JP2022170190A - Positive electrode for secondary battery - Google Patents

Abstract

To provide means for improving the cycle durability in a secondary battery such as an all-solid lithium secondary battery.SOLUTION: A surface of particles of a positive electrode active material is coated with particles of lithium ion conductive oxide at a coating rate of 50% or more, the positive electrode active material containing Li(Ni-Mn-Co)O2 and lithium transition metal composite oxide (NMC composite oxide) that has a composition in which some of these transition metals are substituted with the other elements. The resultant is blended into a positive electrode active material layer together with solid electrolyte.SELECTED DRAWING: None

Description

The present invention relates to a positive electrode for secondary batteries.

In recent years, in order to cope with global warming, reduction of carbon dioxide emissions is earnestly desired. In the automobile industry, expectations are gathering for the reduction of carbon dioxide emissions through the introduction of electric vehicles (EV) and hybrid electric vehicles (HEV). Development of electrolyte secondary batteries has been actively carried out.

Secondary batteries for motor driving are required to have extremely high output characteristics and high energy compared to consumer lithium-ion secondary batteries used in mobile phones, notebook computers, and the like. Therefore, lithium-ion secondary batteries, which have the highest theoretical energy among all practical batteries, have attracted attention and are being rapidly developed.

Lithium ion secondary batteries, which are currently in widespread use, use a combustible organic electrolyte as an electrolyte. Such a liquid type lithium ion secondary battery requires stricter safety measures against liquid leakage, short circuit, overcharge, etc. than other batteries.

Therefore, in recent years, research and development on all-solid-state batteries such as all-solid-state lithium-ion secondary batteries using oxide-based or sulfide-based solid electrolytes have been actively carried out. A solid electrolyte is a material composed mainly of an ionic conductor capable of conducting ions in a solid. Therefore, in all-solid-state lithium ion secondary batteries, in principle, various problems due to combustible organic electrolytes do not occur unlike conventional liquid-type lithium ion secondary batteries. In general, the use of a high-potential, large-capacity positive electrode material and a large-capacity negative electrode material can significantly improve the output density and energy density of the battery.

As a positive electrode active material for lithium ion secondary batteries, a lithium metal composite oxide or the like is sometimes used. It may consist of Conventionally, a powdery lithium-transition metal composite oxide of monodisperse primary particles (single particles) containing one element selected from the group of cobalt, nickel, and manganese as main components and lithium as a main component has a grain boundary. It has been proposed that there is an advantage that cracks and breakage are less likely to occur during molding of the positive electrode material.

On the other hand, when such a composite oxide is used as a positive electrode active material in an all-solid secondary battery that includes a sulfide solid electrolyte in the positive electrode active material layer, the positive electrode active material and the sulfide solid electrolyte are separated from each other as charging and discharging proceed. is known to react to form a high-resistance layer at the interface between them, increasing the interface resistance and gradually decreasing the battery capacity.

Here, in Patent Document 1, by arranging a coating layer composed of an oxide essentially containing lithium and boron on the surface of a core material particle composed of a lithium transition metal oxide, the positive electrode active material as described above The formation of a high-resistance layer at the interface between the solid electrolyte and the solid electrolyte and the resulting reduction in cycle durability are suppressed.

WO2020/022305 Pamphlet

However, according to the studies of the present inventors, it has been found that sufficient cycle durability may not be obtained even if the technology described in Patent Document 1 is adopted.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide means for improving the cycle durability of a secondary battery such as an all-solid lithium secondary battery.

The inventors of the present invention have made intensive studies in view of the above problems. As a result, positive electrode active materials containing Li(Ni—Mn—Co)O ₂ and lithium transition metal composite oxides (NMC composite oxides) having a composition in which a portion of these transition metals are replaced by other elements The inventors have found that the above problems can be solved by coating the surface of the particles with particles of a lithium ion conductive oxide at a coverage rate of 50% or more and blending this together with the solid electrolyte into the positive electrode active material layer. I have completed my invention.

According to one aspect of the present invention, a positive electrode active material containing Li(Ni—Mn—Co)O ₂ and a lithium-transition metal composite oxide having a composition in which a part of these transition metals are replaced by other elements; A positive electrode for a secondary battery comprising a positive electrode active material layer containing a solid electrolyte, wherein the surfaces of the positive electrode active material particles are coated with lithium ion conductive oxide particles at a coverage rate of 50% or more. A coated positive electrode for a secondary battery is provided.

ADVANTAGE OF THE INVENTION According to the positive electrode for secondary batteries which concerns on one form of this invention, it is a secondary battery, such as an all-solid-state lithium secondary battery. WHEREIN: Cycle durability can be improved.

FIG. 1 is a perspective view showing the appearance of a flat laminated all-solid lithium ion secondary battery, which is one embodiment of a secondary battery provided with a positive electrode for a secondary battery according to the present invention. FIG. 2 is a cross-sectional view along line 2-2 shown in FIG. FIG. 3 is a schematic diagram schematically showing the structure of the positive electrode active material used in the positive electrode for secondary battery according to one embodiment of the present invention.

《Secondary battery》
One aspect of the present invention is a positive electrode active material containing Li(Ni—Mn—Co)O ₂ and a lithium-transition metal composite oxide having a composition in which a portion of these transition metals are replaced by other elements; A positive electrode for a secondary battery comprising a positive electrode active material layer containing an electrolyte,
The positive electrode for a secondary battery, wherein the surfaces of the particles of the positive electrode active material are covered with the particles of the lithium ion conductive oxide at a coverage rate of 50% or more.

Hereinafter, embodiments of the positive electrode for a secondary battery according to the present embodiment described above will be described with reference to the drawings. is not limited to only the form of Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may differ from the actual ratios.

FIG. 1 is a perspective view showing the appearance of a flat laminated all-solid lithium ion secondary battery, which is one embodiment of a secondary battery provided with a positive electrode for a secondary battery according to the present invention. FIG. 2 is a cross-sectional view along line 2-2 shown in FIG. By using a laminate type, the battery can be made compact and have a high capacity. In the present specification, a flat laminated non-bipolar lithium ion secondary battery (hereinafter also simply referred to as a "laminated battery") shown in FIGS. 1 and 2 will be described in detail as an example. However, when viewed from the electrical connection form (electrode structure) inside the lithium ion secondary battery according to the present embodiment, it can be either a non-bipolar type (internal parallel connection type) battery or a bipolar type (internal series connection type) battery. It can also be applied to

As shown in FIG. 1, the laminated battery 10a has a rectangular flat shape, and from both sides thereof, a negative electrode collector plate 25 and a positive electrode collector plate 27 for extracting electric power are pulled out. there is The power generation element 21 is wrapped by the battery exterior material (laminate film 29) of the laminated battery 10a, and the periphery thereof is heat-sealed. It is sealed in the pulled out state.

Note that the lithium-ion secondary battery according to the present embodiment is not limited to a laminated flat shape. The wound type lithium ion secondary battery may have a cylindrical shape, or may have a rectangular flat shape obtained by deforming such a cylindrical shape. etc. is not particularly limited. In the case of the above-mentioned cylindrical shape, a laminate film may be used as the exterior material, or a conventional cylindrical can (metal can) may be used, and there is no particular limitation. Preferably, the power generation element is housed inside a laminate film containing aluminum. Weight reduction can be achieved by this configuration.

In addition, there are no particular restrictions on how the collector plates (25, 27) shown in FIG. 1 can be taken out. The negative electrode current collector plate 25 and the positive electrode current collector plate 27 may be pulled out from the same side, or the negative electrode current collector plate 25 and the positive electrode current collector plate 27 may be divided into a plurality of pieces and pulled out from each side. It is not limited to what is shown in FIG. 1, such as good. In a wound type lithium ion battery, terminals may be formed using, for example, cylindrical cans (metal cans) instead of tabs.

As shown in FIG. 2, the laminate type battery 10a of the present embodiment has a structure in which a flat and substantially rectangular power generation element 21 in which charge/discharge reactions actually progress is sealed inside a laminate film 29 that is a battery exterior material. have Here, the power generation element 21 has a structure in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are laminated. The positive electrode has a structure in which positive electrode active material layers 15 containing a positive electrode active material are arranged on both sides of a positive electrode current collector 11″. The negative electrode contains a negative electrode active material on both sides of a negative electrode current collector 11′. It has a structure in which active material layers 13 are arranged, specifically, one positive electrode active material layer 15 and an adjacent negative electrode active material layer 13 face each other with a solid electrolyte layer 17 interposed therebetween. A positive electrode, a solid electrolyte layer, and a negative electrode are stacked in this order, so that the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute one single cell layer 19. Therefore, the stacked battery 10a shown in FIG. It can also be said that a plurality of unit cell layers 19 are stacked to have a configuration in which they are electrically connected in parallel.

As shown in FIG. 2, the outermost negative electrode current collectors positioned on both outermost layers of the power generating element 21 have the negative electrode active material layer 13 only on one side, but the active material layer is provided on both sides. may be That is, a current collector having active material layers on both sides may be used as it is as the current collector for the outermost layer, instead of using a current collector exclusively for the outermost layer having an active material layer on only one side. In some cases, the negative electrode active material layer 13 and the positive electrode active material layer 15 may be used as the negative electrode and the positive electrode, respectively, without using the current collectors (11', 11'').

The negative electrode current collector 11′ and the positive electrode current collector 11″ are attached with a negative electrode current collector plate (tab) 25 and a positive electrode current collector plate (tab) 27, which are electrically connected to the electrodes (positive electrode and negative electrode), respectively. It has a structure in which it is sandwiched between the ends of the laminate film 29, which is the material, and led out of the laminate film 29. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 are respectively connected to the positive electrode as necessary. It may be attached to the positive electrode current collector 11″ and the negative electrode current collector 11′ of each electrode by ultrasonic welding, resistance welding, or the like via a lead and a negative electrode lead (not shown).

Main constituent members of a secondary battery to which the positive electrode for a secondary battery according to the present embodiment is applied are described below.

[Current collector]
The current collector has a function of mediating transfer of electrons from the electrode active material layer. There are no particular restrictions on the material that constitutes the current collector. As the constituent material of the current collector, for example, a metal or a conductive resin can be used.

Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition to these, a clad material of nickel and aluminum, a clad material of copper and aluminum, or the like may be used. Alternatively, a foil in which a metal surface is coated with aluminum may be used. Among them, aluminum, stainless steel, copper, and nickel are preferable from the viewpoint of electronic conductivity, battery operating potential, adhesion of the negative electrode active material to the current collector by sputtering, and the like.

As the latter conductive resin, a resin obtained by adding a conductive filler to a non-conductive polymeric material can be used.

Examples of non-conductive polymer materials include polyethylene (PE; high-density polyethylene (HDPE), low-density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), and polyimide. (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethylacrylate (PMA), polymethylmethacrylate (PMMA) , polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), or polystyrene (PS). Such non-conductive polymeric materials can have excellent electrical potential or solvent resistance.

Any electrically conductive filler can be used without particular limitation. Examples of materials having excellent conductivity, potential resistance, or lithium ion blocking properties include metals and conductive carbon. The metal is not particularly limited, but includes at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb, or these metals. It preferably contains alloys or metal oxides. Also, the conductive carbon is not particularly limited. Preferably, it is selected from the group consisting of acetylene black, Vulcan (registered trademark), Black Pearl (registered trademark), carbon nanofiber, Ketjenblack (registered trademark), carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene. At least one type is included.

The amount of the conductive filler added is not particularly limited as long as it is an amount that can impart sufficient conductivity to the current collector, and is generally 5 to 80% by mass with respect to 100% by mass of the total mass of the current collector. is.

The current collector may have a single-layer structure made of a single material, or may have a laminated structure in which layers made of these materials are appropriately combined. From the viewpoint of reducing the weight of the current collector, it is preferable to include at least a conductive resin layer made of a resin having conductivity. Moreover, from the viewpoint of blocking movement of lithium ions between the single cell layers, a metal layer may be provided on a part of the current collector. Furthermore, if the negative electrode active material layer and the positive electrode active material layer to be described later themselves have conductivity and can exhibit a current collecting function, a current collector is used as a member separate from these electrode active material layers. No need. In such a form, the negative electrode active material layer to be described later constitutes the negative electrode as it is, and the positive electrode active material layer to be described later constitutes the positive electrode as it is.

[Positive electrode active material layer]
In the positive electrode for a secondary battery according to this embodiment, the positive electrode active material layer contains a positive electrode active material. The positive electrode active material contained in the positive electrode active material layer includes Li(Ni—Mn—Co) O ₂ and a lithium-transition metal composite oxide ( hereinafter also simply referred to as "NMC composite oxide").

The NMC composite oxide has a layered crystal structure in which a lithium atomic layer and a transition metal (Ni, Mn and Co are arranged in an orderly manner) atomic layer are alternately stacked via an oxygen atomic layer. It contains one Li atom, and the amount of Li that can be extracted is twice that of the spinel-based lithium manganese oxide, that is, the supply capacity is doubled, and a high capacity can be obtained.

NMC composite oxides also include composite oxides in which a portion of the transition metal is replaced with another element, as described above. Other elements in that case include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, and Cu. , Ag, Zn, etc., preferably Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, more preferably Ti, Zr, P, Al, Mg, Cr, more preferably Ti, Zr, Al, Mg, or Cr from the viewpoint of improving cycle characteristics.

Since the NMC composite oxide has a high theoretical discharge capacity, it is preferably represented by general formula (1):
_{LiaNibMncCodMxO2 ( 1 ) _ _}
It has a composition represented by

In the general formula (1), a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x is Represents the atomic ratio of M. Further, a, b, c, d, and x are 0.98≦a≦1.2, 0.6≦b≦0.9, 0<c≦0.4, 0<d≦0.4, 0 ≦x≦0.3 and b+c+d+x=1 are satisfied. M is at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr and Cr.

The positive electrode active material layer may further contain a positive electrode active material other than the NMC composite oxide. Examples of positive electrode active materials other than NMC composite oxides include metal oxides other than NMC composite oxides, elemental sulfur, and the like. Metal oxides other than NMC composite oxides are not particularly limited, but layered rock salt type active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ and LiVO ₂ , LiMn ₂ O ₄ , LiNi _0.5 Mn _1.5 O ₄ , olivine-type active materials such as LiFePO ₄ and LiMnPO ₄ , and Si-containing active materials such as Li ₂ FeSiO ₄ and Li ₂ MnSiO ₄ . Moreover, as metal _oxides other than the above, _Li4Ti5O12 is _mentioned , for example.

Here, the ratio of the content of the NMC composite oxide to 100% by mass of the total amount of the positive electrode active material is preferably 50% by mass or more, more preferably 70% by mass or more, and still more preferably 80% by mass or more. , more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

In the positive electrode for a secondary battery according to this embodiment, the NMC composite oxide contained in the positive electrode active material layer preferably contains primary particles and secondary particles composed of the primary particles. Here, "secondary particles" mean aggregates formed by aggregating primary particles (single particles). In a preferred embodiment in which the NMC composite oxide contains primary particles and secondary particles composed of the primary particles, the average particle diameter (arithmetic mean diameter) of the secondary particles constituting the NMC composite oxide is 4.9 μm. and the average particle diameter (arithmetic mean diameter) of the primary particles constituting the secondary particles is controlled to 1.2 μm or more. The particles of the NMC composite oxide having such a structure have a shape close to that of a single particle, and expand and contract relatively large with the progress of charging and discharging. Therefore, by using the NMC composite oxide particles having the above-described particle size profile, there is an advantage that the effects of the configuration of the present embodiment can be more remarkably exhibited. In addition, by adopting the above particle size profile, even when a large press pressure is applied during the production of the electrode (positive electrode), it is possible to suppress particle cracking of the positive electrode active material, resulting in particle cracking. A decrease in battery capacity can be prevented. In this specification, the term "particle diameter" means the maximum distance between any two points on the contour line of the observed particle, and the average particle diameter of the active material. As the value, the value measured by the method described in the section of Examples to be described later shall be adopted. Also, the lower limit of the average particle size of the secondary particles constituting the NMC composite oxide is not particularly limited, but it is usually 3.0 μm or more, preferably 3.6 μm or more. Also, the upper limit of the average particle size of the primary particles constituting the secondary particles is not particularly limited, and is usually 2.7 μm or less, preferably 2.3 μm or less.

However, in the positive electrode for a secondary battery according to the present embodiment, the NMC composite oxide contained in the positive electrode active material layer may consist only of primary particles (that is, do not constitute secondary particles). Even in this case, in a preferred embodiment, the average particle diameter (arithmetic mean diameter) of the primary particles is controlled to 1.2 μm or more.

In addition, the ratio of the number of particles composed of a single crystallite to the primary particles constituting the secondary particles is preferably 10% or more, more preferably 20% or more, and still more preferably 30% or more. be. Such a configuration is preferable because it facilitates obtaining an NMC composite oxide having a particle size profile as described above.

The method of controlling the average particle size of the constituent particles of the NMC composite oxide to the configuration described above is not particularly limited, and conventionally known knowledge that can control the average particle size of the particles of the NMC composite oxide can be appropriately referred to. sell. Specifically, for example, patent documents such as Patent No. 6574222 and Patent No. 5702289, Solid State Ionics, Volume 345, February 2020, 115200, Journal of The Electrochemical Society, 165 (5) A1038-A1045 (20 Techniques disclosed in non-patent literature can be referred to.

In the positive electrode for a secondary battery according to the present embodiment, the surface of the particles of the lithium transition metal composite oxide (NMC composite oxide) contained in the positive electrode active material layer is 50% or more of the particles of the lithium ion conductive oxide. It is characterized in that it is covered with a coverage rate.

FIG. 3 is a schematic diagram schematically showing the structure of the positive electrode active material used in the positive electrode for a secondary battery according to this embodiment. As shown in FIG. 3 , in the positive electrode active material 100 used in the positive electrode for a secondary battery according to the present embodiment, the surfaces of particles 110 of NMC composite oxide, which is a lithium transition metal composite oxide, are lithium ion conductive oxides. are coated with particles 120 of And the coverage at that time is 50% or more. Here, the “coverage ratio” means the percentage of the area of the portion covered with the lithium ion conductive oxide particles 120 out of the surface area of the NMC composite oxide particles 110 . Therefore, in the positive electrode active material 100 according to the present embodiment, half or more of the surface area of the NMC composite oxide particles 110 is covered with the lithium ion conductive oxide particles 120 . From the viewpoint of more effectively expressing the effects of the present invention, the value of the coverage is preferably 60% or more, more preferably 70% or more, and still more preferably 80% or more. and particularly preferably 90% or more. Also, the upper limit of the coverage is not particularly limited, but it is usually 99% or less, preferably 95% or less. The value of the above-mentioned "coverage" can be calculated from an image observed when the positive electrode active material is observed using a scanning electron microscope (SEM), as described in the Examples section below. It is possible.

Here, as described above with reference to FIG. 3, in the positive electrode for a secondary battery according to the present embodiment, the surfaces of the particles of the lithium transition metal composite oxide (NMC composite oxide) included in the positive electrode active material layer are , is coated with "particles" of lithium-ion-conducting oxide. In other words, even if the surfaces of the NMC composite oxide particles are coated with a "coating layer" in which the lithium ion conductive oxide is continuously extended, such coated portions are not covered by the present invention. It is not considered in the calculation of the "coverage" related to the form. Therefore, although the existence of such a "coating layer" is not completely excluded, a coverage rate of 50% or more considering only coating with "particles" of lithium ion conductive oxide as shown in FIG. If so, it is considered to be within the scope of the present invention.

In this embodiment, as the lithium ion conductive oxide coating the surfaces of the particles of the NMC composite oxide, any oxide having lithium ion conductivity can be used without particular limitation. As an example, from the viewpoint of the effect of improving cycle durability, the lithium ion conductive oxide has the following general formula (2):
_{LixAOy (} 2)
It is preferable to have a composition represented by.

In general formula (2), A is Al, Zr, Nb, Ti, Ta, La, P, B, C, Si, S, Mo or W. Also, x and y each independently represent a positive integer.

Specific examples of the lithium ion conductive oxide represented by the general formula (2) include Li ₂ ZrO ₃ , LiNbO ₃ , LiAlO ₂ , Li ₂ TiO ₃ , Li ₄ Ti ₅ O ₁₂ , Li ₂ Ti _{2 O5} , _LiTaO3 , _{Li3PO4 , Li3BO3 , LiBO2 , Li2CO3 , Li4SiO4 , Li2SiO3} , _Li2SO4 , _{Li2MoO4 and Li2WO4 , etc.} mentioned. Among them, at least one of Li ₂ ZrO ₃ , LiNbO ₃ and LiAlO ₂ is preferably used, and at least one of Li ₂ ZrO ₃ and LiNbO ₃ is used from the viewpoint of particularly excellent effect of improving cycle durability. More preferably, Li ₂ ZrO ₃ is used.

The average particle diameter (arithmetic mean diameter) of the lithium ion conductive oxide particles covering the surfaces of the NMC composite oxide particles is not particularly limited, but is preferably 2 to 20 nm, more preferably 2 to 15 nm. more preferably 2 to 10 nm, particularly preferably 3 to 8 nm. When the average particle size of the particles of the lithium ion conductive oxide is within this range, the relative volume of the coated particles to the volume of the NMC composite oxide is very small. A decrease in the volumetric energy density of the secondary battery can be minimized. Further, by setting the average particle size within the above range, the surfaces of the particles of the NMC composite oxide can be easily coated with the particles of the lithium ion conductive oxide at a high coverage rate.

The method of coating the surface of the NMC composite oxide particles with the lithium ion conductive oxide particles to obtain the positive electrode active material according to the present embodiment is not particularly limited, and a large number of other particles are coated on the surface of the particles. A conventionally known method for attaching (attaching) can be appropriately adopted. As an example of such a technique, for example, a milling device such as a planetary ball mill device can be used. In some cases, a mechanofusion system manufactured by Hosokawa Micron Corporation may be used.

According to the studies of the present inventors, as described above, the surface of the NMC composite oxide particles is coated with the lithium ion conductive oxide particles at a coverage rate of a predetermined value or more. It has been found that the cycle durability of the secondary battery can be improved by configuring the secondary battery. Although the mechanism by which the effects specified in the present application are exhibited by the configuration according to the present embodiment is not completely clear, the following mechanism is presumed. That is, generally, in the charging process, the NMC composite oxide contracts as lithium ions are released from the NMC composite oxide. At this time, as in the technique described in Patent Document 1, if the particles of the NMC composite oxide are coated with a "coating layer" of the lithium ion conductive oxide, the coating layer causes the shrinkage of the NMC composite oxide. try to follow. However, since the lithium ion conductive oxide cannot shrink as much as the NMC composite oxide, the coating layer may float from the particles of the NMC composite oxide, or in some cases, the coating layer may peel off or fall off from the NMC composite oxide. do. Further, if the NMC composite oxide expands during the subsequent discharge process, cracks may occur in the coating layer. In this way, if the coating layer made of the lithium ion conductive oxide peels off from the NMC composite oxide, or cracks occur in the coating layer, the positive electrode active material (NMC composite oxide), which is the original role of the coating layer, ) and the solid electrolyte cannot be suppressed. As a result, the reaction between the positive electrode active material and the solid electrolyte progresses with the progress of charging and discharging, forming a high-resistance layer at the interface between them, resulting in a decrease in battery capacity.

On the other hand, in the positive electrode active material according to the present embodiment, unlike Patent Document 1, it is not premised to be coated with a "coating layer", but the particles of the NMC composite oxide are covered by the "particles" of the lithium ion conductive oxide. covered. Therefore, even if the NMC composite oxide shrinks during the charging process, the lithium ion conductive oxide particles covering it can sufficiently follow this shrinkage. As a result, peeling and falling off of the coated particles and an increase in interfacial resistance due to reaction between the positive electrode active material and the solid electrolyte are suppressed, which is considered to contribute to improvement of cycle durability. Techniques for providing a coating layer made of various materials on the surfaces of particles of a positive electrode active material have been proposed for conventional liquid-type lithium ion secondary batteries. In the secondary battery, since the problem of reaction due to solid-solid contact between the positive electrode active material and the solid electrolyte does not exist in the first place, it can be said that there was no problem of preventing such a reaction. It should be noted that the mechanism described above is purely based on speculation, and its correctness or wrongness does not affect the technical scope of the present invention.

The positive electrode active material layer further includes a solid electrolyte. By including the solid electrolyte in the positive electrode active material layer, the ion conductivity of the positive electrode active material layer can be improved. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes, but sulfide solid electrolytes are preferable from the viewpoint of excellent ion conductivity and further improvement of battery performance.

Examples of sulfide solid electrolytes include LiI-Li ₂ S-SiS ₂ , LiI-Li ₂ SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , Li ₂ SP ₂ S ₅ , LiI _{- Li3PS4} , LiI _- LiBr _{- Li3PS4 , Li3PS4} , _{Li2SP2S5 - LiI} , _{Li2SP2S5 - Li2O} , _Li2SP 2S5 _- Li2O _- LiI, _{Li2S - SiS2} , Li2S _- SiS2 _- LiI, Li2S _- SiS2 _- LiBr, Li2S _- SiS2-LiCl, _{Li2S - SiS2} -B ₂ S ₃ -LiI, Li ₂ S-SiS ₂ -P ₂ S ₅ -LiI, Li ₂ S-B ₂ S ₃ , Li ₂ SP ₂ S ₅ -Z _m S _n (where m, n is a positive number and Z is one of Ge, Zn and Ga), Li ₂ S—GeS ₂ , Li ₂ S—SiS ₂ —Li ₃ PO ₄ , Li ₂ S—SiS ₂ —Li _x MO _y (where x and y are positive numbers and M is any one of P, Si, Ge, B, Al, Ga and In) and the like. The description of “Li ₂ SP _{2 S 5 ” means a sulfide solid electrolyte using a raw material composition containing Li 2 S and P 2 S 5 , and} the same applies to other descriptions.

The sulfide solid electrolyte may have, for example, a Li ₃ PS ₄ skeleton, a Li ₄ P ₂ S ₇ skeleton, or a Li ₄ P ₂ S ₆ skeleton. . Examples of sulfide solid electrolytes having a Li ₃ PS ₄ skeleton include LiI--Li ₃ PS ₄ , LiI--LiBr--Li ₃ PS ₄ and Li ₃ PS ₄ . Examples of sulfide solid electrolytes having a Li ₄ P ₂ S ₇ skeleton include Li—P—S solid electrolytes called LPS (eg, Li ₇ P ₃ S ₁₁ ). As the sulfide solid electrolyte, for example, LGPS represented by Li _(4-x) Ge _(1-x) P _x S ₄ (where x satisfies 0<x<1) may be used. Among them, the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing the element P, and more preferably a material containing Li ₂ SP ₂ S ₅ as a main component. Furthermore, the sulfide solid electrolyte may contain halogens (F, Cl, Br, I). Halogen-containing sulfide solid electrolytes include aldirodite-type solid electrolytes (Li ₆ PS ₅ Cl and Li ₆ PS ₅ Br), which are also materials that can be preferably used.

When the sulfide solid electrolyte is Li ₂ S—P ₂ S ₅ system, the molar ratio of Li ₂ S and P ₂ S ₅ is Li ₂ S:P ₂ S ₅ =50:50 to 100. :0, and more preferably Li ₂ S:P ₂ S ₅ =70:30 to 80:20.

The sulfide solid electrolyte may be sulfide glass, crystallized sulfide glass, or a crystalline material obtained by a solid phase method. The sulfide glass can be obtained, for example, by subjecting the raw material composition to mechanical milling (such as a ball mill). Crystallized sulfide glass can be obtained, for example, by heat-treating sulfide glass at a temperature equal to or higher than the crystallization temperature. Further, the ionic conductivity (eg, Li ion conductivity) of the sulfide solid electrolyte at room temperature (25° C.) is, for example, preferably 1×10 ⁻⁵ S/cm or more, and 1×10 ⁻⁴ S/cm. cm or more is more preferable. Incidentally, the value of the ionic conductivity of the solid electrolyte can be measured by the AC impedance method.

Examples of oxide solid electrolytes include compounds having a NASICON structure. Examples of compounds having a NASICON structure include compounds represented by the general formula Li _1+x Al _x Ge _2-x (PO ₄ ) ₃ (0≦x≦2) (LAGP) and general formula Li _1+x Al _x Ti _{2 -x} (PO ₄ ) ₃ (0≦x≦2) (LATP) and the like. Other examples of oxide solid electrolytes include LiLaTiO (e.g., Li _0.34 La _0.51 TiO ₃ ), LiPON (e.g., Li _2.9 PO _3.3 N _0.46 ), LiLaZrO (e.g., , Li ₇ La ₃ Zr ₂ O ₁₂ ) and the like.

The shape of the solid electrolyte is, for example, a particle shape such as a perfect sphere or an ellipsoid. The average particle size (arithmetic mean size) of the primary particles of the solid electrolyte contained in the positive electrode active material layer is not particularly limited, but is preferably 0.8 μm or less. On the other hand, the lower limit of the average particle size of the primary particles of the solid electrolyte is not particularly limited, and is usually 0.5 μm or more.

The positive electrode active material layer may further contain at least one of a conductive aid and a binder in addition to the positive electrode active material and solid electrolyte described above.

Examples of conductive aids include metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys containing these metals, and metal oxides; carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fiber, pitch-based carbon fiber, rayon-based carbon fiber, activated carbon fiber, etc.), carbon nanotubes (CNT), carbon black (specifically, acetylene black, Ketjenblack (registered trademark) , furnace black, channel black, thermal lamp black, etc.), but are not limited thereto. In addition, a particulate ceramic material or resin material coated with the above metal material by plating or the like can also be used as a conductive aid. Among these conductive aids, from the viewpoint of electrical stability, it is preferable to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, copper, titanium, and carbon. , silver, gold, and carbon, and more preferably at least one carbon. These conductive aids may be used alone or in combination of two or more.

The shape of the conductive aid is preferably particulate or fibrous. When the conductive agent is particulate, the shape of the particles is not particularly limited, and may be powdery, spherical, rod-like, needle-like, plate-like, columnar, amorphous, scaly, spindle-like, or the like. I don't mind.

The average particle size (primary particle size) of the conductive additive in the form of particles is not particularly limited, but is preferably 0.01 to 10 μm from the viewpoint of the electrical characteristics of the battery.

When the positive electrode active material layer contains a conductive aid, the content of the conductive aid in the positive electrode active material layer is not particularly limited, but is preferably 0 to 10% by mass with respect to the total weight of the positive electrode active material layer. , more preferably 2 to 8% by mass, more preferably 4 to 7% by mass. Within such a range, it is possible to form a stronger electron conduction path in the positive electrode active material layer, which can effectively contribute to the improvement of battery characteristics.

On the other hand, the binder is not particularly limited, but includes, for example, the following materials.

Polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are replaced with other halogen elements), carboxymethylcellulose, polyethylene, polypropylene, polymethylpentene, polybutene, polyethernitrile, polytetra Fluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its Hydrogenated products, thermoplastic polymers such as styrene/isoprene/styrene block copolymers and their hydrogenated products, tetrafluoroethylene/hexafluoropropylene copolymers (FEP), tetrafluoroethylene/perfluoroalkyl vinyl ether copolymers (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) and other fluorine resins, vinylidene Fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene Vinylidene fluoride fluororubbers such as fluororubbers (VDF-PFMVE-TFE fluororubbers), vinylidene fluoride-chlorotrifluoroethylene fluororubbers (VDF-CTFE fluororubbers), epoxy resins, and the like. Among them, polyimide, styrene-butadiene rubber, carboxymethylcellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable.

Although the thickness of the positive electrode active material layer varies depending on the configuration of the intended all-solid-state battery, it is preferably in the range of 0.1 to 1000 μm, for example.

[Negative electrode (negative electrode active material layer)]
In the secondary battery according to this embodiment, the negative electrode active material layer 13 contains a negative electrode active material. The types of negative electrode active materials are not particularly limited, but include carbon materials, metal oxides and metal active materials. A metal containing lithium may also be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is an active material containing lithium, and examples thereof include metallic lithium and lithium-containing alloys. Lithium-containing alloys include, for example, alloys of Li and at least one of In, Al, Si and Sn. In some cases, two or more kinds of negative electrode active materials may be used together. Needless to say, negative electrode active materials other than those described above may be used. The negative electrode active material preferably contains metallic lithium or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and particularly preferably contains metallic lithium or a lithium-containing alloy. When metal lithium or a lithium-containing alloy is used as the negative electrode active material, the lithium secondary battery as an electrical device deposits lithium metal as the negative electrode active material on the negative electrode current collector during the charging process, a so-called lithium deposition type. can be of Therefore, in such a form, the thickness of the negative electrode active material layer increases as the charging process progresses, and the thickness of the negative electrode active material layer decreases as the discharging process progresses. Although the negative electrode active material layer does not have to exist at the time of complete discharge, depending on the situation, a certain amount of the negative electrode active material layer made of lithium metal may be arranged at the time of complete discharge.

Examples of the shape of the negative electrode active material include particulate (spherical, fibrous), thin film, and the like. When the negative electrode active material is in the form of particles, the average particle size thereof is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, and even more preferably in the range of 100 nm to 20 μm. within the range of 1 to 20 μm.

The content of the negative electrode active material in the negative electrode active material layer is not particularly limited. more preferred. The negative electrode active material layer may further contain a solid electrolyte, a conductive aid and/or a binder, and specific forms and preferred forms thereof are described in the section on the positive electrode active material layer above. can be adopted as well.

Although the thickness of the negative electrode active material layer varies depending on the configuration of the intended all-solid-state battery, it is preferably in the range of 0.1 to 1000 μm, for example.

[Solid electrolyte layer]
In the secondary battery according to the present embodiment, the solid electrolyte layer is a layer interposed between the positive electrode active material layer and the negative electrode active material layer and essentially containing a solid electrolyte. The specific form of the solid electrolyte contained in the solid electrolyte layer is not particularly limited, and the exemplified and preferred forms described in the column of the positive electrode active material layer are similarly employed. That is, the solid electrolyte layer preferably contains a sulfide solid electrolyte from the viewpoint of excellent ion conductivity and further improvement of battery performance. Alternatively, it may contain only a solid electrolyte other than the sulfide solid electrolyte.

The solid electrolyte layer may further contain a binder in addition to the solid electrolyte described above. As for the binder that can be contained in the solid electrolyte layer, the examples and preferred forms described in the column of the positive electrode active material layer can be similarly adopted.

The thickness of the solid electrolyte layer varies depending on the intended configuration of the lithium ion secondary battery, but from the viewpoint of improving the volume energy density of the battery, it is preferably 600 μm or less, more preferably 500 μm or less. , and more preferably 400 μm or less. On the other hand, the lower limit of the thickness of the solid electrolyte layer is not particularly limited, but is preferably 10 μm or more, more preferably 50 μm or more, and still more preferably 100 μm or more.

[Positive collector plate and negative collector plate]
The material constituting the current collectors (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collectors for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable as the constituent material of the current collector plate. From the viewpoint of light weight, corrosion resistance and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The same material or different materials may be used for the positive electrode collector plate 27 and the negative electrode collector plate 25 .

[Positive lead and negative lead]
Although not shown, the current collectors (11″, 11′) and the current collector plates (27, 25) may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent material of the negative electrode lead, materials used in known lithium-ion secondary batteries can be similarly employed.The portion taken out from the exterior may contact peripheral devices, wiring, etc. and cause electric leakage. It is preferable to cover with a heat-resistant insulating heat-shrinkable tube or the like so as not to affect products (for example, automobile parts, especially electronic devices).

[Battery exterior material]
As the battery exterior material, a known metal can case can be used, and as shown in FIGS. 1 and 2, a bag-like case using a laminate film 29 containing aluminum that can cover the power generation element is used. can be The laminated film may be, for example, a laminated film having a three-layer structure in which PP, aluminum and nylon are laminated in this order, but is not limited thereto. A laminate film is desirable from the viewpoint that it is excellent in high power output and cooling performance and can be suitably used for batteries for large equipment for EV and HEV. Further, the exterior material is more preferably a laminate film containing aluminum, since the group pressure applied to the power generating element from the outside can be easily adjusted.

The stacked battery according to this embodiment has a structure in which a plurality of single cell layers are connected in parallel, so that it has a high capacity and excellent cycle durability. Therefore, the stacked battery according to this embodiment is suitably used as a power source for driving EVs and HEVs.

Although one embodiment of the lithium ion secondary battery has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be appropriately modified based on the description of the claims. It is possible.

For example, as a type of battery to which the lithium ion secondary battery according to the present invention is applied, a positive electrode active material layer electrically coupled to one surface of the current collector and an electrically Bipolar batteries are also included, which include a bipolar electrode having a negative electrode active material layer bonded to a cathode.

Further, the secondary battery according to this embodiment does not have to be an all-solid-state battery. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution). The amount of the liquid electrolyte (electrolyte solution) that can be contained in the solid electrolyte layer is not particularly limited, but the amount is such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and the liquid electrolyte (electrolyte solution) does not leak. is preferably the amount of

A liquid electrolyte (electrolytic solution) that can be used has a form in which a lithium salt is dissolved in an organic solvent. Examples of organic solvents used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), and methyl formate. (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL). Among them, the organic solvent is preferably a chain carbonate, more preferably diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), from the viewpoint that the rapid charging characteristics and output characteristics can be further improved. It is at least one selected from the group consisting of, more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

Lithium salts include Li(FSO ₂ ) ₂ N (lithium bis(fluorosulfonyl)imide; LiFSI), Li(C ₂ F ₅ SO ₂ ) ₂ N, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF _{3 SO3} and the like. Among them, the lithium salt is preferably Li(FSO ₂ ) ₂ N (LiFSI) from the viewpoint of battery output and charge/discharge cycle characteristics.

The liquid electrolyte (electrolytic solution) may further contain additives other than the components described above. Specific examples of such compounds include ethylene carbonate, vinylene carbonate, methylvinylene carbonate, dimethylvinylene carbonate, phenylvinylene carbonate, diphenylvinylene carbonate, ethylvinylene carbonate, diethylvinylene carbonate, vinylethylene carbonate, 1,2- Divinylethylene carbonate, 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethylethylene carbonate, allyloxymethylethylene carbonate, acryloxymethylethylene carbonate, methacryloxymethylethylene carbonate, ethynylethylene carbonate, propargylethylene carbonate, ethynyloxymethylethylene carbonate, propargyloxyethylene carbonate, methyleneethylene carbonate, 1 , 1-dimethyl-2-methylene ethylene carbonate and the like. These additives may be used alone or in combination of two or more. In addition, when the additive is used in the electrolytic solution, the amount used can be appropriately adjusted.

[Assembled battery]
An assembled battery is an object configured by connecting a plurality of batteries. Specifically, at least two or more are used, and serialization or parallelization or both of them are used. By connecting in series and in parallel, it is possible to freely adjust the capacity and voltage.

A plurality of batteries can be connected in series or in parallel to form a small detachable assembled battery. A plurality of these detachable small battery packs are further connected in series or in parallel to form a large-capacity, large-capacity battery suitable for vehicle drive power sources and auxiliary power sources that require high volumetric energy density and high volumetric output density. It is also possible to form assembled batteries (battery modules, battery packs, etc.) with output. How many batteries are connected to make an assembled battery, and how many stages of small assembled batteries are stacked to make a large-capacity assembled battery, depends on the battery capacity of the vehicle (electric vehicle) in which it is installed. It should be decided according to the output.

[vehicle]
A battery or an assembled battery formed by combining a plurality of these batteries can be mounted on the vehicle. In the present invention, since a long-life battery with excellent long-term reliability can be configured, a plug-in hybrid electric vehicle with a long EV driving range and an electric vehicle with a long driving range per charge can be configured by mounting such a battery. Batteries or assembled batteries made by combining a plurality of these, for example, in the case of automobiles, hybrid vehicles, fuel cell vehicles, electric vehicles (all four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, buses, light vehicles, etc.) , two-wheeled vehicles (motorcycles) and three-wheeled vehicles) will provide a long-lasting and highly reliable automobile. However, the application is not limited to automobiles, for example, it can be applied to various power sources for other vehicles, such as moving bodies such as trains, and power sources for installation such as uninterruptible power supplies. It can also be used as

The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited only to the following examples.

<<Preparation example of positive electrode>>
[Comparative Example 1]
(Preparation of coated positive electrode active material)
First, a lithium transition metal oxide (composition=LiNi _0.8 Mn _0.1 Co _0.1 O) was prepared as a positive electrode active material. In addition, fine powder of LiNbO ₃ , which is lithium ion conductive oxide particles, was separately prepared. Next, using a tumbling fluidized granulation coating apparatus, LiNbO ₃ is uniformly deposited on the surface of the NMC composite oxide particles and dried to form a coating layer of LiNbO ₃ on the surface of the NMC composite oxide particles. Then, a coated positive electrode active material was prepared.

Here, the average particle size of the secondary particles of the coated positive electrode active material obtained as described above, the average particle size of the primary particles constituting the secondary particles, and the thickness of the coating layer are measured by the following method. Measurements were 4.9 μm, 2.3 μm and 5 nm, respectively.

(Method for measuring average particle size)
A cross section of the coated positive electrode active material prepared above was observed and evaluated using a scanning electron microscope (SEM).

The average particle diameter (arithmetic mean diameter) of the primary particles of the positive electrode active material is the particle diameter of 50 or more particles without grain boundaries that can be confirmed from the cross section of the positive electrode active material (any two points on the observed particle contour line Among the distances between the two, the maximum distance) was measured, and the arithmetic mean value was calculated. In addition, the average particle diameter (arithmetic mean diameter) of the secondary particles of the positive electrode active material is obtained by measuring the particle diameters of 50 or more primary particle groups in which there is no other substance between them, and calculating the arithmetic mean value. obtained by Furthermore, the thickness of the coating layer was obtained by measuring the thickness of each of 50 or more locations of the coating layer in the observed image and calculating the arithmetic mean value.

(Preparation of positive electrode)
Next, an aldirodite-type solid electrolyte (Li ₆ PS ₅ Cl), which is a lithium ion conductive sulfide solid electrolyte, was prepared. Also, acetylene black, which is a conductive agent, was prepared. Predetermined amounts of the coated positive electrode active material, the sulfide solid electrolyte, and the conductive aid prepared above were weighed, and beads were further added and roller-mixed to obtain a positive electrode mixture. The compounding ratio (mass ratio) of each component was set to coated positive electrode active material:sulfide solid electrolyte:conductive aid=60:34:6.

(Preparation of cell for evaluation)
An evaluation cell (lithium precipitation type all-solid lithium secondary battery) was produced by placing the positive electrode mixture prepared above and the negative electrode current collector opposite to each other and interposing a solid electrolyte layer therebetween.

Specifically, 100 mg of the same sulfide solid electrolyte (Li ₆ PS ₅ Cl) as above was weighed, placed in a ceramic tube, smoothed on the surface, and subjected to a pressure of 200 [MPa] using a fastening jig. A solid electrolyte pellet (φ10 mm) was produced by pressing.

After that, the fastening jig is pulled out, a predetermined amount of the positive electrode mixture obtained above is placed on one side of the pellet, and a pressure of 200 [MPa] is applied using the fastening jig to form a positive electrode active material layer (thickness: 25 μm). was made. On the other side of the pellet, a nickel foil was placed as a negative electrode current collector and pressed at 100 [MPa] using a fastening jig to prepare an evaluation cell of this comparative example.

[Comparative Example 2]
Comparative Example 1 described above _, except that the amount of LiNbO3 added when preparing the coated positive electrode active material was adjusted so that the thickness of the coating layer of LiNbO3 _, which is the lithium ion conductive oxide particles, was 10 nm. A positive electrode and an evaluation cell of this comparative example were produced in the same manner as in .

[Example 1]
(Preparation of coated positive electrode active material)
First, a lithium transition metal oxide (composition=LiNi _0.8 Mn _0.1 Co _0.1 O) was prepared as a positive electrode active material. In addition, fine powder of LiNbO ₃ , which is lithium ion conductive oxide particles, was separately prepared. Next, using a planetary ball mill, a large number of LiNbO ₃ particles were attached to the surfaces of the NMC composite oxide particles to prepare a coated positive electrode active material.

Here, the average particle diameter of the secondary particles of the coated positive electrode active material obtained as described above, the average particle diameter of the primary particles constituting the secondary particles, and the average particle diameter of the LiNbO ₃ particles are calculated as above. When measured by the same method as described above, they were 4.9 μm, 2.3 μm and 5 nm, respectively. The average particle diameter (arithmetic mean diameter) of the LiNbO ₃ particles is the particle diameter of 50 or more particles that can be confirmed from the cross section of the positive electrode active material (out of the distance between any two points on the observed particle contour line) , maximum distance) were measured, and the arithmetic mean was calculated.

In addition, when the surface coverage of the NMC composite oxide particles with the LiNbO ₃ particles was measured from the SEM observation image used for the measurement of the average particle size described above, it was 80%.

A positive electrode and an evaluation cell of this example were produced in the same manner as in Comparative Example 1 described above, except that the coated positive electrode active material thus prepared was used.

[Example 2]
A positive electrode and an evaluation cell of this example were produced in the same manner as in Example 1 described above, except that LiAlO ₂ was used instead of LiNbO ₃ as the lithium ion conductive oxide.

[Example 3]
A positive electrode and an evaluation cell of this example were produced in the same manner as in Example _{1 ,} except that _Li2ZrO3 was used instead of LiNbO3 as the lithium ion conductive oxide.

[Example 4]
Except that the amount of Li ₂ ZrO ₃ added in preparing the coated positive electrode active material was adjusted so that the coverage of the surface of the NMC composite oxide particles with the Li ₂ ZrO ₃ particles was 50%. A positive electrode and an evaluation cell of this comparative example were produced in the same manner as in Example 3.

<<Evaluation example of positive electrode>>
Cycle charge-discharge tests were carried out under the following charge-discharge test conditions using the evaluation cells produced in the examples and comparative examples described above, and the respective positive electrodes produced above were evaluated.

(Charge/discharge test conditions)
1) Charging and discharging conditions [Voltage range] 2.5 to 4.3 V
[Charging process] CCCV
[Discharge process] CCCV
[Charging and discharging rate] 0.05C
(Pause for 30 minutes each after charging and discharging)
[Charging/discharging cycle] 50 cycles 2) Evaluation temperature: 298K (25°C).

Using a charge/discharge tester, the evaluation cell was placed in a constant temperature bath set at the above evaluation temperature, and during the charging process (lithium metal was deposited on the negative electrode current collector), constant current and constant voltage ( CCCV) mode, and charged from 2.5V to 4.3V at 0.05C. After that, in the discharge process (lithium metal on the negative electrode current collector is dissolved), the battery was discharged from 4.3V to 2.5V at 0.05C in a constant current/constant voltage (CCCV) mode. Here, 1C is a current value at which the battery is fully charged (100% charged) when charged for one hour at that current value.

After completion of the cycle charge/discharge test, the percentage [%] of the discharge capacity at the 50th cycle to the discharge capacity at the 1st cycle was calculated as the discharge capacity retention rate, and used as an index of cycle durability. The results are shown in Table 1 below.

From the results shown in Table 1, it can be seen that the use of the positive electrode according to one embodiment of the present invention improves the cycle durability. Moreover, it can be seen that a particularly high discharge capacity retention rate is achieved in Example 3, which has a high coverage rate of 80%, compared to Example 4, which has a coverage rate of 50%. On the other hand, in Comparative Examples 1 and 2 in which a similar lithium ion conductive oxide is used, but the particles are not adhered to the surface of the NMC composite oxide particles, but a coating layer is formed therefrom. , it can also be seen that a sufficient discharge capacity retention rate could not be obtained. This is because the coating layer cannot sufficiently follow the contraction of the NMC composite oxide particles during the charging process, and the coating layer peels off and falls off from the surface of the active material particles. This is considered to be due to the increase in resistance.

10a laminated battery,
11' negative electrode current collector,
11″ cathode current collector,
13 negative electrode active material layer,
15 positive electrode active material layer,
17 solid electrolyte layer,
19 cell layer,
21 power generation element,
25 negative electrode current collector,
27 positive current collector,
29 laminate film,
100 positive electrode active material,
110 Particles of NMC composite oxide 120 Particles of lithium ion conductive oxide.

Claims (9)

A positive electrode containing a positive electrode active material containing Li(Ni—Mn—Co)O ₂ and a lithium-transition metal composite oxide having a composition in which a portion of these transition metals are replaced with other elements, and a solid electrolyte. A positive electrode for a secondary battery comprising an active material layer,
A positive electrode for a secondary battery, wherein the surfaces of the particles of the lithium-transition metal composite oxide are coated with the particles of the lithium-ion conductive oxide at a coverage rate of 50% or more. 2. The positive electrode for a secondary battery according to claim 1, wherein said coverage is 80% or more. The lithium-transition metal composite oxide contains primary particles and secondary particles composed of the primary particles, the average particle size of the secondary particles is 4.9 μm or less, and the average particle size of the primary particles constituting the secondary particles. 3. The positive electrode for a secondary battery according to claim 1, having a diameter of 1.2 [mu]m or more. The lithium ion conductive oxide has the following general formula (2):
_{LixAOy (} 2)
In general formula (2),
A is Al, Zr, Nb, Ti, Ta, La, P, B, C, Si, S, Mo or W;
x and y each independently represent a positive integer;
The positive electrode for a secondary battery according to any one of claims 1 to 3, having a composition represented by: _The lithium ion conductive _oxides include _Li2ZrO3 , _LiNbO3 , _LiAlO2 , _Li2TiO3 , _{Li4Ti5O12 , Li2Ti2O5 , LiTaO3 , Li3PO4 , Li3BO . 5. The} positive electrode for a _secondary battery _{according to claim 4 ,} which is _{LiBO2 , Li2CO3 , Li4SiO4} , _Li2SiO3 , _Li2SO4 , _Li2MoO4 or _Li2WO4 . The positive electrode for a secondary battery according to any one of claims 1 to 5, wherein the particles of the lithium ion conductive oxide have an average particle size of 2 to 20 nm. The positive electrode for a secondary battery according to any one of claims 1 to 6, wherein said solid electrolyte contains a sulfide solid electrolyte. A secondary battery comprising the positive electrode for a secondary battery according to any one of claims 1 to 7. The secondary battery according to claim 8, which is an all-solid lithium secondary battery.

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