JP5482582B2 - Negative electrode active material - Google Patents

Description

  The present invention relates to a negative electrode active material. More specifically, the present invention relates to an improvement for achieving a long life of a lithium ion secondary battery.

  In recent years, in order to cope with global warming, reduction of the amount of carbon dioxide is eagerly desired. In the automobile industry, there is a great expectation for reducing carbon dioxide emissions by introducing electric vehicles (EV) and hybrid electric vehicles (HEV), and the development of secondary batteries for motor drive that holds the key to commercialization of these is thriving. Has been done.

  As a secondary battery for driving a motor, it is required to exhibit extremely high output characteristics and high energy density as compared with a consumer-use lithium ion secondary battery used in a mobile phone, a notebook computer, or the like. Therefore, lithium ion secondary batteries having the highest theoretical energy among all the batteries are attracting attention, and are currently being developed rapidly.

  Generally, a lithium ion secondary battery includes a positive electrode in which a positive electrode active material or the like is applied to the surface of a current collector, and a negative electrode in which a negative electrode active material or the like is applied to the surface of the current collector through an electrolyte layer containing an electrolytic solution. Connected and housed in a battery case.

  The electrolyte solution contained in the electrolyte layer is generally formed by dissolving a lithium salt in an aprotic polar solvent (hereinafter also simply referred to as “solvent”), and the solvent functions as a lithium ion transfer medium. . Since the electrostatic attractive force between positive and negative ions is inversely proportional to the relative dielectric constant of the medium, it is preferable that the solvent has a higher dielectric constant from the viewpoint of promoting ion dissociation. On the other hand, since the mobility of ions is inversely proportional to the viscosity of the solvent, it is preferable that the viscosity of the solvent is low. However, since it is difficult to achieve a high dielectric constant and low viscosity with a single solvent, a mixed solvent of a cyclic carbonate that is a high dielectric constant solvent and a chain carbonate that is a low viscosity solvent is usually used. (Hereinafter, “carbonate ester” is also referred to as carbonate).

  However, it is known that these carbonate-based solvents undergo reductive decomposition on the surface of the negative electrode active material during charging, thereby reducing cycle characteristics. For this reason, the technique for suppressing decomposition | disassembly of a carbonate type solvent is searched.

  For example, Patent Document 1 proposes a method of adding vinylene carbonate to an electrolytic solution containing propylene carbonate, which is a cyclic carbonate, and chain carbonate. Vinylene carbonate has a double bond at the C—C bond portion of ethylene carbonate, and is polymerized like electrolytic polymerization at the time of initial charge to form a solid electrolyte film (Solid Electrolyte Interface: SEI) on the surface of the negative electrode active material. . According to the document, the reductive decomposition of propylene carbonate can be suppressed by the formation of SEI.

JP-A-11-67266

  However, among carbonate solvents, chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate generate gases such as methane and ethane by reductive decomposition. Generation of such gas may cause expansion of the battery, deterioration of cycle characteristics, and the like, and may shorten the battery life. In the technology of Patent Document 1, since the generation of such a solvent-derived gas could not be sufficiently prevented during initial charging, further improvement has been desired.

  In view of the above, an object of the present invention is to provide a negative electrode active material capable of suppressing the generation of gas that occurs when a solvent in an electrolyte solution of a lithium ion secondary battery is reductively decomposed.

  The negative electrode active material of the present invention is formed by coating at least part of the surface of the negative electrode active material main body with a polymer having a carbonate structure represented by the following chemical formula 1;

In the formula, n represents an integer of 2 to 100, each R ¹ independently represents an alkyl group having 1 to 10 carbon atoms, and each R ² independently represents an alkylene group having 2 to 5 carbon atoms. .

  According to the negative electrode active material of the present invention, the polymer having a carbonate structure that covers the surface of the negative electrode active material main body is reductively decomposed before the solvent at the time of initial charging. A polymer having a carbonate structure forms SEI with almost no gas even when reductively decomposed. The formation of SEI suppresses further reductive decomposition of the solvent. Therefore, it is possible to effectively suppress the generation of gas caused by the reductive decomposition of the solvent in the electrolyte solution of the lithium ion secondary battery.

It is the cross-sectional schematic which represented typically the negative electrode active material material which concerns on one Embodiment of this invention. 1 is a schematic cross-sectional view schematically showing the overall structure of a stacked lithium ion secondary battery according to an embodiment of the present invention.

  Hereinafter, preferred modes of the present invention will be described. However, the technical scope of the present invention should be determined by the description of the scope of claims, and is not limited to the following modes. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may be different from the actual ratios.

<Negative electrode active material>
FIG. 1 is a cross-sectional view schematically showing a negative electrode active material according to an embodiment of the present invention. In the negative electrode active material 1 of FIG. 1, the entire surface of graphite 3 as a negative electrode active material is coated with a polymer 5 having a carbonate structure. Hereinafter, each member which comprises the negative electrode active material of this form is demonstrated.

[Negative electrode active material]
The negative electrode active material has a composition capable of releasing ions during discharge and storing ions during charging. In this specification, in order to distinguish the negative electrode active material from a negative electrode active material coated with a polymer having a carbonate structure (hereinafter also abbreviated as “carbonate polymer”), “negative electrode active material” May be referred to as a “negative electrode active material body”.

The negative electrode active material is not particularly limited as long as it can reversibly occlude and release lithium. Examples of the negative electrode active material include metals such as Si and Sn, TiO, Ti ₂ O ₃ , TiO ₂ , or Metal oxides such as SiO ₂ , SiO, SnO ₂ , complex oxides of lithium and transition metals such as Li _4/3 Ti _5/3 O ₄ or Li ₇ MnN, Li—Pb alloys, Li—Al alloys , Li, or carbon materials such as graphite (graphite), carbon black, activated carbon, carbon fiber, coke, soft carbon, or hard carbon. Moreover, it is preferable that a negative electrode active material contains the element alloyed with lithium. By using an element that forms an alloy with lithium, it is possible to obtain a battery having a high capacity and an excellent output characteristic having a higher energy density than that of a conventional carbon-based material. The negative electrode active material may be used alone or in the form of a mixture of two or more.

  The element alloying with lithium is not limited to the following, but specifically, 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. Among these, from the viewpoint that a battery excellent in capacity and energy density can be configured, at least one selected from the group consisting of carbon materials and / or Si, Ge, Sn, Pb, Al, In, and Zn. It is preferable that a carbon material, Si, or Sn element is included. These may be used individually by 1 type and may use 2 or more types together.

  The average particle size of the negative electrode active material is not particularly limited, but is preferably 1 to 100 μm, more preferably 1 to 20 μm from the viewpoint of increasing the capacity, reactivity, and cycle durability of the negative electrode active material. Within such a range, the secondary battery can suppress an increase in the internal resistance of the battery during charging and discharging under high output conditions, and can extract a sufficient current. In addition, when the negative electrode active material is secondary particles, it can be said that the average particle diameter of primary particles constituting the secondary particles is desirably in the range of 10 nm to 1 μm. It is not limited to. However, it goes without saying that, depending on the manufacturing method, the negative electrode active material may not be a secondary particle formed by aggregation, lump or the like. As the particle size of the negative electrode active material and the particle size of the primary particles, the median diameter obtained using a laser diffraction method can be used. The shape of the negative electrode active material varies depending on the type and manufacturing method, and examples thereof include a spherical shape (powdered shape), a plate shape, a needle shape, a column shape, and a square shape, but are not limited thereto. Any shape can be used without any problems. Preferably, an optimal shape that can improve battery characteristics such as charge / discharge characteristics is appropriately selected.

[Polymer having carbonate structure]
The polymer having a carbonate structure (carbonate polymer) in this embodiment has a function of suppressing reductive decomposition of the solvent in the electrolytic solution on the surface of the negative electrode active material.

  The carbonate polymer has a structure represented by the following chemical formula 1.

In the formula, n represents an integer of 2 to 100, each R ¹ independently represents an alkyl group having 1 to 10 carbon atoms, and each R ² independently represents an alkylene group having 2 to 5 carbon atoms. . In this case, the plurality of R ¹ or R ² may be the same or different.

The alkyl group having 1 to 10 carbon atoms represented by R ¹ is not particularly limited. For example, a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, sec- Butyl group, tert-butyl group, n-pentyl group, isopentyl group, neopentyl group, 1,2-dimethylpropyl group, n-hexyl group, cyclohexyl group, 1,3-dimethylbutyl group, 1-isopropylpropyl group, 1 , 2-dimethylbutyl group, n-heptyl group, 1,4-dimethylpentyl group, 2-methyl-1-isopropylpropyl group, 1-ethyl-3-methylbutyl group, n-octyl group, 2-ethylhexyl group, n -Nonyl group, n-decyl group, etc. are mentioned. Among these, R ¹ is preferably an alkyl group having 1 to 4 carbon atoms, and more preferably an alkyl group having 3 or 4 carbon atoms having a larger steric hindrance.

The alkylene group having 2 to 5 carbon atoms represented by R ² is not particularly limited, and examples thereof include an ethylene group, trimethylene group, methylethylene group, tetramethylene group, 1,2-dimethylethylene group, and pentane. A methylene group etc. are mentioned. Among these, R ² is preferably an alkylene group having 2 or 3 carbon atoms, and more preferably an ethylene group. In addition, these carbonate polymers may be used individually by 1 type, and 2 or more types may be used in combination.

  The carbonate polymer represented by the chemical formula 1 can be produced, for example, by a reaction represented by the following chemical formula 2.

In Formula 2, R ¹ in Chemical Formula 1 is an isopropyl group, synthesis examples of the carbonate polymer when R ² is an ethylene group.

As shown in Chemical Formula 2, first, ethylene glycol (HO—CH ₂ CH ₂ —OH) is contacted with carbon monoxide (CO) and oxygen (O ₂ ) in the presence of a metal catalyst such as Pd. Thereby, ethylene glycol is polymerized via a carbonate bond, and water is by-produced accordingly. Then, after removing the catalyst from the obtained reaction solution by filtration, water produced as a by-product from the filtrate is removed by distillation or the like to obtain a polymer having a carbonate skeleton having a hydroxy group (—OH) at the terminal.

  Next, the polymer, isopropyl alcohol (i-PrOH), carbon monoxide, and oxygen are contacted in the presence of a metal catalyst such as Pd as in the above reaction. By this reaction, an i-Pr group is bonded to the end of the polymer via a carbonate group. And after removing a catalyst from the obtained reaction solution by filtration, the target carbonate polymer can be obtained by removing by-product water from the filtrate by distillation or the like. In addition, there is no restriction | limiting in particular in the addition amount of each raw material used for reaction, reaction conditions, etc., A well-known method can be referred suitably.

  The carbonate polymer that covers the surface of the negative electrode active material undergoes reductive decomposition during initial charging and becomes SEI. In general, the SEI is preferably thin and dense on the surface of the negative electrode active material, and is preferably formed in a short time. Therefore, it is preferable that the surface of the negative electrode active material is uniformly covered with a necessary amount of the carbonate polymer.

  Specifically, the amount of the carbonate polymer covering the negative electrode active material is preferably 1 to 10% by mass with respect to the total mass of the negative electrode active material. Generation | occurrence | production of gas can be effectively prevented as the coating amount is 1 mass% or more. On the other hand, when the coating amount is 10% by mass or less, an increase in battery resistance can be prevented.

  Further, from the viewpoint of effectively preventing gas generation, it is preferable that the carbonate polymer covers a wider range of the surface area of the negative electrode active material body. Specifically, the area covered by the carbonate polymer is preferably 70 to 100%, more preferably 90 to 100%, and still more preferably 95 to 100% with respect to the surface area of the negative electrode active material.

  The negative electrode active material of this embodiment is produced by coating the negative electrode active material with a carbonate polymer. The coating method is not particularly limited. For example, a carbonate polymer solution prepared by diluting a carbonate polymer with an appropriate solvent is prepared, and after mixing the carbonate polymer solution and the negative electrode active material, an excess carbonate weight is prepared. It can be obtained by removing the combined solution and drying. The coating amount of the carbonate polymer on the negative electrode active material can be adjusted by repeatedly performing the concentration of the carbonate polymer solution and the above coating operation.

In the negative electrode active material of this embodiment, at least a part of the surface of the negative electrode active material is coated with a carbonate polymer, so that decomposition of the solvent constituting the electrolytic solution can be suppressed and gas generation can be prevented. . When a conventional negative electrode active material not coated with a carbonate polymer is used, as shown in the following chemical formula 3, the solvent is reduced and decomposed on the surface of the negative electrode active material during initial charging, and the R—O bond is cleaved. ^* And ROCOOLi are formed. Since ROCOOLi is insoluble in the solvent, SEI is formed on the surface of the negative electrode active material. ROCOOOLi can be further reduced and decomposed to form R ^* and LiOCOOLi (lithium carbonate). Since lithium carbonate is also insoluble in the solvent, SEI is formed on the surface of the negative electrode active material. Further, R ^* generated by reductive decomposition of the solvent reacts with hydrogen (H ₂ ) in the system to generate RH, or two R ^{* s} react to generate RR. Since RH or RR is a gas, the expansion of the battery can occur due to these gases.

On the other hand, when the negative electrode active material of this embodiment is used, as shown in the following chemical formula 4, the carbonate polymer coated on the surface of the negative electrode active material during initial charging is reduced and decomposed before the solvent. At this time, the R ¹ —O bond or the R ² —O bond may be cleaved, but the number of R ² —O bonds is larger than the R ¹ —O bond in one molecule of the carbonate polymer. The probability of cleavage of the R ² —O bond is higher than that of the terminal R ¹ —O bond, and the R ¹ —O bond is hardly cleaved. When the R ² —O bond is cleaved, a lithium salt represented by A in Chemical Formula 4 and a carbonate polymer represented by B in Chemical Formula 4 are produced. Even if the carbonate polymer represented by B in Chemical Formula 4 reacts with hydrogen or the like, it does not turn into a gas because of its large molecular weight, and thus can prevent battery expansion. In addition, since deterioration due to reductive decomposition of the solvent can be prevented, deterioration of cycle characteristics can also be suppressed. In addition, once the surface of the negative electrode active material is covered with SEI, the solvent is not reduced and decomposed on the surface of the negative electrode active material, so that the generation of gas can be suppressed and the deterioration of cycle characteristics can be prevented even after the initial charge. it can.

<Lithium ion secondary battery>
Next, the lithium ion secondary battery of this embodiment will be described. The lithium ion secondary battery of this embodiment includes the above-described negative electrode active material.

  FIG. 2 is a schematic cross-sectional view schematically showing the overall structure of a stacked lithium ion secondary battery according to an embodiment of the present invention. As shown in FIG. 2, in the lithium ion secondary battery 10 of the present embodiment, a substantially rectangular power generation element 17 in which a charge / discharge reaction actually proceeds is sealed inside a laminate film 22 that is a battery exterior material. It has a structure. Specifically, the power generation element 17 is housed and sealed by using a polymer-metal composite laminate film as a battery exterior material and joining all of its peripheral parts by thermal fusion.

  The power generation element 17 includes a negative electrode in which the negative electrode active material layer 12 is disposed on both sides of the negative electrode current collector 11 (only one side for the lowermost layer and the uppermost layer of the power generation element), an electrolyte layer 13, and a positive electrode current collector 14. And a positive electrode in which the positive electrode active material layer 15 is disposed on both sides. Specifically, the negative electrode, the electrolyte layer 13 and the positive electrode are laminated in this order so that one negative electrode active material layer 12 and the positive electrode active material layer 15 adjacent to the negative electrode active material layer 15 face each other with the electrolyte layer 13 therebetween. Yes.

  Thus, the adjacent negative electrode, electrolyte layer 13 and positive electrode constitute one single cell layer 16. Therefore, it can be said that the lithium ion secondary battery 10 of the present embodiment has a configuration in which a plurality of single battery layers 16 are stacked and electrically connected in parallel. Further, a seal portion (insulating layer) (not shown) for insulating between the adjacent negative electrode current collector 11 and the positive electrode current collector 14 may be provided on the outer periphery of the unit cell layer 16. . The negative electrode active material layer 12 is disposed only on one side of the outermost layer negative electrode current collector 11 a located in both outermost layers of the power generation element 17. Note that the arrangement of the negative electrode and the positive electrode is reversed from that in FIG. 2 so that the outermost positive electrode current collector is positioned in both outermost layers of the power generation element 17, and the positive electrode is formed on only one side of the outermost layer positive electrode current collector. An active material layer may be arranged.

  The negative electrode current collector 11 and the positive electrode current collector 14 are attached with a negative electrode current collector plate 18 and a positive electrode current collector plate 19 that are electrically connected to the respective electrodes (negative electrode and positive electrode), and are sandwiched between ends of the laminate film 22. Thus, it has a structure led out of the laminate film 22. The negative electrode current collector 18 and the positive electrode current collector 19 are connected to the negative electrode current collector 11 and the positive electrode current collector 14 of each electrode by ultrasonic welding or resistance via a negative electrode terminal lead 20 and a positive electrode terminal lead 21 as necessary. It may be attached by welding or the like (this form is shown in FIG. 2). However, the negative electrode current collector 11 may be extended to form the negative electrode current collector plate 18 and may be led out from the laminate film 22. Similarly, the positive electrode current collector 14 may be extended to form a positive electrode current collector plate 19, which may be similarly derived from the battery exterior material 22.

  Another example of the lithium ion secondary battery is a bipolar lithium ion secondary battery. A bipolar lithium ion secondary battery has a bipolar electrode in which a positive electrode active material layer is formed on one surface of a current collector and a negative electrode active material layer is formed on the other surface of the current collector, with an electrolyte layer interposed therebetween. The power generation element is laminated. With respect to the constituent requirements and the manufacturing method of the lithium ion secondary battery 10 and the bipolar lithium ion secondary battery, basically, except that the electrical connection form (electrode structure) in both batteries is different. Is the same. Therefore, it demonstrates below centering on each component of the lithium ion secondary battery 10 mentioned above. However, it goes without saying that the constituent elements and the manufacturing method of the bipolar lithium ion secondary battery can be configured or manufactured by appropriately using the same constituent elements and manufacturing method.

  Hereinafter, although the member which comprises the lithium ion secondary battery of this form is demonstrated easily, it is not restrict | limited only to the following form, A conventionally well-known form may be employ | adopted similarly.

(Current collector)
The current collector is made of a conductive material, and active material layers are arranged on both sides thereof to constitute a battery electrode.

  There is no particular limitation on the material constituting 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, a clad material of nickel and aluminum, a clad material of copper and aluminum, or a plating material combining these metals can be preferably used. Moreover, the foil by which aluminum is coat | covered on the metal surface may be sufficient. Of these, aluminum, stainless steel, and copper are preferable from the viewpoints of electronic conductivity and battery operating potential.

  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.

  Examples of non-conductive polymer materials include polyethylene (PE; high density polyethylene (HDPE), low density polyethylene (LDPE)), 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), Examples include polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), and polystyrene (PS). Such a non-conductive polymer material may have excellent potential resistance or solvent resistance.

  A conductive filler may be added to the conductive polymer material or the non-conductive polymer material as necessary. In particular, when the resin used as the base material of the current collector is made of only a non-conductive polymer, a conductive filler is inevitably necessary 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, metals, conductive carbon, etc. are mentioned as a material excellent in electroconductivity, electric potential resistance, or lithium ion barrier | blocking property. The metal is not particularly limited, but 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 It is preferable to contain an alloy or metal oxide containing. The conductive carbon is not particularly limited, but is at least selected from the group consisting of acetylene black, vulcan, black pearl, carbon nanofiber, ketjen black, carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene. It is preferable that 1 type is included. 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.

  The size of the current collector is 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. Although there is no restriction | limiting in particular also about the thickness of a collector, Usually, it is about 1-100 micrometers.

(Positive electrode active material layer)
The positive electrode active material layer includes a positive electrode active material. The positive electrode active material has a composition that occludes ions during discharging and releases ions during charging. A preferable example is a lithium-transition metal composite oxide that is a composite oxide of a transition metal and lithium. Specifically, Li · Co-based composite oxide such as LiCoO _2, Li · Ni-based composite oxide such as LiNiO _2, Li · Mn-based composite oxide such as spinel LiMn ₂ O _4, Li · such LiFeO ₂ Fe-based composite oxides and those obtained by replacing some of these transition metals with other elements can be used. These lithium-transition metal composite oxides are excellent in reactivity and cycle characteristics, and are low-cost materials. Therefore, it is possible to form a battery having excellent output characteristics by using these materials for electrodes. In addition, examples of the positive electrode active material include transition metal oxides such as LiFePO ₄ and lithium phosphate compounds and sulfuric acid compounds; transition metal oxides such as V ₂ O ₅ , MnO ₂ , TiS ₂ , MoS ₂ , and MoO ₃ , and sulfides. Materials; PbO ₂ , AgO, NiOOH, etc. can also be used. The positive electrode active material may be used alone or in the form of a mixture of two or more.

  In addition, the particle diameter and shape of the positive electrode active material are not particularly limited, and can take the same form as the above-described negative electrode active material, and thus detailed description thereof is omitted here.

  The active material layer may contain other materials if necessary. For example, a conductive aid, a binder, and the like can be included. When an ion conductive polymer is included, a polymerization initiator for polymerizing the polymer may be included.

  A conductive support agent means the additive mix | blended in order to improve the electroconductivity of an active material layer. Examples of the conductive assistant include carbon powders such as acetylene black, carbon black, ketjen black, and graphite, various carbon fibers such as vapor grown carbon fiber (VGCF; registered trademark), expanded graphite, and the like. However, it goes without saying that the conductive aid is not limited to these.

  Examples of the binder include polyvinylidene fluoride (PVdF), polyimide, PTFE, SBR, and a synthetic rubber binder. However, it goes without saying that the binder is not limited to these. When the binder and the matrix polymer used as the gel electrolyte are the same, it is not necessary to use a binder.

  The compounding ratio of the components contained in the active material layer is not particularly limited. The blending ratio can be adjusted by appropriately referring to known knowledge about the lithium ion secondary battery. The thickness of the active material layer is not particularly limited, and conventionally known knowledge about the lithium ion secondary battery can be appropriately referred to. As an example, the thickness of the active material layer is preferably about 10 to 100 μm, and more preferably 20 to 50 μm. If the active material layer is about 10 μm or more, the battery capacity can be sufficiently secured. On the other hand, if the active material layer is about 100 μm or less, it is possible to suppress the occurrence of the problem of an increase in internal resistance due to the difficulty in diffusing lithium ions in the electrode deep part (current collector side).

  The method for forming the positive electrode active material layer (or the negative electrode active material layer) on the current collector surface is not particularly limited, and known methods can be used in the same manner. For example, as described above, a positive electrode active material (or a negative electrode active material), and if necessary, an electrolyte salt for increasing ionic conductivity, a conductive auxiliary agent for increasing electronic conductivity, and a binder are appropriately used. A positive electrode active material slurry (or a negative electrode active material slurry) is prepared by dispersing or dissolving in an appropriate solvent. This is applied onto a current collector, dried to remove the solvent, and then pressed to form a positive electrode active material layer (or negative electrode active material layer) on the current collector. In this case, the solvent is not particularly limited, and N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methylformamide, cyclohexane, hexane and the like can be used. When adopting polyvinylidene fluoride (PVdF) as a binder, NMP is preferably used as a solvent.

  In the above method, the positive electrode active material slurry (or the negative electrode active material material slurry) is applied onto the current collector, dried, and then pressed. At this time, the porosity of the positive electrode active material layer (or the negative electrode active material layer) can be controlled by adjusting the pressing conditions.

  Specific means and press conditions for the press treatment are not particularly limited, and can be appropriately adjusted so that the porosity of the positive electrode active material layer (or the negative electrode active material layer) after the press treatment becomes a desired value. Specific examples of the press process include a hot press machine and a calendar roll press machine. Also, the pressing conditions (temperature, pressure, etc.) are not particularly limited, and conventionally known knowledge can be referred to as appropriate.

[Electrolyte layer]
The electrolyte layer functions as a medium when lithium ions move between the electrodes. There is no restriction | limiting in particular in the electrolyte which comprises an electrolyte layer, A liquid electrolyte (electrolytic solution), a polymer gel electrolyte, etc. can be used suitably.

  The liquid electrolyte is obtained by dissolving a lithium salt as a supporting salt in a solvent. The solvent is not particularly limited. For example, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC) ), Etc .; cyclic carbonate solvents such as ethylene carbonate (EC) and propylene carbonate (PC); methyl propionate (MP), ethyl propionate (EP), methyl acetate (MA), ethyl acetate ( And ester solvents such as EA). Since these carbonate-based solvents or ester-based solvents generate a significant amount of gas due to reductive decomposition on the surface of the negative electrode active material, the use of the negative electrode active material of this embodiment when using these solvents is even more prominent. Effects can be obtained. These solvents may be used alone or as a mixture of two or more.

  In addition, additives such as vinylene carbonate (VC), propane sultone (PS), fluoroethylene carbonate, phosphazene, or lithium bisoxalate borate (LiBOB) may be added to the liquid electrolyte as necessary. These additives may be used individually by 1 type, and may use 2 or more types together.

The supporting salt (lithium salt) is not particularly _{limited, LiPF 6, LiBF 4, LiClO} 4, LiAsF 6, LiTaF 6, LiSbF 6, LiAlCl 4, Li 2 B 10 Cl 10, LiI, LiBr, LiCl, LiAlCl , LiHF ₂ , LiSCN, and other inorganic acid anion salts, LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiBOB (lithium bisoxalate borate), LiBETI (lithium bis (perfluoroethylenesulfonylimide); Li Organic acid anion salts such as (also described as (C ₂ F ₅ SO ₂ ) ₂ N). These electrolyte salts may be used alone or in the form of a mixture of two or more.

  On the other hand, the polymer gel electrolyte has a configuration in which the above liquid electrolyte is injected into a matrix polymer having lithium ion conductivity. Examples of the matrix polymer having lithium ion conductivity include a polymer having polyethylene oxide in the main chain or side chain (PEO), a polymer having polypropylene oxide in the main chain or side chain (PPO), polyethylene glycol (PEG), poly Acrylonitrile (PAN), polymethacrylic acid ester, polyvinylidene fluoride (PVdF), copolymer of polyvinylidene fluoride and hexafluoropropylene (PVdF-HFP), polyacrylonitrile (PAN), poly (methyl acrylate) (PMA), poly (Methyl methacrylate) (PMMA) etc. are mentioned. In addition, mixtures such as the above polymers, modified products, derivatives, random copolymers, alternating copolymers, graft copolymers, block copolymers, and the like can also be used. Among these, it is desirable to use PEO, PPO and their copolymers, PVdF, PVdF-HFP. In such a matrix polymer, a lithium salt can be well dissolved. In addition, the matrix polymer can exhibit excellent mechanical strength by forming a crosslinked structure.

  In addition, when an electrolyte layer is comprised from a liquid electrolyte or a gel electrolyte, you may use a separator for an electrolyte layer. Specific examples of the separator include a microporous film made of polyolefin such as polyethylene and polypropylene, hydrocarbon such as polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), glass fiber, and the like.

[Current collector]
In a lithium ion secondary battery, for the purpose of extracting current outside the battery, a current collector plate (a positive current collector plate and a negative current collector plate) electrically connected to a current collector is external to the laminate film. Has been taken out.

  The material which comprises a current collector plate is not specifically limited, The well-known highly electroconductive material conventionally used as a current collector plate for lithium ion secondary batteries can be used. As a constituent material of the current collector plate, for example, metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable. Note that the same material may be used for the positive electrode current collector plate and the negative electrode current collector plate, or different materials may be used.

[Positive terminal and negative terminal lead]
In the lithium ion secondary battery 10 shown in FIG. 2, the current collector is electrically connected to the current collector plate via the negative electrode terminal lead 20 and the positive electrode terminal lead 21.

  As a material of the positive electrode and the negative electrode terminal lead, a lead used in a known lithium ion secondary battery can be used. In addition, the parts removed from the battery exterior material must be heat-insulating so that they do not affect products (for example, automobile parts, especially electronic devices) by touching peripheral devices or wiring and causing leakage. It is preferable to coat with a heat shrinkable tube or the like.

[Exterior material]
A conventionally known metal can case can be used as the exterior material. In addition, the power generation element 17 may be packed using a laminate film 22 as shown in FIG. The laminate film can be configured as a three-layer structure in which, for example, polypropylene, aluminum, and nylon are laminated in this order.

  In the lithium ion secondary battery of this embodiment, generation of gas due to reductive decomposition of the solvent contained in the electrolyte is suppressed by using the negative electrode active material in the negative electrode active material layer. Therefore, expansion of the battery after sealing and deterioration of cycle characteristics due to decomposition of the solvent can be effectively prevented, and the life of the battery can be extended.

  The effect of this invention is demonstrated using a following example and a comparative example. However, the technical scope of the present invention is not limited only to the following examples.

<Production of lithium ion secondary battery>
[Example 1]
(Synthesis of carbonate polymer)
Ethylene glycol (110 mmol) and a Pd / Cu catalyst (catalytic amount) were charged into a 1 L SUS stirred autoclave. While stirring these at 150 ° C., carbon monoxide (120 mmol) and oxygen (60 mmol) were added equimolarly while adjusting the pressure in the autoclave to 0.5 MPa or less using a dropping cylinder. . After the entire amount of carbon monoxide and oxygen was added and stirred for 10 hours, the temperature was lowered to 25 ° C., the unreacted gas in the autoclave was removed, and the reaction solution was taken out. Then, the reaction solution was distilled to remove moisture, thereby obtaining a polymer having a carbonate skeleton having a hydroxyl group (—OH) at the end.

Next, the polymer, methanol (24 mmol), and Pd / Cu catalyst (catalytic amount) were charged into a stirring autoclave. And as above, while stirring at 150 ° C., carbon monoxide (24 mmol) and oxygen (12 mmol) were adjusted using a dropping cylinder so that the pressure in the autoclave was 0.5 MPa or less. Mole was added. After completion of the reaction, the Pd / Cu catalyst is removed by filtration, and the filtrate is distilled to remove water to remove the carbonate polymer (in Chemical Formula 1, R ¹ is a methyl group and R ² is an ethylene group). Got. The number average molecular weight of the carbonate was 1000 (average polymerization degree n = 10). The number average molecular weight was measured by GPC (gel permeation chromatography) using polystyrene as a standard substance.

(Preparation of negative electrode active material)
A carbonate polymer solution was prepared by diluting 1 part by mass of the above carbonate polymer with 10 parts by mass of acetonitrile. 50 g of graphite (average particle size: 10 μm) as a negative electrode active material and 100 g of the above polycarbonate solution were mixed. The powder component in the mixture was recovered by filtration, and the carbonate polymer was coated on the graphite surface by removing excess carbonate polymer and acetonitrile adhering to the powder component. The mass of the obtained powder component was measured, and the above coating operation was repeated until 5 parts by mass of the carbonate polymer was coated on 95 parts by mass of graphite before coating. Then, the negative electrode active material was obtained by vacuum-drying the powder component at 80 ° C. for 8 hours.

(Preparation of negative electrode)
A solid content composed of 90% by mass of the negative electrode active material and 10% by mass of polyvinylidene fluoride (PVdF) as a binder was prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP), which is a slurry viscosity adjusting solvent, was added to the solid content to prepare a negative electrode active material slurry. The obtained negative electrode active material slurry was applied to one surface of a copper foil (thickness: 10 μm) as a negative electrode current collector, dried, and then pressed. This produced the negative electrode (outermost layer negative electrode) which has a negative electrode active material layer in the one surface of a collector. Separately, after applying and drying the negative electrode active material slurry on one surface of the copper foil, the negative electrode active material slurry is also applied to the other surface and dried, followed by press treatment, A negative electrode having a negative electrode active material layer on both surfaces was prepared.

(Preparation of positive electrode)
A solid content of 85% by mass of LiMnO ₂ (average particle size: 10 μm) as a positive electrode active material, 5% by mass of acetylene black (average particle size: 0.1 μm) as a conductive additive, and 10% by mass of PVdF as a binder was prepared. An appropriate amount of NMP, which is a slurry viscosity adjusting solvent, was added to the solid content to prepare a positive electrode active material slurry. The obtained positive electrode active material slurry was applied to one side of an aluminum foil (thickness: 10 μm) as a positive electrode current collector and dried, and then applied to the other side and dried, followed by press treatment. To produce a positive electrode having a positive electrode active material layer on both sides.

(Preparation of electrolyte)
As a solvent, a mixed solvent (40:60 (volume ratio)) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was prepared. To this solvent, LiPF ₆ as a lithium salt was added at a concentration of 1.0 M to prepare an electrolytic solution.

(Battery assembly)
The positive electrode and the negative electrode prepared above are arranged so that the active material layers face each other through a separator (polyethylene microporous film, thickness: 25 μm), the outermost layer negative electrode, separator, positive electrode, separator, negative electrode... Positive electrode, The separator and the outermost layer negative electrode were laminated in this order. As a result, a laminated body (5 positive electrodes, 4 negative electrodes, 2 outermost negative electrodes) in which 10 unit cell layers were laminated was obtained.

  A current collector plate was welded to each of the positive electrode and the negative electrode of the obtained laminate, and sealed in an exterior made of an aluminum laminate film. After drying this, the electrolytic solution prepared above was injected and vacuum-sealed to complete a stacked lithium ion secondary battery.

[Example 2]
In (Synthesis of carbonate polymer), a lithium ion secondary battery was produced in the same manner as in Example 1 except that ethanol was used instead of methanol. Incidentally, carbonate polymers, R ¹ in Chemical Formula 1 is a methyl group, having the structure R ² is an ethylene group, a number average molecular weight of 1000 (average degree of polymerization n = 10).

[Example 3]
In (Synthesis of carbonate polymer), a lithium ion secondary battery was produced in the same manner as in Example 1 except that n-propanol was used instead of methanol. Incidentally, carbonate polymers, ^{R 1} in Chemical Formula 1 is a n- propyl group, has the structure ^{R 2} is an ethylene group, a number average molecular weight of 1000 (average degree of polymerization n = 10).

[Example 4]
In (Synthesis of carbonate polymer), a lithium ion secondary battery was produced in the same manner as in Example 1 except that isopropanol was used instead of methanol. Incidentally, carbonate polymers, R ¹ in Chemical Formula 1 is an isopropyl group, has the structure R ² is an ethylene group, a number average molecular weight of 1000 (average degree of polymerization n = 10).

[Example 5]
In (Synthesis of carbonate polymer), a lithium ion secondary battery was produced in the same manner as in Example 1 except that n-butanol was used instead of methanol. Incidentally, carbonate polymers, ^{R 1} in Chemical Formula 1 is an n- butyl group, has the structure ^{R 2} is an ethylene group, a number average molecular weight of 1100 (average degree of polymerization n = 10).

[Example 6]
In (Synthesis of carbonate polymer), a lithium ion secondary battery was produced in the same manner as in Example 1 except that isobutanol was used instead of methanol. Incidentally, carbonate polymers, R ¹ in Chemical Formula 1 is isobutyl group, has the structure R ² is an ethylene group, a number average molecular weight of 1100 (average degree of polymerization n = 10).

[Example 7]
In (Synthesis of carbonate polymer), a lithium ion secondary battery was produced in the same manner as in Example 1 except that sec-butanol was used instead of methanol. Incidentally, carbonate polymers, ^{R 1} in Chemical Formula 1 is a sec- butyl group, has the structure ^{R 2} is an ethylene group, a number average molecular weight of 1100 (average degree of polymerization n = 10).

[Example 8]
In (Synthesis of carbonate polymer), a lithium ion secondary battery was produced in the same manner as in Example 1 except that tert-butanol was used instead of methanol. Incidentally, carbonate polymers, ^{R 1} in Chemical Formula 1 is a tert- butyl group, has the structure ^{R 2} is an ethylene group, a number average molecular weight of 1100 (average degree of polymerization n = 10).

[Example 9]
In (Synthesis of carbonate polymer), a lithium ion secondary battery was produced in the same manner as in Example 4 except that 1,2-propanediol was used instead of ethylene glycol. Incidentally, carbonate polymers, ^{R 1} in Chemical Formula 1 is an isopropyl group, ^{R 2} is a methyl ethylene group _{(-CH (CH 3) CH 2} -) has a structure that is, the number-average molecular weight of 1100 (average polymerization Degree n = 10).

[Example 10]
In (Synthesis of carbonate polymer), a lithium ion secondary battery was produced in the same manner as in Example 4 except that 1,3-propanediol was used instead of ethylene glycol. Incidentally, carbonate polymers, ^{R 1} in Chemical Formula 1 is an isopropyl group, ^{R 2} is a trimethylene group _{(-CH 2 CH 2 CH 2 -} ) has a structure that is, the number-average molecular weight of 1100 (average degree of polymerization n = 10).

[Example 11]
A lithium ion secondary battery was produced in the same manner as in Example 4 except that diethyl carbonate (DEC) was used instead of ethyl methyl carbonate (EMC) in (Preparation of electrolyte solution).

[Example 12]
(Preparation of electrolytic solution) In place of a mixed solvent (40:60 (volume ratio)) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) A lithium ion secondary battery was produced in the same manner as in Example 4 except that a mixed solvent (40:30:30 (volume ratio)) with ethyl methyl carbonate (DMC) was used.

[Example 13]
(Preparation of electrolytic solution) In place of a mixed solvent (40:60 (volume ratio)) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) A lithium ion secondary battery was produced in the same manner as in Example 4 except that a mixed solvent (40:30:30 (volume ratio)) with diethyl carbonate (DEC) was used.

[Example 14]
(Preparation of electrolyte) In place of a mixed solvent (40:60 (volume ratio)) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl A lithium ion secondary battery was produced in the same manner as in Example 4 except that a mixed solvent of methyl carbonate (EMC) and dimethyl carbonate (DMC) (40: 20: 20: 20 (volume ratio)) was used. Was made.

[Example 15]
A lithium ion secondary battery was prepared in the same manner as in Example 4 except that 1% by mass of vinylene carbonate (VC) was further added to the electrolyte prepared in (Preparation of electrolyte).

[Example 16]
A lithium ion secondary battery was produced in the same manner as in Example 4 except that 1% by mass of propane sultone (PS) was further added to the electrolyte produced in (Production of Electrolyte).

[Comparative Example 1]
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the carbonate polymer was not coated in (Production of negative electrode).

[Comparative Example 2]
In (Preparation of negative electrode), a lithium ion secondary battery was prepared in the same manner as in Example 11 except that the carbonate polymer was not coated.

[Comparative Example 3]
In (Preparation of negative electrode), a lithium ion secondary battery was prepared in the same manner as in Example 12 except that the carbonate polymer was not coated.

[Comparative Example 4]
In (Preparation of negative electrode), a lithium ion secondary battery was prepared in the same manner as in Example 13 except that the carbonate polymer was not coated.

[Comparative Example 5]
A lithium ion secondary battery was produced in the same manner as in Example 15 except that in (Production of negative electrode), the carbonate polymer was not coated.

[Comparative Example 6]
In (Preparation of negative electrode), a lithium ion secondary battery was prepared in the same manner as in Example 16 except that the carbonate polymer was not coated.

<Evaluation of battery performance>
(Initial discharge capacity measurement)
About the lithium ion secondary battery produced by the said Example and the comparative example, after performing initial charge by charging for 20 hours (CCCV, upper limit voltage 4.2V) at 0.1C, it discharges at 0.1C (CC , Lower limit voltage 2.5V). During this initial charge, the carbonate polymer coated on the surface of the negative electrode active material is reduced and decomposed, and SEI is formed on the surface of the negative electrode active material.

  After 24 hours at 25 ° C., charge for 8 hours at 0.2 C (CCCV, upper limit voltage 4.2 V), then discharge at 0.2 C (CC, lower limit voltage 2.5 V), The discharge capacity at the time was measured, and this was used as the initial discharge capacity.

  Moreover, the battery volume was measured by the Archimedes method and set as the initial battery volume.

(Battery resistance measurement)
The lithium ion secondary battery is charged to a state of charge (SOC) of 50%, and the potential difference ΔE when discharged at 1 C for 10 seconds is divided by the value of the flowing current. The value obtained by multiplying the area of the portion facing the negative electrode was defined as the battery resistance. The battery resistance measurement was performed at -20 ° C.

(Cycle test)
About the said lithium ion secondary battery, in the thermostat set to 55 degreeC, after charging 2.5 hours at 1C (CCCV, upper limit 4.2V), it rests for 10 minutes, and discharges at 1C (CC, The lower limit was 2.5V). The cycle test was performed by repeating this operation as one cycle for 300 cycles.

  Then, after 24 hours at 25 ° C., after charging for 8 hours at 0.2 C (CCCV, upper limit voltage 4.2 V), discharging at 0.2 C (CC, lower limit voltage 2.5 V), The discharge capacity after the cycle test was measured. And the ratio of the discharge capacity after the cycle test with respect to the initial discharge capacity was expressed as a percentage and used as the capacity maintenance rate.

  Moreover, the battery volume after the cycle test was measured by the Archimedes method, and the ratio of the volume after the cycle test to the initial battery volume was expressed as a percentage, which was defined as the volume expansion rate (the degree of cell swelling). The results are shown in Table 1.

  According to Table 1, in Examples 1-16 using the negative electrode active material material which coat | covers the surface of a negative electrode active material with a carbonate polymer, compared with Comparative Examples 1-6 which did not perform the coating with a carbonate polymer. It was shown that the volume expansion rate was low and the capacity retention rate was high. This is because the carbonate polymer that covers the surface of the negative electrode active material during initial charging is reductively decomposed before the solvent to form SEI, thereby preventing reductive decomposition of the solvent and suppressing gas generation. It was considered a thing.

The volume R ¹ in Formula 1 in which Examples 3-16 alkyl group having a carbon number of 3 or 4, compared R ¹ is as in Example 1 or 2 is a methyl group or an ethyl group, more excellent An expansion rate and a capacity maintenance rate were obtained.

1 negative electrode active material,
3 negative electrode active material,
5 a polymer having a carbonate structure;
10 Lithium ion secondary battery,
11 negative electrode current collector,
11a outermost layer negative electrode current collector,
12 negative electrode active material layer,
13 electrolyte layer,
14 positive electrode current collector,
15 positive electrode active material layer,
16 cell layer,
17 Power generation elements,
18 negative electrode current collector plate,
19 positive current collector,
20 Negative terminal lead,
21 positive terminal lead,
22 Laminated film.

Claims (4)

A negative electrode active material, wherein at least a part of the surface of the negative electrode active material body is coated with a polymer having a carbonate structure represented by the following chemical formula 1;

In the formula, n represents an integer of 2 to 100, each R ¹ independently represents an alkyl group having 1 to 10 carbon atoms, and each R ² independently represents an alkylene group having 2 to 5 carbon atoms. . 2. The negative electrode according to claim 1, wherein in Formula 1, each R ¹ independently represents an alkyl group having 1 to 4 carbon atoms, and each R ² independently represents an alkylene group having 2 or 3 carbon atoms. Active material. 3. The negative electrode active material according to claim 2, wherein in Chemical Formula 1, R ¹ independently represents an alkyl group having 3 or 4 carbon atoms, and R ² represents an ethylene group. A current collector,
A positive electrode active material layer formed on the surface of the current collector;
A negative electrode active material layer formed on the surface of the current collector;
A lithium ion secondary battery having a power generation element formed by laminating an electrolyte layer containing an electrolyte solution interposed between the positive electrode active material layer and the negative electrode active material layer,
The lithium ion secondary battery in which the said negative electrode active material layer contains the negative electrode active material material of any one of Claims 1-3.

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