JP6946719B2 - Negative electrode active material for lithium ion secondary battery, negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Description

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

Due to its high energy density, lithium-ion secondary batteries are widely used as power sources for portable electronic devices such as notebook personal computers, communication devices such as mobile phones, and power sources for automobiles. This lithium ion secondary battery is generally composed of a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte for allowing ions to move between the positive electrode and the negative electrode. .. Recently, from the viewpoint of miniaturization and weight reduction of portable electronic devices and communication devices, there is a strong demand for lithium ion secondary batteries having a higher energy density. In addition, there is a strong demand for longer life of automobile batteries.

By the way, a metal such as silicon (Si) or a silicon oxide (SiO _x ) or a negative electrode active material particle such as a metal oxide has a theoretical capacity much higher than the theoretical capacity of 372 mAh / g of graphite currently in practical use. From the above, it is the most promising material for increasing the energy density of batteries. However, silicon and silicon oxide have a large theoretical capacity and a large expansion and contraction at the time of insertion and desorption of lithium ions, so that micronization occurs. Since the active material that has been pulverized and dropped off cannot contribute to the charge / discharge reaction due to isolation, there is a problem that the charge / discharge efficiency is lowered. Further, as the charge / discharge efficiency decreases, a sufficient charge / discharge cycle cannot be obtained.

To solve such a problem, it has been studied to coat the surface of the negative electrode active material particles with a coating film. In Patent Documents 1 and 2, the surface of the negative electrode active material particles is an oxide containing at least one element selected from the group consisting of silicon (Si), titanium (Ti), aluminum (Al) and zirconium (Zr). It has been proposed to coat with ceramics such as nitrides or carbides. Further, Patent Document 3 proposes to coat the surface of the negative electrode active material particles with a metal oxide such as _{TiO 2} or ZrO _2.

Japanese Unexamined Patent Publication No. 2004-335334 Japanese Unexamined Patent Publication No. 2004-335335 Japanese Unexamined Patent Publication No. 2007-141666

However, according to the study by the present inventor, when the surface of the negative electrode active material particles is coated with the coating film described in Patent Documents 1 to 3, the initial charge / discharge efficiency may decrease.
An object of the present invention has been made in view of the above circumstances, and to provide a negative electrode active material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery having excellent initial charge / discharge efficiency. It is in.

The present inventor has determined that the charge / discharge efficiency of the negative electrode active material particles is improved by coating at least a part of the negative electrode active material particles with a coating film containing a specific lithium-containing titanium oxide containing titanium and lithium. I found it.
That is, the present invention provides the following means for solving the above problems.

(1) The negative electrode active material for a lithium ion secondary battery according to the first aspect includes active material particles capable of occluding and releasing lithium ions, and a coating film provided on at least a part of the surface of the active material particles. The coating film contains a lithium-containing titanium oxide represented by the following formula (1).
Ti _{1-x / 4-a / 4-2b / 4-3c / 4-d} Li _x A _a B _b C _c D _d O ₂ ... (1)
However, in the formula (1), A is one or more monovalent metals selected from the group consisting of K and Na, and B is one or more divalent metals selected from the group consisting of Mg, Ca and Sr. A metal, C is one or more trivalent metals selected from the group consisting of Al, Y, La and Bi, and D is one selected from the group consisting of Zr, Si, Hf, Sn and Ge. In the above tetravalent metal, x is a number satisfying 0 <x ≦ 0.60, a is a number satisfying 0 ≦ a ≦ 0.27, and b is a number satisfying 0 ≦ b ≦ 0.27. C is a number satisfying 0 ≦ c ≦ 0.27, and d is a number satisfying 0 ≦ d ≦ 0.27.

(2) In the negative electrode active material for a lithium ion secondary battery according to the above aspect, the lithium-containing titanium oxide may contain at least one metal of Mg, Ca, Al, Zr, and Na.

(3) The negative electrode active material for a lithium ion secondary battery according to the above aspect is a number in which the a in the formula (1) satisfies 0 <a≤0.27, and the ratio x / of the x to the a. a may be in the range of 0.07 or more and 60 or less.

(3) The negative electrode active material for a lithium ion secondary battery according to the above aspect is a number in which the b in the formula (1) satisfies 0 <b ≦ 0.27, and the ratio x / of the x to the b. b may be in the range of 0.07 or more and 60 or less.

(4) The negative electrode active material for a lithium ion secondary battery according to the above aspect is a number in which the c in the formula (1) satisfies 0 <c ≦ 0.27, and the ratio x / of the x to the c. c may be in the range of 0.07 or more and 60 or less.

(5) The negative electrode active material for a lithium ion secondary battery according to the above aspect is a number in which the d in the formula (1) satisfies 0 <d ≦ 0.27, and the ratio x / of the x to the d. d may be in the range of 0.07 or more and 60 or less.

(6) The negative electrode for a lithium ion secondary battery according to the second aspect is a lithium ion secondary battery having a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The negative electrode for use, wherein the negative electrode active material layer contains the negative electrode active material for a lithium ion secondary battery according to the first aspect.

(7) The lithium ion secondary battery according to the third aspect is a lithium ion secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. The negative electrode is the negative electrode for a lithium ion secondary battery according to the second aspect.

According to the present invention, it is possible to provide a negative electrode active material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery having excellent initial charge / discharge efficiency.

It is sectional drawing of the lithium ion secondary battery which concerns on this embodiment.

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

[Lithium-ion secondary battery]
FIG. 1 is a schematic cross-sectional view of the lithium ion secondary battery according to the present embodiment. The lithium ion secondary battery 100 shown in FIG. 1 mainly includes a laminate 40, a case 50 that houses the laminate 40 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 40. Although not shown, the electrolyte is housed in the case 50 together with the laminate 40.

In the laminated body 40, the positive electrode 20 and the negative electrode 30 are arranged so as to face each other with the separator 10 interposed therebetween. The positive electrode 20 has a positive electrode active material layer 24 provided on a plate-shaped (film-shaped) positive electrode current collector 22. The negative electrode 30 has a negative electrode active material layer 34 provided on a plate-shaped (film-shaped) negative electrode current collector 32.

The positive electrode active material layer 24 and the negative electrode active material layer 34 are in contact with both sides of the separator 10, respectively. Leads 62 and 60 are connected to the ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the ends of the leads 60 and 62 extend to the outside of the case 50. In FIG. 1, a case where one laminated body 40 is provided in the case 50 is illustrated, but a plurality of laminated bodies 40 may be laminated.

"Negative electrode"
The negative electrode 30 has a negative electrode current collector 32 and a negative electrode active material layer 34 provided on the negative electrode current collector 32.

(Negative electrode current collector)
The negative electrode current collector 32 may be any conductive plate material, and for example, a thin metal plate of aluminum foil, electrolytic copper foil, rolled copper foil, nickel foil, or SUS foil can be used.

(Negative electrode active material layer)
The negative electrode active material layer 34 has a negative electrode active material and a negative electrode binder, and optionally has a negative electrode conductive material.

(Negative electrode active material)
The negative electrode active material for a lithium ion secondary battery of the present embodiment has active material particles capable of occluding and releasing lithium ions, and a coating film provided on at least a part of the surface of the active material particles. The coating is preferably provided over the entire surface of the active material particles. The presence or absence of the coating and the composition can be confirmed by XPS (X-ray photoelectron spectroscopy).

As the material of the negative electrode active material particles, various materials used as the negative electrode active material for a known lithium ion secondary battery can be used. Examples of materials for the negative electrode active material include carbon materials, silicon, _{silicon-containing compounds such as silicon oxide represented by SiO x} (0 <x <2), metallic lithium, metals forming alloys with lithium, and metals. Examples thereof include these alloys, amorphous compounds mainly composed of oxides such as tin dioxide, and lithium titanate (Li ₄ Ti ₅ O ₁₂ ). Examples of carbon materials include graphite (natural graphite, artificial graphite), carbon nanotubes, non-graphitized carbon, easily graphitized carbon, low-temperature calcined carbon, and the like. Examples of metals that form alloys with metallic lithium include aluminum, silicon, and tin. The negative electrode active material particles preferably have an average particle diameter in the range of 0.01 μm or more and 50 μm or less. More preferably, it is in the range of 0.1 μm or more and 10 μm or less.

The film provided on the surface of the active material particles contains a lithium-containing titanium oxide represented by the following formula (1).
Ti _{1-x / 4-a / 4-2b / 4-3c / 4-d} Li _x A _a B _b C _c D _d O ₂ ... (1)

In formula (1), A is one or more monovalent metals selected from the group consisting of K and Na. B is one or more divalent metals selected from the group consisting of Mg, Ca and Sr. C is one or more trivalent metals selected from the group consisting of Al, Y, La and Bi. D is one or more tetravalent metals selected from the group consisting of Zr, Si, Hf, Sn and Ge.

x is a number that satisfies 0 <x ≦ 0.60. Therefore, the coating film contains Li as an essential component.
a is a number satisfying 0 ≦ a ≦ 0.27, b is a number satisfying 0 ≦ b ≦ 0.27, c is a number satisfying 0 ≦ c ≦ 0.27, and d is a number satisfying 0 ≦ a ≦ 0.27. , 0 ≦ d ≦ 0.27.

The reason why the initial charge / discharge efficiency is improved by using the negative electrode active material of the present embodiment having the above lithium-containing titanium oxide on the surface in the lithium ion secondary battery is considered as follows.
The above lithium-containing titanium oxide has a Ti—O—Ti bond as a skeleton, and a part of Ti is substituted with Li to form a Ti—O—Li bond. Since O and Li in the Ti—O—Li bond have a strong ionic bond, the lithium-containing titanium oxide has higher conductivity than titanium oxide. As described above, since the negative electrode active material of the present embodiment has high conductivity, the initial charge / discharge efficiency is improved.
In addition, Li in the Ti—O—Li bond of the lithium-containing titanium oxide is easily desorbed. When Li in the Ti—O—Li bond is desorbed, Li ions easily enter the lithium-containing titanium oxide, and the conductivity of Li ions between the negative electrode active material and the electrolyte is improved. As described above, since the negative electrode active material of the present embodiment has high conductivity of Li ions, the initial charge / discharge efficiency is improved.

If the Li content of the lithium-containing titanium oxide is too small, the above effects may not be easily exhibited. On the other hand, if the Li content is too high, it may be difficult for Li in the Ti—O—Li bond to be detached, and it may be difficult for Li ions to enter the lithium-containing titanium oxide. Therefore, in the present embodiment, the Li content is a number in which x in the formula (1) satisfies 0 <x ≦ 0.64.

The lithium-containing titanium oxide of the present embodiment may contain a monovalent to tetravalent metal in addition to Li.

When the lithium-containing titanium oxide contains a monovalent metal such as K or Na, the atomic number ratio of lithium to the monovalent metal (lithium / monovalent metal) is preferably in the range of 0.07 or more and 60 or less. That is, in the lithium-containing titanium oxide, when a in the formula (1) is a number satisfying 0 <a≤0.50, the ratio x / a of x and a is in the range of 0.07 or more and 60 or less. Is preferable. In this case, the initial charge / discharge efficiency of the negative electrode active material can be improved more reliably.

When the lithium-containing titanium oxide contains a divalent metal such as Mg, Ca, Sr, the atomic number ratio of lithium to the divalent metal (lithium / divalent metal) may be in the range of 0.07 or more and 60 or less. preferable. That is, when b in the formula (1) is a number satisfying 0 <b ≦ 0.27, the ratio x / b of x and b is preferably in the range of 0.07 or more and 60 or less. In this case, the initial charge / discharge efficiency of the negative electrode active material can be improved more reliably.

When the lithium-containing titanium oxide contains a trivalent metal such as Al, Y, La, Bi, the atomic number ratio of lithium to the trivalent metal (lithium / trivalent metal) is in the range of 0.07 or more and 60 or less. Is preferable. That is, when c in the formula (1) is a number satisfying 0 <c ≦ 0.27, the ratio x / c of x and c is preferably in the range of 0.07 or more and 60 or less. In this case, the initial charge / discharge efficiency of the negative electrode active material can be improved more reliably.

When the lithium-containing titanium oxide contains a tetravalent metal such as Zr, Si, Hf, Sn, and Ge, the atomic number ratio of lithium to the tetravalent metal (lithium / tetravalent metal) is in the range of 0.07 or more and 60 or less. It is preferable to be in. That is, when d in the formula (1) is a number satisfying 0 <d ≦ 0.27, the ratio x / d of x and d is preferably in the range of 0.07 or more and 60 or less. In this case, the initial charge / discharge efficiency of the negative electrode active material can be improved more reliably.

Among the above monovalent to tetravalent metals, at least one of Mg, Ca, Al, and Zr may be contained. In this case, the initial charge / discharge efficiency of the negative electrode active material can be further reliably improved.

The lithium-containing titanium oxide is preferably, for example, a compound represented by the following formulas (2) to (6).
(2) Ti _{1-x / 4} Li _x O ₂ (0 <x ≦ 0.60)
(3) Ti _{1-x / 4-2b / 4} Li _x Mg _b O ₂ (0 <x ≦ 0.60, 0 <b ≦ 0.27, 0.07 ≦ x / b ≦ 60)
_{(4) Ti 1-x /} 4-2b / 4 Li x Ca b O 2 (0 <x ≦ 0.60,0 <b ≦ 0.27,0.07 ≦ x / b ≦ 60)
(5) Ti _{1-x / 4-3c / 4} Li _x Al _c O ₂ (0 <x ≦ 0.60, 0 <c ≦ 0.27, 0.07 ≦ x / c ≦ 60)
(6) Ti _{1-x / 4-d} Li _x Zr _d O ₂ (0 <x ≦ 0.60, 0 <d ≦ 0.27, 0.07 ≦ x / d ≦ 60)

(Negative electrode active material)
The negative electrode active material of the present embodiment can be produced, for example, as follows.
A solution (Liquid A) for forming a film is prepared by dissolving a titanium alkoxide and a monovalent to tetravalent metal alkoxide in a solvent. In addition, an active material particle dispersion liquid (solution B) in which a trace amount of water and active material particles capable of occluding and releasing lithium ions are dispersed in a solvent is prepared. Next, while stirring the solution B, the solution A was added to the solution B, and then the alkoxide of titanium and the alkoxide of the added metal were hydrolyzed and condensed polymerized to form a monovalent to tetravalent alkoxide on the surface of the active material particles. A film of titanium oxide sol containing metal is formed. The active material particles on which the sol film is formed are recovered, washed with pure water or alcohol, and then dried under reduced pressure to form a film with a titanium oxide film containing a monovalent to tetravalent metal. Obtain material particles.

Next, the active material particles with a coating are immersed in an aqueous hydrochloric acid solution to elute all or part of the monovalent to tetravalent metals (excluding titanium) in the coating as metal ions. The elution amount of metal ions can be adjusted by adjusting the concentration of the aqueous hydrochloric acid solution and the immersion time. Then, the active material particles with a film are recovered from the aqueous hydrochloric acid solution and thoroughly washed with pure water.

Next, the active material particles with a film are immersed in an aqueous solution of lithium hydroxide, and lithium ions are inserted into the defective sites generated by elution of metal ions. The pH of the lithium hydroxide aqueous solution is preferably in the range of 9 or more and 14 or less. Then, the film-coated active material particles into which lithium ions are inserted are recovered from the lithium hydroxide aqueous solution, washed with pure water or alcohol, and then dried under reduced pressure.

(Negative electrode conductive material)
Examples of the conductive material include carbon powder such as carbon black, carbon nanotubes, carbon material, metal fine powder such as copper, nickel, stainless steel and iron, a mixture of carbon material and metal fine powder, and conductive oxide such as ITO. Be done. Among these, carbon powders such as acetylene black and ethylene black are particularly preferable. When sufficient conductivity can be ensured only by the negative electrode active material 36, the lithium ion secondary battery 100 does not have to contain the conductive material.

(Negative electrode binder)
The binder binds the active materials to each other and also binds the active material to the negative electrode current collector 32. The binder may be any one capable of the above-mentioned bonds, for example, polyfluorinated vinylidene (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-. Perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) ) And other fluororesins.

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

Further, an electron conductive conductive polymer or an ionic conductive polymer may be used as the binder. Examples of the electron-conducting conductive polymer include polyacetylene and the like. In this case, since the binder also functions as a conductive material, it is not necessary to add the conductive material. Examples of the ionic conductive polymer include monomers of polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide, polyphosphazene, etc.), LiClO ₄ , LiBF ₄ , LiPF _{6, and the} like. Examples thereof include a compound of a lithium salt or an alkali metal salt mainly composed of lithium. Examples of the polymerization initiator used for the complexing include a photopolymerization initiator or a thermal polymerization initiator compatible with the above-mentioned monomers.

In addition to this, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamide resin, polyamideimide resin, polyacrylic resin and the like may be used as the binder.

The contents of the negative electrode active material 36, the conductive material and the binder in the negative electrode active material layer 34 are not particularly limited. The composition ratio of the negative electrode active material 36 in the negative electrode active material layer 34 is preferably 65% or more and 98% or less in terms of mass ratio. The composition ratio of the conductive material in the negative electrode active material layer 34 is preferably 0% or more and 20% or less in terms of mass ratio, and the composition ratio of the binder in the negative electrode active material layer 34 is 2.0% or more and 35% or more in mass ratio. % Or less is preferable.

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

"Positive electrode"
The positive electrode 20 has a positive electrode current collector 22 and a positive electrode active material layer 24 provided on the positive electrode current collector 22.

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

(Positive electrode active material layer)
The positive electrode active material used for the positive electrode active material layer 24 includes storage and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or counter anions of lithium ions and lithium ions (for example, PF ^6- ). An electrode active material capable of reversibly advancing the doping and dedoping of the above can be used.

For example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), lithium manganese spinel (LiMn ₂ O _4), and the general _{formula: LiNi x Co y Mn z M} a O ₂ (x + y + z + a = 1, 0 ≦ x <1, 0 ≦ y <1, 0 ≦ z <1, 0 ≦ a <1, M is one type selected from Al, Mg, Nb, Ti, Cu, Zn, Cr. Composite metal oxide represented by the above elements), lithium vanadium compound (LiV ₂ O ₅ ), olivine type LiMPO ₄ (however, M is Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr. more shows one or more elements or VO selected), lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1) mixed metal oxide, such as Examples include compounds, polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene and the like.

(Conductive material)
Examples of the conductive material include carbon powder such as carbon black, carbon nanotubes, carbon material, metal fine powder such as copper, nickel, stainless steel and iron, a mixture of carbon material and metal fine powder, and conductive oxide such as ITO. .. When sufficient conductivity can be ensured only by the positive electrode active material, the lithium ion secondary battery 100 does not have to contain the conductive material.

(Positive binder)
The binder used for the positive electrode can be the same as that used for the negative electrode.

The composition ratio of the positive electrode active material in the positive electrode active material layer 24 is preferably 80% or more and 96% or less in terms of mass ratio. The composition ratio of the conductive material in the positive electrode active material layer 24 is preferably 2.0% or more and 10% or less in terms of mass ratio, and the composition ratio of the binder in the positive electrode active material layer 24 is 2.0% by mass ratio. It is preferably 10% or more.

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

"Electrolytes"
The electrolyte is contained inside the positive electrode active material layer 24, the negative electrode active material layer 34, and the separator 10. The electrolyte is not particularly limited, and for example, in the present embodiment, an electrolyte solution containing a lithium salt (an aqueous electrolyte solution, an electrolyte solution using an organic solvent) can be used. However, the aqueous electrolyte solution is preferably an electrolytic solution (non-aqueous electrolyte solution) that uses an organic solvent because the withstand voltage during charging is limited to be low due to the electrochemically low decomposition voltage. As the electrolytic solution, one in which a lithium salt is dissolved in a non-aqueous solvent (organic solvent) is preferably used. The lithium salt is not particularly limited, and a lithium salt used as an electrolyte of a lithium ion secondary battery can be used. For example, LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) _2. Lithium salts such as LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB can be used. One of these lithium salts may be used alone, or two or more thereof may be used in combination. In particular, from the viewpoint of the degree of ionization, it is preferable to contain _{LiPF 6.}

Examples of the organic solvent include aprotic high dielectric constant solvents such as ethylene carbonate, propylene carbonate and fluoroethylene carbonate, and aprotics such as acetate esters such as dimethyl carbonate and ethyl methyl carbonate or propionic acid esters. Examples thereof include low-viscosity solvents. It is desirable to use these aprotic high dielectric constant solvents and aprotic low viscosity solvents together in an appropriate mixing ratio. Furthermore, ionic liquids using imidazolium, ammonium, and pyridinium type cations can be used. Although counter anion is not particularly ^{_{limited, BF 4 -, PF 6 -}} , (CF 3 SO 2) 2 N - , and the like. The ionic liquid can be used by mixing with the above-mentioned organic solvent.
The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 2.0 M from the viewpoint of electrical conductivity. The conductivity of this electrolyte at a temperature of 25 ° C. is preferably 0.01 S / m or more, and is adjusted according to the type of electrolyte salt or its concentration.

When the electrolyte is a solid electrolyte or a gel electrolyte, poly (vinylidene fluoride) or the like can be contained as a polymer material.
Further, various additives may be added to the electrolytic solution of the present embodiment, if necessary. Examples of the additive include vinylene carbonate, methylvinylene carbonate, etc. for the purpose of improving the cycle life, biphenyl, alkylbiphenyl, etc. for the purpose of preventing overcharging, various carbonate compounds for the purpose of deoxidation and dehydration, and various types. Examples include carboxylic acid anhydrides, various nitrogen-containing and sulfur-containing compounds.

"Case"
The case 50 seals the laminate 40 and the electrolytic solution inside the case 50. The case 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and invasion of water or the like into the inside of the lithium ion secondary battery 100 from the outside.

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

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

[Manufacturing method of lithium ion secondary battery]
Next, a method for manufacturing the lithium ion secondary battery 100 will be specifically described.

First, a negative electrode active material, a binder and a solvent are mixed to prepare a paint. If necessary, a conductive material may be further added. As the solvent, for example, water, N-methyl-2-pyrrolidone and the like can be used. The composition ratio of the negative electrode active material, the conductive material, and the binder is preferably 65 wt% to 98 wt%: 0 wt% to 20 wt%: 2.0 wt% to 35 wt% in terms of mass ratio. These mass ratios are adjusted so as to be 100 wt% as a whole.

The method of mixing these components constituting the paint is not particularly limited, and the mixing order is also not particularly limited. The above paint is applied to the negative electrode current collector 32. The coating method is not particularly limited, and a method usually adopted when producing an electrode can be used. For example, the slit die coat method and the doctor blade method can be mentioned. Similarly, for the positive electrode, a paint for the positive electrode is applied on the positive electrode current collector 22.

Subsequently, the solvent in the paint applied on the positive electrode current collector 22 and the negative electrode current collector 32 is removed. The removal method is not particularly limited. For example, the positive electrode current collector 22 and the negative electrode current collector 32 coated with the paint may be dried in an atmosphere of 80 ° C. to 150 ° C.

Then, the electrodes on which the positive electrode active material layer 24 and the negative electrode active material layer 34 are formed in this way are pressed by a roll press device or the like, if necessary.

Next, the positive electrode 20 having the positive electrode active material layer 24, the negative electrode 30 having the negative electrode active material layer 34, the separator 10 interposed between the positive electrode and the negative electrode, and the electrolytic solution are sealed in the case 50.

For example, the positive electrode 20, the negative electrode 30, and the separator 10 are laminated, and the positive electrode 20 and the negative electrode 30 are heated and pressed by a press instrument from a direction perpendicular to the stacking direction, and the positive electrode 20, the separator 10, and the negative electrode 30 are heated and pressed. Adhere. Then, for example, the laminated body 40 is put into a bag-shaped case 50 prepared in advance.

Finally, the lithium ion secondary battery is manufactured by injecting the electrolytic solution into the case 50. Instead of injecting the electrolytic solution into the case, the laminate 40 may be impregnated with the electrolytic solution.

As described above, the negative electrode active material according to the present embodiment is conductive because at least a part of the negative electrode active material particles is coated with a film containing a specific lithium-containing titanium oxide containing titanium and lithium. It is considered that the conductivity of Li ions is increased. Therefore, the lithium ion secondary battery 100 using the negative electrode 30 containing the negative electrode active material according to the present embodiment is excellent in initial charge / discharge efficiency.

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

[Example 1]
In 2-ethoxyethanol 100 mL, acetonitrile 30mL it was charged, and then magnesium ethoxide _{[Mg (OC 2 H 5)} 2] was poured 0.015 g, was stirred with a magnetic stirrer. After the magnesium ethoxydo is dissolved, 5.0 g of titanium (IV) tetrabutoxide and monomer [Ti (OC ₄ H ₉ ) ₄ ] are added, and the mixture is stirred for 1 hour to mix magnesium (IV) tetrabutoxide. The solution was prepared. This solution was designated as solution A.
30 mL of acetonitrile was added to 100 mL of 2-ethoxyethanol, then 100 g of Si powder (average particle size 1 μm) was added, and the mixture was stirred for 1 hour. Next, 7 mL of 28% aqueous ammonia was added and stirred for 5 minutes to prepare a dispersion of Si powder. This dispersion was designated as Liquid B.

Next, while stirring the solution B, the solution A was added to the solution B, and then the stirring was continued for another hour to hydrolyze and polymerize magnesium ethoxide and titanium (IV) tetrabutoxide by the water content in the ammonia water. Then, a film of a sol of titanium oxide containing magnesium was formed on the surface of the Si powder. After stirring, the mixture was allowed to stand at 25 ° C. for 30 minutes. Next, the Si powder on which the sol film was formed was collected by filtration, washed with pure water and ethanol, and then dried in a vacuum drying oven at 150 ° C. with a film having a titanium oxide film containing magnesium. Si powder was obtained.

Next, the obtained Si powder with a coating was put into 100 mL of a hydrochloric acid aqueous solution having a concentration of 0.1 mol / L and immersed for 15 minutes to elute all of the magnesium in the coating as magnesium ions. Then, the Si powder was recovered by filtration and washed with pure water for 5 minutes. Next, the washed Si powder with a coating film was put into 100 mL of a lithium hydroxide aqueous solution having a pH adjusted to 10 and having a concentration of 0.04 mol / L, and immersed for 15 minutes. Ion was inserted. Then, the coated Si powder was recovered by filtration. The recovered coated Si powder was washed with pure water for 2 minutes and then dried in a vacuum drying oven at 150 ° C., which was used as the negative electrode active material of Example 1. The work so far was performed in the dry room.

[Comparative Example 1]
The Si powder (average particle size 1 μm) used in Example 1 was used as the negative electrode active material of Comparative Example 1.

[Comparative Example 2]
To 100 mL of 2-ethoxyethanol, 30 mL of acetonitrile is added, _{5.0 g of titanium (IV) tetrabutoxide and a monomer [Ti (OC 4} H ₉ ) ₄ ] are added, and the mixture is stirred for 1 hour to obtain titanium (IV) tetrabutoxide. A solution was prepared. This solution was designated as solution A.
A dispersion liquid of Si powder was prepared in the same manner as in Example 1, and this dispersion liquid was designated as liquid B.
Next, while stirring the B solution, the A solution is added to the B solution, and then stirring is continued for another 1 hour to hydrolyze and shrink-polymerize titanium (IV) tetrabutoxide with the water content in the ammonia water to obtain Si. A titanium oxide sol film was formed on the surface of the powder. After stirring, the mixture was allowed to stand at 25 ° C. for 30 minutes. Next, the Si powder on which the sol film was formed was collected by filtration, washed with pure water and ethanol, and then dried in a vacuum drying oven at 150 ° C. to obtain a coated Si powder having a titanium oxide film. It was prepared and used as the negative electrode active material of Comparative Example 2.

[Examples 2 to 8, Comparative Example 3]
By changing the input amount of magnesium ethoxydo, a coated Si powder having a coating of a titanium oxide containing magnesium obtained in Example 1 and a coated Si powder having a different magnesium content were obtained, and this was obtained. A coated Si powder was prepared in the same manner as in Example 1 except that it was immersed in an aqueous solution of hydrochloric acid and an aqueous solution of lithium hydroxide, and this was used as the negative electrode active material of Examples 2 to 8 and Comparative Example 3.

[evaluation]
The composition of the coating film and the charge / discharge characteristics of the negative electrode active materials obtained in Examples 1 to 8 and Comparative Examples 1 to 3 were measured. The results are shown in Table 1 below. However, the composition of the coating film of the negative electrode active material of Comparative Example 1 was not measured.
The composition of the coating film was measured by the following method. As for the charge / discharge characteristics, a negative electrode was prepared using the negative electrode active material by the following method, and the charge / discharge characteristics of the half cell prepared using this negative electrode were measured.

(1) Composition of coating film The contents of Ti, Li, Mg and O in the coating film were measured using XPS, and the content (atomic%) with respect to the total amount of each element was calculated. Then, the composition formula of the coating film formed on the surface of the Si powder was obtained from the obtained content (atomic%) of each element.

(2) Preparation of Negative Electrode The negative electrode active material and acetylene black (AB) as a conductive auxiliary agent were mixed to obtain a mixed powder. Further, a polyamide-imide resin [manufactured by Hitachi Kasei Co., Ltd., HPC-1000] as a binder was dissolved in N-methylpyrrolidone (NMP) to prepare a binder solution. A mixed powder of the negative electrode active material and AB, and a binder solution were mixed to prepare a slurry. The blending ratio of the negative electrode active material, AB and the binder (solid content) was 80: 5: 15 in terms of mass ratio. The prepared slurry was applied to the surface of a rolled copper foil (current collector) having a thickness of 10 μm using a doctor blade to form a negative electrode active material layer on the rolled copper foil. Then, it was dried at 110 ° C. for 1 hour, and NMP was volatilized and removed from the negative electrode active material layer. After drying, the current collector and the negative electrode active material layer were firmly tight-junctioned by a roll press machine. This was heat-cured at 300 ° C. for 1 hour to obtain an electrode having an active material layer having a thickness of about 10 μm.

(3) Preparation of Half Cell A lithium ion secondary battery (half cell) was prepared using the negative electrode prepared in (2) above as an evaluation electrode. The opposite electrode was a metallic lithium foil. Then, a separator (material: polypropylene) was sandwiched between the evaluation electrode and the counter electrode to form a laminated body. This laminate was housed in a battery case. _{Further, in the battery case, a non-aqueous electrolyte in which LiPF 6} was dissolved at a concentration of 1 M was injected into a mixed solvent in which fluoroethylene carbonate and dimethyl carbonate were mixed at a ratio of 4: 6 (volume ratio). The battery case was sealed to obtain a half cell.

(4) Evaluation of Charge / Discharge of Negative Electrode Active Material The half cell produced in (3) above was subjected to a charge / discharge test [TOSCAT-3000 manufactured by Toyo System Co., Ltd.] to evaluate the initial charge / discharge capacity. First, under a temperature environment of 25 ° C., charging is performed with a constant current of 0.05C up to a final charging voltage of 0.01V based on metal Li, and then discharging is performed with a constant current of 0.05C up to a final discharge voltage of 2.0V. The operation to be performed was set to one cycle, and this cycle was carried out for two cycles.
Then, charge / discharge evaluation was performed with a constant current of 0.2 C. In the charge / discharge test, one cycle was an operation of charging with a constant current of 0.2 C up to a final charge voltage of 0.01 V and then discharging with a constant current of 0.2 C up to a final discharge voltage of 2.0 V. Table 1 shows the charge capacity, discharge capacity, and initial efficiency (= discharge capacity / charge capacity × 100) of the first cycle. "Charging" is the direction in which the active material of the evaluation electrode stores Li, and "discharging" is the direction in which the active material of the evaluation electrode releases Li. Further, the charge capacity and the discharge capacity are capacities per unit mass of the negative electrode active material, respectively.

As is clear from Table 1, in Examples 1 to 8 in which the Si powder coated with the lithium titanium oxide containing Li in the range of the present invention was used as the negative electrode active material, the Si powder having no coating was used as the negative electrode active material. Compared with Comparative Example 1 in which Si powder coated with titanium oxide was used as the negative electrode active material, the initial charge / discharge efficiency was improved and the discharge capacity per unit mass was improved. On the other hand, Comparative Example 3 in which Si powder coated with lithium titanium oxide containing more Li ions than the range of the present invention was used as the negative electrode active material was earlier than in Examples 1 to 8. The charge / discharge efficiency and the charge / discharge capacity per unit mass decreased.

[Example 9]
In 2-ethoxyethanol 100 mL, acetonitrile 30mL it was charged, and then after turning the magnesium ethoxide _{[Mg (OC 2 H 5)} 2] 0.5g, was stirred with a magnetic stirrer. After the magnesium ethoxydo is dissolved, 5.0 g of titanium (IV) tetrabutoxide and monomer [Ti (OC ₄ H ₉ ) ₄ ] are added, and the mixture is stirred for 1 hour to mix magnesium (IV) tetrabutoxide. The solution was prepared. This solution was designated as solution A.
A dispersion liquid of Si powder was prepared in the same manner as in Example 1, and this dispersion liquid was designated as liquid B.

Next, while stirring the solution B, the solution A was added to the solution B, and then the stirring was continued for another hour to hydrolyze and polymerize magnesium ethoxide and titanium (IV) tetrabutoxide by the water content in the ammonia water. Then, a film of a sol of titanium oxide containing magnesium was formed on the surface of the Si powder. After stirring, the mixture was allowed to stand at 25 ° C. for 30 minutes. Next, the Si powder on which the sol film was formed was collected by filtration, washed with pure water and ethanol, and then dried in a vacuum drying oven at 30 ° C. with a film having a titanium oxide film containing magnesium. Si powder was obtained.

Next, the obtained Si powder with a coating was put into 100 mL of a hydrochloric acid aqueous solution having a concentration of 0.01 mol / L and immersed for 15 minutes to elute a part of magnesium in the coating as magnesium ions. Then, the Si powder was recovered by filtration and washed with pure water for 5 minutes. Next, the washed Si powder with a coating film was put into 100 mL of a lithium hydroxide aqueous solution having a pH adjusted to 10 and having a concentration of 0.04 mol / L, and immersed for 15 minutes. Ion was inserted. Then, the coated Si powder was recovered by filtration. The recovered coated Si powder was washed with pure water for 2 minutes and then dried in a vacuum drying oven at 150 ° C., which was used as the negative electrode active material of Example 9. The work so far was performed in the dry room.

[Examples 10 to 16]
The coating film was formed in the same manner as in Example 9 except that the Li / Mg molar ratio was adjusted to the value shown in Table 2 below by changing the immersion time in the hydrochloric acid aqueous solution having a concentration of 0.01 mol / L. The Si powder of the above was prepared, and this was used as the negative electrode active material of Examples 10.
[evaluation]
With respect to the negative electrode active materials obtained in Examples 9 to 16, the composition of the coating film and the charge / discharge characteristics were measured by the above method. The results are shown in Table 2 below.

As is clear from Table 2, in Examples 9 to 16 in which Si powder coated with lithium titanium oxide containing lithium and magnesium within the range of the present invention was used as the negative electrode active material, the initial charge / discharge efficiency was improved. However, it was confirmed that the discharge capacity per unit mass was improved.

[Example 17]
30 ml of acetonitrile was added to 100 mL of 2-ethoxyethanol, and then _{1.15 g of aluminum sec-butoxide [Al (OCHCH 3} CH ₂ CH ₃ ) ₃ ] was added, and the mixture was stirred with a magnetic stirrer. After the aluminum sec-butoxide is dissolved, 5.0 g of titanium (IV) tetrabutoxide and monomer [Ti (OC ₄ H ₉ ) ₄ ] are added, and the mixture is stirred for 1 hour to add aluminum sec-butoxide and titanium (IV) tetrabutoxide. A mixed solution of This solution was designated as solution A.
A dispersion liquid of Si powder was prepared in the same manner as in Example 1, and this dispersion liquid was designated as liquid B.

Next, while stirring the B solution, the A solution is added to the B solution, and then the stirring is continued for another 1 hour to hydrolyze and shrink the aluminum sec-butoxide and the titanium (IV) tetrabutoxide by the water content in the ammonia water. It was polymerized to form a film of a titanium oxide sol containing aluminum on the surface of the Si powder. After stirring, the mixture was allowed to stand at 25 ° C. for 30 minutes. Next, the Si powder on which the sol film was formed was collected by filtration, washed with pure water and ethanol, and then dried in a vacuum drying oven at 30 ° C. to have a film having a titanium oxide film containing aluminum. Si powder was obtained.

Next, the obtained Si powder with a coating was put into 100 mL of a hydrochloric acid aqueous solution having a concentration of 0.01 mol / L and immersed for 15 minutes to elute a part of aluminum in the coating as aluminum ions. Then, the coated Si powder was recovered by filtration and washed with pure water for 5 minutes. Next, the washed Si powder with a coating film was put into 100 mL of a lithium hydroxide aqueous solution having a pH adjusted to 10 and having a concentration of 0.04 mol / L, and immersed for 15 minutes. Ion was inserted. Then, the coated Si powder was recovered by filtration. The recovered coated Si powder was washed with pure water for 2 minutes and then dried in a vacuum drying oven at 150 ° C., which was used as the negative electrode active material of Example 17. The work so far was performed in the dry room.

[Examples 18 to 24]
The coating film was formed in the same manner as in Example 17 except that the Li / Al molar ratio was adjusted to the value shown in Table 3 below by changing the immersion time in the hydrochloric acid aqueous solution having a concentration of 0.01 mol / L. The Si powder was prepared and used as the negative electrode active material of Examples 18 to 24.
[evaluation]
With respect to the negative electrode active materials obtained in Examples 17 to 24, the composition of the coating film and the charge / discharge characteristics were measured by the above method. The results are shown in Table 3 below.

As is clear from Table 3, in Examples 17 to 24 in which Si powder coated with lithium titanium oxide containing lithium and aluminum within the range of the present invention was used as the negative electrode active material, the initial charge / discharge efficiency was improved. However, it was confirmed that the discharge capacity per unit mass was improved.

[Example 25]
30 ml of acetonitrile was added to 100 mL of 2-ethoxyethanol, then _{0.44 g of calcium methoxydo [Ca (OCH 3} ) ₂ ] was added, and the mixture was stirred with a magnetic stirrer. After the calcium methoxyde is dissolved, 5.0 g of titanium (IV) tetrabutoxide and monomer [Ti (OC ₄ H ₉ ) ₄ ] are added, and the mixture is stirred for 1 hour to mix calcium methoxyde and titanium (IV) tetrabutoxide. The solution was prepared. This solution was designated as solution A.
A dispersion liquid of Si powder was prepared in the same manner as in Example 1, and this dispersion liquid was designated as liquid B.

Next, while stirring the B solution, the A solution is added to the B solution, and then the stirring is continued for another 1 hour to hydrolyze and shrink-polymerize calcium methoxyde and titanium (IV) tetrabutoxide by the water content in the ammonia water. Then, a film of a titanium oxide sol containing calcium was formed on the surface of the Si powder. After stirring, the mixture was allowed to stand at 25 ° C. for 30 minutes. Next, the Si powder on which the sol film was formed was collected by filtration, washed with pure water and ethanol, and then dried in a vacuum drying oven at 30 ° C. with a film having a titanium oxide film containing calcium. Si powder was obtained.

Next, the obtained Si powder with a coating was put into 100 mL of a hydrochloric acid aqueous solution having a concentration of 0.01 mol / L and immersed for 15 minutes to elute a part of calcium in the coating as calcium ions. Then, the coated Si powder was recovered by filtration and washed with pure water for 5 minutes. Next, the washed Si powder was put into 100 mL of a lithium hydroxide aqueous solution having a pH adjusted to 10 and having a concentration of 0.04 mol / L, and immersed for 15 minutes to add lithium ions to the defective sites generated by elution of calcium ions. I inserted it. Then, the coated Si powder was recovered by filtration. The recovered coated Si powder was washed with pure water for 2 minutes and then dried in a vacuum drying oven at 150 ° C., which was used as the negative electrode active material of Example 25. The work so far was performed in the dry room.

[Examples 26 to 32]
The coating film was formed in the same manner as in Example 25, except that the Li / Ca molar ratio was adjusted to the value shown in Table 4 below by changing the immersion time in the hydrochloric acid aqueous solution having a concentration of 0.01 mol / L. The Si powder of the above was prepared, and this was used as the negative electrode active material of Examples 26 to 32.
[evaluation]
With respect to the negative electrode active materials obtained in Examples 25 to 32, the composition of the coating film and the charge / discharge characteristics were measured by the above method. The results are shown in Table 4 below.

As is clear from Table 4, in Examples 25 to 32 in which Si powder coated with lithium titanium oxide containing lithium and calcium within the range of the present invention was used as the negative electrode active material, the initial charge / discharge efficiency was improved. However, it was confirmed that the discharge capacity per unit mass was improved.

[Example 33]
In 2-ethoxyethanol 100 mL, acetonitrile 30ml were charged, and then after turning the zirconium (IV) tetrabutoxide _{[Zr (C 4 H 9 O} ) 4] 1.98g, was stirred with a magnetic stirrer. After the zirconium (IV) tetrabutoxide is dissolved, 5.0 g of titanium (IV) tetrabutoxide and monomer [Ti (OC ₄ H ₉ ) ₄ ] are added, and the mixture is stirred for 1 hour to add zirconium (IV) tetrabutoxide and titanium ( IV) A mixed solution of tetrabutoxide was prepared. This solution was designated as solution A.
A dispersion liquid of Si powder was prepared in the same manner as in Example 1, and this dispersion liquid was designated as liquid B.

Next, while stirring the solution B, the solution A is added to the solution B, and then the stirring is continued for another hour to hydrolyze the zirconium (IV) tetrabutoxide and the titanium (IV) tetrabutoxide by the water content in the ammonia water. And shrink polymerization to form a sol film of titanium oxide containing zirconium on the surface of the Si powder. After stirring, the mixture was allowed to stand at 25 ° C. for 30 minutes. Next, the Si powder on which the sol film was formed was collected by filtration, washed with pure water and ethanol, and then dried in a vacuum drying oven at 30 ° C. with a film having a titanium oxide film containing zirconium. Si powder was obtained.

Next, the obtained Si powder with a coating was put into 100 mL of a hydrochloric acid aqueous solution having a concentration of 0.01 mol / L and immersed for 15 minutes to elute a part of zirconium in the coating as zirconium ions. Then, the coated Si powder was recovered by filtration and washed with pure water for 5 minutes. Next, the washed Si powder with a coating film was put into 100 mL of a lithium hydroxide aqueous solution having a pH adjusted to 10 and having a concentration of 0.04 mol / L, and immersed for 15 minutes. Ions were inserted. Then, the coated Si powder was recovered by filtration. The recovered coated Si powder was washed with pure water for 2 minutes and then dried in a vacuum drying oven at 150 ° C., which was used as the negative electrode active material of Example 33. The work so far was performed in the dry room.

[Examples 34 to 40]
The coating film was formed in the same manner as in Example 33, except that the immersion time in the hydrochloric acid aqueous solution having a concentration of 0.01 mol / L was changed so that the Li / Zr molar ratio was adjusted to the value shown in Table 5 below. The Si powder was prepared and used as the negative electrode active material of Examples 31 to 36.
[evaluation]
With respect to the negative electrode active materials obtained in Examples 34 to 40, the composition of the coating film and the charge / discharge characteristics were measured by the above method. The results are shown in Table 5 below.

As is clear from Table 5, in Examples 33 to 40 in which Si powder coated with lithium titanium oxide containing lithium and zirconium within the range of the present invention was used as the negative electrode active material, the initial charge / discharge efficiency was improved. However, it was confirmed that the discharge capacity per unit mass was improved.

[Example 41]
In 2-ethoxyethanol 100 mL, acetonitrile 30ml were charged, followed by sodium ethoxide (NaOC ₂ H ₅₎ was poured 0.27 g, was stirred with a magnetic stirrer. After the sodium ethoxide is dissolved, 5.0 g of titanium (IV) tetrabutoxide and monomer [Ti (OC ₄ H ₉ ) ₄ ] are added, and the mixture is stirred for 1 hour to mix sodium ethoxide and titanium (IV) tetrabutoxide. The solution was prepared. This solution was designated as solution A.
A dispersion liquid of Si powder was prepared in the same manner as in Example 1, and this dispersion liquid was designated as liquid B.

Next, while stirring the solution B, the solution A was added to the solution B, and then the stirring was continued for another hour to hydrolyze and polymerize sodium ethoxyde and titanium (IV) tetrabutoxide by the water content in the ammonia water. Then, a film of a sol of titanium oxide containing sodium was formed on the surface of the Si powder. After stirring, the mixture was allowed to stand at 25 ° C. for 30 minutes. Next, the Si powder on which the sol film was formed was collected by filtration, washed with pure water and ethanol, and then dried in a vacuum drying oven at 30 ° C. with a film having a titanium oxide film containing sodium. Si powder was obtained.

Next, the obtained Si powder with a coating was put into 100 mL of a hydrochloric acid aqueous solution having a concentration of 0.01 mol / L and immersed for 15 minutes to elute a part of sodium in the coating as sodium ions. Then, the coated Si powder was recovered by filtration and washed with pure water for 5 minutes. Next, the washed Si powder with a coating film was put into 100 mL of a lithium hydroxide aqueous solution having a pH adjusted to 10 and having a concentration of 0.04 mol / L, and immersed for 15 minutes. Ion was inserted. Then, the coated Si powder was recovered by filtration. The recovered coated Si powder was washed with pure water for 2 minutes and then dried in a vacuum drying oven at 150 ° C., which was used as the negative electrode active material of Example 41. The work so far was performed in the dry room.

[Examples 42 to 48]
The coating film was formed in the same manner as in Example 33, except that the immersion time in the hydrochloric acid aqueous solution having a concentration of 0.01 mol / L was changed so that the Li / Zr molar ratio was adjusted to the value shown in Table 6 below. The Si powder of the above was prepared, and this was used as the negative electrode active material of Examples 42 to 48.
[evaluation]
With respect to the negative electrode active materials obtained in Examples 41 to 48, the composition of the coating film and the charge / discharge characteristics were measured by the above method. The results are shown in Table 6 below.

As is clear from Table 6, Examples 41 to 48 using Si powder coated with lithium titanium oxide containing lithium and sodium within the range of the present invention as the negative electrode active material have improved initial charge / discharge efficiency. However, it was confirmed that the discharge capacity per unit mass was improved.

10 ... Separator, 20 ... Positive electrode, 22 ... Positive electrode current collector, 24 ... Positive electrode active material layer, 30 ... Negative electrode, 32 ... Negative electrode current collector, 34 ... Negative electrode active material layer, 40 ... Laminated body, 50 ... Case, 52 ... Metal foil, 54 ... Polymer film, 60, 62 ... Lead, 100 ... Lithium ion secondary battery

Claims (7)

It has active material particles capable of occluding and releasing lithium ions, and a coating provided on at least a part of the surface of the active material particles.
The coating film contains a lithium-containing titanium oxide represented by the following formula (1).
The lithium-containing titanium oxide, see contains Mg, Ca, Al, at least one metal of Zr,
Negative electrode active material for lithium ion secondary batteries in which the active material particles are Si:
Ti _{1-x / 4-a / 4-2b / 4-3c / 4-d} Li _x A _a B _b C _c D _d O ₂ ... (1)
However, in the formula (1), A is one or more monovalent metals selected from the group consisting of K and Na, and B is one or more divalent metals selected from the group consisting of Mg, Ca and Sr. A metal, C is one or more trivalent metals selected from the group consisting of Al, Y, La and Bi, and D is one selected from the group consisting of Zr, Si, Hf, Sn and Ge. In the above tetravalent metal, x is a number satisfying 0 <x ≦ 0.60, a is a number satisfying 0 ≦ a ≦ 0.27, and b is a number satisfying 0 ≦ b ≦ 0.27. C is a number satisfying 0 ≦ c ≦ 0.27, and d is a number satisfying 0 ≦ d ≦ 0.27. The first aspect of claim 1, wherein the a in the formula (1) is a number satisfying 0 <a≤0.27, and the ratio x / a of the x to the a is in the range of 0.07 or more and 60 or less. Negative electrode active material for lithium ion secondary batteries. The first aspect of claim 1, wherein the b in the formula (1) is a number satisfying 0 <b ≦ 0.27, and the ratio x / b of the x to the b is in the range of 0.07 or more and 60 or less. Negative electrode active material for lithium ion secondary batteries. The first aspect of claim 1, wherein the c in the formula (1) is a number satisfying 0 <c ≦ 0.27, and the ratio x / c of the x to the c is in the range of 0.07 or more and 60 or less. Negative electrode active material for lithium ion secondary batteries. The first aspect of claim 1, wherein the d in the formula (1) is a number satisfying 0 <d ≦ 0.27, and the ratio x / d of the x to the d is in the range of 0.07 or more and 60 or less. Negative electrode active material for lithium ion secondary batteries. A negative electrode having a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector.
A negative electrode for a lithium ion secondary battery, wherein the negative electrode active material layer contains the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5. A lithium ion secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.
A lithium ion secondary battery in which the negative electrode is the negative electrode for the lithium ion secondary battery according to claim 6.

Images