more preferably 5 to 45%, and particularly preferably 7 to 35%. When the content of P ₂ O ₅ is too large, the water resistance tends to lower. In addition, when a water-based electrode paste is produced, unwanted heterogeneous crystals are likely to be generated, and the P ₂ O ₅ network is cut by the heterogeneous crystals, so that the cycle characteristics are liable to deteriorate.

_Although not limited particularly content of B 2 _{O 3,} in consideration of the balance with other components, it is preferable preferably from 0-50%, in particular 3-40%.

The molar ratio of the content of Fe ₂ O ₃ and SiO ₂ + B ₂ O ₃ + P ₂ O ₅ (Fe ₂ O ₃ / (SiO ₂ + B ₂ O ₃ + P ₂ O ₅ )) is 0.1 to 5 More preferably, it is 0.2-4, 0.3-3, especially 0.4-2. If Fe ₂ O ₃ / (SiO ₂ + B ₂ O ₃ + P ₂ O ₅ ) is too small, the discharge capacity per unit mass of the negative electrode active material tends to be small, and the charge / discharge efficiency during the initial charge / discharge tends to decrease. There is. On the other hand, if Fe ₂ O ₃ / (SiO ₂ + B ₂ O ₃ + P ₂ O ₅ ) is too large, the volume change of Fe ions accompanying charge / discharge cannot be relaxed, and the cycle characteristics tend to deteriorate.

The negative electrode active material for an electricity storage device of the present invention preferably contains a Li 2 _O or Na 2 _O as alkali metal oxide component. The negative electrode active material for an electricity storage device of the present invention occludes and releases alkali ions with charge / discharge, but some of the alkali ions may not be released while being occluded in the negative electrode active material. The alkali ions lead to irreversible capacity and cause a decrease in initial discharge capacity. Therefore, by previously containing Li ₂ O or Na ₂ O in the negative electrode active material, it becomes difficult for the negative electrode active material to absorb alkali ions during the initial charge (a state in which the alkali ions are not released while being occluded). It is possible to improve the initial discharge capacity. Further, since the anode active material contains Li 2 _O or Na 2 _O, enhanced alkali ion conductivity, it is possible to improve the cycle characteristics. The content of Li ₂ O + Na ₂ O is preferably 1 to 50%, more preferably 2 to 45%, 3 to 40%, and particularly preferably 4 to 35%. When li ₂ O + Na content ₂ O is too small, the effect is difficult to obtain. On the other hand, if the content of Li ₂ O + Na ₂ O is too large, a large amount of different crystals such as Li ₃ PO ₄ , Li ₄ SiO ₄ , Na ₃ PO ₄ are formed, and the cycle characteristics are likely to deteriorate. The content of each component of Li ₂ O and Na ₂ O is preferably 1 to 50%, more preferably 2 to 45%, 3 to 40%, and particularly preferably 4 to 35%.

Li 2 _O and Na 2 _O may be contained alone or may contain both. The ion to be occluded and released along with the charging and discharging (when the negative electrode active material i.e. for a lithium ion secondary battery) when a lithium ion is preferable to contain Li 2 _O, occlusion due to charge and discharge and When the ions to be released are sodium ions (that is, a negative electrode active material for a sodium ion secondary battery), it is preferable to contain Na ₂ O.

Further, various components can be added in addition to the above components within the range not impairing the effects of the present invention. For example, in order to facilitate vitrification, the total amount of MgO, CaO, SrO, BaO, ZnO, CuO, Al ₂ O ₃ , Bi ₂ O ₃ , GeO ₂ , ZrO ₂ , V ₂ O _{5 and the} like is preferably 0. You may make it contain -40%, 0.1-36%, Furthermore, 0.5-30%.

  The negative electrode active material for an electricity storage device of the present invention preferably contains an amorphous phase. Since the amorphous phase has a function of relaxing the volume change of Fe ions accompanying charging / discharging, it becomes possible to improve cycle characteristics. Further, since the amorphous phase is excellent in alkali ion conductivity, it also has a function of improving rapid charge / discharge characteristics.

  Basically, the smaller the degree of crystallinity, the greater the proportion of the amorphous phase, which is preferable. Specifically, the 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, 50% or less, 30% or less, and particularly preferably 10% or less. .

  The degree of crystallinity is obtained by separating a diffraction line profile of 10 to 60 ° with 2θ value obtained by powder X-ray diffraction measurement using CuKα ray into a crystalline diffraction line and an amorphous halo. . Specifically, the integrated intensity obtained by peak-separating a broad diffraction line (amorphous halo) at 10 to 45 ° 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 for an electricity storage device of the present invention is in a powder form, the average particle size is preferably 0.1 to 20 μm, more preferably 0.2 to 15 μm, 0.3 to 10 μm, particularly 0.5 to 5 μm. It is preferable that The maximum particle size is preferably 150 μm or less, more preferably 100 μm or less, 75 μm or less, and particularly preferably 55 μm or less. If the average particle size or the maximum particle size of the negative electrode active material is too large, the volume change of the negative electrode active material accompanying occlusion and release of alkali ions during charge / discharge cannot be mitigated, and it becomes easy to peel off from the current collector. There is a tendency for the properties to decrease significantly. On the other hand, if the average particle size of the negative electrode active material is too small, the dispersion state of the powder tends to deteriorate when it is made into a paste for producing an electrode forming paste, and it tends to be difficult to produce a homogeneous electrode. is there. In addition, since the specific surface area becomes too large and the dispersion of the negative electrode active material powder is inferior when producing a paste for forming an electrode, a large amount of a binder and a 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, as the median diameter of primary particles, and were measured by a laser diffraction particle size distribution analyzer. Value.

Moreover, it is preferable that the specific surface area by BET method of a powdered negative electrode active material is 0.1-20 m < ² > / g, Furthermore, it is 0.15-15 m < ² > / g, Especially it is 0.2-10 m < ² > / g. It is preferable. If the specific surface area of the negative electrode active material is too small, alkali ions cannot be absorbed and released quickly, and the charge / discharge time tends to be long. On the other hand, when the specific surface area of the negative electrode active material is too large, when the paste for forming an electrode is produced, the dispersion state is inferior, and a large amount of binder and solvent tend to be required. Moreover, it is inferior to the applicability | paintability of the electrode formation paste, and it becomes difficult to form the negative electrode which has uniform thickness.

In addition, after charging / discharging the electricity storage device using the negative electrode active material for the electricity storage device of the present invention, lithium oxide, sodium oxide, iron-lithium composite oxide, iron-sodium composite oxide, or metallic iron May contain. For example, when the discharge of the electricity storage device using the negative electrode active material of the present invention is completed, the negative electrode active material for the electricity storage device is mol% in oxide notation, Fe ₂ O ₃ 10 to 90%, SiO ₂ + B ₂ O ₃ + _P 2 _O 5 _5~85%, containing _{1~80% Li 2 O + Na 2} O. Here, “when discharging is completed” means that the negative electrode material for an electricity storage device including the negative electrode active material for an electricity storage device of the present invention is used for the negative electrode, metallic sodium as the positive electrode, and 1M NaPF ₆ solution / EC: DEC = 1 as the electrolytic solution. In a test battery using 1 (EC = ethylene carbonate, DEC = diethyl carbonate, volume ratio), it is charged at a constant current of 0.2 mA up to 0.5 V and then discharged to 2.5 V.

  The negative electrode active material for an electricity storage device of the present invention can be produced, for example, by heating and melting raw material powder to vitrify, and then pulverizing and classifying as necessary. For pulverization and classification, a mortar, ball mill, vibrating ball mill, satellite ball mill, planetary ball mill, jet mill, sieve, centrifuge, air classifier, or the like is used.

  The negative electrode active material for an electricity storage device of the present invention is used as an anode material for an electricity storage device by adding a binder, a conductive auxiliary agent or the like. As binders, cellulose derivatives such as carboxymethylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose, hydroxyethylcellulose, ethylcellulose, hydroxymethylcellulose, water-soluble polymers such as polyvinyl alcohol; polyimide resins, phenol resins, epoxy resins, urea resins, Thermosetting resins such as melamine resin, unsaturated polyester resin and polyurethane; and polyvinylidene fluoride.

  Examples of the conductive assistant include highly conductive carbon black such as acetylene black and ketjen black, carbon powder such as graphite, and carbon fiber.

  The negative electrode material for an electricity storage device can be used as an anode for an electricity storage device by applying it to the surface of a metal foil or the like that serves as a current collector.

The aqueous electrolyte is obtained by dissolving an electrolyte salt in water. Examples of the electrolyte salt include LiNO ₃ , LiOH, LiF, LiCl, LiBr, LiI, LiClO ₄ , Li ₂ SO ₄ , CH ₃ COOLi, LiBF ₄ , LiPF _{6, and the} like when the alkali ion supplied from the positive electrode is lithium. In the case of sodium, NaNO ₃ , Na ₂ SO ₄ , NaOH, NaCl, CH ₃ COONa and the like are mentioned. In the case of potassium ion, KNO ₃ , KOH, KF, KCl, KBr, KI, KClO ₄ , _{K 2 SO 4, CH 3 COOK} , KBF 4, KPF 6 , and the like. These electrolyte salts may be used alone or in combination of two or more. In general, the electrolyte salt concentration is appropriately adjusted within a range of 0.1 M or more and a saturation concentration or less.

  The non-aqueous electrolyte includes an organic solvent and / or ionic liquid that is a non-aqueous solvent, and an electrolyte salt dissolved in the non-aqueous solvent. Specific examples of the organic solvent, ionic liquid, and electrolyte salt are listed below. In addition, an abbreviation is shown in [] after the following compound name.

  Examples of the organic solvent include propylene carbonate [PC], ethylene carbonate [EC], 1,2-dimethoxyethane [DME], γ-butyrolactone [GBL], tetrahydrofuran [THF], 2-methyltetrahydrofuran [2-MeHF], 1 , 3-dioxolane, sulfolane, acetonitrile [AN], diethyl carbonate [DEC], dimethyl carbonate [DMC], methyl ethyl carbonate [MEC], dipropyl carbonate [DPC] and the like. These organic solvents may be used alone or in combination of two or more. Of these, propylene carbonate having excellent low-temperature characteristics is preferable.

  Examples of ionic liquids include N, N, N-trimethyl-N-propylammonium bis (trifluoromethanesulfonyl) imide [TMPA-TFSI], N-methyl-N-propylpiperidinium bis (trifluoromethanesulfonyl) imide [PP13- TFSI], N-methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonyl) imide [P13-TFSI], N-methyl-N-butylpyrrolidinium bis (trifluoromethanesulfonyl) imide [P14-TFSI], etc. Aliphatic quaternary ammonium salts; 1-methyl-3-ethylimidazolium tetrafluoroborate [EMIBF4], 1-methyl-3-ethylimidazolium bis (trifluoromethanesulfonyl) imide [EMITFSI], 1-allyl-3-ethyli Dazolium bromide [AEImBr], 1-allyl-3-ethylimidazolium tetrafluoroborate [AEImBF4], 1-allyl-3-ethylimidazolium bis (trifluoromethanesulfonyl) imide [AEImTFSI], 1,3-diallylimidazolo Alkyl imidazolium quaternary salts such as lithium bromide [AAImBr], 1,3-diallylimidazolium tetrafluoroborate [AAImBF4], 1,3-diallylimidazolium bis (trifluoromethanesulfonyl) imide [AAImTFSI] .

As the electrolyte ^{_{salt, PF 6 -, BF 4 -}} , (CF 3 SO 2) 2 N - [TFSI], CF 3 SO 3 - [TFS], (C 2 F 5 SO 2) 2 N - [BETI], ^{_{ClO 4 -, AsF 6 -,}} SbF 6 -, B (C 2 O 4) 2 - [BOB], BF 2 OCOOC (CF 3) 3 - lithium salts such as [B (HHIB)], sodium salt, potassium salt Is mentioned. These electrolyte salts may be used alone or in combination of two or more. In particular, inexpensive lithium salts, sodium salts, and potassium salts of PF ₆ ⁻ and BF ₄ ⁻ are preferable. In general, the electrolyte salt concentration is appropriately adjusted within a range of 0.5 to 3 M or less.

  In addition, you may contain additives, such as vinylene carbonate [VC], vinylene acetate [VA], vinylene butyrate, vinylene hexanate, vinylene crotonate, catechol carbonate, in a nonaqueous electrolyte. These additives have a role of forming a protective film (such as LiCOx) on the surface of the negative electrode active material. The amount of the additive is preferably 0.1 to 3 parts by mass, and particularly preferably 0.5 to 1 part by mass with respect to 100 parts by mass of the nonaqueous electrolyte. If the amount of the additive is too small, it is difficult to obtain the above effect. On the other hand, even if the amount of the additive is too large, it is difficult to obtain further effects.

As the solid electrolyte, when the alkali ions supplied from the positive electrode to the negative electrode are lithium ions, lithium β-alumina, lithium β ″ -alumina, Li ₂ S—P ₂ S ₅ glass or crystallized glass, Li _{1 + x} Al _x Ge _2−x (PO ₄ ) ₃ crystal or crystallized glass, Li ₁₄ Al _0.4 (Ge _2−x Ti _x ) _1.6 (PO ₄ ) ₃ crystal or crystallized glass, Li _3x La _{2/3 x} TiO ₃ crystal or crystallized glass, Li _0.8 La _0.6 Zr ₂ (PO ₄ ) ₃ crystal or crystallized glass, Li _{1 + x} Ti _2-x Al _x (PO ₄ ) ₃ crystal or crystallized glass, Li _{1 + x + y Ti 2-} x Al x Si y (PO 4) 3-y crystal or crystalline _{glass, LiTi x Zr 2-x (} PO 4) 3 crystal or crystallization moth It scans and the like, when a sodium ion, sodium β- alumina, sodium beta "- _{alumina, Na 1 + x Zr 2 Si} x P 3-x O 12 crystalline or crystallized _{glass, Na} 3.12 _Si 2 _{Zr 1. 88} Y _0.12 PO ₁₂ crystal or crystallized glass, Na _5.9 Sm _0.6 Al _0.1 P _0.3 Si _3.6 O ₉ crystallized glass, and the like. β-alumina, potassium β ″ -alumina and the like can be mentioned.

  Of the above electrolytes, non-aqueous electrolytes and solid electrolytes are preferred because of their wide potential windows. In particular, since the solid electrolyte having alkali ion conductivity has a wide potential window, the generation of gas accompanying the decomposition of the electrolyte during charging and discharging hardly occurs, and the power storage device is excellent in safety.

  As described above, the case where the power storage device is mainly a lithium ion secondary battery or a sodium ion secondary battery has been described. However, the negative electrode active material for the power storage device of the present invention is not limited to this, and other non-aqueous secondary batteries. Secondary battery, aqueous secondary battery using aqueous electrolyte, all solid battery using solid electrolyte, and negative electrode active material and non-aqueous electric double layer used for lithium ion secondary battery or sodium ion secondary battery The present invention can also be applied to a hybrid capacitor combined with a positive electrode material for a capacitor.

  A lithium ion capacitor and a sodium ion capacitor, which are hybrid capacitors, are a kind of asymmetric capacitors having different positive and negative charge / discharge principles. The lithium ion capacitor has a structure in which a negative electrode for a lithium ion secondary battery and a positive electrode for an electric double layer capacitor are combined. The sodium ion capacitor has a structure in which a negative electrode for a sodium ion secondary battery and a positive electrode for an electric double layer capacitor are combined. 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, a negative electrode material containing the negative electrode active material for an electricity storage device of the present invention can be used for the negative electrode. 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 similar to a lithium ion secondary battery or a sodium ion secondary battery, It is charged and discharged by a chemical reaction (occlusion and release) of lithium ions or sodium ions.

  In addition, when using the negative electrode active material of this invention for a lithium ion capacitor or a sodium ion capacitor, it is necessary to occlude lithium ion or sodium ion and an electron beforehand in a negative electrode active material. The means is not particularly limited. For example, a metal lithium electrode or a metal sodium electrode, which is a supply source of lithium ions or sodium ions and electrons, is arranged in the capacitor cell, and directly or electrically conductive with the negative electrode containing the negative electrode active material of the present invention. The negative electrode active material of the present invention may be preliminarily occluded with lithium ions, sodium ions, and electrons in another cell and then incorporated into a capacitor cell.

  Hereinafter, examples applied to a sodium ion secondary battery will be described as an example of the negative electrode active material for an electricity storage device of the present invention, but the present invention is not limited to these examples.

  Tables 1 and 2 show Examples 1 to 10 and Comparative Examples.

(1) Preparation of negative electrode active material Raw material powders were prepared using various oxides, carbonates and the like so as to have the compositions shown in Tables 1 and 2. The raw material powder was put into a platinum crucible and melted at 1500 ° C. for 60 minutes in the atmosphere using an electric furnace to vitrify it.

  Next, the molten glass was poured out between a pair of rotating rollers and molded while rapidly cooling to obtain a film-like sample having a thickness of 0.1 to 2 mm. This film sample was pulverized by a ball mill and then passed through a sieve having an opening of 20 μm to obtain a glass powder having an average particle diameter of 3 μm.

  15 parts by weight of acetylene black (Denka Black) was added as a conductive aid to 85 parts by weight of the glass powder obtained above, and the mixture was coated with carbon by mixing at 800 rpm for 40 minutes using a planetary ball mill. A negative electrode active material powder was obtained.

  About the obtained negative electrode active material powder, the structure was identified by performing powder X-ray-diffraction measurement. The results are shown in Tables 1 and 2. For Examples 1 to 3, crystalline diffraction lines and amorphous halos derived from the crystals shown in Table 1 were confirmed. Moreover, about Examples 4-10, the amorphous halo was detected and the crystalline diffraction line was not detected. On the other hand, for the powder of Comparative Example 1, no amorphous halo was detected, and only crystalline diffraction lines were detected.

(2) Production of negative electrode for sodium ion secondary battery For the negative electrode active material obtained above, conductive carbon black (SuperC65, manufactured by Timcal) as a conductive auxiliary agent, polyvinylidene fluoride as a binder, powder: Conductive aid: binder = 80: 5: 15 (mass ratio), weighed to be dispersed in N-methylpyrrolidone, and then sufficiently stirred with a rotation / revolution mixer to obtain a slurry (negative electrode material) It was. Next, using a doctor blade with a gap of 125 μm, the obtained slurry was coated on a copper foil with a thickness of 20 μm as a negative electrode current collector, vacuum-dried with a dryer at 70 ° C., and then between a pair of rotating rollers. An electrode sheet was obtained by pressing through. This electrode sheet was punched to a diameter of 11 mm with an electrode punching machine and dried under reduced pressure at a temperature of 150 ° C. for 8 hours to obtain a circular negative electrode.

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

(4) Charge / Discharge Test The test battery was subjected to CC (constant current) charge (sodium ion occlusion in the negative electrode active material) at 30 ° C. from an open circuit voltage to 0.5V. Next, CC discharge (sodium ion release from the negative electrode active material) was performed from 0.5 V to 2.5 V, and the amount of electricity discharged from the negative electrode active material per unit mass (initial discharge capacity) was determined. The C rate was 0.1C. Thereafter, the charge / discharge cycle was repeated under the same conditions. Tables 1 and 2 show the results of the charge / discharge characteristics. The discharge capacity retention rate was evaluated by the ratio of the discharge capacity at the 50th cycle to the initial discharge capacity.

  As shown in Tables 1 and 2, the initial discharge capacities in Examples 1 to 10 were 80.9 mAh / g or more, and the discharge capacity retention rate was 92% or more. On the other hand, in the comparative example, the initial discharge capacity was as high as 332 mAh / g, but the discharge capacity retention rate was as low as 48%.

  The negative electrode active material for an electricity storage device of the present invention is a negative electrode in an electricity storage device such as a lithium ion secondary battery, a sodium ion secondary battery, and a hybrid capacitor used for portable electronic devices, electric vehicles, electric tools, backup emergency power supplies, etc. Suitable as a constituent material.

Claims (7)

A negative electrode active material for an electricity storage device, comprising at least one selected from the group of SiO ₂ , B ₂ O ₃ and P ₂ O ₅ and Fe ₂ O ₃ as a composition represented by an oxide. _{2. The} negative electrode active for an electricity storage device according to claim 1, comprising 10% to 90% of Fe ₂ O _{3 and 5} % to 85% of SiO ₂ + B ₂ O ₃ + P ₂ O _{5 in} terms of mol% of oxide. material. The negative electrode active material for an electricity storage device according to claim 1, wherein Fe ₂ O ₃ / (SiO ₂ + B ₂ O ₃ + P ₂ O ₅ ) is 0.1 to 5 in terms of a molar ratio. Negative electrode active material for an electricity storage device according to any one of claims 1 to 3 in mole percent on the oxide _notation characterized by containing _{Li 2 O + Na 2 O 1~50} %. Negative electrode active material for an electricity storage device according to any one of claims 1 to 3 in mole percent on the oxide notation characterized by containing Na ₂ O 1 to 50%.   The negative electrode active material for an electricity storage device according to claim 1, comprising an amorphous phase.   It is for sodium ion secondary batteries, The negative electrode active material for electrical storage devices as described in any one of Claims 1-6 characterized by the above-mentioned.