JP5383530B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium ion battery.

  A non-aqueous electrolyte secondary battery represented by a lithium ion battery has a high energy density, and thus has attracted attention as a battery for electric vehicles. The target electric vehicles include zero-emission electric vehicles that are not equipped with an engine, hybrid electric vehicles that are equipped with both an engine and a secondary battery, and plug-in hybrid electric vehicles that are directly charged from a system power source. The nonaqueous electrolyte secondary battery is also expected to be used as a stationary power storage device that stores power and supplies power in an emergency when the power system is cut off.

  Lithium ion batteries are required to have excellent durability for such various uses. That is, even when the environmental temperature is high, the rate of decrease in chargeable capacity is low and the capacity maintenance rate of the battery is high over a long period of time. In particular, a lithium ion battery for an electric vehicle has important characteristics required for storage characteristics and cycle life in a high temperature environment of 40 to 70 ° C. due to radiant heat from the road surface or heat transfer from the inside of the vehicle.

  Various techniques, such as a highly durable electrode material or electrolyte solution, are studied as a prior art for suppressing the capacity | capacitance fall or cycle deterioration at the time of high temperature storage. Under a high temperature environment, defects such as inhibition of lithium occlusion / release performance due to the growth of the coating on the negative electrode surface and poor contact between particles of the negative electrode active material are likely to occur. In particular, when the temperature is high, the diffusion rate of the electrolyte that permeates the surface coating increases, and the solvent that reaches the surface of the negative electrode active material is likely to undergo a reduction reaction. As a result, it is considered that the growth rate of the surface film is accelerated as the temperature becomes higher, which is a main factor for the decrease in battery capacity.

  For this reason, a method of forming a stable film excellent in high-temperature durability on the negative electrode surface and using it as a protective film is conceivable, and many proposals such as Patent Document 1 have been made regarding this method. . Moreover, the technique which deposits a metal on the negative electrode active material surface and improves the charging / discharging characteristic and lifetime of a negative electrode is also known, and is described in patent documents 2-4.

  For example, Patent Document 1 discloses an invention that improves high-efficiency discharge characteristics after long-term storage by using a non-aqueous electrolyte obtained by dissolving an electrolyte in a mixed solvent obtained by adding a low-boiling solvent to a cyclic carbonate. Yes. Patent Document 2 discloses that a negative electrode mainly composed of graphite contains a metal such as gold, silver, or copper to reduce the polarization of the negative electrode in a low-temperature environment, and that lithium metal is deposited on the negative electrode surface during low-temperature charging. An invention to prevent is disclosed. Patent Document 3 discloses an invention in which metal particles such as copper are supported on a carbon surface, dendrite formation is suppressed at the end of charging, and the safety of the lithium ion battery is improved. Patent Document 4 discloses an invention in which a carbon surface is coated with particles containing an alkali metal or an alkaline earth metal to improve input / output characteristics and cycle characteristics of a lithium ion battery.

JP-A-4-95362 JP-A-8-45548 JP 2000-67863 A JP 2009-16245 A

  The present inventors have conducted many experiments and researches on non-aqueous electrolyte secondary batteries including lithium ion batteries. From the viewpoint of stopping the decomposition reaction of the electrolytic solution by forming a stable film excellent in the thickness and thereby suppressing the decrease in the capacity of the battery, it has been experienced that there is still room for improvement.

The present invention has been made through the above experience, and it is an object to disclose a non-aqueous electrolyte secondary battery having superior high-temperature durability than before by stabilizing the coating film formed on the negative electrode surface. to.

  In order to solve the above problems, the present inventors further conducted experiments and research. For example, in the case of a lithium ion battery, a metal ion other than lithium ions contributing to charge / discharge is added to the electrolyte in a small amount, and the negative electrode By doping the graphite constituting the metal ion, more preferably a metal inert to lithium, the film formed on the negative electrode surface is stabilized, and the high-temperature durability is excellent due to the stabilization of the film. It has been found that a lithium ion battery can be obtained. The present invention has been made based on the above findings.

  The nonaqueous electrolyte secondary battery according to the present invention is a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing graphite, and a nonaqueous electrolyte solution containing a nonaqueous electrolyte, and contributes to charge and discharge in the vicinity of the surface of the graphite. Metal ions other than the ions to be doped are doped.

  In a preferred embodiment, the non-aqueous electrolyte secondary battery is a lithium ion battery, and the metal ions include any one of copper ions, iron ions, nickel ions, and zinc ions. More preferably, the metal ion is added to the non-aqueous electrolyte as an iodide salt. The amount of the metal ion added is 0.05 to 1 mmol / g per unit weight of graphite.

  The non-aqueous electrolyte secondary battery manufacturing method according to the present invention is also characterized in that when the metal ions are doped into the graphite, the negative electrode potential is reduced to the reduction potential using a charger, and the metal ions are doped into the graphite. It is characterized by doing. In another manufacturing method, when doping the metal ions into the graphite, a voltage is applied to the battery container and the negative electrode using a charger so that the potential of the negative electrode becomes a reduction potential. It is characterized by doping.

  According to the present invention, the capacity retention rate of the nonaqueous electrolyte secondary battery can be increased. In particular, the present invention is suitable for application to a lithium ion battery.

The figure which shows typically the internal structure of the lithium ion battery which is an example of a nonaqueous electrolyte secondary battery. The figure which shows the battery system in this invention which connected two lithium ion batteries in series.

  Hereinafter, the contents of the present invention will be described in detail with reference to the drawings.

  FIG. 1 schematically shows the internal structure of a lithium ion battery which is an example of a nonaqueous electrolyte secondary battery. Non-aqueous electrolyte secondary battery is a general term for electrochemical devices that can store and use electrical energy by occluding and releasing ions to and from the electrode in the non-aqueous electrolyte. Will be described.

  In the lithium ion battery 101 of FIG. 1, an electrode group including a positive electrode 107, a negative electrode 108, and a separator 109 inserted between both electrodes is housed in a sealed state in the battery container 102. A lid 103 is provided on the upper part of the battery container 102, and a positive electrode external terminal 104, a negative electrode external terminal 105, and a liquid inlet 106 are attached to the lid 103. The positive electrode 107 is connected to the positive electrode external terminal 104 via the positive electrode lead wire 110, and the negative electrode 108 is connected to the negative electrode external terminal 105 via the negative electrode lead wire 111. Note that the lead wires 110 and 111 can take any shape such as a wire shape or a plate shape. The lead wires 110 and 111 may have any shape and material as long as the material can reduce ohmic loss when a current is passed and the material does not react with the electrolyte. As a representative example, aluminum can be used for the positive electrode lead wire 110, and copper or nickel can be used for the negative electrode lead wire 111.

  After the electrode group is stored in the battery container 102, the cover 103 is placed on the battery container 102, and the outer periphery of the cover 103 is welded to be integrated with the battery container 102. For attachment of the lid 103 to the battery container 102, other methods such as caulking and adhesion can be employed in addition to welding.

The positive electrode 107 includes a positive electrode active material, a conductive agent, a binder, and a current collector. Illustrative examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ . In addition, LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , Li ₄ Mn ₅ O ₁₂ , LiMn _2-x M _x O ₂ (However, M = Co, Ni, Fe, Cr, Zn, Ta, x = 0.01 to 0.2), Li ₂ Mn ₃ MO ₈ (where M = Fe, Co, Ni, Cu, Zn), Li _1-x A _x Mn ₂ O ₄ (where A = Mg, B, Al, Fe, Co, Ni, Cr, Zn, Ca, x = 0.01 to 0.1), LiNi _1-x M _x O ₂ (However, M = Co, Fe, Ga, x = 0.01 to 0.2), LiFeO ₂ , Fe ₂ (SO ₄ ) ₃ , LiCo _1-x M _x O ₂ (where M = Ni, Fe, Mn, x = 0.01 to 0.2), LiNi _1-x M _x O ₂ (where M = Mn, Fe, Co, Al, Ga , Ca, Mg, x = 0.01 to 0.2), Fe (MoO ₄ ) ₃ , FeF ₃ , LiFePO ₄ , LiMnPO _{4 and the} like can be listed. In this example shown below, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ was used as the positive electrode active material. However, the present invention is not limited to the positive electrode material, and is not limited to these materials.

  The particle size of the positive electrode active material is defined to be equal to or less than the thickness of the mixture layer. When there are coarse particles having a size larger than the thickness of the mixture layer in the positive electrode active material powder, the coarse particles are removed in advance by sieving classification, wind classification or the like to produce particles having a thickness of the mixture layer or less.

  Further, since the positive electrode active material is an oxide type and has high electric resistance, it is preferable to use a conductive agent made of carbon powder for supplementing their electric conductivity. Since both the positive electrode active material and the conductive agent are powders, a binder is mixed with the powders so that the powders are bonded together and simultaneously bonded to the current collector.

  For the current collector, an aluminum foil having a thickness of 10 to 100 μm, an aluminum perforated foil having a thickness of 10 to 100 μm and a hole diameter of 0.1 to 10 mm, an expanded metal, a metal foam plate, and the like are used. In addition, stainless steel, titanium, etc. are also applicable. In the present invention, any current collector can be used without being limited by the material, shape, manufacturing method and the like.

  A positive electrode slurry in which a positive electrode active material, a conductive agent, a binder, and an organic solvent are mixed is attached to a current collector by a doctor blade method, a dipping method, a spray method, etc., and then the organic solvent is dried, and the positive electrode is removed by a roll press. A positive electrode can be produced by pressure molding. In addition, a plurality of mixture layers can be laminated on the current collector by performing a plurality of times from application to drying.

  The negative electrode 108 includes a negative electrode active material, a binder, and a current collector. The negative electrode active material is preferably a carbon material having a graphene structure. That is, natural graphite, artificial graphite, mesophase carbon, expanded graphite, carbon fiber, vapor-grown carbon fiber, pitch-based carbonaceous material, needle coke, petroleum coke capable of electrochemically occluding and releasing lithium ions, Uses carbonaceous materials such as polyacrylonitrile-based carbon fiber and carbon black, or amorphous carbon materials synthesized by thermal decomposition of 5-membered or 6-membered cyclic hydrocarbons or cyclic oxygen-containing organic compounds. Is possible.

  Natural graphite, artificial graphite, mesophase carbon, carbon fiber, pitch-based carbon and the like having a graphene structure at least on the surface are more suitable, and the (002) plane spacing in the X-ray structural analysis is 0.33 to 0.39 nm. It is desirable to include carbon in the range. In particular, the present invention works effectively for highly crystalline carbon, that is, graphite having a (002) spacing of 0.335 to 0.345 nm, or a material close thereto.

  This is because the metal ion doping effect of the present invention on the negative electrode penetrates into the interlayer near the surface of the graphene structure, and the metal ions reduce the change width of the layer spacing. When the layer interval is widened from the beginning, the expansion between the layers accompanying charging is reduced, and the change width of the layer interval tends to be small even without doping with metal ions.

  In the present invention, metal ions need only be doped in the vicinity of the surface of the negative electrode active material, and do not need to be doped to the inside of the negative electrode active material. This is because the place where the coating covering the negative electrode active material is subjected to stress during charging / discharging is the interface with the active material surface where the spacing (ie, (002) plane spacing) of the graphene layers accompanying the insertion and extraction of lithium ions changes. Because.

  A mixed negative electrode of a material such as graphite, graphitizable carbon, and non-graphitizable carbon, or a metal that forms an alloy with lithium on the carbon material may be added. If at least graphite is contained in the main component, there is no obstacle to carrying out the present invention. Any carbon material having at least a part of the graphene structure may be used. In the present invention, the negative electrode active material is not particularly limited, and materials other than those described above can be used. As described above, the present invention is particularly effective when a negative electrode mainly composed of a carbonaceous material having a graphene structure on the surface is used.

  Note that the metal or the like that forms an alloy with lithium is a metal such as aluminum, tin, or silicon and an alloy containing these elements, and is a material that can take in lithium by charging.

  Further, in order to reduce the electric resistance inside the negative electrode, addition of carbon black, metal powder that does not form an alloy with lithium, or the like does not cause any obstacle to the implementation of the present invention.

  A conductive agent may be added to the negative electrode when high rate charge / discharge is required. Since the conductive agent does not participate in occlusion / release of lithium ions and acts as an electron medium, it does not affect the occlusion / release reaction of lithium ions in the negative electrode active material of the present invention.

  Furthermore, a conductive polymer material made of polyacene, polyparaphenylene, polyaniline, or polyacetylene can also be used for the negative electrode. The present invention can be implemented by combining these materials with a carbon material having a graphene structure such as graphite, graphitizable carbon, and non-graphitizable carbon.

  As long as the negative electrode includes a carbon material having a graphene structure on at least the surface, the present invention can be applied even if other materials are used at the same time. In particular, the present invention is effective when the carbon material is graphite.

  Since the negative electrode active material generally used is a powder, a binder is mixed with the negative electrode active material so that the powders are bonded together and simultaneously bonded to the current collector. In the negative electrode of the present invention, it is desirable that the particle size of the negative electrode active material is not more than the thickness of the mixture layer. When there are coarse particles having a size larger than the thickness of the mixture layer in the negative electrode active material powder, the coarse particles are previously removed by sieving classification, wind classification or the like, and particles having a thickness of the mixture layer thickness or less are used.

  For the current collector, a copper foil having a thickness of 10 to 100 μm, a copper perforated foil having a thickness of 10 to 100 μm and a hole diameter of 0.1 to 10 mm, an expanded metal, a metal foam plate, etc. are used. In addition, stainless steel, titanium, nickel, and the like are also applicable. In the present invention, any current collector can be used without being limited by the material, shape, manufacturing method and the like.

  A negative electrode slurry in which a negative electrode active material, a binder, and an organic solvent are mixed is attached to a current collector by a doctor blade method, a dipping method, a spray method, etc., then the organic solvent is dried, and the negative electrode is pressure-formed by a roll press. By doing so, a negative electrode can also be produced. Moreover, it is also possible to form a multilayer mixture layer on the current collector by carrying out a plurality of times from application to drying.

  A separator 109 is inserted between the positive electrode 107 and the negative electrode 108 manufactured as described above, and a short circuit between the positive electrode 107 and the negative electrode 108 is prevented. It is possible to use a polyolefin polymer sheet made of polyethylene, polypropylene, or the like, or a separator 109 having a multilayer structure in which a polyolefin polymer and a fluorine polymer sheet typified by tetrafluoropolyethylene are welded. A mixture of ceramics and a binder may be formed in a thin layer on the surface of the separator 109 so that the separator 109 does not contract when the battery temperature increases. These separators 109 need to allow lithium ions to pass through when charging and discharging the battery. Generally, if the pore diameter is 0.01 to 10 μm and the porosity is 20 to 90%, the lithium ion battery 101 can be used.

  The separator 109 is also inserted between the electrode disposed at the end of the electrode group and the battery container 102 so that the positive electrode 107 and the negative electrode 108 are not short-circuited through the battery container 102. An electrolyte solution composed of an electrolyte and a non-aqueous solvent is held on the surfaces of the separator 109 and the electrodes 106 and 107 and inside the pores.

  In the lithium ion battery 101 described above, an insulating sealing material 112 is inserted between the positive electrode external terminal 104 or the negative electrode external terminal 105 and the battery container 102 so as not to short-circuit both terminals. The insulating sealing material 112 can be selected from a fluororesin, a thermosetting resin, a glass hermetic seal, and the like, and any material that does not react with the electrolytic solution and has excellent airtightness can be used.

  Further, a positive temperature coefficient (PTC; Positive) is provided in the middle of the positive electrode lead wire 110 or the negative electrode lead wire 111, or at the connection portion between the positive electrode lead wire 110 and the positive electrode external terminal 104, or at the connection portion between the negative electrode lead wire 111 and the negative electrode external terminal 105. temperature coefficient) A current interruption mechanism using a resistance element may be provided. In that case, when the temperature inside the battery becomes high, charging / discharging of the lithium ion battery 101 can be stopped to protect the battery. The lead wires 110 and 111 can have any shape such as a foil shape or a plate shape.

  The structure of the electrode group can be various shapes such as those obtained by laminating the strip-shaped electrodes shown in FIG. 1 and wound in an arbitrary shape such as a cylindrical shape or a flat shape. The shape of the battery container 102 may be selected from shapes such as a cylindrical shape, a flat oval shape, and a square shape according to the shape of the electrode group.

  The material of the battery container 102 is selected from materials that are corrosion resistant to the non-aqueous electrolyte, such as aluminum, stainless steel, and nickel-plated steel. When the battery container 102 is electrically connected to the positive electrode lead wire 110 or the negative electrode lead wire 111, the material is altered by corrosion of the battery container or alloying with lithium ions in the portion in contact with the nonaqueous electrolyte. Select the lead wire material to prevent this from occurring.

As a representative example of an electrolytic solution that can be used in the present invention, a solvent obtained by mixing dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and the like with ethylene carbonate, lithium hexafluorophosphate (LiPF ₆ ), or lithium borofluoride ( There is a solution in which LiBF ₄ ) is dissolved. The present invention is not limited to the type of solvent and electrolyte, and the mixing ratio of solvents, and other electrolytes can be used.

  The electrolyte can also be used in a state of being contained in an ion conductive polymer such as polyvinylidene fluoride and polyethylene oxide. This is the case with so-called gel electrolytes. In such a case, the separator 109 becomes unnecessary.

  Solvents that can be used for the electrolyte are propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, 1,2-dimethoxyethane, 2-methyltetrahydrofuran, dimethyl Sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, methyl propionate, ethyl propionate, phosphoric acid triester, trimethoxymethane, dioxolane, diethyl ether, sulfolane, 3-methyl-2-oxazolidinone, tetrahydrofuran, 1 There are non-aqueous solvents such as 2-diethoxyethane, chloroethylene carbonate, and chloropropylene carbonate. Other solvents may be used as long as they do not decompose on the positive electrode or the negative electrode incorporated in the battery of the present invention.

Further, the electrolyte, the chemical formula _{LiPF 6, LiBF 4, LiClO 4} , LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, or multi such imide lithium salts represented by lithium trifluoromethane sulfonimide There are different types of lithium salts. A non-aqueous electrolytic solution obtained by dissolving these salts in the above-described solvent can be used as a battery electrolytic solution. An electrolyte other than this may be used as long as it does not decompose on the positive electrode or the negative electrode incorporated in the battery of the present invention.

  When a solid polymer electrolyte (polymer electrolyte) is used, an ion conductive polymer such as ethylene oxide, acrylonitrile, polyvinylidene fluoride, methyl methacrylate, and hexafluoropropylene polyethylene oxide can be used as the electrolyte. When these solid polymer electrolytes are used, there is an advantage that the separator 109 can be omitted.

Furthermore, an ionic liquid can be used. For example, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF ₄ ), mixed complex of lithium salt LiN (SO ₂ CF ₃ ) ₂ (LiTFSI), triglyme and tetraglyme), cyclic quaternary ammonium cation (N- Select a combination that does not decompose at the positive electrode and the negative electrode from methyl-N-propylpyrrolidinium and imide anions (exemplified by bis (fluorosulfonyl) imide), and the non-aqueous electrolyte secondary battery of the present invention Can be used.

  As a method for injecting the electrolyte, there are a method in which the lid 103 is removed from the battery container 102 and added directly to the electrode group, or a method in which the lid 103 is added through the liquid injection port 106 installed in the lid 103.

  The liquid injection port 106 of the lithium ion battery shown in FIG. 1 is installed on the upper surface of the battery container 102. After the electrode group is accommodated in the battery container 102 and sealed, an electrolyte solution composed of an electrolyte and a non-aqueous solvent is dropped from the injection port 106, and after filling a predetermined amount of the electrolyte solution, the injection port 106 is sealed.

  The ion doping source of the present invention is a salt of metal ions such as copper ions, nickel ions and iron ions. These metal elements are elements that do not form an alloy with lithium in the metal state or do not substantially alloy with lithium. The metal ions that can serve as the ion doping source of the present invention are hereinafter referred to as inert metal ions.

  Here, substantially not alloyed with lithium is at least 1/10 of the charge / discharge capacity density (expressed in units of mAh / g) of a carbon material having a graphene structure on the surface. For example, it is a metal or alloy having a very low charge / discharge capacity density of 37 mAh / g or less with respect to the theoretical charge / discharge capacity density of 372 mAh / g of graphite. Since such a material has a small lithium composition, even if lithium occluded in the negative electrode due to a side reaction with the electrolytic solution is consumed, the capacity reduction of the negative electrode is not significantly affected. In addition, since the lithium composition is low, the electrolyte decomposition reaction rate on the lithium occluded alloy is slowed, and it also has a function of suppressing the growth of the coating that hinders the diffusion of lithium ions into the negative electrode active material.

  For example, iodides and iodates such as copper ions, iron ions, and nickel ions are particularly preferable for use in the present invention because they are soluble in the electrolyte. The concentration of these salts is added to the electrolyte so as to be 0.01 to 1 mmol / g with respect to the weight of graphite contained in the negative electrode. When the concentration is 0.1 to 1 mmol / g, the amount of doping between graphite layers increases, which is more desirable. As the counter ion (anion) of the inert metal ion, any chemical species other than the above-mentioned anion can be selected. The counter ion is arbitrary as long as it is difficult to decompose on the positive electrode and the negative electrode, is electrochemically stable, and can be dissolved to the above concentration.

  Not forming an alloy with lithium is not an absolute meaning to exclude the elements forming the alloy even a little. That is, even an element that forms an alloy with lithium can be used in the present invention as long as the amount deposited on the negative electrode surface is suppressed as much as possible and the element can be doped between the graphite layers. It is. For example, if the negative electrode potential is made higher than the oxidation-reduction potential of metal ions, the amount deposited on the negative electrode can be suppressed, and metal ions can be doped between graphite layers. Such an ion doping source that can be used conditionally is referred to as a quasi-inert metal ion, and these ions are also included in the inert metal ion of the present invention.

  Examples of the quasi-inert metal include iodides such as zinc ions, tin ions, and silver ions, or iodates. Since these are soluble in the electrolytic solution, they can be used in the present invention. The concentration of these salts is added to the electrolyte so as to be 0.01 to 1 mmol / g with respect to the weight of graphite contained in the negative electrode. If the concentration is lower than the concentration of the inert metal ions described above, metal deposition such as zinc can be avoided, and the capacity reduction of the negative electrode can be suppressed. When the concentration is 0.01 to 0.1 mmol / g, metal deposition can be avoided.

  The inert metal ion desirably has an ionic radius equal to or less than that of the lithium ion. Based on the literature value of the ionic radius in the aqueous solution, it can be selected from the transition metal ions in the third and fourth periods. If the ionic radius is equal to or smaller than the lithium ionic radius, it can be inserted between graphite layers to produce an intercalation compound.

  The inert metal ion dope is electrochemically inserted between the graphite layers by setting the reduction potential. An intercalation compound can be obtained by mixing graphite and the above ion doping source in an inert gas such as argon gas and utilizing a known technique such as heating.

  Instead of the non-aqueous electrolyte, a solid polymer electrolyte (polymer electrolyte) or a gel electrolyte can be used. As the solid polymer electrolyte, a known polymer electrolyte such as polyethylene oxide or a mixture (gel electrolyte) of polyvinylidene fluoride and a nonaqueous electrolytic solution can be used. Further, an ionic liquid may be used.

  It is also possible to add a safety mechanism to the inlet 106. As a safety mechanism, a pressure valve for releasing the pressure inside the battery container may be provided.

  FIG. 2 shows a battery system of the present invention in which two lithium ion batteries 201a and 201b are connected in series. Each of the lithium ion batteries 201a and 201b has an electrode group having the same specifications including a positive electrode 207, a negative electrode 208, and a separator 209, and a positive external terminal 204 and a negative external terminal 205 are provided on the upper part. An insulating seal member 212 is inserted between each external terminal and the battery container so that the external terminals are not short-circuited. In FIG. 2, the positive electrode lead wire 110 and the negative electrode lead wire 111 are omitted, but the internal structure of the lithium ion batteries 201 a and 201 b is the same as that in FIG. 1. , The numbers in the 200s are attached.

  A negative external terminal 205 of the lithium ion battery 201 a is connected to a negative input terminal of the charge controller 216 by a power cable 213. The positive external terminal 204 of the lithium ion battery 201a is connected to the negative external terminal 205 of the lithium ion battery 201b via the power cable 214. A positive external terminal 204 of the lithium ion battery 201 b is connected to a positive input terminal of the charge controller 216 by a power cable 215. With such a wiring configuration, the two lithium ion batteries 201a and 201b can be charged or discharged. Although FIG. 2 shows a series connection composed of two batteries, the number of batteries in series and the number of parallel connections are arbitrary.

  The charge / discharge controller 216 exchanges power with an externally installed device (hereinafter referred to as an external device) 219 via the power cables 217 and 218. The external device 219 includes various electric devices such as an external power source and a regenerative motor for supplying power to the charge / discharge controller 216, and an inverter, a converter, and a load that supply power from the system. An inverter or the like may be provided in accordance with the types of AC and DC that the external device supports. As these devices, known devices can be arbitrarily applied.

An electrolyte solution in which a salt of the above-described inert metal ion or quasi-inert metal ion is dissolved is prepared, and the electrolyte solution is added from the liquid injection ports 206 of the lithium ion batteries 201a and 201b. Thereafter, the liquid injection port 206 was sealed, and the system of FIG. 2 was assembled. Next, a treatment for doping the negative electrode with metal ions is performed. In order to dope the negative electrode with metal ions, the potential of the negative electrode may be made lower than the redox potential of the metal ions. The potential is controlled with reference to a reference electrode of lithium metal (−3.05 V with respect to the standard hydrogen electrode potential) in a lithium ion battery with reference to the oxidation-reduction potential in an aqueous solution. The oxidation-reduction potential E ⁰ of the metal ions listed above is a standard potential based on the above-mentioned reference electrode potential (−3.04 V) based on the numerical values described in the non-patent literature described later.

Cu ⁺ + e ⁻ → Cu E ⁰ = 3.56 V (Formula 1)
Fe ²⁺ + 2e ⁻ → Fe E ⁰ = 2.60 V (Formula 2)
Ni ²⁺ + 2e ⁻ → Ni E ⁰ = 3.28 V (Formula 3)
Zn ²⁺ + 2e ⁻ → Zn E ⁰ = 2.28 V (Formula 4)
(Non-Patent Document) Electrochemical Society, 4th edition, Electrochemical manual, pages 71-74

  The present invention is not limited to the above metals, and can be applied to the present invention as long as it is a metal ion having an oxidation-reduction potential in at least the above-listed potential range (2.2 to 3.6 V on the basis of the lithium metal reference electrode potential). Is possible. Among these, copper ions, iron ions, and nickel ions are particularly effective for extending the life of the negative electrode. Also, the valence of ions is trivalent (3+) to bivalent (2+), and bivalent to monovalent (+) cations are stably doped into the graphite layer.

Next, an ion doping method for the negative electrode will be described. First, the system of FIG. 2 is assembled. The charge / discharge controller 216 is operated and set to a voltage lower than the above-described oxidation-reduction potential (E ⁰ ). At this time, since there is no reference electrode serving as a reference, the negative electrode potential (Ea) is set to E ⁰ or less with reference to the positive electrode potential (Ec).

  The positive electrode potential (Ec) is a charge / discharge test of the positive electrode alone before the lithium ion batteries 101, 201a, 201b of the present invention are manufactured. At that time, a cell having a counter electrode made of lithium metal and a reference electrode made of lithium metal can be assembled, and the charge capacity density (Qc) and potential (Ec) of the positive electrode can be measured. Here, Qc is the amount of electricity (mAh / g) per weight of the positive electrode active material. From this test data, a functional expression (formula 5) of the positive electrode potential (Ec) with respect to the charge capacity density (Qc) of the positive electrode can be obtained.

Ec = f (Qc) (Formula 5)
Here, f represents a function having a variable in ().

  The positive electrode potential (Ec) in the actual lithium ion batteries 101, 201a, 201b is based on the charge capacity density obtained by dividing the charge electricity amount of the battery by the positive electrode active material weight (Mc) (formula 5). It is calculated from. When the positive electrode potential (Ec) is determined, the negative electrode potential (Ea) is obtained by subtracting the positive electrode potential (Ec) from the battery voltage (Eb) (Equation 6).

  Ea = Eb−Ec (Formula 6)

  Therefore, using Equation 6, it is possible to predict the negative electrode potential (Ea) from the charge capacity of the battery.

  Further, the ion doping amount (Qd) can be calculated by subtracting the irreversible capacity (Qi) from the total amount of electricity (Qt) up to the charged negative electrode potential (Ea) (Formula 7). Here, the irreversible capacity (Qi) means a charge loss capacity resulting from an electrolyte decomposition reaction on the negative electrode in the process of taking lithium ions into the negative electrode. The charge / discharge controller 216 may execute a control program based on a value obtained by converting these three values per graphite weight.

  Qd = Qt−Qi (Formula 7)

  It is known that the reaction for irreversible capacity on graphite usually proceeds at 1 V or less on the basis of a lithium metal reference electrode. Therefore, when the negative electrode potential (Ea) is held at a potential higher than 1 V, the ion doping amount (Qd) can be calculated by setting Qi to zero. In addition, the number of moles of metal ions doped in graphite is determined by assigning the ion doping amount (Qd) by the number of transferred electrons in the oxidation-reduction reaction equation shown in Equations 1 to 4 and the Faraday constant (96500C). Desired.

  Based on the concept described above, the charge / discharge controller 216 is operated, and the negative electrode 208 is doped with an inert metal ion or a quasi-inert metal ion in the graphite layer. The charging time is the time until the amount of electricity corresponding to the target ion doping amount in Equation 7 flows. More practically, it is desirable to record in the microcomputer so that the charge / discharge controller 216 is charged for a certain time.

  After the ion doping process is completed, the lithium ion batteries 201a and 201b are charged in a normal manner to obtain a rated capacity. For example, after charging at a constant current of 1 hour rate, constant voltage charging of 4.1 V or 4.2 V can be performed for 0.5 to 1 hour. Since the charging conditions are determined by the design of the material and amount of use of the lithium ion battery, the conditions are optimal for each battery specification.

  After charging the lithium ion batteries 201a and 201b, the charge / discharge controller 216 is switched to the discharge mode to discharge each battery. Normally, the discharge is stopped when a certain lower limit voltage is reached.

  During the charging / discharging operation described above, the external device 219 supplies power to the batteries 201a and 201b via the charge / discharge controller 216 during charging, and consumes power supplied from the 201a and 201b during discharging. .

  Based on the contents described above, specific examples will be shown below to clarify the effects of the present invention. In addition, you may change a concrete structural material, components, etc. in the range which does not change the summary of this invention. In addition, if the constituent elements of the present invention are included, a known technique can be added or replaced with a known technique. Further, FIG. 2 shows the battery system of the present invention in which two lithium ion batteries 201a and 201b are connected in series, but this is an example, and the non-aqueous electrolyte secondary battery is a single system. Even in a system in which three or more are connected in series, or in parallel connection, the metal ion doping effect on the negative electrode is achieved by the present invention.

(Examples 1-3)
In the lithium ion battery shown in FIG. 1, an electrolyte prepared by mixing an electrolyte LiPF ₆ to a concentration of 1 mol / liter in a solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a volume ratio of 1: 1. Prepared. The capacity of the battery is 10 Ah. To the electrolyte, copper iodide (CuI) was added to the electrolyte so as to be 0.05, 0.1, 1 mmol / g per weight of the negative electrode active material (graphite).

  The system of FIG. 2 is configured so that the battery voltage (Eb) of the lithium ion batteries 201a and 201b is in the range of 1.5 to 2.0V, that is, the negative electrode potential (Ea) is 1.5 to 1.0V. The battery was charged while controlling the current so as to be controlled, and the negative electrodes of the lithium ion batteries 201a and 201b were doped with copper ions. Subsequently, after the battery voltage reached 1.2 V at a current equivalent to an hourly rate (10 A), constant voltage charging was performed for 0.5 hour at that voltage. A 10 minute rest period was provided after the end of charging, discharging was performed at an hour rate, and the capacities of the lithium ion batteries 201a and 201b were measured.

  Furthermore, the charge / discharge controller 216 was used to perform 2000 hour charge / discharge cycles. The ambient temperature was room temperature. The initial capacity and the capacity retention after 2000 cycles are shown in Examples 1 to 3 in Table 1. As shown in Table 1, the smaller the amount of copper iodide added, the higher the initial capacity, and the higher the concentration, the slightly lower the capacity. This is presumably because when copper ions become excessive, they precipitate on the negative electrode surface and inhibit the lithium ion storage / desorption reaction.

( Reference Examples 1 to 4 )
Instead of copper iodide (CuI), iron iodide (FeI ₂ ) 0.1 mmol / g, nickel iodide (NiI ₂ ) 0.1 mmol / g, zinc iodide (ZnI ₂ ) 0.1 mmol / g and 0. Four types of lithium ion batteries added with 05 mmol / g were fabricated and incorporated into the system of FIG. Then, the initial capacity and capacity retention rate were measured in the same manner as in Examples 1 to 3. The results are shown in Reference Examples 1 to 4 in Table 1. When the addition amount of metal ions was 0.05 to 1 mmol / g, a large capacity could be maintained.

  When the addition amount of copper iodide, iron iodide, nickel iodide, and zinc iodide became 0.02 to 0.04 mmol / g or less, the capacity retention ratio began to decrease and became 74 to 77%. Conversely, when the addition amount was 2 mmol / g or more, the initial capacity started to decrease and decreased to 6 to 7 Ah.

(Comparative Example 1)
As a comparative example, a similar test was performed on a battery to which no metal salt was added. The results are shown in Comparative Example 1 in Table 1.

(Discussion)
The initial capacity was highest in the case of no addition (Comparative Example 1). This value was the same as in Example 1 when very diluted copper iodide was added (0.05 mmol / g). However, the effect of the added metal salt was manifested in the capacity retention rate after 2000 cycles. That is, as in Reference Example 1 and Reference Example 2 , when iron iodide or nickel iodide was added, the capacity retention rate after 2000 cycles increased in all cases.

In the case of zinc iodide, the capacity retention rate was low at a high concentration of 0.1 mmol / g ( Reference Example 3 ). This is because zinc was deposited, an alloy with lithium was formed, and reaction products with the electrolyte increased on the negative electrode. However, as shown in Reference Example 4, when the zinc iodide concentration was reduced, a capacity retention rate equivalent to that of other ion doping sources was obtained.

  In addition, after this test, the battery doped with metal ions was disassembled and the negative electrode was taken out, and then X-ray photoelectron spectroscopy measurement was performed. As a result, it was confirmed that copper ions, iron ions, zinc ions, and nickel ions were present on the surface layer of the graphite particles.

  As described above, it can be seen that the capacity retention rate of the lithium ion battery can be increased by applying the method of the present invention.

101 non-aqueous electrolyte secondary battery (lithium ion battery), 102 battery container, 103 lid, 104 positive external terminal, 105 negative external terminal, 106 injection port, 107 positive electrode, 108 negative electrode, 109 separator, 110 positive electrode lead wire, 111 Negative electrode lead wire, 112 Insulating sealing material, 201a Nonaqueous electrolyte secondary battery, 201b Nonaqueous electrolyte secondary battery, 202 Battery container, 204 Positive electrode external terminal, 205 Negative electrode external terminal, 206 Injection port, 207 Positive electrode, 208 Negative electrode , 209 Separator, 212 Insulating seal material, 213 Power cable, 214 Power cable, 215 Power cable, 216 Charge / discharge controller, 217 Power cable, 218 Power cable, 219 External equipment

Claims (1)

In a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing graphite, and a non-aqueous electrolyte containing a non-aqueous electrolyte,
Wherein a lithium ion battery nonaqueous electrolyte secondary battery, the nonaqueous copper iodide in the electrolytic liquid is added in amount per unit weight 0.05 to 1 mmol / g of graphite, copper near the surface of the graphite A non-aqueous electrolyte secondary battery characterized by being doped with ions .

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