JP6962154B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

Generally, a lithium ion secondary battery includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and an electrolytic solution as main components. Then, an appropriate electrolyte is added to the electrolytic solution in an appropriate concentration range. _{Lithium salts such as LiClO 4} , LiAsF ₆ , LiPF ₆ , LiBF ₄ , CF ₃ SO ₃ Li, and (CF ₃ SO ₂ ) ₂ NLi are generally added to the electrolyte of the lithium ion secondary battery as an electrolyte. Here, the concentration of the lithium salt in the electrolytic solution is generally set to about 1 mol / L.

Further, as the organic solvent used in the electrolytic solution, an organic solvent having a high relative permittivity such as ethylene carbonate or propylene carbonate and a high dipole moment is mixed in an amount of about 30% by volume or more in order to preferably dissolve the electrolyte. Is common.

In fact, Patent Document 1 discloses a lithium ion secondary battery using a mixed organic solvent containing 33% by volume of ethylene carbonate _{and using an electrolytic solution containing LiPF 6 at a concentration of 1 mol / L.}

Recently, Patent Document 2 and Patent Document 3 have reported an electrolytic solution containing a metal salt as an electrolyte at a high concentration and a lithium ion secondary battery including the electrolytic solution.

Patent Document 2 specifically describes an electrolytic solution containing _{(FSO 2} ) ₂ NLi as a metal salt and dimethyl carbonate as an organic solvent at _{a molar ratio of dimethyl carbonate to (FSO 2} ) _{2 NLi of 2 or 3.} The lithium ion secondary battery provided with the electrolytic solution is also specifically described.

Patent Document 3 describes that an electrolytic solution having a molar ratio of a specific organic solvent to a specific metal salt of 3 to 5 is excellent in physical properties, and further, a lithium ion secondary battery including the electrolytic solution is described. It is specifically described.

Japanese Unexamined Patent Publication No. 2013-149477 International Publication No. 2015/045389 International Publication No. 2016/063468

By the way, the industry demands a lithium ion secondary battery having excellent characteristics.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a lithium ion secondary battery having excellent characteristics.

The present inventor has aimed to provide a high-capacity lithium ion secondary battery using an electrolytic solution containing an electrolyte at a high concentration. As a means for that purpose, a lithium metal composite oxide having a layered rock salt structure containing lithium, nickel, cobalt, and aluminum or manganese was selected as the positive electrode active material because of its high capacity and excellent durability.

Further, it is generally known that a reversible capacity and an irreversible capacity are observed in the negative electrode active material. The reversible capacity is the chargeable capacity that can be discharged, and the irreversible capacity is the chargeable capacity that cannot be discharged.
It is not preferable to supply lithium corresponding to the irreversible capacity of the negative electrode active material from the positive electrode active material at the time of charging because lithium that can be a charge carrier in the positive electrode active material is consumed. Therefore, the present inventor has aimed to reduce or eliminate the irreversible capacity of the negative electrode active material by introducing lithium into the negative electrode active material in advance. By introducing lithium into the negative electrode active material in advance, lithium supplied from the positive electrode active material can efficiently contribute to charging and discharging.

Then, when the present inventor actually manufactured and tested a lithium ion secondary battery having a lithium metal composite oxide having a layered rock salt structure as a positive electrode active material, a discharge in a low SOC (State of Charge) region was performed. We found that the resistance was significantly higher.

Further studies by the present inventor have found that a lithium ion secondary battery using a lithium metal composite oxide having a layered rock salt structure having a high nickel ratio can relatively suppress discharge resistance in a low SOC region. ..
Based on the above findings, the present inventor has completed the present invention.

The lithium ion secondary battery of the present invention
As a cathode active material, the general formula of the layered rock salt _structure: in _{Li a Ni b Co c M d} D e O f ( the general formula, 0.2 ≦ a ≦ 2,0.85 ≦ b ≦ 0.9,0 < Satisfy c <0.15, 0 <d <0.15, 0 ≦ e <0.15, b + c + d + e = 1, 1.7 ≦ f ≦ 3. M is Al and / or Mn. D is W. , Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru , Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V).
A negative electrode with a negative electrode active material in which lithium has been introduced in advance, and a negative electrode
It is characterized by having.

The lithium ion secondary battery of the present invention has a high capacity and is excellent in discharge in a low SOC region.

It is a graph which shows the relationship between the positive electrode potential and the discharge resistance in the evaluation example 1. It is the graph which enlarged the low potential region of the graph of FIG. It is a graph which shows both the charge / discharge curve of the positive electrode of Production Example 2 and the charge / discharge curve of the negative electrode provided with the Si-containing negative electrode active material. It is a graph which showed the charge / discharge curve of the positive electrode of Production Example 2 in the case of operating in the lithium ion secondary battery which includes the negative electrode active material which introduced lithium in advance.

Hereinafter, embodiments for carrying out the present invention will be described. Unless otherwise specified, the numerical range "a to b" described in the present specification includes the lower limit a and the upper limit b in the range. Then, a numerical range can be constructed by arbitrarily combining these upper and lower limit values and the numerical values listed in the examples. Further, a numerical value arbitrarily selected from these numerical values can be used as a new upper limit value or lower limit value.

The lithium ion secondary battery of the present invention
As a cathode active material, the general formula of the layered rock salt _structure: in _{Li a Ni b Co c M d} D e O f ( the general formula, 0.2 ≦ a ≦ 2,0.85 ≦ b ≦ 0.9,0 < Satisfy c <0.15, 0 <d <0.15, 0 ≦ e <0.15, b + c + d + e = 1, 1.7 ≦ f ≦ 3. M is Al and / or Mn. D is W. , Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru , Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V).
A negative electrode with a negative electrode active material in which lithium has been introduced in advance, and a negative electrode
It is characterized by having.

First, the positive electrode will be described.
Nickel, cobalt, aluminum, and manganese are considered to have the following roles in the lithium metal composite oxide having a layered rock salt structure.

Ni: Most active during charge / discharge reaction. The higher the Ni content in the active material, the higher the capacity.
Al or Mn: Most inert during the charge / discharge reaction. The higher the Al or Mn content in the active material, the lower the volume, but on the other hand, the higher the Al or Mn content in the active material, the more stable the crystal structure of the active material.
Co: The activity during the charge / discharge reaction is between Ni and Al or Mn. The capacity and degree of stability with respect to the content in the active material are also intermediate between Ni and Al or Mn.

Therefore, it can be said that the lithium ion secondary battery of the present invention provided with the lithium metal composite oxide having a high nickel ratio is excellent in capacity. Further, it was confirmed that the lithium metal composite oxide having a high nickel ratio has a lower discharge resistance in the low SOC region than the lithium metal composite oxide having a low nickel ratio. Therefore, it was confirmed that the lithium metal composite oxide having a high nickel ratio The lithium ion secondary battery of the present invention provided with an oxide is advantageous for discharging in a low SOC region.

However, in the case of a lithium metal composite oxide having an excessively high nickel ratio, the ratio of cobalt and aluminum or manganese is low, so that it can be said that there is a difficulty in its structural stability. Specifically, a lithium metal composite oxide having an excessively high nickel ratio makes it difficult to maintain a layered rock salt structure during charge and discharge. Therefore, a lithium ion secondary battery including a lithium metal composite oxide having an excessively high nickel ratio has a drawback of having a short life.

The reason why the range of 0.85 ≦ b ≦ 0.9 is preferable for b showing the nickel ratio in the lithium metal composite oxide is as described above. As a more preferable range of b, 0.86 ≦ b ≦ 0.89 and 0.87 ≦ b ≦ 0.88 can be shown.

In the above general formula of the lithium metal composite oxide, the values of c and d are not particularly limited as long as they satisfy the above conditions, but 0.03 ≦ c ≦ 0.12 and 0.05 ≦ with respect to c. C ≦ 0.1, 0.01 ≦ d ≦ 0.12 and 0.02 ≦ d ≦ 0.1 with respect to d can be exemplified as suitable ranges, respectively.

In the above general formula of the lithium metal composite oxide, a, e, and f may be values within the range specified by the general formula, and a is preferably within the range of 0.5 ≦ a ≦ 1.5. The range of 0.7 ≦ a ≦ 1.3 is more preferable, and the range of 0.9 ≦ a ≦ 1.2 is even more preferable. For e and f, 0 ≦ e ≦ 0.1, 0 ≦ e ≦ 0.05, e = 0, 1.8 ≦ f ≦ 2.5, 1.9 ≦ f ≦ 2.2, f = 2. It can be exemplified.

The shape of the lithium metal composite oxide is not particularly limited, but it is preferably in the form of powder. The average particle size of the lithium metal composite oxide is exemplified in the range of 0.5 to 20 μm, 1 to 15 μm, and 3 to 10 μm. In the present specification, the average particle size means _{D 50} when the sample is measured with a general laser diffraction type particle size distribution meter.

Specifically, the positive electrode includes a current collector and a positive electrode active material layer formed on the surface of the current collector.

A current collector is a chemically inert electron conductor that keeps current flowing through the electrodes during the discharge or charging of a lithium-ion secondary battery. As the current collector, at least one selected from silver, copper, gold, aluminum, magnesium, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel. Metallic materials such as, etc. can be exemplified.

When the potential of the positive electrode is 4 V or more based on lithium, it is preferable to use aluminum as the current collector for the positive electrode.

Specifically, it is preferable to use a current collector for the positive electrode made of aluminum or an aluminum alloy. Here, aluminum refers to pure aluminum, and aluminum having a purity of 99.0% or more is referred to as pure aluminum. An alloy obtained by adding various elements to pure aluminum is called an aluminum alloy. Examples of the aluminum alloy include Al—Cu, Al—Mn, Al—Fe, Al—Si, Al—Mg, Al—Mg—Si, and Al—Zn—Mg.

Further, as aluminum or an aluminum alloy, specifically, for example, A1000 series alloys such as JIS A1085 and A1N30 (pure aluminum series), A3000 series alloys such as JIS A3003 and A3004 (Al-Mn series), JIS A8079, A8021 and the like. A8000 series alloy (Al—Fe series) can be mentioned.

The current collector may be coated with a known protective layer. Further, a current collector whose surface is treated by a known method may be used as the current collector.

The current collector can take the form of foil, sheet, film, linear, rod, mesh or the like. Therefore, as the current collector, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, or a stainless foil can be preferably used. When the current collector is in the form of a foil, a sheet, or a film, the thickness thereof is preferably in the range of 1 μm to 100 μm.

The positive electrode active material layer contains the above-mentioned lithium metal composite oxide as the positive electrode active material, and may further contain additives such as a conductive auxiliary agent, a binder, and a dispersant. A known positive electrode active material may be used in combination as long as the gist of the present invention is not deviated.

The amount of the lithium metal composite oxide with respect to the entire positive electrode active material layer is preferably in the range of 70 to 99% by mass, more preferably in the range of 80 to 98% by mass, and particularly preferably in the range of 90 to 97% by mass.

The conductive auxiliary agent is added to increase the conductivity of the electrode. Therefore, the conductive auxiliary agent may be arbitrarily added when the conductivity of the electrode is insufficient, and may not be added when the conductivity of the electrode is sufficiently excellent.

The conductive auxiliary agent may be a chemically inert electronic conductor, and examples thereof include carbon black, which are carbonaceous fine particles, graphite, Vapor Grown Carbon Fiber, and various metal particles. Examples of carbon black include acetylene black, Ketjen black (registered trademark), furnace black, and channel black. These conductive auxiliary agents can be added to the positive electrode active material layer alone or in combination of two or more.

The shape of the conductive auxiliary agent is not particularly limited, but from the viewpoint of its role, it is preferable that the average particle size of the conductive auxiliary agent is small. A preferable average particle size of the conductive auxiliary agent is 10 μm or less, and a more preferable average particle size is in the range of 0.01 to 1 μm.

The blending amount of the conductive auxiliary agent is not particularly limited, but the blending amount of the conductive auxiliary agent in the positive electrode active material layer is preferably in the range of 0.5 to 10% by mass, preferably in the range of 1 to 7% by mass. More preferably, it is particularly preferably in the range of 1.5 to 5% by mass.

The binder plays a role of binding the positive electrode active material and the conductive auxiliary agent to the surface of the current collector. Examples of the binder include fluororesins such as polyvinylidene fluoride, polytetrafluoroethylene and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, and alkoxysilyl group-containing resins. be able to. Further, a polymer having a hydrophilic group may be adopted as the binder. Examples of the hydrophilic group of the polymer having a hydrophilic group include a carboxyl group, a sulfo group, a silanol group, an amino group, a hydroxyl group, and a phosphoric acid group. Specific examples of the polymer having a hydrophilic group include polyacrylic acid, carboxymethyl cellulose, polymethacrylic acid, and poly (p-styrenesulfonic acid).

The blending amount of the binder in the positive electrode active material layer is preferably in the range of 0.5 to 10% by mass, more preferably in the range of 1 to 7% by mass, and particularly in the range of 1.5 to 5% by mass. preferable. If the amount of the binder compounded is too small, the moldability of the positive electrode active material layer may decrease. Further, if the blending amount of the binder is too large, the amount of the positive electrode active material in the positive electrode active material layer is relatively reduced, which is not preferable.

Known additives such as dispersants other than the conductive auxiliary agent and the binder can be adopted.

Next, the negative electrode will be described. Specifically, the negative electrode includes a current collector and a negative electrode active material layer formed on the surface of the current collector. As the current collector, the one described in the positive electrode may be appropriately adopted. The negative electrode active material layer contains a negative electrode active material into which lithium has been introduced in advance.

The "negative electrode active material into which lithium has been introduced in advance" means a material in which lithium is introduced into the negative electrode active material before the introduction of lithium ions by the positive electrode active material during charging. Examples of the lithium introduction method for the negative electrode active material include the methods shown in D-1 to D-4 below.

D-1) A method of attaching a lithium foil to the surface of the negative electrode active material layer and introducing lithium into the negative electrode active material D-2) A composition layer containing lithium powder on the surface of the negative electrode active material layer (hereinafter, lithium) A method of forming a powder-containing layer) and introducing lithium into the negative electrode active material D-3) Using a cell different from the lithium ion secondary battery of the present invention (hereinafter referred to as a lithium predoping cell). D-4) A method in which a lithium doping agent is mixed in the positive electrode active material layer and lithium is introduced into the negative electrode active material by using the lithium doping agent.

D-1 and D-2 will be described.

When a lithium foil is attached to the surface of the negative electrode active material layer or a lithium powder-containing layer is formed, lithium in the lithium foil or the lithium powder-containing layer is introduced into the negative electrode active material of the negative electrode active material layer over time. When it is desired to increase the introduction rate of lithium, the negative electrode to which the lithium foil is attached or the negative electrode on which the lithium powder-containing layer is formed is pressurized to improve the adhesion between the lithium foil or the lithium powder-containing layer and the negative electrode active material layer. Alternatively, the negative electrode to which the lithium foil is attached or the negative electrode on which the lithium powder-containing layer is formed may be immersed in the electrolytic solution. The amount and thickness of the lithium foil or the lithium powder-containing layer may be appropriately determined according to the desired amount of lithium introduced.

The lithium powder-containing layer can be formed by applying a slurry containing lithium powder, a solvent, a binder, and the like onto the negative electrode active material layer and drying the slurry, or the lithium powder-containing layer prepared in advance. It may be formed by attaching the layer to the negative electrode active material layer.

D-3 will be described.
As the lithium predoping cell, a cell having a negative electrode and metallic lithium as electrodes and an electrolytic solution is preferable. By applying a voltage to the lithium predoping cell in the direction of doping the negative electrode with lithium of the metallic lithium electrode, lithium can be doped into the negative electrode active material.

D-4 will be described.
Preferred lithium dope agents include Li ₂ O, Li (Fe _1-x , Mn _x ) PO ₄ (where 0 ≦ x ≦ 1), Li ₅ FeO ₄ , Li ₆ MnO ₄ , Li ₆ CoO ₄ , and Li _6. ZnO ₄ , Li ₅ AlO ₄ , and Li ₅ GaO ₄ can be exemplified. As the lithium doping agent, it is preferable to use one coated with carbon to improve the conductivity.
Lithium can be introduced into the negative electrode active material by applying a voltage at which the lithium doping agent emits lithium ions to the lithium ion secondary battery provided with the positive electrode active material layer containing the lithium doping agent.

It is preferable to introduce lithium in an amount corresponding to the irreversible capacity of the negative electrode active material as the negative electrode active material.

As the negative electrode active material, a known material can be adopted. The action and effect of the present invention are remarkably exhibited when a negative electrode active material having a large irreversible capacity is used. Therefore, as the negative electrode active material, it is preferable to use a Si-containing negative electrode active material containing silicon, which has a large theoretical capacity and a large irreversible capacity.

Examples of the Si-containing negative electrode active material include elemental silicon, SiOx (0.3 ≦ x ≦ 1.6), alloys of Si with other metals, and the silicon material described in International Publication No. 2014/08608. The Si-containing negative electrode active material may be coated with carbon. The carbon-coated Si-containing negative electrode active material has excellent conductivity.

The silicon material described in International Publication No. 2014/08608 (hereinafter, may be simply referred to as “silicon material”) will be described in detail. The silicon material has a structure in which a plurality of plate-shaped silicon bodies are laminated in the thickness direction. For example, CaSi ₂ is reacted with an acid to synthesize a layered silicon compound containing polysilane as a main component. It is produced through a step and a step of heating the layered silicon compound at 300 ° C. or higher to release hydrogen.

The ideal reaction formula when hydrogen chloride is used as the acid shows the method for producing the silicon material as follows.
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl _{2
Si 6 H 6 → 6Si + 3H} 2 ↑

_{However, in the upper reaction for synthesizing Si 6} H ₆ which is polysilane, water is usually used as the reaction solvent from the viewpoint of removing by-products and impurities. Since Si ₆ H ₆ can react with water, the layered silicon compound is rarely produced as containing only _{Si 6} H ₆ in the step of synthesizing the layered silicon compound including the reaction in the upper stage, and is layered. Silicon compounds are represented by Si ₆ H _s (OH) _t X _u (X is an element or group derived from an anion of an acid, s + t + u = 6, 0 <s <6, 0 <t <6, 0 <u <6). Manufactured as a compound. In the above chemical formula, unavoidable impurities such as Ca that may remain are not considered. The silicon material obtained by heating the layered silicon compound also contains an element derived from an anion of oxygen or acid.

As described above, the silicon material has a structure in which a plurality of plate-shaped silicon bodies are laminated in the thickness direction. In order to efficiently occlude and release a charge carrier such as lithium ion, the plate-shaped silicon body preferably has a thickness in the range of 10 nm to 100 nm, and more preferably in the range of 20 nm to 50 nm. The length of the plate-shaped silicon body in the longitudinal direction is preferably in the range of 0.1 μm to 50 μm. Further, the plate-shaped silicon body preferably has a (length in the longitudinal direction) / (thickness) in the range of 2 to 1000. The laminated structure of the plate-shaped silicon body can be confirmed by observation with a scanning electron microscope or the like. Further, this laminated structure is considered to be a remnant of the Si layer in _{the raw material CaSi 2.}

The silicon material preferably contains amorphous silicon and / or silicon crystallites. In particular, in the plate-shaped silicon body, it is preferable that amorphous silicon is used as a matrix and silicon crystallites are scattered in the matrix. The size of the silicon crystallite is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, further preferably in the range of 1 nm to 50 nm, and particularly preferably in the range of 1 nm to 10 nm. The size of the silicon crystallite is calculated from Scherrer's equation using the half width of the diffraction peak on the Si (111) plane of the obtained X-ray diffraction chart obtained by performing X-ray diffraction measurement on the silicon material. ..

The abundance and size of the plate-shaped silicon body, amorphous silicon, and silicon crystallites contained in the silicon material mainly depend on the heating temperature and heating time. The heating temperature is preferably in the range of 350 ° C. to 950 ° C., more preferably in the range of 400 ° C. to 900 ° C., and even more preferably in the range of 500 ° C. to 800 ° C.

In addition to the negative electrode active material, the negative electrode active material layer may further contain additives such as a conductive auxiliary agent, a binder, and a dispersant. As the dispersant, known ones can be adopted.

The amount of the negative electrode active material with respect to the entire negative electrode active material layer is preferably in the range of 60 to 90% by mass, more preferably in the range of 70 to 80% by mass, in relation to the amount of the conductive auxiliary agent and the binder.

Since the Si-containing negative electrode active material is a semiconductor, a relatively large amount of conductive auxiliary agent is required for the negative electrode active material layer in order to smoothly transfer electrons between the Si-containing negative electrode active material and the negative electrode current collector. May become. The blending amount of the conductive auxiliary agent in the negative electrode active material layer is preferably in the range of 5 to 20% by mass, more preferably in the range of 7 to 15% by mass, and further preferably in the range of 10 to 15% by mass. As the conductive auxiliary agent, those described for the positive electrode may be appropriately adopted.

Further, the Si-containing negative electrode active material has a large degree of expansion and contraction due to charging and discharging. Therefore, in order to prevent the Si-containing negative electrode active material from falling off from the negative electrode due to expansion and contraction, a relatively large amount of binder is required for the negative electrode active material layer. The blending amount of the binder in the negative electrode active material layer is preferably in the range of 5 to 20% by mass, more preferably in the range of 10 to 15% by mass.

As the binder for the negative electrode, one or a plurality of known binders such as the binder described for the positive electrode and styrene-butadiene rubber may be used.

Further, a crosslinked polymer obtained by cross-linking a carboxyl group-containing polymer such as polyacrylic acid or polymethacrylic acid with a polyamine such as diamine disclosed in International Publication No. 2016/06382 may be used as a binder. By using the crosslinked polymer as a binder, it can be expected that the characteristics of the lithium ion secondary battery of the present invention will become more suitable.

Examples of the diamine used in the crosslinked polymer include alkylenediamines such as ethylenediamine, propylenediamine and hexamethylenediamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophoronediamine and bis (4-aminocyclohexyl) methane. Saturated carbocyclic diamine, m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, bis (4-aminophenyl) sulfone, benzidine, o-trizine, 2,4- Examples thereof include aromatic diamines such as tolylene diamine, 2,6-tolylene diamine, xylylene diamine and naphthalene diamine.

In order to form the active material layer on the surface of the current collector, conventionally known methods such as a roll coating method, a die coating method, a dip coating method, a doctor blade method, a spray coating method, and a curtain coating method may be used. Specifically, an active material, a solvent, and if necessary, a binder and a conductive auxiliary agent are mixed to form a slurry, and then the slurry is applied to the surface of a current collector and then dried. Examples of the solvent include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, and water. The dried one may be compressed in order to increase the electrode density.

The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, and an electrolytic solution, and further includes a separator, if necessary.

The electrolytic solution contains an electrolyte and a non-aqueous solvent. The electrolytic solution preferably contains a lithium salt, which is an electrolyte, at a high concentration. The concentration of the lithium salt in the electrolytic solution is preferably 1.5 to 4 mol / L, more preferably 1.6 to 3.5 mol / L, further preferably 1.7 to 3 mol / L, and 1.8 to 2.5 mol. / L is particularly preferable.
If the electrolytic solution contains a high concentration of lithium salt, the transfer of lithium ions can be suitably realized in the charging and discharging of a lithium ion secondary battery including a positive electrode active material having a high capacity specification and a negative electrode active material having a high capacity specification. ..

Lithium salts include LiPF ₆ , LiBF ₄ , LiAsF ₆ , Li ₂ SiF ₆ , (FSO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, FSO ₂ (CF). ₃ SO ₂ ) NLi, (SO ₂ CF ₂ CF ₂ SO ₂ ) NLi, (SO ₂ CF ₂ CF ₂ CF ₂ SO ₂ ) NLi, FSO ₂ (CH ₃ SO ₂ ) NLi, FSO ₂ (C ₂ F ₅ SO) ₂ ) NLi, or FSO ₂ (C ₂ H ₅ SO ₂ ) NLi, (OCOCO ₂ ) ₂ BLi, (OCOCO ₂ ) BF ₂ Li can be exemplified.

As the non-aqueous solvent, cyclic carbonates, cyclic esters, chain carbonates, chain esters, ethers and the like can be used. Examples of the cyclic carbonate include fluoroethylene carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate, and examples of the cyclic ester include gamma-butyrolactone, 2-methyl-gamma-butyrolactone, acetyl-gamma-butyrolactone, and gamma-valerolactone. .. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, and ethyl methyl carbonate, and examples of the chain ester include propionic acid alkyl ester, malonic acid dialkyl ester, and acetate alkyl ester. Examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane. As the non-aqueous solvent, a compound in which some or all of the chemical structure of the specific solvent is replaced with fluorine may be adopted.

As the non-aqueous solvent, it is preferable to use a cyclic carbonate and a chain carbonate in combination. The cyclic carbonate is particularly preferably fluoroethylene carbonate, and the chain carbonate is particularly preferably dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.
The volume ratio of the cyclic carbonate to the chain carbonate is preferably 10:90 to 30:70, more preferably 15:85 to 25:75. By using the cyclic carbonate, the formation of an SEI (Solid electrolyte Interface) film by reduction decomposition at the negative electrode interface can be expected. Further, by using the chain carbonate as the main solvent, the viscosity of the electrolytic solution can be reduced.

If necessary, a separator is used in the lithium ion secondary battery of the present invention. The separator separates the positive electrode and the negative electrode, and allows lithium ions to pass through while preventing a short circuit due to contact between the two electrodes. As the separator, a known one may be adopted, and synthetic resins such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (Aromatic polyamide), polyester and polyacrylonitrile, polysaccharides such as cellulose and amylose, and fibroin , Natural polymers such as keratin, lignin, and suberin, porous materials using one or more electrically insulating materials such as ceramics, non-woven fabrics, and woven fabrics. Further, the separator may have a single layer or a multi-layer structure.

An electrolytic solution containing a high concentration of lithium salt, which is an electrolyte, has a relatively high viscosity. Therefore, among the lithium ion secondary batteries of the present invention, those provided with an electrolytic solution containing a high concentration of lithium salt should adopt a single-layer separator so that lithium ions can move smoothly. preferable.

A specific method for manufacturing the lithium ion secondary battery of the present invention will be described.
A separator is sandwiched between the positive electrode and the negative electrode as needed to form an electrode body. The electrode body may be any of a laminated type in which a positive electrode, a separator and a negative electrode are stacked, or a wound type in which a positive electrode, a separator and a negative electrode are wound. After connecting the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal leading to the outside using a current collecting lead or the like, the electrolytic solution of the present invention is added to the electrode body to add a lithium ion secondary. Use a battery. Further, the lithium ion secondary battery of the present invention may be charged and discharged in a voltage range suitable for the type of active material contained in the electrode.

From the point of view of the present invention, it can be said that the lithium ion secondary battery of the present invention is preferably discharged in a low SOC region. As the low SOC region, a positive electrode potential based on metallic lithium can be exemplified in the range of 3V to 3.6V, in the range of 3.2V to 3.57V, and in the range of 3.4V to 3.55V.
It can be said that the advantageous characteristics of the lithium ion secondary battery of the present invention are effectively exhibited when discharging is performed in a region where the positive electrode potential based on metallic lithium is 3.6 V or less.

The shape of the lithium ion secondary battery of the present invention is not particularly limited, and various shapes such as a cylindrical type, a square type, a coin type, and a laminated type can be adopted.

The lithium ion secondary battery of the present invention may be mounted on a vehicle. The vehicle may be a vehicle that uses electric energy from a lithium ion secondary battery for all or part of its power source, and may be, for example, an electric vehicle or a hybrid vehicle. When a lithium ion secondary battery is mounted on a vehicle, a plurality of lithium ion secondary batteries may be connected in series to form an assembled battery. In addition to vehicles, devices equipped with lithium-ion secondary batteries include various battery-powered home appliances such as personal computers and mobile communication devices, office devices, and industrial devices. Further, the lithium ion secondary battery of the present invention includes wind power generation, solar power generation, hydraulic power generation, other power storage devices and power smoothing devices for electric power systems, power supply sources for power and / or auxiliary machinery such as ships, aircraft, and so on. Power supply sources for spacecraft and / or auxiliary equipment, auxiliary power sources for vehicles that do not use electricity as power sources, power sources for mobile household robots, power sources for system backup, power sources for non-disruptive power sources, It may be used as a power storage device that temporarily stores the electric power required for charging in a charging station for an electric vehicle or the like.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. As long as it does not deviate from the gist of the present invention, it can be carried out in various forms with modifications, improvements, etc. that can be made by those skilled in the art.

Hereinafter, the present invention will be specifically described with reference to production examples, evaluation examples, and the like. The present invention is not limited to these specific examples.

(Manufacturing Example 1)
94 parts by mass of lithium metal composite oxide having a layered rock salt structure represented by LiNi _0.8 Co _0.1 Al _0.1 O ₂ , 3 parts by mass of acetylene black as a conductive auxiliary agent, and 3 parts by mass of polyvinylidene fluoride as a binder. A slurry was prepared by mixing the parts and an appropriate amount of N-methyl-2-pyrrolidone. Aluminum foil was prepared as a current collector for the positive electrode. The above slurry was applied to the surface of the aluminum foil using a doctor blade so as to form a film. N-Methyl-2-pyrrolidone was removed by drying the aluminum foil coated with the slurry. Then, the aluminum foil was pressed to obtain a joint. The obtained bonded product was heated and dried in a vacuum dryer to produce a positive electrode of Production Example 1 in which a positive electrode active material layer was formed.

A copper foil to which lithium foil was bound was prepared, and this was used as the negative electrode. An _{electrolytic solution containing LiPF 6} at 2 mol / L and containing fluoroethylene carbonate and dimethyl carbonate at a volume ratio of 19:81 was prepared. A polyolefin porous membrane was prepared as a separator.

A separator was sandwiched between the positive electrode and the negative electrode of Production Example 1 to form a group of electrode plates. This group of plates was covered with a set of two laminated films, the three sides were sealed, and then the electrolytic solution was injected into the bag-shaped laminated film. Then, by sealing the remaining one side, the four sides were hermetically sealed, and the electrode plate group and the electrolytic solution were sealed to produce the lithium ion secondary battery of Production Example 1.

(Manufacturing Example 2)
As the lithium metal composite oxide, the positive electrode of Production Example 2 was used in the same manner as in Production Example 1 except that a layered rock salt structure represented by _{LiNi 0.82} Co _0.15 Al _0.03 O _{2 was used.} And a lithium ion secondary battery was manufactured.

(Manufacturing Example 3)
As the lithium metal composite oxide, the positive electrode of Production Example 3 was used in the same manner as in Production Example 1 except that a layered rock salt structure represented by _{LiNi 0.87} Co _0.1 Al _0.03 O _{2 was used.} And a lithium ion secondary battery was manufactured.

(Manufacturing Example 4)
As the lithium metal composite oxide, the positive electrode of Production Example 4 was used in the same manner as in Production Example 1 except that a layered rock salt structure represented by _{LiNi 0.88} Co _0.09 Al _0.03 O _{2 was used.} And a lithium ion secondary battery was manufactured.

(Evaluation example 1)
Each lithium ion secondary battery is charged at a constant current of 25 ° C. and 1 C to a positive electrode potential of 4 V based on metallic lithium, and then at a constant current of 1 C, a positive electrode potential based on metallic lithium. It was discharged to 5V. The discharge resistance was calculated from the amount of voltage change and current measured per unit time during discharge.
The resulting graph is shown in FIG. 1, and an enlarged graph for the low potential region is shown in FIG.

From FIG. 1, it can be seen that the discharge resistance in the high potential region is substantially constant regardless of the potential and the nickel composition ratio. From FIGS. 1 and 2, it can be seen that the discharge resistance in the low potential region increases as the potential side increases, and that the lower the nickel composition ratio, the greater the degree of increase in the discharge resistance in the low potential region.
It can be said that the positive electrode active material satisfying the nickel composition ratio of 0.85 ≦ b ≦ 0.9 is suitable for discharge in the low potential region.

(Evaluation example 2)
In this evaluation example, the technical significance of the present invention will be supplementarily described.
FIG. 3 shows the charge / discharge curve of the positive electrode of Production Example 2 and the charge / discharge curve of the negative electrode provided with the Si-containing negative electrode active material in which lithium is not introduced in advance. The negative electrode provided with the Si-containing negative electrode active material was manufactured as follows.

72.5 parts by mass of carbon-coated silicon material, 13.5 parts by mass of acetylene black as a conductive aid, 14 parts by mass of a mixture of polyacrylic acid and 4,4'-diaminodiphenylmethane as a binder, and , An appropriate amount of N-methyl-2-pyrrolidone was mixed to prepare a slurry. A copper foil was prepared as a current collector for the negative electrode. The slurry was applied in a film form on the surface of the copper foil using a doctor blade. N-methyl-2-pyrrolidone was removed by drying the copper foil coated with the slurry to produce a negative electrode on which a Si-containing negative electrode active material layer was formed. The mixture of polyacrylic acid used as a binder and 4,4'-diaminodiphenylmethane was a crosslinked polymer in which the dehydration reaction proceeded by drying and the polyacrylic acid was crosslinked with 4,4'-diaminodiphenylmethane. Changes to.

In FIG. 3, the broken line in the vertical direction corresponds to SOC 0%, SOC 15%, and SOC 100% from the left. Here, SOC 0% means a point where the voltage between the positive electrode and the negative electrode is 3 V, and SOC 100% means a point where the voltage between the positive electrode and the negative electrode is 4.2 V.

From the charge / discharge curve of the negative electrode in FIG. 3, it can be understood that the irreversible capacitance of the silicon material is large. It can also be seen that most of the lithium ions supplied from the positive electrode active material in the initial charge are consumed as the irreversible capacity of the silicon material. As a result, the region where the positive electrode potential is low cannot contribute to charging / discharging from SOC 0% to SOC 100%, which is the actual capacity. Therefore, it can be said that it is extremely difficult to secure a high capacity in a lithium ion secondary battery including a negative electrode active material in which lithium is not introduced in advance.

FIG. 4 shows a charge / discharge curve of the positive electrode of Production Example 2 when operated in a lithium ion secondary battery including a negative electrode active material into which lithium has been introduced in advance.
In addition, in FIG. 4, for easy understanding, the charge / discharge curve of the negative electrode provided with the Si-containing negative electrode active material in which lithium is not introduced in advance is also shown.

In the charge / discharge curve of the positive electrode of FIG. 4, most of the irreversible capacity of the negative electrode active material is canceled by introducing lithium in advance, so that high capacity charge / discharge is guaranteed. That is, even in the region where the positive electrode potential is low, it is possible to contribute to charging / discharging from SOC 0% to SOC 100%, which is the actual capacity.
Since the irreversible capacitance of the negative electrode active material is canceled and the region where the positive electrode potential is low can also contribute to normal charging and discharging, the magnitude of the discharge resistance in the low potential region of the positive electrode potential has emerged as an issue. The lithium ion secondary battery of the present invention solves this problem.

Claims (2)

As a cathode active material, the general formula of the layered rock salt structure: LiNi _b Co _c Al _d In O ₂ (the general formula, 0.85 ≦ b ≦ 0.9,0 <c <0.15,0 <d <0. 15. A positive electrode comprising a lithium metal composite oxide represented by b + c + d = 1) and
A negative electrode equipped with a Si-containing negative electrode active material in which lithium has been introduced in advance, and a negative electrode
Per lithium ion secondary battery in which the concentration of the lithium salt comprises a electrolytic solution is 1.5~4mol / L,
A method for controlling a lithium ion secondary battery in which discharge is performed in a region where the positive electrode potential based on metallic lithium is 3.6 V or less . The method for controlling a lithium ion secondary battery according to claim 1, wherein b satisfies 0.86 ≦ b ≦ 0.89.

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