WO2012124602A1

WO2012124602A1 - Method for pre-processing lithium ion secondary battery

Info

Publication number: WO2012124602A1
Application number: PCT/JP2012/055975
Authority: WO
Inventors: 大澤　康彦; 建三押原; 伊藤　淳史; 智裕蕪木; 松本　太; 佐藤　祐一; 明尋渡邉
Original assignee: 日産自動車株式会社; 学校法人神奈川大学
Priority date: 2011-03-16
Filing date: 2012-03-08
Publication date: 2012-09-20
Also published as: JP2012195126A

Abstract

This method for pre-processing a lithium ion secondary battery is provided with: a step for preparing a lithium ion secondary battery containing the cathode active material represented by chemical formula (1); and a step for charging with the upper limit potential of the lithium ion secondary battery being at least 4.5 V and less than 5.0 V in terms of a lithium counter electrode, and discharging with the lower limit potential being less than 4.0 V in terms of a lithium counter electrode. Also, the charging and discharging is characterized by being performed at a current rate of 0.1-1.3 C inclusive. Formula 1: aLi[Li_1/3Mn_2/3]O₂·(1-a)LiMO₂ (In the formula: a is a value above 0 and less than 1; and LiMO₂ is a lithium complex oxide containing Ni and Mn.)

Description

Pretreatment method for lithium ion secondary battery

The present invention relates to a pretreatment method for a lithium ion secondary battery using a solid solution system material made of a lithium composite oxide as a positive electrode active material. Moreover, it is related with the lithium ion secondary battery processed by this pre-processing method.

In recent years, various electric vehicles including hybrid type vehicles have been popularized with the aim of solving environmental problems and energy problems. In order to widely disseminate such electric vehicles, there is no doubt that it is necessary to improve the performance of the battery, which is the power source for driving the motors of these vehicles, and to reduce the price. In particular, since it is necessary to improve the mileage of an electric vehicle that can be achieved by a single charge as much as that of a gasoline engine vehicle, development of a higher energy battery is desired. As such a secondary battery for driving a motor, a lithium ion secondary battery having a high theoretical energy among various secondary batteries has attracted attention.

Generally, in order to increase the energy density of a lithium ion secondary battery, it is necessary to increase the amount of electricity stored per unit mass of the positive electrode and the negative electrode. A so-called solid solution positive electrode material has attracted attention as a positive electrode material that may satisfy such requirements. Among these solid solution system materials, the electrochemically inactive layered Li ₂ MnO ₃ and the electrochemically active layered LiMO ₂ (wherein M is a transition metal such as Co or Ni) Solid solutions are expected to exhibit large electrical capacities exceeding 200 mAh / g.

Although such a solid solution positive electrode material is remarkably large in capacity, when it is used as a high-capacity positive electrode with a high charge / discharge potential, there is a problem that it easily deteriorates due to repeated charge / discharge. Therefore, as a battery pretreatment for improving cycle durability, the lower limit voltage is set to 2.0 V, the upper limit voltage is set to 4.5 V, and 4.6 V at a current density of 0.2 mA / cm ² (equivalent to 1 / 12C). , 4.7V, 4.8V, and charging / discharging in a stepwise manner have been proposed (see Patent Document 1).

JP 2008-270201 A

However, according to the pretreatment of Patent Document 1 that employs the method of repeating the charge / discharge treatment while increasing the upper limit voltage stepwise, although the cycle characteristics can be greatly improved, for example, an extremely long treatment time exceeding 5 days is required. There was a problem that it took.

The present invention has been made in view of the above problems in a lithium ion secondary battery using the above solid solution system material as a positive electrode active material. Then, an object of the present invention is to provide a pretreatment method of a lithium ion secondary battery that can obtain an effect equivalent to or better than the pretreatment described above in a short time.

That is, the pretreatment method of the lithium ion secondary battery of the present invention includes a step of preparing a lithium ion secondary battery containing a positive electrode active material represented by the following chemical formula 1:
[Formula 1] aLi [Li _1/3 Mn _2/3 ] O _2. (1-a) LiMO ₂
(A in the formula is a numerical value greater than 0 and less than 1, and LiMO ₂ is a lithium composite oxide containing Ni and Mn)
Charging the upper limit potential of the lithium ion secondary battery to 4.5 V or more and less than 5.0 V in terms of a lithium counter electrode, and discharging the lower limit potential to less than 4.0 V in terms of a lithium counter electrode. . The charging and discharging are performed at a current rate of 0.1 C or more and 1.3 C or less. The lithium ion secondary battery of the present invention is characterized by being processed by the pretreatment method of the present invention.

FIG. 1 is a schematic cross-sectional view showing an example of a lithium ion secondary battery according to an embodiment of the present invention.

Hereinafter, the pretreatment method of the lithium ion secondary battery of the present invention and the lithium ion secondary battery processed by this method will be described in detail.

[Pretreatment method of lithium ion secondary battery]
A pretreatment method for a lithium ion secondary battery according to an embodiment of the present invention will be described in detail.

The pretreatment method of this embodiment is performed on a lithium ion secondary battery using a solid solution system positive electrode active material represented by the following chemical formula 1.
[Chemical 1]
aLi [Li _1/3 Mn _2/3 ] O _2. (1-a) LiMO ₂
(A in the formula is a numerical value greater than 0 and less than 1, and LiMO ₂ is a lithium composite oxide containing Ni and Mn)
That is, the upper limit potential is 4.5 V or more and less than 5.0 V converted to the lithium counter electrode, the lower limit potential is also converted to the lithium counter electrode and less than 4.0 V, and the current rate is in the range of 0.1 C to 1.3 C. Perform charge / discharge treatment. By using such a charge / discharge rate, the processing time can be significantly shortened while maintaining the effect obtained by the pre-charge / discharge pretreatment characterized by increasing the conventional upper limit voltage stepwise.

In the pretreatment method of the lithium ion secondary battery of the present invention, when the lower limit potential during charging and discharging is 4.0 V or more, sufficient Li is not inserted. Therefore, it is considered that an appropriate structural change that should occur in the positive electrode active material is suppressed, and as a result, the effect of improving the cycle durability by the pretreatment is greatly reduced. Therefore, it is preferable to discharge with the lower limit potential being less than 4.0V.

In addition, when the upper limit potential during charging is less than 4.5V, the positive electrode active material is not electrochemically activated. When the upper limit potential is 5.0V or more, the electrolyte used is decomposed. There arises a problem that the battery characteristics deteriorate. Therefore, the upper limit potential is preferably 4.5 V or more and less than 5.0 V.

Further, even when the charge / discharge rate exceeds 1.3 C, the effect of improving the cycle durability by the pretreatment is greatly reduced. On the other hand, when the charge / discharge rate is less than 0.1 C, the time required for the pretreatment cannot be shortened, and the original effect of the present invention cannot be obtained. Therefore, the charge / discharge rate is preferably 0.1 C or more and 1.3 C or less.

The mechanism by which the charge / discharge rate affects the effects of the present invention has not yet been clarified, but is considered as follows. In the first charge / discharge after assembling the battery, the crystal structure of the positive electrode material is disturbed by the process of charging at 4.5 V or higher. This disorder of the crystal structure originates from the fact that oxygen ions constituting the crystal at the positive electrode are partially oxidized by charge / discharge and a part thereof is released out of the crystal. Thus, since the solid solution positive electrode active material is activated and a high capacity can be developed, such a charge / discharge process is an essential process for increasing the capacity of the battery. In addition, by discharging at a stage where this reaction is partially performed and returning at least a part of Li ⁺ into the crystal, the disordered crystal structure is restored. It is considered that the repair mechanism of this crystal structure depends not only on the amount of Li ⁺ returning into the crystal but also on the return speed. Here, the amount of Li ⁺ returning into the crystal depends on the lower limit potential, and the return speed depends on the current rate.

In the pretreatment method for a lithium ion secondary battery of the present invention, it is desirable to repeat the charge / discharge treatment at least several times. When the above-described charging / discharging is performed rapidly at a time, the degree of damage to the crystal structure accompanying the oxidation of oxygen ions due to charging at 4.5 V or more may be increased and may not be repaired. Therefore, it is preferable to perform the process partially in a plurality of times.

Particularly in this case, from the viewpoint of avoiding sudden crystal structure disturbance as much as possible, the upper limit potential is increased stepwise when charging and discharging are repeated, that is, the upper limit potential is initially started from a relatively low potential. It is desirable to gradually increase the upper limit potential until a predetermined potential of less than 0V is reached.

On the other hand, with regard to the charge / discharge rate in the pretreatment, the effect of reducing the pretreatment time while avoiding a sudden change in the crystal structure of the positive electrode active material and maintaining the battery cycle durability at a high level is more certain. In view of the above, it is more desirable to set the range of 0.2C to 0.6C.

When the pretreatment method of the present invention is applied to the assembled lithium ion secondary battery, first, the upper and lower potentials for charging and discharging are determined. These potentials need to be values converted to the lithium counter electrode based on the charge and discharge curves of the positive and negative electrodes measured in advance. In addition to this, the potential control method may be performed using a reference electrode. Furthermore, as a pretreatment method by such charge / discharge, control may be performed with an electric quantity corresponding to the electric quantity of each charge / discharge when the potential is controlled. In that case, if batteries of the same standard are connected in series, a large number of batteries can be activated at once by this electric quantity control method.

Next, the configuration and material of the lithium ion secondary battery according to one embodiment of the present invention will be described in detail with reference to the drawings. In addition, the dimension ratio of drawing is exaggerated on account of description, and may differ from an actual ratio.

The lithium ion secondary battery 1 of the present invention uses a solid solution system material represented by the following chemical formula 1 as a positive electrode active material.
[Chemical 1]
aLi [Li _1/3 Mn _2/3 ] O _2. (1-a) LiMO ₂
(A in the formula is a numerical value greater than 0 and less than 1, and LiMO ₂ is a lithium composite oxide containing Ni and Mn)
And the lithium ion secondary battery 1 of this invention performs a charging / discharging process by the above upper limit and lower limit electric potential range, and a charging / discharging rate as a pretreatment after a battery assembly. Due to the effect of the pretreatment described above, the lithium ion secondary battery 1 of the present invention can be obtained in a short time and exhibits excellent cycle durability.

[Configuration of lithium ion secondary battery]
FIG. 1 shows an example of a lithium ion secondary battery according to an embodiment of the present invention. As shown in FIG. 1, the lithium ion secondary battery 1 of this embodiment has a configuration in which a battery element 10 to which a positive electrode tab 21 and a negative electrode tab 22 are attached is enclosed in an exterior body 30. In the present embodiment, the positive electrode tab 21 and the negative electrode tab 22 are led out in the opposite directions from the inside of the exterior body 30 toward the outside. Although not shown, the positive electrode tab and the negative electrode tab may be led out in the same direction from the inside of the exterior body toward the outside. Moreover, such a positive electrode tab and a negative electrode tab can be attached to the positive electrode collector and negative electrode collector which are mentioned later by ultrasonic welding, resistance welding, etc., for example.

[Positive electrode tab and negative electrode tab]
The positive electrode tab 21 and the negative electrode tab 22 are made of materials such as aluminum (Al), copper (Cu), titanium (Ti), nickel (Ni), stainless steel (SUS), and alloys thereof. However, the material is not limited thereto, and a conventionally known material that can be used as a tab for a lithium ion secondary battery can be used. The positive electrode tab and the negative electrode tab may be made of the same material or different materials. Further, as in the present embodiment, a separately prepared tab may be connected to a positive electrode current collector and a negative electrode current collector described later, and each positive electrode current collector and each negative electrode current collector described later are in a foil shape. In some cases, tabs may be formed by extending each one.

[Exterior body]
It is preferable that the said exterior body 30 is formed with the film-shaped exterior material from a viewpoint of size reduction and weight reduction, for example. However, it is not limited to this, What was formed with the conventionally well-known material which can be used for the exterior body for lithium ion secondary batteries can be used. When applying to automobiles, for example, a polymer-metal composite laminate sheet with excellent thermal conductivity should be used to efficiently transfer heat from the heat source of the automobile and to quickly heat the inside of the battery to the battery operating temperature. Is preferred.

[Battery element]
As shown in FIG. 1, the battery element 10 in the lithium ion secondary battery 1 of the present embodiment has a configuration in which a plurality of unit cell layers 14 including a positive electrode 11, an electrolyte layer 13, and a negative electrode 12 are stacked. Yes. The positive electrode 11 has a configuration in which a positive electrode active material layer 11B is formed on both main surfaces of the positive electrode current collector 11A. The negative electrode 12 has a configuration in which a negative electrode active material layer 12B is formed on both main surfaces of the negative electrode current collector 12A.

At this time, the positive electrode active material layer 11B formed on one main surface of the positive electrode current collector 11A in one positive electrode 11 and the one main surface of the negative electrode current collector 12A in the negative electrode 12 adjacent to the positive electrode 11 The negative electrode active material layer 12 </ b> B formed on the opposite side is opposed to the electrolyte layer 13. In this way, a plurality of positive electrodes, electrolyte layers, and negative electrodes are laminated in this order, and the adjacent positive electrode active material layer 11B, electrolyte layer 13, and negative electrode active material layer 12B constitute one single battery layer. That is, the lithium ion secondary battery 1 according to the present embodiment has a configuration in which a plurality of single battery layers 14 are stacked and electrically connected in parallel. Note that the negative electrode current collector 12A located on the outermost layer of the battery element 10 has a negative electrode active material layer 12B formed only on one side.

Further, an insulating layer (not shown) may be provided on the outer periphery of the unit cell layer 14 in order to insulate between the adjacent positive electrode current collector and negative electrode current collector. Such an insulating layer is preferably formed on the outer periphery of the unit cell layer by a material capable of holding the electrolyte contained in the electrolyte layer and preventing electrolyte leakage. Specifically, general-purpose plastics such as polypropylene (PP), polyethylene (PE), polyurethane (PUR), polyamide resin (PA), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and polystyrene (PS) Can be used. Moreover, thermoplastic olefin rubber, silicone rubber, etc. can also be used.

[Positive electrode current collector and negative electrode current collector]
The positive electrode current collector 11A and the negative electrode current collector 12A are made of a conductive material such as foil or mesh aluminum, copper, stainless steel (SUS), for example. However, the material is not limited to these, and a conventionally known material that can be used as a current collector for a lithium ion secondary battery can be used. The size of the current collector can be determined according to the intended use of the battery. For example, if it is used for a large battery that requires a high energy density, a current collector having a large area is used. There is no particular limitation on the thickness of the current collector. The thickness of the current collector is usually about 1 to 100 μm. The shape of the current collector is not particularly limited. In the battery element 10 shown in FIG. 1, in addition to the current collector foil, a mesh shape (expanded grid or the like) can be used. In addition, when forming the thin film alloy which is a negative electrode active material directly on the negative electrode collector 12A by sputtering method etc., it is desirable to use current collection foil.

There are no particular restrictions on the materials that make up the current collector. For example, a metal or a resin in which a conductive filler is added to a conductive polymer material or a non-conductive polymer material can be employed. Specifically, examples of the metal include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition to these, it is preferable to use a clad material of nickel and aluminum, a clad material of copper and aluminum, or a plating material of a combination of these metals. Moreover, the foil by which aluminum is coat | covered on the metal surface may be sufficient. Among these, aluminum, stainless steel, copper, and nickel are preferable from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material by sputtering to the current collector.

Examples of the conductive polymer material include polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, polyphenylene vinylene, polyacrylonitrile, and polyoxadiazole. Since such a conductive polymer material has sufficient conductivity without adding a conductive filler, it is advantageous in terms of facilitating the manufacturing process or reducing the weight of the current collector.

Non-conductive polymer materials include, for example, polyethylene (PE; high density polyethylene (HDPE), low density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyether nitrile (PEN), polyimide (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA) , Polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), polystyrene (PS), and the like. Such a non-conductive polymer material has excellent potential resistance or solvent resistance.

A conductive filler can be added to the conductive polymer material or the non-conductive polymer material as necessary. In particular, when the resin serving as the base material of the current collector is composed of only a non-conductive polymer, a conductive filler is essential to impart conductivity to the resin. The conductive filler can be used without particular limitation as long as it is a substance having conductivity. For example, a metal, conductive carbon, etc. are mentioned as a material excellent in electroconductivity, electric potential resistance, or lithium ion interruption | blocking property. The metal is not particularly limited, but includes at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, Sb, and K, or these metals. An alloy or metal oxide is preferably included. The conductive carbon is not particularly limited, but preferably acetylene black, Vulcan (registered trademark), black pearl (registered trademark), carbon nanofiber, ketjen black (registered trademark), carbon nanotube, carbon nanohorn, carbon It contains at least one selected from the group consisting of nanoballoons and fullerenes. The amount of the conductive filler added is not particularly limited as long as it is an amount capable of imparting sufficient conductivity to the current collector, and is generally about 5 to 35% by mass of the entire current collector.

However, the material is not limited to these, and a conventionally known material used as a current collector for a lithium ion secondary battery can be used.

[Positive electrode]
In the lithium ion secondary battery, the positive electrode is configured by forming a positive electrode active material layer on one or both sides of a positive electrode current collector made of a conductive material such as an aluminum foil, a copper foil, a nickel foil, or a stainless steel foil.

[Positive electrode active material layer]
The positive electrode active material layer 11B includes any one or more of positive electrode materials capable of inserting and extracting lithium as a positive electrode active material, and includes a conductive auxiliary agent and a binder as necessary. May be. In addition, it is not specifically limited as a compounding ratio of these positive electrode active materials, a conductive support agent, and a binder in a positive electrode active material layer.

In the lithium ion secondary battery as the pretreatment target of the present invention, a solid solution system material represented by the following chemical formula 1 is used as the positive electrode active material.
[Chemical 1]
aLi [Li _1/3 Mn _2/3 ] O _2. (1-a) LiMO ₂
(A in the formula is a numerical value greater than 0 and less than 1, and LiMO ₂ is a lithium composite oxide containing Ni and Mn)
As long as such a solid solution positive electrode active material is contained as an essential component, there is no problem even if other positive electrode active materials are used in combination. Examples of such known positive electrode active materials include lithium-transition metal composite oxides, lithium-transition metal phosphate compounds, lithium-transition metal sulfate compounds, ternary systems, NiMn systems, NiCo systems, and spinel Mn systems. Can be mentioned.

As the positive electrode active material made of the solid solution system material represented by the above chemical formula 1, for example, a material synthesized by a solid phase method or a solution method can be used. Examples of the solution method include a mixed hydroxide method, a composite carbonate method, and an organic acid method. Among these synthesis methods, it is desirable to employ a composite carbonate method. According to this method, since the yield is high and the aqueous solution system is used, a uniform composition can be obtained and the composition can be easily controlled. In addition, it can be produced by a general synthesis method such as a coprecipitation method, a sol-gel method, or a PVA method.

In the composition formula representing the solid solution positive electrode active material, the lithium composite oxide represented by LiMO ₂ contains Ni and Mn as essential components. However, as other components, for example, one or more transition metals selected from Co, Al, Ti, Fe, Cu, Mg and the like can also be contained. The addition of Co can be expected to improve the conductivity of the active material, and the addition of Al, Ti, Fe, Cu, Mg can be expected to improve the durability by stabilizing the crystal structure.

The particle size of the positive electrode active material is not particularly limited, but generally finer is more desirable. In particular, considering the work efficiency and ease of handling, the average particle diameter may be about 1 to 30 μm, and more preferably about 5 to 20 μm.

Examples of the lithium-transition metal composite oxide include LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , Li (Ni, Mn, Co) O ₂ , Li (Li, Ni, Mn, Co) O ₂ , LiFePO ₄ and Examples include those in which some of these transition metals are substituted with other elements. Examples of the ternary system include nickel / cobalt / manganese composite cathode materials. Examples of the spinel Mn system include LiMn ₂ O ₄ . Examples of the NiMn system include LiNi _0.5 Mn _1.5 O ₄ . Examples of the NiCo system include Li (NiCo) O ₂ . These positive electrode active materials can also be used in combination.

In addition, when the optimum particle diameter is different for each of these positive electrode active materials to exhibit a unique effect, the optimum particle diameters may be blended and used for expressing each unique effect. That is, it is not always necessary to make the particle sizes of all the active materials uniform.

The binder is added for the purpose of maintaining the electrode structure by binding the active materials or the active material and the current collector. Examples of such a binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl acetate, polyimide (PI), polyamide (PA), polyvinyl chloride (PVC), polymethyl acrylate (PMA), Thermosetting resins such as polymethyl methacrylate (PMMA), polyether nitrile (PEN), polyethylene (PE), polypropylene (PP) and polyacrylonitrile (PAN), epoxy resins, polyurethane resins, and urea resins In addition, rubber-based materials such as styrene butadiene rubber (SBR) can be used.

The conductive assistant is also referred to as a conductive agent, and refers to a conductive additive that is blended to improve conductivity. The conductive auxiliary agent used in the present invention is not particularly limited, and conventionally known ones can be used. For example, carbon materials such as carbon black such as acetylene black, graphite, and carbon fiber can be given. By containing a conductive additive, an electronic network inside the active material layer is effectively formed, which contributes to improving the output characteristics of the battery and improving reliability by improving the liquid retention of the electrolytic solution.

[Negative electrode]
On the other hand, the negative electrode is configured by forming a negative electrode active material layer on one side or both sides of a negative electrode current collector made of a conductive material as described above, similarly to the positive electrode.

[Negative electrode active material layer]
The negative electrode active material layer 12B includes one or more negative electrode materials capable of occluding and releasing lithium as the negative electrode active material, and, if necessary, the above-described positive electrode active material. The same conductive assistant and binder may be included. In addition, it is not specifically limited as a compounding ratio of these negative electrode active materials, a conductive support agent, and a binder in a negative electrode active material layer.

The negative electrode active material applied to the lithium ion secondary battery of the present invention is not particularly limited as long as it can reversibly occlude and release lithium, and a conventionally known negative electrode active material can be used. Other negative electrode active materials include, for example, graphite (natural graphite, artificial graphite, etc.), which is highly crystalline carbon, low crystalline carbon (soft carbon, hard carbon), carbon black (Ketjen black, acetylene black, channel black) , Lamp black, oil furnace black, thermal black, etc.), carbon materials such as fullerene, carbon nanotube, carbon nanofiber, carbon nanohorn, and carbon fibril. Further, as the negative electrode active material, Si, Ge, Sn, Pb, Al, In, Zn, H, Ca, Sr, Ba, Ru, Rh, Ir, Pd, Pt, Ag, Au, Cd, Hg, Ga, Tl , C, N, Sb, Bi, O, S, Se, Te, Cl, and the like, simple elements that form an alloy with lithium, oxides and carbides containing these elements, and the like can also be mentioned. Examples of such oxides include silicon monoxide (SiO), SiO _x (0 <x <2), tin dioxide (SnO ₂ ), SnO _x (0 <x <2), SnSiO _{3 and the} like. Examples of the carbide include silicon carbide (SiC). Further, examples of the negative electrode active material include metal materials such as lithium metal and lithium-transition metal composite oxides such as lithium-titanium composite oxide (lithium titanate: Li ₄ Ti ₅ O ₁₂ ). In addition, these negative electrode active materials can be used alone or in the form of a mixture of two or more.

In the above description, the positive electrode active material layer and the negative electrode active material layer are formed on one surface or both surfaces of each current collector, as described above. The negative electrode active material layer can also be formed on the other surface. Such an electrode can be applied to a bipolar battery.

(Electrolyte layer)
The electrolyte layer is a layer containing a non-aqueous electrolyte. The non-aqueous electrolyte contained in the electrolyte layer functions as a lithium ion carrier that moves between the positive and negative electrodes during charge and discharge. The thickness of the electrolyte layer is preferably as thin as possible from the viewpoint of reducing internal resistance, and is usually in the range of about 1 to 100 μm, preferably 5 to 50 μm.

The nonaqueous electrolyte is not particularly limited as long as it can exhibit such a function, and a liquid electrolyte or a polymer electrolyte can be used.

The liquid electrolyte has a form in which a lithium salt (electrolyte salt) is dissolved in an organic solvent. Examples of the organic solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), Examples include carbonates such as methylpropyl carbonate (MPC). As the lithium _{_{_{_{salt, Li (CF 3 SO 2)}}}} 2 N, Li (C 2 F 5 SO 2) 2 N, LiPF 6, LiBF 4, LiAsF 6, LiTaF 6, LiClO 4, LiCF 3 SO 3 , etc. A compound that can be added to the active material layer of the electrode can be employed.

On the other hand, polymer electrolytes are classified into a gel polymer electrolyte containing an electrolytic solution (gel electrolyte) and an intrinsic polymer electrolyte containing no electrolytic solution. The gel polymer electrolyte is preferably configured by injecting the liquid electrolyte into a matrix polymer (host polymer) made of an ion conductive polymer. When a gel polymer electrolyte is used as the electrolyte, there is an advantage that the fluidity of the electrolyte is lost and it is easy to block ion conduction between the layers.

The ion conductive polymer used as the matrix polymer (host polymer) is not particularly limited. For example, polyethylene oxide (PEO), polypropylene oxide (PPO), polyvinylidene fluoride (PVDF), a copolymer of polyvinylidene fluoride and hexafluoropropylene (PVDF-HFP), polyethylene glycol (PEG), polyacrylonitrile (PAN), Examples thereof include polymethyl methacrylate (PMMA) and copolymers thereof. Here, the ion conductive polymer may be the same as or different from the ion conductive polymer used as the electrolyte in the active material layer, but is preferably the same. The type of the electrolyte solution composed of the lithium salt and the organic solvent is not particularly limited, and an electrolyte salt such as the lithium salt exemplified above and an organic solvent such as carbonates are used.

Authentic polymer electrolyte has a lithium salt dissolved in the above matrix polymer and does not contain an organic solvent. Thus, by using an intrinsic polymer electrolyte as the electrolyte, there is no fear of liquid leakage from the battery, and the reliability of the battery is improved.

The matrix polymer of the gel polymer electrolyte or the intrinsic polymer electrolyte can exhibit excellent mechanical strength by forming a crosslinked structure. In order to form such a crosslinked structure, a polymerization process such as thermal polymerization, ultraviolet polymerization, radiation polymerization, or electron beam polymerization is performed on a polymerizable polymer for forming a polymer electrolyte using an appropriate polymerization initiator. That's fine. Examples of the polymer electrolyte resting polymerizable polymer include PEO and PPO. The non-aqueous electrolyte contained in these electrolyte layers may be used alone or in combination of two or more.

When the electrolyte layer is composed of a liquid electrolyte or a gel polymer electrolyte, a separator is used for the electrolyte layer. Specific examples of the separator include a microporous film made of polyolefin such as polyethylene or polypropylene.

[Battery shape]
A lithium ion secondary battery has a structure in which a battery element is housed in a battery case such as a can or a laminate container (packaging body). The battery element (electrode structure) is configured by connecting a positive electrode and a negative electrode via an electrolyte layer. The battery element is roughly divided into a wound type battery having a structure in which a positive electrode, an electrolyte layer and a negative electrode are wound, and a stacked type battery in which a positive electrode, an electrolyte layer and a negative electrode are stacked. Has a mold structure. Moreover, it may be called what is called a coin cell, a button battery, a laminate battery, etc. according to the shape and structure of a battery case.

Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples.

[1] Synthesis of solid solution based positive electrode active material A solid solution based material composed of a lithium-manganese composite oxide and a lithium-nickel-cobalt-manganese composite oxide was synthesized by a citric acid method as a positive electrode active material. First, as starting materials, nickel acetate ((CH ₃ COO) ₂ Ni · 4H ₂ O), manganese acetate ((CH ₃ COO) ₂ Mn · 4H ₂ O), cobalt acetate ((CH ₃ COO) ₂ Co · 4H ₂ O) and lithium acetate (CH ₃ COOLi · 2H ₂ O) were used, and these were accurately weighed so as to have a predetermined molar ratio (atomic ratio).

Next, the total amount of the above four kinds of metal acetates and citric acid were accurately weighed so that the molar ratio was 1: 1. These were put into a sample beaker and dissolved in ultrapure water to obtain an aqueous solution. Then, it applied to the spray-drying apparatus and the mixed precursor of the powder was obtained by the spray-drying method. The obtained mixed precursor sample was put in a crucible and calcined at 450 ° C. for 10 hours in the air. Then, it put into the mortar and grind | pulverized for 45 minutes, and 3 tons of pressure was applied and formed into the pellet form using the hand press.

Then, the pelletized sample was subjected to main firing in the air at a firing temperature of 900 ° C. for 12 hours in the air with a heating time of 7 hours. Then, quenching (rapid cooling) is performed using liquid nitrogen, and 0.6Li [Li _1/3 Mn _2/3 ] O ₂ .0.4Li [Ni _0.4575 Co _0.0825 Mn _0.4575 ] O ₂ is used. The solid solution positive electrode active material represented was obtained.

[2] Analysis of solid solution positive electrode active material The composition ratio of the obtained positive electrode active material sample was confirmed to be the above composition by inductively coupled plasma (ICP) elemental analysis. Further, the crystal structure of the obtained sample was examined by a powder X-ray diffraction method. As a result, it was confirmed that when a superlattice peak with 2θ of 21 ° to 25 ° was removed, indexing was possible with the space group R-3m, and the above solid solution system compound was obtained.

[3] Preparation of Evaluation Cell 20 mg of the positive electrode active material obtained in the above [1] and 12 mg of TAB-2 as a conductive binder were weighed out. These were put in an agate mortar and kneaded to form pellets having a diameter of 16 mm. This was placed on a stainless steel mesh (current collector) having the same diameter, and pressed with a pressure of 2 tons (0.99 ton / cm ² ). This was dried under vacuum at 120 ° C. for 4 hours to produce an electrode (working electrode). The conductive binder TAB-2 is composed of a composition (mass ratio) of Teflon (registered trademark) -processed acetylene black: graphite = 2: 1.

And with respect to the said electrode (positive electrode) produced as a working electrode, the metal lithium foil (negative electrode) of diameter 15mm was used as a counter electrode, and the cell was assembled through the glass filter paper as a separator. As the electrolytic solution, a solution containing lithium hexafluorophosphate (LiPF ₆ ) in a solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1: 2 was used. Using this, an evaluation cell (coin battery) was produced in a glove box in a dry argon atmosphere.

[4] Pretreatment Each of the evaluation cells prepared above was pretreated at room temperature and at a constant current. Specifically, the voltage range or current rate was changed for each cell, and the time required for completion of the pretreatment was determined for each cell. Note that the current rate used here was measured with 270 mA / g as 1C.

Example 1
(1) The charge / discharge rate was set to 0.50C. Further, charging was performed at a constant current, with the positive electrode potential being 4.5 V relative to the lithium counter electrode, with the charge upper limit potential. Thereafter, the discharge was performed until the potential of the positive electrode was equivalent to 2.0 V with respect to the lithium counter electrode, with the discharge lower limit potential. This charge / discharge operation was repeated twice.
(2) Next, the battery was charged until the potential of the positive electrode reached 4.6 V relative to the lithium counter electrode, with the upper limit potential being charged. Thereafter, the discharge was performed until the potential of the positive electrode was equivalent to 2.0 V with respect to the lithium counter electrode, with the discharge lower limit potential. This charge / discharge operation was repeated twice in the same manner.
(3) Further, the battery was charged as a charge upper limit potential until the positive electrode potential became 4.7V. Thereafter, the battery was discharged at a discharge lower limit potential corresponding to 2.0V. This charge / discharge operation was repeated twice in the same manner.
(4) That is, the charging / discharging process was performed 6 times in total.

(Example 2)
A charge / discharge treatment similar to that in Example 1 was performed on the evaluation cell except that the charge / discharge rate was set to 0.33C.

(Example 3)
A charge / discharge treatment similar to that of Example 2 was performed on the evaluation cell except that the lower limit discharge potential was set to 3.45V.

Example 4
The evaluation cell was subjected to the same charge / discharge treatment as in Example 1 except that the discharge lower limit potential was set to 3.00V.

(Example 5)
As preconditions, the charge / discharge rate is set to 0.67C, the first two charge upper limit potentials are 4.6V, the next two charge upper limit potentials are 4.7V, and the last two times. Was set to 4.8V. Except for these changes, the same charge / discharge treatment as in Example 1 was performed on the evaluation cell.

(Comparative Example 1)
A charge / discharge treatment similar to that in Example 1 was performed on the evaluation cell except that the charge / discharge rate was set to 0.083C.

(Comparative Example 2)
The evaluation cell was directly subjected to a cycle characteristic evaluation test described later without any pretreatment.

(Comparative Example 3)
A charge / discharge treatment similar to that in Example 1 was performed on the evaluation cell except that the charge / discharge rate was set to 1.33C.

(Comparative Example 4)
The same charge / discharge treatment as that of Comparative Example 1 was performed on the evaluation cell, except that the lower limit discharge potential was set to 4.10V.

[5] Evaluation of cycle characteristics A cycle endurance test was carried out on each of the cells that had been subjected to the pretreatments of the above examples and comparative examples. In this cycle endurance test, the charge / discharge voltage range was set to 2.0 to 4.8 V, the current rate was set to 1/12 C, and the storage capacity after 30 cycles was measured. Table 1 shows a comparison of these measurement results. The pretreatment conditions for each example are also shown in Table 1. The criteria for comprehensive evaluation in Table 1 were that the time required for pretreatment could be reduced to 1 day or less without the capacity after 30 cycles being less than 260 mAh / g. Examples that meet this criterion were recognized as having excellent durability and were evaluated as “◯”. On the other hand, those not meeting this standard were recognized as being inferior in cycle characteristics and evaluated as “x”.

From the results of comprehensive evaluation in Table 1, the processing conditions for shortening the time required for the pretreatment were found without impairing the effect of the conventional pretreatment corresponding to the result of Comparative Example 1. As such processing conditions, the current rate of the preprocessing is set to be larger than that in the conventional method and within an appropriate numerical range, and the discharge lower limit potential may be set to an appropriate range. That is, the current rate during charge / discharge is a charge / discharge rate of 0.1 C to 1.3 C. Further, the upper limit potential is converted to a lithium counter electrode and set to 4.5 V or more and less than 5.0 V, and the lower limit potential is converted to a lithium counter electrode and set to less than 4.0 V. It has been clarified that by performing such pretreatment, it is possible to manufacture a high-energy lithium ion secondary battery excellent in durability with high productivity.

The entire contents of Japanese Patent Application No. 2011-057446 (filing date: March 16, 2011) are cited herein.

Although the contents of the present invention have been described according to the embodiments and examples, the present invention is not limited to these descriptions, and it is obvious to those skilled in the art that various modifications and improvements can be made. It is.

According to the present invention, during the pretreatment of the lithium ion secondary battery containing a predetermined solid solution system positive electrode active material, charging and discharging are performed at a predetermined upper limit potential, lower limit potential and current rate. Therefore, cycle durability can be improved by short-time processing.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 10 Battery element 11 Positive electrode 11A Positive electrode collector 11B Positive electrode active material layer 12 Negative electrode 12A Negative electrode collector 12B Negative electrode active material layer 13 Electrolyte layer 14 Single battery layer 21 Positive electrode tab 22 Negative electrode tab 30 Exterior body

Claims

Preparing a lithium ion secondary battery containing a positive electrode active material represented by Chemical Formula 1,
Charging the upper limit potential of the lithium ion secondary battery as a lithium counter electrode as 4.5 V or more and less than 5.0 V, and discharging the lower limit potential as a lithium counter electrode as less than 4.0 V;
With
The pretreatment method for a lithium ion secondary battery, wherein the charging and discharging are performed at a current rate of 0.1 C to 1.3 C.
[Chemical 1]
aLi [Li 1/3 Mn 2/3 ] O 2. (1-a) LiMO 2
(A in the formula is a numerical value greater than 0 and less than 1, and LiMO 2 is a lithium composite oxide containing Ni and Mn)
The method for pretreatment of a lithium ion secondary battery according to claim 1, wherein the charging and discharging are repeated a plurality of times.
3. The method of pretreating a lithium ion secondary battery according to claim 2, wherein when the charging and discharging are repeated a plurality of times, the upper limit potential is increased stepwise.
The pretreatment method for a lithium ion secondary battery according to any one of claims 1 to 3, wherein a current rate in the charging and discharging is 0.2 C or more and 0.6 C or less.
A lithium ion secondary battery, which is processed by the pretreatment method according to any one of claims 1 to 4.