JP7340024B2 - Pre-dopant for power storage device and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a pre-dopant used in electric storage devices such as lithium ion batteries, lithium ion capacitors and electric double layer capacitors.

In electricity storage devices such as lithium-ion batteries, lithium-ion capacitors, and electric double layer capacitors, it is known that pre-doping the negative electrode with lithium ions to lower the potential of the negative electrode can increase the capacity of the electricity storage device. . In recent years, a method has been proposed in which a metal foil with a plurality of through-holes is used as a current collector, and lithium ions are pre-doped into the negative electrode via an electrolytic solution by arranging the metallic lithium foil in an electrode in which a large number of positive and negative electrodes are stacked. It is In recent years, a pre-doping method that does not use a metallic lithium foil has also been proposed.

In Patent Document 1, an electrode comprising an electrode current collector having a plurality of through holes and an electrode mixture layer provided on the electrode current collector; an ion supply source for supplying ions to the layer, and the electrode current collector includes a first region having a predetermined through-hole opening ratio and a second region having a through-hole opening ratio larger than that of the first region. is provided, the first region is the edge portion of the electrode current collector, and the second region is the central portion of the electrode current collector. A lithium electrode is incorporated in the electricity storage device, and the lithium electrode has a lithium electrode current collector to which a metallic lithium foil is crimped as an ion supply source. Pre-doping lithium ions from the electrode to the negative electrode is described. According to this, it is possible to adjust the permeation state of the electrolytic solution, and it is possible to uniformly dope ions to the electrode. However, in the pre-doping method described in Patent Document 1, a metal foil having a plurality of through-holes and a metal lithium foil are used for the current collector, which increases the production cost and further reduces the volumetric energy density of the electricity storage device. There was a problem of hoarding.

In Patent Document 2, the formula: Li _a Me _b O _c (4.5 ≤ a ≤ 6.5, 0.5 ≤ b ≤ 1.5, 3.5 ≤ c ≤ 4.5, Me: Co, Mn , Fe, and one or more selected from the group of Al). It is According to this, the lithium metal composite oxide decomposes under high voltage to release lithium, but because of its large irreversible capacity, it releases a large amount of lithium during charging and absorbs little lithium during discharging. However, it is described that it is a material capable of doping a large amount of lithium into the negative electrode. In addition to the lithium metal composite oxide, Patent Document 2 also describes a pre-dopant containing a carbon material. By increasing the contact area between the lithium metal composite oxide and the carbon material, electrons can be effectively supplied to the lithium metal composite oxide through the highly conductive carbon material. It is described that, as a result, the decomposition reaction of the lithium metal composite oxide is actively performed, and a large amount of lithium can be released from the lithium metal composite oxide. However, there is no description that a pre-dopant having a large irreversible capacity can be obtained by using a lithium iron oxide having a specific composition and a half width of a diffraction peak in X-ray diffraction measurement. In addition, in Patent Document 2, the positive electrode potential at the time of the initial charge is preferably 4.3 V (Li reference electrode standard) or more, and further preferably 4.5 V (Li reference electrode standard) or more. Since a weak electrolytic solution is oxidatively decomposed, there is a problem that the performance deterioration of an electricity storage device such as a lithium ion capacitor is accelerated, and an improvement has been desired.

Patent Document 3 discloses a pre-dopant obtained by combining a lithium-manganese-based oxide having a basic composition of Li ₆ MnO ₄ and a carbon material, and a battery having a positive electrode having a positive electrode active material, a negative electrode, and an electrolyte. A lithium ion secondary characterized by causing lithium ions released from the positive electrode active material and the pre-dopant to be occluded in the negative electrode active material and to generate manganese oxide from the pre-dopant by performing an initial charge. It describes a method for manufacturing a lithium ion secondary battery in which the positive electrode potential at the initial charge is 4.5 V (Li counter electrode standard) or higher. According to this, by combining the carbon material and the lithium manganese oxide, many conductive paths to the lithium manganese oxide are formed, and the lithium manganese oxide is easily decomposed during charging. Are listed. The pre-dopant described in Patent Document 3 is said to have a large irreversible capacity. However, since the positive electrode potential at the time of the first charge is 4.5 V (Li counter electrode reference) or more, it is oxidized and decomposed in a general electrolytic solution, so the performance deterioration of an electricity storage device such as a lithium ion capacitor is accelerated. There was a problem that it was stored, and improvement was desired.

Patent No. 5220510 JP 2016-12620 Patent No. 6217990

The present invention has been made to solve the above problems, and it is possible to suppress the decrease in the volume energy density of the electricity storage device, reduce the manufacturing cost, and pre-dope lithium ions at a lower charging voltage. It is an object of the present invention to provide a pre-dopant for an electricity storage device having a large irreversible capacity, which can be suitably used as an electricity storage device having a high charge depth and a high discharge capacity by suppressing the decomposition of the electrolytic solution. be.

The above object is to obtain a lithium iron oxide represented by the following formula (1), which has a diffraction angle (2θ) of 23.6±0.5° and a half width of 0.5° in X-ray diffraction measurement. The problem is solved by providing a pre-dopant for electric storage devices characterized by a temperature of 06 to 0.17°.
LixFeOy (1)
[In formula (1), x satisfies 3.5≦x≦4.7, and y satisfies 3.25≦y≦3.85. ]

At this time, the space group of the crystal structure is Pbca, the crystal lattice constants a and c satisfy 9.140 Å ≤ a ≤ 9.205 Å and 9.180 Å ≤ c ≤ 9.220 Å, respectively, and the lattice volume V satisfies 773 Å ³ ≤ V ≤ 781 Å ³ .

Further, at this time, a positive electrode for an electricity storage device containing the pre-dopant and a positive electrode active material is a preferred embodiment, and the content of the pre-dopant is 1 to 1 with respect to the total weight of the pre-dopant and the positive electrode active material. A positive electrode that is 60% by weight is a preferred embodiment. An electricity storage device having the positive electrode as a component is also a preferred embodiment.

Further, the above-described problem is a method for producing a pre-dopant for an electric storage device, which is composed of a lithium iron oxide obtained by mixing and firing an iron raw material, a lithium raw material, and a carbon raw material, wherein the iron raw material, the lithium raw material, and the carbon Provided is a method for producing a pre-dopant for an electric storage device, comprising mixing raw materials, firing in an oxygen-free atmosphere at 650 to 1000° C. for 2 to 100 hours, and pulverizing the resulting powdery product to obtain lithium iron oxide. is also solved by

According to the present invention, a lithium iron oxide having a specific composition, in X-ray diffraction measurement, the half width of the diffraction peak at a diffraction angle (2θ) of 23.6 ± 0.5 ° is within a certain range, A pre-dopant for an electricity storage device having a large irreversible capacity can be provided. As a result, pre-doping can be performed without using a metallic lithium foil, so that a decrease in the volume energy density of the electricity storage device can be suppressed and the manufacturing cost can be reduced. In addition, since lithium ions can be pre-doped at a lower charging voltage, the decomposition of the electrolytic solution can be suppressed, and it can be suitably used as an electricity storage device with a high charge depth and a high discharge capacity.

The pre-dopant for an electricity storage device of the present invention (hereinafter sometimes abbreviated as "pre-dopant") is composed of a lithium iron oxide represented by the following formula (1), and has a diffraction angle ( 2θ) is 23.6±0.5°, and the half width of the diffraction peak is 0.06 to 0.17°.
LixFeOy (1)
[In formula (1), x satisfies 3.5≦x≦4.7, and y satisfies 3.25≦y≦3.85. ]

As a result of extensive studies by the present inventors, it was found that a lithium iron oxide having a specific composition has a diffraction peak with a diffraction angle (2θ) of 23.6 ± 0.5° in X-ray diffraction measurement. It has been clarified that a pre-dopant for an electricity storage device having a large irreversible capacity can be obtained by setting the half-value width to 0.06 to 0.17°. As a result, the pre-doping process can be performed even at a low charging voltage, and decomposition of the electrolytic solution can be suppressed. From the viewpoint of suppressing decomposition of the electrolytic solution, the initial charging voltage for pre-doping is preferably 3.0 to 4.2V. Moreover, since the pre-dopant of the present invention has a large irreversible capacity, it is possible to reduce the amount of the pre-dopant used when applying it to the positive electrode. Therefore, the ratio of the positive electrode active material can be increased, and the capacity of the electricity storage device can be increased.

The pre-dopant of the present invention comprises a lithium iron oxide represented by the above formula (1), where x satisfies 3.5≦x≦4.7 and y satisfies 3.25≦y≦3.85. It is. In the lithium iron oxide represented by the above formula (1), the composition ratio x is synonymous with Li/Fe (molar ratio). When x is less than 3.5, the amount of lithium is small, so the amount of lithium doped into the negative electrode may be insufficient, and sufficient irreversible capacity may not be obtained. x is preferably 3.7 or more, more preferably 3.8 or more, and even more preferably 3.9 or more. If x exceeds 4.7, the crystal lattice shrinks, lithium is less likely to be released, and the pre-dopant may have a small irreversible capacity as the charge capacity decreases. x is preferably 4.6 or less, more preferably 4.5 or less, and even more preferably 4.3 or less. If y is less than 3.25, unreacted lithium and iron raw materials may remain. y is preferably 3.3 or more, more preferably 3.4 or more, and even more preferably 3.45 or more. If y exceeds 3.85, the iron in the lithium iron oxide will be in a highly oxidized state, possibly reducing the charge capacity. y is preferably 3.8 or less, more preferably 3.75 or less, and even more preferably 3.65 or less.

As is clear from the comparison between Examples and Comparative Examples to be described later, even when the lithium iron oxide represented by the above formula (1) has a specific composition ratio, X-ray diffraction measurement shows no diffraction. It was confirmed that Comparative Example 5, in which the half-value width of the diffraction peak with a folding angle (2θ) of 23.6±0.5° did not satisfy the range of 0.06 to 0.17°, was a pre-dopant with a small irreversible capacity. . In addition, it was confirmed that the power storage device manufactured using the pre-dopant of Comparative Example 5 had a low charge depth and a low discharge capacity. On the other hand, in Examples 1 to 5 in which the half-value width is in the range of 0.06 to 0.17°, the irreversible capacity is large, and the electric storage device manufactured using this has a high charge depth and a high discharge capacity. confirmed to be high. Therefore, in X-ray diffraction measurement, there is great significance in adopting a configuration in which the half-value width is 0.06 to 0.17°, and the pre-dopant of the present invention makes it possible to reduce manufacturing costs and lower charging. Since lithium ions can be pre-doped with voltage, decomposition of the electrolytic solution can be suppressed, and an electricity storage device with a high charge depth and a high discharge capacity can be provided. If the half width is less than 0.06°, the resulting powdery product may be hard and difficult to pulverize. In addition, the particles after pulverization are also coarse, which may make it difficult to apply to electrodes. The half width is preferably 0.07° or more, more preferably 0.12° or more, and even more preferably 0.13° or more. If the half width exceeds 0.17°, a polymorphic lithium iron oxide is produced, which may reduce the irreversible capacity. The half width is preferably 0.16° or less, more preferably 0.15° or less, and even more preferably 0.14° or less.

In the pre-dopant of the present invention, the space group of the crystal structure is Pbca, and the crystal lattice constants a and c satisfy 9.140 Å ≤ a ≤ 9.205 Å and 9.180 Å ≤ c ≤ 9.220 Å, respectively. , the lattice volume V satisfies 773 Å ³ ≤ V ≤ 781 Å ³ . A study by the present inventors confirmed that there is a correlation between the crystal lattice constants a and c and Li/Fe (molar ratio). That is, the present inventors presume that when the value of Li/Fe (molar ratio) decreases, the crystal lattice constants a and c expand to increase the lattice volume, thereby facilitating the release of lithium. If the crystal lattice constant a is less than 9.140 Å, the lattice volume V becomes small, which may make it difficult to release lithium. The crystal lattice constant a is preferably 9.140 Å or more, more preferably 9.150 Å or more, and even more preferably 9.165 Å or more. On the other hand, if the crystal lattice constant a exceeds 9.205 Å, the amount of lithium decreases, so that lithium doped into the negative electrode may be insufficient, and sufficient irreversible capacity may not be obtained. The crystal lattice constant a is preferably 9.205 Å or less, more preferably 9.195 Å or less, even more preferably 9.185 Å or less. If the crystal lattice constant c is less than 9.180 Å, the lattice volume V becomes small, which may make it difficult to release lithium. The crystal lattice constant c is preferably 9.180 Å or more, more preferably 9.190 Å or more, and even more preferably 9.195 Å or more. On the other hand, when the crystal lattice constant c exceeds 9.220 Å, the amount of lithium doped into the negative electrode is insufficient, so that sufficient irreversible capacity may not be obtained. The crystal lattice constant c is preferably 9.220 Å or less, more preferably 9.213 Å or less, and even more preferably 9.205 Å or less. When the lattice volume V is less than 773 Å ³ , lithium may be difficult to be released because the lattice volume V is small. The lattice volume V is preferably 773 Å ³ or more, more preferably 774 Å ³ or more, and even more preferably 776 Å ³ or more. If the lattice volume V exceeds 781 Å ³ , the amount of lithium is so small that the lithium doped into the negative electrode may be insufficient and sufficient irreversible capacity may not be obtained. The lattice volume V is preferably 781 Å ³ or less, more preferably 780 Å ³ or less, and even more preferably 778 Å ³ or less.

The pre-dopant of the present invention preferably has a volume resistivity of 9.0×10 ⁴ to 3.0×10 ⁷ Ω·cm. When the volume resistivity is less than 9.0 × 10 ⁴ Ω·cm, electrons preferentially move to the pre-dopant in the electricity storage device system, so only the positive electrode active material in the vicinity of the pre-dopant is used, and the local There is a risk that the capacity will decrease due to a negative reaction. The volume resistivity is preferably 9.0×10 ⁴ Ω·cm or more, more preferably 2.0×10 ⁵ Ω·cm or more, and 5.0×10 ⁵ Ω·cm or more. is more preferred. If the volume resistivity exceeds 3.0×10 ⁷ Ω·cm, the movement of electrons is hindered, lithium is not sufficiently released from the pre-dopant, and the irreversible capacity may decrease. The volume resistivity is preferably 3.0×10 ⁷ Ωcm or less, more preferably 1.0×10 ⁷ Ωcm or less, and 2.0×10 ⁶ Ωcm or less. is more preferred.

The pre-dopant of the present invention preferably has a specific surface area of 2 to 10 m ² /g. When the specific surface area is less than 2 m ² /g, the reaction area is small, lithium is not sufficiently released from the pre-dopant, and the charge capacity may decrease and the irreversible capacity may decrease. The specific surface area is preferably 2 m ² /g or more, more preferably 3 m ² /g or more, and even more preferably 4.3 m ² /g or more. If the specific surface area is 10 m ² /g or more, side reactions may occur. The specific surface area is preferably 9 m ² /g or less, more preferably 8 m ² /g or less, even more preferably 7 m ² /g or less.

The method for producing the pre-dopant of the present invention is not particularly limited. A method for producing a pre-dopant for an electric storage device comprising a lithium iron oxide obtained by mixing and firing an iron raw material, a lithium raw material and a carbon raw material, wherein the iron raw material, the lithium raw material and the carbon raw material are mixed (hereinafter , sometimes abbreviated as “mixing step”), calcined at 650 to 1000 ° C. for 2 to 100 hours in an oxygen-free atmosphere (hereinafter sometimes abbreviated as “firing step”), and the resulting powdery product Lithium iron oxide can be preferably obtained by pulverizing the material.

The present inventors have studied that the pre-dopant for an electricity storage device comprising the lithium iron oxide of the present invention can be obtained by mixing the iron raw material, the lithium raw material and the carbon raw material and firing the mixture at a specific temperature for a specific time. Confirmed by In particular, when the iron raw material and the lithium raw material are reacted to produce lithium iron oxide, it is important to mix a certain amount of the carbon raw material. As is clear from the comparison between Examples and Comparative Examples to be described later, in Comparative Example 5 in which no carbon raw material was used, the pre-dopant had a small irreversible capacity. In the device, we have confirmed that the charge depth is low and the discharge capacity is also low.

The iron raw material used in the present invention is not particularly limited, and includes iron oxide hydroxide (III), iron oxide (II), iron oxide (III), ferrous sulfate (II), ferric sulfate (III), Iron (II) hydroxide, iron (III) hydroxide and the like are preferably used.

The lithium raw material used in the present invention is not particularly limited, and lithium hydroxide, lithium carbonate, lithium acetate, lithium nitrate, lithium oxide and the like are preferably used. These may be hydrates or anhydrides. Among them, lithium hydroxide is more preferably used.

The carbon raw material used in the present invention is not particularly limited, and activated carbon, acetylene black, polyvinyl alcohol, carbon nanotubes, carbon nanofibers, graphene, and the like are preferably used. The blending amount of the carbon raw material is preferably 1 to 50% by weight with respect to the total weight of the iron raw material, the lithium raw material and the carbon raw material. If the content of the carbon raw material is less than 1% by weight, the reaction between lithium and iron becomes non-uniform, producing a polymorphic lithium iron oxide and reducing the irreversible capacity. Also, it becomes difficult to pulverize the obtained powdery product. The blending amount of the carbon raw material is preferably 5% by weight or more. Moreover, if the amount of the carbon raw material to be blended exceeds 50% by weight, the manufacturing cost increases, which is not preferable. More preferably, the blending amount of the carbon raw material is 30% by weight or less.

In the mixing step, the iron raw material, the lithium raw material, and the carbon raw material are mixed. Mixing may be performed by a dry method or by a wet method, but mixing by a dry method is preferable. Among them, it is a preferred embodiment to mix the iron raw material, the lithium raw material and the carbon raw material in a powder state.

In the firing step, firing is preferably performed in an oxygen-free atmosphere, for example, an inert gas atmosphere, a hydrogen gas atmosphere, or a hydrogen-inert gas atmosphere. Examples of inert gases include nitrogen, argon, helium, neon, and krypton. preferably used. Moreover, the firing temperature in the firing step is preferably 650 to 1000°C. If the firing temperature is lower than 650° C., unreacted raw materials remain, so there is a possibility that the lithium iron oxide represented by the above formula (1) cannot be obtained. Further, there is a possibility that the lithium iron oxide does not satisfy the range of 0.06 to 0.17° in the half width of the diffraction peak with the diffraction angle (2θ) of 23.6±0.5°. The firing temperature is more preferably 680° C. or higher, still more preferably 725° C. or higher, and particularly preferably 775° C. or higher. If the firing temperature exceeds 1000° C., the resulting powdery product may become hard and difficult to pulverize. In addition, the particles after pulverization become coarse, which may make it difficult to apply the powder to the electrode. Further, there is a possibility that the lithium iron oxide does not satisfy the range of 0.06 to 0.17° in the half width of the diffraction peak with the diffraction angle (2θ) of 23.6±0.5°. The firing temperature is more preferably 950° C. or lower, still more preferably 900° C. or lower, and particularly preferably 880° C. or lower.

The firing time in the firing step is preferably 2 to 100 hours. If the firing time is less than 2 hours, unreacted raw materials remain, and the lithium iron oxide represented by the above formula (1) may not be obtained. Further, there is a possibility that the lithium iron oxide does not satisfy the range of 0.06 to 0.17° in the half width of the diffraction peak with the diffraction angle (2θ) of 23.6±0.5°. The firing time is preferably 2 hours or longer, more preferably 10 hours or longer, still more preferably 24 hours or longer, particularly preferably 48 hours or longer, and most preferably 60 hours or longer. preferable. On the other hand, if the baking time exceeds 100 hours, productivity may decrease. More preferably, the firing time is 90 hours or less.

By using the lithium iron oxide obtained as described above as a pre-dopant for an electricity storage device of the present invention, pre-doping can be performed without using a metallic lithium foil, so that the volume energy density of the electricity storage device does not decrease. It is possible to reduce the manufacturing cost and provide an electricity storage device with a high charge depth and a high discharge capacity. Among them, a positive electrode for an electricity storage device comprising the pre-dopant of the present invention and a positive electrode active material is a preferred embodiment. As the positive electrode active material, materials used in lithium ion batteries and lithium ion capacitors can be used. , Co, Mn, Ti, Fe, Cr, Zn, and Cu; spinel-type lithium oxides containing transition metal elements selected from the group consisting of: _LiFePO4 ; olivine-type lithium phosphate compounds; , graphene sheets, and other carbon-based materials can be suitably used. Examples of the layered rock salt type lithium oxide include LiNiO ₂ , LiCoO ₂ , Li ₂ MnO ₃ , Li(Ni _1/3 Co _1/3 Mn _1/3 )O ₂ , Li(Ni _0.5 Co _0.2 Mn _0.3 ) _{O2 ,} Li( _{Ni0.6Co0.2Mn0.2} ) _{O2 ,} Li _{( Ni0.8Co0.1Mn0.1 ) O2} , Li( _{Ni0.8Co 0.15Al 0.05} )O ₂ and the like, and the spinel-type lithium oxides include LiMn ₂ O ₄ and LiMn _1.5 Ni _0.5 O ₄ .

In the positive electrode, the content of the pre-dopant is preferably 1 to 60% by weight with respect to the total weight of the pre-dopant and the positive electrode active material. If the content of the pre-dopant is less than 1% by weight, the irreversible capacity is small, and the potential of the negative electrode made of graphite, silicon, etc. may not be lowered, and the content of the pre-dopant is 5% by weight or more. is more preferable, and 10% by weight or more is even more preferable. On the other hand, when the content of the pre-dopant exceeds 60% by weight, the energy density may decrease due to the decrease in the content of the positive electrode active material, and the content of the pre-dopant is preferably 55% by weight or less. preferable.

In the present invention, an electricity storage device having the positive electrode as a component is a more preferred embodiment. Carbon-based materials such as graphite and activated carbon, silicon-based materials such as silicon and silicon monoxide, metal materials such as tin, aluminum and germanium, and sulfur can be suitably used as the negative electrode in the electricity storage device. Further, as the electrolyte in the electricity storage device, an electrolytic solution (liquid electrolyte) obtained by dissolving a lithium salt such as LiPF ₆ , LiBF ₄ or LiClO ₄ in an organic solvent (liquid electrolyte), a solid electrolyte, or the like can be suitably used. The type of power storage device is not particularly limited, and at least one power storage device selected from the group consisting of lithium ion batteries, all-solid batteries, lithium ion capacitors, and electric double layer capacitors is suitable. Among them, at least one electricity storage device selected from the group consisting of lithium ion batteries and lithium ion capacitors is more preferable. Among lithium ion capacitors, a graphite-based lithium ion capacitor using graphite for the negative electrode is a more preferable embodiment.

[Preparation of pre-dopant (LFO)]
(Example 1)
Iron hydroxide oxide (III) (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 100 g, lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 176 g and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 31 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 850° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=3.72 (molar ratio)) which is the pre-dopant of Example 1.

(Example 2)
100 g of iron hydroxide (III) oxide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 180 g of lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 31 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 850° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=3.84 (molar ratio)) as a pre-dopant of Example 2.

(Example 3)
100 g of iron hydroxide oxide (III) (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 198 g of lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 33 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 850° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=4.19 (molar ratio)) as a pre-dopant of Example 3.

(Example 4)
Iron hydroxide oxide (III) (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 100 g, lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 207 g and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 34 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 850° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=4.39 (molar ratio)) as a pre-dopant of Example 4.

(Example 5)
100 g of iron hydroxide (III) oxide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 216 g of lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 35 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 850° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=4.58 (molar ratio)) as a pre-dopant of Example 5.

(Example 6)
Iron hydroxide oxide (III) (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 100 g, lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 198 g and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 33 g were dry mixed using a mixer. The resulting mixture was fired in a nitrogen atmosphere at 900° C. for 10 hours using a firing furnace to obtain a powdery product. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=4.19 (molar ratio)) as a pre-dopant of Example 6.

(Example 7)
Iron hydroxide oxide (III) (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 100 g, lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 198 g and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 33 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 950° C. for 5 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=4.19 (molar ratio)) as a pre-dopant of Example 7.

(Example 8)
Iron hydroxide oxide (III) (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 100 g, lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 198 g and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 33 g were dry mixed using a mixer. The obtained mixture was fired in a nitrogen atmosphere at 800° C. for 72 hours using a firing furnace to obtain a powdery product. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=4.19 (molar ratio)) as a pre-dopant of Example 8.

(Example 9)
Iron hydroxide oxide (III) (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 100 g, lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 198 g and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 33 g were dry mixed using a mixer. The obtained mixture was fired in a nitrogen atmosphere at 750° C. for 72 hours using a firing furnace to obtain a powdery product. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=4.19 (molar ratio)) as a pre-dopant of Example 9.

(Example 10)
Iron hydroxide oxide (III) (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 100 g, lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) 198 g and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 33 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 700° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=4.19 (molar ratio)) as a pre-dopant of Example 10.

(Comparative example 1)
100 g of iron hydroxide (III) oxide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 152 g of lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 28 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 850° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=3.21 (molar ratio)) as a pre-dopant of Comparative Example 1.

(Comparative example 2)
100 g of iron hydroxide (III) oxide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 229 g of lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 37 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 850° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=4.85 (molar ratio)) as a pre-dopant of Comparative Example 2.

(Comparative Example 3)
100 g of iron hydroxide (III) oxide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 239 g of lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 38 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 850° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=5.06 (molar ratio)) as a pre-dopant of Comparative Example 3.

(Comparative Example 4)
100 g of iron hydroxide (III) oxide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 289 g of lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and activated carbon ("Kuraray Coal" manufactured by Kuraray Co., Ltd.) 43 g were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 850° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=6.12 (molar ratio)) as a pre-dopant of Comparative Example 4.

(Comparative Example 5)
100 g of iron oxide hydroxide (III) (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and 198 g of lithium hydroxide monohydrate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) were dry mixed using a mixer. A powdery product was obtained by firing the obtained mixture at 850° C. for 72 hours in a nitrogen atmosphere using a firing furnace. Then, the obtained powdery product was pulverized to obtain a lithium iron oxide (Li/Fe=4.19 (molar ratio)) as a pre-dopant of Comparative Example 5.

[Evaluation of pre-dopant]
(composition analysis)
The Li/Fe molar ratio of each pre-dopant obtained in Examples and Comparative Examples was measured by ICP emission spectrometry using a plasma emission spectrometer "SPECTRO ARCOS" manufactured by Hitachi High-Tech Science Co., Ltd. Table 1 shows the results.

(Calculation of crystal lattice constant, lattice volume and half width)
Using an XRD apparatus "X'pert-PRO" manufactured by Philips, the peak position and half-value width of each pre-dopant obtained in Examples and Comparative Examples were measured with the Kα line of Cu. At this time, Si powder (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) was mixed as an internal standard substance to each pre-dopant in a weight ratio of 2 wt % and measured. Analysis software (HighScore Plus) was used to refine the crystal lattice constant and lattice volume. The analysis was performed by Rietveld method analysis (<Phase fit> Default Rietveld using Li ₅ FeO ₄ (ICSD: 01-075-1253) as an approximate structural model after correcting the peak position based on the Si (111) peak. ) to calculate the crystal lattice constant and the lattice volume V. Table 1 shows the results.

(Method for measuring volume resistivity)
The volume resistivity of each pre-dopant obtained in Examples and Comparative Examples was measured using a high resistivity meter (Hiresta UX, manufactured by Mitsubishi Chemical Analytech Co., Ltd.). The results were calculated based on the resistance value at 5 kN. Table 1 shows the results.

(Method for measuring specific surface area)
The specific surface area of each pre-dopant obtained in Examples and Comparative Examples was measured by the BET method using a fully automatic specific surface area measuring device (Macsorb HM model-1208 manufactured by Mountec Co., Ltd.). The degassing step was performed under the conditions of 150°C and 20 minutes. Table 1 shows the results.

[Evaluation of power storage device]
(Preparation of coin-type battery for electrochemical evaluation)
58 wt% of each pre-dopant obtained in Examples and Comparative Examples, 30 wt% of acetylene black (“Denka Black” manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive aid, and polyvinylidene fluoride (PVDF, stock "KF Polymer" manufactured by Kureha Co., Ltd.) was dissolved in N-methylpyrrolidone to prepare a slurry. The above slurry was applied to an etched aluminum foil (JCC-20CB manufactured by Nippon Denki Kogyo Co., Ltd.) as a current collector and dried at 130° C. for 5 minutes. An evaluation electrode (positive electrode) was produced by punching out the dried sheet with a punching machine. Metallic lithium was used as the counter electrode, and a metal lithium foil punched out was used. A separator made of polypropylene was sandwiched between the electrode for evaluation and the counter electrode to form an electrode, which was placed in a coin-shaped battery container. Then, ethylene carbonate ( _EC ) and diethyl carbonate (DEC) were mixed in a volume ratio of EC:DEC = 1:1. A coin cell for electrochemical evaluation was produced by sealing the container.

(Charging and discharging test)
Using the coin-type battery produced above, constant current charging was performed at a current density of 8.67 mA / g (per weight of active material) until the charge termination voltage was 4.0 V, and then constant voltage charging (terminating condition: 0.867 mA /g (per weight of active material). A 3 minute rest step was then performed. Then, constant current discharge was performed at a current density of 8.67 mA/g (per active material weight) until the voltage reached 2.3V. Table 1 shows the obtained charge capacity, discharge capacity and irreversible capacity values.

(Preparation of lithium ion capacitor and pre-doping treatment)
Using the pre-dopants obtained in Examples and Comparative Examples, lithium ion capacitors were produced and pre-doped.

(Production example 1)
(Preparation of positive electrode)
First, activated carbon (“Kuraray Coal” manufactured by Kuraray Co., Ltd.) as a positive electrode active material, the pre-dopant of Example 1 as a pre-dopant, acetylene black (“Denka Black” manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive aid, and a binder. As a positive electrode paint, polyvinylidene fluoride (PVDF, “KF Polymer” manufactured by Kureha Co., Ltd.) was dissolved in N-methylpyrrolidone.
The content of the pre-dopant at this time was adjusted to be 30% with respect to the total mass of the positive electrode active material and the pre-dopant, as indicated by the following formula.
Pre-dopant content (%)=[mass of pre-dopant/(mass of positive electrode active material+mass of pre-dopant)]×100
Furthermore, the mass ratio of the sum of positive electrode active material and pre-dopant/conductive aid/binder was adjusted to 77/14/9. In other words, the mass ratio of the positive electrode active material/pre-dopant/conductive aid/binder was adjusted to 53.9/23.1/14/9.
Finally, the prepared positive electrode paint was applied to an etched aluminum foil (“JCC-20CB” manufactured by Nippon Capacitor Industry Co., Ltd.) as a current collector, dried at 130 ° C. for 5 minutes, and then reduced to a size of 3 cm × 4 cm. A positive electrode was produced by cutting. The design capacity at this time is 2.3 mAh.

(Preparation of negative electrode)
For the negative electrode, a spherulitic graphite electrode (“HS-LIB-N-Gr-001” manufactured by Hosen Co., Ltd., nominal capacity: 1.6 mAh/cm ² ) was used, and the size was 3.3 cm × 4.3 cm. A negative electrode was produced by cutting. The design capacity at this time is 22.7 mAh.

(Production of lithium ion capacitor)
After stacking the positive and negative electrodes prepared above and a separator (manufactured by Nippon Kodo Paper Industry Co., Ltd.), they were housed in an aluminum laminate case.
Next, after injecting 1M LiPF ₆ in EC/DEC=1/1 (manufactured by Kishida Chemical Co., Ltd.) as an electrolytic solution, the lithium ion capacitor of Production Example 1 was produced by vacuum sealing.
In the lithium ion capacitor of Production Example 1, the positive electrode had an electric capacity of 2.3 mAh, the negative electrode had an electric capacity of 22.7 mAh, and the positive/negative electrode capacity ratio (negative electrode/positive electrode) was 9.9.

(pre-dope treatment)
Next, the produced lithium ion capacitor of Production Example 1 was measured at a current density of 0.02 mA/cm ² in an environment of 25° C. up to 4.0 V using a charge/discharge measuring device (manufactured by Hokuto Denko Co., Ltd.). Current charging was performed, and then constant voltage charging (end condition: current value of 0.002 mA/cm ² ) was performed. A 3 minute rest step was then performed. Pre-doping was then performed by discharging to 2.2V.

(Production Examples 2-5, 9-13)
In the production of the positive electrode, lithium ion capacitors of Production Examples 2 to 5 and 9 to 13 were produced and pre-doped in the same manner as in Production Example 1, except that the pre-dopant was changed as shown in Table 2.

(Production example 6)
The positive electrode was produced in the same manner as in Production Example 1, except that the pre-dopant of Example 3 was used and the content of the pre-dopant was adjusted to 20% of the total mass of the positive electrode active material and the pre-dopant. Then, a lithium ion capacitor of Production Example 6 was produced, and pre-doping was performed. That is, the mass ratio of the positive electrode active material/pre-dopant/conduction aid/binder was adjusted to 61.6/15.4/14/9.

(Production example 7)
The positive electrode was produced in the same manner as in Production Example 1, except that the pre-dopant of Example 3 was used and the content of the pre-dopant was adjusted to 40% of the total mass of the positive electrode active material and the pre-dopant. Then, a lithium ion capacitor of Production Example 7 was produced, and pre-doping was performed. That is, the mass ratio of the positive electrode active material/pre-dopant/conduction aid/binder was adjusted to 46.2/30.8/14/9.

(Production example 8)
(Preparation of negative electrode)
Silicon (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a negative electrode active material, acetylene black ("Denka Black" manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive agent, polyvinylidene fluoride (PVDF, manufactured by Kureha Co., Ltd.) as a binder "KF Polymer") was dissolved in N-methylpyrrolidone to prepare a negative electrode paint. The mass ratio of the negative electrode active material/conduction aid/binder was adjusted to 80/10/10.
Next, the prepared negative electrode paint was applied to a copper foil (manufactured by Fukuda Metal Foil & Powder Co., Ltd.) as a current collector, dried at 130 ° C. for 5 minutes, and then reduced to a size of 3.3 cm × 4.3 cm. A negative electrode was produced by cutting. The design capacity at this time is 22.7 mAh.
A lithium ion capacitor of Production Example 8 was produced in the same manner as in Production Example 3 except that the negative electrode was changed to the above electrode, and pre-doping treatment was performed.

(Comparative production examples 1 to 5)
Lithium ion capacitors of Comparative Preparation Examples 1 to 5 were prepared and pre-doped in the same manner as in Preparation Example 1 except that the pre-dopant was changed as shown in Table 2 in the preparation of the positive electrode.

(Preparation of lithium ion battery and pre-doping treatment)
Using the pre-dopants obtained in Examples and Comparative Examples, lithium ion batteries were produced and subjected to pre-doping treatment.

(Production example 14)
(Preparation of positive electrode)
First, Li(Ni _0.8 Co _0.1 Mn _0.1 )O ₂ (manufactured by Hosen Co., Ltd.) as a positive electrode active material, the pre-dopant of Example 3 as a pre-dopant, and acetylene black (electrochemical "Denka Black" manufactured by Kogyo Co., Ltd.) and polyvinylidene fluoride (PVDF, "KF Polymer" manufactured by Kureha Co., Ltd.) as a binder were dissolved in N-methylpyrrolidone to prepare a positive electrode paint.
The content of the pre-dopant at this time was adjusted to be 12% with respect to the total mass of the positive electrode active material and the pre-dopant, as indicated by the following formula.
Pre-dopant content (%)=[mass of pre-dopant/(mass of positive electrode active material+mass of pre-dopant)]×100
Furthermore, the mass ratio of the sum of the positive electrode active material and the pre-dopant/the conductive aid/the binder was adjusted to 83/11/6. That is, the mass ratio of the positive electrode active material/pre-dopant/conduction aid/binder was adjusted to 73/10/11/6.
Finally, the prepared positive electrode paint was applied to an etched aluminum foil (“JCC-20CB” manufactured by Nippon Capacitor Industry Co., Ltd.) as a current collector, dried at 130 ° C. for 5 minutes, and then reduced to a size of 3 cm × 4 cm. A positive electrode was produced by cutting. The design capacity at this time is 8.2 mAh.

(Preparation of negative electrode)
First, silicon as a negative electrode active material (“silgrain e-si” manufactured by Elchem Co., Ltd.), acetylene black (“Denka Black” manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive agent, and polyvinylidene fluoride (PVDF, Co., Ltd.) as a binder. Kureha's "KF Polymer") was dissolved in N-methylpyrrolidone to prepare a negative electrode paint.
The mass ratio of the negative electrode active material/conductive aid/binder was adjusted to 50/25/25.
Finally, the prepared negative electrode paint is applied to a copper foil (manufactured by Fukuda Metal Foil Industry Co., Ltd.) as a current collector, dried at 130 ° C. for 5 minutes, and then cut into a size of 3.3 cm × 4.3 cm. A negative electrode was thus produced. The design capacity at this time is 12.7 mAh.

(Production of lithium ion battery)
After laminating the positive electrode, the negative electrode, and the polyethylene separator prepared above, they were housed in an aluminum laminate case.
Next, after injecting 1M LiPF ₆ in PC (manufactured by Kishida Chemical Co., Ltd.) as an electrolytic solution, the lithium ion battery of Production Example 14 was produced by vacuum sealing.
In the lithium ion battery of Production Example 14, the positive electrode had an electric capacity of 8.2 mAh, the negative electrode had an electric capacity of 12.7 mAh, and the positive/negative electrode capacity ratio (negative electrode/positive electrode) was 1.5.

(pre-dope treatment)
Next, the prepared lithium ion battery of Preparation Example 14 was measured at a current density of 0.02 mA/cm ² in an environment of 25° C. up to 4.2 V using a charge/discharge measuring device (manufactured by Hokuto Denko Co., Ltd.). Current charging was performed, and then constant voltage charging (end condition: current value of 0.002 mA/cm ² ) was performed. A 3 minute rest step was then performed. Then, pre-doping was performed by discharging to 3.2V.

(Comparative example 6)
A lithium ion battery of Comparative Preparation Example 6 was prepared in the same manner as in Preparation Example 14, except that the pre-dopant was changed as shown in Table 2, and pre-doping was performed.

(Measurement of charging depth)
The charging depth of the graphite or silicon negative electrode after pre-doping treatment was measured as follows. After discharging to 2.2 V in the above Preparation Examples 1 to 13 and Comparative Preparation Examples 1 to 5, the lithium ion capacitors were disassembled, and graphite or silicon negative electrodes were taken out and used as evaluation electrodes. Metallic lithium was used as the counter electrode, and a polypropylene separator was sandwiched between the electrode for evaluation and the counter electrode to form an electrode. The electrode was placed in a coin-shaped battery container. Then, an electrolytic solution prepared by dissolving 1M LiPF ₆ in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of EC:DEC=1:1 was injected into the battery container. After that, the battery container was sealed to produce a coin-type battery for electrochemical evaluation. Charge depth was confirmed by charging a coin cell for electrochemical evaluation to 3.0V.
Here, the depth of charge is a value indicating what percentage of the design capacity (22.7 mAh) of the negative electrode was charged by the charge capacity measured by the above charging operation, and is calculated by the following formula.
Depth of charge (%) = [charge capacity (mAh)/negative electrode design capacity 22.7 (mAh)] x 100

(Evaluation of capacitor characteristics (discharge capacity))
Capacitor characteristics (discharge capacity) of the lithium ion capacitors of Production Examples 1 to 13 and Comparative Production Examples 1 to 5 were evaluated. Specifically, using a charge/discharge measuring device (manufactured by Hokuto Denko Co., Ltd.), charge/discharge was performed in a range of 2.2 to 3.8 V in an environment of 25°C. Also, the charge/discharge rate was 1C per weight of the positive electrode active material. The current density at a charge/discharge rate of 1C was 40 mA/g (per active material weight).

(Measurement of charging depth)
The charging depth of the silicon negative electrode after pre-doping was measured as follows. After discharging to 3.2 V in Preparation Example 14 and Comparative Preparation Example 6, the lithium ion battery was disassembled, and the silicon negative electrode was taken out and used as an evaluation electrode. Metallic lithium was used as the counter electrode, and a polypropylene separator was sandwiched between the electrode for evaluation and the counter electrode to form an electrode. The electrode was placed in a coin-shaped battery container. An electrolytic solution prepared by dissolving 1M LiPF ₆ in a propylene carbonate (PC) solvent was poured into the battery container, and then the battery container was sealed to manufacture a coin-type battery for electrochemical evaluation. Charge depth was confirmed by charging a coin cell for electrochemical evaluation to 3.0V.
Here, the depth of charge is a value indicating what percentage of the design capacity (12.7 mAh) of the negative electrode was charged by the charge capacity measured by the above charging operation, and is calculated by the following formula.
Depth of charge (%) = [charge capacity (mAh)/negative electrode design capacity 12.7 (mAh)] x 100

(Evaluation of battery characteristics (discharge capacity))
The battery characteristics (discharge capacity) of the lithium ion batteries of Preparation Example 14 and Comparative Preparation Example 6 were evaluated. Specifically, using a charge/discharge measuring device (manufactured by Hokuto Denko Co., Ltd.), charge/discharge was performed in the range of 3.2 to 4.2 V in an environment of 25°C. Also, the charge/discharge rate was 1C per weight of the positive electrode active material. The current density at a charge/discharge rate of 1C was 160 mA/g (per active material weight).

Claims (5)

It is composed of a lithium iron oxide represented by the following formula (1), and in X-ray diffraction measurement, the half width of the diffraction peak at a diffraction angle (2θ) of 23.6±0.5° is 0.06 to 0.06. 17°, the space group of the crystal structure is Pbca, the crystal lattice constants a and c satisfy 9.140 Å ≤ a ≤ 9.205 Å and 9.180 Å ≤ c ≤ 9.220 Å, respectively; A pre-dopant for an electric storage device , wherein a lattice volume V satisfies 773 Å ³ ≤ V ≤ 781 Å ³ .
LixFeOy (1)
[In formula (1), x satisfies 3.5≦x≦4.7, and y satisfies 3.25≦y≦3.85. ] A positive electrode for an electricity storage device, comprising the pre-dopant according to claim 1 and a positive electrode active material. 3. The positive electrode according to claim 2 , wherein the content of said pre-dopant is 1 to 60% by weight with respect to the total weight of said pre-dopant and said positive electrode active material. An electricity storage device comprising the positive electrode according to claim 2 or 3 as a component. A method for producing a pre-dopant for an electric storage device comprising a lithium iron oxide obtained by mixing and firing an iron raw material, a lithium raw material and a carbon raw material, wherein the iron raw material, the lithium raw material and the carbon raw material are mixed and 2. The method for producing a pre-dopant for an electric storage device according to claim 1 , wherein the pre-dopant for an electric storage device is obtained by firing in an oxygen atmosphere at 650 to 1000° C. for 2 to 100 hours and pulverizing the obtained powdery product to obtain a lithium iron oxide.