JP2014149989A - Active material for lithium ion secondary battery, electrode for lithium ion secondary battery including the same, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an active material for a lithium ion secondary battery, with which a lithium ion secondary battery having excellent charge/discharge capacity and excellent cycle characteristics can be obtained, and a lithium ion secondary battery including the active material.SOLUTION: An active material for a lithium ion secondary battery includes an active material body and a coat layer formed on the surface of the active material body. The coat layer comprises a resin having a binding function, and has a film thickness of 1 nm or more and 50 nm or less. When the total surface area of the active material body is 100%, the percentage of an area occupied by the coat layer is 70% or more.

Description

  The present invention relates to an active material for a lithium ion secondary battery, an electrode for a lithium ion secondary battery having the same, and a lithium ion secondary battery.

In recent years, along with the development of portable electronic devices such as mobile phones and notebook computers, and the practical application of electric vehicles, secondary batteries with small and light weight and high capacity are required. Currently, lithium ion secondary batteries using lithium cobalt oxide (LiCoO ₂ ) as a positive electrode material and a carbon-based material as a negative electrode material are commercialized as high capacity secondary batteries.

  Lithium ion secondary batteries are required to have higher capacities, and various studies have been conducted on active materials used in lithium ion secondary batteries. In general, a material capable of reversibly inserting and extracting lithium is used as the active material. Therefore, the active material is required to have a stable structure even when lithium is inserted or extracted during charge and discharge. The active material has a high surface activity. Therefore, the electrolytic solution that contacts the surface of the active material is decomposed by the active material. When the battery is repeatedly charged and discharged, the electrolytic solution is decomposed and the charge / discharge capacity of the battery is reduced.

  In order to enhance the structural stability of the active material and to suppress the decomposition of the electrolyte solution by the active material, studies have been made to form a surface treatment layer or a coating on the surface of the active material as follows.

In Patent Document 1, MXO _k (M is at least one element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, and rare earth elements, and X is oxygen and An active material in which a surface treatment layer containing a compound represented by the chemical formula (wherein k is an element capable of forming a double bond and k is 2 to 4) is described. The Examples of Patent Document 1, _LiNi 0.8 _Mn 0.2 _{O 2} which LiCoO ₂ and AlPO ₄ layers AlPO ₄ layer is formed is formed is described. Patent Document 1 describes that a battery having an active material on which the surface treatment layer is formed has improved rate characteristics and cycle characteristics and is also excellent in thermal stability. In Patent Document 1, a preferable thickness of the surface treatment layer is described as 0.01 μm to 2 μm. In the example of Patent Document 1, the thickness of the surface treatment layer is described as about 1 μm.

Patent Document 2 discloses that at least part of the surface of the positive electrode active material has a chemical formula M ₁ P _m O _n (wherein M is at least one element that can take a divalent valence, and l, m And n is an integer in a range satisfying 2l + 5m = 2n). A non-aqueous electrolyte secondary battery is described in which a surface treatment layer containing a compound represented by the following formula is formed. It is described that by forming the surface treatment layer on the positive electrode active material, the non-aqueous electrolyte secondary battery has improved cycle characteristics and can suppress a decrease in initial efficiency. Examples of the element M include Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, and Zn. In the examples, phosphorus such as Mg ₂ P ₂ O ₇ and Mg (PO ₃ ) ₂ is used as the compound. Magnesium acid is used. Patent Document 2 does not describe the thickness of the surface treatment layer.

  Patent Document 3 describes positive electrode active material particles coated with a polyamide resin (particularly, a wholly aromatic polyamide resin such as an aramid resin). By this coating, elution of the transition metal from the positive electrode active material is suppressed. It is described that the lithium ion secondary battery having this positive electrode active material has excellent cycle characteristics even when charging / discharging at high voltage and high temperature. Patent Document 3 describes that the thickness of the polyamide resin coating layer is preferably 500 nm or less. Patent Document 3 describes that the polyamide resin is selected because it is excellent in heat resistance. Moreover, in the Example of patent document 3, the active material which the aramid resin coat | covered was obtained by adding an active material in the solution which dissolved the aramid resin 0.1wt%-3wt% in N-methyl-2-pyrrolidone. Yes. However, the example of Patent Document 3 does not describe how much aramid resin is coated on the active material and the thickness of the coating layer made of the aramid resin.

  Patent Document 4 describes a non-aqueous electrolyte secondary battery in which a coating including a polymer having a methyl methacrylate unit and inorganic fine particles is provided on the surface of a positive electrode. According to this configuration, the positive electrode active material on the surface of the positive electrode is covered with the coating. Therefore, the oxidative decomposition of the nonaqueous electrolytic solution by the positive electrode active material on the surface of the positive electrode at a high potential is suppressed. Further, Patent Document 4 describes that inclusion of inorganic fine particles in a polymer creates a microscopic gap in the film, thereby ensuring ionic conductivity of the film. On the other hand, Patent Document 4 describes that the thickness of the coating is preferably 0.1 μm to 10 μm. And in patent document 4, in the battery of Example 12 whose film thickness is 0.05 micrometer, since a uniform film is not formed, the effect of suppressing the oxidative decomposition | disassembly of nonaqueous electrolyte solution in high electric potential became weak. It is described.

JP 2003-7299 A JP 2006-127932 A JP 2010-129494 A JP 2007-134279 A

  As described above, various studies for forming a surface treatment layer and a film on the surface of an active material have been conducted. Here, the surface treatment layer or the coating itself becomes a resistance component with respect to ion conduction. For this reason, it is desirable that the thickness of the surface treatment layer and the coating is thinner from the viewpoint of the charge / discharge capacity of the battery. The present invention has been made in view of such circumstances, and has an active material for a lithium ion secondary battery that has a coating layer thinner than the conventional one and has an effect of improving cycle characteristics. It aims at providing the electrode for secondary batteries, and a lithium ion secondary battery.

  As a result of intensive studies by the present inventors, a thin film made of a resin having a binding function could be attached to the surface of the active material body. And it discovered that cycling characteristics could be improved by using the active material in which the thin film was formed, without reducing the charge-and-discharge capacity of a lithium ion secondary battery.

  That is, the active material for a lithium ion secondary battery of the present invention has an active material body and a coat layer formed on the surface of the active material body, and the coat layer is made of a resin having a binding function, The film thickness is 1 nm or more and 50 nm or less, and the ratio of the area occupied when the entire surface area of the active material body is 100% is 70% or more.

  The surface of the active material body is preferably modified so that the wettability with the resin is improved.

  The active material body preferably has a hydrophobic surface. In this case, it is particularly preferable that the resin is polyvinylidene fluoride (PVDF).

Active material body is represented by the general _{formula: LiCo p Ni q Mn r D} s O 2 (D is the doping component, at least one element selected Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe and Na P + q + r + s = 1, 0 <p ≦ 1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1), and the resin is polyfluorinated. It is preferably at least one selected from vinylidene (PVDF), polytetrafluoroethylene (PTFE), ethylene vinyl alcohol and polyacrylate.

  An electrode for a lithium ion secondary battery according to the present invention includes a current collector, an active material layer formed on the surface of the current collector, the active material layer including the lithium ion secondary battery active material and an active material layer forming binder. It is characterized by having.

  The lithium ion secondary battery of this invention has the said electrode for lithium ion secondary batteries, It is characterized by the above-mentioned.

  In the active material for a lithium ion secondary battery of the present invention, a coat layer is formed on the surface of the active material body.

  The thickness of this coat layer is 1 nm or more and 50 nm or less and is very thin. Therefore, the coat layer is difficult to prevent insertion or desorption of Li ions from the active material during charge / discharge. Therefore, the charge / discharge capacity of the lithium ion secondary battery having the active material for a lithium ion secondary battery of the present invention is unlikely to decrease.

  Moreover, according to the active material for a lithium ion secondary battery in which a coating layer made of a resin having a binding function is formed, the binding force between the active materials and between the active material and the active material layer forming binder is increased. . Therefore, the cycle characteristics of the lithium ion secondary battery using the lithium ion secondary battery active material are improved.

  The coat layer has an area ratio of 70% or more when the entire surface area of the active material body is 100%. The coat layer covers most of the surface of the active material body. Therefore, the coat layer suppresses the active material from coming into direct contact with the electrolytic solution. Therefore, the cycle characteristics of the lithium ion secondary battery using the active material for a lithium ion secondary battery of the present invention are improved.

It is a schematic cross section explaining the active material for lithium ion secondary batteries of this embodiment. It is a cross-sectional photograph of the electrode B by a scanning electron microscope (SEM). It is a cross-sectional photograph by the scanning electron microscope (SEM) of the electrode A. It is the cross-sectional observation result by the energy dispersive X-ray spectroscopy (EDX: Energy Dispersive X-ray Spectroscopy) of the electrode A. FIG. 6 is a graph showing measurement results of initial capacity and post-cycle capacity for lithium ion secondary batteries of Example 1 and Comparative Example 1.

<Active materials for lithium ion secondary batteries>
The active material for a lithium ion secondary battery of the present invention is characterized by having an active material body and a coat layer made of a resin having a binding function formed on the surface of the active material body.

  The active material for a lithium ion secondary battery of the present invention can be applied to both a positive electrode active material and a negative electrode active material. That is, the active material body can be applied to both the positive electrode active material body and the negative electrode active material body.

  As a positive electrode active material main body, what consists of a lithium containing compound or another metal compound can be used.

Examples of the lithium-containing compound include a lithium nickel composite oxide having a layered structure, a lithium manganese composite oxide having a spinel structure, a general formula: LiCo _p Ni _q Mn _r D _S O ₂ (D is a doping component, Al , Mg, Ti, Sn, Zn, W, Zr, Mo, Fe and Na, p + q + r + s = 1, 0 <p ≦ 1, 0 ≦ q <1, 0 ≦ r <1, 0 Lithium cobalt-containing composite metal oxide having a layered structure represented by ≦ s <1), an olivine-type lithium phosphate composite oxide represented by the general formula: LiMPO ₄ (M is one of Mn, Fe, Co, and Ni) at least one), the general _formula: at least one of Li 2 MPO ₄ fluoride olivine-type lithium phosphate compound oxide represented by F (M is Mn, Fe, Co and Ni , General _formula: Li 2 MSiO silicate lithium composite oxide represented by ₄ (M is _{one or more of} at least one of Mn, Fe, Co and Ni) can be used.

  Examples of other metal compounds include oxides such as titanium oxide, vanadium oxide, and manganese dioxide, and disulfides such as titanium sulfide and molybdenum sulfide.

The positive electrode active material body has a general formula: LiCo _p Ni _q Mn _r D _S O ₂ (D is a doping component, and at least selected from Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe, and Na) And a lithium cobalt-containing composite metal oxide having a layered structure represented by p + q + r + s = 1, 0 <p ≦ 1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1) It is preferable. Here, in particular, the above-mentioned p, q, and r are preferably in the ranges of 0 <p <1, 0 <q <1, and 0 <r <1, respectively.

  The lithium cobalt-containing composite metal oxide is excellent in thermal stability and low in cost. By using the lithium cobalt-containing composite metal oxide as a positive electrode active material, a lithium ion secondary battery with high thermal stability and low cost can be obtained.

Examples of the lithium cobalt-containing composite metal oxide include LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , and LiNi _0.5 Co _0.2 Mn. _0.3 O ₂ , LiCoO ₂ , LiNi _0.8 Co _0.2 O ₂ , LiCoMnO ₂ can be used. Among these, LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ and LiNi _0.5 Co _0.2 Mn _0.3 O ₂ are preferable in terms of thermal stability.

  As the negative electrode active material body, a carbon-based material capable of occluding and releasing lithium, an element capable of being alloyed with lithium, or an elemental compound having an element capable of being alloyed with lithium can be used.

  Examples of the carbon-based material include non-graphitizable carbon, artificial graphite, coke, graphite, glassy carbon, organic polymer compound fired body, carbon fiber, activated carbon, or carbon black. Here, the organic polymer compound fired body refers to a material obtained by firing and carbonizing a polymer material such as phenols and furans at an appropriate temperature.

  Elements that can be alloyed with lithium are Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Si, Ge, Sn. , Pb, Sb, Bi. Among these, silicon (Si) or tin (Sn) is preferable as an element that can be alloyed with lithium.

Examples of elemental compounds having elements that can be alloyed with lithium include ZnLiAl, AlSb, SiB ₄ , SiB ₆ , Mg ₂ Si, Mg ₂ Sn, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , _{CaSi 2, CrSi 2, Cu 5} Si, FeSi 2, MnSi 2, NbSi 2, TaSi 2, VSi 2, WSi 2, ZnSi 2, SiC, Si 3 N 4, Si 2 N 2 O, SiO v (0 <v ≦ 2), SnO _w (0 <w ≦ 2), SnSiO ₃ , LiSiO or LiSnO can be used. As the elemental compound having an element that can be alloyed with lithium, a silicon compound or a tin compound is preferable. As the silicon compound, SiO _x (0.5 ≦ x ≦ 1.5) is preferable. As the tin compound, for example, a tin alloy (Cu—Sn alloy, Co—Sn alloy, etc.) can be used.

The negative electrode active material body is preferably made of a carbon-based material or SiO _x . Carbon-based materials are used in the majority of consumer lithium-ion secondary batteries and have a long history of use. Further, SiO _x is a substance that exhibits a high battery capacity and is abundant in resources and is stably supplied.

  The coat layer is made of a resin having a binding function. The resin having the binding function may be any resin having a function of binding the active material main body to another active material main body or a binder for forming an active material layer, a conductive auxiliary agent, or the like. As such a resin, a resin generally used as a binder for forming an active material layer in an electrode of a lithium ion secondary battery can be adopted.

  Since the active material bodies are coated with a resin having a binding function, the binding force between the active material bodies and between the active material bodies and the active material layer forming binder or the conductive assistant is increased. Therefore, the cycle characteristics of the lithium ion secondary battery using the active material for a lithium ion secondary battery of the present invention are improved.

  The resin having a binding function is preferably one having an affinity for the active material body. For example, if the active material body is hydrophilic, the resin having a binding function is preferably aqueous, and if the active material body is hydrophobic, the resin having a binding function is preferably solvent-based. .

  Such an affinity can also be determined, for example, by evaluating the wettability between the active material body and the resin. The wettability can be evaluated by measuring the contact angle between the active material body and the resin. For example, the contact angle can be measured by pressing the active material body into a pellet and dropping a resin solution prepared by dissolving the resin in an appropriate solvent. If the contact angle is less than 90 °, it is determined that the active material body and the resin have affinity. The smaller the contact angle between the active material body and the resin, that is, the higher the wettability, the more uniformly and thinly the coat layer is formed on the surface of the active material body.

  When the positive electrode active material main body described above is used, as the resin having a binding function, for example, a solvent-based resin selected from polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), polyacrylate, ethylene vinyl alcohol, An aqueous resin selected from carboxymethyl cellulose (CMC) and sodium polyacrylate can be used.

  When using the negative electrode active material body described above, the resin having a binding function includes a solvent-based resin made of polyvinylidene fluoride (PVDF), a styrene butadiene copolymer (SBR), a polyacrylate, a polyurethane, and carboxymethyl cellulose (CMC). ) Can be used.

  The resin having a binding function is preferably a binder suitable for use in an electrode of a lithium ion secondary battery driven at a high voltage. Examples of such a binder include PVDF, PTFE, ethylene vinyl alcohol, and polyacrylate.

For example, the active material body has the general formula: LiCo _p Ni _q Mn _r D _s O ₂ (D is a doping component, and at least one selected from Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe and Na) And p + q + r + s = 1, 0 <p ≦ 1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1) using a lithium cobalt-containing composite metal oxide having a layered structure The resin is preferably at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene vinyl alcohol and polyacrylate.

  When the active material body is a carbon-based material, the resin is preferably PVDF.

When the active material body is SiO _x , the resin is preferably at least one selected from styrene butadiene copolymer (SBR), polyacrylate, polyurethane, and carboxymethyl cellulose (CMC).

  Furthermore, the surface of the active material body may be modified so that the wettability with the resin having a binding function is improved. The method for modifying the surface of the active material body for improving the wettability with the resin having a binding function is not particularly limited.

  For example, when a water-based resin such as carboxymethyl cellulose (CMC) is used as the resin having a binding function, it is preferable to hydrophilize the surface of the active material body. When a solvent-based resin such as polyvinylidene fluoride (PVDF) is used as the resin having a binding function, the surface of the active material body is preferably subjected to a hydrophobic treatment. A well-known method can be used for the hydrophilic treatment and the hydrophobic treatment of the surface of the active material body.

  It is also conceivable that wettability is improved by a combination of a functional group possessed by the resin having a binding function and a functional group or substance present on the surface of the active material body. Therefore, modification may be performed so that a functional group or a substance having a good affinity with the functional group possessed by the resin having a binding function is present on the surface of the active material body. This reforming method is not particularly limited.

  The coating layer has a thickness of 1 nm to 50 nm. The thickness of this coat layer is very thin, 1 nm to 50 nm. If the film thickness of the coat layer is 50 nm or less, the coat layer is unlikely to prevent insertion or desorption of Li ions from the active material body during charge / discharge. Therefore, the charge / discharge capacity of the lithium ion secondary battery having the lithium ion secondary battery active material is unlikely to decrease.

  The thickness of this coat layer is, for example, TEM-EDX (TEM (Transmission Electron Microscope) equipped with EDX, EDX is Energy Dispersive X-ray Spectroscopy) Etc. can be measured. Since this coat layer is very thin, it is difficult to measure the film thickness by observation with a scanning electron microscope (SEM).

  The coat layer has an area ratio of 70% or more when the entire surface area of the active material body is 100%. The ratio of the area of the coat layer to the entire surface area of the active material body can be measured using a TEM-EDX apparatus in the same manner as the film thickness.

  If the ratio of the area of the coat layer to the entire surface area of the active material body is 70% or more, the coat layer covers most of the surface of the active material body. The coat layer suppresses the surface of the active material body from coming into direct contact with the electrolytic solution. Therefore, the cycle characteristics of the lithium ion secondary battery using the above-described lithium ion secondary battery active material are improved.

  FIG. 1 is a schematic cross-sectional view illustrating an active material for a lithium ion secondary battery according to this embodiment. As shown in FIG. 1, in the active material 3 for a lithium ion secondary battery, the surface of the active material body 1 is covered with a coat layer 2. For this reason, the adjacent active material 3 for lithium ion secondary batteries contacts a part of each coat layer 2. The coat layer 2 is made of a resin having a binding function. Therefore, adjacent active materials 3 for lithium ion secondary batteries are reliably bound through each other's coat layer 2. Moreover, since the active material 3 for lithium ion secondary batteries has the coating layer 2 on the surface, it is easy to bind | conclude with the binder 4 for active material layer formation. In FIG. 1, the coat layer 2 coats the entire surface of the active material body 1. If 70% or more of the surface of the active material body 1 is coated, a coat layer is formed on a part of the surface of the active material body 1. There may be a place where 2 is not formed.

  The method for forming the coat layer on the active material body is not particularly limited. The active material main body and the resin having the binding function may be mixed, and the resin may be cured using an optimal curing method for each resin.

  The resin that forms the coat layer may be the same as the binder for forming the active material layer, or may be different from the binder for forming the active material layer. When the binder for forming the active material layer and the resin forming the coat layer are the same, it is not always necessary to form a coat layer on the active material body in advance, and the active material body is coated with resin on the active material body at the same time as the electrode is produced. A layer can be formed. When the binder for forming the active material layer and the resin for forming the coat layer are different, the active material body is coated with the resin in advance, and the active material having the coat layer formed thereon and the binder for forming the active material layer An electrode may be prepared using an agent.

<Electrode for lithium ion secondary battery>
An electrode for a lithium ion secondary battery according to the present invention includes a current collector, an active material layer formed on the surface of the current collector, the active material layer including the lithium ion secondary battery active material and an active material layer forming binder. It is characterized by having.

  The current collector refers to a chemically inert electronic high conductor that keeps a current flowing through an electrode during discharge or charging of a lithium ion secondary battery. Examples of materials that can be used for the current collector include metal materials such as stainless steel, titanium, nickel, aluminum, and copper, or conductive resins. The current collector can take the form of a foil, a sheet, a film, or the like. Therefore, metal foils, such as copper foil, nickel foil, aluminum foil, stainless steel foil, can be used suitably as a collector.

  The current collector preferably has a thickness of 10 μm to 100 μm.

  The binder for forming an active material layer plays a role of connecting the active material for a lithium ion secondary battery to a current collector. As the binder for forming the active material layer, for example, fluorine-containing resins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTEF) and fluororubber, polypropylene, polyethylene, ethylene vinyl alcohol and polyvinyl acetate resin, etc. Rubbers such as thermoplastic resins, acrylic resins such as polyacrylate and sodium polyacrylate, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, polyurethane, carboxymethylcellulose (CMC) and styrene butadiene copolymer (SBR) Can be used.

  The active material layer may further contain a conductive additive.

  The conductive assistant is added to increase the conductivity of the electrode. Examples of the conductive assistant include carbon black, graphite, acetylene black (AB), ketjen black (registered trademark) (KB), and vapor grown carbon fiber (VGCF) which are carbonaceous fine particles. These conductive assistants can be used alone or in combination of two or more. Although there are no particular limitations on the amount of conductive aid used, for example, it can be about 1 to 30 parts by mass with respect to 100 parts by mass of the active material contained in the electrode.

  The electrode can be formed as follows. First, a composition for forming an active material layer containing an active material for a lithium ion secondary battery, a binder for forming an active material layer and, if necessary, a conductive additive is prepared, and an appropriate solvent is added to the composition. And paste it. The paste composition is applied to the surface of the current collector and then dried to form an active material layer on the current collector. The current collector on which the active material layer is formed is compressed as necessary to increase the electrode density. Thus, an electrode is formed.

  As a method for applying the composition for forming an active material layer, a conventionally known method such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method may be used.

  As a solvent for adjusting the viscosity, N-methyl-2-pyrrolidone (NMP), methanol, methyl isobutyl ketone (MIBK) and the like can be used.

<Lithium ion secondary battery>
The lithium ion secondary battery of this invention has the electrode for lithium ion secondary batteries mentioned above. In the lithium ion secondary battery of the present invention, either the positive electrode or the negative electrode may be the above-described electrode for a lithium ion secondary battery, or both electrodes may be the above-described electrode for a lithium ion secondary battery. Good.

  The lithium ion secondary battery of the present invention includes a separator and an electrolytic solution as a battery component in addition to the above-described electrodes.

  The separator separates the positive electrode and the negative electrode and allows lithium ions to pass through while preventing a short circuit of current due to contact between the two electrodes. As the separator, for example, a porous film made of synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramics can be used.

  The electrolytic solution includes a solvent and an electrolyte dissolved in the solvent.

  As the solvent, for example, cyclic esters, chain esters, and ethers can be used. Examples of cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, and gamma valerolactone. Examples of the chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester. Examples of ethers that can be used include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane.

Moreover, as an electrolyte dissolved in the electrolytic solution, for example, a lithium salt such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ can be used.

As the electrolytic solution, for example, a lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 in} a solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, dimethyl carbonate, or the like is from 0.5 mol / l to 1.7 mol / l. A solution dissolved at a certain concentration can be used.

  The lithium ion secondary battery can be mounted on a vehicle. The lithium ion secondary battery has a large battery charge / discharge capacity and excellent cycle characteristics. Therefore, a vehicle equipped with the lithium ion secondary battery has high performance in terms of life and output.

  The vehicle may be a vehicle that uses electric energy from a battery for all or part of a power source. Examples of the vehicle include an electric vehicle, a hybrid vehicle, a plug-in hybrid vehicle, a hybrid railway vehicle, an electric forklift, an electric wheelchair, an electric assist bicycle, and an electric motorcycle.

  The embodiments of the active material for a lithium ion secondary battery, the electrode for a lithium ion secondary battery, and the lithium ion secondary battery of the present invention have been described above. However, the present invention is not limited to the above embodiment. The present invention can be carried out in various forms that have been modified or improved by those skilled in the art without departing from the scope of the present invention.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
[Preparation of positive electrode active material for lithium ion secondary battery]
(Surface treatment of positive electrode active material)
A lithium composite metal oxide represented by LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ was prepared as the positive electrode active material body.

The lithium composite metal oxide was brought into contact with an aqueous solution in which the surface treatment material was dissolved in water. The surface treatment material is composed of (NH ₄ ) ₂ HPO ₄ and Mg (NO ₃ ) ₂ . When the aqueous solution was 100% by mass, (NH ₄ ) ₂ HPO ₄ was dissolved in water at a concentration of 4.0% by mass and Mg (NO ₃ ) ₂ was dissolved in water at 5.8% by mass. The molar ratio of (NH ₄ ) ₂ HPO ₄ and Mg (NO ₃ ) ₂ was 3: 2. The lithium composite metal oxide was immersed in an aqueous solution containing a surface treatment material and mixed by stirring. The immersion time was 30 minutes. The temperature of the aqueous solution at the time of immersion was room temperature.

Next, the lithium composite metal oxide after immersion was dried at 130 ° C. for 6 hours. Thereafter, the dried lithium composite metal oxide was heated at 700 ° C. in an air atmosphere for 5 hours to obtain a surface-treated LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ .

  By applying this surface treatment, the lithium composite metal oxide is improved in affinity with the solvent-based resin. That is, it is presumed that the surface of the positive electrode active material main body was hydrophobized by this surface treatment. This will be described later in cross-sectional observation of the positive electrode.

(Preparation of positive electrode)
LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ after the surface treatment described above as the positive electrode active material, acetylene black as the conductive auxiliary agent, and polyvinylidene fluoride (PVDF) as the binder for forming the active material layer Were mixed as 88 parts by mass, 6 parts by mass, and 6 parts by mass, respectively. This mixture was dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare a slurry.

An aluminum foil having a thickness of 20 μm was prepared as a current collector. The slurry was placed on the current collector, and the slurry was applied to the current collector using a doctor blade so that the slurry became a film. The current collector coated with the obtained slurry was dried at 80 ° C. for 20 minutes, and NMP was volatilized and removed from the slurry. Thereafter, the current collector and the coated material on the current collector were firmly and closely joined by a roll press. At this time, the electrode density was set to 12 mg / cm ² . The joined product was heated with a vacuum dryer at 120 ° C. for 6 hours, and cut into a predetermined shape (rectangular shape of 25 mm × 30 mm). The obtained electrode was designated as positive electrode A. The thickness of the positive electrode A was about 60 μm. In the positive electrode A, the resin that forms the coat layer and the binder for forming the active material layer are the same PVDF.

A positive electrode B was produced in the same manner as the positive electrode A except that LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ which was not surface-treated was used as the positive electrode active material.

(Cross-section observation of positive electrode)
The cross sections of the positive electrode A and the positive electrode B were observed with SEM (manufactured by Hitachi High-Technologies Corporation: S-4800). Furthermore, in the cross section of the positive electrode A, the element distribution of the F component was measured at an acceleration voltage of 200 kV using a TEM-EDX apparatus (manufactured by JEOL Ltd.).

  An SEM image of the positive electrode B is shown in FIG. 2, and an SEM image of the positive electrode A is shown in FIG. In FIG. 2, a powdery active material layer forming binder 4 was observed on the surface of the positive electrode active material 1. In addition, it was observed that a part of the binder 4 for forming the active material layer had an elongated shape as if a thread was pulled. It was also observed that the active material layer forming binder 4 was present on the conductive auxiliary agent 5. On the other hand, in FIG. 3, the elongated active material layer forming binder 4 was observed, but the powdery active material layer forming binder 4 was not observed on the surface of the positive electrode active material 1. It was.

  Here, although it is not clearly observed in the SEM image shown in FIG. 3, it was found from the measurement result of TEM-EDX that the coating layer 2 was uniformly coated on the surface of the positive electrode active material 1 with a film thickness of about 10 nm. .

  The measurement result of TEM-EDX is shown in FIG. Specifically, FIG. 4 shows an element distribution image of the F component on the surface of the positive electrode active material of the positive electrode A. It was detected that the F component almost uniformly covered the entire surface of the positive electrode active material with a film thickness of about 10 nm. The F component is a component derived from PVDF. From this, it was found that the PVDF film having a thickness of about 10 nm, that is, the coat layer 2 was formed almost uniformly on the surface of the positive electrode active material.

  When comparing the positive electrode A and the positive electrode B, it was found that a uniform thin PVDF film was formed on the surface of the positive electrode active material by subjecting the positive electrode active material to surface treatment. Since the surface treatment of the positive electrode active material has improved the affinity between the surface of the positive electrode active material and PVDF, in other words, the wettability has been improved, and it is assumed that a uniform PVDF film was formed on the surface of the positive electrode active material. .

  A fluorine-based resin such as PVDF generally has water repellency and oil repellency, and has a property to dislike water and oil. However, since PVDF dissolves in NMP, which is an organic solvent, PVDF is judged to be hydrophobic. The surface of the positive electrode active material subjected to the surface treatment was cleanly covered with this PVDF. From this, it is inferred that the surface-treated positive electrode active material and PVDF had an affinity due to hydrophobic interaction. Therefore, it is estimated that the surface of the positive electrode active material main body was hydrophobized by the surface treatment.

[Production of laminated lithium ion lithium ion secondary battery]
The laminated lithium ion secondary battery of Example 1 was produced as follows.

  Positive electrode A was used as the positive electrode. The negative electrode was produced as follows. 97 parts by mass of graphite powder, 1 part by mass of acetylene black as a conductive additive, 1 part by mass of styrene-butadiene rubber (SBR) and 1 part by mass of carboxymethylcellulose (CMC) as a binder are mixed, and this mixture is mixed. A slurry was prepared by dispersing in an appropriate amount of ion-exchanged water. This slurry was applied to a copper foil having a thickness of 20 μm, which is a negative electrode current collector, in a film shape using a doctor blade. The current collector coated with the slurry was dried and then pressed. The joined product was heated with a vacuum dryer at 200 ° C. for 2 hours, and cut into a predetermined shape (rectangular shape of 25 mm × 30 mm). In this way, a negative electrode was obtained. The thickness of the negative electrode was about 45 μm.

A laminate type lithium ion secondary battery was manufactured using the positive electrode A and the negative electrode. Specifically, a rectangular sheet (27 × 32 mm, thickness 25 μm) made of polypropylene resin was sandwiched between the positive electrode A and the negative electrode to form an electrode plate group. The electrode plate group was covered with a set of two laminated films, and the three sides were sealed, and then an electrolyte solution was injected into the bag-like laminated film. As an electrolytic solution, a solution obtained by dissolving 1 mol of LiPF ₆ in a solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at EC: DEC = 3: 7 (volume ratio) was used. Thereafter, the remaining one side was sealed to obtain a laminate type lithium ion secondary battery in which the four sides were hermetically sealed and the electrode plate group and the electrolyte were sealed. Note that the positive electrode and the negative electrode have a tab that can be electrically connected to the outside, and a part of the tab extends to the outside of the laminated lithium ion secondary battery. The laminated lithium ion secondary battery of Example 1 was produced through the above steps.

(Comparative Example 1)
A laminated lithium ion secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 except that the positive electrode B was used instead of the positive electrode A.

<Initial capacity measurement>
The initial capacities of the laminated lithium ion secondary batteries of Example 1 and Comparative Example 1 were measured. In Example 1, the initial capacity was measured at N = 3. When charging, CC charging (constant current constant voltage charging) was performed at 25 ° C. at a 1C rate and a voltage of 4.5V. CV charging was held at a voltage of 4.5V for 1 hour. When discharging, CC discharge (constant current discharge) was performed at a rate of 0.33 C up to 3.0 V. The discharge capacity at this time was measured and used as the initial capacity.

<Cycle characteristic evaluation>
The cycle characteristics of the laminated lithium ion secondary batteries of Example 1 and Comparative Example 1 were evaluated. As an evaluation of the cycle characteristics, a cycle test in which charge and discharge were repeated under the following conditions was performed. At the time of charging, CCCV charging (constant current constant voltage charging) was performed at a voltage of 4.5 V at a 1C rate at 25 ° C. CV charging was held at a voltage of 4.5V for 1 hour. When discharging, CC discharge (constant current discharge) was performed at a rate of 3.0 V and 0.33 C. This charging / discharging was made into 1 cycle, and it repeated to 100 cycles. The discharge capacity at the 0.33C rate after the first time and after 100 cycles was measured. The initial discharge capacity was the initial capacity and the discharge capacity after 100 cycles was the post-cycle capacity. The capacity retention rate (%) was obtained by the following formula.
Capacity maintenance rate (%) = capacity after cycle / initial capacity × 100

  In the initial capacity measurement and the cycle characteristic evaluation, three laminated lithium ion secondary batteries of Example 1 were produced, and the initial capacity and the cycle characteristics were evaluated for all three. FIG. 5 shows the measurement results of all initial capacities of N = 3 and all post-cycle capacities of Example 1, and the measurement results of initial capacity and post-cycle capacities of Comparative Example 1.

  Compared with the result of N = 3 of Example 1 and the result of Comparative Example 1 from FIG. 5, the initial capacity of Comparative Example 1 is 100%, and the initial capacity of Example 1 is 2. It decreased only 29 ± 0.78%. Further, the capacity retention rate after the cycle of Example 1 was 76.9 ± 2.5%. The capacity retention rate of Comparative Example 1 was 48.3%. Therefore, the post-cycle capacity of Example 1 was improved by 28% or more in capacity retention under high voltage conditions as compared with the post-cycle capacity of Comparative Example 1.

  From this result, a coating layer made of a resin having a binding function with a film thickness of about 10 nm is formed on the surface of the active material main body, so that the initial capacity of the lithium ion secondary battery is hardly reduced. It was found that the cycle characteristics could be improved even under the conditions.

  1: active material body, 2: coat layer, 3: active material for lithium ion secondary battery, 4: binder for forming active material layer, 5: conductive additive.

Claims (7)

An active material body,
A coat layer formed on the surface of the active material body;
Have
The coat layer is made of a resin having a binding function, the film thickness is 1 nm or more and 50 nm or less, and the ratio of the area occupied when the entire surface area of the active material body is 100% is 70% or more. An active material for a lithium ion secondary battery.   The active material for a lithium ion secondary battery according to claim 1, wherein a surface of the active material main body is modified so that wettability with the resin is improved.   The active material for a lithium ion secondary battery according to claim 2, wherein a surface of the active material main body is hydrophobized. The active material body is represented by the general _{formula: LiCo p Ni q Mn r D} s O 2 (D is the doping component, at least Al, Mg, Ti, Sn, Zn, W, Zr, Mo, selected from Fe and Na And a lithium-cobalt-containing composite metal oxide represented by p + q + r + s = 1, 0 <p ≦ 1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1),
3. The lithium ion secondary battery according to claim 1, wherein the resin is at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene vinyl alcohol, and polyacrylate. 4. Active material.   The active material for a lithium ion secondary battery according to claim 3, wherein the resin is polyvinylidene fluoride (PVDF). A current collector,
An active material layer formed on a surface of the current collector, the active material layer including the active material for a lithium ion secondary battery according to any one of claims 1 to 5 and a binder for forming an active material layer,
An electrode for a lithium ion secondary battery.   The lithium ion secondary battery which has an electrode for lithium ion secondary batteries of Claim 6.