JP5369447B2 - Lithium titanium iron composite oxide, method for using the same, lithium ion secondary battery, and method for producing lithium titanium iron composite oxide - Google Patents

Description

  The present invention relates to a lithium titanium iron composite oxide, a method for using the same, a lithium ion secondary battery, and a method for producing a lithium titanium iron composite oxide.

Conventionally, as lithium-titanium-iron composite oxides, lithium carbonate, ferric oxide and titanium dioxide are mixed and fired, and used as the negative electrode active material for lithium secondary batteries, improving cycle characteristics by repeated charge and discharge. What has been proposed is proposed (see, for example, Patent Document 1).
JP-A-10-188976

  However, the lithium-titanium-iron composite oxide described in Patent Document 1 has a relatively high reduction potential of 2.2 V to 2.8 V, and even if the positive electrode is combined with a 4 V or 5 V class positive electrode active material, It has been desired to lower the voltage and increase the energy density.

  This invention is made | formed in view of such a subject, Lithium titanium iron complex oxide which can raise energy density more, its use method, lithium ion secondary battery, and manufacture of lithium titanium iron complex oxide The main purpose is to provide a method.

  As a result of earnest research to achieve the above-mentioned object, the present inventors can obtain a suitable negative electrode active material when the lithium compound and the iron compound are sufficiently mixed, and the lithium compound and the iron compound can be obtained. It has been found that the energy density can be further increased by using a hydroxide, and the present invention has been completed.

That is, the lithium titanium iron composite oxide of the present invention is
A lithium titanium iron composite oxide used as an active material of a lithium ion secondary battery,
The reduction potential for metallic lithium is a predetermined first voltage range including 1.2V, and the oxidation potential for metallic lithium is a predetermined second voltage range including 2.3V.

In addition, the method of using the lithium titanium iron composite oxide of the present invention,
A method of using a lithium titanium iron composite oxide used as an active material of a lithium ion secondary battery,
The lithium titanium iron composite oxide according to any one of claims 1 to 6 is used as a negative electrode active material of a lithium ion secondary battery, and a terminal voltage with respect to metallic lithium of discharge in a charge / discharge cycle is lower than the first voltage range. The lithium titanium iron composite oxide is used.

The lithium ion secondary battery of the present invention is
A negative electrode having a negative electrode active material containing the lithium titanium iron composite oxide described above;
An ion conductive medium that is interposed between the positive electrode and the negative electrode and conducts lithium ions;
It is equipped with.

Moreover, the method for producing the lithium titanium iron composite oxide of the present invention includes
A method for producing a lithium titanium iron composite oxide used as an active material of a lithium ion secondary battery,
A dehydration step of heat-treating a mixture of iron hydroxide, lithium hydroxide, and titanium compound at a dehydration temperature at which water resulting from the hydroxide is generated and generation of lithium titanium iron composite oxide is not promoted;
A firing step of firing the dehydrated mixture at a firing temperature that is higher than the dehydration temperature and promotes generation of a lithium titanium iron composite oxide;
Is included.

  In this lithium titanium iron composite oxide, its method of use, lithium ion secondary battery, and method for producing lithium titanium iron composite oxide, the energy density can be further increased. The reason why such an effect is obtained is not clear, but is presumed as follows. For example, in the lithium titanium iron composite oxide of the present invention, it is considered that each component of the raw material is sufficiently mixed and has a more uniform composition. In addition, when iron hydroxide and lithium hydroxide are used as raw materials, it is considered that each component of the raw materials is further sufficiently mixed to obtain a more uniform composition and further increase the energy density.

  The lithium ion secondary battery of the present invention includes a positive electrode having a positive electrode active material, a negative electrode using lithium titanium iron composite oxide as a negative electrode active material, and an ion conductive medium that is interposed between the positive electrode and the negative electrode to conduct lithium ions. And.

  The positive electrode of the lithium ion secondary battery of the present invention is prepared by mixing a positive electrode active material, a conductive material, and a binder, and adding a suitable solvent to form a paste-like positive electrode material on the surface of the current collector. It may be formed by coating and drying, and compressing to increase the electrode density as necessary. As the positive electrode active material, an oxide containing lithium and a transition metal element can be used. Specifically, for example, lithium cobalt composite oxide, lithium titanium iron composite oxide, lithium manganese composite oxide, lithium iron composite phosphorus oxide, and those obtained by adding other elements to these composite oxides. The conductive material is for ensuring the electrical conductivity of the positive electrode. For example, one or two kinds of graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, and amorphous carbon such as needle coke are used. What mixed the above can be used. The binder plays a role of connecting the active material particles and the conductive material particles. For example, the binder is a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. Etc. can be used. In addition, an aqueous dispersion of a cellulose-based or styrene-butadiene rubber that is an aqueous binder can also be used. Examples of the solvent for dispersing the positive electrode active material, the conductive material, and the binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, and N, N-dimethylaminopropyl. Organic solvents such as amine, ethylene oxide, and tetrahydrofuran can be used. As the current collector, foil of aluminum, stainless steel, nickel plated steel, or the like can be used.

In the lithium ion secondary battery of the present invention, for example, a liquid organic solvent electrolyte, an ionic liquid, a solid polymer solid electrolyte, an inorganic solid electrolyte, a gel electrolyte, or the like can be used as the ion conductive medium. Of these, it is preferable to use a liquid one, particularly a non-aqueous electrolyte containing a supporting salt. The supporting salt is not particularly limited. For example, LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , Li (CF ₃ SO ₂ ) ₂ N, Li (CF ₃ SO ₃ ), LiN (C ₂ ) Known supporting salts such as F ₅ SO ₂ ) can be used. These supporting salts may be used alone or in combination. The concentration of the supporting salt is preferably 0.1 to 2.0M, and more preferably 0.8 to 1.2M. As the electrolytic solution, an aprotic organic solvent can be used. Examples of such an organic solvent include cyclic carbonates, chain carbonates, cyclic esters, cyclic ethers, chain ethers, and the like. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and vinyl carbonate. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Examples of the cyclic ester carbonate include gamma butyrolactone and gamma valerolactone. Examples of the cyclic ether include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of the chain ether include dimethoxyethane and ethylene glycol dimethyl ether. These may be used alone or in combination. Of these, it is preferable to use a mixture of ethyl carbonate (EC) and diethyl carbonate (DEC).

  The lithium ion secondary battery of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it is a material that can withstand the use range of the secondary battery, such as a microporous film of a polymer compound. For example, polyethers such as polyethylene, polypropylene, polyvinylidene fluoride, polyvinylidene chloride, polyacrylonitrile, polyacrylamide, polytetrafluoroethylene, polysulfone, polyethersulfone, polycarbonate, polyamide, polyimide, polyethylene oxide, carboxymethylcellulose and hydroxypropyl Examples thereof include celluloses such as cellulose, polymer compounds mainly composed of poly (meth) acrylic acid and other esters and derivatives thereof, and films made of copolymers or mixtures thereof. These may be used alone or in combination. Further, for example, an additive for enhancing ion conductivity and various additives for enhancing strength and corrosion resistance may be added to these films. Of these microporous films, polyethylene, polypropylene, polyvinylidene fluoride, polysulfone and the like are preferably used. This separator is preferably microporous so that the non-aqueous electrolyte can penetrate and ions can easily pass therethrough.

  The negative electrode of the lithium ion secondary battery of the present invention is prepared by mixing a negative electrode active material, a conductive material, and a binder, and adding a suitable solvent to form a paste-like negative electrode material on the surface of the current collector. It may be formed by coating and drying, and compressing to increase the electrode density as necessary. In addition, as the conductive material, binder, solvent, and the like used for the negative electrode, those exemplified for the positive electrode can be used. For the current collector of the negative electrode, a foil such as copper, nickel, stainless steel, or nickel-plated steel can be used.

  In the negative electrode of the lithium ion secondary battery of the present invention, the negative electrode active material contains a lithium titanium iron composite oxide. The lithium-titanium-iron composite oxide has a predetermined first voltage range in which the reduction potential with respect to metallic lithium includes 1.2 V, and a predetermined second voltage range in which the oxidation potential with respect to metallic lithium includes 2.3 V. The first voltage range is preferably in the range of 0.9V to 1.3V. Moreover, it is preferable that the 2nd voltage range is the range of 2.2V or more and 2.4V or less. The lithium titanium iron composite oxide has a reduction potential for metal lithium lower than the oxidation potential and including 2.1 V when the terminal voltage for discharge of metal lithium in the charge / discharge cycle exceeds the first voltage range. The third voltage range may be used. If it carries out like this, it can utilize in a 3rd voltage range by making termination voltage into a voltage exceeding the 1st voltage range at the time of discharge. The third voltage range may be a range of 2.0V to 2.2V. If this lithium ion secondary battery is repeatedly used at a voltage lower than the third voltage range for metallic lithium, the reduction potential for metallic lithium becomes the first voltage range, but if the open-circuit voltage is measured, it is within the third voltage range. Thus, it can be determined that the lithium-titanium-iron composite oxide of the present invention is based on the open-circuit voltage.

In addition, the lithium titanium iron composite oxide preferably has a basic composition formula of Li _{1/2 + 1 / 2x} Fe _{5 / 2-3 / 2x} Ti _x O ₄ (1 ≦ x <5/3), More preferably, x ≦ 4/3. The range of 1 ≦ x <5/3 is preferable because the lattice constant of the lithium titanium iron composite oxide hardly changes, that is, the volume change can be further suppressed. This lattice constant can be obtained from the peak position of the X-ray diffraction measurement. The lithium titanium iron composite oxide satisfying this basic composition formula may be prepared by, for example, preparing raw materials at a composition ratio satisfying this basic composition, or LiFeTiO ₄ and Li (Li _1/3 Ti _{5 / 3} ) It may be prepared by dissolving O ₄ in solid solution so as to have the desired composition. In this lithium-titanium-iron composite oxide, when the Fe at the 8a site is A (mol) and the Fe at the 16d site is B (mol), the occupation ratio S = A / (A + B) of the Fe at the 8a site is 0.48. It is preferably 0.52 or less, more preferably 0.49 or more and 0.51 or less. By doing so, the position occupied by Fe becomes more appropriate, the change in the lattice constant accompanying charge / discharge can be further suppressed, and the energy density can be further increased. The occupation ratio S can be obtained from the peak intensity ratio of the X-ray diffraction measurement.

The shape of the lithium ion secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. An example of this lithium ion secondary battery is shown in FIG. FIG. 1 is a cross-sectional view schematically showing the configuration of the coin-type battery 20. The coin-type battery 20 includes a cup-shaped battery case 21, a positive electrode 22 provided below the battery case 21, a negative electrode 23 provided at a position facing the positive electrode 22 with a separator 24 interposed therebetween, A nonaqueous electrolytic solution 28 containing LiPF ₆ as a supporting salt, a gasket 25 formed of an insulating material, a sealing plate 26 disposed in the opening of the battery case 21 and sealing the battery case 21 via the gasket 25 It is equipped with. The negative electrode 23 contains the above-described lithium titanium iron composite oxide as a negative electrode active material.

  A lithium ion secondary battery according to the present invention uses the above-described lithium titanium iron composite oxide as a negative electrode active material of a lithium ion secondary battery, and a voltage lower than a first voltage range for a terminal voltage with respect to metallic lithium during discharge in a charge / discharge cycle. It may be used in such a state. In this way, since the reduction potential with respect to metallic lithium can be used in the first voltage range including 1.2 V, the lithium ion secondary battery can be operated at a higher battery voltage, and the energy density is increased. Can do. Alternatively, in the lithium ion secondary battery of the present invention, the above-described lithium titanium iron composite oxide is used as a negative electrode active material of a lithium ion secondary battery, and the termination voltage with respect to metallic lithium in discharge in the charge / discharge cycle is set in the first voltage range. It is good also as what is used in the state which becomes a voltage exceeding. If it carries out like this, a lithium ion secondary battery can be operate | moved with the battery voltage different from the case where it uses in the state in which a termination voltage becomes a voltage which falls below a 1st voltage range.

Next, an example of the manufacturing method of lithium titanium iron complex oxide as a negative electrode active material is demonstrated. This manufacturing method includes (1) a raw material preparation step, (2) a dehydration step, (3) a firing step, and the like. Here, a description will be mainly given of a process for producing a lithium-titanium-iron composite oxide using iron oxide hydroxide (FeO (OH)), lithium hydroxide (LiOH), and titanium dioxide (anatase type TiO ₂ ) as raw materials. .

(1) Raw material preparation process In this process, the raw material (inorganic material) used as lithium titanium iron complex oxide is measured and mixed. As the lithium raw material, lithium hydroxide, lithium carbonate, lithium chloride, lithium nitrate, and the like can be used. Of these, lithium hydroxide is preferably used because of its high reactivity. As the iron raw material, iron hydroxide, iron oxide, or the like can be used. Examples of the iron hydroxide include iron oxide hydroxide (FeO (OH)). Examples of the iron oxide include diiron trioxide (Fe ₂ O ₃ ) and triiron tetroxide (Fe ₃ O ₄ ). And iron oxide (FeO). Of these, iron hydroxide (iron oxide hydroxide) is preferred. As the titanium raw material, titanium hydroxide, titanium oxide, or the like can be used. Examples of the titanium hydroxide include titanium hydroxide (Ti (OH) ₄ ). Examples of the titanium oxide include anatase type and rutile type titanium dioxide (TiO ₂ ). Here, as a combination of raw materials, it is more preferable to use lithium, iron, and titanium in the same series of raw materials. For example, when iron is used as iron hydroxide, lithium hydroxide and titanium hydroxide are preferably used. When iron is used as iron oxide, it is more preferable to use lithium carbonate and titanium oxide. Among these, it is more preferable to use a combination of iron oxide hydroxide and lithium hydroxide. Next, these raw materials are appropriately weighed and mixed so as to have a target composition. The composition is preferably Li _{1/2 + 1 / 2x} Fe _{5 / 2-3 / 2x} Ti _x O ₄ (1 ≦ x <5/3). Moreover, it is good also as what preliminarily grind | pulverizes raw material powder as pre-processing before mixing of a raw material. For example, when lithium carbonate is used as a raw material, preliminary pulverization is performed using a predetermined pulverization method. In this way, a lithium titanium iron composite oxide having better active material characteristics can be obtained. The preliminary pulverization may be performed using a pulverizing mixer such as a dry ball mill, an attritor, a planetary mill, or a mortar. This is preferable because the reactivity of the pre-ground inorganic material can be increased. For example, when preliminary pulverization is performed using a ball mill, it can be performed for 1 hour or more and 10 hours or less using 30 g of inorganic material and 0.2 L of ball media in a 0.3 L pot. The mixing of the raw materials may be performed using a pulverizing mixer such as a dry ball mill, an attritor or a planetary mill, or using a mortar. Thus, when prepared by mixing sufficiently, the reduction potential for metallic lithium is in a predetermined first voltage range including 1.2 V, and the oxidation potential for metallic lithium is in a predetermined second voltage range including 2.3 V. A certain lithium titanium iron composite oxide can be produced. The mixed inorganic material is preferably formed into a predetermined shape. This is preferable because the raw materials contained in the firing step are easily reacted. The molding may be performed, for example, in a disk shape or a pellet shape. Molding pressure may be, for example, ^{100kgf / cm 2 ~400kgf / cm 2} . This molding may be omitted, and the next step may be performed with the powder as it is.

(2) Dehydration step This step is a step performed when an inorganic material containing a hydroxide is used as a raw material in the raw material preparation step described above. When a hydroxide is used as a raw material, water resulting from the hydroxide is generated, and thus, by performing this dehydration step, a lithium titanium iron composite oxide having a more uniform composition is generated. Note that this step may be performed even when an inorganic material not containing hydroxide is used as a raw material in the raw material preparation step. In this dehydration step, heat treatment is performed at a dehydration temperature at which water resulting from the hydroxide is generated and generation of the lithium titanium iron composite oxide is not promoted. The dehydration temperature is preferably a predetermined temperature within a temperature range of 350 ° C. or more and 600 ° C. or less, and more preferably 400 ° C. or more and 500 ° C. or less. When the dehydration temperature is 350 ° C. or higher, the hydroxide dehydration reaction is promoted, and when the dehydration temperature is 600 ° C. or lower, generation of lithium titanium-iron composite oxide during the dehydration reaction can be further suppressed. This dehydration step is preferably performed in the presence of oxygen, and may be performed in air. In addition, this dehydration treatment is preferably carried out over a period of time obtained by empirically obtaining a time for sufficiently removing water from the inorganic material.

(3) Firing step Next, the inorganic material prepared in the raw material preparation step is fired. In this firing step, the dehydrated mixture is fired at a firing temperature that is higher than the dehydration temperature and promotes the formation of the lithium titanium iron composite oxide. The firing step is performed in an oxygen atmosphere, and the oxygen atmosphere may be in the air or in oxygen. This baking step may be a batch method in which an oxidation treatment is performed in an initial introduction atmosphere or an air flow method in which an oxygen atmosphere is continuously supplied, but the latter is more preferable. The firing temperature is preferably 700 ° C. or higher and 1000 ° C. or lower, and more preferably 800 ° C. or higher and 900 ° C. or lower. When the firing temperature is 700 ° C. or higher, the formation of lithium titanium iron composite oxide is preferably promoted, and when it is 1000 ° C. or lower, energy consumption can be suppressed, which is preferable. The firing time may be determined empirically from the degree of formation of the lithium titanium iron composite oxide.

The lithium titanium iron composite oxide thus obtained can be used as a negative electrode active material of a lithium ion secondary battery, and at this time, the energy density can be further increased. Moreover, when an iron hydroxide is used as a raw material, the energy density can be further increased. Moreover, when charging / discharging is repeated, a reduction in charge / discharge capacity can be further suppressed. Moreover, when charging / discharging is repeated, the resistance increase rate of a lithium ion secondary battery can be made lower. The reason for this is not clear, but the following reasons are possible. For example, it is presumed that the lithium titanium iron composite oxide has a more uniform composition by sufficiently mixing the inorganic material as the raw material. Moreover, it is guessed that a lithium titanium iron complex oxide becomes a more uniform composition by performing the dehydration process which fully dehydrates. Thus, with a uniform composition, the lithium-titanium-iron composite oxide has a predetermined first voltage range in which the reduction potential for metallic lithium includes 1.2 V, and a predetermined potential in which the oxidation potential for metallic lithium includes 2.3 V. The second voltage range is considered. Here, lithium titanium oxide represented by Li (Li _1/3 Ti _5/3 ) O ₄ (referred to as LTO) is known as a negative electrode active material. This LTO is an active material with almost no change in lattice constant due to insertion / extraction of lithium ions (T. Ohzuku, A. Ueda, and N. Yamamoto, J. Electrochem. Soc. 142 (1995) 1431). It is known that it is an active material that has better thermal stability than lithium in the state of containing lithium ions (J. Jiang, KWEberman, LJ Krause, and, JRDahn, J. Electrochem. Soc. 152 (2005 ) A566), is expected as a highly safe electrode material to replace carbon materials. Since the lattice constant of the lithium titanium iron composite oxide of the present invention hardly changes even when the insertion and release of lithium ions are repeated, the cycle characteristics are high like LTO, and the reduction potential of LTO (about 1.55 V). ), The energy density can be further increased as compared with LTO. Further, when the lithium titanium iron composite oxide is made from iron hydroxide, the above-mentioned 8a site Fe occupancy S = A / (A + B) is in the range of 0.48 to 0.52, that is, Therefore, it is presumed that the energy density can be further increased because lithium ions can be more stably inserted and desorbed.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

Below, the example which produced lithium titanium iron complex oxide concretely is demonstrated using an experiment example. Here, a lithium-titanium-iron composite oxide represented by the basic composition formula Li _{1/2 + 1 / 2x} Fe _{5 / 2-3 / 2x} Ti _x O ₄ is prepared, and the counter electrode is a lithium metal. The coin-type evaluation battery shown was produced and the battery characteristics were evaluated.

[Experiment 1]
The raw material was iron hydroxide, and a lithium titanium iron composite oxide having a basic composition of LiFeTiO ₄ (x = 1) was produced. First, lithium hydroxide (LiOH.H ₂ O), iron oxide hydroxide (FeO (OH)), anatase-type titanium dioxide is used as the raw material inorganic material, and Li: Fe: Ti is 1: 1: 1. Were mixed and ground using a mortar. This mixed powder was molded into a pellet shape having a diameter of 20 mm and a thickness of 5 mm with a pressing pressure of 100 kgf / cm ² . Next, the compact was held in an air stream at 400 ° C. for 3 hours, and water was dehydrated due to hydroxide. The compact was calcined at 800 ° C. for 12 hours as it was, and the calcined product was sufficiently pulverized with a mortar to obtain the lithium titanium iron composite oxide of Experimental Example 1.

[Experiment 2]
The raw material was iron hydroxide, and a lithium titanium-iron composite oxide having a basic composition of Li _1.17 Fe _0.5 Ti _1.33 O ₄ (x = _4/3 ) was produced. Lithium hydroxide (LiOH.H ₂ O), iron oxide hydroxide (FeO (OH)), anatase-type titanium dioxide is used as the raw material inorganic material, and Li: Fe: Ti is 1.167: 0.5: 1. The lithium titanium iron composite oxide of Experimental Example 2 was obtained through the same steps as in Experimental Example 1, except that the weight was adjusted to be .333.

[Experiment 3]
A lithium titanium composite oxide (LTO) having a basic composition of Li _1.33 Ti _1.67 O ₄ (x = 5/3) was produced. As the raw material inorganic material, lithium hydroxide (LiOH.H ₂ O), anatase-type titanium dioxide was used, and it was weighed so that Li: Ti was 4: 5. The lithium titanium composite oxide of Experimental Example 3 was obtained.

[Experimental Example 4]
A lithium titanium-iron composite oxide having an iron oxide as a raw material and a basic composition of LiFeTiO ₄ (x = 1) was produced. Lithium carbonate (Li ₂ CO ₃ ), iron oxide (Fe ₂ O ₃ ), and anatase titanium dioxide were used as the raw material inorganic material. This lithium carbonate was preliminarily pulverized by a ball mill in an alumina pot having a volume of 0.3 L under the conditions of 2 hours using 15 g of lithium carbonate and 0.2 L of ball media. Except for using these inorganic materials, the lithium titanium iron composite oxide of Experimental Example 4 was obtained through the same steps as in Experimental Example 1.

[Production of coin-type battery]
A coin-type battery 20 shown in FIG. 1 was produced using the lithium manganese composite oxides of Experimental Examples 1 to 4 described above as the negative electrode active material. First, 95% by weight of the negative electrode active material, 5% by weight of polyvinylidene fluoride (manufactured by Kureha Chemical Co., Ltd.) as a binder, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a dispersing agent are added and dispersed. Thus, a slurry-like negative electrode mixture was obtained. This negative electrode mixture was applied to an aluminum foil current collector with a thickness of 20 μm, vacuum-dried at 120 ° C. for 12 hours, then densified with a roll press, and cut into a shape with a diameter of 15 mm to obtain a sheet-like negative electrode 23 It was. In addition, the adhesion amount of the negative electrode active material was about 30 mg. Metal Li was used for the counter electrode (positive electrode). A polyethylene separator 24 was sandwiched between the negative electrode 23 and the metal Li positive electrode 22, placed in the battery case 21, and a non-aqueous electrolyte solution 28 was accommodated to produce a 2016 coin-type battery 20. The nonaqueous electrolytic solution 28 was prepared by dissolving 1M LiPF ₆ in an electrolytic solution in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 1: 1. The coin-type battery 20 was manufactured in a glove box under an Ar atmosphere.

[X-ray diffraction measurement]
X-ray diffraction measurement of the electrode powders of Experimental Examples 1 to 4 was performed with CuKα rays using an X-ray diffractometer (RINT-2200 manufactured by Rigaku Corporation). FIG. 2 shows X-ray diffraction measurement results of the powder of Experimental Example 1 before using the battery and the powder of Experimental Example 1 after discharging as a battery. The X-ray diffraction measurement of the powder of Experimental Example 1 after discharging as a battery was performed by discharging the lithium-titanium-iron composite oxide as the negative electrode active material to 1.0 V using the above-described coin-type battery with the counter electrode being metallic lithium. After that, the negative electrode active material was taken out and sealed in a polyethylene film in a state of blocking air. The discharge capacity at this time was 140 mAh / g. As shown in FIG. 2, the powder (LiFeTiO ₄ ) of Experimental Example 1 before using the battery was attributed to a single-phase spinel structure (space group Fd3m), and the lattice constant a was 8.360 Å. Moreover, the powder (Li _1.91 FeTiO ₄ ) of Experimental Example 1 after battery discharge was attributed to a single-phase spinel structure, and the lattice constant a was 8.374%. Therefore, it was found that the volume change accompanying the discharge was about 1%, and the volume change was extremely small compared to graphite (about 10%). From the X-ray diffraction measurement results of Experimental Examples 2 to 4, the range of 1 ≦ x ≦ _5/3 when the basic composition formula is Li _{1/2 + 1 / 2x} Fe _{5 / 2-3 / 2x} Ti _x O ₄ Then, it was found that the lattice constant a was almost unchanged at about 8.36 mm. In Experimental Examples 2 to 4, it was presumed that the lattice constant a did not substantially change with the insertion / extraction of lithium ions, and the same results as in Experimental Example 1 were obtained. Here, the crystal structure of LiFeTiO ₄ is a spinel structure represented by the space group Fd3m, Li ions are 50% each of 4-coordinate 8a site and 6-coordinate 16d site, and Fe ions are 8a site and 16d site. 50% each and Ti ions occupy only 16d sites. Therefore, the composition formula of LiFeTiO ₄ is represented by (Li _1/2 Fe _1/2 ) ^8a (Li _1/2 Fe _1/2 Ti) ^16d O ₄ . Thus, the position occupied by Li ions and Fe is expected to have a great influence on the electrochemical characteristics. This is because the charge / discharge reaction of the lithium titanium iron composite oxide passes through the 16d site as follows. For example, taking LiFeTiO ₄ as an example, the charge / discharge reaction is represented by the following formula (1), and the Li ⁺ ions and Fe ³⁺ ions at the 8a site move to the 16c site. In a material such as LiMn ₂ O _{4 where} the 8a site is usually occupied only by Li ⁺ ions, it is possible to move between the 8a site and the 16d site, but as in LiFeTiO ₄ , the 8a site is transitioned. In the material occupied by the metal, it is considered that the reaction hardly occurs unless Li ⁺ ions and Fe ^{3 +} ions at the 16d site move from the conduction path. Therefore, it is considered that Fe ions always move reversibly at the time of reaction in materials represented by Li _{1/2 + 1 / 2x} Fe _{5 / 2-3 / 2x} Ti _x O ₄ , including LiFeTiO _4. It is considered that the ratio of Fe ions at the 8a site and the 16d site (before the charge / discharge test) greatly affects the electrochemical characteristics. Here, with respect to Experimental Examples 1, 2, and 4, powder X-ray diffraction measurement was performed, and the occupation rate S of the 8a site of Fe ions was calculated. As a result, assuming that Fe at the 8a site is A (mol) and Fe at the 16d site is B (mol), the occupation ratio S = A / (A + B) of Fe at the 8a site is 0.49 in Experimental Example 1. Yes, Experimental Example 2 was 0.51, and Experimental Example 4 was 0.45. Experimental Example 2 can be represented by (Li _3/4 Fe _1/4 ) ^8a (Li _5/12 Fe _1/4 Ti _4/3 ) ^16d O ₄ , although the composition is different. Thus, in Experimental Examples 1 and 2, it was found that lithium and iron occupy the 8a site and the 16d site at a value close to 1: 1.
^{_{(Li 1/2 Fe 1/2) 8a [}} Li 1/2 Fe 1/2 Ti] 16d O 4 + Li + + e - ← → (Li 3/2 Fe 1/2) 16c [Li 1/2 Fe _1/2 Ti] ^16d O ₄ ... formula (1)

[Charge / discharge cycle test, capacity maintenance rate]
The lithium ion secondary batteries of Experimental Examples 1 to 4 were discharged at a constant current to 1 V with respect to metal Li at a test temperature of 25 ° C. and a current value of 0.5 mA (corresponding to a current density of about 0.28 mA / cm ² ), and then A charge / discharge cycle test was performed in which charge / discharge at a constant current of 0.5 mA (0.28 mA / cm ² ) and constant current charge up to 3.0 V was defined as one cycle, and this cycle was performed for a total of 100 cycles. At this time, the discharge capacity in the first cycle is the initial capacity W0 (mAh / g), the discharge capacity in the 100th cycle is the post-cycle capacity W100 (mAh / g), and the capacity is maintained by the following equation (2). The rate Wma was calculated.
Capacity maintenance ratio Wma (%) = W100 / W0 × 100 (2)

[Resistance increase rate]
Using the lithium ion secondary batteries of Experimental Examples 1 to 4, the resistance increase rate Rin when the charge / discharge cycle was repeated was determined. The battery resistance is set to 0.5 mA, 1.0 mA, 2 mA after charging at constant current / constant voltage for 24 hours at 25 ° C. and at a charging current of 0.5 mA up to 3.0 V before charging / discharging cycle. 0.0 mA, 4.0 mA, and 8.0 mA were applied to measure the voltage 10 seconds later. The applied current and voltage were plotted and linearly approximated to obtain the initial resistance R0 (Ω) from the slope. Next, after performing the above-described charge / discharge cycle test, the battery resistance was similarly obtained, and this value was set as the post-cycle resistance R100 (Ω), and the resistance increase rate Rin (%) was calculated by the following equation (3). .
Resistance increase rate Rin (%) = (R100−R0) / R0 × 100 (3)

[Measurement result]
Basic composition, initial capacity (mAh / g), capacity retention rate Wma (%), resistance increase rate Rin (%), oxidation potential (V) after charge / discharge cycle, reduction potential (charge / discharge cycle) The measurement results of V) are shown in Table 1. 3 is an initial charge / discharge curve of the coin-type battery of Experimental Example 1, FIG. 4 is a charge / discharge curve in a charge / discharge cycle test of the coin-type battery of Experimental Example 1, and FIG. FIG. 6 is a discharge curve after the charge / discharge cycle test of Experimental Examples 1 to 4. FIG. As shown in FIG. 3, Experimental Example 1 has a discharge reaction (Fe reduction reaction) of 2.05 V to 2.15 V (third voltage range including 2.1 V), and a charge reaction (Fe oxidation reaction). ) Was 2.28 V to 2.34 V (second voltage range including 2.3 V), and a discharge capacity of 94% of the theoretical capacity was obtained. For this reason, it turned out that it is a substance completely different from the lithium titanium iron complex oxide of Unexamined-Japanese-Patent No. 10-188976. Moreover, as shown in FIG. 4, when a charge / discharge cycle is performed, in Experimental Example 1, the detailed reaction mechanism is unknown, but as the cycle is repeated, the cycle is repeated. The discharge reaction around 2.1 V shifted to the low potential side, and a reduction potential of 0.95 V to 1.25 V (first voltage range including 1.2 V) was exhibited. At this time, as shown in Table 1, in Experimental Example 1, it was found that a decrease in capacity and an increase in battery resistance associated with the charge / discharge cycle were further suppressed. From the results shown in FIG. 4, the lithium titanium iron composite oxide of Experimental Example 1 shows that when the terminal voltage with respect to metallic lithium of discharge in the charge / discharge cycle is used at a voltage lower than the first voltage range (about 1.2 V), It was speculated that the reduction potential for lithium could be used in the first voltage range (about 1.2 V). In addition, when the termination voltage for discharge metal lithium is used at a voltage exceeding the first voltage range (about 1.2 V), the reduction potential for metal lithium can be used in the third voltage range (about 2.1 V). Inferred. As shown in FIGS. 5 and 6, in Experimental Example 2 which is a composition between Experimental Example 1 and Experimental Example 3, behavior characteristics of both of these reduction potentials were observed. In Experimental Example 3, the decrease in capacity and the increase in battery resistance were relatively suppressed in the charge / discharge cycle, but the reduction potential of Fe was about 1.5 V, which is higher than the other experimental examples (about 1.2 V). The value is shown. In Experimental Example 4, the initial capacity was lower than that of Experimental Example 1, and the capacity reduction and resistance increase were large, but the reduction potential of charge / discharge was the same value as in Experimental Example 1.
The reason for this is not clear, but Experimental Example 4 shows the same reduction potential as Experimental Example 1 because the raw materials are sufficiently mixed and the composition is uniform. It was speculated that the occupation rate S was low, which caused the initial capacity, capacity decrease after charge / discharge cycle, and battery resistance increase. Thus, in the lithium-titanium-iron composite oxides of Experimental Examples 1, 2, and 4, almost no volume change due to insertion / desorption of Li ions was observed, and the reduction potential was lower than that of LTO of Experimental Example 3, It became clear that it is promising as a negative electrode active material that can replace carbon materials. In particular, in Experimental Examples 1 and 2, lithium and iron occupy the 8a site and the 16d site at values close to 1: 1, and the decrease in capacity and increase in battery resistance associated with the charge / discharge cycle are further suppressed. Therefore, it became clear that it was extremely promising.

2 is a cross-sectional view illustrating a schematic configuration of a coin-type battery 20. FIG. It is a X-ray-diffraction measurement result of the powder of Experimental example 1 before battery utilization, and the powder of Experimental example 1 after discharge as a battery. 3 is an initial charge / discharge curve of the coin-type battery of Experimental Example 1. It is a charging / discharging curve in the charging / discharging cycle test of the coin-type battery of Experimental Example 1. It is an initial discharge curve of Experimental Examples 1-4. It is a discharge curve after the charging / discharging cycle test of Experimental Examples 1-4.

Explanation of symbols

  20 coin cell, 21 battery case, 22 positive electrode, 23 negative electrode, 24 separator, 25 gasket, 26 sealing plate, 28 non-aqueous electrolyte.

Claims (10)

A lithium titanium iron composite oxide used as an active material of a lithium ion secondary battery,
A reduction potential with respect to metallic lithium is a first voltage range of 0.9V to 1.3V, and an oxidation potential with respect to metallic lithium is a second voltage range of 2.2V to 2.4V;
The basic composition formula is Li _{1/2 + 1 / 2x} Fe _{5 / 2-3 / 2x} Ti _x O ₄ (1 ≦ x <5/3),
Made using a mixture of iron hydroxide, lithium hydroxide and titanium compound,
Lithium titanium iron composite oxide.   When the termination voltage for discharge of metallic lithium in the charge / discharge cycle is set to a voltage exceeding the first voltage range, the reduction potential for metallic lithium is lower than the oxidation potential and is a predetermined third voltage range including 2.1 V. The lithium titanium iron composite oxide according to claim 1.   3. The lithium titanium iron composite oxide according to claim 2, wherein the third voltage range is a range of 2.0 V or more and 2.2 V or less.   When the 8a site Fe is A (mol) and the 16d site Fe is B (mol), the 8a site Fe occupancy S = A / (A + B) is 0.48 or more and 0.52 or less, Item 4. The lithium titanium iron composite oxide according to any one of Items 1 to 3. A method of using a lithium titanium iron composite oxide used as an active material of a lithium ion secondary battery,
The lithium titanium iron composite oxide according to any one of claims 1 to 4 is used as a negative electrode active material of a lithium ion secondary battery, and a termination voltage with respect to metallic lithium of discharge in a charge / discharge cycle is set to the first voltage range. Using the lithium titanium iron composite oxide at a voltage lower than,
How to use lithium titanium iron composite oxide. A positive electrode having a positive electrode active material;
A negative electrode having a negative electrode active material containing the lithium titanium iron composite oxide according to any one of claims 1 to 4,
An ion conductive medium that is interposed between the positive electrode and the negative electrode and conducts lithium ions;
Lithium ion secondary battery equipped with. It is a manufacturing method of the lithium titanium iron complex oxide of any one of Claims 1-4 used as an active material of a lithium ion secondary battery,
A dehydration step of heat-treating a mixture of iron hydroxide, lithium hydroxide, and titanium compound at a dehydration temperature at which water resulting from the hydroxide is generated and generation of lithium titanium iron composite oxide is not promoted;
A firing step of firing the dehydrated mixture at a firing temperature that is higher than the dehydration temperature and promotes generation of a lithium titanium iron composite oxide;
The manufacturing method of the lithium titanium iron complex oxide containing this.   The method for producing a lithium titanium iron composite oxide according to claim 7, wherein in the dehydration step, heat treatment is performed at a dehydration temperature of 350 ° C. or higher and 600 ° C. or lower.   The method for producing a lithium-titanium-iron composite oxide according to claim 7 or 8, wherein in the firing step, a firing treatment is performed at a firing temperature of 700 ° C or higher and 1000 ° C or lower.   The method for producing a lithium-titanium-iron composite oxide according to any one of claims 7 to 9, wherein in the firing step, a firing treatment is performed at a firing temperature of 800 ° C or higher and 900 ° C or lower.

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