JP5742402B2 - Lithium secondary battery and manufacturing method thereof - Google Patents

Description

  The present invention relates to a lithium secondary battery having a large energy capacity, reduced deterioration due to repeated charge and discharge, high cycle characteristics, and high safety, and a method for manufacturing the same.

  Lithium secondary batteries that use organic solvents, reversibly occlude and release lithium ions at the positive and negative electrodes, and can be repeatedly charged and discharged are portable electronic devices, personal computers, and motors for hybrid electric vehicles. Widely used in driving batteries and the like. While these lithium secondary batteries are required to be further reduced in size and weight, on the other hand, reversible occlusion and release amount of lithium ions in the positive electrode and negative electrode is increased to increase the capacity and cycle deterioration due to charge / discharge. Reduction is an important issue.

  In lithium secondary batteries, silicon as the negative electrode active material has a high capacity and a large amount of lithium ion storage and release per unit volume, but the volume expansion and contraction associated with the storage and release of lithium ions is large. The reversible occlusion and release amount of lithium ions in subsequent charge / discharge is reduced. For this reason, an unused part also arises on the positive electrode side, and as a result, a high capacity cannot be obtained. As a negative electrode active material that suppresses the volume change of silicon accompanying occlusion and release of lithium ions and reduces the irreversible capacity generated by the first charge / discharge, silicon oxide (Patent Document 1), carbon materials for silicon and silicon oxide Have been reported (see Patent Document 2). However, silicon oxide has low electrical conductivity and low current collection, resulting in a large irreversible capacity during charging and discharging. Also, the initial charging and discharging of particles in which silicon and silicon oxide are combined with a carbon material. The efficiency improvement is insufficient.

  On the other hand, olivine compounds such as lithium iron phosphate and lithium manganese phosphate as a positive electrode active material for lithium batteries are less likely to deteriorate due to repeated charge and discharge, can perform stable charge and discharge, and have excellent cycle characteristics. It has been known.

  However, since the olivine compound itself has low conductivity, there is a problem that the internal resistance of the battery is increased and the rate characteristics are lowered. For this reason, a positive electrode for a secondary battery in which olivine-type lithium manganese phosphate and spinel-type lithium manganate are mixed (Patent Document 3) has also been reported. However, a silicon-based material having a large irreversible capacity of lithium ions in the first charge is used. When the negative electrode used as the active material is used, the unused capacity of the positive electrode may increase during subsequent charging / discharging, and deterioration may occur due to repeated charging / discharging, resulting in insufficient cycle characteristics. In some cases.

  In addition, for a negative electrode using composite particles containing carbon and silicon and silicon oxide dispersed therein, olivine type lithium iron phosphate and lithium copper oxide are used as a positive electrode active material, and Li / Li +, the initial charge capacity per area of the negative electrode when charged to 0.03V is larger than the initial charge capacity per area of the positive electrode when charged to 4.1V, and when charged to 4.25V A secondary battery (Patent Document 4) has been disclosed in which the reaction of lithium copper oxide releasing oxygen is suppressed by making it smaller than the initial charge capacity per area of the positive electrode, and safety and longevity are improved. Yes.

However, in the secondary battery described in Patent Document 4, repeated charging and discharging may cause deterioration of the positive electrode itself and the electrolyte due to lithium copper oxide contained in the positive electrode, and further stability. There is a demand for longer life. Moreover, it is used in the range of Li ₂ CuO _{2 to} Li _1.2 CuO ₂ , and a part of lithium related to charge / discharge is not used to make maximum use of Li ₂ CuO ₂ .

  Moreover, although the lithium ion battery (patent document 5) using the olivine type compound containing a specific element for a positive electrode is reported, sufficient capacity | capacitance may not be obtained.

  Even when an olivine compound having a large specific gravity is used as the positive electrode active material and a negative electrode active material with a large irreversible storage amount of lithium ions is used during the initial charge, the energy density per unit volume of the positive electrode is reduced during subsequent charging and discharging. There is a need for a long-life lithium secondary battery that suppresses this, has high safety, and has good charge / discharge efficiency.

  Furthermore, when the positive electrode active material contains an olivine compound and the negative electrode active material is a silicon-based material having a large lithium ion storage capacity and a low specific gravity, the lithium storage and discharge capacities associated with charge and discharge of the positive electrode and the negative electrode are the same. Therefore, it is necessary to increase the thickness of the positive electrode active material layer. When the active material layer is formed by the coating process, there is a limit to the adjustment of the thickness of the coating film, and when the active material layer is thick, when a large current is passed, the occlusion and release of lithium cannot follow, In some cases, the occlusion / release capacity of the resin may decrease, and handling may become difficult due to the difference in layer thickness, resulting in a decrease in production efficiency. Therefore, there is a demand for a method for producing a lithium secondary battery that can easily produce a lithium secondary battery using an olivine compound as a positive electrode active material and a negative electrode active material having a large specific gravity difference.

JP 06-325765 A JP2004-139886 JP 2006-278256 A JP2010-80196 JP2007-134274A

  The object of the present invention is to use an olivine compound that has a large specific gravity but excellent stability during charge and discharge as the positive electrode active material, and a negative electrode active material that has a large irreversible capacity of lithium ions during the first charge and discharge. However, in subsequent charging and discharging, the decrease in energy density per unit volume of the positive electrode active material is suppressed, the capacity is high, and the decrease in production efficiency due to the difference in the layer thickness between the positive electrode and the negative electrode can be suppressed. Therefore, it is to provide a lithium secondary battery having a high life, good charge / discharge efficiency, and a long life, and a method for producing the same.

  As a result of diligent research, the present inventors have been able to irreversibly supply lithium ions that have a specific gravity smaller than that of the olivine compound and are irreversibly occluded by the negative electrode during the first charge / discharge in the positive electrode using the olivine compound as the positive electrode active material. The lithium transition metal compound represented by Formula (2) was found as a compound. The lithium transition metal compound represented by the formula (2) releases lithium ions and changes to a lithium irreversible transition metal oxide at the first charge. Lithium irreversible transition metal oxide reduces the weight of the positive electrode by discharging the generated oxygen to the outside after the first charge to produce a battery, and in the charge and discharge after the first charge, The knowledge that it contributes to stabilization of the lithium ion occlusion-release reaction was obtained. Based on this knowledge, a lithium secondary metal compound that uses lithium transition metal compounds that supply lithium ions only when the first negative electrode is charged together with the olivine compound as the positive electrode active material to improve cycle characteristics and extend the life. The present invention concerning a battery was completed.

In the present invention, the positive electrode before the first charge is represented by the formula (1)
LiGPO ₄ (1)
(Wherein G represents Fe or Mn), and the formula (2)
Li ₂ MO ₂ (2)
(Wherein, M represents Cu or Ni), and a positive electrode active material containing a lithium transition metal oxide represented by
The positive electrode active material after the first charge includes a lithium irreversible transition metal oxide formed by releasing all lithium that can be released by the lithium transition metal oxide represented by the formula (2), The present invention relates to a lithium secondary battery obtained by sealing by discharging oxygen gas generated during charging .

Further, the present invention provides the formula (1) at the time of initial charge.
LiGPO ₄ (1)
(Wherein G represents Fe or Mn), and the formula (2)
Li ₂ MO ₂ (2)
(In the formula, M represents Cu or Ni.) Lithium ions are released from the positive electrode containing a lithium transition metal oxide represented by the formula (2) as a positive electrode active material, and lithium transition metal oxidation represented by the formula (2) The present invention relates to a method for manufacturing a lithium secondary battery, characterized in that a lithium irreversible transition metal oxide is used, and then oxygen gas is discharged and sealed.

  The lithium secondary battery of the present invention uses an olivine compound having high specific gravity as the positive electrode active material but excellent in stability during charging and discharging, and an active material having a large irreversible storage amount of lithium ions during the initial charging in the negative electrode. Even when it is used, the subsequent decrease in energy density per unit volume of the positive electrode active material is suppressed, the capacity is high, the safety is high, the charge / discharge efficiency is good, and the life is long. The method for producing a lithium secondary battery of the present invention has a small difference in the layer thickness between the positive electrode and the negative electrode, and is easy to handle them. In addition, the positive electrode active material can be obtained without the complicated process of reassembling the battery after the first charge / discharge. A lithium secondary battery using an olivine compound as a substance can be efficiently manufactured.

It is a block diagram which shows the structure of an example of the lithium secondary battery of this invention.

  The lithium secondary battery of the present invention has a positive electrode, a negative electrode, and an electrolytic solution impregnating them.

[Positive electrode]
The positive electrode before the first charge / discharge is represented by the formula (1)
LiGPO ₄ (1)
(Wherein, G represents Fe or Mn) and the olivine compound (hereinafter also referred to as olivine compound (1)), and the formula (2)
Li ₂ MO ₂ (2)
(Wherein, M represents Cu or Ni), and a positive electrode active material containing a lithium transition metal oxide (hereinafter also referred to as lithium transition metal oxide (2)).

The olivine compound (1) reversibly occludes and releases lithium ions by charge and discharge, and has a strong bond between phosphorus atoms and oxygen, and release of oxygen atoms even when lithium ion is occluded and released by charge and discharge. Small amount and stable cycling characteristics. The olivine compound (1) is LiMnPO ₄ or LiFePO ₄ , which may be either one or both.

  Further, the lithium transition metal oxide (2) contained in the positive electrode before the first charge / discharge is as shown in the structural formula (3):

A planar four-coordinate GO ₄ is formed, and has a planar belt-like structure sharing a side (a side formed by two oxygen atoms) facing each other. Such a structure can be confirmed by X-ray diffraction measurement. Specifically, the lithium transition metal oxide (2) is Li ₂ CuO ₂ or Li ₂ NiO ₂ , and either of these may include both. These have low reactivity, are easy to handle in work, and have high safety and stability. In particular, Li ₂ CuO ₂ is preferable because it has water resistance, is easy to handle, and can be manufactured in the air.

  The lithium transition metal oxide (2) has a charge capacity density B per unit mass larger than the charge / discharge capacity density A per unit mass of the olivine compound (1), and satisfies the relationship of A <B. For this reason, compared with the case where only the olivine compound (1) is used for the positive electrode active material, the initial charge capacity can be increased by containing the lithium transition metal oxide (2).

  Here, the charge / discharge capacity density is the unit of the substance to be measured by performing the initial charge between 4.3 V and 3.0 V and measuring the initial charge capacity in a model cell in which the substance to be measured is counter electrode of metallic lithium. The charge capacity density per mass can be calculated and obtained.

  Such a lithium transition metal oxide (2) is changed to a lithium irreversible transition metal oxide that releases all lithium ions that can be released by first charging the negative electrode and releasing lithium ions. . For example, the first charge of the negative electrode can be charged by supplying a current that is 1/10 of the current that uses up the initial charge capacity of the positive electrode in one hour. In the subsequent charge / discharge, it will be present in the positive electrode active material as a lithium irreversible transition metal oxide that does not participate in occlusion / release of lithium. By performing the initial charge of the negative electrode under the above conditions, the lithium transition metal oxide (2) is changed to copper oxide or nickel oxide, which is a lithium irreversible transition metal oxide that does not participate in occlusion and release of lithium in subsequent charging and discharging. However, in some molecules of the lithium transition metal oxide (2), the lithium transition metal oxide from which one atom of lithium is released may remain as an impurity. When the lithium transition metal oxide (2) changes to a lithium irreversible transition metal oxide, oxygen is generated, but the oxygen is removed out of the battery.

  When the positive electrode active material contains a lithium irreversible transition metal oxide after the initial charge, the capacity of the battery can be increased. The reason is as follows.

  Compared with the amount of lithium ions released from the olivine compound (1) and lithium transition metal oxide (2) of the positive electrode active material during the first charge, that is, compared to the amount of lithium ions occluded in the negative electrode during the first charge, The amount of lithium ions released from the negative electrode during the first discharge is less than the amount corresponding to the irreversible capacity for irreversibly storing lithium ions in the negative electrode. If all of the positive electrode active materials are olivine compounds (1), the amount of lithium ions corresponding to the irreversible capacity of the negative electrode is insufficient with respect to the amount of lithium ions occluded by the olivine compound (1) during the first discharge, Olivine compounds that do not participate in the storage and release of lithium ions are produced. The lithium transition metal oxide (2) contained in the positive electrode active material before the first charge is released into lithium ions and converted to a lithium irreversible transition metal oxide during the first charge. This lithium irreversible transition metal oxide has a smaller specific gravity than the olivine compound (1), and is contained as a positive electrode active material that does not participate in charge / discharge after the first charge / discharge, thereby reducing the mass of the positive electrode active material. Thus, the lithium ion storage / release amount per unit mass of the positive electrode active material can be increased.

  The content of the lithium transition metal oxide (2) contained in the positive electrode active material before the first charge is determined by the amount of lithium ions γ released by the lithium transition metal oxide (2) of the positive electrode active material and the negative electrode during the first charge. The amount of lithium ions α irreversibly occluded is preferably an amount satisfying the relationship of γ ≦ α, and more preferably γ = α. The amount of lithium ions released from the lithium transition metal oxide (2) of the positive electrode active material during the initial charge is less than the amount of lithium ions irreversibly occluded in the negative electrode during the initial charge. It is possible to suppress generation of lithium ions that are not occluded in the olivine compound (1) as a positive electrode active material that occludes lithium ions released from the negative electrode, and to suppress reduction of the lithium ion occlusion capacity of the positive electrode active material. it can. If γ = α, the lithium ions released from the lithium transition metal oxide (2) in the first charge are used as the lithium ions irreversibly occluded in the negative electrode. The olivine compound (1) involved in the occlusion / release of ions can be used without excess or deficiency.

  Here, the lithium ion amount α irreversibly stored in the negative electrode during the first charge is charged between 1.5 V and 0.02 V between them in a model cell in which metallic lithium is used as a negative electrode for the negative electrode active material. The first charge capacity of the negative electrode and the capacity discharged by the first discharge following the first charge are measured, and the difference between the capacity, that is, the irreversible capacity corresponding to the irreversible storage amount of lithium ions can be obtained. Further, the lithium ion amount γ released by the lithium transition metal oxide (2) is charged between 4.3 V and 3.0 V in a model cell in which the lithium metal is a counter electrode with respect to the lithium transition metal oxide, The initial charge capacity can be obtained as a value corresponding to the lithium ion release amount γ.

  This lithium irreversible transition metal oxide does not function as a positive electrode active material in the subsequent charge / discharge, but suppresses deterioration of the olivine compound (1) of the positive electrode active material, increases the capacity, and has an excellent cycle. A battery with characteristics, stability and long life can be provided.

  In the initial charge / discharge, the initial charge capacity Z of the positive electrode preferably satisfies Z ≦ Y with respect to the initial charge capacity Y of the negative electrode. If Z ≦ Y, all of the positive electrode active material can be used for charging and discharging, and a high-capacity battery can be obtained.

  As the initial charge capacity Z of the positive electrode, a measured value of the initial charge capacity when charging is performed between 4.3 V and 3.0 V in a model cell in which the positive electrode active material is metal lithium as a counter electrode can be adopted. Further, as the initial charge capacity Y of the negative electrode, a measurement value of the initial charge capacity when charging is performed between 1.5 V and 0.02 V in a model cell in which the negative electrode active material is metal lithium as a counter electrode can be adopted.

As the positive electrode active material, other positive electrode active materials may be included as long as the functions of the olivine compound (1), the lithium transition metal oxide (2), and the lithium irreversible transition metal oxide are not impaired. Other positive electrode active material, _{specifically, LiM1 x Mn 2-x O} 4 (M1: elements other than Mn, 0 <x <0.4) , LiCoO 2, Li (M2 x Mn 1-x) O ₂ (M2: element other than Mn), Li (M3 _x Ni _1-x ) O ₂ (M3: element other than Ni), Li ₂ MSiO ₄ (M: at least one of Mn, Fe, Co), etc. Can be mentioned. These can be used alone or in combination of two or more.

Specific surface areas of the olivine compound (1) and the lithium transition metal oxide (2) can include, for example, 0.01 to ⁵ m ² / g, preferably 0.05 to ⁴ m ² / g, and more. Preferably it is 0.2-2 m < ² > / g. By setting the specific surface area to 4 m ² / g or less, the contact area with the electrolyte can be adjusted to an appropriate range, and in the positive electrode active material layer, the movement of lithium ions accompanying charge / discharge is facilitated, and the resistance is reduced. It can be further reduced. Further, by setting the specific surface area to 0.05 m ² / g or more, decomposition of the electrolytic solution can be suppressed, and elution of the constituent elements of the active material into the electrolytic solution can be suppressed.

  As the specific surface area, a value measured by a specific surface area measuring device using a gas adsorption method can be adopted.

  The central particle size of the olivine compound (1) and the lithium transition metal oxide (2) is preferably 0.1 to 50 μm, and more preferably 0.2 to 40 μm. By setting the central particle size of the olivine compound (1) and the lithium transition metal oxide (2) to 0.1 μm or more, elution of the constituent elements into the electrolytic solution is suppressed, and deterioration of the positive electrode due to contact with the electrolytic solution is prevented. It can be suppressed more. In addition, by setting the central particle size of the olivine compound (1) and the lithium transition metal oxide (2) to 50 μm or less, lithium ion insertion / extraction in the positive electrode accompanying charge / discharge is facilitated, and resistance is further reduced. Can do.

  As the central particle size of the lithium manganese composite oxide, a value measured by a laser diffraction / scattering particle size distribution analyzer can be adopted.

  The positive electrode active material is formed as a positive electrode active material layer integrally bound on the positive electrode current collector together with a conductive agent and a positive electrode binder.

  The conductive agent lowers the impedance of the positive electrode active material, and carbon black, acetylene black, or the like can be used. As content of a electrically conductive agent, 3-5 mass parts can be mentioned with respect to 100 mass parts of positive electrode active materials.

  Examples of the binder for the positive electrode include polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer rubber, and polytetrafluoroethylene. , Polypropylene, polyethylene, polyimide, polyamideimide and the like. Among these, polyvinylidene fluoride is preferable from the viewpoint of versatility and low cost. The amount of the positive electrode binder used is preferably 2 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material in terms of adjusting the energy density and the binding force.

  The positive electrode current collector may be any material that supports the positive electrode active material layer including the positive electrode active material integrated by the binder and has electrical conductivity that enables electrical connection with the external terminal. In view of electrochemical stability, aluminum, nickel, copper, silver, or an alloy thereof is preferable. Examples of the shape include foil, flat plate, and mesh.

  The thickness of the positive electrode current collector is preferably a thickness that can maintain the strength capable of supporting the positive electrode active material layer, and is preferably 4 to 100 μm, for example, and 5 to 30 μm to increase the energy density. It is more preferable.

The electrode density of the positive electrode active material layer is preferably 1.0 g / cm ³ or more and 3.0 g / cm ³ or less. When the electrode density of the positive electrode is 1.0 g / cm ³ or more, the absolute value of the discharge capacity can be suppressed from becoming small. On the other hand, when the electrode density of the positive electrode is 3.0 g / cm ³ or less, it is possible to suppress the electrolyte from being easily impregnated into the electrode and the discharge capacity from being lowered.

  Such a positive electrode active material layer includes a positive electrode active material including a powder of the olivine compound (1) and a powder of a lithium transition metal oxide (2), a conductive agent powder as necessary, and a positive electrode binder. , N-methyl-2-pyrrolidone (NMP), dispersed in a solvent such as dehydrated toluene, and kneaded the material for the positive electrode active material layer on the positive electrode current collector by the doctor blade method, the die coater method, etc. It can be prepared by coating and drying in a high temperature atmosphere. Examples of the method for producing the positive electrode active material layer include a CVD method, a sputtering method, and the like in addition to the coating method. After forming a positive electrode active material layer in advance, a thin film of aluminum, nickel, or an alloy thereof may be formed by a method such as vapor deposition or sputtering to form a positive electrode current collector.

[Negative electrode]
Examples of the negative electrode include those having a structure in which a conductive agent together with a negative electrode active material is integrally bound on a negative electrode current collector by a negative electrode binder.

The negative electrode active material may be any silicon-based material, carbon, or a material containing both of these, and the more lithium ions that are irreversibly stored in the initial charge, the more the effects of the present invention are obtained. be able to. It is preferable to use at least one selected from silicon, silicon oxide, and carbon as the negative electrode active material. Examples of the silicon oxide, may be mentioned SiO, the SiO _2. Examples of carbon include graphite and hard carbon. These may be used alone or in combination of two or more.

As the negative electrode active material, in addition to the above, Al, Si, Pb, S, Zn, Cd, Sb, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, La and other metals, these two types The above alloys, or an alloy of these metals or alloys and lithium may be included. Also, aluminum oxide, tin oxide, indium oxide, zinc oxide, lithium oxide, lithium iron oxide, tungsten oxide, molybdenum oxide, copper oxide, tin oxide such as SnO, SnO ₂ , niobium oxide, Li _x Ti _2-x O ₄ (1 ≦ x ≦ 4/3), metal oxides such as lead oxide such as PbO ₂ and Pb ₂ O ₅ , metal sulfides such as SnS and FeS ₂ , polyacene or polythiophene, or Li ₅ (Li ₃ N) Li ₇ MnN ₄ , Li ₃ FeN ₂ , Li _2.5 Co _0.5 N, or lithium nitride such as Li ₃ CoN may be included.

  The conductive agent used for the negative electrode can be the same as that specifically exemplified in the positive electrode, and the amount used can be 1 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material. it can. As the negative electrode binder, thermosetting resins such as polyimide, polyamide, polyamideimide, polyacrylic acid resin, and polymethacrylic acid resin can be used. The amount of the negative electrode binder to be used is preferably in the range of 1 to 30% by mass, more preferably 2 to 25% by mass with respect to the total amount of the negative electrode active material and the negative electrode binder. By making the content of the negative electrode binder 1% by mass or more, the adhesion between the active materials or between the active material and the current collector is improved, the cycle characteristics are improved, and the content is made 30% by mass or less. Thus, the active material ratio can be improved and the negative electrode capacity can be improved.

  The negative electrode current collector may be any material as long as it has conductivity that supports the negative electrode active material layer including the negative electrode active material integrated with the binder and enables conduction with the external terminal. Specifically, the same thing as the said positive electrode electrical power collector can be mentioned.

  The thickness of the negative electrode current collector is preferably a thickness that can maintain the strength capable of supporting the negative electrode active material layer, and can be the same thickness as the positive electrode current collector.

The electrode density of the negative electrode active material layer is preferably 0.5 g / cm ³ or more and 2.0 g / cm ³ or less. If the electrode density of the negative electrode is 0.5 g / cm ³ or more, the absolute value of the discharge capacity can be suppressed from decreasing. On the other hand, when the electrode density of the negative electrode is 2.0 g / cm ³ or less, it is possible to suppress the electrolyte from being easily impregnated into the electrode and the discharge capacity from being lowered.

  Such a negative electrode active material layer was obtained by kneading a negative electrode active material powder and a negative electrode binder with a conductive agent or a solvent such as N-methyl-2-pyrrolidone (NMP) as necessary. The negative electrode active material layer material can be coated on a negative electrode current collector such as a copper foil and rolled into a coating-type electrode plate or directly pressed to obtain a pressure-molded electrode plate. After the process, the coating film can be dried in a high temperature atmosphere to produce a negative electrode active material layer. As another method for manufacturing the negative electrode active material layer, a method similar to the method for manufacturing the positive electrode active material layer can be given.

[Electrolyte]
The electrolytic solution is a solution in which an electrolyte is dissolved in a non-aqueous organic solvent, and is a solution capable of dissolving lithium ions. In order to enable occlusion and release of lithium in the positive electrode and negative electrode during charge and discharge, the positive electrode and the negative electrode are connected. It is provided soaked.

  The solvent of the electrolytic solution can suppress the decomposition of the electrolytic solution even by repeated charge and discharge, and has fluidity so that the positive electrode and the negative electrode can be sufficiently immersed, thereby extending the life of the battery. Therefore, it is preferable. Specific examples of the electrolyte solvent include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and vinylene carbonate (VC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). Chain carbonates such as ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC), aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate, and γ-lactones such as γ-butyrolactone, Chain ethers such as 2-ethoxyethane (DEE) and ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, Dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2 -Aprotic organic solvents such as oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone and the like can be mentioned. These can be used alone or in combination of two or more.

As an electrolyte contained in the electrolytic solution, a lithium salt is preferable. Specific examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ CO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN ( CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiB ₁₀ Cl ₁₀ , lower aliphatic lithium carboxylate, lithium chloroborane, lithium tetraphenylborate, LiBr, LiI, LiSCN, LiCl, imides, Examples thereof include boron fluorides. These can be used alone or in combination of two or more.

  Further, a polymer electrolyte, an inorganic solid electrolyte, an ionic liquid, or the like may be used instead of the electrolytic solution.

  The concentration of the electrolyte in the electrolytic solution is preferably 0.01 mol / L or more and 3 mol / L or less, more preferably 0.5 mol / L or more and 1.5 mol / L or less. When the electrolyte concentration is within this range, safety can be improved, and a battery having high reliability and contributing to reduction of environmental load can be obtained.

[Separator]
Any separator may be used as long as it suppresses the contact between the positive electrode and the negative electrode, does not inhibit the permeation of the charged body, and has durability against the electrolytic solution. Specific examples of the material that can be used include polyolefin microporous membranes such as polypropylene and polyethylene, cellulose, polyethylene terephthalate, polyimide, and polyvinylidene fluoride. These can be used as porous films, woven fabrics, non-woven fabrics and the like.

[Cell exterior body]
As the outer package, those having a strength capable of stably holding the positive electrode, the negative electrode, the separator, and the electrolytic solution, electrochemically stable and watertight with respect to these substances are preferable. Specifically, for example, stainless steel, nickel-plated iron, aluminum, titanium, or alloys thereof, plated materials, metal laminate resins, and the like can be used, and the resin used for the metal laminate resin is polyethylene. Polypropylene, polyethylene terephthalate, etc. can be used. These may be a structure of one layer or two or more layers.

[Lithium secondary battery]
The shape of the lithium secondary battery may be any of a cylindrical shape, a flat wound rectangular shape, a laminated rectangular shape, a coin shape, a wound laminate type, a flat wound laminate type, a laminated laminate type, and the like.

  As an example of the lithium secondary battery, a laminated laminate type secondary battery shown in FIG. 1 can be given. This laminated laminate type secondary battery 10 includes a negative electrode 3 in which a negative electrode current collector 2 such as a copper foil and a negative electrode active material layer 1 are laminated, a positive electrode current collector 5 such as an aluminum foil, and a positive electrode active material layer 4. The stacked positive electrodes 6 are disposed to face each other via a separator 7 that avoids these contacts, and these are accommodated in a laminate film outer package 8. The laminate film is filled with an electrolytic solution, and the negative electrode lead tab 9a connected to the negative electrode current collector 2 and the positive electrode lead tab 9c connected to the positive electrode current collector 5 are respectively drawn out of the laminate film 8, and the electrode terminals and Is done.

[Charge / Discharge]
The charge / discharge after the initial charge / discharge in the lithium secondary battery is preferably performed in the range where the discharge end voltage value is 2.5 V or more and 3.5 V or less. If the discharge end voltage value is 2.5 V or more, deterioration of the discharge capacity due to repeated charge and discharge can be suppressed, and circuit design is easy. On the other hand, if the discharge end voltage value is 3.5 V or less, the absolute value of the discharge capacity can be suppressed from being reduced, and the discharge capacity of the negative electrode active material can be fully utilized.

[Production method]
In the method for producing a lithium secondary battery of the present invention, a lithium transition metal oxide (2) is obtained at the time of initial charge with respect to a positive electrode including a positive electrode active material containing an olivine compound (1) and a lithium transition metal oxide (2). All the releasable lithium is released into a lithium irreversible metal oxide, and then oxygen is released to seal.

  In the method for producing a lithium secondary battery according to the present invention, a positive electrode and a negative electrode containing the olivine compound (1) and the lithium transition metal oxide (2) as a positive electrode active material are introduced via a separator. It is preferable to arrange and set in the exterior body and perform the first charge / discharge between 4.2V and 2.5V. By the first charge, lithium ions are released from the olivine compound (1) and the lithium transition metal oxide (2) and occluded in the negative electrode active material. At this time, the lithium transition metal oxide (2) generates oxygen along with the release of lithium ions, and becomes a lithium irreversible transition metal oxide.

  Release of the generated oxygen gas is performed by making a hole in the laminate part of the secondary battery, venting the gas, and sealing the holed part under vacuum, and the production of the lithium secondary battery is completed. By releasing oxygen from the battery, it is possible to suppress a decrease in the lithium ion storage / release capacity during subsequent charge / discharge, and the capacity of the negative electrode active material having a large lithium ion storage / release capacity can be fully utilized. be able to.

Hereinafter, the lithium secondary battery of the present invention will be described in detail.
[Example 1]
[Production of negative electrode and initial charge / discharge capacity]
As the negative electrode active material, powder having a molar ratio of Si, SiO ₂ and carbon of 1: 1: 0.8 was used, and these were mixed to obtain a negative electrode active material. A negative electrode material in which polyimide as a binder and NMP as a solvent were mixed with this negative electrode active material was applied onto a copper foil having a thickness of 10 μm and dried at 125 ° C. for 5 minutes. Then, compression molding was performed with a roll press, and drying treatment was performed again in a drying furnace at 350 ° C. for 30 minutes in an N ₂ atmosphere. The copper foil on which the negative electrode active material layer was formed was punched out to 30 × 28 mm to produce a negative electrode. To this negative electrode, a nickel negative electrode lead tab for extracting electric charge was fused by ultrasonic waves.

  The initial charge capacity of the obtained negative electrode active material was measured. Charging was performed between 1.5 V and 0.02 V with a model cell using metallic lithium as a counter electrode, and the initial charge capacity was measured to determine the initial charge capacity per gram of the negative electrode active material. The initial charge capacity Y per 1 g of the negative electrode active material is 2500 mAh / g and the subsequent initial discharge capacity is 1650 mAh / g. From these values, the irreversible capacity per 1 g of the negative electrode active material in the initial charge is calculated, and 850 mAh / g. Got. This irreversible capacity corresponds to the amount of lithium ions α irreversibly stored per gram of the negative electrode active material by the first charge. The irreversible capacity was 34% with respect to the initial charge capacity.

[Production of positive electrode and initial charge capacity]
Li ₂ CuO ₂ was prepared by the following method. CuO and Li ₂ CO ₃ were mixed in a predetermined amount, calcined in air at 650 ° C. for 24 hours, and then calcined at 800 ° C. for 48 hours to obtain a sintered body of Li ₂ CuO ₂ . This was pulverized to obtain Li ₂ CuO ₂ powder. The steps from mixing to pulverization were performed in low humidity (dew point -30 ° C or lower). The powder X-ray diffraction measurement of the Li ₂ CuO ₂ powder revealed no impurity peak, the formation of a planar four-coordinated MO ₄ in the structure, and the sides facing the planar four-coordinated structure (sides formed by two oxygen atoms) ) Was confirmed to form a one-dimensional chain.

The initial charge capacity of the obtained Li ₂ CuO ₂ was measured. Charging was performed between 4.3 V and 3.0 V using a model cell with metallic lithium as a counter electrode, and the initial charge capacity was measured to determine the initial charge capacity per 1 g of Li ₂ CuO ₂ . The initial charge capacity per 1 g of Li ₂ CuO ₂ was 400 mAh / g. This charge capacity corresponds to the amount of lithium ions γ released per gram of Li ₂ CuO ₂ by the initial charge of the negative electrode.

Next, the initial charge capacity of LiMnPO ₄ was measured. Charging was performed between 4.3 V and 3.0 V using a model cell with metallic lithium as a counter electrode, and the initial charge capacity was measured to determine the initial charge capacity per gram of LiMnPO ₄ . The initial charge capacity per gram of LiMnPO ₄ was 160 mAh / g.

The initial charge capacity B per gram of Li ₂ CuO ₂ is 400 mAh / g, which is larger than the initial charge capacity A of 160 mAh / g per gram of LiMnPO ₄ olivine compound, A <B, and the olivine compound It can be seen that a high-capacity battery can be obtained by using Li ₂ CuO ₂ instead.

A positive electrode material prepared by mixing Li ₂ CuO ₂ and LiMnPO ₄ with polyvinylidene fluoride as a binder and NMP as a solvent is applied onto an aluminum foil having a thickness of 20 μm, dried at 125 ° C. for 5 minutes, and positive electrode active material A layer was made. This was punched out to 30 × 28 mm to produce a positive electrode. An aluminum positive electrode lead tab for extracting electric charge was fused to the positive electrode by ultrasonic waves.

  The initial charge capacity of the obtained positive electrode active material was measured. Charging between 4.3 V and 3.0 V using a model cell with metallic lithium as the counter electrode for the positive electrode active material, measuring the initial charge capacity, and the initial charge capacity Z (mAh / g per gram of the positive electrode active material) )

After laminating the negative electrode, the separator, and the positive electrode in this order so that each active material layer faces the separator, the laminate film is sandwiched between laminate films, injected with an electrolyte, and sealed under vacuum. A secondary battery was produced. As the electrolytic solution, one obtained by dissolving 1 mol / L LiPF ₆ in a mixed solvent of EC, DEC, and EMC in a volume ratio of 3: 5: 2 was used. The mass ratio of the negative electrode active material and the positive electrode active material was selected so that the initial charge capacity Y of the negative electrode and the initial charge capacity Z of the positive electrode were the same. Furthermore, in order to make the amount of lithium ions α irreversibly occluded during the first charge and the amount of lithium ions γ released by Li ₂ CuO _{2 the same} as the first charge, the first irreversible capacity of the negative electrode and the Li ₂ The mass ratio between the negative electrode active material and Li ₂ CuO ₂ was adjusted so that the initial charge capacity resulting from CuO ₂ would be the same.

  The charge / discharge test of the obtained laminated laminate type secondary battery was performed at a constant current of 3 mA, the charge end voltage was 4.2V, and the discharge end voltage was 2.5V. After the first charge, make a hole in the laminate part of the secondary battery, degas, seal the holed part under vacuum, then perform the first discharge and the second charge, the first and second charge The charge capacity of the positive electrode during charging was measured. The results are shown in Table 1. In addition, the charge capacity of the negative electrode during the second charge was measured, and the charge capacity per gram of the negative electrode active material was determined. The results are shown in Table 2.

[Examples 2 to 4]
The mass ratio of the negative electrode to the positive electrode was adjusted so that the initial charge capacity Y of the negative electrode and the initial charge capacity Z of the positive electrode were in the ratio shown in Table 1. Otherwise, a laminated laminate type secondary battery was prepared in the same manner as in Example 1, and the charge capacity of the positive electrode during the first and second charging was measured. The results are shown in Tables 1 and 2.

  In the laminated laminate type secondary batteries of Examples 1 to 4, the difference between the initial charge capacity of the positive electrode and the second charge capacity is small, and it can be seen that the decrease in capacity due to the irreversible capacity of the negative electrode is suppressed. Moreover, it turns out that the charge capacity density per 1g of the negative electrode active material at the time of the second charge is so large that the initial charge capacity Z of the positive electrode is close to the same amount as the initial charge amount of the negative electrode.

  The charge capacity at the time of the second charge of the negative electrode using the carbon material of the lithium secondary battery currently in practical use as the active material is about 300 to 370 mAh / g, and the lithium secondary battery according to the present invention has an extremely high capacity. In other words, it was confirmed to be a secondary battery having a high energy density.

[Examples 5 to 8]
Li ₂ NiO ₂ was prepared by the following method. NiO and Li ₂ O were mixed in a predetermined amount and heated in a reducing atmosphere at 700 ° C. for 48 hours to obtain a target sintered body of Li ₂ NiO ₂ . This was pulverized to obtain Li ₂ NiO ₂ powder. The steps from mixing to pulverization were performed in low humidity (dew point -30 ° C or lower). When the powder X-ray diffraction measurement of the Li ₂ NiO ₂ powder was performed, there was no impurity peak, a planar four-coordinated MO ₄ was formed in the structure, and the sides of the planar four-coordinated structure facing each other (sides formed by two oxygen atoms) ) Was confirmed to form a one-dimensional chain.

The initial charge capacity of the obtained Li ₂ NiO ₂ was measured. Charging was performed between 4.3 V and 3.0 V using a model cell with metallic lithium as a counter electrode, and the initial charge capacity was measured to determine the initial charge capacity per 1 g of Li ₂ NiO ₂ . The initial charge capacity B per gram of Li ₂ NiO ₂ is 450 mAh / g, which is larger than the initial charge capacity A of 160 mAh / g per gram of the olivine compound, A <B, and higher capacity than LiMnPO ₄ . It can be seen that a battery is obtained. This charge capacity corresponds to the amount of lithium ions γ released per gram of Li ₂ NiO ₂ by the initial charge of the negative electrode.

As the positive electrode active material, using Li ₂ NiO ₂ in place of Li ₂ CuO _2, the initial charge capacity Y of the negative electrode to the initial charge capacity Z of the cathode so that the ratio shown in Table 3, the anode active material and the positive electrode active material A laminated laminate type secondary battery was produced in the same manner as in Example 1 except that the mass ratio was adjusted, and the charge capacity of the positive electrode during the first and second charging was measured. The results are shown in Table 3.

As shown in Table 3, in the laminated laminate type secondary batteries of Examples 5 to 8, the difference between the initial charge capacity and the second charge capacity is small, and the decrease in capacity due to the irreversible capacity of the negative electrode is suppressed. I understand that.

[Examples 9 to 12]
The mass ratio of the negative electrode active material and Li ₂ CuO ₂ so that the amount of lithium ions α irreversibly occluded in the negative electrode and the amount of lithium ions γ released by Li ₂ CuO ₂ during the initial charge is α = 2γ. In the same manner as in Example 1, except that the mass ratio of the negative electrode active material and the positive electrode active material was adjusted so that the initial charge capacity Y of the negative electrode with respect to the initial charge capacity Z of the positive electrode became the ratio shown in Table 4. A laminated laminate type secondary battery was prepared, and the charge capacity of the positive electrode during the first and second charge was measured. The results are shown in Table 4.

[Examples 13 to 16]
The mass ratio of the negative electrode active material and Li ₂ CuO ₂ so that the amount of lithium ions α irreversibly occluded in the negative electrode and the amount of lithium ions γ released by Li ₂ CuO ₂ during the initial charge is α = 3γ. As in Example 1, except that the mass ratio of the negative electrode active material and the positive electrode active material was adjusted so that the initial charge capacity Y of the negative electrode with respect to the initial charge capacity Z of the positive electrode became the ratio shown in Table 5. A laminated laminate type secondary battery was prepared, and the charge capacity of the positive electrode during the first and second charge was measured. The results are shown in Table 5.

Even when the amount of lithium ions α irreversibly occluded in the negative electrode for the first time and the amount of lithium ions γ released by Li ₂ CuO ₂ during the first charge are γ <α, a high capacity battery with a second charge capacity can be obtained. It was.

[Example 17]
A battery was prepared in the same manner as in Example 1 except that a laminate type secondary battery in which a laminate of a positive electrode, a separator, and a negative electrode was wound, and the charge capacity of the positive electrode during the first and second charging was measured. The results are shown in Table 6.

  In the wound laminate type battery, the decrease in charge capacity of the positive electrode after the first time was suppressed, but the effect of suppressing the decrease in capacity was smaller than that in Example 1.

[Examples 18 to 21]
The relationship between the initial charge capacity Y of the negative electrode and the initial charge capacity Z of the positive electrode Y: laminated laminate in the same manner as in Example 1 except that the mass ratio of the negative electrode and the positive electrode was adjusted so that Z was a value shown in Table 7 Type secondary batteries were prepared, and the charge capacity of the positive electrode during the first and second charge was measured. The results are shown in Table 7.

In the secondary batteries according to Examples 18 to 21, the initial charge capacity Y of the negative electrode and the initial charge capacity Z of the positive electrode have a relationship of Z> Y, but compared with Example 1 where Y = Z, Although both the initial charge capacity and the second charge capacity of the positive electrode were reduced, it was found that there was no practical problem.

[Examples 22 to 25]
The mass ratio of the negative electrode active material and Li ₂ CuO ₂ so that the amount of lithium ions α irreversibly occluded in the negative electrode for the first time and the amount of lithium ions γ released during the initial charge of Li ₂ CuO ₂ is 2α = γ. As in Example 1, except that the mass ratio of the negative electrode active material and the positive electrode active material was adjusted so that the initial charge capacity Y of the negative electrode with respect to the initial charge capacity Z of the positive electrode became the ratio shown in Table 8. A laminated laminate type lithium secondary battery was prepared, and the charge capacity of the positive electrode during the first and second charge was measured. The results are shown in Table 8.

In the secondary batteries according to Examples 22 to 25, the lithium ion amount γ released by Li ₂ CuO ₂ during the initial charge with respect to the lithium ion amount α stored in the negative electrode during the initial charge corresponding to the initial irreversible capacity of the negative electrode, Although it has a relationship of γ> α, it has been found that although both the initial charge capacity and the second charge capacity of the positive electrode are decreased, there is no practical problem.

[Examples 26 to 29]
The mass ratio between the negative electrode active material and Li ₂ CuO ₂ so that the amount of lithium ions α irreversibly occluded in the negative electrode and the amount of lithium ions γ released by Li ₂ CuO ₂ during the initial charge is 5α = 2γ. In the same manner as in Example 1 except that the mass ratio of the negative electrode active material and the positive electrode active material was adjusted so that the initial charge capacity Y of the negative electrode with respect to the initial charge capacity Z of the positive electrode became the ratio shown in Table 9. A laminated laminate type lithium secondary battery was prepared, and the charge capacity of the positive electrode during the first and second charge was measured. The results are shown in Table 9.

In the secondary batteries according to Examples 26 to 29, the amount of lithium ions γ released by Li ₂ CuO ₂ during the initial charge with respect to the amount of lithium ions α stored in the negative electrode during the initial charge corresponding to the initial irreversible capacity of the negative electrode, Although it has a relationship of γ> α, it has been found that although both the initial charge capacity and the second charge capacity of the positive electrode are decreased, there is no practical problem.

[Comparative Examples 1 and 2]
A laminated laminated lithium secondary battery was prepared in the same manner as in Examples 1 and 2 except that only the olivine-type lithium manganese phosphate was used as the positive electrode active material, and the charge capacity of the positive electrode during the first and second charge Was measured. The results are shown in Table 10.

Compared with the corresponding secondary batteries of Examples 1 and 2, the charge capacity of the positive electrode during the first and second charge decreased.

[Comparative Examples 3 and 4]
A laminated laminate type lithium secondary battery was prepared in the same manner as in Example 1 and Example 2 except that the degassing step at the first charge was omitted, and the charge capacity of the positive electrode at the first charge and the second charge was measured. The results are shown in Table 11.

As the positive electrode active material, the lithium secondary battery including the olivine compound (1) and a part of the olivine compound (1) in place of the lithium transition metal oxide (2) is a positive electrode active material consumed by the initial irreversible capacity inherent to the negative electrode active material. By using a higher capacity lithium transition metal oxide (2), the mass of the positive electrode active material that is not used for charge / discharge after the first discharge can be reduced, and by degassing during the first charge, a higher capacity can be obtained. In addition, a substance that is not involved in the storage and release of lithium in the charge and discharge after the initial charge contributes to safety and long life, and is extremely safe and can provide a lithium secondary battery with excellent cycle characteristics. It was confirmed.

  The present invention can be used in all industrial fields that require a power source, as well as industrial fields related to the transportation, storage and supply of electrical energy. Specifically, it can be used as a power source for mobile devices such as mobile phones and notebook computers.

DESCRIPTION OF SYMBOLS 1 Negative electrode active material layer 2 Negative electrode collector 3 Negative electrode 4 Positive electrode active material layer 5 Positive electrode collector 6 Positive electrode 7 Separator 8 Laminate film exterior 10 Laminated laminate type secondary battery

Claims (8)

The positive electrode before the first charge is the formula (1)
LiGPO ₄ (1)
(Wherein G represents Fe or Mn), and the formula (2)
Li ₂ MO ₂ (2)
(Wherein, M represents Cu or Ni), and a positive electrode active material containing a lithium transition metal oxide represented by
The positive electrode active material after the first charge includes a lithium irreversible transition metal oxide formed by releasing all lithium that can be released by the lithium transition metal oxide represented by the formula (2), A lithium secondary battery obtained by discharging and sealing oxygen gas generated during charging .   The content of the lithium transition metal oxide represented by the formula (2) in the positive electrode is released from the lithium transition metal oxide (2) with respect to the amount of lithium ions α irreversibly occluded in the negative electrode during the initial charge. The lithium secondary battery according to claim 1, wherein a lithium ion amount γ is an amount satisfying a relationship of γ ≦ α.   3. The lithium secondary battery according to claim 1, wherein the initial charge capacity Z of the positive electrode during the initial charge satisfies Z ≦ Y with respect to the initial charge capacity Y of the negative electrode. The lithium secondary battery according to claim 2 , wherein the negative electrode contains a silicon-based material as a negative electrode active material. When charging for the first time, formula (1)
LiGPO ₄ (1)
(Wherein G represents Fe or Mn), and the formula (2)
Li ₂ MO ₂ (2)
(In the formula, M represents Cu or Ni.) Lithium ions are released from the positive electrode containing a lithium transition metal oxide represented by the formula (2) as a positive electrode active material, and lithium transition metal oxidation represented by the formula (2) A method for producing a lithium secondary battery, characterized in that a lithium irreversible transition metal oxide is used, and then oxygen gas is discharged and sealed.   The content of the lithium transition metal oxide represented by the formula (2) in the positive electrode is released from the lithium transition metal oxide (2) with respect to the amount of lithium ions α irreversibly occluded in the negative electrode during the initial charge. 6. The method for producing a lithium secondary battery according to claim 5, wherein the amount of lithium ions γ to be satisfied is an amount satisfying a relationship of γ ≦ α.   The method for producing a lithium secondary battery according to claim 5, wherein the initial charge capacity Z of the positive electrode during the initial charge satisfies Z ≦ Y with respect to the initial charge capacity Y of the negative electrode.   The method for producing a lithium secondary battery according to claim 5, wherein the negative electrode contains a silicon-based material as an active material.

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