JP7017108B2 - Active materials, electrodes and lithium-ion secondary batteries - Google Patents

Description

The present invention relates to active materials, electrodes and lithium ion secondary batteries.

Lithium-ion secondary batteries are lighter and have higher capacity than nickel-cadmium batteries, nickel-metal hydride batteries, and the like, and are widely used as power sources for portable electronic devices. Lithium-ion secondary batteries are also promising candidates for power sources for hybrid and electric vehicles. The demand for miniaturization, high functionality, and high capacity of power sources for automobiles is increasing year by year, and it is expected that the capacity of lithium ion batteries will be further increased.

Since the performance of a lithium ion secondary battery largely depends on the active material of the electrode, studies are being conducted to improve the performance of the active material. For example, Patent Document 1 describes active material particles whose surface is coated with a solid electrolyte. By coating the surface with a solid electrolyte having high mechanical strength and excellent ionic conductivity, expansion and contraction of active material particles are suppressed, and the cycle characteristics of the lithium ion secondary battery are improved.

Further, Patent Document 2 describes active material particles whose surface is coated with oxide glass. The oxide glass relaxes the expansion and contraction of the active material particles and suppresses the peeling of the contact surface between the solid electrolyte and the active material particles.

Japanese Unexamined Patent Publication No. 2003-59492 Japanese Unexamined Patent Publication No. 2014-22204

However, the active materials described in Patent Documents 1 and 2 could not sufficiently improve the charge / discharge characteristics of the lithium ion secondary battery. In particular, it was not possible to sufficiently suppress the decrease in discharge capacity when high-rate discharge was performed after the cycle had elapsed.

For example, the active material particles described in Patent Document 1 use a crystalline coating layer in order to increase the mechanical strength. However, since a crystalline coating layer is used, the discharge capacity is reduced due to the modification of the composition of the coating layer as the number of cycles increases.

Further, for example, the active material particles described in Patent Document 2 use a low melting point oxide glass as a coating layer in order to alleviate volume changes due to expansion and contraction. However, in this method, since the solid electrolyte is amorphous, the conductivity of lithium ions is not sufficient, and sufficient characteristics cannot be obtained by charging and discharging at a high rate.

The present invention has been made in view of the above problems, and provides an active material capable of improving charge / discharge characteristics, and an electrode and a lithium ion secondary battery having excellent charge / discharge characteristics by using this active material. The purpose.

As a result of diligent studies, the present inventors have found that the charge / discharge characteristics of a lithium ion secondary battery can be improved by using an amorphous composite oxide having a small amount of lithium on the surface of the active material.
That is, in order to solve the above problems, the following means are provided.

(1) The active material according to the first aspect has a core portion and a coating layer covering the core portion, and the coating layer is an amorphous composite oxide containing Li, Ti and P. The composition ratio of these elements in the amorphous composite oxide satisfies the relationship of the following general formula (1).
Li: Al: Ti: P = (1 + ab): a: (2-a): 3 ... (1)
(However, 0 ≦ a ≦ 1, 0 <b <1)

(2) In the general formula (1) of the active material according to the above embodiment, a may be in the range of 0 <a ≦ 1.

(3) The film thickness of the coating layer in the active material according to the above aspect may be 30 nm or more and 70 nm or less.

(4) The electrode according to the second aspect contains the active material according to the above aspect.

(5) The lithium ion secondary battery according to the third aspect includes an electrode according to the above aspect and a counter electrode facing the electrode.

The active material according to the above aspect can improve the charge / discharge characteristics of the lithium ion secondary battery.

It is sectional drawing of the lithium ion secondary battery which concerns on this embodiment. It is sectional drawing of the positive electrode which concerns on this embodiment. It is sectional drawing of the positive electrode active material which concerns on this embodiment. It is a figure which showed typically the mobility of lithium ion in the vicinity of the interface between a positive electrode active material which does not have a coating layer, and an electrolytic solution. It is a figure which showed schematically the mobility of lithium ion in the vicinity of the interface between a positive electrode active material which has a crystalline coating layer, and an electrolytic solution. It is a figure which shows typically the mobility of lithium ion in the vicinity of the interface between a positive electrode active material and an electrolytic solution which concerns on this embodiment.

Hereinafter, the present embodiment will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience in order to make the features of the present invention easy to understand, and the dimensional ratios of each component may differ from the actual ones. be. The materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

[Lithium-ion secondary battery]
FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery according to the present embodiment. The lithium ion secondary battery 100 shown in FIG. 1 includes a laminated body 40, an exterior body 50 that houses the laminated body 40 in a sealed state, and a pair of leads 60 and 62 connected to the laminated body 40. The ends of the leads 60 and 62 extend to the outside of the exterior body 50. Further, although not shown, the electrolytic solution is housed in the exterior body 50 together with the laminated body 40.

(Laminated body)
The laminate 40 has a positive electrode 20, a negative electrode 30, and a separator 10. The positive electrode 20 and the negative electrode 30 are arranged so as to face each other with the separator 10 interposed therebetween. In FIG. 1, the case where one laminated body 40 is provided in the exterior body 50 is illustrated, but a plurality of laminated bodies 40 may be laminated. Further, a wound body may be used instead of the laminated body 40.

"Positive electrode"
The positive electrode 20 has a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is arranged on both sides of the positive electrode current collector 22 except for the uppermost surface or the lowermost surface of the laminated body 40. Since the positive electrode 20 arranged on the uppermost surface or the lowermost surface of the laminated body 40 does not have the negative electrode 30 facing the positive electrode 20, it is not necessary to provide the positive electrode active material layer 24.

<Positive current collector>
The positive electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

<Positive electrode active material layer>
FIG. 2 is an enlarged cross-sectional schematic view of a main part in the vicinity of the positive electrode according to the present embodiment. The cross section of FIG. 2 is an arbitrary surface orthogonal to the surface on which the positive electrode 20 extends. As shown in FIG. 2, the positive electrode active material layer 24 has a positive electrode active material 26, a conductive auxiliary agent 28, and a binder (not shown). The space K between the positive electrode active material 26 and the conductive auxiliary agent 28 is impregnated with the electrolytic solution.

[Positive electrode active material]
FIG. 3 is a schematic cross-sectional view of the positive electrode active material 26 according to the present embodiment. In FIG. 3, the cross section of the positive electrode active material 26 is shown as a circle for simplicity, but the shape of the positive electrode active material 26 can be any shape.

As shown in FIG. 3, the positive electrode active material 26 has a core portion 26C and a coating layer 26S. The coating layer 26S may cover at least a part of the core portion 26C, and preferably covers the entire surface of the core portion 26C.

Lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or doping and dedoping of lithium ion and lithium ion counter anion (for example, PF6-) are reversed in the core portion 26C. An electrode active material that can be promoted is used.

For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and general formula: LiNi _{x Coy Mn z Ma 2} ( x + y + z + a = 1, 0≤x <1, 0≤y <1, 0≤z <1, 0≤a <1, M is one or more types selected from Al, Mg, Nb, Ti, Cu, Zn, Cr. Composite metal oxide represented by element), lithium vanadium compound (LiV ₂ O ₅ ), olivine type LiMPO ₄ (however, M is selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr). (Representing one or more elements or VOs), composite metal oxides such as lithium titanate (Li ₄ Ti ₅ O ₁₂ ), LiNi _x Coy Al _z O ₂ (0.9 <x + _y + z <1.1), Polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene and the like can be mentioned as substances constituting the core portion 26C.

In general, in a positive electrode active material, an irreversible structural phase transition occurs when the Li ion desorption reaction is locally concentrated. The portion where this irreversible structural phase transition occurs cannot contribute to the charge / discharge capacity as an active material, which causes capacity deterioration. LiNi _x Coy Al _z O ₂ (0.9 <x + _y + z <1.1, hereinafter referred to as "NCA") has a larger amount of Li ions that can be desorbed than other active substances, and is brought into direct contact with the electrolytic solution. In this case, irreversible structural phase transitions are likely to occur locally near the surface of the active material. Therefore, if NCA is used for the core portion 26C and the core portion 26C is coated with the coating layer 26S, the desorption reaction of lithium ions via the coating layer 26S becomes possible, and even during high-rate charging / discharging, the entire active material is evenly distributed. It becomes possible to desorb lithium ions. That is, when NCA is used for the core portion 26C, the effect of providing the covering layer 26S can be remarkably obtained.

The coating layer 26S is an amorphous composite oxide containing Li, Ti and P. The amorphous coating layer 26S alleviates the difference in lithium ion concentration between the crystalline core portion 26C and the electrolytic solution, and an irreversible structural phase transition portion is generated near the surface of the active material during high-rate charging / discharging. In order to prevent this, it is possible to suppress a decrease in charge / discharge capacity after a high rate cycle.

FIG. 4 is a diagram schematically showing the concentration of lithium ions in the vicinity of the interface between the positive electrode active material 26'without a coating layer and the electrolytic solution 70. The solid line in FIG. 4 represents the lithium ion concentration immediately after the start of operation of high-rate discharge. The one-point chain line means the interface between the positive electrode active material 26'and the electrolytic solution 70, the left side is the positive electrode active material 26', and the right side is the electrolytic solution 70. The electric double layer 71 is formed between the alternate long and short dash line in the electrolytic solution, and the diffusion layer 72 is formed on the outside thereof.

In the positive electrode active material 26', lithium ions move between the crystal lattices of the layered structure. On the other hand, in the electrolytic solution 70, the lithium ions are solvated in the vicinity of the electric double layer 71 so that they can move freely. Since the positive electrode active material 26'is a crystal, the difference in concentration of lithium ions becomes large at the interface with the electric double layer 71. Therefore, during high-rate charging, lithium ions are intensively desorbed from the vicinity of the surface of the positive electrode active material 26', and the concentration of lithium ions decreases near the surface of the positive electrode active material 26'. When the lithium ion concentration near the surface of the positive electrode active material 26'is lower than the lithium ion concentration inside the positive electrode active material when fully charged (reference numeral B1), it is irreversible near the surface of the positive electrode active material 26'. A structural phase transition occurs. At the location where the irreversible structural phase transition occurs, the decrease in lithium ion concentration cannot be resolved.

On the other hand, at the time of high-rate discharge, the decrease in the lithium ion concentration near the surface of the electric double layer 71 and the positive electrode active material 26 ′ has an advantageous effect of drawing in the lithium ions diffused in the electrolytic solution 70. For this reason, the capacity deterioration due to the irreversible structural phase transition does not become apparent in the discharge capacity at the time of high-rate discharge, but the capacity deterioration becomes apparent for the first time by comparing the discharge capacities after about 100 cycles. do.

That is, first, the portion where the irreversible structural phase transition occurs near the surface of the positive electrode active material cannot contribute to the charge / discharge capacity as the active material, and the capacity deteriorates. Secondly, during charge / discharge after a high rate cycle, the capacity decrease due to the irreversible structural phase transition near the surface of the positive electrode active material 26'becomes apparent. Deterioration is noticeable.

Here, the high rate means the charge / discharge rate. Assuming that a cell having a capacity of a nominal capacity value is discharged with a constant current and the current value at which charging / discharging is completed in 1 hour is 1C, a high-rate charging / discharging rate means a case where the cell is discharged with a current larger than 1C.

FIG. 5 is a diagram schematically showing the lithium ion concentration in the vicinity of the interface between the positive electrode active material having the crystalline coating layer 26S'and the electrolytic solution. The solid line in FIG. 5 represents the lithium ion concentration immediately after the start of operation during high-rate discharge. The alternate long and short dash line means the interface between the core portion 26C'of the positive electrode active material and the coating layer 26S', and the long chain line means the interface between the coating layer 26S'of the positive electrode active material and the electrolytic solution 70. The electric double layer 71 is formed in the intermediate portion between the long chain wire and the alternate long and short dash line in the electrolytic solution, and the diffusion layer 72 is formed on the outside thereof.

As shown in FIG. 5, when the crystalline coating layer 26S'is provided, an interface between the crystals is formed, so that the difference in lithium ion concentration is small, and the surface of the core portion 26C' is charged even at a high rate. No decrease in lithium ion concentration occurs in the vicinity. (Reference B2) Therefore, the capacity reduction peculiar to high-rate charging does not occur.

However, since the coating layer 26S'is a crystal that does not contribute to charging and discharging, it has a large lithium ion concentration difference at the interface between the coating layer 26S'and the electric double layer 71 at the time of discharging. Therefore, in the positive electrode active material having the crystalline coating layer 26S', this difference in lithium ion concentration becomes a barrier when discharging, and lithium ions are transferred from the electrolytic solution 70 to the coating layer 26S'of the positive electrode active material. It becomes difficult to conduct ions. Therefore, the capacity cannot be obtained at the time of discharging. In particular, capacity deterioration becomes remarkable in high-rate discharge accompanied by a sudden change in lithium ion concentration.

On the other hand, FIG. 6 is a diagram schematically showing the concentration of lithium ions in the vicinity of the interface between the positive electrode active material and the electrolytic solution according to the present embodiment. The solid line in FIG. 6 represents the lithium ion concentration at the time of high rate charging. The alternate long and short dash line means the interface between the core portion 26C of the positive electrode active material 26 and the coating layer 26S, and the long chain wire means the interface between the coating layer 26S of the positive electrode active material 26 and the electrolytic solution 70. The electric double layer 71 is formed in the intermediate portion between the long chain wire and the alternate long and short dash line in the electrolytic solution, and the diffusion layer 72 is formed on the outside thereof.

As shown in FIG. 6, when the amorphous coating layer 26S is provided, the lithium ion concentration difference between the vicinity of the surface of the core portion 26C and the coating layer 26S forms a direct interface with the electric double layer 71 as shown in FIG. The change in the lithium ion concentration is small in the vicinity of the surface of the core portion 26C during high-rate charging, as in the case where the crystalline coating layer 26S'is provided. (Reference B3) Therefore, an irreversible structural phase transition does not occur near the surface of the core portion 26C. Therefore, the capacity reduction peculiar to high-rate charging does not occur.

Further, the amorphous coating layer 26S according to the present embodiment is formed of an amorphous composite oxide satisfying the relationship of the following general formula (1).
Li: Al: Ti: P = (1 + ab): a: (2-a): 3 ... (1)
(However, 0 ≦ a ≦ 1, 0 <b ≦ 1)

Li _{1 + x} Al _x Ti _2-x (PO ₄ ) ₃ is known as a crystalline composite oxide containing Li, Ti and P. In the case of a crystalline composite oxide, the range of crystals that can be taken by each element is crystallographically limited, but in the case of an amorphous composite oxide, the range of lithium that can be taken by the crystal structure is limited. do not have. That is, the coating layer 26S according to the present embodiment is produced by a predetermined method capable of adjusting the valence of Ti, so that the coating layer 26S satisfies the relationship of the general formula (1) by b minutes with respect to the crystalline composition. Lithium-deficient compositions can also be formed.

By forming the coating layer 26S with a composition having a low lithium ion concentration by the above method, the lithium ion concentration of the coating layer 26S is lower than the lithium ion concentration inside the core portion 26C in the discharged state, as shown in FIG. can do. Therefore, in the positive electrode active material having the amorphous coating layer 26S, the difference in lithium ion concentration at the interface between the coating layer 26S of the positive electrode active material and the electric double layer 71 is suppressed to a low level when high-rate discharge is performed. Compared with the crystalline coating layer 26S'as shown in FIG. 5, the lithium ion concentration barrier is low, and it is easy to obtain a discharge capacity even at a high rate. Therefore, the difference in discharge capacity becomes remarkable especially in the discharge after the high rate cycle.

The thickness of the coating layer 26S is preferably 30 nm or more and 70 nm or less. When the thickness of the coating layer 26S is within the range, the coating layer 26S can sufficiently alleviate the difference in the concentration of lithium ions between the electrolytic solution 70 and the core portion 26C. At the same time, it is possible to suppress the occurrence of defects such as cracks in the coating film 26S, and it is possible to form a stable film.

Further, in the amorphous composite oxide of the above general formula (1) forming the amorphous coating layer 26S according to the present embodiment, a may be in the range of 0 <a ≦ 1.

The fact that a is in the range of 0 <a≤1 means that the amorphous composite oxide of the general formula (1) forming the amorphous coating layer 26S contains Al, and has such a configuration. This improves the stability of the amorphous composite oxide.

As described above, according to the positive electrode active material 26 according to the present embodiment, the occurrence of irreversible phase transition near the surface of the core portion 26C can be suppressed, and the coating layer 26S causes lithium ions between the electrolytic solution 70 and the core portion 26C. The difference in concentration can be sufficiently alleviated. Further, the lithium content of the coating layer 26S constituting the positive electrode active material 26 can be reduced, and the high-rate discharge capacity can be increased by relaxing the difference in the concentration of lithium ions between the coating layer 26S and the electrolytic solution 70. can.

[Conductive aid (positive electrode)]
Examples of the conductive auxiliary agent include carbon powder such as carbon black, carbon nanotube, carbon material, metal fine powder such as copper, nickel, stainless steel and iron, a mixture of carbon material and metal fine powder, and conductive oxide such as ITO. Be done. Among these, a carbon material such as carbon black is preferable. When sufficient conductivity can be ensured only by the positive electrode active material, the lithium ion secondary battery 100 does not have to contain a conductive auxiliary agent.

[Binder (positive electrode)]
The binder binds the active materials to each other and also binds the active material to the positive electrode current collector 22. The binder may be any one capable of the above-mentioned binding, for example, polyfluorinated vinylidene (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-. Perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) ) And the like.

In addition to the above, as binders, for example, vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-). TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), Vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluororubber (VDF-PFMVE-TFE fluororubber), vinylidene fluoride-chlorotrifluoroethylene fluororubber (VDF-CTFE fluororubber), etc. A fluororubber may be used.

The composition ratio of the positive electrode active material in the positive electrode active material layer 24 is preferably 94.0% or more and 97.0% or less in terms of mass ratio. The composition ratio of the conductive auxiliary agent in the positive electrode active material layer 24 is preferably 1.0% or more and 3.0% or less in terms of mass ratio, and the composition ratio of the binder in the positive electrode active material layer 24 is 1 in terms of mass ratio. It is preferably 8.8% or more and 2.8% or less.

"Negative electrode"
The negative electrode has a negative electrode current collector 32 and a negative electrode active material layer 34 (see FIG. 1). The negative electrode active material layer 34 has a negative electrode active material, and further has a negative electrode binder and a conductive auxiliary agent, if necessary.

<Negative electrode current collector>
The negative electrode current collector 32 may be a conductive plate material, and for example, a thin metal plate made of aluminum, copper, or nickel foil can be used.

<Negative electrode active material layer>
[Negative electrode active material]
The negative electrode active material may be any compound that can occlude and release lithium ions, and a known negative electrode active material for a lithium secondary battery can be used. Examples of the negative electrode active material include metallic lithium, carbon materials such as graphite capable of storing and releasing lithium ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, easily graphitized carbon, and low-temperature calcined carbon. Metals that can be combined with lithium such as aluminum, silicon, and tin, SiO _x (0 <x <2), amorphous compounds mainly composed of oxides such as tin dioxide, lithium titanate (Li ₄ Ti ₅ ) Examples thereof include particles containing O ₁₂ ) and the like.

[Conductive aid (negative electrode)]
Examples of the conductive auxiliary agent include carbon powder such as carbon black, carbon nanotube, carbon material, metal fine powder such as copper, nickel, stainless steel and iron, a mixture of carbon material and metal fine powder, and conductive oxide such as ITO. Can be mentioned.

[Binder (negative electrode)]
The binder used for the negative electrode can be the same as that used for the positive electrode. In addition to this, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin and the like may be used as the binder.

Further, as the binder, an electron conductive conductive polymer or an ion conductive conductive polymer may be used. Examples of the electron-conducting conductive polymer include polyacetylene and the like. In this case, since the binder also exerts the function of the conductive auxiliary agent particles, it is not necessary to add the conductive auxiliary agent. As the ionic conductive polymer, for example, one having ionic conductivity such as lithium ion can be used, and for example, a polymer compound (polyether-based polymer compound such as polyethylene oxide and polypropylene oxide) can be used. , Polyphosphazene, etc.) and a lithium salt such as LiClO ₄ , LiBF ₄ , LiPF ₆ or an alkali metal salt mainly composed of lithium, and the like. Examples of the polymerization initiator used for the composite include a photopolymerization initiator or a thermal polymerization initiator compatible with the above-mentioned monomers.

The contents of the negative electrode active material, the conductive material and the binder in the negative electrode active material layer 34 are not particularly limited. The composition ratio of the negative electrode active material in the negative electrode active material layer 34 is preferably 70% or more and 98% or less in terms of mass ratio. The composition ratio of the conductive material in the negative electrode active material layer 34 is preferably 1% or more and 20% or less in terms of mass ratio, and the composition ratio of the binder in the negative electrode active material layer 34 is 1% or more and 10% or less in terms of mass ratio. Is preferable.

By setting the contents of the negative electrode active material and the binder within the above range, it is possible to suppress the tendency that the amount of the binder is too small to form a strong negative electrode active material layer in the obtained negative electrode active material layer 34. In addition, the amount of the binder that does not contribute to the electric capacity increases, and the tendency that it becomes difficult to obtain a sufficient volumetric energy density can be suppressed.

"Separator"
The separator 10 may be formed of an electrically insulating porous structure, for example, a monolayer of a film made of polyethylene, polypropylene or polyolefin, a laminated body or a stretched film of a mixture of the above resins, or cellulose, polyester and Examples thereof include fibrous nonwoven fabrics made of at least one constituent material selected from the group consisting of polypropylene.

"Electrolytic solution"
As the electrolytic solution, an electrolyte solution containing a lithium salt (an aqueous electrolyte solution, an electrolyte solution using an organic solvent) can be used. However, since the decomposition voltage of the aqueous electrolyte solution is electrochemically low, the withstand voltage during charging is low and limited. Therefore, it is preferable to use an electrolyte solution (non-aqueous electrolyte solution) that uses an organic solvent.

The non-aqueous electrolyte solution has an electrolyte dissolved in a non-aqueous solvent, and may contain a cyclic carbonate and a chain carbonate as the non-aqueous solvent.

As the cyclic carbonate, one capable of solvating an electrolyte can be used. For example, ethylene carbonate, propylene carbonate, butylene carbonate and the like can be used.

The chain carbonate can reduce the viscosity of the cyclic carbonate. For example, diethyl carbonate, dimethyl carbonate and ethylmethyl carbonate can be mentioned. In addition, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane and the like may be mixed and used.

The ratio of cyclic carbonate to chain carbonate in the non-aqueous solvent is preferably 1: 9 to 1: 1 in volume.

Examples of the electrolyte include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiN (CF ₃ CF). ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB and other lithium salts can be used. It should be noted that one of these lithium salts may be used alone, or two or more thereof may be used in combination. In particular, from the viewpoint of the degree of ionization, it is preferable to contain LiPF ₆ .

When dissolving LiPF6 in a non-aqueous solvent, it is preferable to adjust the concentration of the electrolyte in the non-aqueous electrolyte solution to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the lithium ion concentration of the non-aqueous electrolyte solution can be sufficiently secured, and a sufficient capacity can be easily obtained at the time of charging / discharging. Further, by suppressing the concentration of the electrolyte to 2.0 mol / L or less, it is possible to suppress the increase in the viscosity of the non-aqueous electrolyte solution, sufficiently secure the mobility of lithium ions, and obtain a sufficient capacity during charging and discharging. It will be easier.

Even when LiPF 6 is mixed with other electrolytes, it is preferable to adjust the lithium ion concentration in the non-aqueous electrolyte solution to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol% or more thereof. It is more preferable that it is contained.

"Exterior body"
The exterior body 50 seals the laminated body 40 and the electrolytic solution inside the exterior body 50. The exterior body 50 suppresses leakage of the electrolytic solution to the outside and invasion of water and the like into the inside of the lithium ion secondary battery 100 from the outside.

For example, as the exterior body 50, as shown in FIG. 1, a metal laminating film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. As the metal foil 52, for example, an aluminum foil can be used, and as the polymer film 54, a film such as polypropylene can be used. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, for example, polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferable.

"Lead"
The leads 60 and 62 are made of a conductive material such as aluminum. Then, the leads 60 and 62 are welded to the positive electrode current collector 22 and the negative electrode current collector 32, respectively, by a known method, and a separator is provided between the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 34 of the negative electrode 30. With the 10 sandwiched between them, they are inserted into the exterior body 50 together with the electrolytic solution to seal the entrance of the exterior body 50.

As described above, since the lithium ion secondary battery according to the present embodiment contains a predetermined positive electrode active material, the conductivity of lithium ions between the positive electrode active material and the electrolytic solution in the positive electrode is high. That is, the lithium ion secondary battery according to the present embodiment is excellent in charge / discharge characteristics, and particularly excellent in high-rate discharge characteristics.

[Manufacturing method of lithium ion secondary battery]
First, the positive electrode active material 26 is prepared (see FIG. 3). The core portion 26C of the positive electrode active material 26 can be produced by a known method such as a solid phase reaction method. Next, the surface of the core portion 26C is covered with the coating layer 26S.

The coating layer 26S is formed by using a dry manufacturing method such as a powder sputtering method (also referred to as a barrel sputtering method). In the powder sputtering method, a target is placed in the center of the barrel, and lithium composite compound particles constituting the core portion 26C are placed on the inner peripheral surface of the barrel. Then, by sputtering in a vacuum while rotating the barrel, the coating layer 26S is formed on the surface of the core portion 26C, and the positive electrode active material 26 according to the present embodiment is obtained.

The thickness, film quality, etc. of the coating film 26S to be formed can be freely designed by changing the ultimate pressure, applied voltage, gas flow rate, and target-barrel distance.

By using a dry production method such as a powder sputtering method, an amorphous coating film having a large amount of Li can be formed. Ceramic materials have a very high melting point and are difficult to amorphize using the liquid quenching method. Therefore, in order to form an amorphous coating film, it is preferable to use a dry method such as a sputtering method or a CVD method.

In the dry manufacturing method, all the elements used as raw materials are ionized once in a vacuum environment. Therefore, a coating film can be obtained by separating the electrons in the raw material to generate plasma and recombination with the plasma electrons. Therefore, in the case of Ti having a plurality of valences, it is easy to obtain an oxidation number different from that of the raw material, and it is easy to form a coating film containing a large amount of Li ions.

Further, when the powder sputtering method is used, it is possible to form a film in which the amount of lightweight Li ions is selectively reduced by changing the production conditions such as gas conditions and electric power. For example, if the amount of Ar gas at the time of film formation is increased, the lightweight Li is diffused in the chamber, and the composition ratio can be reduced. On the other hand, if the amount of Ar gas is increased too much, the film forming speed is lowered and the productivity is lowered. In order to secure the film formation rate while reducing the Li composition ratio in the coating film 26S, it is preferable to set the Ar gas amount at the time of film formation to about 60 sccm.

As described above, it is preferable to use the powder sputtering method in order to form an amorphous coating film in which the Li content of b represented by the general formula (1) is reduced.

Next, the positive electrode 20 is manufactured using the prepared positive electrode active material 26. A paint is prepared by mixing the positive electrode active material 26, a binder and a solvent. If necessary, a conductive auxiliary agent may be further added. As the solvent, for example, water, N-methyl-2-pyrrolidone, N, N-dimethylformamide and the like can be used. The composition ratio of the positive electrode active material, the conductive auxiliary agent, and the binder is preferably 80 wt% to 90 wt%: 0.1 wt% to 10 wt%: 0.1 wt% to 10 wt% in terms of mass ratio. These mass ratios are adjusted to be 100 wt% as a whole.

The method of mixing these components constituting the paint is not particularly limited, and the mixing order is also not particularly limited. The above paint is applied to the positive electrode current collector. The coating method is not particularly limited, and a method usually adopted when manufacturing an electrode can be used. For example, a slit die coat method and a doctor blade method can be mentioned.

Subsequently, the solvent in the paint applied on the positive electrode current collector and the negative electrode current collector is removed. The removal method is not particularly limited. For example, the positive electrode current collector and the negative electrode current collector coated with the paint may be dried in an atmosphere of 80 ° C. to 150 ° C. The positive electrode 20 and the negative electrode 30 differ only in the material used as the active material, and the negative electrode 30 can also be manufactured by the same manufacturing method.

The positive electrode 20, the negative electrode 30, and the separator 10 are laminated so that the separator 10 is between the positive electrode 20 and the negative electrode 30 to form the laminated body 40.

Finally, the laminated body 40 is enclosed in the exterior body 50. The non-aqueous electrolyte solution may be injected into the exterior body 50, or the laminated body 40 may be impregnated into the non-aqueous electrolyte solution. Then, heat or the like is applied to the exterior body 50, and the exterior body 50 is sealed by laminating to produce a non-aqueous electrolyte secondary battery 100.

Although the embodiments of the present invention have been described in detail with reference to the drawings, the configurations and combinations thereof in each embodiment are examples, and the configurations may be added or omitted within a range not deviating from the gist of the present invention. , Replacements, and other changes are possible.

"Example 1"
First, a positive electrode active material was prepared. 50 g of LiNi _0.8 Co _{0.15 Al 0.05} O ₂ was prepared as the core portion of the positive electrode active material.

Then, a coating film was formed on the surface of the prepared core portion by using the powder sputtering method. Li _1.0 Ti _2.0 (PO ₄ ) ₃ was used as the target. Then, under the conditions of an ultimate pressure of 6.0 × 10 ^-4 Pa, an Ar gas flow rate of 60 sccm, RF, 400 W, and a target-barrel distance of 20 cm, film formation was performed for 1 hour while rotating the barrel.

Immediately after the production of the positive electrode active material, a film was formed on the Si wafer under the same sputtering conditions (however, the rollers were stopped). The composition and film thickness of the coating film were identified using the film formed on this Si wafer. The film thickness was measured using a profilometer and the composition was measured by ICP analysis. The results are shown in Table 1.

Next, the prepared positive electrode active material and acetylene black were mixed, and polyvinylidene fluoride (PVDF) dissolved in n-methylpyrrolidone (NMP) was added as a binder to prepare a slurry. The mixing ratio of the positive electrode active material, acetylene black, and PVDF was 97.5: 1.0: 1.5 (weight ratio). This slurry was applied to an Al current collector, dried, and then pressed to prepare a positive electrode.

Next, a half-cell lithium-ion secondary battery was produced using this positive electrode as a working electrode. For the counter electrode facing the working electrode, Li foil was attached on a copper foil having a thickness of 16 μm. A polyethylene microporous membrane was used as the separator. As the external lead-out terminal, an aluminum lead (width 4 mm, length 40 mm, thickness 100 μm) and a nickel lead (width 4 mm, length 40 mm, thickness 100 μm) were used. As the electrolytic solution, a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 3: 7 and an electrolytic solution containing lithium hexafluorophosphate (LiPF6) at a concentration of 1 mol / L were used. .. Then, these were vacuum-sealed inside the exterior to prepare the lithium ion secondary battery of Example 1.

Then, the electrical characteristics of the manufactured lithium ion secondary battery were measured. In a constant temperature bath at 25 ° C., the battery was charged with a constant current up to 4.3 V at a current density corresponding to 3 C, and then charged with a constant voltage at 4.3 V. Constant voltage charging was continued until the current density dropped to a value corresponding to 0.05 C. Then, a constant current discharge was performed up to 3.0 V at a current density of 3C, and this was repeated for 100 cycles. The discharge capacity of Example 1 was 26.2 mAh / g. The results are shown in Table 1.

Further, by observation with a transmission electron microscope (TEM), it was confirmed that a coating layer having a diameter of 50 nm was present in the positive electrode active material, and a lattice pattern could not be obtained from the lattice image of this coating layer. From this, it can be said that the coating layer is composed of an amorphous substance containing a composition ratio of Li _0.9 Ti _2.0 (PO ₄ ) ₃ obtained from the ICP analysis result.

(Example 2 and Examples 9 to 10)
Examples 2 and 9 to 10 differ from Example 1 in that the film thickness of the coating layer is changed. The film thickness of the coating layer was changed by changing the film forming time.

In Example 2, the Ar gas flow rates are 100 sccm and 60 sccm, the applied power is 400 W, the target is Li _1.0 Ti _2.0 (PO ₄ ) ₃ , and the film formation time is 10 hours, respectively. It was set to 3 hours and 7 hours, respectively.

Then, the other conditions were the same as in Example 1, and the electrical characteristics of the lithium ion secondary battery were measured. The results are shown in Table 1.

(Comparative Example 1)
Comparative Example 1 is different from Example 1 in that the coating layer of the positive electrode active material is changed to a crystalline substance. Specifically, using NOB-MINI manufactured by Hosokawa Micron Co., Ltd., LiNi _0.8 Co _0.15 Al _0.05 O ₂ (particle size 20 μm, 500 g) and Li _1.3 Al _0.3 Ti _{1. 7} (PO ₄ ) ₃ was added at a ratio of 3 wt%, and the coating was performed by compounding. A surface-coated positive electrode active material was obtained by performing a compounding treatment at a rotation speed of 1800 rpm for 20 minutes.

Further, by observing the positive electrode active material particles of Comparative Example 1 with a transmission electron microscope (TEM), it was confirmed that a coating layer having a diameter of 50 nm was present in the positive electrode active material, and the lattice image of the coating layer showed a lattice pattern. was gotten. From this, it can be said that the coating layer is a crystal composed of the composition ratio of Li _1.3 Al _0.3 Ti _1.3 (PO ₄ ) ₃ obtained by ICP analysis.

Then, the other conditions were the same as in Example 1, and the electrical characteristics of the lithium ion secondary battery were measured. The results are shown in Table 1.

(Comparative Examples 2 and 3)
Comparative Examples 2 and 3 are different from Comparative Example 2 in that the composition of the coating layer is changed. In Comparative Example 2, Li _0.5 Ti _2.0 (PO ₄ ) ₃ was used instead of Li _1.3 Al _0.3 Ti _1.3 (PO ₄ ) ₃ , and in Comparative Example 3, Li _1.3 was used. Li _1.0 Al _1.0 Ti _1.0 (PO ₄ ) ₃ was used instead of Al _0.3 Ti _1.3 (PO ₄ ) ₃ . The other conditions were the same as in Example 1, and the electrical characteristics of the lithium ion secondary battery were measured. The results are shown in Table 1.

(Examples 3 to 4 and Examples 11 to 12)
Examples 3 to 4 and Examples 11 to 12 differ from Examples 1 and 2 and Examples 11 to 12 in that the composition of the coating layer is changed. In Examples 3 to 4 and Examples 11 to 12, the composition of the coating layer was changed by changing the gas amount condition at the time of film formation, and the film thickness was changed by changing the film formation time.

In Examples 3 to 4 and Examples 11 to 12, the Ar gas flow rate was 100 sccm, the applied power was 400 W, the target was Li _1.0 Ti _2.0 (PO ₄ ) ₃ , and the film formation time was 2 hours, respectively. It was set to 20 hours, 6 hours, and 14 hours.

Then, the other conditions were the same as in Example 1, and the electrical characteristics of the lithium ion secondary battery were measured. The results are shown in Table 1.

(Examples 5 to 6 and 13 and 17 to 18)
Examples 5 to 6 and Examples 13 and 17 to 18 are different from Example 1 in that the composition and the film thickness of the coating layer are changed. In Examples 5 to 6, 13 and 17 to 18, the composition of the coating layer was changed by changing the target, and the film thickness was changed by changing the film forming time.

In Examples 5 to 6 and Examples 13 and 17 to 18, the Ar gas flow rate is 60 sccm, the applied power is 400 W, the target is Li _1.1 Al _0.1 Ti _1.9 (PO ₄ ) ₃ , and the film is formed. The time was set to 1 hour, 10 hours, 5 hours, 3 hours, and 7 hours, respectively.

Then, the other conditions were the same as in Example 1, and the electrical characteristics of the lithium ion secondary battery were measured. The results are shown in Table 1.

(Examples 7 to 8 and 14 and 19 to 20)
Examples 7 to 8 and Examples 14 and 19 to 20 are different from Example 1 in that the composition and the film thickness of the coating layer are changed. In Examples 7 to 8 and 14 and 19 to 20, the composition of the coating layer was changed by changing the target, and the film thickness was changed by changing the film forming time.

In Examples 7 to 8 and Examples 14 and 19 to 20, the Ar gas flow rate was 100 sccm, the applied power was 400 W, and the target was Li _2.0 Al _1.0 Ti _1.0 (PO ₄ ) ₃ . The membrane time was set to 2 hours, 20 hours, 10 hours, 6 hours, and 14 hours, respectively.

Then, the other conditions were the same as in Example 1, and the electrical characteristics of the lithium ion secondary battery were measured. The results are shown in Table 1.

(Examples 15 to 16)
Examples 15 to 16 differ from Examples 13 to 14 in that the composition of the coating layer is changed. In Examples 15-16, the composition of the coating layer was changed by changing the target.

In Examples 15 to 16, the Ar gas flow rate is 80 sccm, the applied power is 400 W, and the targets are Li _1.5 Al _0.5 Ti _1.5 (PO ₄ ) ₃ , Li _1.3 Al _0.3 Ti _{1. 7} (PO ₄ ) ₃ was set, and the film forming time was set to 8 hours.

Then, the other conditions were the same as in Example 1, and the electrical characteristics of the lithium ion secondary battery were measured. The results are shown in Table 1.

(Comparative Example 4)
Comparative Example 4 is different from Example 1 in that the composition of the coating layer is changed. The composition of the coating layer was changed by changing the film formation target.

In Comparative Example 4, the Ar gas flow rate was 50 sccm, the applied power was 400 W, the target was Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , and the film formation time was 5 hours.
And said. ancestor

Other conditions were the same as in Example 1, and the electrical characteristics of each lithium ion secondary battery were measured. The results are shown in Table 1.

As shown in Table 1, Examples 1 to 11 having an amorphous coating layer were excellent in high-rate charge / discharge characteristics as compared with Comparative Examples 1 to 3 having a crystalline coating layer. Further, even in Comparative Example 4 in which the composition of the coating layer was not within the predetermined range and the Li content was high, it could not be said that the high-rate charge / discharge characteristics were sufficient.

10 ... Separator, 20 ... Positive electrode, 22 ... Positive electrode current collector, 24 ... Positive electrode active material layer, 26, 26'... Positive electrode active material, 26C, 26C' ... Core part, 26S, 26S' ... Coating film, 28 ... Conductive aid, 30 ... Negative electrode, 32 ... Negative electrode current collector, 34 ... Negative electrode active material layer, 40 ... Laminated body, 50 ... Exterior body, 52 ... Metal layer, 54 ... Polymer film, 60, 62 ... Lead, 70 ... Electrode electrode, 71 ... Electric double layer, 72 ... Diffusion layer, 100 ... Lithium ion secondary battery

Claims (5)

The core part that can occlude and release lithium ions ,
It has a coating layer that covers the core portion, and has.
The coating layer is an amorphous composite oxide containing Li, Ti and P, and is an amorphous composite oxide.
An active material for a lithium ion secondary battery in which the composition ratio of these elements in the amorphous composite oxide satisfies the relationship of the following general formula (1).
Li: Al: Ti: P = (1 + ab): a: (2-a): 3 ... (1)
(However, 0 ≦ a ≦ 1, 0.1 ≦ b ≦ 1) The active material according to claim 1, wherein a is in the range of 0 <a≤1 in the general formula (1). The active material according to claim 1 or 2, wherein the coating layer has a film thickness of 30 nm or more and 70 nm or less. An electrode containing the active material according to any one of claims 1 to 3. A lithium ion secondary battery comprising the electrode according to claim 4 and a counter electrode facing the electrode.

Images