JP7322776B2 - lithium ion secondary battery - Google Patents

Description

The present invention relates to lithium ion secondary batteries.

Lithium ion secondary batteries are also widely used as power sources for mobile devices such as mobile phones and laptop computers, and hybrid cars. With the development of these fields, lithium ion secondary batteries are required to have higher performance.

One of the performances is the cycle characteristics of lithium ion secondary batteries. Cycle characteristics are an index of deterioration when a lithium ion secondary battery is charged and discharged. Patent Document 1 describes a lithium ion secondary battery having excellent cycle characteristics. The lithium ion secondary battery described in Patent Document 1 has an active material in which core particles are coated with a first coating layer and a second coating layer. The first coating layer relaxes the distortion of the lattice and crystallites of the core particles due to the intercalation/deintercalation reaction of lithium ions, and the second coating layer suppresses metal elution from the core particles. The second coating layer prevents metal elution from the core particles, thereby improving the cycle characteristics of the lithium ion secondary battery.

Japanese Unexamined Patent Application Publication No. 2016-33902

However, even when the active material described in Patent Document 1 is used, the cycle characteristics of the lithium ion secondary battery may not be sufficient.

The present disclosure has been made in view of the above problems, and an object thereof is to provide a lithium ion secondary battery capable of improving cycle characteristics.

As a result of intensive studies, the present inventors have concluded that the deterioration of the cycle characteristics of the lithium-ion secondary battery of Patent Document 1 is caused by cracks occurring in the second coating layer. When cracks occur in the second coating layer, metal is eluted from the cracks, and the cycle characteristics of the lithium ion secondary battery deteriorate. In the second coating layer, cracks occur due to contraction and expansion of the core particles due to the intercalation/deintercalation reaction of lithium ions.

The present inventors have found that the use of a phosphate-containing non-aqueous electrolyte can repair cracks and suppress deterioration in the cycle characteristics of a lithium-ion secondary battery.
That is, in order to solve the above problems, the following means are provided.

(1) A lithium ion secondary battery according to a first aspect includes a positive electrode containing an active material, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The active material has a core containing a lithium composite oxide and a coating layer covering at least a part of the surface of the core, and the coating layer comprises an oxide or fluoride different from the lithium composite oxide. and the non-aqueous electrolyte contains a phosphate.

(2) The coating layer of the lithium ion secondary battery according to the above aspect may not insert and remove lithium ions at the charging and discharging voltage of the positive electrode.

(3) In the lithium ion secondary battery according to the above aspect, the active material has an intermediate layer on at least part of the surface between the core and the coating layer, and the intermediate layer is a lithium composite oxide and the lithium composite oxide contained in the intermediate layer has substantially the same composition as the lithium composite oxide contained in the core, and has lower crystallinity than the lithium composite oxide contained in the core. good too.

(4) In the lithium ion secondary battery according to the aspect described above, the coating layer may have cracks, and the cracks may be coated with a lithium compound having a composition different from that of the coating layer.

(5) In the lithium ion secondary battery according to the above aspect, the phosphate may be difluorophosphate.

(6) In the lithium ion secondary battery according to the aspect described above, the concentration of the phosphate in the non-aqueous electrolyte may be 10 ppm or more and 130000 ppm or less.

(7) In the lithium ion secondary battery according to the above aspect, the negative electrode may contain silicon.

(8) In the lithium ion secondary battery according to the above aspect, the negative electrode may contain metallic lithium.

The lithium ion secondary battery according to the aspect described above has improved cycle characteristics.

1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment; FIG. 1 is a cross-sectional view of a positive electrode active material according to a first embodiment; FIG. 4 shows a state in the vicinity of the positive electrode active material during operation of the lithium ion secondary battery according to the first embodiment; FIG. 5 is a cross-sectional view of a positive electrode active material according to a first modified example; FIG. 10 is a cross-sectional view of a positive electrode active material according to a second modified example;

Hereinafter, embodiments will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, characteristic portions may be enlarged for convenience in order to make the characteristics easier to understand, and the dimensional ratios and the like of each component may differ from the actual. 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 implemented with appropriate modifications without changing the gist of the invention.

"Lithium-ion secondary battery"
FIG. 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. A lithium-ion secondary battery 100 shown in FIG. 1 includes a power generation element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). The exterior body 50 covers the periphery of the power generation element 40 . The power generation element 40 is connected to the outside by a pair of connected terminals 60 and 62 . A non-aqueous electrolyte is contained in the exterior body 50 .

(power generation element)
The power generation element 40 includes a positive electrode 20 , a negative electrode 30 and a separator 10 .

<Separator>
Separator 10 is sandwiched between positive electrode 20 and negative electrode 30 . The separator 10 separates the positive electrode 20 and the negative electrode 30 and prevents short circuit between the positive electrode 20 and the negative electrode 30 . Lithium ions can pass through the separator 10 .

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer of a film made of polyolefin such as polyethylene or polypropylene, a stretched film of a laminate or a mixture of the above resins, or selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene and polypropylene. A fibrous nonwoven fabric made of at least one constituent material can be mentioned.

Separator 10 may be, for example, a solid electrolyte. Solid electrolytes are polymer solid electrolytes, oxide-based solid electrolytes, and sulfide-based solid electrolytes, for example. The polymer solid electrolyte is, for example, a polyethylene oxide polymer dissolved in an alkali metal salt. Examples of oxide-based solid electrolytes include Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ (Nasicon type), Li _1.07 Al _0.69 Ti _1.46 (PO ₄ ) ₃ (glass ceramics ₎ , _{Li0.34La0.51TiO2.94} ( _{perovskite type ) , Li7La3Zr2O12} (garnet type), _{Li2.9PO3.3N0.46} ( _amorphous , LIPON ₎ , 50Li ₄ SiO ₄ .50Li ₂ BO ₃ (glass), and 90Li ₃ BO ₃ .10Li ₂ SO ₄ (glass ceramics). Sulfide-based solid electrolytes include, for example, Li _3.25 Ge _0.25 P _0.75 S ₄ (crystal), Li ₁₀ GeP ₂ S ₁₂ (crystal, LGPS), Li ₆ PS ₅ Cl (crystal, aldirodite type) , Li _9.54 Si _1.74 P _1.44 S _11.7 Cl _0.3 (crystal), Li _3.25 P _0.95 S ₄ (glass ceramics), Li ₇ P ₃ S ₁₁ (glass ceramics) , 70Li ₂ S.30P ₂ S ₅ (glass), 30Li ₂ S.26B _{2 S} 3.44LiI (glass), 50Li ₂ S.17P ₂ S 5.33LiBH ₄ (glass) _, 63Li ₂ S.36SiS _{2.Li 3PO4 (} glass), _{57Li2S.38SiS2.5Li4SiO4} ( _{glass ) .}

<Positive electrode>
The positive electrode 20 has a positive electrode current collector 22 and a positive electrode active material layer 24 . The positive electrode active material layer 24 is formed on at least one surface of the positive electrode current collector 22 .

[Positive collector]
The positive electrode current collector 22 is, for example, a conductive plate. The positive electrode current collector 22 is, for example, a metal thin plate made of aluminum, copper, nickel, titanium, stainless steel, or the like.

[Positive electrode active material layer]
The positive electrode active material layer 24 has, for example, a positive electrode active material, a conductive aid, and a binder.

The positive electrode active material can reversibly absorb and desorb lithium ions, desorb and insert (intercalate) lithium ions, or dope and dedope lithium ions and counter anions.

FIG. 2 is a cross-sectional view of the positive electrode active material according to the first embodiment. Positive electrode active material 25 has core 26 , coating layer 27 and intermediate layer 28 .

The core 26 is a core portion of the positive electrode active material 25 . The shape of the core 26 is not particularly limited. The core 26 has, for example, a spherical shape, an ellipsoidal shape, a needle shape, a plate shape, a scale shape, a tube shape, a wire shape, a rod shape, or an amorphous shape.

Core 26 contains a lithium composite oxide. A lithium composite oxide can occlude and release lithium ions. The core 26 expands and contracts in volume when absorbing and desorbing lithium ions. Lithium composite oxide deinserts lithium ions by, for example, crystal phase transition. The lithium composite oxide is crystalline or a mixture of crystalline and amorphous. A lithium composite oxide contains, for example, lithium, one or more transition metal elements, and oxygen. Lithium composite oxides have, for example, a layered rock salt structure, an olivine structure, and a spinel structure.

A lithium composite oxide having a layered rock salt structure is represented by, for example, the following compositional formulas (1) and (2).

_{LiwMxNyO2 - zXz} ( _{1 )}
In the composition formula (1), w satisfies 0.8<w<1.2, x+y satisfies 0.9<x+y<1.1, y satisfies 0≦y<0.1, and z is 0 ≦z<0.05 is satisfied. In composition formula (1), M is at least one of Ti, V, Cr, Mn, Fe, Co, Ni, Mn and Cu. In composition formula (1), N is at least one of Na, Mg, Al, Si, K, Ca, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ba, La, W and Bi. In composition formula (1), X is at least one of F, Cl and S. The composition of lithium varies depending on the state of charge and discharge, and the value of w represents the value in the fully discharged state. Lithium manganate (LiMnO ₂ ) and lithium nickelate (LiNiO ₂ ) are examples of composition formula (1).

_{LixCo1 - yMyO2 -zXz (} 2)
In composition formula (2), x satisfies 0.8<w<1.2, y satisfies 0≦y<0.15, and z satisfies 0≦z<0.05. In composition formula (2), M is Ti, V, Cr, Mn, Fe, Ni, Mn, Cu, Na, Mg, Al, Si, K, Ca, Zn, Ga, Sr, Y, Zr, Nb, Mo, At least one of Ba, La and W. In composition formula (2), X is at least one of F, Cl and S. The composition of lithium varies depending on the state of charge and discharge, and the value of x represents the value in the fully discharged state. Lithium cobalt oxide (LiCoO ₂ ) is an example of composition formula (2).

A lithium composite oxide having an olivine structure is represented, for example, by the following compositional formula (3).
_{LiaMbPO4 ( 3} )
In composition formula (3), a satisfies 0≦a≦2.0, and b satisfies 0.5≦b≦2.0. In the composition formula (3), M is at least one element selected from Groups 2 to 15. The composition of lithium varies depending on the state of charge and discharge, and the value of a represents the value in the fully discharged state.

A lithium composite oxide having a spinel structure is represented, for example, by the following compositional formula (4).
_{LivMn2 - wMwOxFy ( 4} )
In the composition formula (4), v satisfies 0.9 ≤ v ≤ 1.1, w satisfies 1.0 ≤ w ≤ 0.6, x satisfies 3.7 ≤ x ≤ 4.1, and y satisfies It satisfies 0≦y≦0.1. M in the composition formula (4) is at least one of Co, Ni, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ca, Sr and W. The composition of lithium varies depending on the state of charge and discharge, and the value of v represents the value in the fully discharged state. Lithium manganese spinel (LiMn ₂ O ₄ ) is an example of composition formula (4).

Other examples of lithium composite oxides include lithium vanadium compounds (LiV ₂ O ₅ ), lithium vanadium compounds (LiV ₂ O ₅ ), and lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

A covering layer 27 covers the core 26 . The covering layer 27 covers at least part of the core 26 . The coating layer 27 may cover the entire outer surface of the core 26 or may cover a portion of the core 26 .

The coating layer 27 contains oxide or fluoride. The oxide forming coating layer 27 is different from the lithium composite oxide forming core 26 . The coating layer 27 may be amorphous or crystalline.

Examples of oxides forming the coating layer 27 include Li, Ti, V, Cr, Mn, Fe, Co, Ni, Mn, Cu, Na, Mg, Al, Si, K, Ca, Zn, Ga, Sr, At least one element selected from Y, Zr, Nb, Mo, Ba, La, W, Bi, P and B is included. The oxide forming the coating layer 27 is, for example, an oxide containing the above elements, such as LiO ₂ , Al ₂ O ₃ , MgO, NiO, Mn ₂ O ₃ , ZrO ₂ , TiO ₂ , B ₂ O ₃ , P ₂ O ₅ . The oxide forming the coating layer 27 is, for example, a lithium composite oxide different from the lithium composite oxide forming the core 26, such as LiMn ₂ O ₄ or LiCoO ₂ .

Fluorides constituting the coating layer 27 include Li, Ti, V, Cr, Mn, Fe, Co, Ni, Mn, Cu, Na, Mg, Al, Si, K, Ca, Zn, Ga, Sr, Y, At least one element selected from Zr, Nb, Mo, Ba, La, W, Bi, P and B is included. Fluorides are, for example, LiF and MgF.

The coating layer 27 is preferably, for example, an oxide or a fluoride that does not deintercalate lithium ions at the charging/discharging voltage of the positive electrode 20 . The charge/discharge voltage of the positive electrode 20 is the charge/discharge voltage during normal operation, and is, for example, 2.0 V or more and 4.5 V or less. In this case, the coating layer 27 does not deinsert lithium during charging and discharging, and the volume change during charging and discharging is small.

The coating layer 27 may partially have cracks 27A. The crack 27A is caused, for example, by a difference in volume expansion coefficient between the coating layer 27 and the core 26 when lithium ions are intercalated and deintercalated. Further, for example, the crack 27A may be a portion where the coating layer 27 was not formed when the coating layer 27 was formed.

A portion of crack 27A is covered with lithium compound 29 . The lithium compound 29 is formed by a phosphate contained in a non-aqueous electrolyte, which will be described later, and metal ions eluted from the core 26 . Lithium compound 29 differs in composition from coating layer 27 .

The presence of lithium compound 29 can be confirmed by infrared spectroscopy (nano-IR) with nanoscale resolution. For example, the positive electrode 20 is taken out from the lithium ion secondary battery 100, washed with dimethyl carbonate or the like, and then the surface (coating layer 27) of the positive electrode active material 25 is measured by nano-IR. If the results of nano-IR analysis show vibration peaks generated when the P=O bond expands and contracts and peaks observed due to the PF bond, it can be said that the coating layer 27 contains the lithium compound 29 . A vibrational peak generated when the P=O bond expands and contracts is a peak at a wave number of 1150 cm ⁻¹ or more and 1362 cm ⁻¹ or less. The PF bond peak is a peak with a wave number of 815 cm ⁻¹ or more and 940 cm ⁻¹ or less, or a wave number of 740 cm ⁻¹ or more and 800 cm ⁻¹ or less.

The lithium compound 29 is considered to be, for example, a metal ion coordinated with a phosphate. Phosphates, for example, coordinate like ionomers to metal ions. Ionomers are synthetic resins in which macromolecules aggregate using the cohesive force of metal ions. Phosphate aggregates with metal ions instead of macromolecules. The negatively charged oxygen ions of the phosphate coordinate to the positively charged metal ions. Lithium compound 29 prevents elution of metal ions from core 26 .

The ratio of the coating layer 27 to the entire positive electrode active material 25 is preferably 0.1 mol % or more and 10 mol % or less. If the ratio of the covering layer 27 to the entire positive electrode active material 25 is small, the covering layer 27 cannot sufficiently cover the core 26, and the probability of metal elution from the core 26 increases. When the ratio of the coating layer 27 to the entire positive electrode active material 25 is high, the contribution of the coating layer 27 to charge/discharge increases, and the capacity of the lithium ion secondary battery 100 changes.

The average thickness of the coating layer 27 is, for example, 10 nm or more and 1 μm or less, preferably 10 nm or more and 500 nm or less. If the coating layer 27 is too thick, cracks 27A are likely to occur, and if it is too thin, the elution of metal ions cannot be sufficiently suppressed. The average thickness of the coating layer 27 differs from, for example, the average thickness of the lithium compound 29 covering the cracks 27A. A step is formed between the coating layer 27 and the lithium compound 29 . The thicknesses of the coating layer 27 and the lithium compound 29 can be confirmed using, for example, a scanning electron microscope (SEM).

Intermediate layer 28 is located between core 26 and cover layer 27 . The intermediate layer 28 is formed on the surface of the core 26, for example. The intermediate layer 28 may cover all or part of the outer side of the core 26 .

Intermediate layer 28 contains a lithium composite oxide. The lithium composite oxide contained in the intermediate layer 28 has substantially the same composition as the lithium composite oxide contained in the core 26 . Approximately the same composition means that the constituent elements constituting the lithium composite oxide are the same, and the atomic ratio of each constituent element is approximately the same. Approximately the same atomic ratio means that the atomic ratio is within a range in which the lithium composite oxide has a similar crystal structure in the deinsertion reaction of lithium ions. The difference in the atomic ratio of each constituent element between the lithium composite oxide contained in the intermediate layer 28 and the lithium composite oxide contained in the core 26 is preferably within 10 atomic %, more preferably within 3 atomic %. more preferred.

The composition of the intermediate layer 28 may change discontinuously at the boundary with the core 26 or may change continuously. If the composition of the intermediate layer 28 changes continuously with respect to the composition of the core 26, it is difficult to clearly identify the boundary between the intermediate layer 28 and the core 26.

The crystallinity of the lithium composite oxide contained in the intermediate layer 28 is, for example, lower than the crystallinity of the lithium composite oxide contained in the core 26 . For example, when the lithium composite oxide contained in the core 26 is crystalline and the lithium composite oxide contained in the intermediate layer 28 is amorphous or a mixture of amorphous and crystalline, the core 26 and the intermediate layer 28 is a mixture of crystalline and amorphous, and the lithium composite oxide contained in the intermediate layer 28 has a higher amorphous ratio, the lithium contained in the intermediate layer 28 The crystallinity of the composite oxide is lower than the crystallinity of the lithium composite oxide contained in the core 26 .

The crystallinity of the lithium composite oxide can be confirmed by, for example, a cross-sectional image by a transmission electron microscope (TEM) or Raman spectroscopy. Crystallinity is confirmed by Raman spectroscopy by measuring the Raman spectrum of the positive electrode active material 25 and obtaining the peak intensity ratio E/A between the peak intensity A of the first peak and the peak intensity E of the second peak. The first peak is in the range of 550 cm ⁻¹ to 650 cm ⁻¹ , and the second peak is in the range of 450 cm ⁻¹ to 500 cm ⁻¹ . The first peak is due to Co—O vibrations parallel to the c-axis, and the second peak is due to vibrations parallel to the Li layer. When the peak intensity A of the first peak is smaller than the peak intensity E of the second peak, it indicates that the crystallinity of the measurement location is low.

The intermediate layer 28 preferably has a peak intensity ratio E/A between the first peak and the second peak that satisfies 0.1≦E/A≦0.35. When the peak intensity ratio E/A is less than 0.1, the elution of metal particles in the core 26 increases due to diffusion within the solid. If E/A is greater than 0.35, the intermediate layer 28 cannot sufficiently mitigate the expansion and contraction of the volume of the core 26 during the insertion and removal of lithium ions.

The average thickness of the intermediate layer 28 is, for example, 1 nm or more and 50 nm or less. If the average thickness of the intermediate layer 28 is 1 nm or more, the uniformity of the thickness of the intermediate layer 28 is enhanced. The reversibility of lithium ion desorption/insertion is enhanced, and the cycle characteristics of the lithium ion secondary battery 100 are improved. In addition, if the average thickness of the intermediate layer 28 is 50 nm or less, the intermediate layer 28 has lower crystallinity than the core 26 and the metal is more likely to be eluted than the core 26, and the existence ratio of the intermediate layer 28 in the positive electrode active material 25 can be reduced. Elution volume is reduced.

The binder binds together the positive electrode active materials 25 in the positive electrode active material layer 24 . A known binder can be used. The binder is, for example, fluororesin. Fluororesins include, 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, binders include, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride Vinylidene fluoride-based fluorine such as Ride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber) Rubber may be used.

The conductive aid enhances the conductivity between the positive electrode active materials 25 in the positive electrode active material layer 24 . Conductive agents include, for example, carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. . A carbon material such as carbon black is preferable as the conductive aid. If the active material alone can ensure sufficient conductivity, the positive electrode active material layer 24 does not need to contain a conductive aid.

Also, the positive electrode active material layer 24 may contain a solid electrolyte or a gel electrolyte. The solid electrolyte is similar to those that can be used for the separator, for example.

<Negative Electrode>
The negative electrode 30 has, for example, a negative electrode current collector 32 and a negative electrode active material layer 34 . The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32 .

[Negative electrode current collector]
The negative electrode current collector 32 is, for example, a conductive plate. The negative electrode current collector 32 can be the same as the positive electrode current collector 22 .

[Negative electrode active material layer]
The negative electrode active material layer 34 contains a negative electrode active material. Moreover, a conductive material, a binder, and a solid electrolyte may be included as necessary.

The negative electrode active material may be any compound that can occlude and release ions, and known negative electrode active materials used in lithium ion secondary batteries can be used. Examples of the negative electrode active material include carbon materials such as metallic lithium, lithium alloys, graphite capable of absorbing and releasing ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, and low-temperature fired carbon. , aluminum, silicon, tin, germanium and other metals that can combine with lithium and other metals, SiO _x (0<x<2), amorphous compounds mainly composed of oxides such as tin dioxide, titanic acid Particles containing lithium (Li ₄ Ti ₅ O ₁₂ ) and the like.

The negative electrode active material layer 34 may contain, for example, silicon, tin, germanium, as described above. Silicon, tin, and germanium may exist as single elements or as compounds. Compounds are, for example, alloys, oxides, and the like. As an example, if the negative electrode active material 34 is silicon, the negative electrode 30 is sometimes referred to as a Si negative electrode. The negative electrode active material may be, for example, silicon, tin, or germanium alone or a mixture of a compound and a carbon material. The carbon material is, for example, natural graphite. Further, the negative electrode active material may be, for example, silicon, tin, germanium, or a compound whose surface is coated with carbon. The carbon material and coated carbon enhance the electrical conductivity between the negative electrode active material and the conductive aid. When the negative electrode active material layer contains silicon, tin, and germanium, the capacity of the lithium ion secondary battery 100 increases.

The negative electrode active material layer 34 may contain lithium, for example, as described above. Lithium may be metallic lithium or a lithium alloy. The negative electrode active material layer 34 may be metallic lithium or a lithium alloy. Lithium alloys include, for example, Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, An alloy of one or more elements selected from the group consisting of Ra, Ge, and Al, and lithium. As an example, when the negative electrode active material is metallic lithium, negative electrode 30 may be referred to as a Li negative electrode. The negative electrode active material layer 34 may be a sheet of lithium.

The negative electrode 30 may include only the negative electrode current collector 32 without the negative electrode active material layer 34 at the time of fabrication. When the lithium ion secondary battery 100 is charged, metallic lithium is deposited on the surface of the negative electrode current collector 32 . The metallic lithium is a single lithium in which lithium ions are deposited, and the metallic lithium functions as the negative electrode active material layer 34 .

The same conductive material and binder as those used for the positive electrode 20 can be used. The binder in the negative electrode 30 may be, for example, cellulose, styrene/butadiene rubber, ethylene/propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, etc., in addition to those listed for the positive electrode 20 . The cellulose may be, for example, carboxymethylcellulose (CMC).

(Terminal)
Terminals 60 and 62 are connected to positive electrode 20 and negative electrode 30, respectively. A terminal 60 connected to the positive electrode 20 is a positive terminal, and a terminal 62 connected to the negative electrode 30 is a negative terminal. Terminals 60 and 62 are responsible for electrical connection with the outside. Terminals 60, 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. Terminals 60, 62 are preferably protected with insulating tape to prevent short circuits.

(Exterior body)
The exterior body 50 seals the power generation element 40 and the non-aqueous electrolyte therein. The exterior body 50 prevents the leakage of the non-aqueous electrolyte to the outside and the intrusion of moisture into the inside of the lithium ion secondary battery 100 from the outside.

The exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52, as shown in FIG. 1, for example. The exterior body 50 is a metal laminate film in which a metal foil 52 is coated from both sides with polymer films (resin layers 54).

For example, aluminum foil can be used as the metal foil 52 . A polymer film such as polypropylene can be used for the resin layer 54 . The material forming the resin layer 54 may be different between the inner side and the outer side. For example, a polymer with a high melting point such as polyethylene terephthalate (PET) or polyamide (PA) is used as the outer material, and polyethylene (PE) or polypropylene (PP) is used as the inner polymer film material. be able to.

(Non-aqueous electrolyte)
The non-aqueous electrolyte is enclosed in the exterior body 50 and impregnates the power generating element 40 .
The non-aqueous electrolyte has, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in a non-aqueous solvent.

Non-aqueous solvents include, for example, cyclic carbonates and chain carbonates. Cyclic carbonates solvate electrolytes. Cyclic carbonates are, for example, ethylene carbonate, propylene carbonate and butylene carbonate. The cyclic carbonate preferably contains at least propylene carbonate. Chain carbonates reduce the viscosity of cyclic carbonates. Chain carbonates are, for example, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate. Non-aqueous solvents may also include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like. .

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

The non-aqueous electrolyte contains a phosphate as an electrolyte. The non-aqueous electrolyte may have electrolytes other than phosphate. For _example , _nonaqueous electrolytes include _{LiPF6 ,} LiClO4, _LiBF4 , _{LiCF3SO3 , LiCF3CF2SO3} , LiC( _CF3SO2 ) ₃ , LiN( _CF3SO2 ) _{2 , LiN} (CF Lithium salts such _{as 3CF2SO2 ) 2} , _LiN ( _{CF3SO2 )} ( _{C4F9SO2 )} , LiN( _{CF3CF2CO ) 2} , LiBOB may be included along with the phosphate.

Phosphates include, for example, LiPO ₂ F ₂ (lithium difluorophosphate), Li ₂ PO ₃ F (lithium monofluorophosphate), Li ₃ PO ₄ (trilithium phosphate), Na ₃ PO ₄ (trilithium phosphate), sodium), lithium dibutyl phosphate, lithium diphenyl phosphate, disodium phenyl phosphate, disodium 1-naphthyl phosphate. The phosphate is preferably LiPO ₂ F ₂ (lithium difluorophosphate). The phosphate combines with metal eluted from core 26 to form lithium compound 29 .

The concentration of phosphate in the non-aqueous electrolyte is preferably 10 ppm or more and 130000 ppm or less. The concentration of phosphate is more preferably 10 ppm or more and 10000 ppm or less, still more preferably 10 ppm or more and 1000 ppm or less, and particularly preferably 100 ppm or more and 1000 ppm or less.

In addition to the phosphate, the nonaqueous electrolyte includes 1,3-propanesultone, 1,4-butanesultone, 1-propene 1,3sultone, 1,3,2-dioxathiolane 2,2-dioxide (ethylenesultone fert) and other sultone compounds, and nitrile compounds such as adiponitrile, succinonitrile and pimelonitrile.

"Manufacturing method of lithium ion secondary battery"
First, the positive electrode active material 25 is produced. The core 26 can be produced by a solid phase reaction method or the like. The intermediate layer 28 and the covering layer 27 are formed by a compounding process. The compounding treatment is, for example, a mechanical alloying method. First, the powder of the material that will form the intermediate layer 28 and the core 26 are mechanically mixed to produce particles in which the intermediate layer 28 is formed on the surface of the core 26 . Next, by mechanically mixing the produced particles and the powder of the material that will form the coating layer 27 , the coating layer 27 is formed on the surface of the intermediate layer 28 and the positive electrode active material 25 is obtained. The powder of the material that forms the intermediate layer 28 and the powder of the material that forms the coating layer 27 can each be produced by a solid phase reaction method or the like. Alternatively, the covering layer 27 may be formed by coating. For coating, the core 26 with the intermediate layer 28 formed thereon is placed in a coating liquid and heat-treated.

Next, the positive electrode 20 is produced. The positive electrode 20 is produced by mixing a positive electrode active material, a binder and a solvent to prepare a pasty positive electrode slurry. The mixing method of these components constituting the positive electrode slurry is not particularly limited, and the mixing order is also not particularly limited. Next, the positive electrode slurry is applied to the positive electrode current collector 22 . The coating method is not particularly limited. For example, a slit die coating method and a doctor blade method can be used.

Subsequently, the solvent in the positive electrode slurry applied on the positive electrode current collector 22 is removed. A removal method is not particularly limited. For example, the positive electrode current collector 22 coated with the positive electrode slurry is dried in an atmosphere of 80.degree. C. to 150.degree. Then, the positive electrode 20 is obtained by pressing the obtained coating film to increase the density of the positive electrode active material layer 24 . As a means of pressing, for example, a roll press machine, a hydrostatic press machine, or the like can be used.

Next, the negative electrode 30 is produced. The negative electrode 30 can be produced in the same manner as the positive electrode 20 . The negative electrode 30 is produced by mixing a negative electrode active material, a binder and a solvent to prepare a pasty negative electrode slurry. The negative electrode 30 is obtained by applying the negative electrode slurry to the negative electrode current collector 32 and drying it. When the negative electrode active material is metallic lithium, lithium foil may be attached to the negative electrode current collector 32 .

Next, the positive electrode 20 and the negative electrode 30 are laminated so that the separator 10 is positioned between them to produce the power generation element 40 . When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and the separator 10 are wound around one end side of the separator.

Finally, the power generation element 40 is enclosed in the exterior body 50 . A non-aqueous electrolyte is injected into the exterior body 50 . After injecting the non-aqueous electrolyte, the power generation element 40 is impregnated with the non-aqueous electrolyte by depressurizing, heating, or the like. The lithium ion secondary battery 100 is obtained by applying heat or the like to seal the exterior body 50 .

The lithium ion secondary battery 100 according to the first embodiment can suppress deterioration in cycle characteristics. FIG. 3 shows the vicinity of the positive electrode active material during operation of the lithium ion secondary battery according to the first embodiment.

The positive electrode active material 25 has a composition different from that of the core 26 and the coating layer 27 . When the lithium-ion secondary battery 100 is charged and discharged, the core 26 and the covering layer 27 expand and contract differently. For example, if the coating layer 27 does not deintercalate lithium ions, the volume of the coating layer 27 hardly changes due to charging and discharging. On the other hand, the core 26 changes its volume as lithium ions are inserted/desorbed by charging/discharging. As shown in FIG. 3, if the coating layer 27 cannot alleviate the volume change of the core 26, the coating layer 27 will have cracks 27A.

The non-aqueous electrolyte reaches the core 26 when the crack 27A occurs. The metal M contained in the core 26 is eluted from the core 26 into the non-aqueous electrolyte as ions. Elution of the metal M from the core 26 degrades the cycle characteristics of the lithium ion secondary battery.

When the non-aqueous electrolyte contains phosphate, the metal M eluted from the core 26 reacts with the decomposition product of the phosphate. The phosphate coordinates to the metal M like an ionomer. The reaction between the metal M and the phosphate occurs in the vicinity of the crack 27A where the metal M is easily eluted. As a result, the crack 27A is covered with a lithium compound 29, which is a reaction product of the metal M and the phosphate, and repaired. The lithium compound 29 inhibits contact between the non-aqueous electrolyte and the core 26 and prevents the metal M from eluting.

That is, in the lithium-ion secondary battery 100 according to the first embodiment, even if cracks 27A occur in the positive electrode active material 25, the cracks 27A are self-repaired and the elution of the metal M is suppressed. As a result, the lithium ion secondary battery 100 according to the first embodiment can suppress degradation in cycle characteristics.

In addition, when the positive electrode active material 25 has the intermediate layer 28 , the intermediate layer 28 relaxes lattice and crystallite distortion associated with deinsertion and deinsertion of lithium ions from the core 26 . This is because the intermediate layer 28 has a lower crystallinity than the core 26 . When the intermediate layer 28 relaxes the strain caused by the volume change of the core 26, the crack 27A is less likely to occur in the covering layer 27. FIG. If the frequency of occurrence of cracks 27A is reduced, the metal M is less likely to be eluted, and the cycle characteristics of the lithium ion secondary battery 100 are further improved.

Further, when the phosphate is difluorophosphate, the lithium compound 29 is easily formed, and the crack 27A is easily repaired. Furthermore, if the concentration of the phosphate in the non-aqueous electrolyte is 10 ppm or more and 130000 ppm or less, the eluted metal M can be sufficiently captured, and the elution of the metal M can be effectively suppressed.

Also, the metal M eluted from the core 26 moves to the surface of the negative electrode 30 . When the metal M is deposited on the surface of the negative electrode 30, the negative electrode 30 tends to self-discharge. When the negative electrode 30 contains silicon, tin, or germanium (for example, in the case of a Si negative electrode), and when the negative electrode 30 contains lithium (for example, in the case of a Li negative electrode), the deposition of the metal M has a large effect on self-discharge characteristics. . For example, when the negative electrode 30 is a Si negative electrode or a Li negative electrode, preventing the elution of the metal M can particularly suppress the deterioration of the characteristics (for example, self-discharge characteristics) of the lithium ion secondary battery 100 .

As described above, the embodiments of the present invention have been described in detail with reference to the drawings. , substitutions, and other modifications are possible.

For example, FIG. 4 is a cross-sectional view of a positive electrode active material 25A according to a first modified example. A positive electrode active material 25A according to the first modified example differs from the positive electrode active material 25 according to the first embodiment in that the intermediate layer 28 is not provided. In FIG. 4, the same components as those of the positive electrode active material 25 are denoted by the same reference numerals.

The positive electrode active material 25A cannot relax strain due to volume change of the core 26 more than the positive electrode active material 25, and cracks 27A tend to occur in the coating layer 27. FIG. However, the crack 27A is repaired by the lithium compound 29 which is the reaction product of the metal M and the phosphate decomposition product. Therefore, even in the lithium-ion secondary battery using the positive electrode active material 25A according to the first modified example, deterioration in cycle characteristics can be suppressed.

Further, for example, FIG. 5 is a cross-sectional view of a positive electrode active material 25B according to a second modification. The positive electrode active material 25B according to the second modification is related to the first embodiment in that the core 26 and the intermediate layer 28 aggregate to form secondary particles, and the coating layer 27 covers the periphery of the secondary particles. It differs from the positive electrode active material 25 . In FIG. 5, the same components as those of the positive electrode active material 25 are denoted by the same reference numerals.

Even if a crack 27A occurs in the coating layer 27 of the positive electrode active material 25B, the crack 27A is repaired by the lithium compound 29, which is a reaction product of the metal M and the decomposition product of the phosphate. Therefore, even in the lithium-ion secondary battery using the positive electrode active material 25B according to the second modified example, deterioration in cycle characteristics can be suppressed.

"Example 1"
(Preparation of positive electrode)
First, a positive electrode active material was produced. Cobalt oxide and lithium carbonate were mixed so that the molar ratio of Co and Li was 1:1. A material for the intermediate layer 28 was produced by the same procedure except that the cooling conditions after the core 26 and firing were rapid cooling. Particles in which 97% by mass of the powder of the core 26 and 3% by mass of the material for the intermediate layer 28 are mixed while mechanically rubbing each other using a mixer manufactured by Hosokawa Micron Corporation, and the intermediate layer 28 covers the core 26. was made.

LiF particles were then prepared and mixed with the particles with the intermediate layer 28 covering the core 26 . The mass ratio at the time of mixing was 97% by mass of the particles in which the core 26 was covered with the intermediate layer 28 and 3% by mass of the LiF particles. Mixing was performed by mechanically rubbing each other using a mixer manufactured by Hosokawa Micron Corporation. As a result, a positive electrode active material 25 was obtained.

Next, the positive electrode active material, the conductive material, and the binder were mixed to prepare a positive electrode mixture. Carbon black was used as the conductive material, and polyvinylidene fluoride (PVDF) was used as the binder. The mass ratio of the positive electrode active material, the conductive material, and the binder was 90:5:5. This positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode slurry. Then, the positive electrode slurry was applied to one surface of an aluminum foil having a thickness of 15 μm. After the application, it was dried at 100° C. to remove the solvent and form a positive electrode active material layer.

(Preparation of negative electrode)
A negative electrode mixture was prepared by mixing a negative electrode active material, a conductive material, and a binder. Graphite was used as the negative electrode active material, carbon black was used as the conductive material, and carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) were used as the binder. The mass ratio of the negative electrode active material, the conductive material, and the binder was 90:5:5. This negative electrode mixture was dispersed in distilled water to prepare a negative electrode slurry. Then, the negative electrode slurry was applied to one surface of a copper foil having a thickness of 10 μm. After coating, the coating was dried at 100° C. to remove the solvent and form a negative electrode active material layer.

(Production of cells)
The produced negative electrode and positive electrode were punched into a predetermined shape, and laminated alternately via a polypropylene separator having a thickness of 25 μm to produce a laminate by laminating 9 negative electrodes and 8 positive electrodes.

An opening was formed by inserting the laminate into an outer package made of an aluminum laminate film and heat-sealing the laminate except for one peripheral portion. A non-aqueous electrolyte was injected into the exterior body. The non-aqueous electrolyte is obtained by dissolving 1.0 MOL/L of lithium hexafluorophosphate (LiPF ₆ ) in a solvent in which equal amounts of ethylene carbonate (EC) and dimethyl carbonate (DEC) are mixed. LiPO ₂ F ₂ ) was added at 100 ppm. Then, the remaining one portion was heat-sealed while the pressure was reduced by a vacuum sealer, and a lithium ion secondary battery according to Example 1 was produced.

(Structural analysis and evaluation of batteries)
The cycle characteristics of the produced lithium ion battery were evaluated. Cycle characteristics were evaluated by 1C/1C charge/discharge cycles.

(Measurement of charge-discharge cycle characteristics)
Cycle characteristics were measured using a secondary battery charge/discharge tester. The voltage range was from 4.3V to 3.0V. First, as a pretreatment, only the initial charging was performed by 0.2C constant current charging. After that, charging and discharging were performed to obtain cycle characteristics. Charging was performed at constant current and constant voltage. Charging was performed at a current value of 1.0 C, and after reaching 4.3 V, the charging was completed when the current value reached 5% of the 1 C current value. The discharge was performed under the condition of discharging at a current value of 1.0C. The cycle characteristics were evaluated as a capacity retention rate (%). The capacity retention rate (%) is the ratio of the discharge capacity after 500 cycles to the initial discharge capacity, with the discharge capacity at the first cycle as the initial discharge capacity. The capacity retention rate (%) is represented by the following formula.
Capacity retention rate (%) = (“Discharge capacity after 500 cycles”/“Discharge capacity at 1st cycle”) × 100

1C is a current value at which charging and discharging of a battery cell having a capacity of the nominal capacity value is completed in exactly 1 hour when constant-current charging or constant-current discharging is performed. A higher capacity retention rate means better cycle characteristics.

The lithium ion secondary battery produced in Example 1 was repeatedly charged and discharged under the above conditions, and the capacity retention rate after 500 cycles was evaluated as cycle characteristics. The results are shown in Table 1.

"Examples 2-6"
Examples 2 to 6 differ from Example 1 in that the materials of the core and the intermediate layer of the positive electrode active material were changed. In Example 2, LiNi _0.5 Co _0.2 Mn _0.3 (denoted as NCM523 in Table 1) was used for the core and intermediate layer of the positive electrode active material.
In Example 3, LiNi _0.6 Co _0.2 Mn _0.2 (denoted as NCM622 in Table 1) was used for the core and intermediate layer of the positive electrode active material.
In Example 4, LiNi _0.8 Co _0.1 Mn _0.1 (denoted as NCM811 in Table 1) was used for the core and intermediate layer of the positive electrode active material.
In Example 5, LiNi _0.85 Co _0.12 Al _0.03 (denoted as NCA in Table 1) was used for the core and intermediate layer of the positive electrode active material.
In Example 6, LiFePO ₄ (denoted as LFP in Table 1) was used for the core and intermediate layer of the positive electrode active material.
The cycle characteristics of Examples 2 to 6 were also evaluated, and the results are shown in Table 1.

"Examples 7-9"
Examples 7 to 9 differ from Example 1 in that the phosphate contained in the non-aqueous electrolyte was changed.
In Example 7, monofluorolithium phosphate (Li ₂ PO ₃ F) was used as the phosphate.
In Example 8, trilithium phosphate (Li ₃ PO ₄ ) was used as the phosphate.
Example 9 used trisodium phosphate (Na ₃ PO ₄ ) as the phosphate.
The cycle characteristics of Examples 7 to 9 were also evaluated, and the results are shown in Table 1.

"Example 10"
Example 10 differs from Example 1 in that only the coating layer was formed without providing the intermediate layer on the positive electrode active material. Example 10 was also evaluated for cycle characteristics, and the results are shown in Table 1.

"Examples 11-17"
In Examples 11 to 17, the amount of lithium difluorophosphate (LiPO ₂ F ₂ ) added to the non-aqueous electrolyte was changed.
In Example 11, the amount added was 10 ppm.
In Example 12, the added amount was 1000 ppm.
In Example 13, the added amount was 5000 ppm.
In Example 14, the amount added was 10000 ppm.
In Example 15, the added amount was 75000 ppm.
In Example 16, the added amount was 130000 ppm.
In Example 17, the added amount was 150000 ppm.
The cycle characteristics of Examples 11 to 17 were also evaluated, and the results are shown in Table 1.

"Examples 18-24"
Examples 18 to 25 differ from Example 1 in that the material of the coating layer of the positive electrode active material was changed.
In Example 18, Al ₂ O ₃ was used as the coating layer of the positive electrode active material.
In Example 19, TiO ₂ was used as the coating layer of the positive electrode active material.
In Example 20, Li ₂ O was used as the coating layer of the positive electrode active material.
In Example 21, MgO was used as the coating layer of the positive electrode active material.
In Example 22, the coating layer of the positive electrode active material was _MgF2 .
In Example 23, NCA was used as the coating layer of the positive electrode active material.
In Example 24, NCM523 was used as the coating layer of the positive electrode active material.
The cycle characteristics of Examples 11 to 17 were also evaluated, and the results are shown in Table 1.

"Comparative Examples 1 to 4"
Comparative Example 1 differs from Example 1 in that lithium difluorophosphate (LiPO ₂ F ₂ ) was not added to the non-aqueous electrolyte.
Comparative Example 2 differs from Example 1 in that lithium difluorophosphate (LiPO ₂ F ₂ ) was not added to the non-aqueous electrolyte and no intermediate layer was provided.
Comparative Example 3 differs from Example 1 in that lithium carbonate was added to the non-aqueous electrolyte instead of lithium difluorophosphate (LiPO ₂ F ₂ ). The amount of lithium carbonate added to the non-aqueous electrolyte was 100 ppm.
Comparative Example 1 differs from Example 1 in that lithium hydroxide was added to the non-aqueous electrolyte instead of lithium difluorophosphate (LiPO ₂ F ₂ ). The amount of lithium hydroxide added to the non-aqueous electrolyte was 100 ppm.
Cycle characteristics were also evaluated for Comparative Examples 1 to 4, and the results are shown in Table 1.

As shown in Table 1, Examples 1-24 had improved cycle characteristics compared to Comparative Examples 1-4 containing no phosphate. It was also confirmed that the cycle characteristics of the lithium ion secondary battery were improved even when the type of active material, the type of phosphate, and the type of coating layer were changed. It was also confirmed that the cycle characteristics were changed by changing the amount of phosphate in the non-aqueous electrolyte.

"Examples 25-27"
Examples 25 to 27 differ from Example 5 in that the phosphate contained in the non-aqueous electrolyte was changed.
Example 25 used lithium monofluorophosphate (Li ₂ PO ₃ F) as the phosphate.
Example 26 used trilithium phosphate (Li ₃ PO ₄ ) as the phosphate.
Example 27 used trisodium phosphate (Na ₃ PO ₄ ) as the phosphate.
Cycle characteristics were also evaluated for Examples 25 to 27, and the results are shown in Table 2.

"Example 28"
Example 28 differs from Example 5 in that only the covering layer was formed without providing the intermediate layer on the positive electrode active material. The cycle characteristics of Example 28 were also evaluated, and the results are shown in Table 2.

"Examples 29-35"
In Examples 29 to 35, the amount of lithium difluorophosphate (LiPO ₂ F ₂ ) added to the non-aqueous electrolyte was changed.
In Example 29, the amount added was 10 ppm.
In Example 30, the amount added was 1000 ppm.
In Example 31, the amount added was 5000 ppm.
In Example 32, the amount added was 10000 ppm.
In Example 33, the amount added was 75000 ppm.
In Example 34, the amount added was 130000 ppm.
In Example 35, the added amount was 150000 ppm.
The cycle characteristics of Examples 29 to 35 were also evaluated, and the results are shown in Table 2.

"Examples 36-42"
Examples 36 to 42 differ from Example 5 in that the material of the coating layer of the positive electrode active material was changed.
In Example 36, the coating layer of the positive electrode active material was Al ₂ O ₃ .
In Example 37, TiO ₂ was used as the coating layer of the positive electrode active material.
In Example 38, Li ₂ O was used as the coating layer of the positive electrode active material.
In Example 39, MgO was used as the coating layer of the positive electrode active material.
In Example 40, the coating layer of the positive electrode active material was _MgF2 .
In Example 41, NCA was used as the coating layer of the positive electrode active material.
In Example 42, NCM523 was used as the coating layer of the positive electrode active material.
Cycle characteristics were also evaluated for Examples 36 to 42, and the results are shown in Table 2.

"Comparative Examples 5-8"
Comparative Example 5 differs from Example 5 in that lithium difluorophosphate (LiPO ₂ F ₂ ) was not added to the non-aqueous electrolyte.
Comparative Example 6 differs from Example 5 in that lithium difluorophosphate (LiPO ₂ F ₂ ) was not added to the non-aqueous electrolyte and no intermediate layer was provided.
Comparative Example 7 differs from Example 5 in that lithium carbonate was added to the non-aqueous electrolyte instead of lithium difluorophosphate (LiPO ₂ F ₂ ). The amount of lithium carbonate added to the non-aqueous electrolyte was 100 ppm.
Comparative Example 8 differs from Example 5 in that instead of lithium difluorophosphate (LiPO ₂ F ₂ ), lithium hydroxide was added to the non-aqueous electrolyte. The amount of lithium hydroxide added to the non-aqueous electrolyte was 100 ppm.
Cycle characteristics were also evaluated for Comparative Examples 5 to 8, and the results are shown in Table 2.

As shown in Table 2, Examples 5 and 25 to 42 had improved cycle characteristics compared to Comparative Examples 1 to 4 containing no phosphate. Even when the active material was changed to NCA, the effect of phosphate on the cycle characteristics was the same as when the active material was LCO.

"Examples 43-47"
In Examples 43 to 47, the negative electrode active material was changed from graphite.
Example 43 differs from Example 1 in that a Li foil was attached to one surface of a copper foil (negative electrode current collector) without preparing a negative electrode slurry. In Example 43, the negative electrode is a Li negative electrode.
Example 44 differs from Example 1 in that silicon (Si) is used as the negative electrode active material. In Example 44, the negative electrode is a Si negative electrode.
Example 45 differs from Example 1 in that silicon (Si) and graphite were used as the negative electrode active material. In Example 45, the negative electrode is a mixed negative electrode of Si and graphite.
Example 46 differs from Example 1 in that silicon oxide (SiO) was used as the negative electrode active material.
Example 47 differs from Example 1 in that silicon oxide (SiO) and graphite were used as the negative electrode active material.
The cycle characteristics of Examples 43 to 47 were also evaluated, and the results are shown in Table 3.

In Examples 43 to 47, self-discharge characteristics were also evaluated. Self-discharge characteristics were measured by the following procedure.
The lithium ion secondary battery was charged at a constant current up to 3.7V at a current value corresponding to 0.2C as a current density in a constant temperature bath at 25°C, and then charged at a constant voltage at 3.7V. Constant voltage charging was continued until the current density dropped to a value corresponding to 0.05C.
Thereafter, the voltage of the lithium ion secondary battery after waiting at 25° C. for one week was recorded as V1, and the voltage after storage at 25° C. for 24 hours was recorded as V2. The self-discharge characteristic was defined as (V1-V2)/24 from the amount of voltage drop after 24 hours. Lithium ion secondary batteries are more excellent in self-discharge characteristics as the amount of voltage drop is smaller. Table 3 shows the results.

"Comparative Examples 9-11"
Comparative Example 9 differs from Example 43 in that lithium difluorophosphate (LiPO ₂ F ₂ ) was not added to the non-aqueous electrolyte.
Comparative Example 10 differs from Example 44 in that lithium difluorophosphate (LiPO ₂ F ₂ ) was not added to the non-aqueous electrolyte.
Comparative Example 11 differs from Example 46 in that lithium difluorophosphate (LiPO ₂ F ₂ ) was not added to the non-aqueous electrolyte.
Cycle characteristics were also evaluated for Comparative Examples 9 to 11, and the results are shown in Table 3.

As shown in Table 3, even when the negative electrode was made of a material other than graphite, the addition of the phosphate to the non-aqueous electrolyte improved the cycle characteristics. Also, by changing the negative electrode to a material other than graphite, the self-discharge characteristics were improved. Presumably, this is because the prevention of elution of metal from the positive electrode active material inhibited deposition of the eluted metal on the surface of the negative electrode.

10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layers 25, 25A, 25B Positive electrode active material 26 Core 27 Coating layer 27A Crack 28 Intermediate layer 29 Lithium compound 30 Negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generation element 50 exterior body 52 metal foil 54 resin layers 60, 62 terminal 100 lithium ion secondary battery M metal

Claims (7)

a positive electrode comprising an active material;
a negative electrode;
a separator sandwiched between the positive electrode and the negative electrode;
and a non-aqueous electrolyte,
The active material has a core containing a lithium composite oxide and a coating layer covering at least part of the surface of the core,
The coating layer contains an oxide or fluoride different from the lithium composite oxide,
The non-aqueous electrolyte contains a phosphate,
the active material has an intermediate layer on at least part of the surface between the core and the coating layer;
The intermediate layer contains a lithium composite oxide,
The lithium composite oxide contained in the intermediate layer has substantially the same composition as the lithium composite oxide contained in the core, and has lower crystallinity than the lithium composite oxide contained in the core. next battery. 2. The lithium ion secondary battery according to claim 1, wherein said coating layer does not deintercalate lithium ions at the charge/discharge voltage of said positive electrode. The coating layer has cracks,
3. The lithium ion secondary battery according to claim 1 , wherein said crack is coated with a lithium compound having a composition different from that of said coating layer. The lithium ion secondary battery according to any one of claims 1 to 3 , wherein said phosphate is difluorophosphate. The lithium ion secondary battery according to any one of claims 1 to 4 , wherein the concentration of said phosphate in said non-aqueous electrolyte is 10 ppm or more and 130000 ppm or less. The lithium ion secondary battery according to any one of claims 1 to 5 , wherein the negative electrode contains silicon. The lithium ion secondary battery according to any one of claims 1 to 6 , wherein the negative electrode contains metallic lithium.

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