JP2018170099A - Active material, electrode, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an active material capable of improving charge and discharge characteristics.SOLUTION: An active material according to an embodiment of the present invention includes a core portion and a coating layer covering the core portion, and the coating layer is an amorphous composite oxide containing Li, Al, Ti, and P, and the composition ratio of these elements in the amorphous composite oxide satisfies Li:Al:Ti:P=(1+x+y):x:(2-x):3 (1) (0≤x≤1, 0<y≤4).SELECTED DRAWING: Figure 3

Description

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

  Lithium ion secondary batteries are lighter and have a higher capacity than nickel cadmium batteries and nickel metal hydride batteries, and are widely used as power sources for portable electronic devices. Lithium ion secondary batteries are also promising candidates as power sources for hybrid vehicles and electric vehicles. The demand for miniaturization and high functionality of portable electronic devices and high capacity of power supplies for automobiles is increasing year by year, and further increase in capacity of lithium ion batteries is expected.

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

  Patent Document 2 describes active material particles whose surfaces are covered with oxide glass. The oxide glass relaxes the expansion and contraction of the active material particles and suppresses separation of the contact surface between the solid electrolyte and the active material particles.

JP 2003-59492 A JP 2014-22204 A

  However, the active materials described in Patent Documents 1 and 2 have not been able to sufficiently improve the charge / discharge characteristics of the lithium ion secondary battery. In particular, the decrease in discharge capacity when the discharge rate was high could not be sufficiently suppressed.

  For example, the active material particles described in Patent Document 1 use a crystalline coating layer in order to increase mechanical strength. Crystalline solid electrolytes and electrolytes differ greatly in ion mobility. Therefore, ions receive resistance when passing through these interfaces, and charge / discharge characteristics deteriorate. In particular, at the time of high-rate discharge, a sufficient ion conduction path cannot be secured, and the discharge capacity decreases.

  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 relieve volume change due to expansion and contraction. The oxide glass is coated by mixing an active material in the molten oxide glass. In this method, the ion mobility of the oxide glass is low, and sufficient charge / discharge characteristics cannot be obtained.

  This invention is made | formed in view of the said problem, and provides the active material which can improve charging / discharging characteristics, and the electrode and lithium ion secondary battery which were excellent in charging / discharging characteristics by using this active material. Objective.

As a result of intensive studies, the present inventors have found that the charge / discharge characteristics of a lithium ion secondary battery can be enhanced by using an amorphous composite oxide having a large 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 includes a core portion and a coating layer that covers the core portion, and the coating layer includes an amorphous composite containing Li, Al, Ti, and P. It is an oxide, and 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 + x + y): x: (2-x): 3 (1)
(However, 0 ≦ x ≦ 1, 0 <y ≦ 4)

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

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

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

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

  The active material which concerns on the said aspect can improve the charging / discharging characteristic of a lithium ion secondary battery.

It is a cross-sectional schematic diagram of the lithium ion secondary battery concerning this embodiment. It is a cross-sectional schematic diagram of the positive electrode concerning this embodiment. It is a cross-sectional schematic diagram of the positive electrode active material concerning this embodiment. It is the figure which showed typically the mobility of the lithium ion in the interface vicinity of the positive electrode active material which does not have a coating layer, and electrolyte solution. It is the figure which showed typically the mobility of the lithium ion in the interface vicinity of the positive electrode active material which has a crystalline coating layer, and electrolyte solution. It is the figure which showed typically the mobility of the lithium ion in the interface vicinity of the positive electrode active material concerning this embodiment, and electrolyte solution.

  Hereinafter, the present embodiment will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, in order to make the characteristics of the present invention easier to understand, there are cases where the characteristic parts are enlarged for the sake of convenience, and the dimensional ratios of the respective components are different from actual ones. is there. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to them, and can be appropriately modified and implemented without departing from the scope of the invention.

[Lithium ion secondary battery]
FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery according to this embodiment. A lithium ion secondary battery 100 illustrated in FIG. 1 includes a stacked body 40, an exterior body 50 that houses the stacked body 40 in a sealed state, and a pair of leads 60 and 62 connected to the stacked body 40. The ends of the leads 60 and 62 extend to the outside of the exterior body 50. Although not shown, the electrolyte solution is accommodated in the outer package 50 together with the laminate 40.

(Laminate)
The stacked body 40 includes the positive electrode 20, the negative electrode 30, and the separator 10. The positive electrode 20 and the negative electrode 30 are disposed to face each other with the separator 10 interposed therebetween. In FIG. 1, the case where there is one laminated body 40 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 includes a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is disposed on both surfaces of the positive electrode current collector 22 except for the uppermost surface or the lowermost surface of the laminate 40. Since the positive electrode 20 disposed on the uppermost surface or the lowermost surface of the stacked body 40 does not have the opposing negative electrode 30, it is not necessary to provide the positive electrode active material layer 24.

<Positive electrode 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 schematic cross-sectional view of the 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 includes a positive electrode active material 26, a conductive additive 28, and a binder (not shown). The space K between the positive electrode active material 26 and the conductive additive 28 is impregnated with an 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 illustrated 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 only needs to cover at least part of the core portion 26C, and preferably covers the entire surface of the core portion 26C.

The core portion 26C is subjected to occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and lithium ion counter anions (for example, PF ⁶⁻ ). An electrode active material capable of proceeding reversibly is used.

For example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), lithium manganese spinel (LiMn ₂ O _4), and the general _{formula: LiNi x Co y Mn z M} a2 ( x + y + z + a = 1, 0 ≦ x <1, 0 ≦ y <1, 0 ≦ z <1, 0 ≦ a <1, M is one or more selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr Element)), lithium vanadium compound (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (where M is selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr) are shown one or more elements or VO), lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1 1) a composite metal oxide such as polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene and the like, are mentioned as material constituting the core portion 26C.

In general, in the positive electrode active material, irreversible structural phase transition occurs when the Li ion elimination reaction is locally concentrated. The part where the irreversible structural phase transition occurs cannot contribute as charge / discharge capacity as an active material, and causes capacity deterioration. LiNi _x Co _y Al _z O ₂ (0.9 <x + y + z <1.1, hereinafter referred to as “NCA”) has a larger amount of desorbable Li ions than other active materials, and is brought into direct contact with the electrolyte. In this case, an irreversible structural phase transition is likely to occur locally near the surface of the active material. Therefore, when NCA is used for the core portion 26C and the core portion 26C is covered with the coating layer 26S, Li ions can be eliminated through the coating layer 26S, and Li ions can be evenly removed from the entire active material even at a high rate. Can be separated. That is, when NCA is used for the core part 26C, the effect by providing the coating layer 26S can be remarkably obtained.

  The coating layer 26S is an amorphous composite oxide containing Li, Al, Ti, and P. The amorphous coating layer 26S can reduce the interface resistance between the crystalline core part 26C and the electrolytic solution, and suppress the deterioration of the charge / discharge characteristics.

  FIG. 4 is a diagram schematically showing the mobility of lithium ions in the vicinity of the interface between the positive electrode active material 26 ′ having no coating layer and the electrolytic solution 70. A dotted line in FIG. 4 means an interface between the positive electrode active material 26 ′ and the electrolytic solution 70, and the left side is the positive electrode active material 26 ′ and the right side is the electrolytic solution 70.

  In the positive electrode active material 26 ′, lithium ions move between layered crystal lattices. On the other hand, lithium ions move freely in the electrolytic solution 70. Therefore, the mobility of lithium ions in the positive electrode active material 26 ′ is lower than the mobility of lithium ions in the electrolytic solution 70.

  At the interface between the positive electrode active material 26 ′ and the electrolytic solution 70, the mobility of lithium ions varies greatly. Therefore, even if lithium ions try to enter the positive electrode active material 26 ′ from the electrolytic solution 70, the path through which the lithium ions can conduct is insufficient, and the lithium ions cannot enter the positive electrode active material 26 ′. As a result, an interface resistance is generated at the interface between the positive electrode active material 26 ′ and the electrolytic solution 70, and the lithium ion mobility is lowest at the interface between the positive electrode active material 26 ′ and the electrolytic solution 70 (reference numeral B <b> 1).

  If there is a portion in the lithium ion secondary battery 100 where the mobility of lithium ions is low, the charge / discharge characteristics are limited by the mobility. That is, the minimum value B1 of the lithium ion mobility becomes a bottleneck of the charge / discharge characteristics of the lithium ion secondary battery 100.

  The minimum value B1 of the lithium ion mobility becomes low particularly during high-rate discharge. During the charge / discharge at a high rate, the movement speed of lithium ions is increased. Therefore, the congestion of lithium ions becomes larger at the interface. As a result, the interface resistance increases and the minimum mobility value B1 at the interface becomes lower. That is, the charge / discharge capacity at the time of high rate charge / discharge is particularly reduced.

  Here, a high rate means a discharge rate. If a cell having a nominal capacity value is discharged at a constant current, and the current value at which discharge is completed in 1 hour is 1 C, the high rate discharge rate is a case of discharging at a voltage higher than 1 C.

  FIG. 5 is a diagram schematically showing the mobility of lithium ions in the vicinity of the interface between the positive electrode active material having the crystalline coating layer 26S ′ and the electrolytic solution. The dotted line in FIG. 5 means the interface between the positive electrode active material coating layer 26S 'and the electrolyte solution 70, and the alternate long and short dash line means the interface between the positive electrode active material core 26C' and the coating layer 26S '.

  As shown in FIG. 5, when the crystalline coating layer 26 </ b> S ′ is provided, there are two interfaces, and the locations where lithium ions concentrate are dispersed from one location to two locations. As a result, the minimum value B2 of lithium ion mobility in FIG. 5 is higher than the minimum value B1 of lithium ion mobility in FIG.

  On the other hand, the difference in the mobility of lithium ions at the interface between the coating layer 26S ′ and the electrolytic solution 70 and the interface between the core portion 26C ′ and the coating layer 26S ′ is large. Absent. Therefore, it is not possible to say that the charge / discharge characteristics are sufficient only by providing the crystalline coating layer 26S ', and it is not possible to provide a high rate discharge capacity.

  On the other hand, FIG. 6 is a diagram schematically showing the mobility of lithium ions in the vicinity of the interface between the positive electrode active material and the electrolytic solution according to the present embodiment. 6 indicate the interface between the coating layer 26S of the positive electrode active material 26 and the electrolytic solution 70 and the interface between the core portion 26C of the positive electrode active material 26 and the coating layer 26S.

  As shown in FIG. 6, the amorphous coating layer 26S has more gaps and higher lithium ion mobility than the crystalline core portion 26C. Therefore, the mobility can be reduced stepwise in the order of the electrolytic solution 70, the coating layer 26S, and the core portion 26C, and the interface resistance at each interface can be reduced. As a result, the minimum value B3 of lithium ion mobility in FIG. 6 is higher than the minimum value B1 of lithium ion mobility in FIG. 4 and the minimum value B2 of lithium ion mobility in FIG.

The amorphous coating layer 26S according to the present embodiment is formed of an amorphous composite oxide film that satisfies the relationship of the following general formula (1).
Li: Al: Ti: P = (1 + x + y): x: (2-x): 3 (1)
(However, 0 ≦ x ≦ 1, 0 <y ≦ 4)

Li _{1 + x} Al _x Ti _2−x P ₃ O ₁₂ is known as a crystalline composite oxide containing Li, Al, Ti, and P. In the case of a crystalline composite oxide, the range that each element can take is limited crystallographically.

  On the other hand, the amorphous composite oxide film satisfying the relationship of the general formula (1) can contain abundant lithium by y. The coating layer 26S according to the present embodiment is amorphous, and the range of lithium that can be taken is not limited by the crystal structure. Further, 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 amount of lithium that can be contained is large. Since Ti is a transition metal element, it can be stably reduced up to +2, and the maximum value that y can take is 4.

  When the content of lithium in the coating layer 26S increases, the concentration of lithium ions that function as carriers in the coating layer 26S increases, and the mobility m1 of lithium ions in the coating layer 26S increases. That is, the lithium ion mobility of the entire lithium ion secondary battery 100 is raised, and the charge / discharge characteristics of the lithium ion secondary battery 100 are improved. Further, the interface resistance between the electrolytic solution 70 and the coating layer 26S is reduced.

  If the coating layer 26S contains trivalent aluminum, the tetravalent titanium is replaced with aluminum. Then, the covering layer 26S can take in lithium by the amount of the replaced insufficient valence. When the coating layer 26S contains more lithium, the concentration of lithium ions that function as carriers in the coating layer 26S increases, and the mobility m1 of lithium ions in the coating layer 26S increases. That is, the lithium ion mobility of the entire lithium ion secondary battery 100 is raised, and the charge / discharge characteristics of the lithium ion secondary battery 100 are improved. Further, the interface resistance between the electrolytic solution 70 and the coating layer 26S is reduced.

  The thickness of the coating layer 26S is preferably 3 nm or more and 30 nm or less. If the thickness of the coating layer 26S is within the range, the coating layer 26S can alleviate the difference in lithium ion mobility between the electrolytic solution 70 and the core portion 26C. Further, generation of defects such as cracks in the coating film 26S is suppressed, and a stable film can be formed.

  As described above, according to the positive electrode active material 26 according to the present embodiment, the interface resistance between the positive electrode active material 26 and the electrolytic solution 70 can be reduced. Further, the lithium content of the coating layer 26S constituting the positive electrode active material 26 can be increased, and the mobility of lithium ions in the coating layer 26S can be increased.

[Conductive auxiliary agent (positive electrode)]
Examples of the conductive assistant include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO. It is done. Among these, carbon materials such as carbon black are preferable. In the case where sufficient conductivity can be ensured with only the positive electrode active material, the lithium ion secondary battery 100 may not include a conductive additive.

[Binder (Positive electrode)]
The binder bonds the active materials to each other and bonds the active material to the positive electrode current collector 22. The binder is not particularly limited as long as the above-described bonding is possible. For example, polyvinylidene fluoride (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 the binder, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-) TFE fluorine rubber), vinylidene fluoride-pentafluoropropylene fluorine rubber (VDF-PFP fluorine rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluorine rubber (VDF-PFP-TFE fluorine rubber), Vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluoro rubber (VDF-PFMVE-TFE fluoro rubber), vinylidene fluoride-chlorotrifluoroethylene fluoro rubber The containing rubbers (VDF-CTFE-based fluorine rubber) vinylidene fluoride-based fluorine rubbers such as may be used.

  The constituent 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 by mass ratio. The constituent ratio of the conductive additive in the positive electrode active material layer 24 is preferably 1.0% to 3.0% in terms of mass ratio, and the constituent ratio of the binder in the positive electrode active material layer 24 is 1 in mass ratio. It is preferable that it is 0.8% 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 includes a negative electrode active material, and further includes a negative electrode binder and a conductive additive as necessary.

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

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

[Conductive auxiliary agent (negative electrode)]
Examples of the conductive assistant include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO. Can be mentioned.

[Binder (negative electrode)]
The binder used for a negative electrode can use the same thing as a positive electrode. In addition, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, or the like may be used as the binder.

Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene. In this case, since the binder also exhibits the function of the conductive assistant particles, it is not necessary to add the conductive assistant. As the ion-conductive conductive polymer, for example, those having ion conductivity such as lithium ion can be used. For example, polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide) , 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 combination include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-described monomer.

  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 constituent ratio of the negative electrode active material in the negative electrode active material layer 34 is preferably 70% or more and 98% or less by mass ratio. The constituent ratio of the conductive material in the negative electrode active material layer 34 is preferably 1% or more and 20% or less by mass ratio, and the constituent ratio of the binder in the negative electrode active material layer 34 is 1% or more and 10% or less by mass ratio. It is preferable that

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

"Separator"
The separator 10 only needs to be formed of an electrically insulating porous structure, for example, a single layer of a film made of polyethylene, polypropylene, or polyolefin, a stretched film of a laminate or a mixture of the above resins, or cellulose, polyester, and Examples thereof include a fiber nonwoven fabric made of at least one constituent material selected from the group consisting of polypropylene.

"Electrolyte"
As the electrolytic solution, an electrolyte solution containing lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, since the electrolytic aqueous solution has a low decomposition voltage electrochemically, the withstand voltage during charging is limited to be low. Therefore, an electrolyte solution (nonaqueous electrolyte solution) using an organic solvent is preferable.

  In the non-aqueous electrolyte solution, an electrolyte is dissolved in a non-aqueous solvent, and a cyclic carbonate and a chain carbonate may be contained as a non-aqueous solvent.

  As cyclic carbonate, what can solvate 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. Examples thereof include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. 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 the cyclic carbonate and the chain carbonate in the non-aqueous solvent is preferably 1: 9 to 1: 1 by 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 , 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, lithium salts such as LiBOB can be used. In addition, these lithium salts may be used individually by 1 type, and may use 2 or more types together. In particular, LiPF ₆ is preferably included from the viewpoint of the degree of ionization.

When LiPF ₆ is dissolved in a non-aqueous solvent, the concentration of the electrolyte in the non-aqueous electrolyte solution is preferably adjusted 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 nonaqueous electrolytic solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging and discharging. Moreover, by suppressing the electrolyte concentration to within 2.0 mol / L, it is possible to suppress an increase in the viscosity of the non-aqueous electrolyte, to sufficiently secure the mobility of lithium ions, and to obtain a sufficient capacity during charging and discharging. It becomes easy.

Even when LiPF ₆ is mixed with another electrolyte, the lithium ion concentration in the non-aqueous electrolyte solution is preferably adjusted to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol%. More preferably, it is contained.

"Exterior body"
The exterior body 50 seals the laminated body 40 and the electrolytic solution therein. The outer package 50 suppresses leakage of the electrolytic solution to the outside, penetration of moisture and the like into the lithium ion secondary battery 100 from the outside, and the like.

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

"Lead"
The leads 60 and 62 are made of a conductive material such as aluminum. Then, the leads 60 and 62 are respectively welded to the positive electrode current collector 22 and the negative electrode current collector 32 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. 10 is inserted into the exterior body 50 together with the electrolyte, and the entrance of the exterior body 50 is sealed.

  As described above, since the lithium ion secondary battery according to this embodiment includes a predetermined positive electrode active material, the lithium ion conductivity between the positive electrode active material and the electrolyte in the positive electrode is high. That is, the lithium ion secondary battery according to this embodiment is excellent in charge / discharge characteristics, and particularly excellent in high-rate discharge characteristics.

[Method for producing lithium ion secondary battery]
First, the positive electrode active material 26 is produced (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 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 installed in the center of the barrel, and lithium composite compound particles constituting the core portion 26C are installed on the inner peripheral surface of the barrel. And by rotating in a vacuum while rotating a barrel, the coating layer 26S is formed on the surface of the core part 26C, and the positive electrode active material 26 according to the present embodiment is obtained.

  The thickness, film quality, and the like 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.

  When a dry manufacturing method such as a powder sputtering method is used, an amorphous coating film rich in Li can be formed. Ceramic materials have a very high melting point, and it is difficult to make them amorphous using a liquid quenching method. For this reason, it is preferable to use a dry manufacturing method such as a sputtering method or a CVD method in order to form an amorphous coating film.

  In the dry manufacturing method, all elements that are raw materials are once ionized in a vacuum environment. Therefore, the coating film is obtained by separating the electrons in the raw material, generating plasma, and recombining with the plasma electrons. Therefore, in the case of Ti having a plurality of valences, an oxidation number different from that of the raw material is easily obtained, and a coating film containing a large amount of Li ions is easily formed.

  Furthermore, when the powder sputtering method is used, it is possible to selectively form light Li ions by changing manufacturing conditions such as gas conditions and electric power. For example, when the distance between the target and the barrel is increased, the composition ratio of lightweight Li can be increased. On the other hand, if the distance between the target and the barrel is too large, the film formation rate decreases and the productivity decreases. In order to secure the film formation speed while increasing the Li composition ratio in the coating film 26S, the distance between the target and the barrel is preferably about 30 cm.

  As described above, it is preferable to use a powder sputtering method in order to form an amorphous coating film containing Li for + y shown in the general formula (1) in the coating film.

  Next, the positive electrode 20 is produced using the produced positive electrode active material 26. The positive electrode active material 26, a binder and a solvent are mixed to prepare a paint. You may add a conductive support agent further as needed. As the solvent, for example, water, N-methyl-2-pyrrolidone, N, N-dimethylformamide or the like can be used. The constituent ratio of the positive electrode active material, the conductive additive, 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 so as to be 100 wt% as a whole.

  The mixing method of these components constituting the paint is not particularly limited, and the mixing order is not particularly limited. The paint is applied to the positive electrode current collector. There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. Examples thereof include a slit die coating method and a doctor blade method.

  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 to which the paint is applied 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 that becomes 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 stacked so that the separator 10 is between the positive electrode 20 and the negative electrode 30, thereby forming a stacked body 40.

  Finally, the laminated body 40 is sealed in the exterior body 50. The nonaqueous electrolyte solution may be injected into the outer package 50, or the laminate 40 may be impregnated with the nonaqueous electrolyte solution. And heat etc. are applied to the exterior body 50, it seals by laminating, and the nonaqueous electrolyte secondary battery 100 is produced.

  The embodiments of the present invention have been described in detail with reference to the drawings. However, the configurations and combinations of the embodiments in the embodiments are examples, and the addition and the omission of the configurations are within the scope not departing from the gist of the present invention. , Substitutions, 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 a core part of the positive electrode active material.

And the coating film was formed in the surface of the prepared core part using the powder sputtering method. Li _1.0 Ti _2.0 (PO ₄ ) ₃ was used as a target. Then, film formation was performed for 4 hours while rotating the barrel under the conditions of ultimate pressure of 3.0 × 10 ⁻⁴ Pa, Ar gas flow rate of 50 sccm, RF, 400 W, and target-barrel distance of 30 cm.

  Immediately after the production of the positive electrode active material, a film was formed on a 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 step gauge, and the composition was measured by XPS 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). The slurry was applied to an Al current collector, dried, and pressed to prepare a positive electrode.

Next, a half-cell lithium ion secondary battery was fabricated using this positive electrode as a working electrode. The counter electrode facing the working electrode was obtained by attaching a Li foil on a copper foil having a thickness of 16 μm. The separator used was a polyethylene microporous membrane. As the external lead terminals, aluminum leads (width 4 mm, length 40 mm, thickness 100 μm) and nickel leads (width 4 mm, length 40 mm, thickness 100 μm) were used. As the electrolytic solution, a mixed solvent obtained by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 3: 7 and an electrolytic solution containing lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1 mol / L are used. It was. And these were vacuum-sealed in the exterior body, and the lithium ion secondary battery of Example 1 was produced.

  And the electrical property of the produced lithium ion secondary battery was measured. In a constant temperature bath at 25 ° C., the battery was charged at a constant current up to 4.3 V with a current value corresponding to 3 C as the current density, and then charged at a constant voltage of 4.3 V. Constant voltage charging was continued until the current density dropped to a value corresponding to 0.05C. Thereafter, constant current discharge was performed up to 3.0 V at a current density of 3 C, and this was repeated 10 cycles. The discharge capacity of Example 1 was 30.9 mAh / g. The results are shown in Table 1.

(Comparative Example 1)
Comparative Example 1 is different from Example 1 in that no coating layer was provided on the positive electrode active material. 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 2)
Comparative Example 2 differs from Example 1 in that the coating layer of the positive electrode active material is changed to a crystalline material. Specifically, using a Hosokawa Micron Co., Ltd. _NOB-MINI, the _{LiNi 0.8 Co 0.15 Al 0.05 O 2} ( particle size _{20μm, 500g), Li 1.3 Al} 0.3 Ti 1. ₇ (PO ₄ ) ₃ was added at a rate of 3 wt%, and coating was performed by compounding. By performing a composite treatment for 20 minutes at a rotation speed of 1800 rpm, a surface-covered positive electrode active material was obtained.

Further, an X-ray diffraction (XRD) pattern of the positive electrode active material of Comparative Example 2 was measured, and a diffraction pattern that could be attributed to LiNi _0.8 Co _0.15 Al _0.05 O ₂ and Li _1.3 Al _0.3 Ti _1.3 (PO ₄ ) A diffraction pattern attributable to ₃ was confirmed. That is, the coating layer is crystalline Li _1.3 Al _0.3 Ti _1.3 (PO ₄ ) ₃ . Moreover, the thickness of the coating layer of the positive electrode active material was confirmed by a scanning electron microscope (SEM), and was 100 nm on average.

  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 3 and 4)
Comparative Examples 3 and 4 differ from Comparative Example 2 in that the composition of the coating layer was changed. In Comparative Example 3, Li _1.0 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 4, Li _1.3 Ti _1.3 (PO ₄ ) ₃ was used. Li _2.0 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 2 to 5)
Examples 2 to 5 differ from Example 1 in that the composition and film thickness of the coating layer were changed. The composition of the coating layer was changed by changing the film forming conditions, and the film thickness was changed by changing the film forming time.

In Examples 2 to 5, the Ar gas flow rate is 60 sccm, the applied voltage is 450 W, the target is Li _2.0 Al _1.0 Ti _1.0 (PO ₄ ) ₃ , and the film formation times are 8 hours, 6 hours, 4 hours, respectively. The time was 3.5 hours.

  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 6-7 and Comparative Examples 5-8)
Examples 6 to 7 and Comparative Examples 5 to 8 differ from Example 1 in that the composition of the coating layer was changed. The composition of the coating layer was changed by changing the film formation conditions.

In Example 6, the target was Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ and in Example 7, the target was Li _1.5 Al _0.5 Ti _1.5 (PO ₄ ) ₃ . .

In Comparative Examples 5 to 7, the Ar gas flow rate was 30 sccm, the applied voltage was 350 W, and the film formation time was 5 hours. In Comparative Example 5, the target was Li _1.0 Ti _2.0 (PO ₄ ) _3, and in Comparative Example 6, the target was Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) _3. In No. 7, the target was Li _2.0 Al _1.0 Ti _1.0 (PO ₄ ) ₃ .

  The 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 1-7, in which the coating layer is amorphous, have higher rates compared to Comparative Example 1 having no coating layer and Comparative Examples 2 to 4 having a crystalline coating layer. The charge / discharge characteristics were excellent. Further, Comparative Examples 5 to 7 in which the composition of the coating layer was not in the predetermined range and the content of Li was small also could not be said to have sufficient high rate charge / discharge characteristics.

DESCRIPTION OF SYMBOLS 10 ... Separator, 20 ... Positive electrode, 22 ... Positive electrode 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 ... outer package, 52 ... metal layer, 54 ... polymer film, 60, 62 ... lead, 100 ... Lithium ion secondary battery

Claims (5)

The core,
A coating layer covering the core portion,
The coating layer is an amorphous composite oxide containing Li, Al, Ti and P,
An active material 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 + x + y): x: (2-x): 3 (1)
(However, 0 ≦ x ≦ 1, 0 <y ≦ 4)   The active material according to claim 1, wherein in the general formula (1), x is in a range of 0 <x ≦ 1.   The active material according to claim 1 or 2, wherein the coating layer has a thickness of 3 nm or more and 30 nm or less.   The electrode containing the active material concerning any one of Claims 1-3.   A lithium ion secondary battery comprising the electrode according to claim 4 and a counter electrode facing the electrode.

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