JP5159048B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte.

  Currently, non-aqueous electrolyte secondary batteries that use a non-aqueous electrolyte and charge and discharge by moving lithium ions between the positive electrode and the negative electrode are widely used as secondary batteries with high energy density. .

In such a non-aqueous electrolyte secondary battery, a lithium transition metal composite oxide such as lithium cobaltate (LiCoO ₂ ) is generally used as the positive electrode, and a carbon material capable of occluding and releasing lithium as the negative electrode, lithium metal, lithium An alloy or the like is used (for example, see Patent Document 1).

In addition, a non-aqueous electrolyte in which an electrolyte salt such as lithium tetrafluoroborate (LiBF ₄ ) or lithium hexafluorophosphate (LiPF ₆ ) is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate is used. ing.

  In recent years, such non-aqueous electrolyte secondary batteries have been used as power sources for various portable devices, etc., but with the increase in power consumption due to multifunctional portable devices, non-aqueous electrolytes with higher energy density Secondary batteries are desired.

  In addition, diversification of uses of nonaqueous electrolyte secondary batteries has led to a demand for materials that are richer and less expensive than cobalt (Co).

  As such a material, lithium manganate is used. It is known that a high energy density can be obtained by using lithium manganate as a positive electrode active material.

The discharge reaction when lithium manganate is used as the positive electrode active material occurs in two stages as shown in the following equation. The reaction of the following formula (1) occurs at a voltage near 4 V (vs. Li ^/ Li ⁺ ), and the reaction of the following formula (2) occurs at a voltage near 3 V (vs. Li ^/ Li ⁺ ) (for example, non-patent Reference 1).

Mn ₂ O ₄ + Li ⁺ + e ⁻ → LiMn ₂ O ₄ (1)
LiMn ₂ O ₄ + Li ⁺ + e ⁻ → Li ₂ Mn ₂ O ₄ (2)
Thus, a large theoretical capacity density close to 300 mAh / g can be obtained by using LiMn ₂ O ₄ as a positive electrode active material and utilizing voltages in the 4 V region and the 3 V region.

Further, since manganese (Mn) is richer and less expensive than cobalt, lithium manganate is desirable as a positive electrode active material instead of lithium cobaltate.
Japanese Patent Laid-Open No. 10-116615 Journal of The Electrochemical Society, 137 (3) 769-775 (1990)

However, when lithium manganate is used as the positive electrode active material, relatively good charge / discharge cycle characteristics can be obtained in the reaction of the above formula (1), but in the reaction of the above formula (2), the crystal system of the positive electrode active material is obtained. Since the (crystal structure) changes from cubic LiMn ₂ O ₄ to tetragonal Li ₂ Mn ₂ O ₄ , it is known that the charge / discharge cycle characteristics are not good.

  Therefore, research on non-aqueous electrolyte secondary batteries using only the reaction of the above formula (1) has been actively conducted. However, non-aqueous electrolyte secondary batteries using only the reaction of the above formula (1) have high energy. The density cannot be obtained.

  An object of the present invention is to provide a nonaqueous electrolyte secondary battery capable of obtaining good charge / discharge cycle characteristics and obtaining a high energy density.

The non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode including a positive electrode active material, a negative electrode, and a non-aqueous electrolyte, and the positive electrode active material is tetragonal Li _{2 + x} Mn _{2-y in} a fully discharged state. M _z O _{4 + q} is included, x and q are greater than −1 and less than 1, y and z are 0 or more and 0.5 or less, and M is from chromium, iron, cobalt, nickel, copper, aluminum and magnesium One or more selected from the group consisting of at least part of the surface of the positive electrode active material is covered with a coating material composed of primary particles smaller than the size of the primary particles of the positive electrode active material, Includes lithium iron phosphate .

In the non-aqueous electrolyte secondary battery according to the present invention, the positive electrode active material contains tetragonal Li _{2 + x} Mn _{2 -y} M _z O _{4 + q} in the fully discharged state, and the surface of the positive electrode active material The following formula (1) which occurs when lithium manganate is used as the positive electrode active material by being at least partially covered with a coating material composed of primary particles smaller than the size of the primary particles of the positive electrode active material. Good charge / discharge cycle characteristics can be obtained in the two-stage discharge reaction of (2). Thereby, a high energy density can be obtained.

Mn ₂ O ₄ + Li ⁺ + e ⁻ → LiMn ₂ O ₄ (1)
LiMn ₂ O ₄ + Li ⁺ + e ⁻ → Li ₂ Mn ₂ O ₄ (2)
The coating material may have one or both of electronic conductivity and lithium ion conductivity. In this case, it can be expected that the electron conductivity of the positive electrode active material is improved during the charge / discharge reaction, or that lithium ions are occluded and released smoothly. Thereby, charging / discharging cycle characteristics become better.

  At least a part of the surface of the positive electrode active material may be covered with a coating material by a mechanical milling process. In this case, at least part of the surface of the positive electrode active material can be satisfactorily covered with the coating material in a short time by mechanical milling.

  The potential of the positive electrode active material based on lithium metal at the end of discharge may be 2.8 V or less. In this case, the discharge reaction of the above formula (2) can be used.

  The potential of the positive electrode active material based on lithium metal at the end of charging may be 4.2 V or higher. In this case, the discharge reaction of the above formula (1) can be used.

  The positive electrode active material having at least a part of the surface covered with the coating material may be heat-treated at a temperature lower than 400 ° C.

  When at least a part of the surface of the positive electrode active material is covered with a coating material by the mechanical milling process, the specific surface area of the positive electrode active material may increase. Then, after covering at least a part of the surface of the positive electrode active material with a coating material by mechanical milling treatment, the specific surface area of the positive electrode active material is reduced by heat-treating the positive electrode active material at a temperature of less than 400 ° C. Thus, a positive electrode active material capable of obtaining charge / discharge cycle characteristics can be generated.

  The coating material may include carbon. In this case, it becomes easy to cover the positive electrode active material by using carbon, and the conductivity of the positive electrode active material covered with carbon can be improved.

The positive electrode active material in the stage before performing the first charge / discharge has a cubic LiMn ₂ O ₄ having a basic skeleton in which at least part of the surface is covered with carbon, and the negative electrode in the stage before the first charge / discharge is Sufficient lithium may be included so that the basic skeleton of the positive electrode active material changes to tetragonal Li ₂ Mn ₂ O ₄ in a fully discharged state.

In this case, the basic skeleton of the positive electrode active material is changed to tetragonal Li ₂ Mn ₂ O ₄ in the fully discharged state, even if the lithium content in the positive electrode active material in the stage before the first charge / discharge is relatively small. By using a negative electrode containing sufficient lithium for the purpose, good charge / discharge cycle characteristics can be obtained.

The coating material includes lithium iron phosphate. In this case, by using lithium iron phosphate, the positive electrode active material can be easily covered, and the conductivity of the positive electrode active material covered with lithium iron phosphate can be improved.

The positive electrode active material in the stage before performing the first charge / discharge is based on cubic LiMn ₂ O ₄ having a cubic structure in which at least part of the surface is covered with lithium iron phosphate, and in the stage before the first charge / discharge. The negative electrode may contain sufficient lithium so that the basic skeleton of the positive electrode active material changes to tetragonal Li ₂ Mn ₂ O ₄ in a fully discharged state.

In this case, the basic skeleton of the positive electrode active material is changed to tetragonal Li ₂ Mn ₂ O ₄ in the fully discharged state, even if the lithium content in the positive electrode active material in the stage before the first charge / discharge is relatively small. By using a negative electrode containing sufficient lithium for the purpose, good charge / discharge cycle characteristics can be obtained.

  According to the present invention, good charge / discharge cycle characteristics can be obtained, and a high energy density can be obtained.

  Hereinafter, the nonaqueous electrolyte secondary battery according to the present embodiment will be described. The nonaqueous electrolyte secondary battery according to the present embodiment includes a positive electrode, a negative electrode, and a nonaqueous electrolyte.

(1) Production of positive electrode The positive electrode active material of the present embodiment includes lithium manganate (Li ₂ Mn ₂ O ₄ ) having a tetragonal crystal system (crystal structure) as a basic skeleton in a fully discharged state.

  Here, as described in the background art above, the discharge reaction when lithium manganate is used as the positive electrode active material occurs in two stages as shown in the following formulas (1) and (2).

Mn ₂ O ₄ + Li ⁺ + e ⁻ → LiMn ₂ O ₄ (1)
LiMn ₂ O ₄ + Li ⁺ + e ⁻ → Li ₂ Mn ₂ O ₄ (2)
Further, at least a part of the surface of the positive electrode active material Li ₂ Mn ₂ O ₄ is covered with a material (hereinafter referred to as a coating material) composed of primary particles smaller than the size (diameter) of the primary particles of the positive electrode active material. It has been broken. Thus, it becomes easy to cover the surface of the positive electrode active material by using primary particles smaller than the size of the primary particles of the positive electrode active material.

If a negative electrode containing enough lithium to cause the reaction of the above formula (2) is used, the Li ₂ Mn ₂ O ₄ can be used as the positive electrode active material. Instead, a positive electrode active material having a cubic LiMn ₂ O ₄ crystal system as a basic skeleton can be used. Also in this case, at least a part of the surface of the positive electrode active material LiMn ₂ O ₄ is covered with the coating material.

  As the negative electrode containing lithium sufficient to cause the reaction of the above formula (2), lithium metal, a lithium-carbon intercalation compound, a lithium alloy, or the like can be used.

The composition of the positive electrode active material having tetragonal Li ₂ Mn ₂ O ₄ as a basic skeleton is represented by Li _{2 + x} Mn _2−y M _z O _{4 + q} . Note that -1 <x <1, 0 ≦ y ≦ 0.5, 0 ≦ z ≦ 0.5, and −1 <q <1, and M is chromium (Cr), iron (Fe), cobalt (Co ), Nickel (Ni), copper (Cu), aluminum (Al), and magnesium (Mg).

The composition of the positive electrode active material having cubic LiMn ₂ O ₄ as a basic skeleton is represented by Li _1-x Mn _{2 -y Mz} O _{4 + q} . In addition, 0 ≦ x ≦ 1, 0 ≦ y ≦ 0.5, 0 ≦ z ≦ 0.5, and −1 <q <1, and M is from chromium, iron, cobalt, nickel, copper, aluminum, and magnesium One or more selected from the group consisting of:

  Hereinafter, a method of covering the positive electrode active material with the coating material and details of the coating material will be described.

  Examples of methods for covering the positive electrode active material with a coating material include mechanical milling, treatment with a hybridization system, treatment with an attritor (solid phase reactor), treatment with a rotary flow device, treatment with a laser ablation system, etc. In addition to physical treatment, chemical treatment such as treatment for firing the positive electrode active material and the coating material, and treatment for firing the starting material for the positive electrode active material and the starting material for the coating material, and the like can be given. In the present embodiment, it is preferable to perform a mechanical milling process.

  The coating material preferably has one or both of electronic conductivity and lithium ion conductivity. By covering the positive electrode active material with a coating material having such properties, when this positive electrode active material is used in a non-aqueous electrolyte secondary battery, the electron conductivity of the positive electrode active material is improved during the charge / discharge reaction. Or occlusion and release of lithium ions can be expected to be performed smoothly. Thereby, charge / discharge cycle characteristics are improved.

  Examples of the coating material having one or both of electron conductivity and lithium ion conductivity include the following. In addition, only 1 type may be used from the following example and it may use it combining several things.

Examples of the coating material include hexagonal crystal systems (crystal systems belonging to the space group R3m) such as carbon (C), LiCoO ₂ and LiNiO ₂ , and spinel structures such as LiMn ₂ O _4. The transition metal oxide which has is mentioned.

Examples of other coating materials include LiTi ₂ (PO ₄ ) ₃ , Fe ₂ (SO ₄ ) ₃ , Fe ₂ (MoO ₄ ) ₃ , Fe ₂ (SO ₄ ) ₂ (PO ₄ ), Li ₃ Fe _2. (PO ₄ ) ₃ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₃ Fe ₂ (AsO ₄ ) ₃ , TiNb (PO ₄ ) ₃ , LiFeNb (PO ₄ ) ₃ , Li ₂ FeTi (PO ₄ ) ₃ , Li ₂ CrTi (PO ₄ ) ₃ , Fe ₄ (P ₂ O ₇ ) ₃ , Li _{1 + x} Al _x Ti _2−x (PO ₄ ) ₃ (0 ≦ x ≦ 2), Li _{1 + x} Sc _x Ti ₂₋ Examples include Nasicon-based materials such as _x (PO ₄ ) ₃ (0 ≦ x ≦ 2) and Li ₃ M ₂ (PO ₄ ) ₃ (M represents yttrium (Y) or lanthanum (La)). It is done.

Examples of other coating materials include Fe ₄ (P ₂ O ₇ ) ₃ , LiFeP ₂ O ₇ , TiP ₂ O ₇ , LiVP ₂ O ₇ , MoP ₂ O ₇ , and Mo ₂ P ₂ O ₁₁ . Examples thereof include phosphoric acid / condensed salt-based materials and Oliveline-based materials such as LiFePO ₄ , LiCoPO ₄ , and LiMnPO ₄ .

Further, examples of other coating _{materials, MoOPO 4, VOPO 4, LiVOPO} 4, VOSO 4, VOAsO 4, and Aobo ₄ based materials such as Li ₂ VOSiO _4, Brannerite based materials such as LiVMoO _6, Fe ₃ BO ₆ , Borate-based materials such as FeBO ₃ and VBO ₃ , metal nitrides such as Li ₃ N, TiN, ZrN and TaN, metal carbides such as TiC, ZrC, TaC and WC, and TiB ₂ , ZrB ₂ And metal borides such as TaB ₂ .

Examples of other coating materials include Li ₃ M ₂ (PO ₄ ) ₃ (M represents scandium (Sc), indium (In), chromium (Cr), or iron (Fe)), and Li _x. (M _1-y M ′ _y ) ₂ (PO ₄ ) ₃ (M represents In or Sc, M ′ represents zirconium (Zr), niobium (Nb), 1 ≦ x ≦ 3, 0 ≦ y Β-Fe ₂ (SO ₄ ) ₃ type material such as ≦ 1, and Li ₁₄ Zn (GeO ₄ ) ₄ .

Further, as other coating materials, Li ₃ PS ₄ , Li ₂ SiS ₄ , Li ₄ SiS ₄ , Li _{4 + x + δ} (Ge _1-δ′-x Ga _x ) S ₄ (0 ≦ x ≦ 1,0) ≦ δ ′ ≦ 1, 0 ≦ x + δ ′ ≦ 1, 0 ≦ δ ≦ 3), Li _4-x Ge _1-x P _x S ₄ (0 ≦ x ≦ 1), and Li _4-x Si _1-x P Examples thereof include thio LISICON type materials such as _x S ₄ (0 ≦ x ≦ 1).

Further, as another coating material, LiPON (Li-P-O -N), LiX-Li 2 O-M m O n (X is iodine (I), shows the bromine (Br), or chlorine (Cl) , M _m O _n represents B ₂ O ₃ , P ₂ O ₅ , or GeO _2. ), Li ₄ SiO ₄ —Li ₃ BO ₃ , Li ₂ O—SiO ₂ —B ₂ O ₃ , and Li ₂ An oxide glass such as O—SiO ₂ —ZrO ₂ may be used.

Furthermore, as other coating materials, Li ₂ S—P ₂ S ₅ , Li ₂ S—GeS ₂ , Li ₂ S—B ₂ S ₃ , Li ₂ S—SiS ₂ , Li ₂ S—SiS ₂ —LiX (X Represents I, Br, or Cl.), Sulfide glasses such as Li ₂ S—SiS ₂ —Li ₄ SiO ₄ , and Li ₂ S—SiS ₂ —Li ₃ PO ₄ .

  In this embodiment, carbon having electron conductivity is used as the coating material.

  When performing a mechanical milling process, it is preferable to use a planetary ball mill. In the planetary ball mill, the pot rotates and the platform rotates revolvingly. With such a configuration, very high impact energy can be generated efficiently, so that at least a part of the surface of the positive electrode active material can be covered with the coating material in a relatively short time.

  However, when at least a part of the surface of the positive electrode active material is covered with carbon as a coating material by mechanical milling, the specific surface area of the positive electrode active material may increase.

  Therefore, it is preferable that at least a part of the surface of the positive electrode active material is covered with a coating material by mechanical milling, and then the positive electrode active material is fired at a temperature of, for example, less than 400 ° C. Thereby, the specific surface area of the positive electrode active material is reduced, and a positive electrode active material capable of obtaining excellent charge / discharge cycle characteristics can be generated. However, if the firing temperature is too high, a part of the positive electrode active material reacts with a part of the carbon that is the coating material, and good charge / discharge cycle characteristics cannot be obtained.

  Binders (binders) added when producing the positive electrode are polytetrafluoroethylene, polyvinylidene fluoride, polyethylene oxide, polyvinyl acetate, polymethacrylate, polyacrylate, polyacrylonitrile, polyvinyl alcohol, styrene-butadiene rubber and carboxymethylcellulose. At least one selected from the above can be used.

  In addition, when there is much addition amount of a binder, since the ratio of the positive electrode active material contained in a positive electrode will decrease, a high energy density will no longer be obtained. Therefore, the addition amount of the binder is in the range of 0 to 30% by weight, preferably in the range of 0 to 20% by weight, and more preferably in the range of 0 to 10% by weight of the whole positive electrode.

  Here, although the positive electrode active material of the present embodiment has conductivity, it is preferable to add a conductive agent when producing the positive electrode in order to further improve the conductivity and obtain good charge / discharge characteristics.

  As the conductive agent, a carbon material or the like can be used. Examples of the carbon material having excellent conductivity include ketjen black, acetylene black, and graphite.

  In addition, when the addition amount of the conductive agent is small, it is difficult to sufficiently improve the conductivity of the positive electrode. On the other hand, when the addition amount of the binder is large, the proportion of the positive electrode active material contained in the positive electrode decreases. High energy density cannot be obtained. Therefore, the addition amount of the conductive agent is in the range of 0 to 30% by weight of the whole positive electrode, preferably in the range of 0 to 20% by weight, more preferably in the range of 0 to 10% by weight.

(2) Production of non-aqueous electrolyte As the non-aqueous electrolyte, an electrolyte salt dissolved in a non-aqueous solvent can be used.

  Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles, amides, and the like, which are usually used as non-aqueous solvents for batteries. Is mentioned.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, etc., and those in which some or all of these hydrogen groups are fluorinated can be used. For example, trifluoropropylene carbonate, fluoro Examples include ethyl carbonate.

  Examples of the chain carbonic acid ester include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, and the like. Some of these hydrogen groups are fluorinated. It is possible to use.

  Examples of the esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3,5. -Trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether, etc. are mentioned.

  As chain ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl Ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1 -Dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethy Glycols dimethyl, and the like.

  Nitriles include acetonitrile and the like, and amides include dimethylformamide and the like.

Examples of the electrolyte salt include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiN. (C ₂ F ₅ SO ₂ ) ₂ , LiAsF ₆ , and a non-peroxide that is not soluble in a non-aqueous solvent selected from the group consisting of lithium difluoro (oxalato) borate represented by the following formula (3) Use the one with high.

  In addition, you may use 1 type in the said electrolyte salt, and may use it in combination of 2 or more type.

  In this embodiment, as a nonaqueous electrolyte, a nonaqueous solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 30:70, and lithium hexafluorophosphate as an electrolyte salt to a concentration of 1 mol / l. What was added so that it may become is used.

(3) Configuration of negative electrode In this embodiment, a material capable of inserting and extracting lithium ions is used. Examples of this material include one or more materials selected from the group consisting of lithium, silicon, carbon, tin, germanium, aluminum, lead, indium, gallium, lithium-containing alloys, and compounds containing lithium and carbon. Can be used. An example of the above compound containing lithium and carbon is a graphite intercalation compound in which lithium is inserted between graphite layers.

(4) Production of Nonaqueous Electrolyte Secondary Battery A nonaqueous electrolyte secondary battery is produced using the positive electrode, the negative electrode, and the nonaqueous electrolyte as described below.

  FIG. 1 is a schematic explanatory diagram of a test cell of a nonaqueous electrolyte secondary battery according to the present embodiment.

  As shown in FIG. 1, a working electrode 2 is obtained by attaching a lead to the positive electrode in an inert atmosphere, and a counter electrode 1 is obtained by attaching a lead to the negative electrode made of lithium metal.

  Next, the separator 4 is inserted between the working electrode 2 and the counter electrode 1, and the working electrode 2, the counter electrode 1, and the reference electrode 3 made of, for example, lithium metal are disposed in the cell container 10. And the nonaqueous electrolyte secondary battery as a test cell is produced by inject | pouring the said nonaqueous electrolyte 5 in the cell container 10. FIG.

  Note that when the charge / discharge test is performed using the nonaqueous electrolyte secondary battery manufactured in this embodiment, the potential of the working electrode 2 with reference to the reference electrode 3 is set to 4.2 V or more at the end of charging. Therefore, the discharge reaction of the above formula (1) can be used, and the potential of the working electrode 2 with respect to the reference electrode 3 at the end of the discharge is set to 2.8 V or less, whereby the above formula (2) A discharge reaction can be used.

In addition, a tetragonal crystal formed by chemically or electrochemically inserting lithium ions into a positive electrode active material obtained by mechanically milling a mixture of lithium manganate (LiMn ₂ O ₄ ) and carbon Li ₂ Mn ₂ O ₄ of the system may be used as the positive electrode active material.

(5) Effects in the present embodiment In the present embodiment, by using a positive electrode active material obtained by mechanical milling a mixture of lithium manganate (LiMn ₂ O ₄ ) and carbon, the above formula ( It is possible to obtain good charge / discharge cycle characteristics in the discharge reaction of 1) and the above formula (2). Thereby, a high energy density can be obtained. In addition, it is preferable that the time of a mechanical milling process is 30 minutes or less.

  Moreover, it is possible to obtain the positive electrode active material which has the further outstanding charging / discharging cycling characteristics by baking this mixture at the temperature below 400 degreeC after the mechanical milling process of a mixture.

(6) Examples in which carbon is used as a coating material Hereinafter, the nonaqueous electrolyte secondary battery of each example and a charge / discharge test using the same will be described in detail.

(6-a) Example 1 and Comparative Example 1
(6-a-1) Nonaqueous Electrolyte Secondary Battery of Example 1 After weighing lithium manganate having a spinel structure (LiMn ₂ O ₄ ) and carbon of the coating material in a ratio of 90: 5, the above A mixture of lithium manganate and carbon was prepared.

  A positive electrode active material was obtained by subjecting this mixture to mechanical milling using a planetary ball mill at a rotation speed of 300 rotations / minute. In addition, the time of the mechanical milling process was set to four types of 15 minutes, 30 minutes, 60 minutes, and 120 minutes.

  In a solution prepared by dissolving polyvinylidene fluoride as a binder in N-methyl-2-pyrrolidone, the weight ratio of the obtained positive electrode active material, the binder, and the carbon conductive agent is 90: 5: 5. A slurry as a positive electrode mixture was prepared by mixing after each addition.

  Then, after apply | coating the produced slurry on the aluminum foil of a positive electrode collector with the doctor blade method, it dried in vacuum and formed the positive electrode active material layer by rolling with a rolling roller. And the working electrode 2 was completed by attaching a positive electrode tab on the area | region of the aluminum foil which did not form a positive electrode active material layer. Note that lithium metal was used for the counter electrode 1 and the reference electrode 3.

  Further, as the non-aqueous electrolyte 5, lithium hexafluorophosphate as an electrolyte salt has a concentration of 1 mol / l in a non-aqueous solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 30:70. What was added to was used.

  Using the thus obtained working electrode 2, counter electrode 1, reference electrode 3, and nonaqueous electrolyte 5, a test cell of a nonaqueous electrolyte secondary battery was fabricated based on the above embodiment (FIG. 1).

(6-a-2) Nonaqueous electrolyte secondary battery of Comparative Example 1 The nonaqueous electrolyte secondary battery of Comparative Example 1 differs from the nonaqueous electrolyte secondary battery of Example 1 in that mechanical milling treatment using carbon is performed. In this case, lithium manganate (LiMn ₂ O ₄ ) itself was used as the positive electrode active material.

(6-a-3) Charging / discharging test and its evaluation In the nonaqueous electrolyte secondary battery produced in Example 1 and Comparative Example 1, the reference electrode 3 was used as a standard at a constant current of 5 mA in a temperature environment of 25 ° C. After charging until the potential of the working electrode 2 reached 4.3V, 10 cycles of discharging were performed until the potential of the working electrode 2 with respect to the reference electrode 3 reached 1.0V.

  Table 1 shows the discharge capacity densities and the discharge capacity density retention rates of the first and tenth cycles in the nonaqueous electrolyte secondary batteries of Example 1 and Comparative Example 1. The discharge capacity density maintenance rate is defined by the ratio of the discharge capacity density at the 10th cycle to the discharge capacity density at the 1st cycle. Moreover, the charging / discharging characteristic in the nonaqueous electrolyte secondary battery of Example 1 (4 types of mechanical milling process time is set) and the comparative example 1 is shown in FIGS.

As shown in Table 1, by using a positive electrode active material obtained by performing mechanical milling treatment for 15 minutes or 30 minutes on a mixture of lithium manganate (LiMn ₂ O ₄ ) and carbon as a coating material, A discharge capacity density maintenance rate higher than the discharge capacity density maintenance rate of the nonaqueous electrolyte secondary battery of Comparative Example 1 could be obtained.

  In addition, the discharge capacity density retention rate when the mechanical milling treatment was 60 minutes or longer was smaller than the discharge capacity density retention rate of Comparative Example 1. This is presumably because the mechanical milling process gives a severe impact to the positive electrode active material, and thus, when such a process is performed for a long time, the charge / discharge characteristics of the positive electrode active material deteriorate.

  FIG. 2 is a graph showing the charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by mechanical milling for 15 minutes in Example 1, and FIG. FIG. 4 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by mechanical milling of FIG. 4, and FIG. 4 is a positive electrode active material obtained by mechanical milling for 60 minutes in Example 1 It is the graph which showed the charging / discharging characteristic of the non-aqueous electrolyte secondary battery using A.

  5 is a graph showing the charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by mechanical milling for 120 minutes in Example 1, and FIG. It is the graph which showed the charging / discharging characteristic of the nonaqueous electrolyte secondary battery.

As shown in FIGS. 2 to 6, plateaus (potential flat portions) were confirmed in the 4V region and the 3V region in the discharge curve of each first cycle. These curves are almost the same as the discharge curves in Non-Patent Document 1 described in the background art section. From this, similar to Non-Patent Document 1, the reaction of the above formula (1) occurs at a voltage in the vicinity of 4 V (vs. Li ^/ Li ⁺ ), and the reaction of the above formula (2) is 3 V (vs. Li / Li). ⁺ ) Probably caused by nearby voltage. In the reaction of the above formula (2), the crystal system of the positive electrode active material is considered to have changed from cubic LiMn ₂ O ₄ to tetragonal Li ₂ Mn ₂ O ₄ .

(6-b) Example 2 and Comparative Example 2
(6-b-1) Each positive electrode active material of Example 2 and Comparative Example 2 In Example 2, 90: 5 of lithium manganate (LiMn ₂ O ₄ ) having a spinel structure and carbon of the coating material were used. After weighing in proportion, a mixture of the lithium manganate and carbon was prepared.

  A positive electrode active material of Example 2 was obtained by subjecting this mixture to mechanical milling for 15 minutes using a planetary ball mill at a rotation speed of 300 rotations / minute.

  On the other hand, in Comparative Example 2, the same lithium manganate and carbon as described above were weighed at a ratio of 90: 5, and then sufficiently mixed in a mortar to obtain the positive electrode active material of Comparative Example 2.

(6-b-2) Observation of each positive electrode active material and its evaluation Each positive electrode active material of Example 2 and Comparative Example 2 was observed with a scanning electron microscope (SEM).

  FIG. 7 is an enlarged photograph by SEM of the positive electrode active material obtained by mechanical milling, and FIG. 8 is an enlarged photograph by SEM of the positive electrode active material obtained by mixing in a mortar. In FIGS. 7 and 8, the large particles are lithium manganate, and the small particles are carbon.

  From FIG. 7, it was found that a part of lithium manganate was covered with carbon. On the other hand, FIG. 8 shows that lithium manganate and carbon are dispersed.

As described above, it was found that at least a part of the surface of the lithium manganate can be covered with carbon by performing mechanical milling treatment on the mixture of lithium manganate (LiMn ₂ O ₄ ) and carbon.

(6-c) Example 3 and Comparative Example 3
(6-c-1) Nonaqueous Electrolyte Secondary Battery of Example 3 After weighing lithium manganate (LiMn ₂ O ₄ ) having a spinel structure and carbon of the coating material in a ratio of 90: 5, the above A mixture of lithium manganate and carbon was prepared.

  The mixture was mechanically milled for 15 minutes at a rotational speed of 300 revolutions / minute using a planetary ball mill, and then fired in the atmosphere. The firing time was 6 hours, and the firing temperature was set to four types of 200 ° C., 300 ° C., 400 ° C. and 500 ° C.

  In a solution prepared by dissolving polyvinylidene fluoride as a binder in N-methyl-2-pyrrolidone, the weight ratio of the obtained positive electrode active material, the binder, and the carbon conductive agent is 90: 5: 5. A slurry as a positive electrode mixture was prepared by mixing after each addition.

  Then, after apply | coating the produced slurry on the aluminum foil of a positive electrode collector with the doctor blade method, it dried in vacuum and formed the positive electrode active material layer by rolling with a rolling roller. And the working electrode 2 was completed by attaching a positive electrode tab on the area | region of the aluminum foil which did not form a positive electrode active material layer. Note that lithium metal was used for the counter electrode 1 and the reference electrode 3.

  Further, as the non-aqueous electrolyte 5, lithium hexafluorophosphate as an electrolyte salt has a concentration of 1 mol / l in a non-aqueous solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 30:70. What was added to was used.

  Using the thus obtained working electrode 2, counter electrode 1, reference electrode 3, and nonaqueous electrolyte 5, a test cell of a nonaqueous electrolyte secondary battery was fabricated based on the above embodiment (FIG. 1).

(6-c-2) Nonaqueous Electrolyte Secondary Battery of Comparative Example 3 The nonaqueous electrolyte secondary battery of Comparative Example 3 is different from the nonaqueous electrolyte secondary battery of Example 3 in that mechanical milling treatment using carbon is performed. In this case, lithium manganate (LiMn ₂ O ₄ ) itself was used as the positive electrode active material.

(6-c-3) Charge / Discharge Test and its Evaluation A charge / discharge test was conducted in the same manner as in Example 1 and Comparative Example 1 except that the discharge end voltage was 1.5V.

  Table 2 shows the discharge capacity densities and the discharge capacity density retention ratios at the first cycle and the tenth cycle in the nonaqueous electrolyte secondary batteries of Example 3 and Comparative Example 3. Moreover, the charge / discharge characteristics in the nonaqueous electrolyte secondary battery of Example 3 (four kinds of firing temperatures are set) and Comparative Example 3 are shown in FIGS.

As shown in Table 2, the mixture of lithium manganate (LiMn ₂ O ₄ ) and carbon as a coating material was subjected to mechanical milling for 15 minutes, and then this was performed in an air atmosphere at 200 ° C. or 300 ° C. By using the positive electrode active material obtained by firing for 6 hours, a discharge capacity density maintenance rate significantly higher than the discharge capacity density maintenance rate of the nonaqueous electrolyte secondary battery of Comparative Example 1 could be obtained.

  In addition, the discharge capacity density maintenance rate when the firing temperature was 400 ° C. or higher was smaller than the discharge capacity density maintenance rate of Comparative Example 1. This is presumably because manganese in the positive electrode active material is reduced at a high temperature of 400 ° C. or higher, and the charge / discharge characteristics of the positive electrode active material deteriorate.

  FIG. 9 is a graph showing the charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by firing in an air atmosphere at 200 ° C. in Example 3, and FIG. FIG. 11 is a graph showing the charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by firing in an air atmosphere at 300 ° C. in FIG. It is the graph which showed the charging / discharging characteristic of the nonaqueous electrolyte secondary battery using the positive electrode active material obtained by baking by.

  FIG. 12 is a graph showing the charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by firing in an air atmosphere at 500 ° C. in Example 3, and FIG. 5 is a graph showing charge / discharge characteristics of the nonaqueous electrolyte secondary battery of Example 3. FIG.

As shown in FIGS. 9 to 13, plateaus (potential flat portions) were confirmed in the 4V region and the 3V region in the discharge curve of each first cycle. These curves are almost the same as the discharge curves in Non-Patent Document 1 described in the background art section. From this, similar to Non-Patent Document 1, the reaction of the above formula (1) occurs at a voltage in the vicinity of 4 V (vs. Li ^/ Li ⁺ ), and the reaction of the above formula (2) is 3 V (vs. Li / Li). ⁺ ) Probably caused by nearby voltage. In the reaction of the above formula (2), the crystal system of the positive electrode active material is considered to have changed from cubic LiMn ₂ O ₄ to tetragonal Li ₂ Mn ₂ O ₄ .

(6-d) Comprehensive evaluation of Examples 1 to 3 By using a positive electrode active material obtained by mechanical milling a mixture of lithium manganate (LiMn ₂ O ₄ ) and carbon, the above formula (1) and It was found that good charge / discharge cycle characteristics can be obtained in the discharge reaction of the above formula (2). Thereby, it was found that a high energy density can be obtained. In addition, when using carbon as a coating | covering material, it turned out that it is preferable that the time of the mechanical milling process is 30 minutes or less.

  Moreover, it turned out that the positive electrode active material which has the further outstanding charging / discharging cycling characteristics can be obtained by baking this mixture at the temperature below 400 degreeC after the mechanical milling process of a mixture.

(7) Other Embodiments In the above embodiment and Examples 1 to 3, the case where carbon is used as the coating material has been described. Hereinafter, the case where lithium iron phosphate (LiFePO ₄ ) is used as the coating material. explain.

A positive electrode is produced in the same manner as in the above embodiment except that lithium iron phosphate (LiFePO ₄ ) is used as a coating material covering at least a part of the surface of the positive electrode active material LiMn ₂ O ₄ .

It is produced by chemically or electrochemically inserting lithium ions into the positive electrode active material obtained by mechanical milling a mixture of lithium manganate (LiMn ₂ O ₄ ) and lithium iron phosphate. Tetragonal Li ₂ Mn ₂ O ₄ may be used as the positive electrode active material.

  Further, similarly to the above embodiment, a nonaqueous electrolyte and a negative electrode are produced, and a nonaqueous electrolyte secondary battery is produced using them.

(8) Effect in Other Embodiments By using a positive electrode active material obtained by subjecting a mixture of lithium manganate (LiMn ₂ O ₄ ) and lithium iron phosphate (LiFePO ₄ ) to mechanical milling, the above formula It is possible to obtain good charge / discharge cycle characteristics in the discharge reaction of (1) and the above formula (2). Thereby, a high energy density can be obtained. The mechanical milling time is preferably about 120 minutes.

(9) Example in which lithium iron phosphate is used as coating material (9-a) Example 4 and Comparative Example 4
(9-a-1) Nonaqueous electrolyte secondary battery of Example 4 Lithium manganate (LiMn ₂ O ₄ ) having a spinel structure and lithium iron phosphate (LiFePO ₄ ) as a coating material were mixed at 90: 5 After weighing in proportion, a mixture of the above lithium manganate and lithium iron phosphate was prepared.

Here, the particle diameters of lithium manganate (LiMn ₂ O ₄ ) and lithium iron phosphate (LiFePO ₄ ) were examined. FIG. 14 is an enlarged photograph of lithium manganate (LiMn ₂ O ₄ ) by SEM, and FIG. 15 is an enlarged photograph of lithium iron phosphate (LiFePO ₄ ) by SEM.

As shown in FIG. 14, the primary particle diameter of lithium manganate (LiMn ₂ O ₄ ) was about 0.5 to 1.0 μm. On the other hand, as shown in FIG. 15, the primary particle diameter of lithium iron phosphate (LiFePO ₄ ) was about 0.1 to 0.2 μm. Thus, the primary particle diameter of lithium iron phosphate (LiFePO ₄₎ was found to be smaller than the primary particle diameter of the lithium manganate (LiMn ₂ O _4).

  Next, a positive electrode active material was obtained by subjecting a mixture of lithium manganate and lithium iron phosphate to a mechanical milling treatment at a rotational speed of 300 revolutions / minute using a planetary ball mill. The time for the mechanical milling process was set to 120 minutes.

  Here, the positive electrode active material obtained by the mechanical milling process was observed by SEM.

FIG. 16 is an SEM enlarged photograph of a positive electrode active material obtained by subjecting a mixture of lithium manganate (LiMn ₂ O ₄ ) and lithium iron phosphate (LiFePO ₄ ) to mechanical milling.

  As shown in FIG. 16, a part of the lithium manganate having a large particle size (particle diameter of about 0.5 to 1.0 μm) is a small particle (a particle diameter of about 0.1 to 0.2 μm) phosphorous. It was found to be covered with lithium iron oxide.

  Next, a weight ratio of the obtained positive electrode active material, the binder, and the carbon conductive agent is 90: 5: 5 in a solution obtained by dissolving polyvinylidene fluoride as a binder in N-methyl-2-pyrrolidone. Thus, the slurry as a positive electrode mixture was produced by mixing after adding each.

  Then, after apply | coating the produced slurry on the aluminum foil of a positive electrode collector with the doctor blade method, it dried in vacuum and formed the positive electrode active material layer by rolling with a rolling roller. And the working electrode 2 was completed by attaching a positive electrode tab on the area | region of the aluminum foil which did not form a positive electrode active material layer. Note that lithium metal was used for the counter electrode 1 and the reference electrode 3.

  Further, as the non-aqueous electrolyte 5, lithium hexafluorophosphate as an electrolyte salt has a concentration of 1 mol / l in a non-aqueous solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 30:70. What was added to was used.

  Using the thus obtained working electrode 2, counter electrode 1, reference electrode 3, and nonaqueous electrolyte 5, a test cell of a nonaqueous electrolyte secondary battery was fabricated based on the above embodiment (FIG. 1).

(9-a-2) Nonaqueous Electrolyte Secondary Battery of Comparative Example 4 The nonaqueous electrolyte secondary battery of Comparative Example 4 differs from the nonaqueous electrolyte secondary battery of Example 4 in that lithium iron phosphate (LiFePO ₄ ) Using lithium manganate (LiMn ₂ O ₄ ) itself as a positive electrode active material.

(9-a-3) Charge / Discharge Test and Evaluation The charge / discharge test was conducted in the same manner as in Example 1 and Comparative Example 1 except that the discharge end voltage was 1.5V.

  Table 3 shows the discharge capacity densities and the discharge capacity density retention rates at the first and tenth cycles in the nonaqueous electrolyte secondary batteries of Example 4 and Comparative Example 4. The charge / discharge characteristics of the nonaqueous electrolyte secondary batteries of Example 4 and Comparative Example 4 are shown in FIGS.

As shown in Table 3, a positive electrode active material obtained by subjecting a mixture of lithium manganate (LiMn ₂ O ₄ ) and lithium iron phosphate (LiFePO ₄ ) as a coating material to mechanical milling for 120 minutes By using it, a discharge capacity density maintenance rate significantly higher than the discharge capacity density maintenance rate of the nonaqueous electrolyte secondary battery of Comparative Example 4 could be obtained.

  In addition, when carbon was used as the coating material, a high discharge capacity density retention rate could be obtained when the mechanical milling time was 30 minutes or less, but when lithium iron phosphate was used as the coating material. In this case, a high discharge capacity density retention rate could be obtained by setting the mechanical milling time to 120 minutes. This is considered to be because lithium manganate and lithium iron phosphate are both ceramics and thus are hardly mixed with each other.

Further, as shown in FIGS. 17 and 18, plateaus (potential flat portions) were confirmed in the 4V region and the 3V region in the discharge curve of each first cycle. These curves are almost the same as the discharge curves in Non-Patent Document 1 described in the background art section. From this, similar to Non-Patent Document 1, the reaction of the above formula (1) occurs at a voltage in the vicinity of 4 V (vs. Li ^/ Li ⁺ ), and the reaction of the above formula (2) is 3 V (vs. Li / Li). ⁺ ) Probably caused by nearby voltage. In the reaction of the above formula (2), the crystal system of the positive electrode active material is considered to have changed from cubic LiMn ₂ O ₄ to tetragonal Li ₂ Mn ₂ O ₄ .

(9-b) Evaluation of Example 4 By using a positive electrode active material obtained by mechanically milling a mixture of lithium manganate (LiMn ₂ O ₄ ) and lithium iron phosphate (LiFePO ₄ ), the above formula It was found that good charge / discharge cycle characteristics can be obtained in the discharge reaction of (1) and the above formula (2). Thereby, it was found that a high energy density can be obtained. In addition, it turned out that it is preferable that the time of the mechanical milling process in the case of using lithium iron phosphate as a coating material is 120 minutes.

  The nonaqueous electrolyte secondary battery according to the present invention can be used as various power sources such as a portable power source and an automotive power source.

It is a schematic explanatory drawing of the test cell of the nonaqueous electrolyte secondary battery which concerns on this Embodiment. 2 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by mechanical milling for 15 minutes in Example 1. FIG. 2 is a graph showing charge / discharge characteristics of a non-aqueous electrolyte secondary battery using a positive electrode active material obtained by mechanical milling for 30 minutes in Example 1. FIG. 2 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by mechanical milling for 60 minutes in Example 1. FIG. 4 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by mechanical milling treatment for 120 minutes in Example 1. FIG. 5 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery of Comparative Example 1. It is an enlarged photograph by SEM of the positive electrode active material obtained by the mechanical milling process. It is an enlarged photograph by SEM of the positive electrode active material obtained by mixing with a mortar. 4 is a graph showing charge / discharge characteristics of a non-aqueous electrolyte secondary battery using a positive electrode active material obtained by firing in an air atmosphere at 200 ° C. in Example 3. 6 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by firing in an air atmosphere at 300 ° C. in Example 3. 6 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by firing in an air atmosphere at 400 ° C. in Example 3. FIG. 6 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery using a positive electrode active material obtained by firing in an air atmosphere at 500 ° C. in Example 3. FIG. 5 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery of Comparative Example 3. It is an enlarged photograph by SEM of lithium manganate. It is an enlarged photograph by SEM of lithium iron phosphate. It is an enlarged photograph by SEM of the positive electrode active material obtained by performing the mechanical milling process to the mixture of lithium manganate and lithium iron phosphate. It is the graph which showed the charge / discharge characteristic of the nonaqueous electrolyte secondary battery using the positive electrode active material obtained by the mechanical milling process in Example 4. 6 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery of Comparative Example 4.

Explanation of symbols

1 Counter electrode 2 Working electrode 3 Reference electrode 4 Separator 5 Nonaqueous electrolyte 10 Cell container

Claims (6)

A positive electrode containing a positive electrode active material, a negative electrode, and a non-aqueous electrolyte,
The positive active material during fully discharged state includes tetragonal _{Li 2 + x Mn 2-y} M z O 4 + q, the x and the q is less than 1 greater than -1, the y and the z is 0 or more and 0.5 or less, and the M includes one or more selected from the group consisting of chromium, iron, cobalt, nickel, copper, aluminum, and magnesium,
At least a part of the surface of the positive electrode active material is covered with a coating material composed of primary particles smaller than the size of the primary particles of the positive electrode active material ,
The non-aqueous electrolyte secondary battery , wherein the coating material contains lithium iron phosphate . The nonaqueous electrolyte secondary battery according to claim 1 , wherein at least a part of the surface of the positive electrode active material is covered with the coating material by a mechanical milling process. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the potential of the positive electrode active material based on lithium metal at the end of discharge is 2.8 V or less. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, the potential of the positive electrode active material relative to the lithium metal at the time of end of charge is equal to or is 4.2V or more. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4 , wherein the positive electrode active material having at least a part of the surface covered with the coating material is heat-treated at a temperature of less than 400 ° C. The positive electrode active material in the stage before the first charge / discharge is based on cubic LiMn ₂ O ₄ in a cubic system in which at least a part of the surface is covered with lithium iron phosphate,
The negative electrode in a stage before performing the first charge / discharge includes a sufficient amount of lithium so that the basic skeleton of the positive electrode active material changes to tetragonal Li ₂ Mn ₂ O ₄ in a full discharge state. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 5 .

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